Composites of Spherical Polyelectrolyte Brushes and Nanoparticles – Synthesis, Characterization and Their Use in Catalysis

Composites of Spherical Polyelectrolyte Brushes and Nanoparticles – Synthesis, Characterization and Their Use in Catalysis DISSERTATION Zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) im Fach Chemie der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth Vorgelegt von Frank Polzer Geboren in Erlangen, Deutschland Bayreuth 2011 I Table of Contents 1.Introduction ................................................................................................ 1 1.1.Manganese Oxide Nanoparticles ............................................................................. 2 1.2.Nanoparticles in Catalysis ........................................................................................ 4 1.3.Stabilization of Nanoparticles in Solution ............................................................... 7 1.4.Spherical Polyelectrolyte Brushes for the Stabilization of Nanoparticles in Solution .................................................................................................................... 8 1.5.X-Ray Absorption Fine Structure Spectroscopy on Nanosized Materials ............. 10 1.6.Cryogenic Transmission Electron Microscopy on Nanosized Materials ............... 13 1.7.Objective of this Thesis ......................................................................................... 14 1.8.References .............................................................................................................. 15 2.Overview ................................................................................................... 23 2.1.Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes.......................................................................................... 25 2.2.Structural Analysis of Colloidal MnOx Composites .............................................. 27 2.3.Catalytic Oxidation of an Organic Dye by MnOx Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes ......................................................................... 29 2.4.Kinetic Analysis of the Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes ......................... 31 2.5.Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution ................................................................................................... 33 2.6.Individual Contributions to Joint Publications ...................................................... 35 3.Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes .......................................................... 39 3.1.Abstract .................................................................................................................. 40 3.2.Introduction ............................................................................................................ 41 3.3.Experimental Section ............................................................................................. 42 3.4.Results and Discussion .......................................................................................... 43 3.5.Conclusions ............................................................................................................ 50 3.6.Acknowledgements ................................................................................................ 50 II 3.7.Supporting Information .......................................................................................... 51 3.8.References .............................................................................................................. 52 4.Structural Analysis of Colloidal MnOx Composites ............................. 57 4.1.Abstract .................................................................................................................. 58 4.2.Introduction ............................................................................................................ 59 4.3.Experimental Section ............................................................................................. 60 4.4.Results and Discussion .......................................................................................... 62 4.5.Conclusion ............................................................................................................. 75 4.6.Acknowledgements ................................................................................................ 76 4.7.Supporting Information .......................................................................................... 76 4.8.References .............................................................................................................. 77 5.Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles ...... 83 5.1.Abstract .................................................................................................................. 84 5.2.Introduction ............................................................................................................ 85 5.3.Experimental Section ............................................................................................. 87 5.4.Results and Discussion .......................................................................................... 88 5.5.Conclusions ............................................................................................................ 98 5.6.Acknowledgements ................................................................................................ 98 5.7.Supporting information .......................................................................................... 99 5.8.References ............................................................................................................ 100 6.Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes .... 103 6.1.Abstract ................................................................................................................ 104 6.2.Introduction .......................................................................................................... 105 6.3.Experimental Section ........................................................................................... 107 6.4.Results and Discussion ........................................................................................ 108 6.5.Conclusion ........................................................................................................... 117 6.6.Acknowledgment ................................................................................................. 117 6.7.References ............................................................................................................ 117 7.Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution ................................................................ 121 III 7.1.Abstract ................................................................................................................ 122 7.2.Introduction .......................................................................................................... 123 7.3.Experimental Section ........................................................................................... 124 7.4.Results and Discussion ........................................................................................ 126 7.5.Conclusion ........................................................................................................... 134 7.6.Acknowledgements .............................................................................................. 135 7.7.Supporting Information ........................................................................................ 135 7.8.References ............................................................................................................ 137 8.Summary/Zusammenfassung ............................................................... 141 Summary ............................................................................................................................. 141 Zusammenfassung .............................................................................................................. 142 AList of Publications ................................................................................ 145 A1.Publications of this Thesis ................................................................................... 145 A2.Publications as a co-Author ................................................................................. 145 A3.Patents submitted during the Course of the Thesis .............................................. 146 BPresentations at International Conferences and Meetings ................ 147 CAbbreviations ......................................................................................... 149 DDanksagung ............................................................................................ 151 ESchlusserklärung .................................................................................... 153 IV V Die vorliegende Arbeit wurde in der Zeit von August 2007 bis Februar 2011 in Bayreuth am Lehrstuhl Physikalische Chemie I und am Helmholtz Zentrum Berlin für Materialien und Energie unter Betreuung von Herrn Prof. Dr. Matthias Ballauff angefertigt. Vollständiger Abdruck der von der Fakultät Biologie, Chemie und Geowissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) Dissertation eingereicht am: 16. Februar 2011 Tag der Zulassung durch die Prüfungskommission: 25. Juni 2011 Tag des wissenschaftlichen Kolloquiums: 05. Juli 2011 Amtierender Dekan: Prof. Dr. Stephan Clemens Prüfungsausschuss: Prof. Dr. Matthias Ballauff (Erstgutachter) Prof. Dr. Josef Breu (Zweitgutachter) Prof. Dr. Axel H. E. Müller (Vorsitz) Prof. Dr. George Papastavrou VI VII Well there’s those that do And those that just do talking We’re all going through hell It’s burn or keep on walking Randy Blythe Manganese Oxide Nanoparticles 2 shape.19,20,21 This fact has directly led to a huge number of studies investigating the control of the dimensions as well as of the morphology of nanoparticles.22,23,24 In principle, nanoparticles and their assemblies can be subdivided according to their dimensionality into 1D, 2D and 3D particles.25 Due to the huge variety of different nanoparticular materials available, the characteristics of these particles differ strongly. In general, the properties can be divided into three groups. Surface-dependent properties are properties the bulk material also possess but that become dominant at high surface-to-volume ratios, e.g. for catalytic applications of nanoparticles (see Figure 1.0.2a). Second, size-dependent properties that are directly related to the small size, e.g. in Bragg stacks or in photonic crystals.26,27 Lastly, there exist size-dependent quantum effects. This means a direct influence of the size of nanoparticles on their electronic structure, e.g. the size-dependent surface plasmon shift in metal nanoparticles (Figure 1.0.2b).28,29,30 (a) (b) Figure 1.0.2. (a) Dependence of the size of gold nanoparticles onto the ratio of inner atoms to atoms at the surface.31 (b) Illustration of the influence of the size of metallic nanoparticles onto the density of states. Particles with sizes between the size of small molecules and that of bulk metal display electronic structures, reflecting the electronic band structure of the nanoparticles, owing to quantum-mechanical rules.32 Applications of nanoparticles are wide-ranging and include medical applications like drug delivery33 and sensing34, coatings35, waste water treatment36, energy conversion37 and storage38 and many others more.39 Besides all of these interesting applications, the use of nanoparticles in catalysis seems to be one of the most promising.40,41,42 This will be discussed further in section 1.2. 1.1. Manganese Oxide Nanoparticles Manganese oxide nanoparticles include all common oxidation states of manganese, such as MnO, Mn2O3, Mn3O4, MnO2 and also mixed valent compounds (MnOxNP).43,44,45,46,47,48 There exist numerous reports about the synthesis of all different kinds of MnOxNP including 1D, 2D Introduction 3 and 3D structures.49 Applications of manganese oxide nanostructured materials mostly focus on the use as electrode materials for energy storage, for magnetic data storage, as ion exchange materials, and for catalysis.50,51,52,53 With its wide range of polymorphs, MnO2 has received the most attention among the different manganese oxides. All different polymorphs are built from the same basic unit, that is the MnO6 octahedra (see Figure 1.1.1a). Due to the different linkage of the MnO6 octahedra, α-, β-, γand δ-MnO2 also possess distinct properties.54 δ-MnO2 consists predominantly of edge-sharing MnO6 units that form a layered topology similar to that of clay minerals like silicates or aluminosilicates.55 One of the most intensely studied compounds in the family of layered manganese oxides is birnessite.56,57,58,59,60 This phyllomanganate is composed of hexagonal sheets of predominantly edge-shared MnO6 octahedra.61,62 Typically, the interlayers between single sheets bear hydrolyzable cations.63 The cations balance the negative charges of the sheets.64 These negative surface charges are due to the presence of vacancies within the ab-plane of the sheets.65,66 Furthermore, a net charge can arise by the incorporation of a certain amount of corner-shared Mn3+ octahedra.67,68 The intercalation can be used for the sorption of other positively charge compounds which directly leads to applications in waste water treatment.69,70 In addition, the cation exchange of alkali metal ions by more bulky cations, e.g. organic cations, surfactants, polyelectrolytes, etc., leads to a swelling of the interlayer distance in aqueous solution.71,72,73 This effect can be used for adjacent delamination of the hexagonal sheets.74,75 Exfoliation refers to the generation of single layers of sheets which is mostly achieved by multi-step processes.76,77,78,79 There exist only a few reports on the synthesis of single lamellae of layered manganese oxides, e.g. birnessite.80,81 The generation of single layered nanosheets is desirable because of their large specific surface area.80 The surface can directly serve as a highly active catalyst or it can be further functionalized by organic ligands. Furthermore, octahedral layered (OL) manganese oxides can be used as building blocks for layer-by-layer self-assemblies and pillared MnOx structures.82,83 OL manganese oxides can also be converted into 1D tunnel structures that are termed octahedral molecular sieves (OMS).84,85,86 Some common crystallographic structures of these materials are shown in Figure 1.1.1. OMS and OL materials offer a broad range of different properties depending on their architecture, the average oxidation state and the interlayer cations.87 These parameters are very sensitive to the synthetic route and proved to have major impact on the catalytic properties.88,89,90 Nanoparticles in Catalysis 4 (a) (b) (c) (d) (e) (f) Figure 1.1.1. Crystallographic structures of different OL and OMS structures. (a) Structure of MnO6 octahedra (Mn blue sphere, O red sphere) which is the building block of all OL and OMS materials. (b) Na-Birnessite (Na red sphere); (c) Pyrolusite; (d) K-Cryptomelane (K grey sphere); (e) Mg-Todorokite (Mg brown sphere) and (f) Ba-Romanechite (Ba green sphere). 1.2. Nanoparticles in Catalysis Nanoparticles made their breakthrough as catalyst materials after Haruta et al.91,92 reported on the high performance of gold nanoparticles on the CO oxidation in the late 1980s. At the same time Hutchings et al. discovered the activity of gold nanoparticles for the hydrochlorination of acetylene.93 Since then, a vast amount of studies have been conducted on the catalytic activity of nanoparticles.9,94 Those include most kinds of nanoparticles and different catalytic reactions, e.g. oxidation reactions, hydrogenation reactions, reductions, coupling reactions, photocatalytic reactions, etc.11,33,42 Introduction 5 Besides the numerous works on applied catalysis with nanoparticles many studies have been conducted to elucidate the activity of nanosized matter. Hence, different kinds of model reactions for detailed investigations have been established. These include reduction or oxidation reactions of a variety of different dye molecules which enables in situ investigations by UV/visible spectroscopy (UV/vis).95,96,97 One aspect of these studies was to elucidate the size-dependence of the catalytic activity of nanoparticles, e.g. gold nanoparticles.98,99,100 Another focus has been laid on the influence of the shape of nanoparticles on their catalytic activity.101,102 Though there exists a high number of different investigations, there is still a lack of comprehension of the mechanism of reactions catalyzed by nanoparticles. In principle, surface catalyzed reactions can be divided into two distinct mechanisms: the Eley-Rideal mechanism (ER) and the Langmuir-Hinshelwood mechanism (LH).103 The basic catalyst cycle for a LH mechanism is depicted in Figure 1.2.1. Figure 1.2.1. Illustration of a catalytic cycle of a bimolecular surface reaction via a Langmuir-Hinshelwood mechanism. Reactant A and B concomitantly adsorb onto the catalyst surface in a reversible step (a & b) which is followed by an irreversible bimolecular surface reaction of A and B (c). The reaction product P desorbs from the catalyst surface (d) and leaves free active sites on the catalyst (e). In the end, the catalytic cycle can start again by the adsorption of further reactants (f & a). Nanoparticles in Catalysis 6 The main characteristic of LH reactions is that both reactants have to be adsorbed onto the catalyst surface.104 The adsorption process is described by a Langmuir isotherm = 1+ (1.2.1) where θi is the surface coverage of the reactant i, Ki represents the adsorption constant of i and ci its concentration respectively.105 The adsorbed reactants undergo a surface reaction, which is the rate determining step of the cycle. Subsequently the reaction product desorbs from the catalyst surface, leaving free sites on the catalyst for the start of a new cycle. This is schematically depicted in Figure 1.2.1. In contrast, in an ER mechanism, only one of the reactants is adsorbed onto the catalyst surface. This species reacts via a collision with a molecule from the bulk phase. There exist marked differences between the two mechanisms concerning the dependency of concentration of the reactants on the rate of reaction (r). For a LH mechanism, the scheme in Figure 1.2.2 shows the course of r with increasing concentration of reactant A. At a low surface coverage θ of A, the surface is predominantly occupied by B and the reaction between A and B is hindered. The curve goes through a maximum, since the adsorption of A becomes more likely with an increasing concentration. At higher concentrations of A, r is decreasing again since the surface is blocked by this species and surface reaction between A and B is hindered again (see Figure 1.2.2).103 Figure 1.2.2. Illustration of the dependence of the concentration of reactant A on the rate of the reaction r according to a LH mechanism assuming one type of active site. If the surface is predominantly occupied by one of the species, the surface reaction is hindered and therefore r decreases. Since the surface coverage θ is determined by Langmuir isotherms it depends on the concentration of the reactants and their adsorption constants, respectively. An irreversible, bimolecular surface reaction where both reactants are adsorbed onto the surface (see Figure 1.2.2) can be described by the following equation:103 Introduction 7 = ()() (1++) (1.2.2) Here, S represents the total surface area of the catalyst, KA and KB are the adsorption constants for A and B. The concentrations of A and B are expressed by c A and cB, respectively. The kinetic constant of the surface reaction of A and B is expressed by k. For an ER mechanism there is no such dependence described by Eq. 1.2.2. In fact, the course follows that of a Langmuir isotherm since at some point an increase of B does not lead a further increase of r. This is because B has to react with absorbed A and if the concentration of B exceeds the concentration of adsorbed A, neither an increase nor a decrease of r follows.103 The number of studies investigating the mechanism of catalyzed benchmark reactions with nanoparticles is scarce so far.106,107 The main reasons for that are the limited stability and the detailed characterization of small nanoparticles.41 Therefore, new ways for the stabilization of very small nanoparticles in solution are required. It would be an asset if the stabilizing agent did not block the active surface of the nanoparticles since this would significantly affect the catalytic studies.108 If these requirements are fulfilled, more detailed insights of the catalytic mechanisms will be possible in combination with state of the art characterization techniques. 1.3. Stabilization of Nanoparticles in Solution Since most of the methods for the generation of nanoparticles are conducted in aqueous medium or in an organic solvent, the stabilization of these particles against coagulation is of great importance.19,109 If no stabilization is provided, uncontrolled growth with subsequent Ostwald ripening will occur. This leads to agglomeration and precipitation. The agglomeration is caused by van der Waals forces that lead to an attraction of particles at short particle distances. If no repulsive forces are present nanoparticles tend to aggregate.110 The stabilization mechanism of surface charged colloidal particles is described by the theory of Derjaguin, Landau, Verwey and Overbeek (DLVO theory).111,112,113 For most applications, agglomeration leads to a loss of the functionality of nanoparticles due to the strong relation between the properties of the nanoparticles to their size and shape, e.g. in catalysis.19 In general, nanoparticles can be stabilized in two different ways. Either the nanoparticles are modified at their surface or they are immobilized onto support particles that provide sufficient stabilization against coagulation.40 For both principles two different kinds of stabilization mechanisms and a combination of both can be distinguished. A schematic representation of electrostatic, steric and electrosteric stabilization is given in Figure 1.3.1.40,114 Spherical Polyelectrolyte Brushes for the Stabilization of Nanoparticles in Solution 8 Figure 1.3.1. Illustration of the different principles of stabilizing colloidal particles. (a) Electrostatic stabilization by charges that are either chemically bound on the surface (surface functionalization) or affixed by selective adsorption. (b) Steric stabilization can either be achieved by adsorption or by grafting of polymer chains onto the colloidal particles. (c) Electrosteric stabilization of colloidal particles by charged polymer chains represents a combination of both stabilization mechanisms. As already mentioned, nanosized materials can also be stabilized by the immobilization onto support particles. This can be achieved either by adsorption of the nanoparticles or by in situ generation of the nanoparticles on/inside of the support. The principles of the stabilization mechanisms of the support particles remain identical to those discussed in the section above. If nanoparticles are immobilized onto support particles, so-called synergistic effects (also support effects) have to be taken into account.115 This term is related to an enhancement of any distinct property of the nanoparticles due to the interactions with the support material.116,117 1.4. Spherical Polyelectrolyte Brushes for the Stabilization of Nanoparticles in Solution A spherical polyelectrolyte brush (SPB) is a spherical, solid particle onto which long polyelectrolyte chains are densely grafted (see Figure 1.4.1).118 Hereby, the distance between two neighboring grafted chains has to be lower than the chains radius of gyration Rg in a good solvent.119,120 Figure 1.4.1 shows a schematic illustration of a SPB with its main characteristic parameters. Introduction 9 Figure 1.4.1. Illustration of a spherical polyelectrolyte brush. The core consists of a hydrophobic polymer, e.g. poly(styrene), onto which polyelectrolyte chains are densely grafted. Rcore represents the radius of the core particle, Rh is the hydrodynamic radius of the SPB, L is the hydrodynamic thickness of the shell layer (= Rh - Rcore) and D is the distance between the grafted chain ends. The polyelectrolyte chains are strongly stretched in aqueous solution due to the osmotic pressure inside of the shell layer. This can be seen in the cryoTEM micrograph of anionic SPBs with a pSS shell.121 In principal, SPBs can be classified by the type of polyelectrolyte present in the shell. An annealed SPB is a brush particle with a weak polyelectrolyte forming the shell, e.g. poly(acrylic acid). In contrast, the shell of a quenched SPB consists of a strong polyelectrolyte, e.g. poly(sodium styrene sulfonate). The difference of the two classes of SPB is that the charge density of an annealed SPB can be influenced by changing the pH, whereas for a quenched SPB no pH dependence is present.118 The high electrostatic interaction of the densely grafted polyelectrolyte chains leads to a number of new properties in comparison to uncharged grafted macromolecules. An essential characteristic is given by the confinement of the counter ions of the polyelectrolyte chains within the shell layer.118,122,123 This leads to a swelling of the polyelectrolyte shell due to the high osmotic pressure of the confined counter ions in salt free solution. The effect of the confinement of the counter ions has successfully been used for the generation of nanoparticles within the shell of the SPB.124 Charged molecular precursors can be introduced into the shell by a controlled exchange of the counter ions. In a second step, the precursors can be reduced to nanoparticles.125,126 The ion exchange can either be enhanced by the introduction of multivalent counter ions or by ions that possess specific interactions with the charged groups of the shell. After reduction of the charged precursors, nanoparticles are directly generated and immobilized within the shell of the SPB. Using this approach, a number of different noble metal nanoparticles and their alloys have been successfully synthesized.127,128,129,130 X-Ray Absorption Fine Structure Spectroscopy on Nanosized Materials 10 This method proved to have a number of advantages. In general, the nanoparticles generated in SPBs are strongly bound to the the carrier particle and are of small size. The small size evidently leads to a high surface to volume ratio whereas the immobilization helps to prevent the uncontrolled release of nanoparticles into the surrounding media.124 Therefore, the immobilization of nanoparticles onto colloidal stable particles simplifies the handling of nanoparticles, which is one of the main purposes of so-called mesostructured materials. In particular, this becomes important because nanosized materials are suspected to cause harm to living tissue. Furthermore, the immobilization of nanoparticles on colloidal particles is advantageous for applications in catalysis because it helps to separate the catalyst from the reaction solution, e.g. by filtration.40,124 This improves the recyclability of the catalytic active composite particles and also it prevents the leaching of nanoparticles in the reaction products.40 The catalytic activity of composite materials of nanoparticles at SPBs has been shown in various studies.124,126 These include hydrogenation reactions, oxidation and epoxidation reactions as well as coupling reactions like the Heck-Suzuki reaction.131,132,133,134 1.5. X-Ray Absorption Fine Structure Spectroscopy on Nanosized Materials The characterization of nanoparticles is sometimes challenging, especially if they are immobilized onto support particles. This is due to the fact that nanoparticles synthesized at mild conditions are often highly disordered materials exhibiting poor diffraction patterns obtained by PXRD.135 In addition, the small particle size leads to a broadening of the reflections of the lattice planes. Furthermore, the presence of support particles is often accompanied by a high amorphous background of the composite material which complicates investigations by methods like PXRD or high resolution TEM (HRTEM). Due to these difficulties, additional methods should be considered for the analysis of nanostructured composite materials. Since X-ray fine structure (XAFS) measurements can be made on elements of minority and even trace abundance it provides a unique and direct measurement of the chemical and physical state of dilute species in a variety of systems. XAFS spectra can be measured for essentially every element on the periodic table. Importantly, no long range order within the compounds is required for XAFS measurements. This makes XAFS one of the few structural probes available for non-crystalline and highly disordered materials, even including solutions. XAFS refers to the details of how X-rays are absorbed by an atom at energies near and above the core-level binding energies of that atom. These spectra are especially sensitive to the coordination chemistry, formal oxidation state, and the distances, coordination number and species of the atoms immediately surrounding the selected element. Because of this dependence, XAFS provides a practical way to determine the chemical state and local atomic structure for a selected atomic species.136 The X-ray absorption spectrum is typically divided into two regimes: The X-ray absorption near-edge spectroscopy (XANES) with its typical range of up to 100 eV from the absorption Introduction 11 edge. Secondly, the extended X-ray absorption fine structure spectroscopy (EXAFS) spanning a range from the XANES region up to 1000 eV above the absorption edge. The two regions have the same physical origin, but a distinction is convenient for the interpretation. Whereas XANES is strongly sensitive to the formal oxidation state and the coordination chemistry of the absorbing atom, the EXAFS is used to investigate the distances, coordination number, and species of the neighbors of the absorbing atom.137,138 In simple terms, for XAFS the dependence of the energy on the absorption coefficient µ at and above the binding energy of a known core level of a known atomic species is measured. According to Lambert-Beers’ law, µ is related to the X-ray intensity as follows: = µ  (1.5.1) Here, I0 represents the incident X-ray intensity, I is the transmitted X-ray intensity and t is the thickness of the sample. The absorption coefficient is related to the energy of the X-ray beam E by µ≈   (1.5.2) where ρ and Z are the sample density and the atomic number of the absorbing element. A represents the atomic mass of the absorber. Due to the Z4 dependence, µ is very sensitive to the chemical nature of the absorber.136 For EXAFS, the oscillation above the absorption edge is of major interest and the EXAFS function can be written as:  ()=µ()−µ() Δµ(E) (1.5.3) µ(E) is the measured absorption coefficient, µ0(E) is a smooth background function representing the absorption of an isolated atom, and Δµ0 is the measured jump in the absorption µ(E) at the threshold energy E0. χ(E) is converted into χ(k) because the absorption process of EXAFS is treated best by the wave behavior of the photoelectron. 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W.; Breu, J.; Ballauff, M. J. Phys. Chem. B 2007, 111, 7676-7681. 128 Schrinner, M.; Polzer, F.; Mei, Y.; Lu, Y.; Haupt, B.; Göldel, A.; Drechsler, M.; Preussner, J.; Glatzel, U.; Ballauff, M. Macromol. Chem. Phys. 2007, 208, 1542-1547. 129 Mei, Y.; Lu, Y.; Polzer, F.; Drechsler, M.; Ballauff, M. Chem. Mater. 2007, 19, 10621069. References 20 130 Schrinner, M.; Möller, M.; Thun, J.; Kauffmann, Y.; Breu, J.; Talmon, Y.; Ballauff, M. Science 2009, 323, 617-620. 131 Sharma, G.; Mei, Y.; Lu, Y.; Ballauff, M.; Irrgang, T.; Kempe, R. J. Catal. 2007, 246, 1014. 132 Proch, S.; Mei, Y.; Rivera Villanueva, J. M.; Lu, Y.; Karpov, A.; Ballauff, M.; Kempe, R. Adv. Synth. Catal. 2008, 350, 493-500. 133 Schrinner, M.; Proch, S.; Mei, Y.; Kempe, R.; Miyajima, N.; Ballauff, M. Adv. Mater. 2008, 20, 1928-1933. 134 Malysheva, Y. B.; Gushchin, A.V.; Mei, Y.; Lu, Y.; Ballauff, M.; Proch, S.; Kempe, R.; Eur. J. Inorg. Chem. 2008, 3, 379-383. 135 Hara, D.; Shirakawa, J.; Ikuta, H.; Uchimoto, Y.; Wakihara, M.; Miyanaga, T.; Watanabe, I. J. Mater. Chem. 2003, 13, 897-903. 136 Kelly, S. D.; Hesterberg, D.; Ravel, B. Analysis of soils and minerals using X-ray absorption spectroscopy. Methods of soil analysis, Part 5 - Mineralogical methods., (Eds.: Ulery, A. L. and Drees, L. R.); Soil Science Society of America: Madison, WI, 2008. 137 Stöhr, J. NEXAFS Spectroscopy; Springer Verlag: Berlin, 1992. 138 Koningsberger, D. C.; Prins, R. X-ray Absorption: Principles, Applications, Techniques of EXAFS SEXAFS, and XANES; Wiley: New York, 1988. 139 Kobayashi, S.; Kottegoda, R. M.; Uchimoto, Y.; Wakihara, M. J. Mater. Chem. 2004, 14, 1843-1848. 140 Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Tech. 1988, 10, 87-111. 141 Almgren, M.; Edwards, K.; Karlsson, J. Colloids Surf. A 2000, 174, 3-21. 142 Talmon, Y. in Modern Characterization Methods of Surfactant Systems (Ed.: Binks, B. P.); Marcel Dekker Inc: New York, 1999. 143 Almgren, M.; Edwards, K.; Gustafsson, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 270278. 144 Frederik, P. M.; Sommerdijk, N. Curr. Opin. Colloid Interface Sci. 2005, 10, 245-249. 145 Bremer, A.; Henn, C.; Engel, A.; Baumeister, W.; Aebi, U. Ultramicroscopy 1992, 46, 85111. 146 Nudelmann, F.; de With, G.; Sommerdijk, N. A. J. M. Soft Matter 2011, 7, 17-24. 147 Crassous, J. J.; Rochette, C. N.; Wittemann, A.; Schrinner, M.; Drechsler, M.; Ballauff, M. Langmuir 2010, 25, 7862-7871. 148 Friedrich, H.; Frederik, P. M.; de With, G.; Sommerdijk, N. A. J. M. Angew. Chem. Int. Ed. 2010, 49, 7850-7858. 149 Dubochet, J.; Mc Dowall, A. W. J. Microsc. 1981, 124, 3-4. 150 Dubochet, J.; Chang, J. J.; Freeman, R.; Lepault, J.; Mc Dowall, A. W. Ultramicroscopy 1982, 10, 55-61. Introduction 21 151 Falls, A. H.; Wellinghoff, S. T.; Talmon, Y.; Thomas, E. L. J. Mater. Sci. 1983, 18, 27522764. 152 Vinson, P. K.; Bellare, J. R.; Davis, H. T.; Miller, W. G.; Scriven, L. E. Colloid Interface Sci. 1991, 142, 74-91. 153 Cui, H.; Hodgdon, T. K.; Klaer, E. W.; Abezgauz, L.; Danino, D.; Lubovsky, M.; Talmon, Y.; Pochan, D. J. Soft Matter 2007, 3, 945-955. 154 Crassous, J. J.; Wittemann, A.; Siebenbürger, M.; Schrinner, M.; Drechsler, M.; Ballauff, M. Colloid Interface Sci. 2008, 286, 805-812. 155 Samokhina, L.; Schrinner, M.; Ballauff, M. Langmuir 2007, 23, 3615-3619. 156 Lu, Y.; Spyra, P.; Mei, Y.; Pich, A.; Ballauff, M. Macromol. Chem. Phys. 2007, 208, 254261. 157 Lu, Y.; Mei, Y.; Walker, R.; Drechsler, M.; Ballauff, M. Polymer 2006, 47, 4985-4995. References 22 Overview 23 2. Overview The main objective of this thesis was the synthesis of MnOxNP and to immobilize these nanoparticles onto a colloidal stable carrier particle. The support particles consist of a PS core particle with a typical diameter of about 100 nm onto which long polyelectrolyte chains are chemically grafted. These systems are denoted as SPBs and possess a high colloidal stability in aqueous solution.  In Chapter 3, a detailed description of the synthesis of the composite material is presented. In addition, the composite material was characterized extensively by TEM and cryoTEM to elucidate its structure. A comparison of PXRD patterns of different reference compounds to the diffraction pattern of the composite material gave first insights into the crystallographic structure of the MnOxNP immobilized on SPBs which led to the development of a model for the composite system.  Chapter 4 is dedicated to a detailed analysis of the crystallographic structure of the MnOxNP by XAFS measurements to overcome the problems of the missing long-range order of the nanoparticles which hampered the characterization by PXRD. Special emphasis has been laid on the local structure of the MnOxNP around the Mn absorber with regards to differences of the structure of the composite material in the dried and in the dispersed state. Additionally, a new kind of composite material composed of starshaped pTMAEMC homopolymer and MnOxNP was synthesized and characterized.  The composite material of MnOxNP immobilized on SPBs was tested for its catalytic activity on the oxidation of morin by hydrogen peroxide which is presented in Chapter 5. Therefore, the kinetic model of a Langmuir-Hinshelwood mechanism for heterogeneous catalyzed reactions has been applied to the oxidation reaction.  In analogy to that, Chapter 6 deals with the analysis of the reduction of 4-nitrophenol by sodium borohydride in the presence of composite particles of SPB and gold and platinum nanoparticles. A Langmuir-Hinshelwood model was applied to this catalytic reaction. Furthermore, the induction period observed during the investigations was analyzed in detail and could be subscribed to a surface reconstruction of the nanoparticles.  Chapter 7 presents the synthesis and characterization of novel SPB particles with a zwitterionic shell. The synthesis of the shell was conducted by aqueous ATRP. A combination of DLS, TEM and cryoTEM measurements lead to the conclusion that the zwitterionic shell of pMEDSAH is predominantly in a collapsed state with a minor part of the chains reaching out of the collapsed layer. This has been assigned to an internal phase separation of the zwitterionic shell. Temperatureand salt-dependent DLS Overview 24 measurements proved the responsive behavior to external stimuli of the zwitterionic shell that leads to a swelling of the latter. This doctoral thesis comprises five publications given in the Chapters 3, 4, 5, 6 and 7. Overview 25 2.1. Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes We studied the in-situ generation of MnOxNP on cationic SPBs with a shell of poly(2trimethyl ammoniumethyl methacrylate chloride) (pTMAEMC). First, we investigated the synthesis of the MnOxNP by varying the parameters like the amount of added KMnO4, the speed of the addition and the temperature. This study showed that no reducing agent is needed to generate the MnOxNP. In addition, the reduction by the monomer units of the polyelectrolyte shell could be ruled out. Based on these findings we developed the mechanism of a basic catalyzed reduction of the MnO4ions within the brush layer. Furthermore, DLS measurements showed that the brush thickness decreases upon the immobilization of the MnOxNP indicating the interaction of the negatively charged nanoparticles with the positively charged polyelectrolyte chains of the shell. This was confirmed by zeta potential measurements and proves the excellent stabilization of the MnOxNP by the pTMAEMC shell. A comparison of the composite particles by TEM and cryoTEM revealed that there is a significant difference in the morphology of the MnOxNP immobilized onto SPBs between the dried state and the aqueous dispersed state (see Figure 2.1.1). This was assigned to a collapse of the ultrathin platelet-like nanoparticles onto the PS core of the SPB upon drying (see Figure 2.1.1a). In contrast to that, the MnOxNP are also present in the polyelectrolyte shell of the SPB which is confirmed by the cryoTEM micrograph in Figure 2.1.1b. (a) (b) Figure 2.1.1. (a) TEM micrograph and (b) cryoTEM micrograph of the composite material TMAEMC-MnOx-5 comprised of cationic SPB particles and MnOxNP. Marked differences of the composite materials exist between the dried state and the aqueous dispersed state as can be seen by comparing the two micrographs. Overview 26 A comparison of the PXRD patterns of the composite material to reference compounds, e.g. H + -birnessite and K + -birnessite, lead to the conclusion that the MnO x NP are composed of hexagonal lamellae of birnessite (see Figure 2.1.2). This mixed valent manganese oxide possesses a layered topology and is composed of predominantly edge-shared MnO 6 octahedra. The missing 00l reflections of the composite material in combination with the electron micrographs prove that the MnO x NP are composed of single or only a few stacks of lamellae of birnessite. Additionally, the hk bands at 36.5° and 65° in 2θ prove a disorder along the stacking axis of the lamellae. (a) (b) Figure 2.1.2. (a) PXRD patterns of the composite material TMAEMC-MnO x -5, H + -birnessite and K + -birnessite. The missing 00l reflections and the hk bands at 36.5° and 65° in 2θ in combination with the electron micrographs prove the generation of ultrathin platelet-like MnO x NP exhibiting a disorder along the c-axis. (b) Schematic model of the composite material TMAEMC-MnO x -5. The negatively charged MnO x NP are stabilized by the cationic pTMAEMC chains. The MnO x NP are composed of edge-shared MnO 6 octahedra (Mn white spheres, O red spheres) that form into layers with intercalated K + -ions and water molecules (blue spheres) between the layers. The full publication can be found in Chapter 3. Overview 27 2.2. Structural Analysis of Colloidal MnOx Composites This work reports the first synthesis of MnOxNP with a layered topology stabilized by starshaped pTMAEMC homopolymer. The reduction of KMnO4 was started by the addition of 2butanol since no in-situ generation of MnOxNP was observed, as found for the synthesis of MnOxNP@SPB particles. The disk-like nanoparticles exhibit an average diameter of about 5 nm as shown in the HRTEM micrograph in Figure 2.2.1. PXRD measurements reveal a similar structure of the MnOxNP stabilized by star-shaped pTMAEMC compared to that of the composite particles described in Chapter 2.1. Figure 2.2.1. HRTEM micrograph of the composite material composed of star-shaped pTMAEMC homopolymer and MnOxNP. The image shows the MnOxNP visible as dark, disk-like particles embedded in the polymer. At some spots, lattice planes of the MnOxNP are visible. The lack of long-range order in the MnOxNP complicated the detailed analysis of the inorganic material by conventional PXRD measurements. Therefore, XAFS measurements were conducted to probe the local structure around the Mn atom within the MnOxNP stabilized either by cationic SPBs or by star-shaped pTMAEMC homopolymer. Firstly, the XANES spectra were used to determine the average oxidation states of the Mn of the MnOxNP by comparing the position of the absorption edge to that of different manganese oxide reference compounds. This showed that the average oxidation state of Mn of the composite material is between 3.5 - 3.7 indicating the mixed valency of Mn within the crystallographic structure. A qualitative comparison of the EXAFS spectra of the different composite materials to those of H+-birnessite and K+-birnessite prove similarities in the crystallographic structure as already expected by PXRD analysis. The χ(k)k3 spectra as well as the Fourier transformed spectra of all composite materials and of birnessite reference compounds are shown in Figure 2.2.2. Overview 34 The presence of an upper critical solution temperature was investigated by temperaturedependent DLS measurements. The results shown in Figure 2.5.2b prove that there is a distinct swelling of the zwitterionic corona in the range between 20 °C and 75 °C which could be further enhanced by the addition of KCl. The swelling is completely reversible. (a) (b) Figure 2.5.2. (a) Influence of the concentration of NaCl on the hydrodynamic brush thickness L of the zwitterionic SPB. The addition of salt leads to a pronounced swelling of the zwitterionic corona at salt concentration above 0.5 M. (b) Influence of the temperature on the hydrodynamic brush thickness L without the addition of KCl ( heating,  cooling), 1 M KCl ( heating,  cooling) and 2 M KCl ( heating,  cooling). The zwitterionic corona is swelling upon heating of the sample whereas adjacent cooling leads to a re-shrinking. The effect can be significantly enhanced by the addition of salt. Additional zeta potential measurements could show that the negative surface potential of the zwitterionic SPB is due to the presence of negative charges of the PS-co-DVB core particles. These charges could be traced back to the presence of remaining surfactant and of incorporated fragments of initiator molecules due to the core synthesis. The full publication can be found in Chapter 7. Overview 35 2.6. Individual Contributions to Joint Publications The results presented in this thesis were obtained in close collaboration with different coworkers and published or submitted as indicated below. The individual contribution of each co-author to the publications is listed. The asterisk denotes the corresponding author. Chapter 3 This work has been published in Chemistry of Materials under the title “Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes” by Frank Polzer, Daniel A. Kunz, Josef Breu and Matthias Ballauff*.  I conducted the synthesis and characterization of the cationic SPBs. Furthermore, I did all the synthetic work for the in-situ generation of the birnessite-type nanoparticles and all TEM and cryoTEM measurements. Furthermore, I wrote the paper.  Daniel A. Kunz assisted during the synthesis of the manganese oxide reference compounds. He conducted the PXRD measurements and contributed to their discussion.  Prof. Josef Breu wrote the discussion of the PXRD results. He also contributed to the scientific discussion.  Prof. Matthias Ballauff contributed to the scientific discussion. Chapter 4 This work has been submitted to the Journal of Colloids and Polymer Science under the title “Structural Analysis of Composites of MnOx and a Polymer Colloid” by Frank Polzer, Elisabeta Holub-Krappe, Hermann Rossner, Alexei Erko, Holm Kirmse, Felix Plamper, Alexander Schmalz, Axel H. E. Müller and Matthias Ballauff*.  I conducted the synthesis of the composite materials and the reference compounds and their characterization including cryoTEM imaging. Furthermore, I wrote the publication.  Felix Plamper and Alexander Schmalz synthesized and characterized the star-shaped p(TMAEMC) homopolymer. Overview 36  Hermann Rossner assisted with the EXAFS evaluation and contributed to the discussion of the XANES and EXAFS section of the publication. Furthermore, he wrote the description of the Bayes-Turchin approach for the EXAFS evaluation.  Elisabeta Holub-Krappe contributed to the scientific discussion on XANES and EXAFS.  Prof. Alexei Erko contributed to the discussion of the XAFS measurements and introduced me into the KMC2 beamline at BESSY II.  Holm Kirmse introduced me into the technique of HRTEM and contributed to the discussion of this method.  Prof. Matthias Ballauff and Prof. Axel H. E. Müller contributed to the scientific discussion. Chapter 5 This work is accepted by the Journal of Catalysis under the title “Catalytic Oxidation of an Organic Dye by MnOx Nanoparticles” by Frank Polzer, Stefanie Wunder and Matthias Ballauff*.  I conducted the synthesis of the composite particles and its characterization. The UV/vis measurements and their evaluation considering the activation energy, the LH kinetics at room temperature were done by me. Furthermore, I wrote the paper.  Stefanie Wunder proved the reproducibility of the synthesis and the UV/vis measurements. Furthermore, she contributed the study of the influence of the buffer concentration, the temperature dependence of the LH kinetics as well as the effect of oxygen on the reaction kinetics. She also took part in the scientific discussion.  Prof. Matthias Ballauff contributed to the scientific discussion. Chapter 6 This work has been published in the Journal of Physical Chemistry C under the title “Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes” by Stefanie Wunder, Frank Polzer, Yan Lu, Yu Mei and Matthias Ballauff*. Overview 37  I conducted the synthesis and characterization of the gold nanoparticles immobilized onto SPBs. Furthermore, I did all TEM measurements of the composite materials. All UV/vis measurements concerning kinetic reduction of 4-nitrophenol using composite particles with gold nanoparticles were conducted and evaluated by myself. I established the Langmuir-Hinshelwood model for the description of the reaction mechanism.  Stefanie Wunder and Yan Lu synthesized the platinum nanoparticle immobilized on SPBs. Furthermore the Langmuir-Hinshelwood model was modified by Stefanie Wunder taking into account the Freundlich exponents of the adsorbents. Stefanie Wunder conducted all the measurements of the Pt nanoparticle composite materials concerning the kinetic reduction of 4-nitrophenol. The data for Pt and Au nanoparticles were evaluated using the Langmuir-Freundlich model by Stefanie Wunder.  Prof. Matthias Ballauff contributed to the scientific discussion. Chapter 7 This work has been published in Macromolecules under the title “Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution” by Frank Polzer, Johannes Heigl, Christian Schneider, Oleg Borisov and Matthias Ballauff*.  I conducted the synthesis of the zwitterionic SPB and all measurements of this work including TEM, cryoTEM, DLS and zeta potential measurements.  Johannes Heigl assisted the work during the course of his bachelor work under my supervision.  Christian Schneider provided the fits of the zeta potential measurements and helped with their discussion.  Prof. Matthias Ballauff and Oleg Borisov contributed to the scientific discussion. Overview 38 Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes 39 3. Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes Frank Polzer, § Daniel A. Kunz, ‡Josef Breu, ‡ Matthias Ballauff §* §Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, and Department of Physics, Humboldt University Berlin, Newtonstr. 15, 12489 Berlin, Germany ‡ Department of Inorganic Chemistry I, University of Bayreuth, 95440 Bayreuth, Germany Email: Matthias.Ballau[email protected] Published in Chemistry of Materials Reproduced with permission from Chemistry of Materials, 2010, 22, 2916. © 2010 American Chemical Society. DOI: 10.1021/cm100226h Abstract 40 3.1. Abstract A new route of in situ formation and stabilization of ultrathin, needle-like manganese dioxide nanoparticles (MnO 2 NP) in aqueous solution by using spherical polyelectrolyte brush (SPB) particles is presented. The SPBs that act as carrier particles consist of a solid polystyrene core of about 50 nm radius onto which long chains of the positively charged polyelectrolyte poly(2-trimethyl ammonium ethyl methacylate chloride) (pTMAEMC) are grafted to yield a overall radius of about 85 nm. Potassium permanganate (KMnO 4 ) is directly reduced within the brush layer of these particles due to the basic environment within this layer. This mechanism seems to limit the size of the MnO 2 NP to the dimensions of the brush layer. Powder X-ray diffraction, (PXRD), transmission electron microscopy (TEM) and cryogenic transmission electron microscopy (cryoTEM) prove that birnessite-type MnO 2 NP with a c*-disorder are generated on the SPB without adding any reducing agent. The birnessite nanoparticles have an average length of 20 nm and a breadth of ca. 1.6 nm. They are composed of single lamellae or of ultrathin stacks of very few lamellae. Energy-dispersive X-ray spectroscopy (EDX) demonstrates that most of the charges of the thin birnessite platelets are balance by potassium ions. The excellent stabilization by the SPB carrier particles in aqueous solution can be traced back to a strong interaction of the birnessite particles with the positively charged pTMAEMC chains of the SPB. Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes 41 3.2. Introduction Manganese oxide materials and especially manganese dioxide (MnO2) that appears in a wide variety of polymorphs such as α-, β-, γ-, and δ-MnO2 have attracted great interest recently. This is due to the possible applications as e.g. electrode materials,1,2 catalysts,3,4 ion exchange,5 and magnetic materials.6,7 All different polymorphs are based on the MnO6 octahedron and differ in the linkage of these basic units.8 In principle, the synthesis can be achieved by the oxidation of Mn2+, by the reduction of permanganate (MnO4-) or by direct conversion of manganese oxides (Mn2O3, MnOOH, etc.).9,8,10,11 The physical and chemical properties of these materials change when downsized to the nanoscopic scale and great efforts have been made to design MnO2 nanometer-sized structures of different size and shape.12,13,14,15,16 Hence, a number of different morphologies have been realized, as e.g. onedimensional (1D) structures (nanorods, nanowires, nanofibers, etc.), two-dimensional (2D) structures (nanosheets, etc.) and three-dimensional (3D) structures (ball-like core-corona particles, nanodisks, etc.).8,17,18,19,20 Among these manganese oxides, birnessite has attracted particular attention because of its unique properties and its use as an intermediate for the preparation of other MnO2 based materials such as octahedral molecular sieves.21,22 Birnessite is a phyllomanganate meaning a layered structure of hydrous manganese oxide comprised of edge-sharing octahedra containing predominantly Mn4+ cations as central ions. Due to the presence of Mn3+ cations and/or vacant octahedral sites a net layer charge arises that is compensated by the incorporation of different cations into the interlamellar space. Typically, these interlayer cations are hydrated.23,24,25,26 The interlayer cations can be exchanged against various other ions such as tetraalkylammonium ions or positively charged aluminium based oligo-cations such as Keggin ions.27,28,29,30 The intercalation of bulky counter ions can furthermore be used to expand the interlayer space and finally delaminate the hexagonal sheets to gain single lamellae of birnessite.31 This delamination process is well known from layered silicates or similarly structured materials, e.g. layered double hydroxides. Moreover, it is important for the generation of ultrathin films and for the building of layer-by-layer structures.32 The generation of delaminated or exfoliated (stacks of only a few lamellae) birnessites is usually a tedious multistep process which involves the intercalation of bulky ions and subsequent delamination.28,33,34 To our best knowledge there is only one report in literature of single step routes to created ultrathin birnessite materials.35 Aqueous suspensions of these particles do not exhibit a high colloidal stability and are difficult to handle because of their high surface area and the large lateral dimensions of the platelets of up to 0.5 µm. Applications in e.g. catalysis, however, require stable colloidal systems that can be easily synthesized in at kg scale. In this paper we present the synthesis and comprehensive characterization of ultrathin birnessite nano-needles that are affixed to spherical polyelectrolyte brushes (SPBs).36 The SPBs used herein consist of a solid polystyrene (PS) core onto which long polyelectrolyte chains (PE chains) are densely grafted (see Figure 3.2.1). Here we use the strong polyelectrolyte poly(2-trimethylammonium ethyl methacrylate chloride) (pTMAEMC) which carries positive charges. Recent work has shown that the immobilization of metallic or oxidic Experimental Section 42 nanoparticles on SPB is a promising way to colloidal stable composite particles with a high catalytic activity.37,38,39,41 Moreover, cationic polyelectrolytes have recently been used successfully to create and stabilize thin films of birnessite.42,43 Figure 3.2.1. Scheme of a cationic spherical polyelectrolyte brush with brush monomer 2trimethylammonium ethyl methacrylate chloride (TMAEMC). Here R represents the hydrodynamic radius of the polystyrene core, L stands for the contour length of the polyelectrolyte chains and D is the average distance of junctions of polyelectrolyte chains on the surface of the core particle. Here we demonstrate that cationic spherical polyelectrolyte brushes can be used to prepare and immobilize ultrathin birnessite needles by adding KMnO4 solution to the aqueous suspension of the SPB. The resulting composite particles exhibit an excellent colloidal stability and open new venues for the use of birnessite as catalyst in aqueous systems. 3.3. Experimental Section Materials. All chemicals were of analytical grade and used without further purification. 2trimethylammonium ethyl methacrylate chloride (TMAEMC) was received from Polysciences. KMnO4 was purchased from Fluka and used as received. Water used in all of our work described here was 18 MΩ Millipore water. Synthesis of the Cationic SPB. Cationic SPB TMAEMC-40 was synthesized and characterized as described recently.44 In a typical run, 62.5 g of 2-[p-(2-hydroxy-2methylpropiophe-none)]-ethylene glycol-methacrylate (HMEM) functionalized polystyrene latex were dispersed in water to give a solid content of 3.5 wt %. After addition of 56.0 g of TMAEMC to the dispersion and the mixture was degassed and cooled down to 8 °C. The polymerization was started by irradiation of the dispersion by UV light.45 The reaction was irradiated and cooled for 30 minutes. The dispersion was purified by ultrafiltration (UF) until the conductivity of the serum reached values lower than 3 µScm-1. Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes 43 Synthesis of the MnO2NP immobilized on SPB. The dispersion of SPB was diluted with water to a solid content of about 1.0 wt %. The mixture was bubbled with nitrogen under stirring for half an hour to remove oxygen. Afterwards, 20 mL of a 0.04 M solution of KMnO4 were injected and the solution was stirred for 12 h. The composite particles were cleaned with water by UF until the conductivity of the serum reached a value of lower than 3 µScm-1. Synthesis of the H+-birnessite. This compound was synthesized according to McKenzie et al.46 In a typical reaction, 1.58 g KMnO4 were dissolved in 100 mL water and 1.64 mL of concentrated hydrochloric acid (HCl) were added dropwise to the solution. The precipitate was filtered and cleaned by dialysis against water. Synthesis of the K+-birnessite. K +-birnessite was synthesized by the thermal decomposition of KMnO4 at 800 °C for 16 h. In a typical run, 5.0 g of KMnO4 were heated at 2 °Cmin-1 to 800 °C and kept for 16 h at this temperature, before cooling down with 1 °Cmin-1. The product was washed with water until the filtrate became clear.47 Methods. Transmission electron microscopy (TEM) and cryogenic transmission electron microscopy (cryoTEM) measurements were conducted with a Zeiss EM922 EFTEM (Zeiss NTS GmbH, Oberkochen, Germany) as described recently.48 Dynamic light scattering (DLS) was performed with an ALV 4000 (Peters) light scattering goniometer. Samples for powder X-ray diffraction (PXRD) were prepared onto a silicon zero-background plate via a backloading technique to minimize textural effects. PXRD patterns were obtained using nickel filtered Cu-Kα radiation (1.54187 Å) on a Bragg-Brentano-type diffractometer (Panalytical XPERT-PRO) equipped with an X’Celerator Scientific RTMS detector. Energy dispersive Xray spectroscopy (EDX) was conducted with a Zeiss 1530 FESEM. The number of amino groups of the SPB, and therefore the core to shell ratio, was determined by potentiometric titration of TMAEMC-40 with 0.01 M silver nitrate (AgNO3) standard solution (Merck) using a WTW cond 197i conductometer. The amount of MnO2 immobilized on the SPB was determined by thermal gravimetric analysis (TGA) using a Mettler Toledo STARe system. Zeta potential measurements were performed with a Malvern Zetasizer Nano ZS. 3.4. Results and Discussion Synthesis. The synthesis of the cationic spherical polyelectrolyte brushes was conducted as described in previous work.44 The hydrodynamic radius Rh of the polystyrene-co-HMEM core due to DLS measurements is 42.7 nm ± 0.3 nm. After the photoemulsion polymerization, Rh of the cationic SPB increased to 84.7 nm ± 0.5 nm due to the grafting-from process of polyelectrolyte chains consisting of pTMAEMC onto the core particles. The pTMAEMC shell thus grafted from the surface of the core particles has a Rh of 42.0 nm ± 0.8 nm. Potentiometric Titration with 0.01 M AgNO3 standard solution gives the total number of charges on one particle. This analysis showed that the SPB used in this study had a core-toshell mass weighted ratio of 6.6 to 1. Thus, the weight fraction of the shell is approximately Conclusions 50 Birnessite-type single lamellae and ultra-thin, turbostratically disordered stacks of only very few lamellae are sandwiched between cationic pTMAEMC chains and thus are tightly immobilized by electrostatic interactions between the SPB carrier and the negative surface charge of MnO2NP. Nevertheless, the high contents of K+ ions found by EDX analysis prove that most of the negative charges of the internal and external surfaces of the birnessite-type nanoparticles are not balanced by pTMAEMC chains bound to the external surface but instead by intercalated and surface-adsorbed K+ ions, respectively. A penetration/intercalation of the pTMAEMC chains between birnessite lamellae seems not to occur, since steric hindrance of large ions leads to a decreasing probability for intercalation with increasing ion radius.28 Instead the pTMAEMC chains interact with the external surface charge only. This is in good agreement with the finding that the length of the nanoparticles does not exceed the size of the polyelectrolyte chains. 3.5. Conclusions A facile route has been developed for the preparation of ultrathin birnessite-type nanoneedles within SPBs by in situ reduction of KMnO4 precursor molecules. We conclude that the KMnO4 reacts with the quaternized ammonium groups of the brush polymer to form a p(TMAEM MnO4) precursor which gets directly reduced by the basic environment within the brush layer. This leads to a polyelectrolyte-directed growth and stabilization of the nanoparticles which causes a size limitation of the MnO2NP by the brush extention. TEM mircrographs reveal a collapsed structure of the birnessite nanoparticles immobilized on SPB due to drying effects and the loss of interfacial bound water between the single lamellae. In contrast to that, cryoTEM images confirm the needle-like nanoparticles with an average length of 20 nm and a breadth of 1.6 nm are well distributed among the carrier particle and that no MnO2NP are in free solution. CryoTEM and the PXRD analysis point out that the needles exhibit a c*-disordered birnessite type structure of the MnO2. The excess charge of these nano-needles is mostly balanced by K+ ions which could be inferred from the EDX analysis. Hence, we obtained ultrathin birnessite-type nanostructures without further delamination processes. Moreover, the composites of the SPB and the nanoparticles exhibit an excellent colloidal stability. These properties make the composite particles a promising material for applications in catalysis. Work along this direction is under way. 3.6. Acknowledgements We thank the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 840 Bayreuth and the Henkel AG & Co. KGaA for the financial support. The authors are indebted to Benjamin Goßler for EDX measurements. Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes 51 3.7. Supporting Information Figure S3.1. Proposed mechanism for the MnO2NP generation at SPB. By adding KMnO4 solution to the SPB dispersion, a TAA MnO4 precursor is formed which gets instantly reduced inside the brush layer. As a consequence, birnessite-type MnO2NP (red hexagon) are generated which are stabilized by the PE chains of the SPBs. Figure S3.2. PXRD patterns of the bare SPB TMAEMC-40 and the composite systems SPBMnO2-5, SPB-MnO2-8 and SPB-MnO2-9 from undermost to uppermost curve. The hkreflections of the composite systems refer to the birnessite nano-needles that exhibit only a 2D order within the ab-layer plane. References 52 (a) (b) Figure S3.3. (a) SEM image of the sample SPB-MnO2-8 and (b) the corresponding EDX pattern that indicates the formation of the MnO2NP with K+-ions inside the interlayers of birnessite. 3.8. References 1 Débart, A.; Paterson, A. J.; Bao, J.; Bruce, P. G. Angew. Chem. 2008, 120, 1-5. 2 Fischer, A. E.; Pettigrew, K. A.; Rolison, D. R.; Stroud, R. M.; Long, J. W. Nano Letters 2007, 7, 281-286. 3 Son, Y. C.; Makwana, V. D.; Howell, A. R.; Suib, S. L. Angew. Chem. 2001, 40, 4280-4283. 4 Espinal, L.; Suib, S. L.; Rusling, J. F. J. Am. Chem. Soc. 2004, 126, 7676-7682. 5 Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 511-515. 6 Zhu, H. T.; Luo, J.; Yang, H. X.; Laing, J. K.; Rao, G. H.; Li, J. B.; Du, Z. M. J. Phys. Chem. C 2008, 112, 17089-17094. 7 Ge. J.; Zhou, L.; Yang, F.; Tang, B.; Wu, L.; Tung, C. J. Phys. Chem. B 2006, 110, 1785417859. 8 Wang, N.; Cao, X.; He, L.; Zhang, W.; Gou, L.; Chen, C.; Wang, R.; Yang, S. J. Phys. Chem. 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Ed. 2007, 46, 4951-4955. 19 Fukuda, K.; Nakai, I.; Ebina, Y.; Tananka, M.; Mori, T.; Sasaki, T. J. Phys. Chem. B 2006, 110, 17070-17075. 20 Portevault, D.; Cassaignon, S.; Baudrin, E.; Jolivet, J.-P. Chem. Mater. 2008, 20, 61406147. 21 Cai, J.; Liu, J.; Suib, S. L. Chem. Mater. 2002, 14, 2071-2077. 22 Yang., D. S.; Wang, M. K. Chem. Mater. 2001, 13, 2589-2594. 23 Post, J. E.; Veblen, D. R. Am. Miner. 1990, 75, 477-489. 24 Ching, S.; Petrovay, D. J.; Jorgensen, M. L.; Suib, S. L. Inorg. Chem. 1997, 36, 883-890. 25 Gaillot, A.-C.; Drits, V. A.; Plancon, A.; Lanson, B. Chem.Mater. 2004, 16, 1890-1905. 26 Gaillot, A.-C.; Flot, D.; Drits, V. A.; Manceau, A.; Burghammer, M.; Lanson, B. Chem. Mater. 2003, 15, 4666–4678. 27 Tang, W. P.; Kanoh, H.; Yang, X. J.; Ooi, K. Chem. Mater. 2000, 12, 3271-3279. 28 Liu, Z. H.; Ooi, K.; Kanoh, H.; Tang, W.-P.; Tomida, T. Langmuir 2000, 16, 4154-4164. 29 Brock, S. L.; Sanabria, M.; Urban, V.; Thiyagarajan, P.; Potter, D. I.; Suib, S. L. J. Phys. Chem. B 1999, 103, 7416-7428. 30 Wong, S. T.; Cheng, S. Inorg. Chem. 1992, 31, 1164-1172. 31 Liu, Z.; Ma, R.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Mater. 2007, 19, 6504-6512. 32 Möller, M. W.; Handge, U. A.; Kunz, D. A.; Lunkenbein, T.; Altstädt, V.; Breu, J. ASC Nano 2010, 4, 717-724. 33 Gao Q.; Giraldo, O.; Tong, W.; Suib, S. L. Chem. Mater. 2001, 13, 778-786. 34 Omomo, Y.; Sasaki, T.; Wang, L. Z.; Watanabe, M. J. Am. Chem. Soc. 2003, 125, 35683575. 35 Kai, K.; Yoshida, Y.; Kageyama, H.; Saito, G.; Ishigaki, T.; Furukawa, Y.; Kawamata, J. J. Am. Chem. Soc. 2008, 130, 15938-15943. 36 Ballauff, M. Prog. Polym. Sci., 2007, 32, 1135-1151. 37 Lu, Y.; Wittemann, A.; Ballauff, M. Macromol. Rapid Commun. 2009, 30, 806-815. 38 Schrinner, M.; Polzer, F.; Mei, Y.; Lu, Y.; Haupt, B.; Göldel, A.; Drechsler, M.; Preussner, J.; Glatzel, U.; Ballauff, M. Macromol. Chem. Phys. 2007, 208, 1542-1547. 39 Schrinner, M.; Proch, S.; Mei, Y.; Kempe, R.; Miyajima, N.; Ballauff, M. Adv. Mater., 2008, 20, 1928-1933. 40 Mei, Y.; Sharma, G.; Lu, Y.; Drechsler, M.; Irrgang, T.; Kempe, R.; Ballauff, M. Langmuir 2005, 21, 12229-12234. References 54 41 Schrinner, M.; Möller, M.; Thun, J.; Kauffmann, Y.; Breu, J.; Talmon, Y.; Ballauff, M. Science 2009, 323, 617-620. 42 Nakayama, M.; Tagashira, H. Langmuir 2006, 22, 3864-3869. 43 Lvov, Y.; Munge, B.; Giraldo, O.; Ichinose, I.; Suib, S. L.; Rusling, J. F. Langmuir 2000, 16, 8850-8857. 44 Sharma, G.; Ballauff, M. Macromol. Rapid Commun. 2004, 25, 547-557. 45 Schrinner, M.; Haupt, B.; Wittemann, A. Chem. Eng. J. 2008, 144, 138-145. 46 McKenzie, R. M. Mineral. Mag. 1978, 38, 493-502. 47 Kim, S. H.; Kim, S. J.; Oh, S. M. Chem. Mater. 1999, 11, 557-563. 48 Wittemann, A.; Drechsler, M.; Talmon, Y.; Ballauff, M. J. Am. Chem. Soc., 2005, 127, 9688-9689. 49 Lu, Y; Spyra, P.; Mei, Y; Pich, A.; Ballauff, M. Macromol. Chem. Phys. 2007, 208, 254261. Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes 55 References 56 Structural Analysis of Colloidal MnOx Composites 57 4. Structural Analysis of Colloidal MnOx Composites Frank Polzer,1 Elizabeta Holub-Krappe,1 Hermann Rossner,1 Alexei Erko,1 Holm Kirmse,2 Felix Plamper,3 Alexander Schmalz,4 Axel H. E. Müller,4 Matthias Ballauff 1* 1Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany and Department of Physics, Humboldt University Berlin, Newtonstr. 15, 12489 Berlin, Germany 2Department of Physics, Humboldt University Berlin, Newtonstr. 15, 12489 Berlin 3Physical Chemistry II, RWTH Aachen, Landoltweg 2, 52056 Aachen 4Macromolecular Chemistry II, University of Bayreuth, Universitätsstr. 30, 95447 Bayreuth Email: Matthias.Ballau[email protected] Submitted to the Journal of Colloid and Polymer Science Abstract 58 4.1. Abstract We report on the detailed structure of MnO x nanoparticles (MnO x NP) which are either stabilized by cationic spherical polyelectrolyte brushes or by star-shaped cationic polyelectrolyte chains. In both cases, the polycation is composed of 2- (trimethylammonium)ethyl methacrylate chloride (TMAEMC). The analysis by transmission electron microscopy (TEM), cryogenic transmission electron microscopy (cryoTEM) and powder X-ray diffraction (PXRD) leads to the conclusion that the MnO x nanoparticles in aqueous dispersed state are composed of only a few or even single lamellae of c-disordered potassium birnessite (K + -birnessite). Using star-shaped pTMAEMC homopolymer for the synthesis of composite particles we obtain MnO x NP with an average diameter of about 5 nm. MnO x NP immobilized on cationic spherical polyelectrolyte brush (SPB) have a length of about 20 nm and a width of 1.6 nm. Comparison of the extended X-ray absorption fine structure (EXAFS) spectra of the MnO x composites with reference spectra leads to the conclusion that all materials include c-disordered birnessite-type nanoparticles. A comparison of the energy shift of Mn K-edge absorption peak of the X-ray absorption near edge structure (XANES) spectra of different manganese oxide reference materials with the different MnO x NP revealed an average oxidation state of about 3.5 - 3.7 for synthesized compounds. No distinct structural difference is found when comparing the dried samples to samples dispersed in water. A comparison of the EXAFS data of the birnessite nanoparticles with the crystal structure of macroscopic systems showed a compression in the c-direction accompanied by a slight elongation within the ab-plane of the layered material. Structural Analysis of Colloidal MnOx Composites 59 4.2. Introduction Mixed valent manganese oxides (MnOx) with layered topologies have been studied in literature extensively in recent years.1,2 These structures can be synthesized in a wide-spread variety by changing the key parameters such as the porosity, the nature of interlayered cations, the degree of interlayer hydration, the average oxidation state of manganese and the number of cation vacancies within the layer.3,4,5 The control over these parameters has led to a number of applications of layered MnOx, most notably as catalysts and as new types of electrode materials for lithium ion secondary batteries.6,7,8,9,10 One extensively studied material is birnessite, a phyllomanganate composed of predominantly edge-sharing MnO6 octahedra.11,12 The mixed valence of such materials due to cation vacancies and/or the presence of Mn3+ cations within the MnOx layers leads to a negative charge in the layers.13,14,15,16 This excess charge is balanced by the incorporation of different cations within the interlayers which makes these oxides suitable for heavy metal sorption from waste water or similar systems.17 There are several reports in literature about the synthesis of nanometer-sized layered MnOx materials.18 Most of the methods that create highly delaminated or even exfoliated layers of MnOx are low temperature solution-based methods because of the strong tendency of these structures for coagulation and precipitation.19,20,21,22 These mild reaction conditions lead mostly to highly disordered materials. Disorder may be a favourable side effect in some cases, e.g. for applications as cathode materials.23 However, the lack of a long-range ordered crystalline phase and the nano-scale size complicates the analysis of such oxides by conventional methods such as PXRD.24,25 Therefore investigations have been conducted using X-ray absorption fine structure (XAFS) measurements in order to identify and analyze highly disordered nanometer sized MnOx materials.26,27,28,29 Thus, Fukuda and co-workers studied the local structure of Mn by XANES and EXAFS of exfoliated unilamellar crystallites of manganese oxide nanosheets.30 MnOxNP in different bacteria have been characterized by XAFS successfully by Saratovsky et al. and Grangeon et al.31,32 Ressler and co-workers presented a detailed analysis of MnOx colloids with intercalated tetraalkylammonium ions.33 Therein it was shown that EXAFS can be used to investigate structural changes of MnOx colloids due to sol-gel transition processes. All studies proved that EXAFS is well suited for the analysis of dispersed MnOx colloids. We have recently shown that by adding potassium permanganate (KMnO4) to a dispersion of cationic SPB, ultrathin platelet-like MnOxNP are generated in-situ.34 Figure 4.2.1 displays the synthesis of these composite particles in a schematic fashion: The SPB consists of a solid polystyrene (PS) core onto which long cationic chains of TMAEMC are chemically grafted. The addition of KMnO4 leads to an ion exchange of MnO4against the chloride counter ions of the brush layer. The reduction of the MnO4precursor in the basic environment of the brush layer leads to the generation of MnOxNP of layered topology which was proven by TEM and cryoTEM studies recently.34 The ultra-thin platelets are stabilized Results and Discussion 66 KMnO 4 . This is in good agreement with recent findings that the generation of MnO x NP inside a pTMAEMC brush layer is catalyzed by OH - ions inside the cationic brush layer of SPBs. 34 Due to the higher charge density, and therefore the higher exchange capacity of a SPB in comparison to a star-shaped homopolymer, the OH - concentration is significantly higher within the brush layer of a SPB. Hence, in case of the pTMAEMC star the pH is not sufficiently high enough to reduce the MnO 4- -ions. Thus, the generation of the nanoparticles stabilized by pTMAEMC star has been induced by the addition of 2-butanol. The redox process can be followed by a fading of the purplish color of KMnO 4 to brown, indicating the generation of MnO x NP. (a) (b) (c) (d) Figure 4.4.3. (a) HRTEM micrograph of MnO x NP@pTMAEMC star on a lacey carbon grid in the dried state. The MnO x NP are visible as dark objects embedded in the TMAEMC star matrix. The lattice planes of the MnO x NP are clearly visible. The white square in Figure 4.4.3a shows the selected area for the micrograph in Figure 4.4.3b and its corresponding diffractogram in Figure 4.4.3c. Schematic representation of the composite material composed of pTMAEMC star and MnO x NP. The negatively charged, platelet-like MnO x NP with a typical diameter of 2-5 nm are stabilized by the cationic pTMAEMC star polymer due to electrosteric stabilization. The crystallographic structure of a birnessite composed of Mn central atoms (white spheres) surrounded by 6 oxygen atoms (red spheres) is displayed next to the composite material. The predominantly edge-shared MnO 6 octahedra of the birnessite form into layers with intercalated K + -ions and water molecules (blue spheres) between these layers. HRTEM micrographs in Figure 4.4.3 show the MnO x NP together with the pTMAEMCstar in the dried state. The nanoparticles are displayed as dark objects with a disk-like shape. Structural Analysis of Colloidal MnOx Composites 67 Lattice planes are clearly visible. As already discussed, there are a large number of similar dspacings within the crystallographic structure of the birnessite. The lattice planes in Figure 4.4.3b show a d-spacing of 2.24 Å, most probably referring to (112) or (202) lattice planes of a [111] orientation according to the discussion of the HRTEM micrographs for MnOx@SPB. A second lattice plane is indicated by the diffractogram in Figure 4.4.3c but could not be resolved by the HRTEM imaging. The weak contrast of the MnOxNP in the TEM micrographs points to the fact that the particles are composed of a low number of stacks of lamellae only. Based on the investigations by TEM and the high colloidal stability of the composite material in water, the negatively charged, disk-like MnOxNP (typical diameter ca. 2-5 nm) are stabilized by the cationic pTMAEMCstar that adsorbs onto the MnOxNP due to Coulomb interactions (Figure 4.4.3d). The adsorbed pTMAEMCstar stabilizes the small nanoparticles against coagulation in aqueous solution due to steric and electrostatic interactions. Due to the stabilization by pTMAEMCstar the MnOxNP exhibit a high colloidal stability over several months. The size is in the same magnitude observed for MnOxNP synthesized in the presence tetraalkyl ammonium ions as reported previously by Brock et al.27 The MnOxNP generated in the presence of pTMAEMCstar are significantly smaller than those synthesized within the cationic SPB. This could be evidence for a polymer directed growth of the MnOxNP since the pTMAEMC chains of the star shaped polymer are shorter compared to the pTMAEMC chains of the SPB. Since the HRTEM studies only represent a small local area of the sample and due to the fact that a large number of different lattice planes with similar d-spacing are present in birnessite structures, further investigations by PXRD and by XAFS are necessary to get complete information about the crystallographic structure of the MnOxNP. The results are shown in Figure 4.4.4. In principal, the diffraction pattern for the MnOxNP@pTMAEMCstar material proves that the nanoparticles can be characterized as cdisordered birnessite with a small crystal size. Here, we would like to add that extremely small birnessite crystallites composed of only a few, randomly stacked lamellae are sometimes termed as -MnO2 in literature.42,53,54,55 Due to the absence of 00l reflections, which correspond to the interlayer distance of stacked lamellae in the c-axis, the MnOxNP are composed only of a few or even of single lamellae (see Figure S4.1). This agrees with the low electron contrast of the MnOxNP in the TEM micrographs in Figure 4.4.1. As already discussed in a previous work, the maximum of the hk bands at about 36° and 65° in 2  of MnOxNP@SPB matches with the corresponding reflections of a birnessite whereas the maximum of the hk bands of the c-disordered H+-birnessite is slightly shifted to 36.3° and 65.4° in 2  . This slight shift can either be traced back to small differences of d-values of the corresponding lattice planes or might be an effect of the small crystallite size. It is well known for turbostratically disordered smectites, which also possess a 2D layered topology that the maxima of the strongly asymmetric bands do not superimpose with the d-value of a specific reflection anymore.56 The reason for this fact is that the structure factor influences the shape of the hk bands when the crystallites come to nanoscopic dimensions so that the maxima of the hk bands do not match the reflections of the corresponding lattice planes. Nevertheless, Results and Discussion 68 the maxima of the hk bands fit well with the reflections of the corresponding c-ordered birnessite as shown in Figure 4.4.4. The pattern of the MnOxNP@pTMAEMCstar composite matches with that of the MnOxNP@SPB and therefore can also be assigned to a c-disordered birnessite structure. Since both composites show structural similarities according to the PXRD patterns we conclude that the pTMAEMC chains play an important role in the particle generation mechanism. Figure 4.4.4. PXRD pattern of MnO xNP@pTMAEMCstar, MnOxNP@SPB, -MnO2, H+- birnessite and birnessite. The reflection at around 36.5° and 65° in 2θ refer to the hk bands due to a disorder in c-direction of the composite materials and the H+-birnessite of the hexagonal sheets composed of MnO6 octahedra. Figure S4.1 shows the PXRD patterns of the compounds from 15° to 70° in 2θ. However, no detailed structural analysis as e.g. interatomic distances is possible by analyzing the very weak and broad hk bands in PXRD. For this purpose, XAFS measurements were conducted to elucidate the local structure around the manganese atoms. X-ray Absorption Near Edge Structure (XANES) The background corrected and normalized XANES spectra measured at the Mn K-edge for the MnOxNP@SPB and for K+- and H+-birnessite are presented in Figure 4.4.5. The spectra contain several characteristics like a pre-edge peak, the white line, the high energy shoulder and a second peak at 6580 eV. The pre-edge peak arises from dipole forbidden quadrupole allowed Mn 1s  3d inner atomic transition in an octahedral environment.57,58 The intensity of the pre-edge peak is enhanced if Mn(III)O6 octahedra with reduced inversion symmetry due to Jahn-Teller distortion are present.33 Structural Analysis of Colloidal MnOx Composites 69 Figure 4.4.5. XANES spectra of composite samples and reference compounds H+- birnessite, birnessite and -MnO2. MnOxNP@SPB and MnOxNP@pTMAEMCstar. All samples show the characteristic pre-edge feature below the Mn K-edge due to the octahedral crystal field splitting between eg and t2g orbitals.39 The XANES region can be also used for a first differentiation of the samples from the huge amount of different layered MnOx structures known so far. Therefore the XANES spectra of MnOxNP@SPB and MnOxNP@pTMAEMCstar were compared to the most probable candidates for crystallographic similarity based on the PXRD study. Figure 4.4.5 shows a comparison of the XANES and for K+- and H+-birnessite and -MnO2, and proves the excellent agreement of the MnOxNP of the composite materials with the reference compounds. This implies that these structures are closely related and that MnOxNP@SPB and MnOxNP@pTMAEMCstar are mainly composed of edge-sharing MnO6 octahedra that form a layered topology. As it can be seen from Figure 4.4.5, the shape and the position of the main peak and the high energy shoulder of the XANES of MnOx@SPB shows better agreement with the one of -MnO2. MnOxNP@pTMAEMCstar composite matches better to the spectrum of a triclinic K+-birnessite. This is in compliance with the results of Ressler et al. which showed that MnOx nanoparticles, generated by the reduction of KMnO4 with alcohols in the presence of tetraalkylammonium ions, possess similar structure than triclinic K+-birnessite.33 The XANES spectrum is very sensitive to the oxidation state of the sample and its coordination chemistry.49 Due to the diversity of different layered minerals composed of MnO6 octahedra and their differences in the average oxidation state, the analysis of the XANES region can give important a priori information on the sample. Therefore a comparison of the K-edge positions of the samples and of the reference compounds with welldefined average oxidation state has been done. The energy of the X-ray absorption edge is increasing with increasing oxidation state. This is due to the fact that the successive removal of electrons from the absorbing atom is raising the electron binding energy.59 The linear relationship of the edge position determined by the maximum of the first derivative and the Mn oxidation state is shown in Figure S4.2.33 This plot proves the excellent linear dependency of the average oxidation state of Mn on the Mn K-edge positions for various manganese oxides as expected. The average oxidation states of the MnOxNP samples were determined Results and Discussion 70 based on this calibration and the results are summarized in Table 4.1. Though the pre-edge feature is also sensitive to the average oxidation state, an analysis of the edge position is more straightforward and less ambiguous.33 The average oxidation state of MnOx@SPB and of MnOxNP@pTMAEMCstar is about 3.7 and 3.5, respectively. This indicates that the samples are predominantly of Mn4+ cations including a fraction of manganese cations in a lower oxidation state.39,60 This finding points towards a birnessite structure that includes also Mn3+ sites and not only Mn4+ sites as found for systems with hexagonal sheet symmetry, e.g. -MnO2.42 The accuracy of the average oxidation state determination via the X-ray absorption edge has an error of about 10%.31,61 The error is higher than for methods like titration reduction/oxidation techniques.62,63,64,65 However, the analysis of composite materials by titration techniques may be problematic since it is not assured that only the inorganic part, namely MnOx, is exclusively oxidized or reduced by the titrant.31 Table 4.1. Edge Positions and Average Mn Oxidation States of Manganese Oxide Compounds sample absorption edge position [eV] average oxidation state of Mn H+-birnessite 6550.5 3.7 ± 0.3 birnessite 6549.1 3.3 ± 0.3 MnOxNP@SPB (powder) 6550.7 3.7 ± 0.3 MnOxNP@pTMAEMCstar 6549.9 3.5 ± 0.3 Extended X-ray Absorption Fine Structure (EXAFS) Since layered structured MnOx materials exist in a great variety, a qualitative comparison with reference compounds is necessary for choosing a proper crystallographic model for the polymer supported MnOxNP samples. In our previous work we could already show that the MnOxNP have similar PXRD patterns than that of phyllomanganates like H+- and K+- birnessite.34 The experimental χ(k) spectra of those reference compounds and of the composite materials are displayed in Figure 4.4.6 in a range of 0 Å-1 ≤ k ≤ 11 Å-1. It should be noted at this point that in our case EXAFS only probes the local environment around the Mn central atom within a distance of about 6 Å. Therefore, neighboring atoms in adjacent layers along the crystallographic c-axis are not considered in this analysis. In general, the all spectra show a good agreement in this range indicating similarity in the crystallographic structure, making a differentiation rather difficult. It proves that the birnessite materials are closely related to each other. For MnOxNP@pTMAEMCstar, K+-birnessite shows slightly Structural Analysis of Colloidal MnOx Composites 71 more spectral similarities according to Figure 4.4.6. The MnOxNP@SPB matches best to - MnO2 in between 2 Å-1 and 5 Å-1 compared to the other two reference compounds. Figure 4.4.6. Experimental Mn K-edge EXAFS signal, χ(k)k3, of composite particles MnOxNP@SPB and MnOxNP@pTMAEMCstar and of reference compounds K+- and H+- birnessite and -MnO2. The composite materials show a good agreement with the reference samples in the range of 2 Å-1 ≤ k ≤ 11 Å-1. Figure 4.4.7 shows the Fourier transformed (FT) k2-weighted χ(k) functions of H+-birnessite, K+-birnessite, -MnO2, MnOxNP@SPB and MnOxNP@pTMAEMCstar. The spectra are not phase corrected for the phase shift associated with the scattering process of the photoelectron so that the peak distances in this plot shifted to lower R values of about 0.4 Å. Therefore the first peak corresponding to the six oxygen atoms of the first shell located at a distance of ~1.9 Å appears at ~1.5 Å in Figure 4.4.7. The peak at around 2.5 Å represents the distance between to edge-shared MnO6 octahedra. The spectra in R-space are dominated by the first two shells around the scattering center which is common for phyllomanganats. Another important feature of layered manganese oxide with a mixed valency is an additional peak at about 3.1 Å which is due to the presence of Mn-Mn corner sharing. There is no significant signal around this peak distance for the composite materials displayed in Figure 4.4.7 leading to the conclusion that the amount of corner-shared MnO6 units is negligible in these samples. The average oxidation states for the two composite materials of about 3.5 - 3.7 for the composite materials (see Table 4.1) then might be traced back to the presence of Mn3+ ions acting as interlayer, charge compensating cations. This is common for poorly crystalline hexagonal birnessite structures which mostly are generated at mild reaction conditions like it is the case for both composite materials and the H+-birnessite. The peak at around 5.2 Å is due to the focusing effect and refers to a Mn-Mn distance of three edge-shared MnO6 units located along a center line.66,67 The forward scattering through the center atom increases the backscattering power and therefore the amplitude contribution of χ(k) of the third atom in the row.68 The amplitude of the backscattering signal of the third atom is strongly dependent on the dihedral angle between the three atoms in the row. The peak is decreasing with increasing deviation from the dihedral angle from 180° and hence the intensity of the focusing peak can be used as Results and Discussion 72 a measure for the dihedral angle.69,70 This was shown by Ressler et al. based on theoretical calculations of Fourier transformed XAFS of four edge-shared MnO6 octahedra.33 There is no pronounced focusing peak for the composite materials presented in Figure 4.4.7 which indicates a deviation of a collinear arrangement of neighboring MnO6 octahedra. This finding matches well with the disordered structure found by PXRD and HRTEM measurements which do not suggest a long-range ordered material. After a qualitative discussion of the XAFS data we now turn to the quantitative analysis of the EXAFS of MnOxNP@SPB. The comparison of TEM and cryoTEM micrographs revealed significant differences in the morphology of the MnOxNP in the dried and the dispersed state as already shown. Therefore the local structure of the MnOxNP was investigated in both states by EXAFS measurements to analyze if differences exist. The comparison of XANES and qualitative EXAFS spectra revealed a good agreement of the composite material MnOxNP@SPB with the birnessite reference compounds. We chose the crystallographic structure of a monoclinic birnessite according to Post et al. for the theoretical EXAFS calculations.13 Please note that we also tried fitting the data with a hexagonal P63/mmc but within the limits of error a differentiation between both structures could not be achieved. A list of the parameters of the first two nearest neighboring shells of the crystallographic structure is given in Table 4.2. Figure 4.4.7. Fourier transformed experimental Mn K-edge χ(k)k2 functions of MnOxNP composites and of H+- and K+-birnessite and -MnO2 (non-phase corrected). The first peak corresponds to the Mn-O distance whereas the second peak at about 2.5 Å corresponds to the Mn-Mn distance between two edge-shared MnO6 octahedra. For the quantitative EXAFS analysis the Bayes-Turchin method was used,71 based on the standard EXAFS equation.72 This approach compares the measured absorption coefficient  exp with the corresponding model data computed by the FEFF code.51 The Bayes-Turchin method uses correction parameters for the atomic-like background absorption besides the usual structure parameters and yields fit parameters with uncertainties consistently calculated from the experimental and model uncertainties. These were assigned as follows:   exp/  exp = 0.5 % for k < 10 Å-1 and 0.75 % for k ≥ 10 Å-1, fj/fj = 7 % and   j = 0.07 rad for scattering Structural Analysis of Colloidal MnOx Composites 73 amplitudes and phases for each path j, respectively, and the uncertainty of the mean free electron path was set to     = 10 %. A total number of 86 scattering paths were used and the experimental and model EXAFS oscillations were Fourier filtered with R-window between 0 Å and 6 Å. The truncation error, which describes the uncertainty of the fit with respect to the truncation of the multiple-scattering series, was calculated for relative scattering amplitude of 4 %. In an EXAFS analysis usually the Debye-Waller factor   j is fitted for each scattering path or estimated from Debye models. Here we use the approach that   j has a thermal disorder component   j,therm and a structural disorder component   j,struc. For a perfect crystal structure the latter component should be small and independent of temperature and usually is neglected at elevated temperatures. In our case we have to consider significant disorder and therefore both components were treated as fitting parameters, where   j, struc just depends on the number of atoms Nj in path j,   j, struc =   struc Nj /2. At the beginning   struc was set to 0.001 [Å2] and   j, therm was calculated according to the Debye model with Debye temperature of ΘDebye = 400 K. During the fitting procedure j, therm of the two oxygen bonds at R1 ≈ R2 ≈ 1.9 Å and the two manganese bonds at R3 ≈ R4 ≈ 2.8 Å were adjusted with restrictions   1, therm =   2, therm and   3, therm =   4, therm. Table 4.2. Crystallographic Structure of Monoclinic Birnessite to Derive Theoretical EXAFS Phases and Amplitudes (C2/m, a = 5.0 Å, b= 2.850 Å, c = 7.336 Å, , β = 103.18°)13 shellsa pairsb CNc Rd [Å] first shell Mn-O 4 1.908 Mn-O 2 1.910 second shell Mn-Mn 2 2.850 Mn-Mn 4 2.878 a nearest neighbor shells around the Mn absorber; b corresponding atom pair of the shell; c coordination number of the atom pairs; d radial distance of the absorber and the scattering atoms The data reduction of the µ(E) spectra of both samples was conducted in the same way to minimize an influence on the extracted χ(k). The number of fitting parameters was minimized by the introduction of two independent cell expansion factors: αab represents the expansion of the atom coordinates within the ab-plane whereas αc is the expansion factor along the c-axis of the atom coordinates that is, the stacking axis of MnOx sheets. The coordination numbers were fixed to the values from the crystallographic information file.13 The amplitude reduction factor S02 was set to a value of 0.9. The EXAFS analysis was conducted in k-space and restricted to a distance in R-space of 6 Å. As a consequence, adjacent layers of hydrous MnOxNP are not included in the EXAFS fits since they are typically more than 7 Å apart. Results and Discussion 74 Hence, the investigations by EXAFS presented herein are limited to the ab-plane within individual hexagonal sheets. This restriction plays no role here inasmuch the long-range order of the MnOxNP has already been deduced from PXRD measurements. The results of the least square fits for the MnOxNP@SPB in the dried and in the dispersed state are summarized in Table 4.3, and the fits of the experimental data are presented in Figure 4.4.8. The expansion factors indicate that the MnOx nanosheets are compressed along the c-axis of about 4 % whereas they are slightly elongated within the ab-plane, that is, the two dimensional expansion of the hexagonal sheets composed of the MnO6 octahedra. The error correlations between the parameters shown in Table 4.3 stayed below 0.5 even for σ2struc and j, therm R-factors are below 0.006 and summarized in Table 4.3. (a) powder (b) liquid Figure 4.4.8. Experimental EXAFS functions (k) weighted by wave number k are shown by open green circles together with the uncertainties. The solid curves are the most probable curves resulting from the fits. To verify the new geometrical parameters the fitting procedure was repeated using the new crystal parameters a = 5.023 Å, b = 2.86289 Å, c = 7.044 Å for the solid sample of MnOxNP@SPB, and a = 5.015 Å, b = 2.858 Å, c = 7.052 Å for the liquid sample of MnOxNP@SPB. All other starting values were not changed. The results shown in Table 4.3 were reproduced within standard deviations. Fukuda et al. observed an elongation within the ab-plane of layered MnOx nanosheets after delamination.30 They claimed that this is due to a decrease in the average oxidation state of the MnOxNP during the delamination process. The authors also find an expansion in the thickness of the platelets in c-direction, that is, an overall expansion of the crystallographic volume. As already mentioned, we have herein observed a slight elongation along the abplane which is accompanied by a compression with regards to the c-direction of a single platelet, that is, along the stacking direction of the layered MnOxNP. This is a new finding for MnOxNP with layered topology which might be an effect of the small crystallite size together with the high degree of delamination of the platelets. -0.8 -0.4 0.0 0.4  (k) * k 141210864 k [Å-1] exp fit powder -0.8 -0.4 0.0 0.4  (k) * k 141210864 k [Å-1] exp fit liquid Structural Analysis of Colloidal MnOx Composites 75 Table 4.3. EXAFS Best Fit Results of MnOxNP@SPB in Dried and in Aqueous Dispersed State for the First Two Shells. parameter MnOx@SPB (powder) MnOx@SPB (liquid) E0 [eV] 6543.0 ± 0.3 6543.1 ± 0.3 αaba 1.004 ± 0.005 1.009 ± 0.005 αcb 0.960 ± 0.005 0.961 ± 0.006 σ2(Mn-O)c [Å 2] 0.00124 ± 0.00075 0.00263 ± 0.00088 σ2(Mn-Mn)d [Å 2] 0.00414 ± 0.00078 0.00572 ± 0.00090 σ2struce [Å 2]0.00083 ± 0.00036 0.00153 ± 0.00040 R-factorf 0.0059 0.0034 a cell expansion factor for the ab-plane, b cell expansion factor for the c-axis, c DebyeWaller factor for the first shell, d Debye-Waller factor for the second shell, e structural disorder component. f standard goodness-of-fit parameter Despite the significant differences observed by a comparison of the morphology of the composite particles in TEM and cryoTEM, the EXAFS analysis could prove that no significant difference in the local environment of the Mn of the MnOxNP in the dried and in the aqueous dispersed state is present. The Debye-Waller factors σ2 are higher for the liquid sample indicating a higher disorder in comparison to the powder sample. The σ2 includes the structural disorder σstruc2 as well as the thermal disorder. Since no temperature dependent measurements of σ2 are available and correlations of the thermal and structural disorder are present, no detailed interpretation of the σ2 can be made at this point. 4.5. Conclusion We presented the analysis of birnessite nanoparticles supported by cationic star-shaped pTMAEMC homopolymer. The MnOxNP stabilized by pTMAEMCstar are of disk-like shape with a diameter between 2 - 5 nm. A combination of HRTEM, PXRD and XAFS analysis revealed that the MnOxNP@pTMAEMCstar shows a structure closely related to the MnOxNP@SPB which has been identified as randomly stacked birnessite-type nanoparticles of small crystallite size. In addition, a XANES analysis proved the findings of the PXRD measurements and revealed oxidation states between 3.5 - 3.7 for the composite materials. The analysis of the EXAFS data of MnOxNP@SPB in the dried and in the dispersed state showed that no significant effect on the crystallographic parameters is observed, despite the significant structural difference between the TEM and cryoTEM micrographs of the References 82 Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles 83 5. Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles Frank Polzer, † Stefanie Wunder, † Yan Lu, † Matthias Ballauff† * †Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, and Department of Physics, Humboldt University Berlin, Newtonstr. 15, 12489 Berlin, Germany Email: Matthias.Ballau[email protected] Accepted by the Journal of Catalysis Reproduced with permission from Journal of Catalysis, 2012 © 2012 Elsevier. DOI: http://dx.doi.org/10.1016/j.jcat.2012.01.016 Abstract 84 5.1. Abstract We present a study on the catalytic oxidation of the organic dye morin by hydrogen peroxide in the presence of manganese oxide nanoparticles in aqueous solution. The ultrathin manganese oxide nanoparticles consist of c*-disordered potassium birnessite and are immobilized on spherical polyelectrolyte brushes. The catalytic activity of these composite particles was investigated using the oxidation of morin by hydrogen peroxide as a model reaction. The oxidative degradation of morin was followed by UV/vis spectroscopy leading to an apparent rate constant kapp. We propose a modeling of the results in terms of a LangmuirHinshelwood model. kapp can be related to the kinetic constant k and to the apparent adsorption constants of H2O2 and morin. Based on this model, the dependence of kapp on temperature can be traced back to the activation energy of the rate constant k and the adsorption enthalpies of both educts on the surface of the nanoparticles. Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles 85 5.2. Introduction In the last decade, manganese oxide nanostructures have become systems of particular interest as catalysts for oxidation reactions.1,2,3,4,5 There is a wide variety of polymorphs of Mn(IV) oxides such as α-, β-, γand δ-type MnO2 that differ in their respective linkage of the basic structure, the [MnO6] octahedron.6 In general, manganese oxide nanoparticles (MnOxNP) are good catalysts for the catalytic decomposition of hydrogen peroxide (H2O2). 7,8,9 For layered manganese oxides, the total surface can be increased by exfoliation which leads to an enhanced catalytic activity.10,11 However, only a few synthetic routes are known by now which create delaminated or highly exfoliated birnessite nanoparticles, most of them being multi-step approaches.12,13,14 Since nanoparticles are mostly generated by solutionbased methods, aggregation may occur under the conditions of catalytic reactions. This process may lead to a marked decrease of the catalytic activity with time. Recently, we presented a new and facile room temperature method to generate and stabilize nanometer scale layered MnOxNP onto cationic spherical polyelectrolyte brushes (SPB). 15 Figure 1 displays the schematic representation of the composite particles MnOxNP@SPB with a corresponding cryogenic transmission electron microscopy (cryoTEM) micrograph. The SPB consist of a solid polystyrene (PS) core onto which cationic polyelectrolyte chains are densely grafted.16 By adding potassium permanganate to an cationic SPB at room temperature MnOxNP are formed directly on the carrier particles. The reduction of KMnO4 leads to platelets of birnessite that are affixed to the core particles by interaction with the cationic chains. This fixation prevents the coagulation or coarsening of the nanoparticles in an effective way.16 The composite particles that consist of the SPB together with the immobilized MnOxNP exhibit an excellent colloidal stability.15 Introduction 86 (a) (b) Figure 5.2.1. (a) Scheme of the composite material consisting of cationic spherical polyelectrolyte brushes with brush monomer 2-trimethylammonium ethyl methacrylate chloride and birnessite nanoparticles (MnO x NP@SPB) used in the catalytic studies. The negatively charged birnessite nanoparticles are bound to SPB carrier particles by the cationic polyelectrolyte chains. (b) CryoTEM image of the composite material MnO x NP@SPB. Thin plate like birnessite particles are bound to the core particles. The polyelectrolyte chains are not visible because of their low contrast (see Ref. 15). Here we present a study of the catalytic activity of MnO x NP@SPB. 15 The oxidation of morin with H 2 O 2 was chosen as a model reaction to analyze the mechanism of the catalysis in presence of MnO x NP@SPB. Morin belongs to a group of flavonoid plant dyes (see Figure 5.2.2). 17 Figure 5.2.2. Structure of 2’,3,4’,5,7-pentahydroxyflavone (morin) which belongs to the group of flavonoid plant dyes. The oxidation of this polyphenolic dye is a benchmark reaction for the catalytic activity of bleach catalysts. These polyphenolic dyes are present in tea, fruits and vegetables and can be used as model compounds for studying bleaching processes in laundry detergents. 18 Previous work has demonstrated that the oxidation of dyes by hydrogen peroxide is catalyzed by manganese oxide. Moreover, clear evidence was found that this oxidation is related to the surface of the Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles 87 particles. For example, Segal et al. demonstrated that cyanine dyes can be decomposed using well-defined manganese oxide catalysts and concluded that the reaction takes place on the surface of the particles.19 Gemeay et al. studied the oxidative decolorization of organic dyes on polyaniline/MnO2 composites.20 During this study, evidence for the competing adsorption of both the dye and H2O2 on the surface of the catalyst was given. This was argued from the fact the rate constant goes through a distinct maximum as the function of the concentration of H2O2 while it decreases with increasing concentration of the dye. Zhang et al. analyzed the oxidation of methylene blue with H2O2 on β-MnO2 nanorods 21 and concluded that methylene blue and H2O2 adsorb onto the manganese oxide surface, where the reaction takes place. It is also interesting to note that the heterogeneous epoxidation of various alkenes with H2O2 in presence of manganese oxides studied by Qi et al. is surface-controlled as well.22 Here a reaction mechanism which involves a surface bound Mn2+ was proposed. Up to now, however, a full kinetic study of the oxidation of a dye on manganese oxide is still missing. The present investigation gives a full kinetic study of the decomposition of a dye in presence of MnOx-nanoparticles. Since the composite particle MnOxNP@SPB exhibit an excellent colloidal stability, a precise analysis of their catalytic activity in solution can be done. No problems as e.g. coagulation or coarsening which would lead to a much smaller active surface are hampering the analysis. Hence, the influence of the surface on the kinetics can be studied quantitatively. Moreover, all results will be compared to recent studies on the catalytic activity of manganese ions in aqueous solution.Fehler! Textmarke nicht definiert.,23,24,25 The kinetic study presented here will also provide a firm basis for future technical application of these systems in detergent formulations. 5.3. Experimental Section Materials: All chemicals were of analytical grade and used without further purification. 2trimethylammonium ethyl methacrylate chloride (TMAEMC) was received from Polysciences. KMnO4, Na2CO3 and NaHCO3 were purchased by Fluka and H2O2 and morin hydrate were received from Sigma-Aldrich. The boric acid buffer solution was purchased from Carl Roth.The water used here was 18 MΩ Millipore water. Synthesis of MnOxNP immobilized on SPB. The cationic SPB TMAEMC-40 was synthesized and characterized as described recently.26 The synthesis of the composite particles was conducted as described in previous work.15 The dispersion of SPB was diluted with water to give a solid content of about 1 wt %. Afterwards, 20 mL of a 0.04 molar solution of KMnO4 were injected and the solution was stirred for 12 hours. The composite particles were cleaned with water by ultrafiltration against pure water until the conductivity of the serum reached a value of lower than 3 µS·cm-1. The overall amount of quaternized ammonium groups in the polyelectrolyte shell and the core-to-shell mass ratio were determined as 6.6 to 1 by conductivity titration. The composite particles were analyzed by inductively-coupled plasma optical emission spectroscopy (ICP-OES; Varian Vista-Pro Radial) for their Results and Discussion 88 manganese content. Additionally, transmission electron microscopy (TEM) and cryogenic TEM ( Zeiss LEO 922, Zeiss NTS GmbH, Oberkochen, Germany) and powder X-ray diffraction (PXRD; Panalytical XPERT-PRO) were used for characterization. CryoTEM samples were prepared as described recently.27 Catalysis. All catalytic runs were performed in 3 mL optical quartz cells (Hellma). The reactions were carried out in a carbonate buffer system of sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3) which was adjusted by the addition of hydrochloric acid to pH 10. In order to investigate a possible influence of this buffer onto the reaction, a set of experiments was done using a borate buffer. A freshly prepared 0.4 mM morin solution was diluted with buffer to result in morin solutions with concentrations between 0.01 to 0.15 mM for all experiments. Then the desired amount of catalyst solution with a solid content of 0.1 wt % was added. In the following, the catalyst concentration is always related to the Mn content since the manganese oxide is the active species. The solution in the reference cell contained the same concentration of catalyst to subtract its weak but noticeable UV/vis-absorption. At last the required amount of H2O2 solution was added to start the catalytic oxidation of morin. The mixture was instantaneously filled into the optical cell and the measurement was started. All solutions were kept at a given temperature before mixing. The measurements were carried out with a Lambda 650 (PerkinElmer) UV/vis spectrometer at a fixed wavelength of 410 nm, which is the absorption maximum of morin at pH 10. 5.4. Results and Discussion Synthesis of MnOxNP@SPB The synthesis and characterization of the colloidal carrier particles has been described in detail in earlier work.15 TEM and dynamic light scattering analysis gave a PS core radius of 42.7 ± 0.3 nm and an average thickness of the polyelectrolyte shell of 42.0 ± 0.8 nm. After the addition of KMnO4 solution to the aqueous dispersion of the cationic SPB, the onset of nanoparticle formation could directly be followed by a change in the color from purple to brown due to OH– catalyzed reduction of the MnO4ions confined in the brush layer. No further reducing agent needs to be added for this reaction. The total amount of manganese in the composite particles determined by ICP-OES gave a value of 3.85 wt % for the sample used here. CryoTEM micrographs revealed an average length of 20 nm and a width of about 1.6 nm of the MnOxNP platelets immobilized on SPB. PXRD measurements show a c*- disordered K+-birnessite modification of the nanoparticles.15 Catalysis. Previous work has demonstrated that the SPBs present an ideal carrier system inasmuch as they present no diffusion barrier for reactants that need to diffuse to the nanoparticles located inside the brush layer.28 Moreover, the suspensions of these hybrid particles can be purified from soluble species in the aqueous phase by prolonged ultrafiltration.28 As in previous Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles 89 studies, only initial reaction rates were used for the kinetic analysis.28 Given these prerequisites, the apparent rate constant kapp is:  ][][][ 221 MorinkOHMorink dt Morind app  (5.4.1) The pH was kept constant at pH 10 during the reactions using a carbonate buffer because the absorbance of morin depends on the pH. The pKa values of morin are 3.5 and 8.1.29 The oxidation of morin is increasing with the pH due to the higher deprotonation of morin.17, 30 Moreover, the pH will influence the decomposition rate of H2O2. 31 The use of a basic buffer system was necessary because MnOxNP may dissolve under acidic conditions. The leaching of MnO2 at pH values higher than 9 were found to be around 1 ppb only which means that any interference of the catalytic study with manganese ions in solution can be neglected 9 and any interference of the catalytic study with manganese ions in solution can be neglected. The oxidation of morin can be monitored by measuring the time resolved absorbance at 410 nm by UV/vis spectroscopy. Without any catalyst, the oxidative degradation of morin with H2O2 in the carbonate buffer does not proceed in the time frame of the experiments. The corresponding spectra can be found in Figure S5.1 in the supporting information. Figure 5.4.1 shows the characteristic decrease of the absorption maximum of morin at 410 nm. In the course of the oxidation reaction a new peak at 321 nm appears which is increasing with time. The isosbestic points in Figure 5.4.1 at 361 nm and 286 nm clearly show that only a single reaction product is formed and that for a rather short time (t < 12min) morin is oxidized without any side products. After a longer time, the isosbestic points vanish and the peak at 321 nm is decreasing again. This finding points to a secondary reaction, most probably a further oxidation of the products. However, since we use only the initial rates for the kinetic analysis, this later stage of the reaction is of no concern and we only look into the early stage where morin is oxidized to a single species. The peak at 321 nm has been assigned to an intermediate product, a substituted benzofuranone which decomposes further to 2,4 dihydroxy benzoic acid and 2,4,6 trihydroxy benzoic acid.32,33 This reaction pathway is quite similar to the one of quercetin which differs with regard to only the position of one OH group. Oxidation of quercetin leads to the formation of 2,3 dihydroxy benzoic acid and 2,4,6 trihydroxy benzoic acid.34 Similar findings were reported recently by Rothbart et al. who studied the catalytic activity of manganese complexes in solution.25 Here it was shown that the oxidation of morin with H2O2 in carbonate buffer solution leads to an increase of the peak at 321 nm in the beginning. After a few minutes the peak decreases again. In this case isosbestic points were visible for the first 5 spectra of the oxidation. Without any catalyst, the oxidative degradation of morin with H2O2 in the carbonate buffer does not proceed in the time frame of the experiments. The corresponding spectra can be found in Figure S5.1 in the supporting information. Results and Discussion 90 (a) (b) Figure 5.4.1. UV/vis spectra of a 0.1 mM morin solution in 50 mM carbonate buffer solution at pH 10 with a concentration of 10 mM H2O2. The spectrum shows a decrease of the characteristic absorption maximum at 410 nm of morin with time due to the decomposition of the polyphenolic dye by catalytic oxidation. The spectrum on the left hand side shows the reaction in the initial stage (spectra every two minutes). The isosbestic points are marked with dashed lines. On the right hand side the reaction is shown over a period of one hour with spectra taken every 6 minutes. Topalovic and co-workers have also measured the increasing peak at 321 nm in their mechanistic study on morin oxidation with manganese 1,4,7-trimethyl-1,4,7triazacyclononane complexes by air oxygen from air.23 We also found that morin undergoes slow decomposition in aqueous solutions at pH 10 in presence of MnOx@SPB if oxygen is present (see Figure S5.2). Under this condition, the peak at 321 nm is steadily increasing. To exclude the role of air oxygen experiments were conducted after all solutions were purged with nitrogen to exclude oxygen. No difference in the reaction rate was found by using the purged solutions. This suggested that the oxidation by air is not relevant in presence of H2O2. The relative decrease of the adsorption A of morin at 410 nm could be used best to determine the kinetics of the reaction since it gives a direct measure for the decrease of the morin concentration. Typical kinetic runs showing the relative adsorption A/A0 as the function of reaction time are given in Figure 5.4.2a. The reaction immediately starts after addition of the MnOxNP and the normalized absorption follows a linear decrease which can be well described with a first order rate law as shown in Eq. (5.4.1). The initial rate was taken from the slopes of these curves at t =0. Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles 91 (a) (b) Figure 5.4.2. Kinetics of the oxidation of morin. (a). Relative absorption A/A0 recorded at 410 nm as the function of the reaction time. The parameters of the runs are: 0.1 mM morin solution in 10 mM carbonate buffer solution at pH 10; concentration of H2O2: 10 mM. The concentrations of the catalyst are: □ 0.019 mg·L-1, ○ 0.039 mg·L-1,  0.077 mg·L-1,  0.116 mg·L-1 and  0.154 mg·L-1. (b) Influence of the concentration of catalyst on the apparent rate constant kapp of the oxidation of morin with H2O2 taken from 5.4.2a in 12.5 mM carbonate buffer. The apparent rate constant is proportional to the amount of MnOxNP in the system as shown in Figure 5.4.2b. Here the apparent reaction rate kapp obtained from Figure 5.4.2a is plotted against the amount of catalyst present in the system. A strictly linear relation is found. Since leaching of Mn-species can be ruled out with the present experimental protocol, this finding is a clear indication for the involvement of the surface of the MnOx-nanoparticles in the rate-determining step. Figure 5.4.3. Dependence of the buffer concentration on the rate constant at 20°C for two carbonate buffer concentrations (black squares) and boric acid buffer (blue dots). The concentration of morin is 0.1 mM at pH 10 with a concentration of H2O2 = 10 mM and a concentration of Mn in the MnOxNP = 0.39 mg·L-1. The arrows mark the carbonate concentrations used in the mechanistic study. Conclusions 98 Table 5.2: Summary of the Temperature Dependence of the Rate Constants and Adsorption Constants. EAa [kJ  mol-1] ΔHb [kJ  mol-1] ΔSc [Jmol-1K-1] k*S [molm2L-2s-1] 45.8 ± 7.0 - - KMorind [Lmol-1] - -19.8 ± 8.7 0.2± 29.6 KH2O2e [Lmol-1] - -20.8 ± 7.1 -26.1 ± 10.7 a Ea: activation energy. b ΔH: enthalpy. c ΔS: enthropy. d Kmorin: adsorption constant of morin. e KH2O2: adsorption constant of H2O2. 5.5. Conclusions We presented a kinetic study of the catalytic oxidation of morin by H2O2 in aqueous solution using c*-disordered birnessite nanoparticles immobilized on cationic SPB as catalyst. The analysis of the kinetic data suggested that the rate determining step takes place on the surface of the nanoparticles. Both reactants need to be adsorbed onto the surface of the catalyst in order to react. The adsorption process and the surface reaction are described by the thermodynamic adsorption constants K of both reactants and the kinetic constant k, respectively. The mechanism found for colloidal MnOx-particles is hence determined by the surface of the particles. Applications therefore must ensure a sufficient colloidal stabilization in order to keep the catalyst in an active form. 5.6. Acknowledgements We thank the Deutsche Forschungsgemeinschaft, and the Henkel AG & Co. KGaA for the financial support. The authors are indebted to W. von Rybinski and A. Hätzelt for helpful discussion. Oxidation of an Organic Dye Catalyzed by MnOx Nanoparticles 99 5.7. Supporting information Figure S5.1. Oxidation of morin with H2O2 without MnOx-NP in 50 mM carbonate buffer. The spectra were recorded every 2 minutes. Figure S5.2. Oxidation of morin in presence of MnOx@SPB without H2O2 in 50 mM carbonate buffer. Left hand side: solutions were purged with nitrogen before the mixing of the reactants. Right hand side: solutions were not purged with nitrogen before the mixing of the reactants. The spectra were recorded every 2 minutes. References 100 5.8. References 1 Débart, A.; Paterson, A. J.; Bao, J.; Bruce, P. G. Angew. Chem. Int. Ed. 2008, 47, 45214524. 2 Tang, W. P.; Kanoh, H.; Yang, X. J.; Ooi, K. Chem. Mater. 2000, 12, 3271-3279. 3 Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.; Potter, D.I.; O’Young, C.L. Science 1993, 260, 511-515. 4 Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.;. Kim, S. H; Ri, H.-C. J. Am. Chem. Soc. 2002, 124, 12094-12095. 5 Cao, H.; Suib, S. L. J. Am. Chem. Soc. 1994, 116, 5334-5342. 6 Cheng, F. Y.; Chen, J.; Gou, X. L.; Shen, P. W. Adv. Mater. 2005, 17, 2753-2756. 7 Zhang, W.; Wang, H.; Yang, Z.; Wang, F. Colloid Surf. A 2007, 304, 60-66. 8 Broughton, D. B.; Wentworth, R. L. J. Am. Chem. Soc. 1947, 69, 741-744. 9 Do, S.-H.; Batchelor, B.; Lee, H.-K.; Kong, S.-H. Chemosphere 2009, 75, 8-12. 10 Osada, M.; Sasaki, T. J. Mater. 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Chem. 1978, 82, 1505-1509. 32 Topalovic, T.; Catalytic Bleaching of Cotton: Molecular and Macroscopic Aspects, University of Twente, Netherlands, 2007. 33 Colombini, M.P.; Andreotti, A.; Baraldi, C.; Degano, I.; Łucejko, J.J. Microchem. J. 2007, 85, 174. 34 Zhou, A. L.; Sadik, O. A. J. Agric. Food. Chem. 2008, 56, 12081. 35 Vannice, M. A. Reactions; Springer Science + Business Media: Philadelphia, PA, 2005. 36 Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. C 2010, 114, 88148820. 37 Zhou, H.; Shen, Y. F.; Wang, J. Y.; O’Young, C.-L.; Suib, S. L. J. Catalysis 1998, 176, 321-328. 38 Rill, C.; Kolar, Z. I.; Kickelbick, G.;. Wolterbeek, H. T; Peters, J.A. Langmuir 2009, 25, 2294-2301. 39 László, K.; Podkościelny, P.; Daborwski, A. Langmuir 2003, 19, 5287-5294. 40 Zhou, X. W.; Xu, G.; Liu, D.; Panda, P. Chen, J. Am. Chem. Soc. 2010, 132, 138-146. 41 Ressler, T.; Brock, S.L.; Wong, J.; Suib, S. L. J. Phys. Chem. B 1999, 103, 6407-6420. 42 Saratovsky, I.; Wightman, P.G.; Pastén, P.A.; Gaillard, J.-F.; Poeppelmeier, K. R. J. Am. Chem. Soc. 2006, 128, 11188-11198. References 102 Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes 103 6. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes Stefanie Wunder, Frank Polzer, Yan Lu, Yu Mei, Matthias Ballauff 1 1Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, and Department of Physics, Humboldt University Berlin, Newtonstr. 15, 12489 Berlin, Germany E-mail address: [email protected] Published in the Journal of Physical Chemistry C Reproduced with permission from Journal of Physical Chemistry C, 2010, 114, 8814. © 2010 American Chemical Society. DOI: 10.1021/jp101125j Abstract 104 6.1. Abstract We present a study on the catalytic reduction of 4-nitrophenol by sodium borohydride in the presence of metal nanoparticles. The nanoparticles are embedded in spherical polyelectrolyte brushes (SPBs), which consist of a polystyrene (PS) core onto which a dense layer of cationic polyelectrolyte brushes are grafted. The average size of the nanoparticles is approximately 2 nm. The kinetic data obtained by monitoring the reduction of 4-nitrophenol by UV/visspectroscopy could be explained in terms of the Langmuir-Hinshelwood model: The borohydride ions transfer a surface-hydrogen species in a reversible manner to the surface. Concomitantly 4-nitrophenol is adsorbed and the rate-determining step consists of the reduction of nitrophenol by the surface-hydrogen species. The apparent reaction rate can therefore be related to the total surface S of the nanoparticles, to the kinetic constant k related to the rate-determining step and to the adsorption constants KNip and KBH4 of nitrophenol and of borohydride, respectively. In all cases, an induction time t0 was observed of the order of minutes. The reciprocal induction time can be treated as a reaction rate that is directly related to the kinetics of the surface reaction because there is a linear relation between 1/(k t0) and the concentration of nitrophenol in the solution. All data obtained for t0 so far and a comparison with data from literature indicates that the induction time is related to a slow surface reconstruction of the nanoparticles, the rate of which is directly related to the surface reaction. BH BH 4 4¯ ¯ Nip Nip Amp Amp 300 350 400 450 500 0.0 0.5 1.0 1.5 2.0 absorption wavelength [nm] time Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes 105 6.2. Introduction Metallic nanoparticles (NP) have been the subject of intense research during the recent years because of their potential use in catalysis.1,2,3,4 In particular, redox reactions catalyzed by nanoparticles have been extensively studied.5,6,7,8,9 A central problem in this field is the quantification of the catalytic activity of the nanoparticles affixed to various carrier systems. A model reaction suitable for this purpose should be well-defined, that is, no by-products should be formed. Moreover, the degree of conversion should be easily monitored by a simple and fast technique. Pal and coworkers were the first to identify the reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium borohydride (BH4¯) as such a model reaction.10 This reaction is catalyzed by free or immobilized nanoparticles and proceeds in aqueous solution at ambient temperature. Moreover, it can be easily monitored via UV/Visspectroscopy by the decrease of the strong adsorption of 4-nitrophenolate anion at 400nm, leading directly to the rate constant.11 Several isosbestic points in the spectra of the reacting mixtures demonstrate that no side reaction occurs.12 This reaction has been used frequently to check the catalytic activity of the nanoparticles.13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34 The number of studies directly related to the mechanism of this reaction, however, is much smaller. Esumi et al. investigated the catalytic activity of dendrimer-stabilized nanoparticles.25 These authors concluded that the reaction is diffusion-controlled. A systematic study of the kinetics of the reaction has been presented recently by Saha et al. by varying the initial concentrations of borohydride, of Nip, and of the metal nanoparticles.32 From their data these authors could demonstrate that the reduction of Nip must take place on the surface of the nanoparticles. Zeng et al. investigated the catalytic properties of Au-based nanocages, nanoboxes and particles using this model reaction in order to elucidate the role of particle morphology for the activity of the catalyst.31 Special attention was paid to the induction time t0 after which the reaction starts. This induction time has been observed by a number of authors with different carrier systems 10,11,16,31,36 and was interpreted in terms of the time needed for the reactants to diffuse to the surface of the particles.16 A mechanistic explanation of the surface reaction was provided by Zhang et al. by investigating the catalytic activity of Ag nanoclusters supported on TiO2.30 These authors assume that a surface hydrogen species is first transferred to the Ag-nanoparticles by borohydride. This species reacts then with the Nip to yield the product Amp. This model would imply that the kinetics of the reaction must be modeled in terms of a LangmuirHinshelwood mechanism, that is, both reactants need to be adsorbed on the surface prior to reaction. However, Khalavka et al. recently came to the conclusion that only hydrogen needs to be adsorbed onto the surface (Eley-Rideal-mechanism).35 A number of authors have also studied the activation energy EA of this reaction by carrying out kinetic runs at different temperatures.12,17,22,24,28,31,32,33,35,36,39,43 Zheng et al. related EA to diffusion barriers in the system.31 Thus, although the reduction of Nip by borohydride has become one of the most used benchmarks for the catalytic activity of metal nanoparticles, a comprehensive kinetic analysis of this reaction is still lacking. Introduction 106 Recently, we showed that SPBs are excellent systems for the generation and immobilization of metal nanoparticles. 12,36,37,38 Figure 6.2.1 demonstrates the SPBs as carriers for nanoparticles in a schematic fashion: Figure 6.2.1. Scheme of the spherical polyelectrolyte brushes used in this study. Two different polyelectrolytes have been used for the synthesis of the metal nanparticles: For the synthesis of platinum NP we used poly[2-(methylacryloyloxy)ethyl-trimethylammonium chloride] affixed to the polystyrene cores whereas Au-NP were synthesized using poly[(2aminoethyl)-methacrylate hydrochloride]. 36,39 The right-hand side displays the TEM micrographs of the composite particles. The SPBs consist of a solid PS core onto which long chains of polyelectrolyte chains have been chemically grafted. This brush layer can be used to immobilize ions of noble metals as e.g. gold or platinum. Subsequent reduction then leads to metal nanoparticles of 1 - 3 nm in diameter that are firmly embedded in a dense mesh of the polyelectrolyte chains. 39 The composites of the nanoparticles and the SPBs exhibit a high colloidal stability and can be used repeatedly even under harsh conditions as e.g. in the phase-transfer hydrogenation or the Heckand Suzuki-reaction. 40,41 The reduction of Nip in the presence of these composites has been used previously in order to compare the catalytic activity of different metal nanoparticles immobilized in the same system. 42 In addition, this reaction has been used to test the catalytic activity of metal nanoparticles immobilized in other carrier systems like core-shell microgels or tree-like brushes. 12,43 Previous work demonstrated that Nip is reduced to Amp only in the presence of the composite particles; no reaction takes place in absence of the nanoparticles. 36 If an excess of borohydride is used, the reaction is first order in the concentration of 4-nitrophenol c Nip . Moreover, the apparent kinetic rate constant k app is strictly proportional to the total surface S of all metal nanoparticles. 12,29,36 Hence, the kinetic constants k app and k 1 can be defined through: NipNipapp Nip cSkck dt dc  1 (6.2.1) Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes 107 Here we aim at a full analysis of the heterogeneous reduction of Nip in the presence of metallic nanoparticles. We shall demonstrate that the reaction is surface-controlled and can be analyzed in terms of the Langmuir-Hinshelwood mechanism.44,45,46 In this mechanism it is assumed that both reactants need to be adsorbed on the surface of the catalyst. The ratedetermining step is given by the reaction of the adsorbed species. The adsorption/desorption equilibrium is assumed to be much faster and is modelled in terms of a Langmuir isotherm. The influence of SPB carrier particles on the reduction of 4-nitrophenol will be excluded for the study due to the open structure of the brush system.12 In addition to the analysis of the reaction rate, the dependence of the induction time t 0 on the various parameters will be analyzed quantitatively and related to parameters derived from the Langmuir-Hinshelwood analysis. 6.3. Experimental Section The spherical polyelectrolyte brushes and the metal nanoparticles were synthesized as described previously.37,36,39 To 0.1 g of latex particles, 10 mL of an aqueous solution of 2.55 mM metal salt was added dropwise. Afterwards the mixture was stirred for 30 minutes under N2 to remove the oxygen from the liquid and then the metal ions were reduced by a threefold excess of BH4¯. Thereafter the latex was purified via ultra filtration. Transmission electron microscopy was done using a Zeiss EM922 Omega transmission electron microscope. The TEM-micrographs of both systems are shown in Figure 6.2.1. The radius of the PS-core is 45 nm and the polyelectrolyte layer has a thickness of 86 nm in case of the Pt-NP. The carrier particles of the Au-NP have a PS-core with a radius of 43 nm. The thickness of the polyelectrolyte layer is 76 nm. The amount of metal immobilized on the SPBs was determined by TGA using a Mettler Toledo STARe system. The samples were first dried under vacuum at 50 °C. Then ca. 8 mg of the solid composite particles was heated to 800 °C under a 60 mLmin-1 nitrogen flow with a heating rate of 10 °Cmin-1 and holding temperature at 800 °C for about 30 minutes. The size of the metal nanoparticles was calculated from the TEM-micrographs. Approximately 400 nanoparticles were measured in order to obtain the average size. The specific surface area of the nanoparticles was calculated from the average radius thus obtained and to their total mass per particle. The density of the platinum nanoparticle was taken from literature (Pt: 21.45 gcm-3; Ref.36; Au: 19.32 gcm-3 Ref.47). The catalytic runs were performed in 3 mL optical cells made from quartz. The solutions had been purged prior to the run with N2 in order to remove O2. After mixing fresh solutions of BH4¯ and Nip, a given amount of solutions of the composite particles was added. The solution was carefully mixed by shaking shortly before the measurement. The extinction of Nip was subsequently detected via UV/vis spectroscopy using a Lambda 650 spectrometer (Perkin Elmer) at a constant pH value of 10. Results and Discussion 114 (a) (b) Figure 6.4.5. Langmuir-Hinshelwood kinetics of the reduction of nitrophenol. The product of the apparent rate kapp and the concentration of Nip is plotted against the product of θBH4 θNip according to eq.(6.4.2). The lines are the product of the rate constant k and the surface S of the different metallic nanoparticles. The squares are related to the Pt-NP (filled squares for the surface area of 0.00687 m2 L-1 and open squares for a surface area of 0.00481 m2 L-1), whereas the filled circles represent the data deduced for the Au-NP (surface area of 0.0107 m2 L-1). The left hand side of the diagram displays the data referring to the variation of nitrophenol, while the right hand side displays the diagram referring to the variation of sodium borohydride. As mentioned above, the activation energy of this reaction has been measured by several groups.12,17,22,24,28,31,32,35,36,39,43 Figure 6.4.6 displays the Arrhenius plot of k1 and of the reciprocal induction time 1/t0 of the Pt-NP used in the present work. The concentration of Nip and BH4¯ was 0.1 and 10 mM, respectively. These conditions were chosen to ensure a meaningful comparison with previous work.12,36,39 Figure 6.4.6. Arrhenius plot of the surface normalized rate constant k1 (eq. (6.2.1)) and the inverse induction time obtained for Pt nanoparticles. The concentration of Nip and BH4¯ was 0.1 and 10 mM, respectively. The black squares belong to the surface normalized rate constant k1 whereas the red circles refer to the invers induction period t0. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes 115 The activation energy EA for the normalized rate constant was 40 kJmol-1. This is in good agreement with our previous work for metal nanoparticles where EA was found to be 44 kJ.12,36,39 However, the data obtained for EA for different systems differ appreciably. For example, Khalavka et al. found the activation energy for the CTAB-stabilized gold-rods to be 38 kJmol-1.35 Chang et al. reported EA of 52 kJmol-1 for the magnetically recoverable AuNP.22 Mahmoud et al. compared Pt-nanocubes with nanocubes immobilized on PSmicrospheres and obtained an EA of 14 kJmol-1 and 12 kJmol-1, respectively.24 Moreover, Pal and coworkers pointed out that the activation energy is dependent on the surface of the catalyst. Thus, smaller particles exhibit a higher activity due to an increase in the roughness of the available surface.29 An activation energy of 31 kJmol-1 was found for citrate-stabilized Au-NP and 21 kJmol-1 for calcium alginate stabilized Au-NP.29,32 Zeng et al. came to comparable conclusions in the course of measurements of the activation energy of differently shaped Au-NP. The activation energy varied from partially hollow Au-nanoboxes (55 kJmol-1), hollow Au-nanoboxes (44 kJmol-1) to Au-nanocages (28 kJmol-1).31 The present analysis demonstrated clearly that EA reflects the temperature dependence of the kinetic constants k and of the two thermodynamic adsorption constants KNip and KBH4. Evidently, the latter constants may depend quite strongly on the method of immobilizing or stabilizing the nanoparticles. The activation energy is therefore difficult to interpret and additional measurements of the reaction kinetics at different temperatures are necessary. Induction time t0 It rests to explain the induction time t0 and its relation to the various parameters. Xia et al. have also observed this induction time and they assumed that the rate of adsorption of Nip is a dominant factor to t0.31 However, as already discussed diffusion control can be definitively ruled out for the present system. In the following, t0 is related to the rate constants k derived from the above analysis by. Thus, 1/t0 is treated as a reaction rate. This assumption is valid because the dependence of 1/t0 on temperature can be treated by an Arrhenius law. Moreover, the activation energy of 1/t0 is practically the same as EA of the apparent rate constant.12,36,39,43 This was also observed here and the Arrhenius diagram for the Pt-NP used in present study is shown in Figure 6.4.6. It is interesting to note that Zhang et al. have observed a similar induction period in the catalytic hydrolysis of borohydride with Ru-NP.55 They also found that the inverse induction period follows an Arrhenius behaviour which is in accord with the present results. In Figure 6.4.7a we plot t0 for all experiments as the function of the concentration of borohydride. Within the present limits of error, the induction period is independent of the concentration of sodium borohydride. This finding clearly rules out that the initial step related to t0 is attributed to any reaction involving borohydride such as the transfer of a surfacehydrogen species to the metal nanoparticles. However, Figure 6.4.7b demonstrates that the rate 1/t0 normalized to the rate constant k (see Table 6.1) is linearly dependent on the concentration of Nip, giving a master curve for both Auand Pt-NP within the experimental Results and Discussion 116 error. This finding suggests that t0 is related to a slow surface reconstruction that is related to the kinetic constant k found previously for the stationary surface reaction (see Table 6.1). (a) (b) Figure 6.4.7. Induction time versus the concentration of BH4¯ (a) and of Nip (b). The squares show the data obtained for the Pt-NP. Filled squares refer to the surface area of 0.00687 m2 L1 while open squares refer to a surface area of 0.00481 m2 L-1. The filled circles represent data obtained for the Au-NP (surface area: 0.0107 m2 L-1). Figure 6.4.7a shows that the induction period is independent of the concentration of BH4¯ (a). The black and dark blue color represents a concentration of 0.1 mM Nip while the red and light blue color refer to the concentration of 0.05 mM Nip. Figure 6.4.7b demonstrates that the inverse induction time normalized by the kinetic constant k (see Table 6.1) scales linearly with the concentration of Nip., Here the black and dark blue color represents the BH4¯ concentration of 10 mM and the red and light blue color a concentration of 5 mM. The dashed line in Figure 6.4.7b presents a least-square fit of all data indicating a small but finite intercept. This finding can be explained as follows: Recent work by Zhou et al. demonstrated that a time scale in the order of minutes may be caused by processes related to a dynamic restructuring of the surface of the nanoparticles.59 These authors found a time scale for spontaneous surface restructuring of ~ 60 – 250 s which is in the range of the time scale of t0 found for the present reaction in our study and by others. Moreover, Zhou et al. could clearly demonstrate that the surface restructuring is related to the rate of reaction, that is, the rate of restructuring is directly coupled to the catalysis.59 In absence of catalytic activity, there is a finite rate of spontaneous surface restructuring which is more notable for small particles. These findings can be directly compared to Figure 6.4.7b where a finite intercept suggests a spontaneous effect on t0 in absence of Nip as well. Thus, a surface restructuring of the nanoparticles related to the presence of Nip and to the kinetic constant k seems to be a plausible explanation of the long induction periods for this reaction. However, the nature of the surface restructuring is not known. It may be related to a shift of single atoms or to a concerted rearrangement of surface atoms.59 The present data only indicate that the restructuring is alleviated by the presence of Nip. Moreover, it is the necessary step that activates the nanoparticles. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes 117 6.5. Conclusion In conclusion, we have demonstrated that the catalytic reduction of Nip by borohydride in the presence of metallic nanoparticles (Pt, Au) can be modeled in terms of the LangmuirHinshelwood model that assumes the adsorption of both reactants on the surface of the catalyst. The kinetics of the reaction can therefore be described in terms of three constants, a kinetic constant k describing the surface reactivity of the adsorbed species and the thermodynamic adsorption constants for both components, namely KNip for nitrophenol and KBH4 for borohydride. The induction periods t0, that may be of the order of minutes, could be directly related to the rate constant k found for the rate determining step of the stationary reaction. Most probably, t0 is related to a slow surface restructuring of the nanoparticles that is directly related to their catalytic activity. 6.6. Acknowledgment Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. 6.7. References 1 Astruc, D.; Nanoparticles and Catalysis, Wiley-VCH, 2008. 2 Narayanan, R.; El-Sayed, M.; A. J. Phys. Chem. B 2005, 109, 12663-12676. 3 Hashmi, A. S.; Hutchings, G. J. Angew. Chem. Int. Ed. 2006, 45, 7896-7936. 4 Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem. Int. 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Science 2009, 323, 617-620. 39 Schrinner, S.; Polzer, F.; Mei, Y.; Lu, Y.; Haupt, B.; Ballauff, M.; Göldel, A.; Drechsler, M.; Preussner, J.; Glatzel, U. Macromol. Chem. Phys. 2007, 208, 1542-1547. 40 Sharma, G.; Mei, Y.; Lu, Y.; Ballauff, M.; Irrgang, T.; Proch, S.; Kempe, R. J. Catal. 2007, 246, 10-14. 41 Proch, S.; Mei, Y.; Villanueva, J. M. R.; Lu, Y.; Karpov, A.; Ballauff, M.; Kempe, R. Adv. Synth. Catal. 2008, 350, 493-500. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes 119 42 Lu, Y.; Mei, Y.; Schrinner, M.; Ballauff, M.; Möller, M. W.; Breu, J. J. Phys. Chem. C 2007, 111, 7676-7681. 43 Lu, Y.; Mei, Y.; Walker, R.; Ballauff, M.; Drechsler, M. Polymer 2006, 47, 4985-4995. 44 Vannice, M. A. Reactions, Springer Science + Business Media 2005. 45 Xu, W.; Kong, J. S.; Yeh, Y.-T. E.; Chen, P. Nature Mater. 2008, 7, 992-996. 46 Xu, W.; Kong, J. S.; Chen, P. J. Phys. Chem. 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I.; Kickelbick, G.; Wolterbeek, H. T.; Peters, J. A. Langmuir 2009, 25, 2294-2301. 59 Zhou, X.; Xu, W.; Liu, G.; Panda, D.; Chen, P. J. Am. Chem. Soc. 2010, 132, 138-146. References 120 Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution 121 7. Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution Frank Polzer, Johannes Heigl, Christian Schneider, Matthias Ballauff* *Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany, and Department of Physics, Humboldt University Berlin, Newtonstr. 15, 12489 Berlin, Germany Oleg V. Borisov Institut Pluridisciplinaire de Recherche sur l'Environnement et les Matériaux, UMR 5254 CNRS/UPPA, Pau, France, and Institute of Macromolecular Compounds of the Russian Academy of Sciences, 199004 St. Petersburg, Russia Email: Matthias.Ballau[email protected] Published in Macromolecules Reproduced with permission from Macromolecules, 2011, 44, 1654. © 2011 American Chemical Society. DOI: 10.1021/ma102927c Abstract 122 7.1. Abstract We present the synthesis and characterization of spherical polyelectrolyte brush (SPB) particles carrying zwitterionic polyelectrolyte chains. The colloidal particles consist of a divinyl benzene crosslinked poly(styrene) core (PS-co-DVB core) of about 100 nm in diameter onto which linear zwitterionic poly(2-(methacryloyloxy)ethyl dimethyl-(3sulfopropyl)ammonium hydroxide) (pMEDSAH) chains are chemically grafted via ATRP. Zeta potential measurements demonstrated that the SPB has an electrophoretic mobility due to the net charge of the PS-co-DVB core particles. There is an increase of the brush thickness L of the zwitterionic brush at high concentrations of sodium chloride at room temperature. Temperature-dependent measurements by dynamic light scattering (DLS) showed that the zwitterionic SPBs swell reversibly with increasing temperature because of the upper critical solution temperature (UCST) of the pMEDSAH chains in water. This effect could be enhanced by the addition of salt. Cryogenic transmission electron microscopy (cryoTEM) showed that the shell of the particles is quite compact at room temperature. However, the hydrodynamic radius as measured by DLS was significantly larger than the particles radius inferred from microscopy. This result is explained in terms of a model in which the shell of the zwitterionic SPB undergoes a phase separation into a dense phase and a few chains sticking out into the aqueous phase. Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution 123 7.2. Introduction Polyampholytes in general and specifically zwitterionic polymers have become of great importance in the last decades due to their possible applications.1,2,3 This includes ultra-low fouling coatings and high-tech applications as biocompatible components for drug delivery.4,5,6 Colloidal polymer brushes in which the radius of gyration of the grafted polymer chains exceeds the average distance between the joints of the polymer chains can be prepared by surface polymerization of attached initiators (grafting-from).7,8 Up to now, there is a large number of studies devoted to non-charged and charged polymer brushes.7,8 However, there exists much less work on zwitterionic polyelectrolyte brushes so far. To the authors’ best knowledge, there are only a few studies of planar zwitterionic brushes: Azzaroni and coworkers successfully synthesized zwitterionic polymer brushes consisting of poly(2- (Methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)ammonium hydroxide) (pMEDSAH) by grafting-from bromide functionalized gold and silicon dioxide surfaces.9 They observed hydrophilic and hydrophobic brush regimes depending on the height of the synthesized brush. In a further investigation they showed that one can tune the brush behavior by changing the temperature, which was explained by the upper critical solution temperature (UCST) of polysulfobetains.10 Using atomic force microscopy and neutron reflectometry Terayama et al. showed that planar brushes made from poly(3-dimethyl(methacryloyloxyethyl)ammonium propane (pMPDSAH) swell by the addition of salt in aqueous solution.11 The interaction of proteins with planar pMEDSAH and poly(1-carboxy-N,N-dimethyl-N-(2’- methacryloyloxyethyl) methanaminium inner salt) (pCBMA) brushes was thoroughly studied by Zhang and co-workers.12 They observed that these systems possess a high resistance against nonspecific protein adsorption.13 Moreover, planar brushes of poly(2methacryloyloxyethyl phosphorylcholine) (pMPC) provide excellent lubrication in aqueous media which makes them promising candidates for applications as boundary lubricants in artificial joints or similar systems.14, 15 All systems mentioned so far are planar systems. Since one of the most interesting properties of zwitterionic polymer brushes is their resistance against nonspecific protein adsorption and their biocompatibility, a promising field of application is drug delivery.16,17,18 Therefore these polyzwitterions were used as coatings for inorganic or organic nanoparticles and colloids such as gold, magnetite or silica nanoparticles, quantum dots, carbon nanotubes or even DNA.19,20,21,22,23,24 However, the number of systematical studies investigating the solution behavior of zwitterionic SPB in aqueous medium is scarce. Matsuda et al. investigated the interactions of a zwitterionic SPB with a silica nanoparticle core and pMPC chains.25 They observed no salt induced changes of the brush layer of the zwitterionic SPB investigated by DLS. Since pMEDSAH possesses a UCST, temperature dependent measurements were conducted in previous investigations on planar carboxybetaine and sulfobetaine brushes.26 Results and Discussion 130 (a) (b) (c) Figure 7.4.3. TEM micrographs of the zwitterionic SPB prepared on a carbon support. A 0.1 M CsI solution has been used to enhance the contrast of the shell. The lower part displays the structure of the particles on the surface in a schematic fashion. To obtain detailed microscopic information of the zwitterionic SPB in aqueous solution, cryoTEM measurements have been conducted.40,41 Figure 7.4.4 shows cryoTEM micrographs of the zwitterionic system dispersed in salt-free solution and in 0.1 M CsI solutions. Figure 7.4.4a shows the dispersed zwitterionic SPB in non-saline environment. The radius of the particles increased compared to the radius of the bare core particles. Additionally, the surface of the SPB is corrugated because of the pMEDSAH shell on the core particles. Vitrifying the particles in 0.1 M CsI solution leads to a significant increase in the electron density of the shell as it can be seen in the cryoTEM micrographs in Figure 7.4.4b. In this way the shell can be visualized in a much better way. However, subsequent DLS data (see the discussion of Figure 7.4.4 below) demonstrates that the addition of salt in this concentration regime does not alter the conformation of the shell. In Figure 7.4.4b the pMEDSAH chains of the shell are clearly visible due to the presence of CsI. The radius of the particles is about 80 nm. Figure 7.4.4b demonstrates that a closed shell of pMEDSAH is grafted onto the PS-co-DVB/BIEM core-shell latex particles. That directly proves that a closed shell of BIEM has been generated by the emulsion polymerization under starved conditions which is in agreement with past studies.7 The deviation between the results for L and the shell thickness observed by cryoTEM gives important information about the conformation of the pMEDSAH shell of the zwitterionic SPB. The cryoTEM images show a shell thickness of pMEDSAH chains of about 32 nm, whereas L obtained by DLS is about 48 Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution 131 nm. The fact that the shell thickness according to cryoTEM and DLS differs can be explained by the fact that in light scattering the particle size is determined by the longest chains of the SPB. 42 The longest or most stretched pMEDSAH chains are not visible in the cryoTEM images because of the poor contrast of single polymer chains even after introducing CsI to the solution. (a) (b) Figure 7.4.4. CryoTEM micrographs of the zwitterionic SPB in aqueous solution without any salt (a), and in 0.1 M CsI solution (b). Figure 7.4.4b includes hydrodynamic radii of the bare core particles R core and of the core-shell particles R h as determined by DLS. This leads us to the conclusion that the zwitterionic shell of the SPB is not fully stretched in the dispersed state. Most of the chains are collapsed so that a layer of about 40 nm thickness results. Only a few chains stretch further away from the core and thus cause the measured L in the DLS experiment. This behavior is schematically depicted in Figure 7.4.5 and is in qualitative agreement with the model derived from Wagner et al. 43 These authors considered a collapse transition in a polymer brush caused by formation of clusters comprising 3n monomer groups. 43 In our case, we can expect association of 3n dipole groups inside the brush into stable clusters. Furthermore, the formation of stable clusters can also be induced by hydrophobic interactions of the polymer backbone of the pMEDSAH chains. Both effects lead to a collapse transition accompanied by the microphase segregation inside the brush: A dense phase is formed close to the grafting surface whereas the sparse periphery of the brush is formed by more extended chains. Thus, this phase separation causes a bimodal distribution of the polymer chains with respect to their extension. A similar trend has also been predicted for the complexation of polymer brushes with surfactants. 44 In the present case, water represents the poor solvent for the pMEDSAH chains, which leads to a collapse of the shell polymer. 45 A part of the pMEDSAH chains is not included in the surface-near layer leading to an internal phase separation which causes a lateral inhomogeneity. The chains in the dilute swollen layer Results and Discussion 132 of the shell extend further out into the solution and cause a significant contribution to L in the DLS experiments. Figure 7.4.5. Model for the zwitterionic SPB in aqueous solution. In a poor solvent, e.g. in water, most of the chains are in a collapsed state. Only a small portion of the chains is stretched further away into solution. This fact is revealed by comparing the shell thickness observed in cryoTEM micrographs with the results for L determined by DLS. Thus, the shell of the zwitterionic SPB undergoes a phase separation into a condensed phase near the surface of the core particles and a dilute swollen layer of the shell which extends far into the solution. Since in the model shown in Figure 7.4.5 the majority of the pMEDSAH chains are in a collapsed state, investigations have been conducted to elucidate if this structure can be influenced by external stimuli. Therefore DLS measurements of the core-shell particles at different concentrations of NaCl have been done. Figure 7.4.6 shows the results of the saltdependent measurements. There is no notable increase in L of the zwitterionic SPB within the limits of error upon salt addition up to concentrations of 0.5 mol·L-1. These results are in good agreement with those of Matsuda and co-workers who also did not observe a swelling of the zwitterionic pMPC shell upon the addition of up to 0.5 mol·L-1 salt.25 They conclude that the chains are already fully extended even in non-saline solution due to the excluded volume effect of densely packed polymer chains in polymer brushes. However, Figure 7.4.6 indicates an increase of L starting at salt concentrations higher than 0.5 mol·L-1, which results in a 40 % higher L at 2 mol·L-1 as compared to the non-saline state. The increase in L shows that the SPB shell is not fully extended in the non-saline state, which is in full accordance with the model proposed in Figure 7.4.5. Figure 7.4.6 demonstrates that the solution behavior of the pMEDSAH chains is changed, if the salt concentration is sufficiently high. The observation that the onset of the swelling of the shell takes place at concentrations higher than 0.5 mol·L-1 indicates that the swelling cannot be Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution 133 related to the conventional anti-polyelectrolyte effect. The anti-polyelectrolyte effect is generally understood as a Coulomb screening effect which is typically observed for salt concentration up to 0.01 mol·L-1.46 The response of the pMEDSAH shell at salt concentrations higher than 0.5 mol·L-1 suggests that ion-specific and hydrophobic interactions may play a role in these systems.47 An alternative explanation may be sought in the breaking of salt bridges in the zwitterionic layer that occurs only at high salt concentrations.48 Figure 7.4.6. Salt dependent measurements of L of the zwitterionic SPB via DLS at a temperature of 25 °C. The pMEDSAH shell shows a swelling of L at concentrations of NaCl higher than 0.5 mol·L-1. We now turn to the investigations of the temperature-dependent behavior of the zwitterionic SPB. Figure 7.4.7 demonstrates that there is an increase in L of about 7 nm upon heating. The results for the cooling and reheating fully agree and show a good reproducibility. The stretching of the shell at high temperatures is due to the UCST behavior of the pMEDSAH chains. At higher temperatures, the solvent quality will increase for pMEDSAH chains due to their UCST temperature. This has been found by different groups in earlier works on planar brushes.9,10 In the system under consideration here, the expansion of the shell is not very pronounced as compared to the salt-dependent measurements presented in Figure 7.4.6. Since the results of the previous paragraph showed that the addition of high amounts of salt significantly increased L at room temperature, temperature-dependent DLS measurements at different salt concentrations have been conducted. The results of these measurements are also presented in Figure 7.4.7 and show two important effects: On the one hand, L significantly increases at room temperature at salt concentrations higher than 1 mol·L-1. This finding has been shown earlier in Figure 7.4.6. Additionally, temperature cycles at different salt concentrations reveal a drastic swelling of the zwitterionic shell upon heating. This is due to the increase of the solvent quality for the zwitterionic polymer chains. The UCST behavior gets more pronounced after the addition of salt which was expected since both, the salt concentrations and the temperature, are increasing the solubility of the pMEDSAH chains. The influence of the amount of added salt onto the UCST of pMEDSAH homopolymer was also observed by Mary et al.26 Conclusion 134 Figure 7.4.7. Temperature dependent measurements of L of the zwitterionic SPB with pMEDSAH chains by DLS. Increasing the temperature from 20 °C to 75 °C leads to a swelling of the brush layer of the zwitterionic SPB. The behavior is completely reversible, which was shown by subsequent cooling of the system. Therefore it can be assigned to the UCST of the pMEDSAH chains. The effect of swelling can be significantly enhanced by the addition of high amounts of salt. The lowest dataset represents a heating cycle of the zwitterionic SPB in salt-free solution (■ heating and □ cooling), the second curve shows the swelling of the zwitterionic SPB in 1 mol·L-1 NaCl solution ( heating and  cooling) and 2 mol·L-1 NaCl solution for the uppermost curve ( heating and ○ cooling). 7.5. Conclusion We presented a method for the synthesis of colloidal stable spherical polymer brushes with a zwitterionic brush layer of pMEDSAH chains. The extension of the shell can be influenced upon the addition of salt which may be due to ion-specific interactions. Furthermore, the zwitterionic shell showed a fully reversible swelling upon heating due to the UCST behavior of the pMEDSAH chains. This effect could be enhanced upon the addition of salt. By a combination of DLS, TEM and cryoTEM measurements we propose a model for the zwitterionic SPB including an internal phase separation of the pMEDSAH shell according to Wagner et al.43 In this model the shell is mostly collapsed in a condensed state near the surface of the core particles whereas only a small portion of the shell is in a dilute swollen state with the pMEDSAH chains extending far out into solution. Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution 135 7.6. Acknowledgements We thank the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 840 Bayreuth. C.S. thanks the Elite Study Program Macromolecular Science in the Elite Network Bavaria and the Bavarian Graduate Support Program for financial support. We thank R. Hill for the extensive support provided for using the MPEK software and J. Dzubiella for helpful discussions. 7.7. Supporting Information Figure S7.1. H1-NMR of BIEM (250 MHz, CDCl3). δ (ppm) = 6.07 (1H, s), 5.53 (1H, s), 4.35 (4H, t), 1.87 (3H, dd), 1.86 (6H, s). 13C-NMR (62.5 MHz, CDCl3) d (ppm) = 170.2, 135.7, 126.2, 63.4, 61.7, 54.9, 30.4, 18.3. Supporting Information 136 (a) (b) Figure S7.2. TEM micrographs of PS/BIEM1 (a) and PS-co-DVB/BIEM2 (b) core particles. The particles show a narrow size distribution with an average Rcore of 48.1 ± 0.2 nm for the core particles PS-co-DVB/BIEM1 and 59.9 ± 0.2 nm for PS-co-DVB/BIEM2 according to DLS. Figure S7.3. GPC curve of the cleaved chains of the brush polymer with a PDI of 1.16. The GPC was calibrated with pMAA standard. Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution 137 7.8. References 1 Kudaibergenov, S. E.; Cifferi, A. Macromol. Rapid Comm. 2007, 28, 1969-1986. 2 Kudaibergenov, S. E.; Jaeger, W.; Laschewsky, A. Adv. Polym. Sci. 2006, 201, 157-224. 3 Das, M.; Sanson, N.; Kumacheva, E. Chem. 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