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Erasmus Mundus Master in Membrane Engineering 2012 - 2014 FINAL MASTER PROJECT 3D structuration of MOF layers for gas sensors enhancement and its application in microreactors Cheryl Maria Karman June 24th 2014 Supervisors: Dr. Reyes Mallada Dr. Miguel Urbiztondo Abstract “In order to integrate ZIF-8 layers in the fabrication of microdevices and membrane, ZIF-8 layer synthesis was conducted on silicon and polymer based supports. The synthesis of ZIF-8 layer was conducted in room temperature by repeated immersion of supports in the precursor solution. The synthesis method was compatible to the fabrication of ZIF-8 layer based microsensors and micro preconcentrator. The fabrication of free-standing micromembrane was not achieved. Study of PBI/ZIF8 composite membrane synthesis has obtained promising results.” 2 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Acknowledgement I would like to thank Dr Reyes Mallada as the main supervisor of this project, for her support, guidance, ideas and suggestions from the beginning until the crucial moments of completing the experiments. Also I would like to thank Dr Miguel Urbitzondo as co-supervisor for his inputs and suggestions. Not to forget Dr Ismael Pellejero for his help during the research project, especially for teaching me how to work in the clean room (UV lithography, RIE, and profilemeter utilization) and the device preliminary trials. I would like to give thanks to Dr. Carlos Cuestas Ayllón for teaching me how to use the Scanning Electron Microscopy. I also am grateful for the opportunity given by the EM3E Consortium to be a part of the the master program. I also appreciate all the help, suggestions, teaching and training given by all of my colleagues and professors at Institute of Nanotechnology Aragon, Spain. 3 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) DISCLAMER EN - This project has been funded with support from the European Commission. This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein. BG - Този проект е финансиран с подкрепата на Европейската комисия.Тази публикация отразява само личните виждания на нейния автор и от Комисията не може да бъде търсена отговорност за използването на съдържащата се в нея информация. CS - Tento projekt byl realizován za finanční podpory Evropské unie. Za obsah publikací odpovídá výlučně autor. Publikace nereprezentují názory Evropské komise a Evropská komise neodpovídá za použití informací, jež jsou jejich obsahem. DA - Dette projekt er finansieret med støtte fra Europa-Kommissionen. Denne publikation forpligter kun forfatteren, og Kommissionen kan ikke drages til ansvar for brug af oplysningerne heri. DE - Dieses Projekt wurde mit Unterstützung der Europäischen Kommission finanziert. Die Verantwortung für den Inhalt dieser Veröffentlichung trägt allein der Verfasser; die Kommission haftet nicht für die weitere Verwendung der darin enthaltenen Angaben. ΕΛ - Το σχέδιο αυτό χρηματοδοτήθηκε με την υποστήριξη της Ευρωπαϊκής Επιτροπής. Η παρούσα δημοσίευση δεσμεύει μόνο τον συντάκη της και η Επιτροπή δεν ευθύνεται για τυχόν χρήση των πληροφοριών που περιέχονται σε αυτήν. ES - El presente proyecto ha sido financiado con el apoyo de la Comisión Europea. Esta publicación es responsabilidad exclusiva de su autor. La Comisión no es responsable del uso que pueda hacerse de la información aquí difundida. 4 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) ET - Projekti on rahaliselt toetanud Euroopa Komisjon. Publikatsiooni sisu peegeldab autori seisukohti ja Euroopa Komisjon ei ole vastutav selles sisalduva informatsiooni kasutamise eest. FI - Hanke on rahoitettu Euroopan komission tuella. Tästä julkaisusta (tiedotteesta) vastaa ainoastaan sen laatija, eikä komissio ole vastuussa siihen sisältyvien tietojen mahdollisesta käytöstä. FR - Ce projet a été financé avec le soutien de la Commission européenne. Cette publication (communication) n’engage que son auteur et la Commission n’est pas responsable de l’usage qui pourrait être fait des informations qui y sont contenues. GA - Maoiníodh an tionscadal seo le tacaíocht ón gCoimisiún Eorpach. Tuairimí an údair amháin atá san fhoilseachán [scéala] seo, agus ní bheidh an Coimisiún freagrach as aon úsáid a d’fhéadfaí a bhaint as an eolas atá ann. HU - Az Európai Bizottság támogatást nyújtott ennek a projektnek a költségeihez. Ez a kiadvány (közlemény) a szerzõ nézeteit tükrözi, és az Európai Bizottság nem tehetõ felelõssé az abban foglaltak bárminemû felhasználásért. IT - Il presente progetto è finanziato con il sostegno della Commissione europea. L’autore è il solo responsabile di questa pubblicazione (comunicazione) e la Commissione declina ogni responsabilità sull’uso che potrà essere fatto delle informazioni in essa contenute. NL - Dit project werd gefinancierd met de steun van de Europese Commissie. De verantwoordelijkheid voor deze publicatie (mededeling) ligt uitsluitend bij de auteur; de Commissie kan niet aansprakelijk worden gesteld voor het gebruik van de informatie die erin is vervat. 5 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) LT - Šis projektas finansuojamas remiant Europos Komisijai. Šis leidinys [pranešimas] atspindi tik autoriaus požiūrį, todėl Komisija negali būti laikoma atsakinga už bet kokį jame pateikiamos informacijos naudojimą. LV - Šis projekts tika finansēts ar Eiropas Komisijas atbalstu. Šī publikācija [paziņojums] atspoguļo vienīgi autora uzskatus, un Komisijai nevar uzlikt atbildību par tajā ietvertās informācijas jebkuru iespējamo izlietojumu. MT - Dan il-proġett ġie finanzjat bl-għajnuna tal-Kummissjoni Ewropea. Din il-publikazzjoni tirrifletti (Dan il-komunikat jirrifletti) l-opinjonijiet ta’ l-awtur biss, u l-Kummissjoni ma tistax tinżamm responsabbli għal kull tip ta’ uzu li jista’ jsir mill-informazzjoni li tinsab fiha ( fih). PL - Ten projekt został zrealizowany przy wsparciu finansowym Komisji Europejskiej. Projekt lub publikacja odzwierciedlają jedynie stanowisko ich autora i Komisja Europejska nie ponosi odpowiedzialności za umieszczoną w niej zawartość merytoryczną. PT - Projecto financiado com o apoio da Comissão Europeia. A informação contida nesta publicação (comunicação) vincula exclusivamente o autor, não sendo a Comissão responsável pela utilização que dela possa ser feita. RO - Acest proiect a fost finanţat cu sprijinul Comisiei Europene.<0}Această publicaţie (comunicare) reflectă numai punctul de vedere al autorului şi Comisia nu este responsabilă pentru eventuala utilizare a informaţiilor pe care le conţine. SK -Tento projekt bol financovaný s podporou Európskej Komisie. Táto publikácia (dokument) reprezentuje výlučne názor autora a Komisia nezodpovedá za akékoľvek použitie informácií obsiahnutých v tejto publikácii (dokumente). SL - Izvedba tega projekta je financirana s strani Evropske komisije. Vsebina publikacije (komunikacije) je izključno odgovornost avtorja in v nobenem primeru ne predstavlja stališč Evropske komisije. 6 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) SV - Projektet genomförs med ekonomiskt stöd från Europeiska kommissionen. För uppgifterna i denna publikation (som är ett meddelande) ansvarar endast upphovsmannen. Europeiska kommissionen tar inget ansvar för hur dessa uppgifter kan komma att användas 7 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Contents Abstract ....................................................................................................................................................... 11 1. Introduction ......................................................................................................................................... 12 1.1 Microporous materials in the micro scale ................................................................................... 12 1.2 Metal – Organic Framework ....................................................................................................... 14 1.3 Zeolitic-Imidazolate Framework ................................................................................................. 16 1.4 Metal-Organic Framework patterning......................................................................................... 21 2. Objectives ........................................................................................................................................... 23 3. Experimental ....................................................................................................................................... 25 3.1 Materials ..................................................................................................................................... 25 3.2 ZIF-8 layers on silicon based substrates ..................................................................................... 25 3.2.1 ZIF-8 layer synthesis on silicon substrate ........................................................................... 25 3.2.2 ZIF-8 layer synthesis on Si3N4 and SiO2 substrates ............................................................ 28 3.2.3 Patterning of ZIF-8 layer ..................................................................................................... 28 3.2.4 ZIF-8 layer synthesis on patterned Si substrate .................................................................. 32 3.3 ZIF-8 layers on polymeric substrates .......................................................................................... 34 3.3.1 Growth of ZIF-8 layers on SU-8 substrates ........................................................................ 34 3.3.2 Growth of ZIF-8 layers on porous polybenzimidazole (PBI) membranes .......................... 35 3.4 Characterization .......................................................................................................................... 37 3.4.1 Scanning Electron Microscopy (SEM) ............................................................................... 37 3.4.2 Profilemeter ......................................................................................................................... 37 3.4.3 X-Ray Diffraction (XRD) ................................................................................................... 38 3.5 Distillation of the methanolic waste solution .............................................................................. 38 4. Results and discussion ........................................................................................................................ 39 4.1 ZIF-8 layers on silicon based substrates ..................................................................................... 39 4.1.1 ZIF-8 layer synthesis on silicon support ............................................................................. 39 4.1.2 ZIF-8 layer synthesis on Si3N4 and SiO2 substrates ............................................................ 46 4.1.3 Patterning of ZIF-8 layer ..................................................................................................... 48 4.1.4 ZIF-8 layer synthesis on patterned Si substrate .................................................................. 58 4.2 ZIF-8 layers on polymeric substrates .......................................................................................... 61 4.2.1 Growth of ZIF-8 layers on SU-8 substrates ........................................................................ 61 4.2.2 Growth of ZIF-8 layers on porous PBI membranes ............................................................ 63 8 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 5. Conclusion and future work suggestions ............................................................................................ 68 5.1 ZIF-8 layer on silicon based supports ......................................................................................... 68 5.2 ZIF-8 layer on polymer based supports ...................................................................................... 69 Reference .................................................................................................................................................... 71 Appendix ..................................................................................................................................................... 78 9 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figures Figure 1 Diagram of zeolite films applications in the micro-scale[6] ......................................................... 12 Figure 2 Silicon micro preconcentrator ((a) micro pillars, (b) micro cavities) filled with DAY zeolite [7] .................................................................................................................................................................... 13 Figure 3 Schematic representation of SURMOF-2 structure consisting of metal nodes and organic ligand[13] .................................................................................................................................................... 14 Figure 4 (a) ZIF-8 structure viewed along the [100] direction with the crystallographic spacing d110 of 1.20 nm. Red: Zn; Blue: Nitrogen atom; Green: mIm cyclic molecule; H atoms have been omitted for clarity.[21]; (b) ZIF-8 SOD structure tiling [33] ......................................................................................... 17 Figure 5 Schematic goals of the Final Master Project in ZIF-8 layer integration in microdevices and membrane .................................................................................................................................................... 24 Figure 6 Experimental Set-up of ZIF-8 layer growth on silicon wafer pieces ............................................ 26 Figure 7 Silicon wafer containers. (a) PTFE, (b) glass and (c) PA holders ................................................ 27 Figure 8 Schematic cross section of SiO2 or Si3N4 support (without grids) ................................................ 28 Figure 9 Schematic cross section and top view of SiO2 or Si3N4 support (with grids) ............................... 28 Figure 10 Schematic representation of UV lithography test of ZIF-8 layer ............................................... 30 Figure 11 Schematic representation of UV lithography steps in patterning silicon wafer, continued by ZIF-8 layer growth on the patterned support .............................................................................................. 32 Figure 12 (a) Electrospunned PBI membrane and (b) asymetric porous PBI membrane ........................... 36 Figure 13 Experimental set-up of ZIF-8 layer synthesis on PBI supports. (a) small-scale trials and (b) bigscale synthesis using PA O-ring strecher frame .......................................................................................... 36 Figure 14Correlation between total free energy versus the radius of the clusters [68] ............................... 39 Figure 15 Comparison of 20-cycle ZIF-8 layer syntheses on (a) cleaned and (b) uncleaned silicon substrates ..................................................................................................................................................... 40 Figure 16 ZIF-8 layer thickness correlation the with number of cycles of synthesis (Lu et al.[55] and experimental) .............................................................................................................................................. 42 Figure 17 SEM images (cross section – left, top view – right) of ZIF-8 layers for (a) 10-cycle, (b) 15cycle and (c) 20-cycle syntheses ................................................................................................................. 43 Figure 18 X-Ray difractogram of ZIF-8 layer experimental sample and Lu et al.’s experiments .............. 44 Figure 19 ZIF-8 crystal and its crystallographic orientations [71] ............................................................. 45 Figure 20 2-cycle synthesis of ZIF-8 layer up-scaled growth in different synthesis containers. (a) Round PTFE, (b) up-right PA and (c) glass containers. Heterogenous depositions are shown in red circles. ....... 46 Figure 21 (a) Top-view of ZIF-8 layer on non-grid Si3N4 support. (b) Cross-section of ZIF-8 layer on grid Si3N4 support ............................................................................................................................................... 47 Figure 22 Top-view and cross-section of samples after wet etching using diluted HNO3 solution for (a) 5 seconds, (b) 10 seconds (c) 15 seconds and (d) 20 seconds ........................................................................ 50 Figure 23 ZIF-8 patterning results obtained by Lu et al.[63], showing pattern edge roughness of 500 nm 51 Figure 24 Correlation between etching time and thickness decrease of ZIF-8 layer .................................. 52 Figure 25 SEM top-view images of etched ZIF-8 layers using RIE method for (a) 1 minute, (b) 3 minutes and (c) 5 minutes ......................................................................................................................................... 52 16 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) MOF thin films and membranes have been utilized widely in the application of selective gas adsorption and gas separation for their properties in size exclusion and adsorbate-MOF surface interaction based separations.[18] There have been plenty of research on this matter, for example H2 purification and recovery using MOF layers such as MOF-5, HKUST-1, ZIF-8, etc. The others are CO2 separation and capture with ZIF-69 and ZIF-8 membranes, olefin/parafin separation using ZIF-8 membranes, and liquid separations using various types of MOF such as MIL-53, ZIF-8, ZIF-71, ZIF-78, etc.[17] Another application of MOF thin films is in the field of chemical sensor, especially in sensing vapors and gases in small concentration, such as nitro-containing molecules for explosives (TNT, DNT and DMNB), xylene, etc. The means of signal transduction of MOF sensors can be varied from electromechanical transduction (micro cantilever, quartz micro balance, surface acoustic wave devices), to luminescence transduction for MOFs that have luminescence effect under the adsorption of certain molecules (e.g. Zn3btc2 film for sensing TNT).[12] 1.3 Zeolitic-Imidazolate Framework Zeolitic-Imidazolate Framework (ZIF) is a subclass of MOF which has a crystalline structure consisting of Zn or Co metal nodes in the form of tetrahedral metal ions and imidazolate organic ligand linkages.[19,20,21] The term zeolitic refers to ZIFs similiarity to zeolite structure, due to the same bond angle of (M – Im – M) in ZIF and (Si – O – Si) in zeolites which is 145o.[20] There are different types of ZIFs varied by different metal nodes and functional group of imidazolate ligands, some of them have been extensively investigated in scientific journals and listed in Table 1. Comparing to other subclasses of MOFs, ZIFs hold higher chemical and thermal stability, making it useful for broad applications, such as gas separations, pervaporation and functional devices for ZIFs in the form of thin films.[20] 17 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Table 1 Various types of ZIFs [20] ZIF type Molecular structure Topology Pore size (nm) Ref ZIF-7 Zn(benzimidazole)2 SOD 0.3 21, 22 23, 24 23 ZIF-8 Zn(2-methylimidazole)2 SOD 0.34 ZIF-9 Co(benzilimidazole)2 SOD <0.30 ZIF-22 Zn(5-azabenzimidazolate)2 LTA 0.3 25 ZIF-69 Zn(5-chlorobenzimidazole)(2-nitroimidazole) GME 0.44 26 ZIF-71 Zn(4,5-dichloroimidazole)2 RHO 0.42 27 ZIF-78 Zn(5-nitrobenzimidazole)(2-nitroimidazole) GME 0.38 28 ZIF-90 Zn(imidazolate-2-carboxaldehyde)2 SOD 0.35 29 ZIF-95 Zn(5-chlorobenzimidazole)2 POZ 0.37 30 SIM-1 Zn(4-methyl-5-imidazolecarboxaldehyde)2 SOD <0.34 31 ZIF-9-67 Co(benzimidazole)(2-methylimidazole) SOD <0.34 32 In Figure 4(a), it is shown the crystallographic structure of ZIF-8, as a part of ZIF subclass, which is specifically containing tetrahedral corners (Zn(mIm)4 units) with each Zn2+ ions linked with two (mIm)- ligands to form a 3D sodalite (SOD) structure with a pore size of 3.4 Å.[21] The SOD structure is similiar to the topology of Zeolite A and will form tiling, shown in Figure 4(b). Figure 4 (a) ZIF-8 structure viewed along the [100] direction with the crystallographic spacing d110 of 1.20 nm. Red: Zn; Blue: Nitrogen atom; Green: mIm cyclic molecule; H atoms have been omitted for clarity.[21]; (b) ZIF-8 SOD structure tiling [33] a b 18 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) ZIF-8 thin film is synthesized based on direct synthesis and secondary growth method, as well as other methods summarized in Table 2, 3 and 4. The direct synthesis method is straight forward and does not include many different steps. Nevertheless, the resulted thin films often have intercrystal voids which are unfavorable in some applications, especially in gas separation membrane fabrication.[20] On the other hand, the secondary growth have been known to be more effective in thickness and crystal orientation control.[20] The drawbacks of secondary growth is more synthesis steps of ZIF-8 seed crystal which is done by solvothermal methods such as the one produced by Park et al.[34] ZIF-8 layers have been showing good performance in the application of H2 separation and recovery due to ZIF-8 small pore size, so that it is able to act as molecular sieve for the selectivity of H2 molecules (kinetic diameter of 2.9 Å) from other bigger molecules.[17] ZIF-8 is also known to exclude selectively water molecules for its hydrophobic properties.[23] Other applications such as, hydrocarbon separation and chemical vapors detection have also been tested by researchers shown in Table 3 and 4. Table 2 ZIF-8 syntheses by direct synthesis method Substrate Solvent Synthesis temperature (oC) Heating media Film thickness (µm) Synthesis time (h) Application Ref N/A (ZIF-8 nanoparticles) DMF 140 Oven - 4 Hydrogen adsorption 34 Titania Methanol 100 Microwave oven 30 4 H 2 /CH 4 gas separation (H2 selective) 23 ZnO deposited alumina Methanol 120 Oven 25 4 N/A 35 The main advantage of using methanol as solvent in the synthesis is that it has smaller kinetic diameter and weaker interaction with ZIF-8 framework compared to DMF. Therefore, methanol removal from the pore network is easier.[23] ZIF-8 precursors are more soluble in methanol in room temperature compared to water. Moreover, methanol has high volatility for the ease of film drying after the synthesis.[20] 19 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Table 3 ZIF-8 syntheses by secondary growth method Substrate Solvent Seeding method Synthesis temperature (oC) Heating media Film thickness (µm) Synthesis time (h) Application Ref Alumina Water ZIF-8 particles seeding 30 Oven 2.5 6 C2/C3 hydrocarbon gas mixtures separation (ethane/propane, ethylene/propylene and ethylene/propane) 36 Alumina hollow fiber Water ZIF-8 particles seeding 25 - 100 Oven 1.4 6 - 72 Single membrane permeation test of H2, N2 37 Alumina tube Methanol ZIF-8 particles seeding 110 Oven 5 4 H2/CO2, H2/N2 and H2/CH4 gas separation 38 Yttriastabilized zirconia hollow fiber Water ZIF-8 particles seeding 30 Oven 2 6 Highly H2 permeable membrane 39 Alumina Methanol ZIF-8 particles seeding 25 - ~1 <6 C 3 H 6 /C 3 H 8 and CH 4 /n-C 4 H 10 gas separation 40 Polyethersulfone Methanol ZIF-8 particles seeding 90 Oven 7.2 6 H2 separation membrane 41 Carbon nanotube Methanol ZIF-8 particles seeding 25 - 5 - 6 3 - 6 Gas separation enhancer of the verticallyaligned carbon nanotubes 42 Anodic aluminium oxide membrane Methanol Fast in situ seeding 25 Ultrasound - 4 Gas separation membrane (H2/CO2 and H2/N2) 43 APTES functionalized alumina hollow fiber Water Cyclic flow (continuous) secondary growth 30 Oven 2 6 H2 recovery 44 Alumina Methanol Hot support seeding 200 (seeding) Oven 12 0.3 H2/CH4 gas separation (H2 selective) 45 120 36 Alumina hollow fiber Methanol Hot support seeding 150 (seeding) Oven 20 4 High H2 permselective membrane 46 25 - 0.3 20 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Table 4 ZIF-8 syntheses by other methods Substrate Solvent Synthesis method Synthesis temperature (oC) Heating media Film thickness (µm) Synthesis time (h) Application Ref Alumina holow fiber Methanol Repeated growth 150 Oven 6 5 per cycle H 2 /CO 2 , N 2 /CO 2 and CH 4 /CO 2 gas separation 47 Polyethersulfone Methanol Layer-by-layer growth 25 - 10 30 Gas separation membrane (H2/CO2 and H2/C3H6) 48 Alumina Methanol Precursor infiltration 50 Oven 10 4 H2 selective membrane 49 Alumina holow fiber Methanol ZnO deposition and reactive seeding 100 Oven 8 5 Gas permeation and permselectivity 50 Nylon Methanol Counter-diffusion 25 - 16 16 - 72 Gas separation membrane (H2/N2) 51 Nylon Water Counter-diffusion 25 - 2.5 16 - 48 H 2 selective membrane 52 APTES functionalized alumina tube Methanol Counter-diffusion 150 Oven 2 5 H2 selective membrane 53 Alumina hollow fiber Methanol Counter-diffusion 50 Oven 70 12 - 96 Propylene/propane separation 54 Silicon wafer Methanol Repeated growth 25 - <5 0.5 per cycle Selective sensor for chemical vapors and gases (nhexane/cyclohexane, ethanol/water) 55 Silicon wafer N/A (solvent free) ZnO deposition and thermal synthesis 160 Oven 1 - 5 0.3 N/A 56 21 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Ceramic membranes, e.g. alumina and titania have been utilized in ultrafiltration application[57] and incorporation of ZIF-8 layers are intended to improve the material selectivity towards certain gases, especially H2. The ceramic hollow fiber and tube shaped supports, have the advantage of easy assembly into compact modules which is applicable directly in industrial separation processes.[20] Polymeric supports are favorable due to the good compatibility between ZIF-8 and polymeric compounds taking into account the interaction between the polymers and the organic ligands.[20] Silicon wafer is an option for ZIF-8 thin film support for the application in microdevices, in regards to the ability to produce microdevices using integrated zeolite films on silicon wafers[3,4,6,7,10], and the synthesis method obtained by Lu et al.[55] Other synthesis methods shown in Table 4, were conducted to improve direct and seeded growth syntheses. For the application in microsensors, Lu et al.[55] have conducted a method of synthesis in room temperature for the compatibility to the working condition in the clean room. Stassen et al.[56] have also improved Lu et al. methods by a solvent-free synthesis. Nevertheless, higher reaction temperature is needed for this method. Counter-diffusion method, in which the two precursor solutions are separated by porous supports, can be used to grow intergrown layer without the complexity of secondary growth processes.[58] 1.4 Metal-Organic Framework patterning For the objective of higher surface-to-volume ratio of MOF layers, which will lead to higher adsorption accessibility of gases, layer patterning is needed.[59] Several experiments have been conducted to achieve controlled patterns of MOF deposition. The growth of MOF-5, for example was successfuly controlled by previously functionalizing the support with SelfAssembly Monolayer (SAM) which has carboxylic acid functional groups that bond better with MOF-5 precursors. SAM was obtained by soft lithographic micro contact printing (µCP) or electron beam lithography (EBL).[16] Other functionalization is selective seeding of the support with ZIF-7 crystals to controlly grow ZIF-7 pattern.[60] 22 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Another method of patterning involve external control of the growth of MOF. Electrochemical induced synthesis was done by Ameloot et al.[61] for patterned deposition of HKUST-1 on copper substrates. Controlled deposition of HKUST-1 solution drops on patterned wells was also conducted by using dip-pen nanolithography.[62] Patterning can also be done after the growth of the MOF layer, for example the patterning of ZIF-8 layer by photolithography done by Lu et al.[63] and ZIF-9 layer using X-Ray lithography by Dimitrakakis et al.[64] 23 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 2. Objectives This Final Master Project consists of studies of Metal-Organic Framework layer. ZIF-8 layer has been chosen, taking into account of its thermal stability and its applications in hydrogen purification and gas sensor, also considering its ease of synthesis which can be conducted in room temperature, using methanolic precursor solutions; a method that could be directly implemented in clean room processes. The goal of this Final Master Project is to integrate ZIF-8 layer in various microdevices (mainly microsensor, micro preconcentrator and self-standing micro membrane) and a composite membrane. The challenge is to utilize the already developed methods in the literature for the synthesis of ZIF-8 together with the experiences of patterning zeolites developed in Nanostructured Films and Particles Group (NFP) of Instituto de Nanociencia de Aragon (INA) to reach the goal. The specific goals are described in Figure 5 and consist of: - ZIF-8 layer on silicon based supports; A. ZIF-8 microsensor B. ZIF-8 micro preconcentrator C. ZIF-8 free-standing micro membrane - ZIF-8 layer on polymer based supports; D. ZIF-8 free-standing micro membrane E. PBI/ZIF-8 composite membrane In order to reach the goals, the work is divided into the following tasks; a. Synthesis of ZIF-8 layer on silicon support • ZIF-8 layer growth • Chemical stability tests of ZIF-8 layer (etching and ultrasound treatments of the layer) • Patterning ZIF-8 layer by UV lithography b. Synthesis of ZIF-8 layer on patterned silicon support • Patterning silicon support by UV lithography • ZIF-8 layer growth on the patterned support 24 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) • Packaging the layer and building the micro preconcentrator device c. Synthesis of defect-free ZIF-8 layer on Si3N4 and SiO2 grid supports • ZIF-8 layer growth on the grid supports • Back-etching silicon layer and formation of free-standing ZIF-8 membrane d. Synthesis of defect-free ZIF-8 layer on SU-8 support • ZIF-8 layer growth on the patterned SU-8 support • Back-etching the support and formation of free-standing ZIF-8 membrane e. Synthesis of PBI/ZIF-8 composite membrane Figure 5 Schematic goals of the Final Master Project in ZIF-8 layer integration in microdevices and membrane 25 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 3. Experimental 3.1 Materials Precursors of ZIF-8 layer synthesis, zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and 2methylimidazole (mIm), and anhydrous methanol 99.8% (MeOH) was purchased from SigmaAldrich. The UV Lithography process used commercial adhesion promoter (TI Prime) and reversible photoresist (TI 35 ES) obtained from Microchemicals. The photo developer was purchased from AZ Electronic Materials and acetone from Panreac. Etching solutions consisted of tetramethyl-ammonium hydroxide 25% (TMAOH) was obtained from Fluka-Analytical, and potassium hydroxide 45% (KOH) and nitric acid 64 - 66% (HNO3) were from Sigma-Aldrich. 3.2 ZIF-8 layers on silicon based substrates Based on the objectives of integrating ZIF-8 layers in microdevices, silicon substrates were chosen, taking into account of the previous experiments by Lu et al.[55], the well establishment of silicon manufacturing technology in the semiconductor industry and the availability of developed micromachining methods of silicon substrates in Instituto de Nanociencia de Aragon (INA). 3.2.1 ZIF-8 layer synthesis on silicon substrate The syntheses of ZIF-8 layers were done by following the synthesis method established by Lu et al.[55], begun with small scale growth to obtain a reproducible results and continued with up-scaling synthesis on 3” silicon wafers. Silicon wafers (ptype and double polished with the crystal orientation of 100) were obtained from INA. 32 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 3.2.4 ZIF-8 layer synthesis on patterned Si substrate Substrate patterning and ZIF-8 layer synthesis For the purpose of integrating ZIF-8 layers in the fabricaton of micro preconcentrator, ZIF-8 layer was synthesized on patterned channels of the silicon support. The UV lithography method of channeling silicon wafers was the same of the one to pattern ZIF-8 layer, with differences in the order of the protocol and some parameters of the RIE process. The steps and process parameters of this experiment is shown in the scheme in Figure 11 and in Table 7. Figure 11 Schematic representation of UV lithography steps in patterning silicon wafer, continued by ZIF-8 layer growth on the patterned support 33 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Table 7 Patterning silicon wafer and ZIF-8 layer growth protocol Step Process Process Parameters Equipment 1. UV Lithography Substrate Heating T = 120 ºC; t = 10 min SUSS MicroTec Delta 20T2 Adhesive deposition Spin coating 4000 rpm Adhesive activation T = 120 ºC; t= 2 min Positive resist deposition Spin coating 4000 rpm Soft bake T = 100º C ; t = 2 min UV exposition Hard contact t = 25 sec Mask = oxygenator SUSS MicroTec MA6/BA6 Waiting time 1 min - Pattern revealing Diluted AZ Developer : H 2 O = 1:1 t = ± 50 s - Inspection under optical microscope - ZEISS Hard bake T = 140º C; t = 2 min SUSS MicroTec Delta 20T2 2. Reactive Ion Etching (RIE) Reactive = SF 6 Pf = 0.19 mbar RF = 200 watts Bias = 5 V Gas flux = 200 sccm t = 30 min SISTEC – RIE 600 3. ZIF-8 Layer Synthesis 20 cycles - 4. Lift-Off Process Method = mechanical scratch of resist layer and rinsing with acetone - 34 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Micro preconcentrator device packaging and test Device was closed by anodic bonding of silicon wafer with Pyrex glass which was available in the Clean Room Class 1000. The process was done in a homemade apparatus mounted by Servicio de Apoyo de Investigacion de Universidad de Zaragoza (SAI). The device closure was done by heating the silicon substrate at 300 oC and putting the Pyrex glass wafer on top of the silicon wafer and applying electrical voltage of 1000 volts. The closed device was then connected to pipes by first of all making a hole on the glass, at the inlet and outlet of the channel patterns, by sand blaster (alumina sand with the size of 125µm). 3.3 ZIF-8 layers on polymeric substrates ZIFs are known to have good compatibility with polymeric substances due to the organic content of the framework. Therefore, for the objective of fabricating intergrown, defect-free micro membranes, synthesis of ZIF-8 layers on polymeric substrates were experimented. 3.3.1 Growth of ZIF-8 layers on SU-8 substrates SU-8 is a type of polymeric materials which have high thermal stability (±300 oC).[65] Based on this property, the affinity of ZIF-8 crystals to polymeric materials and also the availability of patterned SU-8 supports in INA, the synthesize of ZIF-8 layer on this polymeric support was tested for the objectives of micro membrane fabrication. SU-8 support was made in Nanostructured Films and Particles (NFP) Group in INA by template molding method. ZIF-8 layer synthesis was done by following the method described in Section 3.2.1 for 20 cycles of synthesis. 35 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 3.3.2 Growth of ZIF-8 layers on porous polybenzimidazole (PBI) membranes PBI/ZIF-8 mixed matrix membranes (MMMs) have been investigated by Yang et al. [66] for the application of high temperature hydrogen purification in the petroleum and chemical industries, taking into account the high thermal stability of both materials individually and the compatibility between the organic ligand of ZIF-8 (2-methyl imidazole) and PBI. Nevertheless, MMMs are known to have drawbacks in poor fillerpolymer interaction, filler sedimentation and agglomeration[67]. Research in growing the ZIF-8 layer on the polymeric support has been done to improve the composite membrane properties. Growth of ZIF-8 layer on porous asymmetric polysulfone was investigated by Cacho-Bailo et al.[67] by repeated solvothermal synthesis method. A thick ZIF-8 intergrown layer (± 35 µm) deposition on the support was obtained and achieved high H2 selectivity among reported polymer-supported ZIF-8 membranes. Both Cacho-Bailo et al. and Yang et al. synthesized the composite membrane by incorporation of solvothermal-based ZIF-8 synthesis. In this experiment, a novel method of ZIF-8 layer synthesis on PBI was conducted, following the synthesis method done by Lu et al.[55] that was done in room temperature. Both porous PBI membranes (see Figure 12) were produced in NFP group in INA. The non-woven electrospunned PBI membranes were obtained by electrospinning method and have average thickness of 45 µm. The assymetric porous PBI membranes were produced by phase separation method of the homogenous polymer solution into a polymer rich and polymer lean phase and have average pore size of 100 nm and thickness of 60 µm. 36 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 12 (a) Electrospunned PBI membrane and (b) asymetric porous PBI membrane The growth was done by the method explained in Section 3.2.1 without Teflon holder and varied for 10, 15, and 20 cycles of synthesis. The synthesis set-ups are shown in Figure 13. The up-scale growth was done using an O-ring stretcher frame from a material of Polyamide (PA) to keep the membrane straight during the synthesis. Figure 13 Experimental set-up of ZIF-8 layer synthesis on PBI supports. (a) small-scale trials and (b) big-scale synthesis using PA O-ring strecher frame a b small scale big scale 37 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) The resulted composite membrane was then dried in 120 oC for 2.5 hours for methanol removal from the framework, and placed in the membrane module of the permeation test equipment available in INA. 3.4 Characterization 3.4.1 Scanning Electron Microscopy (SEM) SEM (FEI Inspect) was used to characterize the surface of the ZIF-8 layer and also the layer thickness by cross-sectional observation. The samples were placed on a stepped holder by carbon tape. Due to the high porosity of MOFs, these materials are known to be beam sensitive and the accelerated electron beam damages them. Therefore, samples were coated by platinum coating which was deposited by sputtering method using EMITECH CA 7620 Sputter Coater with an electric current (I) of 15 mA for 90 seconds. Pt-coated samples were observed in the SEM at low voltage 2 kV and spot size 2.0 to minimize the risk of framework collapsing due to electron beam exposure. In order to make sure about the layer boundaries between the support and the ZIF-8 layer, Energy Dispersive X-Ray Analyzer (EDX: INCA PentaFET X3) attached to the microscope was utilized. 3.4.2 Profilemeter The Profilemeter (KLA Tencor P-6) was mainly used to check the thickness difference between one part of a sample which was etched (by several etching methods; RIE using SF6 and O2 as reactive, wet etching using diluted HNO3) and the part which was not due to layer coverage using Kapton tape during the etching process. 38 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 3.4.3 X-Ray Diffraction (XRD) XRD was used to check the crystallography of the deposited layers on the silicon substrate, whether it matches the crystallography of ZIF-8 thin films or not. XRD characterization was done in Servicio de Difracción de Rayos X y Análisis por Fluorescencia del Servicio General de Apoyo a la Investigación de la Universidad de Zaragoza. The datas were obtained at ambient temperature using D-Max Rigaku difractometer equipped by a rotating anode and worked at 40 kV voltages and 80 mA current. Measurement conditions were: 2θ = 2.5 – 40o, step = 0.03o and t = 1 s/step. 3.5 Distillation of the methanolic waste solution Methanolic waste of ZIF-8 layer synthesis and washing was kept to be distillated in order to recover clean methanol. Distillation was done using rotavapor (Heidolph HEI-VAP Advantage) equipped with a spiral tube condenser. The waste solution was heated in oil bath at 110oC and 50 rpm rotation speed. The process was done without vacuum pump to avoid rapid vaporization and loss of methanol. The methanol vapor was condensed by cooling tap water and accumulated in the rounded glass container. The connections of the apparatuses in the distillation system were closed by Teflon tape instead of vacuum grease, so that there was not any organic contaminant in both waste and distilled methanol. The distillation process was repeated three times to maximize the purification of methanol. Distilled methanol was then used to synthesize ZIF-8 layers based on the 15 cycle process as explained in section 3.2.1, in order to proof if it can be recycled. Recovery of methanol from the methanolic waste can be successfully conducted by a 3-cycle distillation method. Synthesis of ZIF-8 layer using recycled methanol has less homogenous thickness due to impurities in the distilled methanol container and the laboratory environment. 39 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 4. Results and discussion 4.1 ZIF-8 layers on silicon based substrates The synthesis method proposed by Lu et al.[55] was followed and the reproducibility of the method was obtained by taking into account of details, such as cleaning of silicon supports, utilization of fresh mixture of solution and clean containers. 4.1.1 ZIF-8 layer synthesis on silicon support Importance of silicon support cleaning The cleaning step of substrate preparation plays an important role in terms of homogenous growth of the ZIF-8 layer. This is related to the energetic of nucleation stage of crystals to form stable clusters. The energy needed for the system to form stable clusters (with the radius r* in Figure 11) is defined as critical energy barrier (ΔGT*) in Equation 1. [68] Equation 1 Total free energy of nucleation process [68] ΔG𝑇𝑇= −4 3𝜋𝜋𝑟𝑟3 𝑉𝑉∆𝜇𝜇 + 4𝜋𝜋𝑟𝑟2𝜎𝜎 Figure 14Correlation between total free energy versus the radius of the clusters [68] 40 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) ΔGT is described as the total free energy. The term (4 3𝜋𝜋𝑟𝑟3 𝑉𝑉) is defined as the number of molecules contained in each nucleus, assuming spherical shape of the nucleus, ∆𝜇𝜇 is the difference in chemical potential between a molecule in solution and that in the bulk of the crystal. The term 4𝜋𝜋𝑟𝑟2 was the surface area of the spherical nucleus and 𝜎𝜎 is the surface free energy between the solid-liquid interfaces. In the case of impurities in the experiments, the presence of organic molecule on the surface of the substrate changes the surface free energy of some parts of the substrate, hence inducing heterogeneous nucleation of ZIF-8 on diverse area across the surface of the support, and formation of crystals with wide distribution of size. Therefore, imperfectly cleaned silicon substrate surface will induce less homogenous layer shown in Figure 12(b). Figure 15 Comparison of 20-cycle ZIF-8 layer syntheses on (a) cleaned and (b) uncleaned silicon substrates Importance of clean beaker glass In every cycle of the synthesis, it is also important to use clean glass container instead of the same container as the previous cycles. This is due to heterogenous nucleation of some of the fresh precursors on the ZIF-8 crystals deposited on the glass surface, so that the layer growth on the substrate is not maximized. a b 500 µm 500 µm 41 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Importance of utilization of fresh mixture of precursor solution Each cycle of ZIF-8 synthesis was done using fresh mixture of precursor solution, so that the layer growth could take place. Based on the experiments of two syntheses of ZIF-8 layers, one using fresh mixture of precursor solution every cycle and the other using the same solution in each cycle, the result of thickness characterization using profilemeter shows that the ZIF-8 layer synthesized using the same solution was not grown as much as the other sample. This is validated by the layer thickness of only 85.5 nm. This thickness profile was in accordance to only one-cycle synthesis. The number of moles in the precursor solution mixture was calculated and shows that stoichiometrically there are enough Zn(mIm)2 molecules to form 0.41 mm of ZIF-8 layer on a 2 x 2 cm2 support (calculation described in Apendix 1). Despite the excess amount of precursor molecules, it does not affect the thickness of the layer due to completed crystals formation of the precursors after ± 30 minutes of reaction[56]. Lu et al. [56] used Quartz Crystal Microbalance and exposed the crystal firstly to the Zn(NO3)2 solution until it reached saturation, then to the mIm solution. The changes in frequency, i.e. mass gain, indicated that after 30 minutes the reaction is completed. The increasing number of cycles, instead, was just increasing the density of the thin film layer according to experiments done by Tian et al.[69] Small scale growth ZIF-8 layer growth was conducted for different number of cycles in order to find its effect on layer thickness. Layer thickness was measured by SEM crosssectional observation, listed in Table 8 and shown in Figure 17. The top-view observation of SEM image shows intergrown layers of ZIF-8 crystals. The 48 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) The cracks contribute to membrane defects. Therefore, the layer cannot be integrated in the fabrication of micro membranes, even though that the layer growth was perfectly covering the grid step shown in Figure 26(b), which is one of the good processing factor in producing self-standing membranes. 4.1.3 Patterning of ZIF-8 layer Experiments were conducted before patterning the ZIF-8 layer to test the chemical stability of the layer towards processes in patterning method, such as etching and lift-off process. Wet etching of ZIF-8 layer - Wet etching using KOH and TMAOH solutions Wet etching using KOH and TMAOH solutions were used for the purpose of integrating ZIF-8 layer in the fabrication of free-standing micro membranes on Si3N4 and SiO2 grids. Back-etching of silicon support using basic solutions to build free-standing layer has been widely used. Pellejero et al.[75] fabricated free-standing silicalite cantilever by etching the silicon support using TMAOH solution (25%) at 70 oC for 3 hours. Altena et al. [76] used KOH solution for producing free-standing Si3N4 membrane. Stability of ZIF-8 layer was tested in KOH and TMAOH solution. The etching solutions of KOH and TMAOH is considered to be too reactive for ZIF-8 layers because after 5 seconds of sample immersion, all the ZIF-8 layer reacts with the etching reactant and cleaned silicon supports are obtained. This result can lead to loss of ZIF-8 layer during back-etching process of the silicon support. 49 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) - Wet etching using diluted HNO3 solutions Integration of thin films in the fabrication of microsensor has been investigated. Pellejero et al. [75] used HF solution (20 %wt) to pattern silicalite layer as microcantilever sensor. To etch ZIF-8 layer for microsensor fabrication, Lu et al.[63] used diluted HNO3. Etching was done to a 200 nm-thick layer, on which some parts were covered by photoresist as a part of UV lithography method. ZIF-8 layer which was exposed to the solution was completely reacted after 5 seconds of etching. Wet etching test was done using diluted HNO3 solution (purity 64 – 66%; 1000:1 ratio of H2O:HNO3) for ZIF-8 layers produced from a 15-cycle synthesis. The purpose of the experiment is to have a controlled etching method for patterning the ZIF-8 layer. SEM images of etching results are shown in Figure 22. The thickness decreases of wet etching process are listed in Table 9. Table 9 Wet etching of ZIF-8 layer using HNO3 solution results Etching time (seconds) Layer thickness (µm) Thickness decrease (µm) 0 (initial layer) 1.42 ± 0.07 - 5 1.05 ± 0.03 0.36 10 0.95 ± 0.02 0.46 15 0.94 ± 0.01 0.47 20 0.95 ± 0.03 0.46 50 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 22 Top-view and cross-section of samples after wet etching using diluted HNO3 solution for (a) 5 seconds, (b) 10 seconds (c) 15 seconds and (d) 20 seconds a 10 µm 3 µm 5 seconds 5 seconds 10 seconds 10 seconds b c d 15 seconds 15 seconds 20 seconds 20 seconds 10 µm 3 µm 10 µm 3 µm 10 µm 3 µm 51 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) The etching results show that it is not possible to establish a controlled etching rate. This is due to ZIF-8 etching mechanism which tends to have individual crystals detachment instead of lateral thickness decrease mechanism. This is in accordance to the tendency of silicalite thin film etched by HF solution that is reported by Pellejero et al.[75] Lu et al.[63] did not develop experiments to determine the etching rate of ZIF-8 layer. Nevertheless, it is observed that 5 seconds immersion in HNO3 solution can successfully etch away 200 nm of exposed ZIF-8 layer and make a pattern with pattern side roughness reaching 500 nm. The roughness shown in Figure 19 is possibly due to under etching phenomena and ZIF-8 etching mechanism in acid solution as also observed in the experiments (see Figure 23). Figure 23 ZIF-8 patterning results obtained by Lu et al.[63], showing pattern edge roughness of 500 nm Reactive Ion Etching (RIE) of ZIF-8 layer The experiments were conducted using a 10-cycle synthesized ZIF-8 layer with the layer thickness of 0.63 ± 0.02 µm. The etched layer thickness was measured using profilemeter and thickness decrease was clearly obtained (see Figure 24). However, the etching process produced etched ZIF-8 layer with large rugosity (± 500 nm) as measured using profilemeter and observed using SEM shown in Figure 25. 52 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 24 Correlation between etching time and thickness decrease of ZIF-8 layer Figure 25 SEM top-view images of etched ZIF-8 layers using RIE method for (a) 1 minute, (b) 3 minutes and (c) 5 minutes 73,8 165,5 296,7 Δthickness = 58,68 etching time R² = 0,986 0 50 100 150 200 250 300 350 0123456 Thickness decrease (nm) Etching time (minutes) Reactive Ion Etching (O2) of ZIF-8 layer a b c 5 µm 1-minute RIE 3-minute RIE 5-minute RIE 5 µm 5 µm 53 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Lu et al.[63] have conducted a novel patterning method for ZIF-8 layer using photo lithography method. In this experiment, a novel method of ZIF-8 layer etching using RIE was tested in order to obtain a method with controlled etching rate. Pellejero et al. [75] used SF6 as RIE reactant to etch silicalite layer and resulted in under-etching phenomena and rapid etching rate of silicon support. Reactive ion etching using oxygen plasma has been utilized to etch organic compounds.[77] ZIF-8 layer etching used O2 plasma RIE to attack the organic ligands in ZIF-8 framework and to avoid etching of silicon parts of the samples which happens in SF6 plasma.[75] Based on the results obtained, ZIF-8 layer etching using RIE with O2 plasma is considered to give more control in thickness decrease by an establishment of an etching rate. Moreover, the distribution of layer holes obtained by RIE is more homogenous compared to the one of the wet etching, especially layer hole shown in Figure 22(d). Effect of lift-off process on ZIF-8 layers Lift-off process in UV lithography is the last step in the patterning method to remove the photoresist layer deposition from the substrate. The process is done by dissolution of photoresist layer in acetone. To accelerate the rate of photoresist removal, ultrasound is utilized. Therefore, ZIF-8 layer stability towards ultrasound and acetone were tested. 54 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) - Effect of ultrasound on ZIF-8 layer Thompson et al.[78] have conducted experiments about the effect of ultrasound on ZIF-8 nanoparticles. The results show the phenomena of Ostwald ripening of the nanoparticles. Ostwald ripening of ZIF-8 particles happens due to higher temperature in some area (hot spots) that induces ZIF-8 constituents on the surface of the particles to dissolve and be recrystalized on the bigger particles’ surface. Therefore, the big particles become bigger and the small ones smaller or even vanished. The prefential growth of bigger crystals happens due to lower thermodynamic stability and surface-to-volume ratio of the smaller particles in the solution. A sample of a 10-cycle ZIF-8 layer on silicon supports were immersed in methanol and treated for 2, 5, and 15 minutes of ultrasound. The observation in SEM is shown in Figure 26. The top-view SEM images of the sonicated layers, show changes in the crystal shape compared to the one before sonication shown in Figure 17(a). Some cracks are also observed and even loss of layer after 15 minutes of sonication. Cross section images of the three samples are also represented in Figure 18. The thickness of the ZIF-8 layers after 2 and 5 minutes of sonication was not significantly decreased. The thickness of the layer before sonication was 0.63 ± 0.02 µm. On the other hand, the layer sonicated for 15 minutes had significant layer thickness decrease due to layer detachment as shown in the top-view image. SEM images show that there are parts which became denser and the other loss the crystals, which explains the cracks, layer detachment and crystal deformation. From these observations, it can be concluded that Ostwald ripening phenomena also appears in ZIF-8 thin layer. 55 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 26 Comparison between ZIF-8 layers (top-view and cross-section view) after sonification in methanol for (a) 2 minutes, (b) 5 minutes and (c) 15 minute - Effect of acetone (with and without ultrasound) on ZIF-8 layer Acetone and ultrasound effect on ZIF-8 layer was tested to observe the adhesion of ZIF-8 layer on the silicon support and the stability of the layer during the lift-off process. The results are shown in Figure 23. 2 minutes 2 minutes a b c 5 minutes 5 minutes 15 minutes 15 minutes 2 µm 2 µm 2 µm 3 µm 3 µm 3 µm 56 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 27 Effect of acetone to ZIF-8 layer (a) without sonication and with sonication for (b) 1 minute and (c) 2 minutes Immersion of ZIF-8 layer in acetone (Figure 27(a)) does not affect the stability of the ZIF-8 layer. The effect of sonication in methanol and acetone shows the same phenomena which is the Ostwald ripening of ZIF-8 crystals on the layer. ZIF-8 layer patterning by UV lithography Based on the preliminary tests done for etching ZIF-8 layer and investigating the effect of acetone and ultrasound to the stability of the layer, UV lithography of ZIF-8 a b c w/o ultrasound 2 µm 2 µm 2 µm 1 min ultrasound 2 min ultrasound 57 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) layer was done by RIE etching using O2 plasma, and lift-off process using acetone without ultrasound treatment. The result of UV exposure on ZIF-8 layer after development using AZ Developper solution is shown in Figure 28(a) where it can be seen clearly the pattern in which the black part was the photoresist which remained unexposed to the UV light. Positive photoresist (TI 35 ES) from Microchemicals consists of compounds of the diazonaphtho-quinone-sulphonates (DNQ) group, which transforms into carboxylic acid during UV exposure and become more soluble in basic solution, e.g. AZ developper solution.[79] The positive resist deposition on the ZIF-8 layer had the thickness of around 2.5 µm, as measured by profilemeter. Photoresist is also etched by O2 plasma in RIE process with an etching rate of ±1 µm/min. Therefore, etching of ZIF-8 layer was done for 10 minutes, to avoid complete etching of resist layer. Lift-off process was done by washing with acetone and obtained pattern (Figure 28(b)) was observed using profilemeter by measurement of layer step between the etched and non-etched ZIF-8 layer. The thickness decrease is 559.4 nm that is more or less is in accordance to the etching rate obtained in preliminary RIE test, with a standard error of 4.67%. Patterning ZIF-8 layer using UV lithography with a novel etching method by O2 plasma RIE has been conducted successfully. The method can be implemented in the fabrication of ZIF-8 microcantilever sensors, as substitute for zeolite cantilevers [80] that require hydrothermal synthesis. 64 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 34 SEM top-view (left) and cross section (right) of ZIF-8 deposition on electrospunned PBI membrane for (a,b) 10 cycle, (c,d) 15 cycle and (e,f) 20 cycle of synthesis a b c d e f 2 µm 2 µm 2 µm 5 µm 5 µm 4 µm 10 cycles 10 cycles 15 cycles 15 cycles 20 cycles 20 cycles 65 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) As the number of synthesis cycle increases, the density of the hybrid membrane increases. After 20 cycle of synthesis, the ZIF-8 layer started to grow on the surface of the membrane which is already covered by ZIF-8 crystals. This is due to the less porosity of the composite membrane and selective ZIF-8 nucleation on the deposited ZIF-8 crystals. Compared to PBI/ZIF-8 membrane obtained by Cacho-Bailo et al.[67], more deposition of ZIF-8 crystals were obtained inside the porous electrospunned PBI membrane, due to the high pore size of the PBI membrane. Moreover, the deposition of ZIF-8 layer was achieved on both sides of the PBI membrane (shown in Figure 35). Thicker ZIF-8 membrane (45 µm) was produced, compared to the 35 µm layer obtained by Cacho-Bailo et al. These results lead to denser membrane, i.e. more membrane retention and lower permeation flux. Figure 35 Cross section of PBI/ZIF-8 membrane from 20-cycle ZIF-8 layer synthesis Up-scale growth Based on the result of small scale trials, porous electrospunned PBI membrane was used. The stretching of the O-ring frame resulted to tear of the membrane on the edge part of the frame during the first cycle of the synthesis. Therefore, the synthesis was continued by direct immersion of the membrane without using stretcher (see Figure 36). 100 µm 45 µm 66 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 36 Final experimental set-up of big scale ZIF-8 synthesis on electrospunned PBI membrane Although the electrospunned PBI membrane could remain stable throughout the ZIF-8 layer synthesis, the dry composite membrane also shows brittleness especially during the placement of the membrane in the membrane module of the permeation test. Therefore composite membrane for the application of hydrogen separation in high temperature and pressure condition cannot be achieved. Distillation of methanolic waste solution from the synthesis and washing process of ZIF-8 layer growth, was successfully carried out in a more elevated temperature (110oC) from the boiling point of methanol[84] due to the elevated pressure in the distillation system which induce the increase of methanol boiling point. After 3 cycles of distillation, clear methanol was obtained. Synthesis of ZIF-8 layer for 15 numbers of cycles using distilled methanol produced layer deposition shown in Figure 37. The dots shown in the top view SEM image are caused by excessive Pt coating which was due to instable electrical current of the device. The thickness of the layer is less homogenous compared to the one obtained by utilizing anhydrous methanol due to impurities in the distilled methanol container and laboratory environment. 67 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) Figure 37 SEM images of ZIF-8 layer synthesized using distilled methanol The crystallography of the obtained ZIF-8 layer was also tested using XRD analysis and the difractogram is shown in Figure 38. The difractogram peaks pattern obtained from synthesis using distilled methanol are identical with the synthesis with anhydrous methanol. The result also shows the same preferential crystal orientation into (110) direction. Figure 38 X-Ray difractogram of ZIF-8 layer experimental (using anhydrous and distilled methanol) and Lu et al.’s results 2 µm 3 µm 68 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) 5. Conclusion and future work suggestions In this Final Master Project, experiments on ZIF-8 layer integration in microdevices and membrane fabrication have been conducted. The tasks were divided into layer synthesis on different supports. 5.1 ZIF-8 layer on silicon based supports ZIF-8 layer on silicon support ZIF-8 layer on silicon support has been successfully synthesized, based on the synthesis method established by Lu et al.[55] The average layer thickness which can be obtained each cycle of synthesis is 68.6 nm. The ZIF-8 crystallography was confirmed by XRD and has the crystal orientation preference of (110). Etching rate of ZIF-8 layer cannot be obtained from wet etching method using diluted HNO3 solution. However, an etching rate 58.7 nm/min can be developed by Reactive Ion Etching (RIE) using O2 plasma. Ultrasound has the effect of Ostwald ripening to the layer, creating defects such as cracks and layer detachment. The first goal of ZIF-8 layer patterning was successfully done by UV lithography method, incorporating O2 plasma RIE for the etching step and acetone washing without ultrasound in the lift-off process. ZIF-8 layer pattern has a step thickness of 559.4 nm. Results of ZIF-8 layer patterning shows that it is possible to pattern the layer by UV lithography, giving opportunity to exploit the layer in the application of microsensors. Future works in integration on ZIF-8 layer patterning in fabrication of ZIF-8 layer on microcantilevers and in its sensing ability can be carried out. Moreover, development of ZIF-8 layer synthesis inside micro continuous reactors by continuous precursor solution flow through the supports should be carried out in order to have a process that consumes less methanol and produces less waste. 69 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) ZIF-8 layer on patterned silicon support ZIF-8 layer has been successfully grown on channel-shaped patterned silicon wafer. Liftoff process cannot be conducted by acetone washing, but is able to be carried out mechanically. The goal of integrating the ZIF-8 layer in micro preconcentrator was established by closing the device using anodic bonding with Pyrex glass. Future works in building device connections to adsorption test equipment and adsorption test of several gases can be carried out for the purpose of preconcentrator of low concentration analytes, in the range of ppt, such as explosives. ZIF-8 layer on Si3N4 and SiO2 supports ZIF-8 layers on Si3N4 and SiO2 supports show cracks. Etching rate of ZIF-8 layer KOH and TMAOH solutions is too high, resulting in complete reaction of the layer after 5 seconds exposure in the solutions. Therefore the goal of fabricating ZIF-8 layer as a free-standing micro membrane on Si3N4 and SiO2 grids cannot be achieved. Future work on ZnO seed deposition before layer growing of ZIF-8 can be exploited to have better adhesion of the ZIF-8 layer on the Si3N4 and SiO2 supports. 5.2 ZIF-8 layer on polymer based supports ZIF-8 layer on SU-8 support ZIF-8 layer on SU-8 support also shows cracks and nonhomogeneity. The goal of integrating ZIF-8 layer in free-standing micro membrane using SU-8 support cannot be achieved. Future experiments on experimental set-up improvements should be conducted in order to avoid SU-8 support folding during the synthesis and avoid cracking of the ZIF-8 layer. 70 “The EM3E Master is an Education Programme supported by the European Commision, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centers and universities” (www.em3e.eu) ZIF-8 layer on porous PBI membranes Porous asymmetric PBI membrane is too brittle to endure the synthesis of ZIF-8 layer. The porous electrospunned PBI membrane shows stability during the synthesis of the ZIF-8 layer. ZIF-8 crystals have good adhesion to the PBI fibers and the increase of synthesis’ number of cycles increases the density of the membrane. The ZIF-8 crystals grow on both sides of the PBI support. 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