Dynamics of heuristics selection for cooperative behaviour

nanomaterials Article Incorporation of NiO into SiO2, TiO2, Al2O3, and Na4.2Ca2.8(Si6O18) Matrices: Medium Effect on the Optical Properties and Catalytic Degradation of Methylene Blue Carlos Diaz 1,*, María L. Valenzuela 2, Olga Cifuentes-Vaca 3, Marjorie Segovia 1and Miguel A. Laguna-Bercero 4,* 1Departamento de Química, Facultad de Química, Universidad de Chile, La Palmeras 3425, Nuñoa, Casilla 653, 7800003 Santiago de Chile, Chile; [email protected] 2 Inorganic Chemistry and Molecular Material Center, Facultad de Ingenier í a, Instituto de Ciencias Qu í micas Aplicadas, Universidad Autónoma de Chile, Av. El Llano Subercaseaux 2801, San Miguel, 8910060 Santiago de Chile, Chile; [email protected] 3Departamento Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Sede Concepción, Autopista Concepción-Talcahuano, 7100 Talcahuano, Chile; [email protected] 4Instituto de Nanociencia y Materiales de Aragón (INMA), CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain *Correspondence: [email protected] (C.D.); [email protected] (M.A.L.-B.) Received: 13 November 2020; Accepted: 3 December 2020; Published: 10 December 2020   Abstract: The medium effect of the optical and catalytic degradation of methylene blue was studied in the NiO/SiO 2 , NiO/TiO 2 , NiO/Al 2 O 3 , and NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) composites, which were prepared by a solid-state method. The new composites were characterized by XRD (X-ray diffraction of powder), SEM/EDS, TEM, and HR-TEM. The size of the NiO nanoparticles obtained from the PSP-4-PVP (polyvinylpyrrolidone) precursors inside the different matrices follow the order of SiO 2 >TiO 2 >Al 2 O 3 . However, NiO nanoparticles obtained from the chitosan precursor does not present an effect on the particle size. It was found that the medium effect of the matrices (SiO 2 , TiO 2 , Al 2 O 3 , and Na 4.2 Ca 2.8 (Si 6 O 18 )) on the photocatalytic methylene blue degradation, can be described as a specific interaction of the NiO material acting as a semiconductor with the MxOymaterials through a possible p-n junction. The highest catalytic activity was found for the TiO 2 and glass composites where a favorable p-n junction was formed. The isolating character of Al 2 O 3 and SiO 2 and their non-semiconductor behavior preclude this interaction to form a p-n junction, and thus a lower catalytic activity. NiO/SiO 2 and NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) showed a similar photocatalytic behavior. On the other hand, the effect of the matrix on the optical properties for the NiO/SiO 2 , NiO/TiO 2 , NiO/Al 2 O 3 , and NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) composites can be described by the different dielectric constants of the SiO 2 , TiO 2 , Al 2 O 3 , Na 4.2 Ca 2.8 (Si 6 O 18 ) matrices. The maxima absorption of the composites ( λmax ) exhibit a direct relationship with the dielectric constants, while their semiconductor bandgap (E g ) present an inverse relationship with the dielectric constants. A direct relationship between λmax and E g was found from these correlations. The effect of the polymer precursor on the particle size can explain some deviations from this relationship, as the correlation between the particle size and absorption is well known. Finally, the NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) composite was reported in this work for the first time. Keywords: nickel oxide; photocatalysis; chitosan; polyvinylpyrrolidone; optical properties Nanomaterials 2020,10, 2470; doi:10.3390/nano10122470 www.mdpi.com/journal/nanomaterials Nanomaterials 2020,10, 2470 2 of 17 1. Introduction Metal oxide nanoparticles are widely used in many applications such as coatings, catalysis, electrode materials, or sensors [ 1 ]. It is important to remark that their physical and chemical properties are strongly influenced by their agglomeration [ 2 ]. In this sense, it is well known that the incorporation of metal oxides onto inert support materials with high surface areas could help prevent particle agglomeration and also improve their reactivity and stability [3,4]. NiO is a p-type semiconductor with E G =3.5 eV presenting multiple practical applications [ 4 – 6 ]. However, their band gap can be modified by doping with other metal oxide semiconductors, and thus changing their photocatalytic properties [ 5 , 6 ]. NiO has been widely used in catalysis, battery cathodes, fuel cell electrodes, electrochromic films, electrochemical supercapacitors, or magnetic materials [ 4 – 6 ]. In this sense, Bonomo et al. [ 7 ] recently reported on the electrochemical and opto-electrochemical properties of nanostructured NiO for photoconversion applications. Although these applications are determined by their band-gap, which depend on the environment [ 8 , 9 ], no systematic studies have been reported regarding the effect of the medium on the band-gap behavior [ 10 – 12 ]. In this sense, it is well known that the dielectric medium affects the optical properties of nanoparticles, as previously observed for Au and Ag systems [ 10 ]. The optical properties of Au nanoparticles embedded into TiO 2 , ZrO 2 , and Al 2 O 3 have been also studied qualitatively [ 10 ]. In addition, the effect of SiO 2 , TiO 2 , and ZrO 2 supports was recently analyzed showing that MoO 3 /SiO 2 is the most efficient epoxidation catalyst [12]. The Na 4.2 Ca 2.8 (Si 6 O 18 ) compound (combeite) is a crystalline phase normally obtained from the fusion of precursor Na 2 O · CaO · SiO 2 glasses [ 13 – 15 ]. In this sense, there are no reported metal oxides using Na4.2Ca2.8(Si6O18) as a solid matrix. In previous works, we have reported a method to prepare metal and metal oxide nanostructured materials from a thermal treatment of the Chitosan (MLn)xand PS-co-4-PVP (MLn)xmacromolecular complexes [ 16 – 18 ]. The method consists of two steps: (1) Formation of both macromolecular complexes by a solvent assisted reaction between the respective polymer and the metallic salt; and (2) a thermal process of the solid under air atmosphere. The M ◦ and M x O y nanostructures can be easily incorporated into SiO 2 matrices using a similar approach by different thermal treatments of the solid-state precursors: Chitosan (ML n ) x/ /SiO 2 and PS-co-4-PVP (ML n ) x/ /SiO 2 affording M x O Y //SiO 2 composites [ 19 , 20 ]. This method can be also used to prepare NiO//M x O y composites using SiO 2 , TiO 2 , Al 2 O 3 , or Na 4.2 Ca 2.8 (Si 6 O 18 ) matrices. Although a few methods were proposed to prepare NiO/SiO 2 [ 4 , 21 , 22 ], NiO/TiO 2 [ 6 , 23 – 25 ], NiO/Al 2 O 3 [ 26 – 29 ] composites, none of them is as general and simple as the one described here. As for the Na 4.2 Ca 2.8 (Si 6 O 18 ) case, although this particular composition has not been reported, similar nickel oxide doped with silica matrices have been successfully synthesized via a sol–gel process [ 30 ]. Furthermore, this solid state method has been used for other systems [ 31 ]. A summary of the proposed fabrication route is shown in Figure 1[13]. Nanomaterials 2020, 10, x FOR PEER REVIEW 2 of 17 1. Introduction Metal oxide nanoparticles are widely used in many applications such as coatings, catalysis, electrode materials, or sensors [1]. It is important to remark that their physical and chemical properties are strongly influenced by their agglomeration [2]. In this sense, it is well known that the incorporation of metal oxides onto inert support materials with high surface areas could help prevent particle agglomeration and also improve their reactivity and stability [3,4]. NiO is a p-type semiconductor with E G = 3.5 eV presenting multiple practical applications [4–6]. However, their band gap can be modified by doping with other metal oxide semiconductors, and thus changing their photocatalytic properties [5,6]. NiO has been widely used in catalysis, battery cathodes, fuel cell electrodes, electrochromic films, electrochemical supercapacitors, or magnetic materials [4–6]. In this sense, Bonomo et al. [7] recently reported on the electrochemical and opto-electrochemical properties of nanostructured NiO for photoconversion applications. Although these applications are determined by their band-gap, which depend on the environment [8,9], no systematic studies have been reported regarding the effect of the medium on the band-gap behavior [10–12]. In this sense, it is well known that the dielectric medium affects the optical properties of nanoparticles, as previously observed for Au and Ag systems [10]. The optical properties of Au nanoparticles embedded into TiO 2 , ZrO 2 , and Al 2 O 3 have been also studied qualitatively [10]. In addition, the effect of SiO 2 , TiO 2 , and ZrO 2 supports was recently analyzed showing that MoO 3 /SiO 2 is the most efficient epoxidation catalyst [12]. The Na 4.2 Ca 2.8 (Si 6 O 18 ) compound (combeite) is a crystalline phase normally obtained from the fusion of precursor Na 2 O⋅CaO⋅SiO 2 glasses [13–15]. In this sense, there are no reported metal oxides using Na 4.2 Ca 2.8 (Si 6 O 18 ) as a solid matrix. In previous works, we have reported a method to prepare metal and metal oxide nanostructured materials from a thermal treatment of the Chitosan (ML n ) x and PS-co-4-PVP (ML n ) x macromolecular complexes [16–18]. The method consists of two steps: (1) Formation of both macromolecular complexes by a solvent assisted reaction between the respective polymer and the metallic salt; and (2) a thermal process of the solid under air atmosphere. The M° and M x O y nanostructures can be easily incorporated into SiO 2 matrices using a similar approach by different thermal treatments of the solid-state precursors: Chitosan (ML n ) x/ /SiO 2 and PS-co-4-PVP (ML n ) x/ /SiO 2 affording M x O Y //SiO 2 composites [19,20]. This method can be also used to prepare NiO//M x O y composites using SiO 2 , TiO 2 , Al 2 O 3 , or Na 4.2 Ca 2.8 (Si 6 O 18 ) matrices. Although a few methods were proposed to prepare NiO/SiO 2 [4,21,22], NiO/TiO 2 [6,23–25], NiO/Al 2 O 3 [26–29] composites, none of them is as general and simple as the one described here. As for the Na 4.2 Ca 2.8 (Si 6 O 18 ) case, although this particular composition has not been reported, similar nickel oxide doped with silica matrices have been successfully synthesized via a sol–gel process [30]. Furthermore, this solid state method has been used for other systems [31]. A summary of the proposed fabrication route is shown in Figure 1 [13]. In addition, the effect of the different matrices on the optical properties will also be studied and discussed. Figure 1. Schematic representation of the preparation method of metallic M° and metal oxides M x O y nanoparticles inside M’ x O´ y matrices. Figure 1. Schematic representation of the preparation method of metallic M ◦ and metal oxides M x O y nanoparticles inside M0xO0ymatrices. Nanomaterials 2020,10, 2470 3 of 17 In addition, the effect of the different matrices on the optical properties will also be studied and discussed. 2. Materials and Methods NiCl 2· 6H 2 O, tetraethyl orthosilicate (TEOS), chitosan, poly(styrene-co-4-vinilpyridine) PS-co-4-PVP, ethyl alcohol, acetic acid, and dichloromethane were supplied from Sigma-Aldrich and were used as received. 2.1. Preparation of the NiO/SiO2, NiO/TiO2, NiO/Al2O3Composites SiO 2 was prepared according to the literature procedures [ 19 , 20 ]. Briefly, tetraethoxysilane (TEOS), ethanol, and acetic acid were mixed in a molar ratio of 1:4:4 with water (nanopure milli-Q), and added over the dichloromethane solution of the previously prepared chitosan (NiCl 2· 6H 2 O) x and PS-co-4-PVP (NiCl 2· 6H 2 O) x . The mixture was stirred for 3 days. The obtained gel was dried at 100 ◦ C under a vacuum. The chitosan (NiCl 2· 6H 2 O) x //SiO 2 and PS-co-4-PVP (NiCl 2· 6H 2 O) x //SiO 2 precursors were finally calcined at 800 ◦C for 2 h under air. 2.2. Preparation of the Chitosan (NiCl2·6H2O)x//TiO2and PS-co-4-PVP (NiCl2)x//TiO2Precursors TiO 2 was prepared according to the literature procedures [ 19 , 20 ]. Briefly, titanium tetra-isopropoxide (Ti(OC 3 H 7 ) 4 , TTIP) ethanol and acetic acid were mixed in a molar ratio of 1:4:4 with water (nanopure milli-Q), and added over the dichloromethane solution of the previously prepared chitosan (NiCl 2· 6H 2 O) x and PS-co-4-PVP (NiCl 2· 6H 2 O) x . The mixture was stirred for 3 days. The obtained gel was dried at 100 ◦ C under a vacuum. The solid chitosan (NiCl 2· 6H 2 O) x/ /TiO 2 and PS-co-4-PVP (NiCl2·6H2O)x//TiO2precursors were calcined at 800 ◦C for 2 h under air. 2.3. Preparation of the Chitosan (NiCl2·6H2O)x//Al2O3and PS-co-4-PVP (NiCl2)x//Al2O3Precursors Al 2 O 3 was prepared according to the literature procedures [ 27 – 30 ]. Briefly, AlCl 3, ethanol, and acetic acid were mixed in a molar ratio of 1:4:4 with water (nanopure milli-Q), and added over the dichloromethane solution of the previously prepared chitosan (NiCl 2· 6H 2 O) x and PS-co-4-PVP (NiCl 2· 6H 2 O) x . The mixture was stirred for 3 days. The obtained gel was dried at 100 ◦ C under a vacuum. The solid chitosan (NiCl 2· 6H 2 O) x //Al 2 O 3 and PS-co-4-PVP (NiCl 2· 6H 2 O) x //Al 2 O 3 precursors were calcined at 800 ◦C for 2 h under air. 2.4. Preparation of the Precursors: Chitosan (NiCl2·6H2O)x//NiO/Na4.2Ca2.8(Si6O18) and PS-co-4-PVP (NiCl2)x//NiO/Na4.2Ca2.8(Si6O18) The compounds were prepared according to the literature procedures [ 28 ]. Briefly, tetraethoxysilane (TEOS), ethanol, and acetic acid were mixed in a molar ratio of 1:4:4 with water (nanopure milli-Q), then Na 2 O, CaO, and SiO 2 solids (in mol% of 14:1.5:73) were added over the dichloromethane solution of the previously prepared chitosan (NiCl 2· 6H 2 O) x and PS-co-4-PVP (NiCl 2· 6H 2 O) x . The mixture was stirred for 3 days. The obtained gel was dried at 100 ◦ C under a vacuum. The solid chitosan (NiCl 2· 6H 2 O) x //Na 2 O CaO SiO 2 and PS-co-4-PVP (NiCl 2· 6H 2 O) x //Na 2 O CaO SiO 2 precursors were calcined at 800 ◦C for 2 h under air. The coordination of the polymer was confirmed by IR analysis, as the broad ν (OH)+ ν (NH) band observed at 3448 cm −1 for free chitosan becomes unfolded upon coordination, shifting in the range of 3345–3393 cm −1 . On the other hand, the ν (py) band is shifting to high frequencies upon coordination [16–18]. Finally, polymer-metal complexes were placed into a box furnace (lab tech) using a pyrolysis temperature of 180 ◦ C for the precursor complexes and 800 ◦ C for the polymer complexes. Additional experimental conditions are summarized in Table 1. Nanomaterials 2020,10, 2470 4 of 17 Table 1. Composition of the pyrolytic products from the respective precursors. Precursor Precursor Formula Matrix Composite Composite Number (1) Chitosan·NiCl2(chitosan) - NiO C1 (2) PSP-4-PVP·NiCl2(PVP) - NiO C2 (3) Chitosan·NiCl2SiO2NiO/SiO2C3 (4) PSP-4-PVP·NiCl2SiO2NiO/SiO2C4 (5) Chitosan·NiCl2TiO2NiO/TiO2C5 (6) PSP-4-PVP·NiCl2TiO2NiO/TiO2C6 (7) Chitosan·NiCl2Al2O3NiO/Al2O3C7 (8) Chitosan·NiCl2Na4.2Ca2.8(Si6O18) NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) C8 2.5. Characterization IR spectra were recorded with a FT-IR Jasco 4600 spectrophotometer (Jasco Inc., Easton, MD, USA). Scanning electron microscopy (SEM) was performed on a JEOL 5410 scanning electron microscope (JEOL Ltd., Tokyo, Japan). Elemental microanalysis was performed by energy dispersive X-ray (EDS) analysis using a NORAN Instrument micro-probe attached to the SEM (Thermo Scientific, Waltham, MA, USA). High-resolution transmission electron microscopy (HR-TEM) was performed using a JEOL 2000FX TEM microscope (JEOL Ltd., Tokyo, Japan)at 200 kV to characterize the average particle size, distribution, and elemental and crystal composition. EDS analysis was performed in individual particles in order to discriminate NiO from the matrix. Average particle sizes were calculated using the Digital Micrograph software (Gatan, Inc., Pleasanton, CA, US). Methylene blue (MB) was used as a model compound to test the photocatalytic properties at 655 nm under UV-Vis illumination (Shimadzu UV-2600 spectrophotometer, Shimadzu Coorporation, Kyoto, Japan) using a xenon lamp (150 W) positioned 20 cm away from the photoreactor in a 330–680 nm range at room temperature, to avoid the self-degradation and thermal catalytic effects of cationic dye. Suspensions were stirred in the dark for 60 min to establish an adsorption/desorption equilibrium, after which the photocatalytic discoloration of MB was initiated. 3. Results and Discussion 3.1. Composite NiO/SiO2 The X-ray diffraction pattern of the as-synthesized NiO/SiO 2 composite for the material from the chitosan precursor is shown in Figure 2a. All the reflection peaks of the XRD pattern can be indexed to NiO and SiO 2 phases [ 19 ] (JPDS no. 03-065-2901 for NiO and JPDS no. 01-088-1535 for SiO 2 ). The broad feature appearing at 22 ◦ corresponds to amorphous silica [ 19 ]. Similar X-ray diffraction patterns for NiO from the PVP precursor were obtained. The SEM analysis (Figure 2b) shows irregular particle agglomerates, as typically observed from the preparation of nanoparticles using the solid-state thermal route [ 30 ]. From the TEM analysis, the agglomeration of NiO nanoparticles embedded into a mesh of SiO 2 can be observed in Figure 2c, where these agglomerates are composed of fused NiO nanoparticles. The size of these nanoparticles are in the range of 14 nm with a mean size of 25 nm (Figure 2c). Detailed HR-TEM images in Figure 2e,f show a homogeneous dispersion of NiO over the silica network. However, it was not possible to acquire high resolution images in order to study the interfaces between NiO and the different matrices. In any case, as also confirmed by SEM-EDS mapping (Figure 2g), there is a uniform distribution of NiO and SiO 2 particles. Similar results were observed for NiO obtained from the PVP precursor (see Supplementary Materials, Figure S1). The only difference is that NiO particles are bigger in size ca. 100 nm. Nanomaterials 2020,10, 2470 5 of 17 Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 17 Figure 2. (a) XRD pattern; (b) SEM image; (c) TEM image; (d) particle histogram; (e,f) HRTEM images; and (g) SEM element mapping of the pyrolytic NiO compound obtained using the chitosan precursor. The SEM analysis (Figure 2b) shows irregular particle agglomerates, as typically observed from the preparation of nanoparticles using the solid-state thermal route [30]. From the TEM analysis, the agglomeration of NiO nanoparticles embedded into a mesh of SiO 2 can be observed in Figure 2c, where these agglomerates are composed of fused NiO nanoparticles. The size of these nanoparticles are in the range of 14 nm with a mean size of 25 nm (Figure 2c). Detailed HR-TEM images in Figure 2e,f show a homogeneous dispersion of NiO over the silica network. However, it was not possible to acquire high resolution images in order to study the interfaces between NiO and the different matrices. In any case, as also confirmed by SEM-EDS mapping (Figure 2g), there is a uniform distribution of NiO and SiO 2 particles. Similar results were observed for NiO obtained from the PVP Figure 2. ( a ) XRD pattern; ( b ) SEM image; ( c ) TEM image; ( d ) particle histogram; ( e , f ) HRTEM images; and ( g ) SEM element mapping of the pyrolytic NiO compound obtained using the chitosan precursor. 3.2. NiO/TiO2 Figure 3shows the XRD pattern of the NiO/TiO 2 nanocomposite from the chitosan precursor, where the anatase phase and NiO are observed as single phases. Using this method, the pure TiO 2 anatase phase was obtained, in contrast with other solution methods, where a mixture of anatase and rutile in the NiO/TiO 2 composite was obtained [ 22 ]. The NiO/TiO 2 composite shows a “cotton” type morphology from the chitosan precursor (Figure 3b), whereas the morphology from the PVP precursor presents a more densified structure, as shown in Figure 3c. The SEM-EDS mapping, shown in Nanomaterials 2020,10, 2470 6 of 17 Figure 2g, indicates an homogeneous distribution of NiO and TiO 2 . Similar results were obtained for the NiO/TiO2from the PVP precursor (see Supplementary Materials, Figure S2). Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 17 precursor (see Supplementary Materials, Figure S1). The only difference is that NiO particles are bigger in size ca. 100 nm. 3.2. NiO/TiO 2 Figure 3 shows the XRD pattern of the NiO/TiO 2 nanocomposite from the chitosan precursor, where the anatase phase and NiO are observed as single phases. Using this method, the pure TiO 2 anatase phase was obtained, in contrast with other solution methods, where a mixture of anatase and rutile in the NiO/TiO 2 composite was obtained [22]. The NiO/TiO 2 composite shows a “cotton” type morphology from the chitosan precursor (Figure 3b), whereas the morphology from the PVP precursor presents a more densified structure, as shown in Figure 3c. The SEM-EDS mapping, shown in Figure 2g, indicates an homogeneous distribution of NiO and TiO 2 . Similar results were obtained for the NiO/TiO 2 from the PVP precursor (see Supplementary Materials, Figure S2). The TEM analysis (Figure 3d,e) presents a “spider web” TiO 2 network where the NiO nucleates forming agglomerated nanoparticles. They present a mean particle size of 25 nm (Figure 2f). A similar TEM analysis was observed for NiO/TiO 2 obtained from the PVP precursor (Figure S1b and Supplementary Materials, Figure S2). Figure 3. (a) XRD pattern; (b) SEM image of NiO from chitosan and (c) from PVP; (d,e) TEM images of NiO from chitosan and (f) their histogram; and (g) SEM mapping element for NiO from the chitosan precursor. Figure 3. ( a ) XRD pattern; ( b ) SEM image of NiO from chitosan and ( c ) from PVP; ( d , e ) TEM images of NiO from chitosan and ( f ) their histogram; and ( g ) SEM mapping element for NiO from the chitosan precursor. The TEM analysis (Figure 3d,e) presents a “spider web” TiO 2 network where the NiO nucleates forming agglomerated nanoparticles. They present a mean particle size of 25 nm (Figure 2f). A similar TEM analysis was observed for NiO/TiO 2 obtained from the PVP precursor (Figure 3b and Supplementary Materials, Figure S2). 3.3. NiO/Al2O3 Figure 4a shows the XRD pattern of the NiO/Al 2 O 3 composite from the chitosan precursor where the corresponding peaks of γ-Al2O3and NiO can be observed. Nanomaterials 2020,10, 2470 7 of 17 Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 17 3.3. NiO/Al 2 O 3 Figure 4a shows the XRD pattern of the NiO/Al 2 O 3 composite from the chitosan precursor where the corresponding peaks of γ-Al 2 O 3 and NiO can be observed. Figure 4. (a) XRD pattern of NiO/Al 2 O 3 from the chitosan precursor; (b) SEM image of NiO from chitosan and (c) from PVP; (d) TEM image of NiO from chitosan and (e) from PVP; (f) EDS mapping of NiO from chitosan. Figure 4. ( a ) XRD pattern of NiO/Al 2 O 3 from the chitosan precursor; ( b ) SEM image of NiO from chitosan and ( c ) from PVP; ( d ) TEM image of NiO from chitosan and ( e ) from PVP; ( f ) EDS mapping of NiO from chitosan. The effect of the polymer template on the morphology can be observed in Figure 4b,c. The chitosan precursor induces a “cotton” type morphology, while the PVP precursor also combines dense and Nanomaterials 2020,10, 2470 8 of 17 irregular zones. Figure 4f shows an elemental mapping image demonstrating that NiO is well dispersed inside Al2O3. A complete characterization is shown in Supplementary Materials, Figure S3. As observed for the NiO/TiO 2 system, the TEM analysis (Figure 4e) shows a “spider web” network of Al 2 O 3 where the NiO nucleates form agglomerates. The histogram (Supplementary Materials, Figure S3) shows a particle mean size of 17 nm. The HRTEM image of the NiO/Al 2 O 3 from the PVP precursor is shown in Supplementary Materials, Figure 3c, where it can be observed that the medium particle size is 32 nm. 3.4. NiO/Na4.2Ca2.8(Si6O18) The XRD pattern of the NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) composite prepared from the chitosan precursor indicates the formation of NiO inside the glass Na 4.2 Ca 2.8 (Si 6 O 18 ) (see Figure 5a). The XRD pattern is in agreement with those reported in the literature [ 13 – 15 ]. The observed morphology is similar to the one previously reported [ 13 – 15 ] (see Figure 5b,c), also presenting a uniform distribution of NiO inside the Na 4.2 Ca 2.8 (Si 6 O 18 ) (Figure 5d). Similar conclusions can be deduced for the PVP precursor (see Supplementary Materials, Figure S4). Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 17 The effect of the polymer template on the morphology can be observed in Figure 4b,c. The chitosan precursor induces a “cotton” type morphology, while the PVP precursor also combines dense and irregular zones. Figure 4f shows an elemental mapping image demonstrating that NiO is well dispersed inside Al 2 O 3 . A complete characterization is shown in Supplementary Materials, Figure S3. As observed for the NiO/TiO 2 system, the TEM analysis (Figure 4e) shows a “spider web” network of Al 2 O 3 where the NiO nucleates form agglomerates. The histogram (Supplementary Material, Figure S3) shows a particle mean size of 17 nm. The HRTEM image of the NiO/Al 2 O 3 from the PVP precursor is shown in Supplementary Materials, Figure 3c, where it can be observed that the medium particle size is 32 nm. 3.4. NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) The XRD pattern of the NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ) composite prepared from the chitosan precursor indicates the formation of NiO inside the glass Na 4.2 Ca 2.8 (Si 6 O 18 ) (see Figure 5a). The XRD pattern is in agreement with those reported in the literature [13–15]. The observed morphology is similar to the one previously reported [13–15] (see Figure 5b,c), also presenting a uniform distribution of NiO inside the Na 4.2 Ca 2.8 (Si 6 O 18 ) (Figure 5d). Similar conclusions can be deduced for the PVP precursor (see Supplementary Materials, Figure S4). Figure 5. (a) XRD pattern of NiO inside Na 4.2 Ca 2.8 (Si 6 O 18 ); (b) and (c) SEM images; and (d) EDS mapping by an element of the composite NiO/Na 4.2 Ca 2.8 (Si 6 O 18 ). Figure 5. ( a ) XRD pattern of NiO inside Na 4.2 Ca 2.8 (Si 6 O 18 ); ( b ) and ( c ) SEM images; and ( d ) EDS mapping by an element of the composite NiO/Na4.2Ca2.8(Si6O18). Nanomaterials 2020,10, 2470 9 of 17 A summary of the medium particle sizes for NiO included into the different matrices is presented in Table 2, where the effect of the matrix and that of the polymer precursors on the final particle sizes can be observed. Table 2. Nanoparticle size for the composites. Composite Precursor Formula Particle Size (nm) Reference NiO Chitosan·NiCl2>50 [17] NiO PSP-4-PVP·NiCl2>50 [17] NiO/SiO2Chitosan·NiCl225 This work NiO/SiO2PSP-4-PVP·NiCl2100 This work NiO/TiO2Chitosan·NiCl225 This work NiO/TiO2PSP-4-PVP·NiCl263 This work NiO/Al2O3Chitosan·NiCl230 This work NiO/Al2O3Chitosan·NiCl217 This work NiO/Na4.2Ca2.8(Si6O18) Chitosan·NiCl2Not measured This work The nanoparticle size of NiO obtained from the PVP precursor inside the matrices follow the order of SiO 2 >TiO 2 >Al 2 O 3 , while that for the NiO from the chitosan precursor does not present a significant effect on the nanoparticle size. 3.5. Photocatalytic Behavior Although the main applied property of NiO is in the field of electrochemistry as Li-ion batteries [ 32 ] and supercapacitors applications, [ 33 ] its application as a photocatalytic activity toward organic dyes have also been suggested [ 34 ]. In any case, reports on the photocatalytic activity toward organic dyes using NiO/matrices are scarce. Yu et al. [ 6 ] found a higher photocatalytic activity for NiO/TiO 2 than for pure NiO, towards the photodegradation of p-chlorophenol. Regarding the photocatalytic efficiency when using composites, important parameters to be considered include the formation of hierarchical porous structures, the dispersion of the catalytic semiconductor on the matrix surface, and the p-n junction in a NiO/M x O y composite, where a new band gap will be formed with a most favorable value for the photodegradation chemical processes. 3.6. NiO Methylene blue (MB) is extensively used as an organic dye in coloring paper, temporary hair colorant, dyeing cottons, and coating for paper stock [ 35 ]. The removal of this hazardous dye is considered as one of the growing requirements in recent years. The photocatalytic experiments were carried on the sample with definite dye concentration under dark conditions and UV irradiation. The band-gap of the NiO is 5.0 and 5.2 eV, when it is prepared from chitosan and PVP precursors, respectively. For the semiconductor metal oxides, their band gap value dictates their photocatalytic activity [ 35 , 36 ]. For this reason, the band gap of the C 3 –C 8 composites was determined. These values are: 5.0, 5.2, and 5.4 eV for the NiO/SiO 2 , NiO/TiO 2 , NiO/Al 2 O 3 composites, respectively, all obtained from the chitosan precursors. The values for the PVP precursor are: 5.5 eV, 5.2 eV for the NiO/SiO 2 , NiO/TiO 2 composites, respectively. Those values do not change significantly, and are slightly higher than those reported previously, which can be due to their bigger particle sizes [ 34 ] (see Supplementary Materials, Figure S5). The changes in the absorption spectra of the MB aqueous solution exposed to UV light for various times in the presence of NiO are shown in Supplementary Materials, Figure S6. The peak at 655 nm is characteristic of methylene blue and decreases with the irradiation time. Figure 6shows the plot of time vs. concentration of methylene blue measured as C/C o for NiO arising from both Nanomaterials 2020,10, 2470 16 of 17 23. Yu, J.H.; Yang, H.; Jung, R.H.; Lee, J.W.; Boo, J.H. Hierarchical NiO/TiO2 composite structures for enhanced electrochromic durability. Thin Solid Films 2018,664, 1–5. [CrossRef] 24. Chen, J.-Z.; Chen, T.-H.; Lai, L.-W.; Li, P.-Y.; Liu, H.-W.; Hong, Y.-Y.; Liu, D.-S. Preparation and Characterization of Suface Photocatalytic Activity with NiO/TiO 2 Nanocomposite Structure. 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