An Investigation on the Synthesis of Molybdenum Oxide and Its Silica Nanoparticle Composites for Dye Degradation

The molybdenum oxide (MoO3) and MoO3@SiO2 nanoparticles were successfully prepared using the chemical bath deposition (CBD) method. The photocatalytic activities of molybdenum oxide (MoO3), SiO2, and MoO3@SiO2 nanoparticles composite have shown a synergistic photocatalytic effect of SiO2 combined with MoO3. The first-order degradation rate constants for MoO3, SiO2, and MoO3@SiO2 nanocomposite were 10.3 × 10−3 min−1, 15.1 × 10−3 min−1, and 16.3 × 10−3 min−1, respectively. The MoO3@SiO2 composite showed degradation efficiencies in the methylene blue solution close to 100% after 60 min of UV irradiation. The X-ray diffraction (XRD) showed that the MoO3 powder has a hexagonal crystal structure and the silica is the tridymite type of SiO2. The crystallite size was about 94 nm, 32 nm, and 125 nm for MoO3, silica, and MoO3@SiO2, respectively, as calculated by the Scherrer equation. The scanning electron microscopy (SEM) images revealed that the MoO3 powder consisted of a uniform hexagonal structure; the silica showed a rod-like micro-flake morphology and the MoO3@SiO2 composite had the appearance of coral-like structures.


Introduction
Catalytic materials were developed over the years to purify the polluted water and air, where the heterogeneous photocatalysis plays an important role [1][2][3][4][5]. The well-known transition metal oxide photocatalysts are ZnO [6], TiO 2 [7], WO 3 [8], and MoO 3 [9]. The oxidation process of heterogeneous photocatalysis is achieved by using light to activate the catalyst and to generate highly reactive free radicals, which then reduce particular organic compounds [6,7,[9][10][11]. When the mineralization process is finished, the outcome consists of H 2 O and CO 2 .
The photocatalytic mechanism is activated when the oxide semiconductor is immersed in a liquid or placed in a gaseous medium and then irradiated with light of an energy that is equal to or greater than its bandgap [11]. In this case, electron-hole pairs are generated on the surface of the semiconductor and subsequent chemical reactions with the environmental media lead to the production of free radicals, which degrade the organic pollutants [10,11].
Photocatalysis requires a large surface area for reaction. The exterior of natural silica is covered with a network of pores to optimize the capture of light. This feature of silica has attracted the attention of nanotechnologists. Moreover, silica is a phylum of unicellular microalgae (from 2 µm to 1 mm) present in all aquatic environments, and the majorities are in biofilms (with a preference for cold water) and enveloped by a siliceous external skeleton. The degradation of MB under the visible light has been demonstrated for photocatalysts prepared using green and renewable resources [12]. Mesoporous silica impregnated with Pt-Porphyrin or PtNPs [13] and titania sensitized with porphyrin [14] or magnetic photocatalyst porphyrin [15] has also been shown visible-light-driven photocatalysis. MoO 3 is one of the most promising metal oxides because of its many advantages, such as its non-toxic nature, and can be widely used in an organic light-emitting diode, gas sensing, catalysis, transistors, and solar cells [16][17][18]. The smaller the particle size of MoO 3 , the larger the surface area, which potentially increases the adsorption methylene blue and photoactive sites, resulting in enhanced photocatalytic activity [9,16,19]. Various methods have been used to optimize the preparation and processing technology of molybdenum oxide films, such as thermal evaporation, sol-gel deposition, and chemical vapor deposition [20][21][22]. In our work, we used a simple and inexpensive chemical bath deposition (CBD) technique because the film properties can be optimized through various deposition process parameters. The CBD technique has received great consideration from the research community for the production of low-cost semiconductor photocatalyst.
In our search for novel and sustainable photocatalysts, we used the unique architecture of silica and high surface area to increase the degradation efficiency of the organic dyes. Because MoO 3 has shown good photocatalytic degradation properties [23][24][25], present in this study the synthesis of a novel MoO 3 and SiO 2 nanoparticle composites, labeled MoO 3 @SiO 2 , and their photocatalytic properties. The main objective of this work is to study the photocatalytic activities of the films for the photodegradation of methylene blue (MB) under UV light, as well as examine the physical properties of the MoO 3 films.

Experimental and Characterization Details
The chemical bath deposition (CBD) method was performed to synthesize nanocrystalline MoO 3 and MoO 3 on SiO 2 , as illustrated in Figure 1. In a typical synthesis, an aqueous solution of 15 mL of 0.05 M (NH 4 ) 6 Mo 7 O 24 ·4H 2 O (99%, Merck, Kenilworth, NJ, USA) solution was mixed with SiO 2 in a reaction bath. The temperature of the reaction bath was slowly increased to 50 • C. Then, 5 mL of concentrated HNO 3 (ACS reagent, ≥90.0%) was added dropwise with constant stirring until the pH of the solution was 2.2, and a clear solution was obtained. Then, after the solution was stirred for 15 min, the temperature of the reaction bath was raised to 70 • C, where the initial seeds started to form. The reaction bath was held at 70 • C for 30 min, during which time a white precipitate of h-MoO 3 nanoparticles was observed. When the synthesis was complete, the white precipitate was filtered using deionized water and then dried in an oven at a constant temperature of 110 • C for 1 h [26,27].
The nanoparticle composites were analyzed by X-ray diffraction (XRD) scanning electron microscopy (SEM), and UV-Vis spectroscopy. The XRD patterns were recorded on an X'pert PRO X-ray diffractometer (Malvern Panalytical Ltd, Malvern, UK) with graphite monochromatized Cu Kα radiation source (1.5406 Å). Morphologies of nanopowders were examined using a JEOL-JSM-6490 LV scanning electron microscope (SEMTech Solutions, Inc., North Billerica, MA, USA) and absorbance measurements were performed using a Perkin Elmer Lambda 950 spectrometer (Perkin Elmer, Waltham, MA, USA). The UV irradiation was performed at 254 nm wavelength using an 8 W power lamp (Philips Germicidal Ultraviolet-C, Philips Lightning, Eindhoven, The Netherlands). The nanoparticle composites were analyzed by X-ray diffraction (XRD) scanning electron microscopy (SEM), and UV-Vis spectroscopy. The XRD patterns were recorded on an X'pert PRO X-ray diffractometer (Malvern Panalytical Ltd, Malvern, UK) with graphite monochromatized Cu Kα radiation source (1.5406 Å). Morphologies of nanopowders were examined using a JEOL-JSM-6490 LV scanning electron microscope (SEMTech Solutions, Inc., North Billerica, MA, USA) and absorbance measurements were performed using a Perkin Elmer Lambda 950 spectrometer (Perkin Elmer, Waltham, MA, USA). The UV irradiation was performed at 254 nm wavelength using an 8 W power lamp (Philips Germicidal Ultraviolet-C, Philips Lightning, Eindhoven, The Netherlands).

Structural Properties
Both MoO3 nanoparticles and SiO2 were analyzed by X-ray diffraction analysis and compared to the MoO3@SiO2 nanoparticles composite. Figure 2 shows the XRD patterns of the MoO3, SiO2, and MoO3@SiO2 nanoparticles grown by chemical bath deposition. The diffraction patterns correspond to the h-MoO3 phase for the MoO3 nanoparticles and to tridymite, which is the monoclinic phase corresponding to SiO2 for the silica [28].

Structural Properties
Both MoO 3 nanoparticles and SiO 2 were analyzed by X-ray diffraction analysis and compared to the MoO 3 @SiO 2 nanoparticles composite. Figure 2 shows the XRD patterns of the MoO 3 , SiO 2, and MoO 3 @SiO 2 nanoparticles grown by chemical bath deposition. The diffraction patterns correspond to the h-MoO 3 phase for the MoO 3 nanoparticles and to tridymite, which is the monoclinic phase corresponding to SiO 2 for the silica [28]. Tridymite is a species of mineral of the tectosilicate family, and one of the polymorphs of silica with quartz, coesite, cristobalite, stishovite, having the chemical formula of SiO₂ and containing traces of titanium, aluminum, iron, manganese, magnesium, calcium, sodium, and potassium.
The crystallite size can be estimated from the full width half maximum (FWHM) values obtained from the predominant (210) for MoO 3 and MoO 3 @SiO 2 diffraction peak at 2θ = 25.7 • according to the following Debye-Sherrer equation [30][31][32]: where λ is the wavelength of Cu-Kα1 radiation (1.5406 Å) and θ is the Bragg diffraction angle. The crystallite sizes calculated with Equation (1) were around 94, 32, and 125 nm for MoO 3 , SiO 2 , and MoO 3 @SiO 2 , respectively. The observed broadening of the SiO 2 peak(004) in the MoO 3 @SiO 2 spectrum is attributed to the size and strain effect between MoO 3 and SiO 2 [19].
In the XRD spectra of the MoO 3 @SiO 2 composite, all peaks attributed to the MoO 3 phase are observed, which confirms that the MoO 3 nanoparticles are well grown on the SiO 2 surface. It was also observed that the intensity for all MoO 3 peaks decreases for MoO 3 @SiO 2 composite compared to those of MoO 3 . Note the existence of the preferential SiO 2 peak at 2θ = 21.6 • . All MoO 3 @SiO 2 peaks decreased in intensity, which may be due to the fact that MoO 3 nanoparticles are well-formed on the surface of SiO 2 , but in a dispersed manner.

Morphological Analysis
Surface morphology of the nanoparticles composite and constituent nanoparticles were investigated by using SEM analyses. The nanoparticles of MoO 3 consist of a uniform hexagonal rod-like morphology. The regular faceted surface of each hexagonal rod [33] is clearly seen in Figure 3a. In Figure 3b, stems have developed out of a central point [34], with flower-like clusters of hexagonal MoO 3 stem-shaped petals. The SEM images of SiO 2 ( Figure 3c) shows a similar morphology as SiO 2 [35]. The morphology of SiO 2 depicts mostly micro-flake and irregular rod-shaped with particles agglomeration. Regarding the MoO 3 @SiO 2 composite (Figure 3d,e), we observed the appearance of coral-like structures in the form of hexagonal rods. The morphology of this composite indicates the incorporation of MoO 3 into the SiO 2 in the MoO 3 @SiO 2 composite, which is in agreement with the XRD analysis in Figure 2. SEM observation shows that the specific surface area increased for the MoO 3 @SiO 2 composite compared to those for MoO 3 or SiO 2 . Increasing the specific surface area, especially in the case of MoO 3 @SiO 2 , could play an important role in improving sensitivity in optoelectronic applications like photocatalysis and gas sensors.
rod-shaped with particles agglomeration. Regarding the MoO3@SiO2 composite (Figure 3d,e), we observed the appearance of coral-like structures in the form of hexagonal rods. The morphology of this composite indicates the incorporation of MoO3 into the SiO2 in the MoO3@SiO2 composite, which is in agreement with the XRD analysis in Figure 2. SEM observation shows that the specific surface area increased for the MoO3@SiO2 composite compared to those for MoO3 or SiO2. Increasing the specific surface area, especially in the case of MoO3@SiO2, could play an important role in improving sensitivity in optoelectronic applications like photocatalysis and gas sensors.

Photocatalytic Studies
MoO3 nanoparticles were used as photocatalyst for the degradation of methylene blue (MB), which was used as a model compound. It was found that there was no MB degradation in the dark and in the presence of MoO3, SiO2, and MoO3@SiO2. In this work, we have monitored the MB degradation under UV light at different times with MoO3, SiO2, and MoO3@SiO2 nanoparticle catalysts. Figure 4 presents the UV-Vis absorption spectra of MoO3 nanoparticles, SiO2 and MoO3@SiO2 nanoparticles exposed to UV light for different times.
There are two different absorption bands for the aqueous cationic MB dye solution, i.e., at 293

Photocatalytic Studies
MoO 3 nanoparticles were used as photocatalyst for the degradation of methylene blue (MB), which was used as a model compound. It was found that there was no MB degradation in the dark and in the presence of MoO 3 , SiO 2 , and MoO 3 @SiO 2 . In this work, we have monitored the MB degradation under UV light at different times with MoO 3 , SiO 2 , and MoO 3 @SiO 2 nanoparticle catalysts. Figure 4 presents the UV-Vis absorption spectra of MoO 3 nanoparticles, SiO 2 and MoO 3 @SiO 2 nanoparticles exposed to UV light for different times.

Photocatalytic Studies
MoO3 nanoparticles were used as photocatalyst for the degradation of methylene blue (MB), which was used as a model compound. It was found that there was no MB degradation in the dark and in the presence of MoO3, SiO2, and MoO3@SiO2. In this work, we have monitored the MB degradation under UV light at different times with MoO3, SiO2, and MoO3@SiO2 nanoparticle catalysts. Figure 4 presents the UV-Vis absorption spectra of MoO3 nanoparticles, SiO2 and MoO3@SiO2 nanoparticles exposed to UV light for different times.
There are two different absorption bands for the aqueous cationic MB dye solution, i.e., at 293 nm ( - * ) and 664 nm ( - * ) [34]. In this work, the intensities of the absorption peaks at 664 nm decrease with increasing the time of irradiation, compared to the catalyst-free solution. The degradation of the MB solution containing h-MoO3 catalyst synthesized by CBD was 90% [34].  During photocatalysis, the electrons in the valence band of the oxide semiconductor are excited under UV light radiation and leave holes in the valence band after they jump to the conduction band. The holes combine with H2O to produce · H and · OH radicals. In the meanwhile, the electrons in the conduction band scattered towards the adsorbed O2 to generate activated · O2 [36] with the consequent transformation of the water molecules into · OH radicals.
The mechanism of photocatalytic degradation for MoO3nanoparticles is similar to that of a metal oxide semiconductor [37], as follows: There are two different absorption bands for the aqueous cationic MB dye solution, i.e., at 293 nm (π-π * ) and 664 nm (n-π * ) [34]. In this work, the intensities of the absorption peaks at 664 nm decrease with increasing the time of irradiation, compared to the catalyst-free solution. The degradation of the MB solution containing h-MoO 3 catalyst synthesized by CBD was 90% [34].
During photocatalysis, the electrons in the valence band of the oxide semiconductor are excited under UV light radiation and leave holes in the valence band after they jump to the conduction band. The holes combine with H 2 O to produce · H and · OH radicals. In the meanwhile, the electrons in the conduction band scattered towards the adsorbed O 2 to generate activated · O 2 [36] with the consequent transformation of the water molecules into · OH radicals.
The mechanism of photocatalytic degradation for MoO 3 nanoparticles is similar to that of a metal oxide semiconductor [37], as follows: These oxidizing species can degrade the MB dye into chemical forms of CO 2 and H 2 O, which is a better solution to water remediation treatments [36]. If the photocatalytic processes do not take place, the recombination of the (e − + h + ) pairs happens, and heat is generated in the materials. The photocatalytic activity depends on various factors, including the structure and the dimension of the particles, degree of crystallinity, specific surface area, adsorbed water molecules, and hydroxyl groups [38][39][40][41].
The degradation efficiency was further studied in the presence of h-MoO 3 , SiO 2 , and MoO 3 @SiO 2 nanoparticle composite in MB dye, and the results are presented in Figure 5. The degradation efficiency was calculated using the following equation [42,43]: where C 0 is the initial dye concentration in the solution, and C is the dye concentration in the solution after irradiation, for a given time interval [42].
Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 12 These oxidizing species can degrade the MB dye into chemical forms of CO2 and H2O, which is a better solution to water remediation treatments [36]. If the photocatalytic processes do not take place, the recombination of the (e − + h + ) pairs happens, and heat is generated in the materials. The photocatalytic activity depends on various factors, including the structure and the dimension of the particles, degree of crystallinity, specific surface area, adsorbed water molecules, and hydroxyl groups [38][39][40][41].
The degradation efficiency was further studied in the presence of h-MoO3, SiO2, and MoO3@SiO2 nanoparticle composite in MB dye, and the results are presented in Figure 5. The degradation efficiency was calculated using the following equation [42,43]: where C0 is the initial dye concentration in the solution, and C is the dye concentration in the solution after irradiation, for a given time interval [42]. Figure 5 shows that the degradation efficiency increases with exposure time under UV-light. The MoO3@SiO2 composite showed degradation efficiencies in the MB solution close to 100% after 60 min of UV irradiation. The MoO3@SiO2 composite showed stable rates of MB photodegradation up to six cycles. The rate kinetics analysis, an important parameter in the degradation studies, was performed to predict the rate at which MB is removed from the aqueous solution [42]. In these experiments, different amounts of MoO3, SiO2, and MoO3@SiO2 composite were used with a fixed concentration of MB. The reaction kinetics was calculated with Equation (8) Figure 5 shows that the degradation efficiency increases with exposure time under UV-light. The MoO 3 @SiO 2 composite showed degradation efficiencies in the MB solution close to 100% after 60 min of UV irradiation. The MoO 3 @SiO 2 composite showed stable rates of MB photodegradation up to six cycles.
The rate kinetics analysis, an important parameter in the degradation studies, was performed to predict the rate at which MB is removed from the aqueous solution [42]. In these experiments, different amounts of MoO 3 , SiO 2, and MoO 3 @SiO 2 composite were used with a fixed concentration of MB. The reaction kinetics was calculated with Equation (8) [42]: where C 0 and C were defined for Equation (7). The graph of the natural logarithm, Ln(C/C 0 ) for MB dye versus time in the presence of MoO 3 , SiO 2 , and MoO 3 @SiO 2 nanocomposite is presented in Figure 6.
Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 12 where C0 and C were defined for Equation (7). The graph of the natural logarithm, Ln(C/C0) for MB dye versus time in the presence of MoO3, SiO2, and MoO3@SiO2 nanocomposite is presented in Figure  6. The MB concentration presented in log scale in Figure 6 varies practically linearly with time, indicating that the photodegradation of MB dye follows the first-order kinetics [42]. The kinetic rate constants (k) were determined from the slope of fitted curves. The first-order degradation rate constants for MoO3, SiO2, and MoO3@SiO2 nanocomposite were 10.3 × 10 −3 min −1 , 15.1 × 10 −3 min −1 , and 16.3 × 10 −3 min −1 , respectively. Table 1 presents the rate constants for MB degradation obtained in this work in comparison to other literature data. The degradation rate of MB is faster for MoO3@SiO2 nanocomposite compared to MoO3 or SiO2.  The MB concentration presented in log scale in Figure 6 varies practically linearly with time, indicating that the photodegradation of MB dye follows the first-order kinetics [42]. The kinetic rate constants (k) were determined from the slope of fitted curves. The first-order degradation rate constants for MoO 3 , SiO 2 , and MoO 3 @SiO 2 nanocomposite were 10.3 × 10 −3 min −1 , 15.1 × 10 −3 min −1 , and 16.3 × 10 −3 min −1 , respectively. Table 1 presents the rate constants for MB degradation obtained in this work in comparison to other literature data. The degradation rate of MB is faster for MoO 3 @SiO 2 nanocomposite compared to MoO 3 or SiO 2 .

Conclusions
We have synthesized MoO 3 @SiO 2 nanoparticle composite using chemical bath deposition. The diffraction patterns are in good agreement with the hexagonal phase MoO 3 with the lattice parameters of a = 10.53 Å and c = 14.876 Å. The SiO 2 has the following lattice parameters: a = 25.93 Å, b = 5.01 Å, and c = 18.54 Å. The XRD analysis showed that MoO 3 , silica, and MoO 3 @SiO 2 nanoparticle composite have crystalline characteristics phase with an average crystallite size of about 94 nm, 32 nm, and 125 nm, respectively. SiO 2 showed micro-flakes morphology with agglomeration as confirmed by SEM analysis, irregular rod-shaped for MoO 3 , and coral-like structure for MoO 3 @SiO 2 . The optimum photocatalytic activity was found for MoO 3 @SiO 2 nanoparticles, with an efficiency of about 100% after 60 min of exposure to the UV-light, while the degradation efficiency for the same UV exposure time was about 90% and 70% for SiO 2 and MoO 3 , respectively. The degradation rate constants for MoO 3 , SiO 2 , and MoO 3 @SiO 2 nanocomposite were 10.3 × 10 −3 min −1 , 15.1 × 10 −3 min −1 , and 16.3 × 10 −3 min −1 , respectively. These results show that SiO 2 particles have a beneficial photocatalytic effect combined with MoO 3 in the MoO 3 @SiO 2 composite in the photocatalytic processes.