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

Article type: Research Article

Keywords: MoO, silica, nanoparticle composite, structural properties, photocatalytic properties, methylene blue

Authors: Olfa Kamoun ⚬, Abdelaziz Gassoumi, Salah Kouass ⚬, Badriyah Alhalaili, Ruxandra Vidu ⚬, Najoua Turki-Kamoun

Affiliations: Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université de Tunis El Manar, Tunis 2092, Tunisia; o.kamoun@yahoo.fr (O.K.); n.najouakamoun@gmail.com (N.T.-K.); Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia; Laboratoire Matériaux Utiles, Institut National de Recherche et d’Analyse Physico-Chimique (INRAP) Sidi Thabet, Ariana 2020, Tunisia; kouasssa@gmail.com; Nanotechnology and Advanced Materials Program, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait; bhalaili@kisr.edu.kw; Department of Electrical and Computer Engineering, University of California Davis, Davis, CA 95616, USA; Faculty of Materials Science and Engineering, University POLITEHNICA of Bucharest, 313 Splaiul Independentei, RO-060042 Bucharest, Romania

License: © 2020 by the authors. CC BY 4.0 Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Article links: DOI: 10.3390/nano10122409  |  PubMed: 33276515  |  PMC: PMC7761561

Relevance: Relevant: mentioned in keywords or abstract

Full text: PDF (2.4 MB)

1. Introduction

Catalytic materials were developed over the years to purify the polluted water and air, where the heterogeneous photocatalysis plays an important role [ref. 1,ref. 2,ref. 3,ref. 4,ref. 5]. The well-known transition metal oxide photocatalysts are ZnO [ref. 6], TiO₂ [ref. 7], WO₃ [ref. 8], and MoO₃ [ref. 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 [ref. 6,ref. 7,ref. 9,ref. 10,ref. 11]. When the mineralization process is finished, the outcome consists of H₂O and CO₂.

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 [ref. 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 [ref. 10,ref. 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 [ref. 12]. Mesoporous silica impregnated with Pt-Porphyrin or PtNPs [ref. 13] and titania sensitized with porphyrin [ref. 14] or magnetic photocatalyst porphyrin [ref. 15] has also been shown visible-light-driven photocatalysis. MoO₃ 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 [ref. 16,ref. 17,ref. 18]. The smaller the particle size of MoO₃, the larger the surface area, which potentially increases the adsorption methylene blue and photoactive sites, resulting in enhanced photocatalytic activity [ref. 9,ref. 16,ref. 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 [ref. 20,ref. 21,ref. 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 MoO₃ has shown good photocatalytic degradation properties [ref. 23,ref. 24,ref. 25], present in this study the synthesis of a novel MoO₃ and SiO₂ nanoparticle composites, labeled MoO₃@SiO₂, 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 MoO₃ films.

2. Experimental and Characterization Details

The chemical bath deposition (CBD) method was performed to synthesize nanocrystalline MoO₃ and MoO₃ on SiO₂, as illustrated in Figure 1. In a typical synthesis, an aqueous solution of 15 mL of 0.05 M (NH₄)₆Mo₇O₂₄·4H₂O (99%, Merck, Kenilworth, NJ, USA) solution was mixed with SiO₂ in a reaction bath. The temperature of the reaction bath was slowly increased to 50 °C. Then, 5 mL of concentrated HNO₃ (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-MoO₃ 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 [ref. 26,ref. 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).

3. Results and Discussion

3.1. 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, SiO_2, 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 [ref. 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.

MoO₃ has the following lattice parameters: a = 10.53 Å and c = 14.876 Å (JCPDS card no. 21-0569) [ref. 19]. The SiO₂ has the following lattice parameters: a = 25.93 Å, b = 5.01 Å, and c = 18.54 Å, with the highest intensity at 2θ = 21.6°, matching the reference R090042 [ref. 29].

The crystallite size can be estimated from the full width half maximum (FWHM) values obtained from the predominant (210) for MoO₃ and MoO₃@SiO₂ diffraction peak at 2θ = 25.7° according to the following Debye–Sherrer equation [ref. 30,ref. 31,ref. 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 MoO₃, SiO_2, and MoO₃@SiO₂, respectively. The observed broadening of the SiO₂ peak(004) in the MoO₃@SiO₂ spectrum is attributed to the size and strain effect between MoO₃ and SiO₂ [ref. 19].

In the XRD spectra of the MoO₃@SiO₂ composite, all peaks attributed to the MoO₃phase are observed, which confirms that the MoO₃ nanoparticles are well grown on the SiO₂ surface. It was also observed that the intensity for all MoO₃ peaks decreases for MoO₃@SiO₂ composite compared to those of MoO₃. Note the existence of the preferential SiO₂ peak at 2θ = 21.6°. All MoO₃@SiO₂ peaks decreased in intensity, which may be due to the fact that MoO₃ nanoparticles are well-formed on the surface of SiO₂, but in a dispersed manner.

3.2. Morphological Analysis

Surface morphology of the nanoparticles composite and constituent nanoparticles were investigated by using SEM analyses. The nanoparticles of MoO₃ consist of a uniform hexagonal rod-like morphology. The regular faceted surface of each hexagonal rod [ref. 33] is clearly seen in Figure 3a. In Figure 3b, stems have developed out of a central point [ref. 34], with flower-like clusters of hexagonal MoO₃ stem-shaped petals. The SEM images of SiO₂ (Figure 3c) shows a similar morphology as SiO₂ [ref. 35]. The morphology of SiO₂ depicts mostly micro-flake and irregular 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.

3.3. 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 (π-π∗) and 664 nm (n-π∗) [ref. 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% [ref. 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 [ref. 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 [ref. 37], as follows:

These oxidizing species can degrade the MB dye into chemical forms of CO₂ and H₂O, which is a better solution to water remediation treatments [ref. 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 [ref. 38,ref. 39,ref. 40,ref. 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 [ref. 42,ref. 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 [ref. 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 [ref. 42]. In these experiments, different amounts of MoO₃, SiO_2, and MoO₃@SiO₂ composite were used with a fixed concentration of MB. The reaction kinetics was calculated with Equation (8) [ref. 42]:

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 [ref. 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.

4. Conclusions

We have synthesized MoO₃@SiO₂ nanoparticle composite using chemical bath deposition. The diffraction patterns are in good agreement with the hexagonal phase MoO₃ with the lattice parameters of a = 10.53 Å and c = 14.876 Å. The SiO₂ has the following lattice parameters: a = 25.93 Å, b = 5.01 Å, and c = 18.54 Å. The XRD analysis showed that MoO₃, silica, and MoO₃@SiO₂ nanoparticle composite have crystalline characteristics phase with an average crystallite size of about 94 nm, 32 nm, and 125 nm, respectively. SiO₂ showed micro-flakes morphology with agglomeration as confirmed by SEM analysis, irregular rod-shaped for MoO₃, and coral-like structure for MoO₃@SiO₂. The optimum photocatalytic activity was found for MoO₃@SiO₂ 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 SiO₂ and MoO₃, respectively. The 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. These results show that SiO₂ particles have a beneficial photocatalytic effect combined with MoO₃ in the MoO₃@SiO₂ composite in the photocatalytic processes.

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