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Fabrication of WO3 Nanowrinkles on Silica Bead for Photocatalysis under Visible Light
Applied Science and Convergence Technology 2019;28:122-125
Published online July 31, 2019;  https://doi.org/10.5757/ASCT.2019.28.4.122
© 2019 The Korean Vacuum Society.

Hyun Sung Kim*

Department of Chemistry, Pukyong National University, Busan 48513, Republic of Korea
Correspondence to: E-mail: kimhs75@pknu.ac.kr
Received June 10, 2019; Revised June 18, 2019; Accepted June 20, 2019.
cc This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Abstract

WO3 nanowrinkles were successfully synthesized on SiO2 beads via a hydrothermal reaction. The as-prepared WO3-SiO2 core shell composites were characterized by X-ray diffraction, ultraviolet-visible spectroscopy, Brunauer-Emmett-Teller measurements, and scanning electron microscopy. The photocatalytic activity of nanowrinkled WO3-SiO2 core shell composites toward O2 evolution was two times higher than that of commercial WO3 nanoparticles. The Rhodamine-6G degradation efficiency of nanowrinkled WO3-SiO2 composites was two times higher than that of commercial WO3 nanoparticles. The fabrication method used in this study can be used for developing novel photocatalytically active WO3 composites for the photodissociation of water under visible light.

Keywords : Tungsten oxide, Silica bead, Core-shell structure, Photocatalyst, Dye degradation, O2 evolution
I. Introduction

Photocatalysts utilize solar energy to photodissociate water and purify environmental organic pollutants. Titanium dioxide (TiO2) is the most commonly used photocatalyst and has been studied extensively [1]. However, the wide bandgap (3.2 eV) of TiO2 photocatalysts limits their practical applications, especially those requiring visible light [24]. Another promising metal oxide photocatalyst, tungsten trioxide (WO3), has been used extensively in catalytically active materials and electrochromic devices owing to its wide bandgap range of 2.4–2.8 eV. This wide bandgap range makes WO3 highly active photocatalytically under visible light [57].

Recently, core-shell nanostructured materials have gained immense attention owing to their fascinating properties and wide range of catalytic applications. Various efforts have been made to improve the photocatalytic activity of WO3, one of which is by fabricating different types of substrate-supported WO3 cores. To improve the photocatalytic efficiency of WO3, different types of functional metal oxide substrates are used as core materials [8,9]. These functional materials not only act as robust supporters for effective separation after the reaction, but also increase the surface area of the photocatalyst. In this study, core-shell structured WO3 nanowrinkles were fabricated on silicon dioxide (SiO2) beads, which acted as the support. The as-prepared WO3-SiO2 core-shell composites showed higher photocatalytic efficiency for Rhodamine-6G degradation under visible light illumination than that of WO3 nanoparticles. In addition, the photocatalytic oxygen evolution properties of the resulting nanowrinkled WO3@SiO2 core-shell beads were evaluated by measuring the amount of oxygen evolved, which is a result of water splitting, and then compared them with those of WO3 nanoparticles.

II. Experiments

Preparation of SiO2 beads

According to the previous reports [10], silica beads with the size of 300–1000 nm were prepared by carrying out the hydrolysis of tetraethylorthosilicate (TEOS, 10 mL) in ethanol (150 mL) in the presence of 35 % ammonium hydroxide solution (NH4OH, 35 mL) at room temperature. The size of the beads was controlled by changing the concentrations of TEOS and NH4OH. For instance, in the case of 500-nm silica beads, 20 mL of TEOS was added to a solution containing 350 mL of ethanol and 75 mL of 35 % NH4OH, and then the mixture was stirred for 3 h at room temperature. To increase the size of the silica beads from 500 to 700 nm, 16 mL of TEOS and 8 mL of NH4OH were simultaneously added drop-wise into the solution containing 500-nm silica beads. Small silica beads were obtained by decreasing the amounts of TEOS and NH4OH.

Surface functionalization of SiO2 bead with amine group

The prepared SiO2 beads (1.0 g) were introduced into a two necks round bottom flask (250 mL). The flask was then evacuated under vacuum below 10−2 torr for 2 h and was charged with high purity argon. Dry toluene (50 mL) was added to the flask and was sonicated for 5 min to disperse the SiO2 beads in the toluene. 3-aminopropyl trimethoxysilane (1 mL), which is a surface-silylating agent, was added and refluxed for 6 h with continuous stirring. After reaction, the reactor was cooled down to the room temperature and was washed with fresh toluene to remove extra surface-silylating agent completely. After washing, surfaced modified SiO2 beads were dried by evacuation for 20 min.

Preparation of flake wall type-shell beads

Silica beads (0.06 g) were added to vials containing dimethylformamide (DMF) (5 mL) and were dispersed ultrasonically for 3 min. Then, 1,4-phenylenediamine (0.06 mol, 0.06 g) was dissolved in these vials containing DMF (5 mL). Tungsten hexachloride (WCl6) (0.06 mol, 0.24 g) was then added to a 25 mL-round bottom flask containing 10 mL of DMF. The resulting solution was heated at 140 °C for 10 min with continuous stirring. The solution of amine-functionalized silica beads was added to the WCl6 solution with continuous stirring for 10 min. The resultant powders were calcined at 500 °C for 3 h in air at a heating rate of 5 °C/min to remove the organic components and to obtain the WO3@SiO2 beads.

Photocatalytic dye degradation

The photocatalytic activity of the WO3@SiO2 core-shell beads in aqueous solution was evaluated by carrying out Rhodamine-6G (5 mg/L) degradation under visible-light irradiation (300 W Xe lamp) by using a cut-off filter (> 420 nm). Prior to the irradiation, the suspensions of the WO3@SiO2 photocatalyst (30 mg) in Rhodamine-6G (50 mL) were stirred for 5 min in the dark. Subsequently, the suspension (approximately 2 mL) was extracted after every 20 min and the change in the concentration of the solution was monitored by gauging the maximum absorbance of Rhodamine-6G at 550 nm by using a UV-vis spectrophotometer. The degradation efficiency of the photocatalyst was calculated using the equation:

D=ln (Ct/C0)×100%,

where C0 is the initial concentration and Ct is the concentration after a certain time according to the Beer-Lambert law [11,12].

Oxygen evolution analysis

The photocatalytic reaction was carried out in an inner irradiation quartz cell (5 mL). The reaction cell was connected to a closed gas circulation system and the gases evolved were analyzed by using an on-line thermal conductivity detector (TCD) gas chromatograph (model: Agilent5890 seriesII, helium carrier) [6]. Similar to a typical photocatalytic reaction, 10 mg of the WO3 powder, loaded with 0.3 wt% Pt nanoparticles, was suspended in 5 mL of an aqueous KIO3 solution (5 mmol dm−3) under magnetic stirring. The reaction vessel was evacuated several times to remove the air completely. The mixture was then irradiated with light using a 200 W high-pressure mercury lamp. The temperature of the reaction medium was maintained at 298 K by circulating cooling water during the reaction.

III. Results and Discussion

Uniform-sized (~500 nm) SiO2 beads were prepared using the Stober method, as shown in Figs. 1(a) and 1(b). The procedure for the fabrication of WO3 nanowrinkles on the surface of the silica beads is illustrated in Scheme 1. The morphology of the pristine core-shell WO3@SiO2 beads was examined using scanning electron microscopy and transmission electron microscopy [Figs. 1(c) and 1(d)]. The WO3@SiO2 bead microspheres produced after calcination at 773 K remained intact and maintained the fine nanowrinkled structure of the WO3 shell with an average diameter of ca. 3 μm. The crystal structure of the WO3@SiO2 beads was examined using X-ray diffraction (XRD), as shown in Fig. 2. Because the silica beads (core) were amorphous, all the exhibited peaks can be attributed to the shell material, WO3. Calcined samples exhibit broad diffraction patterns characteristic of the monoclinic structure (JCPDS card 43–1035). In addition, the XRD peaks of the WO3@SiO2 bead microspheres could be indexed to those of the commercial WO3 nanoparticles. This indicates that the WO3 shells on the silica beads did not contain any impurity.

The surface areas of WO3@SiO2 beads and WO3 nanoparticles were 411.2 and 314.2 m2/g, respectively (Fig. 3), as revealed by their N2 adsorption isotherms at 77 K and Brunauer–Emmett–Teller analysis. The surface area of the WO3@SiO2 beads was larger than that of the WO3 nanoparticles because of their nanowrinkled structure. This unique nanostructure of the WO3@SiO2 beads contributed to their improved catalyst performance.

The corresponding absorption spectra (represented by the Kubelka–Munk function) are displayed in Fig. 4. This indicates that at approximately 400 nm, WO3@ SiO2 beads and WO3 nanoparticles absorb the entire light.

The photocatalytic activities of the WO3@SiO2 beads and nanocrystalline WO3 were evaluated by carrying out dye degradation. Figure 5(a) shows the C/C0 vs. exposure time (to visible light irradiation) of the samples. Here, C represents the dye concentration at each time interval and C0 represents the initial concentration. From the photolysis curves, it is evident that the dye degradation occurs only in the presence of the catalyst (WO3@SiO2 bead and nanocrystalline WO3). The WO3@SiO2 beads degraded 80 % of the dye over a total test time of 3 h, while the nanocrystalline WO3 degraded 62 % of the dye. This indicates that the WO3@SiO2 beads were approximately 1.3 times more efficient than the WO3 nanoparticles.

Photocatalytic water oxidation reactions were carried out using the WO3@SiO2 beads and nanocrystalline WO3. Figure 5(b) shows the amount of O2 evolved from an aqueous solution containing KIO3 as the electron acceptor in the presence of each of the photocatalysts (WO3@SiO2 beads and nanocrystalline WO3) by visiblelight irradiation. The amount of O2 evolved from the aqueous solution after 2 h for the WO3@SiO2 beads were 7.9 mL/g, which is more than 6 times of that evolved for nanocrystalline WO3.

IV. Summary

Nanowrinkled WO3 was fabricated on the surface of the uniform-sized SiO2 beads (WO3@SiO2 beads). Owing to their unique morphology, the WO3@SiO2 beads showed higher dye degradation efficiency and O2 evolution (from water splitting) than those of the nanocrystalline WO3. Therefore, this fabrication method for WO3 nanostructures offers a new pathway for developing photocatalytically active WO3 composites for the photodissociation of water under visible light.

Acknowledgements

This work was supported by a Research Grant of Pukyong National University (2017 year).

Figures
Fig. 1. (Color online) SEM images: (a), (b) silica beads as core material with different magnifications and (c), (d) wrinkled WO3@SiO2 bead composites.

Fig. 2. XRD pattern of wrinkled WO3@SiO2 bead composites, WO3 nanoparticles and SiO2 beads as indicated.

Fig. 3. (Color online) Surface area from BET analysis of (a) wrinkled WO3@SiO2 bead composites and (b) WO3 nanoparticles as indicated.

Fig. 4. (Color online) UV-vis absorption spectra of wrinkled WO3@SiO2 bead composites and WO3 nanoparticles as indicated.

Fig. 5. (a) Comparison of Rhodamine-6G degradation under visible light (>400 nm) (b) the amount of photocatalytic O2 evolution on wrinkled WO3@SiO2 bead composites and WO3 nanoparticles as indicated.

Fig. 6. (Color online) Schematic of fabrication procedure of wrinkled WO3@SiO2 bead composites.

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