Microwave assisted synthesis of high-surface area WO₃ particles decorated with mosaic patterns via hydrochloric acid treatment of Bi₂W₂O₉

Abstract

Monoclinic WO₃ particles with mosaic structures on the planes of the particles were synthesized from layered bismuth tungstate (Bi₂W₂O₉) with the alternate stacked structure of Bi₂O₂²⁺ layers and W₂O₇²⁻ layers via a hydrothermal technique using hydrochloric acid at 200 °C under microwave heating. These particles possessed high surface areas, giving a high photocatalytic activity in the degradation of gaseous acetaldehyde. Sequential SEM observations have clarified the dynamic transformations of the structures of Bi₂W₂O₉ under microwave heating in comparison with conventional heating. The WO₃ production through the reaction of Bi₂W₂O₉ with HCl consists of two reaction steps, i.e., H₂W₂O₇ generation via replacement of Bi₂O₂²⁺ with H⁺ (the first step) and conversion of H₂W₂O₇ to WO₃ through dehydration of H₂W₂O₇ (the second step). The first step proceeds even at room temperature, while the second reaction requires temperatures above 180 °C. To investigate the microwave heating effect on the first step (the replacement of Bi₂O₂²⁺), the reaction of Bi₂W₂O₉ and HCl was carried out at 80 °C under both microwave heating and conventional heating. It has been found that the replacement of Bi₂O₂²⁺ with H⁺ is accelerated by the microwave selective heating effect. Interestingly, the WO₃ particles with mosaic patterns were produced only under microwave heating. On the other hand, conventional heating of Bi₂W₂O₉ in the presence of HCl resulted in the formation of plate-like WO₃ particles without mosaic patterns.

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Introduction

Tungsten trioxide (WO₃) is an n-type semiconductor with a band gap of 2.6 eV, resulting in visible light absorption. Therefore it is widely used as a visible light responsive photo-catalyst and an anode electrode material of photovoltaic devices.^1–10 In particular, WO₃ is recognized as a characteristic catalyst enabling oxygen generation in Z-scheme type photo-catalytic systems.^6–10 The photo-catalytic activity of WO₃ is as high as TiO₂ under UV light irradiation, and higher than N-doped TiO₂ under visible light irradiation.¹¹ Although this compound is considered to be conventional, it has still been attracting much interests of the researchers working on photocatalysis. Actually some researches produced the WO₃ particles with high surface areas by controlling the geometry or morphology of particles in order to improve the photo-catalytic activity of WO₃ particles.^12–15 Recently, Ueda et al. obtained three dimensionally ordered macroporous (3DOM) tungsten oxides with high surface area (up to 62 m² g⁻¹) by a high temperature treatment of tungsten precursor ((NH₄)₃H₂W₁₂O₄₀, WCl₆, W(OEt)₅ or H₃PW₁₂O₄₀) coated on poly-methyl methacrylate (PMMA).¹⁵ The photo-catalytic performance of the 3DOM WO₃ was uplifted relative to WO₃ particles prepared without PMMA. Other metal oxides (Fe₂O₃, CeO₂, ZnO, etc.) with 3DOM structures were also prepared by the similar procedure to 3DOM.¹⁶ Furthermore, WO₃ nanoparticles with high surface area were synthesized from (NH₄)₃H₂W₁₂O₄₀ or Na₂WO₄ via using mesoporous silica as a substrate¹⁷ or a sol–gel process.¹⁸

In this report, we have paid our attention to layered bismuth tungstate, Bi₂W₂O₉, as a precursor of WO₃ particles with high surface area. Bi₂W₂O₉ consists of alternately stacking anionic tungstate layers, W₂O₇²⁻, and cationic bismuth oxide layer, Bi₂O₂²⁺.^19,20 Nano-sized W₂O₇²⁻ layer having ReO₃ structure type tungsten trioxides interacts with nano-sized Bi₂O₂²⁺ layer through the Coulomb's force, resulting in the alternate piled structures of Bi₂W₂O₉.

Bi₂W₂O₉ gives layered tungstate, H₂W₂O₇, by a treatment with hydrochloric acid.²¹ (eqn (1))

Bi₂W₂O₉ + 2HCl → H₂W₂O₇ + Bi₂O₂²⁺ + 2Cl⁻ (1)

where bismuth oxide layers are eluted into a liquid phase. According to the TG data of H₂W₂O₇, the loss of interlayer hydration water occurs below 120 °C and H₂W₂O₇ is converted into WO₃ via dehydration of the hydroxyl groups above 180 °C.²¹ (eqn (2))

H₂W₂O₇ → 2WO₃ + H₂O (2)

Moreover, we have conceived an idea that microwave irradiation can assist the preparation of the WO₃ particles with high surface area. Microwave irradiation as a heating method was popularly used for inorganic synthesis, exhibiting some advantages such as shortening the reaction time^22,23 or controlling of morphology of the produced compounds.^24–26 These advantages of microwave methods are attributed to characters of microwave heating; rapid heating or substance-selective heating. The rapid heating is caused by heat generation in the irradiated substances through the loss of microwave energy. The selective heating is observed for an object consisting of two or more substances that have much different microwave loss factors (tan δ). When such object is irradiated with microwaves, a substance with greater tan δ value is heated more efficiently than that with lower tan δ, resulting in a large temperature gradient in the object. This phenomenon was directly observed by Raman spectroscopy in Tsukahara's work.²⁷ Recently, Maitani et al. found a specific microwave heating phenomenon observed at the interface of two different materials.²⁸ TiO₂ paste coated on a fluorine-doped tin oxide film is heated up efficiently under microwave irradiation, in which the heat is assumed to be generated locally at the interface between the two substances. This characteristic heating by microwave irradiation might be related to “interfacial dipolar relaxation” proposed elsewhere.²⁹

We can find an example in which microwaves enhances synthesis of WO₃.³⁰ Spitzer et al. reported that the nanoparticles synthesized using the microwave heating have a larger specific surface area and smaller particle sizes than by the conventional heating in the hydrothermal method using WCl₆ as a precursor.³¹ Particle size control of WO₃ by microwaves can be attributed to selective heating of seed crystals of WO₃, when it is taken into account that tungsten trioxides (WO₃) have great tan δ value (0.5 at 1.8 GHz, 0.436 at 9.14 GHz).^32,33

This paper reports a new synthesis of WO₃ particles using Bi₂W₂O₉ as a starting material under microwave irradiation. The effects of microwave heating on the morphology of the generated WO₃ particles are investigated in comparison with conventional heating. The replacement of Bi₂O₂²⁺ with protons is accelerated by the effects of microwave heating, giving H₂W₂O_7, which is further converted to WO₃ at the last stage. A mechanism of the chemical transformation of Bi₂W₂O₉ under microwave irradiation is proposed based on the sequential observations of Bi₂W₂O₉ by SEM. WO₃ particles with large surface area synthesized under microwave irradiation show high photo-catalytic activity for decomposition of gaseous acetaldehyde. Specific effects of microwaves on the chemical transformation of Bi₂W₂O₉ are discussed especially focused on the substance-selective heating and interfacial dipole heating for the layered Bi₂W₂O₉ with extremely large interfacial area between W₂O₇²⁻ and Bi₂O₂²⁺.

Experimental

Chemical reagents

All the chemicals were reagent grade, and used as supplied. Bismuth(III) oxide (99.9%), hydrochloric acid, octylamine (98.0+%), heptane (99.0+%), methanol and H₂PtCl₆·6H₂O were purchased from Wako Pure Chemical Industries, Ltd. Tungsten(VI) oxide (99.99%) was purchased from High Purity Materials KOJUNDO CHEMICAL LABORATORY Co., Ltd. Water was purified by a Millipore Direct-Q 3 Ultrapure Water System to a resistivity of 18.2 MΩ cm.

Preparation of Bi₂W₂O₉

Bi₂W₂O₉ was obtained from the reported procedure.¹⁹ Bismuth(III) oxide (3.02 g) and tungsten(VI) oxide (3.00 g) were mixed by a pestle and mortar. The mixture powder was put in a platinum crucible and calcined at 800 °C for 48 h. Then, the resulting powder was identified as Bi₂W₂O₉ by XRD measurement as shown in Fig. 1a. The EDX spectrum shown therein shows only the sharp peaks attributed to Bi₂W₂O₉, ensuring the high purity of Bi₂W₂O₉ (Fig. S2†).

Fig. 1 (a) XRD patterns of (i) Bi₂W₂O₉, (ii) Bi₂W₂O₉ treated with HCl for 1 h at 80 °C under microwave heating and (iii) Bi₂W₂O₉ treated with HCl for 1 h at 80 °C under conventional heating. (b) SEM image of (i) Bi₂W₂O₉, (ii) Bi₂W₂O₉ treated with HCl for 1 h at 80 °C under microwave heating and (iii) Bi₂W₂O₉ treated with HCl for 1 h at 80 °C under conventional heating.

Hydrochloric acid treatment at 200 °C

Bi₂W₂O₉ (200 mg) suspended in 6 M hydrochloric acid (40 mL) was put into a Teflon (R) autoclave reactor with the volume of 80 mL. Then the reactor was heated at 200 °C. Microwave heating was carried out by Microsynth (Milestone Inc.). The rate of temperature rise was 40 °C min⁻¹ under microwaves. After the temperature reached 200 °C, the samples were maintained at the temperature during various minutes, and then cooled to room temperature. Conventional heating was carried out using a hydrothermal synthesis reactor unit (HIRO COMPANY). The sealed Teflon (R) autoclave was put into the reactor unit heated at 200 °C. After various hours, the autoclave was cooled to room temperature. The reaction product was filtered and dried at 40 °C under vacuum several hours, and then the acid treated samples were obtained.

Hydrochloric acid treatment at 80 °C

Bi₂W₂O₉ (20 mg) suspended in water (2.0 mL) was pre-heated at 80 °C. Then, 12 M hydrochloric acid (2.0 mL) was injected into the above suspension solution. The sample solutions were collected 1, 2, 3, 5 min after HCl was injected. The sampled solutions were centrifuged and dried at 40 °C under vacuum several hours, and then the acid treated samples were obtained. The reaction vessels were Pyrex (R) test tubes with 18 mm inner diameter. A microwave irradiation system with an ellipsoidal chamber (CHRONICS) was used for microwave heating and, on the other hand, an oil bath was used for conventional heating. The reaction solutions were stirred by a mechanical stirrer with a stirring bar shaped like a flat-blade screwdriver. The stirring speed was set as 200 rpm. Reaction temperatures were measured by a fiber-optic thermometer (Opsens, Picosens).

n-Octylamine intercalation

n-Octylamine was intercalated into layered tungstate as previously reported.²¹ The powder obtained by the hydrochloric acid treatment (100 mg) was suspended in a mixture of heptane (2.0 mL) and n-octylamine (1.0 mL), and stirred at room temperature. After 4 h, the suspension was centrifuged and washed by acetone several times and dried at 40 °C under vacuum several hours, and then the intercalated samples were obtained.

Photo-catalytic reaction

Visible light irradiation was performed by a 300 W Xe lamp (MAX-302; Asahi Spectra Co., Ltd) equipped with a 400 nm cut-off filter. Pt co-catalyst was deposited on synthesized WO₃ particles by the reported photo-deposition method.¹¹ The acid treated WO₃ particles (100 mg) were dispersed into water, and H₂PtCl₂·H₂O (2.65 mg corresponding to the amount of 1 wt% of Pt on WO₃) was added into the dispersion. The dispersion was stirred and irradiated with λ > 400 nm light. After 2 hour irradiation, methanol was added and then the dispersion was irradiated with λ > 400 nm light for additional 2 hours. The dispersion was centrifuged, washed by acetone several times and dried at 40 °C under vacuum several hours, and then Pt supported WO₃ particles were obtained.

Photo-catalytic decomposition of gaseous acetaldehyde was performed in a Pyrex reaction vessel with the internal volume of 320 mL. The Pt supported WO₃ particles were spread on the flat bottom of the reaction vessel. Liquid acetaldehyde (15 µmol corresponding to the concentration of ca. 1000 ppm) was introduced into the vessel. The Pt supported WO₃ particles spread on the bottom was irradiated with λ > 400 nm light through the bottom. The components in the gas phase were analysed by FID gas chromatography (Simadzu, GC-16A equipped with an Inart Cap Wax capillary column).

Characterization

X-ray diffraction spectra were collected by using desktop X-ray diffractometer Miniflex (RIGAKU) with bent monochromated Cu Kα radiation. Scanning electron microscopy (SEM) images were collected by a Hitachi S-5500 scanning electron microscope equipped with an energy dispersive X-ray spectroscope (EDX). Nitrogen adsorption measurements were performed on a Belsorp-mini (BEL JAPAN) sorption analyser. Prior to the sorption measurements, the samples were purged by N₂ gas at 423 K.

Results and discussion

H₂W₂O₇ is produced through eqn (1) when Bi₂W₂O₉ is treated with HCl at 80 °C under ambient pressure. This treatment induces elution of the Bi₂O₂²⁺ layers and its substitution with H⁺. We carried out the treatment experiments at 80 °C to slow down the rate of eqn (1) involved as the first step in the whole reaction from Bi₂W₂O₉ + 2HCl to WO₃ for discussing the whole reaction performed at 200 °C described later. Fig. 1 shows the XRD patterns and SEM images of the starting material of Bi₂W₂O₉ and the products after the HCl treatment at 80 °C for 1 h under both microwave heating and conventional heating. A peak at 2θ = 7.5° (d = 1.2 nm) observed in the XRD pattern of Bi₂W₂O₉ (Fig. 1a(i)) was attributed to reflection of the layered structure of Bi₂W₂O₉.¹⁹ This pattern was changed to those shown in Fig. 1a(ii) and (iii) after the treatment with HCl at 80 °C for 1 h under both microwave heating and conventional heating, respectively. A new peak observed at 2θ = 9.1° (d = 0.96 nm) was attributed to the layered structure of H₂W₂O₇ formed by the elution of the Bi₂O₂²⁺ layers and its substitution with H⁺.²¹ The decrease of the d value after the treatment with HCl was caused by replacement of Bi₂O₂²⁺ to H⁺.

In order to confirm the layered structures of the resulting H₂W₂O₇, it was kept in contact with n-octylamine (Fig. S1(a) and (b)†). The treatment of H₂W₂O₇ with n-octylamine induced the shift of the peak at 2θ = 9.1° to 3.3°, corresponding to the expansion of the interlayer gap through the intercalation with n-octylamine by 1.7 nm. Fig. 1b shows the SEM images of the same samples as examined by XRD. No difference in the appearances of the particles was observed after the treatment with HCl at 80 °C for 1 h under both microwave heating and conventional heating.

WO₃ is produced successively through eqn (1) and (2) when Bi₂W₂O₉ is treated with HCl at 200 °C. This treatment induces elution of Bi₂O₂²⁺ layers (eqn (1)) and successive dehydration reaction of H₂W₂O₇ (eqn (2)).²¹ Fig. 2 shows the change of the XRD patterns of the products after the HCl treatment at 200 °C under microwave heating for 1 hour and conventional heating for 4 hour. All the peaks in these XRD patterns were attributed to monoclinic WO₃ (JCPDS 43-1035). These XRD patterns were not changed after treatment with n-octylamine (Fig. S1c†), indicating that the products possessed no layered structure. A peak at 2θ = 18° in Fig. 2a(i) can be also attributed to the (004) plain of H₂W₂O₇ since the peaks observed at 2θ = 18° in Fig. 1a(ii) and (iii) are attributed to the (004) plain of H₂W₂O₇. The (004) peak of H₂W₂O₇ is derived from the layered structure. Because the intermediate of the reaction of Bi₂W₂O₉ with HCl at 200 °C is H₂W₂O₇ (see eqn (1)), the sample obtained by hydrothermal reaction of Bi₂W₂O₉ with HCl at 200 °C under microwaves can contain H₂W₂O₇ as an intermediate.

Fig. 2 (a) XRD patterns of (i) Bi₂W₂O₉ treated with HCl for 1 h at 200 °C under microwave heating and (ii) Bi₂W₂O₉ treated with HCl for 4 h at 200 °C under conventional heating. (b) SEM images of (i) Bi₂W₂O₉ treated with HCl for 1 h at 200 °C under microwave heating and (ii) Bi₂W₂O₉ treated with HCl for 4 h at 200 °C under conventional heating.

Then, we observed the morphological variations of WO₃ produced by Bi₂W₂O₉ treated with HCl at 200 °C by SEM. The SEM image shown in Fig. 3a shows two cracks on the surface of the products obtained by the treatment for 5 min under microwaves. The enlarged SEM image (Fig. 3b) demonstrates more clearly the cracks and striped patterns derived from the layered structure of the precursor remaining at the lateral face of the particles with the plate-like shape. These cracks were thought to be generated by volumetric shrinkage during the dehydration reaction of H₂W₂O₇. After 30 min treatment under microwaves (Fig. 3c and d), the product had perforated structure with many rectangular holes. The particles looked like a sediment of the nano-size platelets with the width of ca. 100 nm (Fig. 3d). After 60 min treatment under microwaves, mosaic patterns with many rectangular holes at the whole area of the particles were observed (Fig. 3e) on the planes of the particles. We denote this product as mosaic-patterned WO₃ considering the identification of the compound by XRD shown in Fig. 2. By comparing with the products obtained by 30 min treatment (Fig. 3c), the mosaic-patterned WO₃ particles obtained by 60 min treatment had the fine and neat patterns of rectangular holes on the surface. The particles looked like a sediment of the nano-size platelet with the width of ca. 50 nm (Fig. 3f). The morphology of the product was extremely changed by 120 min treatment. Fig. 3g shows that flake-ball like WO₃ particles with a diameter of 500 nm were aggregated on the large WO₃ particles. The flake-ball like particles should be formed by re-aggregation of dispersed flake chips produced through the formation of the rectangular holes.

Fig. 3 SEM images of WO₃ treated at 200 °C by the hydrothermal synthesis method under microwave heating for 5 min (a and b), 30 min (c and d), 60 min (e and f), 120 min (g and h), and under conventional heating for 2 h (i and j), 4 h (k and l), 6 h (m and n), 12 h (o and p).

The SEM images in Fig. 3i–p show the morphology variations of WO₃ produced under conventional heating at 200 °C. The products obtained by 2, 4 or 6 hour treatment under conventional heating had the rectangular holes at the surface of WO₃ particles (Fig. 3i, k and m). However, comparing with WO₃ particles obtained by microwave heating, extremely few holes were found on the surface. The particles observed in these images had flat surface and plate-like structures. After 12 hour treatment under conventional heating, the flake-ball like particles were partially formed (Fig. 3o and p).

The sequential SEM observations indicated that the morphology of the produced WO₃ particles was changed according to the reaction time and heating method. Taking into account these observations, we propose a scheme of the morphological variation of Bi₂W₂O₉ treated with HCl under microwave heating and conventional heating (Fig. 4). By microwave heating, the starting material, Bi₂W₂O₉, was changed to WO₃ particles with plate-like structure at the very early stage of the reaction. And then, the morphology of the WO₃ was transformed into the mosaic structure over 60 min of the treatment. After 120 min of the treatment under microwaves, the WO₃ particles with flake-ball structure were produced. On the other hand, under conventional heating, WO₃ particles with plate-like structure were also produced at first, and then these were converted to WO₃ particles with flake-ball structure over 12 h of the reaction. Under conventional heating, WO₃ particles with mosaic patterns were not absolutely observed. This is the first microwave effect observed in the reaction of Bi₂W₂O₉ to WO₃ particles in acidic conditions. Furthermore, the accelerated transformation of WO₃ particles with plate-like structure to WO₃ particles with flake-ball structure should be emphasized as another second microwave effect.

Fig. 4 Scheme for morphological variation of WO₃ produced by Bi₂W₂O₉ treated with hydrochloric acid under microwave heating and conventional heating.

We examined the effect of microwave irradiation on the elution of Bi₂O₂²⁺ (eqn (1)). The elution rate of Bi₂O₂²⁺ under microwave heating was compared with the rate under conventional heating. The Bi/W ratios in the samples were determined using the peak areas of Bi Lα line at 10.84 eV and W Lα line at 8.40 eV in the EDX spectrum, considering the detection sensitivities of both elements. The Bi/W ratio of 1.0 was obtained for Bi₂W₂O₉ as the starting compound using the EDX spectrum shown in Fig. S2† and decreased with the progress of the treatment time. Here, the Bi/W ratio values were the mean value for the five particles observed in the SEM image. It should be noticed here that the EDX spectrum reflects the elemental information of the surface layer with the depth of 1 µm of the measurement samples.

The Bi/W ratios determined by the above EDX method for the samples treated with HCl were decreased with the progress of the reaction time under microwave heating and conventional heating as shown as in Fig. 5, indicating the elution of Bi₂O₂²⁺ during the treatment. The initial rate of the decrease in the Bi/W ratio under microwave heating was about 2 times faster than that under conventional heating. The acceleration of the elution reaction of Bi₂O₂²⁺ by microwave heating can be an origin of the microwave effects observed in the morphology variations observed above in the SEM images.

Fig. 5 Time variation of the Bi/W ratios over hydrochloric acid treatment of Bi₂W₂O₉ under microwave heating (red plots) and oil-bath heating (blue plots). Inset: temperature changes of the reaction solution containing 6 M hydrochloric acid (4.0 mL) and Bi₂W₂O₉ (20 mg) under microwave heating and oil bath heating.

The temperature of the reaction mixture was measured by using a fiber-optic thermometer (Opsens, Picosens) during microwave and conventional heating. Changes of the temperatures are shown in Fig. 5 inset with the heating time. Before starting the reaction by injecting HCl aq, the temperatures of the reaction mixtures were maintained at 80.0–80.8 °C. When the HCl was injected into the reaction mixtures at 0 min (the reaction was started at this time), the temperatures of the reaction mixtures were dropped to 73–76 °C. However, the temperatures of both reaction mixtures under microwave irradiation and conventional heating were recovered to 80 °C within 2 minutes and precisely controlled at 80 ± 1.5 °C. We would add that the temperature of the reaction mixture under microwaves was even lower than conventional heating, except for 0.5–1.5 min. Then, the accelerated elution of the Bi₂O₂²⁺ under microwave irradiation can be hardly explained as due to the temperature difference of the reaction mixtures. We propose here that the acceleration should be attributed to the selective heating at W₂O₇²⁻ layers or the interface between W₂O₇²⁻ layers and Bi₂O₂²⁺ layers. Our group reported the special heating of the interface between a TiO₂ paste and a fluorine doped SnO₂ film.²⁸ The similar interfacial heating can be induced for the present case under microwave irradiation. The temperature distribution caused by such interfacial heating under microwaves can induce higher temperature of the Bi₂O₂²⁺ layers than the surroundings, resulting in the accelerated elution of Bi₂O₂²⁺ layers (eqn (1)). Furthermore, Bi₂W₂O₉ particle might experience the selective heating effect by the interaction with microwave alternative electromagnetic field. The temperature of the inner part of the particles can be higher than the outer surface through the interactions.^34,35

On the other hand, under conventional heating, the Bi₂W₂O₉ particles were heated by heat transfer from the surroundings, and then the temperature distribution of the particles and the surroundings should be homogeneous supposing that the heat equilibrium is reached. The difference in the temperature distribution between microwave irradiation and conventional heating should lead to the difference in the dynamics of the elution of Bi₂O₂²⁺ (eqn (1)) and the dehydration reaction of H₂W₂O₇ (eqn (2)) at the inner particle and outer surface. It might explain why WO₃ particles with mosaic patterns was produced only under microwaves.

The BET specific surface areas and morphologies of the WO₃ particles obtained by treatment of Bi₂W₂O₉ with HCl at 200 °C are summarized in Table 1. The adsorption isotherms of the WO₃ particles produced by microwave and conventional heating are shown in Fig. S3 and S4,† respectively. Under microwave heating, as the reaction time increased from 5 min to 60 min, the surface area of the WO₃ particles increased from 12.3 to 51.8 m² g⁻¹. This increase in the surface area agreed to the changes in the morphology of the particles observed in the SEM images. However, the surface area of the WO₃ particles obtained by 120 min acid treatment was smaller than that of 60 min treatment. In the SEM observation, the structure of the WO₃ particles obtained by 60 min treatment was mosaic, while that obtained by 120 min treatment was flake ball. Therefore, it is mentioned that the structure with the mosaic patterns has large surface area compared to flake ball structure, probably due to the much porosity of the structure with the mosaic patterns.

Table 1 BET surface area, morphology and photocatalytic activity data of WO₃ produced by Bi₂W₂O₉ treated with HCl under at 200 °C by the hydrothermal synthesis method

Synthesis method	Treatment time	BET surf. area/m^2 g^−1	Morphology	Photocatalytic acetaldehyde decomposition rate/µmol h^−1 g^−1
Microwave hydrothermal	5 min	12.3	Plate-like	427
	30 min	20.8	Plate-like
	60 min	51.8	Mosaic
	120 min	18.6	Flake-ball
Conventional hydrothermal	2 h	11.1	Plate-like	408
	4 h	32.2	Plate-like
	6 h	11.3	Plate-like
	12 h	22.2	Flake-ball
Purchased WO3		7.54		147

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The diffuse reflectance spectra of flake-ball WO₃ produced by Bi₂W₂O₉ treated with HCl for 12 h at 200 °C under conventional heating and mosaic-patterned WO₃ produced by Bi₂W₂O₉ treated with HCl for 60 min at 200 °C under microwave heating are shown in Fig. 6. The band gap energy of the flake-ball (2.71 eV) WO₃ was larger than that of mosaic-patterned WO₃ (2.62 eV). The band gap energy of bulk WO₃ is 2.6 eV.¹⁷ Increase in the band gap energy of flake-ball WO₃ should be attributed to the quantum size effect. Then, the average crystallite diameter D(hkl) was estimated from XRD pattern by using Scherrer's equation.

where is λ is the X-ray wavelength, β is the full width at half maximum of X-ray diffraction peak in radian and θ is the Bragg's angle of the diffraction peak. Here, the (002) peak with the highest intensity was selected to evaluate the crystallite diameter. The crystallite diameters of mosaic-patterned WO₃ and flake-ball WO₃ were calculated to be 31.3 nm and 21.6 nm, respectively, suggesting occurrence of the quantum size effect for flake-ball WO₃.

Fig. 6 Diffuse reflectance spectra of mosaic-patterned WO₃ particles produced by Bi₂W₂O₉ treated with HCl for 60 min at 200 °C under microwave heating (red), and WO₃ flake-ball particles produced by Bi₂W₂O₉ treated with HCl for 12 h at 200 °C under conventional heating (blue).

To evaluate the photocatalytic activity of the WO₃ particles prepared in this work, the decomposition reaction of gaseous acetaldehyde was investigated for the samples obtained by microwave heating for 60 min (mosaic-patterned WO₃ with the BET surface area 51.8 m² g⁻¹), obtained by the conventional heating for 4 h (BET surface area 32.2 m² g⁻¹) and commercially available WO₃ particles (BET surface area 7.54 m² g⁻¹) (Fig. 7). 1 wt% Pt particles of about 5 nm particle size were supported on these WO₃ particles by a pre-known method.¹¹ Both WO₃ particles obtained by microwave and conventional heating showed higher photocatalytic activity than the commercial WO₃ particles. The high catalytic activities were attributed to the high specific surface area of the particles prepared in this work. No large difference in the photocatalytic activity was observed between both particles obtained microwave irradiation and conventional heating.

Fig. 7 Photocatalytic decomposition of gaseous acetaldehyde over purchased WO₃ (green plots), mosaic-patterned WO₃ particles by Bi₂W₂O₉ treated with HCl for 1 h at 200 °C under microwave heating (red plots) and plate-like WO₃ particles produced by Bi₂W₂O₉ treated with HCl for 4 h at 200 °C under conventional heating (blue plots).

Conclusions

WO₃ particles of mosaic patterns with high specific surface area (51.8 m² g⁻¹) were obtained by hydrochloric acid treatment of Bi₂W₂O₉ at 200 °C under microwave heating. Mosaic-patterned WO₃ particles obtained by only microwave heating were converted into WO₃ particles with flake-ball shape by the prolonged irradiation. Flake-ball WO₃ was obtained both for microwave heating and conventional heating, but the whole process of the transformation from Bi₂W₂O₉ to WO₃ was much shortened by using microwave irradiation. We have demonstrated that the elution of Bi₂O₂²⁺ layer was accelerated by microwave heating. This accelerated elusion observed under microwave irradiation can be attributed to the selective heating at W₂O₇²⁻ layers or the interface between W₂O₇²⁻ layer and Bi₂O₂²⁺ layer through the interaction of Bi₂W₂O₉ with microwave alternative electromagnetic field. Photocatalytic decomposition of gaseous acetaldehyde under visible light irradiation was examined for the WO₃ particles prepared in this work. Mosaic-patterned WO₃ particles obtained by microwave heating showed 2.9 times higher photocatalytic activity than a commercially available WO₃ particles (BET surface area 7.54 m² g⁻¹) and 1.05 times higher the photocatalytic activity than WO₃ particles obtained by conventional heating (BET surface area 32.2 m² g⁻¹).

Acknowledgements

We thank S. Genseki and K. Hori (Tokyo Institute of Technology) for TEM observations. This study was supported in part by Grant-in-Aid for Scientific Research (A) 25249113, Grant-in-Aid for Exploratory Research, and Grant-in-Aid for Young Scientists (B) from MEXT, Japan, ASPIRE League Research Grant 2014 and 2015, Tokyo Tech, Research Grant of TEPCO Memorial Foundation, and Demonstration and Standardization Project Using New Electric Devices, NEDO.

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Footnotes

† Electronic supplementary information (ESI) available: Adsorption isotherms of WO₃ crystals, XRD patterns of H₂W₂O₇ and mosaic WO₃ reacted with n-octylamine, time change of temperature of reaction mixture at 80 °C, EDX spectrum of Bi₂W₂O₉. See DOI: 10.1039/c5ra15167h

‡ Present address: Interdisciplinary Cluster for Cutting Edge Research, Center for Energy and Environmental Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan.

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