Synthesis and photocatalytic properties of α-ZnWO₄ nanocrystals in tungsten zinc borate glasses

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.

Abstract

Tungsten oxide (WO₃)-containing glasses of WO₃–ZnO–B₂O₃ were prepared using a conventional melt quenching method, and α-ZnWO₄ nanocrystals were synthesized through the crystallization of glasses. A glass with the composition of 20WO₃–50ZnO–30B₂O₃ showed the bulk crystallization of α-ZnWO₄ nanocrystals with a diameter of ∼10 nm. Broad and asymmetric emission peaks were observed at the wavelength of λ ∼ 475 nm, i.e., blue emissions, in the photoluminescence spectra for the samples with α-ZnWO₄ nanocrystals. From the degradation of the intensity of optical absorption under ultraviolet light (λ = 254 nm) irradiations for the solution consisting of crystallized particles with α-ZnWO₄ nanocrystals and methylene blue, it was clarified that α-ZnWO₄ nanocrystals formed have photocatalytic activities. The formation of α-ZnWO₄ nanocrystals is discussed from the viewpoint of the glass-forming tendency.

Keywords:

• Glass
• Crystallization
• α-ZnWO₄
• Nanocrystals
• Photoluminescence
• Photocatalytic activity

1 Introduction

Tungstate compounds have received much attention because of their unique functions, e.g., ZnWO₄ for scintillator [1] and photocatalyst [2], Sc₂(WO₄)₃ showing negative thermal expansion [3], Eu³⁺-doped (La,Gd)₂(WO₄)₃ for phosphors [4], and KYb(WO₄)₂ for solid state laser host [5]. It is of importance to control the morphology, size, and quality of tungstate crystals for practical applications. On the other hand, glasses containing tungsten oxide (WO₃) such as WO₃–TeO₂ and K₂O–WO₃–TeO₂ have been synthesized, and their structure and properties have also received much attention [6–13]. For instance, the electronic polarizability of oxide ions (i.e., optical basicity) in WO₃ is very high [14,15], and thus, WO₃-based glasses show large third-order optical nonlinearities [10,16].

It is known that the crystallization of glass is an effective method for the fabrication of transparent materials with desired shapes and also nanostructures [17–19], and the crystallization technique has been applied to WO₃-based glasses in order to fabricate glass-ceramics (crystallized glasses) consisting of functional WO₃-based compounds [20–23]. For instance, Maczka et al. [20] reported that a glass in the Cs₂O–MgO–WO₃–P₂O₅ system exhibits a prominent nanocrystallization during thermal treatments, providing Cs(Mg_0.25W_1.75)O₆ nanocrystals with the size of 4–10 nm. Aleksandrov et al. [21] synthesized glass–crystal composite materials with the LaBWO₆ crystalline phase through the crystallization of 25La₂O₃–50WO₃–25B₂O₃ glass. It is of interest to synthesize nano-scaled tungstate compounds through the crystallization of glasses and to clarify physical and chemical properties for practical device applications.

In this study, we focus our attention on the synthesis and optical properties of ZnWO₄ nanocrystals through the crystallization of glasses. Unfortunately, for ZnWO₄, it is impossible to prepare bulky glasses (not very thin or small flakes) with the same composition as this crystal, i.e., a glass of 50ZnO–50WO₃. In order to design the crystallization of ZnWO₄, therefore, glass-forming oxides such as SiO₂ and B₂O₃ must be added. In the present study, we use the ternary system of WO₃–ZnO–B₂O₃, although its glass-forming region is narrow as reported by Imaoka [24]. ZnWO₄ crystals have been synthesized by using various techniques such as solid state reaction and mechanochemical methods [1,2,25–27]. However, there has been no report on the crystallization of ZnWO₄ in glasses. Some electrical, thermal, and optical properties of WO₃–ZnO–B₂O₃ glasses have been reported so far [6,9], but any crystallization behavior has not been reported. Recently, Aleksandrov et al. [28] examined the structure and crystallization behavior of MoO₃–ZnO–B₂O₃ glasses and found that some glasses show the bulk crystallization of α-ZnMoO₄ nanocrystals with a diameter of ∼5 nm and Eu³⁺-doped α-ZnMoO₄ nanocrystals show enhanced photoluminescence emissions. Considering the chemical similarity of tungsten (W) and molybdenum (Mo) elements, the crystallization of ZnWO₄ nanocrystals would be expected in WO₃–ZnO–B₂O₃ glasses. As one of the optical properties of ZnWO₄ nanocrystals synthesized in this study, photocatalytic activities for the decomposition of methylene blue under ultraviolet (UV) light were examined. There have been some reports on the photocatalytic activities of ZnWO₄ powders [2,29–31]. For instance, Huang and Zhu [2] reported that ZnWO₄ powders prepared by calcining co-precipitated precursors exhibit excellent photocatalytic activities. Garadkar et al. [29] synthesized ZnWO₄ nanoparticles (∼10 nm) by using a simple microwave assisted technique and found that they exhibit highly enhanced photocatalytic activity toward rhodamine B and methylene blue samples.

2 Experimental

Compositions (mol%) examined in this study are 10WO₃–50ZnO–40B₂O₃, 20WO₃–50ZnO–30B₂O₃, 30WO₃–50ZnO–20B₂O₃, 30WO₃–40ZnO–30B₂O₃, and 30WO₃–30ZnO–40B₂O₃. Commercial powders of reagent grade WO₃, ZnO, and B₂O₃ were used as starting materials and were melted in a platinum crucible at 1250 °C for 20 min in an electric furnace. The melts were poured onto an iron plate and pressed to a thickness of ∼1.5 mm by another iron plate. The glass transition, T_g, and crystallization peak, T_p, temperatures were determined using differential thermal analysis (DTA) (Rigaku, TG8120) at a heating rate of 10 K/min. Glass samples were heat-treated at different temperatures in an electric furnace in air, and the crystalline phase present in the crystallized samples was identified by X-ray diffraction (XRD) analysis (CuKα radiation). Morphologies of crystals in the crystallized samples were examined from high resolution transmission electron microscope (HRTEM, JEM-2100F) observations.

Photoluminescence (PL) emission and excitation (PLE) spectra were measured at room temperature with a spectrofluorophotometer (Shimadzu RF-5300PC). Photocatalytic activities of heat-treated samples were evaluated by the degradation of methylene blue (MB) under UV light (wavelength: λ = 254 nm). First, heat-treated samples were pulverized by using an agate mortar, and powders (100 mg) were mixed with a methylene blue solution (concentration: 5 ppm, volume: 100 mL) in a beaker. UV light (UV lamp; Toshiba, GL15-F) was irradiated onto the mixtures, and optical absorption spectra of solutions were measured by using a spectrophotometer (Shimadzu UV-3150). The total irradiation time of UV light was 180 min, and solutions were stirred by using a magnetic stirrer during UV light irradiations.

3 Results and discussion

3.1 Formation of α-ZnWO₄ nanocrystals

The melt-quenched samples with the compositions of 30WO₃–50ZnO–20B₂O₃, 30WO₃–40ZnO–30B₂O₃, and 30WO₃–30ZnO–40B₂O₃ include some amounts of crystals. On the other hand, the melt-quenched samples of 10WO₃–50ZnO–40B₂O₃ and 20WO₃–50ZnO–30B₂O₃ have a good optical transparency. In the XRD patterns for these two samples, only broad halo peaks being typical for glassy materials were observed. These results about the glass-forming tendency in the ternary WO₃–ZnO–B₂O₃ system is well consistent with that reported by Imaoka [24]. It is, therefore, confirmed from the present study that the glass-forming tendency in the samples with WO₃ contents of more than 30 mol% is weak. Considering the purpose of this study, it is favorable to use glasses with WO₃ and ZnO contents as large as possible. We, therefore, determined to use 20WO₃–50ZnO–30B₂O₃ glass (designated here as 20WZB glass) and to examine its crystallization behavior.

Fig. 1 shows the DTA pattern for the bulk 20WZB glass. An endothermic dip corresponding to the glass transition and three exothermic peaks corresponding to the crystallization are observed clearly, providing the values of T_g = 490 °C, T_p1 = 667 °C, T_p2 = 748 °C, and T_p3 = 811 °C. The optical photograph for the 20WZB glass is included in Fig. 1, indicating colorless in the visible light region. This suggests that the valence of W ions in the 20WZB glass is mainly W⁶⁺. Even if W⁵⁺ ions providing a color of brown are present in the 20WZB glass, their amount would be very small. From the optical absorption spectrum, it was estimated that the 20WZB glass has an optical absorption edge of the wavelength of λ = 354 nm (i.e., 3.50 eV).

Fig. 1 DTA curve of bulk 20WO₃–50ZnO–30B₂O₃ glass. Heating rate was 10 K/min. T_g and T_p are the glass transition and crystallization peak temperatures, respectively. The optical photograph for the bulk glass is also included.

Fig. 2 shows the XRD patterns for the samples obtained by different heat treatment temperatures in the 20WZB glass. The XRD peaks observed for the sample heat-treated at 680 °C for 3 h are assigned to α-ZnWO₄ crystals (ICDD: No. 01-075-8594). For the samples heat-treated at 760 and 815 °C, the formation of Zn₄O(BO₂)₆ crystals is observed besides α-ZnWO₄. The results shown in Fig. 2 indicate that the initial crystalline phase appearing in the crystallization of 20WZB glass is α-ZnWO₄. In order to examine the crystallization behavior of α-ZnWO₄ crystals, the 20WZB glass was heat-treated at temperatures lower than the first crystallization peak temperature of T_p1 = 667 °C, and the XRD patterns for the samples obtained are shown in Fig. 3. It is seen that the samples heat-treated at 570 and 610 °C for 3 h show XRD peaks assigned to α-ZnWO₄ crystals. It should be pointed out that XRD peaks are very broad, suggesting that the size of α-ZnWO₄ crystals might be small. The average particle size of α-ZnWO₄ crystals formed in the heat-treated (610 °C, 3 h) sample was estimated from the peak width of XRD patterns using the Scherrer's equation, in which the (1 1 1) plane was used, and the value of ∼13 nm was obtained. A TEM photograph for the heat-treated (610 °C, 3 h) sample is shown in Fig. 4. The sample for TEM observations was prepared by grinding plate-shaped crystallized glass, and the edge of grain particles was observed. It is seen that crystals having a diameter of around 10 nm are present, being consistent with the average size (∼13 nm) of α-ZnWO₄ crystals estimated from the Scherrer's equation. The XRD and TEM results, therefore, demonstrate that α-ZnWO₄ nanocrystals are formed initially in the crystallization of 20WZB glass. The optical absorption spectrum for the transparent heat-treated (570 °C, 3 h) sample with α-ZnWO₄ nanocrystals was measured, and an optical absorption edge of λ = 350 nm (i.e., 3.54 eV) was obtained. This value is almost the same as that (λ = 354 nm) for the base 20WZB glass. Garadkar et al. [29] reported the optical band gap (E_g) of E_g = 3.4 eV for ZnWO₄ nanoparticles (∼10 nm). Huang and Zhu [2] reported the value of E_g = 3.02 eV for ZnWO₄ powders with a diameter of 200–500 nm. The sample obtained in this study is composed of the glassy phase and α-ZnWO₄ nanocrystals, and further studies would be necessary to determine the optical band gap of α-ZnWO₄ nanocrystals themselves. At this moment, the amount of α-ZnWO₄ nanocrystals has not been determined. However, considering the chemical composition of 20WO₃–50ZnO–30B₂O₃ glass, after the formation of α-ZnWO₄ nanocrystals with the ratio of ZnO/WO₃ = 1, the chemical composition of the residual glassy phase would be close to 30ZnO–30B₂O₃ with the ratio of ZnO/B₂O₃ = 1. This would be a reason for the formation of Zn₄O(BO₂)₆ crystals with the ratio of ZnO/B₂O₃ = 0.67 as the second crystalline phase.

Fig. 2 XRD powder patterns for the samples heat-treated at 680, 760, and 815 °C for 3 h in 20WO₃–50ZnO–30B₂O₃ glass.

Fig. 3 XRD powder patterns for the samples heat-treated at 530, 570, and 610 °C for 3 h in 20WO₃–50ZnO–30B₂O₃ glass. The optical photographs for the heat-treated samples are also included.

Fig. 4 TEM photograph for the sample heat-treated at 610 °C for 3 h in 20WO₃–50ZnO–30B₂O₃ glass.

The glass of 20WO₃–50ZnO–30B₂O₃ includes a relatively small amount (i.e., 30 mol%) of B₂O₃ which is a glass former. The stability of borate structures depends on the delocalization of formal charges of B_nO_m anions as a result of their polymerization [32]. Considering the amount of B₂O₃ content, it is expected that the degree of the polymerization of B–O–B bonds would be small (i.e., short). Furthermore, it is recognized that the polymerized borates are often less stable for high-valence and small-sized cations [32,33]. Cations of higher valence such as WO₃, MoO₃, and Nb₂O₅ can form rather rigid coordination polyhedra with oxygen atoms. In the binary systems such as WO₃–B₂O₃, MoO₃–B₂O₃, and Nb₂O₅–B₂O₃, it is expected that the glass formation would be extremely difficult. Indeed, any wide glass-forming region has not been reported in these binary borate systems [28]. On the other hand, the glass formation has been confirmed in the binary system of ZnO–B₂O₃ [28], i.e., in the compositions of (43.9–63.6)ZnO–(56.1–36.4)B₂O₃ [28], consequently providing the glass formation in the ternary system of WO₃–ZnO–B₂O₃. This information on the glass-forming tendency suggests that it might be difficult to create rigid network structure with strong chemical bonds such as W–O–B in the ternary WO₃–ZnO–B₂O₃ system. Furthermore, it is considered that the formation of compounds containing both WO₃ and ZnO such as α-ZnWO₄, but containing no boron atoms, would be enhanced in WO₃–ZnO–B₂O₃ glasses. The above structural concept in glasses is an important clue for the design and control of crystallization in borate glasses. Indeed, various functional nanocrystals have been formed easily through the crystallization of borate glasses, e.g., α-ZnMoO₄ nanocrystals in MoO₃–ZnO–B₂O₃ glasses [28], ferroelectric Sr_xBa_1−xNb₂O₆ nanocrystals in SrO–BaO–Nb₂O₅–B₂O₃ glasses [34], ferroelectric Ba_1−xRE_2x/3Nb₂O₆ nanocrystals (RE: Sm, Eu, Gd, Dy, Er) in RE₂O₃–BaO–Nb₂O₅–B₂O₃ glasses [17], and CaF₂ nanocrystals in CaF₂–NaF–CaO–Al₂O₃–B₂O₃–SiO₂ glasses [35].

3.2 Photoluminescence of α-ZnWO₄ nanocrystals

The excitation spectrum of the heat-treated (664 °C, 3 h) sample containing α-ZnWO₄ nanocrystals is shown in Fig. 5(a), in which the emission of λ = 482 nm was monitored. A broad excitation emission peaked at λ ∼ 270 nm is observed. A PL spectrum at the excitation of λ = 267 nm for the sample is shown in Fig. 5(b). A broad and asymmetric emission peaked at λ ∼ 475 nm is observed, providing a blue color (Fig. 5(c)). A blue emission in α-ZnWO₄ has been considered to be related to WO₆ polyhedra [36–38]. A molecular orbital model for the octahedral (WO₆)⁶⁻ oxyanion complex (O_h point symmetry) shows that the highest occupied state with a t_1g symmetry is associated with the 2p orbital of oxygen and the excited state consists of an electron at the W 5d orbital with a t_2g symmetry [38]. That is, the excitation and emission spectra in α-ZnWO₄ are related to an electronic transition between the O(2p) and W(5d) states in the WO₆ group. Zinc tungstate α-ZnWO₄ has the wolframite crystal structure, in which the WO₆ octahedral complex has an asymmetric shape with short, medium, and long W–O bond distances [38]. The deformation of WO₆ octahedra in α-ZnWO₄ would be related to the appearance of the broad and asymmetric emission and PL spectra shown in Fig. 5. It is, therefore, concluded that α-ZnWO₄ nanocrystals formed through the crystallization of 20WO₃–50ZnO–30B₂O₃ glass have the electronic state being similar to α-ZnWO₄ crystals synthesized by other preparation techniques.

Fig. 5 Excitation spectrum (a) monitored by the emission of λ = 482 nm and photoluminescence spectrum (b) at the excitation of λ = 267 nm for the heat-treated (664 °C, 3 h) sample containing α-ZnWO₄ nanocrystals in 20WO₃–50ZnO–30B₂O₃ glass. The optical photograph (c) for the heat-treated sample with a blue photoluminescence is also included.

3.3 Photocatalytic properties of α-ZnWO₄ nanocrystals

For the photocatalytic activity of α-ZnWO₄ nanocrystals, the crystallized samples obtained by a heat treatment at 610 °C for 3 h were used. The samples were pulverized by a agate mortar, and the SEM photograph for the particles obtained is shown in Fig. 6(a), indicating the particle size of 1–10 μm and sharp surfaces. UV light (λ = 254 nm) was irradiated onto the solution consisting of these particles and methylene blue. However, any degradation of methylene blue, i.e., the decrease in the intensity of optical absorptions, was not observed. That is, any photocatalytic activity was not confirmed in the particles of the crystallized sample with α-ZnWO₄ nanocrystals. In the crystallized samples themselves, it is considered that the area of α-ZnWO₄ nanocrystals exposing at the surface would be small, because many α-ZnWO₄ nanocrystals are surrounded by the residual glassy phase and also are present in the interior of the particles.

Fig. 6 SEM photographs for the pulverized particles (a) for the heat-treated (610 °C, 3 h) sample and for particles (b) obtained by etching in 1 N HNO₃ solution for the heat-treated sample in 20WO₃–50ZnO–30B₂O₃ glass.

In order to increase the exposure area of α-ZnWO₄ nanocrystals at the surface (i.e., the elimination of the glassy phase), the pulverized particles were etched by using a 1 N HNO₃ solution, and the SEM photograph for the particles obtained is shown in Fig. 6(b). It is seen that the surface state of the etched particles (Fig. 6(b)) changes largely in comparison with that of non-etched particles (Fig. 6(a)). The appearance of the rough surface shown in Fig. 6(b) indicates that the glass of 20WO₃–50ZnO–30B₂O₃ is etched effectively by a 1 N HNO₃ solution. UV light (λ = 254 nm) was irradiated onto the solution of these etched particles and methylene blue, and the results on the optical absorption spectra in the visible light region at room temperature are shown in Fig. 7. It is seen that the intensity of optical absorption due to the methylene blue decreases with increasing UV light irradiation time. In particular, for the irradiation time of 180 min, it is considered that the methylene blue present in the solution is almost decomposed. The results shown in Fig. 7, therefore, indicate that α-ZnWO₄ nanocrystals formed through the crystallization of 20WO₃–50ZnO–30B₂O₃ glass have photocatalytic activities. Similar experiments were carried out for TiO₂ particles (Nippon Aerosil Co. P25, particle size: ∼21 nm) in order to confirm the validity of our experiments applied in this study, and it was found that the methylene blue present in the solution was almost decomposed in the UV (λ = 254 nm) irradiation time of 90 min. Garadkar et al. [29] examined the photocatalytic activity of ZnWO₄ nanoparticles (∼10 nm) synthesized by a simple microwave assisted technique for the degradation of methylene blue in a solution (100 mL, 1.0 × 10⁻⁵ mol/L) under UV light (λ = 254 nm) and found that the UV light irradiation time of 60 min is enough for the decomposition of methylene blue. The photocatalytic activity of α-ZnWO₄ nanocrystals formed through the crystallization of glasses in this study is, therefore, not so high compared with the data reported so far [29]. In this study, the glassy phase that remained in the crystallized glasses were etched by using a 1 N HNO₃ solution, and α-ZnWO₄ nanocrystals were taken. It is required to clarify the surface state of α-ZnWO₄ nanocrystals taken by the etching process. Furthermore, it is of interest to study photocatalytic properties and behaviors including intermediate products for our samples in more detail. It is also strongly desired to pattern α-ZnWO₄ nanocrystals more densely only at the surface of bulk WO₃–ZnO–B₂O₃ glasses for practical applications, for instance, by using laser-induced crystallization techniques [39].

Fig. 7 Optical absorption spectra in the visible light region at room temperature for the solution of etched (1 N HNO₃) powders and methylene blue under different UV light (λ = 254 nm) irradiation times.

4 Conclusions

The WO₃-containing glasses based on the WO₃–ZnO–B₂O₃ system were prepared using a conventional melt quenching method in order to design crystallized glasses with α-ZnWO₄ nanocrystals. It was difficult to prepare glasses with a large amount (∼30 mol%) of WO₃, but the melt-quenched sample with the composition of 20WO₃–50ZnO–30B₂O₃ showed an optically transparent glass (20WZB glass). The 20WZB glass showed the bulk crystallization of α-ZnWO₄ nanocrystals with a diameter of ∼10 nm. Broad and asymmetric emissions peaking at the wavelength of ∼475 nm, i.e., blue emissions, in the photoluminescence spectrum were observed in the samples with α-ZnWO₄ nanocrystals. From the degradation of the absorption of methylene blue color under UV light irradiations, it was clarified that α-ZnWO₄ nanocrystals formed have photocatalytic activities.

Acknowledgements

This study was supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sport, Culture, and Technology, Japan (No. 23246114), and by Program for High Reliable Materials Design and Manufacturing in Nagaoka University of Technology.

Notes

Peer review under responsibility of The Ceramic Society of Japan and the Korean Ceramic Society.