Er<sup>3+</sup>掺杂ZnWO<sub>4</sub>的合成及光催化活性研究

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周宇, 张志洁, 徐家跃, 储耀卿, 尤明江. Er³⁺掺杂ZnWO₄的合成及光催化活性研究 . 无机材料学报, 2015, 30(5): 549-554
ZHOU Yu, ZHANG Zhi-Jie, XU Jia-Yue, CHU Yao-Qing, YOU Ming-Jiang. Synthesis and Enhanced Photocatalytic Activity of Er³⁺-doped ZnWO₄ . Journal of lnorganic Materials,2015, 30(5): 549-554 复制到剪切板

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Er³⁺掺杂ZnWO₄的合成及光催化活性研究

周宇, 张志洁, 徐家跃, 储耀卿, 尤明江

上海应用技术学院材料科学与工程学院, 晶体生长研究所, 上海201418

摘要

通过水热法合成了不同浓度Er³⁺掺杂ZnWO₄纳米棒, 并通过XRD、TEM和DRS等对其进行了表征。通过在模拟太阳光照射下光降解RhB的速度来检测ZnWO₄样品的光催化活性, 研究了Er³⁺掺杂浓度对ZnWO₄催化活性的影响。实验结果表明, 当Er³⁺掺杂浓度为2mol%时, 其光催化性能最好, 因为引进Er³⁺后, Er³⁺加快了电荷分离效率。

关键词: Er3+掺杂钨酸锌; 光催化剂; 罗丹明B; 电荷分离

中图分类号:TQ174 文献标志码:A 文章编号:1000-324X(2015)05-0549-06

Synthesis and Enhanced Photocatalytic Activity of Er³⁺-doped ZnWO₄

ZHOU Yu, ZHANG Zhi-Jie, XU Jia-Yue, CHU Yao-Qing, YOU Ming-Jiang

Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China

Biography: ZHOU Yu(1989-), male, candidate of master degree. E-mail: zhouyu564022786@163.com

Corresponding author: ZHANG Zhi-Jie, PhD. E-mail: zjzhang@sit.edu.cn; XU Jia-Yue, Professor. E-mail: xujiayue@sit.edu.cn

Fund:Foundation item: National Natural Science Foundation of China (51342007, 51402194); Shanghai Science and Technology Committee (11JC1412400, 14YF1410700)

Abstract

Er³⁺-doped ZnWO₄ nanorods with different doping concentrations were synthesized by a hydrothermal method. The as-prepared products were characterized by XRD, TEM and DRS. The photocatalytic activities of the as-prepared ZnWO₄ samples were evaluated by photo-degradation of RhB under simulated solar light irradiation and the effects of Er³⁺ doping on the photocatalytic activity of ZnWO₄ were investigated. The result showed that the sample with the doping concentration of 2mol% exhibited the best photocatalytic performance, which could be ascribed to the promoted charge separation efficiency of Er³⁺-doped ZnWO₄.

Keyword: Er3+-doped ZnWO4; photocatalyst; Rhodamine B; charge separation

Show Figures

Water pollution is one of the most serious environmental problems in modern society. Semiconductor- mediated photocatalytic degradation is a promising technique for wastewater treatment. In recent years, semiconductor-based photocatalysis has attracted more and more attention for solving environment problems, especially for the removal of organic contaminants under sunlight^{[1, 2, 3, 4, 5, 6, 7, 8]}.

Tungstates are widely used functional materials, and it has been confirmed that tungstates are a class of promising photocatalysts^{[9, 10, 11]}. ZnWO₄ as an important member of tungstates, due to its photocatalytic properties and potential applications, has received intense research attention^{[12, 13]}. Up to now, ZnWO₄has been used for water splitting and decomposition of organic pollutants under UV irradiation^{[14, 15, 16, 17]}. However, improvement of photocatalytic activity of ZnWO₄is still indispensable because the photo-activity of ZnWO₄ is not high enough for the requirements of practical application.

It is known that the photocatalytic activity depends strongly on the separation efficiency of photo-generated electron-hole pairs. Up to date, great efforts have been made to facilitate the electron-hole separation and enhance the photocatalytic activity of ZnWO₄, including cation doping^{[18, 19]}, anion doping such as fluorine or chlorine^{[12, 20, 21]}, and coupling with other semiconductors such as Bi₂WO₆^[22] or ZnO^[23]. Especially, doping of lanthanide ions with 4f electron configuration has been proposed as an effective way to eliminate the recombination of electron-hole pairs. For example, it has been reported that doping with Eu³⁺, Er³⁺ or Ce⁴⁺ could improve the photocatalytic activity of TiO₂^{[24, 25]}, and doping with Er³⁺ could enhance the photocatalytic activity of Bi₂WO₆^[26]. Inspired by those progresses, we intended to design a doped ZnWO₄photocatalyst by introducing lanthanide ion Er³⁺. To the best of our knowledge, there are no reports on the photocatalytic activity of Er³⁺-doped ZnWO₄.

In this work, Er³⁺-doped ZnWO₄ photocatalysts were synthesized by a simple hydrothermal process. The product synthesized by this method had uniform phase and good crystallization. Moreover, the shape and size of the product could be controlled. The photo-activity evaluation, via the degradation of RhB under simulated solar light, demonstrated that doping appropriate amount of Er³⁺could enhance the photocatalytic performance of ZnWO₄effectively. Moreover, the mechanism of the photocatalytic process for Er³⁺-doped ZnWO₄was discussed in detail.

1 Experimental procedures

1.1 Synthesis

All chemicals used were analytic grade reagents without further purification. The experimental process of synthesizing Er³⁺-doped ZnWO₄ photocatalysts was as follows: 1 mmol of Na₂WO₄· 2H₂O was dissolved in 10 mL of deionized water to obtain solution A. Meanwhile, 1 mmol of Zn(NO₃)₂· 6H₂O was dissolved in 10 mL of deionized water to obtain solution B. Then an appropriate amount of Er(NO₃)₃ solution was slowly added to solution A under stirring. After that, solution B was added to the mixed solution to obtain a white suspension. The pH value of the suspension was adjusted to 8 by the addition of NaOH solution, then sealed in a Teflon-lined stainless steel autoclave and heated at 433 K for 16 h under autogenous pressure. The obtained solid product was washed thoroughly with deionized water and dried at 333 K. In this work, the doping concentrations of Er³⁺were 0, 1mol%, 2mol%, 2.5mol%, 3.5mol%, 5mol%, and 7mol%, respectively.

1.2 Characterization

The phase and composition of as-prepared samples were measured by the powder X-ray diffractometer operated at 40 kV and 30 mA using Cu Kα radiation (λ =0.15418 nm). The morphologies of the products were obtained by transmission electron microscope (FEI tecnai G2F30). UV-Vis DRS was performed on a Cary 5000 UV-Vis spectrophotometer.

1.3 Photocatalytic test

The photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic degradation of RhB under simulated solar light using a 500 W Xe lamp as the light source. The experiments were performed at room temperature as follows: 0.05 g of photocatalyst was added into 50 mL of RhB solution (1× 10^-5mol/L). The suspensions were magnetically stirred in the dark for 0.5 h in order to ensure the adsorption/desorption equilibrium between the photocatalyst powders and the solution. 3 mL of suspensions were sampled every 0.5 h and centrifuged to remove the photocatalyst powders. Evaluation of the photocatalytic activities of the photocatalysts was conducted by recording the variations of the absorption band maximum through a UV-Vis spectrophotometer (Cary 5000) and the concentration of RhB was analyzed by recording the variations of the absorption band maximum (552 nm).

2 Results and discussion

2.1 Crystal structure

The XRD patterns of the as-prepared Er³⁺-doped ZnWO₄ samples with different doping concentrations were shown in Fig. 1. All the peaks in the patterns matched well with the characteristic reflections of zinc tungstate (JCPDS 15-0774). No signals for any other phases were observed, which indicated that the products were pure phase and Zn²⁺ was successfully replaced by Er³⁺ and Er³⁺ was uniformly incorporated into the lattice.

2.2 UV-Vis spectra

Figure 2 showed the diffuse reflectance spectra (DRS) of pure ZnWO₄ and the 2mol% Er³⁺-doped ZnWO₄ sample. Fig. 1 Powder X-ray diffraction patterns for Er³⁺-doped ZnWO₄ with different doping concentrations.It indicated that pure ZnWO₄ presented an absorption edge around 400 nm. For the Er³⁺-doped ZnWO₄ sample, there were also some sharp absorption bands originated from characteristic f-f or f-d transition of the Er³⁺ion. The transition peaks, which centered at 365, 378, 489, 520, 548, 653, and 795 nm, were attributed to the transitions of ⁴I_15/2→ ⁴G_9/2, ⁴I_15/2→ ⁴G_11/2, ⁴I_15/2→ ⁴F_7/2, ⁴I_15/2→ ⁴H_11/2, ⁴I_15/2→ ⁴S_3/2, ⁴I_15/2→ ⁴F_9/2, ⁴I_15/2→ ⁴I_9/2, respectively^[27].

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	Fig. 1 Powder X-ray diffraction patterns for Er³⁺-doped ZnWO₄ with different doping concentrations

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	Fig. 2 UV-Vis diffuses reflectance spectra of the as-prepared ZnWO₄ samples

2.3 Morphology

The morphologies and microstructures of pure ZnWO₄ and the 2mol% Er³⁺-doped ZnWO₄ samples were investigated with TEM, as shown in Fig. 3. It could be seen clearly that the as-prepared samples exhibited nanorods structure, which indicated that Er³⁺ doping did not change the morphologies of ZnWO₄. Moreover, pure ZnWO₄nanorods were 35 nm in diameter and 40-60 nm in length (Fig. 3(a)) and the Er³⁺-doped ZnWO₄ were 10 nm in diameter and 30-70 nm in length (Fig. 3(c)). This is because that doping Er³⁺ could influence the growing interface and finally change the diameter of ZnWO₄ nanorods according to the chemical bonding theory of single crystal growth^{[28, 29]}. The HRTEM images showed clear lattice fringes of d=0.247 nm for pure ZnWO₄(Fig. 3(b)) and d=0.253 nm for Er³⁺-doped ZnWO₄(Fig. 3(d)), which matched the d-spacing of the (021) plane of ZnWO₄.

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	Fig. 3 Typical TEM and HRTEM images of the as-synthesized ZnWO₄ samples (a, b) Pure ZnWO₄; (c, d) 2mol% Er³⁺-doped ZnWO₄

2.4 Photocatalytic activities of the catalysts

The absorption spectra of RhB after degradation by the 2mol% Er³⁺-doped ZnWO₄ sample was shown in Fig. 4(a). It was clear that the absorption of RhB was negligible after 3 h, which indicated that RhB was degraded completely. The photocatalytic activities of the Er³⁺-doped ZnWO₄ samples were determined by comparing the degradation efficiency of RhB under simulated solar light irradiation (Fig. 4 (b)). Blank test showed that the degradation of RhB was negligible in the absence of photocatalyst under light illumination. It could be seen that the photocatalytic activity of ZnWO₄was enhanced with the introduction of Er³⁺ and reached a maximum with the doping concentration of 2mol%. Then the photocatalytic activity decreased when the doping concentration of Er³⁺ further increased.

Moreover, the comparison of the apparent rate constantkin Fig. 5 showed the 2mol% Er³⁺-doped ZnWO₄ sample had the highest k value in the degradation of RhB (1.411 h^-1), while that of pure ZnWO₄ was only 0.534 h^-1. In other words, the photocatalytic activity of the 2mol% Er³⁺-doped ZnWO₄ sample was 2.64 times that of pure ZnWO₄. This result indicated that Er³⁺ doping played an important role in the enhancement of the photocatalytic activity.

The photocatalytic activities of the 2mol% Er³⁺-doped ZnWO₄ sample under different conditions were shown in Fig. 6. It could be seen clearly that when the pH value of the RhB solution was 3, the sample showed the highest photocatalytic activity (Fig. 6(a)). This is because the pH value can change the surface electrical property of ZnWO₄ and its adsorption ability. Moreover, it could be seen from Fig. 6(b) that when the amount of the photocatalyst and the concentration of RhB were not changed, the more amount of RhB solution was, the weaker the photocatalytic activity was. When the amount of RhB solution increased, fewer photons could penetrate through the solution, which led to the decreased photocatalytic activity.

2.5 Mechanism of the enhanced photo-activities

It’ s known that the photocatalysis efficiency is determined by the competition between the charge separation process and the charge recombination process, and a desired photocatalyst is expected to promote the charge transfer process while suppressing the recombination process. The introduction of Er³⁺could facilitate the charge separation process in such a way: As illustrated in Fig. 7, the presence of Er³⁺ ions might cause the following steps^{[30, 31]}.

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	Fig. 4 The absorption spectra of RhB after degradation by the 2mol% Er³⁺-doped ZnWO₄ sample (a) and degradation efficiency of RhB by the as-prepared samples under a 500 W Xe lamp as a function of time (b)

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	Fig. 5 The rate constant k of Er³⁺-doped ZnWO₄ with different doping concentrations

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	Fig. 6 Degradation efficiency of RhB by the 2mol% Er³⁺-doped ZnWO₄ sample under different conditions (a) Different pH values of the RhB solution; (b) Different amounts of the RhB solution

ZnWO₄ + hv → ZnWO₄(e^-+h⁺) (1)

Er³⁺ + e^- → Er²⁺(2)

Er²⁺ + O₂→ Er³⁺ + O• ₂- (3)

h⁺, O• ₂- + RhB → degraded products (4)

In the photocatalytic process, the Er³⁺ ion trapped the photo-generated electrons and formed a reduction species Er²⁺ ion, and then the Er²⁺ could be oxidized back to Er³⁺ by the adsorbed oxygen. The electron-hole recombination rate could be reduced due to the recycle reactions. In addition, when Er³⁺ was introduced into the system, the substitution of Er³⁺ for Zn²⁺ led to the formation of [Er_Zn]⁺ defects. Meanwhile, [V_Zn]^2-cation vacancies were generated for charge compensation. The {2[Er_Zn]⁺- [V_Zn]^2-} defect groups could act as electron and hole traps, so as to inhibit the recombination and extend the lifetime of charge carriers. Therefore, appropriate doping amount of Er³⁺could enhance the photocatalytic process by suppressing the charge recombination effectively. However, when the doping concentration of Er³⁺ increased further, excess of Er³⁺could act as electron/hole recombination centers, which led to a decreased photocatalytic activity. So there existed an optimum doping concentration for Er³⁺.

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	Fig. 7 Schematic for the photocatalytic mechanism of Er³⁺- doped ZnWO₄

3 Conclusion

Er³⁺-doped ZnWO₄ photocatalysts were synthesized by a hydrothermal method. The effects of Er³⁺ doping on the photocatalytic activity of ZnWO₄ catalysts were investigated. 2mol% Er³⁺-doped ZnWO₄sample exhibited the highest photocatalytic activity, which was 2.64 times that of pure ZnWO₄. The enhanced photocatalytic activity could be attributed to the higher charge separation efficiency brought by Er³⁺ doping. Our work indicates that the introduction of lanthanide ion could be a plausible strategy to develop efficient photocatalysts for environmental remediation and is worth to be extended to the preparation of other photocatalysts with high activity.

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1995

41.298

0.0

Chemical Reviews. 1995, 95(1):49-68

Light-induced redox reactions in nanocrystalline systems.

HAGFELDT A , GRAETZEL M.

... In recent years, semiconductor-based photocatalysis has attracted more and more attention for solving environment problems, especially for the removal of organic contaminants under sunlight^{[1,2,3,4,5,6,7,8]} ...

1995

41.298

0.0

Chemical Reviews. 1995, 95(1):69-96

Environmental applications of semiconductor photocatalysis.

HOFFMANN M R , MARTIN S T , CHOI W

2001

2.145

0.0

Chemical Physics Letters. 2001, 343(3):303-308

Photocatalytic and photophysical properties of a novel series of solid photocatalysts, BiTa₁-_xNb_xO₄ (0#cod#x02264;x#cod#x02264;1).

ZOU Z , YE J , SAYAMA K

solid photocatalysts were prepared by solid-state reaction and characterized by powder X-ray diffraction and Rietveld structure refinement. The structure of BiTa 1− x Nb x O 4 ( x =0.0 and 0.5) is triclinic. However, the structures of x =0.2, 0.8 and 1.0 are orthorhombic. The H 2 evolution was obtained from an aqueous CH 3 OH/H 2 O solution and from pure H 2 O with under UV irradiation. The orthorhombic photocatalysts exhibit much higher activity than that of triclinic photocatalysts. The orthorhombic photocatalyst at x =0.2 showed the highest activity. An UV–vis diffuse reflectance spectroscopy measurement revealed that the band gap of orthorhombic photocatalysts is more narrow than that of triclinic photocatalysts.

2006

8.238

0.0

Chemistry of Materials. 2006, 18(7):1969-1975

Photocatalytic hydrogen evolution on ZnS-CuInS₂-AgInS₂ solid solution photocatalysts with wide visible light absorption bands.

TSUJI I , KATO H , KUDO A.

Pt-loaded ZnS−CuInS 2 −AgInS 2 solid solutions showed photocatalytic activities for H 2 evolution from aqueous solutions containing SO 3 2 - and S 2 - as sacrificial reagents under visible light irradiation (λ ≥ 420 nm). The crystal structures of the solid solutions were either zinc blende or wurtzite, depending on the composition. The diffuse reflectance and photoluminescence spectra of the solid solutions were shifted monotonically to longer wavelengths as the ratio of MInS 2 (M = Cu and Ag) to ZnS increased, indicating that the energy band structure of the sold solution was controllable by the change in the composition. The ZnS−CuInS 2 −AgInS 2 solid solutions exhibited absorption bands that were longer in wavelength than those for the ZnS−CuInS 2 and ZnS−AgInS 2 solid solutions, which is probably due to interactions between the Cu 3d and Ag 4d orbitals. The photocatalytic activity for H 2 evolution was much improved by low-temperature synthesis (773−873 K) and the loading of Ru cocatalysts. The Ru-loaded Cu 0.25 Ag 0.25 In 0.5 ZnS 2 solid solution, which had a wide absorption band (band gap, 1.77 eV) in the visible light region, showed high activity for H 2 production, even under simulated solar irradiation (AM 1.5).

2009

0.0

2010

3.925

0.0

Journal of Hazardous Materials. 2010, 180(1):466-473

Photocatalytic performance of ZnO coated tubular reactor.

AKYOL A , BAYRAMOGLU M.

The recovery of photocatalyst particles constitutes a serious drawback of slurry reactors. Therefore, depositing the catalyst mass on a suitable support without loss of photocatalytic activity is of practical importance. In this study, a chemical deposition method was applied to coat the inner surface of quartz tubes of various diameters with thin ZnO film. The method is economical, energy efficient and easy to apply. ZnO film is mechanically durable and resistant to chemical dissolution. Various physical tests were conducted to characterize the film. ZnO thin film exhibits the crystal wurtzite structure with band gap calculated as 3.24 eV. The film exhibits highly hydrophilic behavior. The average ZnO crystallite size is estimated as 72 nm. SEM analysis shows that the ZnO film has granular morphology with uniform particle size of about 300–400 nm. The film thickness is calculated as 1.41 μm after 20 coating cycles and the increase of the thickness of the thin film per cycle was approximately 70 nm. Photochemical activity tests were performed by measuring photo decolorization rate of a commercial azo dye (Rem Red F3B) solution in tubular reactor and as well as in slurry reactor for comparative purpose. First order rate constants were correlated to principal process parameters. The results showed that thin ZnO film has high photocatalytic activity comparable to that of ZnO powder.

2011

5.825

0.0

Applied Catalysis B: Environmental. 2011, 104(3):234-238

Facile fabrication of hierarchical porous TiO₂ hollow microspheres with high photocatalytic activity for water purification.

LIU Z , BAI H , SUN D.

Hierarchical porous TiO 2 hollow microspheres were synthesized by a facile approach from a one-step template method. This novel approach avoids tedious multiple steps from conventional template method for fabricating hierarchical porous and hollow microspheres. The as-prepared microspheres were characterized by XRD, SEM, TEM, nitrogen adsorption, and UV–vis DRS. Its photocatalytic activity was demonstrated in the photocatalytic degradation of Rhodamine B. Structural characterization indicates that the as-prepared microspheres had a hollow interior and a hierarchical porous structure with both large and small mesopores on its surface. The average diameters and BET surface areas of the hierarchical porous TiO 2 hollow microspheres were 1.5 μm and 117 m 2 /g, respectively. Optical adsorption investigation shows that the hierarchical porous TiO 2 hollow microspheres possessed the optical band gap of 3.42 eV. A possible formation mechanism for the hierarchical porous TiO 2 hollow microspheres was discussed. The hierarchical porous TiO 2 hollow microspheres exhibited much higher photocatalytic activity than the commercial P25 TiO 2 . In addition, the as-prepared microspheres can be easily recycling for reuse. All these advantages show a bright future for this microspheric photocatalyst in environmental purification.

2012

4.814

0.0

Journal of Physical Chemistry C. 2012, 116(49):25898-25903

Photocatalysis coupled with thermal effect Induced by SPR on Ag-loaded Bi₂WO₆ with enhanced photocatalytic activity.

ZHANG Z , WANG W , GAO E

The temperature dependence of the photocatalytic activity of Bi 2 WO 6 was studied and the results showed that a higher reaction temperature can facilitate the photocatalytic process. On the other hand, it is well-known that the excitation of localized plasmon polaritons at well-defined wavelengths on the surface of silver (Ag) nanoparticles causes a tremendous thermal effect. Thus, it is expected that the photocatalytic activity of Bi 2 WO 6 would be enhanced if it was coupled with the thermal effect induced by surface plasmon resonance (SPR) of Ag. Herein, this idea is realized. The measured photocatalytic activity under simulated solar light, monitored by the decomposition of phenol, demonstrated that Ag loading could significantly enhance the photocatalytic activity of Bi 2 WO 6 . The enhanced photoactivity can be ascribed to the synergistic effect brought by electronic effect of Ag nanoparticles and thermal effect induced by SPR. This work provides some insights into the rational design and development of high-performance photocatalysts.

2003

3.607

0.0

Journal of Physical Chemistry B. 2003, 107(51):14265-14269

Photophysical and photocatalytic properties of AgInW₂O₈.

TANG J , ZOU Z , YE J.

A photocatalyst, AgInW 2 O 8 , with a layered structure was synthesized. The physical and photophysical properties of the photocatalyst were characterized by XRD, BET measurement, UV−visible diffuse reflectance, and photoluminescence, respectively. The photocatalytic properties of the photocatalyst for H 2 evolution with Pt cocatalyst from CH 3 OH/H 2 O solution and O 2 evolution from AgNO 3 solution were observed under UV and visible light irradiation. The AgInW 2 O 8 photocatalyst presented not only the activity of O 2 evolution but also the activity of H 2 evolution. Also the photocatalyst showed the ability to evolve H 2 from pure water under full arc irradiation. On the basis of all the results, the band structure of the photocatalyst was discussed.

... Tungstates are widely used functional materials, and it has been confirmed that tungstates are a class of promising photocatalysts^[9,10,11] ...

2006

5.825

0.0

Applied Catalysis B: Environmental. 2006, 66(1):100-110

Photocatalytic properties of nanosized Bi₂WO₆ catalysts synthesized via a hydrothermal process.

FU H , ZHANG L , YAO W

Nanosized Bi 2 WO 6 was synthesized by a hydrothermal crystallization process. The as-prepared samples were characterized by X-ray diffraction, Brunauer–Emmet–Teller surface area and porosity measurements, transmission electron microscopy, Raman spectra, and diffuse reflectance spectroscopy. The photoactivities of the as-prepared samples for the rhodamine-B photodegradation were investigated systematically. As a result, the sample prepared at 180 °C exhibited the highest photochemical activity under visible-light irradiation. The further experiments revealed that the catalyst was active in a wide spectral range. Density functional theory calculations suggested that the visible-light response was due to the transition from the valence band formed by the hybrid orbitals of Bi 6s and O 2p to the conduction band of W 5d. The photoactivity of the catalyst in relationship with the hydrothermal temperature, the crystal and band structure were also discussed in detail.

... Tungstates are widely used functional materials, and it has been confirmed that tungstates are a class of promising photocatalysts^[9,10,11] ...

2010

3.925

0.0

Journal of Hazardous Materials. 2010, 177(1):1013-1018

Low-temperature combustion synthesis of B_i2W_O6 nanoparticles as a visible-light-driven photocatalyst.

ZHANG Z , WANG W , SHANG M

Visible-light-induced Bi 2 WO 6 photocatalyst has been successfully synthesized via a facile low-temperature combustion synthesis method, using glycine as the fuel. The photocatalytic activities of the as-synthesized samples were evaluated by the photodegradation of rhodamine B (RhB) and phenol under visible-light irradiation ( λ > 420 nm). The results showed that the molar ratio of fuel to oxidizer had an important influence on the photocatalytic activities of the products. When the molar ratio of fuel to oxidizer was 1, the photocatalyst exhibited the highest degradation efficiency, which can completely degrade RhB with a concentration up to 10 −4 M within 75 min. Besides decoloring, the markable reduction of chemical oxygen demand (COD) was also observed in the degradation of RhB, further demonstrating the photocatalytic performance of Bi 2 WO 6 . Additionally, the photocatalyst showed much enhanced visible photocatalytic efficiency, up to 94.2% in 4 h, than the bulk Bi 2 WO 6 powder (SSR) in the degradation of phenol.

... Tungstates are widely used functional materials, and it has been confirmed that tungstates are a class of promising photocatalysts^[9,10,11] ...

2007

4.814

0.0

Journal of Physical Chemistry C. 2007, 111(32):11952-11958

Enhanced photocatalytic activity of ZnWO₄ catalyst via fluorine doping.

HUANG G , ZHU Y.

Fluorine interstitially doped ZnWO 4 (F−ZnWO 4 ) was synthesized by fluorine-ion addition in a hydrothermal process. The effects of fluorine doping on crystal structure, optical properties, and photocatalytic activity of ZnWO 4 catalyst were investigated. The results showed that the doped fluorine ions could increase the coordination sphere around the W atom in a WO 6 octahedron and cause the distortion of the WO 6 octahedron in a ZnWO 4 crystal. Moreover, the strength of absorption in the UV region and transfer rate of photogenerated electrons to the surface increased after doping with fluorine. With an increase of fluorine doping concentration, the photoactivities of samples were enhanced greatly. The photocatalytic activity can be enhanced 2.6 times for the degradation of RhB when the optimal atomic ratio of F/Zn was 0.4. The enhanced photocatalytic activity originated from the increase of transfer rate of photogenerated electrons to the photocatalyst surface.

... ZnWO₄ as an important member of tungstates, due to its photocatalytic properties and potential applications, has received intense research attention^[12,13] ...

... Up to date, great efforts have been made to facilitate the electron-hole separation and enhance the photocatalytic activity of ZnWO₄, including cation doping^[18,19], anion doping such as fluorine or chlorine^{[12, 20,21]}, and coupling with other semiconductors such as Bi₂WO₆^[22] or ZnO^[23] ...

2013

1.913

0.0

Materials Research Bulletin. 2013, 48(3):1105-1109

A facile synthesis of ZnWO₄ nanoparticles by microwave assisted technique and its application in photocatalysis.

GARADKAR K M , GHULE L A , SAPNAR K B

A simple microwave assisted technique has been successfully developed to synthesize ZnWO 4 nanoparticles. The X-ray diffraction results indicated that the synthesized nanoparticles exhibited only wolframite structure. Structural, morphological and optical properties of ZnWO 4 nanoparticles have been analyzed by XRD, SEM, TEM EDAX, UV–vis and FT-IR spectral measurements. The transmission electron microscopy (TEM) image revealed that particle size of ZnWO 4 nanoparticles was found to be 10 nm, the band-gap of ZnWO 4 nanoparticles was found to be 3.4 eV. The photocatalytic activities for aqueous Rhodamine B and Methylene Blue samples were investigated and observed that ZnWO 4 nanoparticles exhibited highly enhanced photocatalytic activity towards RhB than MB.

... ZnWO₄ as an important member of tungstates, due to its photocatalytic properties and potential applications, has received intense research attention^[12,13] ...

2002

22.929

0.0

Physics Reports. 2002, 361(1):1-56

The neutrinoless double beta decay from a modern perspective.

VERGADOS J D.

Neutrinoless double beta decay is a very important process both from the particle and nuclear physics point of view. From the elementary particle point of view it pops up in almost every model, giving rise, among others, to the following mechanisms: (a) The traditional contributions like the light neutrino mass mechanism as well as the j L – j R leptonic interference ( λ and η terms). (b) The exotic R -parity violating supersymmetric (SUSY) contributions. In this scheme, the currents are only left handed and the intermediate particles normally are very heavy. There exists, however, the possibility of light intermediate neutrinos arising from the combination of V–A and P–S currents at the quark level. This leads to the same structure as the above λ term. Similar considerations apply to its sister lepton and muon number violating muon to positron conversion in the presence of nuclei. Anyway, regardless of the dominant mechanism, the observation of neutrinoless double betas decay, which is the most important of the two from an experimental point of view, will severely constrain the existing models and will signal that the neutrinos are massive Majorana particles. From the nuclear physics point of view it is challenging, because: (1) The nuclei, which can undergo double beta decay, have a complicated nuclear structure. (2) The energetically allowed transitions are suppressed (exhaust a small part of the entire strength). (3) Since in some mechanisms the intermediate particles are very heavy, one must cope with the short distance behavior of the transition operators. Thus novel effects, like the double beta decay of pions in flight between nucleons, have to be considered. In SUSY models this mechanism is more important than the standard two nucleon mechanism. (4) The intermediate momenta involved are quite high (about ). Thus, one has to take into account possible momentum-dependent terms of the nucleon current, like the modification of the axial current due to PCAC, weak magnetism terms, etc. We find that, for the mass mechanism, such modifications of the nucleon current for light neutrinos reduce the nuclear matrix elements by about 25%, almost regardless of the nuclear model. In the case of heavy neutrino the effect is much larger and model dependent. Taking the above effects into account the needed nuclear matrix elements have become available for all the experimentally interesting nuclei A=76,82,96,100,116,128,130,136 and 150. Some of them have been obtained in the large basis shell model but most of them in various versions of QRPA. Then using the best presently available experimental limits on the half-life of the 0 νββ -decay, we have extracted new limits on the various lepton violating parameters. In particular we find and, for reasonable choices of the parameters of SUSY models in the allowed SUSY parameter space, we get a stringent limit on the R -parity violating parameter λ 111 ′ −3 .

... Up to now, ZnWO₄has been used for water splitting and decomposition of organic pollutants under UV irradiation^{[14,15,16,17]} ...

2012

0.0

rystal Research and Technology. 2012, 47(12):1279-1283

Microwave hydrothermal synthesis and photocatalytic properties of ZnWO₄ nanorods. C

TAN G , ZHANG L , WEI S

Cd 2+ -doped ZnWO 4 nanorods have been synthesized at 200 °C with microwave hydrothermal method, using Zn(NO 3 ) 2 ·6H 2 O, Na 2 WO 4 ·2H 2 O and CdCl 2 as raw materials. The effects of Cd 2+ doping contents on the structure and morphology of the product were studied. The results show that Cd 2+ doping into the crystal lattice of ZnWO 4 nanopowder makes the powder orientationally grow along (010), (110) and (200) crystal planes to form the nanorods. The bigger Cd 2+ doping contents are, the more obviously ZnWO 4 nanorods grow. Meanwhile, the nanopowder is gradually transformed from monoclinic phase into the orthogonal phase. As the charge transference medium between the interfaces, Cd 2+ restrains the combination of holes and electrons. After doped, the photocatalytic properties of ZnWO 4 nanorods are increased. When Cd 2+ doping content is 20%, the Cd 2+ -doped ZnWO 4 nanorods showed the highest degradation rate up to 98% in 2 h.

... Up to now, ZnWO₄has been used for water splitting and decomposition of organic pollutants under UV irradiation^{[14,15,16,17]} ...

2007

2.39

0.0

Journal of Alloys and Compounds. 2007, 432(1):269-276

ZnWO₄ photocatalyst with high activity for degradation of organic contaminants.

HUANG G , ZHANG C , ZHU Y.

ZnWO 4 nanoparticle photocatalyst has been prepared by annealing treatment of hydrothermal sample, and the high photocatalytic activity was explored. The crystalline of ZnWO 4 photocatalysts were improved as the increase of annealing temperature and time. It was found that ZnWO 4 nanoparticle catalysts exhibited a high photocatalytic activity, not only in the decomposition of formaldehyde in gaseous, but also in the degradation of RhB in water. The highest photocatalytic activity both appeared at 450 °C for 1 h due to good crystallization and high surface area. The photocatalytic activity of ZnWO 4 was a little higher than that of P-25 (Degussa) in gaseous and lower than that of P-25 (Degussa) in aqueous. The main reason is that the dispersion of ZnWO 4 powders in aqueous is not good due to its high density. The surface area, crystal size, dispersion and crystalline are important factors, which influence the photocatalytic activities of ZnWO 4 catalysts.

... Up to now, ZnWO₄has been used for water splitting and decomposition of organic pollutants under UV irradiation^{[14,15,16,17]} ...

2010

5.825

0.0

Applied Catalysis B: Environmental. 2010, 100(1):173-178

Synthesis of ZnWO₄ nanorods with [100] orientation and enhanced photocatalytic properties.

SHI R , WANG Y , LI D

ZnWO 4 photocatalysts with various morphologies were synthesized by a template-free hydrothermal synthesis process. The morphologies had a significant influence on the photocatalytic activity for methylene blue and Rhodamine B degradation. ZnWO 4 nanorods with a highly [1 0 0] preferred orientation, obtained at pH 10, exhibited the highest photocatalytic activity among all the samples. This kind of nanorods had the most planes along the [1 0 0] direction—(0 1 0) and (0 1 1) planes, which meant most (0 1 0) and (0 1 1) planes exposed in ZnWO 4 phase. Basis on the analysis of crystal plane and hydrogen-related defects, the (0 1 0) and (0 1 1) planes could speculate to be more active for photodegradation.

... Up to now, ZnWO₄has been used for water splitting and decomposition of organic pollutants under UV irradiation^{[14,15,16,17]} ...

2008

1.913

0.0

Materials Research Bulletin. 2008, 43(7):1694-1701

Characterizations and properties of Eu³+-doped ZnWO₄ prepared via a facile self-propagating combustion method.

DONG T , LI Z , DING Z

ZnWO 4 and Eu 3+ -doped ZnWO 4 were prepared for the first time via a facile self-propagating combustion method. The structure, morphology and luminescence of the as-prepared ZnWO 4 and Eu 3+ -doped ZnWO 4 were characterized. The photoluminescent property of Eu 3+ -doped ZnWO 4 complexes indicated an energy transfer from WO 4 2− groups to Eu 3+ and suggested an effective doping of Eu 3+ into the lattice of ZnWO 4 . The photocatalytic activity of ZnWO 4 and Eu 3+ -doped ZnWO 4 was investigated by the photodegradation of rhodamine B (RhB). Eu 3+ -doped ZnWO 4 showed enhanced photocatalytic activity in the photodegradation of RhB.

2012

4.814

0.0

Journal of Physical Chemistry C. 2012, 116(34):18508-18517

Particle size and structural control of ZnWO₄ nanocrystals via Sn²+ doping for tunable optical and visible photocatalytic properties.

SU Y , ZHU B , GUAN K

In this work, we report on the microwave-assisted hydrothermal synthesis of Sn 2+ -doped ZnWO 4 nanocrystals with controlled particle sizes and lattice structures for tunable optical and photocatalytic properties. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, UV–vis diffuse reflectance spectroscopy, and Barrett–Emmett–Teller technique. The effects of Sn 2+ doping in ZnWO 4 lattice on the crystal structure, electronic structure, and photodegradation of methylene orange dye solution were investigated both experimentally and theoretically. It is found that part of the Sn 2+ ions were homogeneously incorporated in the ZnWO 4 host lattice, leading to a monotonous lattice expansion, and part of Sn 2+ ions were expelled at surface sites for decreased crystallinity and particle size reduction. By Sn 2+ doping, ZnWO 4 nanocrystals showed a significant XPS binding energy shift of Zn 2p, W 4f, and O 1s, which is attributed to the combination of electronegativity between Sn 2+ and Zn 2+ , lattice variation, and particle size reduction. Meanwhile, the BET surface areas were also greatly enlarged from 40.1 to ∼110 m 2 ·g –1 . Contrary to the theoretical predictions of the quantum size effect, Sn 2+ -doped ZnWO 4 nanocrystals showed an abnormal band gap narrowing, which can be well-defined as a consequence of bulk and surface doping effects as well as lattice variations. With well-controlled particle size, crystallinity, and electronic structure via Sn 2+ doping, the photocatalytic performance of Sn 2+ -doped ZnWO 4 nanocrystals was optimized at Sn 2+ doping level of 0.451.

2010

4.814

0.0

Journal of Physical Chemistry C. 2010, 114(17):7680-7688

Experimental and theoretical studies on the enhanced photocatalytic activity of ZnWO₄ nanorods by fluorine doping.

CHEN S , SUN S , SUN H

We present a combined study of the photocatalysis of the F interstitially doped ZnWO 4 (F i -ZnWO 4 ) nanocrystals by means of experiment and DFT band structure calculations, including an analysis of the chemical bonding. The F i -ZnWO 4 nanorods were prepared via a two-step hydrothermal process by adjusting the pH of the reaction solution. The results showed that the F i -ZnWO 4 samples exhibited stronger absorption in the UV−visible range with a red shift in the band-gap transition. The photocatalytic activity of the F i -ZnWO 4 samples was highly enhanced for the photocatalytic decomposition of RhB under ultraviolet light irradiation. The morphology and crystallinity of F i -ZnWO 4 have a significant influence on the photocatalytic activity; the F i -ZnWO 4 nanorod showed much higher photocatalytic activity than the nanoparticle one. The DFT calculations show that the mixing of F 2p and O 2p states in the top of the valence band induces the valence band to extend to higher energy, which accounts for the slight red shift of the optical absorption edge observed in the experiment. In addition, the interstitial F impurity creates a new half-filled state in the band gap, which can supply a hole carrier to improve the photocurrent density for the F-doped ZnWO 4 under UV radiation, thus enhancing the photocatalytic activity of ZnWO 4 . The excellent agreement between the measured spectra and calculated electronic structure can provide a solid basis for understanding the enhanced photocatalytic activity of F i -ZnWO 4 nanocrystals.

2012

3.842

0.0

CrystEngComm. 2012, 14(23):8076-8082

Synthesis and photoactivity enhancement of ZnWO₄ photocatalysts doped with chlorine.

HUANG G , ZHU Y.

ZnWO 4 photocatalysts doped with chlorine were synthesized by a simple hydrothermal process. The influences of chlorine doping on the crystal structure, optical properties and photocatalytic activities of the ZnWO 4 photocatalyst were investigated. ZnWO 4 doped with chlorine showed enhanced photocatalytic activity for the degradation of methylene blue (MB), rhodamine B and 4-chlorophenol. The photocatalytic activity of ZnWO 4 with R Cl = 0.3 can be enhanced approximately 4 times for the degradation of MB. The enhanced photocatalytic activity originated from the increase of the transfer rate of photogenerated electrons to the photocatalyst surface, which was promoted by chlorine doping. The photodegradation process of MB can be described as N-demethylation and oxidative degradation. In the case of chlorine doping, an effective separation of the photogenerated electron–hole pairs and the fast interfacial charge transfer to the electron donor/electron acceptor occurred.

2011

0.0

2014

5.825

0.0

2002

2.224

0.0

Materials Letters. 2002, 57(4):794-801

Titanium dioxide nanoparticles co-doped with Fe³+ and Eu³+ ions for photocatalysis.

YANG P , LU C , HUA N

Nanoparticles of titanium dioxide co-doped with Fe 3+ and Eu 3+ were prepared using the sol–gel method. The photocatalytic reactivities were evaluated by photodegradation of chloroform in solution. Nanocrystalline TiO 2 co-doped with Eu 3+ and Fe 3+ at optimal concentration (1 at.% Fe 3+ , 0.5 at.% Eu 3+ ) shows a synergistic effect, which significantly increases the photodegradation activity of nano-TiO 2 . The effects of the dopants in nanocrystalline TiO 2 particles on the photocatalysis were studied using the photo-electrochemical method. Two dopants of Fe 3+ and Eu 3+ in nanocrystalline TiO 2 play different roles in improving the photoinduced charge separation in the nanostructured semiconductor and in the interfacial charge transfer process at the semiconductor/solution interface. Fe 3+ serves as a hole trap and Eu 3+ as an electron trap in nanocrystalline TiO 2 , respectively. The separation of the charge carriers is attributed to such trappings. In the meantime, Fe 3+ and Eu 3+ also speed up anodic and cathodic processes via improving interfacial charge transfers. Superior photocatalysis of nanocrystalline TiO 2 co-doped with Fe 3+ and Eu 3+ ions was related to the cooperative actions of the two dopants.

... For example, it has been reported that doping with Eu³⁺, Er³⁺ or Ce⁴⁺ could improve the photocatalytic activity of TiO₂^[24,25], and doping with Er³⁺ could enhance the photocatalytic activity of Bi₂WO₆^[26] ...

2002

5.787

0.0

Journal of Catalysis. 2002, 207(2):151-157

The preparation, characterization, and their photocatalytic activities of rare-earth-doped TiO₂ nanoparticles.

XU A W , GAO Y , LIU H Q.

RE/TiO 2 photocatalysts were prepared by the sol–gel method using rare earth (RE=La 3+ , Ce 3+ , Er 3+ , Pr 3+ , Gd 3+ , Nd 3+ , Sm 3+ ) metal salts and tetra- n -butyl titanate as precursors, and were characterized by XRD, IR, UV–vis diffuse reflection, and transient absorption spectra. Their photocatalytic activities were evaluated using nitrite as a decomposition objective. As a result, suitable content of doping rare earth in TiO 2 can efficiently extend the light absorption properties to the visible region. At the same time, it is beneficial to NO 2 − adsorption over the catalysts due to rare earth doping. RE/TiO 2 samples can enhance the photocatalytic activity to some extent as compared with naked TiO 2 . The increase in photoactivity is probably due to the higher adsorption, red shifts to a longer wavelength, and the increase in the interfacial electron transfer rate. Nitrite is almost completely degraded over RE/TiO 2 catalysts after longer irradiation, which is different from Degussa P-25 with a plateau of activity after ca. 20 min irradiation. Gd 3+ -doped TiO 2 showed the highest reaction activity among all concerned RE-doped samples because of its specific characteristics. The amount of RE doping was an important factor affecting photocatalytic activity; the optimum amount of RE doping is ca. 0.5 wt%, at which each RE/TiO 2 sample shows the most reactivity. The photocatalytic degradation reaction of nitrite over Gd 3+ -doped samples and P-25 follows apparent first order kinetics, which is different from that of Sm 3+ , Ce 3+ , Er 3+ , Pr 3+ , La 3+ , and Nd 3+ -doped TiO 2 catalysts, which obey zero-order kinetics, indicating that these processes were dominated by electron–hole recombination.

2010

0.0

Applied Catalysis B: Environmental. 2010, 101(1):68-73

Inducing photocatalysis by visible light beyond the absorption edge: effect of upconversion agent on the photocatalytic activity of Bi₂WO₆.

ZHANG Z , WANG W , YIN W

In order to harvest the solar energy more efficiently, Bi 2 WO 6 photocatalysts doped with Er 3+ were synthesized, in which Er 3+ with upconversion properties could transform visible light beyond the absorption edge of Bi 2 WO 6 into ultraviolet light and activate Bi 2 WO 6 . The photocatalytic activities of the photocatalysts were evaluated by the decolorization of rhodamine B (RhB) and the degradation of a typical colorless model pollutant, phenol under simulated solar light irradiation ( λ > 290 nm). The results showed that the photocatalytic activity of Er 3+ -doped Bi 2 WO 6 was much higher than the undoped one. In addition, Er 3+ -doped Bi 2 WO 6 exhibited photocatalytic activity under a 3 W LED with the wavelength beyond the absorption edge of Bi 2 WO 6 , further testifying the upconversion effect of Er 3+ on the enhanced photocatalytic activity of Bi 2 WO 6 .

2007

2.144

0.0

Journal of Luminescence. 2007, 126(2):623-628

Growth and optical property of ZnWO₄: Er³+ crystal.

YANG F , TU C , LI J

Good quality crystals ZnWO 4 activated with Er 3+ have been grown by means of Czochralski method and characterized using optical spectroscopy techniques. XRD, absorption spectra, fluorescence spectrum are presented and the Judd–Ofelt intensity parameters Ω 2 , Ω 4 , and Ω 6 are obtained to be 6.76×10 –20 , 0.37×10 –20 , and 0.50×10 –20 cm 2 , respectively. Along crystallographic axes, refractive indices are presented. The fluorescence decay time of the 4 I 13/2 level has also been investigated and shows an exponential behavior with a lifetime value of 5.52 ms. The crystal is potentially used for green and infrared eye-safe lasers.

... ⁴I_9/2, respectively^[27] ...

2014

0.0

... nanorods according to the chemical bonding theory of single crystal growth^[28,29] ...

2014

3.842

0.0

CrystEngComm. 2014, 16(11):2129-2135

Chemical bonding theory of single crystal growth and its application to #cod#x003d5; 3#cod#x02032;#cod#x02032; YAG bulk crystal.

SUN C , XUE D.

The growth of YAG bulk crystals was studied using both theoretical calculations based on the anisotropic chemical bonding conditions and practical growth via the Czochralski (Cz) method. The chemical bonding theory of single crystal growth quantitatively describes the anisotropic bonding behaviors of constituent atoms during crystallizing, which can be applied to the thermodynamic growth of YAG single crystals. Both bonding conditions and crystal symmetry determine the projection configuration along the pulling direction and crystal ridges in the crystal shoulder of YAG grown along [111] direction. During Cz growth process of YAG single crystals, the relative low growth rate along directions results in the exposure of surfaces normal to directions. However, the chemical bonding energy density at the intersection of two adjacent growth directions is higher, leading to the exposure of surfaces normal to directions and the truncated-hexagon configuration of YAG along [111] direction. ϕ 3′′ YAG single crystal was successfully grown. Our present work provides a promising approach to achieve controllable growth for functional bulk crystal via both thermodynamic and kinetic controls.

... nanorods according to the chemical bonding theory of single crystal growth^[28,29] ...

2003

8.238

0.0

Chemistry of materials. 2003, 15(3):632-635

Intercalation behavior of n-alkylamines into a protonated form of a layered perovskite derived from aurivillius phase Bi₂SrTa₂O₉.

TSUNODA Y , SUGIMOTO W , SUGAHARA Y.

Intercalation behavior of n -alkylamines into a protonated form of a layered perovskite, H 1.8 [Sr 0.8 Bi 0.2 Ta 2 O 7 ], derived from an Aurivillius phase, Bi 2 SrTa 2 O 9 , has been investigated. H 1.8 [Sr 0.8 Bi 0.2 Ta 2 O 7 ] can accommodate n -alkylamines (C m H m +1 NH 2 ; m = 4, 8, 12, 18) to form intercalation compounds via an acid−base mechanism. The interlayer distance increases to 2.071(2) ( m = 4), 2.840(9) ( m = 8 ), 3.83(1) ( m = 12), and 5.17(2) ( m = 18) nm. In contrast, H 1.8 [Sr 0.8 Bi 0.2 Ta 2 O 7 ] does not form any intercalation compound with pyridine, which is a weaker base, indicating that the protons in H 1.8 [Sr 0.8 Bi 0.2 Ta 2 O 7 ] are weakly acidic. The IR spectra of the intercalation compounds with n -alkylamines ( m = 12 and 18) clearly show that n -alkyl chains possess an all-trans conformation. A linear relationship is observed between the interlayer distance and the number of carbon atoms in n -alkyl chains, and this corresponds to a bilayer arrangement of the n -alkyl chains with a tilt angle of 60°. Despite the relatively high proton content (1.8 H + per [Sr 0.8 Bi 0.2 Ta 2 O 7 ]), only 0.9−1.0 mol of n -alkylamines per [Sr 0.8 Bi 0.2 Ta 2 O 7 ] is intercalated. This observation can be reasonably interpreted based on the surface geometry of the perovskite-like slab and the size of n -alkylamines.

2012

2.39

0.0

Journal of Alloys and Compounds. 2012, 537:24-28

A superior visible light-driven photocatalyst: europium-doped bismuth tungstate hierarchical microspheres.

TIAN Y , ZHANG L , ZHANG J.

Novel visible light-driven photocatalyst europium-doped bismuth tungstate (Eu/Bi 2 WO 6 ) hierarchical microspheres has been synthesized by a hydrothermal route. The obtained products were characterized by SEM, EDX, XRD, XPS and DRS technologies. Their photocatalytic activities were determined by oxidative decomposition of Rhodamine-B (RhB) in aqueous solution under visible light irradiation. The results revealed that the europium doping greatly improved the photocatalytic efficiency of Bi 2 WO 6 microspheres. The photodegradation reactions of RhB by Eu/Bi 2 WO 6 composites followed the first-order kinetics. The possible reasons for Eu/Bi 2 WO 6 composites showing higher activity than pure Bi 2 WO 6 are mainly ascribed to the enhanced absorption in the visible region and the suppressed recombination between photogenerated electrons and holes in composite samples.

0.0