J. Phys. Chem. C 2008, 112, 20393–20397
20393
Ternary Wide Band Gap p-Block Metal Semiconductor ZnGa2O4 for Photocatalytic Benzene Degradation Xun Chen, Hun Xue,† Zhaohui Li,* Ling Wu, Xuxu Wang, and Xianzhi Fu* Research Institute of Photocatalysis, Fuzhou UniVersity, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou 350002, People’s Republic of China ReceiVed: September 15, 2008; ReVised Manuscript ReceiVed: October 26, 2008
Nanocrystalline ternary wide band gap p-block metal semiconductor ZnGa2O4 was successfully prepared via a coprecipitation method. The as-prepared sample was characterized by X-ray diffraction, N2-sorption BET surface area, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and Fourier transformation infrared spectroscopy. ZnGa2O4 showed superior photocatalytic activity and stability for benzene degradation as compared to commercial TiO2. However, its activity was lower than another wide band gap p-block metal semiconductor photocatalyst Sr2Sb2O7. The difference in the photocatalytic activity between ZnGa2O4 and Sr2Sb2O7 can be well explained by their different geometric structures. This result gave some new insight in the development of new ternary wide band gap p-block semiconductor photocatalysts for benzene degradation. 1. Introduction Benzene is a widespread environmental pollutant. Due to its high toxicity, confirmed carcinogenicity, and environmental persistence, it is regarded as the priority hazardous substance.1 The development of an efficient and mild technology to remove benzene from the ambient environment is absolutely necessarily. TiO2-based photocatalytic oxidation (PCO) has been established to be one of the most promising technologies for the environment remediation.2 The PCO technology is environmentally friendly, is capable of performing at room temperature, and can treat organic pollutants at extremely low concentrations. Although numerous VOCs (volatile organic compounds) can be readily photocatalyzed to CO2 and H2O on TiO2, PCO meets with limited success in the treatment of aromatic compounds like benzene due to the deactivation of TiO2 resulting from the accumulation of stable reaction intermediates on the surface.3,4 Loading of a noble metal (Pt, Pd, or Rh) has been used to enhance the performance of TiO2 for photocatalytic oxidation of benzene in gas phase.5–7 Rh/TiO2 was shown to be the best one, but there is concern about its stability because of the oxidation of Rh nanoparticles on the surface of TiO2.7 Sitkiewitz et al.8 and Einaga et al.9 have improved the efficiency of TiO2 photocatalyst toward complete oxidization of benzene by adding a sufficient amount of H2O ([H2O] ) 22000 ppm) in the reaction feed gas to suppress the formation of reaction intermediates on the surface of TiO2. Recently, it has been demonstrated that the introduction of a magnetic field10 or H211,12 into the photochemical reaction system can greatly improve the efficiency of Pt/TiO2 for the photodecomposition of benzene at room temperature. However, it is not easy to realize such a complicated hybrid system for photocatalytic air purification. Therefore the development of new photocatalysts with high performance for benzene degradation is indispensable in view * Authors to whom all correspondence should be addressed. E-mail:
[email protected] (Z. Li);
[email protected] (X. Fu). † Current affiliation: College of Chemistry & Materials Science, Fujian Normal University, Fuzhou, 350007, People’s Republic of China.
of the application of photocatalysis for benzene treatment, yet it remains a great challenge until now. Recently, some wide band gap p-block metal semiconductors like Ga2O3,13 In(OH)3,14 InOOH,15 Sr2Sb2O7,16 and Zn2GeO417 have been reported to show high performance in the photocatalytic degradation of benzene. In addition to their high activity for benzene degradation, they are extremely stable and no deactivation has been observed during prolonged photocatalytic reaction. It is proposed that the high photocatalytic performance observed over these wide band gap p-block metal semiconductors is related to their peculiar electronic structure. The dispersive conduction band of these wide band gap p-block semiconductors due to the hybridization of the orbitals can promote the mobility of the photogenerated electrons and enhance the charge separation.15,16 In addition to this, the wide band gap endows the photogenerated charge carriers in these semiconductors with strong enough redox ability to react with surface adsorbed H2O to produce •OH radicals. In this way, the degradation of benzene can proceed via the •OH radical way, which makes these wide band gap p-block semiconductors capable of maintaining a clean catalyst surface and maintains a long stability during the photocatalytic reaction.15,16 Although previous studies have implied that the wide band gap p-block metal semiconductors are a new generation of photocatalysts for benzene degradation, the already known wide band gap p-block metal semiconductor photocatalysts are limited. Besides this, except the common characteristics like wide band gap and the dispersive conduction band, other factors influencing their photocatalytic activity for benzene degradation remain largely unclear. To study these influencing factors is important not only for the development of new wide band gap p-block semiconductor photocatalysts with high efficiency for practical application in benzene degradation but also for the elucidation of their photocatalytic mechanism. For this purpose, more wide-band gap p-block metal semiconductors, especially ternary ones due to their diversified crystallographic and electronic structure and their photocatalytic activity, should be investigated.
10.1021/jp808194r CCC: $40.75 2008 American Chemical Society Published on Web 12/02/2008
20394 J. Phys. Chem. C, Vol. 112, No. 51, 2008 ZnGa2O4 is a p-block metal semiconductor with a band gap of 4.4-5.0 eV,18 which has been used as the transparent conducting material,18,19 phosphor material,20,21 and photocatalyst for water splitting.22 Theoretical calculation reveals that its conduction band is highly dispersive with the bottom of the conduction band composed of the hybridization of Ga 4s, 4p and Zn 4s, 4p orbitals.22 These characteristics meet the basic requirements of photocatalyst for benzene degradation. However the application of ZnGa2O4 for photocatalytic benzene degradation has never been explored. The previously reported synthetic methods for ZnGa2O4 include a high-temperature solid-state reaction,22,23 a thermal evaporation method,18,20 a hydrothermal method,24,25 a multistage precipitation method,26 direct precipitation,27 a flux method,28 a chemical solution method,21 a solution combustion method,23 etc. Herein we reported the preparation of nanocrystalline ZnGa2O4 via a coprecipitation method and the study of its photocatalytic activity for benzene degradation. The coprecipitation method is favorable for large-scale synthesis and can provide uniform nanocrystalline samples with small particle and high specific surface area. The as-prepared ZnGa2O4 showed superior photocatalytic activity and stability for benzene degradation to commercial TiO2. However, its activity is lower as compared to another ternary wide band gap p-block metal semiconductor photocatalyst Sr2Sb2O7. The difference of the photocatalytic activity between ZnGa2O4 and Sr2Sb2O7 can be well explained by their different geometric structures. 2. Experimental Section 2.1. Synthesis. Nanocrystalline ZnGa2O4 was prepared by a coprecipitation method. Aqueous solutions of Zn(NO3)2 · 6H2O and Ga(NO3)3 · xH2O were mixed in the molar ratio of 1:2. Under vigorous stirring, ammonia solutions were dropped simultaneously into the mixed nitrate solutions until the pH values were in the range of 7-8. Then the precipitate slurries were thoroughly stirred. After that, the precipitates were separated and washed several times with distilled water and absolute ethanol. The precipitations obtained were dried in air at 80 °C and then ground and sintered at 600 °C for 5 h in air. 2.2. Characterizations. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Data were recorded at a scanning rate of 0.004° 2θ s-1 in 2θ ranging from 10° to 70°. It was used to identify the phase present and their crystallite size. The crystallite size was calculated from X-ray line broadening analysis by the Scherer equation: D ) 0.89λ/β cos θ, where D is the crystal size in nanometers, λ is the Cu Ka wavelength (0.15406 nm), β is the half-width of the peak in radians, and θ is the corresponding diffraction angle. UV-visible diffuse reflectance spectra (UV-DRS) of the powders were obtained for the dry-pressed disk samples using a UV-visible spectrophotometer (Cary 500 Scan Spectrophotometers, Varian, USA). BaSO4 was used as a reflectance standard in the UV-visible diffuse reflectance experiment. Fourier transform infrared (FT-IR) spectra on the pellets of the samples were recorded on a Nicolet Magna 670 FTIR spectrometer at a resolution of 4 cm-1. The specific surface area of the samples was measured by nitrogen sorption at 77 K on an ASAP 2020 instrument and calculated by the BET method. 2.3. Photocatalytic Activity Measurements. The photocatalytic oxidation experiment of benzene was conducted in a tubular quartz microreactor operating in a continuous-flow mode. The reactor was surrounded by four 4 W UV lamps (wavelength
Chen et al.
Figure 1. XRD patterns of the nanocrystalline ZnGa2O4.
254 nm). The catalyst loading was 0.3 g (50-70 mesh). Benzene was diluted with a zero air stream (21% oxygen, 79% nitrogen, H2O < 5 ppm, total hydrocarbons
