J. Phys. Chem. C 2007, 111, 4727-4733
4727
Studies on In(OH)ySz Solid Solutions: Syntheses, Characterizations, Electronic Structure, and Visible-Light-Driven Photocatalytic Activities Zhaohui Li, Tiaotiao Dong, Yongfan Zhang, Ling Wu, Junqian Li, Xuxu Wang, and Xianzhi Fu* Research Institute of Photocatalysis, Fuzhou UniVersity, Fuzhou 350002, People’s Republic of China ReceiVed: October 11, 2006; In Final Form: February 7, 2007
Nanocrystalline In(OH)ySz solid solutions were synthesized from In(NO3)3 and thiourea in an aqueous solution of ethylenediamine via a facile hydrothermal method. The samples were characterized by X-ray diffraction (XRD), N2-sorption (BET surface area), UV-vis diffuse reflectance spectra (DRS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectra (EDS), and X-ray photoelectron spectroscopy (XPS). The position of the valence band for In(OH)ySz was established by ultraviolet photoelectron spectra (UPS). Electronic band calculations based on density functional theory (DFT) revealed that upon being doped, several doping states derived from S 3p orbitals appeared in the band gap of undoped In(OH)3. In(OH)ySz showed visible-light-driven photocatalytic activity in the decomposition of gaseous acetone. A possible sub-band-transition mechanism of the photocatalysis on In(OH)ySz solid solutions under visible light irradiation was also proposed in this work.
1. Introduction Semiconductor photocatalysts have been widely employed in the treatment of all kinds of organic contaminants. Photocatalysis has many advantages over other treatment methods. It is environmentally friendly, capable of performing at room temperature, and can treat organic pollutants at extremely low concentrations. However, with a relatively wide band gap (3.2 eV), the currently used photocatalyst TiO2 can only absorb a small fraction of solar energy and thus restrict its practical applications.1-9 To develop a photocatalyst with high activities under visible light is indispensable in view of the efficient utilization of solar energy. Over the past several years, considerable effort has been made to increase the visible-light-response and visible-light-driven photocatalytic activities of the photocatalysts.10-13 Two strategies have been employed in the design of visible-light-driven photocatalysts. One is to develop new single-phase oxide photocatalysts with visible-light-driven photocatalytic activities. CaBi2O4,14 MIn2O4 (M ) Ca, Sr and Ba),15 PbBi2Nb2O9,16 etc. have been prepared and their activities in the photocatalytic degradation of organic pollutants under visible light irradiation have been explored. The other strategy involves the chemical modifications on a UV-active photocatalyst, including doping of foreign elements or coupling with a narrow band gap semiconductor. The modifications on TiO2 have been extensively studied. Combinations of TiO2 with various narrow band gap semiconductors like CdS, Fe2O3, Cu2O, Bi2O3, and ZnMn2O4 have been built17,18 and various transition metals19-24 or nonmetal atoms25-31 have been doped into TiO2 to enhance the photocatalytic activities of TiO2 under visible light. Among these modifications, the doping of nonmetal atoms into TiO2 has been proved to be an effective method. Asahi et al. first reported that N-doped TiO2 showed enhanced photocatalytic activities for the degrada* Author to whom all correspondence should be addressed. E-mail:
[email protected]. Phone (fax): 86-591-83738608.
tion of methylene blue and gaseous acetaldehyde.10 Khan et al. reported a C-doped TiO2 with a higher photocurrent density and photoconversion efficiency for water splitting.11 Asai et al. also reported enhanced photocatalytic activities under visible light on S-doped TiO2.29 The band gap narrowing phenomenon observed on S-doped TiO2 is obvious since the energy level of the S 3p orbital is significantly higher than that of the O 2p state. However, different origins for this band gap narrowing have been proposed. One is attributed to the mixing of S 3p with O 2p states and the other is the generation of a localized S 3p state above the original valence band.32-34 Except for TiO2, little work has been done to explore the S-doping effects on the visible-light-driven photocatalytic activities of other UV-active photocatalysts, partly due to the scarcity of such materials. Our recent experiments showed that In(OH)3 exhibited superior photocatalytic activities in the photodegradation of benzene than P25 under 254 nm UV irradiations. However, In(OH)3 is a wide band gap semiconductor and cannot respond to visible light. Since its wide band gap is related to the deep potential of O 2p orbitals, we anticipate that partial substitution of HO- by S2- can narrow the band gap effectively. Although recently Lei et al. reported that S-doped In(OH)3 showed promising application in the visiblelight-driven photocatalytic water splitting,35 the origin of the band gap narrowing upon the sulfur substitution remains unclear. Besides this, the photocatalytic activities in the degradation of organic pollutants on this S-doped In(OH)3 has never been explored. So in this paper, we studied in detail the preparation, characterizations, and electronic structure of In(OH)ySz solid solutions and the visible-light-driven photocatalytic activities as evaluated by the degradation of gaseous acetone. A possible mechanism for the photocatalysis on In(OH)ySz solid solutions under visible light irradiation was also proposed.
10.1021/jp066671m CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
4728 J. Phys. Chem. C, Vol. 111, No. 12, 2007 2. Experimental Section 2.1. Preparations of Photocatalysts. All of the reagents were analytical grade and used without further purification. In(OH)ySz was prepared by a modified method.35 In a typical synthesis, In(NO3)3‚4.5H2O (0.57 g, 1.5 mmol) and the desired amount of thiourea (S/In ) 0, 0.5, 1.0, 1.5, and 2.0 in the synthesis solution, hereafter simplified as the S/In atomic ratio) were dissolved in 1 mL of ethylenediamine and 14 mL of doubledistilled water. The solution (pH ca. 10) was transferred to a 25-mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 20 h. The final products were collected by filtration and washed several times with distilled water and ethanol. Yellow powders were obtained after drying at 80 °C. 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 scan rate of 0.02° 2θ s-1 in the 2θ range of 10° to 80°. The crystallite size was calculated from X-ray line broadening by the following Scherrer equation: D ) 0.89λ/β cosθ, where D is the average crystal size in nm, λ is the Cu KR wavelength (0.15406 nm), β is the full-width at half-maximum, and θ is the diffraction angle. The peak at 22.38° was used for the calculation of the crystal size. The Brunauer-Emmett-Teller (BET) surface area (SBET) was determined by nitrogen adsorption-desorption isotherm measurements at 77K on a Quantachrome NOVA-4200E system. The samples were degassed in vacuum at 110 °C until the pressure lower than 10-6 Torr was reached before the actual measurements were taken. UVvis diffuse reflectance spectra (DRS) were recorded on a Varian Cary 500 Scan UV-vis-NIR spectrometer with BaSO4 as the background between the range 200 and 800 nm. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by a JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine-mesh copper grid. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system with a monochromatic Al KR source and a charge neutralizer. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Ultraviolet photoelectron spectra (UPS) were carried out in an ultrahigh vacuum (UHV) apparatus of a VG ADES-400 electron energy spectrometer with a base pressure of
