ZrO 2 -Modified Mesoporous Nanocrystalline TiO 2-x N x as Efficient Visible Light Photocatalysts XINCHEN WANG,* , JIMMY C. YU,* ,‡ YILIN CHEN, LING WU, AND XIANZHI FU Research Institute of Photocatalysis, College of Chemistry & Chemical Engineering, Fuzhou University, Fuzhou 350002, China, and Department of Chemistry and Environmental Science Programme, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong Mesoporous nanocrystalline TiO 2-x N x and TiO 2-x N x /ZrO 2 visible-light photocatalysts have been prepared by a sol- gel method. The photocatalysts were characterized by XRD, N 2 adsorption-desorption, TEM, XPS, UV/Vis, and IR spectroscopy. The photocatalytic activity of the samples was evaluated by the decomposition of ethylene in air under visible light (λ > 450 nm) illumination. Results revealed that nitrogen was doped into the lattice of TiO 2 by the thermal treatment of NH 3 -adsorbed TiO 2 hydrous gels, converting the TiO 2 into a visible-light responsive catalyst. The introduction of ZrO 2 into TiO 2-x N x considerably inhibits the undesirable crystal growth during calcination. Consequently, the ZrO 2 - modified TiO 2-x N x displays higher porosity, higher specific surface area, and an improved thermal stability over the corresponding unmodified TiO 2-x N x samples. 1. Introduction Photocatalysts absorb light to initiate chemical reactions for generating hydrogen gas from water splitting, or for de- composing harmful environmental contaminants. Numerous photocatalysts have been proposed, but most of them (e.g., TiO2, ZnO, SnO2) can only be activated by ultraviolet irradi- ation (1). As a result, the abundant visible light in solar spec- trum or artificial light sources cannot be utilized. The develop- ment of visible-light photocatalysts, therefore, has become one of the most important topics in photocatalysis research (2). The conversion of UV-active semiconductors into visible- light photocatalysts by substitutional doping of framework heteroatoms is one of the major approaches to exploit visible- light photocatalysts. A number of visible-light photocatalysts have been developed including V-, Fe-, or Mn-doped TiO2 (3), TiO2-xYx (Y ) N, C, S, or B) (4), N-doped Ta2O5 (5), and Sm2Ti2O5S2 (6). Calcination of the photocatalysts at high temperatures is indispensable for crystallization and for achieving effective doping of cations/anions in the lattices of photocatalysts (particularly in the case of preparing solid- solution visible-light photocatalysts). Unfortunately, such a high temperature treatment often leads to a loss of surface area due to the grain growth. Hence, the photocatalysts retain very low specific surface area after calcination, greatly reducing their light-harvesting capability. Incorporating pores/cavities in bulk materials is a com- mon approach to fabricate porous materials with a large surface-to-volume ratio. Methods for the preparation of porous materials include sol-gel (7), membrane-templated (8), and surfactant-templated syntheses (9). Pore and particle sizes in the order of nanometers can be routinely obtained in these syntheses. Such porous architectures with large surface area and interwoven porous network would improve the photoabsorption and the mass-transfer of materials. For instance, macro/mesoporous TiO2 photocatalysts were proven to be much more active than nonporous TiO2 (10). However, the integrity of the porous architecture is difficult to maintain if a catalyst is subsequently doped with heteroatoms at elevated temperatures. The porous framework often collapses when a catalyst is sintered at high temperatures (10). The use of structural stabilizers to promote the anti- sintering properties of materials has been broadly employed in the production of porous catalysts with sufficient thermo- mechanical strength for high-temperature applications, such as treating automotive exhaust (11). A similar preparation approach has also been adopted in the fabrication of mixed metal oxide photocatalysts, including TiO2/SiO2, TiO2/ZrO2, and TiO2/Al2O3 (12). This study investigated the role of a potential promoter (ZrO2) in enhancing a visible-light photocatalyst (TiO2-xNx) for the oxidation of gaseous organic compounds. The nitrogen-doped photocatalysts are synthesized by reacting amorphous metal oxide xerogels with an ammonia solution, followed by calcination of the products. The calcination temperature could be as low as 400 °C, which is much lower than the >500 °C required for the conventional nitridation of crystalline TiO2 with NH3 gas (13). Moreover, this solution- based nitridation is a lot safer to implement, as it avoids the use of a toxic NH3 gas. 2. Experimental Section Preparation of Catalysts. TiO2/ZrO2 composite xerogels (Zr/Ti molar ratio ) 0.08) were prepared according to the literature (14). The xerogels were treated with an ammonia solution (37%), followed by calcination at 400-600 °C for 4 h. This converted the white xerogels into yellow solids. As a comparison, pure TiO2 and ZrO2 xerogels were also nitridated by using this method. The nitridation product of TiO2 was yellow and was denoted as TiO2-xNx, whereas for the ZrO2 the product was white. Since the amount of ZrO2 was small in the TiO2/ZrO2 hybrid system, the nitridation product of TiO2/ZrO2 was termed as TiO2-xNx/ZrO2 for simplicity. Characterization. X-ray diffraction (XRD) diagrams were collected in θ-θ mode using a Bruker D8 Advance X-ray diffractometer (Cu KR1 irradiation, λ ) 1.5406 Å). Transmis- sion electron microscopy (TEM) and high-resolution trans- mission electron microscopy (HRTEM) were recorded on a JEOL 2010F microscope. Nitrogen adsorption-desorption isotherms were collected at 77 K using Micromeritics ASAP 2010 equipment. UV-vis spectra were recorded on a Varian Cary 500 Scan UV-visible system. FT-IR spectra on pellets of the samples mixed with KBr were recorded on a Nicolet Magna 560 FT-IR spectrometer. X-ray photoelectron spectra (XPS) were recorded 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. Activity Testing. The catalyst (0.28 g) was packed into a 30 × 15 × 2 mm fixed bed plane reactor operated in a single- pass mode. A 450 W high-pressure mercury lamp (λ > 300 nm) with a 450 nm cutoff filter was used as a visible light * Address correspondence to either author. E-mail: xcwang@ fzu.edu.cn (X.W.); jimyu@cuhk.edu.hk (J.C.Y.). Fuzhou University. ‡ The Chinese University of Hong Kong. Environ. Sci. Technol. 2006, 40, 2369-2374 10.1021/es052000a CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2369 Published on Web 02/25/2006