Photocatalytic Activity of Sol-Gel-Derived Nanocrystalline Titania K. V. Baiju, S. Shukla,* K. S. Sandhya, J. James, and K. G. K. Warrier Ceramic Technology Department, Materials and Minerals DiVision (MMD), National Institute for Interdisciplinary Science and Technology (NIST) (Formerly Regional Research Laboratory (RRL)), Council of Scientific and Industrial Research (CSIR), Industrial Estate P.O., Pappanamcode, ThiruVananthapuram, Kerala 695019, India ReceiVed: January 18, 2007; In Final Form: March 27, 2007 Nanocrystalline titania (TiO 2 ) powders have been synthesized via sol-gel, using an alkoxide precursor, under different processing conditions, and their photocatalytic activity has been investigated as a function of processing and material parameters through the decomposition of the methylene blue (MB) dye under exposure to the ultraviolet (UV) radiation (λ ) 200-400 nm) in an aqueous solution. The nanocrystalline TiO 2 powders with different morphology, crystallinity, average nanocrystallite size, surface area, and phase structure are obtained by controlling the ratio of molar concentrations of water and alkoxide (R) within the range of 5-60 and calcining the as-synthesized amorphous powders at higher temperatures (400-800 °C). The nanocrystalline TiO 2 powders have been characterized using the scanning electron microscope (SEM), X-ray diffraction (XRD), and the Brunauer, Emmett, and Teller (BET) surface area measurement techniques while their photocatalytic activity was monitored using a UV-visible spectrometer. The photocatalytic activity of sol-gel-derived nanocrystalline TiO 2 is observed to be a function of R and calcination temperature. The maximum photocatalytic activity is observed for the largest R value and the intermediate calcination temperature as an optimum effect produced by the variation in the morphology, the average nanocrystallite size, the surface area, the phase structure, and the crystallinity of the powders. The dependence of photocatalytic activity on the average nanocrystallite size reveals the existence of a critical size (∼15 nm), below and above which the photocatalytic activity is observed to be reduced. The observed photocatalytic characteristics of sol-gel-derived nanocrystalline TiO 2 have been explained based on the existing mechanism associated with the photocatalytic decomposition of organic molecules using semiconductor oxides. Introduction Titania (TiO 2 ) is a well-known, wide-band-gap, n-type semiconductor oxide used as a photocatalyst for the removal of highly toxic and non-biodegradable pollutants normally present in air and wastewater via photocatalysis, which is a low temperature, non-energy intensive process for the chemical waste remediation, 1 involving the migration of a photon-induced electron (e - ) and hole (h + ) to the particle surface, which serve as redox sites for the destruction of the surface-adsorbed pollutants. Being more photocatalytically active, chemically stable, environmentally friendly, and cheaper, TiO 2 has been the most promising one for the photocatalysis compared to other semiconductors such as tin oxide (SnO 2 ), 2 zinc oxide (ZnO), 3 ceria (CeO 2 ), 4-6 and cadmium sulfide (CdS). 7 Nanocrystalline TiO 2 has been used in the thin film, 8-10 thick film, 11 and powder 12-24 forms for the measurement of the photocatalytic activity by using the various synthesis approaches such as chemical vapor deposition (CVD), 8 sputtering, 9,10 plasma spraying, 11 coprecipitation, 12 microemulsion, 13 hydrothermal, 14 and sol-gel. 15-24 Among the various synthesis approaches, the sol-gel technique has received more attention due to the ease of controlling various material parameters such as the powder morphology, the surface area, the average nanocrystallite size, the crystallinity, and the phase structure, which significantly affect the photocatalytic activity of nanocrystalline TiO 2 . It was demonstrated earlier that 25,26 the nanocrystalline ceramic oxides such as zirconia (ZrO 2 ) could be synthesized, with different average nanocrystallite size, morphology, and phase structure, by varying the key sol-gel processing param- eters such as the ratio of number of moles of water and the alkoxide precursor (R) and the calcination temperature. Hence, in the present investigation, we use the sol-gel process, which utilizes an alkoxide precursor, to prepare the nanocrystalline TiO 2 powders with controlled morphology, surface area, average nanocrystallite size, crystallinity, and phase structure by varying the R and the calcination temperature. We also study their effect on the photocatalytic activity of the sol-gel-derived nanocrys- talline TiO 2 . Such systematic study demonstrating the effect of both R and the calcination temperature on the photocatalytic activity of the sol-gel-derived nanocrystalline TiO 2 is presently not available in the open literature. Experimental Section Chemicals. Titanium(IV) isopropoxide (Ti[OC 3 H 7 ] 4 ) and anhydrous 2-propanol were purchased from Alfa Aesar, U.S.A. and methylene blue (MB) (AR Grade) was purchased from Qualigens Fine Chemicals, India. All of the chemicals were used as received without any further purification. Sol-Gel Processing. The nanocrystalline TiO 2 powders were synthesized via sol-gel process using the hydrolysis and the condensation of titanium(IV) propoxide in an anhydrous alcohol medium. For this purpose, a measured quantity of water was first dissolved in 125 mL of 2-propanol. A second solution * To whom correspondence should be addressed. Phone: +91-471- 2515282. Fax: +91-471-2491712. E-mail: satyajit_shukla@csrrltrd.ren.nic.in. 7612 J. Phys. Chem. C 2007, 111, 7612-7622 10.1021/jp070452z CCC: $37.00 © 2007 American Chemical Society Published on Web 05/05/2007