Preparation, Characterization, and Photoactivity of Polycrystalline Nanostructured TiO 2 Catalysts Maurizio Addamo, ² Vincenzo Augugliaro, ² Agatino Di Paola, ² Elisa Garcı ´a-Lo ´ pez, ² Vittorio Loddo, ² Giuseppe Marcı `, ² Raffaele Molinari, ‡,§ Leonardo Palmisano,* ,² and Mario Schiavello ² Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, UniVersita ` di Palermo, Viale delle Scienze, 90128 Palermo, Italy, and Dipartimento di Ingegneria Chimica e dei Materiali, UniVersita ` della Calabria, Via P. Bucci, 87030 Rende (CS), Italy. ReceiVed: December 8, 2003 Various preparations of nanostructured TiO 2 starting from Ti(iso-OC 3 H 7 ) 4 or TiCl 4 are reported. The samples were characterized by X-ray diffractometry, specific surface area and porosity determinations, scanning and transmission electron microscopy, and diffuse reflectance spectroscopy. 4-Nitrophenol photodegradation in aqueous medium was employed as a probe reaction to test the photoactivity of the catalysts. The photoactivity of some samples derived from Ti(iso-OC 3 H 7 ) 4 was found comparable with that of commercial powders. Calcination after the hydrolysis process was necessary to achieve crystallization of the particles before using them as photocatalysts for the reaction studied. The samples deriving from TiCl 4 were the most photoactive among the home-prepared catalysts, and neither filtration nor calcination was needed to obtain a highly photoactive anatase phase. Introduction Heterogeneous photocatalysis has been found effective for the achievement of the photooxidation of many organic pollut- ants present in liquid effluents 1-4 or in air as VOCs. 5,6 This method offers some advantages compared to the traditional ones because it is not selective; the reactions usually occur at room temperature and atmospheric pressure, and UV radiations of low intensity are needed. TiO 2 is the most used photocatalyst, due to its (photo)stability and low cost. 7 The preparation methods are reported to influence significantly morphological, structural, surface physicochemical, and electronic properties that are related to the extent of photoactivity. 8,9 Unfortunately, near-UV photons (band gaps equal to 3.2 and 3.0 eV for anatase and rutile phases, respectively) are needed for the excitation of TiO 2 . This drawback can be overcome by using not stoichiometric or doped TiO 2 because a bathochromic shift in the light absorption is generally observed in those cases. Nevertheless this phenomenon does not give rise always to an enhancement of the photoactivity of the material. 10-13 Recently, many studies have been reported on the preparation and characterization of various nanostructured semiconductors because it has been found that the nanoparticles exhibit special photochemical characteristics. 14-19 In particular, the band gap of the nanoparticles increases with the decrease of the size and other important properties such as optical and physical absorp- tion and luminescence emission undergo drastic changes. TiO 2 nanoparticles have been recently employed in photo- catalysis for photodegradation of noxious species in aqueous medium, 20-24 although the existence of stable nanoparticles is known to be not easy, due to their tendency to agglomerate. Even if in most cases the samples used did not consist of discrete nanoparticles but only of nanostructured particles, this kind of photocatalysts is reported to show a high photoactivity compared with that of many commercial samples. 23-28 On the other hand, the presence of very small particles cannot be excluded when the photoreactivity experiments are carried out in mixed liquid- solid systems. The use of submicrometric particles is possible today by means of membrane photoreactors. 29 In this paper different TiO 2 samples, consisting of nanostruc- tured particles, were prepared by taking into account the up- to-date literature 27-31 but changing the starting precursors, Ti(iso-OC 3 H 7 ) 4 or TiCl 4 , and/or modifying some operative parameters such as molar ratio of the reagents, heating and calcination temperatures, and times. Bulk and surface characterizations of the powders were carried out by means of X-ray diffractometry (XRD), determi- nation of the BET specific surface areas, porosity measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and diffuse reflectance spectroscopy (DRS) techniques. 4-Nitrophenol photodegradation was employed as a probe reaction in order to test the photoactivity of the home- prepared samples compared with that of some commercial samples. Experimental Section Catalysts Preparation. Titanium isopropoxide, titanium tetrachloride, ethanol, and 2-propanol were obtained from Carlo Erba. All chemicals were used as received. The commercial TiO 2 samples were Degussa P25, Merck and Tioxide. The preparation methods for various nanostructured TiO 2 samples were the following: Preparation 1. Titanium hydroxide was prepared by hydroly- sis of titanium isopropoxide carried out at room temperature * Corresponding author. Tel: +39-091-6567246. Fax: +39-091- 6567280. E-mail: palmisan@dicpm.unipa.it. ² Universita ` di Palermo. ‡ Universita ` della Calabria. § Tel: +39-0984-492090. Fax: +39-0984-492058. E-mail: r.molinari@ unical.it. 3303 J. Phys. Chem. B 2004, 108, 3303-3310 10.1021/jp0312924 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/11/2004