Tailored Anatase/Brookite Nanocrystalline TiO 2 . The Optimal Particle Features for Liquid- and Gas-Phase Photocatalytic Reactions Silvia Ardizzone,* ,²,‡ Claudia L. Bianchi, ²,‡ Giuseppe Cappelletti, ²,‡ Stefano Gialanella, § Carlo Pirola, ² and Vittorio Ragaini ² Department of Physical Chemistry and Electrochemistry, UniVersity of Milan, Via Golgi 19 - 20133 Milano, Italy, Consorzio INSTM, Via Giusti 9 - 50121 Firenze, Italy, and Department of Materials Engineering and Industrial Technologies, UniVersity of Trento, 38050 Mesiano (TN), Italy ReceiVed: May 28, 2007; In Final Form: June 29, 2007 Anatase-brookite composite nanocrystals were synthesized successfully by a controlled sol-gel reaction followed by a prolonged hydrothermal aging or by mild calcinations (300 and 450 °C). The physicochemical and photocatalytic properties of the synthesized TiO 2 composites were studied along with several commercially available nanocrystalline TiO 2 samples showing different features. Rietveld refinements of the powder X-ray diffraction pattern were used to track the brookite content systematically and to generally assess the phase composition of the different samples and their crystallite sizes. SEM, TEM, and HRTEM were used to characterize the particle morphology, size, and surface faceting. BET/BJH analyses combined with mercury porosimetry determinations were employed to characterize the surface area, porosity, and pore size distribution. The surface state of the TiO 2 samples was analyzed by XPS by studying, in detail, the region of oxygen 1s to produce the OH/O tot surface ratio. The photocatalytic activity of all of the samples was tested both for degradation of NO x in the gas phase and for the oxidation of 2-chlorophenol in the liquid phase. The different samples showed the same sequence of activity for the two reactions. The highest degradation and mineralization efficiencies were achieved in the case of samples showing smaller crystallite sizes and larger surface areas. The photocatalytic activity of the anatase-brookite composite, submitted to the hydrothermal treatment, was found to be the highest for both reactions, even greater than that of a single-phase anatase sample showing a much-larger surface area. The different contributions to the photocatalytic performance of the TiO 2 nanocrystals are critically discussed. Introduction A great deal of effort has been devoted, in recent years, to developing oxide semiconductor photocatalysts with high activi- ties for environmental protection and remediation procedures such as air and water purification, water disinfection, and hazardous waste remediation. 1-4 TiO 2 is considered to be a very- promising photocatalyst because, for several reactions, it exhibits higher activity compared with that of other semiconductors and, at the same time, it shows excellent chemical stability and nontoxicity. 4-14 The photocatalytic activity of titania is strongly affected by the particles’ physicochemical features, with respect to both structural and morphological characteristics. 6 Structurally, TiO 2 can crystallize in three polymorphic forms: anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic). The anatase polymorph is generally reported to show the highest photo- activity compared to the brookite or rutile polymorphs because of the low recombination rate of its photogenerated electrons and holes. 9-10 Concerning this latter aspect, some authors 4-6 report that the composite oxide made by two titania phases can show enhanced photocatalytic activity just because of the suppressed recombination of photogenerated electrons and holes. The use and performance of mixed TiO 2 polymorphs in photocatalytic applications are reported to be, in their turn, strongly influenced by the final oxide microstructure. 6 There appears to be no general agreement on the effect of the particle size on the photocatalytic activity of TiO 2 . Several authors report a peak efficiency, for the given reaction, in correspondence of an optimal particle size. A few examples can be mentioned. Maira et al. 12 in the photocatalytic degradation of trichloroethylene in the gas phase with particles in the 2.3- 27 nm range, found an optimum particle size of 7 nm. Also, Zhang et al. 14 in the oxidation of trichloromethene reported the best efficiency for an anatase size of 11 nm. Almquist and Biswas, 13 in the photodegradation of phenol, instead report a much larger optimal particle size in the 25-40 nm range. Furthermore, other authors report a continuous increase in the photocatalytic activity with lowering of the particle size. For example, Anpo et al. 11 in the hydrogenation of CH 3 COH report an increase in conversion when the particle size of anatase TiO 2 decreased from 11 to 3.8 nm. Relevant to this debate are the recent results by Lin et al.: 5 the band gap of anatase TiO 2 was observed to decrease monotonically from 3.239 to 3.173 eV when the particle size decreased from 29 to 17 nm and then to increase from 3.173 to 3.289 eV as the particle size decreased from 17 to 3.8 nm, in agreement with the red and blue shifts of the band gap reported by other researchers. 11,15 Alternatively, their results of photocatalytic oxidation of 2-chlorophenol showed that the smaller the particle size, the faster the * Corresponding author. Tel: +39/0250314253; fax +39/0250314300; e-mail: silvia.ardizzone@unimi.it. ² University of Milan. ‡ Consorzio INSTM. § University of Trento. 13222 J. Phys. Chem. C 2007, 111, 13222-13231 10.1021/jp0741096 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/11/2007