Preparation of bimodal micro–mesoporous TiO 2 with tailored crystalline properties David P. Serrano,* Guillermo Calleja, Raúl Sanz and Patricia Pizarro Department of Chemical, Environmental and Materials Technology, Rey Juan Carlos University, Móstoles (Madrid), Spain. E-mail: d.serrano@escet.urjc.es; Fax: +34 91 664 7490; Tel: +34 91 664 7450 Received (in Cambridge, UK) 28th November 2003, Accepted 23rd February 2004 First published as an Advance Article on the web 17th March 2004 A new mild crystallization procedure has been applied after a synthesis route in the presence of a non-ionic surfactant, leading to the preparation of bimodal micro–mesoporous TiO 2 , with remarkable textural properties and pore walls formed by anatase nanocrystals, which exhibit photocatalytic activity. Sol–gel synthesis assisted by non-ionic surfactants has been demonstrated to provide mesoporous TiO 2 with high surface area by a N 0 I 0 assembly mechanism. 1,2 Nevertheless, the amorphous nature of the pore walls makes these materials non-feasible for many applications, such as electronic devices and photocatalysis, where crystalline titania is required. 3–5 Traditionally, crystallinity of TiO 2 has been developed by severe treatments, such as calcination and hydrothermal processes, which frequently result in the partial or total collapse of the porous structure. 2,6,7 Thus, Yue and Gao 8 recently prepared crystalline mesoporous TiO 2 by a neutral templating route where crystallinity was generated by a hydrothermal post-synthesis treatment. The material so obtained was chemically modified with cerium and presented a surface area at best of 205 m 2 g 21 . Here we report a new and more controllable method leading to bimodal micro–mesoporous TiO 2 photocatalysts (bm-TiO 2 ) with high surface area, mesoscopic ordering and crystalline walls. The presence of a bimodal pore distribution can be considered advantageous as the micropores provides the material with a high surface area and may lead to a strong interaction and adsorption of the reactant molecules, while the mesopores are responsible for a fast intraparticle mass transport. The synthesis method is based on a two-step procedure: i) synthesis of the amorphous porous TiO 2 , ii) controlled crystalliza- tion of the pore walls by acid treatment. The first step takes place by a neutral templating route 6,7 using a titanium alkoxide and a non- ionic surfactant as starting materials. This procedure provides mesostructured materials with highly porous properties but with amorphous titania walls. The crystallization of the pore walls is carried out in a second step, through a mild acid treatment, which allows for a controlled phase transition into the anatase form while the high textural properties previously produced are greatly preserved. In a typical synthesis, Pluronic P-123 (PEO 20 PPO 70 PEO 20 ) was dissolved at ambient temperature in 2-propanol containing HCl diluted in water. The mixture was slowly added to a solution of titanium isopropoxide in 2-propanol at 40 °C under continuous stirring, a gel being formed after around 4 h. The molar gel composition was 1.0 Ti(Pr i O) 4 : 34 C 3 H 7 O : 0.04 HCl : 3 H 2 O : 0.02 P-123. The gel was aged at 40 °C for 16 h, and then vacuum dried and washed with ethanol, at ambient temperature. The solid so obtained was crystallized by treatment with refluxing acid–ethanol mixtures at different compositions (0–5% w/w) during 24 h and, afterwards, it was centrifuged and dried in an opened Petri dish at ambient conditions. The crystallization step was studied using four inorganic acids in the refluxing treatment: HCl, HNO 3 , H 2 SO 4 and H 3 PO 4 . In all cases, the surfactant was efficiently removed from the solid. For 1% acid compositions, the main TiO 2 properties deduced from XRD and nitrogen adsorption–desorption analyses are registered in Table 1. Only HCl and HNO 3 were capable of inducing the phase transformation of the titania walls into pure anatase phase, which is the most interesting one in applications such as photocatalysis, due to its high photoactivity. The occurrence of strong interactions between titania and sulfuric or phosphoric acid may account for the crystallization inhibition and the low values of surface area and pore volume. Porosity of the samples treated with both HCl and HNO 3 are quite similar with high surface areas. By variation of the acid concentration in the crystallization step, bm-TiO 2 materials with surface area up to 260 m 2 g 21 have been obtained. As an example, Fig. 1 illustrates the nitrogen adsorption–desorption analysis of a bm-TiO 2 sample crystallized using 1% HCl. The isotherm is apparently of type IV, typical of mesoporous materials, although with a remarkable adsorption at low relative pressures, suggesting the presence also of micropores. This result is confirmed by the pore size distribution, which exhibits a clear bimodal porosity. The inset of Fig. 1 illustrates the pore size distribution derived for one of the HCl-extracted samples by applying the DFT-Plus software using the Harkins and Jura thickness model. In this figure two families of pores, with sizes centered at 1.4 and 2.9 nm, respectively, are discernible. The DFT-Plus method provides also the relative contribution of both types of porosity to the overall surface area, with values of 146 and 100 m 2 g 21 for the micropore and mesopore surface area, respectively. The presence of micropores has been also detected in silica- based mesostructured materials, such as SBA-15, prepared using block copolymers as surfactants. 9 A similar reason may be also considered for explaining the bm-TiO 2 microporosity, although the Table 1 Properties of bm-TiO 2 after different acid–ethanol treatments Acid S BET / m 2 g 21 V pore / cm 3 g 2 1 Phase D crystal / nm HCl 246 0.136 Anatase 5.2 HNO 3 213 0.108 Anatase 6.2 H 3 PO 4 65 0.032 Amorphous — H 2 SO 4 89 0.070 Amorphous — Fig. 1 N 2 adsorption–desorption isotherm and pore size distribution of bm- TiO 2 after treatment with 1% HCl/EtOH. This journal is © The Royal Society of Chemistry 2004 DOI: 10.1039/b315493a 1000 Chem. Commun., 2004, 1000–1001