PAPER www.rsc.org/dalton | Dalton Transactions Self-assembled TiO 2 nanoparticles: mesoporosity, optical and catalytic properties† Swapan K. Das, Manas K. Bhunia and Asim Bhaumik* Received 7th January 2010, Accepted 10th March 2010 First published as an Advance Article on the web 24th March 2010 DOI: 10.1039/c000317d Herein, we explore the idea of self-assembly of nearly monodisperse nanoparticles as uniform building blocks to design highly crystalline mesoporous TiO 2 nanoparticles, through evaporation-induced self-assembly (EISA) and hydrothermal methods by using non-ionic Pluronic F127 and anionic surfactant SDS, respectively as structure directing agents. The small- and wide-angle powder X-ray diffraction and transmission electron microscopy (TEM) are used to characterize the mesophases. N 2 adsorption–desorption studies and high-resolution TEM results further reveal that mesopores are formed by the arrangement of the nanoparticles of size ca. 4.0–5.0 nm for SDS-templated and 8.0–9.0 nm for F127-templated TiO 2 nanoparticles with broad interparticle pore size distribution. Optical properties of these nanomaterials are studied by UV-visible diffuse-reflectance spectroscopy, photoluminescence (PL) and time-resolved fluorescence (TCSPC). These nanostructured titania exhibited excellent catalytic activity in the photodegradation of ecologically abundant dyes Methylene blue and Rose Bengal under UV-visible light irradiation. Introduction Mesoporous materials have attracted widespread interests in different frontier areas of science owing to their distinctive properties like high surface area, uniform pore size distribution, and well-defined pore topology. 1 Convenient syntheses through different chemical routes, large diversity of framework structures and novel properties have made them an extremely active area of research for over one and half decades, especially in catalysis, 2 ion-exchange, 3 adsorption, 4 biomolecular separation 5 and drug delivery. 6 The unique feature for the synthesis of mesoporous materials is the use of supramolecular assembly of surfactant molecules as structure-directing agents. 7-10 Soft templates such as block copolymers and hard templates such as activated carbon, porous silica etc., have also found a crucial role in directing orga- nized porous structures through self-aggregated superstructures in the solution phase. 7-11 The synthesis of mesoporous silica-based materials has been extended to nonsilicious oxides comprising of semicrystalline domains/clusters within their thick pore walls. 12 However, synthesis of fully crystalline mesoporous semiconductor oxide networks is still a big challenge today. Recently, researchers have focused a great deal of concentra- tion on the synthesis of self-assembled nanoparticles to form mesoporous frameworks of CeO 2 , 13 ZrO 2 , 14-15 SnO 2 , 16 SrTiO 3 , 17 ZnS, 18 WZrO 2 , 19 Ag, Ag 2 S, Ag 2 Se 20 etc. To the best of our knowledge, there are only a few reports of self-assembled titania Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India. E-mail: msab@iacs.res.in; Fax: +91-33-2473-2805 † Electronic supplementary information (ESI) available: Fig. S1–S6 shows HR TEM, band gap, photoluminescence spectra, time resolved fluores- cence, FT-IR spectra and TG-DTA, respectively. Tables S1 and S2 illustrate the results on time resolved fluorescence and Friedel–Craft benzylation reaction, respectively. See DOI: 10.1039/c000317d nanoparticles to form mesoporous TiO 2 with crystalline pore walls. 21-25 Niederberger et al. have synthesized nanocrystalline titania in non-aqueous media at ambient conditions through a sol–gel route. 26 Self-assembly through non-covalent interactions, like H-bonding interaction, 27 p–p interaction, 28 etc. is an effective technique that has proven successful in forming different nanos- tructured materials. Bosc et al. have prepared an anatase hydrosol of titania in strong acidic aqueous medium using titanium(IV) isopropoxide and these nanoparticles are self-assembled in the presence of tri-block copolymer surfactants (Pluronic P123, F127) accomplishing mesoporous nanocrystalline anatase titania thin films. 25 Zhou et al. synthesized very small TiO 2 nanoparticles at room-temperature and these nanoparticles are self-assembled toward mesoporous spherical aggregates in the presence of ionic liquid. 24 However, in most of the cases catalytic application of these nanostructured TiO 2 materials are not explored. Semiconductors having particle sizes ranging 1–10 nm absorbed a greater number of photons on the photocatalyst surface because of their higher surface-to-volume ratio and quantum size effects vis-` a-vis the respective bulk semiconductor oxide. 29 Thus, to design the self- assembled titanium oxide nanoparticles with controllable pore structures and pore walls composed of crystalline nanoparticles using a surfactant templating pathway is challenging as it could open new opportunities in dye-sensitized solar cells, 30 electrolumi- nescent hybrid devices 31 and advanced photocatalysis. 32-36 Environmental pollution caused by industrial effluents is an- other major concern of today. Textile and other related industrial processes release several organic dyes being one of the major water pollutants. 34 There are several reports on photocatalytic degradation of different organic dyes over titanium containing nanostructured materials. 36 Herein, we exploit the concept of self-assembly of nearly monodisperse nanoparticles as uniform building blocks to construct highly crystalline TiO 2 nanoparticles with mesoscopic dimensions through EISA and hydrothermal 4382 | Dalton Trans., 2010, 39, 4382–4390 This journal is © The Royal Society of Chemistry 2010 Downloaded by Indian Association for the Cultivation of Science on 18 March 2011 Published on 24 March 2010 on http://pubs.rsc.org | doi:10.1039/C000317D View Online