Synthesis of recrystallized anatase TiO 2 mesocrystals with Wulff shape assisted by oriented attachment† Rafael O. Da Silva, a Ricardo H. Gonc ¸alves, a Daniel G. Stroppa, bc Antonio J. Ramirez b and Edson R. Leite * a Received 22nd December 2010, Accepted 16th February 2011 DOI: 10.1039/c0nr01016b In this work, we describe a kinetically controlled crystallization process assisted by an oriented attachment (OA) mechanism based on a nonaqueous sol–gel synthetic method (specifically, the reaction of titanium(IV) chloride (TiCl 4 ) with n-octanol) to prepare re-crystallized anatase TiO 2 mesocrystals (single crystal). The kinetics study revealed a multi-step and hierarchical process controlled by OA, and a high resolution transmission electron microscopy (HRTEM) analysis clearly shows that the synthesized mesocrystal presents a truncated bipyramidal Wulff shape, indicating that its surface is dominated by {101} facets. This shape is developed during the recrystallization step. The material developed here displayed superior photocatalytic activity under visible light irradiation compared to TiO 2 –P25 as a benchmarking. Introduction The development of new synthetic routes aiming to obtain nanocrystals and mesocrystals with controlled shapes and reac- tive surfaces is of great scientific and technological interest, especially for chemical and photochemical heterogeneous cata- lytic processes. 1–5 For instance, recently Shen and co-authors have demonstrated that the morphological control of Co 3 O 4 was fundamental to improve the activity and stability of this oxide for CO oxidation. 4 Following the same trend, the development of other nanostructured metal oxide with controlled morphology (or facets exposed on the surface) must result in materials with superior activity and stability for chemical and photochemical reactions. 2 The titanium(IV) oxide (TiO 2 ) in the anatase phase is a key functional material with interesting sensing, photocatalytic and photovoltaic 6 surface dependent properties. The TiO 2 anatase crystal is usually dominated by {101} facets, which present the lowest surface energy. 7 Considering other facets, theoretical studies have demonstrated the following sequence for surface relative energies: {101} < {100} < {001}. An inversion in the sequence position between the {101} and {100} facets can occur; e.g., in oxygenated surfaces, {100} facets are mostly stable whereas under clear and hydrogenated conditions, {101} facets are more stable. However, the {001} has the highest energy facets. The surface relative energy variations can basically be explained by the different chemical compositions of the facets, resulting in diverse degrees of broken chemical bonds on the surface. Recently, a breakthrough in the synthesis of anatase TiO 2 crystals with {001} facets was achieved by Lu and co-authors. 8 They reported the preparation of anatase microcrystals with surfaces formed preferentially by {001} facets. On the basis of first-principle quantum chemistry calculations, the strategy used by them was the reversal of the relative stability of the facets through the use of fluoride ions during the synthesis. The pres- ence of fluoride ions favors the formation of high F–Ti bonding energy at the surface, leading to a decrease in the (001) surface energy, which results in more stability than the (101) surface. Then Zheng and co-workers used a similar approach to synthe- size anatase TiO 2 nanosheets with exposed {001} facets and excellent photocatalytic performance. 9 The crystallization mechanism of the anatase with {001} exposed facets synthesized in hydrofluoric acid solution (under hydrothermal conditions) should be related to monomer-by-monomer assembly; i.e., the attachment of ions/molecules to a primary nucleus. The fluorine ions must act as selective surface poisoning agents following a classical and thus predictable crystallization process. Ohtani and co-workers 10 also recently reported the formation of anatase TiO 2 with exposed {001} facets and high photocatalytic activity; however, they used a gas phase reaction process and TiCl 4 as a titanium precursor. The authors did not discuss the surface energy stabilization, but the presence of chloride can promote a Chemistry Department, Federal University of Sa˜o Carlos, Sa˜o Carlos, SP, 13656-905, Brazil. E-mail: derl@power.ufscar.br; Fax: +55 16 33615215; Tel: +55 16 33519567 b Brazilian Synchrotron Light Laboratory, Campinas, SP, 13083-970, Brazil. E-mail: antonio.ramirez@lnls.br; Fax: +55 19 35121004; Tel: +55 19 35183108 c Materials Engineering Department, Mechanical Engineering School, University of Campinas, Campinas, SP, 13083-860, Brazil † Electronic supplementary information (ESI) available: HRTEM images of the material synthesized with a magnetic stirrer (Fig. S1), XRD analysis of the material synthesized with a magnetic stirrer (Fig. S2) and EDS-TEM analysis showing the low chloride concentration of the material synthesized at 100  C (40 h of treatment time) without a magnetic stirrer. See DOI: 10.1039/conr01016b 1910 | Nanoscale, 2011, 3, 1910–1916 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2011, 3, 1910 www.rsc.org/nanoscale PAPER Published on 18 March 2011. Downloaded by Leibniz-Institut f&#252;r Katalyse e. V. (LIKAT Rostock) on 21/05/2015 19:18:37. View Article Online / Journal Homepage / Table of Contents for this issue