Synthesis of Cuboid-Shaped Single-Crystalline TiO 2 Nanocrystals with High-Energy Facets {001} and Its Dye-Sensitized Solar Cell Application Astam K. Patra, Arghya Dutta, and Asim Bhaumik* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India ABSTRACT: A new facile chemical approach has been developed to produce cuboid- shaped TiO 2 nanocrystals with high-energy facets {001} under stream-assisted ionothermal method. The structural features, crystallinity, purity, mesophase, and morphology of the nanostructured TiO 2 were investigated by powder X-ray diffraction, high-resolution transmission electron microscopy, selected-area electron diffraction, Brunauer-Emmett- Teller N 2 sorption measurement, ultraviolet-visible light diffuse reflectance spectroscopy studies, and photoluminescence spectroscopy. N 2 adsorption-desorption studies showed high surface areas (118.5-124.3 m 2 g -1 ) for these TiO 2 samples. These mesoporous structure facilities the high dye-loading property and light harvesting in dye-sensitized solar cells. Short-circuit current density (J sc ) of 12.6 mA cm -2 and an overall power conversion efficiency of 4.21% have been achieved by utilizing these cuboid-shaped TiO 2 photoelectrode in the DSSC with an open-circuit voltage (V oc ) of 0.67 V. ■ INTRODUCTION Dye-sensitized solar cells (DSSCs) have been recognized as promising devices for energy applications because of their advantages like low cost, ease to manufacture, potential for large-scale solar energy conversion, and involvement of low- toxicity materials. 1-6 A simple DSSC is assembled of photoanode and counter electrode with an electrolyte solution. The photoanode is constructed by casting a dye-sensitized semiconductor nanomaterial film on a transparent conducting glass and to absorb and convert photons to electrons, to transport, and then to collect the photoinduced electrons. 7 Titanium dioxide (TiO 2 ) is recognized as one of most promising wide band gap semiconducting materials for dye- sensitized solar cells application since 1991. 8 To date, the highest photoconversion efficiency in the DSSC with power conversion efficiency over 11% has been achieved with 20 nm titanium dioxide nanocrystalline film sensitized by ruthenium- based dyes. 9 In particular, TiO 2 nanostructured materials have attracted much attention because of their extensive uses in photo- catalysis, hydrogen production, dye-sensitized solar cells, self- cleaning agents, Li ion battery, gas sensing, supercapacitors, catalysis, and also commercially used in a white pigment and in sunscreens, paints, ointments, toothpaste, and in many other applications. 10-23 TiO 2 nanocrystals with a particular shape and size are studied intensively. 24 The properties of these nanocrystals strongly depend on their exposed surfaces, which are related to their surface atomic arrangement. 25 It has been reported that anatase TiO 2 with the {001} surface is much more reactive than the thermodynamically more stable {101} surface. 26,27 So there has been recent increasing interest in the preparation of anatase TiO 2 single crystals with exposed {001} facets. In 2008, Lu and coworkers successfully synthesized anatase TiO 2 microcrystals with a large percentage of reactive {001} facets using TiF 4 as titanium source and hydrofluoric (HF) acid as a capping and shape-controlling agent. 28 Since then, several research groups have prepared anatase TiO 2 single crystals with exposed {001} facets from different titanium source and HF used as a shape-controlling agent, 29-33 but HF is an environmentally toxic reagent (both liquid and vapor forms) and extremely corrosive chemical. 34 Then, Liu et al. synthesized anatase TiO 2 sheets with a high percentage of {001} facets from titanium nitride. 35 Yu and coworkers synthesized single-crystalline anatase TiO 2 microsheets with 80% exposed {001} facets by the hydrothermal method by using tetrafluoroborate-based ionic liquid. 36 Ohtani and coworkers developed a more efficient route to prepare {001} facets-dominated anatase TiO 2 via gas-phase hydrolysis of TiCl 4 with rapid heating and quenching. 37 It is still a challenge to synthesize nanometer-sized anatase TiO 2 single crystals with environmentally friendly process and exposed {001} facets. Very recently Pradhan and coworkers synthesized cuboid TiO 2 nanocrystals with high-energy {001} facets by varying the reaction duration using diethanolamine. 38 In this context, we have synthesized few-nanometer-sized cuboid-shaped TiO 2 nanocrystals with {001} facets by stream-assisted ionothermal method. Here ionic liquids control the shape of the materials, and they are environmental friendly solvent because of their unique properties such as extremely low volatility, good thermal stability, high dissolving ability, wide liquid temperature range, Special Issue: Michael Grä tzel Festschrift Received: December 27, 2013 Revised: February 28, 2014 Published: March 7, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 16703 dx.doi.org/10.1021/jp412674g | J. Phys. Chem. C 2014, 118, 16703-16709