Spontaneous and Photoinduced Conversion of CO 2 on TiO 2 Anatase (001)/(101) Surfaces Martin Ferus, †,‡ Ladislav Kavan, † Marke ́ ta Zukalova ́ , † Arnos ̌ t Zukal, † Mariana Klementova ́ , § and Svatopluk Civis ̌ * ,† † J. Heyrovsky ́ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejs ̌ kova 3, 18223 Prague 8, Czech Republic ‡ Institute of Biophysics, v.v.i., Academy of Sciences of the Czech Republic, Kra ́ lovopolska ́ 135, 612 65 Brno, Czech Republic § Institute of Inorganic Chemistry of the AS CR, v.v.i. CZ-250 68 Husinec-Rez 1001, Czech Republic ABSTRACT: High-resolution FT-IR spectroscopy was used to study the kinetics of oxygen mobility between gaseous isotopically labeled C 18 O 2 and solid phase Ti 16 O 2 nanocrystalline anatase (001). Analysis of the isotopic composition of the gases produced has revealed that anatase (001) calcinated at temperatures under 500 °C is a very weakly reactive material and exhibits a low exchange mobility of oxygen atoms between the gas phase molecules of CO 2 and the TiO 2 lattice. Isotope exchange is blocked by both residual HF adsorbed onto the TiO 2 surface and TiOF 2 impurities. The presence of TiOF 2 was shown by TEM and X-ray diffraction. However, the anatase (001) sample became slightly active after annealing at temperatures higher than 500 °C and upon UV illumination. Nevertheless, the effective rate constant is still 3.45 times lower than that observed for the spontaneous exchange between C 18 O 2 and anatase (101) material calcinated at 500 °C. ■ INTRODUCTION Morphological control and the design of crystal facets are often used as a strategy to optimize the performance of various crystalline semiconductors. 1,2 The fundamental basis of this strategy is that atomic surface configuration and atomic coordination, which essentially determine heterogeneous reactivity, can be excellently tuned by controlling the crystal’s morphology. 3 In general, the catalytic activity of inorganic (nano)crystals is governed not only by their surface composition but also by the physicochemical properties of their exposed surfaces, which are related to their surface atomic arrangement and coordination. 4,5 A typical example is titanium dioxide (TiO 2 ), which has promising energy and environmental applications. 6 For example, (001) facets of anatase TiO 2 are considered to be more reactive than (101). 2,7 The predicted shape of anatase crystals under equilibrium conditions is a tetragonal bipyramid, which might be slightly truncated, exposing a majority of (101) and a minority of (001) facets. 2,8 In contrast to (101) facets with only 50% five-coordinate Ti (Ti5c) atoms, (001) facets with 100% Ti5c atoms were sometimes considered to be more reactive in heterogeneous reactions. 8 Yang et al. 2 pioneered the preparation of anatase nanocrystals with a dramatically increased (001)-to-(101) ratio, which allowed for fundamental follow-up studies. 9-12 The key for controlling the percentage of crystallographic facets of anatase crystals is to change the relative stability of each facet during crystal growth, which is intrinsically determined by the surface energies of the facets. 5 Surface adsorbed fluorine atoms are known to be very effective in changing the surface energy of TiO 2 facets. Previous results have shown that the percentage of (001) facets of anatase crystals can be increased up to approximately 90%, or even close to 100%, 13 depending on the different synthesis routes. 2 Recently, Selloni and Selcuk 14 used density functional theory calculations and first-principles molecular dynamics simulations to investigate the structure and reactivity of TiO 2 (001) facets. According to their results, the (1 × 4) reconstructed surfaces exhibit weak reactivity. Such a result is consistent with our recent experimental observations. 15-19 We have discovered, by investigating a set of anatase crystals with predominantly either (101) or (001) facets, that pure (001) exhibits lower reactivity than (101) in a common reaction with carbon dioxide. ■ EXPERIMENTAL SECTION Synthesis of Materials. Anatase (001) was prepared as follows: 2.4 mL of hydrofluoric acid (48%; Sigma-Aldrich) was added to 20 mL of titanium(IV) butoxide (purum, ≥97.0%, Sigma-Aldrich) with vigorous stirring. The mixture was autoclaved at 200 °C for 24 h. The sample was collected after 24 h of heating and washed with copious amounts of Milli- Q water. The resulting solid was dried at 100 °C for 5 h. The as-received material contained between 6.5 and 20 wt % of F as determined by energy-dispersive X-ray spectroscopy (EDS) analysis. After calcination (500 °C, 1 h) the F content dropped Received: September 8, 2014 Revised: October 24, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp5090668 | J. Phys. Chem. C XXXX, XXX, XXX-XXX