Anatase TiO2 crystals with exposed high-index facets.

Put your best face forward: The performance of TiO2 anatase crystals in energy and environmental applications is normally correlated with the TiO2 crystal facets exposed, and increasing the percentage of highly reactive surfaces is extremely important. A new gas-phase oxidation process using TiCl4 as precursor now yields anatase TiO2 single crystals with primarily high-index {105} facets, which can cleave water photocatalytically. Inorganic functional materials with tailor-made crystal facets have attracted great research interest owing to their applications in catalysis, sensors, batteries, and environmental remediation.1–6 Unfortunately, the surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. Thus, increasing the percentage of known highly reactive surfaces or creating new favorable surfaces is highly desirable. Crystalline titanium dioxide (TiO2) in the anatase phase is one of the most important semiconducting metal oxides, owing to its many promising energy and environmental applications.7–9 Conventionally, anatase TiO2 crystals are dominated by the thermodynamically stable {101} facets (ca. 94 percent, according to the Wulff construction) and a minority of {001} facets.10 Recently, we developed a new strategy to synthesize anatase TiO2 crystals with a large percentage of highly reactive {001} facets using fluorine-containing compounds, such as hydrofluoric acid, as capping agents, which made {001} energetically preferable to {101}.4 Gas-phase reactions with rapid heating and quenching were also reported recently to generate {001}-faceted decahedral anatase TiO2 crystals.11 Most recently, photocatalytically active {100} facets of anatase TiO2 crystals were synthesized using solid sodium titanates as the titanium source under hydrothermal conditions.12 However, all these breakthroughs contribute to the increase of the percentage of known low-index {001} or {100} facets only, which are the basic crystal surfaces in the Wulff construction model of anatase in a thermodynamically stable state and have been evidenced theoretically and experimentally.13 Because they usually have unique surface atomic structures, such as a high density of atomic steps, dangling bonds, kinks, and ledges, that can act as active sites, high-index planes of anatase may have the capability to be used in clean-energy and environmental applications. Unfortunately, owing to the high surface energies, which can lead to the elimination of high-index crystal planes, it is still an open challenge to synthesize tailor-made anatase TiO2 crystals bounded by high-index facets. Herein we report a facile process to prepare well-defined anatase TiO2 crystals with predominantly exposed high-index {105} facets, which have never been realized experimentally before. The anatase TiO2 crystals with exposed high-index {105} facets were prepared by a modified hightemperature gas-phase oxidation route using titanium tetrachloride (TiCl4) as the Ti source.11 A schematic reaction apparatus is given in Figure S1 in the Supporting Information. A straight static furnace pipe and a thin spiral tube were used as reactor and reactant feeder, respectively. In a typical experiment, the vapor-phase TiCl4 was liberated by bubbling oxygen (0.2 L min−1) into TiCl4 liquid at 98 °C and then passed through the furnace pipe at a temperature of 1000 °C. The experimental process was shown to be quite robust, and the reproducible synthesis of the anatase TiO2 crystals with exposed high-index {105} facets was also confirmed. Moreover, key synthesis conditions such as concentration of titanium precursor, reaction temperature, and oxygen flow were also explored extensively. In all experiments, the final white products were collected downstream by a bag filter and washed with deionized water three times to remove the adsorbed chlorine-containing species on the surface. Gram-scale production can be easily achieved if a furnace pipe with a diameter of about 5 cm is used (Figure S2 in the Supporting Information for digital camera images of the final white powder). Figure 1 shows the X-ray diffraction (XRD) pattern of the as-synthesized TiO2 crystals with exposed high-index {105} facets. All the main diffraction peaks can be indexed to the anatase crystal phase (space group I41/amd, JCPDS No. 21-1272), and only a very small amount of rutile impurity can be detected. Moreover, the peak indexed to {105} facets exhibits a higher intensity than in the calculated diffraction pattern of bulk anatase, which indicates that more {105} facets have been exposed (the corresponding peak has been marked with an asterisk (*) in Figure 1). Scanning electron microscopy (SEM) images in Figure 2 a–c show that the synthesized anatase TiO2 crystals display bipyramidal morphology with an average length of 2.42 μm (Figure S3 for the size distribution of these TiO2 crystals). The 3D schematic shape of a typical anatase TiO2 bipyramidal crystal with only high-index {105} facets exposed is shown in Figure 2 d. Statistically, the average interfacial angle indicated in Figure 2 d is 26.67°, which is close to that of {105} and {001} facets. The surfaces of all the crystals are very smooth, and some minority {101} facets can also be found occasionally, as indicated in Figure 2 b and Figure S4 in the Supporting Information. According to the symmetries of anatase TiO2, it can be concluded that the eight triangular surfaces in the bipyramidal crystals must be the high-index (105) facets. A transmission electron microscopy (TEM) image of a free-standing anatase TiO2 bipyramidal crystal and its corresponding selectedarea electron diffraction (SAED) pattern (Figure 3 a, b) demonstrate the single-crystal characteristics. The high-magnification TEM image in Figure 3 c clearly shows the (200) and (020) atomic planes with a lattice spacing of 0.189 nm. It should be noted that both the SAED pattern and the high-magnification TEM image were indexed along the [001] crystallographic direction of