The development of semiconductor crystals with highly reactive facets has been one of the most effective strategies to improve their photocatalytic activity ^{[1, 2, 3, 4, 5, 6, 7]}. Since the pioneering work of synthesizing anatase TiO₂ single crystals with a high percentage of reactive {001} facets by Yang et al ^[8]. in 2008, an unprecedented progress in the controllable synthesis of the faceted TiO₂ crystals has been achieved. So far, various modified strategies have been developed to prepare anatase TiO₂ crystals with different exposed facets, and the facet dependent photocatalytic properties have been well investigated ^{[9, 10, 11, 12, 13]}. Indeed, the selective exposure of specific reactive facets has been demonstrated to significantly improve the activity or change the reaction preference of photocatalysts ^{[14, 15]}. However, the single crystals with well-defined facets generally have a large particle size of several micrometers, which would lead to the prolonged bulk diffusion length of carriers, increased bulk recombination, and decreased surface area. In this sense, the development of porous single crystals with reactive facets exposed is expected to be an ideal solution to address these problems. Crossland et al. ^[16] developed a facile seeded template method to prepare mesoporous single crystal (MSC) anatase TiO₂ with well-developed facets, which delivered enhanced electron mobility and optoelectronic device performance relative to the conventionally used nanocrystalline TiO₂ film. This breakthrough opened up new opportunities of optimizing the performance of metal oxide semiconductors in various applications ^{[17, 18, 19]}. In comparison with anatase TiO₂, rutile TiO₂ possesses additional advantages in photocatalysis applications such as overall water splitting ^[20]. However, there have only been several reports on the synthesis of porous rutile TiO₂ singe crystals ^{[18, 21, 22]}. In particular, to the best of our knowledge, the controllable synthesis of mesoporous rutile TiO₂ with preferential {111} facets remains a challenge owing to the high surface energy of the {111} facets.

In this study, we synthesized mesoporous rutile TiO₂ single crystals with different ratios of {111}/{110} facets, and investigated the facet dependent activity in photocatalytic hydrogen evolution. The mesoporous single crystals were prepared through the seeded-template method by the hydrothermal treatment of TiCl₄ in the HCl solution containing NaF at 220 °C for 12 h. The corresponding control experiment without using a template was also performed to synthesize the solid single crystal (SSC), to enable the comparative study of both samples. NaF has been demonstrated to be an effective facet controlling agent in stabilizing the high energy facet {111} of rutile TiO₂ ^[23]. A series of mesoporous and solid rutile TiO₂ single crystals, with different exposed ratios of {110} and {111} facets, could be obtained by tuning the concentration of NaF. According to the different amount of NaF (20, 40, and 80 mg) added to Ti-containing precursor solution, these as-prepared samples were denoted as MSC-20, MSC-40, MSC-80 for mesoporous single crystals and SSC-20, SSC-40, SSC-80 for solid single crystals.

Seeded silica template was prepared based on the method given in the previous literature ^[16]. 50 nm silica spheres were synthesized by adding 32 mL H₂O, 18 mL ammonium hydroxide (30%) and 99 mL tetraethyl orthosilicate (TEOS, Sigma Aldrich) into 750 mL ethanol and stirred at 700 r/min for 24 h. The reaction solution was centrifuged at 5000 rpm for 5 h to form a quasi-close-packed bead template in a translucent solid. The unwashed solid was collected and sintered at 500 °C for 30 min. To seed the silica template, 25 g of the sintered template was immersed in 150 mL 15 mmol/L TiCl₄ solution, obtained by diluting a 2 mol/L TiCl₄ aqueous stock solution (the stock solution was prepared by diluting 100 mL TiCl₄ in 350 mL H₂O containing 1 mL 35 % HCl in an ice/water bath), and held at 70 °C for 1 h followed by thoroughly rinsing with deionized water. The dried template was resintered at 500 °C for 30 min.

Mesoporous rutile TiO₂ single crystal was prepared through a facile hydrothermal method ^{[18, 23]}. The Ti-containing precursor solution was prepared by dropping 1.64 mL of 15 mmol/L TiCl₄ into 291 mL HCl solution under strong stirring in ice/water bath and further adding deionized water to 1 L. To prepare mesoporous rutile TiO₂ single crystal with different exposed {111} facets, 2 g of the seeded silica template was added to 40 mL of precursor solution containing 20, 40, and 80 mg NaF, respectively. The suspension was transferred to a Teflon-lined autoclave and heated at 220 °C for 12 h. The template product was collected by centrifugation and fully washed with deionized water. To selectively etch the silica template, the sample was heated at 80 °C in a 2 mol/L NaOH solution for 1 h and then washed with deionized water and ethanol. In corresponding control experiment, solid rutile TiO₂ single crystal with different exposed {111} facets was prepared without template. The samples were heated at 600 °C for 2 h to remove surface-terminated F/Cl impurities.

X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku diffractometer using Cu K_α irradiation. Sample morphology and microstructure were characterized by scanning electron microscopy (SEM, Nova NanoSEM 430) and transmission electron microscopy (TEM, JEOL 2010). Brunauer-Emmett-Teller (BET) specific surface area was determined by nitrogen adsorption-desorption isotherm measurements at -196 °C (ASAP 2010). The optical absorbance spectra of the samples were recorded in a UV-visible spectrophotometer (JACSCO-550).

Photocatalytic hydrogen evolution reactions were carried out in a top-irradiation vessel connected to a glass-enclosed gas circulation system. 100 mg of the single crystal TiO₂ powder was dispersed in 300 mL aqueous solution containing 10 vol% methanol. The deposition of 1 wt% Pt cocatalyst was conducted by directly dissolving H₂PtCl₆ in the above 300 mL reaction solution. The reaction temperature was maintained about 10 °C. Under irradiation of the 300 W Xe lamp, the amount of H₂ evolved was determined using gas chromatography (Agilent 6890).

The amount of NaF dependent morphology evolution is shown in Fig. 1. It was clearly shown from the scanning electron microscopy (SEM) images that the porous samples exhibited a very similar morphology to that of the clearly faceted solid rutile TiO₂ single crystals, indicating the successful synthesis of porous single crystals with well-developed facets. According to the crystallographic symmetries of rutile TiO₂, the four lateral rectangular facets in the middle part of the crystals are recognized as {110} facets and the eight triangular facets at the two ends of the crystals are {111} facets ^[12]. It was shown that the thermodynamically most stable rutile {110} facets disappeared gradually with increasing the amount of NaF, and eventually the rutile single crystals with wholly {111} facets were obtained when the amount of NaF was increased up to 80 mg. In contrast to the smooth facets in solid single crystals, many pores with a diameter of approximately 50 nm were observed after removing the silica beads, as expected. The type IV N₂ sorption isotherm curves and pore size distribution centered at about 50 nm confirmed the mesoporous structure (Fig. 2). It is worth noting that, although the morphology of the mesoporous single crystals with different exposed facet ratios greatly changes, the specific surface area of these mesoporous samples are similar (24, 25, 28 m²/g for MSC-20, MSC-40, MSC-80, respectively). In addition, mesoporous single crystals possess an average particle size of 300-500 nm, an order of magnitude smaller than that of solid single crystals (~3-5 µm). Apparently, this particle size difference could be attributed to the seeded nucleation and confinement of growth inside the mesoporous template. Note that several special single crystal particles, composed of a partial mesoporous and solid single crystal, were observed in all three mesoporous samples. This indicates the growth of single crystals from a seed located on or close to an external surface of the mesoporous template, which occurred inevitably in the synthesis of mesoporous single crystals using the seeded template method ^[16].

Fig. 1. Low and high magnification SEM images for MSC-20 (a, d), MSC-40 (b, e) andMSC-80 (c, f) and high magnification SEM images for SSC-20 (g), SSC-40 (h) and SSC-80 (i).

The microstructure of the mesoporous single crystals was further investigated by transmission electron microscopy (TEM). Fig. 3 presents the distinct shapes of one crystal particle in the TEM images which clearly show the morphology transformation from the high aspect ratio of {110}/{111} to the wholly {111} facets. This is similar to the observed changes of solid single crystals (Fig. 1). Also, it reveals the uniformly distributed mesopores with a diameter of about 50 nm, consistent with the value of the pore distribution (Fig. 2), indicating the formation of a connected channel network throughout the whole framework. The corresponding selected area electron diffraction (SAED) patterns, in the insets of Fig. 3 recorded for the whole particle, show a set of sharp spots that are indexed to rutile TiO₂, indicating the single crystal nature of the particle. The clear lattice fringes in the high resolution TEM images consistently suggest the high crystallinity of the mesoporous crystals.

Fig. 2. Nitrogen adsorption/desorption isotherms of mesoporous single crystal (MSC): (left) MSC-20, (middle) MSC-40 and (right) MSC-80, the pore size distributions of MSC are shown in the insets.

Fig. 3. TEM images of samples MSC-20(a), MSC-40(c) and MSC-80(e). High resolution TEM images of MSC-20(b), MSC-40(d) and MSC-80(f). The corresponding selected area electron diffraction (SAED) patterns are shown in the insets.

The phase identity and purity of the solid and mesoporous single crystal TiO₂ were investigated by X-ray diffraction (XRD). For solid single crystals, all the diffraction peaks of these three samples can be assigned to rutile TiO₂ and are very sharp, consistent with the solid particles comprising a single rutile crystal domain. The diffraction peaks of MSC-20 are identical to that of solid single crystal when 20 mg of NaF was added, as shown in Fig. 4. With increasing the amount of NaF to 40 and 80 mg, besides the dominant diffraction peaks of rutile TiO₂, a very weak peak at 25.3° originating from the {101} planes of anatase TiO₂ was detected. The presence of trace anatase phase could originate from the synergistic effect of the promoted nucleation of the anatase phase and suppressed phase transformation from anatase to rutile by the high concentration of F^- ions ^[12]. Importantly, the diffraction peaks of all mesoporous rutile crystals are as sharp as those of the solid single crystals, in agreement with the porous particles being single crystal domains. This is also consistent with the TEM results. Overall, all these results suggest the synthesis of mesoporous rutile TiO₂ single crystals with tunable ratios of exposed facets.

Fig. 4. XRD patterns of samples MSC-20(1), MSC-40(2) and MSC-80(3). A: anatase; R: rutile.

Photocatalytic activities of these mesoporous and solid rutile TiO₂ single crystals were investigated by estimating the photocatalytic hydrogen generation from an aqueous solution containing methanol as a sacrificial agent. The light irradiation time was 5 h. Note that, prior to the test, all the samples were calcined in air at 600 °C for 2 h to clean the surface without altering the morphology ^{[16, 18]}. As shown in Fig. 5, the mesoporous rutile TiO₂ single crystals with wholly {111} facets exhibited the highest hydrogen generation rate among the three mesoporous single crystal samples. As expected, the mesoporous single crystals with the most stable major {110} facets gave the lowest hydrogen evolution rate, which was only half of that with wholly {111} facets. The similar changing trend in photocatalytic activity was also observed in the solid single crystals. Evidently, these improvements could be dominantly attributed to the large exposure of reactive {111} facets because they possessed the similar phase, microstructure, specific surface area and light absorption capability (Fig. 6(b)). However, the surface of these mesoporous crystals studied here were partially covered by fluorine species, which usually lowers the photocatalytic activity of TiO₂ by reducing the number of the unsaturated surface titanium atoms ^{[4, 11, 12]}. Although heating at around 600 °C can effectively remove surface fluorine species from the samples, the collapse of the mesoporous structure is currently hard to avoid. It is anticipated that the photocatalytic activity of these mesoporous crystals can be greatly improved if a suitable method to remove surface fluorine without destroying mesoporous structure can be found. Notably, all mesoporous rutile single crystals show a much superior photocatalytic activity, by at least one order of magnitude, compared with their solid rutile single crystal counterparts. This improvement could be related to the mesoporous structure features enabling an increased surface area for abundant surface active sites, shortened bulk diffusion length of carriers for the decreased bulk recombination and increased light absorbance from the connected mesoporous network, as indicated in Fig. 6.

Fig. 5. Comparison of photocatalytic hydrogen generation after 5 h UV-visible irradiation over mesoporous and solid rutile TiO2 single crystals with different percentages of exposed {111} facets loaded with 1 wt% Pt in the presence of methanol as electron donor.

Fig. 6. UV-visible absorption spectra of solid single crystal rutile TiO2 (a) and mesoporous single crystal rutile TiO2 (b).

In summary, mesoporous single crystal rutile TiO₂ with different exposed facets was synthesized through a seeded template method by changing the amount of NaF morphology controlling agent. The mesoporous single crystal rutile TiO₂ with wholly exposed {111} reactive facets exhibited a greatly enhanced photocatalytic activity relative to the mesoporous single crystals with a minority of {111} facets as well as the solid single crystals. This improvement is attributed to the synergistic effects of the increased surface reactive sites, long-range electron transport and light absorbance.