3. Results and discussion
Ultrafine TiO2 was prepared via a two-step route including a facile hydrothermal process and followed calcination. According to XRD analysis (Fig. 1), the as-synthesized powder has already been identified as composite structures of anatase TiO2 (JCPDS no. 211272) and (NH4)2TiOF4 (JCPDS No. 82-1330). XRD patterns of the sintered products at 300 and 500 1C are also provided in Fig. 1. After annealing at 300 1C for 0.5 h, its XRD pattern presents that all the diffraction peaks are in consistence with that of standard anatase TiO2 (JCPDS No. 21-1272), indicating that the anatase phase was derived from (NH4)2TiOF4 phase by the structural transformation. The product sintered at 500 1C for 0.5 h, which is the calcination temperature of TiO2 photoanode, indicates a purified and stable anatase structure based on the XRD patterns. The crystal transformation from (NH4)2TiOF4 to TiO2 may provide a chance to fabricate nanopores or surface for desired structures.
The ultrafine TiO2 (o20 nm) obtained after annealing at 500 1C for half hour can be observed in FESEM images (Fig. 2(a)). To further study its formation mechanism, the TEM images of the intermediate composite and dice-like TiO2 are presented in Figs. 2(b) and 3(c) respectively. Fig. 2(b) shows that the obtained nanoparticles with an average diameter of 20 nm are assembled by sub-particles with smaller size. After annealing, the dice-like structure can be further confirmed in TEM image (Fig. 2(c)). With the structure transition, nano-concavities were formed on the surface of anatase nano-cubes (Fig. 2(d)). The possible synthetic method is schematically shown in Fig. 2(e), which is powerfully supported by the above mentioned X-ray diffraction (XRD) patterns and TEM images. To check the crystal structure of the ultrafine TiO2, The HRTEM image exhibits a set of clear lattice fringes for the single crystal structure with d-spacing of 0.352 nm and also shows the facet to facet joint in interparticles in Fig. 2(f).
Surface features of dice-like TiO2 were characterized using Quantachrome automated N2 sorption system at 77 K. Its Brunauer–