One-Step Hydrothermal Synthesis of TiO2@MoO3 Core–Shell Nanomaterial: Microstructure, Growth Mechanism, and Improved Photochromic Property

Photochromic TiO2@MoO3 core–shell (TM) nanopowder was synthesized by a one-step hydrothermal method and characterized with XRD, TEM, Raman, and a spectrophotometer. The nanopowder has a very small particle size of 5–10 nm in diameter, and a well-defined core–shell structure with the tight interface; that is, an anatase TiO2 core is tightly surrounded with amorphous MoO3, to form an ideal heterojunction structure. The growth mechanism was proposed with a surface-induced nucleation of amorphous MoO3 from a well-mixed precursor, where the mutual inhibition between core and shell led to the confined core size. The photochromic performance of TiO2@MoO3 was evaluated in solution as well as casted thin film on glass using a spectrophotometer, with a comparison to that of single MoO3. A significant enhancement in the photochromic properties was demonstrated for TM; that is, the change of the absorbance in water at 600 nm is about 30 times larger than that of α-MoO3, and a 20% transmittance regulation at 500–800 nm was obtained for the TM-based thin film. Meanwhile, the samples in ethanol showed much stronger photochromic coloration efficiency than that in water by 66.6% enhancement in the absorption at 600 nm. Such a significant photochromic enhancement is considered to the formation of a relatively ideal heterojunction at the interface, leading to an efficient electron–hole separation. The size effect of TM provides a high specific surface area for cation insertion/extraction and diffusion and decreases the distance of electron transfer. Furthermore, a shift of excitation wavelengths from UV to visible was also observed due to the appearance of extra defect bands by Mo doping into TiO2 in their interface, which was supported by XRD and Raman measurements. The enhanced photochromic performance of TiO2@MoO3 is expected to be used for color displays and smart windows.