Effects of Annealing Parameters on Optical Properties of Thermochromic VO2 Films Prepared in Aqueous Solution

This work confirmed experimentally that the microstructure (grain boundaries, grain sizes, and size distributions) of VO2 films has significant effects on the features of the semiconductor-metal (S-M) transition. This feature enables us to wisely regulate the parameters of the phase transition, which is of great importance in achieving practical applications. Thermochromic VO2 films with various optical properties and phase transition parameters (for example: hysteresis widths ranging from 12 to 50 degrees C) have been synthesized on fused silica substrates via a simple solution process with inorganic precursors and polyvinylpyrrolidone (PVP). The widths and slopes of the hysteresis loops (i.e., the temperature sensitivity of the transition) can be regulated by controlling grain sizes and grain boundary conditions, which are believed to dominate the generation of the elementary hysteresis loop of each grain and the propagation of the S-M transition, respectively. A film consisting of qusai-isolated small particles shows a wide hysteresis loop due to the large interfacial energies and the lack of defects for nucleating. Besides, grain boundaries can decrease the interfacial energies and favor the phase propagation. In addition, particle sizes seem to influence the visible transmittances of these films effectively without deterioration of the infrared regulation ability. This experimental phenomenon is assigned to the blue shift of absorption edges (at around 500 nm) accompanying decreases in particle size.