Photo-thermochromic properties of oxygen-containing yttrium hydride and tungsten oxide composite films

Monolayer oxygen-containing yttrium hydride (YHx:O) and tungsten oxide (WO3) films show photochromic properties and have many potential energy-saving applications. YHx:O/WO3 composite films have better solar energy regulation properties than their monolayers because of the photochromic synergistic effects between the YHx:O and WO3 thin films. However, the reversion of these films to their initial transparent state is slow at room temperature, which limits their use in energy-saving windows. In this work, we developed a photo-thermochromic optical switching method that reversibly changes YHx:O/WO3 films between their transparent and infrared (IR) absorption states smoothly. The YHx:O/WO3 composite films were prepared by DC magnetron sputtering, and the reversible optical switching properties and photochromic mechanism were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Fourier transform infrared (FTIR) spectroscopy. The composite films changed from their transparent state to their absorption state under illumination within 10 min, and returned to their transparent state when they were heated at 60–100 °C for 5 min. Compared with the single-layer YHx:O film, the YHx:O/WO3 composite films had a high color change rate and high solar transmittance (ΔTsol = 50.46%). We speculate that the mobile H in the YHx:O layer diffuses to the WO3 layer after illumination, converting WO3 to a tungsten bronze structure, increasing the IR absorption of the composite films. The diffusion of H in the YHx:O and WO3 layers was reversible and had good switching repeatability, and FTIR spectroscopy confirmed that H moved reversibly in films and played an important role in the photochromic process.