Constructing a hexagonal/orthorhombic WO3 phase junction for enhanced photochromism

Mixed-phase composites with an optimal phase ratio show greater light utilization than their monophasic counterparts. This approach can be applied to develop mixed-phase photochromic tungsten oxide (WO3), which shows a rapid photoresponse and high-contrast coloring. Therefore, this paper optimized the phase ratio of hexagonal WO3 (h-WO3·0.33H2O) and orthorhombic WO3·H2O (o-WO3·H2O) to form a homojunction. To enhance its photochromism, nanostructured WO3 was produced via a one-step hydrothermal strategy, in which its phase structure and morphology were controlled by regulating the reaction temperature during precursor synthesis. Various material characterization techniques were carried out to reveal the complex crystal nucleation and growth of phase junctions. h-WO3·0.33H2O nanoparticles dominated the systems, while o-WO3·H2O nanoblocks also appeared from 41.77 wt% to 56.50 wt% at a low hydrothermal synthesis temperature, producing a mixture of both polymorphs. The improved photochromic properties (from light yellow to black-green) of the mixed-phase WO3 composite compared with pure h-WO3·0.33H2O (from gray-white to dark gray) was attributed to the formation of a phase junction between h-WO3·0.33H2O and o-WO3·H2O. The photoelectrochemical results showed that this resulted in highly efficient charge carrier separation and transfer. The strong visual rendering of the black-green state may be useful for developing intelligent displays and similar devices.