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Abstract
Materials with excellent photochromic (PC) characteristics are promising to be applied in energy-saving smart windows, however little progress was achieved in the past decade. Herein we propose a heterojunction strategy based on a MoO3@TiO2 (MT) core/shell nanorods system to obtain remarkably enhanced photochromic property. MT core/shell nanorods have a crystalline MoO3 core with a chemically bonded amorphous TiO2 shell. They were synthesized by a scalable water-bath method and characterized by using x-ray diffraction (XRD), transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), electron energy loss spectrum (EELS) and ultraviolet-visible spectrophotometry. The MT photochromic response extends from the visible to the infrared and depends on the transmittance of the MT core/shell nanorods. The photochromic optical modulation of MT core/shell nanorods is faster and more efficient than that of MoO3 nanorods, derived from remarkable structural and chemical changes, as revealed by XRD and Raman spectroscopy. XPS analysis combined with EELS probing on a single rod indicates that the photochromic-enhancement mechanism is highly correlated to the charge transfer between Ti and Mo species, especially across the MoO3/TiO2 interface. Oxidation of Ti3+occurs simultaneously with the PC reductive transition of the Mo center, which indicates that Ti3+ plays an important role either as electron source or by favoring electron transfer by charge rectification through hole scavenging. The significant role of Ti3+ is further confirmed by annealing the material in air, which quenches the Ti3+ defects and thereby reduces PC enhancement to negligible levels compared with the unannealed samples. By controlling the shell defect state rather than only considering the bulk band structure, this work provides a new paradigm for modulating photo-driven charge transfer across core/shell heterojunctions.
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