W–O bond shortening by doping of first-row transition metal ions that enhances its catalytic potency

Here, we report thorough spectroscopic characterization of tungsten oxide (WO₃) nanoparticles (NPs) and WO₃ NPs surface doped with transition metal (TM) ions (TM@WO₃; TM = Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, and Ni²⁺) by using various surface analysis techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), high-resolution X-ray photoelectron spectroscopy (HRXPS), and extended X-ray absorption fine structure (EXAFS). At first glance, the morphologically distinct features were not perceivable across TM dopants. However, the detailed XPS analysis confirmed that the WO₃ NPs doped with Cr and Fe ions formed a relatively large number of surface defects. Moreover, we found through EXAFS analysis that the chromium and iron ions doping on WO₃ NPs promoted the dramatic shortening of the W–O bond length as compared to pristine WO₃ and other metal ion doped WO₃ NPs. Much enhanced photocatalytic and electrocatalytic activities of Cr@WO₃ and Fe@WO₃, which relied on the different density of surface defects created by the charge and size of the TM dopants, were demonstrated in the photocatalytic degradation reactions of 4-chlorophenol and the electrocatalytic hydrogen evolution reaction. The present result can serve as a practical guideline for atomically precise control of oxygen vacancy toward the development of high-performance metal oxide photocatalysts.