Mesoporous SiO2/BiVO4/CuOx nanospheres for Z-scheme, visible light aerobic C–N coupling and dehydrogenation

Abstract

In light of the growing demand to use renewable energy, we have synthesized a mesoporous bismuth vanadate-copper oxide-silica photocatalyst (SiO2/BiVO4/CuOx) to act as a chromophore in a Z-scheme system. These photocatalysts are intended for effective light harvesting and charge separation in the synthesis of solar chemicals, using air as an environmentally benign oxidant. Full characterization of the SiO2/BiVO4/CuOx was conducted, including X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), which confirm the presence of CuOx at the heterojunction of the nanostructures. The other characterization methods that were employed included powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), elemental mapping, UV-vis diffuse reflectance spectroscopy (UV-vis DRS), N2 sorption experiments, photoluminescence spectroscopy (PL), electrochemical impedance spectroscopy (EIS), and photocurrent measurements. Notably, the SiO2/BiVO4/CuOx nanospheres perform seven times faster than bulk BiVO4 and provide higher yields for the oxidative coupling of amines to imines, which are valuable precursors for agrochemicals and active pharmaceutical ingredients. The superior photocatalysis of SiO2/BiVO4/CuOx is attributed to the surface CuOx nanoparticles that increase the average PL lifetime from 2.3 to 4.5 ns, which improved the charge separation and decreased the unproductive recombinations of electron-hole pairs. In addition, the photocurrent density of the SiO2/BiVO4/CuOx electrode was about 3.5 times higher than that of SiO2/BiVO4, while lower charge transfer resistance was observed by EIS. Meanwhile, chemical scavenging experiments revealed that holes and superoxide radicals were the main reactive oxygen species in the photocatalytic reaction. The nanospheres also show broad functional group tolerance, good recyclability with high conversions, and high to moderate yields for the oxidatively coupled imine products after eight runs. Thus, we demonstrate that an effective and green approach in artificial photosynthesis is applicable for organic synthesis as well.