Copolymerization Approach to Improving Ru(II)-Complex/C3N4 Hybrid Photocatalysts for Visible-Light CO2 Reduction

Copolymerized carbon nitride nanosheets (NS-C3N4) were used as a light absorbing unit while paired with a Ru(II) complex that served as a catalyst for CO2 reduction, forming a hybrid photocatalytic system. Copolymerization with urea and phenylurea in air at 823 K resulted in a carbon nitride material that had wide visible light absorption extending to 650 nm, significantly red-shifted compared to the absorption edge of pristine NS-C3N4, an analogue prepared with only urea (ca. 435 nm). While a hybrid system consisting of pristine NS-C3N4 was found to be inactive under longer wavelength visible light (lambda > 500 nm) due to its large band gap, the copolymerized material was able to catalytically convert CO2 to HCOOH under lambda > 500 nm irradiation. Furthermore, its activity toward HCOOH production is doubled under lambda > 400 nm irradiation after 5 h compared to pristine NS-C3N4. Transient absorption spectroscopy clearly showed improved lifetime of photogenerated free (and/or shallowly trapped) electrons, which should be the key to enhancing the photocatalytic activity of this hybrid system even under shorter wavelength visible light.