g-C₃N₄/(Cu/TiO₂) nanocomposite for enhanced photoreduction of CO₂ to CH₃OH and HCOOH under UV/visible light

Graphical abstract: Cu-modified graphitic carbon nitride (g-C₃N₄) and titanium dioxide (TiO₂) nanocomposites for enhanced photocatalytic CO₂ reduction with H₂O under UV and visible light irradiations have been investigated. The photocatalysts, prepared by pyrolysis and impregnation were characterized by XRD, FE-SEM, TEM, FT-IR, N₂ adsorption-desorption, XPS, UV-vis DRS and PL spectroscopy. The Cu-metal loaded over TiO₂ and g-C₃N₄ enhanced CO₂ reduction efficiency to CH₃OH and HCOOH by fostering carrier charge separation. The Cu-metal in the composite as well as the wt.% ratio of g-C₃N₄ and TiO₂ also influenced the photoactivity and products selectivity. The low band gap, electronic structure and visible light absorption capacity of g-C₃N₄ facilitated the transfer of photo-generated electrons to Cu/TiO₂ in the composite. Moreover, the position of the metal in the composite affected the electrons distribution and hence enhanced the photoactivity. The maximum yield of the products detected under visible light were 2574 and 5069μmol/g.cat of CH₃OH and HCOOH, respectively. The yield of CH₃OH under visible light was four fold higher compared to UV-light irradiation. The ratio (30:70) of g-C₃N₄ and Cu/TiO₂ in the composite and the use of visible light improved the efficiency of the photocatalytic system. The stability of the photocatalyst prevailed in continuous CH₃OH production under visible light irradiation compared to UV-light in cyclic runs. Possible reaction mechanisms were proposed to understand the movement of electrons and holes and the function of both UV and visible light for CO₂ reduction over the g-C₃N₄/TiO₂ (30:70) photocatalyst.