CO2 reduction CdS CuS@CuGaS2 H2 evolution H2 generation MoS2 MoSe2 Niobium(V) oxide Technology: general issues TiO2 Z-Scheme band gap density-functional theory disordered surface electron-hole recombination energy g-C3N4 graphitic carbon nitride heterojunction heterostructures hydrogen evolution hydrogen production hydrothermal synthesis methanol photo-splitting perovskite oxynitride photocatalysis photocatalysts photoelectrochemical deposition rapid-thermal annealing solar energy