Journal Article carbon nitride oxygen reduction reaction metal-free nitrogen doping porous carbon self-assembly CO2 photoreduction Co,N-co-doped carbon Research Support, Non-U.S. Gov't Review UV light electrocatalysts hydrogen evolution inorganic nanoparticles layered double hydroxides light mesoporous silica-protected calcination nanoparticles nanovesicles nitrogen deficiency photocatalysis photocatalytic water splitting surface defects ultrathin nanosheets visible light zeolitic imidazolate frameworks