Mixed cobalt-nitrides Co N and Ta2N bifunction-modified Ta3N5 nanosheets for enhanced photocatalytic water-splitting into hydrogen

A novel mixed cobalt nitrides CoxN (x = 1, 2, and 5.47) and Ta2N bifunction-modified Ta3N5 nanosheet photocatalyst was successfully synthesized through a high-temperature ammonolysis of Co2+/Co3+-adsorbed Ta3N5@Ta2O5 nanoparticles. Effects of Co2+ or/and Co3+ modifying on the structural, electronic, optical, and photoelectrochemical performances of Ta3N5 were investigated systematically. Co-modifying extended the absorption over the whole visible region due to the sub-band gap optical absorption from the increased heterojunction interfaces of CoxN/Ta3N5 or/and Ta2N/Ta3N5. Co2+/Co3+ dual-modifying inhibited the crystallite growth of Ta3N5. Uniformly dispersed Co elements on the Ta3N5 surface provided the more surface active sites. Co3+ played a key role in controlling the modifying amounts of both Ta2N and Co5.47N. Co2+/Co3+ dual-modifying produced a synergetic effect on enhancing the charge carrier separation, decreasing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) overpotentials of Ta3N5, and then efficiently increasing the photocatalytic H2-evolution activity. 1.5%Co2+/1.5%Co3+-TN possessed an excellent photocatalytic H2-evolution activity (75.69 μmol g−1 h−1) under visible light irradiation, 3.48 times higher than Ta3N5@Ta2O5 nanoparticles. Co5.47N and Ta2N as HER cocatalysts, and CoN and Co2N as OER cocatalysts, decreased the HER and OER overpotentials of Ta3N5, accelerating the kinetics of surface catalytic reaction. A synergetic mechanism for enhanced photocatalytic H2-evolution performance was discussed in detail. Moreover, self-oxidation of Ta3N5 can be suppressed by surface-modified cobalt nitrides and Ta2N. This work provides a promising photocatalyst and a new strategy for develop efficient and stable Ta3N5-based photocatalysts for solar energy conversion.