Preparation And Visible-light-driven Photocatalytic Performance Of Noble Metal Decorated Graphitic Carbon Nitride

Photocatalytic water splitting driven by solar light with the aid of semiconductor photocatalysts has been recognized as an efficient strategy for renewable hydrogen resource development and energy shortage solution.Among a lot of photocatalysts,graphitic carbon nitride?g-C₃N₄?emerging as a new polymeric semiconductor photocatalyst for water splitting,possesses the advantages of chemical,thermal and optical stability,a narrow band gap to harvest visible light and appropriate redox band positions,thus it has received extensive research in recent years.However,bare g-C₃N₄ can only absorb part of solar energy below the wavelength of 450 nm,leading to the lack of solar utilization.Moreover,owing to the fast recombination of photogenerated electrons and holes,the quantum efficiency for hydrogen?H₂?evolution of g-C₃N₄ is found to be quite low.Such shortcomings seriously hinder its application in the field of photocatalysis.Therefore,this work mainly focuses on the modification of g-C₃N₄ with noble metal cocatalysts to enhance the photocatalytic H₂ evolution activity and investigates the properties and photocatalytic mechanism on the basis of detailed analysis.Firstly,g-C₃N₄ photocatalysts with Au and PtO cocatalysts supported together on the surface are prepared using a facile photodeposition method.The obtained g-C₃N₄ composites with co-existed Au and PtO cocatalysts have an enhanced visible-light absorption and exhibit a much higher photocatalytic hydrogen evolution activity than individual Au/g-C₃N₄ and PtO/g-C₃N₄ photocatalysts.Besides,by comparing the photocatalytic activities of different Au to Pt loading mass ratio,it is found that when the ratio was 4:6,the Au-PtO/g-C₃N₄ photocatalyst shows the highest H₂ production rate.It is demonstrated that PtO nanoparticles can reduce the overpotential during H₂ generation reaction,effectively lower the recombination rate of photoinduced carriers and moreover,oxidized PtO is considered to suppress the H₂ back-oxidization reaction.With the co-existence of Au nanoparticles in the g-C₃N₄-based system,the surface plasmon resonance effect?SPR?of Au nanoparticles can generate a local electric field,which further promotes the separation of photogenerated carriers.Thus the synergetic action between PtO and Au dual cocatalysts plays the critical role to greatly improve the photocatalytic performance of g-C₃N₄ under visible light.Secondly,cubic,octahedral and irregular spherical Pt nanocrystals are ex-situ deposited on g-C₃N₄ substrate by electrostatic adsorption-deposition method for visible-light-driven photocatalytic hydrogen evolution reactions.These Pt nanoparticles of different shapes have similar sizes around 10 nm but are exposed with different facets.It is found that the visible light-driven photocatalytic activities for the three types of Pt/g-C₃N₄ composites follow the order as:cubic Pt/g-C₃N₄<octahedral Pt/g-C₃N₄<spherical Pt/g-C₃N₄,revealing that the photocatalytic hydrogen production activity of Pt modified g-C₃N₄ under visible light is affected by the shape of Pt cocatalysts.The shape-sensitive activity has correlations with the surface atom structures of different exposed facets of Pt nanocrystals,which further influence the active sites and adsorption energies in photocatalytic reactions.Cubic Pt nanocrystal is composed of six?100?facets,while Pt octahedron exposed with eight?111?facets has more sharp edges and corners with unsaturated coordination in the structure,which provide more active sites for photocatalytic H₂ production reaction.The adsorption energy of H₂O molecule on Pt?100?and?111?facets is also calculated and the results clearly reveal that H₂O molecules tend to adsorb on octahedral Pt particles rather than cubic Pt surface for H₂ evolution reactions.Therefore,more efficient photocatalysts with improved activity and selectivity can be rationally designed by modulating the shapes of cocatalyst nanoparticles.