Modified Ni₅P₄ Co-catalyst For Enhancing The Photocatalytic Hydrogen Production Performance Of CdS

Semiconductor photocatalytic hydrogen production is an ideal way to address environmental degradation and energy crisis.However,the low efficiency,severe photogenerated charge recombination and narrow light absorption range of single semiconductor photocatalyst restrict its large-scale application.Loading cocatalyst on the semiconductor surface is one of the effective measures to promote its photocatalytic performance.The cocatalyst not only captures the electrons in the semiconductor and promotes charge separation,but also provides active sites for the catalytic reaction.Non-noble metal hydrogen-producing cocatalyst with a certain capacitance value has a capacitance catalytic effect,that is,its capacitance can store photoexcited electrons of semiconductors(positive effect)and promote the separation and transfer of photogenerated charges,but over large capacitance will restrain the timely release of electrons(negative effect),which is not conducive to hydrogen production.Introducing a little amount noble metals on the surface of non-noble metal cocatalyst can inhibit the negative effect of capacitance catalysis and improve the catalytic effect,but the existence of noble metal prevents such catalyst from being used on a large scale.Therefore,the advancement of efficient and low-cost co-catalyst is of great significance to promote the industrialization of photocatalytic hydrogen production technology.In this paper,two composite photocatalyst models,Zn-Ni5P4/CdS and C-Ni5P4/CdS,were constructed with semiconductor CdS as the host catalyst and Ni5P4 by Zn and C modified as the co-catalysts.The photocatalytic water splitting for hydrogen production were carried out under visible light,and their photocatalytic hydrogen production mechanisms were investigated deeply.The specific research contents are as follows:(1)The effect of Zn-Ni5P4 on the photocatalytic hydrogen production of CdS was studied.The co-catalyst Zn-Ni5P4 was synthesized by doping Ni5P4 with Zn,and then loaded onto CdS to construct the Zn-Ni5P4/CdS composite photocatalyst.The highest hydrogen production rate of Zn-Ni5P4/CdS was 8969 μmol/g/h,which is 2.4 times that of Ni5P4/CdS(3668 μmol/g/h).The photocurrent response test showed that Zn-Ni5P4/CdS with excellent photogenerated charge separation efficiency.And the surface photovoltage results showed that Zn-Ni5P4,as an electron acceptor,leading to a large amount of electrons enrichment on the surface of Zn-Ni5P4/CdS,which is conducive to electron-hole separation.The results of linear sweep voltammetry and electrochemical impedance tests revealed that Zn-Ni5P4/CdS had strong reduce ability and excellent electron conduction property.The cyclic voltammetry test found that Zn doping reduced the capacitance of Ni5P4 from 680 mF/g to 540 mF/g,and the galvanostatic charge-discharge tests further confirmed that Zn doping reduced the capacitance of Ni5P4.The capacitance of Ni5P4 can store the photoexcited electrons of CdS,and Zn doping reduced its capacitance,effectively inhibited the negative effect of Ni5P4 capacitance catalysis,and promoted the efficient electron-hole separation.Therefore,Zn-Ni5P4 significantly improved the hydrogen production activity of CdS under visible irradiation.(2)The co-catalyst C-Ni5P4 of graphitic carbon modified Ni5P4 was synthesized by phosphating precursor Ni-MOF,and then loaded onto CdS surface to synthesize C-Ni5P4/CdS photocatalyst,and the mechanism of photocatalytic hydrogen production was studied.The AQY of C-Ni5P4/CdS was 3.2%(λ=420 nm),and the highest hydrogen production activity of 12283 μmol/h/g,which is 3.8 times that of Ni5P4/CdS.The linear sweep voltammetry curves and electrochemical impedance tests showed that the hydrogen production overpotential and impedance of C-Ni5P4 were smaller than that of Ni5P4,which greatly improved the reduction capacity and charge transfer efficiency of CdS.The higher photocurrent density and lower PL signal of C-Ni5P4/CdS indicated that it had excellent photogenerated charge separation efficiency.The BET test showed that the specific surface area of C-Ni5P4/CdS was 131.67 m2/g,which provided abundant active sites for photocatalytic H+reduction reaction.The cyclic voltammetry curve test showed that specific capacitance(1.22 mF/cm2)of C-Ni5P4 was smaller than that of Ni5P4(1.30 mF/cm2),which proved that the appropriate capacitance value of C-Ni5P4 can store photoexcited electrons,and the presence of graphitic carbon weakened of the constraint effect of its capacitance on electrons,electrons can be quickly released to the active sites on the catalyst surface for H+proton reduction to produce hydrogen,so C-Ni5P4 had a strong capacitance catalytic effect.Therefore,the synergistic effect of the large surface area of C-Ni5P4/CdS and the strong capacitance catalytic effect of C-Ni5P4 makes C-Ni5P4/CdS have superior photocatalytic performance.