| Hydrogen energy has aroused wide attention because of its-clean and eco-friendliness.Water splitting into hydrogen is an effective strategy,using the inexhaustible solar energy.Among the reported photocatalysts,non-metal g-C3N4 is a favorite photocatalyst in photocatalytic field due to its stable chemical and thermal property,suitable energy band structure,non-toxicity and eco-friendliness.However,the quantum efficiency of pristine g-C3N4 is poor,which results from the severe recombination of photogenerated electron and hole.For this reason,researchers have devoted themselves to improve the photocatalytic activity of g-C3N4,mainly through doping element,constructing heterojunction and loading cocatalyst.Although the photocatalytic activity is considerable,while most of those need to add Pt as a cocatalyst in the photocatalytic reaction.Therefore,it is necessary to look for cocatalyst with low-price and high efficiency for improving quantum efficiency in water splitting into hydrogen.Three steps are contained in photocatalytic reaction:the generation of electron and hole through the light absorbance;the separation,transmission and recombination of photogenerated charges;the participation in photocatalytic reaction of photogenerated charges which transport to the surface of photocatalyst.The second step is the main step in photocatalytic reaction,which could reflect the transfer mechanism of photogenerated charges.However,the photocatalytic mechanism is still not unambiguous with the limited measurements and incoordination of those methods.As a result,it is indispensable to explore the generation,separation,transfer and recombination of photogenerated charges in water splitting into hydrogen,employing appropriate characterization means.We use the transition metal nickel-based compounds with low price and rich-content in earth to modify g-C3N4,aiming at improving the separation efficiency of photogenerated charges and exploring the photocatalytic mechanism,which obtains the enhanced rate of photocatalytic hydrogen production in stepwise growth.The characterized methods of surface photovoltage,transient photovoltage,surface photocurrent,photolumiesence,photoacoustic response and electrochemical testing was used to study the generation,separation,transmission and recombination of photogenerated charges in photocatalyst system.Meanwhile,the photocatalytic mechanism was proposed and discussed in detail.This study mainly contains four areas:1.g-C3N4 modified with non-noble metal Ni,in which interfacial electric field was constructured from g-C3N4 to Ni.The photogenerated charges could move in certain direction,therefore,the separation efficiency of photogenerated charges is improved and the photocatalytic hydrogen production rate is further enhanced.In order to explore the species of photogenerated charges that Ni accepted in photocatalytic reaction,Ni/CM-C3N4 with good electrical conductivity was sysnthesized.Photogenerated electrons was captured by Ni that much more photogenerated electrons could transfer to the surface of CM-C3N4 and participate in the photocatalytic hydrogen evolution reaction,resulting in the rate of hydrogen production of 313mmol g-11 h-1.2.NiCoP with low overpotential was in-suit grown on the surface of g-C3N4 to accelerate the kinetic process.The result illustrates that the recombination of photogenerated charges in NiCoP/g-C3N4 is effectively suppressed,which reveals the better photocatalytic activity,achieving 1643mmol g-11 h-1.More importantly,the testing system of surface photovoltage and transient photovoltage was designed to simulate the actual atmosphere of photocatalytic hydrogen production by adding triethanolamine-water soluation,confirming that NiCoP could induce much more photogenerated electrons transfer to the surface of g-C3N4,which is benefical for the photocatalytic water reduction reaction.3.PtNix modified g-C3N4 was synthesized with the introduction of Pt into Ni,further improve the rate of photocatalytic hydrogen,achieving 8456mmol g-11 h-1.The excellent photocatalytic activity of PtNix/g-C3N4 could be attributed to the electron acceptor of PtNix with the synergistic effect of Pt and Ni,thus the recombination of photogenerated charges could be effectively suppressed.Morewhile,the modification of PtNix with low overpotential could reduce charge transfer resistance,which the charge transfer is more unobstructed.Therefore,much more photogenerated electrons participate in the photocatalytic reaction.4.Based on section three,we cut down the content of Pt in PtNix cocatalyst to reduce the cost of experiment,and then NiSx was obtained through sulfidizing to lower activation energy.The photocatalytic activity of S-PtNix/g-C3N4 obtained by the subsequent sulfidizing is considerable,achieving 4966mmol g-11 h-1.The improved photocatalytic hydrogen production rate of 0.5S-PtNix/g-C3N4 is attributed to the synergistic effect of electron channel of PtNix and electron acceptor of NiSx. |
