Controllable Synthesis Of Nickel-based Phosphides And Its Co-catalysis Performance In Photocatalytic Hydrogen Production

Ni-based phosphide?NiP_x?is a type of interstitial metal phosphide that has significant catalytic effect on hydrogen-related reactions.It can replace noble metal catalysts for reactions such as hydrodesulfurization,denitrification,and photo/electric hydrogen evolution.Traditional NiP_x is mainly prepared by using a large amount of red phosphorus or NaH₂PO₂ as a phosphorus source through a calcination reaction under high-temperature hydrogen atmosphere or water/solvothermal reaction,etc.The reaction conditions are harsh,and the product is easily polluted by excessive phosphorus precursors.A new solvothermal method with milder reaction conditions was developed to regulate the synthesis of NiP_x.With EDA as the solvent and NaBH₄as the auxiliary agent,a series of NiP_x can be synthesized by controlling the amount of Ni?H₂PO₂?₂·6H₂O and NaH₂PO₂,including Ni,Ni₁₂P₅,Ni₂P/Ni₁₂P₅,Ni/Ni₂P and Ni₂P.The catalytic performance of these Ni-based phosphides on g-C₃N₄photocatalytic hydrogen production?HER?was further investigated.The results show that:1).NaBH₄ is indispensable in the synthesis of NiP_x and plays a role in reducing Ni²⁺to Ni in the early stage of the reaction;2).?H₂PO₂?₂^2-in the precursor Ni?H₂PO₂?₂ is insufficient to provide sufficient P.So the phosphating reaction requires additional addition of NaH₂PO₂;3).Ni simple substance were prepared with the molar ratio of Ni?H₂PO₂?₂·6H₂O:NaBH₄:NaH₂PO₂ is 3.4:5.3:0,Ni₁₂P₅ were prepared with the molar ratio is 3.4:5.3:3.4,Ni₂P/Ni₁₂P₅ were prepared with the molar of when the ratio is 3.4:5.3:17,Ni/Ni₂P were prepared with the molar ratio is 3.4:10.6:17,and Ni₂P were prepared with the molar ratio is 3.4:10.6:28.4.The general trend is that as the input of NaBH₄and NaH₂PO₂ increases,the product changes in the order of Ni?Ni₁₂P₅?Ni₂P/Ni₁₂P₅?Ni/Ni₂P?Ni₂P,and the phosphatization is gradually complete;4.Theoptimal conditions for preparation are solvent heat treatment at 160°C for 24 h.The solvent heat treatment process goes through the following three reaction stages:First,NaBH₄ decomposes at high temperature to release atomic hydrogen[H].These[H]subsequently reduce Ni²⁺to Ni⁰ and adsorb on the Ni⁰ formed by the reduction.Finally,[H₂PO₂]^-reacts with Ni⁰-[H]_x adsorbed on the Ni surface to form P atoms.These P atoms are then inserted in situ into the Ni lattice to form NiP_x;5.All NiP_x can improve the performance of g-C₃N₄ photocatalytic HER,and the hydrogen production activity increases with the increase of P content.Ni₂P has the largest hydrogen production rate.In order to show the best activity,the hydrogen production rate is(215.1?mol·g^-1·h^-1),which is almost pure g-C₃N₄(35.6?mol·g^-1·h^-1)six times;6.The increase in the activity of g-C₃N₄ by NiP_x is achieved by accelerating the separation of photogenerated carriers and providing a lower hydrogen evolution overpotential active site.This part of the work will provide valuable reference for the"green"and selective synthesis of NiP_x and the optimization of efficient photocatalytic synthesis of NiP_x.Based on the above work,we were extended the developed phosphating method for one-pot synthesis of Ni₂P modified CdS nanorod photocatalysts.It is expected that the contact quality of Ni₂P and the substrate CdS nanorods will be improved by the phosphating deposition in a one-pot process,and then its HER performance will be improved.The research systematically investigated the effect of Ni₂P deposition on hydrogen production activity,and compared the performance with samples obtained by other phosphating methods.The HER performance improvement mechanism of the samples was finally studied.The results show that the performance of CdS first increases and then decreases with the loading amount of Ni₂P.The sample with 2%Ni₂P shows the best performance.The hydrogen production rate at room temperature is as high as 23.5 mmol g^-1 h^-1,corresponding to an apparent quantum efficiency of 56%?irradiation at?=435 nm?.The hydrogen production performance of the sample obtained by the developed phosopidation procee is obviously better than that obtained by the traditional synthesis methods.At a load of 2%,the hydrogen production rate of these samples is according to this method(23.5 mmol·g^-1·h^-1)>High-temperaturesolid-phase phosphatization method(10.52 mmol·g^-1·h^-1)>EDA-red P system(6.88mmol·g^-1·h^-1)>The order of H₂O-red P system(3.01 mmol·g^-1·h^-1)decreased.The activity is significantly higher than the samples prepared by the conventional phosphidation processes and the classical Pt?2%?modified CdS(3.95 mmol·g^-1·h^-1).The remarkable HER performance can be ascribed to the loading of intimately contacted Ni₂P which can promote the separation of photoinduced charge carriers and simultaneously reduce the overpotential for H₂ evolution.Furthermore,a reservoir role for photoinduced e^-was observed on the deposited Ni₂P beyond as a cocatalyst.The enrichment of e^-in Ni₂P indicates its potential application in some multiple-electrons related conversions,such as the photocatalytic CO₂ reduction and the N₂ fixation.The work offers a new and effective strategy for the fabrication of TMPs and their loading on photocatalyst to replace expensive noble metals for photocatalytic or electrocatalytic HER.