Photocatalyst Modified With Ni,Co And Their Phosphides

With the enhanced awareness of environmental protection and the energy dilemma,the development of renewable energy has attracted tremendous attention.H₂ has lots of advantages,such as combustion value,renewable and non-polluting products,that gets the attention.Solar energy as a new energy source has been paid more and more attention.The water is abundant in the earth and contains a lot of H element.The photocatalytic water splitting reaction is regarded as one of the most promising routes for solar energy utilization and H₂ evolution.It is well known that without catalyst it is difficult to spilitting water into H₂and O₂ under the sunlight.Therefore,the key of photocatalytic water spilitting is design and prepared efficient,stable and inexpensive composite photocatalyst.It was found that the noble modified semiconductor is one of an excellent photocatalyst.However,given that noble metals are rare and expensive,it is necessary to develop novel cocatalysts made of earth-abundant elements.Recently,it was found that transition metal and transition metal phosphides modified semiconductor can be used as a kind of high efficiency,stable and inexpensive composite photocatalyst.At the same time,taking into account safety and energy saving,it is greatly necessary and significant to design novel preparation strategy for transition metal and transition metal phosphides.The noble metal modified semiconductor was prepared by photo-deposition strategy,which means that photo-deposition strategy is facile and efficient method for preparing composite photocatalyst.So far,the photo-deposition method is limited to preparation of the noble metal,and it is a challenge that the use of photo-deposition method to produce small electronegativity of metal elements and its phosphide.Therefore,this paper is devoted to the development of facile and efficient photo-deposition strategy for the preparation of transition metal and their phosphide modified photocatalyst,and applied it to solar photocatalytic H₂ evolution from water.The main research contents are as follows:1.The nickel-based compound modified graphene carbonitride?g-C₃N₄?is an ideal composite photocatalyst of inexpensive,stable and composed of earth-abundant elements.In this paper,a rapid photo-deposition strategy was proposed to prepare effectively combined Ni/g-C₃N₄ composite photocatalysts.In this process,the Ni?0?formed from the reduction of Ni?II?by photogenerated electrons and was loaded on the electron outlet points of g-C₃N₄within 30 min,and the ratio of Ni/g-C₃N₄ can be adjusted by changing the addition of nickle salt.When the loaded Ni was 7.40 wt%,a high photocatalytic H₂ production rate of 4318?mol g^-1 h^-1 was obtained and no noticeable decrease of photocatalytic H₂ evolution rate was observed after four runs for 48 h in triethanolamine aqueous solution.Furthermore,the Ni/g-C₃N₄ composite photocatalyst presented sunlight-driven H₂ production ability of 4000?mol g^-1 h^-1 under natural sunlight outdoors.Finally,the mechanisms for the efficient and stable activity of Ni/g-C₃N₄ hybrid photocatalyst were investigated in detail through photoelectrochemical experiments and steady-state photoluminescence spectra.It was found that,Ni effectively prevented the recombination of photogenerated electrons and holes of g-C₃N₄ and improved the H₂ evolution activity.2.Composed of earth-abundant elements,metal phosphides have revealed remarkable advantages as photocatalytic hydrogen generation cocatalysts for their outstanding performance,stability and low cost.Taking the importance for solar energy utilization and current synthetic methods of metal phosphides into consideration together,we focus on the safe and energy-saving preparation of metal phosphide as cocatalyst for photocatalytic hydrogen evolution.In this paper,taking Co_xP/CdS as a case,a novel photochemical strategy to synthesize Co_xP at room temperature was proposed and realized.In this process,Co salt and NaH₂PO₂ were used as the source of Co and P,and the optimal photocatalytic activity of Co_xP/CdS composite photocatalyst can be obtained by irradiation for 50 minutes.Moreover,the optimized hydrogen evolution rate of as-prepared Co_xP/CdS NRs was about 500 mmol g^-1h^-1 under visible light,which is among the most robust photocatalytic HER systems currently.Furthermore,the mechanisms for the efficient and stable activity of Co_xP/CdS hybrid photocatalyst were investigated in detail through surface photovoltage and steady-state photoluminescence spectra.It was found that,Co_xP effectively prevented the recombination of photogenerated electrons and holes of CdS and improved the H₂ evolution activity.The photochemical synthesis route proposed could open a door for facile preparation and practical application of many other metal phosphides.3.Up to now,most of based on transition metal compounds modified photocatalyst is instability under acidic conditions for photocatalytic H₂ evolution.Therefore,the development of a stable photocatalyst under acidic conditions has become one of the focuses of this paper.In this work,the facile and efficient photo-deposition strategy was used to prepare Ni_xP/g-C₃N₄ composite photocatast,which is stable under acidic conditions,and Ni_xP was loaded on the electron outlet points of g-C₃N₄.In this process,Ni salt and NaH₂PO₂ were used as the source of Ni and P,and the optimal photocatalytic activity of Ni_xP/g-C₃N₄composite photocatalyst can be obtained by irradiation for 20 minutes.Moreover,the optimized hydrogen evolution rate of as-prepared Ni_xP/g-C₃N₄ composite photocatalyst was about 8585?mol g^-1 h^-1 under TEOA solution.More importantly,due to the intrinsic acid-resistant properties of Ni_xP and g-C₃N₄,Ni_xP/g-C₃N₄ composite demonstrated excellent acid-stable photocatalytic activity for H₂ evolution?stable run for 75 h under acidic solution of pH=2?.Furthermore,the mechanism for photocatalytic activity of g-C₃N₄ enhanced by Ni_xP was investigated in detail by steady-state photoluminescence spectra and surface photovoltage spectra,which indicated that separation efficiency of photogenerated carriers from g-C₃N₄ was effectively enhanced by Ni_xP.