Photochemical Preparation And Performance Studies Of Nickel-based Cocatalysts

Photocatalytic hydrogen evolution by water splitting is an environment-friendly strategy for converting inexhaustible solar energy into available chemical energy,and further relieving the energy crisis and environmental pollution.The photocatalytic process is composed of three steps:light absorption,charge transport and surface reaction,and the third step is the speed control step.Loading cocatalysts on the photo-active semiconductors can accelerate the surface reaction and improve photocatalytic efficiency.Many studies have demonstrated that transition metals and their compounds can serve as efficient cocatalysts for hydrogen production.Photochemical deposition is a gentle and efficient method to prepare transition metal based cocatalysts.However,transition metal based cocatalysts are difficult to be prepared by photochemical methods because of the small electronegativity and low reduction potential of their ions.Therefore,it is of great significance to design photochemical deposition strategies for non-precious metal based cocatalysts.The influential factors on photochemical preparation of transition metal-based cocatalysts are not clear,which limits the development and application of this method.In-depth study of the influential factors of photochemical deposition process can provide reasonable guidance for the preparation of transition metal-based cocatalysts.This paper mainly explores the influence of the solution environment of nickel ions in the photochemical process on the species,size,distribution,chemical state and catalytic activity of nickel deposits.The main research contents are as follows:1.In this paper,NiS modified g-C₃N₄ nanosheet hybrid photocatalysts were prepared by a facile,rapid and scalable photodeposition method.The generation of NiS resulted from the reduction of adsorbed complex of Ni?II?ions and thiourea?TU?by photogenerated electrons from g-C₃N₄ nanosheets under light irradiation.The content of deposited NiS can be simply adjusted by the change of irradiation time.When the preparation time was 20 minutes,the highest photocatalytic hydrogen evolution rate mounted up to 16400?mol g^-1 h^-1 with 0.76wt%NiS loading,which was about 2500 times higher than that of pure g-C₃N₄.The photocatalytic H₂ evolution rate was not markedly decline after 10 runs for 40 hours.The turnover number?TON?reached 1230000 in 52 hours with a turnover frequency?TOF?of23600 for NiS.Furthermore,the hydrogen evolution of the NiS-20/g-C₃N₄ composite photocatalyst reached 28.3 mmol g^-1 during 7 hours under natural sunlight.A possible mechanism on the enhanced photocatalytic activity for NiS/g-C₃N₄ was proposed that the presence of NiS cocatalyst can efficiently promote the separation of photogenerated electron-hole pairs of g-C₃N₄,which was supported by the steady-state photoluminescence spectroscopy and photoelectrochemical analyses.2.This paper explored the effects of alcohols?sacrificial reagents?and triethanolamine?complexing agent?on the Ni deposits.The solution environment of nickle ions in the photochemical process fundamentally determined the size,dispersion and the photocatalytic activity of the Ni deposits.Under optimal experimental conditions?40 vol%isopropanol and20 vol%triethanolamine?,an efficient Ni-Ni?OH?₂/CdS photocatalyst was successfully prepared.The optimal hydrogen production rate was 428 mmol g^-1 h^-1 under visible light and remained essentially unchanged after 24 hours.The photoinduced interfacial charge dynamics was investigated by photoluminescence spectra and surface photovoltage spectra.Ni and Ni?OH?₂ were excellent cocatalysts for promoting the charge transfer,and improving the photocatalytic activity.3.In this paper,atomically dispersed Ni modified CdS nanorods?NRs?hybrid photocatalyst?Ni₁/CdS?with Ni loading up to 2.85 wt%was prepared by a simple and facile photochemical method.Under optimal conditions,the highest rate for H₂ evolution is 630.1mmol g^-1 h^-1 under visible light,which is one of the most robust photocatalytic HER systems based on CdS currently.Furthermore,the Ni₁/CdS catalyst exhibits good stability and durability for HER and outstanding photocatalytic activity under sunlight and aerobic conditions,indicating the great practical value of present reaction system.The introduction of single Ni atom on the CdS can improve the hydrogen binding energy and electronic properties,thus greatly boosting the photocatalytic H₂ production activity.