| With the increasing global energy demand,mankind urgently needs to find a clean and efficient new energy source,and hydrogen is undoubtedly the most ideal choice.However,the current methods of producing hydrogen still rely on traditional energy sources such as fossil fuels.These methods will inevitably pollute the environment and damage the ecosystem.The photocatalytic decomposition of water to produce hydrogen is the direct conversion of inexhaustible solar energy into available hydrogen energy,which has attracted great attention on researchers.In the system of photocatalytic decomposition of water to produce hydrogen,the use of a co-catalyst is very important.It can effectively separate photo-generated electrons and holes and reduce the overpotential required for hydrogen evolution,thereby greatly improving the hydrogen production efficiency.At present,the most efficient co-catalyst for hydrogen evolution is a Pt-based precious metal,but it is difficult to obtain large-scale applications due to its scarce content.Transition metals(such as Fe,Co,Ni,etc.)have become the protagonists of non-precious metal co-catalyst research in recent years due to their abundant content in the earth’s crust and relatively easy access to raw materials.For example,its carbides and hydroxides are widely used in the fields of photocatalysis and electrocatalysis.The other star material,graphene,has a large specific surface area,excellent electron transport ability,and low preparation cost.It is often used as a carrier and co-catalyst for semiconductors.In this paper,we have synthesized a series of graphene-based transition metal composites using salts of transition metals and graphene as raw materials for dye-sensitized decomposition of water to produce hydrogen.This work mainly includes the following two parts:(1)First,a graphene oxide(GO)sol was prepared by an improved Hummers method,and then a CoCl2·6H2O solution of different proportions was added to obtain a Co2+/GO precursor.The precursor was calcined in a N2 atmosphere at different temperatures.2 h,a porous cobalt-based compound/RGO composite material was prepared(labeled as CG-T).Analysis by XRD,XPS,and HRTEM showed that the composite was obtained at a lower calcination temperature(350,400 ℃).Co2(OH)3Cl/cobalt carbide/RGO is the main component,and its morphology is porous and amorphous;while the main component of the sample calcined at high temperature(500,600℃)is CoO/Co/cobalt carbide/RGO,which has a very different morphology from the former,is a spherical hollow structure.The hydrogen production activity of the sample obtained at low temperature is obviously higher than that of the sample prepared at high temperature.Among them,the AQY of CG-350 composite at 420 nm reached 28.3%,and the hydrogen production was 23.1%of that of single Co2(OH)3Cl and RGO.161.8 times,showing excellent hydrogen evolution synergy effect.This can be attributed to the formation of cobalt carbide and Co2(OH)3C1 by RGO and Co2+in the sample obtained at low temperature,and the excellent charge transport performance between the two.(2)Using graphene sol as raw material,add transition metal Co and Cu salt to it respectively,first hydrothermally treat at low temperature(120℃)for two hours,and then pass Ar gas at appropriate temperature(200,250 and 300℃)after calcining for 2 h,a CuO/Co(OH)2/RGO composite(labeled CCG-T)was obtained.The CCG-250 composite showed excellent photocatalytic hydrogen production activity.At a wavelength of 420 nm,The apparent quantum efficiency can be as high as 30.4%.At the same time,the complex has a small hydrogen evolution overpotential and a Tafel slope.When the current density is 10 mA·cm-2,the hydrogen evolution overpotential is only 200 mV,and the Tafel slope is 115.2 mV·dec-1.This excellent hydrogen production performance can be attributed to the uniform dispersion of Co and Cu species on the graphene level,which can effectively separate photo-generated charges and promote hydrogen production activity. |
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