Preparation And Photoelectrocatalytic Hydrogenation Production Property Of Cobalt Phosphite/Metal Sulfide/Titanium Dioxide Nanotube Arrays

Energy is the material basis of sustainable development of human society.However,the rapid consumption of traditional energy has caused energy shortage and environmental pollution,threatening human's production activities and social development.Therefore,the development of new clean and renewable energy is the top priority of current social development.Among the numerous renewable energies,solar energy has the advantages of abundant reserves,environment friendly,and economic features.Therefore,solar-driven photoelectrocatalytic?PEC?hydrogen evolution becomes an effective method to alleviate the problem of energy shortage.TiO₂ nanotube arrays?TNA?has the characteristics of low-cost,eco-friendly,non-toxic,stable properties and so on.In particular,its one-dimensional array structure is conducive to transfer charge,so being considered as a promising PEC material.However,low visible light utilization rate and high recombination rate of photogenerated charge limited its PEC performance.In order to improve these shortcomings of TNA,a variety of modification methods have been attempted.Narrow band gap semiconductor coupling not only improve visible light absorption of TNA but form heterostructure between coupled semiconductors with TNA,whose reasonable band alignment can effectively promote the separation of photogenerated charge.Moreover,loading cocatalyst is used to provide more reactive sites,and reduce the reaction activation energy,thus accelerating the redox reaction.Herein,in this paper,the modification of TNA based on the above two aspects was investigated.As a common narrowband gap semiconductor,metal sulfide has a good photores-ponse in the visible light region,but it was easy to be oxidized by holes,leading to its unstable catalytic performance.In addition,transition metal phosphide?TMP?is an effective electrocatalyst for water splitting hydrogen and oxygen evolution,which has the advantages of low cost and high activity and is as a trap for charge to facilit-ate the migration of photogenerated holes from metal sulfide.thus hindering the photocorrosion of metal sulfide.TMP can also be used as a cocatalyst to accelerate the reaction of PEC hydrogen production and improve the catalytic performance of composite photoanode.According to the above reasons,in this paper,two ternary composite photoelectrode CoP/CdS/TNA and CoP/Cd_0.5Zn_0.5S/TNA?CoP/CZS/TNA?were prepared,in which TNA was as the substrate to be deposited by CdS and Cd_0.5Zn_0.5S on the surface,and then loaded CoP as cocatalys.The morphology,optical propert0 ies,and PEC hydrogen production performance of the as-prepared photoelectrodes were systematically studied.The enhanced mechanism of CoP as cocatalyst about PEC activity and stability of the ternary composite photoelectrode was particularly discussed.The specific work was as follows:1.Firsty,TNA was prepared by two-step anodic oxidation method,and then CdS was directly deposited on the surface of TNA to form nanoparticles by succession ion layer adsorption reaction.Finally,CoP was loaded on the modified-TNA photoelectrode by a simple drop-coating method,thus the ternary composite photoelectrode of CoP/CdS/TNA was obtained.CdS/TNA,CoP/CdS/TNA-50,CoP/CdS/TNA-100 and CoP/CdS/TNA-150 were respectively obtained by changing the loading amount of CoP.Among TNA and modified TNA photoelectrodes,CoP/CdS/TNA-100 showed the best visible-light response and decreasing recombination rate of photogenerated carriers,which were testified by PEC tests.The photocurrent density of CoP/CdS/TNA-100 was 9.6 mA·crm^-2 at bias voltage of 1.23 V vs.RHE under visible light irradiation??>420 nm?while PEC hydrogen production rate was 447.3?mol·cm^-2·h^-1 under the identical conditions.The further analysis showed that in CoP/CdS/TNA ternary composite photoelectrode,the existence of CdS widened the absorption of visible light and enhanced the intensity of light response,thus increasing the number of photogenerated electrons significantly.Moreover,CoP,as a cocatalyst,was prone to trap photogenerated holes to realize spatially opposite separation of photogenerated carriers,effectively inhibiting recombination of photogenerated carriers.Therefore,in CoP/CdS/TNA ternary composite photoelec-trode,PEC hydrogen production rate and stability were further improved from the two aspects:increasing the number of photoelectrons and reducing the recombinati-on efficiency of photogenerated carriers.2.Cd_0.5Zn_0.5S nanoparticles were deposited on the surface of TNA substrate instead of CdS,and CoP was loaded on the composite photoelectrode to obtain CoP/CZS/TNA ternary composite photoelectrode.CZS/TNA,CoP/CZS/TNA-50,CoP/CZS/TNA-100 and CoP/CZS/TNA-150 were respectively obtained by changing the loading amount of CoP.The experimental results showed that compared to bare TNA and modified TNA composite photoelectrodes,CoP/CZS/TNA-100 had an absorption region of visible light and inhibited recombination efficiency of photogenerated carriers.In PEC tests,the photocurrent density of CoP/CZS/TNA-100 was 1_0.5 mA·cm^-2 while its PEC hydrogen production rate was 558.3 ?mol-cm^-2·h^-1 at bias voltage 1.23 V vs.RHE under visible light irradiation??>420 nm?.The analysis showed that in the CoP/CZS/TNA ternary composite photoelectrode,the presence of Cd_0.5Zn_0.5S significantly increased the number of photogenerated electrons involved in PEC hydrogen production.Moreover,CoP,as a cocatalyst,reduced the recombination rate of photogenerated carriers.In addition,the introduction of Zn²⁺ resulted in forming Cd_0.5Zn_0.5S solid solution.CoP/CZS/TNA ternary composite photoelectrode avoided the photocorrosion of photoelectrode to make itself more stalble.At the same time,due to more negative conduction band potential of Cd_0.5Zn_0.5S the ternary composite photoelectrode had stronger reduction potential that benefiting hydrogen production.As a result,PEC hydrogen production rate and stability of CoP/CZS/TNA were further enhanced.