Fabrication Of WO₃-Based Composite Photoelectrodes And Their Photocatalytic And Photoelectrochemical Properties

Semiconductor photocatalytic organic pollutant degradation and hydrogen production by photoelectrochemical（PEC）water splitting are one of the most important fields of green energy development.Tungsten trioxide（WO₃）as a kind of n-type semiconductor material has been widely researched in photocatalysis field due to its moderate band gap,appropriate band edge position and good chemical stability.However,the photocatalytic activity of traditional WO₃ is still low for degradation of pollutants because of its easy to aggregate,high recombination rate of photogenerated carriers and poor conductivity of bulk charge.In addition,the WO₃ photoelectrode suffers from high combination rate of photogenerated electron and hole pairs,slow photogenerated hole oxidation reaction kinetics and surface photo corrosion,which severely limits its practical application in the field of photoelectrochemical.In order to address these issues,this paper starts from the construction of WO₃films photocatalyst with ordered nanostructure,achieving high-efficient electron transport through controlling the morphology and constructing Z-type heterojunction,exploration of the relationship between the morphology,charge transfer path,photoelectrochemical performance of the films photocatalyst and its photocatalytic degradation activity.On this basis,the visible light absorption range is widened and the separation and transfer of photogenerated charge carriers are accelerated through the constructing p-n type heterojunction and ternary hybrid heterojunction on the surface of WO₃,and thus improve the PEC water splitting activity.The mechanism of heterojunction interface regulation on the improvement of charge separation efficiency and PEC performance is revealed.The main research contents are as follows:1.By adjusting the acidity of precursor solution and without introduction of any additives or surfactants,a series of WO₃ nanoarray films with different morphologies have been successfully prepared on FTO conductive glass substrate by one-step hydrothermal method.The effect of the amount of hydrochloric acid on the morphology regulation of WO₃ was investigated,the nucleation and growth mechanisms of the WO₃ film on FTO substrate were analyzed,and the kinetics of photocatalytic degradation of methylene blue were also studied.The experimental results showed that the obtained WO₃ films were stable monoclinic phase structure and has a high crystallinity.Among them,the three-dimensional WO₃ nanoflower with hexagonal structure exhibited the highest photocatalytic activity with the reaction rate constant is 9.51×10^-3 min^-1,and has favorable stability.According to the radical scavenger experiments,the photogenerated holes were the main active species in photocatalytic degradation reaction.It can be concluded that the improvement in photocatalytic activity could be attributed to the three-dimensionally hierarchical heterostructures,which would facilitate the charge transfer and inhibits the recombination of photogenerated electron-hole pairs.2.Firstly,the WO₃ nanorod arrays with one-dimensional structure were prepared by a simple hydrothermal method,then the Cu₂O nanoparticles were deposited on the surface of WO₃ nanorods by electrodeposition technique and ultimately constructed the Z-scheme WO₃-Cu₂O heterojunction composite photocatalysts.The effects of electrodeposition time on the structure,morphology,and photocatalytic performance of the products have been investigated in detail.Furthermore,on the basis of optimal electrodeposition time,the as-obtained WO₃-Cu₂O-120s heterojunction photocatalyst shows the highest photocatalytic activity,and the reaction rate constant is 12.57×10^-3min^-1.The Z-scheme charge transfer mechanism was proposed by the active radical quenchers and the EPR DMPO radical trapping experiments.Moreover,the results verify that the superoxide radical（·O₂^-）is the primary active species in the degradation of methylene blue.This excellent photocatalytic performance due to the enhanced light absorption capacity and the effective migration of carriers at the heterogeneous interface,as well as the redox potentials of photogenerated electrons and holes by-scheme heterojunctions.3.The layer of Cu O nanoparticles was uniformly deposited on the surface of WO₃nanorods by electrodeposition technology and heat treatment method,and the visible-light responding WO₃-Cu O p-n heterojunction photoelectrode films were fabricated for efficient photoelectrochemical water splitting to hydrogen.The effect of Cu O load on WO₃-Cu O heterojunction films was investigated.The experimental results show that the WO₃-Cu O heterojunction expands the light absorption range and exhibited light absorption intensity.Meanwhile,the WO₃-Cu O-90s heterojunction photoelectrodes achieved the highest photocurrent density of 1.84 m A?cm^-2 at 1.23 V vs.RHE.Based on the density of states,band structure and charge density difference results from density functional theory calculations,the WO₃-Cu O heterojunction can accelerate the electrons transfer at the interface and optimize the redistribution of interfacial charges.The theoretical overpotential（η）of WO₃-Cu O heterojunction is only 0.63 V,which accelerates the kinetics of water oxidation and improves the activity of photoelectrochemical water splitting.The improvement of photoelectrochemical performance was mainly due to changes the electronic properties of the space charge region in p-n heterojunction,which promote the electrons transfer and accelerate the separation of carriers.4.By controlling the interface reaction between WO₃ and Cu₂O,Cu WO₄nanocrystals were grown in situ at the interface after heat treatment,and the WO₃-Cu WO₄-Cu O ternary heterojunction photoelectrode with short-distance electron transport and multistage electron transfer was constructed.The synergistic effect between Cu WO₄ and Cu O can improve the efficiency of photon absorption,and the ternary type II heterojunction with energy band structure can accelerates the charge separation at the interface,thus improve the efficiency of photoelectrochemical water splitting.The experimental characterizations demonstrate that the ternary hybrid WO₃-Cu WO₄-Cu O photoelectrode exhibited excellent light absorption properties and highly efficient photoelectrochemical activity.The WO₃-Cu WO₄-Cu O heterojunction photoelectrode achieved a photocurrent density of 2.24 m A?cm^-2 at 1.23V vs.RHE,the onset potential shifted negatively by 110 m V compared with WO₃.Construction of type II WO₃-Cu WO₄-Cu O photoelectrode with the cascade structured band alignment,which significantly improve the efficiency of photogenerated electron-hole pairs separation and effectively reduce the recombination of photogenerated carrier,as a result in accelerate the kinetics of water oxidation on the photoelectrode surface.