Porphyrin/Phthalocyanine Decorated BiVO₄ For Enhanced Photocatalytic Water Splitting Properties

Artificial photocatalysis,which mimics biological photosynthesis,is one of the many innovative technologies that have been developed.It breaks down water into its component elements of hydrogen and oxygen.A semiconductor substance called bismuth vanadate（BiVO₄）is utilized as the leaf in the analogy of photosynthesis because of its visible light sensitivity.Additionally,it is environmentally safe,inexpensive to make,and has a relatively low bandgap energy of 2.4 e V.Unfortunately,the bandgap is too wide for effective PEC water splitting,the oxygen evolution process is slow,and the carrier diffusion length is short,all of which limit the overall BiVO₄ film needed for adequate light absorption.Because of the material’s limited capacity for light absorption and PEC water splitting,we developed three types of cocatalysts（MTPP,TAPC and NiCOP）to efficient and synergistically increase its PEC water splitting capacity.Firstly,we found that decorating semiconducting materials with sensitizers that have great light harvesting capabilities,such as porphyrins,is a viable strategy for creating photocatalysts that encourage electron injection and charge separation at photoelectrolytic interfaces.In this chapter,we produced three BiVO₄/MTPP composite catalysts,which are composed of metalloporphyrins（MTPP,M=Ni,Co,and Ni）.We found that the production of BiVO₄/MTPP composites increased light absorption using a range of spectroscopic techniques.A trend in PEC water splitting was observed where BiVO₄/Co TPP>BiVO₄/NiTPP>BiVO₄/Cu TPP>BiVO₄ in increasing performance tests.The LSV photocurrent density of BiVO₄ was enhanced from 1.01 m A cm^-2 to3.9 m A cm^-2 at 1.23 V vs.RHE under the same condition of AM 1.5G light irradiation.The charge injection efficacy and charge separation efficiency of the prepared samples were also determined,where they followed the mentioned trend,with BiVO₄/Co TPP experiencing the highest in both tests,exhibiting 80%and 65%,respectively.The samples’water oxidation performance using double-layer capacitance(C_dl)found BiVO₄/Co TPP had a significantly larger C_dl value of 241.3μF cm^-2 compared to that of pristine BiVO₄ at 150.8μF cm^-2.A possible PEC water splitting mechanism was proposed for the enhanced photoanode composites.Then,a tetra（amino）phthalocyanine（TAPC）was prepared and also incorporated on the surface of BiVO₄ to form a composite photoanode BiVO₄/TAPC.The photocurrent density of the pristine BiVO₄ was found to have been boosted with the introduction of TAPC from 1.7 to 3.1 m A cm^-2.An increase in surface topology was observed using SEM imaging after the addition of NiCOP compared to the pristine BiVO₄.This was further supported by the electrochemical active surface area（ECSA）test,where BiVO₄/TAPC was estimated at 130μF cm^-2 compared to 76μF cm^-2 for BiVO₄.The compared stability also showed that BiVO₄/TAPC was better than pristine BiVO₄.Overall,improved PEC water splitting was observed with the use of TAPC as a cocatalyst,and the mechanism was also proposed tailored to the proprieties of BiVO₄/TAPC.Finally,BiVO₄ was cocatalyzed with a nickel porphyrin covalent organic polymer（NiCOP）connected by phosphonitrile groups to enhance its light-harvesting and PEC water-splitting capabilities.The obtained BiVO₄/NiCOP exhibited excellent light harvesting ability,having a photocurrent density of 6.13 m A cm^-2at 1.23 V vs.RHE compared to a pristine 1.88 m A cm^-2for pure BiVO₄ at the same condition.The maximum efficiency of the photoelectrochemical cell of the applied bias photon-to-current efficiency（ABPE）values was tested and found to be 2.4%and0.5%at 0.7 V bias for BiVO₄/NiCOP and BiVO₄,respectively.The electrochemical impedance spectroscopy（EIS）measurements were consisted with the photoelectrochemical studies,which suggested that the addition of the NiCOP layer enhanced charge transfer kinetics and lowered the recombination rate.