Fabrication And Photoelectrochemical Water Splitting Study Of[001]-Oriented Sb₂Se₃-based Thin-films

Photoelectrochemical(PEC)water splitting directly converts solar energy into clean and sustainable hydrogen,representing a promising but challenging technique for solving the world's energy crisis.Antimony selenide(Sb₂Se₃)has emerged as a promising photocathode material because of its electronic band structures suitable for visible light absorption and water splitting.Sb₂Se₃ exhibits anisotropic physical and chemical properties due to its anisotropic crystalline structures.Controlling crystallographic orientation of Sb₂Se₃ film is highly desired,but remains a challenge.In the present work,we designed and fabricated highly[001]-oriented Sb₂Se₃ films as photocathodes for PEC water splitting.[001]-oriented Sb₂Se₃ thin-films possess the highest carrier mobility along[001]orientation and low recombination loss between self-passivated van der Waals gaps during carrier extraction.Therefore,this work presents a new insights into adjusting the[001]-orientation for high-efficient PEC water splitting system.The main contents in this paper are as follows:(1)Layered Sb₂Se₃ consists of infinite chains of repeating Sb₄Se₆ unit stacked along the c-axis via Sb-Se covalent bonds whereas linked by van der Waals force along the a-axis and b-axis.It usually tends to grow along[hk0]orientation with low surface energy.The[001]-oriented growth that efficiently enhances charge transport is often considered unstable due to high surface energy.Here,we propose a universal growth method to fabricate Sb₂Se₃ thin-film with preferential[001]-orientation on various substrates.Combined experiments disclosed the kinetically controlled growth mechanism of preferential[001]orientation.At high temperature,the selenization reaction occurs much faster than the diffusion of Se atoms into the film,thereby making Se diffusion as the rate-determining step.Due to the anisotropic structure of Sb₂Se₃,Se diffusion along the[001]-orientation through van der Waals gaps is expected to be much faster than diffusion across the van der Waals gap of the[hkl,l=0]and[hkl,l?0]orientation due to shortest travel distance.Accordingly,the nanoribbons naturally oriented perpendicular to the substrate,exposed the shortest pathway of van der Waals gaps for faster Se diffusion and selenization reaction.(2)We have successfully fabricated high optoelectronic quality[001]-oriented Sb₂Se₃ film by selenization strategy,as a promising photocathode,exhibiting a photocurrent density of-12.2 m A cm^-2 at 0 V vs.RHE.The combined experiments verified and quantified the positive correlation between highly[001]-oriented Sb₂Se₃film and carriers extraction efficiency,demonstrated that vertically oriented ribbon growth provided perfect carrier-migration channel and defect-free benign GBs among ribbons minimizing the bulk recombination rate,thus enabling an exceptionally high efficiency for PEC water splitting.Moreover,post annealing treatments eliminate the excess Se defects from preparation process and maximize the advantage of ribbon-assisted carrier transport.The material design and synthesis strategy demonstrated in this work not only demonstrate a potential application of[001]-oriented Sb₂Se₃ for PEC water splitting but also provide an new platform for designing future functionalized optoelectronic devices.(3)A TiO₂/Sb₂Se₃ buried junction was successfully fabricated by atomic layer deposition.Buried junction can provide a larger band bending at Sb₂Se₃ region,leading to a larger built-in electric field.This can efficiently improve charge separation.Moreover,20 nm ultrathin TiO₂ does not block electron transport and cause significant optical loss.Thus,TiO₂ deposition can simultaneously improve photocurrent density and fill factor.the optimized TiO₂/Sb₂Se₃ photocathode exhibits an impressive photocurrent density of-20.5 m A cm^-2 at 0 V vs.RHE and a maximum photocurrent density of-28 m A cm^-2 at-0.2 V vs.RHE under 1-sun AM 1.5 G illumination.(4)We fabricated S doped Sb₂Se₃ nanowire arrays with gradient S concentration(grad-S:Sb₂Se₃).Gradient S doping creating a cascade valance band structure in Sb₂Se₃simultaneously enhances its onset potential and fill factor.Light-trapping effects in nanowire geometry improves the light harvesting efficiency.A further deposition of a thin TiO₂ layer is utilized to passivate the detrimental surface defects and form buried junction with grad-S:Sb₂Se₃.By employing the prepared grad-S:Sb₂Se₃ nanowire arrays as photocathode for water reduction,we achieved a high onset potential of 0.42 V versus RHE.a fill factor of 34%and a record high half-cell solar-to-hydrogen conversion efficiency of 2%from the Sb₂Se₃-based photocathodes.