Construction Of SnX₂（X=S,Se） Based Photoanode And Its Photoelectrocatalyic Performance

Energy crisis and Environmental pollution are becoming two major global problems in the21st century,and the whole world is facing unprecedented challenge.Exploring and developing the earth’s abundant,low-cost,non-toxic and pollution-free,green and environment-friendly,renewable energy is one of the effective way to solve the deterioration of human living environment,which is of long-term and strategic significance.Hydrogen with high energy density is renewable clean energy,and has attracted much more attention due to its advantages of easy storage,transportation and wide applicability.Photoelectrochemical technology has been regarded as the cheapest,efficient,convenient and advanced tenchnology,which rapidly converts solar energy into chemical energy by oxidation/reduction reactions between photoanode and photocathode,and then stores in the chemical bonds.This process is similar with photosynthesis in nature.The efficiency,stability and repeatability of photoelecrode material is one of the hot issues in photoelectrochemical processes.Semiconductor photoelectrochemical processes mainly involve light absorption,charge separation and migration,and redox reactions at surface.So it is necessary to develop photoelectrode structures with the wide spectral response,fast charge carriers separation and efficient chemical reactions.Among many semiconductor materials,SnS₂ and SnSe₂ in the family of IV-VI group compounds show the great potential and application prospects in the field of photoelectric catalysis because of their unique layered structure with fast charge carrier mobility,good photoelectric response to UV-Vis-NIR spectra,and high photoelectric catalytic efficiency.In this thesis,the research objects are mainly to build a highly efficient SnX₂（X=S,Se）based photoanode and explore the charge transportation mechanism.By strategies of optimizing the morphology,controlling the crystal plane,constructing the heterojunction and introducing the defect level,they are effective to expand the light spectral absorption,enhance the ability of trapping light,accelerate photogenerated carrier separation and migration,promote the electrode/electrolyte interface chemical reaction rate,and improve the photoelectric catalysis efficiency of photoelectrode,further clarifying the transportation mechanism of carrier and the enhancement mechanism of photoelectrochemical performance.Then,SnS₂ and SnSe₂ based photoanodes are applied to photoelectrochemical water splitting and photoelectrocatalytic degradation of organic pollutants（Such as MB,RhB and Cr（VI））,respectively.The specific innovative research contents are as follows:（1）Novel 2D/1D SnS₂ NSs/TiO₂ NTs heterojunction photoanode combined with 2D SnS₂NSs and 1D TiO₂ NTs structures was constructed by anodic oxidation and solvothermal method for photoelectrochemical water splittig.2D/1D Sn S₂ NSs/TiO₂ NTs heterojunction prepared at180 ^oC（ST-180）presents the photocurrent density of 1.05 mA cm^-2 at 1.23 V（vs.RHE）,which is 4.6 times higher than pure TiO₂ NTs(0.23 mA cm^-2).Furthermore,the generation rates of H₂and O₂ for ST-180 photoanode are 47.2 and 23.1μmol cm^-2 h^-1,and the corresponding faraday efficiency of about 80.1%and 78.3%.After analysis,the main factors for the enhancement of photoelectrochemical performance are as follows:（i）the trap-like 2D SnS₂ NSs on the top can reduce the light reflection at materials surface and increase the light transmission path,so as to achieve efficient light harvesting and enhance absorption efficiency of light;（ii）the electron-transmission channel-like 1D TiO₂ NTs on the bottom can effectively achieve the fast and directional transmission of photogenerated electrons,enhancing the efficiency of electron migration;（iii）the inner electric field（E_ξ）at the 2D/1D SnS₂ NSs/TiO₂ NTs interface can promote the separation efficiency of photogenerated electron-hole pairs,and synergistically promote SnS₂ NSs/TiO₂ NTs heterojunction to achieve efficient photoelectrochemical water splitting.This work provides new research ideas and experimental basis for the design of novel heterojunction structures.（2）Strong oxidizability 0D/2D SnO₂ NPs/SnS₂ NSs Step-heterojunction（S-type）was innovatively constructed by combining solvothermal and in-situ oxidation methods for photoelectrochemical water splitting.In N₂/H₂（95:5）atmosphere,the surface and edges of 2D SnS₂ NSs annealed at 400 ^oC（SS-400）appear a proper amount of 0D SnO₂ NPs,which significantly improves the photoelectric performance and photoelectrochemical water splitting efficiency.The steady-state photocurrent density of SS-400 is 0.33 mA cm^-2 at 1.23 V（vs.RHE）,which is about 1.9 and 1.2 times higher than SS-300(0.17 mA cm^-2)and SS-500(0.27 mA cm^-2).Furthermore,the SS-400 shows the H₂ and O₂ evolution of 5.5 and 2.7μmol cm^-2 h^-1,and the corresponding faradaic efficiencies of around 89.4%and 87.7%,respectively.Comprehensive analysis the catalytic performance enhancement factor is 0D/2D SnO₂NPs/SnS₂ NSs Step-heterojunction,which with the novel S-type charge transport mechanism promotes the recombination of electons with weaker reduction ability in CB of SnO₂ and holes with weaker oxidation ability in VB of SnS₂,so as to enhance the separation of holes with stronger oxidation ability in VB of SnO₂ and electrons with stronger reduction ability in CB of SnS₂.Therefore,the strong oxidizability of SS-400 photoanode enhances the chemical reaction rate at the surface/interface.（3）The light absorption and carriers migration characteristics of Al₂O₃/SnS₂ composite heterojunction were enhanced by the effect of Al₂O₃ on the high reflectance of visible light and the defect energy level.The carrier transport mechanism and performance enhancement factors are analyzed by studying the influence of Al₂O₃ contents on the optical,photoelectric and catalytic performances of Al₂O₃/SnS₂ composite heterojunction.After testing,Al₂O₃ mass ratio of 11%-Al₂O₃/SnS₂（AOSS-11）composite heterojunction has the best photoelectrochemical degradation performance of MB.Under visible light（λ≥420 nm）and 0.49 V（vs.Ag/AgCl）bias,AOSS-11 shows the highest photocurrent density of about 37μA cm^-2 and photoelectrocatalytic efficiency of around 85.9%,which are about 1.23 and 1.18 times than SnS₂ NSs,respectively.The mian factors of performancem enhancement of Al₂O₃/SnS₂ heterojunction are as follows:（i）Al₂O₃ with high reflectivity（～98%）promotes light multiple reflection and absorption for Al₂O₃/SnS₂ heterojunction under the appropriate ratio of Al₂O₃ and SnS₂,and improves the utilization efficiency of light;（ii）the defect level of Al₂O₃ captures the photogenerated electrons,which enhances the separation efficiency of electron-hole pairs;（iii）Al₂O₃ increases the surface area of Al₂O₃/SnS₂ heterojunction,which enhances the migration efficiency and chemical reaction rate of carriers,further realizing the efficient and stable PEC degradation of MB.（4）Based on the wide spectral response characteristic of SnSe₂,high-crystalline 2D SnSe₂/FTO photoanode was prepared by Chemical Vapor Deposition.The distances（d）from FTO to SnCl₄·5H₂O and the carrier-gas flowing rates（f）are mainly influence factors to the purity,crystallinity,and（001）/（101）crystal plane exposure ratio of 2D SnSe₂,further exploring the photoelectrochemical simultaneous degradation for RhB and Cr（VI）.When d=10 cm and f=50 sccm,the photocurrent densities of 2D SnSe₂/FTO photoanode are 0.32（full spectrum）and 0.24（λ≥420 nm）mA cm^-2 at 1.23 V（vs.RHE）,respectively.The maximum ABPE efficiency of 2D SnSe₂/FTO（d=10 cm,f=50 sccm）is 0.04%,which is 2.4 times than 2D SnSe₂/FTO（d=12 cm,f=50 sccm）.The measurement revealed that the main factors for enhancing the photoelectric performance of 2D SnSe₂/FTO photoanode are as follows:（i）purity and crystallinity of 2D SnSe₂/FTO photoanode;（ii）Se^2-defect reduces the band gap of SnSe₂and broadens the spectral absorption range to near-infrared light;（iii）the VB defect of SnSe₂captures the photogenerated holes,and enhances the separation efficiency of carriers;（iv）Se^2-defect provides more reactive sites and accelerates the chemical reaction rate at surface/interface;（v）the suitable（001）/（101）crystal plane ratio of 2D SnSe₂.This work provides an important experimental basis for the preparation of high-crystalline 2D SnSe₂/FTO photoelectrode,and has certain scientific reference value.（5）Combining the directional transmission characteristic of TiO₂ NTs and the wide spectral response characteristics of Se and SnSe₂,the stable of Se/TiO₂,SnSe₂/TiO₂ and Se/SnSe₂/TiO₂ multiple heterojunctions were constructed and applied in photoelectrochemical degradation of Rh B and Cr（VI）mixed solution,so as to explore the carrier transport mechanism and photoelectrochemical degradation mechanism between structures.Through the optical,photoelectric and catalytic performances measurement,the optimal photocurrent density of Se/SnSe₂/TiO₂ multiple heterojunction is 0.94 m A cm^-2 at 1.23 V（vs.RHE）,which is 1.7,1.4and 1.3 times than TiO₂ NTs,Se/TiO₂ and SnSe₂/TiO₂.The optimal catalytic efficiency of simultaneous degradation of RhB and Cr（VI）by Se/SnSe₂/TiO₂ heterojunction are 95.3%and72.6%,respectively.After analysis,the main factors for enhancing the catalytic performance of Se/SnSe₂/TiO₂ multiple heterojunction are as follows:（i）broadening the absorption spectrum range and increasing the absorption intensity,further enhancing the light absorption efficiency;（ii）exciting more photogenerated electron-hole pairs and extending the carriers life;（iii）the inner electric field（E_ξ）enhancing the separation efficiency of electron-hole pairs at the interface;and（iv）increasing the specific surface and active sites numbers,further accelerating the chemical reaction rate at surface/interface.