Surface And Interface Modification Of CuInS₂ Colloidal Quantum Dots For Light-driven Hydrogen Production

In order to meet the increasing demand for sustainable and clean energy in human society,the light-driven hydrogen production technology provides a promising approach,which can directly convert solar energy into hydrogen with the highest energy density per unit mass and clean characteristics.Light-driven hydrogen production technology mainly includes photoelectrochemical and photocatalytic hydrogen production,both of which rely on semiconductor materials driving light-energy conversion.Colloidal quantum dots（QDs）have unique optoelectronic properties due to their quantum effects.In addition,due to their high surface atomic proportion and specific surface area,their surface effects are more pronounced,regarded as potential semiconductor materials for the development of light-driven hydrogen production technology.However,the most commonly used QDs at present are binary II-VI QDs containing toxic heavy metals（such as Cd Se,Pb S,etc.）.Considering the future scale and environmental pollution issues in the field of light-driven hydrogen production,as a new type of environmentally friendly QDs material,ternary I-III-VI₂ Cu In S2 QDs have become a key focus in the research field.Unfortunately,the stability of CuInS₂ QDs are relatively poor,and due to the diverse elemental composition,there are a large number of defects formed on the surface of the QDs during the synthesis process,which lead to the recombination of photo-generated carriers on their surface.All of above limit the further development of CuInS₂ QDs for light-driven hydrogen production.This dissertation addresses the above issues in CuInS₂ QDs and adopts various surface and interface modification strategies,including core-shell structural design,passivation layer modification,surface ligand optimization,semiconductor heterostructure construction,co-catalyst loading,and defect engineering.These surface and interface modification strategies optimize the utilization of solar energy,photoelectric performance and stability of CuInS₂ QDs and their optoelectronic devices,achieving effective regulation of defect states and efficient separation of photo-generated charges.Finally,the optimized CuInS₂QDs and their optoelectronic devices were applied to construct photoelectrochemical and photocatalytic hydrogen production systems.Moreover,the performance of light-driven hydrogen production and optimization mechanism were systematically explored.The main research contents are as follows:1.Near-infrared Se-doped CuInS₂ QDs（Cu In Se S）were synthesized.By constructing the Zn S surface passivation shell layer to regulate the band structure of Cu In Se S QDs,the core-shell Cu In Se S@Zn S QDs can sensitize the N-type semiconductor Bi VO₄ photoanode.On the one hand,the Zn S shell can passivate the surface defects of Cu In Se S QDs,and thus suppress the non-radiative recombination process caused by defects.On the other hand,the passivation shell layer can also adjust the band edge position of Cu In Se S QDs,thereby forming a suitable energy band match with Bi VO₄photoanode,which can construct a photo-generated hole transfer channel for water oxidation.In addition,X-ray photoelectron spectroscopy（XPS）revealed the existence of chemical bonding contact between the QDs and Bi VO₄.Ultraviolet photoelectron spectroscopy（UPS）analysis revealed that there is a type-II heterojunction between the QDs and Bi VO₄,which is conducive to the separation and transfer of photo-generated charges.Femtosecond transient absorption（fs-TA）and fluorescence（PL）spectra also proved the existence of ultrafast charge transfer between the QDs and Bi VO₄.Finally,the constructed photoelectrochemical cell achieved a photocurrent density of 3.17 m A cm^-2under a bias voltage of 1.23 V vs.RHE and standard sunlight irradiation.Compared to the Bi VO₄ photoelectrode without modification of QDs,the sensitized Bi VO₄-QDs photoelectrode shows an outstanding improved hydrogen production performance by nearly four times,and its stability for two-hour continuous hydrogen production is also about three times improved.2.Based on the first work,continue to explore the universality of CuInS₂ QDs-sensitized photoelectrodes and construct a bias-free photoelectrochemical hydrogen production system.The cationic Zn-doped CuInS₂ QDs（ZCIS）was prepared as the research subject.The dual functional effects were investigated by adjusting the surface ligands on QDs,which sensitized both P-type semiconductor Cu₂O photocathode and Bi VO₄ photocathode.UPS analysis and density functional theory（DFT）calculations have demonstrated that ZCIS QDs can form a type-II heterojunction with N-type Bi VO₄photocathode and a P-N junction with P-type Cu₂O photocathodes,thereby promoting photo-generated charge transfer.PL spectroscopy and electrochemical impedance spectroscopy（EIS）show that the functional groups of the surface ligand affect the contact between the QDs and the metal-oxide photoelectrode.The monodentate ligand dodecyl mercaptan（DDT）promotes the formation of direct contact between the QDs and metal-oxide photoelectrode,while the bidentate ligand mercaptopropionic acid（MPA）hinders the contact between the QDs and the metal-oxide photoelectrode just with indirect contact through organic ligand.At last,the optimized tandem BVO-ZCIS_DDT‖Cu₂O-ZCIS_DDTQDs-based composite photoelectrode achieved a solar-to-hydrogen energy conversion efficiency(η_STH)of 0.59%under bias-free conditions and can stably produce hydrogen for up to 120 minutes.3.On the basis of the ZCIS QDs optimized in the second work mentioned above,V_Cu-high ZCIS QDs rich in copper vacancy defects(V_Cu′)were constructed by changing the copper-deficient synthesis environment and the doping amount of divalent Zn²⁺.The V_Cu′and high valent copper defects（Cu*）on the surface of the QDs were successfully regulated,including their defect concentration,defect distribution,and defect-defect interaction.Transmission electron microscopy（TEM）and electron paramagnetic resonance（EPR）experiments confirmed the existence of V_Cu′.DFT result and TEM data demonstrated that V_Cu′defects preferentially form in the face of the QDs rather than on the edge.In addition,XPS and PL spectra can determine the presence of high valence Cu*defects in ZCIS QDs.DFT calculations shown that the distribution of high valence Cu*defects will broaden in band as the concentration of V_Cu′defects increase.The fs-TA spectrum revealed that V_Cu′can capture holes,and thus suppress ultrafast（～178 fs）hot electron Auger process,decouple electron-hole pairs.Under the influence of phonon-bottleneck effect,the Cu*defects can generate long-lived（～90 ps）hot electrons through photoinduced absorption,which is helpful for photocatalytic hydrogen production.In terms of catalytic mechanism,the high valence Cu*defect is close to the Fermi level,which increases the mobility of photo-generated charge.Gibbs free energy calculations showed that the on-edge high valence Cu*,as the catalytic site,contribute to reduce the kinetic energy barrier of hydrogen evolution reaction.Finally,the optimized QDs-based photocatalyst achieved a superior photocatalytic hydrogen production rate of 50.4 mmol g^-1 h^-1 without any noble-metal loading,and could operate continuously and stably for 24hours under standard sunlight irradiation.