Optimization Construction Of CQDs/Au/TiO₂ Composite Photoelectrode And Its Characteristics Of Photoelectricphotothermal Hydrogen Production

Photoelectrochemical decomposition of water（PEC）is a promising method for hydrogen production using solar energy,and the preparation of high-performance photoanode materials is an important step to achieve efficient PEC decomposition of water.Among them,taking TiO2 semiconductor as the substrate and supporting sensitizer to expand the light absorption range of composite photoelectrode and promote carrier transfer is one of the current research hotspots.Meanwhile,the role of photothermal effect produced by sensitizers such as plasmon metal in PEC reaction system has gradually attracted attention.In this dissertation,TiO2 nanorods array electrode was co-modified with Au nanoparticles（Au NPs）and carbon quantum dots（CQDs）,which have both photoelectric and photothermal effects.Combined with experiments and simulation,the optimal loading mode of nanoparticles,the local characteristics of photothermal effect in PEC system,and photoelectric and photothermal synergistic mechanism of Au NPs and CQDs were investigated.The main work and results of this dissertation are summarized as follows:The construction and optimization of TiO2 composite photoelectrode modified with Au NPS and CQDs were studied.Firstly,the electric field was simulated by COMSOL based on finite element method,and the optimal metal loading method was found in theory.Experimentally,Au NPs with different particle sizes were deposited on TiO2 nanorods by adjusting the ultraviolet reduction time.Among them,The Au/TiO2 electrode with a photoreduction time of 4 min was closest to the above ideal distribution and had the best corresponding photoelectric performance.Carbon quantum dots were prepared by one-step basic-assisted electrochemical method and deposited on TiO2 by impregnation method.The effect of impregnation time on the photoelectric properties of CQDs/TiO2 electrode was studied.On this basis,CQDs/Au/TiO2（CAT）and Au/CQDs/TiO2（ACT）composite photoelectrodes co-modified with Au NPs and CQDs were prepared by changing the deposition sequence and adjusting the photoreduction and impregnation time.Finally,the CAT（3min/12h）sample has the best performance,and the photocurrent reaches 1.31 m A/cm²,which is5.2 times of that of pure TiO2（0.25 m A/cm²）.The synergistic effect and charge transfer mechanism of Au NPs and CQDs were investigated.Hydrogen production rate of the samples was measured under simulated sunlight（AM 1.5）,and UV-Vis DRS,I-T and LSV characterization tests were carried out.It was found that the photoabsorption activity of Au NPs and CQDs modified separately and co-modified was better than that of TiO2.CAT（3min/12h）has the best vision-near-infrared light efficiency,which is promoted by the up-conversion fluorescence characteristics of CQDs and the Local Surface Plasmon Resonance（LSPR）effect of Au NPs.Through the analysis of charge transfer performance test and electric field simulation of EIS,IMPS and PL,the thesis that CQDs acts as hole collector in PEC system is put forward,and the reason for the low photoelectric performance of ACT is explained.In CAT,Au NPs injected hot electrons into TiO2and captured photogenerated electrons through the Shottky barrier,while CQDs was responsible for collecting holes.The two cooperated to achieve the highest charge separation efficiency.For accurate characterization of thermal effects in the intensity and characteristic in the PEC system,a macro equivalent simplified temperature field model was innovatively constructed based on the collective photothermal effect theory.By considering the structure characteristics of PEC system and photoelectrode,the simulation results are consistent with the experiment to the maximum extent,which solves the problem that the previous photothermal simulation research is far away from the real experimental conditions.The simulation results confirm that Au NPs and CQDs have significant local photothermal effects in PEC system,and can effectively affect the local water environment and TiO2 substrate.Time domain temperature simulation shows that the CQDs/Au/TiO2 photoelectrode increases from25 to 45.5°C in 0-60 min,which is more accurate than the temperature measurement result of thermal imager.On this basis,through I-t,LSV,EIS and PL tests of composite photoelectrode at different temperatures,the influence of photothermal effect on the photoelectric activity of small polaron TiO2 semiconductor and carrier migration rate in PEC system was explored.The catalytic effect of photothermal effect on REDOX water decomposition was analyzed from the perspective of kinetics and thermodynamics.Considering the photophotothermal synergistic effect of Au NPs and CQDs,the hydrogen evolution rate of CAT（3min/12h）sample under AM 1.5 light is 641.9μL h^-1 cm^-2,5 times that of TiO₂(135.2μL h^-1 cm^-2),and it has good stability.