Tuning The Physical Properties Of Eco-friendly InP-based Quantum Dots For High Performance Photoelectrochemical Cells

Economic globalization has brought not only development but also pollution to humanity.Moreover,the energy demand increases each year.More importantly,traditional fossil fuels will be confronted with depletion,which is not conducive to the grand goal of sustainable development.Therefore,it is urgent that new clean energy is exploited to promote the continuous development of human society.Hydrogen is a clean candidate energy carrier for driving the development of the new era because it has the advantages of high energy density and only produces pollution-free water after combustion.The photoelectrochemical（PEC）driven hydrogen production technology relies on semiconductor materials and assists with a small amount of bias to achieve sustainable hydrogen production.Colloidal quantum dots（QDs）,as semiconductor materials with quantum effects,have shown great potential in photoelectrochemically driven hydrogen production technology and have great significance for large-scale application.However,common QDs used in PEC hydrogen generation,have some drawbacks such as high surface sensitivity,sluggish carrier dynamics,and even some containing Pb/Cd elements that are adverse effects for human health or the environment.Considering the sustainable development of PEC driven hydrogen production technology,InP QDs are fairly concerned as a new type of semiconductor material that is environmentally friendly,has outstanding photoelectric conversion ability and multiple synthesis methods.However,due to influences of large specific surface area and abundant surface defect states,bare InP QDs always exhibit weak stability and severe nonradiative recombination of photogenerated carriers,which has affected its further assembly into optoelectronic devices and limited the further development of InP QDs-based PEC（QDs-PEC）hydrogen generation technology.Aiming the above-mentioned issues for bare InP QDs,this article adopts various strategies to tune its electronic structure,including core-shell structure modification,doping state formation,interface layer decoration,semiconductor heterostructure construction,and passivation layer growth.It optimizes the photoelectric conversion ability of InP QDs and obtains the durable stability of corresponding devices,thereby achieving boosted separation of photo generated carriers,remission of surface defect states of InP QDs and efficient conversion of solar energy to hydrogen energy.Finally,the optimized InP based QDs were applied to PEC hydrogen production technology,and the underlying mechanisms of the corresponding device’s photoelectric conversion were systematically studied.The main research contents are as follows:1.Firstly,by tuning the growth time,InP QDs with different particle sizes are obtained;Then,ZnSe was selected as the shell of InP QDs to formed InP/ZnSe core/shell QDs because ZnSe has a low lattice mismatch with InP;Finally,the above-mentioned different InP/ZnSe QDs were combined with Ti O2 photoanodes through electrochemical deposition technology.To further reduce the photo/chemical-corrosion of the obtained QDs-based photoanode and achieve a more durable stability,the Zn S passivation layer is formed on the surface of the QDs-based photoanode through successive ionic layer adsorption and reaction（SILAR）technology.In addition,UV-vis absorption spectroscopy confirmed that the InP/ZnSe QDs/Ti O2 photoanode exhibited a broadened absorption.UV photoelectron spectroscopy（UPS）confirmed that InP/ZnSe QDs and Ti O2 can form a type II band structure that is conducive to the separation and transfer of photogenerated charges in PEC cells systems.Time resolved fluorescence spectroscopy（TRPL）of the photoanode device also confirmed the ultrafast carrier dynamics of InP/ZnSe QDs based devices.Finally,with an optimized InP core growth time of 60minutes,PEC cells based on InP60/ZnSe QDs exhibited a high photocurrent density of 4.9mA cm^-2 at a bias voltage of 0.8 V（vs.RHE）,which was 9.8 times higher than PEC cells based on bare InP QDs.2.To further optimize the photoelectric conversion ability of InP-based QDs,this section continues to advance the research on modification strategies of InP/ZnSe core-shell QDs.Using Cu ion doped InP/ZnSe（InP/ZnSe:Cu）QDs as the research subject,the capture behavior of Cu ion on photo-generated holes was investigated by tuning the concentration of Cu ions doped into the ZnSe shell to obtain the photoelectric conversion ability of InP-based QDs devices with the optimal Cu doping concentration.UPS analysis and cyclic voltammetry（CV）methods have demonstrated that the appropriate concentration of Cu⁺doping entering the ZnSe shell of InP/ZnSe core-shell QDs can capture photo generated holes and enhance the effective separation of electron-hole pairs.However,excessive introduction of Cu⁺can form hole scattering centers and hinder the transport of charge carriers.The decay curves of TRPL and electrochemical impedance spectroscopy（EIS）confirm that Cu⁺doping can not only increase the radiative recombination lifetime of charge carriers,but also effectively increase the carrier transport rate in PEC processes.Finally,the optimized InP/ZnSe:Cu0.2 QDs sensitized Ti O2 photoanode device delivered a saturated photocurrent density of 7.4 mA cm^-2 and long-term stability under standard solar irradiation(AM 1.5G,100 m W cm^-2).3.To improve the transfer ability of photoexcited carriers in InP/ZnSe QDs from InP core to ZnSe shell,continue to optimize the carrier transport channels of InP based QDs and construct a PEC cell for hydrogen generation without hole sacrificial agents.Using Ga P interface layer modified InP/ZnSe（InP/Ga P/ZnSe）QDs as the research object,the influence of gradient energy level construction on carrier transfer and separation was investigated by tuning the thickness of Ga P interface layer.Transmission electron microscopy（TEM）and inductively coupled plasma mass spectroscopy（ICP-MS）confirmed that the Ga P interface layer was introduced through cation exchange（CE）procedure but the particle size of InP/ZnSe QDs remains unchanged during the process,forming a gradient energy level structure in situ.TRPL analysis and fluorescence quantum yield（QY）demonstrated that the introduction of Ga P interface layer can regulate the optical behavior of InP/ZnSe QDs and lead to more efficient transfer of photo-generated carries from the core to shell region.Finally,under one standard solar irradiation(AM 1.5G,100 m W cm^-2),the prepared InP/Ga P/ZnSe（30 min）QDs-based photoanode exhibited a saturated photocurrent density of up to 4.1 mA cm^-2 and excellent long-term stability at a bias of 1.23 V relative to RHE in PEC cells without hole sacrificial agents as electrolytes.