Growth Regulation and Photoelectrochemical Water Splitting Properties of InGaN Nanorods

Author:Xu Zhen Zhu

Supervisor:li guo qiang


Degree Year:2019





Solar-powered photoelectrochemical(PEC)water splitting is a renewable approach to directly store abundant solar energy in the form of energy-rich(143 MJ/kg)and carbon-free hydrogen fuel,which is an attractive route for addressing the rising global energy demand and environmental issues.One dimensional InGaN nanorods(NRs)are the ideal material for PEC water splitting,whose band gap can be tuned from 0.7 to 3.4 eV by changing the In content.Despite recent interest,there are some critical problems to realize high-performance InGaN NRs-based PEC water splitting.Firstly,the spontaneous growth of InGaN NRs is inherently complicated,and the composition-and morphology-controllable growth of InGaN NRs is difficult.Secondly,due to the high density of surface states introduced in the nanorods growth process and subsequent exposure to air,electrons will be trapped in these states and recombine with photo-generated holes,which leads to a high loss of photo-generated carriers and creates a large onset potential for carrier transport.Thirdly,the fast charge recombination in InGaN NRs hinders the PEC performance of NRs,and effective charge separation needs to be realized.This thesis aims at developing efficient PEC water splitting.The complex growth mechanism of InGaN NRs,PEC water splitting performance of as-grown NRs,surface passivation of InGaN NRs for enhanced PEC performance,and construction of InGaN NRs heterojunction are systematically investigated.The main achievements are as follows:Firstly,the mechanism underlying morphology evolution and composition incorporation mechanism of InGaN NRs has been systematically investigated.The increased Ga flux enhances both the axial and the radial growth at the growth stage.However,the changed Ga flux influences not only the growth but also the nucleation of InGaN NRs.At the nucleation stage,the increased Ga flux shortens the delay time for NR formation,and prolongs the growth stage for a fixed total growth time.Those two aspects result in the increase of NR diameter and height with the supplied Ga flux.In addition,the continuous nucleation is ended much earlier due to the accelerated saturation of substrate area with the increased Ga flux,resulting in a decreased final NR density.The increased Ga flux would leads to an enhanced Ga incorporation due to the higher binding energy of Ga-N compared to In-N,and thus decrease the In content of InGaN NRs.Interestingly,the In distribution of InGaN NRs depends critically on the NR diameter along the NR growth direction,and the NRs show a morphology-dependent In incorporation.An In-assisted growth of well-separated(In)GaN NRs has also been investigated.In the In-rich condition,the supplied In flux decreases the density of(In)GaN NRs due to the increased substrate surface migration of Ga adatoms at nucleation stage.According to the theoretical calculations,the increase of In content of the NRs can enhance Ga diffusion on the NR sidewalls,which results in an increased axial growth rate of the NRs.Consequently,the coalescence between the neighboring NRs is significantly suppressed.The structural and elemental analysis shows that the(In)GaN NRs without coalescence are high crystalline quality and no large variations in the In concentration,revealing high quality of the NRs.Secondly,correlations among morphology,composition,and PEC water splitting properties of as-grown InGaN NRs have been carefully investigated.The results reveal that the photocurrent density of the InGaN NRs improves effectively with the increased total surface area,despite the In content of the NRs is gradually reduced.It can be concluded that for improving the PEC water splitting performance of the InGaN NRs,one can take action by increasing the total surface area through controlling their morphology,in addition to enhance the spectral absorption range of the InGaN NRs by increasing the In content or/and taking advantage of the radial Stark effect.Thirdly,surface passivation of as-grown InGaN NRs using H3PO4 has been investigated to eliminate the adverse effects from those of surface oxides,In surface segregation,and dangling bonds formed on the surface of NRs.Using optimized surface passivation condition,surface trapping states in as-grown InGaN nanorods can be removed.Hence,carrier recombination mediated by these surface states is weakened,as well as applied bias potential required for carrier transport is lowered.Resultantly,a significant negative shift of onset potential from 0.7 to 0.3 V vs.RHE for the treated photoanode is achieved.Moreover,the optimized H3PO4-treated photoanode exhibits a high photocurrent density of 18 mA/cm2 at1.23 V vs.RHE and a maximum applied bias photon-to-current efficiency value of 1.09%,whereas as-grown InGaN photoanode reaches only 0.64%.Last,semiconducting heterostructures has been designed with rational engineering of energy bands and interfaces to accelerate electron-hole separation.Hence,InGaN nanorods(NRs)/C3N4 heterojunction photoanode has been constructed by directly loading C3N4 on the InGaN NRs surface through a simple chemical vapor deposition method.The unique heterojunction exhibits efficient charge separation through the potential gradient and enhanced interfacial charge transfer due to the surface passivation.Eventually,the photocurrent density of the InGaN NRs/C3N4 heterojunction photoanode with loading weight ratio of 0.38%reaches up to 13.9 mA/cm2 at 1.23 V vs.RHE,which is 2 times higher than that of the pristine InGaN NRs.The applied bias photon-to-current efficiency of the designed herterojunction can achieve as high as 2.26%at 0.9 V vs.RHE,1.65 times higher than the bare InGaN NRs(1.37%).Moreover,the InGaN NRs/C3N4 heterojunction exhibits an obviously improved stability against photocorrosion due to the efficient interfacial charge transfer.In summary,after the research on growth mechanism of InGaN NRs,the PEC water splitting performance of as-grown InGaN NRs has been studied in detail.The enhanced PEC performance is achieved by surface passivation of InGaN NRs and the rational design and construction of heterojunction based photoelectrode.