Controllable Synthesis Of Ternary CuGaS₂ And Quaternary Cu-Ga-Zn-S Nanocrystals And Their Photocatalytic Hydrogen Evolution

In recent years,multinary copper-based chalcogenides have became an extending application in the field of photodiodes,photovoltaic devices photodetectors and photocatalysis due to their complex crystal structure,controllable morphology,and numerous composition elements,which can control their optical band gaps and improve effective electron-hole separation.Meanwhile,photocatalytic hydrogen evolution is the one of the important methods to produce hydrogen energy.Due to the intrinsic properties of semiconductor materials,photogenerated electron-hole pairs are generated under light conditions.Photogenerated electrons with strong reduction activity can reduce water into hydrogen.In this paper,ternary CuGaS2 and quaternary Cu-Ga-Zn-S semiconductor nanocrystals were synthesized using a colloidal method,and the formation mechanisms of the two types of nanocrystals were studied.Furthermore,photocatalytic hydrogen evolution was measured to evaluate their photocatalytic activity.The main contents of this paper are as follows:Firstly,high-quality wurtzite CuGaS2 nanocrystals were synthesized by using a one-step heating-up methods.Their morphology could be tuned from one-dimensional to two-dimensional by precise choice of surface ligands and gallium precursors.Furthermore,the formation mechanism of CuGaS2 nanocrystals was studied comprehensively by means of the temporal-evolution of the morphology,crystal structure and optical absorption results.The reaction started from djurleite monoclinic Cu31S16 nanocrystals,and then proceeded with the formation of Cu3iS16-CuGaS2 heteronanostructures accompanying inter-diffusion of Ga3+ cations,and finally the transformation from Cu31S16-CuGaS2 heteronanostructures to monophasic CuGaS2 nanorods took place with prolonging of the synthesis time.The optical bandgap and the energy level of the different-dimensional CuGaS2 nanocrystals exhibited a strong dependence on the morphology change.The theoretical calculation based on density functional theory revealed that the CuGaS2 {001} facets facilitated the charge transport rather than the {100} facets,and three electrochemical impedance spectroscopy can further support this viewpoint within experimental results.Comparing with one-dimensional CuGaS2 nanocrystals and quasi-two-dimensional CuGaS2 nanocrystals,two-dimensional CuGaS2 nanocrystals with more exposed {001} facets exhibited an attractive photocatalytic hydrogen production activity under simulated solar illumination This study demonstrates that controlling over the dimension of ?-?-?group semiconductor nanocrystals could lead to a significant improvement of the photocatalytic hydrogen evolution.Secondly,quaternary L-shaped wurtzite Cu-Ga-Zn-S alloyed nanorod can successfully be obtained by using a simple direat heating method,which exhibit an improved photocatalytic hydrogen evolution compared with the binary Cu31S16 and ternary CuGaS2 nanocrystals.To clearly characterize as-synthesized Cu-Ga-Zn-S nanocrystals,they are the uniform spatial composition distribution L-shaped wurtzite nanorods.To investigate the growth process,the Cu31S16 nanospheres can be formed as seeds firstly,and then the successive incorporation of zinc and gallium ions into Cu31S16 seeds leads to formation of Cu31S16-ZnS and Cu31S16-(Cu-Ga-Zn-S)heteronanostructures and finally results in the formation of Cu-Ga-Zn-S alloyed nanorods.The features of L-shaped wurtzite Cu-Ga-Zn-S nanorods promote the spatial charge separation and electrical transport efficiently,which contribute synergistically to improve photocatalytic activity.This study provides a new way to develop novel multinary copper-based chalcogenides with complex nanostructures for efficient photocatalytic hydrogen production.