| As the energy requirement increasing rapidly with the development of human society,the environmental crisis caused by the consumption of fossil fuels in large quantities spurs the appetite of renewable and clean energy around the world.Being able to directly transfer the solar energy to chemical energy and producing H2,photocatalytic water splitting attracts attention worldwide.As an important part of photocatalytic water splitting,photoelectrochemical water splitting shows advantages in the separation of H2 and O2.Therefore,photoelectrochemical water splitting has the prospect of commercial application in large scaleIn order to commercialize photoelectrochemical water splitting,higher efficiency,better stability and lower cost are needed.Extending the absorption range,increase the reactivity and suppressing the charge carrier recombination rate are the effective ways to promote photoelectrochemical water splitting efficiency.Choosing the semiconductors with narrow band gap can increase the light absorption dramatically.Depositing chemical stable materials on the surfaces of photoelectrodes is a common method to make the photoelectrodes more durable.To enhance the kinetic of water splitting reactions,the deposition of co-catalyst is essential,and this can also improve the stability to some extent.It is widely accepted that the high resistances of the semiconductors will severly suppress the photoelectrochemical water splitting capabilities.Thereofore,we focus on the transport of charge carriers,and investigate the effects of ferroelectric and heterojunction on the separation of charge carriers.In our study,LiNbO3 single crystal thin films and BiVO4/Bi2O3 heterjunction were used as models,and our main contents are as followsIdentifying the influences of ferroelectric polarization on the charge carrier separation of LiNbO3 photoanodes.We studied(0 0 1)orientated LiNbO3 single crystal thin films with different polarization directions.In photoelectrochemcial water oxidation testing,we found that the c-LiNbO3 exhibited a better performance than the c+ LiNbO3,and their photocurrents were 153.6 and 47.3 μA cm-2 at 1.23 V vs.Reversible Hydrogen Electrode(RHE),individually.The following Kelvin probe force microscopy and open circuit potential testing revealed that the c-LiNbO3 has an advantage in separating charge carriers over the c+LiNbO3.Mott-Schottky testing was aimed at analyzing space charge region,which is essential for charge carrier separation.The results showed that the c-LiNbO3 had a wider space charge region when contacting with electrolyte.In electrochemical impedance spectroscopy measurement we found that the bulk transport resistance of the c-LiNbO3 was much lower than that of the c+LiNbO3.Identifying the effects of BiVO4/Bi2O3 heterjunction on the charge carrier separation of BiVO4 photoanodes.We applied hydrothermal alkali treatments to BiVO4 photoanodes aiming at forming BiVO4/Bi2O3 heterojunction at the surfaces.Obvious loss of vanadium was detected through X-ray photoelectron spectroscopy testing,and a stronger rectification effect was observed in I-V testing.The following photoelectrochemical Na2SO3 oxidation testing showed that the charge separation efficiency of the 2-hour treated BiVO4 sample was higher than the as-prepared BiVO4 sample.When applied a bias of 1.23 V vs.RHE,their photocurrent densities were 1.51 and 1.23 mA cm-2,respectively.In addition,the onset potential decreased owing to the treatment.Open circuit potential,Mott-Schottky and electrochemical impedance spectroscopy testing results revealed that hydrothermal alkali treatment could promote both the charge separation and the charge transport of the samples.Moreover,this method has a similar effect on the WO3/BiVO4 samples,as the onset potential exhibited a cathodic shift of 90 mV after a 2-hour treatment. |
