3.8 Electrochemical analysis
In this portion, in order to demonstrate the significant function of NiSe2 in facilitating the electron hole separation efficiency, we performed a series of electrochemical characterizations of the MCS, NiSe2 and MCNS-10 samples. Fig. 8a shown the transient photocurrent response spectra of MCS, NiSe2 and MCNS-10 in periodic on/off lamp state. Composite catalyst MCNS-10 reveal the highest photocurrent responses, demonstrating that the existence of NiSe2promoted photogenerated charge separation rate and enhanced visible-light response ability [48]. All samples showed a decreasing trend with the increase of test time, on account of accumulation of holes [8, 24].
The LSV was often used to study the carrier characteristics of semiconductor. At higher current densities (-0.6 V), the LSV curves was shown inFig. 8b . Obviously, NiSe2 acted as co-catalyst can decrease the hydrogen evolution overpotential of MCS . This clearly shown that loading NiSe2 could increase photoinduced electrons participating in hydrogen reaction. Moreover, the Nyquist plots of EIS of MCS, NiSe2 and MCNS-10 were shown inFig.8C , the insert was the equivalent circuit diagram, where charge transfer take place in the arched high-frequency region. The diameter of the arc for NiSe2 electrode was the smallest, this was due to the excellent electrical conductivity of NiSe2 [6, 49]. Secondly, a smaller arc had been detected for composite MCNS-10 compared to that of MCS, indicating a low impedance during charge transfer.
In Fig. 8d , Mott-Schottky plots of MCS and NiSe2 were used to investigate the electrons transport acted of photocatalysis. MCS and MCNS-10 exhibits a positive slopes of the E-C-2 plots, indicating that they were characteristics n-type semiconductor [16]. The flat-band potential (Efb ) of MCS and NiSe2 could be calculated to be -0.89 and -0.64 V versus the saturated calomel electrode (SCE), respectively. According to previous reports, the conduction band (ECB ) values of n-type semiconductor was more negative about 0.1-0.2 V than theEfb , and combined with the calculation formula:ENHE = ESCE + 0.24 V [34]. Therefore, the ECB of MCS and NiSe2 were calculated as -0.85 and -0.6 eV versus NHE (normal hydrogen electrode). Because ECB value of MCS has a more negative than NiSe2, this indicate that the electron transfer direction was from MCS to NiSe2.