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.