Fig. 6 H2 evolution amount of (a) pure NiSe2, MCS with different weight loading ratios of NiSe2; (b) AQE of MCNS-10 in the 400-600 nm wavelength range. (c) The stability test of hydrogen production cycle of MCNS-10; (d) XRD of MCNS-10 composite catalyst after hydrogen production.
3.7 The PL and TRPL spectra
In order to study the crucial role of NiSe2 cocatalyst in the boosting the processes of charge separation, transfer and recombination in the composite material MCSN system, the all samples were tested by PL and TRPL spectra.Fig. 7a shown PL spectra of all as-synthesized catalysts with an excitation and emission wavelength of 390 and 533 nm, respectively. The no fluorescence peak was detected in NiSe2, indicating that NiSe2 could not excited by irradiation. The composite catalyst MCNS-10 exhibited the lowest PL intensity compared with other binary composites, indicating that higher separation efficiency of electron-hole pair.
The fluorescence lifetime generally refer to the average time required for an electron to transfer from an excited state to a ground state. The TRPL spectra shown inFig. 7b further indicated that the charge carriers lifetime of MCNS-10 (2.21 ns) was significantly lower than that of MCS (2.46 ns), MCNS-3 (2.4 ns), MCNS-5 (2.3 ns), MCNS-15 (2.25 ns), MCNS-20 (2.38 ns). Generally, the electron lifetime was negatively correlated with electron transfer rate. The consequences show that the electron transit time of MCNS-10 composite is short [47], this was because NiSe2 could provide additional decay channel to the excited states of the MCNS [43]. We further studied the catalyst performance from two aspects values of electron transfer rate constant (KET) and the electron injection efficiency (ηinj).
The parameters of emission decay of the samples as the shownTable 2 . It was clearly observed that KET and ηinj value of MCNS-10 were larger than that MCS and other composite catalysts, indicating that NiSe2 acted as electron traps can effectively capture electrons attached on the surface of MCS [6,45,46]. Further, three dimensional topography of NiSe2 exposed more active sites and improved the separation efficiency of electrons and hole pairs. These analyses together shown that the NiSe2 play an important role in hydrogen production experiments [25].