3.3 Photoluminescence analysis
The steady-state fluorescence spectrum was used to test the fluorescence performance of the catalyst in EY solution, and the electron transfer and excited state interactions between EY and photocatalyst were discussed. As shown in Figure 8(a), the pure EY solution produces the strongest fluorescence under the excitation of 480 nm light. Different fluorescence intensity indicates different degrees of electron-hole recombination. When the fluorescence intensity is higher, the peak value is higher, showing that the number of excited electrons returned to the ground state by radiation decay transition is higher, which leads to higher electron recombination rate [42, 43]. With the addition of photocatalyst materials, the signal decreases, indicating that the recombination rate of electron-hole pairs is reduced [44], and it is also shown that the electron-hole pair recombination of the catalyst is suppressed [45, 46]. Figure S5(a) shows the steady-state fluorescence curves of Cu-MOFs@ZIF-9 catalysts with different degrees of phosphatization. The Cu-Co-2P-2 photocatalyst has the lowest fluorescence peak intensity, indicating that the recombination rate of electron-hole pairs in Cu-Co-2P-2 is most significantly reduced. After adjusting the amount of internal Cu-MOFs, the fluorescence pattern of Cu-Co-2P-x (x = 1.2.3.4) prepared under the same phosphating conditions is shown in Figure S5(b). With the increase of the mass of Cu-MOFs, the intensity of the fluorescence peak decreases at first and then increases. This indicates that thereexists a good ratio of Cu-MOFs to ZIF-9(Co) and good phosphation degree while preparing the composite photocatalyst. At the same time, in order to further test the fluorescence performance of Cu-Co-2P-2 catalyst, it was also compared with Cu-P and Co-P derived from pure Cu-MOFs and Co-MOFs. The results are shown in Figure 8(a). In Cu-Co-2P-2 composite samples, the presence of Cu3P@CoP p-n type heterojunctions greatly promotes electron transport and improves charge separation efficiency, so the fluorescence peak intensity is the lowest.
The dynamic conditions of photogenic carriers were detected by transient fluorescence, as shown in Figure 8(b) and Figure S5(c, d). The corresponding time resolved photoluminescence (TRPL) was fitted by using the three-exponential attenuation model.
\begin{equation} \mathbf{I}\left(\mathbf{t}\right)\mathbf{=\ B+}\sum_{\mathbf{i=1,\ 2,\ 3}}\mathbf{A}_{\mathbf{\text{i\ }}}\mathbf{e}^{\frac{\mathbf{-}\mathbf{t}}{\mathbf{\tau}_{\mathbf{i}}}}\mathbf{\text{\ \ \ \ \ \ \ }}\left(\mathbf{1}\right)\mathbf{;}\nonumber \\ \end{equation}\begin{equation} \tau\ \text{is}\ \text{emission}\ lifetimes;\ A\ \text{is}\ \text{the}\ \text{corresponding}\ \text{amplitude}.\nonumber \\ \end{equation}
The transient fluorescence lifetimes of Cu-P, Co-P and Cu-Co-P semiconductor catalysts are shown in Table 1. It can be clearly seen that due to the interaction between the dye molecule EY and the photocatalyst, the formed complex has weak fluorescence. At the same time, in order to gain deeper understanding the reasons for different phosphatization degrees and the difference in hydrogen production activity of various precursor ratios,instantaneous fluorescence test of Cu-Co-xP-2 and Cu-Co-2P-x (x = 1. 3. 4.) composite catalyst have been alsodone, and correlated analysis results are shown in Table S1.
The average life (τave) can be calculated by formula (2):
\begin{equation} <\mathbf{\tau}_{\mathbf{\text{ave}}}\mathbf{>}\ =\ \frac{\sum_{\mathbf{i=1,\ 2,\ 3}}{\mathbf{A}_{\mathbf{i}}\mathbf{\ }\mathbf{\tau}_{\mathbf{i}}^{\mathbf{2}}}}{\sum_{\mathbf{i=1,\ 2,\ 3}}{\mathbf{A}_{\mathbf{i}}\mathbf{\ }\mathbf{\tau}_{\mathbf{i}}}}\mathbf{\text{\ \ \ }}\left(\mathbf{2}\right)\mathbf{;}\nonumber \\ \end{equation}\begin{equation} \tau\ \text{is}\ \text{emission}\ lifetimes;\ A\ \text{is}\ \text{the}\ \text{corresponding}\ \text{amplitude}.\nonumber \\ \end{equation}
It is clear from Table 1 and Table S2 that the average lifespan of all samples varies widely. The simple EY life is 0.3579 ns, and the average life has been greatly improved after adding the photocatalyst material. The average lifetimes of Cu-P and Co-P are 0.2831 ns and 0.2884 ns, respectively. In particular, Cu-Co-2P-2 composites have the lowest average lifetime of 0.1325 ns, which indicates that Cu3P@CoP p-n type heterojunctions can enhance electron transfer and significantly reduce the recombination of photogenerated charges [46].
The electron transfer rate constant is calculated by formula (3).
\begin{equation} \mathbf{k}_{\mathbf{\text{et\ }}}\mathbf{=\ }\frac{\mathbf{1}}{\mathbf{\tau}_{\mathbf{F,s}}}\mathbf{-\ }\frac{\mathbf{1}}{\mathbf{\tau}_{\mathbf{F,l}}}\mathbf{\text{\ \ \ }}\left(\mathbf{3}\right)\mathbf{.}\nonumber \\ \end{equation}\begin{equation} k_{\text{et\ }}\ is\ electron\ transfer\ rate\ constants;\ \tau_{F,s\ }is\ short\ lifetimes;\ \tau_{F,l}\text{\ is\ long\ lifetimes.}\nonumber \\ \end{equation}
Based on the difference between the long life and short life of the carriers in the sample, the specific charge transfer rate constants (ket) of all the catalyst samples are calculated in Table 1 and Table S2. The ket of Cu-Co-2P-2 is 1.198×1010, which is the largest among all catalysts. The maximum transfer rate means the fastest photo-generated charge transfer, the lowest recombination efficiency of photo-generated carriers, and the highest photocatalytic activity of hydrogen evolution. This is consistent with the results of the hydrogen evolution kinetics.