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.