3.2 Photocatalytic hydrogen evolution activity of the catalyst
The photocatalytic hydrogen production reaction of the catalyst was performed in a reaction system of 15 vt% triethanolamine as a sacrificial reagent. The use of EY sensitizer enhances the catalyst’s ability to absorb light. In the dark state (without light), no hydrogen was generated in all samples. The activity ratio experiments of composite photocatalysts with different ratios and degrees of phosphatization were performed in TEOA solution at pH = 10. It can be clearly seen from Figure 6(a) that Cu-P and Co-P produced by complete phosphation of Cu-MOFs and ZIF-9(Co) show a certain photocatalytic hydrogen release activity, and the activity of Co-P is better than that of Cu-P. The photocatalytic activity of Cu-Co-xP-2 (x = 1, 2, 3, 4 .) composite photocatalysts was obtained from Cu-MOFs@ZIF-9(Co) composites with different degrees of phosphating treatment, as shown in Figure 6(b). Under EY sensitized conditions, the in-situ phosphation was used to control the phosphorization of the Cu-MOFs@ZIF-9(Co) core-shell structure. The results show that the photocatalytic activity of the composite catalyst constructed by introducing a certain amount of P on the surface of Cu-MOFs@ZIF-9(Co) has been significantly improved. The activity of Cu-Co-1P-2 and Cu-Co-2P-2 samples was significantly improved during slight phosphating. However, after further phosphating, the activity of Cu-Co-3P-2 and Cu-Co-4P-2 composite photocatalysts began to decrease. Combined with XRD (Figure 2(c)), it can be seen that the production of CoP on the surface of Cu-MOFs@ZIF-9(Co) and the exposure of unphosphorylated Cu-MoFs on the inner layer are the main reasons for the change in the activity of the composite photocatalyst. In order to adjust the internal and external phosphating ratio of Cu-MOFs@ZIF-9(Co) core-shell structure, Cu-Co-x (x = 1, 2, 3, 4.) materials with different Cu-Co ratios were constructed by adjusting the amount of Cu-MOFs. Cu-Co-2P-x (x = 1, 2, 3, 4.) photocatalyst was obtained under the same phosphating treatment. As shown in Figure 6(c), the hydrogen-producing activity of Cu-Co-2P-x (x = 1, 2, 3, 4.) composite catalysts with different contents of Cu-MOFs was explored. The highest hydrogen-producing activity was the Cu-Co-2P-2 photocatalyst. With the increase of Cu-MOFs content, the phosphatization degree of the whole sample also increased, so there is almost no Cu-MOFs peak in Cu-Co-2P-4. It can be seen that the excellent photocatalytic activity of Cu-Co-2P-2 composite catalyst can be derived from two aspects, on the one hand, the synergistic effect of CoP and Cu3P,which were derived from MOFs phosphation, play an important role. On the other hand, because the nucleated Cu-MOFs is not completely phosphated, it retains the three-dimensional structure as a reaction carrier. We further explored the effect of pH of system on the catalytic activity of Cu-Co-2P-2 composite photocatalyst. It can be seen from Figure 6(d) that the Cu-Co-2P-2 composite photocatalyst exhibits different selectivity to the reaction environment and has the best activity at pH = 10. This shows that a mild alkaline environment is the most favorable factor for a hydrogen production system [29]. The system with extra acid orbase is not conducive to theprocess of the hydrogen precipitation reaction [30]. This is because the neutral solution contains a large amount of H+, which may cause the protonation of the triethanolamine solution and serious decrease of the electron donor effectiveness [38, 39]. When the reaction conditions become strongly basic, the thermodynamic driving force for hydrogen evolution is obviously insufficient due to the lack of H+ [40]. In addition, the adsorption of EY molecules on the photocatalyst is also affected by the solution pH of the reaction system.