1. Introduction
Facing the excessive use of fossil energy and the increasingly serious environmental pollution problem, the use of infinite solar energy is a possible and promising solution [1, 2]. Since Fujishima and Honda’s research on the photoelectrochemical hydrogen production of titanium dioxide (TiO2) photoanode in 1972, semiconductor-based photocatalysts have received widespread attention worldwide [3]. And many photocatalysts have been developed, such as TiO2[4], CdS [5, 6], Zn0.5Cd0.5S [7, 8], BiVO4 [9], g-C3N4 [10, 11], MOFs and their derivatives [12]. However, the light conversion efficiency and photocatalytic performance have not improved significantly. In order to synthesize advanced high-efficiency photocatalytic materials, the research interest of semiconductor materials based on MOFs and its derivatives is increasing.
As an kind of emerging material, metal organic frameworks (MOFs) are important and potential materials in heterogeneous catalytic reactions due to its rich pore structure, large specific surface, high structural diversity, adjustable chemical properties, and highly dispersed metal sites [13, 14]. MOFs are used as precursors for the synthesis of MOF-based hybrid structures, which will provide more opportunities for the comprehensive conversion and application of MOFs and develop more unique potentials about MOFs. Low-temperature phosphating of different kinds of MOFs can lead to excellent performance of transition metal phosphides, such as Ni2P [15, 16], CoP [17], FeP [18, 19] and NiCoP [20]. These phosphides, as n-type semiconductors, are favorable for accepting photo-generated electrons. In previous research reports, transition metal phosphides were mainly used as co-catalysts to enhance the photocatalytic H2release to improve the separation and transport efficiency of electron-hole pairs [21, 22, 23]. In fact, the transition metal phosphide alone exhibits better photocatalytic performance under dye sensitization conditions, mainly due to its narrow band gap structure and strong light absorption capacity [24, 25].
A promising approach to enhance the catalytic performance of MOFs and their derivatives is to change MOFs into a core-shell structure [26]. Here, we have developed a novel in-situ growth scheme that combines the advantages of ZIF-9 and Cu3(BTC)2, and uses Cu-MOFs@ZIF-9(Co) core-shell material as a precursor to derive Cu3P@CoP composite catalyst. Since Cu3P is a p-type semiconductor and CoP is a n-type semiconductor, the Cu3P@CoP composite catalyst obtained from Cu-MOFs@ZIF-9(Co) core-shell material not only has a layered structure, but also builds a p-n heterogeneity at the interface. In order to understand this phosphation more deeply, we control the degree of phosphation of Cu-MOFs@ZIF-9(Co) material on the one hand, and adjust the ratio of Cu to Co by changing the content of Cu-MOFs to explore its reasons and rules for the formation of photocatalytic activity. The unique structure and composition of Cu3P@CoP can promote charge separation and migration, provide large surface area and rich active sites to drive water photolysis. The results show that the optimized Cu-Co-2P-2 photocatalyst exhibits generation activity of 469.95 μmol H2 and has high stability. Our work will provide a new strategy for the rational design of efficient catalysts for MOFs derivatives.