The citation frequency of the article can intuitively show the degree of recognition of scholars. However, a simple analysis of the citation frequency of the article ignores the internal relationship between different documents. In bibliometrics, if two papers are cited together by other papers, we call them coreferences. Figure shows the above relationship, and there is a co-citation relationship between papers A and B because they are cited by papers C, D, E and F. Therefore, the "co-citation" index can examine the similarity of research topics in different papers. The CiteSpace software is used to analyze the co-citation of 1382 articles. A huge co-citation network is synthesized, which intuitively shows the evolution process, research fields and hotspots of the literature. To show a clear co-citation network, we exclude the citation frequency ranking of 100 papers while only allowing the extraction of references within ten years before each paper was published. The co-citation network is shown in Figure , which shows nine clusters with different topic centres in which the order of labels is determined by the number of references contained in each cluster. In the co-citation network, each node represents a specific paper, and the node's size represents the citation frequency. In each cluster, references with high citation frequency will be extracted and displayed in the form of the first author and publication year. Among them, the colour change of different clusters represents the evolution of the literature citation trend. Figure shows the evolution of POM-based photocatalysts research focus since 1991. Cluster #4 and cluster #5 correspond to the research hotspots of scholars before 2000. Their research mainly focuses on the synthesis and structural characterization of POMs, and exploring their performance and application is not deep enough. A few experiments had been conducted on the degradation of chlorophenol and 4chlorophenol decomposition by POMs. Since Fujishima et al. reported n-type semiconductor TiO2 as a photocatalyst to decompose water into hydrogen in 1972, researchers have been working to improve its photocatalytic efficiency. Yoon M et al. believed that it was difficult for TiO2 particles to be used as the real simulation of plant photosynthetic reaction centre. To improve the light-induced interfacial electron transfer efficiency, they doped heteropoly acid in TiO2 colloid as a photocatalyst to catalyze the reduction of methyl orange under near-UV irradiation. In 2001, Anastasia Hiskia et al. confirmed that POMs and metal oxide aggregates had similar photoredox properties. Ozer R et al. reported using TiO2-PW12O40 3-, TiO2-SiW12O40 4-and TiO2-W10O32 4-as photocatalysts to drive the oxidative degradation of 1,2-dichlorobenzene. Subsequently, they loaded POMs on NaY Zeolite to achieve photocatalytic oxidation of 1,2-dichlorobenzene in water. Researchers had opened up in-depth studies on POM-based photocatalysts involving kinetics, reaction mechanism and catalytic types, and cluster #6, cluster #7 and cluster #8 represent the research focus of this period.