*2.1. Data Source*

Literature databases commonly used by international experts and scholars include Google Scholar, Scopus, PubMed, and Web of Science. Each of these databases has advantages and disadvantages. Google Scholar has a broad coverage of literature data and plentiful literature types, but it has low data quality and many duplicate data [22]. PubMed is a free biomedical information retrieval system developed by the NCBI. PubMed has a rich literature in the medical field but lacks literature data in other subject areas. Scopus and Web of Science are comprehensive databases, and there is not much difference in data coverage between the two. However, some scholars have shown that when using CiteSpace software for visual analysis, the knowledge map made by the literature of the Web of Science database is better [23,24]. Therefore, this study uses the Web of Science to retrieve the literature data needed for analysis. To ensure the reliability of the scientific metrological analysis, we chose the Web of Science core collection, and the indexes are SCI-EXPANDED, SSCI, CCR-EXPANDED, and Index Chemicus. The detailed retrieval strategy is shown in Figure 1. After preliminary searching, 14,692

pieces of literature data were obtained. For some reason, there are differences and ambiguities in the expressions of author names, institution names, and country names in literature data. In response to this, before conducting scientific measurement analysis, we used CiteSpace's data deduplication and name merge functions to standardize the data. The literature search date is December 30 December 2019.

**Figure 1.** Research framework.
