Mapping the Field of Value Chain: A Bibliometric and Visualization Analysis

: In recent years, the interest in academic research in the ﬁeld of the value chain has increased rapidly. However, there is a lack of bibliometric and visualization research on this subject. This paper aims to analyze the main trends of the value chain in multidomain-related literature in the past decade. Thus, we used bibliometric 2002 documents data from the Scopus to generate ﬁgures like the trend, co-occurrence and proportion of the value chain by using Rstudio and VOSviewer. The results not only explained the main modern trends under the time series, but also showed the evolution of the development of the theme of the value chain. Additionally, the paper also determines the impact of the value chain in different journals and documents and analyzes the impact of themes, countries and keywords on the publication of the value chain literature. After identifying the most popular themes and keywords in the past decade, we predicted the trend and direction of future value chain research. Due to the lack of literature for data analysis on value-chain-related innovation, this study is a unique contribution to the literature on the measurement method as a supplement. This study also provides a visual and schematic framework for the relevant research in the ﬁeld of the value chain and summarizes the trend and trajectory. This may help researchers understand the current trend of the research on the value chain and grasp the future research direction.


Introduction
In recent years, with the development of transportation and communication technologies, the relevant product costs have been reducing.This has led to drastic changes in many fields, especially in manufacturing industries.Additionally, COVID-19 has been wreaking havoc around the world for two years.It has seriously affected the health of millions of people and has had a huge impact on production and trade activities [1].It also has directly changed the market and trade policies of every government and organization around the world.Enterprises have tried to compete for the promotion of product value through their own optimization and resource integration under the changes to global markets, as well as value promotion so to realize the sustainable development of the enterprises.Overall, the value chain is mainly a concept emerging after the construction and popularization of international production networks.It is also one of the main research contents of global industrial upgrading and in enterprise sustainable development.It contains the concept of economic upgrading, derived from international trade theory, which is a dynamic process of continuous upgrading to create value.Value chain theory mainly studies the global production division network formed by the fragmented spatial separation of different production links in the process of economic globalization.Compared with the supply chain, the flow in the supply chain that reflects the good operation of enterprises is the value distribution.Through the integration of resources in the supply chain, the optimal distribution of value in each enterprise and the link of the supply chain is highlighted.
In other words, the core of the supply chain is the application and maximization of the value chain.
In addition to being widely used in production, the value chain also balances the relationship between enterprises and financial capital [2].This is also an important reason for the gradual increase of value chain research topics in recent years.The value chain takes the pursuit of value as the goal, which changes the oriented profit goal of traditional enterprises [3].Relevant research has also changed from the initial competitive advantage within a single enterprise to the value network theory with the deepening of trade relations between enterprises [4].It has gradually formed an alliance operation mode with the central risk, information and benefit sharing borne by the core enterprises, breaking the organizational boundary restrictions of enterprises.The financing generated by trade between participants constitutes the basis for the value chain to affect the financial capital of enterprises, and the capital structure is the result of financing.Therefore, it maintains a reasonable capital structure of industries up and down the value chain in order to maximize the allocation of resources.Capital realization enhances the stability of the value chain and achieves the purpose of value appreciation.The European Union and North America have begun to standardize value chain management in enterprises [5], for example, the hydrogen energy value chain, the marine development value chain and the IT industry value chain [6].
Because of the relevance of the value chain, there has been a tremendous increase in studies in this area in the previous several years.Because of this rapid expansion, it is becoming increasingly difficult to grasp the present trends and future directions of value chain research.A systematic analysis of the published literature could help the reader and other stakeholders to understand the current trends and trajectories of the value chain.To address this issue, in this study we used bibliometric analysis to outline the current trends of value chain research.Using the performance analysis aspect of the bibliometric technique, the study outlines the most productive sources of the value chain.Second, the study draws out the intellectual networking of value chain research.In so doing, the study contributes to the literature in several ways.

Literature Review 2.1. Value Chain
In 1985, the concept of the value chain was first proposed by Michael Porter, a professor at Harvard Business School [7].Porter's theory indicated that the production and operation activities of each enterprise are a collection of product design, production and sales.All these activities can be represented by a value chain.The value creation of enterprises is composed of a series of activities, which can be divided into basic activities and auxiliary activities.
Cox applied it to the spatial layout analysis of enterprise production and discussed the relationship between the value chain governance mode and industrial spatial transfer [8].Garry Gereffi put forward the concept of the "global commodity chain" for the first time [9] in 1999.He believed that under the guidance of value maximization, global enterprises are closely linked to the global production chain of commodities, which makes the production activities in the world economy show the characteristics of networking.On this basis, in 2001, Bair and Gereffi further proposed the concept of the "global value chain" to describe the cross-regional production activities dominated by multinational corporations [10].In 2002, the United Nations Industrial Development Organization (UNIDO) defined the "value chain" as a transnational production network connecting production, processing, sales and recycling all over the world.Under the connection of the global value chain, the country attribute of products is becoming more and more blurred.A product may contain added value from multiple countries [11].At the same time, the profitability of each link of the value chain is different.Many participating enterprises capture different profits by undertaking different links.There are always some strategic links in each global value chain that can create higher profits.
In recent years, the development of international production and the value chain of goods and services has had a profound impact on the distribution of benefits obtained by countries participating in the global value chain [12].Specifically, the value chain distinguishes four kinds of upgrading for sustainable economic development: process upgrading, product upgrading, functional upgrading and intersectoral upgrading.Process upgrading is based on the improvement of efficiency and productivity, reflecting sustainable development [13].Product upgrading is based on turning to more sophisticated products in the existing value chain, reflecting sustainable development [1].Functional upgrading is based on the shift to more sophisticated production tasks, reflecting sustainable development [2].Intersectoral upgrading refers to the transfer of the original department to a new value chain to reflect sustainable development [14].

What Can Bibliometric Analysis Do?
Literature is an important carrier of scientific knowledge and has the function of scientific evaluation [15][16][17].Its content, form, quantity, dissemination and citation reflect many aspects of the frontier direction of scientific research [18,19].Therefore, bibliometrics can be used to evaluate and predict scientific knowledge [20,21].
Bibliometrics uses scientific literature as the research object [22].Through the comprehensive use of statistical analysis, social network analysis and mathematical modeling, it studies the characteristics and relationship of literature quantitatively [23].Bibliometrics is widely used.The microapplication includes determining the core documents, evaluating the relevance of keywords, investigating the utilization rate of documents and realizing the scientific management of research disciplines.Macroapplications include analyzing the related fields of research objects, predicting research directions and developing and perfecting basic theories.Bibliometrics provides theoretical and methodological support for the reliable evaluation of scientific research productivity, academic influence and the scientific frontier progress of scientists, scientific research teams, scientific research institutions and countries/regions [24].Considering the feasibility and objectivity of this method, the analysis and control of the research output results based on literature measurement is becoming a new way for scholars to effectively carry out science and technology management [25,26].
This paper mainly uses bibliometric methods to analyze the literature related to the value chain.The hotspots of value chain research can be reflected by the performance of the literature, which can be reflected by the number of references and times the literature has been cited.Scientific mapping represents the field and structure of dynamic and visual value chain development.For the sake of the realization of these research objectives, this paper meticulously analyzes value chain literature of the following contents:

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The Literature of Value Chain Citation and Cocitation Research.

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The Literature of Value Chain Bibliographic Coupling of Multielement.

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The Literature of Value Chain Keyword Research.
The literature on value chain citation and bibliographic research mainly reflects the significance of the literature in the field of the value chain and the degree of correlation between the main contents.At the same time, citation research enables scholars to fully understand the influence of journals, papers and authors on the published papers in the field of the value chain, while cocitation research simplifies the linear relationship between multiple papers into the relationship between multiple countries or regions through econometric analysis, which can intuitively express the internal relationship between literature.Keyword co-occurrence analysis mainly studies the relevance of keywords in the value chain literature.By analyzing the tightness of keywords, we can identify the current research hotspots of the value chain and infer the current research trend.

Relevant Definitions and Formulas
The specific contents of bibliometrics include research institutions, institutional heterogeneity, cooperation mode, interdisciplinary, hot topics and research directions [27,28].
In addition, considering the dynamic and regional nature of knowledge production, this paper adds two dimensions of time and space to the framework.This paper finds that the analysis dimension reflects the characteristics of the scientific knowledge production model from the external and internal characteristics of knowledge research and can respond to the new model theory.Considering the quantitative evaluation of the characteristics of scientific development in different stages and countries/regions [29], this paper quantifies the model using various indicators.
The application of statistics and forecasting is a novelty of this study that creates the potential to predict and interpret the results obtained [30].
According to the results that the citation, bibliographic coupling and keyword research need to output, this paper quantifies the corresponding indicators.The related concepts are expressed in Table 1.
Table 1.Main Indicators and Formulas.

Indicators
Relevant Definitions and Formulas Among them, p i , p j represents the proportion of theme i and j cited in a specific paper and q ij is the similarity between the cited documents i and j.

Data scale (L)
The number of data nodes in the data, which is used to investigate the size of the selected data.

Connection
The number of data nodes that can be removed from the connection between the start and end point, which is used to measure the accessibility of the selected data.

Cliqueness
Local characteristic variables.It refers to the probability that multiple nodes connected to a standard node also directly intersect.It is used to measure the collectivization degree of the selected data.

Degree, d
The degree of a node is equal to the concentration of the connection associated with the node.The degree of a node determines the influence and authority of the node in the whole network.Describes the resource acquisition capability of the node in the network.The calculation method is as follows:

Density
The ratio between the actual number of connections and the theoretical number of connections in the literature network connection reflects the link function between nodes.The calculation method is as follows:

Degree Centrality
Refers to the number of nodes directly connected to this node.Similarity centrality is a standardized form of offset centrality, which refers to the ratio of the offset centrality of the node to the maximum degree of nodes in the network.The relative centrality is calculated as follows:

Closeness Centrality
The distance of each node in the grid is expressed in the proximity of the node to all other points in the network and reflects the control force of each node on the whole network.The calculation method is as follows: The number of times of all shortest paths passing through the node in the data network refers to the network location of the measurement node.The more obvious the intermediary centrality is, it shows that this node is on the shortcut of many other nodes.The calculation method is as follows:

Selection of Database and Data Clean
The selection of the database is the primary and most important segment of bibliometric analysis.At present, the databases most commonly used by scholars include Web of Science (WOS), Scopus and Google Scholar [31,32].These databases differ in data volume, quality and function.WOS includes citation data from the 1900s and covers 12,000 major journals worldwide.However, the number of publishers and journals is less than that of Scopus, and the results are biased when used in the bibliometric software.
Although Google Scholar has a large amount of literature data, its downloaded data format is complex and structurally unstable, which is not suitable for the software analysis of bibliometrics.Compared with WOS and Google Scholar, Scopus contains 19,000 source journals from 4000 publishers around the world, which is the world's largest database of abstracts and citations.Therefore, this study selects the Scopus core collection database with the most journals and high data quality as the data source.This database has been used for bibliometric analysis in several previous studies [20,33].
As a prevalent topic in the research of supply chain-related fields in recent years, the value chain involves extensive and complex disciplinary knowledge, and is still expanding in the context of interdisciplinary research.New theories and methods are also being created, and it is more complex to divide its fields clearly.Therefore, on the basis of the relevant research, this study will determine the target literature through subject retrieval.The search terms include value chain, supply chain and global value chain.This paper mainly studies the influence of value chain in the economic field.Therefore, the discipline is selected as the economic category.Considering the late start time of value-chain-related expansion research, this paper further limits the research time to the past decade; that is, 2012-2021.Considering the accuracy of the literature, this paper only selects the literature in the English language, and the publication status is published.The data processing method is based on Boolean operators.Table 2 shows the process of data selection.The Boolean operators "AND" and "OR" were used to ensure the selected documents focus on both social capital and innovation, not only on either side independently.Moreover, the search process is shown as follows: TITLE-ABS-KEY ("value chain") AND (LIMIT-TO (SUBJAREA, "ECON") ) AND (LIMIT-TO (DOCTYPE, "ar")) AND (LIMIT-TO (PUBYEAR, 2021) OR LIMIT-TO (PUBYEAR, 2020) OR LIMIT-TO (PUBYEAR, 2019) OR LIMIT-TO (PUBYEAR, 2018) OR LIMIT-TO (PUBYEAR, 2017) OR LIMIT-TO (PUBYEAR, 2016) OR LIMIT-TO (PUBYEAR, 2015) OR LIMIT-TO (PUBYEAR, 2014) OR LIMIT-TO (PUBYEAR, 2013) OR LIMIT-TO (PUBYEAR, 2012)) AND (LIMIT-TO (LANGUAGE, "En-glish")) AND (LIMIT-TO (PUBSTAGE, "final")).Based on this retrieval method, this paper selects 2002 documents from Scopus.

Selection of Technical Software
Because it is difficult to use search websites such as Web of Science (WOS), Scopus and Google Scholar to present citation and cocitation research, the bibliographic coupling of multielement and the research of bibliometrics needs the support of software to analyze the results.The paper mainly uses 2 technical software to output results: VOSviewer and R-Studio.VOSviewer is a software suitable for data analysis.This Java is good at generating various types of text maps [25].In addition, it can carry out the image of cooperative network analysis, co-occurrence analysis, citation analysis, document coupling analysis and cocitation analysis.In addition, the insufficient part of VOSviewer and the common and linear analysis data output part are completed by R-Studio.R-Studio usually was used for direct code execution and tools for plotting, history, debugging and workspace management, which is an integrated development environment (IDE) for R. The study mainly uses the subprogram biblioshiny to output the images of the bibliometric analysis.A combination of VOSviewer and R-Studio generates visual images and tables of the literature data from Scopus.In the three years from 2012 to 2014, the literature on the value chain showed a slow upward trend.In 2015, the publication of literature on the value chain decreased compared with the previous year.From 2016 to 2018, the publication of literature on the value chain showed a rapid upward trend and declined until 2019.However, the total number of articles published has increased in the past three years, reaching a maximum of 354 in 2021, which is likely to be related to the academic impact of COVID-19 on enterprise management.
Based on Table 2, the number of citations indexed reached a peak in 2018, reaching In the three years from 2012 to 2014, the literature on the value chain showed a slow upward trend.In 2015, the publication of literature on the value chain decreased compared with the previous year.From 2016 to 2018, the publication of literature on the value chain showed a rapid upward trend and declined until 2019.However, the total number of articles published has increased in the past three years, reaching a maximum of 354 in 2021, which is likely to be related to the academic impact of COVID-19 on enterprise management.
Based on Table 2, the number of citations indexed reached a peak in 2018, reaching 4318 times.This is related to the theoretical and reality research put forward from the coupling analysis of the value chain and the supply chain in 2018.In addition, the concept of the world circular economy has been explored as a popular research field.Except for 2018, the number of citations before 2018 is in the range from 2000 to 3500.The number of citations after 2018 is lower than that in the previous period, 1719 in 2020 and 1620 in 2021.The number of citations has not become a positive proportional relationship with the number of published citations in recent years, which is mainly due to the delayed nature of literature citations.According to the research heat represented by the trend of the number of publications, such as the increasing number of publications from 2019 to 2021, it can be predicted that the citation trend will rise in the future.

Contributions and Relationship of Countries and Regions in Research
On the whole, the research on the value chain is generally advanced by the western region, composed of European countries and the United States.As shown in Table 3, the total number of documents in the USA is 340 and the number of citations is 7806, which are the highest.The UK has a total of 246 articles and 5192 citations, followed by the UK.Regional economies of other developed countries, such as Germany, Italy and France, as well as countries with many achievements in world economics, rank high in the research field of the value chain by virtue of their rich enterprise management experience and theoretical research advantages.In addition to the research results of economically developed regions, developing countries with large economies such as China and India have also contributed to the research in the field of value chain.China's 159 publications and 1820 citations are among the top.However, it is worth noting that the cited ratios of China and India are not high.The average number of citations per document can be regarded as the quality of references and the relevance to the research directions in other regions.The larger the number, the more attention is paid by other scholars.As a contrast, the countries with an average number of citations per document of more than 20 are the USA, UK, Netherlands, Denmark and Norway, which are all belong to developed economies.
This shows that the research on the value chain in developed economic regions is closer and more valued by researchers in other regions.In order to more intuitively see the degree of correlation between countries and regions to study the topics related to the value chain, this paper draws into the bibliographic coupling of Countries/Regions of countries from the result by VOSviewer.
Figure 2 of the bibliographic coupling of Countries/Regions is based on the concepts of time and space and the exponential formula quoted in the Methods part of this paper, and the resulting diagram is generated by the VOSviewer software.Figure 2 shows the bibliographic coupling of value chain research countries based on global cooperation.The network consists of 48 nodes, each representing a country, and the connection between nodes represents the cooperative relationship between countries, and the size of nodes represents the numbers of cooperation with other countries.The larger the node is, the more international the cooperation the country participates in.The node color represents the clustering generated and the direction of the value chain research according to the strength of the cooperative relationship.The results in Figure 2 show that the United States and the United Kingdom have the largest circles, which means that the aggregation of research of value chain topics in the United Kingdom and the United States is the strongest.Corresponding to the color, the research direction of the value chain in developed economies is generally the same.The USA and the UK have the most connections with other countries, and their research direction of the value chain is followed by most countries, which is shown in the figure as the circle with most red lines.The figure also shows that the links between regions are very close.For example, the connections of several countries in Asia, such as China, Japan, South Korea and Australia, are green, which shows that the research directions of these countries are the same when studying the value chain.

Leading Journals in Value Chain Research
Based on the identification and analysis of value chain literature and the main research directions, this paper explores the discipline distribution characteristics of value-chainrelated research.The specific research contents will include the following points: Identifying the main journals publishing value chain literature based on the number and change trend of the literature.Based on the discipline of the journal, identifying the disciplinary fields covered by the value chain research and their evolution trend.Based on the main cited literature, measuring its interdisciplinary nature and its change trend.Identifying the literature of the value chain research based on citation analysis.
According to These journals are related to the world economy and the world business operation mode.Therefore, the recent core research of the value chain is mainly composed of the role of the value chain in the world economy recycling and business activities.Figure 3 shows the trend chart of journals published in the past decade, respectively, which was drawn by bibiloshiny from Rstudio.This means that what themes and directions the value chain is related to is the focus of these journals.According to the trend chart of the literature published in journals, Marine Policy and Resources, Conservation and Recycling show an obvious upward trend, and the Journal of Agribusiness in Developing and Emerging Economies and the Review of International Political Economy show an obvious downward trend.This means that the topics related to value chain research in the future will be specific to various fields, such as value chain application in marine and food fields.For the research on the relationship between agribusiness and the value chain in developing and emerging economies, as well as the positioning and application of the value chain in the field of international politics, the overall research trend is not as high as before.

Most Influential Publications
"Circular economy-from review of theories and practices to development of implementation tools" has the most citations, reaching 424, which was published in 2018 based on the database.This paper describes the theoretical methods, strategies and implementation cases of circular economy.After analyzing different methods and basic theories of the circular economy, this paper mainly discusses the main objectives of the tools to develop circular economy.According to Table 5, the main content of the most citations is the circular economy strategy database, which includes several circular economy strategies suitable for different parts of the value chain.The second citation of documents is "Global value chains in a post-Washington Consensus world", which was published in 2014.This paper introduces a more differentiated type of governance structure, which focuses on the new coordination in the global value chain.The organization of the global economy is entering the stage of the buyer-driven and producer-driven commodity chain, and the governance structure of the global value chain and global capitalism is changing at all levels.In addition to this article, there are some references related to the world value chain, which also rank high.This shows that, for scholars, the focus of value chain research in the past was on globalization and international cooperation.There are other topics about the value chain cited by many scholars, such as food fields, global production networks, the global supply chain and supply chain management.

Most Influential Publications
"Circular economy-from review of theories and practices to development of implementation tools" has the most citations, reaching 424, which was published in 2018 based on the database.This paper describes the theoretical methods, strategies and implementation cases of circular economy.After analyzing different methods and basic theories of the circular economy, this paper mainly discusses the main objectives of the tools to develop circular economy.According to Table 5, the main content of the most citations is the circular economy strategy database, which includes several circular economy strategies suitable for different parts of the value chain.The second citation of documents is "Global value chains in a post-Washington Consensus world", which was published in 2014.This paper introduces a more differentiated type of governance structure, which focuses on the new coordination in the global value chain.The organization of the global economy is entering the stage of the buyer-driven and producer-driven commodity chain, and the governance structure of the global value chain and global capitalism is changing at all levels.In addition to this article, there are some references related to the world value chain, which also rank high.This shows that, for scholars, the focus of value chain research in the past was on globalization and international cooperation.There are other topics about the value chain cited by many scholars, such as food fields, global production networks, the global supply chain and supply chain management.The Figure 4 shows the theme keywords about the value chain based on the database drawn by the VOSviewer software.The keywords of the value chain papers are interpretive and also have the function of summarizing the whole research direction in the field of the value chain.They carry the most important and core information of the value chain research.They are not only the source of obtaining more information but also the key to mastering the important information of the value chain literature.Figure 4 describes five clusters to distinguish the relationship and closeness between different keywords, including red clusters, blue clusters, green clusters, blue clusters and purple clusters.Through the analysis of high-frequency keyword clusters of the value chain, it shows the research points and hotspots in the field of the value chain so as to present its microlevel knowledge distribution and structural characteristics.
Like the bibliographic coupling of countries, the figure of the co-occurrence of keywords is based on the concepts of time and space and the exponential formula quoted in the Methods part of this paper, and the resulting diagram is generated by the VOSviewer software.The value chain literature collection of this study contains 1859 keywords, with a total frequency of 5315 times.There are differences in the frequency distribution between words; that is, the number distribution of knowledge points represented by the keywords is not very balanced.Figure 4 shows the co-occurrence of keywords of the 37 keywords, with the highest frequency with other literature.The larger the circle where the font is located, the higher the frequency of the word.In addition, the color of a keyword indicates the cluster it belongs to.The distance between the words represents the degree of relevance of two words; that is, the closer the distance, the higher the relevance.There are five clusters in Figure 4 including red clusters, blue clusters, green clusters, and purple clusters, and each cluster represents an independent research direction.These clusters also have strong and weak connections with various links.For the red clusters, they contain the keywords supply chain, innovation, sustainability, competitiveness, supply chain management, circular economy, strategy, entrepreneurship, developing countries and agriculture.These keywords take supply chain and sustainable development as the core and are closely connected in the direction of the circular economy and supply chain industrial upgrading.Additionally, the red clustering is a cluster that has a connection relationship with the other clusters.It can be seen that the qualitative analysis of the value chain is inseparable from the supply chain upgrading and sustainable economic circular development in both developed and developing economies.
This part mainly believes that the current link upgrading of the value chain mainly refers to functional upgrading.Due to the transfer of production to higher value links, the general production of this process is accompanied by the improvement of labor productivity.From the macroeconomy view, the value chain upgrading of the national economy or industrial sectors is often reflected in the improvement of value-added acquisition ability.Some empirical tests have tested the relationship between value chain participation and national economic upgrading, and found that the degree of national value chain participation is positively correlated with domestic value-added [36,37].Some studies have also described the relationship between global value chain participation and unit capital GDP and found that the global value chain participation will increase unit capital GDP, thus driving the upgrading of the value chain of the national economy [38,39].In addition, the integration of the manufacturing industry into the global value chain division of labor has significantly promoted the improvement of total factor productivity, which is an important embodiment of the upgrading of the manufacturing value chain in the process of global economic transformation.As one of the important manifestations of value chain upgrading, the improvement of total factor productivity has also attracted the attention of academic circles at the urban level.
For the blue clusters, they contain the keywords global value chains, productivity, exports, trade in value added, trade policy, Indonesia, globalization, economic development and industrial policy.These keywords take global value chain, industrial production and transaction as the main core, and the research direction mainly focuses on the global value growth and remodeling of the value chain for productivity.Blue clustering is also closely related to other clustering.In addition to studying the circular economy through the global value chain and the supply chain upgrading, together with red cluster, the blue cluster is related to the trade development in the purple cluster and Asian countries in the green cluster because of its concern for products and exports.
This part mainly believes that for the blue cluster associated by related keywords connected with other clusters, the regions with a high embeddedness of the global value chain have a greater positive impact of international trade on total factor productivity.As the specific impact of the embedding degree of the urban global value chain on total factor productivity, it is reflected in that the embedding degree of the global value chain will significantly promote the improvement of urban total factor productivity and has a significant spatial positive correlation.Product quality is also a key and prominent part of the global value chain.From the perspective of related keywords, the supply chain of products upgraded in the value chain applied to import and export trade can be closely combined with value chain management.
For the green clusters, they contain the keywords India, upgrading, governance, Asia, institutions, contract farming and Vietnam.These keywords mainly focus on the value chain development of Asian countries, especially South Asian countries India and Vietnam, and their internal relationship is guided by industrial upgrading and the relationship between government departments.For the green cluster, this part is closely related to the blue cluster, especially the international value chain.At the same time, it is also related to the purple cluster in China and Africa.
This part mainly believes that the theoretical framework of the value chain pays more attention to the participation of developing countries, especially manufacturing countries in Asia.The embedding of the manufacturing industry in the value chain and its impact on the upgrading of the value chain have attracted the attention of academic circles.From the perspective of studying the global value chain, Asian manufacturing development plays a positive role in the trade division under the upgrading of the value chain, not only because it can promote more exports, but also because it reduces the disconnection between productivity progress and exports caused by liquidity constraints, which directly leads to the fact that the productivity of microenterprises in developing countries is a more dominant factor in enterprise export decision-making [40].The development of the value chain needs investment support related to technological innovation, while labor-intensive industries in developing countries also need relatively more investment in marketing and brand development.Therefore, enterprises in developing countries are more vulnerable to the insufficient application of external value chain links and are locked in low value-added manufacturing links.
For the yellow clusters, they contain the keywords international trade, employment, manufacturing, economic growth, offshoring and outsourcing.These value chain keywords take international trade, offshore and outsourcing as the main core, and their internal value chain correlation is used to be related to the outsourcing part of trade, especially the production and manufacturing of developing economies.Therefore, yellow clustering is related to almost all development keywords and has a strong connection with contract farming of the green clustering.This shows that the value chain has a strong connection between external production and offshore transactions in world trade.
This part mainly believes that international trade-related matters will indeed have an impact on the research of the value chain.The phenomenon, focused by international trade, is an attempt to integrate the research of the value chain and the research of the world urban network.On the one hand, the chain network model based on flow space theory uses enterprises data, and the quantitative results tend to reflect the economic attributes of cities.On the other hand, the global value chain theory emphasizes the division and agglomeration of value-added links, and cities are often the carrier of these value-added links.From these two aspects, offshore and outsourcing trade plays an important role in the two theoretical frameworks, which leads to the integration of the world urban network and the global value chain [41].This part of the cluster also includes manufacturing and employment that are highly related to outsourcing trade.The increasing participation of Asian countries in the global industrial chain is closely related to the rise of manufacturing in these countries.Since 1995, the proportion of the manufacturing industry in most economic systems in Asia has exceeded the global average by 20%.Among them, China accounts for the highest proportion, at nearly 40%.In addition to increasing participation, the productivity of East Asia, Southeast Asia and South Asia in the global industrial chain is also increasing.
For the purple clusters, they contain the keywords China, development, trade, COVID-19, Africa and Ethiopia.Compared with other clusters, the purple cluster mainly focus on the direction of current affairs.China was first affected by COVID-19, so the keywords China and COVID-19 belong to the same cluster.In addition, China is closely integrated with development, trade and Africa.This is mainly because China is Africa's largest trading partner, and the trade between China and Africa is also the focus of scholars' research from the perspective of value chain.
This part mainly believes that China's position in the global value chain has gradually changed.In recent years, the dominant position of China's world factory has weakened.The rise of labor costs has gradually lost China's advantage in labor-intensive production [42].At the same time, China's manufacturing industry has achieved a certain degree of technological transformation and upgrading.In the industrial chain of general-purpose special equipment, capital-intensive and technology-intensive manufacturing, it continues to climb to the high value-added link of the value chain.Smart devices, big data and Internet of Things technologies are developing rapidly in China, and there has been a high level of scientific and technological development.However, COVID-19 has a great impact on the manufacturing industry in China, and even the world, especially in the upgrading of the value chain industry, which will shackle the progress of developing economies such as China.Therefore, the value chain issues related to COVID-19 and developing economies have attracted scholars' attention.

Analysis of Proportion of Keywords
The proportion of keywords related to the value chain is shown in Figure 5, which represents the frequency of keywords in the value chain.This figure is 44 squares generated by biblioshiny in Rstudio through the database.Each square represents a high-frequency keyword.The larger the square, the higher ratio of the proportion.Overall, the frequency of keywords related to the value chain is relatively balanced.Supply chain management accounts for the highest proportion, reaching 6% of value chain keywords, and the keywords that belong to industrial category with supply chain management are sustainability (3%), innovation (3%), sustainable development (2%), waste management (2%), competitiveness (2%) and supply chains (1%).The keywords that belong to large category in the field of economic development are international trade (4%), export (4%), input-output analysis (3%), developing world (3%), commerce (2%), globalization (2%), economic development (2%), trade flow (2%), import (2%), environmental economics (2%), marketing (1%) and investment (1%).The keywords that belong to the large category in the field of countries or regions are China (5%), European Union (2%), Europe (2%), India (1%), Germany (1%) and United Kingdom (1%).The keywords that belong to the large category in the field of industry are manufacturing (2%), food security (2%), industrial production (2%), food market (1%) and agricultural market (1%).
From the keywords weight distribution presented in this part, the research related to the value chain focuses more on the upgrading and management of the value chain under supply chain and international trade.The value chain keywords are associated with countries that are in economically developed or in fast-growing economic areas such as Europe and China.In addition, the research on industrial relevance involving the value chain is related to food and industrial upgrading strategies.
From the keywords weight distribution presented in this part, the research related to the value chain focuses more on the upgrading and management of the value chain under supply chain and international trade.The value chain keywords are associated with countries that are in economically developed or in fast-growing economic areas such as Europe and China.In addition, the research on industrial relevance involving the value chain is related to food and industrial upgrading strategies.

Analysis of Co-Occurrence of Keywords over Time
The co-occurrence of keywords over time related to the value chain is shown in Figure 6, which represents the time series of keywords in the value chain.This figure is 37 squares generated by VOSviewer through the database.The color of each square in the figure ranges from deep blue to yellow and from dark to light.The lighter the color, the closer it is to the keyword of yellow, and the closer it is to the present time to research.Supply chain management, agriculture and outsourcing have the darkest box color; that is, the direction of value chain research.The keywords at these levels were very popular in previous years and decreased in recent years.At the same time, the color of squares, such as manufacturing, industrial policy, offshore trade and upgrading, is between blue and yellow, which shows that the research on the relationship between the value chain and these levels is mainly focused on 2016-2018.After 2018, the keywords related to the value chain are represented by yellow lexical chunks, including circular economy, trade in value-added, supply chain, global value chains, economic growth and COVID-19.

Analysis of Co-Occurrence of Keywords over Time
The co-occurrence of keywords over time related to the value chain is shown in Figure 6, which represents the time series of keywords in the value chain.This figure is 37 squares generated by VOSviewer through the database.The color of each square in the figure ranges from deep blue to yellow and from dark to light.The lighter the color, the closer it is to the keyword of yellow, and the closer it is to the present time to research.Supply chain management, agriculture and outsourcing have the darkest box color; that is, the direction of value chain research.The keywords at these levels were very popular in previous years and decreased in recent years.At the same time, the color of squares, such as manufacturing, industrial policy, offshore trade and upgrading, is between blue and yellow, which shows that the research on the relationship between the value chain and these levels is mainly focused on 2016-2018.After 2018, the keywords related to the value chain are represented by yellow lexical chunks, including circular economy, trade in value-added, supply chain, global value chains, economic growth and COVID-19.
The results show that, globally, value chain research involves a wide range of fields, covers a variety of topics and the scope of keyword relevance continues to grow with the passage of time.Among them, before 2016, the promotion of industrial upgrading by the value chain under supply chain management was the main field of value chain research, with a large number of papers and the strongest relevance of research topics.In recent years, with the gradual opening and development of the world after the epidemic, the scale of economic development is growing at a high speed in geometric progression [43].The relevant research of the value chain can play an important role in promoting and restoring economic development, which will lead to a larger number of value chain literature moving closer to the direction of promoting enterprises, changing global trade patterns and sustainable economic development after the epidemic.In this context, the global value chain will force the change of value chain research so as to promote the continuous strengthening of technological development and the innovation of enterprises and manufacturing in all fields of the world.

Potential Future Research Directions
According to the chart analysis in the previous section, this paper has obtained some research hotspots and past research trends on the value chain, especially combined with the commonness analysis of the co-occurrence of keywords, the proportion of keywords and the co-occurrence of keywords over time.The potential future research of the value chain predicted includes four aspects: the global value chain under COVID-19, the governance and upgrading of the value chain, the global supply chain and the value chain, and the circular economy and sustainable development.

Global Value Chain under COVID-19
One of the important characteristics of modern economic development is the globalization of production realized in the way of the global value chain [44].The global value chain has always been an important part of the research on value chain issues between 2012 and 2021.In the past, the research on the global value chain mainly relied on global trade and international trade policy.With the effective control of the COVID-19 epidemic, the global manufacturing industry resumed production, thereby realizing in the way the global value chain.The topic of global value chain research is likely to shift to production manufacturing.On the one hand, it benefits from the enhancement of the scale capacity of the manufacturing industry in developing economies; on the other hand, it is the integrated management of internal integration of multinational corporations by developed economies.
The progress in the above two aspects is conducive to the economic entities to concentrate their limited resources on the strategic links with high added-value [45].The stripping and integration of a large number of noncore and core businesses provide a theoretical basis for the value chain theory to study the development of enterprise value maximization in developed economies.In essence, globalization in the field of value chain research not only explains the expansion of economic ties in the geographical scope, but also clarifies how decentralized economic behavior realizes the coordination and integration in the functional sense.Therefore, for developing countries, researching the economic trend of value chain upgrading is the import and export part of industrial upgrading and offshore trade.In summary, the global value chain under COVID-19 may be in a popular theme to be researched by scholars in the next few years.

Governance and Upgrading of Value Chain
The essence of value chain upgrading is the process of obtaining a higher value chain by improving production efficiency, improving industrial technology content and optimizing product quality.In other words, value chain upgrading is a process in which enterprises upgrade to areas with high capital and technology intensity to obtain more excess profits.In recent years, the research on value chain upgrading mainly discusses the level of enterprises in the process of value chain upgrading, but it has not pointed out the specific path of value chain upgrading for different types of enterprises [13].Therefore, with the gradual improvement of the theoretical framework of the value chain, scholars will further define the upgrading of the value chain as the process of countries or regions moving to higher value-added links of the value chain, so as to improve their global production income.In summary, governance and upgrading of the value chain may be a popular theme to be researched by scholars in the next few years.

Global Supply Chain and Value Chain
In the context of increasingly segmented global supply chains, some phenomena cannot be explained.For example, the distribution benefits of economies represented by trade scale cannot explain the distortion of factor value.The rapid expansion of manufacturing trade scale cannot explain the imbalance of resource allocation [46].Therefore, it is considered that with the rapid expansion of international trade, the imbalance of trade interests in some countries is becoming more and more prominent, and the organizational and governance forces contained in the value chain are also shaping and affecting the final profitability of the industry.Especially under the global supply chain revolution of international vertical division of labor, one of the important characteristics of international trade is the change of governance structure and the benefit distribution pattern under the production division system of the value chain.The relevant economic measurement methods can no longer reflect the actual situation of the current international trade based on the global supply chain.On the contrary, there is a certain degree of distortion in the reflection of the trade imbalance of various countries.Based on this background, it generates a more rigorous method of using the value chain to optimize the links of enterprise production, manufacturing and sales, which can more objectively reflect a country's real position in the global supply chain.By identifying the embedding path of global supply chain, scholars can more deeply analyze the behavior of heterogeneous economic subjects in the value chain.In summary, the global supply chain and the value chain may be popular themes to be researched by scholars in the next few years.

Circular Economy and Sustainable Development
The theme of the circular economy and sustainable development are mainly from the perspective of the value chain.The rise of emerging economies has brought new opportunities.In recent years, the market demand of developed countries has been weak, and global trade has become more and more dependent on emerging economies, especially the Asian region with a developed manufacturing industry, which shows that Asian emerging economies have become a new driving force for global economic growth [47].The gap between emerging economies and developed countries is gradually narrowing, trade links are gradually strengthening and mutual direct investment is growing steadily.All these have brought new growth points to the research on the circular economy and sustainable development from the perspective of the value chain.For the technology supporting sustainable development, the new technology revolution is currently in the incubation period, which provides a prerequisite for the value chain to promote the development of new technology applications in industrial enterprises [48].While developed countries are actively promoting reindustrialization, developing countries are also vigorously developing intelligent technology and next-generation Internet technology [49].Emerging economies, represented by China, South Korea, India and Russia, are becoming an important force in scientific and technological progress [50].The substantial increase in research and development investment, and the institutional reform that gave birth to new organizations and new business forms, provide good conditions for late developing countries to catch up with and surpass developed countries.Therefore, the research of the internal industrial structure adjustment of these developing countries has created a new opportunity for the circular development of the world economy from the perspective of the value chain.At the same time, the adjustment of economic links makes the traditional international industrial division face changes.Emerging economies are expected to break the dilemma limited to the low-end links of the value chain and gradually transform to capital-and technology-intensive industries and service industries.The popularization of intelligence and automation will liberate a large number of the labor force and bring opportunities for the development of consumption and the service industry.In summary, the circular economy and sustainable development from the perspective of the value chain may be popular themes to be researched by scholars in the next few years.

Discussion
The research on the value chain is gradually increasing with the trend of enterprise transformation and world economic recovery.Its field also involves the research on the relevance of industrial manufacturing in different industries, economies with different economic conditions in various countries and regions and supply chains to enterprise upgrading.Some existing value chain studies focus more on a single field and lack the direction of value chain research from the perspective of time series and bibliometric methods [51][52][53].Some existing bibliometric research mainly focuses on the business model upgrading under the supply chain and technological innovation in the field of the green supply chain [19,54,55].Therefore, there is a lack of judgment on the research trends and future research hotspots in the field of the value chain.Through the bibliometric analysis of the papers published in the field of the value chain in the past decade, this paper makes up for the blank of using the bibliometric method and time series number generation in the value chain.This has theoretical and practical implications for the research of value chain.

Theoretical Implications
This bibliometric analysis contributes to the literature and practice in several ways.In regard to its theoretical contribution, this study may provide an objective discovery of knowledge clusters on the value chain, especially the networking.There is widespread agreement among researchers that such findings benefit scientific communication and future information retrieval procedures, both of which are essential to scientific development [56].Many bibliographic data sources and science-mapping approaches (bibliographic coupling, cocitations, etc.) are used to discover the various sorts of knowledge clusters that may exist in the subject of study in question.Secondly, this analysis may also be helpful in understanding the social networking of different actors of value chain research.The processes of information creation, diffusion and expansion are inextricably intertwined within a social network.Cocitation or coauthorship analyses may be used in scientific mapping to help us understand social patterns, such as the structure of academic networks and their connections (countries and institutions), that indicate relational qualities among scholarly organizations.In order to uncover the social processes that assist, cocreate, communicate and absorb information inside and beyond clusters, we first should acquire this understanding, because "individuals within a cluster develop a common vocabulary and a shared language".Third, this study tracks the evolutionary nuances.Scientific knowledge advances when new ideas are added, current ideas are borrowed and outmoded concepts are eliminated.This development represents evolutionary variations, and bibliometric research aids us in capturing and visualizing evolutionary nuances over time and space.Finally, this analysis provides researchers some ideas for future research.Using science mapping techniques, prospective researchers may identify critical knowledge gaps in the literature on the value chain and place potential contributions against known research streams.

Practical Implications
After the COVID-19, the global economy is facing the demand for recovery and regrowth, which requires diversity for different enterprises in developed and developing countries or regions.Through the figure of bibliographic coupling of countries/ regions, this paper finds that the current research area of the value chain is mainly composed of developed countries such as the United States and the United Kingdom.Through the figure of co-occurrence of keywords, this paper finds five main research fields, including supply chain innovation and sustainable development, industrial upgrading and transformation, international value chain and high value-added products, policy impact of developing countries, international labor and economic growth.Another contribution of this paper is represented by the keyword time series of the value chain from the figure of co-occurrence of keywords over time.By discussing the hotspots of previous research on the value chain, this paper discriminates the development of international trade under resource mismatch.By analyzing the research directions of the future value chain affecting enterprise production, economic circular development and international trade, this paper contributes to the future research direction of scholars in the field of the value chain.In addition, this study may help the practitioners in conducting the objective assessment and reporting of the research actors such as authors, countries and institutions.The measures that are used in bibliometric research also provide an objective way to evaluate relative performance.This might be an input for decision makers who are attempting to assess the ability of prospects (researchers, institutions, etc.) in comparison to that of others working in the sector.

Conclusions
Through the bibliometric and time series analysis of the literature in the field of the value chain in the past ten years, generated by the software Rstudio and VOSviewer and by the database from Scopus, the main conclusion are as follows.
In the past decade, the publication of literature on the value chain has generally shown an upward trend.From the research of different countries, the research papers of developed economic countries such as the USA and UK have the most published and cited papers.Marine Policy, Resources, Conservation and Recycling and World Development are the journals that publish the most articles related to the value chain.However, the most influential journals are Resources, Conservation and Recycling, World Development and the Review of International Political Economy.This paper also divides the keywords related to the value chain into five clusters.Each cluster represents a research direction and is related to each other.Additionally, the proportion and co-occurrence of keywords over time were studied to provide evidence to support future trend research.The predicted potential future research of the value chain mainly involves the fields of the economic, industry and global supply chain.In the field of economic development, future research will mainly focus on the application of the value chain in sustainable development and the circular economy.In the industrial field, future research will mainly focus on the application of the value chain in the process of industrial governance and upgrading.In the field of the international

Figure 1 .
Figure 1.Trends in the Number of Articles Published.

Figure 1 .
Figure 1.Trends in the Number of Articles Published.
Between 2016 and 2018, the research on the theme of value chain is more inclined to the research on the change of enterprise management mode of value chain in global trade.This kind of research has been very popular until now, thanks to the change of global trade.Since 2018, due to the influence of the COVID-19 virus, the global economic development has been greatly restricted, so the research direction of the value chain is closely connected with the epidemic.The development of the antiglobalization economic model also gives birth to the research types of the circular economy from the perspective of the value chain.

Figure 6 .
Figure 6.Co-occurrence of Keywords Over Time.

Table 2 .
Data Selection in Scopus.

Table 3 .
Trends of Publications and Citations.

Table 4
, the most published journal in the last ten years is the Marine Policy, which has published 68 articles.Marine Policy, Resources, Conservation and Recycling and World Development have published more than 40 documents.However, it is worth noting that the journals Resources, Conservation and Recycling, World Development and the Review of International Political Economy are cited the most times in total.At the same time, the average number of citations per document values of World Development, the Review of International Political Economy, the Journal of Business Ethics and the Journal of International Business Studies exceeded 40, reaching 40.91, 45.83, 47.00 and 48.78, respectively.

Table 4 .
Leading Countries in Value Chain Research.
AC/D: Average number of citations per document.

Table 5 .
Leading Journals in Value Chain Research.
AC/D: Average number of citations per document.