Sustainable Development and Industry 4.0: A Bibliometric Analysis Identifying Key Scientiﬁc Problems of the Sustainable Industry 4.0

: The main aim of the manuscript is the identiﬁcation of key research problems in the ﬁeld of sustainable development, in the era of implementing the Industry 4.0 concept. The manuscript presents results of the bibliometric analysis in the subject: “Sustainable Industry 4.0”. The bibliometric analysis was realized in three segments: Sustainability, Industry 4.0 and Sustainable Industry 4.0. In the analysis, the following databases were used: Web of Science (WoS), Scopus, Google Scholar. The main purpose of the analysis was to outline the dynamics of publications in the categories: citation, author, country, type document, science ﬁeld, research area. The review of sources carried out in this way allowed us to identify key research areas and conﬁrm the research thesis adopted in the manuscript. The research thesis: Sustainable Industry 4.0 allows the integration of the Industry 4.0 concept with sustainable development goals. The article is dedicated especially to scientists looking for still unsolved research problems in the implementation of sustainable Industry 4.0. Furthermore, the manuscript could be an inspiration for scientists, stakeholders, practitioners and governments to complete today’s knowledge about the problems of sustainability in Industry 4.0.


Introduction
In recent years, modern enterprises have been increasingly implementing new technologies belonging to the Industry 4.0 concept. Industry 4.0 is understood as a fourth industrial revolution, and is a consequence of the advances in ICT (information and communication technologies) being implemented in industry. Industrial changes are focused on industrial production and information technology. The combination of industry and information technology is the basis of the new Industry 4.0 (abbreviation: I 4.0). The term "Industry 4.0" was first used by Henning Kagermann at the 2011 Hanover Fair. Kagermann is a professor of physics, was CEO of SAP AG, and was involved in changing the development strategy of German industry [1,2]. Therefore, in order to define the key competences for the preceding period, the focus should be on research before this symbolic date. The Industry 4.0 concept can be understood as the technical integration of virtual-physical systems called cyber-physical systems (CPS) in production and logistics systems, and the application of the Industrial Internet of Things (IIoT) and the Internet of Services (IoS) in industry [3]. The Industry 4.0 concept means the formation of new value chains, changes in business models and the reorganization of processes of service provision and work [4][5][6][7][8]. Klaus Schwab, president of the World Economic Forum, published a Industry 4.0 implementation. The main aim of the manuscript is the identification of key research problems in the field of sustainable development, in the era of implementing the Industry 4.0 concept.

Materials and Methods
A research thesis was formulated in the manuscript: Sustainable Industry 4.0 enables the integration of the concept of Industry 4.0 with sustainable development objectives. A bibliometric analysis was used to prove the thesis. The bibliometric analysis is a method of evaluation of the research results, and a comprehensive view of current scientific achievements. It is based on the use and quantification of quantitative data on scientific papers, and the use of quantitative indicators of various databases. It is a measure of texts and information [29]. Bibliometric information related to a given scientific articles includes the author, affiliation, quotations from other publications, use by the reader and related key words. It may describe research related to a specific field or similarly describe the quantity and purpose of research in a given organization [30]. As an assessment method, it may help to determine the impact of technology or the effectiveness of the author or research organization. Finally, it serves as a monitoring tool, as it allows one to track the level of activity in a given field of research over a specific period of time.
Bibliometric studies are often used by researchers, public institutions, universities and businesses. The subjects in bibliometric analyses are determined by researchers. Researchers who identify research subjects also decide on the criteria for analysing databases. Quantitative bibliometric studies can be used to analyse market trends [31] and the development dynamics of individual thematic studies: sustainable logistics and sustainable production [32]; Industry 4.0 [33], supply chain sustainability [34], research at flexibility [35], green supply chains [36], smart factory concept [25]; reverse logistics considerations [37]; urban logistics [38]; renewable energy [39], etc. The results of the analysis are most often presented numerically (researchers use mathematical and statistical methods), or descriptively, using semantic maps, matrices, clusters, trends, etc. [32,33,39,40]. The bibliometric analysis was carried out according to the stages of systematic literature review (SLR) [41][42][43][44][45][46]. According to the methodology adopted in the study, the following stages were carried out: planning, implementation, reporting ( Figure 1). The structure of the applied methodology is presented in Table 1.
bibliometric analysis in the subject: "Sustainable Industry 4.0" Step 1: Planning research subject, research goal, information sourcesdatabases, research segments, time period, keywords, indicators and other criteria of analysis Step 2: Executing search forms (automatic, manual), collecting and ordering data, compilation of data and final data Step 3: Reporting analysis of dynamics, presentation of results (tables, figures), description of obtained results   • for segment S1: S1 KW1: sustainability S1 KW2: sustainable business S1 KW3: sustainable production used keywords in the layered analysis were fixed on based experts knowledge automatic (by using program Publish or Perish), manual manual form was used for preparing a discussion as the part of the publication search results: S1 KW1 I1, S1 KW1 I2, S1 KW1 I3 . . . etc. for each segments (S1, S2, S3) results of searching data bases were organized into three parts of the paper according to analyzed segments: S1, S2, S3 The research focused on three thematic/research areas: • (S1)-Sustainability; it was the largest area due to the age/maturity of the concept (1987-the Bruntland report-was the beginning of the concept of "Sustainability"). The time span of the analysed period is 1987-2019.
• (S2)-Industry 4.0, whose time span in the analyzed period was 2011-2019. The beginning of this research area is 2011, when the German government introduced an industrial development strategy based on high and intelligent technology. The author of the name "Industry 4.0" is Henning Kagermann (2011) [1].
• (S3)-Sustainable Industry 4.0; the time span of the analysed period is 2014-2019. It was assumed that "Sustainable Industry 4.0" is an evolved form of Industry 4.0 towards sustainable development.
The structure of the stratified analyses, which was carried out in three segments, is shown in Figure 2.
" "   In the adopted research methodology, the keyword list is a closed list. The search method adopted in the study allowed us to achieve the aim of the article, which was to signal a new, common research area called "Sustainable Industry 4.0".
According to the adopted methodology, the focus was on a homogeneous set of research, which is "Sustainable Industry 4.0", and not on parallel sets: Industry 4.0 AND Sustainable Development. The separation of separate collections of scientific articles according to keywords: Sustainability (S1) and Industry 4.0 (S2) were used by the authors to show the evolution of the publication dynamics. Supplementing the list of keywords with additional synonyms related to "Industry 4.0" and "Sustainable Development" in a bibliometric analysis would result in a significant increase in the number of selected scientific papers, and the need for an additional qualitative analysis of their content.

Results of the Bibliometric Analysis in the Segment (S1): Sustainability
Based on bibliometric analysis, it was found that the most search results concerned the keyword "sustainability" (which in the manuscript is marked with code S1 KW1): For the second keyword: "sustainable business" (S1 KW2): • The average number of authors per publication (paper) was 2.35 (median 2), • The Hirsch index for "Sustainable business" was: 168 (a = 4.33, m = 6.22).
The number of scientific papers (total) in particular databases is shown in Figure 3.
The dynamics of publications for keyword "Sustainability" in the period from 1987 to 2019 for particular keywords (KW) in the segment S1 "Sustainability" The dynamics of publications for keyword "Sustainability" in the period from 1987 to 2019 are shown in Figure 4.
The dynamics of publications for keyword "Sustainability" in the period from 1987 to 2019 for particular keywords (KW) in the segment S1 "Sustainability" Figure 4. Dynamics of publishing for particular keywords (KW) in the segment S1 "Sustainability". Source: Own study based on the databases: Web of Science and Scopus. Moreover, the number of scientific papers in recent years was compiled in Table 2. The next analyzed category was "document type". Among different types, the most share belonged to scientific articles. For the keyword "sustainability", the share was 64% (Scopus) and 72% (WoS). In second place were proceedings paper/conference paper (above 20%); the next positions were: reviews, book chapters and materials editorial. In the "source title" category, the most results were achieved for journal "Sustainability": in the Scopus database: 6135 results; in the WoS database: 5630. The next items in the "source title" category were occupied by the journal: Journal of Cleaner Production (3720 results in the database Scopus and 4319 results in the database WoS); and Ecological Economics (1040 results in the Scopus database and 1086 results in the WoS database).
For the keyword "sustainable business" in both databases using the filter "type document", these also brought up the most scientific articles. The share of scientific articles in the WoS database was 64%, and in the Scopus database was 60%. In second place were conference papers: 21% in the Scopus database and 29% in the WoS database. The next places belonged to book chapters, reviews and editorial materials. In the Scopus database, the most popular journals were: Journal of Cleaner Production (127), Sustainability Switzerland (109), Business Strategy and the Environment (29), and in the WoS database: Journal of Cleaner Production (120), Sustainability (106), Business Strategy and the Environment (23).
In the Scopus database for the keyword "sustainable production", 64% of all documents belonged to the category "article"; the second category was "conference paper" with 14%, and third was "review" with 5.5%. Other types of documents were: book chapters (7.5%), and books (1.4%). In the WoS database, the following shares were obtained: 72% for the category: "article", 17% for the category "proceeding paper" and 11% for the category "review". Categories: book chapters (4%) and books (2%) had less than 5% share in the total number of scientific papers. In the WoS database for keyword "sustainable production", most of the results were obtained by journals: Journal of Cleaner Production (225), Acta Horticulturae (151) and Sustainability (82), and in the Scopus database: Journal of Cleaner Production (222), Acta Horticulturae (170) and Sustainability (78). Figure 5 shows the document types in two databases (Scopus-B1 and WoS-B2) for particular keywords. Table 3 shows an overview of the most important scientific journals (Top 3), with the largest number of papers included "sustainable", "sustainable business" or "sustainable production". The order of presentation in the table is related to scientific impact.
Energies 2020, 13, 4254 9 of 27 "Sustainability" "sustainable", "sustainable business" or "sustainable production" --" " Figure 5. Types of published scientific papers for particular keywords (KW) in the segment S1 "Sustainability". Source: Own study based on the databases: Web of Science and Scopus. Table 3. The most important scientific journals with the largest number of scientific papers which include keywords: "sustainable", "sustainable business" or "sustainable production" (TOP 3). In both databases for the three categories of keywords, the most popular journals were: Sustainability and Journal of Cleaner Production. These journals have achieved, together, in all categories, the following results: Sustainability-6322 results in the Scopus database and 5818 results in the WoS database; Journal of Cleaner Production-4069 in the Scopus database and 4664 results in the WoS database.
The next analyzed category was "author name": • In Google Scholar, the most citations for the keyword "Sustainability" were for: Dierickx Results for the filter "author name" are shown in Table 4 (TOP 5). By country category, the publications searched for contained all the key words from the segment (S1); these most often came from the USA, China or EU countries. Table 5 presents details of this search, including scientific papers from India and Australia. Over 95% of all papers in the WoS and Scopus databases were written in English.   [33] in Figure 8 are shown. Table 6 presents the number of publications in the period from 2011 to 2019.   The analysis of publications by category "document type" showed the popularity of conference papers. In the Scopus database, for keyword "Industry 4.0" (S2 KW1), there were: 4831 conference papers, 2528 articles, 270 book chapters, 207 conference reviews, 168 reviews, and others. The share of conference papers in all publications (total 8241) was 59%, and the share of articles was 31%.
for particular keywords (KW) in the segment S2 "Industry 4.0" he keywords: "Industry 4.0", "smart factory" or "smart production" Results of own analysis for the subject: "Industry 4.0" in the Scopus database are presented in Table 8. According to Google Scholar, among the quoted items on the subject of "Industry 4.0", the first place was taken by the publication: Hermann, M., Pentek, T. Otto, B. Design principles for Industry 4.0 scenarios, published in 2016 during the 49th International Hawaiian Conference [8]. These papers received 1614 quotations (12 December 2019) (a total of 19,600 search results for "Industry 4.0" in Google Scholar). Table 7. Top 3 most important sources with the largest number of articles for the keywords: "Industry 4.0", "smart factory" or "smart production".  In the analyzed segment (S2) for the keyword Industry 4.0, the most popular country was Germany, in second place among countries was Italy, and on the third, was the United States. For the keyword "smart factory", in second place was South Korea, and in third place was China. For the keyword "smart production", such results were found: Germany, China, Austria (TOP 3). In the WoS database publications ("Industry 4.0"), realized by German authors in 2011-2019, were 992 papers. Second place in the WoS: Italy-498, third place: China-408, fourth place: Spain-290, fifth place: US-268. The next keywords in the first place were German authors too. Most of the scientific papers were written in English language, 94% (Scopus); in second place was German language (5%). Table 9 presents more information about scientific papers by using the filter: country.

Results of the Bibliometric Analysis in the Segment (S3): Sustainable Industry 4.0
The analysis of this segment in the Scopus and WoS databases was limited to two keywords: "sustainable Industry 4.0" and "sustainable smart production". The databases were also searched using other keywords, but no significant results were obtained. The shortest analysis period was adopted because the topic "Sustainable Industry 4.0" is the newest of the analyzed topics, and had a short history.

Discussion
Based on the bibliometric analysis, it was found that most science articles concerned the topic of "sustainable development". This is due to the long period of research on this issue (from the end of the last century to the present day). A smaller set of publications consists of the subject of "Industry 4.0", and the smallest one is "Sustainable Industry 4.0" (see diagram Venna). Even if the same analysis period 1987-2019 was used for all three segments, significant changes in the results would not be achieved, as both the concepts of "Industry 4.0" and "Sustainable Industry 4.0" are new concepts. The concept of "Sustainable Industry 4.0" is gradually being built, using the pillars of Industry 4.0 to achieve sustainable development. The results of the bibliometric analysis confirm the thesis that: Sustainable Industry 4.0 allows the integration of the Industry 4.0 concept with the goals of sustainable development. Contemporary business organizations recognize changes in the Industry 4.0 concept as important for sustainable development. This means that "Sustainable Industry 4.0" is a developed form of "Industry 4.0", taking into account the aspects of sustainable development (see Figure 10).
" " " " " -" " " " " " " " " " Moreover, when analyzing the content of scientific articles in bibliometric analysis, it can be concluded that three keywords: sustainable business, sustainable production and sustainable enterprise with intelligent innovations, create a new research area: "Sustainable Industry 4.0" (see Figure 11).
" " " " " " " " " " " " " " Using Venn diagrams (Figure 12), the set of scientific articles from the research area "Sustainable Industry 4.0" was presented as a subset of a larger set of "Sustainability". The set of articles from the research area "Industry 4.0" was inside the larger set of "Sustainable development", and its further evolution in the set led to the creation of another subset "Sustainable Industry 4.0". The discussion about the directions of evolution of the Industry 4.0 concept in a sustainable environment concerns the search for an answer to the question: How sustainable is smart and how smart is sustainable? [64]. Figure 12 shows the evolution of the interest of scientists in particular areas of research. In our opinion, the share of sustainable Industry 4.0 will fill the Sustainability area, while the interest in Industry 4.0 will decrease in the future. Based on the bibliometric analysis, the collections presented in Figure 12a) are those that best reflect the current situation with regard to the topics of scientific papers and research areas. The authors who deal with the topic of "Sustainability" try to explore a new area, Sustainable Industry 4.0, using the solutions offered by Industry 4.0 to achieve a sustainable but modern environment. Another set relationship, marked as Figure 12b) in Figure 12, is the detailing of set Figure 12a), where only "smart" technology is extracted from Industry 4.0 in order to apply it for sustainability. Figure 12c) illustrates a situation where there are scientific papers that fall into both sets, i.e., the research set called "Sustainability" and called "Industry 4.0", but present a new thinking pattern that can be classified as Sustainable Industry 4.0. The last collection (part d) of Figure 12 is the future, when the authors' research on Sustainable Industry 4.0 will be so extensive that the currently popular area of research "Industry 4.0" will be included in the collection of research on Sustainable Industry 4.0. Sustainability is likely to remain the largest collection, due to the wide range of research on what it contains. An interesting issue of the considerations is the level of interest in the issues of "Sustainable Industry 4.0" in the future. This requires an answer to the following question: whether "Sustainable Industry 4.0" will be a more extensive area of research and analysis than "Industry 4.0". Future research may lead to the expansion of the Sustainable Industry 4.0 concept, due to the fact that sustainable development is a popular and fashionable direction of change in the world economy. The "Sustainable Industry 4.0" as a research area is the result of the penetration of a strong set: "Sustainable" with the smaller (currently) set "Industry 4.0". Sustainable Industry 4.0 has a common part with the two sets:" Sustainability "and" Industry 4.0". The common part is "smart" [65]. The discussions about the role of Industry 4.0 in sustainable development and the evolution of Industry 4.0 to sustainable Industry 4.0 were provided in scientific papers [66][67][68]. Similarly, the problem of the distinctiveness of the features of "Sustainable Industry 4.0" and "Industry 4.0" was considered in the work of Erol (2016) [69]. In addition, other authors (2019) [70] propose a new approach to Industry 4.0 organizations and business models that are able to lead to a more sustainable society.
set of "smart" technology is extracted from Industry 4.0 in order situation where exist both sets, i.e., the research set called "Sustainability" and called "Industry 4.0", ion presenting that the authors' research on Sustainable Industry 4.0 will be so extensive that the currently popular area of research "Industry 4.0" will be included in the collection of research on "Sustainable Industry 4.0". " " " " (c) A situation where exist both sets, i.e., the research set called "Sustainability" and called "Industry 4.0", but present a new thinking pattern that can be classified as "Sustainable Industry 4.0". 12d) Expected situation presenting that the authors' research on Sustainable Industry 4.0 will be so extensive that the currently popular area of research "Industry 4.0" will be included in the collection of research on "Sustainable Industry 4.0". Source: Own study based on bibliometric analysis.
As a result of the conducted research, common research areas were distinguished for the scientific papers under consideration. A detailed analysis of the content of the selected papers shows that the Industry 4.0 implementation in the sustainable environment most often concerns urban spaces and creates a research area: "Sustainable smart cities" or "Sustainable smart urbanization". Detailed research in these areas is presented in the works of: Farag, SG (2019) [ [85]. The authors of scientific papers have formulated the following problems: Do cities want to become smart or sustainable? [86,87]; What are the components of smart cities? [84], etc. They have defined the components of smart cities as smart tourism, smart transport [88,89], smart buildings, smart public services, smart eco-system, etc.
The second large area of research of scientists is the use of Industry 4.0 solutions for energy management. This area was the subject of the following works: Hidayatno, Destyanto, and Hulu (2019) [90], Wang, L. (2014) [91], Kakegawa, M. (2019) [92]. Among the scientific papers about sustainable and smart energy, there is also a paper about using sustainable energy for smart cities [93]. An interesting problem was considered in the paper [63], where the authors have presented the approach to reduce waste and energy consumption, within an interesting approach called "multi-stage smart sustainable biofuel production system".
The third area of research is "Sustainable Smart Manufacturing". The topic was presented in the following papers: Rodrigo Goyannes Gusmăo Caiado, Osvaldo Luiz Gonçalves Quelhas (2019) [ [26]. Authors show the use of Internet of Things technologies, Big Data and CPPS (cyber-physical systems) in industrial environments, to create smart factories. The intensive development of new technologies is heading towards the functioning of artificial intelligence in a sustainable environment. Modern sustainable production uses IT solutions, new materials (smart, green, renewable materials) [100] and new products.
A detailed analysis of scientific papers allowed one to define current research areas in the field of sustainable development of economies, markets and individual industries. Using the "Sustainable Industry 4.0" filter, the following areas of research were found: smart sustainable tourist [101]), sustainable smart agriculture [102], smart ecosystem [103], sustainable smart service design [104], smart sustainable data [105,106] and intelligent supply chain [107][108][109]. Based on the number of scientific papers, different sizes of research areas were determined for "sustainable Industry 4.0". Scientists undertake research in the following areas: industrial sector (e.g., food, agriculture, energy), urban development, economy and society (towards society 5.0 and more connected society and economy). The social aspect "Sustainable Smart Society" was presented by Ferro Particular clusters can be grouped by micro-and macroeconomic areas and/or by levels. Such a division was made by authors Müller et al. [116], who define hypothesized relationships of Industry 4.0-related opportunities and challenges with the implementation of Industry 4.0 in sustainability. The authors examined three levels of opportunities: strategy, operations, environment and people, and identified three types of challenges: competitiveness and future viability, organizational and production fit, employee qualification and acceptance. The individual areas presented in Figure 13 can also be considered in the layout proposed by Machado et al. [99], that is, from the pillars of Industry 4.0 through the scope of sustainable manufacturing and business models, and sustainable models of development. In our analysis, the presented areas are only the final result of bibliometric analysis and may be further studied in the context of their grouping and relations between them.

Conclusions
Sustainability in Industry 4.0 is a new concept in the literature, and was created from the combination of sustainable development goals, in the process of implementing Industry 4.0 solutions in economies, society and enterprises. The Sustainable Industry 4.0 concept integrates sustainability and Industry 4.0, and is an area of research currently increasingly conducted by scientists and practitioners. Due to the research gap in the area of the lack of identification of research problems related to the Sustainable Industry 4.0 implementation, a bibliographic analysis was performed.
The article confirms the thesis that: Sustainable Industry 4.0 enables the integration of the Industry 4.0 concept with the goals of sustainable development. The identified new research area is poorly recognized in the literature, as are its separate components, such as Industry 4.0 or sustainability. According to conducted analyses, the sustainability has been present in scientific papers since the 1990s, that is, from the moment when the discussion about sustainability intensified. Starting from 2011, the Industry 4.0 concept appeared in scientific papers as new technological and IT solutions of the fourth industrial revolution. These two areas create a new area of research called "Sustainable Industry 4.0".
The new research area has already existed in scientific articles in analyzed databases. The scientific papers highlight a number of scientific problems in the area of Sustainable Industry 4.0, as follows: how to prepare society to implement Industry 4.0 in a sustainable environment? What skills and knowledge of employees will be required in the Economy 4.0? How will Industry 4.0 affect sustainable consumption and sustainable urban planning and smart city with smart transport? What kind of materials and resources will be used for Sustainable Industry 4.0? What kind of Industry 4.0 technologies are fit for use in sustainable development? Which industries and sectors of the economy are most susceptible to the implementation of Sustainable Industry 4.0? What smart technologies can be used in the circular economy?
In analyzed papers, the following areas are most often considered: sustainable smart city, sustainable smart urban planning, smart energy, green and smart materials, sustainable smart manufacturing, energy efficiency, cybersecurity. Presented research problems should encourage researchers and practitioners working on Industry 4.0 implementation to conduct research, especially in area economic, social and environmental benefits from the Sustainable Industry 4.0 implementation in different contexts.