Scientometric Trends and Knowledge Gaps of Zero-Emission Campuses

Abstract

As the urgency of addressing climate change grows, strategies such as developing zero-emission campuses to achieve carbon neutrality are becoming increasingly crucial. Yet, research in this field remains somewhat underdeveloped and fragmented. This study aims to bridge this gap, providing a scientometric analysis of the research conducted on zero-emission campuses from 1997 to 2023, using data from the Web of Science Core Collection. The study analyzed 1009 bibliographic records with the aid of CiteSpace software, focusing on identifying key co-authors, co-words, co-citations, and clusters. The findings indicate a rapid increase in research in the field of zero-emission campuses, with a significant surge in the number of publications in recent years, culminating in 174 in 2021 alone. The leading universities in terms of publication count were the University of California System, Egyptian Knowledge Bank, and the Chinese Academy of Sciences. Furthermore, the United States, China, and the United Kingdom were identified as the main contributing countries/regions to publishing in this field, indicating a broad, global collaboration. The scope of research has broadened from technical elements, such as energy, to encompass social factors that influence sustainability. Emerging research areas were identified, including education and sustainability, renewable energy and energy efficiency, campus planning and design, waste management and recycling, policy support, and pro-environmental behavior. This study provides a structured overview of the research landscape in the field of zero-emission campuses, offering valuable guidance for academics and encouraging further collaboration. The identified research clusters, notable authors, and influential institutions hold significant implications for policy decisions, industry practices, and the implementation of zero-emission strategies on campuses, aiding in the broader pursuit of sustainability.

1. Introduction

1.1. Background

Climate change inarguably remains mankind’s biggest challenge, and has now become a reality people must deal with. The anthropogenic emissions of CO₂ in the atmosphere have negatively impacted the environment to levels that have prompted humanity to take immediate action towards carbon neutrality [1]. As a strategy for combating this global threat, there has been discussion about intergenerational equity and developing a sustainable lifestyle [2]. Because sustainability is a difficult concept to grasp, there are numerous perspectives on what it means to be sustainable [3]. One of these includes offsetting the anthropogenic CO₂ emissions through carbon capture while adopting clean and renewable energy sources thus attaining net zero emissions [4].

The fulcrum of global attention being paid to the phenomenon of net zero emissions was the Paris agreement of 2015, which aimed to keep the increase in the global average temperature to well below 2 °C above pre-industrial levels and to limit global warming to less than 1.5 °C Celsius; immediate action is required to reduce global greenhouse gas (GHG) emissions by 44% below 2010 levels by 2030, with net zero emissions by 2050 [4,5,6]. Due to the increasing urgency of the climate crisis, it is now one of the most discussed environmental issues, and it has become a part of nearly every school’s curriculum in order to give future generations the best chance of combating climate change [7,8,9,10,11].

Cities around the world have recognized their critical role in the fight against global warming, and have committed to reducing emissions and becoming carbon neutral despite these challenges [12,13,14]. At the end of the 2019 United Nations climate action summit, this resolution was ratified [15]. But, academics have criticized their climate change initiatives for lacking enthusiasm and ambition [16,17]. However, these criticisms are weakened by the lack of macro-studies that evaluate the efforts of major cities around the world in the fight against climate change [18].

According to the IPCC [19], cities around the world can achieve net zero GHG emissions if they phase out fossil fuels, use renewable energy sources (RESs), encourage behavior change, improve energy efficiency (EE) for both supply and demand, and implement negative emissions measures. The journey to net zero emissions necessitates the active commitment and coordination of public bodies at all levels of government, as well as non-state [20,21] and other societal actors [22,23,24]. If local governments and non-state actors are not dedicated to or equipped for carrying out their responsibilities, national policies may be undermined. Furthermore, insufficient coordination among various governance actors may prevent local actors from responding appropriately to emerging threats [25,26].

In an effort to promote sustainable development through systemic institutional changes at the local level, universities all over the world are implementing local initiatives known as “zero emission universities” within cities [27,28,29,30,31,32,33,34,35]. Universities are ideally suited to take the lead in the movement toward carbon neutrality due to their high energy consumption, extensive research capabilities [32,36], and ability to train tomorrow’s citizens and climate leaders via practical experience [37,38,39,40]. As a result of their commitment to and the implementation of sustainable development policies [41,42], adoption of renewables, and adaptation within their operations [43,44], zero-emission universities can impact responses to climate change.

1.2. Zero-Emission Campuses

Universities are pivotal in driving decarbonization and sustainable development, given their crucial societal and educational responsibilities in shaping the minds of future leaders [45]. With the swift expansion of university infrastructures globally, it is imperative to adopt strategies that emphasize low-carbon emissions and energy efficiency. Ali et al. [46] examined the influence of South African University Campuses on greenhouse gas emissions and the generation of renewable energy, concluding that the existing engagement is insufficiently effective. This underscores the necessity for fresh initiatives for eco-friendly campuses and policy direction from African governments. Mohamed et al. [47] conducted an analysis of sustainability indicator adoption at three University Campuses in Malaysia, utilizing the UI GreenMetric World University Ranking. Their focus was on key indicators, including infrastructure setting, energy and climate change policy, waste management, water usage, transportation systems, and sustainability in education. However, the primary challenge for universities is to figure out how to reduce their carbon footprint for decarbonization. Carbon emissions from universities fall into three categories: scope 1, scope 2, and scope 3. Scope 1 includes direct emissions from university facilities and vehicles, while scope 2 includes direct emissions primarily from electricity purchases. Scope 3 includes indirect emissions from the institution’s operations, also known as value chain emissions. This includes everything from on-site-manufactured goods to student/staff transportation to waste generated in operations.

Scope 3 emissions are said to be a major problem for universities due to the high population density on campus and the fact that they are practically impossible to measure, making them difficult to regulate [48]. Additionally, universities’ energy-intensive facilities make meeting their proposed emission targets difficult [49]. Numerous studies on the subject have primarily concentrated on reducing scope 1 and 2 emissions [50,51] while discussing incentives for reducing scope 3 emissions [52,53,54,55,56]. These, on the other hand, indicate universities’ ability to achieve operational carbon neutrality by reducing scope 1 and 2 emissions by 2030 [49] and scope 3 emissions by 2050, hence becoming zero emission campuses. Cano et al. [57] calculated the carbon footprint of emissions corresponding to scopes 1, 2, and 3 of urban Columbian universities. The authors identified the transportation process as the largest source of greenhouse gas emissions (58.51%), followed by the wastewater process (17.01%), electricity consumption (14.03%), and e-mails sent (6.51%). Also, Mustafa et al. [58] estimated the carbon footprint of NED university of engineering and technology in Pakistan and identified a carbon footprint of 21,500 metric tons of equivalent CO₂, with scope 1 and 2 emissions contributing 7% of carbon footprint, while scope 3 emissions accounted for 85.6% of the carbon footprint. However, in order to examine recent advances in the interdisciplinary fields of carbon emissions, carbon footprints, and advanced management methods, the zero-carbon campus terminology must be thoroughly examined and broadly interpreted.

1.3. Rationale for Scientometric Analysis

Scientometric Analysis serves as an excellent method for consolidating discoveries from recent studies, pinpointing gaps in research, and offering critical perspectives on evolving trends in scholarly inquiry. Sen et al. [33] conducted a systematic review of some targeted higher education institutions that are part of Australia’s efforts to achieve carbon neutrality. Helmers et al. [59] compared the carbon footprint at several universities objectively using standardized carbon footprint metrics. They identified only one zero-emission university (Leuphana University in Germany) and advocated for higher education institutions around the world to undergo rapid transformation. Kourgiozou et al. [55] outlined the recent scholarly work concerning smart building principles and smart energy systems on university campuses in the UK higher education sector. The paper then delved into the opportunities and challenges of integrating smart energy systems into university campuses, examining them through the lenses of policy and technology. Shboul et al. [54] endeavored to perform a statistical analysis of the campus as an integrated system, incorporating viewpoints from faculty, staff, and students (the campus users), aiming for a net-zero carbon footprint. This was pursued by creating an index to gauge the likelihood of adopting sustainable behavioral changes.

While previous studies have focused on quantifying and reducing the carbon footprint of universities, this study will map the linkage or working relationships among clusters of zero-emission campus research and their network to analyze the emerging trend during the last decades. Additionally, no prior research has analyzed its research scope to such depth, including bibliometric indicators like co-citation clusters, keywords, or research clusters. Thus, this research represents an original contribution to academia by filling a gap in the literature regarding the systemic institutional changes needed for zero-emission campuses.

1.4. Knowledge Gap, Research Objectives, and Value

In light of the foregoing, the purpose of this study is to conduct a thorough bibliometric review in order to identify and comprehend the development and context of zero-emission campuses, with the ultimate goal of furnishing researchers and practitioners with a comprehensive comprehension of the status quo and research in its research, with a concentration on carbon neutrality. For this reason, we will use the four scientometric methods described in Section 2 to (i) track the development of the field of zero-emission campus research and (ii) identify leading scholars and academic institutions. Furthermore, this study aims to (iii) determine the most important subject areas, (iv) isolate relevant research keywords and co-citation clusters, and (v) extrapolate the most important and rapidly developing trends in the field. The study’s results are expected to add to the existing body of knowledge by illuminating the trend and pattern of the zero-emission research field, identifying its research themes and clusters, outlining the network of key zero emissions researchers and institutions, and suggesting avenues for future research.

Furthermore, the findings of this study have the potential to significantly contribute to the global effort to combat climate change and promote sustainable development. By providing a comprehensive understanding of the current state of research on zero-emission campuses, this study could help inform policy makers in their efforts to combat climate change by identifying gaps in knowledge and areas for further research. It may also shed light on the most effective ways for universities and other institutions to reduce their carbon footprint and achieve carbon neutrality.

Meanwhile, 1009 bibliographic records of the Web of Science (WoS) Core Collection would be examined between 1997 and 2023. Methodology and literature search/indexing procedures are discussed in Section 2. Following this is a discussion of the scientometric review’s findings, as well as a breakdown of the key research clusters in Section 3, and, finally, a summary and recommendations for the way forward in Section 4.

2. Materials and Methods

To achieve its predefined research objectives, this study employed scientometric review, analyses, and visualization. The aim was to provide a comprehensive understanding of the structure (clusters), research areas, and trending topics in zero-emission campus studies to academics and industry practitioners, using illustrative diagrams and maps.

Scientometric analysis is a method that facilitates the comprehensive and concise capture and mapping of a scientific knowledge domain. This technique analyzes a body of scientific literature using mathematical models and diagrams [60] and assesses the growth and performance of research conducted by individuals, institutions, countries, and even scholarly journals within a certain research field [61].

A variety of methods, including bibliometric analysis [62,63], content analysis [64], literature reviews [65], and scientometric analysis [60,61] have been explored by numerous scholars in fields like energy, sustainability, waste management, and green building and innovation.

Four distinct scientometric methods are utilized in this research:

(1)

Co-author analysis: This examines the repeated occurrences of the same authors, countries, or institutions in scholarly publications.

(2)

Co-word analysis: This identifies links between keywords and topics by evaluating their co-occurrence in a body of literature.

(3)

Co-citation analysis: This involves assessing how often certain authors, articles, and journals are cited together.

(4)

Clusters analysis: This includes both silhouette metric analysis and burst detection analysis. The silhouette metric analysis evaluates the effectiveness of data clustering, whereas the burst detection analysis detects rapid surges or “bursts” in activity within the data.

To execute the four aforementioned scientometric analyses and their visualization, the software package “CiteSpace”, developed by Chaomei Chen, was used. As stated by Chen [66], CiteSpace version 6.2.R2 (64bit) Advanced was used to analyze articles from the indexed corpus, as it aids in knowledge domain mapping and facilitates the illustration of such maps through visual representations. For detailed instructions on conducting scientometric reviews of a research field using “CiteSpace”, refer to Chen [66,67,68,69] and Chen and Song [70]. The research strategy of the study is illustrated in Figure 1.

2.1. Research Design and Data

A comprehensive exploration of the knowledge domain surrounding zero-emission campuses requires not only the identification of pertinent scientific publications, but also the elucidation of trends and patterns within the research, as depicted in Figure 1. The initial critical step is the selection of suitable scientific databases. Scopus, Web of Science (WoS), and Google Scholar are recognized as three of the most extensive databases for scholarly works. A comparative study by Olawumi et al. [62], evaluated these databases based on their coverage, indexing policies, and search capabilities.

For this study, the Web of Science Core Collection was selected due to its focus on influential and rigorous research. It indexes influential journals renowned for their quality and offers balanced coverage across natural sciences, social sciences, and arts and humanities [60,71].

To identify relevant publications, a search string was developed, combining relevant terms: (zero emission* OR “low carbon” OR “zero carbon” OR “zero waste” OR “net zero” emission OR “climate neutral” OR “carbon neutral” OR “green”) AND (campus* OR university* OR college OR “Higher Education Institutions”) AND (“sustainability” OR “environmental” OR “renewable energy” OR “energy efficiency” OR “green building” OR “sustainable transportation” OR “sustainable food” OR “waste reduction” OR “carbon footprint”), as visualized in Figure 2. The asterisk (*) functions as a fuzzy search mechanism, accounting for variations in terminology.

The search spanned publications from 1997 to 2023, with the aim of capturing trends over a period of 26 years. This timeline encompasses the emergence of global climate consciousness, which was marked by the Kyoto Protocol in 1997 [72]. Journal articles, typically providing more detail than other types of publications [73], are considered credible sources of research [74]. Therefore, to narrow down the results to the area of interest, only articles classified as “Environmental Sciences”, “Green Sustainable Science Technology”, “Environmental studies”, “Energy Fuels”, and “Engineering Environmental” were retained.

As of November 2023, the search had resulted in 1009 bibliographic records. These references were downloaded and subsequently indexed using Mendeley reference management software. CiteSpace, a Java application designed for the visualization and analysis of scientific literature trends, was set up to scrutinize the data, following the guidelines detailed in the CiteSpace user’s guide [66]. Considering the extensive time span of the data, one-year time slices were employed to inspect citation patterns and relationships over time. Within each time slice, co-author analysis, co-word analysis, co-citation analysis, and cluster analysis were executed. The Pathfinder tool was utilized to prune redundant links, thereby refining network connections.

2.2. Network Analysis Techniques

As suggested by Chen [66] network structural properties such as modularity, mean silhouette, and betweenness centrality scores are instrumental in verifying the validity and quality of clusters identified through network analysis. In this study, the modularity score, mean silhouette score, and betweenness centrality were employed to gain a deeper insight into research trends, knowledge gaps, and collaboration patterns within the crucial and swiftly evolving field of zero-emission campuses.

2.2.1. Modularity Analysis

Modularity of a network, which leverages connectivity patterns to detect clusters of nodes, is an analytical tool that measures the degree of clustering and offers an overall reference for a given network’s decomposition [75]. The modularity score, with a range from 0 to 1, reflects the level of clustering within the network, with higher scores indicating more clustering. In this study, modularity analysis is applied to identify research topics that closely relate to each other and may share similar research themes. The clusters that are identified will be scrutinized to assess the extent of research collaboration and co-authorship within each.

2.2.2. Mean Silhouette

The mean silhouette score, a measure that assesses the similarity of nodes within a cluster compared to nodes in other clusters, ranges in value from −1 to 1, with the highest value signifying an ideal solution [66]. In this study, the mean silhouette score is employed to assess the quality of the clusters identified through the modularity analysis.

2.2.3. Betweenness Centrality

The betweenness centrality of a node within a network quantifies the extent to which that node is connected through paths linking any two nodes in the network [68]. In this study, the analysis of betweenness centrality is utilized to identify the most crucial and influential nodes within the co-authorship network of zero-emission campuses’ publications.

3. Results and Discussion

This section delves deeper into the scientometric analysis carried out for this study, analyzing its numerous facets and findings (see Figure 1 for an overview).

Tracing the origins of zero-emission campus research, it is observed that the first article was published in 1998, and it focused on the establishment of a zero-pollution-emissions building at Montana State University in the United States [76].

Today, the assessment of 1009 bibliographic records spanning the years 1997 to 2023 shows a positive trend (see Figure 3). Between 2012 and 2022, there is a significant increase in the number of papers devoted to the field of zero-emission campuses. The annual output of such publications had surpassed 174 by 2021, demonstrating increased interest and research intensity in this vital area of study.

The following sub-sections will dive into co-author analysis, co-word analysis, co-citation analysis, and cluster analysis to further examine this emerging discipline. These comprehensive analyses will provide insights into the important contributors, thematic trends, seminal works, and emerging clusters in the zero-emission campus research ecosystem.

3.1. Co-Authorship Analysis

Records from the WoS in this study area provide valuable insights into the authors, which can be leveraged to identify key influencers, universities, and countries/regions. Consequently, this information can be further utilized to analyze authorship networks, geographical distribution, and institutional affiliations, offering a comprehensive view of the research landscape.

3.1.1. Co-Authorship Network

A vast range of researchers contribute to the research landscape of zero-emission campuses. Walter Leal Filho of Manchester Metropolitan University, José Baltazar Andrade Guerra of Universidade do Sul de Santa Catarina, and Fabrizio Ascione of the University of Pisa lead the list in terms of publications (see

Table 1. Top 15 Leading authors with their H-index.

Authors	Institution	Country	Record Count	H-Index 1	% of 1009
Walter Leal Filho	Manchester Metropolitan University	England	9	45	0.892
José Baltazar Andrade Guerra	Universidade do Sul de Santa Catarina	Brasil	6	20	0.595
Fabrizio Ascione	University of Naples Federico II	Italy	4	37	0.396
Carlos Rogerio Montenegro De Lima	Universidade do Sul de Santa Catarina	Brasil	4	6	0.396
Rosa Francesca De Masi	University of Sannio	Italy	4	24	0.396
Francisco J. Rey-Martinez	University of Valladolid	Spain	4	8	0.396
Thiago Coelho Soares	Universidade do Sul de Santa Catarina	Brasil	4	5	0.396
Giuseppe Peter Vanoli	University of Molise	Italy	4	36	0.396
Fengqi You	Cornell University	United States	4	67	0.396
Ismaila Rimi Abubakar	Imam Abdulrahman Bin Faisal University	Saudi Arabia	3	22	0.297
Cecilia Marcelo Aldaz	Autonomous University of Madrid	Spain	3	3	0.297
Chia-chien Chang	National Taiwan University	Taiwan	3	11	0.297
Ye Chen	Jilin University	China	3	4	0.297
Marco Contri	University of Pisa	Italy	3	1	0.297
Liziane Araujo Da Silva	Universidade do Sul de Santa Catarina	Brasil	3	1	0.297

¹ The H-index of the authors are based on Web of Science calculation.

).

The co-authorship network study, which includes 507 nodes and 493 links, yields a modularity (Q) of 0.9772 and a mean silhouette (S) of 1. A high modularity score above 0.7141 suggests that our network is divided into loosely coupled clusters, indicating a healthy level of diversity and specialization within the research field. The perfect mean silhouette score of 1 indicates that the nodes within each cluster have similar characteristics, implying significant collaboration links and related research interests among these clusters. This implies that the co-authorship network is well-structured, with distinct and relevant clusters.

However, an assessment of citation bursts within this co-authorship network indicated a lack of such bursts. This indicates that the research outputs in this field have yet to attract substantial attention from the broader scientific community. This lack of citation bursts could be attributed to the field’s early stages of development, the relatively small size of the clusters, and the high level of interdisciplinarity. While these characteristics contribute to the field’s complexity and richness, they may also pose challenges to gaining immediate, widespread recognition. Nonetheless, they represent interesting potential for future research and collaboration.

3.1.2. Network of Co-Authors’ Institutions and Countries/Regions

This network consists of 441 nodes and 834 links, and provides detailed information about the countries, regions, and institutions that have contributed to the study of zero-emission campuses. As seen in Figure 4, the network’s modularity, Q = 0.6758, and high mean silhouette value, S = 0.8932, imply a topology in which nodes are densely packed within distinct clusters. This shows that these clusters have strong collaborations and knowledge exchange.

The United States appears as a prominent contributor, with 190 papers, closely followed by China, with 175 articles. England (84 articles), Italy (68 articles), and Spain (62 articles) also make significant contributions to the research area, each with more than fifty articles. The substantial output from the United States and China, each with over 150 publications, demonstrates their leadership in this field.

Furthermore, the analysis suggests strong worldwide collaboration, particularly among scholars from the United States, China, England, Italy, Spain, Germany, and Malaysia. Such relationships are essential to the global pursuit of zero-emission campuses. At the institutional level, the University of California System leads with 18 articles, followed by the Egyptian Knowledge Bank with 13, the Chinese Academy of Sciences with 12, and both Tianjin University and Universidade Do Sul De Santa Catarina with 10 each. These institutions are essential knowledge hubs in the field, contributing significantly to the growing corpus of research on zero-emission campuses.

In

Table 2. Citation bursts for countries and institutions.

Country/Regions	Burst Strength	Time Period	Institution	Burst Strength	Time Period
USA	12.22	2009–2017	Egyptian Knowledge Bank	2.82	2021–2023
Taiwan	4.34	2014–2019	Universidade da Coruna	2.8	2020–2021
Thailand	3.37	2019–2021	University of California Sytem	2.53	2015–2016
Canada	3.17	2008–2012	State University System of Florida	2.33	2020–2021
Italy	2.26	2016–2019	Autonomous University of Madrid	2.14	2019–2022
Japan	2.26	2007–2013	University System of Georgia	1.87	2006–2013
India	2.14	2017–2018	Southwestern University of Finance and Economics—China	1.79	2021–2023
Israel	1.87	2015–2020	University of California Berkeley	1.7	2015–2019
England	1.83	2014–2016	Lappeenranta-Lahti University of Technology	1.64	2016–2018
Indonesia	1.76	2020–2021	Lund University	1.62	2005–2013

, the results reveal that the United States (burst strength = 12.22, 2009–2017), Taiwan (burst strength = 4.34, 2014–2019), Thailand (burst strength = 3.37, 2019–2021), and Canada (3.17, 2008–2012) are the countries that have demonstrated the highest citation bursts in research related to zero-emission campuses. This indicates a concentrated period of intense interest and scholarly activity within these geographical regions.

On an institutional level, the Egyptian Knowledge Bank (burst strength = 2.82, 2021–2023), Universidade da Coruna (burst strength = 2.8, 2020–2021), University of California System (burst strength = 2.53, 2015–2016), State University System of Florida (burst strength = 2.33, 2020–2021), and Autonomous University of Madrid (burst strength = 2.14, 2019–2022) have experienced the highest citation bursts. These institutions appear to have significantly contributed to the body of knowledge on zero-emission campuses during these periods.

These citation bursts, particularly noticed between 2015 and 2021, align with the growing popularity of research on zero-emission campuses. This surge is consistent with the increased global awareness of climate change, punctuated by the 2015 Paris agreement. It underscores the recognition of university campuses as potential exemplars for sustainable development and the urgency of transitioning towards zero-emission societies.

The further analysis of our network, characterized by purple trims, points towards nodes with high betweenness centrality (see Figure 4). These central nodes represent pivotal points in the network, indicating that they have been influential in the flow and diffusion of knowledge.

In the context of countries, England (centrality = 0.32, 2001), the United States (centrality = 0.26, 2000), Germany (centrality = 0.21, 2002), Spain (centrality = 0.16, 2007), Italy (centrality = 0.15, 2010), Malaysia (centrality = 0.11, 2013), and China (centrality = 0.10, 2001) have emerged as key contributors. These countries, during the years studies, have played significant roles in curating and creating the research landscape related to zero-emission campuses.

At the institutional level, the University of California System (centrality = 0.03, 2002), Chinese Academy of Sciences (centrality = 0.02, 2002), California State University System (centrality = 0.02, 2013), and Arizona State University (centrality = 0.01, 2013) have exhibited high betweenness centrality. These institutions stand as major nodes in the research network, proving their prominent positions in driving and connecting research in the domain of zero-emission campuses.

3.2. Co-Occuring Author Keywords and Keyword Plus

Over the years, a convergence of diverse disciplines has formed recognized trends and novel areas in zero-emission campus research. This finding is based on the analysis of bibliometric data from the WoS database, with a focus on keywords and subject categories related to zero emissions.

Within the generated WoS dataset, subject categories were classified based on the scope of the related journals. However, an article could be assigned to more than one category. Over 100 zero-emission related publications were discovered in six subject categories: Environmental sciences (568 articles), Green Sustainable Science Technology (537 articles), Environmental studies (341 articles), Energy fuels (189 articles), Engineering Environmental (159 articles), and Education Educational Research (117 articles).

Additionally examined were keywords, which served as important reference words. They play a crucial role in the identification of key topics, concepts, and themes within publications, hence aiding in the categorization and retrieval of research data. This study focused on examining two distinct categories of keywords, author keywords, often provided within the articles, and keywords plus, which are automatically generated by databases such as Web of Science, based on the titles of the articles cited in the paper. These keywords are merged in the CiteSpace software to form a network of co-occurring keywords (Figure 5).

The resulting network incorporated 542 nodes and 1840 links, exhibiting a modularity of Q = 0.6071 and a mean silhouette of S = 0.7838. High-frequency keywords within the dataset included “performance” (count = 105), “university” (count = 93), “sustainable development” (count = 86), sustainability (count = 86), “management” (count = 78), “attitudes” (count = 65), “climate change” (count = 59), “impact” (count = 57), “behaviour” (count = 52), and “renewable energy” (count = 51).

As seen in

Table 3. Citation bursts for keywords.

Keywords	Strength	Time Period
Green building	5.44	2006–2016
Energy efficiency	4.85	2008–2015
Tool	4.45	2020–2021
Behaviour	3.88	2015–2019
Pro-environmental behaviour	3.65	2019–2021
Green space	3.65	2019–2021
Conservation	3.58	2016–2018
Climate change	3.55	2018–2019
Intention	3.23	2020–2021
Theory of planned behaviour	3.23	2020–2021
Urban	3.18	2021–2023
Environments	3.01	2019–2020
Green buildings	2.84	2021–2023
Efficiency	2.83	2014–2018
Feedback	2.82	2017–2018

, the citation bursts pointed to 15 keywords that comprise the network’s nexus. These keywords comprise emerging issues and themes in zero-emissions research, reflecting shifting academic and institutional priorities throughout time.

Historically, keywords such as “green buildings,” “energy efficiency,” “behavior,” “pro-environmental behavior,” “green space,” “climate change,” “Theory of Planned Behavior,” and “urban” have been prominent on university campuses. However, the keywords “green building” and “energy efficiency” have been constantly prevalent since 2006, with the longest citation burst duration of approximately 10 years.

From 2006 to 2016, for example, “green building” experienced a strong citation burst (strength of 5.44), showing the increased emphasis on sustainable building practices. Similarly, the keyword “Energy efficiency” experienced a substantial citation burst (strength of 4.85) between 2008 and 2015, indicating the increasing importance of energy saving on college campuses. The relevance of the keyword “Tool” (strength of 4.45) has increased in recent years (2020–2021), indicating the development or use of novel methodologies in this research area. The keywords “Behaviour” and “Pro environmental behaviour”, with bursts from 2015 to 2019 and 2019 to 2021, respectively, signal a growing interest in understanding and promoting environmentally friendly actions.

Other terms such as “Green space,” “Conservation”, “Climate change,” “Intention”, “Theory of Planned Behavior”, “Urban”, “Environments”, “Green buildings”, “Efficiency”, and “Feedback” all experienced citation bursts at various periods, highlighting the field’s multidisciplinary nature, which combines urban planning, environmental science, construction, and social psychology. These bursts highlight the diverse and complex landscape of research interests on the path towards zero-emission campuses.

3.3. Co-Citation Analysis

Co-citation refers to the process where two or more research papers are collectively cited by a subsequent study, a feature which enhances the accuracy of author identification [68]. It aims to identify the influential works, authors, or research themes within a specific field, and to reveal patterns of intellectual impact and collaboration. Journal, author, and document co-citation networks were generated by analyzing indexed bibliographic records from the WoS database.

3.3.1. Journal Co-Citation Networks

This study analyzed 1009 bibliographic records from WoS, encompassing 203 journals. Remarkably, three journals each contributed at least fifty articles to the research corpus.

Table 4. The 9 most prominent academic journals used in the database.

Publication Titles	Host Country	Impact Factor 1	H-Index	Publisher	Record Count
Sustainability	Switzerland	3.889	136	MDPI	210
International Journal of Sustainability in Higher Education	United Kingdom	4.12	72	Emerald Group Publishing Ltd.	104
Journal of Cleaner Production	United Kingdom	11.07	268	Elsevier Ltd.	94
Energies	Switzerland	2.841	132	MDPI	32
International Journal of Environmental Research and Public Health	Switzerland	4.598	167	MDPI	23
Energy and Buildings	Netherlands	9.238	214	Elsevier	20
Environmental Science and Pollution Research	Germany	3.986	154	Springer Nature	19
Applied Energy	United Kingdom	11.745	264	Elsevier	18
Building and Environment	United Kingdom	8.003	189	Elsevier	17

¹ Impact factor in the year 2023.

details the top nine journals contributing to zero-emission research, as well as their respective H-indices and impact factors.

As shown in Figure 6, the journal co-citation network included 925 nodes and 2917 linkages. The network had a modularity of Q = 0.7148 and a mean silhouette of S = 0.8547, indicating that it had a robust community structure. Each node’s size shows the frequency of co-citations between journals in the dataset. The most frequently co-cited journals in the dataset are the Journal of Cleaner Production (frequency = 558), Sustainability (frequency = 417), International Journal of Sustainability in Higher Education (frequency = 286), Renewable and Sustainable Energy Reviews (frequency = 245), Energy Policy (frequency = 185), and Applied Energy (frequency = 183). These journals are frequently cited due to their significant contributions to the field of zero-emission research.

As shown in

Table 5. Burst strengths of co-cited journals.

Cited Journals	Burst Strength	Time Period
Sustainability	13.78	2011–2019
Energy Policy	12.72	2013–2018
Energy and Buildings	11.57	2011–2018
International Journal of Sustainability in Higher Education	9.05	2011–2016
Journal of Social Issues	7.74	2013–2018
Atmospheric Environment	7.23	2013–2019
Journal of Environmental Education	5.91	2013–2018
Solar Energy	5.78	2013–2017
Health Place	5.64	2021–2021
Journal of Environment International	5.55	2020–2023
Journal of Marketing	5.45	2015–2017
Environment	5.42	2014–2020
Environmental Research	5.2	2021–2023
Ecological Economics	5.11	2016–2018

, the study identified fourteen journals that experienced citation bursts, three of which had burst strengths more than 10.0. These journals, which received numerous citations in relatively short periods of time (less than two years), are indicative of active scholarly collaborations and research interests during those periods.

The strongest citation burst was observed in Sustainability from 2011 to 2019. This correlates with similar citation bursts in journals such as Energy Policy and Energy and Buildings. This pattern highlights the critical role of sustainability studies, policy analysis, and building design in the research field of zero-emission campuses.

Furthermore, the citation bursts observed in other academic journals, including but not limited to the Journal of Social Issues, Atmospheric Environment, The Journal of Environmental Education, Solar Energy, Health and Place, and Ecological Economics, highlight the multidisciplinary nature of zero-emission campus research. These sources cover a wide range of fields, from social concerns and atmospheric studies to environmental education and ecological economics, demonstrating the complex and multifaceted character of research toward zero-emission campuses.

As illustrated in Figure 6, certain journals were found to have high betweenness centrality scores, as indicated by the purple links within the network. These journals include Environmental Science and Technology (centrality = 0.16, 2002), The International Journal of Life Cycle Assessment (centrality = 0.11, 2004), Energy (centrality = 0.10, 2001), Ecological Economics (centrality = 0.08, 2008), Environment and Behavior (centrality = 0.08, 2001), and Social Science & Medicine (centrality = 0.08, 1997). The high centrality scores suggest that these journals enabled the integration and cross-pollination of zero-emission campus concepts, theories, and methodologies.

Additionally, the range of topics covered by these journals, from environmental science to social science, emphasizes the interdisciplinary nature of zero-emission campus research. This suggests the need to take a diverse approach to tackling the challenges and utilizing the opportunities associated with developing and managing zero-emission campuses.

3.3.2. Author Co-Citation Network

The author co-citation network, as illustrated in Figure 7, assesses author relationships, and it is generated based on the frequency of their works being cited together in the same research article. The network, which was generated from the Web of Science (WoS) dataset, is composed of 1003 nodes and 2675 links, exhibiting a modularity of Q = 0.8089 and a mean silhouette of S = 0.9077. The sizes of the nodes in this dataset are indicative of the co-citation frequency of authors, and the links between nodes serve as an informal representation of cooperation.

Among the 30 most-cited authors identified within the network, four are supranational organizations: the United Nations (frequency = 50), the European Commission (frequency = 45), UNESCO (frequency = 39), and IPCC (frequency = 26). Their participation contributes to the credibility, standardization, and global impact of the research. Furthermore, the diversity based on the authors’ affiliations further substantiates the evolution of zero-emissions research.

In the network, certain authors were identified as having experienced significant citation bursts, indicative of a rapid increase in the number of citations to their articles over a short span of time. These authors include Stephens J.C. (burst strength = 7.01, 2016–2019), Kollmuss A. (burst strength = 5.68, 2018–2019), Findler F. (burst strength = 4.98, 2020–2023), Abrahamase W. (burst strength = 4.93, 2016–2018), and Trencher G. (burst strength = 4.8, 2020–2021).

Furthermore, certain nodes in the network were observed to have high betweenness centrality scores, as indicated by the purple links. The authors with the highest centrality, which suggests they serve as crucial connectors within the network, include Stern P.C. (centrality = 0.11, 2001), Cortese A.D. (centrality = 0.09, 2003), Alshuwaikhat H.M. (centrality = 0.07, 2008), Ajzen I. (centrality = 0.05, 2007), Jabbour CJC (centrality = 0.05, 2013), and Wright T.S. (centrality = 0.05, 2012).

3.3.3. Document Co-Citation Network

In the quest to understand the intellectual structure of zero-emission campus research, 1009 bibliographic records were examined for document co-citations. An analysis of the WoS dataset showed six of the ten most cited publications, each with over ten citations, were published in the Journal of Cleaner Production, underlining the journal’s crucial role in this field of study. Co-cited documents, represented as nodes and links in the network, provide valuable insights into the foundational literature in the field. The five most frequently co-cited documents are works by Dagiliute et al. (2015) [37] (counts = 35), Hair J.F. et al. (2009) [77] (counts = 31), Lozano et al. (2015) [78] (counts = 23), Lozano et al. (2013) [79] (counts = 20), Aleixo et al. (2018) [80] (counts = 17), and Findler et al. (2019) [81] (counts = 13).

As shown in Figure 8, the network of document co-citations contained 2817 links and 1118 nodes, with a mean silhouette of 0.9455 and a modality Q of 0.9169. In addition, it should be noted that every node and link within the network diagram represents a cited document and co-citation relationship, respectively. These documents are appropriately labeled with the last name of the author and the year of publication. Moreover, the diagram also captures the co-citation relationship that exists between two distinct articles.

The articles with the highest citation bursts have been identified as those authored by Lozano et al. [78] (burst strength = 9.79, 2013–2018), Lozano [79] (burst strength = 9.24, 2016–2020), Dagiliute et al. [37] (burst strength = 7.24, 2019–2023), Zsoka et al. [82] (burst strength = 5.61, 2016–2018), and Findler et al. [83] (bursts strength = 5, 2020–2023).

Dagiliute et al. [37] conducted an in-depth assessment of the disparities in sustainability performance between “green” and “non-green” universities from students’ perspectives. This research received the highest number of citations, with a total of 35. The paper suggests the adoption of official declarations and commitments in certain campus activities and the utilization of information campaigns as means to enhance sustainability measures in a more comprehensive manner. Furthermore, an investigation was conducted by Lozano et al. [78] into whether commitments to declarations, charters, and other initiatives by academic institutions lead to more sustainable development within these institutions. A call was made for higher education systems to integrate sustainability policies and strategies and commit to institutionalization to ensure the system-wide implementation of sustainable development. Furthermore, it was suggested by Zsoka et al. [82] that environmental education plays a significant role in shaping attitudes towards sustainable consumption.

The top four articles in terms of betweenness centrality were identified as Hair et al. [84] (centrality = 0.03), Hair J.F. [77] (centrality = 0.02), Dagiliute et al. [37] (centrality = 0.02), and Avila et al. [85] (centrality = 0.02). These articles are particularly useful for those researching the transition to zero-emission campuses, as they address the main issues that have been raised thus far, such as students’ attitudes towards sustainability in green and non-green universities, the implementation of net zero waste in universities, and the practices of environmental management systems in these institutions.

3.4. Cluster Analysis

In this study, an exploratory data-mining technique known as cluster analysis was utilized to discern and scrutinize key concepts, trends, and connections in the domain of zero-emission campus research. This technique allows the objective segmentation of substantial volumes of research data into more digestible categories, from which insights can be extracted.

Citespace software was employed in order to distinguish the distribution and structures of research themes and topics. Moreover, within Citespace, the log likelihood ratio (LLR) clustering technique was utilized to identify relationships between research terms or themes (such as keywords, authors, and journals) within the WoS corpus that display high intra-class and low inter-class characteristics.

Two types of clusters were identified in this study: keyword clusters, which were derived from author keywords and keyword plus, and document citation clusters, which were derived from cited references. This approach offers a valuable perspective on the evolving trends and central themes in the field of zero-emission campus research.

3.4.1. Keyword Cluster

Utilizing the LLR algorithm of CiteSpace, thirty-four (34) keyword clusters were identified within the network, and the ten largest of these clusters are summarized in

Table 6. Keyword citation cluster for zero-emission campus research.

Cluster ID	Size	Silhouette	Cluster Label (LLR)	Alternative Label	Average Year
0	53	0.646	low-carbon economy (371.73, 1.0 × 10−4)	education institution; sustainable economy	2016
2	51	0.712	education institution (718.55, 1.0 × 10−4)	education institution; sustainanle economy	2016
1	49	0.613	pro-environmental behavior (629.27, 1.0 × 10−4)	university student; sustainable economy	2017
3	34	0.747	neighborhood greenness (430.75, 1.0 × 10−4)	neighborhood greennesss; sustainable economy	2017
4	30	0.715	systematic review (276.6, 1.0 × 10−4)	case study; energy policy target	2018
5	18	0.909	using photovoice (182.86, 1.0 × 10−4)	life cycle assessment; polygeneration system	2010
6	18	0.853	soil surface (110.93, 1.0 × 10−4)	case study; early life exposure	2010
7	18	0.87	pharmaceutical compound (197.64, 1.0 × 10−4)	sustainable development; sustainable economy	2008
9	11	0.865	environmental practice (127.36, 1.0 × 10−4)	biomedical research facilities; renewable building energy supply-a case	2010
8	10	0.981	hydrologic behaviour (144.73, 1.0 × 10−4)	university campus; sustainable economy	2008

. The size of each cluster is represented by the number of its members. The average year of article publication within each cluster is provided, offering insight into whether the articles forming a cluster are recent or from an earlier period.

In the conducted analysis, the mean silhouette scores varied from 0.646 to a perfect 1, implying a robust fit of members within their respective clusters. This finding suggests a high level of cohesion and similarity within the identified clusters. It has been observed that Clusters #0, #2, #1, and #3 occur between 2015 and 2021, following the implementation of the sustainable development goals. On the contrary, Clusters #12, #11, #8, and #13 represent the earlier stages of zero-emission campus research, as they are composed of older articles.

Figure 9 demonstrates that the development of zero-emission campus research is predominantly focused on Clusters #0, #1, #2, and #3.

The keyword cluster analysis further identifies “Education and sustainable development”, “Renewable energy and energy efficiency”, and “Campus planning and design” as the most significant and active research themes within this field. Cluster #0, encompassing the keywords “education institution”, “low-carbon economy”, and “sustainable economy”, has the largest number of members, at 53. This indicates a strong scholarly interest in developing and implementing educational programs and resources aimed at enabling students and staff to learn about and apply sustainable practices. This trend is particularly noticeable in the most-cited paper within this cluster by Marques et al. [86], which proposes a ten-step framework for conducting an environmental impact assessment in universities, emphasizing the use of computer simulation for analyzing environmental metrics.

Cluster #3, consisting of 34 members and characterized by keywords such as “neighborhood greenness,” “sustainability,” and “environmental quality,” indicates a notable interest in exploring the relationship between local greenness and the advancement of zero-emission campus initiatives. The studies conducted within this cluster largely investigate the impact of neighborhood greenness on the behavior of students and staff, as well as its potential in assisting schools in mitigating their environmental footprint. This phenomenon is demonstrated in the highly referenced study within this research cluster conducted by Prieto-Sandoval [87], which examined the proclivity of students towards sustainable consumption both prior and subsequent to their enrollment in a course focused on “Sustainability and Circular Economy”. The results of the study provided confirmation that the course had a favorable impact on the students’ attitudes and behaviors related to sustainable consumption.

3.4.2. Document Co-Citation Cluster

Through the analysis, one hundred and eighty-two (182) document co-citation clusters were formed, with a summary of the ten largest clusters provided in

Table 7. Document co-citation cluster in zero-emission research.

Cluster ID	Size	Silhouette	Label (LLR)	Alternative Label	Average Year
0	127	0.875	education institution (107.83, 1.0 × 10−4)	education institution; biorefinery	2018
1	105	0.924	green universities (88.39, 1.0 × 10−4)	case study; environmental sustainability awareness	2013
2	57	1	chemical (13.8, 1.0× 10−4)	new trends for design towards sustainability chemical engineering, green engineering; education institution	2003
3	48	0.932	green entrepreneurial intention (74.25, 1.0 × 10−4)	environmental performance; psychological resource	2019
4	47	1	biorefinery (13.8, 1.0 × 10−4)	biorefinery: conversion of woody biomass to chemicals, energy and materials; education institution	2005
5	46	1	undergraduate teaching laboratory (14.98, 1.0 × 10−4)	a review of aqueous organic reactions for the undergraduate teaching laboratory; education institution	2005
6	29	1	hydrologic behavior (16.03, 1.0 × 10−4)	hydrologic behaviour of vegetated roofs; education institution	2003
11	24	0.964	policy support (107.51, 1.0 × 10−4)	policy support; green innovation investment	2014
12	21	0.964	pro-environmental behaviour (139.26, 0.005)	pro-environmental behavior: zero-waste campus framework	2018
28	8	0.999	photovoltaic plant (39.01, 1.0 × 10−4)	economic analysis; education institution	2018

. The average publication year of each cluster implies the relatively recent emergence of the field of zero-emission campuses, with most research published within the last decade. Nonetheless, a rising interest in this field is evidenced by the increasing number of recent publications, painting a picture of the gradual adoption of sustainable practices by universities over the past decade.

The silhouette metric scores for the ten document clusters range from 0.875 to 1.000. These scores are notably higher than those of the keyword clusters, illustrating a strong consistency among cluster members. Furthermore, each cluster has a representative document—a journal article with the highest co-citation frequency within each cluster (see Figure 10). These representative documents not only influence the labeling of clusters, but also the articles cited in the field, making them noteworthy for further follow up.

Cluster #0, labeled “Education Institution”, is the largest, with 127 members and a silhouette value of 0.875, suggesting education as a focal theme in zero-emission campus research. The research within this cluster primarily revolves around student and staff behavior, along with the development of new educational tools and resources aimed at reducing campuses’ environmental impact. The most-cited paper within this cluster, by Du et al. [18], underscores the need for sustainable assessment tools specifically designed for Chinese Higher Education Institutions in their current stage of sustainable development.

Cluster #1, the second largest, comprises 105 members. This cluster’s research is primarily focused on emission reduction and the crucial role of environmental sustainability awareness in the development of zero-emission campuses. The most-cited paper in this cluster, by Daguilite [37], reveals that students from green universities are more likely to perceive their institutions as environmentally friendly and engage in sustainability activities compared to their counterparts from non-green universities.

The other clusters generated from the analysis also highlight critical research areas within the zero-emission campus field. For instance, the “Chemical and Environmental Performance” cluster underscores the importance of assessing the environmental impact of various zero-emission campus initiatives. The “Psychological Resource and Green Entrepreneurial Intention” cluster suggests that psychological resources, such as self-efficacy and optimism, could foster green entrepreneurial intentions among students and staff on zero-emission campuses.

4. Conclusions

Global interest in zero-emission campuses has increased substantially in recent years among government agencies, academics, practitioners, and international organizations. Universities are particularly well-positioned to lead the movement toward carbon neutrality due to their high energy consumption, extensive research capabilities, and capacity to train future citizens and climate leaders. While the concept of zero-emission campuses was first introduced in 1998, the topic has gained increasing attention following the 2015 Paris Climate Agreement.

To understand the current state of research on zero-emission campuses and highlight opportunities for advancing the field, a comprehensive scientometric analysis of 1009 research articles from the Web of Science Core Collection published between 1997 and 2023 was conducted. The analysis employed four scientometric approaches to examine publication trends, leading authors and institutions, key research areas and topics, and emerging trends. Through this analysis, research gaps and opportunities to advance the field were identified.

This research identified the most influential contributors to zero-emission campus research, with Walter Leal Filho, University of California System, and United States standing out as the key author, institution, and country, respectively. Key research themes were identified, including education and sustainability, renewable energy and energy efficiency, campus planning and design, and waste management and recycling, policy support, and pro-environmental behavior. Emerging research clusters in the field include green universities, green innovation and investment, psychological resources and green entrepreneurial intention, biorefinery and environmental sustainability awareness, and policy support and pro-environmental behavior.

This study does have its limitations, one being its exclusive reliance on publications indexed in the Web of Science Core Collection, potentially omitting relevant studies from other databases such as Scopus and Google Scholar. The research also primarily focuses on the structural properties of the research network, which, while providing valuable insights, does not delve into the content quality of individual studies.

For future research, it would be valuable to broaden the scientometric analysis to incorporate other databases like Scopus and Google Scholar, providing a more expansive view of the research landscape. Complementing the current methodology with a content analysis could provide a deeper understanding of the themes, methodologies, and quality of research in the zero-emission campus domain. Lastly, conducting a comparative study of zero-emission campus research across different countries and regions could help to identify best practices and areas needing improvement.