The traditional industrial agglomeration mostly focuses on the single industrial agglomeration, such as manufacturing agglomeration or producer services agglomeration, it mainly reflects the economic phenomenon that enterprises in the same industry choose to close to each other due to the constraints of production factors, transaction costs and location advantages. However, in reality, the industrial activities are more manifested in the phenomenon of co-agglomeration among different industries. With the upgrading of industrial structure, the refinement of social division of labor, the improvement of specialization level, and the transformation from "industrial economy" to "service economy", the collaborative development trend between manufacturing and producer services is becoming more and more obvious (Ke et al., 2014). Industrial co-agglomeration is a higher stage of industrial agglomeration, which not only multiplies the knowledge spillover effect and crowding effect of single industrial agglomeration, but also produces the economic, technological and knowledge linkages due to the vertical connection among heterogeneous industries, then will have an impact on total factor energy efficiency. The co-agglomeration driven by double-wheel of manufacturing and producer services is of great significance for changing the economic growth mode driven by a single industry, integrating and extending regional resources and industries, and improving the total factor energy efficiency.
On the one hand, manufacturing and producer services industrial co-agglomeration can realize industry division and cooperation through the mechanisms of mutual dependence, cooperation and mutual supplement, refining division of labor and resource sharing, which is not only conducive to establishing long-term and stable cooperative relations, so as to reduce the search time and transactions cost among enterprises, simplify the transaction process and improve the transaction efficiency (Pandit et al., 2001). But also it can promote industrial linkage and spatial linkage, effectively solve the problem of industrial resources mismatch, guide the optimal allocation of industrial resources, and then produce scale economy effect, technology spillover effect and centralized governance effect, thus directly promote the improvement of total factor energy efficiency. Firstly, industrial co-agglomeration can bring increasing returns to scale to enterprises by forming scale economy effect, which is conducive to comprehensively improving factor production efficiency and effectively reducing the average production and transaction costs. Moreover, the loss of raw materials in production is further reduced, and the in-transit loss of intermediate inputs is continuously reduced, so as to greatly improve the utilization efficiency of resources, greatly reduce the unit energy consumption and improve the energy efficiency (Hosoe and Naito, 2006; Zeng and Zhao, 2009; Liu et al., 2017; Wang et al., 2020b). Secondly, industrial co-agglomeration is also conducive to strengthening the inter-regional agglomeration and flow of innovative elements such as talent, capital, information and technology, thus resulting in technology spillover effect and accelerating the process of new technology R&D and technology diffusion (Teece, 1986). Therefore, it can improve the technical efficiency, effectively reduce the capital, personnel and energy inputs in the production process, as well as resource waste, save production costs and energy consumption, then the undesired output in production activities is significantly reduced, the desired output is effectively improved, and finally the total factor energy efficiency of enterprises will be improved (Yang et al., 2020). In addition, industrial co-agglomeration is also conducive for government departments to adopting interview, rectification, punishment and other means to carry out centralized control of environmental pollution, and promoting the centralized governance effect, then making it possible to deal with pollutants on a large scale. Therefore, it can help to reduce the governance cost and energy input per unit pollutant, so as to reduce pollutant emissions and energy consumption, further improve resource utilization efficiency, and promote energy-efficient recycling, finally promote the continuous improvement of total factor energy efficiency (Liu et al., 2017).
On the other hand, as the same with single industrial agglomeration, manufacturing and producer services industrial co-agglomeration also comes from the four key factors proposed by Marshall, which are the relationship between intermediate inputs and final product suppliers, the sharing of the labor market, increas opportunities for information exchange and innovation. Through the agglomeration of related industries and supporting industries, industrial co-agglomeration can strengthen the connection between peripheral industries and central industries, so as to promote the "collective efficiency" and "external economy" level, improve the "knowledge content" in the industrial agglomeration zone and promote green technological innovation (Ehrenfeld, 2003; Howard et al., 2016), then bringing about the energy-saving technology progress, promote the green transformation of the economy, optimize and adjust the energy structure and effectively reduce energy consumption. Therefore, it can improve energy technical efficiency and accelerate energy technological progress, promote the effective increase of desirable output and the significant decrease of undesirable output in the production activities, and finally indirectly drive the rapid improvement of total factor energy efficiency.
In summary, industrial co-agglomeration not only has a significant direct impact on total factor energy efficiency, but also has a significant indirect effect on total factor energy efficiency through promoting green technological innovation. The influence mechanism of industrial co-agglomeration on total factor energy efficiency can be illustrated by Fig. 1.
However, it can not be ignored that in the initial stage of producer services and manufacturing industrial co-agglomeration, the proportion of manufacturing industries is usually high. With the continuous agglomeration of producer services industries, the production cost of manufacturing industries can be effectively reduced, and is also convenient to the massive flow of talents, information and technology in the industrial agglomeration zone (Yang et al., 2021). Therefore, enterprises have the motivation to expand production scale, which may produce a "crowding effect", then leads to an increase in energy consumption and a significant increase in pollutant emissions (Verhoef and Nijkamp, 2002; Akbostanci et al., 2007; Shen et al., 2019), finally the improvement of total factor energy efficiency will be restrained. That is to say, industrial co-agglomeration also has a certain negative effect on total factor energy efficiency. Moreover, the spatial co-agglomeration of industries in the specific regions may also lead to frequent "free-riding" by some enterprises, resulting in the failure of government policies, which leads to they are not willing to make efforts to save the energy and improve the environment while enjoying government's preferential policies, making it difficult to achieve the required level of energy intensive utilization and environmental centralized governance in the agglomeration zone, finally the total factor energy efficiency will be decreased (Cheng, 2016; Han et al., 2018). What is more important, in the early stage of industrial co-agglomeration, the scale of industrial co-agglomeration is relatively small, and the resource allocation has yet been optimally allocated, which not only leads to a serious waste of resources, but also easily leads to the superposition of cross regional pollutants, then leads to the increase of the cost of environmental pollution control, and the decrease of energy efficiency. Last but not least, the positive effects of industrial co-agglomeration on total factor energy efficiency have obvious hysteresis (Henderson, 2003). Therefore, in the initial stage of industrial co-agglomeration, its negative impact on total factor energy efficiency is dominant. With the growing co-agglomeration between manufacturing and producer services industry, producer services industry gradually shows the characteristics of low energy consumption and low pollution by its high-tech and high value-added attributes. By providing clean outsourcing services, it can reduce the production and transaction costs of manufacturing industry and improve the energy utilization efficiency, thus the promotion effect of industrial co-agglomeration on total factor energy efficiency begins to rise and gradually occupied a dominant position (Zheng and Lin, 2018).
In summary, this paper proposes the hypothesis H1: There may be a U-shaped curve relationship between industrial co-agglomeration and total factor energy efficiency. That is, with the improvement of industrial co-agglomeration degree, it first shows a certain inhibitory effect on the total factor energy efficiency, and then shows an obvious promoting effect.
At present, the green technological innovation ability of Chinese enterprises is not strong, the resource utilization efficiency is low, and the pollutant emission is serious. In the initial stage of industrial co-agglomeration, it is often manifested in the single agglomeration of manufacturing or producer services industry in the spatial dimension, and there may be a spatial mismatch between manufacturing and producer service industry, resulting in enterprises spending money on the information search and matching process, inhibiting enterprises' green technological innovation, which is not conducive to the improvement of total factor energy efficiency. With the deepening of manufacturing and producer services industrial co-agglomeration degree, producer services industries, which are matched with manufacturing, pour into the agglomeration region continuously. Industrial co-agglomeration is bound to strengthen the information and knowledge dissemination among enterprises, which is conducive to the transfer of tacit knowledge and the generation of technology spillover effect, so as to promote green scientific and technological innovation of enterprises, bring about energy-saving technological progress, and thus effectively reduce the energy consumption and improve the energy utilization efficiency, and reduce the pollutant emission, finally improve the total factor energy efficiency of enterprises.
Therefore, this paper proposes the hypothesis that H2: industrial co-agglomeration has a significant indirect impact on total factor energy efficiency through green technological innovation. That is to say, green technological innovation has an obvious mediating effect on the impact of industrial co-agglomeration on total factor energy efficiency.