## 3 Results and Discussion
### Science Mapping and Visualization
Researchers started to explore more alternatives that conformed to the energy efficiency and aesthetic requirements, both indoors and outdoors, and there was increasing interest in materials with the appropriate thermal emissivity/absorption spectrum. For this purpose, the careful selection of nanoparticles and pigments was developed to optimize the thermal and optical performances of materials, such as radiative cooling painting. Several works were led in this sense to develop high-reflective paints known as \"cool paints\", such as smart coatings with high NIR reflectance to reduce the solar heat gain and the energy consumption [56, 57, 58, 59]. In addition, the coatings were developed in order to achieve the color and efficient radiative cooling requirements in a simple, low-cost, and scalable way [60]. Since the third period (2011-2020) was the most prolific for this field of research, it is analyzed in detail below. To achieve this, Figure 7 was created, which shows a strategic diagram of a two-dimensional space that was built by plotting the themes according to their centralities and their density rank values. This includes four quadrants, each containing a specific theme: (i) Motor themes, in the upper-right quadrant; (ii) Basic and transversal themes, in the lower-right quadrant; (iii) Highly developed and isolated themes, in the upper-left quadrant; and (iv) Emerging or declining themes, in the lower-left quadrant [61]. According to Figure 7a, almost 70% of the published records represent the roofs, urban-area, and coatings themes, and they have the highest impacts (Figure 7b). It can be seen that roofs and urban-area are the most cited themes, with 7079 and 3497 citations, respectively, followed at a distance by coatings, with 1442 citations (Figure 7c). In the case of the roofs theme, its origins show an association with the concepts, building, solar-radiation, solar energy, and urban areas (Figure 6), and it represents the most applied domain of cool materials, as is shown in Figure 4. It is noteworthy that the urban-area motor theme is associated with all of the specific themes of the third period as an origin theme. The most detailed analysis of the cluster network allowed for the obtainment of information regarding the materials applied for cool surface purposes. Thus, the coatings theme showed a strong relation with the development of pigments (Figure 6a), especially the ones performing in the NIR spectrum as reflective materials, as can be seen in its cluster network (Figure 8d). In terms of the isolated membranes theme (Figure 8c), it mainly concerns the latent strategy of the thermal energy storage resumed in the PCMs. Finally, the two emerging themes of building-materials and cool-materials (Figure 8a,d, respectively) discuss ceramic  
Figure 7: Strategic diagram of the third period, the volume of the spheres is proportional to the number of documents published (**a**), to the h-index (**b**) and to the number of citations (**c**) in the third period associated with each theme. materials and glass, and retroreflective materials (RR materials), respectively. The most relevant contributions to these topics are analyzed in the next section for the review of materials applied for cool surfaces.