On the combination of quantum dots with near-infrared reflective base coats to maximize their urban overheating mitigation potential

Highlights

• Demonstration of near-infrared permeability of QDs coatings as an interesting feature for the urban overheating application.

• Combining near-infrared reflection and fluorescent cooling techniques to improve cooling potential of QDs coating.

• Estimating the combined cooling potential of near-infrared reflection and fluorescent cooling methods.

Abstract

Application of highly absorptive construction materials is proved to be one of leading causes of urban overheating in big cities. To avoid the excessive heat by the conventional construction materials, several advanced heat-rejecting coating technologies were developed during the last decades. The main idea behind heat-rejecting coatings is to have colder coatings with the same appearance and colour of conventional coatings. One of the existing technologies for heat-rejecting coatings are advanced coatings with high solar reflection in the infrared range or so-called cool coatings. Recently, re-emission of the visible-range light by nano-scale semiconductors, known as Quantum Dots (QDs), were introduced as another effective heat-rejecting technology. In this paper, we showed that QDs also demonstrate a very high solar transmission in the near-infrared range, and therefore, a highly near-infrared reflective base layer can significantly improve their cooling potential. The high transmission value in the near-infrared range is due to the low absorption coefficient in the wavelengths longer than absorption edge wavelength (i.e. the wavelength corresponding to the bandgap energy) in semiconductors. We show that surface temperature reduction potential of CdSe/ZnS QDs film through fluorescent cooling is about 2.5 °C, which could be increased by another 8.1 °C with a highly near-infrared reflective base layer in a typical sunny day.

Introduction

The urban overheating effect is an ongoing environmental concern caused by synergistic combination of local and global climate change phenomena. Reduced number of trees, higher production of anthropogenic heat, and highly absorptive construction materials are the main drivers of the local climate change in big cities (Santamouris, 2013a, Akbari and Kolokotsa, 2016). According to the available literature, local urban overheating is observed in more than 400 cities in the world (Santamouris, 2007, Santamouris, 2015b, Santamouris, 2020, Founda and Santamouris, 2017). A comprehensive analysis on 100 Asian and Australian cities revealed that the local urban overheating in Australian and Asian metropolitan cities is quite significant, with an average and maximum magnitude of 4.1 °C and 11.0 °C, respectively (Santamouris, 2015a). A recent study also showed that the peak ambient temperature of inland western Sydney during heatwaves of January 2018 was 6–8 °C higher than that at the airport (Santamouris et al., 2020). In addition to the local climate change caused by urbanization, the ambient temperatures will also increase as a result of global climate change phenomenon. A recent study demonstrated that the peak and average summer ambient temperatures will increase by 0.8 °C and 1.6 °C by 2050 in Western Sydney due to the ongoing global climate change condition (Garshasbi et al., 2020b).

Application of greeneries, water-based technologies, and heat-rejecting materials including cool coatings, fluorescent materials, and thermochromic materials are the main strategies to reduce the urban overheating magnitude in big cities (Santamouris, 2013b, Cotana et al., 2014, Akbari and Kolokotsa, 2016, Wang and Akbari, 2016, Garshasbi and Santamouris, 2019, Ulpiani, 2019). Cool materials are advanced coatings presenting much lower surface temperature than their corresponding colour-matched conventional coatings due to their high solar reflection in the near-infrared range (Bretz and Akbari, 1997, Akbari et al., 2005, Levinson et al., 2007, Synnefa et al., 2007). A study on the climate and energy impacts of different mitigation strategies including highly reflective cool materials, green roofs, and green pavements showed that application of the cool materials is the most effective strategy with a peak ambient temperature reduction of 0.5 °C per 0.10 increase in the albedo and 3.5% peak cooling demand savings for a typical residential building in Sydney (Santamouris et al., 2018). The so-called cool coatings are made with highly near-infrared reflective metallic oxide pigments such as titanium oxide, chromium oxide, cobalt oxide, and barium oxide as additives. While cool coatings technology shows significant cooling potential, it’s complicated to achieve dark-coloured cool coatings with high solar reflection. This is mainly due to the limitations for addition of the light-coloured metallic oxides (Genjima and Mochizuki, 2002). As reported by Syneffa et al., the solar reflection of cool black-coloured coatings ranges between 0.12 and 0.27. Among other cool-coloured pigments examined in this study, the highest solar reflection is reported for cool orange-coloured coating with a solar reflection of 0.63 (Synnefa, Santamouris and Apostolakis, 2007). In another study by Levinson et al., the dark-coloured paints with high near-infrared reflection property are reported to attain an albedo of up to 0.35–0.4 (Levinson et al., 2007). Some research indicates that a two-layered coating composed of a near-infrared permeable topcoat and a near-infrared reflective base coat can be used to achieve dark-coloured coatings with high reflection (Levinson et al., 2016). As defined in the US patent by Genjima et al., the highly near-infrared reflective base coat layer should have a solar reflection of 60% or more, and a solar transmission of 25% or less; while the permeable topcoat should have a solar reflection of 60% or less, absorbance of less than 50%, and solar transmission of 30% or more in the near-infrared wavelength range between 800 and 2600 nm (Genjima and Mochizuki, 2002). The near-infrared reflective basecoat may be a metal (particularly aluminium flakes) or a white-coloured substrate (Brady and Wake, 1992, Genjima and Mochizuki, 2002), while the near-infrared permeable layer can be made by resins and permeable pigments such as phthalocyanine blues and greens, and carbazole dioxazine violet organic pigments (Brady and Wake, 1992). The study performed by Levinson et al. showed that application of a layer of cool black pigment increased solar reflection of the roof shingles from 0.04 to 0.12; and adding a thick white basecoat as near-infrared reflective layer showed another 0.06 solar reflection increase (Levinson et al., 2007).

The application of fluorescent materials as novel heat rejecting coatings has recently gained a lot of attention due to their capability to re-emit the visible-range solar irradiations (Berdahl et al., 2016, Berdahl et al., 2018, Garshasbi et al., 2020a, Kousis et al., 2020). The fluorescent cooling effect of QDs is attributed to the non-thermal relaxation of the excited electrons from conduction band to valence band (Garshasbi and Santamouris, 2019, Garshasbi et al., 2020a). A unique advantage of nano-scale fluorescent materials or so-called Quantum Dots (QDs) over their corresponding bulk fluorescent materials is their potentially higher fluorescent cooling power. The higher fluorescent cooling potential is due to the emergence of adjustable fluorescent/photoluminescence (PL) properties (such as quantum yield (QY) and absorption edge wavelength (λ_AEW)) at nano scale (Garshasbi and Santamouris, 2019, Garshasbi et al., 2020a). An interesting study on PbS QDs showed that the absorption peak wavelength can be tuned from 870 nm to 1400 nm by adjusting reaction parameters (such as reaction time and injection temperature) (Zhang et al., 2017). Another study reported photoluminescence (PL) peak wavelength tuning of CdS QDs from 431 nm to 547 nm by changing the PH value from 7.5 to 9.5 (Mo et al., 2012). The quantum efficiency of QDs as another key fluorescent property can be also improved by surface passivation methods like growing a semiconductor shell of larger bandgap on QDs surface, ligand exchange, etc. (Vasudevan et al., 2015). An interesting study on the impact of CdS shell thickness on quantum yield and stability of PbS QDs has reported a significant improvement of quantum yield from 20 to 40% for shell-free QDs to up to 67% quantum yield at the optimal shell thickness of 0.7 nm (Zhao et al., 2011). In our previous paper, we have developed an advanced fluorescent cooling algorithm for the precise optimization of fluorescent and optical properties of QDs for the cooling application for the first time (Garshasbi et al., 2020a). This model is also a helpful tool for calculating the complex correlation between PL effect and other heat loss mechanisms including reflection, emission, and convection.

In this paper, we demonstrated the high near-infrared penetrability of QDs coatings as another intriguing feature for the cooling application. The main reason behind near-infrared penetrability of QDs is that photons of light with an energy level less than bandgap energy cannot excite electrons in the valence band up to the conduction band and get absorbed, therefore, QDs as nano-scale semiconductor materials have a very sharp edge in their absorption coefficient at the wavelengths corresponding to their bandgap energy (Levinson et al., 2016). Given the high near-infrared penetrability of QDs, this paper examines the idea of using a two-layered coating composed of a near-infrared reflective basecoat layer and a near-infrared permeable QDs top layer. More precisely, this study aims to evaluate the combined cooling potential of fluorescent and near-infrared reflective coatings. Taking the magnitude of local and global climate change into account, development of super cool coatings with combined cooling mechanisms seems to be an effective response to the ongoing overheating challenge in major cities. Also, since QDs have been intensively investigated as a building-integrated photovoltaics (BIPV) technique (Chen et al., 2017, Liu et al., 2020), the combination of building cooling and BIPV can be considered as another promising application of QDs coatings.