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Review

Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings

Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden
*
Author to whom correspondence should be addressed.
Buildings 2017, 7(1), 3; https://doi.org/10.3390/buildings7010003
Submission received: 24 November 2016 / Revised: 16 December 2016 / Accepted: 20 December 2016 / Published: 29 December 2016
(This article belongs to the Special Issue Building Refurbishment and Energy Performance)

Abstract

:
Today’s advances in materials science and technology can lead to better buildings with improved energy efficiency and indoor conditions. Particular attention should be directed towards windows and glass facades—jointly known as “glazings”—since current practices often lead to huge energy expenditures related to excessive inflow or outflow of energy which need to be balanced by energy-intensive cooling or heating. This review article outlines recent progress in thermochromics, i.e., it deals with materials whose optical properties are strongly dependent on temperature. In particular, we discuss oxide-based thin surface coatings (thin films) and nanoparticle composites which can be deposited onto glass and are able to regulate the throughput of solar energy while the luminous (visible) properties remain more or less unaltered. Another implementation embodies lamination materials incorporating thermochromic (TC) nanoparticles. The thin films and nanocomposites are based on vanadium dioxide (VO2), which is able to change its properties within a narrow temperature range in the vicinity of room temperature and either reflects or absorbs infrared light at elevated temperatures, whereas the reflectance or absorptance is much smaller at lower temperatures. The review outlines the state of the art for these thin films and nanocomposites with particular attention to recent developments that have taken place in laboratories worldwide. Specifically, we first set the scene by discussing environmental challenges and their relationship with TC glazings. Then enters VO2 and we present its key properties in thin-film form and as nanoparticles. The next part of the article gives perspectives on the manufacturing of these films and particles. We point out that the properties of pure VO2 may not be fully adequate for buildings and we elaborate how additives, antireflection layers, nanostructuring and protective over-coatings can be employed to yield improved performance and durability that make TC glazings of considerable interest for building-related applications. Finally, we briefly describe recent developments towards TC light scattering and draw some final conclusions.

1. Introduction

The current energy–environment nexus, and the challenges it imposes on society, serves as a driving force behind many endeavours in research and development. At the basis of these challenges lies the fact that the measured concentration of carbon dioxide in the Earth’s atmosphere is rising dramatically; it was ~315 ppm at the end of the 1950s and now regularly exceeds ~400 ppm [1]. Furthermore, the rate of increase has almost tripled during this time span. The increased amount of CO2 has its origin in energy generation—specifically unrestrained burning of coal, oil and gas—and is widely believed to lead to global warming and rising sea levels [2]. There may also be many secondary harmful impacts of climate change related to, for example, socioeconomic effects and increased risk for human conflict [3,4,5].
Furthermore, the world’s population is growing rapidly both in absolute and relative numbers and is predicted to be 50% larger by the year 2100 than it is today [6]. This population is increasingly agglomerated in urban centres, and it is expected that 70% of all people will be living in such areas by the year 2050 [7]. The centres behave as “urban heat islands” and may reach temperatures that can be several degrees in excess of those in the surrounding countryside [8]. Hence, the “urban heat island” effect exacerbates CO2-induced climate change [9] for the majority of the world’s population. The effects of human activities are already pervasive enough that the current geological era—i.e., the Anthropocene—is considered distinctly different from the period prior to the Industrial Revolution, known as the Holocene [10].
It is evident that the global energy sector must be decarbonized, and this realization points sharply to the importance of improving buildings which are responsible for 30%–40% of the worldwide use of primary energy [11]. In fact, the impact of buildings is presently increasing in many parts of the world, and their part of the energy use in the USA, for example, was 34% in 1980 and as large as 41% in 2010 [12], and nothing indicates that this trend is changing. Another reason to consider buildings ensues from the fact that, in industrialized countries, people spend as much as 80%–90% of their time indoors [13].
Energy efficiency in the built environment is too often disregarded as an opportunity for CO2 abatement [14]. However, there are many “green” technologies, often with nano-attributes, that can be harnessed [15,16,17,18,19,20,21,22,23], and energy-efficient glazings—i.e., windows and glass facades—are one of the most important options. Today’s glazings frequently permit excessive energy flows to enter or exit a building, which then necessitates energy-wasteful cooling or heating. One principle solution to this conundrum is to make the glazings small, but this is not acceptable in practice since precious indoors–outdoors contact and daylighting are then curtailed, and both of these features are essential for human well-being and task performance [24,25,26]. However, energy efficiency can be achieved with glazings permitting variable amounts of solar energy and visible light to be transmitted. These glazings are often referred to as “intelligent” or “smart” and are based on stimulus-responsive “chromogenic” materials [27,28].
This review article deals with thermochromic (TC) materials, which are characterized by temperature-dependent properties. Thin films of oxide-based materials of this type can let through more solar energy at low temperatures than at high temperatures, which means that energy-efficient glazings can introduce solar energy primarily when it is needed. TC-based fenestration was suggested already some three decades ago [29,30] but has not yet made it to the marketplace in the case of oxide-based materials for reasons that are discussed below. However, there have been a number of recent advances in thermochromics that indicate that practical implementation is feasible in buildings [31,32,33,34,35,36,37,38,39,40].
It is important to realize already from the outset that TC devices for buildings must be compatible with truly large-scale manufacturing. In order to get a feel for the magnitude, one should contemplate that flat-glass production—mostly by the float process—has been predicted to be 9.2 × 109 square meters per year in 2016 [41]. Having TC-based functionality on only a small fraction of the glazings of the world requires production units capable of handling square-kilometer-sized areas per year.
TC materials and devices need to be characterized with regard to their ability to transmit and reflect luminous (i.e., visible) and solar radiation. These properties are conveniently introduced by considering a number of spectra illustrated in Figure 1. Thermal radiation, as shown in Figure 1a, is governed by blackbody curves—which are shown for four values of the temperature τ—multiplied by a materials-specific emittance which is less than unity; it is apparent that this radiation lies at λ > 2 μm for normal ambient temperatures, where λ is wavelength. Solar radiation impinging upon the Earth’s atmospheric envelope can be approximated by blackbody-like radiation pertaining to the sun’s surface temperature (5505 °C) and falls at 0.2 < λ < 3 μm. At ground level and for typical clear-weather conditions, this radiation is representatively the one illustrated in Figure 1b, where the sharp minima are a consequence of molecular absorption in the air. Luminous radiation, finally, is represented by the bell-shaped curve in Figure 1b which extends over the 0.4 < λ < 0.7 μm range and has a peak at 0.55 μm. Quantitative information for luminous (lum) and solar (sol) transmittance, indicated as Tlum(τ) and Tsol(τ), respectively, are obtained from
Tlum,sol(τ) = ∫ ϕlum,sol(λ) T(λ,τ)/∫ ϕlum,sol(λ),
where T(λ,τ) is temperature-dependent spectral transmittance, ϕlum is the spectral sensitivity of the human eye [42], and ϕsol denotes the “air mass 1.5” solar irradiance spectrum (for the sun at 37° above the horizon) [43]. Analogous formulas apply to reflectance R.
The TC materials are predominantly modulating the solar energy throughput, whereas the luminous performance is more or less temperature-independent, which implies that the modulation is conveniently characterized by
ΔTsolTsol(τ < τc) – Tsol(τ > τc),
where τc indicates the “critical” temperature at which the temperature-dependent properties are changed.
There are many reviews of TC materials and devices. The present one serves as an update and extension of some recent articles [40,44,45,46].

2. Basic Properties of Vanadium Dioxide Thin Films and Nanoparticles

There are many inorganic and organic thermochromic materials, but there is a limited number of possibilities for glazings, especially if the focus is on oxide-based thin films and nanoparticle composites suitable for large-area applications. Figure 2 shows data on several materials, specifically on temperature-dependent electrical conductivity of oxides and some other materials [29]. Abrupt changes appear at well-defined temperatures and originate from structural transformations. Vanadium dioxide (VO2) is of special interest since τc ≈ 68 °C, i.e., it is not vastly different from ordinary room temperature, as was discovered almost 60 years ago [47]. This material hence is the most promising alternative for developing TC glazings and, in fact, most research efforts for this application make use of VO2, at least as a point of departure. The switching at τc takes place between two states: (i) a low-temperature semiconducting phase characterized by low infrared absorption—and therefore large infrared throughput as a thin film—and monoclinic crystal structure; and (ii) a high-temperature metal-like phase having large infrared reflectance and rutile-type crystal structure. The physical nature of the switching at τc has been debated for decades and continues to be the subject of scientific inquiry as apparent from an ongoing steady stream of publications [48,49,50,51].
Figure 3 summarizes the most important optical properties of VO2 thin films and nanoparticle composites. Figure 3a,c show spectral transmittance (a) and reflectance (c) for 50 nm-thick VO2 films on glass for wavelengths pertinent to solar radiation. It is apparent that T(λ,τ) is much higher in the semiconducting state below τc than in the metallic state above τc as long as λ > 1 μm. The difference in the transmittance between low and high temperature is enhanced for increasing wavelength. This type of variation in T(λ,τ) clearly is the desired one, and a glazing with a VO2-based TC thin film transmits more energy below τc than above τc. Similar properties have been recorded many times [31,32,33,34,35,36,37,38,39,40,52,53,54]. Figure 3 also demonstrates that corresponding data on R(λ) increase monotonically towards long wavelengths for the metallic-like state above τc, which is the expected behavior. The reflectance seems to level off at ~40% for the 50 nm-thick film, and a higher reflectance would have been reached in a thicker film. Figure 3b,d apply to a dilute suspension of VO2 nanospheres with a mass thickness of 50 nm. Comparing these data with those for the film and the solar spectrum in Figure 1 makes it evident that Tlum(τ) and ΔTsol are enlarged and that Rlum(τ) and Rsol(τ) are decreased and almost temperature-independent. These results will be discussed in more detail in Section 4 below.

3. On the Preparation of Vanadium Dioxide (VO2) Thin Films and Nanoparticles

Techniques for making VO2 thin films and nanoparticles are of much interest and are presently undergoing rapid development. Vanadium has high oxygen affinity and is able to form compounds with the metal in oxidation states being +5, +4, +3 and +2. Therefore, it is hardly surprising that the vanadium–oxygen phase diagram, shown in Figure 4, is complex and includes almost 20 different phases, frequently with only minor compositional differences [55,56,57,58]. The challenges for the synthesis of VO2 are related to the co-existence of these various oxide forms and also with the existence of various polymorphs. Therefore, it is rarely straightforward to make phase-pure VO2, which is produced in a very narrow interval of oxygen partial pressure.
Elemental solid (s) vanadium reacts with oxygen gas (g) in a simple and direct manner and yields VO2 via the overall reaction
V(s) + O2(g) → VO2(s).
This reaction constitutes the basis for various physical vapor deposition techniques—such as thermal evaporation [59], pulsed laser deposition [60] and sputtering [40,61,62]—which synthesize VO2 thin films by deposition in an ambience with controlled oxygen partial pressure and at high temperature. The temperature is typically ~450 °C although lower temperatures can be used for special sputtering techniques employing pulsed plasma [63,64,65,66,67]. The required process control is difficult, and vanadium is rapidly oxidized at high temperature and transforms from tetragonal VO2 to V6O13 and V3O7, and it ultimately reaches thermodynamically stable orthorhombic V2O5 in agreement with the phase diagram in Figure 4 (we come back to this point in Section 4).
More facile thin-film preparation might be accomplished by use of a mild oxidation agent that could serve as an alternative to the oxygen in reaction (3). One possibility is to use SO2, as discussed recently [40,62], in order to exploit the overall reaction
V(s) + SO2(g) → VO2(s) + S(g),
presumably in conjunction with a catalyst for boosting the kinetics. If SO2 is fully decomposed, the reaction products would be VO2 and S; the sulfur can then be removed through vaporization at a high-enough temperature.
There are numerous other methods to manufacture VO2-based thin films for glazings, such as chemical vapor deposition [68,69] and sol–gel technology [70]. Several of the deposition techniques were surveyed recently [54].
We now focus on VO2-based nanoparticles, which can be prepared by a number of techniques, physical as well as chemical. This large body of work is cited, for example, in References [61,71]. Recent work has demonstrated that VO2 nanoparticles can be prepared by direct and continuous synthesis via a two-step process incorporating hydrothermal flow synthesis followed by a short heat-treatment step [72]. Nanoparticles can be more or less well rounded and they can also be rod- and wire-like. Figure 5 shows scanning electron microscopy (SEM) images of nanorods prepared by sputter deposition onto glass heated to ~460 °C in an atmosphere of of oxygen and argon [61]. We focus on sputtering since this technique has excellent scalability and is commonly employed for glazing applictions.
The substrate material can have a significant influence on the growth of sputter-deposited particles [73], and glass precoated with electrically conducting In2O3:Sn (indium–tin oxide, ITO) gave more well-defined particles than in the case of analogous deposition onto bare glass or glass precoated with non-conducting SnO2. The reason for the influence of the substrate is not known in detail, but surface energy and the fact that an electrically conducting substrate avoids charge-induced repulsion between adjacent particles are expected to play a role. The sublayer of ITO on the glass substrate is of significance also because it broadens the process window for making VO2 deposits with TC properties. Figure 6 shows images taken by SEM and atomic force microscopy (AFM) of a particulate VO2 film on ITO-coated glass and demonstrates a layer of highly irregular nanoparticles with sizes of the order of 1 μm [74]; we come back to these films in Section 5 below.
It is possible to embed the VO2 nanoparticles in a host material, and Figure 7 shows preliminary results on a VO2–SiO2 composite prepared by reactive magnetron sputtering [75]. SEM micrographs displayed rounded nanoparticles with sizes of 100–300 nm, and energy-dispersive X-ray (EDX) studies indicated that these particles were vanadium-rich. In the case of an alternative deposition process, tiny VO2-based crystallites were formed in an Al2O3 matrix by reactive co-sputtering onto a substrate kept at ~400 °C, as evidenced by the high-resolution transmission electron microscopy (HRTEM) image in Figure 8 [76].
Vanadium-dioxide-based thin films and nanoparticle composites for buildings integration are normally backed by rigid glass plates or panes. However, thin films can be prepared also on ultrathin flexible glass and on temperature-resistent polymer foil [65].

4. Towards VO2-Based Thin Films for Glazings

The data for T(λ,τ) in the left-hand images of Figure 3 are interesting with regard to glazings, but the performance is hardly sufficient for practical implementation in buildings. Nevertheless, the properties of VO2 are sufficiently near to the wanted ones that it makes good sense to make a concerted attempt to modify this material [32]. In particular, the following items demand careful consideration for VO2-based coatings and are discussed in the remainder of the present section: (i) τc takes place at ~68 °C which clearly is too high for buildings; (ii) Tlum is only ~40% and too low for most glazings (Tlum would be higher for a thinner film but the consequence is then that ΔTsol becomes minuscule); (iii) temperature-dependent transmittance modulation is large mainly for wavelengths where the solar irradiation is weak and this limits ΔTsol to no more than ~10%; and (iv) VO2 is not thermodynamically stable but further oxidation leads to non-TC V2O5.
We first consider whether τc can occur around room temperature. In fact, it was well known already in the early 1970s that an addition of tungsten and a number of other elements to bulk VO2 can give a depression of τc [77,78,79], and the same effect can be found in thin films [80]. Other additions can enhance τc to some extent [77]. Figure 9 reports that ~2% of W displaces τc to approximately room temperature [81], and numerous other datasets give analogous results provided that the oxide film is well crystallized and homogeneous [32].
Next we look at ways to increase Tlum, or rather to lower the absorptance of visible light. Figure 3 demonstrates that strong optical absorption occurs at wavelengths below 0.5–0.6 μm, which is related to the fact that the optical band gap in VO2 is smaller than desired. A partial solution to this problem can be reached by replacing some of the vanadium by magnesium [81,82,83,84,85] in order to widen the optical band gap, and this effect can be reconciled with computational results [86,87]. Figure 10 illustrates the effect of Mg doping by showing the spectral transmittance for 50 nm-thick films of doped and pure VO2 at τ < τc and τ > τc. It is evident that the dopant displaces the high-transmittance region towards shorter wavelengths, but it also has the unwanted effect of diminishing ΔTsol. It is apparent from Figure 10 that τc drops as the Mg content is enhanced albeit not as rapidly as for W doping. Figure 11 indicates that Tlum is increased from 39% to 51% as the amount of Mg goes from zero to 7.2%. Band-gap widening can be accomplished also by adding other alkaline earth metals [83], zinc [88] or terbium [89], or by replacing some of the oxygen with fluorine [90,91,92].
The third item under scrutiny is how to enhance ΔTsol. An important step towards this goal was taken when it was discovered through computing that a layer of VO2 nanoparticles—rather than a corresponding continuous thin film of this material—could yield the wanted properties [71,93]. Additional calculations showed that the particle size should not be larger than ~20 nm in order to prevent optical scattering (“haze”) [94]. However, refractive-index-matching between VO2 nanoparticles and an embedding material—as seen in Figure 7—can suppress scattering efficiently. “Nanothermochromism” [71] is in fact demonstrated in Figure 3b,d, which illustrate T(λ) and R(λ) for a 5 μm-thick layer of a transparent medium, whose refractive index is characteristic of glass and polymers, including 1 vol.% of well-dispersed and non-scattering nanospheres of VO2. This choice of parameters produces an equivalent VO2 thickness of 50 nm so that that the data for the nanospheres and for the continuous thin film can be readily compared.
Judging from Figure 3, it is clear that there are important qualitative differences between optical data for the nanoparticle composites and the corresponding thin films. Thus (i) the nanosphere-containing material is considerably more transparent than the corresponding thin film; (ii) metallic-like nanoparticles display a clear-cut minimum in the transmittance for the 0.7 < λ < 1.5 μm wavelength interval which strongly limits Tsol since the solar irradiance is intense for these wavelengths, whereas the visible optical performance is barely affected at all; and (iii) the nanospheres absorb rather than reflect. The striking transmittance minimum—which corresponds to an absorption maximum—in the infrared is due to localized plasmon resonance of free electrons in VO2 nanoparticles and takes place around λ ≈ 1.2 μm. Clearly, this feature has an almost ideal position for absorbing infrared solar radiation without influencing the visible performance (cf. Figure 1). A plasmonic character of the near-infrared absorption has been observed and discussed several times in prior work [95,96,97,98].
Figure 12 gives clear evidence for nanothermochromism in a film comprised of VO2–SiO2 and was recorded on the sample illustrated in Figure 7. The spectral absorptance A(λ), derived according to
A(λ) = 1 − T(λ) − R(λ),
displays a conspicuous maximum for near-infrared solar radiation when the VO2 nanoparticles are in their metallic-like state at τ > τc, whereas no feature of this kind can be discerned for the semiconductor state at τ < τc [75].
It is evident that various combinations of Tlum and ΔTsol can be obtained in VO2-based materials, and Figure 13 illustrates the current state of the art for Tlum(τ < τc) and ΔTsol [34]. It should be pointed out that Tlum is approximately the same at τ < τc and τ > τc. Thin films of pure VO2 give the lowest magnitudes of Tlum(τ < τc) and ΔTsol, but the optical performance can be boosted by antireflection (AR) layers. Particularly good properties were obtained with multilayers of TiO2 and VO2 [99,100,101]. “Bio-inspired” cone-shaped surfaces represent another possibility to increase Tlum [102]. Mg-containing VO2 films have improved properties, which can be further enhanced by AR layers. However, optimum performance can be reached with VO2 nanoparticles which, for example, can yield Tlum(τ < τc) ≈ 60% and ΔTsol > 20%. It is also seen that Mg-doped nanoparticles are not superior to nanoparticles of pure VO2; the reason for this behavior is that the doping lowers the solar modulation, as seen in Figure 10, to a degree that is not balanced by the higher short-wavelength transmittance. Nanothermochromism is in a state of rapid development and, for example, recent work has demonstrated excellent TC properties in composite films comprised of VO2 nanoparticles and ionic-liquid–nickel–chlorine complexes [103]. VO2-based core–shell particles arranged so as to make two-dimensional photonic crystals can achieve static tunability of Tlum while thermochromism is exhibited with regard to Tsol [104].
Finally, we look at the durability of VO2-based thin films and nanoparticles, which is an issue of great practical interest since glazings must last for many years without essential performance losses. It must be remembered that V2O5 is thermodynamically stable, as is apparent from the phase diagram in Figure 4. Figure 14 reports data on 80 nm-thick sputter-deposited VO2 thin films kept in dry air at 300 °C for one hour [105]. The as-deposited film demonstrates the expected TC properties (cf. Figure 3 and Figure 10), whereas the heat-treated film displays a very different spectral transmittance which is indicative of non-TC V2O5 [106,107]. This transition is expected to be much slower at room temperature, but the data in Figure 14 nevertheless show that VO2-based materials must be shielded against further oxidation. The durability of VO2 is also dependent on the deposition technique and is improved for films with high density and large grain size [66,108].
Figure 15 depicts T(λ) for similar VO2 films over-coated with 10 nm and 30 nm of sputter deposited alumina and heat treated at 300 °C for periods in the range 1 < th < 30 h [105]. Clearly the top layer provides good protection, and only the thinnest Al2O3 layer and the longest heat-treatment period yielded some minor decline of the thermochromism. Al-nitride top layers were able to provide a similar protection for VO2 films [109]. The data in Figure 15 strongly indicate that VO2-based materials can be used for long times in, for example, energy-efficient glazings. Over-coatings similar to the ones above gave efficient protection also under conditions of high humidity [105]. Recent work has demonstrated results that are similar to those above for the case of VO2-based core–shell nanoparticles [110].

5. Recent Development: Thermochromic Light Scattering

Light scattering from VO2 particles was investigated for samples with particle sizes of the order of 1 μm, which is comparable with the wavelengths for visible light and solar radiation. Specifically, the study employed the particulate layer depicted in Figure 6 [74], and the shown AFM image was quantified with regard to individual particle volumes Vp and surface areas Sp by use of available computer code [111]. In order to formulate a theoretical model of light-scattering data, the individual particles were modelled by use of two different sets of equivalent sphere radii: either equal-volume spheres represented by req = (3Vp/4π) or equal-volume-to-area spheres represented by req = 3Vp/Sp. Earlier work, rooted in atmospheric science, has demonstrated that the latter representation can work well for light scattering from highly irregular objects [112]. The particle number is not conserved in the latter case, and the number of equivalent spheres is larger than the number of real particles. Figure 16 presents actual data on the radial distributions for req and req.
Specular and diffuse reflectance as well as direct and diffuse transmittance were recorded at τ < τc and τ > τc for the particulate sample, and these results were used to derive approximate absorption and scattering coefficients—denoted αabs and αsca, respectively—as shown in Figure 17. Corresponding computed quantities—denoted K*abs and K*sca, respectively—were then obtained from Lorenz–Mie theory [113] applied to the particle radii distributions in Figure 16. It is evident from Figure 17 that the calculations based on the equal-volume-to-area model can be reconciled with experimental data at λ < 0.7 μm for absorption as well as scattering and at both low and high temperature. The equal-volume approximation worked less well. The conclusion is that a semi-quantitative description of TC light scattering appears possible even from highly irregular particles, but more work is needed to fully vindicate this result.
Glazing-related applications of TC light scattering can be envisaged, and other uses may include functional fibre mats [114,115] and elastomeric composites [116].

6. Conclusions

We have given an overview of recent progress on materials for thermochromic glazings for applications in energy-efficient buildings. The presentation was confined to VO2-based thin coatings and nanoparticle composites, but it should be kept in mind that there are other thermochromic materials, especially among the organics [117,118]. In the case of vanadium dioxide (VO2)-based thin films, switching occurs between states with low and high solar energy throughput at high and low temperature, respectively. The transition temperature can be brought close to room temperature by adding some tungsten atoms, and luminous transmittance can be enhanced by antireflection layers as well as by doping with magnesium. Nanoparticle composites with VO2 dispersed in a transparent host are able to combine large luminous transmittance with a high modulation of solar energy transmittance. Durability under heating and at high humidity can be accomplished by use of protective over-coatings.
With regard to practical application in thermochromic glazings, VO2-based thin films can be employed in insulated glass units in the same way as in current technology which normally uses metal-based or doped-semiconductor-based thin films with static optical properties [15,119]. The thin films must be produced by a method that allows large-scale high-throughput manufacturing, with reactive DC magnetron sputtering and chemical vapor deposition (spray pyrolysis) being established technologies. Thermochromic nanoparticles—probably with VO2 cores surrounded by protective shells—can be dispersed in polymeric lamination materials. Another development may be to employ such thermochromic laminates as electrolytes in electrochromic devices capable of modulating the transmittance of visible light and solar energy when electrical charge is transported between thin films based on, for example, nickel oxide and tungsten oxide [120,121,122]. Furthermore, thermochromism can be combined with photocatalytic self-cleaning in multifunctional glazings [123].
We note, finally, that technologies such as those discussed in this article may lead to a radical change of the very concept of a building—it must no longer be static, and in need of vast quantities of energy to achieve an adequate indoor environment, but instead can be dynamic and able to adapt to varying ambience, thereby minimizing energy expenditure and enhancing indoor comfort.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Earthscan System Research Laboratory, Global Monitoring Division. Available online: http://www.esrl. noaa.gov/gmd/ccgg/trends/index.html (accessed on 14 November 2016).
  2. Stocker, T.F.; Qin, D.; Plattner, G.-K.; Tignor, M.M.B.; Allen, S.K.; Boschung, J.; Nauels, A.; Xia, Y.; Bex, V.; Midgley, P.M. (Eds.) Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the Fifth Assessment Report on the Intergovermental Panel on Climate Change; Cambridge University Press: New York, NY, USA, 2013.
  3. Burke, M.; Hsiang, S.M.; Miguel, E. Global non-linear effect of temperature on economic production. Nature 2015, 527, 235–239. [Google Scholar] [CrossRef] [PubMed]
  4. Kelley, C.P.; Mohtadi, S.; Cane, M.A.; Seager, R.; Kushnir, Y. Climate change in the Fertile Crescent and implications of the recent Syrian drought. Proc. Natl. Acad. Sci. USA 2015, 112, 3241–3246. [Google Scholar] [CrossRef] [PubMed]
  5. Carleton, T.A.; Hsiang, S.M. Social and economic impacts of climate. Science 2016, 353, aad9837/1–aad9837/15. [Google Scholar] [CrossRef] [PubMed]
  6. United Nations, Department of Economic and Social Affairs. World Population Prospects: The 2015 Revision; United Nations: New York, NY, USA, 2015. [Google Scholar]
  7. United Nations, Department of Economic and Social Affairs. World Urbanization Prospects: The 2014 Revision; United Nations: New York, NY, USA, 2014. [Google Scholar]
  8. Akbari, H.; Cartalis, C.; Kolokotsa, D.; Muscio, A.; Pisello, A.L.; Rossi, F.; Santamouris, M.; Synnefa, A.; Wong, N.H.; Zinzi, M. Local climate change and urban heat island mitigation techniques: The state of the art. J. Civil Eng. Manag. 2016, 22, 1–16. [Google Scholar] [CrossRef]
  9. Sachindra, D.A.; Ng, A.W.M.; Muthukumaran, S.; Perera, B.J.C. Impact of climate change on urban heat island effect and extreme temperatures: A case-study. Quart. J. R. Meterorol. Soc. 2016, 142, 172–186. [Google Scholar] [CrossRef]
  10. Waters, C.N.; Zalasiewicz, J.; Summerhayes, C.; Barnosky, A.D.; Poirer, C.; Galuszka, A.; Cearreta, A.; Edgeworth, M.; Ellis, E.C.; Ellis, M.; et al. The Antropocene is functionally and stratigraphically distinct from the Holocene. Science 2016, 351, aad2622/1–aad2622/10. [Google Scholar] [CrossRef] [PubMed]
  11. United Nations Environmental Programme. Buildings and Climate Change: Summary for Decision-Makers; UNDP Sustainable Buildings & Climate Initiative: Paris, France, 2009. [Google Scholar]
  12. U.S. Department of Energy. 2011 Buildings Energy Data Book; U.S. Department of Energy: Washington, DC, USA, 2012.
  13. Leech, J.A.; Nelson, W.C.; Burnett, R.T.; Aaron, S.; Raizenne, M.E. It’s about time: A comparison of Canadian and American time–activity patterns. J. Exposure Anal. Environm. Epidemiol. 2002, 12, 427–432. [Google Scholar] [CrossRef] [PubMed]
  14. Richter, B.; Goldston, D.; Crabtree, G.; Glicksman, L.; Goldstein, D.; Greene, D.; Kammen, D.; Levine, M.; Lubell, M.; Savitz, M.; et al. How America can look within to achieve energy security and reduce global warming. Rev. Mod. Phys. 2008, 80, S1–S107. [Google Scholar] [CrossRef]
  15. Smith, G.B.; Granqvist, C.G. Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment; CRC Press: Boca Raton, FL, USA, 2010. [Google Scholar]
  16. Ginley, D.S.; Cahen, D. (Eds.) Fundamentals of Materials for Energy and Environmental Sustainability; Cambridge University Press: Cambridge, UK, 2012.
  17. García-Martínez, J. (Ed.) Nanotechnology for the Energy Challenge; Wiley-VCH: Weinheim, Germany, 2013.
  18. Pacheco-Torgal, F.; Diamanti, M.V.; Nazari, A.; Granqvist, C.G. (Eds.) Nanotechnology in Eco-Efficient Construction; Woodhead: Cambridge, UK, 2013.
  19. Pacheco-Torgal, F.; Mistretta, M.; Kaklauskas, A.; Granqvist, C.G.; Cabeza, L.F. (Eds.) Nearly Zero Energy Building Refurbishment; Springer: London, UK, 2013.
  20. Pacheco-Torgal, F.; Labrincha, J.A.; Cabeza, L.F.; Granqvist, C.G. (Eds.) Eco-Efficient Materials for Mitigating Building Cooling Needs: Design, Properties and Applications; Woodhead: Cambridge, UK, 2015.
  21. Pacheco-Torgal, F.; Buratti, C.; Kalaiselvam, S.; Granqvist, C.G.; Ivanov, V. (Eds.) Nano and Biotech Based Materials for Energy Building Efficiency; Springer: Cham, Switzerland, 2016.
  22. Pacheco-Torgal, F.; Rasmussen, E.; Granqvist, C.G.; Ivanov, V.; Kaklauskas, A.; Makonin, S. (Eds.) Start-Up Creation: The Smart Eco-Efficient Built Environment; Woodhead: Cambridge, UK, 2016.
  23. Pacheco-Torgal, F.; Granqvist, C.G.; Jelle, B.P.; Vanoli, G.P.; Bianco, N.; Kurnitski, J. (Eds.) Cost-Effective Energy Efficient Building Retrofitting: Materials, Technologies, Optimization and Case Studies; Woodhead: Cambridge, UK, 2017.
  24. Heschong, L.; Wright, R.L.; Okura, S. Daylighting impacts on retail sales performance. J. Illum. Engr. Soc. 2002, 31, 21–25. [Google Scholar] [CrossRef]
  25. Heschong, L.; Wright, R.L.; Okura, S. Daylighting impacts on human performance in school. J. Illum. Engr. Soc. 2002, 31, 101–114. [Google Scholar] [CrossRef]
  26. Altomonte, S. Daylight for energy savings and psycho-physiological well-being in sustainable built environments. J. Sustain. Dev. 2008, 1, 3–16. [Google Scholar] [CrossRef]
  27. Granqvist, C.G. Chromogenic materials for transmittance control of large-area windows. Crit. Rev. Solid State Phys. Mater. Sci. 1990, 16, 291–308. [Google Scholar] [CrossRef]
  28. Lampert, C.M.; Granqvist, C.G. (Eds.) Large-Area Chromogenics: Materials and Devices for Transmittance Control; SPIE Institutes for Advanced Optical Technologies; SPIE Optical Engineering Press: Bellingham, WA, USA, 1990; Volume IS4.
  29. Jorgenson, G.V.; Lee, J.C. Doped vanadium oxide for optical switching films. Sol. Energy Mater. 1986, 14, 205–214. [Google Scholar] [CrossRef]
  30. Babulanam, S.M.; Eriksson, T.S.; Niklasson, G.A.; Granqvist, C.G. Thermochromic VO2 films for energy-efficient windows. Sol. Energy Mater. 1987, 16, 347–363. [Google Scholar] [CrossRef]
  31. Gao, Y.; Luo, H.; Zhang, Z.; Kang, L.; Chen, Z.; Du, J.; Kanehira, M.; Cao, C. Nanoceramic VO2 thermochromic smart glass: A review on progress in solution processing. Nano Energy 2012, 1, 221–246. [Google Scholar] [CrossRef]
  32. Li, S.-Y.; Niklasson, G.A.; Granqvist, C.G. Thermochromic fenestration with VO2-based materials: Three challenges and how they can be met. Thin Solid Films 2012, 520, 3823–3828. [Google Scholar] [CrossRef]
  33. Li, S.-Y. VO2-Based Thermochromic and Nanothermochromic Materials for Energy-Efficient Windows: Computational and Experimental Studies; Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1095; Acta Universitatis Upsaliensis: Uppsala, Sweden, 2013. [Google Scholar]
  34. Li, S.-Y.; Niklasson, G.A.; Granqvist, C.G. Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows. J. Appl. Phys. 2014, 115, 053513/1–053513/10. [Google Scholar] [CrossRef]
  35. Hoffmann, S.; Lee, E.S.; Clavero, C. Examination of the technical potential of near-infrared switching thermochromic windows for commercial building applications. Sol. Energy Mater. Sol. Cells 2014, 123, 65–80. [Google Scholar] [CrossRef]
  36. Warwick, M.E.A.; Binions, R. Advances in thermochromic vanadium dioxide films. J. Mater. Chem. A 2014, 2, 3275–3292. [Google Scholar] [CrossRef]
  37. Anderson, A.-L.; Chen, S.; Romero, L.; Binions, R. Thin films for advanced glazing applications. Buildings 2016, 6, 37. [Google Scholar] [CrossRef]
  38. Warwick, M.E.A.; Ridley, I.; Binions, R. Variation of thermochromic glazing systems transition temperature, hysteresis gradient and width on energy efficiency. Buildings 2016, 6, 22. [Google Scholar] [CrossRef]
  39. Warwick, M.E.A.; Ridley, I.; Binions, R. Thermochromic vanadium dioxide thin films prepared by electric field assisted atmospheric pressure chemical vapour deposition for intelligent glazing application and their energy demand reduction properties. Sol. Energy Mater. Sol. Cells 2016, 157, 686–694. [Google Scholar] [CrossRef]
  40. Ji, Y.-X.; Boman, M.; Niklasson, G.A.; Granqvist, C.G. Thermochromics for energy-efficient buildings: Thin surface coatings and nanoparticle composites. In Nano and Biotech Based Materials for Energy Building Efficiency; Pacheco-Torgal, F., Buratti, C., Kalaiselvam, S., Granqvist, C.G., Ivanov, V., Eds.; Springer: Cham, Switzerland, 2016; pp. 71–96. [Google Scholar]
  41. The Freedonia Group. World Flat Glass to 2016: Industry Market Research, Market Share, Market Size, Sales, Demand Forecast, Market Leaders, Company Profiles, Industry Trends; The Freedonia Group: Cleveland, OH, USA, 2013. [Google Scholar]
  42. Wyszecki, J.; Stiles, W.S. Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed.; Wiley: New York, NY, USA, 2000. [Google Scholar]
  43. American Society for Testing and Materials (ASTM). American Society for Testing and Materials (ASTM). G173-03 Standard tables of reference solar spectral irradiances: Direct normal and hemispherical on a 37° tilted surface. In Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, USA, 2012. [Google Scholar]
  44. Granqvist, C.G. Oxide-based chromogenic coatings and devices for energy efficient fenestration: Brief survey and update on thermochromics and electrochromics. J. Vac. Sci. Technol. B 2014, 32, 060801/1–060801/13. [Google Scholar] [CrossRef]
  45. Granqvist, C.G. Recent progress in thermochromics and electrochromics: A brief survey. Thin Solid Films 2016, 614, 90–96. [Google Scholar] [CrossRef]
  46. Granqvist, C.G.; Ji, Y.-X.; Montero, J.; Niklasson, G.A. Thermochromic vanadium-dioxide-based thin films and nanoparticles: Survey of some buildings-related advances. J. Phys. Conf. Ser. 2016, 764, 012002/1–012002/11. [Google Scholar] [CrossRef]
  47. Morin, F.J. Oxides which show a metal-to-insulator transition at the Neel temperature. Phys. Rev. Lett. 1959, 3, 34–36. [Google Scholar] [CrossRef]
  48. Chen, S.; Liu, J.; Luo, H.; Gao, Y. Calculation evidence of staged Mott and Peierls transitions in VO2 revealed by mapping reduced-dimension potential energy surface. Phys. Chem. Lett. 2015, 6, 3650–3656. [Google Scholar] [CrossRef] [PubMed]
  49. Zheng, H.; Wagner, L.K. Computation of the correlated metal–insulator transition in vanadium dioxide from first principles. Phys. Rev. Lett. 2015, 114, 176401/1–176401/4. [Google Scholar] [CrossRef] [PubMed]
  50. Kumar, S.; Strachan, J.P.; Kilcoyne, A.L.D.; Tyliszczak, T.; Pickett, M.D.; Santori, C.; Gibson, G.; Williams, R.S. The phase transition in VO2 probed using X-rays, visible and infrared radiations. Appl. Phys. Lett. 2016, 108, 073102/1–073102/4. [Google Scholar] [CrossRef]
  51. Liu, S.; Phillabaum, B.; Carlson, E.W.; Dahmen, K.A.; Vidhyadhiraja, N.S.; Qazilbash, M.M.; Basov, D.N. Random field driven spatial complexity at the Mott transition in VO2. Phys. Rev. Lett. 2016, 116, 036401/1–036401/5. [Google Scholar] [CrossRef] [PubMed]
  52. Saeli, M.; Piccirillo, C.; Parkin, I.P.; Binions, R.; Ridley, I. Energy modelling studies of thermochromic glazing. Energy Build. 2010, 42, 1666–1673. [Google Scholar] [CrossRef]
  53. Saeli, M.; Piccirillo, C.; Parkin, I.P.; Ridley, I.; Binions, R. Nano-composite thermochromic thin films and their application in energy-efficient glazing. Sol. Energy Mater. Sol. Cells 2010, 94, 141–151. [Google Scholar] [CrossRef]
  54. Wang, S.; Liu, M.; Kong, L.; Long, Y.; Jiang, X.; Yu, A. Recent progress in VO2 smart coatings: Strategies to improve the thermochromic properties. Progr. Mater. Sci. 2016, 81, 1–54. [Google Scholar] [CrossRef]
  55. Smith, J.F. (Ed.) Phase diagrams of binary vanadium alloys. In Monograph Series on Alloy Phase Diagrams; ASM International: Metals Park, OH, USA, 1989.
  56. Wriedt, H.A. The O–V (oxygen–vanadium) system. Bull. Alloy Phase Diagr. 1989, 10, 271–277. [Google Scholar] [CrossRef]
  57. Kang, Y.-B. Critical evaluation and thermodynamic optimization of the VO–VO2.5 system. J. Eur. Ceram. Soc. 2012, 32, 3187–3198. [Google Scholar] [CrossRef]
  58. Bahlawane, N.; Lenoble, D. Vanadium oxide compounds: Structure, properties, and growth from the gas phase. Chem. Vapor Depos. 2014, 20, 299–311. [Google Scholar] [CrossRef]
  59. Marvel, R.E.; Appavoo, K.; Choi, B.K.; Nag, J.; Haglund, R.F., Jr. Electron-beam deposition of vanadium dioxide thin films. Appl. Phys. A 2013, 111, 975–981. [Google Scholar] [CrossRef]
  60. Jian, J.; Chen, A.; Zhang, W.; Wang, H. Sharp metal-to-insulator transition of VO2 thin films on glass substrates. J. Appl. Phys. 2013, 114, 244301/1–244301/6. [Google Scholar] [CrossRef]
  61. Li, S.-Y.; Namura, K.; Suzuki, M.; Niklasson, G.A.; Granqvist, C.G. Thermochromic VO2 nanorods made by sputter deposition: Growth conditions and optical modeling. J. Appl. Phys. 2013, 114, 033516/1–033516/11. [Google Scholar] [CrossRef] [Green Version]
  62. Ji, Y.-X.; Niklasson, G.A.; Granqvist, C.G.; Boman, M. Thermochromic VO2 films by thermal oxidation of vanadium in SO2. Sol. Energy Mater. Sol. Cells 2016, 144, 713–716. [Google Scholar] [CrossRef]
  63. Fortier, J.-P.; Baloukas, B.; Zabeida, O.; Klemberg-Sapieha, J.E.; Martinu, L. Thermochromic VO2 thin films deposited by HiPIMS. Sol. Energy Mater. Sol. Cells 2014, 125, 291–296. [Google Scholar] [CrossRef]
  64. Aijaz, A.; Ji, Y.-X.; Montero, J.; Niklasson, G.A.; Granqvist, C.G.; Kubart, T. Low-temperature synthesis of thermochromic vanadium dioxide thin films by reactive high power impulse magnetron sputtering. Sol. Energy Mater. Sol. Cells 2016, 149, 137–144. [Google Scholar] [CrossRef]
  65. Loquai, S.; Baloukas, B.; Zabeida, O.; Klemberg-Sapieha, J.E.; Martinu, L. HiPIMS-deposited thermochromic VO2 films on polymeric substrates. Sol. Energy Mater. Sol. Cells 2016, 155, 60–69. [Google Scholar] [CrossRef]
  66. Loquai, S.; Baloukas, B.; Klemberg-Sapieha, J.E.; Martinu, L. HiPIMS-deposited thermochromic VO2 films with high environmental stability. Sol. Energy Mater. Sol. Cells 2017, 160, 217–224. [Google Scholar] [CrossRef]
  67. Houska, J.; Kolenaty, D.; Rezek, J.; Vlcek, J. Characterization of thermochromic VO2 (prepared at 250 °C) in a wide temperature range by spectroscopic ellipsometry. Appl. Surf. Sci. 2017, in press. [Google Scholar] [CrossRef]
  68. Drosos, C.; Vernardou, D. Perspectives of energy materials grown by APCVD. Sol. Energy Mater. Sol. Cells 2015, 140, 1–8. [Google Scholar] [CrossRef]
  69. Louloudakis, D.; Vernardou, D.; Spanakis, E.; Suchea, M.; Kenanakis, G.; Pemble, M.; Savvakis, C.; Katsarakis, N.; Koudoumas, E.; Kiriakidis, G. Atmospheric pressure chemical vapor deposition of amorphous tungsten doped vanadium dioxide for smart window applications. Adv. Mater. Lett. 2016, 7, 10–15. [Google Scholar] [CrossRef]
  70. Seyfouri, M.M.; Binions, R. Sol–gel approaches to thermochromic vanadium dioxide coating for smart glazing application. Sol. Energy Mater. Sol. Cells 2017, 159, 52–65. [Google Scholar] [CrossRef]
  71. Li, S.-Y.; Niklasson, G.A.; Granqvist, C.G. Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation. J. Appl. Phys. 2010, 108, 063525/1–063525/5. [Google Scholar] [CrossRef]
  72. Powell, M.J.; Marchand, P.; Denis, C.J.; Bear, J.C.; Darr, J.A.; Parkin, I.P. Direct and continuous synthesis of VO2 nanoparticles. Nanoscale 2015, 7, 18686–18693. [Google Scholar] [CrossRef] [PubMed]
  73. Montero, J.; Ji, Y.-X.; Li, S.-Y.; Niklasson, G.A.; Granqvist, C.G. Sputter deposition of thermochromic VO2 films on In2O3:Sn, SnO2 and glass: Structure and composition versus oxygen partial pressure. J. Vac. Sci. Technol. B 2015, 33, 031805/1–031804/7. [Google Scholar] [CrossRef]
  74. Montero, J.; Ji, Y.-X.; Granqvist, C.G.; Niklasson, G.A. Thermochromic light scattering from particulate VO2 layers. J. Appl. Phys. 2016, 119, 085302/1–085302/6. [Google Scholar] [CrossRef]
  75. Ji, Y.-X.; Niklasson, G.A.; Granqvist, C.G. Direct formation of thermochromic composite films of VO2 nanoparticles in SiO2 hosts. In Proceedings of the IEEE Nano 2016: 16th International Conference on Nanotechnology, Sendai, Japan, 22–26 August 2016; pp. 823–825.
  76. Romanyuk, A.; Steiner, R.; Marot, L.; Spassov, V.; Oelhafen, P. nc-VO2/Al2O3 nanocomposite films prepared by dual target magnetron sputtering. Thin Solid Films 2008, 516, 8513–8516. [Google Scholar] [CrossRef]
  77. Goodenough, J.B. The two components of the crystallographic transition in VO2. J. Solid State Chem. 1971, 3, 490–500. [Google Scholar] [CrossRef]
  78. Hörlin, T.; Niklewski, T.; Nygren, M. Electrical and magnetic properties of V1–xWxO2, 0 ≤ x ≤ 0.060. Mater. Res. Bull. 1972, 7, 1515–1524. [Google Scholar] [CrossRef]
  79. Reyes, J.M.; Lynch, G.F.; Sayer, M.; McBride, S.L.; Hutchison, T.S. Electrical, optical, magnetic resonance and microhardness properties of tungsten-doped VO2. J. Can. Ceram. Soc. 1972, 41, 69–75. [Google Scholar]
  80. Greenberg, C.B. Undoped and doped VO2 films grown from VO(OC3H7)3. Thin Solid Films 1983, 110, 7–82. [Google Scholar] [CrossRef]
  81. Mlyuka, N.R.; Niklasson, G.A.; Granqvist, C.G. Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal–insulator transition temperature. Appl. Phys. Lett. 2009, 95, 171909/1–171909/3. [Google Scholar] [CrossRef]
  82. Li, S.-Y.; Mlyuka, N.R.; Primetzhofer, D.; Hallén, A.; Possnert, G.; Niklasson, G.A.; Granqvist, C.G. Bandgap widening in thermochromic Mg-doped VO2 thin films: Quantitative data based on optical absorption. Appl. Phys. Lett. 2013, 103, 161907/1–161907/4. [Google Scholar] [CrossRef]
  83. Dietrich, M.K.; Kramm, B.G.; Becker, M.; Meyer, B.K.; Polity, A.; Klar, P.J. Influence of doping with alkaline earth metals on the optical properties of thermochromic VO2. J. Appl. Phys. 2015, 117, 185301/1–185301/8. [Google Scholar] [CrossRef]
  84. Gagaoudakis, E.; Kortidis, I.; Michail, G.; Tsagaraki, K.; Binas, V.; Kiriakidis, G.; Aperathitis, E. Study of low temperature rf-sputtered Mg-doped vanadium dioxide thermochromic films deposited on low-emissivity substrates. Thin Solid Films 2016, 601, 99–105. [Google Scholar] [CrossRef]
  85. Panagopoulou, M.; Gagaoudakis, E.; Boukos, N.; Aperathitis, E.; Kiriakidis, G.; Tsoukalas, D.; Raptis, Y.S. Thermochromic performance of Mg-doped VO2 thin films on functional substrates for glazing applications. Sol. Energy Mater. Sol. Cells 2016, 157, 1004–1010. [Google Scholar] [CrossRef]
  86. Hu, S.; Li, S.-Y.; Ahuja, R.; Granqvist, C.G.; Hermansson, K.; Niklasson, G.A.; Scheicher, R.H. Optical properties of Mg-doped VO2: Absorption measurements and hybrid functional calculations. Appl. Phys. Lett. 2012, 101, 201902/1–201902/4. [Google Scholar]
  87. Abdellaoui, I.; Merad, G.; Maaza, M.; Si Abdelkader, H. Electronic and optical properties of Mg-, F-doped and Mg/F-codoped M1-VO2 via hybrid density functional calculations. J. Alloys Cpds. 2016, 658, 569–575. [Google Scholar] [CrossRef]
  88. Jiang, M.; Bao, S.; Cao, X.; Li, Y.; Li, S.; Zhou, H.; Luo, H.; Jin, P. Improved luminous transmittance and diminished yellow color in VO2 energy efficient smart thin films by Zn doping. Ceram. Int. 2014, 40, 6331–6334. [Google Scholar] [CrossRef]
  89. Wang, N.; Duchamp, M.; Dunin-Borokowski, R.E.; Liu, S.; Zeng, X.T.; Cao, X.; Long, Y. Terbium-doped VO2 thin films: Reduced phase transition temperature and largely enhanced luminous transmittance. Langmuir 2016, 32, 759–764. [Google Scholar] [CrossRef] [PubMed]
  90. Khan, K.A.; Granqvist, C.G. Thermochromic sputter-deposited vanadium oxyfluoride coatings with low luminous absorptance. Appl. Phys. Lett. 1989, 55, 4–6. [Google Scholar] [CrossRef]
  91. Burkhardt, W.; Christmann, T.; Franke, S.; Kriegseis, W.; Meister, D.; Meyer, B.K.; Niessner, W.; Schalch, D.; Scharmann, A. Tungsten and fluorine co-doping of VO2 films. Thin Solid Films 2002, 402, 226–231. [Google Scholar] [CrossRef]
  92. Kiri, P.; Warwick, M.E.A.; Binions, B. Fluorine doped vanadium dioxide thin films for smart windows. Thin Solid Films 2011, 520, 1363–1366. [Google Scholar] [CrossRef]
  93. Li, S.-Y.; Niklasson, G.A.; Granqvist, C.G. Nanothermochromics with VO2-based core–shell structures: Calculated luminous and solar optical properties. J. Appl. Phys. 2011, 109, 113515/1–113515/8. [Google Scholar] [CrossRef]
  94. Laaksonen, K.; Li, S.-Y.; Puisto, S.R.; Rostedt, N.K.J.; Ala-Nissila, T.; Granqvist, C.G.; Nieminen, R.M.; Niklasson, G.A. Nanoparticles of TiO2 and VO2 in dielectric media: Conditions for low optical scattering, and comparison between effective medium and four-flux theories. Sol. Energy Mater. Sol. Cells 2014, 130, 132–137. [Google Scholar] [CrossRef]
  95. Lopez, R.; Boatner, L.A.; Haynes, T.E.; Feldman, L.C.; Haglund, R.F., Jr. Synthesis and characterization of size-controlled vanadium-dioxide nanocrystals in a fused silica matrix. J. Appl. Phys. 2002, 92, 4031–4036. [Google Scholar] [CrossRef]
  96. Lopez, R.; Haynes, T.E.; Boatner, L.A.; Feldman, L.C.; Haglund, R.F., Jr. Temperature-controlled surface plasmon resonance in VO2 nanorods. Opt. Lett. 2002, 27, 1327–1329. [Google Scholar] [CrossRef] [PubMed]
  97. Gentle, A.; Maaroof, A.I.; Smith, G.B. Nanograin VO2 in the metal phase: A plasmonic system with falling resistivity as temperature rises. Nanotechnology 2007, 18, 025202/1–025202/7. [Google Scholar] [CrossRef]
  98. Bai, H.; Cortie, M.B.; Maaroof, A.I.; Dowd, A.; Kealley, C.; Smith, G.B. The preparation of a plasmonically resonant VO2 thermochromic pigment. Nanotechnology 2009, 20, 085607/1–085607/9. [Google Scholar] [CrossRef] [PubMed]
  99. Mlyuka, N.R.; Niklasson, G.A.; Granqvist, C.G. Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance. Sol. Energy Mater. Sol. Cells 2009, 93, 1685–1687. [Google Scholar] [CrossRef]
  100. Mlyuka, N.R.; Niklasson, G.A.; Granqvist, C.G. Thermochromic VO2-based multilayer films with enhanced luminous transmittance and solar modulation. Phys. Status Solidi A 2009, 206, 2155–2160. [Google Scholar] [CrossRef]
  101. Chen, Z.; Gao, Y.; Kang, L.; Du, J.; Zhang, Z.; Luo, H.; Miao, H. VO2-based double-layered films for smart windows: Optical design, all-solution preparation and improved properties. Sol. Energy Mater. Sol. Cells 2011, 95, 2677–2684. [Google Scholar] [CrossRef]
  102. Taylor, A.; Parkin, I.; Noor, N.; Tummeltshammer, C.; Brown, M.S.; Papakonstantinou, I. A bioinspired solution for spectrally selective thermochromic VO2 coated intelligent glazing. Opt. Expr. 2013, 21, A750–A764. [Google Scholar] [CrossRef] [PubMed]
  103. Zhu, J.; Huang, A.; Ma, H.; Ma, Y.; Tong, K.; Ji, S.; Bao, S.; Cao, X.; Jin, P. Composite film of vanadium dioxide nanoparticles and ionic liquid–nickel–chlorine complexes with excellent visible thermochromic performance. ACS Appl. Mater. Interfaces 2016, 8, 29742–29748. [Google Scholar] [CrossRef] [PubMed]
  104. Ke, Y.; Balin, I.; Wang, N.; Lu, Q.; Tok, A.I.Y.; White, T.J.; Magdassi, S.; Abdulhalim, I.; Long, Y. Two-dimensional SiO2/VO2 photonic crystals with statically visible and dynamically infrared modulated for smart window deployment. ACS Appl. Mater. Interfaces 2016, 8, 33112–33120. [Google Scholar] [CrossRef] [PubMed]
  105. Ji, Y.-X.; Li, S.-Y.; Niklasson, G.A.; Granqvist, C.G. Durability of thermochromic VO2 thin films under heating and humidity: Effect of Al oxide top coatings. Thin Solid Films 2014, 562, 568–573. [Google Scholar] [CrossRef]
  106. Talledo, A.; Granqvist, C.G. Electrochromic vanadium-pentoxide-based films: Structural, electrochemical, and optical properties. J. Appl. Phys. 1995, 77, 4655–4666. [Google Scholar] [CrossRef]
  107. Lykissa, I.; Li, S.-Y.; Ramzan, M.; Chakraborty, S.; Ahuja, R.; Granqvist, C.G.; Niklasson, G.A. Electronic density-of-states of amorphous vanadium pentoxide films: Electrochemical data and density functional theory calculations. J. Appl. Phys. 2014, 115, 183701/1–183701/5. [Google Scholar] [CrossRef]
  108. Vernardou, D.; Louloudakis, D.; Spanakis, E.; Katsarakis, N.; Koudoumas, E. Thermochromic amorphous VO2 coatings grown by APCVD using a single-precursor. Sol. Energy Mater. Sol. Cells 2014, 128, 36–40. [Google Scholar] [CrossRef]
  109. Ji, Y.-X.; Niklasson, G.A.; Granqvist, C.G. Durability of VO2-based thin films at elevated temperature: Towards thermochromic fenestration. J. Phys. Conf. Proc. 2014, 559, 012005. [Google Scholar] [CrossRef]
  110. Tong, K.; Li, R.; Zhu, J.; Yao, H.; Zhou, H.; Zeng, X.; Ji, S.; Jin, P. Preparation of VO2/Al-O core–shell structure with enhanced weathering resistance for smart window. Ceram. Int. 2016. [Google Scholar] [CrossRef]
  111. Nečas, D.; Klapetek, P. Gwyddion: An open-source software for SPM data analysis. Cent. Eur. J. Phys. 2012, 10, 181–188. [Google Scholar] [CrossRef]
  112. Grenfell, T.C.; Warren, S.G. Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation. J. Geophys. Res. 1999, 104, 31697–31709. [Google Scholar] [CrossRef]
  113. Bohren, C.F.; Huffman, D.R. Absorption and Light Scattering by Small Particles; Wiley: New York, NY, USA, 1983. [Google Scholar]
  114. Li, S.; Li, Y.; Qian, K.; Ji, S.; Luo, H.; Gao, Y.; Jin, P. Functional fiber mats with tunable diffuse reflectance composed of electrospun VO2/PVP composite fibers. ACS Appl. Mater. Interfaces 2014, 6, 9–13. [Google Scholar] [CrossRef] [PubMed]
  115. Qian, K.; Li, S.; Ji, S.; Li, W.; Li, Y.; Chen, R.; Jin, P. Fabrication of VO2 nanorods/PVP composite fiber mats and their unique optical diffuse reflectance properties. Ceram. Int. 2014, 40, 14517–14521. [Google Scholar] [CrossRef]
  116. Moot, T.; Palin, C.; Mitran, S.; Cahoon, J.F.; Lopez, R. Designing plasmon-enhanced thermochromic films using a vanadium dioxide nanoparticle elastomeric composite. Adv. Opt. Mater. 2016, 4, 578–583. [Google Scholar] [CrossRef]
  117. Seeboth, A.; Lötzsch, D. Thermochromic and Thermotropic Materials; Pan Stanford Publishing: Singapore, 2014. [Google Scholar]
  118. Anderson, A.D.; Broekhuis, M.D.; Byker, H.J.; DeJong, S.J. Color Neutral Thermochromic Layers and Laminates. U.S. Patent No. 9,465,239, 11 October 2016. [Google Scholar]
  119. Jelle, B.P.; Hynd, A.; Gustavsen, A.; Arasteh, D.; Goudey, H.; Hart, R. Fenestration today and tomorrow: A state-of-the-art review and future research oppoprtunities. Sol. Energy Mater. Sol. Cells 2012, 96, 1–28. [Google Scholar] [CrossRef]
  120. Niklasson, G.A.; Granqvist, C.G. Electrochromics for smart windows: Thin films of tungsten oxide and nickel oxide, and devices based on these. J. Mater. Chem. 2007, 17, 127–156. [Google Scholar] [CrossRef]
  121. Granqvist, C.G. Electrochromics for smart windows: Oxide-based thin films and devices. Thin Solid Films 2014, 564, 1–38. [Google Scholar] [CrossRef]
  122. Mortimer, R.J.; Rosseinsky, D.R.; Monk, P.M.S. Electrochromic Materials and Devices; Wiley-VCH: Weinberg, Germany, 2015. [Google Scholar]
  123. Powell, M.J.; Quesada-Cabrera, R.; Taylor, A.; Teixeira, D.; Papakostantinou, I.; Palgrave, R.G.; Sankar, G.; Parkin, I.P. Intelligent multifunctional VO2/SiO2/TiO2 coatings for self-cleaning, energy-saving window panels. Chem. Mater. 2016, 28, 1369–1376. [Google Scholar] [CrossRef]
Figure 1. (a) Blackbody spectra for the indicated temperatures, which include the sun’s surface temperature (the vertical scales differ for the spectra); (b) Typical solar irradiance spectrum for clear weather, and relative spectral sensitivity of the light-adapted human eye [33].
Figure 1. (a) Blackbody spectra for the indicated temperatures, which include the sun’s surface temperature (the vertical scales differ for the spectra); (b) Typical solar irradiance spectrum for clear weather, and relative spectral sensitivity of the light-adapted human eye [33].
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Figure 2. Electrical conductivity as a function of reciprocal temperature (lower horizontal axis) and temperature (upper horizontal axis) for the shown metal-based compounds. The vertical line denotes room temperature. Arrows on the vertical lines signify hysteretic transitions [29].
Figure 2. Electrical conductivity as a function of reciprocal temperature (lower horizontal axis) and temperature (upper horizontal axis) for the shown metal-based compounds. The vertical line denotes room temperature. Arrows on the vertical lines signify hysteretic transitions [29].
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Figure 3. Spectral transmittance (a,b) and reflectance (c,d) for a 50 nm-thick thin film of VO2 (a,c) and for a layer comprising a dilute dispersion of VO2 nanospheres, with an equivalent VO2 thickness of 50 nm, in a medium mimicking transparent glass or polymer (b,d).
Figure 3. Spectral transmittance (a,b) and reflectance (c,d) for a 50 nm-thick thin film of VO2 (a,c) and for a layer comprising a dilute dispersion of VO2 nanospheres, with an equivalent VO2 thickness of 50 nm, in a medium mimicking transparent glass or polymer (b,d).
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Figure 4. Phase diagram for the VO–VO2.5 system [57].
Figure 4. Phase diagram for the VO–VO2.5 system [57].
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Figure 5. Scanning electron microscopy (SEM) images of a VO2 layer deposited onto heated glass to an (equivalent) thickness of 102 nm. (ac) images indicate a top-view, imaging with 70° between the electron beam and the sample’s normal, and a cross-sectional picture, respectively. Note that the magnifications are different among the images [61].
Figure 5. Scanning electron microscopy (SEM) images of a VO2 layer deposited onto heated glass to an (equivalent) thickness of 102 nm. (ac) images indicate a top-view, imaging with 70° between the electron beam and the sample’s normal, and a cross-sectional picture, respectively. Note that the magnifications are different among the images [61].
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Figure 6. SEM top-view (a) and atomic force microscopy (AFM) image (b) of a particulate VO2 layer on ITO-coated glass. The maximum peak height of the deposit was ~190 nm [74].
Figure 6. SEM top-view (a) and atomic force microscopy (AFM) image (b) of a particulate VO2 layer on ITO-coated glass. The maximum peak height of the deposit was ~190 nm [74].
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Figure 7. SEM (a) and energy-dispersive X-ray (EDX) (b) data for a 220 nm-thick VO2–SiO2 film prepared by sputter deposition; green dots signify vanadium [75].
Figure 7. SEM (a) and energy-dispersive X-ray (EDX) (b) data for a 220 nm-thick VO2–SiO2 film prepared by sputter deposition; green dots signify vanadium [75].
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Figure 8. High-resolution transmission electron microscopy (HRTEM) image of a nanocrystalline VO2–Al2O3 composite film with encircled areas indicating VO2 [76].
Figure 8. High-resolution transmission electron microscopy (HRTEM) image of a nanocrystalline VO2–Al2O3 composite film with encircled areas indicating VO2 [76].
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Figure 9. Transition termperature τc between semiconducting and metallic-like states in VO2 films containing W or Mg [81]. The data are based on electrical measurements.
Figure 9. Transition termperature τc between semiconducting and metallic-like states in VO2 films containing W or Mg [81]. The data are based on electrical measurements.
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Figure 10. Spectral transmittance for 50 nm-thick Mg-doped and undoped VO2 films in semiconducting (τ = 22 °C) and metallic (τ = 100 °C) states [81].
Figure 10. Spectral transmittance for 50 nm-thick Mg-doped and undoped VO2 films in semiconducting (τ = 22 °C) and metallic (τ = 100 °C) states [81].
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Figure 11. Luminous transmittance as a function of Mg doping in 50 nm-thick VO2-based films as measured at τ = 22 °C. Line was drawn to guide the eye [81].
Figure 11. Luminous transmittance as a function of Mg doping in 50 nm-thick VO2-based films as measured at τ = 22 °C. Line was drawn to guide the eye [81].
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Figure 12. Spectral absorptance for the VO2–SiO2 deposit shown in Figure 7 as recorded at the stated temperatures. Also shown is a solar spectrum analogous to that in Figure 1b [75].
Figure 12. Spectral absorptance for the VO2–SiO2 deposit shown in Figure 7 as recorded at the stated temperatures. Also shown is a solar spectrum analogous to that in Figure 1b [75].
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Figure 13. Schematic performance limits for VO2-based thin films and nanoparticle composites with regard to TC glazings [34].
Figure 13. Schematic performance limits for VO2-based thin films and nanoparticle composites with regard to TC glazings [34].
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Figure 14. Spectral transmittance for an 80 nm-thick VO2 film in as-deposited state and after subsequent heating at 300 °C for one hour. Data pertain to films in (a) semiconducting (τ = 25 °C) and (b) metallic-like (τ = 100 °C) states [105].
Figure 14. Spectral transmittance for an 80 nm-thick VO2 film in as-deposited state and after subsequent heating at 300 °C for one hour. Data pertain to films in (a) semiconducting (τ = 25 °C) and (b) metallic-like (τ = 100 °C) states [105].
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Figure 15. Spectral transmittance for 80 nm-thick VO2 films, over-coated with 10 nm (a,b) and 30 nm (c,d) of Al2O3, in as-deposited state and after heating at 300 °C for the shown time periods th. Data were recorded for films in semiconducting (τ = 25 °C) and metallic-like (τ = 100 °C) states [105].
Figure 15. Spectral transmittance for 80 nm-thick VO2 films, over-coated with 10 nm (a,b) and 30 nm (c,d) of Al2O3, in as-deposited state and after heating at 300 °C for the shown time periods th. Data were recorded for films in semiconducting (τ = 25 °C) and metallic-like (τ = 100 °C) states [105].
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Figure 16. Distributions of the number Neq of equivalent sphere radii for the data shown in Figure 6b [74].
Figure 16. Distributions of the number Neq of equivalent sphere radii for the data shown in Figure 6b [74].
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Figure 17. Experimental (αabs) and theoretical (K*abs) spectral absorption data, and corresponding results for experimental (αsca) and theoretical (K*sca) spectral scattering data, for the particulate VO2 sample shown in Figure 6. The calculations used the equal-volume and equal-volume-to-area approximations in Figure 16. Data are given for the semiconducting state at τ < τc (a,c) and the metallic state at τ > τc (b,d) [74].
Figure 17. Experimental (αabs) and theoretical (K*abs) spectral absorption data, and corresponding results for experimental (αsca) and theoretical (K*sca) spectral scattering data, for the particulate VO2 sample shown in Figure 6. The calculations used the equal-volume and equal-volume-to-area approximations in Figure 16. Data are given for the semiconducting state at τ < τc (a,c) and the metallic state at τ > τc (b,d) [74].
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Granqvist, C.G.; Niklasson, G.A. Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings. Buildings 2017, 7, 3. https://doi.org/10.3390/buildings7010003

AMA Style

Granqvist CG, Niklasson GA. Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings. Buildings. 2017; 7(1):3. https://doi.org/10.3390/buildings7010003

Chicago/Turabian Style

Granqvist, Claes G., and Gunnar A. Niklasson. 2017. "Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings" Buildings 7, no. 1: 3. https://doi.org/10.3390/buildings7010003

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