Vanadium dioxide is a promising thermochromic material, seemed as the great candidate for smart window applications. The real application of VO2 requires high visible transmission (Tlum) as well as large solar modulating abilities (∆Tsol), which could not be achieved by pristine VO2 materials due to the trade-off between Tlum and ∆Tsol. Here in, the porosity design is thoroughly reviewed from the effect on modulating the thermochromic performance to the porous control and preparation. To begin with, the history, advantages, challenges and approaches to tackle the issues comprised of antireflection multilayer structure, nanothermochromism, patterning and porous design is introduced in detail. Then, the effect of porosity on improving the thermochromic performance of VO2 thin films is demonstrated using the newest experimental and simulation results. In the following, the porous control and structural synthesis, including the polymer-assisted deposition (PAD), freeze-drying, colloidal lithography as well as the dual phase transformation is summarized. Fourthly, the characterization methods, composed of scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy as well as UV-Vis-NIR spectroscopy are demonstrated. Finally, the challenges that the porous design faces and possible approaches to optimize the performance are presented.
- vanadium dioxide
- smart window
- energy saving
In recent decades, the usage of traditional energy materials, including the oil and the coal meets more and more challenges due to the increase of air pollution, energy shortage and the global warming. Therefore, the concepts of sustainable and environment-friendly production were raised by scientists for energy-saving, and various clean energy technologies have been proposed for industries, for example, fuel cell [1, 2, 3, 4], solar cell [5, 6, 7] and wind turbines [8, 9, 10, 11]. On the other hand, the alternative energy-saving approach is to develop green-energy buildings equipped with state-of-the-art smart windows, for example, electrochromic/thermochromic smart windows [12, 13, 14, 15, 16, 17].
Vanadium dioxide (VO2), as a promising coating material for thermochromic smart windows have been investigated for half a century, since Morin found the intrinsic metal-to-insulator transition (MIT) of VO2 in 1959 . Below a critical temperature (τc) ~ 68°C, VO2 shows the monoclinic insulating phase (VO2(M)) with zig-zag V-V chains along the c-axis (P21/c, V-V separation is 0.262, 0.316 nm) . Above the τc, VO2 is transformed to rutile metallic phase (VO2(R)) with linear V-V chains along the c-axis (P42/mnnm, V-V separation is 0.288 nm) . The increase of the electrical resistance across the MIT is always in 3–5 orders of magnitude, and the first-order transition could occur simultaneously with the time less than 500 fs . Along with the MIT, the IR transmittance of VO2 could also be modulated by a large magnitude owing to the change of the optical parameters (refractive ‘n’ and extinction coefficient ‘k’) . As a coating material, VO2 shows the high IR transmittance at the cold state while exhibits the large absorption as well as the strong reflection at the hot state, which gives rise to large IR modulating ability [22, 23, 24, 25]. Due to the little difference of optical parameters in the visible region, VO2 shows the little transmittance difference in the visible region [26, 27, 28]. The solar modulating ability especially in the IR region makes VO2 a promising coating material for thermochromic smart windows.
The VO2 thermochromic smart windows have various advantages in energy saving. To begin with, the phase transition temperature (τc) of VO2 is close to the room temperature, which cannot be found in other phase transition materials (τc(V2O3) = −123°C, τc(V2O5) = 257°C, τc(V6O13) = −123°C, τc(TinO2n+1) = 127–377°C) . Secondly, the τc of VO2 could be further reduced to ambient temperature through doping with other high valence metal cations, for example, W6+ [22, 29, 30, 31, 32, 33], Mo6+ [34, 35, 36]. Finally, several synthetic methods, for example, atmospheric pressure CVD [36, 37, 38, 39, 40], magnetron sputtering [41, 42, 43, 44, 45], sol-gel [35, 46, 47] and hydrothermal assembly [48, 49, 50], have been developed to fabricate VO2 nanostructures for applications. However, for thermochromic applications, VO2 still meets several challenges. Firstly, it is hard to achieve the high visible transmittance (Tlum) and the large solar modulating abilities (∆Tsol) simultaneously, since there is always a tradeoff between the Tlum and ∆Tsol . Secondly, the thermochromic property is hard to maintain when reducing the τc to room temperatures via doping . Finally, the VO2 coating is not stable in the air .
In order to improve the thermochromic performance of VO2 coating, several interesting strategies, including nanoporosity, nanothermochromism, patterning as well as multilayer structures have been investigated by the scientists. Gao’s group reported the enhanced luminous transmittance (Tlum = ~40%) and improved thermochromic properties (∆Tsol = ~14%) of nanoporous VO2 thin films with low optical constants, and the optical calculations suggested that the further improved performance could be expected by increasing the thin film porosity . Li et al.  calculated the nanothermochromics of VO2 nanocomposite by dispersing VO2 nanoparticles in the dielectric host, which revealed that the thermochromic performance could be largely enhanced (Tlum = ~65%, ∆Tsol = ~20%) with spherical morphologies of the VO2nanoparticles in the nanocomposite. Long’s group investigated the micropatterning  and nanopatterning  of VO2 thin films, which both benefited the VO2 thin films with improved Tlum and ∆Tsol. Mlyuka et al. reported the five-layer TiO2/VO2/TiO2/VO2/TiO2 structure, which showed the high Tlum (~43%) and the large ∆Tsol (~12%). Across the strategies, the nanoporous design showed the advantages in easy-to-handling, low usage of VO2 materials as well as the thickness control.
2. Enhanced thermochromic properties of VO2 with porous structure
As is well known, the porous structure could effectively increase the specific area of materials and thus supply large active areas under low loading. On the other hand, the porous design could also reduce the optical constants (refractive index ‘n’ and the extinction coefficient ‘k’), which could benefit the materials with enhanced visible transmittance. The optical calculations of nanoporous VO2 thin films could be performed with an optical-admittance recursive method, based on the assumption that the optical constants should be linearly dependent on the volume fraction or the ‘n’ and ‘k’ is linearly decreased with the porosity. As shown in Figure 1, as for the random distributed nanoporous VO2 thin films (Figure 1a), the porous structure gave rise to an obvious decrease of optical constants (n, k) compared with the normal thin film, and the optical calculations revealed the largely enhanced Tlum and ∆Tsol with increasing the porosity of the thin films.
With respect to the porous structure of VO2 thin films, there are normally the random distributed and the periodic porous structures. In the random case, as reported by Gao’s group  and Long’s group , the thermochromic properties could be enhanced to Tlum > 40% and ∆Tsol > 14%. In contrast, for the periodic porous structure, as reported by Xie’s group  and further developed by Long’s group [59, 60], the visible transmittance could be above 46% while maintaining the ∆Tsol above 13%. Actually, the periodic nanoporous design is more efficient in controlling the porosity and optimizing the thermochromic properties than the random counterpart, since the porosity could be easily estimated from the structure design.
3. Porous control and synthetic methods
In the nanoporous design for thermochromic VO2 thin films, there are mainly four different approaches to synthesize and control the porosity, including the polymer assistant deposition (PAD) , freeze-drying preparation , colloidal lithography assembly  as well as the dual-phase transformation .
To begin with the PAD, it is a powerful technique to get the continuous nanoporous VO2 thin films. The polymer used in the PAD process could be cetyltrimethyl ammonium bromide (CTAB) , cetyltrimethylammonium vanadate (CTAV) , polyvinylpyrrolidone (PVP) [53, 64, 65] or polyethylenimine (PEI) [66, 67]. Take CTAB as an example, when the vanadium precursor was modified by the amphiphilic polymer, the nuclear could be effectively isolated and the nanopores could be formed during the annealing process (Figure 2) . It should be noted that the control of the polymer addition is critical to optimize the shape and size of the nanopores.
Freeze-drying is also an efficient way to prepare the nanoporous VO2 thin films. For a normal sol-gel process, it is hard to get a film with high porosity. When the precursor is frozen and then dried in vacuum, the solvents could sublime and be removed quickly from the structure, which therefore gives rise to the in-situ formation of nanoporous structure (Figure 3d). In a typical process for fabricating nanoporous VO2 thin films with freeze-drying, the V2O5-H2O2-ox (oxalic acid) precursor was firstly dip coated onto fused silica substrates for gelation, and then a pre-freezing process was performed with a following freeze-drying at −80°C and 0.01 mbar . After a post-annealing process under Ar atmosphere at 550°C for 2 h, the nanoporous VO2 thin films were subsequently obtained (Figure 3a–c).
Colloidal lithography assembly is an alternative approach to get the nanoporous VO2 thin films, especially for the periodic porous design. The close packed monolayer colloidal crystal (MCC) template has been the usual sacrificing template for colloidal lithography assembly, which make it a facile way to prepare the periodic nanoporous structure. In a typical colloidal lithography assembly for nanoporous VO2 thin films, the polystyrene (PS) MCC template was firstly infiltrated by VOSO4 solution, then the infiltration with NH4HCO3 solution as precipitator was performed to confirm the coating of vanadium source on the template. Finally, the template was picked up by a clean substrate, and then the periodic nanoporous VO2 thin films were attained though annealing in nitrogen gas . The nanoporous structure could be further modulated by changing the layer number and/or the concentration of the precursor, which could help to optimize the thermochromic properties of the thin films.
More systematically, colloidal lithography was explored to prepare the two-dimensional patterned VO2 films with tunable periodicity and diverse nanostructures including nanoparticle, nanonet and nanodome arrays . The fabrication process is more flexible via introducing of the plasma etching (PE) technology and controlling the precursor viscosity. They concluded the synthesis routes in Figure 4. When short PE duration applied, nanoparticle and nanodome arrays are produced using low (Route 1) and high (Route 2) viscosity precursors, respectively. Nanonet arrays are fabricated via prolonging PE duration and using low viscosity precursor (Route 3). Produced two-dimensional patterned VO2 arrays are highly uniform (Figure 5). For the first time, hexagonally patterned VO2 nanoparticle array with the average diameter down to 60 nm and the periodicity of 160 nm has been fabricated (Figure 5a). It is of great interest that such structure gives rise to tunable peak positon and intensity of the localized surface plasmon resonance (LSPR) at different temperature. The LSPR was also found a red-shift with increase of the particle size and the media reflective index, respectively, and these results fit well with the tendency calculated using 3D finite-difference time-domain (FDTD). Besides decent thermochromic performance (up to ΔTsol = 13.2% and Tlum = 46%) achieved, the 2D patterned VO2 films have been demonstrated as an efficient smart thermal radiation filter to remote control the lower critical solution temperature (LCST) behavior of poly N-isopropylacrylamine (PNIPAm) hydrogel. Comparing with template-free method, periodic films produced by nanosphere lithography technique offer more uniform periodicity (less periodic defect) as well as smaller individual nanostructure that is able down to sub-100 nm.
An interesting study using colloidal lithography was to develop photonic structures, consisted of two-dimensional SiO2-VO2 core-shell monolayer (Figure 6a and b) . The structures with periodicity in visible range are demonstrated with the ability to modulate the visible transmittance by selectively reflecting the light with certain color (Figure 6c). Benefiting from this ability, smart windows based on such structures display controllable appearances as well as good thermochromic performance, which is up to Tlum = 49.6% and ΔTsol = 11.0% calculated by 3D FDTD. This statically visible and dynamically near-infrared modulation is further proved by experiments. However, the optimized thermochromic performance is much lower than that in simulation, which is attributed to the sol-gel method where the perfect core-shell structure cannot be produced in experiment as in simulation. Thus, other more controllable methods, such as physical vapor deposition or chemical vapor deposition, could be proposed as a better way for the fabrication of such two-dimensional core-shell structures.
The dual-phase transformation is a newly developed template-free method to prepare the nanoporous VO2 thin films with ultrahigh visible transmittance. As depicted in Figure 7, this method is based on the transformation between the colloids and ionic states stimulated by the moisture. Firstly, the precursor (VOCl2 + HCl + H2O + N2H4) was spin coated onto fused silica substrates, and then the hydrous colloids were formed through water evaporation. After a quick annealing at 300°C to solidify the film, and an additional annealing at 500°C in N2, the honeycomb-like nanoporous VO2 structures were finally obtained with high visible transmittance (~700 nm) above 90% as well as a decent solar modulating ability (∆Tsol = ~5.5%). The critical factor for forming the initial hydrous spheres (colloids) is the ratio control between the HCl and the N2H4 .
In order to fully characterize the structure and the thermochromic properties of VO2 nanoporous thin films, the advanced techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy as well as UV-Vis-NIR spectroscopy could be utilized in the investigation.
With respect to the nanoporous morphology, SEM is a powerful technique to observe the size, shape and the distributions of the nanopores on the surface in a large scale vision, while the details within the pore could be determined using the TEM in a cross-section view. Due to the non-destructive advantage, AFM is also an efficient way to scan the pore distribution on the surface although some artifacts always appear in the AFM images.
Regarding to the thermochromic properties, the VO2 phase could be firstly confirmed though the XRD and Raman scan, and then the solar modulation ability could be determined with temperature dependent UV-Vis-NIR characterization. As for the XRD, VO2 (M, P21/c) will show the crystalline planes (011)/(−211)/(220)/(022)/(202) at the 2θ positions 28°/37°/55.5°/57.5°/65°, while the VO2 (R, P42/mnm) will show the crystalline planes (110)/(101)/(211)/(220)/(002) at the 2θ positions 28°/37°/55.5°/57.5°/65° [22, 70]. For the Raman scan [58, 71], the VO2 (M) phase will show the Ag peaks at the Raman shift positions 192/222/302/392/611 cm−1 and the Bg peak at 258 cm−1. In the measurement of thermochromic performance, the transmittance of the normal incidence is recorded at the wavelength range 250–2500 nm at the temperature below and above the τc, and the integrated luminous transmission (Tlum, 380 nm < λ < 780 nm) and the integrated solar modulating abilities (∆Tsol, 250 nm < λ < 2500 nm) could be calculated from the expression
where φlum is the standard luminous efficiency function for the photopic vision of human eyes , and the φsol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon) . ∆Tsol is calculated from Tsol(τ < τc) − Tsol(τ > τc).
5. Concluding remarks and outlook
In this chapter, we have elaborated the fabrication of nanoporous VO2 nanomaterials and the effect of porosity on enhancing the thermochromic properties. Compared with the other property enhancement methods, such as ARC multilayers, biomimetic patterning, nanothermochromism and periodic patterning (Figure 8), the porous design shows the advantages in easy-to-handling, low usage of VO2 materials as well as the thickness control, which could reduce the cost in the real applications. In the fabrication of nanoporous VO2 thin films, the PAD, freeze-drying as well as the dual-phase transformation are the three main methods for random nanoporous structures, while the colloidal lithography with the MCC template is an effective approach for periodic nanoporous structures. The calculations reveal that the nanoporous structure could result in the decrease of optical constants and thus lead to the enhancement of visible transmission while maintain the decent solar modulating abilities.
Although many efforts have been dedicated to optimize the effect of nanoporous structure on enhancing the thermochromic performance of VO2 thin films, the low visible transmission (<~80%) and the low solar modulating ability (<~30%) restrict the real applications in thermochromic smart windows. From the viewpoint of materials design, the periodic nanoporous VO2 thin films with the periodicity below 100 nm should give rise to the largely enhanced visible transmission as well as the highly reduced scattering, which could greatly improve the thermochromic performance for smart window applications.
This research is supported by National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme.