**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) [53], freeze-drying preparation [57], colloidal lithography assembly [58] as well as the dual-phase transformation [61].

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) [62], cetyltrimethylammonium vanadate (CTAV) [63], 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**) [62]. 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 VO<sup>2</sup> 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 V<sup>2</sup> O5 - H2 O2 -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 [57]. 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 VOSO<sup>4</sup> solution, then the infiltration with NH<sup>4</sup> HCO3 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 [58]. 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.

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

**Figure 2.** Modification of vanadium precursor by the CTAB. (a) Initial step for adding the CTAB into the vanadium precursor. (b) and (c) Two forms of separation for the nuclear functionalized by the CTAB after strong stirring [62].

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 redshift 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 Δ*T*sol = 13.2% and *T*lum = 46%) achieved,

arrays are highly uniform (**Figure 5**).

Controlled Porosity in Thermochromic Coatings http://dx.doi.org/10.5772/intechopen.70890 97

nanoparticle array with the average diameter

(Route 3). Produced two-dimensional patterned VO<sup>2</sup>

For the first time, hexagonally patterned VO<sup>2</sup>

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 [59]. The fabrication process is more flexible via introducing of the plasma etching (PE) technology and controlling the precursor viscosity. They concluded

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.

approaches to synthesize and control the porosity, including the polymer assistant deposition (PAD) [53], freeze-drying preparation [57], colloidal lithography assembly [58] as well as the

films. The polymer used in the PAD process could be cetyltrimethyl ammonium bromide (CTAB) [62], cetyltrimethylammonium vanadate (CTAV) [63], 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**) [62]. It should be noted that the control of the polymer addition is critical to optimize the shape and size of

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**).


To begin with the PAD, it is a powerful technique to get the continuous nanoporous VO2

Freeze-drying is also an efficient way to prepare the nanoporous VO<sup>2</sup>

thin films were subsequently obtained (**Figure 3a**–**c**).

which could help to optimize the thermochromic properties of the thin films.

Colloidal lithography assembly is an alternative approach to get the nanoporous VO2

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

cipitator 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

films were attained though annealing in nitrogen gas [58]. The nanoporous structure could be further modulated by changing the layer number and/or the concentration of the precursor,

More systematically, colloidal lithography was explored to prepare the two-dimensional pat-

nanonet and nanodome arrays [59]. The fabrication process is more flexible via introducing of the plasma etching (PE) technology and controlling the precursor viscosity. They concluded

solution, then the infiltration with NH<sup>4</sup>

films with tunable periodicity and diverse nanostructures including nanoparticle,

In a typical process for fabricating nanoporous VO2

lithography assembly for nanoporous VO2

was firstly infiltrated by VOSO<sup>4</sup>

thin films, there are mainly four different

thin films with freeze-drying, the V<sup>2</sup>

thin films, the polystyrene (PS) MCC template

HCO3

thin

O5 -

thin

thin

solution as pre-

thin films. For a normal

**3. Porous control and synthetic methods**

In the nanoporous design for thermochromic VO2

dual-phase transformation [61].

96 Porosity - Process, Technologies and Applications

the nanopores.

H2 O2

porous VO2

terned VO2

**Figure 2.** Modification of vanadium precursor by the CTAB. (a) Initial step for adding the CTAB into the vanadium precursor. (b) and (c) Two forms of separation for the nuclear functionalized by the CTAB after strong stirring [62].

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 VO<sup>2</sup> arrays are highly uniform (**Figure 5**). For the first time, hexagonally patterned VO<sup>2</sup> 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 redshift 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 Δ*T*sol = 13.2% and *T*lum = 46%) achieved,

**Figure 3.** Field-emission scanning electron microscopy (FESEM) image for the freeze-dried nanoporous VO2 films with 7.5 mL of H2 O2 (a) and 17.5 mL of H2 O2 (b) in the precursor. (c) TEM image of (b) and the corresponding SAED (inset). (d) Schematic illustration of the freeze-drying process for the nanoporous design [57].

the 2D patterned VO<sup>2</sup>

[59].

**Figure 5.** FESEM images of periodic VO2

of two-dimensional SiO2


films have been demonstrated as an efficient smart thermal radia-

films. (a–c) Nanoparticle, (d–f) nanonet, and (g–i) tilted-views of nanodome

Controlled Porosity in Thermochromic Coatings http://dx.doi.org/10.5772/intechopen.70890 99

core-shell monolayer (**Figure 6a** and **b**) [60]. The structures

tion 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

arrays with periodicity of 160, 490 and 830 nm from left to right, respectively. The insert of (h) is high magnitude tiltedview image of 490 nm periodic nanodome on edge. Yellow hexagons in (a–c) are illustrations for hexagons patterning

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

**Figure 4.** Effect of synthesis conditions on the morphology evolution. Route 1: nanoparticle arrays are prepared via short PE duration and low viscosity precursor; Route 2: nanodome arrays are produced, using high viscosity precursor that can stick on the tops of PS spheres; Route 3, nanonets are fabricated by controlling the interval space between adjacent spheres via long PE duration [59].

**Figure 5.** FESEM images of periodic VO2 films. (a–c) Nanoparticle, (d–f) nanonet, and (g–i) tilted-views of nanodome arrays with periodicity of 160, 490 and 830 nm from left to right, respectively. The insert of (h) is high magnitude tiltedview image of 490 nm periodic nanodome on edge. Yellow hexagons in (a–c) are illustrations for hexagons patterning [59].

the 2D patterned VO<sup>2</sup> 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**) [60]. 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

**Figure 4.** Effect of synthesis conditions on the morphology evolution. Route 1: nanoparticle arrays are prepared via short PE duration and low viscosity precursor; Route 2: nanodome arrays are produced, using high viscosity precursor that can stick on the tops of PS spheres; Route 3, nanonets are fabricated by controlling the interval space between adjacent

**Figure 3.** Field-emission scanning electron microscopy (FESEM) image for the freeze-dried nanoporous VO2

(b) in the precursor. (c) TEM image of (b) and the corresponding SAED (inset).

films with

spheres via long PE duration [59].

7.5 mL of H2

O2

(a) and 17.5 mL of H2

98 Porosity - Process, Technologies and Applications

O2

(d) Schematic illustration of the freeze-drying process for the nanoporous design [57].

this ability, smart windows based on such structures display controllable appearances as well as good thermochromic performance, which is up to *T*lum = 49.6% and Δ*T*sol = 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 + H2 O + N2 H4 ) 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

**Figure 6.** (a) Illustration of how color-changed thermochromic smart window works. (b) Illustration of designed structures for simulation. (c) Calculated transmittance spectrum. The colorful background in (c) denotes the visible spectrum from 370 to 770 nm [60].

500°C in N2

became hollow VO(OH)2

rous thin films.

, the honeycomb-like nanoporous VO2

ratio control between the HCl and the N2

after SSTA process. SEM images of (g) captured hollow VO(OH)2

**Figure 7.** Formation of nanoporous VO2

high visible transmittance (~700 nm) above 90% as well as a decent solar modulating ability (∆*T*sol = ~5.5%). The critical factor for forming the initial hydrous spheres (colloids) is the

based precursor film was deposited at room condition (25 °C, 50% RH). (b) Precursor was spontaneously self-templated and assembled (SSTA) into hydrous sphere arrays after water evaporation in dry nitrogen (25 °C, ∼0 RH). (c) Hydrous spheres

being heated at a rate of 2 °C and maintained at 500 °C for 1 h. Microscopic photos of (e) the precursor film and (f) the film

H4 [61]. Apart from the above methods, the approaches including, but not limited to the chemical

etching [68] and reactive ion etching [69] could also be utilized to produce the VO2

structures were finally obtained with

Controlled Porosity in Thermochromic Coatings http://dx.doi.org/10.5772/intechopen.70890 101

thin films through dual phase transformation [61]. (a) Homogeneous, fully solution-

spheres and (h) final honeycomb structures.

spheres after instant heating to 300 °C and (d) finally collapsed to honeycomb structures after

nanopo-

this ability, smart windows based on such structures display controllable appearances as well as good thermochromic performance, which is up to *T*lum = 49.6% and Δ*T*sol = 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

The dual-phase transformation is a newly developed template-free method to prepare the

this method is based on the transformation between the colloids and ionic states stimulated

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

**Figure 6.** (a) Illustration of how color-changed thermochromic smart window works. (b) Illustration of designed structures for simulation. (c) Calculated transmittance spectrum. The colorful background in (c) denotes the visible

thin films with ultrahigh visible transmittance. As depicted in **Figure 7**,

O + N2

H4

) was spin coated onto

as a better way for the fabrication of such two-dimensional core-shell structures.

by the moisture. Firstly, the precursor (VOCl2 + HCl + H2

nanoporous VO2

100 Porosity - Process, Technologies and Applications

spectrum from 370 to 770 nm [60].

**Figure 7.** Formation of nanoporous VO2 thin films through dual phase transformation [61]. (a) Homogeneous, fully solutionbased precursor film was deposited at room condition (25 °C, 50% RH). (b) Precursor was spontaneously self-templated and assembled (SSTA) into hydrous sphere arrays after water evaporation in dry nitrogen (25 °C, ∼0 RH). (c) Hydrous spheres became hollow VO(OH)2 spheres after instant heating to 300 °C and (d) finally collapsed to honeycomb structures after being heated at a rate of 2 °C and maintained at 500 °C for 1 h. Microscopic photos of (e) the precursor film and (f) the film after SSTA process. SEM images of (g) captured hollow VO(OH)2 spheres and (h) final honeycomb structures.

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 (∆*T*sol = ~5.5%). The critical factor for forming the initial hydrous spheres (colloids) is the ratio control between the HCl and the N2 H4 [61].

Apart from the above methods, the approaches including, but not limited to the chemical etching [68] and reactive ion etching [69] could also be utilized to produce the VO2 nanoporous thin films.
