Vanadium pentoxide sol-gel prepared thin films were deposited on indium-tin-oxide (ITO) substrates by dip-coating at a subzero temperature (−10°C). The structure, morphology, and optical and electrochromic properties of dense and porous vanadium oxide films coated at low temperature were determined and compared to those of the corresponding films deposited under room-temperature conditions. The results indicated that, in the films coated at −10°C, a residual compressive stress exists that would originate from the formation of microvoids during the deposition. These microvoids are preserved during the heat treatment of the films. The microvoid morphology would favor the formation of nanostructures that would be responsible for the improved electrochromic properties of the subzero dip-coated films. Low-temperature coated films, heated at 450°C for several hours, undergo the transformation from a layered to a highly uniform nanorod structure that would be an important feature for different applications.
Due to its fundamental and technological interest, vanadium pentoxide has been an important topic of research for many years. Because of the high charge intercalation capacity of vanadium pentoxide films, they are successfully used in energy storage/conversion devices, as cathodes for secondary lithium ion batteries and electrochromic devices. Other interesting applications encompass optical filters, reflectance mirrors, and surfaces with tunable emittance for temperature control of space vehicles [
Vanadium oxide thin films, as many other transition metal oxides, have been prepared using physical techniques such as sputtering [
The thermal evaporation and sputtering methods were among the first methods that were used for the fabrication and characterization of vanadium pentoxide films and they continue to be used today for specific applications. However, after the development of sol-gel route, many of the studies in the field of electrochromism (EC) were based on sol-gel methods because of the simplicity and the milder conditions. Sol-gel synthesis has proven to be one of the most convenient methods to prepare electrochromic nanostructured vanadium pentoxide films. In the sol-gel technique, the deposition sol is prepared by the hydrolysis and condensation of a vanadium oxide precursor solution. The method is relatively simple and the concentration of reactants can be controlled accurately. There is today a plethora of precursor molecules that are both organic and inorganic, and, generally, the chemistry of the reactions is well known. The solutions can be easily coated on indium-tin-oxide (ITO) substrates by dip- and spin-coating, by using relatively simple equipment [
Kim et al. [
In our previous work, porous vanadium pentoxide films were prepared by using template methods. It has been reported that template sol-gel prepared vanadium oxide films yield, after removing the template, meso- (pores in the size range of 2 to 20 nm) or macroporous films with significantly enhanced electrochromic properties [
The preparation and deposition of V2O5 thin films are described in detail in our previous paper [
Conditions used for the fabrication of V2O5 dense and porous films.
Sample | Annealing/drying temperature (°C) | Type of the template |
---|---|---|
A | 27 | No |
B | 300 | No |
C | 450 | No |
D | 500 | No |
E | 450 | Triblock copolymer, 20% |
F | 450 | PS 600 nm |
The nonionic surfactant, EO20/PO70/EO20 (Pluronic P123), was dissolved in the coating solution (concentration of 20%). To remove the triblock copolymer from the film, the samples were soaked in a mixture of water-ethanol (1 : 1) for 1 h, or they were heat-treated at high temperatures. Table
Vanadium oxide films were characterized morphologically by using Scanning Electron Microscopy (SEM). Atomic Force Microscopy (AFM) was used to measure the thickness of the film and the crystalline structure was determined by using X-ray diffraction (XRD). XRD measurements were carried out in symmetric reflection mode with a custom-built
Vanadium pentoxide has an orthorhombic structure and crystallizes in the Pmmn space group, with the unit cell parameters:
The layered structure of orthorhombic V2O5 is built up of edge-sharing VO5 square pyramids. As shown in Figure
Coordination of vanadium with oxygen in vanadium pentoxide.
Figure
Crystallographic data corresponding to subzero dip-coated vanadium pentoxide films.
Sample | Deposition temperature |
|
|
|
Unit cell volume, nm3 | Size, nm |
---|---|---|---|---|---|---|
300°C | −10°C | 1.1477 | 0.35745 | 0.43858 | 0.1799 | 14.54 |
RT | 1.14400 | 0.35925 | 0.4375 | 0.1798 | 12.27 | |
|
||||||
500°C | −10°C | 1.1478 | 0.3521 | 0.4380 | 0.1770 | 23.14 |
RT | 1.14540 | 0.36087 | 0.4387 | 0.1813 | 23.07 | |
|
||||||
Literature data | 1.1516 | 0.35656 | 0.43727 | 0.17955 |
XRD pattern of the vanadium oxide films, dip-coated at −10°C and annealed at 300°C (a) and 500°C (b). X-ray diffraction signal of an orthorhombic V2O5 film (ICDD 41-1426) on an ITO covered glass substrate, annealed at 500°C for one hour. The signal from the glass substrate has been subtracted. The fit (red line) includes the contributions from V2O5 and ITO to the signal.
In general, we find that the lattice parameters of the vanadium oxide crystallites are close to the literature values. However, the parameter
In order to account for the changes observed in the crystallographic data, the Raman spectra of the film dip-coated at −10°C have been investigated. Figure
Position of the Raman bands in the spectra corresponding to the subzero and room-temperature dip-coating.
Position of Raman bands (cm−1) | Assignment | ||
---|---|---|---|
RT |
LT |
| |
997.49 | 997.96 | +0.47 |
|
708.44 | 711.26 | +2.82 |
|
528.15 | 532.16 | +4.01 |
|
484.75 | 486.71 | +1.96 |
|
406.06 | 408.03 | +1.97 |
|
303.32 | 303.66 | +0.34 | — |
283.97 | 286.95 | +2.98 |
|
Raman spectrum of the V2O5 film dip-coated at −10°C and annealed at 500°C. The Raman bands were assigned according to [
Table
SEM image of the V2O5 film dip-coated at −10°C and annealed at 300°C (a) and the film dip-coated at room temperature and annealed at 300°C (b).
However, after heating the subzero coated film to 450°C for 3 hours, the formation of nanorods can be observed as it has been noted in the case of the room-temperature coated films.
The SEM image in Figure
SEM image of the V2O5 nanorod film, dip-coated at room temperature (a) and −10°C, (b) both annealed at 450°C (sample template with the triblock copolymer).
The results show that the subzero temperature deposition of the V2O5 films significantly varied its photoluminescence properties, as seen in Figure
Photoluminescence spectra of the subzero temperature dip-coated film (a) and of the RT deposited film (b), both annealed at 27°C (green), 300°C (red), and 500°C (black), respectively.
Figure
The PL spectrum corresponding to the room-temperature coated film (Figure
In order to investigate the mechanism of transformation of the layered structure in nanorods, in the case of the subzero temperature dip-coated films, the time of annealing was varied between 1 and 3 hours. The SEM images of the resultant films are shown in Figure
SEM image of the film template with PS microspheres (600 nm), dip-coated at subzero temperature, and annealed at 450°C for 1 h (a), 2 hours (b), and 3 hours (c).
In this section, we compare the electrochromic (EC) properties, namely, the coloration efficiency and diffusion coefficients of the subzero coated films, with those of the films deposited under the same conditions at room temperature, thoroughly investigated in our previous work [
Coloration efficiency (CE) of the subzero coated dense films, subsequently annealed at different temperatures.
Sample | Temperature of drying/annealing (°C) |
|
CE (cm2/C) (600 nm) | CE (cm2/C) (843 nm) | |||
---|---|---|---|---|---|---|---|
RT |
SZT |
RT |
SZT |
RT |
SZT | ||
A | RT | 9 | 14 | 19 | 32 | 3 | 13 |
B | 300°C | 18 | 16 | 28 | 4 | 28 | 44 |
C | 450°C | 23 | 19 | 22 | 23 | 51.9 | 73 |
D | 500°C | 13 | 19 | 24 | 53 | 68 | 38 |
Coloration efficiency (CE) and diffusion coefficient of Li+ in porous films prepared with structure directing agents.
Sample | Template film |
|
CE (cm2/C) (600 nm) | CE (cm2/C) (843 nm) |
|
||||
---|---|---|---|---|---|---|---|---|---|
RT | SZT | RT | SZT | RT | SZT | RT | SZT | ||
E | Copolymer (20%) extracted with a mixture of water and ethanol | 18 | 13 | 34 | 15 | 28 | 35 | 2.8 × 10−10 | 2.0 × 10−10 |
|
|||||||||
F | PS (600 nm) annealing at 450°C | 9 | 15 | 10 | 15 | 33 | 57 | 8.3 × 10−11 | 1.5 × 10−10 |
The thickness and the roughness of both films were measured by scratched films, by using AFM measurements. The results indicated that, indeed, the thickness of films dip-coated at −10°C, under precisely the same conditions, is almost twice, compared with the RT-deposited films (around 130 versus 66 nm, resp.). This result is in agreement with the data found for the subzero coated ZnO films and accounted for by a higher deposition rate because of the increased viscosity of the coating solution at low temperatures [
Figure
Cyclic voltammogram (a) and optical modulation (b) annealed at 500°C. The modulation is shown at 600 nm (red curve) and at 843 nm (green curve).
The coloration efficiency of the subzero (SZT) coated films are compared with those of the RT-deposited films. The CE values shown in Table
In the case of porous films (Table
The equivalent circuit used to obtain the fitting is shown in Figure
Equivalent circuits used to obtain the fitting parameters [
In Figure
The electrochemical impedance spectra (Nyquist plots) corresponding to the copolymer template samples (see Table
Fitting parameters corresponding to the subzero temperature dip-coated films.
Sample |
dc voltage |
|
|
|
|
|
|
|
| |
---|---|---|---|---|---|---|---|---|---|---|
SZT | RT | |||||||||
D |
|
378 | 91 | 3.6 × 10−3 | 246 | 225 | 82 × 10−6 | 700 × 10−3 | 69 × 10−9 | 809 × 10−3 |
|
343 | 123 | 482 × 10−6 | 1158 | — | 135 × 10−6 | 650 × 10−3 | 61 × 10−9 | 817 × 10−3 | |
|
43 × 10−9 | 502 | 42 × 10−6 | 15004 | 206 | 49 × 10−6 | 804 × 10−3 | 7.1 × 10−6 | 303 × 10−3 | |
|
5 × 10−6 | 493 | 28 × 10−6 | 307 | — | 55 × 10−6 | 820 × 10−3 | 1.7 × 10−6 | 514 × 10−3 | |
E |
|
289 | 97 | 12 × 10−3 | 40 | 173 | 410 × 10−6 | 594 × 10−3 | 743 × 10−9 | 720 × 10−3 |
|
36 | 357 | 1.1 × 10−3 | 101 | 351 | 768 × 10−6 | 678 × 10−3 | 1.14 × 10−6 | 475 × 10−3 | |
|
257 | 155 | 764 × 10−9 | 110 | — | 469 × 10−6 | 624 × 10−3 | 645 × 10−9 | 697 × 10−3 |
Nyquist plots corresponding to the triblock copolymer template sample subjected to potentials of −0.50 V (a) and −1.0 V (b), respectively.
When compared to the spectrum measured at −0.5 V, the curve measured at −1.0 V is displaced towards lower resistances—associated with faster kinetics.
The measured electrochemical impedance spectra at different electrode potentials were analyzed. The results show that the Nyquist plot impedance spectra consist of two medium frequency depressed arcs and a low frequency straight line. The low frequency line with a phase angle of 45° corresponds to the diffusion of lithium ion through the vanadium oxide phase.
In Table
In Table
Vanadium oxide thin films have been prepared by a subzero temperature dip-coating method. Structural properties as well as Raman spectroscopy and the morphology characteristics have indicated the presence of a residual compressive stress in the films dip-coated at −10°C. The stress is believed to exist due to the formation of microvoids during the deposition process, microvoids that are preserved during the heat treatment of the films. The microvoids are pores that provide additional sites for the intercalation of lithium ions in both dense and porous films, and their presence may also favor the formation of nanostructures that would be responsible for the enhanced electrochromic properties of the vanadium oxide films.
Highly uniform nanorods, interesting for potential applications such as field emission displays and interconnections, have been obtained by annealing at 450°C of the subzero coated films. The transformation of the layered film into nanorods is thought to happen in a slower fashion and through a mechanism that seems to be different from the transformation of room-temperature deposited films.
The fabrication method proposed is simple and may be used for large surface deposition.
The authors declare that there is no conflict of interests regarding the publication of this paper.