Commitment Between Roughness and Crystallite Size in the Vanadium Oxide Thin Film opto-electrochemical Properties

Abstract

The V₂O₅ thin films has been widely studied because it has application as ionic host in electrochromic and lithium-ion batteries, two technologies that have an intimate connection with sustainability as substitutes for fossil energies and as agents for improving energy efficiency. In electrochromic technology, V₂O₅ is applied as a passive electrode due to its high transmittance and small contrast, and its reversibility on electrochemical reactions. To contribute to increase the optical and charge efficiency of V₂O₅ thin film passive electrodes, were investigated in this work the influence of the morphological properties, crystallite size and roughness, on the reversible specific charge capacity and the respective optical responses. The films morphological properties were modified by varying their thickness to the nanoscale. The films were deposited by thermal evaporation from powdered V₂O₅. The crystallite size and surface roughness were measured respectively by XRD and AFM. The results showed that the charge capacity is directly proportional to the surface roughness and inversely proportional to the crystallite size. The film optical contrast and the nominal transmittance shows to be improved according to their morphological properties. In conclusion, the V₂O₅ opto-electrochemical properties can be improved, increasing the efficiency on the light control processes.

Keywords
Thermal evaporation; vanadium oxide; electrochromism; lithium intercalation

1. Introduction

Electrochromic materials have been widely studied, since their inception ¹, considering the characteristics of this material, which might be easily integrated into many scientific and technological applications ^2-4. Nowadays its commercial application focuses on smart windows, which replace curtains in the control of light and heat entering a room that has an intimate connection with sustainability as agents for improving energy efficiency ⁵.

Electrochromic devices are similar to a battery; they are composed of two electrodes separated by an ionic electrolyte. They present a difference of electric potential and charge and discharge electronic capacity. The electric current in an electrochromic device is a result of the ionic intercalation of the electrolyte species in the host oxide structure, altering its optical properties. The optical changes arise from the increased ratio between reduced and oxidized species within the electrode and vary according to the amount of intercalated charge, which promotes, for most cathode materials ^6-10, the appearance of states of energy in the form of small polarons.

Electrochromic materials can be classified into three types: 1) active cathodic, 2) active anodic, and 3) the passive (although it does not vary their optical absorption with the ionic intercalation). The fundamental characteristic of an active-type electrochromic material is its ability to modulate the absorption of radiation due to the co-intercalation of electron and ionic species into its electronic and crystallographic structures respectively. While the cathodic active electrode (WO₃, TiO₂, MoO₃, V₂O₅ etc.) darkens when it receives ions and electrons in its structure, the anodic electrode (NiO, MnO₂, Co₃O₄, V₂O₅ etc.) becomes transparent and vice versa as the ions and electrons leave their structures.

An electrochromic device can be constructed in two ways, Figure 1. First, with opposite optical modulation electrodes, one of the anodic type and the other of the cathodic type, which means that while one electrode is clear the other is also and the same two dims together. Second, by changing the anodic electrode to one of the passive type, whose main characteristics are the high transparency and the small optical contrast. All electrochromic component has to be transparent, the electrolyte and the electronic conductor (ITO).

Figure 1
Basic design of an electrochromic device ¹¹

V₂O₅ films can be prepared by physical and chemical routes ^12-15. Although physical deposition, such as resistive thermal evaporation, has a high cost when compared to chemical deposition, it offers a better control to the thin film composition, and structural and morphological aspects ^16,17. Thus, the deposition by thermal evaporation is a suitable technique to investigate the influence of the microstructural V₂O₅ thin films properties on their electrochemical charge capacity performance and optical behavior ¹⁸.

Several works showed that V₂O₅ thin films deposited by physical process with controlled thickness, varying from few hundreds to thousands of nanometers, presented maximum transmittance around 90% and optical contrast of about 20%. For a passive film integrating a electrochromic device, these values has to be improved ^19,20. With regard to the reversible ion storage capacity, for applications as a rechargeable battery electrode, V₂O₅ presents 294 mAhg^-1 theoretical charge capacity, which value is higher than the electrodes usually used in batteries such as LiCoO₂ (274 mAhg^-1), and its electrochemical performance can increases when it is made of nanoparticles ²¹.

Film thickness has a great impact on your own stoichiometry, shape, crystallites sizes and orientation and optical properties ²²,²³. The impact of the thickness on the stoichiometry of the film is due to the variability of oxides that may be formed, which also explains why the optical variations may not follow the Beer Lambert Law ^24-26.The influence of thickness on crystallite size has been related due to shortening of the unit cell parameters ²⁷, and due to the V₂O₅ film doping ²⁸.

To contribute to increase the efficiency of V₂O₅ thin film passive electrodes, that is, a film that exhibit high transmittance with small contrast and high charge capacity, were investigated in this work the influence of the morphological properties, crystallite size and roughness, on its opto-electrochemical performance. The films morphological properties were modified by varying the film thickness to the nanoscale.

2. Methodology

Vanadium oxide thin films were deposited by resistive thermal evaporation under high vacuum in an HHV (AUTO 306) system. Glass plates covered with ITO were used as substrates. The pellet V₂O₅ powder evaporation source was obtained from SIGMA-ALDRICH, 99.9%. The glow-discharge was performed to ensure substrate surface cleaning before the depositions. The base pressure was 7.89x10^-6 mbar and 1.15x10^-5 mbar during deposition. The samples (Figure 2) were thermally treated in an oxidizing atmosphere (O₂) at 400 ºC by 2 hours.

Figure 2
Samples images of the films studied with different thicknesses after heat treatment.

The films thicknesses were measured indirectly by the calcium Kα intensity attenuation using a portable X-ray fluorescence system (PXRF), whose diagram is shown in Figure 3. ²⁹ Table 1 shows the thickness and surface density against the mass of the evaporating V₂O₅ powder.

Figure 3
Schematic of the CaKα attenuation for the film thickness measurement.

The CaKα comes from borosilicate substrate, and the values were shown in Table 1.

Table 1
Thickness of V₂O₅ samples calculated by attenuation of the calcium Kα line.

Sample	Deposited mass (mg)	Ca-Kα net area (arb. unit)	Thickness (nm)	Surface density (μg/cm2)
A	26 ± 3	10008	50 ± 7	16.1 ± 3.3
B	56 ± 3	9081	110 ± 3	36.9 ± 3.0
C	82 ± 3	8501	160 ± 5	53.7 ± 3.8

The samples morphology were analyzed by atomic force microscopy (AFM), (Nanosurf), in the intermittent contact mode. Images were acquired covering 30 x 30 µm² with 512 points per line. The RMS (root mean square) was obtained by Gwyddion software. The X-ray diffraction (XRD) measurements were performed on a Bruker D8 diffractometer with CuKα radiation, θ-2θ geometry, angle range from 18 to 25º (2θ), the step size of 0.05 º and 5 s for counting time per point. The X-ray tube voltage and current were 40 kV and 40 mA, respectively. The crystallite size (S) was calculated from the (001) peak of the samples, using Scherrer equation:

(1) $\mathit{S}=\frac{\mathit{K}\mathit{\lambda }}{{\mathit{\beta }}_{\left(2\mathit{\theta }\right)}\cos \mathit{\theta }}$

where K is the form factor choose as 0.94 ³⁰, λ is the CuKα wavelength, β_(2θ) is the peak FWHM, and θ is the diffraction angle, both in radians for the same (hkl) reflection plane. β_(2θ) was estimated using a gaussian function and corrected by the equation 2, where β_instrumental was obtained from silicon polycrystalline standart at the same instrumental configuration.

(2) ${\mathit{\beta }}_{\left(2\mathit{\theta }\right)}=\sqrt{{\mathit{\beta }}_{\mathit{measured}}^2-{\mathit{\beta }}_{\mathit{instrumental}}^2}$

Opto-electrochemistry measurements were performed in situ in a three-electrode cell using a 632.8 nm monochromatic laser. The working electrode (WT) was the ITO/V₂O₅, and both the counter electrode (CE) and the reference electrode (RE) are metallic lithium wires. The electrolyte was a solution of lithium perchlorate (LiClO₄) dissolved in propylene carbonate (PC) at 1.0 mol/L. Cyclic voltammetry and chronopotentiometry were performed in a potentiostat/galvanostat (VoltaLab10, Radiometer Analytical). Voltammetry was done within a 2.0 to 3.8 V potential range and 0.1 mV/s for the scanning speed. Chronopotentiometry was applied to the cell at ± 1µA from 2.0 to 3.8 V.

3. Results and Discussions

3.1 Atomic force microscopy and X-ray diffraction

Figure 4 shows the AFM images of the V₂O₅ films of samples A, B and C. The AFM images show, in the used scale, that the roughness increases as the thickness decreases. The RMS decreased from 13 to 11.2 and to 6.7 nm for the A, B and C samples respectively (50, 110 and 160 nm). It is in agreement with the increase on surface density showed in table 1. It is evident that with the increase of the film thickness, the morphology of the film became more uniform, reducing its surface roughness and increasing its density.

Figure 4
Morphological images of the surface of the films studied with different thicknesses.

The X-ray diffractograms of the heat treated V₂O₅ thin films deposited on the ITO-covered glass substrate are shown in Figure 5. It can be observed, for all samples, a Gaussian like peak centered at 2θ = 20.17º, and a peak in 2θ = 21.50º only for sample C (high thickness), which are attributed to the orthorhombic phase of V₂O₅ , with orthorhombic crystal system (Pmmn 59) ^31,32. The decrease in peak intensity is proportional to the decrease in its thickness and the peaks that appear in 2θ = 24.25º and 2θ = 26.23º refers to the ITO film. From Scherrer the crystallite size values were 12.2 nm, 15.8 nm and 18.2 nm, from the lowest to the highest thickness, respectively. Nanocrystalline and continuous films can be understood as consisting of the crystallite volume, and its boundary volume ³³. The excess free volume associated with the crystallite boundaries were estimated using the equation (3) and the values are, 8%, 6% and 5%, respectively to A, B and C samples. It is another indicator that the amorphous phase is higher in A sample (smallest thickness).

(3) $\Delta {\mathit{V}}_{\mathit{F}}=\frac{{\left(\mathit{L}+\raisebox{1ex}{$\mathit{d}$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\right)}^2-{\mathit{L}}^2}{{\mathit{L}}^2}$

Figure 5
Diffraction of the thermally treated V₂O₅ films.

3.2 Electrochemical properties

The films voltammograms present three pairs of current peaks, corresponding to the reduction (intercalation), at potentials of 2.33, 3.20 and 3.41 V, and oxidation (deintercalation) at 2.64, 3.23 and 3.44 V (vs. Li|Li⁺). The peaks can be attributed to different phase transitions of V₂O₅ , due to the weak van der Waals forces between the adjacent layers, that are showed in the Figure 6 ^13,34,35.

Figure 6
Voltammograms for V₂O₅ thin films with different thicknesses, showed the phases transitions.

Chronopotentiometry shows that the normalized amount of intercalated charge, and reversibly deintercalated, is higher for the thinner film (Figure 7).

Figure 7
Potential of the cell (black line) and optical transmittance (red line) versus the stoichiometry of Li⁺ intercalated in the films of vanadium oxide.

The transmittance monitoring with 632.80 nm light showed the effect of double monochromatic coloration, whose maximum optical absorption occurs as the percentage of V⁺⁴ and V⁺⁵ is of 50%, since the small polaron center needs the pair to exist. The double spectral coloration also occurs in the film and will be shown below. With the intercalation of lithium ions in the film, the vanadium valence state changes from +5 to +4 reversely according the equation 4.

(4) ${\mathit{V}}_2^{+5}{\mathit{O}}_5^{-2}+{\mathit{xe}}^{-}+{\mathit{xLi}}^{+}\underset{\mathit{oxidation}}{\overset{\mathit{reduction}}{\rightleftarrows }}{\mathit{Li}}_{\mathit{x}}^{+}\left[{\mathit{V}}_{2-\mathit{x}}^{+5}{\mathit{V}}_{\mathit{x}}^{+4}\right]{\mathit{O}}_5^{-2\left|\right|}$

In the intercalation process, while lithium ions occupy crystallographic sites, or structural defects, the electrons populate the V3d band causing increase on gap energy and the centers of color absorption (Figure 8) ^11,36.

Figure 8
Banding structure scheme for non-intercalated V2O5 (a) and intercalated (b) [Adapted from Wu et al. ¹⁶].

For high energy photons, E > 2.0 eV, the optical absorption occurs due to the interband electronic transition, O2p to V3d (split). At low energies, E < 2.0 eV, absorption occurs due to small polarons absorption that is the electron hopping between two different vanadium oxidation state ions (electrons going from V⁺⁴ to V⁺⁵). The spectrophotometric curves showed in Figure 9 clearly shows these two phenomena separated by an isosbestic point ³⁷. While in high energies the absorption increases, in low it decreases and vice versa, as electrode is intercalated or deintercalated. On the inset is showed the inverse behavior on transmittance between high energy and low energy photons. In another paper, a more detailed investigation of double coloration electrochromism will be shown.

Figure 9
Transmittance as a function of wavelength during the ionic intercalation process (Inset: Transmittance as a function of potential showing regions of optical absorption for blue light (400 nm) and red light (632 nm)).

In Figure 10 (a) and (b) are shown the correlation between crystallite size, roughness, and the specific charge capacity, versus the films thicknesses. While the specific charge capacity is linearly dependent on the roughness, it is inversely proportional to the crystallite size. As the thickness decreases the transmittance naturally increases, but the contrast percentage has also been shown to be dependent mainly on the crystallite size.

Figure 10
(a) The correlation between the crystallite size, the roughness, and the charge capacity, versus the thickness of the films. (b) The transmittance, in bleached and dark states versus the film thickness.

4. Conclusion

In this work the influence of the morphological properties, crystallite size and roughness, on the reversible charge capacity and the respective optical responses were investigated. The films morphological properties were modified by varying their thickness to the nanoscale. The electrochemical experiments showed that the specific charge capacity is directly proportional to the surface roughness and inversely proportional to the crystallite size. With the increase of the film thickness, the morphology of the film became more uniform, reducing its surface roughness and increasing its density. The film optical contrast and the nominal transmittance shows to be improved according to their morphological properties. There are arguments to conclude that the optimization of the opto-electrochemical properties of V₂O₅ thin films can be achieved by decreasing the size of its crystallites and increasing their roughness, optimizing and increasing the efficiency on the light control processes.

5. Acknowledgements

To Laboratory of Nanostructured Films and Spectroscopy at São Paulo State University (UNESP), campus in Presidente Prudente, for the measurements of atomic force microscopy. To the Multi-user Laboratory of the IFGW at UNICAMP, for the measurements of XRD and Caroline Amorin by the graphics editing. CAPES and CNPq for financial support.

Conflicts of Interest: The authors declare that they have no conflicts of interest.

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Publication Dates

• Publication in this collection
  01 Nov 2018
• Date of issue
  2019

History

• Received
  02 Apr 2018
• Reviewed
  27 July 2018
• Accepted
  04 Oct 2018