Preparation and electrochromic performance of Nb-Doped Tungsten oxide nanowires

Different niobium (Nb)-doped tungsten oxide (WO3) nanowire films were prepared by magnetron sputtering and hydrothermal methods, and their performances were analyzed. The experimental results revealed that Nb doping caused the tungsten oxide nanowire structure to become loose, increased the interplanar spacing, and raised the oxygen vacancy concentration, significantly improving the electrochromic properties. However, with increasing Nb doping levels, the cyclic stability of the samples decreased, possibly due to lattice distortion induced by Nb. The analysis demonstrated that the sample doped with 0.5% Nb exhibited optimal electrochromic performance and displayed a 5.68% coloring transmittance and an 83.3% optical modulation range at a wavelength of 633 nm. This study elucidated the influence of Nb doping on the electrochromic properties of WO3 nanowire films.


Introduction
Electrochromic materials possess the ability to adjust optical properties such as color, transmittance, and reflectance based on an applied electric field.Tungsten oxide (WO3), as a representative material in this category, demonstrates excellent stability and adjustability in optics [1][2] .However, it suffers from drawbacks such as poor color selectivity, slow response speed, and low efficiency.To address these limitations, metal doping has been considered an effective strategy.Introducing metal ions such as Mo [3- 4] , Au [5] , Fe [6] , Ti [7] , Cu [8] , Sb [9] , and Li [10] into electrochromic films can modulate the physicochemical state of WO3, thereby improving its optical performance.
Among metallic elements, the minimal difference in ionic radii between niobium (Nb) and tungsten (W) enables Nb to easily substitute for W positions in the WO3 crystal, leading to lattice expansion and affecting the electrochromic characteristics of WO3.In this study, a combination of magnetron sputtering and hydrothermal methods was employed to prepare Nb-doped WO3 nanowire films, investigating the influence of different Nb doping levels on the microstructure and electrochromic performance of WO3.

Preparation of sample films
Under vacuum conditions, FTO glass, cleaned and dried, was fixed on the sample holder, and a tungsten trioxide target was installed.With a pressure of 2×10 -4 Pa, an Ar:O2 flow rate of 50 sccm:50 sccm was used for sputtering for approximately 10 minutes (pressure of 0.8 Pa, power of 30 W), followed by annealing at 400℃ for 2 hours, resulting in a crystalline WO3 thin layer.
Different proportions of tungstic acid and niobium oxide precursor solutions (both at a concentration of 0.05 M), mixed with HCl, deionized water, and acetonitrile, were treated at 180℃ in a high-pressure vessel for 12 hours.After cooling, rinsing, and annealing at 200℃ for 1 hour, WO3 nanowire films doped with different Nb proportions (Nb-to-W atomic ratios of 0%, 0.5%, and 1.5%) were prepared.The preparation process is shown in Figure 1.

Morphology and crystal structure of samples
Figure 2 presents SEM images of tungsten oxide surfaces doped with different Nb levels, showing thin film composed of WO3 nanowire arrays with diameters of approximately 15-40 nanometers, relatively evenly distributed.In Figure 2 (a), nanowires connect to form a blocky structure, whereas in Figure 2 (b), 0.5% Nb doping displays gradually dispersed nanowires.This dispersion increases the surface area of nanowires in contact with the electrolyte, potentially enhancing electrochromic performance.Figure 2 (c) shows that nanowires reconnect after 1.5% Nb doping, potentially negatively impacting electrochromic performance.

Transmission electron microscopy analysis
To further understand the influence of Nb at different doping concentrations on the crystal structure and micro-morphology of WO3, the TEM analysis was performed.Figure 3 displays the TEM images of 0.5% Nb-doped sample nanowires.sample.Figure 3 (c) reveals an interplanar spacing of 0.315 nanometers for Nb-doped nanowires, slightly larger than the 0.301 nanometers of WO3 nanowires, suggesting a minor lattice expansion.Energy dispersive spectroscopy analysis in Figures 3 (d-g) confirms the presence of Nb, indicating Nb is likely a substitute for some tungsten positions in the crystal lattice.

X-ray photoelectron spectroscopy analysis
In analyzing the W and O element XPS fitted graphs before and after Nb doping of WO3, no significant changes were observed in the W and O peak graphs.For a deeper understanding of the W element state, peak fitting for W was conducted.Figure 4 (a) displays two higher peaks at 35.9 and 38.0 eV for WO3 samples, representing W4f7/2 and W4f5/2, indicating a W oxidation state of 6+.Additionally, there are two lower peaks at 34.3 and 36.4 eV, corresponding to a W oxidation state of 5+, signifying a mixed state of W6+ and W5+ in WO3.
Comparatively, Figure 4 (b) for 0.5% Nb-doped samples indicates that while the positions of W6+ and W5+ peaks did not shift, the W5+/(W6++W5+) area ratio increased from 25.9% to 32.5%.This suggests a change in the W oxidation state due to Nb doping, with some W6+ transforming into W5+, possibly accompanied by the release of oxygen ions from the lattice, forming oxygen vacancies.4 (d) indicates a 70.83% increase in oxygen vacancy/oxygen total area ratio (from 12.0% to 20.5%) for 0.5% Nb-doped samples.Thus, Nb doping assists in promoting the formation of oxygen vacancies, potentially enhancing material conductivity and improving electrochromic performance.

Electrochemical and electrochromic performance testing and analysis
We conducted cyclic voltammetry (CV) testing on WO3 with Nb doping using a three-electrode system.Figure 5 (a) illustrates that the CV curve area initially increases and then decreases with increasing Nb doping.The oxidation peak potential presents a trend of decreasing to a minimum of -0.75 V before rising to -0.5 V, potentially affecting the diffusion of lithium ions.
Visible and near-infrared spectroscopy testing Figure 5 (b-d) revealed that at 633 nm, the sample's fading transmittance decreased from 94.7% to 70.5% with increasing Nb doping, exhibiting a transition from transparent to white.Under negative voltage, the coloring transmittance decreased from 28.7% to 3.0%, displaying a deeper blue.Nb doping increased the nanowire's interplanar spacing and oxygen vacancy concentration, enhanced electrochromic performance, and enabled better accommodation of lithium ions.Further investigation using a three-electrode system explored the influence of Nb doping on the speed and stability of WO3 electrochromic performance, with experimental results recorded in Figure 6 and Table 1.As Nb doping increased, the sample's response speed initially increased and then decreased, correlating with changes in the oxidation peak potential during electrochromism.
Subsequent research revealed that the 0.5% Nb-doped sample retained only 55.4% of the optical modulation range after 2000 cycles, while pure WO3 samples maintained 98.7% (Table 1).A comparison in Figure 7 (a) and (b) demonstrated a far greater decrease in charge quantity for Nb-doped samples compared to pure WO3, suggesting that Nb doping reduced the cyclic stability of the film.This might be due to Nb's larger ionic radius than W's, leading to lattice distortion and decreasing the sample's crystal structure stability.

Conclusion
(1) The addition of a small amount of Nb significantly improved the coloring effect of tungsten oxide and broadened the optical modulation range.This effect is closely related to Nb doping, causing the loosening of the nanowire structure, increasing interplanar spacing, and enhancing oxygen vacancy concentration.
(2) Nb doping reduced the cyclic stability of the samples, potentially linked to lattice distortion caused by Nb, leading to decreased crystal structure stability.
(3) The optimal electrochromic performance was observed at an Nb doping level of 0.5%.At a wavelength of 633 nm, it exhibited a 5.68% coloring transmittance and an 83.3% optical modulation range (a 26% increase).

Figure 1
Figure 1 Preparation process of niobium-doped tungsten oxide thin film.

Figure 3 (
a) illustrates the diameters of Nb-doped nanowires ranging from 15 to 40 nanometers, similar to previous WO3 nanowires.In the diffraction pattern in Figure 3 (b), diffraction points of crystal planes (200) and (002) for Nb-doped nanowires are confirmed.

Figure 3
Figure 3 (a) Low-power TEM of WO3-Nb0.5 sample, (b) Diffraction pattern of WO3-Nb0.5 sample, (c) High-resolution TEM of WO3-Nb0.5 sample, (d-g)EDS diagram of W, O and B of WO3-Nb0.5sample.Figure3(c) reveals an interplanar spacing of 0.315 nanometers for Nb-doped nanowires, slightly larger than the 0.301 nanometers of WO3 nanowires, suggesting a minor lattice expansion.Energy dispersive spectroscopy analysis in Figures3 (d-g) confirms the presence of Nb, indicating Nb is likely a substitute for some tungsten positions in the crystal lattice.