A. J. More1*
1 Department of Physics,
Vidya Vikas Mandal’s Art’s, Science and Commerce College Sakri, Tal. Sakri,
Dist. Dhule, MS, India
Abstract
Tungsten oxide (WO₃) thin films were
successfully deposited on indium tin oxide (ITO) coated glass substrates using
a pulse electrodeposition method. Peroxotungstic acid (PTA) precursor solution
was synthesized by the controlled reaction of tungsten metal powder with
hydrogen peroxide. The prepared films were characterized using X-ray
diffraction (XRD), UV–Visible spectroscopy, and scanning electron microscopy
(SEM) to investigate their structural, optical, and surface morphological
properties. XRD study shows that the as deposited WO3 samples are
amorphous. Confirmation of material is from the micro-Raman study. Optical
studies revealed high transparency in the visible region with a well-defined
absorbance edge. Scanning electron microscope (SEM) micrographs showed uniform
and compact surface morphology. These results demonstrate that the pulse
deposition technique is an effective and low-cost method for the fabrication of
high-quality WO₃
thin films suitable for optoelectronic and electrochromic applications.
Keywords: WO₃ thin films, Pulse
deposition, Peroxotungstic acid, Optical properties, SEM, XRD
*Corresponding
author: dranupmore@gmail.com
Introduction
Transition metal oxide (TMO) thin
films have attracted considerable attention due to their wide range of
applications in electrochromic devices, gas sensors, smart windows, photocatalysis,
and energy storage systems. Among them, tungsten oxide (WO₃) is a well-known
n-type semiconductor exhibiting excellent electrochromic behavior, high
chemical stability, and good optical transparency in the visible region [1].
Various techniques such as sol–gel,
sputtering, chemical vapor deposition, spray pyrolysis, and electrodeposition
have been employed for the preparation of WO₃ thin films.
However, many of these methods require high vacuum systems, elevated
temperatures, or complex instrumentation. Solution-based deposition techniques
offer advantages such as low cost, large-area coating capability, and ease of
composition control [2].
Pulse deposition is an emerging
technique that allows controlled growth of thin films by alternating deposition
and relaxation cycles. In this work, WO₃ thin films were
prepared using a pulse deposition method with peroxotungstic acid (PTA) as the
precursor. The structural, optical, and morphological properties of the pulse
electrodeposited films were systematically investigated.
Experimental
Details
PTA precursor solution was
synthesized by reacting tungsten metal powder with hydrogen peroxide, 9.192 g
of tungsten metal powder (99% purity, Sigma-Aldrich) was gradually added to 100
ml of 30% hydrogen peroxide (H₂O₂) at room
temperature. The reaction is highly exothermic; therefore, tungsten powder was
added slowly under continuous stirring, and the reaction vessel was maintained
in an ice-cold bath to control the temperature. The solution was stirred
continuously for 48 hours, during which complete dissolution of tungsten
occurred, forming a clear and colorless PTA solution. The resulting PTA can be
represented as WO₃·nH₂O₂·mH₂O, depending on
synthesis conditions [3]. To prepare the deposition solution, 50 ml of PTA was
mixed with 60% ethanol and 40% double-distilled water. The solution was
refluxed at 60 °C for 25 minutes to remove excess water and hydrogen peroxide.
After refluxing, a yellow-colored acid solution was obtained, indicating the
formation of a suitable precursor for WO₃ thin-film
deposition.
Deposition
of WO₃
Thin Films
WO₃ thin films were
deposited on ITO-coated glass substrates using the pulse deposition (50:50)
method. In this technique, equal on-time and off-time pulses were applied to
ensure uniform film growth and controlled nucleation. The substrates were
thoroughly cleaned prior to deposition to ensure good adhesion and film
uniformity.
Results
and Discussion
Figure
1 shows the XRD pattern of the WO₃ thin film
deposited using the pulse deposition (50:50) method. The XRD spectrum exhibit a
broad hump in the low 2θ region due to the glass and the major peaks are of ITO
(JCPDS card # 00-039-1058), all peaks have been assigned to the characteristics
of ITO, confirming that WO3 thin films are of amorphous nature.
Figure 1
XRD pattern of the WO₃
thin film
micro-Raman Spectroscopy
Figure 2 shows the micro-Raman
spectrum of the WO₃
thin film. The chemical structure and functional groups of galvanostatically
electrodeposited WO3 thin films were studied with the help of
micro-Raman spectroscopy. The peaks provide the information about a-WO3
thin films. Figure 2 shows the micro-Raman spectrum of the WO₃ thin film. An
intense peak centered at ~950 cm-1 due to ν(W=O) structure of WO3
[4-5]. For the electrodeposition, ITO
substrate is used, so the other peaks generated are due to ITO. The peaks at
302, 452, 560, 779 and 1092 cm-1 are attributed to ITO substrate.
The peak 302 cm-1 is attributed to In-O-In [6], 452 cm-1 is attributed to δ (Si–Ob–Si) [7], 560 cm-1 is attributed to δ
(Si–Ob–Si) [7], 779 cm-1
is attributed to Si–O stretching [8] and
1092 cm-1 is attributed to antisymmetric stretching of C-O [9]. Two additional peaks are seen at 240 cm-1
[10-11] and 376 cm-1 [12] assigned to WO3.H2O
ν(W–OH2) stretching mode which confirming WO3 with
hydrated content. The peak at 950 cm-1 which is found in WO3
samples, is of particular relevance and deserves special consideration. A
feature around 950 cm−1, due to W=O stretching vibration, is
associated to film nanostructuring [13]:
the involved bonds are mainly located at surface sites, thus a comparatively
high intensity. From micro-Raman analysis, it is confirmed that the deposited
thin film is of tungsten oxide with hydrated content is present.
Figure 2 micro-Raman
spectrum of the WO₃
thin film
Optical
Properties
Figure 3 presents the UV–Visible absorbance spectrum
of the WO₃
thin film with respect to the ITO substrate. The film shows strong absorbance
in the UV region and low absorbance in the visible region, indicating good
transparency.
Figure
3 UV–Visible absorbance spectrum of the WO₃ thin film
Figure
4 shows the transmittance spectrum, revealing high optical transmittance in the
visible range, which is a desirable property for smart window and display
applications. The sharp absorption edge suggests good optical quality and
uniform film thickness.
Figure 4 Transmittance spectrum of the WO₃ thin film
The optical behavior of the films can
be attributed to the electronic transitions between the valence band and
conduction band of WO₃,
as well as the nanocrystalline structure obtained through pulse deposition.
Surface
Morphology (SEM)
SEM
micrographs of the WO₃
thin film are shown in Figure 5. The images reveal a uniform, dense, and
crack-free surface morphology. The grains are well distributed over the
substrate, indicating homogeneous nucleation and growth during pulse
deposition. Such compact morphology enhances the optical transparency and
improves the electrochemical stability of the films, making them suitable for
device applications.
Figure 5 SEM micrographs of the WO₃ thin film
Conclusion
WO₃ thin films were
successfully fabricated on ITO substrates using a pulse electrodeposition
(50:50) method with peroxotungstic acid precursor. XRD study shows that the as
deposited WO3 sample is amorphous. Formation of WO3 with
presence of hydrated content is evident from the micro-Raman study. Optical
studies showed high transparency in the visible region, while SEM analysis
revealed uniform and compact surface morphology. The results demonstrate that
pulse deposition is a simple, cost-effective, and efficient technique for
preparing high-quality WO₃
thin films for optoelectronic, electrochromic, and sensing applications.
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