05-08-2014, 11:00 AM
Characterization of Electrochromic Vanadium
Pentoxide Thin Films Prepared By Spray Pyrolysis
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. Introduction:
Due to the diverse structural, magnetic, electrical and optical
properties of transition metal oxides, they found widespread applications in
different fields like, optoelectronics, microelectronics, solid state ionics [1-3],
microbatteries and electrochromic display devices [3,4]. Vanadium oxygen
systems (V2O5, VO2) have been studied intensively by theoretical and
experimental techniques [3,5]. They show metal-semiconductor transition,
which implies an abrupt change in optical and electrical properties [6]. That is
why this oxide is used in thermal sensing and switching. Vanadium pentoxide
. Results and Discussion:
The as grown Vanadium pentoxide films have uniform yellow colour
similar to those prepared by other techniques. Such yellow color could indicate
that vanadium was incorporated as V+5 in V2O5 lattice, because, it is known that
V
+4 presents a brown or black color [29]. However, it was shown that, films
Optical Properties:
The spectral transmittance and reflectance, measured in the range 300-
2500nm, of vanadium pentoxide films sprayed onto glass substrates kept at Ts
=
400°C are shown in Fig. (6). The results indicate that the transmission over the
whole spectral range investigated, is lowered with increasing solution
concentration. This is due to the higher film absorption associated with larger
film thickness. Films prepared with concentration 0.1M, show higher
transmission exceeding 80% over the whole spectral range investigated, with a
well defined absorption edge lying in the UV region. For higher molarities, 0.2-
0.5M, besides the reduction in intensity, shifts in the transmission threshold to
lower energy is observed. Similar observations have been found for films
sprayed at 200°C from vanadium trichloride solutions of different molarities
and reported recently [35]. The reflectance spectra for the same films showed
similar trends as transmittance curves, films prepared from higher solution
concentrations shows lower reflectance, However, beyond the absorption edge,
the summation of T(✁
) + R(✁
) does not equal to one, indicating presence of
either light scattering, absorption or both. This is consistence with the visual
appearance of the films surfaces particularly for larger thickness. Therefore, for
subsequent computation of optical constants, the reflectance values were
Conclusion:
Films sprayed from ammonium meta-vanadat of different concentration
and at temperatures Ts
=250°C are amorphous, while films deposited at Ts
=
400°C showed to be polycrystalline with orthorhombic symmetry having aaxis prefered growth orientation. Single phase V2O5 is formed from
concentrations 0.2M. With the lowest concentration (0.1M), beside the
formation of V2O5, a V4O9 appeared was identified. Increasing the solution
concentration increases the crystallite size from 24 to 40 nm and the strain from
1.5x10-3 to 13 x10-3. The increase in film strain may be related to the temperature miss-match between the formed underlayer and the newly sprayed
solution accompanied with film relaxation due to water release. The spectral
variation of refractive index, showed anomalous dispersion in the blue region
followed by normal variation obeying Sellmeir relation. Analysis of the absorption curves revealed allowed direct transition with optical energy gap
2.34eV. Electrochromic investigation showed that, the changes in the films
colouration are consistent with the optical absorption in the wavelengthes range
500-900nm. Also, the absorption edge shifts towards lower energies. The film colouration showed to be stable over several tens of hours claiming high
colouration memory. Due to its limited optical modulation, V2O5 films may be
useful as a passive counter electrode in conjunction with an active working
electrode in coplementary electrochromic devices.