Effects of substrate on the structure and properties of V2O5 thin films prepared by the sol-gel method

Vanadium pentoxide (V2O5) thin films were prepared by sol-gel spin coating on Si and glass substrates. X-ray diffraction indicated that the annealed V2O5 film grown on Si substrate was a α-phase orthorhombic structure, while the annealed V2O5 film grown on glass substrates was a β-phase monoclinic structure. Raman spectroscopy revealed the formation of a V-O bond on both phase films. Scanning electron microscopy (SEM) showed that the annealed film on the Si substrate exhibited more uniform rod-like morphology compared with glass substrate film. Electrical measurements indicated the typical n-type semiconducting behavior of both annealed films coated on Si and glass substrates.Vanadium pentoxide (V2O5) thin films were prepared by sol-gel spin coating on Si and glass substrates. X-ray diffraction indicated that the annealed V2O5 film grown on Si substrate was a α-phase orthorhombic structure, while the annealed V2O5 film grown on glass substrates was a β-phase monoclinic structure. Raman spectroscopy revealed the formation of a V-O bond on both phase films. Scanning electron microscopy (SEM) showed that the annealed film on the Si substrate exhibited more uniform rod-like morphology compared with glass substrate film. Electrical measurements indicated the typical n-type semiconducting behavior of both annealed films coated on Si and glass substrates.


I. INTRODUCTION
The layered structure of V 2 O5 has attracted special attention in recent years due to its electrochemical activity, good specific energy, special layer structure as well as excellent thermoelectric property. 1 2 O5 has some polymorphs, such as α-V 2 O5 (orthorhombic), β-V 2 O5 (monoclinic or tetragonal) and γ-V 2 O5 (orthorhombic).V 2 O5 thin films have been studied extensively and prepared by various methods, such as sputtering, sol-gel process, chemical vapor deposition and spray pyrolysis technique.[2][3][4] The sol-gel technique has become one of the best method for preparation of V 2 O5 films because of its low cost and simple procedure.Bhat et al. 5 obtained α-V 2 O5 films on quartz, Si, and alumina substrates and amorphous V 2 O5 films on glass substrates by the sol-gel method whereas Vasanth Raj et al. 6 prepared β-V 2 O5 films on glass substrates by sol-gel technique.Prześniak-Welenc et al. 7 fabricated V 2 O5 films exhibiting mixture structure of α and β phases by the sol-gel method on both quartz glass and silicon substrates.The most attentions of those previous reports were paid to the effects of annealing temperature on the structure and properties of V 2 O5 films.However, the influences of substrate on the phase structure and properties of V 2 O5 films have been rarely reported by other authors.We have carried out optimization of annealing temperature of V 2 O5 films.Under the same experimental conditions, both α-V 2 O5 and β-V 2 O5 films were prepared at the same time by the sol-gel method.Also the effect of the substrate on the film growth was considered.
In this study, highly oriented α-V 2 O5 and β-V 2 O5 thin films were prepared on Si and glass substrates under the same experimental conditions by the sol-gel method, respectively.The influence of the substrates on crystal structure, morphology and electrical properties of V 2 O5 films was investigated.

II. EXPERIMENTAL
1g of 99% purity V 2 O5 powder was melted at 850 ○ C, and then the melt was poured into 25g of double distilled water under vigorous stirring to obtain V 2 O5⋅nH 2 O sol.The spin coating was used to cast the thin films on silicon (111) and glass substrates.The asdeposited films were annealed in air at the optimum temperature (450 ○ C) for 60 min.The structural characterization of the samples was performed by X-ray diffraction (XRD) using a RIGAKU D/Max-2400 powder diffractometer.Raman scattering spectra were obtained with a Horiba JY-HR800 Raman Spectrometer.Morphological features were detected by a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800).The electrical conductivity of the samples with respect to temperature was studied using a fourpoint probe equipped with a heater.The film thickness was measured using multiple-beam Fizeau fringes at reflection either with mono or polychromatic light.

III. RESULTS AND DISCUSSION
Fig. 1a shows XRD patterns of as-grown and annealed V 2 O5 films deposited on Si substrates.The XRD pattern of as-grown V 2 O5 film does not have any evident peaks.It indicates that the as-grown film is amorphous in nature.For annealed film, the peaks appearing in the diffraction spectrum at 2θ values of 20.31 ○ and 41.12 ○ correspond to reflections in the ( 001) and (002) planes of the α-V 2 O5 phase, respectively.It matches well with the orthorhombic crystal structure of α-V 2 O5 according to the JCPDS FILE (85-0601). 8The strongest peak of (001) plan indicates that the annealed films grow preferentially along the c-axis direction and the (a, b)-plane parallel to the substrate surface.The mean grain size calculated using Debye-Scherrer formula takes a value of about 62 nm for the annealed film.
Fig. 1b shows XRD patterns of as-grown and annealed V 2 O5 films deposited on glass substrates, respectively.The XRD pattern of the as-grown films does not show any diffraction peak, indicating that the as-grown films are amorphous.However the annealed films has a very sharp diffraction peak at 12.56 ○ .It implies that the annealed films have a high preferential orientation along the (200) plane corresponding to β-V 2 O5 tetragonal system phase (JCPDS FILE number 45-1074).The average crystallite sizes of β-V 2 O5 films is about 20 nm.In the reports of Jeyalakshmi et al., 9 β-V 2 O5 films were prepared on glass substrates by sol-gel spin coating method after annealing at 350 ○ C. Vasanth Raj et al 6 obtained β-V 2 O5 films on glass substrates by sol-gel dip coating method after annealing at 500 ○ C. In our work, V 2 O5 films deposited on glass substrates after annealing at 450 ○ C show the β-V 2 O5 phase.This difference in annealing temperature might be related to precursor.
Fig. 1c shows the Raman spectra of as-grown and annealed V 2 O5 films deposited onto Si substrates.Raman spectra of the asgrown V 2 O5 film demonstrates the amorphous character of the film, because it has no Raman active peaks corresponding to the Raman signature of the V 2 O5 crystal.The Raman spectra of V 2 O5 film annealed at 450 ○ C shows sharp crystalline features which have strong Raman peak at 144 and 993 cm −1 .The peak about 144cm −1 in the low-frequency region is a rigid layer-like mode and corresponds to the vibrations of -O-V-O-V-O-chemical bond.The peak about of 993 cm −1 in the high-frequency region is attributed to vanadyl mode and corresponds to the vibration of V=O atoms.The modes of vibrations observed of about 144 and 993 cm −1 are significant for the characterization of orthorhombic α-V 2 O5 films.
Fig. 1d shows the Raman spectra of as-grown and annealed V 2 O5 films deposited on glass substrates.The as-grown V 2 O5 films also are amorphous.Comparing with the α-V 2 O5 Raman spectra, the intensity of the Raman peaks of annealed V 2 O5 films deposited on glass substrates decreases and the 993cm −1 peak is absent.However, in the lower-frequency region the peak at 141 cm −1 still appeared, and some new peaks located at 382, 484 and 804 cm −1 present, which are more nearly in accord with the Raman signature of β-V 2 O5 reported by Grzechnik et al. 10 According to the XRD patterns and Raman spectra analysis, it can be concluded that α-V 2 O5 phase and β-V 2 O5 phase films are prepared on Si and glass substrates, respectively.
Fig. 2a and 2b show the SEM images of V 2 O5 thin films deposited onto Si substrates.Smooth surface and high density have been observed for the as-grown V 2 O5 thin film (Fig. 2a).On the other hand, the V 2 O5 thin film annealed at 450 ○ C (Fig. 2b) exhibits a porous surface and consists of numerous tangled rod-like microcrystalline structures.It might be assumed that those features represent clusters of crystalline α-V 2 O5 grains.Fig. 2c and 2d show the SEM images of V 2 O5 thin films deposited on glass substrates.Apparently, the substrate has no influence on the morphology of as-grown thin films.The amorphous film (Fig. 2c) has a smooth surface and is quite dense as observed for the amorphous thin film deposited onto Si substrate (Fig. 2a).On the contrary, the SEM image of V 2 O5 thin film annealing at 450 ○ C (Fig. 2d) shows that the morphology changed drastically from a smooth and dense surface to a non-contiguous surface formed by numerous tangled stick-like microcrystalline structures.It is noticed from the SEM images that the substrate has a significant effect on the morphology of crystalline V 2 O5 films.It could be explained that the different crystal structures and morphologies of the V 2 O5 films on Si and glass substrates should be related to the thermal expansion coefficients and lattice structure of substrates, which significantly influence the initial nucleation and growth. 11he conductivity (σ) at room temperature for the Si and glass substrates annealed films were 2.4×10 -1 and 3.2×10 -2 Ω -1 cm -1 respectively.Since experimental parameters except substrates are the same in our work, we consider that the conductivity correlates to the microstructures of films, which strongly depend on the substrates.At different temperatures, the variation of conductivity versus reciprocal absolute temperature was also investigated.Fig. 3 shows the temperature dependence of the conductivity measurements, σ (T) for the annealed V 2 O5 thin films on both substrates.It is observed that the conductivity of both α-V 2 O5 and β-V 2 O5 thin films increases with increase in temperature indicating semiconducting behavior.This fits well with the following relation.
Where σo is the conductivity at room temperature, W is the activation energy, k is the Boltzmann constant, and T is the absolute temperature.The electrical conductivity takes place by electronic hopping from low to high valence states (V 4+ and V 5+ ).The thermal activation energy was derived according to plots in Fig. 3  and β-V 2 O5 thin films, respectively.It indicates that the electrons of α-V 2 O5 film need less activation energy to jump from a vanadium site to another one compared with the activation energy for β-V 2 O5 films.The reason for this difference could be connected with the film growth mechanism resulting from substrates.The high value of activation energy might result from the creation of more defects in β-V 2 O5 films.The hopping of electrons rather than ions might result in the low value of activation energy in the film.As for Si substrate, thermally activated atoms join more closely, and the fewer defects are produced because of the fusing of islands, which leads to the less hopping energy related to the thermally assisted tunneling process.

IV. CONCLUSION
In this paper, Nanocrystalline V 2 O5 thin films have been obtained by the sol-gel spin coating method on Si and glass substrates after annealing.The results revealed that the annealed film deposited on Si substrate has the highest c-axis orientation corresponding to α-V 2 O5 phase, while the annealed film deposited on Si substrate has high preferential orientation along (200) plan corresponding to β-V 2 O5 phase.It is due to the difference between the thermal expansion coefficients and/or lattice constants of the substrates and film material.Although the annealed films coated on Si and glass substrates show significant difference in microstructure and morphology, both of them exhibit typical n-type semiconducting behavior.

FIG. 1 .FIG. 2 .
FIG. 1. XRD patterns of V 2 O 5 films deposited on Si (a) and glass (b) substrates, and Raman spectra of V 2 O 5 films deposited on Si (c) and glass (d) substrates.