Microstructural and Electrical Properties of (WO3)1-x(MoO3)x Thin Films Synthesized by Spray Pyrolysis Technique

Abstract

New composite oxides of (WO3)1-x(MoO3)x thin films were deposited by spray pyrolysis method. Microstructural and electrical properties of the films as-deposited and after annealing at different MoO3 concentrations were investigated. XRD analysis, as-deposited films were amorphous, but after annealing, the crystallinity of WO3 and MoO3 were improved with a hexagonal structure with an increase in the MoO3, consisted of three non-stiochiometric phases, which have a monoclinic structure. Microstructure of a mixture based on the proportion of MoO3 in the film. The electrical conductivity mechanisms appear to have two behaviors; extrinsic and intrinsic conductivities at lower and higher temperatures, respectively.

Keywords

Mixed oxides, XRD, Spray pyrolysis, Microstructural, Electrical properties, Thin films

Introduction

Transition metal oxides have been widely used as gas-sensing, opto-electronic devices and inorganic electrochromic materials [1]. Tungsten trioxide (WO₃) and molybdenum oxide (MoO₃) thin film are one of the most promising material in Electrochromic and gas-sensing studies, and a common gasochromic window because their sensitivity to various gases, such as NO, NO₂, CO, NH₃ and H₂, which is due to the physical and chemical properties of these oxides [2-4]. The reason for choosing WO₃ as a doping with MoO₃ is because its film has a high transparency, high electrochemical activity and stability. The interaction between WO₃ and MoO₃ is unique due to their similar ionic radii and nearly-identical structures in their highest oxidation state. In addition, they were used in the electrochromic process as a cathode material due to the following reasons, they are (i) high specific energy density [5], (ii) undergo reversible potactic reaction with ions [6] and (iii) they have a higher electrochemical activity with the highest stability. In order to improve the quality of sensitivity of gas detection, the variation of gas-sensing and electrochromic devices, the binary combinations of oxides will modify and improve the characteristic of different oxides.

Composite trioxides of (WO₃)_1-x(MoO₃)_x films have been prepared by different techniques; such as the vacuum evaporation [7,8], electro-deposition [9,10], sputtering [11], electron-beam deposition [12] and sol-gel deposition [13,14]. The spray pyrolysis process offers several advantages over conventional deposition techniques for the control of stoichiometry and film structure.

This work is aimed to present mixed metal oxide sprayed synthesis of new molybdenum trioxide doped tungsten trioxide film, WO₃-MoO₃ and study the microstructral and electrical characteristics. The extent of a mixture concentration of MoO₃ ranges from 10-40 mol% in this study. A detailed structural characterization of the WO₃-MoO₃ thin films was presented by X-ray diffraction (XRD) and as well as the study of the electrical properties was done. Also they have been studying the influence of thermal annealing on the properties of different concentrations. Structural and electrical properties correlation was aimed to study.

Experimental Work

Samples Preparation

Molybdenum trioxide doped tungsten trioxide films were prepared using a chemical spray pyrolysis technique. The precursor solutions were prepared by dissolving ammonium paratungstate and ammonium paramolybdate with a solution molarity of 0.005 M in a hot distilled water of 333K. The molybdenum trioxide concentration is coincident with the percentage in atomic weight of ammonium paramolybdate dissolved in the sprayed solution. Different atomic percentages quantities, namely 10, 20, 30 and 40 mol% were considered to get a composite of (WO₃)_1-x(MoO₃)_x thin films. These solutions were mixed together accorded with atomic ratios to the deposition onto the heated glass substrate with change percentages referred to in Table 1. Equations expected the chemical reactions to get the compound as follows;

Table 1: The weights of the ammonium paratungstate and ammonium paramolybdate powder used in the preparation of samples for a mixture oxides of WO₃ and MoO₃ with different atomic percentage, which was dissolved in 100 ml of distilled water.

(NH₄)10[H₂W₁₂O₄₂].4H₂O →12WO₃ + 10NH₃↑ + 10 H₂O↑ (1)

(NH₄)₆Mo₇O₂4·4H₂O → 7MoO₃ +6 NH₃ ↑+ 7H₂O↑ (2)

The mixture of aqueous solution sprayed on the heated surface of glass substrate will be re-composed of WO₃ and MoO₃ which yield in the form of mixed (WO₃)_1-x-(MoO₃)_x thin films according to the following reaction equation;

(NH₄)10[H₂W₁₂O₄₂].4H₂O + (NH₄)₆Mo₇O₂4·4H₂O → 12WO₃ + 7MoO₃ + 16NH₃↑ + 17 H₂O↑ (3)

The substrate temperature (T_sub) and film thickness were kept constant at 673K and 650 nm, respectively. The prepared samples of mixed WO₃-MoO₃ were then annealed at temperature of 723K in the air for two hours. Thin films were carefully preserved in a desiccator to be used later in studying of their characterization. Films deposited on the glass substrates were used for structural and electrical characterization.

Stylus mechanical method was used for thickness measurements. In stylus instruments, a diamond needle of very small dimensions is used as the arm of an electromechanical pick-up. The position of the pickup is determined by a slider which is pressed on the area surrounding the needle to give the mean zero position.

Microstructural Characterization

X-ray diffraction (XRD) patterns were taken to collect by X-ray diffractometer (JEOL model JSDX- 60PA materials research), at room temperature using Ni-filtered Cu K_α- radiation (λ = 0.15418 nm). Continuous scanning was applied with a slow scanning speed (1/min) and a small time constant (one second) for the two theta range from 4 to 80°. X-ray diffraction line profile analysis (XRDLPA) was used to study the microstructure of the prepared samples. The line broadening is a result of the small size of the crystalline [15-18] as well as the non-uniform displacements strain of the atoms with respect to their reference lattice positions [18]. The observed integral breadth of the sample is a convolution of an instrument and a physical factor. This can be represented as follows;

I_obs(2θ) = I_exp(2θ) ٭ I_S(2θ) + background (4)

where (٭) is a convolution operator. The functions I_obs, I_exp and I_S are the broadening profiles of (i) the observed (B), (ii) the instrumental or standard sample (b) and (iii) the pure sample (β), noting that, these three broadening profiles are functions in the diffraction angle 2θ. The corrected Lorentzian (Couchy) and Gaussian components of the integral breadth are given by

β_L = B - b (5)

where B and b are the broadening and instrumental factors, respectively. The average crystallite size (D) is calculated by using Scherrer's equation as follows [19];

D = Kλ/βcosθ (7)

Where the constant K is taken to be 0.94, λ the wavelength of X-ray used and β the broadening of pure specimen correction (full width at half maximum) of the diffraction peak. The internal micorstrain, <ε> in the film is calculated according to the relation;

Electrical Characterizations

Electrical resistivity measurements: The electrical resistance of the thin films was measured by using two-point probe technique. Two aluminum electrodes were deposited on the samples with leaving an uncoated trip in the middle of the film. Loadspring was fixed to two copper rods used for better contact with the two aluminum electrodes. The resistance of the sample was measured using a digital Keithly electrometer (model 616) and a stabilized power supply. The temperature of the specimen was recorded with chromel alumel in close thermal contact with the specimen surface. The temperature dependence of d.c. electrical resistivity, ρ_dc for the film was measured in the temperature range from 300 to 523K. It was calculated by;

where d is the distance between two copper electrodes (1.4 cm), R is the resistance of thin film (Ω), w is the electrode length covered by the film (width of the thin film is 2.5 cm) and t is the film thickness (cm).

Sheet resistance measurement: The sheet resistance of semiconductor is considered as a widely used technique to characterize the thin films. It measures the resistance of thin films across a square area to identify the nature of the roughness of film surface and thickness homogeneity. To calculate the sheet resistance of thin films is implied that the current passes through the sheet plane and is not perpendicular to it. Also, the resistivity of bulk material, ρ can be calculated by multiplying the film thickness, t and the sheet resistance R_sh as given by the following formula [20];

Results and Discussion

Structural Characteristics

The structure of as-synthesized of composite WO₃-MoO₃ thin films: XRD patterns of the as-deposited samples as well as samples of a composite (WO₃)_1-x(MoO₃)_x films with different concentrations of MoO₃ are shown in Figure 1. Results showed that, the investigated samples of tungsten and molybdenum trioxide have a polycrystalline structure. All the diffraction peaks of pre-annealed WO₃ in the lower diffraction pattern can be indexed to the hexagonal structure of WO₃ with lattice constants of a=b=0.73025 nm, c=0.38970 nm, α=β=90°, γ=120° (PDF card, 75-2187). No peaks of any other phase or impurities were observed in the XRD patterns. Also, the second diffraction pattern is perfectly with the MoO₃, which have a hexagonal structure (PDF card, 05-0508). When the MoO₃ mixed with WO₃, all the diffraction patterns completely amorphous form of all doped concentrations. This is may be due to, the properties of composites of oxide materials depend not only on the chemical and physical nature of of each compound, but also on the interface and synergy between them.

The effect of annealing on the structure of WO₃-MoO₃ thin films: The change of crystallinity and structural identification after thermal annealing of WO₃ thin films with the different concentration of MoO₃ were studied by using XRD technique [21-23]. The diffractograms of the thin films after annealing at temperature 723K for two hours using XRD are shown in Figure 2. All the investigated samples were transformed into a polycrystalline structure. The strong and sharp diffraction peaks also indicate a good crystallinity of the samples. Compositions and phase identification process of the samples were recorded in Tables 2 and 3. It can be revealed that, the three non-stiochiometric phases Mo-W-O₃, Mo_9.35W_1.65O₃₂ and W_0.71Mo_0.29O₃ are present. The phases are depending on the doping concentration. At concentration of 10mol%, the peaks expected corresponding to the a hexagonal structure as well as for un-doped WO₃ [24]. The characteristic diffraction lines corresponding to (100), (110), (111), and (200) planes of WO₃ was observed and coincides with PDF number 75–2187. However, at higher concentrations of 20% and 30%, the composition of the films has a monoclinic structure with characteristic lines corresponding to (201), (010) and (411) planes of Mo_9.35W_1.65O₃₂. This is matched with the planes of PDF 73-2198. Furthermore, the composition of the films at higher concentration of MoO₃ (40%) has a monoclinic structure with lines (111), (200) and (112) planes of W0.71Mo0.29O₃ which coincide with the planes of PDF 76-1279 [25,26].

Table 2: X-ray diffraction results of thin films sprayed from pure trioxides of tungsten and molybdenum, as well as from different ratios of a mixture of MoO₃ and WO₃ after thermal annealing at a temperature of 723K for two hours.

Table 3: The electrical resistivity and sheet resistance at three different temperature zones with different doping concentrations.

The chemical reaction mechanism of the composite W-Mo-O₃ can be explained according to electrochemical activity series of elements W and Mo, where WO₃ and MoO₃ have a hexagonal structure. It was found that, the excess of Mo content increases the unit cell volume of the formed WO₃-MoO₃ network. The structure of composite W-Mo-O₃ alloys was found to be a non-stiochiometric phase of Mo_9.35W_1.65O₃₂ which has a monoclinic structure in the concentration range 10 to 30% of MoO₃. The observed three peaks at Brag's angles are 2θ=11.00, 21.90 and 33.00o and corresponding the Millar indices are (201), (010) and (411). XRD data reveal that the composite system of W-Mo-O₃ phase becomes no stable and its formation is independent of MoO₃ concentration. This result indicates that the stoichiometric ratio is verified [27,28].

The effect of annealing on the crystallite size and internal microstrain: The variation of crystallite/domain size with varying doping percentage is shown in Figure 3. It is observed that, the crystallite size has nearly the same value up to a concentration of 30%, while at a concentration value of 40%, the crystallite size was suddenly increased. This is due to, the composite of WO₃ and MoO₃ are the same of atomic volume. So the rate of agglomeration growth of thin films is kept constant. But at higher concentration of MoO₃, the rate of growth was increased. These values of crystallite size are in good agreement with the data estimated by Kawar et al. for a similar phase and prepared by spray pyrolysis technique and other techniques [29-31].

The effect of doping percentage on the internal microstrain is depicted in Figure 4. It can be seen that, the internal microstrain depending on the MoO₃ concentration. Since the internal microstrain is equivalent to variations in the d-spacing within domains by an amount depending on the elastic constants of the material and the nature of internal stresses. This is due to, the growth of MoO₃ inserted the host unit cell of WO₃, consequently the inter planer spacing was shifted. This shift in the plane position or the internal microstrain is produced as a result of the growth process of the samples and the preparation conditions. Note from the Figure 4, the value of internal microstrain nearly the same value at MoO₃ concentrations of 10 to 30%, while suddenly increased when the concentration 40%. It is expected that the addition of more MoO₃ causes imperfections to the original lattice of WO₃ and thus increases the amount of internal stress, which in turn increases the amount of internal microstrain. This confirms that, the WO₃ phase has a good crystallinity formation in the studied range from 0 to 30% [32].

Electrical Properties of WO₃-MoO₃ Thin Films

The resistivity of semiconductor depends on several parameters, e.g. impurity concentrations, crystal defect concentration, temperature (lattice vibrations and phonon), and electron-hole concentrations. The electrical characterization of the as deposited WO₃–MoO₃ thin films at different doping concentrations of 0%, 10%, 20%, 30% and 40 mol% was carried out by means of the resistivity-temperature measurements.

Effect of temperature on electrical resistivity: The electrical resistivity of mixed WO₃–MoO₃ thin films as a function of temperature according to the Pertritz model. The thermal activation energies, ΔE, were calculated by using the following relation [33];

Where ρ_o and ρ1 are pre-exponential constants, KB the Boltzmann constant and T is the absolute temperature. Figure 5 shows the representation of the Lnρ_dc and the reciprocal absolute temperature (1000/T). It is clear that, the electrical resistivity exponentially decreased with increasing temperature for all samples regardless of the doping concentration. Furthermore, the electrical resistivity is found to be strongly dependent on the composition of thin films. This is explained by increasing of the number of thermally excited electrons [33,34].

On the other hand, the dark electrical resistivity (ρ_dc), for temperature at three different zones for WO₃-MoO₃ thin films was recorded in Table 3. It is observed that, the electrical resistivity is changed randomly with the doping percentages of MoO₃. But generally, a significant increase in the resistivity is observed at any MoO₃ concentration reaches a higher value at a concentration of 30% and then returning to decrease again at a concentration 40%. Where the electrical resistivity of the compound (WO₃)_1-x(MoO₃)_x does not take a certain behavior, due to the non-stiochiometric phase formation of the film as a result of increasing impurities (Mo atoms).

Effect of temperature on sheet resistance: The sheet resistance of doping semiconductor is considered a widely used technique to characterize the thin films. It measures the resistance of thin films across a square area to identify the nature of the roughness of film surface and thickness homogeneity. Measurements of electrical sheet resistance, R_sh of the WO₃ mixed with MoO₃ thin films were carried out in the temperature range 300-523K, as shown in Figure 6. It is observed that, the sheet resistance for all ratios of MoO₃ additives is the exponential decreasing behavior as a function of temperature. Also, its value of R_sh does not only depend on the ratio of adding MoO₃, but also depends on the morphology of the film surface. Furthermore, the sheet resistance, R_sh, values at temperatures in three different zones for mixed thin films of (WO₃)_1-x(MoO₃)_x was listed in Table 3. It was observed that, the sheet resistance decreased with increasing MoO₃ concentration. The decreases in R_sh may be due to substitution of Mo-atoms instead of the W-atoms in the unit cell, which causes a decrease of energy gap consequently leads to the increase of electrical conductivity.

Conductivity mechanisms and activation energy: The representation of logarithmic resistivity, Ln ρ_dc versus the reciprocal absolute temperature (1000/T) with the different doping percentage is shown in Figure 7. It is clear that, the two different linear regions and their dependence upon the selected temperature range. The first region is identified at low temperature (LT). This region extends also upwards as far as the impurity exhaustion temperatures and is identified as an extrinsic conductivity of semiconductor due to the ionization of impurity atoms. At the low temperature range, the data followed Pertritz expression indicated that the conduction attributed to the thermally assisted hopping of carriers between localized states closes to the Fermi level [34,35]. The second region (higher temperature region, HT) is identified with the intrinsic conduction of semiconductor. Within this region, the density of carriers is equal to that of intrinsic carriers [31]. This can be explained due to the lower degree of crystallinity and the small crystallite size. Also, the presence of defects such as structural disorders, dislocations and surface imperfections also play an essential role in decreasing the conductivity as reported earlier [31]. As, the transition here depends on two parameters; the temperature and lattice vibrations which create phonons, will produce electron-phonon interaction. This interaction causes hindering for electrons, so electrons need more energy to be activated. This interprets the increasing of activation energy values of the second region [34]. The corresponding activation energies of the two regions were listed in Table 4. It is observed that, the obtained activation energy in the lattice vibration region (HT) is approximately equal to one half of the optical energy gap [36,37]. Also, ΔE values for all of WO₃ mixed with MoO₃ thin films indicate that the prepared samples were semiconductors materials. On the other hand, our all present data were consonant with the results of the other semiconducting materials [34,37-39].

Table 4: The residual resistivity, activation energy and the temperature change of conductivity mechanisms with different MoO₃ doping percentage.

The change in the transition temperature (Tc) in conductivity mechanism as a function of MoO₃ concentration is represented in Figure 8 and has been recorded in Table 4. Represented measured data was fitted to be obtained straight lines satisfying of the following equation [31,36];

T_c= -1.07 X + 121 ± 0.62 (12)

These results can be interpreted as follows, since the Mo atom is an inserted within the host unit cell of WO₃ instead of the W atom because the size of Mo atom is equal to the size of W atom. So the dimensions of the unit cell are remaining constant in the three directions for the same extension. So, the shift of transition temperature, T_c is due to increase in the doping percentage, which represent the impurities in the composite where the conductivity mechanism depends on the temperature, impurities and crystal defects. Therefore, the shift in T_c can be attributed to the change in the impurity content which is affecting the conductivity mechanism.

Conclusion

1. Composite trioxide (WO₃)_1-x(MoO₃)_x thin films were deposited on the previous glass heating substrates by using the spray pyrolysis technique.

2. The thermal annealing process in the air at 723 K for two hours was done.

3. XRD reveals that, composite W-Mo-O₃ alloys were found to be non-stoichiometries phases of Mo-W-O₃, Mo_9.35W_1.65O₃₂ and W0.71Mo0.29O₃ which have a monoclinic structure.

4. The microstructure (crystallite size and internal microstrain) was estimated as a function of MoO₃ content.

5. The electrical properties (resistivity, sheet resistance and activation energies) of a composite (WO₃)_1-x(MoO₃)_x were carried out.

6. The conductivity mechanisms divided into the two processes; an extrinsic conductivity and intrinsic conduction.

7. Transition temperature of the conductivity mechanism was increased with MoO₃ concentrations.

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