Abstract
Nanostructure and electrical properties of iridium oxide () thin films prepared by spray pyrolysis technique (SPT) have been experimentally characterized. The effect of solution molarity (SM) and substrate temperature () on the nanostructure features and electrical conductivity of these films has been investigated. The results of X-ray diffraction (XRD) showed that all samples prepared at with different SM, appear almost in amorphous form. XRD revealed that the films deposited at were tetragonal structures with a preferential orientation along direction. Moreover, the degree of crystallinity was improved by solution molarity. Single order Voigt profile method has been used to determine the nanostructure parameters at different SM and . The dark conductivity measurements at room temperature as a function of SM were observed and the value of conductivity were slightly increases at higher SM, reaching the bulk value of 20 . The values of activation energy of and of were found to be 0.21 eV and ·, respectively.
1. Introduction
Among the transition metal oxide, iridium oxide has attracted considerable interest over the years owing to its wide range of applications. The interest of iridium oxide has been growing due to its remarkable chemical, electrochemical, and physical properties, including pH- sensing [1, 2], chlorine, ammonia, or oxygen evaluation [3], neural stimulation [4], field emission cathode [5], advanced memory technology [6], and electrochromic devices and optical information storage [7]. Furthermore, it is used as coating for electrodes in functional electrical stimulation [8]. The electrodes are used to electrically excite nerve cells. The application draws on biocompatibility, high electrochemical activity, and corrosion resistance. The technology of the electrochromic thin films becomes more and more attractive for researchers. The interest for electrochromic materials is based on their potential applications to smart windows, gas sensors and high contrast nonemissive information display system, and devices for thermal control [9]. IrO2 thin film is one of the most promising electrochromic electrodes. One of the advantages of iridium oxide film is the influence of doping on the crystallite size and hence the coloration efficiency [10]. The disadvantage of Ir-oxide layer is its small coloration efficiency (CE) which is about in comparison to that of tungsten oxide, [11].
Several more adequate deposition techniques were employed for synthesis of thin films of Ir-oxide such as sputtering [12], liquid delivery metal-organic (MOCD) [13], sol-gel process [14], pulsed laser deposition [15], thermal evaporation process [16], and spray pyrolysis [17]. Most of these techniques are either not feasible or too expensive for industrial applications. The spray pyrolysis process is an attractive low-cost, versatile method for synthesis of thin films. Furthermore, this method has been used intensively lately for the preparation of different types of thin film, especially of oxides and sulfides of many metals and semiconductors [18, 19]. The advantage of spray pyrolysis is manifold; microstructure of the film can be altered depending on the spray condition, repeatability, adherent deposition, and easy doping.
The aims of the present study are as follows:(i)to investigate the effect of the and SM on the synthesis of a single phase of IrO2 thin films were deposited by using spray pyrolysis iridium chloride (IrCl3. 3H2O) as starting solution onto heated glass substrate.(ii)to investigate and discuses the influence of the and SM on the crystalline growth of nanostructure and electrical properties of the obtained samples were studied in order to determine the suitable temperature and molarity to produce a single phase of IrO2.
2. Experimental Details
Thin films of Ir-oxide were deposited by spray pyrolysis technique (SPT) from aqueous solution of iridium chloride (IrCl3. 3H2O) onto preheated glass substrate at two temperatures 350 and C. The solution at different concentrations (0.005–0.03 M) was sprayed through glass nozzle using air as a carrier gas onto the ultrasonically cleaned substrate. The overall reaction process was expressed as thermal decomposition of iridium chloride to form clusters of Ir-oxide, in the presence of the water, as follows: The heat annealed process at C for 120 minutes was made of all prepared samples at different SM and . The chemical reaction that took place was as follows: The resulting films were found to be uniform, strongly adherent to the substrates, and blackish in colour. A controller of temperature was used to measure the using the resistance heater via a thermocouple. In order to get homogeneity thin films, the height of the spraying nozzle and the rate of the spray process were kept constant during the deposition process at 25 cm and 5 cm3/min, respectively.
The structure characteristics of the films were studied using by X-ray diffractometer (JEOL model JSDX-60PA) with attached Ni-filtered Cu- radiation (nm). Continuous scanning was applied with a slow scanning speed (/min) and a small time constant (1 second). A range of two theta (from to ) was scanned, to insure the detection of all possible diffraction peaks detected. Nanostructure and the phase identification of the polycrystalline thin film of IrO2 at different SM and were investigated. The crystallite size and macrostrain of the films were determined using single order Voigt profile analysis [20]. The average values of FWHM of the characterized peaks were measured after correction of instrumental factor. Numerical calculation was developed using the refractive indices of crystalline, amorphous, and single crystal of IrO2 as in (6) to estimate the crystalline volume fraction. Also, the film thicknesses of the prepared samples were measured using mechanical stylus method (MSM) (Sloan Dektak, model 11A) [21].
The DC of electrical conductivity measurement was performed on the films using by four-point probe technique of Van der Pauw [22] at room temperature with the required correction tables [23]. Two gold electrodes were deposited on the iridium oxide/glass samples leaving an uncoated trip in the middle. A constant-load spring was used for the contact. The current-voltage characteristics were measured using Keithely digital electrometer (Model 616).
3. Results and Discussion
3.1. Film Thickness Measurements
Ir-oxide film thicknesses prepared at two substrate temperatures (350 and C) with different SM and constant deposition time (DT) of 10 minutes were measured by MSM. Figure 1 shows the variation of the film thickness of IrO2 as a function of solution molarity (SM). It is clear that, the film thicknesses of the prepared samples at C have a higher value than those at another temperature. This is because the evaporation rate was increased at higher temperature, leading to diminished mass transport outwards the substrates, which is a decrease of film thickness. Furthermore, at rise of the solution molarity, the values of film thickness increased. Each data point is the average of five measurements taken at different locations on the film surface; the calculated error was found to be %. Consequently, these results may suggest that the different temperature and SM of sprayed films have a significant influence on the film thickness of the IrO2 films. Similar results were previously described using NiO2 film on glass substrate [18].
3.2. Formation of Single Phase Ir-Oxide
Figure 2 depicts the variation of the diffraction patterns as a function of SM while the of C and DT of 10 minutes were kept constant. The results of XRD for all samples as-deposited at C show that the film structure was noncrystalline phase with different SM, while at C, the films were fully transformed to polycrystalline phase of IrO2 which had a tetragonal structure when SM varies (Figure 3). The presence of the characteristic lines corresponding to (110), (101), (200), and (211) planes of IrO2 was observed which coincide with ICDD number 15–870. The increase in the amount and the crystallinity of the phase were exhibited at raising the SM which is indicated by the enhancement of intensity and decrease of FWHM in peaks. Also, the increase in the intensity of the peaks may be attributed to either grain growth associated with smallest thickness or the increase in the degree of crystallinity by increasing the temperature or both.
The average values of calculated lattice parameters were found to be and which coincide with the bulk of IrO2 [24]. Moreover, the films show preferred growth along direction and as SM increases. Also, no other additional peaks corresponding to other phases have emerged. This suggests that IrO2 phase is stable and its formation is independent of SM.
3.3. Nanostructure Characteristics of IrO2 Films
The IrO2 thin films were analyzed by XRD technique to study nanostructural parameters (crystallite/domain size and macrostrain) under several SM and temperatures. The crystallite/domain size is by definition measured in the direction normal to the diffraction plane and consequently perpendicular to the substrate. Therefore, the observed increase in the crystallite/domain size may be interpreted in terms of a columnar grain growth. The macrostrain is equivalent to a variation in interplanar spacing within domains by amount which depends on the elastic constants of the material and the nature of internal stresses. Viogt method for single-order profile analysis was used to calculate the crystallite/domain size and macrostrain of the polycrystalline IrO2 films [25]. The parameter of interest with the Voigt function is “the shape parameter” which is defined as [/integral breadth]. The value of the shape parameter, ranging from 0.6366 to 0.9395, was used to determine the fractional Lorentzian (Chuchy) and Gaussian components in the convolute. The Lorentzian (Chuchy) and Gaussian components of the integral breadth of pure specimen profile are given by where h, f, and g are the observed, specimen, and instrumental profile function, respectively. The apparent crystallite/domain size is The calculated average crystallite/domain size according to the four diffraction planes (110), (101), (200), and (211) at different SM using by Lorentzian as well as Gaussian component is given in Table 1. It is clear that the value of crystallite size is slightly increased at different SM. This means that the variation in crystallite size was observed due to the mobility and concentration of ions which is appropriate for crystal growing on the substrate. Kawar et al. reported the same value of grain size for the like phase prepared by spray pyrolysis [26].
The macrostrain of investigated samples was determined using Voigt method formula: Where is the standard value of interplanar spacing taken from JCPDS data file . The macrostrain was calculated as an average of fractional change, , in the interplanar spacing, d, of the four diffraction planes (110), (101), (200), and (211). The calculated macrostrain for all prepared samples at different SM is given in Table 1. It is clear that the macrostrain was decreased as a function of SM, which indicates that the sprayed molecules are ejected with higher kinetic energy and momentum. Therefore, these molecules recombine with higher mobility energy causing variations to occur within their internal stress and consequent macrostrain decrement.
3.4. Crystalline Volume Fraction
The crystalline volume fraction of IrO2 thin film as a function of SM was estimated by the analysis of the optical reflectivity spectra in the ultraviolet region [27, 28]. The crystalline volume fraction, , is defined by the following equation: where , , and are the complex refractive indices for nanocrystalline, amorphous, and single crystalline IrO2, respectively. The volume fraction of the crystalline IrO2 was obtained by fitting the calculated reflectivity spectra to experimental reflectivity spectra. The crystalline volume fraction, , was estimated for nanocrystalline IrO2 films at different SM listed in Table 2. It can be observed that the increase in the SM induces a change in the amorphous-crystalline transition, showing an increase in in the investigated samples. This fact is related to a crystallization of the material for higher substrate temperature and SM [29].
3.5. Dark Electrical Conductivity of IrO2 Films
To measure the electrical conductivity, two ways of investigation were done, one of them at room temperature and the other at different temperature. The values of DC electrical conductivity as a function of solution molarity of IrO2 films prepared at 350 and C were measured at room temperature, using four-point probe technique shown in Figure 4. It was found that the values of electrical conductivity of amorphous are lower than nanocrystalline structure, and it increases rapidly with increasing SM for both substrate temperatures. This is because the film thickness increases as concentration increases. Thus the increase in crystallite size resulted in the reduction of grain boundary associated with electrons scattering and thence increase in electrical conductivity. Accordingly, the increment of electrical conductivity was correlated with the film morphology change of IrO2 film. Similar results have been previously reported [15, 30]. Also, the value of conductivity was found to be from 15.9 to when SM varies from 0.005 to 0.03 M, respectively.
3.6. Effect of Temperature on the Conductivity
At different temperature, the variation of conductivity against reciprocal absolute temperature was investigated. In iridium oxide, the dependence of conductivity on temperature is more complex because of different chemistry of decomposition. IrO2 has been found to be nonstoichiometric with excess of oxygen [31]. The temperature dependence of electrical conductivity is given by the Arrhenius relation: where σo is the pre-exponential factor, is the activation energy for DC conduction, is the Boltzmann’s constant, and is the absolute temperature. The temperature dependent dc dark conductivity of IrO2 thin films is shown in Figure 5. The plots of Log versus 1000/T were found to be straight line indicating that conduction is performed through an activated process having single activation energy in the temperature range from 303 to 350 K. This is attributed to the thermally assisted hopping of carriers between localized states closes to Fermi level [30]. The values of and were calculated from slope of the curve and were found to be 0.21 eV and , respectively. The behavior of conductivity of these samples as a function of temperature was matched with sputtered iridium oxide thin film [25]. The various magnitude values of electrical conductivity that we found may indicate possible transformation of IrO2 films from semiconducting phase to metallic one. This transition is conceived to be due to the change in oxidation state of iridium [31, 32], as a result of which the Fermi energy level, , crosses the conduction band edge .
4. Conclusions
Ir-oxide films have been prepared onto heated glass substrates from iridium chloride solution using SPT at different solution molarity and two substrate temperatures. As revealed by XRD, at C when SM is different, the amorphous iridium oxide has been formed. While at C, the films were fully transformed to nanocrystalline single phase of IrO2 at any concentration. Also, the increase in the intensity of the peaks with increasing the SM may be attributed to either grain growth or the increase in the degree of crystallinity or both. The variation in crystallite size was observed at different SM, because the concentration and mobility of ions were appropriate for crystal growing on the substrate. Also, it was mentioned to maximum value at higher SM that this is probably because the mobility and collision of ions are sufficient to recombine the crystal agglomerate. The behavior of electrical conductivity as a function of concentration was observed and the IrO2 films may be possible transformation from semiconducting phase to metallic one. The values of and were found to be 0.21 eV and , respectively.