IS STUDY OF POROUS INDIUM TIN OXIDE-BASED GAS SENSOR
It was reported that many transparent conductive oxides, such as ZnO, In2Oj, and Sn02 can be used in a gas sensor (Hsueh, & Hsu, 2008). Their popularity as gas-sensitive materials is due to their suitable physicochemical properties, such as natural nonstoichiometric structures (Kaur, Kumar, & Bhatnagar, 2007), and free electrons arising from oxygen vacancies contribute to a change in electronic conductivity with a change in the composition of the surrounding atmosphere (Chu, Zeng, Jiang, & Masuda, 2009). In Saadoun, Boujmil, El Mir, and Bessais (2009), the gas-sensitive properties of 1TO films were studied in DC measurements, which provide information on the global response of the sensor. The study of AC impedance is necessary to understand the nature of the conductivity processes and the gas/ solid interaction mechanism).
To investigate the ITO sensing properties toward N02, Madhi. Saadoun, and Bessais (2012) have studied the nature of conduction processes and model-sensitive mechanisms in the impedance of AC of ITO films obtained by screen printing. All impedance spectra appear to be single semicircles without displacement along the Z' axis (Figures 3.19-3.21).
FIGURE 3.18 Inductive artifact in the electrode impedance in a three-electrode cell with a porous Pt electrode with a significant larger polarization resistance than the LSCF working electrode.
Reproduced with permission from Boukamp et al. (2011). Copyright 2011, Elsevier.
FIGURE 3.19 Nyquist diagram at various operating temperatures in ambient air. Reproduced with permission from Madhi ct al. (2012). Copyright 2012, Elsevier.
FIGURE 3.20 Impedance variation consecutives to repetitive excitation by 200 ppm of N02. Reproduced with permission from Madhi ct al. (2012). Copyright 2012, Elsevier.
The impedance spectra have a semicycle shape indicating the homogeneity of the grains (Figure 3.19). The spectrum remains quasi-stable and no memory effects were observed (Figure 3.20). Figure 3.21 illustrates the effect of N02 concentration on a Nyquist pattern. The impedance (Z') increases when exposed to N02 and begins to saturate when the concentration exceeds 160 ppm. The equivalent circuit can be decomposed in the simplest case in the form of a parallel R-C circuit, similar to other transparent conductive oxides.
STUDIES OF PROPAGATION MECHANISM IN SOLID IONIC MATRICES
Heli. Sattarahmady, and Majdi (2012) made a composite electrode of graphite, Nujol, and nanoparticles of the core of Fe2C>3 - hexacyano-cobalt ferrate shell was prepared and charge
FIGURE 3.21 IS of the screen-printed ITO film vs. N02 concentration. Reproduced with permission from Madhi et al. (2012). Copyright 2012, Elsevier.
transfer processes in the volume of this composite were studied. The electrode/solution interface was assumed to be binary electrolyte, the charge transfers of which occurred between the redox sites of nanoparticles present in the composite and cations found in solution. Using cyclic voltammetry, diffusion of the oncoming cation in the shell has been investigated. The use of chronoamperometry, effective diffusion coefficient, and its dependence on the applied potential has been gained. The Nyquist diagrams are different time constants appeared in relation to different physical and electrochemical processes. Percolation of the electrons in the shell of the nanoparticles appeared at very high frequencies and showed a diffusion feature process with passing boundary condition at the interface. Many studies have been devoted to the understanding of charge propagation mechanism in solid ionic matrices (Heli, Sattarah- mady, & Majdi, 2012: Heli & Yadegari, 2010; Inzelt, 1994). A solid ionic material-containing redox sites in contact with a liquid electrolyte is a binary electrolyte whose charge transport is occurred between redox sites of the solid material and the charge compensation is performed by the counterions originated from an infinite space of the liquid electrolyte. On the other side, the solid material itself, bears both ionic and electroreactive species. Depending on the experimental conditions (e.g., type of solid matrix, nature and concentration of redox centers, and mobility and availability of counterions), the overall charge propagation process can be controlled by a variety of phenomena such as electron self-exchange rate, counterion migration. and ion pairing (Dalton et al., 1990; Surridge et al., 1989).
However, lower slope line appears at low frequencies in the Nyquist diagrams (see Figure 2 in Heli et al., 2012). This behavior can be characterized using CPE instead of the pure capacitance.
The roughness at the electrode/interface (local inhomogeneity presents at the electrode surface and nonuniform distribution of local capacities) and consequently, a distribution of activation energies of the processes occurring in the double layer causes the appearance of CPE behavior. The Nyquist diagrams in this DC potential range can be characterized with the electrical equivalent circuit as shown in Figures 2 and 6 in Heli et al. (2012). In this circuit, R$, W, and CPE are the solution resistance, semi-infinite Warburg element related to the electron diffusion, and a CPE related to the double-layer capacitance, respectively.
MESOPOROUS C/-Fe203 Films
Cummings, Marken, Peter, Upul Wijayantha, and Tahir (2011) studied thin mesoporous «-Fe203 films that were obtained on conductive glass substrates using self-layer assembly of ~4 nm aqueous oxide nanoparticles followed by calcination. As noted above, «-Fe203 (hematite) has been widely studied as a potential photoanode material for cells that destroys water (Dare-Edwards et al., 1983). Although its forbidden band (2.0 eV) is suitable for collecting visible light, and its valence band has sufficiently low energy for holes to oxidize water, an external bias is necessary in order to raise the energy of free electrons in the conduction band sufficiently for control reaction of hydrogen evolution at the counter electrode for water splitting The necessary external voltage bias can be provided by the solar cell in the configuration of the tandem cell (Brillet, Gratzel, & Sivula, 2010). Performance limitations imposed by high doping lengths and short diffusion holes in hematite films were damaged to increase the probability of holes that reach the interface between the oxide/electrolyte interfaces (Brillet et ah, 2010). Electrodes were used to study the oxygen evolution reaction (OER) in the dark and when illuminated using in situ potentially modulated absorption spectroscopy (PMAS) and light modulated absorption spectroscopy (LMAS) in combination with IS. The formation of surface-bound valence types of iron (or “surface traps” holes) was derived from the PMAS spectra measured in the region of the beginning of the OER. Similar LMAS spectra were obtained at more negative potentials in the region of the onset of photoelectrochemical OER, which indicates attraction of the same intermediaries. The original solution in the work was to attract the impedance characteristics of the mesoporous electrodes a-Fe203. Frequency allowed measurements of PMAS and LMAS revealed slow relaxation, which may be related to the impedance response and that indicates that the lifetime of the intermediates (or trapped holes) involved in the OER is extremely long.
Electronic transport and electron transmission can be represented in a transmission line model as shown in Figure 3.22a (additional series resistance, R, arising mainly from substrate and fluorine-doped tin oxide contacts, not shown). The model describes charge transfer in the electrode material as well as transfer and storage of charge on the electrode/electrolyte interface: rtrans is a distributed transport resistance, Rcl is the charge transfer resistance, and Csurf is the capacitance of
CPE BEHAVIOR CAUSED BY RESISTIVITY DISTRIBUTIONS
Further, our discussion will concern such an important element of equivalent impedance circuits as CPE. Generally, the analytic expression for the resulting impedance provides a useful relationship between system properties and CPE parameters. Application of such an approach to experimental data is reported in a paper by Hirschorn et al. (2010). Hirschorn et al. (2010) argue what behavior of CPE the normal distribution of resistivity will give for a system with a uniform dielectric constant. They show that under the assumption that the dielectric constant is position independent, the normal power distribution of local resistivity is consistent with
FIGURE 3.22 (a) Finite transmission line representing the impedance of the porous a-Fe203 electrodes. (b) Equivalent circuit in the low frequency limit. The additional series resistance is not shown in either circuit, (c) Typical impedance response for the transmission line circuit including a series resistance. Note the linear region at high frequencies, which is characteristic of transmission line behavior. The combination of spectroscopic and impedance techniques has given a deeper insight into the mechanism and kinetics of oxygen evolution at a-Fe203.
CPE. The original postulate is that does not take into account the formation of a space charge region for a material exhibiting semiconductor properties.
The power-law distribution of resistivity provides a physically reasonable interpretation of CPE for a wide class of systems where properties are expected to change in the direction perpendicular to the electrode. Excellent results have been described, for example, for applying a power-law distribution of resistivity to obtain the properties of a passive film on Fe-17Cr (Hirschorn et al., 2010). With comparison to the Young model, the impedance corresponding to a power-law distribution of resistivity was provided. The Young resistivity distribution can be expressed as p(x) = pQ ехр(-лЛк), where p0 is the resistivity at the surface and X represents a characteristic length. Extraction of physical parameters: the frequency ranges, for which the impedance response is consistent with the CPE, are presented in Figure 3.23.