Chapter-VIII: Performance of Undoped & Doped ZnO Films.. Gas Sensor

Transcription

1 8.1 Introduction The solid state gas sensors based on ZnO, CuO, SnO 2 and other metal oxide compounds have important applications in gas alarms for the detection and monitor the various toxic & inflammable gases present at ppm concentration in the atmosphere [1, 2]. Apart from the gas sensing applications, it also includes barrier layer for solar cell, optoelectronic devices, liquid crystal displays and so on; due to their interesting properties, such as high electrical conductivity, high transparency, in the visible region and high thermal, mechanical and chemical stabilities [3]. Naayoshi Taguchi who developed a series of ceramic sensors called Taguchi Gas sensors (TGS). His company Figaro Gas Sensors manufactures millions of sensors annually. The principle behind gas detection is the surface reaction that takes place between the adsorbed oxygen and the gas to be detected. The conductivity of the sensor varies in the presence of oxidizing and reducing gases. Among the adsorbed species O - species is more prominent in gas sensors. The gas sensing properties are influenced by many factors, such as the operating temperature, microstructure of the powders used for the sensor fabrication, preparative parameters and the sensor geometry [4]. The desirable characteristics of a gas sensor are high sensitivity, fast response and recovery times, long term stability and the selectivity [5-8]. The sensitivity and selectivity can be tuned by doping it with various noble metals. While working with metal oxide loaded with noble metals, homogeneous distribution of noble metal atoms through the metal oxide would be used to introduce them and on the subsequent thermal treatments performed [9]. In order to enhance the gas performance, morphological features of the materials should be controlled during chemical synthesis resulting in high specific surface areas and uniform systems for the large pores leading to higher probability for a gas to interact with the semiconductor increasing the sensitivity of the materials [10]. 131

2 The combustion of fossil fuels results in the emission of gases, such as CO, NOx and hydrocarbons (HCS). It is imperative to monitor such gases in situ, during combustion process at higher temperature in order to improve the efficiency of combustion and to reduce the emission of hazardous gases, various types of gas sensors have been developed for the in situ monitoring of emitted gases. For example, optical techniques based upon ultraviolet and infrared adsorption have been used for gas detection [11]. Optical sensors generally gives good spatial and temporal resolution and non-intrusive. Today lots of attempts are made to develop chemi-resistive sensors made of semiconductors, such as SnO 2, ZnO, MoO 3, TiO 2 and Fe 3 O 4 [12-14]. However most of them can t be used at higher temperatures, For example sensitivity of SnO 2 sensor vanishes, when the temperature exceeds 723K. For harsh high temperature environments, semiconductor sensors are less attractive than solid state sensors with Yttri stabilized Zirconia (YSZ) as electrode, which have greater selectivity. In the present chapter the Zn x Cu 1-x O, Al doped ZnO and Pd doped ZnO films deposited by spray pyrolysis technique are characterized for low ppm H 2 S gas sensing applications. The gas sensing characteristics such as sensitivity, response and recovery time that determines the performance of H 2 S gas sensor are studied at length. 8.2 Experimental The samples of ZnO, CuO, Zn x Cu 1-x O, Al doped ZnO and Pd doped ZnO were prepared by using the optimized conditions obtained in the previous chapters. The ZnO samples prepared under optimized conditions by spray pyrolysis method are then tested for sensor application. The ZnO thin film is mounted in the gas chamber. Hydrogen sulphide gas of fixed concentration (ppm) is passed in the chamber. The change in resistance of the film with respect to time is recorded. Then by plotting the graph of resistance versus time one can determine the response and recovery time. By knowing the resistance of the film in air (R a ) and 132

3 in the presence of gas (R g ) one can determine the sensitivity. By comparing the values, higher sensitivity, fast response and recovery time, the best optimized sample for sensing H 2 S gas is selected. With the same procedure, the optimized films of Zn x Cu 1-x O, Al doped and Pd doped ZnO were tested for H 2 S gas sensor characteristics. 8.3 Results and Discussion Gas sensing performance of Zn x Cu 1-x O thin films Fig.8.1 represents the change in resistance of ZnO, Zn 0.6 Cu 0.4 O and CuO samples as a function of exposure time to H 2 S and subsequent exhaust of H 2 S gas. Generally resistive sensors such as ZnO and CuO are highly resistive at room temperature due to the lower carrier concentrations and adsorption of atmospheric oxygen on the film surface that develops a potential barrier (ф b ) to charge transport. Even at working temperatures below 573K, the response of the films to reducing H 2 S gas is limited by the rate of the chemical reaction; since the gas molecules do not have sufficient thermal energy to react with the surface adsorbed oxygen species. However, at 523K the thermal energy acquired by resistors especially ZnO [Zn x Cu 1-x O at x=1] thin film is high enough to reduce the potential barrier (ф b ) and H 2 S reacts to the film surface to inject the electrons into conduction band so as to increase the electron concentration substantially [15]. This in turn leads to a decrease in film resistance. The H 2 S sensing reaction [16] concerned herein can be seen as follows, H 2 S (g) + 2O - (ads) H 2 (g) + SO 2 (g) + 2e -... (8.1) Schematically this reaction is depicted in fig. 8.2 (a). Additionally, according to Dayan et al. the intrinsic defects on the ZnO surface can ionize and contribute electrons to decrease in the resistance of sensor [17]. Zn 2+ i Zn + i + e -... (8.2) On the other hand, increasing composition values x from 0 to 1 in Zn x Cu 1-x O alters the material electrical properties to increase the potential barrier. So, extra thermal energy is needed to overcome this barrier and resultantly H 2 S reduction 133

4 reactions occur at higher temperatures for Zn 0.6 Cu 0.4 O and CuO as compared to undoped ZnO. Therefore, both these sensors show change in resistance values due to reducing H 2 S atmosphere at 553K. The resistance of ZnO, Zn 0.6 Cu 0.4 O and CuO film increases with time when H 2 S gas is exhaust, due to adsorption of molecular oxygen from atmosphere on the film surface that captures electrons from the conduction band. Fig. 8.1 Variation in resistance of (a) ZnO, (b) Zn 0.6 Cu 0.4 O and (c) CuO as a function of time for a 10ppm H 2 S gas exposure and back. In this, the oxygen is adsorbed in the form of O - depending on the operating temperature [18]. O - 2 (ads) + e- 2O- (ads).. T= K. (8.3) The ample adsorption of oxygen creates depletion layers at the surface regions of ZnO, Zn 0.6 Cu 0.4 O and CuO films; thereby decreases carrier concentration and 134

5 increase the potential barrier (ф b ) and hence increases the resistance of the sensor film. This oxygen adsorption is represented in fig. 8.2 (b). Fig.8.2 Schematic illustration of chemical reactions on ZnO surface underlying H 2 S-sensing mechanism of ZnO nanocrystals, where denotes the potential barrier (ф b ). 135

6 Virtually, the usefulness of the metal oxide semiconductor gas sensors is determined by the response time; which is the time required to reach 90% response signal. Whereas sensor material should recover from the gas-surface reaction rapidly. This recovery time is defined as the time needed 90% of the original baseline signal. From Fig.8.2 comparison of response and recovery time of three different sensors under 10 ppm exposure to H 2 S, is possible. Fig. 8.3 exhibits that response and recovery times becomes shorter with increasing composition x values. i. e. CuO sensor response time is 95 sec and it recovers much slowly, with recovery time of 243 sec. Further Zn 0.6 Cu 0.4 O samples both response and recovery time reduces to 77 and 110 sec respectively. The ZnO film response time is 34sec and recovery time is 42 sec. Fig. 8.3 Response and recovery times (sec) for the ZnO, Zn 0.6 Cu 0.4 O and CuO films. 136

7 The Variation of the film resistance noted with time is used and sensitivity (S) for AZO films is evaluated using following formula, S R a R a R g (8.4) Where, R a is resistance in air, R g is resistance in testing gas i. e. H 2 S. Fig.8.3 shows the H 2 S sensing response with time for ZnO, Zn 0.6 Cu 0.4 O and CuO films with different composition values x. It is seen that the sensing response increase with increasing zinc content in the composite films. The CuO film exhibits minimum of 30% sensitivity at operating temperature of 553K. Whereas in case of Zn 0.6 Cu 0.4 O film the H 2 S sensitivity is enhanced to 74% at the same operating temperature. It is further seen that, film exhibits 97% sensitivity at comparatively lower temperature (553K). Fig. 8.4 Variation in sensitivity as a function of time for Zn x Cu 1-x O films with different x values. 137

8 This considerable enhancement in the sensitivity can be explained on the basis of structural properties. As per the XRD analysis, the undoped CuO thin films are almost amorphous in nature with about submicron sized grains; while with the increase in the composition x, phase transforms to the polycrystalline with nano sized crystallites and ZnO film exhibits minimum crystallite size. With decrease in grain size the specific surface area of film that exposes to the reducing H 2 S gas atmosphere increases; which in turn improves the sensitivity for ZnO film [19, 20] Gas sensing performance of ZnO: Al (AZO) thin films Fig. 8.5 exhibits variation of resistance of the Al doped ZnO films with time, when exposed to H 2 S gas of 10 ppm at an operating temperature of 473K. Fig. 8.5, it is observed that the resistances of all samples decrease exponentially with time (H 2 S ON). This happens because of the reducing action of the H 2 S; which introduces electrons in the conduction band. This leads to the increase in the carrier concentration of AZO; thereby diminishing potential barrier (ф b ) and decrease in resistances of sensors is observed [15]. The reaction involved in this case is similar to the one given by the eq. (8.1) and can be represented by Fig. 8.2(a). Besides, the ZnO surface intrinsic defects can also ionize and decrease the resistance [17]. However, the factor by which the resistance changes due to H 2 S exposure; differs from sample to sample. This is due to the intrinsic electrical properties of AZO thin films; that depend on the growth (substrate) temperature. On the other hand, H 2 S is removed (H 2 S OFF) molecular oxygen from the surrounding adsorbs on the AZO surface that raises AZO film resistance by trapping the conduction band electrons. The depletion layers formed on the surface of AZO film after sufficient oxygen adsorption decreases carrier concentration and hence increases the resistance of the film sensor. This increase in the potential barrier due to oxygen adsorption as represented in Fig. 8.2(b). 138

9 Fig.8.5 Electrical resistance (ohm) versus time (sec) plot for AZO thin films prepared at different substrate temperature, under 10 ppm H 2 S exposure and at 473K operating temperature. Fig. 8.6 illustrates the variations in these response and recovery time for the AZO film synthesized at various substrate temperatures. Under the exposure of 10 ppm H 2 S gas at 473K operating temperature, the AZO film deposited at 698K exhibits a fastest response time of 27 sec and the response is slowed down for substrate temperatures and AZO film deposited at 773K showed maximum response time of 40 sec. For further higher substrate temperature the response time decreased. The recovery characteristics of AZO sensors are recorded by draining out the H 2 S gas from the gas chamber. The recovery time is high (299sec) at lower substrate temperature and it decreases with increase in substrate temperature. It is minimum at 773K substrate temperature & for further higher substrate temperature it increase. The change in activation energy of adsorbed species description. 139

10 Fig. 8.6 Response and recovery times (sec) for the AZO thin films deposited at various substrate temperatures. The H 2 S sensitivities of AZO films evaluated using eq. (8.4), for 10 ppm H 2 S gas exposure and at 473K operating temperature are measured as a function of time as shown in fig The AZO thin film prepared at 698K shows a maximum sensitivity of 49%. However, the sensor sensitivity decreases with the increasing growth temperature and the sample prepared at 773K exhibits least sensitivity of 16%. For 798K, the sensitivity again increases to 33%. This change in the sensitivity depends upon the grain/crystallite size variations. According to V. Musat et al the gas sensitivity increases when the grain size decreases [19]. The ZnO: Al film prepared at 698K with minimum crystallite size possesses larger 140

11 specific surface area, which results in better oxygen adsorption and higher sensitivity to H 2 S [20]. Fig. 8.7 Sensitivity (%) as a function of the time (sec) for AZO films prepared at different substrate temperature Gas sensing performance of ZnO: Pd thin films The change in resistance of various ZnO: Pd samples as a function of time for 10 ppm H 2 S gas exposure at an operating temperature of 473K is shown in fig The resistance is decreased for each sample, when a sample is subjected to a reducing H 2 S atmosphere. Upon introduction to H 2 S, the surface oxygen is consumed due to the chemical reaction (eq. 8.1), contributing a few electrons reversely and thus declining potential barrier [15] [similar to fig. 8.2(a)] and then electrical resistance. 141

12 Fig.8.8 Electrical resistance (ohm) versus time (sec) plot for Pd: ZnO thin films prepared at (a)0.5, (b)1.0, (c)1.5 and (d)2.0 wt% palladium concentrations, under 10 ppm H 2 S exposure and at 473K operating temperature. Moreover, with the removal of H 2 S, molecular oxygen gets adsorbed [eq. 8.3] on the film surface to trap the electrons and increases the potential barrier as can be perceived from fig. 8.2(b). Consequently an enhancement in the film sheet resistance is observed for all samples. It is also observed that the recovery time increases with increase in Pd doping. 142

13 Fig. 8.9 Response and recovery times (sec) for the ZnO: Pd thin films deposited at various palladium concentrations (wt %). Fig.8.9 gives the response and recovery times of different ZnO: Pd resistors for a gas inlet of 10 ppm H 2 S. At the working temperature of 473K, the response time goes on decreasing with the increasing Pd doping concentration and becomes least of 72 sec for 1 wt%. However for further increase in doping concentration, the response was delayed. On the other hand, the recovery time for the film is much slower compared to response time in all samples. The 1.0 wt% Pd doped ZnO thin film recovered much faster than the other samples. 143

14 Fig Sensitivity (%) as a function of the time (sec) for ZnO: Pd films prepared at different palladium concentrations, in a H 2 S atmosphere of 10 ppm and at 473K. Fig.8.10 represents the sensitivity characteristic of different Pd-doped ZnO films as a function of time with 10 ppm H 2 S exposure and operating temperature of 473K. it is seen that the sensitivity is increased with the increasing Pd concentration in ZnO and reaches maximum of 49% for the 1.5 wt% Pd doped film. For further increase in the Pd concentration the sensitivity to H 2 S is reduced to 29%. These observations indicate that the decrease in the operating temperature and the variations in sensitivity are influenced by the palladium content in ZnO. In this case, the changes in grain size have proportionally altered the H 2 S response of ZnO: Pd resistors [19, 20]. 144

15 References 1. G. Behr, W. Fliegel, Sens. Actuators B (26-27) (1995) M. S. Berberich, J. G. Zhang, U. Weimer, W. Gopel, N. Baran, E. Pentia, A. Tomenscu Sens. Actuators B 31(1996) G. Faglia, E. Comini, A. Cristalli, G. Sberveglieri, L. Bori, Sens. Actuators B 55(1999) T. Aste, D. Berudi, R. Botter, C. Ciccarelli, M. Givedani, P. Pizzolini, Sens. Actuators B 55(1994) Oomman K., Varghese, L. F. Malhotra, G. L. Sharma, Sens. Actuators B 55(1999) Y. Zhao, Z. Feng, Y. Liong, Sens. Actuators B 56(1999) S. Mishra, C. Ghanshyam, N. Ram, S.Singh, R. P. Bajpai, R. K. Bedi, Bull. Mater. Sci. 25 (2002) C. Bittencourt, E. Llobet, P. Ivanov, X. Correig, X. Vilanaio, J. Brezmer, J. Hubalek, K. Malysz, J. J. Pireauxg, J. Calderer, Sens. Actuators B 97 (2004) P. Ivanov, E. Llobet, X. Vilanova, J. Brozmer, J. Hiebalet, X. Correig, Sens. Actuators B 99 (2004) N. Schweizer Berberich, S. Strathmann, U. Weinar, R. Sharma, A. Seube, a. Peyre Lavigne, W. Gopel, Sens. Actuators B 58(1999) H. Teeicheri, T. Fernhotz, Ebert V, Appl. Opt, 42(2003) N. Yamazoe, N. Miura, IEEE Packing Manuf. Tech. A 18 (1995) S. Seal, S. Shukla, JOM 54 (2002) F. Menil, V. Coillard, Sensors and Actuators B 67( 2000 ) H. Windischmann, P. Mark, J. Electrochem. Soc. 126 (1979) S. Kar, B. N. Pal, S. Chaudhuri, D. Chakravorty, J. Phys. Chem. B 110 (2006)

16 17. N. J. Dayan, S. R. Sainkar, R. N. Karekar, R. C. Aiyer, Thin Solid Films 325 (1998) K. Arshak, I. Gaidan, Mater. Sci. Eng. B118 (2005) V. Musat, A. M. Rego, R. Monteiro, E. Fortunate, Thin Solid Films 516 (2008) Y. Zhu, Y. Fu, Q.-Q. Ni, Appl. Surf. Sci. 257 (2011)