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Simultaneous Adsorption-Desorption Processes in the Conductance Transient of Anatase Titania for Sensing Ethanol: A Distinctive Feature with Kinetic Perceptive Priyanka Das, Biswanath Mondal, and Kalisadhan Mukherjee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10041 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016
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The Journal of Physical Chemistry
Simultaneous Adsorption-Desorption Processes in the Conductance Transient of Anatase Titania for Sensing Ethanol: A Distinctive Feature with Kinetic Perceptive Priyanka Dasa,b, Biswanath Mondala,b, Kalisadhan Mukherjeea,b* a
Centre for Advanced Materials Processing, CSIR-Central Mechanical Engineering Research Institute, Durgapur713209, India b
Academy of Scientific and Innovative Research, CSIR-Central Mechanical Engineering Research Institute, Durgapur-713209, India ABSTRACT: A distinctive feature in the operating temperature dependent conductance transients of titania sensor is identified during chemi-resistive type detection of ethanol vapor. Precisely, simultaneous adsorption of ethanol and desorption of the corresponding oxidized product over anatase titania sensor are reflected systematically in the response transients measured with the rise in operating temperature. An attempt is made here to understand the origin of the said feature based on the reaction sequences of ethanol over sensor surface. For a fixed ethanol concentration (200 ppm), the conductance transients obtained for response process are modelled using Langmuir–Hinshelwood reaction mechanism and the characteristic time constants are estimated for adsorption and desorption of ethanol. From the temperature variations of these characteristic time constants, the activation energies for the adsorption and desorption of ethanol over sensor surface is estimated. In addition, the general ethanol sensing characteristics (response %, response time etc) of the anatase titania is also reported by varying the ethanol concentration (50-500 ppm) and sensor operating temperature (275-375oC).
INTRODUCTION Semiconducting metal oxide (SMO) based chemi-resistors (e.g. SnO2, ZnO, TiO2, Fe2O3, ZnFe2O4 etc) are popular as simple, cost effective and sensitive sensors for the detection of various toxic as well as inflammable gases and volatile organics (e.g. H2, CO, NOx, CH4, C2H5OH, HCHO, CH3OCH3 etc).1-5 The principle of such chemi-resistive sensors is simply based on their change in surface resistance/conductance when exposed to reducing/ oxidizing gases/ vapors.6-7 For an ‘n’ type SMO, the resistance of the sensor in air (Rair) decreases in exposure to reducing gases/vapors finally to attain steady state value (Rvap).8 Thus, the resulted resistance/ conductance transients are basically the reflection of interactions (i.e. adsorption, desorption, diffusion etc) between gases and sensor surface. Depending on the nature of gas-sensor interactions, the patterns of resistance/ conductance transients may change. In most of the gas sensing studies, the consequences of such gas-sensor interactions in originating the resistance/ conductance transients are not much nurtured by the researchers. On contrary, attention is paid mostly to evaluate the response (%) (S), response/recovery times from the resistance/ conductance transients of the sensing element by varying the sensor operating temperature and concentration of target vapor. 8-10 The correlation between the interactions of gases on sensor surface and the nature of associated electrical response is not been fully understood till date and thus it remains a complicated but interesting topic of research. In this context, herein, we have addressed a generic feature of SMO chemi-resistive sensors citing the experimentally obtained resistance transients for anatase titania based SMO sensors towards the detection of ethanol vapor. It has been found that for the detection of fixed con-
centration (200 ppm) of ethanol, the pattern of resistance transients changes significantly with the variation of operating temperature. More specifically, adsorptiondesorption of ethanol vapor on sensor surface are found simultaneous and a systematic variation in the patterns of resistance transients are noted with the rise in sensor operating temperature. For a fixed ethanol concentration (200 ppm), the conductance transients obtained for response process (in the studied operating temperature range) are modelled using Langmuir-Hinshelwood reaction mechanism and the characteristic time constants are estimated for adsorption of ethanol and desorption of corresponding oxidized product. From the temperature variations of these characteristic time constants, the activation energies for the adsorption and desorption of ethanol are estimated. To the best of our knowledge and belief, identification as well as kinetic studies of aforesaid feature in resistance transients of SMO gas sensors is not reported till date. For a fixed sensor, if the said feature can be identified as a characteristic feature of a specific gas, it may provide valuable information towards the development of a selective sensor. Apart from addressing the aforementioned feature and modeling the conductance transients, in the present article, we have also reported the operating temperature (275-375oC) and concentration (50-500 ppm) dependent ethanol sensing characteristics (response %, response time etc) of anatase titania sensor.
EXPERIMENTAL Titania particles are synthesized by hydrolyzing the Tiisopropoxide in pure aqueous medium. The resultant precipitate is filtered, dried at room temperature and finally calcined at 450oC to achieve the anatase phase titania particles. The surface morphology and the phase formation behavior of the synthesized particles are studied us-
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ing electron microscope and X-ray diffractometer. The calcined particles are compacted in the form of circular disc (12 mm diameter, 2 mm thickness) and chemiresistive response towards ethanol vapor is measured using a static mode gas sensing chamber developed in the laboratory. The schematic (Figure S1) and the description of the gas sensing set up are provided as supporting information. The sensing characteristics are measured by varying the operating temperature (275-375oC) of the sensor and concentration (50-500 ppm) of ethanol. The details of the measurement set-up and the sensing measurement protocols are already described elsewhere.11 The response % (S) of the developed titania based sensing element for the detection of ethanol is estimated using Eq. 1.12
S
Rair REtOH 100 R air
(1)
where Rair and REtOH are the equilibrated resistance of the sensor in presence of air and ethanol.
RESULTS AND DISCUSSIONS Phase and morphological features of synthesized titania particles: Since the present article is mainly concerned with the evolution and kinetic study of a special feature in the resistance transients of the prepared sensing element, in depth analysis on the structural properties of the samples are not illustrated here. However, to identify the phase and morphology, we have included here the powder XRD pattern and FESEM images of the samples.
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Figure 1a shows the X-ray diffraction (XRD) pattern of prepared titania particles. The XRD pattern resembles well with the standard powder XRD pattern (JCPDS-card84-1286) of anatase phase titania particles. The FESEM image of the prepared particles is shown in Figure 1b which represents the nano size of the particles. Principle of observations:
gas
sensing
and
experimental
Prior to illustrate the experimentally obtained features of resistance transients for anatase titania sensor, it seems important to describe the principle of SMO based chemiresistive sensors for the convenience of readers. In Figure 2, the sensing mechanism of SMO sensors is explained schematically by presenting simultaneously the gassensor interaction and associated electrical response. As presented, the metal oxide based sensing element is composed of innumerable number of tiny particles. At elevated temperature, these particles become electronically depleted due to the chemi-adsorption of atmospheric oxygen and as a consequence, the resistance of the sensor becomes high.6,11 The oxygen chemiadsorbed sensor surface and the associated electrical response are symbolized as ‘A’ in figure. The reducing gases (Rgas) in exposure to the chemi-adsorbed oxygen become oxidized during response process and release the trapped electrons to the conductance band of the sensing material decreasing the resistance of the sensor.
(a)
Figure 2: Schematic representation of the gas-sensor interaction (left panel) and electrical response (right panel) for semiconducting metal oxide sensor.
(b)
Diminishing of electron depleted layer in sensing particles due to the interaction of reducing gases (Rgas) with the chemi-adsorbed oxygens (O-/ O2-) and associated resistance change are represented through the transition from state ‘A’ to ‘B’ in the figure. As denoted in the transformation from ‘B’ to ‘C’, the recovery of the sensor is achieved when it is exposed further in air. In ideal situation when the sensor reinstate completely during recovery, state ‘C’ should be equivalent to state ‘A’. The chemical reactions occur for response process i.e. the transformation from ‘A’ to ‘B’ is presented in Eq. 2. The
Figure 1: (a) XRD pattern and (b) FESEM image of TiO2 nano particles calcined at 450oC.
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reactions for recovery process i.e. the transformation from‘B’ to ‘C’ can be presented by Eq. 3 and 4 respectively:13 R gas Oad ROad e
(2)
ROad ROgas
(3)
O2 e O / O2 (chemi-adsorption)
(4)
Similarly, the reactions of ethanol on various metal oxide surface are also reported elsewhere.14-19 As mentioned by M. Rumyantseva et al., ethanol is oxidized on the metal oxide surface and ultimately produces CO2 and H2O through the formation of acetaldehyde (CH3CHO) or ethylene (C2H4) intermediates.16 C. Wang et al. in their published article also demonstrated the formation of CO2 and H2O due to the oxidation of ethanol on titania surface.18 The decomposition of oxidized intermediate of ethanol into CO2 and H2O on the perovskite type metal oxide surface is further reported by X. H. Wu et al.14 Similar studies on the oxidation of ethanol followed by the decomposition to CO2 and H2O on spinel structured metal oxide surface is reported by M. Cao et al.19 As reported in the literature, for the oxidation of ethanol following reaction sequences (Eq. 5 and 6) are operative on the metal oxide surface.18-19
C2 H5OH + O- /O2- (C2 H5 )2O
(5)
(C2 H5 )2O CO2 + H2O
(6)
In line to the description reported in the above literatures, we have also reported that ethanol (Rgas) is
oxidized on the titania surface resulting the product ROad which ultimately desorbed from the surface as RO gas. The ROad here represents (C2H5)2O and ROgas corresponds to the mixture of CO2 and H2O respectively. The consideration of CO2 and H2O gas as two different products may bring the influence of competitive adsorption-desorption phenomenon in our study which will further complicate the model. To make the model general and less complicated we have defined ROgas as the mixture of CO2 and H2O.
The chemi-adsorption of oxygen during recovery process (Eq. 4) can proceed by flushing air and follows the reaction for desorption of ROad to ROgas (Eq. 3). Hence, Eq. 3 can be considered as rate determining step for recovery process. Figure 3a-Figure 3e represent the experimentally obtained resistance transients of the sensor for the detection of 200 ppm ethanol at operating temperature ranging from 275-375oC. In each operating temperature, 2 nos of repeated response and recovery cycles are recorded. Figure 3f shows the operating temperature dependent response % of anatase titania for sensing 200 ppm ethanol. As reflected from the figure, within the studied operating temperature range, the response of the sensor increases with operating temperature and achieves maximum at ~ 375oC. It is important to mention here that due to the lack of experimental facilities, the operating temperature of the sensor can not be raised beyond 375oC. Critically comparing the response cycles presented in Figure 3a-Figure 3e), it is observed that the resistance transients starts to become inclined upwards systematically with the rise in operating temperature at equilibrium point. As described in schematic Figure 2, the upward inclination of resistance transient during
3 Figure 3: (a)-(e) Resistance transients of the titania sensor towards 200 ppm ethanol at different operating temperature (mentioned within the figures); Systematic change in the features of resistance transients are highlighted in figures. (f) Dependence of response % with the sensor operating temperature.
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response process is not expected. In the forthcoming section, we made an attempt to correlate the above mentioned feature based on the interaction of gases on sensor surface. Hypothesis on the origin of feature based on the principle of gas sensing and Lagmuir-Hinshelwood mechanism: The upward inclination of resistance transient at saturation point is assumed due to the simultaneous adsorption of ethanol and desorption of it’s oxidized product during response cycle. As discussed previously the desorption of oxidized products is generally feasible during recovery when the sensor is exposed in fresh air. However, the systematic change of resistance during response reflects that the desorption of oxidized products is possible at higher operating temperature.
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reaction where the formation as well as desorption of oxidized products occur consecutively. Here we have proposed that depending on the surface temperature of the sensor the reactions presented in Eq. 2, 3 may proceed consequitively as represented in Eq. 7. ka kd R gas Oad ROad ROgas
(7)
The schematic illustrations on the interaction of reducing gas (Rgas) with the sensor surface, formation of products and the intermediate reaction stages during response and recovery processes can be described in the energy diagram shown in Figure 5. The schematic resembles the Langmuir Hinshelwood mechanism where both reactants are first adsorbed on the sensing surface and then they collide to form the reaction products.20
Figure 5: Energy diagram for the response and recovery process of SMO sensor following LangmuirHinshelwood reaction mechanism.
Figure 4: Schematic of various types of resistance transients during response process (lower panel) originate due to nature of gas-chemiresistor interaction (upper panel). The excess air present within the sensing chamber assists the desorption process at high temperature during response process. The phenomenon can be represented schematically using Figure 4. As presented, during response process, the higher temperature may extend the usual transformation from ‘A’ to ‘B’ upto the partial formation of state ‘C’ (C1 / C2) and accordingly lead to the change in features of respective resistance transients. Thus as observed the reactions occurred during response at higher temperature can be described as consecutive
As described in the figure the formation of state A to state B during response process is feasible via the transition state # 1 (TS #1). The presence of reducing gas (Rgas) and high surface temperature of the sensor favor the transformation of state A to B. The formation of state C from B via the transition state #2 (TS # 2) takes place when the sensor is further exposed in fresh air during recovery. For the observed feature presented in Figure 3aFigure 3e, here we have proposed that with the rise in operating temperature only, a part of oxidized product (ROgas) attains energy to overcome the energy barrier of TS #2 facilitating the desorption of ROgas from sensor surface and chemi-adsorption of excess oxygen present within the sensing chamber. As a consequence, the resistance transients show upward inclination at the equilibrium region of response process. Modelling of experimental results:
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Considering the formation of ROad and desorption to ROgas over sensor surface during response cycle is an operating temperature dependent consecutive process, we have proposed earlier that the sensor follows the reaction mentioned in Eq. 7. In accordance to the description presented elsewhere, it is assumed here that the site fraction θ is covered by O-ad on the sensor surface.20 Available unoccupied space is adsorbed by reducing gas (Rgas) i.e. ethanol which reacts with O-ad to form reaction product ROad. The rate constant for the 1st step formation of ROad in Eq. 7 is defined as ka. The rate constant for the 2nd step of Eq. 7 i.e. desorption of ROad to ROgas is taken as kd. Therefore the rate for the net formation of ROad can be presented as follows (Eq. 8) d[ROad ] dt k a [Oad ]R gas k d [ROad ]
are fitted using Eq. 9 and the fittings are shown in Figure 7a-Figure 7e respectively. The kinetic parameters (G0, G1, G1́, τ1, τ2) for 1st and 2nd step reactions estimated from the fitted response conductance transients are summarized in Table 1. As envisaged from the table, the change in conductance and characteristics time constants for the transients measured at 275 and 300oC are not mentioned since at these temperatures the desorption of products and upward resistance change are not observed.
(8)
For the studied titania sensor the formation of RO ad is linearly dependent with it’s transient conductance (G(t)) change. In line to the linear relation of RO ad with the sensor conductance value reported elsewhere one can represent the surface reactions of Eq. 8 in terms of conductance change as follows (Eq. 9) 21
G(t)response G0 G1[1 exp( t / 1 ) ] G1' exp( t / 2 )
(9) st
where τ1 and τ2 are the characteristic time constants for 1 and 2nd step reactions respectively. G0 is the initial conductance of the sensor and G1 and G1΄ are the value of conductance change for 1st and 2nd step reactions respectively
Figure 7: Fitting of conductance transients measured at (a) 275 (b) 300 (c) 325 (d) 350 and (e) 375oC. Table 1. Fitted parameters estimated from fitting of the conductance transients shown in Figure 7 Oper. Temp (oC) 275 300 325 350 375
G0 (S)
G1 (S)
G1́ (S)
τ1 (s)
τ2 (s)
R2 value
3.43e-09 4e-09 1.5e-09 1.5e-09 1.5e-09
4.2e-09 4.9e-09 6.7e-09 9.4e-09 1.41e-08
…… …… 2.4e-09 4.2e-09 5.2e-09
159 96 70 46 29
…… …… 321 208 170
0.96 0.99 0.99 0.98 0.95
Dependence of kinetic parameters with operating temperature and estimation of activation energy for adsorption and desorption:
Figure 6: Operating temperature dependent conductance transients of the sensor for detection 200 ppm ethanol. For the prepared anatase phase titania sensor, conductance transients recorded during response towards 200 ppm ethanol in the operating temperature range between 275-3750C are shown in Figure 6. These temperature dependent response conductance transients
As described, the oxidation of reducing gases with the chemi-adsorbed oxygen predominantly occur in the 1st step whereas desorption of oxidized product is prevalent in the 2nd step of the response cycle. The characteristic time constant for the 1st and 2nd step reactions usually follow the temperature dependence mentioned in Eq. 10 and Eq. 11 respectively.
1 0 exp[(EA / kT]
(10)
2 '0 exp[(ED / 2kT]
(11)
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where EA is the activation energy for the oxidation of reducing gas with chemi-adsorbed oxygen and ED is the activation energy for the desorption of oxidized product. From a linear fit between ln1/ ln2 vs 1000/T; the activation energies EA and ED can be estimated from the slope of plots presented in Figure 8a and Figure 8b. As envisaged from the figures the variations of ln1/ ln2 with 1000/T fit well in linear relationship. The activation energy for the oxidation of 200 ppm reducing gas with chemi-adsorbed oxygen (EA) and desorption of oxidized product (EB) are estimated 0.505 eV and 0.426 eV. The proximity in the values of EA and EB favors the desorption of oxidized product in the response cycle at high operating temperature.
Figure 8: Linear variation of characteristics time constants estimated for the (a) 1st step and (b) 2nd step of response process with operating temperature.
Figure 9: Resistance transients of the anatase titania sensor towards different concentrations (50-500 ppm) of ethanol.
The simultaneous oxidation of ethanol and desorption of oxidized product is observed also in presence of different concentrations of gases at higher operating temperature. Resistance transients of the titania sensing element (kept at ~ 375oC) for the detection of 50-500 ppm ethanol are shown in Figure 9. It is reflected from the figure that the similar feature i.e. the simultaneous oxidation of ethanol
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and desorption of oxidized product is also exhibited in the resistance transients of response cycle recorded for the detection of different concentrations of ethanol. However, the correlation between the observed features and the concentration of gas is not yet understood till date. The calculated response % (S) values for the detection of different concentrations of gases are mentioned in figure. It is notable that the in presence of different concentrations of ethanol, change in sensor resistance is ditinguishable and the response % for the detection of 50 ppm ethanol is ~ 38%. The response time of the sensing element for the detection of 50-500 ppm ethanol at ~ 375oC is also found within a minute. 4. Conclusions: In chemi-resistive gas sensing, the resulted resistance transients actually reflect the interaction between target gas and sensor surface. The correlation between the interactions of gases on sensor surface and the nature of associated resistance transients is not been fully understood till date and thus it remains a interesting topic of research. Features of resistance transient for a fixed sensor may be a gas specific characteristic which can provide also valuable information towards the development of sensor selective for a definite gas. The present work has identified a distinctive and systematic feature in operating temperature dependent resistance transients of anatase titania sensor during detection of 200 ppm ethanol. Precisely, simultaneous adsorption of ethanol and desorption of the respective oxidized product over sensor surface are reflected clearly in the response transient of sensor with the rise in operating temperature from 275 to 375oC. The underlying mechanisms for originating the aforementioned feature are discussed and the operating temperature dependent resistance transients are modelled in accordance to the LangmuirHinshelwood reaction. From the fitting of the model, characteristic time constants are estimated for adsorption of ethanol and desorption of respective oxidized product. The empirical relations of the estimated time costants with the operating temperature provide the values of activation energies for the adsorption of ethanol (~ 0.505 eV) and desorption of respective oxidized product (~ 0.426 eV) over sensor surface. Additionally, the present work also provides the operating temperature and ethanol concentration dependent response % of the developed titania sensor. The sensor promisingly detects ethanol vapor and the estimated value of response % for the detection of 50 ppm vapor is ~ 38 %. Supporting Information Schematic and the description of the gas sensing set-up is provided as supporting information.
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ACKNOWLEDGMENT The authors would like to express their gratitude to Director, CSIR-CMERI for his kind support to carry out the work. KM thanks DST, Govt. of India for providing him Inspire Faculty fellowship (Ref. DST/ IFA12-CH-43) and associated research grant. PD is thankful to DST, Govt. of India for supporting her fellowship. The authors also wish to acknowledge Central Research Facility, CSIRCMERI for providing the FESEM facility.
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Corresponding Author (15)
*Corresponding Author, Email:
[email protected];
[email protected] (16)
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