Hydrogen and ethanol sensing properties of molybdenum oxide nanorods based thin films: Effect of electrode metallization and humid ambience

Sensors and Actuators B 187 (2013) 611–621

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Hydrogen and ethanol sensing properties of molybdenum oxide nanorods based thin films: Effect of electrode metallization and humid ambience Navas Illyaskutty a,b,∗ , Heinz Kohler a,∗∗ , Thomas Trautmann a , Matthias Schwotzer c , V.P. Mahadevan Pillai b a

Institute for Sensorics and Information Systems (ISIS), Karlsruhe University of Applied Sciences, Moltkestr. 30, D-76133 Karlsruhe, Germany Department of Optoelectronics, University of Kerala, Kariavattom, Trivandrum 695581, Kerala, India c Institute of Functional Interfaces, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Karlsruhe, Germany b

a r t i c l e

i n f o

Article history: Received 20 July 2012 Received in revised form 26 March 2013 Accepted 27 May 2013 Available online 7 June 2013 Keywords: MoO3 nanorods Hydrogen/ethanol sensing Catalytic activity Non-dissociative/dissociative adsorption Spillover Electrode material Humidity

a b s t r a c t Nanostructured molybdenum oxide (MoO3 ) gas sensitive layers were prepared via RF magnetron sputtering and controlled post deposition annealing on Au and Pt inter-digitated electrodes (IDE), which are integrated onto alumina substrates. Sensitivity test measurements towards hydrogen and ethanol vapour at different concentrations in synthetic air under non-humid and humid ambience at isothermal (200 ◦ C and 300 ◦ C) conditions are presented. Extremely different response behaviour to the analytes depending on morphology of the sensing layer, operating temperature, background humidity and electrode material was observed. The humid ambience does not significantly change the sensitivity to H2 , however, it drastically diminishes the sensitivity to ethanol. At higher temperature (300 ◦ C), influence of electrode material (catalytic effect of Pt from the Pt-IDE) on the gas sensing performance of MoO3 layer is identified. Non-dissociative and dissociative adsorption of analytes on the sensing layer, reaction of the adsorbed analyte species with lattice oxygen and diffusion effects due to different layer morphologies were taken into account in order to account the diverse sensing behaviour. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The modern industrialized world is fuelling the need for efficient, miniaturized, reliable and low cost chemical sensing devices for various applications. Considerable efforts have been directed towards exploring novel potential sensing materials which can offer better sensitivity, stability, selectivity, low operating temperature and least moisture dependence. Recent advances in nanotechnology have facilitated the development of novel classes of metal oxide nanomaterials which can render enhanced gas sensing capabilities owing to their improved adsorption and surface reaction kinetics [1–4]. Moreover, nanostructured materials can offer reduced working temperature, miniaturized size and low power consumption for sensors [3]. The current trend has

∗ Corresponding author at: Institute for Sensorics and Information Systems (ISIS), Karlsruhe University of Applied Sciences, Moltkestr. 30, D-76133 Karlsruhe, Germany. Tel.: +49 7219251282; fax: +49 7219251301. ∗∗ Corresponding author. Tel.: +49 7219251282; fax: +49 7219251301. E-mail addresses: [email protected], [email protected] (N. Illyaskutty), [email protected] (H. Kohler). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.092

concentrated more towards the development and design of the novel forms of low dimensional materials including nanorods [1], nanowires [2], nanotubes [4], etc. for gas sensing applications. Nanostructured molybdenum oxide (MoO3 ) thin films have attracted augmented research interest for using as active elements in conductance-type gas sensors as it can form these types of nanomaterials due to its peculiar layered structure [5–8]. MoO3 has been subjected to investigate the gas sensing capabilities towards a variety of gases like H2 , CO, CH4 , C2 H5 OH, NO, NO2 , NH3 , etc. [5–23]. A brief summary of the recent attempts to study the gas sensing properties of molybdenum oxide and its additives is given in Table 1. However, relatively small effort has been made to examine the sensing properties and exact mechanism of sensing of MoO3 , compared to other semiconducting oxides like SnO2 or ZnO, because it exhibits a number of limitations (e.g. relatively low evaporating temperature ∼795 ◦ C compared to SnO2 (1127 ◦ C) and high resistivity (∼1010 m in film form)) [9,10]. Nevertheless, in the lower temperature range, it shows excellent sensing response and high selectivity in certain cases [5–7]. Moreover, it is reported that the sensing characteristics of MoO3 nanostructures can be modulated/improved by varying the preparation (fabrication techniques, introducing foreign materials) and operating environments

612

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Table 1 Summary of previous studies on the gas sensing properties of molybdenum oxide. Sensor material

Preparation method

Morphology

Operating temperature (◦ C)

Target gas

Year and reference

Pt/MoO3 /La2 O3 MoO3 ␣-MoO3 /SnO2 Single crystalline ␣-MoO3 MoO3 MoO3 MoO3 MoO3 MoO3 ␣-MoO3 MoO3

Thermal evaporation Ultrasonic synthesis Hydrothermal/wet chemical Topochemical Solvothermal Probe sonication route Thermal evaporation Hydrothermal Commercial Infrared irradiation of Mo foil Sol–gel

Nanoplatelets Nanorods Nanobelts Nanoplates Nanoparticle Microrods Lamellar/nanobelts Nanorods – Nanorods –

25–300 290 120–300 260–400 375 200–300 275 240–420 200–500 200 275–400

H2 NO2 Ethanol Ethanol Various gases Trimethylamine NO2 and H2 NO2 and NH3 NO, NO2 and CH4 CO and CH3 OH NH3 , H2 and LPG

2012 [5] 2012 [6] 2011 [7] 2011 [8] 2010 [9] 2010 [10] 2010 [11] 2006 [12] 2006 [13] 2005 [14] 2004 [15]

(operating temperature, various ambience) [5,6,12,13]. Concerning the sensing mechanism of MoO3 , unlike other metal oxides like SnO2 or ZnO, the gas detection by MoO3 mainly directs by the lattice oxygen rather than chemisorbed oxygen as MoO3 does not chemisorb oxygen owing to its peculiar orthorhombic structure with double layers of distorted MoO6 octahedra. The lattice oxygen from MoO3 layer catalytically oxidizes the analyte gas and simultaneously reduces, which determines the change in conductivity [9,14–16]. In the present work, molybdenum oxide nanostructures are subjected to gas sensing studies with special emphasis given to the influence of morphological variation, operating temperature, background humidity and inter-digitated electrode (IDE) metallization (Au and Pt) on the sensitivity towards different gas components, such as hydrogen (H2 ) and ethanol (C2 H5 OH or EtOH) vapor, at non-humid and humid conditions. Moreover, substantial interests have been directed towards the determination of parameters that contribute to the electrical conductivity and gas response behaviour of the MoO3 sensing layers, with aim to improve the performance and to understand a more detailed

way of the reaction mechanism involved in the gas detection process. The effect of electrode materials has to be taken into account while investigating the gas detection process and mechanism of sensing of metal oxide gas sensors. The metallic electrode materials (usually Au or Pt) used in semiconductor gas sensors are also utilized as catalytic activators by means of admixing or decorating the layer surfaces etc. [24,25]. Therefore, it is quite possible that the electrode materials act as catalysts can influence the overall conductance and gas sensing performances [26–28]. It is reported that the presence of electrode material results in a contact resistance in the electrode-metal oxide interface which can have significant contribution to the response of the sensors [28,29]. Therefore, any difference in the transport and sensing properties has to be attributed to the electrodes or to the interaction between the electrode metal and the metal oxide sensing layer as well. However, the effect of electrode material on the sensing mechanism and characteristics of sensor performance of MoO3 has not been fully investigated. In the present study, metal IDEs are used to get practically low resistance values of MoO3 sensing layers, besides, the

Fig. 1. MoO3 thin film sensing layers on (a) Au and (b) Pt–IDEs. Sensor chip size: 7 mm × 7 mm, IDE-spacing/IDE finger width: 60 ␮m, finger length: 4.825 mm. (c) Pt-heater structure with contact pads at the lower left and upper right corner. (d) Sensor chip mounted on TO 8 header.

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Fig. 2. Gas exposure sequence over time applied for the sensitivity measurements. The analyte gases are mixed with synthetic air at various concentration steps 250, 500 and 1000 ppm for duration of 90 min each under non-humid and humid (75% rH at 21 ◦ C) ambience.

influence of electrodes on the sensing properties is also investigated by comparing samples deposited on different electrode materials, namely Au and Pt. Moreover, the present study also aimed to explore the effect of humidity on the sensing performance of the MoO3 layers. 2. Experimental The sensor test structures comprising Au and Pt inter-digitated electrodes (IDEs) on one side (Fig. 1a and b) and platinum heater on the reverse side (Fig. 1c) of alumina substrates are fabricated in miniaturized form of size 7 mm × 7 mm by thin film vacuum technique (DC magnetron sputtering), photolithography and plasma etching. Nanostructured MoO3 sensing layers are deposited on the IDEs by RF magnetron sputtering and controlled subsequent annealing at 500 ◦ C for 60 min in atmospheric air. The annealing temperature variations are imparted very slowly by maintaining the raise and drop of temperature at a rate of 10 and 5 K/min, respectively using a programmed furnace. Pressed powder of molybdenum oxide (Merck, purity 99.99%) was used as target. The sputtering was carried out in pure Ar atmosphere (∼0.05 mbar) with RF power 150 W operating at 13.56 MHz. The optimized parameters for the preparation of the nanostructures by RF magnetron sputtering are substrate to target distance – 5 cm, deposition time – 60 min, and base pressure –10−6 mbar.

613

Surface morphology of the nanostructures on the Au and Pt IDEs was analyzed using an environmental scanning electron microscope, ESEM XL 30 FEG (Philips). The compositional analysis was carried out using energy dispersive X-ray (EDX) set up attached to ESEM. Crystalline structure and crystallographic orientation of the MoO3 nanostructures were investigated by using Bruker D8 Advance X-ray diffractometer equipped with X-ray source of wavelength  = 0.154 nm (Cu K-alpha). Micro-Raman spectra of the films were recorded using a Horibo Jobin Yvon LabRAM HR 800 Spectrometer of spectral resolution of about 1 cm−1 equipped with an Ar+ ion laser at an excitation wavelength, 488 nm. Micro welding technique was used to contact gold wires (50 ␮m diameter) to the IDEs and heater pads, and then to mount the sensor test structures on TO8 headers (Fig. 1d). An automated standard gas sensing measurement setup [30] was used for the characterization of the MoO3 gas sensitive layers towards H2 and EtOH vapour at different concentrations in synthetic air under non-humid and humid conditions. The flow-through technique was used to test the gas sensing properties of the layers; a constant flux of synthetic air (100 ml/min) was used as gas carrier, into which the desired concentration of analytes were mixed. The Pt heaters were calibrated for the desired operating temperatures by applying voltages according to the heater resistances, and the homogenous distribution of temperature over the sensing layers was affirmed by IR camera (DIAS PYROVIEW 380L) analysis. The test structures were kept at ageing for one week in atmospheric air and one week at constant synthetic air flow before starting the gas exposure experiments. The gas exposure sequence over time used at two operating temperatures, 200 ◦ C and 300 ◦ C for the sensing test of H2 and EtOH is given in Fig. 2. The sensors were operated continuously ∼3–6 months without instability of the sensing behaviours at given operating temperatures. However, the operating temperatures above 350 ◦ C were found to distress the layer stability when operated for longer periods.

3. Results and discussion 3.1. Morphology and crystallography The ESEM analysis (Fig. 3) shows that the electrode metallization has profound influence on the formation of nanostructures on the Au and Pt-IDEs. The film on Au-IDE exhibits the presence of uniformly distributed densely packed tiny nanorods over IDE finger and alumina with high porosity, whereas, the layer on Pt-IDE shows smooth dense morphology on alumina and bigger grains over Pt finger. The nanostructures formed on the alumina and Au finger (Fig. 3) are similar suggesting that inter-grain migration over the alumina and Au fingers is possible during growth i.e. the presence of Au fingers did not affect the grain growth. The dissimilar formation of grains on alumina and Pt-IDE finger suggests that the presence

Fig. 3. ESEM images of the MoO3 sensing layers distributed over different regions of gold (a) and platinum (b) IDEs. ‘Al’ represents the alumina spacing between the fingers of Au/Pt.

614

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Fig. 4. ESEM images of the MoO3 sensing layers after measurements for approximately 6 months. (a) Layer on Au-IDE and (b) layer on Pt-IDE.

of Pt electrode influenced the growth of MoO3 layer, which has to be considered for further investigations. Compared with conventional thin/thick films, the porous nanostructured films are well defined building blocks for enhanced gas sensing. The nano-porous morphology of the layers may give advantages of high surface-tovolume ratio and results in enhanced adsorption cross section to analytes and short detection time. Moreover, the large porosities may act as active sensing sites and effective diffusion pathways of analyte molecules into the sensing layer [7,12]. The sensor layer morphology was analyzed again after the gas sensitive measurements for a period of ∼6 months and is shown in Fig. 4. Interestingly, a tendency of degradation of the quality of nanorod morphology of the layer on Au-IDE is observed, however, considerable change in the morphology of the layer on Pt-IDE is not seen. The XRD analysis (Fig. 5) supports the dissimilar morphology of the layers on different IDEs. The XRD peaks of MoO3 layers on both type of IDEs can be indexed to orthorhombic ␣-MoO3 [Ref. Code: 00-005-508; space group Pbnm; orthorhombic symmetry, lattice constants a = 0.3962 nm, b = 1.3858 nm and c = 0.3697 nm]. Basal spacing d0 1 0 /2 = b/2 of the layer on Au-IDE indicates the anisotropic growth of nanostructures in (0 k 0) direction [31]. The change in morphology of the layer on the Pt-IDE might be due to the change in predominant direction of growth (0 k l) (Fig. 5c) which is more similar to the XRD of the MoO3 powder (Fig. 5a). Typical microRaman spectra of the MoO3 nanostructures on different substrates are shown in Fig. 6. All the layers exhibit sharp and strong Raman

Fig. 5. XRD diffractograms of (a) MoO3 pure powder and MoO3 thin-film gas sensitive layer over (b) Au and (c) Pt. The shaded peaks belong to alumina substrate.

Fig. 6. Micro-Raman spectra of the MoO3 nanostructured gas sensitive layers on Au (a) and Pt (b) IDEs.

bands at ∼996, 820 and 666 cm−1 , and several week bands correspond to characteristics of orthorhombic ␣-MoO3 phase [32] which is consistent with the results of XRD analysis. Detailed Raman vibrational analysis of MoO3 bands can be found elsewhere [31,32]. The EDX spectra (measured over the layer on alumina part of IDEs) displaying the characteristic X-ray peaks originating from Mo and O are shown in Fig. 7.

Fig. 7. EDX spectra of the MoO3 gas sensitive layers on Au and Pt IDEs. The area selected for the EDS analysis is not shown in the ESEM images as it is analyzed through wide area scan over the layers.

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

3.2. Gas sensing 3.2.1. Hydrogen sensing The gas sensing response of the MoO3 layers on Au and Pt IDEs to hydrogen at two operating temperatures 200 ◦ C and 300 ◦ C under the gas exposure sequence (Fig. 2) are given in Fig. 8. At 300 ◦ C, by the primary exposure of synthetic air, the sensor baselines of the layers on both types of IDEs stabilize after few min, whereas, the baselines seem to need more time to get stabilized at 200 ◦ C. As temperature increases from 200 ◦ C to 300 ◦ C the initial conductance baseline enhances approximately an order of magnitude for the layer on Au-IDE from 3.55 × 10−8 to 3.72 × 10−7 , whereas in the case of Pt-IDE the variation is from 2.11 × 10−8 to 2.82 × 10−7 . This enhancement in the conductance baseline may be attributed to the temperature dependent concentration of oxygen vacancies. The oxygen vacancies in MoO3 result in the formation of intragap states lying just below the conduction-band edge which can act as shallow donors. At lower temperature, a large fraction of these states are not ionized which results in high resistance. The intragap states ionize at higher temperatures and the free-carrier electron density increases. Moreover, phonon-assisted tunnelling is also expected to enhance the conductance at elevated temperatures [33]. At 200 ◦ C, the sensing layers show good responses to H2 at 0% and 75% rH which are quite stable and moderately reversible at the raising and dropping concentration steps for the layer on Au-IDE, and however, not well reversible for the layer on Pt-IDE. At both temperatures, upon the exposure of H2 and humidity the layer on Au-IDE shows an initial quick response and it gets rather equilibrated over time. The response times of the layer on Au-IDE at

615

200 ◦ C and 300 ◦ C are 270 and 170 s, respectively. The response time of the layer on Pt-IDE is much higher compared to the layer on AuIDE at both operating temperatures. At higher temperature (300 ◦ C) by the introduction of water vapour, the signal of the sensing layer on Au-IDE reaches maximum value within 90 s and the conductance saturates at a value lower than the maximum. Moreover, the subsequent sensitivity action to H2 shows more or less good reversibility at the stepwise changes of the H2 gas concentrations at both non-humid and humid conditions. At both temperatures, the response of the layer on Pt-IDE to H2 shows different conductance behaviour compared to the layer on Au-IDE; a slow gradual increase of conductance instead of the initial hike observed at Au-IDE and the signal hardly equilibrates. The presence of humidity shifts the conductance baseline value of the sensing layers to higher values. The sensing layer on Au-IDE shows comparable sensitivity to H2 at both operating temperatures at humid and non-humid conditions, however, in the case of the layer on Pt-IDE the sensitivity increases dramatically at 300 ◦ C compared to lower temperature (Fig. 9a and b). At 200 ◦ C, the sensitivity of the layers on Au and PtIDEs are comparable, whereas, the layer on Pt-IDE shows a dramatic enhancement in sensitivity at 300 ◦ C which is much higher than the sensitivity of the layer on Au-IDE (Fig. 9b). The sensor response of the layer on Pt-IDE at 300 ◦ C shows that the sensitivity profile is reproducible at humid and non-humid conditions, with higher conductance at humid conditions. The sensing layer on the Au-IDE recovers 75% of the conductance within very short time (100 s) by the removal of H2 /humidity which is much faster as compared to the sensing activity of the layer on Pt-IDE. For the interpretation of the gas response behaviour to the analytes, probably surface morphology of the MoO3 layers on different

Fig. 8. Gas sensitivity response of MoO3 layers on Au and Pt IDEs towards hydrogen at different concentrations, humidities (0% and 75% rH at 21 ◦ C) and operating temperatures (200 ◦ C and 300 ◦ C) as per the concentration sequence profile given in Fig. 2.

616

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Fig. 9. Sensor response of MoO3 layers on Au and Pt IDEs towards Hydrogen and ethanol at different operating temperatures (200 ◦ C (a) and 300 ◦ C (b)), analyte concentrations and relative humidities (0% and 75% rH at 21 ◦ C).

IDEs and competitive reactions of analyte species with MoO3 surface layer at non-humid and humid ambience, which proceed with different kinetics at 200 ◦ C and 300 ◦ C, have to be considered. Moreover the catalytic effect of platinum has to be taken into account in the case of sensing properties of the layer on Pt-IDE. The following models are proposed in order to interpret the conductance behaviour of the MoO3 sensing layers to the exposure of H2 or/and H2 O (Fig. 8). Two different types of adsorption (non-dissociative and dissociative) of hydrogen and water vapour are assumed to be taking place on the sensing layers since the gas detection process in MoO3 is mainly directed by the lattice oxygen. The initial quick hike in conductance observed with the layer on Au-IDE by the exposure of H2 in dry air at both operating temperatures can be attributed to the donor effect of adsorbed hydrogen [34,35]. These fast reactions of H2 at the surface layer can be represented as H2(gas) → H2(ads) → H(ads) + H(ads)

(1)

H(ads) → H+ + e− (ads)

(2)

This quick process seems to be activated on the surface of the layer of Pt-IDE at both operating temperatures as well. The absence of the initial hike in conductance from the layer on Pt-IDE can be attributed to the slow equilibration reaction to H2 due to slower gas diffusion into the layer, which is a consequence of its more dense morphology compared to the nano-porous layer on Au-IDE (Fig. 4), where the diffusion is assumed to be much faster. At 200 ◦ C, after initial hike in conductance of the layer on AuIDE, the fast reaction equilibrates and the signal stabilizes at higher level, however, at higher temperature (300 ◦ C) the sensor signal seems not to be equilibrated and the conductance stabilizes at lower values. This sensitivity action may attribute to a slow reaction which proceeds with higher temperature activated interaction of H+ with lattice oxygen to form either surface hydroxyl groups or hydrogen molybdenum bronze (Hx MoO3 ) [36,37]. The hence forming hydroxyl groups may have an electron acceptor effect (Eq. (3)) which would explain the reduction in conductance after initial hike (Fig. 8). 2H+ + 2e− + 2Oo(s) → 2OH(ads) (ads)

− 2H(ads) + Oo(s) → H2 O(ads) + V2+ o + 2e

(5)

After this process (Eq. (5)), the MoO3 sensing layer becomes nonstoichiometric and can be represented as MoO3 → MoO3−x

(6)

The electrical conductance of the non-stoichiometric MoO3 is expected to be higher compared to stoichiometric MoO3 because of the presence of lower valent molybdenum ions, i.e. the formation of oxygen vacancies generates free electrons to the bulk. This would explain the conductance stabilization just below the maximum conductance value obtained at the initial hike. Moreover, the possibility of formation of hydrogen molybdenum bronze (Hx MoO3 ) along with the sub-stoichiometric MoO3 has to be considered, as MoO3 can form Hx MoO3 as an intermediate phase during the catalytic reduction of MoO3 by H2 [36,37]. The electrical conductance of Hx MoO3 is accounted to be higher compared to stoichiometric MoO3 [39,40] which may also contribute to the stabilization of the sensor signal after initial hike. The reactions (3) and (4) are believed to be less prominent at the layer on Pt-IDE at 300 ◦ C, where the sensitivity action dominantly proceeds through the reaction (2), which may be favoured by excess hydrogen spilled, from the catalytic dissociation of hydrogen molecule over Pt, across Pt/MoO3 interface. The recovery process of the sensing layers after withdrawal of H2 involves kinetics of desorption of the reaction product from the oxide surface and subsequent adsorption kinetics of oxygen molecules from the ambient [15]. Depletion of oxygen at the surface is replenished with movement of oxygen planes from interior of the grains towards the surface accompanied by the formation of shear planes in the MoO3 structure [15,41]. As an alternative, Raju et al. and Prasad et al. reported that the oxygen vacancies in MoO3 could be replenished by re-oxidation of the oxide surface with gaseous oxygen through the reaction [19,23]. V2+ o +

1 O2 + 2e− ↔ Oo(s) 2

(7)

(3)

where, (s) and (ads) indicate surface sites and adsorbed species, respectively. The sensitivity action may further progress through desorption of hydroxyl groups by forming water (Eq. (4)), leaving oxygen vacancies which may result in the formation of reduced form of MoO3 (Eq. (6)) [38]. − 2OH(ads) → H2 O(ads) + Oo(s) + V2+ o + 2e

The overall reaction directing the sensitivity action [(3)+(4)] can be stated as

(4)

3.2.2. Hydrogen sensing in humid ambience The MoO3 layer on Au-IDE shows an initial quick response upon the introduction of water vapour at both operating temperatures, which is subsequently followed by the saturation of the signal. This quick response in conductance indicates a fast process by nondissociative adsorption of water (Eq. (8)) in its molecular form with donor effect [34,35], H2 O(gas) → H2 O(ads) → H2 O+ + e− (ads)

(8)

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

which is assumed to be limited to a pure surface process. Réti et al. [35], who observed similar resistance-change behaviour with humidity on sputtered ␤-Ga2 O3 thin-film layers, assumed a non-dissociative adsorption of water according to the reaction (8). By this surface process the adsorbed molecular water generates free electrons to the MoO3 sensitive layer which results in initial sharp response. This quick process is assumed to be activated on the surface of the layer on Pt-IDE at both operating temperatures as well, however, not well prominent due to the limited diffusion through the dense layer morphology of the layer of Pt-IDE. After the initial hike, the conductance behaviour of the layer on Au-IDE progresses differently at the two operating temperatures. At 200 ◦ C, the sensor signal seems to be stabilized at a higher conductance level indicating the equilibration of reaction (8), whereas, at 300 ◦ C, the conductance drifts to lower values and then stabilizes below the highest value. This suggests that the quick response to humidity is superposed by a slow reaction process at 300 ◦ C, which seems not to be equilibrated even after having finished the sequence with 75% rH exposure. This conductance behaviour is observed with the layer on Pt-IDE at 300 ◦ C as well, however, not prominent compared to the layer on Au-IDE. The slow decrease of conductance following the quick step at 300 ◦ C, which is not observed at 200 ◦ C, can be attributed to a second slow reaction step which progresses simultaneously with process (8). In this slow reaction, the adsorbed water molecule is assumed to dissociate to form hydrogen and hydroxyl groups [34,35,38]. H2 O(ads) → OH(ads) + H(ads)

(9)

Indeed, both kinds of water adsorption, the free undissociated water and the Mo-OH strongly adsorbed states, were found simultaneously at MoO3 thin films by IR reflectance spectroscopy after exposure to humidity [42]. In the former process, water vapour was assumed to penetrate into the voids of MoO3 films [42,43] and the high porosity of the MoO3 thin film layer favours the fast and high quantity adsorption at the surface of the nanorods. It was shown that this process is a pure surface adsorption effect [44]. According to the investigations on MoO3 films [42,45] and WO3 films [46] the second step reaction with water vapour forming adsorbed OH groups (Eq. (10)) is assumed to be a slow volume chemical reaction. −Mo − O − Mo − O + H2 O → HO(ads) − Mo − O − Mo − OH(ads) (10) The adsorption of the OH groups on molybdenum ion may be associated with an acceptor effect (Eq. (11)) as already assumed for Ga2 O3 in [35] which could explain the slow decrease of conductance at humid atmosphere after initial hike (Fig. 8). −

−

OH(ads) + e → OH

(11)

In addition, the hydrogen formed during the dissociation of water (Eq. (9)) may react with the lattice oxygen according to the following reactions [38]. − H(ads) + Oo(s) → OH(ads) + V2+ o + 2e

(12)

+ − − OH(ads) + V2+ o + 2e → (OH) + e

(13)

The overall reaction directing the sensitivity action can be given by [(9)+(12)] − H2 O(ads) + Oo(s) → 2OH(ads) + V2+ o + 2e

(14)

Hence the processes (9)–(14) direct the equilibration of conductance after the initial hike to lower values higher than the base line of MoO3 .

617

If the reactions (9)–(14) are assumed to proceed into the bulk through diffusion, the formation of MoO3−x (OH)x hydrolyzed structures has to be considered as well, when humidified gas is exposed for longer time. This means, in the presence of water vapour, the MoO3 layer is assumed to change to the hydrolyzed form in a rather slow process according to Eq. (15) [43]. MoO3 + xH2 O → MoO3−x (OH)x

(15)

This hydrolyzed form of MoO3 is assumed to show enhanced conductance [15,23] compared to MoO3 in good agreement with the response behaviour (Fig. 8); however, the film conductance reduces over time at humid atmosphere. This may be due to the slowly changing equilibrium state with undissociated adsorbed water (Eq. (8)). When H2 is introduced simultaneously with humidity, a competitive reaction of H2 O with H2 on the sensing layer may take place, which results in the further reduction of MoO3 according to Eq. (16) [47]. MoO3−x (OH)x + yH2 → MoO3−(x+y) + zH2 O

(16)

The reaction of hydrogen with the hydrolyzed molybdenum oxide (Eq. (16)) may further enhance the conductance (Fig. 8) as the nonstoichiometric MoO3 is expected to show better conductance compared to stoichiometric MoO3 because of the formation of lower valent molybdenum ions [16]. At 200 ◦ C, the layer on Au-IDE shows better response to hydrogen in the absence and presence of humidity compared to the layer on Pt-IDE (Fig. 9a). Since the sensitivity is governed by surface reactions, morphology/porosity of the layers might play a key role in the gas sensing behaviour. The sensing layer on Au-IDE exhibits high porosity with well distributed and oriented nanorods compared to smooth and large grain nature of the layer on Pt-IDE. The nanoporous morphology offers high surface to volume ratio and hence large adsorption cross section to the analyte species, which result in higher sensitivity of the layer on Au-IDE. Moreover, the catalytic effect of Pt is not activated at this lower temperature. At higher temperature (300 ◦ C), the layer on Pt-IDE shows much higher sensitivity to H2 than the layer on the Au-IDE (Fig. 9b) which can be attributed to the activated catalytic effect of Pt. The adsorbed H2 molecules are assumed to be dissociated to more reactive species upon interacting with Pt at higher temperature and subsequently ‘spill over’ across the Pt-MoO3 interface via adsorption process, which can presumably enhance the sensitivity [48]. As the water vapour injection is withdrawn, the starting conductance value is quickly attained at the layer on Au-IDE suggesting that the desorption process is also fast at these nanoscaled thinfilms [23]. The water desorption process can be stated as 2OH(ads) → H2 O(ads) + Oo(s) → H2 O(gas) + Oo(s)

(17)

The desorption process of water is very slow at the layer on Pt-IDE compared to the layer on Au-IDE which can be due to the difference in layer morphology. 3.2.3. Ethanol sensing The ethanol vapour sensing responses of the MoO3 layers on Au and Pt IDEs at different operating temperatures are illustrated in Fig. 10. The layers show extremely different sensing behaviour to ethanol compared to hydrogen at 200 ◦ C and 300 ◦ C under nonhumid and humid conditions. The layers on both IDEs exhibit much higher sensitivity (one to three orders of magnitude conductance changes at non-humid conditions) to ethanol compared to hydrogen (Fig. 9a and b). At both temperatures, the conductance profile over concentration sequence to ethanol at non-humid conditions seems to be much better reproducible compared to hydrogen response profiles, at the repeated gas exposure sequence before and

618

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Fig. 10. Gas sensitivity response of MoO3 layers on Au and Pt IDEs towards ethanol at different concentrations, humidities (0% and 75% rH at 21 ◦ C) and operating temperatures (200 ◦ C and 300 ◦ C) as per the concentration sequence profile given in Fig. 2.

after the exposure of humidity (Fig. 10). In non-humid conditions, at both temperatures, the conductance increases with increase in ethanol concentration and the signal does not get stabilized at each concentration step. However, at humid ambience, the sensitivity to ethanol is drastically reduced and the response is found to be rather stable and well reversible (at the dropping of concentration steps) compared to the conductance response at non-humid conditions. The sensor signals seem to be maintaining the same baseline while stepping up the concentration from 0 to 1000 ppm and vice versa in the presence of water vapour except for the layer on Pt-IDE at 200 ◦ C. At lower temperature (200 ◦ C), the layer on Au-IDE shows better sensitivity compared to the layer on Pt-IDE, whereas, the layer on Pt-IDE shows a dramatic enhancement in response (∼628 for 1000 ppm) at 300 ◦ C which is much higher than the response (∼185 for 1000 ppm) of the layer on Au-IDE at 300 ◦ C, especially at non-humid conditions (Fig. 9b). Ethanol sensing mechanism over MoO3 was interpreted as the transformation of ethanol to acetaldehyde through catalytic interaction of ethanol vapour with the oxide surface layer [14,18,49,50]. In the present case, two different types of adsorption (nondissociative and dissociative) of ethanol have to be considered in order to interpret the sensing behaviour of the layers on Pt and Au-IDEs at different temperatures. At non-humid conditions, the sensing layer on Au-IDE shows an initial sudden response to ethanol as seen in the case of H2 and H2 O sensing behaviour and the conductance gradually increases to higher values. The gaseous ethanol molecules are first chemisorbed on the oxide sensing layer with a donor effect and inject electrons into the n-type oxide semiconductor according to Eq. (18) [51]. C2 H5 OH → C2 H5 OH(ads) → C2 H5 OH+ + e− (ads)

(18)

This process seems to be activated on the layer of Pt-IDE as well, however, not prominent due to the limited diffusion through more dense morphology compared to the layer on Au-IDE. The sensitivity action may further progress through the following reactions of adsorbed ethanol with the sensing layer of both types of IDEs, occurring simultaneously with reaction 18 at both temperatures with different kinetics. The O H bond of ethanol dissociates heterolytically to yield ethoxide groups and hydrogen as follows [52,53]. C2 H5 OH(ads) → C2 H5 O(ads) + H(ads)

(19)

The adsorbed hydrogen atoms inject electrons into the solid, and again diminish the resistance by forming a negative space charge region through the relation given by Eq. (2). The three processes (Eqs. (2), (18) and (19)) are assumed to take place simultaneously on the sensing layer upon the introduction of ethanol vapour and contribute to the sensitivity enormously. This may be the reason for the high increase in sensitivity exhibited by the sensing layers towards ethanol compared to hydrogen. Subsequently, the ethoxide group forms ionic bond with the unsaturated metal site and H atom is bound with nearby lattice oxygen anion, as given in the representation (20) [49,53]. C2 H5 O − Mo − O − Mo − OH(ads)

(20)

Ethoxides then undergo dehydrogenation with subsequent proton donation to the cation, which is possible due to the reducible–re-oxidizable cationic nature of Mo, yielding acetaldehyde as follows [51,53]. C2 H5 O − Mo − O − Mo − OH → CH3 CHO + H − Mo − O − Mo − OH

(21)

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Hence, when an alcohol molecule is catalytically oxidized by MoO3 , two hydrogens are liberated, (1) removed during the initial dissociative chemisorption of the alcohol and (2) formed when the surface ethoxide species decomposes to the acetaldehyde. Both liberated protons are assumed to desorb as water, which originates from the recombination of OH and Mo H. The hence formed acetaldehyde and water desorb by leaving a surface oxygen vacancy and a partially reduced metal (Eq. (22)) which will re-oxidize in presence of gas phase O2 [52,53]. The overall reaction can be written as C2 H5 OH + Oo(s) → CH3 CHO + H2 O + V2+ + 2e− 0

(22)

Therefore, it can be concluded that EtOH undergoes a reducing behaviour by the catalytic oxidation which results in electron transfer to the metallic site. As mentioned above, the electrical conductance of the non-stoichiometric MoO3 is expected to be high because of the presence of lower valent molybdenum ions. Moreover the formation of oxygen vacancies generates free electrons to the bulk (Eq. (22)), hence conductance increases with ethanol vapour concentration. 3.2.4. Ethanol sensing in humid ambience The introduction of water vapour (75% rH) resulted in an increase in conductance which subsequently saturated at lower values as described in the case of H2 sensing. The presence of water shows a dramatic influence on the sensor performance towards ethanol vapour; the sensitivity decreases drastically, however the sensor behaviour seems to be more stable and reversible compared to the signal at non-humid ambience. It is reported that the presence of water decreases the formation of acetaldehyde during the catalytic oxidation of ethanol [54,55]. In their study of ethanol oxidation, Medeiros et al. [54] proposed that two factors may be responsible for the decrease of sensitivity towards EtOH after addition of water vapour (1) site blocking by hydroxyl groups precludes the formation of bridging species (dissociated products of ethanol) and (2) water molecules compete with the adsorbed species, which reduces superficial concentrations hence prevent catalytic oxidation of ethanol (Eq. (22)) to acetaldehyde. This may be the reason for the reduction of sensitivity as well as, stability and well-reversibility of the signal in humid ambience. Similar observations have been reported in the sensor responses of other metal oxides to ethanol [55]. At lower temperature (200 ◦ C), the sensing layer on Au-IDE shows better response (Fig. 9a) to ethanol than the layer on the Pt-IDE which can be attributed to the nanoporous morphology of the well-distributed and oriented nanorods on Au-IDE. The catalytic enhancement of sensitivity by Pt-IDE at higher temperature (300 ◦ C) is observed towards EtOH, and however, the catalytic activity seems not to be activated at lower temperature which is obvious from Fig. 10. It is argued [56,57] that the improved sensitivity of certain metal oxides to EtOH by the presence of Pt is due to the ‘spillover’ effect (high catalysis of noble metals on the evolution of hydrogen), which may lead to more hydrogen to penetrate into the metal oxide sensing layer. In the present case, probably two actions are expected upon the introduction of ethanol in presence of Pt catalyst; (1) readily occurred dehydrogenation of ethanol which may result in the spill over of hydrogen species to MoO3 sensing layer and (2) enhanced catalytic oxidation of ethanol over MoO3 according to Eq. (22) [54,58], both can reasonably contribute to the sensitivity. 4. Concluding remarks and outlook The hydrogen and ethanol sensing behaviours of MoO3 nanostructured thin films at non-humid and humid conditions give away various characteristic sensitivity features which have been

619

discussed in context with common reaction models available in literature. Preparation of the MoO3 layers on Pt- as well as Au-IDE at similar conditions resulted in different layer morphologies, indicating that the electrode material has profound influence on the formation of nanostructures. The operating temperature has dramatic influence on the reaction kinetics leading to diverse sensing performance of the MoO3 sensing layers towards H2 , ethanol and humidity. Fast surface (non-dissociative adsorption) and slow bulk (dissociative adsorption and reaction with lattice oxygen) reaction processes, which may take place during the interaction of analytes (hydrogen, water vapour and ethanol) with the sensing layer, have to be considered in order to interpret the response behaviour. The slow bulk reactions are assumed to result in the formation of hydrogen molybdenum bronze (Hx MoO3 ) and hydrolyzed form of MoO3 , (MoO3−x (OH)x ) during prolonged interaction with H2 and H2 O, respectively. At 200 ◦ C, the layer on Au-IDE shows better sensitivity to hydrogen and ethanol in the absence and presence of humidity compared to the layer on Pt-IDE and can be attributed to the highly porous and well distributed nanorod morphology of the layer on Au-IDE, where the diffusion is much faster. At higher temperature (300 ◦ C), the sensitivity action is activated by the catalytic effect of platinum and the layer on Pt-IDE therefore shows better sensitivity to hydrogen and ethanol compared to the layer on Au-IDE. The presence of water ambience does not significantly change the sensitivity to H2 , however, it dramatically diminishes the sensing performance towards ethanol vapour, which can be attributed to site blocking nature of hydroxyl groups. The ethanol response of the layers seems to be stable and well-reversible under humid ambience. The optimal temperature of the sensors in the present study is found to be 300 ◦ C as the sensing layers on both kind of IDEs show good sensor response to H2 and ethanol under non-humid and humid ambiences. For better understanding of the surface chemistry and mechanisms of sensing, in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic studies under gas exposure will be included in our future work. Comparison of the sensing performance of low-dimensional nanostructured MoO3 thin-films with sol–gel prepared low dimensional MoO3 thick-films are also planned in order to study the influence of layer thickness and morphology. Acknowledgements N. Illyaskutty would like to thank German Academic Exchange Service (DAAD) for the fellowship (PKZ: A/09/74480) throughout the course of this work. The valuable support of UGC-DAE, CSR, Indore, India by providing XRD and micro-Raman facilities is also acknowledged. References [1] H. Huang, S. Tian, J. Xu, Z. Xie, D. Zeng, D. Chen, G. Shen, Needle-like Zn-doped SnO2 nanorods with enhanced photocatalytic and gas sensing properties, Nanotechnology 23 (2012) 105502. [2] J. Wang, L. Wei, L. Zhang, C. Jiang, E. Kong, Y. Zhang, Preparation of high aspect ratio nickel oxide nanowires and their gas sensing devices with fast response and high sensitivity, Journal of Materials Chemistry 22 (2012) 8327–8335. [3] G.J. Cadena, J. Riu, F.X. Rius, Gas sensors based on nanostructured materials, Analyst 132 (2007) 1083–1099. [4] G. Chen, T.M. Paronyan, E.M. Pigos, A.R. Harutyunyan, Enhanced gas sensing in pristine carbon nanotubes under continuous ultraviolet light illumination, Nature Scientific Reports 2 (2012) 343. [5] M. Shafieia, J. Yu, G. Chen, P.T. Lai, N. Motta, W. Wlodarski, K. Kalantar-zadeh, Improving the hydrogen gas sensing performance of Pt/MoO3 nanoplatelets using a nano thick layer of La2 O3 , Sensors and Actuators B (2012), http://dx.doi.org/10.1016/j.snb.2012.11.019 [6] S. Bai, S. Chen, L. Chen, K. Zhang, R. Luo, D. Li, C.C. Liu, Ultrasonic synthesis of MoO3 nanorods and their gas sensing properties, Sensors and Actuators B 174 (2012) 51–58.

620

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

[7] L.L. Xing, S. Yuan, Z.H. Chen, X.Y. Xue, Enhanced gas sensing performance of SnO2 /␣-MoO3 heterostructure nanobelts, Nanotechnology 22 (2011) 225502. [8] D. Chen, M. Liu, L. Yin, T. Li, Z. Yang, X. Li, B. Fan, H. Wang, R. Zhang, Z. Li, H. Xu, L. Gao, Single-crystalline MoO3 nanoplates: topochemical synthesis and enhanced ethanol-sensing performance, Journal of Materials Chemistry 21 (2011) 9332. [9] W. Kim, H. Kim, S. Hong, Gas sensing properties of MoO3 nanoparticles synthesized by solvothermal method, Journal of Nanoparticle Research 12 (2010) 1889–1896. [10] X. Chu, S. Liang, W. Sun, W. Zhang, T. Chen, Q. Zhang, Trimethylamine sensing properties of sensors based on MoO3 microrods, Sensors and Actuators B 148 (2010) 399–403. [11] M.B. Rahmani, S.H. Keshmiri, J. Yu, A.Z. Sadekc, L. Al-Mashat, A. Moafi, K. Lathamd, Y.X. Li, W. Wlodarski, K. Kalantar-zadeh, Gas sensing properties of thermally evaporated lamellar MoO3 , Sensors and Actuators B 145 (2010) 13–19. [12] A.M. Taurino, A. Forleo, L. Francioso, P. Siciliano, M. Stalder, R. Nesper, Synthesis electrical characterization, and gas sensing properties of molybdenum oxide nanorods, Applied Physics Letters 88 (2006) 152111. [13] S. Barazzouk, R.P. Tandon, S. Hotchandani, MoO3 -based sensor for NO, NO2 and CH4 detection, Sensors and Actuators B 119 (2006) 691–694. [14] E. Comini, L. Yubao, Y. Brando, G. Sberveglieri, Gas sensing properties of MoO3 nanorods to CO and CH3 OH, Chemical Physics Letters 407 (2005) 368–371. [15] S.S. Sunu, E. Prabhu, V. Jayaraman, K.I. Gnanasekar, T.K. Seshagiri, T. Gnanasekaran, Electrical conductivity and gas sensing properties of MoO3 , Sensors and Actuators B 101 (2004) 161–174. [16] S.S. Sunu, E. Prabhu, V. Jayaraman, K.I. Gnanasekar, T. Gnanasekaran, Gas sensing properties of PLD made MoO3 films, Sensors and Actuators B 94 (2003) 189. [17] A.K. Prasad, P.I. Gouma, D.J. Kubinski, J.H. Visser, R.E. Soltis, P.J. Schmitz, Reactively sputtered MoO3 films for ammonia sensing, Thin Solid Films 436 (2003) 46–51. [18] E. Comini, G. Sberveglieri, M. Ferroni, V. Guidi, G. Martinelli, Response to ethanol of thin films based on Mo and Ti oxides deposited by sputtering, Sensors and Actuators B 93 (2003) 409–415. [19] A.K. Prasad, D.J. Kubinski, P.I. Gouma, Comparison of sol–gel and ion beam deposited MoO3 thin film gas sensors for selective ammonia detection, Sensors and Actuators B 93 (2003) 25–30. [20] V. Guidi, D. Boscarino, L. Casarotto, E. Comini, M. Ferroni, G. Martinelli, G. Sberveglieri, Nanosized Ti-doped MoO3 thin films for gas-sensing application, Sensors and Actuators B 77 (2001) 555–560. [21] C. Imawan, H. Steffes, F. Solzbacher, E. Obermeier, A new preparation method for sputtered MoO3 multilayers for the application in gas sensors, Sensors and Actuators B 78 (2001) 119–125. [22] M. Ferroni, V. Guidi, M. Sacerdoti, P. Nelli, G. Sberveglieri, MoO3 -based sputtered thin films for fast NO2 detection, Sensors and Actuators B 48 (1998) 285–288. [23] A. Raju, C. Rao, MoO3 /TiO2 and Bi2 MoO6 as ammonia sensors, Sensors and Actuators B 21 (1994), 23 ±26. [24] T. Itoh, I. Matsubara, M. Kadosaki, Y. Sakai, W. Shin, N. Izu, M. Nishibori, Effects of high-humidity aging on platinum, palladium, and gold loaded tin oxide—volatile organic compound sensors, Sensors 10 (2010) 6513–6521. [25] J. Nah, S.B. Kumar, H. Fang, Y. Chen, E. Plis, Y. Chueh, S. Krishna, J. Guo, A. Javey, Quantum size effects on the chemical sensing performance of two-dimensional semiconductors, The Journal of Physical Chemistry C 116 (2012) 9750–9754. [26] S. Saukko, V. Lantto, Influence of electrode material on properties of SnO2 -based gas sensor, Thin Solid Films 436 (2003) 137–140. [27] S. Capone, P. Siciliano, F. Quaranta, R. Rella, M. Epifani, L. Vasanelli, Moisture influence and geometry effect of Au and Pt electrodes on CO sensing response of SnO2 microsensors based on sol–gel thin film, Sensors and Actuators B 77 (2001) 503–511. [28] G. Faglia, E. Comini, G. Sberveglieri, R. Rella, P. Siciliano, L. Vasanelli, Square and collinear four probe array and Hall measurements on metal oxide thin film gas sensors, Sensors and Actuators B 53 (1998) 69–75. [29] K. Fukui, M. Nakane, Effects of tin oxide semiconductor-electrode interface on gas sensitivity characteristics, Sensors and Actuators B 13 (14) (1993) 589–590. [30] A. Jerger, H. Kohler, F. Becker, H.B. Keller, R. Seifert, New applications of tin oxide gas sensors: II. Intelligent sensor system for reliable monitoring of ammonia leakages, Sensors and Actuators B 81 (2002) 301–307. [31] I. Navas, R. Vinodkumar, K.J. Lethy, V. Ganesan, V. Sathe, V.P. Mahadevan Pillai, Growth and characterization of molybdenum oxide nanorods by RF magnetron sputtering and subsequent annealing, Journal of Physics D: Applied Physics 42 (2009) 175305. [32] G. Mestl, In situ Raman spectroscopy for the characterization of MoVW mixed oxide catalysts, Journal of Raman Spectroscopy 33 (2002) 333–347. [33] P. Feng, Q. Wan, T.H. Wang, Contact-controlled sensing properties of flowerlike ZnO nanostructures, Applied Physics Letters 87 (2005) 213111. [34] J. Gerblinger, U. Lampe, H. Meixner, J. Giber, Cross-sensitivity of various doped strontium titanate films to CO CO2 , H2 , H2 O and CH4 , Sensors and Actuators B 18–19 (1994) 529–534. [35] F. Réti, M. Fleischer, H. Meixner, J. Giber, Effect of coadsorption of reducing gases on the conductivity of ␤-Ga2 O3 thin films in the presence of O2 , Sensors and Actuators B 19 (1994) 573–577. [36] J.Z. Ou, J.L. Campbell, D. Yao, W. Wlodarski, K. Kalantar-zadeh, In situ Raman spectroscopy of H2 gas interaction with layered MoO3 , The Journal of Physical Chemistry C 115 (2011) 10757–10763.

[37] D. Mei, A.M. Karim, Y. Wang, Density functional theory study of acetaldehyde hydrodeoxygenation on MoO3 , The Journal of Physical Chemistry C 115 (2011) 8155–8164. [38] T. Seiyama, N. Yamazoe, H. Arai, Ceramic humidity sensors, Sensors and Actuators B 4 (1983) 85. [39] T.M. Barbara, G. Gammie, J.W. Lyding, J. Jonas, Single-crystal conductivity measurements of hydrogen molybdenum bronzes: Hx MoO3 , Journal of Solid State Chemistry 75 (1988) 183–187. [40] X.K. Hu, Y.T. Qian, Z.T. Song, J.R. Huang, R. Cao, J.Q. Xiao, Comparative study on MoO3 and Hx MoO3 nanobelts: structure and electric transport, Chemistry of Materials 20 (2008) 1527–1533. [41] P.I. Gouma, A.K. Prasad, K.K. Iyer, Selective nanoprobes for ‘signalling gases’, Nanotechnology 17 (2006) S48–S53. [42] T.S. Sian, G.B. Reddy, Effect of adsorbed water vapor on Mg intercalation in electrochromic a-MoO3 films, Electrochimica Acta 49 (2004) 5223–5226. [43] T.C. Arnoldussen, Electrochromism and photochromism in MoO3 films, Journal of The Electrochemical Society 123 (1983) 527. [44] A. Papakondylis, P. Sautet, Ab initio study of the structure of the ␣-MoO3 solid and study of the adsorption of H2 O and CO molecules on its (1 0 0) surface, The Journal of Physical Chemistry 100 (1996) 10681. [45] T.S. Sian, G.B. Reddy, Effect of stoichiometry and microstructure on hydrolysis in MoO3 films, Chemical Physics Letters 418 (2006) 170–173. [46] N. Yoshiike, S. Kondo, Electrochemical properties of WO3−x (H2 O), Journal of The Electrochemical Society 130 (1983) 2283. [47] Y.J. Lee, W.T. Nichols, D.G. Kim, Y.D. Kim, Chemical vapour transport synthesis and optical characterization of MoO3 thin films, Journal of Physics D: Applied Physics 42 (2009) 115419. [48] J.G. Kim, J.R. Regalbuto, The effect of calcination on H2 spillover in Pt/MoO3 : II. Kinetic modeling, Journal of Catalysis 139 (1993) 175–190. [49] H. Idriss, E.G. Seebauer, Reactions of ethanol over metal oxides, Journal of Molecular Catalysis A: Chemical 152 (2000) 201–212. [50] N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catalysis Surveys from Asia 7 (2003) 63–75. [51] F. Réti, M. Fleischer, I.V. Perczel, H. Meixner, J. Giber, Detection of reducing gases in air by ␤-Ga2 O3 thin films using self-heated and externally (oven-) heated operation modes, Sensors and Actuators B 34 (1996) 378–382. [52] R.L. Smith, G.S. Rohrer, The protonation of MoO3 during the partial oxidation of alcohols, Journal of Catalysis 173 (1998) 219–228. [53] B.C. Gates, H. Knözinger, Impact of Surface Science on Catalysis, Academic Press, USA, 2000. [54] P.R.S. Medeiros, J.G. Eon, L.G. Appel, The role of water in ethanol oxidation over SnO2 -supported molybdenum oxides, Catalysis Letters 69 (2000) 79–82. [55] F. Pourfayaz, A. Khodadadi, Y. Mortazavi, S.S. Mohajerzadeh, CeO2 doped SnO2 sensor selective to ethanol in presence of CO, LPG and CH4 , Sensors and Actuators B 108 (2005) 172–176. [56] P. Montmeat, C. Pijolat, B. Riviere, G. Tournier, J. Viricelle, Effect of a platinum membrane on the sensing properties of materials based on thin and thick tin dioxide films, Materials Science and Engineering: C 21 (2002) 113–123. [57] M. Epifani, T. Andreu, R. Zamani, J. Arbiol, E. Comini, P. Siciliano, G. Faglia, J.R. Morante, Pt doping triggers growth of TiO2 nanorods: nanocomposite synthesis and gas-sensing properties, CrystEngComm 14 (2012) 3882–3887. [58] Z. Xu, Y. Wang, Effects of alloyed metal on the catalysis activity of pt for ethanol partial oxidation: adsorption and dehydrogenation on Pt3 M (M = Pt, Ru, Sn, Re, Rh, and Pd), The Journal of Physical Chemistry C 115 (2011) 20565–20571.

Biographies

Navas Illyaskutty is currently working as postdoctoral fellow at Institute for Sensorics and Information Systems, Karlsruhe University of Applied Sciences, Karlsruhe, Germany. He was a recipient of German Academic exchange service (DAAD) sandwich model fellowship in 2010. He received Ph.D. in Optoelectronics (2012), M.Phil. in Photonics (2007) and M.Sc. in Physics (2005) from University of Kerala, India. His current research interests span to material science, nanostructures, nanophotonics, sensor system technology, chemical sensors and nanotechnology enabled sensors.

Heinz Kohler graduated in physics (1975–1981) at Karlsruhe Technical University, Germany. Afterwards, he had a doctoral fellowship at the Max Planck Institute of Solid State Research, Stuttgart (1981–1984) and received his doctoral degree from Technical University of Karlsruhe, Germany in 1984. Since 1992, he has been working as Professor of Physics and Chemical Sensors at Karlsruhe University of Applied Sciences, Germany. His present research interests include chemical sensors, sensor technology and intelligent gas sensor systems.

N. Illyaskutty et al. / Sensors and Actuators B 187 (2013) 611–621

Thomas Trautmann was a scientific engineer at Institute for Sensorics and Information Systems, Karlsruhe University of Applied Sciences, Karlsruhe, Germany from 2008 till 2011. He received his Dipl. Ing (FH) degree in Sensor System Technology in 2006 from Karlsruhe University of Applied Sciences and gained experience in fabrication of metal oxide based conductometric chemical sensors during his employment tenure. He is currently working as a scientific teacher for System and Information Technology at Ehrhardt Schott Technical School, Schwetzingen (Heidelberg).

Dr. Matthias Schwotzer is graduated in mineralogy and received his Ph.D. in 2008 from Karlsruhe Institute of Technology (KIT), Germany for his research on the mechanisms of deterioration reactions of cement based materials exposed to various aqueous environments. Currently, he is the team leader of the research group “reactive transport processes” at the Institute of Functional Interfaces at KIT. His research interests are durability and functionalization of porous mineral materials, with focus on the chemical stability in aggressive aqueous environments. He is a cofounder of the spin of company IONYS AG – chemistry in engineering for durable constructions.

621

Dr. V.P. Mahadevan Pillai is the Professor and Head of Department of Optoelectronics, University of Kerala, Kariavattom, Thiruvananthapuram, India. He is presently acting as the Dean, Faculty of Applied Sciences and Technology, University of Kerala. He took his M.Sc. degree in Physics (1982) and M.Phil. degree in Physics (1992) and Ph.D. degree in Physics (1996) from University of Kerala. His current research interests are material science, nanophotonics, sensor technology, holography, laser technology and laser remote sensing. He has published 90 research papers in peer reviewed journals and produced 13 Ph.D.s so far.