Highly selective and sensitive response of 30.5 % of sprayed molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection

Accepted Manuscript Highly selective and sensitive response of 30.5 % of sprayed molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection A.A. Mane, M.P. Suryawanshi, J.H. Kim, A.V. Moholkar PII: DOI: Reference:

S0021-9797(16)30586-0 http://dx.doi.org/10.1016/j.jcis.2016.08.031 YJCIS 21499

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

4 June 2016 5 August 2016 12 August 2016

Please cite this article as: A.A. Mane, M.P. Suryawanshi, J.H. Kim, A.V. Moholkar, Highly selective and sensitive response of 30.5 % of sprayed molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.08.031

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Highly selective and sensitive response of 30.5 % of sprayed molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection A. A. Mane a,b, M. P. Suryawanshi c, J. H. Kim c, A. V. Moholkar *, a a

Thin Film Nanomaterials Laboratory, Department of Physics, Shivaji University, Kolhapur 416

004, India b

General Science and Humanities Department, Sant Gajanan Maharaj College of Engineering,

Mahagaon, 416 503, India c

Department of Materials Science and Engineering, Chonnam National University, 300

Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea * Corresponding author: [email protected]

Abstract The molybdenum trioxide (MoO3) thin films have been successfully deposited onto the glass substrates using chemical spray pyrolysis (CSP) deposition technique at various substrate temperatures ranging from 300 °C to 450 °C with an interval of 50 °C. The effect of substrate temperature on the structural, morphological, optical and gas sensing properties of MoO3 thin films has been thoroughly investigated. The X-ray diffraction analysis reveals that all the films have an orthorhombic crystal structure and are polycrystalline in nature. The FE-SEM micrographs depict the formation of nanobelts-like morphology. The AFM study reveals that RMS surface roughness of MoO3 thin films increases from 8.6 nm to 12 nm with increase in substrate temperature from 300 ºC to 400 ºC and then decreases to 11.5 nm for substrate temperature of 450 ºC. Optical results show that the band gap of MoO3 thin films decreases from 3.92 eV to 3.44 eV. The selectivity studies show that the gas response of various gases varies as NH3 < SO2 < CO2 < CO < H2S < NO2. Moreover, typical MoO3 film is highly selective and sensitive for detection of NO2 gas in comparison with other gases. The maximum response of 30.5 % is obtained for MoO3 thin film deposited at substrate temperature of 400 ºC towards 100 ppm NO2 gas concentration at an operating temperature of 200 °C with response and recovery times of 20 s and 160 s, respectively. Finally, NO2 gas sensing mechanism model based on the chemisorption process is discussed.

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Keywords: Molybdenum trioxide thin films; Chemical spray pyrolysis; Optical properties; selectivity; sensitivity; NO2 gas sensor.

1. Introduction The gas sensor is a device that detects the presence of combustible, flammable and toxic gases in the surrounding atmosphere [1]. The toxic pollutant gases released from industrial manufacturing processes, power plants, automobiles exhausts and combustion of fossil fuels causes air pollution [2]. Among various pollutant gases (such as NO2, CO, CO2, SO2, H2S, NH3), the detection of nitrogen dioxide (NO2) gas has become a major environmental issue because it contributes to the formation of photochemical smog, acid rain and particulate matter through chemical reactions in the atmosphere [3]. The long-term exposure of NO2 gas causes harmful effects on human health. The maximum concentration of NO2 gas in the air is expressed less than 3 parts per million (ppm). Therefore, to avoid adverse effects on human health, NO2 gas sensors are timely needed [4, 5]. The nanostructured metal oxide semiconductors (MOS) based gas sensors comparising different types of morphologies such as nanowires, nanobelts, nanoparticles, nanorods, nanotubes etc., have high gas response due to their high surface area to volume ratio [6, 7]. Recently, the significant efforts have been focused on improving the gas response, selectivity, sensitivity, stability and reproducibility of MOS gas sensors. The toxic effect of chemicals and gases on an environment and human being led to rapid development of MOS based gas sensors for monitoring and detection of toxic, pollutant, combustible gases and organic vapors. Such type of gas sensors has attracted tremendous attention because of their low cost, facile fabrication, small size, high response and very low power consumption. In chemical gas sensors, the chemical reactions occurring between analyte gas and adsorbed oxygen species on the surface of sensor led to change in electron concentration of sensing material and thereby, change in electrical resistance is observed [8]. Among various transition metal oxides, n-type conductivity of molybdenum trioxide (MoO3) due to oxygen vacancies dominates the electronic and chemical properties and therefore, received significant attention due to its wide band gap, diverse structures and functional properties [9]. Currently, MoO3 is used for novel scientific and technological applications in various fields such as buffer layer for organic photovoltaic’s (OPVs) [10], gas sensors [11], supercapacitor electrode [12], optoelectronic devices [13] and anode in organic light emitting 2

diodes (OLEDs) [14]. There are several reports available on the preparation of MoO3 thin films including sol-gel dip coating [15], DC magnetron sputtering [16], pulsed laser deposition [17] and spray pyrolysis [18]. Despite of various synthesis techniques, chemical spray pyrolysis (CSP) deposition technique is a simple and cost effectiveness alternative to deposit MOS thin films for various applications [19]. The MoO3 exists in at least three phases i.e., thermodynamically stable orthorhombic α-MoO3 phase, metastable monoclinic β-MoO3 phase and hexagonal metastable h-MoO3 phase [20]. The MoO3 thin film is found to be very sensitive for detecting variety of gases such as NO, NO2, CO, H2 and NH3 in the temperature range of 300 °C to 600 °C [21]. Prasad et al. [22] have reported NH3 sensing properties of ion beam sputtered MoO3 thin films. Chang et al. [23] have investigated post deposition annealing control of phase and texture for sputtered MoO3 films and observed various crystalline polymorphs. Below 350 °C, a pure metastable monoclinic β-MoO3 phase; between 350 °C to 400 °C, both α-and β-MoO3 composed phases and at 450 °C, thermodynamically stable pure α-MoO3 phase have been obtained. The β-MoO3 phase has shown best response while α-MoO3 phase with planar texture has shown the shortest recovery time towards NH3 gas. Wu et al. [24] have synthesized porous In2TiO5–rutile composite nanotubes using electrospinning approach for detection of NO2 gas at room temperature. In the work of Bai et al. [25], MoO3 nanorods prepared using probe ultrasonic method showed NO2 gas sensing properties at a working temperature of 290 ºC. It was also reported that [26], the MoO3 rectangular plates prepared using thermal evaporation method showed response towards NO2 gas at an operating temperature of 225 ºC. In our present work, the NO2 gas sensing response is obtained at lower operating temperature of 200 ºC using MoO 3 nanobelts. It can also be derived that the response performance of MoO 3 based sensor is comparable to these published results. In present study, MoO3 thin films have been successfully deposited onto the glass substrates at various substrate temperatures using a cost effective CSP deposition technique. The effect of substrate temperature on physicochemical and gas sensing properties of MoO3 thin films has been investigated. The NO2 sensing properties of MoO3 thin film at different operating temperatures and gas concentrations has been reported. The proposed MoO3 nanobelts based gas sensor shows higher response, selectivity and fast response/recovery times towards NO2 gas and provides a promising avenue for improving the sensing performance.

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2. Materials and method Firstly, the soda lime glass (SLG) substrates were rinsed in double distilled water (DDW) followed by ultrasonic cleaning to remove surface contaminants and then followed by alcohol (methanol and acetone) drying. The molybdenum pentachloride (MoCl5) powder procured from Alfa Aesar, Mumbai without any treatment was used for preparation of 10 mM of spraying solution. The 10 mM of MoCl5 solution was prepared by dissolving 0.07 g of MoCl5 powder in 25 ml of DDW at room temperature. The dark blue colored MoCl5 solution was then sprayed onto

the preheated glass substrates maintained at various substrate temperatures ranging from 300 °C to 450 °C with an interval of 50 °C. The schematic of CSP deposition technique is shown in our previous report [27]. As deposited films were allowed to cool naturally and then were used for structural, morphological, optical characterization and gas sensing measurement. During the deposition process, all typical spray parameters such as spray nozzle to glass substrate distance (27.5 cm), solution spray rate (1.5 ml/min.) and solution quantity (25 ml) were kept constant. The compressed ambient air was used as a carrier gas. The thermogravimetric analysis-differential thermal analysis (TGA-DTA) of MoCl5 powder was carried out using thermal analysis (TA) instrument model SDT Q600 V20.9 in air at a heating rate of 10 °C/min. from room temperature to 1000 °C to study their thermal decomposition behavior. The thickness of MoO3 thin films was measured using AMBIOS, USA XP-I stylus surface profiler. The X-ray diffraction (XRD) analysis of thin films was carried out using Bruker D2 Phaser with Cu Kα radiation (λ = 1.5406 Å). The surface morphological analysis was carried out using field emission-scanning electron microscopy (FE-SEM), Mira-3, Tescan, Brno-Czech Republic. The surface topography of MoO3 thin films was analyzed using atomic force microscopy (AFM), INNOVA 1B3BE, Bruker, USA. The optical properties of MoO3 thin films were studied using a double beam Shimadzu UV-Vis 3600 spectrophotometer in the wavelength range from 200 nm to 1100 nm to determine the band gap energy. The gas sensing measurements of MoO3 thin films were carried out using locally fabricated gas sensing unit equipped with Keithley electrometer. The canisters of various gases such as NH3, H2S, CO, CO2, SO2 and NO2 with 5,000 ppm gas concentration were procured from Shreyaa Enterprises Pvt. Ltd. Mumbai, India. The MoO3 thin film sensor of size 1 cm × 1 cm with silver electrical contacts was kept on the copper hot plate and then mounted into the 250 ml airtight metallic gas chamber. The digital temperature controller was used to set the sensor at 4

desired temperature through a thermocouple. The sensor film was heated until its resistance stabilizes and then exposed to analyte gas. The stabilized value of sensor resistance was taken as reference resistance Ra i.e. resistance of film in air for calculating the gas response. The known concentration of analyte gas was then injected into the test chamber using a microsyringe through a small opening septum with rubber gasket and change in resistance of the film monitored and measured as a function of time till a steady value of resistance was reached. The chamber was then purged with air and the sensor was allowed to reach the initial value of resistance before the next cycle of experiment carried out. The gas sensing response is calculated using the formula:

where, Ra is the resistance of film when exposed to air and Rg is the resistance of film when exposed to analyte gas. After each successive measurement, fresh air was passed into gas chamber. The gas sensing measurement was carried out at different operating temperatures and gas concentrations. The selectivity study was carried out by introducing various gases such as NH3, H2S, CO, CO2, SO2 and NO2 of known concentration into gas chamber. The selectivity of sensor is measured in terms of selectivity coefficient (K) using the formula:

where, St and Si are the gas responses towards target gas and interfering gas, respectively.

3. Results and discussion 3.1 Reaction mechanism and film thickness measurement Initially, the MoCl5 solution is converted into aerosols by spray nozzle. When fine aerosols of MoCl5 solution passes through temperature gradient in CSP deposition unit, the pyrolytic decomposition and evaporation of solvent takes place. The evaporation of solvent leads to formation of precipitate as aerosols approaches towards the substrates. The melting and vaporization of precipitate occurs in succession. When vapors of precipitate diffuse onto the surface of substrate, the growth of nuclei leads to formation of continuous thin layer of MoO3. The proposed pyrolytic chemical reaction is as follows:

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The deposited MoO3 thin films are uniform, pinhole free and well adherent to glass substrates. The thickness values of MoO3 thin films deposited at various substrate temperatures are listed in Table 1. It is observed that the film thickness decreases from 529 nm to 235 nm with increase in substrate temperature from 300 ºC to 450 ºC. For MoO3 thin film deposited at substrate temperature of 300 ºC, the higher deposition rate results into the maximum film thickness of 529 nm. At substrate temperature of 350 ºC, the increase in evaporation rate of precursor solution results into the decrease in film thickness to 430 nm. For MoO3 thin film deposited at substrate temperature of 400 ºC, the thermal energy gained by droplets of MoCl5 solution is such that it vaporizes just above the surface of substrate and gives well adherent film of thickness 350 nm. With further increase in substrate temperature to 450 ºC, aerosols completely vaporize above the nucleation centre of film and thereby, diminishing the deposition efficiency which result into the formation of powdery film of thickness 235 nm. Thus, with increase in substrate temperature from 300 ºC to 450 ºC, the increase in evaporation rate of precursor solution results into less amount of mass transport towards the substrates and therefore, the film thickness decreases.

3.2 TGA-DTA studies Fig. 1 shows the TGA-DTA curves of MoCl5 powder. In TGA curve, weight loss of 27.33 % occur from room temperature to 125.90 °C (region I) due to desorption of physically absorbed moisture from MoCl5 powder. The decomposition over the temperature range from 125.90 °C to 211 °C (region II) gives the weight loss of 42.87 % and it is assigned to elimination of chlorine from MoCl5 powder. Between 211 °C to 781.72 °C (region III), a small weight loss is ascribed to pyrolytic decomposition to form MoO3. In DTA curve, an endothermic peak appearing at 125.90 °C is attributed to release of hygroscopically bound water from MoCl5 powder. Similarly, an endothermic peak appearing at 211 °C is attributed to release of chlorine gas from MoCl5. The subsequent broad exothermic peak over the temperature range from 211 °C to 538.07 °C is attributed to an oxidation of molybdenum followed by crystallization of MoO3. Finally, an endothermic peak appearing at 781.72 °C is ascribed to melting of remaining residue [28]. Thus, by observing TGA-DTA curves, the MoO3 thin films are deposited in the temperature range from 300 °C to 450 °C using CSP deposition technique. 6

3.3 XRD studies The XRD patterns of MoO3 thin films deposited at various substrate temperatures are shown in Fig. 2. The XRD patterns of thin films matches well with Joint Committee for Powder Diffraction Standards (JCPDS) card No.00-005-0508 (cell parameters: a = 3.96 Å, b = 13.85 Å, c = 3.69 Å) which confirms that MoO3 thin films have an orthorhombic crystal structure. It is seen that the XRD peak intensity of (020) and (040) reflection is relatively higher than those of other reflections. This implies that MoO3 films grows along [020] and [040] directions and exhibit the dominant b-axis orientation. For MoO3 thin films deposited at substrate temperatures of 300 ºC, less intense peaks for (020), (040) and (060) planes are observed due to insufficient thermal energy for decomposition of precursor solution. At substrate temperature of 350 ºC, the intensities of (020), (040) and (060) diffraction peaks increases, indicating further improvement in crystallinity. With further increase in substrate temperature to 400 ºC, the intensities of (020), (040) and (060) diffraction peaks is higher due to sufficient thermal energy required for decomposition of precursor solution, indicating maximum improvement in crystallinity. At substrate temperature of 450 ºC, the precursor solution decomposes before reaching the substrate surface due to high thermophoretic force which results into the decrease in intensities of (020), (040) and (060) planes. The strong intense diffraction peaks observed for (020), (040) and (060) crystal planes proves the existence of lamellar structure and has largest probability to expose the environment [29, 30]. The {010} MoO3 facets exhibit stronger preference for chemisorptions and catalytic activity [31]. The weak intense peaks appearing at 2θ values of 23.32°, 27.33° and 67.52º corresponds to (110), (021) and (0100) planes, respectively indicates that crystallite growth is slow in these directions. For MoO3 thin films deposited at substrate temperatures of 400 ºC and 450 ºC, the extra intense peak is observed at 2θ values of 49.24º corresponding to (002) plane due to sufficient thermal energy for decomposition of precursor solution [32]. The intensity this (002) plane is less than the intensity of (060) plane at substrate temperature of 400 ºC. However, the intensity (002) plane is nearly similar to the intensity of (060) plane at substrate temperature of 450 ºC. This shows that substrate temperature play an important role in growth of thin film. The crystallite size (D) is calculated for most intense (040) plane using Scherrer’s formula [33]:

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where, k is a dimensionless constant which is related to shape and distribution of crystallites usually taken as 0.9, λ is the wavelength of X-ray (1.5406 Å), β is the full width half maximum (FWHM) in radian and θ is Bragg’s angle of diffraction. The crystallite size for (040) plane of MoO3 thin films deposited at various substrate temperatures is mentioned in Table 1. It is seen that the crystallite size for (040) plane increases from 50.4 nm to 59.1 nm with increase in substrate temperature from 300 ºC to 400 ºC and then decreases to 51.7 nm for 450 ºC, respectively. The increase in substrate temperature allows the crystallite to nucleate, grow along specific growth sites and arrange orderly. Thus, the increase in crystallite size with increase in substrate temperature indicates improvement in crystallinity of films. For the film deposited at substrate temperature of 450 ºC, the decrease in crystallite size for (040) plane is attributed to formation of less adherent powdery film onto the glass substrate.

3.4 FE-SEM studies Fig. 3 shows the FE-SEM micrographs of MoO3 thin films deposited at various substrate temperatures. For MoO3 thin films deposited at substrate temperature of 300 ºC, the nanobelts of varying length from 0.5 μm to 1 μm and width of 90 nm to 140 nm are observed. With further increase in substrate temperature to 350 ºC, the aggregation and coalescences of nanobelts is observed. At substrate temperature of 400 ºC, the nanobelts are densely grown with uniform length and similar shape. The small pores of varying sizes are also observed over the surface of film which helps to the diffusion of gas into pores and thereby, increases the gas response. For the films deposited at substrate temperatures of 450 ºC, the large amount of coalescence of neighbouring crystallites and agglomeration of nanobelts takes place which results into decrease in porous structure.

3.5 AFM studies The three dimensional AFM micrographs of MoO3 thin films deposited at various substrate temperatures are shown in Fig. 4. It is seen that with increase in substrate temperature from 300 ºC to 450 ºC, the larger grains of same shape are formed with smaller grains between them in the valleys. For MoO3 thin films deposited at substrate temperatures of 300 ºC and 350 ºC, the increase in substrate temperature causes increase in mobility of atoms on the substrate surface which results into formation of larger particles. The same process is happened for film 8

deposited at 400 ºC and larger grains are formed onto the surface of substrate. At substrate temperature of 450 ºC, the smaller particles are formed due to formation of powdery film onto the substrate. The average particle size and root mean square (RMS) surface roughness of MoO3 thin films deposited at various substrate temperatures are presented in Table 1. It is seen that the particle size increases from 160 nm to 260 nm with increase in substrate temperature from 300 ºC to 400 ºC and is ascribed to coalescences of smaller grains to form larger grains on the surface of films. The maximum particle growth of 260 nm is observed for film deposited at 400 ºC due to availability of more nucleation centers. At substrate temperature of 400 ºC, the deposition rate decreases to large extent due to higher evaporation rate of precursor solution which results into non-uniform growth of thin film with decrease in particle size to 225 nm. It is also seen that RMS surface roughness of MoO3 thin films increases from 8.6 nm to 12 nm with increase in substrate temperature from 300 ºC to 400 ºC and then decreases to 11.5 nm for 450 ºC. For the film deposited at 400 ºC, the nanobelts and small pores make the surface rough which increases the effective surface area for gas adsorption. For the film deposited at 450 ºC, the deposition rate decreases due to higher evaporation rate and complete decomposition of precursor solution occurs before reaching the substrate surface which results into the randomly accumulated and agglomerated nanobelts with decrease in surface roughness to 11.5 nm.

3.6 Optical properties Fig. 5 shows optical absorption spectra of MoO3 thin films deposited at various substrate temperatures. It is observed that the optical absorption slightly shifted towards lower wavelength side with increase in substrate temperature from 300 ºC to 450 ºC. This is due to decrease in lattice oxygen and thickness of thin films. The optical band gap energy of MoO3 thin films is calculated using the equation [27]:

where, α0 is constant, hν is the photon energy in eV, h is the Planck’s constant in eV, Eg is the band gap energy in eV and n is the constant which take values such as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively. Fig. 6 shows the plots of (αhν)2 versus photon energy (hν) of MoO3 thin films deposited at various substrate temperatures. For n = 1/2, the plots exhibit almost linear behavior in high energy region and follows exponential behavior as photon energy decreases. The linear behavior 9

in high energy region is due to strong absorption at the absorption edge with charge transition from highest filled band to lowest empty band. The exponential form of tail in the lower energy region is ascribed to existence of defects or disorder in the films. The red shift in the phonon wavelength is attributed to presence of defects such as point defects, twins, and stacking fault which leads to formation of sub-stoichiometric MoO3-X thin film which is the case with spray deposited thin films [34, 35]. The extrapolation at zero absorption coefficient on energy axis shows that band gap energy (Eg) decreases with increase in substrate temperature. The ‘E g’ values of MoO3 thin films are found to be 3.92 eV, 3.62 eV, 3.65 eV and 3.44 eV for MoO3 thin

films deposited at 300 ºC, 350 ºC, 400 ºC and 450 ºC, respectively. The observed ‘Eg’ values are in close agreement with the values reported in literature the previously [15]. It should also be noted that the scattering in reported values of band gap energy is attributed to many factors such as method of preparation, deposition conditions, post-deposition annealing, crystallinity etc. The decrease in optical band gap of MoO3 thin films from 3.92 eV to 3.62 eV with increase in substrate temperature from 300 ºC to 350 ºC is attributed to increase in oxygen ion vacancies which capture one or two electrons [36]. For MoO3 thin films deposited at 400 ºC, the increase in optical band gap to 3.65 eV is attributed to partial filling of oxygen ion vacancies [16]. For the films deposited at 450 ºC, the band gap value decreases to 3.44 eV. This may be due to formation of more oxygen vacancies. Generally, the absorption in MoO3 originates from charge transfer transition in Mo–O band. The sub-stoichiometric MoO3-X thin films with oxygen deficiencies contain excess molybdenum atoms which act as doping centers. These centers then control the electrical and optical properties of MoO3 thin films. The dissociation of lattice oxygen

in MoO3 lattice

with formation of sub-stoichiometric MoO3-X is represented as [36]:

where, x is the vacancy concentration and

is lattice oxygen ion vacancy. The decrease in

band gap is due to loss of oxygen with creation of positively charged oxygen ion vacancies with leaving of electrons in the MoO3 lattice. These positively charged oxygen ion vacancies are structural defects in the film which form a narrow donor band below the conduction band known

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as donor levels. Such positively charged oxygen ion vacancies act as donor centers and trap electrons which are responsible for broad band absorption [37].

3.7 Gas sensing properties of MoO3 thin films 3.7.1 Stabilization of sensor resistance Fig. 7 shows the resistance stabilization curve at 200 ºC for MoO3 thin film deposited at substrate temperature of 400 ºC. It is seen that the film resistance decreases with increase in time. The decrease in resistance is attributed to formation of oxygen ion vacancies and production of free charge carriers as a result of temperature. Also, the decrease in resistance involve the oxidative adsorption of water molecules or other gas species onto the surface of film due to electron transfer from adsorbates to MoO3 [38]. The more detailed experiments under well controlled environment are needed to elucidate the nature of oxidized adsorbates on the surface of MoO3 film. The equilibration of chemisorption process i.e., appropriate rate for oxygen adsorption and desorption, results into stabilization of sensor resistance. The time required for stabilization of sensor resistance is around 50 min.

3.7.2 Selectivity study The gas sensors for practical applications have required not only strong response, quick response time and recovery time, but also have a very good selectivity to target gas [39]. The selectivity of a sensor for specific gas is the ability to discriminate between mixture of gases i.e., cross sensitivity to all other gases. Therefore, most desirable sensor is sensitive to only single gas and not affected by others at all. However, most of MOS sensors suffer from lack of gas selectivity. To investigate the selectivity of MoO3 thin film, the response to various gases such as NH3, H2S, CO, CO2, SO2 and NO2 are examined. The gas response of MoO3 thin film deposited at 400 ºC is tested at different operating temperatures by introducing various gases such as NH3, H2S, CO, CO2, SO2 and NO2 of 100 ppm concentration each into test chamber. Fig. 8 shows the selectivity studies of MoO3 thin film deposited at 400 ºC for different operating temperatures towards 100 ppm concentration of various gases. At an operating temperature of 50 ºC, no response is observed to NH3, H2S, CO, CO2, SO2 and NO2 gas due to insufficient thermal energy for adsorption of gas molecules on the surface of sensor film. At an operating temperature of 200 ºC, the responses towards NH3, SO2, 11

CO2, CO, H2S and NO2 gases are found to be 3.8 %, 4 %, 4.3 %, 6 %, 8 % and 30.5 %, respectively. The MoO3 film has relatively large response of 30.5 % to NO2 gas than other gases (NH3, SO2, CO2, CO and H2S) at an operating temperature of 200 ºC. This result shows that the response of gases varies as NH3 < SO2 < CO2 < CO < H2S < NO2 at an operating temperature of 200 ºC. The higher gas response towards NO2 gas is attributed to different reaction dynamics of NO2 gas and other gases with adsorbed oxygen species on the surface of film. It is also concluded that the same film could be used for detection of different gases by operating it at particular temperature for a typical gas because different gases have different energies for adsorption, desorption and reaction on the surface of metal oxide [40]. Thus, by setting operating temperature of sensor, one can use the sensor for particular detection of gas. The gas response and selectivity coefficient studies of MoO3 thin film deposited at 400 ºC towards 100 ppm concentration of various gases at an operating temperature of 200 °C are shown in Fig. 9. The selectivity coefficient study shows that MoO3 film is more sensitive and selective towards NO2 gas. This is due to higher electron affinity of NO2 (2.28 eV) in comparison with pre-adsorbed oxygen (0.43 eV) [41]. In NO2, nitrogen (N) contains one unpaired electron after forming the covalent bond between nitrogen and oxygen. This single unpaired electron in nitrogen is the reason why chemisorption of NO2 gas is more likely in comparison of other gases. This unpaired electron in N of NO2 forms bond with the surface oxygen of MoO3 and subsequently, promote the chemisorption [42].

3.7.3 The NO2 gas response of MoO3 thin films deposited at various substrate temperatures Fig. 10 shows the transient response curves towards 100 ppm NO2 gas concentration at an operating temperature of 200 °C for MoO3 thin films deposited at various substrate temperatures. The MoO3 thin films show an increase in resistance during exposure of an oxidizing NO2 gas, indicating n-type behavior of films. The film deposited at substrate temperature of 300 ºC shows low gas response of 21.5 % due to lower surface roughness and crystallinity. For MoO3 thin film deposited at 350 ºC, the increased NO2 gas response of 23 % is attributed to improved surface roughness and crystallinity of film. For MoO3 thin film deposited at 400 ºC, the maximum NO2 gas response of 30.5 % is observed. This is due to nanobelts-like morphology, higher surface roughness which increases the scattering of charge carriers from the surface of film and increases its resistance by capturing oxygen atoms. The higher porosity 12

provides larger contact area for diffusion of more gas molecules. From FE-SEM micrograph of film deposited at 400 ºC, it is seen that the nanobelts-like network is connected deep inside the porous structure. Therefore, such kind of morphology helps the NO2 gas molecules to react on outer surface as well as inside porous surface more efficiently. Thus, effective surface area for NO2 gas adsorption and reaction increases which increases the response evidently. For MoO3 thin film deposited at 450 ºC, the agglomeration of nanobelts over entire substrate surface reduces the active surface area for adsorption and hence, the gas response decreases to 26 %. The response and recovery times are two important factors of sensor that determines the real time detection of gas species [28]. The response time of sensor involve the diffusion of interacting species to oxide surface from ambient followed by its adsorption on the surface and subsequent chemical interaction with surface lattice oxygen which results into formation of nonstoichiometric oxide. The recovery time of sensor depends on the kinetics of desorption of reaction products from the oxide surface and subsequent adsorption kinetics of oxygen molecule from ambient to surface of film [43]. The response and recovery times of MoO3 thin films deposited at various substrate temperatures towards 100 ppm NO2 gas concentration at an operating temperature of 200 ºC are summarized in Table 2.

3.7.4 Determination of operating temperature The operating temperature of sensor is most important parameter that affects the gas response and response kinetics. The adsorption, desorption, co-adsorption, surface coverage, chemical decomposition and other reactions are temperature activated processes which results into different static characteristics at different operating temperatures. The resistance of n-type MOS is significantly increases by the formation of an electron depletion layer near the surface due to adsorption of oxygen atoms in the form of

,

, and

species [44]. The ionized

species of adsorption depends upon the sensing temperature. The increase in sensor resistance upon exposure to NO2 gas in the present study is attributed to decrease in electron concentration due to filling of lattice oxygen vacancies and adsorption of NO2 gas. As operating temperature is an important parameter affecting the gas sensing performance, the gas response of MoO3 thin film deposited at 400 ºC is studied at different operating temperatures. At appropriate operating temperature, more active sites are available on surface of material. It is well known that at low temperature, the activation energy is low and the hopping conductivity is attributable to 13

formation of large number of oxygen ion vacancies in the film. At higher temperature, the activation energy is higher and the electrical conduction in the film is due to band-to-band transitions [45]. To determine an optimum operating temperature of MoO3 thin film, the response of film is measured towards 100 ppm concentration of NO2 gas at different operating temperatures ranging from 50 °C to 250 °C. The variation in NO2 gas sensing response for MoO3 thin film deposited at 400 ºC towards 100 ppm concentration at different operating temperatures is shown in Fig. 11. It is observed that at lower operating temperature of 50 ºC, there is no gas response. This is due to gas molecules do not have sufficient thermal energy to react with adsorbed oxygen species at the surface. The NO2 gas response increases from 13.8 % to 30.5 % with increase in operating temperature from 100 ºC to 200 ºC and then decreases to 12.2 % for 250 ºC. The MoO3 thin film deposited at 400 ºC shows low response of 13.8 % and 14 % at an operating temperature of 100 °C and 150 °C, respectively. This is attributed to less number of charge carriers participating in hopping process due to insufficient thermal energy which leads to less adsorption, dissociation and reaction of NO2 gas molecules on the surface of film. It means that at lower operating temperatures, the sensor response is restricted by speed of chemical reactions. At an operating temperature of 200 °C, the maximum NO2 gas response of 30.5 % is obtained. This is due to sufficient thermal energy for reactions between adsorbed gas and thermally excited charge carriers. At an operating temperature of 200 °C, more charge carriers overcome the activation energy barrier i.e. trap levels below the conduction band and participate in electrical conduction. Therefore, the speed values of two processes (diffusion of gas molecules and chemical reaction) become equilibrium and the sensor response reaches its maximum value. However, at higher operating temperature, the gas response is restricted by the speed of diffusion of gas molecules because the gas adsorption is too difficult due to increased surface reactivity. Therefore, if rate of reaction is too large than that of diffusion rate of gas, gas molecules cannot access the grains and leaving them un-utilized for gas sensing which results into loss of sensor response [46]. At an operating temperature of 250 °C, the number of charge carries increases, while the pre-adsorbed oxygen and adsorbed NO2 gas species gets reduced from the sensing sites of sensor due to dominant desorption rate than adsorption rate of NO2 gas. This decreases the NO2 response to 12.2 %. Thus, when oxygen desorbs from the sensor surface, it gives electrons back to the sensor surface. Therefore, adsorption/desorption of oxygen increases/decreases the 14

resistivity of MoO3 thin film. This shows that operating temperature has significant effect on gas sensing properties of MoO3 thin film. As the MoO3 thin film shows highest NO2 gas response at an operating temperature of 200 °C and on either sides of this temperature gas response decreases, an optimum operating temperature of 200 °C has been used in further measurements. The transient response curves for MoO3 thin film deposited at 400 ºC towards 100 ppm NO2 gas concentration at different operating temperatures are shown in Fig. 12. The response and recovery times for MoO3 thin film deposited at 400 ºC towards 100 ppm NO2 gas concentration at different operating temperatures are summarized in Table 3. Both response and recovery times decreases as operating temperature increases. The recovery time is found to be longer than response time. The larger recovery time at lower operating temperature is due to more prominently adsorbed

species on the surface which are less reactive as compared

to other species of oxygen such as

and

.

3.7.5 Effect of NO2 concentration on gas response The concentration of analyte gas depends on the concentration of active site on the surface of sensing material. It means that the response is proportional to active site concentrations on the surface of sensing material [47]. After fixing an operating temperature to 200 °C, the gas response is studied by varying NO2 gas concentration from 20 ppm to 100 ppm. The transient response curves for MoO3 thin film deposited at 400 ºC towards various NO2 gas concentrations at an operating temperature of 200 °C are shown in Fig. 13. The response of MoO3 thin film to NO2 gas shows a good linear increase with increase in NO2 gas concentration from 20 ppm to 100 ppm. This is attributed to increase in chemically adsorbed gas molecules as well as sufficient availability of adsorption sites on the sensor surface. The lower response of 10.3 % for 20 ppm gas concentration is attributed to smaller surface coverage by NO2 gas molecules on the film surface. The response is found to be about 14.5 %, 18.2 %, 25.1 % and 30.5 % towards 40 ppm, 60 ppm, 80 ppm and 100 ppm NO2 gas concentration, respectively. The increase in response is attributed to increase in surface reactions due to larger surface coverage by adsorbed NO2 gas molecules. The response and recovery times of MoO3 thin film deposited at 400 ºC towards various NO2 gas concentrations at an operating temperature of 200 °C are summarized in Table 4.

15

It is seen that the response and recovery times increases with an increase in NO 2 gas concentration. The sensitivity of sensor is the slope of graph of sensor response versus concentration of NO2 gas. Fig. 14 shows the plot of gas response as a function of NO2 gas concentration. It is well known that the surface reactions are linearly dependent on the concentration of adsorbed gas as long as adsorption sites are enough [48]. A nearly linear response over the concentration range from 20 ppm to 100 ppm of NO2 is due to almost larger number of availability of surface adsorption sites. The sensitivity of MoO 3 thin film sensor is found to be 0.25 per ppm.

3.7. 6 NO2 gas sensing mechanism The MoO3 is n-type semiconductor due to oxygen vacancies and interstitial molybdenum atoms. The free molybdenum atoms not bounded to oxygen atoms act as donors and provides free electrons. Due to these free electrons, the MoO3 film has high charge carrier density. However, if number of oxygen atoms integrated within the molybdenum atoms increases, the number of free electrons decreases and thereby, the resistance of film increases [49]. It is well known that the chemisorption of oxygen species such as

,

and

on the MOS surface

strongly depends on the temperature [50]:

where, subscript (ads.) stands for adsorption. The monomolecular adsorption of oxygen occurs below 100 oC, while adsorption by dissociation occurs in the temperature range of 100 oC to 300 oC. When gas arrives at the surface of sensor, it interacts both physically and chemically. The adsorption of gas leads to charge exchange between adsorbate layer and the material itself which results into variation in free electron concentration available for conduction [51]. When MoO3 film is exposed to air, oxygen molecules in the air chemisorbed onto the surface of film by capturing electrons from the conductance band of MoO3. This results into increase in resistance of film and leads to formation of an electron depleted space charge region at the surface of MoO3

16

nanobelts as shown in Fig. 15 (a). The space charge region created at the surface of MoO3 nanobelts causes decrease in concentration of electrons in the film [52]. When MoO3 film is exposed to NO2, the NO2 gas molecules capture electrons from conduction band of MoO3 and adsorbed as

. This results into further increase in electron

depleted space charge region at the surface of MoO3 nanobelts shown in Fig. 15 (b). The high surface area to volume ratio of MoO3 nanobelts provides larger number of atoms on the surface which increases the adsorption of NO2 gas molecules. Thus, increase in space charge region causes further decrease in concentration of electrons which results into again increase in resistance of film. The proposed NO2 gas adsorption reactions are as follows:

The monomolecular adsorption of NO2 gas at oxidized sensing surface layer takes place as follows:

A dissociative adsorption of NO2 gas on the sensor surface takes place at the lattice oxygen vacancies and forms adsorbed oxygen species as follows:

where,

is the oxygen vacancy. Thus, the adsorbed

ions are desorbed as NO gas immediately when input flow of

NO2 gas is stopped and consequently, the process of recovery of initial conditions takes place. The recovery process of sensing layer involves the kinetics of desorption of reaction products from the MoO3 surface, electron filling at oxygen vacancy sites and subsequent adsorption kinetics of oxygen molecules from the atmosphere. Thus, oxygen vacancies in MoO3 are replenished by re-oxidation of oxide surface with gaseous oxygen through the reaction as follows [26]:

The NO gas released in equations (14) and (15) further gets oxidized in air by reacting with atmospheric oxygen with forming NO2 gas.

17

4. Conclusions The MoO3 thin films have been successfully synthesized using a simple and cost effective CSP deposition technique. The effect of substrate temperature on physicochemical and gas sensing properties of MoO3 thin films has been studied. The physicochemical characterization of thin films indicates that crystallinity, surface morphology, surface roughness, film thickness and absorption edge are strongly dependent on the substrate temperature. The XRD studies confirm the formation of MoO3 thin film with an orthorhombic crystal structure. The FE-SEM micrographs show the formation of nanobelts-like morphology which provides high surface area to volume ratio for gas adsorption. The small pores of varying sizes observed due to random orientation of nanobelts over the surface of film provide larger contact area for gas adsorption. The AFM study reveals that the increase in RMS surface roughness of MoO3 thin films provides more adsorption sites for gas adsorption and increases the gas response. The optical study shows that optical band gap of MoO3 thin films decreases from 3.92 eV to 3.44 eV. The selectivity study shows that the films are more sensitive and selective towards NO2 gas. The maximum NO2 gas response of 30.5 % is obtained for film deposited at substrate temperature of 400 ºC with response and recovery times of 20 s and 160 s, respectively. The response and recovery times are found to be increased with increase in NO2 gas concentration from 20 ppm to 100 ppm. Thus, the higher response, selectivity and fast response/recovery times of MoO3 nanobelts based sensor towards NO2 gas has potential application for the development of novel NO2 gas sensor. In addition, nanobelts-like morphology of MoO3 with high surface area to volume ratio can open novel scientific, environmental and technological applications in various fields such as solar cells, supercapacitor electrodes, optoelectronic devices and water splitting cells.

Acknowledgements The authors are very much thankful to DAE-BRNS, Mumbai, India for financial support through major research project No. 2013/36/29-BRNS/2351 and physics instrumentation facility centre (PIFC), Department of Physics, Shivaji University, Kolhapur for providing characterization facilities.

18

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Figure captions: Fig. 1 The TGA-DTA curves of MoCl5 powder. Fig. 2 The XRD patterns of MoO3 thin films deposited at various substrate temperatures (a) 300 ºC, (b) 350 ºC, (c) 400 ºC and (d) 450 ºC. Fig. 3 The FE-SEM micrographs of MoO3 thin films deposited at various substrate temperatures (a) 300 ºC, (b) 350 ºC, (c) 400 ºC and (d) 450 ºC. Fig. 4 The three dimensional AFM micrographs of MoO3 thin films deposited at various substrate temperatures (a) 300 ºC, (b) 350 ºC, (c) 400 ºC and (d) 450 ºC. Fig. 5 Optical absorption spectra of MoO3 thin films deposited at various substrate temperatures (a) 300 ºC, (b) 350 ºC, (c) 400 ºC and (d) 450 ºC. Fig. 6 The plots of (αhν)2 versus photon energy (hν) of MoO3 thin films deposited at various substrate temperatures (a) 300 ºC, (b) 350 ºC, (c) 400 ºC and (d) 450 ºC. Fig. 7 The resistance stabilization curve of at 200 ºC for MoO3 thin film deposited at substrate temperature of 400 ºC. Fig. 8 The selectivity studies of MoO3 thin film deposited at 400 ºC for different operating temperatures towards 100 ppm concentration of various gases. Fig. 9 The gas response and selectivity coefficient studies of MoO3 thin film deposited at 400 ºC towards 100 ppm concentration of various gases at an operating temperature of 200 °C. Fig. 10 The transient response curves towards 100 ppm NO2 gas concentration at an operating temperature of 200 °C for MoO3 thin films deposited at various substrate temperatures (a) 300 ºC, (b) 350 ºC, (c) 400 ºC and (d) 450 ºC. Fig. 11 The variation in NO2 gas sensing response for MoO3 thin film deposited at 400 ºC towards 100 ppm concentration at different operating temperatures Fig. 12 The transient response curves for MoO3 thin film deposited at 400 ºC towards 100 ppm NO2 gas concentration at different operating temperatures (a) 100 ºC, (b) 150 ºC, (c) 200 ºC and (d) 250 ºC. Fig. 13 The transient response curves for MoO3 thin film deposited at 400 ºC towards various NO2 gas concentrations at an operating temperature of 200 °C. Fig. 14 The plot of gas response as a function of NO2 gas concentration. Fig. 15 The proposed schematic gas sensing mechanism of MoO3 nanobelts (a) oxygen adsorption (b) NO2 gas adsorption. 24

Table captions: Table 1 The film thickness, crystallite size, average particle size and RMS surface roughness values of MoO3 thin films deposited at various substrate temperatures. Table 2 The response and recovery times of MoO3 thin films deposited at various substrate temperatures towards 100 ppm NO2 gas concentration at operating temperature of 200 ºC. Table 3 The response and recovery times for MoO3 thin film deposited at 400 ºC towards 100 ppm NO2 gas concentration at different operating temperatures. Table 4 The gas response, response and recovery times of MoO3 thin film deposited at 400 ºC towards various NO2 gas concentrations at an operating temperature of 200 °C.

25

Fig. 1

26

Fig. 2

27

Fig. 3

28

Fig. 4

29

Fig. 5

30

Fig. 6

31

Fig. 7

32

Fig. 8

33

Fig. 9

34

Fig. 10

35

Fig. 11

36

Fig. 12

37

Fig. 13

38

Fig. 14

39

Fig. 15

40

Table 1

XRD results

Substrate

Film

temperature

thickness

Crystallite size

(ºC)

(nm)

for (040) plane (nm)

AFM results Average

RMS

particle

roughness

size (nm)

(nm)

300

529

50.4

160

8.6

350

430

51.6

205

11

400

350

59.1

260

12

450

235

51.7

225

11.5

Response time (s)

Recovery time (s)

Table 2

Substrate

NO2 gas

temperature (ºC)

response (%)

300

21.5

52

> 985

350

23

44

485

400

30.5

20

160

450

26

74

633

41

Table 3

Operating

NO2 gas

temperature (°C)

response (%)

Response time (s)

Recovery time (s)

50

0

---

---

100

13.8

222

> 1060

150

14

92

>1123

200

30.5

20

160

250

12.2

18

60

NO2 gas

NO2 gas

concentration (ppm)

response (%)

20

Table 4

Response time (s)

Recovery time (s)

10.3

8

64

40

14.5

10

99

60

18.5

12

128

80

25.1

17

136

100

30.5

20

160

42

Graphical abstract

43