Electrical conductivity and gas sensing properties of MoO31

Sensors and Actuators B 101 (2004) 161–174

Electrical conductivity and gas sensing properties of MoO31 S.S. Sunu a , E. Prabhu a , V. Jayaraman a , K.I. Gnanasekar a , T.K. Seshagiri b , T. Gnanasekaran a,∗ a

Materials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India b Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Received 13 August 2003; received in revised form 23 February 2004; accepted 27 February 2004 Available online 22 April 2004

Abstract Electrical conductivity and gas sensing properties of MoO3 are investigated. The electrical conductivity is found to be independent of oxygen partial pressure in the temperature range 510–773 K. Two distinct conduction processes were identified from the conductivity experiments carried out under ambient air, moist oxygen, and moist argon. The conductivity in the low temperature range (510–578 K) are attributed to species arising from the reversibly inserted water molecules into MoO3 lattice. The conduction process in the high temperature region (578–773 K) are attributed to the non-stoichiometry existing in the sample due to the presence of Mo5+ ions which was confirmed by EPR and XPS investigations. Sensing characteristics of MoO3 towards NH3 , H2 , and LPG were studied. Experiments showed that the ammonia sensing mechanism of MoO3 involved the formation of molybdenum suboxides and nitride. © 2004 Elsevier B.V. All rights reserved. Keywords: Molybdenum oxide; Electrical conductivity; Gas sensors

1. Introduction Transition metal oxides attract considerable interest due to their suitability for use in electronic and magnetic devices, in heterogeneous catalysis and in a number of other applications including gas sensors. These compounds are characterized by the incomplete d-shell of the metal cations, which makes them exhibit wide variety of properties. TiO2 [1], a variety of niobates and tantalates [2], and WO3 [3] are some of the transition metal oxides which are explored as gas sensor materials. Molybdenum trioxide has been well known for its application as catalyst in oxidation of hydrocarbons and reduction of NOx [4,5]. Efforts have been made to examine the gas sensing properties of MoO3 and MoO3 based systems [6–10]. Since modulation of electrical conductivity of the oxide in the presence and absence of analyte gas is exploited for gas sensing applications, detailed studies on the electrical properties of porous pellets of MoO3 under different oxygen partial pressures have been carried out by us as a first step in understanding its gas sensing mechanism. Effect of moisture on the electrical ∗ Corresponding author. Tel.: +91-4114-280-098; fax: +91-4114-280-065. E-mail address: [email protected] (T. Gnanasekaran). 1 Dedicated to Prof. Adolf Mikula, University of Vienna on the occasion of his 60th birthday.

0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.02.048

conductivity of MoO3 has been measured and supported by infrared and thermogravimetric measurements. Its sensing characteristics towards various analyte gases such as NH3 , LPG, and H2 have been investigated at different operating temperatures. Experiments were designed to identify the intermediate phases formed during ammonia sensing by MoO3 . Isothermal mass loss characteristics and conductivity changes in air–ammonia mixtures were also studied to elucidate the mechanism of ammonia sensing by MoO3 . The results of these experiments are presented in this paper.

2. Experimental Molybdenum trioxide was prepared by heating ammonium heptamolybdate tetrahydrate at 673 K for 30 h to constant weight in air. Complete decomposition of ammonium heptamolybdate to MoO3 under these conditions was previously confirmed by thermogravimetric and differential thermal analysis (model STA 1500, Rheometric Scientific, UK). This product was further heated at 773 K for 24 h in air and stored under ambient conditions. The product, which is henceforth referred to as as-prepared MoO3 , was analyzed by X-ray powder diffraction (model D 500 of M/s Siemens, Germany) using Cu K␣ radiation. EPR spectra of the as-prepared MoO3 samples were recorded at room

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temperature using Bruker ESP-300 spectrometer operated at X-band frequency 9.5 GHz using 100 kHz field modulation. Diphenyl picryl hydrazyl radical (DPPH) was used as a field marker. X-ray photoelectron spectra of these samples were also recorded using VGESCA LAB MK II spectrophotometer with 150 mm hemispherical analyzer at a band pass energy of 20 eV. Al K␣ X-ray radiation of 1486.6 eV was used. Samples deliberately mixed with 1000 ppm of ultra high pure carbon were supported on indium foil. XPS analyses were carried out at room temperature under a vacuum of 2.3 × 10−8 mbar. The binding energy value of C 1s was also recorded for correction arising out of charging. Weight change characteristics of the as-prepared MoO3 powders were determined by thermogravimetric studies by heating the sample in air ambient from room temperature to 773 K at a heating rate of 5 K min−1 . These experiments were also conducted by heating another sample of the as-prepared MoO3 in air ambient at a heating rate of 10 K min−1 up to 873 K followed by holding the sample isothermally at 873 K for 2 h. Pellets (10 mm diameter and 2–3 mm thickness) of the as-prepared MoO3 powders were made after adding polyvinyl alcohol as binder and were heated at 773 K in air for 5 h. The densities of the sintered pellets were measured by Pycnometry. One such pellet was mounted between two spring-loaded platinum foils of identical dimensions and placed inside a quartz chamber which in turn was heated by a furnace. Conductivity of the pellet sample was measured in the temperature range of 510–773 K using a frequency response analyzer (Model SI 1255 of Solartron, M/s Schlumberger, UK) coupled with an electrochemical interface (Model 1286 of Solartron, M/s Schlumberger, UK) in the frequency range of 1 Hz–1 MHz. Conductivity was measured in flowing ambient air (flow rate ∼30 ml min−1 and RH ∼50%) and in air dried by purging it through silica gel. To understand the effect of oxygen partial pressure on the conductivity of the as-prepared MoO3 sample, measurements were carried out under flowing argon (∼10 ppm O2 ) and 100% O2 in the same range of temperature (flow rates in each atmosphere 30 ml min−1 ). These experiments were repeated under moist argon and oxygen obtained by bubbling them through water at ambient temperature. To separate the contribution of electrode–electrolyte interfacial phenomenon from those of bulk and grain boundary conductions, measurements were made using gold and silver electrodes also. To evaluate the dependence of conductivity characteristics of MoO3 on the temperature at which it was prepared in air from the starting materials, as-prepared MoO3 powders were heated in air at 973 K for 10 h. The powders were then pelletized as described above. Electrical conductivity of this pellet specimen was also measured as a function of temperature in ambient air in the temperature range of 510–773 K. Using an FTIR MB-100 spectrometer of M/s Bomem, Canada, infrared spectra of the as-prepared MoO3 samples were recorded. IR spectra were also recorded for (a) the sam-

ples heated at 373, 473, and 573 K in flowing argon atmosphere for 5 h and (b) the sample which was first heated at 573 K under flowing argon and then stored in ambient air for a few days. Specimens for IR spectra were prepared by dispersing about 1–2 mg of the samples in ∼100 mg of moisture free KBr (spectroscopy grade of M/s E-Merck, Germany) in argon atmosphere glove box and compacting into thin discs. Differential scanning calorimetric experiments (M/s Mettler Toledo, USA) were carried out with as-prepared MoO3 samples (∼50 mg) in static air up to 773 K. For sensor studies, a pellet of the as-prepared MoO3 sample was sandwiched between two platinum electrodes and was housed in a glass test chamber. To optimize the sensor operating temperature, studies were carried out with 500 ppm of NH3 in air at temperatures between 503 and 673 K as per the procedure described in reference [11]. The sensor assembly was heated by a resistance furnace and its temperature was controlled to ±1 K. Resistance of the sensor was measured using an electrometer (Model 616, M/s Keithley Instruments, USA). At a chosen experimental temperature, the sensor was equilibrated till a steady baseline resistance in air was reached. Adequate quantity of ammonia was then injected into the test chamber so that resulting ammonia concentration was 500 ppm. Change in resistance of the pellet was measured as a function of time till a steady value was reached. The chamber was then purged with air for about 2 min and the sensor allowed to reach the initial value of the resistance before the next experiment was carried out. The sensitivity for the chosen concentration of ammonia viz. 500 ppm was calculated as below:
 R a − Rg sensitivity(%) = × 100 Ra where Ra is resistance of the sensor in air and Rg the resistance of the sensor in presence of the analyte gas. Minimum three tests were made at each sensor operating temperature. This cycle of testing as a function of temperature lasted for ca. 24 h and was followed by two more cycles to check the reproducibility and stability of the sensor. Sensitivity for 500 ppm of ammonia in air was calculated by taking the arithmetic mean of the values (which were within ±3%) obtained from individual tests made at each temperature in these cycles. In order to check the reproducibility, the experiments were repeated with minimum two more pellets. Similar studies were carried out with 500 ppm each of H2 and LPG in air. Sensitivities were also measured for different concentrations of NH3 and H2 (10–500 ppm) in air at the sensor operating temperature of 623 K. In order to examine the effect of electrode materials on ammonia sensing characteristics, experiments were also carried out with gold and graphite electrodes. To unravel the ammonia sensing mechanism of MoO3 , compounds formed during its interaction with ammonia containing air were characterized. For this purpose three pellets of as-prepared MoO3 sample were placed inside a glass chamber of known volume and heated to 673 K in static air.

S.S. Sunu et al. / Sensors and Actuators B 101 (2004) 161–174

(021)

3000

10

20

30

(200) (210)

(150)

0

(141)

(101)

(130)

(041)

(111)

(060)

1000

40

(002) (230) (170) (211) (221) (112) (042) (171) (081) (260) (251) (062) (190) (0100)

(040)

(110)

2000

(020)

Intensity (a.u.)

NH3 gas corresponding to its final concentration of 6.67% was injected into the chamber. The chamber was then allowed to cool at the rate of 10–15 K min−1 to room temperature. Pellets were removed and one of them was subjected to XRD analysis immediately. This pellet was reheated in air at 673 K for 3 h and again analyzed by XRD. The other two pellets were used for XPS and SEM studies. Thin films of MoO3 (∼1 ␮m thickness) were deposited on polycrystalline alumina substrates by pulsed laser deposition, details of which are given in our earlier publication [12]. One such film was housed in a glass test chamber and heated to 673 K. It was then exposed to a continuous flow of 1500 ppm of ammonia in air for 90 min and cooled in the same environment to room temperature. The film was analyzed by XRD. It was reheated in air at 673 K for 3 h and analyzed by XRD. To study the characteristics of the intermediates formed during the sensing of ammonia by MoO3 , isothermal thermogravimetric experiments under an ambient of flowing 50% NH3 –air mixture and electrical conductivity measurements under alternating ambients of air and 6.67% NH3 –air mixture were conducted. For isothermal thermogravimetric studies, 50 mg of the as-prepared MoO3 sample was maintained at 673 K in the thermal analyzer under a flow of synthetic air (flow rate = 60 ml min−1 ). The gas flowing into the equipment was then switched over from air to 1:1 air–ammonia mixture (flow rate = 60 ml min−1 ). The weight loss by the sample under this gas environment was continuously monitored. For electrical conductivity studies, a pellet of the as-prepared MoO3 sample was sandwiched between two platinum electrodes and placed inside a glass chamber. The pellet was maintained at 673 K and its stable resistance in air was measured. A known quantity of ammonia corresponding to a final concentration of 6.67% ammonia in air was injected into the chamber and the variation of resistance of the pellet with time was recorded. On attaining a stable resistance, the chamber was purged with air and gain in resistance of the pellet was monitored. All these experiments were repeated several times for ensuring reproducibility.

163

50

60

70

2θ / degrees Fig. 1. XRD pattern of the as-prepared MoO3 sample prepared from ammonium heptamolybdate.

it. EPR spectra recorded at room temperature is shown in Fig. 2. Two types of signals were observed. The signal with g⊥ = 2.09 and g|| = 2.00 was tentatively assigned to O− on the basis of the g values reported in the literature [16]. However, it should be noted that the g⊥ values are highly matrix dependent and vary over a wide range. The other signal with g⊥ = 1.9685 and g|| = 1.8684 is of Mo5+ (d1 ) species [17]. This signal is associated to Mo5+ in the vicinity of an oxygen vacancy in the hexa-coordinated MoO6 octahedra in MoO3 lattice. Fig. 3 shows the XPS spectra of Mo 3d levels for the as-prepared MoO3 sample. The doublet pattern observed for MoO3 is due to the spin orbit splitting of Mo 3d levels giving rise to Mo 3d5/2 and Mo 3d3/2 levels with an energy separation of 3.2 eV [18]. Binding energy of Mo

3. Results and discussion 3.1. Characterization of MoO3 samples Fig. 1 shows the XRD pattern of the as-prepared MoO3 sample and the observed lines are indexed in terms of the orthorhombic phase of MoO3 (space group: Pbnm, JCPDS card number 5–508). Stoichiometry of MoO3 is known to be significantly dependent upon the method of preparation employed [13,14]. It is also known from literature that the orthorhombic structure of the phase is retained till the oxygen to metal ratio is reduced down to 2.89 and further reduction of oxygen content leads to separation of Mo9 O26 phase [15]. EPR spectroscopy studies with the as-prepared MoO3 powder had confirmed the presence of Mo5+ ions in

Fig. 2. EPR spectrum of as-prepared MoO3 sample recorded at room temperature: (a) perpendicular component of O− , (b) parallel component of O− , (c) perpendicular component of Mo5+ , (d) parallel component of Mo5+ .

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8k

Mo3d 5/2

Mo3d in MoO 3 Fitted Curve

37.42

Mo 5+ Mo 6+

Intensity (a.u.)

6k

37.40

Mo3d 3/2

Loss due to moisture

Weight / mg

37.38

4k

2k

37.36

Weight pick up due to oxygen intake

873 K

37.34 37.32

0

Evaporation loss (45 g/h)

733 K 673 K

240

238

236

234

232

230

37.30

228

Binding energy (eV)

Isothermally maintained at 873 K

37.28 40

Fig. 3. XPS spectra of the Mo 3d doublet in MoO3 as-prepared sample.

3d3/2 and Mo 3d5/2 levels are observed at 235.7 (FWHM ∼ 2.1 eV) and 232.5 eV (FWHM ∼ 2.0 eV), respectively. The values are close to the standard values of Mo6+ ions (Mo 3d5/2 : 232.5 ± 0.2 and Mo 3d3/2 : 235.7 ± 0.2) and confirm that Mo is present mainly in +6 oxidation state listed in Table 1 when it is coordinated with oxygen ions. A careful examination of the figure shows the presence of a shoulder at ∼231.5 eV implying the existence of Mo ions with different valence states. In fact, large values of FWHMs (∼2.0 eV) observed for Mo 3d5/2 and Mo 3d3/2 levels also indicate a strong overlap of the spectra arising out of different valence states. The spectra obtained were deconvoluted using a splitting parameter of 3.2 eV and an intensity ratio (Mo 3d5/2 /Mo 3d3/2 ) of 1.5. In the deconvoluted spectra, a clear pattern of Mo 3d levels with binding energies values at 231.5 eV for Mo 3d5/2 level and 234.7 eV for Mo 3d3/2 level are obtained in addition to the characteristic pattern of Mo6+ ions. Comparison of these binding energy values with the standard values reported in literature for different valence states of Mo (Table 1) shows that they correspond to Mo5+ ions in the lattice. These results of EPR and XPS studies confirm the presence of Mo5+ ions and hence the existence of non-stoichiometry in the as-prepared MoO3 sample. The non-stoichiometry of the as-prepared MoO3 sample was estimated from the results of thermogravimetric studies conducted. During these studies a known quantity of

80

120

160

Time / min Fig. 4. Thermogravimetric analysis of the as-prepared MoO3 sample.

the as-prepared MoO3 sample (37.413 mg) was heated in air at 10 K min−1 up to 873 K followed by maintaining it isothermally at 873 K for 2 h. The thermogravimetric pattern obtained is shown in Fig. 4. This experiment showed that a mass loss of 0.23 wt.% occurred up to 673 K due to removal of moisture (this aspect is discussed in the next section). Thereafter the sample mass remained constant (37.327 mg) till its temperature reached 733 K. In the following 14 min, a low but definite increase in sample mass was observed till it reached a maximum. After attaining this maximum, the sample mass started decreasing with the loss rate increasing with time. The sample mass again became equal to the value that prevailed in the temperature range of 673–733 K viz. 37.327 mg in ∼51 min and thereafter the mass loss rate remained constant. Since diffusion coefficient of oxygen in MoO3 lattice would be significant at these experimental temperatures [19], an increase in sample mass due to annihilation of its non-stoichiometry by oxygen pick up from ambient air is expected. On the other hand, a significant mass loss is also expected since the vapor pressure of MoO3 is appreciable at these temperatures (1 Pa at 873 K) [20]. Rate of increase of sample mass would be dependent on the extent of non-stoichiometry and this would be high at the initial stages. The rate of increase of mass would decrease with

Table 1 Standard binding energies of Mo 3d levels for molybdenum at different oxidation states Serial No.

Oxidation state

1. 2. 3. 4. 5. 6.

Mo6+ Mo5+ Mo4+ Mo3+ Mo2+ Mo0 a b

Standard (reported) values (in eV) for

References

Mo 3d5/2 (FWHM)

Mo 3d3/2 (FWHM)

232.5 231.5 230.1 229.3 228.4 227.7

235.7 234.7 233.3 232.5 232.6 230.9

(±0.02): (±0.02): (±0.02): (±0.02): (±0.02): (±0.02):

1.7 1.7 1.6 1.5 1.4 1.2

Binding energies reported for Mo6+ , Mo5+ , and Mo4+ ions bonded with oxygen atoms. Binding energies reported for Mo3+ and Mo2+ ions bonded with nitrogen atoms.

(±0.02): (±0.02): (±0.02): (±0.02): (±0.02): (±0.02):

1.7 1.7 1.6 1.5 1.4 1.2

[18]a [18]a [18]a [35–39]b [35–39]b [18]

S.S. Sunu et al. / Sensors and Actuators B 101 (2004) 161–174

3.2. Electrical properties Fig. 5 shows the typical impedance pattern of as-prepared MoO3 at 750 K in ambient air using platinum as electrodes. The data was analyzed by deconvolution technique using Boukamp analysis programme [21]. The data obtained at temperatures above 698 K could be fitted to two semicircles. To separate the contribution of electrode–electrolyte interfacial processes, measurements were made using different amplitudes of voltage signal viz. 50, 100, 150, and 250 mV and also with gold and silver as electrodes. We observed a reaction between silver electrode and MoO3 under these conditions while gold electrode was compatible. Hence, further experiments with silver electrode were not conducted. These changes of the amplitude of the applied voltage and of the electrode material to gold led to the alteration of capacitance and resistance of the semicircle corresponding to the low frequency region while those of the semicircle at the high frequency region remained unaltered. The capacitance value of the semicircle in the low frequency region was a few microfarad indicating it to be due to electrode–electrolyte interfacial process [22]. The semicircle

-60

Z" / k ohms

time since the non-stoichiometry of the sample would fall with time by oxygen pick up. But, the rate of mass loss due to evaporation of the sample would increase first, since the sample temperature would have increased from 733 to 873 K and thereafter remained constant during the isothermal heating. Due to these opposing factors, the mass of the sample first increased and later started decreasing. If the sample is assumed to have become stoichiometric between the time period when its mass started increasing and when it again became equal to the pre-increase mass (viz. 65 min), the time independent mass loss rate observed afterwards would be determined by the steady evaporation of the sample. The expected increase in sample mass by oxygen pick up to annihilate the non-stoichiometry in it would then be equal to mass lost in this period due to evaporation. This mass pick up was calculated from the observed steady mass loss rate of 45 ␮g h−1 by assuming that the sample temperature remained at 873 K (although it varied from 733 to 873 K in the first 14 min and thereafter remained constant at 873 K) throughout the period when oxygen pick up occurred. Using this calculated mass pick up and taking the mass of the sample that remained steady between 673 and 733 K as the mass of the moisture free as-prepared MoO3 sample, the stoichiometry of the as-prepared sample was estimated as MoO2.988 . Dieterle [14] had prepared MoO3 samples by heating ammonium heptamolybdate tetrahydrate first in nitrogen for 4 h followed by heating the ensuing product in air at 673 K for 1 h. He had determined the stoichiometry of the sample by Raman spectroscopy as 2.946. In the present work, ammonium heptamolybdate tetrahydrate was heated in air at 773 K for 24 h and hence stoichiometery of the as-prepared MoO3 samples is expected to be higher than that obtained by Dieterle.

165

-40

ω -20

1 Hz 1 MHz

0 0

30

60

90

Z' / k ohms Fig. 5. Typical impedance spectrum of as-prepared MoO3 sample at 750 K in air (RH ∼ 50%).

observed at high frequency region therefore corresponds to either bulk or grain boundary conduction or a combination of both the processes, which could not be resolved due to overlap of relaxation frequencies. The data obtained below 698 K could not be resolved into two semicircles because the electrode–electrolyte interfacial process was measurable at high temperatures only. The conductivity of as-prepared MoO3 sample as a function of temperature in air (RH ∼ 50%) is shown in Fig. 6a. The conductivity values were reproducible during heating and cooling cycles. The observed variation of conductivity in air with respect to temperature could be represented as a superposition of two straight lines (i) log σ1 = A1 − E1 /kT for the low temperature region (up to 578 K) and (ii) log σ2 = A2 − E2 /kT for the high temperature region (578–773 K), where A1 and A2 are Arrhenius constants, E1 and E2 activation energies for the conduction processes, k is the Boltzmann constant and T is temperature in Kelvin. The activation energies calculated are 0.71 ± 0.02 eV in the high temperature region and 0.27 ± 0.03 eV in the low temperature region. The conductivity studies carried out under moist O2 and moist argon also showed a slope change in the Arrhenius plot at 578 K (Fig. 6b) as in the case of the measurements in ambient air. When the conductivity experiments were carried out in dry air, oxygen, and argon, only a single linear trace was observed in the entire temperature range of investigation (Fig. 6c). The slope change in the plot of log σ versus reciprocal temperature at 578 K under humid environments could be due to either of the following reasons: (a) a crystallographic transformation of MoO3 occurs at that temperature leading to a change in conduction mechanism of the conducting species or (b) electrical conduction in MoO3 could be due to two different species each dominating at different temperature regimes. To identify any phase

166

S.S. Sunu et al. / Sensors and Actuators B 101 (2004) 161–174 -5.5

-6.0

log(σ) / scm-1

-6.5

-7.0

-7.5

(A) -8.0

(B) -8.5 1.2

1.4

1.6 1.8 1000/T (K)

(a)

2.0

2.2

-5.5

-6.5

log(σ) /

s cm

-1

-6.0

578 K

-7.0

-7.5

-8.0

-8.5 1.2

(b)

1.5

1000/T(K)

1.8

2.1

-5.5

log(σ) / S Cm-1

-6.0

-6.5

-7.0

-7.5

-8.0

-8.5 1.2

(c)

1.5

1.8

1000/T (K)

2.1

transformation in MoO3 at around 578 K, differential scanning calorimetric experiments were carried out under ambient air. These studies showed clearly that no phase transition occurred in MoO3 in the temperature range of 300–650 K ruling out this reason for the observed slope change in the conductivity plot. These results indicate that the conduction mode in MoO3 up to 578 K is influenced by water molecules. Under humid environment additional conducting species arise probably due to insertion of water molecules in the MoO3 lattice. When the experiments were carried out under dry conditions, such insertion of water molecules is not possible and hence a change of slope was not observed. In order to estimate the water that got inserted into the lattice, thermogravimetric analyses were carried out in air at heating rates of (1) 5 K min−1 up to 773 K and (2) at a heating rate of 10 K min−1 up to 873 K followed by isothermally holding at 873 K (as discussed in Section 3.1). The results of both experiments showed progressive mass loss up to 673 K. In both experiments, the mass loss up to 673 K was ca. 0.23%, thereafter sample mass remained constant up to 733 K. Mass change characteristics of the sample at temperatures above 733 K have been discussed in the earlier section. Further to establish the presence of moisture in the as-prepared MoO3 sample, infrared spectra of this sample and those of the samples subjected to different heat treatments were recorded. All the spectra normalized for path length and sample mass are shown in Fig. 7. In the IR spectrum of the as-prepared MoO3 sample (a in Fig. 7), a broad band is observed between 3300 and 3700 cm−1 in the figure and is indicative of hydrogen bonded OH groups. A small peak at 1650 cm−1 indicates the bending mode vibration of H–O–H bond. Infrared spectra for MoO3 sample heated at 373 and 473 K (b and c in Fig. 7) under flowing argon also showed similar features, indicating the presence of moisture. But, the sample heated at 573 K under argon flow did not show these features (d in Fig. 7). The absence of these features which are characteristics of hydrogen bonded OH groups and H–O–H can be attributed either to near complete desorption of water at 573 K from the sample or its presence below the detection limit of the instrument. But the sample heated at 573 K and subsequently exposed to ambient air at room temperature showed (not presented in the figure) the presence of hydrogen bonded OH groups. All these observations establish the presence of water in as-prepared MoO3 . The enhanced conductivity observed from room temperature to 578 K is due to species formed from the reversibly inserted water molecules.

Fig. 6. (a) Arrhenius plot for electrical conductivity of MoO3 in air (RH ∼ 50%): (䊏) heating and (䊉) cooling (lines shown are the least square fitted ones of the experimental points); curve A: conductivity of as-prepared sample; curve B: conductivity of the as-prepared sample after annealing in air at 973 K for 10 h. (b) Arrhenius plot for electrical conductivity of MoO3 under humid (䊏) oxygen, (䊊) argon, and (䉱) air ambients. (c) Arrhenius plot for electrical conductivity of MoO3 under dry (䊏) argon, (䊊) oxygen, and (䉱) air environments.

S.S. Sunu et al. / Sensors and Actuators B 101 (2004) 161–174

1.05

(a)

Absorbance

0.90

(b) 0.75

(c)

0.60

(d)

0.45 1000

1500

2000

2500

3000

Wavenumber / (cm

3500 -1

4000

)

Fig. 7. (a) Infrared spectrum of MoO3 sample prepared from ammonium heptamolybdate and stored in air for prolonged durations. Infrared spectra of MoO3 sample heated in argon at (b) 373, (c) 473, and (d) 573 K and cooled to room temperature in argon.

Data available in literature on electrical conductivity of MoO3 are limited [23–27]. While studies reported by Ioffe et al. [23] and Pandit et al. [24] were for single crystal samples, those of Deb [25] were for polycrystalline samples. Galatsis et al. [26] and Prasad et al. [27] have reported conductivity characteristics of thin films of MoO3 . As mentioned earlier, the oxygen stoichiometry in MoO3 is highly sensitive to the preparation conditions employed. The non-stoichiometry present in these samples as Mo5+ ions at donor levels between the valence and conduction bands is postulated to be involved in the conduction process whose activation energy is low (viz. 0.71 eV). It is to be pointed out that MoO3 is a n-type semiconductor whose band gap energy is ∼3.5 eV and the activation energy for the conduction process involving intrinsic electrons is expected to be ∼1.75 eV (=1/2 Eband gap ). Deb measured conductivities of pellet samples of MoO3 and reported an activation energy of 0.56 eV when temperature was between 450 and 650 K and the activation energy was 1.83 eV when temperature was above 650 K. The authors had prepared the pellet samples by compacting commercially procured MoO3 powders followed by sintering in oxygen at 1073 K and hence stoichiometry of MoO3 used by them is expected to be higher than that employed in the present work. Deb reported the stoichiometry of their samples as 2.999. Conductivity of their samples were lower than the conductivity of the as-prepared MoO3 samples of this work. Similar trend is shown by the conductivity of the as-prepared MoO3 sample after annealing it in air at 973 K for 10 h (curve B in Fig. 6a). The conductivity of this sample was lower than that of the untreated as-prepared MoO3 at all temperatures, showing that the former sample was of higher stoichiometry. It is to be noted that

167

the conductivity patterns of as-prepared MoO3 samples in humid environments were reversible in cooling and heating runs, indicating the reversible insertion and removal of water. These data have also shown that water inserted into the samples also dictates the electrical conductivity in addition to the Mo5+ ions and probably a direct relation exists between them. The extent of water insertion was probably high in the as-prepared MoO3 samples used in the present work as exemplified by the present IR and TG studies and therefore the contribution to electrical conductivity due to the species arising from water insertion persisted up to 578 K. Closer examination of the conductivity data reported by Deb shows that at temperatures below 450 K another conductivity process with lower activation energy persisted, though this aspect had not been discussed in their publication. This conduction mode with low activation energy is probably due to the water insertion in the MoO3 lattice. Since the sample used by them was highly stoichiometric, water inserted is anticipated to be low and this would explain their observation of the conduction with low activation energy only up to 450 K. Arrhenius plot for the as-prepared MoO3 sample of the present work after annealing in air at 973 K for 10 h indicates that the break in the plot appears at a lower temperature (550 K) than in the case of untreated sample and this is in agreement with the observations made above. Ioffe et al. [23] also observed a break in the Arrhenius plot of the conductivity values of a single crystal sample measured in argon atmosphere and the activation energies for the high and low temperature processes were 0.65 and 0.22 eV, respectively. However, the break was observed at about 373 K itself indicating very low amount of moisture in the sample. Although the authors had grown the samples in argon, moisture probably got introduced into it during its handling in air. After the first heating cycle, this break was not observed by them. In the present studies, when the conductivity experiments were carried out in 100% dry oxygen and dry argon (∼10 ppm O2 ), linear traces were observed in the whole temperature range of studies for log σ versus 1/T plots. These results are compared in Fig. 6c with those obtained under dry air conditions. It is seen that conductivity values in all ambients are same. The calculated activation energies for conduction in 100%, 10 ppm oxygen, and dry air are 0.68 ± 0.05, 0.69 ± 0.01, and 0.69 ± 0.08 eV, respectively. These results show the conductivity of MoO3 to be independent of oxygen partial pressure in the surrounding gas ambient. These results also show that the non-stoichiometry existing in the as-prepared sample does not get altered by subsequently heating in air, oxygen, and argon containing 10 ppm of oxygen up to 773 K. However, on heating it at high temperatures in air this non-stoichiometry gets reduced as discussed in Section 3.1. Galatsis et al. [26] had observed change in electrical conductivity of thin films of MoO3 when exposed to different concentrations of oxygen. However, the authors have reported that this property was not reproducible particularly when the temperatures were above 673 K. It is to be pointed out that they had prepared

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% Sensitivity

75

50

25

0 500

550

600

650

Temperature / K Fig. 8. Sensing characteristics of MoO3 in the form of porous pellet towards 500 ppm each of (䉱) LPG, (䊉)H2 , and (䊏) NH3 in air as a function of temperature.

3.3. Sensor characteristics Fig. 8 shows the typical temperature dependence of sensitivities of as-prepared MoO3 pellet sensors (density ca. 90% theoretical density) with platinum electrodes towards 500 ppm each of NH3 , H2 , and LPG. For 500 ppm ammonia, the sensitivity was around 15% at 503 K. It increased to 30–35% in the temperature range of 548–602 K while it was 60% at 623 K. Maximum sensitivity of 69% was observed at 673 K. For 500 ppm H2 , sensitivities observed at 623 and 673 K were 23 and 42.5%, respectively. Sensitivity remained at 5–10% in the temperature range of 548–602 K and was very low at 503 K. For LPG, sensitivity increased with temperature; it was 61% at 673 K, but, was insignificant when

temperature was below 553 K. As the sensitivities towards all gases particularly for NH3 and H2 were high at 623 K, this temperature was chosen for calibration experiments. Typical concentration dependence of the sensitivity of the sensor for ammonia and hydrogen (10–500 ppm) at 623 K is shown in Fig. 9. For ammonia, sensitivity increased linearly with concentration above 25 ppm. For 25 ppm of NH3 , the observed sensitivity was around 20% and below this concentration, the sensitivities observed were very low (9% for 10 ppm). As seen from the figure, the sensitivities for hydrogen also increased with concentration. For hydrogen, (a) 60

% Sensitivity

thin films of MoO3 by a sol–gel technique but the films were not structurally characterized. It is probable that these films initially were with high non-stoichiometry which got annealed out on exposure to air at high temperatures and hence the observation of irreproducible results. Recently Prasad et al. [27] have studied the ammonia sensing characteristics of MoO3 films prepared by RF sputtering. Although the baseline resistance of the MoO3 film was found to be weakly dependent on oxygen level in the environment gas, authors have indicated the possibility of residual ammonia lingering in the test chamber between experiments and this could have played a role for their observation. All these considerations and our experimental observations indicate that the electrical conductivity of MoO3 is not dependent on oxygen partial pressures within the temperature and pressure ranges of present investigation.

(b)

40

20

0 0

200 400 Concentration (ppm)

Fig. 9. Variation of sensitivity as a function of concentration: (a) ammonia and (b) hydrogen in air. Sensor temperature: 623 K.

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169

Resistance(MΩ)

20

15

10

5 0

100

200

300

400

500

Time (sec) Fig. 10. Typical response and retrace characteristics of porous pellet sensors of as-prepared MoO3 towards 500 ppm of ammonia in air at 673 K.

the observed sensitivity remained nearly 20% for concentrations between 100 and 500 ppm. The sensitivity was 14% for 50 ppm hydrogen but was less than 5% for lower concentrations. Typical response and recovery characteristics of the pellet sensor operating at 623 K towards 500 ppm of ammonia are presented in Fig. 10. The response time is about 60 s and the recovery time is about 180 s, indicating reasonably fast response and retracing properties of the sensor towards ammonia. Fig. 11 shows the temperature dependence of sensitivities toward 500 ppm of NH3 when different electrodes namely

Fig. 11. Sensing characteristics of MoO3 towards 500 ppm NH3 in air using different electrode materials: (a) platinum, (b) gold, and (c) graphite.

gold, graphite, and platinum were used. Variation of sensitivity with temperature with these electrodes was similar although there were small differences in actual sensitivity values. Sensitivities were in the range of 60–70% at 623 K while they were lower (15–35%) at all other temperatures. These studies show that gold, platinum, and graphite electrodes do not interfere with the ammonia sensing process and can be effectively employed in a sensor assembly. 3.4. Mechanism of ammonia sensing Sensing mechanism of the semiconducting oxides generally involve chemisorbed oxide ions which modulate the carrier concentration in the surface [28]. If the sensing mechanism in MoO3 also involved chemisorbed surface oxide ions, variation of its electrical conductivity as a function of oxygen partial pressure is expected. As discussed earlier, no significant change in conductivity was observed with change in ambient oxygen partial pressures. Hence, the gas sensing action of MoO3 does not involve the general mechanism operating in semiconducting oxide sensors. Molybdenum oxides are known to be very good catalysts in oxidation reactions. Bielanski and Haber [29] also showed that MoO3 does not chemisorb oxygen and only its lattice oxygen is involved in various catalytic oxidation reactions of hydrocarbons. MoO3 has orthorhombic structure with double layers of linked, distorted MoO6 octahedra, parallel to [0 1 0] direction. Each double layer consists of zig-zag chains along [0 0 1] of octahedra sharing edges, neighboring chains being linked by octahedra sharing corners to form the

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[(111)γ-Mo2N ]

[(311)γ-Mo2N ] (0100) (042)

(211) (112) (221) (171) (081)

(002) (230)

(200) (210)

(101) (111)

(150)

(021)

[(111) MoO2 ]

200

(060)

400

[(200)γ-Mo2N ]

(040)

600

[(211) χ- Mo4O11] (110)

layers [30]. The successive layers are held together by van der Waals forces, i.e. in the (0 1 0) plane of the orthorhombic unit cell (Fig. 12a). One of the characteristic features of this structure is its ability to form shear structures on removal of oxygen and this is related to the fascile rearrangement of the coordination polyhedra linked together. This rearrangement normally involves a change from corner-sharing octahedra either to edge-sharing or face-sharing octahedra (Fig. 12b and c). For MoO6 octahedra, energy of the edge-linked system is considerably lower than the corner-linked system, making the reaction highly favorable [31]. Mars and van Krevelen [32] had proposed a mechanism involving the lattice oxygen for the catalytic activity of these oxides towards various organic compounds. According to this mechanism, reduction of the surface layer of the oxide takes place in the absence of gas phase oxidants. After the removal of the product(s), oxygen vacancy formed at the surface is replenished by the diffusion of oxygen from the bulk. Desorption of the product is accompanied by the simultaneous rearrangement of MoO6 octahedra which is known to proceed readily. The same mechanism can be extrapolated for sensing action also. The change in conduction during sensing action of MoO3 would be due to the involvement of its lattice oxygen unlike in other semiconducting oxide sensors where the chemisorbed oxygen participates. Reaction between pure ammonia gas and solid MoO3 under a temperature ramp has been studied by different authors [33,34]. In the temperature range of 620–650 K, the reaction led to the appearance of surface domains consisting of slightly reduced MoO3 by formation of crystallographic shear planes in the oxide [33]. The reaction became signifi-

(020)

Fig. 12. (a) Crystal structure of MoO3 showing layered arrangement of MoO6 octahedra, (b) schematic diagram of an idealized MoO3 lattice of corner-shared octahedra which undergoes reconstruction through the formation of crystallographic shear planes (arrows indicate the direction of shear process), and (c) structure after the formation of shear plane.

cant only at temperatures above 660 K and the final products of the reaction were MoO2 , Mo2 N, and MoN. An oxynitride intermediate was also reported to be formed during this reaction. The nature of the products and their proportions were found to be dependent on the heating rate employed and space velocities of the reactant gas over the solid MoO3 . Fraction of MoO2 formed was high at high heating rates while the fraction of the nitrides was high at low heating rates. MoO2 , which appears as a reaction product, further reacts with ammonia at temperatures above 873 K only [34]. {1 0 0} planes of Mo2 N are shown to be parallel to {0 1 0} planes of MoO3 and hence the transformation of MoO3 to Mo2 N is proposed to occur topotactically [33]. Electrical conductivities of molybdenum nitrides are known to be high. While stoichiometric MoO3 has very low electrical conductivity, the electrical conductivities of suboxides of molybdenum, represented by general formula Mon O3n−1 where n = 4, 8, 9, 13, and 17, would increase progressively with decrease in the value n because of the increase of molybdenum ions with lower valance states accompanied by Mo–Mo bonds [13]. In fact, MoO2 has very high electrical conductivity. These data and the results of the present work can be used to deduce the mechanism of ammonia sensing by MoO3 . XRD pattern of the products formed on exposure of an as-prepared MoO3 pellet sample to air containing 6.67% ammonia at 673 K and subsequently cooled to room temperature at 10–15 K min−1 is shown in Fig. 13. It is to be mentioned that high concentrations of ammonia were used to obtain the products at X-ray detectable level. Analysis of the XRD pattern shows the presence of a mixture of Mo4 O11 , MoO2 (P21 /n, monoclinic), ␥–Mo2 N (Pm3m, cubic, Z = 2) and some unreacted MoO3 . It was also observed that the surface of the pellet was darkened. This discoloration had not penetrated into the pellet and the color of the interior

Intensity (a.u.)

170

0

20

40

60

2θ / Degrees Fig. 13. XRD pattern of an as-prepared MoO3 pellet after exposure to air containing 6.67% ammonia at 673 K.

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171

Fig. 14. SEM images of (a) as-prepared MoO3 pellet specimen, (b) specimen exposed to air containing 6.66% of NH3 at 673 K.

of the pellet remained unaltered. Fig. 14a shows the scanning electron microscopic image (SEM) of the as-prepared MoO3 sample in the form of pellet and Fig. 14b shows that of the same after exposure to NH3 in air. Prior to exposure to NH3 containing air, the surface consisted of MoO3 exhibiting platelet type structure, which is characteristic of orthorhombic MoO3 . Lengths of the platelets were in the range of 1–3 ␮m while their widths and thickness were approximately 0.5 and 0.3 ␮m, respectively. After exposure to NH3 , the morphology had changed from platelet to spherical granules with very small spikes appearing around the grains (Fig. 14b). The irregular shaped grains could be suboxides or ␥-Mo2 N or both. Immediately after exposure to air–ammonia mixture, the MoO3 pellet had been cooled and its temperature dropped to 573 K (where no reaction between NH3 and MoO3 is reported to take place) in about 10 min. Using the data on diffusion coefficient of oxygen in MoO3 and by assuming that the sample was isothermal at 673 K for 5 min, the depth up to which the non-stoichiometry generated on the surface of a platelet on exposure to the test gas spread into it (95%) was calculated as 0.32 ␮m. This showed that on exposure to the test gas, a near complete change of the stoichiometry of the platelets of MoO3 on the pellet sur-

face would have occurred (since their widths and thickness are ca. 0.5 and 0.3 ␮m) and led to the formation of the reaction products. As the test gas contained large quantities of air, the oxide formed on the surface is expected to be a mixture of suboxides although the oxide formed during the reaction between pure ammonia and MoO3 was MoO2 only. The MoO3 detected in XRD pattern could have been due to unreacted portion of MoO3 beneath the surface. XRD pattern of the thin film of MoO3 after isothermal exposure to flowing air 1500 ppm of ammonia at 673 K for 90 min and subsequently cooling to room temperature in the same environment is shown in Fig. 15a. This pattern also indicates the formation the MoO2 , Mo4 O11 , and Mo2 N. Under the conditions of deposition and heat treatment following deposition, the film had been characterized to consist of MoO3 only [12]. It is to be pointed out that MoO3 was absent in the products, indicating the complete conversion of MoO3 in the film to products. Since the film thickness was ca. 1 ␮m and time for which the sample was exposed to test gas was 90 min, complete conversion of MoO3 to products is expected from considerations of the diffusion coefficient of oxygen. This result also shows that the products formed when MoO3 is exposed to air containing large and low concentrations of

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25k Mo-3d in MoO3 + NH3 Fitted Curve Mo2+, Mo 3+

20k

η-Mo4O11

4000

Mo4+ Mo5+ Mo6+

KCPS

3000

10k

2000

* 5k

**

MoO

*

0

* *

MoO2

MoO2 MoO2

*

2 Mo 2N

MoO2MoO

MoO2

MoO2

*

-Mo 4O11

Mo 2N

Mo 2N

1000

Mo 2N

*

2

MoO2

Intensity (a.u.)

15k

240

20

40

60

80

2θ / (Degrees)

(a)

(021)

*

(040)

*

(110)

500

*

* * *

(062)

(221) (112) (042) (081)

(150)

(200) (210) (002)

*

(060)

(101) (111)

(020)

Intensity (a.u.)

1000

0 20

40

238

236

234

232

230

228

Binding Energy (eV)

Fig. 16. XPS pattern for Mo 3d level of molybdenum after exposure to 6.67% of ammonia in air.

*

(b)

0

60

2θ / Degrees

Fig. 15. (a) XRD pattern of the thin film of MoO3 exposed to 1500 ppm of ammonia in air at 673 K. (b) The same film after heating in air at 673 K.

ammonia are same although their relative quantities could be different. These products were converted back to MoO3 on heating them in clean air at 673 K and this is evident from the XRD pattern shown in Fig. 15b. Same results were deduced from analysis of the XRD pattern (not shown) of the as-prepared MoO3 pellet sample when heated in air at 673 K after its exposure to air containing 6.67% ammonia. It is to be mentioned that during these experiments, the oxynitride of molybdenum which is proposed as an intermediate during MoO3 –NH3 reaction [34] has not been identified. However, contamination of nitride by dissolved oxygen could be deduced from XPS results of the products. Valency of molybdenum in the products formed on exposure of a pellet of the as-prepared MoO3 sample with air containing 6.67% NH3 was probed by XPS. Fig. 16 shows the

XPS pattern for Mo 3d level of molybdenum after exposure to the test gas. A broad pattern with binding energy values of 232.5 eV (Mo 3d5/2 ) and 235.7 (Mo 3d3/2 ), which is characteristic of Mo6+ ions bonded to oxygen atoms is observed (for comparison the standard binding energy values [18] are shown in Table 1). In addition, a new peak with a binding energy value of 229.1 eV is also seen clearly. A binding energy value of ∼228.8 eV would correspond to that of Moδ+ state bonded to nitrogen atoms where 2+ < δ+ < 3+ (for comparison the standard binding energy values for molybdenum in nitrides [35–39] are shown in Table 1). Taking all these observations into account, a spectrum was simulated assuming the presence of molybdenum in +2 to +6 states, which is shown in Fig. 16. The overall, synthesized curve based on the simulated individual peaks fitted well with the experimental data, with a correlation coefficient higher than 0.98. This shows that the spectrum is the combination of patterns arising from Moδ+ , Mo4+ , Mo5+ , and Mo6+ with the respective Mo 3d5/2 binding energy values of 229.1, 230.2, 231.3, and 232.5 eV along with corresponding energies for Mo 3d3/2 level. A binding energy value of 229.1 eV is slightly higher than the standard value reported for Mo2+ ions in pure molybdenum nitride but less than that of Mo3+ observed in oxygen contaminated nitrides. Possibly a mixture of pure and oxygen-contaminated nitrides were formed under the conditions of the present experiment and this could have been the reason for this observation of higher values of binding energy. Electrical conductivity of the pellets of the as-prepared MoO3 samples in alternating gaseous environments of air and 6.67% NH3 in air at 673 K are shown in Fig. 17. It is seen that the resistance of the sample in air dropped by more than two orders of magnitude on switching the environment to ammonia–air mixture and reached a steady value within ∼60 s. On re-exposure to air, the resistance traced

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Resistance / (M ohms)

30 25 20 15 10 5 0 0

200

400

600

800

1000

Time / s Fig. 17. Electrical conductivity of the pellets of the as-prepared MoO3 samples in alternating gaseous environments of air and 6.67% NH3 in air at 673 K.

back to its original value and the recovery time was ∼180 s. The response and recovery times of the thin film [12] on exposure to 500 ppm of ammonia in air were also in the same order. Fig. 18 shows the isothermal thermogravimetric curve of as-prepared MoO3 powder at 673 K under a flowing gas ambient of 1:1 air–ammonia mixture. Here again large concentration of ammonia was employed so that the resulting changes in mass during the reaction would be measurable. Mass of MoO3 sample, which remained constant in air sharply decreased on exposure to air–ammonia mixture. A mass loss of about 6% occurred in about 3 min. The rate of mass loss thereafter slowed down and the mass of the sample reached a constant minimum corresponding to ca. 11% mass loss after about 13 min. It is to be pointed that the change in electrical conductivity of the sample came to

% weight loss

100

96

92 13 mins

88 0

40

80

Time / (mins) Fig. 18. Isothermal thermogravimetric curve of as-prepared MoO3 powder at 673 K under a flowing gas ambient of 1:1 air–ammonia mixture.

173

an end within ca. 1 min while mass decrease prolonged for another about 16 min. On exposure to ammonia containing air, suboxides, and some amount nitrides of molybdenum would be formed on the surface of MoO3 and the thickness of the layer containing these electrically conducting products would grow with time. Within the observed sensor response time (∼60 s), this layer probably becomes sufficiently thick on the MoO3 platelets present in the film and those present on the surface of the pellet. Consequently electrical conductivity would drop and any further growth of this conducting layer would not result in any significant increase in conductivity in view of the insulating nature of MoO3 . Assuming the layer to consist of suboxides only, the thickness of the transformed layer can be calculated using the diffusion coefficient of oxygen in MoO3 and is ∼0.15 ␮m. Time needed for the entire platelet/film to attain the stoichiometry generated on its surface would be higher and mass loss would continue during this period also. Calculations showed that this period would be in the order of 10 min which is in agreement with the isothermal gravimetric experimental results in view of the simplifying assumptions made. The extent of non-stoichiometry on the surface would be determined by the concentration of ammonia in air. When 1:1 air–ammonia mixture was employed, product containing an appropriate mixture of suboxides and nitrides which would correspond to the 11% mass loss would have formed. At lower concentrations of ammonia, as would be the case during gas sensing applications, the stoichiometry of the suboxides and the relative quantities of suboxides and nitrides is expected to vary as a function of concentration of ammonia. This would lead to the observed concentration dependence of the sensitivity of the sensors.

4. Conclusions From the present investigations, it is found that the electrical conductivity exhibited by MoO3 is independent of oxygen partial pressure in the temperature range of 510–773 K. Two distinct conduction processes were identified when the conductivity experiments were carried out under ambient air, moist oxygen, and moist argon. The slope change in the Arrhenius plot was found at 578 K. The conductivity in the lower temperature range is attributed to species arising due to the reversibly inserted water molecules into MoO3 lattice. The presence of water in the sample was established by thermogravimetric and infrared spectroscopic studies. The conduction process in the high temperature region could be due to the non-stoichiometry existing in the sample due to presence of Mo5+ ions which was confirmed by EPR and XPS studies. It has been shown that MoO3 can be used as sensor material for sensing ammonia down to 25 ppm. Gold, platinum, and graphite electrodes are found to be compatible with MoO3 for sensor studies. Mechanism of ammonia sensing by MoO3 is shown to involve formation of suboxides of molybdenum and Mo2 N.

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