Gas Sensing Properties of MoO₃ ^†

Abstract

In the present work, we studied the controlled growth of MoO₃ nanostructures and their gas sensing performance. MoO₃ NS were synthesized using thermal evaporation method. FE-SEM (LEO 1525) and XRD were used to characterize the morphology and crystallinity of MoO₃ NS. Sensing devices were mounted on TO packages using electro-soldered gold wires, and they were tested toward some chemical compounds. These NS give reversible response to Acetone, Ethanol and H₂ in a temperature range of 300–550 °C. MoO₃ sensors exhibit a high response ∆G/G = 45 towards 50 ppm of Ethanol at 450 °C.

1. Introduction

Semiconductor metal oxide nanostructures (NS) have been exploited in literature as chemical gas sensors due to their low cost easy fabrication, consistence stability, high sensitivity and simple interface electronics comparing to other type of gas sensors [1]. SnO₂ and ZnO are the most common metal oxide used for gas sensing even if, recently, there is an increasing trend of using different metal oxides for gas sensing application. Among other metal oxides, MoO₃ is an n-type semiconductor with 2.9 eV band gap, which can potentially cover a wide range of application including catalysis, electrochromic devices and gas sensing.

MoO₃ thin films are widely used as chemical gas sensors, and show high reversible response toward some gases [2]. However, there are few literature works reporting the use of MoO₃ NS for gas sensing application [1]. It is well known that, for chemisorption, gas sensitivity is mainly associated with the size and morphology of the metal oxide material; NS materials have high surface to volume ratio and provides rapid diffusion of gas molecule. There are plenty of synthesis methods for NS and among them; VLS is one of the leading technique because of low risk of contamination and produce highly crystalline structures. Also this technique has capability to growth NS on the substrate and without using expensive and time consuming transferring methods.

In this study, we report the synthesis of MoO₃ by evaporation condensation method_, characterization by SEM and XRD and gas sensing performance of this NS.

2. Materials and Method

Ultrasonically cleaned (in acetone bath for 15 min) Alumina (99.9% purity, 2 mm × 2 mm, Kyocera, Japan) substrates were used to growth MoO₃ NS. A thin layer of Au catalyst was deposited on the alumina substrates by RF magnetron sputtering (75 W argon plasma 5.5 × 10⁻³ mbar, room temperate) to promote the growth of NS. Then MoO₃ powder was placed in middle of the tubular furnace and heat up to 700 °C to promote the evaporation of MoO₃. To growth MoO₃ NS, Au coated alumina substrate was placed in temperate zone around 450–550 °C. Pressure was maintained at 10 mbar during the process and 10SSCM of Ar was used as carrier gas to transport MoO₃ vapor towards the substrates. The deposition time was 30 min and it was cooled down to room temperature by naturally. In order to investigate the gas sensing performance of the MoO₃ materials, contacts and heaters were prepared on top of metal oxide and the back side of the alumina substrate. First TiW adhesion layer was deposited by DC magnetron sputtering (70 W argon plasma, 7SCCm argon flow, 5.5 × 10⁻³ mbar presser in room temperature) then the platinum contacts were deposited on the TiW adhesion layer by using same condition used to for TiW. Platinum heaters were deposited on back side of the alumina. This alumina substrate was mounted on TO packages using electro-soldered gold wires. To investigate the condutrometric response for the gas species, the sensors were placed in homemade test chamber. The total gas flow was maintain at 200SCCM and relative humidity was set to 40%. A fixed voltage (1V) applied to the sensor and conductance of the sensor was measured using dedicated picoammeters and the sensors were tested in the temperature range of 200 °C to 550 °C.

3. Results

3.1. Characterization

MoO₃ were successfully grown on Au-catalyzed alumina substrate by thermal evaporation method. The morphology of growth marital was investigated by FE-SEM. MoO₃ NS have grown on alumina substrates in flakes-like shape as illustrated in Figure 1a,b. The condensation temperature has a strong influence on the groth of NS. The NS grown in high temperature range Figure 1b were thin and longer compared to NS grown in low temperature range Figure 1a. The SEM images clearly show that the thickness of the flakes is less than 200 nm, and that they grow homogeneously on the alumina substrate.

Figure 1. The SEM image of the MoO₃ nano structures at evaporation temperature 700 °C, pressure 10 mbar and 10SCCM Ar flow: (a) At low temperate zone (450 °C–500 °C); (b) At high temperature zone (500 °C–550 °C).

Figure 1. The SEM image of the MoO₃ nano structures at evaporation temperature 700 °C, pressure 10 mbar and 10SCCM Ar flow: (a) At low temperate zone (450 °C–500 °C); (b) At high temperature zone (500 °C–550 °C).

Figure 2. The XRD spectra of thermally evaporated MoO₃.

Figure 2. The XRD spectra of thermally evaporated MoO₃.

Figure 3, instead, shows the XRD spectrum of MoO₃ NS, conforming the orthorhombic symmetry of crystalline structure. The diffraction peaks 12.77, 25.71 belongs to (020), (040) prove the presencs of lamellar structure of the MoO₃. Also other peacks at 23.34, 27.30 and 33.77 are belogs to the (110), (021) and (111) respectivly.

3.2. Gas Sensing

The gas sensing properties of MoO₃ were also investigated, and Figure 3a reports the dynamic responce performed on the device. We can observe an increase of the electrical conductance in presence of Acetone, Ethanol and H₂, while sensors recover the baseline when exposed to synthetic air. Figure 3b shows the temperature dependence of the MoO₃ NS and MoO₃ NS gives high response in temperature range between 400 °C–550 °C. Ethanol and Acetone show a high response at two different working temperatures. The optimal operation temperature is 450 °C for 50 ppm of Ethanol and 500 ppm of H₂. This flakes like structure shows high response ∆G/G = 45.05 at 450 °C towards 50 ppm of Ethanol also response 14.71 and 22.26 for 500 ppm of H₂ and 100 ppm of Acetone respectively. This MoO₃ can be used to detect low concentrations of Ethanol and acetone and the highest response is varying with working temperature for different gases thus this can be used to identify the different gas species Figure 4 demonstrates the calibration curves and it’s in line with power low. The detection limit for ethanol and Acetone in ppb level, but detection limit for H₂ is high compare to the other gas. All measurements were performed at a relative humidity of 40% at 20 °C, with an applied voltage equal to 1 V.

4. Conclusions

In this study we successfully synthesed MoO₃ by thermal evaporation method and measured the sensing behavior towards different chemical gases. This evaporation condensation method has potential to produce MoO₃ NS in large quantities with tunable MoO₃ NS. Dimensions can be changed mainly according to the condensation temperature and deposition time. XRD conform the orthorhombic symmetry of crystalline structure and SEM shows the morphological structure of the MoO_3. This NS give high response to Ethanol and H₂ in 450 °C and for Acetone at 400 °C. The detection limit for Acetone and Ethanol is lower than 1 ppm.