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Molybdenum trioxide nanopaper as a dual gas sensor for detecting trimethylamine and hydrogen sulfide

A free-standing, flexible, and semi-transparent MoO3 nanopaper was fabricated using ultralong MoO3 nanobelts (length ∼ 200 μm; width 200–400 nm), and its gas-sensing characteristics were investigated. The sensor exhibited high responses (resistance ratio) of 49 to 5 parts per million (ppm) hydrogen sulfide (H2S) at 250 °C and 121 to 5 ppm trimethylamine (TMA) at 325 °C with excellent gas selectivity, demonstrating its dual function for gas detection. Moreover, the sensor showed promising potential for the all-in-one detection of three representative offensive odors (TMA, H2S, and NH3) simply by tuning of the sensing temperature. This particular performance is attributed to the high chemical affinity of MoO3 to H2S and the acid–base interaction between basic TMA/NH3 and acidic MoO3. The mechanism underlying the control of gas selectivity by modulating the sensor temperature was investigated by Diffuse Reflectance Infrared Fourier Transform (DRIFT) measurements.


Introduction
Trimethylamine (TMA) and hydrogen sulde (H 2 S), which are generated by the decomposition of plants and animals, are colorless, ammable, and poisonous gases with unpleasant odors that are described as "shy" and "rotten-eggs", respectively. [1][2][3][4] Although the smell is initially pungent, it rapidly paralyzes the olfactory system, resulting in unawareness of its presence. Exposure to a certain concentration ($10 parts per million (ppm)) of either TMA or H 2 S can cause not only discomfort but also headaches, nausea, and irritation to the eyes and respiratory system. 5,6 TMA and H 2 S are trace biomarkers (sub-ppm level)-detected in human exhaled breath-of renal-system imbalance and periodontal disease, respectively. 7,8 Accordingly, the detection of sub-ppm to ppm levels of TMA and H 2 S is of vital importance in food quality control, environmental monitoring, and medical diagnosis.
Metal-oxide semiconductors are widely used in gas sensors to detect trace concentrations of reducing gases because of their high response, simple structure, facile integration into small devices, and cost effectiveness. [9][10][11] However, oxide-semiconductor gas sensors oen exhibit similar gas responses to a range of different gases, which hampers the selective detection of a specic gas. Many efforts have been made to enhance the selectivity, including the modulation of the sensor temperature, 12 the loading of noble-metal catalysts, 13,14 the doping of catalytic oxides, 15,16 and control of acid-base interactions. 17,18 Although pattern recognition using an array of sensors is one solution for gas discrimination, the number of sensors should be minimized to reduce the size, cost, power consumption, and sensing circuit of devices. 19,20 From this viewpoint, the detection of multiple gases using a single sensor is the ideal approach for realizing simple and reliable gas sensors.
Yang et al. 34 fabricated a gas sensor by drop coating a slurry containing MoO 3 nanobelts onto a substrate and demonstrated its potential for detecting TMA. However, neither the control of the selectivity to different offensive odors such as TMA, H 2 S, and NH 3 nor the detailed gas-sensing mechanism was investigated. In the present study, MoO 3 nanobelts with an average length of 200 mm were synthesized by a modied hydrothermal method and fabricated into a free-standing, exible, semitransparent ultrathin paper to increase the gas accessibility and the gas-sensing reliability. The sensor using MoO 3 nanopapers showed the dual functionality to detect H 2 S at 250 C and TMA 325 C both with excellent selectivity. The control of the gas selectivity, the gas response, and dual gas sensing by the modulation of the sensor temperature was investigated in relation to the gas-sensing mechanism and surface-reaction analysis using Diffuse Reectance Infrared Fourier Transform (DRIFT) spectra.

Experimental
Ultralong MoO 3 nanobelts were synthesized via a modied hydrothermal method. 35 2 g of molybdenum powder (Aladdin, 99.5%) to 10 mL of deionized (DI) water and 20 mL of 30 wt% H 2 O 2 until the solution became light yellow. The solution was stirred for 30 min to react thoroughly and then transferred to a Teon-lined stainless-steel autoclave and heated to 220 C for 100 h. The precipitate was ltered and rinsed by DI water and ethanol several times.
Vacuum ltration was employed to fabricate MoO 3 nanopapers. First, 0.82 mg of MoO 3 nanobelts was dispersed into 50 mL of DI water to form a homogeneous suspension. The asprepared suspension was ltered through a membrane (pore size of 220 nm). Aer ltration, the nanopaper formed on the membrane was dried in air for 5 min and then peeled from the lter membrane to obtain a free-standing and semi-transparent MoO 3 nanopaper.
The MoO 3 nanopaper was cut into $1.5 mm Â 1.5 mm pieces using scissors. A SiO 2 /Si substrate (area ¼ 10 Â 10 mm 2 , thickness ¼ 0.5 mm) with two platinum electrodes on its top surface (electrode widths of 50 mm and separation of 100 mm) was used as the sensor substrate. To enhance the adhesion between the MoO 3 nanopaper and the silicon-wafer substrate, a droplet of DI water (20 mL) was dropped on the substrate using a micro-pipette, and then the MoO 3 nanopaper was placed on the substrate. Aer drying at 60 C for 20 min, the sensor was annealed at 450 C for 2 h to remove any residual organic content and to stabilize the sensor at the operation temperature. A schematic image of the MoO 3 -nanopaper sensor is shown in Fig. 1a.
The crystal structure of the MoO 3 nanopaper was investigated using an X-ray diffractometer (Philips X'Pert Pro) with Cu Ka radiation (l ¼ 1.54184Å). The morphologies of the sensor were characterized using scanning electron microscopy (SEM, JEOL JSM-6060) and transmission electron microscopy (TEM, Tecnai G20). The reaction between the gas molecules and the sensing materials was analyzed using DRIFT spectra, which were recorded with a VERTEX 70-FTIR spectrometer equipped with a SMART collector and an MCT detector.
The sensors were placed in a quartz tube, and their gassensing characteristics were measured at 100-350 C. A owthrough technique with a constant ow rate of 200 cm 3 min À1 was used. The gas concentrations were controlled by changing the mixing ratio of the parent gases (TMA, H 2 S, NH 3 , NO 2 , acetone, benzene, ethanol, formaldehyde, toluene, xylene; all 5 ppm in dry synthetic air balance) and dry synthetic air. The interference gases are chosen for the practical applications of sensors in food quality control, environment monitoring and medical diagnosis. A thermocouple was placed near the sample to accurately monitor the sensor temperature. The directcurrent two-point probe resistance of the sensor was measured using an electrometer with a computer interface.

Results and discussion
According to the XRD pattern shown in Fig. 1b, the nanopaper was identied as orthorhombic MoO 3 (JCPDS no. 05-0508) with a highly ordered orientation. SEM images of the MoO 3 nanobelt sensor ( Fig. 1c-f) conrm that uniform MoO 3 nanopapers consisting of highly interconnected MoO 3 nanobelts were coated on the electrodes. The MoO 3 nanobelts were 200-400 nm wide and 100-200 mm long, indicating that almost every MoO 3 nanobelt could bridge two electrodes over the gap (100 mm). The highly interconnected and gas-accessible structures surely promote the gas-sensing reaction and carrier transportation. TEM and SAED images of the MoO 3 nanobelts (Fig. 2) reveal that the nanobelts were single-crystalline with interplanar spacings of 0.37 and 0.40 nm along the perpendicular directions, which are consistent with the (002) and (200) d-spaces of MoO 3 .
The responses of the MoO 3 nanopaper to 5 ppm H 2 S and TMA at 250 and 325 C are shown in Fig. S1. † The sensor resistance decreased when reducing analyte gas was introduced, which indicates typical n-type sensing. The gas response (S) is dened as R a /R g for reducing gas; and ÀR g /R a for oxidizing gas, where R a and R g are the sensor resistances in air and gas, respectively. The response and recovery time is dened as the time required to reach 90% variation of the sensor resistance upon exposure to the analyte gas or air. To better illustrate the sensor performance, the gas-sensing characteristics, including the gas response and response and recovery time, are listed in Table S1. † 15,[23][24][25]31,32,34 The MoO 3 -nanopaper sensor shows relatively rapid gas responses and complete recovery upon exposure to air.
The gas responses of the MoO 3 -nanopaper sensor to 5 ppm TMA and H 2 S were measured at 100-350 C (Fig. 3). The maximum responses to TMA and H 2 S occurred at 325 and 250 C, respectively. The selectivity of the sensors at different operating temperatures is presented in Fig. 4. At 250 and 325 C, the MoO 3 -nanopaper sensor showed good selectivity toward H 2 S and TMA, respectively ( Fig. 4b and d). The sensor showed a relatively high response to NH 3 in the entire range of the sensor temperature (225-325 C). This demonstrates the potential of the sensor to detect offensive odors. In particular, the comparable and high responses to 5 ppm TMA, H 2 S, and NH 3 at 225 and 275 C (Fig. 4a and c) indicate that the sensor can be used to detect total irritant gases with unpleasant smells in a selective manner. The all-in-one detection of TMA, H 2 S, and NH 3 can be used in environment monitoring and food quality control.
The response-recovery curves of the MoO 3 -nanopaper sensor for different gas concentrations are shown in Fig. 5. At 250 C, the sensor detected 250 ppb H 2 S gas with a response of 1.53. At 325 C, the response to 25 ppb TMA was 4.26, demonstrating that the detection limit was lower. According to the U.S. National Institute for Occupational Safety and Health, the permissible exposure limits of TMA and H 2 S are both 10 ppm. 36,37 Although humans can recognize the presence of H 2 S at concentrations as low as 10-50 ppb, the human nose cannot quantify the concentration of H 2 S and is susceptible to olfactory fatigue. 20 The presence of both gases at a concentration higher than 10 ppm in the ambient atmosphere is known to cause severe irritation to human beings. The presence of TMA and H 2 S in human exhaled breath (0.1-0.2 ppm for TMA, 0.1-0.5 ppm for H 2 S) indicates renal-system imbalance and periodontal disease, respectively. 7,8 Thus, a MoO 3 -nanopaper sensor with an ultralow gas-detection limit, such as the proposed   senor, can be used in various applications, including food quality control, environmental monitoring, and medical diagnosis. The MoO 3 sensor showed stable and reversible sensing characteristics upon repetitive exposure to 5 ppm H 2 S and TMA, as shown in Fig. S2, † conrming its good stability and reversibility. In general, CuO-loaded n-type semiconductors (SnO 2 , ZnO, and WO 3 ) show high response and selectivity to H 2 S because of the strong chemical interaction between CuO and H 2 S and the conversion of semiconducting CuO into metallic CuS. 26,29,38 However, the recovery is sluggish and oen incomplete, which is mainly ascribed to the sluggish oxidation of CuS into CuO and/or the deterioration of the sensing surface by sulfur compounds. 39,40 In contrast, MoO 3 is known to form nonreactive adsorption compounds such as MoO 3 $SH 2 and MoO 2 $S at 100-300 C, and the material surface can be regenerated by the desorption of sulfur-containing surface species. 41 This shows that MoO 3 is a promising material for detecting H 2 S in a highly reversible manner. Moreover, acidic MoO 3 can enable the detection of other offensive odors with alkaline nature, such as those of TMA and NH 3 .
The surface reaction of the MoO 3 nanopaper was investigated using DRIFT to investigate the gas-sensing mechanism (Fig. 6). Note that gas concentrations 10 times higher (50 ppm H 2 S and 50 ppm TMA) were used during the DRIFT measurements in order to observe the interaction between the analyte gas and the sensor surface more clearly. The absorption bands around 2200 and 1750 cm À1 are related to the H-S bond; the bands around 1250 cm À1 and 1600 cm À1 are related to the SO 2 and H 2 O, respectively (Fig. 6a). 42,43 The absorption bands at 1750, 1600, and 1250 cm À1 at 250 C are substantially larger than those at 325 C. The absorption at 1750 cm À1 suggests the active adsorption of H 2 S, and the absorption at 1600 and 1250 cm À1 indicates the promotion of the following gas-sensing reaction to form H 2 O and SO 2 : which is consistent with the literature. 31,44 In Fig. 6b, the absorption band around 1450 cm À1 is related to the methyl groups, 45 and that around 2280 cm À1 shows the presence of O]C]O bonds, which were due to the asymmetric stretching vibration of CO 2 molecules. 46 As explained in Fig. 6a, the absorption band around 1600 cm À1 is related to H 2 O molecules. The intensities of the three bands are substantially higher at 325 C than at 250 C, indicating that an active surface reaction involving the methyl group occurred at 325 C. The oxidation of TMA into CO 2 and H 2 O by the reaction with negatively charged surface oxygen can be expressed as a probable sensing reaction: This accords with gas-chromatography results from the literature. 34 The basic mechanism of chemiresistive metal-oxide gas sensing includes the adsorption and desorption of gas molecules, surface reaction, and charge transportation. The MoO 3 nanopaper comprised a network of nanobelts that facilitated the adsorption, surface reaction, and desorption of the testing/  This journal is © The Royal Society of Chemistry 2017 product gases because of the high gas accessibility and large surface-to-volume ratio. When the MoO 3 was exposed to air, oxygen molecules absorbed on the surface and formed chemisorbed oxygen species. Thus, a depletion layer was formed, which led to a high sensor resistance. When the MoO 3 was exposed to H 2 S gas, its high chemical affinity to H 2 S promoted the adsorption of H 2 S and the gas-sensing reaction (1). DRIFT results clearly show that the H 2 S-sensing reaction involving the adsorption and oxidation of H 2 S was more active at 250 C (Fig. 6a). When the MoO 3 was exposed to TMA, the nitrogen atoms in TMA had a lone pair of electrons that could be donated to form bonds with the Lewis-acid (Mo ion) sites of MoO 3 . In addition, the methyl group in TMA easily reacted with the negatively charged oxygen on the surface. According to the DRIFT results (Fig. 6b), the gas-sensing reaction (2) was active at 325 C, conrming that the MoO 3 nanopaper had the best response to TMA at 325 C.

Conclusions
Free-standing, semi-transparent, exible MoO 3 nanopapers were synthesized by a modied hydrothermal method and ltration. The nanopapers exhibited excellent dual-sensing properties toward H 2 S and TMA at different sensing temperatures. The sensor also demonstrated the potential for the all-inone detection of three representative offensive odors (TMA, H 2 S, and NH 3 ). The high chemical affinity of MoO 3 to H 2 S and the regenerative refreshing of the sensor surface are suggested as reasons for the selective, sensitive, and reversible detection of H 2 S at 250 C, while the acid-base interaction between acidic MoO 3 and basic TMA was responsible for the selective and sensitive detection of TMA. The mechanism underlying the dual sensing was elucidated using DRIFT analysis. The proposed sensor provides a simple solution to detect various irritant gases in a highly selective manner.