Nitrogen Oxide ( NO ) Gas-Sensing Properties of Bi 2 MoO 6 Nanosheets Synthesized by a Hydrothermal Method

Bi2MoO6 nanosheets were synthesized by a hydrothermal method. Morphology and structure of the Bi2MoO6 nanosheets were analyzed by SEM, XRD, N2 adsorption techniques and XPS. Gassensing properties of the as-prepared Bi2MoO6 sensors were also systematically investigated. The results showed the reaction temperature greatly affected the morphology and structure of as-prepared Bi2MoO6 nanosheets. When the reaction temperature reached 170 oC, the morphology of the Bi2MoO6 nanosheets tended to regular, and pure Bi2MoO6 nanosheets were obtained. The operating temperature determined the gas-sensing properties of the Bi2MoO6 sensor. At this optimal operating temperature of 300 oC, the sensitivity of the Bi2MoO6 sensor towards 20 ppm nitrogen oxide (NO) reached a maximum of 3.13. With the increase of the nitrogen oxide (NO) concentration, the sensitivity of the Bi2MoO6 sensor also rapidly increased, and displayed an almost linear relationship between them. Additionally, the Bi2MoO6 sensor demonstrated excellent selectivity with respect to several typical interfering gases.


Introduction
In the last several years, with the increasing concerns about public safety and human health, there are great demands on precise detection and early warning for environmentally hazardous gases such as NO, NO 2 , N 2 O, NH 3 , H 2 S, SO 2 , CO, CO 2 , and CH 4 . 1 Among them, nitrogen oxide (NO) is a typical hazardous gas produced from combustion chemical plants and automobiles, and can cause acid rains, photochemical smog and production of ozone. 2 Therefore, the fast and real-time detection for nitrogen oxide (NO) is very crucial to reduce its harmful effect to environmental and human beings.Currently, traditional detection technologies including optical spectroscopy and gas chromatography usually employ expensive, bulky instruments and complicated sample preparation processes.Therefore, developing miniaturized smart sensors for fast and real-time detection is attracting a great deal of interest. 3As we know, several kinds of gas sensors have been explored according to different sensing materials and various transduction platforms. 4][7][8][9] Recently, with the fast development of nanotechnology, a large number of nanostructured materials including SnO 2 , WO 3 , ZnO, Co 3 O 4 , and La 2 O 3 have been employed for gassensing applications. 10However, compared to these simple binary oxides, multicomponent oxides have greater advantages.For example, they have more freedom to tune the chemical and physical properties of nanomaterials by altering their compositions, which make them more suitable to be adopted as gas sensors. 11Bi 2 MoO 6 is a typical multicomponent metal oxide with a band gap of approximately 2.66 eV.As one of the most important members in the Aurivillius family constructed by (Bi 2 O 2 ) 2+ layers sandwiched between (MoO 4 ) 2− slabs, Bi 2 MoO 6 has aroused extensive attention owing to its potential applications on ferroelectricity, oxide anion conductivity, and non-linear dielectric. 12,13  is well known that the morphologies of nanomaterials have significant effects on their physical and chemical properties.5][16][17][18][19][20][21] These previous studies reveal that the exploration of Bi 2 MoO 6 nanomaterials with different morphologies can achieve versatile properties, which can meet a variety of application needs.In the following, we reported the synthesis of Bi 2 MoO 6 nanosheets using a simple hydrothermal method, and investigated the morphologies and gas-sensing properties of Bi 2 MoO 6 nanosheets at different reaction temperatures.Finally, we evaluated the practicability

Preparation of Bi 2 MoO 6 nanomaterials
0.06 mol of Bi(NO 3 ) 3 ▪5H 2 O and 0.03 mol of Na 2 MoO 4 ▪2H 2 O were added to 100 ml deionized water under stirring.Then 3M HCl was added in the mixed solution until the pH of the solution reached 1.0.Then, the mixture was sealed in a 100 mL Teflon-lined autoclave and heated at 140 ºC, 170 ºC and 200 ºC for 20 h, respectively, and then cooled to room temperature.The resulting precipitates were collected by centrifugation and washed three times by deionized water and ethanol to remove possible impurities.After drying at 60 o C for 10 h, Bi 2 MoO 6 nanomaterials were obtained.

Fabrication and measurement of Bi 2 MoO 6 gas sensor
Bi 2 MoO 6 nanomaterials were mixed with several drops of ethanol to form a slurry, and then the slurry was brush-coated onto the surfaces of an alumina tube with two Au electrodes and four Pt wires.A Ni-Cr heating wire was inserted into the alumina tube and used as a heater.The alumina tube was then welded onto a pedestal with six probes to obtain the final sensor unit.Gas sensing tests were performed on a WS-30A static gas-sensing system (HanWei Electronics Co., Ltd., Henan, China) using ambient air as the dilute and reference gas.The test gas with a calculated volume was introduced into the test chamber by a microsyringe. 22he sensor sensitivity is defined as follows, where R a and R g are the electrical resistance of the sensor in air and in test gas, respectively.

Gas-sensing properties of Bi 2 MoO 6 nanomaterials
The operating temperature has a great influence on the response of the Bi 2 MoO 6 sensor towards nitrogen oxide (NO).   ) and release the trapped electrons back to the conduction band, resulting in the decrease of the electrical resistance of the Bi 2 MoO 6 sensor. 28n order to investigate the practicability of the Bi 2 MoO 6 sensor for nitrogen oxide (NO) detection, the gas-sensing performances of the Bi 2 MoO 6 sensor towards other interfering gases were also carried out under the same conditions.Figure 7 presents the nitrogen oxide (NO)-selective characteristics of the Bi 2 MoO 6 sensor with respect to several typical interfering gases including CO, CO 2 , CH 4 , and NH 3 .The gas sensitivity of the Bi 2 MoO 6 sensor to 100 ppm nitrogen oxide (NO) is 3.94, which is significantly higher than all the other gases.All those indicate that the Bi 2 MoO 6 sensor processes excellent selectivity.The response of the Bi 2 MoO 6 sensor towards nitrogen oxide (NO) can be interpreted by the change in the electrical resistance of the Bi 2 MoO 6 sensor, which is usually adopted as a response signal to detect the tested gas.In air, oxygen molecules adsorbed on the surface of the Bi 2 MoO 6 sensor will capture electrons from the conduction band and form

Conclusions
We have fabricated the Bi 2 MoO 6 nanosheets using a simple hydrothermal method.The XRD and SEM analysis revealed that pure and regular nanosheet-type Bi 2 MoO 6 nanomaterials were obtained at the reaction temperature of

Acknowledgement
This work was supported by the National Natural Science Foundation of China (21476034) and the Fundamental Research Funds for the Central Universities (3132016327).

X
-ray diffraction (XRD) patterns of Bi 2 MoO 6 nanomaterials were recorded using a D/Max-2400 diffractometer (Cu Kα radiation, λ =1.54055Å) in a range of diffraction angle 2θ from 5 o to 85 o to analyze the diffraction peaks of Bi 2 MoO 6 nanomaterials.The morphologies of Bi 2 MoO 6 nanomaterials were observed by a scanning electron microscope (SEM) (Philips XL30 FEG).X-ray photoelectron spectroscopy (XPS) of Bi 2 MoO 6 nanomaterials was carried out on Thermo SCIENTIFIC ESCALAB 250 spectrometer with a monochromatic Al Ka source.The porous structure of Bi 2 MoO 6 nanomaterials was characterized by nitrogen sorption technique (Quantachrome Autosorb-iQ).

Figure 1
Figure 1 provides the representative XRD patterns of the Bi 2 MoO 6 nanomaterials synthesized at the three reaction temperatures (140 o C, 170 o C, and 200 o C).As observed, when the reaction temperatures are 140 o C and 170 o C, the Bi 2 MoO 6 nanomaterials show the diffraction peaks at 2θ values 23.5 o , 28.2 o , 32.5 o , 33.2 o , 36.1 o , 46.8 o , 47.2 o , 55.5 o , 56.4 o and 58.4 o , which are assigned to (111), (131), (002), (060), (151), (202), (260), (133), (191) and (262) planes of Bi 2 MoO 6 (JCPDS No. 76-2388), suggesting pure Bi 2 MoO 6 nanomaterials are obtained.23The sharp and strong intensity of XRD peaks at the two reaction temperatures suggest that the samples have better crystallinity.As reaction temperature is increased to 200 o C, many extra peaks arising from other phases are detected, indicating impurity phase is introduced in the Bi 2 MoO 6 nanomaterials.The morphologies of the as-synthesized samples at different reaction temperatures were investigated by SEM (Figure2).As observed, when the reaction temperature is controlled at 140 o C, aggregated and irregular nanosheets and nanoparticles are formed.Increasing the reaction temperature to 170 o C, the morphology of the Bi 2 MoO 6 nanomaterials tended to regular nanosheets.When the reaction temperature reaches 200 o C, the shape of nanosheets grows more regular, and the sizes of these regular nanosheets are rough several hundreds nanometer.

Figure 3
Figure 3 displays N 2 adsorption-desorption isotherms of the Bi 2 MoO 6 samples synthesized at the three reaction temperatures (140 o C, 170 o C, and 200 o C).Obviously, typical IV-type isothermal curves with a hysteresis loop are observed, which implies these materials have mesoporous structure.As measured, the BET specific surface areas of the three Bi 2 MoO 6 samples are found to be 16.99 m 2 •g -1 , 10.43 m 2 •g -1 , and 8.68 m 2 •g -1 , respectively, which may result from their different microstructure synthesized at the three temperatures.

Figure 5
presents the sensitivity of the Bi 2 MoO 6 sensors obtained at 140 o C, 170 o C and 200 o C to 20 ppm nitrogen oxide (NO) as a function of the operating temperature.As observed, the Bi 2 MoO 6 sensor synthesized at the reaction temperature of 170 o C demonstrates high gas sensitivity, which may be ascribed to its regular nonosheet-type structure and high BET specific surface area.The increase in the operating temperature significantly improves the sensitivity of the Bi 2 MoO 6 sensor, which reaches a maximum of 3.13 at 300 o C and then decreases.The phenomenon can be explained by the kinetics and thermodynamics of gas adsorption and desorption on the surface of Bi 2 MoO 6 nonosheets.When the operating temperature is lower than 300 o C, nitrogen oxide

Figure 5 .
Figure 5. Sensitivity of the Bi 2 MoO 6 sensor at different operating temperatures.

Figure 6
Figure6provides the relationship between the sensitivity of the Bi 2 MoO 6 sensor and nitrogen oxide (NO) concentration (from 10 ppm to 50 ppm).It can be seen that with the increase of the nitrogen oxide (NO) concentration, the sensitivity of the Bi 2 MoO 6 sensor also rapidly increases, and displays an almost linear relationship between them, implying the Bi 2 MoO 6 sensor is more favorable for detecting low concentration of nitrogen oxide (NO) in real application.27

Figure 6 .
Figure 6.Sensitivity of the Bi 2 MoO 6 sensor to different nitrogen oxide (NO) concentrations.

Figure 7 .
Figure 7. Sensitivity of the Bi 2 MoO 6 sensor to other interference gases.
170 o C. The XPS wide scan spectra confirmed the presence of Bi, Mo and O in Bi 2 MoO 6 nanosheets.The sensitivity of the as-prepared Bi 2 MoO 6 sensor towards 20 ppm nitrogen oxide (NO) increased with the increasing operating temperature and then reached a maximum at 300 o C. The Bi 2 MoO 6 sensor showed almost linear relationships between the nitrogen oxide (NO) concentration and sensitivity of the Bi 2 MoO 6 sensor.In the presence of other interference gases (including CO, CO 2 , CH 4 , and NH 3 ), the Bi 2 MoO 6 sensor still demonstrated excellent selectivity.
MoO 6 nanomaterials applied as gas sensors for nitrogen oxide (NO) detection.