3.1 Materials characterization
XRD characterization diagrams of MoSe2, MoO3 and MoO3/MoSe2 heterostructure are shown in Fig. 2, with a test angle range from 10° to 60°. From the XRD pattern of MoO3, three high-intensity diffraction peaks can be observed, which indicates that MoO3 is a highly ordered orthorhombic crystal [23]. The angles of all diffraction peaks of MoO3 are 13.2°, 23.8°, 26.1°, 27.4°, 38.6°, 42.3°, 46.6° and 53.3°, respectively, which correspond to the (020), (110), (040), (021), (060), (141), (200) and (211) crystal planes of MoO3 [24]. The XRD confirms that the synthesized MoO3 contains no other impurities. In the XRD pattern of MoSe2, the peaks at 14.1°, 33.2°, 37.1° and 55.9° correspond to the (002), (100), (103) and (110) crystal planes of MoSe2 [25]. The black line in the figure contains the characteristic diffraction peaks of MoSe2 and MoO3, which shows that the MoO3/MoSe2 heterostructure has been successfully synthesized.
SEM characterization was performed to further verify the successful preparation and visually observe the morphology of the prepared materials. Figure 3 shows the SEM characterization results of MoSe2, MoO3 and MoO3/MoSe2 heterostructure. It can be found from Fig. 3 (a) that MoSe2 is flower-shaped structure as a whole, and it has a porous structure. The surface of MoO3 in Fig. 3 (b) has a high degree of crystallinity, and its overall shape is a long rod. Figures 3 (c-d) are SEM characterizations of the MoO3/MoSe2 heterostructure. We can clearly observe the tightly coupled morphology of the two, indicating the successful synthesis of the two materials.
TEM characterization of the prepared materials is shown in Fig. 4. Figure 4 (a) is the micro-morphology of the MoO3/MoSe2 heterostructure. It can be observed that the aggregated flower-like shaded part is MoSe2, and the rod-shaped object on the upper right is MoO3, which shows that the two substances have been combined together closely. Figure 4 (b) is the TEM characterization diagram of MoO3. The rod-shaped MoO3 morphology can be clearly seen, and its width is about 400 nm. The inset is the electron diffraction pattern (SAED) of partial region of single MoO3. Figure 4 (c) is TEM characterization diagram of MoSe2. It can be found that MoSe2 is flower-like shape composed of thin layers. The inset is the SAED of partial area of single MoSe2. Figures 4 (d-e) are HRTEM characterization diagrams of MoO3 and MoSe2, respectively. The lattice fringe spacings of 0.33 and 0.41 nm correspond to the (021) and (200) crystal planes of MoO3, respectively. The spacings of 0.28 and 0.67 nm correspond to the (100) and (002) crystal planes of MoSe2, respectively [26 − 27]. In order to clearly observe the atomic distribution on the surface of MoSe2, some designated areas in Fig. 4 (e) are enlarged, as shown in Fig. 4 (f). The MoSe2 surface exhibits periodic honeycomb structure, which shows that the prepared MoSe2 has a graphene-like planar structure [25].
Figure 5 is XPS characterization result of MoO3/MoSe2 heterostructure. The XPS full spectrum in Fig. 5 (a) indicates that the sample contains elements of Mo, Se, O, and a small amount of C. The source of C may be some organic contaminants remaining on the surface of the sample. Figure 5 (b) shows the spectrum of Mo element. The characteristic peaks appearing at 232.17 and 229.02 eV correspond to the Mo 3d3/2 and Mo 3d5/2 orbitals of molybdenum tetravalent, respectively, and it is because that MoSe2 contains Mo-Se bonds [28]. While in MoO3, the characteristic peaks at 236.18 and 233.05 eV correspond to Mo 3d3/2 and Mo 3d5/2 orbitals of hexavalent molybdenum, respectively [29]. The characteristic peaks at 55.35 and 54.45 eV in Fig. 5 (c) respectively correspond to the Se 3d3/2 and Se 3p5/2 orbits of Se2− [28]. Figure 5 (d) is the spectrum of O element, in which the low binding energy of 530.75 eV is derived from the Mo-O bond in MoO3, and the high binding energy of 531.25 eV corresponds to the oxygen adsorbed on the sample surface [29]. The XPS characterization results also prove the successful synthesis of the composite materials.
3.2 Sensing performance
The experiment results of the sensors on exposure to different concentrations of trimethylamine at RT are shown in Fig. 6. Figure 6 (a) is the dynamic response diagram of MoSe2 sensor and MoO3/MoSe2 heterostructure sensor switching back and forth in air and TMA from 20 to 1000 ppb. Both sensors have good response and recovery characteristics, but MoO3/MoSe2 nanocomposite sensor shows higher response values at the same concentration of TMA gas. Moreover, MoO3/MoSe2 sensor can detect TMA as low as 20 ppb with high sensitivity, whose response is as high as 19%. Figure 6 (b) shows the fitting curves of the response values and gas concentrations of the two. Among them, the fitting curve of MoO3/MoSe2 heterostructure sensor is Y = 6.1861X0.3544, and the regression coefficient R2 is 0.9912. The response fitting curve of MoSe2 sensor is Y = 1.6909X0.4396, and R2 is 0.9856. Figure 6 (c) is the dynamic resistance change of MoO3/MoSe2 nanocomposite sensor. When the sensor is switched from air to trimethylamine environment, its resistance values emerge a downward trend, which shows that the sensor as a whole is n-type. Figure 6 (d) shows the response/recovery characteristics of two sensors to 1000 ppb TMA. It is not difficult to find that MoO3/MoSe2 nanocomposite sensor not only has a higher response value, but also has a faster response speed, which further illustrates that the MoO3/MoSe2 heterostructure helps to improve the gas sensing performance.
Figure 7 (a) shows the long-term stability of MoO3/MoSe2 nanocomposite sensor. Every two days, the sensor was tested on exposure to different concentrations (50, 250 and 500 ppb) of TMA, and the results show that the sensor is stable enough. When studying sensor performances, it is necessary to explore the effect of humidity on sensor response. Figure 7 (b) is the fitting curve of the response of MoO3/MoSe2 heterostructure sensor to 250 ppb TMA under different humidity conditions. The fitting equation is Y = 55.672 − 0.215X, where the horizontal axis X is relative humidity, vertical axis Y is the response value of the sensor, and the regression coefficient R2 is 0.939, which shows that the influence of humidity on the sensor is regular. The repeatability test was also performed on the sensor, as shown in Fig. 7 (c). MoO3/MoSe2 sensor was tested in air and fixed concentration of TMA gas for five cycles, respectively. The experimental results show that the sensor has good recoverability. The selectivity of the sensor to target gas is very important. As shown in Fig. 7 (d), compared with other interfering gases, MoO3/MoSe2 nanocomposite sensor and MoSe2 sensor have higher response values to TMA, indicating that test sensors have good selectivity to TMA.
Table 1 lists the comparison of TMA response characteristics among MoO3/MoSe2 based sensor in this work and previously reported sensors. It can be found that compared with the previous reported TMA sensors, the MoO3/MoSe2 sensor not only has a very low detection limit and higher response, but also has a lower working temperature, which can be used in room temperature. Therefore, the sensor proposed in this paper can be an excellent candidate for TMA detection.
Table 1
Comparison of TMA sensing performance of as-fabricated MoO3/MoSe2 based sensor against previous reported results.
Materials | Working Temp. | Response | LOD | Ref. |
Pd-ZnO | 300oC | 2.9 (1 ppm) | 1 ppm | [1] |
α-MoO3 | 133oC | 1.25 (20 ppb) | 20 ppb | [12] |
V2O5 | 240oC | 2.8 (100 ppm) | 10 ppm | [30] |
MoO3/Bi2Mo3O12 | 170oC | 7.2 (10 ppm) | 100 ppb | [32] |
V2O3−Cu2O | RT | 1.08 (3 ppm) | 3 ppm | [33] |
Co3O4/SnO2 | 175oC | 9.3 (5 ppm) | 1 ppm | [34] |
3D rGO/In2O3 | RT | 9.3% (100 ppm) | 100 ppm | [35] |
CdO:Al | RT | 32.12% (300 ppm) | 50 ppm | [36] |
MoO3/MoSe2 | RT | 19% (20 ppb) | 20 ppb | This work |
3.3 Gas sensing mechanism
The schematic diagram of the TMA sensing mechanism of MoO3/MoSe2 sensor is shown in Fig. 8. As a typical n-type semiconductor, the carriers of MoO3 are electrons. It is known from the above gas-sensitivity test results that the MoSe2 based sensor also exhibits n-type semiconductor characteristics to TMA. In gas sensing process, the MoO3/MoSe2 heterostructure does not participate in the reaction, it is the oxygen ions adsorbed on the surface that do. In other words, the gas sensing mechanism benefits from the adsorption and desorption of oxygen on the sensor surface. Possible reactions are as follows [31, 36]:
O2 (gas) → O2 (ads) (1)
O2 (ads) + e− → O2− (ads) (2)
4(CH3)3N + 21O2− (ads) → 2N2 + 12CO2 + 18H2O + 21e− (3)
It can be seen that when the sensor device is transferred from air to TMA environment, a large amount of electrons will be released, so the sensor exhibits lower resistance value. There may be two reasons for the enhanced gas sensing performance of MoO3/MoSe2 heterostructure sensor. Firstly, MoO3/MoSe2 composite structure has a large specific surface area [37 − 38], which is conducive to the diffusion of TMA molecules and makes TMA react with adsorbed oxygen thoroughly, thereby enhancing the gas sensing response. Secondly, the heterostructure formed between MoO3 and MoSe2 has synergistic effect. The formation of the heterostructure has an effect on the conductivity of the sensor. After MoO3 is modified with MoSe2 to form the heterostructure, the resistance value of the sensor in air decreases, meaning that the conductivity of the device is improved, which is conducive to the rapid conduction of carriers. The energy band diagram at the interface between MoO3 and MoSe2 and the formation process of the heterostructure are studied, as shown in Fig. 9. Figure 9 (a) is the energy band diagram before the two substances are combined, in which the band gap of MoO3 and MoSe2 are 3.2 and 1.3 eV, the work function is 6.9 and 4.6 eV, and the electron affinity is 6.7 and 3.9 eV, respectively [39 − 40]. Figure 9 (b) shows the formation process of the n-n heterostructure in air. At this time, the electron transfer between the two causes the energy band to bend. The change process of the heterostructure in TMA is shown in Fig. 9 (c). The width of the depletion layer is narrowed due to the reaction between TMA and the adsorbed oxygen, so that the sensor shows higher response value and faster response speed.