English

• 0.   Introduction

  Trimethylamine (TMA) is a basic tertiary amine compound with the chemical formula N(CH₃)₃^[1]. TMA can cause headaches, nausea, and irritation to the eyes as well as to the respiratory system^[2]. Besides, TMA is known to be existent in dead fish^[3], it could be a good method to evaluate the freshness of fish by testing the TMA concentration released from fish^[4]. In the evaluation process of fish freshness, 0~10 μL·L^-1 of TMA is regarded as fresh, whereas more than 10 μL·L^-1 of TMA is regarded as decayed^[5].

  Metal oxide semiconductor gas sensors have been investigated by many researchers due to their high sensitivity, fast response, simple fabrication, and low cost^[6]. Many metal oxides, such as ZnO^[7], SnO₂^[8-9], TiO₂^[10] and MoO₃^[11], have been reported to exhibit high response to TMA. Among these metal oxides, MoO₃ has been considered as one of the most promising gas sensing materials to different type of gases^[11]. MoO₃ can be used in many fields such as gas sensors^[12], ion batteries^[13], and photocatalysis^[14] due to its wide bandgap (2.39~2.9 eV). Yang et al. prepared MoO₃ nanoribbons by a simple hydrothermal method; the sensor based on MoO₃ nanoribbons shows high response to 1 000 μL· L^-1 H₂ at high operation temperature of 300 ℃; the response to 1 000 μL·L^-1 H₂ is 17.3 at 300 ℃; while the response time and recovery time for 1 000 μL·L^-1 H₂ are 10.9 and 30.4 s, respectively^[15]. Imawan et al. prepared sputtered MoO₃ multilayers; the sensors based on MoO₃ multilayers expose a very high response to H₂ with a good signal linearity for high concentrations in the range of 2 000 to 9 000 μL·L^{-1 [16]}. Hussain et al. prepared MoO₃ thin films by activated reactive evaporation technique; the sensor based on MoO₃ thin films shows the response to NH₃ and CO gases at concentrations lower than 10 μL·L^-1 in dry air; the response time and recovery time for 100 μL·L^-1 NH₃ are about 2 min and less than 10 min, while the response time and recovery time for 100 μL·L^-1 CO are 1 and 20 min, respectively^[17]. Therefore, it still need to enhance the gas sensing properties of sensors based on MoO₃ materials.

  Graphene quantum dots (GQDs) are known as nanoparticles that are made from the fragment of few layers of graphene, which present unique properties due to their quantum confinement effects and these are expected to apply in many fields such as field effect transistors (FETs), capacitors, Li-ion batteries, electrodes, and solar cells^[18-20]. Graphene (G) has been considered as promising candidates for sensing materials that can detect extremely low concentrations of gases such as CO₂^[21], NH₃^[22], H₂^[23], TMA^[24]. Chu et al. prepared GQDs/ZnFe₂O₄ composites via hydrothermal method; the responses of the sensors based on pure ZnFe₂O₄ (S-0) and ZnFe₂O₄/GQDs (S-15) to 1 000 μL·L^-1 acetone are 1.1 and 13.3, at room temperature respectively; the response time and the recovery time for 1 000 and 5 μL·L^-1 acetone are all shorter than 12 s^[25]. Hu et al. prepared GQDs/α-Fe₂O₃ composites via a onestep facile hydrothermal method, the responses of the sensors based on pure α-Fe₂O₃ (S-0) and GQDs/α Fe₂O₃ (S-15) to 1 000 μL·L^-1 TMA are 5.5 and 1 033.0, respectively^[26]. Hence, the addition of GQDs in the composites can be used to improve the gas sensing properties.

  In this paper, we prepared MoO₃-GQDs composites by hydrothermal method. The as-prepared samples were characterized through various techniques and their gas sensing properties were studied. The results showed that the addition of GQDs in the MoO₃ -GQDs composites improved gas sensing response and gas sensing selectivity to TMA at 230 ℃.

  1.   Experimental

  GQDs were prepared by hydrothermal method. The preparation process was as follows: 2.0 g citric acid monohydrate was dissolved with 50 mL of deionized water and stirred for 30 min until the solution was clear. Then the solution was transformed into a 100 mL Teflon-lined stainless steel autoclave and heated at 200 ℃ for 5 h. The GQDs suspension was obtained after the reactor was cooled down to room temperature.

  GQDs-MoO₃ nanocomposites were prepared by hydrothermal method. The typical synthesis process was as follows: the different amounts of GQDs suspension (0, 2.0, 4.0, 6.0, and 8.0 mL) was diluted with deionized water, then 2.0 g ammonium molybdate tetrahydrate crystals ((NH₄)₆Mo₇O₂₄·4H₂O) were dissolved in the diluted suspension under vigorous stirring for 20 min, and the mixed suspension was sonicated for 30 min; then concentrated nitric acid (HNO₃) solution having the mass concentration of 65.0%~68.0% was added dropwise to the suspension until the pH of the mixed reaction solution reached 2.0 under vigorous stirring for 30 min; finally, the above mixture was transferred into 100 mL of Teflon-lined stainless steel autoclave, which was sealed tightly before placed in oven. Then the autoclave was heated at 180 ℃ for 24 h, and cooled down to the room temperature naturally. The obtained products were filtered, washed with deionized water and anhydrous ethanol several times, and dried at 80 ℃ for 12 h. The samples were labeled as S-0, S-2, S4, S-6 and, S-8, respectively.

  The as-prepared materials were uniformly ground in a mortar with two or three drops of terpineol to form a slurry. The slurry was coated onto the outer surface of an Al₂O₃ ceramic tube (4 mm in length, 1.2 mm in external diameter and 0.8 mm internal diameter, with a pair of Au electrodes and four Pt wires) uniformly with a small brush and dried at 90 ℃ for 2~3 h in a vacuum oven to remove terpineol. The Ni-Cr heating wire was inserted into the Al₂O₃ tube was used to control the operating temperature in the range of 20~450 ℃. The response of the sensor (S) was defined as the ratio (R_a/R_g) of the stable electrical resistance of gas sensor in air (R_a) to that in the test gases (R_g). The response time and recovery time were defined as the time for a sensor to reach 90% value of the final signal, respectively.

  A series of methods were used to characterize MoO₃ and GQDs-MoO₃ composites. The phase composition of nanocomposites was analyzed by X-ray diffraction (XRD, Bruker D8 Advance, Cu target Kα radiation, λ=0.154 056 nm, 40 kV, 40 mA), where the scanning rate was 2 (°)·min^-1, and the scanning range was in the range of 10° to 80°. The scanning electron microscopy (SEM) images were obtained on a Hitachi S4800 with an accelerating voltage of 10 kV. The transmission electron microscopy images were obtained on JEM-1200EX with an accelerating voltage of 120 kV. High-resolution transmission electron microscopy (HRTEM) images were obtained on Tecnai G2 F20 STWIN. Thermogravimetric analysis (TG) was carried out using a Netzsch STA449F3 system at a heating rate of 10 ℃ ·min^-1. Raman spectra were acquired on the Renishaw Invia Raman microscope. Surface bonding and functional groupings of the composites were studied by Fourier transform infrared (FTIR) spectroscopy using a Nicolet 6700 FTIR spectrometer in the range 400~4 000 cm^-1, with the KBr pellet technique. X-ray photoelectron spectra (XPS) measurements were performed on the ESCALAB250Xi photoelectron spectrometer.

  2.   Results and discussions

  Fig. 1 shows the X-ray diffraction patterns of pure MoO₃ and GQDs-MoO₃ composites with different contents of GQDs. By comparison, it was observed that the XRD diffraction peaks of all the samples were consistent with the diffraction peaks of the orthogonal type α-MoO₃ (PDF No. 05-0508). All the characteristic peaks at 12.9°, 23.2°, 25.8°, 27.5°, 39.1°, 49.4°, 55.3°, 57.9°, and 59.0° are attributed to the (020), (110), (040), (021), (060), (002), (112), (042), and (081) crystal planes of orthogonal α-MoO₃. The strong and sharp peaks in the XRD patterns showed that the sample were well crystallized. There was no peak of impurity in the XRD patterns of all products. With the increase of GQDs amount, the intensity of the diffraction peaks of (020), (040), and (060) gradually increased, which manifested that GQDs affected the growth of crystal face. The diffraction peaks of GQDs were not observed in the XRD patterns of GQDs/MoO₃ composites, which might result from the low content and relatively low diffraction intensity of GQDs.

  Figure 1

  

  Figure 1.  XRD patterns of S-0 and GQDs-MoO₃ composites

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  The morphology of the as-prepared samples was characterized by SEM and TEM. The SEM images of S-0 are shown in Fig. 2a and 2b, the surface of these micro-rods was relatively smooth; the length of these rods was mainly distributed in the range of 6~12 μm, and the width of these micro-rods was in the range of 200~300 nm. The SEM image of S-6 composite are shown in Fig. 2c, the length of the micro-rods in S-6 composite was around 6 μm. The TEM image of S-6 composite is shown in Fig. 2d, the width of a single nanorod was about 150~200 nm. The HRTEM images of S-6 are shown in Fig. 2(e, f), a very clear and well-defined lattice spacing of 0.262 nm in HRTEM image corresponds to the (101) crystal planes of graphene^[27]; the plane spacings of 0.373 and 0.24 nm correspond to the (001) and (201) facet of α-MoO₃^[28], respectively. These results confirmed that there were GQDs and MoO₃ in the as-prepared composite (S-6).

  Figure 2

  

  Figure 2.  (a, b) SEM images of S-0; (c) SEM image of S-6; (d) TEM image of S-6; (e, f) HRTEM images of S-6

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  Fig. 3 shows the TG curves of different samples (S-0, S-2, S-4, S-6, and S-8). As shown in Fig. 3, there was weight losses for all samples between 30 and 400 ℃, which resulted from the evaporation of water molecules adsorbed on the surface of the sample^[29]. When the temperature was higher than 400 ℃, an obvious weight loss appeared in the TG curves of S-2, S-4, S-6, and S-8, the weight loss was caused by the pyrolysis of the carbon skeleton of graphene quantum dots present in the samples^[30]. Weight loss in the temperature range of 400~450 ℃ certified the presence of GQDs in the composites. The sample tended to be stable from 460 to 760 ℃, α-MoO₃ reached the thermodynamic stable phase^[31]. When the temperature further increased to 790 ℃, the sharp weight losses occurred in the TG curves of all samples, which can be ascribed to sublimation of MoO₃^[32]. Weight loss curve showed that the content of GQDs in S-0, S-2, S-4, S-6, and S-8 estimated from TG curves were 0%, 1%, 2%, 3%, and 4%, respectively.

  Figure 3

  

  Figure 3.  TG curves of S-0, S-2, S-4, S-6 and S-8

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  Raman spectra of S-6 and S-0 composites are shown in Fig. 4. There were many characteristic peaks in the range of 100~400 cm^-1, which belonged to the various modes of bending vibration of pure α-MoO₃^[33-34]. There were three peaks at 991, 663, and 815 cm^-1 in the Raman spectra of S-0 and S-6; the characteristic peak at 991 cm^-1 can be assigned to the asymmetric stretching mode of terminal oxygen interaction (Mo⁶⁺＝O)^[35]; the peak observed at 815 cm^-1 can be ascribed to the doubly coordinated oxygen atoms to Mo (Mo＝O symmetric stretching) atoms^[36] while the peak located at 663 cm^-1 can be attributed to triply coordinated oxygen atoms to Mo (O—Mo—O stretching) atoms^[37]. The characteristic peaks at 1 345 and 1 585 cm^-1 in the Raman spectrum of S-6 correspond to the D peak and G peak of graphene respectively, which further confirmed the existence of GQDs in S-6 sample^[38].

  Figure 4

  

  Figure 4.  Raman spectra of S-6 and S-0

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  Fig. 5 shows the FTIR spectra of S-0 and S-6. There were two peaks at 3 430 and 1 620 cm^-1 that can be attributed to the stretching vibration and bending vibration of O—H of absorbed water on the surface of material^[39-41]. Due to the different molybdenum and oxygen atom linking modes in the MoO₃ octahedral, there were three infrared vibration modes, the characteristic peaks at 995 cm^-1 in the FTIR spectrum of S-0 and 998 cm^-1 in the spectrum of S-6 are the stretching vibration of the Mo＝O double bond^[42]; the characteristic peaks at 864 cm^-1 in the spectrum of S-0 and 870 cm^-1 in the spectrum of S-6 correspond to the Mo—O—Mo vibrational mode of Mo⁶⁺; the characteristic peaks at 544 cm^-1 in the spectrum of S-0 and 550 cm^-1 in the spectrum of S-6 are due to the bending vibration of Mo— O—Mo bond, where each O^2- is shared by three Mo⁶⁺. The peaks located at 1 726, 1 402, and 1 120 cm^-1 in the FTIR spectrum of GQDs-MoO₃ (S-6) can be assigned to characteristic bands of C＝O stretching vibrations of COOH groups, the stretching vibration of C—O (carboxyl), and stretching vibration of C—O (alkoxy), respectively^[43], which further verified the presence of GQDs in S-6.

  Figure 5

  

  Figure 5.  FTIR spectra of S-0 and S-6

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  The XPS spectra of S-6 composite are shown in Fig. 6. It could be found from the full survey spectrum that the composite was composed of Mo, C, and O elements. The XPS spectrum of Mo3d exhibited two peaks at 232.8 and 236.1 eV corresponding to Mo3d_5/2 and Mo3d_3/2 respectively, the binding energy difference between Mo3d_5/2 and Mo3d_3/2 was found to be 3.3 eV. This revealed the presence of Mo in S-6 composite as Mo⁶⁺ oxidation state^[44]. The C1s spectrum showed that there were three peaks at 284.9, 285.7, and 288.8 eV. The characteristic peaks at 284.9, 285.7, and 288.8 eV correspond to the sp² hybrid functional groups of carbon (C ＝C and C—C) in GQDs, sp³ C hybrid functional groups and C＝O bonds, respectively^[45]. From the deconvoluted peaks of O1s spectrum centered at 532.56, 531.26, and 530.7 eV in Fig. 6d, the presence of O^2-, O^- and O₂^- species were confirmed respectively^[46]. XPS spectrum results showed that the GQDs were present in S-6 composite.

  Figure 6

  

  Figure 6.  XPS spectra of S-6: (a) survey; (b) Mo3d; (c) C1s; (d) O1s

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  Fig. 7 shows the responses of the sensors based on pure MoO₃ and GQDs-MoO₃ composites (S-2, S-4, S-6, and S-8) to 1 000 μL·L^-1 TMA at different operating temperatures. The responses of all sensors to 1 000 μL·L^-1 TMA were very low when the operating temperatures were lower than 150 ℃. The response of the sensor based on pure MoO₃ to 1 000 μL·L^-1 TMA increased with the operating temperature increasing in the temperature range of 25~310 ℃, the response was 13.8 when the operating temperature was 310 ℃. When the operating temperature was 230 ℃, the responses of GQDs-MoO₃ composites (S-2, S-4, S-6, and S-8) were higher than those of pure MoO₃, the responses of composite materials to TMA increased first and then decreased with the increase of the content of graphene quantum dots; the responses of sensors based on S-2, S-4, S-6, and S-8 nanocomposites were 10.97, 15.2, 74.08, and 48.43, respectively. Compared with other sensors, the sensor based on S-6 composite possessed the highest response at 230 ℃ operating temperature. As the temperature beyond the optimum operating temperature (at which the sensor response was highest), the response decreased because of the low adsorption ability of the TMA molecules, which caused a low utilization rate of the sensing material^[47].

  Figure 7

  

  Figure 7.  Response of the sensors based on S-0, S-2, S-4, S-6, and S-8 to 1 000 μL·L^-1 TMA at different operating temperatures

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  The response of S-6 to 1 000 μL·L^-1 of various gases at different operating temperatures were depicted in Fig. 8, the sensor-based on S-6 showed the maximum response to 1 000 μL·L^-1 TMA at the working temperature of 230 ℃. At an operating temperature of 230 ℃, the responses of the sensor based on the nano-composite (S-6) to 1 000 μL·L^-1 TMA, ethanol, acetone, ammonia, acetic acid and acetaldehyde were 74.08, 17.84, 7.92, 4.85, 2.1, and 1.3, respectively; the sensor showed high response and good gas sensing selectivity to TMA; the response of the sensor to 1 000 μL·L^-1 TMA was 74.08. When detecting TMA, ethanol was usually the interfering gas, so the response ratio of S_TMA to S_ethanol could be used as a gas sensing selectivity index; the TMA sensing performances of the materials reported in some literature and this work are shown in Table 1, the ratio of the response to 1 000 μL·L^-1 TMA of S-6 to that of 1 000 μL·L^-1 ethanol attained 74.08/ 17.84=4.15, which indicated that the selectivity to TMA was increased greatly.

  Figure 8

  

  Figure 8.  Response of the S-6 sample to 1 000 μL·L^-1 of various gases at different operating temperatures

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  Table 1

  

  Table 1.  Comparison of TMA sensing performance of as-fabricated GQDs-MoO₃ based sensor against previously reported results

  下载: 导出CSV

  Sensing material	TMA / (μL·L-1)	STMA (Ra/Rg)	STMA/Sethanol	Temperature / ℃	Reference
  Cr2O3-decorated SnO2 nanowires	5	9.87	1.97	450	[49]
  Fe2O3/TiO2	50	13.9	—	250	[50]
  ZnO-In2O3 composite nanofibers	5	133.9	3.27	300	[51]
  Flower-like In2O3	5	6	1.75	340	[52]
  MoO3 nanoplate	5	5	—	300	[53]
  Rh-doped SnO2 hollow spheres	5	1 177.5	1.42	450	[54]
  Ru-SnO2 hollow spheres	5	99.5	1.38	400	[55]
  GQDs-MoO3	1 000	74.08	4.15	230	This work
  	10	5.9	—	230	This work
  “—”means that there is no data in the literature, STMA (Ra/Rg) is the response of the sensor to 1 000 μL·L-1 TMA, STMA/Sethanol is the selectivity of sensor to 1 000 μL·L-1 TMA.

  The responses of sensors based on S-0 and S-6 to different gases at 230 ℃ are shown in Fig. 9, the responses of the sensor based on S-0 to 1 000 μL·L^-1 acetic acid, acetaldehyde, ethanol, acetone, TMA and ammonia, were 2.87, 1.4, 1.3, 3.67, 5.32, and 1.15, respectively; but the responses of the sensor based on S-6 to 1 000 μL·L^-1 acetic acid, acetaldehyde, ethanol, acetone, TMA and ammonia, were 2.96, 2.22, 17.84, 7.92, 74.08, and 4.85, respectively. S-0 composite had a response of 1.3 and 5.32, to 1 000 μL·L^-1 ethanol and TMA, whereas S-6 composite had a response of 17.84 and 74.08 to 1 000 μL·L^-1 ethanol and TMA, which proved that the modification of GQDs not only changed the response to TMA but also improved gas sensing selectivity.

  Figure 9

  

  Figure 9.  Responses of sensors based on S-0 and S-6 to different gases at 230 ℃

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  The response transients of the sensor based on sample S-6 composite to TMA (1 000, 500, 100, 10, and 1 μL·L^-1) at 230 ℃ were shown in Fig. 10. The response times for 1 000, 500, 100, 10, and 1 μL·L^-1 TMA were 73, 87, 50, 20, and 21 s, respectively. The recovery times for 1 000, 500, 100, 10, and 1 μL·L^-1 were 34, 41, 37, 26, and 23 s, respectively. The minimum detection limit of the sensor based on sample S-6 composite for TMA was 1 μL·L^-1. This showed that the sensor based on sample S-6 composite exhibited a large detection range for TMA vapor.

  Figure 10

  

  Figure 10.  Response transients of the sensor based on S-6 to TMA (1 000, 500, 100, 10, and 1 μL·L^-1) at 230 ℃

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  The TMA sensing mechanism of metal oxide gas sensing materials was reported by many researchers^{[5, 12, 15, 27]}, the TMA sensing mechanism on the surface of GQDs/MoO₃ was based on the reaction between TMA and adsorbed oxygen on the surface of GQDs/MoO₃ and formed adsorbed oxygen species, which led to the decrease of electrons concentration and the increase of the sensor resistance. Moreover, the GQDs in GQDs/ MoO₃ nanocomposites played an important role in enhancing the gas sensing performances. Firstly, GQDs can enhance the conductivity of the sensors based on GQDs-MoO₃ composites comparing with pure MoO₃. Secondly, the addition of GQDs in composites facilitates the electron transfer from GQDs/MoO₃ conducting channel to TMA to form N₂ and CO₂. Thirdly, the improvement of gas sensing properties is related to the interaction between MoO₃ and GQDs^{[25, 48]}. When the sensor is in air ambient, the oxygen molecules adsorbs on the surface of GQDs/MoO₃ nanocomposites and captures electrons from the conduction band of GQDs/MoO₃ and formed O₂^- (ads); the formation of O₂^- (ads) causes the increase of the sensor resistance. When the sensor is exposed to TMA atmosphere, TMA molecules react with the adsorbed oxygen species and give the captured electrons back to the conduction band of MoO₃, which lowers the electrical resistance of the sensor device. The reaction can be expressed as:

  $ {{\rm{O}}_{\rm{2}}}\left( {{\rm{air}}} \right){\rm{ + }}{{\rm{e}}^{\rm{ - }}} \to {\rm{ }}{{\rm{O}}_{\rm{2}}}^{\rm{ - }}\left( {{\rm{ads}}} \right) $

  (1)

  $ \begin{array}{l} {\rm{4(C}}{{\rm{H}}_{\rm{3}}}{{\rm{)}}_{\rm{3}}}{\rm{N + 21}}{{\rm{O}}_{\rm{2}}}^{\rm{ - }}\left( {{\rm{ads}}} \right){\rm{ }} \to \\ \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{2}}{{\rm{N}}_{\rm{2}}}{\rm{ + 12C}}{{\rm{O}}_{\rm{2}}}{\rm{ + 18}}{{\rm{H}}_{\rm{2}}}{\rm{O + 21}}{{\rm{e}}^{\rm{ - }}} \end{array} $

  (2)

  Figure 11

  

  Figure 11.  Schematic drawing of the TMA sensing mechanism of GQDs-MoO₃ nanocomposites in air and TMA ambient

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  3.   Conclusions

  In summary, GQDs and GQDs-MoO₃ composites with different amounts of GQDs were synthesized by a hydrothermal method. The synthesized GQDs-MoO₃ nanocomposites were found to be more efficient for the detection of TMA at the operating temperature of 230 ℃. The sensor-based on nano-composite (S-6) exhibited good response and good selectivity to TMA vapor. The sensor of GQDs-MoO₃ composites could be operated at 230 ℃, and showed a higher response to TMA than pure MoO₃ sensor; the response of the sensor to 1 000 μL·L^-1 TMA reached 74.08. The response times for 1 000, 500, 100, 10, and 1 μL·L^-1 TMA were 73, 87, 50, 20, and 21 s, respectively. The recovery times for 1 000, 500, 100, 10, and 1 μL·L^-1 were 34, 41, 37, 26, and 23 s, respectively. The sensor of MoO₃-GQDs (S-6) composite could detect TMA as low as 1 μL·L^-1.

  
  Acknowledgments: The authors are grateful to the financial support from the National Natural Science Foundation of China (Grant No.61671019, 61971003).
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• 

  Figure 1  XRD patterns of S-0 and GQDs-MoO₃ composites

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  Figure 2  (a, b) SEM images of S-0; (c) SEM image of S-6; (d) TEM image of S-6; (e, f) HRTEM images of S-6

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  Figure 3  TG curves of S-0, S-2, S-4, S-6 and S-8

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  Figure 4  Raman spectra of S-6 and S-0

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  Figure 5  FTIR spectra of S-0 and S-6

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  Figure 6  XPS spectra of S-6: (a) survey; (b) Mo3d; (c) C1s; (d) O1s

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  Figure 7  Response of the sensors based on S-0, S-2, S-4, S-6, and S-8 to 1 000 μL·L^-1 TMA at different operating temperatures

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  Figure 8  Response of the S-6 sample to 1 000 μL·L^-1 of various gases at different operating temperatures

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  Figure 9  Responses of sensors based on S-0 and S-6 to different gases at 230 ℃

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  Figure 10  Response transients of the sensor based on S-6 to TMA (1 000, 500, 100, 10, and 1 μL·L^-1) at 230 ℃

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  Figure 11  Schematic drawing of the TMA sensing mechanism of GQDs-MoO₃ nanocomposites in air and TMA ambient

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  Table 1.  Comparison of TMA sensing performance of as-fabricated GQDs-MoO₃ based sensor against previously reported results

  Sensing material	TMA / (μL·L-1)	STMA (Ra/Rg)	STMA/Sethanol	Temperature / ℃	Reference
  Cr2O3-decorated SnO2 nanowires	5	9.87	1.97	450	[49]
  Fe2O3/TiO2	50	13.9	—	250	[50]
  ZnO-In2O3 composite nanofibers	5	133.9	3.27	300	[51]
  Flower-like In2O3	5	6	1.75	340	[52]
  MoO3 nanoplate	5	5	—	300	[53]
  Rh-doped SnO2 hollow spheres	5	1 177.5	1.42	450	[54]
  Ru-SnO2 hollow spheres	5	99.5	1.38	400	[55]
  GQDs-MoO3	1 000	74.08	4.15	230	This work
  	10	5.9	—	230	This work
  “—”means that there is no data in the literature, STMA (Ra/Rg) is the response of the sensor to 1 000 μL·L-1 TMA, STMA/Sethanol is the selectivity of sensor to 1 000 μL·L-1 TMA.

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•