Acetylene Gas-Sensing Properties of Layer-by-Layer Self-Assembled Ag-Decorated Tin Dioxide/Graphene Nanocomposite Film

This paper demonstrates an acetylene gas sensor based on an Ag-decorated tin dioxide/reduced graphene oxide (Ag–SnO2/rGO) nanocomposite film, prepared by layer-by-layer (LbL) self-assembly technology. The as-prepared Ag–SnO2/rGO nanocomposite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectrum. The acetylene sensing properties were investigated using different working temperatures and gas concentrations. An optimal temperature of 90 °C was determined, and the Ag–SnO2/rGO nanocomposite sensor exhibited excellent sensing behaviors towards acetylene, in terms of response, repeatability, stability and response/recovery characteristics, which were superior to the pure SnO2 and SnO2/rGO film sensors. The sensing mechanism of the Ag–SnO2/rGO sensor was attributed to the synergistic effect of the ternary nanomaterials, and the heterojunctions created at the interfaces between SnO2 and rGO. This work indicates that the Ag–SnO2/rGO nanocomposite is a good candidate for constructing a low-temperature acetylene sensor.


Introduction
Acetylene (C 2 H 2 ) is a colorless and highly combustible gaseous hydrocarbon, widely used as a fuel in many industrial fields, such as metal welding [1], polyacetylene preparation [2], lithium-ion batteries [3], and conductive plastic manufacturing [4]. However, acetylene is unstable and there is a huge potential risk of fire or explosive accidents during its compression and heat treatment, or due to leakage. Furthermore, the dissolved content of acetylene gas in power transformer oil is critical to the safety and reliability of the transformer system [5,6]. Therefore, reliable, economical and portable acetylene gas sensors are of great importance to many applications. In recent years, a lot of interest has been attracted surrounding the development of effective techniques and sensitive methods for acetylene gas detection, such as photoacoustic spectroscopy [7], optical fiber [6,8] and metal-oxide semiconductors (MOS) and nanomaterial-based sensors (i.e., PdO-decorated SnO 2 [9], Au/multi-wall carbon nanotubes [10], Sm 2 O 3 -decorated SnO 2 [11], Ag-loaded ZnO [12][13][14] and NiO/SnO 2 heterostructures [15]). Among them, metal oxides have become important candidates Figure 1a illustrates the hydrothermal synthesis of SnO 2 . Firstly, 24 mg of SnCl 4 ·5H 2 O was dissolved in 30 mL of deionized water and stirred for 1 h. After that, the solution was hydrothermally treated at 120 • C for 12 h, and then washed with deionized water and ethanol several times. The SnO 2 aqueous solution was obtained after being ultrasonicated for 1 h and centrifugated for 15 min. A substrate with interdigitated electrodes for resistive sensing and heating elements was fabricated. The Ag-SnO 2 /rGO nanocomposite was deposited on the sensing electrodes using the layer-by-layer (LbL) self-assembly technique, which is shown in Figure 1b. Two bi-layers of PDDA/PSS were firstly self-assembled as the precursor layer, followed by alternative immersion into SnO 2 , GO, SnO 2 and Ag suspensions. The immersing time here used was 10 min for the polyelectrolytes and 15 min for the SnO 2 , GO and Ag suspensions. Intermediate rinsing with deionized water and drying with nitrogen gas were required after each monolayer assembly, to reinforce the interconnection between the layers. The film was formed due to the interaction of electrostatic forces between the positively and negatively charged nanoparticles. The first SnO 2 layer (positively charged) was designed for the intermediate bonding between PSS (negatively charged) and GO (negatively charged). The Ag-SnO 2 /rGO nanocomposite sensor was obtained via the thermal reduction of GO into rGO at 220 • C for 5 h in an oven. Furthermore, the pure SnO 2 and SnO 2 /rGO film sensor were fabricated to allow a comparison between the drop-casting and LbL self-assembly methods, respectively. aqueous solution was obtained after being ultrasonicated for 1 h and centrifugated for 15 min. A substrate with interdigitated electrodes for resistive sensing and heating elements was fabricated. The Ag-SnO2/rGO nanocomposite was deposited on the sensing electrodes using the layer-by-layer (LbL) self-assembly technique, which is shown in Figure 1b. Two bi-layers of PDDA/PSS were firstly self-assembled as the precursor layer, followed by alternative immersion into SnO2, GO, SnO2 and Ag suspensions. The immersing time here used was 10 min for the polyelectrolytes and 15 min for the SnO2, GO and Ag suspensions. Intermediate rinsing with deionized water and drying with nitrogen gas were required after each monolayer assembly, to reinforce the interconnection between the layers. The film was formed due to the interaction of electrostatic forces between the positively and negatively charged nanoparticles. The first SnO2 layer (positively charged) was designed for the intermediate bonding between PSS (negatively charged) and GO (negatively charged). The Ag-SnO2/rGO nanocomposite sensor was obtained via the thermal reduction of GO into rGO at 220 °C for 5 h in an oven. Furthermore, the pure SnO2 and SnO2/rGO film sensor were fabricated to allow a comparison between the drop-casting and LbL self-assembly methods, respectively.

Instruments and Analysis
Surface microscopy of the Ag-SnO2/rGO sample was carried out with a scanning electron microscope (SEM, Hitachi S-4800, Tokyo, Japan). The X-ray diffraction (XRD) spectrum of the samples was examined with an X-ray diffractometer (Rigaku D/Max 2500PC, Tokyo, Japan) using Cu Kα radiation with a wavelength of 1.5418 Å. The lattice fringes of Ag-SnO2/rGO were inspected with a transmission electron microscope (FEI Tecnai G2 F20, Shanghai, China). The nanostructural and compositional features of the Ag-SnO2/rGO and SnO2/rGO samples were characterized by Raman spectra (RamLab-010, Horiba Jobin Yvon, Paris, France).
A schematic of the experimental setup for acetylene sensing is shown in Figure 2. The acetylene gas sensing properties were investigated by exposing the sensor to various concentrations of acetylene gas, and the desired gas concentration was obtained by injecting the required quantity of acetylene into a sealed chamber using a syringe. The working temperature for the sensor was adjusted through applying varying voltages to the heating electrodes with a power source (Gwinstek GPD-4303S, New Taipei, Taiwan). The heater resistor (RH), heating voltage (VH), sensor resistance (RS)

Instruments and Analysis
Surface microscopy of the Ag-SnO 2 /rGO sample was carried out with a scanning electron microscope (SEM, Hitachi S-4800, Tokyo, Japan). The X-ray diffraction (XRD) spectrum of the samples was examined with an X-ray diffractometer (Rigaku D/Max 2500PC, Tokyo, Japan) using Cu Kα radiation with a wavelength of 1.5418 Å. The lattice fringes of Ag-SnO 2 /rGO were inspected with a transmission electron microscope (FEI Tecnai G2 F20, Shanghai, China). The nanostructural and compositional features of the Ag-SnO 2 /rGO and SnO 2 /rGO samples were characterized by Raman spectra (RamLab-010, Horiba Jobin Yvon, Paris, France).
A schematic of the experimental setup for acetylene sensing is shown in Figure 2. The acetylene gas sensing properties were investigated by exposing the sensor to various concentrations of acetylene gas, and the desired gas concentration was obtained by injecting the required quantity of acetylene Nanomaterials 2017, 7, 278 4 of 12 into a sealed chamber using a syringe. The working temperature for the sensor was adjusted through applying varying voltages to the heating electrodes with a power source (Gwinstek GPD-4303S, New Taipei, Taiwan). The heater resistor (R H ), heating voltage (V H ), sensor resistance (R S ) and protection resistor (R L ) made up the simplified circuit. The sensor resistance was recorded using a data logger (Agilent 34970A, San Jose, CA, USA), connected to a computer via a recommended standard (RS)-232 interface. The sensitivity of the sensor was defined as S = (R 0 − R g )/R 0 × 100%, where R 0 and R g were the sensor resistances in dry air and acetylene gas, respectively. The time taken by a sensor to achieve 90% of the total resistance change was defined as the response or recovery time. and protection resistor (RL) made up the simplified circuit. The sensor resistance was recorded using a data logger (Agilent 34970A, San Jose, CA, USA), connected to a computer via a recommended standard (RS)-232 interface. The sensitivity of the sensor was defined as S = (R0 − Rg)/R0 × 100%, where R0 and Rg were the sensor resistances in dry air and acetylene gas, respectively. The time taken by a sensor to achieve 90% of the total resistance change was defined as the response or recovery time.  Figure 3a shows the SEM image of the Ag-SnO2/rGO nanocomposite film. SnO2 microspheres and Ag nanoparticles (NPs) attached to the surface of rGO sheets are clearly observed. Figure 3b shows a high-resolution TEM image of the Ag-SnO2/rGO nanocomposite, and lattice fringe spacings of 0.35, 0.33 and 0.23 nm for rGO, SnO2 and Ag, respectively, are measured. Figure 3c indicates the XRD spectrum for the GO, rGO, SnO2 and Ag-SnO2/rGO nanocomposite films. Obvious peaks at 2θ angles of 10.78° and 24.7° are observed for the GO and thermally treated rGO, respectively, which is in agreement with previously published results, and further confirms the successful reduction of GO via thermal treatment [29][30][31]. The XRD spectrum of SnO2 indicates several peaks at 26.41°, 33.82°, 37.60°, 51.73° and 65.68°, indexed to the (110), (101), (200), (211) and (301) planes of rutile SnO2, which is in accordance with the data from JCPDS Card no. 41-1445 [32], and confirms the successful formation of SnO2 nanocrystals. Apart from the characteristic peaks attributed to SnO2, the XRD spectrum of the Ag-SnO2/rGO nanocomposite exhibited distinct peaks at 38.10°, 44.37° and 64.17°, which indexed to the (111), (200) and (220) planes of Ag crystallines, respectively [33]. However, the broad peak of rGO is not obvious in the XRD pattern of the Ag-SnO2/rGO nanocomposite, probably because the weak peak of rGO is swamped by the high intensity peak of the SnO2 at the 2θ angle of 26.41° [34,35]. Figure 3d shows the Raman spectrum of the SnO2/rGO and Ag-SnO2/rGO nanocomposites. The peaks located at 633 cm −1 in the SnO2/rGO and Ag-SnO2/rGO nanocomposites are typical Raman peaks of SnO2. The weak peaks located at 796 and 1588 cm −1 for the two samples are attributed to the Ag NPs. The peaks located at 1363 and 1640 cm −1 are attributed to defects and disorder in the graphite layer.  Figure 3a shows the SEM image of the Ag-SnO 2 /rGO nanocomposite film. SnO 2 microspheres and Ag nanoparticles (NPs) attached to the surface of rGO sheets are clearly observed. Figure 3b shows a high-resolution TEM image of the Ag-SnO 2 /rGO nanocomposite, and lattice fringe spacings of 0.35, 0.33 and 0.23 nm for rGO, SnO 2 and Ag, respectively, are measured. Figure 3c indicates the XRD spectrum for the GO, rGO, SnO 2 and Ag-SnO 2 /rGO nanocomposite films. Obvious peaks at 2θ angles of 10.78 • and 24.7 • are observed for the GO and thermally treated rGO, respectively, which is in agreement with previously published results, and further confirms the successful reduction of GO via thermal treatment [29][30][31]. The XRD spectrum of SnO 2 indicates several peaks at 26.41 • , 33.82 • , 37.60 • , 51.73 • and 65.68 • , indexed to the (110), (101), (200), (211) and (301) planes of rutile SnO 2 , which is in accordance with the data from JCPDS Card no. 41-1445 [32], and confirms the successful formation of SnO 2 nanocrystals. Apart from the characteristic peaks attributed to SnO 2 , the XRD spectrum of the Ag-SnO 2 /rGO nanocomposite exhibited distinct peaks at 38.10 • , 44.37 • and 64.17 • , which indexed to the (111), (200) and (220) planes of Ag crystallines, respectively [33]. However, the broad peak of rGO is not obvious in the XRD pattern of the Ag-SnO 2 /rGO nanocomposite, probably because the weak peak of rGO is swamped by the high intensity peak of the SnO 2 at the 2θ angle of 26.41 • [34,35]. Figure 3d shows the Raman spectrum of the SnO 2 /rGO and Ag-SnO 2 /rGO nanocomposites. The peaks located at 633 cm −1 in the SnO 2 /rGO and Ag-SnO 2 /rGO nanocomposites are typical Raman peaks of SnO 2 . The weak peaks located at 796 and 1588 cm −1 for the two samples are attributed to the Ag NPs. The peaks located at 1363 and 1640 cm −1 are attributed to defects and disorder in the graphite layer.  Figure 4 shows the sensitivity of the Ag-SnO2/rGO nanocomposite sensor to 100 ppm acetylene under operating temperatures of 25 to 170 °C. The sensor sensitivity increases and reaches its highest value at 90 °C, and then decreases with any further increase in temperature. This can be explained by the fact that the appropriate operating temperature improves sensor sensitivity, but higher temperatures lower the binding energies of gas molecules and sensing film. An optimal temperature of 90 °C was determined. Therefore, the operating temperature of 90 °C was selected for the sensor in the following experiments.   Figure 4 shows the sensitivity of the Ag-SnO 2 /rGO nanocomposite sensor to 100 ppm acetylene under operating temperatures of 25 to 170 • C. The sensor sensitivity increases and reaches its highest value at 90 • C, and then decreases with any further increase in temperature. This can be explained by the fact that the appropriate operating temperature improves sensor sensitivity, but higher temperatures lower the binding energies of gas molecules and sensing film. An optimal temperature of 90 • C was determined. Therefore, the operating temperature of 90 • C was selected for the sensor in the following experiments.  Figure 4 shows the sensitivity of the Ag-SnO2/rGO nanocomposite sensor to 100 ppm acetylene under operating temperatures of 25 to 170 °C. The sensor sensitivity increases and reaches its highest value at 90 °C, and then decreases with any further increase in temperature. This can be explained by the fact that the appropriate operating temperature improves sensor sensitivity, but higher temperatures lower the binding energies of gas molecules and sensing film. An optimal temperature of 90 °C was determined. Therefore, the operating temperature of 90 °C was selected for the sensor in the following experiments.   Figure 5 shows the sensitivity of pure SnO 2 , SnO 2 /rGO and Ag-SnO 2 /rGO film sensors to 50 ppm acetylene at 90 • C. We found that the Ag-SnO 2 /rGO film sensor yielded the highest sensitivity among the three sensors. Moreover, the Ag-SnO 2 /rGO film sensor showed a shorter response/recovery time than the other two sensors. A response time and recovery time of 235 and 160 s, respectively, were observed for the Ag-SnO 2 /rGO film sensor upon exposure to 50 ppm acetylene.

Acetylene Sensing Properties
Nanomaterials 2017, 7, 278 6 of 12 Figure 5 shows the sensitivity of pure SnO2, SnO2/rGO and Ag-SnO2/rGO film sensors to 50 ppm acetylene at 90 °C. We found that the Ag-SnO2/rGO film sensor yielded the highest sensitivity among the three sensors. Moreover, the Ag-SnO2/rGO film sensor showed a shorter response/recovery time than the other two sensors. A response time and recovery time of 235 and 160 s, respectively, were observed for the Ag-SnO2/rGO film sensor upon exposure to 50 ppm acetylene.      Figure 5 shows the sensitivity of pure SnO2, SnO2/rGO and Ag-SnO2/rGO film sensors to 50 ppm acetylene at 90 °C. We found that the Ag-SnO2/rGO film sensor yielded the highest sensitivity among the three sensors. Moreover, the Ag-SnO2/rGO film sensor showed a shorter response/recovery time than the other two sensors. A response time and recovery time of 235 and 160 s, respectively, were observed for the Ag-SnO2/rGO film sensor upon exposure to 50 ppm acetylene.     Figure 7 shows the repeatability of the Ag-SnO 2 /rGO film sensor with concentrations of 5, 150 and 500 ppm acetylene at 90 • C. There were no significant changes in sensor sensitivity during the repeated exposure/recovery cycles, indicating an acceptable repeatability for acetylene sensing. Figure 8 demonstrates the typical response and recovery curves of the Ag-SnO 2 /rGO film sensor towards an acetylene pulse, at concentrations between 0 and 500 ppm, exhibiting good response/recovery behavior. Nanomaterials 2017, 7, 278 7 of 12 Figure 7 shows the repeatability of the Ag-SnO2/rGO film sensor with concentrations of 5, 150 and 500 ppm acetylene at 90 °C. There were no significant changes in sensor sensitivity during the repeated exposure/recovery cycles, indicating an acceptable repeatability for acetylene sensing. Figure 8 demonstrates the typical response and recovery curves of the Ag-SnO2/rGO film sensor towards an acetylene pulse, at concentrations between 0 and 500 ppm, exhibiting good response/recovery behavior.   Figure 9 shows the response of the Ag-SnO2/rGO nanocomposite film sensor to concentrations of 5, 150, and 500 ppm acetylene gas, measured every five days for over 30 days. It was clearly shown that the sensor response does not vary significantly with time, confirming that the Ag-SnO2/rGO nanocomposite film sensor has good long-term stability. Figure 10 shows the experimental current-  Figure 7 shows the repeatability of the Ag-SnO2/rGO film sensor with concentrations of 5, 150 and 500 ppm acetylene at 90 °C. There were no significant changes in sensor sensitivity during the repeated exposure/recovery cycles, indicating an acceptable repeatability for acetylene sensing. Figure 8 demonstrates the typical response and recovery curves of the Ag-SnO2/rGO film sensor towards an acetylene pulse, at concentrations between 0 and 500 ppm, exhibiting good response/recovery behavior.   Figure 9 shows the response of the Ag-SnO2/rGO nanocomposite film sensor to concentrations of 5, 150, and 500 ppm acetylene gas, measured every five days for over 30 days. It was clearly shown that the sensor response does not vary significantly with time, confirming that the Ag-SnO2/rGO nanocomposite film sensor has good long-term stability. Figure 10 shows the experimental current-  Figure 9 shows the response of the Ag-SnO 2 /rGO nanocomposite film sensor to concentrations of 5, 150, and 500 ppm acetylene gas, measured every five days for over 30 days. It was clearly shown that the sensor response does not vary significantly with time, confirming that the Ag-SnO 2 /rGO nanocomposite film sensor has good long-term stability. Figure 10 shows the experimental current-voltage (I-V) curves for the SnO 2 , SnO 2 /rGO and Ag-SnO 2 /rGO film devices, measured by applying voltages from −4 to 4 V. It is clearly shown that the measurement results indicate good Ohmic contact to n-type Nanomaterials 2017, 7, 278 8 of 12 semiconductors for the three devices. A larger current passes through the Ag-SnO 2 /rGO sensor than that of the SnO 2 /rGO and SnO 2 sensors. This is because the doping of Ag and rGO results in a decrease in the resistance of the Ag-SnO 2 /rGO film. Table 1 presents the performance of the proposed acetylene gas sensor in comparison with previous reported works [12,13,15,23,[36][37][38]. The working temperature and responses for the prepared sensor are comparable to metal oxide-based sensors made by hydrothermal-RF magnetron sputtering, hydrothermal, electrospinning, spin-coating and sol-gel methods. The presented Ag-SnO 2 /rGO film sensor exhibited a much higher response and a lower working temperature than that of its MOS-based counterparts. voltage (I-V) curves for the SnO2, SnO2/rGO and Ag-SnO2/rGO film devices, measured by applying voltages from −4 to 4 V. It is clearly shown that the measurement results indicate good Ohmic contact to n-type semiconductors for the three devices. A larger current passes through the Ag-SnO2/rGO sensor than that of the SnO2/rGO and SnO2 sensors. This is because the doping of Ag and rGO results in a decrease in the resistance of the Ag-SnO2/rGO film. Table 1 presents the performance of the proposed acetylene gas sensor in comparison with previous reported works [12,13,15,23,[36][37][38]. The working temperature and responses for the prepared sensor are comparable to metal oxide-based sensors made by hydrothermal-RF magnetron sputtering, hydrothermal, electrospinning, spincoating and sol-gel methods. The presented Ag-SnO2/rGO film sensor exhibited a much higher response and a lower working temperature than that of its MOS-based counterparts.   voltage (I-V) curves for the SnO2, SnO2/rGO and Ag-SnO2/rGO film devices, measured by applying voltages from −4 to 4 V. It is clearly shown that the measurement results indicate good Ohmic contact to n-type semiconductors for the three devices. A larger current passes through the Ag-SnO2/rGO sensor than that of the SnO2/rGO and SnO2 sensors. This is because the doping of Ag and rGO results in a decrease in the resistance of the Ag-SnO2/rGO film. Table 1 presents the performance of the proposed acetylene gas sensor in comparison with previous reported works [12,13,15,23,[36][37][38]. The working temperature and responses for the prepared sensor are comparable to metal oxide-based sensors made by hydrothermal-RF magnetron sputtering, hydrothermal, electrospinning, spincoating and sol-gel methods. The presented Ag-SnO2/rGO film sensor exhibited a much higher response and a lower working temperature than that of its MOS-based counterparts.

Acetylene-Sensing Mechanism
The Ag-SnO 2 /rGO ternary nanocomposite film demonstrated excellent sensing properties towards acetylene gas at low temperatures. Its sensitive mechanism can be attributed to the synergistic effect of the ternary hybrids and the created potential barrier. Pristine SnO 2 is an n-type semiconductor and electrons are majority carriers. Graphene, as one of the emerging 2D nanomaterials, has a unique layered structure, a large surface area-to-volume ratio and excellent electrical properties, which greatly facilitate the absorption and diffusion of acetylene gas molecules. Figure 11 shows the sensing mechanism of the Ag-SnO 2 /rGO nanocomposite film in air and acetylene gas. The oxygen molecules adsorbed on the conduction band of n-type SnO 2 are ionized to oxygen negative ions through the trapping of free electrons from the surface of the SnO 2 [39]. When the sensor is exposed to acetylene gas, the adsorbed acetylene interacts with the oxygen's negative ions and produces carbon dioxide, water molecules and free electrons. The reaction is expressed as C 2 H 2 (ads) + O − (ads) → CO 2 + H 2 O + e − , which leads to a decrease in the sensor's resistance [14,28].

Acetylene-Sensing Mechanism
The Ag-SnO2/rGO ternary nanocomposite film demonstrated excellent sensing properties towards acetylene gas at low temperatures. Its sensitive mechanism can be attributed to the synergistic effect of the ternary hybrids and the created potential barrier. Pristine SnO2 is an n-type semiconductor and electrons are majority carriers. Graphene, as one of the emerging 2D nanomaterials, has a unique layered structure, a large surface area-to-volume ratio and excellent electrical properties, which greatly facilitate the absorption and diffusion of acetylene gas molecules. Figure 11 shows the sensing mechanism of the Ag-SnO2/rGO nanocomposite film in air and acetylene gas. The oxygen molecules adsorbed on the conduction band of n-type SnO2 are ionized to oxygen negative ions through the trapping of free electrons from the surface of the SnO2 [39]. When the sensor is exposed to acetylene gas, the adsorbed acetylene interacts with the oxygen's negative ions and produces carbon dioxide, water molecules and free electrons. The reaction is expressed as C2H2 (ads) + O − (ads) → CO2 + H2O + e − , which leads to a decrease in the sensor's resistance [14,28]. The formation of a heterojunction at the interface between rGO and SnO2 is beneficial, because it enhances the acetylene-sensing properties. Figure 12a shows a schematic of an energy band The formation of a heterojunction at the interface between rGO and SnO 2 is beneficial, because it enhances the acetylene-sensing properties. Figure 12a shows a schematic of an energy band diagram of the SnO 2 /rGO heterojunction. The band-gaps for n-type SnO 2 and p-type rGO are 3.6 and 0.4 eV, respectively [34] and their work functions are 4.5 and 5.1 eV for SnO 2 and rGO, respectively [40,41]. Because the Fermi energies are not at the same level and the rGO has a higher work function, when SnO 2 and rGO come into contact with each other, electrons transfer from SnO 2 to rGO, and holes flow in the opposite direction until a dynamic equilibrium state is reached, and thus a depletion layer is formed at the interface [42]. Figure 12b shows the variation of depletion layer thickness for the SnO 2 /rGO heterojunction. When the sensor was exposed to acetylene gas, the interaction between adsorbed O − and acetylene molecules released free electrons, with the released free electrons increasing the n-type doping of SnO 2 . Higher SnO 2 doping results in a reduced depletion layer in SnO 2 , thereby decreasing the sensor resistance during acetylene gas exposure. diagram of the SnO2/rGO heterojunction. The band-gaps for n-type SnO2 and p-type rGO are 3.6 and 0.4 eV, respectively [34] and their work functions are 4.5 and 5.1 eV for SnO2 and rGO, respectively [40,41]. Because the Fermi energies are not at the same level and the rGO has a higher work function, when SnO2 and rGO come into contact with each other, electrons transfer from SnO2 to rGO, and holes flow in the opposite direction until a dynamic equilibrium state is reached, and thus a depletion layer is formed at the interface [42]. Figure 12b shows the variation of depletion layer thickness for the SnO2/rGO heterojunction. When the sensor was exposed to acetylene gas, the interaction between adsorbed O − and acetylene molecules released free electrons, with the released free electrons increasing the n-type doping of SnO2. Higher SnO2 doping results in a reduced depletion layer in SnO2, thereby decreasing the sensor resistance during acetylene gas exposure. The Ag NPs in the nanocomposite have a positive effect on electronic sensitization during gas adsorption [43,44]. A potential barrier can be created at the contact interface between SnO2 and Ag, which is beneficial to the enhancement of gas-sensing. Electrons transfer from SnO2 to Ag NPs and form a highly-resistive "barrier layer" in air, and Ag NPs become centers for electron accumulation. When the sensor is exposed to acetylene gas, electrons transfer from acetylene gas to the Ag NPs and then to SnO2; the highly-resistive "barrier layer" is transformed into a highly-conductive "anti-barrier layer", which improves the electron mobility and sensing performance of the Ag-SnO2/rGO nanocomposite sensor.

Conclusions
In this work, an acetylene gas sensor, based on Ag-SnO2/rGO nanocomposite film was fabricated by layer-by-layer self-assembly technology. Successful preparation of the Ag-SnO2/rGO nanocomposite was characterized and confirmed by means of SEM, TEM, XRD and Raman spectrum. The Ag-SnO2/rGO nanocomposite sensor exhibited excellent sensing behaviors towards acetylene in terms of response, repeatability, stability and response/recovery characteristics at an optimal temperature of 90 °C, which are superior to pure SnO2 and SnO2/rGO film sensors. The underlying sensing mechanism of the Ag-SnO2/rGO sensor was explored. This work provides guidance for an acetylene sensor based on Ag-SnO2/rGO ternary nanocomposites. The Ag NPs in the nanocomposite have a positive effect on electronic sensitization during gas adsorption [43,44]. A potential barrier can be created at the contact interface between SnO 2 and Ag, which is beneficial to the enhancement of gas-sensing. Electrons transfer from SnO 2 to Ag NPs and form a highly-resistive "barrier layer" in air, and Ag NPs become centers for electron accumulation. When the sensor is exposed to acetylene gas, electrons transfer from acetylene gas to the Ag NPs and then to SnO 2 ; the highly-resistive "barrier layer" is transformed into a highly-conductive "anti-barrier layer", which improves the electron mobility and sensing performance of the Ag-SnO 2 /rGO nanocomposite sensor.

Conclusions
In this work, an acetylene gas sensor, based on Ag-SnO 2 /rGO nanocomposite film was fabricated by layer-by-layer self-assembly technology. Successful preparation of the Ag-SnO 2 /rGO nanocomposite was characterized and confirmed by means of SEM, TEM, XRD and Raman spectrum. The Ag-SnO 2 /rGO nanocomposite sensor exhibited excellent sensing behaviors towards acetylene in terms of response, repeatability, stability and response/recovery characteristics at an optimal temperature of 90 • C, which are superior to pure SnO 2 and SnO 2 /rGO film sensors. The underlying sensing mechanism of the Ag-SnO 2 /rGO sensor was explored. This work provides guidance for an acetylene sensor based on Ag-SnO 2 /rGO ternary nanocomposites.