High selectivity to ethanol gas sensor based on ZnO nanosheets decorated with Ag nanoparticles by aqueous solution and photochemical deposition


 We demonstrated the fabrication of Ag NPs decorated ZnO NSs on a glass substrate using the aqueous solution and photochemical deposition method. The synthesis process is room temperature. This two-dimensional ZnO NSs can increase the performance of gas sensors due to increase the surface-to-volume ratio. Through increasing the surface-to-volume ratio of nanostructures, it is possible to increase the absorption potion of the target gas. In addition, noble metal NPs attached to the ZnO nanostructure can also increase the performance of gas sensors because Ag NPs act as strong electron acceptors, it will be induced enhanced electron depletion layer. As a result, the Ag NPs decorated ZnO NSs gas sensor has high selectivity to ethanol, as well as improved gas sensing performance to ethanol as compared to ZnO.

surface morphology and structure can affect performance of the gas sensor, so that bulk materials are too difficult to achieve highly sensitive properties. To improve the performance of gas sensors, we can proceed from structure of metal oxide semiconductor. For example, adjust structure defects or fabricate composite structure, including decorating noble palladium, platinum, aurum, silver (Ag) nanoparticles on the surface of metal-oxide semiconductor [30][31][32][33] . These factors can be changed gas sensing performance, such as sensitivity of detecting gas, force on the detection of target gas, decreasing detection response/recovery times and power consumption for sensor operating temperature 34,35 . Hsueh et al. reported that Pt modified ZnO NWs-based ammonia sensors exhibited a better response than pure ZnO NWs 36 . Zou et al. reported that Au modified ZnO microwires-based ethanol sensors exhibited a better response than pure ZnO microwires 37 .
However, the price of noble metals above is more expensive.
In the paper, the Ag NPs decorated ZnO NSs ethanol gas sensor was prepared on glass substrate by aqueous solution method and photochemical method. The physical, and electrical properties of the Ag NPs decorated ZnO NSs gas sensor were discussed. The both sensors gas sensing mechanism study among the ZnO and Ag NPs decorated ZnO NSs were performed. Our results indicate that Ag NPs decorated ZnO NSs sensor exhibited excellent performance for ethanol detection.

EXPERTIMENTAL
In this work, the main precursor include silver nitrate (AgNO3, purity 99%), zinc nitrate hexahydrate (Zn(NO3)2 ▪ 6H2O, purity 99%), sodium hydroxide (NaOH) and sodium citrate dihydrate (Na3C6H5O7．H2O) to fabricate Ag NPs decorated ZnO NSs on glass substrate. All experiments are used to distillated water as solvent. Before fabricating Ag NPs decorated ZnO NSs on glass substrate, a seed layer (ZnO) was deposited by radio frequency sputter. The seed layer was prepared on glass substrate according to previously reported procedures 22 . Then, the precursor solution was prepared for the synthesis of ZnO NSs, which the proportion of precursors were 0.1 M of Zn(NO3)2 ▪ 6H2O and 0.4 M of NaOH. The two precursors were separately dissolved in 75 mL distillated water and stirring for 10 min. The ZnO/glass substrate is set on a self-made holder. The two precursors solution was poured slowly into a 150 mL beaker. After stirring for 1 min, the ZnO/glass substrate was immersed in the synthetic aqueous solution. During synthesis process, the synthesis solution kept stirring for 1 hr. Finally, the ZnO NSs was washed several times with distillated water. The ZnO NSs was prepared on glass substrate according to previously reported procedures 38 . Ag NPs was synthesized by using photochemical method at room temperature. The precursor solution was prepared by 0.5 mM of Na3C6H5O7．H2O and 0.5 mM of AgNO3 solution in distillated water stirring for 10 min. The sample was immersed in solution and irradiated under UV light (4W, 254 nm) for 15 min. After the completion of adsorption process, the sample was removed from synthetic solution and cleaned in distillated water. Finally, both samples were dried for 24 hr.
The ZnO and Ag NPs decorated ZnO NSs samples were measured by X-ray diffraction (XRD) on a Bruker D8 Discover diffractometer (source: Cu-Kα radiation). The range of scanning is between 20̊ to 80. The morphology of synthesis materials was investigated on a field-emission scanning electron microscope (SEM, Hitachi S-4800 type I). Then, the lattice and crystal of ZnO and Ag NPs were recorded on a high-resolution transmission electron microscope (HRTEM, Philips Technai-F20-G2 FEI-TEM). The two electron microscopes were carried on energy dispersive identification to analyze elemental composition of the sample. On the other hand, the element composition can also be verified by X-ray photoelectron spectroscopy (XPS).
The gas sensing measurement of ZnO NSs and Ag NPs decorated ZnO NSs sensors were carried on 10.6 L chamber. The chamber equipped with a heater, fan, mass flow controller and multimeter (MFC). Before measured gas sensor, the ZnO NSs and Ag NPs decorated ZnO NSs samples were coated silver paste to both ends of sample and dried in the oven at room temperature for 24 hr, which the sensor working area was 25 mm 2 . Then, the sensor was placed on the heater in the chamber and connected to semiconductor multimeter analyzer (Keithley 2410). The schematic diagram of gas measurement system as shown Figure 1. Before injecting the target gas, ZnO NSs and Ag NPs decorated ZnO NSs sensors were preheated until resistance stability of sensor. The working temperature is set in the range of 150 to 300 ℃, which each measurement is interval at 30 ℃. During measurement, the initial environment of chamber was filled air until resistance stability of sensor Subsequently, the target gases were introduced to the chamber by MFC and recorded resistance and current value of sensors from semiconductor multimeter analyzer. The response (R) of ZnO NSs and Ag NPs decorated ZnO NSs sensors were defined as R = Igas/Iair, where Iair and Igas were initial air and injection of target gases current value, respectively.

RESULTS AND DISCUSSION
Currently, there are very many techniques for photochemical reduction of Ag NPs. 39,40 In our case, the method is use sodium citrate as a reducing agent. The citrate ions can effectively reduce silver ions. And then it can be increased reduction rate for silver ions by irradiated with ultraviolet light. The citrate-assisted mechanism of photochemical reduction of Ag + can be expressed as 41,42 :  It is important factor which the operating temperature of metal oxide gas sensors because operating temperature of sensor directly affect the sensor performance. Thus, we design different operating temperature to find the optimal working conditions of the sensor. Figure 6 ( ZnO NSs sensor has the highest response at operating temperature of 270℃, then decrease gradually. Therefore, the optimal operating temperature for both sensors were set at 270℃.  ZnO sensor provides high response sensitivity than pure sensor, and the sensitivity rapidly increases with increasing ethanol concentration. As shown Figure 6 (c), the dynamic response/recovery curve in seven period, which the ZnO NSs and Ag NPs decorated ZnO NSs ethanol gas sensor at 270 ℃ (ethanol as 100 ppm). It can be observed that each cycle is very stability and reproducibility.
To explain the working mechanism of the Ag NPs decorated ZnO NSs gas sensor, we have prepared some models to explain these phenomena as shown Figure 6 (d). When sample exposed to the atmosphere, the surface of ZnO NSs will adhere to ambient gas molecules, including oxygen, water vapor and other gas. As the increase in operating temperature, larger amount oxygen species reactions process can be expresses as follows: It can act as a catalytic activity to target gases and oxygen for a noble metal decorated on ZnO NSs. Among them, oxygen species( 2 − , − , 2− ) can be lightly adsorbed on the surface of Ag NPs, resulting in many electrons to be captured from the conduction band of ZnO. As mentioned above, oxygen molecules captured electrons and generated an electron depletion layer from surface of ZnO NSs 54 . After Ag NPs adsorption, the electron depletion layer will also increase from the surface of Ag/ZnO. From results observed the current value of Ag NPs decorated ZnO NSs sensor is lower than ZnO NSs sensor before the target gas is not injected. (In the atmosphere) as shown To compare with other types of reducing gas, we analyze four types of VOC gases, including methanol, acetone, ethanol and isopropanol. Figure 7