Abstract: ZnO nanoplates with hexagonal wurtzitestructure were synthesized by hydrothermal treatment. The average dimension andaverage thickness of the plates were approximately 200´400 nm and 40 nm, respectively. ZnO nanoplates weredeposited on Pt interdigitatedelectrodes to fabricate gas sensing devices. The ethanol sensing properties ofthe devices were investigated under dark versus ultra-violet (UV) illumination.Under the UV illumination, the optimal operating temperature of the devices canbe reduced from 237 oC down to 164 oC, and the responseof the device was increased from 2.8 to 8.
5 toward 1500 ppm ethanol vapor.Keywords: Hydrothermal,UV illumination, ZnO nanoplate, ethanol vapor sensing.1 IntroductionZinc oxide (ZnO) is a type II–VI semiconductor with awide and direct band gap of 3.37 eV (at 300 K) and large free exciton bindingenergy of 60 meV 1. ZnO is one of the most promising materials for gassensors, especially for detecting ethanol vapor 1,2. The sensor operation-based ZnO nanomaterial exhibitsthe resistance change during the injection of target gas on the ZnO surface.The sensing properties of ZnO are directly related to its morphology andoperating temperature 3. Nevertheless, ZnO exhibit limitations, such as highoperating temperature and poor sensitivity, which probably limit itsapplications.
These drawbacks can be addressed by doping transition elements 4,5 and noble metals 6 or irradiation of ultraviolet (UV) light 7,8. These approaches enhance the gas-sensing performanceof the devices. In addition, UV illumination is a potential solution to reducethe working temperature.
For sensing enhancement using UV light activation,B.P.J. de Lacy Costello et al. showed that a sensor based on particulate ZnOactivated with UV LED at a peak wavelength of 400 nm and incident lightintensity of 2.2 mW/cm2 can detect very low concentrations ofacetone and acetaldehyde (1 ppb) 9. J.
Zhai et al. revealed that C-doped ZnO under 500°C calcination exhibits excellent UV-activated room-temperature gas-sensingactivity for ethanol detection 10. These studies were conducted at room temperature. Arecent study reported the influence of UV light combined with thermal energy onsensing materials 11-13. However, research on this field remains limited.Inthis paper, we report a free-catalyzed synthesis of ZnO nanoplates byhydrothermal treatment.
The ethanol sensing properties of the ZnO nanoplates duringthe induction of UV light were investigated. The ethanol sensing mechanism ofthe material under UV illumination at various temperatures was also proposedand discussed.2 Experimental ZnOnanoplates were synthesized by hydrothermal treatment at 180 oC for24h using zinc nitrate hexahydrate Zn(NO3)2·6H2O(99%, China), potassium hydroxide (KOH 85%, China), absolute ethanol (99.6%,China) as starting materials. The procedure has been reported in 13.
The crystal structure of ZnO nanoplates was identified usingBruker-AXS (Siemens) D5005 with CuKa radiation (l = 1.54065 Å) at a scanning rate of 0.03°/2swithin the 2qrange of 30°–70°. The morphology of the nanopowder was identified by scanningelectron microscope (Nova nanoSEM 450). For the gas sensing measurement, the sensor device were preparedfollowing procedure: at first, the obtained ZnO powder was dispersed withassistance of polyethylene glycol (PEG, 4000) into distillated water at roomtemperature, then dropped onto Pt-interdigitated electrodes (the electrode gapis 20 mm) and spin coated at1800 rpm for 60 s. The device was dried in air at 80 °C for 24 h and heated at600 °C for 2 h to evaporate organic species. The sensor devicewas placed in a plate of external electric heater inside a glasschamber. A UV LED (wavelength, 365 nm; power, 1 W) was placed in the oppositedirection above the device.
The current–voltage (I–V) characteristic andethanol sensing performance of the device were determined using a static gas-sensingsystem. The response of the device toward 125–1500 ppm ethanol was tested at100–300 °C. 3 Results and discussionsHigh-density ZnOnanoplates with smooth surfaces were synthesized (Fig. 1a). The ZnO nanoplatesexhibit regular shapes with an average size of 200 nm ´400 nm and thickness of approximately 40 nm. The typical XRD pattern of thenanoplates (Fig. 1b) depicts that all the diffraction peaks can be assigned tothe hexagonal wurtzite of ZnO with lattice constants of a = b =0.
3249 nm and c = 0.5206 nm (JCPDS 36-1451). The strong and narrowdiffraction peaks indicate that the material possesses good crystallinity.
Thecrystallite size of the nanoplate is 24.8 nm, which is calculated for the mostintense peak (101) by using the Scherrer equation 14: where D is the crystallite size (nm), bis the full width of the diffraction line at half of the maximum intensity i.e.(101) in radians, l is the X-ray wavelength of CuKa= 1.54065 Å and q is the Bragg’s angle.First,we tested the response of the ZnO nanoplate film to UV light by three cycles at138 °C (Fig. 2).
For example, the dynamic response is stable and reproduciblewith good on/off current ration. The voltage on the film increases intensivelyunder UV illumination. This effect can be attributed to the fact that UVlight stimulates carrier generation and consequently increases the density offree electron–hole pairs in the ZnO film through below reactions 12: Figure 3 shows the I–V characteristics of the ZnO nanoplatefilm at various working temperatures in the dark (Fig.
3a) and under UV illumination (Fig.3b). The I–V curves of the device under UV illumination display higher slopethan those of the device in the dark. This observation indicates that theresistance of the ZnO thin film decreases under UV illumination. In addition,the I-V curve at 92 °C under UV illumination is less sloped than ones. Thetransient response of ZnO nanoplate film in Fig. 4 depicts the influence of UVillumination on the sensitivity of sample at 164 °C.
The resistive curve isadequately clear. The response time for 125 ppm ethanol vapor is 154.4 s, andthe recovery time for 1500 ppm ethanol vapor is 114.2 s. Hence, we can observethat ethanol molecules are quickly desorbed on the film.The Figure 5 presents the responses of ZnO nanoplate film to ethanol vapor (125–1500ppm) atvarious working temperatures in the dark (Fig. 5a) and under UV illumination(Fig.
5b). We can observe that the film response reaches the maximalvalue at 237 °C and 164 °C for all ethanol vapor concentration levels in thedark and under UV illumination, respectively. The film response to 1500 ppmethanol vapor is 8.5 at 164 °C under UV illumination and 2.8 at 237 °C in thedark.
This result indicates that the optimal working temperature of the filmdecreases, and the film response increases under UV illumination. Moreover, theresponse of ZnO nanoplate to vapor ethanol can be determined at temperatureshigher than 92 °C under UV illumination. In the dark, the response can bedetermined only at temperatures higher than 164 °C. A possible explanation isrelated to the photoconduction of ZnO nanoplate that is affected with oxygenadsorption under UV illumination.Figure 6 illustrates that the response of ZnO nanoplate film can be determined from the responsecurve at 138 °C under UV illumination (Fig. 6b). Although the resistive curve is slightlyunclear, the film response can be calculated. In the dark, the filmshows no response to ethanol vapor at all concentration levels (Fig.
6a). For ethanol gas sensing, oxygen adsorptioninfluences the electrical transport properties of ZnO nanoplates. On thesurface of ZnO nanoplates, reactive oxygen ions, such as , , and , are mainlychemisorbed with the aid of thermal energy in the dark. The reactions kineticscan be described as follows 15: ; ; , and wherethe subscripts “gas”, “ads”, and “lat” mean gas, adsorbed and lattice,respectively. These oxygen ionsreact with ethanol vapor introduced into the glass chamber by the followingreaction 15: These reactions release electrons back to the conduction band of ZnOwhich decrease the surface depletion layer width, and subsequently increase theelectrical conductivity of the sensor sample.
Therefore, the film response evidentlyincreases when the film is exposed to ethanol vapor. Under UVillumination, the free electron–hole pairs could be generated by reaction (2).Besides, the photoinduced hole may interact with chemisorbed oxygen ion, causing the oxygen to be desorbed from the ZnOfilm surface by reaction 7: Photogeneratedoxygen ions are created due to the reaction of ambient oxygen molecules withphotoelectrons, as follows 7: In contrast to thechemisorbed oxygen ions, which are strongly attached to the ZnO surface, thesephotogenerated oxygen ions are weaklybound to ZnO and can be easily removed 8.
The ethanolmolecules will react rapidly with these additional photogenerated oxygen ionson the ZnO surface according to the following reaction 11,16: The decreased filmresistance at 160 °C may be related to the desorption of water molecules in thesurface of the film 17. Since the placeswhich were occupied by water molecules can be exposed to UV irradiation. Itcould cause that the amount of photogenerated oxygen ions and the amount ofactivating sites located in the film surface can be raised. In addition, it canalso increase the amount of ethanol molecules which were available to interactdirectly with the film surface. Hence, the sensitivity of the ZnO nanoplatesample under UV illumination reaches the maximum at 164 °C.4 ConclusionsZnO nanoplates withwurtzite structure and average thickness of 40 nm were successfully synthesizedby a wet chemical process through hydrothermal technique at 180 °C for 20 h. UV illuminationsignificantly enhances the sensitivity of ethanol vapor sensor based on ZnO nanoplates.In addition, illuminating the zinc oxide-based gas sensor with energy radiationcan reduce the operating temperature of the sensor compared with the bandgap ofthe metal oxide.
This sensor can be used in areas where working at hightemperatures is unfeasible, thereby significantly enhancing its applicability. AcknowledgmentThis work was supported by the project codeB2017-BKA-49.