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Abstract:

ZnO nanoplates with hexagonal wurtzite
structure were synthesized by hydrothermal treatment. The average dimension and
average thickness of the plates were approximately 200´400 nm and 40 nm, respectively. ZnO nanoplates were
deposited on Pt interdigitated
electrodes to fabricate gas sensing devices. The ethanol sensing properties of
the devices were investigated under dark versus ultra-violet (UV) illumination.
Under the UV illumination, the optimal operating temperature of the devices can
be reduced from 237 oC down to 164 oC, and the response
of the device was increased from 2.8 to 8.5 toward 1500 ppm ethanol vapor.

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Keywords: Hydrothermal,
UV illumination, ZnO nanoplate, ethanol vapor sensing.

1 Introduction

Zinc oxide (ZnO) is a type II–VI semiconductor with a
wide and direct band gap of 3.37 eV (at 300 K) and large free exciton binding
energy of 60 meV 1. ZnO is one of the most promising materials for gas
sensors, especially for detecting ethanol vapor 1,2. The sensor operation-based ZnO nanomaterial exhibits
the resistance change during the injection of target gas on the ZnO surface.
The sensing properties of ZnO are directly related to its morphology and
operating temperature 3. Nevertheless, ZnO exhibit limitations, such as high
operating temperature and poor sensitivity, which probably limit its
applications. 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 performance
of the devices. In addition, UV illumination is a potential solution to reduce
the working temperature.

For sensing enhancement using UV light activation,
B.P.J. de Lacy Costello et al. showed that a sensor based on particulate ZnO
activated with UV LED at a peak wavelength of 400 nm and incident light
intensity of 2.2 mW/cm2 can detect very low concentrations of
acetone 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-sensing
activity for ethanol detection 10. These studies were conducted at room temperature. A
recent study reported the influence of UV light combined with thermal energy on
sensing materials 11-13. However, research on this field remains limited.

In
this paper, we report a free-catalyzed synthesis of ZnO nanoplates by
hydrothermal treatment. The ethanol sensing properties of the ZnO nanoplates during
the induction of UV light were investigated. The ethanol sensing mechanism of
the material under UV illumination at various temperatures was also proposed
and discussed.

2 Experimental

ZnO
nanoplates were synthesized by hydrothermal treatment at 180 oC for
24h 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 using
Bruker-AXS (Siemens) D5005 with CuKa radiation (l = 1.54065 Å) at a scanning rate of 0.03°/2s
within the 2q
range of 30°–70°. The morphology of the nanopowder was identified by scanning
electron microscope (Nova nanoSEM 450).

 

For the gas sensing measurement, the sensor device were prepared
following procedure: at first, the obtained ZnO powder was dispersed with
assistance of polyethylene glycol (PEG, 4000) into distillated water at room
temperature, then dropped onto Pt-interdigitated electrodes (the electrode gap
is 20 mm) and spin coated at
1800 rpm for 60 s. The device was dried in air at 80 °C for 24 h and heated at
600 °C for 2 h to evaporate organic species. The sensor device
was placed in a plate of external electric heater inside a glass
chamber. A UV LED (wavelength, 365 nm; power, 1 W) was placed in the opposite
direction above the device. The current–voltage (I–V) characteristic and
ethanol sensing performance of the device were determined using a static gas-sensing
system. The response of the device toward 125–1500 ppm ethanol was tested at
100–300 °C.

3 Results and discussions

High-density ZnO
nanoplates with smooth surfaces were synthesized (Fig. 1a). The ZnO nanoplates
exhibit regular shapes with an average size of 200 nm ´
400 nm and thickness of approximately 40 nm. The typical XRD pattern of the
nanoplates (Fig. 1b) depicts that all the diffraction peaks can be assigned to
the hexagonal wurtzite of ZnO with lattice constants of a = b =
0.3249 nm and c = 0.5206 nm (JCPDS 36-1451). The strong and narrow
diffraction peaks indicate that the material possesses good crystallinity. The
crystallite size of the nanoplate is 24.8 nm, which is calculated for the most
intense peak (101) by using the Scherrer equation 14:

                                                                                                     

 

where D is the crystallite size (nm), b
is 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 at
138 °C (Fig. 2). For example, the dynamic response is stable and reproducible
with good on/off current ration. The voltage on the film increases intensively
under UV illumination. This effect can be attributed to the fact that UV
light stimulates carrier generation and consequently increases the density of
free electron–hole pairs in the ZnO film through below reactions 12:

                                                                                                 

Figure 3 shows the I–V characteristics of the ZnO nanoplate
film 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 slope
than those of the device in the dark. This observation indicates that the
resistance 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.

The
transient response of ZnO nanoplate film in Fig. 4 depicts the influence of UV
illumination on the sensitivity of sample at 164 °C. The resistive curve is
adequately clear. The response time for 125 ppm ethanol vapor is 154.4 s, and
the recovery time for 1500 ppm ethanol vapor is 114.2 s. Hence, we can observe
that ethanol molecules are quickly desorbed on the film.

The Figure 5 presents the responses of ZnO nanoplate film to ethanol vapor (125–1500
ppm) at
various working temperatures in the dark (Fig. 5a) and under UV illumination
(Fig. 5b). We can observe that the film response reaches the maximal
value at 237 °C and 164 °C for all ethanol vapor concentration levels in the
dark and under UV illumination, respectively. The film response to 1500 ppm
ethanol vapor is 8.5 at 164 °C under UV illumination and 2.8 at 237 °C in the
dark. This result indicates that the optimal working temperature of the film
decreases, and the film response increases under UV illumination. Moreover, the
response of ZnO nanoplate to vapor ethanol can be determined at temperatures
higher than 92 °C under UV illumination. In the dark, the response can be
determined only at temperatures higher than 164 °C. A possible explanation is
related to the photoconduction of ZnO nanoplate that is affected with oxygen
adsorption under UV illumination.

Figure 6 illustrates that the response of ZnO nanoplate film can be determined from the response
curve at 138 °C under UV illumination (Fig. 6b). Although the resistive curve is slightly
unclear, the film response can be calculated. In the dark, the film
shows no response to ethanol vapor at all concentration levels (Fig. 6a).

For ethanol gas sensing, oxygen adsorption
influences the electrical transport properties of ZnO nanoplates. On the
surface of ZnO nanoplates, reactive oxygen ions, such as , , and , are mainly
chemisorbed with the aid of thermal energy in the dark. The reactions kinetics
can be described as follows 15:

 

                                                    ;                                              

                                                 ;                                           

                                              , and                                         

                                                                                                

 

where
the subscripts “gas”, “ads”, and “lat” mean gas, adsorbed and lattice,
respectively.

These oxygen ions
react with ethanol vapor introduced into the glass chamber by the following
reaction 15:

                                                        

These reactions release electrons back to the conduction band of ZnO
which decrease the surface depletion layer width, and subsequently increase the
electrical conductivity of the sensor sample. Therefore, the film response evidently
increases when the film is exposed to ethanol vapor.

Under UV
illumination, 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 ZnO
film surface by reaction 7:

                                                                                            

Photogenerated
oxygen ions are created due to the reaction of ambient oxygen molecules with
photoelectrons, as follows 7:

                                                                                      

In contrast to the
chemisorbed oxygen ions, which are strongly attached to the ZnO surface, these
photogenerated oxygen ions   are weakly
bound to ZnO and can be easily removed 8. The ethanol
molecules will react rapidly with these additional photogenerated oxygen ions
on the ZnO surface according to the following reaction 11,16:

                                                 

The decreased film
resistance at 160 °C may be related to the desorption of water molecules in the
surface of the film 17. Since the places
which were occupied by water molecules can be exposed to UV irradiation. It
could cause that the amount of photogenerated oxygen ions and the amount of
activating sites located in the film surface can be raised. In addition, it can
also increase the amount of ethanol molecules which were available to interact
directly with the film surface. Hence, the sensitivity of the ZnO nanoplate
sample under UV illumination reaches the maximum at 164 °C.

4 Conclusions

ZnO nanoplates with
wurtzite structure and average thickness of 40 nm were successfully synthesized
by a wet chemical process through hydrothermal technique at 180 °C for 20 h. UV illumination
significantly enhances the sensitivity of ethanol vapor sensor based on ZnO nanoplates.
In addition, illuminating the zinc oxide-based gas sensor with energy radiation
can reduce the operating temperature of the sensor compared with the bandgap of
the metal oxide. This sensor can be used in areas where working at high
temperatures is unfeasible, thereby significantly enhancing its applicability.

Acknowledgment

This work was supported by the project code
B2017-BKA-49.

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