ZnO nanowires (ZnO-NWs) and Pd-decorated ZnO nanowires (Pd-ZnO-NWs) were prepared by hydrothermal growth and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). When used for gas sensing, both types of nanowires showed a good selectivity to ethanol but a higher sensitivity and lower operating temperature were found with Pd-ZnO-NWs sensors comparing to those of ZnO-NWs sensor. When exposed to 200 ppm ethanol, our ZnO-NWs sensor showed a sensitivity of about 2.69 at 425°C whereas 1.3 at. % Pd-ZnO-NWs sensor provided a 57% more detection sensibility at 325°C. In addition, both response and recovery times of Pd-ZnO-NWs sensors were significantly reduced (9 s) comparing to the ZnO-NWs. Finally, Pd-ZnO-NWs sensor also showed a much lower detection limit of about 1 ppm. The sensing mechanism of Pd-ZnO-NWs sensors has also been clarified, thereby providing a new perspective for further improvement of the sensing performance of ethanol sensors.
Semiconducting metal oxides are widely used in environmental monitoring, industrial safety monitoring and medical diagnosis [
Comparing to the bulk material, micro- and nanostructured ZnO can provide a much improved performance for device applications [
In this work, we studied the sensing performance of ZnO-NWs with or without Pd decoration. Vertical ZnO-NWs with an average diameter of 80 nm and a length of 5
Vertically aligned ZnO-NWs arrays were grown on glass substrate by two-step hydrothermal method as previously reported [
To obtain Pd-ZnO-NWs, the ZnO-NWs were dipped in an ethanol solution of 9.6 mM palladium chloride (PdCl2, 99.9%, Sigma-Aldrich) for 8 s and then dried using an air gun. The cycle was repeated several times. The Pd-ZnO-NWs were annealed at 500°C for 1 hour and then cooled down to room temperature. To improve the stability and the repeatability of the gas sensors, samples of ZnO-NWs and Pd-ZnO-NWs were aged at 200°C for 10 days. Finally, a conductive silver paste was uniformly coated on the sample to form electrodes to ensure the electrical contact between the nanowires and probes.
The morphology of ZnO-NWs and Pd-ZnO-NWs was observed using scanning electron microscope (SEM, Hitachi S-3400N), equipped with X-ray energy dispersive spectroscope (EDS, Bruker). The crystal structure and components of samples were characterized by X-ray diffraction technique (XRD, DRIGC-Y 2000A) with Cu K
The gas sensing properties of the fabricated sensors were tested using the intelligent analysis system (CGS-1TP, Beijing Elite Tech Co., Ltd, China). Figure
Schematic illustration of a set-up for gas sensitivity analyses. The insert shows a typical device configuration of ZnO-NWs sensor.
The target gas at different concentrations was prepared by a static state method. The whole process of gas dilution could be approximately considered as an isobaric process, resulting in the following relationship between the volume of injected liquid, and the concentration of target gas [
Considering the n-type semiconductor characteristic of ZnO, the response value of gas sensor to reductive gas can be defined as
Figures
(a) Top-view and (b) side-view SEM images of ZnO-NWs grown on the substrate, (c) magnified top-view SEM images of ZnO-NWs and (d) Pd-ZnO-NWs (the inserted image shows the nanoparticles of Pd on ZnO-NWs), (e) EDS spectrum of Pd-ZnO-NWs, and (f) the XRD spectra of ZnO-NWs and Pd-ZnO-NWs.
To investigate the crystalline structures of ZnO-NWs and Pd-ZnO-NWs, XRD spectra of the samples are obtained as shown in Figure
In order to confirm a good gas selectivity of the sensors, we have performed sensing studies of ZnO-NWs and Pd-ZnO-NWs for various gases such as CH4, CO, CH3OH, CH3COCH3, and C2H5OH (Figure
Response of (a) ZnO-NWs and (b) Pd-ZnO-NWs to different gases of 200 ppm concentration at various operating temperatures.
To evaluate the gas selectivity of sensors, we define a selectivity factor (Q) as
Selectivity factor (
Target gases | |||||
---|---|---|---|---|---|
C2H5OH | CH4 | CO | CH3OH | CH3COCH3 | |
|
1 | 2.69 | 2.69 | 2.07 | 1.60 |
|
1 | 2.88 | 3.02 | 1.99 | 1.52 |
To investigate the sensing properties of ZnO-NWs and Pd-ZnO-NWs for ethanol gas, the ZnO-NWs grown on the substrate were dipped in an ethanol solution of 9.6 mM palladium chloride and this process was repeated 3, 5, and 7 times, respectively. We refer to these samples as Pd-ZnO-NWs-3, Pd-ZnO-NWs-5, and Pd-ZnO-NWs-7, representing 0.5%, 1.3%, and 3.1% atomic percentage of Pd on the surface of ZnO-NWs. Figure
The Comparison with the fabrication parameters of Pd-ZnO-NWs sensors reported in the literature and our work.
Structure | Diameter/length (nm/ |
Concentration (ppm) | Operating temperature (°C) | Responsea (%) | Response/recovery time (s) |
---|---|---|---|---|---|
Pd-ZnO-NWs-5 (our work) | 70–100/5 | 200 | 325 | 76.4 | 9/9 |
Pd-decorated ZnO nanorods [ |
70/0.5 | 1530 | 200 | 94 | 14/70 |
Pd-sensitized ZnO-NWs [ |
50/3.5 | 500 | 230 | 61.5 | a few seconds/more than 100 seconds |
Pd-functionalized individual ZnO microwire [ |
1000/50–200 | 200 | 400 | 13 | −/− |
Pd/ZnO spheroidal and rod-like nanoparticles [ |
10–20 |
250 | 400 | 65.7 | 15/within minutes |
Pd incorporated ZnO nanoparticles [ |
17 | 200 | 170 | 68 | 60/10 |
Transient responses of ZnO-NWs and Pd-ZnO-NWs-
Figure
Dynamic responses of ZnO-NWs and Pd-ZnO-NWs-5 sensors to ethanol in the detection range from 1 to 800 ppm. The inserted image shows the partial magnified response curve of sensors exposed to 1 ppm ethanol.
Figure
(a) Response of ZnO-NWs and Pd-ZnO-NWs-5 versus ethanol concentrations (the inserted image shows the partial magnified response curve of the two sensors exposed to 1–100 ppm ethanol), (b) linear relationship between
It is well known that chemisorbed oxygen plays a key role in electrical transmission of semiconducting metal oxides [
Our data have shown that the additive Pd can improve the gas sensing properties of the ZnO sensors to ethanol. At high temperature, oxygen molecules can weakly bond to the catalytic atoms Pd [
Consider the following:
Meanwhile, because of the PdO attached on the surface of ZnO, a heterojunction at the interface between ZnO (n-type semiconductor) and PdO (p-type semiconductor) will be formed as shown in Figure
(a) Diagram of energy band structure for p-type PdO/n-type ZnO heterojunction, (b) schematic model of the ZnO-NWs and Pd-ZnO-NWs sensors exposed to ethanol gas.
Simultaneously, ethanol molecule can also combine with the hole in PdO (
Consider the following:
ZnO-NWs were prepared by hydrothermal growth, showing high quality crystallinity and
The authors thank the National Natural Science Foundation of China (Grant nos. 51205273 and 51205274), the Shanxi Province Science Foundation for Youths (Grant no. 2013021017-2), the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (Grant no. 20120007), the Shanxi Scholarship Council of China (Grant no. 2013-035), the Excellent Innovation Programs for Postgraduate in Shanxi Province (Grant no. 20133028), the Special/Youth Foundation of Taiyuan University of Technology (Grant nos. 2012L034 and 2012L011), and also the Excellent Graduate’s Science and Technology Innovation Foundation of Taiyuan University of Technology (Grant no. 2013A005).