A novel gas ionization sensor using Pd nanoparticle-capped ZnO
© Wang et al; licensee Springer. 2011
Received: 9 June 2011
Accepted: 30 September 2011
Published: 30 September 2011
A novel gas ionization sensor using Pd nanoparticle-capped ZnO (Pd/ZnO) nanorods as the anode is proposed. The Pd/ZnO nanorod-based sensors, compared with the bare ZnO nanorod, show lower breakdown voltage for the detected gases with good sensitivity and selectivity. Moreover, the sensors exhibit stable performance after more than 200 tests for both inert and active gases. The simple, low-cost, Pd/ZnO nanorod-based field-ionization gas sensors presented in this study have potential applications in the field of gas sensor devices.
Gas sensors have attracted considerable attention in recent years because of their huge potential applications, such as pollution detection, environment protection, gas detection for counter-terrorism, etc. . There are two types of gas sensors, chemical type operated by gas adsorption-desorption and physical type operated by field ionization. In different gas varies and concentrations atmosphere, the chemical type gas sensor can detect the modifications of the electronic properties in the active layer, such as carbon nanotubes (CNTs) [2–4], porous silicon , and metal oxides . However, the application of the chemical type gas sensor is limited by several disadvantages, such as the potential difficulties in detecting gases with low adsorption energies, the high working temperature (except for the CNTs based sensors), and the higher power consumption.
Recent efforts have been directed to the physical type of gas senor based on the field ionization, which works by fingerprinting the ionization characteristics of distinct gases. This type sensor can detect gases regardless of their adsorption energies. A novel physical gas sensor based on CNTs has been demonstrated with low breakdown voltage due to its extremely sharp radii [7, 8]. This sensor can detect many gases, such as Air, He, Ar, and gas mixtures, by the strong electric fields generated at the tips to strip electrons from the various gas molecules . However, the CNTs show poor stability because it could easily be oxidized and degraded in the oxygen-contained atmosphere [9, 10].
Recently, gas ionization sensors using a sparse array of vertically aligned gold nanorods as substitutes for CNTs have successfully been prepared for the first time [11–14]. Owing to the chemical stability of one-dimensional ZnO (1D ZnO) nanowires at room temperature, they also have been used for stable field-ionization gas sensors instead of CNTs . However, 1D ZnO nanostructures with relatively smooth surface and larger tip radii, compared with CNTs, need higher breakdown voltage. Therefore, the modification of the surface of 1D ZnO nanostructures to obtain lower breakdown voltage is one of the key issues for gas sensor applications. In this study, we introduce a physical gas sensor using palladium (Pd) nanoparticle-capped ZnO (Pd/ZnO) nanorods as the anode. The results show that the breakdown voltage decreases for the Pd/ZnO nanorod-based sensor, compared with the bare ZnO nanorod. This study investigates the potential applications of such physical ionization gas sensors.
2. Experimental section
ZnO nanorods were grown on silicon substrates through a reactive vapor deposition method as reported in detail elsewhere . The Pd nanoparticles were deposited by a dc sputtering system at a fixed current of 40 mA for 120 s. The morphology of ZnO nanorods was characterized by a Sirion FEG scanning electron microscopy (SEM) and JEM-2010FET transmission electron microscope system operated at 200 kV, respectively. All the data were obtained from the same ZnO nanorods sample, which was cut into two pieces before Pd sputtering.
3. Results and discussion
Figure 1b shows the top view SEM image of the bare ZnO nanorods. The ZnO nanorods have a diameter of 30-40 nm and length of 1 μm. Figure 1c, d shows the typical individual bare ZnO nanorods and Pd/ZnO nanorods with Pd capping by 120 s sputtering, respectively. It is clearly seen that the bare ZnO nanorod has a rather smooth surface and the surface of Pd/ZnO nanorod is distributed with Pd nanoparticles with diameters of about 5 nm.
Figure 2b shows the breakdown voltage of the sensor using Pd/ZnO nanorods for Ar, He/CH4 (60%/40%), Air, and N2, respectively. All the tests were performed at room temperature and at a chamber pressure of 100 Pa. It can be seen that each gas exhibits a distinct breakdown voltage: Ar shows the lowest breakdown voltage and N2 displays the highest one. This precise breakdown voltage is a fingerprinting property for individual gas. The stability of sensor using Pd/ZnO nanorods was tested for air and insert gas of Ar, as shown in Figure 2c. The breakdown voltages of both gases are maintained up to 200 cycles without any significant change. It shows that the sensor exhibits much better stability than that of CNTs . The good performance indicates that Pd/ZnO nanorods could be a better candidate for the field-ionization gas sensor.
In conclusion, a novel field-ionization gas sensor using ZnO nanorods was demonstrated. The sensors using Pd nanoparticle-capped ZnO nanorods, compared with the bare ZnO nanorods, showed lower breakdown voltage. Besides that, the sensors showed good sensitivity and selectivity. Moreover, the breakdown voltage of Pd/ZnO nanorod-based sensors was maintained without any significant change during 200 cycle tests. The simple, low-cost devices presented in this study might be expected to expand the applications of gas sensor.
- 1D ZnO:
Pd nanoparticle-capped ZnO
scanning electron microscopy.
This work was supported by the National Natural Science Foundation of China under contract 61106124 and 11075121, the International Science and Technology Cooperation Program of China under contract 2010DFA02010, and the Fundamental Research Funds for the Central Universities.
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