ARTICLE pubs.acs.org/JPCC
Optimizing Hydrogen Sensing Behavior by Controlling the Coverage in Pd Nanoparticle Films Bo Xie,† Linlin Liu,† Xing Peng,† Yue Zhang,† Qian Xu,† Mengyang Zheng,† Toshio Takiya,‡ and Min Han†,* †
National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China ‡ Hitachi Zosen Corporation, 2-2-11 Funamachi, Taisho, Osaka 551-0022, Japan ABSTRACT: The response of quantum-conductance-based hydrogen sensors fabricated by controllable deposition of closely spaced Pd nanoparticle films between interdigital electrodes was investigated. Three typical response regions with different conductance hydrogen pressure correlations were observed. The response characteristics of the devices were found to depend strongly on the nanoparticle coverage. In the low H2 pressure region, higher coverage gives higher sensitivity. In the high H2 pressure region, quantitative sensing can only be realized with low nanoparticle coverage. Optimizing the coverage allows the attainment of highly sensitive hydrogen sensors with a very wide quantitative working range, extending far beyond the hydrogen pressure region associated with the R-to-β phase transition of Pd.
1. INTRODUCTION Hydrogen is an important new alternative energy source that has great potential in various fields, including chemical, metallurgical, and electronic industries, and fuel cells. With the development of these applications, hydrogen detection becomes an important issue. Because of the broad range of fields of application, sensitive hydrogen detectors that can work over a very wide range of temperatures (including room temperature) and pressures or hydrogen gas concentrations are critically required not only for leak detection but also for monitoring and controlling flow and hydrogen purity. Although some commercial hydrogen sensors are currently available on the market, a universal hydrogen detector that can meet the above requirement is still lacking. Metallic nanostructures on substrates have interesting conduction properties and have been investigated extensively. Recent publications have demonstrated that hydrogen sensing based on the quantum conductance of Pd nanostructures may be able to exceed the performance of existing hydrogen sensors in both response speed and sensitivity.1 6 The sensing mechanism is based on thermally activated hopping or tunneling of electrons, which dominates current transport across the barrier separating neighboring nanoelements, assuming small gaps are contained between them. Changes in the gap size owing to lattice expansion or dwindling of Pd during hydrogen absorption or desorption result in changes in the electron barrier, leading to a measurable change in the conductance of the nanostructures. A hydrogen sensor fabricated from arrays of palladium nanowires was first demonstrated by Penner et al., who showed a rapid response time (as short as 2 kPa. The legends give the base conductances (G0) of each specimen.
is thus related directly to the number of the electron transport paths the nanoparticle film contains. The existence of a huge number of thermally activated conducting paths results in a welldeveloped statistical distribution of the interparticle spacing, which leads to high sensitivities and wide response ranges for the hydrogen sensors fabricated on the Pd-nanoparticle filmcovered interdigital electrodes. However, there will be a sensitive dependence of the conductance on the nanoparticle coverage 16165
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The Journal of Physical Chemistry C when the latter is close to the effective percolation threshold of system size L.7 Films with higher nanoparticle coverage are favorable to form more transport paths at moderate hydrogen pressure so as to generate larger conductance; therefore, their hydrogen sensitivities are higher at lower PH2 or even at high PH2 for those films with moderate coverage. However, at higher hydrogen pressure, more and more gaps between neighboring nanoparticles will be closed, which may result in saturation of the thermally activated transport paths if the films have excessive nanoparticle coverage so that the rate of the conductance change induced by hydrogen absorption decreases more and more. In the worst case, the response of ΔG/G0 to PH2 may lose linearity even at PH2 < 1000 Pa, as in the case of specimen f, which has a base conductance as high as 9.00 μS. In conclusion, thermally activated conductance-based hydrogen sensors with very high sensitivity were fabricated by controllable deposition of closely spaced Pd nanoparticle films on interdigital electrodes in the gas phase. Three typical response regions with different ΔG/G0 versus PH2 dependences, separated by an interjacent region corresponding to the R-to-β phase transition of Pd, were observed. The devices have distinctive response characteristics at different nanoparticle coverage. Sensitive and quantitative hydrogen response can be realized with moderate nanoparticle coverage. Higher coverage is favorable for low-level hydrogen detection, whereas quantitative sensing at high hydrogen pressure can be realized only with the lower nanoparticle coverage devices. We demonstrate that by optimizing the nanoparticle coverage, the device not only can work for hydrogen sensing with very high sensitivity in the pressure region associated with the R-to-β phase transition of Pd, which is required for most quantum conductance-based hydrogen sensors being studied, but also can be available for quantitative hydrogen detection over a widely extended hydrogen pressure region far beyond this narrow phase-transition region.
ARTICLE
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’ AUTHOR INFORMATION Corresponding Author
*Tel: 86-25-83686248. Fax: 86-25-83686248. E-mail: sjhanmin@ nju.edu.cn.
’ ACKNOWLEDGMENT We acknowledge the financial support from NSFC (grant nos. 10974092 and 10674063), the National Basic Research Program of China (973 Program, contract no. 2009CB930501), the Fundamental Research Funds for the Central Universities (grant no. 1114021303), and the Industrialization Promotion of University Research Program in Jiangsu Province under contract no. JH10-2. This research was also supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. ’ REFERENCES (1) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (2) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 1546. (3) Walter, E. C.; Penner, R. M.; Liu, H.; Ng, K. H.; Zach, M. P.; Favier, F. Surf. Interface Anal. 2002, 34, 409. (4) Cherevko, S.; Kulyk, N.; Fu, J.; Chung, C. H. Sens. Actuators, B 2009, 136, 388. 16166
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