Outstanding H 2 Sensing Performance of Pd Nanoparticle-Decorated ZnO Nanorod Arrays and the Temperature-Dependent Sensing Mechanisms Chia-Ming Chang, † Min-Hsiung Hon, †,‡ and Ing-Chi Leu §, * † Department of Materials Science and Engineering and ‡ Research Center for Energy Technology and Strategy, National Cheng Kung University, 1, Ta-Hsueh Road, Tainan 701, Taiwan, ROC § Department of Materials Science, National University of Tainan, 33, Sec.2, Shu-Lin St., Tainan 700, Taiwan, ROC * S Supporting Information ABSTRACT: The nearly monodispersed Pd nanoparticles with controllable density on ZnO nanorod arrays were prepared by the unique PVP-mediated photochemical deposition (PCD). The changes in morphology and dispersion of Pd on ZnO surface are ascribed to the stabilizing property and self-assembly characteristic of PVP being exploited during PCD. There are three temperature-dependent H 2 sensing mechanisms in those Pd/ZnO NRs, including general oxygen adsorption/desorption mode within 200-300 °C, surface conductivity mode at 60-120 °C and palladium hydride (PdH x ) formation at room temperature, which causes a significant discrepancy in sensitivity variations as a function of Pd density. It is also verified that the electronic sensitization related to the transition of Pd 2+ /Pd 0 redox couple predominates the promoting mechanism in Pd/ZnO NRs used for sensing H 2 at 200-300 °C. Therefore, the gas sensitivity to 500 ppm H 2 of Pd/ZnO NRs can be significantly improved by around 553-fold (S, R a /R g = 1106) at 260 °C through decorating an adequate amount of discrete Pd nanoparticles instead of the Pd clusters, moreover, the corresponding sensitivity at room temperature is 16.9 that is superior to some promising devices reported in the literatures. KEYWORDS: ZnO nanorod arrays, Pd nanoparticle, photochemical deposition, sensing mechanism 1. INTRODUCTION The ZnO nanorod/nanowire arrays (NRs/NWs) have been considered as ideal building blocks for high sensitivity gas sensor in virtue of the high surface-to-volume ratio and a readily gas accessible three-dimensional structure. Consequently, ZnO NRs/NWs have been used for sensing various gas analytes, comprising O 2 ,O 3 ,H 2 ,C 2 H 5 OH, C 3 H 6 O, CO, NH 3 , and so on. 1-7 Among those gases, hydrogen has recently attracted considerable attention as an energy carrier or source for fuel cell. However, there is serious concern about its safe production, storage, and usage because it is explosive when being mixed with air above 4 vol %. 8 Therefore, the effective and accurate detection of hydrogen gas is essential for exploiting it as a renewable energy resource. But the reported sensitivities to H 2 gases in the blank ZnO-based gas sensor are generally not good enough in the literatures 9,10 and the lower sensitivities should be due to the relatively inert surface state for H 2 adsorption on those ZnO nanostructures 11 and the nonpreference to H 2 gases of the ZnO gas sensor by the donating effect. 12 The surface state consisting of surface defects and surface adsorption is highly responsible for gas sensing performance of metal-oxide semiconductor nanostructures 13 and consequently its activity is usually promoted by means of decoration with catalytic noble metals like Pd, 7,14 Pt, 15 Au, 16 Ag, 17 etc. Besides, the palladium not only has a synergistic effect of electronic and chemical sensitization in metal-oxide semiconductor gas sensor, 7,18,19 but also has been commonly used as H 2 sensing material with high sensitivity and selectivity because of its high hydrogen solubility at room temper- ature. 20,21 Hence, decorating ZnO NRs with Pd nanoparticles is expected to harvest a high-performance H 2 gas sensor. It is noticeable that the sensing properties of this kind of composite structures are manifestly influenced by the dispersion, size distribution and density of metal nanoparticles on oxide surface. 15,17,22 Yet it is difficult to regulate the size of metal nanoparticles and their distribution on the supporting structures through facile solution-based procedures 23,24 as well as the photochemical deposition (PCD) proposed in our earlier study. 7 Though the gas-phase approaches may give effective ways to tune those features, 17,18 they are stuck with some inevitable drawbacks, such as high processing cost, low yield, and complex adjustment course. Thus it is necessary to exploit Received: October 11, 2012 Accepted: December 11, 2012 Published: December 11, 2012 Research Article www.acsami.org © 2012 American Chemical Society 135 dx.doi.org/10.1021/am302294v | ACS Appl. Mater. Interfaces 2013, 5, 135-143