Elsevier

Chemical Physics Letters

Volume 661, 16 September 2016, Pages 191-195
Chemical Physics Letters

Research paper
Effects of hydriding and ageing of Pd nanoparticles to contact between nanoparticles and quartz and contacts among nanoparticles investigated by the pump-probe technique

https://doi.org/10.1016/j.cplett.2016.08.074Get rights and content

Highlights

  • Pd NPs are uniformly deposited on quartz sheets and closely contiguous.

  • The ageing in air or hydriding in 0.9% of hydrogen cause little change of the NPs.

  • The ageing and hydriding loose the connection between the NPs and quartz.

  • The connection among NPs looses with ageing whereas compresses with hydriding.

Abstract

Closely contacted Pd nanoparticles with average size of 8 nm are uniformly deposited on quartz. The ageing, hydriding, hydriding and then ageing of the nanoparticles are characterized by the transmission electron microscopy, optical extinction spectrum, and X-ray photoelectron spectroscopy, which show little chemical and crystallographic change. The contact between the nanoparticles and quartz and contacts among the nanoparticles are investigated by the pump-probe technique. The contact between nanoparticles and quartz looses with the ageing, hydriding, hydriding and then ageing of the nanoparticles, and the contacts among nanoparticles loose with the ageing whereas compresses with the hydriding.

Introduction

As an important chemical raw material and ideal green energy resource, hydrogen is widely used in petrochemical industry and fuel cell. Due to the explosiveness of hydrogen, the hydrogen sensor is very important for the utilization of hydrogen. Pd is a frequently-used hydrogen sensitive materials in hydrogen sensors [1]. Although metal oxides such as ZnO, SnO2, and WO3 can also be used as hydrogen sensitive materials, Pd is still necessary as the sensitizing agent to enhance the sensitivity of the materials [2], [3], [4]. Currently, nano-structured Pd and metal oxides are prevalent hydrogen sensitive materials due to the advantages of shorter response time and higher sensitivity comparing to the bulk counterparts. For these materials, Pd nanostructures are typically fabricated on a substrate or deposited on metal oxide nanostructures. The interaction between hydrogen and Pd leads to the changes of the linear optical and/or electric properties of the materials, which are the research focus and basis of hydrogen sensing [1], [2], [3], [4], [5], [6], [7]. Today various hydrogen sensing methods have been proposed, whereas most of the corresponding hydrogen sensors are far from mature and applicable. The main reason is the bad repeatability of these hydrogen sensors, therefore the specific improvement of the sensors is highly required. To perform the improvement, most importantly, the hydriding and ageing of Pd nanostructures, the contact between the nanostructures and substrate or metal oxide, and the contacts among the nanostructures, should be investigated sortably. However such investigations have rarely been reported specially.

On the other hand, the pump-probe technique is powerful in investigation of the contact characteristic between heterogeneous and/or homogeneous materials [8]. A typical ultrafast dynamic of metal nanoparticles (NPs) revealed by the pump-probe technique can be divide into four stages according to time [9]. The first stage is the excitation of electrons and collision between the excited and unexcited electrons, which leads to the thermalization of the electron gas. The second stage is the thermalized electron gas interacts with the lattice of NP, leading to the thermalization of the NP. And the third stage is the ultrafast heating of the lattice, leading to the expansion of NP, which subsequently undergoes radial contractions and expansions. This is the acoustic breathing vibration [10]. Finally, the thermalized NPs with high temperature interact with the substrate and contacted surrounding NPs (if any) with low temperature through the phonon-phonon interaction, which heats the substrate and surrounding NPs by consuming the thermal energy of the NPs [11]. Obviously, a longer characteristic time of phonon-phonon interaction reveals a looser contact between the NPs and the substrate or looser contacts among NPs. Therefore the pump-probe technique can provide perfect insights to the contact between Pd NPs and substrate and the contacts among Pd NPs, which is highly desired for the principle research of hydrogen sensitive materials and significantly valuable for the specific improvement of hydrogen sensors. Note that the contact between Pd NPs and substrate and the contacts among Pd NPs have rarely been investigated by the pump-probe technique. In this letter, the hydriding and ageing of Pd NPs are investigated. The contact between Pd NPs and substrate and the contacts among the NPs are investigated by the pump-probe technique. And the effects of ageing and hydriding of Pd NPs to these contacts are also investigated.

Section snippets

Experimental details

Pd NPs are deposited on a quartz sheet with size of 10 × 10 × 1 mm3 (thickness 1 mm) by means of low energy cluster beam deposition method [12]. This is a perfect method that can fabricate metal and semiconductor NPs on various substrates, nanostructures, fringe electrodes, and so on with desired density. As shown in Fig. 1, briefly, a Pd target in the chamber 1 (120 Pa) is magnetically spurted by argon (sputter gas) with a gas flow of 70 sccm and a sputtering power of 35 W. During the sputtering

Results and discussions

A TEM image of the new sample is shown in Fig. 2(a), which shows that the NPs are uniformly deposited and many NPs are closely contacted with the coverage of NPs is about 80%. The close contact enables the construction of a novel hydrogen sensor [13], [14]. The size distribution histogram of the NPs is shown in the inset of Fig. 2(a), which shows that the sizes of the NPs are narrowly distributed with an average size of 8 nm. The HRTEM image of the new sample is shown in the inset of Fig. 2(a),

Conclusion

Pd NPs with average size of 8 nm are uniformly deposited on quartz sheets with the NPs are closely contacted. The ageing, hydriding, hydriding and then ageing of the NPs are investigated, which show inappreciable chemical and crystallographic change except that the hydriding and then ageing changes the crystal structure of Pd NP from monocrystal to polycrystal and causes slight oxidation of the Pd NPs. The contact between the NPs and quartz and the contacts among the NPs are investigated by

Acknowledgments

This research is supported by the Natural Scientific Research Innovation Foundation in Harbin Institute of Technology (Project HIT. NSRIF. 2014140), and the Project of Shandong Province Higher Educational Science and Technology Program (Project J14LJ54), as well as the National Natural Science Foundation of China (Grant Nos. 11574064, 11304064, 61301015, 51171077, 11374077).

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