Pd Nanoparticles and Thin Films for Room Temperature Hydrogen Sensor
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We report the application of palladium nanoparticles and thin films for hydrogen sensor. Electrochemically grown palladium particles with spherical shapes deposited on Si substrate and sputter deposited Pd thin films were used to detect hydrogen at room temperature. Grain size dependence of H2sensing behavior has been discussed for both types of Pd films. The electrochemically grown Pd nanoparticles were observed to show better hydrogen sensing response than the sputtered palladium thin films. The demonstration of size dependent room temperature H2sensing paves the ways to fabricate the room temperature metallic and metal–metal oxide semiconductor sensor by tuning the size of metal catalyst in mixed systems. H2sensing by the Pd nanostructures is attributed to the chemical and electronic sensitization mechanisms.
KeywordsPalladium Hydrogen sensors
Current trend of research in hydrogen sensor technology is dedicated towards the development of room temperature sensors. Various types of sensors have been widely studied and used for sensing the oxidizing and reducing gases [1, 2, 3, 4, 5]. However, it has been a difficult task to detect the gases at room temperature even after adding the metal catalyst into the semiconductors. Recently, H2 sensors have gained increased interest due to its application in many significant areas. Various methods have been reported for the development of H2 sensors based on metal-oxide [4, 5], nanowires [6, 7], acoustic wave [8, 9], thin film metal and semiconductor [10, 11]. Most of the approaches utilize Pd as a catalyst, since Pd has great affinity towards H2 absorption [12, 13, 14]. Pd undergoes a change in physical properties by adsorbing H2, which can be restored by removing H2 gas from the ambient. Palladium based hydrogen sensors are based on the increase of electrical resistivity due to the increased electron scattering on hydrogen incorporation [15, 16, 17, 18]. Penner et al.  have reported a novel mechanism of hydrogen gas detection using resistive palladium mesowire arrays that change their resistivity upon exposure to hydrogen virtually instantaneously. In an another very important work on hydrogen sensors Yushi et al. have fabricated and demonstrated the use of a single metallic nanowire as a hydrogen sensor with extremely high sensitivity .
In our recent work we have used Pd nanoparticles for synthesis of multiwall carbon nanotube for scanning probe microscopy application [21, 22]. In order to extend the application of chemically grown Pd nanoparticles we used these particles for hydrogen detection at room temperature. In this article we demonstrate the room temperature detection of hydrogen using Pd nanoparticles with different sizes. The hydrogen sensing results of electrochemically grown Pd are compared with the hydrogen sensing results obtained for the sputtered Pd thin films.
Electrochemical Deposition of Pd Nanoparticles on Silicon Substrates
Palladium nanostructures were deposited on conductive Si (100) substrates (resistivity = 0.005 Ω-cm) through potentiostatic electrodeposition [21, 22]. An amount of the electrolyte, consists of Pd sulfate and sulfuric acid, was taken into an electrochemical cell for the growth of nanoparticles on silicon substrate at working electrode. Three-electrode configuration system was used for the growth of Pd nanostructures. Pt was used as counter electrode, Ag/AgCl as the reference electrode and silicon substrate as working electrode for electrochemical deposition of Pd on Si substrates. The Si substrates were cleaned using deionizer water, ethanol and additionally treated with HF solution for removing the native oxide layers just before the electrochemical growth. The three electrodes, with silicon substrate as working electrode, were dipped into the electrolyte solution and an appropriate bias was applied to the system. After the chosen deposition time the substrates were taken out of the solution and dried and tested for their hydrogen sensing response. Surface morphology of electrochemically grown Pd nanostructures was investigated extensively using the field emission scanning electron microscopy (FESEM). Crystallographic structure of the Pd nanostructure grown on silicon substrates was studied using glancing angle X-ray diffraction. Average grain size of electrochemically grown nanoparticles was estimated using X-ray diffraction data and given in our previous article .
Sputtering Deposition of Pd Thin Films
Palladium thin films were prepared by DC-magnetron sputtering system on oxidized silicon substrate with SiO2layer thickness of 100 nm on Si. The Pd films were utilized, as-grown, to detect hydrogen at room temperature. A custom built DC-magnetron sputtering system was utilized to deposit the nanocrystalline palladium thin films. Deposition process was carried out at a vacuum of 10−6Torr. A 99.95% pure Pd metal was used as the target material. The distance between the target and substrates was kept at 10 cm and the substrate was rotated at a uniform rate during the deposition. The chamber was pumped down using a cryopump and ultra high pure argon was introduced into the chamber through a mass flow controller. Pre-sputtering was carried out for 5 min to etch away any dirt present in the target and then the metal was deposited on the substrate. A regulated DC power source with a constant input power of 60 W was used to deposit thin film nanocrystalline Pd. Pd films were deposited at pressures varying from 22 to 120 m Torr. In sputtered Pd films it was observed that on increasing the deposition pressure from 22 to 120 m Torr the average grain size varied from 10 to 30 nm. XRD and atomic force microscopy (AFM) was used to study structure and morphology of the films.
Hydrogen Sensor Characterization
Results and Discussion
Variation of average grain size with pressure during film deposition by sputtering
Deposition pressure (m Torr)
Average grain size (nm)
As the Pd particle size approaches the nanoscale dimensions, the probability of forming Pd-hydride increases due to a very strong affinity of Pd toward H2 absorption which should be more in the nanodimension due to high specific surface area of Pd . The formation of Pd-hydride, which is a process of lattice modifications , results in the band structure alteration and changing the work function on hydrogen exposure. We have observed a systematic change in the work function on H2 exposure to the Pd nanoparticles and thin films using X-ray photoelectron spectroscopy techniques. The experimentally observed very higher value of recovery times for these sensors can be correlated to the formation of Pd-hydride. Once Pd is transformed as Pd-hydride, the lattice structure of the palladium changes, in terms of expansion. This causes the sensor to recover more slowly to the baseline resistance. The change in work function due to gas adsorption suggests the presence of electronic sensitization on the films [24, 25]. Therefore, we believe that hydrogen sensing by Pd is attributed to the dual (chemical and electronic) sensitization mechanism. Enhancement in sensing response with lower particle size is attributed to the chemical sensitization whereas the improved sensing due to experimentally observed change in work function on hydrogen exposure is attributed to electronic sensitization.
Electrochemically grown Pd nanoparticles and sputtered Pd thin films have been used to detect hydrogen at room temperature. Comparative study shows that electrochemically grown Pd nanoparticles have better H2sensing characteristics than the sputtered palladium thin films. Gas sensing response has been observed to improve with lower grain size in the both type of systems. Chemical and electronic sensitization mechanisms were observed to be responsible for the enhanced gas sensing behavior with lower grain size. The Pd based conductive sensors, tested for different concentration of H2, were observed to be highly stable and reproducible.
This work is supported by National Science Foundation through NIRT # ECS 0404137. We are grateful to Mr. Tom Gage and Mr. Tony of Engineering Machine Shop at the University of South Florida for making gas sensor sample holder.
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