The effects of Ni contents on hydrogen sensing response of closely spaced Pd–Ni alloy nanoparticle films
Graphical abstract
Introduction
Utilization of hydrogen as an important new alternative energy source requires comprehensive safety management during its storage, handling and use. Hydrogen is highly flammable and explosive when its concentration exceeds the lower explosion limit in air of 4% at room temperature [1]. It is often desirable to have very small, low-power, fast-response, remotely fieldable devices that can work over a very wide range of hydrogen pressures or concentrations. Although numerous approaches have been reported for hydrogen sensors, those sensors in their early stage of development suffered from a number of drawbacks, there is still a lack of universal hydrogen detectors that can meet the above requirements.
Over the past few decades, use of palladium as a hydrogen sensor material has attracted much attention because of the high sensitivity it can achieve [2], [3], [4], [5]. Recently, Pd has been incorporated into nano-wires, nano-tubes, and nano-chains to reduce size, decrease power consumption, and enhance sensing properties [6], [7], [8], [9], [10], [11]. Although the hydrogen sensors based on nanostructured Pd have superior sensitivity and other advantages, they still have several drawbacks. For example, the concentration range of hydrogen adsorption on pure palladium is limited, the device signals are virtually saturated at low H2 partial pressures [12]. In addition, pure palladium undergoes structural phase transition from α phase to the β phase palladium hydride at fairly low hydrogen partial pressure (about 900 Pa at 300 K). The α–β phase transition leads to irreversible process in Pd [13], [14], which is responsible for irreproducibility and hysteresis in the detection of hydrogen at concentrations above 1% H2. To overcome such drawbacks, the sensing materials have been modified by introducing a second metal to make Pd-based alloys, such as Pd-Al [15], Pd–Ag [16], Pd–Mg [17], and Pd–Ni [18], [19], [20] systems.
Recently, quantum-conductance-based hydrogen sensors consisting of films of closely spaced Pd nanoparticles have been developed [9], [11]. The nanoparticle films were produced by gas phase cluster beam deposition so that the devices could be fabricated in a well-controlled manner. By optimizing the nanoparticle coverage, very high sensitivity, wide quantitative response range, and sub-second response time could be realized. However, the α to β-phase transition of Pd in the hydrogen concentration region near the lower explosion limit (4%) induced irregular electrical response and reduced response speed [21], [22]. Previous researches showed that the presence of nickel in the Pd sensing element suppressed the α to β-phase transition so that good stability could be achieved [20]. In this paper, we study the modification of the α to β-phase transition properties of Pd nanoparticles by introducing nickel additions with different contents. We fabricate Pd–Ni alloy nanoparticles by gas aggregation process and examine the hydrogen sensing behavior of the closely spaced Pd–Ni nanoparticle films deposited with controlled coverage.
Section snippets
Material and methods
We prepared Pd–Ni nanoparticle films by performing gas phase cluster deposition. The nanoparticles were formed with a gas aggregation process in a cluster condensation chamber equipped with two magnetron discharge heads, installed with either a Pd target or a Ni target. The magnetron discharges were operated at a pressure of about 100 Pa in argon stream in the condensation chamber [23]. The magnetron discharges of the two targets were operated separately with controlled powers. Atoms were
Results and discussion
Fig. 1a shows TEM microimage of a film of Pd–Ni nanoparticles with 16% Ni content. As shown, isolated nanoparticles are randomly distributed in the film and form numerous closely spaced particle assembling areas. The size distribution of the nanoparticles was calculated from the TEM image and is shown in Fig. 1b. It fits a log- normal distribution with a mean diameter of 6.9 nm and a standard deviation of about 2.8 nm. From the high magnification TEM image shown in Fig. 1c, it can be identified
Conclusions
Pd100-xNix (0 ≤ x ≤ 60) nanoparticles were synthesized with a gas aggregation cluster source equipped with two magnetron discharge heads. The synthesized nanoparticles are of alloy form, with a crystalline state and cuboid shape. The mean size of the alloy nanoparticles was found to decrease with Ni content, from 7.8 nm for pure Pd to 6.2 nm at 45% Ni. Quantum conductance-based hydrogen sensors were fabricated by gas-phase deposition of closely spaced Pd–Ni nanoparticle films with well
Acknowledgments
The authors thank the National Natural Science Foundation of China (Grant nos. 51171077, 61301015). This research was also supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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2021, Sensors and Actuators, B: ChemicalCitation Excerpt :Further extending the activation time to 10 min (Fig. 9d) or even 30 min (Fig. 10e), or increasing the number of activation (four 10-minute activations, Fig. 10f), the resistance of Pd-ONF in hydrogen-free atmosphere becomes almost constant after a sharp decrease, and the response to 200 ppb H2 is increased to a stable value. The nucleation and growth of β-phase Pd-H spend more time than the generation of α-phase Pd-H, according to the researches of Lang et al. [15]. If the activation process lasting a short time, the irreversible deformation and lots of defects may have not fully generated in Pd, thus, the expanding of Pd nanoparticles and the decrease of the sensor’s resistance have a tendency to recover, and the response to 200 ppb H2 is negligible.