A recent article published in Nanomaterials demonstrated the fabrication of ultrathin platinum-gadolinium (PtGd) alloy films for resistive hydrogen gas sensing, highlighting the enhanced hydrogen sensing capabilities of rare earth metals.
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Background
Hydrogen is a universal energy source integral to various industries, including chemical synthesis (such as petroleum refining, ammonia production, and metal purification), fuel cells, and propulsion systems for terrestrial and extraterrestrial travel. With rising environmental concerns, hydrogen is recognized as a clean, renewable, and green energy alternative.
However, its potent reducing characteristics and ability to penetrate diverse materials require rigorous control. Being invisible and combustible, hydrogen poses a significant safety risk, as it can lead to explosive reactions at concentrations above 4 % in air. Hence, there is a growing demand for hydrogen sensors to prevent hazardous air mixtures and potential explosions.
The physical and structural properties of rare earth elements change dramatically when interacting with varying hydrogen levels. Notably, rare-earth-based alloys can switch between reflective and transparent states with reversible hydrogenation and dehydrogenation. This study examined the functional characteristics of 2 nm thick PtxGd100-x (x = 25, 50, and 75) alloy films and analyzed their hydrogen gas sensing properties.
Methods
PtxGd100-x films were deposited via the co-sputtering method on 0.5 mm thick silicon substrates coated with a 300 nm oxide layer. High-purity targets of Pt and Gd were mounted in a magnetron sputtering system (5.5×10−8 Torr base pressure) to synthesize the films.
A surface profilometer was employed to calibrate Gd and Pt deposition rates using thicker films. Subsequently, sub-monolayers of Pt and Gd were deposited in multiple cycles according to the desired film thickness. Finally, the films were annealed at 300 °C for two hours under vacuum conditions of 10−2 millibar to form the alloy and ensure homogeneous film thickness.
The PtGd alloy films were structurally characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) techniques.
Silver contacts (5×2 mm2) with 1 mm separation were deposited on the PtGd films by thermal evaporation to fabricate a resistive hydrogen sensor. The electrical resistance of these sensors was constantly monitored in different environmental compositions using a custom-built measurement cell. A heater varied the cell's temperature between 25 °C and 150 °C. Hydrogen gas, in concentrations ranging from 10 ppm to 5 % (50,000 ppm), was introduced into the cell using dry air as the carrier gas.
Results and Discussion
XPS results confirmed successful alloy formation, showing notable shifts in the principal peaks of Pt and Gd. SEM images depicted the films' smooth surface across all compositions without any segregation. These results validated the efficacy of the proposed low-rate deposition followed by annealing in fabricating the targeted alloy structure with high uniformity and compositional precision.
Additionally, the temperature-dependent electrical properties of the fabricated PtGd alloys exhibited a strong correlation with Nordheim’s rule.
The PtGd alloy films exhibited varying sensitivity to hydrogen at different temperatures. The resistance of all the films decreased with increasing concentrations of hydrogen in the measurement cell, which was attributed to the surface scattering phenomenon. At 150 °C, the sensitivity of the Pt75Gd25, Pt50Gd50, and Pt25Gd75 films toward 10000 ppm of hydrogen was 6.1, 5.8, and 4.9, respectively.
The response time of the PtGd hydrogen sensors decreased with increasing hydrogen concentrations and temperatures. This was attributed to the diffusion of hydrogen and absorbance onto the film surface. Among all, Pt25Gd75 film exhibited minimum response time.
The researchers also explored the absorption isotherm characteristics of the PtGd films through multiple renowned models, including Henry, Temkin, Freundlich, and Langmuir adsorption isotherms. Among these, Langmuir’s absorption isotherm depicted the closest alignment with the results of fabricated PtGd thin film resistive hydrogen sensors.
Conclusion and Future Prospects
The study comprehensively examined the relationship between alloy composition, sensing temperature, and hydrogen concentration on the resistive sensing capabilities of ultrathin PtGd films.
The hydrogen gas sensing mechanisms of PtGd alloy films were explained by surface scattering. Gd, known for its high spin density and intrinsic magnetic moment due to its half-filled 4f orbitals, significantly influences hydrogen absorption even under thin Pd coatings.
Thus, the unique combination of Pt and Gd demonstrated promising results for gas sensing applications with high reactivity to hydrogen gas and tunable sensitivity according to the alloy composition.
These findings can aid in designing and optimizing hydrogen gas sensors with improved performance characteristics. Moreover, the PtGd alloy shows significant potential in enhancing the selectivity of hydrogen gas sensors, which is crucial for accurate and reliable detection in practical applications.
Journal Reference
Kilinc, N., Cardoso, S., Erkovan, M. (2024). Rare Earth Material for Hydrogen Gas Sensing: PtGd Alloy Thin Films as a Promising Frontier. Nanomaterials. DOI: 10.3390/nano1413109
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