High-Performance Nanostructured Palladium-Based Hydrogen Sensors-Current Limitations and Strategies for Their Mitigation

Enhanced Pd/a-WO3 /VO2 Hydrogen Gas Sensor Based on VO2 Phase Transition Layer

The utilization of clean hydrogen energy is becoming more feasible for the sustainable development of this society. Considering the safety issues in the hydrogen production, storage, and utilization, a sensitive hydrogen sensor for reliable detection is essential and highly important. Though various gas sensor devices are developed based on tin oxide, tungsten trioxide, or other oxides, the relatively high working temperature, unsatisfactory response time, and detection limitation still affect the extensive applications. In the current study, an amorphous tungsten trioxide (a-WO₃ ) layer is deposited on a phase-change vanadium dioxide film to fabricate a phase transition controlled Pd/a-WO₃ /VO₂ hydrogen sensor for hydrogen detection. Results show that both the response time and sensitivity of the hydrogen sensor are improved greatly if increasing the working temperature over the transition temperature of VO₂ . Theoretical calculations also reveal that the charge transfer at VO₂ /a-WO₃ interface becomes more pronounced once the VO₂ layer transforms to the metal state, which will affect the migration barrier of H atoms in a-WO₃ layer and thus improve the sensor performance. The current study not only realizes a hydrogen sensor with ultrahigh performance based on VO₂ layer, but also provides some clues for designing other gas sensors with phase-change material.

Multilevel Self-Assembly of Block Copolymers and Polymer Colloids for a Transparent and Sensitive Gas Sensor Platform

The recent emerging significance of the Internet of Things (IoT) demands sensor devices to be integrated with many different functional structures and devices while conserving their original functionalities. To this end, optical transparency and mechanical flexibility of sensor devices are critical requirements for optimal integration as well as high sensitivity. In this work, a transparent, flexible, and sensitive gas sensor building platform is introduced by using multilevel self-assembly of block copolymers (BCPs) and polystyrene (PS) colloids. For the demonstration of an H₂ gas sensor, a hierarchically porous Pd metal mesh structure is obtained by overlaying the two different patterned template structures with synergistic, distinctive characteristic length scales. The hierarchical Pd mesh shows not only high transparency over 90% but also superior sensing performance in terms of response and recovery time owing to enhanced Pd-to-hydride ratio and short H₂ diffusion lengths from the enlarged active surface areas. The hierarchical morphology also endows high mechanical flexibility while securing reliable sensing performance even under severe mechanical deformation cycles. Our scalable self-assembly based multiscale nanopatterning offers an intriguing generalized platform for many different multifunctional devices requiring hidden in situ monitoring of environmental signals.

Bias-Modified Schottky Barrier Height-Dependent Graphene/ReSe2 van der Waals Heterostructures for Excellent Photodetector and NO2 Gas Sensing Applications

Herein, we reported a unique photo device consisting of monolayer graphene and a few-layer rhenium diselenide (ReSe₂) heterojunction. The prepared Gr/ReSe₂-HS demonstrated an excellent mobility of 380 cm²/Vs, current on/off ratio ~ 10⁴, photoresponsivity (R ~ 74 AW^-1 @ 82 mW cm^-2), detectivity (D^* ~ 1.25 × 10¹¹ Jones), external quantum efficiency (EQE ~ 173%) and rapid photoresponse (rise/fall time ~ 75/3 µs) significantly higher to an individual ReSe₂ device (mobility = 36 cm² V^-1s^-1, Ion/Ioff ratio = 1.4 × 10⁵-1.8 × 10⁵, R = 11.2 AW^-1, D* = 1.02 × 10¹⁰, EQE ~ 26.1%, rise/fall time = 2.37/5.03 s). Additionally, gate-bias dependent Schottky barrier height (SBH) estimation for individual ReSe₂ (45 meV at V_bg = 40 V) and Gr/ReSe₂-HS (9.02 meV at V_bg = 40 V) revealed a low value for the heterostructure, confirming dry transfer technique to be successful in fabricating an interfacial defects-free junction. In addition, HS is fully capable to demonstrate an excellent gas sensing response with rapid response/recovery time (39/126 s for NO₂ at 200 ppb) and is operational at room temperature (26.85 °C). The proposed Gr/ReSe₂-HS is capable of demonstrating excellent electro-optical, as well as gas sensing, performance simultaneously and, therefore, can be used as a building block to fabricate next-generation photodetectors and gas sensors.

Inverse designed plasmonic metasurface with parts per billion optical hydrogen detection

Plasmonic sensors rely on optical resonances in metal nanoparticles and are typically limited by their broad spectral features. This constraint is particularly taxing for optical hydrogen sensors, in which hydrogen is absorbed inside optically-lossy Pd nanostructures and for which state-of-the-art detection limits are only at the low parts-per-million (ppm) range. Here, we overcome this limitation by inversely designing a plasmonic metasurface based on a periodic array of Pd nanoparticles. Guided by a particle swarm optimization algorithm, we numerically identify and experimentally demonstrate a sensor with an optimal balance between a narrow spectral linewidth and a large field enhancement inside the nanoparticles, enabling a measured hydrogen detection limit of 250 parts-per-billion (ppb). Our work significantly improves current plasmonic hydrogen sensor capabilities and, in a broader context, highlights the power of inverse design of plasmonic metasurfaces for ultrasensitive optical (gas) detection.

Plasmonic hydrogen sensors have limited sensitivity due to broad spectral features. Here, the authors use a particle swarm optimization algorithm to inversely design a plasmonic metasurface based on a periodic array of Pd nanoparticles, and demonstrate hydrogen detection limit of 250 ppb.

Hydrogen-Induced Aggregation of Au@Pd Nanoparticles for Eye-Readable Plasmonic Hydrogen Sensors

Plasmonic materials provide a promising platform for optical hydrogen detection, but their sensitivities remain limited. Herein, a new type of eye-readable H₂ sensor based on Au@Pd core-shell nanoparticle arrays (NAs) is reported. After exposed to 2% H₂, Au@Pd (16/2) NAs demonstrate a dramatic decrease in the optical extinction intensity, along with an obvious color change from turquoise to gray. Experimental results and theoretical calculations prove that the huge optical change resulted from the H₂-induced aggregation of Au@Pd nanoparticles (NPs), which remarkably alters the plasmon coupling effect between NPs. Moreover, we optimize the sensing behavior from two aspects. The first is selecting appropriate substrates (either rigid glass substrate or flexible polyethylene terephthalate substrate) to offer moderate adhesion force to NAs, ensuring an efficient aggregation of Au@Pd NPs upon H₂ exposure. The second is adjusting the Pd shell thickness to control the extent of NP aggregation and thus the detection range of the as-prepared sensors. This work highlights the advantage of designing eye-readable plasmonic H₂ sensors from the aspect of tuning the interparticle plasmonic coupling in NP assemblies. Au@Pd NAs presented here have several advantages in terms of simple fabrication method, eye-readability in air background at room temperature, tunable detection range, and high cost-effectiveness.

Morphological Evolution Trajectory of Multifaceted Palladium Nanoparticles

Precise control over the morphology and facets of Pd nanomaterials has great importance in catalytic and sensing applications. In this study, we synthesized Pd nanoparticles with multiple types of low-Miller-index-faceted morphologies by systematically defining the synthesis conditions of the seed-mediated colloidal preparation method. We discovered the morphological evolution of Pd nanoparticles by following the trajectory of the surface Miller indices, which were determined by the cooperative effects of cetyltrimethylammonium bromide and ascorbic acid. By precise control of the morphological trajectory, Pd nanoparticles with a new cuborhombicube morphology, composed of 36 facets and concave edges, were discovered. This study provides important insight into the design of the surface Miller indices and morphologies of functional nanomaterials.

Optics-mechanics synergistic fiber optic sensor for hydrogen detection

As a carbon-free energy carrier and an attractive alternative energy source, hydrogen energy has great development potential for future considerations, and it may be the ultimate answer to the global energy crisis. Due to the high combustibility of hydrogen, hydrogen sensors will be a vital component of safe use of hydrogen. Among the various sensors, the optical hydrogen sensor can meet the requirements of intrinsic safety, online detection, surrounding immunity, and lack of spark. Hence, we demonstrate a miniature optics-mechanics synergistic fiber optic hydrogen sensor by using Pd nanofilm, it has a large response range (0.5%-3.5%), high sensitivity of -0.334 nm/1% concentration and a short response time (10s)/recovery time (25s). Experimental results reveal that the proposed optics-mechanics synergistic fiber optic hydrogen sensor is reusable, durable, and low temperature sensitive. In this optics-mechanics synergistic fiber optic hydrogen sensor, nano Pd film with a large surface-to-volume ratio allows for rapid hydrogen dissociation, and Pd lattice expansion caused by Pd-hydrogen reaction is effectively transduced into optical change. This proposed sensor integrated Pd nanofilm with optical fiber by using an optics-mechanics synergistic strategy, resulting in a compact and all-optical solution for the safe measurement of hydrogen concentration, which can be used in hazardous or space-limited environments.

Wireless and Linear Hydrogen Detection up to 4% with High Sensitivity through Phase-Transition-Inhibited Pd Nanowires

Palladium (Pd) has been drawing increasing attention as a hydrogen (H₂) detecting material due to its highly selective sensitivity to H₂. However, at H₂ concentrations above 2%, Pd undergoes an inevitable phase transition, causing undesirable electrical and mechanical alterations. In particular, nonlinear gas response (ΔR/R₀) that accompanies phase transition has been a great bottleneck for detecting H₂ in high concentrations, which is especially important as there is a risk of explosion over 4% H₂. Here, we propose a phase-transition-inhibited Pd nanowire H₂ sensor that can detect up to 4% H₂ with high linearity and high sensitivity. Based on the calculation of the change in free energy, we designed Pd nanowires that are highly adhered to the substrate to withstand the stress that leads to phase transition. We theoretically optimized the Pd nanowire dimensions using a finite element method simulation and then experimentally fabricated the proposed sensor by exploiting a developed nanofabrication method. The proposed sensor exhibits a high sensing linearity (98.9%) with high and stable sensitivity (ΔR/R₀/[H₂] = 875%·bar^-1) over a full range of H₂ concentrations (0.1-4%). Using the fabricated Pd sensors, we have successfully demonstrated a wireless sensor module that can detect H₂ with high linearity, notifying real-time H₂ leakage through remote communication. Overall, our work suggests a nanostructuring strategy for detecting H₂ with a phase-transition-inhibited pure Pd H₂ sensor with rigorous scientific exploration.

Graphene-Promoted Adhesion-Reduced Expansion of Discontinuous Palladium Nanowires upon Hydrogenation

Monitoring conductivity changes of discontinuous palladium (Pd) nanostructures upon hydrogenation is becoming one of the most promising approaches toward hydrogen sensing. Development of sensors in this type has long been impeded due to strong ubiquitous interfacial adhesion which could distinctly restrict Pd expansion so as to hinder the closing of a nanogap. Herein, graphene underlayers were applied in the fabrication of nanogap-based hydrogen sensors to promote the lateral expansion of a Pd nanowire upon hydrogenation by reducing the adhesion between the metal and the substrate. In order to clarify details as well as mechanisms underlaid of graphene-enhanced Pd expansion, nanowire samples with serial lengths (6-48 μm) and gaps (0-260 nm in width) were controllably prepared on single-layer graphene (SLG), double-layer graphene (DLG), and quadruple-layer graphene (QLG, DLG × 2) via the combination of electron beam lithography (EBL) and electron beam deposition (EBD) technology. Response features and intrinsic analysis in physical sense of the graphene-based discontinuous Pd circuits upon hydrogen were established, in light of which the effects of underlayers on Pd expansion and on nanogap closing process were investigated. Such graphene-promoted expansion was demonstrated through the achievement of the closure of a large gap threshold (Gt) up to 260 nm as well as the systematical investigation of its influence on the sensing performance.

Ultralow detection limit and ultrafast response/recovery of the H2 gas sensor based on Pd-doped rGO/ZnO-SnO2 from hydrothermal synthesis

Hydrogen (H2) sensors are of great significance in hydrogen energy development and hydrogen safety monitoring. However, achieving fast and effective detection of low concentrations of hydrogen is a key problem to be solved in hydrogen sensing. In this work, we combined the excellent gas sensing properties of tin(IV) oxide (SnO2) and zinc oxide (ZnO) with the outstanding electrical properties of reduced graphene oxide (rGO) and prepared palladium (Pd)-doped rGO/ZnO-SnO2 nanocomposites by a hydrothermal method. The crystal structure, structural morphology, and elemental composition of the material were characterized by FE-SEM, TEM, XRD, XPS, Raman spectroscopy, and N2 adsorption-desorption. The results showed that the Pd-doped ZnO-SnO2 composites were successfully synthesized and uniformly coated on the surface of the rGO. The hydrogen gas sensing performance of the sensor prepared in this work was investigated, and the results showed that, compared with the pure Pd-doped ZnO-SnO2 sensor, the Pd-doped rGO/ZnO-SnO2 sensor modified with 3 wt% rGO had better hydrogen (H2)-sensing response of 9.4-100 ppm H2 at 380 °C. In addition, this sensor had extremely low time parameters (the response time and recovery time for 100 ppm H2 at 380 °C were 4 s and 8 s, respectively) and an extremely low detection limit (50 ppb). Moreover, the sensor exhibited outstanding repeatability and restoration. According to the analysis of the sensing mechanism of this nanocomposite, the enhanced sensing performance of the Pd-doped rGO/ZnO-SnO2 sensor is mainly due to the heterostructure of rGO, ZnO, and SnO2, the excellent electrical and physical properties of rGO and the synergy between rGO and Pd.

Noble Metal-Decorated Nanostructured Zinc Oxide: Strategies to Advance Chemiresistive Hydrogen Gas Sensing

Hydrogen (H₂ ) is known as the key player in the alternative and renewable energy revolution and henceforth H₂ production, transportation, storage and usage have been a major interest of current research. However, due to severe safety concerns, strategies are indispensable to devise superior H₂ sensors, particularly selective and sensitive H₂ sensors. In this personal account, three specific gas sensing constructs; zinc oxide (ZnO) nanostructures-, noble metal nanoparticles-decorated ZnO- and noble metal nanoparticles-decorated ZnO nanostructures on reduced graphene oxide (rGO)-based H₂ sensors have been demonstrated. The dynamic response and H₂ sensing characteristics of ZnO nanostructures-based H₂ sensors were found to be improved compared to those of pristine ZnO. High-resolution field emission scanning electron microscopy (FESEM) confirmed the flower-like nanostructures that had higher surface area around the nanoscale petals. The mechanism behind the superior sensing characteristics of ZnO nanostructures-based H₂ sensor has been demonstrated. Decoration of ZnO nanostructures with noble metal nanoparticles, particularly platinum (Pt) and gold (Au) was observed to be useful in achieving better H₂ sensing performance compared to that of ZnO nanostructures. The Pt- and Au-decorated ZnO nanostructures followed the well-known "Spill-over" mechanism in enhancing the H₂ sensing characteristics. Abundant free electrons/holes generation and higher conductivity are two important parameters for designing selective and sensitive gas sensors. In this context, a hybrid nanocomposite, rGO-ZnO has been developed and decorated with noble metal nanoparticles, particularly Pt and Au. The ultimate sensing material has been characterized and compared to those of pristine ZnO, ZnO nanostructures and Pt- and Au-decorated ZnO for H₂ gas sensing applications. Such systemic and focus strategies is critical not only for developing efficient H₂ gas sensors but also for better understanding the mechanisms underlying such superior performance.

Highly Responsive Pd-Decorated MoO3 Nanowall H2 Gas Sensors Obtained from In-Situ-Controlled Thermal Oxidation of Sputtered MoS2 Films

Among transition metal oxides, MoO₃ is a promising material due to its layered structure and different oxidation states, making it suitable for different device applications. One of the methods used to grow MoO₃ is radio frequency magnetron sputtering (RFMS), which is the most compatible method in industry. However, obtaining nanostructures by RFMS for metal oxides is challenging because of compact morphology film formation. In this study, α-MoO₃ with vertical nanowalls is obtained by a two-step process; deposition of magnetron-sputtered MoS₂ vertical nanowalls and postoxidation of these structures without changing the morphology. In situ transmittance and electrical measurements are performed to control the oxidation process, which shed light on understanding the oxidation of MoS₂ nanowalls. The transition from MoS₂ to α-MoO₃ is investigated with partially oxidized MoS₂/MoO₃ samples with different thicknesses. It is also concluded that oxidation starts from nanowalls perpendicular to the substrate and lasts with oxidation of basal planes. Four different thicknesses of α-MoO₃ nanowall samples are fabricated for H₂ gas sensors. Also, the effect of Pd deposition on the H₂-sensing properties of sensors is deeply investigated. An outstanding response of 3.3 × 10⁵ as well as the response and recovery times of 379 and 304 s, respectively, are achieved from the thinnest Pd-loaded sample. Also, the gas-sensing mechanism is explored by gasochromic measurements to investigate the sensor behaviors under the conditions of dry air and N₂ gas as the carrier gas.

Metal Nanocluster-Metal Organic Framework-Polymer Hybrid Nanomaterials for Improved Hydrogen Detection

The development of hydrogen sensors is of paramount importance for timely leak detection and remains a crucial unmet need. Palladium-based materials, well known as hydrogen sensors, still suffer from poisoning and deactivation. Here, a hybrid hydrogen sensor consisting of a Pd nanocluster (NC) film, a metal-organic framework (MOF), and a polymer, are proposed. The polymer coating, as a protection layer, endows the sensor with excellent H₂ selectivity and CO-poisoning resistance. The MOF serves as an interface layer between the Pd NC film and the polymer layer, which alters the nature of the interaction with hydrogen and leads to significant sensing performance improvements, owing to the interfacial electronic coupling between Pd NCs and the MOF. The strategy overcomes the shortcomings of retarded response speed and degraded sensitivity induced by the polymer coating of a Pd NC film-polymer hybrid system. This is the first exhibition of a hydrogen-sensing enhancement mechanism achieved by engineering the electronic coupling between Pd and a MOF. The work establishes a deep understanding of the hydrogen-sensing enhancement mechanism at the nanoscale and provides a feasible strategy to engineer next-generation gas-sensing nanodevices with superior sensing figures of merit via hybrid material systems.

Plasmonic Hydrogen Sensors

Hydrogen is regarded as the ultimate fuel and energy carrier with a high theoretical energy density and universality of sourcing. However, hydrogen is easy to leak and has a wide flammability range in air. For safely handling hydrogen, robust sensors are in high demand. Plasmonic hydrogen sensors (PHS) are attracting growing interest due to the advantages of high sensitivity, fast response speed, miniaturization, and high-degree of integration, etc. In this review, the mechanism and recent development (mainly after the year 2015) of hydrogen sensors based on plasmonic nanostructures are presented. The working principle of PHS is introduced. The sensing properties and the effects of resonance mode, configuration, material, and structure of the plasmonic nanostructures on the sensing performances are discussed. The merit and demerit of different types of plasmonic nanostructures are summarized and potential development directions are proposed. The aim of this review is not only to clarify the current strategies for PHS, but also to give a comprehensive understanding of the working principle of PHS, which may inspire more ingenious designs and execution of plasmonics for advanced hydrogen sensors.

In Situ TEM Technique Revealing the Deactivation Mechanism of Bimetallic Pd-Ag Nanoparticles in Hydrogen Sensors

Bimetallic Pd-Ag alloy nanoparticles exhibit satisfactory H₂-sensing improvements and show application potential for H₂ sensor construction. However, the long-term stability of the H₂ sensor with Pd-Ag nanoparticles as the catalyst is found to dramatically decrease during operation. Herein, gas-cell in situ transmission electron microscopy (TEM) is used to investigate the failure mechanisms of Pd-Ag nanoparticles under operation conditions. Based on the in situ TEM results, the Pd-Ag nanoparticles have two failure mechanisms: particles coalescence at 300 °C and phase segregation at 500 °C. Guided by the failure mechanisms, the H₂ sensor is comprehensively optimized based on the working temperature and the amount of Pd-Ag alloy nanoparticles. The optimized sensor exhibits satisfactory H₂-sensing properties, and the response decline of the sensor after 1 month is negligible. The revealing of the failure mechanisms with in situ TEM technology provides a valuable route for developing gas sensors with high long-term stability.

Controllable Fabrication of PdO-PdAu Ternary Hollow Shells: Synergistic Acceleration of H2 -Sensing Speed via Morphology Regulation and Electronic Structure Modulation

Designing ultrafast H₂ sensors is of particular importance for practical applications of hydrogen energy but still quite challenging. Herein, PdO decorated PdAu ternary hollow shells (PdO-PdAu HSs) exhibiting an ultrafast response of ≈0.9 s to 1% H₂ in air at room temperature are presented. PdO-PdAu HSs are fabricated by calcinating PdAu bimetallic HSs in air to form PdO-Au binary HSs, which are then partially reduced by NaBH₄ solution, forming PdO-PdAu HSs. This ternary hybrid material takes advantage of multiple aspects to synergistically accelerate the sensing speed. The HS morphology promises high gas accessibility and high surface area for H₂ adsorption, and decoration of Au and PdO alters the electronic state of Pd and reduces the energy barrier for hydrogen diffusing from the surface site of Pd into the subsurface site. The content of Au and PdO in the ternary HSs can be simply tuned, which offers the possibility to optimize their promotion effects to reach the best performance. The proposed fabrication strategy sheds light on the rational design of ultrafast Pd-based H₂ sensors by controlling the sensor structure and engineering the electronic state of active species.

Wafer-scale, highly uniform, and well-arrayed suspended nanostructures for enhancing the performance of electronic devices

Suspended nanostructures play an important role in enhancing the performance of a diverse group of nanodevices. However, realizing a good arrangement and suspension for nanostructures of various shapes remains a significant challenge. Herein, a rapid and simple method for fabricating wafer-scale, highly uniform, well-arrayed suspended nanostructures via nanowelding lithography is reported. Suspended nanostructures with various shapes (nanowires, nanoholes, nanomesh, and nanofilms) and materials (gold, silver, and palladium metals) were employed to demonstrate the applicability of our method. Moreover, gas sensors and thermoacoustic speakers with suspended nanowires outperformed those with unsuspended nanostructures. The proposed method is expected to help advance the development of future nanodevices based on suspended nanostructures.

Reaction of H2 with polyoxometalate supported Rhodium(0) and Iridium(0) nanoparticles in aqueous suspensions: a kinetic study

The mechanism of the reaction between Rh0 and Ir0 NPs with H2 was measured in the absence of an electrical bias via monitoring the catalytic reduction of PW12O403− and it was compared to the previous results of Pt0 NPs.

Synthesis of a Pd2In-Pd3In bi-continuous nanoporous structure by electrochemical dealloying in molten salts

A study of the high-temperature electrochemical selective dissolution of PdIn intermetallics in molten mixtures of alkali chlorides with the formation of nanoporous structures has been carried out to obtain a coherent structure consisting of Pd₂In and Pd₃In intermetallics. The smallest pore size (approximately 100 nanometers) with a bi-continuous structure of Pd₂In-Pd₃In is obtained from the PdIn intermetallic phase at a temperature of 450 °C and a current density of 50 mA cm^-2 in a molten LiCl-KCl eutectic. It has been shown that the temperature of the anodic dissolution process is the most important factor in controlling the pore size and structural morphology. The morphology of pores and ligaments in the sample at 600 °C is a 3D hierarchy with pore sizes from several hundred nanometers to a micron-scale but with the same Pd₂In-Pd₃In (2 : 1) composition.

Suppression of the Phase Coexistence of the fcc-fct Transition in Hafnium-Hydride Thin Films

Metal hydrides may play a paramount role in a future hydrogen economy. While most applications are based on nanostructured and confined materials, studies considering the structural response of these materials to hydrogen concentrate on bulk material. Here, using in situ in- and out-of-plane X-ray diffraction and reflectometry, we study the fcc ↔ fct transition in Hf thin films, an optical hydrogen-sensing material. We show that the confinement of Hf affects this transition: compared to bulk Hf, the transition is pushed to a higher hydrogen-to-metal ratio, the tetragonality of the fct phase is reduced, and phase coexistence is suppressed. These nanoconfinement effects ensure the hysteresis-free response of hafnium to hydrogen, enabling its remarkable performance as a hydrogen-sensing material. In a wider perspective, the results highlight the profound influences of the nanostructuring and nanoconfinement of metal hydrides on their structural response to hydrogen with a significant impact on their applicability in future devices.

Thin Film and Nanostructured Pd-Based Materials for Optical H2 Sensors: A Review

In this review paper, we provide an overview of state-of-the-art Pd-based materials for optical H2 sensors. The first part of the manuscript introduces the operating principles, providing background information on the thermodynamics and the primary mechanisms of optical detection. Optical H2 sensors using thin films (i.e., films without any nanostructuring) are discussed first, followed by those employing nanostructured materials based on aggregated or isolated nanoparticles (ANPs and INPs, respectively), as well as complex nanostructured (CN) architectures. The different material types are discussed on the basis of the properties they can attribute to the resulting sensors, including their limit of detection, sensitivity, and response time. Limitations induced by cracking and the hysteresis effect, which reduce the repeatability and reliability of the sensors, as well as by CO poisoning that deteriorates their performance in the long run, are also discussed together with an overview of manufacturing approaches (e.g., tailoring the composition and/or applying functionalizing coatings) for addressing these issues.

Enhancing the Hydrogen-Sensing Performance of p-Type PdO by Modulating the Conduction Model

The implementation of the p-type metal oxide semiconductor (MOS) in modern sensing systems requires a strategy to effectively enhance its inherent low response. However, for p-type MOS sensors, conventional methods such as catalyst nanoparticle (NP) decoration and grain size regulation do not work as effectively as they do for n-type MOS sensors, which is basically due to the fact that the p-type MOS adopts an unfavorable parallel conduction model. Herein, taking Au@PdO as an example, we demonstrate that the conduction model of the p-type MOS can be manipulated into the series conduction model by inserting a high-conductive metallic core into less-conductive p-type MOS NPs. This unique series conduction model makes the sensor response of Au@PdO nanoparticle arrays (NAs) very sensitive to the catalyst NP decoration as well as the change of structural parameters. For example, Au@PdO NAs demonstrate an ∼9000 times increase in sensor response when decorated with Pd NPs, whereas there is only ∼100 times increase for PdO NAs. This greatly improved response value outperforms all previously reported PdO-based (and most other p-type semiconductor-based) H2 sensors, which helps the obtained sensor to achieve an ultralow detection limit of ∼0.1 ppm at room temperature. Additionally, Au@PdO NAs inherit the high surface reactivity and gas adsorption property of p-type PdO. As a result, the as-prepared sensor exhibits high humidity-resistive property and excellent selectivity. This work provides a new strategy to significantly enhance the sensing performance of p-type gas sensors by manipulating their conduction model.

Hydrogenation Kinetics of Metal Hydride Catalytic Layers

Catalyzing capping layers on metal hydrides are employed to enhance the hydrogenation kinetics of metal hydride-based systems such as hydrogen sensors. Here, we use a novel experimental method to study the hydrogenation kinetics of catalyzing capping layers composed of several alloys of Pd and Au as well as Pt, Ni, and Ru, all with and without an additional PTFE polymer protection layer and under the same set of experimental conditions. In particular, we employ a thin Ta film as an optical indicator to study the kinetics of the catalytic layers deposited on top of it and which allows one to determine the absolute hydrogenation rates. Our results demonstrate that doping Pd with Au results in significantly faster hydrogenation kinetics, with response times up to five times shorter than Pd through enhanced diffusion and a reduction in the activation energy. On the other hand, the kinetics of non-Pd-based materials turn out to be significantly slower and mainly limited by the diffusion through the capping layer itself. Surprisingly, the additional PTFE layer was only found to improve the kinetics of Pd-based capping materials and has no significant effect on the kinetics of Pt, Ni, and Ru. Taken together, the experimental results aid in rationally choosing a suitable capping material for the application of metal hydrides and other materials in a hydrogen economy. In addition, the used method can be applied to simultaneously study the hydrogenation kinetics in thin-film materials for a wide set of experimental conditions.

Accelerating Hydrogen Absorption and Desorption Rates in Palladium Nanocubes with an Ultrathin Surface Modification

Exploiting the high surface-area-to-volume ratio of nanomaterials to store energy in the form of electrochemical alloys is an exceptionally promising route for achieving high-rate energy storage and delivery. Nanoscale palladium hydride is an excellent model system for understanding how nanoscale-specific properties affect the absorption and desorption of energy carrying equivalents. Hydrogen absorption and desorption in shape-controlled Pd nanostructures does not occur uniformly across the entire nanoparticle surface. Instead, hydrogen absorption and desorption proceed selectively through high-activity sites at the corners and edges. Such a mechanism hinders the hydrogen absorption rates and greatly reduces the benefit of nanoscaling the dimensions of the palladium. To solve this, we modify the surface of palladium with an ultrathin platinum shell. This modification nearly removes the barrier for hydrogen absorption (89 kJ/mol without a Pt shell and 1.8 kJ/mol with a Pt shell) and enables diffusion through the entire Pd/Pt surface.

2-step reaction kinetics for hydrogen absorption into bulk material via dissociative adsorption on the surface

We have demonstrated that the process of hydrogen absorption into a solid experimentally follows a Langmuir-type (hyperbolic) function instead of Sieverts law. This can be explained by independent two theories. One is the well-known solubility theory which is the basis of Sieverts law. It explains that the amount of hydrogen absorption can be expressed as a Langmuir-type (hyperbolic) function of the square root of the hydrogen pressure. We have succeeded in drawing the same conclusion from the other theory. It is a 2-step reaction kinetics (2sRK) model that expresses absorption into the bulk via adsorption on the surface. The 2sRK model has an advantage to the solubility theory: Since it can describe the dynamic process, it can be used to discuss both the amount of hydrogen absorption and the absorption rate. Some phenomena with absorption via adsorption can be understood in a unified manner by the 2sRK model.

Grain-growth mediated hydrogen sorption kinetics and compensation effect in single Pd nanoparticles

Grains constitute the building blocks of polycrystalline materials and their boundaries determine bulk physical properties like electrical conductivity, diffusivity and ductility. However, the structure and evolution of grains in nanostructured materials and the role of grain boundaries in reaction or phase transformation kinetics are poorly understood, despite likely importance in catalysis, batteries and hydrogen energy technology applications. Here we report an investigation of the kinetics of (de)hydriding phase transformations in individual Pd nanoparticles. We find dramatic evolution of single particle grain morphology upon cyclic exposure to hydrogen, which we identify as the reason for the observed rapidly slowing sorption kinetics, and as the origin of the observed kinetic compensation effect. These results shed light on the impact of grain growth on kinetic processes occurring inside nanoparticles, and provide mechanistic insight in the observed kinetic compensation effect.

Grains are the building blocks of crystalline solids. Here the authors show how hydrogen-sorption induced grain-growth in Pd nanoparticles slows down the hydrogen sorption kinetics and constitutes the physical origin of corresponding kinetic compensation.

Highly Permeable Fluorinated Polymer Nanocomposites for Plasmonic Hydrogen Sensing

Hydrogen (H₂) sensors that can be produced en masse with cost-effective manufacturing tools are critical for enabling safety in the emerging hydrogen economy. The use of melt-processed nanocomposites in this context would allow the combination of the advantages of plasmonic hydrogen detection with polymer technology; an approach which is held back by the slow diffusion of H₂ through the polymer matrix. Here, we show that the use of an amorphous fluorinated polymer, compounded with colloidal Pd nanoparticles prepared by highly scalable continuous flow synthesis, results in nanocomposites that display a high H₂ diffusion coefficient in the order of 10^-5 cm² s^-1. As a result, plasmonic optical hydrogen detection with melt-pressed fluorinated polymer nanocomposites is no longer limited by the diffusion of the H₂ analyte to the Pd nanoparticle transducer elements, despite a thickness of up to 100 μm, thereby enabling response times as short as 2.5 s at 100 mbar (≡10 vol. %) H₂. Evidently, plasmonic sensors with a fast response time can be fabricated with thick, melt-processed nanocomposites, which paves the way for a new generation of robust H₂ sensors.