Palladium Nanosheet-Based Dual Gas Sensors for Sensitive Room-Temperature Hydrogen and Carbon Monoxide Detection

Atomic Layer-Modified 3D Pd Nanochannels for High-Performance Hydrogen Sensing

Palladium (Pd), known for its excellent H2 adsorption properties and ability to form palladium hydride (PdHx), is extensively utilized as a key material in hydrogen (H2) sensing technologies. Nevertheless, conventional Pd-based H2 sensors have shown limited performance enhancements due to challenges in precisely controlling the microscopic interfaces between Pd nanograins, which determine the total resistance signal of the sensors. This limitation arises from the lack of a technique capable of precisely manipulating these interfaces at the atomic level. In this study, we develop an atomic layer etching (ALE) technique to enhance the performance of Pd-based H2 sensors by enabling precise atomic-scale control over the surface of Pd nanochannels. We fabricated 3D Pd nanopatterns with ultrasmall grain sizes through a top-down nanolithography process, followed by an ALE process that achieved atomic-level precision (10 Å resolution) without compromising material crystallinity. Our two-step ALE process, comprising surface modification with Cl2 plasma and removal with NH3 ligand addition, enables uniform etching across a 4 in. wafer with less than 1% variation in etch per cycle (EPC). This atomic-level modulation of Pd nanochannels resulted in significantly enhanced H2 sensitivity, demonstrating a maximum 130-fold increase in response to 1% H2 concentration compared to nonatomically controlled sensors. Such substantial enhancement has been difficult to achieve through conventional structural tuning methods and is attributed to the maximized volume change of PdHx resulting from the expanded gaps between Pd grains. This platform provides a promising avenue for developing high-performance H2 sensors and other noble-metal-based applications requiring atomic-level structural precision.

Pd nanoparticles-functionalized In2O3 based gas sensor for highly selective detection of toluene

Given the threat posed by toluene to human health and environmental safety, real-time and efficient detection of toluene assumes paramount importance. However, the low chemical reactivity and structural similarity of benzene, toluene, and xylene (BTX) gases impede the attainment of highly selective toluene detection. Herein, palladium-loaded indium oxide nanospheres were successfully synthesized through a combination of solvothermal and post-reduction methods. And the sensor based on 0.75 wt% Pd-In2O3 exhibits the response to the concentration of 100 ppm toluene (Ra/Rg = 21) that is approximately four times better compared to pure indium oxide (Ra/Rg = 4) at their respective optimum operating temperatures. Moreover, this sensor exhibited enhanced sensing performance towards toluene, including a low operating temperature of 160 °C, exceptional selectivity, and good stability. Furthermore, an investigation into the sensing mechanism of toluene by the Pd-In2O3-based sensor was conducted. The chemical and electron sensitization effects of palladium result in the more chemisorbed oxygen of the sensing material, which improves the toluene sensing performance by enhancing the reaction with more toluene molecules. Additionally, the moderate catalytic activation of toluene by palladium plays a crucial role in improving the selectivity. Overall, this work provides a basis for the rational design of metal oxide semiconductor sensors with catalytic properties for the highly selective detection of toluene.

Relationship between Density Changes and Electrical Properties of Chemically Self-Assembled Monolayer Single-Walled Carbon Nanotube Networks by Controlling Anchoring Density

Single-walled carbon nanotubes (SWNTs) are valued for their high carrier mobility, tunable band gaps, and strong mechanical properties, making them promising for electronic applications. However, the presence of metallic SWNTs in mixtures impairs device performance, requiring the isolation of semiconducting SWNTs (sc-SWNTs). Conjugated polymer wrapping is a leading technique for this selective separation owing to its simplicity and high selectivity; however, challenges persist in achieving optimal SWNT density, uniformity, and reproducibility. In this study, chemically self-assembled monolayer sc-SWNTs are fabricated using a click reaction on prepatterned alkyne-functional adhesion layers. We elucidated the effect of variations in the azide content on the sc-SWNT selectivity, SWNT number density, uniformity, and network distribution, as well as its subsequent effect on the field-effect transistor (FET) performance. In addition, we propose gradually reducing azide functionalization in wrapping polymer side chains to enhance the sc-SWNT selectivity while maintaining effective chemical self-assembly. The sc-SWNT purity, film density, and FET performance were significantly improved when the azide content was reduced to a certain level. This study offers a pathway to enhance sc-SWNT selectivity, purity, and device performance via azide functionalization optimization, advancing the commercialization of SWNT-based electronics.

The Selective and Sensitive Fluorogenic Detection of Hydrogen Gas Using an Azomethine-H Dye

Hydrogen (H2(g)) is a viable green fossil fuel alternative, as its combustion yields only water and energy. However, H2(g) is highly flammable, explosive, and lacks odor. These characteristics warrant sensitive and specific detection methods for its widespread use as an alternative fuel source. Recently, there has been growing interest in the development of H2(g) sensors, particularly those that are easy to use, environmentally friendly, and sensitive. Here, we show the first example of an optical fluorogenic hydrogen sensing platform, which employs a readily available dye azomethine-H (Az-H, 4-hydroxy-5-(2-hydroxy-benzylideneamino)-naphthalene-2,7-disulfonic acid) and a hydrogen-transferring compound [{Ir(Cp*)(Cl)}2(thbpym)](Cl)2 (IrCp*, (Cp* = C5Me5-, thbpym = 4,4',6,6'-tetrahydroxy-2,2'-bipyrimidine)) to engineer H2(g) gas selectivity with high sensitivity at room temperature and pressure. This system yields ∼47-fold fluorescence enhancement when exposed to H2(g) in aqueous solution or ∼2.4-fold in a carboxymethyl cellulose (CMC) hydrogel matrix, with an estimated detection limit of ∼0.5% H2(g) with no cross-reactivity observed for potentially contaminating gases such as nitrogen (N2(g)), oxygen (O2(g)), or air.

Recent advances in nanomaterial-enabled chemiresistive hydrogen sensors

With the growing adoption of hydrogen energy and the rapid advancement of Internet of Things (IoT) technologies, there is an increasing demand for high-performance hydrogen gas (H2) sensors. Among various sensor types, chemiresistive H2 sensors have emerged as particularly promising due to their excellent sensitivity, fast response times, cost-effectiveness, and portability. This review comprehensively examines the recent progress in chemiresistive H2 sensors, focusing on developments over the past five years in nanostructured materials such as metals, metal oxide semiconductors, and emerging alternatives. This review delves into the underlying sensing mechanisms, highlighting the enhancement strategies that have been employed to improve sensing performance. Finally, current challenges are identified, and future research directions are proposed to address the limitations of existing chemiresistive H2 sensor technologies. This work provides a critical synthesis of the most recent advancements, offering valuable insights into both current challenges and future directions. Its emphasis on innovative material designs and sensing strategies will significantly contribute to the ongoing development of next-generation H2 sensors, fostering safer and more efficient energy applications.

Long-term reliable wireless H2 gas sensor via repeatable thermal refreshing of palladium nanowire

The increasing significance of hydrogen (H2) gas as a clean energy source has prompted the development of high-performance H2 gas sensors. Palladium (Pd)-based sensors, with their advantages of selectivity, scalability, and cost-effectiveness, have shown promise in this regard. However, the long-term stability and reliability of Pd-based sensors remain a challenge. This study not only identifies the exact cause for performance degradation in palladium (Pd) nanowire H2 sensors, but also implements and optimizes a cost-effective recovery method. The results from density functional theory (DFT) calculations and material analysis confirm the presence of C = O bonds, indicating performance degradation due to carbon dioxide (CO2) accumulation on the Pd surface. Based on the molecular behavior calculation in high temperatures, we optimized the thermal treatment method of 200 °C for 10 minutes to remove the C = O contaminants, resulting in nearly 100% recovery of the sensor's initial performance even after 2 months of contamination.

Hydrogen Sensing with Palladium-Based Materials: Mechanisms, Challenges, and Opportunities

Palladium morphologies are prominently used in Hydrogen gas sensing applications owing to their unique characteristics and properties. In this review article, Palladium nanoparticles, thin films, and alloys were designated as the scope of Palladium morphologies. The aim of this review article is to explore Hydrogen sensing using Palladium, focusing on the recent advancements in the field.. The principles underlying Hydrogen sensing mechanisms with Palladium are discussed initially, highlighting the unique properties of Palladium that make it a promising material for this purpose. Special attention is given to the surface interactions and structural modifications that influence the sensitivity and selectivity of Palladium-based sensors The study also addresses key challenges and recent innovations in the field which contribute to the enhancement of Palladium-based Hydrogen sensing capabilities. The current state of research is critically examined to identify gaps in knowledge and future research directions are highlighted. The prospects and challenges associated with the use of Palladium for Hydrogen sensing, emphasizing its pivotal role in advancing sensor technologies for Hydrogen detection are also discussed.

Study on the Gas-Chromic Character of Pd/TiO2 for Fast Room-Temperature CO Detection

As a widely used support, TiO2 has often been combined with Pd to form highly sensitive gas-chromic materials. Herein, we prepared a series of Pd/TiO2 catalysts with different Pd content (from 0.1 to 5 wt.%) by the impregnation method for their utilization in fast room-temperature CO detection. The detection was simply based on visible color change when the Pd/TiO2 was exposed to CO. The sample with 1 wt.% Pd/TiO2 presented an excellent CO gasochromic character, associated with a maximum chromatic aberration value of 90 before and after CO exposure. Systematic catalyst characterizations of XPS, FT-IR, CO-TPD, and N2 adsorption-desorption and density functional theory calculations for the CO adsorption and charge transfer over the Pd and PdO surfaces were further carried out. It was found that the interaction between CO and the Pd surface was strong, associated with a large adsorption energy of -1.99 eV and charge transfer of 0.196 e. The color change was caused by a reduction in Pd2+ to metallic Pd0 over the Pd/TiO2 surface after CO exposure.

Decorating Pd-Au Nanodots Around Porous In2O3 Nanocubes for Tolerant H2 Sensing Against Switching Response and H2S Poisoning

With the recently-booming hydrogen (H2) economy by green H2 as the energy carriers and the newly-emerged exhaled diagnosis by human organ-metabolized H2 as a biomarker, H2 sensing is simultaneously required with fast response, low detection limit, and tolerant stability against humidity, switching, and poisoning. Here, reliable H2 sensing has been developed by utilizing indium oxide nanocubes decorated with palladium and gold nanodots (Pd-Au NDs/In2O3 NCBs), which have been synthesized by combined hydrothermal reaction, annealing, and chemical bath deposition. As-prepared Pd-Au NDs/In2O3 NCBs are observed with surface-enriched NDs and nanopores. Beneficially, Pd-Au NDs/In2O3 NCBs show 300 ppb-low detection limit, 5 s-fast response to 500 ppm H2, 75%RH-high humidity tolerance, and 56 days-long stability at 280 °C. Further, Pd-Au NDs/In2O3 NCBs show excellent stability against switching sensing response, and are tolerant to H2S poisoning even being exposed to 10 ppm H2S at 280 °C. Such excellent H2 sensing may be attributed to the synergistic effect of the boosted Pd-Au NDs' spillover effect and interfacial electron transfer, increased adsorption sites over the porous NCBs' surface, and utilized Pd NDs' affinity with H2 and H2S. Practically, Pd-Au NDs/In2O3 NCBs are integrated into the H2 sensing device, which can reliably communicate with a smartphone.

Palladium Nanoparticles and Lung Health: Assessing Morphology-Dependent Subacute Toxicity in Rats and Toxicity Modulation by Naringin - Paving the Way for Cleaner Vehicular Emissions

The release of palladium nanoparticles (PdNPs) from autocatalytic converters has raised concerns regarding public health and the environment due to their emergence as anthropogenic contaminants. With growing vehicular population, there is an urgent need for comprehensive toxicological studies of PdNPs to mitigate their risk. The present study aims to investigate the effects of spherical PdNPs with average sizes of 20 and 80 nm, as well as Pd nanorods, on the lung function of female Wistar rats following oral exposure to environmentally relevant doses (1 and 10 μg/kg) over a period of 28 days. Various biological parameters were evaluated, including liver and kidney biochemical changes, lung oxidative stress markers (SOD, CAT, GSH, LPO), lung inflammatory markers (IL-1β, IL-8, IL-6, and TNF-α), and histopathological alterations in the lungs. Additionally, the potential mitigating effects of naringin on PdNPs-induced toxicity were examined. The results demonstrate a significant increase in oxidative stress, the onset of inflammation, and histological changes in lung alveolar sacs upon exposure to all tested particles. Specifically, 20@PdNPs and PdNRs exhibited higher cytotoxicity and pro-inflammatory properties compared to 80@PdNPs. Naringin effectively attenuated the pulmonary toxicity induced by PdNPs by modulating oxidative and inflammatory pathways. These findings contribute to the sustainable development of PdNPs for their future applications in the biomedical and environmental sectors, ensuring the advancement of safe and sustainable nanotechnology.

Surface modification of Co3O4 nanosheets through Cd-doping for enhanced CO sensing performance

The detection of hazardous CO gas is an important research content in the domain of the Internet of Things (IoT). Herein, we introduced a facile metal-organic frameworks (MOFs)-templated strategy to synthesize Cd-doped Co3O4 nanosheets (Cd-Co3O4 NSs) aimed at boosting the CO-sensing performance. The synthesized Cd-Co3O4 NSs feature a multihole nanomeshes structure and a large specific surface area (106.579 m2·g-1), which endows the sensing materials with favorable gas diffusion and interaction ability. Furthermore, compared with unadulterated Co3O4, the 2 mol % Cd-doped Co3O4 (2% Cd-Co3O4) sensor exhibits enhanced sensitivity (244%) to 100 ppm CO at 200 °C and a comparatively low experimental limit of detection (0.5 ppm/experimental value). The 2% Cd-Co3O4 NSs show good selectivity, reproducibility, and long-term stability. The improved CO sensitivity signal is probably owing to the stable nanomeshes construction, high surface area, and rich oxygen vacancies caused by cadmium doping. This study presents a facile avenue to promote the sensing performance of p-type metal oxide semiconductors by enhancing the surface activity of Co3O4 combined with morphology control and component regulation.

Ultrafast (∼0.6 s), Robust, and Highly Linear Hydrogen Detection up to 10% Using Fully Suspended Pure Pd Nanowire

The high explosiveness of hydrogen gas in the air necessitates prompt detection in settings where hydrogen is used. For this reason, hydrogen sensors are required to offer rapid detection and possess superior sensing characteristics in terms of measurement range, linearity, selectivity, lifetime, and environment insensitivity according to the publicized protocol. However, previous approaches have only partially achieved the standardized requirements and have been limited in their capability to develop reliable materials for spatially accessible systems. Here, an electrical hydrogen sensor with an ultrafast response (∼0.6 s) satisfying all demands for hydrogen detection is demonstrated. Tailoring structural engineering based on the reaction kinetics of hydrogen and palladium, an optimized heating architecture that thermally activates fully suspended palladium (Pd) nanowires at a uniform temperature is designed. The developed Pd nanostructure, at a designated temperature distribution, rapidly reacts with hydrogen, enabling a hysteresis-free response from 0.1% to 10% and durable characteristics in mechanical shock and repetitive operation (>10,000 cycles). Moreover, the device selectively detects hydrogen without performance degradation in humid or carbon-based interfering gas circumstances. Finally, to verify spatial accessibility, the wireless hydrogen detection system has been demonstrated, detecting and reporting hydrogen leakage in real-time within just 1 s.

Structure and Technological Parameters' Effect on MISFET-Based Hydrogen Sensors' Characteristics

The influence of structure and technological parameters (STPs) on the metrological characteristics of hydrogen sensors based on MISFETs has been investigated. Compact electrophysical and electrical models connecting the drain current, the voltage between the drain and the source and the voltage between the gate and the substrate with the technological parameters of the n-channel MISFET as a sensitive element of the hydrogen sensor are proposed in a general form. Unlike the majority of works, in which the hydrogen sensitivity of only the threshold voltage of the MISFET is investigated, the proposed models allow us to simulate the hydrogen sensitivity of gate voltages or drain currents in weak and strong inversion modes, taking into account changes in the MIS structure charges. A quantitative assessment of the effect of STPs on MISFET performances (conversion function, hydrogen sensitivity, gas concentration measurement errors, sensitivity threshold and operating range) is given for a MISFET with a Pd-Ta₂O₅-SiO₂-Si structure. In the calculations, the parameters of the models obtained on the basis of the previous experimental results were used. It was shown how STPs and their technological variations, taking into account the electrical parameters, can affect the characteristics of MISFET-based hydrogen sensors. It is noted, in particular, that for MISFET with submicron two-layer gate insulators, the key influencing parameters are their type and thickness. Proposed approaches and compact refined models can be used to predict performances of MISFET-based gas analysis devices and micro-systems.

Paper-Based Hydrogen Sensors Using Ultrathin Palladium Nanowires

Hydrogen (H₂), as a chemical energy carrier, is a cleaner alternative to conventional fossil fuels with zero carbon emission and high energy density. The development of fast, low-cost, and sensitive H₂ detection systems is important for the widespread adoption of H₂ technologies. Paper is an environment-friendly, porous, and flexible material with great potential for use in sustainable electronics. Here, we report a paper-based sensor for room-temperature H₂ detection using ultrathin palladium nanowires (PdNWs). To elucidate the sensing mechanism, we compare the performance of polycrystalline and quasi-single-crystalline PdNWs. The polycrystalline PdNWs showed a response of 4.3% to 1 vol % H₂ with response and recovery times of 4.9 and 10.6 s, while quasi-single-crystalline PdNWs showed a response of 8% to 1 vol % H₂ with response and recovery times of 9.3 and 13.0 s, respectively. The polycrystalline PdNWs show excellent selectivity, stability, and sensitivity, with a limit of detection of 10 ppm H₂ in air. The fast response of ultrathin polycrystalline PdNW paper-based sensors arises from the synergistic effects of their ultrasmall diameter, high-index surface facets, strain-coupled grain boundaries, and porous paper substrate. This paper-based sensor is one of the fastest chemiresistive H₂ sensors reported and is potentially orders of magnitude less expensive than current state-of-the-art H₂-sensing solutions. This brings low-cost, room-temperature chemiresistive H₂ sensing closer to the performance of ultrafast optical sensors and high-temperature metal oxide-based sensors.

Formation of biologically influenced palladium microstructures by Desulfovibrio desulfuricans and Desulfovibrio ferrophilus IS5

A range of Desulfovibrio spp. can reduce metal ions to form metallic nanoparticles that remain attached to their surfaces. The bioreduction of palladium (Pd) has been given considerable attention due to its extensive use in areas of catalysis and electronics and other technological domains. In this study we report, for the first time, evidence for Pd(II) reduction by the highly corrosive Desulfovibrio ferrophilus IS5 strain to form surface attached Pd nanoparticles, as well as rapid formation of Pd(0) coated microbial nanowires. These filaments reached up to 8 µm in length and led to the formation of a tightly bound group of interconnected cells with enhanced ability to attach to a low carbon steel surface. Moreover, when supplied with high concentrations of Pd (≥ 100 mmol Pd(II) g^-1 dry cells), both Desulfovibrio desulfuricans and D. ferrophilus IS5 formed bacteria/Pd hybrid porous microstructures comprising millions of cells. These three-dimensional structures reached up to 3 mm in diameter with a dose of 1200 mmol Pd(II) g^-1 dry cells. Under suitable hydrodynamic conditions during reduction, two-dimensional nanosheets of Pd metal were formed that were up to several cm in length. Lower dosing of Pd(II) for promoting rapid synthesis of metal coated nanowires and enhanced attachment of cells onto metal surfaces could improve the efficiency of various biotechnological applications such as microbial fuel cells. Formation of biologically stimulated Pd microstructures could lead to a novel way to produce metal scaffolds or nanosheets for a wide variety of applications.

Room-Temperature Semiconductor Gas Sensors: Challenges and Opportunities

Our demand for ubiquitous and reliable gas detection is spurring the design of intelligent and enabling gas sensors for the next-generation Internet of Things and Artificial Intelligence. The desire to introduce gas sensors everywhere is fueled by opportunities to create room-temperature semiconductor gas sensors with ultralow power consumption. In this Perspective, we provide an overview of the recent achievement of room-temperature gas sensors that have been translated from the advances in the design of the chemical and physical properties of low-dimensional semiconductor nanomaterials. The emergence of solution-processable nanomaterials opens up remarkable opportunities to integrate into high-performance and flexible room-temperature gas sensors by using low-temperature, large-area, solution-based methods instead of costly, high-vacuum, high-temperature device manufacturing processes. We review the fundamental factors which affect the receptor and transducer functions of semiconductor gas sensors. We also discuss challenges that must be addressed in the move to the continuous miniaturization and evolution of semiconductor gas sensors.

Humidity-Tolerant Room-Temperature Selective Dual Sensing and Discrimination of NH3 and NO Using a WS2/MWCNT Composite

Continuous detection of toxic and hazardous gases like nitric oxide (NO) and ammonia (NH₃) is needed for environmental management and noninvasive diagnosis of various diseases. However, to the best of our knowledge, dual detection of these two gases has not been previously reported. To address the challenge, we demonstrate the design and fabrication of low-cost NH₃ and NO dual gas sensors using tungsten disulfide/multiwall carbon nanotube (WS₂/MWCNT) nanocomposites as sensing channels which maintained their performance in a humid environment. The composite-based device has shown successful dual detection at temperatures down to 18 °C and relative humidity of 90%. For 0.1 ppm ammonia, it exhibited a p-type conduction with response and recovery times of 102 and 261 s, respectively; on the other hand, with NO (10 ppb, n-type), these times were 285 and 198 s, respectively. The device with 5 mg MWCNTs possesses a superior selectivity along with a relative response of ≈7% (5 ppb) and ≈5% (0.1 ppm) for NO and NH₃, respectively, at 18 °C. The response is less affected by relative humidity, and this is attributed to the presence of MWCNTs that are hydrophobic in nature. Upon simultaneous exposure to NO (5-10 ppb) and NH₃ (0.1-5 ppm), the response was dominated by NO, implying clear discrimination to the simultaneous presence of these two gases. We propose a sensing mechanism based on adsorption/desportion and accompanied charge transfer between the adsorbed gas molecules and sensing surface. The results suggest that an optimized weight ratio of WS₂ and MWCNTs could govern favorable sensing conditions for a particular gas molecule.

An Improved Humidity Sensor with GO-Mn-Doped ZnO Nanocomposite and Dimensional Orchestration of Comb Electrode for Effective Bulk Manufacturing

The measurement and control of humidity is a major challenge that affects the sensing properties of sensors used in high-precision equipment manufacturing industries. Graphene Oxide(GO)-based materials have been extensively explored in humidity sensing applications because of their high surface area and functional groups. However, there is a lack of effective bulk-manufacturing processes for the synthesis of 2D-based nanocomposites with comb electrodes. Moreover, water intercalation within the layers of 2D materials increases recovery time. This work demonstrates the enhanced sensing characteristics of a capacitive/resistive GO-MnZnO nanocomposite humidity sensor produced using a cost-effective single-pot synthesis process. The in-plane sensing layer consistently improves sensitivity and reduces response time for a wide range of relative humidity measurements (10% to 90%). Interdigitated gold electrodes with varying numbers of fingers and spacing were fabricated using photolithography on a Si/SiO₂ for a consistent sensor device platform. The choice of nanomaterials, dimension of the sensor, and fabrication method influence the performance of the humidity sensor in a controlled environment. GO nanocomposites show significant improvement in response time (82.67 times greater at 40% RH) and sensitivity (95.7 times more at 60% RH). The response time of 4.5 s and recovery time of 21 s was significantly better for a wider range of relative humidity compared to the reduced GO-sensing layer and ZnMnO. An optimized 6 mm × 3 mm dimension sensor with a 28-fingers comb was fabricated with a metal-etching process. This is one of the most effective methods for bulk manufacturing. The performance of the sensing layer is comparable to established sensing nanomaterials that are currently used in humidity sensors, and hence can be extended for optimal bulk manufacturing with minimum electrochemical treatments.