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

Single-step aerosol-based synthesis of nanostructured thin films for hydrogen sensing

This article describes a single-step method for synthesizing nanostructured materials using evaporation-condensation synthesis and inertial impaction of aerosol nanoparticles. The as-deposited films exhibit anisotropic vertical and horizontal sintering of their palladium nanoparticle building blocks, yielding vertical structures. The electrical conductivity of the films is stable and highly sensitive to the presence of hydrogen in the overlaying gas, at concentrations that range from a few hundreds parts per million to a few percent.

Tuning atomic-scale mixing of nanoparticles produced by atmospheric-pressure spark ablation

Nanoparticles (NPs) mixed at the atomic scale have been synthesized by atmospheric-pressure spark ablation using pairs of Pd and Hf electrodes. Gravimetric analysis of the electrodes showed that the fraction of each material in the resulting mixed NPs can be varied from ca. 15-85 at% to 85-15 at% by employing different combinations of electrode polarities and thicknesses. These results were also qualitatively corroborated by microscopy and elemental analysis of the produced NPs. When using pairs of electrodes having the same diameter, the material from the one at negative polarity was represented at a substantially higher fraction in the mixed NPs regardless of whether a pair of thin or thick electrodes were employed. This can be attributed to the higher ablation rate of the electrodes at the negative polarity, as already known from earlier experiments. When using electrodes of different diameters, the fraction of the element from the thinner electrode was always higher. This is because thinner electrodes are ablated more effectively due to, at least in part, the increased importance of the associated heat losses compared to its thicker counterpart. In those cases, the polarity of the electrodes had a significantly smaller effect. Overall, our results demonstrate, for the first time, that spark ablation can be used to control atomic scale mixing and thus produce alloyed NPs with compositions that can be tuned to a good extent by simply using different combinations of electrode diameters and polarities. This expands the capabilities of the technique for producing mixed nanoparticle building blocks of well-defined composition that are highly desired for a wide range of applications.

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.

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.

Metal oxide semiconducting nanomaterials for air quality gas sensors: operating principles, performance, and synthesis techniques

To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of  a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring.