Confined Synthesis of Noble Metal Clusters Assisted by Liquid Film for Photocatalytic CO2 Reduction

The important concept of confined synthesis is considered a promising strategy for the design and synthesis of definable nanostructured materials with controllable compositions and specific morphology, such as highly loaded single-atom catalysts capable of providing abundant active sites for photocatalytic reactions. In recent years, researchers have been working on developing new confined reaction systems and searching for new confined spaces. Here, we present for the first time the concept of a bubble liquid film as a novel confined space. The liquid film has a typical sandwich structure consisting of a water layer, sandwiched between the upper and lower surfactant layers, with the thickness of the intermediate water layer at the micro- and nanometer scales, which can serve as a good confinement. Based on the above understanding and combined with the photodeposition method, we successfully confined synthesized Ag/TiO2, Au/TiO2, and Pd/TiO2 photocatalysts in liquid film. By HAADF-STEM, it can be seen that the noble metal morphologies are all nanoclusters of about 1 nm and are highly uniformly dispersed on the TiO2 surface. Compared with photodeposition in solution, we believe that the surfactant molecular layer restricts a limited amount of precursor to the liquid film, avoiding the accumulation of noble metals and the formation of large particle size nanoparticles. The liquid film, meanwhile, restricts the migration path of noble metal precursors, allowing for thorough in situ photodeposition and enables the complete and uniform dispersion of noble metal precursors, greatly reducing the photodeposition time. The uniform loading of the three noble metals proved the universality of the method, and the catalysts showed high activity for photocatalytic CO2 reduction. The rates of reduction of CO2 to CO over the Ag/TiO2 photocatalytic reached 230 μmol g-1 h-1.This study provides a new idea for the expansion of the confined reaction system and a reference for the study of liquid film as the confined space.

Microheater Integrated Nanotube Array Gas Sensor for Parts-Per-Trillion Level Gas Detection and Single Sensor-Based Gas Discrimination

Real-time monitoring of health threatening gases for chemical safety and human health protection requires detection and discrimination of trace gases with proper gas sensors. In many applications, costly, bulky, and power-hungry devices, normally employing optical gas sensors and electrochemical gas sensors, are used for this purpose. Using a single miniature low-power semiconductor gas sensor to achieve this goal is hardly possible, mostly due to its selectivity issue. Herein, we report a dual-mode microheater integrated nanotube array gas sensor (MINA sensor). The MINA sensor can detect hydrogen, acetone, toluene, and formaldehyde with the lowest measured limits of detection (LODs) as 40 parts-per-trillion (ppt) and the theoretical LODs of ∼7 ppt, under the continuous heating (CH) mode, owing to the nanotubular architecture with large sensing area and excellent surface catalytic activity. Intriguingly, unlike the conventional electronic noses that use arrays of gas sensors for gas discrimination, we discovered that when driven by the pulse heating (PH) mode, a single MINA sensor possesses discrimination capability of multiple gases through a transient feature extraction method. These above features of our MINA sensors make them highly attractive for distributed low-power sensor networks and battery-powered mobile sensing systems for chemical/environmental safety and healthcare applications.

Ab Initio Studies of Work Function Changes of CO Adsorption on Clean and Pd-Doped ZnGa2O4(111) Surfaces for Gas Sensors

We performed first-principles calculations to study the adsorption of the CO molecules on both clean and Pd-doped ZnGa2O4(111) surfaces. The adsorption reaction and work function of the CO adsorption models were examined. The CO molecules on the clean and Pd-doped ZnGa2O4(111) surfaces exhibit maximum work function changes of −0.55 eV and −0.79 eV, respectively. The work function change of Pd-doped ZnGa2O4(111) for detecting CO is 1.43 times higher than that of the clean ZnGa2O4(111). In addition, the adsorption energy is also significantly reduced from −1.88 eV to −3.36 eV without and with Pd atoms, respectively. The results demonstrate ZnGa2O4-based gas sensors doped by palladium can improve the sensitivity of detecting CO molecules.

An Operando X-ray Absorption Spectroscopy Study on Sensing Characteristics of Vertically Aligned ZnO Thin Film for Methane Gas Sensors

In this work, a simple, facile growth approach for a vertically aligned ZnO thin film is fabricated and its application towards methane gas sensors is demonstrated. ZnO thin film was prepared by a combination of hydrothermal and sputtering methods. First, a ZnO seed layer was prepared on the substrate through a sputtering technique, then a ZnO nanorod was fabricated using a hydrothermal method. The surface morphology of the ZnO film was observed by scanning electron microscopy (SEM). A ZnO nanorod coated on the dense seed layer is clearly visible in the SEM image. The average size of the hexagonal-shaped ZnO rod was around 50 nm in diameter, with a thickness of about 1 mm. X-ray absorption near-edge structures (XANES) were recorded to characterize the structural properties of the prepared film. The obtained normalized Zn K-edge XANES of the film showed the characteristic features of ZnO, which agreed well with the standard ZnO sample. The measurement of Zn K-edge XANES was performed simultaneously with the sensing response. The results showed a good correlation between sensor response and ZnO structure under optimal conditions.

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.

Functionalization of Zinc Oxide Nanoflowers with Palladium Nanoparticles via Microwave Absorption for Room Temperature-Operating Hydrogen Gas Sensors in the ppb Level

Microwave-assisted functionalization of zinc oxide nanoflowers (ZnO NFs) with palladium nanoparticles (Pd NPs) is demonstrated to realize high-performance chemiresistive-type hydrogen (H₂) gas sensors operating at room temperature (RT). The developed gas sensors exhibit a high response of up to 70% at 50 ppm and a theoretical detection limit of 10 ppb. The formation of ZnO NFs with an enhanced specific surface area and their functionalization with Pd NPs are investigated through various characterizations. Furthermore, the optimization of microwave absorption upon the structural incorporations between nanostructures (NF-NPs) is investigated for solution-based functionalization at low temperatures (below 120 °C) for short process times (within 1 min), compared to the conventional thermal annealing at 250 °C for 1 h. Highly sensitive and selective ZnO-based gas sensors enabling the detection of H₂ gas molecules at 300 ppb concentration at RT exhibit a short response/recovery time of below 3 min and a good selectivity toward different gases including nitric oxide, carbon monoxide, and oxygen. The successful functionalization of nanostructured metal oxide semiconductors (MOSs) with metal NPs via effective and practical microwave absorption enhances the potential on highly sensitive and selective chemiresistive-type MOS-based gas sensors operating at RT without additional heaters or photogenerators.

The 1,2-propanediol-sensing properties of one-dimension Tb2O3-modified ZnO nanowires synthesized by water-glycerol binary thermal route

One-dimension Tb₂O₃-modified ZnO nanowires were synthesized via water-glycerol binary thermal route. The X-ray diffraction (XRD) patterns demonstrated that the Tb₂O₃-modified ZnO was pure phase with high crystallinity. The Energy Dispersive Spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS) confirmed the chemical compositions of Tb₂O₃-modified ZnO. The images of field-emission electron microscopy (FESEM) indicated that the Tb₂O₃-modified ZnO was one-dimension nanowires with a diameter of ∼40-100 nm. N₂ adsorption/desorption measurements and BET analysis were used to revealed the specific surface area and pore size of Tb₂O₃-modified ZnO. The images of Transmission Electron Microscope (TEM) and High-Resolution Transmission Electron Microscopy (HRTEM) further showed that the highly uniform heterojunctions had been successfully obtained. The sensitivity and response/recovery time of 3 at% Tb₂O₃-modified ZnO tested at 200 °C were ∼123 and 73s/50s to 50 ppm 1,2-propanediol, respectively. In addition, the detection limit was as low as 1 ppm. Under UV irradiation, the sensitivity was further improved to 152 while the response/recovery time was shortened to 67s/23s. The morphology of one-dimension Tb₂O₃-modified ZnO nanowires, the increase of oxygen-deficient region in ZnO and the formation of p-n heterojunction enhanced the properties of 1,2-propanediol-sensing synergistically.

Electrospun ZnO/Pd Nanofibers: CO Sensing and Humidity Effect

Variable air humidity affects the characteristics of semiconductor metal oxides, which complicates the reliable and reproducible determination of CO content in ambient air by resistive gas sensors. In this work, we determined the sensor properties of electrospun ZnO and ZnO/Pd nanofibers in the detection of CO in dry and humid air, and investigated the sensing mechanism. The microstructure of the samples, palladium content, and oxidation state, type, and concentration of surface groups were characterized using complementary techniques: X-ray fluorescent spectroscopy, XRD, high-resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray (EDX) mapping, XPS, and FTIR spectroscopy. The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100-450 °C in the concentration range of 5-15 ppm CO in dry (RH₂₅ = 0%) and humid (RH₂₅ = 60%) air. It was found that under humid conditions, ZnO completely loses its sensitivity to CO, while ZnO/Pd retains a high sensor response. On the basis of in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) results, it was concluded that high sensor response of ZnO/Pd nanofibers in dry and humid air was due to the electronic sensitization effect, which was not influenced by humidity change.

Pd-Decorated PdO Hollow Shells: A H2-Sensing System in Which Catalyst Nanoparticle and Semiconductor Support are Interconvertible

Developing a simple strategy to fabricate high-performance hydrogen sensors with long-term stability remains quite challenging. Here, we report the H₂-sensing performance of Pd-decorated PdO hollow shells (Pd/PdO HSs). In this novel system, the catalyst nanoparticles (Pd NPs) and semiconductor support (PdO) are interconvertible, which is different from traditional hydrogen-sensing systems such as Pd/TiO₂ and Pd/ZnO. This Pd/PdO system exhibits multiple unique properties. First, well-distributed Pd NPs with controllable density can be decorated on PdO support through a one-step NaBH₄ treatment during which PdO is partially reduced into Pd. Second, the decorated Pd NPs are physically inlaid in the PdO support, which not only prevents the agglomeration or detachment of Pd NPs but also enhances the electron transfer between Pd NPs and PdO. Third, Pd/PdO HSs can be reoxidized into PdO HSs once their sensing performance degrades, which repeatedly manipulates Pd/PdO HSs under the initial reduction process, leading to the reactivation of the sensing performance. With all these advantages, Pd/PdO HSs demonstrate a detection limit lower than 1 ppm, a response/recovery time to 1% H₂ of 5 s/32 s at room temperature, and a repeatable reactivation ability. The strategy presented here is convenient and time saving and has no need to prefunctionalize the PdO surface for the decoration of catalyst NPs. Moreover, the unique reactivation ability of Pd/PdO system opens a new strategy toward extending the lifetime of H₂ sensors.

Vanadyl Phthalocyanine Films and Their Hybrid Structures with Pd Nanoparticles: Structure and Sensing Properties

In this work, thin films of vanadyl phthalocyanines (VOPc and VOPcF₄) are studied as active layers for the detection of gaseous ammonia and hydrogen. The effect of F-substituents on the structural features of vanadyl phthalocyanine films and their sensor response toward ammonia (10–50 ppm) and hydrogen (100–500 ppm) is investigated by X-ray diffraction (XRD) and chemiresistive methods, respectively. It is shown that the sensor response of VOPcF₄ films to ammonia is 2–3 times higher than that of VOPc films. By contrast, the sensor response to hydrogen is higher in the case of VOPc films. Apart from this, the hybrid structures of vanadyl phthalocyanine films with Pd nanoparticles deposited on their surface by a chemical vapor deposition method are also tested to reveal the effect of Pd nanoparticles on the sensitivity of VOPc films to hydrogen. Deposition of Pd nanoparticles on the surface of VOPc films leads to the noticeable increase of their sensitivity to hydrogen.

Gamma radiation-assisted in situ synthesis of palladium nanoparticles supported on ethylenediamine-functionalized polypropylene fabric as an efficient catalyst for reduction of 4-nitrophenol

Immobilization of Pd nanometals on a functionalized non-woven polypropylene fabric offers heterogenous catalytic activity in many chemical transformations and convenient separation from the reaction mixture.

A highly responsive NH3 sensor based on Pd-loaded ZnO nanoparticles prepared via a chemical precipitation approach

The gas-detecting ability of nanostructured ZnO has led to significant attention being paid to the development of a unique and effective approach to its synthesis. However, its poor sensitivity, cross-sensitivity to humidity, long response/recovery times and poor selectivity hinder its practical use in environmental and health monitoring. In this context, the addition of noble metals, as dopants or catalysts to modify the ZnO surface has been examined to enhance its sensing performance. Herein, we report preparation of Pd-loaded ZnO nanoparticles via a chemical precipitation approach. Various Pd loadings were employed to produce surface-modified ZnO nanostructure sensors, and their resulting NH₃ sensing capabilities both in dry and humid environments were investigated. Through a comparative gas sensing study between the pure and Pd-loaded ZnO sensors upon exposure to NH₃ at an optimal operating temperature of 350 °C, the Pd-loaded ZnO sensors were found to exhibit enhanced sensor responses and fast response/recovery times. The influence of Pd loading and its successful incorporation into ZnO nanostructure was examined by X-ray diffraction, high resolution-transmission electron microscopy, and X-ray photoelectron spectroscopy. XPS studies demonstrated that in all samples, Pd existed in two chemical states, namely Pd° and Pd²⁺. The possible sensing mechanism related to NH₃ gas is also discussed in detail.

High performance and low temperature coal mine gas sensor activated by UV-irradiation

In this work, well-aligned vertical ZnO nanorod (ZnO NRs) on p-type Si substrate was fabricated by a microwave-assisted hydrothermal reactor to study the coal mine methane (CMM) gas sensing properties. The XRD diffraction peaks and Raman spectra of the ZnO NRs confirmed the hexagonal wurtzite structure with strong preferential orientation along the c axis and well crystal quality. SEM analysis showed NRs with 100 nm average diameter and ~600 nm length. The variations of the sensor electrical resistance in the presence of CMM were investigated at different gas concentrations and various temperatures in the dark and under UV light. The selectivity and response time of the sensor to CMM gas were improved under UV irradiation. The optimal operating temperatures were 225 °C and 100 °C in dark and exposing UV-irradiation, respectively. Also the response of ZnO NRs sensor under UV excitation in humid condition was higher. The sensor was more selective to CMM than CO₂. The sensor stability was considered by repeating CMM detection for 90 days.

Controlled synthesis of Pt and Co3O4 dual-functionalized In2O3 nanoassemblies for room temperature detection of carbon monoxide

Innovative Pt and Co₃O₄ nanostructure co-decorated In₂O₃ nanobundles have been successfully developed and demonstrated as high-performance room-temperature CO gas sensors.

Hybridization of Zinc Oxide Tetrapods for Selective Gas Sensing Applications

In this work, the exceptionally improved sensing capability of highly porous three-dimensional (3-D) hybrid ceramic networks toward reducing gases is demonstrated for the first time. The 3-D hybrid ceramic networks are based on doped metal oxides (Me_xO_y and Zn_xMe_1-xO_y, Me = Fe, Cu, Al) and alloyed zinc oxide tetrapods (ZnO-T) forming numerous junctions and heterojunctions. A change in morphology of the samples and formation of different complex microstructures is achieved by mixing the metallic (Fe, Cu, Al) microparticles with ZnO-T grown by the flame transport synthesis (FTS) in different weight ratios (ZnO-T:Me, e.g., 20:1) followed by subsequent thermal annealing in air. The gas sensing studies reveal the possibility to control and change/tune the selectivity of the materials, depending on the elemental content ratio and the type of added metal oxide in the 3-D ZnO-T hybrid networks. While pristine ZnO-T networks showed a good response to H₂ gas, a change/tune in selectivity to ethanol vapor with a decrease in optimal operating temperature was observed in the networks hybridized with Fe-oxide and Cu-oxide. In the case of hybridization with ZnAl₂O₄, an improvement of H₂ gas response (to ∼7.5) was reached at lower doping concentrations (20:1), whereas the increase in concentration of ZnAl₂O₄ (ZnO-T:Al, 10:1), the selectivity changes to methane CH₄ gas (response is about 28). Selectivity tuning to different gases is attributed to the catalytic properties of the metal oxides after hybridization, while the gas sensitivity improvement is mainly associated with additional modulation of the electrical resistance by the built-in potential barriers between n-n and n-p heterojunctions, during adsorption and desorption of gaseous species. Density functional theory based calculations provided the mechanistic insights into the interactions between different hybrid networks and gas molecules to support the experimentally observed results. The studied networked materials and sensor structures performances would provide particular advantages in the field of fundamental research, applied physics studies, and industrial and ecological applications.

One-pot synthesis of La-doped SnO2 layered nanoarrays with an enhanced gas-sensing performance toward acetone

Acetone-sensing properties were greatly enhanced by La-doped SnO₂ nanoarray with a novel nanostructure of double nanorod layers synthesized by hydrothermal method.

Physical synthesis methodology and enhanced gas sensing and photoelectrochemical performance of 1D serrated zinc oxide-zinc ferrite nanocomposites

We successfully prepared one-dimensional ZnO-ZnFe2O4 (ZFO) heterostructures for acetone gas-sensing and photoelectrochemical applications, by using sputter deposition of ZFO crystallites on ZnO nanostructure templates. The nanoscale ZFO crystallites were homogeneously coated on the surfaces of the ZnO nanostructures. Electron microscope images revealed that the ZnO-ZFO heterostructures exhibited a serrated surface morphology. Coating the ZnO nanostructures with a ZFO aggregated layer appreciably enhanced their acetone gas-sensing capability at 250 °C in comparison with pure ZnO nanostructures. The presence of many depleted nanoscale ZFO crystallites, the rugged surface of the heterostructures, and electron depletion at the ZnO/ZFO interface might contribute to the enhanced acetone gas-sensing response. Furthermore, the larger surface area and higher light absorption of ZnO-ZFO relative to the surface area and light absorption of ZnO were correlated with a substantial enhancement of the photocurrent value of ZnO-ZFO in photoelectrochemical tests produced by the simulated solar light irradiation.

Hydrothermal Fabrication of WO₃ Hierarchical Architectures: Structure, Growth and Response

Recently hierarchical architectures, consisting of two-dimensional (2D) nanostructures, are of great interest for potential applications in energy and environmental. Here, novel rose-like WO₃ hierarchical architectures were successfully synthesized via a facile hydrothermal method. The as-prepared WO₃ hierarchical architectures were in fact assembled by numerous nanosheets with an average thickness of ~30 nm. We found that the oxalic acid played a significant role in governing morphologies of WO₃ during hydrothermal process. Based on comparative studies, a possible formation mechanism was also proposed in detail. Furthermore, gas-sensing measurement showed that the well-defined 3D WO₃ hierarchical architectures exhibited the excellent gas sensing properties towards CO.

Single CuO nanowires decorated with size-selected Pd nanoparticles for CO sensing in humid atmosphere

We report on conductometric gas sensors based on single CuO nanowires and compare the carbon monoxide (CO) sensing properties of pristine as well as Pd nanoparticle decorated devices in humid atmosphere. Magnetron sputter inert gas aggregation combined with a quadrupole mass filter for cluster size selection was used for single-step Pd nanoparticle deposition in the soft landing regime. Uniformly dispersed, crystalline Pd nanoparticles with size-selected diameters around 5 nm were deposited on single CuO nanowire devices in a four point configuration. During gas sensing experiments in humid synthetic air, significantly enhanced CO response for CuO nanowires decorated with Pd nanoparticles was observed, which validates that magnetron sputter gas aggregation is very well suited for the realization of nanoparticle-functionalized sensors with improved performance.

Synthesis and highly enhanced acetylene sensing properties of Au nanoparticle-decorated hexagonal ZnO nanorings

Au nanoparticles with a size of 3–10 nm were decorated discretely on the surface of ZnO nanorings with the porous Au–ZnO nanorings showing highly enhanced acetylene-sensing properties.

Ag-decorated ZnO nanorods prepared by photochemical deposition and their high selectivity to ethanol using conducting oxide electrodes

A novel gas sensor structure consisting of LaNiO₃ thin film electrodes and gas sensitive Ag nanoparticle-decorated ZnO nanorods is presented in this work.

Hexagonal ZnO nanorings: synthesis, formation mechanism and trimethylamine sensing properties

ZnO nanorings are synthesized by the Ostwald ripening of ZnO nanoplates and the porous film formed by ZnO nanorings shows an excellent trimethylamine-sensing property.

A new type of acetylene gas sensor based on a hollow heterostructure

A new type of acetylene gas sensor based on the hollow NiO/SnO₂ heterostructure synthesized by a two-step hydrothermal method followed by calcination was fabricated.

Synthesis and enhanced humidity detection response of nanoscale Au-particle-decorated ZnS spheres

We successfully prepared Au-nanoparticle-decorated ZnS (ZnS-Au) spheres by sputtering Au ultrathin films on surfaces of hydrothermally synthesized ZnS spheres and subsequently postannealed the samples in a high-vacuum atmosphere. The Au nanoparticles were distributed on ZnS surfaces without substantial aggregation. The Au nanoparticle diameter range was 5 to 10 nm. Structural information showed that the surface of the annealed ZnS-Au spheres became more irregular and rough. A humidity sensor constructed using the Au-nanoparticle-decorated ZnS spheres demonstrated a substantially improved response to the cyclic change in humidity from 11% relative humidity (RH) to 33% to 95% RH at room temperature. The improved response was associated with the enhanced efficiency of water molecule adsorption onto the surfaces of the ZnS because of the surface modification of the ZnS spheres through noble-metal nanoparticle decoration.

Effective decoration of Pd nanoparticles on the surface of SnO2 nanowires for enhancement of CO gas-sensing performance

Decoration of noble metal nanoparticles (NPs) on the surface of semiconducting metal oxide nanowires (NWs) to enhance material characteristics, functionalization, and sensing abilities has attracted increasing interests from researchers worldwide. In this study, we introduce an effective method for the decoration of Pd NPs on the surface of SnO2 NWs to enhance CO gas-sensing performance. Single-crystal SnO2 NWs were fabricated by chemical vapor deposition, whereas Pd NPs were decorated on the surface of SnO2 NWs by in situ reduction of the Pd complex at room temperature without using any linker or reduction agent excepting the copolymer P123. The materials were characterized by advanced techniques, such as high-resolution transmission electron microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The Pd NPs were effectively decorated on the surface of SnO2 NWs. As an example, the CO sensing characteristics of SnO2 NWs decorated with Pd NPs were investigated at different temperatures. Results revealed that the gas sensor exhibited excellent sensing performance to CO at low concentration (1-25ppm) with ultrafast response-recovery time (in seconds), high responsivity, good stability, and reproducibility.

Controlling the hydrothermal growth and the properties of ZnO nanorod arrays by pre-treating the seed layer

Annealing or plasma pre-treating the ZnO seed layer influences the nucleation and hydrothermal growth of ZnO nanorods and their photoluminescence.

Structural, optical and fluorescence properties of wet chemically synthesized ZnO:Pd2+ nanocrystals

This paper presents the structural, optical and photoluminescence properties of wet chemically synthesized ZnO:Pd2+ colloidal nanocrystals characterised by X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopic techniques. Increase in lattice parameters from diffraction data indicates the incorporation of Pd2+ in the ZnO crystal lattice. A small amount of dopant favours the formation of stoichiometric ZnO nanoparticles; otherwise, non-stoichiometric nanocrystal formation was observed from the EDS data. The optical gap was found to decrease with the doping concentration, except for the small dopant level of 0.05% of Pd2+ where an increase in the optical gap was observed. Intensities of characteristic luminescence bands for pure ZnO nanocrystals (357, 387 and 420 nm) were found to decrease with the increasing Pd2+ concentration, and two new bands centred at 528 and 581 nm for 0.5% Pd2+ concentration were observed. These results have been explained on the basis of change in the oxygen vacancy-related defects and/or formation of new trap states which in turn affect the luminescence properties of ZnO:Pd2+ nanocrystals, which are important in the realisation of visible light-emitting solid-state devices.

Outstanding H2 sensing performance of Pd nanoparticle-decorated ZnO nanorod arrays and the temperature-dependent sensing mechanisms

The nearly monodispersed Pd nanoparticles with controllable density on ZnO nanorod arrays were prepared by the unique PVP-mediated photochemical deposition (PCD). The changes in morphology and dispersion of Pd on ZnO surface are ascribed to the stabilizing property and self-assembly characteristic of PVP being exploited during PCD. There are three temperature-dependent H(2) sensing mechanisms in those Pd/ZnO NRs, including general oxygen adsorption/desorption mode within 200-300 °C, surface conductivity mode at 60-120 °C and palladium hydride (PdH(x)) formation at room temperature, which causes a significant discrepancy in sensitivity variations as a function of Pd density. It is also verified that the electronic sensitization related to the transition of Pd(2+)/Pd(0) redox couple predominates the promoting mechanism in Pd/ZnO NRs used for sensing H(2) at 200-300 °C. Therefore, the gas sensitivity to 500 ppm H(2) of Pd/ZnO NRs can be significantly improved by around 553-fold (S, R(a)/R(g) = 1106) at 260 °C through decorating an adequate amount of discrete Pd nanoparticles instead of the Pd clusters, moreover, the corresponding sensitivity at room temperature is 16.9 that is superior to some promising devices reported in the literatures.