Analysis of the kinetics of surface reactions on a zinc oxide nanosheet-based carbon monoxide sensor using an Eley–Rideal model

Herein, we experimentally test a mathematical model of the reactions on the surface of a zinc oxide nanosheet-based carbon monoxide sensor. The carbon monoxide is assumed to react with surface oxygen via an Eley–Rideal mechanism, considering only the direct reaction between the two species. We demonstrate that the measured resistance responses of the system are well described by the model, facilitating further analysis of the physical rate constants in the system. By initially considering the system in the absence of any reducing gas, it is shown that various reaction parameters may be precisely estimated. For instance, fitting the model to response curves obtained at different temperatures shows the activation energy of the reaction between oxygen ions and carbon monoxide to be 54 ± 9 kJ mol−1, whereas the recovery curves yield an estimate of 42 ± 7 kJ mol−1. Similarly, the energy barrier for the formation of oxygen ions is found to equal 72 ± 9 kJ mol−1 from the sensor response and 63 ± 10 kJ mol−1 from the recovery. These estimates are in agreement with values quoted elsewhere in the literature, corroborating the validity of the model. In the absence of surface ions, the energy difference between the Fermi level and the conduction band minimum at the surface is estimated as 590 ± 90 meV

High performance tunable piezoresistive pressure sensor based on direct contact between printed graphene nanoplatelet composite layers

This article details the development of a thin film piezoresistive screen printed pressure sensor on a flexible substrate using a composite ink based on functionalised graphene nanoplatelets (GNPs). The sensor operates through direct interfacial contact between two distinct films of the composite ink deposited over conductive substrates, without requiring any intermediate gap through spacers. The sensors showed consistent results and sensitivity forces ranging between 10 N to 2000 N. The piezoresistive range of the sensor can be tuned with the number of layers deposited per side

The role of probe oxide in local surface conductivity measurements

Local probe methods can be used to measure nanoscale surface conductivity, but some techniques including nanoscale four point probe rely on at least two of the probes forming the same low resistivity non-rectifying contact to the sample. Here, the role of probe shank oxide has been examined by carrying out contact and non-contact I V measurements on GaAs when the probe oxide has been controllably reduced, both experimentally and in simulation. In contact the barrier height is pinned but the barrier shape changes with probe shank oxide dimensions. In non-contact measurements, the oxide modifies the electrostatic interaction inducing a quantum dot that alters the tunneling behavior. For both, the contact resistance change is dependent on polarity, which violates the assumption required for four point probe to remove probe contact resistance from the measured conductivity. This has implications for all nanoscale surface probe measurements and macroscopic four point probe, both in air and vacuum, where the role of probe oxide contamination is not well understood

XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media

The redox potential (Eh) in a cementitious nuclear waste repository is critical to the retardation behavior of redox-sensitive radionuclides (RNs), and largely controlled by embedded steel corrosion but hard to be determined experimentally. Here, we propose an innovative Eh determination method based on chemical/spectroscopic measurements. Oxidized nuclides (UVI, SeIV, MoVI, and SbV) were employed as species probes to detect the Eh values imposed by steel (Fe0) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe0, in decreasing Kd order for U > Sb > Se > Mo under both N2 and H2 atmospheres. The reduced nuclide species were identified as UO2, U4O9, FeSe, FeSe2, Se0, Sb0, and Sb2O3, but no redox transformation occurred for Mo. Eh values were obtained by using the Nernst equation. Remarkably, their values fell in a small range centered around −456 mV at pH ∼ 13.5 for both Fe0 and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)2/Fe(OH)3 or (Fe1–x,Cax)(OH)2/Fe(OH)3 couple, whose presence was confirmed by pair distribution function analyses. This approach could pave the way for describing the Eh gradient in reinforced concrete where traditional Eh measurements are not feasible

The effects of vacuum annealing on the conduction characteristics of ZnO nanorods

Optimised ZnO nanorods characteristics are essential for novel devices to operate efficiently. The effects of vacuum annealing on the electrical transport properties and defect chemistry of ZnO nanorods have been studied. Annealing to 500 °C removed surface contamination causing reduced resistance while annealing to 600 °C created acceptor defects, changing the contact type from ohmic to rectifying. At 700 °C donor defects reduced leading to increased resistance while annealing to 800 °C caused a reduction in all defects and decreased resistance. This suggests that contact resistance is the major contributor to the system’s resistance rather than the inherent material resistance alone. The results indicate that contact type can be controlled by manipulating the defect chemistry via controlled annealing

XPS investigation of titanium contact formation to ZnO nanowires

Ti is often used to form an initial Ohmic interface between ZnO and Au due to its low work function, and the TiO2/ZnO heterojunction is also of great importance for many practical applications of nanoparticles. Here, Ti has been controllably deposited onto hydrothermally grown ZnO nanowires and the formation of metal–semiconductor contact has been investigated using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy and scanning electron microscopy. XPS results showed that that the Ti initially reacts with surface oxygen species to form TiO2, and further deposition results in the formation of oxides with oxidation state numbers lower than four, and eventually metallic Ti on top of the TiO2. The formation of TiC was also observed. XPS showed that the onset of metallic Ti coincided with a Zn 3p core level shift to lower binding energy, indicating upwards band bending and the formation of a rectifying contact. Annealing caused a near-complete conversion of the metallic Ti to TiO2 and caused the Zn 3p to shift back to its original higher binding energy, resulting in downwards band bending and a more Ohmic contact. PL measurements showed that the optical properties of the nanowires are not affected by the contact formation

In-situ synthesis of magnetic iron-oxide nanoparticle-nanofibre composites using electrospinning

We demonstrate a facile, one-step process to form polymer scaffolds composed of magnetic iron oxide nanoparticles (MNPs) contained within electrospun nano- and micro-fibres of two biocompatible polymers, Poly(ethylene oxide) (PEO) and Poly(vinyl pyrrolidone) (PVP). This was achieved with both needle and free-surface electrospinning systems demonstrating the scalability of the composite fibre manufacture; a 228 fold increase in fibre fabrication was observed for the free-surface system. In all cases the nanoparticle-nanofibre composite scaffolds displayed morphological properties as good as or better than those previously described and fabricated using complex multi-stage techniques. Fibres produced had an average diameter (Needle-spun: 125 ± 18 nm (PEO) and 1.58 ± 0.28 μm (PVP); Free-surface electrospun: 155 ± 31 nm (PEO)) similar to that reported previously, were smooth with no bead defects. Nanoparticle-nanofibre composites were characterised using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), dynamic light scattering (DLS) (Nanoparticle average diameter ranging from 8 ± 3 nm to 27 ± 5 nm), XRD (Phase of iron oxide nanoparticles identified as magnetite) and nuclear magnetic resonance relaxation measurements (NMR) (T1/T2: 32.44 for PEO fibres containing MNPs) were used to verify the magnetic behaviour of MNPs. This study represents a significant step forward for production rates of magnetic nanoparticle-nanofibre composite scaffolds by the electrospinning technique

Formation of metal-gallium nitride contacts.

The influence of pre-metallisation surface preparation on the structural, chemical, and electrical properties of metal-nGaN interfaces has been investigated by X-ray Photoemission Spectroscopy (XPS), current-voltage measurement (I-V) and cross section Transmission Electron Microscopy (TEM). XPS analysis showed that the three GaN substrate treatments investigated, ex-situ HF etch, in-situ anneal in Ultra-High-Vacuum (UHV), and in-situ Ga reflux cleaning in UHV result in surfaces increasingly free of contaminants. Additionally, the three treatments are found to induce increasingly larger upward band bending. Ag-nGaN contacts formed after Ga reflux cleaning exhibit a Schottky barrier height of 0.80 eV and an ideality factor of 1.56, as determined by I-V.XPS and TEM characterisation of Au-nGaN formed after the three pre-metallisation surface treatments show that HF etching and UHV annealing produce abrupt, well-defined interfaces. Conversely, GaN substrate cleaning in a Ga flux results in Au/GaN intermixing. I-V characterisation of Au-nGaN contacts yield a Schottky barrier height of 1.25 eV with very low ideality factor and very good contact uniformity for the pre-metallisation UHV anneal while the Ga reflux cleaning result in a much lower barrier (0.85 eV), with poor ideality and uniformity. I-V and XPS results suggest a high density of acceptor states at the surface, which is further enhanced by UHV annealing. The mechanisms of Ga-nGaN, Ag-nGaN and Au-nGaN Schottky barrier formation are discussed in the context of the Metal-Induced Gap States model (MIGS) Unified Defect Model (UDM) and Cowley-Sze model