XPS,TDS, and AFM studies of surface chemistry and morphology of Ag-covered L-CVD SnO2 nanolayers

This is well known that the selectivity and sensitivity of tin dioxide (SnO₂) thin film sensors for the detection of low concentration of volatile sulfides such as H₂S in air can be improved by small amount of Ag additives. In this paper we present the results of comparative X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS), and atomic force microscopy (AFM) studies of the surface chemistry and morphology of SnO₂ nanolayers obtained by laser-enhanced chemical vapor deposition (L-CVD) additionally covered with 1 monolayer (ML) of Ag. For as deposited SnO₂ nanolayers, a mixture of tin oxide (SnO) and tin dioxide (SnO₂) with the [C]/[Sn] ratio of approximately 1.3 was observed. After dry air exposure, the [O]/[Sn] ratio slightly increased to approximately 1.55. Moreover, an evident increasing of C contamination was observed with [C]/[Sn] ratio of approximately 3.5. After TDS experiment, the [O]/[Sn] ratio goes back to 1.3, whereas C contamination evidently decreases (by factor of 3). Simultaneously, the Ag concentration after air exposure and TDS experiment subsequently decreased (finally by factor of approximately 2), which was caused by the diffusion of Ag atoms into the subsurface layers related to the grain-type surface morphology of Ag-covered L-CVD SnO₂ nanolayers, as confirmed by XPS ion depth profiling studies. The variation of surface chemistry of the Ag-covered L-CVD SnO₂ after air exposure observed by XPS was in a good correlation with the desorption of residual gases from these nanolayers observed in TDS experiments.     

Considerable Enhancement of Field Emission of SnO2 Nanowires by Post-Annealing Process in Oxygen at High Temperature

The field emission properties of SnO₂ nanowires fabricated by chemical vapor deposition with metallic catalyst-assistance were investigated. For the as-fabricated SnO₂ nanowires, the turn-on and threshold field were 4.03 and 5.4 V/μm, respectively. Considerable enhancement of field emission of SnO₂ nanowires was obtained by a post-annealing process in oxygen at high temperature. When the SnO₂ nanowires were post-annealed at 1,000 °C in oxygen, the turn-on and threshold field were decreased to 3.77 and 4.4 V/μm, respectively, and the current density was increased to 6.58 from 0.3 mA/cm² at the same applied electric field of 5.0 V/μm.     

SnO2 Nanowire Arrays and Electrical Properties Synthesized by Fast Heating a Mixture of SnO2 and CNTs Waste Soot

SnO₂ nanowire arrays were synthesized by fast heating a mixture of SnO₂ and the carbon nanotubes waste soot by high-frequency induction heating. The resultant SnO₂ nanowires possess diameters from 50 to 100 nm and lengths up to tens of mircrometers. The field-effect transistors based on single SnO₂ nanowire exhibit that as-synthesized nanowires have better transistor performance in terms of transconductance and on/off ratio. This work demonstrates a simple technique to the growth of nanomaterials for application in future nanoelectronic devices.     

Preparation and Characterization of SnO2-Based Composite Metal Oxides: Active and Thermally Stable Catalysts for CH4 Oxidation

A series of SnO₂-based catalysts modified by Fe, Cr and Mn were prepared by the combination of redox reaction and co-precipitation methods, and applied to catalytic CH₄ oxidation. The modified catalysts show generally higher activity than the unmodified SnO₂. XRD analysis indicates that Fe, Cr and Mn cations could be incorporated into the lattice of rutile SnO₂ (cassiterite) to form solid solution structure. As a result, more reducible and active oxygen species was formed in the samples, as substantiated by the H₂-TPR results. Moreover, the specific surface areas of the modified catalysts are much higher than that of pure SnO₂ and their crystallite sizes are smaller, indicating they are more resistant to thermal sintering. Indeed, the high specific surface areas and the formation of more active oxygen species in the modified samples are believed to be the predominant reasons leading to their enhanced CH₄ oxidation activity. Eventually, it is noted that SnCrO displays not only remarkable CH₄ oxidation activity, but also potent resistance to SO₂ and water deactivation, which makes it a promising catalyst with the potential to be applied in some real CH₄ oxidation processes.     

Synthesis and high sensing properties of a single Pd-doped SnO2 nanoribbon

Monocrystal SnO₂ and Pd-SnO₂ nanoribbons have been successfully synthesized by thermal evaporation, and novel ethanol sensors based on a single Pd-SnO₂ nanoribbon and a single SnO₂ nanoribbon were fabricated. The sensing properties of SnO₂ nanoribbon (SnO₂ NB) and Pd-doped SnO₂ nanoribbon (Pd-SnO₂ NB) sensors were investigated. The results indicated that the SnO₂ NB showed a high sensitivity to ethanol and the Pd-SnO₂ NB has a much higher sensitivity of 4.3 at 1,000 ppm of ethanol at 230°C, which is the highest sensitivity for a SnO₂-based NB. Pd-SnO₂ NB can detect ethanol in a wide range of concentration (1 ~ 1,000 ppm) with a relatively quick response (recovery) time of 8 s (9 s) at a temperature from 100°C to 300°C. In the meantime, the sensing capabilities of the Pd-SnO₂ NB under 1 ppm of ethanol at 230°C will help to promote the sensitivity of a single nanoribbon sensor. Excellent performances of such a sensor make it a promising candidate for a device design toward ever-shrinking dimensions because a single nanoribbon device is easily integrated in the electronic devices.     

Synthesis of ternary zinc spinel oxides and their application in the photodegradation of organic pollutant

Ternary zinc spinel oxides such as Zn₂SnO₄, ZnAl₂O₄ and ZnFe₂O₄ were synthesized and characterized, and their activities in the photodegradation of phenol molecules were investigated. Zn₂SnO₄, ZnAl₂O₄ and ZnFe₂O₄ powders were synthesized by hydrothermal, metal–chitosan complexation and solvothermal routes, respectively. The face-centered cubic spinel structure of each material was confirmed by powder X-ray diffractometry (XRD) and its porous structure by N₂ adsorption–desorption isotherms. The characterization of spinels was complemented with Fourier transform infrared spectroscopy (FTIR) and X-rays fluorescence (XRF), revealing the formation of spinel structures with high purity. The photocatalytic activity in the degradation of phenol was observed only with Zn₂SnO₄ oxide. Mineralization degree of phenol molecules by Zn₂SnO₄ photocatalyst determined by total organic carbon analysis (TOC) reached 80% at 360 min under sunlight.     

Synthesis and characterization of bionanocomposites of poly(lactic acid) and TiO2 nanowires by in situ polymerization

Bionanocomposites from biopolymers and inorganic nanoparticles are of great interest for packaging materials due to their enhanced physical, thermal, mechanical, and processing characteristics. In this study, poly(lactic acid) (PLA) nanocomposites with covalent bonding between TiO₂ nanowire surface and PLA chains were synthesized through in situ melt polycondensation. Molecular weight, structure, morphology, and thermal properties were characterized. Fourier transform infrared spectroscopy confirmed that PLA chains were covalently grafted onto TiO₂ nanowire surface. Transmission electron microscopy images also revealed clearly a third phase presence on the nanowires after the grafting process. Those grafted PLA chains exhibited significantly increased glass transition temperature and thermal stability, compared with pure PLA. The weight-average molecular weight of PLA/2% TiO₂ nanowire bulk nanocomposites increased by 66% compared with that of pure PLA. The electron microscopy results showed that strong interfacial interaction and homogeneous distribution were achieved between inorganic nanowires and organic PLA matrix in the bulk nanocomposites. The PLA matrix in bulk nanocomposites exhibited elevated glass transition temperature and decreased crystallization ability as the TiO₂ nanowire concentrations were increased from 0 to 2%.     

Interaction of H (D) atoms with surfaces of glassy carbon: adsorption, abstraction, and etching

The interaction of thermal (2000 K) H and D atoms with glassy carbon (GC) surfaces was investigated in ultrahigh vacuum environment using thermal desorption and reaction kinetics mass spectroscopy. Virgin GC surfaces (not previously subjected to stationary etching by H atoms) exhibit a remarkably low reactivity with respect to adsorption of D. The saturation coverage of D on GC is about a factor of ten smaller than on (0 0 0 1) graphite surfaces (HOPG or natural single crystal). Thermal desorption spectra indicate that D atoms on virgin GC surfaces are adsorbed on distorted graphite basal planes, i.e. the recombinative desorption features of D on GC between 400 and 600 K are broadened as compared to those measured on graphite. Upon heating of D-covered virgin GC surfaces, CD₃ surface groups desorb between 500 and 1000 K. Stationary etching of GC by a flux of H atoms is most efficient around 600 K, as was previously observed on other carbon materials, a-C:H thin films and graphite, and as expected from the etching mechanism on C substrates. GC surfaces repeatedly etched by H exhibit an increasing density of C atoms located at edge sites which are capable to adsorb D via formation of spⁿ C-D bonds. D from these sites desorbs recombinatively around 830 K and competitive desorption of C₂ deuterocarbons at 780 K occurs. The graphite-like fraction of the surface is unaffected by etching. Abstraction of D on virgin GC by H exhibits the same phenomenology as on graphite: Eley-Rideal mechanism and large abstraction cross-section at small D coverages.     

Growth of In2O3 Nanowires Catalyzed by Cu via a Solid–Liquid–Solid Mechanism

In₂O₃ nanowires that are 10–50 nm in diameter and several hundred nanometers to micrometers in length have been synthesized by simply annealing Cu–In compound at a relatively low temperature of 550°C. The catalysis of Cu on the growth of In₂O₃ nanowires is investigated. It is believed that the growth of In₂O₃ nanowires is via a solid–liquid–solid (SLS) mechanism. Moreover, photoluminescence (PL) peaks of In₂O₃ nanowires at 412 and 523 nm were observed at room temperature, and their mechanism is also discussed.     

Synthesis of porous Al2O3/ZrO2 nanocomposites by chemical vapour deposition

Al₂O₃/ZrO₂ one-dimensional nanocomposite structures were synthesised by chemical vapour deposition using Al₂O₃ nanowires and a ZrCl₄ powder source at a temperature of 800?°C and a pressure of 130?Pa. The samples were characterised using X-ray diffraction, the scanning electron microscopy, the transmission electron microscopy, and N₂ adsorption–desorption. The results revealed that Al₂O₃/ZrO₂ composite nanowires coated with surface-embedded ZrO₂ nanocrystals were formed and that the ZrO₂ macroporous and mesoporous structures changed as the ZrO₂ deposition time increased. The pore structure and surface area were also elucidated from the N₂ adsorption–desorption measurements.     

Growth and Characterization of ZnO, SnO2 and ZnO/SnO2 Nanostructures from the Vapor Phase

Zinc oxide (ZnO), tin dioxide (SnO₂) and compounds ZnO/SnO₂ (ZTO) nanostructures have been synthesized successfully from the vapor phase without a catalyst using three different approaches. XRD analyses showed that ZnO with a wurtzite crystal structure, SnO₂ with a rutile crystal structure and zinc stannate (ZnSnO₃) and/or dizinc stannate (Zn₂SnO₄) were condensed from the vapor phase when Zn and/or Sn metal powders or their oxides individually or mixed were used as the starting materials. The formation of either zinc or dizinc stannate was controlled by the Zn/Sn ratio and growth technique. SEM and TEM investigations showed that ZnO grew mainly in the form of wires, rods and belts. These are believed to be originated from the common tetrapod structure of ZnO. While SnO₂ grew in the form of tetragonal rods with rectangle-like cross section and nanoparticles, ZTO grew in the form of nanobelts. The final length, width and thickness were as low as 40, 10 and 5 nm, respectively. The driving forces for growth of nanowires, nanorods, nanobelts, and nanoparticles were found to be vapor density or supersaturation, temperature, pressure and location of deposition from the source materials. The optical absorbance and photoluminescence spectra of all samples showed excitonic character at room temperature implying good crystal quality, and high photocurrent properties suggesting possible applications in nanoscaled functional devices such as optoelectronics and gas sensors.     

Ambient contamination of poly-crystalline diamond surfaces studied by high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy

In this work we provide direct evidence of hydrogen, carbon and oxygen contamination of poly-crystalline diamond surfaces from ambient conditions and their thermal stability upon vacuum annealing. Deuterated diamond films were exposed to ambient conditions for ~ 3 months and then studied by high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy. Hydrocarbon contaminations posses at least two different binding states which desorb upon annealing to ~ 300 °C and ~ 600 °C. Oxygen contaminations gradually desorb upon annealing to 700–800 °C. It is shown that thermal desorption of contaminations creates sp² carbon atoms on the diamond film surface.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Synthesis of SnO2/Mg2SnO4 nanoparticles and their electrochemical performance for use in Li-ion battery electrodes

Uniform crystalline MgSn(OH)₆ nanocubes were synthesized by a hydrothermal method. The influences of reaction conditions were investigated and the results showed that the solvent constituents significantly affected the shape and size of MgSn(OH)₆·SnO₂/Mg₂SnO₄ has been obtained by thermal treatment at 850 °C for 8 h under a nitrogen atmosphere using MgSn(OH)₆ as the precursor. The electrochemical tests of SnO₂/Mg₂SnO₄ revealed that SnO₂/Mg₂SnO₄ has a higher capacity and better cyclability compared to pure SnO₂ or Mg₂SnO₄. The electrochemical performance of SnO₂/Mg₂SnO₄ was sensitive to the size of the nanoparticles. The lithium-driven structural and morphological changes of the electrode after cycling were also studied by the ex-situ XRD pattern and TEM tests. This work indicates that SnO₂/Mg₂SnO₄ is a promising anode material candidate for application in Li-ion batteries.     

Giant Persistent Photoconductivity of the WO3 Nanowires in Vacuum Condition

A giant persistent photoconductivity (PPC) phenomenon has been observed in vacuum condition based on a single WO₃ nanowire and presents some interesting results in the experiments. With the decay time lasting for 1 × 10⁴ s, no obvious current change can be found in vacuum, and a decreasing current can be only observed in air condition. When the WO₃ nanowires were coated with 200 nm SiO₂ layer, the photoresponse almost disappeared. And the high bias and high electric field effect could not reduce the current in vacuum condition. These results show that the photoconductivity of WO₃ nanowires is mainly related to the oxygen adsorption and desorption, and the semiconductor photoconductivity properties are very weak. The giant PPC effect in vacuum condition was caused by the absence of oxygen molecular. And the thermal effect combining with oxygen re-adsorption can reduce the intensity of PPC.     

Behavior of Water and Oxygen Molecules on the Surface of Tin Dioxide Films

Polycrystalline films of compositions SnO₂, SnO_1.93F_0.07, 0.93SnO₂ · 0.07Sb₂O₃, and 0.97SnO₂ · 0.03CuO, which are of interest for use in gas-sensitive sensors, are prepared by the hydropyrolytic method from metal chlorides on a 22-XC ceramic substrate. The films prepared are studied using mass spectrometry, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) at different temperatures. The results of mass spectrometric investigations demonstrate that, except for water (18 amu), no compounds in the mass range 10–70 amu are desorbed from all the films. The differential thermal and thermogravimetric analyses have revealed that desorption of water molecules from the film surface occurs in the temperature range 50–125°C and the oxidation processes associated with the oxygen chemisorption proceed beginning from a temperature of 125°C.     

Thermal coarsening of individual titanate nanowires and their assemblies: Surface vs. bulk diffusion

The thermal coarsening of titanate nanowires was investigated from room temperature to 1000 °C by AFM, SEM, XRD and Raman spectroscopy. Phase transformation kinetics and the external morphology were studied in two configurations: i) as individual and suspended titanate nanowires on SiO₂ surface (denoted as 2D geometry); and ii) as large assembly in form of sintered pellets (denoted as 3D geometry). The individual titanates nanowires were coarsened far below the melting temperature (1843 °C) of TiO₂ and two temperature dependent geometrical transformation changes were identified. In the first one, the height decreases around 200 °C (~20% shrinkage) due to the dehydration of the layered titanate, followed by the titanate to anatase recrystallization until 600 °C. Interestingly, the surface area of the SiO₂ supported high temperature stabilized individual nanowires increased or remained constant, which may be due to the topotactic effects and epitaxial strain stabilization. In the second one, at sintering temperature of 600–1000 °C, an intense nanowire coarsening was observed driven by a surface diffusion mechanism on substrate and a surprising stabilization of the anatase TiO₂. In the case of a 3D assembly of the nanowires the densification with significant particle coarsening is accompanied by the phase transformation from anatase to rutile around the known phase transition temperature. These results suggest that the surface area evolution of individual titanium oxide nanowires on SiO₂ appreciably differs from the 3D assembly, and the surfaces may induce an extra stabilization effect and deviation from the classical Ostwald ripening surface diffusion model.     

Properties of sol-gel SnO2/TiO2 electrodes and their photoelectrocatalytic activities under UV and visible light illumination

A visible light active binary SnO₂-TiO₂ composite was successfully prepared by a sol-gel method and deposited on Ti sheet as a photoanode to degrade orange II dye. Titanium and SnO₂ can promote the development of rutile phase of TiO₂ and inhibit the formation of anatase phase of TiO₂. Formation of SnO₂ crystalline is insignificant even when the calcination temperature increases to 700 °C. Heterogenized interface between SnO₂ and TiO₂ inhibits growth of TiO₂ linkage and leads to the particle-filled surface morphology of SnO₂-containing films. The carbonaceous, Ti-O-C bonds and Ti³⁺ species are likely to account for the photoabsorption and photoelectrocatalytic (PEC) activity under visible light illumination. The electrode with 30% SnO₂ exhibits higher photocurrent when compared with those in the region of 0-50%. The 600 °C-calcined SnO₂-TiO₂ electrode indicates higher activity when compared with those at 400, 500, 700 and 800 °C. PEC degradation of orange II follows the Langmuir-Hinshelwood model and takes place much effectively in a solution of pH 3.0 than those in pH 7.0 and pH 11.0.     

The current image of single SnO2 nanobelt nanodevice studied by conductive atomic force microscopy

A single SnO₂ nanobelt was assembled on a pair of Au electrodes by electric-field assembly method. The electronic transport property of single SnO₂ nanobelt was studied by conductive atomic force microscopy (C-AFM). Back-to-back Schottky barrier-type junctions were created between AFM tip/SnO₂ nanobelt/Au electrode which can be concluded from the I-V curve. The current images of single SnO₂ nanobelt nanodevices were also studied by C-AFM techniques, which showed stripes patterns on the nanobelt surface. The current images of the nanobelt devices correlate the microscopy with separate transport properties measurement together.     

Controllable Synthesis of Single-Crystalline CdO and Cd(OH)<Subscript>2</Subscript>Nanowires by a Simple Hydrothermal Approach

Single-crystalline Cd(OH)₂ or CdO nanowires can be selectively synthesized at 150 °C by a simple hydrothermal method using aqueous Cd(NO₃)₂ as precursor. The method is biosafe, and compared to the conventional oil-water surfactant approach, more environmental-benign. As revealed by the XRD results, CdO or Cd(OH)₂ nanowires can be generated in high purity by varying the time of synthesis. The results of FESEM and HRTEM analysis show that the CdO nanowires are formed in bundles. Over the CdO-nanowire bundles, photoluminescence at ~517 nm attributable to near band-edge emission of CdO was recorded. Based on the experimental results, a possible growth mechanism of the products is proposed.