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.     

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.     

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.     

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.     

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.     

SO2-Assisted Simultaneous Reduction of SO2 and NO by CO on SnO2-TiO2 Solid Solution

Sn_0.5Ti_0.5O₂ shows excellent catalytic performance both for the CO-SO₂ reaction and the CO-SO₂-NO reaction. At 350 ° C, 525 ppm SO₂/520 ppm NO/2085 ppm CO, SV = 3000 h^-1, the conversion of SO₂ is nearly complete in the CO-SO₂ reaction and above 89% in the CO-SO₂-NO reaction; NO conversion is above 98% in the latter reaction. The selectivities of S and N₂ are both close to 100%. SO₂ shows a significant promoting effect on the activity of the Sn_0.5Ti_0.5O₂ catalyst for NO reduction by CO. Combining transient response experiments, catalytic tests and TPD results, we propose a SO₂-assisted NO-CO reaction concept. The existence of a surface sulfur species, which was formed during the CO-SO₂ or CO-SO₂-NO reaction, is proved by XPS analysis. It is the active site for NO reduction in the CO-SO₂-NO reaction, and through which SO₂ accomplishes its promoter role. On the basis of the results obtained, the SO₂-assisted redox mechanism of simultaneous reduction of SO₂ and NO by CO is proposed.     

Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries

SnO₂ nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO₂ nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO₂ was determined to be around 5 nm. The as-synthesized SnO₂/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO₂ nanoparticles. The SnO₂/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g⁻¹ and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.     

Growth and characterization of TiO2 nanotubes from sputtered Ti film on Si substrate

In this paper, we present the synthesis of self-organized TiO₂ nanotube arrays formed by anodization of thin Ti film deposited on Si wafers by direct current (D.C.) sputtering. Organic electrolyte was used to demonstrate the growth of stable nanotubes at room temperature with voltages varying from 10 to 60 V (D.C.). The tubes were about 1.4 times longer than the thickness of the sputtered Ti film, showing little undesired dissolution of the metal in the electrolyte during anodization. By varying the thickness of the deposited Ti film, the length of the nanotubes could be controlled precisely irrespective of longer anodization time and/or anodization voltage. Scanning electron microscopy, atomic force microscopy, diffuse-reflectance UV–vis spectroscopy, and X-ray diffraction were used to characterize the thin film nanotubes. The tubes exhibited good adhesion to the wafer and did not peel off after annealing in air at 350 °C to form anatase TiO₂. With TiO₂ nanotubes on planar/stable Si substrates, one can envision their integration with the current micro-fabrication technique large-scale fabrication of TiO₂ nanotube-based devices.     

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.     

Combustion of Methane at Low Temperature over Pd and Pt Catalysts Supported on Al2O3, SnO2 and Al2O3-Grafted SnO2

Pd and Pt catalysts supported on alumina, tin(IV) oxide and tin(IV) oxide grafted on alumina were prepared, characterised and tested with respect to the low-temperature combustion of methane after reduction in H₂ and ageing under reactants at 600°C. In the case of Pd, the use of SnO₂ or SnO₂-based supports led to catalysts slightly less active than Pd/Al₂O₃. In contrast, SnO₂ was found to strongly promote the oxidation of methane over Pt catalysts with respect to Pt/Al₂O₃, even after ageing under reactants. When Pt was supported on SnO₂ grafted on Al₂O₃, the activity was found at most similar to or, after ageing, lower than Pt/Al₂O₃. This negative effect was discussed, being partly related to the sintering of SnO₂ under reactants observed by FTIR and XRD.     

Preparation and characterization of Bi2Se3 nanowires by electrodeposition

The electrodeposition of Bi₂Se₃ nanowires on an anodic aluminum oxide template was investigated by cyclic voltammetry in a tartaric acid aqueous solution. The electrochemical behavior of the Bi₂Se₃ nanowires in the electrolytic solution was also investigated using cyclic voltammetry, and the underpotential deposition mechanism of the Bi₂Se₃ nanowires was determined. According to the cyclic voltammetric curves, −0.20 V vs. SCE (saturated calomel electrode) was chosen as the deposition potential of the Bi₂Se₃ nanowires. The ratio of Bi to Se is nearly 2:3, verified by energy-dispersive X-ray spectroscopy and with the addition of surfactant. X-ray diffraction, scanning electron microscopy, selected-area electron diffraction and high-resolution transmission electron microscopy indicate that annealing can improve the crystallinity and chemical composition of Bi₂Se₃ nanowires. Surfactant can also improve the surface morphology and composition of the Bi₂Se₃ nanowires.     

A comparative study on the anisole methylation activity of lanthanum-promoted SnO2 catalyst and its sulfate-doped analogue

A comparative study on the anisole methylation with methanol over lanthanum-promoted SnO₂ catalyst and its sulfate-doped analogue is presented. A maximum 2,6-xylenol selectivity of 82% was achieved at 400°C under optimized conditions at an anisole conversion of 65% over lanthanum-promoted SnO₂ catalyst. The sulfate modification resulted in the dealkylation of anisole to phenol followed by several unselective side reactions due to the creation of strong acid sites. The activity of lanthanum-modified tin oxide catalysts in the selective formation of 2,6-xylenol is ascribed to the presence of weak Lewis acid sites and comparatively stronger basic sites.     

A planar catalytic combustion sensor using nano-crystalline F-doped SnO2 as a supporting material for hydrogen detection

A planar catlytic combustion gas sensor based on Pd/Pt catalyst supported on F-doped SnO₂ nano-crystalline materials has been designed and fabricated for hydrogen detection. The sensor consists of platinum heaters on an alumina plate coated with a catalytic layer and compensating layer. This sensor exhibited better performance than that of the sensors employing sensing material of Pd/Pt catalyst on γ-Al₂O₃ and of Pd/Pt catalyst on nano-crystalline SnO₂. The detection limit of the sensor at 370 °C is in the concentration range of 0.5–5% (v/v), with an excellent linearity of signal voltage to the hydrogen gas concentration.     

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.     

An XPS study of dispersion and valence state of TiO2 supported vanadium oxide catalysts

Monolayer vanadium species are mainly in the V(V) valence state, but with XPS a small fraction of V⁴⁺ species are identified. Prolonged analysis treatment increases the V⁴⁺ concentration. With increasing vanadium concentration, a monolayer coverage corresponding to 1 mg V₂O₅ per m² develops, and it contains additional layers with a thickness of about 250 Å at 4 mg V₂O₅ per m², covering 3% of its surface area.     

XPS and DRS of Au/TiO2 catalysts: effect of pretreatment

TiO₂ impregnated with AuCl₃ was subjected to different pretreatments and then characterized by XPS and DRS. After drying at 298 K under vacuum, the catalyst contains highly dispersed, nonmetallic Au species; whereas drying at 393 K in an oven caused the Au to be partially reduced and agglomerate. Further treatments of the oven-dried sample at higher temperatures resulted in the disappearance of Au signals in XPS except the one after a HTR/C/LTR (high-temperature reduction/calcination/low-temperature reduction) sequence. The high-temperature reduction at 773 K shifted the plasmon resonance peak in DRS to higher wavelength, and the following C and LTR treatment did not change the peak position. This peak shifting is interpreted as a change in the electronic status of the Au. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrochemical decomposition of bisphenol A using Pt/Ti and SnO2/Ti anodes

Methodology for the electrochemical decomposition of bisphenol A is described. The electrochemical behaviour of bisphenol A at a Pt electrode was investigated by means of cyclic voltammetric techniques. The electrochemical oxidation of bisphenol A led to the deactivation of the electrode as a result of the deposition of an electropolymerized film. However the electrochemical decomposition of bisphenol A could be achieved by the use of a platinum coated titanium (Pt/Ti) electrode and a tin dioxide coated (SnO₂/Ti) electrode. The electrolysis was carried out galvanostatically at a constant current of 0.3 A. The mineralization of bisphenol A was monitored by determining the amount of total organic carbon. Furthermore, the generation and nature of intermediates produced in the electrochemical reactions was investigated. Although large amounts of aliphatic acids were generated by electrolysis with the Pt/Ti anode, they were produced only to a small extent in at the SnO₂/Ti anode. In the case of the SnO₂/Ti anode, bisphenol A is rapidly oxidized to carbon dioxide and water, compared to the Pt/Ti anode.     

XPS Characterization of CeO2 Catalyst for Hydrogenation of Benzoic Acid to Benzaldehyde

The CeO₂ catalysts for hydrogenation of benzoic acid to benzaldehyde were characterized by X-ray photoelectron spectroscopy (XPS). The results indicate the catalyst deactivation is due to the coke formation and the valence changes of Ce over the catalyst.     

Co-doping effect of Zn and Sb in SnO2: Valence stabilization of Sb and expanded solubility limit

In this study, the co-doping effect of ZnO and Sb₂O₅ on the solubility limit in SnO₂ was investigated. When ZnO was added to SnO₂, its solubility limit was around 3 at%, while that of Sb₂O₅ could not be evaluated due to the severe evaporation of Sb₂O₅ during sintering. For the co-doping of ZnO and Sb₂O₅, the ZnSb₂O₆ phase was used for the source of Zn and Sb dopants to prevent the evaporation of Sb₂O₅. When ZnO and Sb₂O₅ were co-doped by ZnSb₂O₆, the solubility limit expanded to 60 at%. XPS analysis of the Sb revealed that Sb⁵⁺ is stable when Zn is co-doped. The extended solubility limit is explained by electrostatic and strain energy minimization in the lattice.     

Fabrication and photocatalytic properties of silicon nanowires by metal-assisted chemical etching: effect of H2O2 concentration

In the current study, monocrystalline silicon nanowire arrays (SiNWs) were prepared through a metal-assisted chemical etching method of silicon wafers in an etching solution composed of HF and H₂O₂. Photoelectric properties of the monocrystalline SiNWs are improved greatly with the formation of the nanostructure on the silicon wafers. By controlling the hydrogen peroxide concentration in the etching solution, SiNWs with different morphologies and surface characteristics are obtained. A reasonable mechanism of the etching process was proposed. Photocatalytic experiment shows that SiNWs prepared by 20% H₂O₂ etching solution exhibit the best activity in the decomposition of the target organic pollutant, Rhodamine B (RhB), under Xe arc lamp irradiation for its appropriate Si nanowire density with the effect of Si content and contact area of photocatalyst and RhB optimized.