Formation mechanisms of nanocomposite layers based on multiwalled carbon nanotubes and non-stoichiometric tin oxide

Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnO _x ) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume, fixed by SnO _x islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands, which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite layers to (NO₂)⁻ molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs.     

Enhanced gas-sensing behaviour of Ru-doped SnO2 surface: A periodic density functional approach

A theoretical study on Ru-doped rutile SnO₂(1 1 0) surface has been carried out by means of periodic density functional theory (DFT) at generalized gradient approximation (GGA-RPBE) level with a periodic supercell approach. Electronic structure analysis was performed based on the band structure and partial density of states. The results provide evidence that the electronic structures of SnO₂(1 1 0) surface are modified by the surface Ru dopant, in which Ru 4d orbital are located at the edge of the band gap region. It is demonstrated that molecular oxygen adsorption characteristics on stoichiometric SnO₂(1 1 0) surface are changed from endothermic to exothermic due to the existence of surface Ru dopant. The dissociative adsorption of molecular oxygen on the Ru_5c/SnO₂(1 1 0) surface is exothermic, which indicates that Ru could act as an active site to increase the oxygen atom species on SnO₂(1 1 0) surface. Our present study reveals that the Ru dopant on surface is playing both electronic and chemical role in promoting the SnO₂ gas-sensing property.     

A computational study on nanocrystalline SnO2: Adsorption of CO and O2 onto defective nanograins

This work presents a study of the adsorption properties of defective nanostructures. The calculations have quantum mechanical detail and are based on a semi-empirical Hamiltonian, which is applied to the evaluation of both the electronic structure and of the conductance. The material considered in this study, i.e. SnO₂, has a widespread use as gas sensor and oxygen vacancies are known to act as active catalytic sites for the adsorption of small molecules. In the following calculations crystalline SnO₂ nanograins, with a size and shape comparable with the experimental ones, have been considered. The grains lattice, which has the rutile structure of the bulk material, includes oxygen vacancies and the adsorbed system is generated by depositing a gaseous molecule, either CO or O₂, above an atom on the grain surface. The calculations show that the presence of the defects enhances the grain cohesion and favors adsorption. The conductance has a functional relationship with the structure and the defective state of the nanograins and its dependence on these quantities parallels the one of the binding energy.     

Ab-initio study of fluorine-doped tin dioxide: A prospective catalyst support for water electrolysis

In an attempt to identify new electrochemically stable catalyst supports for electrolysis of water, the electronic structure of SnO₂ doped with different fluorine concentrations has been calculated using the Vienna ab-initio simulation package (VASP) in the projector-augmented wave (PAW) method with the general gradient approximation (GGA) for conducting the exchange-correlation corrections. The role of fluorine in improving the electronic conductivity is discussed. An increase in the density of electronic states at the Fermi level with increase in the concentration of fluorine incorporated into the main SnO₂ matrix agrees well with published experimental observations. Despite a gradual decrease in the cohesive energies for the fluorine-doped tin oxide with increase in fluorine concentration, the doped material still remains an appropriate candidate for use as catalyst supports in water electrolysis warranting further experimental validation.     

Density functional theory study of the NO2-sensing mechanism on a WO3 (0 0 1) surface: the role of surface oxygen vacancies in the formation of NO and NO3

The trapping and detection of nitrogen oxide with tungsten trioxide has become a popular research topic in recent years. Knowledge of the complete reaction mechanism for nitrogen oxide adsorption is necessary to improve detector performance. In this work, we used density functional theory (DFT) calculations to study the adsorption characteristics and electron transfer of nitrogen dioxide on an oxygen-deficient monoclinic WO₃ (0 0 1) surface. We observed different reactions of NO₂ on slabs with different O- and WO-terminated WO₃ (0 0 1) surfaces with oxygen vacancies. Our calculations show that the bridging oxygen atom on an oxygen defect on an O-terminated WO₃ (0 0 1) surface is the active site where an NO₂ molecule is oxidised into nitrate and is adsorbed onto the surface. On a WO-terminated (0 0 1) surface, one of the oxygen atoms from the NO₂ molecule fills the oxygen vacancy, and the resulting NO fragment is adsorbed onto a W atom. Both of these adsorption models can cause an increase in the electrical resistance of WO₃. We also calculated the adsorption energies of NO₂ on slabs with different oxygen-deficient WO₃ surfaces.     

Resistive switching effect in metal–oxide–metal structures with ZnO:Li oxide layer

The resistive switching effect in metal–oxide–metal (MOM) structures has been investigated, where the 10% Li-doped ZnO layer was used as an oxide layer, as well as Pt and 20% fluorine doped SnO₂ (SnO₂:F) were used as a bottom electrodes. The current–voltage (I–V) and switching (I–t) characteristics of Ag/ZnO:Li/Pt and Ag/ZnO:Li/SnO₂:F structures were investigated. The unipolar resistive switching is detected in the structures with the Pt, while the use of transparent conductive SnO₂:F electrode instead of Pt, results to the bipolar memory effect.     

Adsorption of oxygen molecules and carbon monoxide molecules on tin dioxide

The effect of oxygen and carbon oxide on the SnO₂ surface conductivity is considered in the framework of the early model of gas adsorption on semiconductor oxides. A change in the work function of the adsorption system is calculated.     

Fabrication of hydrophobic surface of titanium dioxide films by successive ionic layer adsorption and reaction (SILAR) method

Titanium dioxide (TiO₂) films were fabricated on fluorine doped tin oxide (FTO) coated glass substrate using successive ionic layer adsorption and reaction (SILAR) method. The X-ray diffraction, scanning electron microscopy, transmission electron microscopy, optical absorption and contact angle measurement were applied to study the structural, surface morphological, optical and surface wettability properties of the as-deposited and annealed TiO₂ films. The X-ray diffraction studies revealed both as-deposited and annealed TiO₂ films are amorphous. Irregular shaped spherical grains of random size and well covered to the fluorine doped tin oxide coated glass substrates were observed from SEM studies with some cracks after annealing. The optical band gap values of virgin TiO_2, annealed, methyl violet and rose bengal sensitized TiO₂ were found to be 3.6, 3.5, 2.87 and 2.95 eV, respectively. Surface wettability studied in contact with liquid interface, showed hydrophobic nature as water contact angles were greater than 90°. The adsorption of dyes, as confirmed by the photographs, is one of the prime requirements for dye sensitized solar cells (DSSC).     

SnO2 nanograins Au-doped: A quantum mechanical evaluation of CO adsorption

In this study an analysis is made of the adsorption properties of nanocrystalline SnO₂ containing a metallic dopant. The analysis is based on semi-empirical Hartree–Fock and scattering theories and the structures considered are SnO₂ grains, with a rutile lattice, whose size and shape are comparable with the nanograins and nanowires produced in experiments. The grains contain rows of gold atoms located externally, on the grain surface, or in an endohedral position, in the grain interior, and the adsorbed system is generated by depositing CO molecules on the grain surface. The calculations illustrate the dependence of the binding energies and of the conductance on the grain size and on the location of the metallic additives in both the clean and in the CO-adsorbed grains. These results show that adsorption and current transport are determined by the intrinsic electronic structure of the adsorbing grains.     

Electron enhanced sorption of fluorine by silver surfaces

Quartz crystal microbalance measurements indicate a multilayer adsorption of fluorine occurs at a relatively high rate during exposure of a silver surface to F₂orXeF₂. As more fluorine is adsorbed our results indicate that it reacts chemically with the silver forming AgF. The rate of fluorine adsorption from XeF₂ increases whereas the rate of adsorption of F₂ remains constant, when this occurs. When a flux of electrons is simultaneously incident on a silver surface along with a flux of F₂orXeF₂ the fluorine adsorption rates increase greatly. 1000 Å thin films of ThF₄, MgF₂, SiO₂andAl₂O₃ present relatively minimal barriers to fluorine uptake. The fluorine uptake actually occurs faster if a fluoride film is deposited on silver as opposed to an oxide film or no film at all. Gold films were not found to exhibit this behavior.     

Effect of solvent volume on the physical properties of undoped and fluorine doped tin oxide films deposited using a low-cost spray technique

Undoped and fluorine doped tin oxide films were deposited from starting solutions having different values of solvent volume (10-50 ml) by employing a low cost and simplified spray technique using perfume atomizer. X-ray diffraction studies showed that there was a change in the preferential orientation from (2 1 1) plane to (1 1 0) plane as the volume of the solvent was increased. The sheet resistance (R_sh) of undoped SnO₂ film was found to be minimum (13.58 KΩ/□) when the solvent volume was lesser (10 ml) and there was a sharp increase in R_sh for higher values of solvent volume. Interestingly, it was observed that while the R_sh increases sharply with the increase in solvent volume for undoped SnO₂ films, it decreases gradually in the case of fluorine doped SnO₂ films. The quantitative analysis of EDAX confirmed that the electrical resistivity of the sprayed tin oxide film was mainly governed by the number of oxygen vacancies and the interstitial incorporation of Sn atoms which in turn was governed by the impinging flux on the hot substrate. The films were found to have good optical characteristics suitable for opto-electronic devices.     

Surface properties of superparamagnetic iron oxide MR contrast agents: Ferumoxides,ferumoxtran, ferumoxsil

The surface properties of the active ingredients in AMI colloidal, superparamagnetic iron oxide magnetic resonance (MR) contrast agents are described. Scanning electron microscopy/energy dispersive X-ray elemental analyses and diffuse reflectance Fourier transform infrared spectroscopy (FTIR) spectra of ferumoxsil (AMI-121 drug substance) were consistent with the presence of a monolayer of H₂NCH₂CH₂NHCH₂CH₂CH₂Si(O⁻)₃ siloxane monomer or dimer. The X-ray photoelectron spectra (XPS) of ferumoxsil are also consistent with complete coverage of the iron oxide surface with a monolayer of siloxane. The static secondary ion mass spectra (SSIMS) of ferumoxsil showed that the siloxane film is covalently bonded (i.e., SiOFe bonds) to the iron oxide surface. The FTIR of ferumoxides (AMI-25) and Ferumoxtran (AMI-227) showed only adsorbed dextran. The XPS spectra of the dextrancoated colloids showed that Ferumoxtran has a thicker layer of dextran than ferumoxides iron oxide particles (∼5 and ∼3 nm, respectively). The SSIMS spectra of these dextran-coated colloids showed only low mass fragments due to the adsorbed dextran. The nature of the interactions of the dextran coating with the iron oxide surfaces of ferumoxides and Ferumoxtran is discussed.     

Electrical, structural and surface properties of fluorine doped tin oxide films

Fluorine (F) incorporated polycrystalline SnO₂ films have been deposited onto glass substrates by ultrasonic spray pyrolysis technique. To possess information about the electrical properties of all films, their electrical conductivities were investigated depending on the temperature, and their activation and trap energies were analyzed. The crystalline structure, surface properties and elemental analysis of the SnO₂ films were examined to determine the effect of the F element. After all investigations, it was concluded that each fluorine incorporation rate has a different and important effect on the physical properties, and SnO₂:F (3 at%) films were found to be the most promising sample for energy conversion devices, especially as conducting electrode in solar cells with its improved structural and electrical properties as compared to others.     

Deposition of hybrid structures of reduced graphene oxide and tin dioxide thin films,and persistent photoconductivity observation

Reduced graphene oxide (rGO) is deposited on glass substrate by dripping and sol-gel-coating methods giving rise to nanostructures. When in combination with thin films of SnO₂, they form a heterostructure SnO₂:2 at% Eu/rGO, which alters the surface electrical conductivity. SnO₂ and rGO were also combined as a composite, with conductivity strongly affected by ultraviolet excitation, and shows persistent photoconductivity (PPC) phenomenon even very close to room temperature. Both sort o hybrid structures can be applied in electronic devices. The SnO₂ films are deposited via chemical route by sol-gel or by a mixed technique that combines powders generated by drying the sol-gel solution with resistive evaporation of this powder. Resistivity measured as a function of temperature show that the SnO₂:2 at%Eu sample behaves very similarly to the SnO₂:2 at%Eu/rGO heterostructure sample, with the same energy level for the dominant defect, 172 meV, coincident with ionization of oxygen vacancies. Despite not changing the position of this level, the presence of rGO on the surface of the SnO₂ film induces a decrease in conductivity in vacuum, demonstrating the surface interaction.     

Conformation and energetics of benzene adsorbate on SnO2(110) surfaces: A first principles study

Adsorption of benzene on oxygen rich and reduced SnO₂ surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals approach. Rather than preferential adsorption sites, it is found that the adsorption potential energy surface is flat at both surfaces. The bridging oxygen atoms on the stoichiometric surface induce both covalent and ionic bonding leading to weak chemisorptions, whereas bonding on the reduce surface is closer to physisorption. Deformation of the benzene adsorbate due to adsorption is negligible and only small opposite charge transfer is found explaining the differences between the two surfaces.     

Density functional theory description of the mechanism of ferromagnetism in nitrogen-doped SnO2

Based on first-principles calculations, we have studied the occurrence of spin polarization in the magnetic metal oxide SnO₂ doped with nonmagnetic nitrogen (N) impurities. It was found that the local magnetic moments are localized mainly on the N dopant, causing a total moment of 0.95μB per cell. The long-range magnetic coupling of N-doped SnO₂ may be attributed to a p-p coupling interaction between the N impurity and host valence states.     

Influence of Matrix Nature on the Structural Characteristics of In<Subscript>2</Subscript>O<Subscript>3</Subscript>–CeO<Subscript>2</Subscript> and SnO<Subscript>2</Subscript>–CeO<Subscript>2</Subscript> Composites Fabricated by the Impregnation Method

The structural characteristics, valence states, and distribution of cerium ions between the components in In₂O₃–CeO₂ and SnO₂–CeO₂ nanocomposites fabricated using the impregnation method were studied. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) were used to show that, during impregnation, cerium ions are not included into In₂O₃ crystals and are disposed only on their surface in the form of nano-sized crystallites or amorphous clusters. On the other side, under the contact of CeO₂ clusters with a surface of SnO₂ matrix crystals, cerium ions penetrate into the surface layer of these crystals. In contrast to an In₂O₃–CeO₂ system, where the addition of CeO₂ does not affect the conduction activation energy, where cerium oxide is added to SnO₂, the observed increase in the resistance of a SnO₂–CeO₂ composite is accompanied by a sufficient increase in activation energy. These data and the XPS spectra confirm the modification of the surface layers of conductive SnO₂ crystals as, a result of the penetration of cerium ions into these layers.     

A sonochemical assisted synthesis of hollow sphere structured tin (IV) oxide on graphene oxide sheets for the low-level detection of environmental pollutant mercury in biological samples and foodstuffs

In modern approaches for nanomaterials synthesis, ultrasonication plays an important role in providing the larger surface area and smaller crystalline size properties that are favorable to electrochemical techniques. Herein, we report the tin (IV) oxide on graphene oxide nanoparticles were synthesized (SnO₂@GO NPs) by ultrasonic methodology (UZ SONOPULS HD 3400 Ultrasonic homogenizer) with the total power of 400 W and the (frequency of 20 kHz; 140 W/dm³). The formation of as-prepared SnO₂@GO NPs and its surface morphology were scrutinized over XRD, XPS, TEM, and FESEM. Besides, the sonochemically prepared SnO₂@GO NPs were employed for the determination of environmental hazardous mercury (Hg). As a result, the modified electrode acquired a very low-level detection limit of 1.2 nM with a wider range of 0.01–10.41-µM and 14.52–225.4-µM for the detection of Hg. Finally, the practical applicability of SnO₂@GO NPs in spiked human blood serum and tuna fish samples shows appreciable found and recovery values..     

Adsorption of γ-mercaptopropyltrimethoxysilane on zinc: A study of the competition between thiol and silanol functions related to the age of the siloxane solution,its pH and the oxidation state of the surface

We describe the adsorption of γ–mercaptopropyltrimethoxysilane (γ-MPS) on zinc under various experimental conditions, including the age of the siloxane solution (t_ag), its pH (7 or 4), and the mode of preparation of the surface (RCA treatment or in situ polishing). It is shown by XPS studies that the structure of the adsorbed monolayer varies dramatically with the pH of the solution. At the natural pH of the siloxane solution (pH 7) where no hydrolysis of the SiOCH₃ group occurs, adsorption proceeds through the SH moiety and not through SiOCH₃ groups. This preferential attachment through SH is found whatever the age of the solution and the treatment of the zinc. It is confirmed by the fact that n-propyltrimethoxysilane (PSi) does not interact with the surface in the case of very old solutions (adsorption is not observed when Zn is polished in situ and only occurs with RCA zinc treatment for t_ag > 40 min). With siloxane solutions at pH 4, adsorption of γ-MPS is more complex and the structure of the adsorbed layer depends mainly on the age of the solution. With a fresh solution, hydrolysis is not very advanced and, as mentioned previously, adsorption occurs through the SH group. With older solutions and as a consequence of the progressive hydrolysis of the SiOCH₃ group to SiOH, the density of the grafted siloxane monolayer increases (6 min < t_ag < 10 min), followed by a mixed adsorption through SH and SiOH (10 min < t_ag < 40–50 min) revealed by the decrease in the normalised (Si2p/S2p)* intensity ratio. Finally, adsorption of dimers and oligomers is observed with still older siloxane solutions. In contrast to PSi whose adsorption on zinc is favoured by the RCA treatment, neither treatment of the surface changes the results significantly in the case of γ-MPS. Comparison with alkanethiols confirms the transition from monomer to dimer adsorption and IRRAS studies clearly indicate a condensation reaction between OH and SH groups.     

Synthesis and enhanced acetone gas-sensing performance of ZnSnO<Subscript>3</Subscript>/SnO<Subscript>2</Subscript> hollow urchin nanostructures

A kind of novel ZnSnO₃/SnO₂ hollow urchin nanostructure was synthesized by a facile, eco-friendly two-step liquid-phase process. The structure, morphology, and composition of samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption–desorption techniques. The results revealed that many tiny needle-like SnO₂ nanowires with the average diameter of 5 nm uniformly grew on the surface of the ZnSnO₃ hollow microspheres and the ZnSnO₃/SnO₂ hollow urchin nanostructures with different SnO₂ content also were successfully prepared. In order to comprehend the evolution process of the ZnSnO₃/SnO₂ hollow urchin nanostructures, the possible growth mechanism of samples was illustrated via several experiments in different reaction conditions. Moreover, the gas-sensing performance of as-prepared samples was investigated. The results showed that ZnSnO₃/SnO₂ hollow urchin nanostructures with high response to various concentration levels of acetone enhanced selectivity, satisfying repeatability, and good long-term stability for acetone detection. Specially, the 10 wt% ZnSnO₃/SnO₂ hollow urchin nanostructure exhibited the best gas sensitivity (17.03 for 50 ppm acetone) may be a reliable biomarker for the diabetes patients, which could be ascribed to its large specific surface area, complete pore permeability, and increase of chemisorbed oxygen due to the doping of SnO₂.