TiO2 coated SnO2 nanosheet films for dye-sensitized solar cells

TiO₂-coated SnO₂ nanosheet (TiO₂-SnO₂ NS) films about 300 nm in thickness were fabricated on fluorine-doped tin oxide glass by a two-step process with facile solution-grown approach and subsequent hydrolysis of TiCl₄ aqueous solution. The as-prepared TiO₂-SnO₂ NSs were characterized by scanning electron microscopy and X-ray diffraction. The performances of the dye-sensitized solar cells (DSCs) with TiO₂-SnO₂ NSs were analyzed by current-voltage measurements and electrochemical impedance spectroscopy. Experimental results show that the introduction of TiO₂-SnO₂ NSs can provide an efficient electron transition channel along the SnO₂ nanosheets, increase the short current density, and finally improve the conversion efficiency for the DSCs from 4.52 to 5.71%.     

SnO2:Ga thin films as oxygen gas sensor

Ga-doped SnO₂ thin films deposited by spray pyrolysis were investigated as oxygen gas sensors. Gallium was added to the films to enhance the catalytic activity of the surface’s film to oxygen. Film resistance was studied in an environment of dry air loaded with oxygen in excess at partial pressures in the range from 0 to 8.78×10³ Pa. The best sensitivity lies close to partial pressures of 133.3 Pa. Film sensitivity reach a maximum at 350 °C. For this temperature and a doping concentration of 3 at.% of Ga in the starting solution, a sensitivity up to 2.1 was obtained.     

氧化锡薄膜场致发射研究

利用直流磁控反应溅射法,制备氧化锡薄膜,利用扫描电镜等方法对氧化锡薄膜微观结构进行分析。在低真空下,对不同厚度的氧化锡薄膜进行场致发射测试,结果显示,在氧化锡薄膜厚度为60nm时,场致发射性能最佳,当电流密度为10μA/m2时,开启电压为4.5 V/μm,阴阳两极电场为7 V/μm时,有较佳的场发射密度,同时发光亮度达到2180 cd/m2,结果表明,氧化锡薄膜在场发射平板显示及真空电子器件方面具有较好的应用潜力。     

Dependence of La2O3 content on the nonlinear electrical behaviour of ZnO, CoO and Ta2O5 doped SnO2 varistors

The effects of La₂O₃ on the properties of (Zn, Co, Ta) doped SnO₂ varistors were investigated in this study. The samples with different La₂O₃ concentrations were sintered at 1400 °C for 2 h and their properties were characterized by XRD, SEM, I-V and impedance spectroscopy. The grain size was found to decrease from 13 μm to 9 μm with increasing La₂O₃ content. The addition of rare earth element leads to increase the nonlinear coefficient and the breakdown voltage. The enhancement was expected to arise from the possible segregation of lanthanide ion due to its larger ionic radius to the grain boundaries, thereby modifying its electrical characteristics. Furthermore, the dopants such as La may help in the adsorption of O′ to O″ at the grain boundaries characteristics.     

Properties of SnO2 films fabricated using a rectangular filtered vacuum arc plasma source

Transparent and conducting SnO₂ films of 57–200nm thickness were deposited on microscope glass slide substrates, using a rectangular filtered vacuum arc deposition system. The 40 glass slides were equally distributed on a 400 × 420mm substrate carriage, and were exposed to a Sn plasma beam, produced by a rectangular vacuum arc plasma gun with a Sn cathode, and passed through a rectangular magnetic macroparticle filter towards the substrates. The carriage with the substrates was transported past the 94 × 494mm filter outlet. The SnO₂ films were fabricated on the glass substrates at room temperature by maintaining the chamber oxygen background pressure at 0.52Pa. The film composition, and electrical and optical properties were studied as a function of the film thickness. The films were stored under ambient air conditions, and their electrical resistance was measured as a function of storage time over a period of several months.

The average resistivity of films was 10–17mΩ cm for films with thickness (t) less than 100nm, but that of t > 100nm it was 5–9mΩ cm. The resistivity of the films with t > 100nm did not change significantly after 8months of storage in ambient air. The optical transmittance of the films in the visible spectrum was in the range of 75–90%. The optical constants, i.e., the refractive index and the extinction coefficient of the films at wavelength λ = 550nm were in the range of 2.02–2.09 and 0.013–0.023, respectively, and the optical band gap energy was 4.15–4.21eV. Unlike the electrical resistivity, the optical parameters weakly depended on t.     

Water-assisted growth and characterization of SnO2 nanobelts

SnO₂ nanobelts have been synthesized by water-assisted growth at 850 °C using high pure Sn powders as the source materials. The as-synthesized products were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy(TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersed X-ray spectroscopy (EDX), infrared spectrum (IR) and room-temperature photoluminescence (PL) spectrum. XRD pattern of the sample is quite in accord with the standard pattern of the tetragonal rutile SnO₂; SEM and TEM images show that the uniform single-crystalline SnO₂ nanobelts are about tens of micrometers in length, 70-100 nm in width and 5-8 nm in thickness, and is smooth in surface. The special IR and PL properties were also detected by IR and PL testing. The growth mechanisms and special properties relative to the SnO₂ nanostructures are discussed.     

Cathodoluminescence study of SnO2 powders aimed for gas sensor applications

In this paper we report on cathodoluminescence (CL) spectra of SnO₂ powders, synthesized using the wet chemical route. The analysis of influence of the modes of calcination (T_an-450–800 °C), and doping by both Pd and Pt (0.01–10.0 wt.%) on CL spectra was made. It was found that the measurement of CL spectra could be an effective research method of nanostructured metal oxides, aimed for gas sensor applications. It was established that in nanocrystalline SnO₂ the same system of energy levels, associated with radiative recombination, as in single crystalline and polycrystalline SnO₂, is retained. It was found that doping by both Pd and Pt modifies the structural properties of SnO₂ grains. Also, there is an optimum doping; near 0.1–0.2 wt.%, at which a maximum intensity of cathodoluminescence is reached. It was concluded that for low concentrations of both Pd and Pt additives in SnO₂ an improvement of the material's crystal structure is promoted, and is associated with a decrease in the non-radiating recombination rate.     

Effect of density and thickness on H2-gas sensing property of sputtered SnO2 films

Undoped and Pd-doped SnO₂ films were deposited under various conditions for the investigation of the effect of Pd doping, porosity, and thickness on their H₂ gas sensing properties. The temperature of the substrate and the pressure of the discharge gas were varied. All films formed were composed of columns with thicknesses between 20 and 30 nm. The film density decreased as the discharge gas pressure increased and the substrate temperature decreased. It showed values between 4.2×10³ and 7.0×10³ kg/m³ depending on the deposition condition. Low film density and Pd doping resulted in high sensitivity and fast response. The largest sensitivity was observed for a Pd-doped film with a low density of 4.7×10³ kg/m³ and a thickness of 20 nm.     

Polarization dependence of the optical response in SnO2 and the effects from heavily F doping

The optical properties of intrinsic SnO₂ (TO) and fluorine doped (FTO) are characterized in terms of the dielectric function ε(ħω) = ε₁(ħω) + iε₂(ħω) by electronic structure calculations. The intrinsic TO shows intriguing absorption characteristics in the 3.0–8.0 eV region: (i) the low energy region of the fundamental band gap (3.2 < ħω < 3.9 eV), the optical transitions Г₃⁺ → Г₁⁺ (valence-band maximum to conduction-band minimum) is symmetry forbidden, and the band-edge absorption is therefore extremely weak. (ii) In the higher energy region (3.9 < ħω < 5.1 eV) the Г₅⁻ → Г₁⁺ transitions (from the second uppermost valence band) is strongly polarized perpendicular to the main c axis. (iii) Transitions with polarization axis parallel to c axis are generated from Г₂⁻ → Г₁⁺ transitions (from the third uppermost valence bands), and dominates at high energies (5.1 < ħω eV). Heavily F doped TO (FTO) with doping concentrations n_F = 4 × 10²⁰ cm^− 3 changes the absorption significantly: (iv) Substitutional F_O generates strong inter-conduction band absorption at 0.8, 2.2, and 3.8 eV which affects also the high frequency dielectric constant ε_∞. (v) Interstitial F_i is inactive as a single dopant, but act as a compensating acceptor in highly n-type FTO. This explains the measured non-linear dependence of the resistivity with respect to F concentration.     

Preparation of nanocrystalline SnO2 thin film coated Al2O3 ultrafine particles by fluidized chemical vapor deposition

Fluidized chemical vapor deposition (FCVD) technology was developed for coating SnO₂ thin film on Al₂O₃ ultrafine particles. TEM and HREM analysis found that SnO₂ films with different structures were deposited by controlling the coating temperature, reactant concentration, etc. Nanocrystalline SnO₂ film was coated at 573.15 K by gas phase reaction of SnCl₄ with H₂O. EPMA and EDS studies indicated that the distribution of SnO₂ inner and outer of the agglomerates was uniform. Nucleation and film deposition were coexisted mechanism during the FCVD coating process. The fraction of SnO₂ in the composite particles increased with increasing coating temperature, SnCl₄ concentration, and coating time. The mass fraction of SnO₂ in the composite particles increased strongly with the ratio of P_H2O and P_SnCl4 at low mole ratio of H₂O with SnCl₄, but increased little under the conditions of excess H₂O with respect to SnCl₄.     

Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption

The correlations between microstructure of nanocrystalline TCO SnO₂ and In₂O₃ and parameters of oxygen chemisorption are analyzed. Nanocrystalline SnO₂ and In₂O₃ were prepared by wet chemical method. The sample's microstructure was characterized by TEM, XRD and low-temperature nitrogen adsorption. Electrical properties of TCO were studied at 200-400 °C depending on the oxygen partial pressure. Increase of TCO grain size leads to the increase of the fraction of atomic forms of chemisorbed oxygen at the fixed temperature. It could be due to the decrease of surface barrier resulting in the decrease of activation energy of dissociation of molecular ion O_2(ads)⁻.     

H2S sensing property of porous SnO2 sputtered films coated with various doping films

Pure SnO₂ films and Ag-, Cu-, Pt-, and Pd-doped SnO₂ films were investigated for H₂S sensing properties. SnO₂ films were deposited by DC magnetron sputtering at various substrate temperatures and discharge gas pressures. As the discharge gas pressure increased and the substrate temperature decreased, the film became porous. Doping with Cu or Ag film improved the sensitivity, and the highest sensitivity was obtained in the porous SnO₂ film coated with an Ag film 16 nm thick. According to the X-ray diffraction (XRD) pattern, Ag deposited on SnO₂ film transformed to Ag₂S upon exposure to H₂S. When the Ag-doped film sensor was operated at a low temperature, the sensitivity was extremely high, but the recovery was insufficient. By increasing the operation temperature, the recovery was improved but the sensitivity decreased.     

氮掺杂对SnO2薄膜光电性能的影响

SnO2薄膜具有透明导电的特性,因而被制成透明电极而广泛应用于平板显示器和太阳能电池中。研究表明,经掺杂的薄膜具有更优异的光电性能,然而传统的掺杂元素Sb,Te或F较为昂贵且有毒性,因此,掺氮将有望解决上述问题。本文利用反应射频磁控溅射法制备出不同氧含量的SnO2以及氮掺杂SnO2薄膜,并分析了薄膜的形貌结构及光电性能。结果表明:薄膜沉积过程中氧分压和氮掺杂对薄膜性能影响较大。在SnO2薄膜中,晶粒呈包状形态,随着氧分压的增加,晶粒取向从(101)转向(110)方向,晶粒尺寸逐渐变小,可见光透光率提升到80%以上,光学带隙增加到4.05 eV;在氮掺杂SnO2薄膜中,晶粒呈四棱锥形态,晶粒取向为(101)方向,随着氧分压的增加,可见光的透过率同样提升到80%以上,光学带隙增加到3.99 eV。SnO2薄膜和氮掺杂SnO2薄膜的电阻率最低分别达到1.5×10-1和4.8×10-3Ω.cm。     

Opto-electronic properties of SnO2 layers obtained by SPD and ECD techniques

The paper presents a comparative approach concerning the properties of SnO₂ thin layers obtained via spray pyrolysis deposition (SPD) and electro-chemical deposition (ECD). The influences of crystalline structure (X-ray diffraction), morphology (atomic force microscopy, contact angle) on the electric (electrical conductivity) properties of the layers were studied. The SPD samples present a porous morphology with high surface energy compared with ECD samples characterized by a dense morphology. The photocatalytic efficiency of the samples was tested in the photodegradation of methylene blue and the higher values (57%) correspond to SPD samples.     

The influence of methanol addition during the film growth of SnO2 by atmospheric pressure chemical vapor deposition

Undoped tin oxide (SnO₂) thin films have been deposited in a stagnant point flow chemical vapor deposition reactor from a water/tin tetrachloride mixture. By adding methanol during the deposition process the film electrical properties change significantly: ten times more conductive SnO₂ films are obtained, with remarkably high mobility values of up to 55 cm²/V s. The investigations on the morphological and structural properties indicate that the main effect of methanol is the densification of the SnO₂ films, which probably causes the improvement in the electrical properties. In all conditions the nucleation and coalescence phases take place very early in the growth. Below 10 nm the films are already very conductive, which is very beneficial to applications that have strict requirements in terms of film transparency.     

Structural properties of porous silicon/SnO2:F heterostructures

In this work we present structural studies made on SnO₂ deposited on macroporous silicon structures. The porous silicon substrates were prepared by anodization of p-type silicon wafers. The SnO₂ doped layers were synthesized by the sol-gel method from SnCl₄·5H₂O-ethanolic precursor, where the effect of fluorine doping level on structural properties was investigated. The fundamental structural parameters of tin oxide such as the lattice parameter and the crystallite size were studied in correlation with the dopant concentration. In addition, the effect of fluorine incorporation into the structure of tin oxide was analyzed on the basis of theoretical calculations that take into account the structural factor. The results obtained indicate that incorporation of fluorine occurs only at substitutional sites for SnO₂ deposited on porous silicon.     

The influence of the precursor concentration on the properties of SnO2 thin films

The paper presents a comparative approach concerning the properties of SnO₂ thin layers obtained via spray pyrolysis deposition (SPD) using precursors with different molar concentrations. The photocatalytic efficiency of the samples was tested in the photodegradation of methylene blue. The sample obtained from 0.001 M precursor exhibit higher electrical conductivity (current-voltage curves) and photocatalytic properties compared with the samples obtained from precursors with lower molar concentrations. Regardless of the molar concentrations the samples present tetragonal structure (X-ray diffraction) and granular morphology (atomic force microscopy).     

Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.