Laser ablation doping of poly crystalline tin dioxide films with palladium

Laser ablation of palladium was studied and velocity (energy) distributions of palladium ions evaporated by an Kr-F laser in a vacuum were obtained. The optimum values of energy fluence (fluence rate) of laser radiation for doping tin dioxide films, at which neither multiply charged Pd^N+ ions nor ionized clusters Pd _N ⁺ , occur in a plasma, were determined. From time-of-flight probe measurement data, Pd⁺ implantation depths in SnO₂ films were calculated, which qualitatively agree with the results obtained by secondary neutral mass spectrometry. Electric conductivity measurements on the obtained films in a gas phase showed that introduction of palladium into polycrystalline SnO₂ films by laser ablation significantly enhanced their gas sensitivity to hydrogen.     

Dopants in nanocrystalline tin dioxide

The review surveys studies aimed at constructing new materials for gas sensors based on nanocrystalline tin dioxide. The influence of doping with various impurities (Pt, Pd, Ru, Rh, Cu, Ni, or Fe) on the composition, microstructure, and electrophysical and sensor properties of nanocrystalline SnO₂ was discussed. The conditions for the preparation of powders and thick and thin SnO₂ films by the wet chemical method and aerosol pyrolysis of organometallic compounds are reported. The mechanism of interaction of pure and doped nanocrystalline SnO₂ with a gas phase was analyzed based on the data from Mossbauer, Auger electron, and X-ray photoelectron spectroscopy and the results of in situ Raman spectroscopy, XANES, and conductivity measurements.     

Study of the properties of in situ Pt-doped SnO2 thin films prepared by metal-organic chemical vapour deposition

Pt-SnO₂ thin films were directly deposited by MOCVD in the temperature range 320–440°C using the reactive gas mixture SnEt4/O₂/Pt(hfa)₂ (hfa being bis(1,1,1,5,5,5-hexafluoroacetylacetonato). The Pt content of the films increases by increasing either the Pt(hfa)₂ mole fraction or the growth temperature. Platinum is uniformly incorporated through the thickness of the films although a small excess was found on the surface. In as-deposited films, Pt is in the metallic form on the surface whereas it is mainly in the form of PtO in the bulk of the layers. Pt doping decreases the resistivity by more than an order of magnitude and improves the thermal stability of the films. The detection sensitivity to ethanol in dry air of in situ Pt-doped SnO₂ is significantly enhanced compared to undoped layers. Another beneficial effect of Pt doping is the lowering of the optimum detection temperature of ethanol.     

SnOx Thin Film Gas Sensors Prepared by Plasma Deposition from Tetrabutyltin

Amorphous ultrafine tin-containing organic thin films have been prepared by means of radio frequency (RF) glow-discharge plasma polymerization of tetrabutyltin (TBT). After being annealed, the films turned into ultrafine SnOx thin films, which possessed high gas sensitivity. The influences of the thermal annealing process on the film's composition, microstructure, and gas sensitivity to such reductive gases as EtOH, H ₂ , CH ₄ , and CO were investigated, and the effects of doping silver into the film on its gas sensitivity were also discussed in detail. By doping with Ag, the sensitivity and selectivity of the film obviously increased, the optimum operating temperature decreased, and the film color apparently changed. Based on these facts, the catalysis mechanism of doped Ag is addressed.     

Optical fiber evanescent wave absorption spectrometry of nanocrystalline tin oxide thin films for selective hydrogen sensing in high temperature gas samples

SnO₂ nanocrystalline material was prepared with a sol-gel process and thin films of the nanocrystalline SnO₂ were coated on the surface of bent optical fiber cores for gas sensing. The UV/vis absorption spectrometry of the porous SnO₂ coating on the surface of the bent optical fiber core exposed to reducing gases was investigated with a fiber optical spectrometric method. The SnO₂ film causes optical absorption signal in UV region with peak absorption wavelength at around 320 nm when contacting H₂-N₂ samples at high temperatures. This SnO₂ thin film does not respond to other reducing gases, such as CO, CH₄ and other hydrocarbons, at high temperatures within the tested temperature range from 300 °C to 800 °C. The response of the sensing probe is fast (within seconds). Replenishing of the oxygen in tin oxide was demonstrated by switching the gas flow from H₂-N₂ mixture to pure nitrogen and compressed air. It takes about 20 min for the absorption signal to decrease to the baseline after the gas sample was switched to pure nitrogen, while the absorption signal decreased quickly (in 5 min) to the baseline after switching to compressed air. The adhesion of tin oxide thin films is found to be improved by pre-coating a thin layer of silica gel on the optical fiber. Adhesion increases due to increase interaction of optical fiber surface and the coated silica gel and tin oxide film. Optical absorption spectra of SnO₂ coating doped with 5 wt% MoO₃ were observed to change and red-shifted from 320 nm to 600 nm. SnO₂ thin film promoted with 1 wt% Pt was found to be sensitive to CH₄ containing gas.     

Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique

Al doped SnO₂ thin films have been synthesized by a sol-gel dip coating technique with different percentages of Al on glass and silicon substrates. X-ray diffraction studies confirmed the proper phase formation in the films and atomic percentage of aluminium doping in the films was obtained by energy dispersive X-ray studies. SEM studies showed the particle sizes lying in the range 100–150 nm for the undoped films and it decreased with increase of Al doping. Optical transmittance spectra of the films showed high transparency (∼80%) in the visible region and the transparency increases with the increase of Al doping in the films. The direct allowed bandgap of the films have been measured for different Al concentration and they lie within the range of 3.87–4.21 eV. FTIR studies depicted the presence of Sn–O, Al–O, bonding within the films. The room temperature electrical conductivities of the films are obtained in the range of 0.21 S cm⁻¹ to 1.36 S cm⁻¹ for variation of Al doping in the films 2.31–18.56%. Room temperature Seebeck coefficients, S_RT of the films were found in the range +56.0 μVK⁻¹ to −23.3 μVK⁻¹ for variation of Al doping in the films 18.56–8.16%. It is observed that the Seebeck coefficient changes its sign at 12.05% of Al in the films indicating that below 12.05% of Al doping, SnO₂:Al behaves as an n-type material and above this percentage it is a p-type material.     

电沉积三维多孔Pt/SnO2薄膜及其对甲醇的电催化氧化

在高电流密度下以阴极析出的氢气泡为“模板”电沉积三维多孔Sn薄膜, 经在200 ℃ 2 h和400 ℃ 2 h热处理氧化后电沉积金属Pt, 制得三维多孔的Pt/SnO₂ (3D-Pt/SnO₂)薄膜. 通过扫描电镜(SEM)和X射线衍射(XRD)分析了薄膜的形貌和结构. 结果显示Pt主要沉积在SnO₂枝晶上, 形成Pt_shell/SnO_2core结构的枝晶. 在0.5 mol•dm^－3 H₂SO₄＋1.0 mol•dm^－3 CH₃OH溶液中的循环伏安结果表明, 3D-Pt/SnO₂薄膜电极在酸性溶液中电催化氧化甲醇的性能优于电沉积的纯铂电极, 而且具有较高的稳定性.     

Atomic Force Microscopy Studies of SnO2 Thin Film Microstruc tures Deposited by Atomic Layer Epitaxy

 ALE-grown, rutile-type SnO₂ thin films and gas sensor structures based thereupon were studied by AFM with main emphasis on cross-sectional investigations (X-AFM). On glass substrates the polycrystalline films showed a preferred orientation which depended on the film thickness and growth temperature while on single crystal sapphire () the growth was heteroepitaxial. For the X-AFM studies various sample preparation techniques were investigated but only ion beam etching gave satisfactory results and revealed substructures in the sensor structure consisting of Pt and SnO₂ layers on a silicon substrate.     

Mechanism of silicon film deposition in the RF plasma reduction of silicon tetrachloride

Plasma-chemical reduction of SiCl₄ in mixtures with H₂ and Ar has been studied by optical emission spectroscopy (OES) and laser interferometry techniques. It has been found that the Ar:H₂ ratio strongly affects the plasma composition as well as the deposition (r _D) and etch (r _E) rates of Si: H, Cl films and that the electron impact dissociation is the most important channel for the production of SiCl_x species, which are the precursors of the film growth. Chemisorption of SiCl_x and the reactive surface reaction SiCl_x+H–SiCl_(x–1)0+HCl are important steps in the deposition process. The suggested deposition model givesr _D [SiCl_x][H], in agreement with the experimental data. Etching of Si: H, Cl films occurs at high Ar: H₂ ratio when Cl atoms in the gas phase become appreciable and increases with increasing Cl concentration. The etch rate is controlled by the Cl atom chemisorption step.     

Photocatalytic Degradation of Glyphosate in Water by N‐Doped SnO2/TiO2 Thin‐Film‐Coated Glass Fibers

Photocatalytic degradation of glyphosate contaminated in water was investigated. The N‐doped SnO₂/TiO₂ films were prepared via sol–gel method, and coated on glass fibers by dipping method. The effects of nitrogen doping on coating morphology, physical properties and glyphosate degradation rates were experimentally determined. Main variable was the concentration of nitrogen doping in range 0–40 mol%. Nitrogen doping results in shifting the absorption wavelengths and narrowing the band gap energy those lead to enhancement of photocatalytic performance. The near optimal 20N/SnO₂/TiO₂ composite thin film exhibited about two‐ and four‐folds of glyphosate degradation rates compared to the undoped SnO₂/TiO₂ and TiO₂ films when photocatalytic treatment were performed under UV and solar irradiations, respectively, due to its narrowest band gap energy (optical absorption wavelength shifting to visible light region) and smallest crystallite size influenced by N‐doping.     

Patterning Effect of a Sol-Gel TiO2 Overlayer on the Photocatalytic Activity of a TiO2/SnO2 Bilayer-Type Photocatalyst

TiO₂ films with a thickness of 75 ± 5 nm (anatase) were formed on SnO₂-film (580 ± 80 nm) coated soda-lime glass substrates (SnO₂/SL-glass) by a sol-gel method. Although the photocatalytic activity for CH₃CHO oxidation (_ex > 300 nm) significantly exceeded that of a standard TiO₂/quartz sample, it decayed with illumination time (t) at t > 0.75 h. Stripes of anatase TiO₂ films of 40 nm in thickness and 1 mm in width were prepared on the SnO₂/SL-glass substrate in a 1-mm pitch by photolysis of an organically modified sol-gel film. The TiO₂ patterning further increased the photocatalytic activity by a factor of 4.1 as compared to the non-patterned sample, and it was also maintained at 0 < t < 2 h. The flat band potentials of the TiO₂ and SnO₂ films are determined to be –0.34 and +0.07 V (vs. SHE), respectively, at pH = 7 by the Mott-Schottky plots. On the basis of the results, the outstanding patterning effects could be rationalized in terms of the vectorial charge separation at the interface between TiO₂ and SnO₂.     

Sol–gel derived mesoporous Pt and Cr-doped WO<Subscript>3</Subscript> thin films: the role played by mesoporosity and metal doping in enhancing the gas sensing properties

Mesoporous Cr or Pt-doped WO₃ thin films to be employed as ammonia gas sensors were prepared by a fast one-step sol–gel procedure, based on the use of triblock copolymer as templating agent. The obtained films were constituted by aggregates of interconnected WO₃ nanocrystals (20–50 nm) separated by mesopores with dimensions ranging between 2 and 15 nm. The doping metals, Pt and Cr, resulted differently hosted in the WO₃ mesoporous matrix. Chromium is homogeneously dispersed in the oxide matrix, mainly as Cr(III) and Cr(V) centers, as revealed by EPR spectroscopy; instead platinum segregated as Pt (0) nanoparticles (4 nm) mainly included inside the WO₃ nanocrystals. The semiconductor layers containing Pt nanoclusters revealed, upon exposure to NH₃, remarkable electrical responses, much higher than Cr-doped and undoped layers, particularly at low ammonia concentration (6.2 ppm). This behavior was attributed to the presence of Pt nanoparticles segregated inside the semiconductor matrix, which act as catalysts of the N–H bond cleavage, decreasing the activation barrier in the ammonia dissociation. The role of the mesoporous structure in influencing the chemisorption and the gas diffusion in the WO₃ matrix appeared less decisive than the electronic differences between the two examined doping metals. The overall results suggest that a careful combination between mesoporous architecture and metal doping can really promote the electrical response of WO₃ toward ammonia.     

Sensors based on SnO<Subscript>2</Subscript> + In<Subscript>2</Subscript>O<Subscript>3</Subscript> composite films for detecting CO in air

The sensor properties of nanostructured films of SnO₂, In₂O₃, and their combinations for detecting CO in air in the temperature range of 330–520°C were investigated. It was found that SnO₂ films show the least sensitivity to CO. Sensitivity grows as the concentration of In₂O₃ in SnO₂ increases, and it reaches its maximum value in pure In₂O₃. At the same time, the maximum of sensitivity to CO in air shifts towards low temperatures. Sensor response time was found to be about 1 s for the studied SnO₂ and In₂O₃ films, and about 0.5 s for the composite film. The mechanism of sensor sensitivity for the studied metal oxide films in detecting CO in air is discussed.     

High-resolution transmission electron microscopy investigation of nanostructures in SnO2 thin films prepared by pulsed laser deposition

Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO₂ thin films onto glass substrates. The nanocrystallites and microstructures in SnO₂ thin films grown by PLD techniques have been investigated in detail by using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The PLD process was carried out at room temperature under a working pressure of about 2×10⁻⁶ mbar. Experimental results indicate that thin films are composed of a polycrystalline SnO₂ and an amorphous SnO phase. In particular, the presence of such an amorphous SnO phase in the thin films greatly limits their practical use as gas-sensing devices. HRTEM observations revealed that SnO₂ nanocrystallites with tetragonal rutile structure embed in an amorphous SnO matrix, which are approximatively equiaxed. These approximatively equiaxed SnO₂ nanocrystallites contain a high density of defects, such as twin boundaries and edge dislocations. The grain growth of SnO₂ thin films may be discussed in terms of the coalescent particle growth mechanism.     

Structure Evolution and Associated Catalytic Properties of PtSn Bimetallic Nanoparticles

Bimetallic nanoparticles (NPs) often show new catalytic properties that are different from those of the parent metals. Carefully exploring the structures of bimetallic NPs is a prerequisite for understanding the structure‐associated properties. Herein, binary Pt?Sn NPs with tunable composition are prepared in a controllable manner. X‐ray characterizations reveal that their structures evolve from SnO_2?x‐patched PtSn alloys to SnO_2?x‐patched Pt clusters when more tin is incorporated. An obvious composition‐dependent catalytic performance is observed for the hydrogenation of α,β‐unsaturated aldehydes: the selectivity to unsaturated alcohol increases substantially at high tin content, whereas the reaction rate follows a volcano shape. Furthermore, Pt sites are responsible for hydrogen dissociation, whereas oxygen vacancy (O_vac) sites, provided by SnO_2?x, drastically enhance the adsorption of carbonyl group.     

Analysis of Antimony Doping in Tin Oxide Thin Films Obtained by the Sol-Gel Method

The antimony doping in SnO₂ thin films prepared by the sol-gel dip-coating method has been studied using two characterization techniques. In order to determine the actual doping level directly in the deposited layers, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) have been used. We found that this doping level is systematically lower than expected from the starting solutions composition, and that two oxidation states are present: Sb³⁺ and Sb⁵⁺. As the antimony content increases, there is a competition between Sb⁵⁺ and Sb³⁺ species.The SnO₂: Sb thin films have also been observed by transmission electron microscopy (TEM), showing that the measured mean size of crystallites decreases as the Sb content increases in the oxide. No precipitates of either Sn or Sb oxides (other than SnO₂) could be detected.     

Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: Thermoelectric and p-type conductivity properties

In this paper, we report structural, electrical, optical, and especially thermoelectrical characterization of iron (Fe) doped tin oxide films, which have been deposited by spray pyrolysis technique. The doping level has changed from 0 to 10 wt% in solution ([Fe]/[Sn] = 0–40 at% in solution). The thermoelectric response versus temperature difference has exhibited a nonlinear behavior, and the Seebeck coefficient has been calculated from its slope in temperature range of 300–500 K. The Hall effect and thermoelectric measurements have shown p-type conductivity in SnO₂:Fe films with [Fe]/[Sn] ≥ 7.8 at%. In doping levels lower than 7.8 at%, SnO₂:Fe films have been n-type with a negative thermoelectric coefficient. The Seebeck coefficient for SnO₂:Fe films with 7.8 at% doping level has been obtained to be as high as +1850 μV/K. The analysis of as-deposited samples with thicknesses ～350 nm by X-ray diffraction (XRD) and scanning electron microscopy (SEM) has shown polycrystalline structure with clear characteristic peak of SnO₂ cassiterite phase in all films. The optical transparency (T%) of SnO₂:Fe films in visible spectra decreases from 90% to 75% and electrical resistivity (ρ) increases from 1.2 × 10^?2 to 3 × 10³ Ω cm for Fe-doping in the range 0–40 at%.     

Optical,electrical, and electrochemical behavior of p-type nanostructured SnO<Subscript>2</Subscript>:Ni (NTO) thin films

The physical and electrochemical properties of sol-gel synthesized nickel-doped tin oxide (NTO) thin films were investigated. The X-ray diffraction results showed that NTO samples exhibited a tetragonal structure. The average crystallite size and the unit cell volume of the films were reduced by Ni increment, while the stacking fault probability was increased. Furthermore, the field-emission scanning electron microscopy images clearly displayed that the worm-like surface morphology of the SnO₂ thin films was altered to the spherical feature in 3 and 10 mol% NTO samples. Moreover, by virtue of Ni incorporation, the average transparency of the SnO₂ thin films rose up from 67 to 85% in the visible region; also, the optical band gap of the SnO₂ sample (3.97 eV) increased and the thin film with 3 mol% dopant concentration showed a maximum value of 4.22 eV. The blue/green emission intensities of photoluminescence spectra of SnO₂ thin film changed via Ni doping. The Hall effect measurements revealed that by Ni addition, the electrical conductivity of tin oxide thin films altered from n- to p-type and the carrier concentration of the films decreased due to the role of Ni²⁺ ions which act as electron acceptors in NTO films. In contrast, 20 mol% Ni-doped sample had the highest mobility about 9.65 cm² (V s)^?1. In addition, the cyclic voltammogram of NTO thin films in KOH electrolyte indicated the charge storage capacity and the surface total charge density of SnO₂ thin films enhanced via Ni doping. Moreover, the diffusion constant of the samples increased from 2?×?10^?15 to 6.5?×?10^?15 cm² s^?1 for undoped and 5 mol% dopant concentration. The electrochemical impedance spectroscopy of the NTO thin films in two different potentials showed the different electrochemical behaviors of n- and p-type thin films. It revealed that the 20 mol% NTO thin film had maximum charge transfer at lower applied potential.     

Crack‐Free Periodic Porous Thin Films Assisted by Plasma Irradiation at Low Temperature and Their Enhanced Gas‐Sensing Performance

Homogenous thin films are preferable for high‐performance gas sensors because of their remarkable reproducibility and long‐term stability. In this work, a low‐temperature fabrication route is presented to prepare crack‐free and homogenous metal oxide periodic porous thin films by oxygen plasma irradiation instead of high temperature annealing by using a sacrificial colloidal template. Rutile SnO₂ is taken as an example to demonstrate the validity of this route. The crack‐free and homogenous porous thin films are successfully synthesized on the substrates in situ with electrodes. The SnO₂ porous thin film obtained by plasma irradiation is rich in surface OH groups and hence superhydrophilic. It exhibits a more homogenous structure and lower resistance than porous films generated by annealing. More importantly, such thin films display higher sensitivity, a lower detection threshold (100 ppb to acetone) and better durability than those that have been directly annealed, resulting in enhanced gas‐sensing performance. The presented method could be applied to synthesize other metal oxide homogenous thin films and to fabricate gas‐sensing devices with high performances.