Optical Gas Sensing Properties of Silica Film Doped with Cobalt Oxide Nanocrystals

SiO₂ sol-gel films doped with cobalt oxide nanocrystals have been fabricated. The nanocrystals precipitate in the glass film at 500_°C, while the film is still porous. The nanocomposite films showed a reversible change in the optical transmittance when exposed to CO in the 250 < < 850 nm range. The effects of the residual porosity and testing temperature have been studied. The gas sensing properties of the cobalt oxide nanocrystals doped films are compared with those of nickel oxide nanocrystals doped silica film, previously reported.     

CO-sensing properties of undoped and doped tin oxide thin films prepared by electron beam evaporation

Undoped thin films of tin oxide and those doped with indium oxide and nickel oxides were deposited by electron beam evaporation. The effects of the film thickness and preparation conditions (films prepared with or without the presence of oxygen environment during deposition) on the optical and carbon monoxide sensing properties of the films were studied. The films were characterized using X-ray diffraction and X-ray photoelectron spectroscopy and optical spectroscopy techniques. All the films were found to be amorphous. It was found that the sensitivity of the films to CO increased with the thickness and the porosity of the films. It was found that their selectivity to CO gas relative to CO₂ and SO₂ gases could be improved upon doping the films with indium (or nickel) oxide.     

Comparison of the surface characteristics of polypropylene films treated by Ar and mixed gas (Ar/O2) atmospheric pressure plasma

In an attempt to modify the hydrophobic surface properties of polypropylene (PP) films, this study examined the optimum process parameters of atmospheric pressure plasma (APP) using Ar gas. Under optimized conditions, the effects of a mixed gas (Ar/O2) plasma treatment on the surface-free energy of a PP film were investigated as a function of the O2 content. The polar contribution of the surface-free energy of the PP film increased with increasing O2 content in the gas mixture. However, slightly more oxygen-containing polar functional groups such as CO, CO, and COO were introduced on the PP film surface by the Ar gas only rather than by the Ar/O2 gas mixture. In addition, AFM analysis showed that the Ar plasma treatment of the PP film produced the smoothest surface as a result of the relatively homogeneous etching process.     

Synthesis of cobalt doped silica thin film for low temperature optical gas sensor

A modified sol–gel method was used to prepare cobalt doped silica thin film with a cobalt content of 10, 20 and 30 mol% (10Co, 20Co and 30Co). The prepared films were annealed at different temperatures in the range 400–1,000 °C, and their structural evolution examined. The mixed valence cobalt oxide, Co₃O₄, crystallizes only in the sample with the higher cobalt content, while cobalt silicate is the only crystalline phase detected in the sample 10Co and 20Co. Both the cobalt content and the temperature of heat treatment resulted to affect the nature of cobalt species dispersed in the silica matrix. The 30Co was selected for further investigations by FTIR spectroscopy to follow the structural evolution of 30Co film as function of the temperature and UV–Vis to get information on the cobalt valence state. The optical gas-sensing properties of 30Co films, containing Co₃O₄ as the major cobalt phase, were studied through the measuring of the film transmittance in dry air and in presence of dry air containing variable concentrations of polluting gases, CO and NO₂. The 30Co samples resulted to be highly sensitive to CO at room temperature. An explanation for the CO sensing characteristics, at low temperature, was proposed by referring to the physisorption-related mechanics of CO.     

氧化锌薄膜/锡掺杂玻璃光波导元件及其对氯苯的气敏性研究

建立了玻璃光波导气敏元件检测氯苯气体的方法.采用浸渍-提拉法将ZnO敏感膜固定在锡掺杂玻璃光波导表面,研制出了检测氯苯气体的ZnO薄膜/锡掺杂玻璃光波导气敏元件,并用该玻璃光波导气敏元件对挥发性有机气体进行了检测.实验结果表明,在室温下,气敏元件对氯苯气体有明显的响应,而对相同浓度的其它挥发性有机气体的响应相对较小,对...     

Nanostructured black cobalt coatings for solar absorbers

Nanocrystalline black cobalt electrically deposited onto a steel substrate from aqueous solution was investigated. The influence of electrolyte composition and operating parameters on the appearance and optical properties of the coat was studied. The deposition conditions that ensure the highest solar absorptance were optimized. The chemical composition of fabricated thin films before and after annealing at 400 °C was determined by energy dispersive X‐ray analysis (EDS) and XPS technique. The crystal structure analysis showed that the bulk composition of the films was mainly cobalt oxide. The surface analysis reveals that the topmost surface layers of the films are made of different cobalt compounds confirming the multivalence state of Co on the surface with an oxidation state of ≥ + 2. Scanning electron microscope (SEM) observation indicated that the surface morphology was changed from dendritic structure to lamellar at higher current density. The black cobalt film showed soft magnetic characteristicsand excellent optical properties to transform solar energy into thermal energy. Copyright © 2008 John Wiley & Sons, Ltd.     

The gas sensing properties of TiO_2 nanotubes synthesized by hydrothermal method

In this paper,the TiO₂ nanotubes were synthesized by hydrothermal method using a 10 mol/L NaOH aqueous solution at 150℃. The structure of prepared materials was characterized by X-ray diffraction（XRD）,transmission electron microscopy（TEM）. scanning electron microscope（SEM） and Brunauer-Emmett-Teller（BET）.The prepared TiO₂ nanotubes were used to prepare thick film gas sensors and the gas sensing properties to various gases were tested.Results show the prepared TiO₂ nanotube gas sensors responses to ethanol under dry condition at 450℃.This could be attributed to the fact that it had high porous morphology and a higher pore volume,which can promote the diffusion of ethanol deep inside the films and improve the sensor response. Moreover,the gas sensor made with nanotubes exhibit high selective response towards ethanol gas compared with H₂,CO,acetone.     

Structural properties of lithium thio-germanate thin film electrolytes grown by radio frequency sputtering

In this study, lithium thio-germanate thin film electrolytes have been successfully prepared by radio frequency (RF) magnetron sputtering deposition in Ar gas atmospheres. The targets for RF sputtering were prepared by milling and pressing appropriate amounts of the melt-quenched starting materials in the nLi(2)S + GeS(2) (n = 1, 2, and 3) binary system. Approximately 1 μm thin films were grown on Ni coated Si (Ni/Si) substrates and pressed CsI pellets using 50 W power and 25 mtorr (～3.3 Pa) Ar gas pressures to prepare samples for Raman and Infrared (IR) spectroscopy, respectively. To improve the adhesion between the silicon substrate and the thin film electrolyte, a sputtered Ni layer (～120 nm) was used. The surface morphologies and thickness of the thin films were determined by field emission scanning electron microscopy (FE-SEM). The structural properties of the starting materials, target materials, and the grown thin films were examined by X-ray diffraction (XRD), Raman, and IR spectroscopy.     

Fabrication,characterization and exploration of cobalt (II) ion doped,modified zinc oxide thick film sensor for gas sensing characteristics of some pernicious gases

The present research deals with the synthesis of zinc oxide (ZnO) nanoparticles by the co-precipitation (CPT) method. The CPT method was successfully utilized for the synthesis of ZnO nanoparticles. The structural properties of undoped ZnO and cobalt doped ZnO were confirmed by employing an X-ray diffraction (XRD) study, from which the average particle size for each prepared material was calculated from the Debye Scherer formula. The average particle size confirms the nano range fabrication of undoped and cobalt doped ZnO material. The surface characteristics, morphology, texture, and porosity properties of undoped ZnO and cobalt doped ZnO were investigated from scanning electron microscopy (SEM). The elemental composition was investigated from energy dispersive spectroscopy (EDS). The High-resolution transmission electron microscopy (HRTEM) results revealed the hexagonal phase of prepared material. Furthermore, the undoped ZnO and 5% cobalt doped ZnO gas sensors prepared by screen printing technology were utilized for gas sensing purposes for testing the gases like H₂S, NO₂, SO₂, and methanol. For the gases examined, the cobalt modified ZnO sensor proved to be quite effective, especially for H₂S and NO₂ gas vapors. The Co²⁺ doped ZnO sensor showed 70.12% sensitivity for H₂S gas at 150 ⁰C and 68.75% gas response for NO₂ gas vapors at 120 ⁰C. In addition, the cobalt modified sensor was also investigated for reusability test to get concrete gas response results with the time interval of 15 days. In conclusion, it can be mentioned that the cobalt doped ZnO thick film sensor is a promising sensor for H₂S and NO₂ gas vapors.     

Gold nanoparticles to boost the gas sensing performance of porous sol–gel thin films

In this paper we review our research work of the last few years on the synthesis and the gas sensing properties of nanocomposite thin films of sensitive materials with a large specific surface area, which consist of porous matrices containing functional nanocrystals of metal oxides and gold. The film porosity provides a path for the gas molecules to reach the active reaction sites on the nanoparticles surface undergoing chemical reactions which nature depends on the nature of the active material. The introduction of Au nanoparticles affects the reactions mechanism improving the sensing process, moreover the Au Surface Plasmon Resonance peak can be used for the realization of selective optical gas sensor. Two different synthetic approaches will be described, each of them characterized by a peculiar control of the final materials morphology, structure and micro-structure.     

Facile Non‐enzymatic Lactic Acid Sensor Based on Cobalt Oxide Nanostructures

In this study, we have investigated the effect of counter anions on the morphology of cobalt oxide nanostructures. The nanostructures of cobalt oxide are prepared by a low temperature aqueous chemical growth method. The morphology of cobalt oxide nanostructure material was investigated by scanning electron microscopy and the crystalline structure was studied by powder X‐ray diffraction technique. The cobalt oxide nanostructures exhibit the nanowire, lump, bundle of the nanowire and flower‐like morphologies. The XRD study has revealed a cubic phase of cobalt oxide nanostructures. The electro‐catalytic properties of cobalt oxide nanostructures were explored through cyclic voltammetry and amperometric techniques by sensing of lactic acid in the alkaline media. The cobalt oxide nanostructures prepared from cobalt nitrate have shown a well‐resolved redox peak. The proposed mechanism for the non‐enzymatic lactic acid sensor is elucidated by considering the morphology and cyclic voltammetry response. The limit of detection for the sensor was found to be 0.006 mM and it exhibits a linear range from 0.05–3 mM of lactic acid as shown by cyclic voltammetry. The amperometric response has shown the excellent current‐concentration response and the linear range of sensor was found to be 0.1 mM to 5.5 mM. The lactic acid sensor is stable, selective and can be used for practical applications. This study provides an excellent alternative analytical tool for the determination of lactic acid.     

CO gas sensing by ultrathin tin oxide films grown by atomic layer deposition using transmission FTIR spectroscopy

Ultrathin tin oxide films were deposited on SiO2 nanoparticles using atomic layer deposition (ALD) techniques with SnCl4 and H2O2 as the reactants. These SnO(x) films were then exposed to O2 and CO gas pressure at 300 degrees C to measure and understand their ability to serve as CO gas sensors. In situ transmission Fourier transform infrared (FTIR) spectroscopy was used to monitor both the charge conduction in the SnO(x) films and the gas-phase species. The background infrared absorbance measured the electrical conductivity of the SnO(x) films based on Drude-Zener theory. O2 pressure was observed to decrease the SnO(x) film conductivity. Addition of CO pressure then increased the SnO(x) film conductivity. Static experiments also monitored the buildup of gas-phase CO2 reaction products as the CO reacted with oxygen species. These results were consistent with both ionosorption and oxygen-vacancy models for chemiresistant semiconductor gas sensors. Additional experiments demonstrated that O2 pressure was not necessary for the SnO(x) films to detect CO pressure. The background infrared absorbance increased with CO pressure in the absence of O2 pressure. These results indicate that CO can produce oxygen vacancies on the SnO(x) surface that ionize and release electrons that increase the SnO(x) film conductivity, as suggested by the oxygen-vacancy model. The time scale of the response of the SnO(x) films to O2 and CO pressure was also measured by using transient experiments. The ultrathin SnO(x) ALD films with a thickness of approximately 10 A were able to respond to O2 within approximately 100 s and to CO within approximately 10 s. These in situ transmission FTIR spectroscopy help confirm the mechanisms for chemiresistant semiconductor gas sensors.     

Synthesis of polypyrrole films for the development of ammonia sensor

In the present investigation, we have synthesized a polypyrrole films by chemical polymerization technique for the development of ammonia sensor. The polypyrrole films were synthesized in an aqueous acidic medium on glass substrate with mild oxidation of ferric chloride at temperature 29°C. The concentrations (molar) of monomer (pyrrole), oxidant (ferric chloride), and dopant (polyvinyl sulfonate) have been optimized for the uniform and porous surface morphology of the synthesized polypyrrole film. The synthesized films were characterized by scanning electron microscopy, ultraviolet‐visible, and Fourier transforms infrared spectroscopy. Ammonia gas sensing behavior of polypyrrole films was studied by using indigenously developed gas sensing chamber. The synthesized polypyrrole film with optimized process parameters shows excellent and reproducible response to low concentration (100 ppm) of ammonia gas. Copyright © 2007 John Wiley & Sons, Ltd.     

热处理对LiFe1-0.01xY0.005xAg0.005xPO4薄膜光波导传感元件气敏性的影响

以FeSO4.7H2O,H3PO4,LiOH.H2O,AgNO3及Y(NO3)3.6H2O为原料,利用水热法一步合成出了LiFe1-0.01xY0.005xAg0.005xPO4粉体(x=0.5,1.0),并将该材料作为敏感试剂,用旋转-甩涂法做成纳米薄膜固定在锡掺杂玻璃光波导表面,在不同温度下进行热处理。采用紫外-可见分光光度计、测厚仪以及自组装的玻璃光波导气敏测试仪研究了热处理对LiFe1-0.01xY0.005xAg0.005xPO4薄膜光学及气敏特性的影响。研究结果表明:在450℃下进行热处理的薄膜元件具有良好的光学透明及较好的气敏特性。相同浓度的不同挥发性有机气体中,该传感元件对二甲苯气体有很好的选择性响应,其检测响应范围为1×10-7～1×10-3(V/V),响应-恢复时间分别小于5和100 s。     

总气压与Ar/O2流量比对直流对向靶磁控溅射TiO2薄膜光催化性能的影响

采用对向靶磁控溅射法在不同气压和Ar/O₂流量比条件下, 以氟化SnO₂ (FTO)导电玻璃为基底制备了多晶TiO₂薄膜. 台阶仪测量结果显示所制备TiO₂薄膜的平均厚度约为200 nm. 随着溅射气压的升高, TiO₂薄膜由锐钛矿与金红石混晶结构转变为纯锐钛矿结构. 分别采用场发射扫描电镜(FESEM)和原子力显微镜(AFM)分析了不同气压和Ar/O₂流量比对TiO₂薄膜表面形貌的影响, 结果显示TiO₂薄膜的表面粗糙度随溅射总气压和Ar/O₂流量比的增加而增大. 以初始浓度为100×10^-6 (体积分数)的异丙醇(IPA)气体为目标物检测所制备TiO₂薄膜的光催化性能, 并分析该气相光催化反应的机理, 在紫外照射条件下异丙醇先氧化为丙酮再被氧化为CO₂.当总溅射气压为2.0 Pa、Ar/O₂流量比为1:1时, 溅射所得TiO₂薄膜具备最优光催化活性并可在IPA降解反应中保持较高的催化活性和稳定性.     

氮掺杂二氧化钛-氧化镍双层薄膜的光电致色特性

以金属钛为靶材、O2/N2/Ar混合气氛为溅射气体,在导电玻璃(ITO)表面磁控溅射一层薄膜,再经300-500℃退火处理制备了氮掺杂TiO2薄膜.采用X射线衍射(XRD)、X光电子能谱(XPS)、扫描电子显微镜(SEM)和紫外-可见吸收光谱等对薄膜的微观结构、光学特性和光电化学性能等进行了研究.进而采用化学沉积的方法在TiO2-xNx薄膜表面沉积上一层多孔NiO薄膜,研究表明,制备的ITO/TiO2-xNx/NiO双层薄膜具有明显的光电致色特性,400℃退火处理的氮掺杂TiO2薄膜具有最高的光电流响应,经氙灯照射1h后,薄膜从无色变成棕色,500nm波长处光透过率从79.0%下降至12.6%.     

Development of an optical ammonia sensor based on polyaniline/epoxy resin (SU-8) composite

Polyaniline (PANI)/glycidyl ether of bisphenol A (SU-8) composite film is elaborated in order to detect ammonia gas. These composite films are characterized by ultraviolet-visible (UV-vis) spectroscopy, Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The sensitivity to ammonia is measured by optical absorption changes. The ammonia sensing properties of PANI/SU-8 composite films are studied, and then are compared to pure PANI films elaborated by chemical way. Experimental results show that the PANI/SU-8 optical sensor has simultaneously a rapid response to ammonia gas and regenerates easily, that is advantageous compared to pure PANI films.     

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.     

Gas sensing mechanism in chemiresistive cobalt and metal-free phthalocyanine thin films

The gas sensing behaviors of cobalt phthalocyanine (CoPc) and metal-free phthalocyanine (H2Pc) thin films were investigated with respect to analyte basicity. Chemiresistive sensors were fabricated by deposition of 50 nm thick films on interdigitated gold electrodes via organic molecular beam epitaxy (OMBE). Time-dependent current responses of the films were measured at constant voltage during exposure to analyte vapor doses. The analytes spanned a range of electron donor and hydrogen-bonding strengths. It was found that, when the analyte exceeded a critical base strength, the device responses for CoPc correlated with Lewis basicity, and device responses for H2Pc correlated with hydrogen-bond basicity. This suggests that the analyte-phthalocyanine interaction is dominated by binding to the central cavity of the phthalocyanine with analyte coordination strength governing CoPc sensor responses and analyte hydrogen-bonding ability governing H2Pc sensor responses. The interactions between the phthalocyanine films and analytes were found to follow first-order kinetics. The influence of O2 on the film response was found to significantly affect sensor response and recovery. The increase of resistance generally observed for analyte binding can be attributed to hole destruction in the semiconductor film by oxygen displacement, as well as hole trapping by electron donor ligands.     

Characterization of the Optical and Gas Sensitivities of a Nickel-Doped Lithium Iron Phosphate Thin Film

The influence of heat-treatment temperature on the optical properties (refractive index, transmittance, and attenuation) and gas sensitivities of nickel-doped lithium iron phosphate (LiFe_0.99Ni_0.01PO₄) thin films were discussed. LiFe_0.99Ni_0.01PO₄ was synthesized in one step using hydrothermal methods and fixed to tin-diffused glass as a sensing film by spin-coating before calcination at different temperatures. The obtained thin films were characterized by refractive index, thickness, attenuation, and porosity, as well as gas sensing performances for benzene, toluene, and xylene. The experimental results indicated that the LiFe_0.99Ni_0.01PO₄ thin films dried at 450°C displayed higher refractive indices, good transparency, and less attenuation; thus, the resulting sensor of a LiFe_0.99Ni_0.01PO₄ thin film/tin-diffused optical wave-guide exhibited a greater response to xylene in the concentration range of 0.1–1000?ppm.