材料专业文献翻译.doc

Four short words sum up what has lifted most successful individuals above the crowd: a little bit more.-author-date材料专业文献翻译材料专业文献翻译Synthesis and Gas Sensitivity of In2O3/CdO CompositeAbstract: Indium oxide (In2O3) was synthesized using a hydrothermal process. The crystallography and microstructure of the synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). The In2O3 had a flower-like hierarchical nanostructure and was composed of tiny near-spherical crystals with a diameter of approximately 20 nm. When In2O3 was mixed with CdO in a 1:1 molar ratio, it was found that the resulting In2O3/CdO composite showed an interesting grape-like porousmicrostructure following calcinations at elevated temperatures. A gas sensor using this In2O3/CdOcomposite as the sensing material showed higher sensitivity to different concentration of formaldehyde than the gas sensor based on pure flower-like In2O3 nanomaterials. The In2O3/CdO-based sensors showed a high sensitivity to a concentration of 0.05×10-6 formaldehyde at the optimized operating temperature of 410 °C and a good level of selectivity over other possible interference gases such as ethanol, toluene, acetone, methanol, and ammonia. The gas sensing mechanism of In2O3/CdO sensor has been discussed in detail.1 IntroductionFormaldehyde (HCHO) is a colorless and strong-smellinggas coming from building materials, interior decoration materials,wood furniture, carpet and so on.HCHO is one of the most dangerous indoor pollutants among volatile organic compounds (VOCs), and is found to be associated with asthma, nasopharyngealcancer, and multiple subjective health complaints. In particular, HCHO is considered as a major cause of sick building syndrome (SBS). World Health Organization (WHO) established a standard of 0.08×10-6 (volume fraction)averaged over 30 min for long-term exposure in formaldehyde vapor. Many methods to detect VOCs have been investigated. Among them, semiconductor gas sensors are widely used since they are cheap and easy to be available. The sensing materials, including SnO2,10-12 ZnO,13 NiO,14 and In2O3,15,16 have been explored for formaldehyde detection.In recent years, nanostructure semiconductor materials havebeen extensively studied due to their exceptional propertiesand potential applications in various fields. Among them, indium oxide (energy gap 3.67 eV, Bohr radius 2.14 nm) material has been widely studied because of its unique optoelectronic properties, such as high electrical conductivity and high UV transparency. It has been widely used in the optoelectronic devices such as solar cells, window heaters, and liquid crystal displays。It has been also explored for sensing materials for detectingO3， HCHO, trimethylamine (TMA),NO2 CO,and NH3。Various vapor-phase or physical template methods were developed to prepare In2O3 nanocrystals. For example, In2O3 nanowires were synthesized by using the vapor-liquid- solid technique。The In2O3 nanowire arrays or nanorods were induced by template-assisted growth, and the In2O3 nanobelts were obtained through thermal evaporation。Besides these physical methods, there are also wet-chemistry methods to prepare specific In2O3 nanostructures. For instance, In2O3 with structures of nanorod bundles, spherelike agglomerates, lotus-root-like, and nanotubes were successfully synthesized by hydrothermal route。Quasi-monodisperse In2O3 nanocrystals were obtained through an organic solution synthetic route。In this work, the flower-like hierachical nanostructure In2O3 composed of the tiny spherical nanocrystallines was fabricated by using the hydrothermal method. Then, the as-synthesized In2O3 powders were mixed with CdO in a molar ratio of 1:1 to form a gas sensing material. The formaldehyde sensing properties of the In2O3/CdO-based sensors were carried out.2 Experimental2.1 Preparation and characterization of materialsInCl3·4H2O (99.5%) was obtained from Sinopharm Chemical Reagent Co., Ltd., China. Ethylene diamine tetra acetic acid (99.5%, EDTA, C10H16N2O8) and CS(NH2)2 (99.0%) were obtained from Tianjin Kermel Chemical Reagent Co., Ltd.,China. CdO (99.5%) powder was analytical grade with 30 nm particle size, and purchased from Haitai Nanometer Materials Co., China. All of the reagents used in the experiments were analytical grade and utilized without further purification.Flower-like In2O3 was synthesized by a hydrothermal method. In a typical procedure, 1 mmol InCl3·4H2O and 2 mmol CS(NH2)2 were dissolved in 30 mL EDTA. A few drops of ammonia were dripped into the solution, and the solution was under the conditions of ultrasonic dispersing and constant stirring alternately for 20 min. Then, the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an electric oven at 180 °C for 18 h. After that, the autoclave was cooled to room temperature naturally. The pink precipitate was collected and washed with ethanol and deionized water alternately for several times. Then it was dried in electric oven at 80 °C and the precursor was generated. Flowerlike In2O3 was obtained by roasting the precursor at 600 °C in muffle for 1 h.X-ray diffraction (XRD) patterns of the powders were examined in 2 region of 20°-80° with Cu K (0.154 nm) radiation on Rigaku, Model D/MAX 2400, Japan. Scanning electron microscopy (SEM) images were examined on a FEI QUANTA200F (United States) microscope equipped with energy dispersive X-ray (EDX) spectroscopy. Transmission electron microscopy (TEM) image was carried out to obtain direct information about the size and structure by Tecnai G220 S-Twin transmission electron microscope.2.2 Fabrication and measurement of gas sensorsThe In2O3 and CdO powders were mixed in a molar radio of 1:1 and ground with deionized water to form a paste. The paste was painted on a clean ceramic tube ( 2 mm × 4 mm) on which a pair of Au electrodes were previously printed, and then sintered at 600 °C for 2 h. A Ni-Cr heating wire with 30 as a heater was inserted through the tube to provide heating for gas sensor. The electrode and heater wires were welded on a base to form gas sensor. The fabricated gas sensors were aged with a heating temperature of 300 °C for 240 h in air.The gas sensing properties of In2O3/CdO composite gas sensors were tested in a sealed chamber. The testing temperature and humidity were 20 °C and 20%RH (relative humidity), respectively. A heating voltage which was provided by a d.c.power supply (GPS-3303C, Guwei Electronic, Taiwan) was supplied to the wire of sensor for providing a operating temperature, and a circuit voltage was supplied across the sensor and the load resistor connected to the sensor in series. The output voltage across the load resister was recorded by a data acquisition card which was connected to a computer to record the real- time data. The whole system was controlled by a computer automatically. 3 Results and discussion3.1 Characterizations of In2O3 and In2O3/CdO compositeThe XRD pattern of the In2O3 is shown in Fig.1. All peaks can be indexed to pure cubic phase of In2O3 (JCPDS No. 65-3170), indicating that a pure phase of In2O3 was obtained by calcining the precursor. The crystalline size of the In2O3 was calculated by using Debye Scherror formula, D=0.89/cos, where D is the average grain size, is the X-ray wavelength (0.154 nm), is full-width at half-maximum, and is diffraction angle. The calculated average crystalline size of the In2O3 is about 22 nm.Fig.2 shows the SEM images of the In2O3 material. It can be seen that the as-prepared In2O3 nanoparticles were congregated together and formed flower-like microstructure with a diameter of several to ten micrometers. Fig.2(b) shows that there are many wrinkles and holes on the“flower”.Fig.3 shows the TEM image of the as-synthesized In2O3. The In2O3 nanoparticles are uniform and the shapes of the crystallites are spherical and ellipse. The grain size of the In2O3 is around 20 nm consistent with the calculated result.From Figs.2 and 3, it can be seen that the as-prepared flower-like In2O3 particles were formed with the tiny crystallites indicating a hierarchical nanostructure in nature. The formation of the flower-like hierarchical structure In2O3 by using the hydrothermal method could be described as follows. The chemical reaction occurred in the InCl3 mixture during the preparation process:In3+EDTA=In(EDTA) (1)Then, the following reactions were presented:NH3·H2O=NH4+OH- (2)CS(NH2)2+2OH-=S2-+CO2+2NH3 (3)2In(EDTA)+3S2-=In2S3+2(EDTA)3- (4)2In2S3+9O2=2In2O3+6SO2 (5)EDTA is a strong complex agent and easily reacts with metalions. EDTA chemical reaction (1) was the complex reaction and resultant was In(EDTA). Then the ammonia ionized to provide an alkaline condition (reaction (2). CS(NH2)2 can easily hydrolyze in the alkaline condition and the reaction (3) occurred to generate S2- ions. A replacement reaction occurred between In(EDTA) and S2- in reaction (4), consequently forming the precursor. The ultrasonic dispersing and stirring process made nano-clusters be uniform. The precursor In2S3 dispersed at a form of nano-clusters. On the other hand, EDTA had strong adsorbability and it was easy to conglomerate. When Teflon-lined stainless steel autoclave provided a high temperature and high pressure situation, some precursor particles conglomerated together and formed some spherical and ellipse structure row materials with different magnitudes in this experiment. Eventually, the row materials were calcined and the inner organic matters became vapors of CO2, SO2, and H2O, and the In2O3 was obtained (reaction (5). The vapors went into surrounding air through some pore canals in the row material, while wrinkles appeared on the surface and formed flower-like In2O3 material.The composite of In2O3/CdO nanoparticles (a mixture of In2O3 and CdO) was examined by XRD as shown in Fig.4. All peaks can be indexed to both pure cubic phase of In2O3 (JCPDS No. 65-3170) and pure cubic CdO (JCPDS No. 65-2908). No new phases appeared. The EDX pattern shown in Fig.5 reveals that the In2O3/CdO nanoparticles are composed of In, Cd,and O. The C peak in the spectrum is attributed to the electric latex of the SEM sample holder.Fig.6 gives the SEM images of In2O3/CdO composite. It can be seen that the composite presents many ball-shape particles, and there are many gaps and holes between the balls, which indicate that there is less conglomeration in the materials. Therefore, the hierarchical structure of the flower-like In2O3 materials was broken and the In2O3 nanoparticles mixed with CdO nanoparticles forming uniform In2O3/CdO composite materials.3.2 Gas sensing propertiesThe sensitivity (S) of a gas sensor is calculated as follows: S=Ra/Rg (6) where Ra and Rg are resistances of a sensor in air and in detected gas, respectively.Fig.7 indicates the sensitivity of the In2O3/CdO gas sensor as a function of operating temperature in a range of 240-490 °C and the formaldehyde concentration is 30×10-6 (volume fraction). The sensitivity of the sensor was highest at 410 °C, and therefore, 410 °C was used as operating temperature during the following In2O3/CdO sensor measurement.Fig.8 shows the sensitivity of gas sensors based on In2O3 nanoparticles, In2O3/CdO composite to different formaldehyde concentrations in a range of 0.05×10-6-30×10-6 at the operating temperature of 410 °C, respectively. An inset figure is the sensitivity of the both kinds of sensors to gas concentration range of 0.05×10-6-1.0×10-6. We can see that the sensitivity of the gas sensor based on In2O3/CdO composite is much higher than that based on In2O3 nanoparticles. The 0.05 × 10-6 is the lowest concentration which the sensors based on In2O3/CdO composite can detect and the corresponding sensitivity is 1.45. Therefore, it can be seen that the In2O3/CdO sensor can well meet the need of the standard of World Health Organization (0.08×10-6 (volume fraction).Fig.9 shows response transient of the In2O3/CdO gas sensor to 0.05 × 10-6 formaldehyde. The change on output voltage of the gas sensor is 380 mV when the surrounding vapor changes from air to 0.05×10-6 formaldehyde.Fig.10 shows the response transient of In2O3/CdO composite gas sensor in 10×10-6 formaldehyde as a function of time. The response time and recovery time of the sensor to 10×10-6 formaldehyde are about 70 and 110 s, respectively. The response time and recovery time are defined as the time for a sensor to attain 90% of the final equilibrium values.Selectivity (cross sensitivity) is an important property for practical gas sensors. The sensitivities of the In2O3/CdO gas sensor to five interference gases including ethanol, toluene, acetone, methanol, and ammonia with a concentration range of 0.1 × 10-6 to 10 × 10-6 were examined, as shown in Fig.11(a). Fig.11(b) gives the sensitivities of the gas sensor to 10×10-6 different gases. The sensitivity of the In2O3/CdO gas sensor to formaldehyde is higher than the sensitivities to the other interference gases. The sensitivities of the gas sensor to low concentration of toluene, acetone, methanol, and ammonia are very small and negligible. Therefore, it can be concluded that the In2O3/CdO gas sensor has a good selectivity to formaldehyde.Fig.12 gives the stability of the In2O3/CdO gas sensor in 10×10-6 formaldehyde vapor with a measurement temperature range of 18-22 ° C. The sensitivity of the sensor does not change very much in this time range.3.3 Sensing mechanismBoth In2O3 and CdO are typical n-type semiconductor materials. When In2O3/CdO composite was exposed to the reducing vapors such as formaldehyde, the electrons will transfer from the vapor to the sensing material by taking away the adsorbed oxygen on the oxide surface. Oxygen in air takes electron from the surface of n-type material and becomes adsorbed oxygen (O2 -ads ). One or both of the following two reactions could happen when In2O3/CdO composite meets formaldehyde molecules: CHOH(g)+O2 -ads CO2(g)+H2O(g)+2e- (7)CHOH(g)+OadsHCOOH(g)+e- (8)The products could be water and CO2 and/or formic acid (HCOOH). Both of the reactions produce electrons, and the electron concentration in the sensing oxides will increase. As a result, the resistance of the In2O3/CdO composite gas sensor will decrease.The mechanism of the stronger response of the In2O3/CdO composite gas sensor to formaldehyde is not quite clear yet. Some possible explanations are as follows. Firstly, the specific surface area of material, especially porosity, has important influences on the ga