In2O3/CdO复合材料的制备及气敏特性

陈鹏鹏 王 兢,* 姚朋军,2 杜海英,3 李晓干

(1大连理工大学电子科学与技术学院,大连116023;2沈阳师范大学教育技术学院,沈阳110000; 3大连民族学院机电信息工程学院,大连116600)

In2O3/CdO复合材料的制备及气敏特性

陈鹏鹏1王 兢1,*姚朋军1,2杜海英1,3李晓干1

(1大连理工大学电子科学与技术学院,大连116023;2沈阳师范大学教育技术学院,沈阳110000;3大连民族学院机电信息工程学院,大连116600)

采用水热合成方法制备了花状In2O3纳米材料.利用X射线衍射(XRD)、扫描电镜(SEM)、能量色散X射线光谱(EDX)及透射电镜(TEM)对材料的结晶学特性及微结构进行了表征.制备的In2O3材料呈现花状,是由粒径约20 nm的椭球状小颗粒构成的分级结构材料.将制备的In2O3与纳米CdO以摩尔比1:1混合后,发现制成的In2O3/CdO复合材料经热处理后呈现葡萄状多孔结构.测试In2O3/CdO复合材料制作的气敏元件处于最佳工作温度(410°C)时,对0.05×10-6(体积分数,φ)的甲醛气体表现出较高的灵敏度.对比测试发现,In2O3/CdO复合材料制作的气敏元件对不同浓度甲醛的灵敏度明显优于纯花状In2O3纳米材料.同时In2O3/CdO复合材料制作的气敏元件在乙醇、甲苯、丙酮、甲醇以及氨气等干扰气体中具有对甲醛良好的选择性.讨论了In2O3/CdO复合材料气敏元件的敏感机理.

复合材料;气敏元件;甲醛气体;纳米材料;多孔结构

1 Introduction

Formaldehyde(HCHO)is a colorless and strong-smelling gas 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,nasopharyngeal cancer,and multiple subjective health complaints.1In particular,HCHO is considered as a major cause of sick building syndrome(SBS).2World Health Organization (WHO)established a standard of 0.08×10-6(volume fraction) averaged over 30 min for long-term exposure in formaldehyde vapor.3Many methods to detect VOCs have been investigated.4-9Among them,semiconductor gas sensors are widely used since they are cheap and easy to be available.The sensing materials,including SnO2,10-12ZnO,13NiO,14and In2O3,15,16have been explored for formaldehyde detection.

In recent years,nanostructure semiconductor materials have been extensively studied due to their exceptional properties and 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.17It has been also explored for sensing materials for detecting O3,18,19HCHO,12,13trimethylamine(TMA),20NO2,21CO,22and NH3.23Various vapor-phase or physical template methods were developed to prepare In2O3nanocrystals.For example, In2O3nanowires were synthesized by using the vapor-liquid-solid technique.24The In2O3nanowire arrays25or nanorods26were induced by template-assisted growth,and the In2O3nanobelts were obtained through thermal evaporation.27

Besides these physical methods,there are also wet-chemistry methods to prepare specific In2O3nanostructures.For instance,In2O3with structures of nanorod bundles,spherelike agglomerates,lotus-root-like,and nanotubes were successfully synthesized by hydrothermal route.28,29Quasi-monodisperse In2O3nanocrystals were obtained through an organic solution synthetic route.30

In this work,the flower-like hierachical nanostructure In2O3composed of the tiny spherical nanocrystallines was fabricated by using the hydrothermal method.Then,the as-synthesized In2O3powders 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 Experimental

2.1 Preparation and characterization of materials

InCl3·4H2O(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 In2O3was synthesized by a hydrothermal method.In a typical procedure,1 mmol InCl3·4H2O and 2 mmol CS(NH2)2were 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 In2O3was obtained by roasting the precursor at 600°C in muffle for 1 h.

X-ray diffraction(XRD)patterns of the powders were examined in 2θ region of 20°-80°with Cu Kα(0.154 nm)radiation on Rigaku,Model D/MAX 2400,Japan.Scanning electron microscopy(SEM)images were examined on a FEI QUANTA 200F(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 sensors

The In2O3and 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(Φ 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 Ω 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 discussion

3.1 Characterizations of In2O3and In2O3/CdO composite

The XRD pattern of the In2O3is 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 In2O3was obtained by calcining the precursor.The crystalline size of the In2O3was calculated by using Debye Scherror formula,D=0.89λ/βcosθ, where D is the average grain size,λ is the X-ray wavelength (0.154 nm),β is full-width at half-maximum,and θ is diffraction angle.The calculated average crystalline size of the In2O3is about 22 nm.

Fig.1 XRD pattern of In2O3powders

Fig.2 shows the SEM images of the In2O3material.It can be seen that the as-prepared In2O3nanoparticles 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 In2O3nanoparticles are uniform and the shapes of the crystallites are spherical and ellipse.The grain size of the In2O3is around 20 nm consistent with the calculated result.From Figs. 2 and 3,it can be seen that the as-prepared flower-like In2O3particles were formed with the tiny crystallites indicating a hierarchical nanostructure in nature.

The formation of the flower-like hierarchical structure In2O3by using the hydrothermal method could be described as follows.The chemical reaction occurred in the InCl3mixture during the preparation process:

Fig.2 SEM images of In2O3material with different magnifications

Fig.3 TEM image of In2O3material

EDTA is a strong complex agent and easily reacts with metal ions.31EDTA 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)2can 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 In2S3dispersed 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 In2O3was 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 In2O3material..

The composite of In2O3/CdO nanoparticles(a mixture of In2O3and 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.4 XRD pattern of In2O3/CdO composite

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 In2O3materials was broken and the In2O3nanoparticles mixed with CdO nanoparticles forming uniform In2O3/CdO composite materials.

Fig.5 EDX pattern of the In2O3/CdO nanoparticles

Fig.6 SEM image of In2O3/CdO composite

3.2 Gas sensing properties

The sensitivity(S)of a gas sensor is calculated as follows:

where Raand Rgare 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.7 Sensitivity(Ra/Rg)of In2O3/CdO gas sensor vs operating temperature

Fig.8 shows the sensitivity of gas sensors based on In2O3nanoparticles,In2O3/CdO composite to different formaldehyde concentrations in a range of 0.05×10-6-30×10-6at 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 In2O3nanoparticles.The 0.05×10-6is 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 0.05×10-6formaldehyde.The change on output voltage of the gas sensor is 380 mV when the surrounding vapor changes from air to 0.05×10-6formaldehyde.

Fig.10 shows the response transient of In2O3/CdO composite gas sensor in 10×10-6formaldehyde as a function of time.The response time and recovery time of the sensor to 10×10-6formaldehyde 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.

Fig.8 Sensitivity of gas sensor based on In2O3,In2O3/CdO vs formaldehyde concentration

Fig.9 Response transient of In2O3/CdO gas sensor to 0.05×10-6formaldehyde

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 0.1×10-6to 10×10-6were examined,as shown in Fig.11(a). Fig.11(b)gives the sensitivities of the gas sensor to 10×10-6different 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.10 Response transient of In2O3/CdO gas sensor to 10×10-6formaldehyde

Fig.12 gives the stability of the In2O3/CdO gas sensor in 10× 10-6formaldehyde vapor with a measurement temperature range of 18-22°C.The sensitivity of the sensor does not change very much in this time range.

Fig.11 Sensitivities of In2O3/CdO gas sensor to different gasesφ:(a)0.1×10-6-10×10-6,(b)10×10-6

Fig.12 Sensitivity of In2O3/CdO gas sensor vs time

3.3 Sensing mechanism

Both In2O3and 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.32Oxygen in air takes electron from the surface of n-type material and becomes adsorbed oxygenOne or both of the following two reactions could happen when In2O3/CdO composite meets formaldehyde molecules:33,34

4 Conclusions

The flower-like hierarchical structured In2O3was synthesized by the hydrothermal process.The as-synthesized In2O3showed a flower-like nanostructure and was assembled by the small crystals(～20 nm).The porous composite of In2O3/CdO was prepared by mixing In2O3and CdO(1:1 in molar ratio). The In2O3/CdO composite showed grapes-like microstructure and indicated a high porosity.The highest sensitivity of the sensor based on the In2O3/CdO composite appeared when an operating temperature was 410°C.The lowest formaldehyde concentration detected by In2O3/CdO gas sensor was 0.05×10-6(volume fraction).The response time,and recovery time of the sensor to 10×10-6formaldehyde were about 70 and 110 s,respectively.The selectivity of the In2O3/CdO gas sensor to formaldehyde was good when the interference gases were ethanol, toluene,acetone,methanol,and ammonia.The porous microstructure in the grape-like In2O3/CdO composite,which could provide more opportunity for vapor to contact with the inner sensing materials,hetero-junction at the interface of In2O3and CdO grains,and stronger adsorption capacity of the composite to formaldehyde could be the possible reasons for the better selectivity achieved for the sensors.

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Synthesis and Gas Sensitivity of In2O3/CdO Composite

CHEN Peng-Peng1WANG Jing1,*YAO Peng-Jun1,2DU Hai-Ying1,3LI Xiao-Gan1
(1School of Electronic Science and Technology,Dalian University of Technology,Dalian 116023,Liaoning Province,P.R.China;2School of Educational Technology,Shenyang Normal University,Shenyang 110000,P.R.China;3Department of Electromechanical Engineering&Information,Dalian Nationalities University,Dalian 116600,Liaoning Province,P.R.China)

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 In2O3had a flower-like hierarchical nanostructure and was composed of tiny near-spherical crystals with a diameter of approximately 20 nm.When In2O3was mixed with CdO in a 1:1 molar ratio,it was found that the resulting In2O3/CdO composite showed an interesting grape-like porous microstructure following calcinations at elevated temperatures.A gas sensor using this In2O3/CdO composite as the sensing material showed higher sensitivity to different concentration of formaldehyde than the gas sensor based on pure flower-like In2O3nanomaterials.The In2O3/CdO-based sensors showed a high sensitivity to a concentration of 0.05×10-6formaldehyde 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.

Composite material;Gas sensor;Formaldehyde vapor;Nano-materials; Porous microstructure

The authors also gratefully thank Mr. CHEN Xing-Ru,Dalian University of Technology,for his help in the experiments.

10.3866/PKU.WHXB201204101

∗Corresponding author.Email:wangjing@dlut.edu.cn;Tel:+86-411-84708382;Fax:+86-411-84706706.

The project was supported by the National Natural Science Foundation of China(61176068,61131004,61001054).

国家自然科学基金(61176068,61131004,61001054)资助项目

O649.4