Highly Sensitive SnO2 Nanorods Ethanol Sensors with the Adsorption of Au Nanoparticles

2012年10月贵金属第33卷增刊1Precious metalsVol. 33. No. SIHighly Sensitive SnO2 Nanorods Ethanol Sensorswith the Adsorption of Au nanoparticlesLI Jin, WANG Yi, MA Guang, LI Yin'e(NorthwestInstituteforNonferrousMetalResearch,Xi'an710016,China.E-maiL:13892899884@163.com)Absract: Flower-liked SnO2 nanorods were prepared by a hydrothermal method. The sensors were fabricatedusing SnO2 nanorods adsorption of Au nanoparticles through sputtering deposition. We found that the loading of asmall amount of Au nanoparticles on the surface of SnO2 nanorods can effectively enhance and functionalize thegas sensing performance of SnO, nanorods, which due to the Au adsorption make the surface-depletion effectmore pronounced. Such enhanced surface depletion increases the sensitivity, lowers the operation temperature anddecreases theKey words: SnOz nanorods; ethanol sensors; Au nanoparticlesCLC number: TB381Document Code: AArticle d:10040676(2012)S1-0107-04In the past decades, solid-state gas sensors attracted considerable attention owing to their application inedical diagnosis, environment monitoring, personal safety and chemical process controlling. Among the varioussolid-state sensors, nanostructures based on semiconductor metal oxide sensors(such as SnO2, Zno, TiO2, Fe203)are the most promising due to their high surface-to-volume ratio, simplicity in device structure, low fabricationcost, robustness in practical applications, and adaptability to a wide variety of toxic and inflammable gasesTheir gas sensing properties are largely based on the surface reaction between the metal-oxides and adsorbed gasspecies. The charge transfer interactions on the surface of such metal oxides, i.e., the adsorption of negativelycharged oxygen and the oxidative/reductive interaction between target gases and adsorbed oxygen, lead to thesignificant variation in electrical conductivity upon exposure to analyze gasesSnO2 is a stable n-type wide band gap semiconductor(Eg =3.6 eV, at 300 K)with excellent optical andelectrical properties. Nanosized SnO2 materials have been extensively investigated as regards their applicationsinchuding in transparent conductive electrodes and transistors, Li ion batteries, dyesensitized solar cells, andchemical gas sensors 2. Some techniques such as chemical and physical vaporization methods, laser ablationelectrochemical deposition, molten salt and plasma treatment, and wet-chemical preparation have beensuccessfully developed for synthesizing SnO2 nanomaterials including nanoparticles, nanowires, nanorods andnanobelts 2l. As an n-type semiconductor, SnO2 has been extensively used as a gas-sensing material. It is wellknown that the sensing mechanism of SnO belongs to the surface controlled type, in which the grainsurface states, and oxygen adsorption quantities play important roles in its gas sensitivity. So far, SnO2 hassuccessfully employed to detect various gases, such as H2S, CH3 CH2OH, O2, NO2 and NH331.Nevertheless,the中国煤化工Received date: 2012-05-01CNMHGPrecious Metalsgas sensor based on undoped SnO2 cannot satisfy current requirements for fast, continuous and trace detectionHow to make a further improvement of the gas sensitivity and selectivity with good stability is the main tendencyof gas sensor development. The researchers try to enhance the gas response by doping and compounding variouselements to the nanomaterials. For instance Kumar et al. successfully synthesized Cu-doped SnO2 nanowires andnanobelts under argon atmosphere at ambient pressure, and seen its higher gas response than bare SnO2anowires[4). However, these techniques often require high temperature and induce impurities in the final productswhen catalysts and templates are introduced to a reaction system. The active sites of doping nanowires are nothigh enough too. Therefore, taking some appropriate process to modify SnO2 nanowires may be an effective wayto enhance the gas sensing properties. Self-assemblies process is a simple and effective method for synthesizing insolution system under low temperature and ambient pressure. Recently, it was reported that ZnO nanowires withPd nanoparticles self-assembled onto their surfaces showed excellent gas sensing properties5.However,theself-assemblies process is relative complicated, and the poly(vinylpyrrolidone)(PVP) plays a key role inself-assemblies process. The relationship of PvP and Pd-ZnO sensor was also unknownIn this paper, the flower- liked SnO2 nanorods were firstly synthesized by a simple hydrothermal approach,then Au nanoparticles embedded-Sno2 nanorods (abbreviated as Au-SnOz nanorods) were obtained throughsputtering deposition. Sensing properties of the Au-SnO2 nanorods to ethanol gas were also discussed1 Experimental section1.1 SynthesisAll reagents were analytically pure, bought from Shanghai Chemical Corp, and used without furtherpurification. The SnO2 nanorods were synthesized by a simple hydrothermal rout similar to previous report. In atypical process, 2 mL SnC14 5H2O (0.5 mol/L)and 5 mL NaoH (5 molL)were mixed with 40 mL alcohol/watersolution. After stirring for about 10 min, the solution was transferred into a 50 mL Teflon- linked autoclave andkept at 190C for 1 day. The resulting white precipitates were collected, washed with distilled water and ethanolseveral times, and then dried at 60C for 12 h in air. the Sno2 nanorods were coated with Au nanoparticlesdeposited by sputtering1.2 CharacterizationX-ray diffraction (XRD) data of the products were obtained on an X'Pert MPD Philips diffractometer withCu Ka radiation. The general morphology of the products was taken on a Zeiss Supra 55 field emission scanningelectron microscopy(FE-SEM)operated at 20 kV. SEM samples were prepared by drying a dispersion of powderon a piece of aluminum foil1.3 Sensor fabrication and testGas sensing measurements were carried out on a computer controlled WS-30A system. The structure,fabrication and testing principle of our gas sensors based on the as-prepared Au-SnO2 nanorods were similar tothat for Fe203 nanotubes!. For comparison, the other gas sensor using the bare SnO2 nanorods were alsofabricated ahd tested. The sensitivity(response magnitude), S, was determined as the ratio, RRg, where Ra isthe resistance in ambient air and R is the resistance in tested gas atmosphere2 Results and discussionThe morphology of the as-synthesized SnO2 was observed via fieYH中国煤化工 on microscopeand is shown in Fig. 1. It can be clearly seen that the diameter and lengthCN MH Gd50-100 and200-500 nm, respectively. Fig. 2 shows the XRD pattern of the obtained SnO2 nanorods. All the diffraction peaksNo SILI Jin et al: Highly Sensitive SnO2 Nanorods Ethanol Sensors with the Adsorption of Au Nanoparticlescan be readily indexed to the rutile symmetry of Sno2(JCPDS file NO. 41-1445). No other impurities weredetected by XRD analysis, indicating the phase purity of the SnO2 nanorodsFig. I Field emission scanning electron microscope morphology of Fig.2 XRD pattern of the as-synthesizedthe as-synthesized SnO2SnO, nanorodsFig3 shows the sensitivity of these sensors to the examined gases, such as, acetone, ammonia, ethanolformaldehyde, 90" gasoline and toluene. All the gases were tested at an operating temperature of 300C with aconcentration of 50 ppm. As expected, the sensor based on Au-SnO2 exhibited enhanced responses for each gascompared with that based on bare SnO2 nanorods. These results strongly prove that the as-prepared Au-SnO2architectures are promising candidates for gas sensing applications. In addition, the sensor based on Au-snO2showed high response detection to ethanolacetoneNommon sSS50 ppmethanolormaldehlueneSNAu-Sno,ResponseTime/sFig3 Responses of the of the sensors fabricated by Fig 4 Typical response curves of the Au-SnO,basedbare SnO2 and Au-SnOz to various gases at 300 c sensors during cycling between increasing(The concentration of all gases was 50 ppm)concentration of ethanol at 300CFig4 shows typical isothermal response curves of Au-SnO2 based sensor when cycled with increasingethanol vapor concentrations from 1 to 100 ppm in ambient air. It can be seen that the sensors fabricated withAu-SnO2 nanorods exhibit excellent sensitivity when exposed to various ethanol concentrations at 300C.Thesensitivity continued to increase gradually with increasing vapor concentration. Their resistance underwent adramatic increase on the injection of ethanol and then rapidly recove v凵中国煤化 T released. Thesensors also show well performance to trace detection, even the ethanolCNMH GPpm Here, thesensitivity is 3.4 for 1 ppm ethanol vapor. The sensitivities are about 12.3, 18.2, 39.9 50.65 and 74.7 to 5, 10, 30110Precious metalsVol 3350 and 100 ppm ethanol, respectively. It indicates that the ethanol sensing properties are strongly enhanced by theabsorption of Au nanoparticles on the surface of SnO2 nanorodsThe enhancement in gas sensing properties on the SnO2 rods adsorption with Au nanoparticles has beenexplained by the modulation model of the depletion layer When theensors are exposed to air, oxygenmolecules can adsorb on the surface of the nanorods and form o".o and oby capturing electrons fromthe conduction band. This leads to the formation of a thick space-charge layer( depletion layer) which increasesthe potential barrier, and therefore, results in a higher resistance(Eq (1-3))O2(ads)+e→O2(ads)O2(ads+CH3CH2OH (gas)-CH3CH2OH (ads)CH3CH2OH (ads )+ 30(ads)- 2CO2(gas)+ 3H2o(gas)+ 6e(5)When the Au nanoparticles are loaded, more oxygen can be absorbed and easy dissociated on the surface ofSnO2 nanorods. The Au nanoparticles catalytically activate the dissociation of molecular oxygen which is knownas a spillover effect in catalysis. Thus, Au nanoparticles significantly increase the quantity of oxygen ions. So, adeeper depletion layer is induced by Au nanoparticles compared to the bare SnO2 nanorods. After the sensors areexposed upon a reducing gas such as ethanol, the gas will react with the adsorbed o" to form CO2 and H2O, andlease the trapped electrons back to the conduction band. This leads to an increasing carrier concentration of thesample and decreasing resistances of sensors3 ConclusionIn summary, sensors with effectively enhanced ethanol-sensing properties were achieved by loading of asmall amount of Au nanoparticles on the surface of SnO2 nanorods. With the sensitization effect of Auimproved sensing was ascribed to the enhanced surface depletion effect due to the au absorptio anorods.Thenanoparticles, the sensors exhibited much higher sensitivity than those fabricated by bare SnO2References[1] Li J, Fan HQ, Jia X H, et al. Enhanced blue-green emission and ethanol sensing of Co-doped ZnO nanocrystals prepared by asolvothermal route[J]. Appl Phys A, 2010, 98: 537-542[2]Qin L, Xu J Q, Dong X W, et al. The template-free synthesis of quare-shaped SnOz nanowires: the temperature effect andacetone gas sensors]. nanotechnology, 2008, 19: 185705-185712[3] Huang X, Choi Y K. Chemical sensors based on nanostructured materials[J]. 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