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Sensors and Actuators B 158 (2011) 199– 207

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

in–copper mixed metal oxide nanowires: Synthesis and sensor response tohemical vapors

iaopeng Lia, Zhiyong Gua,∗, JungHwan Chob, Hongwei Sunc, Pradeep Kurupb

Department of Chemical Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USADepartment of Civil and Environmental Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USADepartment of Mechanical Engineering, University of Massachusetts Lowell, One University Ave., Lowell, MA 01854, USA

r t i c l e i n f o

rticle history:eceived 30 December 2010eceived in revised form 27 May 2011ccepted 1 June 2011vailable online 12 June 2011

eywords:

a b s t r a c t

Tin–copper mixed metal oxide nanowires were successfully prepared by thermally oxidizing elec-trodeposited metallic nanowires (Sn–8 at.% Cu, Sn–43 at.% Cu and Sn–86 at.% Cu). The structure andcomposition of these nanowires before and after thermal oxidation were characterized by scanningelectron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD).Dielectrophoresis was utilized to align the nanowires in contact with pre-fabricated interdigitated elec-trodes to form a chemiresistive gas sensor circuit. The sensitivity variation of the nanowires with different

anowireixed metal oxide

ynthesishemical gas sensor

compositions was tested with acetone, ethanol and ethyl acetate vapors at different concentration levels,and the temperature effect was studied at five operating temperatures, ranging from 200 ◦C to 440 ◦C.All the three mixed metal oxide nanowire sensors exhibited higher sensitivity than that of pure tinoxide nanowire sensor. The sensor performance was also investigated in terms of response/recoverytime and repeatability. An interesting positive/negative response was observed by varying the element

oxid

composition of the mixed

. Introduction

The concept of using semiconducting metal oxide material as aetector for gaseous component was proposed as early as in 1966y Seiyama and Kagawa [1]. Ever since then, various metal oxidesuch as SnO2 [2], CuO/CuO2 [3], TiO2 [4], In2O3 [5], NiO [6], Co3O4 [7]nd ZnO [8], have been synthesized and the semiconductor typedas sensors were used for automotive [9], indoor air quality control10] and homeland security [11]. However, these sensing materialsre normally used in the form of bulk materials or thin films in con-entional sensors, which have draw backs in terms of large poweronsumption and long response time [12].

With the rising tide of nanotechnology, researchers find thathe sensing properties of nanostructured metal oxide materialsiffer substantially from bulk materials of the same element andomposition, and may lead to sensors with high sensitivity, fastesponse/recovery time, low energy consumption and small inize making it ideal for portable devices. To date, semiconduct-ng metal oxide nanostructures have been successfully synthesizednd intensively studied. In addition, in order to further improve

he sensor performance, many efforts have been devoted to theevelopment of modified metal oxide materials featuring multiplelements instead of traditional single element metal oxide material.

∗ Corresponding author. Tel.: +1 978 934 3540; fax: +1 978 934 3047.E-mail address: Zhiyong Gu@uml.edu (Z. Gu).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.06.004

e nanowires.© 2011 Elsevier B.V. All rights reserved.

Among them, p-type copper oxide and n-type tin oxide have drawnmuch attention due to their desirable electrical, ferromagnetic[13], and chemical properties exhibiting high sensitivity [14,15]toward gas sensing, with promising applications in catalyst [16],bio/chemical sensors [17,18] and field effect transistors [19–21].Currently the major methods of creating tin–copper (Sn–Cu) mixedoxide materials include: (1) doping a small amount of copperinto tin oxide [22,23] and (2) fabricating heterostructured copperoxide/tin oxide composites [24–26]. In most cases above, tin oxideis chosen to be the primary component while the molar percent-age of copper is controlled at a relatively low level. However, thehigh mutual solubility of copper in tin makes it possible to expandthe copper fraction, even more than 50%. This may significantlyexpand the sensing window and provide more freedom for sensingability.

In this article, we report a unique method to synthesize mixedmetal oxide nanowires with a wide range of composition whichstarted with synthesis of electrodeposited tin–copper metallicnanowires, followed by a thermal oxidation process. Tin–coppermixed metal oxide nanowires with different compositions wereinvestigated, and dramatic surface morphology difference wasobserved from samples with varying compositions. Further explo-ration on their sensor performance was carried out by both static

and dynamic method, to reveal the sensitivity and repeatability ofthese materials toward chemical exposure. The result also showedan interesting positive/negative response by varying the elementcomposition of the mixed oxide nanowires.

200 X. Li et al. / Sensors and Actuators B 158 (2011) 199– 207

e and

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Fig. 1. Schematic diagram of sensor chip fabricating procedur

. Experimental procedures

.1. Nanowire synthesis and characterization

Polycarbonate (PC) membrane with porous structure of 50 nmach in diameter (nominal size) was utilized to fabricate metal-ic nanowires. One side of this membrane was coated with a thinayer of silver (200 nm) and put into contact with a copper plates cathode, while the other side was immersed in the platingolution with a platinum anode. The growth of nanowires insidehe membrane was achieved by applying current between thelectrodes – a typical electrodeposition process. Later the PC mem-rane was dissolved in dichloromethane to release nanowires. Theetailed fabrication process has been described in a previous papery our group [27]. The plating solution was prepared by mixingommercial plating solution of copper (Cu) (Copper U Bath RTU,echnic, Inc.) and tin (Sn) (Techni Tin) together with a correspond-ng make-up solution and antioxidant (obtained from Techni, Inc.oo). Because copper and tin can be co-deposited at any ratio28], we made appropriate adjustment to the electroplating batholution to control the composition. As a result, mixed metallicanowires of Sn–8 at.% Cu, Sn–43 at.% Cu and Sn–86 at.% Cu were

abricated under the conditions of applied current: 18 mA and timeuration: 70 s. The next step was to convert tin–copper metallicanowires into their corresponding oxides. For that purpose, a ther-al treatment was carried out in a tube furnace (Thermo Scientific

indberg Blue M) at 600–700 ◦C for 5 h with two ends of the quartzube open to the air to ensure sufficient oxygen circulation. Afteranowire fabrication and treatment, the nanowires were charac-erized using a JEOL JSM-7401F field emission scanning electron

icroscope (FE-SEM) and an EDAX Genesis V4.61 energy disperse

-ray detector before and after thermal oxidation, for morphology

nformation and elementary analysis. X-ray diffraction (XRD) pat-erns were obtained using Oxford diffraction Xcalibur PX Ultra withNYX detector for crystal phase study.

top-view of the sensor chip surface (SEM image, bottom left).

2.2. Nanowire sensor integration

A schematic diagram is given in Fig. 1 to depict the nanowire sen-sor fabrication process. In detailed description, after the nanowireswere synthesized and released from the PC membrane they weresubjected to several rinse–wash cycles and then stored in ethanol. Adrop of this nanowire suspension was transferred to a commerciallyavailable micro-electrode substrate (Platinum based materials)with an embedded heater (Heraus, MSP332) under the influenceof dielectrophoresis (DEP). The interdigitated electrodes were con-nected to a function generator (Tektronix FG502) which gave asine wave output as the source of the non-uniform AC electric fieldand dielectrophoretic force. The parameters for this DEP alignmentwere tuned up first and finally fixed at 6 MHz frequency and 6 Vpeak to peak amplitude to make sure that most of the nanowireswere aligned, bridging between the interdigitated electrodes afterethanol evaporated. Then thermal oxidation was conducted byplacing the sensor chip in the tube furnace at 600–700 ◦C for 5 h.The inset picture in Fig. 1 shows an SEM image of the top-view of thesensor chip surface with nanowires assembled on the interdigitatedelectrodes.

2.3. Gas sensing experiments

The gas sensing measurements were conducted in the staticand dynamic modes. The experimental set-up for the static methodconsists of a 4.4 L glass chamber, a sensor board mounting four sen-sor chips with different oxide compositions, and a data acquisitiondevice (USB-1608). The sensor board was sealed inside the chamberwith all the connection wires going out through an airtight port andconnected to the power supply and data acquisition system. The

chamber was purged with dry air before testing and during testing.A certain volume of chemical vapor was extracted from a samplecontainer using a syringe. The sample container was placed in awater bath at constant temperature to maintain a constant equi-

X. Li et al. / Sensors and Actuato

Fin

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ig. 2. (A) SEM images of as-fabricated tin–copper metallic nanowires and a zoom-n view of a single nanowire (inset); (B) EDS spectra layout of tin–copper metallicanowires with three different compositions.

ibrium vapor pressure. The extracted vapor sample was injectednto the testing chamber. Repeating this injecting operation caused

series of step increases in concentration, and the sensor responseas recorded by a computer.

Dynamic test set-up had two gas flow paths controlled bywo mass flow controllers (MFCs). One was directly connectedo a carrier gas (dry air) tank and the other went through aubbler with the liquid chemical inside. The dynamic sensoresponses were obtained from a measurement sequence consistingf vapor delivery and dry-air purging by controlling two three-wayalves connected to the bubbler and MFCs. The total measure-ent sequences and their operation times were controlled by a

irtual instrument (VI) programmed using LabVIEW. According tohe measurement sequence, the dynamic sensor responses werecquired and sent to computer using a data acquisition and controlystem developed using a microcontroller.

. Results and discussion

Morphological characteristics of as-fabricated tin–copper

etallic nanowires are shown in Fig. 2A. Nanowires appear to be

ery uniform, and the typical dimensions of nanowires are 5–6 �mn length and 50–70 nm in diameter. The co-existence of tin andopper elements in the original metallic nanowires was confirmed

rs B 158 (2011) 199– 207 201

by energy-dispersive X-ray spectroscopy (EDS) analysis as a spectralayout shown in Fig. 2B. The EDS analysis also provides quantita-tive information of copper and tin in the nanowires. According totheir atomic fractions of each element, the three types of nanowireswere labeled as Sn14Cu86, Sn57Cu43 and Sn92Cu8 according totheir atomic fractions.

Compositional information after thermal oxidation is shown inFig. 3. The EDS mapping results (Fig. 3A) reveal three major ele-ments colored differently (O, Cu, Sn) and the spectrum (Fig. 3B)shows a strong oxygen peak in addition to the peaks of the metalelements. In general, by performing EDS analysis in spot modealong the entire length of the nanowire and then averaging themeasurements, all three samples have an oxygen fraction slightlylower than the highest amount (stoichiometric ratio) that we canexpect for tin dioxide (SnO2) and copper oxide (CuO), indicating aclose-to-complete oxidation.

In addition to EDS, the X-ray diffraction patterns of nanowiresamples are shown in Fig. 4, where crystallographic informa-tion was studied to verify the compounds existing in this mixedmetal/metal oxide system. The peak intensity and 2� angle ofpossible compounds obtained from Powder Diffraction File (Inter-national Centre for Diffraction Data (ICDD)) are displayed at thebottom as reference. Fig. 4A lists the XRD spectra of nanowire sam-ples before oxidation, from top to bottom: sample Sn92Cu8, sampleSn57Cu43, and sample Sn14Cu86. In sample Sn92Cu8, except forthe characteristic peaks of tin, only slight amount of copper andCu3Sn appear. As the atomic fraction of copper goes up, in sampleSn57Cu43, peak intensity of copper and intermetallic compoundCu3Sn increases, while that of tin lowers in comparison to sam-ple Sn92Cu8. It was also found that small peaks of intermetalliccompound Cu6Sn5 showed up, indicating the second intermetal-lic compound formation in this sample. For sample Sn14Cu86, tinpeaks further decrease, while copper peak intensity shows a notice-able increase as well as intermetallic compound Cu6Sn5 and Cu3Sn.To sum up, for metallic nanowires, major peaks are identified to bethe characteristic peaks of tin and copper, and trend of their inten-sity differing is in good accordance with tin–copper compositionratio in each sample. Intermetallic compound Cu3Sn was observedat low copper concentration; while both Cu3Sn and Cu6Sn5 wereobserved at medium to high copper concentration. The intermetal-lic compound peak intensity increases along with the increase ofcopper ratio in the samples, indicating a higher concentration ofboth intermetallic compounds.

In the layout of XRD spectra after thermal oxidation (Fig. 4B),strong peaks of SnO2 were observed in oxidized sample Sn92Cu8.Small amount of intermediate oxides SnO and Sn2O3 were foundco-existing. The evidence of CuO in the mixed metal oxide systemshows up as peaks with relatively small intensity. No observ-able Cu2O peak was found. In oxidized sample Sn57Cu43, thecharacteristic peaks of Cu2O appear in small intensity, and theheight of CuO peaks increases due to higher copper ratio. In oxi-dized sample Sn14Cu86, the peak intensity of two copper oxides(CuO and Cu2O) further increases. The existence of Cu2O, Sn2O3and SnO help explain the measured oxygen deficiency from theEDS (none stoichiometric, complete oxidation). It is very commonto find intermediate oxides co-existing since they are the maincontributors to the electron donor/acceptor of bulk metal oxide;for example, in some studies tin oxide may also be referred asSnOx/SnO2−x [29,30], or SnxOy [31].

Morphological details of each type of nanowire surface after oxi-dation are shown in Fig. 5. These high-magnification images showthat after oxidation, the nanowires have bumpy surfaces decorated

with island structures, which are very different from the smoothsurfaces before oxidation. Similar bumpy surfaces have beenreported for several metal oxide nanowires [32,33]. We believethat the growth and relief of stresses generated during oxidation

202 X. Li et al. / Sensors and Actuators B 158 (2011) 199– 207

F DS mo

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ig. 3. (A) SEM image of tin–copper mixed oxide nanowires (sample Sn92Cu8) and Ef the same sample.

ay have contributed to the formation of the irregular surfaces.he stresses may come from recrystallization, non-stoichiometricrowth [34] and the volume difference between the oxide andhe metal from which it forms. From the phase diagram, tin andopper alloy system has various phases such as Cu6Sn5, Cu3Sn,u10Sn3, Cu41Sn11, etc.; however, there are only four phases – Sn,u, Cu6Sn5 and Cu3Sn at the room temperature, which compareell with our experimental observation. Due to the limited solu-

ility, the co-existence of multiple solid phases is inevitable duringhe electroplating, and there has to be an excessive amount of tinr copper not incorporated with the other element so that grainsf only tin or copper atoms may form all over the nanowire. Thexistence of some specific sites on the surface where copper con-aining compound or pure copper grains may lead to the formationf oxide islands after thermal oxidation. This is a very commonhenomenon for copper [35,36] and copper containing compos-

tes [37]. Also, for those incorporated atoms, the element which isinority in atomic fraction would be considered a substitutional

mpurity in the fcc structure [38], and such impurities, especiallyhe one on or close to the surfaces may greatly influence the oxideucleation, growth and hence the surface roughness [39]. As a com-arison, bare tin nanowire synthesized through similar procedures

s presented as a control sample and its surface after oxidation iselatively smooth (Fig. 5D). Obviously, the introduction of copper

lement indeed has a dramatic effect during the reconstruction andonversion of metal oxides. The surface change becomes vigorousomparing to the mildness of tin oxide surface. It should be notedhat oxidized sample Sn92Cu8 has island structures of largest size

apping of three major elements (Sn, Cu, and O) on nanowires, and (B) EDS spectrum

and oxidized sample Sn14Cu86 has a higher number of uniformisland structures. Although the oxidation is an important processand the famous Cabrera–Mott model [40,41] has shown consider-able predictive power for projecting chemical order of oxidation ofmetals and alloys on a macro-scale, yet no atomic level details ofthe growth mechanism are available [42]. So the fully understand-ing of what exactly causes the surface change in such a mannerstill needs to be further investigated. However, it should be notedthat such surface roughness features will increase the total surfacearea of the nanowires, and this may be in favor of their sensingperformance.

The response or sensitivity of sensor chips toward chemicalvapors is measured in terms of nanowires’ resistance change andis defined as (Rg − R0)/R0, also expressed as �R/R0, where R0 is theoverall resistance measured from the sensor chip in dry air condi-tion and Rg is the resistance when a chemical vapor is in presence.Our research began with investigating how the sensors behaveupon exposure to ethanol (Fig. 6A) at a fixed operating tempera-ture of 380 ◦C and the summarized data of these nanowires withthree compositions were plotted as a function of concentration(Fig. 6B). Fig. 6A is the static testing result from sample Sn14Cu86when responding to a series of concentration step changes from1 ppm to 500 ppm. Apparently, the change of resistance and theconcentration is not in a linear relationship, which resembles the

general trend of metal oxide sensors with increasing concentration.At lower concentration region (<100 ppm), the response causedby injection of ethanol/ppm is much higher than that in higherconcentration region (>100 ppm). Noticeably, this nanowire had

X. Li et al. / Sensors and Actuato

Fig. 4. X-ray diffraction patterns of nanowire samples (A) before and (B) after oxi-dation, with reference patterns of possible compounds from Powder DiffractionFile.

Fig. 5. SEM images of surface morphology of mixed metal oxide nanowire sample

rs B 158 (2011) 199– 207 203

about 20% augments in resistance at a few ppm level which indi-cated that those sensor can measure ethanol concentration wellbelow ppm level. Fig. 6B shows the sensor responses of the threemixed metal oxide nanowire sensors and that of tin oxide nanowiresensor to ethanol ranging from 1 ppm to 500 ppm. The tin oxidenanowire sensor, as a control sample, was synthesized using thesame method, and the response data were gained from our pre-vious work [43]. All of three copper-containing metal oxides haveexhibited larger response than pure SnO2 nanowires at the sameconcentration level. The sensor response enhancement is signifi-cant for sample Sn92Cu8 and Sn14Cu86 and moderate for sampleSn57Cu43. Interestingly, both positive and negtive responses wereobserved here. For tin dominated compositions, the resistanceof nanowires decreased as the concentration went up, and theresponse was larger with more copper element. However, for sam-ple Sn14Cu86, copper dominated nanowire, its resistance increasedwith increasing concentration.

Another critical parameter – working temperature was alsoinvestigated in the testing toward three chemicals includingethanol, acetone, and ethyl acetate. Five temperature points in therange of 200–440 ◦C were chosen to help understand how the sen-sitivity changes with the working temperature. The response datawere compared in absolute values and put into radar charts to pro-vide a straightforward view of the results. In Fig. 7, radar charts havebeen adopted in sensor related studies for displaying multiple vari-ables in a single figure [44,45]. Here in our study, each chart consistsof five scaled spokes representing the operating temperature, andthe length of the data point is proportional to the magnitude of thesensor response. In Fig. 7A, the response of three nanowire sensors

to ethanol at 500 ppm are presented. Sample Sn57Cu43 displays thelowest sensitivity among the three nanowire sensors, while in mosttemperature points, sample Sn14Cu86 gives the highest, exceptfor 440 ◦C, where the sensor response of sample Sn92Cu slightly

s (A) Sn92Cu and (B) Sn57Cu43 and (C) Sn14Cu86 and (D) SnO2 nanowires.

204 X. Li et al. / Sensors and Actuators B 158 (2011) 199– 207

Fig. 6. (A) Static testing results of sample Sn14Cu86 nanowire sensor respondingtrn

emwoin3twci5iiasSatsmrtbarIi

Fig. 7. The response variation of three types of mixed metal oxide nanowire sensors

o a series of step changes with increasing ethanol concentration and (B) sensoresponse of three types of mixed metal oxide nanowire sensors and that of tin oxideanowire sensor for ethanol from 1 ppm to 500 ppm.

xceeds sample Sn14Cu86. It is interesting to notice that Sn92Cu8etal oxide nanowires behaves most like tin oxide nanowireith respect to the temperature dependence [43]. The sensitivity

f sample Sn92Cu8 changes positively with the increased work-ng temperatures up to 440 ◦C. The other two tin–copper oxideanowires have a preferable working temperature region from20 ◦C to 380 ◦C. By conducting the same experiments to ace-one and ethyl acetate exposure, these three nanowire sensorsere found to have similar trends of sensor response. For all three

hemicals tested, the highest sensor response was evoked by expos-ng sample Sn14Cu86 (working temperature 380 ◦C) to acetone at00 ppm, due to its p-type sensor response, and there is a resistance

ncrease over 100%. For n-type response, sample Sn92Cu8 (work-ng temperature 440 ◦C) gives an over 80% change when exposed tocetone. Generally speaking, the sensitivity order of each nanowireensor toward these three chemicals is: for sample Sn92Cu8 andn57Cu43, acetone > ethyl acetate > ethanol; for sample Sn14Cu86,cetone > ethanol > ethyl acetate. Despite the difference in sensi-ivity, the sensing mechanisms of these three chemicals are veryimilar [46–48]; it basically involved the reactions of chemicalolecules with pre-adsorbed oxygen species, regardless the mate-

ials [49–51]. For these three reducing analytes, when oxidized byhe oxygen ions, the electrical disturbance is mainly from the O–Hond-breaking process [52,53]. From our testing results, acetone

ppeared to be the most active over all, and ethanol caused higheresponse than ethyl acetate in 200–380 ◦C, and lower in 380–440 ◦C.t is also well known that in low temperature region, physisorptions dominant, while chemisorption of oxygen gradually takes charge

for ethanol, acetone and ethyl acetate in correlation with five working temperatureswithin the range of 200–440 ◦C.

when temperature increases [54]. The combination of them deter-mines the sensitivity and resistivity of the material. Another way tolook at the temperature effect and composition effect on sensitivityis that, for three mixed metal oxide materials, the surface adsorp-tion of oxygen species (O2, O2

− and O−) [55] might be different.However, the detailed mechanism behind the preference of eachchemical at different temperatures and the exact chemi- vs. physi-sorption competition of oxygen species on metal oxide materialsurface need to be further investigated.

In the case of metal oxide chemiresistive sensors, the most crit-ical property measured in testing is their resistance/conductancechange when exposed to various chemicals. It is well knownthat the resistance of semiconducting materials is determined bythe number of charge carriers, either electrons (n-type) or holes(p-type). The reason that tin oxide is considered to be an n-type semiconductor is because of its oxygen-deficiency – a typeof intrinsic defects which does not change the overall compo-sition but may cause the semiconducting behavior [56]. On theother hand, oxygen-excess copper oxide shows p-type conduc-

tivity by holes [57]. In dry air condition, the absorbed surfaceoxygen plays a very important role in trapping electrons from thesemiconductor [54,58]. With this extra amount of oxygen getting

X. Li et al. / Sensors and Actuato

Fig. 8. (A) The response of sample Sn92Cu8 for acetone, ethanol and ethyl acetateomt

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f 500 ppm obtained from the dynamic test and (B) the response of three types ofixed metal oxide nanowires toward ethyl acetate of 500 ppm from the dynamic

est.

lectrons from metal oxide materials, the concentration of holesn-type)/electrons (p-type) – essential charge carriers in the semi-onductor – will increase/decrease, hence the material becomeore/less conductive. It is generally accepted that, with the intro-

uction of analytes, oxygen ions tend to react with the hydrogenissociated from adsorbed organic molecules, releasing electronsack to the surface of metal oxide [59]. Thus, for n-type semi-onducting metal oxide, the resistance drops when exposed tohemicals. The opposite holds for p-type semiconducting metalxide. From our observation, among mixed metal oxide nanowireamples, two of them (Sn92Cu8, Sn57Cu43) with higher tin fractionehave like n-type while the sample Sn14Cu86 has a typical p-typeesponse to chemical exposure.

Small amount of copper incorporated in the crystal lattice isonsidered to be electroactive dopant to tin oxide [60], and its pos-essing acceptor action also contributes to the process of energyarrier formation by changing the electron concentration in theulk of tin dioxide due to compensation mechanism [61]. It haseen reported that the incorporation of Cu2+ ions onto Sn4+ sites

eads to the creation of oxygen vacancies, and hence a decrease inhe free electron concentration and an increase in sensitivity of the

aterial [62]. This mechanism may also apply to the situation of

mall amount of tin incorporated in copper, as the substitution ofu2+ by Sn4+ decreases the number of holes. Excessive amount ofin oxide is responsible for sensing, showing a similar but dampedensor response like the sample Sn92Cu8.

rs B 158 (2011) 199– 207 205

The response kinetics and the repeatability of the nanowire sen-sors were investigated through the dynamic testing. A full test cycleincludes exposure of all three sensors to the gas flow of the samecondition, then back to initial state. This cycle was repeated for 5times. Fig. 8A shows the testing results of sample Sn92Cu8 uponexposure to three analytes at a concentration of roughly 500 ppmand a working temperature of 380 ◦C. Fast responses were recordedand the typical response time (90% of the peak value) was calcu-lated to be around 8 s, with a small variation of ±3 s toward differentchemicals and concentrations. Fig. 8B is a comparison of three typesof nanowires responding to 500 ppm of ethyl acetate at a workingtemperature of 380 ◦C. The intrinsic positive and negative resis-tance change were observed for different metal oxides, and goodrepeatability was demonstrated. The recovery process appeared tobe relatively slow comparing to the fast response time, and it usu-ally took about 6 min. The recovery time of samples with higher tinratio (Sn92Cu8 and Sn57Cu43) was faster than that of the sampleSn14Cu86. Besides sensing property of nanowires, the box-shapedgeometry of our test chamber may also affect the recovery time byproviding a fraction of dead volume when it gets flushed with blankcarrier gas. Optimization and better design of the testing chambermay decrease the response/recovery time of the nanowire sensorssynthesized.

4. Conclusion

Tin–copper mixed metal oxide nanowires were synthesized bythermally oxidizing the tin–copper metallic nanowires that havebeen obtained by electrodeposition method using nanoporous tem-plates. The composition of prepared tin and copper elements can betailored in a wide range through changing the electroplating solu-tions. Oxidation process causes a tremendous change on the surfacecondition of the nanowires with different compositions. XRD mea-surement indicated that two intermetallic compounds Cu6Sn5 andCu3Sn exist in the mixed metal nanowires, together with metal-lic tin and copper; while for mixed metal oxide, the existence ofSnO2 and CuO was found to be majority and intermediate oxidesCu2O, SnO and Sn2O3 were minority co-existing in the oxidizedsamples. Testing of these nanowires’ response to chemical vaporsof acetone, ethanol and ethyl acetate revealed that the supplementof copper into the tin nanowires improved the sensitivity as com-pared to the pure tin oxide nanowires. An interesting n-type/p-typesensing properties was observed with the variation of tin–coppercomposition. Significant enhancement on sensitivity is observed onSn92Cu8 and Sn14Cu86 metal oxide nanowires and both positiveand negative resistance change were observed.

Acknowledgements

Financial support from the National Science Foundation (awardnumber ECCS-0731125) is greatly acknowledged. We thank YingWang and Dr. Yu Lei at University of Connecticut for the help onXRD measurements.

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Biographies

Xiaopeng Li is currently a Ph.D. student in the Department of Chemical Engineeringat the University of Massachusetts Lowell, USA. He received his B.S. in Chemistry andB.Eng. in Chemical Engineering in a dual bachelor’s program from Shandong Univer-sity, China, in 2007. His current project is focused on the synthesis and assembly ofmetal oxide nanowires and conducting polymer nanowires for sensor applications.

Zhiyong Gu is an Assistant Professor in the Department of Chemical Engineeringat the University of Massachusetts Lowell. He is also affiliated with the CHN/NCOENanomanufacturing Center. He received his B.E. from Qingdao Institute of ChemicalTechnology, China, in 1996, his M.S. from the University of Notre Dame in 2001,and his Ph.D. from the State University of New York at Buffalo in 2004, respec-

tively. He was a Postdoctoral Fellow at the Johns Hopkins University from 2004 to2006. He has published 4 book chapters and over 40 refereed papers. His currentresearch interests include synthesis of nanoparticles and nanowires, self-assembly,block copolymers, nanocomposites, and nanoscale-integration for electronics andsensors.

ctuato

JeGaSre

HaIPRn

Engineering (1993) from Louisiana State University (LSU). Dr. Kurup has conducted

X. Li et al. / Sensors and A

ungHwan Cho is currently a PostDoc. at the University of Massachusetts Low-ll, U.S.A. He received a B.S. in Instrumentation and Control Engineering fromyeongsang National University, Jinju, South Korea in 2001, then received an M.S.nd a Ph.D. in Electronic Engineering from Kyungpook National University, Daegu,outh Korea in 2003 and 2008, respectively. His research interests include patternecognition techniques, fuzzy systems, and artificial neural networks applied tolectronic noses and gas detection devices.

ongwei Sun is an Assistant Professor in the Department of Mechanical Engineering

t the University of Massachusetts Lowell (UML). He graduated with a Ph.D. fromnstitute of Engineering Thermophysics at Chinese Academy of Science in 1998.rior to joining UML in 2005, he was a postdoctoral researcher at the University ofhode Island and later a research scientist at the Massachusetts Institute of Tech-ology. His research interest is on power microelectromechanical systems (Power

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MEMS), MEMS acoustic sensors, microscale cooling systems. His other interest is inmicro/nano fabrication technology, fundamental understanding of micro/nanoscalefluidics and their applications in biological analysis and energy areas.

Pradeep U. Kurup is a Professor in the Department of Civil and EnvironmentalEngineering at the University of Massachusetts Lowell. He received his B.Tech. inCivil Engineering in 1985 from the University of Kerala and obtained his M.Tech.from the Indian Institute of Technology Madras (1987). He holds a Ph.D. in Civil

extensive research in sensor integration; pattern recognition using intelligent mod-els; multi-sensor data fusion; artificial olfaction; and geotechnical & environmentalsite characterization. Dr. Kurup is a member of several professional societies, and isa registered Professional Engineer in the State of Louisiana.