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_{LECopyright copy 2014 by American Scientific Publishers}

_{All rights reserved}

_{Printed in the United States of America}

_{Energy and Environment FocusVol 3 pp 404ndash410 2014(wwwaspbscomefocus)}

_{Hierarchically Assembled ZnO Nanorods on TiO2}

_{Nanobelts for High Performance Gas SensorZhenhuan Zhao12 Dongzhou Wang1 Xueliang Kang1 Yuanhua Sang1 and Hong Liu12lowast}

_{1State Key Lab of Crystal Materials Shandong University Jinan 250100 China2Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 10085 P R China}

_ABSTRACT

_{Gas sensors based on semiconductors have wide applications In this study TiO2 nanobelts prepared fromalkali-hydrothermal process were used to hierarchically assemble ZnO nanorods to form a surface heterostruc-ture The as-synthesized hierarchical heterostructure was confirmed with characterizations The possible grownmechanism was investigated The hierarchical heterostructure was used to assemble gas sensor The sensingperformance of the assembled gas sensor was explored using different gas and it is found that the hierarchi-cal heterostructure sensor has excellent sensitivity and selectivity upon exposure to trimethylamine gas Thesensitivity was about 25 at the concentration as low as 5 ppm of trimethylamine The best working temperaturewas determined to be 200 C The sensing mechanism was also discussed and proposed to be related to asurface-depletion model}

_{KEYWORDS ZnO Nanorod TiO2 Nanobelt Hierarchical Heterostructure Gas Sensor}

_{1 INTRODUCTIONGas sensors based on semiconducting metal oxides haveattracting much attention for a long period becauseof their small dimensions low cost high sensing per-formance and high compatibility with microelectronicmanipulation1 They are widely employed to detect thetoxic and combustible gases monitor emissions from vehi-cles and quality control in chemicals food and cosmetics2}

_{Trimethylamine is one of the toxic gases in biology andfood industry Trimethylamine is commonly used as tar-get gas for the monitoring of the food quality through gassensors3 However traditional gas sensors are based onmetal oxide film consisted of nanoparticles which has thedisadvantages of low sensitivity and poor selectivityRecent years one dimensional nanostructures such as}

_{nanowires45 nanotubes67 and nanobelts8ndash10 are intensivelyused as the sensing materials for various detection appli-cations due to the ultrahigh surface-to-volume ratio111}

_{Among these metal oxides ZnO in the form of nanorodnanowire and nanobelt is one of the most promising sens-ing materials for gas sensor due to its high mobility ofconduction electrons good chemical and thermal stabilitylow cost and easy manipulation of various nanostructureTiO2 is also an important semiconductor used in various}

_{fields such as photocatalysis12 solar cell13 biosensor14ndash16}

_{lowastAuthor to whom correspondence should be addressedEmail hongliusdueducnReceived 23 May 2014Accepted 18 July 2014}

_{and gas sensor17 because of its low cost chemical stabilityand environmental-friendly It has already been demon-strated that one dimensional TiO2 are the excellent mate-rial for gas sensor1819 However the sensing performanceof these one dimensional nanostructures is limited to avery low level Further improvement in the sensitivity ofthe gas sensor is urgently expected To solve this problemmany methods have been developed and one of the mosteffective approach is to construct surface heterostructureSeveral advantages can be introduced to the heterostructuresuch as increased effective surface area improved chargetransfer and synergistic effect of the different parts in theheterostructure For example the ethanol sensing perfor-mance of TiO2 nanobelts can be significantly enhanced byan acid corrosion method With the aid of acid corrosionthe smooth surface of primary TiO2 nanobelts canbe deco-rated with numerous TiO2 nanoparticles to form a surfaceheterostructure with dramatically increased surface areafor the adsorption of more target gas molecules20 In addi-tion due to the excellent chemical stability and mechan-ical properties TiO2 nanobelts can also be employed asthe supporting material to assemble surface heterostruc-ture by loading noble metal nanoparticles such as Agnanoparticles9 and Pd nanoparticles10 and metal oxide likeSnO2}

_{21 Therefore to assemble surface heterostructure onTiO2 nanobelt should be the potential strategy for gas sens-ing enhancementIn this study alkali-hydrothermal synthesized TiO2}

_{nanobelts were used as the backbone to support ZnOnanorods for the construction of hierarchical surface}

_{404 Energy Environ Focus 2014 Vol 3 No 4 2326-304020143404007 doi101166eef20141118}

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_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LEheterostructure The hierarchical structure was confirmedby several characterization methods The results of thegas sensing experiments indicated that the hierarchicalheterostructure showed improved sensing sensitivity totrimethylamine gas It demonstrates that by assemblingZnO nanorods onto the surface of TiO2 nanobelts to formsurface heterostructure is an effective way to improve thesensor performance}

_{2 EXPERIMENTAL DETAILS21 MaterialsSodium hydroxide (NaOH analytic grade) sulfuric acid(H2SO4) hexahydrate zinc nitrate (Zn(NO3)2) middot6H2O) zincacetate Zn(CH3COOH)2 ethanol hexamethylenetetramine(C6H12N4) Deionized water was used throughout theexperiments Titania P25 (TiO2 ca 80 wt anatase and20 wt rutile was used as the Ti precursor to prepare TiO2}

_nanobelts

_{22 Preparation of TiO2 NanobeltsTiO2 nanobelts were synthesized through an alkalihydrothermal process22 In a typical procedure 60 mL of10 M NaOH aqueous solution and 03 g titania P25 werehomogeneously mixed together and then transferred into a80 mL Teflon-lined autoclave The autoclave was heated at200 C for 72 h The products were collected and washedwith copious deionized water through filtration This stepgave the formation of sodium titanate nanobelt which wastransferred into hydrogen titanate nanobelt through an ionexchange process with 01 M HCl aqueous solution for48 h The obtained hydrogen titanate nanobelts were thenwashed and dried in air at 70 C Finally TiO2 nanobeltswere prepared by thermally annealing hydrogen titanatenanobelts at 600 C for 2 h}

_{23 Preparation of ZnO NDsTiO2 NBsHierarchical Surface Heterostructure}

_{The ZnO NDsTiO2 NBs hierarchical surface het-erostructure was prepared by a hydrothermal processBefore the hydrothermal process the TiO2 nanobeltswere coated with ZnO seeds by a modified dip-coatingmethod2324 In detail 0005 M zinc acetate was firstlydispersed into 40 mL ethanol followed by the additionof 01 g TiO2 nanobelts The mixture was sonicated for20 min and gently stirred for another 20 min The homoge-nous dispersion was then filtrated and dried at 70 CThe dispersing-filtration-drying process was repeated forseveral times By calcination at 350 C for 20 minZnO seeds coated TiO2 nanobelts were obtained Subse-quently ZnO seeds covered TiO2 nanobelts were addedto an aqueous solution of 40 mL containing 0005 Mzinc nitrate and 0005 M hexamethylenetetramine Themixture was sealed into a 50 mL Teflon-lined auto-clave and heated at 90 C for 8 h After reactions}

_{the ZnO nanorodsTiO2 nanobelts hierarchical surfaceheterostructure (ZnO-NDsTiO2-NBs HSH) was finallyobtained by isolating the solution through filtration andwashing with deionized water and ethanol For compar-ison ZnO nanorods were also prepared using the samehydrothermal procedure without the addition of TiO2}

_nanobelts

_{24 Characterization MethodsThe X-ray powder diffraction (XRD) patterns of the sam-ples were recorded with a Bruker D8 Advance pow-der X-ray diffractionmeter with Cu K radiation with = 015406 nm over a 2 scan range between 20 and80 The microstructure and morphology of the sampleswere observed using a HITACHI S-800 field-emissionscanning electron microscope (FE-SEM) The fine struc-ture and crystalline lattice were obtained using the high-resolution transmission electron microscope (HRTEM)on a JEOL JEM 2100F microscope The Fast-Fourier-Transform (FFT) patterns acquired using the DigitalMicro-graph software aligned to the HRTEM microscope}

_{25 Gas Sensing ExperimentsThe as-synthesized ZnO nanorods (ZnO-NDs) TiO2}

_{nanobelts (TiO2-NBs) and ZnO-NDsTiO2-NBs HSHwere used as the sensing materials To assembling thesensors the as-prepared individual sample was uniformlymixed with a calculated amount of water to prepare theslurry which was carefully coated onto the outer surfaceof a ceramic tube and dried in air At the two ends of theceramic tube two gold wires were inlaid as electrodes andconnected to two Pt wires The control over the tempera-ture was achieved by inserting a resistive heating wire intothe ceramic tube Figure 1(a) shows the schematics of thesensorThe gas sensing experiment was carried out with a WS-}

_{30A gas sensing system (Zhengzhou Winsen Electronics}

_{Fig 1 Schematics of (a) the as-assembled gas sensor and (b) the cor-responding equivalent circuit}

_{Energy Environ Focus 3 404ndash410 2014 405}

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_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LETechnology Co Ltd P R China) The ethanol with thedesired amount was injected onto a beating platform in thetest chamber to prepare ethanol-air mixed gas for ethanolsensing Figure 1(b) shows the sensing electrical circuitThe electrical resistance of the sensors in air or in thetarget gas is calculated as R= RLVc minusVoutVout whereR is the sensor resistance RL is the load resistance Vc isthe total loop voltage applied to the electrical circuit andthe Vout is the output voltage across the load resistor Thesensor sensitivity S is defined as S = RaRg where Ra}

_{and Rg is the sensor resistance measured in air and inthe mixed air containing target gas at the same relativehumidity of about 30 respectively9}

_{3 RESULTS AND DISCUSSION31 Structure and MorphologyTo investigate the crystalline phase of the as-synthesizedsamples X-ray diffraction analysis was performedFigure 2 shows the XRD patterns of the as-preparedsamples Curve a and b are drawn from the PDFcards with the number of 79-2205 and 21-1272 of ZnOand TiO2 respectively The XRD patterns of the pri-mary TiO2 NBs (curve c) ZnO NDs (curve d) ZnOseedsTiO2-NBs (curve e) and ZnO-NDsTiO2-NBshierarchical heterostructure were also illustrated for com-parison Obviously the crystal phase of TiO2 nanobeltsbackbone is mainly consist of anatase phase due to the exis-tence of characteristic peaks located at 252 376 478538 55 and 626 (curve c) The weak peak located at282 indicates the existence of a little portion of TiO2-B (JCPDF card no 46ndash1237)25 Curve d shows the XRDpattern of ZnO-NDs All the peaks can be indexed tohexagonal wurtzite ZnO with characteristic peaks at 317343 and 361 which is well consistent with the JCPDFcard (curve b) The very sharp peaks with high intensityindicate the single crystalline feature of the as-prepared}

_{Fig 2 (a) and (b) refer to the corresponding PDF cards with the num-ber of 79-2205 and 21-1272 of ZnO and TiO2 respectively XRD patternsof as-synthesized (c) TiO2-NBs (d) ZnO NDs (e) ZnO seedsTiO2-NBsand (f) ZnO-NDsTiO2-NBs hierarchical heterostructure}

_{ZnO-NDs because no characteristic peaks can be observedfor impurities Curve e shows the XRD pattern of TiO2}

_{nanobelts coated with ZnO seeds The peak located at 252}

_{can be assigned to (101) crystalline plane of TiO2-NBswhereas the sharp peaks located at 317 343 and 361}

_{can be indexed to the crystalline planes of (100) (002)and (101) of wurtzite ZnO Curve f is the XRD patternof ZnO-NDsTiO2-NBs hierarchical heterostructure It isfound that the XRD pattern of the hierarchical heterostruc-ture shows the similar characteristic peaks with that ofZnO seeds coated TiO2 nanobelts meaning the success ofgrowth of ZnO nanorods on TiO2 nanobelt surface Thesingle crystalline feature of ZnO nanorods in the hierar-chical heterostructure is expected to facilitate the chargetransferThe micromorphology of ZnO-NSsTiO2-NBs hierar-}

_{chical heterostructure was investigated by SEM Figure 3shows the SEM images of as-prepared hierarchical het-erostructure primary TiO2 nanobelts and ZnO nanorodsThe primary TiO2 nanobelt with a width of 50ndash200 nm andlength up to several micrometershas a typical smooth sur-face and rectangular cross section (Fig 3(a)) which is con-sistent with our previously prepared TiO2 nanobelts2022}

_{By applying the hydrothermal growth process of ZnO-NDs the initially smooth TiO2 nanobelts branched outforming a hierarchical heterostructure The typical SEMimages of such heterostructure are shown in Figures 3(c)and (d) It is interesting that ZnO nanorods were denselyassembled onto the surface of TiO2 nanobelts The lengthof the secondary ZnO nanorods is up to 500 nm Themace-like morphology of the hierarchical heterostructurehas large exposed ZnO surface area In addition the denseZnO nanorods on TiO2 nanobelts formed numerous micro-channel between the nanorods These advantages of the}

_{Fig 3 SEM images of (a) primary TiO2-NBs (b) ZnO-NDs (c) and(d) ZnO-NDsTiO2-NBs HSH with different magnification The insertimage in (d) shows the end of the ZnO-NDsTiO2-NBs hierarchicalheterostructure}

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_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LEhierarchical heterostructure are expected to improve thegas sensing performance}

_{To further examine the microstructure transmissionelectron microscopy (TEM) was also used Figure 4 showsthe TEM images of the ZnO-NDsTiO2-NBs hierarchicalheterostructure As shown in Figure 4(a) ZnO nanorodswere vertically grown on the TiO2 nanobelt support Theclear crystalline lattice fringes of the ZnO nanorod branchand the TiO2 nanobelt support indicate the single crys-talline feature of both the ZnO and TiO2 This conclu-sion can be well supported by the bright spots of the twodimensional FFT patterns shown in Figures 4(c) and (d) Itshould be pointed out that ZnO nanorods was grown on theZnO seeds which can be clearly seen from Figure 4(b)Both the nanorod and the seed part displayed similarcrystalline lattice fringes The measured lattice spacing is035 nm for TiO2 and 027 nm for ZnO corresponding tothe (101) plane of TiO2 and (0002) plane of ZnO respec-tively Therefore the growth direction of the TiO2 nanobeltsupport and the ZnO nanorod branch was determined tobe along [010] and [0001] respectively}

_{Based on the above analysis the growth process of theZnO-NDsTiO2-NBs hierarchical heterostructure is illus-trated in Figure 5 Firstly the primary TiO2 nanobeltswere coated with a layer of ZnO seed nanoparticlesThese ZnO nanoparticles acted as the specific nucleatesites which can be clearly seen from Figure 5(a) Forthe growth of ZnO nanorod it is necessary to seed theTiO2 nanobelt with ZnO nanoparticles In the initial stageZnO nanorods nucleated and crystallized along the [0001]direction on the ZnO seed sites With the proceeding of}

_{Fig 4 Microstructure characterization of the ZnO-NDsTiO2-NBshierarchical heterostructure (a) and (b) TEM image of the hierarchicalheterostructure taken at different magnification (c) HRTEM of the hier-archical heterostructure taken near the junction (d) and (e) Fast-Fourier-Transform patterns from the selected area in (c) respectively}

_{Fig 5 Growth process of the ZnO-NDsTiO2-NBs hierarchical het-erostructure Top row schematics Bottom row corresponding SEMimages of the (a) bare TiO2 nanobelt (b) ZnO seeded TiO2 nanobelt and(c) ZnO-NDsTiO2-NBs hierarchical heterostructure}

_{the hydrothermal process the initial nanorods absorbedZn2+ and OHminus ions from the precursor solution toform Zn(OH)2 which decomposed to form ZnO Thoughthe TiO2 has six different crystalline planes The ZnOnanorods were still densely grown on the TiO2 nanobeltsurface This may be due to the high zinc salt concentra-tion and extended hydrothermal reaction time}

_{32 Gas Sensing PerformanceA good gas sensor should exhibit fast response once uponexposure to the target gas Meanwhile it also should havethe ability to recognize the target gas at a concentrationof target gas as low as possible Summarily the gas sen-sor should have good selectivity and sensitivity Under thepresent experiments we have investigated the gas sensingperformance of the prepared ZnO-NDsTiO2-NBs hier-archical heterostructure For full comparison the sensingbehavior of TiO2 nanobelts and ZnO nanorods was alsoexamined under the same conditionFigure 6 shows the responserecovery curves at 200 C}

_{of the TiO2 nanobelts sensor ZnO nanorods sensor andthe hierarchical heterostructure sensor The gas concentra-tion of trimethylamine was tuned from 5 ppm to 500 ppmObviously once exposed to the trimethylamine gas allthe three sensors exhibited a remarkable response withdifferent output voltage of Vout However there was stilldifferent response behavior for the three gas sensors Forexample at 500 ppm the output voltage of Vout was 114134 and as high as 40 for TiO2 nanobelts (Fig 6(a))ZnO nanorods (Fig 6(b)) and ZnO-NDsTiO2-NBs hier-archical heterostructure (Fig 6(c)) respectively Oncethe concentration decreased to 100 ppm the Vout valuealso decreased to 0312 0434 and 247 respectivelyThis means that with decreasing the concentration oftrimethylamine gas the output voltage of the threesensors decreased in a different rate suggesting anincreasing order of TiO2 nanobelts ZnO nanorods ZnO-NDsTiO2-NBs hierarchical heterostructure of the sens-ing performance Impressively the sensor made from thehierarchical heterostructure even showed a high outputvalue of about 0956 at the very low concentration of}

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_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LE(a) (b)}

_{(c) (d)}

_{Fig 6 Responserecovery curves gas sensors made from (a) TiO2 nanobelts (b) ZnO nanorods and (c) ZnO-NDsTiO2-NBs hierarchical heterostruc-ture upon exposed to different concentrations of trimethylamine gas at 200 C (d) Calculated sensitivity S as a function of gas concentration}

_{5 ppm indicating the good sensing capability to trimethy-lamine gasThe sensor performance can be directly illustrated in}

_{the sensitivity S The corresponding sensitivity S of thethree sensors are shown in Figure 6(d) According toFigure 6(d) the sensitivity of the three sensors increaseslinearly with the concentration of trimethylamine gas Thesensitivity of the ZnO sensor is slightly higher than that ofTiO2 sensor while notably the sensitivity of the hierarchi-cal heterostructure sensor shows much higher values at allthe gas concentrations Even at the concentration as lowas 5 ppm the sensitivity of the hierarchical heterostruc-ture sensor reaches to about 25 which is much higher thanthose listed in literatures This means by assembling ZnOnanorods onto the surface of TiO2 nanobelts can signifi-cantly improve the sensing capabilityGas sensors also need to detect target gas at differ-}

_{ent working temperatures Therefore it is important toinvestigate the effect of temperature on the sensing per-formance Figure 7 shows responserecovery curves of theZnO-NDsTiO2-NBs hierarchical heterostructure sensorat different temperaturesby exposure to trimethylamine gasat a constant concentration of 300 ppm Clearly as seenfrom Figure 7(a) the sensor shows good responserecoverygas at all the temperatures because the Vout in trimethy-lamine gas recovered to similar value when fresh air waspurged into the testing chamber Notably when the sen-sor was working at 100 C the increase of the outputvoltage Vout of the sensor was the smallest With the}

_{increase of the working temperature the increase of theoutput voltage gradually increased and reached to the high-est value of about 35 at 200 C While the temperatureincreased to 250 C the output voltage Vout decreasedto 18 much lower than 35 at 100 C This indicatesthat the hierarchical heterostructure sensor showed the bestresponserecovery behavior while working at 200 C Thecorresponding sensitivity at different temperatures as thefunction of gas concentration is shown in Figure 7(b) Itis found that at all the concentrations the sensor hardlyexhibited sensing capability at 100 C and 150 C Simi-lar increasing trend can be observed when the sensor wasworking at 200 C and 250 C However the sensitivityobtained at 200 C was much higher than that at 250 CTherefore it is concluded that the ZnO-NDsTiO2-NBshierarchical heterostructure has the best sensing perfor-mance upon exposure to trimethylamine gas at 200 CAs stated previously the selectivity of the gas sensor is}

_{another key parameter Therefore the sensing performanceof the hierarchical heterostructure to different gas was alsoinvestigated at the same gas concentration and workingtemperature Figure 8 shows the responserecovery curvesof the ZnO-NDsTiO2-NBs hierarchical heterostructureupon exposure to trimethylamine gas ammonia gas andCO and ethanol vapor respectivelyat 200 C and constantconcentration of 300 ppm It is found that though the sen-sor shows response to all the detecting gas remarkablydifferent response behavior can be seen from Figure 9The value of the Vout upon exposure to ethanol vapor}

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_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LE(a) (b)}

_{Fig 7 (a) Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure at different temperatures by exposureto trimethylamine gas at a constant concentration of 300 ppm and (b) the corresponding calculated sensitivity S as a function of gas concentration}

_{CO ammonia gas and trimethylamine gas was about 059097 144 and 343 respectively indicating the best detect-ing capability to trimethylamine gas Actually the presentZnO-NDsTiO2-NBs hierarchical heterostructure sensorshows much better sensing performance to trimethylaminegas The sensitivity of the hierarchical heterostructure sen-sor is much higher than those listed in literatures32627}

_{Most impressively the present hierarchical heterostructuresensor has excellent sensing performance at a very lowgas concentration of 5 ppm This gas concentration ismuch lower than 160 ppm of ZnO film sensor28 400 ppmof ZnO and Al film29 and 100 ppm of SnO2 and ZnOnanocomposite30}

_{33 Sensing MechanismMost semiconductor oxide based gas sensors operate onthe basis of the variation of the electrical propertiesof the oxide materials4 This variation of the electricalresistance is thought to be related to a surface deple-tion model Actually the sensing mechanism of the TiO2}

_{nanobelts for detecting reductive gas has been discussedin literature9 Trimethylamine gas belongs to the reductive}

_{Fig 8 Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure by exposure to different gasat the same gas concentration of 300 ppm working at 200 C}

_{gas type Therefore at the present sensor TiO2 nanobeltsfor sensing trimethylamine gas should be based on the sim-ilar mechanism ie the surface-depletion model31 WhenTiO2 nanobelts are exposure to air oxygen molecules areadsorbed onto the surface of TiO2 nanobelt and trap theelectrons from the conduction band of TiO2 to form super-oxo or perpxo-like species This surface reaction bringsabout the formation of a surface depletion region withinthe nanobelt resulting in the increase of the electricalresistance of the nanobelts Once the reductive gas (thetarget gas) were introduced to contact with the surface ofthe nanobelts effective electron transfer happens from thetarget gas to TiO2 nanobelt leading to the decrease of theelectrical resistance of the nanobeltsActually ZnO should obey the similar surface depletion}

_{mechanism The surface physical and chemical propertiesplays important roles in its gas sensing performance11}

_{Though the energy level of the conduction band of TiO2}

_{and ZnO is similar with each other the present hydrother-mally synthesized TiO2 nanobelts dominantly belongs toanatase phase and are known to have many oxygen vacan-cies which will act as the electron trap This will helpto drain electrons from ZnO phase to TiO2 nanobeltsthrough the interfacial heterostructure between ZnO andTiO2 The electron transfer will undoubtedly speed theelectron transfer from the target reductive gas to theZnO and then to TiO2 In addition the electron mobil-ity of ZnO (1175 cm2V middot S) is much higher than that ofTiO2 (02 cm2V middotS)32 Since the charge carriers diffusionfrom bulk to surface of the semiconductor is the rate-limiting step once the hierarchical heterostructure sen-sor is exposed to air the ZnO nanorod part should bemore sensitive than the TiO2 nanobelt part to the oxy-gen molecule adsorption In another word ZnO nanorodsshould be more sensitive to the variation of the con-centration of electrons induced by the oxygen adsorptionand electron transfer between the reductive target gas andthe semiconductor The present hierarchical heterostructuredisplays several advantages which is beneficial for the gassensing The first one is the exposure of both the surface}

_{Energy Environ Focus 3 404ndash410 2014 409}

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_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LEof ZnO nanorod and TiO2 nanobelt These two kinds ofsemiconductors are proved to be effective materials forgas sensor application The second one is the exposure ofmore sensitive surface of ZnO nanorod The third one isthe specific hierarchical structure The charge transport isfaster in one dimensional ZnO nanorod and TiO2 nanobeltwhich both of them are highly crystalized Meanwhile themicrochannel formed between the ZnO nanorods can facil-itate the gas diffusion based on a capillary force There-fore based on the synergistic effect of ZnO and TiO2 thehierarchical heterostructure sensor exhibits excellent sens-ing performance to the reductive trimethylamine gas}

_{4 CONCLUSIONIn this study ZnO nanorods were assembled onto the sur-face of TiO2 nanobelts to form a hierarchical surface het-erostructure through a hydrothermal process with the aidof seeds coating method In the hierarchical heterostruc-ture the ZnO nanorods with single crystalline featurewere perpendicularly grown on the surface of TiO2 Thegrown mechanism has been investigated through XRD andHRTEM The sensing performance of this hierarchical het-erostructure was examined and it was found that the hier-archical heterostructure sensor has excellent sensitivity andselectivity to trimethylamine gas The potential sensingmechanism was also discussed and thought to be relatedto the surface-depletion model}

_{Acknowledgments The authors are thankful for fundsfrom National Natural Science Foundation of China(Grant No 51372142) National Science Fund for Distin-guished Young Scholars (NSFDYS 50925205) InnovationResearch Group (IRG 51321091) and the lsquo100 TalentsProgramrsquo of Chinese Academy of Sciences Thanks for thesupport from the lsquothousands talentsrsquo program for pioneerresearcher and his innovation team China}

_{References and Notes1 K J Choi and H W Jang Sensors 10 4083 (2010)2 I-D Kim A Rothschild and H L Tuller Acta Mater 61 974}

_{(2013)3 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134}

_{403 (2008)4 Q Li Y Liang Q Wan and T Wang Appl Phys Lett 85 6389}

_{(2004)5 Q Wan Q Li Y Chen T-H Wang X He J Li and C Lin Appl}

_{Phys Lett 84 3654 (2004)6 J Chen L-N Xu W-Y Li and X-L Gou Adv Mater 17 582}

₍₂₀₀₅₎

_{7 D R Kauffman and A Star Angew Chem Int Ed 47 6550 (2008)8 Y M Wang G J Du H Liu D Liu S B Qin N Wang C G}

_{Hu X T Tao J Jiao J Y Wang and Z L Wang Adv FunctMater 18 1131 (2008)}

_{9 P Q Hu G J Du W J Zhou J J Cui J J Lin H Liu D LiuJ Y Wang and S W Chen Acs Appl Mater Inter 2 3263 (2010)}

_{10 D Z Wang W J Zhou P G Hu Y Guan L M Chen J HLi G C Wang H Liu J Y Wang G Z Cao and H D JiangJ Colloid Interf Sci 388 144 (2012)}

_{11 L Liao H Lu J Li H He D Wang D Fu C Liu and W ZhangJ Phys Chem C 111 1900 (2007)}

_{12 A Fujishima X Zhang and D Tryk Surface Science Reports 63515 (2008)}

_{13 Z Y Han J C Zhang and W L Cao Mater Lett 84 34 (2012)14 J J Cui Y K Ge S W Chen H S Zhao H Liu Z Huang H D}

_{Jiang and J Chen J Mater Chem B 1 2072 (2013)15 J J Cui D H Sun S W Chen W J Zhou P G Hu H Liu and}

_{Z Huang J Mater Chem 21 10633 (2011)16 J J Cui D H Sun W J Zhou H Liu P G Hu N Ren H M}

_{Qin Z Huang J J Lin and H Y Ma Phys Chem Chem Phys13 9232 (2011)}

_{17 G K Mor M A Carvalho O K Varghese M V Pishko and C AGrimes J Mater Res 19 628 (2004)}

_{18 H F Lu F Li G Liu Z-G Chen D-W Wang H-T Fang G QLu Z H Jiang and H-M Cheng Nanotechnology 19 405504(2008)}

_{19 J Gong Y Li Z Hu Z Zhou and Y Deng J Phys Chem C 1149970 (2010)}

_{20 W J Zhou G J Du P G Hu G H Li D Z Wang H Liu J YWang R I Boughton D Liu and H D Jiang J Mater Chem 217937 (2011)}

_{21 W Zeng T M Liu and Z C Wang J Mater Chem 22 3544(2012)}

_{22 Z Zhao J Tian D Wang X Kang Y Sang H Liu J WangS Chen R I Boughton and H Jiang Journal of Materials Chem-istry 22 23395 (2012)}

_{23 L E Greene M Law D H Tan M Montano J GoldbergerG Somorjai and P Yang Nano Lett 5 1231 (2005)}

_{24 C Cheng B Liu H Yang W Zhou L Sun R Chen S F YuJ Zhang H Gong and H Sun Acs Nano 3 3069 (2009)}

_{25 W J Zhou L G Gai P G Hu J J Cui X Y Liu D Z WangG H Li H D Jiang D Liu H Liu and J Y Wang Cryst Engcomm 13 6643 (2011)}

_{26 Y-Z Lv C-R Li L Guo F-C Wang Y Xu and X-F Chu SensActuators B Chemical 141 85 (2009)}

_{27 H Tang M Yan X Ma H Zhang M Wang and D Yang SensActuators B Chemical 113 324 (2006)}

_{28 S-H Park J-Y Ryu H-H Choi and T-H Kwon Sens ActuatorsB Chemical 46 75 (1998)}

_{29 S Roy and S Basu Journal of Materials Science Materials inElectronics 15 321 (2004)}

_{30 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134403 (2008)}

_{31 X M Yin C C Li M Zhang Q Y Hao S Liu Q H Li L BChen and T H Wang Nanotechnology 20 455503 (2009)}

_{32 P-C Chen G Shen and C Zhou IEEE Transactions on Nanotech-nology 7 668 (2008)}

_{410 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LETechnology Co Ltd P R China) The ethanol with thedesired amount was injected onto a beating platform in thetest chamber to prepare ethanol-air mixed gas for ethanolsensing Figure 1(b) shows the sensing electrical circuitThe electrical resistance of the sensors in air or in thetarget gas is calculated as R= RLVc minusVoutVout whereR is the sensor resistance RL is the load resistance Vc isthe total loop voltage applied to the electrical circuit andthe Vout is the output voltage across the load resistor Thesensor sensitivity S is defined as S = RaRg where Ra}

_{and Rg is the sensor resistance measured in air and inthe mixed air containing target gas at the same relativehumidity of about 30 respectively9}

_{3 RESULTS AND DISCUSSION31 Structure and MorphologyTo investigate the crystalline phase of the as-synthesizedsamples X-ray diffraction analysis was performedFigure 2 shows the XRD patterns of the as-preparedsamples Curve a and b are drawn from the PDFcards with the number of 79-2205 and 21-1272 of ZnOand TiO2 respectively The XRD patterns of the pri-mary TiO2 NBs (curve c) ZnO NDs (curve d) ZnOseedsTiO2-NBs (curve e) and ZnO-NDsTiO2-NBshierarchical heterostructure were also illustrated for com-parison Obviously the crystal phase of TiO2 nanobeltsbackbone is mainly consist of anatase phase due to the exis-tence of characteristic peaks located at 252 376 478538 55 and 626 (curve c) The weak peak located at282 indicates the existence of a little portion of TiO2-B (JCPDF card no 46ndash1237)25 Curve d shows the XRDpattern of ZnO-NDs All the peaks can be indexed tohexagonal wurtzite ZnO with characteristic peaks at 317343 and 361 which is well consistent with the JCPDFcard (curve b) The very sharp peaks with high intensityindicate the single crystalline feature of the as-prepared}

_{Fig 2 (a) and (b) refer to the corresponding PDF cards with the num-ber of 79-2205 and 21-1272 of ZnO and TiO2 respectively XRD patternsof as-synthesized (c) TiO2-NBs (d) ZnO NDs (e) ZnO seedsTiO2-NBsand (f) ZnO-NDsTiO2-NBs hierarchical heterostructure}

_{ZnO-NDs because no characteristic peaks can be observedfor impurities Curve e shows the XRD pattern of TiO2}

_{nanobelts coated with ZnO seeds The peak located at 252}

_{can be assigned to (101) crystalline plane of TiO2-NBswhereas the sharp peaks located at 317 343 and 361}

_{can be indexed to the crystalline planes of (100) (002)and (101) of wurtzite ZnO Curve f is the XRD patternof ZnO-NDsTiO2-NBs hierarchical heterostructure It isfound that the XRD pattern of the hierarchical heterostruc-ture shows the similar characteristic peaks with that ofZnO seeds coated TiO2 nanobelts meaning the success ofgrowth of ZnO nanorods on TiO2 nanobelt surface Thesingle crystalline feature of ZnO nanorods in the hierar-chical heterostructure is expected to facilitate the chargetransferThe micromorphology of ZnO-NSsTiO2-NBs hierar-}

_{chical heterostructure was investigated by SEM Figure 3shows the SEM images of as-prepared hierarchical het-erostructure primary TiO2 nanobelts and ZnO nanorodsThe primary TiO2 nanobelt with a width of 50ndash200 nm andlength up to several micrometershas a typical smooth sur-face and rectangular cross section (Fig 3(a)) which is con-sistent with our previously prepared TiO2 nanobelts2022}

_{By applying the hydrothermal growth process of ZnO-NDs the initially smooth TiO2 nanobelts branched outforming a hierarchical heterostructure The typical SEMimages of such heterostructure are shown in Figures 3(c)and (d) It is interesting that ZnO nanorods were denselyassembled onto the surface of TiO2 nanobelts The lengthof the secondary ZnO nanorods is up to 500 nm Themace-like morphology of the hierarchical heterostructurehas large exposed ZnO surface area In addition the denseZnO nanorods on TiO2 nanobelts formed numerous micro-channel between the nanorods These advantages of the}

_{Fig 3 SEM images of (a) primary TiO2-NBs (b) ZnO-NDs (c) and(d) ZnO-NDsTiO2-NBs HSH with different magnification The insertimage in (d) shows the end of the ZnO-NDsTiO2-NBs hierarchicalheterostructure}

_{406 Energy Environ Focus 3 404ndash410 2014}

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_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LEhierarchical heterostructure are expected to improve thegas sensing performance}

_{To further examine the microstructure transmissionelectron microscopy (TEM) was also used Figure 4 showsthe TEM images of the ZnO-NDsTiO2-NBs hierarchicalheterostructure As shown in Figure 4(a) ZnO nanorodswere vertically grown on the TiO2 nanobelt support Theclear crystalline lattice fringes of the ZnO nanorod branchand the TiO2 nanobelt support indicate the single crys-talline feature of both the ZnO and TiO2 This conclu-sion can be well supported by the bright spots of the twodimensional FFT patterns shown in Figures 4(c) and (d) Itshould be pointed out that ZnO nanorods was grown on theZnO seeds which can be clearly seen from Figure 4(b)Both the nanorod and the seed part displayed similarcrystalline lattice fringes The measured lattice spacing is035 nm for TiO2 and 027 nm for ZnO corresponding tothe (101) plane of TiO2 and (0002) plane of ZnO respec-tively Therefore the growth direction of the TiO2 nanobeltsupport and the ZnO nanorod branch was determined tobe along [010] and [0001] respectively}

_{Based on the above analysis the growth process of theZnO-NDsTiO2-NBs hierarchical heterostructure is illus-trated in Figure 5 Firstly the primary TiO2 nanobeltswere coated with a layer of ZnO seed nanoparticlesThese ZnO nanoparticles acted as the specific nucleatesites which can be clearly seen from Figure 5(a) Forthe growth of ZnO nanorod it is necessary to seed theTiO2 nanobelt with ZnO nanoparticles In the initial stageZnO nanorods nucleated and crystallized along the [0001]direction on the ZnO seed sites With the proceeding of}

_{Fig 4 Microstructure characterization of the ZnO-NDsTiO2-NBshierarchical heterostructure (a) and (b) TEM image of the hierarchicalheterostructure taken at different magnification (c) HRTEM of the hier-archical heterostructure taken near the junction (d) and (e) Fast-Fourier-Transform patterns from the selected area in (c) respectively}

_{Fig 5 Growth process of the ZnO-NDsTiO2-NBs hierarchical het-erostructure Top row schematics Bottom row corresponding SEMimages of the (a) bare TiO2 nanobelt (b) ZnO seeded TiO2 nanobelt and(c) ZnO-NDsTiO2-NBs hierarchical heterostructure}

_{the hydrothermal process the initial nanorods absorbedZn2+ and OHminus ions from the precursor solution toform Zn(OH)2 which decomposed to form ZnO Thoughthe TiO2 has six different crystalline planes The ZnOnanorods were still densely grown on the TiO2 nanobeltsurface This may be due to the high zinc salt concentra-tion and extended hydrothermal reaction time}

_{32 Gas Sensing PerformanceA good gas sensor should exhibit fast response once uponexposure to the target gas Meanwhile it also should havethe ability to recognize the target gas at a concentrationof target gas as low as possible Summarily the gas sen-sor should have good selectivity and sensitivity Under thepresent experiments we have investigated the gas sensingperformance of the prepared ZnO-NDsTiO2-NBs hier-archical heterostructure For full comparison the sensingbehavior of TiO2 nanobelts and ZnO nanorods was alsoexamined under the same conditionFigure 6 shows the responserecovery curves at 200 C}

_{of the TiO2 nanobelts sensor ZnO nanorods sensor andthe hierarchical heterostructure sensor The gas concentra-tion of trimethylamine was tuned from 5 ppm to 500 ppmObviously once exposed to the trimethylamine gas allthe three sensors exhibited a remarkable response withdifferent output voltage of Vout However there was stilldifferent response behavior for the three gas sensors Forexample at 500 ppm the output voltage of Vout was 114134 and as high as 40 for TiO2 nanobelts (Fig 6(a))ZnO nanorods (Fig 6(b)) and ZnO-NDsTiO2-NBs hier-archical heterostructure (Fig 6(c)) respectively Oncethe concentration decreased to 100 ppm the Vout valuealso decreased to 0312 0434 and 247 respectivelyThis means that with decreasing the concentration oftrimethylamine gas the output voltage of the threesensors decreased in a different rate suggesting anincreasing order of TiO2 nanobelts ZnO nanorods ZnO-NDsTiO2-NBs hierarchical heterostructure of the sens-ing performance Impressively the sensor made from thehierarchical heterostructure even showed a high outputvalue of about 0956 at the very low concentration of}

_{Energy Environ Focus 3 404ndash410 2014 407}

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_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LE(a) (b)}

_{(c) (d)}

_{Fig 6 Responserecovery curves gas sensors made from (a) TiO2 nanobelts (b) ZnO nanorods and (c) ZnO-NDsTiO2-NBs hierarchical heterostruc-ture upon exposed to different concentrations of trimethylamine gas at 200 C (d) Calculated sensitivity S as a function of gas concentration}

_{5 ppm indicating the good sensing capability to trimethy-lamine gasThe sensor performance can be directly illustrated in}

_{the sensitivity S The corresponding sensitivity S of thethree sensors are shown in Figure 6(d) According toFigure 6(d) the sensitivity of the three sensors increaseslinearly with the concentration of trimethylamine gas Thesensitivity of the ZnO sensor is slightly higher than that ofTiO2 sensor while notably the sensitivity of the hierarchi-cal heterostructure sensor shows much higher values at allthe gas concentrations Even at the concentration as lowas 5 ppm the sensitivity of the hierarchical heterostruc-ture sensor reaches to about 25 which is much higher thanthose listed in literatures This means by assembling ZnOnanorods onto the surface of TiO2 nanobelts can signifi-cantly improve the sensing capabilityGas sensors also need to detect target gas at differ-}

_{ent working temperatures Therefore it is important toinvestigate the effect of temperature on the sensing per-formance Figure 7 shows responserecovery curves of theZnO-NDsTiO2-NBs hierarchical heterostructure sensorat different temperaturesby exposure to trimethylamine gasat a constant concentration of 300 ppm Clearly as seenfrom Figure 7(a) the sensor shows good responserecoverygas at all the temperatures because the Vout in trimethy-lamine gas recovered to similar value when fresh air waspurged into the testing chamber Notably when the sen-sor was working at 100 C the increase of the outputvoltage Vout of the sensor was the smallest With the}

_{increase of the working temperature the increase of theoutput voltage gradually increased and reached to the high-est value of about 35 at 200 C While the temperatureincreased to 250 C the output voltage Vout decreasedto 18 much lower than 35 at 100 C This indicatesthat the hierarchical heterostructure sensor showed the bestresponserecovery behavior while working at 200 C Thecorresponding sensitivity at different temperatures as thefunction of gas concentration is shown in Figure 7(b) Itis found that at all the concentrations the sensor hardlyexhibited sensing capability at 100 C and 150 C Simi-lar increasing trend can be observed when the sensor wasworking at 200 C and 250 C However the sensitivityobtained at 200 C was much higher than that at 250 CTherefore it is concluded that the ZnO-NDsTiO2-NBshierarchical heterostructure has the best sensing perfor-mance upon exposure to trimethylamine gas at 200 CAs stated previously the selectivity of the gas sensor is}

_{another key parameter Therefore the sensing performanceof the hierarchical heterostructure to different gas was alsoinvestigated at the same gas concentration and workingtemperature Figure 8 shows the responserecovery curvesof the ZnO-NDsTiO2-NBs hierarchical heterostructureupon exposure to trimethylamine gas ammonia gas andCO and ethanol vapor respectivelyat 200 C and constantconcentration of 300 ppm It is found that though the sen-sor shows response to all the detecting gas remarkablydifferent response behavior can be seen from Figure 9The value of the Vout upon exposure to ethanol vapor}

_{408 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LE(a) (b)}

_{Fig 7 (a) Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure at different temperatures by exposureto trimethylamine gas at a constant concentration of 300 ppm and (b) the corresponding calculated sensitivity S as a function of gas concentration}

_{CO ammonia gas and trimethylamine gas was about 059097 144 and 343 respectively indicating the best detect-ing capability to trimethylamine gas Actually the presentZnO-NDsTiO2-NBs hierarchical heterostructure sensorshows much better sensing performance to trimethylaminegas The sensitivity of the hierarchical heterostructure sen-sor is much higher than those listed in literatures32627}

_{Most impressively the present hierarchical heterostructuresensor has excellent sensing performance at a very lowgas concentration of 5 ppm This gas concentration ismuch lower than 160 ppm of ZnO film sensor28 400 ppmof ZnO and Al film29 and 100 ppm of SnO2 and ZnOnanocomposite30}

_{33 Sensing MechanismMost semiconductor oxide based gas sensors operate onthe basis of the variation of the electrical propertiesof the oxide materials4 This variation of the electricalresistance is thought to be related to a surface deple-tion model Actually the sensing mechanism of the TiO2}

_{nanobelts for detecting reductive gas has been discussedin literature9 Trimethylamine gas belongs to the reductive}

_{Fig 8 Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure by exposure to different gasat the same gas concentration of 300 ppm working at 200 C}

_{gas type Therefore at the present sensor TiO2 nanobeltsfor sensing trimethylamine gas should be based on the sim-ilar mechanism ie the surface-depletion model31 WhenTiO2 nanobelts are exposure to air oxygen molecules areadsorbed onto the surface of TiO2 nanobelt and trap theelectrons from the conduction band of TiO2 to form super-oxo or perpxo-like species This surface reaction bringsabout the formation of a surface depletion region withinthe nanobelt resulting in the increase of the electricalresistance of the nanobelts Once the reductive gas (thetarget gas) were introduced to contact with the surface ofthe nanobelts effective electron transfer happens from thetarget gas to TiO2 nanobelt leading to the decrease of theelectrical resistance of the nanobeltsActually ZnO should obey the similar surface depletion}

_{mechanism The surface physical and chemical propertiesplays important roles in its gas sensing performance11}

_{Though the energy level of the conduction band of TiO2}

_{and ZnO is similar with each other the present hydrother-mally synthesized TiO2 nanobelts dominantly belongs toanatase phase and are known to have many oxygen vacan-cies which will act as the electron trap This will helpto drain electrons from ZnO phase to TiO2 nanobeltsthrough the interfacial heterostructure between ZnO andTiO2 The electron transfer will undoubtedly speed theelectron transfer from the target reductive gas to theZnO and then to TiO2 In addition the electron mobil-ity of ZnO (1175 cm2V middot S) is much higher than that ofTiO2 (02 cm2V middotS)32 Since the charge carriers diffusionfrom bulk to surface of the semiconductor is the rate-limiting step once the hierarchical heterostructure sen-sor is exposed to air the ZnO nanorod part should bemore sensitive than the TiO2 nanobelt part to the oxy-gen molecule adsorption In another word ZnO nanorodsshould be more sensitive to the variation of the con-centration of electrons induced by the oxygen adsorptionand electron transfer between the reductive target gas andthe semiconductor The present hierarchical heterostructuredisplays several advantages which is beneficial for the gassensing The first one is the exposure of both the surface}

_{Energy Environ Focus 3 404ndash410 2014 409}

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_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LEof ZnO nanorod and TiO2 nanobelt These two kinds ofsemiconductors are proved to be effective materials forgas sensor application The second one is the exposure ofmore sensitive surface of ZnO nanorod The third one isthe specific hierarchical structure The charge transport isfaster in one dimensional ZnO nanorod and TiO2 nanobeltwhich both of them are highly crystalized Meanwhile themicrochannel formed between the ZnO nanorods can facil-itate the gas diffusion based on a capillary force There-fore based on the synergistic effect of ZnO and TiO2 thehierarchical heterostructure sensor exhibits excellent sens-ing performance to the reductive trimethylamine gas}

_{4 CONCLUSIONIn this study ZnO nanorods were assembled onto the sur-face of TiO2 nanobelts to form a hierarchical surface het-erostructure through a hydrothermal process with the aidof seeds coating method In the hierarchical heterostruc-ture the ZnO nanorods with single crystalline featurewere perpendicularly grown on the surface of TiO2 Thegrown mechanism has been investigated through XRD andHRTEM The sensing performance of this hierarchical het-erostructure was examined and it was found that the hier-archical heterostructure sensor has excellent sensitivity andselectivity to trimethylamine gas The potential sensingmechanism was also discussed and thought to be relatedto the surface-depletion model}

_{Acknowledgments The authors are thankful for fundsfrom National Natural Science Foundation of China(Grant No 51372142) National Science Fund for Distin-guished Young Scholars (NSFDYS 50925205) InnovationResearch Group (IRG 51321091) and the lsquo100 TalentsProgramrsquo of Chinese Academy of Sciences Thanks for thesupport from the lsquothousands talentsrsquo program for pioneerresearcher and his innovation team China}

_{References and Notes1 K J Choi and H W Jang Sensors 10 4083 (2010)2 I-D Kim A Rothschild and H L Tuller Acta Mater 61 974}

_{(2013)3 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134}

_{403 (2008)4 Q Li Y Liang Q Wan and T Wang Appl Phys Lett 85 6389}

_{(2004)5 Q Wan Q Li Y Chen T-H Wang X He J Li and C Lin Appl}

_{Phys Lett 84 3654 (2004)6 J Chen L-N Xu W-Y Li and X-L Gou Adv Mater 17 582}

₍₂₀₀₅₎

_{7 D R Kauffman and A Star Angew Chem Int Ed 47 6550 (2008)8 Y M Wang G J Du H Liu D Liu S B Qin N Wang C G}

_{Hu X T Tao J Jiao J Y Wang and Z L Wang Adv FunctMater 18 1131 (2008)}

_{9 P Q Hu G J Du W J Zhou J J Cui J J Lin H Liu D LiuJ Y Wang and S W Chen Acs Appl Mater Inter 2 3263 (2010)}

_{10 D Z Wang W J Zhou P G Hu Y Guan L M Chen J HLi G C Wang H Liu J Y Wang G Z Cao and H D JiangJ Colloid Interf Sci 388 144 (2012)}

_{11 L Liao H Lu J Li H He D Wang D Fu C Liu and W ZhangJ Phys Chem C 111 1900 (2007)}

_{12 A Fujishima X Zhang and D Tryk Surface Science Reports 63515 (2008)}

_{13 Z Y Han J C Zhang and W L Cao Mater Lett 84 34 (2012)14 J J Cui Y K Ge S W Chen H S Zhao H Liu Z Huang H D}

_{Jiang and J Chen J Mater Chem B 1 2072 (2013)15 J J Cui D H Sun S W Chen W J Zhou P G Hu H Liu and}

_{Z Huang J Mater Chem 21 10633 (2011)16 J J Cui D H Sun W J Zhou H Liu P G Hu N Ren H M}

_{Qin Z Huang J J Lin and H Y Ma Phys Chem Chem Phys13 9232 (2011)}

_{17 G K Mor M A Carvalho O K Varghese M V Pishko and C AGrimes J Mater Res 19 628 (2004)}

_{18 H F Lu F Li G Liu Z-G Chen D-W Wang H-T Fang G QLu Z H Jiang and H-M Cheng Nanotechnology 19 405504(2008)}

_{19 J Gong Y Li Z Hu Z Zhou and Y Deng J Phys Chem C 1149970 (2010)}

_{20 W J Zhou G J Du P G Hu G H Li D Z Wang H Liu J YWang R I Boughton D Liu and H D Jiang J Mater Chem 217937 (2011)}

_{21 W Zeng T M Liu and Z C Wang J Mater Chem 22 3544(2012)}

_{22 Z Zhao J Tian D Wang X Kang Y Sang H Liu J WangS Chen R I Boughton and H Jiang Journal of Materials Chem-istry 22 23395 (2012)}

_{23 L E Greene M Law D H Tan M Montano J GoldbergerG Somorjai and P Yang Nano Lett 5 1231 (2005)}

_{24 C Cheng B Liu H Yang W Zhou L Sun R Chen S F YuJ Zhang H Gong and H Sun Acs Nano 3 3069 (2009)}

_{25 W J Zhou L G Gai P G Hu J J Cui X Y Liu D Z WangG H Li H D Jiang D Liu H Liu and J Y Wang Cryst Engcomm 13 6643 (2011)}

_{26 Y-Z Lv C-R Li L Guo F-C Wang Y Xu and X-F Chu SensActuators B Chemical 141 85 (2009)}

_{27 H Tang M Yan X Ma H Zhang M Wang and D Yang SensActuators B Chemical 113 324 (2006)}

_{28 S-H Park J-Y Ryu H-H Choi and T-H Kwon Sens ActuatorsB Chemical 46 75 (1998)}

_{29 S Roy and S Basu Journal of Materials Science Materials inElectronics 15 321 (2004)}

_{30 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134403 (2008)}

_{31 X M Yin C C Li M Zhang Q Y Hao S Liu Q H Li L BChen and T H Wang Nanotechnology 20 455503 (2009)}

_{32 P-C Chen G Shen and C Zhou IEEE Transactions on Nanotech-nology 7 668 (2008)}

_{410 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LEhierarchical heterostructure are expected to improve thegas sensing performance}

_{To further examine the microstructure transmissionelectron microscopy (TEM) was also used Figure 4 showsthe TEM images of the ZnO-NDsTiO2-NBs hierarchicalheterostructure As shown in Figure 4(a) ZnO nanorodswere vertically grown on the TiO2 nanobelt support Theclear crystalline lattice fringes of the ZnO nanorod branchand the TiO2 nanobelt support indicate the single crys-talline feature of both the ZnO and TiO2 This conclu-sion can be well supported by the bright spots of the twodimensional FFT patterns shown in Figures 4(c) and (d) Itshould be pointed out that ZnO nanorods was grown on theZnO seeds which can be clearly seen from Figure 4(b)Both the nanorod and the seed part displayed similarcrystalline lattice fringes The measured lattice spacing is035 nm for TiO2 and 027 nm for ZnO corresponding tothe (101) plane of TiO2 and (0002) plane of ZnO respec-tively Therefore the growth direction of the TiO2 nanobeltsupport and the ZnO nanorod branch was determined tobe along [010] and [0001] respectively}

_{Based on the above analysis the growth process of theZnO-NDsTiO2-NBs hierarchical heterostructure is illus-trated in Figure 5 Firstly the primary TiO2 nanobeltswere coated with a layer of ZnO seed nanoparticlesThese ZnO nanoparticles acted as the specific nucleatesites which can be clearly seen from Figure 5(a) Forthe growth of ZnO nanorod it is necessary to seed theTiO2 nanobelt with ZnO nanoparticles In the initial stageZnO nanorods nucleated and crystallized along the [0001]direction on the ZnO seed sites With the proceeding of}

_{Fig 4 Microstructure characterization of the ZnO-NDsTiO2-NBshierarchical heterostructure (a) and (b) TEM image of the hierarchicalheterostructure taken at different magnification (c) HRTEM of the hier-archical heterostructure taken near the junction (d) and (e) Fast-Fourier-Transform patterns from the selected area in (c) respectively}

_{Fig 5 Growth process of the ZnO-NDsTiO2-NBs hierarchical het-erostructure Top row schematics Bottom row corresponding SEMimages of the (a) bare TiO2 nanobelt (b) ZnO seeded TiO2 nanobelt and(c) ZnO-NDsTiO2-NBs hierarchical heterostructure}

_{the hydrothermal process the initial nanorods absorbedZn2+ and OHminus ions from the precursor solution toform Zn(OH)2 which decomposed to form ZnO Thoughthe TiO2 has six different crystalline planes The ZnOnanorods were still densely grown on the TiO2 nanobeltsurface This may be due to the high zinc salt concentra-tion and extended hydrothermal reaction time}

_{32 Gas Sensing PerformanceA good gas sensor should exhibit fast response once uponexposure to the target gas Meanwhile it also should havethe ability to recognize the target gas at a concentrationof target gas as low as possible Summarily the gas sen-sor should have good selectivity and sensitivity Under thepresent experiments we have investigated the gas sensingperformance of the prepared ZnO-NDsTiO2-NBs hier-archical heterostructure For full comparison the sensingbehavior of TiO2 nanobelts and ZnO nanorods was alsoexamined under the same conditionFigure 6 shows the responserecovery curves at 200 C}

_{of the TiO2 nanobelts sensor ZnO nanorods sensor andthe hierarchical heterostructure sensor The gas concentra-tion of trimethylamine was tuned from 5 ppm to 500 ppmObviously once exposed to the trimethylamine gas allthe three sensors exhibited a remarkable response withdifferent output voltage of Vout However there was stilldifferent response behavior for the three gas sensors Forexample at 500 ppm the output voltage of Vout was 114134 and as high as 40 for TiO2 nanobelts (Fig 6(a))ZnO nanorods (Fig 6(b)) and ZnO-NDsTiO2-NBs hier-archical heterostructure (Fig 6(c)) respectively Oncethe concentration decreased to 100 ppm the Vout valuealso decreased to 0312 0434 and 247 respectivelyThis means that with decreasing the concentration oftrimethylamine gas the output voltage of the threesensors decreased in a different rate suggesting anincreasing order of TiO2 nanobelts ZnO nanorods ZnO-NDsTiO2-NBs hierarchical heterostructure of the sens-ing performance Impressively the sensor made from thehierarchical heterostructure even showed a high outputvalue of about 0956 at the very low concentration of}

_{Energy Environ Focus 3 404ndash410 2014 407}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LE(a) (b)}

_{(c) (d)}

_{Fig 6 Responserecovery curves gas sensors made from (a) TiO2 nanobelts (b) ZnO nanorods and (c) ZnO-NDsTiO2-NBs hierarchical heterostruc-ture upon exposed to different concentrations of trimethylamine gas at 200 C (d) Calculated sensitivity S as a function of gas concentration}

_{5 ppm indicating the good sensing capability to trimethy-lamine gasThe sensor performance can be directly illustrated in}

_{the sensitivity S The corresponding sensitivity S of thethree sensors are shown in Figure 6(d) According toFigure 6(d) the sensitivity of the three sensors increaseslinearly with the concentration of trimethylamine gas Thesensitivity of the ZnO sensor is slightly higher than that ofTiO2 sensor while notably the sensitivity of the hierarchi-cal heterostructure sensor shows much higher values at allthe gas concentrations Even at the concentration as lowas 5 ppm the sensitivity of the hierarchical heterostruc-ture sensor reaches to about 25 which is much higher thanthose listed in literatures This means by assembling ZnOnanorods onto the surface of TiO2 nanobelts can signifi-cantly improve the sensing capabilityGas sensors also need to detect target gas at differ-}

_{ent working temperatures Therefore it is important toinvestigate the effect of temperature on the sensing per-formance Figure 7 shows responserecovery curves of theZnO-NDsTiO2-NBs hierarchical heterostructure sensorat different temperaturesby exposure to trimethylamine gasat a constant concentration of 300 ppm Clearly as seenfrom Figure 7(a) the sensor shows good responserecoverygas at all the temperatures because the Vout in trimethy-lamine gas recovered to similar value when fresh air waspurged into the testing chamber Notably when the sen-sor was working at 100 C the increase of the outputvoltage Vout of the sensor was the smallest With the}

_{increase of the working temperature the increase of theoutput voltage gradually increased and reached to the high-est value of about 35 at 200 C While the temperatureincreased to 250 C the output voltage Vout decreasedto 18 much lower than 35 at 100 C This indicatesthat the hierarchical heterostructure sensor showed the bestresponserecovery behavior while working at 200 C Thecorresponding sensitivity at different temperatures as thefunction of gas concentration is shown in Figure 7(b) Itis found that at all the concentrations the sensor hardlyexhibited sensing capability at 100 C and 150 C Simi-lar increasing trend can be observed when the sensor wasworking at 200 C and 250 C However the sensitivityobtained at 200 C was much higher than that at 250 CTherefore it is concluded that the ZnO-NDsTiO2-NBshierarchical heterostructure has the best sensing perfor-mance upon exposure to trimethylamine gas at 200 CAs stated previously the selectivity of the gas sensor is}

_{another key parameter Therefore the sensing performanceof the hierarchical heterostructure to different gas was alsoinvestigated at the same gas concentration and workingtemperature Figure 8 shows the responserecovery curvesof the ZnO-NDsTiO2-NBs hierarchical heterostructureupon exposure to trimethylamine gas ammonia gas andCO and ethanol vapor respectivelyat 200 C and constantconcentration of 300 ppm It is found that though the sen-sor shows response to all the detecting gas remarkablydifferent response behavior can be seen from Figure 9The value of the Vout upon exposure to ethanol vapor}

_{408 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LE(a) (b)}

_{Fig 7 (a) Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure at different temperatures by exposureto trimethylamine gas at a constant concentration of 300 ppm and (b) the corresponding calculated sensitivity S as a function of gas concentration}

_{CO ammonia gas and trimethylamine gas was about 059097 144 and 343 respectively indicating the best detect-ing capability to trimethylamine gas Actually the presentZnO-NDsTiO2-NBs hierarchical heterostructure sensorshows much better sensing performance to trimethylaminegas The sensitivity of the hierarchical heterostructure sen-sor is much higher than those listed in literatures32627}

_{Most impressively the present hierarchical heterostructuresensor has excellent sensing performance at a very lowgas concentration of 5 ppm This gas concentration ismuch lower than 160 ppm of ZnO film sensor28 400 ppmof ZnO and Al film29 and 100 ppm of SnO2 and ZnOnanocomposite30}

_{33 Sensing MechanismMost semiconductor oxide based gas sensors operate onthe basis of the variation of the electrical propertiesof the oxide materials4 This variation of the electricalresistance is thought to be related to a surface deple-tion model Actually the sensing mechanism of the TiO2}

_{nanobelts for detecting reductive gas has been discussedin literature9 Trimethylamine gas belongs to the reductive}

_{Fig 8 Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure by exposure to different gasat the same gas concentration of 300 ppm working at 200 C}

_{gas type Therefore at the present sensor TiO2 nanobeltsfor sensing trimethylamine gas should be based on the sim-ilar mechanism ie the surface-depletion model31 WhenTiO2 nanobelts are exposure to air oxygen molecules areadsorbed onto the surface of TiO2 nanobelt and trap theelectrons from the conduction band of TiO2 to form super-oxo or perpxo-like species This surface reaction bringsabout the formation of a surface depletion region withinthe nanobelt resulting in the increase of the electricalresistance of the nanobelts Once the reductive gas (thetarget gas) were introduced to contact with the surface ofthe nanobelts effective electron transfer happens from thetarget gas to TiO2 nanobelt leading to the decrease of theelectrical resistance of the nanobeltsActually ZnO should obey the similar surface depletion}

_{mechanism The surface physical and chemical propertiesplays important roles in its gas sensing performance11}

_{Though the energy level of the conduction band of TiO2}

_{and ZnO is similar with each other the present hydrother-mally synthesized TiO2 nanobelts dominantly belongs toanatase phase and are known to have many oxygen vacan-cies which will act as the electron trap This will helpto drain electrons from ZnO phase to TiO2 nanobeltsthrough the interfacial heterostructure between ZnO andTiO2 The electron transfer will undoubtedly speed theelectron transfer from the target reductive gas to theZnO and then to TiO2 In addition the electron mobil-ity of ZnO (1175 cm2V middot S) is much higher than that ofTiO2 (02 cm2V middotS)32 Since the charge carriers diffusionfrom bulk to surface of the semiconductor is the rate-limiting step once the hierarchical heterostructure sen-sor is exposed to air the ZnO nanorod part should bemore sensitive than the TiO2 nanobelt part to the oxy-gen molecule adsorption In another word ZnO nanorodsshould be more sensitive to the variation of the con-centration of electrons induced by the oxygen adsorptionand electron transfer between the reductive target gas andthe semiconductor The present hierarchical heterostructuredisplays several advantages which is beneficial for the gassensing The first one is the exposure of both the surface}

_{Energy Environ Focus 3 404ndash410 2014 409}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LEof ZnO nanorod and TiO2 nanobelt These two kinds ofsemiconductors are proved to be effective materials forgas sensor application The second one is the exposure ofmore sensitive surface of ZnO nanorod The third one isthe specific hierarchical structure The charge transport isfaster in one dimensional ZnO nanorod and TiO2 nanobeltwhich both of them are highly crystalized Meanwhile themicrochannel formed between the ZnO nanorods can facil-itate the gas diffusion based on a capillary force There-fore based on the synergistic effect of ZnO and TiO2 thehierarchical heterostructure sensor exhibits excellent sens-ing performance to the reductive trimethylamine gas}

_{4 CONCLUSIONIn this study ZnO nanorods were assembled onto the sur-face of TiO2 nanobelts to form a hierarchical surface het-erostructure through a hydrothermal process with the aidof seeds coating method In the hierarchical heterostruc-ture the ZnO nanorods with single crystalline featurewere perpendicularly grown on the surface of TiO2 Thegrown mechanism has been investigated through XRD andHRTEM The sensing performance of this hierarchical het-erostructure was examined and it was found that the hier-archical heterostructure sensor has excellent sensitivity andselectivity to trimethylamine gas The potential sensingmechanism was also discussed and thought to be relatedto the surface-depletion model}

_{Acknowledgments The authors are thankful for fundsfrom National Natural Science Foundation of China(Grant No 51372142) National Science Fund for Distin-guished Young Scholars (NSFDYS 50925205) InnovationResearch Group (IRG 51321091) and the lsquo100 TalentsProgramrsquo of Chinese Academy of Sciences Thanks for thesupport from the lsquothousands talentsrsquo program for pioneerresearcher and his innovation team China}

_{References and Notes1 K J Choi and H W Jang Sensors 10 4083 (2010)2 I-D Kim A Rothschild and H L Tuller Acta Mater 61 974}

_{(2013)3 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134}

_{403 (2008)4 Q Li Y Liang Q Wan and T Wang Appl Phys Lett 85 6389}

_{(2004)5 Q Wan Q Li Y Chen T-H Wang X He J Li and C Lin Appl}

_{Phys Lett 84 3654 (2004)6 J Chen L-N Xu W-Y Li and X-L Gou Adv Mater 17 582}

₍₂₀₀₅₎

_{7 D R Kauffman and A Star Angew Chem Int Ed 47 6550 (2008)8 Y M Wang G J Du H Liu D Liu S B Qin N Wang C G}

_{Hu X T Tao J Jiao J Y Wang and Z L Wang Adv FunctMater 18 1131 (2008)}

_{9 P Q Hu G J Du W J Zhou J J Cui J J Lin H Liu D LiuJ Y Wang and S W Chen Acs Appl Mater Inter 2 3263 (2010)}

_{10 D Z Wang W J Zhou P G Hu Y Guan L M Chen J HLi G C Wang H Liu J Y Wang G Z Cao and H D JiangJ Colloid Interf Sci 388 144 (2012)}

_{11 L Liao H Lu J Li H He D Wang D Fu C Liu and W ZhangJ Phys Chem C 111 1900 (2007)}

_{12 A Fujishima X Zhang and D Tryk Surface Science Reports 63515 (2008)}

_{13 Z Y Han J C Zhang and W L Cao Mater Lett 84 34 (2012)14 J J Cui Y K Ge S W Chen H S Zhao H Liu Z Huang H D}

_{Jiang and J Chen J Mater Chem B 1 2072 (2013)15 J J Cui D H Sun S W Chen W J Zhou P G Hu H Liu and}

_{Z Huang J Mater Chem 21 10633 (2011)16 J J Cui D H Sun W J Zhou H Liu P G Hu N Ren H M}

_{Qin Z Huang J J Lin and H Y Ma Phys Chem Chem Phys13 9232 (2011)}

_{17 G K Mor M A Carvalho O K Varghese M V Pishko and C AGrimes J Mater Res 19 628 (2004)}

_{18 H F Lu F Li G Liu Z-G Chen D-W Wang H-T Fang G QLu Z H Jiang and H-M Cheng Nanotechnology 19 405504(2008)}

_{19 J Gong Y Li Z Hu Z Zhou and Y Deng J Phys Chem C 1149970 (2010)}

_{20 W J Zhou G J Du P G Hu G H Li D Z Wang H Liu J YWang R I Boughton D Liu and H D Jiang J Mater Chem 217937 (2011)}

_{21 W Zeng T M Liu and Z C Wang J Mater Chem 22 3544(2012)}

_{22 Z Zhao J Tian D Wang X Kang Y Sang H Liu J WangS Chen R I Boughton and H Jiang Journal of Materials Chem-istry 22 23395 (2012)}

_{23 L E Greene M Law D H Tan M Montano J GoldbergerG Somorjai and P Yang Nano Lett 5 1231 (2005)}

_{24 C Cheng B Liu H Yang W Zhou L Sun R Chen S F YuJ Zhang H Gong and H Sun Acs Nano 3 3069 (2009)}

_{25 W J Zhou L G Gai P G Hu J J Cui X Y Liu D Z WangG H Li H D Jiang D Liu H Liu and J Y Wang Cryst Engcomm 13 6643 (2011)}

_{26 Y-Z Lv C-R Li L Guo F-C Wang Y Xu and X-F Chu SensActuators B Chemical 141 85 (2009)}

_{27 H Tang M Yan X Ma H Zhang M Wang and D Yang SensActuators B Chemical 113 324 (2006)}

_{28 S-H Park J-Y Ryu H-H Choi and T-H Kwon Sens ActuatorsB Chemical 46 75 (1998)}

_{29 S Roy and S Basu Journal of Materials Science Materials inElectronics 15 321 (2004)}

_{30 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134403 (2008)}

_{31 X M Yin C C Li M Zhang Q Y Hao S Liu Q H Li L BChen and T H Wang Nanotechnology 20 455503 (2009)}

_{32 P-C Chen G Shen and C Zhou IEEE Transactions on Nanotech-nology 7 668 (2008)}

_{410 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LE(a) (b)}

_{(c) (d)}

_{Fig 6 Responserecovery curves gas sensors made from (a) TiO2 nanobelts (b) ZnO nanorods and (c) ZnO-NDsTiO2-NBs hierarchical heterostruc-ture upon exposed to different concentrations of trimethylamine gas at 200 C (d) Calculated sensitivity S as a function of gas concentration}

_{5 ppm indicating the good sensing capability to trimethy-lamine gasThe sensor performance can be directly illustrated in}

_{the sensitivity S The corresponding sensitivity S of thethree sensors are shown in Figure 6(d) According toFigure 6(d) the sensitivity of the three sensors increaseslinearly with the concentration of trimethylamine gas Thesensitivity of the ZnO sensor is slightly higher than that ofTiO2 sensor while notably the sensitivity of the hierarchi-cal heterostructure sensor shows much higher values at allthe gas concentrations Even at the concentration as lowas 5 ppm the sensitivity of the hierarchical heterostruc-ture sensor reaches to about 25 which is much higher thanthose listed in literatures This means by assembling ZnOnanorods onto the surface of TiO2 nanobelts can signifi-cantly improve the sensing capabilityGas sensors also need to detect target gas at differ-}

_{ent working temperatures Therefore it is important toinvestigate the effect of temperature on the sensing per-formance Figure 7 shows responserecovery curves of theZnO-NDsTiO2-NBs hierarchical heterostructure sensorat different temperaturesby exposure to trimethylamine gasat a constant concentration of 300 ppm Clearly as seenfrom Figure 7(a) the sensor shows good responserecoverygas at all the temperatures because the Vout in trimethy-lamine gas recovered to similar value when fresh air waspurged into the testing chamber Notably when the sen-sor was working at 100 C the increase of the outputvoltage Vout of the sensor was the smallest With the}

_{increase of the working temperature the increase of theoutput voltage gradually increased and reached to the high-est value of about 35 at 200 C While the temperatureincreased to 250 C the output voltage Vout decreasedto 18 much lower than 35 at 100 C This indicatesthat the hierarchical heterostructure sensor showed the bestresponserecovery behavior while working at 200 C Thecorresponding sensitivity at different temperatures as thefunction of gas concentration is shown in Figure 7(b) Itis found that at all the concentrations the sensor hardlyexhibited sensing capability at 100 C and 150 C Simi-lar increasing trend can be observed when the sensor wasworking at 200 C and 250 C However the sensitivityobtained at 200 C was much higher than that at 250 CTherefore it is concluded that the ZnO-NDsTiO2-NBshierarchical heterostructure has the best sensing perfor-mance upon exposure to trimethylamine gas at 200 CAs stated previously the selectivity of the gas sensor is}

_{another key parameter Therefore the sensing performanceof the hierarchical heterostructure to different gas was alsoinvestigated at the same gas concentration and workingtemperature Figure 8 shows the responserecovery curvesof the ZnO-NDsTiO2-NBs hierarchical heterostructureupon exposure to trimethylamine gas ammonia gas andCO and ethanol vapor respectivelyat 200 C and constantconcentration of 300 ppm It is found that though the sen-sor shows response to all the detecting gas remarkablydifferent response behavior can be seen from Figure 9The value of the Vout upon exposure to ethanol vapor}

_{408 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LE(a) (b)}

_{Fig 7 (a) Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure at different temperatures by exposureto trimethylamine gas at a constant concentration of 300 ppm and (b) the corresponding calculated sensitivity S as a function of gas concentration}

_{CO ammonia gas and trimethylamine gas was about 059097 144 and 343 respectively indicating the best detect-ing capability to trimethylamine gas Actually the presentZnO-NDsTiO2-NBs hierarchical heterostructure sensorshows much better sensing performance to trimethylaminegas The sensitivity of the hierarchical heterostructure sen-sor is much higher than those listed in literatures32627}

_{Most impressively the present hierarchical heterostructuresensor has excellent sensing performance at a very lowgas concentration of 5 ppm This gas concentration ismuch lower than 160 ppm of ZnO film sensor28 400 ppmof ZnO and Al film29 and 100 ppm of SnO2 and ZnOnanocomposite30}

_{33 Sensing MechanismMost semiconductor oxide based gas sensors operate onthe basis of the variation of the electrical propertiesof the oxide materials4 This variation of the electricalresistance is thought to be related to a surface deple-tion model Actually the sensing mechanism of the TiO2}

_{nanobelts for detecting reductive gas has been discussedin literature9 Trimethylamine gas belongs to the reductive}

_{Fig 8 Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure by exposure to different gasat the same gas concentration of 300 ppm working at 200 C}

_{gas type Therefore at the present sensor TiO2 nanobeltsfor sensing trimethylamine gas should be based on the sim-ilar mechanism ie the surface-depletion model31 WhenTiO2 nanobelts are exposure to air oxygen molecules areadsorbed onto the surface of TiO2 nanobelt and trap theelectrons from the conduction band of TiO2 to form super-oxo or perpxo-like species This surface reaction bringsabout the formation of a surface depletion region withinthe nanobelt resulting in the increase of the electricalresistance of the nanobelts Once the reductive gas (thetarget gas) were introduced to contact with the surface ofthe nanobelts effective electron transfer happens from thetarget gas to TiO2 nanobelt leading to the decrease of theelectrical resistance of the nanobeltsActually ZnO should obey the similar surface depletion}

_{mechanism The surface physical and chemical propertiesplays important roles in its gas sensing performance11}

_{Though the energy level of the conduction band of TiO2}

_{and ZnO is similar with each other the present hydrother-mally synthesized TiO2 nanobelts dominantly belongs toanatase phase and are known to have many oxygen vacan-cies which will act as the electron trap This will helpto drain electrons from ZnO phase to TiO2 nanobeltsthrough the interfacial heterostructure between ZnO andTiO2 The electron transfer will undoubtedly speed theelectron transfer from the target reductive gas to theZnO and then to TiO2 In addition the electron mobil-ity of ZnO (1175 cm2V middot S) is much higher than that ofTiO2 (02 cm2V middotS)32 Since the charge carriers diffusionfrom bulk to surface of the semiconductor is the rate-limiting step once the hierarchical heterostructure sen-sor is exposed to air the ZnO nanorod part should bemore sensitive than the TiO2 nanobelt part to the oxy-gen molecule adsorption In another word ZnO nanorodsshould be more sensitive to the variation of the con-centration of electrons induced by the oxygen adsorptionand electron transfer between the reductive target gas andthe semiconductor The present hierarchical heterostructuredisplays several advantages which is beneficial for the gassensing The first one is the exposure of both the surface}

_{Energy Environ Focus 3 404ndash410 2014 409}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LEof ZnO nanorod and TiO2 nanobelt These two kinds ofsemiconductors are proved to be effective materials forgas sensor application The second one is the exposure ofmore sensitive surface of ZnO nanorod The third one isthe specific hierarchical structure The charge transport isfaster in one dimensional ZnO nanorod and TiO2 nanobeltwhich both of them are highly crystalized Meanwhile themicrochannel formed between the ZnO nanorods can facil-itate the gas diffusion based on a capillary force There-fore based on the synergistic effect of ZnO and TiO2 thehierarchical heterostructure sensor exhibits excellent sens-ing performance to the reductive trimethylamine gas}

_{4 CONCLUSIONIn this study ZnO nanorods were assembled onto the sur-face of TiO2 nanobelts to form a hierarchical surface het-erostructure through a hydrothermal process with the aidof seeds coating method In the hierarchical heterostruc-ture the ZnO nanorods with single crystalline featurewere perpendicularly grown on the surface of TiO2 Thegrown mechanism has been investigated through XRD andHRTEM The sensing performance of this hierarchical het-erostructure was examined and it was found that the hier-archical heterostructure sensor has excellent sensitivity andselectivity to trimethylamine gas The potential sensingmechanism was also discussed and thought to be relatedto the surface-depletion model}

_{Acknowledgments The authors are thankful for fundsfrom National Natural Science Foundation of China(Grant No 51372142) National Science Fund for Distin-guished Young Scholars (NSFDYS 50925205) InnovationResearch Group (IRG 51321091) and the lsquo100 TalentsProgramrsquo of Chinese Academy of Sciences Thanks for thesupport from the lsquothousands talentsrsquo program for pioneerresearcher and his innovation team China}

_{References and Notes1 K J Choi and H W Jang Sensors 10 4083 (2010)2 I-D Kim A Rothschild and H L Tuller Acta Mater 61 974}

_{(2013)3 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134}

_{403 (2008)4 Q Li Y Liang Q Wan and T Wang Appl Phys Lett 85 6389}

_{(2004)5 Q Wan Q Li Y Chen T-H Wang X He J Li and C Lin Appl}

_{Phys Lett 84 3654 (2004)6 J Chen L-N Xu W-Y Li and X-L Gou Adv Mater 17 582}

₍₂₀₀₅₎

_{7 D R Kauffman and A Star Angew Chem Int Ed 47 6550 (2008)8 Y M Wang G J Du H Liu D Liu S B Qin N Wang C G}

_{Hu X T Tao J Jiao J Y Wang and Z L Wang Adv FunctMater 18 1131 (2008)}

_{9 P Q Hu G J Du W J Zhou J J Cui J J Lin H Liu D LiuJ Y Wang and S W Chen Acs Appl Mater Inter 2 3263 (2010)}

_{10 D Z Wang W J Zhou P G Hu Y Guan L M Chen J HLi G C Wang H Liu J Y Wang G Z Cao and H D JiangJ Colloid Interf Sci 388 144 (2012)}

_{11 L Liao H Lu J Li H He D Wang D Fu C Liu and W ZhangJ Phys Chem C 111 1900 (2007)}

_{12 A Fujishima X Zhang and D Tryk Surface Science Reports 63515 (2008)}

_{13 Z Y Han J C Zhang and W L Cao Mater Lett 84 34 (2012)14 J J Cui Y K Ge S W Chen H S Zhao H Liu Z Huang H D}

_{Jiang and J Chen J Mater Chem B 1 2072 (2013)15 J J Cui D H Sun S W Chen W J Zhou P G Hu H Liu and}

_{Z Huang J Mater Chem 21 10633 (2011)16 J J Cui D H Sun W J Zhou H Liu P G Hu N Ren H M}

_{Qin Z Huang J J Lin and H Y Ma Phys Chem Chem Phys13 9232 (2011)}

_{17 G K Mor M A Carvalho O K Varghese M V Pishko and C AGrimes J Mater Res 19 628 (2004)}

_{18 H F Lu F Li G Liu Z-G Chen D-W Wang H-T Fang G QLu Z H Jiang and H-M Cheng Nanotechnology 19 405504(2008)}

_{19 J Gong Y Li Z Hu Z Zhou and Y Deng J Phys Chem C 1149970 (2010)}

_{20 W J Zhou G J Du P G Hu G H Li D Z Wang H Liu J YWang R I Boughton D Liu and H D Jiang J Mater Chem 217937 (2011)}

_{21 W Zeng T M Liu and Z C Wang J Mater Chem 22 3544(2012)}

_{22 Z Zhao J Tian D Wang X Kang Y Sang H Liu J WangS Chen R I Boughton and H Jiang Journal of Materials Chem-istry 22 23395 (2012)}

_{23 L E Greene M Law D H Tan M Montano J GoldbergerG Somorjai and P Yang Nano Lett 5 1231 (2005)}

_{24 C Cheng B Liu H Yang W Zhou L Sun R Chen S F YuJ Zhang H Gong and H Sun Acs Nano 3 3069 (2009)}

_{25 W J Zhou L G Gai P G Hu J J Cui X Y Liu D Z WangG H Li H D Jiang D Liu H Liu and J Y Wang Cryst Engcomm 13 6643 (2011)}

_{26 Y-Z Lv C-R Li L Guo F-C Wang Y Xu and X-F Chu SensActuators B Chemical 141 85 (2009)}

_{27 H Tang M Yan X Ma H Zhang M Wang and D Yang SensActuators B Chemical 113 324 (2006)}

_{28 S-H Park J-Y Ryu H-H Choi and T-H Kwon Sens ActuatorsB Chemical 46 75 (1998)}

_{29 S Roy and S Basu Journal of Materials Science Materials inElectronics 15 321 (2004)}

_{30 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134403 (2008)}

_{31 X M Yin C C Li M Zhang Q Y Hao S Liu Q H Li L BChen and T H Wang Nanotechnology 20 455503 (2009)}

_{32 P-C Chen G Shen and C Zhou IEEE Transactions on Nanotech-nology 7 668 (2008)}

_{410 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Zhao et al Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor}

_ARTIC

_{LE(a) (b)}

_{Fig 7 (a) Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure at different temperatures by exposureto trimethylamine gas at a constant concentration of 300 ppm and (b) the corresponding calculated sensitivity S as a function of gas concentration}

_{CO ammonia gas and trimethylamine gas was about 059097 144 and 343 respectively indicating the best detect-ing capability to trimethylamine gas Actually the presentZnO-NDsTiO2-NBs hierarchical heterostructure sensorshows much better sensing performance to trimethylaminegas The sensitivity of the hierarchical heterostructure sen-sor is much higher than those listed in literatures32627}

_{Most impressively the present hierarchical heterostructuresensor has excellent sensing performance at a very lowgas concentration of 5 ppm This gas concentration ismuch lower than 160 ppm of ZnO film sensor28 400 ppmof ZnO and Al film29 and 100 ppm of SnO2 and ZnOnanocomposite30}

_{33 Sensing MechanismMost semiconductor oxide based gas sensors operate onthe basis of the variation of the electrical propertiesof the oxide materials4 This variation of the electricalresistance is thought to be related to a surface deple-tion model Actually the sensing mechanism of the TiO2}

_{nanobelts for detecting reductive gas has been discussedin literature9 Trimethylamine gas belongs to the reductive}

_{Fig 8 Responserecovery curves of gas sensor made from ZnO-NDsTiO2-NBs hierarchical heterostructure by exposure to different gasat the same gas concentration of 300 ppm working at 200 C}

_{gas type Therefore at the present sensor TiO2 nanobeltsfor sensing trimethylamine gas should be based on the sim-ilar mechanism ie the surface-depletion model31 WhenTiO2 nanobelts are exposure to air oxygen molecules areadsorbed onto the surface of TiO2 nanobelt and trap theelectrons from the conduction band of TiO2 to form super-oxo or perpxo-like species This surface reaction bringsabout the formation of a surface depletion region withinthe nanobelt resulting in the increase of the electricalresistance of the nanobelts Once the reductive gas (thetarget gas) were introduced to contact with the surface ofthe nanobelts effective electron transfer happens from thetarget gas to TiO2 nanobelt leading to the decrease of theelectrical resistance of the nanobeltsActually ZnO should obey the similar surface depletion}

_{mechanism The surface physical and chemical propertiesplays important roles in its gas sensing performance11}

_{Though the energy level of the conduction band of TiO2}

_{and ZnO is similar with each other the present hydrother-mally synthesized TiO2 nanobelts dominantly belongs toanatase phase and are known to have many oxygen vacan-cies which will act as the electron trap This will helpto drain electrons from ZnO phase to TiO2 nanobeltsthrough the interfacial heterostructure between ZnO andTiO2 The electron transfer will undoubtedly speed theelectron transfer from the target reductive gas to theZnO and then to TiO2 In addition the electron mobil-ity of ZnO (1175 cm2V middot S) is much higher than that ofTiO2 (02 cm2V middotS)32 Since the charge carriers diffusionfrom bulk to surface of the semiconductor is the rate-limiting step once the hierarchical heterostructure sen-sor is exposed to air the ZnO nanorod part should bemore sensitive than the TiO2 nanobelt part to the oxy-gen molecule adsorption In another word ZnO nanorodsshould be more sensitive to the variation of the con-centration of electrons induced by the oxygen adsorptionand electron transfer between the reductive target gas andthe semiconductor The present hierarchical heterostructuredisplays several advantages which is beneficial for the gassensing The first one is the exposure of both the surface}

_{Energy Environ Focus 3 404ndash410 2014 409}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LEof ZnO nanorod and TiO2 nanobelt These two kinds ofsemiconductors are proved to be effective materials forgas sensor application The second one is the exposure ofmore sensitive surface of ZnO nanorod The third one isthe specific hierarchical structure The charge transport isfaster in one dimensional ZnO nanorod and TiO2 nanobeltwhich both of them are highly crystalized Meanwhile themicrochannel formed between the ZnO nanorods can facil-itate the gas diffusion based on a capillary force There-fore based on the synergistic effect of ZnO and TiO2 thehierarchical heterostructure sensor exhibits excellent sens-ing performance to the reductive trimethylamine gas}

_{4 CONCLUSIONIn this study ZnO nanorods were assembled onto the sur-face of TiO2 nanobelts to form a hierarchical surface het-erostructure through a hydrothermal process with the aidof seeds coating method In the hierarchical heterostruc-ture the ZnO nanorods with single crystalline featurewere perpendicularly grown on the surface of TiO2 Thegrown mechanism has been investigated through XRD andHRTEM The sensing performance of this hierarchical het-erostructure was examined and it was found that the hier-archical heterostructure sensor has excellent sensitivity andselectivity to trimethylamine gas The potential sensingmechanism was also discussed and thought to be relatedto the surface-depletion model}

_{Acknowledgments The authors are thankful for fundsfrom National Natural Science Foundation of China(Grant No 51372142) National Science Fund for Distin-guished Young Scholars (NSFDYS 50925205) InnovationResearch Group (IRG 51321091) and the lsquo100 TalentsProgramrsquo of Chinese Academy of Sciences Thanks for thesupport from the lsquothousands talentsrsquo program for pioneerresearcher and his innovation team China}

_{References and Notes1 K J Choi and H W Jang Sensors 10 4083 (2010)2 I-D Kim A Rothschild and H L Tuller Acta Mater 61 974}

_{(2013)3 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134}

_{403 (2008)4 Q Li Y Liang Q Wan and T Wang Appl Phys Lett 85 6389}

_{(2004)5 Q Wan Q Li Y Chen T-H Wang X He J Li and C Lin Appl}

_{Phys Lett 84 3654 (2004)6 J Chen L-N Xu W-Y Li and X-L Gou Adv Mater 17 582}

₍₂₀₀₅₎

_{7 D R Kauffman and A Star Angew Chem Int Ed 47 6550 (2008)8 Y M Wang G J Du H Liu D Liu S B Qin N Wang C G}

_{Hu X T Tao J Jiao J Y Wang and Z L Wang Adv FunctMater 18 1131 (2008)}

_{9 P Q Hu G J Du W J Zhou J J Cui J J Lin H Liu D LiuJ Y Wang and S W Chen Acs Appl Mater Inter 2 3263 (2010)}

_{10 D Z Wang W J Zhou P G Hu Y Guan L M Chen J HLi G C Wang H Liu J Y Wang G Z Cao and H D JiangJ Colloid Interf Sci 388 144 (2012)}

_{11 L Liao H Lu J Li H He D Wang D Fu C Liu and W ZhangJ Phys Chem C 111 1900 (2007)}

_{12 A Fujishima X Zhang and D Tryk Surface Science Reports 63515 (2008)}

_{13 Z Y Han J C Zhang and W L Cao Mater Lett 84 34 (2012)14 J J Cui Y K Ge S W Chen H S Zhao H Liu Z Huang H D}

_{Jiang and J Chen J Mater Chem B 1 2072 (2013)15 J J Cui D H Sun S W Chen W J Zhou P G Hu H Liu and}

_{Z Huang J Mater Chem 21 10633 (2011)16 J J Cui D H Sun W J Zhou H Liu P G Hu N Ren H M}

_{Qin Z Huang J J Lin and H Y Ma Phys Chem Chem Phys13 9232 (2011)}

_{17 G K Mor M A Carvalho O K Varghese M V Pishko and C AGrimes J Mater Res 19 628 (2004)}

_{18 H F Lu F Li G Liu Z-G Chen D-W Wang H-T Fang G QLu Z H Jiang and H-M Cheng Nanotechnology 19 405504(2008)}

_{19 J Gong Y Li Z Hu Z Zhou and Y Deng J Phys Chem C 1149970 (2010)}

_{20 W J Zhou G J Du P G Hu G H Li D Z Wang H Liu J YWang R I Boughton D Liu and H D Jiang J Mater Chem 217937 (2011)}

_{21 W Zeng T M Liu and Z C Wang J Mater Chem 22 3544(2012)}

_{22 Z Zhao J Tian D Wang X Kang Y Sang H Liu J WangS Chen R I Boughton and H Jiang Journal of Materials Chem-istry 22 23395 (2012)}

_{23 L E Greene M Law D H Tan M Montano J GoldbergerG Somorjai and P Yang Nano Lett 5 1231 (2005)}

_{24 C Cheng B Liu H Yang W Zhou L Sun R Chen S F YuJ Zhang H Gong and H Sun Acs Nano 3 3069 (2009)}

_{25 W J Zhou L G Gai P G Hu J J Cui X Y Liu D Z WangG H Li H D Jiang D Liu H Liu and J Y Wang Cryst Engcomm 13 6643 (2011)}

_{26 Y-Z Lv C-R Li L Guo F-C Wang Y Xu and X-F Chu SensActuators B Chemical 141 85 (2009)}

_{27 H Tang M Yan X Ma H Zhang M Wang and D Yang SensActuators B Chemical 113 324 (2006)}

_{28 S-H Park J-Y Ryu H-H Choi and T-H Kwon Sens ActuatorsB Chemical 46 75 (1998)}

_{29 S Roy and S Basu Journal of Materials Science Materials inElectronics 15 321 (2004)}

_{30 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134403 (2008)}

_{31 X M Yin C C Li M Zhang Q Y Hao S Liu Q H Li L BChen and T H Wang Nanotechnology 20 455503 (2009)}

_{32 P-C Chen G Shen and C Zhou IEEE Transactions on Nanotech-nology 7 668 (2008)}

_{410 Energy Environ Focus 3 404ndash410 2014}

_{Delivered by Publishing Technology to Guest UserIP 16611112071 On Mon 12 Jan 2015 014730}

_{Copyright American Scientific Publishers}

_{Hierarchically Assembled ZnO Nanorods on TiO2 Nanobelts for High Performance Gas Sensor Zhao et al}

_ARTIC

_{LEof ZnO nanorod and TiO2 nanobelt These two kinds ofsemiconductors are proved to be effective materials forgas sensor application The second one is the exposure ofmore sensitive surface of ZnO nanorod The third one isthe specific hierarchical structure The charge transport isfaster in one dimensional ZnO nanorod and TiO2 nanobeltwhich both of them are highly crystalized Meanwhile themicrochannel formed between the ZnO nanorods can facil-itate the gas diffusion based on a capillary force There-fore based on the synergistic effect of ZnO and TiO2 thehierarchical heterostructure sensor exhibits excellent sens-ing performance to the reductive trimethylamine gas}

_{4 CONCLUSIONIn this study ZnO nanorods were assembled onto the sur-face of TiO2 nanobelts to form a hierarchical surface het-erostructure through a hydrothermal process with the aidof seeds coating method In the hierarchical heterostruc-ture the ZnO nanorods with single crystalline featurewere perpendicularly grown on the surface of TiO2 Thegrown mechanism has been investigated through XRD andHRTEM The sensing performance of this hierarchical het-erostructure was examined and it was found that the hier-archical heterostructure sensor has excellent sensitivity andselectivity to trimethylamine gas The potential sensingmechanism was also discussed and thought to be relatedto the surface-depletion model}

_{Acknowledgments The authors are thankful for fundsfrom National Natural Science Foundation of China(Grant No 51372142) National Science Fund for Distin-guished Young Scholars (NSFDYS 50925205) InnovationResearch Group (IRG 51321091) and the lsquo100 TalentsProgramrsquo of Chinese Academy of Sciences Thanks for thesupport from the lsquothousands talentsrsquo program for pioneerresearcher and his innovation team China}

_{References and Notes1 K J Choi and H W Jang Sensors 10 4083 (2010)2 I-D Kim A Rothschild and H L Tuller Acta Mater 61 974}

_{(2013)3 W-H Zhang and W-D Zhang Sens Actuators B Chemical 134}

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