Brush-Like Hierarchical ZnO Nanostructures:Synthesis,Photoluminescence and Gas Sensor PropertiesYuan Zhang,†,‡Jiaqiang Xu,*,†,§Qun Xiang,†Hui Li,†,‡Qingyi Pan,†and Pengcheng Xu §Department of Chemistry,College of Science,Shanghai Uni V ersity,Shanghai 200444,China,Department of Physics,College of Science,Shanghai Uni V ersity,Shanghai 200444,China,and State Key Laboratory of Transducer Technology,Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences,Shanghai 200050,ChinaRecei V ed:September 12,2008;Re V ised Manuscript Recei V ed:December 28,2008Brush-like hierarchical ZnO nanostructures assembled from initial 1D ZnO nanostructures were prepared from sequential nucleation and growth following a hydrothermal process.The morphology,structure,and optical property of hierarchical ZnO nanostructures were characterized by X-ray diffraction (XRD),field-emission scanning electron microscopy (FE-SEM),and photoluminescence (PL)studies.The FE-SEM images showed that the brush-like hierarchical ZnO nanostructures are composed of 6-fold nanorod-arrays grown on the side surface of core pared with ZnO nanowires,brush-like hierarchical ZnO nanostructures easily fabricated satisfactory ethanol sensors.The main advantages of these sensors are featured in excellent selectivity,fast response (less than 10s),high response (sensitivity),and low detection limit (with detectable ethanol concentration in ppm).1.IntroductionNanoscale materials have stimulated great interest in current materials science due to their importance in such basic scientific researches and potential technology applications as microelec-tronic devices,1chemical and biological sensors,2light-emitting displays,3catalysis,4and energy conversion and storage devices.5Previous studies indicated that the shape of nanoscale materials have a profound influence on their properties.6This has led to intensive investigation on quantum dots,7nanowires,8nano-tubes,9and self-assembled monolayers (SAMs).10In recent years,much attention has been paid to three-dimensional and hierarchical architectures that derive from the above-mentioned low dimensional nanostructures as building blocks,due to various novel applications.For example,the Lieber group has reported that core/shell coaxial silicon nanowires architectures can be employed as solar cells.1Wang reported that a novel hierarchical nanostructure based on Kevlar fibers coated with ZnO nanowires could serve as nanogenerators.11Though tremendous progress has been made in this significant field,there are still great demands on the synthesis of alternative three-dimensional and hierarchical architectures with novel or po-tential applications.Hierarchical ZnO architectures have been extensively pro-duced with gas-phase approaches.12-14With these synthetic methods,nanowire arrays,15nanohelitics,16nanopropeller,17and tower-like nanocolumns 18have been successfully prepared.However,these protocols often require high temperature and induce impurities in the final products when catalysts and templates are introduced to the reaction system.In practice,this made it difficult to obtain organic/inorganic hybrid hierarchical architectures.In addition,although the solution-based synthetic strategies are simple and effective in the production of thebuilding blocks of hierarchical nanostructures such as nanopar-ticles,19nanowires,20and nanorods,21achievement of hierarchical architectures from such techniques remains a challenge.Herein,we developed a simple nucleation and growth strategy to synthesize brush-like hierarchical ZnO nanostructures.The two step seeded-growth approach allows stepwise control and optimization of experimental conditions and provides an op-portunity for rational design and synthesis of controlled architectures in nanostructures.22-24We also investigated the effect of morphology and structure of brush-like hierarchical ZnO nanostructures on its gas sensing responses.The results show that ZnO hierarchical nanostructures display better ethanol sensing property than that of ZnO nanowires.2.Experimental SectionAll reagents employed were analytically pure and used as received from Shanghai Chemical Industrial Co.Ltd.(Shanghai,China)unless otherwise mentioned.2.1.Synthesis.The strategies to fabricate the brush-like hierarchical ZnO nanostructures are summarized as follows.First,the ZnO nanowires with a uniform shape were synthesized as described elsewhere,25which was used as the initial material to grow hierarchical ZnO nanostructures.Subsequently,a saturated solution of Zn(OH)42-was prepared by dissolving excess ZnO in 10mL of NaOH solution (5mol/L)for growing hierarchical ZnO nanostructures.Third,the ZnO nanowires (0.05g)were uniformly suspended in deionized water (37mL)in an ultrasonic bath.The suspension was mixed with the Zn(OH)42-saturated solution (3mL).After the mixture was transferred into a Teflon-lined autoclave (50mL),it was kept at 100°C for 10h.Finally,upon the hydrothermal treatment a white precipitate was formed,which was washed thoroughly with ethanol and distilled water in sequence,and dried at 60°C under vacuum for 6h.2.2.Characterization.The initial ZnO seeds were character-ized by transmission electron microscopy (TEM,JEM-200CX,using an accelerating voltage of 160kV).The morphology of*Corresponding author.Tel:+862166134728.Fax:+862166134725.E-mail:xujiaqiang@.†Department of Chemistry,Shanghai University.‡Department of Physics,Shanghai University.§Chinese Academy of Sciences.J.Phys.Chem.C 2009,113,3430–3435343010.1021/jp8092258CCC:$40.75 2009American Chemical SocietyPublished on Web 02/06/2009hierarchical ZnO nanostructures was observed by field emission scanning electron microscopy (FE-SEM,JSM-6700F).The crystal phase of as-synthesized products was identified by powder X-ray diffraction (XRD)analysis using a D/max 2550V diffractometer with Cu K R radiation (λ) 1.54056Å)(Rigaku,Tokyo,Japan),and the XRD data were collected at a scanning rate of 0.02deg s -1for 2θin a range from 10°to 70°.2.3.Photoluminescence (PL)Measurement.Room tem-perature PL measurements were performed on a Hitachi RF-5301PC spectrofluophotometer using the 350nm Xe laser line as the excitation source.2.4.Gas Sensor Fabrication and Response Test.The ZnO powder was mixed with Terpineol and ground in an agate mortar to form a paste.The resulting paste was coated on an alumina tube-like substrate on which a pair of Au electrodes had been printed previously,followed by drying at 100°C for about 2h and subsequent annealing at 600°C for about 2h.Finally,a small Ni -Cr alloy coil was inserted into the tube as a heater,which provided the working temperature of the gas sensor.The schematic drawing of the as-fabricated gas sensor is shown in Figure 1.In order to improve the long-term stability,the sensors were kept at the working temperature for several days.A stationary state gas distribution method was used for testing gas response (Air humidity:47%).In the measurement of electric circuit for gas sensors (Figure 2),a load resistor (Load resistor value:470k Ω)was connected in series with a gas sensor.The circuit voltage was set at 10V,and the output voltage (V out )was the terminal voltage of the load resistor.The working temperature of a sensor was adjusted through varying the heating voltage.The resistance of a sensor in air or test gas was measured by monitoring V out .The test was operated in a measuring system of HW-30A (Hanwei Electronics Co.Ltd.,P.R.China).Detect-ing gases,such as C 2H 5OH,were injected into a test chamber and mixed with air.The gas response of the sensor in this paper was defined as S )R a /R g (reducing gases)or S )R g /R a (oxidizing gases),where R a and R g were the resistance in air and test gas,respectively.The response or recovery time was expressed as the time taken for the sensor output to reach 90%of its saturation after applying or switching off the gas in a step function.3.Results and Discussion3.1.Structure and Morphology.In the XRD pattern of the as-synthesized products (Figure 3),all of the peaks were well indexed to hexagonal wurtzite ZnO (JCPDS No.36-1451,a )0.3249nm,c )0.5205nm)with high crystallization.No characteristic peaks were observed for impurities.The FE-SEM images of hierarchical ZnO nanostructures (Figure 4)were observed at low and medium magnifications,respectively.From the low magnification image (Figure 4a),the secondary nanorods self-organized into very regular arrays,which mimic brush-like hierarchical nanostructures.These nanorod arrays that grew onto one common central nucleus could also be revealed from the protrudent brush-like structures (Figure 4a).The medium magnification (Figure 4b)clearly indicated that these secondary nanorod arrays were brush-like 6-fold symmetry.MorecarefulFigure 1.Sketch of the gassensor.Figure 2.Measuring electric circuit of the gassensor.Figure 3.XRD pattern of the brush-like hierarchical ZnOnanostructures.Figure 4.FE-SEM images of the brush-like hierarchical ZnO nanostructures:(a)at low magnification and (b)at medium magnification.Brush-Like Hierarchical ZnO Nanostructures J.Phys.Chem.C,Vol.113,No.9,20093431observation of the morphology of hierarchical nanostructures indicated that the nanorod arrays grew on the side surface of core nanowire.The reason for nanorod arrays to grow into 6-fold symmetry may arise from the hexagonal symmetry of the major core.26The central stems provide its six prismatic crystal planes/facets as growth platforms for branching of multipod units.The synthesis of 6-fold ZnO nanostructures have been reported;17,26-28however,the solution-based approach to this structure is rarely disclosed.Under hydrothermal conditions,heteronucleation can take place,and the interfacial energy between crystal nuclei and substrates is usually smaller than that between crystal nuclei and solutions.22Therefore,the secondary rod-like branches can grow on the wire-like ZnO central core.3.2.Impact of the Reaction Conditions on the Growth of ZnO Hierarchical Nanostructures.Our studies suggest that the morphology of ZnO seeds and the concentration of OH -ion in the reaction system are the key factors for the formation of ZnO hierarchical nanostructures.First,ZnO nanostructures with different morphology were used as seeds for the nucleation and growth process.In the synthesis of brush-like ZnO hierarchical nanostructures,high aspect ratio (height/width)ZnO nanowires were used as seeds.These ZnO nanowires have typical diameters of 80-100nm and lengths up to 10µm (Figure 5).In order to study the effect of the seeds morphology on the growth of brush-like ZnO hierarchical nanostructures,a lower aspect ratio of ZnO nanorods (200nm diameter,aspect ratio <5,Figure 6a)was used as substitute seeds in this seeding growth method.When nanorods were used as seeds for further crystal growth,shorter and thicker rod-like ZnO nanostructures were obtained (Figure 6b).Second,the brush-like ZnO hierarchical nanostructures could only be obtained under an optimized concentration (5M)of OH -ion.The morphologies of the as-obtained products varied remarkably with changes in the OH -ion concentration.When the OH -ion concentration dropped to 3M,only a few disorder secondary nanorods grew on each common central nucleus (Figure 7a).As the OH -ion concentration increased to 15M,the secondary rod-like branches structures and the brush-like hierarchical nanostructures became shorter than those at lower concentrations because of the dissolve effect of NaOH (Figure 7b).When the OH -ion concentration increased to 20M,the amount of sub-branch nanorods also increased,and chrysan-themum-like microstructures could be obtained (Figure 7c).It is worth mentioning that these secondary rod-like crystals have a relatively planar bottom at high OH -ion concentration.We noted that the concentration of NaOH (i.e.,the concentra-tion of OH -)was crucial to the formation of Zn(OH)42-precursor.The growth of ZnO did not proceed at low concentra-tion of OH -because no Zn(OH)2was formed in the solution.Accordingly,only a few disordered sub-branch nanorods grew on each common central nucleus.With an increase of OH -concentration,the nucleation of ZnO was accelerated leading to more nanorods growing on the central nucleus.When the concentration of OH -was further increased,the growth of sub-branch nanorods was impeded,29and the average aspect ratio of ZnO nanorods was decreased.However,when excess NaOH was employed,it became not only a source of hydroxyl ions to form ZnO,but a capping agent.30ZnO crystal is a polar solid with a positive polar plane (0001)rich in Zn and a negativepolar plane (0001j )rich in O.31At a high OH -concentration,OH -ions are preferably adsorbed on the (0001)plane of ZnO,29,32and the growth of the ZnO nanocrystallite along the c axis is partially suppressed.It suggests that the OH -ion could act as a surface termination reagent,thus impeding the growth of crystal face (0001).29Accordingly,the secondary rod-like crystals have a flat face.Previous studies indicated that surfactants could control the morphology in the solution phase process.33-36We synthesized the ZnO hierarchical nanostructures V ia a surfactant-free route.But,do surfactants have a positive effect on the morphology of the products?Taking a typical surfactant CTAB (cetyltrimethy-lammonium bromide)as example,when CTAB was introduced into the reaction system,the morphology of the as-obtained products was ZnO microsphere consisting of nanorods(FigureFigure 5.TEM image of the ZnO nanowireseeds.Figure 6.TEM images of the ZnO samples:(a)the initial ZnO nanorods seeds and (b)the secondary crystals from the initial ZnO nanorods.3432J.Phys.Chem.C,Vol.113,No.9,2009Zhang et al.7d).The ability of CTAB to form diversified shapes of microparticles has been exploited extensively.37-41Due to the coulomb force action between [Zn(OH)42-]and CTAB,they interact to form complexes,which are adsorbed on the circum-ference of the ZnO nuclei.42Upon the formation of globules,the zinc complex dissociated to form rod-like nanostructures with the assistance of CTAB,43,44leading to the nanorod-assembled microsphere.In addition,the hydrothermal temperature (100-120°C)and hydrothermal time (10-15h)made no obvious difference in the morphology of ZnO hierarchical nanostructures.3.3.Optical Properties Measurement.The photolumines-cence spectrum of the secondary brush-shaped nanostructures was measured using Xe laser (350nm)as excitation source.We also measured the PL spectrum of the initial nanowires for comparison (Figure 8).Strong emission at ∼389nm was observed for both structures.In addition,we observed that the green emission intensity at ∼558nm for the brush-shaped nanostructures is stronger than that of the initial nanowires.The green emission peak is commonly referred to singly ionized oxygen vacancy in ZnO resulting from the radiative recombina-tion of a photogenerated hole with an electron occupying theoxygen vacancy.15,45The stronger green light emission intensity suggests that there is a greater fraction of oxygen vacancies in the brush-shaped nanostructures.Defects at metal oxide surfaces are believed to significantly influence a variety of surface properties,including chemical adsorption reactivity,46,47such as heterogeneous catalysis,cor-rosion inhibition and gas sensing.The influence of the intrinsic defects on the ZnO surface chemistry and the effects of chemisorption have been addressed by theoretical calculation and experimental data.47-50Moreover,the mechanism of the oxygen vacancies induced gas sensing enhancement properties of ZnO has been explained.51A large quantity of oxygen vacancy results in high adsorptions of oxygen,and in turn enhances the chance of interaction with testing gases.3.4.Gas Sensing Properties.The gas sensing properties of low dimensional ZnO nanostructures (nanoparticles,nanowires,nanorods)have been widely investigated by many research groups;52-55however,the influence of hierarchical ZnO mor-phology on their gas sensing performance has scarcely been investigated.In this paper,we also studied the ethanol gas-sensing properties of ZnO hierarchical nanostructures,and discussed the effect of morphology on the gas sensing responses.Eight typical reducing gases (CH 4,H 2,NH 3,i -C 4H 10,HCHO,CO,C 6H 6,and C 2H 5OH)and two typical oxidizing gases (NO 2and Cl 2)were selected as target gases to investigate the gas response at an optimal operating temperature of 265°C,where the concentration of all the tested gases was 50ppm.The gas sensor based on the ZnO hierarchical nanostructures (Figure 9)showed good selectivity to ethanol with little interference against other gases.ZnO is a very common ethanol sensing material.56-58The ethanol sensing mechanism of the ZnO sensors has also been reported in our previous studies.59However,only few studies indicate that ZnO has higher gas response to ethanol than other commonly used gases.58Most metal oxide semiconductor gas sensors are based on the conductance change arising fromtheFigure 7.The FE-SEM images of the secondary ZnO nanostructures obtained under different conditions a)3M NaOH,b)15M NaOH,c)20M NaOH,d)1gCTAB.Figure 8.Photoluminescence spectra of different ZnO samples (a)brush-like hierarchical nanostructures and (b)nanowires.Brush-Like Hierarchical ZnO Nanostructures J.Phys.Chem.C,Vol.113,No.9,20093433reactions between the oxygen ions and the detecting gas molecules adsorbed on the active surface.60,61We found that the electron donating effect of the testing gas molecule is an important factor in the explanation of this phenomenon (In the several forms of oxygen adsorbates,the O -is more active form of adsorbed oxygen.).For example,the typical testing gas sensing reactions are expressed as follows:CO +O -f CO 2+e -(1)H 2+O -f H 2O +e -(2)HCHO +2O -f H 2O +CO 2+2e -(3)Catalytic oxidation of ethanol gas is known through two different routes depending on the acid or base properties of catalyst surface,i.e.,a dehydrogenation route through CH 3CHO intermediate (on the basic surface)and a dehydration route through a C 2H 4intermediate (on the acidic surface).61,62Since ZnO is a basic oxide,dehydrogenation is favored,providing CH 3CHO as the major which undergoes subsequent oxidation to form CO 2and H 2O.C 2H 5OH f CH 3CHO +H 2(4)CH 3CHO(ad)+5O -(ad)f 2CO 2+2H 2O +5e -(5)According to these equations,the electron donating effect of ethanol is stronger than that of the other gases.Therefore,the gas response of ethanol is higher than that of the other gases at the equivalent concentration.However,owing to the complicated reactivity of some gas molecules on the ZnO surface,the electron donating effect is not the only influential factor on gas response.Other factors (such as reaction of testing gases and sensing material,humidity,temperature,and some additional external fluctuating factor)may exist and compete against each other.Interestingly,the response of brush-like hierarchical nano-structures (Figure 9)is greater than that of initial ZnO nanowires.This observation is general to all the gases examined herein.The gas sensing response enhancement can be attributed to more active centers that are obtained from the enhanced oxygen vacancy defects on the brush-shaped nanostructures.Besides,the formation of initial nanorod/secondary nanorods junctions may be an additional reason for the response enhancement.These junctions are considered as the active sites that canenhance the response of the gas sensors.63Moreover,the nanorod arrays could increase the numbers of the gas channels leading to more effective surface areas (defined as the areas which can contact with the gas).The enhancement in gas response of the brush-like hierarchi-cal nanostructures with the increase of ethanol concentration was observed (Figure 10)in a range of 5-50ppm.The gas response of brush-like hierarchical nanostructures was so high that even if the concentration of ethanol decreased to 5ppm,the gas response increased by 3.0times.In addition,the brush-like hierarchical ZnO nanostructures had very high responses to ethanol,showing a promise as ethanol-sensing materials.The dynamic response of brush-like hierarchical ZnO nanostructures to 10,30,and 50ppm ethanol (Figure 11)displayed impressive response and recovery properties for the brush-like hierarchical ZnO nanostructure-based sensors.All the response time and the recovery time of the sensors at the ethanol concentration of 10,30,and 50ppm were less than 10s.The SEM image (Figure 12)of brush-like hierarchical ZnO nanostructure after sintering at 600°C for 2h suggested that the brush-like hierarchical nanostructures were kept well.The heat treatment did not affect the morphology and sizes of these hierarchical nanostructures,which demonstrated a high stability of the brush-like hierarchical ZnO nanostructures.The structural stability may be in favor of the long-term stability of sensors suitable for practical applications.4.ConclusionIn this paper,we developed a simple hydrothermal route to synthesize brush-like hierarchical ZnO nanostructures under mild conditions.Our results showed that a soft chemical route is promising for rational and structural design of the nanoscale materials.The photoluminescence and gas sensing properties of brush-like hierarchical ZnO nanostructures were studied.The photoluminescence spectrum suggested higher oxygen vacancy defects on the brush-shaped nanostructures thannanowires,Figure 9.Responses of the brush-like hierarchical ZnO nanostructures and the ZnO nanowires to various gases (The concentration of all gases was 50ppm).Figure 10.Relationship between gas response of the brush-like hierarchical ZnO nanostructures and ethanolconcentration.Figure 11.Response of sensor based on brush-like hierarchical ZnO nanostructures to 10,30,and 50ppm ethanol.3434J.Phys.Chem.C,Vol.113,No.9,2009Zhang et al.which combined with special ZnO morphology were able to generate more active centers so as to enhance the gas response.The gas sensing measurements showed that the brush-like hierarchical ZnO nanostructures could serve excellent ethanol sensors.Acknowledgment.We appreciate the financial support of Shanghai NSF (No.07ER14039)and Leading Academic Discipline Project of Shanghai Municipal Education Commis-sion (No.J50102).We also thank the Analysis and Research Center at Shanghai University for sample characterization.References and Notes(1)Lauhon,L.J.;Gudiksen,M.S.;Wang,C.L.;Lieber,C.M.Nature 2002,420,57.(2)Modi,A.;Koratkar,N.;Lass,E.;Wei,B.Q.;Ajayan,P.M.Nature 2003,424,171.(3)Banerjee,D.;Jo,S.H.;Ren,Z.F.Ad V .Mater.2004,16,2028.(4)Bansal,V.;Jani,H.;Du Plessis,J.;Coloe,P.J.;Bhargava,S.K.Ad V .Mater.2008,20,717.(5)Arico, A.S.;Bruce,P.;Scrosati, B.;Tarascon,J.M.;Van 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