In Situ Growth of Self-Assembled and Single In2O3Nanosheets onthe Surface of Indium GrainsHeqing Yang,*,†Ruigang Zhang,†Hongxing Dong,†Jie Yu,†Wenyu Yang,†andDichun Chen‡Key Laboratory of Macromolecular Science of Shaanxi Pro V ince,School of Chemistry and MaterialsScience,Shaanxi Normal Uni V ersity,Xi’an710062,China,and Ad V anced Material Analysis and TestCenter,Xi’an Uni V ersity of Technology,Xi’an710048,ChinaRecei V ed January7,2007;Re V ised Manuscript Recei V ed May15,2008ABSTRACT:Self-assembled In2O3nanosheet networks andflowerlike nanoarchitectures,as well as single In2O3nanosheets,have been grown in situ on indium substrate by heating indium grains at900-950°C under theflow of O2in the presence of a small quantity of P2O5.The as-synthesized In2O3nanosheets were characterized by transmission electron microscopy,scanning electron microscopy,and Raman spectrum.It was found that the In2O3nanosheets were single crystals with body-centered cubic structure and dimensions of about0.5-3µm.A possible mechanism for the In2O3nanosheet growth was also proposed on the basis of the results of the present and previous works.This mechanism not only can explain all the experimental observations but also helps to clarify the growth mechanism of other nanostructures in the gas phase.A strong and narrow photoluminescent(PL)peak at428nm was observed from the nanosheets,which is attributed to radiative recombination between an electron on an oxygen vacancy and a hole on an indium-oxygen vacancy center in the In2O3nanosheets.1.IntroductionSynthesis of different dimensional(D)nanostructures,such as0D quantum dots,1D nanowires and nanotubes,or2D nanosheets and nanodisks are of great importance in studying the physical properties of nanomaterials or constructing func-tional nanodevices.1Indium oxide(In2O3),an n-type semicon-ductor with a wide bandgap of about3.6eV,has been widely used as window heater,solar cell,andflat-panel display materials2and gas sensors.3Since the discovery of indium oxide nanobelts in2001,4research in In2O3nanostructures,including nanowires,nanotubes,nanobelts,octahedrons,nanocubes,and core-shell nanoparticles has been rapidly expanded due to their potential application in high sensitivity sensor,optoelectronic,field emission,and electronic devices.The indium oxide nanowires have been used to fabricatedfield-effect transistors,5,6 nanoscale chemical sensors,7and biosensing devices.8Up to now,many kinds of In2O3nanostructures have been synthesized via thermal evaporation of In2O3,chemical vapor deposition(CVD),pulsed laser deposition(PLD),and wet chemical methods.In2O3nanobelts were synthesized via a thermal evaporation of In2O3powders at1400°C4or via a CVD using thermal oxidation reactions of In.9In2O3nanowires can be obtained by the CVD of thermal oxidations10–12and reductions,6,13,14by a laser ablation of InAs target,15or by triblock copolymer and porous alumina template methods.16 Additionally,single-crystalline In2O3nanotubesfilled with metallic In,17In2O3nanocrystal chains,and nanowire networks18 were synthesized by evaporating a mixture of In/In2O3or C/In2O3.Hollow In2O3nanotubes were grown in porous alumina membranes by a sol-gel process.19Aligned1D In2O3structures with a triangular cross-section were synthesized by a metal-organic chemical vapor deposition method(MOVCD).20In addition to the quasi-1D nanostructures,0D quantum dots such as quasi-monodisperse In2O3nanoparticles,21,22nanocubes,23In2O3octahedron,24,25and highly ordered In2O3coated In core-shell nanoparticles26were prepared via wet chemical methods,21–23CVD,24,25and a three-step oxidation process of In nanoparticle arrays.26However,to our knowledge,synthesis of2D In2O3nanostructures except for thinfilms has not been reported until now.Herein we report the synthesis of self-assembled and single In2O3nanosheets by an in situ thermal oxidation method.These sheet-like nanostructures were directly grown on the surfaces of indium grains by heating indium metal at900-950°C in an oxygen gas atmosphere in the presence of a small quantity of pared with CVD,MOCVD,and PLD,this procedure does not include In or In2O vapor transport and condensation processes and does not require very high temperature and low pressure.Growth mechanism and photoluminescence of the In2O3nanosheets were investigated in detail.2.Experimental Procedures2.1.Sample Preparation.The In2O3nanosheets were synthesized by a simple thermal oxidation of indium metal in a conventionalhorizontal tube furnace.In a typical experiment,In metal grains(purity99.999%)were treated in an aqueous1.0M HCl solution for30s andthen washed with absolute ethanol in an ultrasonic bath for15min.The grain was placed on a silicon wafer,and the silicon wafer wasplaced in a quartz boat containing a small quantity of P2O5(formed byheating a quartz boat containing about2-5mg of red phosphorus to500°C and then maintaining it at500°C for0.5h in an O2gasatmosphere).The boat was placed at the center of a quartz tube thatwas inserted in a horizontal tube furnace,where the temperature andgrowth time were controlled.Prior to heating,high-purity N2(99.999%)was introduced into the quartz tube with a constantflow rate of3.0L/h to purge the O2inside.After20min,the system was heated to900°C for60min under a constantflow of N2gas at a rate of1.0L/h. Afterward,1.0L/h O2was introduced into the chamber,and thetemperature was kept at900°C for2h.After the system cooled toroom temperature under a constantflow of N2gas at a rate of1.0L/h,a large amount of ashen products were found on the surface of theindium grains.2.2.Characterization.The synthesized products were characterized and analyzed by X-ray diffraction(XRD;Rigaku DMX-2550/PC X-ray diffractometer),Raman spectra(Jobin Yvon LabRAM HR800and*Corresponding author.Fax:+86-29-85307774.Tel:+86-29-85303943.E-mail address:hqyang@.†Shaanxi Normal University.‡Xi’an University of Technology.10.1021/cg070019e CCC:$40.75 XXXX American Chemical SocietyPublished on Web 07/23/2008Nicolet Alemga dispersive Raman spectrometer),scanning electron microscopy (SEM;FEI Quanta 200),and high-resolution transmission electron microscopy (HRTEM;JEOL JEM-3010at 300kV).Samples for HRTEM were prepared by dispersing a powdered In 2O 3product on a carbon-coated copper grid.An energy-dispersive X-ray spectros-copy (EDS)facility attached to the SEM and TEM was employed to analyze the chemical composition.Photoluminescent (PL)spectra were measured at room temperature in an Edinburgh FLS920fluorescence spectrophotometer with a Xe lamp using excitation at 380nm.3.Results and DiscussionFigure 1a -c shows typical SEM images of as-prepared samples grown at 900°C for 2h at low and high magnifications.These In 2O 3nanosheets were randomly and fairly uniformly distributed on the surface of the In grain.Figure 1c clearly shows that In 2O 3nanosheets are oriented upward with respect to the underlying substrate and have irregularly shaped morphologies with maximum dimension of about 0.5-3.0µm on the bottom,gradually narrowing to the top.The minimum thickness of the In 2O 3sheets on the top is tens of nanometers.In addition to the dispersed In 2O 3nanosheets,a small quantity of intercrossed In 2O 3nanosheet networks are also observed in some areas on the surface of the In grains.The typical morphology of these self-assembled In 2O 3nanosheets is shown in Figure 2.Figure 2a-b shows the low-and high-magnification SEM images,respectively.Figure 2b clearly shows that the network is constructed of nanosheets with heights ranging from 0.5to 1.2µm.The nanosheets intercross with each other to form complicated networks.When the reaction temperature was increased from 900to 950°C,a few interesting flowerlike In 2O 3nanoarchitectures were observed on the surface of the In grains.Typical SEM micrographs of the flowerlike In 2O 3nanoarchi-tectures at low and high magnifications are presented in Figure 3a,b.From Figure 3a,b,it is evident that In 2O 3nanoflowers consist of sheetlike nanostructures.The In 2O 3nanosheets possessmainly fan-shaped morphologies and are 0.5-5.5µm in length and 0.6-3.2µm in height.Figure 4a shows the XRD pattern of the samples grown at 900°C for 2h in an O 2gas atmosphere.Sixteen peaks at 2θ)30.5°,32.8°,35.3°,37.6°,41.7°,43.6°,45.5°,49.1°,50.9°,52.8°,56.0°,59.0°,60.5°,62.1°,63.5°,and 64.4°are observed from Figure 4a.According to JCPDS card no.06-0461,the products are In 2O 3with body-centered cubic structure,and these peaks are assigned to (222),(321),(400),(411),(332),(422),(431),(521),(440),(433),(611),(541),(622),(631),(444),and (543)diffraction lines of cubic In 2O 3phases,respectively.RamanFigure 1.SEM images of as-prepared samples grown at 900°C for 2h at differentmagnifications.Figure 2.Low-(a)and high-magnification (b)SEM images of the intercrossed In 2O 3nanosheetnetworks.Figure 3.Low-(a)and high-magnification (b)SEM micrographs of the flowerlike In 2O 3nanoarchitectures grown at 950°C for 2h.Figure 4.XRD pattern (a)and Raman spectrum (b)of the samples grown at 900°C for 2h.B Crystal Growth &Design,Vol.xxx,No.xx,XXXX Yang et al.scattering,due to its sensitivity to crystallization in nanostruc-tures,was also measured for the In 2O 3nanosheets.Figure 4b shows the Raman spectrum of the samples grown at 900°C for 2h excited with an Ar +laser at 514nm at room temperature.The five peaks at 126,301,358,489,and 622cm -1can be identified to be those of the cubic In 2O 3.27XRD and Raman indicate that the products obtained are In 2O 3nanosheets with cubic structure.The characterization of individual In 2O 3nanosheets was achieved in further detail using TEM.Figure 5a shows the TEM image of a quasi-rectangular In 2O 3nanosheet.The nanosheet is 650nm in length and 520nm in width.The contrast on a whole sheet is inhomogenous,which indicates that the thick-nesses of the sheet at the root and the center are greater than that at the top and side edges,as observed by SEM.The corresponding selected area electron diffraction (SAED)pattern is shown in Figure 5b;it can be indexed as a cubic In 2O 3along the [125]axis,consistent with the XRD and Raman results.The HRTEM image of the In 2O 3nanosheet is displayed in Figure5c.The fringe spacing is about 0.42nm,corresponding to the (121j )crystal planes of the cubic In 2O 3.The chemical composi-tion of the In 2O 3nanosheet was verified by an EDS facility attached to the TEM.The EDS data curve is shown in Figure 5d,in which In,O,and Cu elements were marked.The Cu-related peak is due to the presence of the Cu grids.So,the nanosheet consists of indium and oxygen.These results indicate that the nanosheets are a single-crystalline with body-centered cubic structure.To identify whether there was P 2O 5on the surface of the In 2O 3nanosheets,an EDS facility attached to the SEM was employed to analyze the chemical composition of the In 2O 3nanosheets obtained by heating indium grains at 900°C for 2h in an O 2gas atmosphere in the presence of a small quality P 2O 5,and the results are shown in Figure 6.Figure 6b-c shows EDS spectra from the marked region and dot in panel a,respectively.The peaks of In,P,and O elements were observed from the EDS spectra.It indicates that there was P 2O 5on the surface of In 2O 3nanosheets.The P-related peak was not found in the EDS spectrum (Figure 5d)obtained by using an EDS facility attached to the TEM.The disappearance of the P-related peak may because that the P 2O 5was dissolved in ethanol during prepara-tion of the samples for TEM analysis.To illuminate the role of P 2O 5in the formation of In 2O 3nanosheets,the products obtained by heating indium grains at 900°C for 2h in an O 2gas atmosphere in the absence of P 2O 5were characterized with SEM and Raman,and results are shown in Figure 7.We found that there are octahedra instead of nanosheets on the surface of the indium grain in the SEM image (Figure 7a),indicating that P 2O 5plays an important role in the growth process of In 2O 3nanosheets.Figure 7b is Raman spectra of the sample.In the Raman spectra,we observed five scattering peaks at 130,306,366,495,and 628cm -1,which can be identified to be those of the cubic In 2O 3.27It indicates that metallic In was oxidized to form an In 2O 3octahedral layer on the surface of In grains when indium grains were heated at 900°C in an O 2gas atmosphere without P 2O 5.In order to understand the formation process of the In 2O 3nanosheets,time-dependent experiments were carried out,and the resultant products were analyzed by SEM and Raman spectra.The representative SEM images of the products prepared at certain reaction time intervals are shown in Figure 8.A large quantity of spherical particles was seen on the surface of the In grains obtained by heating at 900°C for 2min (Figure 8a).When the reaction time was prolonged to10min,these particles aggregated with each other to form large congeries (Figure 8b).In addition to the congeries,a small quantity of In 2O 3nanosheets was observed on the surface of the congeries (Figure 8c).When the reaction time was increased to 30min,a large quantity of In 2O 3nanosheets were observed on the surface of the In grains (Figure 8d).Figure 9shows the Raman spectra from the samples reacted for 2,10,and 30min excited with an Nd:YVO 4laser at 532nm at room temperature,which indicates that the nanoparticles and nanosheets are In 2O 3with a cubic structure.27The growth process for the In 2O 3is similar to the growth of BN nanowires through the reaction of a mixed gas of N 2and NH 3over R -FeB particles 28and the growth of In 2O 3nanowires through the reaction of an O 2gas over indium grains coated on a Au film.29The P 2O 5may be a catalyst for the growth of In 2O 3nanosheets.On the basis of the investigations described above,a possible mechanism to form In 2O 3nanosheets was proposed with reference to the preparation of Ti-doped CeO 2nanopar-ticles,30growth of carbon tubes via surface diffusion,31and growth of silicon nanowires via a solid -liquid -solid(S-L-S)Figure 5.TEM images and SAED pattern of the In 2O 3nanosheets synthesized at 900°C for 2h:(a)typical image of a single In 2O 3nanosheet;(b)corresponding SAED pattern;(c)HRTEM image;(d)EDS spectrum.Self-Assembled and Single In 2O 3Nanosheets on In Grains Crystal Growth &Design,Vol.xxx,No.xx,XXXX Cmechanism.32As illustrated in Figure 10,during heating under the flow of N 2,the In metal was melted to form liquid In (the melting point of In metal is 156.6°C);P 2O 5was vaporized and reacted with surface In of In grains to produce In -P -O liquid-phase layers.When the temperature was increased to 900°C,O 2gas was introduced into the chamber and reacted with the surface In rapidly to produce In 2O 3.According to Feng,30Tian,31a and Hofmann 31b and their co-workers,the In 2O 3congregated,nucleated,and grew into In 2O 3nanoparticles coated with an In -P -O liquid-phase layer on the liquid In surface via surface diffusion (Figure 10c).The In 2O 3nanoparticles coated with an In -P -O liquid-phase layer evolved into large particles driven by the minimization of surface energy.The In -P -O liquid-phase layers reacted with the In 2O 3core to form In -P -O droplets (Figure 10e)on the surface of the large particles.When the In 2O 3in the droplets reach a saturated concentration,crystalline In 2O 3nanosheets begin to grow from the droplets via the S-L-S mechanism (Figure 10f).The growth mechanism may be characterized by the growth at the roots of the nanosheets.The underlying In 2O 3core provides the neces-sary feeding materials for the nanosheet growth.The surrounding In 2O 3of the In 2O 3core was consumed and transported through the surface of In metal for the continuous growth of the In 2O 3nanosheets.The oxidation reaction of the surface In of indium grains provides In 2O 3to maintain the surface diffusion and nanosheet growth.After the reaction,the shape of the In grain is also spherical (Figure 1a),which indicates that the oxidation of metallic In as well as nucleation and growth of In 2O 3nanosheets occurred on the metallic In surface.During the growth of In 2O 3nanosheets in addition to the S-L-S growth mechanism,surrounding In 2O 3of the In 2O 3sheets was trans-ported through the surface of In 2O 3particles and nanosheets to the top and side edges of nanosheets,and nucleated and grew (Figure 10g).There is a concentration gradient of In 2O 3between the top and roots of the In 2O 3sheets to maintain the surface diffusion.The thicknesses of the sheet at the root and center are bigger than those on the top and side edges due to the presence of the In 2O 3concentration gradient.During nucleation and growth of In 2O 3nanostructures,the initially formed nuclei in the droplets are dispersed,and subsequent growth from the nuclei results in the nanosheets.Agglomeration of neighboring nanoscale nuclei is likely to be responsible for self-assembled nanosheets.The initially formed nuclei aggregate to form nuclei with a network structure and nucleus arrays,and subsequent growth from the assembled nuclei results in the intercrossed nanosheet networks and flowerlike nanoarchitectures.When the reaction temperature was increased from 900to 950°C,the agglomeration of neighboring nanoscale nuclei was aggrandized,and thus the flowerlike nanoarchitectures were obtained.As the In 2O 3nanosheets formed,an In 2O 3layer is also obtained on the surface of the In metal.The In 2O 3layer protects the In metal from further oxidation.Therefore,after the reaction,the shape of the In grain is also spherical (Figure 1a).Without P 2O 5during the heating under the flow of N 2,the In metal was melted and vaporized.As O 2gas was introduced into the chamber at 900°C,the In vapor reacted rapidly with O 2to form In 2O 3.TheFigure 6.SEM image and EDS spectra of the products obtained at 900°C for 2h:(a)SEM image;(b,c)EDS spectra from the selected area and spot in panela.Figure 7.SEM image (a)and Raman spectrum (b)of products prepared at 900°C for 2h without P 2O 5.D Crystal Growth &Design,Vol.xxx,No.xx,XXXX Yang et al.In 2O 3directly deposited on the In grain and grew into octahedra via a vapor -solid process.4It is known that bulk In 2O 3cannot emit light at room temperature.33However,Recently,Seo 22and Liu 21and their co-workers observed PL peaks at 325-332,392,and 423nm from In 2O 3nanoparticles.Lee et al.34observed PL peaks at 360,400,and 470nm from In 2O 3nanocubes.Liang 10and Guha 35et al.reported a peak at 470nm from nanofibers and octahedrons of In 2O 3.Lee 36and Li 17and their co-workers observed PL peaks at 637and 617nm from thin films and nanotubes of In 2O 3,respectively.The PL spectra of the In 2O 3nanosheets obtained at 900°C at room temperature are shown in Figure 11a.As can be seen from Figure 11a,a strong and narrow PL peak at 428nm is observed from the nanosheets under excitation at 380nm,which is different from broad blue PL emission spectra observed from In 2O 3nanoparticles and nanowires.21,22,10The full width at half-maximum intensity of the PL peak is 28nm.Figure 11b shows the excitation spectra for blue light emission monitored at 428nm.The excitation spectra have three maximum around 300,322,and 385nm.In general,The UV emission would correspond to the near-band-edge emission.The visible emission matches the deep-level emission,which originated from defects or oxygen vacancies in the lattice sites of the In 2O 3crystals produced in the preparation of thesamples.Figure 8.SEM images of the products prepared at 900°C for different reaction times:(a)2min;(b,c)10min;(d)30min.Figure 9.Raman spectra of the products synthesized at 900°C for different reaction times:(a)2min;(b)10min;(c)30min.Figure 10.Schematic illustration of a possible mechanism for the In 2O 3nanosheet growth:(a)indium grain;(b)formation of In -P -O liquid-phase layers on the In surface;(c)formation of In 2O 3spherical particles coated with a In -P -O liquid-phase layer;(d)congregation of the In 2O 3particles into large particles;(e)formation of In -P -O droplets on the surface of the In 2O 3particles;(f)the nanosheet growth starts from the In -P -O droplets;(g)diffusion of In 2O 3on the surface of the In 2O 3particles and nanosheets;(h)final state of the nanosheets.Self-Assembled and Single In 2O 3Nanosheets on In Grains Crystal Growth &Design,Vol.xxx,No.xx,XXXX EThe intensive blue light emission can be attributed to oxygenvacancy (V O x )and indium -oxygen vacancy centers (V In ,V O )x .35The (V In ,V O )x and the (V O x)may act as the acceptors and thedonors,respectively.An electron in donor level (V O x)may becaptured by a hole on an acceptor {(V In ,V O )x}to form a trapped exciton.The trapped exciton recombines radiatively to produce the observed blue emission.The oxygen vacancy and indium -oxygen vacancy centers should be generated because of partial crystallization during the growth process of In 2O 3nanosheets.4.ConclusionsWe have successfully synthesized self-assembled In 2O 3nanosheet networks and flowerlike nanoarchitectures,as well as single In 2O 3nanosheets,by direct thermal oxidation of In metal with a small quantity of P 2O 5for the first pared with CVD,MOCVD,and PLD,this procedure does not include In or In 2O vapor transport and condensation processes and does not require very high temperature and low pressure.The growth process has been clearly illuminated,which starts from the oxidation of In followed by the sequential growth of the In 2O 3nanoparticles and nanosheets,and a schematic elucidation is presented.This mechanism not only can explain all the experimental observations but also helps to clarify the growth mechanism of other nanostructures in the gas phase.The In 2O 3nanosheets exhibited strong PL emission in the blue region of the spectrum.This emission can be attributed to oxygen vacancy and indium -oxygen vacancy centers.The methodology dem-onstrated here for synthesizing 2D nanosheets may be employed for synthesis of other semiconductor 2D nanostructures and might offer unlimited possibilities in a broad range of fields such as photonics,chemical sensors,catalysis,and nanodevices.Acknowledgment.This work was supported by the National Natural Science 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