NaCo2O4晶体的生长形貌和生长机理英文

第41卷第3期人工晶体学报Vol．41No．32012年6月JOURNAL OF SYNTHETIC CRYSTALS June ，2012Growth ，Morphology and Growth Mechanismof NaCo 2O 4CrystalsHAN Shu-juan 1，WANG Ji-yang 1，LI Jing 1，GUO Yong-jie 1，WANG Yong-zheng 1，ZHAO Lan-ling 1，YAO Shu-hua 2，CHEN Yan-bin 2，Boughton R．I．3（1．State Key Laboratory of Crystal Materials ，Shandong University ，Jinan 250100，China ；2．College of Materials Science and Engineering ，Nanjing University ，Nanjing 210093，China ；3．Department of Physics and Astronomy ，Bowling Green State University ，Bowling Green ，Ohio 43403，USA ）（Received 28September 2011，accepted 5March 2012）Abstract ：Millimetre-sized NaCo 2O 4crystals were grown from a molten flux based on NaCl-Na 2CO 3byspontaneous nucleation method．Details of the preparation and growth procedures are provided．The as-grown crystals were characterized by X-ray powder diffraction （XRPD ）．The morphology and growthmechanism were investigated by scanning electron microscopy （SEM ）and atomic force microscopy（AFM ）．The results show that the obtained crystal are well crystallized and indexed in a hexagonalcrystal system with lattice parameters a =b =0．2842nm ，c =1．0894nm ，and V =0．0761997nm 3．Thegrowth of NaCo 2O 4single crystals was controlled by a two-dimensional （2D ）layer-by-layer mechanismacting along the c -axis．Furthermore ，the morphology of the crystals was also interpreted in the viewpointof anionic coordination polyhedron growth units．Key words ：layered compounds ；crystal growth ；microstructureReceived date ：2011-09-28；accepted date ：2012-03-05Foundation item ：National Natural Science Foundation of China （50872066）；National Fundamental Research Project （2010CB833103）；GraduateIndependent Innovation Foundation of Shandong University （GIIFSDU ）Biography ：HAN Shu-juan （1983-），female ，from Shandong province ，Doctor．E-mail ：shujuanhan84@163．comCorresponding author ：WANG Ji-yang ，professor．E-mail address ：jywang@sdu．edu．cnCLC number ：O78Document code ：A Article ID ：1000-985X （2012）03-0573-05NaCo 2O 4晶体的生长，形貌和生长机理研究韩树娟1，王继扬1，李静1，郭永解1，王永政1，赵兰玲1，姚淑华2，陈延彬2，Boughton R．I．3（1．山东大学晶体材料国家重点实验室，济南250100；2．南京大学材料科学与工程学院，南京210093；3．博林格林州立大学天文和物理学系，俄亥俄博林格林43403）摘要：采用自发成核方法，以NaCl-Na 2CO 3为助熔剂，生长了毫米级的NaCo 2O 4晶体。

通过X 射线衍射对晶体作了表征。

利用扫描电子显微镜和原子力显微镜研究了晶体的形貌和生长机理。

结果表明：所得晶体是NaCo 2O 4，属于六方晶系，晶胞参数：a =b =0．2842nm ，c =1．0894nm ，V =0．0761997nm 3。

NaCo 2O 4晶体是沿c 轴层状生长的，同时从阴离子配位多面体的角度分析了晶体的形貌。

关键词：层状化合物；晶体生长；微观结构574人工晶体学报第41卷1IntroductionRecently ，a number of studies have been carried out to investigate the thermoelectric properties of cobalt oxides as potential candidates for thermoelectric applications ，motivated by their attractive advantages such as high thermal and chemical stability ，excellent oxidation resistance ，low costs and weak toxicity ［1-3］．Layered compounds ，such as NaCo 2O 4［4］and Ca 3Co 4O 9［5］are found to exhibit interesting thermoelectric performance ，which is attributed to their unique layered structure and the presence of mixed Co valences ［6-8］．There has been increasing interest in the growth and characterization of NaCo 2O 4since single crystal NaCo 2O 4was discovered to be a potentially important thermoelectric material by Terasaki ［9］．Ken Kurosaki et al．［10］，Shin Tajima et al．［11］and Jinguang Cheng et al．［12］reported the thermoelectric properties of NaCo 2O 4polycrystal and indicated its potential for high-temperature thermoelectric application．The electronic structure of NaCo 2O 4was studied by Woosuck et al．［13］and Ying Xu et al．［14］using different methods．They reported that the compound showed metallic character and had large effective mass of the charge carrier．The Na ions played the role of providing valence electrons．In addition ，Santi et al．fabricated the NaCo 2O 4nanofibers ［15］by electrospinning．However ，no detailed investigation of the morphology and growth mechanism of NaCo 2O 4crystals has yet been reported．In this paper ，The details of NaCo 2O 4crystals growth from the flux system based on NaCl-Na 2CO 3by the spontaneous nucleation method were reported．The morphology and growth mechanism of the as-grown crystals were investigated by SEM and AFM．And the morphology of the crystals was also interpreted in the viewpoint of anionic coordination polyhedron growth units for the first time．2Experimental 2．1Crystal growthThe NaCo 2O 4crystals were grown from the flux melt based on NaCl-Na 2CO 3．The raw materials were analytical pure reagents Co 2O 3（99．9wt%），Na 2CO 3（99．5wt%）and NaCl （99．5wt%）．The crystal growth experiment was performed in a vertical electric furnace controlled by an FP21digital microprocessor temperature programmer-controller （Programmable PID regulator ，Island Power Company ，Japan ）in air．The temperature was measured using a thermocouple．Fig．1SEM images of the selected NaCo 2O 4crystalsThe starting materials Co 2O 3，Na 2CO 3and NaCl in a weight ratio of 1ʒ6ʒ10were completely mixed and put into a platinum crucible with dimensions of 40mm in diameter and 50mm in height．The platinum crucible was placed in the center of a vertical ，programmable temperature furnace which was not sealed．So the crucible containing the raw materials was open to air．The mixture was heated to 900ħat a rate of 100ħ/h and held at this temperature for two days to provide a mix as homogeneous as possible．Then the temperature was lowered to 830ħat a rate of 9ħ/h．After this stage ，the melt was lowered to 710ħat a rate of 2ħ/h ，and then cooled第3期HAN Shu-juan et al：Growth，Morphology and Growth Mechanism of NaCo2O4Crystals575to room temperature naturally．Finally，millimeter-sized crystals were obtained．The as-grown crystals were mechanically separated from the flux without washing in water．A typical millimeter-sized crystal that was obtained is shown in Fig．1a．2．2X-ray powder diffraction analysisX-ray powder diffraction was used to estimate lattice parameters of the as-grown crystals at room temperature．The crystals were ground into powder form for examination．XRD data was recorded on Japan Burker by Bruker，（which is a German brand，not Japan one）diffractometer system with graphite monochromatized Cu Kαirradiation （λ=0．15418nm），together with a diffractometer scan step size of2θ=0．02ʎ，and a counting time of2s/step，over a2θrange of10ʎ-70ʎ．2．3Surface morphology and microstructureThe surface morphology and microstructure of the NaCo2O4crystals were investigated using scanning electronmicroscopy（SEM，Hitachi，S-4800）and atomic force microscopy（AFM）．The AFM images were collected in ambient atmosphere at room temperature in contact mode using a commercial Nanoscope IIIa MultiMode AFM instrument．A J-type scanner and a standard SiN cantilever integrated with NP tips were used．3Results and discussion3．1Flux growth and characteristics of NaCo2O4crystalFig．2XRD patterns of the as-grown crystalsBy comparison with other crystal growth methods，the flux growth technique is especially preferable because it readily allows crystal growth at a temperature well below the melting point of the solute．In addition，crystals grown from the flux have regular morphology［16］．Blackhexagonal-plate crystals of NaCo2O4were obtained．Theimages obtained from the SEM studies of the selectedNaCo2O4crystals are shown in Fig．1．It can be foundthat the crystals obtained are millimeter-sized．Due tohexagonal layered structure of NaCo2O4crystals，somecrystals are thin platelets and exhibit the expected hexagonal shape，as shown in Fig．1a．However，during the crystal growth，owing to the fluctuation of the temperature gradient or composition，the atomic deposition and arrangement to form the crystal at the melt-crystal interface are very easily interrupted，leading to the forming of the crystals with the unperfect hexagonal shape，such as Fig．1b．3．2Phase identification of the crystalsFig．2shows the X-ray powder diffraction pattern of the as-grown crystals．All of the peaks in Fig．2can beindexed in accordance with the standard JCPDS Card File27-682for NaCo2O4．The lattice parameters were calculated using the program TOPASS from the observed2values（see Table1）．All the results are in agreement with the report of Ken Kurosaki［10］．Table1Lattice parameters of NaCo2O4crystalLattice parameters a/nm b/nm c/nmReference［10］0．28340．28341．0899 This work0．28420．28421．08943．3Surface morphology and growth mechanism analysisFrom observations of the SEM（Fig．3）and AFM（Fig．4）results on the NaCo2O4crystals，it is clear that the576人工晶体学报第41卷crystal growth is controlled by a two-dimensional （2D ），layer-by-layer mechanism acting along the c -axis．NaCo 2O 4crystallizes in hexagonal structure．As reported elsewhere ［4］，the layered hexagonal structure of NaCo 2O 4is described as formed by Na and CoO 2layers arranging themselves alternately along the c -axis ，and shows a high degree of two-dimensionalcharacter．Fig．3SEM images of the as-grown crystals （a ）and （b ）Surface micro-morphological study of the crystals ，（c ）and （d ）Side view of the crystalsSurface micro-morphological examinations （Fig．3a ，b ）reveal that there are some growth steps on the surface ，which indicate that the growth mode of layered steps is formed ［17］．The side views of the crystals （Fig．3c ，d ）indicate that growth takes place via a two-dimensional layer-by-layer mechanism．The growth mechanism of the crystal was further investigated by AFM and growth steps were observed as shown in Fig．4．The AFM measurements reveal a step-and-terrace surface structure with a step height of 40nm and terraces of 100-300nm breadth．Fig．4Growth steps on the surface observed by AFM （a ）Scanning areas are 3．0ˑ3．0μm 2；（b ）Scanning areas are 2．6ˑ2．6μm 2Growth progresses by the propagation of steps in two-dimensions with 2D nuclei acting as the growth center．The appearance of 2D nuclei provides step source forfurther growth and the step terraces can also becomenucleation sites ［18］．Steps with wide terraces are thepreferential positions for nucleation．Step movement ismainly determined by surface diffusion．Initially ，thesolution aroundthe steps has almost the same supersaturation ［19］．Therefore ，steps can advance at anequal speed．However ，if the growth environment haslittle diversity during the growth ，such as the fluctuation of the thermal or composition ，the supersaturation around the steps changes ，the solute will deposit too rapidly at the position with the high supersaturation．So the steps could not move at the same speed．The step lined with hexagonal characterization is destroyed ，leading to the forming of the crystals with the unperfect hexagonal shape ，such as Fig．1b．Zhong and co-workers put forward the theory of growth basic structural unit of negative ion coordination polyhedron ［20］．They put forward that the growth and morphology of coordination polyhedron crystal depend on the link style of negative ion coordination polyhedron growth units ：corner ，edge ，and face of the coordination polyhedron presenting at interface ，which lead to the different growth rate at different interface．The direction of the crystal face with the corner of the coordination polyhedron occurring at the interface has the fastest growth rate ；the direction of the crystal face with the edge of the coordination polyhedron occurring at the interface has the second fastest growth rate ；the direction of the crystal face with the face of the coordination polyhedron occurring at theinterface has the slowest growth rate ［21，22］．It is well accepted that with the corner ，face or edge of the negative ioncoordination polyhedron occurring at interface ，the corresponding crystal face will easily disappear ，appear or occasionally appear ，respectively．NaCo 2O 4belongs to the hexagonal crystal system and has a hexagonal layered第3期HAN Shu-juan et al：Growth，Morphology and Growth Mechanism of NaCo2O4Crystals577structure consisting of the two-dimensional（2D）layers of CoO2and Na+ions．In NaCo2O4structure［4］，CoO6octahedrons，as the growth unit，are connected together by edge to form the CoO2layers．Na+ions are in a prismsite between CoO2blocks．Na+ions and CoO2layers alternately stack along the c axis to form layered hexagonalstructure．In the direction［001］，the face of CoO6octahedron occurs at interface，so growth rate was slowest in the direction［001］and the corresponding face tended to appear．When the supersaturation increases，CoO6octahedrons will combine with each other and form the growth unit which is larger than one unit cell dimension．Such a growth unit can exist steady and already has the feature of nucleus．And it is easy to form an epiphytic crystal（see Fig．1c）［23］or coalescence crystal［24］（see Fig．1d）．Actually，it is an oriented crystallization of surface-induced during the growth units superimpose themselves onto the crystal interface．4ConclusionsTo summarize，NaCo2O4crystals have been grown by the flux method．The crystals are black hexagonalplatelets．Investigations of the SEM and AFM results reveal that the crystal growth is controlled by a two-dimensional（2D）layer-by-layer mechanism along the c-axis．Reference［1］Zhou A J，Zhu T J，Zhao X B．Thermoelectric Properties of Perovskite Oxides La1－x Sr x CoO3Prepared by Polymerlized Complex Method［J］．J Mater．Sci．，2008，43：1520-1524．［2］Singh S K，Kumar J．Ac Conductivity，Dielectric Losses，Permittivity Behavior of Ba x Sr1－x Fe0．8Co0．2O3－d（x=0，0．5and1）Ceramics［J］．J Mater．Sci．，2007，42：2105-2111．［3］Androulakis J，Migiakis P，Giapintzakis J．La0．95Sr0．05CoO3：An 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Q，et al．Investigation of Nucleation Growth Mechanism of MMTC Crystal by AFM［J］．J．Cryst．Growth，2005，280：266-270．（下转第583页）第3期王娇等：冷坩埚法制备α-Al2O3多晶材料的工艺参数研究583 2．6 2．8g/cm3，符合目前国内生长大尺寸蓝宝石晶体对原料纯度和密度的要求。