Large-Scale Synthesis of Single-Crystalline Quasi-Aligned Submicrometer CuO Ribbons
Large-Scale Synthesis of Single-Crystalline
Quasi-Aligned Submicrometer CuO Ribbons
Hongwei Hou,Yi Xie,*and Qing Li
Structure Research Laboratory and Department of Chemistry,University of Science and
Technology of China,Hefei,Anhui230026,People’s Republic of China
Received January14,2004;Revised Manuscript Received September16,2004
ABSTRACT:A new strategy of scrolling a lamellar precursor was designed to form a ribbonlike structure.In a simple chemical route involving an oxidation-dehydration process,single-crystalline quasi-aligned submicrometer CuO ribbons were synthesized on a copper surface.Electron microscopic studies indicate that these ribbons have a width of around250nm and length of up to5μm.
1.Introduction
Cupric oxide(CuO)has been extensively studied because of its close connection to high-T c superconduc-tors.1The valence of Cu and its fluctuation play important roles in the superconductivity of various types of cupric compounds.2As a p-type semiconductor,cupric oxide exhibits a narrow band gap(1.2eV).3Because of its commercial value,CuO has been widely exploited as a powerful heterogeneous catalyst4and in the fabrication of lithium-copper oxide electrochemical cells.5It is reasonable to expect that the ability to process CuO into microstructure materials should en-rich our understanding of its fundamental properties and enhance its performance in currently existing applications.
For fundamental and practical reasons,the synthesis of a one-dimensional CuO structure has attracted considerable attention.CuO nanowires were fabricated by heating copper substrates in air,6and nanostructured copper compound films were shaped on a copper foil by oxidation of copper in solution.7CuO nanotubes and nanorods were synthesized by a solvothermal route in the presence of CTAB.8Recently,flexible CuO nanor-ibbons have been prepared in water-ethanol mixed solvent.9Usually,the ribbons formed in solution inter-twist together,while separate and aligned ribbons may find more direct and potential applications.We consider aligned ribbons may be prepared through the design of the precursors.Up to now,scrolling from appropriate lamellar precursors has been regarded as an effective method to obtain nanotubes.10Inspired by the above strategy,we design a mild and simple method via scrolling a lamellar structure to form a ribbon structure. For CuO ribbons,Cu(OH)2with lamellar structure is an excellent candidate precursor,in that not only does the conversion just involve a simple dehydration pro-cess,but also the control of its scrolling may produce aligned ribbons without intertwist.
2.Experimental Section
A typical synthesis of submicrometer CuO ribbons on a copper foil was performed as follows.An aqueous solution was prepared in a50mL glass bottle by mixing appropriate analytically pure8.4g of KOH(5M),1.08g of K2S2O8(0.13 M),and30mL of water.A piece of99.99%copper foil(5×2.5×0.25mm3),which has been cleaned in30%nitric acid for20
s,rinsed in deionized water,and then ultrasonically cleaned in acetone,was immersed in the solution.After the solution was maintained at40°C for15min,the copper surface was covered with a deep-blue film.The copper foil was then taken out from the solution and rinsed with distilled water.Then the copper foil was put into the center of the quartz tube of a vertical induction furnace.The copper foil was heated in air and kept at250°C for1h.Then temperature of the furnace descended gradually to room temperature,and a sample of black film on the copper foil was obtained.
The phase identification of the samples was carried out on X-ray powder diffraction(XRD)patterns,using a MAC Science Co.Ltd.MXP18AHF X-ray diffractometer with Cu K R radiation(λ)1.54056?).The morphology of the products was measured by field emission scanning electron microscopy (FE-SEM;JEOL JSM-6700F),transmission electron micros-copy(TEM;Hitachi H-800),and selected area electron diffrac-tion(SAED).
3.Results and Discussion
The morphology of the as-prepared sample was ex-amined by FE-SEM.Figure1a,the panoramic morphol-ogy of the sample,shows that the sample consists of submicrometer ribbons in high quantity and that the proportion of the ribbons synthesized by this method is nearly100%.The high-magnification image of the sample shown in Figure1b indicates that the ribbons, with an average width around250nm,are homoge-neous,upright,outward,and quasi-aligned.
The composition of the as-prepared sample was examined by XRD.A typical XRD pattern of the sample is shown in Figure2a,and all the diffraction peaks can be indexed to monoclinic-phase CuO(space group C2/ c),except those marked with an asterisk from the copper https://www.360docs.net/doc/2d16760143.html,pared with the standard diffraction patterns(JCPDS Card No.45-937),no characteristic peaks from impurities,such as Cu(OH)2or Cu2O,are detected.For clear comparison,a typical XRD pattern of the intermediate compound,formed before the oxi-dized copper foil was heat treated for dehydration,is shown in Figure2b.All of the diffraction peaks can be indexed to orthorhombic-phase Cu(OH)2(space group Cmc21,JCPDS Card No.13-420),except those marked with an asterisk from the copper substrate.A typical XRD pattern of the Cu foil(*)is shown in Figure2c, showing its cubic-phase Cu.
*To whom correspondence should be addressed.E-mail:yxielab@ https://www.360docs.net/doc/2d16760143.html,.
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For verifying the formation of pure submicrometer CuO ribbons,the ribbons were scraped off the copper foil for SAED and TEM observation.The SAED pattern taken on the scraped film,as shown in Figure 3a,exhibits a single-crystalline CuO structure,which ac-cords well with the XRD result.Different parts of this sample show exactly the same electron pattern,indicat-ing that the submicrometer CuO ribbons are of single-crystalline structure.The peaks of other crystal planes still appear in the XRD pattern (Figure 2),due to the fact that this XRD pattern was measured on a CuO surface in which the long ribbons were randomly mixed.The TEM image,shown in Figure 3b,indicates that some of the submicrometer ribbons are in the brim of the sample.Several submicrometer CuO ribbons with length of up to 5μm are exhibited in Figure 3c,confirming the ribbonlike CuO with a width of around 250nm.
The formation of submicrometer CuO ribbons on a copper foil includes a simple oxidation -dehydration process,and the involved chemical process is as follows:
The formation of an intermediate compound,Cu(OH)2,was also confirmed by TEM and FE-SEM before the oxidized copper foil was heat treated for dehydration.For verifying the formation of pure Cu(OH)2ribbons,the ribbons were scraped off the copper foil for TEM
observation and SAED,which are shown in Figure 4a.The TEM image of Cu(OH)2accords well with the CuO results.The SAED pattern taken on the scraped film exhibits a single-crystal Cu(OH)2structure.As we expected,the intermediate Cu(OH)2is determinative to the formation of submicrometer CuO ribbons.Figure 4b reveals that the morphology of the intermediate Cu(OH)2ribbons before the dehydration process accords well with the morphology of the final submicrometer CuO ribbons.The inset SAED pattern taken on the Cu(OH)2ribbon also exhibits its single-crystal structure.
Structural analysis can find that the interactional stress in the layers of distorted Cu(OH)6octahedra urges the lamellar sheets to form Cu(OH)2submicrometer ribbons.Under the nonequilibrium of the
oxidation
Figure 1.FE-SEM image of quasi-aligned submicrometer CuO ribbons prepared in solution with [KOH])5M and [K 2S 2O 8])0.13M for 15min at 40°C and then dehydrated at 250°C in air:(a)low-magnification image;(b)high-magnification image.
Cu +2KOH +K 2S 2O 8f Cu(OH)2+2K 2SO 4
(1)Cu(OH)2f CuO +H 2O
(2)
Figure 2.(a)XRD patterns of the as-prepared CuO.(b)XRD patterns of the intermediate Cu(OH)2.(c)XRD patterns of the copper foil (*,Cu).
202Crystal Growth &Design,Vol.5,No.1,2005Hou et al.
reaction system,the formed orthorhombic Cu(OH)2consists of olation chains in the (001)planes,character-ized by square-planar coordination of the Cu 2+ions with strong σbonds.The Cu 2+ions form two other longer
and more ionic bonds with two OH -groups from neighboring chains along the c axis,completing a deformed octahedron in the first coordination sphere of Cu 2+.7
Essentially,the sheet is formed by edge sharing of the distorted Cu(OH)6octahedra,and a 3D Cu(OH)2crystal is formed by stacking of the sheets through hydrogen bonds.11The OH -ligand acts as a nucleophile and undergoes a change in coordination only when it switches from a terminal ligand in a monomer to a bridging ligand in a condensed species.12The growth speed of such a crystal is normally proportional to 1/d hkl .13The interplanar distance of (010)planes is the longest with relatively loose hydrogen bond linkages,whereas the (100)plane is the shortest.Cu(OH)2nano-tubes grown along the [100]direction were reported to form by stacking and scrolling the sheets parallel to (010)planes.14In our approach,similar stacking and scrolling the sheets parallel to (010)induce the forma-tion of a quasi-aligned ribbonlike Cu(OH)2structure.The schematic diagram of the (010)plane projection of corrugated Cu(OH)6octahedra .is shown in Figure 4c.Therefore,it is transformed to its dehydrated product monoclinic CuO by breaking the interplanar hydrogen bonds without changing the essential morphology.According to the depicted mechanism,the unit con-centration of the nucleophile OH -may have an influ-ence on the scrolling of Cu(OH)2and then affect the morphology of the final product.So,we examined the influence of the concentration of KOH and K 2S 2O 8on the scrolling formation of submicrometer CuO ribbons.Under different concentrations of KOH,the CuO submicrometer structures were examined with [K 2S 2O 8])0.13M for 15min at 40°C and then dehydrated at 250°C in air.At a lower [KOH],1M,the oxidation process is slow and the final morphology is a floccule (Figure 5a).When [KOH]) 2.5M,submicrometer ribbons appeared and some epitaxial scrolls grew on the ribbons (Figure 5b).Under different concentrations of K 2S 2O 8,the CuO submicrometer structure was exam-ined with [KOH])5M for 15min at 40°C and then dehydrated at 250°C in air.At a lower [K 2S 2O 8],0.065M,the Cu foil was corraded to a rough surface (Figure 6a).When [K 2S 2O 8])0.26M,submicrometer ribbons appeared and a great deal of epitaxial scrolls grew on the ribbons (Figure 6b).Therefore,the appropriate concentrations of KOH and K 2S 2O 8are important to the scrolling of Cu(OH)2and have a direct influence on the final morphology,which also supports the supposed
mechanism.
Figure 3.TEM images for quasi-aligned submicrometer CuO ribbons prepared in solution with [KOH])5M and [K 2S 2O 8])0.13M for 15min at 40°C and dehydrated at 250°C in air:(a)SAED pattern of a single CuO ribbon;(b)a part of the panoramic image of submicrometer CuO ribbons;(c)several submicrometer CuO
ribbons.
Figure 4.Structure and morphology of intermediate Cu(OH)2submicrometer ribbons formed in solution with [KOH])5M and [K 2S 2O 8])0.13M for 15min at 40°C:(a)TEM images of the as-prepared intermediate Cu(OH)2.(inset:corresponding SAED pattern);(b)FE-SEM morphology of intermediate Cu(OH)2submicrometer ribbons;(c)schematic diagram of the (010)plane projection of corrugated Cu(OH)6octahedra.
Synthesis of Quasi-Aligned CuO Ribbons Crystal Growth &Design,Vol.5,No.1,2005203
Furthermore,we examined the influence of the reac-tion time and the reaction temperature on the scrolling formation of submicrometer CuO ribbons with [KOH])5M and [K 2S 2O 8])0.13M.As shown in Figure 7,according to increasing reaction time,thicker and longer ribbons formed.At room temperature,the oxidation process was slow and a rough surface formed
(Figure
Figure 5.FE-SEM images of the CuO submicrometer struc-ture prepared under different concentrations of KOH with [K 2S 2O 8])0.13M for 15min at 40°C and dehydrated at 250°C in air:(a)[KOH])1M;(b)[KOH])2.5
M.
Figure 6.FE-SEM images of the CuO submicrometer struc-ture prepared in solution under different concentrations of K 2S 2O 8with [KOH])5M for 15min at 40°C and dehydrated at 250°C in air:(a)[K 2S 2O 8])0.065M;(b)[K 2S 2O 8])0.26
M.
Figure 7.FE-SEM images of the CuO submicrometer struc-ture prepared in solution under different reaction times with [KOH])5M and [K 2S 2O 8])0.13M at 40°C and then dehydrated at 250°C in air:(a)8min;(b)25min;(c)35
min.
Figure 8.FE-SEM images of the CuO submicrometer struc-ture prepared in solution under different reaction tempera-tures with [KOH])5M and [K 2S 2O 8])0.13M for 15min and then dehydrated at 250°C in air:(a)room temperature;(b)60°C.
204Crystal Growth &Design,Vol.5,No.1,2005Hou et al.
8a).When the temperature rose to60°C,longer ribbons appeared(Figure8b).
4.Conclusions
In summary,a new strategy of scrolling the lamellar precursor was designed to form a ribbonlike CuO submicrometer structure.In a simple chemical route involving an oxidation-dehydration process,we suc-cessfully synthesized single-crystalline quasi-aligned submicrometer CuO ribbons on a copper foil,which show homogeneous,outward morphology with a width of around250nm and a length of up to5μm.The advantages of the approach for the final practical applications of quasi-aligned cupric oxide on a copper foil are its mild temperature,low cost,and easy control method.It is expected that the novel submicrometer CuO ribbons may offer exciting opportunities for poten-tial applications in catalysis,electrochemistry,and superconductivity.
Acknowledgment.Financial support from the Na-tional Natural Science Foundation of China,the Chinese Academy of Sciences,and the Chinese Ministry of Education is gratefully acknowledged.
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