尚通明 關明云 孫建華 周全法 徐 正

(1江蘇省貴金屬深加工技術及其應用重點實驗室，江蘇技術師范學院化學化工學院，常州 213001)(2南京大學配位化學國家重點實驗室，南京大學化學化工學院，南京 210093)

研究簡報

不同形貌的Cu2O：可控合成及光學性質

尚通明1,2關明云1,2孫建華1,2周全法1徐 正＊,2

(1江蘇省貴金屬深加工技術及其應用重點實驗室，江蘇技術師范學院化學化工學院，常州 213001)(2南京大學配位化學國家重點實驗室，南京大學化學化工學院，南京 210093)

氧化亞銅；六足分級結構；形貌控制；光學性質

The morphology control of crystals with welldefined shapes remains an important goal of modern materials chemistry,since the size,morphology and structure of nanomaterials significantly influence their physical and chemical properties and therefore,their technological applications[1-4].General speaking,the growth of crystal depends on both its intrinsic lattice structure and external conditions.The former one will lead to the equilibrium crystal shape(ECS)with minimum total surface energy,whereas the later one usually forces the crystal growth to deflect from ECS and to develop into various morphologies.Recently,various capping agents have been used to control the growth direction and dimension during crystals growth process because they can selectively interact with various crystallographic planes[5-9].So,it is possible to fabricate some practical materials with desired morphology by using a suitable surfactant.

Cuprous oxide(Cu2O)has been the focus of intensive research owing to its unique optical,magneticproperties and technological applications,such as,solar energy conversion, magnetic storage devices,biosensing,and catalysis[10-21].Cu2O nanoparticles with different shapes have been synthesized by various methods[8-24].Here,we report a simple route to selectively grow Cu2O hexapodal branch structure with high uniformity, flower-like structure, nanocube and nanoplate.Our method involves the use of glucose to reduce copper acetate in the presence of surfactants.The morphologies of Cu2O are relative with the surfactant species and its concentration.

1 Experimental

All the reagents used were analytical grade without further purification.In a typical synthesis,2.5 mmol copper acetate((CH3COO)2Cu)and 2 mmol glucose(C6H12O6)were dissolved in 20 mL aqueous solution in a beaker.10 mL sodium dodecyl sulfate(SDS)(0.14 mol·L-1),or cetyltrimethylammonium(CTAB)(0.014 mol·L-1and 0.14 mol·L-1),or 6-aminohexanoic acid(0.23 mol·L-1)aqueous solution were added into the above solution.The reaction was carried out in a beaker at 100℃for 5 h.The deposits were collected,washed with distilled water and absolute ethanol several times to remove surfactant.

Products were characterized by XRD(Shimadzu XD-3A X-ray diffractmeter with Cu Kα radiation,λ=0.154 18 nm).Transmission electron microscopy(TEM)and high resolution TEM(HRTEM)images and fast-Fourier-transform patterns were obtained on a FEI Tecnai G220 S-TWIN high-resolution transmission electron microscope,using an accelerating voltage of 200 kV.Scanning electron microscopy(SEM)images were taken with a Hitachi S-3400N apparatus.UVVisible absorption spectra were recorded on UV-240/PC UV-Vis recording spectrophotometer.

2 Results and discussion

Fig.1a shows XRD pattern of the samples obtained by using SDS as surfactant.All diffraction peaks are well indexed to cubic phase Cu2O(PDF 05-0667)[7].No other diffraction peaks from metallic Cu or CuO appear in the XRD pattern,indicating the high purity of the sample[21].SEM images of Cu2O sample(Fig.1b and inset)clearly demonstrate large area of the hexapodal branch structure,indicating very high yield and good uniformity of this approach.It also shows that the hexapodal structure grows from the crystallographic equivalent six facets of a crystal nuclear.The TEM image of Cu2O sample further confirms the hexapodal morphology(Fig.1c).The HRTEM image for the tip of one pod reveals that the interplanar spacing is about 0.3 nm,which corresponds to the(110)lattice planes.Its corresponding fast-Fourier-transform pattern(Fig.1(d),inset)further reveals a single crystal entity and the preferential[001]growth direction[10,23].

Fig.1 (a)XRD pattern of the products;(b)Low magnification SEM image of Cu2O sample(inset,High magnification SEM image);(c)TEM image of Cu2O sample;(d)HRTEM image for a tip of one pod(inset,corresponding fast-Fourier-transform pattern)

Fig.2a and 2b are SEM and TEM images of nanocube(CTAB,0.014 mol·L-1)with a few cubic boxes obtained by using CTAB instead of SDS.When concentration of CTAB is up to 0.14 mol·L-1,Fig.2c shows that the morphology of the products is with plate structure.Using 6-aminohexanoic acid(0.23 mol·L-1)instead of SDS,the flower-like morphology shown in Fig.2c is obtained.

Fig.2 (a,b)SEM and TEM images of the sample obtained using CTAB(0.014 mol·L-1).(c)SEM image of the sample obtained using CTAB(0.14 mol·L-1).(d)SEM image of the sample obtained using 6-aminohexanoic acid(0.23 mol·L-1)

Morphologies of crystals are determined by the relative order of growth rates along the different crystallographic directions.The surface energies of different planes of crystal are different,the fastest crystal growth will occur along the direction perpendicular to the face with the highest surface energy[26].A common method to manipulate growth rates along the different crystallographic directions is to employ organic or inorganic additives.Because of anisotropy in adsorption stability,the preferential adsorption lowers the surface energy of the bound plane and prevents the particles from further packing on this plane,resulting in crystal with different morphology.As illustrated by Wang[27],the geometrical shape of a cubicstructure is mainly determined by the ratio(R)of the growth rate along the ＜100＞ to＜111＞ directions.When R value varies from ≤0.58,through 1.15 to 1.73,the cubic,the truncated octahedral and octahedral will be formed,respectively[21,26,28].In order to see the roles of the surfactants in synthesis of various morphologies of Cu2O,we carried out the experiments in absence of additives.The results show that hexapodal branch structure coexists with octahedrons(Fig.3a).It indicates that R value in our reaction system without SDS is in the range of 1.15～1.73,and simultaneously that under high enough supersaturation,the one dimension growth on six｛100｝facets of the truncated octahedron initiates simultaneously to form the hexapodal structure.The results mean that the acetate anion or the oxidation product of the glucose can also selectively stabilize｛111｝ facet to a certain extent.When SDS is added,hexapodal branch structure with high uniformity can be obtained.It indicates that SDS reduces R value from near1.73 to 1.15,and a truncated octahedron nanoparticle is formed.As a nucleus,six ｛100｝ facets of the truncated octahedron initiate 1D growth under high enough supersaturation to form the hexapodal structure[29].SDS is an anionic surfactant while CTAB is a cationic surfactant.They have a different adsorption performance for the different facets of a crystal.Using surfactant CTAB(0.014 mol·L-1),nanocube bound by the(100)plane is formed.It indicates that CTAB is preferentially adsorbed on the {100}facets,thus reducing R value to ≤0.58.CTAB hasbeen systematically studied as a capping agent and/or a“soft” template.CTAB may form spherical,cylindrical micelles or even higher-order phases depending on the solution conditions[30].The concentration of CTAB significantly affects the morphology of products.Fig.3b～e shows SEM images of the sample obtained at various concentrations of CTAB.The morphologies of Cu2O transit from size multi-dispersion particles to nanoplates.When the concentration of CTAB is ≥0.084 mol·L-1,irregular plate-like Cu2O is formed(Fig.2c and Fig.3b～e).The reason might be that CTAB forms a layer liquid crystal in aqueous solution at higher concentration.The flower-like shape of Cu2O composed of nanoparticles is obtained using surfactant 6-aminohexanoic acid.The formation of flower-like shape mightattribute to carboxyl and amino groups of 6-aminohexanoic acid,which is favorable for the nanoparticles self-assembly into flower-like shape.The exact formation mechanism of nanoplate and flower-like shapes are unknown,further work is needed.

Fig.3 (a)SEM image of the sample obtained without surfactant.b,c,d,e)SEM images of the sample obtained using various concentrations of CTAB:(b)0.028 mol·L-1,(c)0.084 mol·L-1,(d)0.168 mol·L-1M,(e)0.224 mol·L-1

The optical properties of Cu2O are investigated with ultraviolet and visible light(UV-Vis)absorption spectroscopy(Fig.4).The results show that the optical properties of Cu2O nanoparticles are shape-dependent.The relative intensity of the UV-Vis absorption bands is different for Cu2O nanoparticles with different shape and size.Fig.4 shows the strongest band located at ca.551 nm(Eg=2.38 eV)for hexapodal branch structure and flower-like structure,but at ca.450 nm(Eg=2.91 eV)for the nanocube and nanoplate.Comparing with the direct band gap(Eg=2.17 eV)[7,31]of bulk Cu2O,the value of hexapodal branch structure and flower-like structure is slight greater than that of bulk Cu2O,and the value of the nanocube and nanoplate is obvious greater than that of bulk Cu2O.It is attributed to the strong quantum confinement effects of Cu2O nanoparticles.

Fig.4 UV-Visible absorption spectra of various shapes of Cu2O

3 Conclusions

In summary,we demonstrate that capping agent plays an important role on the morphology of Cu2O.The shapes of the Cu2O crystal are tuned by varying the species and concentration of surfactants.The possible formation mechanism is discussed.

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Cu2O with Different Morphologies:Controlled Synthesis and Optical Properties

SHANG Tong-Ming1,2GUAN Ming-Yun1,2SUN Jian-Hua1,2ZHOU Quan-Fa1XU Zheng＊,2
(1Jiangsu Key Laboratory of Precious Metals Chemistry,School of Chemistry and Engineering,Jiangsu Teachers University of Technology,Changzhou Jiangsu 213001)(2State Key Laboratory of Coordination Chemistry,School of Chemistry and Chemical Engineering,Nanjing University,Nanjing 210093)

Cuprous oxide(Cu2O)hexapodal branch structure with high uniformity was prepared by a solution phase route using sodium dodecyl sulfate as a capping agent.The shapes of Cu2O crystal(flower-like structure,nanocube and nanoplate)were tuned by varying species and concentrations of surfactants to control the growth rate on different crystal planes of Cu2O.Cu2O nanostructures were characterized by UV-Vis spectroscopy,XRD,TEM and SEM.XRD result shows that the obtained Cu2O belongs to cubic phase.TEM and SEM results demonstrate that specie and concentration of surfactants play a key role in the formation of various morphologies of Cu2O.The formation mechanism is discussed.Moreover,the optical properties of the obtained Cu2O are shape-dependent.

Cu2O;hexapodal branch structure;shape-controlled;optics properties

O614.121

A

1001-4861(2010)07-1294-05

2009-09-20。收修改稿日期：2010-02-22。

國家自然科學基金資助項目(No.90606005,20490210)，江蘇省高校自然科學基金(No.08KJD150003)，常州市青年基金(No.CQ2008011)項目資助。＊

。 E-mail：zhengxu@netra.nju.edu.cn

尚通明，男，53歲，教授；研究方向：無機材料可控合成及性能。