Polycrystalline Cu7Te4 Dendritic Microstructures Constructed by

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Polycrystalline Cu7Te4 Dendritic Microstructures Constructed by Spherical Nanoparticles: Fast Electrodeposition, Influencing Factors, and the Shape Evolution Yongmei Zhang,† Yonghong Ni,*,† Xuemei Wang,† Jun Xia,† and Jianming Hong‡ †

College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, P. R. China ‡ Centers of Modern Analysis, Nanjing University, Nanjing 210093, P. R. China ABSTRACT: Polycrystalline Cu7Te4 dendritic microstructures were successfully synthesized via a simple galvanostatic electrochemical deposition method at room temperature, employing a mixed solution containing Cu(CH3COO)2, Na2TeO3, and nitric acid as the electrolyte. Cu7Te4 dendrites were deposited at the current of 16 mA for 5 min. The phase and morphology of the asprepared product were characterized by means of powder X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), (highresolution) transmission electron microscopy (HR/TEM), and scanning electron microscopy (SEM). Some factors influencing the formation of dendritic Cu7Te4 microstructures were systematically investigated, including the depositing currents, complexants, surfactants, original amounts of Cu(CH3COO)2, and Cu2+ ion sources. Experiments showed that the low deposition current and the concentration of Cu(CH3COO)2 were unfavorable for the formation of dendritic Cu7Te4 microstructures constructed by nanoparticles. A time-dependent shape evolution of polycrystalline Cu7Te4 dendrites was studied.

1. INTRODUCTION Transition-metal telluride semiconductors have aroused extensive attention due to their outstanding optoelectronic and thermodynamic properties,1,2 as well as wide potential applications in solar cells,3 photovoltaic devices,1 thermoelectric materials,4 photodetectors,5 optical filters,6 photovoltaic cells,7 biosensors,8 etc. As one of the typical IVI transition-metal telluride semiconductors, tellurides of copper have attracted much attention due to their unique physicochemical properties9 and a variety of crystal structures including CuTe, Cu4Te3, Cu7Te4, and Cu2Te.10,11 Previous reports mainly focused on the thin films and bulk materials of copper tellurides. For example, Neyvasagam et al. synthesized CuTe thin films with thickness from 50 to 200 nm by a thermal evaporation technique onto well-cleaned glass substrates kept at 300 K under a vacuum greater than 2 105 mbar.9 Pathan and co-workers prepared copper telluride thin films via a chemical bath deposition route.10 Recently, copper telluride nanostructures have attracted much attention. Nanostructured copper tellurides with morphologies of nanoparticles, nanorods, and nanoribbons have been successfully synthesized by many methods including a sonochemical approach,11 an elementdirected aqueous solution route,12 hydrothermal synthesis,13 and template-free electrochemical deposition.14 Different from the above-mentioned nanostructures, branched structures have some advantages including large surface areas, allowing for heterostructures or continuous networks, etc. Therefore, the fabrication of dendritic structures may pave a new pathway for r 2011 American Chemical Society

wide applications of future nanodevices.15 To date, however, no dendritic copper telluride micro- or nanostructures are reported in literature. Recently, the preparation of dendritic micro- or nanostructures attracted our research interest. Some metal and compound dendrites such as Bi, Pb, and PbTe have been successfully synthesized in our group.1618 In the present work, we designed a simple galvanostatic electrodeposition route to successfully synthesize dendritic Cu7Te4 micro- and nanostructures, employing Cu(CH3COO)2 and Na2TeO3 as the Cu and Te sources. The deposition process was carried out in a nitric acid solution without the assistance of any template or surfactant at a depositing current of 16 mA for 5 min at room temperature. SEM observations showed that the as-obtained dendritic Cu7Te4 micro- and nanostructures were constructed by self-assembly of near-spherical nanoparticles. SAED patterns proved that the dendritic Cu7Te4 micro- and nanostructures were polycrystalline, which differs from our previous reports.1618 At the same time, some experimental parameters influencing the formation of polycrystalline Cu7Te4 dendrites were systematically investigated, including electrodeposition currents and durations, the original Cu ion sources, and the original amounts of Cu(II) and additives. Experiments showed that the morphology of the final Received: March 28, 2011 Revised: July 9, 2011 Published: August 10, 2011 4368

dx.doi.org/10.1021/cg200391d | Cryst. Growth Des. 2011, 11, 4368–4377

Crystal Growth & Design

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Figure 1. The XRD pattern (a) and EDS analysis (b) of the product deposited under the galvanostatic model at the current of 16 mA for 5 min.

product could be strongly affected by the deposition currents and the concentrations of Cu(CH3COO)2 and additives. Simultaneously, to investigate the possible formation process of dendritic Cu7Te4 microstructures, a time-dependent shape evolution of the product was studied.

2. EXPERIMENTAL SECTION All reagents were analytically pure, purchased from Shanghai Chemical Company, and used without further purification. The solutions were prepared with twice-distilled water. 2.1. Preparation of Dendritic Cu7Te4 Microstructures. In a typical preparation procedure, a solution containing 0.5 mmol of Cu(CH3COO)2 was added into another solution containing 0.5 mmol of Na2TeO3; a pale-green precipitate appeared immediately. Herein, the pH was measured to be 5.94. Then, 2 mL of nitric acid (4.8 M) was introduced to the above system. The precipitate was dissolved. A pale-green

solution was obtained and further diluted to 30 mL. The pH of the system was reduced to 0.55. To obtain the product, a simple three-electrode cell was used in our experiments, employing a Pt wire as the counter electrode, a saturated Ag/AgCl electrode as the reference electrode, and a pure Cu plate (99.99%, 1.0 cm2) as the working electrode. The electrodeposition experiments were carried out in air under a constant current of 16 mA at room temperature for 5 min. After deposition, the pH changed to 0.25. 2.2. Characterization. X-ray diffraction (XRD) pattern of the electrodeposition was obtained on a Shimadzu XRD-6000 X-ray diffractometer (Cu Kα radiation, λ = 0.154 060 nm), employing a scanning rate of 0.02 deg s1 and 2θ ranges from 10° to 80°. Scanning electron microscopy (SEM) images and energy dispersive spectrum (EDS) of the final product were taken on Hitachi S-4800 field emission scanning electron microscope, employing the accelerating voltage of 5 or 15 kV (15 kV for EDS). High-resolution transmission electron microscopy (HR/TEM) images were carried out on a JEOL-2010 transmission electron microscope, employing an accelerating voltage of 200 kV. Electrochemical 4369

dx.doi.org/10.1021/cg200391d |Cryst. Growth Des. 2011, 11, 4368–4377


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Figure 2. Electron microscopy images of the product: (a) a representative low magnification SEM image, (b) a high magnification SEM image, (c) a TEM image of a subbranch (the inset shown in Figure 2c is the SAED pattern of the product), and (d) a HRTEM image. responses of the solutions after electrodepositing for different times were recorded on CHI 440A electrochemical analyzer (CHI, USA) with a conventional three-electrode cell. A bare glass-carbon electrode was used as the working electrode. A Ag/AgCl electrode and a platinum electrode were used as the reference and the auxiliary electrode, respectively.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology Characterization. The phase of the as-obtained product is determined by XRD analysis. Figure 1a (upper) shows the XRD pattern of the product obtained under the current experimental conditions. Most of diffraction

peaks can be indexed as the hexagonal Cu7Te4 form by comparison with the data of JCPDS card files no. 65-2057 (see Figure 1a, bottom). By comparison with the standard data, however, the intensities of some peaks obviously change. The (012) peak enhances and the (202) and (212) peaks weaken, implying that the final product grows orientedly. Furthermore, two weak peaks centered at 26.2° and 29.3° can be attributed to the orthorhombic Cu4Te3 form (no. 42-1254). Figure 1b gives the EDS analysis of the as-deposited product. The strong Cu and Te peaks can be easily seen, which confirms the formation of copper telluride. The weak C and O peaks should be ascribed to the physical adsorption of carbon and oxygen molecules in air. 4370



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Figure 3. SEM images of the products prepared from the same system at current of 16 mA for different durations: (a) 0 s, (b) 30 s, (c) 60 s, (d) 90 s, and (e) 3 min.

The morphology of the as-synthesized product was characterized by SEM and TEM. Figure 2a depicts a representative lowmagnification SEM image of the product. Mass dendrites of several micrometers in length can be easily seen. Further enlargement shows that both the trunks and the branches of the dendrites are comprised of abundant nanoparticles with sizes from 20 to 50 nm (see Figure 2b). Figure 2c is a typical TEM image of a subbranch. From the prominent edges, the outlines of near-spherical particles can be found. The inset shown in Figure 2c is a SAED pattern of nanoparticles. Annular diffraction dots clearly show the polycrystalline nature of the product and can be indexed as the hexagonal Cu7Te4 form by comparison with the data of JCPDS card files no. 65-2057. Figure 2d gives a high-resolution TEM image of the product, which was taken from the end of the subbranch shown in Figure 2c. The clear stripes indicate good crystallinity of nanoparticles. The distance between neighboring planes is calculated to be ∼0.209 nm, which is very close to 0.2070 nm of the (220) plane of the hexagonal Cu7Te4 form. In addition, an amorphous surface can be seen in Figure 2d. In our experiment, Cu7Te4 was rapidly produced within a very short duration under the present electrodeposition

conditions. It was possible that the freshly produced particles had not enough time for the crystallization. Thus, the amorphous surface of Cu7Te4 was retained. 3.2. The Growth Process of Dendritic Cu7Te4 Microstructures. In order to ascertain the formation process of dendritic Cu7Te4 microstructures built up of abundant nanoparticles, we investigated the shape-evolution process of the product with the deposition time. As shown in Figure 3a, the surface of the substrate (Cu plate) is rather smooth. When the deposition time is 30 s, the surface of the substrate obviously changes, implying the appearance of the product. Figure 3b depicts a typical SEM image of the product, which is comprised of abundant nanoparticles and some irregular grains with sizes from 1 to 3 μm. Further enlargement shows that these irregular grains are formed by small nanoparticles (see the inset in Figure 3b). After 60 s, the irregular grains made of nanoparticles markedly increase (Figure 3c). Upon further extension of the deposition time to 90 s, some dendritic products start to appear (see the arrows in Figure 3d). After deposition for 3 min, abundant dendritic products are formed (Figure 3e). After 5 min, large-scale Cu7Te4 dendrites constructed from nanoparticles are obtained (see Figure 2a). The 4371



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Figure 4. The cyclic voltammograms (CVs) of the glass-carbon electrode in the system containing 0.5 mmol of Cu(CH3COO)2 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3 with different deposition durations.

Figure 6. SEM images of the products prepared from the electrolyte containing 0.5 mmol of Cu(CH3COO)2 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3 with different deposition currents for 5 min: (a) 8, (b) 12, and (c) 20 mA.

Figure 5. SEM images of the products deposited on the ITO substrate from the system containing 0.5 mmol of Cu(CH3COO)2 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3 at the same current for different durations: (a) 5 and (b) 7 min.

above shape evolution process clearly demonstrates the formation of dendritic Cu7Te4 microstructures. In the initial stage, TeO32 and partial Cu(II) ions were first reduced into Te2 and Cu(I) ions under the constant current. The residual Cu(II) and the produced Cu(I) ions combined with Te2 ions to form Cu7Te4, 4372



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Figure 8. SEM images of the products prepared from the systems after HNO3 was replaced by (a) 2 mL of 4.8 M HClO4 or (b) 2 mL of 2.4 M H2SO4 at deposition current of 16 mA for 5 min.

Cu2þ þ 6Cuþ þ 4Te2 ¼ Cu7 Te4

Figure 7. SEM images of the products prepared at the deposition current of 16 mA for 5 min with different amount of 4.8 M HNO3: (a) 1, (b) 3, and (c) 5 mL.

which nucleated on the Cu substrate and grew into irregular particles with various sizes. The related reactions are described as follows: Cu2þ þ e ¼ Cuþ

ð1Þ

TeO3 2 þ 6e þ 6Hþ ¼ Te2 þ 3H2 O

ð2Þ

ð3Þ

Moreover, the above time-dependent shape evolution can be also proven indirectly by the electrochemical responses of the system. Figure 4 depicts the cyclic voltammograms (CVs) of the glass-carbon electrode in the above systems with different deposition durations. Before the deposition, the system has the biggest reductive current owing to the biggest ion concentration. Two peaks centered at 0.087 and 0.025 V should be attributed to the reduction peaks of Cu2+/Cu+ and TeO32+/Te2 pairs,19 respectively. After deposition for 3090 s, the current intensities of two reductive peaks obviously decreased, indicating that the concentration of ions in the solution reduced owing to the formation of the product. However, there was no obvious change in three CVs. This is in good agreement with the results of SEM observations. Here, the morphologies of the products were nanoparticles or grains constructed from nanoparticles (see Figure 3b,c,d). Upon further increase of the deposition time to 3 min, the current intensities of two reductive peaks markedly reduced again. SEM observations showed the appearance of plentiful dendrites (Figure 3e). After deposition for 5 min, the 4373



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Figure 9. SEM images of the products prepared from various systems at deposition current of 16 mA for 5 min: (a) 1.0 mmol of Cu(CH3COO)2 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3, (b) 0.25 mmol of Cu(CH3COO)2 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3; (c) 0.5 mmol of CuSO4 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3, (d) 0.5 mmol of Cu(NO3)2 + 0.5 mmol of Na2TeO3 + 2 mL of 4.8 M HNO3.

morphology of the product was similar to those deposited for 3 min (see Figure 2a). Accordingly, the CV curve of the solution only had slight change compared with that deposited for 3 min. Lyahovitskaya et al.20 considered that nanometer-sized particles could imitate the elastic domains of single crystals to selforganize into polycrystalline macrodomains via a very efficient strain-relieving mechanism, which could be effective in a large variety of nanocrystalline materials. With prolongation of the deposition duration in the current work, the freshly produced Cu7Te4 nanoparticles self-organized on the previously produced particles based on the strain-relieving mechanism. Hence, small nanoparticles gradually disappeared and irregular big grains increased. Since the surface energies of various planes in nanoparticles were different, Cu7Te4 nanoparticles orientedly grew to reduce the surface energy. Thus, the dendritic Cu7Te4 microstructures were finally obtained. Furthermore, dendritic Cu7Te4 microstructures could be also formed under the same deposition conditions when ITO was selected as the substrate instead of the Cu plate. However, the size of the product was smaller, and the dendritic shape was imperfect compared with the ones obtained on the Cu substrate (see Figure 5a). This should be ascribed to the different conductivity of two substrates. ITO has smaller conductivity than the Cu plate. Thus, the deposition and growth rates of the product on ITO substrate are lower than those on the Cu plate within the same deposition current and time. As a result, smaller and imperfect Cu7Te4 dendrites were finally produced. Herein, if the deposition time was prolonged, relatively perfect dendrites could also be obtained (see Figure 5b).

3.3. The Influence of the Deposition Current and the Original Amounts of HNO3. Experiments showed that the

deposition current was an important factor affecting the formation of dendritic Cu7Te4 microstructures under the present conditions. Figure 6 depicts representative SEM images of the products deposited from the same system at various deposition currents for 5 min. When a current of 8 mA was used, no dendritic product was obtained. Here, the product was composed of abundant nanoparticles of ∼40 nm and aggregated grains (see Figure 6a). Upon increase of the current to 12 mA, many dendritic Cu7Te4 microstructures and small amounts of nanoparticles coexisted in the final product (Figure 6b). After increase of the current to 16 mA, many dendritic Cu7Te4 microstructures consisting of nanoparticles 2050 nm in diameter were obtained (see Figure 2b). When a current of 20 mA was employed, the morphology of the product was similar to that shown in Figure 2 (see Figure 6c). The above experiments indicate that low deposition current does not support the formation of dendritic Cu7Te4 microstructures. Furthermore, we investigated the influence of the amount of HNO3 on the morphology of the final product. Experiments showed that no product was prepared when no HNO3 was added to the system. With increase of the HNO3 amount, dendritic Cu7Te4 microstructures could be deposited. Figure 7 exhibits SEM images of the product prepared under the same conditions from the systems with volumes of 4.8 M nitric acid from 1 to 5 mL. The dendritic microstructures can be clearly seen. Nevertheless, the sizes of nanoparticles constructing dendrites gradually increased from ∼20 to ∼80 nm with the increase of nitric 4374



Figure 10. SEM images of the products prepared under the same depositing conditions in the presence of different surfactants: (a) 1 mmol CTAB; (b) 1 mmol PVP; (c) 1 mmol SDBS.

acid amount. During experiments, we had observed that Cu2+ ions could react with TeO32 ions to form a pale-green precipitate in the system without nitric acid. Since free Cu2+ and TeO32 ions decreased, no product was obtained under the same deposition current for 5 min. After certain amounts of nitric acid were added into the system, the above precipitate reaction was restrained. Thus, the electrochemical reaction took place and

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Figure 11. SEM images of the products prepared under the same depositing conditions (current 16 mA for 5 min) with different complexing agents: (a) 2 mmol of tartaric acid; (b) 2 mmol of citric acid; (c) 2 mmol of EDTA2Na.

dendritic Cu7Te4 microstructures constructed from nanoparticles appeared. Since HNO3 could react with CH3COO and TeO32 ions to produce CH3COOH and H2TeO3, which acted as buffer solutions, the pH of the system hardly changed in the range of appropriate HNO3 amount. As a result, the morphology of the final product was little influenced. The above conclusion 4375


Crystal Growth & Design was confirmed by our experiments. When 2 mL of HNO3 of 4.8 M was replaced by perchloric acid or sulfuric acid with the same H+ ion amounts, the pH of the systems changed from 0.52 to 0.48 (for HClO4) and 0.59 to 0.57 (for H2SO4) before and after the electrodeposition. SEM observations showed that dendritic Cu7Te4 microstructures could be also obtained (see Figure 8). 3.4. The Influences of the Original Amounts of Cu(CH3COO)2 and the Cu(II) Sources. It was found that the morphology of the final product could be affected by the original amount of Cu(CH3COO)2. As shown in Figure 9a, the final product was composed of abundant near-spherical particles and a few grains built up of small nanoparticles under the same deposition conditions when the amount of Cu(CH3COO)2 was increased to 1.0 mmol. When the amount of Cu(CH3COO)2 was decreased to 0.25 mmol, some flowerlike aggregates constructed from microrods were obtained (see Figure 9b). The above results imply that the dendritic Cu7Te4 microstructures can be obtained only when an appropriate amount of Cu(CH3COO)2 exists in the system. In the present work, 0.5 mmol of Cu(CH3COO)2 was the optimum amount for the formation of dendritic Cu7Te4 microstructures. In our recent work, a similar experimental phenomenon was also observed during the electrodeposition of thicket-like PbTe microstructures.18 Generally, the number of crystal nucleation sites decrease or increase with the decrease or increase of original metal ion amounts during nucleation. In the current experiment, due to the reduction of the nucleation sites, crystal growth was restricted in certain directions. Thus, flowerlike aggregates constructed from microrods were prepared. The increase of nucleation sites made the crystal grow in more directions, which resulted in the formation of near-spherical particles. Also, we investigated the influence of the Cu(II) ion source on the morphology of the final product. Figure 9c is a representative SEM image of the product deposited under the same conditions employing CuSO4 as Cu2+ ion sources instead of Cu(CH3COO)2. Dendritic Cu7Te4 microstructures could be also obtained. However, no obvious dendritic structures were found when Cu(NO3)2 was used as the Cu2+ ion source (see Figure 9d). The above facts indicate that counteranions play important roles in the formation of dendritic Cu7Te4 microstructures, too. Usually, CH3COO and SO42 ions possess stronger coordinating ability to metal ions than NO3 ions. In the current work, CH3COO and SO42- ions could surround Cu7Te4 nuclei when Cu(CH3COO)2 and CuSO4 were used as the copper sources. This led to the difference of the sites or number of further nucleation and growth of crystals from those when Cu(NO3)2 was used as copper ion sources. As a result of the change of the crystal nucleation and growth environments, no dendritic Cu7Te4 microstructures were obtained when Cu(NO3)2 was used as copper ion sources. 3.5. The Influence of Additives on the Morphology of the Final Product. During the syntheses of materials, complexants or surfactants are usually employed to control the morphology of the final product. In the current work, some complexants and surfactants were also investigated. Figure 10 gives SEM images of the products deposited from systems containing surfactants of cetyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), and sodium dodecyl benzene sulfonate (SDBS), respectively. The morphologies of the products are film-like structures made of nanoparticles, nanoparticle-aggregated grains, and retrogressive dendrites, respectively. The presence of surfactants markedly affects the morphologies of the final products, which should be attributed to the different structures of three

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surfactants. SDBS is an anion surfactant. It can be decomposed to SDB and Na+ ions. Thus, a strong interaction between Cu2+ and DBS ions existed in the system containing SDBS, which resulted in the shape retrogression of the final product. CTAB and PVP as capping agents surrounded the nuclei of Cu7Te4. They held back the oriented growth of Cu7Te4. As a result, Cu7Te4 nanoparticles were obtained. However, PVP contains coordinating N and O atoms. It was probable that the interaction between PVP and Cu7Te4 nuclei caused the formation of micelles with certain shapes. Therefore, discrete nanoparticleaggregated grains were prepared. Moreover, we also investigated the influence of some complexants such as tartaric acid, citric acid, and EDTA2Na on the morphology of the final product. It was found from Figure 11 that the morphologies of the three products were obviously different from that shown in Figure 2 and the amount of dendrites in final products gradually reduced, indicating that the morphology of the final product can be affected by the complexants. The above three complexants can coordinate with Cu2+ ions to form complexes of CuL with the stability constants of 3.24 106, 1.58 1011, and 6.0 1018, respectively.21 The formations of complexes reduced the amounts of free Cu2+ ions, which further decreased the nucleation and growth rate of Cu7Te4. Thus, Cu7Te4 with various shapes were obtained from the systems containing different complexants.

4. CONCLUSIONS In summary, dendritic Cu7Te4 microstructures have been successfully prepared by a simple electrodeposition route in air at room temperature and ambient pressure, employing CuSO4 and Na2TeO3 as the original Cu2+ and Te2 sources. Experimental results showed that high deposition current promoted the formation of dendritic Cu7Te4 microstructures and a certain amount of strong inorganic acid was indispensable in the formation of dendritic Cu7Te4 microstructures. Moreover, the morphology of the final product could be affected by some factors including the original Cu2+ ion amounts and Cu2+ ion sources, surfactants, and complexants. The time-dependent shape evolution experiments proved that dendritic Cu7Te4 microstructures originated from the oriented assembly and growth of nanoparticles. However, few reports are found in the literature on the study of Cu7Te4. Its special properties such as band gap (0.1 eV),22 carrier type, and conductivity still need to be studied. The present electrodeposition route is a simple, rapid, and reliable method for the preparation of dendritic Cu7Te4 microstructures and can be developed for the synthesis of other dendritic microstructures. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 86-553-3869303.

’ ACKNOWLEDGMENT The authors thank Key Foundation of Chinese Ministry of Education (Grnat 210098) for the fund support. ’ REFERENCES (1) Rousset, J.; Olsson, P.; McCandless, B.; Lincot, D. Chem. Mater. 2008, 20, 6550–6555. (2) Teeter, G. Thin Solid Films 2007, 515, 7886–7891. 4376



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Polycrystalline Cu7Te4 Dendritic Microstructures Constructed by

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