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Potentiostatic Electrodeposition Route for Quick Synthesis of Featherlike PbTe Dendrites: Influencing Factors and Shape Evolution Yonghong Ni,*,† Yongmei Zhang,† 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: Featherlike PbTe dendrites were synthesized via a simple potentiostatic electrochemical deposition method at room temperature, employing Pb(NO3)2 and Na2TeO3 as the precursors and tartaric acid as capping molecule. PbTe dendrites were deposited at the potential of 0.2 V for 5 min. This experiment was against the previous report (Chem. Mater. 2008, 20, 33063314), which considered that the potential oscillation led to the formation of PbTe dendrites. The phase and morphology of the as-prepared product were characterized by means of powder X-ray diffraction (XRD), energy dispersive spectrometry (EDS), (high resolution) transmission electron microscopy (HR/TEM), and scanning electron microscopy (SEM). Some factors influencing the formation of featherlike PbTe dendrites were systematically investigated, including the depositing time, potential, complexant, the original amount of tartaric acid, and Pb2þ ion sources. A time-dependent shape evolution process showed that the growth of PbTe underwent three stages from near-spherical nanoparticles, to flowerlike structures, finally to featherlike dendrites.
1. INTRODUCTION Various thermoelectric materials have attracted extensive research interest in recent years based on the consideration of saving energy and cooling microchip.1 As a representative IVVI semiconducting compound, lead telluride (PbTe), with the rock salt structure and a narrow band gap (Eg = 0.31 eV at 300 K), is an important thermoelectric material, due to its high figure of merit and melting point (∼900 K), low vapor pressure, and good chemical stability.24 Also, PbTe nanostructures possess a large Bohr exciton radius (∼46 nm), which allows for strong quantum confinement within a large range of size.5,6 Furthermore, besides thermoelectric applications, PbTe can be extensively used in many fields, such as field effect transistors, solar cells, telecommunication, and biological imaging.713 Many methods have been explored to synthesize PbTe, including microwave-assistant synthesis,14 solvothermal preparation,15 electrodeposition method,16 hydrothermal technology,17 chemical bath route,10 and molecular beam epitaxy approach.18 Among them, the electrodeposition technique attracts much attention, owing to its speediness, simplicity, and low cost. For example, Yang et al.19 synthesized PbTe nanowires using the lithographically patterned electrodeposition (LPNE) method. Qiu and co-workers reported the synthesis of the PbTe nanorods at room-temperature via a constant-current electrodeposition route accompanied by applying ultrasonic pulses in an aqueous solution containing lead nitrate.20 PbTe films on the SiO2 substrate coated by Au films were potentiostatically electrodeposited by Xiao and co-workers from a nitric acid bath, r 2011 American Chemical Society
employing Pb(NO3)2 and TeO2 as the starting reactants.16b Also, it was found that featherlike PbTe dendrites could be obtained from 0.01 M HTeO2þ solution at the potential range from 0.13 to 0.40 V.16b In 2008, Li and co-workers prepared highly symmetrical PbTe dendrites consisted of orderly and regular particles via a galvanostatic electrodeposition route in the presence of tartaric acid with a current density of 3.33 mA/cm2 for 30 min.16c They considered that the formation of PbTe dendrites was attributed to the potential oscillation of the electrode. Simultaneously, the authors simply discussed the influences of tartaric acid, the current density, and the depositing time on the morphology of the product. In the present work, we also successfully electrodeposited featherlike PbTe dendrites from a system similar to Li’s work via a potentiostatic model with a potential of 0.2 V for 5 min, employing a Cu plate as the working electrode and a mixed system of Pb(NO3)2, Na2TeO3, and tartaric acid as the electrolyte. All experimental processes were carried out in air at room temperature. At the same time, we systematically investigated the influences of various parameters such as the depositing time, potential, complexant, the original amount of tartaric acid, and Pb2þ ion sources on the formation of featherlike PbTe dendrites. Comparing the current work with Li’s work,16c one can find some obvious differences: (1) PbTe dendrites were obtained through a Received: October 19, 2010 Revised: April 16, 2011 Published: April 22, 2011 2142
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Figure 1. XRD pattern (a) and EDS analysis (b) of the product deposited under the current potentiostatic model at the potential 0.2 V for 5 min.
simple potentiostatic model, which experimentally reversed the conclusion that the formation of PbTe dendrites was relevant to the electrode potential oscillation. (2) The role of tartaric acid in the formation of featherlike PbTe dendrites was experimentally proved. (3) It spent shorter times (35 min) for the deposition of plentiful featherlike PbTe dendrites, which availed to saving energy. (4) A more complete growth process of PbTe dendrites could be gained based on shorter time-dependent shape evolution, which was favorable for the understanding of the formation mechanism of PbTe dendrites.
2. EXPERIMENTAL SECTION All reagents were analytically pure, purchased from Shanghai Chemical Company, and used without further purification. A simple threeelectrode 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. In a typical experimental procedure, 0.5 mmol of Pb(NO3)2 (99 wt %), 1.0 mmol of Na2TeO3 (98 wt %), and 1.0 g (∼6.7 mmol) of tartaric acid were dissolved into twice-distilled water to form an electrolyte of 30 mL. Featherlike PbTe dendrites were successfully obtained at room temperature by potentiostatic electrolysis with the potential of 0.2 V for 5 min.
Figure 2. Electronic microscopy images of PbTe dendrites: (a) a representative low magnification SEM image, (b) a high magnification SEM image, and (c) a TEM image of a subbranch. Left inset: SAED pattern. Right inset: HRTEM. X-ray diffraction (XRD) patterns of the typical structures of electrodeposition were carried out on a Shimadzu XRD-6000 X-ray diffractometer (Cu KR radiation, λ = 0.154060 nm), employing a scanning rate of 0.02° s1 and 2θ ranges from 20° to 70°. Scanning electron microscopy (SEM) images and an energy dispersive spectrum (EDS) of the final product were taken via Hitachi S-4800 field emission scanning electron microscopy, employing the accelerating voltage of 5 kV 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. 2143
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Figure 3. SEM images of the products prepared from the solution containing 0.5 mmol of Pb(NO3)3 and 1 mmol of Na2TeO3 in the presence of different amounts of tartaric acid: (a) 0.0, (b) 0.15, (c) 0.45, and (d) 0.75 g.
3. RESULTS AND DISCUSSION 3.1. Structures 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 the diffraction peaks can be indexed as the cubic PbTe form by comparison with the data of JCPDS card file no. 38-1435 (see Figure 1a bottom). The other diffraction peaks are attributed to Cu peaks, which come from the Cu substrate. The EDS analysis of the as-deposited product further confirms the formation of PbTe. As shown in Figure 1b, the strong Pb, Te peaks can be easily seen; and the weak peaks of C and O can be ascribed to the residua of tartaric acid on the surfaces of PbTe dendrites. The morphology of the as-synthesized product was characterized by SEM and TEM. A low-magnification SEM image of the product is depicted in Figure 2a, from which a plenty of orderly featherlike dendrites with 1020 μm in length can be easily seen. Obviously, the morphology of the product prepared under the current potentiostatic conditions is the same as that reported in Li’s work16c except for higher yield and smaller sizes. Further enlargement shows that the central trunk is made up of orderly and regular starlike particles with the sizes 300400 nm (see Figure 2b). All the secondary branches are almost parallel to
each other and consist of regular trigonal particles with the sizes of about 100200 nm. Figure 2c is a typical TEM image of a subbranch, which is broken from long dendrites by an ultrasonic wave. Some spindly nanoparticles can be clearly seen in Figure 2c. The left inset shown in Figure 2c is a SAED pattern of spindly nanoparticles, from which regular diffraction dots clearly show the single-crystalline nature of the product. A high resolution TEM image of spindly nanoparticles is given in the right inset in Figure 2c. The clear stripes further confirm the single-crystalline nature of nanoparticles, and the distance of the neighboring planes is measured to be 0.229 nm, which is very close to the 0.22845 nm of the (220) plane of the cubic PbTe form. It was found that many experimental parameters could markedly affect the morphology of the final PbTe, including the original amount of tartaric acid, complexants, Pb2þ ion sources, the depositing times and potentials. 3.2. Specific Role of Tartaric Acid in the Formation of Featherlike PbTe Dendrites. Figure 3 shows SEM images of the products deposited from the electrolytes with different amounts of tartaric acid at the potential of 0.2 V for 5 min. When no tartaric acid was used, only spherical nanoparticles with a mean size of ∼20 nm were prepared (see Figure 3a), which is obviously different from the result reported via the galvanostatic route.16c This should be attributed to the shorter deposition time 2144
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in our work. After 0.15 g of tartaric acid was employed, abundant small dendrites appeared (Figure 3b). With the further increase of the amount of tartaric acid, dendritic nanostructures increased and became longer (see Figure 3c, d). After adding the amount of tartaric acid to 1.0 g, a plenty of perfect PbTe feathers were obtained (see Figure 2a). The above facts clearly show that tartaric acid plays a crucial role in the electrodeposition of PbTe dendrites under the present potentiostatic conditions. In the preparation of the electrolyte, white PbTeO3 precipitates could be produced after mixing Pb(NO3)2 and Na2TeO3 solutions (see eq 1); and the amount of white precipitates did not obviously change even if excess tartaric acid was introduced. Based on chemistry knowledge, Pb2þ ions can react with excess tartaric acid to produce [Pb(C4H4O6)3]4 complex ions (eq 2). However, the stability constant of the complex is too small (Kstable = 5.01 104).21 Thus, white PbTeO3 precipitates cannot be efficiently converted into [Pb(C4H4O6)3]4 ions under the current experimental conditions. Pb2þ þ TeO3 2 ¼ PbTeO3 V ðwhiteÞ
ð1Þ
Pb2þ þ 3C4 H6 O6 ðtartaric acidÞ ¼ ½PbðC4 H4 O6 Þ3 4 þ 6Hþ
ð2Þ
In Li’s work,16c tartaric acid was considered to play two roles in the formation of dendritic PbTe structures: first, as complexant to form complexes with Pb2þ ions, which decreased the concentration of free Pb2þ ions and accordingly reduced the deposition rate of PbTe; second, as capping reagent to control the growth of PbTe due to specific interactions between PbTe nuclei and tartaric acid. Obviously, based on the above experimental results, there were few free Pb2þ ions in the system before tartaric acid was introduced. The introduction of tartaric acid could not efficiently reduce the concentration of free Pb2þ ions. Thus, the first role of tartaric acid could be ignored. Tartaric acid should mainly act as the capping reagent in the formation of featherlike PbTe dendrites. To confirm this conclusion, we prepared PbTe under the same electrodeposition conditions through using other complexants instead of tartaric acid. Parts a and b of Figure 4 depict the classical SEM images of the products obtained from the systems containing 1.0 g of citric acid and the disodium salt of ethylendiaminetetraacetic acid (EDTA-2Na), respectively. According to Li’s opinion, both citric acid and EDTA-2Na can also decrease the concentration of free Pb2þ ions such as tartaric acid, due to their coordination to Pb2þ ions. As a result, the deposition rate of PbTe can be reduced, too. As shown in Figure 4a and b, however, no dendritic PbTe nanostructures are obtained under the two cases. More interesting, after the white precipitates in the tartaric acid system were fully dissolved by proper amounts of diluted HNO3, perfect branchlike PbTe structures could be obtained under the same deposition potential and time (0.2 V, 5 min). The secondary, tertiary, and quartus branches can be clearly seen (Figure 4c). This result is in good agreement with those deposited by the galvanostatic model at the current density of 1.0 mA/m2 for 60 min or 3.33 mA/m2 for 30 min.16c Distinctly, plentiful free Pb2þ ions existed in the system after white precipitates were dissolved by HNO3. Here, it was impossible that the deposition rate of PbTe was decreased. Therefore, the formation of branchlike PbTe structures should be attributed to the capping function of tartaric acid. The above facts imply that tartaric acid should just act as
Figure 4. SEM images of the products prepared from the various systems: (a) 0.5 mmol of Pb(NO3)2 þ 1 mmol of Na2TeO3 þ 1.0 g of citric acid, (b) 0.5 mmol of Pb(NO3)2 þ 1 mmol of Na2TeO3 þ 1.0 g of EDTA-2Na and (c) 0.5 mmol of Pb(NO3)2 þ 1 mmol of Na2TeO3 þ 1.0 g of tartaric acid þ proper amounts of HNO3.
structure-directing reagent in the formation of featherlike PbTe dendrites in the current experiment. As pointed out by Li et al.,16c the specific interaction existed between PbTe nuclei and tartaric acid molecules. However, it was possible that the binding abilities of tartaric acid molecules to various facets of PbTe nuclei were different. Thus, some sites of PbTe nuclei were anchored by abundant tartaric acid molecules, and the other ones by only few molecules or none. In the former case, the growth rates of PbTe crystal were slowed down; and in the latter case, the fast growth rates were still retained. The different growth rates of various facets of PbTe nuclei led to the oriented growth of PbTe. On the other hand, the sites/planes of PbTe crystals anchored by tartaric acid molecules should be consistent during the crystal growth. As a result, PbTe dendrites appeared. 2145
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Figure 5. SEM images of the products prepared under the same conditions with different Pb2þ ion sources: (a) PbSO4 and (b) Pb(CH3COO)2.
3.3. Influences of the Pb2þ Ion Sources and the Deposition Potentials. It is noteworthy that the morphology of the final
product can be affected by the original Pb2þ ion source. Figure 5 shows representative SEM images of products prepared from the systems after Pb(NO3)2 has been replaced by Pb(CH3COO)2 and PbSO4, respectively, under the same depositing conditions. When PbSO4 was selected as lead ion source, the product presented filmlike structures made up of nanoparticles (Figure 5a). While Pb(CH3COO)2 was employed, the product was comprised of aggregated granules and rods with four arrises (Figure 5b). Further enlargement showed these rods were constructed by assembly of abundant nanoparticles (see the inset in Figure 5b). Distinctly, no featherlike PbTe dendrites are obtained in the above two cases. This implies that counteranions can affect the formation of dendritic PbTe structures, too. Usually, there are strong interactions between CH3COO/SO42 ions and Pb2þ ions.22 Thus, the deposition of PbTe in the tartaric acid system is affected by them. Furthermore, we also investigated the influences of depositing potentials on the morphology of the final product. Figure 6 shows representative SEM images of products prepared from the same system with various deposition potentials for 5 min. When the potential of 0.1 V was used, aggregated microspheres dispersed on the substrate and the yield was very low (see Figure 6a). After the potential was decreased to 0.12 V, abundant flowerlike products appeared and the yield markedly increased (Figure 6b). Further reducing the potential to 0.15 V, the product was almost composed of a great deal of featherlike dendrites (Figure 6c). When the potential of 0.2 V was employed, perfect featherlike PbTe dendrites with a high yield
Figure 6. SEM images of the products obtained from the systems containing 0.5 mmol of Pb(NO3)2 þ 1 mmol of Na2TeO3 þ 1.0 g of tartaric acid for 5 min at various potentials: (a) 0.1 V, (b) 0.12 V, and (c) 0.15 V.
were deposited (Figure 2a). The above experimental facts clearly show that the deposition potential can affect the growth of dendritic structures and the yield of PbTe. The lower potential is favorable for the formation of featherlike PbTe structures in the current work. 3.4. Time-Dependent Shape Evolution and the Growth Mechanism of Featherlike PbTe Dendrites. In order to investigate the growth process of featherlike PbTe dendrites under the present potentiostatic model, we observed the timedependent shape evolution of PbTe dendrites using a SEM technique. Figure 7 shows SEM images of the products deposited at the potential of 0.2 V for various durations. When the deposition time was less than 10 s, some connected nearspherical particles with a mean size of ∼200 nm were deposited (Figure 7a). After 20 s, the near-spherical particles disappeared 2146
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Figure 7. SEM of the products deposited from the system with 0.5 mmol of Pb(NO3)2 þ 1.0 mmol of Na2TeO3 þ 1.0 g of tartaric acid at the potential of 0.2 V for various durations: (a) 5 s, (b) 20 s, (c) 1.0 min, and (d) 3 min.
Scheme 1. Possible Growth Process of Featherlike PbTe Dendrites under the Current Potentiostatic Model
and many flowerlike structures appeared (Figure 7b). Namely, PbTe started to grow orientedly. After prolonging the time to 1 min, many long petals could be easily seen from SEM observations (Figure 7c), which should be ascribed to the ceaseless growth of flowerlike structures. Upon further prolonging the time to 3 min, abundant dendritic products had been obtained (Figure 7d). After 5 min, large-scale perfect featherlike PbTe dendrites were deposited (see Figure 2a). The above shape evolution clearly demonstrates the formation process of featherlike PbTe dendrites. In the initial stage, the deposited PbTe nanoparticles nucleated on the Cu substrate and grew into nearspherical particles. These near-spherical particles became the seeds for further growth of freshly produced PbTe. Since the
growth rates of various planes were different, PbTe nanoparticles orientedly grew on near-spherical particles under the assistance of tartaric acid molecules, which led to the appearance of flowerlike products (see Figure 7b and c). As a result of the continuous growth, perfect featherlike PbTe dendrites were finally formed. The potential used in the present work was 0.2 V, which was higher than 0.51 V. Here, in the light of previous literature,22,23 the reductions of Pb2þ to Pb and HTeO2þ to Te could be simultaneous on the surface of the cathode, which resulted in the formation of PbTe, due to a solid-state reaction between Pb and Te. The produced PbTe nucleated, grew, and aggregated into near-spherical particles, owing to big surface energies (see Figure 7a). Then, under the assistance of tartaric acid, these 2147
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4. CONCLUSIONS Featherlike PbTe dendrites have been successfully electrodeposited on Cu substrates at a large scale via a potentiostatic model at the potential of 0.2 V for 5 min under the presence of proper amounts of tartaric acid. It was found that the morphology of the product could be affected by some parameters, including complexants, Pb2þ ion sources, and deposition potentials. Experimental facts proved that tartaric acid mainly acted as the capping reagent in the formation of featherlike PbTe dendrites, which differed from the previous literature.16c Furthermore, under keeping the other conditions constant, increasing the amount of tartaric acid or decreasing the deposition potential availed the formation of featherlike PbTe dendrites. A timedependent shape evolution process showed that the growth of PbTe crystals underwent a process from near-spherical particles, to flowerlike particles, and finally to featherlike dendrites. This shape evolution experiment perfects the growth process of PbTe dendrites, owing to the shorter deposition times, and is favorable for the understanding of the formation mechanism of PbTe dendrites. The present work is a powerful supplement for the electrochemical deposition of PbTe materials, since it experimentally reversed the conclusions that the formation of PbTe dendrites was relevant to the electrode potential oscillation and that tartaric acid acted as both complexants to decrease the deposition rate of PbTe and capping reagents in the formation of PbTe dendrites.16c ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: (þ86)553-3869303.
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