Shell Coaxial Nanowires Prepared by Anodic

Dec 30, 2010 - *Phone/Fax: +86-10-82616517. .... Zinc bis(diethyldithiocarbamate) [Zn(S2CNEt2)2, 99%] was purchased ... Electropolymerization of pyrro...
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ARTICLE pubs.acs.org/JPCC

Polypyrrole/ZnS Core/Shell Coaxial Nanowires Prepared by Anodic Aluminum Oxide Template Methods Dezhong Zhang,†,‡ Liang Luo,†,‡ Qing Liao,† Hao Wang,†,‡ Hongbing Fu,*,† and Jianniao Yao*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China

bS Supporting Information ABSTRACT: The polypyrrole/ZnS core/shell coaxial nanowires are fabricated through a two-step process with the assistance of anodic aluminum oxide (AAO) templates. First, ZnS nanotube arrays are synthesized within AAO templates by using the metal organic chemical vapor deposition (MOCVD) method. Then, polypyrrole (PPy) is electrochemically deposited into as-prepared ZnS nanotubes, creating PPy/ZnS core/shell coaxial nanowires. The morphology and structure of PPy/ZnS coaxial nanowires are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Fourier-transform infrared (FTIR) spectroscopy verifies that the in-plane deformation vibration of the pyrrole (Py) ring shows a blue shift from 1144 cm-1 in PPy nanowires to 1173 cm-1 in the PPy/ZnS coaxial nanowires. In X-ray photoelectron spectroscopy analysis (XPS), the changes of the N 1s peak and S 2p peak reveal an electron transfer from the ZnS shell to the PPy core in PPy/ZnS coaxial nanowires, which lowers the reduction potential of PPy at the interface to -0.2 V as compared with -0.88 V observed for pure PPy nanowires. The current-voltage (I-V) characteristics of the ZnS nanotube show the semiconducting behavior, while ohmic behavior is observed for the PPy nanowire. Remarkably, the I-V characteristics of a single core-shell coaxial nanowire exhibit a rectification behavior, probably due to electron transfer between PPy and ZnS. Therefore, this kind of core/shell coaxial nanowires, which combine properties of core and shell materials of different components, might be applicable for nanosccale optoelectronics.

’ INTRODUCTION In recent years, one-dimensional (1D) organic/inorganic hybrid nanomaterials, such as segmented nanowires,1-4 have attracted a great deal of interest, as they combine inorganic semiconductors with functional organic molecules to produce a new class of materials which have distinct properties that were not observed in the individual components on either the nanosize or bulk scale.5-7 For example, Li et al.8 fabricated CdS-PPy (polypyrrole) segmented nanowires as a light-controlled diode. Mirkin and co-workers9 prepared Au-PPy-Cd-Au multisegmented nanowires, which show either diode or resistor properties depending upon their compositions and spatial distribution of the different compositional blocks. Recently, 1D core/shell nanomaterials of inorganic semiconductors,10 which provide a multiphase anisotropic interface much larger than that in segmented heterostructures, have been widely applied as building blocks for nanoscale optoelectronics.11-14 For example, Mallouk15 and co-workers fabricated Au-CdS-Au/SiO2 core/shell coaxial nanowires and demonstrated a “wrap-around gate” approach to field effect transistors. The Javey group16 has shown the applications of CdTe/CdS coaxially vertical nanopillar arrays in solar energy conversion. Generally, the synthesis of these inorganic core/shell nanomaterials had involved two steps: the formation of the 1D core followed by the r 2010 American Chemical Society

solution-phase or vapor-phase growth of the shell material. Especially, the second step requires a good lattice match between the core and shell semiconductor materials.2 This is rather difficult to be achieved for the organic/inorganic hybrid system. As compared with inorganic core/shell nanomaterials, fabrication of hybrid organic/ inorganic core/shell ones had met with limited success.17 Herein, we focused on PPy (a p-type organic semiconductor) and ZnS (an n-type inorganic semiconductor) core/shell coaxial nanowires, prepared within AAO templates by combining chemical vapor deposition with in situ electrochemical deposition techniques. The length and diameter of the PPy/ZnS can be controlled through the diameter of the template's nanopores and the deposition time. The schematic procedure for the preparation of core/shell nanowires is shown in Scheme 1. After the PPy nanowires electrochemically deposited into as-prepared ZnS nanotubes, electron transfer occurs across the interface from the PPy core to the ZnS shell, resulting in positive shifts in both the redox and oxidation potentials of PPy for PPy/ZnS coaxial nanowires as compared with those for the pure PPy nanowires. Received: August 23, 2010 Revised: December 2, 2010 Published: December 30, 2010 2360

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The Journal of Physical Chemistry C Scheme 1. Procedure for the Preparation of PPy/ZnS Core/ Shell Coaxial Nanowires

Moreover, the single core/shell nanowire exhibits different properties from those of the core or the shell nanostructures. The formation of the P-N junction at the interface between the ZnS nanotube and the PPy nanowire leads the single PPy/ZnS core/shell nanostructures to exhibit the rectification behavior.

’ EXPERIMENTAL SECTION Materials. Zinc bis(diethyldithiocarbamate) [Zn(S2CNEt2)2, 99%] was purchased from Tokyo Chemical Industry China (Shanghai) Co., Ltd. Lithium perchlorate (LiClO4, 99%) was obtained from Alfa Aesar China (Tianjin) Co., Ltd. Platinum (Pt, 99%) foil, pyrrole (C4H5N, A. R.), and acetonitrile (C2H3N, A. R.) were purchased from Beijing Chemical Co., China. Alumina membranes, with a pore diameter of 200 nm and thickness of 60 μm, are commercially available from Whatman Co. (Clifton, NJ). All chemicals were used without further purification. Purified water was obtained through a Millli-Q water purification system (Millipore, Shanghai, China) and had a resistivity of 18.2 ΩM cm-1. Synthesis of ZnS Nanotubes. The synthesis of ZnS nanotubes was carried out in a quartz tube mounted inside a horizontal tube furnace. In a typical experiment,18 Zn(S2CNEt2)2 (0.35 g) in a crucible was placed at the upstream end, and the AAO template was placed vertically at the central high-temperature zone of the quartz tube. Prior to heating, the system was flushed with highpurity N2 gas for 1 h to eliminate O2. The gas flow rate and pressure inside the tube were kept at 45 standard cubic centimeters per minute (sccm) and 60 Pa, respectively, during the process of CVD. The system was then heated to 400 °C at the central region of the furnace at a rate of 40 °C min-1 and held for 4 h. According to the temperature gradient from the center to the end of the furnace, Zn(S2CNEt2)2 was situated at about 150 °C. After deposition, the template was collected when the furnace was cooled to room temperature. Each side of this template was ionetched for 1 h (Ar buffer gas, 100 W, 5 Pa) to remove the surface layer and to expose the tubes within the template by using a reaction ion etching system (L-451D-L, ANELVA). Then, one side of the template was coated with a thin layer of Pt (ca. 20 nm thick) to serve as a working electrode by using a LEICA EM SCD500 High Vacuum Sputter Coater. Synthesis of PPy/ZnS Core/Shell Coaxial Nanowires. Electropolymerization of pyrrole was performed in a three-electrode, single compartment electrochemical cell at 25 °C. The electrolyte solution was 0.l M LiClO4 in acetonitrile. The ZnS nanotubes within templates as a working electrode were connected to a

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potentiostat (660A CHI instruments, Shanghai). Electropolymerization of the 0.5 M pyrrole monomer was performed potentiostatically at a constant potential of 0.85 V versus saturated calomel reference electrode (SCE) by using Pt foil as a counter electrode in acetonitrile solution. After 10 h, polypyrrole was filled into ZnS nanotubes, and the PPy/ZnS core/shell coaxial nanowires were formed. Characterizations. Scanning electron microscope observations were obtained by using a HITACHI F-4300 field emission scanning electron microscope (FESEM) operating at an acceleration voltage of 15 kV. Specimens for FESEM were prepared as follows: after removing the surface layer on the template by ion etching, the sample was fixed to a piece of copper tape by using epoxy and soaked in 6 M aqueous sodium hydroxide for 4 h to remove the alumina template completely. After rinsing with purified water carefully, the tape was attached to an FESEM specimen holder and was sputtered with a layer of platinum to prevent charging during SEM imaging. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed on a JEOL JEM-1011 electron microscope at an acceleration of 100 kV to gain a sufficient transmission. The highangle annular dark-field scanning TEM (HAADF-STEM) and element mapping images were characterized by a Philips Tecnai G2 F20 U-TWIN electron microscope at an acceleration of 200 kV. The TEM samples were prepared by first dissolving the aluminum oxide templates in 6 M NaOH to free the encapsulated nanotubes. The nanotube-containing material was then washed with distilled water, filtered, and suspended in 1 mL of water by applying ultrasonic. A small drop of the suspension was placed on a carbon-coated TEM grid for observation. Crystal structures were identified in an X-ray diffractometer (XRD, X-Pert, PANalytic, Netherlands) with Cu KR radiation (40 kV, 30 mA). Fourier transform infrared (FTIR) spectroscopy was recorded with a BRUKER TENSOR 27 FTIR spectrometer. Prior to analysis, dried samples were mixed with KBr and pressed into a tablet. Cyclic voltammetry (CV) curves were performed on a CHI 660A potentiostat. Testing was carried out at potentials between -1.0 and 0.6 V vs SCE in a three-electrode, single-compartment electrochemical cell using a deoxygenated 0.4 M KCl aqueous electrolyte. X-ray photoelectron spectroscopy data (XPS) were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al KR radiation. The base pressure was about 3  10-9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Current-voltage (I-V) characteristics of devices were recorded with a Keithley 4200 SCS and a Micromanipulator probe station in dark conditions at room temperature.

’ RESULTS AND DISCUSSION Figure 1a and b shows the top views of as-prepared ZnS nanotube arrays before and after dissolving AAO templates. It can be seen that the pores of the nanotubes are uniform and arranged in a continuous, parallel, and well-ordered way, with a diameter of 200-300 nm, consistent with the pore sizes of the AAO template used (Figure S1, see Supporting Information). The length of ZnS nanotubes synthesized here is about 60 μm, as shown in Figure 1c. They correspond well to the length of AAO templates. Figure 1d shows a typical TEM image of a single ZnS nanotube. It can be seen that the nanotube is straight and has uniform diameter along its length. The outer diameter and wall thickness of the nanotube are ca. 250 and 10 nm, respectively. In addition, the corresponding SAED pattern (the inset of 2361

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Figure 3. FTIR spectra of (a) PPy nanowires embedded in AAO and (b) PPy/ZnS core/shell coaxial nanowires embedded in AAO.

Figure 1. SEM images of ZnS nanotubes (a) before and (b) after dissolving the AAO templates. The inset is the enlarged view. (c) Crosssection image of ZnS nanotube arrays after dissolving the AAO templates. (d) TEM image of ZnS nanotube. The inset is the SAED pattern of the ZnS nanotube.

Figure 2. SEM images of PPy/ZnS core/shell coaxial nanowires (a) before and (b) after dissolving the AAO templates. The insets are the enlarged views. (c) TEM image of a single PPy/ZnS core/shell coaxial nanowire. The inset is the SAED pattern of the PPy/ZnS nanowire. (d) HAADF-STEM image of a single PPy/ZnS nanowire. The insets are the STEM-assisted EDS line analysis of element zinc and element nitrogen for the single PPy/ZnS nanowire. (e) Element mapping of a single PPy/ ZnS core/shell coaxial nanowire.

Figure 1d) shows that the ZnS nanotubes are polycrystalline, and the diffraction rings observed in the SAED pattern can be indexed as the (100), (002), and (110) lattice planes of hexagonal Wurtzite ZnS. These results demonstrate that large-scale, high-density, and well-aligned ZnS nanotube arrays have been fabricated by a template-based CVD method using Zn(S2CNEt2)2 as a single source of molecular precursors. Figure 2a and b depicts the top views of PPy/ZnS core/shell coaxial nanowires. The low magnification images show that a

uniform PPy/ZnS nanowire nanoarray is fabricated in large scale. The insets are the enlarged views. It can be seen that PPy components are successfully deposited into as-prepared ZnS nanotubes. Further structure information and characterization of the single PPy/ZnS core/shell nanowire were performed by TEM, as shown in Figure 2c and d. Figure 2c depicts the TEM image of a single PPy/ZnS core/shell coaxial nanowire, which was completely filled with PPy. The inset is the corresponding SAED pattern of the PPy/ZnS nanowire. The diffraction rings agree with those of the ZnS nanotube (the inset of Figure 1d) because the inner PPy core is amorphous in nature. The crystalline properties of ZnS nanotubes and PPy/ZnS coaxial nanowires were also proved by XRD measurements (Figure S2, Supporting Information). Figure 2d shows the HAADF-STEM image of a single PPy/ZnS nanowire. One can clearly observe that the inner PPy nanowires adhere closely to the outer ZnS nanotubes, in good agreement with the SEM results. The insets show the STEMassisted EDS line analysis of element zinc and element nitrogen for the single PPy/ZnS nanowire (EDS line analysis of element sulfur and carbon is in Figure S3, Supporting Information). It can be seen that element zinc is concentrated at the fringe of the ZnS/PPy nanowire, and element nitrogen is concentrated in the middle core of the ZnS/PPy nanowire. Figure 2e shows element mapping image analysis. The yellow and jacinth colored areas show the dispersion of element zinc and element nitrogen, respectively, which confirm the enriched areas of both ZnS and PPy in the core/shell nanowires. These results reveal that polypyrrole has been electrochemically deposited into as-prepared ZnS nanotubes. Moreover, the filling length of PPy components into the ZnS nanotube was tunable by adjusting the total current passed through the alumina membrane as a function of the deposition time. For example, the filling lengths of PPy components into ZnS nanotube were 10, 20, and 40 μm at a deposition time of 2, 4, and 8 h, respectively, in good agreement with the calculations according to the Faraday Law (Figure S5, Supporting Information). In comparison with the PPy/ZnS core/shell coaxial nanowires, pure PPy nanowires were synthesized in AAO templates by electrochemical methods. The SEM image of PPy nanowires is shown in Figure S7 (Supporting Information). The FTIR spectrum of AAO templates and ZnS into AAO templates is shown in Figure S8 (Supporting Information). The FTIR spectra of the above two samples are completely accordant.19-21 Because ZnS has no absorption above 400 cm-1, the FTIR spectrum of ZnS nanotubes into AAO shows the absorption of the AAO. Figure 3a shows the FTIR spectrum of PPy nanowires embedded in AAO. The peaks at 1638 and 1546 cm-1 belong to 2362

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Figure 4. N 1s spectra of (a) PPy nanowires and (c) PPy/ZnS core/ shell coaxial nanowires. The S 2p spectra of (b) ZnS nanotubes and (d) PPy/ ZnS core/shell coaxial nanowires. All samples are embedded in AAO.

the CdC stretching vibration and Py ring skeletal vibration, respectively. The band at 1120 cm-1 is attributable to the C-N stretching vibration.22 The absorption peaks appearing at 1144 cm-1 are assigned to the in-plane deformation vibration of the Py ring.23 The peak at 1038 cm-1 could be assigned to the C-H deformation vibration.24 The characteristic absorption vibrations of ClO4- doping appear at 1090 and 630 cm-1.25,26 In contrast with Figure 3a, the inplane deformation vibration of the Py ring in PPy/ZnS (Figure 3b) shows a blue shift from 1144 to 1173 cm-1. To further confirm the interaction between ZnS and PPy, we have used XPS analytical techniques. In the XPS spectra, the N 1s peak (400.3 eV) in the PPy nanowires (Figure 4a) is characteristic of C-NHþ-C groups. The binding energy at 399.6 eV in Figure 4c is approximate to neutral C-N (399.5 eV).27-29 A pronounced difference is also observed between S 2p XPS specra of the ZnS nanotubes and PPy/ZnS nanowires (Figure 4b and d). Figure 4b shows a sharp and symmetrical S 2p peak (161.9 eV) in ZnS nanotubes, which corresponds to S2- groups. However, the binding energy changes to 163.1 eV in PPy/ZnS nanowires (Figure 4d), which may be assigned to the sulfur element (163.9 eV).30 These data illustrate that the chemical environment of sulfur and nitrogen elements transforms after PPy is deposited inside ZnS nanotubes, which suggests that static attraction interactions similar to hydrogen bonds do occur between the electron pair of the S2- atom and the empty orbital of the Nþ atom within polypyrrole/ZnS core/shell coaxial nanowires. Consequently, the blue shift of the in-plane deformation vibration of the Py ring in the FTIR spectrum can be accounted for by the weakly inductive, mesomeric, and steric effects of neighboring S2- ions. Because S2- ions are not connected with the Py ring in terms of chemical bond but are compounded in nanoscale, mesomeric, inductive, and steric effects which tend to change the electron cloud density of the Py ring cause a blue shift of the in-plane deformation vibration of the Py ring. Figure 5 compares the cyclic voltammogram (CV) results of pure PPy and core/shell PPy/ZnS nanowire arrays within the AAO template. The CV curve of PPy/ZnS naowires (dashed line) was found to be obviously different from that of pure PPy nanowires (solid line). (i) The reduction and oxidation potentials of PPy components shifted from -0.88 and 0.07 V in pure PPy naowires to -0.68 and 0.25 V in PPy/ZnS core/shell structures, respectively. (ii) An additional reduction peak around -0.2 V was observed in PPy/ZnS nanowires. These data suggest that the ZnS shell significantly alters the electrochemical properties of the PPy core in our hybrid structures. Note that the

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Figure 5. Cyclic voltammograms of PPy nanowires (solid line) and PPy/ZnS core/shell coaxial nanowires (dashed line) arrayed in AAO for both samples in 0.4 M KCl aqueous electrolyte at a scan rate of 1 mV/S.

Figure 6. (a) Schematic diagram of a top-contact nanoscale device. (b) Optical image and (c) SEM image of a single PPy/ZnS core/shell coaxial nanowire with top-contact electrodes. (d) I-V characteristic curves of a single PPy nanowire (b), ZnS nanotube (3), and PPy/ZnS nanowire (9) measured in dark conditions at room temperature.

electrolyte ions not only act as the conducting media but also participate in the reaction during the reduction and oxidation process of PPy. Usually, the oxidation and reduction processes of PPy are involved in adsorption and desorption of chlorine anions, respectively, as shown in reactions 1 and 2.31,32 PPy þ Cl - - e f PPy þ Cl ð1Þ PPy þ Cl - þ e f PPy þ Cl -

ð2Þ

Because of the existence of the ZnS shell, the diffusion of chlorine anions in (adsorption) and out of (desorption) the PPy core becomes difficult in core/shell structures. Therefore, both the reduction and oxidation potentials show more positive shifts in core/shell structures than those in pure PPy wires.33-35 As mentioned above, 2363

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The Journal of Physical Chemistry C electron transfer occurs across the interface from the PPy core to the ZnS shell. Although it had not been fully understood yet, the reduction peak at -0.2 V observed in core/shell nanowires probably comes from PPy moieties at the interface, as a result of the charge transfer from the ZnS shell to PPy at the interface. In any event, these results suggest that a combination of ZnS and PPy has a strong influence on the electron exchange properties of core/shell hybrid structures.36 Figure 6a shows a schematic diagram of a top-contact nanoscale device configuration for I-V measurements. Device fabrication was performed on a Micromanipulator probe station.37 The optical image and SEM image shown in Figure 6b and c, respectively, indicate a single-wire device with top-contact electrodes. For comparison, the I-V characteristics of the single ZnS tube and PPy wire were also measured. As shown in Figure 6d, the I-V curve of the ZnS nanotube shows semiconducting behavior (triangle), while an ohmic behavior is observed for the PPy nanowire (circle). From the slope of the I-V curves for the PPy wire, the resistivity of a single PPy nanowire is estimated to be 5.4  106 Ω 3 cm. Remarkably, the I-V characteristic curve of the single core-shell coaxial nanowire (square) shows a rectification behavior due to the formation of the hybrid junction between the outer semiconducting ZnS shell and the inner PPy core.38,39 Electron transfer occurs across the interface from ZnS to PPy and results in an equilibrium between the Fermi level of the PPy and the ZnS.40,41 As a consequence, an electrically charged layer and a built-in electric field were formed at the interface, giving rise to the observed rectification behavior of the single PPy/ZnS nanowire.42

’ CONCLUSIONS In summary, we systematically synthesize multicomponent core/shell coaxial structures that contain inorganic semiconductors and conducting polymers via a template-assisted in metal organic chemical vapor deposition and in situ electrochemical deposition. SEM and TEM results forcefully support the formation of the core/shell structure of PPy/ZnS. The electron transfer between PPy and ZnS is conformed by FTIR and XPS experiments. This electron transfer in PPy/ZnS coaxial nanowires results in positive shifts in the redox potentials of PPy, compared with the pure PPy nanowires. Moreover, this also leads to the rectification behavior in the single PPy/ZnS core/shell nanowires. We believe that this kind of core/shell coaxial nanowire would be valuable for the current efforts to apply the 1D nanomaterials in photoelectronics. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM images of AAO templates and PPy nanowires, XRD patterns, the STEM-assisted EDS line analysis for a single PPy/ZnS nanowire, the detailed tunable procedure, and FTIR spectra of AAO templates and ZnS nanotubes arrayed into AAO. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone/Fax: þ86-10-82616517. E-mail: [email protected]. cn; [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 90301010, 20373077, 20873163,

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90606004), the Chinese Academy of Sciences (``100 Talents'' program), and the National Research Fund for Fundamental Key Project 973 (2006CB806202, 2006CB932101).

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