Electrochemical assembly of nano-organized poly-o-phenylenediamine films

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Abstract

Potential-dependent surface-enhanced Raman scattering spectra reveal that the orientation of o-phenylenediamine (OPD) molecules adsorbed on a Au electrode depends on the electrode potential and electropolymerization of OPD can be carried out at +0.3 V. The electrochemical-assembly technique was successfully applied to the fabrication of a nano-structured o-phenylenediamine oligomer (POPD) film on a bare Au substrate and on a p-aminothiophenol (PATP) self-assembled Au substrate in phosphate buffer solution (pH=7.0). Electrochemistry and scanning tunneling microscopy measurements indicate that a POPD/Au(111) surface displays a nano-scale dot array structure. At the same time, a POPD/PATP/Au(111) surface shows a nano-scale line array structure.

Introduction

We report here a preliminary study of the preparation of an nano-organized o-phenylenediamine (POPD) oligomer film using the electrochemical-assembly (ECA) technique [1]. To our knowledge, this is the first report of a nano-organized POPD film in two dimensions.

Organized conducting oligomers or polymers films have attracted considerable interest because of their outstanding properties and large potential applications as wires, sensors, memories, logic operators, catalysts, light-emitting devices and flexible liquid crystal light valves 2, 3. As one of the main conjugated polymers containing a benzene ring, POPD is a ladder polymer with various characteristic functions, e.g., pH response, electrochemical diode property, electron-transfer mediation, anticorrosion of metals, permselectivity for a dissolved species, protection against photo-corrosion of n-type semiconductor electrodes, electrochromic property, etc. 4, 5. A large amount of work was devoted during the past decade to the characterizations and applications of POPD polymer films in the fields mentioned above. However, the surface topography of the POPD films usually formed by potential cycling in strong acid solution containing OPD monomer exhibits non-organized structure [6].

There is great demand for electrochemists to find a simple, feasible and low-cost technique to fabricate organized two-dimensional (2D) conducting oligomer films on a nano-scale. With this in mind, we designed a potential pulse sequence for the ECA technique to construct OPD oligomer film that is comprised of nano-structured units whose arrangement is organized in two dimensions. This is a novel way of “having many electric pulses but few oxidation charges at each”. We have found that, according to in situ surface-enhanced Raman scattering (SERS) spectra, the orientation of OPD molecules absorbed on Au electrodes very strongly depended on the electrode potential and OPD can be absorbed on the Au surface by a `end-on' way or `tilt-on' way under some definite potential. Therefore, it is possible that a potential pulse sequence is applied to such an OPD absorbed electrode inducing the electropolymerization of the OPD molecules and the arrangement of the oligomer structure units is organized due to the interaction among them. Electrochemistry and scanning tunneling microscope (STM) measurements show that 2D nano-organized OPD oligomer films have been successfully fabricated using the ECA technique. The OPD oligomer film assembled on a bare Au surface shows a nano-scale dot array structure. At the same time, the OPD oligomer films formed on a PATP SAM covering a Au surface exhibits a nano-scale line array structure.

Section snippets

Experimental

OPD (analytical regent) and PATP (purchased from Sigma) were used without further purification. All the other chemicals used were analytical regents and the aqueous solutions were prepared using 18 MΩ Millipore filtered water.

SERS spectra were obtained by a confocal microprobe Raman system (LabRam I) using the excitation line of 632.8 nm from an internal He–Ne laser. Considering high laser power will yield great heat inducing the polymerization of OPD, the laser power on the electrode surface

Results and discussion

Potential-dependent SERS spectra of OPD adsorbed on a Au electrode in 0.2 M PBs are shown in Fig. 1. The spectra were recorded at potentials ranging from −0.9 to +0.3 V. The bands at 1596, 1498 and 1032 cm−1 are assigned to νC–C (the ring stretching vibration), νC–C+δC–H (the in-plane C–H deformation mode) and δC–H, respectively. These three bands almost disappear at +0.1 V implying that the adsorbing orientation of OPD on Au electrode is changed from `end-on' or `tilt-on' to `face-on'. At +0.3

Conclusion

Potential-dependent SERS spectra reveal that the orientation of the OPD molecules adsorbed on a Au electrode is changed from the `end-on' or `tilt-on' way to the `face-on' way at a positive potential up to +0.1 V. In addition, remarkable electro-oxidation of OPD can be carried out at +0.3 V.

Based on the in situ SERS spectra, a potential pulse sequence was designed to electrochemically assemble nano-organized OPD oligomer films in two dimensions. The relationship of electropolymerization versus

Acknowledgements

This work was supported by the National Natural Science Foundation of China (29703006 and 29833060) and Ministry of Education Grants (99177). The authors sincerely thank Prof. Binwei Mao and Miss Jing Tang for their great help in STM measurements. We gratefully thank Prof. Zhongqun Tian and Mr. Jianlin Yao for their kind help in SERS experiments. Our thanks are also extended to Dr. Zhaoxiong Xie for valuable discussions.

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