CdS nanoparticles

PII: S0968-5677(98)00089-3

Supramolecular Science 5 (1998) 623—626  1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/98/$19.00

A new approach to fabrication of a self-organizing film of heterostructured polymer/CdS nanoparticles Huiming Xiong, Zhen Zhou, Zhiqiang Wang, Xi Zhang* and Jiacong Shen Key Lab for Supramolecular Structure and Spectroscopy, Department of Chemistry, Jilin University, Changchun 130023, People’s Republic of China

In this paper, we alternately deposit transition metal Cd> neutralized polyelectrolytes and ligands pyridine contained polymer via the formation of complexes, and by sequential reaction with HS gas, in situ fabricate CdS nanoparticles/polymer heterostructured film. The driving force for the construction of multilayered films is based on covalent coordination.  1998 Elsevier Science Limited. All rights reserved. (Keywords: polyelectrolytes; ion-exchange; heterostructure)

INTRODUCTION Semiconductor nanoparticles have been studied extensively for their special properties such as chemical, electrical, optical, magnetic, and electro-optical properties which differ from either the individual molecules or bulk species, due to the quantum size effect and surface effect. Also the assembly of nanoparticles into inorganic/ organic heterostructure or superlattice is of great importance to design and fabricate advanced electronic, optical, and mechanical materials. There are two well-developed basic approaches to fabrication of heterostructured nanoparticle films. The first involves the preparation of discrete soluble clusters and the modification of their surface with charged species and then alternative deposition with oppositely charged materials, for example polyelectrolytes, to construct multilayered films. The second involves fabricating an organic film containing metal ions which is then exposed to reactive gas, the semiconductor nanoparticles are generated in the film directly as reactive gas diffuses through the film and reacts with the entrained metal ions. This is a general method in using LB films or casting films as matrix to fabricate superlattice. Polymers as multifunctional materials can offer a choice of building up layered structures through different types of interactions, e.g. electrostatic interaction, hydrogen-bonding bridges, charge-transfer complexes, or covalent bonds. In this paper, we used an alternative deposition of transition metal Cd> neutralized polyelectrolytes and ligands pyridine contained polymer via the formation of complexes, and sequential reaction

*Author to whom correspondence should be addressed. Fax: 04318923907; e-mail: [email protected]

with H S gas, in situ construction of CdS nanoparticles/  polymer heterostructured film, which is schematically shown in Figure 1. The driving force for the construction of multilayered films is based on covalent coordination. We believe that this method further develops the Decher’s concept, and a lot of reactive polymer and suitable metal ions can be used to fabricate self-organizing films and finally advanced nanosized hybrid with multi-properties can be achieved. The use of polymeric materials can provide an advantage for the fabrication of multilayered film with reduced defect formation and propagation. Moreover, its individual layer thickness can be easily adjusted by changing concentration, molecular weight and ionic strength and so on.

EXPERIMENTAL 40 mg poly(sodium styrene sulfonate) (molecular weight 70,000) abbreviated as PSS mixed with 44 mg cadmium chloride was dissolved in 20 ml H O for 24 h. The water  is twice-distilled after being additionally purified by a Millipore ion-exchange system. Cd> exchanged with Na> of poly(sodium styrene sulfonate) in solution. Then the solution was dialyzed for 48 h. ICP emission spectroscopy was used to check if the dialysis was completed. The mole ratio S vs Cd> in polyelectrolytes is about 2.4 : 1. Poly(vinyl pyridine) (molecular weight 100,000) abbreviated as PVP was dissolved in methanol with a concentration of 5.3;10\ M (unit mole concentration). Freshly cleaned quartz, CaF slide or silicon were  immersed into a 0.5 vol% aqueous cationic poly(ethyleneimine) (PEI) for 1 h. After rinsing with Milli-Q water, the PEI-modified substrate was immersed into an aqueous solution of cadmium neutralized poly(styrene-4 sulfonate) in a concentration of 5.5;10\ M

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A new approach to heterostructured film: H. Xiong et al.

Figure 1 Schematic illustrations of the alternating PSS(Cd) /PVP multilayers and PSS-CdS nanoparticles/PVP heterostructure 

(unit mole concentration) for 30 min, at pH6, then rinsed with Milli-Q water and dried under filtered N flow.  Further, the resulting substrate was dipped into a solution of poly(vinyl pyridine) for 30 min, rinsed in methanol and dried under filtered N flow. An alternating  PSS(Cd) /PVP multilayered film can be obtained by  repeating these two steps in a cyclic fashion. PSS-CdS nanoparticles/PVP multilayered film was obtained by the exposure of PSS(Cd) /PVP multi layered film in a flow of H S gas for 30 min at room  temperature, then evacuated to remove excess H S gas.  Small angle X-ray diffraction experiment was conducted on Rigaku X-ray diffractometer (D/max cA, using copper Ka-radiation of a wavelength 1.542 A> ). The UV—Vis absorption spectra were recorded on a Shimadzu 31 000 UV—VIS—NIR spectrophotometer. IR spectra were measured with a Bruker IFS-66V FTIR spectrometer.

RESULTS AND DISCUSSION The multilayered films were first prepared by consecutive adsorption of poly(cadmium 4-styrene-sulfonate) (PSS(Cd) ) and poly 4-vinylpyridine (PVP). Because  both PSS(Cd) and PVP have characteristic absorption  in the UV—Vis region, UV—Vis spectroscopy was used to follow the depositing process (Figure 2). Two absorption peaks at 225 and 256 nm correspond to PSS(Cd) and  PVP, respectively. From the spectra we can clearly see that the two polymers were deposited alternately. The linear increase of the absorbance with the number of layers (insert) demonstrated that the consecutive absorption was a uniform process. The driving force for building up PSS(Cd) /PVP  multilayers was identified by infrared spectra. Figure 3 shows the infrared spectra of PSS(Cd) , PVP and  PSS(Cd) /PVP multilayers. There was a new band  peaked at 1613 cm\ for the PSS(Cd) /PVP alternat

Figure 2 UV—Vis absorption spectra of alternating PSS(Cd) /PVP  multilayers. The number of layers deposited is from 3 to 10, from bottom to top. Inset: absorbance vs. number of layers at 225 nm, 256 nm

ing multilayers. It was the aromatic carbon—nitrogen stretch absorption band that shifted to 1613 cm\ region in multilayers. Many groups have studied the intermolecular interaction between specific function groups like pyridine and transition metal neutralized sulfonate linked on two polymers, which can prompt miscibility or partial miscibility from polymers that would otherwise be immiscible due to the formation of coordination bonds. According to these publications, we knew that the 1613 cm\ absorption was directly related to the specific interactions between the pyridine and the transition metal neutralized sulfonate group. In our experiment, it was found that without cadmium ions PSSNa and PVP cannot form alternating multilayered films. It means that nontransition metal counter-ions such as sodium do not

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Figure 4 UV—Vis absorption spectra of PSS(Cd) /PVP film of eight  layers (a) and that of after reaction with H S gas (b) 

Figure 3 IR spectra of PSS(Cd) , PVP and PSS(Cd) /PVP multi  layers from top to bottom

have specific interaction with pyridine. The interaction, which results from the formation of a specific coordination complex, requires a transition metal such as zinc and copper for the counterions. Thus we can conclude that the specific coordination between the pyridine and the Cd> neutralized sulfonate groups is responsible for building up the multilayer. Figure 4 is the UV—Vis spectra of PSS(Cd) /PVP  multilayer after the reaction with H S gas. From the  spectra we can see that besides the characteristic absorption of sulfonate and pyridine groups, there appeared a new absorption edge at about 395 nm. The blue shifted absorption edge compared with bulk CdS indicates the Q-CdS generates in the multilayered film, which is due to the quantum size effect . The small angle X-ray diffraction pattern of multilayers is shown in Figure 5, which provided additional evidence for the presence of a well layered structure in PSS(Cd) /PVP multilayered film and PSS-CdS  nanoparticles/PVP multilayered film. Curve a is the diffraction pattern of PSS(Cd) /PVP multilayered film  and curve b is that of PSS—CdS nanoparticles/PVP multilayered film. From the data of X-ray diffraction we calculated that the d-spacing of PSS(Cd) /PVP and  PSS—CdS nanoparticles/PVP multilayers was about 25.5 and 28 nm, respectively. The spacing of PSS—CdS nanoparticles/PVP multilayered film was larger than that of PSS(Cd) /PVP multilayered film. There were  many orders of diffraction peaks for PSS(Cd) /PVP 

Figure 5 Small angle X-ray diffraction patterns of the alternating PSS(Cd) /PVP multilayered film (curve a) and PSS—CdS nanopar ticles/PVP heterostructure (curve b)

multilayer and PSS—CdS nanoparticles/PVP multilayered film, which indicates that these films possessed a well-ordered layer structure. By alternating the absorption of oppositely charged macromolecules for fabrication of ordered film, Decher et al. used salts such as NaCl, KCl, BaCl , MnCl , MgCl to increase the thick   ness of film in the growth process and explained that the addition of salts led to the formation of coils from chains which, in turn, provided the formation of thick layers during adsorption. In our experiment, the introduction of Cd> may lead to polymer chains aggregation and Cd> acted as a slightly crosslinked point. Therefore it is thought that the larger d-spacing is reasonable in this case. Based on the ion-exchange properties of the sulfonate group, metal ions can be introduced into the polymer

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A new approach to heterostructured film: H. Xiong et al. containing the sulfonate group, then the polymer containing nanoparticles are synthesized after reaction with H S gas. For example, Nafion was used as a polymer  matrix, by ion-exchange with Cd> or Pb> and further reaction with H S gas Nafion/CdS nanoparticles or  Nafion/PbS nanoparticles have been synthesized. By utilizing the ion-exchange property of the sulfonate group to prepare PSS(Cd) , taking advantage of co ordination bond between Cd> and pyridine group to construct a self-organizing multilayer of PSS(Cd) /  PVP, we have then in situ prepared PSS-CdS nanoparticles/PVP hybrid film by diffusing H S gas into the  multilayered films to react with Cd>. Recently, we have successfully fabricated polymer/Cu S nanoparticles and  polymer/PbS nanoparticles heterostructured thin film based on this method. It is anticipated that the method is a powerful strategy for fabrication of ordered inorganic/organic heterostructured thin films.

ACKNOWLEDGEMENTS The authors are thankful for the support by the National Natural Science Foundation of China.

REFERENCES 1 Bard, A.J. Ber Bensen-Ges, Phys. Chem. 1988, 92, 1194; Henglein, A. Chem. Rev. 1989, 89, 1861; Brus, L.E. Appl. Phys. A 1991, 53, 465; Kataoka, T., Tokizaki, T. and Nakamura, A. Phys. Rev. B 1993, 48, 2815; Weller, A. and Eychmu¨ller, A. in ‘Advances in Photochemistry’ Vol. 20 (Eds D.C Neckers, D.H. Volman, and G.V. Bu¨nau), 1995

2 Kimizuka, N. and Kunitake, T. Adv. Mater. 1996, 8, 89; Fendler, J.H. and Meldrum, F.C. Adv. Mater. 1995, 7, 607; Feldheim, D.L., Grabar, K.C., Natan, M.J. and Mallouk, T.E. J. Am. Chem. Soc. 1996, 118, 7640; Peng, X.G., Guan, S.Q., Chai, X.D., Jiang, Y.S. and Li, T.J. J. Phys. Chem. 1992, 96, 3170 3 Fendler, J.H. Chem. Mater. 1996, 8, 1616; Schmitt, J., Decher, G., Dressick, W.J., Brandow, S.L., Geer, R.E., Shashidhar, R. and Calvert, J.M. Adv. Mater. 1997, 9, 61 4 Smotkin, E.S., Lee, C., Bard, A.J., Campion, A., Fox, M.A., Mallouk, T.E., Webber, S.E. and White, J.M. Chem. Phys. ¸ett. 1988, 152, 265; Ichinose, I., Kimizuka, N. and Kunitake, T. J. Phys. Chem. 1995, 99, 3736 5 Decher, G. and Hong, J.D. Makromol. Chem. Macromol. Symp. 1991, 46, 321; Schmitt, J., Gru¨newald, T., Kjaer, K., Pershan, P., Decher, G. and Lo¨sche, M. Macromolecules 1993, 26, 7058; Decher, G. Science 1997, 277, 1232; Gao, M.L., Kong, X.X., Zhang, X. and Shen, J.C. ¹hin Solid Films 1994, 244, 815; Kong, W., Zhang, X., Gao, M.L., Zhou, H., Li, W. and Shen, J.C. Makromol. Chem. Rapid. Commun. 1994, 15, 405 6 Stockton, W.B. and Rubner, M. Macromolecules 1997, 30, 2717 7 Shimazaki, Y., Mitsushi, M., Ito, S. and Yamamoto, M. ¸angmuir 1997, 13, 1385 8 Makowski, H.S., Lundberg, R.D., Westerman, L. and Bock, J. in ‘Ions in Polymers’, (Ed A. Eisenberg), American Chemical Society, Washington, D.C., 1980, Series No. 187, p. 3; Lu, X. and Weiss, R.A. Macromolecules 1991, 24, 5763; Register, R.A., Weiss, R.A., Li, C. and Cooper, S. J. Poly. Sci.: Part B: Polym. Phys. 1989, 27, 1911; Peiffer, D.G., Duvdevani, I., Agarwal, P.K. and Lundberg, R.D. J. Poly. Sci.: Polym. ¸ett. Ed. 1986, 24, 58; Jiang, M., Zhou, C.L. and Zhang, Z.Q. Polymer Bulletin 1993, 30, 455 9 Weller, H., Schmidt, H.M., Koch, U., Fojtik, A., Baral, S., Henglein, A., Kunath E. and Weiss, K. Chem. Phys. ¸ett. 1986, 124, 557; Vossmeyer, T., Katsikas, L., Giersig, M., Chemseddine, A., Diesner, K., Popovic, I.G., Eychmuller, A. and Weller, H. J. Phys. Chem. 1994, 98, 7665; Wang, Y. and Herron, N. J. Phys. Chem. 1991, 95, 525 10 Lvov, Y.M., Decher, G., Crystallography Reports 1994, 39, 696; Lvov, Y.M., Decher, G. and Mo¨hwald, H. ¸angmuir 1993, 9, 481 11 Mau, A.W.H., Huang, C.B., Kakuta, N., Bard, A., Campion, A., Fox, M.A., White, J.M. and Webber, S.E. J. Am. Chem. Soc. 1984, 106, 6537; Wang, Y., Suna, A., Mahler, W. and Kasowski, R. J. Chem, Phys. 1987, 87, 7315; Hilinski, E.F., Lucas, P.A. and Wang, Y. J. Chem. Phys. 1988, 89, 3435; Miyoshi, H., Yamachika, M., Yoneyama, H. and Morl, H. J. Chem. Soc. Faraday ¹rans. 1990, 86, 815

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