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A covalently attached film based on poly(methacrylic acid)-capped Fe3O4 nanoparticles
Thin Solid Films 429 (2003) 167–173
A covalently attached film based on poly(methacrylic acid)-capped Fe3O4 nanoparticles Hao Zhang, Ruibing Wang, Gang Zhang, Bai Yang* Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 119 Jie Fang Road, Changchun 130023, PR China Received 29 October 2002; received in revised form 1 January 2003; accepted 19 January 2003
Abstract Poly(methacrylic acid) (PMAA)-capped Fe3 O4 nanoparticles were prepared by coprecipitation with PMAA in aqueous solution. Fe3O4 nanoparticles were further assembled with 2-nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) to form a photosensitive precursor film, by virtue of the coulombic attraction between the negatively charged PMAA surface capping agent and the cationic polyelectrolyte of NDR. Covalent bonds were formed under ultraviolet irradiation. As a result of polymer capping of the nanoparticles and covalent linkage, a highly stable multilayer structure was formed. Transmission electron micrographs and selected area electron diffraction pattern revealed the Fe3 O4 nanoparticles to be approximately 8 nm in diameter with a cubic phase structure. X-Ray photoelectron spectroscopy provided evidence for the presence of Fe3 O4 nanoparticles and NDR within the ultrathin films. The UV-visible spectroscopy and atomic force microscopy measurements supported the improvement of the stability of the film, which became impervious to polar solvents when the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Iron oxide; Multilayers; Nanostructures; Photo-cross-linking reaction
1. Introduction Recently, a promising area of materials research that deals with synthesis, characterization and assembly of a series of inorganic nanoparticles is rapidly emerging w1– 3x. Due to quantum confinement effects w4–6x, advanced materials based on nanoparticles with optical, electrical and magnetic properties have wide applications for both fundamental and industrial interest w7–9x. Generally speaking, these inorganic nanoparticles will be mostly used in the form of ultrathin films w3,7–10x. The preparation of layered multifunctional assemblies from nanoparticles by using standard layer-by-layer (LbL) assembly techniques provides a relatively easy and universal approach to the preparation of composite multilayer systems w3,7,8,11–16x. Experimentally, the LbL process is an adsorption and desorption balance of colloidal nanoparticles and polyelectrolytes at solidy *Corresponding author. Tel.: q86-431-8924107; fax: q86-4318923907. E-mail address: [email protected] (B. Yang).
liquid interfaces, which depends greatly on ambient factors such as pH value, hydrophilicyhydrophobic interactions and ionic strength as well w17,18x. These factors make controlled assembly difficult and limit the longterm stability of resulting ultrathin films. Thus, harnessing these functional films for real-word applications is still an important challenge, which demands both the strong affinity of particlesypolyelectrolyte in the LbL process and the durability of the resulting films w3,16x. The ability to prepare inorganic nanoparticles in the matrix of polymer gives the opportunity to design and fabricate novel materials, especially for LbL multilayer films w19–21x. Accordingly, the disadvantage of desorption of inorganic nanoparticles in the LbL process can be partly avoided w3x. Moreover, previous reports focusing on a novel method to fabricate covalently attached multilayer films based on polyelectrolytes, provides an efficient way to produce highly stable ultrathin films w22–27x. This method seems to have the effect of reinforcing multilayer structures of negatively charged polyelectrolytes with nucleophilic groups such as
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00059-2
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hydroxyl w22,23x, sulfonic acid w24x and especially carboxylic acid w25–27x. For the sake of fabricating a highly stable magnetic nanoparticle-based thin film, we have prepared typical magnetic nanoparticles of Fe3O4 by coprecipitation with poly(methacrylic acid) (PMAA) in aqueous solution. These PMAA-capped Fe3O4 nanoparticles were further assembled LbL in 2-nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) by virtue of Coulombic attraction, which result in a photosensitive precursor film. The linkages between the PMAA-capped nanoparticles and NDR changed from ionic bonds to covalent bonds under ultraviolet irradiation. As a consequence, the stability of this ultrathin film was improved dramatically.
2.3. Fabrication of a precursor film The LbL films were prepared at room temperature in the dark. A cleaned substrate (quartz slide, and singlecrystal silicon) was first modified by adsorption of PDAC (0.9 vol%) according to reference w14x. Then, these charged substrates were alternately dipped into Fe3O4 suspension and aqueous solutions of NDR (1 mgyml) for 20 min each, interrupted with water washing and N2 drying. The Fe3O4 nanoparticlesypolyelectrolyte multilayer films were fabricated. For AFM measurement, a cleaned negative mica substrate was dipped into aqueous solutions of NDR and Fe3O4 suspension for 20 min each, interrupted with water washing and N2 drying. Then one-bilayer film was prepared.
2. Experimental section
2.4. Characterization
2.1. Materials
Nanoparticle size, morphology and structure were measured via transmission electron micrographs (TEM) and selected area electron diffraction (SAED). A JEOL2010 electron microscope was used. The microscope was operated at 200 kV. Nanoparticles were deposited from dilute solution onto a film of Formvar䉸 supported by 200 mesh copper grids. One drop of dilute Fe3O4 suspension was deposited onto the grid and evaporated. UV-Visible transmission spectra were obtained using a Shimadzu 3100 UV-Vis-near-IR recording spectrophotometer. X-Ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB MK II spectrometer at room temperature. The base pressure was 10y7 Pa. The Al-Ka (1486.5 eV) was used as the excitation source. An analyzing pass energy of 10 eV with a step size of 0.1 eV was used for separate scans. The X-ray incidence angle was 908. Argon ion sputtering was not carried out prior to analysis in order to preserve the chemical composition and chemical bonds at the films. Binding energy calibration was based on C 1s at 284.6 eV. Atomic force microscopy (AFM) observations of the film surfaces were carried out with a commercial instrument (Digital Instrument, Nanoscope IIIa, Multimode), under ambient conditions at room temperature. All tapping mode images were measured at room temperature in air with the microfabricated rectangle crystal silicon cantilevers (from Nanosensor). Height images were obtained using a resonance frequency of approximate 365 kHz for the probe oscillation. For our investigations we adopted a set point at 1.34 V.
Poly(methacrylic acid, sodium salt) (PMAA) (30 wt.% aqueous solution, MW ca. 9500) was from Aldrich. 2-Nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) (Mn ca. 2500) was kindly provided by Prof. Weixiao Cao (College of Chemistry and Molecular Engineering, Peking University, Beijing) w23x. Poly(diallyldimethylammonium chloride) (PDAC) (MW ca. 100 000–200 000, 20 wt.% solution in water) was from Aldrich. Formvar䉸 wPoly(vinyl formal)x (;1 wt.% solution in 1,2-Dichloroethane) was from Fluka. Other reagents were all of analytical grade commercially available. 2.2. Preparation of PMAA-capped Fe3O4 nanoparticles Typical experimental procedures for the preparation of PMAA-capped Fe3O4 nanoparticles are as follows. First, an aqueous colloidal precursor was prepared by diluting a mixture of 0.6627 g FeCl2Ø4H2O, 1.083 g FeCl3, 3.4 ml 12.1 N HCl and 20 ml PMAA to 100 ml with oxygen-free water. The molar ratio of Fe2q yFe3q y PMAA (referring to carboxylate group) was 1:2:15. Then, 12.5-ml precursor solution was added dropwise to 125 ml deoxygenated water in the presence of 7.5 g NaOH under vigorous stirring for 10 min. After the black solid product was isolated with a magnet, the supernatant was decanted, and the precipitate was centrifugated at 4000 rev.ymin for 5 min. The black solid was washed using 200 ml deoxygenated water for three times with a centrifuge at 8000 rev.ymin. The product was washed using 100 ml 0.01 N HCl and centrifugated at 20 000 rev.ymin. The resulting PMAA-capped Fe3O4 nanoparticles were again peptized by water, and the final pH was approximately 4.4.
3. Results and discussion 3.1. Characterization nanoparticles
of
PMAA-capped
Fe3O4
The size distribution and crystalline structure of resulting Fe3O4 nanoparticles were determined by TEM and
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169
Fig. 1. (a) TEM image of PMAA-capped Fe3O4 nanoparticles, inset: the SAED pattern. (b) UV-Vis absorption spectra of PMAA-capped Fe3O4 suspension.
SAED pattern (Fig. 1a). The typical size of PMAAcapped Fe3O4 magnetic nanoparticles was approximately 8 nm. The SAED pattern exhibited broad diffuse rings owning to the small size of the particles, and the indexing of the lattice parameters patterns of SAED was well fitted to the cubic phase structure w28x. Fig. 1b represents the UV-Vis absorption spectra of the PMAAcapped Fe3O4 suspension, which indicates a typical absorption spectra of Fe3O4 nanoparticles w9,29x. This suspension was sensitive to pH values and could be stored over a period of several weeks. Among the advantages of our synthesis method by using PMAA as capping agent, the functionality of resulting nanoparticles should be mentioned. In our approach, the carboxyl acid groups in PMAA can interact with the Fe3O4 particles in acidic range. The PMAA chains wrapping around the Fe3O4 nanoparticles via the interaction between carboxyl and iron, which will provide a stabilizing effect to the Fe3O4. Moreover, the surplus carboxylates in PMAA chains provide multi-surface adsorption sites, which give the opportunity for further assembly with oppositely charged polyelectrolyte. Thus, the suspensions of Fe3O4 nanoparticles with polyanion capping agent can be prepared which is necessary for the deposition of Fe3O4 with low molecular weight polycation. 3.2. Fabrication of Fe3O4 nanoparticleypolycation photosensitive precursor films In previous work, the desorption of nanoparticles in the LbL process greatly limited the applications of these nanoscale films. Desorption occurred in our previous
effort to fabricate a covalently attached polyelectrolytey nanoparticle multilayer composite film that resulted in smaller increases of nanoparticles in each cycle w16x. The proposed mechanism of this desorption can be mainly attributed to the difference in surface wettability between the layers of polyelectrolyte and layers of small molecule-stabilized nanoparticles w18x. The relative hydrophobic polyelectrolyte might replace more highly water-soluble nanoparticles absorbed on substrate, and self-adsorption phenomenon occurs. In order to overcome the desorption of nanoparticles and provide a durable magnetic film, we present here the fabrication of LbL films based on PMAA-capped Fe3O4 nanoparticles and a photosensitive polyelectrolyte. This self-assembly strategy relied on a Coulombic attraction between the polycation of NDR and negatively charged Fe3O4 nanoparticle w19x. Fig. 2 shows the UVVis absorption spectra of a Fe3O4 yNDR multilayer film prepared on a quartz slide with different numbers of bilayers. The appearance of the absorption onsets at 325 and 485 nm illustrated that the Fe3O4 nanoparticle was assembled successfully into the thin film. The absorption peak at 375 nm was assigned to the p – p* transition of the diazonium group of NDR w26x, which provided evidence that NDR was also assembled into this film. In addition, the observed linear increase of diazonium group absorbance vs. the number of bilayers (inset of Fig. 2) indicated a stepwise and uniform assembling process. It can be estimated that the absorbance at 360 nm increased by ca. 0.035 for each LbL cycle. In comparison to previous studies of small molecule-capped nanoparticles w16x, the desorption of nanoparticles became less pronounced in this system due to the
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Fig. 2. UV-Vis absorption spectra of 2, 4, 6, 8, 10, 12, 14 and 16 layers of Fe3O4yNDR self-assembled films on a quartz slide. The first layer is a positively charged PDDA layer, then an Fe3O4 layer (dot) and a NDR layer (solid). Inset: absorbance at 360 and 485 nm of (CdTeyNDR)=n films vs. the number of corresponding layers.
Fig. 3. UV-Vis absorption spectra of a 16-bilayer film irradiated for different times. Irradiation time 0 min (dash), and 30 min (solid).
3.3. Photo-cross-linking reaction in the photosensitive precursor films
diazonium groups decomposed gradually with the decrease of the absorbance at 375 nm (absorption of Nq 2 group) and concomitant increase of the absorbance at 290 nm (Ph–O absorption). This photoreaction finished completely within 40 min and formed a covalent linkage. The proposed scheme of the conversion of ionic bonds to covalent bonds is shown in Fig. 4. FTIR spectroscopy was also applied to investigate the photoreaction between diazonium groups and carboxylate groups taking place in the films (not shown). The peak at 2170 cmy1 belonging to stretching vibration of the diazonium groups disappeared after irradiation, indicating the decomposition of diazonium groups. The absorption of carboxylate at 1585 cmy1 decreased due to the conversion of the ionic bonds to phenyl ester bonds
The advantage of the present approach was that the mechanism of the photo-cross-linking reaction between diazoresin and nucleophile induced by ultraviolet irradiation or heating has been widely studied in previous research w16,22–27x. As for the polycation of NDR used here, diazonium groups were easy to decompose under ultraviolet irradiation to form phenyl cations. These cations could react easily with carboxylates to convert ionic bonds to covalent bonds w23x. The photoreaction in multilayer films has been applied to a variety of chemicals including polyanion w26,27x, dendrimers w25x and nanoparticles with nucleophilic groups w16x. Here, a 16-bilayer film, based on NDR and PMAA-capped Fe3O4 nanoparticles, was selected as an example. A 16W ultraviolet lamp with 254-nm wavelength was used to irradiate the film at a distance of 2 cm for 40 min. The decomposition of diazonium groups and the formation of phenyl ester were recorded by UV-Vis absorption spectra (Fig. 3). Under UV irradiation, the
Fig. 4. The schematic scheme of the photoreaction of NDR and PMAA-capped Fe3O4 nanoparticles in a self-assembled film.
introduction of PMAA matrix. It was reasonable to speculate that PMAA-capped Fe3O4 nanoparticles could provide multi-surface adsorption sites resulting in a better surface adsorption on substrate in the deposition stage. The strong adsorption ability of PMAA-capped particles likely in turn affected the desorption of this deposited layer in the next stage leading to the less pronounced desorption. This result emphasizes the better surface adsorption ability of PMAA-capped Fe3O4 nanoparticles.
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171
Fig. 5. XPS data of a three-bilayer Fe3 O4 yNDR self-assembled film before UV irradiation. (a) N1s, and (b) Fe2p. Inset table: the molar ratio of NyFe in the thin film before and after 30 min UV irradiation.
w16x. Whereas, the increasing of the absorption at 1740 cmy1 indicated the formation of carboxylic ester groups w25–27x. This result indicates that after UV irradiation the original ionic bond in the multilayer films has been destroyed and a covalent linkage formed. The elemental composition of the Fe3O4 yNDR multilayer films was further analyzed by XPS, which was used previously to investigate the chemical nature of nanoparticles in ultrathin films w14–16x. Fig. 5 shows the XPS data for these Fe3O4 yNDR ultrathin films. The appearance of the characteristic XPS N1s peaks and Fe2p peaks at expected positions confirmed the existence of NDR and Fe3O4 nanoparticles in the film w30x. Therefore, this LbL assembly is a practical method for the preparation of magnetic thin films. Before UV irradiation, four kinds of nitrogen atoms with different chemical environments were found in XPS data (Fig. 5a), which is consistent with previous reports w23x. The molar ratio of NyFe in the thin film was approximately 2.5. This result more firmly established the existence of much Fe3O4 nanoparticles in the thin films. Under irradiation, diazonium groups would decompose along with the production of nitrogen gas. The theoretical Ny Fe value after irradiation should be as half as that before irradiation, providing the assumption that diazonium groups decomposed completely. In our experiment, the disappearance of N1s peaks at 402.6 eV confirmed the decomposition of diazonium groups. Moreover, the Ny Fe ratio from XPS data decreased from 2.5 to 1.4 in reasonable agreement with the theoretical result (inset of Fig. 5b).
experiment using a ternary mixture of H2O–DMF– ZnCl2 (3:5:2, wywyw) w16,26x. The amount of film etched out could be estimated easily from the change of the UV-Vis absorption spectra (Fig. 6). For a 16-bilayer film without UV irradiation, an extensive removal was observed after short time etching (such as 5 min), whereas almost no decrease was observed for the multilayer film with 40-min UV irradiation after 24-h etching. It meant the stability of the film towards polar solvents could be improved dramatically when the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. It should be mentioned that the decrease of PMAA-capped Fe3O4
3.4. The stability of covalently attached films The improvement of the stability of the film caused by UV irradiation was evaluated by a solvent etching
Fig. 6. UV-Vis absorption spectra of a 16-bilayer film before etching (solid), and after etching (dot) in a ternary mixture of H2O–DMF– ZnCl2 (3:5:2, wywyw) for 24 h at room temperature.
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Fig. 7. AFM (tapping mode) height images of NDRyFe3O4 one-bilayer film on mica, etched using a ternary mixture of H2O–DMF–ZnCl2 for 40 min (a) without UV irradiation and (b) after 40 min UV irradiation.
nanoparticles in the thin film after UV irradiation was less pronounced than that of previous small moleculecapped nanoparticles w16x. It might reflect the better adsorption nature of polymer-capped nanoparticles, which contributed greatly to the stability of resulting magnetic film, in addition to the covalent linkage existing in the film. The existence of Fe3O4 nanoparticles in the ultrathin film and the changing of linkages between the nanoparticles and NDR from ionic bonds to covalent bonds were further verified by AFM measurement. The Fe3O4 nanoparticles with slight aggregation could be clearly found in one-bilayer film with nearly full surface coverage. The surface coverage had no difference for both of the films with irradiation and without irradiation, which meant that the formation of a uniform multilayer film from NDR and PMAA-capped nanoparticles via Coulombic attraction was practicable. The one-bilayer film with 40-min solvent etching was also investigated by AFM measurement (Fig. 7). There was no obvious change for the film with 40-min UV irradiation, while approximately 70% nanoparticles scaled off for a film without irradiation. This direct proof verified that changing the linkages between the Fe3O4 nanoparticles and NDR from ionic bonds to covalent bonds dramatically enhances the stability of nanoparticle-based films, which was consistent with the UV-Vis spectra result. 4. Conclusions PMAA-capped Fe3O4 nanoparticles were prepared to avoid desorption in the LbL process. Then, Fe3O4 particles and NDR based ultrathin multilayer films were fabricated by the LbL technique. Under UV irradiation,
the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. The covalent linkage dramatically improved the long-term stability of this magnetic film towards polar solvents. Our results reveal that the stability of nanoparticle-based LbL films can be enhanced by two steps: introduction of nanoparticles into polymer matrix; and further formation of a covalent attachment. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 29925412, 200340062). We thank Prof. Dr Weixiao Cao at the College of Chemistry and Molecular Engineering, Peking University, Beijing for the help in providing necessary materials of NDR. References w1x J.H. Fendler, Chem. Mater. 13 (2001) 3196. w2x H. Weller, Angew. Chem. Int. Ed. Engl. 32 (1993) 41. w3x J.W. Ostrander, A.A. Mamedov, N.A. Kotov, J. Am. Chem. Soc. 123 (2001) 1101. w4x A.P. Alivisatos, Science 271 (1996) 933. w5x L.E. Brus, J. Phys. Chem. 98 (1994) 3575. w6x M. Nirma, L. Brus, Acc. Chem. Res. 32 (1999) 407. w7x M. Gao, B. Richter, S. Kirstein, H. Mohwald, ¨ J. Phys. Chem. B 102 (1998) 4096. w8x E. Hao, B. Yang, J. Zhang, X. Zhang, J. Sun, J. Shen, J. Mater. Chem. 8 (1998) 1327. w9x Y. Liu, A. Wang, R.C. Claus, Appl. Phys. Lett. 71 (1997) 2265. w10x B.O. Dabbousi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi, J. Phys. Chem. B 101 (1997) 9463. w11x H. Zhang, B. Yang, Thin Solid Films 418 (2002) 169.
H. Zhang et al. / Thin Solid Films 429 (2003) 167–173 w12x Y.P. Sun, E. Hao, X. Zhang, B. Yang, M.Y. Gao, J. Shen, Chem. Commun. (1996) 2381. w13x M.Y. Gao, M.L. Gao, X. Zhang, B. Yang, J. Shen, Chem. Commun. (1994) 2777. w14x E. Hao, H. Zhang, B. Yang, H. Ren, J. Shen, J. Colloid Interface Sci. 238 (2001) 285. w15x H. Zhang, E. Hao, B. Yang, J. Shen, Chem. J. Chin. Univ. 21 (2000) 1766. w16x H. Zhang, B. Yang, R.B. Wang, G. Zhang, X. Hou, L. Wu, J. Colloid Interface Sci. 247 (2002) 361. w17x G.Z. Mao, Y. Tsao, M. Tirrell, H.T. Davis, V. Hessel, H. Ringsdorf, Langmuir 11 (1995) 942. w18x J. Chen, G. Luo, W.X. Cao, J. Colloid Interface Sci. 238 (2001) 62. w19x L.I. Halaoui, Langmuir 17 (2001) 7130. w20x D.H. Chen, Y.Y. Chen, J. Colloid Interface Sci. 235 (2001) 9. w21x M.Y. Gao, Y. Yang, B. Yang, F. Bian, J. Shen, Chem. Commun. (1994) 2779.
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w22x J. Chen, W.X. Cao, Chem. Commun. (1999) 1711. w23x S. Cao, C. Zhao, W.X. Cao, Polym. Int. 45 (1998) 142. w24x J.Q. Sun, Z.Q. Wang, Y.P. Sun, X. Zhang, J. Shen, Chem. Commun. (1999) 693. w25x J.F. Wang, J.Y. Chen, X.R. Jia, W.X. Cao, M.Q. Li, Chem. Commun. (2000) 511. w26x J.Q. Sun, T. Wu, F. Liu, Z.Q. Wang, X. Zhang, J. Shen, Langmuir 16 (2000) 4620. w27x H. Luo, J. Chen, G. Luo, Y. Chen, W.X. Cao, J. Mater. Chem. 11 (2001) 419. w28x Z.H. Zhou, J. Wang, X. Liu, H.S.O. Chan, J. Mater. Chem. 11 (2001) 1704. w29x R.F. Ziolo, E.P. Giannelis, B.A. Weinstein, M.P. O’Horo, B.N. Ganguly, V. Mehrotra, M.W. Russell, D.R. Huffman, Science 257 (1992) 219. w30x J. Chastain (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Prairie, 1992.
A covalently attached film based on poly(methacrylic acid)-capped Fe3O4 nanoparticles Hao Zhang, Ruibing Wang, Gang Zhang, Bai Yang* Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 119 Jie Fang Road, Changchun 130023, PR China Received 29 October 2002; received in revised form 1 January 2003; accepted 19 January 2003
Abstract Poly(methacrylic acid) (PMAA)-capped Fe3 O4 nanoparticles were prepared by coprecipitation with PMAA in aqueous solution. Fe3O4 nanoparticles were further assembled with 2-nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) to form a photosensitive precursor film, by virtue of the coulombic attraction between the negatively charged PMAA surface capping agent and the cationic polyelectrolyte of NDR. Covalent bonds were formed under ultraviolet irradiation. As a result of polymer capping of the nanoparticles and covalent linkage, a highly stable multilayer structure was formed. Transmission electron micrographs and selected area electron diffraction pattern revealed the Fe3 O4 nanoparticles to be approximately 8 nm in diameter with a cubic phase structure. X-Ray photoelectron spectroscopy provided evidence for the presence of Fe3 O4 nanoparticles and NDR within the ultrathin films. The UV-visible spectroscopy and atomic force microscopy measurements supported the improvement of the stability of the film, which became impervious to polar solvents when the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Iron oxide; Multilayers; Nanostructures; Photo-cross-linking reaction
1. Introduction Recently, a promising area of materials research that deals with synthesis, characterization and assembly of a series of inorganic nanoparticles is rapidly emerging w1– 3x. Due to quantum confinement effects w4–6x, advanced materials based on nanoparticles with optical, electrical and magnetic properties have wide applications for both fundamental and industrial interest w7–9x. Generally speaking, these inorganic nanoparticles will be mostly used in the form of ultrathin films w3,7–10x. The preparation of layered multifunctional assemblies from nanoparticles by using standard layer-by-layer (LbL) assembly techniques provides a relatively easy and universal approach to the preparation of composite multilayer systems w3,7,8,11–16x. Experimentally, the LbL process is an adsorption and desorption balance of colloidal nanoparticles and polyelectrolytes at solidy *Corresponding author. Tel.: q86-431-8924107; fax: q86-4318923907. E-mail address: [email protected] (B. Yang).
liquid interfaces, which depends greatly on ambient factors such as pH value, hydrophilicyhydrophobic interactions and ionic strength as well w17,18x. These factors make controlled assembly difficult and limit the longterm stability of resulting ultrathin films. Thus, harnessing these functional films for real-word applications is still an important challenge, which demands both the strong affinity of particlesypolyelectrolyte in the LbL process and the durability of the resulting films w3,16x. The ability to prepare inorganic nanoparticles in the matrix of polymer gives the opportunity to design and fabricate novel materials, especially for LbL multilayer films w19–21x. Accordingly, the disadvantage of desorption of inorganic nanoparticles in the LbL process can be partly avoided w3x. Moreover, previous reports focusing on a novel method to fabricate covalently attached multilayer films based on polyelectrolytes, provides an efficient way to produce highly stable ultrathin films w22–27x. This method seems to have the effect of reinforcing multilayer structures of negatively charged polyelectrolytes with nucleophilic groups such as
0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00059-2
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H. Zhang et al. / Thin Solid Films 429 (2003) 167–173
hydroxyl w22,23x, sulfonic acid w24x and especially carboxylic acid w25–27x. For the sake of fabricating a highly stable magnetic nanoparticle-based thin film, we have prepared typical magnetic nanoparticles of Fe3O4 by coprecipitation with poly(methacrylic acid) (PMAA) in aqueous solution. These PMAA-capped Fe3O4 nanoparticles were further assembled LbL in 2-nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) by virtue of Coulombic attraction, which result in a photosensitive precursor film. The linkages between the PMAA-capped nanoparticles and NDR changed from ionic bonds to covalent bonds under ultraviolet irradiation. As a consequence, the stability of this ultrathin film was improved dramatically.
2.3. Fabrication of a precursor film The LbL films were prepared at room temperature in the dark. A cleaned substrate (quartz slide, and singlecrystal silicon) was first modified by adsorption of PDAC (0.9 vol%) according to reference w14x. Then, these charged substrates were alternately dipped into Fe3O4 suspension and aqueous solutions of NDR (1 mgyml) for 20 min each, interrupted with water washing and N2 drying. The Fe3O4 nanoparticlesypolyelectrolyte multilayer films were fabricated. For AFM measurement, a cleaned negative mica substrate was dipped into aqueous solutions of NDR and Fe3O4 suspension for 20 min each, interrupted with water washing and N2 drying. Then one-bilayer film was prepared.
2. Experimental section
2.4. Characterization
2.1. Materials
Nanoparticle size, morphology and structure were measured via transmission electron micrographs (TEM) and selected area electron diffraction (SAED). A JEOL2010 electron microscope was used. The microscope was operated at 200 kV. Nanoparticles were deposited from dilute solution onto a film of Formvar䉸 supported by 200 mesh copper grids. One drop of dilute Fe3O4 suspension was deposited onto the grid and evaporated. UV-Visible transmission spectra were obtained using a Shimadzu 3100 UV-Vis-near-IR recording spectrophotometer. X-Ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB MK II spectrometer at room temperature. The base pressure was 10y7 Pa. The Al-Ka (1486.5 eV) was used as the excitation source. An analyzing pass energy of 10 eV with a step size of 0.1 eV was used for separate scans. The X-ray incidence angle was 908. Argon ion sputtering was not carried out prior to analysis in order to preserve the chemical composition and chemical bonds at the films. Binding energy calibration was based on C 1s at 284.6 eV. Atomic force microscopy (AFM) observations of the film surfaces were carried out with a commercial instrument (Digital Instrument, Nanoscope IIIa, Multimode), under ambient conditions at room temperature. All tapping mode images were measured at room temperature in air with the microfabricated rectangle crystal silicon cantilevers (from Nanosensor). Height images were obtained using a resonance frequency of approximate 365 kHz for the probe oscillation. For our investigations we adopted a set point at 1.34 V.
Poly(methacrylic acid, sodium salt) (PMAA) (30 wt.% aqueous solution, MW ca. 9500) was from Aldrich. 2-Nitro-N-methyl-4-diazonium-formaldehyde resin (NDR) (Mn ca. 2500) was kindly provided by Prof. Weixiao Cao (College of Chemistry and Molecular Engineering, Peking University, Beijing) w23x. Poly(diallyldimethylammonium chloride) (PDAC) (MW ca. 100 000–200 000, 20 wt.% solution in water) was from Aldrich. Formvar䉸 wPoly(vinyl formal)x (;1 wt.% solution in 1,2-Dichloroethane) was from Fluka. Other reagents were all of analytical grade commercially available. 2.2. Preparation of PMAA-capped Fe3O4 nanoparticles Typical experimental procedures for the preparation of PMAA-capped Fe3O4 nanoparticles are as follows. First, an aqueous colloidal precursor was prepared by diluting a mixture of 0.6627 g FeCl2Ø4H2O, 1.083 g FeCl3, 3.4 ml 12.1 N HCl and 20 ml PMAA to 100 ml with oxygen-free water. The molar ratio of Fe2q yFe3q y PMAA (referring to carboxylate group) was 1:2:15. Then, 12.5-ml precursor solution was added dropwise to 125 ml deoxygenated water in the presence of 7.5 g NaOH under vigorous stirring for 10 min. After the black solid product was isolated with a magnet, the supernatant was decanted, and the precipitate was centrifugated at 4000 rev.ymin for 5 min. The black solid was washed using 200 ml deoxygenated water for three times with a centrifuge at 8000 rev.ymin. The product was washed using 100 ml 0.01 N HCl and centrifugated at 20 000 rev.ymin. The resulting PMAA-capped Fe3O4 nanoparticles were again peptized by water, and the final pH was approximately 4.4.
3. Results and discussion 3.1. Characterization nanoparticles
of
PMAA-capped
Fe3O4
The size distribution and crystalline structure of resulting Fe3O4 nanoparticles were determined by TEM and
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Fig. 1. (a) TEM image of PMAA-capped Fe3O4 nanoparticles, inset: the SAED pattern. (b) UV-Vis absorption spectra of PMAA-capped Fe3O4 suspension.
SAED pattern (Fig. 1a). The typical size of PMAAcapped Fe3O4 magnetic nanoparticles was approximately 8 nm. The SAED pattern exhibited broad diffuse rings owning to the small size of the particles, and the indexing of the lattice parameters patterns of SAED was well fitted to the cubic phase structure w28x. Fig. 1b represents the UV-Vis absorption spectra of the PMAAcapped Fe3O4 suspension, which indicates a typical absorption spectra of Fe3O4 nanoparticles w9,29x. This suspension was sensitive to pH values and could be stored over a period of several weeks. Among the advantages of our synthesis method by using PMAA as capping agent, the functionality of resulting nanoparticles should be mentioned. In our approach, the carboxyl acid groups in PMAA can interact with the Fe3O4 particles in acidic range. The PMAA chains wrapping around the Fe3O4 nanoparticles via the interaction between carboxyl and iron, which will provide a stabilizing effect to the Fe3O4. Moreover, the surplus carboxylates in PMAA chains provide multi-surface adsorption sites, which give the opportunity for further assembly with oppositely charged polyelectrolyte. Thus, the suspensions of Fe3O4 nanoparticles with polyanion capping agent can be prepared which is necessary for the deposition of Fe3O4 with low molecular weight polycation. 3.2. Fabrication of Fe3O4 nanoparticleypolycation photosensitive precursor films In previous work, the desorption of nanoparticles in the LbL process greatly limited the applications of these nanoscale films. Desorption occurred in our previous
effort to fabricate a covalently attached polyelectrolytey nanoparticle multilayer composite film that resulted in smaller increases of nanoparticles in each cycle w16x. The proposed mechanism of this desorption can be mainly attributed to the difference in surface wettability between the layers of polyelectrolyte and layers of small molecule-stabilized nanoparticles w18x. The relative hydrophobic polyelectrolyte might replace more highly water-soluble nanoparticles absorbed on substrate, and self-adsorption phenomenon occurs. In order to overcome the desorption of nanoparticles and provide a durable magnetic film, we present here the fabrication of LbL films based on PMAA-capped Fe3O4 nanoparticles and a photosensitive polyelectrolyte. This self-assembly strategy relied on a Coulombic attraction between the polycation of NDR and negatively charged Fe3O4 nanoparticle w19x. Fig. 2 shows the UVVis absorption spectra of a Fe3O4 yNDR multilayer film prepared on a quartz slide with different numbers of bilayers. The appearance of the absorption onsets at 325 and 485 nm illustrated that the Fe3O4 nanoparticle was assembled successfully into the thin film. The absorption peak at 375 nm was assigned to the p – p* transition of the diazonium group of NDR w26x, which provided evidence that NDR was also assembled into this film. In addition, the observed linear increase of diazonium group absorbance vs. the number of bilayers (inset of Fig. 2) indicated a stepwise and uniform assembling process. It can be estimated that the absorbance at 360 nm increased by ca. 0.035 for each LbL cycle. In comparison to previous studies of small molecule-capped nanoparticles w16x, the desorption of nanoparticles became less pronounced in this system due to the
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Fig. 2. UV-Vis absorption spectra of 2, 4, 6, 8, 10, 12, 14 and 16 layers of Fe3O4yNDR self-assembled films on a quartz slide. The first layer is a positively charged PDDA layer, then an Fe3O4 layer (dot) and a NDR layer (solid). Inset: absorbance at 360 and 485 nm of (CdTeyNDR)=n films vs. the number of corresponding layers.
Fig. 3. UV-Vis absorption spectra of a 16-bilayer film irradiated for different times. Irradiation time 0 min (dash), and 30 min (solid).
3.3. Photo-cross-linking reaction in the photosensitive precursor films
diazonium groups decomposed gradually with the decrease of the absorbance at 375 nm (absorption of Nq 2 group) and concomitant increase of the absorbance at 290 nm (Ph–O absorption). This photoreaction finished completely within 40 min and formed a covalent linkage. The proposed scheme of the conversion of ionic bonds to covalent bonds is shown in Fig. 4. FTIR spectroscopy was also applied to investigate the photoreaction between diazonium groups and carboxylate groups taking place in the films (not shown). The peak at 2170 cmy1 belonging to stretching vibration of the diazonium groups disappeared after irradiation, indicating the decomposition of diazonium groups. The absorption of carboxylate at 1585 cmy1 decreased due to the conversion of the ionic bonds to phenyl ester bonds
The advantage of the present approach was that the mechanism of the photo-cross-linking reaction between diazoresin and nucleophile induced by ultraviolet irradiation or heating has been widely studied in previous research w16,22–27x. As for the polycation of NDR used here, diazonium groups were easy to decompose under ultraviolet irradiation to form phenyl cations. These cations could react easily with carboxylates to convert ionic bonds to covalent bonds w23x. The photoreaction in multilayer films has been applied to a variety of chemicals including polyanion w26,27x, dendrimers w25x and nanoparticles with nucleophilic groups w16x. Here, a 16-bilayer film, based on NDR and PMAA-capped Fe3O4 nanoparticles, was selected as an example. A 16W ultraviolet lamp with 254-nm wavelength was used to irradiate the film at a distance of 2 cm for 40 min. The decomposition of diazonium groups and the formation of phenyl ester were recorded by UV-Vis absorption spectra (Fig. 3). Under UV irradiation, the
Fig. 4. The schematic scheme of the photoreaction of NDR and PMAA-capped Fe3O4 nanoparticles in a self-assembled film.
introduction of PMAA matrix. It was reasonable to speculate that PMAA-capped Fe3O4 nanoparticles could provide multi-surface adsorption sites resulting in a better surface adsorption on substrate in the deposition stage. The strong adsorption ability of PMAA-capped particles likely in turn affected the desorption of this deposited layer in the next stage leading to the less pronounced desorption. This result emphasizes the better surface adsorption ability of PMAA-capped Fe3O4 nanoparticles.
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Fig. 5. XPS data of a three-bilayer Fe3 O4 yNDR self-assembled film before UV irradiation. (a) N1s, and (b) Fe2p. Inset table: the molar ratio of NyFe in the thin film before and after 30 min UV irradiation.
w16x. Whereas, the increasing of the absorption at 1740 cmy1 indicated the formation of carboxylic ester groups w25–27x. This result indicates that after UV irradiation the original ionic bond in the multilayer films has been destroyed and a covalent linkage formed. The elemental composition of the Fe3O4 yNDR multilayer films was further analyzed by XPS, which was used previously to investigate the chemical nature of nanoparticles in ultrathin films w14–16x. Fig. 5 shows the XPS data for these Fe3O4 yNDR ultrathin films. The appearance of the characteristic XPS N1s peaks and Fe2p peaks at expected positions confirmed the existence of NDR and Fe3O4 nanoparticles in the film w30x. Therefore, this LbL assembly is a practical method for the preparation of magnetic thin films. Before UV irradiation, four kinds of nitrogen atoms with different chemical environments were found in XPS data (Fig. 5a), which is consistent with previous reports w23x. The molar ratio of NyFe in the thin film was approximately 2.5. This result more firmly established the existence of much Fe3O4 nanoparticles in the thin films. Under irradiation, diazonium groups would decompose along with the production of nitrogen gas. The theoretical Ny Fe value after irradiation should be as half as that before irradiation, providing the assumption that diazonium groups decomposed completely. In our experiment, the disappearance of N1s peaks at 402.6 eV confirmed the decomposition of diazonium groups. Moreover, the Ny Fe ratio from XPS data decreased from 2.5 to 1.4 in reasonable agreement with the theoretical result (inset of Fig. 5b).
experiment using a ternary mixture of H2O–DMF– ZnCl2 (3:5:2, wywyw) w16,26x. The amount of film etched out could be estimated easily from the change of the UV-Vis absorption spectra (Fig. 6). For a 16-bilayer film without UV irradiation, an extensive removal was observed after short time etching (such as 5 min), whereas almost no decrease was observed for the multilayer film with 40-min UV irradiation after 24-h etching. It meant the stability of the film towards polar solvents could be improved dramatically when the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. It should be mentioned that the decrease of PMAA-capped Fe3O4
3.4. The stability of covalently attached films The improvement of the stability of the film caused by UV irradiation was evaluated by a solvent etching
Fig. 6. UV-Vis absorption spectra of a 16-bilayer film before etching (solid), and after etching (dot) in a ternary mixture of H2O–DMF– ZnCl2 (3:5:2, wywyw) for 24 h at room temperature.
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Fig. 7. AFM (tapping mode) height images of NDRyFe3O4 one-bilayer film on mica, etched using a ternary mixture of H2O–DMF–ZnCl2 for 40 min (a) without UV irradiation and (b) after 40 min UV irradiation.
nanoparticles in the thin film after UV irradiation was less pronounced than that of previous small moleculecapped nanoparticles w16x. It might reflect the better adsorption nature of polymer-capped nanoparticles, which contributed greatly to the stability of resulting magnetic film, in addition to the covalent linkage existing in the film. The existence of Fe3O4 nanoparticles in the ultrathin film and the changing of linkages between the nanoparticles and NDR from ionic bonds to covalent bonds were further verified by AFM measurement. The Fe3O4 nanoparticles with slight aggregation could be clearly found in one-bilayer film with nearly full surface coverage. The surface coverage had no difference for both of the films with irradiation and without irradiation, which meant that the formation of a uniform multilayer film from NDR and PMAA-capped nanoparticles via Coulombic attraction was practicable. The one-bilayer film with 40-min solvent etching was also investigated by AFM measurement (Fig. 7). There was no obvious change for the film with 40-min UV irradiation, while approximately 70% nanoparticles scaled off for a film without irradiation. This direct proof verified that changing the linkages between the Fe3O4 nanoparticles and NDR from ionic bonds to covalent bonds dramatically enhances the stability of nanoparticle-based films, which was consistent with the UV-Vis spectra result. 4. Conclusions PMAA-capped Fe3O4 nanoparticles were prepared to avoid desorption in the LbL process. Then, Fe3O4 particles and NDR based ultrathin multilayer films were fabricated by the LbL technique. Under UV irradiation,
the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds. The covalent linkage dramatically improved the long-term stability of this magnetic film towards polar solvents. Our results reveal that the stability of nanoparticle-based LbL films can be enhanced by two steps: introduction of nanoparticles into polymer matrix; and further formation of a covalent attachment. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 29925412, 200340062). We thank Prof. Dr Weixiao Cao at the College of Chemistry and Molecular Engineering, Peking University, Beijing for the help in providing necessary materials of NDR. References w1x J.H. Fendler, Chem. Mater. 13 (2001) 3196. w2x H. Weller, Angew. Chem. Int. Ed. Engl. 32 (1993) 41. w3x J.W. Ostrander, A.A. Mamedov, N.A. Kotov, J. Am. Chem. Soc. 123 (2001) 1101. w4x A.P. Alivisatos, Science 271 (1996) 933. w5x L.E. Brus, J. Phys. Chem. 98 (1994) 3575. w6x M. Nirma, L. Brus, Acc. Chem. Res. 32 (1999) 407. w7x M. Gao, B. Richter, S. Kirstein, H. Mohwald, ¨ J. Phys. Chem. B 102 (1998) 4096. w8x E. Hao, B. Yang, J. Zhang, X. Zhang, J. Sun, J. Shen, J. Mater. Chem. 8 (1998) 1327. w9x Y. Liu, A. Wang, R.C. Claus, Appl. Phys. Lett. 71 (1997) 2265. w10x B.O. Dabbousi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi, J. Phys. Chem. B 101 (1997) 9463. w11x H. Zhang, B. Yang, Thin Solid Films 418 (2002) 169.
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