liquid interface

Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 7–12

Formation of a monolayer containing nanoparticles of europium complex at an air/liquid interface Wen-Zhong Lan, Hong-Guo Liu, Kong-Zhang Yang * Institute of Colloid and Interface Chemistry, Shandong University, Jinan, 250100, People’s Republic of China Received 29 October 1996; accepted 10 June 1997

Abstract On a surface of Eu(bpy) (H O)3+ liquid solution, a functional ultrathin film was obtained, by means of the n 2 m electrostatic interaction between arachidic acid (AA) and the rare earth complex ion. The surface pressure–area (p–A) isotherms, the fluorescence spectrum and FT-IR spectrum of LB films indicated that the luminous complex ions were assembled into the films at the air/liquid interface. Round nanoparticles were found in the films through transmission electron microscopy. The diameter of the particles, which varies from 20 to 160 nm, is related to the concentration of Eu(bpy) (H O)3+ , the interval between spreading and compressing t , and the time of film equilibrated at a n 2 m int target surface pressure t . © 1998 Elsevier Science B.V. All rights reserved. equ Keywords: Europium complex; Nanoparticle; Monolayer

1. Introduction Nanoscale particles show a lot of possible applications in many research fields [1]. The nanoparticles of many metal compounds, such as metal oxides, metal sulphides, and metal selenides, etc., which are magnetic and ferrocelectric, have been widely studied [2]. Metal complexes, especially rare earth complexes, display distinctive properties of optics, electronics and magnetism. However, there has been a lack of study on nanoparticles of metal complexes [3]. The Langmuir–Blodgett (LB) technique, which is a useful method to form nanoparticles of metal compounds [2], has been used extensively to study the functional organized molecular assemblies of metal complexes [4–18]. The metal complexes show distinctive spectral properties in LB films compared with those in the * Corresponding author. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 7 ) 0 0 19 3 - 3

bulk phase [10–16 ]. These spectral differences have often been only due to the ordered arrangement of the matel complexes in LB films. It has been shown that the fluorescence emission of nanoparticles of Eu(DBM ) is greatly different from that 3 of the bulk phase, indicating that the size of the particles has a great influence on the properties [3]. Therefore, it is probable that the new features of the spectral properties of metal complexes in LB films are not only related to the ordered arrangement of molecules, but also to the size of the aggregates formed in the film. It is of great importance to investigate the formation of the aggregates of metal complexes in films [18], especially to control the growth of the particles. One of the useful methods to assemble the ordered films containing metal complexes is to utilize the electrostatic interactions between the charged metal complex ions in the subphase and the amphiphilic materials which have an opposite

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W.-Z. Lan et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 7–12

charge at the air/liquid interface [10–12,19]. In this paper, the composite monolayer of arachidic acid (AA)/complex was obtained by utilizing the electrostatic interaction between the cation of europium bipyridine complex in the subphase and the carboxylic group at the air/liquid interface. The nanoparticles of the AA/complex were investigated, and the factors which affect the size are discussed.

2. Experimental section Eu O (99.9%) was purchased from the Yuelong 2 3 Chemical Plant (Shanghai), arachidic acid (AA, >99%) and 2,2∞-bipyridine (bpy, analytical reagent) were purchased from the Shanghai Chemical Reagent Plant and used as-received. Two complex subphases were used in this paper, which were prepared by mixing the ethanol solution of bpy with the aqueous solution of EuCl 3 (obtained from Eu O ) in a molar ratio of 2 3 Eu( III )/bpy of 1:3, and diluting with distilled water. The ethanol/water ratios are less than 0.2% in the subphases. The concentrations of the complex ion in the complex subphases are 5×10−5 M and 1.5×10−4 M. The pH values of the complex subphases are found to be 5.7. The fluorescence emission spectra of the two subphases show the characteristic emission peaks of Eu(III ) compared with the spectrum of aqueous solution of EuCl , confirming that the complex of Eu(III ) 3 with bpy formed in the subphase. The experiments were carried out by using a round trough of NIMA 2000 at 20±1°C. The surface pressure–area (p–A) isotherms were obtained by spreading the chloroform solution of AA onto the subphase surface, and compressing the monolayer after a certain time with the compression rate of 25 cm2 min−1. The area–time (A–t) isobars were measured at a certain target pressure. The solvent was allowed to evaporate for 60 min, then the monolayer was compressed to 25 mN m−1 and equilibrated at the target pressure for 240 min before being transferred onto solid substrates. The Y-type LB films of ten layers on glass plate and 30 layers on CaF plate, for the 2 measurements of fluorescence emission spectrum

and FT-IR spectrum respectively, were fabricated by using the vertical dipping method at 25 mN m−1 with a transfer speed of 10 mm min−1. The transfer ratios are about 0.8–1. The fluorescence emission spectrum of the LB film was obtained on a Hitachi 850 fluorescence spectrophotometer (Japan) with the excitation wavelength of 330 nm. The FT-IR spectrum of the LB film was recorded on a Nicolet 710 FT-IR spectrophotometer ( USA); a blank CaF plate was used 2 as reference. Monolayers for transmission electron microscopy ( TEM ) (Model JEM-100 cx II, Japan) were deposited onto 230-mesh copper grid covered with formvar by the subphase lowing (SL) method.

3. Results and discussion 3.1. Monolayer behaviour of AA on the complex subphase The surface pressure–area (p–A) isotherms of AA monolayers on the complex subphase surface are different from that on pure water surface, as shown in Fig. 1. The collapse pressures are lower and the mean molecular areas are bigger than those on a pure water surface. With a higher concentration of the complex ion in the subphase and a longer interval between spreading and compressing (t ), the difference is more obvious, and int the liquid phase part in the curve becomes longer. These results indicate that the complex ion is

Fig. 1. p–A isotherms of AA monolayers on the surface of the complex subphase of 5×10−5 M (a) and 1.5×10−4 M (b).

W.-Z. Lan et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 7–12

Fig. 2. A–t isobars of AA monolayers on the surface of the 1.5×10−4 M complex subphase at 20 mN m−1: (a) t =480 min; (b) t =10 min. int int

incorporated into the monolayer, probably due to the attraction between the AA in the film and the complex ion in the subphase. The big hydrophilic group (the complex ion) was attached to the carboxylic group of AA due to the reaction. When the monolayer was compressed, the hydrophobic long chains packed loosely, leading to the lower collapse pressure and the longer liquid phase than those of AA on the pure water surface. The area–time (A–t) isobars for AA monolayer on the complex subphase surface at 20 mN m−1 are shown in Fig. 2. The sharp area decrease process at the beginning corresponds to the compression process of the monolayer. The monolayers are stable. It is found that the mean molecular areas are different because of the different t . The result int is consistent with that obtained from the p–A isotherms. 3.2. Characteristics of LB film The fluorescence emission spectrum of the AA/complex LB film prepared on the complex subphase was measured with the excitation wavelength of 330 nm. The characteristic emission peaks of Eu(III ) complex appear at 593 nm, 613 nm and 652 nm, corresponding to the 5D 7F , 5D 7F and 5D 7F transitions 0 1 0 2 0 3 respectively, indicating that the europium bipyridine complex exists in the LB film.

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Fig. 3. The FT-IR spectrum of AA/complex LB film.

The FT-IR spectrum of AA/complex LB film prepared on the complex subphase is shown in Fig. 3; the spectral data are summarized in Table 1. It is found that vibration peaks at 1588, 1569 and 1434 cm−1 appear in the spectrum. Compared with those of pure bipyridine (1578, 1552, 1450 cm−1) [20], the first two peaks shift to high wave number with 10 and 17 cm−1 respectively, and the peak at 1434 cm−1 shifts to low wave number with 16 cm−1, suggesting that the bipyridine coordinated with the Eu(III ) in the subphase. Furthermore, a vibration band at 1490 cm−1 appears in the spectrum, which is a new band corresponding to the complex [20]. Moreover, the vibration peaks at 1636 and 1622 cm−1, which are attributed to vibrations of H O [20], indicate that 2 the complex contains coordinated H O. The for2 mula of the complex ion in the subphase can be deduced as Eu(bpy) (H O)3+ from these results. n 2 m The strong asymmetrical stretch vibration peaks of –COO− appear at 1540 and 1417 cm−1 and the symmetrical stretch vibration peak of –COO− appears at 1453 cm−1, suggesting that a large amount of AA molecules in the monolayer on the complex subphase reacted with the Eu(bpy) (H O)3+ ion. Two weak vibration bands n 2 m at 1731 and 1697 cm−1 show that some AA molecules still exist in the monolayer. 3.3. TEM observations of monolayers The AA monolayer on pure water surface is homogeneous. However, the round particles

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Table 1 FT-IR spectral data (cm−1) of AA/complex LB film Bpy powder [20] LB film Assignment

1578 1731

1697

–COOH

1636 1612 HO 2

1588

appeared in the monolayers on the surfaces of the complex subphases. Fig. 4 shows the TEM micrographs of the AA monolayers on the 1.5×10−4 M complex subphase surface with different t of 15 min and 60 min. Round particles int with almost same diameter of about 20–30 nm appear in the two micrographs. The particles are thicker and more abundant in the micrograph of t =60 min than in that of t =15 min, clearly int int

Fig. 4. The TEM micrographs of monolayers on the complex subphase surface.

1552 1569 1551 bpy

1450 1540 1519 –COO−

1648 1490 complex

1469

1453

1434

–CH – 2

–COO−

bpy

indicating that the t has a great influence on the int formation of particles. This result is consistent with that of p–A isotherms which show that the mean molecular area of AA become bigger with longer t , and indicates that more AA molecules int react with the complex ions. Fig. 5 shows the micrographs of various samples. When t =120 min or 240 min, the equ diameters of the particles formed at the 1.5×10−4 M complex subphase surface are about 80 nm or 160 nm respectively, whereas those at the 5×10−5 M complex subphase surface are about 30 nm or 40–50 nm respectively. This results show that t and the concentration of the comequ plex ion greatly affect the size of the particles. The A–t isobars of AA on the complex subphase surface show that the mean molecular areas neither increase nor decrease with increasing t , indicatequ ing that the complex ion cannot insert into the closely packed monolayer at the target pressure. However, the diameters of the particles obviously increase with increasing t . Probably, it is a equ particle growth process induced by the monolayer. First some AA molecules reacted with the complex ions when the chloroform solution was spreading on the complex subphase. Then the monolayer was compressed, and some tiny particles incorporated into the monolayer. When the monolayer was kept the target pressure, the AA molecules around the tiny particles disassociated gradually, and the carboxyl groups reacted with the complex ions continuously. More and more complex ions were adsorbed under the monolayer with increasing time, resulting in the formation of bigger particles. We have investigated the sample which was obtained by the SL method without spreading AA molecules. The TEM micrograph is homogeneous. No round particles were found. The p–A isotherm

W.-Z. Lan et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 139 (1998) 7–12

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Fig. 5. The TEM micrographs of monolayers on the complex subphase surface.

of the complex subphase surface without spreading AA molecules shows that the surface pressure does not increase with compressing. The surface tension of the complex subphase was found to be close to that of pure water. These results indicate that the complex ion in the subphase does not behave as an amphiphile. It is the reaction between AA molecules and the complex ions at the air/liquid interface that causes the formation of the AA/complex nanoparticles. The pH of the subphase would have great influence on the dissociation of AA molecules. In our experiment, the pH of the subphase was kept at 5.7. If the pure water (pH=5.6) was used as subphase, the AA could not be dissociated. However, if a reaction happened, the aliphatic acid would dissociate gradually even at pH#3 [21]. The AA molecules dissociated gradually to react with the complex cations in the complex subphase. This process is similar to that of the reactions of AA with the

ruthenium bipyridine complexes at a air/liquid interface [10–12]. At a certain pH, the concentration of the complex ion in the subphase and the reaction time would greatly affect the rate and degree of the reaction. The size of the nanoparticles in the monolayer would be controlled by choosing the appropriate conditions.

4. Conclusion The monolayers containing nanoscale particles of luminescent europium complex were obtained at a air/liquid interface by means of the reaction between arachidic acid and europium bipyridine complex ion. The size of the particle can be controlled by choosing the appropriate experimental conditions.

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Acknowledgment The authors are grateful for the financial support of the Climbing Program (a National Fundamental Research Key Project) and an NNSFC Grant.

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