4484
J. Phys. Chem. 1993,97, 4484-4487
Structure Characterization of the Two Kinds of Nanometer Size cu-Fe203-Stearate Altemating Langmuir-Blodgett Films by Fourier Transform Infrared Spectroscopy Jun Yang,'Vt Xiao-Gang Peng,t Yan Zhang,t Hui Wang,? and Tie-Jim Lit Institute of Atomic and Molecular Physics, Jilin University, Changchun 130023, Peoples' Republic of China, and Department of Chemistry, Jilin University, Changchun 130023, Peoples' Republic of China Received: July 28, 1992; In Final Form: December 21, 1992
Structure, infrared optical properties, and molecular orientation of 70- and 20-A nanoparticulate a-Fe2O3stearate alternating Langmuir-Blodgett films were studied by using FTIR absorption spectra and KramersKronig (KK) analysis. The KK analysis results show that the average refractive indexes of the films are 1.83 (70 A) and 1.62 (20 A), respectively. According to the refractive index, extinction spectra, and the linear infrared dichroic spectra of the films, the molecular orientation was calculated. The orientation angles are 5 O and 38O for fatty chains, 80° and 6 5 O for COO- asymmetric mode, and 5 5 O and 39O for COO- symmetric mode of stearate ions in 20- and 70-Aa - F e 2 0 3 S t LB films, respectively. From these results, the carboxylate groups were considered to be a relatively spherical symmetric distribution, and on the contrary, the hydrocarbon chains were found almost perpendicular to the substrate. The band progressions of F e z 0 3 4 LB films in the range of 1360-1 320 cm-1 can be detected, especially in the case of 2 0 4 Fe203St LB films. These indicated that the films possess a high-order alternating structure. The hexagonal-like close packing (hcp) form of the nanoparticles was considered as a possible structure between two stearate ion layers. The obvious hypsochrome shift was observed in the UV-visible spectra of the 20- and 70-A Fe203-St films, indicating the influence of quantum size effect.
Introduction The syntheses and characterization of nanoparticulate semiconductors represent areas of intense recent research.'" Size reduction can result in substantially altered physical and chemical properties. These new properties involve important applications in optics and electronics. For a number of cases, these properties of nanoparticles are researched under random states. Therefore a potential very significant question is if these nanoparticles are orderly organized in matrix, what phenomena will happen and what new properties will appear. We have reported the techniques for the preparation of relative monodispersed a-Fe2O3 nan~particles.~ And then, a new series of nanoparticulate a-Fe203-stearate alternating LangmuirBlodgett multilayer ( F e 2 0 3 S tLB films) was obtained through the Langmuir-Blodgett method using nanoparticulate a-Fe2O3 hydrosol as the subphase directly.* The structure of Fez0343 LB films was proved to be a highly ordered assembly in the range of nanometers, i.e. a kind of three-dimensional superlattice.8 We expected that, by the use of FezO3St LB films these applications as mentioned above can be used and developed fully. In addition, we guessed that some new properties of the superlattice may appear. Because the stearate ions within the Fe203St LB films are orderly bound on the surface of the nanoparticles, the vibration frequencies of the stearate ions will give out some information of order structure of the alternating multilayer. The infrared dichroicbehaviorsofthevibration bandsisoneof the most effective methods which is used to study the order structure of the organic part in the three-dimensionalsuperlattice multilayer. UV-visible spectra can be used to study the quantum size effect and optical properties of nanoparticle in the alternating films. In this paper, we measured FTIR absorption spectra of the films and obtained the average refractive index, extinctionspectra by KK analysis. We calculated the molecular orientation of transition moments by a method proposed by Vandevyverg which was developed by us using the average refractive index of the I
Institute of Atomic and Molecular Physics, Jilin University. Department of Chemistry, Jilin University.
0022-3654/93/2097-4484$04.00/0
NP-organic molecular alternating film system. The difference of the order structure between the two kinds of size Fe203N P S t LB films and size effect were studied. The like-free ferric and tight-binding surface iron ionic states of nanoparticles were discussed. For the two films, the interfaced difference between the two adjacent nanoparticles and between nanoparticles and stearate ions in the alternating structure were considered as a bound mechanism of the two iron ionic states. The optical properties and quantum size effect of the two kinds of films were studied by UV-visible absorption spectra. Experimental Section Two kinds of nanoparticulate a-Fe203-stearate alternating Langmuir-Blodgett films, the size of a-Fe203nanoparticlesbeing 70 and 20 A, respectively, were prepared. The 70-ha-Fe203 nanoparticles were also completely coated with a layer of stearate ions (FNPCS), which was used as a comparing sample. The substrates used to deposit the LB films are CaF2and ZnSeplates. The a-Fe203nanoparticle hydrosol, 20-hFe203St films, 70-h Fe203St LB films, and FNPCS were prepared by the methods described in previous detail.'J In present work, the infrared absorption spectra of these samples were measured by using a Nicolet 5-DX Fourier transform spectrometer (FTIR) at 4 cm-i resolution. The linear infrared dichroic light was obtained by using a Perkin-Elmer 25 mm diameter linear wire grid polarizer; the dichroic spectra were obtained by using a rotating holder which allows two samples rotating to the same position with respect to the incidenceinfrared beam in the sample room of the spectrometer. The spectra of 20-h Fe2O3-St LB films and 70-A F e z 0 3 4 LB films were measured through comparison with the clean substrate used as the deposition of the sample; the spectrum of FNPCS (powders) was obtained by using the KBr pellet method (1OO:l). UVvisible spectra were recorded on a Shimadzu UV-visible 356 spectrometer. Results and Discussion 1. Molecular OrientationandRefractive Index. Figure 1 shows infrared absorption spectra of 20-hand 70-hFeZOsSt LB films 0 1993 American Chemical Society
Structure of Nanoparticulate LB Films
The Journal of Physical Chemistry, Vol. 97, No. 17, I993 4485
a
A I 1390
I
1293
1 1195
I 1098
1000
Wavenumbers (cm-')
Figure 1. FTIR absorption spectra of nanoparticulate a-Fe~03-stearate alternating Langmuir-Blodgett films (Fe203-3 LB films) and a-Fez03 nanoparticles coated with a layer of stearate (FNPCS) in the range of 1390-1000~m-~:(a) 70-AFNPCS, 1:lOO KBr pellet; (b) 25-layer 70-A FezO3St LB films on each side of a CaF2 substrate; (c) 21-layer 20-A Fe203St LB films on each side of a CaFz substrate; (d) 59-layer 20-A F e z 0 3 4 LB films on each side of a ZnSe substrate.
and FWPCS. In the region between 1360 and 1200 cm-I (Figure l), the band progressions arising from CH2 wagging and CH2 twisting vibrations are found in the spectra of F e 2 0 3 S tLB films of two sizes. In the spectrum of 20-A F e 2 0 3 S tLB films (Figure IC),the bands are clear and strong relatively; in 70-A Fe203St LB films, the bands can also be found, but their relative intensities are weaker. However in the spectrum of FNPCS, the band progressions cannot be found. The existence of the band progressions shows that the hydrocarbon chains within the LB films exist in a trans zigzag planar structure.1° Thus the difference of the band progressions between the spectra of 20-A and 70-A F e 2 0 3 S LB t films shows that the order of the hydrocarbonchain on the nanoparticle surface increases with decreasing of the particle size. Therefore we believe that the F e 2 0 , S t alternating multilayer is a kind of high order LB film. In order to evaluate accurately molecular orientation of the alternating films, we used the linear dichroic spectra method by two absorption spectra, which was proposed by Vandevyver et a1.8 but used KK analysis and a effective optical model for the inorganic-rganic alternating LB films, particularly, considered size effect of the nanoparticles. The FTIR linear dichroic spectra of the 20-A F e 2 0 3 S t LB films and 70-A Fe20,St LB films are demonstrated in Figure 2 in the ranges of 3000-2800 and 1800-1000 cm-l. On the basis of the calculation theory and method reported by Umemura et al." and M. Vandevyver et a1.,8 we have developed a calculation method for the inorganio-organic alternating LB films. In this method, we considered the effect of refractive index of inorganic NP in the LB film. Therefore we need to obtain refractive indexes of the film. The refractive index and extinction spectra can be obtained from KK analysis for the absorption spectra of the films. The high-frequency refractive index of fine particles a-Fe2O3 (1 pm) is about no 2.3712 (800 nm) and the index of stearate is about &(St) zz 1.5 (800 nm). Because of the size effect, for 70-A and 20-A a-FezO3 NP no(NP) are 2.32 and 1.59,respectively. After consideringthe effectiveoptical thickness of nanoparticles and stearate of the alternating structure, for the total film system, the high frequency at 800 nm refractive index
=
Wavenumbers (an-')
Figure 2. FTIR linear dichroic spectra of the 20-A FezOl-St LB films and 70-AFezO3St LB films: (a) incident angleOO,21-layer 20-AFe203St LB films on each side of a CaF2 substrate; (b) Incident angle 60°, 21-layer 20-A Fe203St LB films on each side of a substrate; (c) incident angle Oo, 25-layer 70-A FezOsSt LB films on each side of a CaF2 substrate; (d) incident angle 60°, 25-layer 70-A Fe203St LB films on each side of a CaF2 substrate. The electric vector of the incidence is parallel to the dipping direction.
were estimated about 1.83 and 1.62 for 70-A and 20-A NPFe203/St LB films, respectively. The adopted procedure consists in measuring the ratio of the band intensities in extinction spectra B(i) = a~~(i)/cq(O) between the extinction for linearly polarized light arriving onto the film at an angle (i) with respect to N with the electric field parallel to the plane of incidence and the extinction at normal incidence (i = 0) with the same polarization. If we designate with 4 the angle between N and the dipole moment of a nondegenerate molecular transition, the theoryI3 provides an expression that related B(i) to the average value , and , and the refractive index n(v) of the film (v is the frequency of the incident infrared beam, here the average value of n(v) is 1.83 for 70-AFe203LB film and 1.62 for 20-A Fez03 LB film), here o is related to the intensity of ratio of the band between that of the electricity direction of incident infrared beam parallel to the dipping direction and that perpendicular to the dipping direction y = a l ( i = O)/all(i 30). Undercertainconditionsandsimplifying assumptions such as that one is dealing with a weakly absorbing LB film of thickness much smaller than the wavelength of the incident radiation deposited on a substrate material of not too high refractive index, the expression takes on a simplified form
sin2(i)
4>> n:(v)
+ cos(i) cos(r) n,(v)
+
cos(20) = (1 - y)/(y 1) (2) In eq 1 we have accounted for the refractive index difference between the stearate and the a-Fe2O3 nanoparticle layers and the effect of multilayer. And we have assumed that the external medium is air (refractive index n(air) = 1); n,(v) is refractive index of the substrate. The angle of refraction r is given by the usual relation sin(r) = sin(i)/ns(v). Under the restrictive conditions mentioned above, use of eq 1 allows us to obtain estimates of the average orientation 6 of the transition dipole moments with respect to N although it should be emphasized that nothing can be said concerning the width and shape of the orientation distributions. Let the angles between the surface normal and the transition moments of the asymmetric and symmetric CHI stretching and COO- stretching modes be 4as(CHI), ds(CH2),I#J~~(COO-), and &(COO-), respectively. Then the tilt angle, 8, of the hydrocarbon chain axis from the surface normal and COO- groups from the surface on the nanoparticles I#J
Yang et al.
4486 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993 hydrocarbon chain
’11
~~
~
~-
a vector in thefilm plane
Figure 3. Schematic diagram showing the hypothetical structure of the stearate nanoparticle surface.
can be evaluated by the orthogonal relation among &, &, and 0 cos2(4as)
+ COS2(4,) + COS2(0) = 1
(3)
According to data of band intensity in the linear dichroic FTIR spectra (Figure 2), we calculated the orientation of the hydrocarbon chains and COO- group of F e 2 0 3 S tLB films. For 2 0 4 Fe203-StLB films, the tilt angle between the hydrocarbon chains and the substrate is about 5 f 5’; for 70-AFe203St LB films, the fatty chains tilt angle is about 38 f 5’. These results show that the hydrocarbon chains are almost perpendicular to the film plane. The orientation of hydrocarbon chains is also obviously dependent on the size of the nanoparticles. The angle decreases with increasing the size of the nanoparticles. For the 20-Afilm, the COO-asymmetric stretching vibration transition moment orientation is about 87 f 5 O (1585 cm-I) and 72 f 5 O (1530 cm-I) and the symmetric one is about 55 f 5 O ; for the 70-A film, the COO- asymmetric stretching vibration transition moment orientation is about 53 f 5’ (1574cm-I) and 77 f 5O (1527 cm-I) and the symmetric one is about 39 f 5O. We considered that the difference of orientation angles between the fatty acid salt LB films and Fe203St LB films is due to the substrate being a plane for fatty acid salt LB films deposited in the general manner, but the “substrate” of stearate ions within Fe203StLB films is close-packed spherical nanoparticles. Thus weconsidered that it is the spherical surface that made the COOgroups of stearate ions bound on them have a like-spherical symmetric distribution (Figure 3). 2. Close-Packing Structure and Coverage Ratio of Nanoparticles in the Films. Because the films are expected to be a kind of superlattice structure, the high coverage of the nanoparticles implies less defect in the layer of nanoparticles, the close-packing form implies that the quantum wall width (effective optical layer thickness) can be determined. So we discussed the possible close packing structure and coverage ratio of the nanoparticles in the films. The TEM photograph of the Fez03 nanoparticle hydrosol shows that these nanoparticles can be considered as spherical parti~1es.I~For spherical particles, there are a certain number of gaps between the particles no matter which kind of the packing forms are adopted. According to the thickness data of the alternating bilayer from X-ray and the intensity ratio of the C g O stretching vibrations, we guessed that the packing form of nanoparticles in the two size Fez03-St LB films is like hexagonal close packing (hcp) structure. This method to estimate the substrate surface coverage by the particles is supported by two other facts: (1) the area per hydrocarbon chains of FezO3stearate monolayer is close to that of stearic acid LB films; (2) the transfer ratio of Fe203-stearate monolayer is around unity.8 The X-ray and molecular orientation results of hydrocarbon chains show that the thickness of the alternating bilayer is 160 5 As in IO-A Fe203St LB film. Although the thickness of stearate ions bilayer is about 50 A at the long-axis direction, the
*
200
300
400
500
600
700
800
A (nm) Figure 4. UV-visible absorption spectra of 20-A and 70-AFe2OjSt LB
films. thickness at the normal direction is about 40 f 5 A due to the 3 8 O average tilt angle of the long axis. So the nanoparticulate bilayer is 120 f 5 A. We calculated the normal thickness by using a hcplike close-packing bilayer structure in the polar area of the LB films; the result is 120 f 5 A. The X-ray result for 20-Aa-FezO3 LB film is 60 f 5 A. However the ratio of C - 0 vibration band in the film and TEMI4 also show the coverage ratio is about 70%. So the real alternating bilayer thickness was evaluated as 65 A. The thickness of stearate ions bilayer is about 49 A due to 1 3 O average tilt angle. The thickness of nanoparticles bilayer was evaluated as 23 A by using a hcp-like closed-packing structure model; the thickness is 72 A. 3. UV-Visible Spectra. In the new superlattice system, nanoparticles may play a important role for some further possible applications. For the possible quantum size effect of nanoparticulate semiconductors, UV-visible spectroscopy can be used to determine the electronic spectra and relevant optical properties of the NP or the NP-organic amphiphilic molecular alternating films. For the stearate acid molecule, it is difficult to find a distinguishable absorption band in UV-visible range; therefore we can only observe the optical properties of the nanoparticles in this range. UV-visible absorption spectra of the 20-Aand 70-AF e 2 0 3 S tLB films are shown in Figure 4. For the 20-A Fe203St LB film; the extrapolation of the rising portion of the spectra to zero absorption, as shown in the figure, gave the shifting about 30 nm with respect to 70-AFe203-St LB film, about 100 nm with respect to bulk powder Fe2O3.Is This indicates the influence of the quantum size effect. The two narrow shoulder peaks at 300 and 379 nm in the spectrum of the 20-A Fe203St LB film and the one broad shoulder peak at 400nm in the spectrum of the 70-A Fe203St LB film are all the position of biexciton excitation of Fe2O3.I6p8 We can also find a broad shoulder peak at 565 nm in the spectrum of 7 0 4 F e 2 0 3 S tLB film, but it cannot be seen in the spectra of 20-A Fe203St LB film. We found strong optical nonlinear response of the Fez03 hydrosol or Fez03 nanoparticles coated with a layer of surfactant molecules (DBS or St),” and their absorption spectra are similar to that of the alternating films. So we believe the same strong optical nonlinear response may also be observed. Conclusion
Now we have obtained the quantum wall width and defect in the alternating films from estimating of the hcplike closepacking
Structure of Nanoparticulate LB Films structure and coverage ratio of the nanoparticles. We have also obtained refractive index and extinctive spectra. We developed a method of molecular orientation calculation which can be used to calculate the new inorganic NP and organic alternating film system. The molecular orientation result showed that the COOgroups of stearate ions are spherically bound on the surface of the nanoparticles: the hydrocarbon chains are almost perpendicular to the substrate, and the hydrocarbon chains exist in a trans zigzag planar structure. These results imply that the film possess a high-order alternating structure along normal direction. A obvious hypsochrome shift was observed in the UV-visible spectra of the 20-A and 70-A F e 2 0 3 S tfilms, which indicated the quantum size effect. Acknowledgment. The authors wish to thank Professor L. Z. Xiao and Dr. B. S. Zou of the Chemistry Department, and Professor D. X.Tang, Institute of Atomic & Molecular Physics, Jilin University, for their valuable suggestions rendered at the initial stage of this work. We also thank the National Natural Science Foundation of China (NSFC) for the provision of financial support.
The Journal of Physical Chemistry, Vol. 97, No. 17, I993 4487 References and Notes Kayanuma, Y. Phys. Rev. E 1988,38, 9797. Wang, Y.;Herron, N. Phys. Reu. E 1990, 42, 7253. Henglein, A. Chem. Rev. 1989, 89, 1861. Brus, L. E. J . Phys. Chem. 1986, 90, 2555. (5) Watzke, H. J.; Fendler, J. H. J . Phys. Chem. 1987, 91, 854. (6) Fendler, J. H. Chem. Rev. 1987, 87, 877. (7) Zhang, Y.; Zou, B. S.;Xiao, L. Z. Acta Sci. Nor. Uniu. Jilin. 1990, 98, 115. (8) Peng, X. G.; Zhang, Y.; Yang, J.; Zou, B. S.;Xiao, L. Z.; Li, T. J. J . Phys. Chem. 1992,96 (No. 8 ) , 3412. (9) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gigantic, C. J . Collid Interface Sci. 1982, 85, 571. (101 Kimura. K.; Umemura. J.: Takenaka. T. Lanfmuir 1986. 2. 96. (1 1) Umemura, J.; Kimura, F.; Kamata, T.; Kawac T.; Takenaka, T. J . Phys. Chem. 1990, 94,62. (12) Iwauchi, K. Jpn. J . Appl. Phys. 1971, 10, 1520. (131 Yana, J.; Pena. X. G.: Li. T. J. Unuublished work. (14) P e n i X. G.; 2 al. Submitted to C&m. Phys. Lett. (15) Miyoshi, H.; Yoneyama, H. J . Chem. SOC.,Faraday Trans. I 1989, 85, 1873. (16) Sherman, D. M.; Waite, T. D. Am. Mineral. 1985, 70, 1262. (17) (a) Zou, B.; Zhang, Y.; Xiao, L.; Li, T. Chin. J . Semicond. 1991,12, 145. (b) Zou, B.; Xiao, L.; Zhang, C.; Li, T. Paper presented at the Second International Conference of Molecular Electronics and Biocomputers, 8-1 1 September, 1989, Moscow. (1) (2) (3) (4)
