3412
J. Phys. Chem. 1992, 96, 3412-3415
tional energy at the T site is 145 meV in the present calculation and 152 meV in the zeroth-order approximation. The energy at the 0 site is 43 meV in the present calculation and 86 meV in the zeroth-order approximation. By the inelastic neutron scattering measurements the vibrational energy of hydrogen in the bulk a-zirconium at 873 K was obtained to be 144 meV, and the energy spread (fwhm of the peak) is 47 meVn2’It has been found that the calculated vibrational energy at the T site is in good agreement with the experiment. Garrett measured the vibrational energy
of hydrogen on Ti(0001) surface by using high-resolution electron energy loss spectroscopy (HREELS).2S The vibrational energy of H and D is reported to be 120 and 85 meV, respectively. This value is greater than that calculated for hydrogen on zirconium surface. We cannot differentiate that this is due to the difference between Ti and Zr or the uncertainty of this calculation.
Formation of Nanoparticulate Fe,O,-Stearate Method
Multilayer through the Langmuir-Blodgett
(25) Garrett, S. J.; Egdell, R. G.; Riviere, J. C. J . Electron Spectrosc. Relat. Phenom. 1990, 54/55. 1065.
Xiaogang Peng,* Yan Zhang, Jun Yang,+ Bingsuo Zou, Liangzhi Xiao, and Tiejin Li* Department of Chemistry, Jilin University, Changchun, 130023, P . R. China (Received: August 30, 1991)
Stable nanoparticulate a-Fe203stearate(Fe203-St) monolayers can be obtained by using nanoparticulate a-Fe203(diameter 70 A) as the subphase. The area extrapolated to r = 0 is 27 A2 per hydrocarbon chain of the monolayer. The “collapse” pressure is about 28 dynlcm. The transfer ratio is 1.0 f 0.1 (this is a constant within the limit of experiment). The long spacing of the multilayer determined by X-ray diffraction is 160 A. The tilt angle of the hydrocarbon chains revealed by IR is 31° f 5 O . The long CH2 sequence of the hydrocarbon chains of the multilayer is in a trans planar structure. The surface coverage by Fez03 nanoparticles is very large (more than 90%). Stearic acid is converted to stearate ion almost completely and chemical bonds were formed between Fez03 and stearate ions. The particles in the polar plane of the multilayer are close packed in a hexagonal close packed (hcp) bilayer structure. The total thickness of the multilayer can be several thousand angstroms or more. The absorption and transmission properties of isolated nanoparticulate Fe203 hydrosol were maintained. Thus the multilayer can be regarded as a three-dimensional superlattice of quantum size particles.
Introduction It was proved that nanoparticulate semiconductors are a kind of very important optical nonlinear materials.’ One of the most challenging problems2is to arrange these nanoparticles in a defined and functionally useful way while maintaining the nanoparticle mesoscopic physical properties (a three-dimensional quantum superlattice). Recently, some investigators recognized that a LB matrix can provide geometrical control of the nan~particles.~.~ Several kinds of nanoparticulate semiconductors were synthesized in LB films as well as at the interface between the monolayer and water by chemical4” or electrochemical reactions.’ On the other hand, Zhao et al. have reported that cationic magnetic Fe304 ultrasmall particles (50 f 5 A) have been sandwiched between the polar planes of arachidate ion monolayers deposited on oxidizing silicon substrates.* Incident-angle-dependent reflective measurements of seven successive units of arachidate ion sandwiched Fe304 nanoparticles on films have led to an average thickness of 89.2 A for a single-sandwiched unit. The average surface coverage of the substrate by particles is about 35% We have synthesized Q-state PbS monolayers or nanoparticles in the polar planes of stearic acid (SA) LB films by chemical rea~tion.~We observed that the distribution of PbS nanoparticles in the LB films is irregular and that the nanoparticles partly destroy the structure of LB films.9b Thus we considered that well-ordered three-dimensional Q-state superlattice cannot be obtained by chemical reactions. On the basis of the work of Zhao et al.,* we guessed that, by the LB method, nanoparticles can be directly transferred from its hydrosol used as subphase. In this way, a three-dimensional quantum superlattice may be built up. As the first approach, we chose Fe203nanoparticle because we have found that Fe203 nanoparticle exhibits strong optical nonlinear response especially when the particle surface was coated with a layer of surfactant molecules.’o The other reason choosing
’Institute of Atomic and Molecular Physics, Jilin University.
Fe203is that its nanoparticle (69 A) can form supercrysta1.I’ It was clarified by experiments that well-ordered Fe20,-St alternating multilayer can be conventiently deposited on several kinds of substrates by the LB technique. When the particle size of Fe O3 is 70 A, the long spacing of the LB films (Y-type) is 160 determined by X-ray diffraction, the surface coverage of the substrates by particles is about 90%. The total thickness of the LB films can be several thousand angstroms or more. And we observed that the structure and properties of the alternating LB films changed with the change of the size of Fe203 nanoparticle.
Experimtatat Section The hydrosol of a-Fe20!was prepared by the forced hydrolysis method developed by Matijbic and Scheiner.12 Ferric chloride ( I ) Wang, Y.;Herron, N.; Mahler, W.; Suna, A. J . Opt. Soc. Am. E 1989, 6, No. 4, 808.
(2) Stucky, G. D.; MacDougall, J. E. Science 1990, 247, 669. (3) Fendler, J. H.Chem. Rev. 1987, 87, 877. (4) (a) Ruaudel-Teixier. A.; Lcloup, J.; Barraud, A. Mol. Crysf.Uq.Cryst. 1986, 134, 347. (b) Zylberajch, C.; Ruaudel-Teixier, A.; Barraud, A. Thin Solid Films 1989, 179, 9. (5) Smotkin, E. S.; Lee, C.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.;Webber, S. E.; White, J. M.Cham. Phys. Lett. 1988, J52, 265. (6) Yi,K. C.; Fendler, J. H. Langmuir 1990, 6, 1519. (7) Zhao, X. K.; Fendler. J. H. J. Phys. Chem. 1990, 94, 3384. (8) Zhao, X. K.; Xu,S. Q.; Fendler, J. H. J . Phys. Chem. 1990.94, 2573. ( 9 ) (a) Per& X. G.; Wei, Q.; Jiang, Y. S.; Chai, X. D.; Li, T. J.; Shen, J. C. Thin Solid Films, in press. (b) Peng, X. G.; Guan, S . Q.; Chai, X. D.; Jiang, Y. S. J . Phys. Chem., in press. (10) (a) Zou, B. S . ; Zhang, Y.; Xiao, L. Z.; Li, T. J. Chin. J . Semicond. 1991, 12, 145. (b) Zou, B. S.; Xiao, L. Z.; Zhang, C. M.; Li, T. J. Paper
presented at the Second International Conference of Molecular Electronics and Biocomputers, 8-1 I September, 1989, Moscow. ( I 1 ) Bentzon, M. D.; Wontcrghem, J. V.;Morup, S . ; Tholin, A,; Koch, C. J. W . Philos. Mag.E 1989, 60. 169.
0022-36541921209634 12%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3413
Fe203-Stearate Alternating LB Multilayer
5uh
n 4 0 . 0 0 r--
1
r(
5 35 00
40
\
I 3000
I
I
27:
I L
I
i
10
I
20
,
15 00
I
30
I
40
I
50
20'oo
I
60
70
A ( .a'/mOl)
Figure 1. *-A isothermal curve for Fe,O,-St
hydrosol surface.
Results and Discussion All the results mentioned in this section are about the larger particle (70 A) except those with special explanation. 1. Fe203-St Monolayer on the Subphase Surface and the Transfer of Fe203-St Monolayer. Figure 1 shows the *-A isotherm of Fe203-St monolayer formed on the surface of the hydrosol subphase. We considered that AB is the solid-state film and B is the collapse point of the monolayer. The area extrapolated to T = 0 is 27 A2per hydrocarbon chain. It is a little larger than the value of stearic acid monolayer. It means that all hydrocarbon chains within Fe20,-St monolayer are closely packed and parallel to each other; Le., every hydrocarbon chain occupies a certain area. Figure 2 illustrates the A-r isobaric curve (at 15 dyn/cm). No relaxation can be detected. We considered that these results show that a stable solid-state monolayer can be obtained. At the constant pressure of 15 dyn/cm, the monolayer was transferred on to a solid substrate (CaF2, silicon, quartz, ZnSe, and mica). In Table I, the transfer ratios of a sample are listed. The data show that the transfer of the monolayer is possible and complete. No tendency to transfer irregularly was observed till ~~
(1 2)
Matijevit, E.; Scheiner, P. J . Colloid Interface Sei. 1978, 63, 509.
i
10 00 000
monolayer on the Fe203
solution (0.01 M) was mixed with predetermined amounts of hydrochloric acid at room temperature to prevent hydrolysis, and aged 2.5 h or 10 min at 100OC. After aging, the sample was cooled to room temperature. The hydrosol particles were coated with stearic acid dispersed in an organic solvent. The mean diameter of particles coated with a layer of stearic acid was determined by small-angle X-ray scattering. The mean diameter of the particles aged for 2.5 h and 10 min is 70 and 20 A, respectively. The particles aged for 10 min were observed by transmission electron microscopy. The micrograph shows that the particle size is almost homogeneous. The value of the diameter determined by TEM is 20 f 5 A. Fe203nanoparticle hydrosol prepared by the method mentioned above was used as subphase (pH = 3.2). The concentration of iron ion was 2 X lo4 M determined by atomic absorption spectroscopy. Stearic acid was A.R. grade, made in China, and was recrystallized from ethanol. The spreading solutions of 1.0 X lo-, M were prepared by dissolving stearic acid in chloroform (A.R. grade). The LB trough used on this study was a RMC 2T multicompartmental round trough from Mayer-Feintechnic (Germany). The films were deposited (Y-type) onto a solid substrate at constant surface pressure (1 5 dyn/cm) at room temperature. The transfer speed was 1.O cm/min. X-ray diffraction patterns were obtained with the diffraction vector perpendicular to the plane of the films using a Rigaku D/max rA X-ray diffractometer. I R spectra and UV-visible spectra were recorded on a Nicolet 5DX FTIR spectrometer and a Shimadzu W - 3 6 5spectrometer, respectively. XPS spectra were recorded on a VG ESCALAB MKII spectrometer.
1 2000
1000
30.00
4000
50.00
6000
t(min) Figure 2. A-t isobaric curve for Fe,O,-St monolayer on Fe203hydrosol surface at the constant pressure of IS dyn/cm.
r
I
~
I
4000.0
3400. o
I' I
I
2800.
o
I
I
I
2200. o 1900. o 1600. o WAVENUMBERS(cm-l)
I
1300.
o
1000.o
Figure 3. IR transmission spectrum of 21-layer Fe,O,-St LB films on each side of a CaF2 substrate.
TABLE I: Transfer Ratio of Fe203-St Monolayer onto a Mica Substrate
layer no.
transfer ratio
layer no.
transfer ratio
1
1.1 1.1 1 .o 1 .o 1 .o 1 .o 0.9
17 18 19 20 21
1 .o 0.9 0.9 1.1 1 .o
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1 .O
1 .o 0.9 0.9 1.1 1 .o 1 .o 1 .o 0.9
22 23
24 25 26 27
28 29 30 31
1 .O 1 .O
1 .o 1.1 1.1 1.1 1.1 1 .o 1 .o 0.9
the layer number was more than 30 on each side of the substrate. A stable Fe20,-St monolayer of the smaller particle (20 A) can also be prepared. The "collapse" pressure in this case is greater than that of the 70-A particle. 2. IR Spectra of Fe203-St Alternating LB Films. Figure 3 is the IR transmission spectrum of Fe20,-St alternating LB films (4000-1000 cm-I). The band at 3397 cm-l is ascribed to the vibration of OH existing in the Fe203particles. The intensity of this band decreased a little during LB film aging for months. Then, it does not change till the temperature was as high as 100°. All these facts indicate that some OH species initially exist on the surface of the particles and disappear upon aging. The other
3414 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
I
I
3 i u u . 0 3000. 0
I
2900. 0 2800. 0 WAVENUMBERS(cm- ’ 1
Peng et ai.
I
2700.
Figure 4. Polarized IR transmission spectra of the CH2 stretching vibrations. The electric vector of the incidence is perpendicular to the dipping direction. The angle between the incidence and the film plane is (a) 60’ and (b) Oo. The sample is a 21-layer Fe,O,-St LB film deposited on a CaF, substrate.
k (002)
OH species are combined with particles and do not escape at the high temperature. Band progressions (1 300-1 150 cm-l) of CH2wagging vibrations mixed with CH2 twisting and rocking vibrations can be distinguished in Figure 3, although they are all weak. The existence of band progressions shows that the long CH2sequence of stearate ion is in a trans planar structure.” The band progressions are more intense than those of ordinary fatty acid salt LB films.143” The number of the bands of band progressions is eight and differs from the number observed for stearic acid salts in solid state.16 The bands in the region of 1500-1600 and 1400-1445 cm-’ are ascribed to the asymmetric and symmetric stretching vibrations of the carboxylate group, respectively. These bands, at first, show that stearic acid molecules were converted to stearate ions almost completely (there is still a small shoulder at the position of free stearic acid (about 1700 cm-I)). It means that Fe203particles are packed closely; i.e., the surface coverage is very high (more than 90%). If this structure was wrong, the particles having a long distance from each other, there should be more free acid molecules which could not be c o ~ e c t e dwith F%03particla. This structure was also supported by the XPS experiments mentioned later. The asymmetric stretching vibration region includes two bands. The position of the band, 1585 cm-I, is similar to that of F&t,OH solid state.17J8 But the intense band at 1530 cm-l always appears in the spectrum of Fe203 nanoparticles coated with a layer of stearate ions.17 We also found that the intensity of the band at 1585 cm-l (1530 cm-l) of the LB films of the smaller particles (20 A) is smaller (larger) than that of the 70-A particles. This is a interesting phenomenon needed further study. Polarized IR absorption spectra in the C H stretching vibration region of the Fe203-St LB films are demonstrated in Figure 4. From the two spectra, using the method suggested by Barraud‘s group,I9 we evaluate a tilt angle of the hydrocarbon chains of 3 1
* 50.
(13) Rabolt, J. F.;Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J . Chem. Phys. 1983, 78, 946, and references therein. (14) Allara, D. L.; Nuzzo, R. G. Lungmuir 1985. I , 52. (IS) Kimura, F.; Umemura, J.; Takenaka, T. Lungmuir 1986, 2, 96. (16) Meiklejiohn. R. A.; Meyer, R. J.; Aronovic, S. M.; Schvette, H . A.; Meloch, V. W. Anal. Chem. 1957, 29, 329. (17) Yang, J.; Peng, X. G.; Li, T, J. Unpublished results. (18) Yang, K. Z., et al. Thin Solid Films 1989, 178, 341. (19) (a) Vandevyver. M.; Barraud, A.; Ruaudel-Teixier, A. J . Colloid Interface Sci. 1982,85, 571. (b) Richard, J.; Vandevyver, M.; Lesieur, P.; Ruaudel-Teixier, A.; Barraud, A. J . Chem. Phys. 1987, 86, 2428. (20) Takenaka, T.; Nogami, K.; Gotoh, H.; Gotoh. R. J . Colloid Inrerfuce Sci. 1971, 35, 395. (21) Clark, G. L.; Leppla, P. W. J . Am. Chem. SOC.1936, 58, 2199.
A 1. 0 1. 5 2. 3
20 Figure 5. Small-angle X-ray diffraction pattern of a 13-layer alternating Fe,O,-St LB film on a silicon substrate.
I bl
F i 6. Schematics of Fe,O,-St alternating LB films: (a) a lateral view of a unit cell of the LB films; (b) a top view of a Fe,03 nanoparticle bilayer.
3. X-ray Diffraction from Fez03-St Alternating LB Films. Figure 5 is the small-angle X-ray diffraction pattern of a 13-layer Fe20,-St alternating LB films on a silicon substrate. There are two (001) Bragg peaks corresponding to a long spacing of 160 A. This result exhibits that Fe,O,-St alternating LB films form a Y-type structure indeed. As mentioned above, we have learned that the tilt angle of the hydrocarbon chains within the LB films is about 31’. With a tilt angle of 31’ and a length of 25 A for one molecule of stearate ion, the thickness of a stearate bilayer is about 40 A. Adopting this value as the size of bilayer of the stearate ion, the thickness of the nanoparticulate Fe2O3 layer is 120 A. This value is 20 A
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3415
Fe20,-Stearate Alternating LB Multilayer 1
.
0 60
0
~
0 50
0 40 v Y
030 0
" T .
020
0 10
000 4
6
8
IO
12
14
16
18
Number o f layer Figure 8. Absorbance of the samples in Figure 7 a t 300 nm versus their layer number. Figure 7. UV-visible absorption spectra of Fe,O,-St LB films: (a) Slayer, (b) 1 1-layer, and (c) 17-layer on each side of quartz substrate.
smaller than twice the diameter of the particle as mentioned in the experimental part. The supercrystal of 69-A Fe203 particles is a hexagonal close-packed structure (hcp structure)." We imaged that 70-A Fe2O3 particles in the LB films maybe form the hcp structure, too (Figure 6). From the particle diameter and the geometrical relationship shown in Figure 6, we can calculate that the thickness of the F q 0 3 part of the unit cell of the LB films is 127 A. This theoretical value almost equals 120 A determined by IR and X-ray diffraction measurements. From the top view structure (Figure 6b), we can calculate the surface coverage to be 91%. This theoretical surface coverage is virtually consistent with the experimental result o b tained by IR experiments mentioned above. Hence, both of the results support the hcp structure shown in Figure 6. Thus we suggest that this is the structure of Fe20,-St alternating LB films, and we consider that the structure is a well-ordered three-dimensional quantum superlattice because, as mentioned later, the UV-visible absorption and transmission properties of the LB films are practically similar to that of the isolated Fe203nanoparticle hydrosol. IR and X-ray diffraction experiments indicated that the structure of Fe20,-St alternating LB films of 20-A particles is totally different from that of the 70-A particles mentioned above. A typical difference is that the smaller particles (20 A) are closely packed in one single layer in the polar planes of the LB films. The existence of (001) Bragg peaks indicates that Fe20,-St LB films form a perfect layer structure, Le., the roughness of the Fe203particle bilayer is Ysmoothednby the organic layer. 4. XPS Results of Fe203-St Alternating LB Films. Further, we shall discuss some XPS experimental results of Fe203-St LB films which are useful for determining the structure of the LB films. A more detailed analysis will be the subject of another study. Within the experimental limit, Si,, of a 13-layer Fe20,-St LB films on a silicon substrate cannot be detected. This supports the above structure (Figure 6 ) , indicating that Fe.20, particles are closely packed within LB films. Fe,, (93.7 eV), Fezp(711.3 eV), and Fe3, (56.0 eV) all correspond to those of a-Fe203 and are different from those of a-FeOOH.2Z,23Thus the IR absorption band at 3397 cm-' is due (22) Mcintyre, N. S.;Zetaruk, D. G. Anal. Chem. 1977, 49, 1521. (23) Welsh, I. D.; S h e r w d , P. M. A. Phys. Reu. 13 1989, 40, 6386.
to the water molecule absorbed on the surface of the particle and involved in the internal part of the particles. The 3d-4sp satellite in the Fe,,, FeS8,and valence band XPS spectra of Fe203-St LB films is more intense than that of the bulk a-Fe203,which indicated the electron delocalization enhancement in this system. 5. UV-Visible Spectra of Fe203;St LB F i b . Because of the quantum size effects of nanoparticulate semiconductors, UVvisible spectroscopy has become an important investigative tool for determining the optical properties of semiconductor nanoparticle^.^ It was found that iron oxide nanoparticles incorporated in sodium montmorillonite clay interlayers exhibited ca. 0.28 eV greater band gap than cr-Fe.20, bulk powder.2s The diameter of the Fez03 particle in clay interlayers is about 6.6 A according to X-ray diffraction analysis. Figure 7 illustrates the UV-visible transmission absorption spectra of 5-layer, 11-layer, and 17-layer Fe20,-St LB films on each side of the quartz slides. The absorption onset of the spectra is at about 600 nm, shifting about 100 nm with respect to bulk powder Fe203.25 This indicates the influence of quantum size effects. The absorption spectrum of Fe20,-St LB films is similar to that of Fe203 hydrosol or Fe203 nanoparticles coated with a layer of surfactant molecules (DBS or %).lo It implies that strong optical nonlinear response of the LB films may be observed. The shoulder around 300 nm is the position of the biexciton excitation of Fq03.26 Figure 8 is the absorbance at 300 nm versus number of layers. We obtained a linear relationship. This supports the results observed during the film transferring process, that the transfer ratio is a constant. Nonetheless, it shows that the absorption of the sample is mainly due to the absorption of the LB films. These results shown by Figures 7 and 8 indicate that, after Fe203nanoparticles were assembled in the LB films, the absorption and transmission properties of the isolated Fe203nanoparticle remain. Thus we considered that a kind of three-dimensional quantum superlattice was built up by LB technique.
Acknowledgment. We thank Dr. D. Mobius for the careful examination of this paper and the useful discussions. We also thank National Natural Science Foundation of China (NNSFC) for the provision of a financial support. (24) Henglein, A. Chem. Reu. 1989, 89, 1861. (25) Miyoshi, H.; Yoncyama, H. J . Chem. Soc., Faraday Tram. I 1989, 85, 1873. (26) Sherman, D. M.;Waitc, T. D. Am. Mineral. 1985, 70, 1262.
