J. Phys. Chem. B 1998, 102, 2329-2332
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Photovoltaic Properties of Polymer/Fe2O3/Polymer Heterostructured Microspheres Hui Du, Yaan Cao, Yubai Bai,* Peng Zhang, Xinming Qian, Dejun Wang, Tiejin Li, and Xinyi Tang Department of Chemistry, Jilin UniVersity, Changchun, 130023, P.R.C. ReceiVed: September 17, 1997; In Final Form: December 15, 1997
The Fe2O3 particles with diameters of 3-5 nm were encapsulated between the core and the shell of polymer via emulsion polymerization into the heterostructured microspheres, which were observed through TEM images. The photovoltaic response of the composites, which was studied by the electric-field-induced surfacephotovoltage-spectroscopy technique, appeared only if there was an external electric field (EEF) and its decay was much slower than that of Fe2O3 nanoparticles themselves. A reasonable energy band model was supposed to explain this phenomena and indicated that the polymer shell acted as an electric-passivation layer of the semiconductor Fe2O3 particles. The charge-separation process in the struture was also analyzed with this model.
1. Introduction
2. Experimental Section
The application of nanocrystallites in various fields such as nanoelectronics, nonlinear optics, and information storage has aroused much attention. With the development from materials to devices, electronic passivation becomes an important problem. Passivation is the chemical process by which the nanocluster surfaces are bonded to another material with a much larger band gap that acts as a chemical potential for electrons or holes at the interface. This potential confines electrons or holes inside the cluster, much like the “particle in a box” of elementary quantum mechanics.1 Hines and Sionnest recently synthesized ZnS-capped CdSe semiconductor nanocrystals, which exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature.2 Another method for passivation is via organic molecules. One of the most novel structures is the quantum-dot superlattice of colloidal CdSe nanocrystals, passivated with organic surfactants of hexadecyl phosphate, etc.3 The strength of the electronic coupling between adjacent dots could be tuned by variation of the organic passivating molecules so that bands of states could be formed. Polymer materials maybe a good substance for passivation, since they have large band gaps and low conductivity due to the small number of charge carriers with very low mobility and the high trap density. In the past, many inorganic particles were encapsulated in the polymer layer in order to affect the physical properties of such powders, particularly in terms of increasing their dispersity in solvents or in composite phases.4-6 Only Y. Haga et al. found that the photoconductivity of CdS, ZnO, and TiO2 was affected by the interaction between the polymers and the inorganic phases and the effect upon the electron transport process was thought to be especially large.7-9 In this paper, Fe2O3 nanoparticles with diameters of 3-5 nm were encapsulated in the polymer microspheres in a trilayer core-shell heterostructure. The effect of polymers on its properties were investigated by UV spectroscopy and surface-photovoltage spectroscopy. An energy band model was suggested to explain the charge-separation process in the composites.
2.1. Preparation of Polymer/Fe2O3/Polymer Heterostructures. The preparation and the structure of the composite particles with three layers have been reported in ref 10. The seed latex was synthesized via emulsion copolymerization of styrene/butyl acrylate/acrylic acid (8/1/1 in weight ratio). Fe2O3 hydrosols consisting of Fe2O3 particles with diameters of 3-5 nm (tested by TEM H-8100) were prepared by forced hydrolysis of acidified FeCl3 aqueous solution in open vessels according to ref 11. A certain volume of Fe2O3 hydrosols was mixed with the diluted emulsion. Since there were negative charges on the surfaces of the seed latexes due to -SO4- groups of the surfactant SDS (sodium dodecyl sulfate), the Fe2O3 particles that were positive in acidic medium adhered on the seed latex in a shell. Then the mixed monomer (styrene/butyl acrylate/ acrylic acid), the emulsifiers (SDS and Triton-100), and the initiator KPS (potassium persulfate) were added at 75 °C for 1.5 h of encapsulated polymerization reaction. 2.2. Particle Characterization. The morphology of the composite particles was observed by means of a transmission electron microscope (TEM JEM 2000-FX). The absorption spectra were recorded using a Shimadzu UV3100 spectrophotometer. The surface-photovoltage (SPV) effect, which consists of a change in the surface-potential barrier caused by illumination, has successfully been applied to the investigation of electron processes in semiconductors.12,13 The generation of photovoltage arises from the creation of electron-hole pairs followed by the separation under a built-in electric field (the space-charge layer). The former requires semiconductors with suitable band gaps; the latter requires electron-hole pairs that can be separated and transported to the electrode surface efficiently under a considerably strong built-in electric field. Surface-photovoltage-spectroscopy (SPS) measurements were carried out with a solid-junction photovoltaic cell indium tin oxide (ITO)/sample/ITO using a light source-monochromatorlock-in-detection technique. Monochromatic light was obtained by passing light from a 500-W xenon lamp through a doubleprism monochromator (Higher and Watts, D300). A lock-in
S1089-5647(97)03056-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/06/1998
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Figure 1. Scheme of the SPV experimental cell and the sample condition.
amplifier (Brookdeal, 9503-SC), synchronized with a light chopper, was employed to amplify the photovoltage signal. Electric-field-modulated surface-photovoltage spectroscopy (EFMSPS) is a technique that combines the field-effect principle with SPS. Figure 1 shows the scheme of a SPV cell with the composite microsphere multilayers between two ITO electrodes. The external electric field (EEF) is applied to the two sides of the sample and is regarded as positive when the side under illumination is connected to a positive electrode. 3. Results 3.1. Structure and the Morphology of the Composites. Figure 2a is the TEM image for the morphology of the polymer core adhered with Fe2O3 particles in the dark. In Figure 2b, the composite microspheres are encapsulated with another polymer shell in a trilayer structure. To observe the coreshell structure, the particles were stained by phosphotungstic acid. The butyl acrylate in the polymer-shell layer was negatively stained to increase the contrast between the outlayer in the light and the core in the dark. From the images we can see that the polymer shell is about 10 nm thick. The thickness of the second polymer layer can be controlled by the monomer amount added during emulsion polymerization. But the middle layer of Fe2O3 is not very clear. However, it can be followed that Fe2O3 particles are contained in the spheres by the electrondiffraction pattern in Figure 2c, which is attributed to the hexagonal crystals of Fe2O3. The characterization of the heterostructure has been discussed in detail in ref 14. The interaction between Fe2O3 and the polymer core was confirmed by IR spectral changes of the surface groups of the seed latex. Mossbauer spectra gave evidence for the changes of electric density and electric symmetry around Fe2O3 and indicated that Fe2O3 particles were encapsulated in polymer. 3.2. Characteristics of UV-Visible Absorption. According to the electronic structure of Fe2O3 reported previously,15 the optical absorption band in the visible and UV regions were attributed to the transition in the crystal field and the chargetransfer processes, respectively. Figure 3 shows the absorption spectra of the copolymer themselves and the Fe2O3-encapsulated composites. The copolymer has absorption before 300 nm (Figure 3a). In the case of the composites (Figure 3b), the strongest absorption band is assigned to the electron transfer from O 2p to Fe 3d, corresponding to the valance band to the conductive band transition, and the small shoulder at 480 nm is considered to arise from a crystal-field spin-forbidden transition (d-d transition).
Du et al. 3.3. Measurement of SPS and EFISPS. SPS of Fe2O3 powder (Figure 4a) is in good agreement with UV spectra. The peaks at 340 and 470 nm correspond to the electron transfer and the d-d transition, respectively. Similar results in Figure 4b were also obtained for the polymer seed latex adhered with Fe2O3 powder without the second polymer shell. This indicates that the photovoltage response is caused by Fe2O3. But when the encapsulated composites were concerned, no photovoltage response appeared (Figure 4c). Only if an external electric field (EEF) was applied could an apparent SPS response be seen (Figure 5). In addition, it was found that the higher the positive external electric field, the more intensive the photovoltage (inset of Figure 5, a nearly linear relationship between the SPS response and the EEF intensity). In constrast, if an opposite EEF was added, the response decreased. When the positive EEF was removed, as shown in Figure 6, the response decayed slowly with time until it disappeared. The above phenomena is similar to Fe2O3 themselves except that the SPV response of Fe2O3 decayed to the initial intensity very quickly when the EEF was removed. 4. Discussion Bulk Fe2O3 is an n-type semiconductor with a band gap of 2.3 eV (Ec ) -4.8 eV, Ev ) -7.1 eV with respect to the vacuum-energy level).16 While for Fe2O3 nanoparticles, the band gap should be broadened. The threshold of the band-band absorption in Figure 4a is 400 nm corresponding to the band gap of 3.1 eV. So far as the electric property is concerned, polystyrene (PS) can also be considered as a semiconductor with a large band gap of 4.6 eV (Ec ) -2.5 eV, Ev ) -7.1 eV, Ef ) -4.8 eV).17 The Femi level of ITO is -4.6 eV. According to the above data and the SPV measurement cell structure in Figure 1, there are two kinds of energy band models shown in Figure 7. In one of them, Ef(Fe2O3) is higher than -4.8 eV and approaches Ec(Fe2O3) (Figure 7a). In the other, Ef(Fe2O3) is lower than Ef(PS) between -4.8 eV and -5.9 eV(Figure 7b). As shown in the models, there are heterojunctions of Fe2O3/PS and Schottky potential barriers on PS/ITO interfaces. On the conductive band and the valance band of the model, there are several deep electron traps and shallow hole traps, respectively. The built-in electric fields on heterojunction A and B are in opposite direction. But at the balance state without an external electric field, the potential heights of the two kinds of junctions, whether for holes or electrons, are equal to each other. This means that ∆Ea ) ∆Eb, ∆Ed ) ∆Ec. When the sample was under illumination, the photocreated electrons were produced on the conductive band of Fe2O3 while the corresponding holes were left on the valance band. This is attributed to the band-band transition of Fe2O3. But the photogenerated electrons and holes of Fe2O3 cannot be transported through the polymer layer immediately, since there is no external electric field and the conductivity of polystyrene is very low.17 The photogenerated electrons and holes will be recombined gradually. No charge separation takes place and no SPV response of Fe2O3 can be measured as shown in Figure 4c. When a positive external electric field was applied (for example, +1.5 V), the whole energy band became tilted (Figure 7, stable-state model). We choose the band model in Figure 7a as an example to describe the process in detail. Under the EEF, the potential barriers of the holes (∆Ed) and the electrons (∆Ea) on heterojunction A are increased, since the built-in electric field here is in the same direction as the external electric field. In contrast, the potential barriers of the holes (∆Ec) and
Polymer/Fe2O3/Polymer
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Figure 2. (a) TEM image of the polymer cores adhered with Fe2O3 nanoparticles. (b) TEM image of the trilayer particles enapsulated with Fe2O3. (c) Electron-diffraction pattern of the particles in (b).
Figure 3. UV-visible spectra of (a) polymer itself and (b) polymer encapsulated with Fe2O3 nanoparticles.
Figure 6. Photovoltage reponse of polymer/Fe2O3/polymer heterostructure at 340 nm is increased by a positive EEF and decreased by a negative EEF. When the EEF is removed, it decays with time.
Figure 4. SPV spectra without external electric field: (a) Fe2O3 nanoparticles; (b) Fe2O3 adhered on the surface of polymer spheres; (c) Fe2O3 encapsulated in polymers.
Figure 5. SPV spectra of Fe2O3 encapsulated in polymers under different external electric fields: (a) +0.5 V, (b) +1.0 V, and (c) +1.5 V. The inset figure is the photovoltage at 420 nm of encapsulated composites as a function of external electric field.
the electrons (∆Eb) on heterojunction B, where there is an opposite built-in field, are decreased. So the whole energy band model, in which the left is higher and the right is lower, changed into a slope. In this case, the hole traps in the valance band
Figure 7. Energy band model scheme of Fe2O3/PS/ITO multilayer structure under a positive electric field. All energy levels were given vs vaccum energy level. (a) EfFe2O3 is higher than EfPS. (b) EfFe2O3 is lower than EfPS.
become shallower and it is favorable for the holes to move from right to left. In addition, the electronic mobility in polystyrene is largely improved under the external electric field.18 This provides a possibility for carriers to transport through the polymer layer. So under the photoillumination, the photogenerated holes of Fe2O3 moved from right to left along the valance band. Oppositely, the photogenerated electrons can be
2332 J. Phys. Chem. B, Vol. 102, No. 13, 1998 transported through the polymer layers from left to right along the electric-field direction and moved to the ITO surface. Since there are a series of traps representing the molecular folding, bond adjustment, the protrusion of side and end groups in the polymer, and the transport of electrons in polystyrene can be characterized as a trap-modified, space-charge-limited process.17 Under the external electric field, more and more electrons were able to skip the traps and move along the electric-field direction. The electronic mobility in PS was increased. The electrons were transported from the left trap to the right one by penetration through the PS layer. Gradually, they were accumulated in the trap nearest the ITO. At the same time, the electrons in trap D penetrate through the PS layer and move to ITO surface. This results in the SPV response of Fe2O3 as shown in Figure 5. Moreover, with the transportation of holes and electrons, the hole density in trap C becomes higher and higher. While in trap D, there is a heavy electron population. Charge separation was obtained in the whole energy band model. As the EEF is removed, the potential barrier on heterojunction A tends to be decreased and that of heterojunction B tends to be increased. The whole energy band is likely to be returned to the original balance-state model without EEF. But during a certain period, the stable state model in Figure 7a cannot be changed into the balance state completely. This is because when the external electric field was removed, the electronic mobility of polystyrene was decreased and the recombination of electrons and holes needed a rather long time (about 50 min shown in Figure 6). The whole energy band model still has a tilt trend. At this time, the SPV response can still be seen, though it became weaker and weaker. Only after a sufficiently long time did the tilt of the energy model disappear. The photogenerated electrons and holes cannot be transported to the opposite direction, and the SPV response decayed to zero. The whole process of SPV generation and disappearance of the heterostructure was clearly indicated in Figure 6. Although the above analysis is mainly about the first energy band model in Figure 7a, the same results can also be obtained in the second band model in Figure 7b. 5. Conclusions Fe2O3 nanoparticles were encapsulated between the polymer core and the shell into a trilayer heterostructure. The passivation
Du et al. of polymer films on Fe2O3 was investigated by electric-fieldinduced surface-photovoltage spectroscopy. A reasonable energy band model was supposed to analyze the charge-separation process. When there is no external electric field, the photogenerated holes of Fe2O3 cannot be transported through the polymer layer with poor conductivity and no SPV response can be obtained. While under the external electric field, the SPV response is induced by the higher electron mobility of polymer films and the tilt of the whole energy band model. The photocreated electrons and holes are accumulated respectively in electron-potential barriers and hole traps. As the EEF is removed, the SPV response of the composites decays much more slowly than that of Fe2O3 themselves. The charge separation under the external electric field in this process will be significant to the photoelectron information-storage technique. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Hines, M. A.; Sionnest, P. G. J. Phys. Chem. 1996, 100, 468. (3) Murray, C. B.; Kagan, C.; Bawendi, M. G. Science 1995, 270, 1335. (4) Hasagawa, M.; Arai, K.; Saito, S. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 3231. (5) Furusawa, K.; Anzai, C. Colliod Polym. Sci. 1987, 265, 882. (6) Lee, J.; Senna, M. Colloid Polym. Sci. 1995, 273, 76. (7) Haga, Y.; Inoue, S.; Nakajima, M.; Yosomiya, R. Mater. Chem. Phys. 1988, 19, 381. (8) Haga, Y.; Inoue, S.; Sato, T.; Yosomiya, R. Die Angew. Makromol. Chem. 1986, 139, 49. (9) Haga, Y.; Inoue, S.; et al. Die Angew. Makromol. Chem. 1991, 188, 73. (10) Du, H.; Kan, S.; Zhang, G.; Liu, F.; Tang, X.; Li, T. Chem. J. Chin. UniV. 1995, 16 (11), 33 (11) Kan, S.; Yu, S.; Peng, X.; Zhang, X.; Li, T. J. Colloid Interface Sci. 1996, 178, 673. (12) Morrison, S. R. The Chemical Physics of Surfaces; Plenum Press: New York, 1977; Chapters 3 and 9 (13) Wang, D.; Zhang, J.; Shi, T.; Wang, B.; Cao, X.; Li, T. J. Photochem. Photobiol., A 1996, 93, 21. (14) Du, H.; Zhang, P.; Liu, F.; Kan, S.; Wang, D.; Li, T.; Tang, X. Polym. Int. 1997, 43, 274. (15) Marusak, L. A.; Messier, R.; White, W. B. J. Phys. Chem. Solids 1980, 41, 981. (16) Leland, J. K.; Bard, A. J. J. Phys. Chem. 1987, 91, 5076. (17) Fabish, T. J.; Duke, C. B. Polymer Surfaces; Wiley-Interscience: Chichester, 1978; Chapters 4-6. (18) Ma, D. Z.; He, P. S.; Xu, Z. D.; Zhou, Y. Q. Structure and Property of Polymers; Science Publication: Beijing, 1995.
