New Polymer−Inorganic Nanocomposites: PEO−ZnO and PEO−ZnO

J. Phys. Chem. B 2001, 105, 10169-10174

10169

New Polymer-Inorganic Nanocomposites: PEO-ZnO and PEO-ZnO-LiClO4 Films Huan-Ming Xiong, Xu Zhao, and Jie-Sheng Chen* State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, Department of Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China ReceiVed: January 25, 2001; In Final Form: June 22, 2001

Polymer-inorganic nanocomposite films of PEO-ZnO and PEO-ZnO-LiClO4, where PEO stands for poly(ethylene oxide), have been prepared through a film-casting method. Interactions between PEO (MW ) 600 000) and ZnO nanoparticles (average size 3.5 nm, with acetate groups on the surface) decrease the photoluminescence intensity of the PEO film to a great extent. These interactions, which are also manifested by the X-ray diffraction, depend on the concentration and the aggregation of the ZnO nanoparticles in the PEO-ZnO films. The cooperative effect of PEO and LiClO4 changes the unidentate coordination mode of the acetate groups with zinc into three coexisting modes: unidentate, bidentate, and bridging, suggesting that PEO segments, lithium ions, and the acetate groups on ZnO nanoparticle surface form cross-linking structures. Such cross-linking structures, on one hand, reduce Li+ClO4- ion pairs to release more free ions as charge carriers in the PEO-ZnO-LiClO4 film and, on the other, decrease the film crystallinity to produce more amorphous regions for charge carriers to transfer, and finally enhance the conductivity of the film.

Introduction The integration of polymer and inorganic materials, which were thought to be separate disciplines several years ago, has constructed an attractive field in materials science.1 Explorations in this field during the past few years have made exciting progress. A variety of polymer-inorganic nanocomposites, which possess interesting electrical,2-4 optical,5-7 and magnetic8,9 properties usually superior to those of the parent polymer or inorganic species, have been reported in the literature. ZnO nanoparticles were used as a luminescence material for a long time and many methods, such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), metalorganic vapor-phase epitaxy (MOVPE) and sol-gel technique were developed to prepare nanosized ZnO. Among these methods, the sol-gel approach10-14 is widely employed and it is also easy to handle. Anderson and co-workers11-13 developed a method to synthesize colloids of ZnO wurtzite nanocrystals. In their work, zinc acetate was hydrolyzed by reaction with lithium hydroxide to form ZnO colloid in ethanol. Recently, Meulenkamp14 isolated pure ZnO nanoparticles from the colloids through a precipitation-redispersion method. In his report, organic “nonsolvents” (referring to solvents that are miscible with ethanol but cannot dissolve ZnO nanoparticles) were used to precipitate ZnO gels from the colloids, and the gels could be redispersed in solvents. Therefore, ZnO nanoparticles either in solvents or in solid states can be investigated without the influence of impurities. The composites of the PEO (poly(ethylene oxide)) and lithium salts are recommended as the solid electrolytes for future lithium batteries, while their conductivities at room temperature are usually too low to be applied practically in lithium batteries.15-17 There are two main reasons for this. One is that PEO (MW > 30 000) is semicrystalline while charge carriers can only transfer within its amorphous phase; the other is that conductivity is determined by the free ion concentration, while the dissociation degree of the ion pairs decreases as salt concentration increases and the coordination of the PEO segments with the lithium ions prevents the movement of the lithium ions.

To enhance the conductivities of the films, various plasticizers such as ethylene carbonate (EC), propylene carbonate (PC), and tetraethylene glycol (TEG) were added into the films. 18,19 Some of the as-prepared films have conductivities higher than 10-4 S/cm at room temperature. Unfortunately, most of the asprepared films are not suitable for application in lithium batteries because the plasticizers may volatilize, decompose, or react with lithium metal electrode. And the addition of liquid plasticizers would destroy the mechanical stability of the solid electrolytes, which would have been an advantage of the polymer electrolytes.2 Another strategy for conductivity enhancement is to add inorganic fillers such as ferroelectric BaTiO3,20 zeolites,21 and especially ceramic powders.22-26 These fillers are able to increase the mechanical stability as well as the conductivity. Recently, Croce et al.2,27 reported that adding TiO2 and Al2O3 nanoparticles into a PEO-LiClO4 film could raise its conductivity to about 10-5 S/cm at 30 °C. They ascribed such enhancement to Lewis acid-base interactions between Lewis acid sites (or OH groups)28 on the nanoparticle surface and both the ClO4- anions and the PEO segments. These interactions render the nanoparticles centers for PEO cross-linking so as to release more free lithium ions and produce more amorphous regions for charge carriers to transfer. Similar Lewis acid-base effects were also found for the systems containing the fillers SiO2,28 R-Al2O3,29,30 AlCl3, and AlBr3.30 Wieczorek and coworkers further found Al2O3 species with neutral, Lewis-acidic and basic surface groups all can increase the conductivities of the PEG-LiClO4 composite.31 In this paper, we describe the preparation and characterization of a new type of polymer-inorganic nanocomposite materials: PEO-ZnO and PEO-ZnO-LiClO4 films. Although ZnO nanoparticles dispersed in solvents have been studied in detail by many researchers,10-14 the luminescence spectra of the ZnO nanoparticles dispersed in solid substrates have rarely been reported before the present work. We found that the PEOZnO composite exhibited interesting photoluminescence properties. Furthermore, when ZnO nanoparticles were added into PEO-LiClO4 film, the ionic conductivity of the film increased

10.1021/jp0103169 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

10170 J. Phys. Chem. B, Vol. 105, No. 42, 2001 significantly. After evacuation, the only species on ZnO nanoparticle surface are the Lewis-basic acetate groups and the Lewis acid-base interactions within the PEO-ZnO-LiClO4 composite were elucidated through IR spectroscopy. Experimental Section Synthesis of ZnO Nanoparticles. The procedures to synthesize ZnO nanoparticles were similar to those previously reported by Anderson and co-workers. Zinc acetate (Zn(Ac)2‚ 2H2O, AR) was dissolved in absolute ethanol (>99.8%, GR) at about 80 °C, stirred, and refluxed for 3 h to obtain a 0.1 M solution that was subsequently cooled to room temperature. LiOH‚H2O was added into the solution with a molar ratio of [Li]/[Zn] ) 1.4. The mixture was placed into an ultrasonic bath in order to dissolve LiOH‚H2O completely. After 1 h, the mixture was filtered through a glass fiber filter (0.1 µm pore size) to remove the insoluble residue. ZnO colloid was obtained and it emitted fluorescence under ultraviolet light. ZnO nanopaticles were precipitated with cyclohexane (volume ratio: C6H12/C2H5OH ) 3) and centrifuged. The precipitate was redispersed in absolute ethanol ultrasonically and centrifuged at a high speed (104 rounds per minute) to obtain a clear solution. This solution was stable for more than one month at ambient temperature. Its concentration was calculated by drying appropriate volumes of the colloid at 100 °C and weighing the obtained ZnO powder. A considerable amount of ZnO nanoparticle powder was also prepared by rotary evaporation at 30 °C. Preparation of PEO-ZnO and PEO-ZnO-LiClO4 Films. PEO (MW ) 600 000, Aldrich) was dissolved in acetonitrile under stirring for hours and centrifuged at a speed of 104 rounds per minute to get a clear solution. Its concentration was measured in the same way as for the ZnO colloid. LiClO4‚3H2O was dried in an oven at 160 °C for several days to get anhydrous LiClO4, and the latter was kept in a vacuum desiccator. An appropriate amount of anhydrous LiClO4 was dissolved in acetonitrile for use. To prepare the nanocomposite films, PEO solution and LiClO4 solution were mixed together. The molar ratio of the ethylene oxide segments over the lithium ions was maintained at [EO]/[Li] ) 8. Various amounts of ZnO ethanol sol were added into the mixture of PEO and LiClO4 under stirring, and in this way a number of PEO-ZnO-LiClO4 composites were obtained. The final mixtures were stirred at room temperature until they became very thick after solvent evaporation. Then they were cast in PTFE containers and dried in a vacuum desiccator for 1 day to form free-standing homogeneous films of about 150 µm thick. The PEO-ZnO films were prepared in the same way without the addition of the LiClO4 solution. Characterization. A Siemens D5005 X-ray diffractometer was used to record the XRD patterns (scan rate 1°/min CuKR radiation λ ) 1.5418 Å) of the ZnO nanoparticle powder and the as-prepared PEO-ZnO films. The diluted ZnO colloid was dropped onto a small copper mesh and left at room temperature so that the ZnO nanoparticles precipitated homogeneously on the carbon films among the tiny pores of the copper mesh. This sample was used to take TEM images under a Hitachi-8100 IV transmission electron microscope operated at 200 kV. The UV-Vis diffuse reflectance spectra of the ZnO powder sample, PEO film and PEO-ZnO films were recorded on a Perkin-Elmer Lambda 20 UV-Vis spectrometer equipped with an integrating sphere, using BaSO4 as the background material. The scanning range was from 850 to 200 nm, and the resolution

Xiong et al. was 1 nm. The as-prepared PEO-ZnO films, the PEO film and the ZnO nanoparticle powder pellets were mounted on a Hitachi F-4500 fluorescence spectrophotometer, respectively, to measure their fluorescence properties. PEO, PEO-ZnO, and PEO-ZnO-LiClO4 films for IR study were prepared by casting several drops of the solution on KBr pellets because films peeled from PTFE containers were too thick for IR spectroscopic measurements. ZnO powder samples were pressed into pellets with KBr. For the sample of the ZnOLiClO4 mixture, ZnO colloid and LiClO4 solution were mixed and dried at ambient temperature, and the obtained powder was also pressed into pellets with KBr. The IR spectra of the asprepared samples were recorded on a Nicolet Impact 410 FTIR spectrometer within 4000-400 cm-1 wavenumber region. The in-situ IR spectra were recorded within 4000-1000 cm-1 wavenumber region in a cell with CaF2 windows after the samples were evacuated under 10-4 Torr at 100 °C for 5 h. For AC impedance measurements, the PEO-ZnO-LiClO4 films were peeled from PTFE containers and cut into round pieces in the same size as the stainless steel electrodes (diameter 15 mm). The films thus obtained were coated on the surfaces of the electrodes and evacuated under 10-4 Torr at 100 °C for 5 h in order to remove the solvent thoroughly. Afterward, they were sealed in a glass vessel and transferred into a vacuum glovebox and kept in a vacuum. After recrystallization at room temperature for two weeks, the films with electrodes were taken out from the glass vessel in a nitrogen atmosphere, sandwiched between two stainless steel electrodes, and fixed in PTFE tubes. The measurements were performed in an anhydrous environment, using a Solartron SI 1287 electrochemical interface and a Solartron SI 1260 impedance/gain-phase analyzer. The scanning frequency ranged from 106 Hz to 0.1 Hz and the ac voltage applied was adjusted according to the resistance of the film, i.e., 3 V for about 107 Ω, 1 V for about 105 Ω, and 0.5 V for below 103 Ω. The heating rate was kept at 1 °C/min from room temperature to 100 °C. The resistance of the film was obtained from circle fit of the impedance spectrum and the conductivity of the film was calculated by taking into account its thickness and area. For comparison, the pure PEO film and the PEOZnO film (x ) 0.05) was treated and tested in the same way. The as-prepared ZnO powders were pressed into pellets (250 µm thick, 1.2 cm in diameter) and coated with Ag powders on both faces, whose conductivities were measured at 20 °C and 100 °C. Results and Discussion Dispersion and Aggregation of ZnO Nanoparticles in PEO Films. The average size of the ZnO nanoparticles is calculated on the basis of the XRD patterns (Figure 1). The Debye-Scherer formula t ) 0.89λ/(β cos θ) is used, where t stands for the average diameter of the particles, λ for the X-ray wavelength (1.5418 Å), θ for the Bragg diffraction angle (half of the measured diffraction angle), and β for the peak width in radians at half-height. The calculation result is about 3.5 nm, which is in agreement with the TEM image (Figure 2). Figure 2 also shows that the size of the ZnO nanoparticles is homogeneous and the aggregation of the nanoparticles rarely appears in the colloid. The mixing of such small particles in colloidal form with PEO ensures ZnO dispersed homogeneously in the films. The UV-Vis spectra of the ZnO powder and the PEO-ZnO films were recorded in diffuse reflectance values and transformed into Kubelka-Munk units as shown in Figure 3. The obtained spectra indicate that the higher the ZnO concentration in the PEO-ZnO films, the more red-shifted the spectral onset

PEO-ZnO and PEO-ZnO-LiClO4 Films

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10171

Figure 1. Powder X-ray diffraction patterns of the as-prepared ZnO nanoparticles.

Figure 4. X-ray diffraction patterns of (a) the PEO film and the PEOZnO films: (b) x ) 0.01, (c) x ) 0.02, (d) x ) 0.05, (e) x ) 0.1, (f) x ) 0.2, and (g) x ) 0.5.

Figure 2. Transmission electron microscopic images of the ZnO nanoparticles, showing an average particle size of about 3.5 nm.

Figure 3. UV-Vis diffuse reflectance spectra of the PEO-ZnO films with various ZnO/PEO weight ratios (x): (a) x ) 0.01, (b) x ) 0.02, (c) x ) 0.05, (d) x ) 0.1, (e) x ) 0.2, (f) x ) 0.5, (g) the ZnO nanoparticle powder (dotted line), and (h) the PEO film.

(λonset), suggesting that larger ZnO aggregates have formed. When the ZnO/PEO weight ratio is increased to x ) 0.2, the λonset of the composite film is very close to that of the as-prepared ZnO powder. Furthermore, when the weight ratio reaches x ) 0.5, the size of the aggregated ZnO is even larger than that of the as-prepared ZnO powder as indicated by their λonset. Anderson and co-workers reported that the UV-Vis absorption and photoluminescence spectra of the ZnO sol redshifted significantly with the concentration increase,11,13 but such a large red-shift of the spectral onset has never been found in ZnO sol, distinguishing the aggregation of ZnO nanoparticles in PEO substrate from that in solvents. We consider that the aggregation degree during the process from sol to solid state

depends on the time for solvent evaporation. Dipping and spinning methods to form ZnO films only take a few minutes and the film λonset(∼360 nm) is close to the sol λonset.12 Rotary evaporation to obtain powder at 30 °C takes about 1 h and the powder reflectance onset (λonset ) ∼390 nm)10 is red-shifted significantly. PEO-ZnO film preparation takes about 10 h and a high degree of aggregation is expected. In Figure 4, the XRD patterns of PEO-ZnO films with different compositions are compared. It is seen that even a small amount of ZnO nanoparticles (x ) 0.01) decreases the crystallinity of the PEO film sharply (compare Figure 4a and Figure 4b). Interestingly, the XRD intensities of PEO-ZnO films decrease from x ) 0.01 to x ) 0.05 and then increase from x ) 0.05 to x ) 0.1. However, the diffraction intensities of the sample decrease again with the x value being increased from 0.1 to 0.5. This phenomenon results from the complex effects of the concentration and the aggregation of ZnO nanoparticles. The crystallinity decrease of PEO-ZnO films is determined by the interactions between PEO and ZnO. If the x value is relatively small, the higher the ZnO concentration, the stronger the interactions, and as a result, the film crystallinity decreases with x value increase in the range of x ) 0.01-0.05. However, the strength of the interactions between PEO and ZnO is also affected by the aggregation of the ZnO nanoparticles. When the effect of the ZnO aggregation exceeds that of the ZnO concentration, the crystallinity of the film increases as shown from x ) 0.05 to x ) 0.1. In the range x ) 0.1-0.5, the XRD intensity decrease of PEO peaks is mainly because the relative amount of PEO is reduced to a considerable extent. It is noted that in this concentration range the XRD intensity for ZnO reflections increases significantly with ZnO addition. Photoluminescence of PEO-ZnO Films. It is well-known that ZnO nanoparticles exhibit two emission peaks when excited with UV light: one is at about 360 nm due to direct recombination of photogenerated electron-hole pairs between the ZnO band gap,10 and the other is near 520 nm (Figure 5d) whose mechanism seems much more complicated. The most popular two theories on the ZnO green emission are “anion vacancies”

10172 J. Phys. Chem. B, Vol. 105, No. 42, 2001

Xiong et al.

Figure 5. Photoluminescence spectra of PEO film and ZnO nanoparticle powder pellet: (a) PEO excitation spectrum detected at 400 nm, (b) ZnO excitation spectrum detected at 500 nm, (c) PEO emission spectrum excited at 320 nm, and (d) ZnO emission spectrum excited at 360 nm.

Figure 7. IR spectra for (a) the as-prepared ZnO nanoparticle powder, (b) PEO-LiClO4 film, (c) PEO-ZnO-LiClO4 film, and (d) sample (c) after in-situ heating under vacuum.

Figure 6. Photoluminescence excitation spectra detected at 500 nm (left part) and emission spectra excited at 320 nm (right part) of the PEO-ZnO films and the ZnO nanoparticle powder pellet: (a) x ) 0.01, (b) x ) 0.02, (c) x ) 0.05, (d) x ) 0.1, (e) x ) 0.2, (f) x ) 0.5, and (g) ZnO pellet.

(or adsorbed Zn2+ luminescence centers on surface)32-38 and photogenerated electrons tunneling to preexisting, trapped holes.10-12 Figure 5 also shows photoluminescence (PL) spectra of the PEO film which have three excitation peaks in the ultraviolet region and one emission peak at about 410 nm when excited with 320 nm light. To compare the PL intensities and the peak positions of different films, the excitation wavelength is fixed at 320 nm and the emission wavelength for excitation detection is fixed at 500 nm as shown in Figure 6. On one hand, either the excitation or the emission for ZnO in the PEO-ZnO films redshifts when the x value increases, and on the other, the PEO and ZnO emission intensities vary distinctly in the meantime. The excitation peaks and the ZnO green emission peaks increase from x ) 0.01 to x ) 0.05, and then decrease from x ) 0.05 to x ) 0.5. Meanwhile, the PEO blue emission decreases sharply when a small amount of ZnO is added (compare Figure 6a, right part and Figure 5c), and finally disappears as the x value increases to x ) 0.1. Hence the existence of substantial interactions between the host PEO and the guest ZnO is deemed undoubted. We consider the “anion vacancies” mechanism for ZnO green emission is compatible with our results. According to the calculation by Monticone and co-workers on the energy difference between the excited state and the ground state,38 ZnO

green emission red-shifts when the particle size increases. And its intensity depends on the singly ionized oxygen vacancy centers or the adsorbed Zn2+ luminescence centers on nanoparticle surface.37 Therefore, under a given concentration, the more aggregated the ZnO nanoparticles, the smaller the total surface areas and the weaker the emission intensities. This explains, at least partially, the emission intensity changes of the PEO-ZnO films. IR Spectroscopy for the PEO-ZnO-LiClO4 Films. Figure 7 shows the IR spectra of different samples within the region 4000 -1000 cm-1. The as-prepared films and ZnO nanoparticle powder exhibit strong absorption at about 3500 cm-1, indicating that both ZnO nanoparticles (Figure 7a) and PEO-LiClO4 film (Figure 7b) adsorb a large amount of water. But the water molecules can be removed thoroughly from the PEO-ZnOLiClO4 film (Figure 7c) after heating at 100 °C, 10-4 Torr for 5 h (Figure 7d). This point is very important, because trace amount of water can greatly enhance the conductivity of the PEO-LiClO4 film.39 Water reacts with lithium electrodes, and hence such conductivity enhancement must be avoided concerning film application in lithium batteries. In Figure 8 the IR spectra of the as-prepared samples are compared in detail. The triplet band of the C-O-C stretching vibrations with maxima at 1147, 1116, and 1062 cm-1, and the split band of CH2 wagging mode with maxima at 1357 and 1343 cm-1 are evidence for the presence of a crystalline PEO phase.40 They are observed clearly in the IR spectra of the PEO film (Figure 8-I-b) and PEO-ZnO film (Figure 8-I-c). But in the IR spectrum of the PEO-LiClO4 film (Figure 8-I-d) only two resolved peaks (downshifted to 1143 and 1112 cm-1) are observed in the former triplet band region, indicating that the Lewis acid-base interactions between lithium ions and PEO segments weaken the PEO C-O-C stretching vibrations.40 Although the interactions between PEO and ZnO nanoparticles decrease the crystallinity of the PEO according to the XRD analysis, these interactions have no influence on the vibration modes of the PEO molecules (compare Figure 8-I-b and Figure 8-I-c).

PEO-ZnO and PEO-ZnO-LiClO4 Films

J. Phys. Chem. B, Vol. 105, No. 42, 2001 10173

Figure 8. IR spectra of different samples in detail. I: (a) ZnO nanoparticle powder, (b) PEO film, (c) PEO-ZnO film, (d) PEOLiClO4 film, and (e) PEO-ZnO-LiClO4 film. II: (a) ZnO nanoparticle powder, (b) sample (a) after in-situ heating under vacuum, (c) ZnOLiClO4 mixture, (d) PEO-ZnO film, (e) PEO-ZnO-LiClO4 film, and (f) sample (e) after in-situ heating under vacuum.

SCHEME 1: Three Modes of Bonding Structures for Acetate Group and Metal

TABLE 1: Peak Wavenumbers for Various Coordination Modes of Acetate Group with Metal unidentate bidentate bridging

νC)O

νC-O

1579 1547 1600

1425 1454 1441

However, when a small amount of ZnO nanoparticles (x ) 0.05) is added into the PEO-LiClO4 composite, the IR spectrum changes dramatically (compare Figure 8-I-d and Figure 8-I-e). The former triplet band of the C-O-C stretching vibrations becomes a single band with the absorption maximum downshifting from 1116 to 1107 cm-1. This confirms that the interactions in PEO-ZnO-LiClO4 lead to the formation of cross-links that further weaken the C-O bonds. In the meantime, the split band of the CH2 wagging mode at 1357 and 1343 cm-1 of PEO degenerates into a single band, as a result of the composite becoming more amorphous. It is more interesting that the CdO stretching band at 1579 cm-1 and the C-O stretching band at 1425 cm-1 for the acetate groups on ZnO surface (Figure 8-I-a) each split into three absorption peaks (Figure 8-I-e) when the ZnO nanoparticles are dispersed in the PEO-LiClO4 film. The bond structures corresponding to these split bands are illustrated in Scheme 1, and the vibration frequencies in wavenumbers are listed in Table 1.13 Anderson and co-workers reported similar coordination change on ZnO nanoparticle surface after heating the nanopar-

ticles at 190 °C in air for hours.12 They suggested that the bonding structure of acetate with zinc changed from unidentate type to bidentate type because the latter was more stable thermodynamically. However, in the present work heating is not the cause for the bonding-mode change, because the change occurs in the film-preparation process. Figure 8-II-a shows the IR spectrum for the as-prepared ZnO sample. After heating at 100 °C, 10-4 Torr for 5 h, the vibration of CdO and C-O of the acetate groups on ZnO surface remains unchanged (Figure 8-II-b) whereas the absorption bands for water and/or ethanol disappear completely (not shown in this figure). This indicates that the solvent molecules adsorbed on ZnO surface have nothing to do with the unidentate type of the acetate groups, and the above-mentioned procedure cannot remove acetate groups from ZnO surface. The coordination type change of the acetate groups is observed in the IR spectrum of the as-prepared PEO-ZnO-LiClO4 (Figure 8-II-e) and that of the in situ treated PEO-ZnO-LiClO4 (Figure 8-II-f). It should be mentioned that the three peaks at 1600, 1579, and 1547 cm-1 in Figure 8-II-e and Figure 8-II-d are ascribed to the vibration modes of the CdO bonds, whereas the peaks at 1465 and 1454 cm-1 might be due to the CH2 bending mode in PEO.41 Only the peak at 1425 cm-1 is undoubtedly ascribed to the unidentate mode of the C-O bonds and their bridging mode at 1441 cm-1 is not observed in Figure 8-II-e and Figure 8-II-f. Since the splitting of the bands does not occur in the IR spectrum of the ZnO-LiClO4 mixture (Figure 8-II-c) and the PEO-ZnO film (Figure 8-II-d), it is the cooperative effect of the PEO and LiClO4 that changes the coordination modes of the acetate groups with the zinc metal. The lithium ions are assumed to coordinate with PEO segments and the acetate groups on ZnO surface forming “bridges” of [PEO-Li+-Ac]. Hence, the unidentate type of some acetate groups changes to bridging type and the blocks of the PEO segments may press the unidentate mode of some acetate groups into bidentate mode. As a result, the film becomes more amorphous and more ClO4- anions are released from Li+ClO4- ion pairs. Conductivity Enhancement of PEO-ZnO-LiClO4 Films. The conductivities of the PEO8-LiClO4 electrolyte vary in the literature. Croce and co-workers reported a value of about 10-8 S/cm at 30 °C earlier2 and one of about 10-7 S/cm at the same temperature later.28 Wieczorek and co-workers showed a value of about 10-6 S/cm at 25 °C in their paper.40 In fact, the PEO8LiClO4 conductivity depends critically on its crystallinity that is determined by the thermal history of the sample. In our experiments, the conductivity of a PEO8-LiClO4 film is higher than 10-6 S/cm after cooled to room temperature immediately from treating at 100 °C. However, when the sample is kept in a vacuum for two weeks its conductivity falls to about 10-9 S/cm and changes little even after two months. This indicates that the sample reaches its stable state in terms of crystallization by aging for two weeks. Croce and co-workers observed similar phenomena.28 In Figure 9 the ionic conductivities of the PEO-LiClO4 and the PEO-ZnO-LiClO4 films (x ) 0.05) are compared at various temperatures. The conductivities for the latter film are more than 1 order of magnitude higher than those for the ZnOfree film at around room temperature, whereas the conductivities for the two films get close to each other at above 70 °C. The conductivity difference at room temperature is ascribed to the interactions in PEO-ZnO-LiClO4 film. On the basis of the changes of the XRD patterns and IR spectra, the conductivity enhancement can be explained in two aspects. First, the crosslinking in the PEO-ZnO-LiClO4 composite decreases the

10174 J. Phys. Chem. B, Vol. 105, No. 42, 2001

Xiong et al. and the Education Ministry of China. We also thank Wei Xu for assistance in conductivity measurements. References and Notes

Figure 9. Plots of the film conductivities versus temperature: (a) PEO-LiClO4 film, [EO]/[Li] ) 8, (b) PEO-ZnO-LiClO4 film, [EO]/ [Li] ) 8, x ) 0.05.

crystallinity of the films and consequently produces more amorphous regions for charge carriers to transfer. Second, the cooperative effects of PEO and ZnO nanoparticles reduce Li+ClO4- ion pairs and release more free ClO4- anions, which act as charge carriers in the polymer electrolyte. When the temperature increases to above the melting point of PEO (about 70 °C), the dissociation degree of LiClO4 increases and the composite becomes wholly amorphous, so the effect of ZnO nanoparticles becomes negligible. To make clear whether the electronic conductance of ZnO semiconductor contributes to the conductivity enhancement, we tested the conductivities of the as-prepared ZnO powder pellets and the PEO-ZnO film (x ) 0.05) at 20 °C and 100 °C, respectively. The conductivities of the ZnO pellet at both temperatures are lower than 10-9 S/cm, while those of the PEO-ZnO film are about 10-10 S/cm at 20 °C and 10-6 S/cm at 100 °C, well corresponding to the values measured for the pure PEO film. Therefore, the electronic contribution of ZnO nanoparticles to the PEO-ZnO-LiClO4 conductivity enhancement is negligible. It should be noted that the as-prepared ZnO nanoparticles are much different from bulk ZnO semiconductor. There are about 15% acetate groups on the surface of ZnO nanoparticles,12 and it is estimated that the molar ratio of zinc atoms to acetate groups on particle surface is approximately 1:1. The acetate groups on the external surface protect the ZnO cores of the nanoparticles so that the particles are not electronically conducting. Conclusions We have demonstrated that the addition of ZnO nanoparticles to PEO and PEO-LiClO4 films causes property changes of either the host or the guest, and the formation of a new type of polymer-inorganic nanocomposites is envisioned. Many methods were employed to investigate the interactions within the PEO-ZnO films and the PEO-ZnO-LiClO4 films due to Lewis acid-base effects. Although the mechanisms of the property changes caused by ZnO addition are not very clear, the acetate groups on the surface of the ZnO nanoparticles are believed to play an active role in affecting crystallinity and conductivity of the composites. Acknowledgment. We gratefully acknowledge the financial support of the National Natural Science Foundation of China

(1) Maclachlan, M. J.; Manners, I.; Ozin, G. A. AdV. Mater. 2000, 12, 675. (2) Croce, F.; Appetecchi, G. B.; Persi, L.; Scrosati, B. Nature 1998, 394, 456. (3) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (4) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848. (5) Guo, L.; Yang, S.; Yang, C.; Yu, P.; Wang, J.; Ge, W.; Wong, G. K. L. Chem. Mater. 2000, 12, 2268. (6) Bekiari, V.; Lianos, P.; Stangar, U. L.; Orel, B.; Judeinstein, P. Chem. Mater. 2000, 12, 3095. (7) Huynh, W. U.; Peng, X.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (8) Dresco, P. A.; Zaitsev, V. S.; Gambino, R. J.; Chu, B. Langmuir 1999, 15, 1945. (9) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang, F.; Pakhomov, A. B.; Zhang, X. X. Chem. Mater. 1999, 11, 1581. (10) Bahnemann, D. W.; Kromann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789. (11) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. (12) Sakohara, S.; Tickanen, L. D.; Anderson, M. A. J. Phys. Chem. 1992, 96, 11086. (13) Sakohara, S.; Ishida, M.; Anderson, M. A. J. Phys. Chem. B 1998, 102, 10169. (14) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566. (15) Weston, J.; Steele, B. C. H. Solid State Ionics 1982, 7, 75. (16) Bruce, P. G.; Vincent, C. A. J. Chem. Soc., Faraday Trans. 1993, 89, 3187. (17) Kumar, B.; Scanlon, L. G. J. Power Sources 1994, 52, 261. (18) Walker, C. W., Jr.; Salomon, M. J. Electrochem. Soc. 1993, 140, 3409. (19) Chintapalli, S.; Frech, R. Macromolecules 1996, 29, 3499. (20) Sun, H. Y.; Sohn, H.-J.; Yamamoto, O.; Takeda, Y.; Imanishi, N. J. Eletrochem. Soc. 1999, 146, 1672. (21) Munichandraiah, N.; Scanlon, L. G.; Marsh, R. A.; Kumar, B.; Sircar, A. K. J. Appl. Electrochem. 1995, 25, 857. (22) Plocharski, J.; Wieczorek, W.; Przyluski, J.; Such, K. Appl. Phys. 1989, A49, 55. (23) Quartarone, E.; Mustarelli, P.; Magistris, A. Solid State Ionics 1998, 110, 1. (24) Mustarelli, P.; Quartarone, E.; Tomasi, C.; Magistris, A. Solid State Ionics 1996, 86-88, 347. (25) Quartarone, E.; Mustarelli, P.; Tomasi, C.; Magistris, A. J. Phys. Chem. B 1998, 102, 9610. (26) Nagasubramanian, G.; Ahia, A. I.; Halpert, G.; Peled, E. Solid State Ionics 1993, 67, 51. (27) Croce, F.; Curini, R.; Martinelli, A.; Persi, L.; Ronci, F.; Scrosati, B.; Caminiti, R. J. Phys. Chem. B 1999, 103, 10632. (28) Scrosati, B.; Croce, F.; Persi, L. J. Electrochem. Soc. 2000, 147, 1718. (29) Wieczorek, W.; Lipka, P.; Z˙ ukowska, G.; Wycis´lik, H. J. Phys. Chem. B 1998, 102, 6968. (30) Wieczorek, W.; Zalewska, A.; Raducha, D.; Florjan´czyk, Z.; Stevens, J. R. J. Phys. Chem. B 1998, 102, 352. (31) Marcinek, M.; Bac, A.; Lipka, P.; Zalewska, A.; Z˙ ukowska, G.; Borokowska, R.; Wieczorek, W. J. Phys. Chem. B 2000, 104, 11088. (32) Koch, U.; Fojtik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507. (33) Liu, M.; Kitai, A. H.; Mascher, P. J. Lumin. 1992, 54, 35. (34) Vanheusden, K.; Warren, W. L.; Voigt, J. A.; Seager, C. H.; Tallant, D. R. Appl. Phys. Lett. 1995, 67, 1280. (35) Mo, C. M.; Li, Y. H.; Liu, Y. S.; Zhang, Y.; Zhang, L. D. J. Appl. Phys. 1998, 83, 4389. (36) Oba, F.; Adachi, H.; Tanaka, I. J. Mater. Res. 2000, 15, 2167. (37) Vanheusden, K.; Warren, W. L.; Seager, C. H.; Tallant, D. R.; Voigt, J. A.; Gnade, B. E. J. Appl. Phys. 1996, 79, 7983. (38) Monticone, S.; Tufeu, R.; Kanaev, A. V. J. Phys. Chem. B 1998, 102, 2854. (39) Lauenstein, Å.; Tegenfeldt, J.; Kuhn, W. Macromolecules 1998, 31, 3886. (40) Wieczorek, W.; Raducha, D.; Zalewska, A.; Stevens, J. R. J. Phys. Chem. B 1998, 102, 8725. (41) Papke, B. L.; Ratner, M. A.; Shriver, D. F. J. Phys. Chem. Solids 1981, 42, 493.