Preparation and Optical Properties of Novel Transparent Al-Doped

May 4, 2009 - Yong-Song Luo, Jiao-Ping Yang, Xiao-Jun Dai, Yang Yang, and Shao-Yun Fu*. Key Laboratory of Functional Crystals and Laser Technology, Te...
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Preparation and Optical Properties of Novel Transparent Al-Doped-ZnO/Epoxy Nanocomposites Yong-Song Luo, Jiao-Ping Yang, Xiao-Jun Dai, Yang Yang, and Shao-Yun Fu* Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ReceiVed: February 18, 2009; ReVised Manuscript ReceiVed: April 13, 2009

In this paper, preparation and optical properties of novel transparent Al-doped zinc oxide (AZO)/epoxy nanocomposites were reported. First, AZO precursor was synthesized via the homogeneous precipitation method and AZO nanoparticles were obtained by calcination of the precursor at different temperatures. Transparent AZO/epoxy nanocomposites were subsequently prepared from the transparent diglycidyl ether of bisphenol A epoxy resin and the as-prepared AZO nanoparticles via a simple direct dispersion method. Optical properties, namely visible-light transmittance, ultraviolet, and infrared opaqueness, of the AZO/epoxy nanocomposites were studied as a function of the AZO content by using an ultraviolet-visible spectrophotometer and an ultraviolet-visible-nearinfrared spectrophotometer. The results indicated that the nanocomposite with a thickness of about 3.5 mm containing a low AZO content of 0.08 wt % showed an excellent UV-shielding efficiency greater than 85% at ∼360 nm together with a good visible transmittance greater than 70% at ∼650 nm. Moreover, the AZO addition obviously enhanced the infrared light shielding efficiency of the epoxy resin. 1. Introduction Recently much effort has been made toward fabrication of nanocomposites based on organic component combined with nanometer-sized inorganic nanoparticles.1-8 This class of polymer-inorganic functional materials has attracted considerable interest owing to their enhanced optical, mechanical, biological, thermal, magnetic, electronic, and optoelectronic properties compared to their corresponding inorganic or polymer components only.9-20 In particular, with the breaking of the ozone layer and the crisis of energy during the past decades, the development of photoprotective materials has drawn increasing attention because of their capability to allow good transmission in the visible range as well as to reduce the ultraviolet (UV) and infrared (IR) radiation. Therefore, there is a need to develop alternative bulk transparent UV- and IR-shielding materials suitable for applications such as UV- and IR-shielding windows, contact lenses, or heat mirrors. Such materials can be fabricated by incorporating suitable UV-absorbing and IRreflecting materials into a transparent polymeric matrix. Al-doped ZnO (AZO) is promising as a cost-effective replacement for transparent conducting oxide (TCO) materials due to its high thermal stability, low price, and nontoxicity.21-23 Its n-type conductivity is caused by substitution of Zn2+ ions by Al3+ ions releasing excess electrons into the conduction band. Moreover, ZnO has a wide bandgap, and allows good transmission in the visible range but is opaque in the UV range.24 Further, Al-doped ZnO shows metallic properties and hence reflects IR.25 Thus, it has been chosen as plasma display panels26 or energyefficient27 and smart coatings28 for automotive and architectural glazing. Epoxy-based photoprotective materials are excellent candidates for novel radiation-resistant materials as they possess superior transmission in the visible range and can also be * To whom correspondence should be addressed. Phone/fax +86-1082543752. E-mail: [email protected].

modified to limit the amount of UV or IR radiation through using inorganic nanoparticles. The synthesis of this class of materials is tremendously complex, with at least two major obstacles that must be overcome in order to fabricate optically selective transparent nanocomposites: (i) the introduction of nanoparticles with different refractive indices (RIs) relative to the epoxy matrix causes significant light scattering, resulting in opaqueness; and (ii) the capability of nanocomposites is 2-fold: one is to shield UV radiation and the other is to reflect IR radiation. However, most of the previous work has been focused on preparation of UV-shielding-only nanocomposites via addition of UV-light absorbents such as ZnO and TiO2 etc.20,29,30 Our previous work20 reported the preparation of transparent ZnO/epoxy nanocomposites with a high-UV shielding efficiency; Nussbaumer et al.29 reported that the UV shielding property was enhanced at high TiO2 contents by addition of TiO2 particles to polymers; in another work,30 the inorganic-organic nanocomposite coatings were prepared on poly(methyl methacrylate) substrate with the enhanced UV shielding property. However, only limited work has been performed to investigate both UV- and IR-opaque energy efficient composites.31 Moreover, no reports have referred to the transparent AZO/epoxy nanocomposites that can effectively shield the UV- and IR- radiations. In this paper, visible transparent but UV- and IR-opaque AZO/epoxy nanocomposites were prepared by incorporating the as-prepared AZO nanoparticles into a transparent epoxy resin. The precipitation method was used to prepare AZO nanoparticles with variable fine particle sizes. The effects of calcination temperature were studied on the particle size and structural properties of AZO nanoparticles. The optical properties of the AZO/epoxy nanocomposites were examined by an ultravioletvisible (UV-vis) spectrophotometer and an ultraviolet-visiblenear-infrared (UV-vis-NIR) spectrophotometer.

10.1021/jp901501z CCC: $40.75  2009 American Chemical Society Published on Web 05/04/2009

Novel Transparent Al-Doped-ZnO/Epoxy Nanocomposites

Figure 1. (a) XRD patterns of the AZO nanoparticles calcined at different temperatures, where the XRD pattern of ZnO (JCPDS 361451) is also given for easy referencing and (b) the relative crystallinity degree of the AZO nanoparticles as a function of calcination temperature, where the crystallinity degree of the samples calcined at 700 °C is denoted as 100%.

2. Experimental Section AZO nanoparticles were first prepared by a coprecipitation method similar to that described in our previous work.20 First, 0.1 mol mixture of Al(NO3)3 · 9H2O and Zn(NO3)2 · 6H2O (the weight ratio of Al:Zn is 3%) was dissolved in 200 mL distilled water under vigorous stirring at room temperature and this solution is named as Solution A. Second, 0.12 mol Na2CO3 was dissolved in 240 mL distilled water at room temperature, which is denoted as Solution B. Subsequently, Solution A was added to Solution B drop by drop under vigorous stirring. The resulting white precipitates were filtered and washed with distilled water

J. Phys. Chem. C, Vol. 113, No. 21, 2009 9407 for two times. The solids were then washed with ethanol and dried at 70 °C for 6 h. Finally, AZO nanoparticles were obtained after calcination of the solids in air at 200, 300, 400, 450, 500, 550, 600, 650, and 700 °C for 2 h, respectively. The as-prepared AZO nanoparticles were dispersed in methylhexahydro phthalic anhydride (MHHPA) curing agent using the ultrasonic technique for 60 min and the resulting mixture was then mixed with the transparent diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The epoxy and the curing agent were well stirred until a homogeneous mixture was obtained. The mixture was poured into a stainless steel mold and heated in an oven at 130 °C for 2 h. After the curing process was completed, the samples were obtained from the mold. Powder X-ray diffraction (XRD) patterns for AZO nanoparticles were recorded on a Bruker D8 ADVANCE X-ray Diffractometer with graphite monochromatized Cu Ka radiation. Thermogravimetry & Differential Scanning Calorimetry (TGDSC) analysis was performed using a NETZSCH STA 409PC instrument in a nitrogen atmosphere in the temperature range of 30-900 °C at a heating rate of 10 °C/min. Scanning electron microscope (SEM) images and X-ray energy dispersive spectroscopy (EDS) spectra were obtained using a HITACHI S-4300 microscope (Japan). Atomic force microscope (AFM) (Nanoscope IIIa, Digital Instruments Co) was used to investigate the surface morphology and topography of the pure epoxy and epoxy nanocomposites. AFM images were taken with 5 × 5 µm2 scan area from the prepared samples. The optical properties of the transparent AZO/epoxy nanocomposites were studied using an UV-vis spectrophotometer (HITACHI U-3900) and an UV-vis-NIR spectrophotometer (CRAY 5000). 3. Results and Discussion Figure 1a displays the XRD patterns of the AZO nanoparticles calcined in air at different temperatures. The results demonstrate that the nanoparticles calcined at 200-700 °C have similar XRD spectra and all of the diffraction peaks can be indexed to a pure wurtzite structure of ZnO with cell constants of a ) 0.324 982 nm and c ) 0.520 661 nm (JCPDS card No. 36-1451). No diffraction peaks from impurities are found in the samples within the experimental errors. The main peaks of the AZO nanoparticles containing 3% Al are in accordance with the pure ZnO, indicating that the ZnO crystal structure was not changed when combined with a small quantity of Al. Moreover, an increase in calcination temperature brings about sharper diffraction peaks, corresponding to the increase in crystallinity degree (Figure 1b). To study the composition of the AZO nanoparticles in detail, element analysis was performed with EDS and TG-DSC

Figure 2. (a) EDS spectra and (b) TG-DSC curves of the AZO nanoparticles.

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Figure 3. SEM images of the AZO nanoparticles prepared at different calcined temperatures: (a) 200, (b) 300, (c) 400, (d) 500, (e) 600, and (f) 700 °C.

Figure 4. Mean AZO particle size vs. calcination temperature.

techniques. The EDS results of the AZO nanoparticles after calcination at 600 °C are shown in Figure 2a. It shows that the AZO nanoparticles are composed of the elements of C, O, Al, Au, and Zn. The Au element is induced by the spattering Au nanoparticles used for the SEM samples and the C element is also discovered probably due to the insulating tape for adhesive samples. In addition, the atomic ratio of Al and Zn is 2.54: 36.28, which is in accordance with the prescribed weight ratio 3% of Al:Zn. The TG-DSC curves of the prepared AZO nanoparticles are shown in Figure 2b. The TG curve reveals that there are two weight loss stages proceeding in successive stages with increasing temperature and the total weight loss is about 25 wt %. The small exothermic DSC signal in the temperature range of 90-150 °C is mainly induced by the removal of absorbed water of the precursor with a weight loss of 5 wt %. The most significant weight loss occurs in the temperature range of 160-270 °C. Moreover, the exothermic peak occurs at 205.7 °C, which is an indication of the transformation of the precursor to AZO with a large weight loss of 20 wt %. The precursor is a complex of alkaline carbonate (Zn1-1.5xAlx(OH)2-2y(CO3)y),32 where x is the doping ratio of the complex and y denotes the content of (CO3)2- in the precursor. The reaction equation for the transformation of the precursor to AZO is given as follows:

Zn1-1.5xAlx(OH)2-2y(CO3)y f Zn1-1.5xAlxO + (1 y)H2O + yCO2 Figure 3 shows SEM images of the AZO nanoparticles prepared at different calcination temperatures. When the precursors were calcined at different temperatures, the AZO nanoparticles with variable fine particle sizes are obtained. It can be seen from Figure 3 that the particle size increases with increasing the calcination temperature. The particle sizes are evaluated by

Figure 5. UV-vis spectra of the AZO/epoxy nanocomposites: (a) transmittance and (b) absorbance.

using the software SemAfore 4.0 and the relation between the mean AZO particle size versus the calcination temperature is shown in Figure 4. For the samples calcined at relatively high temperatures, AZO particles aggregated into clusters and the average particle sizes are naturally relatively large. The UV-vis transmittance and absorbance spectra of the AZO/epoxy nanocomposites (0.08 wt %) containing AZO calcined at various temperatures are shown in Figure 5. As the calcination temperature increases, the transmittance of UV light generally decreases in the wavelength range of 290-400 nm (Figure 5a). Though the samples calcined at 650 °C or above exhibit excellent UV light shielding efficiency, their transmittance in the visible range is relatively poor. However, it can be seen from Figure 5a that the nanocomposite containing AZO

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Figure 6. (a) Transmittance and (b) absorbance of the AZO/epoxy nanocomposites with different AZO contents and (c) digital photographs of the AZO/epoxy nanocomposites under visible light, where the thickness of the samples is 3.5 mm.

nanoparticles calcined at 600 °C shows not only a high UV light shielding efficiency but also a relatively high-visible light transmittance. Moreover, Figure 5b shows that the UV light absorbance of nanocomposites increase generally with increasing the calcination temperature. This result is in accordance with the transmittance spectra of the AZO/epoxy nanocomposites. Figure 6a,b shows the transmittance and absorbance spectra of the AZO/epoxy nanocomposites with a composite block thickness of 3.5 mm as a function of AZO content, where AZO was calcined at 600 °C. The pure epoxy resin blocks UV light up to a wavelength of about 300 nm. That is why the epoxy matrix and the epoxy nanocomposites are opaque in the wavelength range up to about 300 nm as shown in Figure 6a. However, the epoxy resin does not shield the UV light from about 300 to 400 nm. Introduction of AZO into epoxy matrix leads to the significant increase in the UV-shielding efficiency from 300 to 400 nm. Increasing the AZO content caused a decrease in the transmittance (Figure 6a) and an increase in the absorbance of the nanocomposites (Figure 6b). The nanocomposite containing AZO nanoparticles with a diameter of ∼65.1 nm at the 0.08 wt % content showed the 85% UV-shielding efficiency for the UV light at ∼360 nm and the transmittance greater than 70% for the visible light at ∼650 nm. Colloidal ZnO-quantum dots (ZnO-QDs) with a uniform particle size of around 5 nm have been synthesized and blended with polymethylmethacrylate (PMMA) by solution mixing to prepare PMMA/ZnO-QDs nanocomposite films.33 The UV-vis spectra showed that a small amount (0.80 wt %) of colloidal ZnO-QDs could improve the UV-shielding capability but the UV-shielding efficiency in the UV region at about 360 nm is lower than 50%. In our previous work,20 the ZnO/epoxy nanocomposites showed high UV-shielding efficiencies (about 96% at 320 nm and about 91% at 370 nm) by introduction of 0.07 wt % ZnO nanoparticles with a mean diameter of 26.7 nm into a transparent epoxy resin.

Comparison of the present work with our previous one20 and the literature33 indicates that the UV shielding efficiency would depend on both nanoparticle size and polymer matrix type. The intensity loss of transmitted light in the nanocomposites due to light scattering can be estimated by8,34

[

I/I0 ) exp

(

)]

-3Vpxr3 np -1 nm 4λ4

(1)

which is valid for spherical particles with a radius r and a refractive index np uniformly dispersed in a matrix with a refractive index nm; where I is the intensity of the light passing through the sample, I0 the intensity of the light that would pass the sample without scattering, namely, the intensity of the incoming light for nonabsorbing materials, Vp the volume fraction of the particles, λ the light wavelength, and χ the optical path length. Equation 1 shows that in the absence of matching refractive index the light intensity reduces dramatically as r increases. That is to say, scattering losses can be minimized only for composites containing fine particles 1-2 orders of magnitude smaller than the wavelength of light.35,36 But, as the particle size increases, particle agglomeration becomes severe and deteriorates the particle dispersion in the polymer matrix.37 Such agglomerates persist in the composite and scatter visible light, bringing about a turbidity. Referring to the system discussed here, it can thus be concluded that the reduction in the transmittance of nanocomposites by addition of AZO nanoparticles is mainly caused by the relatively large particle size, the particle agglomerates in the nanocomposite samples, and also the mismatching of refractive index between the nanoparticles and the polymer matrix (the refractive index of the ZnO is 1.9-2.0 and the refractive index of epoxy matrix is 1.54-1.558). Moreover, photographs were taken with a digital Olympus camera for the AZO/epoxy nanocomposites as shown

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Figure 7. AFM images of surface morphology and topography for the pure epoxy (a, b) and the epoxy nanocomposites (c, d) with the 0.08 wt % AZO content.

in Figure 6c. It shows that the transparency decreases with increasing the AZO content, which is consistent with the observation of Figure 6a. The two-dimensional (2D) and three-dimensional (3D) AFM images of the surface topography obtained for the pure epoxy and nanocomposite samples are shown in Figure 7. It can be seen from Figure 7a,b that the sample surface prepared from pure epoxy is quite smooth and thus the light scattering due to the effect of surface roughness would be small on the transmittance. So, a high transmittance of pure epoxy resin has been observed as shown in Figure 5a. On the other hand, the introduction of solid AZO nanoparticles leads to an increase in viscosity, resulting in a rough surface of the nanocomposite samples as shown in Figure 7c,d. The light scattering caused by the surface roughness would to some extent reduce the transmittance of the transparent epoxy matrix. This is also one reason for the decrease of the transmittance of the epoxy nanocomposites with increasing the AZO nanoparticle content. Al-doped ZnO is known to exhibit IR light reflection.38 Figure 8 shows the transmittance of the AZO/epoxy nanocomposite with a composite block thickness of 3.5 mm as a function of the AZO content. By adding the AZO nanoparticles to the epoxy matrix, the transmittance of the nanocomposites was decreased in the whole wavelength region. Figure 8 shows that the introduction of AZO nanoparticles at the 0.08 wt % content obviously improves the IR-shielding efficiency with an enhancement of about 10% in the IR region (∼1300 nm). This result is attributed to the optical characteristics of AZO due to the doped aluminum element.39 Al doping reveals a systematic BursteinMoss increase in bandgap with greater carrier concentration from its intrinsic value of 3.4 to ∼3.75 eV and the Al-doped ZnO

Figure 8. Transmittance of the AZO/epoxy nanocomposites as a function of the AZO content.

shows the IR light absorptive capability.39 Therefore, the nanocomposite containing Al-doped ZnO shows the enhanced IR shielding efficiency compared to that of the epoxy matrix and the nanocomposite containing pure ZnO nanoparticles. 4. Conclusions In summary, the transparent AZO/epoxy nanocomposites with UV- and IR-shielding effects have been reported in this paper. AZO nanoparticles with various sizes were obtained by calcination of the AZO precursor derived from a homogeneous precipitation method. The transparent AZO/epoxy nanocomposites were successfully prepared by incorporation of the as-

Novel Transparent Al-Doped-ZnO/Epoxy Nanocomposites obtained AZO nanoparticles into a transparent epoxy. The effects of calcination temperature on the particle size and structural properties of the AZO nanoparticles were studied. The dependence of the optical properties of the nanocomposites on the AZO particle size and loading was investigated in detail. The results showed that the nanocomposite containing an extremely low content (0.08 wt %) of AZO nanoparticles after calcination at 600 °C would have excellent overall optical properties, viz., high visible light transmittance and good UV- and IR-shielding effects. The as-prepared transparent AZO/epoxy nanocomposites have the potential for uses in a range of engineering applications such as UV- and IR-shielding windows, contact lenses, or heat mirrors. Acknowledgment. We gratefully acknowledge the financial support from the Beijing Municipal Natural Science Foundation (Nos. 2091004 and 2082023), the Chinese Academy of Sciences (No. CXJJ-204), and the National Natural Science Foundation of China (No. 10672161). References and Notes (1) Xu, J.; Wang, J.; Mitchell, M.; Mukherjee, P.; Petrich, J. W.; Lin, Z. Q. J. Am. Chem. Soc. 2007, 129, 12828. (2) Yang, Y.; Li, Y. Q.; Fu, S. Y.; Xiao, H. M. J. Phys. Chem. C 2008, 112, 10553. (3) Convertino, A.; Leo, G.; Striccoli, M.; Marco, G. D.; Curri, M. L. Polymer 2008, 49, 5526. (4) Li, Y. Q.; Yang, Y.; Fu, S. Y.; Yi, X. Y.; Wang, L. C.; Chen, H. D. J. Phys. Chem. C 2008, 112, 18616. (5) Wang, W. S.; Chen, H. S.; Wu, Y. W.; Tsai, T. Y.; Chen-Yang, Y. W. Polymer 2008, 49, 4826. (6) Lin, Y.; Chen, H. B.; Chan, C. M. Macromolecules 2008, 41, 9204. (7) Chen, C. H.; Crisostomo, V. M. B.; Li, W. N.; Xu, L. P.; Suib, S. L. J. Am. Chem. Soc. 2008, 130, 14390. (8) Li, Y. Q.; Fu, S. Y.; Yang, Y.; Mai, Y. W. Chem. Mater. 2008, 20, 2637. (9) Ahmad, Z.; Sarwar, M. I.; Mark, J. E. J. Mater. Chem. 1997, 7, 259. (10) Mora-Sero, I.; Bisquert, J.; Fabregat-Santiago, F.; Garcia-Belmonte, G.; Zoppi, G.; Durose, K.; Proskuryakov, Y.; Oja, I.; Belaidi, A.; Dittrich, T.; Tena-Zaera, R.; Katty, A.; Levy-Clement, C.; Barrioz, V.; Irvine, S. J. C. Nano Lett. 2006, 6, 640. (11) Schmidt, H.; Schloze, H.; Tunker, G. J. Non-Cryst. Solids 2002, 80, 557.

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