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Langmuir 1996, 12, 4618-4620
Photoinitiated Reversible Formation of Small Gold Crystallites in Polymer Gels S. Weaver,† D. Taylor,† W. Gale,‡ and G. Mills*,† Department of Chemistry, Auburn University, Auburn, Alabama 36849, and Materials Engineering Program, Auburn University, Auburn, Alabama 36849 Received April 30, 1996. In Final Form: July 26, 1996X Reversible formation of gold crystallites takes place in methanol-swollen cross-linked polymers of diallyldimethylammonium chloride. Metal particles are generated in air-saturated gels via the photoreduction of Au(III) complexes. Oxidation of the crystallites proceeds through a dark reaction that regenerates the Au(III) species. The particle generation process remains completely reversible throughout several formation and decay cycles, indicating that this gel system behaves in a photoresponsive fashion.
Introduction Photoresponsive systems experience light-induced reversible changes in their physical or chemical properties and are interesting because of their adaptive (or “intelligent”) behavior.1 Photochromic glasses are among the simplest photoresponsive systems.2 Metal particles are produced in the glasses when silver halide crystals are photoreduced. The process is reversible because the Ag particles are slowly oxidized in a concurrent dark reaction to re-form the starting silver halides. Polymer gels containing chromophore molecules are the basis for another type of photoresponsive system.3,4 Reversible changes of the gels are induced when properties of the chromophores are altered by the action of light. A different photoresponsive system is presented here; this system is based on the remarkable reversibility of the light-initiated formation of gold particles inside polymeric gels of diallyldimethylammonium chloride (DADMAC). Au crystallites are formed by photoreducing AuCl4- ions that were incorporated into methanol-swollen cross-linked poly(DADMAC), but the particles are oxidized at room temperature in a slow dark reaction. The system is entirely reversible in the presence or absence of air. Experimental Section NaAuCl4‚2H2O and aqueous stock solutions of DADMAC (Aldrich), as well as methanol (Fisher), were used as received. Polymers of DADMAC were made by γ-irradiating 10 mL of degassed aqueous solutions containing 3.85 M monomer in glass test tubes. Cross-linked polymers were obtained as solid transparent cylinders by exposing the solutions to a dose of 1.5 × 105 Gy. Because the cylindrical solids expanded preferentially in the radial direction during swelling, polymer samples (cylinders, 0.5 cm diameter, 1 cm long) were cut perpendicular to the main axis of the solid materials. The samples were rotated 90° and placed vertically inside modified optical glass tubes.5 With this arrangement the swelling of the samples occurred mainly along their vertical axis. The resulting gels occupied the available internal volume of the tubes; only uniform gels free of voids were * Corresponding author: fax number, (334) 844-6959; e-mail address,
[email protected]. † Department of Chemistry, Auburn University. ‡ Materials Engineering Program, Auburn University. X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) Gahndi, M. V.; Thompson, B. S. Smart Materials and Structures; Chapman & Hall: Suffolk, 1992; p 36. (2) Araujo, R. G. ; Borrelli, N. F. In Optical Properties of Glasses; Uhlmann, R., Kreidl, N. J., Eds.; American Ceramic Society: Westerville, OH, 1991; p 125. (3) Suzuki, A.; Tanaka, T. Nature 1990, 346, 345. (4) Irie, M. Pure Appl. Chem. 1990, 62, 1495.
S0743-7463(96)00431-3 CCC: $12.00
used. Gels containing AuCl4- ions were prepared by swelling the polymers with air-saturated solutions of 1 × 10-2 M NaAuCl4 in methanol, using a constant ratio of polymer mass to solution volume of 3.8 × 10-2 g mL-1. Swelling took place in the dark for 3 days, at room temperature, inside closed optical tubes (to avoid solvent evaporation). A small volume of solution remained on top of the gels after swelling, and metal formation in the solutions was suppressed by shielding the top section from light. Irradiations (λ ) 350 ( 17 or 300 ( 15 nm) were performed by placing the optical tubes vertically near the center of a Rayonet 100 illuminator (light intensity ) 3.1 × 10-6 einstein min-1, T ) 29 °C). The decay of the Au particles was followed at room temperature. UV-vis spectra were measured using a Hitachi U-2000 spectrophotometer. X-ray diffraction (XRD) was carried out with a Siemens D5000 powder diffractometer.
Results DADMAC undergoes free radical cyclopolymerization upon exposure of monomer solutions to ionizing radiation to yield water-soluble cationic linear polymers containing five-membered cyclic structures.6 However, increasing cross-linking of the macromolecules takes place when the dose increases, and insoluble solid polymers were generated at the high doses used in our study. These polymers were swollen by H2O or alcohols to form optically transparent gels at λ > 220 nm. Gels containing AuCl4ions were yellow due to the absorption band of these ions centered at 318 nm ( ) 4500 M-1 cm-1).7 Figure 1 shows the spectrum of a gel with 1 × 10-2 M NaAuCl4; at this high concentration the absorption band of the complexes was strong and broad, with a tail extending beyond 400 nm. Illumination of the gels induced a bleaching of the absorption band since AuCl4- ions are photoreduced in alcohols,7 to initially form AuCl2- ions (λmax ) 246 nm).8,9 The samples turned progressively colorless from bottom to top, and depending on the previous history of the gel this process lasted between 30 and 100 min (the induction period). Included in Figure 1 is the spectrum of a gel (5) Spectrophotometer test tubes (Milton Roy No. 33-17-80, optical path ) 1 cm) flattened at the bottom and with glass tubes (8 cm long, 1 cm diameter) fused on their tops were used for measurements of optical density because optical cells were cracked by the gels after several cycles. The optical tubes were closed by attaching to them, via #9 Chem threads, capillaries with a #7 Ace electrode adapter/Teflon septum/ nylon bushing combination. (6) Huber, E. W.; Heineman, W. R. J. Polym. Sci., Polym. Lett. 1988, 26, 333. (7) (a) Kartuzhanskii, A. L.; Studzhinskii, O. P.; Plachenov, B. T.; Sokolova, I. V. J. Appl. Chem. USSR 1986, 59, 2265. (b) Studzhinskii, O. P.; Kartuzhanskii, A. L.; Plachenov, B. T. J. Appl. Chem. USSR 1985, 55, 597. (8) Kunkely, H.; Vogler, A. Inorg. Chem. 1992, 31, 4539. (9) Detection of the optical signal centered at 246 nm of the AuCl2ions was possible with gels prepared in quartz cells, since the borosilicate optical tubes cut-off light below 300 nm.
© 1996 American Chemical Society
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Figure 1. Evolution of the absorption spectra with illumination time for a gel containing 1 × 10-2 M NaAuCl4 exposed to 350 nm photons.
irradiated for 68 min. The intensity of the peak at 318 nm indicated that the concentration of Au(III) left unreduced at that time was about 1.7 × 10-4 M. A shoulder at 356 nm and a weak signal centered at about 525 nm were also detected, but upon further irradiation this shoulder developed into a well-defined peak as the absorption band of the Au(III) species decreased. At the same time, a broad absorption at λ > 500 nm became increasingly stronger, with λmax shifting from 525 to 550 nm as the photoreaction proceeded. Similar observations were made during the formation of Au colloids in basic methanolic solutions of AuCl4- ions.10 Generation of gold crystallites in the gels was confirmed by XRD data from samples illuminated for 8 h (to generate enough metal particles), followed by drying under vacuum and grinding prior to XRD examination. Reflections due to lattice planes of face-centered cubic (fcc) Au were observed, and from the line width of the signals an average particle diameter of 35 nm was estimated using the Scherrer equation. The formation process started only after the induction period, that is, after the time required for the transformation of most of the AuCl4- ions to AuCl2ions. It occurred exclusively in the presence of light, since storing bleached gels in the dark resulted in a regeneration of the Au(III) complexes. Particles were formed sequentially from bottom to top of the gels. Detection of the optical signals of particles produced on the bottom was achieved by modifying the position of the tubes in the sample holder.11 Layered structures were generated; a typical example is illustrated in Figure 2. The layered patterns consisted of ring like red layers alternating with lighter orange layers. Similar structures were formed with continuous or discontinuous photolysis and irrespective of the polymer orientation in the tubes. Surprisingly, the particles decayed slowly at room temperature once the illumination was terminated. The optical signals of the gold crystallites decreased and λmax shifted back to 525 nm, following the reverse pattern of the formation process shown in Figure 1. Oxidation of the metal particles occurred in a few hours to yield colorless gels. In a second and slower step, lasting more than a week, the gels became yellow as the Au(III) complexes (10) Quinn, M.; Mills, G. J. Phys. Chem. 1994, 98, 9840. (11) The analyzing light beam of the spectrophotometer is 1.1 mm wide, and the center of the beam strikes the sample about 9 mm from the bottom of the tubes. To ensure detection of the Au particles during the early stages of their formation, the tubes were raised by 5 mm from their normal position in the sample holder, since the beam extends only 3 mm above and below its center. This caused the lower section of the beam to enter the samples about 0.5 mm above the bottom of the gels.
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Figure 2. Photo of the lower section of a gel with 1 × 10-2 M NaAuCl4, illuminated for 6 h. The location of dark particle layers (labeled p) in the gel are indicated by the arrows, whereas the letter g indicates a region of the gel that is free of particles.
Figure 3. Plots of optical density at 525 nm as a function of irradiation time for the formation of gold particles.
Figure 4. Plots of optical density at 525 nm versus time for the decay of the crystallites. The symbols indicating the cycle number are the same as for Figure 3.
were regenerated. The photoresponsive behavior of the gels was tested by following the evolution of the gold crystallites through several formation and decay cycles. Presented in Figures 3 and 4 are plots of optical density (at 525 nm) vs time for the first six cycles of formation and decay, respectively. In both cases the reactions followed apparent zero-order rate laws. Seventeen cycles of formation and decay have been followed thus far, without any noticeable loss in the reversibility of the system. Formation rate constants were at least 100 times larger than decay rate constants, (zero-order rate constants are expressed as optical density change per min, ∆OD min-1). Shorter induction periods were observed for cycles 2, 5,
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and 6 in Figure 3, where illumination was resumed a few days after the previous cycle. Complete regeneration of the Au(III) complexes was not achieved in these cases, and formation of particles started earlier because smaller amounts of Au(III) species had to be photoreduced. Also, the formation process was faster in cycles 2, 5, and 6, with rate constants in a range between (3.4 and 5.7) × 10-2 ∆ODmin-1, as compared with 2 × 10-2 ∆OD min-1 for the first cycle. In contrast, the results of Figure 4 indicate that decay rate constants remained essentially unchanged ((1.5 ( 0.3) × 10-4 ∆OD min-1) throughout the cycles. Generation of gold crystallites in gels prepared under N2 occurred without forming layers but with formation and decay processes 10 times slower than in air-saturated systems. Photolysis of polymer-free methanolic solutions containing 2 × 10-4 M AuCl4- ions and air for 1 h yielded AuCl2- ions, without formation of the peak at 356 nm shown in Figure 1. The Au(I) complexes decayed over a few hours to form metal and AuCl4- ions. However, Au precipitated after only 40 min in similar experiments with 1 × 10-2 M AuCl4- ions, at which point less than 50% of the metal ions were reduced. Gels made with H2O as the swelling agent exhibited a pH of 6.3, and although metal is formed when AuCl4- ions react thermally with several polymers at this pH,12 the Au(III) complexes remained unchanged even after extensive irradiation. Discussion Illumination of solutions containing AuCl4- ions and electron donors induces a reduction of the metal complexes,7,8,13,14 and an analogous process takes place in methanol-swollen poly(DADMAC). The optical spectra of Figure 1 exhibit the distinctive plasmon resonance at λ > 500 nm of small gold crystallites.14-16 These results and the XRD data clearly indicate that metallic Au is generated in the photoreaction. Broad plasmon bands are observed, with an increasingly red-shifted maximum. These features may be related to the size evolution of the particles during photolysis14,16 or may result from processes yielding networks of agglomerated small Au particles.10,17 As in the case of ethanol,8 we find that AuCl2- is produced via reduction of the excited Au(III) species by methanol molecules. The Au(I) species are also formed in methanol-swollen polymers, but no such process is observed when water is used instead of the alcohol. Our photochemical experiments showed that the AuCl2- ions decay in homogeneous methanolic solutions to yield metal and AuCl4- ions, presumably by disproportionation,18
3AuCl2- h 2Au + AuCl4- + 2Cl-
(1)
However, gold particles form in the gels exclusively under the influence of light. Interruption of the illumination after most of the AuCl4- ions have decayed induces a slow regeneration of the Au(III) complexes without producing (12) Longenberger, L.; Mills, G. In Nanotechnology: Molecularly Designed Materials; Chow, G.-M., Gonsalves, K. E., Eds; ACS Symposium Series 622, American Chemical Society: Washington DC, 1996; p 128. (13) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (14) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. Chem. Phys. 1993, 98, 9933. (15) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301. (16) Lamber, R.; Wetjen, S.; Schulz-Ekloff, G.; Baalmann, A. J. Phys. Chem. 1995, 99, 13834. (17) (a) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97. (b) Biggs, S.; Chow, M. K.; Zukoski, C. F.; Grieser, F. J. Colloid Interface Sci. 1993, 160, 511. (18) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam, 1978; Chapters 1 and 3.
metal. Thus, formation of Au particles via equilibrium 1 does not occur efficiently in the gels, probably because this reaction is displaced to the left side at the high [Cl-] (>1 M) that exist in the swollen polymers, and also due to the lower mobility (by a factor of 103) of anionic complexes when they are incorporated into poly(DADMAC) gels.19 The mechanism of the crystallite formation is complex, since layers of particles are produced in air-containing samples (Figure 2), which resemble Liesegang rings.20 Generation of particles is fastest in these samples, implying that O2 is involved in the formation process. It is known that a combination of H2O2 and light accelerates the production of Au colloids.21 Peroxides may form in gels containing O2 as byproducts during the photoreduction of the Au(III) complexes. The peroxides may subsequently reduce the Au(I) species to form Au particles, and they may also be involved in the faster formation processes that take place when reoxidation of the Au(I) species is incomplete (cycles 2, 5 and 6 of Figure 3). Although the decay of Ag particles occurs in several systems,2,22-24 a partial decay has only been observed for gold particles prepared in inverse micelles.14 In homogeneous solutions of alcohols containing air and bases, AuCl4- ions are reduced to yield stable Au particles.9,25 The fact that Au crystallites are completely oxidized in the relatively mild environment of the gels is surprising. This process is probably related to size effects in the chemical reactivity of metals. Evidence for size-induced enhancement of the oxidation of Ag particles has been recently presented,22 and we believe that a similar effect operates for small gold particles. The rate of corrosion is 10 times higher in air-saturated gels. We speculate that this is due to the combined effect of O2, Cl- ions, and protons that are generated during the photoxidation of the alcohol by AuCl4- ions. The corrosion of the metal particles is facilitated by the high solubility of oxygen in methanol saturated with air (≈2 × 10-3 M)26 and by the high concentration of chloride ions in the gels. This assumption is supported by results showing that oxidation of Au is promoted by chloride ions.27 The unusual oxidation of gold crystallites in poly(DADMAC) gels is the key step for the reversible behavior of these systems. Improvements in the speed of this process and of the formation reaction may result in photoresponsive systems comparable to erasable photochromic materials.28 Acknowledgment. We thank W. Sandlin for his help in the design and construction of the optical cells. We are grateful to Auburn University for fellowships to S.W. through the GANN and PGOP programs. This work was supported by the NSF EPSCoR Program. LA960431O (19) De Castro, E. S.; Huber, E. W.; Villarroel, D.; Galiatsatos, C.; Mark, J. E.; Heineman, W. R. Anal. Chem. 1988, 59, 134. (20) Henisch, H. K. Crystals in Gels and Liesegang Rings; Cambridge University Press: Cambridge, 1988; Chapters 1 and 5. (21) Weiser, H. B. Inorganic Colloid Chemistry; John Wiley: New York, 1933; Vol I, p 35. (22) Li, W.; Virtanen, J. A.; Penner, R. M. Langmuir 1995, 11, 4361. (23) Serpone, N.; Lawless, D.; Sahyun, M. R. V. Supramol. Chem. 1995, 5, 15. (24) Lee, P. C.; Meisel, D. J. Catal. 1981, 70, 160. (25) Siiman, O.; Hsu, W. P. J. Chem. Soc., Faraday Trans. 1 1986, 82, 851. (26) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data 1984, 13, 563. (27) Trevor, D. J.; Chidsey, C. E. D.; Loiacono, D. N. Phys. Rev. Lett. 1989, 62, 929. (28) Irie, M. Photo-reactive Materials for Ultrahigh Density Optical Memory; Elsevier: Amsterdam, 1994.
