Nanoporous Cellulose as Metal Nanoparticles ... - ACS Publications

Biomacromolecules 2009, 10, 87–94

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Nanoporous Cellulose as Metal Nanoparticles Support Jie Cai,*,†,‡ Satoshi Kimura,† Masahisa Wada,† and Shigenori Kuga*,† Graduate School of Agricultural and Life Sciences, The University of Tokyo, Japan and Department of Chemistry, Wuhan University, Wuhan 430072, China Received August 18, 2008; Revised Manuscript Received October 16, 2008

Despite considerable progress in the field of metal nanoparticles synthesis, major challenges remain in many practical applications of nanoparticles which require their immobilization on solid substrates, presenting additional difficulty in separation and processing. Here, transparent nanoporous cellulose gel obtained from aqueous alkali hydroxide-urea solution was examined as supporting medium for noble metal nanoparticles. Silver, gold, and platinum nanoparticles were synthesized in the gel by hydrothermal reduction by cellulose or by added reductant. Both methods gave nanoparticles embedded with high dispersion in cellulose gels. Supercritical CO2 drying of the metal-carrying gel gave corresponding aerogels with high transmittance, porosity, surface area, moderate thermal stability, and good mechanical strength. The cellulose and metal-cellulose gels were characterized by UV/vis spectroscopy, optical microscopy, SEM, TEM, XRD, nitrogen physisorption, TGA, and tensile testing, systematically.

Introduction Synthesis of metallic nanoparticles and their properties are of strong interests due to their potential usefulness as electronic, optical, sensor, and catalytic materials.1-6 Preparation of metal colloids by reduction is a nominally simple reaction, but control of particle size, shape, and dispersion stability requires careful control of synthetic condition, because the process is sensitive to balances between nucleation and crystal growth.1 One approach to facilitate both synthetic control and immobilization is the use of porous materials as reaction medium. While various inorganic materials, such as mesoporous silica,7,8 have been used for this purpose, use of organic polymers is also gaining interests. For example, block copolymer as soft templates have been used to synthesize mesostructured Pt particles by ligandstabilization and electrochemical deposition.9-11 Another attractive class of substrate is insoluble polysaccharides, which are abundantly available and have wide varieties in structure and properties.12 The pioneering works about the use of cellulose as nanoreactor were reported by Kunitake et al. for noble metals13-15 and oxides.16-19 While these works used the nanospace in swollen cellulose fibers, cellulose can be obtained as intrinsically porous material, that is, regenerated hydrogel. We recently reported the formation of nanoporous cellulose gel from a new solvent system consisting of aqueous alkali hydroxide and urea.20-27 The high open porosity, near transparency, large surface area, and high mechanical strength makes this cellulose gel an attractive candidate for nanoparticle synthesis/support medium. The use of cellulose as reaction medium leads to another aspect in metal nanoparticle synthesis. While the common method of metal nanoparticle synthesis is the use of added reductant such as NaBH4 or aldehydes, simple polyols also are known to be effective, giving rise to the “polyol process”.2,28 While the conventional polyol process uses anhydrous organic * To whom correspondence should be addressed. Tel.: +81-3-5841-5241. Fax: +81-3-5684-0299. E-mail: [email protected], [email protected] ( J.C.); [email protected] (S.K.). † The University of Tokyo. ‡ Wuhan University.

Figure 1. TEM images of disembedded ultrathin section of pure cellulose gel (top, left) and hydrothermally reduced silver-cellulose hydrogel (top, right). UV/vis spectra (bottom) and photographs (inset) of cellulose (A0) and silver-cellulose (A-H) hydrogel films prepared from aqueous AgNO3 of 1 mM (A), 10 mM (B), 50 mM (C), 100 mM (D), 250 mM (E), 500 mM (F), 1 M (G), and 2 M (H) at 80 °C for 24 h, respectively.

polyols, typically ethylene glycol, aqueous polysaccharides also have glycol groups and, therefore, can serve as reductant for metals.29-34 There are several reports on reduction of metals by hydrothermal treatment with cellulose;35-40 in these works, however, cellulose was used as dispersed solid reagent, not the reaction medium. Based on these backgrounds, we attempted here to establish methodology and basic features of metal nanoparticle synthesis in cellulose gel both with and without added reductant.

10.1021/bm800919e CCC: $40.75  2009 American Chemical Society Published on Web 11/24/2008

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Figure 2. TEM images of hydrothermally reduced silver-cellulose hydrogel and particle size histograms of silver nanoparticles corresponding to Figure 1.

Experimental Section Cellulose hydrogel was prepared as in our previous work.27 Briefly, Filter Paper Pulp (Advantec Co., Japan), with a viscosity-average molecular weight (Mη) of 8.6 × 104, was dissolved in precooled (-10 °C) 4.6 wt % LiOH and 15 wt % urea aqueous solution to form a 4 wt % cellulose solution. The solution was spread on a glass plate as 0.5 mm thick layer and coagulated by ethanol to form a gel, followed by thoroughly washing with water. For metal nanoparticle synthesis, metal salt (AgNO3, HAuCl4 · 3H2O, and PtCl4, Aldrich) and other reagents (Wako Pure Chemicals, Japan) were used without further purification. Deionized water was used for all experiments. For hydrothermal reduction, 1.2 g transparent cellulose hydrogel films (water content, 95%) were immersed in 10 mL of 250 mM aqueous AgNO3 solution in a Teflon-sealed glass bottle. No apparent reaction was observed on immersion at room temperature. Heating the mixture at 80 °C for 24 h without stirring resulted in coloration of the gel to yellow-brown. After cooling to room temperature, the gel was thoroughly washed with distilled water. The AgNO3 concentration, heating temperatures, and time were varied from 1 mM to 2 M, 60-100 °C, and 2-165 h, respectively. Heating cellulose hydrogels in 50 mM HAuCl4 solution caused disintegration of the gel, together with weak pink coloration of the gel fragments. On the other hand, heating the hydrogel with 10 mM or 50 mM PtCl4 did not give reduction of platinum (black coloration). Alternatively, the gold- and platinumcellulose gels could be prepared by treating the corresponding saltimpregnated gels with 100 mM NaBH4 at room temperature caused pink to red, or black coloration, respectively, without disintegration of the gels. When these metal-cellulose gels were immersed in water for 1 month, release of the metal nanoparticles from the gels was negligible as confirmed by UV/vis spectroscopy.

Figure 3. UV/vis spectra of hydrothermally reduced silver-cellulose hydrogel prepared from aqueous AgNO3 solution of 10 mM (A), 50 mM (B), 100 mM (C), 250 mM (D), and 500 mM (E) at 60 °C for 48 h, respectively, and particle size histogram of silver nanoparticles from TEM images.

To obtain nanoporous aerogels, the imbibed water of metal-cellulose hydrogels was exchanged to ethanol, and supercritical CO2 drying was

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The X-ray diffractometry of cellulose and metal-cellulose aerogels was performed in reflection mode (Rigaku RINT 2000, Japan) with Ni-filtered Cu KR radiation (λ ) 0.15406 nm). The spherical size of crystalline (D) was estimated by Scherrer’s formula:

D)

Figure 4. Time-course change in UV/vis absorption spectra of silvercellulose hydrogel and particle size histograms from TEM images. Conditions: hydrothermal reduction of aqueous AgNO3 solution of 250 mM at 80 °C for 2, 4, 8, 16, 20 (from bottom to top), 28 (A), 44 (B), 52 (C), 76 (D), 92 (E), and 165 h (F), respectively.

Kλ B cos θ

(1)

where K is a constant (0.94), B is the full-width half-maximum of respective diffraction peak (rad), and 2θ is the peak angle in radian. Nitrogen physisorption measurements at 77 K were performed by a Quantachrome NOVA 4000 (U.S.A.), and Brunauer-EmmettTeller (BET) and Barrett-Joyner-Halenda (BJH) analyses were done by Autosorb software (Quantachrome). The samples were degassed at 105 °C in vacuum to remove all the adsorbed species. The BET analysis was done for relative vapor pressure of 0.05-0.3. The BJH analysis was done from the desorption branch of the isotherm. Thermogravimetric analysis (TGA) was carried out by Ulvac TGD 9600. The sample was placed in a platinum pan and heated from 25 to 600 °C at a rate of 10 °C/min under air or nitrogen atmosphere. UV/vis absorption and transmission spectra were taken at room temperature on a Shimadzu UVmini-1240 spectrophotometer using a quartz cuvette with an optical path of 1 cm. A sheet of metal-cellulose hydrogel or aerogel was set on the cell for each measurement. The thickness of the gels was about 0.4 mm. The tensile strength (σb) and Young’s modules (E) of the aerogels were measured on EZ-test (Shimadzu, Japan) at a speed of 5 mm/min.

Results and Discussion

Figure 5. Plot of the plasmon resonance peak position of the silvercellulose hydrogels as a function of particle size. Horizontal bars show standard deviations. Broken line is the smooth connection of points.

performed by a Hitachi HCP-2, through exchanging ethanol to liquid CO2 at 5.3 MPa at 4 °C for 6 h, then at 10 MPa for 0.5 h at 40 °C, and finally by slow release of CO2 at 40 °C. Scanning electron microscopy (SEM) observation of the surface and cross-section of cellulose and metal-cellulose aerogels was done by a Hitachi S-4000. For transmission electron microscopy (TEM), the imbibed water of hydrogels was exchanged to acetone, and then to a 7:3 (v/v) mixture of methylmethacrylate and butylmethacrylate monomers containing 1.4% (w/v) benzoyl peroxide as initiator. After polymerization by curing at 52 °C for 6 h, the embedded specimen was ultrathin-sectioned by a Leica Ultracut-E using a diamond knife. The section approximately 100 nm thick (golden color) was mounted on a grid with carbon support. To observe both of the structure of cellulose and metal nanoparticles, the ultrathin section was disembedded by removing the resin by acetone on the grid, and examined by TEM (JEOL-2000EX, 80 kV tension) without staining (defocus contrast). Also, the metal nanoparticles in the ultrathin-sections were directly observed without disembedding for assessing the influence of migratory loss of metal particles by disembedding treatments. From the micrographs at high magnification, the size histograms were obtained by iTEM software (Olympus Soft Imaging Solutions GmbH) for 200 metal particles.

Hydrothermal treatment of metal salt-impregnated cellulose gels without other reductant readily caused apparent formation of silver and gold nanoparticles. Higher temperature treatments, however, tended to result in disintegration of the gel. Therefore, the use of mild condition (below 100 °C) is crucial for preparing gels supporting metal particles. Figure 1 shows the TEM images, UV/vis spectra, and photographs of cellulose and hydrothermally reduced silver-cellulose hydrogels (80 °C, 24 h). The reduction of silver ions presumably involves both reducing end groups and hydroxyl groups of cellulose.41,42 The formation of metallic silver nanoparticles could be visually recognized by yellow to brown coloration of cellulose gels (Figure 1, inset). During the hydrothermal process, the AgNO3 solution remained colorless, and no deposition of silver on glass wall was observed. The structure of cellulose and silver-cellulose hydrogels was examined by TEM of ultrathin section of resinembedded hydrogels through removal of the resin on the TEM grid (Figure 1, top). The images show nanostructure had formed in cellulose hydrogels. The network of cellulose fibrils, about 20 nm wide, is preserved and the silver nanoparticles are dispersed well, apparently prevented from aggregation due to the presence of nanoporous structure of cellulose hydrogels; this is in obvious contrast with the silver mirror formation by the reaction without cellulose gels. The examination of ultrathin section before disembedding gave images of silver particles only, because cellulose fibrils could not give contrast against the resin (micrograph not shown). Since the population and size of silver particles in this specimen were apparently the same as in the disembedded section, migratory loss of silver nanoparticles during removal of resin was considered to be negligible. The formation of silver nanoparticles was also demonstrated by UV/vis spectra of silver-cellulose hydrogel (Figure 1, bottom). The appearance of plasmon resonance peak at 406 nm indicates the formation of silver nanoparticles. The peak

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Figure 6. SEM images of surface (A-D, I-L) and inside (E-H, M-P) of the cellulose and hydrothermally reduced silver-cellulose aerogels. (A, E) Images of the cellulose aerogel; (B, F), (C, G), and (D, H) images of the silver-cellulose aerogels formed from aqueous AgNO3 solution of 50 mM, 500 mM, and 1 M at 80 °C for 24 h, respectively, corresponding to Figures 1 and 2. (I, M), (J, N), (K, O), and (L, P) images of the silver-cellulose aerogel formed from aqueous AgNO3 solution of 250 mM at 80 °C for 8, 28, 76, and 165 h, respectively, corresponding to Figure 4. Inset of (K) and (L) show silver nanowires (marked by arrowheads) on the surfaces. Scale bars are 400 nm.

intensified and shifted from 406 to 427 nm with increase in AgNO3 concentration from 1 mM to 2 M, indicating the increase of the amount and particle size of silver.2,43,44 Figure 2 shows the TEM images of silver-cellulose hydrogels and size histograms of silver nanoparticles corresponding to Figure 1. From the analysis of TEM images of cellulose hydrogels at each AgNO3 concentration, silver nanoparticles were synthesized and dispersed well in cellulose gels without aggregation. The size histograms show Gaussian-like distributions. The average particles size increased gradually from 8.0 ( 4.0 to 11.4 ( 5.6 nm with increase in AgNO3 concentration, corresponding to the peak shift and bandwidth increase in absorption spectra (Figure 1). As expected, the reaction of cellulose and AgNO3 slowed down at lower temperature (60 °C, 40 h) (Figure 3). The plasmon resonance intensity decreased, indicating lower population of silver nanoparticles. However, the lower-temperature treatment gave smaller silver particles with average particle size of 7.1-9.6 nm. Change in reaction time caused more pronounced changes in spectra and silver particle size (Figure 4). In the period from 2-28 h, the plasmon resonance peak at 408 nm intensified and shifted slightly to longer wavelengths. As the reaction proceeded from 28-165 h, the peak intensified continuously and shifted from 420-447 nm. This shift corresponds well to the increase in silver particle size from 11.5 ( 5.3 to 44.4 ( 13.7 nm shown in the histograms based on the TEM images. After 44 h of treatment, two shoulders appeared at around 350 and 380 nm. These shoulders can be ascribed to the plasmon resonance of

silver nanowires.45 The relative intensity of these shoulders to the main peak seems to be independent of the level of silver deposition. It is well-known that the surface plasmon characteristics of silver nanoparticles depend on their size, shape, surrounding dielectric medium, coupling of the colloids, and adsorbed solutes.46-48 Figure 5 shows the plot of plasmon resonance peak position of silver-cellulose hydrogels as a function of particle size. The peak position of silver nanoparticles with average size less than 10 nm is less than 410 nm; and the peak shifted to longer wavelengths as the particle size increases by extending reaction time. Thus, the silver-cellulose hydrogels with different particle size and optical characteristics can be easily controlled by the AgNO3 concentration, heating temperature, and reaction time by hydrothermal reduction within nanoporous cellulose gels. Figure 6 shows SEM images of surface (A-D, I-L) and inside (E-H, M-P) of the cellulose and silver-cellulose aerogels. The morphology of cellulose network within the gel corresponds to those by TEM images in Figure 1. The appearance of network on the surface had nanoporous but more open structure, remarkably different from that of inside. This difference is likely to result from the difference in condition of cellulose regeneration; that is, the surface is exposed to pure coagulant, experiencing fast coagulation, while the inner region is subjected to gradual exchange of liquid, from cellulose solvent to coagulant. The inside of silver-cellulose aerogels must contain silver nanoparticles, but they are hard to recognize from


Figure 7. Top: UV/vis absorption spectra of NaBH4-reduced gold(A) and platinum-cellulose (B) hydrogel formed from aqueous HAuCl4 and PtCl4 solution of 10 mM. Insert: TEM images of inner part (C) and interface (D) of NaBH4-reduced platinum-cellulose hydrogel. Bottom: SEM images of surface (E) and inside (G) of the goldcellulose aerogels, surface (F) and inside (H) of the platinum-cellulose aerogels. Scale bars for SEM images are 400 nm.

Figure 8. XRD patterns and photographs (inset) of the cellulose (A), silver- (B), gold- (C), and platinum- (D) cellulose aerogels.

appearance only. On the other hand, the surfaces of heavily silver-treated gels (K and L), contained some wavy strings, about 400 nm wide (inset), which are obviously silver nanowires. This observation is consistent with the shoulder peaks in Figure 4. The formation of nanowires is considered to be possible on the surface only, where the silver nanoparticles can elongate without constraints of gel networks. Gold nanoparticles from HAuCl4 needed higher temperature than that for silver. However, cellulose hydrogels tended to decompose probably by acid hydrolysis. Also, platinum


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could not be formed from PtCl4 by the action of cellulose below 100 °C. Therefore we used NaBH4 as reductant to synthesize gold and platinum nanoparticles in cellulose gels under mild conditions. Figure 7 shows the UV/vis spectra (top), TEM (insert), and SEM (bottom) images of gold- and platinum-cellulose gels thus prepared. The gold-cellulose hydrogel gave an intense peak at 535 nm. That of platinumcellulose hydrogel was continuous over the entire range of 300-900 nm, corresponding to platinum black. Analysis by TEM images indicate that the gold (not shown) and platinum nanoparticles are also dispersed well in cellulose hydrogels. Moreover, a large amount of nanoparticles deposited on the surface probably by the diffusion effect. SEM images reveal that NaBH4-reduced gold- and platinum-cellulose aerogels have similar nanoporous structure as that from hydrothermally reduced silver-cellulose aerogels. Figure 8 shows the XRD patterns and photographs of cellulose aerogels and metalcellulose aerogels. The aerogels were moderately transparent, especially with low metal content. Introduction of metal nanoparticles caused a color change from light blue of cellulose to characteristic yellow for silver nanoparticles, pink to red for gold nanoparticles, and gray to black for platinum nanoparticles. The X-ray diffraction pattern of cellulose aerogel (Figure 8, curve A) gave peaks 12.2, 20.1, and 21.2°, corresponding to (11j 0), (110), and (200) diffractions of cellulose II. Additional small peaks in curves B-D are Bragg reflections from metal crystals, all agreeing to the normal diffraction angles of these metals. Analysis by the Scherrer equation indicated that the average particle sizes were 12.3, 6.5, and 4.4 nm for silver, gold, and platinum, respectively. The values agree well with those determined from TEM images, that is, 11.4 ( 5.6, 7.0 ( 3.2, and 5.6 ( 2.3 nm. Together, TEM, XRD, and SEM analyses provide a consistent picture of metal-cellulose gels that is macroscopically homogeneous and nanoporous structure. Figure 9 shows the nitrogen adsorption isotherms and mesopore size distributions of the cellulose and metalcellulose aerogels. The good agreement of adsorption and desorption branches indicates that these aerogels maintain the open mesopore structure of the original cellulose gel. The SBET values of the cellulose, silver-, gold-, and platinumcellulose aerogels were 404, 363, 406, and 403 m2/g (Table 1), respectively, and the mesopore diameter ranges from 2-70 nm, with the most probable values of 10-40 nm. Thus, the metal-cellulose aerogels maintained initial shapes by drying from supercritical CO2. Mechanical strength is another feature of the cellulose aerogel in contrast to silica aerogel, which is highly brittle. The tensile strength (σb) and Young’s modules (E) of pure cellulose aerogel were 12.3 and 65.8 MPa, respectively, which were remarkably high for a material with a porosity of 95%. The values of hydrothermally reduced silver-cellulose aerogels ranged from 11.8-8.8 MPa for σb, and 42.0-24.2 MPa for E. The results for gold- and platinum-cellulose aerogels obtained by the NaBH4 reduction were basically similar. Optical transmission spectra (Figure 10) show 95, 93, 85, and 24% light transmittance (T) for cellulose, silver-, gold-, and platinum-cellulose hydrogels, and 59, 52, 39, and 34% for aerogels at 600 nm, respectively. The difference was due to the light scattering on metal nanoparticles and porous cellulose matrix, and the different refraction index of media. The absorption peak at 413 and 525 nm for silver- and goldcellulose hydrogels reveals the surface plasmon resonance

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Figure 9. Nitrogen adsorption (open) and desorption (solid) isotherms, and BJH mesopore size distribution (from desorption branch; dp ) pore diameter, Vp ) pore volume) of cellulose (A), silver- (B), gold- (C), and platinum-cellulose (D) aerogels. Table 1. Properties of Cellulose, Silver-, Gold-, and Platinum-Cellulose Aerogelsa samples

F [g cm-3]

SBET [m2 g-1]

Vp [cm3 g-1]

2r [nm]

cellulose aerogel silver-cellulose aerogel gold-cellulose aerogel platinum-cellulose aerogel

0.12 0.14 0.16 0.17

404 363 406 403

1.52 1.44 1.39 1.36

22.9 18.9 16.6 16.5

a F is the density by macroscopic measurement; SBET is the specific surface area from N2 adsorption; Vp and 2r are the mesopore volume and mean pore diameter by BJH method. All samples were prepared from a 0.5 mm thick layer of cellulose solution.

Figure 10. Optical properties of cellulose (A, E), silver- (B, F), gold(C, G), and platinum-cellulose (D, H) hydrogel (real line) and aerogel (broken line), respectively.

of nanoparticles. The peaks remained in their aerogels and shifted to 387 and 518 nm, respectively, indicating the optical characteristics of metal-cellulose gels could be controlled by drying treatment. In view of the importance of thermal stability in many applications of nanoporous materials, we examined thermal decomposition of metal-cellulose aerogels by thermogravimetry (TGA) in an air and nitrogen atmosphere (Figures 11 and 12). In all TGA curves, the small weight losses below 150 °C apparently resulted from evaporation of adsorbed

water. Under nitrogen, the decomposition behavior of the metal-cellulose aerogels was nearly the same as pure cellulose, most weight loss taking place at 300-320 °C by 10 °C/min heating. The presence of silver nanoparticles caused increase in remaining char at 600 °C according to the level of silver deposition (Figure 11). Heating in air represent combustion of cellulose, leaving the metal components. Neglecting oxidation of the metals, we determined the silver content of silver-cellulose aerogels as approximately 5% for hydrothermal treatments. TGA curves of gold- and platinum-cellulose aerogels (Figure 12) revealed characteristic differences in the influences of metal species. In nitrogen, gold-cellulose specimen gave significantly higher char yield at 600 °C than others, indicating certain specific interference of gold on cellulose decomposition, which might be attributed to catalytic effect of gold altering some stage of cellulose decomposition. On the other hand, the features of TGA under air indicate acceleration of cellulose combustion by metal particles. With silver and gold nanoparticles, the secondary stage of cellulose decomposition (burning of char) between 350 and 600 °C shifted to lower temperatures. Especially notable is the behavior of the platinum-cellulose specimen; unlike others, the weight loss took place in nearly one step, at 200-250 °C. This is considered to be an example of a strong catalytic effect of platinum nanoparticles on oxidation of organic materials.

Conclusions The nanoporous materials from silver-, gold-, and platinumcellulose gels could be synthesized by hydrothermal- or NaBH4-reduction in cellulose gel. The nanoparticles were dispersed well and stabilized by the cellulose network, apparently prevented from aggregation due to the presence of nanoporous structure of cellulose hydrogels. The amount and size of metal particles could be controlled through concentration, temperature, and duration of reaction. The average particles size increased gradually from 8.0 ( 4.0 to



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Figure 11. TG and DTG curves of the cellulose (A) and hydrothermally reduced silver-cellulose aerogels (B-F) under an air and nitrogen atmosphere. The silver-cellulose aerogels were prepared from aqueous AgNO3 solution of 10 (B), 50 (C), 100 (D), 250 (E), and 500 mM (F) at 80 °C for 24 h, respectively, dried from supercritical CO2.

Figure 12. TG and DTG curves of the cellulose (A), silver- (B), gold- (C), and platinum- (D) cellulose aerogels under air and nitrogen atmosphere.

11.4 ( 5.6 nm with increase in AgNO3 concentration at 80 °C for 24 h. The lower-temperature treatment gave smaller silver particles with average particle size of 7.1-9.6 nm. Change in reaction time caused more pronounced changes in spectra and silver particle size from 11.5 ( 5.3 to 44.4 ( 13.7 nm. NaBH4-reduced gold- and platinum-cellulose aerogels have similar nanoporous structure as that from hydro-

thermally reduced silver-cellulose aerogels. Supercritical CO2 drying of the metal-carrying gel gave corresponding aerogels with high transmittance, porosity, surface area, moderate thermal stability, and good tensile strength. This facile method would be applicable to other metals for nanoparticle synthesis. The obtained nanostructured materials will be useful as antibacterial, electro-optical, and catalytic applications.

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Acknowledgment. This work was supported by the Japan Society for the Promotion of Science (JSPS) Foreign Researcher Fund of Japan.

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Nanoporous Cellulose as Metal Nanoparticles ... - ACS Publications

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