Abstract
A simple approach to fabricate hollow silica nanorods is reported, using nanocrystalline cellulose (NCC) as templates. Uniform NCC with the length of ca. 100 nm and a diameter of ca. 10 nm were prepared by hydrolysis of microcrystalline cellulose (MCC) in strong sulfuric acid condition. NCC was used as a template to prepare a core-shell composite of silica and NCC. The sol-gel reaction of tetraethyl orthosilicate (TEOS) was employed to coat NCC with a nano thickness of silica in the presence of ammonia. Finally, hollow silica nanorods were obtained by calcination of the composite at 600°C to remove the organic cellulose template completely. The obtained hollow silica nanorods were found to have uniform size and shape: with a length of ca. 100 nm, an inner diameter close to the original diameter of NCC, and a thickness of around 10 to 15 nm. These results suggest that NCC is an excellent template for manufacturing nano hollow materials with uniform shape and size.
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Fabrication of hollow silica nanorods using nanocrystalline cellulose as templates
Guangshuai Fu,a Aijian He,a Yongcan Jin,a Qiang Cheng,b and Junlong Song a, *
A simple approach to fabricate hollow silica nanorods is reported, using nanocrystalline cellulose (NCC) as templates. Uniform NCC with the length of ca. 100 nm and a diameter of ca. 10 nm were prepared by hydrolysis of microcrystalline cellulose (MCC) in strong sulfuric acid condition. NCC was used as a template to prepare a core-shell composite of silica and NCC. The sol-gel reaction of tetraethyl orthosilicate (TEOS) was employed to coat NCC with a nano thickness of silica in the presence of ammonia. Finally, hollow silica nanorods were obtained by calcination of the composite at 600°C to remove the organic cellulose template completely. The obtained hollow silica nanorods were found to have uniform size and shape: with a length of ca. 100 nm, an inner diameter close to the original diameter of NCC, and a thickness of around 10 to 15 nm. These results suggest that NCC is an excellent template for manufacturing nano hollow materials with uniform shape and size.
Keywords: Hollow silica nanorods; Template; Sol–gel method; MCC; NCC
Contact information: a: Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu, 210037 P.R. China; b: Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing, 100091 P.R. China; *Corresponding author: junlong.song@gmail.com
INTRODUCTION
In recent years, considerable effort has been devoted to the design and controlled fabrication of nano-structured materials with functional properties (Caruso 2001). Hollow materials have attracted great interest due to their wide range of applications in drug delivery, catalysts supports, fuel cell, enzyme carrier, fluorescers, and so on (Chen et al. 2004; Xiao et al. 2006; Guo and Cui 2009; Scheel et al. 2009). Templating is the most used method to fabricate hollow materials. Templates used include soft templates (such as emulsion droplets, micelles, etc. (Caruso et al. 1998)), and hard templates (for example, polystyrene microspheres (Kato et al. 2010), calcium carbonate (Zhao et al. 2010), and gold particles (Obare et al. 2001), etc.). However, these templates are mostly used to prepare hollow spherical nanoparticles and are, most of the time, in the scale of micro size. So far, limited reports on fabrication of hollow nano, rod-like materials or other shapes are available (Zollfrank et al. 2007).
Recently, long hollow inorganic nanoparticle nanotubes have been fabricated by templating of electrospun block copolymer fibers with clay mineral platelets followed by interlinking of the platelets using condensation reactions. Finally the polymer template is removed by heat or UV/ozone treatment (Ras et al. 2007). Inorganic nanoparticles with
hollow interiors were also constructed by coating self-assembled polymeric nano-objects with a thin Al2O3 layer, followed by removal of the polymer template upon heating (Ras et al. 2007).
Cellulose is capable of serving as a template for nanotube and nanorod materials. Cellulose is the most abundant and biodegradable natural polymer material on the planet. During its biosynthesis, it is expressed from enzyme rosettes in the form of 3 to 5 nm diameter fibrils that aggregate into larger microfibrils up to 20 nm in diameter (Lucia and Hubbe 2008). Therefore cellulose or fiber can be separated into nanoscale architectures, namely nanocrystalline cellulose (NCC), nanofibrils, and nanoscale cell wall architectures. Recently, NCC and nanofibrils have become widely considered for use in the reinforcement of plastics (Kim et al. 2009; Zoppe et al. 2009; Pei et al. 2011; Zhou et al. 2011).
NCC, the crystalline segments of elementary nanofibril crystalline cellulose fibrils after the amorphous segments have been removed, are usually obtained via treatment with strong acids at elevated temperature or with enzyme treatments. NCC is in the range of 100 to 300 nm long with diameters of several to tens nanometers (Bondeson et al. 2006; Morán et al. 2008; Lucia and Rojas 2009).
However, with respect to its size and shape, NCC can serve as a type of ideal template for nanotube and nanorod materials. Zollfrank and coworkers (2007) first successfully fabricated silica nanotubes with diameters of 10 to 30 nm, inner core diameters of approximately 3 nm, and lengths of up to 500 nm, using NCC as a template. In their procedures, oligopropylamino side chain was introduced onto the surface of NCC in the medium of dimethylsulfoxide (DMSO). Then the sol-gel process of tetraethyl orthosilicate (TEOS) was performed to deposit silica on the surface of NCC in aqueous solution. The oligopropylamino side chain is capable of catalyzing the formation of a silica layer, and therefore is responsible for the control of biosilica deposition in diatoms into the cellulose biopolymer.
In this article, we report a simple approach to fabricate silica nanorods using negatively charged NCC as template. When sulfuric acid is used as the hydrolyzing agent, it also chemically reacts with the surface hydroxyl groups of NCC to yield negatively charged (surface) sulfate groups (Lucia et al. 2010). In this investigation, we took advantage of the negatively charged property of NCC and employed the cationic surfactant cetyltrimethylammonium bromide (CTAB) to convert the surface charge of NCC to positive to promote deposition of silicic acid sol. Then, the polycondensation reaction of TEOS exclusively took place at the particles’ surfaces in the presence of ammonia, following the so-called Stober process (Stöber et al. 1968). As a consequence, silica-coated NCC particles were successfully prepared. Hollow silica nanorods were finally obtained after calcination of the obtained composite of silica/NCC at 600°C for 6 hours to remove the organic template. The obtained hollow silica nanorods have uniform shape and size. They do not agglomerate or collapse after calcination. Hollow silica nanorods may have potential applications in controlled release capsules for drugs, dyes, inks, artificial cells, and fillers because of their lower density, large specific surface area, and optical properties.
EXPERIMENTAL
Materials
Microcrystalline cellulose (MCC) was obtained from Whatman (DP approx. 172). All other chemicals, including sulphuric acid (H2SO4, 95 to 98%), tetraethoxysilane (TEOS, >95%), cetyltrimethylammonium bromide (CTAB, 98%), aqueous ammonia (25% w/w), and ethanol (EtOH, >99.7%), were purchased from Everbright Chemical Inc. (Jiangsu, China). All chemicals were used as received without further purification. All solutions were prepared using distilled/deionized water, which was treated with a Milli-Q system (Millipore Corporation, US).
Preparation of Nanocrystalline Cellulose
The procedure of preparation of nanocrystalline cellulose (NCC) was adopted from Beck-Candanedo’s method (Beck-Candanedo et al. 2005). NCC was obtained as follows: MCC (5 g) and H2SO4 (250 mL, 64 wt%) were charged into a 500-mL three-neck flask equipped with a mechanical stirrer. This mixture was hydrolyzed at 45°C for 30 min under a constant stirring rate of 300 rpm. Immediately following hydrolysis, suspensions were diluted 10-fold to stop the reaction. The suspensions were then centrifuged for 20 min at 12000 rpm, washed once with water, and recentrifuged. The procedure was repeated at least three times, or until the supernatant became turbid, to ensure the removal of excess acid. The suspension, at last, was transferred to a dialysis membrane bag with a molecular weight cutoff of 14,000, and dialyzed against water for several days until the water pH remained constant.
Synthesis of Composite of Silica/NCC
A typical coating process was performed as follows: 0.12g CTAB was dissolved in a mixture of 40 mL of H2O, 20 mL of EtOH, 0.4 mL of aqueous ammonia, and 4 mL of NCC dispersion (0.5%w/v). These chemicals were put into a 250-mL three-neck flask, and the dispersion was stirred by a magnetic stirrer (250 rpm) for 10 min at 30°C. Then, 200 μL of TEOS was charged into the system, leading to a sol-gel reaction. TEOS was slowly added into the NCC+CTAB+NH3 dispersion with stirring at 250 rpm at 30°C for 2 hours to allow the hydrolysis and condensation of TEOS and the formation of the silica/NCC composite. After the reaction, the NCC particles coated by the silica were separated from the excess chemicals and byproducts (silica particles) by centrifugation. Only the coated NCC particles were sedimented, and the supernatant was removed. Subsequently, pure water was added, and the sediment was redispersed. This separation procedure was repeated three times to remove most of the excess chemicals and byproducts. The resultant sediment (complex of silica/NCC) was obtained. The powder of complex of silica/NCC was obtained by drying at 50°C under vacuum for hours.
Fabrication of Hollow Silica Nanorods
The powder of silica/NCC composite was calcined at 600°C for 6 hours using a muffle furnace to remove the organic NCC core. Organic NCC was burned and released as CO2 and water. Silica mineral was retained, and kept the shape under which it had been formed. As a consequence, hollow silica nanorods were finally obtained.
Characterizations
FTIR spectra of NCC, silica-coated NCC, and hollow silica nanorods were obtained with a Nicolet 210 spectrometer. The sample for FTIR analysis was dispersed in KBr and then powdered in a mortar. Approximately 1 mg of the obtained samples was mixed with about 200 mg KBr to obtain the required discs. Both specimens and KBr were carefully dried before disk preparation and were subjected to FTIR analysis immediately afterwards.
A TGA analyzer investigated the thermal stability of the composite of silica/NCC. TGA analysis was performed in N2 from room temperature to 600°C at a heating rate of 20°C min-1.
XRD measurements were performed to check the crystallinity of MCC, NCC, silica-coated NCC, and hollow silica nanorods. The diffracted intensity of Cu Kα radiation (k = 0.1542 nm; 50 kV, and 40 mA) was measured in a 2θ range between 10° and 45° for MCC and NCC at a speed of 0.05°·s-1. In the case of silica-coated NCC and hollow silica nanorods, the corresponding values were 5° to 55° and 0.02°·s-1.
The specific surface area was measured by the BET method for silica-coated NCC and hollow silica nanorods. The degassing temperature was 80°C, and the time was 12 hours.
The diameter and morphology of the particles were examined by transmission electron microscopy (TEM) (JEOL JEM-1011) at an accelerator voltage of 100 kV. To prepare the TEM specimens, the NCC, silica-coated NCC, and hollow silica nanorods were dispersed in absolute ethanol with ultrasonication for 10 min. And then one drop of suspension was dropped on a copper TEM grid with carbon membrane, and the NCC was stained by phosphotungstic acid. Excess water was blotted with a piece of filter paper.
RESULTS AND DISCUSSION
Preparation of Nanocrystalline Cellulose
Nanocrystalline cellulose (NCC) was obtained through hydrolysis of MCC with strong sulfuric acid. The procedure has been addressed in detail elsewhere (Beck-Candanedo et al. 2005). NCC obtained from sulfuric acid hydrolysis was dispersed well in water due to the negative surface charge of NCC, which originated from the reaction of sulfuric acid with the surface hydroxyl groups of NCC. NCC solution was very stable, and no precipitate was observed even after months of storage time. The TEM image of obtained NCC is shown in Fig. 1(a). The image shows that cellulose crystals have uniform shape with a length between 100 and 150 nm and a width about 10 nm.
The infrared (IR) spectrum of NCC is presented in Fig. 1(b) with the comparison of MCC. It can be seen that the IR spectra of MCC and NCC were very similar. The observed strong IR absorption bands at 3300, 2920, 1750, and 1100 cm-1 agree with the cellulose standard. The crystallinity of cellulose can be checked with the FTIR method based on Nelson and O’Connor’s procedures (Nelson and O’Connor 1964a,b). Their theory claims that the strength of peaks at a1429/a893 and a1372/a2900 have relationships with the crystallinity of cellulose, and therefore these two relative strength values can be regarded as two crystallinity indexes of cellulose. Another report compared these two indexes with the crystallinity index checked by XRD and obtained two regressed equations (China National Pulp and Paper Research Institute and Beijing Chemical Plant 1981). According to these two equations, the crystallinity indexes checked by FTIR for MCC and NCC were 0.78 and 0.86, respectively. This indicates that the crystallinity of NCC increased to some extent compared with MCC, which was caused by the removal of amorphous regions of cellulose by hydrolysis. The same effect was also supported by XRD results in Fig. 1(c), where stronger peaks can be observed for NCC. There are three peaks at 14°, 16°, and 22°, respectively, of which 22° is maximum, in agreement with the typical XRD pattern of cellulose I.
Fig. 1. (a) TEM image of nanocrystalline cellulose; (b) FTIR spectra of microcrystalline cellulose and nanocrystalline cellulose; (c) XRD of microcrystalline cellulose and nanocrystalline cellulose
Preparation of Silica-Coated NCC
A sol-gel process was employed to coat the NCC with a layer of silica. The sol-gel process is a wet-chemical technique widely used in the fields of materials science. Such methods are used primarily for the fabrication of materials (typically metal oxides) starting from a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers (Min et al. 2009).
The sol–gel reactions of alkylsilane are as follows:
Alkylsilane hydrolysis in acid or basic conditions:
Si-(OC2H5)4 + 4H2O → Si-(OH)4 + 4C2H5OH (1)
Alcohol condensation:
Si-(OH)4 + Si-(OC2H5)4 → ≡Si-O-Si≡ + 4C2H5OH (2)
Water condensation:
Si-(OH)4 + Si-(OH)4 → ≡Si-O-Si≡ + 4H2O (3)
In our case, NCC carried some negative charges, since MCC became hydrolyzed in the sulfuric acids, and etherification may occur between sulfuric acid and the surface hydroxyl groups of NCC (Lucia et al. 2010). Therefore, CTAB, a cationic polymer, was applied to the system to convert the surface charge of NCC to positive. Alkylsilane was initially hydrolyzed in the presence of ammonia to form silicic acid. Then, the formed silicic acid adsorbed onto the surface of NCC due to electrostatic attraction. Condensa-tion with another alkylsilane or with itself to form cross-linked polysiloxane network was the last step. Therefore, a gel was formed and a composite of silica/NCC was prepared.
The TEM image of silica/NCC composite is presented in Fig. 2(a). It shows that these particles were very uniform as NCC, with the length of around 100 to 150 nm, but with an enlarged diameter about 30 to 40 nm, indicating that the silica layer was successfully coated onto the surface of NCC, and the thickness was about 10 to 15 nm. But it shows that these particles were agglomerated to some extent, similar to NCC.
The spectrum of the silica-coated NCC is shown in Fig. 3. The observed strong IR absorption bands at 1077 cm-1, 470 cm-1 (Si–O–Si), and 800 cm-1 (Si-OH) indicate that the condensation reaction of TEOS took place at the particle surfaces. Sharp bands at 2924 cm-1 and 2853 cm-1 indicate the presence of an organic cellulose core.
Fabrication of Hollow Silica Nanorods
The cellulose core was removed from the composite particles by calcination. The TEM image of hollow silica nanorods is shown in Fig. 2(b), which clearly shows that the silica nanorods were hollow. The inner diameter of hollow silica nanorods was measured to be about 10 nm, which is close to the diameter of NCC. The outer diameter of nanorods was in the range of 30 to 40 nm, which agrees with the diameter of core-shell structured silica-coated NCC. This also reveals that the silica layer outside of NCC can preserve its shape even after removal of the core cellulose crystal template in the course of calcination. Moreover, these hollow silica nanoparticles do not collapse, but agglomerate to some extent.
The spectrum of hollow silica nanorods is presented in Fig. 3 along with the spectra of NCC and silica-coated NCC. From comparison, the characteristic IR absorption bands of hollow silica nanorods at 1077 cm-1, 470 cm-1 (Si–O–Si), and 800 cm-1 (Si-OH) are more pronounced than those of silica-coated NCC, which indicates the polymerization of polysiloxane and more cross-linked Si–O–Si bonds formed at elevated temperatures in the case of calcination. Sharp bands of silica-coated NCC at 2924 cm-1 and 2853 cm-1 totally disappeared this time, indicating that the organic cellulose core was completely removed due to calcination.
Fig. 2. TEM image of (a) silica-coated nanocrystalline cellulose and (b) hollow silica nanorods
Fig. 3. FTIR spectra of nanocrystalline cellulose, silica-coated nanocrystalline cellulose, and hollow silica nanorods
The thermal stability of MCC, NCC, silica-coated NCC, and hollow silica nanorods is presented in Fig. 4. The thermal stability was measured by weight loss, as examined by TGA. The thermal decomposition temperature of NCC started from 250°C. Fast decomposition took place when the temperature was within the range of 250°C to 350°C. Since the reaction occurred in a nitrogen atmosphere, there was 20 percent residual carbon. The TGA curve of MCC was very close with that of NCC except with a higher residual carbon. After coating with silica, the thermal stability of NCC was significantly improved. Silica-coated NCC started to decompose at 250°C, and the weight remained constant after 400°C. However, a slight loss of weight can also be observed if one zooms the image to a larger scale. This can be attributed to the further condensation of Si-OH with the release of some small molecules. The TGA curve of hollow silica nanorods is the most stable one among all the TGA curves, and it exhibits almost a horizontal line in the graph.
Fig. 4. Weight loss of microcrystalline cellulose, nanocrystalline cellulose, silica-coated nanocrystalline cellulose, and hollow silica nanorods
Fig. 5. XRD of silica-coated nanocrystalline cellulose and hollow silica nanorods
XRD results for silica-coated nanocrystalline cellulose and hollow silica nanorods are presented in Fig. 5. From comparison, the silica-coated nanocrystalline cellulose has smaller peaks at 16°, 22°, and 33°; these are the characteristic peaks of cellulose, as presented in Fig. 1(c). Because the proportion of cellulose is very small, one can see that the curve mainly corresponded to that of silica. Hollow silica nanorods exhibited a broad peak near 20°, the same peak as silica reported in the literature.
BET was employed to characterize the obtained silica-coated NCC and hollow silica nanorods. The specific surface area of silica-coated NCC was only 36.4 m2/g, while after calcination it increased up to 1499.7 m2/g. One can see that the specific surface area of hollow silica nanorods was much greater than silica-coated NCC. The increment of specific surface area was caused not only by the exposure of interior surface of nanorod tubes, but also caused by the addition of CTAB, which renders the wall of nanotubes mesoporous (Le et al. 2007). So, hollow nanorods obtained in our experiment gave rise to a large surface area, indicating that the produced hollow silica nanorods can be suitable for a number of promising applications.
CONCLUSIONS
- A simple approach is reported for the fabrication of hollow silica nanorods using nanocrystalline cellulose (NCC) as templates, taking advantage of negatively charged nanocellulose crystals prepared by sulfuric acid hydrolysis.
- The obtained hollow silica nanorods have uniform shape with the length of ca. 100 to 150 nm, inner diameter of ca.10 nm, and the thickness of 10 to 15 nm. The hollow silica nanorods are mesoporous, and the specific surface area can reach up to around 1500 m2/g.
- The results show that NCC is an ideal template for manufacturing nano hollow materials with uniform shape and size.
ACKNOWLEDGMENTS
The authors are grateful for the support of Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20103204120005), Talents Foundation of Nanjing Forestry University (163105003), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Article submitted: November 20, 2011; Peer review completed: January 8, 2012; Revised version received: March 29, 2012; Accepted: March 30, 2012; Published: April 10, 2012.