Bimodal Mesoporous Silica Nanotubes Fabricated by Dual Templates

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Bimodal Mesoporous Silica Nanotubes Fabricated by Dual Templates of CTAB and Bare Nanocrystalline Cellulose Junlong Song,†,‡,* Guangshuai Fu,† Qiang Cheng,§ and Yongcan Jin† †

Jiangsu Provincial Key Lab of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing 210037, People’s Republic of China; ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China; § Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, People’s Republic of China. ABSTRACT: Mesoporous nanomaterials have important applications in the fields of biochemistry and materials and thus have recently attracted much attention. In this investigation, silica nanotubes with bimodal mesopores were fabricated using dual templates of bare nanocrystalline cellulose (NCC) and surfactant cetyltrimethylammonium bromide (CTAB). The prepared mesoporous silica nanotubes coined the shape of NCC, with lengths of around 100−150 nm, outer diameters in the range of 30−40 nm, and thicknesses of shell of about 10−15 nm. BET measurement displayed that the obtained sample has a specific surface area up to 1182 m2/g, while the BJH pore size distribution showed that it has featured bimodal mesoporous structures, with 10 nm inner cores originated from the biotemplate NCC and 3 nm mesopores generated in the wall of nanotubes by CTAB. Due to the superhigh surface area and unique mesoporous structures, silica nanotubes obtained by this approach may have numerous potential applications.

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INTRODUCTION In recent years, considerable effort has been devoted to the design and controlled fabrication of nanostructured materials with functional properties.1 Hollow and mesoporous materials have attracted great interest due to their wide range of applications in drug delivery, catalysts support, fuel cells, enzyme carriers, fluorescers, luminescents, adsorbents, and so on.2−7 Soft templates (e.g., emulsion droplets, micelles, etc.) and hard templates (e.g., polystyrene microspheres, calcium carbonate, and gold particles, etc.) are the two most used methods to fabricate hollow and/or mesoporous materials.8−11 Since cellulose 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 during its biosynthesis,12 it can be readily disassociated into its nanocomponents (namely nanocrystalline cellulose (NCC), nanofibrils, and nanoscale cell wall architectures) when it is subjected to the treatment of acid, enzymes, ultrasound, or a combination thereof.13−17 NCC, the residue of cellulose hydrolysis, usually in the range of 100 to 500 nm in length with widths of several to tens of nanometers,14−20 is suitable for being the biotemplate to fabricate hollow and/or mesoporous materials, particularly for tubular or rod-like materials, due to its size and shape. However, as reported by Zhang et al.,21 it is difficult to prepare mesoporous silica tubes based on bare NCC without any treatment. Zollfrank and co-workers22 first successfully fabricated silica nanotubes using NCCs as templates by modification of the surface of NCC with oligopropylamino side chains; the introduced oligopropylamino side chains were capable of catalyzing the formation of a silica layer. The silica nanotubes obtained by this method were of diameters in the range of 10 to 30 nm, inner core diameters of approximately 3 nm, and lengths of up to 500 nm. Dual templates of native © 2013 American Chemical Society

cellulose substance (filter paper) and cetyltrimethylammonium bromide (CTAB) micelles were employed by Zhang et al.21 to produce mesoporous silica nanotubes. In their procedure, cellulose nanofibers were precoated with an ultrathin layer of titania film to help the adsorption of CTAB, and therefore to facilitate the deposition of TEOS sol and subsequent reactions. In this investigation, we attempted to fabricate mesoporous silica nanotubes using bare NCC with the aid of CTAB. Since the nature of NCC obtained from heavy sulfuric acid hydrolysis is negatively charged,13 it could attract CTAB to its surface and then facilitate the sol−gel reaction of TEOS to prepare the composites of silica/NCC. Mesoporous silica nanotubes were obtained by calcination of the composites and then the specific surface area and pore size distribution of the resulting bimodal mesoporous silica nanotubular samples were characterized by BET method. The results showed that this material is of a featured bimodal mesoporous structure with ca. 10 nm mesopores in the inner core and ca. 3 nm mesopores in the silica nanotubular walls. Since the formation of silica/NCC composites is vital to nanotube fabrication, factors affecting the formation of the composites (i.e., dosages of CTAB, ammonia, TEOS, and colsolvent) were discussed in detail as well in this investigation.

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EXPERIMENTAL SECTION Materials. Microcrystalline cellulose (MCC, DP = 215− 240) provided by Sinopharm Chemical Regent Co. Ltd. (Shanghai, China) was used a raw material for production of Received: Revised: Accepted: Published: 708

October 31, 2013 December 3, 2013 December 6, 2013 December 6, 2013 dx.doi.org/10.1021/ie4036803 | Ind. Eng. Chem. Res. 2014, 53, 708−714

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Figure 1. TEM iamges of silica/NCC composites prepared under the condition of TEOS dosage of 200 μL, aqueous ammonia dosage of 400 μL, in the mixture media of 40 mL water and 20 mL ethanol, while with varied CTAB additions of (a) 0 mg, (b) 12 mg, (c) 30 mg, (d) 60 mg, and (e) 120 mg.

Fabrication of Silica Nanotubes. Silica nanotubes were finally obtained through calcining the powder of silica/NCC composites at 600 °C for 6 h to remove the organic NCC cores releasing as CO2 and water, using a muffle furnace. The organic moieties of silica layer were released as CO2 and water too. Analytic Methods. FTIR spectra of NCC, silica-coated NCC, and silica nanotubes were obtained with a Nicolet 380 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 of KBr to obtain the required discs. Both specimens and KBr were carefully dried before and in the course of disk preparation. Thermal stability of the silica/NCC compositeswas performed on a Shimadzu TG-60A/60AH thermalgravimetric analyzer, which was operated in N2 gas from room temperature to 600 °C at a heating rate of 20 °C min−1. X-ray Diffraction patterns were obtained on an Ultima IV (Rigaku Corp., Japan) to check the crystallinity of MCC, NCC, silica-coated NCC, and silica nanotubes. 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° at a speed of 0.05°·s−1 for MCC and NCC. In the case of silicacoated NCC and silica nanotubes, the corresponding values were 5° to 55° and 0.02°·s−1. The specific surface area and pore-size distribution were determined by the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, using N2 adsorption/desorption isotherms by a Micromeritics ASAP 2020 instrument. The degassing process was carried out at 80 °C for 12 h. The shape and size of specimens were characterized by transmission electron microscopy (TEM, JEOL JEM-1011) at an accelerator voltage of 100 kV. NCC, silica-coated NCC, and silica nanotubes were dispersed in absolute ethanol with ultrasonication for 10 min prior to TEM sample preparation. And then one drop of suspension was dropped on a copper TEM grid with carbon membrane.

nanocrystalline cellulose (NCC). All other chemicals, including sulphuric acid (H2SO4, 95−98%), tetraethoxysilane (TEOS, >95%), cetyltrimethyl ammonium bromide (CTAB, 98%), aqueous ammonia (25% w/w), and ethanol (>99.7%), were purchased from Everbright Chemical Inc. (Nanjing, 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, U.S.). Preparation of Nanocrystalline Cellulose (NCC). The procedure of preparation of NCC was adopted from BeckCandanedo’s method.23 NCC was obtained as follows: MCC (5g) and H2SO4 (250 mL, 64 wt %) were charged into a 500mL three-neck flask equipped with a mechanical stirrer. This mixture was hydrolyzed at 45 °C for 30 min under constant stirring. After hydrolysis, the suspension was subjected to centrifuge to remove excess acid several times until the suspension appeared turbid. It then was transferred into a dialysis bag to dialyze against pure water for several days until the medium pH down to a constant close to pure water. Formation of Silica/NCC Composites. A typical coating process was performed as follows: CTAB and aqueous ammonia were initially charged in the mixture of ethanol and H2O which hold in a 250-mL three-neck flask; and then NCC dispersion (0.5%w/v) was added in the solution. The suspension was stirred with a magnetic stirrer (250 rpm) at 30 °C. After 10 min stirring, TEOS was slowly added into the system. After two hours’ reaction, the NCC particles coated with silica were separated from the excess chemicals and byproducts (silica particles) by high centrifugation (12 000 rpm). Subsequently, the sediment was redispersed in the centrifugal tubes by an addition of pure water and subjected to another centrifugation. This separation procedure was repeated three times to remove most of the excess chemicals and byproducts. The resultant composites of silica/NCC were obtained by drying the sediment at 50 °C under vacuum for hours. 709

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RESULTS AND DISCUSSION The formation of silica/NCC composites is a vital process in the fabrication of nanotubular materials. TEOS initially hydrolyze in acid or basic condition to form silicic acid and then the formed silicic acid condensates with another alkylsilane or with itself to form a cross-linked polysiloxane network; thus, a gel is formed. The hydroxyl groups of polysiloxane can react with the hydroxyl group over the surface of NCC and therefore the gel actually can be covalently attached to NCC. Therefore, to control the relative reaction rate of hydrolysis, alcohol condensation and water condensation is crucial to the formation of composites of silica/NCC. On the basis of the literature and our previous experience, the major parameters influencing the formation of core−shellstructured silica/NCC composites include the dosages of CTAB, TEOS, aqueous ammonia, and cosolvent ethanol, while minor influence of temperature and reaction time, both set at 30 °C and 2 h throughout the investigation. Formation of Composites of Silica/NCC. Effect of CTAB Additions on the Formation of Composites of Silica/NCC. As a surfactant, cetyltrimethyl ammonium bromide (CTAB) can be aligned or adsorb at the interfaces of aqueous solution or form micelles in the solution. As a cationic surfactant, it can also be a bridge of electrostatic attraction between solid surface of NCC and TEOS sol. It plays a crucial role in the formation of core−shell-structured composites of silica/NCC. Therefore, in this experiment, the effect of CTAB on the formation of silica/ NCC composites needed to be first identified. In this experiment, other parameters, such as TEOS dosage of 200 μL, aqueous ammonia dosage of 400 μL, the ratio of water to ethanol 2:1 (i.e., 40 mL water to 20 mL ethanol) were maintained while only the CTAB charges were varied from 0 to 120 mg. TEM images of silica/NCC composites with varying CTAB charges are presented in Figure 1. There was no composite of silica/NCC found in the image when no CTAB was added in; only whisker-like NCC was observed in Figure 1a. It can be explained in that the NCC prepared from sulfuric acid carried some negative charges and TEOS sol could not deposit on the surface of it; therefore, TEOS was hydrolyzed in the aqueous solution and then evenly distributed in the system due to the absence of nuclei center for particle growth. When the CTAB addition was 12 mg, a number of silica particles could be observed; however, there was no composite of silica/NCC present in Figure 1b. It indicates that the amount of CTAB was not enough to cover the surface NCC to act as the bridge to bind the silica layer; but CTAB functioned as the nuclei center for particle growth. When the additions of CTAB were 60 mg and 120 mg, nice silica/NCC composites are presented clearly in Figure 1c,d, respectively. However, with the addition of CTAB increased to 120 mg (equivalent to 5 mmol/L), there were some large, dark spheres observed in the image of Figure 1e. Since the Critical Micelle Concentration (CMC) of CTAB was determined to be 1.14 mmol/L at 306 K,24 this value was very close to the temperature in this investigation (our experiment was conducted at 30 °C, i.e., 303 K). When the concentration of CTAB close to its CMC, a nice layer of cationic layer adsorbed on negatively charged NCC and would sufficiently bind TEOS sol. When the concentration of CTAB increased to a very high concentration above CMC, for example 5 mmol/L, besides covering all of the surface of NCC, excess CTAB formed micelles as well. These micelles could function

as soft templates to form spherical silica particles, which can clearly be seen in Figure 1e. This experiment demonstrated that CTAB dosage is critical to the formation of silica/NCC composites and there is an optimal range of CTAB charge. Below or over that range, the silica/NCC composites cannot be properly formed. As such, 30 mg (equivalent to 1.25 mmol/L) of CTAB was chosen as the optimal dosage for all of the experiments afterward. Effect of TEOS Charges on the Formation of Composites of Silica/NCC. TEOS is the silicon source for composites of silica/NCC, so the amount of TEOS addition is directly relevant to the diameter and thickness of the silica layer on NCC.25 In this experiment, parameters, such as CTAB charge, aqueous ammonia charge, the volumes of water/ethanol were maintained at 30 mg, 40 μL and 40 mL/20 mL, respectively, while only TEOS additions varied to investigate the influence of TEOS on the formation of composites of silica/NCC. The TEM images of silica/NCC composites with varying TEOS charges of 50 μL, 100 μL are presented in Figure 2a,b,

Figure 2. TEM images of silica/NCC composites prepared under the condition of CTAB dosage of 30 mg, aqueous ammonia dosage of 400 μL, in the mixture media of 40 mL water and 20 mL ethanol, while with varied TEOS additions of (a) 50 μL and (b) 100 μL.

respectively, while the image for 200 μL of TEOS dosage is presented in Figure 1c. Diameters of silica/NCC composites were measured to be ca. 40, 60, and 100 nm, respectively. From these images, we can see that with the increase of TEOS addition, the diameter of composites enlarges proportionally. This indicates that the thickness of the silica layer coated on the NCC surface can be controlled by the dosages of TEOS. As a consequence, it is possible to use it as a variable to control the wall thickness of nanotubular silica mesoporous materials. Actually, the hollow structure of the composites prepared with a TEOS addition of 50 μL collapsed after calcination. While all the composites with a TEOS addition greater than 100 μL could preserve the tubular structure after calcination. As such, 100 μL of TEOS addition was chosen as the optimal TEOS charge for all of the experiments afterward. Effect of Ammonia Charges on the Formation of Silica/ NCC Composites. In this experiment, aqueous ammonia was used as the catalyst for the sol−gel reaction of TEOS. To investigate the effects of ammonia charges on the formation of composites of silica/NCC, parameters such as CTAB charge, TEOS charge, and the ratio of water and ethanol were maintained at 30 mg, 100 μL, and 40 mL water/20 mL ethanol, respectively, while only the ammonia additions were varied from 100, 400, to 800 μL. TEM images of silica/NCC composites with varying ammonia charges of 100, 400, to 800 μL were presented in Figure 3. From these images, it can be 710

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Figure 3. TEM images of silica/NCC composites prepared under the condition of CTAB dosage of 30 mg, TEOS dosage of 200 μL, and in the mixture media of 40 mL water and 20 mL ethanol, while with varied ammonia additions of (a) 100 μL, (b) 400 μL, and (c) 800 μL.

Figure 4. TEM images of silica/NCC composites prepared under the condition of CTAB dosage of 30 mg, TEOS dosage of 200 μL, ammonia additions of 100 μL, while with varied ratio of water and ethanol (a) 60/0, (b) 40/20, and (c) 0/60 (v/v) .

of ethanol and water were presented in Figures 4 and 3b. These images demonstrated that significant impacts of cosolvent to the formation of silica/NCC composites. If there was no water present in the solvent (actually there was some water present in the system which is from NCC suspension), then huge spherical silica particles were observed in the image of Figure 4a. The diameter of silica spheres was greater than 200 nm. And there was no NCC observable since the diameter of silica spheres was too large to penetrate by the electron beams of TEM. When the ratio of ethanol and water was 40 mL/20 mL, some smaller spherical silica particles with the diameter in the range of 80 to 200 nm were observed (see Figure 4b). Since the particles could be penetrated by the electron beam of TEM, NCC inside of the particles could clearly be seen. When the ratio of ethanol and water is 20 mL/40 mL, nice silica/NCC composites were prepared (Figure 3b). Only a thin layer of silica was coated on the surface of NCC. If the ratio of ethanol to water is at the other extreme end, i.e., no ethanol present in the system, then some networks of silica/NCC formed, as seen in Figure 4c. However, the coated silica layer was too thin to withstand calcination. Therefore, ethanol to water of 1:2 was regarded as the optimal ratio for the formation of silica/NCC composites. Core−Shell Silica/NCC and Accordingly Bimodal Mesoporous Silica Nanotubes Prepared under Optimum Conditions. Through the experiments conducted above, the effects of CTAB, ammonia, TEOS dosages, and the ratio of water and ethanol to the formation of silica/NCC composites were examined. The optimal condition obtained was as follows: 4 mL of NCC dispersion (0.5%w/v), 30 mg of CTAB concentration, 400 μL of aqueous ammonia, 100 μL of TEOS, react in the mixture of 40 mL of water and 20 mL of ethanol. The TEM images of core−shell silica/NCC composites prepared under this optimal condition were

seen that, as the catalyst, ammonia’s charges affected not only the formation of composites of silica/NCC, but also the morphology of the composites. When the ammonia addition was 100 μL, the silica layer started to form, but the thickness was too thin, indicating that the catalyst amount was not enough to catalyze the reaction under this condition. With the adding volumes of ammonia increased to 400 μL and 0.8 mL, the diameter of the composites enlarged to ca. 40 and 60 nm, respectively. As we addressed previously, these thicknesses were able to preserve the tubular structure after calcination. Therefore, we chose the lower one, i.e., 400 μL as the optimal ammonia charge for all the experiments afterward. Ammonia, as the catalyst, can accelerate the hydrolysis and condensation reaction rate of TEOS. Therefore, it can be used as a variable, along with TEOS charge, to control the thickness of the silica layer coated on NCC under a given reaction time. Effect of Cosolvent System on the Formation of Composites of Silica/NCC. In a regular state, TEOS does not dissolve in water and therefore cannot react evenly with the surface of NCC in aqueous solution. A cosolvent has to be used to facilitate the reaction of TEOS homogeneously with the surface of NCC in aqueous solution. Methanol and ethanol are the two most-used cosolvents for TEOS. In this investigation, ethanol was employed as the cosolvent since TEOS released ethanol during hydrolysis in water. From the reaction formula illustrated in the previous section, the cosolvent system, i.e., the ratio of ethanol and water will also be critical to the formation of silica/NCC composites. In order to investigate the influence of cosolvent to the formation of silica/NCC composites, the ratio of ethanol and water was varied as the only variable of 60/0, 40/20, 20/40, and 0/60, and kept all other parameters constant, i.e., 30 mg of CTAB, 100 μL of TEOS, and 400 μL of aqueous ammonia. The TEM images of silica/NCC composites with varying ratio 711

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shown in Figure 5a. From this image, we can see that the silica layer was well coated on NCC and the NCC core could be

Figure 6. XRD patterns of NCC, silica/NCC composites, and silica nanotubes. Figure 5. TEM images of (a) core−shell-structured silica/NCC composites prepared under optimum conditions and (b) the resulting mesoporous silica nanotubes after calcination. Please note that the scale bar in (a) is 50 nm, whereas in (b), it is 100 nm.

clearly observed in the TEM image. The length of the composite was ca. 200 nm, similar to the length of NCC. The diameter of the composites was ca. 40 nm. The inner core diameter was ca. 10 nm. So the wall thickness of silica nanotubes is ca. 15 nm. In order to obtain nanotubular silica materials, the cellulose core was removed from the composites by calcination, which was performed at the maximum temperature of 600 °C. A higher calcination temperature will lead to sintering of silica nanotubes, as reported by Scheel and co-workers.5 In their experiments, silica nanotubes calcinated at 650 °C showed a larger tubular structure than those calcinated at 900 °C. The TEM image of silica nanotubes was shown in Figure 5b, which clearly exhibited the hollow inner core of silica nanotubes. The inner diameters of hollow silica nanotubes were measured to be about 10 nm, close to the diameters of template NCC. The outer diameters of nanotubes were in the range of 30 to 40 nm, which was also consistent with the diameters of core−shell structured silica-coated NCC very well. This revealed that the silica layer outside of NCC preserved its shape even after removal of the core cellulose crystal template in the course of calcination at 600 °C. Moreover, these silica nanotubes did not collapse, since they were covalently connected by Si−O bonds of silica. Agglomerates of silica nanotubes can be observed in the TEM image as well, which is one of the features of nanomaterials. Bimodal Mesoporous Silica Nanotubes Characterized by XRD and BET. XRD patterns of silica nanotubes, as well as NCC and silica/NCC composites are presented in Figure 6. NCC showed a characteristic XRD pattern of cellulose I with pretty high crystallinity at the positions of 14°, 16°, and 22.1°. Silica nanotubes exhibited a broad peak near 18°, the same position as reported in literature for mesoporous silica,26 while for the XRD pattern of silica-coated NCC, it had three small peaks at 12.5°, 18°, and 20°. The peak at the position of 18° is assigned to the silica layer coated on NCC. The other two peaks at 12.5° and 20° are attributed to allomorphs of cellulose II, indicating that part of the allomorphs of cellulose I of NCC converted into cellulose II in the presence of ammonia during reactions. BET was employed to characterize the obtained silica-coated NCC and silica mesoporous nanotubes. Figure 7a showed the nitrogen adsorption−desorption isothermal plot for the

Figure 7. Nitrogen adsorption−desorption isotherms for mesoporous silica nanotubes (a) and the BJH pore size distribution plots determined from the adsorption branches (b).

mesoporous silica nanotubes. An apparent hysteresis loop was observed at relative pressures (P/P0) between 0.45 and 0.85, indicating that mesopores exist. And the adsorption− desorption isothermal plot would be assigned to type VI, according to IUPAC 13.2, and the shape of isotherm originated from cylindrical mesopores. A flat curve in the relative pressure range of 0.7−0.95 (P/P0) was observed, indicating that there were not many macropores in existence in this material. The specific surface area was only 36.4 m2/g for the composite of silica/NCC. However, after calcination it increased up to 1182 m2/g for silica mesoporous nanotubes. In the mean time, the total pore volume increased to 0.86 cm3/ g. Compared with the mesoporous silica spheres prepared by Wang et al.,6 they had a BET specific surface area of 204.6 m2/g and a total pore volume of 0.264 cm3/g. The specific surface area of the obtained mesoporous silica nanotubes was also greater than that of single-handed coiled silica nanoribbons 712

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with mesopores in their walls prepared by Li et al.27 using a dual-templating approach (chiral low-molecular-weight amphiphiles and the F127 triblock copolymer), whose BET surface reached up to 700 m2/g. The BET surface area of the mesoporous silica nanotube prepared by dual templates of fiber and CTAB with the aid of titania layer by Zhang et al.21 reached 765.5 m2/g and possesses a much larger pore volume (1.039 cm3/g) due to the larger template size. And the specific surface area of the obtained silica mesoporous nanotubes in our experiments was also substantially higher than those mesoporous silica assisted by NCC by Sowri Babu et al.,26 whose surface area was only 952 m2/g. It indicated that the small size of the template (NCC) and the hierarchical structure of mesoporous silica nanotubes contributed a significant effect toward improving the surface area and the total pore volume of the material. BJH pore size distribution plots determined from the adsorption branches are given in Figure 7b, showing bimodal sharp peaks with average diameters at ca. 3 and 10 nm. The latter peak corresponds to the tubular cores originated from templates. The former peak is caused by the CTAB, which renders the wall of nanotubes with mesopores.28 And this value is also close to the size of mesopores (ca. 2 nm) of silica nanotubes by sol−gel reaction of TEOS with only CTAB by Zhang et al.21 or through acid-catalyzed sol−gel reaction by Scheel et al.5 The small size of silica nanotubes, its exposure of both interior and exterior surfaces, and mesopores generated in the walls of nanotubes, render this material with such a higher specific surface area than those of conventional silica nanomaterials, which is typically in the range of 200−1000 m2/g. Owing to the super high surface area, silica nanotubes with bimodal mesoporous structure fabricated from dual templates of CTAB and bare NCC are suitable for a number of promising applications, especially in the biomedical and catalysis field.

Foundation of Nanjing Forestry University (163105003), Open fund of State Key Laboratory of Pulp and Paper Engineering (201134), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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CONCLUSIONS Silica nanotubes with bimodal mesopores were developed using dual templates of CTAB and bare NCC with negative charge. The prepared mesoporous silica nanotubes were quite uniform and coined the shape of NCC, with lengths of around 100−150 nm, outer diameters in the range of 30−40 nm, and thicknesses of shell of about 10−15 nm. Dosages of CTAB, ammonia, TEOS, and cosolvent ethanol are important parameters for the formation of silica/NCC composites and therefore affect the formation of mesoporous silica nanotubes. The BET specific surface area for the tubular silica nanomaterial was up to 1182 m2/g, with featured bimodal mesoporous structures at 3 and 10 nm.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-25-85428163. Fax: +86-25-85428689. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support of National Natural Science Foundation of China (Grant No. 31270613), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20103204120005), Scientific Research Foundation for the Returned Overseas Chinese Scholars, Talents 713

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dx.doi.org/10.1021/ie4036803 | Ind. Eng. Chem. Res. 2014, 53, 708−714