Mesoporous Silica Coating on Carbon Nanotubes: Layer-by-Layer

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Mesoporous Silica Coating on Carbon Nanotubes: Layer-by-Layer Method Xiaoyong Deng, Ping Qin, Man Luo, ErLei Shao, Hui Zhao, Xing Yang, Yanwen Wang, Haifa Shen, Zheng Jiao, and Minghong Wu Langmuir, Just Accepted Manuscript • Publication Date (Web): 14 May 2013 Downloaded from http://pubs.acs.org on May 15, 2013

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Mesoporous Silica Coating on Carbon Nanotubes: Layer-by-Layer Method Xiaoyong Deng,†,§,‡ Ping Qin,†,‡ Man Luo, ╫ Erlei Shao,† Hui Zhao,† Xing Yang,† Yanwen Wang,† Haifa Shen,§,┴,* Zheng Jiao, †,║ and Minghong Wu†,║,* †

Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 200444, PR China §

Department of Nanomedicine, The Methodist Hospital Research Institute, Houston, Texas 77030, United States

║

School of Chemical and Environmental Engineering, Shanghai University, Shanghai 200444, PR China

╫

Zhongshan Hospital, Shanghai Medical College, Fudan University, Shanghai 200032, PR China ┴

Department of Cell and Developmental Biology, Weill Cornell Medical College, New York, New York 10065, United States

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KEYWORDS Multi-walled carbon nanotubes; Core/shell nanostructure; Mesoporous; Layer-by-layer assembly; Silica nanotubes

ABSTRACT

It is of great interest to develop a simple, general and easy-handling procedure for the mesoporous silica coating. A facile and single-step method to coat iron oxide nanoparticles has been reported by the Hyeon group. However, up to present, this method only successfully applied to those zero-dimentional nanostructures heavily capped by cetyltrimethyl ammonium bromide (CTAB); no others are reported. It is unknown how this simple method is feasible in coating those nanostructures not capped by CTAB. Herein, using carbon nanotubes (CNTs) as the model, through analogous layer-by-layer assembly method, much more CTAB molecules were found to anchor to CNTs, so that on which uniform mesoporous silica shells can successfully be formed by Hyeon’s coating method. We believe that this contribution will pave the way for advancing the single-step method to become a general protocol in the mesoporous-silica coating field.

1. INTRODUCTION The development pathway of nanotechnology would cover two foundational phases, from passive nanostructure to active nanomaterial with specific functionalities.1 In the past decade, many methods have been developed to prepare passive nanostructures, for heavily investigating the unique properties of nanosized materials. Nowadays, nanotechnology has been in the second foundational phase, and the R&D is expected to shift toward composing the more sophisticated nanosystems.2,3 Compared with the single-component materials, the hierarchical nanostructures ACS Paragon Plus 2Environment

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are expected to possess multi-functionalities and much more enhanced properties, making them widely apply in the fields of catalysis, electronics, medicine, environment and energy.4 In particular in the biomedical field, these hierarchical particles promise to apply in imaging, controlled drug release, targeted drug delivery, cell labeling, and tissue engineering.5 Self-assembly of small building blocks is usually employed to prepare the clever hierarchical nanosystems,6 using both specific and non-covalent interactions, such as hydrogen-bonding,7 electrostatic interactions,8 van der Waals interactions,9 antibody-antigen recognition,10 conjugating proteins,11 DNA-directed assembly,12 synergetic interactions of polymeric matrix,13 etc. For successful self-assembly, two critical factors should be concerned. The first one is to improve the physicochemical limitations of building blocks, such as low stability, high chemical reactivity, and nondesired aggregation. The other is to prepare the building blocks with tailored size, surface functionality, and capability of carrying much functional molecules. An approach, coating the small blocks with silica, might be able to both overcome those limitations and prepare multifunctional blocks.14 Silica is an excellent coating reagent not only due to its compatible surface which can be easily chemically modified, but also for its good biocompatibility and being accepted as “Generally Recognized As Safe” (GRAS) by the FDA.15 In addition, silica-coated nanoparticles can readily assemble with other particles to generate multifunctional complexes.16 Compared with the amorphous silica surface, the mesoporous shells have attracted more and more attention. Because they not only have high surface area for derivation with functional groups tailored for different applications, but also provide accessible large pore channels for high loading of various small molecules (i.e. therapeutic materials), even functional nanoparticles.17 Furthermore, these pore surface can easily be functionalized with stimuli-responsive groups, which can work as smart gatekeepers for control release of encapsulated small molecules.18 Many types of nanoparticles, such as metals,19 metal oxides,20 semiconductor quantum dots

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and carbon nanotubes,22-27 have

been successfully coated with mesoporous silica. These silica coatings usually are achieved ACS Paragon Plus 3Environment

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through sol-gel method in the aqueous or nonaqueous phase or in microemulsion media, using the cationic surfactants (i.e. cetyltrimethyl ammonium bromide, octadecyltrimethylammonium bromide and so on) or triblock copolymer surfactants (i.e. pluronic P-123, pluronic F-127 and others) as the structure-directing agent.14 In most of these methods, the deposition of a thin intermediate layer on the nanoparticles (e.g., microporous silica28 or poly(vinylpyrrolidone)29) was found to be necessary prior to mesoporous silica growth. In addition, these procedures are complicated and heavily affected by the reaction media, temperature, reactant concentration, pH, and any external applied force.14 Therefore, it is of great research interest to develop a simple, general and easy-handling procedure for the mesoporous coating. In 2006, the Hyeon group reported a very facile and single-step method to coat iron oxide nanoparticles with mesoporous silica shells.30 After iron oxide nanoparticles were transferred from organic medium to aqueous phase by cetyltrimethyl ammonium bromide (CTAB), the uniform mesoporous silica shells could easily form on them. Since then, this synthetic method has been generally applied to other nanoparticles, such as R-FeOOH31, MnO31, gold nanoparticles32,33 and quantum dots33. However, up to present, the simple Hyeon’s method only successfully applied in coating zero-dimentional nanostructures, all those nanoparticles heavily capped by CTAB34; no other kinds of nanostructures are reported to be coated by the same single-step synthesis procedure. How can this simple synthetic process serve as a standard protocol for the fabrication of mesoporous-silica core/shell nanostructures, especially coating those nanostructures which can’t absorb much CTAB molecules? Herein, using the star material in one-dimentional nanostructures, carbon nanotubes (CNTs), as the model, we successfully coated them with mesoporous silica shells via modified Hyeon’s method. 2. EXPERIMENTAL SECTION 2.1. Materials. Pristine multi-walled CNTs (MWCNTs, purity > 95%, diameter 10~30 nm, length 5~50 µm, amorphous carbon < 3%, ash (catalyst residue) < 0.2%), produced by the method of chemical vapor deposition, were purchased from Shenzhen Nanotech Port Co., Ltd., China. ACS Paragon Plus 4Environment

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Oxidized MWCNTs and taurine functionalized MWCNTs were prepared from pristine MWCNTs by oxidization with the mixture of H2SO4 and HNO3, and then further functionalization. The detailed method and characterization was described in our previous papers.35,36 Poly (sodium-p-styrenesulfonate) (PSS, Mw=70,000), poly (diallyldimethylammonium chloride) (PDDA, Mw=100,000-200,000) and tetraethyl orthosilicate (TEOS) were purchased from SigmaAldrich Co., USA. Other chemicals, such as CTAB, sodium hydroxide (NaOH), sodium chloride (NaCl) and others, were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Deionized water (Millipore Milli-Q grade) with resistivity of 18.2 MΩ was used in all the experiments. All chemicals were used as received. 2.2. Method Using analogous layer-by-layer method, coating MWCNTs by PSS/CTAB. Initially, 50 mg of MWCNTs was dispersed in 30 mL 1% PSS solution at room temperature by bath sonication (Branson, 5500S-DTH, 180W, 40 kHz) for 30 min. Then, the mixture was centrifuged (HITACHI, CR21GⅡ) at 15,000 rpm for 30 min. The supernatant was completely discarded for removing the excess PSS, while the precipitate was washed once with 30 mL deionized water by sonication and centrifugation. Next, the PSS coated MWCNTs were again dispersed with 30 mL 0.5% CTAB solution, followed by 5-min tip sonication (SONICS, VX130PB, 52W, 20 kHz) and 10-min bath sonication at room temperature. Also, excess CTAB was removed by high-speed centrifugation and the product, named MWCNTs/(PC)1, was washed once with 30 mL deionized water. In the same way, the next layers of PSS/CTAB were repeatedly assembled onto MWCNTs/(PC)1, and thus MWCNTs/(PC)2, MWCNTs/(PC)3, MWCNTs/(PC)4, MWCNTs/(PC)5 were prepared. The number of coated layers are indicated in the abbreviations, for example, MWCNTs/(PC)3 indicates three layers of PSS and CTAB coated on MWCNTs. Using layer-by-layer method, coating MWCNTs by PSS/PDDA/CTAB. Through the similar process describe above, 50 mg of MWCNTs was initially dispersed in 30 mL 1% PSS solution (or containing 0.4 M NaCl) at room temperature by bath sonication for 30 min. After removing the ACS Paragon Plus 5Environment

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excess PSS and washing twice with deionized water, the PSS-coated MWCNTs were dispersed with 30 mL 1% PDDA solution (or containing 0.4 M NaCl). Excess of PDDA was removed by centrifugation step, followed by washing with deionized water. Then the following product is named MWCNTs/(PD)1. The alternating dispersions of MWCNTs in an anionic polyelectrolyte and a cationic polyelectrolyte were repeated until 5 bilayers of PSS and PDDA were obtained. The number of coated layers are also indicated in the abbreviations, for example, MWCNTs/(PD)3 indicates three bilayers of PSS and PDDA coated on MWCNTs, but with the outermost polyelectrolyte being PSS. In order to coat MWCNTs/(PD)i (i=1-5) with mesoporous silica, MWCNTs/(PD)i were first dispersed in 1% PSS solution (or containing 0.4 M NaCl) by sonication for 30 min. After excess PSS removed, the MWCNTs/(PD)i were dispersed with 0.5% CTAB solution, and excess CTAB was removed by high-speed centrifugation. Then the final CTAB capped product was used in the following silica coating process. Synthesizing MWCNTs@Silica and mesoporous silica nanotubes. 5 mg MWCNTs/(PC)i or MWCNTs/(PD)i (i=1-5) was dispersed by bath sonication in 10 mL CTAB solution (0.03%). Upon stirring, 100 µL of 0.1 M NaOH solution was added and pH was adjusted to 10-11. Following this step, three 6 µL injections of pure TEOS were added under gentle stirring at 30 min intervals, and the mixture was further reacted for 12 h. After the centrifugation, the black precipitate was MWCNTs@Silica and washed three times with anhydrous ethanol. Finally, MWCNTs@Silica were dried in a vacuum drying oven at 60 °C for 24 h. In order to prepare mesoporous silica nanotubes, the black MWCNTs@Silica were burned under air atmosphere at 550 °C for 3 h. The obtained white powder was mesoporous silica nanotubes. 2.3. Characterization. TEM observation. Sample was firstly dispersed in anhydrous ethanol by sonication and then a drop of the respective particle suspension was deposited onto a 300-mesh carbon-coated copper ACS Paragon Plus 6Environment

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grid for TEM (160 kv, JEM 200CX, JEOL Corp.), or a micro grid for HRTEM (200 kv, JEM 2010F, JEOL Corp.) and electron dispersive spectroscopy (EDS, 200 kv, JEM 2010F, JEOL Corp.). The specimen was dried at room temperature overnight. All sample analysis was made at different magnifications. Structure analysis. Surface area determination and pore volume analysis were determined by Brunauer-Emmett-Teller (BET) approach and Barrett-Joyner-Halenda (BJH) model using an ASAP 2010 surface area analyzer (Micromeritic Instrument Corp.), according to the standard method of N2 adsorption/desorption. This analysis was performed at School of Materials Science and Engineering, East China University of Science and Technology. XRD analysis. The small-angle XRD pattern of sample was analyzed by a Rigaku-D/MAX2200VPC x-ray diffractometer (Rigaku Corp., Tokyo, Japan). Zeta-potential analysis. The surface zeta-potential of MWCNTs/(PC)i or MWCNTs/(PD)i (i=1-5) were assessed at pH=7.2 PB solution by a ZetaSizer 3000HSA (Malvern Instruments Ltd, Worcestershire, UK). Thermal gravimetric analysis (TGA). The analyses were performed on TGA Q500 of TA Instruments (Delaware, USA), in Hi-Res (High Resolution) mode with a heating rate of 10 °C min-1 under the nitrogen.

3. RESULTS AND DISCUSSION As CTAB is a common surfactant used in dispersing various nanoparticles and transferring nanoparticles from their organic growth medium to the aqueous phase,37 the CTAB-capped nanoparticle system may thus provide a common foundation for mesoporous silica coating. Furthermore, CTAB is also the most frequent template to synthesis the mesoporous silica-based nanomaterials.38 Therefore, it is very meaningful to develop a simple and general method for mesoporous silica coating based on CTAB system.

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As described above, the simple procedure developed by Hyeon group has successfully applied in coating those CTAB-capped nanoparticles. Since CTAB has been also proved to be an efficient cationic surfactant to wrap and disperse MWCNTs,39 it is supposed that the Hyeon’s method is also valid to form the mesoporous silica shells on the CTAB-wrapped MWCNTs. Thus, at the beginning, MWCNTs were dispersed in 0.5% CTAB solution by sonication for 1 h for absorbing the CTAB molecules. We hoped that just as the CTAB-capped metal nanoparticles, CTAB molecules on MWCNTs can be used as the template for the high-yield coating. However, after trials, there was none of mesoporous silica forming on the MWCNTs according to Hyeon group’s protocol. We infer that there must be lack of CTAB molecules on MWCNTs through this simple and single absorption, and attempt to develop an effective method to solve the problem. The layer-by-layer self-assembly (LBL) is based on the electrostatic attraction between charged species, and has been widely used to synthesize polymeric multicomposites, inorganic and hybrid hollow spheres, polymer nanotubes, and core-shell nanostructures.40 Several synthetic polyelectrolytes, PDDA or poly (allylamine hydrochloride) (PAH), and PSS, have been successfully used to functionalize CNTs by LBL method.41-45 LBL assembly provides the inspiration that we may anchor more CTAB molecules onto MWCNT surface though the analogous method. PSS, which has a high density of negatively charged sulfonate groups and serve as primers for the subsequent adsorption of the cationic ions. Instead of PDDA or PAH, CTAB is used as the cation to form the electropositive layer around PSS-wrapped MWCNTs. It is interesting that this analogous LBL method successfully prepares the CTAB-wrapped MWCNTs, on which mesoporous silica-coated MWCNTs (MWCNTs@Silica) can form by the Hyeon’s method.

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Figure 1. TEM images of the bare MWCNTs (A), MWCNTs@Silica composite prepared from MWCNTs/(PC)1 (B), MWCNTs/(PC)2 (C), MWCNTs/(PC)3 (D), MWCNTs/(PC)4 (E), MWCNTs/(PC)5 (F and G). (G) is the HRTEM image and shows that the thickness of the silica mesoporous shell is ca 12 nm. (H) is the mesoporous silica nanotubes from sample (F) by calcination at 550 °C for 3 h. (Note: PC is the abbreviation of PSS/CTAB.) Upon TEM observation, the surface of pristine MWCNTs is clean (Figure 1A), and there are intermittent silica coating formed on the MWCNTs/(PC)1 (Figure 1B), but not mesoporous silica. (Note: PC is the abbreviation of PSS/CTAB.) Then, as the layer number of PC increases, the ACS Paragon Plus 9Environment

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silica coatings become more and more continuous (Figure 1C and D). When 4 bilayers of PC are wrapped, the clear mesoporous silica shells are formed on the MWCNTs (Figure 1E). And when the layer number of PC reaches five, the mesoporous silica walls become more smooth and uniform, with the thickness of 12 nm (Figure 1F and G). This interesting result verifies that the analogous LBL assembly is effective to produce the CTAB-capped MWCNTs which can be coated with uniform mesoporous silica by Hyeon’s single-step method. After calcination at 550 °C for 3 h for removing the MWCNT cores, PSS and CTAB, the mesoporous silica nanotubes were eventually prepared (Figure 1H).

Figure 2. The HRTEM images and EDS analysis of MWCNTs/(PC)1 (A and B) and MWCNTs/(PC)5 (C and D). The thickness of PSS-CTAB layer on MWCNTs increases from 1.6 nm of one layer to 3.7 nm of five layers, and the content of both Br and S indicating the existence of PC also increase. Then we try to elucidate the possible mechanism and suppose the number of CTAB molecules play the important role in the mesoporous silica formation. In order to verify whether the analogous LBL method makes more CTAB molecules anchor onto MWCNT surface, HRTEM, ACS Paragon Plus10 Environment

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EDS, and zeta potential measurement were carried out to characterize the MWCNTs/(PC)i (i=1-5). Figure 2 shows the HRTEM images and EDS results of MWCNTs/(PC)1 and MWCNTs/(PC)5. It clearly shows that the thickness of PC layer increases from 1.6 nm of MWCNTs/(PC)1 to 3.7 nm of MWCNTs/(PC)5, which means more PSS and CTAB molecules anchored to MWCNT surface. Further, the weight percentage of Br (from CTAB) and S (from PSS) in the MWCNTs/(PC)i composites also increases from 0.04% to 0.22% and 0.34% to 0.45%, respectively. Meanwhile, the surface potential rises from +42.3 mV of MWCNTs/(PC)1 to +58.5 mV of MWCNTs/(PC)5. All the results confirm our initial hypothesis that more CTAB molecules are absorbed onto MWCNTs along with the increased PC layers. In fact, it can be easily rationalized when we assume that the similar LBL-assembly mechanism involves in the wrapping PSS and CTAB around the MWCNTs. As is known, PSS is a kind of polyanionic compounds containing aromatic rings and wildly used to wrap MWCNTs through the π-π interaction.46 In the process, PSS serves not only for solubilizing and dispersing MWCNTs, but also for tethering CTAB onto the surface of MWCNTs through the electrostatic attraction (Scheme1).

Scheme 1. Procedure of synthesizing mesoporous silica shells coated MWCNTs and mesoporous silica nanotubes

CTAB plays an important role in the coating process, which serves not only as the stabilizing surfactant for MWCNTs in the aqueous phase, but also as the organic template for the formation of the mesoporous silica coating.38 To examine the role of CTAB, we have performed the ACS Paragon Plus11 Environment

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experiments with five different CTAB concentrations of 0, 0.01％, 0.03％, 0.06％, 0.09％. The result shows that only at the concentration of ca 0.03％, can a uniform, smooth, continuous mesoporous silica shells be formed on MWCNTs (Data not shown). The result is easy to understand. After TEOS was added to the system, it hydrolyzed and condensed into the silicate polyanions, which interacted with positively charged groups of CTAB. The CTAB molecules concentrate on or around the MWCNTs/(PC)5, which not only facilitates the heterogeneous nucleation process of silica on the MWCNTs/(PC)5 surface, but also inhibits homogeneous nucleation process of silica in the bulk solution, thereby, the smooth, uniform mesoporous silica coating is formed on the MWCNTs/(PC)5 surface. However, if there are no free CTAB molecules in the solution, the composites of silicate polyanion and CTAB micelle cannot form so that the mesoporous structure cannot be shaped on the MWCNTs.

Figure 3. TEM images of the MWCNTs@Silica composite prepared from oxidized MWCNTs (A) and oxidized MWCNTs with annealed treatment (C), taurine-functionalized MWCNTs (B) and taurine-functionalized MWCNTs with annealed treatment (D). ACS Paragon Plus12 Environment

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It is interesting to find that the oxidation and surface functionalization of MWCNTs have effects on forming the uniform mesoporous silica shells. When the oxidized MWCNTs were used to absorb the alternate electrostatic layers of PSS and CTAB and then to coat with silica, the shell is not continuous and just randomly deposits on the MWCNTs (Figure 3A). Furthermore, using the functionalized MWCNTs with 2-aminoethanesulfonic acid (taurine) as the beginning template, the result is the same as the acid oxidized MWCNTs (Figure 3C). The exact reasons are not clear yet, however, the anion groups on the MWCNTs might play the important role. In the above successful experiments, MWCNTs were used as received, without any purification step, which is known to introduce oxygen-containing groups, such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (-C=O).47 Thus, the pristine MWCNTs have a well-defined graphitic structure, on

which homogenous PSS and CTAB layers can form. However, as to oxidized and taurinefunctionalized MWCNTs, the carboxyl and sulfonate groups on their surface would impede PSS homogeneously deposit on the MWCNTs, which have the same type of negative charge. Consequently, CTAB molecules were heterogeneously anchored, which made TEOS hydrolysis and nucleation process inhomogeneously occur on the MWCNTs. To verify our hypothesis, we annealed the oxidized and taurine-functionalized MWCNTs at 800 oC for 4 h under N2 atmosphere to remove the functional groups, and used them again for silica coating. Interesting, the uniform shells formed again and the monodispersed MWCNTs@Silica like beautiful worm appear in the TEM image (Figure 3C and D).

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Figure 4. TEM images of the MWCNTs@Silica composite prepared from MWCNTs/(PD)i LBLfunctionalized with PSS and PDDA performed in water (A-C) and 0.4 M NaCl solution (D-F). (A and D: i=1; B and E: i=3; C and F: i=5; Note: PD is the abbreviation of PSS/PDDA.) Next, we try to figure out whether it is feasible to grow uniform mesoporous silica shells around MWCNTs firstly wrapped with alternate electrostatic polymer layers and then with CTAB layer. As reported previously, PDDA and PSS are two kinds of most frequent polyelectrolytes used in building the multilayers by LBL self-assembly.44 Furthermore, the polymer persistence length, varying with solution ionic strength, was found to play important role in the LBL self-assembly process.43 Therefore, the LBL functionalization of MWCNTs with PSS and PDDA was performed both in water and 0.4 M NaCl solution. Following different number of bilayers of PSS and PDDA deposit, MWCNTs was dispersed in 0.5% CTAB solution by sonication for 30 min to absorb CTAB molecules. Then the modified Hyeon’s method was used to coat these MWCNTs/(PD)i composites. (PD is the abbreviation of PSS/PDDA.) However, as shown in Figure 4, none of MWCNTs/(PD)i are successfully coateded by mesoporous silica shells, regardless of the number of PSS/PDDA layers wrapped and the ionic strength in solution. The reasons are not clear yet. The zeta potential of CTAB-wrapped MWCNTs/(PD)i is from +50 mV (i=1) to +63 mV (i=5), which also shows CTAB have successfully been absorbed on surface of MWCNTs/(PD)i. Maybe the structure of PSS/PDDA bilayers is not suitable for TEOS hydrolysis and nucleation processes. ACS Paragon Plus14 Environment

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The detailed mechanism needs further investigation. It should notes that Pastoriza-Santos et al used polyelectrolyte layers (PSS/PAH) to screen the effect of CTAB to coat gold nanoparticles with amorphous silica layer under certain conditions.48 105

MWCNTs MWCNTs@Silica

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Weight (X100%)

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Temperature ( C)

Figure 5. TGA thermogram of the MWCNTs@Silica obtained at 10 °C min-1 under N2 atmosphere. The TGA plot of pristine MWCNTs sonicated in water for 5 h is displayed for comparison. In order to analyze component of MWCNTs@Silica composite, TGA method was carried out. Figure 5 shows the weight losses of MWCNTs and MWCNTs@Silica upon heating in a nitrogen atmosphere. The result reveals that two degradation steps are involved in MWCNTs@Silica. One takes place at the temperature between 200 °C and 340 °C with 8% weight loss which can be assigned to the decomposition of the CTAB,49 while the other at the temperature between 440 °C and 640 °C with 6.5% weight loss corresponding to the degradation of PSS component.50 These results clearly demonstrate that MWCNTs@Silica composite derived from PSS and CTAB coated MWCNTs. However, the decomposition temperature of PSS in MWCNTs@Silica is slightly higher ( 440 °C) than that of pure PSS ( 350 °C).50 We suggest this change might confirm that PSS are in the core of MWCNTs@Silica composite and protected by the silica shell. When the MWCNTs@Silica composite were burned under air atmosphere at 550 °C for 3 h, the weight loss is about 46% and obtained white powder was mesoporous silica nanotubes. ACS Paragon Plus15 Environment

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Figure 6. (A): Small-angle XRD pattern of MWCNTs, MWCNTs@Silica and mesoporous silica nanotubes. (B): Nitrogen adsorption-desorption isotherms of mesoporous silica nanotubes. Inset: Pore size distribution curves. Though the mesopores of the MWCNTs@Silica and silica nanotubes can be observed directly from the TEM and HRTEM images, the small-angle XRD patterns is an additional tool to characterize the mesopores. As shown in Figure 6A, the small-angle XRD pattern of MWCNTs@Silica has a small peak at around 1.9°. After removing the templates, the peak of silica nanotubes at around 1.9° is much enhanced, indicating that the pores are preserved after calcination and that the mesoporous silica nanotubes have relatively ordered mesopores. Figure 6B shows the nitrogen adsorption-desorption isotherms for mesoporous silica nanotubes. The mesoporous silica nanotubes exhibit a typical type IV adsorption isotherms with a narrow hysteresis loops. The mesoporous silica nanotubes have a high BET surface area of 643 m2g-1, and a pore volume of 0.862 cm3g-1. The pore size distribution is estimated using the BJH model, indicating a sharp peak at 2.5 nm, which is the mean diameter of the pores in the mesoporous silica nanotubes.

4. CONCLUSIONS In summary, we successfully develop the analogous LBL method to anchor more CTAB molecules onto MWCNT surface. The HRTEM images clearly show that the thickness of PSS/ ACS Paragon Plus16 Environment

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CTAB layer increases from 1.6 nm of one bilayer to 3.7 nm of 5 bilayers, and the weight percentage of Br increases from 0.04% to 0.22%. Through this method, the CTAB-wrapped MWCNTs can be successfully coated with uniform mesoporous silica shells via the simple singlestep coating procedure. CTAB concentration and surface chemistry of MWCNTs were found to play the important roles in the whole process. However, when MWCNTs were first wrapped with alternate electrostatic polymer layers of PDDA and PSS and finally with CTAB layer, mesoporous silica shells cannot be formed on MWCNTs. Furthermore, after calcination for removing the MWCNT cores, the mesoporous silica nanotubes were eventually prepared, which exhibit a typical type IV adsorption isotherms with mean pore diameter of 2.5 nm. We believe that this method has a wide range of potential applications in synthesizing hierarchical nanostructures with mesoporous silica coatings and also may pave the way for advancing Hyeon’s single-step method to become a general protocol for mesoporous-silica coating.

AUTHOR INFORMATION Corresponding Author *

E-mail address: [email protected]; [email protected]

Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We are grateful to Prof. Yuanfang Liu from Peking University, Prof. Haijiao Zhang and Prof. Jingbo Yin from Shanghai University, for their great help. This research was supported by the ACS Paragon Plus17 Environment

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