Controllable Architecture of Mesoporous Double-Nanoshell SiO2/TiO2 Hollow Tube Based on Layer by Layer Method

Double-shell tubular on-dimensional structure can be fabricated through a layer by layer method, in which the core template was removed to create the tubular shape. In this paper, we report, for the first time, the double nanoshell SiO2/TiO2 hollow tubes prepared through a layer-by-layer deposition method involving the sol-gel process for the SiO2 and TiO2 generation. During TEOS and TEOT hydrolysis/condensation for the SiO2 and TiO2 shell layer formation, cetyltrimethylammonium bromide (CTAB) is adopted both as the structure-directing template and as the mesopore-channel template distributing around the shell. The obtained double-nanoshell hollow tubes illustrate a large surface area and high pore volume. Also, mesoporous double-nanoshell SiO2/TiO2 hollow tubes have the inner and outer shell thickness of about 80 nm and 120 nm, respectively. Plus, the shell thickness of SiO2 and TiO2 is controllable depending on the used concentration of TEOS and TEOT during their sol-gel process. Therefore, the technique for the preparation of SiO2/TiO2 mesoporous double-nanoshell hollow tubes could provide new insights into the construction of mesoporous double-shell and hollow structure for other multicomponent and hierarchical hybrid systems.


Introduction
Hollow-structured mesoporous materials with unique features of high surface area, high permeability, low density, confined inner cavity, and optical properties have been of great interest and received much research, which makes them a promising application in drug delivery systems [1][2][3], chemical and catalysts [4][5][6][7][8][9][10], biological sensors [11][12][13][14][15][16][17], and solar cells [18][19][20]. For example, the inner cavity of the hollow structure is very essential for drug delivery by offering a large volume transportation for DNA, drugs, and cosmetics [21,22]. In addition, the inner-and outer-shell surface provides more active sites; when contacting with reactant molecules, the hollow-structured materials would display high sensor and catalytic activities [23][24][25][26]. Also, the hollowstructured materials enable to enhance the light-scattering effect through adjusting the refractive indices of the empty inner cavity and solid shell [19,27]. Even with these advantages of hollow-structured material, the design of an optimized structure based on the hollow-structure to further enhance their performance for specific application fields remains a challenge.
To improve the advantages of the hollow structure, multishell hollow-structured materials have recently been considered to be a promising structure owning to their unique properties, such as large surface area, multiple components, and outstanding light-scattering effect [28][29][30][31]. The light scattering was enhanced through repeated reflection and scattering events between the inner and outer shells in the multishell structure. Furthermore, the multishell structure has a larger active surface area when comparing with a single shell one, which is because of the increasing surface area by the additional inner shells. As a result, the fabrication of multishell hollow-structured materials with enhanced performance in various applications has been widely studied. (e) Figure 1: (a) FE-SEM images of (BaSr)CO 3 /SiO 2 core@shell rods and (b) high resolution of (BaSr)CO 3 /SiO 2 core@shell rod surface. (c) FE-TEM image of (BaSr)CO 3 /SiO 2 core@shell rods, and (d) FE-TEM image of the interface of (BaSr)CO 3 /SiO 2 core@shell rod. (e) XPS spectra of (BaSr)CO 3 and (BaSr)CO 3 /SiO 2 core@shell rods. fabricated by using a hydrothermal synthesis method to optimize the performance of lithium-ion batteries [32]. With the double-shell configuration, the battery showed improved performance, which was ascribed to the larger contact area between the electrolyte and electrode generated by the gap and hollow interior between the shells. In addition, multishell hollow spheres of microscale ZnO were prepared via the hydrothermal method by Zhang et al., which presented extraordinary sensitivity for the detection of toluene [33]. Furthermore, the double-shell TiO 2 hollow spheres were prepared via the hydrothermal method by Wu et al., exhibiting a reinforced light-scattering ability in the application of dyesensitized solar cell (DSSC) [34]. However, hydrothermal reactions using an autoclave were mainly adopted to fabricate multishell hollow structures in the previous approach, which easily resulted in the size uncontrollable and particle aggregation inevitable because of high reaction temperature and high pressure involved. Therefore, methods for synthesizing particles with multishell hollow structure at the nanoscale were in growing demand. Recently, layer-by-layer (LBL) assembly has already been proved to be a simple, convenient, and controllable method for the design and fabrication of core-shell/core-double-shell particles with tailored chemical composition and controllable architecture on varied substrate surfaces [35][36][37][38][39][40][41]. Xing et al.
synthesized stable colloidal gold-collagen core-shell nanoconjugates with improved mechanical properties by using the LBL assembly method [42]. In addition, Liao and coworkers the unique TiO 2 -C/MnO 2 core-double-shell nanowires using as anode materials for lithium-ion batteries was prepared by layer-by-layer deposition approach [43]. It is not hard to imagine that the core-double-shell particle could be converted to a double-shell hollow particle, when its core was removed. Thus, the layer-by-layer assembly method was supposed to be an applicative approach to fabricate multishell hollow particles.

Experimental Section
2.1. Synthesis of SiO 2 Hollow Tubes. Firstly, (BaSr)CO 3 was prepared by our previously reported coprecipitation method; the details can be seen in literatures [52][53][54]. Subsequently, 1.0 g (BaSr)CO 3 white powders were dispersed in 40 ml of water and then added 2 ml of aqueous ammonia solution (25~28 wt%), 60 ml of ethanol, and 1.0 g of CTAB. Then,  Figure 3: (a) FE-SEM images of (BaSr)CO 3 /SiO 2 /TiO 2 core@ double-shell rods and (b) high resolution of cross-section view of (BaSr)CO 3 /SiO 2 /TiO 2 core@ double-shell rods. (c) FE-TEM image of (BaSr)CO 3 /SiO 2 /TiO 2 core@ double-shell rods, and (d) FE-TEM image of interface of (BaSr)CO 3 /SiO 2 /TiO 2 core@ double-shell rods. (e) EDS line scanning analysis of Ba, Sr, Si, and Ti in (BaSr)CO 3 /SiO 2 /TiO 2 core@ double-shell rod. slowly fed into the above suspension at a flow rate of 0.2 ml/min under rigorous agitation at 30°C. After complete feeding, the product suspension was continuously stirred for 2 h. The final product suspension was filtered out using a filter paper and washed with water and ethanol several times to attain the (BaSr)CO 3 /SiO 2 core@shell rods. Next, the core@shell rods were added into the 10% HCl solution to generate the SiO 2 hollow tubes.
2.2. Synthesis of (BaSr)CO 3 /SiO 2 /TiO 2 Core@ Double-Nanoshell Rods and SiO 2 /TiO 2 Double-Nanoshell Hollow Tubes. The above (BaSr)CO 3 /SiO 2 core@shell rods were dispersed in 100 ml of ethanol and then mixed 0.25 g of CTAB and 0.2 ml of pure TEOT reagent, followed by slow feeding 2 ml of H 2 O into the above (BaSr)CO 3 /SiO 2 core@shell rod suspension with a pump at a flow rate of 0.1 ml/min under rigorous agitation at 30°C. After complete feeding, the product suspension was continuously stirred for 20 h. The final product suspension was filtered out using a filter paper and washed several times with water and ethanol. Next, the attained (BaSr)CO 3 /SiO 2 /TiO 2 core@ double-nanoshell rods were calcinated at 350°C to remove CTAB and then was added into the 1 M HCl to dissolve (BaSr)CO3 core materials. Finally, the resulting sample was filtered out using a filter paper and washed several times with water and ethanol.

Results and Discussion
3.1. Synthesis of (BaSr)CO 3 /SiO 2 Core@Shell Rods and SiO 2 Hollow Tubes. First, the core@shell structure of (BaSr)CO 3 /-SiO 2 was prepared based on the sol-gel method, as shown in Scheme 1. The structure and composition of (BaSr)CO 3 core have been discussed in the published paper by current authors [54,55]. As shown in Figures 1(b) and 1(d), the rodshaped (BaSr)CO 3 /SiO 2 core@shell with a uniform thickness of around 80 nm was attained. Particularly, from XPS spectra (Figure 1(e)), the peaks for Sr 3d and Ba 3d5 in (BaSr)CO 3 /SiO 2 core-shell rods disappeared, while new peaks appeared for Si 2p assigned to SiO 2 . Therefore, this result also indicated that the (BaSr)CO 3 core was fully covered with SiO 2 shell. In addition, the thickness of SiO 2 shell layer could be controlled by tuning the concentration of TEOS, as shown in Supplementary Figure 1. As a result, the thickness of SiO 2 shell layer increased from 40 nm to 180 nm when increasing the TEOS concentration from 0.2% to 0.8%. After that, the (BaSr)CO 3 core was removed by dissolving in the acid solution. Then, the SiO 2 hexagonal tubes with a rough surface were produced as shown in Figure 2. In this tubular structure of SiO 2 , the outer diameters were around 600 nm~800 nm and the wall thickness was 80 nm. The CTAB was used as the template for the structure-directing polymerization during the SiO 2 and TiO 2 formation. Thus, before dissolving the (BaSr)CO 3 into an acid solution, the CTAB was calcinated at 350°C to remove the CTAB; the mesopores were created on the SiO 2 shell, which was confirmed by the high-resolution TEM analysis (Figure 2(d)). The TEM images showed perpendicularly directed pore channels in the SiO 2 shell.

Synthesis of (BaSr)CO 3 /SiO 2 /TiO 2 Core@ Double-Shell
Rods and SiO 2 /TiO 2 Double-Nanoshell Hollow Tubes. According to the layer-by-layer method, the TiO 2 outer layer was successfully coated on the surface of the (BaSr)CO 3 /SiO 2 core@shell rods by a sol-gel method to form the (BaSr)CO 3 /-SiO 2 /TiO 2 core@ double-shell rods. From the cross-section view (Figure 3(b)), the first boundary between the core and SiO 2 inner layer, and the second boundary between SiO 2 inner layer and TiO 2 outer layer were clearly presented. In 5 Journal of Nanomaterials particular, the inner shell with a thickness of around 80 nm uniformly wrapped the core, and the 120 nm thick outer shell homogenously wrapped the inner shell. In addition, the TEM images confirmed the core@double-nanoshell structure of (BaSr)CO 3 /SiO 2 /TiO 2 (Figures 3(c) and 3(d)). The EDS line detection showed that the metal ions were only found in the core region, and the Si covered the diameter of the core and inner shell, whereas Ti overridden the total diameter of the (BaSr)CO 3 /SiO 2 /TiO 2 core@double-nanoshell rods, strongly suggesting a core@double-nanoshell structure of (BaSr)CO 3 /SiO 2 /TiO 2 . In addition, the XRD pattern ( Figure 4) shows there is no difference among the (BaSr)CO 3 core, (BaSr)CO 3 /SiO 2 , and (BaSr)CO 3 /SiO 2 /TiO 2 , that is because of the amorphous form of the SiO 2 and TiO 2 . Similar to the silica formation, the thickness of the TiO 2 shell also could be adjusted by controlling the concentration of TEOT. The thickness of TiO 2 varies from 20 nm to 160 nm, when increasing the concentration of TEOT from 0.05 ml to Similarly, during the formation of TiO 2 , the CTAB was continually employed as the template for the structuredirecting polymerization of TiO 2 uniform formation, which suggests that the pores would be created in the double-shell of SiO 2 /TiO 2 when removing the CTAB by calcinating at 350°C. Then, nitrogen physisorption characterization was used to confirm the porosity of the SiO 2 /TiO 2 double-shell hollow tubes, which illustrated that the tubes were mesoporous based on the nitrogen adsorption-desorption isotherms ( Figure 6(a)), as identified by the increase of the adsorption amount in the relative pressure (P/P 0 ) range of 0.2-0.4. In addition, the pore size distribution curve calculated from the adsorption branch of the isotherms (Figure 6(b)) was around 1 nm to 14 nm, and the surface area of the SiO 2 tubes was calculated to be 304 m 2 g -1 , demonstrating a highly mesoporous double-nanoshell and high surface area of the tube. The SiO 2 /TiO 2 double-nanoshell hollow tubes not only could be applied as light scattering material for highly efficient dye-sensitized solar cells but also enable to be used as camptothecin (CPT) delivery agents for cancer treatment. Details of their potential application will be reported in due course.

Conclusion
In conclusion, the SiO 2 hollow tubes and SiO 2 /TiO 2 doublenanoshell hollow tubes were successfully prepared based on a layer-by-layer method. The as-prepared SiO 2 /TiO 2 doublenanoshelll layer is highly porous and has large surface area, allowing direct interaction between the inner surface of the tube and its surrounding environment. In addition, the layer thickness both of SiO 2 and TiO 2 is adjustable by controlling their used concentration. Therefore, the technique for the preparation of SiO 2 /TiO 2 double-nanoshell hollow tubes can clearly be extended to other mesoporous double-shell architectures and hollow structure of other dimensions and also could be used as a platform for multicomponent, hierarchical hybrid systems. Finally, the proposed method represents a relevant and directed approach to the design of new and novel hollow particles specialized for various applications at the discretion of the end-users.

Data Availability
All data generated or analyzed during this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.   Journal of Nanomaterials