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

Authors: Zhenzhong Chen; Jia Li; Zheng Zhang; Jun-fa Liang; Qizhi Luo; Xuncai Chen
Journal: Journal of Nanomaterials
 (2021)

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## Abstract

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.


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## Body

## 1. 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 [1–3], chemical and catalysts [4–10], biological sensors [11–17], and solar cells [18–20]. 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 [23–26]. Also, the hollow-structured 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 [28–31]. 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. For example, a double-shell LiMn2O4 hollow sphere was 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 TiO2 hollow spheres were prepared via the hydrothermal method by Wu et al., exhibiting a reinforced light-scattering ability in the application of dye-sensitized 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 [35–41]. 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 TiO2-C/MnO2 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.Herein, we report the synthesis of SiO2/TiO2 mesoporous double-shell hollow tubes from (BaSr)CO3/SiO2/TiO2 core-double-nanoshell rods based on the layer-by-layer method. It should be mentioned that the hybrid SiO2/TiO2 particles have been proved to be a versatile material for various application, such as self-cleaning and antireflective coatings, electrorheological fluids, and photocatalysts [44–54]. In this work, the SiO2/TiO2 double-shell hollow tubes was prepared via selectively removing the core material from the (BaSr)CO3/SiO2/TiO2 core-double-shell rods. In addition, the thickness of SiO2 and TiO2 shell can be easily controlled by regulating the concentration of tetraethyl orthosilicate (TEOS) and tetraethyl titanate (TEOT). Lastly, the formation mechanism for the double-shell SiO2/TiO2 hollow tubes was also studied.


## 2. Experimental Section

### 2.1. Synthesis of SiO2 Hollow Tubes

Firstly, (BaSr)CO3 was prepared by our previously reported coprecipitation method; the details can be seen in literatures [52–54]. Subsequently, 1.0 g (BaSr)CO3 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, 100 ml of TEOS solution (6×10−3 ml-TEOS/ml-H2O) was 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)CO3/SiO2 core@shell rods. Next, the core@shell rods were added into the 10% HCl solution to generate the SiO2 hollow tubes.


### 2.2. Synthesis of (BaSr)CO3/SiO2/TiO2 Core@ Double-Nanoshell Rods and SiO2/TiO2 Double-Nanoshell Hollow Tubes

The above (BaSr)CO3/SiO2 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 H2O into the above (BaSr)CO3/SiO2 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)CO3/SiO2/TiO2 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.


### 2.3. Characterizations

The size and shape of (BaSr)CO3/SiO2 core@shell rods, SiO2 hollow tubes, (BaSr)CO3/SiO2/TiO2 core@ double-shell rods, and SiO2/TiO2 double-shell hollow tubes were measured by using an FE-SEM (LEO SUPRA 55 microscope, Carl Zeiss, Germany) and FE-TEM (using a JEM-2100F microscope operated at 200 kV). Their structure was also analyzed by powder X-ray diffraction (M18XHF-SRA, Mac Science, Japan) with Cu Kα radiation (λ=1.54056Å). The X-ray photoelectron spectroscopy (XPS) spectra were observed using a Quantum 2000 XPS system (Physical Electronics, Inc.). The atomic composition of the rods was analyzed by using EDS element mapping (Oxford INCA Resolution 30 mm2 136 eV at Mn Kα 5 B to 92 U). Finally, the surface areas were calculated using the Brunauer-Emmett-Teller (BET) method, and the pore sizes were calculated using the Barrett-Joyner-Hatenda (BJH) model (BELSORP-max(MP), Japan).


## 2.1. Synthesis of SiO2 Hollow Tubes

Firstly, (BaSr)CO3 was prepared by our previously reported coprecipitation method; the details can be seen in literatures [52–54]. Subsequently, 1.0 g (BaSr)CO3 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, 100 ml of TEOS solution (6×10−3 ml-TEOS/ml-H2O) was 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)CO3/SiO2 core@shell rods. Next, the core@shell rods were added into the 10% HCl solution to generate the SiO2 hollow tubes.


## 2.2. Synthesis of (BaSr)CO3/SiO2/TiO2 Core@ Double-Nanoshell Rods and SiO2/TiO2 Double-Nanoshell Hollow Tubes

The above (BaSr)CO3/SiO2 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 H2O into the above (BaSr)CO3/SiO2 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)CO3/SiO2/TiO2 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.


## 2.3. Characterizations

The size and shape of (BaSr)CO3/SiO2 core@shell rods, SiO2 hollow tubes, (BaSr)CO3/SiO2/TiO2 core@ double-shell rods, and SiO2/TiO2 double-shell hollow tubes were measured by using an FE-SEM (LEO SUPRA 55 microscope, Carl Zeiss, Germany) and FE-TEM (using a JEM-2100F microscope operated at 200 kV). Their structure was also analyzed by powder X-ray diffraction (M18XHF-SRA, Mac Science, Japan) with Cu Kα radiation (λ=1.54056Å). The X-ray photoelectron spectroscopy (XPS) spectra were observed using a Quantum 2000 XPS system (Physical Electronics, Inc.). The atomic composition of the rods was analyzed by using EDS element mapping (Oxford INCA Resolution 30 mm2 136 eV at Mn Kα 5 B to 92 U). Finally, the surface areas were calculated using the Brunauer-Emmett-Teller (BET) method, and the pore sizes were calculated using the Barrett-Joyner-Hatenda (BJH) model (BELSORP-max(MP), Japan).


## 3. Results and Discussion

### 3.1. Synthesis of (BaSr)CO3/SiO2 Core@Shell Rods and SiO2 Hollow Tubes

First, the core@shell structure of (BaSr)CO3/SiO2 was prepared based on the sol-gel method, as shown in Scheme 1. The structure and composition of (BaSr)CO3 core have been discussed in the published paper by current authors [54, 55]. As shown in Figures 1(b) and 1(d), the rod-shaped (BaSr)CO3/SiO2 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)CO3/SiO2 core-shell rods disappeared, while new peaks appeared for Si 2p assigned to SiO2. Therefore, this result also indicated that the (BaSr)CO3 core was fully covered with SiO2 shell. In addition, the thickness of SiO2 shell layer could be controlled by tuning the concentration of TEOS, as shown in Supplementary Figure 1. As a result, the thickness of SiO2 shell layer increased from 40 nm to 180 nm when increasing the TEOS concentration from 0.2% to 0.8%. After that, the (BaSr)CO3 core was removed by dissolving in the acid solution. Then, the SiO2 hexagonal tubes with a rough surface were produced as shown in Figure 2. In this tubular structure of SiO2, 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 SiO2 and TiO2 formation. Thus, before dissolving the (BaSr)CO3 into an acid solution, the CTAB was calcinated at 350°C to remove the CTAB; the mesopores were created on the SiO2 shell, which was confirmed by the high-resolution TEM analysis (Figure 2(d)). The TEM images showed perpendicularly directed pore channels in the SiO2 shell.Scheme 1
Schematic representation of the synthesis of mesoporous double-nanoshell SiO2/TiO2 hollow tube.Figure 1
(a) FE-SEM images of (BaSr)CO3/SiO2 core@shell rods and (b) high resolution of (BaSr)CO3/SiO2 core@shell rod surface. (c) FE-TEM image of (BaSr)CO3/SiO2 core@shell rods, and (d) FE-TEM image of the interface of (BaSr)CO3/SiO2 core@shell rod. (e) XPS spectra of (BaSr)CO3 and (BaSr)CO3/SiO2 core@shell rods.
(a)(b)(c)(d)(e)Figure 2
(a) FE-SEM images of SiO2 hollow tube and (b) high resolution of hollow tube. (c) FE-TEM image of the hollow tube, and (d) FE-TEM image of the interface of hollow tube.
(a)(b)(c)(d)


### 3.2. Synthesis of (BaSr)CO3/SiO2/TiO2 Core@ Double-Shell Rods and SiO2/TiO2 Double-Nanoshell Hollow Tubes

According to the layer-by-layer method, the TiO2 outer layer was successfully coated on the surface of the (BaSr)CO3/SiO2 core@shell rods by a sol-gel method to form the (BaSr)CO3/SiO2/TiO2 core@ double-shell rods. From the cross-section view (Figure 3(b)), the first boundary between the core and SiO2 inner layer, and the second boundary between SiO2 inner layer and TiO2 outer layer were clearly presented. In 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)CO3/SiO2/TiO2 (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)CO3/SiO2/TiO2 core@double-nanoshell rods, strongly suggesting a core@double-nanoshell structure of (BaSr)CO3/SiO2/TiO2. In addition, the XRD pattern (Figure 4) shows there is no difference among the (BaSr)CO3 core, (BaSr)CO3/SiO2, and (BaSr)CO3/SiO2/TiO2, that is because of the amorphous form of the SiO2 and TiO2. Similar to the silica formation, the thickness of the TiO2 shell also could be adjusted by controlling the concentration of TEOT. The thickness of TiO2 varies from 20 nm to 160 nm, when increasing the concentration of TEOT from 0.05 ml to 0.25 ml, as shown in Supplementary Figure 2. Next, the SiO2/TiO2 double-shell hollow tubes were prepared by removing the core template. The boundary between the double-shell was obviously seen in the hollow tube structure (Figures 5(b) and 5(d)). Besides, the TEM analysis also confirms the tubular structure and presented the double shell. The EDS line presented a valley-like intensity profile of Si concentrating on the inner shell and Ti centralized on the outer shell, which further proved the formation of SiO2/TiO2 double-shell hollow tubes. Similarly, during the formation of TiO2, the CTAB was continually employed as the template for the structure-directing polymerization of TiO2 uniform formation, which suggests that the pores would be created in the double-shell of SiO2/TiO2 when removing the CTAB by calcinating at 350°C. Then, nitrogen physisorption characterization was used to confirm the porosity of the SiO2/TiO2 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/P0) 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 SiO2 tubes was calculated to be 304 m2g-1, demonstrating a highly mesoporous double-nanoshell and high surface area of the tube.Figure 3
(a) FE-SEM images of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods and (b) high resolution of cross-section view of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods. (c) FE-TEM image of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods, and (d) FE-TEM image of interface of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods. (e) EDS line scanning analysis of Ba, Sr, Si, and Ti in (BaSr)CO3/SiO2/TiO2 core@ double-shell rod.
(a)(b)(c)(d)(e)Figure 4
XRD patterns of (BaSr)CO3, (BaSr)CO3/SiO2 core@shell rods, (BaSr)CO3/SiO2/TiO2 core@ double-shell rods, and the (BaSr)CO3 reference: JCPDS no. 47-0223.Figure 5
(a) FE-SEM images of SiO2/TiO2 double-nanoshell hollow tubes and (b) high resolution of SiO2/TiO2 double-nanoshell hollow tube edge. (c) FE-TEM image of SiO2/TiO2 double-nanoshell hollow tubes, and (d) FE-TEM image of the interface of SiO2/TiO2 double-nanoshell hollow tubes. (e) EDS line scanning analysis of Si and Ti in SiO2/TiO2 double-nanoshell hollow tubes.
(a)(b)(c)(d)(e)Figure 6
Structural characterization of SiO2/TiO2 double-nanoshell hollow tubes: (a) nitrogen adsorption-desorption isotherms and (b) pore size distribution calculated from the adsorption branch of isotherms.
(a)(b)The SiO2/TiO2 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.


## 3.1. Synthesis of (BaSr)CO3/SiO2 Core@Shell Rods and SiO2 Hollow Tubes

First, the core@shell structure of (BaSr)CO3/SiO2 was prepared based on the sol-gel method, as shown in Scheme 1. The structure and composition of (BaSr)CO3 core have been discussed in the published paper by current authors [54, 55]. As shown in Figures 1(b) and 1(d), the rod-shaped (BaSr)CO3/SiO2 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)CO3/SiO2 core-shell rods disappeared, while new peaks appeared for Si 2p assigned to SiO2. Therefore, this result also indicated that the (BaSr)CO3 core was fully covered with SiO2 shell. In addition, the thickness of SiO2 shell layer could be controlled by tuning the concentration of TEOS, as shown in Supplementary Figure 1. As a result, the thickness of SiO2 shell layer increased from 40 nm to 180 nm when increasing the TEOS concentration from 0.2% to 0.8%. After that, the (BaSr)CO3 core was removed by dissolving in the acid solution. Then, the SiO2 hexagonal tubes with a rough surface were produced as shown in Figure 2. In this tubular structure of SiO2, 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 SiO2 and TiO2 formation. Thus, before dissolving the (BaSr)CO3 into an acid solution, the CTAB was calcinated at 350°C to remove the CTAB; the mesopores were created on the SiO2 shell, which was confirmed by the high-resolution TEM analysis (Figure 2(d)). The TEM images showed perpendicularly directed pore channels in the SiO2 shell.Scheme 1
Schematic representation of the synthesis of mesoporous double-nanoshell SiO2/TiO2 hollow tube.Figure 1
(a) FE-SEM images of (BaSr)CO3/SiO2 core@shell rods and (b) high resolution of (BaSr)CO3/SiO2 core@shell rod surface. (c) FE-TEM image of (BaSr)CO3/SiO2 core@shell rods, and (d) FE-TEM image of the interface of (BaSr)CO3/SiO2 core@shell rod. (e) XPS spectra of (BaSr)CO3 and (BaSr)CO3/SiO2 core@shell rods.
(a)(b)(c)(d)(e)Figure 2
(a) FE-SEM images of SiO2 hollow tube and (b) high resolution of hollow tube. (c) FE-TEM image of the hollow tube, and (d) FE-TEM image of the interface of hollow tube.
(a)(b)(c)(d)


## 3.2. Synthesis of (BaSr)CO3/SiO2/TiO2 Core@ Double-Shell Rods and SiO2/TiO2 Double-Nanoshell Hollow Tubes

According to the layer-by-layer method, the TiO2 outer layer was successfully coated on the surface of the (BaSr)CO3/SiO2 core@shell rods by a sol-gel method to form the (BaSr)CO3/SiO2/TiO2 core@ double-shell rods. From the cross-section view (Figure 3(b)), the first boundary between the core and SiO2 inner layer, and the second boundary between SiO2 inner layer and TiO2 outer layer were clearly presented. In 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)CO3/SiO2/TiO2 (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)CO3/SiO2/TiO2 core@double-nanoshell rods, strongly suggesting a core@double-nanoshell structure of (BaSr)CO3/SiO2/TiO2. In addition, the XRD pattern (Figure 4) shows there is no difference among the (BaSr)CO3 core, (BaSr)CO3/SiO2, and (BaSr)CO3/SiO2/TiO2, that is because of the amorphous form of the SiO2 and TiO2. Similar to the silica formation, the thickness of the TiO2 shell also could be adjusted by controlling the concentration of TEOT. The thickness of TiO2 varies from 20 nm to 160 nm, when increasing the concentration of TEOT from 0.05 ml to 0.25 ml, as shown in Supplementary Figure 2. Next, the SiO2/TiO2 double-shell hollow tubes were prepared by removing the core template. The boundary between the double-shell was obviously seen in the hollow tube structure (Figures 5(b) and 5(d)). Besides, the TEM analysis also confirms the tubular structure and presented the double shell. The EDS line presented a valley-like intensity profile of Si concentrating on the inner shell and Ti centralized on the outer shell, which further proved the formation of SiO2/TiO2 double-shell hollow tubes. Similarly, during the formation of TiO2, the CTAB was continually employed as the template for the structure-directing polymerization of TiO2 uniform formation, which suggests that the pores would be created in the double-shell of SiO2/TiO2 when removing the CTAB by calcinating at 350°C. Then, nitrogen physisorption characterization was used to confirm the porosity of the SiO2/TiO2 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/P0) 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 SiO2 tubes was calculated to be 304 m2g-1, demonstrating a highly mesoporous double-nanoshell and high surface area of the tube.Figure 3
(a) FE-SEM images of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods and (b) high resolution of cross-section view of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods. (c) FE-TEM image of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods, and (d) FE-TEM image of interface of (BaSr)CO3/SiO2/TiO2 core@ double-shell rods. (e) EDS line scanning analysis of Ba, Sr, Si, and Ti in (BaSr)CO3/SiO2/TiO2 core@ double-shell rod.
(a)(b)(c)(d)(e)Figure 4
XRD patterns of (BaSr)CO3, (BaSr)CO3/SiO2 core@shell rods, (BaSr)CO3/SiO2/TiO2 core@ double-shell rods, and the (BaSr)CO3 reference: JCPDS no. 47-0223.Figure 5
(a) FE-SEM images of SiO2/TiO2 double-nanoshell hollow tubes and (b) high resolution of SiO2/TiO2 double-nanoshell hollow tube edge. (c) FE-TEM image of SiO2/TiO2 double-nanoshell hollow tubes, and (d) FE-TEM image of the interface of SiO2/TiO2 double-nanoshell hollow tubes. (e) EDS line scanning analysis of Si and Ti in SiO2/TiO2 double-nanoshell hollow tubes.
(a)(b)(c)(d)(e)Figure 6
Structural characterization of SiO2/TiO2 double-nanoshell hollow tubes: (a) nitrogen adsorption-desorption isotherms and (b) pore size distribution calculated from the adsorption branch of isotherms.
(a)(b)The SiO2/TiO2 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.


## 4. Conclusion

In conclusion, the SiO2 hollow tubes and SiO2/TiO2 double-nanoshell hollow tubes were successfully prepared based on a layer-by-layer method. The as-prepared SiO2/TiO2 double-nanoshelll 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 SiO2 and TiO2 is adjustable by controlling their used concentration. Therefore, the technique for the preparation of SiO2/TiO2 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.



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