Self-Assembly Hierarchical Silica Nanotubes with Vertically Aligned

Research Article pubs.acs.org/journal/ascecg

Self-Assembly Hierarchical Silica Nanotubes with Vertically Aligned Silica Nanorods and Embedded Platinum Nanoparticles Chao Zhang, Yuming Zhou,* Yiwei Zhang,* Shuo Zhao, Jiasheng Fang, Xiaoli Sheng, and Hongxing Zhang School of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, P. R. China S Supporting Information *

ABSTRACT: We report a simple method for the fabrication of hierarchical silica-Pt nanotubes. In the system, initial Pt NPs can be obtained via the reduction of H2PtCl6 with trisodium citrate as reductant. The self-assembled SiO2@Pt@SiO2 spheres were stuck together and etched through the “surface-protected etching” strategy. Many vertically aligned silica branches in situ grew from the inlaid SiO2@Pt@SiO2 spheres, fabricating the hierarchical silicaPt nanotubes automatically. TEM and SEM were conducted to monitor the morphological evolution. The effects of the PVP concentration and molar ratios of NH4OH to TEOS have also been investigated with a series of contrast experiments. Furthermore, in this work, several potential applications of HSNs have been investigated, such as the synthesis of Pt-CeO2 nanotubes and other single or double metal nanotubes. Besides, the hierarchical silica-Pt nanotubes exhibited a high thermal stability and excellent catalytic performance in the reaction of propane dehydrogenation, suggesting their potential application in various hightemperature reactions. KEYWORDS: Hierarchical structure, SiO2, Nanotubes, Pt, Catalyst

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INTRODUCTION Hollow silica nanomaterials (HSNs) integrate the advantages of silica materials with hollow colloids, which gives them several unique features, such as easy etching, good thermal stability, facile surface functionalization accessibility, and controllable particle configuration.1−5 These innovative materials, therefore, present great utilitarian value for drug delivery, catalysis, adsorption, energy storage, and conversion.6−9 Among them, silica nanotubes (SNTs) are of special interest because of their hydrophilic nature, controlled surface properties, and easy colloidal suspension formation.10−13 Over the past ten years, there has been explosive growth in the synthesis, characterization, and application of SNTs. Various synthesis strategies for SNTs have been reported. As the most conventional and common route, the hard-templating method has been extensively studied to prepare SNTs. These synthesizing methods normally need a layer of silica materials coated on the template core, followed by the selective removal of the template. However, tedious and complicated procedures are usually required, which are sometimes unsuitable for the synthesis of SNTs. Moreover, it is almost impossible to remove the hard templates from the precursor products completely, and thus an amount of the templates will unavoidably remain in the products, showing low efficiencies and high cost. So it is necessary and important to develop an easy and direct method to fabricate high-quality silica nanotubes. Up to now, many efforts have been devoted to developing a self-assembly © 2016 American Chemical Society

method. In the synthesis process, no additional templates are needed, and therefore, it may have the advantage of a simple synthetic process and better chemical properties.14,15 Generally, assembling SNTs with metal materials to form nanocomposites is an attractive way to further extend their physical and/or chemical properties. For example, Joo et al. encapsulated acid coated gold nanoparticles (NPs) within SNTs, fabricating an advanced intracellular pH sensor.16 Yin et al. reported a general method for the synthesis of noble metal nanorods, including Au, Ag, Pt, and Pd, based on their seeded growth in silica nanotubes.17 Furthermore, Dai et al. reported a thermal stable catalytic system consisting of Pt NPs that are coated with a porous SiO2 nanotube structure sheath.11 The multilayered catalyst can resist sintering up to 750 °C in air, while retaining the catalytic activity of the Pt nanoparticles. Despite these exciting advances, there are still great challenges in the synthesis of high-quality silica−metal nanotube materials. The metal NPs can hardly be introduced into the silica nanotube systems, and silica species tend to form spheres rather than tube nanostructure during the hydrolysis process. More importantly, constructing novel silica materials with hierarchical nanoarchitecture based on the traditional nanotube structure is very difficult, even though diverse hierarchical nanotubes have Received: October 1, 2016 Revised: December 13, 2016 Published: December 27, 2016 1578

DOI: 10.1021/acssuschemeng.6b02368 ACS Sustainable Chem. Eng. 2017, 5, 1578−1585

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ACS Sustainable Chemistry & Engineering been reported.18−20 But, to the best of our knowledge, very few researchers reported the hierarchical silica nanotubes, let alone assembly with metal NPs. Yin et al. proposed a “surfaceprotected etching” strategy that allows convenient conversion of sol−gel derived silica into hollow structures.21 Poly(vinylpyrrolidone) (PVP) is used to protect the near surface layer, and eventually the core was removed by etching, leaving behind a hollow silica sphere. This gives us a hint that there is a possibility to construct a silica nanotube with the “surfaceprotected etching” strategy. It is well-known that hollow spheres can evolve into nanotube structures. Herein, we have for the first time designed and fabricated the hierarchical silica-Pt nanotubes (HSNs) successfully via a facile self-assembly method. In this unique nanoarchitecture, the nanotubes were assembled with numerous multicore SiO2@ Pt@SiO2 spheres and some of the spheres supported on the nanotubes’ surface. Besides, vertically aligned silica branches in situ grow on silica nanotubes from the surface supported multicore spheres, forming the unique hierarchical silica-Ptembed nanotubes. To investigate the mechanism for the growth of the hierarchical silica nanotubes, we intentionally interrupted the growth process at different times to monitor their structural evolution. After careful observation, the whole formation process can be generally divided into four stages, as schematically illustrated in Figure 1a. Briefly, the introduced Pt NPs and silica species fabricated a SiO2 @Pt@SiO 2 multilayered nanostructure first. Then silica nanorods were assembled with adjacent SiO2@Pt@SiO2 spheres and the inner core could be removed by etching, with the assistance of PVP leaving behind a hollow silica nanotube. Lastly, vertically aligned silica branches in situ grow on nanotubes walls automatically with surface exposed SiO2@Pt@SiO2 spheres as seeds. The fabrication strategy of HSNs is very simple, and no hard templates were used in the preparation process. HSNs show a high specific surface area of 229 m2 g−1 (Figure 7a), larger than that of traditional silica bulk materials, and the potential application of HSNs has been investigated in this work. The as-synthesized silica-Pt nanotubes could serve as hard templates in the preparation of other oxide-Pt nanotubes. More importantly, the introduced noble metal composition is not restricted to Pt, but it can be readily extended to other metals. Bimetallic samples could also be synthesized by adding two kinds of corresponding metal colloidal solutions. Furthermore, the catalytic properties of HSNs were also investigated by taking the high-temperature reaction of propane dehydrogenation as an example. The possible reasons for the enhancement of gas phase catalytic performance compared to other silica nanomaterial counterparts were explored. The synthesis strategy of HSNs could be potentially used to simplify the fabrication process of various hierarchical nanotubes, and the results from this work might be valuable for future research toward the hierarchical reactors, especially their application in gas phase catalytic reactions.

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Figure 1. (a) Formation process of the hierarchical silica-Pt nanotubes (HSNs). (b) Pt NPs. TEM images showing the morphology evolution of HSNs after hydrolysis treatment for different time periods: (c) 2 h, (d, e) 4 h, (f, g) 6 h, (h, i) 12 h, and (j, k) 18 h. The inset in (b) and (c) is the high-resolution TEM image of Pt NPs and magnified SiO2@ Pt nanospheres, respectively.

Synthesis. Noble metal colloidal solutions. Trisodium citrate (0.05 g) was dissolved in 50 mL of boiling deionized water under continuous magnetic stirring to form a clear solution. After 10 min, H2PtCl6 (1 mL), aqueous solution was added to the above solution under continuous magnetic stirring for 1 h, and the solution was cooled naturally for 1 h. Au NPs colloidal solution can be synthesized through the same procedure. Hierarchical silica-Pt nanotubes. PVP solution (0.5 mL, 15.3 mg mL−1) was added to the above solution under vigorous stirring. After 12 h, an isopropyl alcohol (100 mL) and ammonia solution (1 mL) was added into the above solution. Then, 0.5 mL of TEOS was added dropwise to construct the as-designed hierarchical silica-Pt nanotubes under stirring for 12 h. The solution was centrifuged and washed with isopropyl alcohol and methanol five times, respectively. Hierarchical silica-Au nanotubes can be prepared via the same strategy. Pt-CeO2 nanotubes. The previous silica-Pt nanotubes (0.1 g) were dispersed in a mixture of ethanol (90 mL) and Ce(NO3)2·6H2O (0.05 g), HMT (0.4 g), and deionized water (30 mL) under sonication. The solution was stirred at 70 °C for 2.5 h. CeO2/Silica-Pt nanotubes were collected by centrifugal separation and redispersed in 50 mL of water under sonication. Then an aqueous NaOH solution (0.5 M) was added to the above solution at room temperature. After etching for 24

EXPERIMENTAL SECTION

Chemicals and materials. Tetraethyl orthosilicate (TEOS), ethanol, isopropanol, cerium(III) nitrate hexahydrate, polyvinylpyrrolidone (PVP), H2PtCl6 (14 mg mL−1), HAuCl4 (10 mg mL−1), NaBH4, and 4-nitrophenol (4-NP) were purchased from SigmaAldrich. Ammonia solution (28%), hexamethylenetetramine (HMT), and sodium citrate dihydrate were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). All reagents were used without further purification. Deionized water was used in all experiments. 1579


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ACS Sustainable Chemistry & Engineering h, the Pt-CeO2 nanotubes were finally obtained by centrifugation and washed five times with deionized water. Characterization. Transmission electron microscopy (TEM) experiments were conducted on a JEM-1230 microscope operated at 100 kV. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer (Germany) with Cu Kα radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) was performed on a Hitachi S-3400N scanning electron microscope, and energy dispersion X-ray analysis (EDX) was conducted on a JEM-1230 microscope operated at 100 kV. The nitrogen adsorption and desorption isotherms were measured at −196 °C on an ASAP 2020 (Micromertics USA). The specific surface area was determined from the linear part of the BET equation (P/P0 = 0.05−0.25). The pore size distribution was derived from the desorption branch of the N2 isotherm using the Barrett−Joyner−Halenda (BJH) method. Reaction of propane dehydrogenation. The catalyst (mass 2.0 g) was placed into the center of a conventional quartz tubular microreactor reactor, and the temperature was measured with a thermocouple inside a titanium thermowell, at the center of the catalyst bed. Reaction conditions were as follows: 590 °C for reaction temperature, 0.1 MPa pressure, n(H2)/n(C3H8) = 0.25, and the propane weight hourly space velocity (WHSV) was 3.0 h−1. The reaction products were analyzed with an online GC-14C gas chromatograph equipped with an activated alumina packed column and a flame ionization detector (FID).

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RESULTS AND DISCUSSION The morphology and structure of the intermediate and final products were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Typical TEM images obtained after each step are shown in Figures 1 and 2. The initial Pt NPs (ca. 2.4 nm in diameter, Figure 1b) were synthesized via the reduction of H2PtCl6 with trisodium citrate as reductant, and the high-resolution TEM image of Pt NPs shows lattice fringes measured with the spacing of 0.2265 nm, corresponding to the (111) atomic plane of the cubic platinum. After the addition of TEOS to the PVP-Pt solution for 2 h, Pt NPs were absorbed on the surface of the initial SiO2 spheres uniformly (Figure 1c). Then Pt@SiO2 spheres were coated with a SiO2 layer, forming the monodispersed multilayered SiO2@Pt@SiO2 nanospheres (Figure 1d and e). It is well-known that nanoparticles tend to aggregate to reduce surface area, including the hydrolytic process of silica species. As shown in Figure 1f and g, the as-synthesized SiO2@Pt@SiO2 nanospheres stuck together, with the silica hydrolytic process prolonging to 6 h. Some of the silica bulks consisting of singleline linear or irregularly distributed SiO2@Pt@SiO2 nanospheres could be found. The silica bulks were assembled with various numbers of nanospheres and thus exhibit different volumes. The surface-capping PVP polymer serves as structuredirecting surfactant, as demonstrated in the synthesis of hollow silica spheres.21 The presence of PVP on the surface dramatically increases the stability of silica spheres against etching. The small OH− ions can diffuse into the interior of silica spheres and lead to a relatively higher etching rate inside, eventually producing hollow spheres upon continued etching. Similarly, the introduced PVP in the liquid phase could be adsorbed on the surface of the silica bulks; then the inner silica was etched out gradually via the “surface-protected etching” strategy. Just as shown in Figure 2a, few cavities marked with white dashed rings appeared in silica nanorods for the etching effect of ammonia. Besides, few specific species of large SiO2@ Pt@SiO2 nanospheres marked with blue dashed rings could be found in Figure S2 and the dissociative SiO2@Pt@SiO2 nanospheres also show an incompact inner area (Figure 2b).

Figure 2. TEM images of (a, b) nanotubes and SiO2@Pt@SiO2 nanospheres under etching with the protection of PVP, (e, f) the asobtained HSNs after hydrolysis treatment for 24 h. SEM images of (c) silica nanotubes and (d) HSNs. The inset in (f) is the deciduous silica nanorods.

Thus, the original silica nanorods could be transformed into nanotube structures gradually. When further increasing reaction time to 12 h, the as-designed silica nanotubes with different pore diameters started to appear (Figure 1h and 2c). The nanotube shows an uneven wall for the inlaid SiO2@Pt@SiO2 nanospheres on the outside surface, and many Pt NPs were uniformly dispersed in the walls (Figure 1i). The uneven wall and inlaid silica spheres demonstrate that the silica nanotubes were evolved from SiO2@Pt@SiO2 nanospheres. Furthermore, the different pore diameters of the as-synthesized nanotubes were ascribed to the various sized silica bulks. Interestingly, the inlaid SiO2@Pt@SiO2 nanospheres could serve as seeds, and lots of vertically aligned silica nanorods are in situ grown from the seeds automatically with silica walls as substrate (Figure 1j and k). Figure S1 shows the initial in situ grown silica nanorods. As the silica nanorods grow, the inlaid SiO2@Pt@SiO2 spheres gradually move away from the silica walls and distribute on the other end of the nanorods, fabricating the fungiform branches (ca. 200 nm in length). The corresponding SEM image of Figure 2d is consistent with the hierarchical morphology of HSNs. So far, the hierarchical silica-Pt nanotubes with vertically aligned silica nanorods and embedded Pt NPs have been synthesized successfully. In fact, the whole process is just like a tree’s growth, in addition to cell combination, seeds’ 1580


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ACS Sustainable Chemistry & Engineering germination, and flourish, little, of course, not withering. As shown in Figure 2e, after progressing the reaction for 24 h, most of the in situ grown silica branches broke away from the silica wall, scattering around the wilted nanotubes. From the small-scale TEM image of Figure 2f, compared with Figure 1i, the original inlaid SiO2@Pt@SiO2 spheres on the silica walls disappeared for the in situ growth of silica branches. The fracture surfaces marked with blue dashed rings in Figure 2f can be observed easily at the tail end of the nanorods and silica wall surface, fully demonstrating that the scattered silica nanorods are broken away from the hierarchical silica nanotubes. Besides, the in situ grown silica nanorods in the inset of Figure 2f had grown from 200 nm up to 1 μm after a 24 h hydrolysis reaction; meanwhile, the silica walls became very thin for the consumption of the silica species. According to the observed structural evolution, PVP might play a critical role in the fabrication of the hierarchical silica nanostructures. Therefore, to understand the effect of PVP concentration on the formation process of HSNs, a series of contrast PVP-dependent experiments were carried out while keeping all other reaction conditions the same. In the absence of PVP, high-quality multicore Pt@SiO2 nanospheres in a narrow size distribution (Figure 3a, ca. 190 ± 7.5 nm in diameter) were obtained. Numerous Pt NPs mainly distribute

in the central area of the as-obtained silica spheres, and the innermost silica spheres of SiO2@Pt@SiO2 cannot be observed, possibly due to the extremely small size. When the concentration of PVP increased to 0.16 wt %, the silica spheres (Figure 3b, ca. 145 ± 15.3 nm) became obviously uneven in size and the innermost silica spheres could be found easily in big silica spheres. Thus, it might be concluded that the size of the inner silica spheres of SiO2@Pt@SiO2 could be adjusted by turning the concentration of the PVP solution. PVP can be absorbed on the just generated silica NPs for the strong hydrogen bonds between its carbonyl groups and the hydroxyls on the silica surface.22,23 H2O molecules preferred binding with PVP instead of acting as the solvent, which was referred to as bound H2O.24 When TEOS was added, its hydrolysis and condensation intend to take place on the surface of PVP absorbed silica NPs, resulting in a higher reaction rate. Thus, large innermost silica spheres can be fabricated with the presence of PVP. In addition, except for the large amounts of unevenly sized silica spheres, very few extremely spindly silica nanotubes (ca. 25 nm in inner-diameter and 20 μm in length) could be found at a PVP concentration of 0.16 wt % (Figure 3c). However, in the absence of PVP, the spindly silica nanotubes couldn’t be found in the sample. Therefore, the appearance of silica nanotubes indicates that PVP polymer may serve as a structure-directing surfactant during the hydrolysis process of TEOS. Furthermore, it is worth noting that when the PVP concentration increased from 0.64 to 3.2 wt %, the whole fabrication process of HSNs was accelerated a lot for the structure-directing effect of PVP (Figure 2d−f). Analogous to the classic sol−gel process, NH4OH was used as a basic catalyst for hydrolysis of TEOS. In addition, NH4OH has also been reported as a morphological catalyst reagent.25 The amount of NH4OH in the reaction medium has a pronounced influence on the integrity and surface morphology of the synthesized silica nanomaterials. A wide range of molar ratios of NH4OH to TEOS (0, 0.5, 1.05, 2.1, 4.2, 8.4, and 16.8) were investigated in the isopropanol system. As can be seen from Figure S4, the introduced Pt NPs were absorbed on irregular shaped silica particles in the absence of ammonia. At a low molar ratio of NH4OH to TEOS (0.5, Figure 4a), lots of silica NPs and multicore Pt@SiO2 spheres stuck together intricately. As the molar ratio of NH4OH to TEOS increased to 1.05, the above disordered silica NPs disappeared and the multicore Pt@SiO2 spheres gathered regularly, showing a similar long strip morphology. Well-organized silica nanotubes could be prepared on the basis of the regularly gathered multicore Pt@SiO2 spheres (Figure 4c). Therefore, with the increasing of the ratio of NH4OH to TEOS, the silica production exhibits an improved regularity, indicating that NH4OH has an inducing effect on the formation of the nanotube structure. If the molar ratio further increased to 4.2, few sparse silica nanorods started to grow on the silica nanotube surface. It well-known that PVP micelles can be constructed at a suitable pH for the presence of a highly polar amide group within the pyrrolidone ring.26 The formation of hollow silica spheres in Figure 4d might be due to the hydrolysis of TEOS around the interface of the H2O phase and PVP micelles. With a further increase in the ratio to 8.4 and 16.8, the in situ grown silica nanorods became thick and stocky apparently. Besides, in Figure S3, very few deciduous silica nanorods marked with blue dashed rings broke away from the nanotubes. Therefore, NH4OH could stimulate the growth of silica nanorods as well as fertilizing plants with manure.

Figure 3. TEM images of silica-Pt materials obtained at different PVP concentrations: (a) 0 wt %, (b, c) 0.16 wt %, (d) 0.64 wt %, (e) 1.28 wt %, and (f) 3.2 wt %. The inset TEM image in (c) is the magnified thin silica nanotube. 1581


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Figure 5. TEM images of (a, b) CeO2−Pt nanotubes, (c) Au NPs, and (d) hierarchical silica-Au nanotubes. The insets in (c) and (d) are high-resolution TEM images of Au NPs and magnified single-core Au@SiO2 nanospheres, respectively.

Figure 4. TEM images of the as-obtained silica-Pt materials prepared under various molar ratios of NH4OH to TEOS in isopropanol solvent. From a to f, the molar ratio of NH4OH to TEOS is 0.5, 1.05, 2.1, 4.2, 8.4, and 16.8, respectively. The experiment was performed by changing the amount of NH4OH, while other conditions were fixed. The inset in (f) is the deciduous thick silica nanorods.

The present silica-Pt nanotubes can be applied in the synthesis of other oxide-Pt nanotubes. An oxide layer was constructed on the surface of silica nanotubes and then, after removing the hard silica template via the etching process, the oxide-Pt nanotubes can be obtained. Just as shown in Figure 5a and b, various sized CeO2 nanotubes were fabricated successfully with silica-Pt nanotubes as the hard template. The original Pt NPs of SiO2@Pt@SiO2 evenly distributed in the CeO2 walls. In addition, the noble metal existing in HMSs is not restricted to Pt, but can be readily extended to Au. The Au NPs (ca. 11 nm in diameter) were synthesized via the reduction of HAuCl4 with trisodium citrate as reductant (Figure 5c). Hierarchical silica-Au nanotubes can be prepared facilely via the present HMSs fabrication strategy (Figure 5d and S5). As marked with the blue arrow, sparse silica nanorods in situ grow on the silica nanotubes. Furthermore, Many singlecore Au@SiO2 spheres appeared in the silica walls. By analogy, the noble metal composition of the assynthesized silica nanotubes could be extended to two metals by adding two kinds of metal NPs colloidal solutions during the silica hydrolysis process. From the EDX analysis of Figure 6a, Pt and Au species had been introduced in the nanocomposite simultaneously. The Pt/Au molar ratio could be adjusted by

Figure 6. EDX analysis (a) and TEM images (b, c) of silica-PtAu nanotubes. TEM image of (d) calcined silica-PtAu nanotubes at 500 °C in air.

varying the added amounts of corresponding metal colloidal solutions. The silica-PtAu nanotubes exhibit a similar structure as the above silica-Pt nanotubes (Figure 6b). Many SiO2@ PtAu@SiO2 spheres were inlaid in the nanotube walls. Furthermore, as marked with green arrows in Figure 6c, large sized Au NPs (ca. 11.5 nm in diameter) were surrounded with the numerous little Pt NPs. It is well-known that noble metal nanoparticles tend to aggregate to lose specific surface area easily at high temperature.27−29 The dense silica layer can serve 1582


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conversion of propane catalyzed by multicore Pt@SiO2 is 21.1%. Moreover, the catalyst exhibits a high deactivation parameter D of 12.4% (defined as D = [X0 − Xf] × 100%/X0, where X0 is the initial propane conversion and Xf is the final propane conversion). Figure 8 shows the TEM images taken from the catalysts after the reaction of propane dehydrogenation. It can be found that the Pt NPs of multicore Pt@SiO2 sintered heavily and the diameter increased from about 2.4 to 7.51 nm, forming a single-core structure. A similar phenomenon occurred in silica-Pt nanotubes. The diameter of Pt NPs increased to 7.92 nm (Figure 7e). But, compared with multicore Pt@SiO2, silica-Pt nanotubes show a higher initial conversion of propane of 22.5%. The increased catalytic performance of silica-Pt nanotubes might be ascribed to the low stacking density of the nanotube structure. The reactants could get access to the active sites through the inner tube space, resulting in a higher mass transfer rate. Furthermore, in this work, HSNs exhibit the best performance in the reaction of propane dehydrogenation, with an initial propane conversion of 22.9% and deactivation parameter D of 8.5%. The Pt NPs of HSNs could be separated adequately by the innermost silica spheres of SiO2@Pt@SiO2, showing an excellent antisintering ability. As shown in Figure 7f, the Pt NPs (ca. 4.47 nm in diameter) sintered slightly after the high-temperature reaction. Additionally, during the formation process of the hierarchical silica nanostructure, part of the silica species were consumed fabricating a thinner silica layer. The outer silica layer of HSNs is about 25 nm (Figure 7f), thinner than multicore Pt@SiO2 and Silica-Pt nanotubes (Figure 7d and e), leading to a lower mass transfer resistance. Besides, to some extent, the vertically aligned silica nanorods on the walls and high specific surface area of 229 m2 g−1 (Figure 7a) would increase the contact opportunities of reactant and active sites. These data demonstrated the high thermal stability and excellent catalytic performance of HSNs in high temperature reactions. Thus, from the viewpoint of fundamental research, such a hierarchical silica-Pt nanotube synthesizing strategy would provide a new powerful platform for designing novel complex nanomaterials for different applications.

as a physical barrier to limit the migration space of individual metal NPs. As a result, the adjacent different metal NPs will aggregate into multimetal nanoparticle automatically. Just as shown in Figure 6d, the diameter of Au NPs increased to 15.4 nm after calcination for the aggregation of Au and Pt species. The cocatalysis effect of scattered metal NPs would be increased for the enhanced multimetal interaction.30 To demonstrate the unique structure advantages of HSNs, the reaction of propane dehydrogenation has been employed to investigate and compare the catalytic performance of multicore Pt@SiO2, silica-Pt nanotubes, and HSNs. These catalysts were synthesized with the same amount of agent dosage, except for PVP amount, and showed similar Pt mass content of around 25% (Figure S6). As presented in Figure 7a, the initial

Figure 7. (a) Nitrogen adsorption−desorption isotherms of HSNs. (b) Propane conversion vs time on stream of multicore Pt@SiO2, silica-Pt nanotubes, and HSNs.

Figure 8. TEM images of (a, d) multicore Pt@SiO2, (b, e) silica-Pt nanotubes, and (c, f) HSNs. All the samples were used in the reaction of propane dehydrogenation. 1583



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CONCLUSIONS A facile method for the fabrication of hierarchical silica-Pt nanotubes (HSNs) has been developed successfully. The formation procedure involves the synthesis of Pt NPs and a subsequent self-assembly process in an isopropyl system. The silica bulks assembled with SiO2@Pt@SiO2 spheres could be etched via the “surface-protected etching” strategy. The inlaid SiO2@Pt@SiO2 nanospheres serve as seeds and lots of vertically aligned silica nanorods in situ grow from the seeds automatically with silica walls as substrate, fabricating the hierarchical silica-Pt nanotubes eventually. The fabrication process of HSNs can be accelerated with a high PVP concentration. NH4OH could stimulate the growth of silica nanorods on the walls as well as fertilizing plants with manure. Furthermore, the potential applications of HSNs have been investigated in this work. The silica-Pt nanotubes can be applied in the synthesis of other oxide-Pt nanotubes. The present HSNs forming strategy can be extended to other noble metals or even multimetals nanotubes. Besides, the reaction of propane dehydrogenation has been employed to demonstrate the high thermal stability and excellent catalytic performance of HSNs. It is believed that this work would provide a simple and effective strategy to construct other hierarchical oxide-metal nanotubes with high catalysis efficiency for gas phase catalytic reactions.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02368. TEM images of the initial in situ grown silica nanorod, the silica-Pt materials prepared in the absence of ammonia, and hierarchical silica-Au nanotubes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Yuming Zhou) E-mail: [email protected]. Tel: +86 25 52090617. Fax: +86 25 52090617. *(Yiwei Zhang) E-mail: [email protected]. Tel: +86 25 52090617. Fax: +86 25 52090617. ORCID

Chao Zhang: 0000-0002-0143-9504 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of National Natural Science Foundation of China (Grant Nos. 21676056, 51673040, 21376051, and 21306023), ‘‘Six Talents Pinnacle Program’’ of Jiangsu Province of China (JNHB-006), Natural Science Foundation of Jiangsu Province (Grant No. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100), Qing Lan Project of Jiangsu Province (1107040167), The Fundamental Research Funds for the Central Universities (3207045421, 3207046302, 3207046409), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (1107047002). 1584


Research Article

ACS Sustainable Chemistry & Engineering temperature gas sensing properties. ACS Appl. Mater. Interfaces 2014, 6, 9150−9159. (19) Qiu, Y.; Li, G.; Hou, Y.; Pan, Z.; Li, H.; Li, W.; Liu, M.; Ye, F.; Yang, X.; Zhang, Y. Vertically aligned carbon nanotubes on carbon nanofibers: a hierarchical three-dimensional carbon nanostructure for high-energy flexible supercapacitors. Chem. Mater. 2015, 27, 1194− 1200. (20) Wang, Z.; Zhao, J.; Bagal, A.; Dandley, E. C.; Oldham, C. J.; Fang, T.; Parsons, G. N.; Chang, C. H. Wicking enhancement in threedimensional hierarchical nanostructures. Langmuir 2016, 32, 8029− 8033. (21) Zhang, Q.; Zhang, T. R.; Ge, J. P.; Yin, Y. D. Permeable silica shell through surface-protected etching. Nano Lett. 2008, 8, 2867− 2871. (22) Nelson, A.; Jack, K. S.; Cosgrove, T.; Kozak, D. NMR solvent relaxation in studies of multicomponent polymer adsorption. Langmuir 2002, 18, 2750−2755. (23) Gun’ko, V. M.; Zarko, V. I.; Voronin, E. F.; Goncharuk, E. V.; Andriyko, L. S.; Guzenko, N. V.; Nosach, L. V.; Janusz, W. Successive interaction of pairs of soluble organics with nanosilica in aqueous media. J. Colloid Interface Sci. 2006, 300, 20−32. (24) de Dood, M. J. A.; Kalkman, J.; Strohhöfer, C.; Michielsen, J.; van der Elsken, J. Hidden transition in the “Unfreezable Water” region of the PVP−Water system. J. Phys. Chem. B 2003, 107, 5906−5913. (25) Byers, C. H.; Harris, M. T.; Williams, D. F. Controlled microcrystalline growth studies by dynamic laser-light-scattering methods. Ind. Eng. Chem. Res. 1987, 26, 1916−1923. (26) Li, X.; Yang, Y.; Yang, Q. Organo-functionalized silica hollow nanospheres: synthesis and catalytic application. J. Mater. Chem. A 2013, 1, 1525−1535. (27) Dai, Y.; Lim, B.; Yang, Y.; Cobley, C. M.; Li, W.; Cho, E. C.; Grayson, B.; Fanson, P. T.; Campbell, C. T.; Sun, Y.; Xia, Y. Addition: a sinter-resistant catalytic system based on platinum nanoparticles supported on TiO2 nanofibers and covered by porous silica. Angew. Chem., Int. Ed. 2012, 51, 10692−10692. (28) Zhang, Z.; Zhou, Y.; Zhang, Y.; Zhou, S.; Xiang, S.; Sheng, X.; Jiang, P. A highly reactive and magnetic recyclable catalytic system based on AuPt nanoalloys supported on ellipsoidal Fe@SiO2. J. Mater. Chem. A 2015, 3, 4642−4651. (29) Du, C.; Guo, Y.; Guo, Y.; Gong, X. Q.; Lu, G. Polymertemplated synthesis of hollow Pd-CeO2 nanocomposite spheres and their catalytic activity and thermal stability. J. Mater. Chem. A 2015, 3, 23230−23239. (30) Arán-Ais, R. M.; Dionigi, F.; Merzdorf, T.; Gocyla, M.; Heggen, M.; Dunin-Borkowski, R. E.; Gliech, M.; Solla-Gullón, J.; Herrero, E.; Feliu, J. M.; Strasser, P. Elemental anisotropic growth and atomic-scale structure of shape-controlled octahedral Pt−Ni−Co alloy nanocatalysts. Nano Lett. 2015, 15, 7473−7480.

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Self-Assembly Hierarchical Silica Nanotubes with Vertically Aligned

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