Reconstruction of Colloidal Spheres by Targeted Etching: A

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Reconstruction of Colloidal Spheres by Targeted Etching: A Generalized Self-Template Route to Porous Amphoteric Metal Oxide Hollow Spheres Jia Hong Pan, Yuqing Bai, and Qing Wang* Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore S Supporting Information *

ABSTRACT: Despite the significant progress in developing various synthetic strategies for metal oxide hollow spheres (hMO), the so-far explored materials are mostly chemically inert metal oxides. Very few attempts have been made for amphoteric metal oxides such as Al2O3 and ZnO due to the difficulties in the control of the dissolution and recrystallization process. Herein, a facile self-template route to the synthesis of amphoteric h-MO with tunable size and shell thickness is developed by targeted etching via an acid−base reaction. With the protection of polyvinylpyrrolidone (PVP) on the surface, the interior of metal oxide solid colloidal spheres (c-MOs) that possess radially divergent structures could be selectively etched with acid/alkali as an etchant, forming h-MO of Al2O3 and ZnO. Our results also show that a wide variety of metal oxide colloidal spheres can be potential self-templates for targeted etching, which paves the way for developing a generalized strategy for the synthesis of various metal oxide hollow spheres.

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INTRODUCTION Mesoscopic metal oxide hollow spheres (h-MOs) are emerging as an important class of nanomaterials, due to their outstanding properties, such as high surface area, open and low-density structures, monodisperse particle size, and superior optical properties, etc. These have rendered a wide variety of applications in fields of catalysis, electrode materials for energy conversion and storage, sensing, absorption, drug delivery, etc.1−5 h-MOs are usually synthesized by the template method using silica and carbonaceous colloidal spheres so that they could be made with controllable composition and size.2,6−9 In view of the multiple steps and high cost involved in the template method, enormous effort has been devoted to developing template-free routes, of which good progress has been made in hydro-/solvothermal method. Plausible mechanisms underlying the formation of h-MOs, such as Ostwald ripening and Kirkendall effect, have been proposed to explain the evolutions in composition, morphology, and microstructure during the hydro-/solvothermal process.3 Nevertheless, restricted to the great complication in the formation mechanism, chemical diversities of the metal oxides and their precursors, there is still a lack of universality for the template-free hydro-/solvothermal synthetic method. Apart from the template-free method, there are attempts, while very limited, to the development of the self-template route. Unlike the template method, the template removal step is obviated in the self-template route because the resultant hMOs are the direct successor of the template itself. As compared to the template-free route, which involves multiple transitions that are generally competing and sometimes even © 2015 American Chemical Society

irreconcilable, self-template method strategically decouples the sphere formation and hollowing process into two different stages, significantly alleviating the complexity for the synthesis of h-MOs. It is also interesting to note that similar structural transformation from solid to hollow has been widely observed in the template-free method especially for that based on insideout Ostwald ripening mechanism. However, the hollowing process in the self-template method is more often conducted by the chemical reaction, for example, selective etching. Hence, according to the exact nature of the self-template, it is more flexible to choose the suitable etchants for making the hollow interior. In addition, hollow structures can be generated by creating compositional differences between the interior and the shell of metal oxide colloidal spheres (c-MOs). For instance, MnO2 hollow spheres have been synthesized by selectively dissolving the MnCO3 from hydrothermally synthesized MnCO3@MnO2 core−shell spheres.10 Alternatively, heterogeneous crystallization of c-MOs induced by thermal oxidization has been reported to generate h-MOs through outward diffusion.11,12 For instance, CuO hollow spheres have been synthesized by nonequilibrium heat oxidation of the spherical CuS and Cu2S precursors.11 However, it is noteworthy that there are only very few reports for the self-template synthesis so far and there is not a general route reported for the selftemplate method, despite its unique advantage in integrating the merits of both template and template-free methods. Received: February 18, 2015 Revised: March 21, 2015 Published: April 2, 2015 4566

DOI: 10.1021/acs.langmuir.5b00638 Langmuir 2015, 31, 4566−4572

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Figure 1. Schematic diagram illustrating the formation of colloidal metal oxide spheres (c-MOs) and their transformation to hollow spheres (hMOs) via a targeted-etching process based on simple acid−base reactions.

template synthetic strategy to Al2O3 and ZnO, which successfully overcome the issues confronted by other methods. To the best of our knowledge, this is the first report of the selftemplate route for the synthesis of these two important amphoteric metal oxide hollow spheres.

The key requirement for the self-template method is to design appropriate solid spheres as the self-template for selective etching in the subsequent hollowing step, for which an ideal self-template should possess significant differences in physicochemical properties along the radial direction. In the past two decades, the synthesis of various c-MOs from homogeneous precursor solution has been greatly advanced.13,14 As schematically illustrated in Figure 1, their formation mechanism can be simply elucidated as a selfagglomeration process driven by hydrogen bonding, electrostatic attraction, etc.15 The structure of the obtained c-MOs is radially divergent across the colloidal spheres. The primary particles located in the core are rapidly agglomerated with other particles rather than grow bigger. Thus, they are relatively small and more densely packed, thereby having a higher mean curvature and surface energy as compared to those in the vicinity of surface. As a result, they present relatively high reactivity and mobility toward dissolution in the presence of etchants. On the basis of the above considerations, mesoporous TiO2 hollow spheres have been successfully synthesized by using amorphous hydrous TiO2 solid spheres as the selftemplate.16,17 Surprisingly, despite the tremendous progress in developing various template-free synthetic strategies, the so-far explored hMOs are mostly chemically inert metal oxides, such as TiO 2 , 18−21 CeO 2 , 22,23 Co 3 O 4 , 12,24 VO 2 , 25,26 Fe 3 O 4 , 27 MnO2,10,28,29 etc. There are only few reports addressing amphoteric metal oxide, including Al2O330,31 and ZnO,32−34 although these are important materials for various applications. The challenges arise from (1) high solubility of the materials via complexing with organic species in the solution; (2) high sensitivity of the materials to pH under solvo-/hydrothermal conditions; and (3) the presence of hydroxide crystalline phases at low temperature: their monoclinic nature directs the formation of lamina, and as a result the template-free assembled hollow structures for Al2O3 and ZnO are mostly derived from the randomly stacked lamina, inevitably leading to poor uniformity of h-MOs.30,31,34 Herein, we extend our self-

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EXPERIMENTAL SECTION

Chemicals. All of the chemicals were purchased from SigmaAldrich and used as received without further purification. Deionized water with a resistivity of 18.2 MΩ cm at 25 °C was used throughout the experiments. Synthesis of Colloidal Al2O3 Solid Spheres (c-Al2O3). In a typical synthesis, 200 mL of transparent aqueous solution consisting of 1.89 mmol/L Al2(SO4)3, 6.22 mmol/L Al(NO3)3, and 0.10 mol/L urea was prepared in a Pyrex glass bottle with polypropylene screw cap.35,36 Herein, the molar ratio of Al2(SO4)3 and Al(NO3)3 (RAl) was 0.3. The bottle was then placed into a preheated oven and aged at 95 °C for 2 h. After reaction, the supernatant was removed, and white precipitates were collected and repeatedly washed with water. Finally, the obtained c-Al2O3 particles were dried in an oven at 60 °C. By keeping the molar ration of total Al3+ to urea to 0.1, size-controllable synthesis of c-Al2O3 was realized by tuning RAl in the range of 0.2−0.4. Synthesis of Colloidal ZnO Solid Spheres (c-ZnO). 0.10 mol/L zinc acetate dihydrate in 100 mL of diethylene glycol (DEG) was refluxed in a round-bottom flask at 160 °C for 2 h under continuous stirring.37 After reaction, the white c-ZnO were collected and rinsed repeatedly with water to remove residual DEG and impure ions. Self-Template Synthesis of Al2O3 and ZnO Hollow Spheres (h-Al2O3, h-ZnO). c-Al2O3 or c-ZnO was sufficiently dispersed in 20 mL of water before adding polyvinylpyrrolidone (PVP, average molecule weight: 40 000) into the resultant suspension for surface coating. The weight ratio of PVP to metal oxides was controlled to 0.2. After the mixture was stirred for 1 h, calculated dosage of 0.5 mol/L NaOH (to hollow Al2O3) or HCl (to hollow ZnO) etchants were added dropwise under vigorous stirring. The solution turned translucent and near transparent gradually. The typical synthesis was carried out at a molar ratio of etchant to metal oxide (Re) of 1:1. The solution was then aged for 5 min under continuous stirring. The hollow spheres were collected and washed repeatedly with water and acetone mixture to remove the PVP coating. 4567

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Figure 2. TEM micrographs showing the microstructural evolution of c-Al2O3 as a function of reaction time during their hollowing transformation to h-Al2O3 by equimolar NaOH etching (Re = 1.0): (a) 1, (b) 3, and (c) 5 min. Materials Characterization. X-ray diffraction (XRD) patterns were measured on a Bruker D8 Advance X-ray diffractometer using a monochromated high-intensity Cu Kα radiation with a wavelength of 0.15418 nm. The morphology and microstructure were characterized by a Zeiss Supra 40 field-effect scanning electron microscope (SEM) and a JEOL JEM-2010 transmission electron microscope (TEM). Nitrogen adsorption−desorption isotherms were obtained at liquid nitrogen temperature (77 K) measured by an ASAP 2020 system (Micromeritics). Brunauer−Emmett−Teller (BET) equation was used to calculate the surface area from adsorption data obtained at P/P0 = 0.01−0.30. The average pore diameter was estimated using the Barrett−Joyner−Halenda (BJH) method.

increasing the dosage of Al2(SO4)3. Supporting Information Figure S1shows the SEM images of c-Al2O3 synthesized at RAl values of 0.2, 0.3, and 0.4. The formed spheres are solid and have uniform size (Supporting Information Figure S2). As expected, with increasing RAl, the diameter of c-Al2O3 spheres increases from ∼110 to 240 nm. XRD analysis shows no distinct diffraction peaks for all c-Al2O3 samples, suggesting their amorphous nature. The hydrophilic and amorphous features of c-Al2O3 are beneficial for the subsequent PVP coating and etching. The surface hydroxyls of c-Al2O3 form hydrogen bonds with the carbonyl groups of water-soluble PVP, leading to a uniform coating of the latter and hence forming a stable c-Al2O3@PVP core−shell structure.16,39 As a result, the exterior of c-Al2O3 spheres would be protected as compared to the interior in the presence of corrosive etchants. Without PVP coating, the etching process merely occurs at the c-Al2O3/liquid interface. The particle size is thus greatly decreased while the inner structure is still solid without any noticeable change (Supporting Information Figure S3). According to the Pourbaix diagram of Al (Supporting Information Figure S4a), Al2O3 can be dissolved in either an acidic (pH < 4) or an alkaline (pH > 9) solution.40 The relevant reactions are described as follows:

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RESULTS AND DISCUSSION Our self-template method starts with the synthesis of solid colloidal spheres, on which the surface is protected by polyvinylpyrrolidone (PVP) coating; following a subsequent acid−base reaction (Figure 1), conformal PVP coating is employed to prevent the distortion of spherical surface during the hollowing process. Etching agents, such as acid or alkaline, are then introduced to the colloidal solution to selectively etch the center of spheres, leading to the formation of uniform hollow structures. Taking the self-template synthesis of h-Al2O3 as an example, the corresponding self-template, c-Al2O3 was first obtained by homogeneous precipitation from an aqueous Al3+ solution containing excess urea. The molar ratio of Al3+ to urea was fixed at 1:10. Upon heating at 95 °C, a temperature below the boiling point of water, for 3 h, urea was gradually hydrolyzed, thus creating an alkaline medium for the formation of ultrafine hydrous Al2O3 clusters in the diluted condition. Mediated by the electrostatic attractive force, the self-assembly of these asformed clusters to uniform colloidal precipitates of c-Al2O3 was thus spontaneously initiated. In additional to Al3+ and urea, SO42− is believed to also participate in the self-assembly process due to its high ionic strength and high affinity to the surface of Al2O3.13,38 During the chemical solution process, SO42− can help to improve the degree of self-assembly of hydrous Al2O3 clusters, and, accordingly, the particle size of c-Al2O3 could be feasibly tuned by adjusting RAl. Generally a bigger particle size of the resultant c-Al2O3 can be formed with a higher RAl by

Al 2O3(s) + 6H+ → 2Al3 + + 3H 2O (pH < 4)

(1)

Al 2O3(s) + 2OH− → 2AlO2− + H 2O (pH > 9)

(2)

Therefore, simply tuning the pH by introducing acid or alkali is likely able to hollow out the solid c-Al2O3. However, among various etchants studied in our repetitive experiments, such as HCl, HNO3, acetic acid, NaOH, and NH3, only NaOH revealed reproducible and scalable results, which is presumably a result that NaOH dissolves Al2O3 in a straightforward manner without involving multiple steps.41 We choose colloidal Al 2 O 3 solid spheres (c-Al 2 O 3 ) Al2(SO4)3, 6.22 mmol/L Al(NO3)3, and 0.10 mol/L urea synthesized at RAl = 0.3 (∼190 nm in diameter, Supporting Information Figures S1 and S2) as an example to investigate the hollowing process. 0.50 mol/L NaOH solution with a 4568

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Figure 3. TEM micrographs of h-Al2O3 with different shell thickness synthesized at different Re values of (a) 0.5, (b) 0.8, and (c) 1.2.

porous with a limited BET surface area of 7.2 m2/g and BJH adsorption pore volume of 0.0272. After etching, the isotherm shape remained unchanged, except for a much larger and wider hysteresis loop in the high relative pressure range of 0.8−1.0, suggesting significantly increased pore volume, broader pore size distribution, and larger mean pore size. The as-formed hAl2O3 presented a larger surface area of 12.1 m2/g and pore volume of 0.0499, primarily due to the generation of inner surface. h-Al2O3 was calcined at 400 °C for 4 h to remove the residual OH groups and water molecules. Remarkably, h-Al2O3 shows superior structural stability toward thermal treatment. Upon annealing, the spherical and hollow structure was well retained, as shown in Supporting Information Figure S5c,d. The calcined h-Al2O3 exhibited a type H2 hysteresis loop with the capillary condensation steps at high relative pressures (P/P0) of 0.5−1.0 (Supporting Information Figure S6b), which is indicative of the existence of ink-bottled neck shaped pores in h-Al2O3. In addition, it presented a bimodal pore size distribution in both mesoporous and near macroporous regions (Supporting Information Figure S6c). The small mesopores less than 10 nm are related to the fine intra-aggregated pores formed between Al2 O 3 nanocrystallites, and the large mesopores in a range of 50−100 nm are associated with the interaggregated pores due to the generation of hollow interiors in h-Al2O3. The shell thickness and diameter of the spheres could be feasibly controlled in our self-template method. The former is critically dependent on the dosage of etchant, and thus sensitive to the value of Re. Figure 3 shows TEM images of h-Al2O3 that were synthesized from identical c-Al2O3 but with different Re values of 0.5, 0.8, and 1.2. All of the h-Al2O3 were spherical in shape and possessed a hollow interior even at low Re, suggesting an effective targeted etching process. With increasing NaOH dosage, there was a monotonous decrease in the shell thickness, while the diameter of spheres was almost unchanged. Hence, the shell thickness of h-Al2O3 could be easily tuned in the range of 15−60 nm. On the other hand, using c-Al2O3 of different sizes allows for the synthesis of sizecontrollable h-Al2O3. That is, the final size of h-Al2O3 is readily determined by the value of RAl. As shown in Supporting Information Figure S7, h-Al2O3 with an average diameter of 110

calculated volume was added into the c-Al2O3@PVP-containing aqueous suspension under vigorous stirring. The final molar ratio of NaOH to c-Al2O3 in the resulting solution was controlled to 1:1 (Re = 1.0) so that around one-half of the cAl2O3 was to be etched. The etching process was triggered upon the penetration of NaOH through PVP coating. Al2O3 species close to the center of the spheres gradually dissolved, and a structural transform from solid to hollow took place, as revealed by SEM (Supporting Information Figure S5a,b). Concomitant with the structural changes, the suspension became homogeneous and optically translucent gradually. TEM was utilized to investigate the time-dependent structural evolution during the etching process. The intermediates were extracted from the solution and promptly redispersed in a weak acidic solution (pH = 6.5) for TEM characterizations. As shown in Figure 2, the spherical structure of c-Al2O3 was inherited, while the hollow features became more obvious with the elapse of the reaction time. As expected, the hollowing process of cAl2O3 occurred preferentially in the spherical interior rather than the shell due to their inherent divergences in structure and chemical reactivity, as well as the additional protection of PVP on the shell. As evidenced in Figure 2a2, a distinguishable gap (marked with a white borderline) between the dense shell and loose interior appeared in c-Al2O3 at the beginning of NaOH permeation. With the further penetration and attacking of NaOH, the interior loose part of c-Al2O3 was gradually dissolved, creating a hollow structure. The etching-induced hollowing process was accomplished after about 5 min of stirring. The added NaOH seemed to have entirely impregnated and consumed inside the spheres. No observable changes in the spherical and hollow structure happened when the reaction time was prolonged. The formed h-Al2O3, as shown in Figure 2c1 and c2, possessed symmetrical shells with a uniform shell thickness of 25−30 nm. Moreover, the newly generated inner surface was as smooth as that in the outer, implying that the etching process is highly isotropic. N2 sorption analysis was further conducted to reveal the structural evolution. Supporting Information Figure S6a compares the N2 adsorption−desorption isotherms of c-Al2O3 and h-Al2O3, and Supporting Information Table S1 summarizes the calculated textural parameters. c-Al2O3 was nearly non4569

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Figure 4. (a,b,d,e) TEM and (c,f) high-resolution TEM micrographs of (a−c) c-ZnO and (d−f) h-ZnO.

of h-Al2O3. Interestingly, when decreasing Re to 0.8, a core− shell structure was generated in some big h-ZnO spheres (Supporting Information Figure S10d−f). The large particle size seemed to hinder the sufficient penetration of HCl etchant, with which the etching only takes place between the surface and core of the c-ZnO. Besides spherical structure, the crystalline feature was also inherited after etching. Comparing the XRD patterns of c-Al2O3 and h-Al2O3, both samples show wurtzite crystal structures, although the etching process seems to slightly decrease the crystallinity of ZnO (Supporting Information Figure S8). Consistent with XRD analysis, (200) planes with a lattice spacing of 2.6 Å could be clearly observed in both samples from high-resolution TEM images (Figure 4c,f).

and 240 nm can be synthesized by using different-sized c-Al2O3 obtained with RAl of 0.2 and 0.4, respectively. Supporting Information Table S2 summarizes the synthetic parameters for different sized h-Al2O3 by varying Re and RAl. We then generalized the current target etching method for the synthesis of h-ZnO. The employed self-template, c-ZnO, was synthesized by the thermal hydrolysis of zinc acetate dehydrate in DEG at 160 °C. Distinct from Al2O3 that is amorphous, the obtained solid c-ZnO consisted of a number of ZnO nanocrystallites with hexagonal wurtzite phase (ICDD 361451), as revealed by its XRD pattern (Supporting Information Figure S8). It has mixed spherical and elliptical shapes with size distribution of 100−400 nm (Figure 4a−c and Supporting Information Figure S9a,b), presumably a result of anisotropic crystal growth and asymmetric assembly of ZnO nanocrystallites. According to its Pourbaix diagram (Supporting Information Figure S4b), ZnO is stable in the pH range of 7−13.40 Thus, we selected acidic etchant for the hollowing process, considering a much higher dosage of alkali was expected to achieve the dissolution of ZnO nanocrystallites. The etching reaction is described as follows: ZnO(s) + 2H+ → Zn 2 + + H 2O (pH < 7)

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, a self-template method integrating colloidal sphere synthesis and targeted etching via an acid−base reaction has been demonstrated to prepare porous amphoteric metal oxide hollow spheres of Al2O3 and ZnO. The most striking feature of the method is the great controllability of sphere size and shell thickness by decoupling the sphere formation and hollowing process: the sphere diameter is determined by the size of the self-template, while the shell thickness can be feasibly tuned by adjusting the dosage of etchants. Our synthetic strategy also demonstrates that colloidal spheres, although old, are promising self-templates with inherent structural divergence for the hollowing reconstruction by targeted etching. Future work will focus on generalizing the currently developed self-template method for the preparation of hollow spheres for a variety of other metal oxides, which we anticipate for advanced energy, environmental, and other related applications.

(3)

Similar to the synthesis of h-Al2O3, coating of 20 wt % PVP and introduction of HCl etchant (Re =1.0) were successively carried out to induce the hollowing transformation from c-ZnO to h-ZnO. SEM images in Supporting Information Figure S9c,d show that h-ZnO presented the higher-degree porous structure and rougher surface than that of c-ZnO with a rather smooth surface consisting of intimately packed ZnO nanocrystallites. Specifically, certain broken hollow spheres existed in the final samples (Supporting Information Figure S9e,f), which clearly indicate the operation of hollowing transformation. The inner structures of h-ZnO were further appreciated by TEM. Again, almost all c-ZnO spheres became hollow as shown in Figure 4d−f and Supporting Information Figure S10a−c. However, the etching seemed to be somewhat anisotropic, plausibly due to the presence of anisotropic wurtzite phase of ZnO nanocrystallites that are randomly packed as building blocks. As a result, the inner structure of h-ZnO was not as uniform as that

* Supporting Information S

XRD, SEM, BET, and TEM analyses of Al2O3 and ZnO solid and hollow spheres. This material is available free of charge via the Internet at http://pubs.acs.org. 4570

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +65 6516-7118. Fax: +65 6776-3604. E-mail: msewq@ nus.edu.sg. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We thank Dr. Jie Fang for providing the experimental setup for the synthesis of c-ZnO. This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Program (CRP Award no. NRF-CRP8-2011-04).

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DOI: 10.1021/acs.langmuir.5b00638 Langmuir 2015, 31, 4566−4572