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Rationalized Fabrication of Structuretailored Multi-shelled Hollow Silica Spheres Yandong Han, Zilong Guo, Shiyong Teng, Haibing Xia, Dayang Wang, Ming-Yong Han, and Wensheng Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02146 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019
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Chemistry of Materials
Rationalized Fabrication of Structure-tailored Multi-shelled Hollow Silica Spheres ⊥
Yandong Han,† Zilong Guo,‡ Shiyong Teng,† Haibing Xia, Dayang Wang,† Ming-Yong Han§ and Wensheng Yang*,† †State
Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China ‡Institute of Molecular Plus, Tianjin University, Tianjin 300072, China ⊥
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
§Institute
of Materials Research and Engineering, 2 Fusionopolis Way, Singapore 138634
ABSTRACT: This work demonstrates a simple approach to rational fabrication of multi-shelled hollow silica spheres via periodic injections of a given amount of tetramethylammonium hydroxide (TMAH) to catalyze the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol/water mixtures. Upon TMAH injection each time, the silica networks formed at the early stage of the sol-gel reaction of TEOS was found to possess noticeably lower condensation degree than these formed at the late reaction stage. The former could be readily and completely etched away in boiling water whereas the latter remained intact. The using of TMAH as basic catalyst led to temporally clear-cut separation between the early and late stages during the sol-gel reaction of TEOS and made the volume ratio of the loosely condensed silica gel networks, formed at the early reaction stage, to the densely condensed ones, formed at the late reaction stage well defined around 3:2, enabling the formation of multi-shelled hollow spheres with precisely defined shell thickness and spacing between neighboring shells. The present template-free approach will open up new prospect in rational design and fabrication of structure-tailored multi-shelled silica spheres to accommodate to the demand of improved applications.
INTRODUCTION Multi-shelled hollow silica spheres hold immense promise for applications in heterogeneous catalysis,1 hierarchical drug delivery,2 and advanced lithium batteries3 and optical devices.4,5 Their technical performance was found to significantly benefit from the high loading capacity, adjustable mass transport length and high surface-to-volume ratio related to the multi-shelled hollow feature.6,7 For instance, multi-shelled hollow spheres were utilized as carriers to encapsulate catalytic nanoparticles to greatly improve their activity and reusability1 or as anodes to promote the electrode-electrolyte reaction with greatly improved performance in silicon-based lithium battery.3 To enhance and tailor the performance of multi-shelled silica spheres, a number of efficacious fabrication approaches by using hard or soft sacrificial templates were developed in the last decade. In principle, the hard-template approach allows fine modulation of the distances between neighboring silica shells (i.e. the void spaces) by altering the thicknesses and subsequent removal of sacrificial hard template layers (e.g. polymeric or inorganic particles) in between, but often suffers from certain damage on resultant silica shells due to great binding to the template layers.2,3,8 The softtemplate approach has an advantage to easily remove sacrificial soft template layers (e.g. supramolecular micelles and polymer vesicles) via calcination or dissolution, but lacks the capability in tuning the distances between
the neighboring silica shells (i.e. the void spaces) due to the thermodynamic metastability of templates.1,9-15 As summarized in Table S1, multi-shelled hollow silica spheres with well-tailored compartments and void spaces can hardly be produced via either template-assisted or template-free approaches available thus far. Herein we demonstrated a simple, template-free strategy for rational fabrication of multi-shelled hollow spheres with precisely defined shell thickness and inter-shell spacing by periodic addition of tetramethylammonium hydroxide (TMAH), instead of ammonia in conventional Stöber method, to catalyze hydrolysis and condensation of tetraethyl orthosilicate (TEOS). Recently, Wong et al. reported a template-free approach to transform multi-shelled silica solid spheres, formed by conventional Stöber method, into multishelled hollow spheres by post etching treatment and conjectured that the hydrolysis of silica precursor– TEOS – might be slow at the early reaction stage but largely enhanced at the late reaction stage, resulting in the formation of silica gel networks with different chemical composition and thus different chemical stability against etching.16 In contrast, Carcouët et al. demonstrated that during the Stöber process, the hydrolysis and condensation of TEOS were faster at early while slowed down at late reaction stages as a result of the pH reduction of the reaction media with the reaction proceeding, and their nuclear magnetic resonance
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spectroscopic analysis indicated the formation of silica gel networks with low and high condensation degrees.17 This temporal differentiation in condensation degree of the silica network formed over the course of Stöber process should lay the foundation for template-free fabrication of multi-shelled hollow spheres, since the silica networks with low condensation degree, is expected more easily decomposed upon mild chemical etching than these with high condensation degree formed.18,19 Up to date, however, the temporal separation of early and late reaction stages in Stöber method was not sufficient sharp for control of the thicknesses of individual sacrificial and scheduled silica layers and the distances, i.e., void spaces, between neighboring scheduled shells, so the present template-free approach remains of trial-and-error. For base-catalyzed hydrolysis and condensation of TEOS, the chemical composition and condensation degree (i.e. chemical stability) of as-prepared silica gel networks hinge on the active concentration of nucleophilic hydroxyl ions derived from the basic catalysts.17-20 In conventional Stöber reaction, ammonia, a weak base, was utilized as a morphological catalyst causing the formation of the silica particles,21 and with the reaction proceeding, the subtle change in pH of the reaction solution indicates a small difference in the active concentration of hydroxyl ions (Figure S1), thus resulting in poorly distinct temporal separation between early and late reaction stages.22 In comparison, tetramethylammonium hydroxide (TMAH), a strong base which had been widely used as catalyst in preparation of aluminosilicate zeolite in aqueous media,23,24 may produce pronounced pH change in the reaction media while it acted as catalyst instead of ammonia (Figure S1), attributed to the possible interactions between the silica networks and the quaternary ammonium ions from TMAH via ionic bonding and hydrogen bonding.25 Our results showed that the use of TMAH as basic catalyst led to temporally clear-cut separation between the early and late stages during the sol-gel reaction of TEOS and made the volume ratio of the loosely condensed silica gel network, formed at the early reaction stage, to the densely condensed one, formed at the late reaction stage well defined around 3:2, enabling the design and fabrication of multi-shelled hollow spheres with precisely defined shell thickness and spacing between neighboring shells. EXPERIMENTAL SECTION Materials. Tetramethylammonium hydroxide (~25 wt % TMAH in water) and ammonium molybdate was purchased from Sinopharm Chemical Reagent Co., Ltd. Doxorubicin hydrochloride (DOX∙HCl, Reagent Grade) was purchased from Sangon Biotech (Shanghai) Co., Ltd. Ethanol, ammonium hydroxide (25%), and tetraethyl orthosilicate (TEOS) were obtained from Beijing Chemical Int. All the chemicals were of analytical grade. While the other chemicals were used without further purification, TEOS was distilled prior to use. High-purity water (Pall Purelab Plus) with a resistivity of 18 MΩ/cm was used in all the experiments. Characterizations. The conductivity of the reaction media was measured on a METTLER TOLEDO InLab 710 conductivity meter. The pH values of solution were
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obtained by using a METTLER TOLEDO FE20/EL20 Laboratory pH meter, the reaction mixtures were kept in a 25 °C water bath during the measurements. TEM imaging was implemented on a JOEL JEM-2010 electron microscope operating at 200 kV. High-resolution transmission electron microscopy (HR-TEM) images were obtained with a JEOL JEM 2100F transmission electron microscope operating at an acceleration voltage of 200 kV. The mixtures were dropped onto carbon-coated copper grids and dried by tissue paper for TEM observations. Fourier transform infrared (FTIR) spectra were collected on a Nicolet 550 spectrophotometer scanning in the range of 4004000 cm-1. The resulting silica particles were purified by twice repletion of cycle of centrifugation (10000 rpm, 20 min), decanting of the supernatants, and dispersion in pure water. The resulting precipitate was dried at 60 °C for 24 h before FTIR measurements. Content of the dried silica particles in the KBr pellets was fixed to be 2 wt% in all the measurements. UV-visible absorption spectra were taken on a Varian Cary-100 spectrometer. The measurements of 29Si solid-state NMR were collected by using a Varian Infinity plus 400 spectrometer with a spinning frequency of 4 kHz and a pulse delay of 6.0 s. The Raman time-resolved spectra were recorded at 532 nm (He-Ne laser) with a LabRam HR Evolution Raman Spectrometer, typical integration times were 30 s for the samples in the spectral range 200– 2000 cm-1, to prevent ambient light influence, the apparatus was always kept in the dark during measurements. N2 sorption isotherms were measurements by using a Micromeritics ASAP 2420 surface analyzer at 77 K under a continuous adsorption condition. The samples were degassed at 150 °C for 10 h before taking the measurements. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area. The pore-size distributions were derived from the Barrett– Joyner–Halenda (BJH) adsorption branches of isotherms. The total pore volumes were estimated from the adsorbed amounts at a relative pressure of 0.995. Preparation of solid silica spheres. In a typical experiment, 1.25 mL of TEOS (100 mM), 52 mL of ethanol and 1.0 mL of water were first mixed in a three-neck flask. After the addition of 1 mL of 1% TMAH (2.0 mM) in aqueous solution, the mixture was kept in a water bath at 25 °C under magnetic stirring at 250 rpm for 3 h for the growth of silica particles. Afterward, subsequent additions of 1 mL of 1% TMAH (2.0 mM) in aqueous solution at a time interval of 3 h allowed the growth of the silica particles in seeded growth fashion. The resulting solid silica spheres were collected by centrifugation at 10000 rpm for 20 min. In another scenario, 6.25 mL of TEOS (final concentration at 500 mM) was used while containing other conditions for preparing larger silica spheres by adding more cycles of TMAH at predetermined concentration. Fabrication of the multi-shelled hollow silica spheres. The resulting solid silica spheres collected by centrifugation were re-dispersed in hot water (100 °C and 1 h) at a concentration of 1.0 mg/mL to promote the solid-to-hollow structural transformation. After twice repletion of cycle of centrifugation (10000 rpm, 20 min), decanting of the supernatants, and dispersion in pure water, pH of the final dispersion was ca. 10.0, contribut-
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Chemistry of Materials
ed by the TMAH agent remained in the solid silica spheres. After being incubated in boiling water for 1 h, the products were purified by twice repletion of cycle of centrifugation (10000 rpm, 20 min), decanting of the supernatants, and dispersion in ethanol for further TEM observations. Quantitative determination of TEOS. The concentration of free TEOS molecules, remaining in the reaction media, were determined by means of ammonium molybdate spectrophotometric method.26 Generally, the reaction of molybdate with hydrolyzed TEOS (silicic acid) to produce yellow silicomolybdate may undergo completely, and there is a good linear relationship between the absorbance (410 nm) and concentration of the yellow product. In the Stöber reaction system, the content of silicic acid is usually very low,22 and that of the unhydrolyzed TEOS (free TEOS) is high, which may be hydrolyzed completely within 30 min under low pH (
