3168
J. Phys. Chem. C 2009, 113, 3168–3175
A Self-Templated Route to Hollow Silica Microspheres Tierui Zhang,† Qiao Zhang,† Jianping Ge,† James Goebl,† Minwei Sun,‡ Yushan Yan,‡ Yi-sheng Liu,§,| Chinglin Chang,| Jinghua Guo,§ and Yadong Yin†,* Department of Chemistry and Department of Chemical and EnVironmental Engineering, UniVersity of California, RiVerside, California 92521, Department of Physics, Tamkang UniVersity, Tamsui 251, Taiwan, and AdVanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: NoVember 25, 2008; ReVised Manuscript ReceiVed: December 29, 2008
A simple, mild, and effective self-templated approach has been developed to directly convert solid SiO2 microspheres into hollow structures. The reaction involves initial partial dissolution of silica cores in a NaBH4 solution and subsequent shell formation due to the redeposition of the silicate species back onto the colloid surfaces. The increasing concentration of NaBO2 as the result of the slow decomposition of NaBH4 in water is found to be responsible for the regrowth of the silica shell. This method allows the production of hollow silica spheres with sizes ranging from ∼70 nanometers to several micrometers, largely determined by the size of the starting silica colloids. The solid-to-hollow transformation mechanism is investigated in detail by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectrometry, X-ray absorption spectroscopy (XAS), N2 adsorption-desorption, and X-ray diffraction (XRD). We also study the reaction conditions that allow control over the wall thickness, surface morphology, and shell porosity. 1. Introduction Many synthetic procedures have been developed in recent years for the preparation of hollow micro- and nanostructures, largely because of their wide applications in drug delivery, photonic crystals, sensing, energy-storage media, and nanoreactors for catalysis.1-8 Traditional strategies mainly use polymer9-12 and silica beads,7,13,14 micelle and emulsion droplets,15-20 and organic aggregates21,22 as sacrificial templates. When monodisperse polymer and silica beads are used as sacrificial templates, it is possible to obtain highly uniform hollow spheres with controllable shell thicknesses. However, such procedures are usually complicated, unsuitable for large scale synthesis, and therefore expensive for practical applications. The use of so-called “soft-templates” such as micelle and emulsion droplets has the advantage of better scalability although most of such methods still involve complex procedures. Multistep processes are usually needed to form core-shell composites first, and then to remove the templates through selective solvent etching or calcination at high temperatures. More importantly, the control over the size distribution of the products is typically poor in these cases because of the difficulty in maintaining the uniformity of the soft templates in solution. Very recently, the Kirkendall effect,23,24 galvanic replacement,25-27 and well-known physical phenomena such as Ostwald ripening28-30 have been employed as self-templated methods for preparing hollow nanostructures without the involvement of additional sacrificial templates. Herein, we report a simple, effective, and scalable selftemplated method for the synthesis of hollow SiO2 spheres. We recently found that amorphous silica colloids, when dispersed * Corresponding author. E-mail:
[email protected]. † University of California, Department of Chemistry. ‡ University of California, Department of Chemical and Environmental Engineering. § Lawrence Berkeley National Laboratory. | Tamkang University.
in an aqueous solution of NaBH4, undergo a spontaneous morphology change from solid to hollow spheres.31 The formation of hollow silica spheres is based on the spontaneous dissolution and regrowth of silica at the nanometer scale. Such a process is different from the well-demonstrated sacrificial templating procedures in many regards: (a) This method is very simple. The conversion from solid SiO2 spheres to hollow particles is a one-step process that does not require high temperature or high pressure. (b) The process can be performed in air and no inert gas protection is needed during the synthesis. This brings the additional benefit of simple reaction setup. (c) It is highly reproducible. We have had a success rate close to 100% when the concentrations of the reactants are maintained in the appropriate range. (d) The procedure is completely scalable. We have demonstrated the reaction in small glass vials with milligram scale production, and also in large flasks with gram scale production. (e) The final hollow particles are very uniform. The size distribution of the hollow spheres has been found to be dependent on the uniformity of the starting SiO2 solid spheres. (f) The production cost is low. This method converts solid silica spheres directly into hollow particles without the need for additional templates. Both NaBH4 and the optional surfactant polyvinylpyrrolidone (PVP) are commercially available in large quantities, and the solvent is water. (g) More importantly, this process allows the convenient incorporation of functional nanomaterials into the hollow silica particles to form composite nanostructures.31 It has been widely reported that silica can coat many nanomaterials to form core-shell structures.32-34 This method provides an effective means to convert solid core-shell structures into hollow yolk-shell structures so that the core particles can be physically isolated from each other while remaining exposed to outside small molecules through the porous shells. This is especially important for many catalytic or sensing applications where the prevention of aggregation of active nanoparticular components is required.35
10.1021/jp810360a CCC: $40.75 2009 American Chemical Society Published on Web 01/30/2009
Converting SiO2 Microspheres into Hollow Structures In this paper we expand upon the results reported in our previous communication. Convincing experimental observations are provided to clarify the mechanism of transformation from solid to hollow silica spheres. We have also explored the reaction conditions that allow control over the wall thickness, surface morphology, and shell porosity. 2. Experimental Section A. Materials. All chemicals were used as received without further purification. All solvents used were of analytical grade and were purchased from Fisher. Tetraethyl orthosilicate (Si(OC2H5)4 or TEOS, 98%) was purchased from Acros. Ammonium hydroxide solution (NH4OH, ∼28% NH3 in water) and polyvinylpyrrolidone (PVP, K30) were purchased from Fluka. Sodium borohydride (NaBH4, 99%) and sodium metaborate tetrahydrate (NaBO2 · 4H2O, 99%) were purchased from Aldrich. Sodium hydroxide (NaOH) was purchased from Fisher. B. Synthesis. Synthesis of Monodisperse SiO2 Solid Spheres. Monodisperse SiO2 solid spheres 50-800 nm in diameter were prepared by using a slightly modified Sto¨ber process.36-38 In a typical synthesis of 360 nm SiO2 solid spheres, 4.5 mL of TEOS was rapidly added into a mixture of 61.75 mL of ethanol, 24.75 mL of H2O, and 9.0 mL of ammonium. By fixing the concentration of TEOS and H2O, and varying the concentration of ammonium, the resulting particle sizes can be easily adjusted. Larger monodisperse SiO2 solid spheres ∼2.7 µm in diameter were synthesized according to the so-called seeded growth reaction.39 First, SiO2 spheres with a diameter of ∼620 nm were prepared by using the Sto¨ber method. One eighth of this sample was removed from the system and used as seeds for the subsequent growth of 1.2-µm SiO2 spheres. In turn, one-fourth of the ∼1.2-µm SiO2 sphere sample was removed and employed as seeds for the growth of 2.7-µm SiO2 spheres. Synthesis of SiO2 Hollow Spheres with NaBH4. In a typical process, SiO2 spheres (0.3 g) and PVP (0.25 g) were first dispersed in 10 mL of water. NaBH4 (0.6 g) was added to the solution under vigorous stirring and the mixture was heated at 51 °C for 6 h. Aliquots (0.1 mL) were extracted, cleaned several times by centrifugation and water redispersion, and finally dispersed in water or dried into powders for various characterizations. The reaction also occurred at room temperature without the presence of PVP under otherwise similar conditions. Reaction of SiO2 Solid Spheres with NaOH Solution. SiO2 spheres (0.3 g) were first dispersed in 10 mL of water. NaOH (0.063 g) was added to the system under vigorous stirring and the reaction continued at room temperature for 24 h. Reaction of SiO2 Solid Spheres with NaBO2 · 4H2O Solution. SiO2 spheres (0.3 g) and PVP (0.25 g) were first dispersed in 10 mL of water. NaBO2 · 4H2O (0.36 g) was added to the system six times at 30-min intervals under vigorous stirring and the mixture was heated at 51 °C for 24 h. The collection and cleaning procedures of samples are the same as those described above. Reaction of SiO2 Hollow Spheres in a Mixture of NaBH4 and NaOH. SiO2 spheres (0.3 g) and PVP (0.25 g) were first dispersed in 10 mL of water. NaBH4 (0.3 g) and NaOH (0.082 g) were added to the system under vigorous stirring and the mixture was heated at 51 °C for 4 h. The collection and cleaning procedures of samples are the same as those described above. Reprecipitation of DissolWed Silicate Species with NaBO2 · 4H2O. SiO2 spheres (0.3 g) were first dispersed in 10 mL of water. NaOH (0.106 g) was added to the system under vigorous stirring and the mixture was heated at 51 °C for 1 h
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3169 until the solution became colorless. After NaBO2 · 4H2O (1.25 g) was added to the system, white precipitates appeared and the reaction was aged for 3 h. The collection and cleaning procedures of samples are the same as those described above. C. Characterization. The morphology of the nanostructures was investigated with use of a Philips Tecnai 12 transmission electron microscope (TEM) at 120 kV. TEM samples were prepared by drop casting 5-10 µL of ethanol solution of the particles on ultrathin 400 mesh carbon-coated grids. The yields, size distribution, and uniformity of these hollow structures were investigated with a Philips XL30 scanning electron microscope (SEM) equipped with a field emission gun operated at 20 kV. Nitrogen adsorption and desorption isotherm measurements were conducted on powder samples with a Micromeritics Accelerated Surface Area and Porosimetry System 2010 at 77 K. Pore size distribution was calculated from the adsorption branch by using the Barrett-Joyner-Halenda (BJH) method. The small-angle X-ray diffraction (SAXRD) pattern of hollow SiO2 spheres was characterized by a Bruker D8 Advance Powder X-ray diffractometer (40 kV, 40 mA, Cu KR λ ) 1.5406 Å with a graphite monochromator) scanning from 1° to 10° (2θ) at a speed of 0.1 deg/min. Fourier transform infrared (FTIR) spectra of SiO2 powders before and after treatment with NaBH4 were collected with a Bruker Equinox 55 spectrophotometer scanning from 400-4000 cm-1 with a resolution of 4 cm-1 for 64 scans. Measurements were performed with pressed pellets which were made by using KBr powder as a diluent. The pH values of solutions were obtained by a Fisher Scientific Accumet AB15 pH meter. X-ray Absorption Spectroscopy (XAS) measurements were performed at beamline 7.0 of Advanced Light Source, Lawrence Berkeley National Laboratory. The measurements were performed at room temperature, with the base pressure in the experimental chamber of 2.0 × 10-9 mbar. XAS spectra were obtained by measuring both the total electron yield (TEY) and fluorescence yield (FY) from the sample as a function of the incoming photon energy. All spectra were normalized to the photocurrent from a clean gold mesh introduced into the beam. 3. Results and Discussions The solid SiO2 spheres gradually transform into hollow structures with similar sizes upon mixing with NaBH4 solution. Figure 1a shows a typical TEM image of monodisperse SiO2 solid spheres of ∼360 nm in diameter prepared by using the well-known Sto¨ber process. After mixing with NaBH4 at the appropriate concentration at 46 °C for 10 h, all solid SiO2 spheres are converted into well-defined hollow spheres with a diameter of ∼330 nm, as shown in the TEM image in Figure 1b. Although the hollow spheres can be produced without the involvement of any surfactants, PVP is usually added to the reaction system to help prevent interparticle aggregation. A lowmagnification SEM image of these hollow spheres in Figure 1c clearly suggests a narrow size distribution. Careful inspection of the image also reveals that some of the particles have openings in the shell and some are broken, supporting the conclusion of hollowness of the products. The rate of the solid-to-hollow conversion process is found to be very sensitive to reaction temperature. Figure 2a plots the dependence of the amount of time required for the completion of the transformation versus the reaction temperature. At room temperature (25 °C), it takes 10 days to finish the conversion process. Increasing the temperature dramatically shortens the conversion to 10 h at 46 °C, 5 h at 56 °C, and 3 h at 61 °C. As can be seen from Figure 2b-d, the roughness of the shells
3170 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Figure 1. (a) TEM image of as-prepared SiO2 spheres. (b) TEM and (c) SEM images of samples after reacting with 0.06 g/mL of NaBH4 at 46 °C for 10 h. Scale bars are 200 nm in panels a and b and 1 µm in panel c.
Figure 2. (a) Dependence of the conversion time of SiO2 spheres from solid into hollow on the reaction temperature. TEM images of hollow SiO2 spheres formed after reacting with 0.06 g/mL of NaBH4 at (b) 25 °C for 10 d, (c) 51 °C for 6 h, and (d) 61 °C for 3 h. Scale bars are 200 nm.
increases gradually with reaction temperature. In comparison to the relatively smooth shells obtained at 25 °C, those formed at 51 and 61 °C show increased grain size and discernible pores. The tunable shell porosity is promising for many applications such as controlled release and size-selective catalysis. Figure 3a shows the nitrogen adsorption-desorption isotherms and the pore size distribution of the hollow spheres displayed in Figure 2c. The desorption isotherm shows two major capillary condensation steps appearing at relative pressure ranges of 0.4-0.6 and 0.6-0.85. These steps correspond to two distinct peaks at 3.6 and 5.5 nm, respectively, in the pore size distribution curve, as calculated by the BJH method. The Brunauer-Emmett-Teller (BET) specific surface area and pore volume are calculated to be 558 m2/g and 0.56 cm3/g,
Zhang et al.
Figure 3. (a) Nitrogen adsorption and desorption isotherms, pore-size distribution (inset), and (b) SAXD pattern of porous hollow SiO2 spheres shown in Figure 2c.
respectively. The small-angle X-ray diffraction pattern of the hollow samples exhibits a peak positioned at 6.4 nm, which probably corresponds to the wormhole-like mesoporous structures (Figure 3b).16,17 FTIR spectroscopy is used to characterize the structural change of SiO2 spheres upon reaction with NaBH4 solution. In Figure 4a, curve I is the FTIR spectrum of as-synthesized solid SiO2 colloids. The absorption peaks around 3400 and 1630 cm-1 are attributed to the silanol group and adsorbed water. Weak absorption bands attributed to C-H bending vibration in unhydrolyzed -OEt groups are observed between 1350 and 1500 cm-1.40 Bands located at 1190, 1101, 953, and 802 cm-1 are associated with the longitudinal-optical (LO) mode and transverse-optical (TO) mode of the Si-O-Si asymmetric bond stretching vibration, the Si-OH stretching vibration, and the network Si-O-Si symmetric bond stretching vibration, respectively.41 After exposing silica spheres to NaBH4 solution at 25 °C for 10 d, the intensity of the bands related to the C-H bending vibration becomes indistinguishable, indicating that the residual -OEt groups in as-prepared solid SiO2 spheres have been completely hydrolyzed during the reaction (curve II in Figure 4a). The TO mode of the Si-O-Si asymmetric stretching vibration band shows a ∼75 cm-1 red shift from 1101 to 1026 cm-1 during the solid-to-hollow conversion, while the band corresponding to the LO mode does not change significantly. Moreover, the Si-O-Si symmetric stretching vibration band at 802 cm-1 also shifts to 783 cm-1 and decreases in intensity. The red shift of the Si-O-Si band suggests a more open SiO2 network structure with lower internal stress in the newly formed silica shells.41,42 The Si-OH stretching vibration band at 953 cm-1 cannot be easily discerned because of its overlap with the red-shifted Si-O-Si asymmetric stretching vibration band. Compared with the spectrum of as-synthesized SiO2 solid spheres, the spectrum of hollow spheres obtained at elevated temperature, e.g. 51 °C, shows a red shift. (curve III). However,
Converting SiO2 Microspheres into Hollow Structures
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3171
Figure 5. Schematic illustration of the spontaneous formation of hollow SiO2 spheres at room temperature reacting with 0.05 g/mL of NaBH4. All the TEM pictures are 445 (length) nm × 455 (width) nm.
Figure 4. (a) FTIR spectra of as-prepared SiO2 spheres (I), and samples after reacting with 0.06 g/mL NaBH4 at room temperature for 10 d without PVP (II), at 51 °C for 6 h without PVP (III), and at 51 °C for 6 h with PVP (IV). (b) XAS spectra of as-prepared SiO2 spheres (0 d) and samples after reacting with 0.06 g/mL of NaBH4 at room temperature for 3, 6, and 10 d. The inset shows the Gaussian deconvolution of peak I.
the red shift of the Si-O-Si bands in curve III is only 8 cm-1, which indicates that the network structure of hollow spheres synthesized at 51 °C is similar to that of solid spheres. This difference could be due to the faster deposition and condensation of silicate species during higher temperature solid-to-hollow conversion resulting in a denser network structure. Curve IV is the spectrum of hollow spheres synthesized at 51 °C with PVP and is very similar to curve III except for a slight 6 cm-1 blueshift of the Si-O-Si asymmetric stretching vibration band, which indicates that PVP as a dispersion agent has nearly no effect on the backbone structure of the hollow spheres. After the solid-to-hollow conversion at room temperature or higher temperature, no new bands were found in the FTIR spectra of the hollow spheres and some bands had red shifts. These results, together with our previous energy dispersive X-ray spectroscopy characterizations, reveal that the hollow spheres are still composed of Si and O and that the ratio is close to 1:2.31 XAS is a useful tool for the investigation of the unoccupied electronic states of the element analyzed and for obtaining information about the local structure such as coordination numbers for amorphous networks. Figure 4b shows the Si L-edge absorption of as-synthesized SiO2 solid spheres and samples reacted with NaBH4 at 25 °C for different times. There are three sets of main peaks in each of these four spectra. The doublet peak I at ∼105.3-105.9 eV corresponds to a transition of Si 2p electrons to the antibonding a1 state (Si 2p f a1). The 0.6 eV splitting of this peak is attributed to the spin-orbital interaction of Si 2p orbitals, so these two peaks are assigned to Si 2p3/2 f a1 and Si 2p1/2 f a1, respectively. The strong peak II at 108.2 eV is related to the transition of Si 2p electrons to
the antibonding t2 state (Si 2p f t2). The broad peak centered on 115.5 eV is associated with the d-like shape resonance of eg character.43-45 Peak I shifts gradually to higher energy and at the same time decreases in intensity when the reaction of solid SiO2 spheres with NaBH4 is prolonged from 0 to 10 d. The shift and decreased intensity of peak I can be clearly discerned from the Gaussian deconvolution in the inset. The peaks assigned to Si 2p3/2 f a1 and Si 2p1/2 f a1 transitions shift from 105.35 and 105.92 eV to 105.41 and 105.99 eV, respectively. 43 The measured samples were ∼330-360 nm in diameter. The photon penetration depth is typically on the order of 100-200 nm. The decreased Si L-edge absorption intensity indicates the smaller amount of Si being detected within the probed volume, which agrees with the formation of hollow structure in the SiO2 spheres. The shift of peak I can be attributed to the chemical shift of Si 2p levels due to the change of local chemical environment of Si atoms. Such a chemical shift was detected previously for Si-O and Si-OH, where Si 2p in SiO2 has the binding energy of 103.8 eV.46,47 As hydroxyl is an electronwithdrawing group, the binding energy of Si 2p in Si-OH shifts to 105.2 eV. Thus, the chemical shift of XAS features suggests the formation of more Si-OH groups in the hollow spheres compared with the original solid SiO2 spheres, though the total shift is not as big as 1.5 eV due to the coexistence of Si-O and Si-OH in our samples. The XAS experiments further support the conclusion obtained from FTIR analysis that the network structure of the hollow spheres is more open than that of the solid spheres. We choose a room temperature reaction to carefully monitor the solid-to-hollow transformation. Figure 5 shows a complete cycle of morphology change of silica colloids through reaction with NaBH4. Figure 5a is the TEM image of as-synthesized SiO2 spheres. After one day of reacting with 0.050 g/mL of NaBH4 at room temperature, SiO2 spheres do not exhibit significant changes in morphology except for a slightly decreased diameter due to etching at high pH (Figure 5b). After a second day of reaction, the surface of the SiO2 spheres appears to be rougher due to the formation of a thin layer of deposition (Figure 5c). It is noted that the inner cores are still solid and their diameter is almost identical with that of the sample in Figure 5b. On day 4, a core-shell structure can be clearly observed with a thin shell around each solid core (Figure 5d). The diameter of the core is apparently smaller when compared with that of the particle in Figure 5c. With time prolonged to day 9, the core further shrinks while the thickness of the shell increases considerably (Figure 5e). The core eventually disappears after about 21 days of reaction, leaving behind hollow shells with perfectly round shapes and thicker walls (Figure 5f). Longer reaction times (e.g., 27 d) produce no observable changes in the morphology and thickness of the hollow structures.
3172 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Zhang et al.
We have previously suggested two separate processes that contribute to the solid-to-hollow transformation: the dissolution of SiO2 and the regrowth of the silica layer. The dissolution of SiO2 is due to the high basicity of the NaBH4 solution, which can dissociate Si-O bonds and form soluble monosilicate and polysilicate species with various compositions.48-54 The regrowth of silica is facilitated by the presence of BO2- in the supersaturated solution of silicates, as will be discussed in detail later. It is well-known that NaBH4 reacts with water to slowly release H2 and sodium metaborate NaBO2:
NaBH4+2H2O f 4H2 v +NaBO2
(1)
At the initial stage of the reaction, a high pH (>11.0) is quickly established so that the amorphous SiO2 solid spheres shrink in size due to etching. The dissolution of silica produces soluble silicate species with increasing concentration as the reaction time prolongs. The silicate species eventually become supersaturated. Along with the dissolution of silica, the concentration of NaBO2 also increases gradually as a result of the decomposition of NaBH4, causing the silicate species to precipitate and redeposit on the core surfaces. In this case, the deposition of the silicate species on the surface of the remaining silica spheres as the result of heterogeneous nucleation is energetically favored over the formation of new solid particles through homogeneous nucleation. The shells further grow at the expense of the cores through the mechanism of Ostwald ripening, eventually leading to the production of completely hollow SiO2 shells.28 The above solid-to-hollow transformation mechanism has been further supported by several experiments as discussed below. First, we verify that the addition of NaBO2 to a supersaturated silicate solution can cause precipitation of silica. As-synthesized solid spheres are first treated with different concentrations of aqueous NaOH ranging from 1.25 to 9.52 g/L at room temperature. This dissolution process simply results in solid spheres with reduced sizes or a clear solution, but no hollow SiO2. For example, after reacting with 6.34 g/L of NaOH aqueous solution for 1 day, the diameter of the SiO2 spheres is reduced from 345 to 326 nm (Figure 6a,b). When the concentration of NaOH is increased to 9.52 g/L, the milky solution becomes colorless and transparent in 4 h, and no SiO2 particles are found by TEM measurements. At this stage, the gradual introduction of NaBO2 causes the solution to turn from clear to turbid, and eventually produces white precipitates of SiO2 when NaBO2 reaches an appropriate concentration (Figure 6c). Although the exact mechanism at the molecular lever is still under investigation, we believe that NaBO2 plays a critical role in inducing the precipitation of silicate species. The above observations suggest that OH- and BO2- groups are two indispensable factors during the solid-to-hollow transformation of SiO2 spheres. Since NaBO2 is also a strong base, it is expected that the replacement of NaBH4 with NaBO2 may also produce hollow SiO2 spheres. NaBO2 · 4H2O with equivalent concentration of boron to 0.06 g/mL of NaBH4 is used to replace NaBH4 and mixed with a solution of SiO2 spheres for 3 h to mimic the slow decomposition behavior of NaBH4. After reacting with NaBO2 at 51 °C for 3 h, SiO2 spheres are still solid and spherical with their diameter decreasing from 410 to 330 nm due to the etching of OH- groups (Figure 7a,b). After 9 h, the etched rough surface of SiO2 spheres becomes smoother and the diameter of spheres is increased to 340 nm. The newly formed SiO2 layer of ∼5 nm in thickness originates from the deposition and regrowth of silicate species in solution induced by the BO2-
Figure 6. TEM images of (a) as-prepared SiO2 spheres, (b) samples after reacting with 6.34 g/L of NaOH at room temperature for 1 d, and (c) samples after first reacting with 10.65 g/L of NaOH at 51 °C for 1 h and then with 0.219 g/mL of NaBO2 · 4H2O for 6 h. Scale bars are 200 nm.
Figure 7. TEM images of (a) as-prepared SiO2 spheres and (b-d) samples after reacting with 0.219 g/mL of NaBO2 · 4H2O at 51 °C for (b) 3 h, (c) 9 h, and (d) 1 d. Scale bars are 200 nm.
groups (Figure 7c). When the reaction is aged to 1 d, some hollow SiO2 nanostructures are formed because of the continuous core dissolution and shell growth (Figure 7d). The hollow SiO2 nanostructures in Figure 7d are not as well defined as those
Converting SiO2 Microspheres into Hollow Structures
Figure 8. TEM images of (a) as-prepared SiO2 spheres, (b) sample after reacting with 0.03 g/mL NaBH4 at 51 °C for 1 d, and (c) sample after reacting with 0.03 g/mL of NaBH4 and 8.24 g/L of NaOH at 51 °C for 4 h. Scale bars are 200 nm.
in Figure 1b because it is difficult to control the addition rate of NaBO2 to exactly mirror the real decomposition rate of NaBH4. It is therefore believed that the unique properties of NaBH4 (high initial pH and slow decomposition to release BO2groups to induce the deposition and regrowth of silicate species) provide the optimal conditions for the growth of hollow structures. According to the solid-to-hollow transformation mechanism, the concentration of NaBH4 determines whether or not hollow structures form. At a low NaBH4 concentration such as 0.03 g/mL, the SiO2 spheres are still solid after 1 d of reaction at 51 °C with slightly reduced diameters and rougher surfaces (Figure 8a,b), because the low pH value due to the low NaBH4 concentration does not facilitate the formation of supersaturate silicate species for the next deposition and regrowth on the remaining SiO2 spheres. To achieve optimal pH in the solution, 8.24 g/L of NaOH together with 0.03 g/mL of NaBH4 are employed for the conversion of SiO2 solid spheres. Mixed with the combined agents, SiO2 solid spheres are easily transformed into hollow spheres in 4 h (Figure 8c). The surface of the hollow spheres seems to be composed of plate-like SiO2 nanoparticles, which is very different from that shown in Figure 1b. This experiment not only further supports the solid-to-hollow transformation mechanism proposed above but also provides a promising approach to adjust the surface morphology of SiO2 hollow spheres. On the basis of our understanding of the solid-to-hollow transformation mechanism, wall thickness control of SiO2
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3173
Figure 9. TEM images of (a) 0.03, (b) 0.0375, (c) 0.045, and (d) 0.0525 g/mL of solid SiO2 spheres after reacting with 0.06 g/mL of NaBH4 at 51 °C for 6, 9, 13, and 18 h, respectively. (e) Dependence of the wall thickness and the diameter of hollow SiO2 spheres on the initial concentration of solid SiO2 spheres. Scale bars are 200 nm.
hollow spheres can be achieved by simply adjusting the starting concentration of SiO2 spheres. As shown in Figure 9a, the wall thickness is ∼25 nm using 0.03 g/mL of SiO2 solid spheres as starting materials. As the concentration of SiO2 solid spheres is increased to 0.0525 g/mL at a 0.0075 g/mL interval, the wall thickness of the hollow spheres increases to ∼41, ∼66, and ∼88 nm, respectively (Figure 9b-d). The wall thickness has a nearly linear relationship with the concentration of starting SiO2 spheres, Y ) -62.37 + 2852X (Figure 9e). The critical concentration of starting SiO2 solid spheres, 0.0219 g/mL, can be obtained by extrapolating the wall thickness to zero. This predicts that if the concentration of starting SiO2 solid spheres is below 0.0219 g/mL, hollow spheres cannot be formed, which is close to our experimental datum of 0.225 g/mL. With an increase of the concentration of starting SiO2 solid spheres from 0.03 to 0.0525 g/mL, the diameter of the SiO2 hollow spheres increases from 333 to 418 nm (Figure 9e). Because the concentration of NaBH4 is constant in all four solutions, the quantity of dissolved silicate species in each solution is also the same regardless of the different concentrations of SiO2 solid spheres. At higher concentrations of SiO2 spheres, each SiO2 sphere is etched less on average than those in lower concentration solutions. During the core dissolution and shell growth stage of the solid-to-hollow transformation, the remaining SiO2 spheres of larger sizes are transformed into SiO2 hollow spheres
3174 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Figure 10. TEM images of hollow SiO2 spheres formed after reacting with 0.06 g/mL of NaBH4 at 51 °C for 6 h (a) without PVP and with (b) 0.025, (c) 0.05, and (d) 0.1 g/mL of PVP. Scale bars are 200 nm.
with greater outer diameters and thicker walls, the values of which can be easily estimated through simple geometric arguments. In theory, the inner diameters should also increase with the concentration of the initial silica spheres; however, the values calculated from the corresponding thicknesses and outer diameters show an overall trend of decreasing. The discrepancy might come from the inaccuracy in determining the shell thickness due to the blurred inner surface, but it may also suggest that the contribution of growth of the shell at the inner surfaces becomes significant for larger dissolving cores. The surface morphology of the SiO2 hollow spheres can be tailored by altering PVP content. More specifically, moderate concentration of PVP will help to improve the smoothness of the silica shells, but too much PVP will lead to uneven deposition of silica species. Due to the fast deposition of silicate species, the SiO2 hollow spheres produced in the absence of PVP at relatively high temperatures (such as 51 °C) possess rough surfaces with obvious partial bumping or clusters (Figure 10a), while those formed at room temperature have a smoother surface. When 0.025 g/mL of PVP is used, the surface smoothness of the SiO2 hollow spheres improves (Figure 10b). It has been widely noted that PVP can serve as a coupling agent to promote uniform silica coatings onto various colloidal particles because of its strong interactions with TEOS precursors and silicate species.8,32 In a similar way, in our case PVP surfactants are adsorbed to the remaining SiO2 cores resulting in the uniform coating of the newly formed SiO2 layer. However, when the PVP concentration is increased to 0.05 g/mL the surface of the SiO2 hollow spheres becomes a little rougher (Figure 10c). As shown in Figure 10d, when the PVP content is further increased to 0.1 g/mL, the surface of the SiO2 hollow spheres becomes very uneven and a large number of aggregations are observed. We believe the steric effect of PVP molecules plays a critical role in controlling the deposition of SiO2. While a small amount of PVP is helpful for the formation of uniform silica coating, too much PVP prevents the deposition of additional SiO2 to the surface due to its strong steric effect. A similar phenomenon has been observed in previous studies when PVP was used as a dispersion reagent to control the deposition of TiO2 on the surface of PS/SiO2 core-shell spheres.11 It is expected that other polymeric surfactants that
Zhang et al.
Figure 11. TEM images of hollow SiO2 spheres (a) ∼73-100 nm and (b) ∼2.6 µm in diameter. Scale bars in panels a and b are 200 nm and 1 µm, respectively.
bind to the silica surface with reasonable strengths can also be used in controlling the morphology of silica shells. The solid-to-hollow transformation process is general for SiO2 spheres ranging from nanometers to micrometers in diameter. As shown in Figure 11a, nanosized SiO2 hollow spheres ∼73-100 nm in diameter with a wall thickness of ∼14-18 nm are obtained by reacting NaBH4 with ∼77-120 nm sized SiO2 solid spheres. By the same process, 2.6 µm SiO2 hollow spheres also can be prepared from 2.8 µm SiO2 solid spheres (Figure 11b). In fact, the simple process reported here can be conveniently extended to produce hollow nanostructures not only from pure solid SiO2 colloids but also from various silicacoated composite particles with various shapes.31 4. Conclusions In summary, we have developed a facile self-templated route for large-scale synthesis of various-sized SiO2 hollow spheres with high morphological fidelity and low size distribution by the reaction of SiO2 solid spheres with NaBH4 aqueous solutions under very mild conditions ranging from room temperature to 61 °C. After the transformation, the hollow structures are still composed of SiO2 but possess a more open silica network compared to that of the original solid spheres. IR studies show that the silica network of the hollow samples obtained at room temperature is more open than that of samples prepared at higher temperature due to the faster growth of new SiO2 shells. The tunable porosity of the shells makes the hollow SiO2 nanocontainers very promising as controllable loading and release vehicles. The OH- and BO2- ions generated from the gradual decomposition of NaBH4 play critical roles in the formation of SiO2 hollow structures by first contributing to the partial dissolution of the SiO2 cores and then the redeposition of the silicate species back to core surfaces to form shells. According to the transformation mechanism, the approach can also be extended to transform other silica-coated composite materials with different shapes into hollow structures with various functions. It is believed that this process can be extended to other amphoteric metal oxide systems if one can identify a chemical agent that has a similar dual function as NaBH4: it
Converting SiO2 Microspheres into Hollow Structures can gradually dissolve the metal oxide while its own decomposition product can also lead to the reprecipitation of the dissolved species. Acknowledgment. Y.Y. thanks the University of California, Riverside for start-up funds and the Chinese-American Faculty Association of Southern California for the Robert T. Poe Faculty Development Grant. Acknowledgment is also made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We thank Dr. Bozhilov and Mr. McDaniel at the Central Facility for Advanced Microscopy and Microanalysis at UCR for assistance with TEM analysis. The Advanced Light Source is supported by the U.S. Department of Energy under contract No. DE-AC02-05CH11231. References and Notes (1) Li, J.; Zeng, H. C. Angew. Chem., Int. Ed. 2005, 44, 4342–4345. (2) Lou, X. W.; Wang, Y.; Yuan, C. L.; Lee, J. Y.; Archer, L. A. AdV. Mater. 2006, 18, 2325–2329. (3) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111– 1114. (4) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Angew. Chem., Int. Ed. 2005, 44, 5083–5087. (5) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179. (6) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 3827– 3831. (7) Xu, X. L.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940–7945. (8) Zhang, Q.; Zhang, T.; Ge, J.; Yin, Y. Nano Lett. 2008, 8, 2867– 2871. (9) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 1325–1331. (10) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. AdV. Mater. 2006, 18, 801–806. (11) Song, X. F.; Gao, L. J. Phys. Chem. C 2007, 111, 8180–8187. (12) Wu, X. F.; Tian, Y. J.; Cui, Y. B.; Wei, L. Q.; Wang, Q.; Chen, Y. F. J. Phys. Chem. C 2007, 111, 9704–9708. (13) Yu, J. G.; Liu, W.; Yu, H. G. Cryst. Growth Des. 2008, 8, 930– 934. (14) Lou, X. W.; Yuan, C. L.; Archer, L. A. Small 2007, 3, 261–265. (15) Tan, B.; Lehmler, H. J.; Vyas, S. M.; Knutson, B. L.; Rankin, S. E. AdV. Mater. 2005, 17, 2368–2371. (16) Sun, Q. Y.; Kooyman, P. J.; Grossmann, J. G.; Bomans, P. H. H.; Frederik, P. M.; Magusin, P. C. M. M.; Beelen, T. P. M.; van Santen, R. A.; Sommerdijk, N. A. J. M. AdV. Mater. 2003, 15, 1097–1100. (17) Wang, J. G.; Xiao, Q.; Zhou, H. J.; Sun, P. C.; Yuan, Z. Y.; Li, B. H.; Ding, D. T.; Shi, A. C.; Chen, T. H. AdV. Mater. 2006, 18, 3284– 3288. (18) Cong, H. P.; Yu, S. H. AdV. Funct. Mater. 2007, 17, 1814–1820. (19) Zoldesi, C. I.; Imhof, A. AdV. Mater. 2005, 17, 924–928. (20) Walsh, D.; Lebeau, B.; Mann, S. AdV. Mater. 1999, 11, 324–328. (21) Wan, Y.; Yu, S. H. J. Phys. Chem. C 2008, 112, 3641–3647. (22) van Bommel, K. J. C.; Jung, J. H.; Shinkai, S. AdV. Mater. 2001, 13, 1472–1476. (23) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711–714.
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3175 (24) Cabot, A.; Puntes, V. F.; Shevchenko, E.; Yin, Y.; Balcells, L.; Marcus, M. A.; Hughes, S. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2007, 129, 10358–10360. (25) Sun, Y.; Wiley, B.; Li, Z. Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 9399–9406. (26) Chen, J.; Wiley, B.; McLellan, J.; Xiong, Y.; Li, Z. Y.; Xia, Y. Nano Lett. 2005, 5, 2058–2062. (27) Yin, Y.; Erdonmez, C.; Aloni, S.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 12671–12673. (28) Zeng, H. C. Curr. Nanosci. 2007, 3, 177–181. (29) Liu, B.; Zeng, H. C. Small 2005, 1, 566–571. (30) Wang, W. S.; Zhen, L.; Xu, C. Y.; Zhang, B. Y.; Shao, W. Z. J. Phys. Chem. B 2006, 110, 23154–23158. (31) Zhang, T.; Ge, J.; Hu, Y.; Zhang, Q.; Aloni, S.; Yin, Y. Angew. Chem., Int. Ed. 2008, 47, 5806–5811. (32) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693–6700. (33) Mulvaney, P.; Liz-Marzan, L. M.; Giersig, M.; Ung, T. J. Mater. Chem. 2000, 10, 1259–1270. (34) Jana, N. R.; Earhart, C.; Ying, J. Y. Chem. Mater. 2007, 19, 5074– 5082. (35) Arnal, P. M.; Comotti, M.; Schuth, F. Angew. Chem., Int. Ed. 2006, 45, 8224–8227. (36) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (37) Wang, W.; Gu, B. H.; Liang, L. Y.; Hamilton, W. J. Phys. Chem. B 2003, 107, 3400–3404. (38) Lu, L. H.; Capek, R.; Kornowski, A.; Gaponik, N.; Eychmuller, A. Angew. Chem., Int. Ed. 2005, 44, 5997–6001. (39) Chen, S. L.; Dong, P.; Yang, G. H.; Yang, J. J. J. Colloid Interface Sci. 1996, 180, 237–241. (40) Yoshino, H.; Kamiya, K.; Nasu, H. J. Non-Cryst. Solids 1990, 126, 68–78. (41) Bruynooghe, S.; Bertin, F.; Chabli, A.; Gay, J. C.; Blanchard, B.; Couchaud, M. Thin Solid Films 1998, 313, 722–726. (42) Zhu, H.; Ma, Y. G.; Fan, Y. G.; Shen, J. C. Thin Solid Films 2001, 397, 95–101. (43) Lin, L. W.; Tang, Y. H.; Li, X. X.; Pei, L. Z.; Zhang, Y.; Guo, C. J. Appl. Phys. 2007, 101, 014314. (44) Li, D.; Bancroft, G. M.; Kasrai, M.; Fleet, M. E.; Secco, R. A.; Feng, X. H.; Tan, K. H.; Yang, B. X. Am. Mineral. 1994, 79, 622–632. (45) Tallarida, M.; Schmeisser, D. Mater. Sci. Eng., B 2007, 144, 23– 26. (46) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Chanhassen, MN, 1995. (47) Paparazzo, E.; Fanfoni, M.; Severini, E.; Priori, S. J. Vac. Sci. Technol. A 1992, 10, 2892–2896. (48) Sjoberg, S. J. Non-Cryst. Solids 1996, 196, 51–57. (49) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica; Wiley: New York, 1979. (50) Cho, H.; Felmy, A. R.; Craciun, R.; Keenum, J. P.; Shah, N.; Dixon, D. A. J. Am. Chem. Soc. 2006, 128, 2324–2335. (51) Brykov, A. S. Colloid J. 2004, 66, 430–434. (52) Sefcik, J.; McCormick, A. V. AIChE J. 1997, 43, 2773–2784. (53) Liu, S.; Wong, Y.; Wang, Y.; Wang, D.; Han, M. Y. AdV. Funct. Mater. 2007, 17, 3147–3152. (54) Grzelczak, M.; Correa-Duarte, M. A.; Liz-Marza´n, L. M. Small 2006, 2, 1174–1177.
JP810360A
