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A Bottle-Around-a-Ship Method to generate Hollow Thin-Shelled Particles Containing Encapsulated Iron Species with Application to the Environmental Decontamination of Chlorinated Compounds Yang Su, Yingqing Wang, Olasehinde Owoseni, Yueheng Zhang, Vijay John, Gary L. McPherson, David Pierce Gamliel, and Julia A. Valla ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14308 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Materials & Interfaces

A Bottle-Around-a-Ship Method to generate Hollow Thin-Shelled Particles Containing Encapsulated Iron Species with Application to the Environmental Decontamination of Chlorinated Compounds

Yang Su1, Yingqing Wang1, Olasehinde Owoseni1, Yueheng Zhang1, David Pierce Gamliel2, Julia A. Valla2, Gary L. McPherson3, Vijay T. John1 *

1. Department of Chemical & Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States 2. Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, United States 3. Department of Chemistry, Tulane University, 6823 St. Charles Avenue, New Orleans, Louisiana 70118, United States

To whom correspondence may be addressed *Vijay T. John [email protected]

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Abstract Thin-shelled hollow silica particles are synthesized using an aerosol based process where the concentration of a silica precursor tetraethyl orthosilicate (TEOS) determines the shell thickness. The synthesis involves a novel concept of the salt-bridging of an iron salt, FeCl3 to a cationic surfactant, cetyltrimethylammonium bromide (CTAB), which modulates the templating effect of the surfactant on silica porosity. The salt bridging leads to a sequestration of the surfactant in the interior of the droplet with the formation of a dense silica shell around the organic material. Subsequent calcination consistently results in hollow particles with encapsulated iron oxides. Control of the TEOS levels leads to the generation of ultrathinshelled (~ 10 nm) particles which become susceptible to rupture upon exposure to ultrasound. The dense silica shell that is formed is impervious to entry of chemical species. Mesoporosity is restored to the shell through desilication and reassembly, again using CTAB as a template. The mesoporous-shelled hollow particles show good reactivity towards the reductive dichlorination of trichloroethylene (TCE) indicating access of TCE to the particle interior. The ordered mesoporous thin-shelled particles containing active iron species are viable systems for chemical reaction and catalysis. KEYWORDS: Thin-shelled hollow silica particles, shell transformation, mesoporous hollow particles, dechlorination


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Introduction Hollow particles with controlled morphologies are of interest due to their wide applications in encapsulation technologies,1 biomolecule separation,2 catalysis,3-4 supercapacitors,5-6 gas sensing,7 drug delivery

8-10

and energy storage.11-15 The preparation of

hollow particles has the intrinsic appeal of using the interior volume to encapsulate materials or to conduct reactions in confined environments, and there have been several recent and exciting developments in the design of complex structures, such as double shelled,16-18 rattlelike and yolk-shell type morphologies.19-21 A variety of chemical strategies to synthesize such hollow particles have been used, employing soft or hard templates in the process.22-28 In most cases, the synthesis route involves using a sacrificial core around which a layer of the desired material is built. The core is then removed by chemical dissolution or high temperature calcination leading to hollow structures.29 Such multistep procedures are inherently complex and difficult to scale up to larger volume applications in catalysis and delivery. The integration of metal nanoparticles within silica shells is a concept that has been studied in the recent literature30 for a number of applications. Li and coworkers have shown the formation of zerovalent iron nanoparticles covered by dense silica shell which is functionalized with polydopamine for the adsorption of polyaromatics.30 Arruebo and coworkers have incorporated iron species into MCM-41 type materials and then used hydrazine to dissolve away silica in the core to leave clusters of hollow particles.31 In a remarkable recent development, Nie and coworkers used microfluidics to fabricate a rapid throughput of hollow silica particles decorated with magnetic nanoparticles for applications in magnetically guided drug delivery.32 In this paper, we describe a facile method for the large scale synthesis of functional hollow particles using an aerosol based process (or spray pyrolysis), a well-known technique for the rapid production of functional inorganic materials and illustrated schematically in Figure 1a.33 The generation of hollow structures is a useful aspect of the aerosol process and 3 ACS Paragon Plus Environment


earlier work reported by our laboratory

17, 34

proposed a mechanism based on microstructure

disruption which we describe briefly here as introduction to this paper. It is a well-known and important finding that the introduction of a templating surfactant such as cetyl trimethylammonium

Figure 1: (a) Schematic of the aerosol based process used in the generation of structured particles. (b) (i) TEM image of an MCM-41 particle showing mesoporosity; (ii) TEM image of a hollow shell particle obtained by adding FeCl3 to the MCM-41 precursor in the aerosol based process. The scale bar is 20 nm in both images.

bromide (CTAB) into a solution containing a silica precursor such as tetraethyl orthosilicate (TEOS) leads to the formation of highly ordered, high surface area, mesoporous silica as MCM-41.35 A representative particle synthesized through the aerosol process is shown in Figure 1b (i). However, in earlier research we have found that the inclusion of ferric chloride into the surfactant containing precursor solution completely negates the templating effect.34 Rather than template mesoporous silica, the inclusion of the ferric salt in the precursor


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solution leads to the remarkable generation of hollow particles, a result that is very specific to the aerosol based process, with a representative particle shown in Figure 1b (ii). This loss of

Figure 2: (a) Schematic of the salt-bridging that leads to a restriction of the templating effect of the cationic CTAB surfactant. (b) Schematic of the aerosol process: (i) The precursor containing CTAB and FeCl3 in the ethanol/0.1M HCl solution. (ii)-(iv) represent a droplet at various stages in the aerosol process with (iii) representing initial formation of a silica shell and (iv) representing restriction of growth towards the interior. (v) The final morphology of the particle after removal of all organics through calcination. Iron oxide species are deposited in the interior of the particle. templating is attributed to salt bridging with the surfactant as shown in Figure 2a, with the possible formation of aggregates. Our proposed hypothesis is that the binding of the CTAB to the ferric salt through strong electrostatic interactions significantly diminishes the ability of CTAB to template the formation of ordered mesoporous MCM-41.34

Figure 2b illustrates the proposed mechanism for hollow particle generation, from the perspective of the process in an aerosol droplet. Figure 2b (i) denotes the precursor solution containing the iron salt, CTAB and the silica precursor, TEOS. Figure 2b (ii), (iii) and (iv) represent processes occurring in the aerosol droplet as it passes through the heated zone with 5 ACS Paragon Plus Environment


evaporation of the solvent and the formation of silica. Here, the silica is first nucleated at the external surface of the droplet where evaporation takes place and supersaturation sets in (Figure 2b (iii)). Diffusion of TEOS to the shell inner surface leads to propagation of growth towards the interior as a moving boundary of silica. However, the interior contains the phase segregated aggregates of surfactant and iron salts, and the lack of templating may lead to a relatively dense shell with growth stopping as the droplet becomes depleted of TEOS. This results in an interior containing colloidal aggregates of CTAB with the iron salt. Thus, the colloidal aggregates formed by CTAB and FeCl3 are locked within the silica shell while the shell propagates inwards. The growth of the silica shell stops when the aerosol droplet becomes depleted with the TEOS silica precursor. The as-synthesized particles at this stage are particles with silica shells containing colloidal aggregates in the core. Subsequent calcination removes the colloidal aggregates leaving a hollow structure with the iron species being converted to iron oxide nanoparticles, as shown schematically on the right of Figure 1b. Detailed TEM imaging of these particles before and after calcination are in our earlier papers.17, 34 Our present work is based on a further understanding of this concept of pore template elimination with the clear objective of developing potentially useful hollow particles. We try to answer the following questions: (a) if the process of initial silica nucleation at the external surface of the droplet is followed by progress of the moving front towards the interior, then can limiting the precursor level in the droplet lead to control of silica shell thickness? (b) How thin can the shell be made, and if made sufficiently thin, can it become susceptible to application of external stimuli such as ultrasound and shatter? (c) Most importantly, if the shells are dense and essentially nonporous, can they be translated into mesoporous shells thus allowing entry and reaction within the shell? If these questions can be addressed, the applicability of these new classes of materials can be significantly expanded to include stimuli-triggered release and the generation of materials conducive to reaction and catalysis. 6 ACS Paragon Plus Environment

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Thus, we show the development of a simple and efficient aerosol based process to synthesize hollow silica particles with a dense ultrathin shell of thickness approximately 10 nm that can be disrupted through the application of ultrasound. Of relevance to application in chemical reaction, we show that the dense shell can be converted to an ordered mesoporous shell to generate reactive functionality to the encapsulated material (typically a catalytic metal or metal oxide) and prove this through the model reaction of the reductive dechlorination of an environmental contaminant such as trichloroethylene using zerovalent iron nanoparticles sequestered within the pores.36-38 The technology described therefore is a “bottle around a ship” method to generate protective shells of a support material around encapsulated metals and metal oxides particles.

Materials and Methods Materials: All chemicals were purchased from Sigma-Aldrich and used as received: Iron chloride hexahydrate (FeCl3. 6H2O, 97%, ACS reagent), hexadecyltrimethylammonium bromide (CTAB, 95%), tetraethyl orthosilicate (TEOS, 98%, reagent grade), hydrochloric acid (HCl, 37%, ACS reagent), sodium hydroxide (NaOH, ≥97%, pellets, ACS reagent), trichloroethylene (TCE, C2HCl3, ≥99.0%, reagent grade). Deionized (DI) water produced from an Elga water purification system (Medica DV25) to a resistance of 18.2 MΩ was used in all experiments.

Preparation of silica particles: In a typical synthesis, 0.95 g of FeCl3 was first dissolved in 15 mL ethanol (95% v/v) followed by the addition of 1.1 g of cetyltrimethylammonium bromide (CTAB). Then various amounts of TEOS (10, 6, 4.5 and 2 mL) were added to the above solution under vigorous stirring at room temperature. 1.8 mL of a 0.1M HCl solution was also added to the solution after 3 min stirring. The resulting aerosol precursor solution was aged for 30 min and was then atomized to form aerosol droplets, which were then sent through the 7 ACS Paragon Plus Environment


dying zone and heating zone of a quartz tube, as shown in Figure 1a. The flow rate of the carrier gas was approximately 2.5 L/min, and the thermal treatment was carried out in a 100 cm length tube (internal diameter of 4.45 cm) with a furnace length of 38 cm, leading to a superficial velocity of 2.6 cm/s and a residence time of around 15 s. The temperature of the heating zone was held at 400 ºC and the resulting particles were collected by a filter system maintained at 80 ºC. The as-synthesized particles were then calcined at 500 ºC for 3 h. Silica particles were also synthesized without addition of salt. The quantity of ethanol (15 mL), CTAB (1.1g), TEOS (4.5 mL) and 0.1 M HCl was the same for all experiments.

Static Light Scattering (SLS): Static light scattering experiments were carried out at 25 °C using the Simultaneous Multiple Sample Light Scattering (SMSLS) instrument using a 35 mW diode laser emitting vertically polarized light at a wavelength of 660 nm. Evolution of light scattering intensity was monitored in real time for CTAB alone and mixtures of CTAB with each salt. The light scattering experiments were carried out with the same concentrations of CTAB and FeCl3 in ethanol/0.1 M HCl used in synthesizing the particles.

Ultrasonic treatment of ultrathin hollow silica particle: The parameters of ultrasonication were set as 20 kHz (frequency) and 150 W (power output) with the sonication probe dipped into the sample solution. Each sample had 5 mg synthesized particles dispersed in 1.5 mL deionized (DI) water and was exposed to 5 min treatment in a sonic dismembrator (Fisher Scientific, model 550). The samples were then centrifuged for 10 min at 12,000 rpm. The particles were imaged using scanning and transmission electron microscopy to evaluate the rupture effect.

Dye-encapsulation experiment: 1 mg hollow silica particles were dispersed into 1 mL rhodamine solution (0.6 mg/mL) at atmospheric pressure for 48 h. The dye-loaded particles 8 ACS Paragon Plus Environment

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were then washed with DI water. Bright field and fluorescence images of these samples were examined using a confocal microscope (Leica SP2 AOBS 2004).

Electron microscopy characterization of hollow silica particles: The morphology and structures of the particles were evaluated using field emission scanning electron microscopy (SEM, Hitachi S-4700, operated at 20 kV), transmission electron microscopy (TEM), highresolution TEM (HRTEM) (JEOL 2010, operated at 200 kV) and X-ray diffraction (XRD, Siemens, D 500, using Cu Kα radiation at 1.54 Å.). The specimen for TEM examination were obtained by dispersing particles in ethanol (95%, v/v) and drops of particles suspension were added onto a copper TEM grid. The cross-section samples for SEM and TEM were prepared by embedding silica particles within resin (Embed 812) in 70 ºC for 48 h and cut by a Leica Microtome. The porosity of the particles was evaluated by the nitrogen sorption technique at 77K (Micromeritics, ASAP 2010). The surface area was calculated using the BrunauerEmmett-Teller (BET) method.

Model Reaction System Dechlorination reaction was tested in batch experiments. Specifically, 0.08 g of particles were dispersed in 20 ml of water and placed in a 40 ml reaction vial equipped with a Mininert valve. 15 µl of a TCE stock solution (20 g/L TCE in methanol) was spiked into this vial, giving an initial TCE concentration of 15 ppm. The reactions were monitored through headspace analysis38 using a HP 6890 gas chromatograph (GC) equipped with a J&W Scientific capillary column (30 m x 0.32 mm) and a flame ionization detector (FID). The sample was injected splitless at 220 oC. The oven temperature was initially held at 75 oC for 2 min, ramped to 150 oC at a rate of 25 oC /min and then maintained at 150 oC for 8 min to ensure adequate peak separation.



Results and Discussion Evidence of CTAB Template Disruption While the proposed mechanism shown in Figure 2 is primarily based on observation

Figure 3. Evolution of normalized light scattering intensity with time and TEM images of particles obtained for systems containing CTAB alone (a), FeCl3 alone (b), and CTAB and FeCl3 (c), in an ethanol/0.1 M HCl solvent. The scale bars are 20 nm in all images. The hollow particles of (c) are obtained with the combination of CTAB and FeCl3 in the precursor solution. The normalized scattering is the ratio of the scattering at t to the scattering at t=0, so that all systems start with a scattering intensity of unity.

of the final structures formed,17, 34 indirect evidence of the aggregation due to salt bridging can be obtained through turbidity measurements of the precursor solutions. Thus, Figure 3 shows 90o scattering intensities of the precursor solutions: (a) in the absence of FeCl3; (b) in the absence of CTAB; and (c) in the presence of both CTAB and FeCl3. We note that the 10 ACS Paragon Plus Environment

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scattering is done at ambient conditions where the growth of silica is extremely slow over the 25 minutes of data collection. The TEM images of Figure 3 correspond to the morphological characteristics of particles formed through high temperature aerosolization of the systems defined in (a), (b) and (c) respectively. In case (a) mesoporous silicas are formed through the aerosol process through templating by CTAB, in case (b) the silica particles are dense due to the lack of the templating agent, while in case (c) the silica particles obtained have hollow structures after calcination. In both (a) and (b) the 90o scattering intensity at ambient conditions indicates negligible formation of any precursor aggregates. On the other hand, Figure 3 indicates a rapid increase in turbidity for case (c) and is evidence that aggregates are formed in the presence of FeCl3 which we attribute as being due to salt bridging between FeCl3 and CTAB. Since the precursor solution is intensely stirred, the aggregates do not precipitate out but exit through the nozzle and are present in the aerosol droplets during silica formation during passage through the furnace. We propose that the presence of such aggregates in the droplet results in a loss of surfactant templating of silica. The external surface of the droplet is in direct contact with the heated gas flow and silica condensation first occurs on the external surface. The lack of templating leads to the formation of dense silica starting from the surface of the droplet, but the progress of the silica internally is impeded by the aggregates in the droplet which are displaced towards the center of the droplet. Fabrication of Ultrathin-Shelled Particles To understand the effect of TEOS loading on silica shell thickness and to control shell thickness, hollow silica particles were made with progressively lower silica precursor concentrations. Figure 4 (panel a) illustrates the TEM of the reference particles clearly showing the generation of hollow submicron spherical particles with diameter of 100 - 1000 nm. The dark dots in the particles are iron oxide crystals, which are primarily hematite.17, 34



Figure 4. Representative electron micrographs of calcined hollow silica microspheres: (a) low magnification TEM image at Si:Fe ratio of 12.9:1; (b) low mganification TEM image at Si:Fe ratio of 7.8:1; (c) low magnification TEM image at Si:Fe ratio of 5.8:1; (d) low magnification TEM image at Si:Fe ratio of 2.6:1; HRTEM of calcined microspheres (~250 nm in diameter) at Si:Fe ratio of (e) 12.9:1 (black box in panel a showing the location) and (f) 2.6:1 (black box in panel d showing the location) respectively. In all cases, the CTAB and FeCl3 concentrations were kept constant to maintain the loss of templating.

Although the particles are polydisperse in size, an intrinsic aspect of the aerosol process, it is seen that virtually all particles are hollow with a relatively thin shell. Figures 4a through 4d illustrate the effect of reducing TEOS concentration, on the silica shell thickness. In each case, the amount of CTAB and FeCl3 in the precursor solution was maintained constant to retain the loss of templating and thus to generate hollow particles. The data are therefore reported as the Si to Fe atomic ratio in the feed (TEOS to FeCl3 ratios) with the 12 ACS Paragon Plus Environment

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lower ratios indicating the reduction of TEOS while the FeCl3 is kept constant. As observed, the shell thickness of the hollow silica particles decreases with the reduction of TEOS concentration in the precursor solution (Figure 4a, 4b, 4c and 4d). Figure 4d shows the system with the thinnest shells we are able to produce, and a further reduction in TEOS concentration leads to capillary collapse and ill-defined particles (not shown). Indeed, even with the lowest TEOS loading (Si:Fe ratio of 2.6:1), the rupture of a small amount of particles is seen perhaps as a result of shell breakage during calcination. We attribute this to the fact that the ultrathin silica shells of some particles cannot maintain their structures due to the internal pressures that are generated through solvent volatilization, and the decomposition of surfactant CTAB during calcination.17 High resolution TEM images (Figures 4e and 4f) show the silica wall characteristics of particles prepared with the highest and lowest TEOS loadings. Supporting Information Figure S1 provides TEM images of single hollow silica particles made from various TEOS loadings with comparable size (~260 nm in diameter, Figures S1 a-d), indicating a variation of shell thickness from 30 nm to 10 nm. The reduction of shell thickness with lower TEOS level confirms the mechanism of silica formation in an aerosol droplet as an initial layer on the external surface of the droplet which moves inward as the drop progresses through the furnace. The shell stops growing as the droplet becomes depleted of the TEOS precursor. Silica hollow particles prepared at various Si:Fe ratios (12.9:1 to 2.6:1) were subjected to sonication (20 kHz, 150W for 5 mins) to evaluate their rupture properties. Details of the sonication induced rupturing are shown in Figure 5.



Figure 5. Representative electron micrographs of calcined hollow silica particles after ultrasonication treatments. Thick-shelled particles at an Si:Fe ratio of 12.9:1 retain structural integrity while reducing the silica precursor to an Si:Fe ratio of 2.6 leads to thin-shelled particles that are broken by ultrasound. (a) SEM image at low magnification with Si:Fe ratio of 12.9:1; (b) SEM image at high magnification with Si:Fe ratio of 12.9:1; (c) TEM image of a particle with Si:Fe ratio of 12.9:1; (d) SEM image at low magnification with Si:Fe ratio of 2.6:1; (e) SEM image at high magnification with Si:Fe ratio of 2.6:1; (f) TEM image of a microsphere with Si:Fe ratio of 2.6:1.

The top set of panels (Figures 5a, 5b and 5c) represent the particles with thicker shells prepared at the Si:Fe ratio of 12.9:1, and clearly show the intactness of the particles after the sonication. The bottom set of panels (Figures 5d, 5e and 5f) show the ruptured morphologies of the ultrathin shelled silica particles ((Si:Fe ratio = 2.6:1), illustrating that below a critical shell thickness, the particles become susceptible to the cavitation induced by ultrasound.

Transformation from Dense Shells to Ordered Mesoporous Shells


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The loss of templating implies that the shells of the hollow particles are dense and intrinsically nonporous as shown in the TEM image of Figure 6a indicating a well defined shell.

Figure 6. (a) TEM and HRTEM images of thin-shell hollow particles. The shell is relatively dense and the thickness of the shell is around 20 nm. (b) TEM and HRTEM images of treated hollow particles (0.1 M NaOH + 0.05M CTAB, 80 oC, 24 h). The treated hollow particles maintain the spherical shape and the shell of the particles are porous. (c) Nitrogen adsorption-desorption isotherms at 77 K for dense shell hollow particles (black curve, BET surface area = 3.8 m²/g) and mesoporous shell hollow particles (red curve, BET surface area = 379.9 m²/g). (d) XRD pattern of dense shell hollow particles (black curve) and mesoporous shell hollow particles (red curve). The pore size of mesoporous shell hollow particles is calculated to be 3.1 nm.

N2 sorption (Figure 6c) indicates that nonporous silica particles have an extremely low surface area (3.8 m2/g), clearly indicating the lack of shell porosity. Such particles, while 15 ACS Paragon Plus Environment


potentially useful for specific applications as lightweight materials, magnetic materials, or burst release of encapsulates through ultrasonication, are incapable of transporting chemical species across the shell for applications in catalysis. To generate porosity in the shell we have used a novel procedure of surfactant templated desilication and recrystallization, typically applied to generate mesoporosity in zeolite crystals.39-41

The anionic silica species rearrange

around the cationic surfactant CTAB micelles which serves as the template for MCM-41 formation. Such local desilication and re-assembly processes occurs simultaneously to define the mesoporous structure of the shell. The process was first described by Garcia-Martinez and coworkers39 in introducing mesoporosity into zeolite crystals and the role of CTAB is considered critical to prevent loss of the soluble silicate species. We have adapted this procedure to introduce meosporosity into the shell of the hollow particles. The pH of the mixture was measured to be 12.7. From a conceptual perspective, the silica shell is gradually etched by NaOH solution to dissolve silicate species. The adsorption of the cationic surfactant on the anionic silica surface together with rearrangement of the silicate species around the CTAB template results in the generation of porosity and the formation of an ordered structure. The simultaneous occurrence of the desilication and re-assembly processes defines the mesoporous structure of the shell. Subsequent removal of surfactant through calcination results in ordered mesoporous shells. In a typical procedure, 0.31 g of hollow particles were dispersed in 15.5 ml of 0.1 M NaOH solution containing 0.282 g of CTAB (0.05 M). The mixture was stirred and heated at 80 ºC. After 24 h, the mixture was collected and filtered with D.I. water. The solid sample was washed several times and then dried into powders. The dried product was then calcined at 550 ºC for 3 h to remove the occluded templates. Figure 6b shows the TEM images of silica particles after conversion of the dense shells to porous shells. Figure 6c (red curve) indicates the significant increase in N2 adsorption capacity exhibited by the silica hollow particles with a porous shell. As Figure 6c 16 ACS Paragon Plus Environment

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indicates, the isotherm representing the porous shells of hollow particles exhibits a combination of types II and IV with a hysteresis loop in the range of 0.4-1.0 P/P0. The high adsorbed volume at the medium relative pressure (0.4-0.7) indicates the presence of mesopores in the particles.42 The BET surface area of the porous hollow particles is 379.9 m²/g and is evidence of the structural transition of the shells to mesoporosity through desilication and recrystallization. The XRD pattern shown in Figure 6d indicates the formation of ordered mesopores typical of MCM-41 through the Bragg peak at 2θ =2.85o corresponding to 3.1 nm spacings of the (100) planes with the secondary broad band at 2θ =4.3o indicative of the (110) plane of the hexagonal pattern. Supporting Information Figure S2 shows wide-angle XRDs patterns of the silica particles that confirm the presence of well-defined α-Fe2O3 crystals within the particle. The hematite nanoparticles are generated from the FeCl3 in the precursor which becomes occluded in the interior of the hollow particles and is oxidized during calcination. Crystallites of the hematite nanoparticles are also visualized in the TEM images of the hollow particles (Figures 4 and 6). We note that there is clearly a degree of microporosity in the dense shells which allows the products of CTAB burnoff (CO2 and H2O) to exit the particles. Supporting Information Figure S3 provides information on the ability of the mesoporous shells to allow entry of a fluorescent dye (rhodamine) while the dense shells are impervious to the dye. Finally, we note that the α-Fe2O3 rattle type particles in both the dense-shelled particles and the mesoporous-shelled particles can be converted to magnetic zerovalent iron through reduction with H2 making the particles magnetically responsive. Supporting information S4 details the magnetization characteristics and the response to the external field of a bar magnet.



Translation to Application The reductive dechlorination of chlorinated compounds such as trichloroethylene (TCE) using zerovalent iron is considered an environmentally benign way of decontamination of such widespread soil and groundwater contaminants.43-45 The overall redox reaction is written as C2HCl3+ 4 Fe0 +5 H+ → C2H6+ 4 Fe2++ 3 Cland it is noted that the zerovalent iron (ZVI) is not a catalyst but participates in the redox reaction being oxidized to Fe2+. During in-situ remediation of TCE, large amounts of Fe0 need to be injected into groundwater and must transport through groundwater to break down the contaminant. However, the ferromagnetism of ZVI leads to particle aggregation and the aggregates are not capable of moving with contaminant plumes.46-48 Our method of encapsulating ZVI in mesoporous shells prevents such aggregation and it is hoped will eventually lead to the development of engineered particles for in-situ remediation. Our objective at this stage of the research is to demonstrate that the hollow particles with mesoporous shells can be used as a vehicle for reaction using the iron species encapsulated in the hollow voids. To convert the iron oxides in the hollow particles to ZVI, the particles were reduced in hydrogen at 550 ºC for 2 hours.49 The particles were placed in a ceramic boat in a tube furnace which was purged with H2 for 1 hour before elevating the temperature. The sample was heated from room temperature to 550 ºC at a ramping rate of 5 ºC/min. Supporting information Figure S2 shows XRD patterns for particles with dense shells and particles with mesoporous shells indicating that both systems undergo the transformation from hematite to ZVI. It is interesting to note, that small molecules (H2) can therefore enter the particles through the dense shells.


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Figure 7. TCE removal from solution and gas product evolution rates for HP-ZVI composite (a) and MesoHP-ZVI composite (b). M/Mo is the fraction of the original TCE remaining and P/Pf is the ratio of the gas product peak to the gas product peak at the end of reaction. Only 30% of TCE is dechlorinated by the HP-ZVI composite, while the MesoHP-ZVI composite dechlorinates TCE completely in 4 hours.



Figure 7 illustrates the comparison of the removal of TCE from solution by the reduced dense-shelled hollow particles (HP-ZVI) (Figure 7a) and the mesoporous shelled hollow particles (MesoHP-ZVI) (Figure 7b), indicating both removal of TCE and evolution of the gaseous product. The removal of TCE is completed in 4 hours with ethane and ethylene as the major products. A pseudo first order initial rate constant km of 1.55 L g-1h-1 can be calculated based on the mass of zero-valent iron.50 In contrast, the dechlorination reactivity of

Figure 8: Representative GC trace of headspace analyses showing TCE degradation and reaction product evolution at various reaction time for HP-ZVI composite (a) and MesoHP-ZVI composite (b). Only 30% of TCE is dechlorinated by the hollow particles with ZVI nanoparticles. 20 ACS Paragon Plus Environment

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the reduced hollow particles indicates that just 30% of TCE is reacted away over an extended period. The presence of a low level of reactivity may be due to iron nanoparticles embedded on the dense shell allowing a degree of accessibility to TCE. However, the absence of shell porosity blocks the zero-valent iron nanoparticles in the interior from reacting with TCE in solution. Opening up the shell through the desilication and reassembly process allows entry of TCE and efficient reaction. Figure 8 illustrates the time scales of reaction for both type of particles. We also note the minimal amount of intermediates (the dichloroethenes, vinyl chloride) that are formed. In translating the development of the hollow shelled particles towards this application in environmental decontamination, we have only considered reactivity aspects based on demonstration of the transition from a dense shell to a mesoporous shell. In fully developing technology for the in-situ remediation of TCE it is necessary to consider aspects of colloidal stability and transport through groundwater saturated sediments. These are objectives of our continuing study to completely define the application.



Conclusions The interaction of iron salts (FeCl3) and a cationic surfactant (CTAB) in solution has been exploited for the generation of hollow morphologies of silica particles synthesized through an aerosol-based process. Additionally, it is possible to control the thickness of the shell and ultrathin hollow silica particles can be synthesized using a reduced TEOS loading in the precursor solution. With thin shells, these are extremely lightweight particles and are intrinsically buoyant. Since the shells are made of silica, they can be easily functionalized for a variety of applications as magnetically responsive adsorbents. Furthermore, these particles can be used in particle-stabilized emulsions (Pickering emulsions) with a magnetically responsive component, and the surface can be decorated with catalytic nanclusters. The separation of a catalytic functionality on the surface of the particle from the magnetic functionality of the interior may be easily exploited for reaction and magnetic separation where the particles can be recycled. Such hollow particles with ultrathin shells can be easily ruptured by ultrasonication making them a promising material for focused intensity ultrasound triggered release. This may be viable in systems needing long term storage of catalytic materials in the reduced state or in applications requiring a sudden loss of buoyancy. While the shells are initially dense and only allow entry to very small molecules (e.g. H2), they can be converted to mesoporous structures through desilication and reassembly, using CTAB as the template. The access to the iron species allows metal induced chemical reactivity and the reductive dechlorination of trichloroethylene has been used as a model reaction to validate the concept. The reaction is effective and the encapsulation of zerovalent iron in these particles provides a method to reduce aggregation of the ferromagnetic species. Such engineered systems therefore have potential in the in-situ remediation of chlorinated compounds. There are several other applications of such systems that our research is currently addressing. We note that a 10-nm shell implies that hollow particles exclusively made of 22 ACS Paragon Plus Environment

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silica will be buoyant when the particle size is greater than 140 nm (the ratio of the inner radius to the outer radius has to be greater than 0.85 for buoyancy of hollow silica particles with density 2.65 g/ml). Such particle sizes are readily attainable as shown in Figure 4d, although the presence of the iron oxides adds to the mass of the material and reduces buoyancy. The incorporation of functional groups on the silica surface will additionally enhance colloidal stability. Our exploratory research indicates that FeCl3 appears to be the most effective salt to induce salt-bridging and the synthesis of hollow particles, but other metal species can be co-encapsulated. With transformation of the dense shell to a mesoporous shell, this opens a number of applications in iron-based and multi-metal catalysis. The work therefore represents a bottle-around-a-ship method to encapsulate materials within mesoporous thin-shelled hollow particles.

Supporting Information: Variation of the hollow silica particles shell thickness with TEOS loading (S1), X-ray diffraction characteristics of the dense shell hollow particles and mesoporous shell hollow particles before and after H2 reduction (S2), dye-encapsulation characteristics of the dense shell hollow particles and mesoporous shell hollow particles (S3), the magnetization characteristics of both reduced hollow particles and mesoporous hollow particles (S4).

Acknowledgements Funding from the National Science Foundation (Grant 1236089) is gratefully acknowledged.



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