Article pubs.acs.org/Langmuir
Crystallization at Nanodroplet Interfaces in Emulsion Systems: A Soft-Template Strategy for Preparing Porous and Hollow Nanoparticles H. Samet Varol,† Olaia Á lvarez-Bermúdez,†,⊥ Paolo Dolcet,‡ Balati Kuerbanjiang,§,# Silvia Gross,‡,∥ Katharina Landfester,† and Rafael Muñoz-Espí*,†,⊥ †
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Via Marzolo 1, 35131 Padova, Italy § Institut für Mikro- und Nanomaterialien, Universität Ulm, Albert-Einstein-Allee 47, 89081 Ulm, Germany ∥ Istituto di Chimica della Materia Condensata e di Tecnologie per l’Energia − CNR (ICMATE-CNR), Via Marzolo 1, 35131 Padova, Italy ⊥ Institut de Ciència dels Materials, Universitat de València, carrer Catedràtic José Beltrán 2, Paterna, 46980 València, Spain ‡
S Supporting Information *
ABSTRACT: A heterophase method to prepare hollow and/or porous crystalline nanoparticles of metal oxides at room temperature is presented, taking cerium(IV) oxide and γ-iron(III) oxide (i.e., maghemite) as representative cases. The crystallization begins at the oil−water interface in aqueous nanodroplets of the precursor in inverse (water-in-oil) miniemulsion systems, and it may continue toward the inner part of the droplets. A poly(styrene-b-acrylic acid) block copolymer is used as a structuring agent because the ability of the carboxylic groups to bind metal ions improves the inorganic shell formation. A precipitating base is added from the continuous phase, generating hydroxide species at the interface that begin the crystallization. We analyze the effects of the synthetic parameters in terms of colloidal stability and morphology of the resulting materials. In the case of maghemite samples, the prepared dispersions of hollow particles present a distinct magnetofluidic behavior.
1. INTRODUCTION The relationship between functional and geometrical properties plays an important role in many applications of micro- and nanostructured materials.1 The hierarchical assembly of building units offers great opportunities for the spontaneous formation of hollow porous structures in the nanometric range, which can be formed from both organic and inorganic species; the latter being especially important from a functional perspective of the final materials (e.g., magnetic, electrical, thermal, optical, or catalytic properties).2−6 Compared with solid particles, inorganic hollow spheres have a lower density, high surface-to-volume ratio, and low thermal expansion coefficients, which can be advantageous in applications such as biomedicine,7 treatment of waste water,8 lithium-ion batteries,9 catalysis,10−12 and sensing.13 They can also serve as nanocarriers to transport and release encapsulated substances.7,14,15 Controlled synthesis of hollow materials can be carried out by the following two general approaches: (i) templated methods, which include hard-template and soft-template routes, and (ii) non-templated (or template-free) methods. The hard-template route is the most common approach to form micro- and nanosized capsules.2,15,16 However, the removal of the hard © 2016 American Chemical Society
(solid) template is a complicated and energy-consuming process, and it can generate structural degradation of the nanoparticles. The sof t-template route is therefore a valuable alternative to obtain hollow particles by using surface-active agents in heterophase systems. Emulsions (especially mini- and microemulsions) are very convenient systems to produce inorganic structures, in the presence of amphiphilic surfactants with the ability to self-assemble into organized structures.17,18 In recent years, emulsion systems have been used to synthesize a large variety of inorganic particles, commonly solid particles.19−25 One step further, by using soft-template routes, our team demonstrated that sol−gel processes can occur at the droplet interface in miniemulsion systems. In this way, we were able to prepare hydrous zirconia and hafnia26 and YCrO327 hollow nanoparticles. However, the resulting materials were amorphous and could only be converted to crystalline materials upon calcination. More recently, we presented the first results on the preparation of crystalline nanostructures of copper(II) oxide by using a similar synthetic pathway,10 but the strategy Received: August 8, 2016 Revised: October 13, 2016 Published: November 5, 2016 13116
DOI: 10.1021/acs.langmuir.6b02954 Langmuir 2016, 32, 13116−13123
Article
Langmuir
2.2. Preparation of Ceria and Iron Oxide Porous and Hollow Particles. Ceria samples were prepared by inverse miniemulsion with the precursors and surfactants indicated in Table 1. A dispersed phase
was quite specific for the case of copper and required a specific interaction of this metal with amino-containing surfactants. Furthermore, although lower with respect to previous work, temperatures well above room temperature (about 80 °C) were required. In this context, it has remained a challenge for us to achieve the preparation of crystalline metal oxide hollow nanoparticles with a more compact shell at temperatures close to room temperature. In the present work, we address this challenge by taking two different metal oxides as model cases, namely, cerium(IV) oxide and maghemite (i.e., γ-iron(III) oxide). Cerium(IV) oxide (or ceria) is a metal oxide with high industrial potential because of its oxygen storage capacity, oxygen deficiency, and electronic conductivity. These features are useful, for instance, in fields such as heterogeneous catalysis28−31 and chemical sensing.32,33 Morphological and microstructural differences have a great influence on the final performance of the materials. Iron oxides of different compositions (mainly magnetite [Fe3O4] and maghemite [γ-Fe2O3]) are used in a wide range of applications, including biotechnology, medicine, electromagnetic devices, and magnetoelectronics.34,35 Many material scientists working in the field of nanoparticles are nowadays interested in the dependence of properties such as superparamagnetism or catalytic activity on morphology and structure. Gedanken and co-workers36 reported more than a decade ago the preparation of porous Fe2O3 particles by a sonochemical method and demonstrated their good catalytic properties in the oxidation of cyclohexane. Li et al.37 also synthesized Fe2O3 hollow particles with photocatalytic activity by a nontemplated route involving a hydrothermal process. A significant part of the research in biomedicine has focused on the integration of superparamagnetic hollow structures of iron oxides for high-capacity drug loading and targeted drug delivery. In this sense, Cao et al.38 reported a precursortemplated conversion method to synthesize highly magnetic and biocompatible hollow spheres, assembled to Fe3O4 and γFe2O3 nanosheets. In spite of the achievements in recent years, the development of a simple and effective synthetic method to obtain superparamagnetic nanocapsules at low temperatures is still an open challenge. Hence, soft-template pathways have priority among other methods. Herein, we study the precipitation of transition metal and lanthanoid oxides at the droplet interface in water-in-oil miniemulsion systems. We report a very facile and versatile soft-template method to produce hollow nanoparticles of cerium(IV) oxide and iron(III) oxide (maghemite) in the nanometric scale at a temperature as low as room temperature. We analyze thereby the effects of the synthetic parameters, including the type of surfactant and the precursor used.
Table 1. Characteristics of the Samples Presented in This Work sample no. C1 C2 C3 C5 C7
F1 F2 F3
F4
F5 F6
surfactant(s)
a
PIBSP (1 wt %) PGPR (1 wt %) S50-b-AA12 (1 wt %) PGPR (1 wt %) + S50b-AA12 (0.5 wt %) PIBSP (1 wt %) + S50-b-AA12 (0.5 wt %) PIBSP (1 wt %) PGPR (1 wt %) PIBSP (1 wt %) + S50-b-AA12 (0.5 wt %) PGPR (1 wt %) + S50-b-AA12 (0.5 wt %) PGPR (1 wt %) PGPR (1 wt %) + S50-b-AA12 (0.5 wt %)
particle size (nm)c
organic content (wt %)d
Ce(III) Ce(III) Ce(III) Ce(III)
n.d. 120 ± 50 190 ± 70 70 ± 30
17.4 14.0 14.2 13.6
Ce(III)
120 ± 30
n.d.
Fe(II)/Fe(III) Fe(II)/Fe(III) Fe(II)/Fe(III)
n.d. n.d. 120 ± 50
n.d. 29.4 28.5
Fe(II)/Fe(III)
70 ± 30
28.1
Fe(II) Fe(II)
n.d. 50 ± 20
19.0 17.9
precursor
b
a
Amounts of surfactants indicated as a weight percent of the continuous phase. bThe dispersed phase was an aqueous solution of the precursor with a total concentration of 1.3 M for all cases. The molar ratio of Fe2+/Fe3+ for samples F1−F4 was 3:2. cFrom statistical analysis of TEM micrographs. dExperimentally determined by TGA (see Figure S1). n.d.: not determined. was prepared by dissolving Ce(NO3)3·6H2O (2.6 mmol, 1.129 g) in water (2.0 g) and mixed with a continuous phase containing the polymeric surfactant(s) (PIBSP, S50-b-AA12 and/or PGPR) dissolved in toluene (8.0 g). The mixture of the two phases was pre-emulsified by stirring with a magnetic stirrer for 1 h and emulsified subsequently by ultrasonication (Branson Sonifier W450 digital, 1/2″ tip, 4 min at 70% amplitude with a pulse of 1.0 s and a pause of 0.1 s) while cooling down in an ice−water bath. The emulsion was further stirred for 1 h after ultrasonication. Afterward, triethylamine (1088 μL, 3 equiv with respect to the Ce3+ precursor) was added to the emulsion under constant stirring. Iron oxide nanocapsules were prepared in an analogous way by dissolving the appropriate iron salt (FeCl3·6H2O, anhydrous FeCl2, or a mixture of FeCl3·6H2O and anhydrous FeCl2) in water (2.0 g) using the precursors and surfactants indicated in Table 1. The same emulsification conditions were used as in the case of ceria, but a shaker (and not a magnetic stirrer, to avoid the effect of the magnetic field in the forming crystals) was used in the experiments. The triethylamine amount was also the same (i.e., 1088 μL, 3 equiv with respect to the iron precursor). 2.3. Characterization Methods. X-ray diffraction (XRD) measurements were recorded in a Philips PW 1820 diffractometer by using Cu Kα radiation (λ = 1.5418 Å) in the range 2θ = 0−70°. For the measurement, the samples were washed with acetone, centrifuged (3000 rpm, 5 min), and dried in a vacuum oven at room temperature for 12−14 h. Scanning electron microcopy (SEM) images were taken by using a LEO 1530 Gemini microscope at voltages below 1 kV. A Zeiss EM912, a JEOL JEM 1400, and a FEI Tecnai F20 were alternatively used for capturing the transmission electron microscopy (TEM) images. Energy-dispersive X-ray (EDX) spectroscopy analysis was performed in a Hitachi SU8000 scanning electron microscope. Electron microscopy samples were prepared by diluting the resulting
2. EXPERIMENTAL SECTION 2.1. Materials. Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, Fluka, p.a., ≥99.0%), iron(III) chloride hexahydrate (FeCl3·6H2O, Sigma-Aldrich, puriss. p.a., ≥99.0%), anhydrous iron(II) chloride (FeCl2, Sigma-Aldrich, ≥98%), toluene (Sigma-Aldrich, ≥99.7%), the commercial polyisobutylene succinimide pentamine OS85737 (PIBSP, Lubrizol France), poly(styrene-block-acrylic acid) with a chain length ratio of 50:12 referring to the ratio of styrene to acrylic acid chain length (abbreviated as S50-b-AA12, ATRP Solutions, USA; PDI = 1.16, Mn = 6100 g/mol), polyglycerol polyricinoleate (PGPR, Danisco), and triethylamine (Et3N, Fluka, ≥99.5%) were used as received without further purification. Deionized water was used throughout the experiments. 13117
DOI: 10.1021/acs.langmuir.6b02954 Langmuir 2016, 32, 13116−13123
Article
Langmuir dispersions in toluene and drop-casting on a silicon wafer or on carbon-coated copper grids for SEM and TEM measurements. After the drop-casting, some of the silicon wafers were further calcinated at 600 °C (from room temperature to 600 °C with a heating rate of 5 °C/min; plateau of 1 h at 600 °C) to evaluate the morphology of the inorganic particles after removing all organic contents. Droplet and particle sizes were analyzed by dynamic light scattering at 90° in a Malvern Zetasizer Nano-Z instrument. Particle size data were typically obtained by measuring the sizes of at least 200 particles from corresponding TEM micrographs with the help of the processing software Fiji/ImageJ.39 Magnetic measurements were carried out using a vibrating sample magnetometer (VSM, Lakeshore). All of the samples were measured under ambient conditions in the powder form after washing. Thermogravimetric analysis (TGA) was carried out in a Mettler ThermoSTAR TGA/SDTA 851 thermobalance under a nitrogen atmosphere, in the range from room temperature to 1000 °C at 10 °C/min. The measurements were recorded in powder form after washing. X-ray photoelectron spectroscopy (XPS) of samples in the powder form was carried out with a Perkin-Elmer Φ 5600-ci instrument using standard Al Kα radiation (1486.6 eV) operating at 250 W. The working pressure was
