www.advmat.de
www.MaterialsViews.com
Communication
General and Controllable Synthesis of Novel Mesoporous Magnetic Iron Oxide@Carbon Encapsulates for Efficient Arsenic Removal Zhangxiong Wu, Wei Li, Paul A. Webley, and Dongyuan Zhao* Mesoporous carbon materials with interpenetrated and regular mesopore systems have recently triggered enormous research activities[1–3] because of their fascinating features such as high specific surface area, tunable mesostructure and pore size, good chemical and thermal stability, and intrinsic high electrical conductivity. While they possess great potential in adsorption and separation, and energy storage and conversion,[4,5] it is more intriguing to explore the possibility of enhancing and/or extending their properties by the formation of nanocomposites so that targeted applications can be designed based on synergic and cooperative effects between the carbon mesostructures and the well-dispersed active nanoparticles. Nanoparticles with a wide range of compositions can be readily introduced into mesoporous carbon matrices through either a direct synthesis approach or a post loading method.[6–13] Normally, the nanoparticles can be well dispersed and/or confined in the case of a low metal content and/or a low converting temperature leading to a poor crystallinity. However, at a high metal loading level and/ or a high converting temperature, the metal species normally aggregate severely into large particles. It is extremely challenging to fix highly concentrated (e.g., >20 wt%) and uniformly dispersed crystalline nanoparticles into predefined mesopores of carbons without aggregating and blocking the open pore networks. For example, mesoporous carbon materials incorporated with magnetic nanoparticles are of intensive scientific and technological interest.[14] However, most reports show randomly dispersed and/or mesopore-wall embedded or even aggregated large nanoparticles.[15–20] It is fairly difficult to obtain uniformly dispersed crystalline nanoparticles with a high metal content. Such disadvantages are largely a result of a lack of an efficient control to avoid substantial diffusion/aggregation of the metal precursors during their conversion into oxides, which is especially challenging in the case of high loading levels. Moreover, little attention has been paid to simultaneously exploring and utilizing the high mesoporosities of carbon matrices and the confined populated nanoparticles as active components for adsorption and separation. This is because the metal species (e.g., iron oxides) are unlikely to be quite accessible as active Z. X. Wu, Prof. P. A. Webley, Prof. D. Y. Zhao Department of Chemical Engineering Faculty of Engineering Monash University Melbourne, VIC 3800, Australia E-mail:
[email protected] Z. X. Wu, W. Li, Prof. D. Y. Zhao Department of Chemistry and Laboratory of Advanced Materials Fudan University Shanghai, 200433, P. R. China
DOI: 10.1002/adma.201103789 Adv. Mater. 2012, 24, 485–491
components for adsorption because of their enclosure by carbon and possible pore blockage by the nanoparticles resulting from high-temperature carbonization and insufficient pore-wall connections. Recently, magnetic iron oxide nanoparticles have been selectively deposited into the intratubular pores of CMK-5 with an iron content up to ∼12 wt%,[15] enough for the catalytic decomposition of ammonia but not so attractive when adopted as an active component for adsorption and separation. Therefore, it is fairly demanding to synthesize mesoporous carbonbased nanocomposites that encapsulate high-content but uniformly dispersed and spatially separated nanoparticles while retaining an open mesopore system, because such features are highly desirable for adsorption and catalysis which depend on molecular diffusion and transportation for their effectiveness. Arsenic exposure has unfortunately raised serious concern worldwide because of its extremely negative impacts on human health.[21] Arsenic, mainly arsenate AsV and arsenite AsIII, in aqueous streams must be lowered to 10 μg L−1. Adsorption is one of the most attractive approaches among the possible techniques for arsenic removal,[22–24] because of its low cost, high concentration efficiency, ease and safety for processing, and versatility for different water streams.[25–29] A variety of materials capable of removing arsenic have been reported with the porous iron-based adsorbents as the most promising ones given their high adsorption capacity and excellent selectivity.[25,28,30–35] However, there is still a need for a highly efficient arsenic adsorbent that presents large capacity, fast uptake rate, easy separation, and long cyclic stability. Herein, we report a general ammonia-atmosphere pre-hydrolysis post-synthetic route for the construction of ordered mesoporous carbon encapsulating a wide range of metal oxide nanoparticles with high concentration (>40 wt%) homogeneously dispersed in predefined mesopores. This route is demonstrated to be a facile, controllable, and versatile approach for the high loading of well-dispersed and uniform nanoparticles. With the mesoporous iron oxide@ carbon (Fe2O3@C) encapsulates as the representative, the materials obtained possess uniformly dispersed, spatially separated, and exclusively mesopore-confined nanoparticles even at a very high metal oxide content up to 52 wt%. The key concept (Scheme 1) is to selectively load nanoparticles with a very high concentration into the primary mesopores (5.6 nm) of a surfactant-templated ordered bimodal mesoporous carbon matrix[36] while leaving its connected mesopores (2.3 nm) empty to retain an open pore network such that fast molecular diffusion/transportation can be achieved. The synergic effects that combine the high and open mesoporosity with the uniformly dispersed iron oxides of high content make the materials ideal for arsenic removal with large capacities, fast adsorption rates, easy magnetic separation, and long cyclic stability. A simple surface oxidation of the bimodal mesoporous carbon matrix (Scheme 1a) by acidic ammonium persulfate gives rise
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
485
www.advmat.de
www.MaterialsViews.com
Communication
Generally, all the mesoporous Fe2O3@CT encapsulates obtained after pyrolysis at 300–600 °C possess highly ordered meso structures with open pore networks in large domains (Figure S2, Supporting Information) as well as uniformly dispersed nanoparticles across the entire mesopore system (Figure S4, Supporting Information). As the temperature increases, uniformly confined nanoparticles with increasing particle sizes are gradually observed without formation of nanowires or any aggregation or growth of large nanoparticles outside the mesopores even at higher temperatures of 500–600 °C and a high iron oxide content of 52 wt% (Figures S2 and S4, Scheme 1. Illustration of the synthesis and arsenic capture processes for the ordered mesSupporting Information). In the mesoporous oporous Fe2O3@C encapsulates: a) the bimodal mesoporous carbon, b) carbon loaded with encapsulates Fe2O3@C-300, spatially separated hydrated iron nitrate precursor, c) carbon loaded with iron hydroxide obtained by in situ hydrolysis under ammonia atmosphere, d) iron oxide@carbon composites obtained by direct nanoparticles with ultra-small particle sizes pyrolysis, e) the Fe2O3@C encapsulates obtained by pyrolysis following the pre-hydrolysis, (∼3 nm) and a very high content (∼40 wt% f) arsenic capture, and g) arsenic-enriched encapsulates. Fe2O3) are uniformly distributed across the whole ordered mesostructured domains (Figure 1a,b), indicating a quite effective and high-level loading to a hydrophilic carbon surface,[37] leading to an easy and full and fascinating confinement, which has not been previously loading of metal nitrate precursors (e.g., Fe(NO3)3·9H2O) into reported. High-magnification transmission electron microscopy the whole mesopore system without obvious aggregation outside (TEM) images (Figure 1c,d) along the [011] and [110] directions the mesopores (Scheme 1b). The precursors are first converted further reveal that the nanoparticles are spatially and selecinto hydroxides by in situ hydrolysis under ammonia atmosphere tively located in the primary mesopores of the carbon matrix (Scheme 1c),[38,39] which is a key factor, especially in the case of without formation of any nanowires. Along the pore channels, a high metal content, to localize the final nanoparticulate metal the distance between most adjacent nanoparticles is ∼3 nm. oxides (e.g., Fe2O3) exclusively inside the mesopores without transA high-resolution TEM (HRTEM) image (inset in Figure 1d) ferring to or aggregating outside the mesopores. A subsequent shows that the nanoparticles are crystallized to some extent with step of pyrolysis at 300–500 °C leads to nucleation and growth the lattices in a d-spacing of ∼2.54 Å, well-matched to the d311 of nanoparticles and release of porosity, resulting in the novel of γ-Fe2O3. Moreover, three well-resolved diffraction rings are metal oxide@carbon (e.g., Fe2O3@C-T) encapsulates with spadetected in the selected area electron diffraction (SAED) pattially separated nanoparticles uniformly and exclusively deposited tern (inset in Figure 1c), well indexed as the (311), (400), and in the major mesopores of the carbon matrix (Scheme 1e) even (440) planes of γ-Fe2O3. The uniformly dispersed nanoparticles at a very high content of metal oxide up to 40 wt% (Figures S1 in the carbon matrix are further elucidated by high-resolution and S2, Supporting Information). Even much higher contents elemental mapping (Figure S5, Supporting Information). It of metal oxides (52–70 wt%) can be achieved through multiple shows that the Fe, C, and O elements are quite uniform and loading cycles with the uniformly dispersed nature retained. Howfaithfully correlated with the ordered mesostructured domain. ever, direct pyrolysis of the as-loaded composites (conventional An increase of the pyrolysis temperature to 500 °C is accompamethod) without the pre-hydrolysis step results in a considerable nied by an obvious growth of the nanoparticles to ∼5 nm and a amount of large particles aggregating on the external surface of shortened distance (∼2 nm) between adjacent nanoparticles, but the carbon matrix especially in the case of a high metal content still uniformly and separately confined in the major mesopores and/or a high temperature of calcination (Scheme 1d, Figure S3, of the carbon matrix (Figure 1e), without the formation of any Supporting Information). Such a remarkable difference in morlarge particles outside of the mesopores (Figure S6, supporting phology originates from the low melting and boiling points of Information), indicating that the nanoparticles are exclusively the hydrated metal nitrate precursors (e.g., ∼47 and 100 °C for deposited inside the mesopore system. Surface sensitive X-ray the melting and boiling points of hydrated iron nitrates). Consephotoelectron spectra (XPS) (Figure S7, Supporting Information) quently, a considerable amount of metal precursors can transfer of the mesoporous encapsulates Fe2O3@C obtained after pyrolout of the mesopores before they are decomposed to oxides, ysis at 300–500 °C show very limited iron contents (
