Oriented Aggregation of Lepidocrocite and Impact on Surface Charge

Jul 23, 2014 - The impact of lepidocrocite (γ-FeOOH) nanoparticle aggregation on mineral surface charge development was resolved in aqueous solutions...
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Oriented Aggregation of Lepidocrocite and Impact on Surface Charge Development Philipp A. Kozin,† Germàn Salazar-Alvarez,‡ and Jean-François Boily*,† †

Department of Chemistry, Umea University, 901 87 Umeå, Sweden Materials and Environmental Chemistry, Stockholm University, 114 18 Stockholm, Sweden

‡

S Supporting Information *

ABSTRACT: The impact of lepidocrocite (γ-FeOOH) nanoparticle aggregation on mineral surface charge development was resolved in aqueous solutions of NaCl and NaClO4. Synthetic rod-like particles exhibiting charged edge (100) and neutrally/low-charged (010) faces self-aggregated in salt-free solutions. Aggregation was notably imaged by high-resolution transmission electron microscopy, and inferred by decreases in N2(g)-B.E.T. specific surface area from 94 m2/g to 77 m2/g after 12 months, and to 66 m2/g after 33 months storage. Potential determining (H+, OH−) ions loadings in the 4−11 pH range were unchanged only if the particles remained aggregated in NaCl but only if they were disaggregated in NaClO4. These differences, alongside molecular simulations and experimental ion loadings resolved in other studies from our group, point to important controls on background electrolyte ion identity on the aggregation and charge development in lepidocrocite. These results may apply further to other mineral surfaces of comparable surface (hydr)oxo populations.

1. INTRODUCTION Charge development at iron (oxy)hydroxide surfaces in aqueous media is a process arising from the complexation of potential determining ions (p.d.i.; H+, OH−) to (hydr)oxo and Fe groups.1,2 This phenomenon is of crucial importance for a large number of technological, environmental, atmospheric, and geochemical processes where iron oxides can interact with a variety of soluble compounds.3 Mineral surface charging can be considerably controlled by particle morphologies, which can define populations of outcropping (hydr)oxo functional groups, but also by surface irregularities4 and intersecting surface planes,5 which expose additional proton active (hydr)oxo groups and are loci of facilitated solvation. A vast number of synthetic and natural iron (oxy)(hydr)oxides of various sizes, morphologies, and surface roughness has been studied in the past four to five decades to understand the interfacial behavior of these important types of minerals.4,6−10 Effects of particle growth rates and of interacting salts are of particular importance in this regard. For instance, goethite (α-FeOOH) particles of different specific surface area and of contrasting charge-uptake capacities can be made by adjusting the rate of neutralization of ferric salts during the initial stages of particle synthesis. Addition of other salts, such as sodium chloride or sodium nitrate, also affects relative rates of crystal plane growth in iron (oxyhydr)oxide minerals, including lepidocrocite (γ-FeOOH)11 and hematite (αFe2O3).12 Recent developments in this field13,14 also highlighted the impact of cocrystallization and oriented aggregation of incipient © 2014 American Chemical Society

nanosized particles, a concept contrasting with the idea for continuous growth of a single particle from soluble mineralbuilding species. Such mechanisms have notably been documented for the cases of zeolites,15,16 metal oxides,17−20 sulfides,21 and even selenides.22 Oriented aggregation mechanisms from mesocrystals, consisting of self-assembled oriented particles lacking crystalline bonding material, can also lead to secondary crystals.23 Given the propensity for particle selfassembly in iron oxyhydroxides and their potentially long-term metastability, interfacial phenomena involving mesocrystal-like materials warrant further evaluation. This aspect becomes especially relevant when considering the potential impact of ion diffusion into and throughout overlapped double layers of freshly-interacted particles, and perhaps even their entrapment as secondary crystals form. In addition, while mineral surface charging is a central process to this issue, little is known on the impact of particle aggregation on p.d.i. adsorption. Synthetic lepidocrocite particles offer an opportunity in this regard as these are generally composed of coexisting crystallographic planes of strongly contrasting charge uptake capabilities. Particles of controlled shapes can be made by the forced oxidation of ferrous chloride solutions, producing rodshaped (RL) or lath-shaped (LL) particles exhibiting different proportions of the (010) plane, ideally consisting of charge neutral and proton-silent hydroxo groups, coexisting with the Received: February 6, 2014 Revised: July 12, 2014 Published: July 23, 2014 9017

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Figure 1. HRTEM and SAED (left) as well as FFT-filtered imaging (right) of rod- (RL) and lath-shaped (LL) lepidocrocite particles. SAED confirms (010) and (001) as the predominant terminations of these particles. FFT-filtered imaging reveals defects as well as antiphase boundaries in RL. Scale bars on HRTEM images on the left are 50 nm.

note that FFT-filtered imaging unveiled dislocations and antiphase boundaries along the fringes running along the ⟨001⟩ direction. HRTEM imaging and SAED (Figure 1) of LL revealed particles that are predominantly terminated by the (010) face, with minor portions of the (001) edge. These results thus revealed that two planes are prominent features in synthetic lepidocrocite particles, even on preparations dominated by the (010) face. While the (010) face remains ideally uncharged under most environmentally relevant pH conditions, due to the proton inactivity of its terminating μ−OH groups, its reactivity stems from its ability to form strong networks of hydrogen bonds with water26 as well as electrolyte ions.27 It may moreover be responsible for aggregation as its structure is identical to that of the sheets of the lepidocrocite bulk and could thus adopt the same hydrogen bonding pattern with a (010) plane of another particle as in the bulk. Particle aggregation via the (010) plane could thereby essentially extend the periodic stacking of these sheets (Figure 2), first as a mesocrystal but over time as a

highly chargeable (001) edge and (100)/(101) particle tip planes (Figure 1). In this Letter, we provide evidence showing that RL particles aggregate through the (010) plane and show how these interactions affect the particles’ ability at developing surface charge.

2. EXPERIMENTAL SECTION Details of the RL and LL synthesis procedures are reported in the Supporting Information, and are also described in previous studies from our group.24 Briefly, these procedures consist of oxidizing aqueous ferrous chloride solutions under circumneutral conditions at 25 °C. Addition of 0.2 M NaCl background electrolyte and slower oxidation rates induced the formation of LL over RL. Sample characterization methods include (1) powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy for confirmation of phase purity, (2) N2(g) adsorption isotherms for specific surface area determination according to the Brunauer− Emmett−Teller (BET) approach, as well as (3) (high resolution) transmission electron microscopy (HRTEM) for resolving particle morphology and size, and (4) selected area electron diffraction (SAED) for identifying crystallographic orientations. HRTEM and SEAD were collected with a JEOL JEM-2100 microscope with a LaB6 filament working at 200 kV ( f = 2.7 mm, Cs = 1.4 mm, Cc = 1.8 mm, point resolution = 2.5 Å, lattice resolution = 1.4 Å) equipped with a Gatan SC1000 ORIUS camera (pixel size 9 × 9 μm2, used for the acquisition of high-resolution images) and a Gatan ES500W Erlangshen camera (pixel size 6.45 × 6.45 μm2, used for the acquisition of diffractograms). The electron dose and acquisition time were optimized to minimize beam damage. High-precision potentiometric titrations RL and LL suspensions at 25 °C (Supporting Information) provided information in the charge/ potential development at the lepidocrocite particle surfaces. Thermodynamic modeling of these data was carried out using a multisite and multiplane complexation model adapted for the lepidocrocite surface recently developed in our group.25

3. RESULTS AND DISCUSSION Confirmation of the (010) and (100) planes in synthetic RL particles were made by HRTEM (Figure 1). SAED showed that the long 140−230 nm dimension of RL is parallel to the ⟨100⟩ crystallographic direction, while the short 5-15 nm dimension is bound by the (010) and (001) planes. It is also interesting to

Figure 2. Schematic representation of RL particle morphology, bulk and surface structures and composition, as well as a possible oriented assemblage mechanism. 9018

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secondary crystal as interparticle ions and water molecules are expelled from the particle interstices and/or accommodated as bulk nonstoichiometric components. It would also affect the mechanisms through which newly aggregated lepidocrocite particles react with surrounding ionic media and, quite importantly, acquire p.d.i. by the complexation of H+ to and/ or exchange of OH− with available surface (hydr)oxo groups. In an effort to address these issue, this study was devised to determine the impact of lepidocrocite particle aggregation on p.d.i. charge uptake. Freshly-made, disaggregated synthetic RL particles with an initial N2(g)-BET specific surface area of 94 m2/g were aged in a salt-free (dialyzed) aqueous solution. Particles aged for a 12 month period underwent a decrease in N2(g)-BET to 77 m2/g, while those aged for 33 months to 66 m2/g. Note that no such effects were found in LL. Estimates based on the original TEM-derived dimensions of the 94 m2/g suggest that the 12 month aged sample corresponds to a doubling of the particle width, and to a loss of about one-half of the aggregated surfaces contacted to aqueous solutions. This would also suggest that the 33 month aged sample would correspond to a 3-fold increase in width, and therefore to a loss of about two-thirds of the aggregated surface. SAED readily showed that the aggregate surfaces parallel to the carbon coating were the (001) plane, and not the (010) plane as in LL. SAED consequently provides an important piece of evidence showing that particles were predominantly aggregated by the (010) plane, thus exposing the proton-active sites of the edge (001) plane to the aggregated particle surfaces (Figures 1 and 3). Charge development at fresh and aged RL surfaces added further information along this front (Figure 4). High-precision potentiometric titration experiments of freshly disaggregated materials exchanged up to 0.14 C/m2 (1.5 μmol/m2) p.d.i. in 0.1 M NaCl between the point-of-zero-charge (pzc = 7.4) and pH 4, and up to 0.10 C/m2 (1.0 μmol/m2) between the pzc and pH 9. Both 12- and 33-month aged particles acquired the same pH dependence on p.d.i. loadings, but only when the data were normalized to their respective 77 and 66 m2/g N2(g)-BET values. Normalizing p.d.i. uptake data by the original N2(g)-BET value of 94 m2/g gave rise to, on the other hand, substantially lower p.d.i. loadings. We also note that exposing the 66 m2/g particles to pH 3.0 for a 24 h period, during which time acidic conditions could have disaggreagaged the particles, did not alter p.d.i. loadings either. Contrasting results were obtained in 0.1 M NaClO4 solutions. In this case, the particles acquired ∼40% lower p.d.i. loadings of 0.1 M NaCl solution due to the well-known smaller charge-neutralizing capability of the perchlorate ion.27 In both cases, the 12- and 33-month aged RL acquired highly similar p.d.i. loadings as the fresh RL but, unlike NaCl, only when normalized by the original 94 m2/g value and not their new BET value. This was also the case for 33-month aged RL preequilibrated at pH 3.0 for 24 h. Provided the surface chemistry of the aggregated particles was not considerably altered, these results are rather pointing to the possibility that NaClO4, in contrast to NaCl, promotes disaggregation of the aged particles. Evidence along such lines was provided by light scattering measurements (Figure S1 of the Supporting Information) of RL notably revealing a decrease in particle size in NaClO4, in contrast to NaCl and in dialized water. Thermodynamic modeling can be used to test these ideas further. In a recent study from our group,25 a thermodynamic adsorption model was developed to predict p.d.i. loadings on

Figure 3. HRTEM images of aggregated RL particles.

RL particles in NaCl and NaClO4 media. The model accounts for all crystallographic site type and densities of the dominant planes,6 accounts for the molecular-scale speciation of electrolyte species, a variable Stern layer capacitance controlled by interfacial electrolyte ion loadings, and isolates electrostatic contributions from the different planes. This model can only predict p.d.i. loadings if defect sites with a density of ∼1 site/ nm2 are included on the (010) face, thus generating a finite, although small, electrostatic potential at this face. For this study, thermodynamic calculations were carried out on various aggregation patterns (e.g., particle pairs or triplets, aggregated either by the (010) or the (100) face) to investigate the impact of site availability on p.d.i. loadings. These tests readily showed that all sites in the original model are needed to be involved to reproduce the experimental data (Figure 4). Stated otherwise, all proton active sites must be involved regardless of whether particles are aggregated or not. The evidence presented so far can therefore be used to suggest that freshly disaggregated and aged aggregated particles exhibit highly similar charging capacities in NaCl. Interestingly, titrations of aged RL in this medium exhibit considerably longer equilibration periods below the pzc than fresh RL (Figure 5). This may therefore be another indicating factor supporting the concept aged RL particles are indeed aggregated in this medium. These longer equilibration times could in fact be indicative of slower diffusion-controlled H+/Na+ exchange 9019

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Figure 5. Equilibration time (min) per solution composition required to achieve a drift in the electromotive force of the pH electrode of less than 0.6 mV/h. Results are shown with respect to net proton loadings relative to the pzc. Larger equilibration times are required for aged RL particles in NaCl, while no discernible differences are manifested in NaClO4.

Figure 6. Dominant electrolyte ion species formed at the (010) plane at 25 °C, as predicted by molecular dynamics simulations.25

further confirmation along these fronts by showing that considerably lower sodium loadings are achieved RL when exposed to NaCl compared to NaClO4 solutions.

Figure 4. Loadings of p.d.i. at RL surfaces in (left) 0.1 M NaCl and (right) 0.1 M NaClO4 solutions at 25 °C. These data were collected under stringent equilibration constraints, requiring that the a drift of less than 0.6 mV/h in the electromotive force of the pH electrode (cf. Supporting Information). Thermodyamic modeling (full line = disaggregated (94 m2/g); dashed line = aggregated (74 or 66 m2/g) particles) suggest that all proton active sites are involved in charge development, regardless of aggregation state.

4. OUTLOOK The evidence presented in this study shows that lepidocrocite particles form mesocrystal-like features when aged in salt-free aqueous media. These assemblages are readily disaggregated by NaClO4 solutions and retain their intrinsic surface charging properties regardless of their aggregation state, but can exhibit considerably altered equilibration time frames in NaCl. Further work along these lines should attempt to identify the impact of ions of varied charge-to-size ratios and/or structure on the stability of such mineral assemblages. Theoretical models predicting particle aggregation, as well as conversion to secondary crystals in the presence of various (in)organic ions will, moreover, be of considerable interest for understanding the behavior of these important forms of particles in natural and technological settings.

within aggregated mesocrystal/interparticle boundaries, but much smaller times under acidic conditions. Equilibration times in NaClO4 are, in contrast to NaCl, unchanged in both fresh and aged particles (Figure 5). This contrasting behavior can be explained by previously documented differences in the modes of interactions between μ− OH groups of the (010) plane and electrolyte ions (Figure 6).2,25 The driving force for disaggregation of aged RL in NaClO4 would, according to this view, arise from the hydrogenbonded complexes between perchlorate and μ−OH, predicted by molecular dynamics25 (Figure 6). Moreover, considering that previous cryogenic X-ray photoelectron spectroscopy (XPS) work from our group27 revealed a considerable enhancement of sodium loadings in the presence of perchlorate, it may even be possible that synergetic effects between these two ions may be responsible for the preferential disaggregation of aged RL in NaClO4. Aged RL in NaClbearing systems do not, on the other hand, experience the same driving forces due to the nature of Cl− interactions with μ−OH groups (Figure 6). Our recent cryogenic XPS findings27 added

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ASSOCIATED CONTENT

S Supporting Information *

All experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel: +46 73 833 2678. 9020

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Notes

(18) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment. Adv. Mater. 2006, 18 (18), 2426− 2431. (19) Jia, C.; Cheng, Y.; Bao, F.; Chen, D.; Wang, Y. pH valuedependant growth of a-Fe2O3 hierarchical nanostructures. J. Cryst. Growth 2006, 294 (2), 353−357. (20) Wang, J.; Sun, J.; Zhang, G.; Wu, X.; Bao, Y.; Li, H.; Chen, D. Preparation of SnO2 nanorods via oriented aggregation of nanoparticles. Vacuum 2007, 82 (1), 5−8. (21) Lai, H.-Y.; Chen, C.-J. Shape-controlled synthesis of iron sulfide nanostructures via oriented attachment mechanism. J. Cryst. Growth ̀ 2009, 311 (23,Ä i24), 4698−4703. (22) Su, H.; Wang, N.; Dong, Q.; Zhang, D. Incubating lead selenide nanoclusters and nanocubes on the eggshell membrane at room ̀ 7−12. temperature. J. Membr. Sci. 2006, 283 (1,Ä i2), (23) Yuwono, V. M.; Burrows, N. D.; Soltis, J. A.; Penn, R. L. Oriented aggregation: Formation and transformation of mesocrystal intermediates revealed. J. Am. Chem. Soc. 2010, 132 (7), 2163−2165. (24) Song, X.; Boily, J. F. Structural controls on OH site availability and reactivity at iron oxyhydroxide particle surfaces. Phys. Chem. Chem. Phys. 2012, 14 (8), 2579−2586. (25) Boily, J. F. The variable capacitance model: A strategy for treating contrasting charge-neutralizing capabilities of counterions at the mineral water interface. Langmuir 2014, 30, 2009−2018. (26) Song, X.; Boily, J. F. Water vapor interactions with FeOOH particle surfaces. Chem. Phys. Lett. 2013, 560 (0), 1−9. (27) Kozin, P. A.; Shchukarev, A.; Boily, J. F. Electrolyte ion binding at iron oxyhydroxide mineral surfaces. Langmuir 2013, 29 (39), 12129−12137.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Swedish research council (VR 2012-2976). The Knut and Alice Wallenberg (KAW) Foundation is acknowledged for providing the electron microscopy facilities and financial support to G.S.A. under the project 3DEM-NATUR.

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