Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Formation and Aggregation of ZrO2 Nanoparticles on Muscovite (001) Canrong Qiu,† Peter J. Eng,‡ Christoph Hennig,† and Moritz Schmidt*,† †
Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States
‡
S Supporting Information *
ABSTRACT: The aggregation of nanoparticles is a key step in the formation of solid phases and a controlling factor for the behavior of suspended nanoparticles in solution. Using a charged mineral surface [muscovite (001)], we apply the surface X-ray diffraction techniques crystal truncation rod (CTR) measurements and resonant anomalous X-ray reflectivity (RAXR) to investigate the aggregation process of Zr nanoparticles at the sub-nanometer scale. The aggregation process was studied as a function of ionic strength (0, 1, 10, and 100 mM NaCl), and the interfacial particles were characterized by CTR/RAXR and AFM. The observations are consistent with an aggregation process that follows a multistep mechanism, which starts with the 3D aggregation of primary building units to form nanosheets. These sheets continue to grow through addition of building units to their reactive edges at higher ionic strength. Once the size and concentration of aggregates are sufficient, “faceto-face” stacking of nanosheets becomes the preferred aggregation mechanism as this minimizes the electrostatic repulsion of the charge that accumulates along nanosheet edges.
1. INTRODUCTION Recent advances in nucleation theory have led to a deeper understanding of the process of formation of a solid phase. Our understanding of this fundamental chemical process has progressed from a stochastic description of bulk thermodynamics to the description of chemical interactions of nanoscale building units (BUs).1−4 The mechanism of this reaction is of importance due to its implications over a broad range of topics from materials synthesis and design5 to biomineralization6 and the environmental transport of heavy metals.7,8 Understanding the aggregation behavior is key to advancing from trial-anderror materials synthesis to a priori material development,9 how microorganisms control the shape and growth of minerals formed for specific functions,10−12 and whether nanoparticle formation slows down or accelerates the environmental transport of contaminants.13,14 In addition, the field of nanotechnology relies heavily on understanding the fundamental formation process of nanoscale materials to develop reliable and cost efficient synthesis routes for mass production.15,16 The formation of nanoscale materials is typically a multistep process, including e.g. the formation of initial building units as well as their aggregation and assembly. These processes depend on the structural or chemical transformations occurring within the material. A fundamental understanding of such multistep processes hinges on the ability to elucidate critical steps, typically requiring simplified model systems. One component of such a system may be the formation of nanoparticles through © XXXX American Chemical Society
the polymerization of metal cation hydrolysis species by olation (1) or oxolation (2). 2H 2O−Zr → Zr−HO−Zr + H 2O
(1)
Zr−OH−Zr + Zr−OH−Zr → H 2O + Zr4O
(2)
The hydrolysis route as a facile pathway to the formation of nanoparticles has been demonstrated in recent work on Ce(IV).17 Nanoparticle formation via hydrolysis has also been found to be an important process in the chemical and environmental behavior of highly radiotoxic Pu,18,19 where it was found that the process is accelerated at mineral surfaces.20,21 Aqueous Zr(IV) polymerization then is a promising model system for studying such multistep nanoparticle evolution processes, since Zr(IV), bearing a small ionic radius (0.75 Å) and high charge, is prone to hydrolysis in solutions leading to a progressive oligomerization that produces polydisperse Zr nanoparticles. Numerous studies have focused on Zr(IV) hydrolysis and oligomerization reactions in aqueous solutions to gain insights into the structural details of potential Zr(IV) building units as a function of solution chemistry using a variety of techniques, including X-ray absorption fine structure (XAFS) spectroscopy,22−25 small (or wide)-angle X-ray scattering Received: October 12, 2017 Revised: February 2, 2018 Published: February 6, 2018 A
DOI: 10.1021/acs.jpcc.7b10101 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C [S(W)AXS],26−31 high-energy X-ray scattering (HEXS),3 NMR/Raman spectroscopy,3,23,32 electron-spray ionization mass spectrometry (ESI-MS),25,33,34 dynamic light scattering (DLS),24,35 transmission electron microscopy (TEM),36 and Xray diffraction (XRD).31,37,38 While the general consensus in those studies points to the presence of dominant cyclic Zr tetrameric species over a wide range of solution conditions, the mechanism of the construction of higher order Zr oligomer and Zr polymer species based on the Zr tetramer building units remains unclear. In addition, many studies rely on elevated temperatures to accelerate the aggregation process,30,39 which may lead to the formation of a different product compared to room temperature. Hagfeldt et al. studied the internal structure of the tetrameric [Zr4(OH)8(OH2)16]8+ building unit by WAXS and EXAFS.31 They find a ring structure consisting of four Zr4+ cations connected by four double-hydroxo bridges; in addition, each Zr ion is decorated by four water molecules pointing away from the ring. The Zr−O bond distance is 2.133 Å with a Zr− O−Zr angle of 112.8° in the hydroxo bridge; the bond to the water molecule oxygens is slightly longer at 2.260 Å. A Zr nanoparticle aggregation mechanism was first proposed by Clearfield et al.,40 who suggested the construction of a twodimensional Zr network through connecting tetramer units at edge sites. The aggregation then proceeds via an olation reaction that converts terminal water groups to bridging hydroxo groups (eq 1). Clearfield’s mechanism was shown to be a possible route to some observed structures in a SAXS/ XAFS study,30 which suggested a sheetlike Zr network structure consisting of cyclic tetramer units. However, the same oligomerization mechanism could not be used to explain the building up of a 3D Zr nanostructure. Hu et al. postulated that the 3D structure could be formed by stacking nanosheets on top of each other in a face-to-face pattern.39 However, no direct evidence of the face-to-face stacking mechanism was found, and it remains unclear what the driving force for this aggregation behavior is since face-to-face stacking would have to proceed through an oxolation reaction (eq 2) that is energetically less favorable than the olation reaction (eq 1) due to the lack of more reactive coordinated water groups −OH2.32 Problems in the unambiguous characterization of the particles and hence the oligomerization mechanism arise from the particular size range of the Zr oxo-hydroxide products of just below one to a few nanometers. This size range is too small for traditional XRD measurement whereas XAFS provides a local coordination geometry but is unable to fully characterize the particle structure. SAXS has been applied as a useful tool for probing the shape of aqueous nanoparticles of size ranging from sub-nm to greater than 100 nm. Previous SAXS studies have suggested that Zr(IV) tetramer building units in a sol undergo self-assembly to form Zr nanoparticles of numerous shapes including nanorods and three-dimensional nanoparticles,39 nanosheets,30 and extended nanochains.26 However, SAXS cannot determine the internal structure of the particles or how the structure progresses from one shape to another. A critical question in the formation process of a solid phase concerns the coordination symmetry of the formed product. For technical applications cubic ZrO2 is the preferred product,41 but the symmetry found in micro- or nanocrystalline ZrO2 solids is typically tetragonal, and the cubic phase has to be stabilized by the introduction of dopants like Ca2+, Y3+, Sb3+, or Ce4+.41−43 It is commonly assumed that Zr in the smallest oligomers is coordinated by eight O atoms in cubic coordination geometry,44 which transitions to a tetragonal
symmetry with one elongated crystallographic axis upon growth of the particles.38,41 Tyrsted et al. showed by SWAXS and HEXS that Zr is coordinated octahedrally in an initially formed oligomeric precursor, but transitions to a cubic coordination environment in the presence of sufficient amounts of Y3+.45 It is, however, unclear whether the observed structures are representative of a solution or sol/gel process, as the experiments were performed in supercritical methanol. Understanding this critical reaction step could open up novel pathways to a chemical control for fabricating monodisperse and homogeneous ZrO2, e.g., as a high-quality ceramic precursor material. To advance the efforts in understanding the transition from nanoscale building units to macroscopic solid phases, we design an experiment that takes advantage of surface X-ray diffraction techniques to resolve the early stages of Zr nanoparticle formation throughout the aggregation process. In these experiments, we employ a charged mineral interface as a means to accumulate nanoparticles from solution as well as to enable the application of surface specific diffraction techniques. Specifically, we use crystal truncation rod (CTR) measurements and resonant anomalous X-ray reflectivity (RAXR) to explicitly characterize the aggregated fraction of nanoparticles at the water/mineral interface. A charged interface will accumulate the highly positively charged species such as Zr4+ oligomers from solution at the interface, which means that we are capable to perform our experiments at lower [Zr] and without a need for increased temperatures. The degree of aggregation in our experiments is controlled via addition of various amounts of NaCl, exploiting the well-established dependency of nanoparticle aggregation on the ionic strength.46,47 Briefly, addition of ionic strength is thought to shield the surface of charge of the suspended nanoparticles, which reduces electrostatic repulsion and facilitates aggregation to larger entities (DLVO theory). The X-ray measurements are complemented by atomic force microscopy (AFM) to characterize the morphology of the adsorbed nanoparticles.
2. EXPERIMENTAL SECTION 2.1. X-ray CTR Measurements. Solutions were prepared using ZrOCl2·8H2O (Sigma-Aldrich, purity ≥99.5%) as starting material, and [Zr] was adjusted to 0.1 mM for all experiments. Four samples were prepared containing 0, 1, 10, and 100 mM NaCl, respectively, corresponding to ionic strengths of 1.8, 2.9, 12, and 102 mM when assuming no polymerization. The pH of Zr(IV) solutions was adjusted to 2.5 ± 0.1 using NaOH and HCl. Each mica crystal of size 1.25 × 1.25 cm and thickness 0.25 mm (Asheville Schoonmaker Mica Company) was cleaved prior to being incubated in Zr(IV) solutions for ∼24 h. For various steps in the reaction experiments were performed with shorter and/or longer time steps with no apparent kinetic effect (data not shown). In an attempt to characterize the surface charge dependence on the ionic strength, we performed zeta potential measurements. The results are discussed in the Supporting Information. After incubation, each mica sample was transferred to a liquid sample cell that was designed for in situ surface reactions similar to the one used in our previous work (Figure S1).48 For surface X-ray diffraction experiments mica crystals were reacted overnight in a Zr solution, quickly removed from the incubation vial, transferred to the sample cell, and an additional 100 μL of Zr(IV) solution was pipetted on top of the mica surface. A liquid film of thickness ∼5−10 μm was subsequently formed B
DOI: 10.1021/acs.jpcc.7b10101 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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adsorbed ions, and interfacial water. RAXR55 is an elementspecific technique which probes the distribution of a resonant element registered at the scattering interface. Model refinement was performed through fitting calculated CTR and RAXR against experimental data using a least-squares fitting routine. Data were fit by applying a parametrized structural model consisting of the ideal muscovite substrate lattice, the interfacial region, and bulk water. The interfacial region includes the structural relaxation of atoms in the two top unit-cell layers of the muscovite surface and the presence of near-surface species such as adsorbed species and water. The distribution of each adsorbed species is described as a Gaussian peak, whose structure factor is expressed as
and maintained on the crystal by a Kapton membrane. This water layer is sufficiently thin to allow for transmission of Xrays but thick enough to be representative of bulk water at adequate distance from the interface (i.e., >30 Å for this system; see Figure 3). The sample cell was mounted on the diffractometer for X-ray measurement, which was performed at the GSECARS undulator beamline 13IDC at the Advanced Photon Source (APS), Argonne National Laboratory. For verification, all experiments were repeated at the Rossendorf beamline BM-20 at the European Synchrotron Facility (ESRF). At GSECARS, the incoming beam was collimated and reduced to 100 μm × 1000 μm. The energy of the incoming beam was adjusted using a liquid nitrogen cooled double-crystal silicon (111) monochromator. A Kappa geometry Newport diffractometer (4S+2D) was used for sample and detector orientation. Scattered X-ray intensity was measured using a Dectris Pilatus 100 K 2D pixel array detector.49 The X-ray CTR measurements were performed in both vertical and horizontal mode to account for any gravitational accumulation of nanoparticles. No gravitational effects were observed, and results from both beamlines are in excellent agreement. The energy of the incoming X-ray was fixed at 16 000 eV for CTR measurements, which is well away from the Zr K-edge (17 998 eV) to minimize resonant effects. For RAXR measurements, the energy of the incoming X-ray was scanned through the Zr K-edge at 21 selected values of momentum transfer q. The exact position of the Zr K-edge was determined on one sample (0 mM NaCl, 0.1 mM Zr) through fluorescence XANES measurements collected in grazing-incidence mode using an SII Vortex ME4 X-ray fluorescence detector. The same spectrum was used to obtain the anomalous dispersion terms required for RAXR data analysis by applying a difference Kramers−Kronig transformation.50 2.2. AFM Imaging. Each of the reaction steps with the ionic strengths mentioned above was carried out in parallel with a second sample for AFM imaging. Here 200 μL of Zr solution was spread onto a freshly cleaved mica surface. The sample was then incubated for 24 h prior to AFM measurement. AFM imaging was performed in situ at room temperature (∼25 °C). Imaging was performed in tapping mode using an Asylum Research Cypher AFM instrument equipped with BRUKER MSNL-10 silicon tips on nitride cantilevers, between 26 and 50 kHz frequency, and force constant of 0.1 N/m. The size of the particles analyzed in this study is sufficiently small for tip artifacts to have a significant impact on measured particle sizes. All samples were measured with the same type of AFM tip to ensure that any tip artifacts are comparable throughout our sample series. As such, the numerical values given below may differ from the particles’ “real” dimensions, but any observed trends should nonetheless be significant. All reported vertical and lateral dimensions are statistical averages over a large number of particles, and we observed good reproducibility, both for different locations on the same sample and for repeat measurements with different samples in the same conditions. The images were analyzed using the open source software Gwyddion.51 2.3. CTR/RAXR Model Refinement. Both CTR and RAXR have been described in detail elsewhere.52−54 Briefly, the specular (00L) crystal truncation rod53 is a column of scattering intensity oriented perpendicular to the surface connecting Bragg peaks in reciprocal space. Refinement of a structure model to the CTR data will generate the total interfacial electron density that is attributed to surface relaxation,
F=
⎡ q 2u 2 ⎤ ⎢− j ⎥ c f ( q ) exp( iqz ) exp ∑ jj j ⎢⎣ 2 ⎥⎦ j
(3)
where f j(q) is the atomic scattering factor and cj, zj, and uj are the occupancy, height from the surface, and rms width of the jth atom, respectively. Bulk water was expressed by a layered water model.56,57 For RAXR the same strategy is applied, and the Zr distribution was described as individual Gaussian peaks. The resonant structure factor is expressed as ⎡ q 2u 2 ⎤ j ⎥ FR (q) = (f ′(E) + if ″(E)) ∑ cj exp(iqzj) exp⎢ − ⎢ ⎥⎦ 2 ⎣ j (4)
where f′(E) and f ″(E) are the anomalous dispersion terms of the resonant element, here Zr. The quality of fit of each CTR or RAXR model was characterized by a scaled χ2 and an R-factor (see Supporting Information for details).
3. RESULTS AND DISCUSSION 3.1. Crystal Truncation Rods and Resonant Anomalous X-ray Reflectivity. CTR data and the curves corresponding to the best fit models for all four conditions (0, 1, 10, and 100 mM NaCl) are shown in Figure 1; the corresponding model parameters and their uncertainties are compiled in Table S1 (refer to Supporting Information). The momentum transfer q is an energy and substrate independent expression for the scattering angle 2θ: q = 4π sin(2θ/2)/λ. As q is a reciprocal space unit, changes at small values of q relate to changes in large structures in real space, and vice versa. The evolution of the interfacial structure can be clearly observed in several places along the rod, in particular at low momentum transfer (q < 1.5 Å−1). For instance, the shape of the first midzone (q ∼ 0.4 Å−1) changes significantly, and a shoulder emerges with increasing ionic strength. A similar effect is observed at q ∼ 0.9 Å−1, where the midzone is essentially flat in the absence of NaCl, while a shoulder is clearly visible at the higher ionic strengths. Less pronounced differences are also clearly visible in the third midzone around q = 1.5 Å−1. The midzones between Bragg peaks are most sensitive to changes in the interfacial structure. This variation in the CTR profiles serves as a direct indicator that the interfacial structure changes as a function of NaCl concentration, suggesting a changing aggregation behavior of Zr. For values of q greater than approximately 2 Å−1 there is little evolution in the CTRs, indicating that the resolvable real space structural changes occur at length scales greater than 2π/q ≈ 3 Å. Selected RAXR scans with the curves corresponding to the respective best fit models are shown in Figure 2 (see C
DOI: 10.1021/acs.jpcc.7b10101 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Specular CTR data measured from Zr(IV)/mica (001) samples (0.1 mM Zr solution of pH 2.5 reacting with freshly cleaved mica (001) for ∼24 h) with different NaCl concentrations: 0 mM NaCl (green), 1 mM NaCl (orange), 10 mM NaCl (blue), and 100 mM NaCl (red). The associated specular CTR calculated based on the associated best fit models are illustrated as solid lines with the same color as the associated data points. Vertical offset (1 log unit stepwise) was applied to CTRs for NaCl-containing Zr solutions for better comparison. Two vertical dashed lines highlight the remarked CTR variation at low q values (q = 0.4 and 0.9 Å−1). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Figure 2. Representative RAXR data (open dots) as a function of NaCl concentration measured for Zr(IV) adsorption on mica (001) surface as well as the calculated RAXR profiles from the best fit model (solid lines). In each subplot, the data points and calculated lines display the results for the Zr/mica interface with NaCl concentrations of 0 mM (green), 1 mM (orange), 10 mM (blue), and 100 mM (red) from bottom to top, respectively. Each spectrum presents the variation of specular reflectivity as a function of photon energy, E, at fixed momentum transfer q. The specular reflectivity (data point and calculated values) at each q for different cases are offset for better visibility. See the Supporting Information for the complete set of RAXR results.
Supporting Information for the complete sets of RAXR spectra). For all samples strong modulations are found, particularly at low values of q (q ≤ 1 Å−1), which dissipate rapidly at higher q. By the same argument as above, small values of q must relate to large structures in real space, which directly indicates the presence of an extended layer of Zr on the muscovite surface. We observe a continuous increase in amplitude of the modulation with increasing ionic strength, while changes in shape, which are related to the resonant phase shift, are comparatively minor, with the biggest differences at 100 mM NaCl. The increasing modulation amplitudes indicate increasing uptake of Zr, while the similar phase values suggest that changes in average sorption height are relatively minor and mostly occur at lowest and highest ionic strength. 3.2. Interfacial Structure. The best fit electron density profiles normal to the surface are shown for both the CTR (lines) and RAXR (area plots) data in Figure 3. All models presented in Figure 3 have a quality of fit χ2 < 8.6 (see Supporting Information). The largest differences in the total electron density profiles are seen between data collected in the presence of NaCl and those collected without it. All conditions show a peak around 2.5 Å. Past measurements56 of the muscovite surface show adsorbed water layers located at 1.3 and 2.5 Å, with a shift of these layers away from the surface upon ion adsorption.58,59 The profile determined in the absence of NaCl additionally shows two distinct peaks at 6.3 and 8.9 Å, respectively. Extended structures up to 20 Å above the surface are observed for 1 and 10 mM NaCl, with only marginal differences between the two profiles. For 100 mM NaCl the extended feature reaches 25 Å above the surface with an ∼40% larger electron density. The vertical size and intensity of these features indicate the adsorption of nanoparticles.
As presented in Figure 3, the Zr distributions for the solutions containing 0−10 mM NaCl are dominated by one broad peak centered at 8.39 ± 0.05 Å for 0 mM NaCl and 8.95 ± 0.08 and 8.74 ± 0.09 Å for 1 and 10 mM NaCl, respectively. The change in peak position is small in this ionic strength range, but it is accompanied by a broadening of the distributions, with the Zr distributions extending to 12.5, 18, and 17.5 Å for 0, 1, and 10 mM NaCl, respectively, showing some vertical growth of the particles. A sorption height around 9 Å is typical for extended outer sphere complexes (adsorbed ions with two intact hydration shells), as had been previously observed for Th(IV).59 However, the occupancies of Zr do not match the expected behavior for ionic adsorption. The muscovite (001) surface has a constant negative charge of 1 e−/AUC [where AUC is the area of the unit cell, AUC = 46.72 Å2 for muscovite (001), equivalent to 3.43 × 10−19 C/nm2], which is typically compensated by adsorbing cations. This surface charge is independent of pH and, thus, not well represented by zeta potential measurements (see Supporting Information).60 Instead of a coverage θ close to a charge compensating monolayer (θCP = 0.25 Zr4+/AUC), we find significantly higher Zr uptake in all cases: θ0 mM = 1.37 ± 0.03 Zr4+/AUC (0 mM NaCl), θ1 mM = 2.20 ± 0.07 Zr4+/AUC (1 mM NaCl), and θ10 mM = 2.39 ± 0.07 Zr4+/AUC (10 mM NaCl). This increased uptake cannot be explained by “overcharging” which might be caused by highly charged Zr4+ ions61 but is rather indicative of the D
DOI: 10.1021/acs.jpcc.7b10101 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. Comparison of CTR-derived total electron density distributions (solid lines) and model-dependent RAXR-derived interfacial Zr electron density (filled peaks) above the mica (001) surface based on surface modeling on mica samples reacting with Zr(IV) solutions of different NaCl concentrations for 24 h. The electron density values are normalized to that of bulk water ρwater = 0.33 e−/Å3; values for NaCl-containing samples are shifted vertically for better visibility. Dotted lines highlight the common pronounced peaks at 2.5 Å in the z-direction.
Figure 4. AFM images for mica surfaces reacted with Zr solution for 24 h with different ionic strength conditions including 0 mM NaCl (A), 1 mM NaCl (B), 10 mM NaCl (C), and 100 mM NaCl (D). Each image has the same probing range of 250 × 250 nm2, and the height color bar for each image is scaled equally for better comparison of dominant structure features. At the bottom (E), a height distribution analysis of four such micrographs is shown for each condition.
presence of Zr oligomers and their aggregation at the interface. It also becomes evident that introduction of NaCl strongly increases uptake by more than 60%, while the additional increase from 1 to 10 mM NaCl concentration only yields an increase of ∼9%. As the uncertainties for both occupancies are in the range of 3% the increase is significant, but the effect of the ionic strength on the aggregation is obviously small in this concentration range. Increasing the NaCl concentration to 100 mM produces an even larger difference in the Zr distribution. The distribution shows a broad feature similar to the other data sets now centered at 10.3 Å and with a slightly higher peak intensity. In addition, a second weaker feature is observed at 22.3 Å. The occupancy is greatly increased to 3.42 ± 0.14 Zr4+/AUCan increase of more than 40% relative to the next lower ionic strength. The second peak in the distribution is weak and contributes only a small amount of Zr to the overall occupancy (∼0.15 Zr4+/AUC), but omission leads to a significant misfit for several RAXR modulations (see Figures S6 and S7, respectively). 3.3. Atomic Force Microscopy. CTR and RAXR deliver a detailed view of the vertical structure of adsorbed species but cannot probe their lateral size in the specular scattering geometry. To assess the aggregation within the surface plane, we apply AFM under the same conditions as used for the CTR/ RAXR experiments. Figure 4 shows selected images for each condition as well as a height distribution plot obtained by statistical analysis of four images for each condition, covering a total surface area of 500 × 500 nm2. To assess the lateral dimension of the adsorbed nanoparticles, 12 profiles (see Figure S7 for representative profiles) were analyzed for each
condition, and an averaged lateral dimension of ∼100 particles was calculated. As AFM does not yield any chemical information on the topological features, we assume for the sake of this analysis that all features on the surface are adsorbed Zr, which appears reasonable given the featureless appearance of a clean mica surface in the same experiment (see Figure S8). As discussed previously, tip artifacts will affect the numerical values determined for the nanoparticle dimensions. However, the same tip was used for all experiments, so the effect should be comparable for all samples. In combination with the statistical analysis of a large number of particles for each condition, we expect the observed relative changes to be reliable, which is consistent with the observed good reproducibility and reasonable agreement with our X-ray scattering data. The height distribution analysis confirms the findings from our X-ray CTR experiments. In the absence of NaCl the peak in the distribution is found at 6.2 Å with a distinct tailing toward larger particles up to a size of ∼12 Å. Addition of NaCl initially leads to a small increase in the vertical particle size with a peak at 8.2 Å and up to ∼18 Å for 1 mM NaCl. At 10 mM NaCl only small changes are observed relative to the lower ionic strength: the peak position and maximum size are virtually identical with 8.3 Å and up to ∼19 Å, respectively. The most notable difference is the absence of smaller particles below ∼5 Å in the height distribution. Vertical growth is most pronounced at the highest ionic strength, where particles below ∼8 Å are absent and the maximum in the distribution more than doubles to 17 Å, with some larger particles growing beyond 30 Å. The AFM experiments thus verify the observation of minor vertical E
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The Journal of Physical Chemistry C growth going from 0 to 1 mM NaCl, no vertical growth between 1 and 10 mM NaCl, and a strongly increased vertical growth when going to 100 mM NaCl. It should be noted that the CTR measured electron density distribution (Figure 3) is referenced to the bulk crystal structure whereas the height distribution determined from AFM is relative to areas of the image that appear to represent the bare mica surface and the zeros are therefore partially arbitrary. When analyzing the lateral dimension of the observed particles, a different trend is found (Figure S7, see also Table 1). All particles are larger laterally than vertically, giving the Table 1. Relative Changes (%) in Vertical and Lateral Dimensions as Well as Zr Coverage (θ) in the Three Aggregation Steps
a
aggregation step
1.8 → 2.9 mM
2.9 → 12 mM
12 → 102 mM
zmax (RAXR) zcen (AFM) lateral (AFM) θ (RAXR)
36 32 −5.5a 61
0 1.2a 43 8.7
38 105 18 43
Values below or close to the error of the respective measurement.
Figure 5. Zr NP structure evolution as a function of ionic strength (IS) based on lateral size obtained from AFM results and vertical size obtained from CTR/RAXR (A). The dashed arrows track the structure evolution. Schematic diagram of the plausible Zr nanoparticle aggregation route with increasing IS based on the size evolution route (B). The red edges carry net positive charge while the facets in green indicate zero net surface charge.
particles a platelet-like morphology. When no background electrolyte is added, the particles are relatively small but show some variation in their lateral dimension. Some particles are several 10 nm in size laterally, while more particle sizes are only a few nanometers. The average value for all analyzed particles is 5.4 ± 0.5 nm. The lateral dimension is effectively unaffected by addition of 1 mM NaCl, with an average value of 5.1 ± 0.1 nm within error of the value for 0 mM NaCl. It appears that the particles are more homogeneous in size, and the larger units occasionally observed in the absence of NaCl are not present under these conditions. Lateral growth of the adsorbed particles is first observed at 10 mM NaCl, where an average value of 7.3 ± 0.6 nm is found. This is in good agreement with the previously observed absence of vertical growth in AFM and RAXR, while RAXR suggests a 9% increase in adsorbed Zr. At the highest ionic strength, we observe additional lateral growth, and particles now have a lateral dimension of 8.6 ± 0.5 nm. This corresponds to an increase of 17% relative to the next lower ionic strength and is thus less pronounced than the change in vertical size under the same condition. 3.4. Zr Nanoparticle Aggregation Mechanism. On the basis of our measurements and the known aqueous Zr chemistry, we attempt to delineate the evolution of the Zr nanoparticle morphology. Table 1 summarizes the observed relative changes between the four solution conditions. The specific numbers vary between AFM and RAXR due to the reasons discussed above, but a trend becomes clear: upon addition of NaCl we observe a significant increase in Zr4+ coverage and growth is predominantly vertical. Increasing the ionic strength to 12 mM causes little additional Zr4+ adsorption, and growth is only observed in the lateral direction. Finally, increasing the ionic strength to 102 mM again significantly increases the Zr4+ coverage, and we now observe growth, both vertically and laterally, though the vertical growth is more pronounced. Figure 5 shows an illustration of a charged mineral interface with four aggregation steps that are consistent with observations summarized in Table 1: (1) adsorption and aggregation of primary building units, small oligomer, at the mineral−water interface; (2) formation of thicker oligomers in
solution prior to interfacial attachment; (3) lateral growth of nanosheets via addition of building units to edges; and (4) formation of vertical nanosheet stacks accompanied by lateral addition of building units similar to 3 (not shown). To better understand these proposed aggregation steps, we first consider the possible role played by the electrical double layer of the mineral−water interface. The interface’s negative charge will lead to a depletion of negatively charged ions in the interfacial region and the accumulation of positive charges. Any effect of the background electrolyte’s Cl− anions on the aggregation behavior of the Zr nanoparticles must then occur in solution. Second, in the absence of a background electrolyte, the oligomers are only marginally shielded by negatively charged ions limiting aggregation by Coulomb repulsion, resulting in a Zr solution speciation dominated by small oligomers, e.g., tetramers, pentamers, and octamers.25,33 The expectation is that these small oligomers will adsorb at the interface. However, the first particles observed in this study are of a size (∼1 × 5 × 5 nm3) much larger than the oligomeric building blocks expected to be present under these solution conditions, leading us to conclude that the initial interface aggregation can likely be attributed to the shielding of the nanoparticles’ surface charge by the much larger muscovite interface charge. This greatly reduces the repulsive forces between oligomers, thus allowing aggregation at the interface. As the reaction is only favorable for already adsorbed particles, growth will occur predominantly in the surface plane, i.e., laterally. Consequently, this first step is dominated by the presence of the interface, which is not the case for the subsequent reaction steps at higher ionic strength. For the following steps, NaCl is introduced into the system as a background electrolyte. An excess of Cl− anions in solution increases the shielding of the building units’ surface charge reducing their electrostatic repulsion and lowering the energy F
DOI: 10.1021/acs.jpcc.7b10101 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
stacking mechanism. Electrostatically, this approach reduces repulsion at the edges as interaction mainly occurs between charge neutral faces of nanosheets, and possibly between faces and edges, where overlapping occurs, but interactions between two charged edges should be negligible. However, this process requires a sufficient concentration of larger aggregates to occur with significant frequency, which is evidently only achieved at the highest ionic strength. We still observe lateral growth at this stage, though to a lesser extent than vertical. A possible explanation is that the addition of smaller building units continues at this stage coincidental with the sheet stacking responsible for the vertical growth. 3.5. Zr Nanoparticle Aggregation under Mild Conditions. The aggregation of Zr nanoparticles has been studied by a number of techniques previously. Typically, these studies employ higher temperatures and/or high initial Zr concentrations (>22 mM) to enhance the formation of nanoparticles. This frequently leads to the formation of very large particles up to 255 nm in size.35 Alternatively, some studies aim to compensate this effect by stabilizing smaller particles at low pH ([H+] = 10−2−10.5)22,24,25,31 In comparison, the conditions applied in our study are much milder, with low [Zr] = 0.1 mM, mildly acidic pH = 2.5, and no additional need for increased temperatures. Where structures are reported along with particle sizes, three general regimes are observed, small oligomers typically tetramers, hexamers, and octamers,22,25,29 cylindrical units with two short, and one clearly elongated direction,26,28,39 or platelike structures with two long dimensions and one shorter direction.30 The latter had only been observed at rather extreme conditions with very high Zr concentration [Zr] = 1.0 M, low pH = 0.8, and elevated temperature. Yet, these particles are most similar in shape to the particles observed in this study, though they are somewhat smaller than the particles found here, 2.8 × 2.8 × 0.5 nm3 compared to 5.5 × 5.5 × 1.0 nm3. This may be an effect of the very low pH, which is expected to hamper the particles’ aggregation. Cylindrical particles with a radius ∼5 Å and length of 15 to more than 400 Å have been observed in three SAXS studies. Because of the radius, it has been assumed that the cylindrical particles consist of stacks of Zr tetramers. They have only been found in experiments with high Zr concentrations [Zr] > 22 mM26,28,39 and additional high temperature treatment at 85 °C26 and 100 °C,39, respectively. All studies find only very small influence of ionic strength on the growth of these particles. On the basis of our data, it can be excluded that such cylindrical particles formed in our experiments. We can speculate that the milder conditions in our experiments, in particular the lower [Zr], give a larger importance to the charge shielding effect of the background electrolyte relative to more concentrated solutions, where collisions of Zr oligomers are inevitable, especially at increased temperatures close to the boiling point. It could then be speculated that the formation of cylindrical aggregates is the consequence of scenarios involving high collision rates of small oligomers, whereas conditions that allow for fewer collisions are more likely to yield the platelet morphology.
barrier for the aggregation, allowing greater linkage between building units. This results in solution aggregation, which is predominant with respect to aggregation at the mineral interface in the presence of NaCl. Experiments after short reaction times with the mineral surface (
