Role of Water During Crystallization of Amorphous Cobalt Phosphate

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The Role of Water During the Crystallization of Amorphous Cobalt Phosphate Nanoparticles Sven Bach, Martin Panthoefer, Ralf Bienert, Ana de Oliveira Guilherme Buzanich, Franziska Emmerling, and Wolfgang Tremel Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00208 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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Crystal Growth & Design

The Role of Water During the Crystallization of Amorphous Cobalt Phosphate Nanoparticles Sven Bach,†,§ Martin Panthöfer,† Ralf Bienert,$ Ana de Oliveira Guilherme Buzanich,$ Franziska Emmerling,$ and Wolfgang Tremel†,* †

Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10–14, D–55128 Mainz, Germany §

Graduate School Materials Science in Mainz, Staudinger Weg 9, D–55128 Mainz, Germany

$

Federal Institute for Materials Research and Testing, Richard-Willstätter-Straße 11, D–12489 Berlin, Germany

Keywords: cobalt phosphate, amorphous, crystallization, nanoparticles The transformation of amorphous precursors into crystalline solids and the associated mechanisms are still poorly understood. We illuminate the formation and reactivity of an amorphous cobalt phosphate hydrate precursor and the role of water for its crystallization process. Amorphous cobalt phosphate hydrate nanoparticles (ACP) with diameters of ≈20 nm were prepared in the absence of additives from aqueous solutions at low concentrations and with short reaction times. To avoid the kinetically controlled transformation of metastable ACP into crystalline Co3(PO4)2 × 8 H2O (CPO) its separation must be fast. The crystallinity of ACP could be controlled through the temperature during precipitation. A second amorphous phase (HT-ACP) containing less water and anhydrous Co3(PO4)2 were formed at higher temperature by the release of coordinating water. ACP contains approximately five molecules of structural water per formula unit as determined by thermal analysis (TGA) and quantitative IR spectroscopy. The Co2+ coordination in ACP is tetrahedral, as shown by XANES/EXAFS spectroscopy, but octahedral in crystalline CPO. ACP is stable in the absence of water even at 500 °C. In the wet state, the transformation of ACP to CPO is triggered by the diffusion and incorporation of water into the structure. Quantitative in situ IR analysis allowed monitoring the crystallization kinetics of ACP in the presence of water.

1. INTRODUCTION Crystallization is an essential step during the production and processing of materials, e.g., for pharmaceuticals, pigments,1 concrete,2 or catalysts.3,4 In addition, it is crucial in geology and environmental sciences,5 where the formation and behavior of crystalline solids may change with temperature, pressure, and chemical environment,6,7 e.g. in biomineral formation8 or the synthesis of biomaterials.9 In addition, the fundamental mechanistic aspects of phase transformations have been a prime reason for studying crystallization.10,11 It is generally accepted that properties of an emerging solid phase are determined at the onset of precipitation, i.e., at the nanoscale. In addition, there is increasing evidence that crystallization is not always a one-step process from the elementary constituents to the macroscopically stable crystalline phase. It may involve precursor and intermediate species successively transforming into each other.12–14 Building blocks added to the crystal lattice may be molecules, soluble ions, nanometer-sized clusters,15 or crystallites undergoing oriented attachment.16–18 The growth process may affect the morphology so that aggregates of discrete units (mesocrystals) may form rather than atomically flat facets.19,20 Crystals growth starts from an initial disordered phase via solid-phase amorphous-to-crystalline transi-

tions.21 A differentiation between these crystal-growth mechanisms is challenging because the building blocks are typically small, unstable, and short-lived. The most prominent model compound for studying crystallization processes is CaCO3.22 In order to test the viability and the limits of the crystallization pathways proposed for CaCO3 new model compounds are needed. Suitable candidates should contain (i) cations (e.g., Ca2+, Co2+ or Zn2+) that allow a reversible binding of water in the hydration shell, (ii) anions (e.g., phosphates and carbonates), whose hydration enthalpy is compatible with the charge density of the counterions, and/or (iii) anions whose acidity (pKs values) allows protonation because hydrogen bonds play an important role in packing and crystal chemistry. The collapse and reorientation of the resulting structures determine the stability and phase transitions of the hydrated solids. As a result, polymorphs with slight changes in the orientation of coordinated water molecules may show a distinctly different thermal behavior. Different orientations lead to different hydrogen-bonding patterns and small energy differences, which, in turn, lead to the formation of metastable (amorphous) intermediates in multistage crystallization processes23 via nanocrystalline hydrated phases. Aggregation through condensation/dehydration leads to a transformation into thermodynamically stable products.

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The formation of cobalt phosphate or zinc phosphate24-26 nanoparticles as a transient (amorphous) intermediate during the early stages of crystallization of cobalt phosphate has been analyzed theoretically, but there is no experimental evidence so far. Its composition, configuration and stability are not understood. (i) Composition: The assumption of a single and uniform amorphous cobalt phosphate hydrate (ACP) may be too simplified, as more than one composition may exist. Besides impurities or surface active molecules,27 which can stabilize the amorphous phase, water is a key variable. Cobalt phosphate has been reported as octahydrate (CPO),28 tetrahydrate,29 two monohydrates30,31 and an anhydrous cobalt phosphate.32 For example, the water content of amorphous Zn3(PO4)226 spans a range from nominally zero to two water molecules of hydration per formula unit. Solid-state NMR revealed water to occur as a molecular entity with little evidence for proton exchange, but with a reorientation on a time scale of milliseconds.26 (ii) Configuration: The formation and configuration of the amorphous phase are difficult to unravel. An appealing formation model of a hydrated amorphous phase is the agglomeration of small cluster nuclei with solvation water molecules trapped by hydrogen bonding in between. This implies that the loss and/or reorganization of hydration water and the reorganization of the clusters are the important during crystallization. EXAFS may give this information on the coordination of the metal atoms, and pair distribution function (PDF) analysis may provide information on local atomic order (as shown for of ZnO,33 yttria stabilized zirconia,34 or iron oxide35). A PDF study of amorphous calcium carbonate36 has shown little structural resemblance with its crystalline hydrous and anhydrous counterparts. (iii) The inertness of the amorphous phase as a function of the conditions is difficult to define. Although amorphous intermediates are assumed to be metastable with respect to their crystalline counterparts, little is known about the thermodynamics during nucleation and the early stages of nanoparticle formation. Inorganic nanoparticles may choose metastable structures different from the thermodynamically stable ones for small particle sizes. Calorimetric measurements by Navrotsky and coworkers37 have shown the competition between lattice and surface energy to be responsible for this polymorph change,38,39 and it could be shown that the stability regime of metastable polymorphs can be extended significantly for small particle sizes. We have synthesized ACP nanoparticles with diameters of ≈20 nm from aqueous solution. ACP contains ≈ 5 molecules of coordinated water per formula unit); its deepblue color points to a tetrahedral Co2+ coordination. ACP is stable in the absence of water at 500 °C. The crystallization of ACP to CPO in the wet state is triggered by the diffusion and incorporation of water, which occurs within hours and follows an exponential rate law derived from in situ quantitative IR spectroscopy. The fast crystallization in the wet state can be explained by the lower activation energy of surface compared to solid state diffusion.40,41

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2. EXPERIMENTAL SECTION 2.1. Synthesis. Amorphous cobalt phosphate hydrate was prepared by precipitation from solution. Co(NO3)2 × 6 H2O (>99%, Acros) and Na3PO4 (>96%, Sigma-Aldrich) were dissolved in 10 mL of deionized water at room temperature (solution 1 and 2), respectively. Solution 1 was stirred with 400 rpm in a beaker and solution 2 was added. The precipitation was carried out at different temperatures (20–60 °C). The precipitate was separated immediately by centrifugation (9000 rpm, 5 min). Subsequently, the solid was resuspended in and centrifuged from reagent-grade acetone (≥99.5%, SigmaAldrich) 3 times in order to remove adsorbed water. The solids were dried at room temperature and under dynamic vacuum (p=3 × 10-3 mbar) for 2 d. 2.2. Characterization. The ACP nanoparticles were amorphous and contained cobalt and phosphate in a 3:2 ratio (determined by X-ray diffraction (XRD) with synchrotron radiation,42,43 inductively coupled plasma mass spectrometry (ICP-MS) and optical emission spectroscopy (ICPOES), energy-dispersive X-ray spectroscopy (EDX), quantitative IR spectroscopy, and TGA). The density of ACP was determined pycnometrically, the density of CPO by X-ray diffraction. Differences in the short-range order of ACP and CPO were analyzed by IR, UV-Vis, and X-ray absorption spectroscopy (EXAFS and XANES). Morphological differences were analyzed by electron microscopy (SEM and TEM); particle sizes were determined by small angle X-ray scattering (SAXS). The inhibition of the thermally induced crystallization of ACP was probed by DTA/TGA and ex situ analysis of the products. The crystallization kinetics of ACP induced by water were monitored by in situ quantitative IR spectroscopy. Mixtures with known ratios of CPO and ACP were used for calibration. Standards were obtained by normalizing the spectra to the asymmetric stretching mode of phosphate and measuring the extinction of the symmetric stretching mode (only occurring in CPO). Reactions in the wet state were monitored based on the splitting of the asymmetric stretching mode of PO43– as the vibrational bands of water superimposed the symmetric stretch of PO43-. For experimental details see Section S1, Supporting Information. 3. RESULTS AND DISCUSSION In order to illuminate the crystallization mechanism of ACP the following questions are addressed: (i) Can an ACP precursor phase be made? (ii) What is its composition? Is the degree of hydration different from that of CPO? (iii) What is the local order of around the cobalt center compared to the crystalline phase and how is it translated into long-range order? (iv) Is ACP stabilized kinetically or thermodynamically? How strong is the stabilization against crystallization and how can it be triggered? (v) What is the role of water? Is it only a structural building block or does it have a kinetic function as well? (vi) What is to role of the reaction temperature? 3.1. Can an ACP precursor phase by made? To address this question, the very early stages of the precipitation of CPO

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Crystal Growth & Design consumption of ions in solution and is therefore diffusion controlled. Diffusion is enhanced in highly supersaturated solutions which leads to an increased tendency to crystallize (shorter lifetime of amorphous solid).44 Amorphous solids precipitating at high supersaturations are not in conflict with a classical kinetic crystallization pathway,45 but the application of the classical model of equilibrium states may be limited.46 In addition, we performed precipitation experiments with different starting concentrations, reaction times, and temperatures (see Table 1). The reaction proceeds to some extent during centrifugation, but the results were not affected by shorter centrifugation times (1 min and 3 min). Therefore, we can assume that the crystallization is interrupted during the early stages by centrifugation. Table 1. Experimental Conditions during Precipitation reaction time

a

Figure 1. X-ray diffractograms of (A) ACP (measured with high-energy synchrotron radiation) and (B) CPO with its calculated line pattern (C). (D) After heating to 500 °C under argon for 1 week the amorphous phase is still preserved. (E) At 600 °C only anhydrous Co3(PO4)2, the thermodynamically stable phase at that temperature was present. (F) Calculated line pattern of Co3(PO4)2.

(solubility product: 2.05 × 10–35 mol5 L–5) were “trapped” by fast mixing of aqueous solutions of Co(NO3)2 × 6 H2O and Na3PO4. To precipitate specific polymorphs, the rate of nucleation and growth was controlled by the degree of supersaturation. Low supersaturations and long reaction times should favor the formation of the thermodynamically stable compound at a given temperature. According to Ostwald’s rule of stages, the least stable (amorphous) polymorph will form first and transform to CPO, the stable compound at room temperature. Deep-blue ACP was precipitated for short reaction times (≤5 min) and low concentrations (15 mM of Co2+ and 10 mM of PO43-) at room temperature (see Figure 1A). In order to prevent crystallization according to Ostwald’s rule the precipitate was separated quickly by centrifugation, re-suspended in acetone, centrifuged, and dried in vacuo. In contrast, pink crystalline CPO was precipitated from concentrated starting solutions (300 mM Co2+ and 200 mM PO43-) at every temperature even for the shortest reaction times (see Figure 1B). The transformation of ACP to CPO proceeds through the

5s 5 min 5 min

reaction temperature (°C) 25 25 ≥30

concentration 2+ 3– Co /PO4 (mM) 15/10 15/10 15/10

≥20 min

25

15/10

≥5 s

25

300/200

result

amorphous amorphous partially a crystalline partially crystalline partially crystalline

for detailed analysis see Section 3.5.

3.2. What is the composition of this amorphous precursor phase and is the hydration different from that of CPO? The Co:P ratio of 3.00:2.02 in as-synthesized amorphous cobalt phosphate hydrate determined by ICP-MS, ICP-OES and EDX ) is in good agreement with the formula Co3(PO4)2 × n H2O (see Figure S2 and Table S1, Supporting Information). As hydrated intermediates with different water content may occur during the formation of CPO, the quantification of coordinating water in the amorphous solid (determined by quantitative IR spectroscopy and TGA) is an important piece of information. The degree of hydration may be essential for triggering the inertness of ACP. Table 2. Extinctions of the Water Bands of the IR Spectra Normalized to the Asymmetric Phosphate Stretching Mode of Different Cobalt Phosphates and the Resulting Water Content (Formula Units) Compound

E(δ(H-O-H))

Water content

Co3(PO4)2 × 8 H2O ACP ACP, 500 °C Co3(PO4)2

0.097 0.061 0.004 0.000

8.0 5.1 0.3 0.0

From IR spectra (normalized to the asymmetric phosphate stretching mode) of ACP (Figure 2, trace A) and crystalline CPO (trace B) a composition of Co3(PO4)2 × 5.1 H2O was extracted for ACP. The deformation band of coordinating water δ(H–O–H) appears at 1570–1650 cm-1 and its valence band ν(O–H) at 3000–



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3500 cm-1. The extinction of the ν(O–H) valence band allows to determine the amount of coordinating water (Table 2). Detailed results of the IR measurements are provided in Table S2, Supporting Information.

Figure 2. IR spectra normalized to the phosphate stretching mode of ACP (A) and CPO (B). At 500 °C (under argon, 1 week) ACP is still amorphous, but contains less water (C). At 600 °C Co3(PO4)2 (D), the thermodynamically stable phase at that temperature, was formed. The bands of CPO and Co3(PO4)2 are split due to lattice symmetry constraints.

Figure 3. Thermogravimetric (full line) and DTA trace (dashed line) of ACP under argon.

Thermogravimetry (Figure 3, solid line) was used as a second independent tool to determine the hydration of ACP. The weight loss of 20% during the heating process is caused by the release of structural (coordinating) water and suggests a composition of Co3(PO4)2 × 4.8 H2O for ACP. 3.3. What is the local order in ACP around the cobalt center compared to the crystalline phase and how is it translated into long-range order? The coordination number and hydration of the Co2+ centers are reflected in the UV-Vis spectra. The optical spectra of deep-blue (RT-)ACP in trace A and of pink/purple CPO in trace C of Figure 4 are

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compatible with an octahedral coordination of the Co2+ ions in CPO and a tetrahedral (or lower than octahedral) coordination in ACP, because optical transitions are allowed for non-octahedral coordination (in amorphous ACP), but forbidden for octahedral coordination (pink/purple color of crystalline compounds).

Figure 4. UV-Vis spectra of (A) RT-ACP, (B) HT-ACP, (C) CPO, and (D) anhydrous Co3(PO4)2. XANES and EXAFS spectroscopy (using synchrotron radiation) provide quantitative information concerning coordination and bond lengths. The XANES spectra in Figure 5A indicate a tetrahedral Co2+ coordination in ACP. XANES is sensitive to differences in the oxidation state. The XANES spectra show edge positions at 7723 eV for ACP and at 7727 eV for crystalline CPO. The pre-edge structure, a pronounced “white-line” accompanied by a pre-peak, can be interpreted as quadrupole transition from the 1s core state to empty 3d states. In the absence of inversion symmetry the pre-edge gains additional intensity due to the local 3d-4p wave function mixing, thus allowing dipole transitions to the 4p character of the 3dband (Figure S3, Supporting Information). The presence of a strong pre-edge peak and a smooth white line are characteristic for a non-centrosymmetric structure (tetrahedral coordination in ACP), a reduced intensity of a preedge peak and a pronounced white line indicate octahedral symmetry (for CPO). EXAFS is sensitive to the local environment of the probed atom in crystalline solids (and also in liquids, melts or amorphous materials): the distance to its nearest neigh bors (position of the peaks) and the number and type of neighboring atoms (coordination number CN) at a given separation, (amplitude of the peaks). Figure 5B shows EXAFS signals after Fourier transformation of χ(k). The first peak at a distance of R≈2.1 Å corresponds to a single scattering (SS) from the oxygen (O) neighbor and differs only slightly for the two investigated species. There are less oxygen neighbors of Co2+ in ACP in accordance with the results of UV-Vis and XANES spectroscopy. The signals at ≈3.0 Å and between 3.5 and 4.0 Å show that in ACP distances between Co2+ and its next neighbors are longer


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Figure 5. (A) XANES spectrum of CPO and ACP. Inset shows the pre-peak region. (B) EXAFS profile – amplitude of the Fourier transform.

than in the crystalline compound. Procedures for fitting of the CPO data are given in Section S3, Supporting Information.47 Although the complexity of hydrated cobalt phossphate (compared to ZnO33 or zirconia34) prevents an in situ total scattering analysis we can state that ACP is non-crystalline with tetrahedral Co2+ coordination and contains ≈5 water molecules of hydration. The long-range order in ACP at the µm and nm scale was studied by electron microscopy. ACP consists of nearly spherical nanoparticles with an average diameter of 23(1) nm. They have a smooth surface but a high degree of agglomeration (Figure S4, Supporting Information). The average size of the nanoparticles determined by SEM is in harmony with the particle diameter in a bulk sample (SAXS, 24.5 nm, Figure S5, Supporting Information). Samples collected after 10 min from solutions of the precursors with high concentrations (300 mM Co(NO3)2 and 200 mM Na3PO4) showed flower-like agglomerates of CPO (Figure 6 A). Higher magnification images (Figure 6 B) revealed that the “petals” develop by coalescence of the nanomaterial, indicating an oriented attachment process. TEM “snapshots” of ACP (from solutions of the precursors with low concentrations sampled 30 s after mixing and prior to centrifugation) showed coalesced “molten” ACP particles (Figure 6 C and D). Selected area electron diffraction (SAED) confirmed the product to be noncrystalline, but crystallization could be induced by irradiation under the electron beam. A “molten” character of the as-synthesized nanomaterial has been reported in related studies on CaCO3.15,48 3.4. Is ACP stabilized kinetically or thermodynamically? How strong is the stabilization against crystallization and how can it be triggered? The thermal stability of ACP was studied by DTA/TGA under Ar in the absence of water (Figure 3). A nearly continuous weight loss of about 20% between 100 and 700 °C together with a broad endothermic DTA signal between 100 and 200 °C indicated the loss of coordinating water. Surface water was removed by drying the sample under dynamic vacuum. In contrast,

CPO releases structural water in two well-defined steps (Figure S6, Supporting Information).

Figure 6. (A, B) SEM images of CPO sampled 10 min after rapid mixing equal volumes of 15 mM Co(NO3)2 and 10 mM Na3PO4 solutions. (C , D) TEM images of images of precipitated ACP particles sampled 1 min after rapid mixing equal volumes of 15 mM Co(NO3)2 and 10 mM Na3PO4 solutions. The SAED pattern (inset of (D)) is featureless, indicating the amorphous nature of ACP. An exothermic signal at 600 °C showed the crystallization of ACP to anhydrous Co3(PO4)2 as demonstrated by PXRD (Figure 1E) and IR spectroscopy (Figure 2D). Anhydrous Co3(PO4)2 crystallized as rounded particles with an average diameter of ≈ 1 µm (Figure S7, Supporting Information). Anhydrous Co3(PO4)2 transformed to CPO very slowly (>10 d by suspending in water), and the transformation was monitored ex situ (Figure S8, Supporting Information).



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ACP showed a high thermal stability up to 600 °C and remained amorphous after extended annealing at 500 °C under Ar for 1 week (Figure 1D and Figure 2C). The annealed product (HT-ACP) had a much lower water content (Co3(PO4)2 × 0.3 H2O) than ACP at room temperature as quantified by IR spectroscopy (Table 2) and TGA (see Figure S9, Supporting Information). The continuous weight loss upon heating in the TGA showed that the heating process might be interrupted at any time resulting in different amorphous cobalt phosphates with an adjustable degree of hydration. The crystal water content is not a major factor for the crystallinity of cobalt phosphate. For the thermally induced crystallization two scenarios may be envisaged: (i) Solid state transition of ACP to anhydrous Co3(PO4)2. This is an exergonic process above and and endergonic process below 600°C. (ii) Phase separation into CPO and a water deficient amorphous phase. As HT-ACP appears as a single phase upon heating to 500 °C, crystallization of ACP with a concomitant redistribution of coordinating water can be excluded. As water is an essential building block of the CPO structure, the formation of CPO stops in the absence of water; but crystallization of nanoparticular ACP proceeds in the presence of water (in aqueous solution, humid atmosphere etc.).

Figure 7. Crystallization kinetics of ACP in water. The red line is an exponential fit. 3.5. What is the role of water? Is it only a structural building block or does it have a kinetic function as well? The crystallization was monitored by in situ IR spectroscopy (Figure 7). The quantitative analysis was based on standards (cf. experimental part and Figure S10, Supporting Information). The densities of ACP (ρ=2.911±0.016 g/cm³) and CPO (ρ=2.812 g/cm³) were used for the standardization. Amorphous compounds are typically less dense than their crystalline counterparts. However, amorphous and crystalline cobalt phosphate hydrate show different hydration and therefore have different compositions. The van der Waals radius of H2O (3.1 Å) is slightly larger than the thermochemical radius of PO43– (2.3±0.42 Å) and the ionic radius of Co2+ (0.65 Å, coordination number=6). Thus, replacing Co2+ and PO43– with H2O leads to lower

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density.49,50 Quantitative analysis of the IR spectra showed an exponential connection between crystallinity and reaction time in the wet state. In contrast, ACP was stable for months under anhydrous conditions, but it crystallized slowly in air. This behavior can be explained by the lower activation energy for surface diffusion compared to diffusion in the solid state.40,41 The ion mobility of Co2+ and PO43– is caused by adsorbed water because surface diffusion coefficients are nearly equivalent to those for diffusion in solution. Water leads to crystallization, even in a surface water film due to atmospheric moisture.40

Figure 8. X-ray diffractograms of the precipitation products 3– 2+ of 10 mM PO4 and 15 mM Co 5 min. after mixing at (A) room temperature, (B) 30°C, (C) 40°C, (D) 50°C, and (E) 60°C showing the increasing degree of crystallinity. (F) Calculated pattern.

3.6. What is to role of the reaction temperature during precipitation? Crystallization of ACP in the wet state is triggered by the diffusion of additional water and its incorporation into the solid. As higher temperature promotes diffusion and the concomitant transformation from the amorphous to the crystalline state, the precipitation of cobalt phosphate from aqueous solution was performed at different temperatures. X-ray powder diffraction of the precipitates (Figure 8, from 10 mM PO43– and 15 mM Co2+) after 5 min showed the crystallinity of the precipitates to increase with the reaction temperature.


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The IR spectra (Figure 9A) revealed a similar trend. Partially crystalline cobalt phosphates were isolated and dried before the measurement. The IR spectra, recorded in the absence of adsorbed water, showed the symmetric phosphate stretch (present only in CPO), whose extinction allowed determining the ratios of the amorphous and crystalline phases at different temperatures (Figure 9B, mixtures of amorphous and crystalline material were used for standardization; Figure S11, Supporting Information). Figure 9C shows that the ratio of amorphous and crystalline phases depends on the temperature in an exponential manner. We assume a dissolution/recrystallization process to be responsible for the crystallization of ACP, which is accelerated at higher temperature. This is supported by experiments for the synthesis of ACP at higher initial concentrations. Partially crystalline samples were obtained at high initial concentrations (300 mM Co2+ and 200 mM PO43-). Again, enhanced diffusion due to high concentrations lead to faster crystallization. 4. CONCLUSIONS The synthesis of ACP nanoparticles from aqueous solutions supports the hypothesis that amorphous phases form prior to the thermodynamically stable compounds (with higher lattice energy) because surface energy domi-

nates the total energy balance. To avoid the kinetically controlled transformation of metastable ACP into crystalline CPO its separation must be fast. This was demonstrated by re-suspending ACP nanoparticles in water and monitoring the transformation to crystalline CPO. ACP is extraordinary stable in the absence of water up to 500 °C as determined by XRD, electron microscopy, SAXS, vibrational and UV-Vis spectroscopy, and thermal analysis. Annealing RT-ACP (Co3(PO4)2 × 5.1 H2O) at 500 °C lead initially to the formation of a second amorphous polymorph, HT-ACP (Co3(PO4)2 × 0.3 H2O). Anhydrous Co3(PO4)2 was formed at still higher temperature after full release of coordinating water. The high inertness of ACP against crystallization may be attributed to two facts: (i) CPO, the thermodynamically stable crystalline compound, contains more coordinating water than ACP; therefore, ACP requires additional water molecules as structural components for cobalt phosphate hydrates are intriguing examples for studying fundamental crystallization processes. (i) Besides the crystalline compounds CPO and Co3(PO4)2, there are different amorphous cobalt phosphate hydrates: ACP nanoparticles (containing approx. 5 H2O molecules per formula unit) with diameters of approx. 20 nm prepared by direct precipitation from aqueous solutions of Co2+ and PO43– at low concentrations and short reaction times. An amorphous

3–

Figure 9. (A) IR spectra normalized to the phosphate stretching mode of the precipitation products of 10 mM PO4 and 15 mM 2+ Co 5 minutes after mixing at (i) 25 °C, (ii) 30 °C, (iii) 40 °C, (iv) 50 °C, and (v) 60 °C showing the increasing degree of crystal3– 2+ linity. (vi) CPO synthesized with 200 mM PO4 and 300 mM Co 10 min after mixing. The arrow highlights the symmetric stretching mode. The inset shows the color of the samples. (B) Quantification of the degree of crystallinity (mass fraction ω of the crystalline compound in the mixture) by analyzing the extinction of the symmetric stretching mode. (C) Analysis of the dependence of the crystalline/amorphous mass fraction on the temperature showed an exponential dependence.



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HT-phase with a lower water content (0.3 H2O) that was obtained by annealing. (ii) The dependence of the optical absorption spectrum on the coordination and hydration state of Co2+ allowed differentiating between different polymorphs. (iii) The crystal structures of the different cobalt phosphate hydrates are simple enough to get fundamental knowledge of the nucleation and crystallization processes from aqueous solution. (iv) The reconstruction of the structure during the transformation of the amorphous to the crystalline phase occurred via dissolution and recrystallization. During the crystallization of ACP water served as a mineralizer that facilitates the transport of the insoluble “nutrient” to the seed crystal, i.e., it accelerates crystallization by increasing the mobility of the ionic constituents.51 Crystallization upon heating in the absence of water is inhibited because ACP is stabilized kinetically. The degree of crystallinity could be adjusted through the reaction temperature. Higher temperatures accelerate the crystallization process of ACP in solution through enhanced dissolution and crystallization. The charge density of the Co2+ cation, which makes the hydration enthalpy comparable to the binding energy of the counter-anions and the moderate acidity of the phosphate anions which leads to the formation of hydrogenbonded networks induce a multistage crystallization process. This process involves the homogeneous precipitation of nanocrystals of hydrated precursor phases that may aggregate by condensation/dehydration ASSOCIATED CONTENT Section S1 – Experimental Details; Section S2 – Additional Figures and Tables: Figure S2 EDX spectrum of RT-ACP; Figure S3 Possible pre-peak transitions for Co3(PO4)2 × 8 H2O; Figure S4 SEM image of RT-ACP; Figure S5 SAXS data of RT-ACP; Figure S6 DTA/TGA of Co3(PO4)2 × 8 H2O; Figure S7 SEM image of Co3(PO4)2; Figure S8 IR spectrum of Co3(PO4)2 after different treatments; Figure S9 DTA/TGA of HT-ACP; Figure S10 Standards for quantification of the different cobalt phosphate hydrates (splitting of the asymmetric stretching mode); Figure S11 Standards for quantification of the different cobalt phosphate hydrates (extinction of symmetric stretching mode); Table S1 Results of ICP-MS and ICP-OES measurements; Table S2 Results of IR spectroscopy in detail; Section S3 – Fitting EXAFS data with ARTEMIS

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was funded by the Deutsche Forschungsgemeinschaft (DFG) within the priority program 1415. S. B. is recipient of a VCI Fellowship and a collegiate of the MAINZ Graduate School of the State of Rhineland-

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Palatinate. We thank R. Jung-Pothmann for XRD measurements, B. Meermann for ICP-MS and ICP-OES measurements, and M. Schedel for the density measurement. The microscopy equipment is operated by the Electron Microscopy Center, Mainz (EMZM), which was supported by the State Excellence Cluster CINEMA. We are indebted to Prof. H. Frey for access to the IR spectrometer. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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The Role of Water During the Crystallization of Amorphous Cobalt Phosphate Nanoparticles Sven Bach,†,§ Martin Panthöfer,† Ralf Bienert,$ Ana de Oliveira Guilherme Buzanich,$ Franziska Emmerling,$ and Wolfgang Tremel†,*

Synopsis: The transformation of amorphous cobalt phosphate hydrate nanoparticles (ACP) into crystalline Co3(PO4)2 × 8 H2O (CPO) was investigated by thermal analysis (TGA) and quantitative IR spectroscopy. ACP contains 5 molecules of structural water per formula unit. XANES/EXAFS spectroscopy shows the Co2+ coordination in ACP to be tetrahedral and octahedral in crystalline CPO. ACP is stable in the absence of water up to 500 °C. The crystallization kinetics of ACP in the presence of water was monitored by quantitative in situ IR analysis.

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Role of Water During Crystallization of Amorphous Cobalt Phosphate

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