Evolution of Nanostructured Manganese (Oxyhydr)oxides in Water

Apr 9, 2010 - Evolution of Nanostructured Manganese (Oxyhydr)oxides in Water through MnO4− ... E-mail: [email protected]., ⊥...
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DOI: 10.1021/cg901394v

Evolution of Nanostructured Manganese (Oxyhydr)oxides in Water through MnO4- Reduction

2010, Vol. 10 2168–2173

David Portehault,†,‡,^ Sophie Cassaignon,*,†,‡ Emmanuel Baudrin,# and Jean-Pierre Jolivet†,‡ †

UPMC Univ Paris 06, UMR 7574, Chimie de la Mati ere Condens ee de Paris, Coll ege de France, ere 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France, ‡CNRS, UMR 7574, Chimie de la Mati Condens ee de Paris, Coll ege de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France, and # Laboratoire de R eactivit e et Chimie des Solides, UMR CNRS 6007, Universit e de Picardie-Jules Verne, 33 rue Saint-Leu, 80039 Amiens cedex, France. ^ Currently affiliated with the Max Planck Institute of Colloids and Interfaces, Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany. Received November 6, 2009; Revised Manuscript Received January 7, 2010

ABSTRACT: Manganese (oxyhydr)oxides are synthesized at low temperature (60 or 95 C) from MnO4- reduction by S2O32in water. Particles are characterized by X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, and electron diffraction. The reactivity of S2O32- as a reducing agent is tuned by the temperature in order to selectively synthesize manganite γ-MnOOH at 60 C and hausmannite Mn3O4 at 95 C. Feitknechtite β-MnOOH is identified as an intermediate phase during the formation of γ-MnOOH. Twinned triangle-shaped feitknechtite particles are obtained and proposed to grow through nanorod self-assembly. The transformation of feitknechtite nanotriangles into manganite nanowires occurs through dissolution of the feitknechtite particle core and recrystallization on the edge of the feitknechtite nanotriangles.

Introduction Design of nanomaterials consists of adjusting their main characteristics (crystalline structure, size, shape, and texture) and is of major importance to optimize their physical and physicochemical properties in areas such as optics, magnetism, or electrochemistry. The solution route toward these materials seems nowadays to be one of the most versatile and simplest to use as compared to physical ways which imply costly procedures and equipment. Many efforts have been devoted to efficient control of the synthesis, usually through various organic agents such as templates, surfactants, or complexing species.1-4 Despite the high control provided by these processes, the resulting costly recycling procedures urge the scientific community to search for alternative, environmentally friendly methods.4-6 From this standpoint, chemistry in water is highly attractive for oxide synthesis, as demonstrated by the versatility of aqueous chemistry of many technologically interesting elements such as aluminum, iron, or titanium.4,5,7 Manganese also is of great interest because its oxides are potential candidates for various applications such as redox catalysis, energy storage, and sensing.8-11 From a more fundamental point of view, the structural diversity of manganese (oxyhydr)oxides raises many questions about the control of their chemistry and their structural relationship. Among the numerous manganese (oxyhydr)oxides with Mn oxidation states varying from II to IV, birnessite “δMnO2” (Scheme 1) is a layered mixed valency III-IV manganese oxide which contains some alkaline ions between the MnO6 layers.12 Manganite γ-MnOOH (Scheme 1) is made of MnO6 octahedra single chains linked together by the apex and forming rutile-like 1  1 section tunnels inside which protons are located.13 Hausmannite Mn3O4 (Scheme 1) has a direct *To whom correspondence should be addressed. E-mail: sophie.cassaignon@ college-de-france.fr. pubs.acs.org/crystal

Published on Web 04/09/2010

spinel-like structure.14 Both manganite and hausmannite contain Mn(III) cations (d4), and their structures are therefore distorted because of the Jahn-Teller effect. Manganite nanoparticles usually occur as nanowires,15-21 while hausmannite is often obtained as rhomboedral particles through solution routes3,22-28 or nanowires through solid-state reactions.29-32 Manganite and hausmannite are potential catalysts for redox reactions, as well as supercapacitor materials.21,33,34 Feitknechtite β-MnOOH35,36 is a metastable polymorph of which the structure has been described as lamellar,37 despite the difficulties of obtaining well crystalline compounds and thus high quality diffraction data. Until now, synthesis difficulties have impeded studies on its growth and morphology at the nanoscale. We present in this report a low temperature (60-95 C) procedure for selective synthesis of manganite and hausmannite via permanganate MnO4- reduction by thiosulfate ions. Careful control of the synthesis temperature enables one to tune the reactivity of the reducing agent in water. Furthermore, we show evidence for the formation of feitknechtite as an intermediate for the formation of manganite nanowires. The first study of feitknechtite nanoparticles is then provided, and morphological evolution is discussed in terms of selfassembly and solution route processes. Experimental Procedures Synthesis. Manganite, hausmannite, and feitknechtite have been synthesized using the reduction of permanganate MnO4- by thiosulfate S2O32- ions in water with total Mn concentration of 0.1 mol 3 L-1. After nitrogen bubbling during 30 min, the pH of a potassium permanganate KMnO4 aqueous solution (225 mL, 25 mmol, Aldrich) was adjusted to 11 by addition of a ca. 2 mol 3 L-1 KOH solution. A K2S2O3 solution (20 mL, 75 mmol, Aldrich) at pH 11 was then added under vigorous stirring and nitrogen bubbling. The volume was adjusted to 250 mL with an aqueous KOH solution at pH 11. Suspensions were aged in an oven at 25, 60, or 95 C for r 2010 American Chemical Society

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Scheme 1. Representation of the Birnessite δ-MnO2, Manganite γ-MnOOH, and Hausmannite Mn3O4 Frameworks

Figure 2. Powder XRD pattern of a sample obtained after aging for 23 days with Mn:S = 1:6 at 30 C using S2O32-. Pentagon: groutite R-MnOOH, circles: manganite γ-MnOOH, filled diamonds: feitkneichtite β-MnOOH, stars: unindexed peaks. Transmission Electron Microscopy (TEM). The nanoparticle morphology and orientation were studied by TEM and selected area electron diffraction (SAED) using a JEOL 100CX (100 kV) apparatus, and high resolution TEM (HRTEM) using a Philips CM20 (200 kV) apparatus. Samples were prepared by evaporating a drop of aqueous diluted suspension on a carbon-coated copper grid. The d-spacings obtained from SAED patterns were calibrated using the Au pattern. Figure 1. Powder XRD patterns of samples obtained after aging for different durations with Mn:S = 1:6 at 60 (a) or 95 C (b) using S2O32-. Squares: birnessite δ-MnO2, circles: manganite γ-MnOOH, filled diamonds: feitkneichtite β-MnOOH, opened diamonds: hausmannite Mn3O4. 30 days and shaken once per day. Samples were collected at different times (over the duration of 30 days) and recovered by centrifugation. The final pH of the reaction medium was ca. 8. The powders were washed three times with deionized water, first dried at room temperature under nitrogen flux and then in an oven at 95 C overnight. The reaction is quantitative and yields approximately 4 g of solid. Techniques. Elemental Analysis. K, Mn elemental ratios were determined at the CNRS Service Central d’Analyse, USR 59. Average Oxidation State (AOS) Determination. The AOS was determined using a two-step procedure described previously.38 The particles were dissolved in a H2SO4 solution, while MnIII and MnIV were reduced to soluble MnII by Fe2þ. Titrations of the resulting Mn2þ and the excess of Fe2þ enabled us to determine the AOS with a relative error of 5%. X-ray Diffraction (XRD). Powder XRD measurements were performed with a Philips PW1050/25 X-ray diffractometer operating in the reflection mode at Cu KR radiation. The data were collected in the 8-80 range (2θ) with 0.02 steps and a counting time of 10 s. JCPDS files were used to identify manganite γ-MnOOH (41-1379), groutite R-MnOOH (24-0713), and hausmannite Mn3O4 (27-0734). Feitknechtite β-MnOOH was identified according to the pattern reported by Mandernack et al.39 Field Emission Scanning Electron Microscopy (FESEM). FESEM observations and energy dispersive X-ray analysis (EDX) have been performed using an Hitachi apparatus (2 kV) equipped with an Oxford analyzer.

Results MnO4-

reduction by S2O32- ions at pH 11 leads to immediate precipitation of a black solid which is shown by XRD to be poorly ordered (Figure 1a, 0 min). After a short aging duration, a mixture of birnessite δ-MnO2 and feitknechtite β-MnOOH39 is obtained (Figure 1a, 1b, and Figure 2b, 3 h) which evolves toward manganite γ-MnOOH with a pH value of 8 for the aged suspension. Manganite structure is preserved at 60 C, while aging 30 days at 95 C leads to hausmannite Mn3O4. Feitknechtite is always obtained in the presence of other phases. Attempts to obtain pure feitknechtite before evolution to manganite γ-MnOOH or hausmannite Mn3O4, by lowering the temperature (30 C) or modifying the Mn:S initial ratio, resulted in unindexed peaks, together with a peak at 21.2 (2θ), which can be unambiguously assigned to the (101) intense reflection of metastable groutite R-MnOOH phase (tunnels with a 2  1 MnO6 cross section) (Figure 2). Energy dispersive X-ray (EDX) analysis performed during FESEM observations indicates that manganite and hausmannite samples are solely composed of Mn and O. Samples comprising feitknechtite contain variable amounts of K which is likely due to birnessite impurities which contain intercalated Kþ ions. This statement is confirmed by using a smaller EDX probe which highlights heterogeneities with K:Mn varying from 0 to 0.3 within the same sample. Manganite γ-MnOOH is obtained as nanowires with a diameter of ca. 40 nm and length ranging from 200 nm to 2 μm (Figure 3a). Hausmannite Mn3O4 is obtained as polydispersed

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faceted particles with a diameter between 100 and 600 nm and shapes varying from rhomboedra to rods (Figure 3b). Samples obtained after a short aging duration and identified by XRD as a mixture of manganite and feitknechtite exhibit two distinct morphologies (Figure 4): nanowires similar to those observed for pure manganite samples (Figure 3a) and triangles, with edges of ca. 400 nm. Triangle assemblies are also observed, with particles which are stacked on their basal faces (Figure 4a) or head-to-head (Figure 4b,c). Moreover, triangles themselves are composed of laterally stacked nanowires (Figure 4b,c). Selected area electron diffraction (SAED) confirms that nanowires obtained after reduction by thiosulfate ions at 60 C are manganite particles (Figure 5). The wire axis is identified by SAED and HRTEM as the [101] direction, which corresponds to the MnO6 octahedra strings (Figure 5). Feitknechtite triangles exhibit a straight median axis indicating twinning (Figure 6). Whatever the particle, the angle at the top is ca. 32 on both sides of the twin axis. In agreement with FESEM, both edges of each triangles exhibit lateral stacking of nanowires (Figure 6a). The SAED pattern (Figure 6b) recorded on a particle laid on its base (Figure 6c) by selecting

Figure 3. FESEM images of manganite γ-MnOOH (a) and hausmannite Mn3O4 (b) samples obtained by using S2O32- with Mn:S = 1:6 at respectively 60 or 95 C for 30 days.

Portehault et al.

an area without the central twinning has a pseudohexagonal symmetry and can be indexed according to a monoclinic cell, a = 5.63 A˚, b = 2.89 A˚, c = 4.84 A˚, β = 108, in agreement with the XRD patterns. TEM performed after aging 17 h at 60 C indicates laterally stacked nanorods of ca. 10 nm diameter, together with particles which exhibit a “crumpled paper” morphology

Figure 5. (a) TEM image, (c, d) SAED patterns, (e) HRTEM image, (e insert) corresponding Fourier transform, and (b) scheme of the orientation of manganite nanowires obtained after reduction by S2O32- with Mn:S = 1:6.

Figure 4. FESEM images of samples composed of manganite γ-MnOOH and feitknechtite obtained after reduction by S2O32- with Mn:S = 1:6 at 60 C during 4 days.

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Figure 6. (a) TEM image of feitknechtite particles obtained after reduction by S2O32- with Mn:S = 1:6 at 60 C during 4 days. (b, c) SAED pattern and corresponding TEM image of a feitknechtite particle obtained after reduction by S2O32- with Mn:S = 1:6 at 60 C during 2 days. Distances are given in angstr€ oms and the indexation is made according to the monoclinic cell a = 5.63 A˚, b = 2.89 A˚, c = 4.84 A˚, β = 108.

(Figure 7a). The latter has already been observed in various MnOx(OH)y syntheses and corresponds to an early poorly ordered manganese dioxide, which is made of disordered MnO6 layers.40 The SAED pattern (Figure 7b) of laterally aggregated nanorods cannot be indexed as manganite γ-MnOOH or groutite R-MnOOH, and neither as the various MnO2 polymorphs. The assembly between such primary nanorods is still observed within the early triangles (Figure 7c,d) which exhibit streaks running parallel to their edges. The center of the early triangles is composed of a rugged median axis highlighting the nanorod assembly. After aging at 60 C for 2 days (Figure 7f) or 4 days (Figure 6a), the central part of feitknechtite triangles do not exhibit any streaks and is totally smoothed. The edge of such triangles is composed of nanowires with a diameter of ca. 50 nm. This is much larger than the early nanorods and identical to the diameter of the final manganite γ-MnOOH wires. A synthesis was performed with a low Mn:S ratio to slow down the reaction. TEM performed after 14 days (Figure 7g) exhibits manganite nanowire aggregates retaining the triangle morphology without the central part. Discussion Structural Control and Morphological Evolution: Manganite γ-MnOOH and Hausmannite Mn3O4. Simplified Pourbaix diagrams of manganese and sulfur (Figure 8)41,42 show that, from a thermodynamical point of view, evolution at pH 8 with a large excess of S2O32- ions should lead to a reduction of MnO4- ions toward hausmannite Mn3O4, as observed upon aging at 95 C. In this case, reduction of Mn(VII) by thiosulfate proceeds through three steps. First, Mn(IV) ions precipitate as a quasi-amorphous solid which, in the second

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Figure 7. (a-f) TEM images and corresponding SAED pattern of feitknechtite particles obtained after reduction by S2O32- with Mn:S = 1:6 at 60 C upon different aging duration. (g) TEM image of manganite particles obtained after reduction by S2O32- with Mn:S = 1:1 at 60 C during 14 days.

Figure 8. Pourbaix diagrams of manganese and sulfur at 25 C. The sulfur diagram for pH < 7 has been simplified for clarity. For the same purpose, S2O82- ions are not represented.

step, is reduced toward Mn(III) containing feitknechtite and manganite. The third step corresponds to the reduction of Mn(III) to Mn(II) and proceeds slowly at 95 C because all Mn ions are precipitated at an evolution pH 8. Evolution from manganite γ-MnOOH nanowires to large polyhedral hausmannite Mn3O4 particles results from a dissolutionrecrystallization process which is rate limited by the low solubility of the Mn species. Then, at the lower temperature 60 C, reduction reactions are strongly slowed down and the third step is “kinetically decorrelated”. Manganite γ-MnOOH is then obtained as a pure compound. Thus, the milestone for structural control is the temperature which enables one to adjust the kinetics of the involved reactions. Feitknechtite Structure. Interestingly, the XRD pattern (Figure 2) reported in this study differs significantly from the

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Scheme 2. Reaction Scheme for the Formation of Feitknechtite Triangles and Their Evolution toward Manganite Wires

report of Bricker36 who proposed a tetragonal system with lattice parameters a = 8.6 A˚, c = 9.3 A˚. In fact, these values were obtained with a mixture of hausmannite and feitknechtite, where peak overlapping hindered accurate evaluation of the system symmetry as well as the cell parameters. On the contrary, the pattern reported herein is similar to the one reported on pure feitknechtite by Mandernack et al.39 who already pointed out the discrepancy with the previous work,36 presumably because of poor quality diffraction data. Although a monoclinic cell (a = 5.63 A˚, b = 2.89 A˚, c = 4.84 A˚, β = 108) could be proposed in our study according to XRD and SAED, this result cannot account satisfactorily for feitknechtite because unindexed reflections are still present (e.g., 29.7 and 30.8 on the XRD pattern Figure 3). Nevertheless, the cell proposed in this study exhibits a and b parameters close to those of the layered birnessite δ-MnO2 monoclinic cell (a = 5.15 A˚, b = 2.84 A˚, c = 7.17 A˚, β = 100.8) proposed by Post et al.43 In this latter case, the (a,b) plane contains the MnO6 sheet. Therefore, the cell derived from XRD and SAED meets the usual representation of feitknechtite as a lamellar compound,36,39,44 which is usually observed as an intermediate phase between lamellar compounds pyrochroite Mn(OH)2 (brucite structure) and birnessite δ-MnO2, with a lower interplanar distance of 4.6 A˚ compared to the value of 7.1 A˚ for birnessite, in agreement with the absence of interlayer cation. Further works are in progress to provide a more accurate representation of the structure. Feitknechtite Growth and Its Transformation into Manganite. Feitknechtite triangles appear to be formed through lateral aggregation of primary nanorods (Figure 7a-d and Scheme 2). The resulting early triangles exhibit strikes which are usual in assembly directed growth processes. Smoothing of the surface and the twinning area can be explained by Oswald ripening45 which usually occurs at a longer time scale than aggregation.38,46 Figure 7f,g indicates that manganite γ-MnOOH wires nucleate at the edge of the feitknechtite triangles (Scheme 2). The nanowire growth is parallel to the triangle edges. The chevron-like nanowire assembly (Figure 7g) observed after transformation preserves the outer edges of the initial triangles. By taking into account the higher stability of manganite compared to feitknechtite, the central part of metastable feitknechtite triangles dissolves and recrystallizes in a more stable structure at the manganite nanowire tip (Scheme 2). Conclusion This work brings a new contribution to the knowledge of the chemistry of manganese in aqueous solution. Temperature is the key parameter to control the structure between pure γ-MnOOH or Mn3O4 compounds through adjustment of the reduction kinetics. The first report on feitknechtite formation

processes and morphology is presented. Further work is currently under way to elucidate the feitknechtite structure. Such efforts could provide better understanding of the relationship between manganite γ-MnOOH and feitknechtite β-MnOOH structures and morphology. Acknowledgment. The authors are indebted to Dr. Elisabeth Tronc for fruitful discussions. We also acknowledge Patricia Beaunier (UPMC), Stephan Borensztajn (UPMC), and Dominique Jalabert (university of Orleans) for TEM, SEM, and HRTEM. Note Added after ASAP Publication. This manuscript was originally published on the web on April 9, 2010, with changes made in the captions of Figures 2-4, 6, and 7. The corrected version was reposted on April 14, 2010.

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