Formation Mechanisms of Nanocrystalline MnO2 ... - ACS Publications

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Formation mechanisms of nanocrystalline MnO2 polymorphs under hydrothermal conditions Steinar Birgisson, Dipankar Saha, and Bo B. Iversen Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01304 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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Crystal Growth & Design

Formation mechanisms of nanocrystalline MnO2 polymorphs under hydrothermal conditions Steinar Birgisson, Dipankar Saha and Bo B. Iversen* Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, DK-8000 Aarhus C, Denmark.

ABSTRACT: Understanding and controlling the polymorphism of manganese dioxide (MnO2) is of vital importance in many nanoscale applications. Here in situ powder X-ray diffraction (PXRD) in combination with in situ total X-ray scattering are used to reveal the formation mechanism as well as polymorph evolution of MnO2 under hydrothermal synthesis conditions. A “PXRD invisible” amorphous phase with a local structure resembling α-MnO2 (denoted αMnO2(A)) is observed at all reaction stages, and it never fully disappears from the reaction solution. The MnO2 phase evolution involves initial formation of δ-MnO2, which transforms to α-MnO2, and then subsequently to β-MnO2. The phase transformations between different polymorphs do not involve dissolution-recrystallization, but they occur via solid-state mechanisms. However, the amorphous α-MnO2(A) phase plays a key role since it is consumed in growing both the α- and β-MnO2 polymorphs. Overall, the polymorphism of the crystalline product can be controlled through reaction time and temperature to form either nanocrystalline

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and disordered δ-MnO2, nanocrystalline α-MnO2 or nanocrystalline β-MnO2. At the lowest temperature (200oC) the very early growth of α-MnO2 appears to be by oriented attachment along (110) crystal planes of primary nanorods, but this is quickly followed by rapid growth along the c-direction supported by consumption of α-MnO2(A).

Introduction Solvothermal synthesis is an efficient method for producing inorganic nanomaterials, and it potentially provides strong control of properties such as crystalline phase, particle size and morphology.1-4 However, obtaining an understanding of how to control these properties can be challenging, and in order to move beyond tedious parametric “trial and error” studies, in situ experiments are increasingly attempted to obtain real time information about the processes leading to the produced material.5 In situ powder X-ray diffraction (PXRD) is often the preferred in situ method since Rietveld refinement is highly robust providing both atomic scale and microstructure information along with simultaneous quantification of co-existing crystalline phases in the reaction mixture.6-19 In this way in situ PXRD experiments can lead to practical synthesis control, but they may also provide fundamental general information about crystal nucleation and reaction mechanisms. The main drawback of PXRD is that it only follows the crystalline parts of the reaction mixture making pre-nucleation events invisible. However, even final reaction products may contain substantial amounts of amorphous phases affecting both the material properties and the overall reaction economy. Undetected amorphous phases will obviously limit the understanding of a given reaction pathway. Recently, in situ total scattering (TS) with subsequent pair distribution function (PDF) analysis was introduced in studies of


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solvothermal reactions,20-25 and since this technique is a local probe independent of long range order a wealth of information on pre-nucleation structures has emerged.5 Here we follow the development and transformations of different crystal polymorphs in a reaction system of large complexity, namely manganese dioxide (MnO2). It is shown that a PXRD-invisible amorphous phase is present continuously throughout the solvothermal reaction, and that it is involved in the specific polymorph transformations. MnO2 is an inexpensive and environmentally friendly multi-functional material, which has been used in numerous applications. As a catalyst MnO2 can be used for aerobic oxidation of alcohols, oxidation of CO, oxygen reduction in alkaline fuel cells and water electrolysis.26-29 MnO2 is commonly utilized as an electrode material in primary and secondary batteries.30-37 MnO2 can furthermore be used in gas sensors and as a molecular sieve.38, 39 In addition, MnO2 based structures are building blocks for derived materials such as MxMnO2 (M = Li and Na) used for ion battery application.40-43 MnO2 extracted directly from natural sources, such as the ocean floor, often exhibit a mixture of polymorphs, poor crystallinity and variation in chemical composition.38 These material properties have a large influence on the functionality of MnO2.30, 38, 44

Therefore, chemical synthesis of MnO2 is preferred to obtain a homogeneous material

exhibiting the specific material properties.


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Figure 1. Crystal structures of a) β-MnO2 (P42/mnm), b) α-MnO2 (I4/m) and c) δ-MnO2 (C2/m). Pink octahedra: MnO6, purple sphere: K+, red sphere: H2O

MnO2 crystallizes in many different crystalline polymorphs. The basic building blocks of all the phases are MnO6 octahedra that connect either by corners or edges to form tunneled, layered or spinel type structures. The most commonly observed crystal phases are: α (I4/m), β (P42/mnm), γ (C2/m), δ (R3m, P63/mmc or C2/m) and λ (Fd3m).45 β-MnO2 forms in the rutile structure with MnO6 octahedra connecting along edges to form chains. These chains connect via corner sharing to form a tunneled structure, Figure 1a. The tunnels align along the


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crystallographic c-direction and have the dimension of a one by one octahedron (1x1). β-MnO2 is the thermodynamically stable MnO2 polymorph at ambient conditions.45 In α-MnO2, two chains connect via edge sharing, and the double chains then connect through corner-sharing to form two by two (2x2) and 1x1 tunnels extending along the crystallographic c-direction, Figure 1b. The larger 2x2 tunnels are stabilized by large ions (in this case potassium). Therefore, α-MnO2 has the chemical formula KxMnO2 (x ≈ 0.16) and the average oxidation state of the manganese ion is less than four. For δ-MnO2 the MnO6 octahedra connect via edges in two dimensions forming MnO2 sheets. The sheets stack in parallel forming a layered structure, Figure 1c. The structure is relatively open and different ions and molecules can reside in between the MnO2 sheets. The nature of the stacking and the distance between the MnO2 layers can vary, giving phases of different symmetry (space group). Despite of this many MnO2 phases have the same overall structural pattern of stacked MnO2 sheets, and they are all referred to as δ-MnO2. In the present case the chemical formula will be KxMnO2·yH2O and it has, similar to α-MnO2, a lower than four average oxidation state of the manganese ion. δ-MnO2 can additionally exhibit different degrees of disorder including parallel shifts of the MnO2 sheets relative to each other or total misalignment of the sheets.46 In a typical hydrothermal synthesis of MnO2, a reduction-oxidation reaction occurs where potassium permanganate (KMnO4) is reduced using manganese sulfate (MnSO4).

2() + 2 () + 3 () + 3 () + 2 () → 5 () + 2() + () + 4() + 2 ()


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This reaction has been shown to produce α-, β- and δ-MnO2 under hydrothermal and refluxing conditions.2, 3, 47-54 Previous studies on MnO2 synthesis report needle shaped nanocrystals of αand β-MnO2 and sheet-shaped δ-MnO2 nanocrystals. However, the studies differ in their claims about how to control the crystalline phase, e.g. via pH, reaction time and temperature. One of the predominant ways to tune between formation of α-, β- and δ-MnO2 is varying the fraction of potassium in the reaction solution.3, 47 More potassium leads to the more open structure of δMnO2, whereas intermediate potassium content gives α-MnO2 and low amounts give β-MnO2.3, 47

Here, we investigate the stoichiometric reaction with the KMnO4:MnSO4 molar ratio being

equal to 2:3, by in situ PXRD and in situ PDF analysis.

Experimental section In situ experimental setup. The solvothermal reactor for in situ PXRD/PDF measurements has been described in detail by Becker et al.16 In brief, the in situ experimental setup consists of a capillary reactor containing the reaction solution. For the PXRD measurements the capillary was a single crystal sapphire tube with inner diameter of 0.7 mm and outer diameter of 1.6 mm. For the PDF measurements the capillary was a fused silica tube with inner diameter of 0.70 mm and outer diameter of 0.85 mm. The reactor is pressurized using an HPLC pump (Lab Alliance: 902497 Rev A) and the pressure was kept constant at ~250 bar during the measurements. The reaction was initiated by heating with hot air gun. Due to the small size of the reactor and high air flow, the heating rate of the reaction solution was very high and constant temperature was reached in few seconds (see the reaction temperature calibration in Supporting Information). The solvothermal reaction conditions, i.e. heating rate and tube diameter, are similar to those found in


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continuous flow reactors or pulse flow reactors used in lab scale experiments.55, 56 The in situ PXRD experiments were performed at beamline I711, MAXII, MAX-lab, Sweden, using a wavelength of 0.9941 Å. A MAR165 CCD 2D detector was placed ~90 mm behind the reactor giving a typical Q-range of 0.7-4.2 Å-1. The exposure time was 4 seconds and with a detector readout time of 1 second, this gave an overall time resolution of 5 seconds. The in situ TS experiments were performed at beamline P02.1 at PETRA-III, DESY, Germany, with a wavelength of 0.207 Å. A PerkinElmer XRD 1621 amorphous silicon area detector was placed ~200 mm behind the reactor giving a typical Q-range of 0.5-24 Å-1. The exposure time was set to 1 sec and with a detector readout time of few milliseconds this gives a time resolution of ~1 sec. Precursor preparation. Precursor solutions were prepared by dissolving ~1.27 g KMnO4 in 20 ml deionized water (DI) and adding ~2.02 g MnSO4·H2O into the solution under vigorous magnetic stirring. This results in a molar ratio of KMnO4:MnSO4 equal to 2:3, as suggested by the reaction stoichiometry, and an initial manganese concentration of ~1 M. Few moments after mixing, the solution changes color from purple to brownish and formation of particles is observed giving a slurry-like solution. The resulting slurry is stirred for ~20 minutes before being transferred to a 20 ml plastic syringe. Note that different mixing procedures, e.g. dissolving KMnO4 and MnSO4·H2O powders simultaneously in the same beaker or mixing solutions of already dissolved KMnO4 and MnSO4·H2O, gives different results both qualitatively and quantitatively. Therefore, the present results only refer to the exact method of mixing described above. For obtaining TEM images of the precursor material a separate batch of precursor solution was prepared and allowed to precipitate. The solution was then centrifuged, decanted and washed three times using DI water followed by drying in a vacuum oven at 60°C.


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TEM imaging. (High resolution) transmission electron microscopy (HR-TEM) was used to confirm the presence of a mixture of crystalline and amorphous material in the precursor solution. HR-TEM was performed on a TALOS F200A with a TWIN lens system, X-FEG electron source and Ceta 16M Camera. Synthesis conditions. The precursor solution was injected into the in situ capillary reactor via 1/16” Swagelok® tubing, after vigorous shaking of the syringe, few minutes before initiating the experiment. The reactor was pressurized to ~250 bar and kept constant during each experiment. The experiments were conducted in isothermal conditions with the heat gun set point at 200, 225, 250, 275 and 300°C. Due to heat losses from the heat gun nozzle to the capillary reactor the actual reaction temperature is ~10% lower than the set point temperature, see Supporting Information for temperature calibration. For simplicity the experiments are denoted by the set point temperature. Data treatment. The sample to detector distance was determined by measuring data on a NIST LaB6 standard in 0.3 mm glass capillary and calibrating using Fit2D.57 The measured data were integrated using Fit2D. Structural information as a function of reaction time was extracted from the PXRD data by sequential Rietveld refinement58 using FullProf.59 The structural models for the tetragonal αMnO2 phase (space group: I4/m) and tetragonal β-MnO2 phase (space group: P42/mnm) were taken from COD-1008322 and ICSD-393, respectively. The sequentially refined instrumental parameters were zero point displacement and a background using linear interpolation between ~20 points with refinable height chosen away from the Bragg peaks. The sequentially refined structural parameters for both phases were scale factor, unit cell parameters and peak profile using Thompson-Cox-Hastings pseudo-Voigt (TCHpV) profile parameters,59 whereas atomic


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coordinates were kept fixed. The instrumental resolution was taken into account by refining TCHpV profile parameters for the LaB6 standard and including them in the sequential Rietveld refinements. The best peak profile fit was obtained with peak broadening originating purely from anisotropic crystallite size corroborating the formation of nanocrystallites. The anisotropic crystallite size was modelled using a platelet/needle model59 and it showed that both phases were elongated along the crystallographic c direction forming rod shaped crystallites. Additionally, the site occupancy for potassium was refined for α-MnO2. Note that only relative changes of the site occupancy within each in situ experiment are analyzed (see Supporting Information). The time resolved TS was converted into PDF data by Fourier transformation using PDFgetX3.60 TS data measured on a capillary filled with deionized water was used for background subtraction. The utilized Q range was 0.5 – 18 Å-1, rpoly = 0.9 and the PDFs were calculated for r = 1 - 70 Å on a 0.01 Å r-grid. Structural information as a function of reaction time was extracted via sequential real space Rietveld refinements using PDFgui.61 To save computer time, the sequential refinements were performed on a coarser r-grid than the actual PDF data, according to the Nyquist-theorem. This has been shown not to affect the refined parameters.62 Dampening of the PDFs due to instrumental resolution was implemented by keeping Qdamp fixed to the value refined for the LaB6 standard. For both time resolved PDF data sets a small correlation peak at ~1.4 Å is observed throughout the reaction. This peak fits well with the S-O bond distance in a sulfate ion in agreement with SO42- ions being present in high concentration in the reaction solution. A slightly improved fit can be obtained by including a K2SO4 phase with a correlation length corresponding to one SO42- ion in the sequential refinements. However, no significant changes in the refinement results of other phases were observed when K2SO4 was included. The SO42- ion is not included in the final sequential


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refinement due to convergence problems. This is probably due a combination of dynamics in the structure, limitations of real space Rietveld refinements and limited data quality making it difficult to accurately determine structural parameters for such a small structural unit. The structural models for the α- and β-MnO2 phases were the same as in the Rietveld refinements of the PXRD data, whereas the structural model for monoclinic δ-MnO2 (space group: C2/m) was taken from ICSD-68918. The amorphous phase was modelled with the α-MnO2 structure using a small coherence length. The sequentially refined structural parameters for all phases were scale factors, unit cell parameters and spherical crystallite sizes. The PDFgui software only allows a spherical crystallite shape to be refined. However, this is not a problem for the present data since dampening due to instrumental resolution limits the observable crystallite sizes to be below ~11 nm. Thus, only the rod diameter (size along crystallographic a/b direction) is practically observable and it is refined in good agreement with the PXRD data. Due to parameter correlations, the site occupancy for potassium in the α-MnO2 phase, it was kept constant to x = 0.125.

Results and Discussions

Figure 2. a) Time resolved PXRD of the reaction at 200°C, *Bragg peaks from α-MnO2. b) Time resolved PXRD of the reaction at 225°C, °Bragg peaks from β-MnO2. c) PXRD of cold precursor solution, +Bragg peak from intra-layer structure in δ-MnO2. Wavelength: 0.9941 Å.


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In situ powder X-ray diffraction. Time resolved PXRD data at 200°C are presented in Figure 2a. Shortly after the reaction is initiated by heating, Bragg peaks for α-MnO2 appear and they persist until the reaction is stopped after 40 minutes. At higher reaction temperature, Figure 2b and Supporting Information, Bragg peaks from α-MnO2 also appear shortly after initiating the reaction, but after prolonged reaction time the α-MnO2 peaks gradually disappear and a set of new Bragg peaks appears, which can be indexed to β-MnO2. Thus, the reaction system initially forms α-MnO2 crystallites that eventually transform to β-MnO2 at temperatures higher than 225°C. The PXRD patterns collected on the cold precursor solution reveal at least one broad Bragg peak at ~23.5° along with broad and diffuse background features, Figure 2c. This Bragg peak can be explained by the intra-layer structure of layered δ-MnO2, which has been observed as a precursor phase in the synthesis of a wide range of MnO2 crystal phases.3, 15, 63 The lack of Miller indices along the stacking direction indicate a disordered δ-MnO2 structure, where the MnO2 layers do not stack in an ordered manner. The broadness of the Bragg peak at ~23.5° further indicates that the crystalline order in the MnO2 layers has a very short coherence length.


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Figure 3. a) Weight fraction of α-MnO2 as function of reaction time at different reaction temperatures, with the rest being β-MnO2. b) Refined scale factor normalized to chemical formula unit plotted as a function of reaction time for α- and β-MnO2 at 225°C.

Phase formation. The weight fraction of the α-MnO2 phase with respect to the crystalline component estimated by Rietveld refinements at different reaction temperatures is shown as a function of reaction time in Figure 3a. Based on the time resolved data the reaction can be divided into three separate stages. In the first stage, pure or almost pure α-MnO2 is the only crystalline product. In stage two, α-MnO2 transforms completely to β-MnO2 and in stage three, pure β-MnO2 is the only crystalline product. Phase transformation from α- to β-MnO2 can be expected since β-MnO2 is the thermodynamically most stable MnO2 phase. Both the initiation


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and the rate of the phase transformation, i.e. initiation of stage two and three, is temperature dependent, where higher reaction temperature gives higher reaction rate. At 200°C, only the first stage is observed. At 225°C, a pure α-MnO2 phase is initially formed in the first stage followed by complete phase transformation, i.e. stage two and three. At 250°C, almost pure α-MnO2 ( with ~3 wt% of β-MnO2) is formed in the first stage followed by complete phase transformation. At 275 °C and 300 °C, ~7 wt% of β-MnO2 is detected from the start, and the phase transformation initiates immediately, i.e. only stage two and three is observed.. Figure 3b shows the refined scale factors for both α- and β-MnO2 as a function of reaction time at 225°C (plots for higher temperatures are shown in the Supporting Information). Notably, the rate of β-MnO2 formation in stage two is much faster than the rate of disappearance of αMnO2, see inset of Figure 3b. Furthermore, the normalized scale factor of β-MnO2 is much larger than the initial amount of α-MnO2. These observations indicate that the β-MnO2 is not formed purely from the α-MnO2 crystallites but also partially from solid and/or dissolved non-crystalline material in the reaction solution. Thus, analysis of the weight fractions of the crystalline phases can be misleading since non-crystalline material is not accounted for in the PXRD. Crystallite size. The Rietveld refinements show that the α- and the β-MnO2 crystallites form and grow as nanorods with the length parallel to the crystallographic c-axis. Figure 4a shows typical growth curves of both the α- and β-MnO2 nanorods as a function of reaction time at 225°C. All other reaction temperatures show the same trends, see Supporting Information. After the reaction is initiated the α-MnO2 crystallites grow rapidly both in length and diameter followed by a slower growth throughout stage one. In stage two, the α-MnO2 crystallites again start growing rapidly in both directions with accelerating growth rate. The growth of the α-MnO2 crystallites is slower than their disappearance by conversion to β-MnO2 leading to an overall


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decrease in their total scattering volume (scale factor). The fact that the α-MnO2 crystallites grow in size (Figure 4) even though they are disappearing from the reaction solution (Figure 3) indicates that they are not dissolving during the phase transformation. When β-MnO2 crystallites are initially observed their sizes and growth curves roughly match the α-MnO2 curves. An even better correspondence is observed for the crystallite volumes, Figure 4b. These observations imply that the α-MnO2 crystallites present in the reaction solution transform directly into βMnO2 crystallites in a solid-state mechanism, i.e. not by dissolution and recrystallization. The solid state mechanism probably involves a collapse of the 2x2 tunnels in α-MnO2 to the 1x1 tunnels in β-MnO2. The fact that the final β-MnO2 crystallites (in stage three) are larger than the original α-MnO2 crystallites furthermore indicates that non-crystalline material is involved in growing the α- and/or β-MnO2 crystallites during the phase transformation (in stage two) in a process separate from the solid-state transformation.


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Figure 4. a) Volume average α- and β-MnO2 crystallite growth along the length and the diameter of the nanorods at 225°C. b) Crystallite volume as a function of reaction time. Filled symbols are for α-MnO2 and open symbols for β-MnO2. Inset highlights growth of small crystallites. For clarity every third data point is plotted.

In Figure 4b the crystallite volume is plotted as a function of reaction time for the different temperatures (see Supporting Information for plots of the individual temperatures with indication of reaction stages). The final α-MnO2 crystallite volume in stage one orders as 225°C>200°C>250°C. The small volume observed at 250°C possibly can be explained by the short reaction time not allowing for much growth of the crystallites. Since the reaction at 200°C is stopped before the end of stage one the final crystallite volume is not known, and further growth may occur. Therefore, the reaction time, rather than reaction temperature, is likely the


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determining factor for the size of the α-MnO2 crystallites at the end of stage one. In general increasing temperatures produce larger β-MnO2 crystallites at the end of stage two. The reaction at 275°C breaks the trend showing smaller crystallites at the end of stage two than at 250°C. The trend breaking might be explained by the fact that at 275°C no reaction stage one is observed and therefore the initial α-MnO2 crystallites are very small. During the first part of stage three the growth rate of β-MnO2 depends on temperature with higher temperature giving higher rate. Later in stage three the growth rate is almost independent of temperature. Crystallite morphology. The evolution of the nanorod morphology at different reaction temperatures is shown in Figure 5, where the aspect ratio (length/diameter) is plotted as a function of reaction time. Plots of individual reaction temperatures are shown in the Supporting Information. In the very early stage of α-MnO2 crystallite growth at 200°C the aspect ratio decreases, see Figure S6a. α-MnO2 nanorods synthesized under similar conditions have been shown by ex situ TEM to grow by oriented attachment.50 It was shown that the (110) surfaces of primary α-MnO2 nanorods attach in an oriented manner to form secondary nanorods of larger diameter. Because attachment along the (110) surface increases the diameter of the nanorods, but not the length, the decrease in aspect ratio observed at 200°C supports that initial growth takes place predominantly by oriented attachment. After the initial drop of the aspect ratio, the aspect ratio is close to 2.5 and gets slightly larger throughout the reaction indicating that the crystallites grow faster in the crystallographic c direction than the a/b direction. Thus, oriented attachment only seems to dominate in the first moments of α-MnO2 growth. The initial drop in aspect ratio is not observed at higher temperatures. As discussed below, the in situ PDF data show that the initial α-MnO2 formation mechanism is the same at both 200 and 250°C. Therefore, the lack of


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initially decreasing aspect ratio at higher temperatures is probably due to limited time resolution in the data.

Figure 5. Aspect ratio (length/diameter) of the nanorods as a function of reaction time at different reaction temperatures. Filled symbols are for α-MnO2 and open symbols for β-MnO2. Inset shows a zoom of the first 5.5 minutes. For clarity every third data point is plotted.

The rate of change of the aspect ratio during stage one increases at higher temperatures indicating that higher temperature promotes faster growth in the c direction. During stage two, the α-MnO2 aspect ratio increases linearly with time at 250, 275 and 300°C. For the reaction at 225 °C the α-MnO2 aspect ratio is almost constant throughout stage 2. When information about the β-MnO2 crystallite shapes becomes available, the aspect ratio is ~30-40% larger than the final α-MnO2 aspect ratio at all temperatures. During the final moments of stage 2 the β-MnO2 aspect ratio drops at all temperatures showing that growth in the a/b direction is preferred. In stage 3 the β-MnO2 aspect ratio is stable showing the crystallites grow equally in both diameter and length of the nanorods. Ordering the reaction temperature according to the final β-MnO2 aspect ratio gives the following trend, 300°C > 250°C > 275°C > 225°C. Thus, more elongated nanorods are obtained at higher temperatures with the 275°C reaction breaking the trend.


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Potassium loading in α-MnO2. Figure 6a shows the changes in potassium loading, determined from refinement of the site occupancy of the potassium site, relative to the highest observed value (∆x for KxMnO2) as a function of reaction time at 225°C. Similar trends are observed for the other temperatures (see Supporting Information). During initial formation of αMnO2, x increases sharply indicating that the first α-MnO2 crystallites are relatively poor in potassium. x increases continuously throughout stage one showing that the average potassium loading of the α-MnO2 crystallite assembly increases either through intercalation of potassium in already formed crystallites or formation of crystallites with progressively higher potassium loading. During the first half of stage two, x is approximately stable at the highest observed potassium loading. After that it drops until changes in the potassium loading are no longer detectable (∆x ≈ -0.02). If β-MnO2 crystallites are to form through a solid-state transformation, as indicated from the growth curves, potassium has to be removed from the 2x2 tunnels in the αMnO2 crystallites. The most obvious mechanism for this is de-intercalation of the potassium ions via diffusion through the 2x2 tunnels in the α-MnO2 structure. As detailed in Supporting Information the ∆x curve in stage two can be explained by a constant volume reaction zone during the phase transformation. The 2x2 channels of the α-MnO2 are only emptied in a constant volume reaction zone and not in the bulk structure. As seen in Supporting Information, this is supported by plotting the refined potassium loading as a function of α-MnO2 weight fraction in stage two and fitting the data to a function describing the average potassium loading of all the αMnO2 crystallites (both bulk and reaction zone). The function assumes that the reaction zone is potassium free and that the bulk has a constant potassium loading. The fitted parameters are the original potassium loading and the weight fraction of the reaction zone relative to the original amount of α-MnO2 crystallites. The fitting results indicate that the reaction zone is ~2 wt% of the


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original amount of α-MnO2 crystallites and increases slightly with increasing reaction temperature, Figure 6b. The reaction zone model suggests that the solid-state phase transformation is driven by local de-intercalation of potassium forming favorable conditions for conversion to the more thermodynamically stable β-MnO2 polymorph. As discussed below this is one of two mechanisms at play during the phase transformation, the other one being growth of αMnO2, β-MnO2 or both by incorporation of amorphous MnO2.

Figure 6. ∆x in KxMnO2 as a function of reaction time for α-MnO2 at 225°C. b) Calculated reaction zone in weight percent of the original α-MnO2 crystallites at different reaction temperatures.


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In situ total X-ray scattering and pair distribution function. The in situ PXRD data only detects crystalline material. This means that information about the precursor solution is very limited due to the highly disordered structure with short coherence length. The PXRD scale factors indicate that β-MnO2 crystallites form at least partly from non-crystalline material. To investigate the non-crystalline parts of the reaction solution in situ TS data at 200 and 250°C were collected. The derived time resolved PDF plots provide information about short and medium range order in the reaction solution. Figure 7a shows the PDF data at 250°C, while data for 200°C are shown in the Supporting Information. The formation of α-MnO2 and subsequent transformation to β-MnO2 is observed through appearance of new correlation peaks in the short and medium r-range. Information about the structural evolution with reaction time is obtained from sequential real space Rietveld refinements. Note that the limited data quality of individual in situ data sets means that the best fit cannot be determined by simple refinement of one data set. Instead the cumulative data quality of whole in situ data collection has to be utilized through sequential refinements. This means the best fit is determined by refinening the proposed structural model sequentially to check the physical soundness. For instance, a structural model that shows a sudden appearance or disappearance of a structural phase and/or has large variation in correlation length from one dataset to another cannot be accepted.


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Figure 7. a) Time resolved PDF for the reaction at 250°C with the regions where each polymorph is dominant marked. b) PDF modelling of the first data frame at 250°C (before initiating the heating) using either hexagonal or monoclinic δ-MnO2. See Supporting Information for better visual fit by including K2SO4 phase and refinement results.

Precursor solution. Results of the PDF modelling of the cold precursor solutions are summarized in Table 1. One additional data set on the precursor solution is included in the table to show the reproducibility of the precursor structure between different precursor injections into the cold reactor. The best fit to the precursor PDF data is obtained with a model containing a mixture of monoclinic δ-MnO2 and α-MnO2. Both structural phases exhibit very short coherence lengths of ~2 nm and ~1 nm, respectively. This is consistent with the PXRD data of the precursor solution that only showed broad and diffuse features. The PXRD data only suggested the presence of nanocrystalline δ-MnO2 since it was known to form under those conditions.3, 15 On the other hand, the PDF provides direct information about the local and medium range order, and


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this allows identification of the symmetry of the nanocrystalline δ-MnO2 phase. It is noteworthy that good fits can only be obtained using monoclinic δ-MnO2. Refining the data using δ-MnO2 phases with other symmetries (e.g. hexagonal cell) results in the sp-diameter refining to a too small value because the structural model does not fit the medium r-range data (r = 7-15 Å), Figure 7b The PDF data reveal that nanodomains of the α-MnO2 phase co-exist in the precursor solution together with δ-MnO2. The very short 1 nm coherence length of the α-MnO2 phase means it is “X-ray amorphous”, i.e. showing no Bragg peaks. The PDF data allows determination of the structure of the amorphous phase, and it is shown here to have a local structure, which is very similar to α-MnO2. For clarity this phase will be referred to as amorphous α-MnO2 or αMnO2(A). Presence of amorphous MnO2 material in the precursor of similar reaction systems previously has been reported based on SEM data.49, 52 TEM pictures of the washed and dried precursor material used in this study confirm the presence of a mixture of crystalline and amorphous material, Figure 8.

Figure 8. a) Bright field TEM image showing a MnO2 precursor particle consisting of amorphous and crystalline parts. b) Dark field TEM image of the same particle, light parts


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indicate nano crystalline material. c) HR-TEM image showing lattice fringes (and Moire fringes) in the crystalline part of the particle.

Apart from being present in the precursor solution, α-MnO2(A) is shown to be present at all reaction stages and it never fully disappears from the reaction solution, see discussion below. Discrepancies are observed between the measured and modelled PDF peaks in the low r-range when crystalline β-MnO2 is refined against the whole data range of a data set collected after 12 minutes of reaction, Figure 9a. The discrepancy becomes more pronounced if only the high rrange is fitted, corresponding to the crystalline structure, Figure 9b. The discrepancy is minimized for the subset of datasets containing crystalline β-MnO2 by including α-MnO2(A) in the refined structural model, Figure 9c. Similar results are obtained for the subset of datasets containing crystalline α-MnO2, see Supporting Information.


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Figure 9. PDF data and real space Rietveld refinement fits after 12 minutes at 250°C. Left: full data range, right: zoom in to r = 1-10 Å a) Fit to the full data range (r = 1-70 Å) assuming purely crystalline β-MnO2. b) Fit to the long r-range (r = 20-70 Å) assuming purely crystalline β-MnO2 extended to the full r-range. c) Fit to the full data range assuming a mixture of β-MnO2 and αMnO2(A).

The atomic structure of α-MnO2(A) (i.e. relative atomic positions) is not refined but kept identical to the crystalline α-MnO2. This is an approximation to the real structure, which is probably more dynamic in nature, but it gives a good fit to the data. The major difference between α-MnO2(A) and α-MnO2 is that α-MnO2(A) has much shorter coherence length, but there is also a small but significant difference in the refined unit cell parameters, see Supporting Information. The a/b and c axis are slightly smaller and larger, respectively. The spread of the


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sequentially refined unit cell parameters is also significantly larger for α-MnO2(A) than for αMnO2. The reason for the large spread might be a more dynamic structure of α-MnO2(A) or an artifact of the data analysis.

Table 1. Results of real space Rietveld refinements of different precursor solutions. The reported results are the mean (and estimated standard deviation) obtained from sequential refinements of the first 20 data frames, collected before initiating the heating.

rw

200°C

250°C

3rd Experiment

0.53(1)

0.59(1)

0.61(1)

α-MnO2(A) wt%

67(2)

61(3)

66(4)

sp-diameter [Å]

10.3(5)

9.5(7)

7.9(8)

a/b [Å]

9.87(4)

9.73(4)

9.65(6)

c [Å]

2.88(1)

2.88(2)

2.89(2)

δ-MnO2 wt%

32(2)

39(3)

34(3)

sp-diameter [Å]

26(1)

20(1)

19(1)

a [Å]

4.97(1)

4.98(1)

4.98(3)

b [Å]

2.848(6)

2.85(1)

2.85(2)

c [Å]

7.65(3)

7.63(4)

7.71(4)

β [°]

100.4(2)

100.5(7)

100.2(9)

PDFs of three different precursor solutions; before initiating the reaction at 200°C, before initiating the reaction at 250°C and for a 3rd experiment not analyzed futher, are analyzed using real space Rietveld refinements (see raw PDFs in Supporting Information). The precursor


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structures in each experiment are expected to be almost identical since they are all injected into the experimental setup in a similar manner from the same precursor solution. The refinement results presented in Table 1 show that the precursor structures are similar but not identical. The mass ratio between the two phases varies but is close to α-MnO2(A):δ-MnO2 = 2:1. The volume averaged crystallite size also varies between different experiments ranging from 0.8-1.0 nm for α-MnO2(A) and 1.9-2.6 nm for δ-MnO2. Some differences are observed in the a/b unit cell parameters for α-MnO2(A) and the c-axis cell parameter for δ-MnO2, whereas the other unit cell parameters are identical within the estimated standard deviation. The spread of exactly these cell parameters, and not the others, is interesting, and it indicates a difference in the average potassium loading of the different phases in the different precursor solutions. This is because the a/b parameter (but not c) in α-MnO2(A), and the c parameter (but not a or b) in δ-MnO2 are the cell parameters most sensitive to the presence of potassium in the structure. The differences between the precursor datasets show that the precursor solution is not homogeneous. Although care is taken to inject the solution into the experimental setup in reproducible manner, the PDF data show that the starting point of each in situ experiment is not identical. A forthcoming study focusing on the general reproducibility of the in situ setup demonstrates that these differences in the precursor solution lead to differences in the onset time for the α- to β-MnO2 phase transformation and the phase transformation rate. However, there is no indication of differences in the overall picture of α-MnO2 nanorod formation that subsequently transform into β-MnO2. α-MnO2(A) is observed to be present in the reaction solution at all stages and never disappear from it. Therefore, a purely crystalline material is never obtained from the reaction system under the reaction conditions examined. The presence of α-MnO2(A) can be shown by


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fitting individual data frames at t = 3 min (crystalline α-MnO2 present) and t = 12 min (crystalline β-MnO2 present), see Supporting Information for fits and discussions. α-MnO2 formation and growth – Reaction stage one. The inset in Figure 10a shows the refined weight fractions for all the detected phases in the precursor solution and their time evolution at 250°C. The weight fraction presented here is based on all the material phases, both crystalline and amorphous, included in the real space Rietveld refinement of the in situ PDF data. Analysis of the normalized scale factors from the refinements show that a decrease in one scale factor is always accompanied be an increase in another, see Supporting Information. It is therefore likely that all major material phases that participate in the reaction are included in the refinement. The weight fractions from the PDF refinements are therefore a more correct representation of what is happening in the reaction solution compared with the PXRD weight fractions discussed above. For the first 3 seconds of reaction there are no significant changes, but at t = 4 s α-MnO2 crystallites are detected. In the following 3 seconds the weight fraction of αMnO2 increases rapidly with a simultaneous decrease in the δ-MnO2 until no δ-MnO2 is detected in the reaction solution at t = 8 s. The formation of the initial α-MnO2 is roughly one to one with the disappearance of δ-MnO2, whereas the decrease of α-MnO2(A) is negligible. This indicates that the initial α-MnO2 crystallites are formed through a direct transformation from the δ-MnO2 crystallites. After δ-MnO2 has disappeared from the reaction solution (t > 8 s) the weight fraction of α-MnO2 increases slowly with a corresponding decrease of α-MnO2(A) until stage two initiates. Thus, α-MnO2(A) is being consumed in the formation of α-MnO2 crystallites during stage one. At the end of stage one the weight fractions of α-MnO2(A) and α-MnO2 are 52(2)wt% and 48(2)wt%, respectively. The same general trend is observed at 200°C with slightly slower formation of α-MnO2, see Supporting Information.


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During stage one the coherence length of δ-MnO2 grows rapidly from an initial value of ~2 nm in the precursor solution, Figure 10b. The growth occurs even during the transformation to αMnO2, where δ-MnO2 is disappearing from the reaction solution. This excludes a dissolutionrecrystallization

mechanism

for

the

phase

transformation

and

indicates

solid-state

transformation. The almost perfect overlap of the growth curves of δ-MnO2 and α-MnO2, Figure 10b, further supports solid-state transformation mechanism. After δ-MnO2 has disappeared from the reaction solution the coherence length of α-MnO2 keeps increasing slowly through inclusion of α-MnO2(A) until the end of stage one. The coherence length of α-MnO2(A) is rather constant throughout reaction stage one following an initial increase from 0.95(7) nm to 1.2(1) nm. At 200°C α-MnO2(A) has a slightly larger coherence length stabilizing at 1.50(1) nm but otherwise the 200°C synthesis shows the same trends as observed at 250°C. Formation of α-MnO2 from a precursor solution containing δ-MnO2 and subsequent transformation to β-MnO2 is in accordance with Ostwald’s step rule that states that during crystallization less stable structures (here more open) are formed first with a subsequent transformation to more stable structures.64,

65

Formation of tunnelled MnO2 structures from

precursor containing layered δ-MnO2 is common. Many different formation mechanisms, such as rolling of the layers; buckling, dissociation and recombination of the layers; and collapsing the layers into tunnels, have been proposed based on experimental and theoretical considerations.3, 47, 48, 66, 67

Other studies have observed amorphous MnO2 particles, without any details about the

local structure, and proposed α-MnO2 growth from those particles.49, 52 The present data suggest that the initial α-MnO2 crystallites are formed from the δ-MnO2 crystallites, while further growth happens by inclusion of amorphous α-MnO2. It therefore seems that the structural relationship


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between α- and δ-MnO2 dictates that α-MnO2 is formed in the reaction solution before β-MnO2 forms.

Figure 10. a) Weight fraction of all phases detected by PDF as a function of reaction time at 250°C. b) Coherence lengths of all phases detected by PDF as a function of reaction time at 250°C. Insets show a zoom in for the cold precursor solution and first 60 s of reaction.

α- to β-MnO2 phase transformation – Reaction stage two. Reaction stage two is only observed in the 250°C PDF data and it initiates at t = 4 minutes when β-MnO2 crystallites appear in the reaction solution, Figure 10a. Throughout stage two the weight fraction for β-MnO2 rises rapidly with a corresponding decrease of α-MnO2. There is also a decrease in the α-MnO2(A) weight fraction throughout reaction stage two. This leads to the amount of β-MnO2 being larger


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in the end of stage two than the amount of α-MnO2 in end of stage one as was also observed in the in situ PXRD data. These observations indicate that at least two mechanisms are at play in the α- to β-MnO2 phase transformation. One is the transformation of α-MnO2 crystallites into βMnO2 crystallites, while other mechanisms are inclusion of α-MnO2(A) into either α- or β-MnO2 or both. Initially the β-MnO2 crystallites are slightly smaller than the α-MnO2 crystallites (~6 nm and ~7 nm respectively). However, β-MnO2 grows faster than α-MnO2 meaning that they become equal in size when reaching ~9 nm. During the middle part of stage two both phases have the same size, but then α-MnO2 starts growing more rapidly just before reaching the resolution limit at ~11 nm. The continuous increase in coherence length of both phases throughout reaction stage two is consistent with the PXRD data and further supports that the αto β-MnO2 phase transformation is a direct solid-state transformation and not a dissolutionrecrystallization. Inclusion of α-MnO2(A) during the α- to β-MnO2 transformation is supported by the fact that the crystallite sizes of both phases increase through stage two. During reaction stage two the coherence length of α-MnO2(A) grows continuously from 1.2(1) nm in end of stage one to 1.9(1) nm. This means the α-MnO2(A) becomes slightly more crystalline during the phase transformation but it is still too small to be detected by diffraction. β-MnO2 – Reaction stage three. At 250°C, reaction stage 3 initiates 5.65 minutes after all the crystalline α-MnO2 material has disappeared from the reaction solution. At that time the reaction solution consist of ~50wt% crystalline β-MnO2 and ~50wt% α-MnO2(A). In the first moments of reaction stage three, the weight fraction of β-MnO2 grows rapidly corroborating the initial increase in crystallite volume observed in stage three in the PXRD data. This means the initial growth of β-MnO2 in stage three is driven by inclusion of α-MnO2(A). After t = 7.5


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minutes, the weight fractions of β-MnO2 and α-MnO2(A) stabilize at 55(2)wt% and 45(2)wt% respectively. This indicates that the later slower growth of the β-MnO2 in reaction stage three observed in the in situ PXRD data, is not due to inclusion of α-MnO2(A) and that it must progress through a mechanism that conserves the total amount of β-MnO2. Here Ostwald ripening is a more likely growth mechanism than oriented attachment since the PXRD data show a constant β-MnO2 aspect ratio. The coherence length for β-MnO2 refines to ~11 nm throughout reaction stage three, i.e. the resolution limit, in good agreement with the PXRD data. The coherence length of α-MnO2(A) is stable at 1.9(1) nm throughout stage 3. Reaction mechanism – Combining in situ PXRD and PDF results. By combining the results obtained from the in situ PXRD and in situ PDF experiments, a reaction mechanism of the formation of α- and β-MnO2 can be proposed, Figure 11. When KMnO4 is reduced by MnSO4 upon mixing at room temperature a suspension of MnO2 particles is produced. The manganese containing phases in this precursor solution are nanocrystalline and disordered δMnO2 and α-MnO2(A) in a weight ratio of ~1:2. The precursor solution is rather inhomogeneous in the distribution of crystallite sizes and potassium content. The inhomogeneity is observed in our capillary scale experiments, but it is uncertain to what degree local inhomogeneities affect a larger scale reaction. After the hydrothermal synthesis is initiated by heating, the reaction can be divided into three separate stages: α-MnO2 formation and growth (stage one), α- to β-MnO2 transformation (stage two) and β-MnO2 growth (stage three). In stage one the initial δ-MnO2 crystallites transform into α-MnO2 nanorods in a solid-state transformation mechanism without dissolution of δ-MnO2. During and right after the δ- to αMnO2 transformation both phases grow rapidly in size and the data is consistent with the α-


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MnO2 crystallites initially growing by oriented attachment, as also suggested by Yuan et.al.50 For the remainder of stage one, the growth of α-MnO2 is supported by consumption of α-MnO2(A). At this stage of the reaction the α-MnO2 nanorods grow preferentially along the crystallographic c-direction. When the α- to β-MnO2 phase transformation initiates in reaction stage two, it is a solidstate transformation. The PXRD data are consistent with the phase transformation progressing though depletion of potassium ions from the 2x2 tunnels of the α-MnO2 phase in a constant volume (~2 wt%) reaction zone. Simultaneously, a significant amount of α-MnO2(A) is consumed in growing both α- and β-MnO2. In the first moments after α-MnO2 has disappeared from the reaction solution (stage three), β-MnO2 continues growing rapidly through inclusion of α-MnO2(A). Then consumption of αMnO2(A) stops and further crystallite growth of β-MnO2 probably progresses through Ostwald ripening. At this stage only small changes are observed and the weight fractions of α-MnO2(A) is constant thus limiting the yield of the β-MnO2 crystallites to ~55wt% at 250°C.

Figure 11. Schematic representation of the proposed reaction mechanism.


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Conclusions In situ PXRD and in situ PDF experiments have revealed details about the crystalline and amorphous phase evolution of MnO2 under hydrothermal conditions. Three nanocrystalline phases, δ-, α- and β-MnO2, are observed at different stages of the synthesis, and simple manipulation of reaction temperature and time allows all of these phases or mixtures of δ- and αMnO2 or α- and β-MnO2 to be extracted as the final crystalline products.2, 3, 48-51 An amorphous phase with a local structure resembling α-MnO2 (α-MnO2(A)) has for the first time been structurally characterized. The α-MnO2(A) phase is present in relatively high amount at all reaction stages and it accounts for 45wt% of the material product at the end of reaction at 250°C. In other words, the present hydrothermal synthesis never produces a phase pure product. Combining the knowledge from in situ PXRD and PDF data allows deduction of a possible reaction mechanism for the phase evolution from the precursor solution to stable β-MnO2. Nanocrystalline and disordered δ-MnO2 formed upon mixing of the reactants transforms in a solid-state mechanism to α-MnO2 nanorods upon heating. After prolonged reaction time at T ≥ 225°C the α-MnO2 nanorods undergo a solid-state transformation to β-MnO2 nanorods. αMnO2(A) is present in the reaction solution at all reaction stages and it is consumed in growing both α- and β-MnO2 crystallites throughout stage one, two and the beginning of stage three. After the rapid initial growth of β-MnO2 in stage three the consumption of α-MnO2(A) stops, and the β-MnO2 crystallites probably grow by Ostwald ripening.


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Footnotes Corresponding Author. *E-mail: [email protected] Funding Sources. This research was supported by the Danish National Research Foundation (DNRF93), Haldor Topsøe A/S, the ProEco project funded by the Danish Research Council for Technology and Production, and by the Danish Center for Synchrotron and Neutron Research (DanScatt). Acknowledgement. We gratefully acknowledge fruitful discussions with Espen D. Bøjesen, Ann-Christin Dippel and Yanbin Shen. Nils L.N. Broge is thanked for collection HR-TEM images. We thank DESY, Germany, a member of the Helmholtz Association (HGF), and MAXlab, Sweden, for beamtime. Supporting Information. In situ experimental setup, Reaction temperature calibration, Time resolved in situ data, PXRD scale factors, Crystallite sizes, Potassium loading – x in KxMnO2, Constant volume reaction zone – fits and calculations, Precursor solution, PDF fits showing the presence of α-MnO2(A) throughout the synthesis, PDF results for 200°C, PDF scale factors and PDF unit cell parameters.

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Manuscript title Formation mechanisms of nanocrystalline MnO2 polymorphs under hydrothermal conditions

Author List Steinar Birgisson, Dipankar Saha and Bo B. Iversen

TOC graphic

Synopsis The combined information from in situ powder X-ray diffraction and in situ total scattering pair distribution function analysis reveals the formation mechanism behind the polymorph evolution of MnO2 under hydrothermal synthesis conditions. The formation mechanism from disordered δMnO2 to nanocrystalline α-MnO2 and further to nanocrystalline β-MnO2 includes an X-ray amorphous MnO2 phase with a local structure resembling α-MnO2.


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Formation Mechanisms of Nanocrystalline MnO2 ... - ACS Publications

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