Synthesis of Birnessite in the Presence of Phosphate, Silicate, or

Sep 27, 2016 - With increasing P/Mn molar ratio, birnessite particle size decreased, while its Mn(III) content increased, attributed to phosphate adso...
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Synthesis of Birnessite in the Presence of Phosphate, Silicate, or Sulfate Qian Wang,† Xianya Liao,† Wenqian Xu,‡ Yang Ren,§ Kenneth J. Livi,⊥ and Mengqiang Zhu*,† †

Department of Ecosystem Science and Management, University of Wyoming, Laramie, Wyoming 82071, United States Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ Materials Characterization and Processing Center, Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, United States ‡

S Supporting Information *

ABSTRACT: Layered manganese (Mn) oxides, such as birnessite, are versatile materials in industrial applications and common minerals mediating elemental cycling in natural environment. Many of birnessite properties are controlled by Mn(III) concentration and particle sizes. Thus, it is important to synthesize birnessite nanoparticles with controlled Mn(III) concentrations and sizes so that one can tune its properties for many applications. Birnessite was synthesized in the presence of oxyanions (phosphate, silicate, or sulfate) during reductive precipitation of KMnO4 by HCl and characterized using multiple synchrotron X-ray techniques, electron microscopy, and diffuse reflectance UV−vis spectroscopy. Results indicate that all three anions decrease MnO6 sheet sizes, attributed to oxyanion adsorption on edges of the sheets, inhibiting their lateral growth. As a result of decreased sizes, sheets undergo significant structural contraction. Meanwhile, Mn(III) concentration significantly increases with increasing oxyanion/Mn ratio. The increased Mn(III) concentration, along with the decreased size, enlarges both direct and indirect band gaps of birnessite. Phosphate imposes the strongest influence, followed by silicate and then by sulfate, consistent with their decreasing adsorption affinity. Reacting with 1 M KOH solution effectively removed the adsorbed oxyanions while leading to increased sheet sizes, probably due to oriented attachment-driven particle growth mechanisms. The results have important implications for developing highly performed birnessite materials, for example, small size Mn(III)-rich birnessite for photochemical and catalytic applications, as well as for understanding chemical compositional variations of naturally occurring birnessite.

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INTRODUCTION

Both Mn vacancies and Mn(III) significantly affect birnessite properties. Vacancy concentration largely determines birnessite metal sorption capacities12 and can enhance photoconductivity by increasing the concentration of photoinduced electrons.13,14 Mn(III) can significantly affect birnessite oxidation ability in oxidizing NH4+,15 Cr(III),16 phenol, sulfide,17 etc., because of its higher reactivity than Mn(IV).15,18 Mn(III) in birnessite may serve as a source for dissolved Mn(III)-ligand complexes in aquatic environment.19,20 Birnessite rich in Mn(III) is the precursor for the formation of tunneled Mn oxides.21 Mn(III) also controls birnessite band gap,22 which affects birnessite photocurrent generation,4 photochemistry,23 electrical conductivity,24,25 and the efficiency in catalyzing water oxidation by capturing sunlight.22 Accumulation of Mn(III) in birnessite acts as the precursor of water oxidation catalysis to molecular oxygen in both synthetic and biological systems,26−29 attributed to Mn(III) lowering the overpotential for water oxidation.30

Birnessite is the most common and abundant type of Mn oxide in terrestrial deposits and ocean nodules, playing a significant role in biogeochemical elemental cycling in nature owing to its extraordinary adsorption and oxidation properties.1 Synthetic birnessite has been attracting increasing attention due to a wide variety of applications and their low cost.2,3 It is used in semiconductors,3,4 cathodes,5 catalysts,2,6 adsorbents, and oxidizers in water and air purification.7,8 The environmental implications and industrial applications of birnessite are attributed to its unique crystal structure and nonstoichiometric chemical composition. Birnessite consists of stacked edge-sharing MnO6 sheets with more or less stacking disorder.9,10 The Mn site in MnO6 sheets is dominated by Mn(IV) with minor Mn(III) and Mn vacancies. The presence of vacancies and Mn(III) leads to negatively charged sheets that are compensated by hydrated metal cations and/or H+ in the interlayer region.5,11 The basal d-spacing is ∼7 or 10 Å, depending on the number of water layers in the interlayer region.1,6 © XXXX American Chemical Society

Received: June 21, 2016

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DOI: 10.1021/acs.inorgchem.6b01465 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Oxyanion/Mn Molar Ratios and Sample Names sample name

initial ratio

ratio in product

sample name

phosphate (P) series P0.02 P0.05 P0.10 P0.20 P0.30 P0.50

0.02 0.05 0.10 0.20 0.30 0.50

initial ratio

0.016 0.042 0.098 0.162 0.296 0.331

Si0.02 Si0.05 Si0.10 Si0.20 Si1.00

0.02 0.05 0.10 0.20 1.00

sample name

initial ratio

ratio in product

sulfate (S) series 0.007 0.017 0.033 0.061 0.107

S0.02 S0.05 S0.10 S0.20 S0.50 S1.00

0.02 0.05 0.10 0.20 0.50 1.00

0.003 0.003 0.004 0.005 0.008 0.013

The suspension was shaken at 25 rpm and room temperature (22.2 ± 0.5 °C) for 24 h. Then the solids were collected by centrifugation and washed thoroughly with DI water. The above processes were repeated three or four times until the vibration peaks of the oxyanions that remained in the solids were not detected by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. It is noteworthy that green-colored solution resulted during the KOH washing due to formation of potassium manganate via oxidation of Mn(IV) by atmospheric oxygen, as described by the following equation:

Thus, stabilization of Mn(III) in birnessite structure was suggested as a strategy for enhancing birnessite catalytic activity, such as via adsorption of poly(allylamine hydrochloride) on birnessite surfaces.30 Particle sizes are another important factor affecting birnessite electronic and optical properties according to the quantum confinement effects.31 A recent study showed that birnessite with smaller size delivers higher electrochemical capacity.32 In addition, smaller particle sizes enhance oxidative degradation and transformation of organic and inorganic pollutants by birnessite, such as hydrophobic polycyclic aromatic hydrocarbons and arsenite.33,34 However, there is a lack of synthetic methods to control both birnessite particle sizes and Mn(III) concentration for desired properties. In the present study, we synthesized birnessite using reduction of potassium permanganate (KMnO4) by hydrochloric acid (HCl) in the presence of various concentrations of phosphate, silicate, or sulfate. The three oxyanions adsorb on birnessite surfaces with different affinity (phosphate > silicate ≫ sulfate),35,36 allowing for determining the underlying mechanisms of the oxyanion effects. We found that birnessite particle sizes, Mn(III) concentration, and band gap energy can be well-controlled by changing the molar ratio of phosphate to permanganate during synthesis. This work provides new possibilities for synthesis of birnessite for desired properties.

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ratio in product

silicate (Si) series

2MnO2 + 4KOH + O2 → 2K 2MnO4 + 2H 2O

(2)

Chemical Composition. Elemental composition, water content, and Mn average oxidation state (AOS) of the birnessite products were measured to obtain their approximate chemical formula. The elemental composition was determined by dissolving ∼0.01 g of dry powder in a 50 mL solution containing 0.1% NH3OH·HCl and 1% HNO3.11 The concentrations of the dissolved ions were measured using inductively coupled plasma optical emission spectrometry (ICPOES). High-resolution thermal gravimetric analysis (HR-TGA) was performed on a TGA/DSC instrument in N2 atmosphere with a heating rate of 20 °C min−1 from 20 to 800 °C to measure the water content of the samples. The weight loss from 80 to 250 °C was attributed to the dehydration of birnessite structural water.11 Mn AOS of the birnessite samples and respective fractions of Mn(II), Mn(III), and Mn(IV) were determined using linear combination fitting (LCF) analysis of the Mn K-edge X-ray absorption near-edge structure (XANES) spectra (see below). X-ray Diffraction and Atomic Pair Distribution Function Analyses. X-ray diffraction (XRD) data were collected from the products using X-rays of 58.6491 keV (λ = 0.2114 Å) with a sample-todetector distance of ∼95 cm at beamline 11-ID-B at the Advanced Photon Source (APS), Argonne National Laboratory. For collection of high-energy X-ray total scattering data for pair distribution function (PDF) analysis, the sample-to-detector distance was set at ∼15 cm to increase the Q range. X-ray total scattering data of PolyMnO2 were collected using X-rays of 111.06 keV (λ = 0.111 65 Å) at beamline 11ID-C at APS with a sample-to-detector distance of ∼24 cm. All the measurements were performed using the rapid-acquisition PDF technique through a PerkinElmer amorphous silicon detector mounted orthogonally to the beam path. The standard CeO2 was used to calibrate the sample-to-detector distance, beam center position, and tilt angles of the detector relative to the beam path, using the program Fit2D. Radial integration further converted two-dimensional (2D) images of the initially collected data to one-dimensional intensity versus wave vector (Q). The integrated data were further transferred to the program PDFgetX2, to obtain the reduced total structure function F(Q). Direct Fourier transformation of F(Q) gave the pair distribution function G(r). X-ray Absorption Spectroscopy. Mn K-edge extended X-ray absorption fine structure (EXAFS) spectra were collected from the products in transmission mode at beamline 13-ID-E at APS. A Mn metal foil reference was measured concurrently with the samples for internal energy calibration (E0 = 6539 eV). The program SixPack was used for energy calibration, raw spectra average, postedge normalization, and background removal.39 Structural parameters were obtained by fitting k3-weighted EXAFS spectra to the standard EXAFS equation using several single-scattering paths over a k range of

MATERIALS AND METHODS

Birnessite Synthesis. KH2PO4, K2SO4, or Na2SiO3 was dissolved in 300 mL of 0.667 M KMnO4 solution in a 500 mL flat-bottom flask to achieve initial P/Mn molar ratios of 0.02−0.50 and both initial S/ Mn and Si/Mn molar ratios of 0.02−1.00. The flask was immersed in an oil bath (Instatherm) that was maintained at 110 °C, and the solution was boiling. Forty-five milliliters of 6 M HCl was added dropwise into the above boiling solution at a flow rate of 1.0 mL min−1 using a peristaltic pump (Masterflex FH100M). The oil bath was placed on a magnetic stirring plate to vigorously stir the solution in the flask during HCl addition. A reflux system was used to minimize HCl and water evaporation loss during the synthesis. Birnessite synthesized in the absence of oxyanions was used as a control. Upon the completion of the HCl addition, the obtained suspension was boiled for additional 30 min, after which it was cooled to 60 °C under ambient conditions and aged at this temperature for 22 h.37 The solids were collected by centrifugation and washed thoroughly with deionized (DI) water to remove residual dissolved ions. A portion of the wet solid of each sample was dried at 40 °C in oven for 3 d, ground in an agate mortar, passed through a 100 mesh sieve, and stored in a closed polyethylene plastic tube at room temperature. The synthetic conditions and sample names are given in Table 1. Polymeric MnO2 (PolyMnO2), a nanoparticulate birnessite with coherent scattering domain (CSD) size of ∼1.5 nm,11 was synthesized as a reference according to Perez-Benito et al.38 To remove the oxyanions associated with the products to obtain pure birnessite, ∼10 g of wet solid of selected samples was added to a 50 mL polyethylene tube containing 35 mL of 1 M KOH solution. B

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Figure 1. Powder XRD patterns of birnessite products (red) with various initial oxyanion/Mn molar ratios. (A) P. (B) Si and S. The XRD patterns are superimposed with the patterns of the corresponding KOH-washed products (black). 3.0−11.9 Å−1 and an R range of 1−3.0 Å. Phase and amplitude functions for the scattering paths were calculated using FEFF7 based on the chalcophanite structure. In all fits, the number of independent variables included was less than the number of independent data points. The Kaiser−Bessel window was used for Fourier transformation of the EXAFS data. Linear combination fitting analysis of the Mn K-edge XANES spectra was conducted over 6530−6600 eV using the program SixPack to quantify the molar fraction of each Mn oxidation state. The Mn reference compounds included those published in Manceau et al.,40 such as MnSO4, feitknechtite (β-MnOOH), manganite (r-MnOOH), bixbyite (Mn2O3), and potassium birnessite. In addition, δ-MnO2 was included to represent nanoparticulate birnessite phase to account for particle size effects on the spectra. All Mn in δ-MnO2 were approximated to be tetravalent (Mn(IV)).41 Energy shifts were not allowed during the LCF fitting. To determine phosphate speciation in the products synthesized in the presence of phosphate, P K-edge XANES spectra were collected from the products in fluorescence mode using a vortex silicon drift detector mounted into a He-filled sample chamber at beamline 14−3 at the Stanford Synchrotron Radiation Laboratory (SSRL) in Menlo Park, CA. Energy calibration was conducted by setting the position of the first absorption maxima of Na3PO4 at 2146 eV using a monochromator equipped with a Si (111) crystal. The program Sixpack was used for P XANES spectra energy calibration, average, background removal, and postedge normalization. Electron Microscopy. The crystallite morphology of birnessite products was determined by an FEI Quanta FEG 450 field emission scanning electron microscope (FE-SEM) after being coated with a gold evaporated film. In terms of high-resolution transmission electron microscopic (HR-TEM) analysis, a small amount of solid was ultrasonically dispersed in DI water for 5 s. A Cu grid was dipped into the suspension followed by air-drying. HR-TEM analyses were performed on an FEI Tecnai G2 F20 200 kV (S) TEM. Specific Surface Area. The Brunauer−Emmett−Teller (BET) specific surface area (SSA) was measured by nitrogen adsorption at liquid nitrogen temperature using TriStar 3000 BET surface area analyzer. Approximately 0.2 g of powder for each sample was degassed at 110 °C for 3 h under vacuum prior to the N2 adsorption measurement. Ultraviolet−Visible Diffuse Reflectance Spectroscopy. UV− vis diffuse reflectance spectroscopy (DRS) was used to determine the band gaps of the products based on Tauc−Mott (TM) plots. The spectra were collected from the products using a Lambda 950 UV/vis Spectrometer (PerkinElmer). A TM plot shows the photon energy (E)

on the abscissa and the (AE)α on the ordinate, where A is the UV−vis absorbance, and α = 2 or 1/2 for direct or indirect band gap, respectively.24 The abscissa intercept of the linear section of a TM plot, where the (AE)α is 0, defines the optical band gap.

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RESULTS Chemical Composition. The determined oxyanion/Mn molar ratios of the solid products are given in Table 1 with their chemical compositions provided in the Supporting Information (Table S1). At a given oxyanion/Mn ratio, the products synthesized in the presence of PO43− contain the highest amount of the oxyanion (i.e., PO43−), followed by those synthesized in the presence of SiO44− and then of SO42−. The order is consistent with their decreasing adsorption affinity on mineral surfaces.35,36 Water content increases with increasing oxyanion/Mn ratios (Table S1). X-ray Diffraction. Figure 1 shows the XRD patterns of the solid products. The control sample has five characteristic peaks of birnessite at ∼7.23, 3.61, 2.45, 1.41, and 1.22 Å.11,42 The first two peaks correspond to the basal reflections (001) and (002), respectively. The remaining peaks are broadened by convolution of multiple reflections and can be indexed to (11, 20), (31, 02) and (22, 40), respectively.42 The products with P/Mn ≤ 0.3, Si/Mn ≤ 0.2, or S/Mn ≤ 1 possess these characteristic XRD peaks, indicating they are birnessite. The XRD pattern of P0.50 consists of strong reflections (Figure S1A), corresponding to an unknown phase. Both P0.30 and Si1.00 contain a trace amount of such phase (Figure S1B,D). With increasing P/Mn ratio (Figure 1A), (001) and (002) peaks are significantly attenuated and broadened, suggesting a decrease in the number of stacked MnO6 sheets and/or stacking order. By neglecting the stacking disorder, the thicknesses of the stacked sheets were estimated based on the full width at half-maximum (fwhm) of the (001) peak using the Scherrer equation (Table S2).43 The thickness of the control sample is 6.1 nm, while it is reduced to 3.6 and 1.4 nm, respectively, for P0.02 and P0.05, corresponding to ∼6 and ∼3 stacked sheets (Table S2). As the (001) peak is absent for the birnessites with P/Mn > 0.05, these birnessites likely consist of only one or two MnO6 sheets. The thicknesses of the Si and S series are also reduced but not as much as those of the P series C

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Figure 2. X-ray atomic pair distribution functions (G(r)s) of birnessite products (red) with various initial oxyanion/Mn ratios: (A) P. (B) Si and S. The PDFs are superimposed with those of the corresponding KOH-washed products (black).

Figure 3. (A) PDFs within 6.5 Å for the birnessite products synthesized at various P/Mn ratios. (B) PDFs of birnessite products (red) are superimposed with those of the corresponding KOH-washed products (black). The subscripts “L” and “In” stand for “Layer” and “Interlayer”, respectively.

observed for the Si series (Figures 1B and S1C). The effects are barely observable for the S series (Figure S1D). Atomic Pair Distribution Function Analysis. To further characterize the structural changes,9 X-ray atomic PDF analysis was performed. Reduced structural functions F(Q)s are given in Figure S2 for all samples. Selected G(r) functions are given in Figure 2 with others in Figure S3. With increasing oxyanion/ Mn ratio, the PDF peak intensities reduce to statistical noise at a shorter r (Figure 2), indicating a decreasing CSD size.11 Phosphate shows the strongest effect, while sulfate shows the weakest (Figure 2). For a closer examination of the local structural changes, G(r)s within 6.5 Å are presented for the P series in Figure 3A. Because adjacent MnO6 sheets are ∼7 Å apart, the G(r) within 6.5 Å mainly reflects the structure of MnO6 sheets without contribution from the atomic pairs between adjacent sheets. To examine the relative peak intensities, all peaks are normalized by adjusting the Mn−O peaks at 1.90 Å to have

(Table S2). In addition, both (001) and (002) peaks shift to left for all the P (P/Mn ≤ 0.05, Figure 1A), Si (Si/Mn ≤ 0.20, Figure S1C), and S (S/Mn < 1.00, Figure S1D) series, indicating increased interlayer spacing. The (11, 20), (31, 02), and (22, 40) peaks mainly reflect the structure of MnO6 sheets. The symmetric shape of the (31, 02) peaks indicates hexagonal layer symmetry as 31 and 02 contributions otherwise split when the layer symmetry is orthogonal.44,45 However, the (11, 20) to (31, 02) d-spacing ratios, particularly for the P and Si series, slightly deviate from 1.732 (Table S3), the ratio for an ideal hexagonal MnO6 sheet (a/b = √3). It suggests that the MnO6 sheets of these products have slight structural distortion. In addition, the (11, 20), (31, 02), and (22, 40) peaks broaden with increasing P/Mn ratio (Figure 1A), indicating a decrease of the MnO6 sheet size. Meanwhile, these peaks significantly shift toward lower d-spacing, indicating structural contraction of the sheets. Similar but much weaker changes are D

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Figure 4. Manganese K-edge XANES spectra (A) and percentage of each Mn valence state and Mn AOS (B) of birnessite synthesized at various P/ Mn ratios using XANES linear combination fitting. Filled symbols are for the products, while the empty symbols are for the corresponding KOHwashed products.

Figure 5. (A) Magnitude of Fourier-transformed Mn K-edge EXAFS spectra of the P-series birnessites. (B) The spectra of birnessite (red) are superimposed with those of the corresponding KOH-washed products (black).

increasing P/Mn ratio, suggesting increasing Mn(III) concentration.47 The XANES LCF analysis was employed to determine the relative proportions of Mn(II), Mn(III), and Mn(IV) in the products. Two Mn(IV) references, that is, potassium birnessite (KBi) macrocrystals and δ-MnO2 nanoparticles, are used to account for the effect of particle sizes on the XANES spectra. MnSO4 solution and Mn(III) oxides and oxyhydroxides (feitknechtite, manganite, and Mn2O3) were used to represent Mn(II) and Mn(III), respectively. Feitknechtite is the Mn(III) phase giving the best fit for most of the cases, probably due to the layer nature of its structure. The fits and data are in good agreement except for P0.30 (Figure S6). The unsatisfactory fit of P0.30 may be due to its smaller size than that of δ-MnO2. With increasing P/Mn ratio, the Mn(IV) in the products is represented increasingly better with δ-MnO2 than with KBi, suggesting decreasing particle size of the products (Table S4). The obtained Mn(III) content increases from 4% to 39% with a concurrent AOS decrease from 3.90 to 3.61 (Figure 4B). The amounts of Mn(II) are negligible in all samples. Similar observations are made for both the Si and S series (Figures S6 and S7 and Table S4) but with less Mn(III) and higher AOS compared to the P series (Figure S8). Mn K-Edge Extended X-ray Absorption Fine Structure Spectroscopy. The k3-weighted EXAFS spectra in k space of the P series are displayed in Figure S9. For the control sample, two peaks at 8.07 and 9.20 Å−1, respectively, are diagnostic for birnessite.48,49 With increasing P/Mn ratio, these two peaks

approximately the same intensity. The peak intensities of all Mn−Mn and O−O pairs decrease drastically with increasing P/ Mn ratio (Figure 3A), indicating a great size decrease of the sheets. In addition, the (Mn−Mn)L and (O−O)L peaks at 2.87 Å, the (Mn−O)L peaks at 4.46 Å, and the MnL−MnIn at 5.32 Å all shift toward shorter distances, indicating concurrent structural contraction. Similar but weaker changes as described above are also observed for the silicate series, although the changes are weaker (Figure S4). The changes of the sulfate series are very subtle (Figure S4). The oxyanions adsorbed on birnessite also contribute to the PDFs. The peaks at ∼1.54 Å (Figure 3A) can be ascribed to the P−O pairs in the PO4 tetrahedron. The peaks at ∼3.14 Å are most likely due to P−Mn pairs, suggesting that P is chemically bound to Mn. These peaks become stronger with increasing P/ Mn ratio, consistent with more P associated with the products (Table 1). As to the Si series, the peaks at 1.57−1.59 Å could be attributed to the Si−O pair in SiO4; however, Si−Mn pairs are not obvious (Figure S4A). This suggests that not much silicate is chemically bound to Mn, probably because of polymerization of silicate anions. No peaks involving S are observed for the S series (Figure S4C) due to its low concentration in the solids. Mn K-Edge X-ray Absorption Near-Edge Structure Spectroscopy. The Mn XANES spectra of the P series are given in Figure 4A. The intensity of the white-line peak decreases with increasing P/Mn ratio, which can be attributed to decreasing particle size.46 The shoulder peak (6556−6560 eV, the inset in Figure 4A) becomes more pronounced with E

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P0.02 and P0.50 cannot fit the spectra of any samples with P/ Mn between 0.02 and 0.5. This suggests that phosphate mainly forms surface complexes on the birnessite products instead of precipitation at P/Mn ratios less than 0.5. Electron Microscopy. Figure 7 shows the FE-SEM images of the control and the selected birnessite products. The control

become weaker and broader with slight shifts to the left (e.g., 9.20 Å−1 peak). The EXAFS spectra in R space show two prominent peaks corresponding to the O shell in MnO6 at ∼1.5 Å (R + ΔR) and the edge-sharing Mn shell at ∼2.5 Å (R + ΔR; Figure 5 A). A weak atomic shell located at ∼2.9−3.0 Å (R + ΔR) corresponds to Mn(II) and/or Mn(III) adsorbed over vacancies in a triplecorner (TC) binding geometry. With increasing P/Mn ratio, all these peaks decrease in intensities, particularly for the Mn−Mn edge-sharing peak, consistent with decreasing particle size. Meanwhile, the edge-sharing Mn−Mn peak significantly shifts to the left (Figure 5A), suggesting decreased edge-sharing Mn− Mn distances. Both the Si and S series show similar but much subtler changes compared to the P series (Figure S10). To quantitatively determine the Mn local coordination environment, the EXAFS spectra were fitted (Figure S11), and the obtained structural parameters are listed in Table S5. For the control sample, the interatomic distances of Mn−O, edgesharing Mn−Mn and Mn−MnTC shell are 1.92, 2.88, and 3.49 Å, respectively, with corresponding coordination numbers (CNs) of 6.9, 6.5, and 1.0, consistent with the birnessite structure. With increasing P/Mn ratio, the interatomic distances of the three shells decrease to 1.91, 2.84, and 3.43 Å (P/Mn ratio of 0.3), respectively. Meanwhile, the CNs of Mn−O and Mn−Mn, respectively, decrease to 6.0 and 5.3. These changes are consistent with the qualitative analyses of the peaks described above. The decreased interatomic distances are in line with the structural contraction of the sheets, in agreement with the XRD and PDF analyses. P K-Edge X-ray Absorption Near-Edge Structure Spectroscopy. The spectra of P0.50 contain a peak at ∼2156.8 eV, indicating formation of a phosphate solid phase.50 This peak is absent in the spectra of other samples (P/Mn ≤ 0.3), suggesting that phosphate more likely adsorbs on birnessite surfaces in these samples. With increasing P/Mn ratio to 0.3 (Figure 6), the whiteline intensity decreases, and its

Figure 7. FE-SEM images of low- and high-magnification with scale bars of 2 μm and 300 nm (insets) for the control and selected birnessite samples.

sample contains thin nanoplates with a thickness of ∼7 nm or smaller, in good agreement with the estimate by the Scherrer equation. The spherical particle aggregates have an average radius of ∼300 nm. The radius of the aggregates becomes smaller with increasing oxyanion/Mn ratio (Figure S12), indicating fewer stacking MnO6 sheets and decreased sheet sizes. With the same order of magnification, the surface of the aggregates becomes smoother with increasing oxyanion/Mn ratio for each series, consistent with smaller particle sizes. The P series has the smallest aggregate size, followed by the Si series and then the S series. As the HR-TEM images show in Figure 8, the control sample has the typical morphology of birnessite, consisting of 2D diskshaped crystals and three-dimensional hierarchical microspheres. The curled feature of the control sample suggests curved sheets, contributing to the disordered stacking along the c direction11 and the loss of periodicity in the basal plane.52

Figure 6. Phosphorus K-edge XANES spectra of the phosphate associated with the birnessites synthesized in the presence of phosphate. (inset) The pre-edges of the spectra.

position shifts toward high energy for P XANES spectra, which can be ascribed to stronger binding of phosphate to birnessite.50,51 Since phosphate adsorbs on Mn(IV) weaker than on Mn(III), the stronger binding suggests that more phosphate binds to Mn(III), consistent with increased Mn(III) concentration in the structure. The trace amount of the phosphate solid phase in P0.30 may contribute to the right shift of the whiteline. However, a P XANES LCF analysis using

Figure 8. HR-TEM images of the birnessite control and P0.30. F

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Figure 9. (A) Diffuse reflectance UV−vis spectra of the P series samples. (B) The direct and indirect band gaps of the samples (filled symbols) and those of the corresponding KOH-washed products (empty symbols).

the S series with increasing S/Mn ratio (Figure S17B). No indirect band gap is observed for the S series. The DRS of acid birnessite, polyMnO2, and δ-MnO2 have similar shapes, but the slopes of the absorption edges are different (Figure S15F), which could be due to the difference in vacancy and Mn(III) concentration, and particle size as well. The determined direct and indirect band gaps of polyMnO2 (1.28 and 0 eV, respectively) and δ-MnO2 (1.32 and 0 eV, respectively) are given in Figure S18. KOH-Washed Birnessite Products. The KOH-washed samples are virtually S, Si, and P free, suggesting that the washing process effectively removed the oxyanions from the solids. The products after KOH washing contain less water and have slightly smaller surface areas (Table S1). After the KOH wash, the (001) and (002) peaks become sharper and shift toward lower d-spacing, indicating enhanced sheet stacking order and/or number of stacked sheets (Figure 1) and decreased interlayer spacing. In addition, the (11, 20), (31, 02), and (22, 40) peak positions shift toward larger dspacing, indicating lattice expansion of MnO6 sheets. More importantly, the MnO6 sheet sizes are increased, suggested by decreasing fwhm values of these peaks. The above changes caused by KOH washing are more significant for the birnessite products synthesized with higher P/Mn and Si/Mn ratios. Both the particle size increase and the lattice expansion are also reflected in the PDF analysis. The CSD size of P0.30 is significantly increased after KOH washing, as evidenced by the stronger peaks and the larger r at which the peaks diminish (Figure 2A). The peaks at 2.87, 3.44, 4.46, and 4.96 Å shift toward longer distances, suggesting structural expansion. In addition, the P−O and P−Mn peaks disappeared, indicating minimal P remained with the solid, consistent with the chemical composition analysis (Table S1). The above phenomenon, albeit weaker, is also observed for P0.10 and Si1.00 (Figure S4). There are almost no changes observed for the sulfate series (Figure S4). KOH washing barely changed PDF (Figure S3C) and XRD patterns (Figure S1E) of PolyMnO2. As to Mn K-edge XANES spectroscopy, the whiteline peaks become stronger after KOH washing, especially for P0.30 sample (Figure S7), suggesting increased particle size, consistent with a higher contribution of KBi in the LCF results (Table S4). The shoulder on the edge becomes weaker for all washed samples, indicating less Mn(III) in the structure, consistent with the XANES LCF analysis (Figures 4B and S8). Apparently, some Mn(III) was oxidized to Mn(IV) during washing, probably by atmospheric O2.

Figure S13 shows smaller average particle dimension and more aggregated microspheres with increasing P/Mn ratio. The aggregation is too extensive to allow for accurate determination of the individual particle size (Figure 8). The effects of Si and S on the size and morphology of the products are much weaker than that of the P series. The bulky crystals of P0.50 (Figures S12 and S13) correspond to the unknown solid phase identified by XRD. Neither this phase nor other new solid phases were observed by the electron microscopic analyses. Specific Surface Area. The BET SSA of the products determined from N2 adsorption are given in Table S1. The specific surface area of the control sample is 30.1 m2 g−1, close to 39 ± 3 m2 g−1 reported in Villalobos et al.33 The surface area of the Si and S series slightly increases with increasing Si/Mn and S/Mn ratio, and the Si series has larger surface areas than the S series. However, the surface areas of the P series are close to or smaller than that of the control sample (Table S1). The smaller surface area is attributable to the extensive aggregation for the P series, limiting the access of N2 to some surface sites during BET measurement.53 Ultraviolet−Visible Diffuse Reflectance Spectroscopy. Figure 9A shows the UV−vis DRS of the P series. Except P0.50, the spectra of the products have similar shape, suggesting these solids have the same phase (birnessite). However, the edge position of the spectra is blue-shifted with increasing P/Mn ratio, which can be attributed to increased Mn(III) concentrations,30 decreasing particle size at the nanometer level,24,25,54 and/or the lattice contraction due to light absorption moving to higher photon energy driven by faster vibrating of atoms at shorter atomic distance.24,55 In addition, the absorbance in the range of 500−800 nm decreases with increasing P/Mn ratio to 0.3 due to decreasing Mn(IV) concentration, because Mn(IV) has lower spin magnetic moment than Mn(III), resulting in more microwave-absorbing capability.56,57 The direct and indirect band gaps of the P series samples are probed optically via Tauc−Mott plot (Figure S14). With increasing P/Mn ratio, the allowed direct band gap increases from 1.48 to 1.82 eV, while the allowed indirect band gap increases from 0 to 0.27 eV (Figure 9B). The UV−vis DRS data for the Si and S series are given in Figure S15. These samples have similar absorption spectra to those of the P series. But they show weaker blue shifts than the P series, with the S series being the weakest. TM plots for direct and indirect band gaps of the Si and S series are shown in Figure S16. For the Si samples, the direct band gap increases from 1.49 to 1.74 eV (Figure S17A), while the indirect band gaps are close to 0. The direct band gap increases slightly for G

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Effects on Mn Valence States. Our results show that with increasing oxyanion/Mn ratio, the proportion of Mn(III) increases, hence decreasing Mn AOS. The redox reaction for birnessite formation can be described by the following reduction equation: 6Cl− + 8H+ + 2MnO4 − → 2MnO2 + 3Cl 2 + 4H 2O (1)

Although the equation shows Mn(VII) is reduced to Mn(IV), the birnessite products always contain Mn(III), and even Mn(II), because of unavoidable side reactions during the synthesis. One of such reactions is the further reduction of newly formed MnO2 by HCl, leading to formation of Mn(II) and/or Mn(III). Ligands have been shown to complex with Mn(III) and thus increase Mn(III) stability and prevent or retard it from disproportionation to Mn(II) and Mn(IV).60,61 The ligands, such as poly(allylamine hydrochloride) and the carboxyl group of humate,30,62 can also stabilize Mn(III) on birnessite surface. During birnessite formation, the oxyanions can adsorb on the Mn(III) produced from Mn(IV) reduction by HCl, stabilizing Mn(III) in birnessite structure. Thus, the edge sites of birnessite must particularly be enriched in Mn(III), and the bulk likely contains elevated concentration of Mn(III), too, as the edge sites may not be enough to account for the high Mn(III) concentration in the products (e.g., P0.30). The varying Mn(III) concentration in the products at a given oxyanion/Mn ratio for the P, Si, and S series again correlates with their different adsorption affinity. The XRD, EXAFS, and PDF analyses indicate that P0.30, containing ∼40% Mn(III), does not resemble triclinic birnessite containing orderly distributed Mn(III) accounting for one-third of total Mn. Thus, the Mn(III) in the birnessite product may have a different arrangement as compared to triclinic birnessite. Lattice Contraction. Significant lattice contraction was observed for birnessite synthesized in the presence of the oxyanions. On the one hand, this could be caused by the decreasing particle sizes that increase the surface stress of the particles due to the large surface/volume ratio.63 On the other hand, the presence of increasing concentration of Mn(III) in the structure may cause structural expansion because of the larger size of Mn(III) ion as opposed to Mn(IV) ion. The observed lattice contraction suggests that the effect of decreased particle sizes dominates. But the increased Mn(III) concentration may contribute to the observed layer symmetry distortion due to the Jahn−Teller effects of Mn(III) octahedra.22,23 Band gaps. Our results indicate that with increasing oxyanion/Mn ratio, the band gap of the products widens, particularly for the P series (Figures 9B and S17). The determined direct band gap of P0.30 is similar to that (1.8 eV) of triclinic birnessite reported in Sherman et al.,64 based on oxygen X-ray absorption and emission spectroscopy, while narrower than that (2.23 eV) of protonic MnO2 nanosheets.4 Our observed dependence of band gaps on oxyanion/Mn ratios can be attributed to both increasing Mn(III) concentration and decreasing particle size, as both factors affect the electronic structures, hence band gaps.22,23 Particle sizes play a critical role in optical and photophysical properties of nanomaterials.65,66 The widening of band gaps of birnessite with decreasing particle sizes is consistent with the intrinsic size effects, that is, a broadening of the surface plasmon

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DISCUSSION Effects on Particle Sizes. The thorough characterization of the reaction products indicates that the presence of oxyanions during birnessite synthesis leads to decreases in both the MnO6 sheet size and the number of the disorderly stacked sheets (Scheme 1A). The smaller sheet size can contribute to the Scheme 1. Proposed Mechanisms for Particle Size Decrease in the Presence of Oxyanions (A) and Particle Growth after KOH Washing (B)

more disordered stacking and fewer stacked sheets. Similar results were obtained for the synthesis of birnessite in the presence of vanadate.42 The sheet size decrease is caused by the chemical adsorption of the oxyanions on the edge sites,58,59 inhibiting particle growth of the sheets (Scheme 1A).42 Phosphate imposes the strongest impact, followed by silicate and then by sulfate, the order being the same to the order of their decreasing adsorption affinity on metal oxide surfaces, that is, phosphate > silicate ≫ sulfate.35,36 That is to say, the higher adsorption affinity of an oxyanion, the stronger inhibitory effect it has on the growth of birnessite sheets. H

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indicating that the particle growth of δ-MnO2 is probably ascribed to the oriented aggregation mechanism.72,73

resonance,66 rather than the extrinsic size effects, because our birnessite nanoparticles are smaller than 20 nm. The contribution of Mn(III) to the widened band gap of birnessite is due to destabilization of the layers.22 Marafatto et al. indicated that the formation of the Jahn−Teller distorted Mn(III) in the octahedral sheet increases the net quantum yield of direct photoreduction by band gap excitations,23 decreasing the ground-state energy by adopting high-spin d4 electron configuration and splitting of the eg and t2g orbitals. The difference energy between excited state and ground state is identified as band gap. Both direct and indirect band gaps depend on the electronic band structure. Thus, both band gaps can be increased in the presence of Mn(III). Compared to the contribution of particle sizes, the Mn(III) contribution is larger, given that the band gap of polyMnO2 is much narrower than P0.30, although they have similar particle sizes. The presence of Mn vacancies may also alter birnessite band gap. Compared to a hypothetical vacancy-free hexagonal birnessite, the band-gap energy is reduced by a vacancy significantly due to newly introduced states of valence bands, which is attributed to the nearest-neighbor Mn and O atoms around a vacancy.13,14 However, our structural characterization does not provide enough information about the changes of vacancy concentrations with oxyanion/Mn ratios, and thus, the contribution of vacancies to the band gap of the samples is unclear. Effects of KOH Washing. Our results show that KOH washing leads to increased sheet sizes and the number of stacked sheets as well as decreased Mn(III) concentration and increased Mn AOS. The effects are stronger for the product having a smaller size and a higher Mn(III) concentration, for example, P0.30 and Si1.00. Both Ostwald ripening and oriented attachment (OA) can be responsible for the growth of crystals.67,68 Ostwald ripening is a process of dissolution of small crystals and re-deposition of the dissolved species on the surface of larger crystals, because the smaller crystals have higher solubility than the larger ones.68 Birnessite is essentially insoluble in KOH solution, and thus particle growth via Ostwald ripening is disfavored. In the process of OA, particle growth occurs via attachment of smaller particles in certain orientation, and adjacent nanoparticles are docked at a planar interface to eliminate the pairs of highenergy surfaces.69 OA often dominates the particle-growth pathway for crystals of low solubility, such as TiO2, ZnS, etc.68,70 Therefore, OA is more likely to be the pathway for particle growth of the MnO6 sheets during KOH washing (Scheme 1B). The desorption of oxyanions and subsequent Mn(III) oxidation to Mn(IV) play an important role during the particle growth. They increase the surface energy of edges of MnO6 sheets so that the two adjacent sheets tend to connect to each other on the edges to eliminate the high-energy surfaces. Small particle sizes alone do not warrant the growth of sheets via OA upon KOH washing, as polyMnO2 has a comparable size to P0.30 but without particle size changes after KOH washing. Portehault et al. proved that the growth of cryptomelane nanowires, a tunneled Mn(IV) oxide,71 proceeded by OA for the lateral direction using HR-TEM analyses. Unfortunately, the high degree of aggregation of the birnessite particles in our study does not allow for the delineation of particle boundaries, preventing the confirmation of the particle growth mechanism. However, this can be indirectly supported by recent studies

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CONCLUSIONS The presence of adsorptive oxyanions (PO43−, SO42−, or SiO44−) during the synthesis of birnessite decreases MnO6 sheet sizes while increasing Mn(III) concentration in birnessite, leading to enlarged birnessite band gap. The consequence is attributed to the adsorption of the oxyanions on birnessite edge sites, inhibiting sheet growth and stabilizing Mn(III) in birnessite structure. Phosphate shows the strongest effect, while sulfate shows the weakest, correlating with their adsorption affinity on birnessite. KOH washing removed the adsorbed oxyanions, resulting in increases of sheet sizes, which may occur via the oriented attachment. This study provides a method for synthesizing small particle size of birnessite with high concentration of stabile Mn(III), which may enhance birnessite catalytic activity for water oxidation and benefit other applications as well.22,27,74,75 In addition, the oxyanions used in the synthesis are economically viable and environmentally benign compared to vanadate used in a previous study.42 Our results also have important implications for understanding particle size and chemical compositional variations of naturally occurring birnessite minerals. Particle sizes and Mn(III) concentration strongly affect birnessite chemical reactivity and mineralogical transformation to other tunneled Mn oxide phases.15−17,21,33,34 The three anions examined in this study are common and abundant in the environment and can adsorb on the edge sites of birnessite under most natural pH conditions.36,76,77 These anions may impose similar effects on particle size and Mn(III) concentration of naturally occurring birnessite when birnessite forms in the presence of these anions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01465. Chemical compositions and SSA of birnessite products, XRD and PDF analysis, Mn XANES and EXAFS spectra and fitting results, FE-SEM and HR-TEM images, and DRS and Tauc-Mott plots for determining the direct and indirect band gaps (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 307-766-5523. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation under Grant No. EAR-1529937 and the Wyoming Reclamation and Restoration Center at the Univ. of Wyoming. We acknowledge Prof. B. A. Parkinson at the Dept. of Chemistry, Univ. of Wyoming, for his help with UV−vis diffuse reflectance spectra collection. We thank beamline scientists K. W. Chapman, K. A. Beyer, and O. J. Borkiewicz at beamline 11ID-B and M. G. Newville and A. Lanzirotti at beamline 13-ID-E at the Advanced Photon Source (APS), and E. J. Nelson and M. J. Latimer at beamline 14-3 at the Stanford Synchrotron I

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Radiation Lightsource (SSRL) for their technical assistance in data collection. This research used resources of the APS, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the SSRL, SLAC National Accelerator Laboratory, is supported by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515.

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Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01465 Inorg. Chem. XXXX, XXX, XXX−XXX