Ascorbate Induced Facet Dependent Reductive Dissolution of

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Ascorbate Induced Facet Dependent Reductive Dissolution of Hematite Nanocrystals Xiaopeng Huang, Xiaojing Hou, Fahui Song, Jincai Zhao, and Lizhi Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09281 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 3, 2017

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The Journal of Physical Chemistry

Ascorbate Induced Facet Dependent Reductive Dissolution of Hematite Nanocrystals Xiaopeng Huang, Xiaojing Hou, Fahui Song, Jincai Zhao and Lizhi Zhang*

Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental & Applied Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China

*

To whom correspondence should be addressed. E-mail: [email protected]. Phone/Fax: +86-27-6786 7535 1

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ABSTRACT In this study, the interaction between ascorbate and hematite facets was systematically investigated with attenuated total reflectance Fourier transform infrared (ATR−FTIR) spectroscopy, density−functional theory (DFT) calculation, and kinetics model. Results of ATR−FTIR spectroscopy and DFT calculation suggested formation of nonprotonated inner−sphere bidentate mononuclear and monodentate mononuclear iron−ascorbate complexes on the hematite {001} and {012} facets, respectively. The estimated reductive dissolution rate constants at pH 5.0 were (4.04 ± 0.16) × 10−4 min−1 and (1.59 ± 0.14) × 10−4 min−1 for hematite nanoplates and nanocubes, respectively, indicating that the bidentate mononuclear iron−ascorbate complexes on the {001} facets favored the hematite reductive dissolution process than the monodentate mononuclear iron−ascorbate counterparts on the {012} facets. These results also revealed that the hematite facet reduction with ascorbate was strongly dependent on the iron−ascorbate complexes formed on the hematite facets. This study provides new insights into the reductive interaction between ascorbate and hematite facets, and also shed light on the environmental effects of hematite at the atomic level.

1. Introduction Natural minerals containing iron oxide and iron (oxy)hydroxide are reactive and widespread in soils, subsurface sediments, particulates of river water and airborne mineral dusts.1-6 Their dissolutions are major iron redox cycling processes in natural environment, and thus inevitably affect the transformation of environmental pollutants.7-9 Among the iron oxides dissolution mechanisms,10 the reductive dissolution of iron oxides was regarded as an important dissolution mechanism besides the microbial iron reduction.10-13 Various reductants, such as hydrogen sulfide, phenolic compounds, dithionite, fructose and ascorbate, can trigger the reductive dissolution of iron oxides.10-12, 14-18 As 2


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ascorbate is a commonly used antioxidant and widely existed in natural waters,7 while iron oxides are also omnipresent in natural environment, the interaction between ascorbate and iron oxides would definitely influence the reductive dissolution of iron oxides. Therefore, it is essential to investigate the interaction of ascorbate and iron oxides to understand the reductive dissolution of iron oxides. As the most thermodynamically stable and ubiquitous iron oxides, hematite possesses environmentally benign characteristic and outstanding catalytic activity.19-22 In early studies, researchers mainly focused on batch reaction on the interaction of ascorbic acid with iron oxides and the apparent kinetic equations for the reductive dissolution processes.7, 11, 12, 14, 23-25 Recently, various advanced technologies were employed to explore the intrinsic interaction between ascorbate ions and iron oxides. For instance, transmission electron microscopy (TEM) was used to investigate the hematite dissolution process mediated by ascorbate.26 The high angle annular dark field–scanning transmission electron microscope (HAADF−STEM) tomography and high resolution TEM revealed that aggregates of hematite nanoparticles dissolved heterogeneously and holes formed on the surface of crystals in the ascorbate solutions.10 However, these studies were carried out with the particles of irregular shapes, making it difficult to understand the intrinsic mechanism of hematite reductive dissolution at the atomic level. Moreover, the interaction between hematite with exposed facets and ascorbate, which could provide new insights into the hematite reductive dissolution mechanism, was never investigated previously. In this study, we systematically investigate the interaction between ascorbate species and hematite facets with attenuated total reflectance Fourier transform infrared (ATR−FTIR) spectroscopy, density−functional theory (DFT) calculation, and kinetics model, so as to understand the hematite 3



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reductive dissolution mechanism. On the basis of the experimental and theoretical results, we aim to clarify the environmental effects of hematite at the atomic level.

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials. Iron trichloride, sodium acetate, sodium ascorbate, sodium oleate, acetic acid, orthophosphoric acid, ferric ammonium sulfate, sodium hydroxide, hydrochloric acid and sulfuric acid were obtained from Sinopharm Chemical Reagent Co. Ltd., China. Hydroxylamine hydrochloride, 2,2’–bipyridine and 1,10−phenanthroline were obtained from Aladdin Chemistry Co. Ltd., China. Deionized water (18 MΩ·cm) from a Millipore Q water purification system was used throughout the experiments. All the chemicals were analytical grade and used without purification. The ascorbate stock solution was prepared by dissolving sodium ascorbate in deionized and deoxygenized water. All the stock solutions were freshly prepared before use. 2.2 Hematite Nanocrystals Synthesis. Hematite nanoplates (HNPs) were synthesized according to previously reported solvothermal methods.27-29 Briefly, 1.09 g of iron trichloride was dissolved under magnetic stirring in 40.0 mL ethanol. After totally dissolved, 2.8 mL deionized water and 3.2 g of sodium acetate were added and stirred for 2 hours at ambient temperature. The mixture was then transferred into a 100 mL Teflon−lined stainless autoclave. The autoclave was heated in an oven at 180 oC for 12 hours. Upon cooling to ambient temperature, the precipitation was collected and washed with ethanol and deionized water thoroughly and dried at 40 °C over night. Hematite nanocubes (HNCs) were synthesized following the methods reported previously.30 Typically, 2.08 g of iron trichloride was added to the solution which was composed of 6.93 g of sodium oleate, 35 mL ethanol and 4.3 mL oleic acid, and then stirred for 2 hours. The mixture solution was poured into a 4


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100 mL Teflon−lined stainless autoclave. The autoclave was heated at 180 oC for 12 hours and then cooled to ambient temperature. The resulting precipitation was collected and washed with cyclohexane, water and ethanol several times, and dried at 40 oC over night. 2.3 Hematite Nanocrystals Characterization. The powder X−ray diffraction (XRD) measurements were performed on a D/Max−IIIA X−ray diffractometer using Cu Kα radiation (λ = 0.15418 nm). The morphologies of the as−prepared hematite samples were observed with scanning electron microscopy (SEM) and a JEOL−2010 transmission electron microscopy (TEM). The structures of as−prepared samples were observed with the JEOL JSM−2010 high−resolution transmission electron microscopy (HRTEM). The Brunauer−Emmett−Teller (BET) surface areas of two hematite samples were calculated from N2 adsorption/desorption isotherms with a nitrogen−adsorption system (Micromeritics Tristar 3000). High−resolution X−ray photoelectron spectroscopy (XPS) was performed on a Kratos ASIS−HS X−ray photoelectron spectroscope. The standard and monochromatic source was operated at 150 W with 10 mA of emission current and 15 kV of accelerating voltage. 2.4 DFT Calculation. Density functional theory (DFT) calculation was conducted with Fe(III) cluster models of two edge-sharing Fe(III)-octahedra by program Gaussian 09.31 Structures of modeled clusters were energy minimized without symmetry constraints by using unrestricted open shell UB3LYP method with the LANL2DZ basis set.32-34 Additionally, the optimization was repeated in solvent (water) described by the integral equation formalism polarizable continuum model (IEFPCM) model.35 As molecular cluster calculation was effective to predict IR vibrational spectra of hematite surface complexes,28, 36 the frequencies based on the energy−minimized structures were also calculated using Gaussian 09 program. All the calculated values were scaled by a factor of 0.970 5



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to account for anharmonicity and systematic errors associated with correlation effects. 2.5 ATR−FTIR Spectroscopy. A Nicolet iS50 FT−IR spectrometer (Thermo, USA) equipped with a diamond internal reflection element and a sensitive liquid N2−cooled MCT detector was employed to obtain ATR−FTIR spectroscopy. The OMNIC software package (v2.11) was used for data collection and analysis. All the data were recorded from 2000 to 1000 cm−1 and consisted of 1200 averaged scans at a resolution of 4 cm−1. Hematite films were prepared by using the deposition technique with modification.37 Briefly, 10 µL of hematite suspension (4 g/L) was dropped onto the diamond internal reflection element and dried to obtain semitransparent hematite films. A background spectrum was subsequently recorded after the film was equilibrated with an electrolyte solution. The IR spectroscopy was then obtained after the hematite film was reacted with ascorbate concentrations ranging from 2.5 × 10−4 to 5.0 × 10−3 mol/L at pH 5.0. 2.6 Reductive Dissolution Experiments. All the reductive dissolution experiments were performed in 100 mL conical flasks with constant stirring under argon gas at room temperature. Each 50 mL reaction solution with ascorbate was prepared with deionized and deoxygenized water. The 0.1 mol/L hydrochloric acid and 0.1 mol/L sodium hydroxide were used to adjust the initial solution pH at 5.0. The dosage of hematite nanocrystals was 0.4 g/L. During the experiments, the conical flasks were first pumped with high−purity argon gas for half an hour and then sealed with rubber stoppers gas escaping. 2.7 Reductive Dissolution Kinetics. The interaction between hematite and ascorbate can be described by the following three elementary steps. First, surface inner−sphere iron−ascorbate complexes (≡FeIIIAsc) were formed on the hematite facets, then the electron transfer occurred from ascorbate to surface iron (≡FeIIIOH2) within the surface complexes to produce surface bound ferrous 6


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species (≡FeIIOH2), and finally dissolved Fe(II) ions were released from the hematite surface slowly. These processes can be described with the following equations according to the previous reports.11, 12, 14

The first step is to form inner−sphere iron−ascorbate complexes on the surface. k1  → ≡ Fe III Asc + H 2 O ≡ Fe III OH 2 + Asc ←  k-1

(1)

The second step is the electron transfer, producing surface bound ferrous ions (≡FeIIOH2) and ascorbate radicals (•Asc). (2) The third step is the releasing of dissolved Fe(II) ions from the hematite surface slowly. +

H , slow ≡ Fe II OH 2  → new ≡ FeIII OH 2 + Fe(II) k3

(3)

The rate equation for the generation of dissolved Fe(II) ions at a given pH can be expressed as rate=

d[Fe(II)] = k3{≡ Fe II OH 2 } dt

(4)

According to equation 2, the surface density of the ≡FeIIIAsc complexes has strong influence on the surface density of ≡FeIIOH2. Therefore, the rate equation for the ≡FeIIOH2 generation can be expressed as

(5) According to the steady-state approximation,11 the surface density of the ≡FeIIIAsc complexes could determine the surface concentration of ≡FeIIOH2.

(6) From equation 4 and 6, the pseudo first-order rate could be obtained from the following equation.

(7) 7



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As a steady-state was assumed for the intermediate ascorbate radical, the equation (7) could be simplified to the following equation.

rate = ke {≡ Fe III Asc}

(8)

where ke is defined as the empirical reductive dissolution rate constant. The density of surface ascorbate is dependent on the total concentration of ascorbate, the total concentration of all dissolved ascorbate species, the concentration of dissolved Fe(II), and the density of surface ≡FeIIOH2 through the following mass balance equation. [Asc]sur = [Asc]T - [Asc]sol -

1 {[Fe(II)] + { ≡ FeIIOH 2 }} 2

(9)

2.8 Analytical Methods. The concentration of dissolved Fe(II) and total dissolved iron ions were measured by a 1,10−phenanthroline method.38 The accumulated surface bound ferrous ions of the nanocrystals were measured with the HCl extraction technique.39,

40

Briefly, after the reduced

dissolution the hematite slurry was exposed to the HCl solution and magnetically stirred under the protection of argon gas. The obtained supernatant liquor was reacted with 1,10−phenanthroline and then measured by a UV–vis spectrophotometer to obtain the amount of accumulated surface bound ferrous ions. The ascorbate ions concentration was calculated by a modified 2, 2’–bipyridine method,41 which was described in our previous study.42 Briefly, ascorbate solution, orthophosphoric acid, ferric ammonium sulfate were mixed, followed with the addition of 2, 2’–bipyridine at different intervals. The solution samples were analyzed by a UV–vis spectrophotometer after reaction for 30 min. The density of surface coordinated ascorbate was calculated from the total concentration of ascorbate, the total concentration of all dissolved ascorbate species, the concentration of dissolved Fe(II), and the density of surface bound ferrous ions through a mass balance equation. An Orion model PHS−25m pH meter was used to calculate the pH values and the 8


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degree of the accuracy was controlled as ± 0.1 pH in this study.

3. RESULTS AND DISCUSSION 3.1 Morphology and Structure of Hematite Nanocrystals. The XRD patterns (Figure S1a) demonstrated that the as-prepared samples were pure hematite without any impurities.27,

30

The

high−resolution XPS of Fe 2p and O 1s core-level (Figures S1b and S1c) confirmed that the as−prepared hematite nanocrystals were clean without organic compounds after ethanol washing.43 The nitrogen adsorption isotherms (Figure S1d) revealed that the specific surface areas of HNPs and HNCs were ~21.3 and ~20.4 m2/g, respectively. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images demonstrated that the as-prepared HNPs and HNCs were of uniform hexagonal plates and pseudocubes, respectively (Figure 1). The mean width and average thickness of hexagonal HNPs were 80.5 and 14.7 nm, respectively (Figures 1a and 1b, Figure S2). The high−resolution transmission electron microscopy (HRTEM) images (Figure 1e) and the fast Fourier transforms (FFT) pattern (Figure 1c) showed a distinguishable lattice fringe of 0.25 nm, which was consistent with {−120}, {110}, and {−210} planes of HNPs.27 The proportions of {001} and {012} planes in HNPs were calculated with the average width and the mean thickness, which revealed that the dominant facets of HNPs were {001} facets.28,

29

HNCs consisted of

uniformed nanocubes with a mean side length of 30.3 nm from SEM and TEM images, and 27.3 nm from the XRD patterns (Figures 1f and 1g, Table S1). The high magnification TEM images revealed that these nanocubes actually had unique oblique parallelepiped morphologies, in which every nanocube was composed of six similar parallelogram planes (Figures 1g and 1i). The dihedral angles of two exposed adjacent planes were 86° or 94°. These structural features of nanocubes matched the 9



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perfect rhombohedron model enclosed by {012} facets,44, 45 which was confirmed by FFT pattern and HRTEM images (Figures 1h and 1j). Therefore, HNCs were completely exposed with {012} facets.

3.2 ATR−FTIR Vibrational Modes of Ascorbate on Hematite Nanocrystals. The interaction between hematite and ascorbate was investigated by ATR−FTIR spectroscopy, which could provide the information of vibrational frequencies of ascorbate on hematite nanocrystals. Moreover, the assessment of ascorbate coordination via ATR–FTIR spectroscopy relies on a thorough comprehension of the vibrational properties of ascorbate ions. Therefore, we first studied the vibrational properties of ascorbate ions, and found that ascorbate species (pKa1 = 4.25 and pKa2 = 11.79) in the aqueous solution included dibasic acid (C6H8O6), ascorbic anion (C6H7O6−), and dianionic ascorbic acid (C6H6O62−), whose distributions were highly pH−dependent at a given total ascorbate concentration (Figure 2a).46, 47 From pH 3.5 to 7, the dominated ascorbate species was C6H7O6− (Figure 2b). In the ATR−FTIR spectra of aqueous ascorbate solutions as a function of total ascorbate concentration (Figure 2c), the peak at 1718 cm−1 was assigned to the carbonyl stretch (C1=O).48 The strong peak at 1579 cm−1 could be attributed to the coupling vibration of C2=C3 and C1=O,49, 50 and the peak at 1420 cm−1 was arisen from the vibration of C2=C3 and C3−O,50 while the multiple peaks starting from 1140 cm−1 were produced by the five membered ring stretch.48-50 Along with increasing the concentrations of ascorbate ions from 2.5 × 10−4 to 5.0 × 10−3 mol/L, the stretching frequencies of these peaks kept stable and did not shift, indicating that their stretching frequencies were less sensitive to the ascorbate concentration changes. The vibrational properties of ascorbate ions bound on the hematite film were then investigated with a surface sensitive ATR−FTIR accessory (Figure 2d). The ATR-FTIR spectra of wet hematite 10


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nanocrystals revealed that the hematite nanocrystals were not contaminated with oleic acid or acetate groups (Figure S3), consistent with the XPS measurement results. Meanwhile, the wet hematite nanocrystals did not possess any strong absorbance in their IR spectra. Figures 2e and 2f show the ATR−FTIR spectra of ascorbate adsorbed on HNPs and HNCs as a function of total ascorbate concentration. Peak positions of representative surface iron−ascorbate complexes were summarized in Tables 1 and 2. The ATR−FTIR spectra of iron−ascorbate complexes on hematite were found to be different from those of the dissolved ascorbate ions, indicating the formation of inner−sphere iron-ascorbate surface complexes. Moreover, we found that the IR peak positions were not affected by the total ascorbate concentration change in a broad range from 2.5 × 10−4 to 5.0 × 10−3 mol/L. The spectra of ascorbate bound on HNPs possessed the stretch vibrational peaks of C1=O, C2=C3/C1=O, C2=C3/C3−O, and the ring stretch vibrational peaks located at 1682, 1584, 1451, and 1120 cm−1, respectively (Figure 2e, Table 1). Differently, the stretch vibrational peaks of C1=O, C2=C3/C1=O, C2=C3/C3−O and the ring stretch vibrational peaks of ascorbate bound on HNCs were respectively located at 1669, 1559, 1383, and 1130 cm−1 (Figure 2f, Table 2). We therefore proposed that two different types of iron−ascorbate complexes might be formed on the surfaces of HNPs and HNCs respectively exposed with {001} and {012} facets. It was also noticed that the spectra of adsorbed ascorbate on hematite differed from those on ferrihydrite reported in the previous study,49 which might be attributed to the different surface structures of hematite and ferrihydrite. To figure out the effect of ionic strength on the ascorbate complexation, more IR spectra were recorded under different concentrations of NaCl ranging from 0.01 to 0.10 mol/L (Figures S4a and S4b). As expected, the intensity and positions of peaks did not change even at high ionic strengths, 11



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further suggesting that the inner−sphere iron−ascorbate complexes were formed on the hematite facets. Moreover, additional IR spectra were obtained with using D2O as the solvent. It was found that the peaks observed in D2O were approximately the same as those in H2O (Figures S5a and S5b). The frequency of C3–O did not shift, revealing that the C3–O was nonprotonated. The similar effect of H/D exchange was observed during other adsorption processes.36, 51 This deuterium exchange without identifiable frequencies shift suggested that the inner−sphere iron−ascorbate complexes were nonprotonated.

3.3 DFT Calculation. The vibrational frequencies of some possible inner−sphere iron−ascorbate model complexes used in previous literature were then calculated to fit for ascorbate bound on hematite (Figures 3a and 3b).49 Theoretically, IR frequencies of monodentate mononuclear iron−ascorbate complexes appeared at 1662, 1548, 1388 and 1133 cm−1, which were 1668, 1590, 1447 and 1118 cm−1 for the bidentate mononuclear counterparts (Tables 1 and 2). After comparing the calculated vibrational frequencies with the experimental data, we found that the measured IR frequencies of iron−ascorbate complexes bound on HNPs matched better with those of the theoretical bidentate mononuclear ones with only difference of 10 cm−1 in comparison with those of the monodentate mononuclear counterparts. Moreover, the correlations between theoretical and experimental frequencies of surface iron−ascorbate complexes on HNPs confirmed that bidentate mononuclear surface iron−ascorbate complexes were formed on the {001} facets (Figure 3c, Table 1). The IR frequencies of iron−ascorbate complexes on HNCs agreed with those of theoretical monodentate mononuclear ones, whose difference was within 10 cm−1 (Figure 3d, Table 2). This confirmed that monodentate mononuclear iron−ascorbate complexes were formed on the {012} facets. 12


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3.4 The Hematite Reductive Dissolution Kinetics. As the different coordination structures of hematite surface complexes could strongly influence the reductive dissolution processes,52-54 we investigated the interaction between hematite facets with ascorbate by comparing their reductive dissolution processes at different pH. The dissolved Fe(II) concentration decreased with increasing pH (Figure 4a). Meanwhile, the density of surface bound ferrous (≡FeIIOH2) and the density of surface coordinated ascorbate (≡FeIIIAsc) also decreased with increasing pH (Figure 4b). On the basis of the reductive dissolution kinetics, the interaction of hematite facets with ascorbate and the subsequent hematite reductive dissolution could be described as follows. First, the inner−sphere iron−ascorbate complexes were formed on the hematite facets, then the electrons transferred from ascorbate to the surface iron (≡FeIIIOH2) to produce ≡FeIIOH2, and finally dissolved Fe(II) ions were released from the hematite surface slowly.11, 12, 14 Given that the interaction of hematite facets with ascorbate was pH dependent, we systematically investigated the interaction of hematite facets with ascorbate at a given pH 5.0. As expected, the concentration of ascorbate ions decreased as a function of time (Figure 4c), suggesting the ascorbate ions were bound onto the hematite facets. The empirical reductive dissolution rate constant (ke) could be obtained from the corresponding slope of the fitting line by plotting dissolved ferrous releasing rate versus the density of ≡FeIIIAsc (Figure 4d). The estimated reductive dissolution rate constants were (4.04 ± 0.16) × 10−4 and (1.59 ± 0.14) × 10−4 min−1 for HNPs and HNCs, respectively. Although their rates are very low in terms of chemical reaction, the reductive dissolution of Fe(III) bearing minerals by ascorbate could still affect the transformation of environmental pollutants and the geochemical cycling of other redox-active elements in geochemical processes.4, 5 The larger ke of HNPs suggested that their surface reductive dissolution was faster than that of HNCs in the presence of ascorbate ions. Given that bidentate 13



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mononuclear and monodentate mononuclear iron−ascorbate complexes were formed on the hematite {001} and {012} facets, respectively, we concluded that the bidentate mononuclear iron−ascorbate complexes more favored the reductive dissolution of hematite than the monodentate mononuclear counterparts. It is recognized that the reductive dissolution of iron oxides has strong influence on the geochemical cycling of some redox−active elements and the transformation of environmental pollutants, as the reductive dissolution of iron oxides belongs to the iron geochemical cycling processes in natural environment. In geochemical processes, the hematite reductive dissolution was strongly affected by the structures of ascorbate surface complexes on hematite. The DFT calculation could predict the atomic level structures of ascorbate surface complexes on hematite nanocrystals, which were confirmed by their vibrational frequencies, as revealed by the ATR−FTIR spectroscopy characterization. From the combined DFT calculation and ATR−FTIR spectroscopy as well as the macroscopic reductive dissolution kinetics results, we could solidly elucidate the environmental effects of hematite at the atomic level. As the {001} and {012} surfaces are two of the dominant growth faces exposed on natural hematite,19, 55-58 and Fe(II) could interact with hematite through electron transfer and atom exchange to produce Fe(III),59, 60 Fe(II)–Fe(III) interactions might also exist in this system. However, because the hematite reductive dissolution by ascorbate is much faster than those of electron transfer and atom exchange between generated Fe(II) and hematite surface in this system, the influence of Fe(II)–Fe(III) interactions can be neglected in this study.

4. CONCLUSIONS In geochemical processes, the contaminants transformation and the geochemical cycling of other 14


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redox−active elements are strongly affected by iron redox cycling. Therefore, a more thorough understanding of reductive dissolution processes between hematite and ascorbate is of very important environmental significances. Particularly, the atomic level insights into the interaction between the ascorbate and the dominant exposed {001} and {012} faces of hematite minerals and their impacts on the organic contaminants degradation are critical to understand the environmental roles of hematite in controlling the fate and transport of contaminants. In this study, the experimental and theoretical data first revealed that the nonprotonated inner−sphere bidentate mononuclear and monodentate mononuclear iron−ascorbate complexes could be selectively formed on the {001} and {012} facets of hematite, respectively. The hematite reductive dissolution kinetics was strongly dependent on the iron−ascorbate complexes formed on the hematite facets, while the bidentate mononuclear iron−ascorbate complexes favored the hematite reductive dissolution processes than the monodentate mononuclear counterparts. Given that the ascorbate ions and hematite minerals are both ubiquitous in environment, this study improves our understanding of facet dependent reductive dissolution of hematite nanocrystals, and also sheds light on the environmental effects of hematite at the atomic level.

ASSOCIATED CONTENT Supporting Information Characterization of the samples; SEM image of HNPs; ATR-FTIR spectra of wet hematite nanocrystals; effect of ionic strength on ascorbate complexation; the deuterium exchange experiment; physical properties of the hematite nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. 15



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ACKNOWLEDGEMENTS This work was supported by Natural Science Funds for Distinguished Young Scholars (Grant 21425728), National Science Foundation of China (Grant 21677059), Self−Determined Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grant CCNU14Z01001), and Excellent Doctorial Dissertation Cultivation Grant from Central China Normal University (Grant 2015YBZD013 and 2016YBZZ036).

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Figure Captions

Figure 1. Representative morphologies crystalline and structure of HNPs and HNCs. (a) TEM image, (b) two nanoplates, (c) FFT pattern, (d) schematic drawing of a plate, and (e) high−resolution TEM image of HNPs. (f) TEM image, (g) a single nanocube, (g) FFT pattern, (i) schematic drawing of a cube and (j) high−resolution TEM image of HNCs.

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Figure 2. (a) Ascorbate speciation as a function of pH. (b) The structure of ascorbate. (c) ATR−FTIR spectra of aqueous ascorbate solutions as a function of total ascorbate concentration. (d) Scheme for surface sensitive ATR diamond internal reflection element with hematite film in contact with aqueous solution. (e) Isotherm spectra of ascorbate adsorption on HNPs. (f) Isotherm spectra of ascorbate adsorption on HNCs. The initial ascorbate concentrations (bottom to top) were 2.5 × 10−4, 5.0 × 10−4, 1.0 × 10−3, 2.5 × 10−3, and 5.0 × 10−3 mol/L, respectively. The ionic strength of NaCl was 0.01 mol/L. The pH of ascorbate solution was 5.0.

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Figure 3. (a) Monodentate mononuclear and (b) bidentate mononuclear model complexes used to calculate vibrational frequencies of bound ascorbate on hematite. Correlation between theoretical and experimental frequencies of ascorbate surface complexes on (c) HNPs and (d) HNCs.

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Figure 4. (a) Change in dissolved Fe(II) concentration of HNPs and HNCs in ascorbate aqueous solutions with time at pH 3.6, 5.0 and 6.0. (b) The corresponding surface iron-ascorbate and ≡FeIIOH2 of HNPs and HNCs at pH 3.6, 5.0 and 6.0. (c) A comparison of dissolved ascorbate concentration change with time in the presence of ascorbate and hematite nanocrystals at pH 5.0. (d) Fe(II) releasing rate of HNPs and HNCs as a function of the surface concentration of ascorbate at pH 5.0. The initial concentration of ascorbate was 1.0 × 10−3 mol/L; the dosage of hematite nanocrystals was 0.4 g/L; the initial pH was 5.0.

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Table 1. Correlation between theoretical and experimental frequencies of ascorbate surface complexes on HNPs.

Models

Experimental frequencies

Theoretical frequencies

(cm-1)

(cm-1)

Ascorbate/HNPs

MM

BM

ν(C1=O)

1682

1662 (-20)*

1688 (6)

ν(C2=C3)/ν(C1=O)

1584

1548 (-36)

1590 (6)

ν(C2=C3)/ν(C3−O)

1451

1388 (-63)

1447 (-4)

Ring stretch

1120

1133 (13)

1118 (-2)

R2

NA

0.98

1.00

Slope

NA

0.92

1.01

Intercept

NA

77.37

-21.12

Standard deviation

NA

44.10

5.54

*The numbers in parenthesis = calculated value - measured value.

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Table 2. Correlation between theoretical and experimental frequencies of ascorbate surface complexes on HNCs.

Models

Experimental frequencies

Theoretical frequencies

(cm-1)

(cm-1)

Ascorbate/HNCs

MM

BM

ν(C1=O)

1669

1662 (-7)*

1688 (19)

ν(C2=C3)/ν(C1=O)

1550

1548 (-2)

1590 (40)

ν(C2=C3)/ν(C3−O)

1383

1388 (5)

1447 (64)

Ring stretch

1130

1133 (3)

1118 (-12)

R2

NA

1.00

0.98

Slope

NA

0.98

1.06

Intercept

NA

26.40

-56.29

Standard deviation

NA

5.38

45.46

*The numbers in parenthesis = calculated value - measured value.

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TOC Art

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Ascorbate Induced Facet Dependent Reductive Dissolution of

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