Article pubs.acs.org/crystal
Role of the Counteranions on the Formation of Different Crystal Structures of Iron Oxyhydroxides via Redox Reaction Mahmud Diab and Taleb Mokari* Department of Chemistry and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel S Supporting Information *
ABSTRACT: Redox reactions have been employed for the conversion of various metal oxide nanocrystals, where several parameters were examined to understand the reaction mechanism. However, the role of the counteranions still has not yet been studied. Herein, we present the influence of the counteranions (sulfate, chloride, and acetate) on the formation of iron oxyhydroxide nanofiber structure (100 nm × few microns; diameter × length) via redox reaction using iron salt and manganese oxide as a template. We found that different structures of iron oxide/oxyhydroxide (goethite, hydrohematite, and maghemite) were obtained when different anions (sulfate, chloride, and acetate, respectively) were used. Moreover, we studied the effect of various parameters such as the concentration of iron precursor, the time, and the temperature of the reaction. Finally, we converted the iron oxyhydroxide film to hematite and examined its photoelectrochemical properties.
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INTRODUCTION Iron-based oxides are attractive nanomaterials due to their high stability, low cost, low toxicity, and abundance that have been used in a wide range of applications such as superconductors, pigments, catalysts, magnetic materials, and sensors.1−4 Iron oxyhydroxides are a particularly interesting type of iron oxides due to their enhanced capacity to extract metal ions from solution by adsorption of these ions to hydroxide groups on the surface.5 Various methods have been reported for the synthesis of iron oxyhydroxides nanostructures such as hydrolysis of iron salt, oxidation of iron sulfide by H2O2/H2O, and microwaveassisted method.6−8 Conversion chemistry is another approach for the synthesis of nanomaterials, where a certain type of nanostructure serves as template and is converted into a new type of nanostructure with different composition, while morphological changes can occur. The main mechanisms that describe the conversion chemistry at the nanoscale are the Kirkendall effect, anion/cation exchange, and redox reaction.9 Redox reactions are driven by the difference in the electrochemical potentials of two materials. This method has been applied to synthesize hollow metallic nanoparticles (NPs)10−12 and various types of semiconductor nanostructures.13,18−21 Recently, several approaches were developed to convert one type of metal oxide to another kind of metal oxide.14−17 Oh et al. demonstrated a general procedure to form γ-Fe2O3 and SnO2 using templates of Mn3O4 and Co3O4 NPs, respectively.18 Lou and co-workers reported the syntheses of iron oxide hollow structures using different templates such as Cu2O or MnO2.19,20 Redox reaction was also used to convert Cu2O template into manganese oxide with various morphologies.21 The mechanism of the redox reaction has been studied © XXXX American Chemical Society
by altering the reaction parameters such as temperature, time, and concentration of metal precursors.22,23 Most of these reported studies discuss the role of the cations while usually neglecting the role of the anions. The role of the anions was studied in the synthesis of iron oxide/oxyhydroxides structures by hydrolysis and it was found that the structural properties (phase, morphologies, size, and crystallinity) of the iron oxide/ oxyhydroxide are affected by the nature of the anions in the solution (e.g., SO42−, NO3−, F−, Cl−, and Br−).24−27 These results were mainly attributed to the differences in the reactivity of these anions with the FeO6 octahedral unit, which is related to the size of the anions, the pH of the growth solution, and the Fe2+/counterion molar ratio.28 Herein, we report the influence of the counterion of the iron(II) cation on the formation of iron oxide/oxyhydroxide via a redox reaction method, using manganese oxide nanofibers grown on several types of substrates as templates. This technique enables us to achieve both the conversion and the assembly of iron oxide/oxyhydroxide in a single step. In order to gain insight of the anion influence on the redox reaction mechanism we alter various parameters such as precursor concentration, reaction time, and reaction temperature. Additionally, we show the photoelectrochemical (PEC) properties of the hematite fibers obtained by post heating treatment of the iron oxyhydroxide fibers. Received: September 17, 2016 Revised: December 5, 2016
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DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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EXPERIMENTAL SECTION
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RESULTS AND DISCUSSION Mn2O3 nanostructures are easily converted to iron oxide/ oxyhydroxide via redox reaction due to the difference between the reduction potential of Fe3+/Fe2+ (0.77 V, vs standard hydrogen electrode (SHE)) and Mn2O3/Mn2+ (1.48 V, vs SHE). For this reason, they are excellent candidates for redox reaction templates. We synthesized manganese oxide nanofibers on a Si (100) substrate via thermal decomposition of Mncupferrate (MnCup2), and then it was annealed at 350 °C under air. This method yielded 7 μm thick films of nanofibers (Figures 1a and S1) with porous morphology (inset of Figure
Materials. Solvents and reagents were purchased from Alfa Aesa, Sigma-Aldrich, and Strem Chemicals and used without any further purification. Cupferron (97%), iron(II) sulfate heptahydrate (FeSO4· 7H2O, 99%), iron(II) chloride tetrahydrate (FeCl2·4H2O, 99%), octylamine (OA, 99%), and 1-octadecene (ODE, 90%) were purchased from Alfa Aesa. Iron(II) acetate (Fe(ac)2, 99.995%) and potassium hydroxide (NaOH, 90%) were purchased from SigmaAldrich. Manganese(II) chloride (MnCl2, 97%) was purchased from Strem. Deionized (DI) water was purified using a Millipore Direct-Q system (18.2 MW·cm resistivity). Synthesis of Mn-Precursor. Mn-cupferrate (MnCup2) precursor was synthesized according to a previously reported method.29 Briefly, 0.19 M of Cupferron solution was prepared in 100 mL DI water. Then this solution was added dropwise to 350 mL of a 2.5 mM manganese(II) chloride solution. After 15 min, the orange MnCup2 solution was vacuum filtered. Synthesis of Manganese Oxide Nanofibers on Substrates. 5 mg of Mn(Cup)2 was dissolved in 1 mL solution of OA (0.7 mL) mixed with ODE (0.3 mL). 30 μL of the Mn(Cup)2 solution was placed on 0.8 × 2.5 cm2 substrate (Si, glass, fluorine doped tin oxide (FTO) or tin-doped indium oxide (ITO)), which were cleaned by sonication in methanol, acetone, and isopropyl alcohol solutions, 10 min sonication in each solvent, and heated on a hot plate for 10 min at 270 °C inside the glovebox (N2 atmosphere). Subsequently, the substrate was calcinated in air at 350 °C for 12 h to obtain the Mn2O3 phase. Redox Reaction. An oil bath was heated to the 90 °C. In a typical experiment, 10 mL of a 0.1 M iron salt (FeSO4·7H2O, FeCl2·4H2O, Fe(ac)2) aqueous solution was placed inside a 20 mL vial. The substrate with Mn2O3 film was diagonally placed inside the vial, with the face containing the Mn2O3 facing down in order to avoid the precipitation of the iron hydroxide. After 120 min reaction, the orange/yellow film was taken out and cleaned by DI water and then dried at 60 °C. Hematite Nanostructure. This was obtained after annealing the iron oxyhydroxide film at 500 °C in air for 5 h. For PEC measurements, samples were further annealed at 710 °C in air for 15 min. Structural Characterization. Scanning electron microscopy (SEM) was performed using a JEOL SM-7400F ultrahigh resolution with a cold-field emission-gun. SEM instrument was operated at 5 kV. Transmission electron microscopy (TEM) was done using an FEI Tecnai 12 TWIN microscope operated at 120 kV. High-resolution TEM (HRTEM) images were taken using a JEOL 2100 microscope operated at 200 kV. The energy-dispersive X-ray spectroscopy (EDX) was detected by using either EDX which was coupled with the SEM and it was operated at an accelerating voltage of 15 kV or by the JEM2100F high-resolution transmission electron microscope in STEM mode equipped with a JEOL JED-2300T energy dispersive X-ray spectrometer. Phase analysis of the samples was carried out using the X-ray diffraction (XRD) method. The data was collected on Empyrean Powder Diffractometer (Panalytical) equipped with position sensitive (PSD) X′Celerator detector using Cu Kα radiation (λ = 1.5418 Å) and operated at 40 kV and 30 mA. UV−vis absorbance measurements were made using a Cary 5000 UV−vis−NIR spectrophotometer. PEC Measurements. PEC measurements of the film were performed in 1 M NaOH solution using a VersaSTAT 3 potentiostat in a three-electrode system. The hematite film acts as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl in saturated KCl as the reference electrode, separated by glass frits. The voltage was swept between −0.5 and +0.8 V vs Ag/AgCl at a scan rate 20 mV s−1. The measured potential was converted to a reversible hydrogen electrode (RHE) using the Nernst equation. Light-emitting diode (LED, 365 nm wavelength, ∼14 mW) was used as the light source to illuminate the substrate from the back side.
Figure 1. SEM images of the manganese oxide nanofibers (a) used as a template for the growth of iron oxide and oxyhydroxide using FeSO4 (b), FeCl2 (c), and Fe(ac)2 (d). The insets show the corresponding TEM images.
1a). Via XRD analysis of these fibers we determined that they are composed solely by cubic Mn2O3 (JCPDS card no. 041− 1442, see Figure 2a, black trace). In order to study the effect of the counteranions of the iron precursor in the redox reaction, we chose to work with three different iron salts: FeSO4, FeCl2, and Fe(ac)2. Figure 1b−d shows the SEM and TEM images of the iron oxide and oxyhydroxide, synthesized by reacting Mn2O3 nanofibers with FeSO4 (Figure 1b), or FeCl2 (Figure 1c) or Fe(ac)2 (Figure 1d) at 90 °C for 120 min. The morphology of the nanofibers was preserved when FeSO4 and FeCl2 were used as precursors. Interestingly, we also observed that the final nanofibers are decorated with nanobelts of iron oxyhydroxide. The formation of those nanobelts can be controlled by changing the reaction time, temperature, and precursor concentration as discussed below. On the other hand, when Fe(ac)2 was used as precursor, the growth of the iron oxide did not follow the template morphology and only aggregation of NPs was obtained. We believe that this behavior is the result of the relative rate of iron oxide/oxyhydroxide deposition to the template dissolution. Manganese oxides such as Mn2O3 and Mn3O4 are not stable in acidic solution, where they undergo a dissolution process to form MnO2 and Mn2+.30 Since the redox process takes place B
DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. XRD patterns (a) and optical absorption spectra (b) of Mn2O3 (a,b black trace) and of the different redox products when FeSO4 (a,b red trace), FeCl2 (a,b green trace), and Fe(ac)2 (a,b blue trace) were used. Peaks marked with * corresponded to Si substrate.
only at the solid−liquid interface,19,20 an iron hydroxide layer deposits on the surface of the manganese oxide, which can also inhibit the diffusion of the ions and solvent molecules that decreases the dissolution rate of the manganese oxide.31 Due to the higher solubility of FeSO4 and FeCl2 in water compared to Fe(ac)2, the concentration of the available Fe2+ free ions in solution is higher which leads to a faster formation of iron hydroxide layer on the surface of the Mn2O3 that protects the fibers from dissolution and therefore preserves their pattern for longer time during the redox reaction. We verified the crystalline phases of the obtained products by XRD and optical measurements. The reaction of manganese oxide with FeSO4 forms orthorhombic goethite (α-FeOOH, JCPDS card no. 029−0713), Figure 2a, red trace).The αFeOOH is known as a semiconductor with 2.1−2.5 eV band gap32 which is in agreement with the UV−vis spectrum obtained for this sample (Figure 2b, red trace). It presents a broad peak between 1100 and 600 nm and a shoulder at ∼490 nm. The broad peak can be attributed to ligand field transition in the Fe(3d) atomic orbitals of the Fe3+ cations.33 The shoulder can be assigned to the electron pair transition (2(6A1) → 2(4T1)(4G), the band gap), which matches the reported values.34 When we used FeCl2 as a source of iron, a rhombohedral hydrohematite (Fe1.833(OH)0.5O2.5, JCPDS card no. 076−0182) was obtained (Figure 2a, green trace). Fe1.833(OH)0.5O2.5 is considered as hematite with a few percent of hydroxyl groups that partially substitute oxygen anions.35 The UV−vis spectrum (Figure 2b, green trace) shows a shoulder at ∼530 nm and a peak at 360 nm, which agrees with reported values in the literature.36,37 In the case of Fe(ac)2 the XRD pattern (Figure 2a, blue trace) matches either with maghemite (γ-Fe2O3) or magnetite (Fe3O4). Fe3O4 shows a very distinctive absorption feature in the near-IR region that can be attributed to intervalence charge transfer (IVCT) transitions, due to the presence of two oxidation states of iron in the structure (Fe2+ and Fe3+).38 This transition is not observed in the γ-Fe2O3 where only Fe(III) is present in the structure. The UV−vis spectrum of our sample (Figure 2b, blue trace) shows a flat region in the near-IR region indicating the
absence of IVCT and therefore suggesting the sole formation of the γ-Fe2O3.39 The formation of two different phases of iron oxyhydroxide (hydrohematite or goethite) when Cl− or SO42−are used, respectively, can be attributed to the difference in the interaction of the Cl− or SO42− with the FeO6 octahedral unit in the ferric oxide/oxyhydroxides. The smaller Cl− ions can be incorporated into the crystal structure during the growth and stabilize the tunnel structure of β-FeOOH. However, larger ions SO42− lead to a slight distortion of the tunnel structure of the FeO6 unit, and then to the formation of α-FeOOH.24 In our case, using FeCl 2 leads to the formation of Fe1.833(OH)0.5O2.5 phase instead of β-FeOOH. This result can be explained by the fact that the redox reaction process took place at 90 °C and β-FeOOH can be converted to α-Fe2O3 by heating at the temperature range of 90−100 °C.40 The formation of γ-Fe2O3 can be attributed to the further oxidation of aqueous solution of Fe3O4 NPs since it was heated 2 h at 90 °C as shown by Tang et al.38 It was also shown that the formation of Fe3O4 is preceded by the formation of Fe(OH)2,41 and therefore concentration of the OH− ions in the solution plays an important role. We measured the concentration of the OH− in three different growth solutions and found that the OH− concentration is 6.3 × 10−12 M, 5 × 10−12 M, and 1 × 10−9 M when we used Cl−, SO42−, and CH3COO−, respectively. Moreover, it is known that the CH3COO− reacts with water molecules and produces CH3COOH + OH− due to a weak hydrolysis tendency. Consequently, the probability of forming Fe(OH)2 and afterward Fe3O4 followed by the formation of γ-Fe2O3 in the case of CH3COO− is much higher than in the other cases. As a prototype system for understanding the mechanism of the redox reaction, we chose to study the formation of Fe1.833(OH)0.5O2.5 nanostructures. A control experiment was carried out where a clean substrate (without Mn2O3 fibers) was placed in FeCl2 solution at 90 °C for 120 min. Figure S2 shows the results of the control experiment where only aggregated NPs were obtained. We monitored the redox reaction at different stages and observed the dissolution of the MnOx in C
DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. (a) XRD pattern (between 31 and 37 deg) of the original Mn2O3 (black trace), the Fe1.833(OH)0.5O2.5 after 120 min reaction (purple trace), and the intermediate products after 2, 3, 4, and 5 min reaction (red trace, green trace, blue trace, and orange trace, respectively). (b) EDS elemental atomic analysis for Mn and Fe in the film at different stages, 2, 3, 4, and 5 min reaction (red trace, green trace, blue trace, and orange trace, respectively).
the sample by following the amount of Mn by EDX and XRD. XRD measurements of the products at different times show a decrease in the intensity of the Mn2O3 peak at 2θ = 33° (the (222) plane) with the progress of the reaction (Figure 3a). Moreover, EDS of the same samples showed a decrease in the Mn and an increase in the Fe content with time (Figure 3b). The full conversion was achieved within a 5 min reaction (a representative TEM image is shown in Figure 4a); the Mn atomic fraction (measured by EDS) was less than 2% (Figure 3), and XRD measurement of the product revealed broad and weak peaks due to the low crystallinity of the product (Figure S3).19 After 20 min we observed the formation of nanobelts (Figure 4b), thus the evolution of new diffraction peaks (Figure 2a, green trace) can attributed to the formation of the Fe1.833(OH)0.5O2.5 nanobelts structure. We believe that the formation of the iron product occurs in two competitive processes, redox reaction and reaction of iron with water. At the earlier stage, the formation of the iron product is governed by the redox process due to the large difference in the redox potential (Fe3+/Fe2+ (0.77 V, vs SHE) and Mn2O3/Mn2+ (1.48 V, vs SHE)), and at a later stage the reaction of iron with water becomes more dominant due to full consumption of the manganese. Further characterization of the Fe1.833(OH)0.5O2.5 was carried out by HRTEM and EDS elemental mapping. Figure 4c shows the HRTEM image which presents two lattice spacing of 3.67 and 2.73 Å, which correspond to the (012) and (104) planes of rhombohedral Fe1.833(OH)0.5O2.5, respectively. Figure 4d(2−4) shows the elemental mapping of Fe, Mn, and O atoms, respectively. These mappings clearly demonstrate that the final product contains primarily Fe and O atoms while only a negligible amount of Mn atoms are present (less than 2%). To better understand the mechanism of the conversion process, the effect of the iron precursor concentration was examined, where the amount of the FeCl2 was varied between 6 and 400 mg (Figure 5). When 6 mg of the FeCl2 was used, small NPs decorating the nanofibers are observed (Figure 5a). Increasing the amount of the precursor to 25 mg forms
Figure 4. Structural characterization of the iron oxyhydroxide. SEM and TEM images of the redox reaction product using FeCl2 at different reaction time, (a) 5 min and (b) 20 min. (c) HRTEM image of iron oxyhydroxide. The inset shows the select area diffraction (SAD) of iron oxyhydroxide. (d) STEM and EDS elemental mapping of iron oxyhydroxide. (d1) STEM image of the examined area. (d2−4) EDS mapping of: (d2) iron, (d3) manganese, and (d4) oxygen.
elongated NPs that decorate the nanofibers (Figure 5b). A further increase in the precursor amount to 50 mg resulted in the decoration of the nanofibers with nanobelts (Figure 5c). The morphology of the nanobelts was preserved at a higher concentration of the iron precursor (Figure 5d). It should be noted here that using different concentrations of iron salt did D
DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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formed nanofibers covered with small whiskers and 98% of the Mn atoms were replaced. Higher temperature, 70 °C, (Figure 6c) led to a clear growth of nanobelts on the surface of the fibers, while the Mn percentage remained the same as that at 50 °C (less than 2%). Nonetheless, when the reaction was carried out at 110 °C (Figure 6d) the nanofibers changed their morphology to an undefined shape and no Mn was detected. PEC Performance. We evaluated the photocatalytic performance of the samples after converting them to hematite. The conversion process was carried out by annealing the iron oxyhydroxide film at 500 °C for 5 h in air.42,43 The morphology of the iron oxide structure was preserved as shown in Figure 7a. The XRD and optical measurements confirm the full crystal phase conversion from rhombohedral Fe1.833(OH)0.5O2.5 to αFe2O3. Figure 7b shows that all the diffraction peaks of the converted product match with α-Fe2O3 bulk structure (JCPDS card no. 072−0469). The absence of diffraction peaks of manganese oxide or other phases of iron oxide confirms that the full conversion process of the Fe1.833(OH)0.5O2.5 to α-Fe2O3 was achieved. The UV−vis absorption spectrum (Figure 7c) shows an absorption shoulder at ∼540 nm (2.3 eV) and a peak at ∼400 nm (3.1 eV), which is similar to the spectrum of αFe2O3 previously reported in the literature.36 The PEC performance of the obtained hematite structure was examined in a three-electrode system in 1 M NaOH solution. The hematite film acts as the working electrode, a platinum wire as the counter electrode, Ag/AgCl in saturated KCl as the reference electrode, and LED (365 nm wavelength, ∼14 mW) was used as the light source. The hematite was further annealed at 710 °C for 15 min for activation.39,44 Figure 7d shows the J− V scans for hematite under illumination (red trace) yielded a low photocurrent density (0.03 mA/cm−2 at 1.23 V vs RHE) but it shows one of the lowest onset potentials (0.85 V vs RHE) compared to other observed values using hematite photoanode without any surface modification.43,44 This low current can be attributed to the incomplete coverage of the FTO by the nanofibers, which negatively affects the charge transfer to the electrode and the percentage of the absorbed light. Furthermore, the photogenerated carriers cannot be effectively collected by the electrode since the thickness of the hematite film exceeded the optimum thickness for electron transfer of 500 nm.37 The PEC performance was further improved by deposition of Co(NO3)2 onto the hematite surface, which doubled the photocurrent (0.06 mA/cm−2 at 1.23 V vs RHE) and shifted the onset potential more than 100 mV.
Figure 5. Effect of the FeCl2 amount. SEM images of the products after reaction of Mn2O3 with various amount of FeCl2, (a) 6 mg, (b) 25 mg, (c) 50 mg, and (d) 400 mg.
not affect the manganese content in the final film (Mn ≈ 2% in all cases). The effect of the reaction temperature was also studied. Figure 6 presents the TEM images of the products (after 120 min) at four different reaction temperatures, 23, 50, 70, and 110 °C inside a sealed vail. Conducting the redox reaction at 23 °C (Figure 6a) preserved the morphology of the nanofibers but only ∼50% of the Mn was replaced by Fe as was measured by EDS. Increasing the reaction temperature to 50 °C (Figure 6b)
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CONCLUSIONS We studied the role of the counteranions of iron salts in the redox reactions using Mn2O3 nanofibers as template. We found that counteranions have a dominant role in determining the composition, phase, and morphology of the redox reaction products. The differences in the reactivity and the size of counteranions lead to the formation of various products (hydrohematite or goethite). The solubility of the iron salt in the growth solution is crucial in preserving the morphology of the manganese oxide template. The redox reaction process can be divided by into steps: the conversion reaction, followed by the growth of the nanobelts on the fibers surface. The morphology, crystallinity, and composition of the final products can be controlled by the precursor concentration, reaction time, and temperature. Finally, the iron oxyhydroxide nanofibers can be further converted to hematite while preserving the original
Figure 6. Effect of the reaction temperature. SEM images of the products after reaction of Mn2O3 with iron chloride at various reaction temperatures: (a) 23 °C. (b) 50 °C. (c) 70 °C. (d) 110 °C. E
DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 7. Conversion to hematite and its performance in water oxidation process. (a) SEM image of hematite structure. The inset shows a higher magnification image. (b) XRD pattern of the hematite on FTO substrate. Peaks marked with * correspond to the FTO diffraction peaks. (c) UV−vis spectra of hematite. (d) J−V scans for hematite without modification in dark (black trace), in light (red trace), and hematite with Co(NO3)2 over layer (green trace).
Marco Mitrani Family Foundation. The authors wish to thank Dr. Dmitry Mogilynaski for XRD measurements, Dr. Vladimir Ezersky for HRTEM images, Kobi Flomin. Dr. Prashant Kumar and Dr. Mariela Pavan for helpful discussions.
morphology of the converted nanostructures. This can further expand the application of these unique nanostructures to various fields such as water-splitting.
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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.cgd.6b01373. SEM images of the cross section view of the manganese oxide and the iron oxyhydroxide. XRD and SEM mage of the control experiments. (PDF)
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REFERENCES
(1) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W. Synthesis and Characterization of Silica-Coated Iron Oxide Nanoparticles in Microemulsion: The Effect of Nonionic Surfactants. Langmuir 2001, 17, 2900−2906. (2) Oh, H.; Moon, J.; Shin, D.; Moon, C.; Choi, H. J. Brief Review on Iron-Based Superconductors: Are There Clues for Unconventional Superconductivity? Prog. Supercond. 2011, 13, 65−84. (3) Lopes, T.; Andrade, L.; Le Formal, F.; Gratzel, M.; Sivula, K.; Mendes, A. Hematite Photoelectrodes for Water Splitting: Evaluation of the Role of Film Thickness by Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 16515. (4) Chou, S.; Huang, C. Application of a Supported Iron Oxyhydroxide Catalyst in Oxidation of Benzoic Acid by Hydrogen Peroxide. Chemosphere 1999, 38, 2719−2731. (5) Scott, T. B.; Allen, G. C.; Heard, P. J.; Lewis, A. C.; Lee, D. F. The Extraction of Uranium from Groundwaters on Iron Surfaces. Proc. R. Soc. London, Ser. A 2005, 461, 1247−1259. (6) Parsons, J. G.; Luna, C.; Botez, C. E.; Elizalde, J.; GardeaTorresdey, J. L. Microwave-Assisted Synthesis of iron(III) Oxyhydroxides/oxides Characterized Using Transmission Electron Microscopy, X-Ray Diffraction, and X-Ray Absorption Spectroscopy. J. Phys. Chem. Solids 2009, 70, 555−560. (7) Xu, C.; Zeng, Y.; Rui, X.; Zhu, J.; Tan, H.; Guerrero, A.; Toribio, J.; Bisquert, J.; Garcia-Belmonte, G.; Yan, Q. Amorphous Iron
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Taleb Mokari: 0000-0001-7712-1589 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the European Research Council (ERC; starting grant, project no. 278779). M. D. thanks the F
DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
Oxyhydroxide Nanosheets: Synthesis, Li Storage, and Conversion Reaction Kinetics. J. Phys. Chem. C 2013, 117, 17462−17469. (8) Sun, Z.; Feng, X.; Hou, W. Morphology-Controlled Synthesis of α-FeOOH and Its Derivatives. Nanotechnology 2007, 18, 455607. (9) Anderson, B. D.; Tracy, J. B. Nanoparticle Conversion Chemistry: Kirkendall Effect, Galvanic Exchange, and Anion Exchange. Nanoscale 2014, 6, 12195. (10) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (11) Gonzalez, E.; Arbiol, J.; Puntes, V. F. Carving at the Nanoscale: Sequential Galvanic Exchange and Kirkendall Growth at Room Temperature. Science 2011, 334, 1377−1380. (12) Métraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. a. Triangular Nanoframes Made of Gold and Silver. Nano Lett. 2003, 3, 519−522. (13) Xiao, F.; Yoo, B.; Lee, K. H.; Myung, N. V. Synthesis of Bi2Te3 Nanotubes by Galvanic Displacement. J. Am. Chem. Soc. 2007, 129, 10068−10069. (14) Zhang, L.; Wu, H. B.; Madhavi, S.; Hng, H. H.; Lou, X. W. D. Formation of Fe2O3 Microboxes with Hierarchical Shell Structures from Metal−Organic Frameworks and Their Lithium Storage Properties. J. Am. Chem. Soc. 2012, 134, 17388−17391. (15) (1) Zhang, L.; Wu, H. B.; Lou, X. W. Metal-OrganicFrameworks-Derived General Formation of Hollow Structures with High Complexity. J. Am. Chem. Soc. 2013, 135, 10664−10672. (16) Ma, F. X.; Hu, H.; Wu, H. B.; Xu, C. Y.; Xu, Z.; Zhen, L.; Lou, X. W. Formation of Uniform Fe3O4 Hollow Spheres Organized by Ultrathin Nanosheets and Their Excellent Lithium Storage Properties. Adv. Mater. 2015, 27, 4097−4101. (17) Wang, B.; Wu, H. B.; Zhang, L.; Lou, X. W. Self-Supported Construction of Uniform Fe3O4 Hollow Microspheres from Nanoplate Building Blocks. Angew. Chem., Int. Ed. 2013, 52, 4165−4168. (18) Oh, M. H.; Yu, T.; Yu, S.-H.; Lim, B.; Ko, K.-T.; Willinger, M.G.; Seo, D.-H.; Kim, B. H.; Cho, M. G.; Park, J.-H.; et al. Galvanic Replacement Reactions in Metal Oxide Nanocrystals. Science 2013, 340, 964−968. (19) Wang, Z.; Luan, D.; Li, C. M.; Su, F.; Madhavi, S.; Boey, F. Y. C.; Lou, X. W. Engineering Nonspherical Hollow Structures with Complex Interiors by Template-Engaged Redox Etching. J. Am. Chem. Soc. 2010, 132, 16271−16277. (20) Gao, G.; Yu, L.; Wu, H. B.; Lou, X. W. Hierarchical Tubular Structures Constructed by Carbon-Coated α-Fe2O3 Nanorods for Highly Reversible Lithium Storage. Small 2014, 10, 1741−1745. (21) Gu, Y.; Cai, J.; He, M.; Kang, L.; Lei, Z.; Liu, Z. H. Preparation and Capacitance Behavior of Manganese Oxide Hollow Structures with Different Morphologies via Template-Engaged Redox Etching. J. Power Sources 2013, 239, 347−355. (22) Yu, Y.; Zhang, Q.; Yao, Q.; Xie, J.; Lee, J. Y. Guiding Principles in the Galvanic Replacement Reaction of an Underpotentially Deposited Metal Layer for Site-Selective Deposition and Shape and Size Control of Satellite Nanocrystals. Chem. Mater. 2013, 25, 4746− 4756. (23) Zhang, W.; Yang, J.; Lu, X. Tailoring Galvanic Replacement Reaction for the Preparation of Pt/Ag Bimetallic Hollow Nanostructures with Controlled Number of Voids. ACS Nano 2012, 6, 7397− 7405. (24) Yue, J.; Jiang, X.; Yu, A. Experimental and Theoretical Study on the β-FeOOH Nanorods: Growth and Conversion. J. Nanopart. Res. 2011, 13, 3961−3974. (25) Dousma, J.; Van den Hoven, T. J.; De Bruyn, P. L. The Influence of Chloride Ions on the Formation of iron(III) Oxyhydroxide. J. Inorg. Nucl. Chem. 1978, 40, 1089−1093. (26) Dousma, J.; den Ottelander, D.; de Bruyn, P. L. The Influence of Sulfate Ions on the Formation of iron(III) Oxides. J. Inorg. Nucl. Chem. 1979, 41, 1565−1568. (27) Domingo, C.; Rodríguez-Clemente, R.; Blesa, M. Morphological Properties of α-FeOOH, γ-FeOOH and Fe3O4 Obtained by Oxidation of Aqueous Fe(II) Solutions. J. Colloid Interface Sci. 1994, 165, 244−252.
(28) Sun, Z.; Feng, X.; Hou, W. Morphology-Controlled Synthesis of α-FeOOH and Its Derivatives. Nanotechnology 2007, 18, 455607. (29) Diab, M.; Moshofsky, B.; Jen-La Plante, I.; Mokari, T. A Facile One-Step Approach for the Synthesis and Assembly of Copper and Copper-Oxide Nanocrystals. J. Mater. Chem. 2011, 21, 11626−11630. (30) Artamonova, I. V.; Gorichev, I. G.; Godunov, E. B. Kinetics of Manganese Oxides Dissolution in Sulphuric Acid Solutions Containing Oxalic Acid. Engineering 2013, 05, 714−719. (31) Villinski, J. E.; O’Day, P. a.; Corley, T. L.; Conklin, M. H. In Situ Spectroscopic and Solution Analyses of the Reductive Dissolution of MnO2 by Fe(II). Environ. Sci. Technol. 2001, 35, 1157−1163. (32) Cornell, R.; Schwertmann, U. Iron Oxides 2003, DOI: 10.1002/ 3527602097. (33) Sherman, D. M.; Waite, T. D. Electronic Spectra of Fe3+ Oxides and Oxide Hydroxides in the near IR to near UV. Am. Mineral. 1985, 70, 1262−1269. (34) Zhang, H.; Bayne, M.; Fernando, S.; Legg, B.; Zhu, M.; Penn, R. L.; Banfield, J. F. Size-Dependent Bandgap of Nanogoethite. J. Phys. Chem. C 2011, 115, 17704−17710. (35) Wolska, E. The Structure of Hydrohematite. Z. Kristallogr. Cryst. Mater. 1981, 154, 69−75. (36) Marusak, L. A.; Messier, R.; White, W. B. Optical Absorption Spectrum of Hematite, α-Fe2O3 near IR to UV. J. Phys. Chem. Solids 1980, 41, 981−984. (37) Li, L.; Yu, Y.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S. Facile Solution Synthesis of α-FeF3·3H2O Nanowires and Their Conversion to α-Fe2O3 Nanowires for Photoelectrochemical Application. Nano Lett. 2012, 12, 724−731. (38) Tang, J.; Myers, M.; Bosnick, K.; Brus, L. E. Magnetite Fe3O4 Nanocrystals: Spectroscopic Observation of Aqueous Oxidation Kinetics †. J. Phys. Chem. B 2003, 107, 7501−7506. (39) Butler, G.; Ison, H. C. K. The Thermal Transformation of Hydrated Ferric Oxides. Chem. Commun. 1965, 264. (40) Domingo, C.; Rodríguez-Clemente, R.; Blesa, M. Morphological Properties of α-FeOOH, γ-FeOOH and Fe3O4 Obtained by Oxidation of Aqueous Fe(II) Solutions. J. Colloid Interface Sci. 1994, 165, 244−252. (41) Li, Z.; Lai, X.; Wang, H.; Mao, D.; Xing, C.; Wang, D. Direct Hydrothermal Synthesis of Single-Crystalline Hematite Nanorods Assisted by 1,2-Propanediamine. Nanotechnology 2009, 20, 245603− 245612. (42) Wang, G.; Ling, Y.; Wheeler, D. a; George, K. E. N.; Horsley, K.; Heske, C.; Zhang, J. Z.; Li, Y. Facile Synthesis of Highly Photoactive αFe2O3-Based Films for Water Oxidation. Nano Lett. 2011, 11, 3503− 3509. (43) Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J.; Grätzel, M. Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. J. Am. Chem. Soc. 2010, 132, 7436−7444. (44) Duret, A.; Grätzel, M. Visible Light-Induced Water Oxidation on Mesoscopic α-Fe2O3 Films Made by Ultrasonic Spray Pyrolysis. J. Phys. Chem. B 2005, 109, 17184−17191.
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DOI: 10.1021/acs.cgd.6b01373 Cryst. Growth Des. XXXX, XXX, XXX−XXX
