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Morphological Control of r-FeOOH Nanostructures by Electrodeposition Shuhong Jiao, Lifen Xu, Keli Hu, Jingjian Li, Song Gao, and Dongsheng Xu* Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: September 20, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009
Herein, we developed a novel anodic electrodeposition route to prepare shape-controlled R-FeOOH thin films on indium tin oxide (ITO) substrates in aqueous (NH4)2Fe(SO4)2 solutions. Different morphologies of R-FeOOH products, including platelets, rhombohedral rods, square rods, six-fold snowflakes, and hexagonal particles were obtained by controlling the growth rates of various facets of the deposit with appropriate capping agents. Furthermore, we demonstrated that R-FeOOH films can be transferred into R-Fe2O3 nanostructures with the same morphologies through a thermal annealing process. Introduction The synthesis of inorganic nanostructures with well-defined shapes has attracted extraordinary attention due to the importance of dimensionality of materials in determining their properties.1-5 In particular, many researchers6-18 have been interested in controlling the shape-guiding process in electrodeposition because it allows the growth of various semiconducting and metallic crystals directly from a conducting substrate with good electrical contact, which can be easily integrated into devices. Notably, Choi et al. have demonstrated an approach for precise control of the shapes and orientations of Cu2O crystals during the electrodeposition process.12-15 Xu et al. have reported a methodical habit modification of the electrodeposited ZnO crystals by preferential adsorption of low molecular mass or ions.16-18 Although the role of capping agents in controlling the size and even shape of the electrodeposits has been realized, study on the shape evolution of other inorganic crystals by electrodeposition is still scarce. Iron oxides/oxyhydroxides are ubiquitous at and near the Earth’s surface and are relevant in many scientific and technical applications. Among them, goethite (R-FeOOH), an antiferromagnetic iron oxyhydroxide, is known as a common constituent of soils and the atmospheric corrosion product of iron-based alloys and, meanwhile, an important material in the process of biomineralization.19-25 In addition, goethite plays an important role in migration of heavy-metal ions and organic pollutants in the natural environment.26-28 However, there are few reports on the synthesis of R-FeOOH by electrodeposition.29-31 Herein, we report a direct electrochemical route for the preparation of the goethite nanostructures with controlled crystalline morphologies. Importantly, we demonstrated that these R-FeOOH samples can be converted into the R-Fe2O3 nanostructures with the same morphologies through a thermal annealing process. Experimental Section Typically, thin films of R-FeOOH were electrodeposited in aqueous solution containing (NH4)2Fe(SO4)2 and CH3COOK. To control the shape of the deposits, several kinds of additives, such as ammonium fluoride and pyridine, were added to the * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 86-10-62753580. Fax: 86-10-62760360.
electrolyte. The R-FeOOH platelets with different sizes can be obtained by changing the concentration of (NH4)2Fe(SO4)2 and CH3COOK or changing solvent. The other R-FeOOH samples with different shapes can be obtained in the complex solution of (NH4)2Fe(SO4)2 and CH3COOK by adding different amounts of NH4F and pyridine. All electrodepositions were carried out in a configured glass cell at 70 °C, in which an ITO substrate with a sheet resistance of ∼20 Ω/cm2, a platinum plate, and a saturated calomel electrode (SCE) served as the working electrode, counter electrode, and reference electrode, respectively. All electrodepositions were done at a potential of 0.65 V versus SCE. The durations of the depositions were 1 h. The R-Fe2O3 nanostructures were obtained by heating the electrodeposited R-FeOOH films in air at 400 °C for 1 h. The morphologies and the structures of the products were characterized by field-emission scanning electron microscopy (FESEM) (Hitachi S4800, 10.0 kV) and X-ray powder diffraction (XRD, Rigaku D/max-2500 diffractometer with Cu KR radiation, λ ) 1.542 Å, 40 kV, 100 mA). Thermal gravimetric analysis (TGA) was carried out on a Q600SDT TGA-DTADSC thermal analyzer at a heating rate of 10 °C/min from room temperature to 500 °C under a N2 atmosphere. The UV-vis diffuse reflection spectra were recorded on a Hitachai UV-visNIR spectrophotometer (U-4100). Photocurrent-voltage measurements were performed on a Keithley 4200 sourcemeter under the backside irradiation (through the ITO substrate) of a 500 W xenon arc lamp. Magnetic measurements were performed on a Quantum Design MPMS-XL5 SQUID magnetometer. Results and Discussion Figure 1a shows the typical SEM image of the R-FeOOH films electrodeposited from aqueous solution containing 0.01 M (NH4)2Fe(SO4)2 and 0.04 M CH3COOK, indicating that large area and dense single-crystalline platelets are grown onto the ITO substrate perpendicularly. The platelets are smooth and homogeneous in both a thickness of 30-50 nm and a length of 500-800 nm. The thickness of the deposited platelets can be controlled by the concentration of Fe2+ and CH3COO-. With an increase of the concentration of Fe2+ and CH3COO-, the platelets turned thicker, as shown in Figure 1b. For the solution containing 0.1 M Fe2+ and 0.4 M CH3COO-, the dark orangered films were obtained and the R-FeOOH platelets are typically
10.1021/jp909072m 2010 American Chemical Society Published on Web 12/15/2009
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Figure 1. SEM of the R-FeOOH nanostructures by electrodeposition from solutions containing (a) 0.01 M Fe2+ and 0.04 M CH3COO-, (b) 0.1 M Fe2+ and 0.4 M CH3COO-, (c) 0.01 M Fe2+ and 0.04 M CH3COO- with 9:1 water and ethanol, and (d) 0.01 M Fe2+ and 0.04 M CH3COO- with 1:9 water and ethanol. Figure 3. XRD patterns of R-FeOOH nanostructures with different morphologies: (a) platelets, (b) rhombohedral rods, (c) square rods, and (d) snowflake particles. An asterisk indicates the ITO substrate.
Figure 2. SEM images of the R-FeOOH nanostructures by electrodeposition from solutions containing (a) 0.01 M Fe2+, 0.04 M CH3COO-, 0.05 M NH4F; (b) 0.01 M Fe2+, 0.04 M CH3COO-, 0.1 M NH4F; (c) 0.01 M Fe2+, 0.04 M CH3COO-, 1 M NH4F; and (d) 0.01 M Fe2+, 0.04 M pyridine, 0.05 M NH4F.
400-500 nm in thickness. Meanwhile, the thickness of the platelets would decrease when using a mixture of ethanol and water as solvents. Figure 1c shows the SEM image of 20-30 nm thick platelets electrodeposited from the solution containing 0.01 M Fe2+ and 0.04 M CH3COO- in the solvent of water and ethanol with 9: 1 in volume. While the ratio of water and ethanol changes to 1:9, the platelets with a thickness less than 10 nm were produced (Figure 1d). To tune the morphologies of the R-FeOOH deposits, several kinds of capping reagents are added into the mixed solution of Fe2+ and CH3COO-. When a small quantity of NH4F was added into the electroplating solution, the shapes of the deposits changed from platelets (Figure 1a) to rhombohedral rods (Figure 2a). These rhombohedral rods are 0.1-0.5 µm in width and the length-to-width ratios are about 2-8. Further increasing the amount of NH4F would induce the formation of six-fold
snowflakes with rhombohedral rods (Figure 2b). It is found that the length-to-width of the rhombohedral rods of six-fold snowflakes would shorten with an increase of the NH4F concentration, and the particles with a hexagonal shape were obtained at a higher concentration of NH4F (Figure 2c). In addition, when acetate was replaced with pyridine, homogeneous square rods were obtained on the ITO substrates (Figure 2d). Figure 3 shows the XRD pattern of electrodeposited R-FeOOH film with different morphologies. All the diffraction peaks are labeled and can be indexed according to the orthorhombic phase of R-FeOOH. As expected, in Figure 3, pattern a, a substantially higher intensity is observed for the 〈200〉 diffraction peaks in the XRD pattern (normalized to the 〈020〉 line, which usually corresponds to the maximum intensity of R-FeOOH, PDF file No. 73-2326), indicating that the R-FeOOH crystallites are oriented perfectly with their (100) planes being perpendicular to the ITO substrates. In the cases of the samples with morphologies of both rhombohedral and square rods, it is found that the c axis of the orthorhombic phase of R-FeOOH crystals is preferentially aligned along the direction normal to the substrate (Figure 3b,c) (PDF file No. 81-0462). R-FeOOH has a goethite structure, which consists of infinite chains of FeO6 octahedra, linked by hydrogen bonds (Figure 4a). The crystal structure is orthorhombic, with lattice dimensions a ) 4.60 Å, b ) 9.94 Å, and c ) 3.00 Å. The most familiar morphologies in synthetic goethite are prisms elongated in the direction of the c axis (Figure 4b), delimited by {110}, {100}, and {010} faces and capped mostly by {021} or {001} faces.20-22 In the prisms, the energies of each face of the lateral planes are {110} < {100} < {010}. On the basis of the SEM observations and the XRD data, together with their internal habits, we can deduce the representations of the crystal structures of the electrodeposited R-FeOOH. Rectangular prisms in both the platelets and the square nanorods would be delimited by {100} and {010} faces (Figures 1a and 2d). Also, the obtuse angle in the rhombs shown in Figure 2b (130° ( 2°) agrees well with the theoretical angle of 130.4° between the goethite
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Figure 4. (a) The crystal structure of goethite (R-FeOOH). (b) The unit cell of goethite (left) and schematic diagram of the iron atom distribution in different crystal planes (right). The iron atoms are shown in red and the oxygen atoms in blue. Hydrogen atoms are not shown.
(110) and (11j0) planes, indicating that the rhomb shapes likely represent cross sections through crystals displaying the {110} faces. The shape evolution of the electrodeposited crystals from platelets to nanorods with square or rhombic sections and snowflakes could be attributed to the capping effect of the additives.14,17 As we know, when a crystal grows, ions or molecules near the surface are preferentially consumed by the growing crystal. The preferential adsorption of additives lowers the surface energy of the bound plane and hinders the crystal growth perpendicular to this plane, resulting in a change in the final morphology.32 Previous studies show the shape control of the electrodeposits varying the reduction potentials and interpreted the shape formation using a model that minimizes the total surface and twin-boundary energies of an assembly of single-crystal units at constant volume.6,12 However, in our approach, the applied potentials and the concentrations of (NH4)2Fe(SO4)2 are constant for all of the electrodeposition. During the electrodeposition, CH3COO- can provide alkaline surroundings by hydrolyzation and the anode reaction can be described as
Fe2+ - e- f Fe3+ Fe3+ + 3OH- f FeOOH + H2O In our system, when the electrolyte contains Fe2+ and CH3COO-, R-FeOOH can be electrodeposited on the ITO substrate by its own crystal growth habits. In the case of adding NH4F, due to the highest density of Fe atoms on the (110) face (Figure 4b), the strong coordination ability of F- would result in the preferential adsorption of F- on the (110) faces, which lowers the growth rate of (110). Meanwhile, the crystal growth along the 〈100〉 and 〈010〉 directions would be enhanced,
Figure 5. (a) SEM image and (b) XRD pattern of the platelet-like R-FeOOH film after annealing at 400 °C for 1 h in air. An asterisk indicates the ITO substrate.
resulting in the disappearance of the (100) and (010) faces to form a rhombic prism with (110) lateral faces. Also, pyridine would be absorbed preferentially on the (010) surface during the crystal growth, and thus, the crystal growth rates along 〈100〉 and 〈010〉 directions are almost identical. Furthermore, we demonstrated that the above R-FeOOH samples can be converted into the R-Fe2O3 nanostructures with the same morphologies through a thermal annealing process. R-Fe2O3 (hematite) is a typical environmentally friendly semiconductor (Eg ) 2.1 eV) and has received increasing attention due to its extensive applications, such as pigments, anticorrosion paint, gas sensors, catalysts, antiferromagnetic material, and photoassisted electrolysis of water.33-36 Although much effort has been made in the design of various R-Fe2O3 materials with a desired structure and morphology, synthesis of R-Fe2O3 films with defined shapes on transparent conductive substrates remains a challenge. Figure 5a shows the SEM image of the electrodeposited sheetlike R-FeOOH film after annealing at 400 °C for 1 h in air. After annealing, the color of the film would change from orange to dark red, and the annealed samples kept the sheetlike morphology. Figure 5b displays the XRD patterns of the R-FeOOH film after annealing, indicating a rhombohedral hematite R-Fe2O3 structure (PDF file No. 87-1165). A substantially higher intensity of the 〈300〉 diffraction peaks indicates that the R-Fe2O3 crystallites are also oriented with their (100) planes being perpendicular to the substrates. The TGA curve of the as-synthesized R-FeOOH, as shown in Figure 6, can provide further evidence of the component of the R-Fe2O3 product after the thermal process. The weight loss below the temperature 130 °C may be attributed to the
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Figure 8. Short-circuit photocurrent vs irradiation time curve of the R-Fe2O3 film prepared by annealing the electrodeposited R-FeOOH film. Figure 6. TGA curve of the as-prepared R-FeOOH platelets.
Figure 9. Temperature dependence of magnetic susceptibility at an applied field of 100 Oe.
Figure 7. UV-vis diffuse reflection spectra of the platelet-like R-FeOOH film before (a) and after (b) annealing at 400 °C for 1 h in air.
detachment of the physically absorbed water or other solvents. A weight loss (-9.059%) step is observed in a temperature range of 165-380 °C, which results from the decomposition of assynthesized R-FeOOH. With the temperature higher than 380 °C, the weight loss ceased. As a consequence, the stable residue can reasonably be ascribed to the crystallographic phase of R-Fe2O3. Figure 7, spectrum a, presents the UV-visible diffuse reflection spectra of the as-deposited sheetlike film with a band gap of approximately 1.7 eV. After annealing, a notable blue shift of the absorption edge of the film is observed (Figure 7, spectrum b). The band-gap value of approximately 2.1 eV deduced from the diffuse reflection spectra is in good agreement with the literature value for R-Fe2O3.36 Meanwhile, the sharp absorption edge in the region of 2.0-2.3 eV implies that the obtained R-Fe2O3 film is highly crystalline. The photoactivity of the obtained R-Fe2O3 film was investigated by measuring the short-circuit photocurrent. A 3-methoxyproprionitrile solution containing 0.05 M LiI, 0.05 M I2, 0.6 M 1-propyl-3-methylimidazolium iodiode (PMII), and 0.5 M 4-tert-butylpyridine was used as an electrolyte. It can be seen that this R-Fe2O3 sample shows n-type behavior, generating
anodic photocurrent (Figure 8). In this case, the photogenerated holes at the electrode/electrolyte interface would oxidize iodide ions, and meanwhile, the photoexcited electrons would be transferred to the Pt counter electrode to reduce triiodide ions. Moreover, the photocurrent generated by the R-Fe2O3 film is stable during several cycles, indicating that the electrode is free from the photocorrosion. The magnetic property of R-Fe2O3 with a platelet shape on an ITO substrate has been explored. As we know, the Morin transition is a magnetic phase transition in R-Fe2O3 hematite where the antiferromagnetic ordering is reorganized from being aligned perpendicular to the c axis to being aligned parallel to the c axis below TM (Morin transition temperature). Hematite nanomaterials can exhibit unusual magnetic properties that are different from those of bulk materials, because the size and morphology of the sample are essential to determine the Morin transition of hematite. The variable-temperature magnetic susceptibility was measured from 0 to 300 K, as shown in Figure 9. The nanostructured R-Fe2O3 by the electrodeposition method on an ITO substrate has a typically lower TM at about 140 K than bulk hematite at 260 K. The decrease in TM may result from the nanoscale morphology and the synthesis method.37,38 Figure 10 shows the field dependence of magnetization of the as-synthesized nanostructured hematite at room temperature (300 K). Almost no hysteresis was found for the sample, which is characteristically similar to typical superparamagnetic materials, which is in agreement with the data reported for spherical
Morphological Control of R-FeOOH Nanostructures
Figure 10. Magnetic hysteresis loop of the nanostructured R-Fe2O3 at 300 K.
hematite nanoparticles and nanorods.38,39 The superparamagnetic property of the as-synthesized hematite at room temperature is ascribed to the small size that is at the same scale level compared to the single domain of hematite. Conclusions In summary, we have developed a novel electrodeposition route for the preparation of well-defined R-FeOOH nanostructures. The morphologies of the R-FeOOH products evolving from platelets to rhombohedral rods, square rods, six-fold snowflakes, and hexagonal particles have been obtained, with CH3COO-, NH4F, and pyridine as capping agents. The shape evolution of the electrodeposited R-FeOOH could be attributed to the capping effect of the additives, in which the preferential adsorption of additives would lower the surface energy of the bound plane and hinder the crystal growth perpendicular to this plane, resulting in a change in the final morphology. Furthermore, we demonstrated that the above R-FeOOH samples can be transferred into the R-Fe2O3 nanostructures with the same morphologies through a thermal annealing process. Acknowledgment. This work is supported by the NSFC (Grant Nos. 20525309, 20673008, and 50821061) and MSTC (MSBRDP, Grant Nos. 2006CB806102 and 2007CB936201). References and Notes (1) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (2) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293.
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