Supporting Information

Transcription

1 Supporting Information Wiley-VCH Weinheim, Germany

2 Topochemical Synthesis of Monometallic (Co 2+ Co 3+ ) Layered Double Hydroxide and Its Exfoliation into Positively Charged Co(OH) 2 Nanosheets Renzhi Ma, Kazunori Takada, Katsutoshi Fukuda, Nobuo Iyi, Yoshio Bando, Takayoshi Sasaki Nanoscale Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki , Japan [1] Experimental Details: β-co(oh) 2 was synthesized from a solution of cobalt chloride (CoCl 2 6H 2 O) under a hydrolysis of HMT. CoCl 2 and HMT were dissolved in a 1000 cm 3 flask of deionized Milli-Q water to give concentrations of 5 mm and 90 mm, respectively. The solution was heated at a refluxing temperature under continuous magnetic stirring and a nitrogen gas protection. After being heated for 3 hrs, pink-colored solid product was recovered by filtering, washing with degassed Milli-Q water, and finally air-drying at RT. XRD data was recorded by a Rigaku RINT-2000 diffractometer with monochromatic Cu K α radiation (λ = nm). Morphology of the synthesized products was examined using a JEOL JSM 6700F field emission scanning SEM. TEM was performed on a JEOL JEM 3100F energy-filtering (Omega type) transmission microscope. FT-IR spectra in a range of cm 1 were measured on an FTS 45RD Bio-Rad infrared spectrophotometer using the KBr pellet technique. X-ray absorption near edge structure (XANES) for the Co-K edge was measured at the Photon Factory (BL-12C) in the Institute of Materials Science, High Energy Accelerator Research Organization (KEK-PF), Japan. The powder samples were pelleted using BN as the binder. Spectra were recorded in transmission mode from 7360 to 8808 ev for the Co edge. Co content in the LDH samples was determined by inductively coupled plasma (ICP) atomic emission spectroscopy (Seiko SPS1700HVR) after dissolving a weighed amount of sample with an aqueous HCl solution. Br content was determined by ICP after dissolving the sample in H 2 SO 4. The valence number of Co was measured by titration using Na 2 S 2 O 3 after dissolving a weighed sample in an aqueous solution containing H 2 SO 4 and iodide. An excess of iodide (I - ) was added together with H 2 SO 4. I 2 produced from the oxidation of I - by Co 3+ was titrated with Na 2 S 2 O 3. This method is employed rather than conventional (COONa) 2 KMnO 4 titration in which it is difficult to determine an accurate oxidation state of Co due to the side reaction of Br - with KMnO 4. Water content was evaluated by thermogravimetry.

3 Thermogravimetric-differential thermal measurements (TG DTA) were carried out using a Rigaku TGA 8120 instrument in a temperature range of ºC at a heating rate of 5ºC min 1 under air. AFM was used to observe the topography of the nanosheets deposited on the Si wafer. A cleaned Si wafer was immersed in a colloidal formamide suspension of LDH nanosheets for 5 min, which was followed by rinsing with a copious amount of water and drying under an N 2 stream. AFM images were acquired in tapping-mode using a Si-tip cantilever with a force constant of 20 N m 1. [2] Determination of Br Content for the Complete Transformation of β-co(oh) 2 into Co 2+ Co 3+ LDH: Our study verified that oxidation of the β-co(oh) 2 by I 2 /CHCl 3 was almost negligible due to an unremarkable oxidation potential difference with Co(OH) 3 /Co(OH) 2, 0.17 V (the standard oxidation potential of I 2 /I - is V). [1] In contrast, Br 2 /Br - is much more oxidative, V, which served as an effective oxidizing agent for Co 2+ in β-co(oh) 2. The transformation of β-co(oh) 2 into an LDH phase may be formulated as partial oxidation of Co 2+ to Co 3+ by the loss of electrons to Br 2 /Br - and simultaneous intercalation of Br -. The reaction can be expressed as: Co 2+ (OH) 2 + x/2 Br 2 Co 2+ 1-xCo 3+ x(oh) 2 Br x. According to this reaction, the extent/degree of oxidation seems to be theoretically determined by the amount of bromine used. Specifically, oxidizing 1/3 mol Co 2+ into Co 3+ and forming a 2/1 ratio of Co 2+ /Co 3+ requires an x value of 1/3 mol, i.e., 1/6 mol Br 2. The practical experiments, however, found it mandatory to use more than the required amount of bromine. Figure S1. The effects of bromine amount and treatment time on the transformation from a β-co(oh) 2 into Co 2+ Co 3+ LDH. (n: multiple of bromine amount theoretically required for oxidizing 1/3 Co 2+ into Co 3+, i.e., n 1/6 mol bromine was added; t: days of treatment). Arrows indicate the peak position for residual β-co(oh) 2 ).

4 As shown by the XRD patterns in Figure S1a, an exact amount of Br 2 theoretically required for oxidizing 1/3 Co 2+ into Co 3+ was not sufficient to form a single phase of Br - -intercalated Co 2+ Co 3+ LDH. After 1 day of treatment, the transformation was apparently incomplete with a major residual peak of β-co(oh) 2 at 4.6 Å. With increasing bromine content, e.g., > 10 times (> 10 1/6 mol bromine was added), residual β-co(oh) 2 appears almost negligible judging from XRD measurements (see arrow). On the other hand, the oxidizing duration also played a role in the phase transformation (Figure S1b). Generally, longer treatment time helped fulfill the transformation when the same amount of bromine was employed. Based on these results, a typical oxidative intercalation procedure was thus designed as 40 times the required amount of Br 2 with treatment duration of ~5 days, to ensure complete conversion into a single Co 2+ Co 3+ LDH phase. Figure S2. a) XANES of I) β-co(oh) 2 ; II) Co 2+ Co 3+ LDH; III) Reference spectrum of Co 2 O 3. A red line indicates the peak position of Co 2+ Co 3+ LDHs. b) FT-IR spectra of I) β-co(oh) 2 ; II) Br - -intercalated; III) ClO - 4 -intercalated Co 2+ Co 3+ LDHs. [3] Phase Characterization of Co 2+ Co 3+ LDH: Figure S2a depicts the normalized Co K-edge XANES of the β-co(oh) 2, Br - -intercalated Co 2+ Co 3+ LDH together with Co 2 O 3 as a reference for Co 3+. A red line indicates the peak top of Co 2+ Co 3+ LDHs, which falls between the peak positions of brucite-like Co(OH) 2 and Co 2 O 3. The edge features demonstrate that the valence state of Co in Br - -intercalated LDH is somewhat intermediate between β-co(oh) 2 (Co 2+ ) and Co 2 O 3 (Co 3+ ). The phase conversion from a brucite-like structure to an LDH one is also evident in Fourier transform infrared (FT-IR) characterizations (Figure S2b). β-co(oh) 2 exhibits a characteristic sharp band at 3630 cm -1 for OH stretching mode (Spectrum (I)), which evolved into a broad band at 3450 cm -1 after treatment with Br 2 /CH 3 CN (Spectrum II). The change and shift of the OH stretching mode strongly indicates that the OH groups in the LDH structure are coupled with hydrogen bonding through a hydration process intercalating interlayer water molecules. Indeed, a bending mode of

5 water molecules at 1620 cm -1 is identified in Spectrum (II). During our experiments, commercially available acetonitrile (Wako Chemical Ltd., Japan) containing ~50 ppm water was used as the solvent to dissolve bromine, which satisfies such a hydration process incorporating water molecules into the LDH interlayer gallery to establish a hydrogen bonding network. In a 2/1 ratio of Co 2+ /Co 3+, the avoidance of nearest neighboring of Co 3+ in the trigonal (hexagonal) host lattice is possible. The schematic model depicted in Figure S3 shows a possible regular arrangement of Co 3+ cations in a 3 3 super cell. This kind of cation ordering is similar to the superstructural arrangement of trivalent metal cations (e.g., Al 3+, Ga 3+ ) in other LDHs (Mg 2+ Al 3+, Mg 2+ Ga 3+, etc.). [2] Such a thermodynamic tendency for energy-favorable cation ordering without direct neighboring of Co 3+ is believed to be a key factor in forming a stable Co 2+ Co 3+ LDH compound. In other words, β-co(oh) 2 would not be over-oxidized into CoOOH or other phases in this peculiar topochemical procedure. Figure S3. Schematic model for a possible Co 2+ /Co 3+ cation ordering in an LDH host lattice (Cuboids: Co 2+ ; Spheres: Co 3+ ). Dashed lines indicate the superstructural arrangement without direct neighboring of Co 3+. A full-indexed XRD pattern of the converted Br - -intercalated Co 2+ Co 3+ LDH is shown in Figure S4. Lattice refinement resulted in a rhombohedral unit cell of a = 3.110(5) Å and c = 23.18(4) Å. The a parameter is somewhat larger than that of its Co 2+ Al 3+ LDH (3.068 Å) and only slightly smaller than that of its Co 2+ Fe 3+ analogue (3.128 Å). The difference might be explained by the ionic radii of Co 3+ (61.0 pm, high-spin) differing from that of Al 3+ (53.0 pm) but comparable with that of Fe 3+ (64.5 pm). [3]

6 Figure S4. XRD pattern of the converted Br - -intercalated Co 2+ Co 3+ LDH indexed in a rhombohedral unit cell with lattice constants of a = 3.110(5) Å and c = 23.18(4) Å. Figure S5. a) EDS of the Co 2+ Co 3+ LDHs in comparison with starting β-co(oh) 2. The signals of copper (Cu) and carbon (C) originate from the carbon-coated Cu grid supporting TEM samples. b) Typical SAED pattern. Chemical composition and crystal structure of the hexagonal platelets were analyzed by TEM. As shown in Figure S5a, energy dispersive X-ray spectrometer (EDS) measurements on individual platelets reveal the incorporation of bromine for the converted Br - -intercalated Co 2+ Co 3+ LDH during the redox reaction. A typical selected area electron diffraction (SAED) pattern, taken from the platelets lying flat on the TEM grid, is given in Figure S5b. It exhibits hexagonally arranged spots that could be indexed as a rhombohedral unit cell with a lattice constant of a = 3.1 Å viewed along the zone axis of [001]. Spots derived from a 3 3 superstructure are not found in the SAED pattern probably due to the trivial difference in scattering potential of Co 3+ from Co 2+. The obtained Br - -intercalated LDH was anion-exchanged into a ClO - 4 form by treating with sodium perchlorate (NaClO 4 ) and a dilute HCl solution. SEM images of the exchanged product are shown in Figure S6. The hexagonal platelet morphology of the LDH crystals was retained.

7 An interlayer spacing of 9.2 Å was yielded when perchlorate ions were exchanged (Figure 2a(III)). There was no apparent color change of the samples during the anion-exchange (see Figure 2b(III)). The exchanged ClO - 4 -intercalated Co 2+ Co 3+ LDH shows no apparent peak in the vicinity of ~1380 cm -1, implying negligible carbonate anions (Spectrum (III) in Figure S2b). Also in this spectrum, bands characteristic of perchlorate ions in the region centered at 1120 cm -1 are clearly discerned. The substitution of bromide anions into percholate anions (represented by chlorine) was recognized in the EDS spectrum given in Figure S5a. Figure S6. SEM images of ClO 4 - -intercalated Co 2+ Co 3+ LDH. Figure S7 shows the TG curves of the transformed LDHs in comparison with β-co(oh) 2. In contrast with the anion-free β-co(oh) 2, the profiles of LDH samples display different weight changes below 400 o C, which could be assigned to the loss of anionic species (Br -, ClO - 4 ) and intercalated water. Figure S7. Thermogravimetric measurements. a) β-co(oh) 2 ; b) Br - -intercalated Co 2+ Co 3+ LDH; c) ClO - 4 -intercalated Co 2+ Co 3+ LDH. [1] W. M. Latimer, Oxidation Potentials, 2nd ed., Prentice-Hall, Academic Press: New York, [2] M. Bellotto, B. Rebours, O. Clause, J. Lynch, J. Phys. Chem. 1996, 100, [3] R. D. Shannon, C. T. Prewitt, Acta Crystallogr. 1969, B25, 925.