Supporting Information

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1 Supporting Information Topochemical Synthesis, Anion Exchange, and Exfoliation of Co-Ni Layered Double Hydroxides: A Route to Positively Charged Co-Ni Hydroxide Nanosheets with Tunable Composition Jianbo Liang, Renzhi Ma, Nobuo Iyi, Yasuo Ebina, Kazunori Takada, Takayoshi Sasaki * International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki , Japan * Corresponding author. sasaki.takayoshi@nims.go.jp. 1

2 Figure S1. X-ray photoelectron spectra of brucite-like Co 2/3 -Ni 1/3 hydroxides and corresponding LDHs. The Co2p core lines of brucite-like Co 2/3 -Ni 1/3 hydroxides is very similar to that of Co(OH) 2, which are split into Co2p 3/2 (781.3 ev) and Co2p 1/2 (797.4 ev) parts accompanied by satellite lines at and ev respectively. [1,2] The spin-orbit splitting value is calculated as 16.1 ev. A prominent Co2p 3/2 satellite line is typical characteristics of high-spin divalent cobalt ions. After the topochemical reaction, the intensity of the Co2p 3/2 satellite line is noticeably lowered. The spin-orbit splitting value is reduced to 15.0 ev (Co2p 3/2 : ev, Co2p 1/2 :796.5 ev), in accordance with that of CoOOH and Co 3 O 4. [1-3] These changes suggest that a portion of divalent cobalt ions is transformed into trivalent state. In contrast, the Ni2p spectra of both two samples are almost the same to that of Ni(OH) 2 (Ni2p 3/2 : ev, Ni2p 3/2 satellite: ev, Ni2p 1/2 : ev, Ni2p 1/2 satellite: ev), [4] indicating that the nickel ions maintain the divalent state during the topochemical reaction. 2

3 Table S1. Elemental content (mass %) of Co 2/3 -Ni 1/3 LDH samples prepared by different anionexchange routes and corresponding exchange rate. Exchange Media Exchange Rate Element Content (wt %) N C H In water 15% In water with addition of 2 mm acid 55% Ethanol/water=1:1 90% Pristine Co 2/3 -Ni 1/3 -Br LDHs The nitrate exchange rate is calculated based on the nitrogen content and the chemical formula of Co 0.68 Ni 0.32 (OH) 2 (NO 3 ) H 2 O. The carbon content of the pristine Co 2/3 -Ni 1/3 -Br LDHs and the NO 3 - exchanged sample obtained in the mixed solution of ethanol/water was as low as 0.5 wt%, which may be taken as the background. Figure S2. ATR (attenuated total reflection)-ir spectra of the Co 2/3 -Ni 1/3 -Br LDHs (a) and the nitrate forms exchanged with different methods: (b) Normal anion-exchange in water, (c) Anionexchange with the addition of 2 mm acid, (d) Anion-exchange in an ethanol/water mixed solution. The spectra were recorded using the ATR method instead of the standard transmission mode using KBr pellet to avoid possible exchange of NO - 3 /Br - in the KBr matrix, as actually revealed in our recent study. 16c The pristine Br-form did not show the band due to carbonate ions, which is consistent with the low carbon content revealed by the elemental analysis. In spectrum c, the band profile at cm -1 is broader and less symmetrical than that in spectrum d, indicating that the sample obtained with the addition of acid contained more carbonate as contamination than the sample exchanged in the water/ethanol mixed solution. 3

4 Figure S3. XRD patterns of brucite-like Co 1-x -Ni x hydroxides prepared through HMT hydrolysis: a) Cobalt-rich samples; b) Nickel-rich samples. The sharp and intense basal peaks of cobalt-rich samples shown in Figure S2a indicate that the cobalt-rich samples are grown in high quality. The basal peaks broaden as nickel content increases, suggesting that the crystallinity of the nickelrich samples is not as good as that of the cobalt-rich samples. 4

5 Figure S4. Variation of lattice parameters of brucite-like Co 1-x -Ni x hydroxides versus nickel content. Parameters a and c decrease linearly with the nickel content, following the Vegard s law. For some points, the standard deviation is too small to be distinguished in the figure. 5

6 Figure S6. XRD patterns of Co 1-x -Ni x -Br LDHs prepared via topochemical reaction. Pure-phase Co 1-x -Ni x -Br LDHs can be achieved for nickel content of 0.50 and below. Figure S7. Illustration of the stacking sequence of the LDH phase (a) and the brucite phase (b). The space group of the brucite phase is P m, in which the stacking sequence is A, c, B, V, A,, the metal ions occupy the c sites and V represents the vacant site; for the LDH phase with space group of R m, the stacking sequence is A, c, B, b, B, a, C, c, C, b, A, a, A, c, B,..The topotactic feature of the oxidative intercalation process can be viewed from incorporating anions into the V site of the brucite layers with the preservation of the slabs. 6

7 Figure S8. Description of the distance between the interlayer vacant sites and the size of the bromide anions. Taking Co 2/3 -Ni 1/3 -Br as an example, the distance between the neighboring guest sites is nm (equal to parameter a), which is smaller than the diameter of the bromide anions (0.392 nm). Therefore, in the Co 2/3 -Ni 1/3 -Br LDHs prepared via oxidative intercalation reaction, if a vacant site is occupied by one Br -, the first-nearest neighboring vacant site cannot accommodate another Br - due to the limited space, as depicted in Figure S7b. It can fit into the second-nearest neighboring vacant site with the distance of a (0.535 nm). In this case, long-range ordering may evolve and the occupation rate would be 1/3. There are two M 2+ and one M 3+ associated with one Br - in the lattice. Figure S9. XRD patterns of Co 1-x -Ni x -NO 3 LDHs prepared through the ethanol-assisted anionexchange process. Using this approach, Co 1-x -Ni x -Br LDHs can be exchanged into nitrate intercalated form with basal spacing of 0.88 nm. For the sample with x of 0.5, the peak is slightly asymmetric which might have resulted from inadequate exchange. 7

8 Figure S10. XRD patterns of the products after treating brucite-like Co 1-x -Ni x hydroxides (x=0.50, 0.33, 0.10) with different amounts of bromine. The peak marked with an asterisk corresponds to the brucite-like phase. The value n represents the multiple of bromine amount theoretically required to transform the brucite-like Co 1-x -Ni x hydroxides into the LDH phase. 8

9 Figure S11. XRD patterns of products obtained by treating brucite-like Co 1-x -Ni x hydroxides (2 mmol) with the stoichiometric amount of bromine (1/3 mmol) for 24 h. The diffraction peaks marked with L and B represent the LDH and brucite phase, respectively. Figure S12. XRD pattern of the product after treating a mixture of β-co(oh) 2 and β-ni(oh) 2 at a molar ratio of 2:1 with 40 times the amount of bromine for 7 days. Residual brucite phase was still observed, indicating that mechanical mixing of β-co(oh) 2 and β-ni(oh) 2 cannot promote their transformation into the LDH phase. References [1] Okamoto, Y.; Imanaka, T.; Teranishi, S. J. Catal. 1980, 65, 448. [2] Schenck, C. V.; Dillard, J. G. ; Murray, J. W. J. Colloid Interface Sci. 1983, 95, 398. [3] Tan, B. J.; Klabunde, K. J. ; Shewood P. M. A. J. Am. Chem. Soc. 1991, 113, 855. [4] Mclntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47,