Supporting Information

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1 Supporting Information A Superlattice of Alternately Stacked Ni-Fe Hydroxide Nanosheets and Graphene for Efficient Splitting of Water Wei Ma,,, Renzhi Ma,, * Chengxiang Wang, Jianbo Liang, Xiaohe Liu,, * Kechao Zhou,,, * Takayoshi Sasaki International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki , Japan School of Resources Processing and Bioengineering, Central South University, Changsha , PR China State Key Laboratory of Powder Metallurgy, Central South University, Changsha , PR China

2 Figure S1. a) SEM images of as-synthesized AQS intercalating Ni 2/3 Fe 1/3 LDH, b) XRD patterns of AQS intercalating Ni 2/3 Fe 1/3 LDH (i) (*: Cl - intercalating LDH), Ni 3/4 Fe 1/4 LDH (ii), Ni 4/5 Fe 1/5 LDH (iii), and DS - intercalating Ni-Fe LDH (iv), c) EDS spectra of Ni 2/3 Fe 1/3 LDH inserted with AQS anions. Figure S1a presents typical SEM image of resulting Ni 2/3 Fe 1/3 LDH intercalated with AQS obtained at 120 o C for 6 h. The as-synthesized platelets exhibit a lateral size of approximate 400 nm, and a uniform thickness of a few tens of nm. The products with different ratio of Ni and Fe (e.g. 3:1 and 4:1) were also synthesized using the same procedure, and a similar morphology with Ni 2/3 Fe 1/3 LDH product was observed. In this synthetic procedure, AQS plays an important role as an in situ oxidizer (Scheme S1) to transform ferrous anions (Fe 2+ ) into ferric anions (Fe 3+ ) and simultaneously intercalated into the interlayer as anions to balance the positive charge of host layers. The anion-exchange process of as-prepared Ni-Fe LDH intercalated with AQS was monitored by XRD measurements. The interlayer spacing of AQS inserted Ni-Fe LDH is approximately 2 nm (Figure S1bi, ii and iii). In addition, a small impurity

3 phase intercalating chloride anions was occasionally observed. The interlayer spacing of Ni-Fe LDH was expanded into 2.4 nm via a conventional anion-exchange route with SDS at room temperature (Figure S1b iv). Figure S1c exhibits the EDS spectra of as-synthesized Ni 2/3 Fe 1/3 LDH inserted with AQS anions. The atomic ratio of Ni and Fe is estimated to be approximate 61:27, which is very close to the stoichiometry of as-synthesized Ni 2/3 Fe 1/3 LDH. Scheme S1. The reduction and oxidation of AQS

4 Figure S2 a) TG-DTA curves of Ni 2/3 Fe 1/3 LDH intercalated with AQS anions, b) XRD pattern of as-transformed samples obtained at different temperature, (i) original, (ii) 160 o C, (iii) ) 600 o C, (iv) 1000 o C. Figure S2a exhibits TG-DTA curves of Ni 2/3 Fe 1/3 LDH intercalated with AQS anions in temperature range of room temperature ~ 1000 C. The first weight loss of 10.5% below 160 C is ascribed to the evaporation of adsorbed water molecules on the Ni 2/3 Fe 1/3 LDH surface. As shown in Figure S2b(i) and S2b(ii), there is no clear change in the interlayer spacing at this stage. With the increase of temperature to 600 o C, the weight loss of 45.2% attributed to the removal of AQS, and the formation of NiFe 2 O 4 (PDF # ), and NiO (PDF # ) (Figure S2b(iii). When the temperature was raised to 1000 C, the loss weight of 5.8% is due to the removal of hydroxyl group and remanent sulphides. XRD pattern shown in Figure S2b(iv) clearly reveals that NiO and NiFe 2 O 4 were obtained via calcination at 1000 C. According to above result of TGA data, the chemical composition of as-prepared LDH product was estimated as Ni 2/3 Fe 1/3 (OH) 2 (AQS) 1/6 0.5H 2 O.

5 Figure S3 TEM image of unilamellar Ni 2/3 Fe 1/3 LDH nanosheets. Table S1 Calculation of mass ratio between LDH nanosheets and graphene based on an area matching model Compound LDH Graphite Formula Ni 2/3 Fe 1/3 (OH) 2 (DS) 1/3 0.5H 2 O C Model Space Group R-3m P6 3 /mmc Lattice parameters a = 0.31 nm / c = 2.43 nm a = 0.25 nm / c = 0.67 nm In-plane unit cell area sin120 o = nm sin120 o = nm Molar mass Mass ratio of LDH/graphite / = 5.13

6 Figure S4 XRD patterns of natural graphite (i), GO (ii) and rgo (iii). Figure S5 TEM image and SAED pattern of Ni 2/3 Fe 1/3 LDH nanosheets and rgo hybrid.

7 Figure S6 (a) ir-corrected polarization curves of Ni 2/3 Fe 1/3 -rgo, Ni 2/3 Fe 1/3 -GO, Ni 2/3 Fe 1/3 -NS, Ni 2/3 Fe 1/3 -DS and Ni 2/3 Fe 1/3 -AQS in 1M KOH solution, (b) charging current density differences (Δj = j a - j c ) plotted against scan rate. The slope was used to represent the ECSA, which is equal to twice of the double-layer capacitance C dl, (c) current density at 300 mv against ECSA (2C dl ) for the catalysts. The results clearly show that the current density at an overpotential of 300 mv as well as ECSA both increase from Ni 2/3 Fe 1/3 -AQS to Ni 2/3 Fe 1/3 -NS and Ni 2/3 Fe 1/3 -DS due to the exfoliation effect or expansion in interlayer spacing for accessible surface area. Upon hybridizing Ni 2/3 Fe 1/3 -NS with GO/rGO, OER activity dramatically increased (4-10 times). The enhancement in catalytic activity is much remarkable than the increase in ECSA, indicating that the increased ECSA is not the only contributor. The synergistic effect between Ni 2/3 Fe 1/3 -NS and GO/rGO at a molecular scale may play an important role in the OER process.

8 Figure S7 ir-corrected polarization curves (a) and Tafel plots (b) of Ni 2/3 Fe 1/3 -DS, Ni 3/4 Fe 1/4 -DS, Ni 2/3 Fe 1/3 -AQS, Ni 3/4 Fe 1/4 -AQS and Ni 4/5 Fe 1/5 -AQS (inset: histogram of corresponding Tafel plots). Figure S8 ir-corrected polarization curve (a) and Tafel plot (b) of Ni 2/3 Fe 1/3 -rgo at a scan rate of 1 mv s -1. S1. Song, F.; Hu, X. L. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, S2. Vrubel, H.; Moehl, T.; Gratzel, M.; Hu, X. L., Revealing and Accelerating Slow Electron Transport in Amorphous Molybdenum Sulphide Particles for Hydrogen Evolution Reaction. Chem. Commun. 2013, 49,