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 305-0044, Japan School of Resources Processing and Bioengineering, Central South University, Changsha 410083, PR China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China
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
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
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 # 54-0964), and NiO (PDF # 65-5745) (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.
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 0.31 0.31 sin120 o = 8.32 10-2 nm 0.25 0.25 sin120 o =5.41 10-2 nm Molar mass 189.2 24 Mass ratio of LDH/graphite 5.41 10-2 189.2 / 8.32 10-2 24 = 5.13
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.
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.
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, 4477. 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, 8985-8987.