Supporting Information

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1 Supporting Information Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation** Jian Bao, Xiaodong Zhang,* Bo Fan, Jiajia Zhang, Min Zhou, Wenlong Yang, Xin Hu, Hui Wang, Bicai Pan, and Yi Xie* ange_ _sm_miscellaneous_information.pdf

2 Supporting Information Table of Contents S1. Detailed experimental section 1 S2. Additional information of the ultrathin nanosheets..2 S3. Performance of the ultrathin nanosheets..5 S4. Additional calculation information...6 Reference..6 S1.Experimental section 1. Synthesis of different kinds of spinel samples Synthesis of the Ni-Co/Co hydroxides ultrathin nanosheet: Typically, 0.5 M NH 3 solution containing p-aminobenzoic acid was added dropwise into the solution containing 5 mmol of Co(NO 3 ) 2.6H 2 O and 2.5 mmol of Ni(NO 3 ) 2.6H 2 O under vigorous stirring. The solid product formed was centrifuged and then dried at 60 C for 12 h under vacuum. After washed with deionized water for several times, the ph of the supernatant was ~7 and the solid tended to form an opaque colloid. The suspension was sonicated, washed with ethanol and then dried at 60 C for 12 h under vacuum. The Co hydroxides were synthesized similarly without the addition of Ni(NO 3 ) 2.6H 2 O. Synthesis of the Zn-Co hydroxides ultrathin nanosheet: To oxidize divalent cobalt ions, a 30 % aqueous solution of hydrogen peroxide was added as an oxidant with the ratio of H 2 O 2 /(Zn + Co)=0.5. The concentrations of Zn 2+, Co 2+, and NaNO 3.H 2 O were controlled to be 100, 200, and 100 mm, respectively. After mixing all the precursors, the resulting solution was stirred at room temperature under a nitrogen atmosphere. The ph of the reactant solution was adjusted to 8 by adding 1 M NaOH solution. After the completion of the synthesis, the precipitates were collected by centrifugation, washed with ethanol, and dried in a vacuum oven at room temperature. The colloidal suspension of the exfoliated Zn Co hydroxides was prepared by dispersion of 0.1 g of the pristine Zn Co hydroxides powder in 100 ml of formamide and then centrifugated, washed with ethanol for furher uesed. Synthesis of the ultrathin nanosheet with rich oxygen vacancies: The above Ni-Co/Co hydroxides was heated in air at 300 C and ZnCo at 350 C resulting in the formation of ultrathin nanosheet with rich oxygen vacancies, The obtained powders were collected for further characterization. Synthesis of the ultrathin nanosheet with poor oxygen vacancies: The ultrathin nanosheets with poor oxygen vacancies were obtained through heating the ultrathin Ni-Co/Co hydroxides in O 2 atmosphere at 300 C and ZnCo at 350 C, the obtained powders were collected for further characterization. 2. Characterization X-ray diffraction (XRD) was recorded by using a Philips X Pert Pro Super diffractometer with Cu K α radiation (λ= Å). The transmission electron microscopy (TEM) was carried out on a JEM-2100F field emission electron microscope at an acceleration voltage of 200 kv. Atomic force microscopy (AFM) was carried out on the Veeco DI Nano-scope MultiMode V system. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MK II with Mg K α as the excitation source. 3. Calculation method The calculations employed the projected augmented wave method with the Perdew-Burke-Ernzerhof (PBE) GGA functional encoded in the Vienna ab initio simulation package. [1-3] The energy cut-off was set to 500 ev, and the atomic positions are allowed to relax until the energy and force are less than 10-4 ev and ev/å, respectively. Each model is separated by a vacuum region of 15 Å. S1

3 S2. Additional characterization information of the samples Scheme S1. Illustration of the topological transformation from the hydroxide to the spinel structure with oxygen vacancies Figure S1. XRD patterns of the Ni-Co and Zn-Co hydroxides. Figure S2. TEM images of the Ni-Co and Zn-Co hydroxide Figure S3. XRD pattern of the ultrathin NiCo 2 O 4 nanosheet powder which randomly arranged and showed no preferred orientation. S2

4 Figure S4. The Ni2p spectra for ultrathin NiCo 2 O 4 nanosheets with rich/poor oxygen vacancies and bulk counterpart. Ni 2p core level spectra was shown in Figure S4, two kinds of Ni species (Ni 2+ and Ni 3+ ) were detected. The binding energies at and ev are ascribed to Ni 2+, another two fitting peaks at and ev are ascribed to Ni 3+. The intense Ni 2+ peaks indicate that the majority of Ni elements in the crystal lattice are Ni 2+ cations. Figure S5. Photoluminescence (PL) spectra for different NiCo 2 O 4 samples. As shown in Figure S5, the PL emission peak at 410 nm is corresponding to the recombination of holes with two-electron-trapped O-vacancy, and the more intensive the peak is, the more Oxygen vacancies the sample possesses. 4 The blue-shifted peak at 395 nm for the ultrathin NiCo 2 O 4 nanosheets could be mainly attributed to the recombination of holes with an electron occupying the oxygen vacancy, further confirming the presence of oxygen vacancies in the nanosheets. 5 S3

5 Figure S6. XRD patterns of the ultrathin ZnCo 2 O 4 and Co 3 O 4 nanosheets. Figure S7. TEM images of the ultrathin ZnCo 2 O 4 and Co 3 O 4 nanosheets. Figure S8. HRTEM image and FFT patterns of ultrathin NiCo 2 O 4 with rich oxygen vacancies. S4

6 S3. Additional performance information of the samples Figure S9. CVs measured in 1M KOH at scan rates from 0.5 to 9 mv/s for ultrathin NiCo 2 O 4 sheets with poor oxygen vacancies and bulk NiCo 2 O 4 Figure S10. Polarization curve of the ultrathin NiCo 2 O 4 nanosheet with rich oxygen vacancies measured in 0.1 M KOH at scan rates of 5 mv/s. Table 1. OER parameters of various NiCo 2 O 4 samples S5

7 S4. Additional calculation information Figure S11. The adsorption of H 2 O molecule onto the NiCo 2 O 4 structure without oxygen vacancy and the adsorption energy of H 2 O is ev References [1] G. Kresse and D. Joubert, Phys. Rev. B 1999, 59, [2] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 1996, 77, [3] G. Kresse, J. Furthmuller, Phys. Rev. B 1996, 54, [4] Huang, G. S.; Wu, X. L.; Mei, Y. F.; Shao, X. F. J.Appl. Phys. 2003, 93,582. [5] F. Lei, Y. Sun, K. Liu, S. Gao, L. Liang, B. Pan, Y. Xie, J. Am. Chem. Soc. 2014, 136, S6