Supplementary Information Carbon Quantum Dots/NiFe Layered Double Hydroxide Composite as High Efficient Electrocatalyst for Water Oxidation Di Tang, Juan Liu, Xuanyu Wu, Ruihua Liu, Xiao Han, Yuzhi Han, Hui Huang, Yang Liu,* and Zhenhui Kang* Catalyst Characterization The samples were characterized by different analytic techniques. Scanning electron microscopy (SEM) images and energy dispersive X-ray analysis (EDX) spectroscopy were taken on a FEI-quanta 200 scanning electron microscope with acceleration voltage of 20 kv. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a FEI/Philips Tecnai 12 BioTWIN transmission electron microscope and a CM200 FEG transmission electron microscope, respectively. The normal TEM samples were prepared by dropping the solution onto a copper grid covered with carbon film and dried in air. Raman spectra were collected on an HR 800 Raman spectroscope (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. The spectrograph uses 600 g mm -1 gratings and a 633 nm He Ne laser. The crystal structure of the resultant products was characterized by X-ray powder diffraction (XRD) by using a X Pert-ProMPD (Holand) D/max-γA X-ray diffractometer with Cu Kα radiation (λ=0.154178 nm). The Fourier Transform Infrared (FTIR) spectra of CQDs were obtained with a Varian Spectrum GX spectrometer. Room temperature UV Vis absorption was recorded on a Lambda 750 (Perking Elmer) spectrophotometer in the wavelength range of 200 1000 nm. X-ray Photoelectron Spectroscopy (XPS) was obtained by using a KRATOS Axis Ultra-DLD X-ray photoelectron spectrometer with a monochromatised Al Kα X-ray (hν = 1486.6 ev).the powder sample was attached to carbon tape and was set in the XPS chamber. Calibration of binding energy was carried out by setting binding energy of C1s peak to 284.5 ev. Atomic force microscopy (AFM) sample was prepared onto a silicon substrate by drop-drying
the aqueous solution of the sample. AFM measurement was done with a VeccoMultimode VNanoscope in the tapping mode. PL study was carried out on a Fluorolog-TCSPC Luminescence Spectrometer. Electrocatalytic Study Electrochemical measurements were performed at room temperature using a rotating disk working electrode made of glassy carbon (3 mm diameter) connected to a CHI 920C electrochemistry workstation. The glassy carbon electrode was polished to a mirror finish and thoroughly cleaned before use. Pt wire and Ag/AgCl (PINE, 4 M KCl) were used as counter and reference electrodes, respectively. The potentials reported in our work were vs. Ag/AgCl or vs. the reversible hydrogen electrode (RHE) through RHE calibration. 1-2 The preparation method of the working electrodes containing investigated catalysts can be found as follows. In short, 1.4 mg of catalyst was dispersed in 768 µl of water, 200 µl of ethanol and 32 µl of 5 wt% Nafion solution (5 wt%, Sigma-Aldrich), then the mixture was ultrasonicated for at least 30 min to generate a homogeneous ink. Next, 10µl of the dispersion was transferred onto the galssy carbon disk, leading to the catalyst loading ~0.2 mg cm -2. Finally, the as-prepared catalyst film was dried at room temperature. For comparison, bare glassy carbon electrode which has been polished and cleaned was also dried for electrochemical measurement. Before the electrochemical measurement, the electrolyte (0.1 M KOH, ph ~13; 1 M KOH, ph ~13.8) were degassed by bubbling nitrogen for 30 min. The polarization curves were obtained by sweeping the potential from 0 to0.8 V vs. Ag/AgCl at room temperature with a sweep rate of 5 mv s -1 and 0.1 mv/s for Tafel plots. The hybrid catalyst was cycled ~50 times by cyclic voltammetry (CV) until a stable CV curve was developed before measuring polarization curves of CQDs/NiFe-LDH. Chronopotentiometry was carried out under a constant current density of 2.5 macm -2 for RDE. Experiments involving RDE were conducted with the working electrode continuously rotating at 1600 rpm to get rid of the oxygen bubbles.
Figure S1. (a) Three-dimensional tapping-mode AFM image of an exfoliated CQDs/NiFe-LDH nanosheet deposited on a silicon wafer. (b) Height profile of a single CQDs/NiFe-LDH nanoplate. The nanoplate is ~40 nm in width and ~1 nm in thickness. Figure S2. Energy dispersive spectrum (EDS) of (a) NiFe-LDH nanosheet and (b) CQDs/NiFe-LDH hybrid. ( The Ni/Fe atomic ratio was estimated ~4.4 here)
Figure S3. High-resolution (a) C 1s, (b) Fe 2p,(c) Ni 2p and (d) the XPS survey spectra of NiFe-LDH/CQDs hybrid material. Figure S4. The sectional amplifying image of Fig. 3a.
Figure S5. (a) SEM image and (b) XRD pattern of the CQDs/NiFe-LDH catalyst after long time OER measurement. No obvious morphological or structural transformation could be observed during OER catalysis. Figure S6. Raman spectra (λex = 633 nm) of CQDs/NiFe-LDH hybrid and pristine NiFe-LDH nanoplates. (D-band in 1345 cm -1 and G-band in 1580 cm -1 are the characteristic peaks of CQDs.) Figure S7. PL spectra of the pure CQDs, NiFe-LDH and the CQDs/NiFe-LDH composite for comparation.
Figure S8. The height profile of two single pristine NiFe-LDH nanoplates. These nanoplates are ~50 nm in width and ~1.2 nm in thickness. References in Supporting Information 1 Gao, M.-R.; Xu, Y.-F.; Jiang, J.; Zheng, Y.-R.; Yu, S.-H.; Water Oxidation Electrocatalyzed by an Efficient Mn 3 O 4 /CoSe 2 Nanocomposite. J. Am. Chem. Soc., 2012, 134, 2930-2933. 2 Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H.; Co 3 O 4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater., 2011, 10, 780-786.