Supporting information Electrochemical cutting in weak aqueous electrolyte: the strategy for efficient and controllable preparation of graphene quantum dots Haoguang Huang,,&,$ Siwei Yang,,&,$ Qingtian Li,,& Yucheng Yang, &, Gang Wang, Xiaofei You,,& Baohua Mao,,& Huishan Wang,,& Yu Ma, Peng He,,&,* Zhi Liu,,&, Guqiao Ding,,&,* and Xiaoming Xie,&, State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, P. R. China & University of Chinese Academy of Sciences, Beijing, 100049, P. R. China School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, P. R. China Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, P. R. China S1
EXPERIMENTAL SECTION Chemicals. Graphene powders (reduced graphene oxide G-100) were purchased from SIMBATT (Shanghai, China). The ammonium hydroxide (25-28%), sodium hydroxide (NaOH), hydrofluoric acid (HF), sulfuretted hydrogen (H 2 S), sulfuric acid (H 2 SO 4, 95.0 98.9%), potassium sulphate (K 2 SO 4 ), potassium hydroxide (KOH), phosphoric acid (H 3 PO 4 ), sodium chloride (NaCl), magnesium chloride (MgCl 2 ), calcium chloride (CaCl 2 ), zinc chloride (ZnCl 2 ), cadmium chloride (CdCl 2 ), potassium chloride (KCl), potassium bromide (KBr), potassium nitrate (KNO 3 ) and potassium bicarbonate (KHCO 3 ),was supplied by Aladdin Reagent (Shanghai, China) Co., Ltd. Preparation of amino-functionalized graphene quantum dots. The af-gqds were prepared via electrochemical cutting of graphene by utilizing ammonia aqueous solution (100 ml, 0.2 mol/l) as the electrolyte under room temperature (25 o C). 0.6 g graphene powders were compressed into graphene paper (diameter: 3.0 cm, thickness: 0.07 cm) by means of a pressure machine with the pressure of 20MPa. The as-prepared graphene paper was employed as the working electrode, while a Pt sheet (1.0 cm 5.0 cm) was used as counter electrode, and the two electrodes were 2 cm apart. The electrochemical preparation of af-gqds was performed with a DC power of KEYSIGHT N5765A. The constant voltage mode was applied (30.0 V). During the anodization, the electrolyte solution changed from colorless to dark brown rapidly. As obtained solution was centrifuged (10000 r/min) to remove the non-luminescence fraction with large size. The mixture was further rotary evaporated to vaporize ammonia. Then, the GQDs aqueous solution was obtained. Notably, the dialysis process was skipped in our strategy compared with the purification process in NaOH system. Considering that the dialysis process is quite slow and S2
expensive, the ammonia aqueous solution is a green and efficient electrolyte for quickly preparing GQDs. Structural characterization. TEM and HRTEM analysis was done (TEM G220, FEI Tecnai) to ascertain the structural attributions and lattice parameters of the materials. AFM was employed to observe the uniformity and thickness of the af-gqds upon Bruker Dimension Icon with a Nanoscope 8.15 in tapping mode. The sample was prepared by dropping diluted af-gqds ethanol dispersion on a SiO 2 substrate. Raman spectroscopy (include af-gqds, graphene paper, residual portion of the graphene paper electrolyzed in ammonia and NaOH solution) with the excitation laser line of 532 nm was performed using a ThermoFisher DXR Raman Microscope. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo ESCALAB 250Xi spectrometer. N content and the atomic ratios of different oxygen-containing functional groups of the af-gqds were calculated from the XPS spectra after correcting the relative peak areas. FT-IR spectroscopy was performed using a BRUKE Vertex 70 (resolution 0.4 cm -1 ) infrared spectrometer and samples were freeze-dried following by compressed into sheets. Ultraviolet-visible (UV-vis) spectra were recorded on a Carry-100 UV-vis spectrophotometer at room temperature, the sample for UV-vis measurement were prepared into dilute aqueous solution. Characterization of optical properties. PL and PLE spectra were measured by a Perkin Elmer LS55 luminescence spectrometer (Perkin Elmer Instruments, U.K.) at room temperature. Timeresolved PL behavior was measured via the time-correlated single-photon counting (TCSPC) technique (Hydra Harp 400, Pico Quant). The samples were excited by a frequency-doubled titanium: sapphire oscillator laser with a pulse duration of 150 fs and a repetition rate of 80 MHz (Chameleon, Coherent). Fluorescence emission was sent to a spectrometer (ihr550, Horiba S3
Jobin Yvon) with 300/mm grating and then detected by a photomultiplier tube. The timeresolved PL curves were fitted with monoexponential decay. The photostability of af-gqds was evaluated under high power UV-light (365 nm, 500 W) at 25 o C for 140 h, and PL intensity was measured every 20 h. The F and F 0 are PL intensity of af- GQDs when t=0 and at corresponding times, respectively. Ionic stability measurement was performed by dissolving 0.2 μl af-gqds into high concentrated (0.5 M) diversified ions (Na +, K +, Mg 2+, Ca 2+, Zn 2+, Cd 2+, Cl -, Br -, NO - 3 and HCO - 3 ). The F and F 0 are PL intensity of af-gqds in the presence of ions and in the absence of ions, respectively. Preparation of GQDs in HF and H 2 S solutions. 100 ml, 0.2 mol/l hydrofluoric acid or hydrosulphuric acid was used as electrolyte. The graphene paper (diameter 3.0 cm, thickness 0.07 cm) was employed as the working electrode and the Pt sheet (1.0 cm 5.0 cm) was used as counter electrode, while the two electrodes were 2 cm apart. The electrochemical preparation of GQDs was performed on a DC power of KEYSIGHT N5765A. The 30.0 V constant voltage was applied. Free radicals (such as OH) capture of the GQDs. To demonstrate the oxidation mechanism, we added 0.1 g of TA into the ammonia (100 ml, 0.2 mol/l) and NaOH solution respectively to trapped OH. The electrode materials were consistent with previous description in preparation of af-gqds. The 30.0 V constant voltage was applied for 30 min. The TAOH can be realized during the electrochemical process, emitting a particular fluorescence around 435 nm under irradiation by laser (315 nm). Then, we investigate the concentration of OH within those two systems by the PL intensity. Evidently, abundant strong oxidizing free radicals included in weak electrolyte system, whereas limited concentration of OH in strong electrolyte system. We S4
further added TEMPO into the ammonia solution to monitor the concentration of OH. The concentration was calculated by Lambert-Beer Law: A=Kbc. Where A is the absorbancy, K is the molar absorption coefficient, b is the thickness of the absorbed layer, and c is the concentration of the light-absorbing substance. Figure S1. TEM image of the graphene sheets prepared in NaOH aqueous solution via electrochemical method. S5
Figure S2. TEM images of the af-gqds. Figure S3. Raman spectrum of the graphene paper. S6
Figure S4. (a) XPS survey spectra of the precursor (graphene paper) and af-gqds, (b) FT-IR spectra of the graphene paper and af-gqds, (c) O 1s spectrum of af-gqds, (d) O 1s spectrum of graphene paper. Figure S5. (a) Metabolic activity of fibroblast treated with different concentrations of af-gqds, (b) Confocal fluorescence microphotograph of fibroblast incubated with 50 μg/ml af-gqds ( λ ex = 380 nm). S7
Figure S6. (a) TEM image of the graphene paper. Figure S7. The PL intensity of the mixed solution of TA+NH 3 H 2 O (red curve) and TA+H 2 SO 4 (black curve) after electrochemical reaction for 2 h. S8
Figure S8. (a) The yield of GQDs prepared from carbon fiber and (b) carbon nanotube in H 2 S, NH 3 H 2 O, HF solutions, respectively. Figure S9. TEM image of the GQDs prepared from carbon fibers in ammonia solution via electrochemical method. S9
Figure S10. XPS survey spectra of the af-gqds prepared in ammonia solution with concentrations of (a) 0.05, (b) 0.10, (c) 0.15, (d) 0.20 and (e) 0.25 mol/l, respectively. Figure S11. N 1s spectrum of af-gqds prepared in ammonia aqueous solution with concentrations of (a) 0.05, (b) 0.10, (c) 0.15, (d) 0.20 (e) and 0.25 mol/l, respectively. S10
Figure S12. Quantum yield of af-gqds obtained in different concentrations of ammonia solution. S11