Supporting Information

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1 Supporting Information Facile Synthesis of Black Phosphorus: an Efficient Electrocatalyst for the Oxygen Evolving Reaction Qianqian Jiang, Lei Xu, Ning Chen, Han Zhang,* Liming Dai,* and Shuangyin Wang* anie_ _sm_miscellaneous_information.pdf

2 Supporting information Experimental Section Pre-treatment of Red Phosphorus The red phosphorus is usually kept in the kerosene solution in order to prevent it reaction with the oxygen in the air. Although the storage method can reduce the reaction with oxygen, there still exist some oxides or oxide layers on the surface of red phosphorus. Therefore, the red phosphorus pre-treatment is very necessary in our experiment, which has the same effect to the pre-treatment in the literature reports. [1-6] Physicochemical characterization X-ray diffraction (XRD) patterns are collected on a powder X-ray diffractometer at 40 kv and 15 ma using Co-Kα radiation (D/MAX-2200V, Rigaku, Japan). The morphology, selected-area electron diffraction (SAED) and crystal lattice of the sample are characterized by the Transmission Electron Microscope (TEM) (Tecnai g2 F20). Raman spectroscopy is performed (England, invia-reflex) in a backscattering configuration excited with a visible laser light under a 633 nm excitation laser. Scanning electron microscopy (SEM) images are recorded using a scanning electron microscope (Hitachi S-4800, Japan). X-ray photoelectron spectra (XPS) are obtained on an ESCALAB 250Xi photoelectron spectrometer and curve fitting and background subtraction are accomplished. Thermogravimetric analysis (TG) is measured using a Thermogravimetric Analyzer (HENVEN, HTG-1) in argon, over a range from room temperature to 1000 C with a heating rate of 10 C/min. Electrochemical impedance spectroscopy (EIS) measurements are performed by applying an AC voltage with 5 mv amplitude in a frequency range from to 1 Hz and recorded at 1.64 V vs. RHE. Electrochemical characterization The electrocatalytic performance is measured with an electrochemical workstation (CHI 760E, CH Instrument) and a rotating disk electrode apparatus (RDE-3A, ALS, Japan) in a three-electrode system. The counter electrode and the reference electrode are a platinum foil and the saturated calomel electrode (SCE), respectively. BP-Ti was used as the working electrode directly. The BP-CNT electrode was prepared by depositing BP-CNT suspension in ethanol (8.9 μl, 4 mg/ml) onto the glassy carbon electrode (4 mm in diameter), followed by dripping with 5 μl Nafion solution (0.05 wt% in isoproponal, Sigma Aldrich, USA). Linear sweep voltammetry (LSV) is employed using the rotating disk electrode (RDE) at a rotation rate of 1600 rpm in 0.1 M KOH solution saturated with O2. All the tests are carried out at room temperature (about 25 C). Potentials are referenced to a reversible hydrogen electrode (RHE) [7]. All LSV data are presented with IR correction. In order to study the reaction mechanism for OER, rotating ring-disk electrode (RRDE) voltammograms are carried out based on a Pt ring electrode and a glassy carbon disk electrode. The as-prepared BP-CNT catalyst is coated onto the RRDE using above method. To detect the content of the formed peroxide intermediates, the ring potential is fixed constantly at 1.5 V vs. RHE in O2-saturated 0.1 M KOH solution and recorded the data under a rotation rate of 1600 rpm at a scan rate of 5 mv/s -1. Specifically, in order to determine the reaction pathway for OER by detecting the HO2 - formation, the ring potential is held constantly at 1.50 V vs. RHE for oxidizing HO2 - intermediate in O2-saturated 0.1 M KOH; on the other hand, to ensure that the oxidation current originates from oxygen evolution rather than other side reactions and to calculate the Faradaic efficiency of the system, the ring potential is held constantly at 0.54 V vs. RHE to reduce the O2 formed from the catalyst on the disk electrode in N2-saturated 0.1 M KOH solution. The calculation of Faradaic efficiency is as follows: ε = Ir/(IdN), where Id is the disk current, Ir is the ring current, and N is correspond to the current collection efficiency of the RRDE (N = 0.2) (see J. Am. Chem. Soc. 2014, 136, ). To properly calculate the Faradaic efficiency of the system, the disk electrode is held at a relatively small constant current of 125 µa (~1 ma/cm 2 ); this current is sufficiently large to ensure an appreciable O2 production and sufficiently small to minimize local saturation and bubble formation at the disk electrode (see J. Am. Chem. Soc. 2013, 135, 16977).

3 Figure S1. TGA results of BP-Ti (a) and BP-CNT (b), showing the BP contents in the BP-Ti and BP-CNT are 13.41% and 15.46%, respectively. Figure S2. a) TEM image of BP separated from Ti foil, Inset in a1) the SAED pattern taken from the same region as in a); b-d) EDS elemental mapping of sample BP separated from Ti foil, corresponding elemental mappings of e) P and d) Ti. The TEM images for BP-Ti prepared by the efficient TVT method are shown in Figure S2a. It shows the product with the particle size about 70 nm which is consist with the result of the SEM images. The selected area electron diffraction patterns in Figure S2a1 are assigned to the (021), (041) and (002) planes of the BP crystal, respectively, which is consistent with the well-known BP lattice parameters. [8, 9] In order to further confirm the distribution of orthorhombic black phosphorus on the Ti foil, the TEM and EDS mapping images of BP-Ti are tested in Fig.S2b-d.

4 Figure S3. SEM images of BP-Ti with different temperature: a, b) 550 º C; c,d) 650 º C; e,f) 750 º C. In Figure S3, we can see that the reaction temperature can impact the morphology of the black phosphorus on the Ti foil. When the reaction temperature is 550 C, the morphology of BP-Ti (Fig.S2a) is close to the polyhedral structure, but the BP texture (Fig.S3b) is not very obvious. However, the BP-Ti with reaction temperature 650 C shows a surface morphology with well-defined polyhedral structures (Fig. S3c). Under a high-magnification (Fig.S3d), a uniformly-distributed BP texture is clearly evident. When the reaction temperature is increased to 750 C, it can be clearly find that the size of the polyhedral particles (Fig. S3e) become larger than that of BP-Ti (650 C), and the BP texture becomes more tightly. Therefore, we can confirm that the reaction temperature can impact the partial size of BP and further for the BP texture. Figure S4. SEM images BP-Ti with different amount of red phosphorus: a) 0.25 g; b) 0.5 g; c) 0.75 g. In order to further study the influence factor of BP-Ti, SEM images of BP-Ti with different amount of red phosphorus is shown in Fig.S4. We can find that the amount of red phosphorus can affect the morphology of the material, and when the amount of red phosphorus is 0.5 g, the BP-Ti has the best morphology and BP texture, which will be good for its OER activity.

5 Figure S5. SEM images BP-Ti prepared for different time: a) 3 h; b) 5 h; c) 7 h. Figure S5 shows the images of BP-Ti prepared for different time, from 3h to 7h. From the three images, it can be seen that the reaction time can hardly affect the morphology of BP-Ti. Figure S6. a) The photograph of Ti foil and the BP-Ti; b) SEM image of the cut up surface of BP-Ti Figure S7. XRD patterns of BP-Ti prepared at different temperatures. Figure S8. Raman spectra of orthorhombic black phosphorus directly grown on Ti foil: a) with different amount of red phosphorus; b) with different temperature; c) with different time.

6 Figure S9. High-resolution P 2p XPS spectra of a) BP; b) BP-Ti; c) BP-CNT. Figure S10. Polarization curves of orthorhombic black phosphorus directly grown on Ti foil: a) with different amount of red phosphorus; b) with different temperature; c) with different times. Figure S11. a) Cyclic voltammograms (CVs) of BP-Ti measured at different scan rates from 2 to 10 mv/s; b) Plot of the current density at 1.14 V vs. the scan rate. Figure S12. XRD of a) BP-Ti and BP-CNT b) after the chronoamperometric response In order to study the structural change of BP-Ti and BP-CNT after OER testing, XRD patterns of BP-Ti and BP-CNT after the chronoamperometric response are investigated in Figure S12. From the results, we can clearly see that all the diffraction peaks of the BP-Ti except those from Ti or CNT can be indexed to an orthorhombic black phosphorus, which indicates that the structure of BP on both Ti and CNT keep very well. It further illustrates that the BP is relatively stable.

7 Figure S13. SEM images of RP, CNT-650 and BP-CNT with different time: a) RP; b) CNT-650; c,d) 3 h; e,f) 5 h; g,h) 7 h. Figure S14. a) TEM image of BP-CNT; b-d) EDS elemental mapping of sample BP-CNT, corresponding elemental mappings of c) C and (d) P.

8 Figure S15. The polarization curves of commercial IrO2, RuO2 and BP-CNT catalysts for OER. Figure S16 a) Experimental phenomena of the titration experiments with the K3PO4 resolution with different mole content); b) Fourier transform infrared spectroscopy (FTIR) spectrum of the KOH resolution before and after the chronoamperometric response From the result of the chronoamperometric response, we can see that sample BP-Ti is relatively stable, possibly due to the uniform grown on the Ti foil. In order to confirm whether the black phosphorus films dissolute in KOH/O2 solution, the titration experiments and the Fourier transform infrared spectroscopy (FTIR) of BP-Ti after the chronoamperometric response are performed in Figure S16. First of all, we prepared serials of control samples. To make the experimental phenomena more obvious, we put K3PO4 with mole content of 2.5*10-3, 2.5*10-4, 5*10-5, 2.5*10-5, 1.25*10-5 and 2.5*10-6 into the each transparent glass bottle, respectively. We could observe obvious color changes with the control experiments. To examine the electrolyte solution (KOH) in this work before and after the long-time chronoamperometric response, we put 1 ml AgNO3 (0.1 mol/l) into the used KOH solution after the durability testing. It can be seen that the KOH solutions before and after the chronoamperometric response both show similar color, the milk white, which is different from any bottles with the K3PO4 species in the control bottles (Fig. S16a). These phenomena indicate the KOH solution after the chronoamperometric response has no or very little (less than 2.5*10-6 mol) PO4 3- species in the solution after the OER reaction. In order to further confirm whether there is any other P-containing species existing in the KOH solution after the durability testing, FTIR spectra of the KOH solution before and after the chronoamperometric response are collected as shown in Figure S16b. The label shows that only the bands intrinsic to KOH and H2O are observed, which indicates that no impurity phase pollutes the solution. The characteristic peak of P-containing species at cm -1 is not observed in the FTIR of the KOH resolution after the chronoamperometric response (KOH-A). This result can further illustrate that black phosphorus is relatively stable in the KOH/O2 solution. Therefore, there may be very little or no dissolution of BP in KOH/O2 solution.

9 Figure S17. Polarization curves of BP-CNT: a) with different amount of red phosphorus; b) with different temperature; c) with different times. Figure S18. a) Cyclic voltammograms (CVs) of BP-CNT measured at different scan rates from 2 to 10 mv/s; b) Plot of the current density at 1.14 V vs the scan rate. Figure S19. Polarization curves of :a) BP-Ti; b) BP-CNT; c) Tafel plots of BP-Ti; d) Tafel plots of BP-CNT in an O2-saturated 0.1 M and 1 M KOH solution (scan rate: 5 mv/s). BP-Ti (Figure S14a) shows a larger current density and much lower Tafel slope (Figure S14b) of mv/dec in 1 M KOH than that in 0.1 M KOH (91.52 mv/dec). More clearly, BP-CNT has much larger current density and much lower Tafel slope (Figure S14d) of mv/dec in 1 M KOH in comparison to that in 0.1 M KOH (72.88 mv/dec). All the results demonstrate the excellent OER activity of the BP in concentrated alkaline electrolyte solutions.

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