Supporting Information

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1 Supporting Information Au Nanowire-Striped Cu 3 P Platelet Photoelectrocatalysts Anirban Dutta, Aneeya K. Samantara, Samrat Das Adhikari, Bikash Kumar Jena and Narayan Pradhan Department of Materials Science and Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata, India Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India S1

2 EXPERIMENTAL SECTION Materials Copper (I) chloride (CuCl, reagent grade, 97%), Trioctylphosphine (TOP, tech, 90%), Trioctylphosphine Oxide (TOPO, tech, 90%), Oleylamine (OLM, tech.70%), Tetrachloroauric (III) acid (HAuCl 4 3H 2 O, 99.9%), Calcium Phosphide (Ca 3 P 2 ) and Nafion 117 solution (5% in a mixture of lower aliphatic alcohols and water) were purchased from Aldrich and, Hydrochloric Acid (HCl, 35%) were purchased from Merck. Methods Synthesis of copper phosphide platelets: Cu 3 P platelets were synthesized following our previous reported protocol. In a typical process, 0.1 mmol CuCl (0.01g), 2.58 mmol TOPO (1g) and 6 ml oleylamine were taken in a three necked flask and the reaction mixture was degassed by purging Argon gas at 120 o C for 15 min. Then the temperature was increased to 150 o C and 0.6ml TOP was injected and kept at this temperature for further 5min. The reaction mixture was heated to 230 o C and PH 3 gas passed through the reaction mixture. Immediate change of the colour of the solution from yellowish to blackish indicated the formation of Cu 3 P nanoplatelets. The reaction temperature was lowered down to 200 o C and annealed for 15min then cooled to room temperature. Finally, the heterostructures were purified using ethanol followed by acetone as non-solvent and chloroform as solvent, finally dispersed in chloroform. Synthesis of stripy pattern Au growth on copper phosphide platelets: The stripy pattern growth of Au on Cu 3 P platelets was carried out by insitu reduction of Au(III) ions on the surface of the platelets in presence of PH 3. In a typical reaction, after formation of Cu 3 P platelets, the HAuCl 4 solution (0.05 mmol in 2 ml oleylamine) was swiftly injected. S2

3 Further, the reaction mixture was again annealed for 10 min for obtaining the stripy pattern Au growth on Cu 3 P platelets. Samples were also collected at different time interval and the growths were monitored microscopically. Finally, the heterostructures were harvested after cooling the reaction to room temperature and purified using ethanol followed by acetone as non-solvent and chloroform as solvent, finally dispersed in chloroform. Phosphine gas generation: Phosphine gas was prepared by modified literature procedure. In a typical procedure, 3 gm Ca 3 P 2 was loaded in a 100 ml three necked flask and degassed with purging of Argon gas for 15 min. Then 3ml of 4M HCl was added to the flask. Released PH 3 gas was passed through CaCl 2 powder before purging to the reaction medium to remove the moisture. Ligand Exchange: The dispersed nanostructure in chloroform is treated with 1,2-Ethanedithiol and shaken vigorously for 2-5min. The clear solution after shaking become hazy indicates the completion of the process. Finally, the resulting mixture is centrifuged and the precipitate dispersed in acetone for further use. Electrochemical measurements: A two compartment three electrode electrochemical cell was used for all the electrochemical measurements. At first the catalyst ink was prepared by making a proper mixture of as synthesized samples (Cu 3 P or Cu 3 P-Au) and 5% nafion in ethanol by means of bath sonication for 5 minutes. After that the ink was drop casted on the glassy carbon electrode (GCE, the electrode was polished with alumina slurry of 1, 0.3 and 0.05µm alumina powder followed by bath sonication for 15 minute) of surface area 1cm 2 and dried properly. The as modified GCE, a bare platinum wire and Ag/AgCl electrode were taken as the working, counter and reference S3

4 electrodes. All the data were recorded by a computer controlled CHI 760D electrochemical work station. The photoelectric current measurement was carried out by illuminating the working electrode with a 100 W lamp (λ 400nm, 900 lm) at zero bias voltage in 0.1M Phosphate buffer solution (PBS, ph=7.2) as electrolyte. On the other hand the photo electrochemical oxidation of the NADH was carried out in 0.1M PBS as the supporting electrolyte at a scan rate of 10mV/s in presence and absence of the light illumination. The effect of NADH presence was studied by recording the photocurrent in presence and absence of 0.5 mm NADH in 0.1M PBS at the oxidation potential of NADH. Characterizations The X-ray diffraction (XRD) study of the purified samples were carried out using a Bruker D8 Advance powder diffractometer, using Cu Kα(λ= 1.54 Å) as the incident radiation. Transmission electron microscopy (TEM) TEM images were taken in JEOL JEM-1400 plus using 120kv electron source and high-resolution TEM (HR-TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were captured on a UHR FEG-TEM, JEOL JEM-2100F electron microscope using a 200kV electron source. The specimens were prepared by dropping a drop of nanocrystal solution, dispersed in chloroform onto a carbon coated copper grid and the grid was subsequently dried and stored in desiccator.varian-cary UV-Vis-NIR was used to record UV-Vis-NIR spectrum. S4

5 Supporting Figures: Figure S1. HAADF-STEM images of Au nanowire striped Cu 3 P disk heterostructures. (a) Wide view image, (b) a single heterostructured disk showing dense Au nanowires at the edges, (c) Magnified Au nanowires on Cu 3 P disk. Figure S2. Powder X-ray diffraction pattern of stripy pattern growth of Au nanowires on Cu 3 P platelets sample. S5

6 Figure S3. TEM images in different resolutions obtained from the sample collected within 5 sec of Au precursor injection to Cu 3 P platelets. Figure S4. TEM images in different resolution obtained with excess TOP introduction. S6

7 Zim ( Au-Cu 3 P Cu 3 P Zrel (10 3 Figure S5. Nyquist impedance plot for Cu 3 P and Au-Cu 3 P modified electrodes in presence of light. S7