Supporting information

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1 Supporting information Plasmon-Sensitized Graphene/ Inverse Opal Nanostructures with Enhanced Charge Collection Efficiency for Water Splitting Ramireddy Boppella, Saji Thomas Kochuveedu,, Heejun Kim, Myung Jin Jeong, Filipe Marques Mota, Jong Hyeok Park, Dong Ha Kim #,* Department of Chemistry and Nano Science, Division of Molecular and Life Sciences, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Korea. Department of Biological and Chemical Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea # Division of Chemical Engineering and Materials Science, College of Engineering, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Korea Current address: Institute of Applied Physics, Johannes Kepler University Linz, Altenberger Straße 69, 4040 Linz, Austria * To whom correspondence should be addressed: dhkim@ewha.ac.kr; Fax: ; Tel.: S-1

2 Synthesis of PS beads Monodispersed polystyrene (PS) beads were synthesized by emulsion polymerization reaction. Typically, 5 ml styrene monomer and 0.5 gr of polyvinylpyrrolidone (PVP ~55000 Mw) were added to the 45 ml of ethanol in three neck 100ml round bottom flask. The reaction mixture was deoxygenated by bubbling nitrogen gas through it at 70 O C under constant stirring. After 30 min, gr of AIBA was dissolved in deionized water (5mL) was added to the above mixture, and was polymerized at same temperature for 20 hrs to obtain PS beads. The PS beads was collected and washed with ethanol for three time and redispersed in 40ml of ethanol to fabricate 3D opal structures. Synthesis of Graphene Oxide GO was synthesized by modified Hummers method. In brief, 1 gr graphite powder was added to 70 ml H 2 SO 4 (98%), then 3 g KMnO 4 and 0.5 g NaNO 3 were added gradually in an ice bath. The mixture was stirred for 4 h and then 100 ml of DI water was added to the mixture was maintained at that temperature for 30 min. After that, 30% H 2 O 2 solution was added into the solution slowly while stirring until the suspension turned to brilliant brown indicating fully oxidization of graphite. The mixture was washed repeatedly with 5% HCl, DI-water and collected by centrifugation. The obtained graphite oxide powder was added to water and exfoliated by sonication for 6 h followed by centrifugation for 30 min at 3000 rpm to remove precipitates and used to obtain a stable GO-water suspension. Synthesis of Au NPs Citrate-capped Au NPs (15 nm in size) were synthesized according to a well-known citratereduction method. In brief, 1.5 ml of chloroauric acid (10 mg/ml) was added to ml S-2

3 Water and then heated to o C. When the solution began to boil, Na-citrate (0.9 ml, 50 mg/ml) was added and the mixture was heated until the solution turned a wine-red color. Characterization The surface morphology was characterized using scanning electron microscopy (SEM; JEOL JSM6700-F) and transmission electron microscopy (TEM; JEOL JSM-2100F operated at 200kV). The crystalline structure was investigated by X-ray diffraction (XRD) Cu-Ka radiation (D/max RA, Rigaku Co.) in 2θ region of 10 o to 70 o to elucidate pertinent information on the crystallinity and phase structure. Raman spectra were recorded on a T64000 (HORIABA Jobin Yvon, France). UV-vis absorbance and diffuse reflectance spectra (DRS) were measured using Varion 5000 spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were measured on a Thermo Scientific K-Alpha XPS, using a dual beam source and ultra-low energy electron beam for charge compensation Gas evolution test PEC water splitting experiments were carried out in a in an air-tight reactor comprising three electrode system using the photoanode as working electrode, Ag/AgCl as reference and Pt foil as the counter electrode under AM 1.5 illumination (100 mw/cm 2 ) at 1.23 V vs RHE in a 0.5 M Na 2 SO 4 electrolyte. Before the photocelectrochemical water splitting reaction, the reactor filled with Na 2 SO 4 was purged with N 2 gas for 30 min to remove the dissolved oxygen. Then, AM 1.5G (100 mw cm -2 ) simulated solar light was illuminated through the quartz glass window for a period of five hours and the amount of H 2 and O 2 gas was noted every 1 h time. 0.1 ml of gas was extracted by a gas-tight micro syringe and analyzed by gas chromatography (GC) equipped with a thermal conductivity detector (TCD). S-3

4 Calculation of photonic stopband The position of the Bragg reflection peak (stopband position) can be estimated using the modified Bragg equation when the incident light is normal to the 1-3 λmax= λmax= (1) where λmax (nm) is the stopband position for 1st order Bragg diffraction, n eff is the average refractive index of the photonic crystal and θ is the incident angle of incident light with respect to the surface normal. Assuming that the incident angle of incident light with respect to the surface normal θ=0, λmax= = (. + (1 )) / (2) n TiO2 and n void are the refractive index of (2.3) and the voids respectively, f is the phase volume percentage, which is generally taken as According to eqn (2) and considering the shrinkage of the pore size of the inverse opal during the removal of PS template by sintering, the stop band of the inverse opals is calculated to be about 1215 nm. S-4

5 Figure S1. Morphological characterization of the structures. (a) Cross sectional SEM image of TiO2 thin film, (b) Top view image of TiO2 thin film, (c) SEM image of the 3D assembly of PS on TiO2 thin film. The scale bar in figure (a) is equal to 100 nm, figure (b) is 200 nm and figure (c) is 1 µm S-5

6 Figure S2. Morphologies showing cracks of TiO2 inverse opal (a) with and (b) without TiO2 thin film layer. S-6

7 Figure S3. Cross sectional SEM image of (a) TiO2, (b) and (c) The scale represented in all figures are equal to 500 nm TiO2 Counts/s rgo/tio Binding energy (ev) S-7 0

8 Figure S4. The total XPS survey spectra of structures Absorbance (a.u.) (b) 522 nm Au NPs Wavelength (nm) Figure S5. (a) TEM image of Au NPs, the scale bar represented in figure is 100 nm and (b) absorption spectra of Au NPs solution showing the SPR absorption at 520 nm S-8

9 Current Density (ma cm -2 ) thin film E/V vs (RHE) Figure S6. Linear sweep voltammetry measurements of thin film under simulated sunlight illumination. Calculation of charge transfer and charge transport efficiency 4-5 The charge transfer and transport efficiencies were estimated measuring the photocurrent in 0.5 M Na 2 SO M Na 2 SO 3 aqueous solution as functions of applied potential by using under AM1.5G simulated solar light illumination. The key assumption for this method is that the oxidation kinetics of Na 2 SO 3 is very fast and its charge transfer efficiency is 100%, so the ratio of photocurrent density measured in Na 2 SO 4 and Na 2 SO 3 gives the charge transfer efficiency (η transfer) for Na 2 SO 4. (Equation 1) The charge transport efficiency (η transport) was further calculated by dividing photocurrent density in Na 2 SO 3 by the total light absorption efficiency (Jmax) which is obtained from integration of light absorption with respect to the AM1.5G solar light spectrum. = ( ) ( ) (1) S-9

10 = ( ) ( ) (2) η transfer Efficiency (%) / wo film / wo film E/V vs RHE Figure S7. Evaluation of charge transfer efficiency of photoelectrodes with and without thin film S-10

11 Current Density (ma.cm -2 ) @rgo(1) E/V vs RHE Figure S8. Linear sweep voltammetry measurements structures with different rgo coating under simulated sunlight illumination, x represented here is the concentration of GO suspension (ca 0.1, 0.3, 0.5, 0.7 and 1 g/l). S-11

12 Table S1. Photoelectrochemical parameters of different samples with different compositions Serial Number Sample Name Synthesis condition Current Density (ma.cm -2 ) at 1.23 V vs RHE 1 As synthesized IO 0.57 (0.1) (0.3) (0.5) (0.7) (1) (3 h) (6 h) (9 h) IO dipping in 0.1 g/l GO suspension IO dipping in 0.3 g/l GO suspension IO dipping in 0.5 g/l GO suspension IO dipping in 0.7 g/l GO suspension IO dipping in 1 g/l GO suspension Citrate capped Au NPs deposition on IO for 3 h time Citrate capped Au NPs deposition on IO for 6 h time Citrate capped Au NPs deposition on IO for 9 h time (6 h) - citrate removed Au NPs deposition on IO for 6 h time and removal of citrate by heat treatment at 300 o C for 1 h 0.85 S-12

13 Current Density (ma cm -2 ) E/V vs RHE @rgo@au Figure S9. Enlarged view of the figure 5a showing the onset potential in light and dark condition S-13

14 Current Density (ma cm -2 ) @rgo@au E/V vs Pt Figure S10. J-V curves under one sun illumination (100 mw/cm 2 ) and two electrode setup ( as working and Pt disk as counter electrode, scan rate of 10 mv/s). S-14

15 Figure S11. Correlation of linear sweep voltammograms of four kinds of photoelectrodes under simulated sunlight illumination with and without chopped light at a scan rate of 10 mv s -1 (a), and S-15

16 Current Density (ma.cm -2 ) removed E/V vs RHE Figure S12. LSV curves with and without citrate capped at optimized condition measured under simulated sunlight illumination. Eliminating the citrate capping (at 300 o C for 2 h) slightly increased the current density of the material. The use of Au nanoparticles without the ligands or the in-situ deposition of AuNP may arise as perspectives of interest in the design of this ternary system with improved catalytic properties. S-16

17 100 η transfer Efficiency (%) E/V Figure S13. Evaluation of charge transfer efficiency of various samples S-17

18 Table S2: Summary and comparison table relating the water splitting properties of various photoanode composites to similar published system Photoanode Structure Current density simulated light (ma.cm -2 ) Current density under visible light Reference Au/TiO2 Au nanoparticle on Nanowire 1.49 ma.cm -2 (0V vs Ag/AgCl) - 6 Au/ Au nanoparticles (NPs) on a TiO2 bi-layer structure with bottomnanorod array layer and top photonic crystal (PC) layer ~1.6 ma cm -2 (1.23 V vs RHE) 0.47 ma cm -2 (1.23 V vs RHE) 7 Au/ Au NPs on nanotube photonic crystals ~2.2 ma cm -2 (1.23 V vs RHE) 0.15 ma cm -2 (1.23 V vs RHE) 8 Au/ Au NPs on inverse opal structures 0.8 ma cm -2 (1.23 V vs RHE) 0.15 ma cm -2 (1.23 V vs RHE) This work rgo/ rgo coated TiO2 inverse opal 0.72 ma cm -2 (1.23 V vs RHE) 0.05 ma cm -2 (1.23 V vs RHE) This work Au/rGO/ Au/reduced graphene oxide/ inverse opal structures 1.29 ma cm -2 (1.23 V vs RHE) 0.3 ma cm - 2 (1.23 V vs RHE) This work Au/rGO/H- Au/reduced graphene oxide/hydrogenated nanotube arrays - ~0.2 ma cm -2 (1.23 V vs RHE) 9 REFERENCES: S-18

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