Supporting Information

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1 Supporting Information Visible-Light Photocatalytic H 2 Production Activity of β-ni(oh) 2 Modified CdS Mesoporous Nano-Heterojunction Networks Ioannis Vamvasakis, Ioannis T. Papadas,, Theocharis Tzanoudakis, Charalampos Drivas, Stelios A. Choulis, Stella Kennou, Gerasimos S. Armatas,* Department of Materials Science and Technology, University of Crete, Heraklion 71003, Greece Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology, Limassol 3041, Cyprus Department of Chemical Engineering, University of Patras, Patra 26504, Greece * garmatas@materials.uoc.gr Supporting tables Table S1. Nickel characteristics and relative percentages of the 10% Ni/CdS sample before (fresh sample) and after 20 h photocatalytic reaction. Sample (10% Ni/CdS) Ni loading a) (wt %) Ni 0 metallic b) (%) Ni 2+ b) as Ni(OH) 2 (%) Atomic ratio Ni/CdS b) Before catalysis After catalysis a) Based on EDS analysis. b) Based on XPS analysis. S1

2 Table S2. Comparison of the photocatalytic H 2 -production activities and quantum yields of different Ni-modified and Pt-loaded CdS-based photocatalysts. Photocatalyst Cocatalyst Loading method Light source Reaction solution Amount catalyst (g) Activity (µmol h -1 g -1 ) H 2 evolution QY (%) Ref. CdS particles NiOx Photodeposition (λ > 400 nm) 30 vol% methanol (V=100 ml) (400 nm) [24] CdS 3D nanowires NiO Hydrothermal 500 W H (Uv-vis) 0.25 M Na 2 SO 3 /0.35 M Na 2 S (V=50 ml) [25] CdS particles Ni Hydrothermal (λ > 400 nm) 50 vol% lactic acid (V=50 ml) [26] CdS nanorods Ni Chemical reduction & photo-induced deposition 1 M (NH 4 ) 2 SO 3 (V=50 ml) [27] CdS nanorods Ni Photodeposition Laser diode (λ = 447 nm) 10 vol% ethanol/5 M NaOH (V=3 ml) (447 nm) [28] CdS nanoparticles Ni Photodeposition Laser diode (λ = 405 nm) 90 vol% ethanol (V=5 ml) ~ (405 nm) [29] CdS nanoparticles Ni Photodeposition 100 vol% 2-propanol (V=5 ml) (447 nm) [30] Cd 0.5 Zn 0.5 S nanorods NiS x a) In-situ formation (λ > 430 nm) 0.25 M Na 2 SO 3 /0.35 M Na 2 S (V=180 ml) (425 nm) [31] CdS nanorods NiS Hydrothermal 0.25 M Na 2 SO 3 /0.35 M Na 2 S (V=80 ml) [32] S2

3 CdS nanowires β-nis Chemical reduction & hydrothermal deposition (λ 420 nm) 20 vol% lactic acid (V=100 ml) (7 o C) (25 o C) 58 (7 o C, 420 nm) 74 (25 o C, 420 nm) [33] CdS particles NiS Hydrothermal 30 vol% lactic acid (V=100 ml) [34] RGO-CdS composites Ni x S In-situ formation 500 W Hg (λ>400nm) 50 vol% ethanol (V=10 ml) (410 nm) [35] CdS nanorods Ni(OH) 2 Precipitation (λ 420 nm) 25 vol% TEOA b) (V=80 ml) [36] CdS/g-C 3 N 4 nanorods Ni(OH) 2 Hydrothermal 0.35 M Na 2 SO 3 /0.25 M Na 2 S (V=80 ml) (450 nm) [37] CdS nanostructures Pt Photodeposition 0.25 M Na 2 SO 3 /0.35 M Na 2 S (V=200 ml) [22] CdS NC assemblies Pt Photodeposition 10 vol% ethanol/5 M NaOH (V=20 ml) [23] CdS NCAs β-ni(oh) 2 Photodeposition (λ 420 nm) 10 vol% ethanol/5 M NaOH (V=20 ml) This work a) Mixture of Cd 0.5 Zn 0.5 S twinned photocatalyst and an unanchored NiS x co-catalyst. b) TEOA: Triethanolamine. S3

4 Calculation of R ct of Ni-CdS NCAs from EIS data The equivalent circuit model R s (Q f /(R d L ad (Q dl /R ct ))) (see inset of Fig. 5b) was used to simulate the EIS curve of the fabricated Ni-CdS NCAs electrodes. R s represent the electrolyte resistance, R ct and Q dl are the charge transfer resistance and the double layer capacitance (C dl ), respectively, and R d and Q f elements account for the defect resistance (associated with the pores, cracks and grain boundaries in the solid film) and material s solid-film capacitance (C f ), respectively. Note that such a circuit model is a good approximation for studying solid electrodes with different degrees of surface roughness and physical non-uniformity. s1,s2 In addition, an inductor (L ad ) to the proposed circuit model, which account for the pseudoinductive behaviour in the high frequency domain, was also necessary for fitting the experimental results. As previously discussed in the literature, such a high-frequency pseudoinductive behaviour is a rather common feature in electrochemical active materials containing heterogeneity or energy disorder (e.g., conducting metal-oxides and nanostructured semiconductors), causing either by disordered charge-carrier relaxation and/or disordered movement of adsorbed redox species at the surface of electrode (pores, cracks, grain boundaries, etc.). s3-s6 Table S3. EIS equivalent circuit fitted parameters of the mesoporous CdS and Ni-CdS (10 and 15 wt % Ni loadings) NCAs catalysts. R s R d C f L ad R ct C dl Sample x 2 (Ω) (Ω) (F) (H) (Ω) (F) CdS NCAs x x x % Ni-CdS x x x % Ni-CdS x x x S4

5 Supporting figures Figure S1. Electrochemical Mott-Schottky plots for the mesoporous CdS NCAs material using Pt wire (black line) and stainless steel (SS316) (red line) as the counter electrodes. Figure S2. Typical EDS spectra of the mesoporous (a) CdS and (b-d) Ni-modified CdS NCAs catalysts. S5

6 Figure S3. XRD pattern of the mesoporous 20% Ni-CdS NCAs. The peaks with (*) correspond to the hexagonal phase of β-ni(oh) 2 according to the JCPDS no Figure S4. High-resolution XPS (a) Cd 3d and (b) S 2p core-level spectra for the 10% Ni-CdS NCAs catalyst before and after 20 h of photocatalytic reaction. S6

7 Figure S5. Nitrogen adsorption (filled cycles) and desorption (open cycles) isotherms at 196 o C for the mesoporous (a) 5% Ni-CdS, (b) 7% Ni-CdS and (c) 15% Ni-CdS NCAs materials. Insets: The corresponding NLDFT pore size distributions calculated from the adsorption branch of isotherms. S7

8 Figure S6. Tauc plots of the mesoporous CdS and Ni-modified CdS NCAs materials. Figure S7. Optical absorption spectrum of the as-prepared β-ni(oh) 2 microparticles, showing the characteristic d-d interband transitions at 380, 670 and 1150 nm, and the steep absorption bellow 320 nm corresponding to the band-gap transition. Inset: the corresponding Tauc plot for direct band gap semiconductor, showing an energy band gap of ~3.9 ev. S8

9 Figure S8. Photocatalytic H 2 evolution rates for 10% Ni-CdS NCAs catalyst using different sacrificial regents, i.e. ethanol (10% v/v) in alkaline (NaOH at ph 14.7 and 10.0) and neutral solutions and methanol (10% v/v) in a 5 M NaOH (ph 14.7) aqueous solution. All photocatalytic reactions were performed as follows: 40 mg of catalyst dispersed in a 20 ml aqueous solution containing the sacrificial reagent; 300-W Xe light radiation with a long-pass cut-off filter allowing λ 420 nm. Figure S9. UV-vis diffuse reflectance spectrum of the 10% Ni-CdS NCAs catalyst and the apparent QYs of H 2 evolution under different incident lights. The error bars refer to the wavelength range of the incident light. Reaction conditions: 40 mg of catalyst, 20 ml aqueous solution containing 5 M NaOH and 10% (v/v) ethanol, under LED light irradiation. The power density of the incident light was 2.55, 14.3, 14.8, 18.8 and 23.4 mw cm -2 for 365, 420, 440, 510 and 620 nm wavelengths, respectively. S9

10 Figure S10. (a) Typical EDS spectrum and (b) N 2 adsorption-desorption isotherms at 196 o C (inset: the corresponding NLDFT pore size distribution) for the 10% Ni-CdS NCAs catalyst retrieved after 20 h of photocatalytic reaction. Figure S11. Time course of photocatalytic H 2 production over four-times reused 10% Ni-CdS NCAs catalyst. An average rate of H 2 evolution at 1.17 mmol h -1 was obtained during 5 h reaction. S10

11 Figure S12. Mott-Schottky plot of the inverse square space-charge capacitance (1/C SC 2 ) as a function of applied voltage (E) relative to the redox potential of Ag/AgCl (3 M KCl) for the as-prepared β- Ni(OH) 2 particles. The negative slope of the corresponding linear fit indicates a p-type conductivity of β-ni(oh) 2. Figure S13. (a) Comparison of OH formation over the mesoporous CdS NCAs (cyan lines) and 10% Ni-CdS (red lines) samples, illustrated by fluorescence (FL) spectra of 2-hydroxyterephthalic acid (HTA). The increasing FL peak at 440 nm indicates the formation of HTA as a product of the reaction of terephthalic acid (TA) with the hydroxyl radicals. The FL spectrum of TA in the presence of 10% Ni-CdS NCAs catalyst in ph 10 aqueous solution after 3 h irradiation is also given for comparison (blue line). (b) Time evolution of the HTA FL intensity at 440 nm for the CdS NCAs and 10% Ni- CdS NCAs samples. All the OH radical formation tests were performed similarly to the photocatalytic H 2 -evolution reactions, but with adding TA instead of ethanol as electron donor. The exact reaction conditions were as follows: 20 mg of catalyst dispersed in 20 ml NaOH aqueous solution (2.5 M, ph 14.0 or ph 10.0) containing 1.2 M terephthalic acid. Before irradiation, the mixture was purged with argon for at least 30 min to remove any dissolved air and then irradiated with a 440 nm LED light. Samples were collected using a syringe (1 ml), diluted with 9 ml DI water and centrifuged to remove the catalyst. The FL spectra were obtained at room temperature on a Jobin- Yvon Horiba FluoroMax-P (SPEX) spectrofluorimeter using 320 nm excitation wavelength. S11

12 References (s1) Cheong, A. K.; Lasia, A.; Lessard, J. Hydrogen Evolution Reaction at Composite Coated Raney Nickel Electrodes in Aqueous and Aqueous Methanolic Solutions. J. Electrochem. Soc. 1993, 140, (s2) Macdonald, J. R. Impendance Spectroscopy. Ann. Biomed. Eng. 1992, 20, (s3) Bisquert, J.; Randriamahazaka, H.; Garcia-Belmonte, G. Inductive Behaviour by Charge-Transfer and Relaxation in Solid-State Electrochemistry. Electrochim. Acta 2005, 51, (s4) Da Silva, L. M.; De Faria, L. A.; Boodts, J. F. C. Electrochemical Impedance Spectroscopic (EIS) Investigation of the Deactivation Mechanism, Surface and Electrocatalytic Properties of Ti/RuO 2 (x)+co 3 O 4 (1 x) Electrodes. J. Electroanal. Chem. 2002, 532, (s5) Da Silva, L. M.; Fernandes, K. C.; De Faria, L. A.; Boodts, J. F. C. Electrochemical Impedance Spectroscopy Study During Accelerated Life Test of Conductive Oxides: Ti/(Ru + Ti + Ce)O 2 -System. Electrochim. Acta 2004, 49, (s6) Harrington, D. A.; Van den Driessche, P. Mechanism and Equivalent Circuits in Electrochemical Impedance Spectroscopy. Electrochim. Acta 2011, 56, S12