Supporting Information for Effects of Electrochemical Tailoring of Monolayers on a Catalytic Redox Entity: An ON-OFF phenomenon Regulated by the Surrounding Medium Alagar Raja Kottaichamy 1, Harish Makri Nimbegondi Kotresh 2, Mruthyunjayachari Chattanahalli Devendrachari 1, Ravikumar Thimmappa 1, Bhuneshwar Paswan 1, Omshanker Tiwari 1, Vimanshu Chanda 1, Pramod Gaikwad 1, Musthafa Ottakam Thotiyl 1* 1 Indian Institute of Science Education and Research (IISER) Pune, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India 2 Department of Chemistry, Acharya Institute of Technology, Soldevanahalli, Bangalore-560107, India AUTHOR INFORMATION. Corresponding Author *Address: Indian Institute of Science Education and Research (IISER) Pune, Dr. Homi Bhabha Road, Pashan, Pune, 411008, India. Email: musthafa@iiserpune.ac.in Tel: +91(020)25908261 1
1. Experimental: 1.1.Materials and Methods The chemicals such as Sulfuric Acid, Hydrogen peroxide, Dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), Tetrabutylammonium hexafluorophosphate (TBAPF 6 ), cobalt (II) chloride hexahydrate, Ammonium molybdate, Ammonium chloride, Pottasium hexa ferricyanide, Ruthenium hexamine, KH 2 PO 4 and K 2 HPO 4 were procured from Alfa Aesar India. Cobalt tetraminophthalocyanine (1,8,15,22-tetraaminophthalocyanatocobalt(II) (α TACoPc) and 2,9,16,23 tetraaminophthalocyanatocobalt(ii) (β TACoPc) were synthesized as per the literature. 1,2 The electrochemical experiments were carried out using BASi Epsilon potentiosat. A standard three-electrode cell consisting of 2mm diameter gold disk (or 3mm glassy carbon) as the working electrode, Ag/AgCl (sat.kcl) as the reference electrode, and Pt as counter electrode was used for electrochemical measurements. The potential values are converted to Reversible Hydrogen Electrode (RHE) using the following equation. V RHE = E Ag / AgCl + E o Ag / AgCl + 0.059 X P H All solutions were prepared using Millipore water (18.2 MΩ.cm) and the cell temperature was maintained at 25 C. XPS was acquired using Thermo K-Alpha (Thermo Scientific, East Grinstead, UK) using a monochromatic Al Kα source at 100 W. A Shirley background subtraction was employed for all the XPS spectra. Au 4F XPS spectra was employed to correct the spectral shifts. Impedance was acquired in the frequency range 100 khz to 5 mhz with an ac amplitude of 10 mv at a bias voltage of 450 mv vs. RHE in O 2 saturated solutions. Tafel slope measurements were carried out by steady state polarization in the electron transfer limiting region. Exchange current densities were estimated by extrapolating the linear region to zero overpotential. 1.2 Synthesis of α TACoPc and β TACoPc 1.2.1 Preparation of 2,9,16,23-Tetraaminophthalocyanatocobalt(II) / 1,8,15,22Tetraamino phthalocyanatocobalt(ii): 2
Synthesis of 2,9,16,23-Tetranitro Co Phthalocyanine / 1,8,15,22- Tetranitro Co Phthalocyanine was done according to the literature. 1,2 A 250 ml round bottomed flask was charged with finely grounded mixture of 4-Nitro Phthalimide / 3-Nitro Phthalimide (3.86 g), Urea (6.0 g), ammonium molybdate (0.05 g) as a catalyst, ammonium chloride (0.5 g) and 30 ml of nitrobenzene. The reaction mixture was stirred for 10 minutes to get a homogeneous mixture and then added cobalt (II) chloride hexahydrate (1.2 g) followed by subsequent vigorous stirring at 100 o C for about 30 minutes. Then the temperature was increased to 180 o C at the rate of 10 o C/30minutes, and refluxed the reaction mixture at 180 o C for about 4 hours. The mixture was diluted with toluene (50 ml) and the resulting precipitate was filtered, washed thoroughly with hot water and methanol and finally washed with hexane and then dried in an oven at 80 0 C. 1.2.2 Preparation of 2,9,16,23-Tetraaminophthalocyanatocobalt(II) / 1,8,15,22Tetraamino phthalocyanatocobalt(ii) from 2,9,16,23-Tetranitrophthalocyanatocobalt(II) / 1,8,15,22- Tetranitrophthalocyanatcobalt(II): A 250 ml round bottomed flask was charged with Tetra nitro Cobalt Phthalocyanine (0.75 g), sodium sulphide nonhydrate (3.0 g) and 30 ml of N,N, dimethyl formamide. The reaction mixture was heated to 80 o C for about 2 hours. It was then cooled to room temperature and diluted with ice water (200 ml), and the resulting precipitate was filtered off and washed thoroughly with hot water followed by methanol and finally with hexane. The obtained precipitate was dried in an oven to obtain cobalt 2,9,16,23-tetraamino Cobalt phthalocyanine or 1,8,15,22- tetraamino Cobalt phthalocyanine. 1.3 Assembly of α TACoPC and β TACoPC on Au and GC electrodes Assembly of α and β TACoPc molecules were achieved by the self-assembly process. 6 mm α and β TACoPc solutions were prepared in dimethyl formamide (DMF) by ultrasonication. Au and GC electrodes were first polished by alumina polishing powder and then cleaned electrochemically. The polished Au electrode was electrochemically cleaned between the potential -0.2 to 1.6 V in 0.1M H 2 SO 4 using potentiodynamic cycling until reproducible voltammograms were obtained. Similarly the polished GC electrode electrochemically cleaned between the potential 0 V to 1.1 V. The electrodes were washed with copious amount of distilled water and finally with DMF. These electrodes were then incubated in 6 mm α and β TACoPc solution for 24 hours. The modified electrodes were washed with fresh DMF to remove 3
physisorbed molecules and finally washed with distilled water. Drop casted electrodes were made by sonicating the required amounts (3mg/ml) of the monomers in DMF and then 10µL of the suspension was carefully added on the conducting part of the GC electrode. 1. Surface coverage calculation The molecular coverage for modified α TACoPC and β TACoPC electrodes was calculated based on the charge for the anodic wave of Co II /Co III couple in nitrogen saturated 0.5 M sulphuric acid at a scan rate 20 mv/s (figure S1). 3-5 The following equation was used for the calculation Г = Q / nfa.. (1) Where Γ, is the surface coverage of the molecules in mol / cm 2, Q is the charge taken from the integration of the oxidation wave resulting from the Co II /Co III, n is the number of electrons transferred in the electrochemical process (which is equal to 1 in the present case), F is the Faraday constant and A is the geometrical electrode area. We have estimated the surface coverage (θ) using Amatore s model based on the Nyquist plot of bare and modified electrodes and the plot of Z vs. 1/(ω) 1/2. A ferrocyanide/ferricyanide redox couple was employed to study the blocking behavior of monolayer electrodes. 6,7 Using the following equations 2 and 3 we have estimated the θ values and the values were found to be 0.96 reflecting a monolayer coverage... (2) (3) Where R Bare ct is the charge-transfer resistance measured at the bare electrode, and R SAM ct the charge-transfer resistance under the same conditions at the monolayer-covered electrode. From equation (2) coverage can be estimated based on Rct. Using equation 3, the coverage can be estimated by using a model that is based on the pinhole size, where σ W is the Warburg coefficient calculated from the characterization of the pretreated bare Au, and m the slope of the 4
linear interval observed in the high frequencies region of the Z vs. ω -1/2 function obtained at the modified electrode. Scheme S1: Molecular structure of alpha (α TACoPC) and beta (β TACoPC) 5
Figure S1: Cyclic voltammograms of α TACoPC and β TACoPC on Au and GC electrodes in nitrogen saturated 0.5 M sulphuric acid at different scan rates. (a) α TACoPC on Au (b) α TACoPC on GC (c) β TACoPC on Au (d) β TACoPC on GC electrodes. 6
Figure S2: Plot of log (current) vs. log (scan rate) extracted from figure S1. (a) α TACoPC on Au (b) α TACoPC on GC (c) β TACoPC on Au (d) β TACoPC on GC electrodes. 7
Figure S3: Complex plane impedance plot in 0.1M phosphate buffer, ph=7.2 containing 2 mm ferrocyanide/ferricyanide redox couple at the open circuit voltage (Frequency range 100kHz to 1mHz at 10 mv amplitude) for unmodified Au (a), α TACoPC modified Au (b), β TACoPC modified Au (c). The circuit used for fitting the Nyquist plot is shown in the fourth quadrant. 8
Figure S4: The plot of Z vs. 1/(ω) 1/2 for unmodified and modified Au electrode in 0.1M phosphate buffer, ph=7.2 containing 20 mm ferrocyanide/ferricyanide redox couple. 9
Figure S5: Cyclic voltammograms for central metal ion s redox response before and after molecular connection. α TACoPC (a, b) and β TACoPC (c, d) modified Au and GC electrodes in N 2 saturated phosphate buffer, ph=7.2 at a scan rate of 20 mv/s. 10
Figure S6: Cyclic voltammograms of hexaammine ruthenium (1 mm) on β TACoPC (a) and α TACoPC (b) modified Au electrodes before and after molecular connection. Electrolyte is N 2 saturated 0.1 M H 2 SO 4 and the scan rate is 20 mv/s. 11
Figure S7: Linear sweep voltammograms for unmodified Au and GC electrodes compared with β TACoPC anchored Au and GC electrodes in oxygen saturated 0.1 M phosphate buffer (ph=7.2) at a scan rate 20 mv/s. 12
Figure S8: Linear sweep voltammograms for β TACoPC modified GC electrode at a scan rate of 20 mv/s in oxygen saturated 0.1M phosphate buffer (ph=7.2) showing the effect of hydrogen peroxide. Figure S9: Linear sweep voltammograms α TACoPC and β TACoPC modified Au electrodes in oxygen saturated 0.1M phosphate buffer (ph=7.2) at different scan rates. (a) α TACoPC on Au and (b) β TACoPC on Au. 13
Figure S10: Plot showing the ratio of the overall ORR peaks and the 1 st oxygen reduction peak as a function of scan rate. The corresponding LSVs are provided in figure S9. 14
15
Figure S11: Cyclic voltammograms for 2 mm ferricyanide on β TACoPC and α TACoPC modified Au and GC electrodes at a scan rate of 20 mv/s before and after molecular connection. Electrolyte is N 2 saturated 0.1 M Phosphate Buffer (ph=7.2). Figure S12: Linear sweep voltammograms of β TACoPC and α TACoPC modified Au and GC electrodes before and after molecular connection. Electrolyte is oxygen saturated 0.1 M phosphate buffer (ph=7.2) and the scan rate is 20 mv/s. (a) on Au electrode and (b) on GC electrode. 16
Figure S13: Linear sweep voltammograms of α TACoPc and β TACoPc drop casted GC electrodes before and after electrochemical polymerization. Electrolyte is oxygen saturated 0.1 M phosphate buffer, ph=7.2 and the scan rate is 20 mv/s. 17
Figure S14: Linear sweep voltammograms of α TACoPc and β TACoPc modified Au electrodes before and after molecular connection. Electrolyte is oxygen saturated 0.1 M NaOH and the scan rate is 20 mv/s. (a) β TACoPc and (b) α TACoPc on Au electrode. 18
Figure S15: Cyclic voltammograms of 2 mm ferricyanide on β TACoPc and α TACoPc modified Au electrodes at a scan rate of 20 mv/s before and after molecular connection. Electrolyte is N 2 saturated 0.1 M NaOH. 19
Figure S16: Cyclic voltammograms for central metal ion s redox response before and after molecular connection. Electrolyte is nitrogen saturated 0.1 M NaOH and the scan rate is 20 mv/s. 20
Figure S17: Linear sweep voltammograms of α TACoPc and β TACoPc drop casted GC electrodes before and after electrochemical polymerization. Electrolyte is oxygen saturated 0.1 M H 2 SO 4 (a, b), phosphate buffer, ph=7.2 (c, d) and 0.1M NaOH (e, f). The scan rate is 20 mv/s. 21
Figure S18: Cyclic voltammograms for central metal ion s redox response for drop casted GC electrodes before and after bulk polymerization. α TACoPc (a, c) and β TACoPc (b, d) in N 2 saturated 0.1 M H 2 SO 4 (a, b) and N 2 saturated phosphate buffer, ph=7.2 (c, d) at a scan rate of 20 mv/s. 22
Table S1: Monolayer coverage (θ) values of modified Au electrode using equation (2) and (3) using Amatore s model. θ R IS and θ P IS stands for coverage value estimated based on charge transfer resistance (eq 2) and pinhole size (eq 3) respectively. 23
Table S2: R CT values extracted by fitting the Nyquist plot using the R(C(RW)) electronic circuit model for α TACoPc and β TACoPc modified Au and GC electrodes before and after molecular connection in phosphate buffer. 24
Table S3: Comparative ORR parameters for α TACoPc and β TACoPc modified Au and GC electrodes in phosphate buffer (before and after molecular connection) 25
Table S4: Exchange current density and Tafel slopes for α TACoPc and β TACoPc modified Au and GC electrodes in oxygen saturated 0.1M H 2 SO 4 (before and after molecular connection) 26
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by Electrochemical Impedance Spectroscopy. J. Electroanal. Chem. 2006, 586, 112 121. 28