Studies of electrodes modified with zeolites and poly(4 nitro 2 phenylenediamine) and their composite 3

ACKNOWLEDGEMENTS I am indebted to my supervisor, Dr. Khoo Soo Beng for his utmost patience, constant guidance and encouragement throughout the course of this project. Special thanks are given to my fellow researchers in the laboratory, Ms. Fathima Shahitha and Chen Fang for their selfless assistance and valuable suggestions given to me during this project. The provision of Research Scholarship from the National University of Singapore is gratefully acknowledged. i CONTENTS Pages Acknowledgements i Table of Contents ii Summary vii List of Figures ix List of Tables xix Chapter I Introduction 1.1 Preparation of chemically modified electrodes 1.1.1 Electrodes modified with monomolecular layers 1.1.1.1 Chemisorption 1.1.1.2 Covalent bonding 1.1.1.3 Hydrophobic Layers 1.1.2 Electrodes modified with multimolecular layers 1.1.2.1 Polymers 6 1.1.2.2 Inorganic films 10 1.1.3 Electrodes modified with spatially defined and heterogeneous layers 11 1.2 Characterization and analysis of chemically modified electrodes 15 1.2.1 Electrochemical methods 15 1.2.2 Spectroscopy and microscopy methods 18 1.2.3 Quartz crystal microbalance 19 1.3 Applications of chemically modified electrodes 1.3.1 Chemical sensors ii 21 21 Chapter II 1.3.2 Energy-producing devices 22 1.3.3 Electrochromic devices 22 1.3.4 Fundamental chemistry 23 1.4 The objectives of the thesis 25 References 28 Studies of zeolite modified electrodes fabricated by electrophoretic 35 deposition 2.1 Introduction 36 2.2 Experimental 39 2.2.1 Reagents 39 2.2.2 Apparatus 40 2.2.3 Procedure 40 2.3 Results and discussion 43 2.3.1 Zeolite modified electrode fabricated by dc voltage EPD 43 2.3.1.1 Effect of the supporting electrolyte concentration 43 2.3.1.2 Effects of dc voltage and time 44 2.3.1.3 Effects of different supporting electrolytes and solvent 48 2.3.2 Zeolite modified electrode fabricated by pulsed voltage EPD 52 2.3.3 Stability and applicability of zeolite modified electrode fabricated by pulsed EPD 2.4 Conclusions 62 References Chapter III 56 63 Electropolymerization of 4-nitro-1,2-phenylenediamine and iii electrochemical studies of poly(4-nitro-1,2-phenylenediamine) films 65 3.1 Introduction 66 3.2 Experimental 68 3.2.1 Reagents 68 3.2.2 Apparatus 68 3.2.3 Procedure 68 3.3 Results and discussion 70 3.3.1 Electropolymerization of 4NoPD 70 3.3.2 Electrochemical characterization of P4NoPD films 74 3.3.2.1 Films formed using different cycles and different supporting electrolytes 74 3.3.2.2 Scan rate studies 76 3.3.2.3 Stability of P4NoPD film 76 3.3.2.4 pH effect 77 3.3.3 Reduction of the nitro-groups of 4NoPD monomer 80 3.3.4 Electrochemical behaviors of the nitro-groups of P4NoPD film Chapter IV 83 3.4 Conclusions 93 References 94 Electrochemical impedance spectroscopic studies of poly(4-nitro 1,2-phenylenediamine) film modified electrodes 97 4.1 Introduction 98 4.2 Experimental 100 4.2.1 Reagents 100 iv 4.2.2 Apparatus 100 4.2.3 Procedure 100 4.3 Results and discussion Chapter V 102 4.3.1 P4NoPD film in 0.50 M H2SO4 102 4.3.2 P4NoPD films formed in different numbers of cycles 109 4.3.3 pH effect 113 4.3.4 Effect of the reduction of nitro-groups of P4NoPD film 114 4.4 Conclusion 122 Reference 123 Studies of electrodes modified with poly(4-nitro-1,2phenylenediamine) / zeolite composite 125 5.1 Introduction 126 5.2 Experimental 128 5.2.1 Reagents 128 5.2.2 Apparatus 128 5.2.3 Procedure 129 5.3 Results and discussion 131 5.3.1 Characterization of the composites in the media (pH=2) 133 5.3.2 Characterization of the composites in the media containing redox active species 137 5.3.2.1 Effect of the amount of zeolites 142 5.3.2.2 pH effect 145 5.3.2.3 Accumulation of Fe(CN)63- 147 5.3.2.4 Effect of redox active species 147 5.4 Conclusions 154 v References 155 Future work 157 List of publications 158 vi SUMMARY In recent years, a number of works have devoted to the preparation, characterization, and electrochemical behavior of chemically modified electrodes (CMEs). In the 1960s, interest arose in the modification of electrode surfaces by covalent attachment of monolayers to electrode surfaces. Electrodes modified with thicker polymeric films and inorganic layers were introduced later. Here we discuss the chemical and physical routes for the preparation of CMEs and the electrochemical and other consequences of this. Early fabrication of zeolite modified electrodes (ZMEs) generally has been plagued by poor reproducibility, lack of mechanical robustness in a stirred solution, and nonideal electrochemical behavior. Therefore, controllable formation of zeolite thin film needs new processing schemes to improve quality and reproducibility. We have shown here the applicability of a novel approach to controllable zeolite deposition on electrode surface by pulsed electrophoretic deposition. While dc electrophoretic deposition also affords control of zeolite deposition, it is less tuneable and convenient as different solutions have to be used and, more importantly, at the expense of electrode surface integrity. In contrast, pulsed EPD is simple, yet more powerful and convenient to give control of the amount of zeolite deposited (from submonolayer to multilayer) by tuning the pulse widths, heights and number of pulses. With regard to stability of ZMEs, it is our view that long term stability, if the ZME is going to be used and reused with washing, storage and exposure to the atmosphere, is not viable without some means of support/anchoring. Conducting polymers, in particular electrodes modified with conducting polymer film, have enjoyed initial success and recently stimulated extensive activities. New types of polymer materials are still being developed to meet the challenges in the vii wide range of areas. Here, we have shown the anodic electropolymerization of 4nitro-1,2-phenylenediamine (4NoPD), which is dependent on the acidity as well as the nature of anions in supporting electrolyte. It is essential to know and understand the properties of poly(4-nitro-1,2-phenylenediamine) (P4NoPD) film (i.e. film conductivity, charge-carrier transfer parameters, capacitance, etc) and the effects of various factors (pH, film thickness, nitro-groups) on these properties. Therefore, we applied the electrochemical studies of P4NoPD film under different conditions. The P4NoPD film was confirmed to have conductivity which was highly electroactive behavior, dependent on film thickness, acidity of the solution and the redox state of the film. Also, film properties (film resistance, low-frequency capacitance, and charge-carrier diffusion coefficient) were influenced by the presence of the electronwithdrawing nitro-groups. Finally, we have shown the applicability of a novel approach to composite P4NoPD and EPD zeolite film on electrode surface. The results proved the dependence of the properties of the composite film on the amount of zeolites. In addition, the electrochemistry of the composite films in the presence of different redox active probes was studied under the present experimental conditions. All of these techniques give the consistent result that the novel properties of composite films can be derived from the successful combination of the characteristics of P4NoPD and zeolite 13X. viii List of Figures Pages Figure 2.1 SEM images of dc voltage EPD fabricated in 3.00 g l-1 suspension of 46 zeolite 13X, 10-5 M KNO3 for 30 min: (a) E=1.9 V; (b) E=2.0 V; (c) E=2.2 V; (d) E=2.5 V. Figure 2.2 CVs (50 mV s-1) in 1.00 mM Fe(CN)63-, 1.00 M KCl: (a) bare 47 GCE, no EPD; (b) EPD in 3.00 g l-1 suspension of zeolite 13X, 10-5 M KNO3 at 2.2 V for 30 min; (c) EPD in 3.00 g l-1 suspension of zeolite 13X, 10-5 M KNO3 by pulsed voltage (E1=0 V, t1=50 s, E2=2 V, t2=200 s, 10 steps). For (b) and (c), the CVs were obtained after removing the deposited zeolite particles by rinsing strongly and copiously with Millipore water. While (b) and (c) are for close to monolayer depositions, similar results were obtained for all levels of depositions studied. Figure 2.3 (a) Typical SEM image of dc EPD fabricated in 3.00 g l-1 49 suspension of zeolite 13X, 10-5 M KNO3 at +4.50 V for 40 s; (b) amplified image of the selected area. Figure 2.4 Pulse Waveform 53 Figure 2.5 SEM images of pulsed EPD fabricated in 3.00 g l-1 suspension 55 of zeolite 13X, 10-5 M KNO3 for 30 min.: (a) E1=0 V, t1=50 s, E2=2 V, t2=190 s; (b) E1=0 V, t1=50 s, E2=2 V, t2=200 s; (c) E1=0 V, t1=20 s, E2=2 V, t2=200 s; (d) E1=0 V, t1=30 s, E2=2 V, t2=190 s; In all cases, the number of steps was 10. ix Figure 2.6 CVs (50 mV s-1) at ZME (E1=0 V, t1=20 s, E2=2 V, t2=200 s, 58 10 steps, multilayer) in 1.00 mM Fe(CN)63-, 0.10 M KNO3 after accumulation in the same solution (stirred): first scan (⎯); second scan (---). The CVs of (a)-(g) are time series, and (h) CV at the bare GCE Figure 3.1 CVs (50 mV s-1) at the Au electrode in 0.50 M H2SO4 solution, 71 containing 4.50 mM 4NoPD, between -0.15 V and +1.10 V. The cycles shown are numbers 1, 10, 15, 20, and 25 (being the last cycle). Figure 3.2 Plot of i vs. t-1/2 for the chronoamperometric response at the 73 Au electrode in 0.50 M H2SO4 containing 0.05 mM 4NoPD. The potential was stepped from 0.4 V to 1.0 V. Figure 3.3 (a) CVs (50 mV s-1) of three P4NPoD films on Au electrodes 75 (electropolymerizations were performed in 0.50 M H2SO4 containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution: 25cycles film (―); 40-cycles film (----); 70-cycles film (····). (b) CVs (50 mV s-1) in 0.50 M H2SO4 for three P4NPoD films, electropolymerized in different H2SO4-Na2SO4 solutions containing 4.50 mM 4NoPD on Au electrodes: 0.10 M H2SO40.40 M Na2SO4 (―); 0.20 M H2SO4-0.30 M Na2SO4 (----); 0.50 M H2SO4 (····). Figure 3.4 Plots of anodic peak currents ip.a of P4NoPD films on Au electrodes (25-cycles, fabricated in 4.50 mM 4NoPD in 0.50 M H2SO4) under different time and storage regimes: x 78 (a) continuous cycling in 0.50 M H2SO4; (b) in between CVs in 0.50 M H2SO4, electrode was stored in 0.50 M H2SO4; (c) in between CVs in 0.50 M H2SO4, electrode was stored in Millipore water. In all cases, the scan rate was 50 mV s-1. Figure 3.5 Plots of anodic peak potential Ep.a (mV) of P4NoPD film at 79 different pH values, scan rate: 50 mV s-1 in sulfate solution (¡) and in phosphate buffer (c). The ion strength of these solutions is ca. 0.2. Figure 3.6 CVs of the reduction of the nitro-groups of 4.50 mM 4NoPD monomer 84 in 0.50 M H2SO4at the GCE for successive cycles: first cycle (―); second cycle (----); third cycle (····). The scan rate was 50 mV s-1. Figure 3.7 CVs of the reduction of nitro-groups of P4NoPD film on GCE 85 (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4 for successive cycles: first cycle (―); second cycle (----); third cycle (····). The scan rate was 50 mV s-1. Figure 3.8 Redox peaks for the P4NoPD film (25 cycles in 4.50 mM 87 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4; comparison of fresh film (―) with a film in which nitro-groups had been reduced in 0.150 M H2SO4 (see Figure 3.7) (----). CVs were obtained at 50 mV s-1. Figure 3.9 (a) CVs (50 mV s-1) of the P4NoPD film (25 cycles in 4.50 mM xi 88 4NoPD in 0.50 M H2SO4) on GCE in: 0.10 M NaOH (―); 1.00 M NaOH (----). (b) CVs (50 mV s-1) of the P4NoPD film (as above) on GCE in 0.50 M H2SO4, comparison of fresh film (―) and film after reduction of nitro-groups in 1.00 M NaOH (----). Figure 3.10 (a) CV (50 mV s-1) of P4NoPD film (25 cycles in 4.50 mM 91 4NoPD in 0.50 M H2SO4) on GCE in 0.10 M Na2SO4 (pH = 5.41): first cycle (―); second cycle (----); (3) third cycle (····). (b) CVs (50 mV s-1) of P4NoPD films (as above) on GCE in the sulfate media of different pH for first cycle: pH = 0.30 (―); pH= 5.41 (-·-·-·-); pH = 7.90 (----); pH = 11.00 (····). Figure 4.1 The impedance spectra of P4NoPD film (formed with 25 103 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) on GCE in 0.50 M H2SO4: (a)-0.20 V; (b) 0.00 V; (c) 0.20 V. A mV amplitude sine wave at the frequency of 20K was used as perturbation signal. Figure 4.2 The equivalent circuit for the polymer modified electrode: 104 (a) simple model; (b) modified model [23]. Ru is the ohmic resistance of the electrolyte and the surface layer, Cdl is the double-layer capacitance, Rct is the charge transfer resistance, ZD is the diffusion impedance, Co is a constant capacitance and CPE is a constant phase element. Figure 4.3 Comparison of the measured impedance spectra (points) xii 107 obtained from Figure 4.1(a) with simulated curves based on the model in Figure 4.2(b) (solid line): (a) Nyquist plot; (b) Bode plot. The values used for the simulations: Ru (5.2 ohm); Cdl (2.1 µF); Rct (18 ohm); Co (7.8 mF); CPE, Zcpe = (1/ 6.2 × 10-4) (jω) –0.31; ZD = (510/(jω)0.5) coth(2 × 10-2 (jω)0.5) Figure 4.4 (a) Plot of film resistance (Rp) for three P4NPoD films on GCEs 111-112 (formed in 0.50 M H2SO4 containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution. (b) Plot of the charge transport diffusion coefficient (D) for three P4NPoD films on GCEs (formed in 0.50 M H2SO4 containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution. (c) Plot of low-frequency capacitance (CL) for three P4NPoD films on GCEs (formed in 0.50 M H2SO4 containing 4.50 mM 4NoPD) in 0.50 M H2SO4 solution. For all these diagrams, a mV amplitude sine wave at the frequency of 20K was used for resistance measurements: 25cycles film (♦); 40-cycles film (•); 70-cycles film (c). Figure 4.5 (a) Plot of the low-frequency capacitance (CL) for P4NoPD film 115-116 on GCE (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in different H2SO4-Na2SO4 solutions. (b) Plot of film resistance (Rp) for P4NoPD film on GCE (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in different H2SO4-Na2SO4 solutions on GCE. For resistance xiii measurements, a mV amplitude sine wave at the frequency of 20K was used. (c) Plot of the charge transport diffusion coefficient (D) for P4NoPD film on GCE (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in different H2SO4-Na2SO4 solutions. For all above diagrams: 0.01 M H2SO4-0.49 M Na2SO4 (c) (pH=4.0); 0.10 M H2SO4-0.40 M Na2SO4 (•) (pH=1.7); 0.40 M H2SO4-0.10 M Na2SO4 H2SO4 (♦) (pH=0.8). Figure 4.6 Bode plots for P4NoPD film (25 cycles in 4.50 mM 4NoPD 117 in 0.50 M H2SO4) in 0.50 M H2SO4 at 0.00 V. Comparison of fresh film (a) with a film (b) in which nitrogroups had been reduced in 0.50 M H2SO4 Figure 4.7 (a) Plot of the low-frequency capacitance (CL) for the P4NoPD film (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4. (b) Plot of film resistance (Rp) for the P4NoPD film (25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4. For resistance measurements, a mV amplitude sine wave at the frequency of 20 kHz was used. (c) Plot of the charge transport diffusion coefficient (D) for the P4NoPD film (25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4. Comparison of fresh film (•) with a film (▲) in which nitrogroups had been reduced in 0.50 M H2SO4 for diagrams. xiv 119-120 Figure 5.1 (a) SEM images of pulsed EPD fabricated in 3.00 g l-1 132 suspension of zeolite 13X, 10-5 M KNO3 for 30 min: (i) submonolayer, E1=0 V, t1=50 s, E2=2 V, t2=200 s; (ii) monolayer, E1=0 V, t1=20s, E2=2 V, t2=200 s; (iii) multiplayer, E1=0 V, t1=30 s, E2=2 V, t2=190 s. In all cases, the number of steps was 10. (b) SEM images of the composite films: (i) submonolayer coated by P4NoPD; (ii) monolayer coated by P4NoPD; (iii) multilayer coated by P4NoPD. P4NoPD films were formed on the freshly prepared EPD through continuous potential cycling between –0.15 V and +1.10 V at 20 mV s-1 for 50 cycles in 0.50 M HNO3 (pH = 2.00, adjust pH with KOH) containing 4.00 mM 4NoPD. Figure 5.2 CVs (50 mV s-1) of the P4NoPD film (⎯) and the composite 134 films in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3). The P4NoPD film was prepared in solution containing 4.50 mM 4NoPD, 0.50 M HNO3 (pH = 2.00, adjusted by KOH). In all above diagrams: submonolayer zeolite EPD coated by P4NoPD film (----); monolayer zeolite EPD coated by P4NoPD film (⋅⋅⋅⋅⋅); multilayer zeolite EPD coated by P4NoPD film (-⋅-⋅-⋅-). Figure 5.3 CVs (50 mV s-1) of reduction of nitro-groups of the P4NoPD film (⎯) and the composite films (Figure 5.2) in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3). In all above diagrams: submonolayer zeolite EPD coated by P4NoPD film (----); xv 136 monolayer zeolite EPD coated by P4NoPD film (⋅⋅⋅⋅⋅); multilayer zeolite EPD coated by P4NoPD film (-⋅-⋅-⋅-). Figure 5.4 Resistance data a of P4NoPD film b (♦) and the composite film c (▲) 138 in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3). a resistance obtained using a mV amplitude sine wave at the frequency of 20K. b the P4NoPD film was prepared in solution containing 4.00 mM 4NoPD and 0.50 M HNO3 (pH = 2.00, adjusted by KOH). c the composite film was prepared by multilayer zeolite EPD coated by P4NoPD film. Figure 5.5 CVs of the composite film (multilayer zeolite EPD coated 140 by P4NoPD film) in contact with 0.10 M KNO3 (pH = 2.00) (⎯) and 1.00 mM Fe(CN)63-, 0.10 M KNO3 (pH = 2.00) (----). Figure 5.6 Typical impedance diagrams of the composite film (multilayer 141 zeolite EPD coated by P4NoPD film) in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3) (o) and 0.10 M KNO3 (pH = 2.00) containing 1.00 mM Fe(CN)63-, (+): (a) -0.35V; (b) -0.20V; (c) 0.05V. Figure 5.7 CVs (50 mV s-1) of bare GC electrode (a), monolayer zeolite EPD (b), pure P4NoPD film (c), and the composite films: (d) submonolayer zeolite EPD coated by P4NoPD film; (e) monolayer zeolite EPD coated by P4NoPD film; (f) multilayer zeolite EPD coated by P4NoPD film. The solution is 0.10 M KNO3 (pH = 2.00, adjusted by HNO3) containing 1.00 mM Fe(CN)63-. xvi 143-144 Figure 5.8 CVs (50 mV s-1) of the composite film (multilayer zeolite 146 EPD coated by P4NoPD film) in various pH of 1.00 mM Fe(CN)63-, 0.10 M KNO3, and pH values were adjusted by HNO3: (a) pH = 2.00; (b) pH = 3.00; (c) pH = 4.80. Figure 5.9 (a) CVs (50 mV s-1) of the composite film (multilayer zeolite 148 EPD coated by P4NoPD film) in 1.00 mM Fe(CN)63-, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), after the composite film was kept at –0.40 V for different periods prior to scanning: 10 s (····); 100 s (----); 1800 s (⎯). (b) Plot (50 mV s-1) of the anodic peak charges Qp1,a of the composite film (multilayer zeolite EPD coated by P4NoPD film) in 1.0 mM Fe(CN)63-, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), after the composite film was kept at – 0.40 V for different periods prior to scanning. Figure 5.10 CVs (50 mV s-1) of the composite film (multilayer zeolite 149 EPD coated by P4NoPD film) in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), the electrolyte solution, containing: (a) 1.00 mM Fe(CN)63-; (b) 1.00 mM Ru(NH3)63+; (c) 1.00 mM hydroquinone; In all curves, solid line is on the composite film, and dash line is on the bare GCE. Figure 5.11 Plots of anodic peak potential separation ∆Ep=Ep1,a – Ep1,c (mV) of HQ/Q versus scan rate v (mV s-1) on the bare GCE (•) and the composite film (multilayer zeolite EPD coated by P4NoPD film) (▲) in 0.10 M KNO3 (pH = 2.00, adjusted by HNO3) containing 1.00 mM HQ. xvii 151 Figure 5.12 CVs (50 mV s-1) of the composite films (multilayer zeolite EPD coated by P4NoPD film) in 1.00 mM HQ, 0.10 M KNO3 (pH = 2.00, adjusted by HNO3), after the composite film was kept at –0.40 V for different periods prior to scanning: 10 s (····); 100 s (----); 1800 s (⎯). xviii 152 List of Tables Pages Table 2.1 Solution/suspension parameters and their effects on dc EPD 45 Table 2.2 Effect of different supporting electrolytes on dc EPD 50 Table 2.3 Cathodic and anodic charges from CVs (50 mV s-1) of bare GCE 60 and ZME* in 1.00 mM Cu2+, 0.10 M KNO3 after the preconcentrations for different periods in the same solution. Table 3.1 Peak potentials and currents for CVs of P4NoPD film* on Au 81 electrode in sulfate solutions of different pH values. * electropolymerized in 0.50 M H2SO4 containing 4.50 mM 4NoPD with 25 cycles Table 3.2 Resistance data a of P4NoPD film b on Au electrode at the peak 82 potentials of their respective CVs in sulfate solutions of different pH. a resistance obtained using a mV amplitude sine wave at the frequency of 20K. b electropolymerized in 0.50 M H2SO4 containing 4.50 mM 4NoPD with 25 cycles. Table 4.1 Impedance data for the P4NoPD filma,b 110 a electropolymerization in 4.50 mM 4NoPD in 0.50 M H2SO4 for 25 cycles b data obtained from Figure 4.1, obtained from the respective modified electrode in 0.50 M H2SO4 xix [...]... frequency of 20 kHz was used (c) Plot of the charge transport diffusion coefficient (D) for the P4NoPD film (25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4 Comparison of fresh film (•) with a film (▲) in which nitrogroups had been reduced in 0.50 M H2SO4 for diagrams xiv 119- 120 Figure 5.1 (a) SEM images of pulsed EPD fabricated in 3. 00 g l-1 1 32 suspension of zeolite 13X, 10-5 M KNO3 for 30 ... film) in contact with 0.10 M KNO3 (pH = 2. 00) (⎯) and 1.00 mM Fe(CN) 63- , 0.10 M KNO3 (pH = 2. 00) ( ) Figure 5.6 Typical impedance diagrams of the composite film (multilayer 141 zeolite EPD coated by P4NoPD film) in 0.10 M KNO3 (pH = 2. 00, adjusted by HNO3) (o) and 0.10 M KNO3 (pH = 2. 00) containing 1.00 mM Fe(CN) 63- , (+): (a) -0 .35 V; (b) -0 .20 V; (c) 0.05V Figure 5.7 CVs (50 mV s-1) of bare GC electrode... solutions is ca 0 .2 Figure 3. 6 CVs of the reduction of the nitro- groups of 4.50 mM 4NoPD monomer 84 in 0.50 M H2SO4at the GCE for 3 successive cycles: first cycle (―); second cycle ( ); third cycle (····) The scan rate was 50 mV s-1 Figure 3. 7 CVs of the reduction of nitro- groups of P4NoPD film on GCE 85 (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4 for 3 successive cycles:... fabricated in 3. 00 g l-1 1 32 suspension of zeolite 13X, 10-5 M KNO3 for 30 min: (i) submonolayer, E1=0 V, t1=50 s, E2 =2 V, t2 =20 0 s; (ii) monolayer, E1=0 V, t1 =20 s, E2 =2 V, t2 =20 0 s; (iii) multiplayer, E1=0 V, t1 =30 s, E2 =2 V, t2=190 s In all cases, the number of steps was 10 (b) SEM images of the composite films: (i) submonolayer coated by P4NoPD; (ii) monolayer coated by P4NoPD; (iii) multilayer coated... frequency of 20 K was used (c) Plot of the charge transport diffusion coefficient (D) for P4NoPD film on GCE (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in different H2SO4-Na2SO4 solutions For all above diagrams: 0.01 M H2SO4-0.49 M Na2SO4 ( ) (pH=4.0); 0.10 M H2SO4-0.40 M Na2SO4 (•) (pH=1.7); 0.40 M H2SO4-0.10 M Na2SO4 H2SO4 (♦) (pH=0.8) Figure 4.6 Bode plots for P4NoPD film (25 cycles... Table 2. 3 Cathodic and anodic charges from CVs (50 mV s-1) of bare GCE 60 and ZME* in 1.00 mM Cu2+, 0.10 M KNO3 after the preconcentrations for different periods in the same solution Table 3. 1 Peak potentials and currents for CVs of P4NoPD film* on Au 81 electrode in sulfate solutions of different pH values * electropolymerized in 0.50 M H2SO4 containing 4.50 mM 4NoPD with 25 cycles Table 3 .2 Resistance... 0.50 M H2SO4) in 0.50 M H2SO4 at 0.00 V Comparison of fresh film (a) with a film (b) in which nitrogroups had been reduced in 0.50 M H2SO4 Figure 4.7 (a) Plot of the low-frequency capacitance (CL) for the P4NoPD film (formed with 25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4 (b) Plot of film resistance (Rp) for the P4NoPD film (25 cycles in 4.50 mM 4NoPD in 0.50 M H2SO4) in 0.50 M H2SO4... Figure 5. 12 CVs (50 mV s-1) of the composite films (multilayer zeolite EPD coated by P4NoPD film) in 1.00 mM HQ, 0.10 M KNO3 (pH = 2. 00, adjusted by HNO3), after the composite film was kept at –0.40 V for different periods prior to scanning: 10 s (····); 100 s ( ); 1800 s (⎯) xviii 1 52 List of Tables Pages Table 2. 1 Solution/suspension parameters and their effects on dc EPD 45 Table 2. 2 Effect of different... continuous potential cycling between –0.15 V and +1.10 V at 20 mV s-1 for 50 cycles in 0.50 M HNO3 (pH = 2. 00, adjust pH with KOH) containing 4.00 mM 4NoPD Figure 5 .2 CVs (50 mV s-1) of the P4NoPD film (⎯) and the composite 134 films in 0.10 M KNO3 (pH = 2. 00, adjusted by HNO3) The P4NoPD film was prepared in solution containing 4.50 mM 4NoPD, 0.50 M HNO3 (pH = 2. 00, adjusted by KOH) In all above diagrams:... Figure 5 .3 CVs (50 mV s-1) of reduction of nitro- groups of the P4NoPD film (⎯) and the composite films (Figure 5 .2) in 0.10 M KNO3 (pH = 2. 00, adjusted by HNO3) In all above diagrams: submonolayer zeolite EPD coated by P4NoPD film ( ); xv 136 monolayer zeolite EPD coated by P4NoPD film (⋅⋅⋅⋅⋅); multilayer zeolite EPD coated by P4NoPD film (-⋅-⋅-⋅-) Figure 5.4 Resistance data a of P4NoPD film b (♦) and . 3. 3 .2. 2 Scan rate studies 76 3. 3 .2. 3 Stability of P4NoPD film 76 3. 3 .2. 4 pH effect 77 3. 3 .3 Reduction of the nitro- groups of 4NoPD monomer 80 3. 3.4 Electrochemical behaviors of the nitro- groups. numbers of cycles 109 4 .3. 3 pH effect 1 13 4 .3. 4 Effect of the reduction of nitro- groups of P4NoPD film 114 4.4 Conclusion 122 Reference 1 23 Chapter V Studies of electrodes modified with poly(4- nitro- 1 ,2- . 4 -nitro- 1 ,2- phenylenediamine and iii electrochemical studies of poly(4- nitro- 1 ,2- phenylenediamine) films 65 3. 1 Introduction 66 3 .2 Experimental 68 3 .2. 1 Reagents 68 3 .2. 2 Apparatus 68 3 .2. 3 Procedure