Inhibiting Oxidation of Cu (111)

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Randolph Preston Cannon
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1 Ethylenediamine Promotes Cu Nanowire Growth by Inhibiting Oxidation of Cu (111) Myung Jun Kim, 1 Patrick F. Flowers, 1 Ian E. Stewart, 1 Shengrong Ye, 1 Seungyeon Baek, 2 Jae Jeong Kim, 2 and Benjamin J. Wiley 1* 1 Department of Chemistry, Duke University, 124 Science Drive, Box 90354, Durham, North Carolina 27708, United States. 2 School of Chemical and Biological Engineering, Seoul National University, Seoul , Republic of Korea.

2 Supplementary experimental method and calculations Cu nanowire synthesis The Cu nanowire synthesis procedure was based on previously published methods. 1 Prior to the synthesis, the glassware and magnetic stir bar were cleaned with HNO 3 and de-ionized water. The following three aqueous solutions were sequentially added to a 50 ml round bottom flask equipped with a magnetic stir bar: (1) 20 ml of 15 M NaOH (Acros Organics, 98%), (2) 1 ml of 0.1 M Cu(NO 3 ) 2 (Sigma-Aldrich, 98%), and (3) 0.1 ml of ethylenediamine (EDA, Acros Organics, 99%). These solutions were prepared using de-ionized water (>1 MΩ). The flask was then placed in a 70 C water bath with stirring at 250 rpm for 5 min. The temperature of the water bath was controlled by a Fisher Scientific Isotemp hotplate to within 0.1 C. Hydrazine solution (10.5 μl, N 2 H 4, Aldrich, 35 wt.% in H 2 O) was added, and the stirring speed was increased to over 550 rpm. The final concentrations of Cu(NO 3 ) 2, EDA, and N 2 H 4 were 4.74 mm, 70 mm, and 5.5 mm, respectively. After 2.5 min, bubbles formed due to N 2 H 4 oxidation and the solution became transparent. If bubbles remained in the solution, an increase in the stirring speed aided their removal. The stir bar was then quickly removed and the solution was kept at 70 C for min. A glass stopper was used on the round bottom flask during the entire reaction except when N 2 H 4 was added and the stir bar was removed. The synthesized Cu nanowires were washed by 1 wt% N,N-diethylhydroxylamine (DEHA, TCI, 98%), 3 wt% polyvinylypyrrolidone (PVP, Mw=10,000, Sigma-Aldrich) aqueous solution three times and were stored in a 3 wt% aqueous DEHA solution. A small amount of Cu nanowire solution was dropped on a small piece of Si wafer and dried under N 2. Images of Cu nanowires were taken by a field emission scanning S1

3 electron microscope (FESEM, FEI XL30 SEM-FEG), and their average diameter was obtained from more than 100 nanowires. Electrochemical measurements Electrochemical measurements were carried out with a three-electrode system consisting of Cu disk electrodes (polycrystalline, Cu(100), and Cu(111)) as the working electrode, Pt wire (99.95%, 0.8 mm diameter, 5 cm length, the exposed area to the electrolyte = 0.50 cm 2, Alfa Aesar) as the counter electrode, and a Hg/HgO (1 M NaOH) electrode as the reference electrode. The polycrystalline Cu electrode (3 mm diameter, geometric area = cm 2 ) and Hg/HgO reference electrode (CHI152) were purchased from Bio-Logic Science Instruments and CH Instruments, Inc, respectively. Before conducting the measurements, the surface of the polycrystalline Cu electrode was sequentially polished with a 1200 grit Carbimet disk, 1.0 μm and 0.3 μm alumina powders with Nylon polishing pads, and 0.05 μm alumina powder with a Microcloth polishing pad (CH Instruments, Inc.) to minimize the surface roughness and obtain a reproducible surface. Between each polishing step, the electrode surface was rinsed by deionized water. The surface roughness of the polycrystalline Cu electrode was measured with an atomic force microscope (AFM, Digital Instruments Dimension 3100) after the mechanical polishing to have a root-mean-square (RMS) roughness of 11 nm (± 3 nm). After mechanically polishing Cu single crystal electrodes in the same manner, additional electrochemical polishing was performed in concentrated phosphoric acid solution by applying 1.6 V (vs. Pt wire) to minimize defects on the surface. 2 The RMS roughness of single crystal electrodes after the electrochemical polishing was measured by AFM to be below 4.9 nm (± 1 nm). After the polishing processes, a mirror-like shiny surface without any scratches was obtained. The S2

4 pretreated electrodes were stored in the air while heating the electrolyte for 5 min. All electrochemical analyses were performed with a potentiostat (CHI600D, CH Instruments, Inc.). Cu(NO 3 ) 2, EDA, and N 2 H 4 were sequentially added into a 15 M NaOH solution at room temperature. NaOH and Cu(NO 3 ) 2 aqueous solutions were prepared using de-ionized water (>1 MΩ). Between the additions, the solution was vortexed for over 30 s, and the mixture was added to a glass cell and placed in a 70 C water bath for 5 min. The electrodes were then dipped into the electrolyte and the electrochemical measurement was immediately started. The temperature of the electrolyte was kept at 70 C during the electrochemical measurements. Linear sweep voltammetry for N 2 H 4 oxidation was performed by sweeping a potential from -1.3 V to -0.5 V with respect to a Hg/HgO reference electrode. Linear sweep voltammetry for the reduction reaction was carried out by sweeping a potential from -0.5 V to -1.3 V (vs. Hg/HgO). The scan rate was fixed at 20 mv/s. Preparation of (111) and (100) single crystal electrodes Cylindrical (111) and (100) Cu single crystals ( %, <2 ) having a diameter of 3 mm and a height of 2 mm were purchased from Princeton Scientific Corp. The Laue diffraction patterns provided by the vendor are shown in Figure S5. To cover the side of the single crystals, polyether ether ketone (PEEK) cylinders (6 mm diameter, 2mm height) with a hole at the center (3 mm diameter) were fabricated, and the single crystals were inserted into the hollow PEEK cylinders. On the back side of the single crystals, Cu wire was attached with conductive silver epoxy. Finally, the exposed back side of crystals, the conductive silver epoxy, and Cu wire were covered by non-conductive and chemical-resistive epoxy to prevent contact with the electrolyte. S3

5 Electrochemical quartz crystal microbalance (EQCM) The EQCM consisted of a potentiostat (263A, EG&G) and a quartz crystal microbalance (QCM, QCA917, EG&G) to record the changes in the potential and frequency of the electrode at the same time. Hg/HgO (1 M NaOH) electrode acted as both a reference and counter electrode. The working electrode was a polycrystalline Cu film on a 9 MHz AT-cut quartz crystal (QA- A9M-Cu(M), Seiko EG&G). The geometric area of the Cu film was cm 2. Prior to the electrochemical measurement, the surface oxide of the Cu film was removed by applying ma/cm 2 of current density for 300 s in a 0.1 M H 2 SO 4 aqueous solution with a saturated calomel electrode (SCE) as a reference electrode and a Pt wire as a counter electrode. The corresponding potential response during this step is shown in Figure S6. The change in electrode mass (g) can be calculated from the change in the frequency of the quartz crystal resonance using the Sauerbrey equation: mass = -Δf A μ q ρ q 2(f 0 ) 2 where Δf is the change in the resonant frequency measured by the quartz crystal microbalance, A is the active surface area of Cu (0.196 cm 2 ), μ q is the Shear modulus of the quartz crystal (2.947 X g/cm s 2 ), ρ q is the density of quartz (2.65 g/cm 3 ), and f 0 is an initial resonant frequency (9.00 MHz). 3 From the mass change, the current density of Cu(OH) 2 - reduction (ma/cm 2 ) was calculated using the following equation, S4

6 i Cu(OH)2 - = Δmass nf M Cu t A where n is the number of electrons required for a reduction reaction (1, Cu(OH) e - Cu 0 + 2OH - ), F is the Faraday constant (96485 C/mol), M Cu is the atomic mass of Cu ( g/mol), and t is time. The calculation of the coulombic efficiency of Cu(OH) 2 - reduction The coulombic efficiency (CE) of Cu(OH) 2 - reduction can be defined as the ratio of the current of Cu(OH) 2 - reduction to the total current: i Cu(OH)2 - CE - Cu(OH)2 = 100 (%) i - Cu(OH)2 +i Cu oxide where i Cu(OH)2 - is the current for Cu(OH) 2 - reduction and i Cu oxide is the current for Cu oxide reduction. To obtain these reduction currents, we first measured the reaction potentials in 15 M NaOH solutions containing either (1) 70 mm EDA and 5.5 mm N 2 H 4 (red circles, Figures 5a and 5b) or (2) 70 mm EDA, 4.74 mm Cu(NO 3 ) 2, and 5.5 mm N 2 H 4 (blue squares, Figures 5a and 5b). We then used I-V curves for N 2 H 4 oxidation (Figure 4a, 70 mm EDA) to convert the reaction potential to an oxidation current of N 2 H 4. For solution (1), the oxidation current of N 2 H 4 is equivalent to i Cu oxide. For solution (2), the oxidation current of N 2 H 4 is equivalent to i - Cu(OH)2 +i Cu oxide. The reduction of surface oxide caused small negative currents in the linear sweep voltammograms in Figure 4a. After multiple scan of linear sweep voltammetry, zero S5

7 current was observed at -0.9 V, indicating the current from N 2 H 4 oxidation was zero at this potential (see Figure S7). However, multiple LSV scans to remove surface oxide will oxidize N 2 H 4, thus, changing its concentration. To avoid this problem, we shifted the entire scan to set the current at -0.9 V to zero for both the Cu(111) and Cu(100) crystals so as to remove the contribution to the current that comes from reduction of surface oxide. S6

8 Supplementary figures Figure S1. (a) I-V curves for N 2 H 4 oxidation on a polycrystalline Cu electrode. (b) The peak current density from N 2 H 4 oxidation versus the concentration of N 2 H 4. S7

9 Figure S2. I-V curves on a polycrystalline Cu electrode (a) in a 15 M NaOH solution and (b) in 15 M NaOH solutions with 4.74 mm Cu(NO 3 ) 2, 4.74 mm CuSO 4, and 9.48 mm NaNO 3. S8

10 Figure S3. Chronoamperometry for N 2 H 4 oxidation on a Cu(111) surface at the E m measured from Figure 6a. The concentration of N 2 H 4 was 5.5 mm [Cu(NO 3 ) 2 ] because N 2 H 4 was consumed by Cu(OH) e - Cu(OH) OH - during the preparation of the solutions for Figure 6a. S9

11 Figure S4. Schematic diagram of the I-V curves for charge transfer-limited Cu(OH) 2 - reduction and diffusion-limited N 2 H 4 oxidation as a function of the concentration of Cu(OH) 2 -. S10

$The Laue diffraction patterns of Cu(100) and Cu(111) single crystals.$

12 Figure S5. The Laue diffraction patterns of Cu(100) and Cu(111) single crystals. The patterns were provided by Princeton Scientific Corp. S11

13 Figure S6. The potential response during the oxide removal from the Cu electrode used for EQCM. S12

14 Figure S7. I-V curves for N 2 H 4 oxidation on a Cu(111) single crystal electrode in 15 M NaOH solution containing 70 mm EDA and 5.5 mm N 2 H 4. S13

15 Reference (1) Rathmell, A. R.; Wiley, B. J. Adv. Mater. 2011, 23, (2011). (2) Moffat, T. P.; Ou Yang, L.-Y. J. Electrochem. Soc. 2010, 157, D228-D241. (3) Sauerbrey, G. Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. Z. Physik 1959, 155, S14

Supporting Information

Supporting Information Open Circuit (Mixed) Potential Changes Allow the Observation of Single Metal Nanoparticle Collisions with an Ultramicroelectrode Hongjun Zhou, Jun Hui Park, Fu-Ren F. Fan, Allen