Underpotential - Overpotential Phase Transformation Phenomena in Copper Deposition Processes on Glassy Carbon
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(1)
where zF is the molar charge of the electrodepositing ions, c is the bulk concentration , D is the diffusion coefficient and N nuclear number density, k = (8ncM/p)" 2, M and p are the molecular weight and the density of the deposited ions, respectively. For progressive nucleation, the dependence of current density on time is described by the following equation: I = zFc (D/t)1"2 [1 - exp (-J., tk'Dt 2/2)], (2) where k' = 4/3 (87rcM/p)1 /2 and J., is a steady-state nucleation rate. The given model allows to determine the nucleation mechanism; evaluate the diffusion coefficient of ions; calculate steady-state nucleation rate, saturation nuclear number density, N., for progressive nucleation, and the nuclei number density for instantaneous nucleation by the following equations, respectively: 339 Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
2 J't = 4,6733/(.tk'Dt m)
(3)
N, = (JJ,/2k'D)" 2
(4)
N = 1,2564/(nkDtm)
(5)
where tm is a time corresponding to the maximum of the current transient. The purpose of this work is to study the initial stages of copper electrocrystallization on glassy carbon and the influence of the substrate surface state on nuclei rate by the method of potentiostatic current transients and pulse experiments. EXPERIMENT The experiments were carried out from a solution of 1 M CuSO 4 in 0.5 M H2S0 4 which was cleared from various inorganic and organic impurities by commonly used methods. Glassy carbon was used as a cathode which surface was abraded and polished by MgO to a mirror finish until all the visual under the microscope (x 200) defects were removed. Reference electrode was a copper electrode. To bring the glassy carbon surface to its standard state, it was first anodic polarized at 0.56 V for 120 s and then was held at an equilibrium Cu2 ÷/Cu potential for 50 s. To receive current transients, potentiostatic pulse of the overpotential (1 = -0.10 - - 0.40 V) was applied on the cathode recording It-curves. The preliminary surface treatment provided a good reproduction of current transients. To evaluate the influence of underpotential range on kinetics of copper nucleation, the surface state of glassy carbon substrate was varied by linearly scanned potential from E0 = 0.56 V to E1 (0 V :s E1 :s 0.40 V) stabilizing its surface at E1 during a waiting time of 100 s. After this potentiostatic current transients were received. RESULTS
2 10-2 I,A m-
20.8 10.4
0
3
6
t,s
Fig. 1. Potentiostatic current transients for the electrodeposition of copper on glassy carbon from a solution of 1 M CuSO4 in 0.5 M H 2S0 4 at the different cathodic overpotentials indicated in V. 340
Fig. 1 shows that with the increase in cathodic overpotential the time tm decreases and the curves have a more distinct maximum. Experimental potentiostatic current transients were treated according to the model of three-dimensional multiple nucleation with diffusion controlled growth (cf. Table I, Fig. 2). Table I. Kinetic parameters of copper nucleation on glassy carbon
-T1,V
10-2 1m,
0.130 0.
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