Dimensión total

The multi-step deposition technique was used to get a better understanding of the Pt de- position mechanism for the plating bath developed. The general mechanism of nucleation and growth can be described by five individual steps, these are illustrated in Figure 3.16, Venables et al. (1984). Firstly the metal ion complex arrives at the electrode surface either by convection or diffusion. Electrons are then transferred to the ion from the cathode as shown in step (ii). In step (iii) the metal ion is either partially or completely desolvated before further metal ions diffuse to the surface (iv) and coalesce (v) to form a nucleation site.

These steps play a crucial role in determining grain sizes. In order to achieve fine- grained thin film deposits, the generation of these nucleation sites needs to be interrupted

0 0.5 1 −2 −1 0 1 2 3 x 10 Potential/ V Current/ mA 0 0.5 1 −2 0 2 4 x 10−4 Potential/ V Current/ mA −0.5 0 0.5 −3 −2.5 −2 −1.5 −1 −0.5 0 x 10−4 Potential/ V Current/ mA Au in 50 mM H SO _{2 4} Au in 100 mM KCl Au in 100 mM KCl and K PtCl_{2 4}

Figure 3.14: Cyclic voltammograms. Top: Au/ glass substrate electrode in 50 mM H2SO4 solution. Middle: Au/glass substrate in 100 mM KCl and Bottom: Au/glass

substrate in 100 mM KCl in the presence of 10 mM K2PtCl4, reproducible scans were

−0.5 0 0.5 1 −6 −4 −2 0 2 4x 10 −7 Potential/ V Current/ mA Pt in 50 mM H SO _{2 4}

Figure 3.15: Cyclic voltammogram of Pt deposited layer on the Au/ glass substrate electrode in 50 mM H2SO4 solution. The scan rate was set to 50 mV s-1.

Figure 3.16: Schematic illustrating the general mechanism of nucleation and growth. Growth of a large scale deposit proceeds in a number of stages: (i) transport of the metal ion complex to the electrode surface (ii) electron transfer (iii) partial/complete

desolvation (iv) surface diffusion (v) formation of stable nuclei and finally growth of nucleus and overlapping towards film formation. Adapted from Venables et al. (1984).

(Scharifker and Hills, 1983; Scheludko and Todorova, 1952). This can be achieved with the multi-step technique in which a reduction potential, followed by a relaxation potential is applied repeatedly. This oscillation of potential disrupts the flow of electrons which facilitates coalescence of ions that form a nucleation site. If the application of the relaxed and reduction potentials is timed correctly and implemented for a significantly long period, a thin film of metal will eventually form (Sprague and Gilmore, 1994; Venables et al., 1984).

Figure 3.17: Schematic representation of the nucleation potential technique. Potential E1

allows nuclei formation on the substrate and potential E2 is a less negative potential

suitable for controlled nuclei growth.

Typically two types of growth mechanisms can be observed for the growth of electrode- posited thin films; instantaneous and progressive nucleations, Scharifker and Hills (1983). The presence of either depends upon the rate of nucleation. In the case of instantaneous nucleation, the growth rate of a new phase is high but the number of formed active nucle- ation sites is low, Venables et al. (1984). In comparison, whilst progressive nucleation is slow, it occurs on a large number of active sites, resulting in the growth of compact grains. The latter is achieved through strategically interrupting the nuclei formation process us- ing the multi-step technique. Interruptions are short and spaced enough to allow, in a following step, controlled growth of the nuclei (Ueda et al., 2002; Venables et al., 1984).

lustrated in Figure 3.17. The technique (Scharifker and Hills, 1983) is characterised by the pulses E1 and E2 and their corresponding pulse durations t1 and t2. This allows for

an efficient way to control the particle size distributions of electrodeposits. The method is based on the knowledge of the critical potential for the system, Ecrit, which is the minimal

potential that has to be applied in order to allow the formation of nuclei.

The first step that was applied, E1, had to be more negative than Ecrit to ensure re-

duction of PtCl42- to Pt. The second step, E2 needed to be more negative than the open

circuit potential (OCP) to avoid redissolution but more anodic to prevent new nuclei for- mation. Duration of the steps was also important, t1 had to be of the order of milliseconds

to avoid the formation of a great number of nuclei, and t2 had to have values ranging from

40 to 300 s to allow a controlled growth (Penner, 2001; Venables et al., 1984).

In order to determine Ecrit and E1 parameters, a cyclic voltammetry was performed

on a 10 mM K2PtCl4/100 mM KCl system, as shown in Figure 3.14 (bottom panel). This

shows a CV starting on open circuit potential (Vocp) conditions (0 mV) and going on

cathodic direction up to a potential of -500 mV to avoid secondary cathodic process. The potential sweep is then reversed towards the anodic direction at +500 mV to avoid oxygen evolution. Under these conditions the platinum reduction peak at -300 mV is observed and re-oxidation at +50 mV. We observe the beginning of the platinum reduction at the critical potential (Ecrit = -180 mV). Thus, nucleation potential E1 was set between -200

mV and -500 mV to avoid hydrogen evolution and secondary cathodic processes. This potential is applied over a nucleation period t1 which is in the order of milliseconds to

enhance nuclei formation only. The growing potential (E2) was set to to a more negative

value than the Vocp and so it was important it did not exceed Ecrit in such a way that

nuclei formation was inhibited, as well as redissolution of platinum deposit. This growing step potentially enhanced the controlled growth of existing nuclei. Therefore the growing conditions were chosen as; E2= -300 mV and t2= 300 s.

Four different types of experiments were carried out with varying E2 and t2 and t1

parameters, these are illustrated in Figure 3.18. Sample 1 (S1) was used as the control where a constant deposition potential Vdep= -300 mV was applied for a duration of t=

300 s. Further to this, a single nucleation step (sample 2, S2) was tested and the protocol for this is illustrated in Figure 3.18 (middle panel). Here the parameters were set as follows; E1= -500 mV ; E2= -300 mV ; t1= 0.1 ms; t2= 300 s. For sample 3 (S3) a double

Figure 3.18: Schematic representations of the different nucleation and growth experiments carried out on individual Au/glass electrodes. Top: S1, control experiment,

V_dep= -300 mV; t= 300 s. Middle: S2, using a single nucleation step, E1= -500 mV;

E2= -300 mV ; t1= 0.1 ms; t2= 300 s. Bottom: S3, using a double nucleation step, E1=

-500 mV ; t1= 0.1 ms; E1a= -500 mV; t1a= 0.1 ms; E2= -300 mV ; t2= 300 s. Potential

E1 and E1a allows nuclei formation on the substrate and potential E2 is a less negative

0 100 200 300 −0.01 0 0.01 Time/s Current/mA 0 100 200 300 −0.01 0 0.01 Time/s Current/mA 0 100 200 300 −0.01 0 0.01 Time/s Current/mA 0 100 200 300 400 500 −0.01 0 0.01 Time/s Current/mA 0 100 200 300 −5 0 5x 10 −4 Time/s Charge/C 0 100 200 300 −4 −2 0 2x 10 −4 Time/s Charge/C 0 100 200 300 −10 −5 0 5x 10 −4 Time/s Charge/C 0 100 200 300 400 500 −4 −2 0x 10 −3 Time/s Charge/C

Figure 3.19: Current-time transients resulting from the different nucleation and growth steps applied at individual Au electrodes. From top to bottom: S1, S2, S3 and S4 with

Sample Charge/ C Calculated film thickness (nm)

S1 -0.0082 56.50

S2 -0.0052 39.43

S3 -0.0007 5.65

S4 -0.0127 95.80

Table 3.4: Table of calculated charge and thickness values for S1, S2, S3 and S4 experiments at the Au glass electrode substrate.

The additional step, assigned as E1a= -500 mV was set with a time duration of t1a= 0.1

ms. Finally a further control was used where the deposition potential was kept constant as described for S1 (Vdep= -300 mV) but with a longer duration of 500 s.

The results obtained for the control depositions experiments (S1 and S4) are presented in Figure 3.19 (top and bottom panel), respectively. Here the behaviour of the current is shown when a -300 mV potential is applied to the system. The results are similar to what we observe for the typical chronoamperometry measurements as discussed in Section 3.4.1. For the case of S1, the current initially rises within the first few seconds due to discharging effects and then remains constant until there is an increase in surface area of Pt deposits, which corresponds to a progressive increase in current over the 300 s run. For S4 (Figure 3.19), the same behaviour is observed, however after ∼320 s, the current stabilises and remains constant for the remaining time. The change in the current induced by the potential also indicates that at short times (<300 s ) a progressive deposit of particles is achieved. However at longer times (S4, t= 500 s) the growth of the larger grains becomes limited by diffusion due to a depletion layer established around the grains, Venables et al. (1984).

For the multi-step depositions, the behaviour of the current indicates two steps; a ‘nucleation process’ at the start and a ‘growth process’ when it stabilises. For S2, the results for which are plotted in Figure 3.19, it can be observed that during the first step (E1) a typical nucleation curve for electrodeposition is observed, Scharifker and Hills

(1983). This is followed by an increase of the current, indicating a possible growth of the Pt grains. For S3, the same trend is seen with the exception of an additional nucleation step.

Figure 3.20: Optical images of Pt surfaces (samples S1, S2, S3 and S4) at 100 x after depositions including individual images of the electrodes at 20 x magnification.

procedure were also investigated and are shown in Figure 3.20. For nucleation step ex- periments (samples S2 and S3) the images showed uneven films and sparse deposition. Whereas for samples S1 and S4 (where no nucleation step was applied) uniform Pt layers with consistent deposits were achieved. The corresponding charge-time plots for all four measurements are provided in Figure 3.19. For samples S1 and S4, a much higher de- posited charge is obtained, whereas for S2 and S3, the resulting charge is between -0.0007 to 0.00127 C, respectively. By calculating the thickness values from these plots (provided in Table 3.4), it was observed that the constant deposition experiments S1 and S4 also gave the greatest thickness values compared to the S2 and S3 under the same conditions. From this it was concluded that with the constant deposition method, the growth of Pt film were achieved more efficiently than that obtained with the multi-step method.