Supplementary material
Journal of Solid State Electrochemistry
New insights on the doping of ZnO films with elements from the group–IIIA group through electrochemical deposition
D. Ramírez, K. Álvarez, G. Riveros, M. Tejos, M.G. Lobos
Departamento de Química y Bioquímica, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile.
*Corresponding author
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Speciation and solubility diagrams at 25 °C
Table 1 Electrolytes used in each experiment, its pH values and predominance of species at 25 °C in perchlorate media. CV: cyclic voltammetry, PED: Potentiostatic electrochemical deposition. Solutions II to V also contained 0.1 M LiClO4 whereas solutions VI to VIII contained both 0.1 M LiClO4 and 5 mM Zn(II).Electrolyte IV was unstable.
Electrolyte / Exp. / Measured pH / Predominance of species at 25 °CI / 0.1 M LiClO4 / CV / 3.0 – 5.6* / -
II / 5 mM Zn(II) / CV / 3.0 – 5.6* / Zn2+
III / 5 mM & 1 mM Al(III) / CV / 3.8 & 5.0** / Al3+ > AlOH2+ & AlOH2+ > Al3+
IV / 5 mM & 1 mM Ga(III) / CV / 3.0 & 3.4** / GaOH2+ > Ga3+ &
GaOH2+ > Ga3+ > Ga(OH)2+
V / 5 mM & 1 mM In(III) / CV / 3.0 & 4.0** / In3+ > InOH2+
In(OH)2+ > InOH2+ ~ In3+ > In(OH)3
VI / 5 – 15 μM Al(III) / PED / 5.2 (lowest)** / AlOH2+ > Al3+ ~ Al(OH)2+,
Zn2+
VII / 5 – 10 μM Ga(III) / PED / 4.9 (lowest)** / Ga(OH)4– > Ga(OH)3,
Zn2+
VIII / 5 – 100 μM In(III) / PED / 4.0 (lowest)** / In(OH)2+ > InOH2+ ~ In3+ > In(OH)3 Zn2+
Fig. 1 Speciation diagrams in aqueous media at 25 °C for: (a) Zn(II), (b) Al(III), (c) Ga(III) y (d) In(III). Solubility diagrams are depicted in (e) showing the most stable insoluble compounds. Values of log S’ are depicted as function of pH for electrolytes summarized in Table I in CV (e) and PED (f) experiments: Zn (+), Al (×), Ga (·) e In (*). pH values measured by Peulon and Lincot near the surface of the electrode during their CV measurements are also represented for dioxygen reduction in absence (◊) and in presence of Zn(II) (¨).
Potentiodynamic study of FTO in perchlorate media
Figure 2a shows that the electrochemical behavior of FTO under argon at pH 5.6 is substantially different from that at pH 3.0. In fact, in this last value, a drastic increase in the current density at a potential ca. –0.8 V can be observed during the negative direction sweep, whereas by scanning in the positive direction, two distinct oxidation peaks centered ca. –0.4 and +0.15 V are clearly identified. For the solution at pH 5.6, current density is relatively low in the same potential range. This difference could be explained by a shift in the reduction reaction of SnO2 to metallic Sn:
towards less negative potentials when the pH decreased from 5.6 to 3.0. The start of the reoxidation peak of Sn at pH 3.0 matches with the redox potential for the SnO2/Sn couple which is –0.492 V vs Ag/AgClsat at pH 3.0, being its value at pH 6.0 shifted to –0.668 V vs Ag/AgClsat. In fig. 1b, it can be observed in detail that at pH 4.0, the tin reoxidation with two very small current density peaks probably related to this process (see arrows) already can be subtly exposed. This interpretation is supported by experimental observations in which at pH 3.0 the change in appearance of the working electrode turning dark brown can be clearly identified. However, at the same pH value when argon was substituted by molecular oxygen, the electrode showed no change in color, remaining transparent throughout the potential range (Fig. 2a). Although in highly acidic conditions, the oxygen reduction reaction (ORR) does not at all favor the formation of hydroxide ions, apparently at pH 3.0 it appears that a sufficient quantity of such ions can be formed which, under quiescent conditions are able to keep a higher pH in the region of the electrode/electrolyte interphase. In fact, with so low pH values such as this, it has been proved that ZnO films can still be formed by means of electrochemical deposition (25, 26). However, in our case the quality of these films was not good for further studies.
Fig. 2 Potentiodynamic profiles obtained on FTO at different pH values in Electrolyte I and at 80 °C under argon or molecular oxygen. (a) Entire profiles and, (b) same as (a) showing detailed profiles of FTO behaviour at pH 5.6 and 4.0 with two oxidation peaks indicated by arrows. Scan rate: 20 mV s–1.
Potentiodynamic study on glassy carbon and FTO in perchlorate media under quiescent condition
Fig. 3 Potentiodynamic profile performed under quiescent conditions at 80 °C and at pH 4.0 on (a) FTO and (b) Glassy Carbon. Scan rate: 20 mV s–1.
Fig. 4 Potentiodynamic profiles performed under both Ar and quiescent conditions on GC in aqueous media at 80°C. Scan rate: 20 mV s–1. The total concentration of Al(III), Ga(III) and In(III) was set at 5 mM. The preferential species can be identified from Figure 1 in the main text.