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
Email: daniel.ramirez@uv.cl
Phone: +56-32-2508068
Fax: +56-32-2508062
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 LiClO 4 and 5 mM
Zn(II).Electrolyte IV was unstable.
Electrolyte
Exp.
Measured pH
Predominance of species at 25 °C
Zn2+
Al3+ >> AlOH2+ & AlOH2+ >> Al3+
GaOH2+ > Ga3+ &
GaOH2+ >> Ga3+ > Ga(OH)2+
In3+ >> InOH2+ &
In(OH)2+ > InOH2+ ~ In3+ > In(OH)3
AlOH2+ >> Al3+ ~ Al(OH)2+,
Zn2+
Ga(OH)4– >> Ga(OH)3,
Zn2+
In(OH)2+ > InOH2+ ~ In3+ > In(OH)3 Zn2+
I
II
III
IV
0.1 M LiClO4
5 mM Zn(II)
5 mM & 1 mM Al(III)
5 mM & 1 mM Ga(III)
CV
CV
CV
CV
3.0 – 5.6*
3.0 – 5.6*
3.8 & 5.0**
3.0 & 3.4**
V
5 mM & 1 mM In(III)
CV
3.0 & 4.0**
VI
5 – 15 μM Al(III)
PED
5.2 (lowest)**
VII
5 – 10 μM Ga(III)
PED
4.9 (lowest)**
VIII
5 – 100 μM In(III)
PED
4.0 (lowest)**
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:
SnO2(s) +2H2O(l) +4e  Sn (s) +4OH(aq)
(1)
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.
0.6
j / mA cm
-2
0.3
(a)
0.0
-0.3
pH 5,6 under Ar
pH 4,0 under Ar
pH 3.0 under Ar
pH 3.0 under O2
-0.6
-0.9
-1.2
-0.9
-0.6 -0.3 0.0
E / V vs Ag/AgClsat.
0.3
0.025
j / mA cm
-2
0.000
(b)
-0.025
-0.050
-0.075
-0.100
-1.2
pH 5,6 under Ar
pH 4,0 under Ar
pH 3,0 under Ar
-0.9
-0.6
-0.3
0.0
E / V vs Ag/AgClsat.
0.3
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
5
4
(a)
-2
3
Electrolyte II, Ar
Electrolyte II, O2
2
j / mA cm
Electrolyte I, Ar
Electrolyte I, O2
1
0
-1
-2
-3
3
j / mA cm
-2
2
-1.2
(b)
-1.0 -0.8 -0.6 -0.4
E / V vs Ag/AgClsat.
-0.2
Electrolyte I, Ar
Electrolyte I, O2
Electrolyte II, Ar
Electrolyte II, O2
1
0
-1
-2
-1.2 -1.0 -0.8 -0.6 -0.4 -0.2
E / V vs Ag/AgClsat.
0.0
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.
3
2
j / mA cm
-2
1
0
-1
-2
Electrolyte I, pH 3,0
Electrolyte II, pH 4.0
Electrolyte III, pH 3.8
Electrolyte IV, pH 3.0
Electrolyte V, pH 3.0
-3
-4
-5
-2.0
-1.6
-1.2 -0.8 -0.4
E / V vs Ag/AgClsat.
0.0
0.4
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.