Electrochimica Acta 54 (2009) 5239–5245
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Composite PbO2 –TiO2 materials deposited from colloidal electrolyte:
Electrosynthesis, and physicochemical properties
R. Amadelli a,∗ , L. Samiolo a , A.B. Velichenko b,∗∗ , V.A. Knysh b , T.V. Luk’yanenko b , F.I. Danilov b
a
b
ISOF-CNR, c/o Department of Chemistry, University of Ferrara, via L. Borsari 46, 44100 Ferrara, Italy
Department of Physical Chemistry, Ukrainian State University of Chemical Technology, Gagarin ave. 8, Dnepropetrovsk 49005, Ukraine
a r t i c l e
i n f o
Article history:
Received 30 October 2008
Received in revised form 7 April 2009
Accepted 9 April 2009
Available online 18 April 2009
Keywords:
Lead dioxide
Titanium dioxide
Photocurrent
Electrocatalysis
Composite electrodes
a b s t r a c t
Electrodeposition of PbO2 from nitrate solutions in the presence of TiO2 nanoparticles leads to composite
PbO2 –TiO2 films. The content of the dispersed oxide which is finally occluded into the composite PbO2 film
depends on electrodeposition conditions such as pH, the value of the electrodeposition constant current
or potential, the amount of added TiO2 and on temperature. It also depends strongly on the presence of
anionic additives such as sodium dodecyl sulfate (SDS) whose adsorption decreases the positive charge
on the surface of the TiO2 particles.
The photo-electrocatalytic activity of the prepared materials has been tested in the oxidation of oxalic
acid and benzyl alcohol. Electrodes showed a photoresponse to illumination at > 320 nm confirming literature reports on synergistic effects of illumination in electro-oxidation processes at PbO2 -based anodes.
We also established that the life service of these electrodes increases by a factor of about 3 with respect
to traditional PbO2 anodes. On the other hand, the more striking achievement in the present work with
PbO2 –TiO2 electrodes is the enhancement of electroactivity in the dark for oxalic acid, benzyl alcohol as
well as for O2 evolution.
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Oxide composite materials find wide use in various reactions
as catalysts and electro-photo catalysts, in electrochemical synthesis of strong oxidising agents, and in incineration of organic and
inorganic contaminants of water and air [1–10]. Various ways to
produce materials of this type are known, e.g., sol–gel techniques,
plasmochemical method, etc. The electrochemical method should
be distinguished as one of the most promising, which enables wide
control over the composition and properties of composites because
of its simple implementation and possibility of smoothly varying
the technological parameters of the process [3–10].
Lead dioxide is a promising electrocatalyst widely used in practice [11–18]. Electrodeposited pure lead dioxide was demonstrated
to exhibit a moderate electrocatalytic activity toward various
anodic reactions in acidic media. However, this activity can often
be enhanced greatly by incorporation of some ions, for example,
Bi3+ , As3+ , Fe3+ , Cl− , and F− [11–16]. Conversely, for the case of PbO2 ,
there is comparatively less information on effects of polyelectrolyte
and surfactant additives on the process of oxide electrodeposition
and the physicochemical properties of the resulting materials. It
was shown [17–19] that both polyelectrolytes and anionic surfactants are adsorbed on PbO2 and form composite materials with new
physicochemical properties.
Composite materials based on lead dioxide are known, which
additionally contain various oxides and, in particular, A12 O3 , Co3 O4 ,
RuO2 , and TiO2 [3,5,8,9]. Related studies on the cathodic electrodeposition of metals–particles composites are numerous and
have been the object of review papers [20,21]. In general, anodic
or cathodic codeposition of particles into electroplated films is
affected by several factors including concentration, size, surface
charge, solution stirring and current density [22].
In the present study we examine some of the fundamental
aspects of the electrodeposition of oxide composites PbO2 –TiO2
as well as their physicochemical and photo-electro catalytical properties. We employ stable colloidal suspensions of TiO2
nanoparticles, which makes stirring during electrodeposition
unnecessary thereby simplifying the system by avoiding convection problems. In addition, the effect of surface charge of the TiO2
particles is examined through addition of anionic surfactants.
2. Experimental
∗ Corresponding author. Tel.: +39 0532 455161; fax: +39 0532 240709.
∗∗ Corresponding author. Tel.: +38 056 3772974; fax: +38 056 3772974.
E-mail addresses: [email protected] (R. Amadelli), [email protected]
(A.B. Velichenko).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.04.024
The composites were deposited in the galvanostatic mode onto
pre-treated platinum-plated titanium electrodes of area 4 cm2 .
The deposition electrolyte contained: 0.1 mol L−1 Pb(NO3 )2 and
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R. Amadelli et al. / Electrochimica Acta 54 (2009) 5239–5245
0.1 mol L−1 HNO3 . Additionally, TiO2 was introduced into the electrolyte. Nanosized, semiconducting TiO2 was either commercially
available (PC 105, Millenium Chemicals) or, alternatively, samples
with an average particle size of 5 nm were prepared by controlled hydrolysis of titanium tetra-isopropoxide in dilute HNO3 .
We used NaF, surfactant sodium dodecyl sulfate (C12 H25 SO3 Na,
henceforth designated SDS) and polymer Nafion® 117 as anionic
additives to the electrolyte. Nafion® 117 was added to the deposition electrolyte from a 5 wt.% solution in a mixture of lower
aliphatic alcohols and water (Aldrich). Solutions were prepared
from reagents of chemically pure grade and twice-distilled water.
The deposition was performed in most cases at a temperature of
20 ± 2 ◦ C and anodic current density of 5 mA cm−2 ; the coating
deposition time was 30 min. To study the physicochemical properties of the composites, thicker coatings were deposited (at a
deposition time of 2 h). It should be noted that the composite materials under study had a composition independent of the coating
thickness.
Electrodeposition experiments in the presence of nanostructured TiO2 needed no stirring as the particles do not sediment and
are stable for weeks. Suspensions of the electroplating electrolyte
containing commercial TiO2 PC 105 were used in just one experiment. They were first sonicated for 30 min then stirred at 1000 rpm
during electrodeposition.
The adsorption of the anionic surfactant (SDS) on a TiO2 powder
was studied in 0.1 mol L−1 HC1 [23]. The electrokinetic potential
was measured using the electrophoretic technique. The point of
zero charge (pH0 ) was measured in 0.1 mol L−1 KC1.
To determine the composition of the composites, the materials
were dissolved in a 1:1 mixture of 5 mol L−1 HNO3 and 30% hydrogen peroxide. The excess of H2 O2 in the resulting solutions was
removed by boiling with a platinum catalyst. The amount of PbO2
in the composite was calculated from the Pb2+ content of the solution, found by amperometric titration with diethylthiocarbominate.
The amount of TiO2 was evaluated as the difference between the
mass of composite and PbO2 determined by analytical procedure
described above. The amount of titanium dioxide in the codeposits
was also determined by the ICP-AES technique using an OPTIMA
3100XL PerkinElmer spectrometer. Two check experiments were
conducted that gave results in excellent agreement with the above
analytical method.
The surface morphology of the materials obtained was studied
by scanning electron microscopy (SEM) with a LEICA S360 microscope. X-ray diffraction patterns were recorded with a PHILIPS
PW3710 diffractometer (Cu K␣ radiation).
Accelerated lifetime tests of anodes with an active layer of a
PbO2 –TiO2 composite were performed in 1 mol L−1 H2 SO4 at an
anodic current density of 200 mA cm−2 .
Electrocatalytic and photoelectrocatalytic tests were performed
on an EG & G model 273A potentiostat/galvanostat using EG & G
software. The saturated calomel electrode (SCE) was used as reference. The illumination source was a mercury medium pressure
lamp (Helios Italquartz) equipped with a > 320 nm cut-off filter.
The irradiation power was 3 mW cm−2 .
ticle size [22]; specifically, the adsorption state of small particles is
more stable, which provides a rationale for the use of nanoparticles.
In addition to particle size and concentration, an important factor
affecting incorporation of the inert oxide into an electrochemically
grown film is the charge of the particles of the dispersed phase [20].
In the following we examine the effect of charge on particle
incorporation. According to our potentiometric data, the point of
zero charge (pH0 ) of TiO2 is 6.36 and, therefore, the dispersed particles are positively charged in the acidic colloidal electrolyte. Upon
adsorption of an anionic surfactant such as SDS, the point of zero
charge is shifted to higher values, which indicates specific adsorption. Indeed, separate experiments proved that SDS is adsorbed
on TiO2 , with the process satisfactorily described by the Frumkin
isotherm. The adsorption parameters were calculated: a limiting
adsorption of 1.75 × 10−5 mol g−1 , reached at a SDS concentration
of 3.8 × 10−4 mol L−1 ; and an adsorption constant of 673.81 L mol−1 .
Adsorption of SDS on TiO2 is week, as indicated by the rather
low value of the adsorption interaction energy G = −25.6 kJ mol−1
[23].
Additional measurements demonstrated that the electrokinetic
potential of titanium dioxide in 0.1 mol L−1 HNO3 is 0.064 V. This
also points to the presence of a small positive charge on titanium
dioxide particles in the colloidal electrolyte. Such a low value of
the -potential is due to the high concentration of the supporting electrolyte. Addition of SDS causes a shift of the electrokinetic
potential to −0.18 V, and this substantial change in the sign and
magnitude of the electrokinetic potential indicates that the electric double layer of particles of the dispersed phase is recharged
because of the adsorption of the surfactant.
The focus of the subsequent discussion is an analysis of the
results of polarisation measurements that aimed to gain insights on
how particles of the dispersed phase affect the fundamental aspects
of lead dioxide electrodeposition from colloidal electrolytes. The
curves at low polarisation (Fig. 1, region I), when plotted in the Tafel
coordinates (E-log J), are linear, which points to a kinetic control of
lead dioxide deposition [17]. At potentials more positive than 1.5 V
(Fig. 1, region II), the current starts to approach the limiting value,
indicating that the process occurs under diffusion control. At still
higher overpotential (E > 1.65 V) the polarisation curve features an
exponential current growth (Fig. 1, region III) that corresponds to
3. Results and discussion
3.1. Electrodeposition of composites
According to the Guglielmi model [24], the process of particles occlusion involves a two-step adsorption onto the electrode.
Particles reaching the electrode surface are initially present in a
thin solution layer comprising ions and solvent molecules and are
only loosely interacting with the surface. In the subsequent step,
particles re-arrange to become strongly adsorbed. The adsorption
energy of weakly adsorbed particles was found to depend on par-
Fig. 1. Steady-state polarisation curves of Pt–PbO2 in solutions: (1) 0.1 mol L−1
HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1 TiO2 ; (2) 0.1 mol L−1 HNO3 + 0.1 mol L−1
Pb(NO3 )2 + 5.0 g L−1 TiO2 + 0.01 mol L−1 NaF; (3) 0.1 mol L−1 HNO3 + 0.1 mol L−1
Pb(NO3 )2 + 5.0 g L−1 TiO2 + 3 × 10−4 mol L−1 SDS; (4) 0.1 mol L−1 HNO3 + 0.1 mol L−1
Pb(NO3 )2 + 5.0 g L−1 TiO2 + 0.05 wt.% Nafion® .
R. Amadelli et al. / Electrochimica Acta 54 (2009) 5239–5245
the simultaneous reactions of Pb2+ oxidation and oxygen evolution
[14]. In this potential region, oxygen evolution contributes significantly to the total current thus decreasing the current efficiency
of lead dioxide formation with deposition potential or current
density. Upon introduction of anionic additives into the colloidal
solution, the polarisation somewhat increases and the current of
PbO2 electrodeposition decreases (Fig. 1). The results obtained can
be attributed to a too high coverage of strongly adsorbed particles
that decreases the anode area available for codeposition [20,22]
and/or to the adsorption of the anionic additives on lead dioxide
[18]; both phenomena leading to partial blocking of the electrode
surface and, consequently, of the electrodeposition process.
In agreement with literature models [22,24], the content of
titanium dioxide in the composite increases with increasing its
concentration in the colloidal electrolyte (Fig. 2, curves 1 and 2).
This effect is probably observed because the gradient of the partial
concentration of the oxide in the dispersed phase grows with its
content in the electrolyte. We observed that the coating composition also depends on the solution pH and temperature. Typically, an
increase in the HNO3 concentration leads to a decrease in the TiO2
content in the coating that can be ascribed, in the absence of anionic
surfactants, to a rise in the positive charge of particles leading to an
augmented electrostatic repulsion at the anode and, at the same
time, to an increased electrostatic attraction to the cathode. As the
deposition temperature is raised from 20 ◦ C to 60 ◦ C, the content of
particles of the dispersed phase in the coating increases from 6.3%
to 16.3% (0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1 TiO2 ,
j = 5 mA cm−2 ), likely on account of a decrease in the viscosity of
the electrolyte, which, in turn, results in diffusion acceleration.
In Fig. 2 it is shown that as the current rises, the content of the
inert oxide in the composite material grows. The curve describing
the content of the dispersed phase in the composite as a function of
the current density can be divided into two portions. In the first of
these, as the deposition rate of lead dioxide grows, the probability
that TiO2 particles can be captured becomes higher and the content
of the inert oxide in the composite increases. In the second one
(j ≥ 10 mA cm−2 ), the content of TiO2 in the coating remains nearly
constant when the Pb2+ deposition rate reaches its limiting value
(Fig. 1).
It is important to note that at a low current density (2 mA cm−2 ),
in the presence of additives, the content of TiO2 in the composite
decreases (Fig. 3). In this case the charge of the electrode surface
is not very high and electrostatic interaction on the interface does
not play critical role. The phenomenon is rather attributed to the
Fig. 2. Effect of deposition current density (j) on the TiO2 content in the composite material deposited from different solutions: (1) 0.1 mol L−1 HNO3 + 0.1 mol L−1
Pb(NO3 )2 + 2.0 g L−1 TiO2 ; (2) 0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1
TiO2 .
5241
Fig. 3. Effect of deposition current density (j) on the TiO2 content in the composite material deposited from different solutions: (1) 0.1 mol L−1 HNO3 + 0.1 mol L−1
Pb(NO3 )2 + 5.0 g L−1 TiO2 ; (2) 0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1
TiO2 + 0.01 mol L−1 NaF; (3) 0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1
TiO2 + 3 × 10−4 mol L−1 SDS; (4) 0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1
TiO2 + 0.05 wt.% Nafion® .
fact that we observed some particles aggregation due to adsorption, which leads to particles instability at the surface [22]. An
increase in the anodic polarisation led to a higher TiO2 content
in the composite, but the amount does not seemingly approach a
limiting value in every case, and the different behaviour depends
on the nature of the additives adsorbed on TiO2 , coverage and
the resulting surface charge; specifically, the coverage by adsorbed
compounds and charge of dispersed particles increases in the order
F− < SDS < Nafion® . Additionally, we point out that the content of
particles of the redox inert oxide in the composite material grows
with current density also in the presence of the additives, but the
process has a more complex character, and depends on the nature
of additives (Fig. 3, curves 2–4).
To sum up the discussion thus far, it is apparent from the results
reported above that the effect of the particles charge should not
be overlooked [20]. Particles of the dispersed phase are incorporated into the growing PbO2 deposit to give a composite material,
and the content of TiO2 in the composite will be determined by
the stages in which particles are delivered from the electrolyte
bulk to the electrode surface. Because the potential of zero charge
of lead dioxide in 0.1 mol L−1 HNO3 is 0.91 ± 0.1 V vs. SCE [25],
at the deposition potentials (E > 1.4 V) the electrode surface will
be positively charged; nevertheless, TiO2 particles incorporation
into the growing coating does occur, and this can be the result
of contrasting effects such as the favourable dimensions of the
nanoparticles and the adverse field influence. Additives introduced
into the deposition electrolyte are adsorbed on TiO2 and cause the
surface of particles of the dispersed phase to be recharged. The negatively charged TiO2 /surfactant particles are transported toward
the positively charged electrode where, in this case, their incorporation into the growing coating is assisted by the electric field
[20,26].
From the point of view of the electrodeposition mechanism,
it should be noted that polarisation curves obtained in the presence and in the absence of the colloid are essentially the same and,
therefore, the electrodeposition of PbO2 from colloidal electrolytes
can be satisfactorily described by the kinetic scheme previously
reported [18]. Taking into account the formation of hydrated Pb(IV)
species and TiO2 particles during electrodeposition process, the
adsorption of negatively charged additives, the influence of electrolysis conditions and electrolyte composition mentioned above,
we can suggest the colloidal–electrochemical mechanism of the
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R. Amadelli et al. / Electrochimica Acta 54 (2009) 5239–5245
on the anode surface (Eq. (4)):
Pb2+ + 2H2 O → PbO2(vol.) + 4H+ + 2e−
(1)
PbO2(vol.) + R ↔ PbO2 − R ads.(vol.)
(2)
TiO2(vol.) + R ↔ TiO2 − R ads.(vol.)
(3)
PbO2 − R ads.(vol.) + TiO2 − R ads.(vol.) → PbO2 − TiO2 − R .(sur.)
(4)
3.2. Codeposits morphology and electrochemical stability
Fig. 4. Scheme of the transport of colloidal particles to the electrode surface.
composite material formation [17–19]. It includes several electrochemical and chemical stages according to reaction scheme (1)–(4):
(i) electrochemical formation of oxide particles in the solution (Eq.
(1)), (ii) adsorption of inorganic anion, polyelectrolyte or surfactant
(R) on the oxide particles (Eqs. (2 and 3)), (iii) transport to the electrode where adsorption is favoured by the field for particles with
negative -potential (Fig. 4) followed by (iv) further crystallization
Lead dioxide deposited from nitrate solutions is a polycrystalline
formation composed of a mixture of ␣- and ␤-phases with various
crystallographic orientations, where the latter phase predominates
[27]. The presence of TiO2 and additives strongly affects the structure of PbO2 -based materials. The crystallographic orientation of
lead dioxide in the composite material changes substantially. For
deposits containing particles of the inert oxide and the additives,
the intensity of the peaks significantly decreases, which is an indication that the size of lead dioxide crystals decreases as the fraction of
X-ray-amorphous phases in a sample increases. It should be noted
that the X-ray diffraction patterns show no peaks of titanium dioxide, which is expected since the nanosized TiO2 particles are below
X-ray detection limit under our measurement conditions.
To obtain additional information, the surface morphology of the
materials obtained was studied by SEM. In electrodeposition of lead
dioxide from true solutions [18], the coatings are composed of a set
of coarse polycrystalline blocks with a poorly pronounced predominant face orientation. As follows from the data obtained (Fig. 5),
Fig. 5. SEM micrographs of PbO2 –TiO2 films deposited at 10 mA cm−2 from different solutions: (1) 0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1 TiO2 ; (2)
0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1 TiO2 + 0.01 mol L−1 NaF; (3) 0.1 mol L−1 HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1 TiO2 + 3 × 10−4 mol L−1 SDS; (4) 0.1 mol L−1
HNO3 + 0.1 mol L−1 Pb(NO3 )2 + 5.0 g L−1 TiO2 + 0.05 wt.% Nafion® .
R. Amadelli et al. / Electrochimica Acta 54 (2009) 5239–5245
incorporation of titanium dioxide particles into PbO2 brings about
significant changes in the morphology of the deposit. The composite is a heterogeneous system with locally separated zones with
different surface morphologies. The regions with coarser crystals
are composed of lead dioxide particles, while those with fine crystals consist of titanium dioxide partly or completely covered with
fine crystals of lead dioxide. The disappearance of coarse crystals
and polycrystalline blocks, characteristic of lead dioxide, indicates
that the number of crystallisation centres grows as the TiO2 content of the composite increases and the additive is adsorbed on the
electrode.
Accelerated lifetime tests of anodes with an active layer of the
PbO2 –TiO2 composite were performed in 1 mol L−1 H2 SO4 at a current density of 200 mA cm−2 . The service life of the electrodes is
determined by the time of uninterrupted polarisation during which
the potential remains nearly unchanged. A steep rise in the potential
points to failure of an electrode, even though no mechanical damage
to the active coating is visually observed. Such an electrode cannot be used because of the substantial increase in the electrolyser
voltage. As follows from the data obtained, electrodes containing
6.3 wt.% titanium dioxide could operate during 280 h, and lead dioxide electrodes, during only 105 h at the same current density. Thus,
the service life of the composite materials increases approximately
three-fold, compared with the conventional PbO2 anodes. It should
be noted that lead dioxide electrodes are commonly used at current
densities of 20–50 mA cm−2 . In these modes, the service life of the
electrodes will be 10–100 times longer than that in the accelerated
tests. Thus, the composite materials obtained can be recommended
for use as electrocatalysts with a prolonged service life.
3.3. Electro-photo catalytic behaviour
In this section we examine the electrocatalytic behaviour of the
composite PbO2 –TiO2 films described above in the oxidation of
oxalic acid [28] and benzyl alcohol. We also examine the effects
of illumination since, due to the occluded TiO2 , the electrodes
can exhibit photo-effects. Synergistic effects of irradiation in oxidation reactions at PbO2 –TiO2 electrodes have been previously
observed [5,29,30]. Some of these studies [5,29] have dealt with the
degradation of dyes using UV illumination at 254 nm where, admittedly, the organic substrate absorbs light. In addition, the choice of
some experimental conditions is arguable, such as carrying measurements at potentials (0.5 V at pH 2) where the PbO2 matrix is
unstable with respect to reduction [5].
In Fig. 6 the neat photocurrents (Ilight − Idark ) for the oxidation
of oxalic acid in perchloric acid are reported. We see that a constant photocurrent can be observed in the potential range from
1.4 V to 1.65 V for PbO2 –TiO2 electrodes independent of whether
the commercial PC 105 (Millenium Chemicals) or the colloidal TiO2
prepared as described above in this work was used. The photocurrent response decreases markedly at potentials more positive than
1.7 V, likely due to the fact that the dark oxidation of the substrate
sets in at PbO2 sites.
In Fig. 7 the neat photocurrents observed in the case of benzyl
alcohol are shown. In contrast to the case of oxalic acid, photocurrents are observed only at the onset of dark oxidation and increase
as potential increases. The dependence of photocurrents on the
alcohol concentration is relatively small (cfr. curves 1 and 2), and
independent of whether TiO2 is present or not.
While the observation of photocurrents in composites with TiO2
is not surprising, the fact that PbO2 itself responds to illumination needs interpretation. Some earlier literature work reports that
PbO2 has a band gap of 1.4 eV [31] but the commonly employed
oxide is well known to exhibit metallic conductivity rather than
semiconductivity [32]. We are then inclined to exclude that the
observed photocurrents originate from band gap irradiation; rather
5243
Fig. 6. Steady-state photocurrent (Ilight − Idark ) vs. potential for the oxidation of
0.01 mol L−1 oxalic acid in 1 mol L−1 HClO4 at PbO2 –TiO2 electrodes where TiO2 is
commercial PC 105 (Millenium Chemicals) or nanostructured TiO2 . Data are compared with those obtained with pure PbO2 . Irradiation at > 320 nm. Composite
PbO2 –TiO2 films were electrodeposited from suspensions of nanostructured TiO2
(5 g L−1 ). Electrode area: 1 cm2 .
the different behaviour observed for oxalic acid and benzyl alcohol oxidation seems to suggest that the origin of photo-response
of PbO2 is photochemistry of adsorbed species, i.e., some kind of
light induced ligand to metal charge transfer, as illustrated by the
following possible reaction sequence involving oxalic acid:
−Pb(IV) − (C2 H2 O4 )ads + h␯ → − Pb(III) − (C2 H2 O4 )+• ads
+•
−Pb(III) − (C2 H2 O4 )
ads
→ −Pb(IV) + products + e
−
(5)
(6)
Oxalic acid and carboxylic acids, in general, are strongly
adsorbed on both TiO2 and PbO2 and can form strongly reducing
radicals upon absorbing light, through a mechanism that is different
for the two oxides.
Benzyl alcohol is less strongly adsorbed on both oxides components of the composite films and we advance the possible
Fig. 7. Steady-state photocurrent (Ilight − Idark ) vs. potential for the oxidation of
5 × 10−3 mol L−1 (1) and 0.01 mol L−1 (2) benzyl alcohol at PbO2 –TiO2 electrodes
obtained with nanostructured TiO2 . Irradiation at > 320 nm. Composite PbO2 –TiO2
films were electrodeposited from suspensions of nanostructured TiO2 (5 g L−1 ). Electrode area: 1 cm2 .
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R. Amadelli et al. / Electrochimica Acta 54 (2009) 5239–5245
formed upon dissolution–redeposition of the oxide is recognised
to play a key role in the mechanism of electrochemical reactions at
high anodic potentials [33]. Another possible factor that has been
invoked to contribute to an enhanced electrochemical activity is the
degree of oxide non-stoichiometry [34]. Interestingly, it has been
reported that PTFE/PbO2 composite electrodes possess a higher
electrocatalytic activity by reason of a higher number of defects
in comparison with the pure oxide [35].
4. Conclusions
This work shows that PbO2 –TiO2 composite electrodes can be
conveniently obtained by electrodeposition from lead nitrate in
nitric acid in the presence of colloidal TiO2 . Colloidal nanoparticles
form uniform suspensions that are stable for several weeks and
make it possible to carry out accurate investigations without the
problem of concentration changes due to particle sedimentation.
The amount of TiO2 in the composite can be varied from 3 wt.% to
16 wt.% by changing the deposition conditions and the electrolyte
composition.
The physicochemical properties of the composites significantly
differ from those of lead dioxide and are determined by the composition of these materials. The composite has the form of a PbO2
matrix with submicrometer and nanosize crystals, into which TiO2
particles are incorporated. Owing to this circumstance, the composites have a large effective surface area.
The composite electrodes respond to UV illumination as
expected from the presence of TiO2 . On the other hand, photocurrents can be also recorded on pure PbO2 and are attributed to
photochemistry of adsorbed species. The main result concerning
the activity of PbO2 –TiO2 electrodes for the oxidation of organic
compounds is a markedly improved electrocatalytic effect (in the
absence of illumination) with respect to PbO2 .
Fig. 8. (A) Cyclic voltammetry experiments for the oxidation of 0.01 mol L−1 oxalic
acid in 1 mol L−1 HClO4 at PbO2 (1) and PbO2 –TiO2 (2) electrodes in the dark; (B)
Cyclic voltammetry experiments for the oxidation of 5 × 10−3 mol L−1 oxalic acid in
1 mol L−1 HClO4 at PbO2 (1) and PbO2 –TiO2 (2) electrodes in the dark. Scan rate
20 mV s−1 . Composite PbO2 –TiO2 films were electrodeposited from suspensions of
nanostructured TiO2 (5 g L−1 ).
explanation that the small current enhancement due to illumination is ascribed to an increased rate of OH radicals generation,
in agreement with the conclusions of Treimer et al. [32] on light
assisted electro-catalytic degradation of different organic compounds on PbO2 -based materials.
On the whole, the above results confirm the cited literature
works reporting on synergistic effects of illumination on the
electro-oxidation processes at PbO2 electrodes, although we find
that these effects are not particularly large. On the other hand, the
more striking result observed in the present work with PbO2 –TiO2
electrodes is probably the enhancement of electroactivity in the
dark for oxalic acid, benzyl alcohol (Fig. 8A and B) as well as for O2
evolution (not shown). In principle, the effect of an increased stability of PbO2 in the composite oxide (vide supra) on electrochemical
activity can provide a possible explanation, while the presence of an
n-type semiconductor such as TiO2 is not expected to contribute any
particular electrocatalytic effect in the range of anodic potentials of
this study.
We plan further research on the issue whether indeed there
is a close connection between activity and stability of the materials under scrutiny in the present work. At present, only an
informed speculation is possible that is based on the large number of investigations in electrocatalysis devoted to understanding
of relationships among activity, structure and stability of catalyst materials. For PbO2 , in particular, a surface gel layer that is
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