5 research outputs found
Efficient Chemisorption of Organophosphorous Redox Probes on Indium Tin Oxide Surfaces under Mild Conditions
We report a mild and straightforward
one-step chemical surface
functionalization of indium tin oxide (ITO) electrodes by redox-active
molecules bearing an organophosphoryl anchoring group (i.e., alkyl
phosphate or alkyl phosphonate group). The method takes advantage
of simple passive adsorption in an aqueous solution at room temperature.
We show that organophosphorus compounds can adsorb much more strongly
and stably on an ITO surface than analogous redox-active molecules
bearing a carboxylate or a boronate moiety. We provide evidence, through
quantitative electrochemical characterization (i.e., by cyclic voltammetry)
of the adsorbed organophosphoryl redox-active molecules, of the occurrence
of three different adsorbate fractions on ITO, exhibiting different
stabilities on the surface. Among these three fractions, one is observed
to be strongly chemisorbed, exhibiting high stability and resistance
to desorption/hydrolysis in a free-redox probe aqueous buffer. We
attribute this remarkable stability to the formation of chemical bonds
between the organophosphorus anchoring group and the metal oxide surface,
likely occurring through a heterocondensation reaction in water. From
XPS analysis, we also demonstrate that the surface coverage of the
chemisorbed molecules is highly affected by the degree of surface
hydroxylation, a parameter that can be tuned by simply preconditioning
the freshly cleaned ITO surfaces in water. The lower the relative
surface hydroxide density on ITO, the higher was the surface coverage
of the chemisorbed species. This behavior is in line with a chemisorption
mechanism involving coordination of a deprotonated phosphoryl oxygen
atom to the non-hydroxylated acidic metal sites of ITO
Chronoabsorptometry To Investigate Conduction-Band-Mediated Electron Transfer in Mesoporous TiO<sub>2</sub> Thin Films
Chronoabsorptometry
response of transparent mesoporous semiconductive
metal oxide film loaded with a redox-active dye is established in
the framework of fast electron diffusion. It is shown that the effect
of uncompensated resistance is to delay the electron accumulation
in the semiconductive thin film. Consequently, this effect must be
duly taken into account when interfacial charge transfer rates between
the adsorbed redox species and the porous semiconductor are extracted
from chronoabsorptometry responses. This is illustrated with the quantitative
analysis of the chronoabsorptometry responses of a heme-based redox
probe strongly adsorbed in highly ordered mesoporous TiO<sub>2</sub> thin films (prepared from evaporation-induced self-assembly, EISA).
It is shown that the reduction of the redox probe originates from
the conduction band and not from the localized traps in the bandgap.
These results are in quantitative agreement with differential cyclic
voltabsorptometry recently reported by us, thus showing full consistency
of both methods
Multivalent-Ion versus Proton Insertion into Nanostructured Electrochromic WO<sub>3</sub> from Mild Aqueous Electrolytes
Mild aqueous electrolytes containing multivalent metal
salts are
currently scrutinized for the development of ecosustainable energy-related
devices. However, the role of soluble multivalent metal ions in the
electrochemical reactivity of transition metal oxides is a matter
of debate, especially when they are performed in protic aqueous electrolytes.
Here, we have compared, by means of (spectro)electrochemistry, the
reversible electrochromic reduction of transparent nanostructured
γ-WO3 thin films in mild aqueous electrolytes of
various chemical composition and pH. This study reveals that reversible
proton insertion is the only charge storage mechanism over a large
pH range and that it is effective for aqueous electrolytes prepared
from either organic (such as acetic acid) or inorganic (such as solvated
multivalent cations) Bro̷nsted acids. By refuting charge storage
mechanisms relying on the reversible insertion of multivalent metal
ions, notably in aqueous electrolytes based on Al3+ ions
or a mixture of Al3+ and Zn2+ ions, these fundamental
results pave the way for the rational development of electrolytes
and active materials for a range of aqueous-based devices, such as
the emerging concept of an energy-saving smart window, which we also
address in this study
Evidencing Fast, Massive, and Reversible H<sup>+</sup> Insertion in Nanostructured TiO<sub>2</sub> Electrodes at Neutral pH. Where Do Protons Come From?
Ongoing
developments of sustainable energy technologies based on
high-surface-area semiconductive metal oxide electrodes operating
under mild and safe aqueous conditions require deep understanding
of proton and electron transfer/transport throughout their porous
structure. To address this issue, we investigated the electrochemical
reductive protonation of high surface area nanostructured amorphous
TiO<sub>2</sub> electrodes (produced by glancing angle deposition)
in both buffered and unbuffered aqueous solutions. Quantitative analysis
of the two charge storage mechanisms was achieved, allowing proper
deconvolution of the electrical double-layer capacitive charge storage
from the reversible faradaic one resulting from the proton-coupled
reduction of bulk TiO<sub>2</sub>. We evidence that this latter process
occurs reversibly and extensively (up to an intercalation ratio of
20%) not only under strongly acidic pH conditions but also, more interestingly,
under neutral pH with the intercalated proton arising from the buffer
rather than water. Moreover, we show that in comparison with reductive
Li<sup>+</sup> intercalation the proton-coupled electron charge storage
occurs more rapidly (in a few seconds). This important finding suggests
that a high-rate and high-power charge storage device could potentially
be achieved with the reversible H<sup>+</sup>-coupled charge/discharge
process in TiO<sub>2</sub> at neutral pH, opening thus new opportunities
to the development of eco-friendly batteries for electrical energy
storage
Unraveling the Mechanism of Catalytic Reduction of O<sub>2</sub> by Microperoxidase-11 Adsorbed within a Transparent 3D-Nanoporous ITO Film
Nanoporous films of indium tin oxide (ITO), with thicknesses
ranging
from 250 nm to 2 μm, were prepared by Glancing Angle Deposition
(GLAD) and used as highly sensitive transparent 3D-electrodes for
quantitatively interrogating, by time-resolved spectroelectrochemistry,
the reactivity of microperoxidase-11 (MP-11) adsorbed within such
films. The capacitive current densities of these 3D-electrodes as
well as the amount of adsorbed MP-11 were shown to be linearly correlated
to the GLAD ITO film thickness, indicating a homogeneous distribution
of MP-11 across the film as well as homogeneous film porosity. Under
saturating adsorption conditions, MP-11 film concentration as high
as 60 mM was reached. This is equivalent to a stack of 110 monolayers
of MP-11 per micrometer film thickness. This high MP-11 film loading
combined with the excellent ITO film conductivity has allowed the
simultaneous characterization of the heterogeneous one-electron transfer
dynamics of the MP-11 Fe<sup>III</sup>/Fe<sup>II</sup> redox couple
by cyclic voltammetry and cyclic voltabsorptometry, up to a scan rate
of few volts per second with a satisfactory single-scan signal-to-noise
ratio. The potency of the method to unravel complex redox coupled
chemical reactions was also demonstrated with the catalytic reduction
of oxygen by MP-11. In the presence of O<sub>2</sub>, cross-correlation
of electrochemical and spectroscopic data has allowed us to determine
the key kinetics and thermodynamics parameters of the redox catalysis
that otherwise could not be easily extracted using conventional protein
film voltammetry. On the basis of numerical simulations of cyclic
voltammograms and voltabsorptograms and within the framework of different
plausible catalytic reaction schemes including appropriate approximations,
it was shown possible to discriminate between different possible catalytic
pathways and to identify the relevant catalytic cycle. In addition,
from the best fits of simulations to the experimental voltammograms
and voltabsorptograms, the partition coefficient of O<sub>2</sub> for
the ITO film as well as the values of two kinetic rate constants could
be extracted. It was finally concluded that the catalytic reduction
of O<sub>2</sub> by MP-11 adsorbed within nanoporous ITO films occurs
via a 2-electron mechanism with the formation of an intermediate Fe<sup>III</sup>–OOH adduct characterized by a decay rate of 11 s<sup>–1</sup>. The spectroelectroanalytical strategy presented
here opens new opportunities for characterizing complex redox-coupled
chemical reactions not only with redox proteins, but also with redox
biomimetic systems and catalysts. It might also be of great interest
for the development and optimization of new spectroelectrochemical
sensors and biosensors, or eventually new photoelectrocatalytic systems
or biofuel cells