4 research outputs found
A Chemometric Approach for the Sensitization Procedure of ZnO Flowerlike Microstructures for Dye-Sensitized Solar Cells
In
this paper, a methodology for the streamlining of the sensitization
procedure of flowerlike ZnO nanostructures for dye-sensitized solar
cells (DSCs) is reported. The sensitization of ZnO surface with ruthenium-based
complexes is a particularly critical process, since one has to minimize
the dissolution of surface Zn atoms by the protons released from the
dye molecules, leading to the formation of Zn<sup>2+</sup>/dye complexes.
The fine-tuning of the experimental parameters, such as the dye loading
time, the dye concentration, and the pH of the sensitizing solution,
performed through a multivariate optimization by means of a chemometric
approach, is here reported. The dye loading procedure was optimized
using ZnO microparticles with nanostructured protrusions, synthesized
by a simple and low-cost hydrothermal process. Mild reaction conditions
were used, and wurtzite-like crystalline structure with a relatively
high surface area was obtained once the reaction process was completed.
After dispersion of ZnO flowerlike particles in an acetic acid-based
solution, a 14 ÎĽm-thick ZnO layer acting as DSC photoanode was
fabricated. The optimized sensitization procedure allowed minimizing
the instability of ZnO surface in contact with acidic dyes, avoiding
the formation of molecular agglomerates unable to inject electrons
in the ZnO conduction band, achieving good results in the photoconversion
efficiency. Moreover, the photoharvesting properties were further
enhanced by adding <i>N</i>-methylbenzimidazole into the
iodine-based liquid electrolyte. Such an additive was proposed here
for the first time in combination with a ZnO photoelectrode, helping
to reduce an undesired recombination between the photoinjected electrons
and the oxidized redox mediator
First Pseudohalogen Polymer Electrolyte for Dye-Sensitized Solar Cells Promising for <i>In Situ</i> Photopolymerization
The incorporation of selenocianated-based
redox couple in a polymer
electrolyte for dye-sensitized solar cells is reported for the first
time. The pseudohalogen redox mediator was integrated in two kinds
of acrylic/methacrylic membranes prepared by photocopolymerization
of multifunctional monomers. Before activation, the obtained membranes
were transparent, self-standing and flexible, and the physicochemical
characterizations of the films showed the formation of highly crosslinked
architectures. Membranes were activated by swelling in an optimized
solution containing the SeCN<sup>–</sup>/(SeCN)<sub>2</sub> redox mediator with 4-<i>tert</i>-butylpyridine in acetonitrile,
and the electrochemical behavior of the electrolytes revealed fast
charge transfer kinetics. The photovoltaic performances of quasi-solid
dye-sensitized solar cells were evaluated and compared with the results
of the liquid counterpart, showing promising photoharvesting properties.
No diminution in photoconversion efficiencies was evidenced in the
comparison between solid and liquid cells, demonstrating an optimal
kinetics of the redox species in the polymer cage, associated with
a noteworthy increase in device durability, as demonstrated by aging
tests. In addition, the <i>in situ</i> photopolymerization
in the presence of the redox mediator is presented with outstanding
results: this process, hardly feasible for the traditional I<sup>–</sup>/I<sub>3</sub><sup>–</sup> couple (inhibitor of radical polymerization
processes), enables at the same time the creation of an excellent
electrode/electrolyte interface and the sealing of the device
Unravelling Resistive Switching Mechanism in ZnO NW Arrays: The Role of the Polycrystalline Base Layer
The
physical mechanism involved in resistive switching phenomena
occurring in devices based on ZnO nanowire (NW) arrays may vary considerably,
also depending on the structure of the switching layer. In particular,
it is shown here that the formation of a ZnO base layer between the
metallic catalyst substrate and the NW, which is typical of CVD-grown
ZnO NW arrays, should not be neglected when explaining the switching
physical mechanism. The structural and electrical properties of this
layer are investigated after the mechanical removal of NWs. Electrical
measurements were performed in the presence of NWs and, after their
removal, showed that the base alone exhibits resistive switching properties.
The proposed switching mechanism is based on the creation/rupture
of an oxygen vacancies conductive path along grain boundaries of the
polycrystalline base. The creation of the filament is facilitated
by the high concentration of vacancies at the grain boundaries that
are oriented perpendicularly to the electrodes, as a direct consequence
of the ZnO growth along the <i>c</i>-axis of the wurtzite
lattice
Mixed 1T–2H Phase MoS<sub>2</sub>/Reduced Graphene Oxide as Active Electrode for Enhanced Supercapacitive Performance
A hybrid
aerogel, composed of MoS<sub>2</sub> sheets of 1T (distorted octahedral)
and 2H (trigonal prismatic) phases, finely mixed with few layers of
reduced graphene oxide (rGO) and obtained by means of a facile environment-friendly
hydrothermal cosynthesis, is proposed as electrode material for supercapacitors.
By electrochemical characterizations in three- and two-electrode configurations
and symmetric planar devices, unique results have been obtained, with
specific capacitance values up to 416 F g<sup>–1</sup> and
a highly stable capacitance behavior over 50000 charge–discharge
cycles. The in-depth morphological and structural characterizations
through field emission scanning electron microscopy, Raman, X-ray
photoelectron spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller,
and transmission electron microscopy analysis provides the proofs
of the unique assembly of such 3D structured matrix. The unpacked
MoS<sub>2</sub> structure exhibits an excellent distribution of 1T
and 2H phase sheets that are highly exposed to interaction with the
electrolyte, and so available for surface/near-surface redox reactions,
notwithstanding the quite low overall content of MoS<sub>2</sub> embedded
in the reduced graphene oxide (rGO) matrix. A comparison with other
“more conventional” hybrid rGO-MoX<sub>2</sub> electrochemically
active materials, synthesized in the same conditions, is provided
to support the outstanding behavior of the cosynthesized rGO-MoS<sub>2</sub>