6 research outputs found
Shape and Morphology Effects on the Electronic Structure of TiO<sub>2</sub> Nanostructures: From Nanocrystals to Nanorods
We carry out an accurate computational analysis on the nature and distribution of electronic trap states in shape-tailored anatase TiO<sub>2</sub> structures, investigating the effect of the morphology on the electronic structure. Linear nanocrystal models up to 6 nm in length with various morphologies, reproducing both flattened and elongated rod-shaped TiO<sub>2</sub> nanocrystals, have been investigated by DFT calculations, to clarify the effect of the crystal facet percentage on the nanocrystal electronic structure, with particular reference to the energetics and distribution of trap states. The calculated densities of states below the conduction band edge have been very well fitted assuming an exponential distribution of energies and have been correlated with experimental capacitance data. In good agreement with the experimental phenomenology our calculations show that elongated rod-shaped nanocrystals with higher values of the ratio between (100) and (101) facets exhibit a relatively deeper distribution of trap states. Our results point at the crucial role of the nanocrystal morphology on the trap state density, highlighting the importance of a balance between the low-energy (101) and high-energy (100)/(001) surface facets in individual TiO<sub>2</sub> nanocrystals
Self-Cleaning Organic/Inorganic Photo-Sensors
We present the fabrication of a multifunctional,
hybrid organic–inorganic
micropatterned device, which is capable to act as a stable photosensor
and, at the same time, displaying inherent superhydrophobic self-cleaning
wetting characteristics. In this framework several arrays of epoxy
photoresist square micropillars have been fabricated on n-doped crystalline
silicon substrates and subsequently coated with a polyÂ(3-hexylthiophene-2,5-diyl)
(P3HT) layer, giving rise to an array of organic/inorganic p–n
junctions. Their photoconductivity has been measured under a solar
light simulator at different illumination intensities. The current–voltage
(<i>I</i>–<i>V</i>) curves show high rectifying
characteristics, which are found to be directly correlated with the
illumination intensity. The photoresponse occurs in extremely short
times (within few tens of milliseconds range). The influence of the
interpillar distance on the <i>I</i>–<i>V</i> characteristics of the sensors is also discussed. Moreover, the
static and dynamic wetting properties of these organic/inorganic photosensors
can be easily tuned by changing the pattern geometry. Measured static
water contact angles range from 125° to 164°, as the distance
between the pillars is increased from 14 to 120 μm while the
contact angle hysteresis decreases from 36° down to 2°
Electrochemical Assessment of the Band-Edge Positioning in Shape-Tailored TiO<sub>2</sub>‑Nanorod-Based Photoelectrodes for Dye Solar Cells
Three families of linear shaped TiO<sub>2</sub> anatase
nanocrystals
with variable aspect ratio (4, 8, 16) and two sets of branched TiO<sub>2</sub> anatase nanocrystals (in the form of open-framework sheaf-like
nanorods and compact braid-like nanorod bundles, respectively) were
employed to fabricate high-quality mesoporous photoelectrodes and
then implemented into dye-sensitized solar cells to elucidate the
intrinsic correlation holding between the photovoltaic performances
and the structure of the nanocrystal building blocks. To this aim,
the chemical capacitance and the charge-transfer resistance of the
photoelectrodes were extrapolated from electrochemical impedance spectroscopy
measurements and used to draw a quantitative energy diagram of the
dye-sensitized solar cells realized, on the basis of which their photovoltaic
performances have been discussed. It has thus been revealed that photoanodes
made from braid-like branched-nanorod bundles exhibited the most favorable
conditions to minimize recombination at the interface with the electrolyte
due to their deep distribution of trap states, whereas linear-shaped
nanorods with higher aspect-ratios result in more remarkable downshift
of the conduction band edge
NiO/MAPbI<sub>3‑x</sub>Cl<sub><i>x</i></sub>/PCBM: A Model Case for an Improved Understanding of Inverted Mesoscopic Solar Cells
A spectroscopic
investigation focusing on the charge generation and transport in inverted
p-type perovskite-based mesoscopic (Ms) solar cells is provided in
this report. Nanocrystalline nickel oxide and PCBM are employed respectively
as hole transporting scaffold and hole blocking layer to sandwich
a perovskite light harvester. An efficient hole transfer process from
perovskite to nickel oxide is assessed, through time-resolved photoluminescence
and photoinduced absorption analyses, for both the employed absorbing
species, namely MAPbI<sub>3‑<i>x</i></sub>Cl<sub><i>x</i></sub> and MAPbI<sub>3</sub>. A striking relevant
difference
between p-type and n-type perovskite-based solar cells emerges from
the study
Influence of Porphyrinic Structure on Electron Transfer Processes at the Electrolyte/Dye/TiO<sub>2</sub> Interface in PSSCs: a Comparison between meso Push–Pull and β‑Pyrrolic Architectures
Time-resolved photophysical and photoelectrochemical
investigations
have been carried out to compare the electron transfer dynamics of
a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted
diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO<sub>2</sub> interface in PSSCs. Although the meso push–pull structural
arrangement has shown, up to now, to have the best performing architecture
for solar cell applications, we have obtained superior energy conversion
efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the
I<sup>–</sup>/I<sub>3</sub><sup>–</sup>-based electrolyte.
To gain deeper insights about these unexpected results, we have investigated
whether the intrinsic structural features of the two different porphyrinic
dyes can play a key role on electron transfer processes occurring
at the dye-sensitized TiO<sub>2</sub> interface. We have found that
charge injection yields into TiO<sub>2</sub> are quite similar for
both dyes and that the regeneration efficiencies by I<sup>–</sup>, are also comparable and in the range of 75–85%. Moreover,
besides injection quantum yields above 80%, identical dye loading,
for both ZnB and ZnM, has been evidenced by spectrophotometric measurements
on transparent thin TiO<sub>2</sub> layers after the same adsorption
period. Conversely, major differences have emerged by DC and AC (electrochemical
impedance spectroscopy) photoelectrochemical investigations, pointing
out a slower charge recombination rate when ZnB is adsorbed on TiO<sub>2</sub>. This may result from its more sterically hindered macrocyclic
core which, besides guaranteeing a decrease of π-staking aggregation
of the dye, promotes a superior shielding of the TiO<sub>2</sub> surface
against charge recombination involving oxidized species of the electrolyte
Ultrathin TiO<sub>2</sub>(B) Nanorods with Superior Lithium-Ion Storage Performance
The peculiar architecture of a novel
class of anisotropic TiO<sub>2</sub>(B) nanocrystals, which were synthesized
by an surfactant-assisted nonaqueous sol–gel route, was profitably
exploited to fabricate highly efficient mesoporous electrodes for
Li storage. These electrodes are composed of a continuous spongy network
of interconnected nanoscale units with a rod-shaped profile that terminates
into one or two bulgelike or branch-shaped apexes spanning areas of
about 5 × 10 nm<sup>2</sup>. This architecture transcribes into
a superior cycling performance (a charge capacitance of 222 mAh g<sup>–1</sup> was achieved by a carbon-free TiO<sub>2</sub>(B)-nanorods-based
electrode vs 110 mAh g<sup>–1</sup> exhibited by a comparable
TiO<sub>2</sub>-anatase electrode) and good chemical stability (more
than 90% of the initial capacity remains after 100 charging/discharging
cycles). Their outstanding lithiation/delithiation capabilities were
also exploited to fabricate electrochromic devices that revealed
an excellent coloration efficiency (130 cm<sup>2</sup> C<sup>–1</sup> at 800 nm) upon the application of 1.5 V as well as an extremely
fast electrochromic switching (coloration time ∼5 s)