5 research outputs found
Tailoring the Morphology of Mesoporous Titania Thin Films through Biotemplating with Nanocrystalline Cellulose
The tunable porosity of titania thin
films is a key factor for
successful applications in photovoltaics, sensing, and photocatalysis.
Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent
templating agent enabling the straightforward synthesis of mesoporous
titania thin films. The obtained structures are highly porous anatase
morphologies having well-defined, narrow pore size distributions.
By varying the titania-to-template ratio, it is possible to tune the
surface area, pore size, pore anisotropy, and dimensions of titania
crystallites in the films. Moreover, a post-treatment at high humidity
and subsequent slow template removal can be used to achieve pore widening;
this treatment is also beneficial for the multilayer deposition of
thick films. The resulting homogeneous transparent films can be directly
spin- or dip- coated on glass, silicon, and transparent conducting
oxide (TCO) substrates. The mesoporous titania films show very high
activity in the photocatalytic NO conversion and in the degradation
of 4-chlorophenol. Furthermore, the films can be successfully applied
as anodes in dye-sensitized solar cells
Nanocellulose-Assisted Formation of Porous Hematite Nanostructures
We report the formation of porous
iron oxide (hematite) nanostructures
via solāgel transformations of molecular precursors in the
confined space of self-organized nanocrystalline cellulose (NCC) used
as a shape-persistent template. The obtained structures are highly
porous Ī±-Fe<sub>2</sub>O<sub>3</sub> (hematite) morphologies
with a well-defined anisotropic porosity. The character of the porous
nanostructure depends on the iron salt used as the precursor and the
heat treatment. Moreover, a postsynthetic hydrothermal treatment of
the NCC/iron salt composites strongly affects the crystal growth as
well as the porous nanomorphology of the obtained hematite scaffolds.
We demonstrate that the hydrothermal treatment alters the crystallization
mechanism of the molecular iron precursors, which proceeds via the
formation of anisotropic iron oxyhydroxide species. The nanocellulose
templating technique established here enables the straightforward
fabrication of a variety of mesoporous crystalline iron oxide scaffolds
with defined porous structure and is particularly attractive for the
processing of porous hematite films on different substrates
Nanocellulose-Templated Porous Titania Scaffolds Incorporating Presynthesized Titania Nanocrystals
Nanocrystalline
cellulose (NCC) is an abundant biogenic nanomaterial
with unique properties that enables the efficient synthesis of mesoporous
crystalline titania. We significantly enhance the photocatalytic activity
of titania thin films by introducing solvothermally synthesized preformed
anatase nanoparticles into a solāgel based biotemplated titania
scaffold. The resulting dual source titania thin films containing
different amounts of preformed crystalline species were investigated
by time-resolved microwave conductivity (TRMC) measurements and tested
in the photocatalytic conversion of 4-chlorophenol. The gradual addition
of preformed nanoparticles leads to a consistent increase of the mean
size of titania crystalline domains, whereas the porosity of the composite
is well-preserved due to the shape-persistent nature of the NCC template.
Microwave conductivity studies establish increased photoconductivity
of the films containing preformed anatase nanoparticles in comparison
to that of films made without the nanoparticles. The synergistic features
of the dual source titania, namely the improved crystalline properties
brought by the preformed nanocrystals in combination with the high
surface area provided by the NCC-templated solāgel titania,
result in a very high photocatalytic activity of the films in the
photocatalytic decomposition of 4-chlorophenol. In quantitative terms,
the dual source titania films prepared with 75% nanoparticles exhibit
a first order degradation rate constant of 0.53 h<sup>ā1</sup> (1.47 Ć 10<sup>ā4</sup> sec<sup>ā1</sup>), which
strongly outperforms the activity of commercial P90 nanopowder showing
a rate constant of 0.17 h<sup>ā1</sup> (0.47 Ć 10<sup>ā4</sup> sec<sup>ā1</sup>) under the same conditions
Nanostructured Ternary FeCrAl Oxide Photocathodes for Water Photoelectrolysis
A solāgel method for the synthesis
of semiconducting FeCrAl
oxide photocathodes for solar-driven hydrogen production was developed
and applied for the production of meso- and macroporous layers with
the overall stoichiometry Fe<sub>0.84</sub>Cr<sub>1.0</sub>Al<sub>0.16</sub>O<sub>3</sub>. Using transmission electron microscopy and
energy-dispersive X-ray spectroscopy, phase separation into Fe- and
Cr-rich phases was observed for both morphologies. Compared to prior
work and to the mesoporous layer, the macroporous FeCrAl oxide photocathode
had a significantly enhanced photoelectrolysis performance, even at
a very early onset potential of 1.1 V vs RHE. By optimizing the macroporous
electrodes, the device reached current densities of up to 0.68 mA
cm<sup>ā2</sup> at 0.5 V vs RHE under AM 1.5 with an incident
photon-to-current efficiency (IPCE) of 28% at 400 nm without the use
of catalysts. Based on transient measurements, this performance increase
could be attributed to an improved collection efficiency. At a potential
of 0.75 V vs RHE, an electron transfer efficiency of 48.5% was determined
Passivation of PbS Quantum Dot Surface with lāGlutathione in Solid-State Quantum-Dot-Sensitized Solar Cells
Surface oxidation of quantum dots
(QDs) is one of the biggest challenges
in quantum dot-sensitized solar cells (QDSCs), because it introduces
surface states that enhance electronāhole recombination and
degrade device performance. Protection of QDs from surface oxidation
by passivating the surface with organic or inorganic layers can be
one way to overcome this issue. In this study, solid-state QDSCs with
a PbS QD absorber layer were prepared from thin mesoporous TiO<sub>2</sub> layers by the successive ionic layer adsorption/reaction
(SILAR) method. Spiro-OMeTAD was used as the organic p-type hole transporting
material (HTM). The effects on the solar cell performance of passivating
the surface of the PbS QDs with the tripeptide l-glutathione
(GSH) were investigated. Currentāvoltage characteristics and
external quantum efficiency measurements of the solar cell devices
showed that GSH-treatment of the QD-sensitized TiO<sub>2</sub> electrodes
more than doubled the short circuit current and conversion efficiency.
Impedance spectroscopy, intensity-modulated photovoltage and photocurrent
spectroscopy analysis of the devices revealed that the enhancement
in solar cell performance of the GSH-treated cells originates from
improved charge injection from PbS QDs into the conduction band of
TiO<sub>2</sub>. Time-resolved photoluminescence decay measurements
show that passivation of the surface of QDs with GSH ligands increases
the exciton lifetime in the QDs