4 research outputs found
Engendering Long-Term Air and Light Stability of a TiO<sub>2</sub>‑Supported Porphyrinic Dye via Atomic Layer Deposition
Organic
and porphyrin-based chromophores are prevalent in liquid-junction
photovoltaic and photocatalytic solar-cell chemistry; however, their
long-term air and light instability may limit their practicality in
real world technologies. Here, we describe the protection of a zinc
porphyrin dye, adsorbed on nanoparticulate TiO<sub>2</sub>, from air
and light degradation by a protective coating of alumina grown with
a previously developed post-treatment atomic layer deposition (ALD)
technique. The protective Al<sub>2</sub>O<sub>3</sub> ALD layer is
deposited using dimethylaluminum isopropoxide as an Al source; in
contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum
isopropoxide does not degrade the zinc porphyrin dye, as confirmed
by UV–vis measurements. The growth of this protective ALD layer
around the dye can be monitored by an in-reactor quartz crystal microbalance
(QCM). Furthermore, greater than 80% of porphyrin light absorption
is retained over ∼1 month of exposure to air and light when
the protective coating is present, whereas almost complete loss of
porphyrin absorption is observed in less than 2 days in the absence
of the ALD protective layer. Applying the Al<sub>2</sub>O<sub>3</sub> post-treatment technique to the TiO<sub>2</sub>-adsorbed dye allows
the dye to remain in electronic contact with both the semiconductor
surface and a surrounding electrolyte solution, the combination of
which makes this technique promising for numerous other electrochemical
photovoltaic and photocatalytic applications, especially those involving
the dye-sensitized evolution of oxygen
Atomic Layer Deposition of Ultrathin Nickel Sulfide Films and Preliminary Assessment of Their Performance as Hydrogen Evolution Catalysts
Transition
metal sulfides show great promise for applications ranging
from catalysis to electrocatalysis to photovoltaics due to their high
stability and conductivity. Nickel sulfide, particularly known for
its ability to electrochemically reduce protons to hydrogen gas nearly
as efficiently as expensive noble metals, can be challenging to produce
with certain surface site compositions or morphologies, e.g., conformal
thin films. To this end, we employed atomic layer deposition (ALD),
a preeminent method to fabricate uniform and conformal films, to construct
thin films of nickel sulfide (NiS<sub><i>x</i></sub>) using
bisÂ(<i>N</i>,<i>N</i>′-di-<i>tert</i>-butylÂacetamidinato)ÂnickelÂ(II) (NiÂ(amd)<sub>2</sub>)
vapor and hydrogen sulfide gas. Effects of experimental conditions
such as pulse and purge times and temperature on the growth of NiS<sub><i>x</i></sub> were investigated. These revealed a wide
temperature range, 125–225 °C, over which self-limiting
NiS<sub><i>x</i></sub> growth can be observed. <i>In
situ</i> quartz crystal microbalance (QCM) studies revealed conventional
linear growth behavior for NiS<sub><i>x</i></sub> films,
with a growth rate of 9.3 ng/cm<sup>2</sup> per cycle being obtained.
The ALD-synthesized films were characterized using X-ray photoelectron
spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess
the electrocatalyitic activity of NiS<sub><i>x</i></sub> for evolution of molecular hydrogen, films were grown on conductive-glass
supports. Overpotentials at a current density of 10 mA/cm<sup>2</sup> were recorded in both acidic and pH 7 phosphate buffer aqueous reaction
media and found to be 440 and 576 mV, respectively, with very low
NiS<sub><i>x</i></sub> loading. These results hint at the
promise of ALD-grown NiS<sub><i>x</i></sub> materials as
water-compatible electrocatalysts
Photodriven Oxidation of Surface-Bound Iridium-Based Molecular Water-Oxidation Catalysts on Perylene-3,4-dicarboximide-Sensitized TiO<sub>2</sub> Electrodes Protected by an Al<sub>2</sub>O<sub>3</sub> Layer
Improving
stability and slowing charge recombination are some of
the greatest challenges in the development of dye-sensitized photoelectrochemical
cells (DSPECs) for solar fuels production. We have investigated the
effect of encasing dye molecules in varying thicknesses of Al<sub>2</sub>O<sub>3</sub> deposited by atomic layer deposition (ALD) before
catalyst loading on both the stability and the charge transfer dynamics
in organic dye-sensitized TiO<sub>2</sub> photoanodes containing iridium-based
molecular water-oxidation catalysts. In the TiO<sub>2</sub>|dye|Al<sub>2</sub>O<sub>3</sub>|catalyst electrodes, a sufficiently thick ALD
layer protects the perylene-3,4-dicarboximide (PMI) chromophores from
degradation over several weeks of exposure to light. The insulating
capacity of the layer allows a higher photocurrent in the presence
of ALD while initial charge injection is slowed by only 1.6 times,
as observed by femtosecond transient absorption spectroscopy. Rapid
picosecond-scale catalyst oxidation is observed in the presence of
a dinuclear catalyst, IrIr, but is slowed to tens of picoseconds for
a mononuclear catalyst, IrSil, that incorporates a long linker. Photoelectrochemical
experiments demonstrate higher photocurrents with IrSil compared to
IrIr, which show that recombination is slower for IrSil, while higher
photocurrents with IrIr upon addition of ALD layers confirm that ALD
successfully slows charge recombination. These findings demonstrate
that, beyond stability improvements, ALD can contribute to tuning
charge transfer dynamics in photoanodes for solar fuels production
and may be particularly useful for slowing charge recombination and
accounting for varying charge transfer rates based on the molecular
structures of incorporated catalysts
Highly Active NiO Photocathodes for H<sub>2</sub>O<sub>2</sub> Production Enabled via Outer-Sphere Electron Transfer
Tandem dye-sensitized
photoelectrosynthesis cells are promising
architectures for the production of solar fuels and commodity chemicals.
A key bottleneck in the development of these architectures is the
low efficiency of the photocathodes, leading to small current densities.
Herein, we report a new design principle for highly active photocathodes
that relies on the outer-sphere reduction of a substrate from the
dye, generating an unstable radical that proceeds to the desired product.
We show that the direct reduction of dioxygen from dye-sensitized
nickel oxide (NiO) leads to the production of H<sub>2</sub>O<sub>2</sub>. In the presence of oxygen and visible light, NiO photocathodes
sensitized with commercially available porphyrin, coumarin, and ruthenium
dyes exhibit large photocurrents (up to 400 μA/cm<sup>2</sup>) near the thermodynamic potential for O<sub>2</sub>/H<sub>2</sub>O<sub>2</sub> in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized
NiO over 24 h results in millimolar concentrations of H<sub>2</sub>O<sub>2</sub> with essentially 100% faradaic efficiency. To our knowledge,
these are among the most active NiO photocathodes reported for multiproton/multielectron
transformations. The photoelectrosynthesis proceeds by initial formation
of superoxide, which disproportionates to H<sub>2</sub>O<sub>2</sub>. This disproportionation-driven charge separation circumvents the
inherent challenges in separating electron–hole pairs for photocathodes
tethered to inner sphere electrocatalysts and enables new applications
for photoelectrosynthesis cells