3 research outputs found
reaxFF Reactive Force Field for Disulfide Mechanochemistry, Fitted to Multireference ab Initio Data
Mechanochemistry,
in particular in the form of single-molecule
atomic force microscopy experiments, is difficult to model theoretically,
for two reasons: Covalent bond breaking is not captured accurately
by single-determinant, single-reference quantum chemistry methods,
and experimental times of milliseconds or longer are hard to simulate
with any approach. Reactive force fields have the potential to alleviate
both problems, as demonstrated in this work: Using nondeterministic
global parameter optimization by evolutionary algorithms, we have
fitted a reaxFF force field to high-level multireference
ab initio data for disulfides. The resulting force field can be used
to reliably model large, multifunctional mechanochemistry units with
disulfide bonds as designed breaking points. Explorative calculations
show that a significant part of the time scale gap between AFM experiments
and dynamical simulations can be bridged with this approach
Black TiO<sub>2</sub> Nanotubes: Cocatalyst-Free Open-Circuit Hydrogen Generation
Here
we report that TiO<sub>2</sub> nanotube (NT) arrays, converted by
a high pressure H<sub>2</sub> treatment to anatase-like “black
titania”, show a high open-circuit photocatalytic hydrogen
production rate without the presence of a cocatalyst. Tubes converted
to black titania using classic reduction treatments (e.g., atmospheric
pressure H<sub>2</sub>/Ar annealing) do not show this effect. The
main difference caused by the high H<sub>2</sub> pressure annealing
is the resulting room-temperature stable, isolated Ti<sup>3+</sup> defect-structure created in the anatase nanotubes, as evident from
electron spin resonance (ESR) investigations. This feature, absent
for conventional reduction, seems thus to be responsible for activating
intrinsic, cocatalytic centers that enable the observed high open-circuit
hydrogen generation
Forming a Highly Active, Homogeneously Alloyed AuPt Co-catalyst Decoration on TiO<sub>2</sub> Nanotubes Directly During Anodic Growth
Au
and Pt do not form homogeneous bulk alloys as they are thermodynamically
not miscible. However, we show that anodic TiO<sub>2</sub> nanotubes
(NTs) can in situ be uniformly decorated with homogeneous AuPt alloy
nanoparticles (NPs) during their anodic growth. For this, a metallic
Ti substrate containing low amounts of dissolved Au (0.1 atom %) and
Pt (0.1 atom %) is used for anodizing. The matrix metal (Ti) is converted
to oxide, whereas at the oxide/metal interface direct noble metal
particle formation and alloying of Au and Pt takes place; continuously
these particles are then picked up by the growing nanotube wall. In
our experiments, the AuPt alloy NPs have an average size of 4.2 nm,
and at the end of the anodic process, these are regularly dispersed
over the TiO<sub>2</sub> nanotubes. These alloyed AuPt particles act
as excellent co-catalyst in photocatalytic H<sub>2</sub> generation,
with a H<sub>2</sub> production rate of 12.04 μL h<sup>–1</sup> under solar light. This represents a strongly enhanced activity
as compared to TiO<sub>2</sub> NTs decorated with monometallic particles
of Au (7 μL h<sup>–1</sup>) or Pt (9.96 μL h<sup>–1</sup>)