Overall photocatalytic
water splitting is one of the most sought after processes for sustainable
solar-to-chemical energy conversion. The efficiency of this process
strongly depends on charge carrier recombination and interaction with
surface adsorbates at different time scales. Here, we investigated
how hydration of TiO2 P25 affects dynamics of photogenerated
electrons at the millisecond to minute time scale characteristic for
chemical reactions. We used rapid scan diffuse-reflectance infrared
Fourier transform spectroscopy (DRIFTS). The decay of photogenerated
electron absorption was substantially slower in the presence of associated
water. For hydrated samples, the charge carrier recombination rates
followed an Arrhenius-type behavior in the temperature range of 273–423
K; these became temperature-independent when the material was dehydrated
at temperatures above 423 K or cooled below 273 K. A DFT+U analysis revealed that hydrogen bonding with adsorbed water stabilizes
surface-trapped holes at anatase TiO2(101) facet and lowers
the barriers for hole migration. Hence, hole mobility should be higher
in the hydrated material than in the dehydrated system. This demonstrates
that adsorbed associated water can efficiently stabilize photogenerated
charge carriers in nanocrystalline TiO2 and suppress their
recombination at the time scale up to minutes.
A photocatalytic route for the conversion of lactic acid to acetaldehyde in water is demonstrated. Direct UV photolysis of lactic acid yields CO2 and ethanol via a radical mechanism. Pt/TiO2 considerably increases the rate of lactic acid decarboxylation with acetaldehyde, H2 and CO2 as the main products. A concerted photodecarboxylation/dehydrogenation mechanism is proposed.
The
mismatch between short lifetimes of free charge carriers and
slow kinetics of surface redox reactions substantially limits the
efficiency of most photocatalytic systems. Hence, the knowledge of
trapping and recombination of photogenerated electrons and holes at
different time scales is key for a rational optimization of photocatalytic
materials. In this study, we used subsecond time-resolved diffuse-reflectance
FTIR spectroscopy to investigate how energy and intensity of the incident
irradiation affect the dynamics of photogenerated charge carriers
in TiO2 P25 photocatalysts subjected to different pretreatments
and how shallow trapped electrons (STE) are formed under these conditions.
Intensity-dependent measurements demonstrated that electrons and holes
generated by 325 and 409 nm irradiation undergo bimolecular and trap-assisted
recombination, respectively. Analysis of characteristic times of photogenerated
electron absorption rise and decay indicated that the apparent charge
carrier dynamics at the time scale of seconds to minutes relate to
chemical trapping of photogenerated electrons and holes. The presence
of dissociatively adsorbed water on the oxide surface was required
for efficient STE formation. This suggests that STE form at the seconds–minutes
time scale upon surface-mediated self-trapping of electrons.
The formation of Cd-Zn sulfide solid solutions from mixed hydroxides under hydrothermal conditions is investigated in detail. The work specifically aims to understand the formation and the role of nanotwinned mixed sulfide particles that have been reported to show excellent performance in photocatalytic water splitting (Liu, M.; et al. Energy Environ. Sci. 2011, 4, 1372). The influence of additives, pH, autoclave tumbling, and the state of the mixed hydroxide precursor on the mixed sulfides was studied by XRD, XPS, TEM, DR UV-vis, and N2 physisorption. Cd-Zn sulfides are formed via a dissolution-precipitation mechanism. Agitation of the synthetic medium and the formation of soluble intermediate complexes during hydrothermal treatment suppress the formation of a hexagonal wurtzite crystal phase and improve the photocatalytic activity of the mixed sulfides. The role of additives can be understood in terms of complex formation, pH maintenance, and adsorption on the facets of growing crystallites. All Cd-Zn sulfide samples exhibit compositional inhomogeneities, resulting in XRD line broadening and decreased bandgaps as compared with the values predicted by Vegard's law. Detailed TEM analysis revealed that the samples with higher amounts of nanotwinned particles were significantly less active in water reduction. The influence of nanotwinned particles is discussed in terms of extended crystal defects and charge carrier recombination.
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