The behavior of crystalline nanoparticles depends strongly on which facets are exposed. Some facets are more active than others, but it is difficult to selectively isolate particular facets. This study provides fundamental insights into photocatalytic and photoelectrochemical performance of three types of TiO(2) nanoparticles with predominantly exposed {101}, {010}, or {001} facets, where 86-99% of the surface area is the desired facet. Photodegradation of methyl orange reveals that {001}-TiO(2) has 1.79 and 3.22 times higher photocatalytic activity than {010} and {101}-TiO(2), respectively. This suggests that the photochemical performance is highly correlated with the surface energy and the number of under-coordinated surface atoms. In contrast, the photoelectrochemical performance of the faceted TiO(2) nanoparticles sensitized with the commercially available MK-2 dye was highest with {010}-TiO(2) which yielded an overall cell efficiency of 6.1%, compared to 3.2% for {101}-TiO(2) and 2.6% for {001}-TiO(2) prepared under analogous conditions. Measurement of desorption kinetics and accompanying computational modeling suggests a stronger covalent interaction of the dye with the {010} and {101} facets compared with the {001} facet. Time-resolved THz spectroscopy and transient absorption spectroscopy measure faster electron injection dynamics when MK-2 is bound to {010} compared to other facets, consistent with extensive computational simulations which indicate that the {010} facet provides the most efficient and direct pathway for interfacial electron transfer. Our experimental and computational results establish for the first time that photoelectrochemical performance is dependent upon the binding energy of the dye as well as the crystalline structure of the facet, as opposed to surface energy alone.

Thin mesoporous photoconductive layers are critically important for efficient water-spitting solar cells. A detailed understanding of photoconductivity in these materials can be achieved via terahertz transient absorption measurements. Such measurements are commonly interpreted using the thin-film approximation. We compare this approximation with a numerical solution of the transfer function without approximations using experimental results for thin-film mesoporous tin oxide (SnO 2) samples which range in thickness from 3.3 to 12.6 mm. These samples were sensitized with either a ruthenium polypyridyl complex or a porphyrin dye. The two sensitizers have markedly different absorption coefficients, resulting in penetration depths of 15 mm and 1 mm, respectively. The thin-film approximation results are in good agreement with the numerical work-up for the short penetration length dye. For the longer penetration length samples, the thin-film formula fails even for thicknesses of only 3 mm % k/100. The imaginary part of the conductivity calculated using the thin-film formula was significantly larger in magnitude than the value without approximations. This discrepancy between the commonly used thin-film approximation and the numerical solution demonstrates the need for a careful analysis of the thin-film formula.

Time-resolved terahertz (THz) spectroscopy and open-circuit photovoltage measurements were employed to examine the size-dependent charge carrier dynamics of tungsten (VI) oxide (WO 3 ) particles for their use as the photoanode in photoelectrochemical cells. Specifically, films of commercially available WO 3 nanoparticles (NPs) and granular particles (GPs) with diameters of 77 ± 34 nm and 390 ± 260 nm, respectively, were examined in air and while immersed in 0.1 M Na 2 SO 4 electrolyte (pH = 2). Examination of the frequency-dependent transient photoconductivity at short and long timescales indicates the presence of both photoinduced high net transport charge carriers at early times, and in some cases low net transport charge carriers at later times. The high net transport charge carriers dominate the photoconductivity signal for ~100 ps after photoexcitation. Depletion of the short-lived high net transport carriers due to trapping leads to the detection of longer-lived low net transport photoinduced charge carriers that likely contribute to surface chemistry. Photoelectrochemical cells (PECs) convert solar energy into either electrical power or a fuel, and their optimization is a promising pathway in the development of renewable energy sources. 1-3 Metal oxide semiconductors have been widely studied for their application in water splitting cells due to their favorable structural and electronic properties. 4-7 Tungsten (VI) oxide (WO 3 ) is of particular interest because it has a relatively small band gap energy of 2.6 eV (corresponding to a wavelength of 475 nm), enabling it to absorb light in the visible spectrum without the need for sensitizer dyes. 8,9 Its electrochemical properties 10,11 and spectroscopic properties, including transient absorption 12-14 and time-resolved microwave spectroscopy, 15 have been studied previously. However, ultrafast photoinduced charge carrier dynamics in WO 3 have not yet been investigated on the picosecond timescale. Our interests lie in the characterization and dynamics of the nascent photoinduced carriers that are required for efficient oxidation at the metal oxide/ electrolyte interface.We examine for the first time ultrafast photoinduced carrier dynamics in WO 3 particles as a function of particle size utilizing optical pump -THz probe (OPTP) spectroscopy. THz measurements were performed under open-circuit conditions on as-prepared dry films in air, as well as in the presence of the aqueous electrolyte employed during photocatalysis in order to gain insights into the ultrafast dynamics under near-operating conditions. The spectroscopic results, including measurements of photoinduced charge carrier lifetimes and the frequencydependent complex transient photoconductivity, are also correlated with open-circuit photoelectrochemical measurements of two WO 3 particle sizes.

The development of water‐splitting dye‐sensitized photoelectrochemical cells has gained interest owing to their ability to generate renewable fuels from solar energy. In this study, photoanodes were assembled from a SnO2 film sensitized with a combination of a high‐potential CF3‐substituted porphyrin dye with a tetrahydropyranyl‐protected hydroxamic acid surface‐anchoring group and a Cp*Ir (Cp*=pentamethylcyclopentadienyl) water‐oxidation catalyst containing a silatrane anchoring group. The dye/catalyst ratios were varied from 2:1 to 32:1 to optimize the photocatalytic water oxidation. Photoelectrochemical measurements showed not only more stable and reproducible photocurrents for lower dye/catalyst ratios but also improved photostability. O2 production was confirmed in real time over a 20 h period with a Clark electrode. Photoanodes prepared from 2:1 and 8:1 dye/catalyst sensitization solutions provided the most active electrodes for photocatalytic water oxidation and performed approximately 30–35 turnovers in 20 h.

Inverse opal scaffolds are finding widespread use in tissue engineering and regenerative medicine. Herein, the way in which the pore sizes and related physical properties of poly(D,L-lactide-co-glycolide) inverse opal scaffolds are affected by the fabrication conditions is systematically investigated. It is found that the window size of an inverse opal scaffold is mainly determined by the annealing temperature rather than the duration of time, and the surface pore size is largely determined by the concentration of the infiltration solution. Although scaffolds with larger pore or window sizes facilitate faster migration of cells, they show slightly lower compressive moduli than scaffolds with smaller pore or window sizes.