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The photoelectrochemical performance of Ta₃N₅ photoanodes is strongly impacted by the presence of shallow and deep defects within the bandgap. However, the role of such states in defining the stability under operational conditions is not well understood. Here, we use a highly controllable synthesis approach to create homogenous Ta₃N₅ thin films with tailored defect concentrations to establish the relationship between atomic-scale point defects and macroscale stability. Reduced oxygen contents increase long-range structural order but lead to high concentrations of deep-level states, while higher oxygen contents result in reduced structural order but beneficially passivate deep-level defects. Despite the different defect properties, the synthesized photoelectrodes degrade similarly under water oxidation conditions due to formation of a surface oxide layer that blocks interfacial hole injection and accelerates charge recombination. In contrast, under ferrocyanide oxidation conditions, we find that Ta₃N₅ films with high oxygen concentrations exhibit long-term stability, whereas those possessing lower oxygen contents and higher deep-level defect concentrations rapidly degrade. These results indicate that deep-level defects result in rapid trapping of photocarriers and surface oxidation but that shallow oxygen donors can be introduced into Ta₃N₅ to enable kinetic stabilization of the interface.

Ternary nitrides are rapidly emerging as promising compounds for optoelectronic and energy conversion applications, yet comparatively little of this vast composition space has been explored. Furthermore, the crystal structures of these compounds can exhibit a significant amount of disorder, the consequences of which are not yet well understood. Here, the deposition of bixbyite-type ZrTaN3 thin films is demonstrated by reactive magnetron co-sputtering and observed semiconducting character, with a strong optical absorption onset at 1.8 eV and significant photoactivity, with prospective application as functional photoanodes. It is found that Wyckoff-site occupancy of cations is a critical factor in determining these beneficial optoelectronic properties. First-principles calculations show that cation disorder leads to minor deviations in the total energy but modulates the bandgap by 0.5 eV, changing orbital hybridization of valence and conduction band states. In addition to demonstrating that ZrTaN3 is a promising visible light-absorbing semiconductor and active photoanode material, the findings provide important insights regarding the role of cation ordering on the electronic structure of ternary semiconductors. In particular, it is shown that not only cation order, but also the cationic Wyckoff site occupancy has a substantial impact on key optoelectronic properties, which can guide future design and synthesis of advanced semiconductors.

Ta3N5 shows great potential as a semiconductor photoanode for solar water splitting. However, its performance is hindered by poor charge carrier transport and trapping due to a high density of defects that introduce electronic states deep within its bandgap. Here, we demonstrate that controlled Ti doping of Ta3N5 can dramatically reduce the concentration of deep-level defects and enhance its photoelectrochemical performance, yielding a sevenfold increase in photocurrent density and a 300 mV cathodic shift in photocurrent onset potential compared to undoped material. Comprehensive characterization reveals that Ti+4 ions substitute Ta+5 lattice sites, thereby introducing compensating acceptor states, reducing concentrations of nitrogen vacancies and reduced Ta+3 states, and thereby suppressing trapping and recombination. Importantly, Ti doping offers distinct advantages compared to Zr, an intensively investigated dopant of Ta3N5 in the same group of the periodic table. Specifically, Ti+4 and Ta+5 have more similar atomic radii, allowing for substitution without introducing lattice strain, and Ti exhibits a lower affinity for oxygen than Zr, enabling its incorporation without increasing the oxygen donor content. Consequently, we demonstrate that the electrical conductivity can be tuned by over seven orders of magnitude. Thus, Ti doping in Ta3N5 provides a powerful basis for precisely engineering the optoelectronic characteristics of Ta3N5 and to substantially improve its functional characteristics as an advanced photoelectrode for solar fuels applications.

Transition metal oxynitrides are a promising class of functional materials for photoelectrochemical (PEC) applications. Although these compounds are most commonly synthesized via ammonolysis of oxide precursors, such synthetic routes often lead to poorly controlled oxygen-to-nitrogen anion ratios, and the harsh nitridation conditions are incompatible with many substrates, including transparent conductive oxides. Here, we report direct reactive sputter deposition of a family of zirconium oxynitride thin films and the comprehensive characterization of their tunable structural, optical, and functional PEC properties. Systematic increases of the oxygen content in the reactive sputter gas mixture enable access to different crystalline structures within the zirconium oxynitride family. Increasing oxygen contents lead to a transition from metallic to semiconducting to insulating phases. In particular, crystalline Zr2ON2-like films have band gaps in the UV−visible range and are n-type semiconductors. These properties, together with a valence band maximum position located favorably relative to the water oxidation potential, make them viable photoanode candidates. Using chopped linear sweep voltammetry, we indeed confirm that our Zr2ON2 films are PEC-active for the oxygen evolution reaction in alkaline electrolytes. We further show that high-vacuum annealing boosts their PEC performance characteristics. Although the observed photocurrents are low compared to state-of-the-art photoanodes, these dense and planar thin films can offer a valuable platform for studying oxynitride photoelectrodes, as well as for future nanostructuring, band gap engineering, and defect engineering efforts.

The Upper Jurassic – Lower Cretaceous interval represents a prolonged marine deoxygenation period particularly in the Boreal–Arctic basins, the controlling factors of which remain poorly understood. Two drill cores totaling >450 m cover the Kimmeridgian–Barremian succession in contrasting locations in an evolving half-graben system (basin center and near the footwall crest) in Wollaston Forland, NE Greenland; they provide an exceptional ∼20 myr long window into paleoenvironmental development and changes in redox conditions within a detailed tectonostratigraphic framework. Synthesis of a multidisciplinary dataset including sedimentology, inorganic and previously published organic geochemistry indicates that, despite continuous black mudstone accumulation from the Kimmeridgian to the Ryazanian, sea floor anoxia was intermittent in the Kimmeridgian, whereas more sustained anoxia/euxinia occurred in the middle Volgian – early Ryazanian. Correlation to reported contemporaneous successions along the Greenland margin, indicate that protracted rifting and generation of localized sea-floor topography were among the major drivers both of sea-floor deoxygenation and current funneling and amplification during the Jurassic–Cretaceous transition. Consequently, distribution of seaway current activity and dysoxia, anoxia and euxinia varied spatially, allowing fully oxygenated and anoxic pockets to coexist.

Chirality is a desired property in functional semiconductors for optoelectronic, catalytic, and spintronic applications. Here, introducing enantiomerically-pure 3-aminobutyric acid (3-ABA) into thin films of the 1D semiconductor dimethylammonium lead iodide (DMAPbI3) is found to result in strong circular dichroism (CD) in the optical absorption. X-ray diffraction and grazing incidence small angle X-ray scattering (GISAXS) are applied to gain molecular-scale insights into the chirality transfer mechanism, which is attributed to a chiral surface modification of DMAPbI3 crystallites. This study demonstrates that the CD signal strength can be controlled by the amino-acid content relative to the crystallite surface area. The CD intensity is tuned by the composition of the precursor solution and the spin-coating time, thereby achieving anisotropy factors (gabs) as high as 1.75 × 10–2. Grazing incidence wide angle scattering reveals strong preferential ordering that can be suppressed via tailored synthesis conditions. Different contributions to the chiroptical properties are resolved by a detailed analysis of the CD signal utilizing an approach based on the Mueller matrix model. This report of a novel class of chiral hybrid semiconductors with precise control over their optical activity presents a promising approach for the design of circularly polarized light detectors and emitters., QC 20230315

In volcanic rifted margins, the timing of hydrocarbon charge is difficult to predict, but is important in understanding fluid genesis. We investigated whether igneous activity was linked to hydrocarbon charge in the prolific South Atlantic pre-salt petroleum system. To do this, we applied in situ carbonate U-Pb geochronology, a relatively novel tool for dating hydrocarbon migration, to bituminous veins in pre-salt travertines from the rifted onshore Namibe Basin (Angola). To test if fluid flow was synchronous with known volcanic pulses, we also obtained new 40Ar/39Ar geochronology from a nearby volcanic complex. Bitumen is associated with calcite in a first generation of veins and vugs, and with dolomite in younger veins. The dated calcite veins yielded a pooled U-Pb age of 86.2 ± 2.4 Ma, which overlaps the volcanism 40Ar/39Ar age of 89.9 ± 1.8 Ma. The overlapping dates and the localized bitumen occurrence around the dated volcanic center show a clear genetic relationship between Late Cretaceous igneous activity and hydrocarbon charge. The dolomite was dated at 56.8 ± 4.8 Ma, revealing a previously unknown Paleocene/Eocene fluid-flow phase in the basin.