We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spectroscopy, we monitor hydrogen abstraction by a functionalized heptazine (Hz) from substituted phenols in condensed-phase hydrogen-bonded complexes. Hz is the monomer unit of the ubiquitous organic polymeric photocatalyst graphitic carbon nitride (g-C₃N₄). Previously, we reported that the Hz derivative 2,5,8-tris(4-methoxyphenyl)-1,3,5,6,7,9,9b-heptaazaphenalene (TAHz) can photochemically abstract H atoms from water, in addition to exhibiting photocatalytic activity for H₂ evolution matching that of g-C₃N₄ in aqueous suspensions. In the present work, we combine ultrafast multipulse TA spectroscopy with predictive wave function-based ab initio electronic-structure calculations to explore the role of mixed nπ*/ππ* upper excited states in directing H atom abstraction from hydroxylic compounds. We use an ultraviolet (365 nm) laser pulse to photoexcite TAHz to a bright upper excited state, and, after a relaxation period of roughly 6 ps, we use a near-infrared (NIR) (1150 nm) pulse to “push” the chromophore from the long-lived S₁ state to a higher-lying excited state. When phenol is present, the NIR push induces a persistent decrease (ΔΔOD) in the S₁ TA signal magnitude, indicating an impulsively driven change in photochemical branching ratios. In the presence of substituted phenols with electron-donating moieties, the magnitude of ΔΔOD diminishes markedly due to the increased excited-state reactivity of these complexes that accompanies the cathodic shift in phenol oxidation potential. In the latter case, H atom abstraction proceeds unaided by additional energy from the push pulse. These results reveal new insight into branching mechanisms among unreactive locally excited states and reactive intermolecular charge-transfer states. They also suggest molecular design strategies for functionalizing aza-aromatics to drive important photoreactions, such as H atom abstraction from water. More generally, this study demonstrates an avidly desired achievement in the field of photochemistry, rationally redirecting excited-state reactivity with light.
We demonstrate chemical tuning and laser-driven control of intermolecular H atom abstraction from protic solvent molecules. Using multipulse ultrafast pump-push-probe transient absorption (TA) spectroscopy, we monitor hydrogen abstraction by a functionalized heptazine (Hz) from substituted phenols in condensed-phase hydrogen-bonded complexes. Hz is the monomer unit of the ubiquitous organic polymeric photocatalyst graphitic carbon nitride (g-C3N4). Previously, we reported that the Hz derivative 2,5,8-tris(4-methoxyphenyl)-1,3,5,6,7,9,9b-heptaazaphenalene (TAHz) can photochemically abstract H atoms from water, in addition to exhibiting photocatalytic activity for H2 evolution matching that of g-C3N4 in aqueous suspensions. In the present work, we combine ultrafast multipulse TA spectroscopy with predictive wave function-based ab initio electronic-structure calculations to explore the role of mixed nπ*/ππ* upper excited states in directing H atom abstraction from hydroxylic compounds. We use an ultraviolet (365 nm) laser pulse to photoexcite TAHz to a bright upper excited state, and, after a relaxation period of roughly 6 ps, we use a near-infrared (NIR) (1150 nm) pulse to "push" the chromophore from the long-lived S1 state to a higher-lying excited state. When phenol is present, the NIR push induces a persistent decrease (ΔΔOD) in the S1 TA signal magnitude, indicating an impulsively driven change in photochemical branching ratios. In the presence of substituted phenols with electron-donating moieties, the magnitude of ΔΔOD diminishes markedly due to the increased excited-state reactivity of these complexes that accompanies the cathodic shift in phenol oxidation potential. In the latter case, H atom abstraction proceeds unaided by additional energy from the push pulse. These results reveal new insight into branching mechanisms among unreactive locally excited states and reactive intermolecular charge-transfer states. They also suggest molecular design strategies for functionalizing aza-aromatics to drive important photoreactions, such as H atom abstraction from water. More generally, this study demonstrates an avidly desired achievement in the field of photochemistry, rationally redirecting excited-state reactivity with light.
Polyethylene glycol (PEO) is a commonly used polymer in the field of batteries for achieving flat and uniform electrodes to enhance the performance of batteries, such as lithium batteries and zinc (Zn) batteries. However, the impact of PEO on the electrochemical deposition of Zn metal on electrodes remains uncertain. In this study, we selected ZnSO 4 solution as electrolyte and copper (Cu) substrates as electrodes, which are widely applied in Zn batteries. We used in situ electrochemical atomic force microscopy (EC-AFM) to observe the nucleation and growth of Zn metal plates on the Cu substrate in the presence of different concentrations of ZnSO 4 and PEO additives. Our results indicate that PEO biases the crystallographic orientation of the initially deposited Zn metal nuclei, but does not have an obvious influence on subsequent growth. Based on our findings, we hypothesize that PEO primarily interacts with the Cu substrate to adjust the interfacial energy of the Cu-electrolyte interfaces. In contrast, due to the lack of apparent change in the Zn growth rate, we postulate that PEO does not affect the delivery rate of Zn 2+ to the electrode surface. The consistent aspect ratio of the Zn plates combined with the lack of an effect on growth rates further suggests that PEO does not interact significantly with the surface of the newly formed Zn plates. The Zn metal will undergo surface reorganization in a mildly acidic aqueous solution as a result of oxidation of Zn, which is not affected by PEO adsorption. Our findings provide both insight into the underlying mechanism by which PEO promotes electrode flattening in Zn batteries and a standardized protocol for elucidating the impact of additives on the morphological evolution of interfaces during electrochemical deposition. Work by C.O. was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Cesium bismuth halides (Cs-Bi-X) have recently been the subject of considerable attention as possible less-toxic alternatives to lead halide perovskites for luminescence and photovoltaics applications, but the full range of synthetically accessible Cs-Bi-X compositions has not been thoroughly explored, and some inconsistent results have appeared in the literature. Here, we have used a combination of hot-injection synthesis and post-synthetic anion exchange to prepare Cs-Bi-X nanocrystals with many structures and compositions, including several that have not previously been characterized. The structural and optical properties of Cs3BiX6 (X = Cl, Br, I) nanocrystals and Cs3Bi2X9 (X = Cl, Br, I) nanoplatelets are reported, and interconversion of these structures between different halide compositions is demonstrated through anion exchange using trimethylsilyl halides. Notably, we find that anion exchange can be used to access structural polymorphs not readily prepared through direct synthesis. In particular, a new structural modification of Cs3Bi2I9 has been accessed; whereas hot injection gives the previously reported "zero-dimensional" nonperovskite structure, anion exchange provides access to a "two-dimensional" layered, ordered-vacancy perovskite phase with a red-shifted absorption spectrum and distinctly different photoluminescence. Spectroscopic and computational characterization of these materials provides insight into structure/property relationships, including properties of the layered Cs3Bi2I9 material, that may be advantageous for optoelectronic applications.
Abstract Measuring and controlling the density of states (DOS) and defect states of two-dimensional van der Waals materials is of profound importance for understanding their unique physical properties, and for advancing their future practical applications. However, probing their defect states typically requires experiments performed at cryogenic temperatures and/or in ultra-high vacuum conditions, severely constraining efforts to monitor the electronic structure evolution of these materials under useful device operating conditions. Here, we develop a new electrochemical quantum capacitance spectroscopy (EQCS) technique for detecting the absolute energies of defect states and band edges in an ambient environment. We demonstrate the viability of this method with a variety of two-dimensional material systems, with the ability to easily extend to many more. The highest energy resolution achieved at room temperature, 116 meV, approaches the theoretical limit of 91 meV (3.5kBT). The in-situ EQCS platform can be further used to monitor and manipulate the DOS in real-time, enabling a controlled enhancement of electrochemical reactions. Notably, band shifts driven by as little as ≈ 1% mechanical strain can increase of the catalytic activity for hydrogen generation by half an order of magnitude. The EQCS platform provides a powerful new method for probing and manipulating the intrinsic DOS and defect states of 2D materials in ambient environments.
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