Ground and excited state charge transfer at aqueous nanodiamonds

Nanodiamonds (NDs) are unique carbonaceous materials with exceptionally high stability, hardness, and notable electronic properties. Their applications in photocatalysis, biomedicine, and energy materials are usually carried out in aqueous environments, where they interact with aqueous adsorbates. Especially, electron density may rearrange from the diamond material toward oxidative adsorbates such as oxygen, which is known as charge transfer doping. In this article, we quantify the charge transfer doping for NDs with inhomogeneous surface coverings (hydroxyl, fluorine, and amorphous carbon), as well as NDs doped with heteroatoms (B, Si, N) using hybrid density functional theory (DFT) calculations. The transfer doping magnitude is largely determined by the NDs' highest occupied molecular orbital energies, which can in turn be modified by the surface covering and doping. However, local modifications of the ND structures do not have any local effects on the magnitude of the charge transfer. We furthermore analyze the impact of aqueous adsorbates on the excited states of an aqueous ND in the context of photocatalysis via time-dependent DFT. Here, we find that the excited electrons are biased to move in the direction of the respective oxidative adsorbate. Surprisingly, we find that also unreactive species such as nitrous oxide may attract the excited electrons, which is probably due to the positive partial charge that is induced by the local N2 O solvation geometry.

Machine Learning Frontier Orbital Energies of Nanodiamonds

Nanodiamonds have a wide range of applications including catalysis, sensing, tribology, and biomedicine. To leverage nanodiamond design via machine learning, we introduce the new data set ND5k, consisting of 5089 diamondoid and nanodiamond structures and their frontier orbital energies. ND5k structures are optimized via tight-binding density functional theory (DFTB) and their frontier orbital energies are computed using density functional theory (DFT) with the PBE0 hybrid functional. From this data set we derive a qualitative design suggestion for nanodiamonds in photocatalysis. We also compare recent machine learning models for predicting frontier orbital energies for similar structures as they have been trained on (interpolation on ND5k), and we test their abilities to extrapolate predictions to larger structures. For both the interpolation and extrapolation task, we find the best performance using the equivariant message passing neural network PaiNN. The second best results are achieved with a message passing neural network using a tailored set of atomic descriptors proposed here.

Highly Occupied Surface States at Deuterium-Grown Boron-Doped Diamond Interfaces for Efficient Photoelectrochemistry

Polycrystalline boron-doped diamond is a promising material for high-power aqueous electrochemical applications in bioanalytics, catalysis, and energy storage. The chemical vapor deposition (CVD) process of diamond formation and doping is totally diversified by using high kinetic energies of deuterium substituting habitually applied hydrogen. The high concentration of deuterium in plasma induces atomic arrangements and steric hindrance during synthesis reactions, which in consequence leads to a preferential (111) texture and more effective boron incorporation into the lattice, reaching a one order of magnitude higher density of charge carriers. This provides the surface reconstruction impacting surficial populations of CC dimers, CH, CO groups, and COOH termination along with enhanced kinetics of their abstraction, as revealed by high-resolution core-level spectroscopies. A series of local densities of states were computed, showing a rich set of highly occupied and localized surface states for samples deposited in deuterium, negating the connotations of band bending. The introduction of enhanced incorporation of boron into (111) facet of diamond leads to the manifestation of surface electronic states below the Fermi level and above the bulk valence band edge. This unique electronic band structure affects the charge transfer kinetics, electron affinity, and diffusion field geometry critical for efficient electrolysis, electrocatalysis, and photoelectrochemistry.

Degradation of Perfluorooctanoic Acid with Hydrated Electron by a Heterogeneous Catalytic System

Hydrated electron (e_aq^-)-induced reduction protocols have bright prospects for the decomposition of recalcitrant organic pollutants. However, traditional e_aq^- production involves homogeneous sulfite photolysis, which has a pH-dependent reaction activity and might have potential secondary pollution risks. In this study, a heterogeneous UV/diamond catalytic system was proposed to decompose of a typical persistent organic pollutant, perfluorooctanoic acid (PFOA). In contrast to the rate constant of the advanced reduction process (ARP) of a UV/SO₃^2-, the k_obs of PFOA decomposition in the UV/diamond system showed only minor pH dependence, ranging from 0.01823 ± 0.0014 min^-1 to 0.02208 ± 0.0013 min^-1 (pH 2 to pH 11). As suggested by the electron affinity (EA) and electron configuration of the diamond catalyst, the diamond catalyst yields facile energetic photogenerated electron emission into water without a high energy barrier after photoexcitation, thus inducing e_aq^- production. The impact of radical scavengers, electron spin resonance (ESR), and transient absorption (TA) measurements verified the formation of e_aq^- in the UV/diamond system. The investigation of diamond for ejection of energetic photoelectrons into a water matrix represents a new paradigm for ARPs and would facilitate future applications of heterogeneous catalytic processes for efficient recalcitrant pollutant removal by e_aq^-.

Detection of Aqueous Solvated Electrons Produced by Photoemission from Solids Using Transient Absorption Measurements

Solvated electrons in water have long been of interest to chemists. While readily produced using intense multiphoton excitation of water and/or irradiation with high-energy particles, the possible role of solvated electrons in electrochemical and photoelectrochemical reactions at electrodes has been controversial. Recent studies showed that excitation of electrons to the conduction band of diamond leads to barrier-free emission of electrons into water. While these electrons can be inferred from the reactions they induce, direct detection by transient absorption measurements provides more direct evidence. Here, we present studies demonstrating direct detection of solvated electrons produced at diamond electrode surfaces and the influence of electrochemical potential and solution-phase electron scavengers. We further present a more detailed analysis of experimental conditions needed to detect solvated electrons emitted from diamond and other solid materials through transient optical absorption measurements.

Ultrathin Diamond Nanofilms-Development, Challenges, and Applications

Diamond is a highly attractive material for ample applications in material science, engineering, chemistry, and biology because of its favorable properties. The advent of conductive diamond coatings and the steady demand for miniaturization in a plethora of economic and scientific fields resulted in the impetus for interdisciplinary research to develop intricate deposition techniques for thin (≤1000 nm) and ultra-thin (≤100 nm) diamond films on non-diamond substrates. By virtue of the lowered thickness, diamond coatings feature high optical transparency in UV-IR range. Combined with their semi-conductivity and mechanical robustness, they are promising candidates for solar cells, optical devices, transparent electrodes, and photochemical applications. In this review, the difficulty of (ultra-thin) diamond film development and production, introduction of important stepping stones for thin diamond synthesis, and summarization of the main nucleation procedures for diamond film synthesis are elucidated. Thereafter, applications of thin diamond coatings are highlighted with a focus on applications relying on ultrathin diamond coatings, and the excellent properties of the diamond exploited in said applications are discussed, thus guiding the reader and enabling the reader to quickly get acquainted with the research field of ultrathin diamond coatings.

Nanosecond transient absorption studies of the pH-dependent hydrated electron quenching by HSO3. Photochem Photobiol Sci 2019;18:1526-1532. [PMID: 30984955 DOI: 10.1039/c9pp00063a]

The large standard reduction potential of an aqueous solvated electron (eaq-, E° = -2.9 V) makes it an attractive candidate for reductive treatment of wastewater contaminants. Using transient absorption spectroscopy, the nanosecond to microsecond dynamics of eaq- generated from 10 mM solutions of Na2SO3 at pH 4 to 11 in H2O and D2O are characterized, resulting in the determination that between pH 4 and 9 it is the HSO3-, and not H+ as previously postulated by others, that effectively quenches eaq-. The observed bimolecular quenching rate constant (k = 1.2 × 108 M-1 s-1) for eaq- deactivation by HSO3- is found to be consistent with a Brønsted acid catalysis mechanism resulting in formation of H˙ and SO32-. A large solvent isotope effect is observed from the lifetimes of the eaq- in H2O compared to D2O (kH2O/kD2O = 4.4). In addition, the bimolecular rate constant for eaq- deactivation by DSO3- (k = 2.7 × 107 M-1 s-1) is found to be an order of magnitude lower than by HSO3-. These results highlight the role of acids, such as HSO3-, in competition with organic contaminant targets for eaq- and, by extension, that knowledge of the pKa of eaq- sources can be a predictive measure of the effective pH range for the treatment of wastewater contaminants.

Beyond fossil fuel-driven nitrogen transformations

Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.

Hydroxyl radical production induced by plasma hydrogenated nanodiamonds under X-ray irradiation

For the first time, overproduction of hydroxyl radicals (HO˙) induced by plasma hydrogenated detonation nanodiamonds (H-NDs) under X-ray irradiation is reported. Using coumarin (COU) as a fluorescent probe, we reveal a significant increase of 40% of the HO˙ production in the presence of H-NDs (6-100 μg ml^-1) compared with water alone. This effect is related to the negative electron affinity of the hydrogenated nanodiamonds and illustrates the ability of H-NDs to produce reactive oxygen species probably via electron emission in water under X-ray irradiation.

Fixation of atmospheric nitrogen by nanodiamonds

Very dilute aqueous colloids of a detonation-produced nanodiamond or an ultrafine synthetic diamond react with N₂ to yield solids containing fixed nitrogen.

Highlights from Faraday Discussion 172: Carbon in Electrochemistry, Sheffield, UK, July 2014

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Concluding remarks: there's nowt so queer as carbon electrodes

This contribution provides a personal overview and summary of Faraday Discussion 172 on “Carbon in Electrochemistry”, covering some of the key points made at the meeting within the broader context of other recent developments on carbon materials for electrochemical applications. Although carbon electrodes have a long history of use in electrochemistry, methods and techniques are only just becoming available that can test long-established models and identify key features for further exploration. This Discussion has highlighted the need for a better understanding of the impact of surface structure, defects, local density of electronic states, and surface functionality and contamination, in order to advance fundamental knowledge of various electrochemical processes and phenomena at carbon electrodes. These developments cut across important materials such as graphene, carbon nanotubes, conducting diamond and high surface area carbon materials. With more detailed pictures of structural and electronic controls of electrochemistry at carbon electrodes (and electrodes generally), will come rational advances in various technological applications, from sensors to energy technology (particularly batteries, supercapacitors and fuel cells), that have been well-illustrated at this Discussion.