Data Mining Practical Machine Learning Tools and Techniques

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

We demonstrate first room temperature, and ultrabright single photon emission from a color center in two-dimensional multilayer hexagonal boron nitride. Density Functional Theory calculations indicate that vacancy-related centers are a likely source of the emission.

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Quantum emission from hexagonal boron nitride monolayers

Single-atom photocatalysts have shown their compelling potential and arguably become the most active research direction in photocatalysis due to their fascinating strengths in enhancing light-harvesting, charge transfer dynamics, and surface reactions of a photocatalytic system. While numerous comprehensions about the single-atom photocatalysts have recently been amassed, advanced characterization techniques and vital theoretical studies are strengthening our understanding on these fascinating materials, allowing us to forecast their working mechanisms and applications in photocatalysis. In this review, we begin by describing the general background and definition of the single-atom photocatalysts. A brief discussion of the metal-support interactions on the single-atom photocatalysts is then provided. Thereafter, the current available characterization techniques for single-atom photocatalysts are summarized. After having some fundamental understanding on the single-atom photocatalysts, their advantages and applications in photocatalysis are discussed. Finally, we end this review with a look into the remaining challenges and future perspectives of single-atom photocatalysts. We anticipate that this review will provide some inspiration for the future discovery of the single-atom photocatalysts, manifestly stimulating the development in this emerging research area.

Heterogeneous Single-Atom Photocatalysts: Fundamentals and Applications.

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Single-Atom Catalysts across the Periodic Table.

Dispersed atomic catalysts can achieve high catalytic efficiency and have the potential to enable chemical transformation of inert molecules like CO2. The effect of surface defects on photocatalytic reduction of CO2 using supported single atom catalysts however requires clarification. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom Cu catalysts and CO2 reduction. Adsorption of Cu was strong to the TiO2 surface, and charges of the Cu atoms were highly dependent on whether surface defects were present. Cu atoms with Ov aided in the adsorption of activated bent CO2, which is key to CO2 reduction. Our results also show that CO2 dissociation (CO2* → CO* + O*), which is a proposed initial step of CO2 reduction to hydrocarbon products, occurs very readily for a single Cu atom in an Ov, with barriers of ∼0.19 eV. Such low barriers do not occur with Cu over a stoichiometric surface. Furth...

Synergy between Defects, Photoexcited Electrons, and Supported Single Atom Catalysts for CO2 Reduction

Catalytic reduction of carbon dioxide to useful chemicals is a potent way to mitigate this greenhouse gas, but the challenge lies in finding active reduction catalysts. Using density functional theory we studied CO2 activation over TiO2-supported Cu clusters of size 1–4 atoms. The linear to bent transformation of CO2 is necessary for activation, and we found that all the clusters stabilized bent CO2, along with a significant gain of electrons on the CO2 (indicative of activation). On all the TiO2 supported Cu clusters, the interfacial sites were found to stabilize the bent CO2 adsorption, where the active site of adsorption on Cu dimer, trimer and tetramer was on the Cu atom farthest away from the TiO2 surface. Particularly, the Cu dimer stabilized bent CO2 very strongly, although this species was found to be unstable on the surface. A synthesis technique that could stabilize the Cu dimer could therefore lead to a very active catalyst. Furthermore we found (using vibrational and charge analysis) that the active sites for the CO2 activation predominantly had 0 and +1 oxidation states; the oxidation state of Cu is known to directly affect CO2 reduction activity. Our study shows TiO2-supported small Cu clusters can be active catalysts for CO2 reduction and also provides further motivation for theoretical and experimental studies of metal clusters.

Activation of CO2 by supported Cu clusters.

Liquid-metal interfaces occur in a number of surface processes, and it is fundamentally important to accurately model these interfaces. We have systematically determined how the presence of water affects several processes using density functional theory with implicit solvation models. We modeled adsorption of 41 common adsorbates, as well as four catalytic reactions in both vacuum and water over the Pt(111) surface. Our results show that adsorption energies for some species can change significantly in the presence of water (up to 0.44 eV). We further show that solvation effects may be explained and predicted by analyzing simple chemical descriptors such as dipole moment and adsorbate charge. We also report models from artificial neural networks using several potential descriptors, including gas-phase solvation energy, adsorbate charge, dipole moment, and surface area. When water was present reaction energies changed by up to 0.23 eV, although it appeared that water solvent negligibly affected several elementary reaction steps. Our results show that hydrogen bonding can be important for a number of reactions, which are largely absent in the implicit solvation models. Furthermore we also modeled other solvents besides water and found that when a solvent has low dielectric constant, then small solvation effects occurred. Our work provides guidelines on when solvation effects may be important for surface chemistry, and also provides valuable insights on modeling such effects.

Evaluating Solvent Effects at the Aqueous/Pt(111) Interface.

Supported single metal atoms embody active, efficient catalysts. Much effort in the literature has focused on late transition metals, but understanding the reactivity and stability trends across al...

Quantifying Support Interactions and Reactivity Trends of Single Metal Atom Catalysts over TiO2

Bi2S3 is a non-toxic n-type semiconductor, which has been commonly synthesized in the form of quantum dots or nanocrystalline films by solution deposition methods Despite a favorable optical band gap of ∼13 eV, such films have not achieved high solar energy conversion efficiencies to date We hypothesize that this is in part due to the presence of sulfur vacancies that, according to our density functional theory calculations, form a deep trap state in the band gap of Bi2S3, which can act as a strong recombination channel for photoexcited charges Here, we report a microcrystalline Bi2S3 thin-film synthesized by annealing solution-deposited nanocrystalline Bi2S3 in a sulfur vapor environment at 445 °C, which simultaneously increases the grain size and phase purity of Bi2S3, fills in sulfur vacancies, and improves optical absorption Time-resolved terahertz spectroscopy (TRTS) reveals that sulfur annealing increases the photoexcited carrier lifetime from sub-picosecond to ∼30 picoseconds, while the internal quantum efficiency of a photoelectrochemical solar cell device is increased 4-fold from ∼10% to ∼40% In addition, TRTS reveals that the intra-grain carrier mobility in the sulfur-annealed films is ∼165 cm2 V−1 s−1 and the long-range mobility is ∼111 cm2 V−1 s−1 at short times, indicating that carriers are able to hop across grain boundaries These results indicate that annealing in sulfur vapor can produce simultaneously high light absorption and charge separation efficiencies by achieving carrier diffusion length that is comparable to the light absorption depth, leading to high solar energy conversion efficiencies in Bi2S3

Satish Kumar Iyemperumal

Papers

Synergy between Defects, Photoexcited Electrons, and Supported Single Atom Catalysts for CO2 Reduction

Activation of CO2 by supported Cu clusters.

Evaluating Solvent Effects at the Aqueous/Pt(111) Interface.

Quantifying Support Interactions and Reactivity Trends of Single Metal Atom Catalysts over TiO2

Enhancing the solar energy conversion efficiency of solution-deposited Bi2S3 thin films by annealing in sulfur vapor at elevated temperature