Forming Bonds While Breaking Old Ones: Isomer-Dependent Formation of H3O+ from Aminobenzoic Acid During X-ray-Induced Fragmentation.

Intramolecular hydrogen transfer, a reaction where donor and acceptor sites of a hydrogen atom are part of the same molecule, is a ubiquitous reaction in biochemistry and organic synthesis. In this work, we report hydronium ion (H3O+) production from aminobenzoic acid (ABA) after core-level ionization with soft X-ray synchrotron radiation. The formation of H3O+ during the fragmentation requires that at least two hydrogen atoms migrate to one of the oxygen atoms within the molecule. The comparison of two structural isomers, ortho- and meta-ABA, revealed that the production of H3O+ depends strongly on the structure of the molecule, the ortho-isomer being much more prone to produce H3O+. The isomer-dependency suggests that the amine group acts as a donor in the hydrogen transfer process. In the case of ortho-ABA, detailed H3O+ production pathways were investigated using photoelectron-photoion-photoion coincidence (PEPIPICO) spectroscopy. It was found that H3O+ can result from a direct two-body dissociation but also from sequential fragmentation processes.

Re-examining the giant magnetization density in α''-Fe16N2 with the SCAN+U method.

We present an in-depth discussion of the magnetic ground state of α''-Fe16N2 within the framework of the density functional theory (DFT). The exchange-correlation effects are treated using a variety of schemes, including the local-spin-density approximation, the generalized-gradient approximation, and the Strongly-Constrained-and-Appropriately-Normed (SCAN) scheme. We also delineate effects of adding an on-site interaction parameter U on the Fe sites. Among all the schemes considered, only SCAN+U is found to capture the surprisingly large magnetization density in α''-Fe16N2 that has been observed experimentally. Our study shows how the combination of SCAN and self-interaction corrections applied on different Fe sites through the parameter U can reproduce both the correct equilibrium volume and the giant magnetization density of α''-Fe16N2.

Metallic Contact between MoS2 and Ni via Au Nanoglue.

A critical factor for electronics based on inorganic layered crystals stems from the electrical contact mode between the semiconducting crystals and the metal counterparts in the electric circuit. Here, a materials tailoring strategy via nanocomposite decoration is carried out to reach metallic contact between MoS2 matrix and transition metal nanoparticles. Nickel nanoparticles (NiNPs) are successfully joined to the sides of a layered MoS2 crystal through gold nanobuffers, forming semiconducting and magnetic NiNPs@MoS2 complexes. The intrinsic semiconducting property of MoS2 remains unchanged, and it can be lowered to only few layers. Chemical bonding of the Ni to the MoS2 host is verified by synchrotron radiation based photoemission electron microscopy, and further proved by first-principles calculations. Following the system's band alignment, new electron migration channels between metal and the semiconducting side contribute to the metallic contact mechanism, while semiconductor-metal heterojunctions enhance the photocatalytic ability.

Adatom Extraction from Pristine Metal Terraces by Dissociative Oxygen Adsorption: Combined STM and Density Functional Theory Investigation of O/Ag(110).

The reconstruction and modification of metal surfaces upon O_{2} adsorption plays an important role in oxidation processes and in gauging their catalytic activity. Here, we show by employing scanning tunneling microscopy and the ab initio density functional theory that Ag atoms are extracted from pristine (110) terraces upon O_{2} dissociation, resulting in vacancies and in Ag-O complexes. The substrate roughening generates undercoordinated atoms and opens pathways to the Ag subsurface layer. With increasing O coverage, multiple vacancies give rise to remarkable structures. The mechanism is expected to be very general depending on the delicate interplay of energy and entropy, so that it may be active for other materials at different temperatures.

A DFT study of the effect of SO4 groups on the properties of TiO2 nanoparticles.

We present a study of the optical, electronic, and structural properties of TiO2 anatase-structured nanoparticles upon adsorption of SO4 groups, which are always present on the surface of the particles during the sulfate manufacturing method. Structural and electronic properties were studied using the density functional theory method (DFT), and optical properties were obtained by time-dependent DFT. It was found that SO4 groups alter both the geometric and electronic structure of TiO2 nanoparticles and change the photoabsorption characteristics. In particular, we find that η2-O2 type O-O moieties are formed due to the adsorption of 3 and 4SO4 groups.

A DFT study of the effect of OH groups on the optical, electronic, and structural properties of TiO2 nanoparticles.

The effects of on-surface OH groups on the structural and optical properties of small TiO2 particles have been studied in order to obtain knowledge about the optical behaviour of the TiO2 nanoparticles in solutions. The standard density functional theory was used to model the structural changes, and time-dependent density functional theory was used to address the changes in the photoabsorption characteristics of an anatase-structured (TiO2)16 cluster. It was shown that the OH groups can alter both the geometric and electronic structure of the clusters, resulting in changes in the optical properties. The large blue shift, obtained in earlier calculations for TiO2 nanoparticles as compared with bulk TiO2, is shown to be reduced by OH adsorption.

Understanding and optimizing the sensitization of anatase titanium dioxide surface with hematite clusters.

The presence of hematite (Fe2O3) clusters at low coverage on titanium dioxide (TiO2) surface has been observed to enhance photocatalytic activity, while excess loading of hematite is detrimental. We conduct a comprehensive density functional theory study of Fe2O3clusters adsorbed on the anatase TiO2(101) surface to investigate the effect of Fe2O3on TiO2. Our study shows that TiO2exhibits improved photocatalytic properties with hematite clusters at low coverage, as evidenced by a systematic study conducted by increasing the number of cluster adsorbates. The adsorption of the clusters generates impurity states in the band gap improving light absorption and consequently affecting the charge transfer dynamics. Furthermore, the presence of hematite clusters enhances the activity of TiO2in the hydrogen evolution reaction. The Fe valence mixing present in some clusters leads to a significant increase in H2evolution rate compared with the fixed +3 valence of Fe in hematite. We also investigate the effect of oxygen defects and find extensive modifications in the electronic properties and local magnetism of the TiO2-Fe2O3system, demonstrating the wide-ranging effect of oxygen defects in the combined system.

Tuning the Electronic Properties of Two-Dimensional Lepidocrocite Titanium Dioxide-Based Heterojunctions.

Two-dimensional (2D) heterostructures reveal novel physicochemical phenomena at different length scales that are highly desirable for technological applications. We present a comprehensive density functional theory study of van der Waals (vdW) heterostructures constructed by stacking 2D TiO2 and 2D MoSSe monolayers to form the TiO2-MoSSe heterojunction. The heterostructure formation is found to be exothermic, indicating stability. We find that by varying the atomic species at the interfaces, the electronic structure can be considerably altered due to the differences in charge transfer arising from the inherent electronegativity of the atoms. We demonstrate that the heterostructures possess a type II or type III band alignment, depending on the atomic termination of MoSSe at the interface. The observed charge transfer occurs from MoSSe to TiO2. Our results suggest that the Janus interface enables the tuning of electronic properties, providing an understanding of the possible applications of the TiO2-MoSSe heterostructure.

Adsorption of CO2 on the ω-Fe (0001) surface: insights from density functional theory.

The stabilization of a hexagonal phase known as the ω-phase in steel has recently been identified. The presence of C in steel samples is found to be helping the formation of this otherwise meta stable phase. This indicates that the probability of degradation of the surface is high in steel samples containing the ω-phase, through surface adsorption. Here we calculate the adsorption process of CO2 on the ω-Fe(0001) surface, for different sites and find that it strongly adsorbs horizontally with a bent configuration. The adsorption is characterized by significant charge transfer from the surface Fe atoms to the CO2 molecule, and structural modification of the molecule is occurring. The density of states calculations indicate that hybridization and subsequent charge transfer is probable between the d orbitals of Fe and p orbitals of CO2, resulting in strong chemisorption, that further leads to spontaneous dissociation of the molecule.

Incorporation of Si atoms into CrCoNiFe high-entropy alloy: a DFT study.

Density functional theory based computational study has been conducted in order to investigate the effect of substitution of Cr and Co components by Si on the structure, mechanical, electronic, and magnetic properties of the high entropy alloy CrCoNiFe. It is found that the presence of a moderate concentration of Si substitutes (up to 12.5%) does not significantly reduce the structural and mechanical stability of CrCoNiFe while it may modify its electronic and magnetic properties. Based on that, Si is proposed as a cheap and functional material for partial substitution of Cr or Co in CrCoNiFe.

Synergistic effect of Ni-Ag-rutile TiO2 ternary nanocomposite for efficient visible-light-driven photocatalytic activity.

P25 comprising of mixed anatase and rutile phases is known to be highly photocatalytically active compared to the individual phases. Using a facile wet chemical method, we demonstrate a ternary nanocomposite consisting of Ni and Ag nanoparticles, decorated on the surface of XTiO2 (X: P25, rutile (R)) as an efficient visible-light-driven photocatalyst. Contrary to the current perspective, RTiO2-based Ni-Ag-RTiO2 shows the highest activity with the H2 evolution rate of ∼86 µmol g-1 W-1 h-1@535 nm. Together with quantitative assessment of active Ni, Ag and XTiO2 in these ternary systems using high energy synchrotron X-ray diffraction, transmission electron microscopy coupled energy dispersive spectroscopy mapping evidences the metal to semiconductor contact via Ag. The robust photocatalytic activity is attributed to the improved visible light absorption, as noted by the observed band edge of ∼2.67 eV corroborating well with the occurrence of Ti3+ in Ti 2p XPS. The effective charge separation due to intimate contact between Ni and RTiO2 via Ag is further evidenced by the plasmon loss peak in Ag 3d XPS. Moreover, density functional theory calculations revealed enhanced adsorption of H2 on Ti8O16 clusters when both Ag and Ni are simultaneously present, owing to the hybridization of the metal atoms with d orbitals of Ti and p orbitals of O leading to enhanced bonding characteristics, as substantiated by the density of states. Additionally, the variation in the electronegativity in Bader charge analysis indicates the possibility of hydrogen evolution at the Ni sites, in agreement with the experimental observations.

Surface Morphology and Sulfur Reduction Pathways of MoS2 Mo Edges of the Monolayer and (100) and (103) Surfaces by Molecular Hydrogen: A DFT Study.

We have performed a density functional theory study of the MoS2 monolayer and the MoS2 (100) and (103) surfaces in relation to the early stages of the hydrodesulfurization reaction. In many X-ray diffraction (XRD) results, the (103) surface exhibits a higher peak than the (100) surface, yet one of the most frequently occurring surface has not been studied extensively. By analyzing experimental studies, we conclude that the (103) surface of MoS2 is the most frequently occurring edge surface when the sample size is thicker than ∼10-15 nm. Herein, we report the first comparison of reaction paths for the formation of a sulfur vacancy on the (103) surface of MoS2, monolayer, and (100) surface of MoS2. The reason for the occurence of the (103) surface in the XRD patterns has been established. We point out the similarity in the reaction barriers for the monolayer and (100) and (103) surfaces and discuss the reason for it. Moreover, we found a more energetically favorable step in the reaction pathway for the formation of a sulfur vacancy, which allowed us to refine the previously established pathway.

Synthesis, characterization and light harvesting properties of nickel(II) diimine dithiolate complexes.

Four Ni(II) diimine dithiolato complexes viz. [Ni{(S2C2Ph2)(1,10-Phenanthroline)}] (2), [Ni{(S2C2Ph2)(3,3'-dicarboxy-2,2'-bipyridyl)}] (3), [Ni{(S2C2Ph2)(4,4'-dicarboxy-2,2'-bipyridyl)}] (4) [Ni{(S2C2Ph2)(2,2'-bipyridyl)}] (5) have been prepared from [Ni(S2C2Ph2)2] (1) and characterized by microanalyses, UV-Vis, IR, (1)H and (13)C NMR. Attempts have been made to explain the nature of charge transfer in these molecules through quantum chemical calculations. The light harvesting properties of all the compounds have been studied using these compounds as photosensitizers in TiO2-based DSSC. The change in position of anchoring group on diimine derivative leads to different structural, electronic and light harvesting properties about the Ni(II) diimine dithiolate dyes.