The heterolayered Au(111)/Graphene/Alkanethiol and Au(111)/Alkanethiol/Graphene structures have been investigated using van der Waals supplemented density functional theory (vdW-DFT) calculations considering striped phases of decanethiol and octanethiol self-assembled monolayers (SAMs) with (6 x root 3) and (12 x root 3) superlattices of graphene and Au(111). For the thiol-based SAMs on gold, graphene not only acts as a protective top layer but also enhances the adsorption energy of alkanes and modifies surface electronic properties. Deposition of alkanethiol molecules on Au(111)-supported graphene is likely to show distinctive flexibility and adsorption characteristics. Electronically, graphene preserves its Dirac conicals and becomes a p-type material in the combined systems. The work functions of the resulting geometries mainly depend on the ordering and the number of the heterolayers as well as on the molecular arrangements.

Carboranedithiol isomers adsorbing with opposite orientations of their dipoles on surfaces are self-assembled together to form mixed monolayers where both lateral dipole-dipole and lateral thiol-thiolate (S-HS) interactions provide enhanced stability over single-component monolayers. We demonstrate the first instance of the ability to map individual isomers in a mixed monolayer using the model system carboranedithiols on Au{111}. The addition of methyl groups to one isomer provides both an enhanced dipole moment and extra apparent height for differentiation via scanning tunneling microscopy (STM). Associated computational investigations rationalize favorable interactions of mixed pairs and the associated stability changes that arise from these interactions. Both STM images and Monte Carlo simulations yield similarly structured mixed monolayers, where approximately 10% of the molecules have reversed dipole moment orientations but no direct chemical attachment to the surface, leading to homogeneous monolayers with no apparent phase separation. Deprotonating the thiols by depositing the molecules under basic conditions eliminates the lateral S-HS interactions while accentuating the dipole-dipole forces. The molecular system investigated is composed of isomeric molecules with opposite orientations of dipoles and identical surface packing, which enables the mapping of individual molecules within the mixed monolayers and enables analyses of the contributions of the relatively weak lateral interactions to the overall stability of the assemblies.

Hydrogenation of the N-N bond under ambient conditions over 1 wt% Ru/Vulcan was monitored through operando Diffuse Reflectance Infrared Spectroscopy (DRIFTS) and DFT. IR signals centered at 3017 cm(-1) and 1302 cm(-1) were visible with attributes similar to the asymmetric stretching and bending vibrations of gas phase ammonia at 3381 cm(-1) and 1650 cm(-1). The intensities of the signals increased with consecutive H-2 : Ar and N-2 flow cycles at room temperature and atmospheric pressure due to accumulation of the formed NHX on the catalyst surface. DFT estimations revealed that a compound with a molecular stoichiometry of N-NH3 can give rise to an IR signal centered at 3051.9 cm(-1). The results of this study, combined with the known vapor liquid phase behavior of ammonia, suggest that under subcritical conditions, the bottlenecks of ammonia synthesis are both N-N bond dissociation and ammonia desorption from the pores of the catalyst.

An experimental and DFT study was conducted to investigate the chemical nature of alpha and beta PdH phases in Pd/ TiO2 through the detailed analysis of their 1H NMR chemical shifts. alpha-PdH states are identified with shielded 1H NMR resonances at frequencies around -30 ppm from TMS. In situ NMR measurements revealed that the appearance of this shielded peak was only observable if PdO was partially reduced. DFT estimations revealed similar NMR chemical shifts due to hydrogen adsorbed at the interface between the metal and surface segregated, i.e. the top, oxide layers of palladium. For fully reduced Pd, DFT estimations relaxed into a single beta phase, similar results were obtained through quantitative in situ NMR measurements. NMR chemical shift DFT calculations over beta phase revealed a linear dependency of 1H chemical shifts for 0.02 <= H/Pd <= 1, due to the anionic character of hydrogen atom within the local electronic structure. This finding was also supported by the experimental data. Anionic character hydrogen atom is due to well localized core hydrogen 1s orbitals at low concentrations which are dispersed as H content increases. These results indicate that the presence of trace oxygen can be involved in the alpha phase of PdH.

Positional isomers of dicarba-doso-dodecaboranethiols with various functional groups (-NO2, -CHO, -CONH2, -F, -Cl, and -OH) were considered on Au(111) forming (3 x 3) and (5 x 5) structures. Dispersion corrected density functional theory calculations reveal the influence of functional groups on the adsorption characteristics of these carboranethiols depending on the coverage. Functionalized molecules not only possess fascinating chemical and electronic properties but also show stronger chemisorption profiles on gold in relation to the pristine precursors. Molecular dipole moments which can be modified by functional groups were found to be effective in tunability of the work function of deposited surfaces. Results indicate that densely packed adsorption geometries in which functional groups lean toward back of adjacent molecules allowing H-bonding are energetically favorable and enhance stability and ordering of constituents in self-assembled monolayers. Carboranethiols with functional groups are promising to enrich the surface electrochemical properties of resulting surfaces.

An experimental and DFT study was conducted to investigate the chemical nature of the 1H NMR chemical shifts of α and β PdH phases in Pd/TiO2. α-PdH states are identified with shielded 1H NMR resonances at frequencies ~ > -30ppm from TMS. . In situ NMR measurements revealed that the appearance of the shielded peak at was only observable if PdO was partially reduced. DFT estimations revealed similar NMR chemical shifts due to hydrogen adsorbed at the interface between the metal and surface segregated oxide layers of palladium. For fully reduced Pd, DFT estimations relaxed into a single β phase, similar results were obtained through quantitative in situ NMR measurements. NMR chemical shift DFT calculations over β phase revealed a linear dependency of 1H chemical shifts for 0.02≤H/Pd ≤1, due to the anionic character of hydrogen atom within the local electronic structure. This finding was also supported by the experimental data. Anionic character hydrogen atom is due to well localized core hydrogen 1s orbitals at low concentrations which are dispersed as H content increases. These results suggest that the presence of trace oxygen can be involved in the α phase of PdH.

In this work, using the state-of-the-art first principles calculations based on density functional theory, we found that the concentration as well as coordination of surface oxygen vacancies with respect to each other were critical for direct water-splitting reaction on the (001) surfaces of PbTiO$_3$ and TiO$_2$. For the water-splitting reaction to happen on TiO$_2$-terminated surfaces, it is necessary to have two neighboring O-vacancies acting as active sites that host two adsorbing water molecules. However, eventual dissociation of O-H bonds is possible only in the presence of an additional nearest-neighbor O-vacancy. Unfortunately, this necessary third vacancy inhibits the formation of molecular hydrogen by trapping the dissociated H atoms over TiO$_2$-teminated surfaces. Formation of up to 3 O-vacancies, is energetically less costly on both terminations of PbTiO$_3$ (001) surfaces compared with that of TiO$_2$, the presence of Pb leads to weaker O bonds over these surfaces. Molecular hydrogen formation is more favorable over the PbO-terminated surface of PbTiO$_3$, requiring only two neighboring oxygen vacancies. However, hydrogen molecule is retained near the surface by weak van der Waals forces. Our study indicates two barriers leading to low productivity of direct water splitting processes. First and foremost, there is an entropic barrier imposed by the requirement of at least two nearest-neighbor O-vacancies, sterically hindering the process. Furthermore, there are also enthalpic barriers of formation over TiO$_2$-terminated surfaces, or removal of H$_2$ molecules from the PbO-terminated surface.

The morphological and electronic properties of isolated and monolayer phases of carboxyl- and amine-functionalized carboranethiols on unreconstructed Au(111) were determined using density functional theory calculations by including van der Waals interactions. The groups are effective in the assembly of pristine adlayers on gold and also offer functionality when exposed at the SAM-environment interface. Anisotropy brought by both functional groups increases tilting of carboranethiols relative to the surface normal and absolute values of the dissociative chemisorption energies. Positional isomerization and the functional groups modify the molecular dipole moments which influnce the adsorption characteristics. Even though carboxylic acid and amine groups have different chemical properties, they have similar effects on the adsoprtion characteristics of carboranethiols. Dense packing favors intermolecular interactions which gives a stronger binding relative to isolated adsorption. The carboranethiols with the functional groups can be arranged in the same lateral direction or in a dimer conformation with molecues facing each other. Carboxyl and amine groups offer functionalization of cabranethiol SAMs and in conjuction with positional isomerization shift tunability ranges of the work function of gold to higher energies.

The structure and function of self-assembled monolayers (SAMs) at the nanoscale are determined by the steric and electronic effects of their building blocks. Carboranethiol molecules form pristine monolayers that provide tunable two-dimensional systems to probe lateral and interfacial interactions. Additional omega-functionality, such as carboxyl groups, can be introduced to change the properties of the exposed surfaces. Here, two geometrically similar isomeric m-carborane analogues of m-mercaptobenzoic acid, 1-COOH-7-SH-1,7-C2B10H10 and racem-1-COOH-9-SH-1,7-C2B10H10, are characterized and their SAMs on Au{111} are examined. The latter isomer belongs to the rare group of chiral cage molecules and becomes, to our knowledge, the first example assembled on Au{111}. Although different in symmetry, molecules of both isomers assemble into similar hexagonal surface patterns. The nearest-neighbor spacing of 8.4 +/- 0.4 angstrom is larger than that of non-carboxylated isomers, consistent with the increased steric demands of the carboxyl groups. Computational modeling reproduced this spacing and suggests a tilt relative to the surface normal. However, tilt domains are not observed experimentally, suggesting the presence of strong lateral interactions. Analyses of the influence of the functional groups through the pseudo-aromatic m-carborane skeleton showed that the thiol group attached to either carbon or boron atoms increases the carboxyl group acidity in solution. In contrast, the acidity of the exposed carboxyl group in the SAMs decreases upon surface attachment; computational analyses suggest that the driving force of this shift is the dielectric of the environment in the monolayer as a result of confined intermolecular interactions, proximity to the Au surface, and partial desolvation.

Hydrogen storage is a critical step for commercialisation of hydrogen consumed energy production. Among other storage methods, solid state storage of hydrogen attracts much attention and requires extensive research. This study rationally and systematically designs novel solid state hydrides; Li2CaH4 (GHD is obtained as -6.95 wt %) and Li2SrH4 (GHD is obtained as -3.83 wt %) using computational method. As a first step, we suggest and predict crystal structures of solid state Li2CaH4 and Li2SrH4 hydrides and look for synthesizability. Then, the mechanical stabilities of hydrides are identified using elastic constants. Both hydrides fulfil the well-known Born stability criteria, indicating that both Li2CaH4 and Li2SrH4 are mechanically stable materials. Several critical parameters, bulk modulus, shear modulus, Cauchy pressures, anisotropy factors of hydrides and bonding characteristics are obtained and evaluated. Furthermore, electronic and optical band structures of hydrides are computed. Both Li2CaH4 and Li2SrH4 have indirect bands gaps as 96 eV (Gamma-U) and 1.10 eV (Gamma-R). Thus, both materials are electronically semiconducting. Also, Bader charge analysis of hydrides have been carried out. Charge density distribution suggests an ionic-like (or polarized covalent) bonding interaction between the atoms. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The effects of Co addition on the chemical and electronic structure of PbTiO$_3$ were explored both by theory and through experiment. Cobalt was incorporated to PbTiO$_3$ during sol gel process. The XRD data of the compounds confirmed the perovskite structure for the pure samples. The XRD lines broadened and showed emerging cubic-like features as the Co incorporation increased. The changes in the XRD pattern were interpreted as double perovskite structure formation. $^{207}$Pb NMR measurements revealed a growing isotropic component in the presence of Co. In line with the experiments, DFT calculated chemical-shift values corroborate isotropic coordination of Pb suggesting the formation of cubic Pb$_2$CoTiO$_6$ domains in the prepared samples. The state-of-the-art hybrid functional first-principles calculations indicate formation of Pb$_2$CoTiO$_6$ with cubic structure and confirms that Co addition can decrease oxygen binding energy significantly. Experimental UV-Vis spectroscopy results indicate that upon addition of Co, the band gap is shifted towards visible wavelengths which was confirmed by the energy bands and absorption spectra calculations. The oxygen binding energies were determined by temperature programmed reduction (TPR) measurements. Upon addition of Co, TPR lines shifted to lower temperatures and new features appeared in the TPR patterns. This shift was interpreted as weakening of oxygen cobalt bond strength. The change in the electronic structure by the alterations of oxygen vacancy formation energy and bond lengths upon Co insertion are determined by DFT calculations.

In this study, single phase DyxLa1-xBO3, DyxCeyLa1-x-yBO3, DyxEuyLa1-x-yBO3, and TbxCeyLa1-x-yBO3 phosphors were successfully synthesized through the microwave assisted solid state method with urea. Synthesized compounds were characterized by X-Ray Diffraction, Fourier Transform Infrared spectroscopy, Scanning Electron Microscope, and Fluorescence spectroscopy. Thermal stabilities of the phosphors were studied with temperature dependent fluorescence measurements. In addition, density functional theory calculations were performed to investigate preferential Dy3+ ion doping sites in LaBO3 lattice. The findings of the study revealed over ten times increase in the emission intensities of phosphors were observed with Ce3+ codoping. The highest luminescence intensities were recorded for Dy0.09Ce0.05La0.86BO3 and Tb0.07Ce0.05La0.88BO3 phosphors. Furthermore, energy transfers were supported by luminescence lifetime measurements. Temperature dependent fluorescence measurements showed their thermal stabilities. According to the CIE 1931 color space, the colors of the Dy3+ only doped, Ce3+/Dy3+; Ce3+/Tb3+ and Dy3+/Eu3+ codoped phosphors were determined as off-white, white, green and yellow-orange, respectively. Intensities of the bands and the colors of the phosphors can be tuned with the choice of codopant ions or with adjustments in their concentration ratio.

Graphene adsorption on the Au(111) surface was explored to identify its common surface structures by means of van der Waals corrected density functional theory calculations. The alignment of graphene in the form of certain rotational angles on the gold surface has an important role in lattice matching, which causes Moiré patterns, and in the electronic properties of the resulting common cell structures. Dispersive weak interactions between carbon and gold layers lead to a downward shift of Fermi energy of the adsorption system with respect to the Dirac point of graphene showing a p-type doping character. Moreover, the shift was shown to depend on the rotational angle of graphene on Au(111).

In this study, oxygen vacancies and adatoms have been considered on the surface of both hexagonal and triangular ZnO nanowires. Their effect on the electronic structure and optical spectra of the nanowires have been investigated using the exact exchange hybrid density functional theory calculations. A surface oxygen vacancy gives rise to appearance of a band gap state at almost 0.7 eV above the valence band of the both types of the nanowires while an oxygen adatom show bulk-like electronic properties. A shape dependence is also indicated by the calculated physical quantities of oxygen related point defects on ZnO nanowires.

We explore the effects of Co-doping on the electronic properties of lead titanate via DFT calculations and experimental studies. Powder diffractometry data are consistent with Co-ions replacing Pb-ions within the perovskite framework. DFT shows a reduced oxygen binding energy and decreased band gap as a result of doping within the PbTiO$_3$ lattice. Both $^{207}$Pb NMR and UV-vis spectroscopy are consistent with the DFT calculations, though the relativistic effects associated with the Pb nucleus limit the agreement between NMR observations and calculations.

The energetics and structures of physisorbed and chemisorbed alkanethiols on Au(111) have been systematically investigated up to 10 carbon atoms using van der Waals (vdW) corrected density functional theory (DFT) calculations. The role of chain length, tilting angle and coverage on the adsorption characteristics has been examined to elucidate the energetics and plausible transformation mechanisms between lying down and standing up phases. Coverage and size dependent chain-chain electronic interactions counteract with the alkyl chain-gold surface interactions and the surface relaxation of the metal in the formation of standing up monolayer structures. For the striped phases of long chain alkanethiols, however, our calculations on decanethiol indicate alkyl chain-gold surface interactions to be strong enough to force the molecule to be perfectly parallel to the surface by lifting a gold atom up, in agreement with the proposed models for this film in the literature.

Isolated and full monolayer adsorption of various carboranethiol (C_2B_10H_12S) isomers on gold (111) surface have been investigated using both the standard and van der Waals density functional theoretical calculations. The effect of differing molecular dipole moment orientations on the low energy adlayer geometries, the binding characteristics and the electronic properties of the self-assembled monolayers of these isomers have been studied. Specifically, the binding energy and work function changes associated with different molecules show a correlation with their dipole moments. The adsorption is favored for the isomers with dipole moments parallel to the surface. Of the two possible unit cell structures, the (5×5) was found to be more stable than the (√(19)×√(19))R23.4^o one.

Isolated and full monolayer adsorption of various carboranethiol (C2B10H12S) isomers on the gold(111) surface has been investigated using both the standard and van der Waals density functional theory calculations. The effect of different molecular dipole moment orientations on the low energy adlayer geometries, the binding characteristics and the electronic properties of the self-assembled monolayers of these isomers has been studied. Specifically, the binding energy and work function changes associated with different molecules show a correlation with their dipole moments. The adsorption is favored for the isomers with dipole moments parallel to the surface. Of the two possible unit cell structures, (5 × 5) was found to be more stable than .

The adsorption of two different organic molecules cyanidin glucoside (C21O11H20) and TA-St-CA on anatase (101) and (001) nanowires has been investigated using the standard and the range separated hybrid density functional theory calculations. The electronic structures and optical spectra of resulting dye–nanowire combined systems show distinct features for these types of photochromophores. The lowest unoccupied molecular orbital of the natural dye cyanidin glucoside is located below the conduction band of the semiconductor while, in the case of TA-St-CA, it resonates with the states inside the conduction band. The wide-bandgap anatase nanowires can be functionalized for solar cells through electron-hole generation and subsequent charge injection by these dye sensitizers. The intermolecular charge transfer character of Donor-π-Acceptor type dye TA-St-CA is substantially modified by its adsorption on TiO2 surfaces. Cyanidin glucoside exhibits relatively stronger anchoring on the nanowires through its hydroxyl groups. The atomic structures of dye–nanowire systems re-optimized with the inclusion of nonlinear solvation effects showed that the binding strengths of both dyes remain moderate even in ionic solutions.