For the past decades, the scientific community has attempted to develop photocatalysts to obtain green hydrogen to diversify the current energy vectors, largely based on fossil fuels. In this context, many researchers have focused on modifying well-known photocatalysts such as TiO2 using other transition metals to boost photocatalytic activity in H2 production. However, powdered materials are difficult to reuse after prolonged exposure to liquid media in photocatalytic reactors. In this work, we have taken an alternative approach by developing structured catalysts based on TiO2 thin films and different Cu species. Photoluminescence analysis showed that incorporating Cu species on the TiO2 thin film decreases the e--h+ recombination rate. The photocatalytic activity of the nanostructured thin films is 298 µmol g−1 h−1 which is comparable to the reports described in the literature. Additionally, the thin films have simple and reproducible manufacturing, are easy to handle, are reusable and cost-effective. All these facts, make them a significantly more efficient technology than the counterpart powder materials.

The full description of the mechanisms for the diffusion of substitutional impurities requires an account of the correlation of the atomic jumps. This study investigated the diffusion of phosphorus (P) and sulfur (S) in the fcc copper (Cu) single crystal using density functional theory (DFT). Vacancy formation energies and impurity–vacancy interactions were calculated, revealing attractive interactions of P and S with the vacancies. The attractive interactions between S and a vacancy were roughly twice as strong as those between P and a vacancy. The 5-frequency—or 5-jump—model was employed to describe the correlation effects during diffusion. The potential energy profiles and activation energies were determined for the different jump paths necessary for the model and to account for all the correlation effects in substitutional impurity diffusion in the single crystal. The results indicated that S diffuses significantly faster than P in Cu, primarily due to lower activation energies for certain jump paths and a more favorable vacancy–impurity interaction. This occurs because when bonding with the crystal, S tends to prefer atomic sites with larger volumes and more asymmetric geometric arrangements when compared to P. This favors the interactions between S and the vacancies, and reduces friction with the matrix during the diffusion of S. The effective diffusion coefficients were calculated and compared with experimental data. The findings provide insights into the diffusion mechanisms of P and S in Cu and how these can be affected by the presence of extended defects such as grain boundaries.

The microstructure, grain size and grain boundary (GB) structure of copper in canisters for encapsulation of spent nuclear fuel have been characterized. Coincident site lattice (CSL) GBs were found to be more common than random high angle GBs. Apart from Σ3, the Σ9 and Σ27 GBs show a higher frequency of occurrence than the other CSL GBs. This effect is more pronounced for the material in the canister lid that is slightly deformed. The high fraction of CSL boundaries of 60–65% is of the same order as for grain boundary engineered material. One critical property for the canister is the creep ductility that is improved by the addition of phosphorus (P) to the copper (Cu-OFP). The structure and segregation energies for P onto CSL boundaries have been determined with quantum mechanical calculations. In comparison to a previous study where literature data for the frequency of occurrence of CSL GBs was used, the absolute values of the segregation energies and the occupancy of P at GBs are significantly increased when using data for the canister copper. The presence of P reduces the amount of creep cavitation, which controls the ductility during brittle creep rupture. The creation of cavities and creep ductility are predicted. The computed creep ductility is slightly higher than in a previous study mainly due to the frequent occurrence of the Σ9 GB, which leads to strong segregation of P. The influence of grain size on the creep ductility has also been analysed. A large reduction in ductility with grain size is found for Cu without P which agrees with observations. For Cu-OFP a reduction in creep ductility for grain sizes up to 300 µm is also predicted, but no further reduction is obtained for still larger grain sizes.

The successful large-scale implementation of hydrogen as an energy vector requires high performance electrodes and catalysts made of abundant materials. Rational materials design strategies are the most efficient means of reaching this goal. Here we present a study on the adsorption of H-atoms onto fcc transition metal surfaces and propose descriptors for the rational design of electrodes and catalysts by means of correlations between fundamental properties of the materials and among other properties, their experimentally measured performance as hydrogen evolution electrodes (HEE). A large set of quantum mechanical modelling data at the DFT level was produced, covering the adsorption of H-atoms onto the most stable surfaces (100), (110) and (111) of: Ag, Au, Co, Cu, Ir, Ni, Pd, Pt and Rh. For each material and surface, a coverage dependent set of minimum energy structures was produced and chemical potentials for adsorption of H-atoms were obtained. Averaging procedures are here proposed to approach modelling to the experiments. Several correlations between the computed data and experimentally measured quantities are done to validate our methodology: surface plane dependent adsorption energies, chemical potentials and experimentally determined surface energies and work functions. We search for descriptors of catalytic activity by testing correlations between the DFT data obtained from our averaging procedures and experimental data on HEE performance. Our methodology allows us to obtain linear correlations between the adsorption energy of H-atoms and the exchange current density (i0) in a HEE, avoiding the volcano-like plots. We show that the chemical potential has limitations as a descriptor of i0 because it reaches an early plateau in terms of i0. Simple quantities obtained from database data such as the first stage electronegativity (χ) as devised by Mulliken has a strong linear correlation i0. With a quantity we denominate modified second-stage electronegativity (χ2m) we can reproduce the typical volcano plot in a correlation with i0. A theoretical and conceptual framework is presented. It shows that both χ and χ2m, that depend on the first ionization potential, second ionization potential and electron affinity of the elements can be used as descriptors in rational design of electrodes or of catalysts for hydrogen systems.

Materials design and performance prediction relies on detailed knowledge of the distribution of solutes that can affect the materials properties. Here we report a study on the distribution and mechanisms of interaction of S and P atoms at four relevant grain boundaries (GBs) of fcc Cu: Σ3, Σ5, Σ9 and Σ11. Segregation site preference was investigated for single atoms and compared to the case where impurities segregate as pairs. Both the driving force for segregation—local minima—and the interactions between impurities at the GBs—global minima—have been investigated. An analysis of geometric and electronic structure effects in segregation was performed. Single S-atoms bind more strongly to GB sites than single P-atoms. For all GBs the driving force for segregation decays fast with distance from the planes and reaches the bulk values already at ≈ 4 Å. In the near vicinity of the GBs, with increased concentration, the interactions between S-atoms are mostly attractive, while for the same sites the interactions between P-atoms are mostly repulsive. S-atoms are capable of displacing P-atoms and the accumulation of P is not favorable at the GB planes, while the accumulation of S is favorable. We also performed geometric and electronic structure analyses using symmetry quantifying indicators and developed a descriptor of electronic structure effects called “impurity projected density of states (DOS)” for analyzing the bonding between the impurities and the Cu matrix. Overall, segregation is favored by an increase in the asymmetry of the segregation sites. S binds more asymmetrically to those sites preferring an off-center position while P-atoms bind more symmetrically adopting a central position. At GBs with the same excess volume, S-atoms bind stronger than P-atoms and fitting functions that describe these trends were obtained. The balance between bonding states, antibonding states, and the covalent contributions to the bonding between the impurities and the copper matrix are at the origin of the observed preferences.

Fumaric acid and alkyl fumarates are a family of structurally related compounds with a wide spectrum of potential or effective therapeutic applications. The series consisting of fumaric acid (FA), monomethyl fumarate (MMF), dimethyl fumarate (DMF), monoethyl fumarate (MEF), and diethyl fumarate (DEF) was studied in this work to address the following main questions: how does the number of OH···O hydrogen bonds that may be established due to systematic differences in molecular structure impacts on the molecular packing and lattice energetics? Is there evidence of a cooperative hydrogen bond strengthening when infinite 1D chains sustained by OH···O hydrogen bonds are formed? How well can the structural and energetic features of this series of related molecules be predicted by state-of-the art force field and periodic DFT procedures that are used in the rationalization or prediction of crystal structures and physical properties of molecular organic solids? By combining results from a variety of experimental (X-ray diffraction, Raman spectroscopy, DSC, Calvet drop-sublimation calorimetry) and theoretical (quantum mechanical, molecular dynamics simulations) methods, it was found that (i) in all cases, the molecular packing leads to layered solids, where each layer consists of 1D chain motifs linked to each other through C–H···O interactions. (ii) The 1D arrangements are determined by two main motifs: the R22(8) carboxyl dimer, typically found in mono- and di-n-alkyl carboxylic acids, and the staggered CH3···H3C synthon, which is present in mono-n-alkyl carboxylic acids and n-alkanes. This leads to the formation of carboxyl–carboxyl and alkyl–alkyl domains that are structurally isolated from each other. (iii) The lattice energy, as measured by the enthalpy of sublimation (ΔsubHmo), varies according to FA > MMF ∼ MEF > DMF ∼ DEF and is linearly correlated with the number of OH···O hydrogen bonds present in the structures. (iv) The larger enthalpy of sublimation of FA compared to MMF and MEF is linked to the number of OH···O hydrogen bonds but does not seem to be related to their individual strength. Examination of O···O distance and C═O stretching frequency as well as theoretically computed dissociation energies of dimeric FA, MMF, and MEF species suggests that the OH···O interaction is weaker in FA than in MMF and MEF. As such, the present study showed no evidence of a cooperative OH···O bond strengthening in FA, relative to MMF and MEF, due to the presence of infinite 1D chains sustained by carboxylic acid dimers. (v) No evident connection between ΔsubHmo and compactness indicators such as density or Kitaigorodski packing index was also found. Finally, (vi) MD simulations and periodic DFT calculations were both able to reproduce the above-mentioned ΔsubHmo trend and capture the main structural features of the family of crystalline materials studied in this work. In terms of accuracy, better overall performance was observed for the force field method developed for this particular type of compounds.

The use of sacrificial agents in photocatalysis is a powerful resource to enhance the performance of photoactive materials. Despite its importance, the effect of the amount of sacrificial agent is not properly described in the literature. In this paper, we have focused on the role of EtOH in the photoreduction of CO2 to CH4 using Cu-P25 photocatalysts in a flow reactor. We found that the production of CH4 increased with the concentration of EtOH, achieving an outstanding CH4 production yield of 235 mu mol/(g & sdot;h) for a flow of 0.25 mu mol/min of EtOH in the gas stream, hinting at the important role of the sacrificial agent in the reaction. The catalytic results together with the characterization of the materials highlight the need to achieve a minimum surface coverage of EtOH on the surface of the catalyst to control the reaction pathway. The adsorption of EtOH is a key factor in boosting the catalytic activity of the best-performing catalyst and producing CH4 from CO2 photoreduction and C2H4O from the photooxidation of EtOH, obtaining two easily separable interesting products for industrial applications in one reaction.

The diffusion of H-atoms is relevant for innumerous physical–chemical processes in metals. A detailed understanding of diffusion in a polycrystalline material requires the knowledge of the activation energies (ΔEa’s) for diffusion at different defects. Here, we report a study of the diffusion of H-atoms at the Σ9 and Σ5 grain boundaries (GBs) of fcc Cu that are relevant for practical applications of the material. The complete set of possible diffusion pathways was determined for each GB and we compared the ΔEa at bulk fcc Cu with the landscape of ΔEa’s at these defects. We found that while a number of diffusion pathways at the GBs have high tortuosity, there are also many paths with very low tortuosity because of specific structural features of the interstitial GB sites. These data show that the diffusion of H-atoms at these GBs is highly directional but can be fast because at certain paths the ΔEa can be as low as 0.05 eV. The lowest energy paths for diffusion of H-atoms through the whole GB models are ΔEa = 0.05 eV for the Σ9 and ΔEa = 0.20 eV at Σ5 which compare with ΔEa = 0.42 eV for the bulk fcc crystal. This shows that H-atoms will be able to diffuse very fast at these defects. With the Laguerre–Voronoi tessellation method, we studied how the local atomic structure of the interstitial sites of the GBs leads to different ΔEa’s for diffusion of H-atoms. We found that the volume expansions and the coordination numbers alone cannot account for the magnitude of the ΔEa’s. Hence, we developed a symmetry quantifying parameter that measures the deviation of symmetry of the GB sites from that of the bulk octahedral site and hence accounts for the distortion at the GB site. Only when this parameter is introduced together with the volume expansions and the coordination numbers, it is possible to correlate the local structure with the ΔEa’s and to obtain descriptors of diffusion. The complete set of data shows that the extrapolation of diffusion data for H-atoms between different types of GBs is non-trivial and should be done with care.

Single atomic layer or monolayer (ML) 2D materials have unique features that can be relevant for protection and functionalization of high-performance surfaces. Here we report a combined density functional theory and ab initio molecular dynamics study on the stability of graphene (Gr), phosphorene (Pp) and silicene (Sl) coatings at the most stable (0001) surface of the metals that have hcp structure at room temperature: Hf, Mg, Os, Re, Ru, Sc, Ti, Zn and Zr. We found that the three 2D materials are stable at the surfaces of many of these metals: Gr is stable at Os, Re, Ti, Zn and Zr; Pp is stable in the blue form at Hf, Mg, Ti and Zr; Sl is stable in the 2D zigzag structure at Hf, Mg and Ti and in the planar form at the Zr surface. For the remaining metals Gr, Pp and Sl do not form van der Waals bonded MLs. In those cases we found structures such as 2D arrangements of mixed hexagons and pentagons, trigonal pyramidal structures and 2 half-monolayers. Many of these structures are very stable coatings covalently bound to the surfaces with considerable adsorption energies that can reach − 2 eV per atom of the coating. Gr imparts hydrophobicity to the surfaces of all materials here studied and increases the distance between these and the first H2O layer by as much as 75 % for Gr coated Ti. These H₂O layers have increased rigidity despite being located further apart from the surfaces. These effects could be explored for the protection or functionalization of high-performance systems such as metallic electrodes for example.

Aluminum is envisioned to be an important material in future hydrogen-based energy systems. Here we report an ab initio investigation on the interactions between H-atoms and common grain boundaries (GBs) of fcc Al: sigma 9, sigma 5, sigma 11 and sigma 3. We found that upon segregation to the GBs, single H-atoms can cause displacement of Al-atoms. Increasing their concentration revealed large cooperative effects between H-atoms that favor the segregation when other H-atoms are bound at neighboring sites. This makes these GBs able to accommodate high concentrations of H-atoms with considerable segregation energies per atom. Structural analyses derived from Laguerre-Voronoi tessellations show that these GBs have many interstitial sites with higher symmetry than the bulk tetrahedral interstitial site. Many of those sites have also large volumes and higher coordination numbers than the bulk sites. These factors are the increased driving force for H-atom segregation at the studied GBs in Al when compared to other metals. These GBs can accommodate a higher concentration of H-atoms which indicates a likely uniform distribution of H-atoms at GBs in the real material. This suggests that attempting to mitigate hydrogen uptake solely by controlling the occurrence of certain GBs may not be the most efficient strategy for Al.