Fabrication of ZnO Scaffolded CdS Nanostructured Photoanodes with Enhanced Photoelectrochemical Water Splitting Activity under Visible Light.

CdS, characterized by its comparatively narrow energy band gap (∼2.4 eV), is an appropriate material for prospective use as a photoanode in photoelectrochemical water splitting. Regrettably, it encounters several obstacles for practical and large-scale applications, including issues such as bulk carrier recombination and diminished conductivity. Here, we have tried to address these challenges by fabricating a novel photoelectrode (ZnO/CdS) composed of one-dimensional ZnO nanorods (NRs) decorated with two-dimensional CdS nanosheets (NSs). A facile two-step chemical method comprising electrodeposition along with chemical bath deposition is employed to synthesize the ZnO NRs, CdS NSs, and ZnO/CdS nanostructures. The prepared nanostructures have been investigated by UV-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy. The fabricated ZnO/CdS nanostructures have shown enhanced photoelectrochemical properties due to the improvement of the semiconductor junction surface area and thereby enhanced visible light absorption. The incorporation of CdS NSs has been further found to promote the rate of the charge separation and transfer process. Subsequently, the fabricated ZnO/CdS photoelectrodes achieved a photocurrent conversion efficiency 3 times higher than that of a planar ZnO NR photoanode and showed excellent performance under visible light irradiation. The highest applied bias photon-to-current conversion efficiency (% ABPE) of about ∼0.63% has been obtained for the sample with thicker CdS NSs on ZnO NRs with a photocurrent density of ∼1.87 mA/cm2 under AM 1.5 G illumination. The newly synthesized nanostructures further demonstrate that the full photovoltaic capacity of nanomaterials is yet to be exhausted.

Unraveling the origin of the high photocatalytic properties of earth-abundant TiO2/FeS2 heterojunctions: insights from first-principles density functional theory.

Herein, first-principles density functional theory calculations have been employed to unravel the interfacial geometries (composition and stability), electronic properties (density of states and differential charge densities), and charge carrier transfers (work function and energy band alignment) of a TiO2(001)/FeS2(100) heterojunction. Analyses of the structure and electronic properties reveal the formation of strong interfacial Fe-O and Ti-S ionic bonds, which stabilize the interface with an adhesion energy of -0.26 eV Å-2. The work function of the TiO2(001)/FeS2(100) heterojunction is predicted to be much smaller than those of the isolated FeS2(100) and TiO2(001) layers, indicating that less energy will be needed for electrons to transfer from the ground state to the surface to promote photochemical reactions. The difference in the work function between the FeS2(100) and TiO2(001) heterojunction components caused an electron density rearrangement at the heterojunction interface, which induces an electric field that separates the photo-generated electrons and holes. Consistently, a staggered band alignment is predicted at the interface with the conduction band edge and the valence-band edge of FeS2 lying 0.37 and 2.62 eV above those of anatase. These results point to efficient charge carrier separation in the TiO2(001)/FeS2(100) heterojunction, wherein photoinduced electrons would transfer from the FeS2 to the TiO2 layer. The atomistic insights into the mechanism of enhanced charge separation and transfer across the interface rationalize the observed high photocatalytic activity of the mixed TiO2(001)/FeS2(100) heterojunction over the individual components.

Lead-Free Solid State Mechanochemical Synthesis of Cs2NaBi1-xFexCl6 Double Perovskite: Reduces Band Gap and Enhances Optical Properties.

Efficient and stable lead-free halide double perovskites (DPs) have attracted great attention for the future generation of electronic devices. Herein, we have developed a doping approach to incorporate Fe3+ ions into the Cs2NaBiCl6 crystal unit and reveal a crystallographic and optoelectronic study of the Cs2NaBi1-xFexCl6 double perovskite. We report a simple solid-state mechanochemical method that has a solvent-free, one-step, green chemistry approach for the synthesis of Cs2NaBi1-xFexCl6 phosphor. The analysis of powder X-ray diffraction (XRD) data determines the contraction of the lattice due to the incorporation of Fe3+ cations, and this effect is well supported by X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, and solid-state nuclear magnetic resonance spectroscopy (ss-NMR). The band gap is reduced with increasing Fe content owing to the strong overlap of the Fe-3d orbitals with Cl-3p orbitals and shift of the valence band maxima (VBM) toward higher energies, as confirmed by ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) analyses. Photoluminescence (PL) studies of Cs2NaBi1-xFexCl6 phosphors exhibit a large Stokes shift, broadband emission, and increased PL intensity more than ten times for 15% of Fe content phosphor with enhancement in the average decay lifetimes (up to 38 ns) compared to pristine Cs2NaBiCl6 DP. These results indicate that the transition of dark self-trapping of excitons (STEs) into bright STEs enhances yellow emission. XRD, UV, and thermo-gravimetric analysis (TGA) confirmed that the Cs2NaB1-xFexCl6 DPs have good structural and thermal stabilities. Our findings indicate that the doping of Fe3+ cations into the Cs2NaBiCl6 lattice is a constructive strategy to enhance significantly the optoelectronic properties of these phosphors.

Interfacial band offset engineering with barium-doping towards enhanced performance of all inorganic CsPbI2Br perovskite solar cells.

This study investigates the incorporation of Ba2+ at a low concentration into CsPbI2Br, resulting in the formation of mixed CsPb1-xBaxI2Br perovskite films. Photovoltaic devices utilizing these Ba-doped CsPbI2Br (Ba-CsPbI2Br) perovskite films achieved a higher stabilized power conversion efficiency of 14.07% compared to 11.60% for pure CsPbI2Br films. First-principles density functional theory calculations indicate that the improved device performance can be attributed to the efficient transport of conduction electrons across the interface between Ba-CsPbI2Br and the TiO2 electron transporting layer (ETL). The Ba-CsPbI2Br/TiO2 interface exhibits a type-II staggered band alignment with a smaller conduction band offset (CBO) of 0.25 eV, in contrast to the CsPbI2Br/TiO2 interface with a CBO of 0.48 eV. The reduced CBO at the Ba-CsPbI2Br/TiO2 interface diminishes the barrier for conduction electrons to transfer from the Ba-CsPbI2Br layer to the TiO2 layer, facilitating efficient charge transport.

The Transfer Hydrogenation of Cinnamaldehyde Using Homogeneous Cobalt(II) and Nickel(II) (E)-1-(Pyridin-2-yl)-N-(3-(triethoxysilyl)propyl)methanimine and the Complexes Anchored on Fe3O4 Support as Pre-Catalysts: An Experimental and In Silico Approach.

The imino pyridine Schiff base cobalt(II) and nickel(II) complexes (C1 and C2) and their functionalised γ-Fe3O4 counterparts (Fe3O4@C1 and Fe3O4@C2) were synthesised and characterised using IR, elemental analysis, and ESI-MS for C1 and C2, and single crystal X-ray diffraction for C1, while the functionalised materials Fe3O4@C1 and Fe3O4@C2 were characterized using IR, XRD, SEM, TEM, EDS, ICP-OES, XPS and TGA. Complexes C1, C2 and the functionalised materials Fe3O4@C1 and Fe3O4@C2 were tested as catalysts for the selective transfer hydrogenation of cinnamaldehyde and all four pre-catalysts showed excellent catalytic activity. Complexes C1 and C2 acted as homogeneous catalysts with high selectivity towards the formation of hydrocinnamaldehyde (88.7% and 92.6%, respectively) while Fe3O4@C1 and Fe3O4@C2 acted as heterogeneous catalysts with high selectivity towards cinnamyl alcohol (89.7% and 87.7%, respectively). Through in silico studies of the adsorption energies, we were able to account for the different products formed using the homogeneous and the heterogeneous catalysts which we attribute to the preferred interaction of the C=C moiety in the substrate with the Ni centre in C2 (-0.79 eV) rather than the C=O (-0.58 eV).

Improved photocatalytic activity of TiO2 nanoparticles through nitrogen and phosphorus co-doped carbon quantum dots: an experimental and theoretical study.

In this work, we develop a photocatalyst wherein nitrogen and phosphorus co-doped carbon quantum dots are scaffolded onto TiO2 nanoparticles (NPCQD/TiO2), denoted as NPCT hereafter. The developed NPCT photocatalyst exhibits an enhanced visible light photocatalytic hydrogen production of 533 µmol h-1 g-1 compared to nitrogen doped CQD/TiO2 (478 µmol h-1 g-1), phosphorus doped CQD/TiO2 (451 µmol h-1 g-1) and pure CQD/TiO2 (427 µmol h-1 g-1) photocatalysts. The enhanced photocatalytic activity of the NPCT photocatalyst is attributed to the excellent synergy between NPCQDs and TiO2 nanoparticles, which results in the creation of virtual energy levels, a decrease in work function and suppressed recombination rates, thereby increasing the lifetime of photogenerated electrons. A detailed mechanism is proposed for the enhancement in visible light hydrogen production by the NPCT photocatalyst from the experimental results, Mott-Schottky plots and ultraviolet photoelectron spectroscopy results. Further, first-principles density functional theory (DFT) simulations are carried out which predict the decrease in the work function and band gap, and the increase in the density of states of NPCT as the factors responsible for the observed enhancement in visible light photocatalytic hydrogen production.

Experimental and Theoretical Investigation of the Structural and Opto-electronic Properties of Fe-Doped Lead-Free Cs2 AgBiCl6 Double Perovskite.

Lead-free double perovskites have emerged as stable and non-toxic alternatives to Pb-halide perovskites. Herein, the synthesis of Fe-doped Cs2 AgBiCl6 lead-free double perovskites are reported that display blue emission using an antisolvent method. The crystal structure, morphology, optical properties, band structure, and stability of the Fe-doped double perovskites were investigated systematically. Formation of the Fe-doped Cs2 AgBiCl6 double perovskite is confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. XRD and thermo-gravimetric analysis (TGA) shows that the Cs2 AgBiCl6 double perovskite has high structural and thermal stability, respectively. Field emission scanning electron microscopy (FE-SEM) analysis revealed the formation of dipyramidal shape Cs2 AgBiCl6 crystals. Furthermore, energy-dispersive X-ray spectroscopy (EDS) mapping shows the overlapping of Cs, Bi, Ag, Fe, and Cl elements and homogenous incorporation of Fe in Cs2 AgBiCl6 double perovskite. The Fe-doped Cs2 AgBiCl6 double perovskite shows a strong absorption at 380 nm. It extends up to 700 nm, suggesting that sub-band gap states transition may originate from the surface defect of the doped perovskite material. The radiative kinetics of the crystals was studied using the time-correlated single-photon counting (TCSPC) technique. Lattice parameters and band gap value of the Fe-doped Cs2 AgBiCl6 double perovskites predicted by the density functional theory (DFT) calculations are confirmed by XRD and UV/Visible spectroscopy analysis. Time-dependent photo-response characteristics of the Fe-doped Cs2 AgBiCl6 double perovskite show fast response and recovery time of charge carriers. We believe that the successful incorporation of Fe in lead-free, environmentally friendly Cs2 AgBiCl6 double perovskite can open a new class of doped double perovskites with significant potential optoelectronics devices fabrication and photocatalytic applications.

Insights from density functional theory calculations into the effects of the adsorption and dissociation of water on the surface properties of zinc diphosphide (ZnP2) nanocrystals.

Zinc phosphides (ZnP2 and Zn3P2) are emerging absorber materials for photovoltaic applications owing to their abundancy and non-toxic nature. Herein, we provide a comprehensive characterisation of the surface structure, composition, stabilities, morphology, and electronic properties of both bare and hydrated/hydroxylated low-Miller index surfaces of ß-ZnP2 by means of density functional theory (DFT) calculations. Mechanistic insights into the fundamental aspects of water adsorption and dissociation, including the adsorption geometries, energetics, and structural parameters along the reaction path are systematically characterised. The stabilities of the surfaces under dry and wet conditions are discussed in detail and the predicted phase diagrams for the water adsorption are presented. Using calculated surface energies, we have derived the equilibrium morphology of the ß-ZnP2 nanocrystals under vacuum and upon hydration or hydroxylation. Atomic-level insights into the origin of the incipient oxidation of ß-ZnP2 surfaces are provided through analysis of Bader charges, which reveal that the Zn sites to which H2O and OH species are bound undergo oxidation due to the transfer of charge to the adsorbed species. Adsorption-induced changes to the electronic properties before and after hydration/hydroxylation were characterised by the work function and partial density of states. The results highlight the need for protection of ß-ZnP2 nanocrystals against possible oxidation in the presence of water through post-synthesis organic functionalisation.

Revealing the electronic structure, heterojunction band offset and alignment of Cu2ZnGeSe4: a combined experimental and computational study towards photovoltaic applications.

Cu2ZnGeSe4 (CZGSe) is a promising earth-abundant and non-toxic semiconductor material for large-scale thin-film solar cell applications. Herein, we have employed a joint computational and experimental approach to characterize and assess the structural, optoelectronic, and heterojunction band offset and alignment properties of a CZGSe solar absorber. The CZGSe films were successfully prepared using DC-sputtering and e-beam evaporation systems and confirmed by XRD and Raman spectroscopy analyses. The CZGSe films exhibit a bandgap of 1.35 eV, as estimated from electrochemical cyclic voltammetry (CV) measurements and validated by first-principles density functional theory (DFT) calculations, which predicts a bandgap of 1.38 eV. A fabricated device based on the CZGSe as a light absorber and CdS as a buffer layer yields power conversion efficiency (PCE) of 4.4% with VOC of 0.69 V, FF of 37.15, and Jsc of 17.12 mA cm-2. Therefore, we suggest that interface and band offset engineering represent promising approaches to improve the performance of CZGSe devices by predicting a type-II staggered band alignment with a small conduction band offset of 0.18 eV at the CZGSe/CdS interface.

Interaction of Aromatic Molecules with Forsterite: Accuracy of the Periodic DFT-D4 Method.

Density functional theory (DFT) has provided deep atomic-level insights into the adsorption behavior of aromatic molecules on solid surfaces. However, modeling the surface phenomena of large molecules on mineral surfaces with accurate plane wave methods (PW) can be orders of magnitude more computationally expensive than localized atomic orbitals (LCAO) methods. In the present work, we propose a less costly approach based on the DFT-D4 method (PBE-D4), using LCAO, to study the interactions of aromatic molecules with the {010} forsterite (Mg2SiO4) surface for their relevance in astrochemistry. We studied the interaction of benzene with the pristine {010} forsterite surface and with transition-metal cations (Fe2+ and Ni2+) using PBE-D4 and a vdW-inclusive density functional (Dion, Rydberg, Schröder, Langreth, and Lundqvist (DRSLL)) with LCAO methods. PBE-D4 shows good agreement with coupled-cluster methods (CCSD(T)) for the binding energy trend of cation complexes and with PW methods for the binding energy of benzene on the forsterite surface with a difference of about 0.03 eV. The basis set superposition error (BSSE) correction is shown to be essential to ensure a correct estimation of the binding energies even when large basis sets are employed for single-point calculations of the optimized structures with smaller basis sets. We also studied the interaction of naphthalene and benzocoronene on pristine and transition-metal-doped {010} forsterite surfaces as a test case for PBE-D4. Yielding results that are in good agreement with the plane wave methods with a difference of about 0.02-0.17 eV, the PBE-D4 method is demonstrated to be effective in unraveling the binding structures and the energetic trends of aromatic molecules on pristine and transition-metal-doped forsterite mineral surfaces. Furthermore, PBE-D4 results are in good agreement with its predecessor PBE-D3(BJM) and with the vdW-inclusive density functionals, as long as transition metals are not involved. Hence, PBE-D4/CP-DZP has been proven to be a robust theory level to study the interaction of aromatic molecules on mineral surfaces.

Unravelling the early oxidation mechanism of zinc phosphide (Zn3P2) surfaces by adsorbed oxygen and water: a first-principles DFT-D3 investigation.

Zinc phosphide (Zn3P2) is a novel earth-abundant photovoltaic material with a direct band gap of 1.5 eV. Herein, the incipient oxidation mechanism of the (001), (101), and (110) Zn3P2 surfaces in the presence of oxygen and water, which severely limits the fabrication of efficient Zn3P2-based photovoltaics, has been investigated in detail by means of dispersion-corrected density functional theory (DFT-D3) calculations. The fundamental aspects of the oxygen and water adsorption, including the initial adsorption geometries, adsorption energies, structural parameters, and electronic properties, are presented and discussed. A chemical picture and origin of the initial steps of Zn3P2 surface oxidation are proposed through analyses of Bader charges, partial density of states, and differential charge density isosurface contours. The results presented show that while water interacts weakly with the Zn ions on the Zn3P2 surfaces, molecular and dissociative oxygen species interact strongly with the (001), (101), and (110) surface species. The adsorption of oxygen is demonstrated to be characterized by a significant charge transfer from the interacting surface species, causing them to be oxidized from Zn2+ to Zn3+ formal oxidation states. Preadsorbed oxygen species are shown to facilitate the O-H bond activation of water towards its dissociation, with the adsorbed hydroxide species (OH-) demonstrated to draw a significant amount of charges from the interacting surface sites. Despite the fact that the semiconducting nature of the different Zn3P2 surfaces is preserved, we observe noticeable adsorption induced changes in their electronic structures, with the covered surface exhibiting smaller band gaps than the naked surfaces. The present study demonstrates the importance of the oxygen-water/solid interface to understand the oxidation mechanism of Zn3P2 in the presence of oxygen and water at the molecular level. The study also highlights the need for Zn3P2 nanoparticles to be protected against possible oxidation in the presence of oxygen and moisture via in situ functionalization, wherein the Zn3P2 nanoparticles are exposed to a vapour of organic functional molecules immediately after synthesis.

Enhancing the electrocatalytic activity of 2H-WS2 for hydrogen evolution via defect engineering.

Transition metal dichalcogenides (TMDs), such as MoS2 and WS2, are promising alternative non-noble metal catalysts to drive the electrocatalytic H2 evolution reaction (HER). However, their catalytic performance is inherently limited by the small number of active sites as well as their poor electrical conductivity. Here, we grow vertically aligned 2H-WS2 on different substrates to expose their edge sites for the HER and introduce a scalable approach to tune these active sites via defect engineering. In a thermal hydrogen treatment procedure, sulfur vacancies and metallic tungsten nanoparticles are formed. The extent of desulfurization, and thus the HER activity, can be tuned via controlling the H2 annealing conditions. The obtained W/WS2-x electrocatalysts are evaluated experimentally and theoretically to arrive at a better understanding of how to modify the inherently inert 2H-WS2 for more efficient HER.

Reactivity of CO2 on the surfaces of magnetite (Fe3O4), greigite (Fe3S4) and mackinawite (FeS).

The growing environmental, industrial and commercial interests in understanding the processes of carbon dioxide (CO2) capture and conversion have led us to simulate, by means of density functional theory calculations, the application of different iron oxide and sulfide minerals to capture, activate and catalytically dissociate this molecule. We have chosen the {001} and {111} surfaces of the spinel-structured magnetite (Fe3O4) and its isostructural sulfide counterpart greigite (Fe3S4), which are both materials with the Fe cations in the 2+/3+ mixed valence state, as well as mackinawite (tetragonal FeS), in which all iron ions are in the ferrous oxidation state. This selection of iron-bearing compounds provides us with understanding of the effect of the composition, stoichiometry, structure and oxidation state on the catalytic activation of CO2 The largest adsorption energies are released for the interaction with the Fe3O4 surfaces, which also corresponds to the biggest conformational changes of the CO2 molecule. Our results suggest that the Fe3S4 surfaces are unable to activate the CO2 molecule, while a major charge transfer takes place on FeS{111}, effectively activating the CO2 molecule. The thermodynamic and kinetic profiles for the catalytic dissociation of CO2 into CO and O show that this process is feasible only on the FeS{111} surface. The findings reported here show that these minerals show promise for future CO2 capture and conversion technologies, ensuring a sustainable future for society.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

Oxidation behaviour of U3Si2: an experimental and first principles investigation.

Uranium-containing metallic systems such as U3Si2 are potential Accident Tolerant Fuels (ATFs) for Light Water Reactors (LWRs) and the next generation of nuclear reactors. Their oxidation behaviour, especially in oxygen and water-enriched environments, plays a critical role in determining their applicability in commercial reactors. In this work, we have investigated the oxidation behaviour of U3Si2 experimentally and by theoretical computation. The appearance of oxide signatures has been established from X-ray diffraction (XRD) and Raman spectroscopic techniques after oxidation of the solid U3Si2 sample in synthetic air (oxygen and nitrogen). We have also studied the changes in the electronic structure as well as the energetics of oxygen interactions on the U3Si2 surfaces using first principles calculations in the Density Functional Theory (DFT) formalism. The detailed charge transfer and bond length analyses revealed the preferential formation of mixed oxides of UO2 and SiO2 on the U3Si2{001} surface as well as UO2 alone on the U3Si2{110} and {111} surfaces. The formation of the peroxo (O22-) state confirmed the dissociation of molecular oxygen before U3Si2 oxidation. Core experimental analyses of the oxidized U3Si2 samples have revealed the formation of higher oxides from Raman spectroscopy and XRD techniques. This work is introduced to further a better understanding of the oxidation of U-Si metallic fuel compounds.

Ab initio investigation of O2 adsorption on Ca-doped LaMnO3 cathodes in solid oxide fuel cells.

We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces. The effect of oxygen vacancies on the adsorption characteristics and energetics has also been investigated. Our results show that the O2 adsorption/reduction process occurs through the formation of superoxide and peroxide intermediates, with the Mn sites found to be generally more active than the La sites. The LCM25{110} surface is found to be more efficient for O2 reduction than the LCM25{100} surface due to its stronger adsorption of O2, with the superoxide and peroxide intermediates shown to be energetically more favorable at the Mn sites than at the Ca sites. Moreover, oxygen vacancy defect sites on both the {100} and {110} surfaces are shown to be more efficient for O2 reduction, as reflected in the higher adsorption energies calculated on the defective surfaces compared to the perfect surfaces. We show from Löwdin population analysis that the O2 adsorption on the pure and 25% Ca-doped LaMnO3 surfaces is characterized by charge transfer from the interacting surface species into the adsorbed oxygen πg orbital, which results in weakening of the O-O bonds and its subsequent reduction. The elongated O-O bonds were confirmed via vibrational frequency analysis.

Structures and Properties of As(OH)3 Adsorption Complexes on Hydrated Mackinawite (FeS) Surfaces: A DFT-D2 Study.

Reactive mineral-water interfaces exert control on the bioavailability of contaminant arsenic species in natural aqueous systems. However, the ability to accurately predict As surface complexation is limited by the lack of molecular-level understanding of As-water-mineral interactions. In the present study, we report the structures and properties of the adsorption complexes of arsenous acid (As(OH)3) on hydrated mackinawite (FeS) surfaces, obtained from density functional theory (DFT) calculations. The fundamental aspects of the adsorption, including the registries of the adsorption complexes, adsorption energies, and structural parameters are presented. The FeS surfaces are shown to be stabilized by hydration, as is perhaps to be expected because the adsorbed water molecules stabilize the low-coordinated surface atoms. As(OH)3 adsorbs weakly at the water-FeS(001) interface through a network of hydrogen-bonded interactions with water molecules on the surface, with the lowest-energy structure calculated to be an As-up outer-sphere complex. Compared to the water-FeS(001) interface, stronger adsorption was calculated for As(OH)3 on the water-FeS(011) and water-FeS(111) interfaces, characterized by strong hybridization between the S-p and O-p states of As(OH)3 and the surface Fe-d states. The As(OH)3 molecule displayed a variety of chemisorption geometries on the water-FeS(011) and water-FeS(111) interfaces, where the most stable configuration at the water-FeS(011) interface is a bidentate Fe-AsO-Fe complex, but on the water-FeS(111) interface, a monodentate Fe-O-Fe complex was found. Detailed information regarding the adsorption mechanisms has been obtained via projected density of states (PDOS) and electron density difference iso-surface analyses and vibrational frequency assignments of the adsorbed As(OH)3 molecule.

Periodic DFT+U investigation of the bulk and surface properties of marcasite (FeS2).

Marcasite FeS2 and its surface properties have been investigated by Hubbard-corrected Density Functional Theory (DFT+U) calculations. The calculated structural parameters, interatomic bond distances, elastic constants and electronic properties of the bulk mineral were determined and compared with earlier theoretical reports and experimental data where available. We have also investigated the relative stabilities, interlayer spacing relaxations, work functions, and electronic structures of the {010}, {101}, {110} and {130} surfaces under dehydrated and hydrated conditions. Using the calculated surface energies, we have derived the equilibrium crystal shape of marcasite from a Wulff construction. The {101} and {010} surfaces dominate the marcasite crystallite surface area under both dehydrated and hydrated conditions, in agreement with their relative stabilities compared to the other surfaces. The simulated scanning tunneling microscopy (STM) images of the {101} and {010} facets are also presented, for comparison with future experiments.

A DFT+U investigation of hydrogen adsorption on the LaFeO3(010) surface.

The ABO3 perovskite lanthanum ferrite (LaFeO3) is a technologically important electrode material for nickel-metal hydride batteries, energy storage and catalysis. However, the electrochemical hydrogen adsorption mechanism on LaFeO3 surfaces remains under debate. In the present study, we have employed spin-polarized density functional theory calculations, with the Hubbard U correction (DFT+U), to unravel the adsorption mechanism of H2 on the LaFeO3(010) surface. We show from our calculated adsorption energies that the preferred site for H2 adsorption is the Fe-O bridge site, with an adsorption energy of -1.18 eV (including the zero point energy), which resulted in the formation of FeOH and FeH surface species. H2 adsorption at the surface oxygen resulted in the formation of a water molecule, which leaves the surface to create an oxygen vacancy. The H2 molecule is found to interact weakly with the Fe and La sites, where it is only physisorbed. The electronic structures of the surface-adsorption systems are discussed via projected density of state and Löwdin population analyses. The implications of the calculated adsorption strengths and structures are discussed in terms of the improved design of nickel-metal hydride (Ni-MH) battery prototypes based on LaFeO3.

CO2 activation and dissociation on the low miller index surfaces of pure and Ni-coated iron metal: a DFT study.

We have used spin polarized density functional theory calculations to perform extensive mechanistic studies of CO2 dissociation into CO and O on the clean Fe(100), (110) and (111) surfaces and on the same surfaces coated by a monolayer of nickel. CO2 chemisorbs on all three bare facets and binds more strongly to the stepped (111) surface than on the open flat (100) and close-packed (110) surfaces, with adsorption energies of -88.7 kJ mol-1, -70.8 kJ mol-1 and -116.8 kJ mol-1 on the (100), (110) and (111) facets, respectively. Compared to the bare Fe surfaces, we found weaker binding of the CO2 molecules on the Ni-deposited surfaces, where the adsorption energies are calculated at +47.2 kJ mol-1, -29.5 kJ mol-1 and -65.0 kJ mol-1 on the Ni-deposited (100), (110) and (111) facets respectively. We have also investigated the thermodynamics and activation energies for CO2 dissociation into CO and O on the bare and Ni-deposited surfaces. Generally, we found that the dissociative adsorption states are thermodynamically preferred over molecular adsorption, with the dissociation most favoured thermodynamically on the close-packed (110) facet. The trends in activation energy barriers were observed to follow that of the trends in surface work functions; consequently, the increased surface work functions observed on the Ni-deposited surfaces resulted in increased dissociation barriers and vice versa. These results suggest that measures to lower the surface work function will kinetically promote the dissociation of CO2 into CO and O, although the instability of the activated CO2 on the Ni-covered surfaces will probably result in CO2 desorption from the nickel-doped iron surfaces, as is also seen on the Fe(110) surface.

A density functional theory study of arsenic immobilization by the Al(III)-modified zeolite clinoptilolite.

We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) and arsenous acid (As(OH)3) on the Al(III)-modified natural zeolite clinoptilolite under anhydrous and hydrated conditions. From our calculated adsorption energies, we show that adsorption of both arsenic species is favorable (associative and exothermic) under anhydrous conditions. When the zeolite is hydrated, adsorption is less favourable, with the water molecules causing dissociation of the arsenic complexes, although exothermic adsorption is still observed for some sites. The strength of interaction of the arsenic complexes is shown to depend sensitively on the Si/Al ratio in the Al(III)-modified clinoptilolite, which decreases as the Si/Al ratio increases. The calculated large adsorption energies indicate the potential of Al(iii)-modified clinoptilolite for arsenic immobilization.