Plane-wave Based Density Functional Theory Research Articles

Observation of tapered multicore fibers based on Nb2CTx for humidity sensing: Experiment and DFT simulation investigations

Recent advancements in the area of human healthcare monitoring and medical diagnosis have sparked a revived interest in humidity sensors. Nb2CTx, characterized by its multilayered structure, large specific surface area, and plentiful hydrophilic groups, actively absorbs a sufficient quantity of water molecules. In this work, the sensing capability and mechanism of water molecules are investigated under different proportions of functional group distribution on the surface of Nb2CTx MXene employing plane-wave based density functional theory calculations, and the provided models can offer guiding principles for the experimental preparation of the highly sensitive humidity sensor based on Nb2CTx. To verify the effectiveness of the model, Nb2CTx material is coated on a Tapered Three-Core Fiber (TTCF), and the high evanescent field characteristics of the sensor are utilized to further improve its performance. As the relative humidity (RH) increased from 35 % to 75 %, the transmitted spectra exhibited a redshift with a sensitivity of 22pm/% RH. Within the relative humidity range of 75 % to 95 %, the Nb2CTx coating heightened water molecule absorption, resulting in a more pronounced redshift in the transmitted spectra. This phase manifests a maximum humidity sensitivity of 299pm/% RH. Noteworthy advantages of this sensor include its uncomplicated structure, cost-effectiveness, and robust stability, rendering it highly suitable a wide variety of potential applications in fields such as biology, chemical and health processing.

The pseudoatomic orbital basis: electronic accuracy and soft-mode distortions in ABO3 perovskites

The perovskite oxides are known to be susceptible to structural distortions over a long wavelength when compared to their parent cubic structures. From an ab initio simulation perspective, this requires accurate calculations including many thousands of atoms; a task well beyond the remit of traditional plane wave-based density functional theory (DFT). We suggest that this void can be filled using the methodology implemented in the large-scale DFT code, CONQUEST, using a local pseudoatomic orbital (PAO) basis. Whilst this basis has been tested before for some structural and energetic properties, none have treated the most fundamental quantity to the theory, the charge density n(r) itself. An accurate description of n(r) is vital to the perovskite oxides due to the crucial role played by short-range restoring forces (characterised by bond covalency) and long range Coulomb forces as suggested by the soft-mode theory of Cochran and Anderson. We find that modestly sized basis sets of PAOs can reproduce the plane-wave charge density to a total integrated error of better than 0.5% and provide Bader partitioned ionic charges, volumes and average charge densities to similar degree of accuracy. Further, the multi-mode antiferroelectric distortion of PbZrO3 and its associated energetics are reproduced by better than 99% when compared to plane-waves. This work suggests that electronic structure calculations using efficient and compact basis sets of pseudoatomic orbitals can achieve the same accuracy as high cutoff energy plane-wave calculations. When paired with the CONQUEST code, calculations with high electronic and structural accuracy can now be performed on many thousands of atoms, even on systems as delicate as the perovskite oxides.

Sorption mechanism and dynamic behavior of graphene oxide as an effective adsorbent for the removal of chlorophenol based environmental-hormones: A DFT and MD simulation study

Chlorophenols are a group of environmental-hormones (EHs) which are carcinogenic, toxic and often precursors to dioxins. Increasing interest in their removal from environment has emerged as a research hotspot in recently years. The adsorption mechanism and dynamic behaviors of four molecules, namely Phenol, 4-chlorophenol (CP), 2,4-dichlorophenol (DCP), and 2,4,6-trichlorophenol (TCP) onto graphene oxide were investigated by means of plane-wave based Density Functional Theory (DFT) and Molecular Dynamics (MD) simulation. The effects of functional groups, solution, pH value, and substituent etc. have been scrutinized through geometric optimization, binding energy calculations and a series of electronic structure analyses. Based on the computational results, we elucidated the adsorption mechanism of chlorophenols onto GOs in aqueous solution was mainly driven by the hydrophobic effect and ultimately stabilized by the hydrogen bonds and π-π interaction. This proposed mechanism was supported by further MD simulations in which a three-state movement was confirmed. It was found that for GOs, the adsorption capacity would be improved by hydroxyl groups which could supply more adsorption sites for the adsorption of chlorophenols. Besides, the adsorption affinity was weakened with increase in number of chloro- groups decorated in the chlorophenols. In addition to the intrinsic properties of the adsorbents and adsorbates, the acidic condition and polar solvent were found to be more favorable, accompanying with the reinforcement of hydrogen-bonds and electrostatic attraction. The combination of DFT method and MD simulations will help to provide insights into the application of graphene oxides for the removal of EHs.

Molecular adsorption and surface formation reactions of HCl, H2 and chlorosilanes on Si(100)-c(4 × 2) with applications for high purity silicon production

The interaction of chlorosilanes and the silicon surface is an important part of reaction processes, such as hydrochlorination and chemical vapor deposition, involved in the production of high purity silicon. We used plane-wave based density functional theory (DFT) to investigate periodic slabs of the Si(100)-c(4 × 2) surface for adsorption of H2, HCl, SiCl2, dichlorosilane (SiH2Cl2 or DCS), trichlorosilane (SiHCl3 or TCS) and silicon tetrachloride (SiCl4 or STC). The effects of surface coverage, molecule orientation, adsorption site and multiple molecule adsorption were studied. All the molecules could undergo dissociative chemisorption in the right orientation and site placement, with HCl and SiCl2 possessing the strongest binding energies. The H2 molecule preferred lower coverage, the HCl molecule was not much affected by coverage while the SiCl2 molecule strongly preferred higher coverage and the STC molecule was affected negatively by both too high or low coverage. The elementary steps leading to transfer of surface crystal silicon atoms to gas phase molecules as part of the chlorination or hydrochlorination process were then looked at through reaction pathway analysis. The formation of SiCl2 from a surface dimer Si atom was found to prefer an intradimer route with a reaction barrier of 3.62 eV (83.48 kcal/mol), going down to 3.10 eV (71.49 kcal/mol) after removal of the first surface Si atom. The subsequent formation of TCS from this SiCl2 was found to have reaction barrier of 1.07 eV (24.68 kcal/mol). STC could also be formed from this SiCl2 molecule with a reaction barrier of 2.86 eV (65.95 kcal/mol).

Enhancement of the selectivity of MXenes (M2C, M = Ti, V, Nb, Mo) via oxygen-functionalization: promising materials for gas-sensing and -separation.

Two-dimensional graphene-like materials, namely MXenes, have been proposed as potential materials for various applications. In this work, the reactivity and selectivity of four MXenes (i.e. M2C (M = Ti, V, Nb, Mo)) and their oxygen-functionalized forms (i.e. O-MXenes or M2CO2) toward gas molecules were investigated by using the plane wave-based Density Functional Theory (DFT) calculations. Small gas molecules, which are commonly found in flue gas streams, are considered herein. Our results demonstrated that MXenes are very reactive. Chemisorption is a predominant process for gas adsorption on MXenes. Simultaneously dissociative adsorption can be observed in most cases. The high reactivity of their non-functionalized surface is attractive for catalytic applications. In contrast, their reactivity is reduced, but the selectivity is improved upon oxygen functionalization. Mo2CO2 and V2CO2 present good selectivity toward NO molecules, while Nb2CO2 and Ti2CO2 show good selectivity toward NH3. The electronic charge properties explain the nature of the substrates and also interactions between them and the adsorbed gases. Our results indicated that O-MXenes are potential materials for gas-separation/capture, -storage, -sensing, etc. Furthermore, their structural stability and SO2-tolerant nature are attractive properties for using them in a wide range of applications. Our finding provides good information to narrow down the choices of materials to be tested in future experimental work.

A DFT study of structural, elastic and lattice dynamical properties of Fe2Zr and FeZr2 intermetallics

We report plane-wave based density functional theory (DFT) investigation of structural, elastic and lattice dynamical properties of Fe-rich C15-Fe2Zr and Zr-rich FeZr2 intermetallic compounds. These intermetallics dominantly appear in the Fe-Zr alloys which are potential candidates for immobilization of nuclear metallic waste. The calculated elastic constants and phonon dispersions show that both phases are mechanically and dynamically stable. The calculated bulk moduli as a function of temperature show that the Fe-rich C15 phase is less compressible and also less sensitive to temperature than the Zr-rich C16 phase. Furthermore, the C15-Fe2Zr phase has a lower thermal expansion co-efficient but higher thermal conductivity as compared to the C16-FeZr2 phase; while the heat capacities (CP and CV) of both the phases are almost identical above 450 K.

Comparison of S-adsorption on (111) and (100) facets of Cu nanoclusters.

In order to gain insight into the nature of chemical bonding of sulfur atoms on coinage metal surfaces, we compare the adsorption energy and structural parameters for sulfur at four-fold hollow (4fh) sites on (100) facets and at three-fold hollow (3fh) sites on (111) facets of Cu nanoclusters. Consistent results are obtained from localized atomic orbital and plane-wave based density functional theory using the same functionals. PBE and its hybrid counterpart (PBE0 or HSE06) also give similar results. 4fh sites are preferred over 3fh sites with stronger bonding by ∼0.6 eV for nanocluster sizes above ∼280 atoms. However, for smaller sizes there are strong variations in the binding strength and the extent of the binding site preference. We show that suitable averaging over clusters of different sizes, or smearing the occupancy of orbitals, provide useful strategies to aid assessment of the behavior in extended surface systems. From site-projected density of states analysis using the smearing technique, we show that S adsorbed on a 4fh site has similar bonding interactions with the substrate as that on a 3fh site, but with much weaker antibonding interactions.

Hydrogen generation from formic acid decomposition on Ni(2 1 1), Pd(2 1 1) and Pt(2 1 1)

The catalytic decomposition of formic acid into carbon dioxide and hydrogen (HCO2H→CO2+H2) on M(211) (M=Ni, Pd, Pt) was investigated by using spin-polarized plane-wave based density functional theory calculations. It is found that on M(211) formic acid prefers the O (OC) atom atop adsorption and the H (HO) atom bridging two neighboring metal atoms, and formate prefers the bidentate adsorption with O atoms atop on metal surface. For M=Ni, Pd, Pt, formic acid has close adsorption energies (−0.69, −0.58, −0.61eV) and also close dissociation barriers (0.42, 0.53, 0.51eV), and the dissociation step is exothermic (−0.95, −0.44, −0.81eV). Formate dissociation into surface CO2 and H (HCOO→CO2+H) is the rate-determining step; and the effective barrier is higher on Ni(211) than on Pd(211) and Pt(211) (1.43 vs. 0.96 and 0.86eV), and formate dissociation is endothermic (0.44eV) on Ni(211), but exothermic on Pd(211) and Pt(211) (−0.09 vs. −0.19eV). On M(211), CO2 has chemisorption (−0.32, −0.13, −0.27eV for M=Ni, Pd, Pt, respectively). For formate dissociation, detailed comparisons between M(211) and M(111) show that Pd(111) and Pt(211) have the smallest effective barriers, while Pt(111) and Ni(211) have the largest effective barriers.

Density Functional Theory Study of Oxygen Reduction Activity on Ultrathin Platinum Nanotubes

The structure, stability, and catalytic activity of a number of single- and double-wall platinum (n,m) nanotubes ranging in diameter from 0.3 to 2.0 nm were studied using plane-wave based density functional theory in the gas phase and water environment. The change in the catalytic activity toward the oxygen reduction reaction (ORR) with the size and chirality of the nanotube was studied by calculating equilibrium adsorption potentials for ORR intermediates and by constructing free energy diagrams in the ORR dissociative mechanism network. In addition, the stability of the platinum nanotubes is investigated in terms of electrochemical dissolution potentials and by determining the most stable state of the material as a function of pH and potential, as represented in Pourbaix diagrams. Our results show that the catalytic activity and the stability toward electrochemical dissolution depend greatly on the diameter and chirality of the nanotube. On the basis of the estimated overpotentials for ORR, we conclude ...

Theoretical calculations of hydrogen adsorption by SnO2 (110) surface: Effect of doping and calcination

A pseudopotential plane-wave based density functional theory simulations of the hydrogen adsorption on rutile SnO2 (110) surface is reported. It is found that on doping with trivalent indium, the surface becomes unstable due to the formation of bridging oxygen vacancies. At sufficiently low doping level, the surface stabilizes at an oxygen vacancy to indium ratio of 1:2. Our calculations predict that at a higher doping level of 9 at. %, this ratio becomes larger, and point out a way to synthesize p-type conducting SnO2 thin films. The binding energy of SnO2 (110) surface with adsorbed hydrogen atoms display a maximum at 3–6 at. % of indium doping. This is in good agreement with the experimental results obtained from the SnO2-based hydrogen sensor’s sensitivity measurements given by Drake et al. [J. Appl. Phys. 101, 104307 (2007)]. The theoretical modeling explains that the calcinations treatment can critically affect the sensitivity of the hydrogen sensor due to the enhancement of the binding energy between the SnO2 surface and the adsorbed hydrogen atoms.

Very large scale wavefunction orthogonalization in Density Functional Theory electronic structure calculations

Enforcing the orthogonality of approximate wavefunctions becomes one of the dominant computational kernels in planewave based Density Functional Theory electronic structure calculations that involve thousands of atoms. In this context, algorithms that enjoy both excellent scalability and single processor performance properties are much needed. In this paper we present block versions of the Gram–Schmidt method and we show that they are excellent candidates for our purposes. We compare the new approach with the state of the art practice in planewave based calculations and find that it has much to offer, especially when applied on massively parallel supercomputers such as the IBM Blue Gene/P Supercomputer. The new method achieves excellent sustained performance that surpasses 73 TFLOPS (67% of peak) on 8 Blue Gene/P 1 racks (32 768 compute cores), while it enables more than a two fold decrease in run time when compared with the best competing methodology.

Optimization methods for finding minimum energy paths

A comparison of chain-of-states based methods for finding minimum energy pathways (MEPs) is presented. In each method, a set of images along an initial pathway between two local minima is relaxed to find a MEP. We compare the nudged elastic band (NEB), doubly nudged elastic band, string, and simplified string methods, each with a set of commonly used optimizers. Our results show that the NEB and string methods are essentially equivalent and the most efficient methods for finding MEPs when coupled with a suitable optimizer. The most efficient optimizer was found to be a form of the limited-memory Broyden-Fletcher-Goldfarb-Shanno method in which the approximate inverse Hessian is constructed globally for all images along the path. The use of a climbing-image allows for finding the saddle point while representing the MEP with as few images as possible. If a highly accurate MEP is desired, it is found to be more efficient to descend from the saddle to the minima than to use a chain-of-states method with many images. Our results are based on a pairwise Morse potential to model rearrangements of a heptamer island on Pt(111), and plane-wave based density functional theory to model a rollover diffusion mechanism of a Pd tetramer on MgO(100) and dissociative adsorption and diffusion of oxygen on Au(111).

Long range interactions on wires: A reciprocal space based formalism

There are many atomic scale systems in materials, chemistry, and biology that can be effectively modeled as finite in two of the physical spatial dimensions and periodically replicated in the third including nanoscale metallic and semiconducting wires, carbon nanotubes, and DNA. However, it is difficult to design techniques to treat long range forces in these systems without truncation or recourse to slowly convergent supercells or computationally inefficient Poisson solvers. In this paper, a rigorous reciprocal space based formalism which permits long range forces on wires to be evaluated simply and easily via a small modification of existing methods for three dimensional periodicity is derived. The formalism is applied to determine long range interactions both between point particles using an Ewald-like approach and the continuous charge distributions that appear in electronic structure calculations. In this way, both empirical force field calculations and, for example, plane-wave based density functional theory computations on wires can be performed easily. The methodology is tested on model and realistic systems including a lithium doped carbon nanotube.

Density functional study of onion-skin-like [As@Ni 12As 20] 3− and [Sb@Pd 12Sb 20] 3− cluster ions

A comprehensive study of the structural and electronic properties of the interesting onion-skin-like [As@Ni 12As 20] 3− cluster ion, characterized by Moses et al. [Science 300 (2003) 778], was carried out using a plane-wave based density functional theory. The calculated interatomic distances agree well with experiment. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for [As@Ni 12As 20] 3− are fivefold-degenerate with h u and h g symmetries, respectively, and its HOMO–LUMO gap is determined to be 0.2 eV lower than that of C 60. The static dipole polarizability of [As@Ni 12As 20] 3− is two times larger than that of C 60. The optical gap of [As@Ni 12As 20] 3− is redshifted by 1.4 eV relative to that of C 60. The possibility of synthesis of [Sb@Pd 12Sb 20] 3− is proposed.

Oxysulfides Ln2Ti2S2O5 as Stable Photocatalysts for Water Oxidation and Reduction under Visible-Light Irradiation

The series Ln2Ti2S2O5 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er) is demonstrated to evolve H2 or O2 from aqueous solutions under visible-light (440 nm ≤ λ ≤ 650 nm) irradiation in the presence of a sacrificial electron donor (Na2S−Na2SO3) or acceptor (Ag+) without noticeable degradation. Ln2Ti2S2O5 is synthesized by sulfurization under H2S flow, and the Sm2Ti2S2O5 form is found to have the highest activity for O2 evolution. X-ray Rietveld refinements reveal that the Ln2Ti2S2O5 framework of the Pr, Nd, and Er forms is distorted from the ideal perovskite structure. The calculations of the electronic band structures of Ln2Ti2S2O5 based on plane-wave based density functional theory indicated that the top of the valence band of [Gd−Er]2Ti2S2O5 is made up of hybridized O2p, S3p, and Ln4f orbitals, whereas Ln4f orbitals are localized in other [Pr−Sm]2Ti2S2O5. In addition, the conduction band of [Gd−Er]2Ti2S2O5 consists of S3p+Ln4f and Ti3d orbitals. The photocatalytic activity is discussed on the basis of the elec...

A new reciprocal space based treatment of long range interactions on surfaces

A new formalism designed to treat long range interactions on surfaces, systems which are infinitely replicated in two spatial directions but have finite extent in the third, is developed. The new formalism is based in reciprocal space and, thus, permits the facile extension of standard plane-wave based density functional theory, Ewald summation, and smooth particle-mesh Ewald methods to handle surfaces efficiently. The method is tested on both model (body centered cubic lattices) and realistic problems (an ice surface with a defect and the 2×1 surface reconstruction of silicon) and found to be accurate, efficient, and a marked improvement on existing formulations in speed, accuracy, and utility.