Density Functional Study Research Articles

Nitrogen-doped graphene quantum dots for extensive production of hydrogen: A hybrid density functional study

Photocatalytic water splitting has emerged as a promising approach for storing solar energy in the form of chemical energy, which can solve the energy crisis and environmental issues. Herein we have explored the potentiality of nitrogen-doped graphene quantum dots for solar-driven hydrogen evolution. The motivation behind applying quantum dots as a photocatalyst is to utilize a major portion of the solar energy spectrum to enhance hydrogen yield. The alignment of frontier molecular orbitals (FMOs) in comparison to the redox potential of water reveals that all the studied quantum dots are eligible for photocatalytic hydrogen evolution. The energy gap between FMOs varies with the size of quantum dots from 3.01 to 2.32 eV, showing absorption of light in the visible range. The Gibbs free energy calculation shows that the formation of intermediate species is energetically unfavorable without exposure to light at pH=7, except for hydrogen adsorption. However, the photogenerated electron and hole drive the steps of oxygen and hydrogen evolution by making those a downhill process. The optical absorption spectra show that all the studied quantum dots absorb visible light.

Continuously tunable the phase transition behavior of VO2 by T-doping and the formation of T-T chain (T = Ta, Nb)

Vanadium dioxide (VO2), a typical strongly correlated transition metal oxide, undergoes a fascinating reversible metal-insulator transition (MIT) at 340 K, attracting increasing interest from both scientific and technological perspectives. Nevertheless, achieving a rational regulation of the phase transition temperature (Tc) and gaining a deeper understanding of the modulation mechanism, as well as clarifying the relationship between the dopants and Tc in VO2, remain significant ongoing challenges. In this work, we present a density functional theoretical study on the structural and electronic properties of T-doped (T = Ta, Nb) VO2. The results demonstrate that the introduction of T (T = Ta, Nb) atoms tends to drive the R phase structure towards a dimerized monoclinic-like structure, while the uniformly aligned V–V chains exhibit alternating short and long V–V dimerization feature. Notably, the R phase VO2 appears to be more susceptible to structural perturbations. Moreover, T (T = Ta, Nb) doping concentration, compressive strain and the numbers of T-T chain cause curious changes in the structural evolution and electronic structures of VO2, acting as effective modulation methods for tuning the phase transition temperature. In addition, the Tc of VO2 could be significantly reduced when the concentration of T (T = Ta, Nb) dopant is less than 6.3%. The current results offer valuable insights into the mechanism of the doping driven MIT in T-doped (T = Ta, Nb) VO2, and contribute to a deeper understanding of the relationship between the dopants and phase transition temperature, further providing valuable insights into its metal-insulator transition behavior.

Phosphorescence by Trapping Defects in Boric Acid Induced by Thermal Processing

AbstractThe phosphorescence of boric acid (BA, H3BO3) at room temperature is a puzzling phenomenon subject to controversial interpretations although the role of structural defects has not yet been considered. Heat treatments of boric acid cause its transformation into the metaboric phase and amorphous boron oxide (B2O3). The structural changes after thermal processing can create defects that become centers of luminescence and recombination channels in the visible range. In the present work, commercial boric acid is thermally processed at different temperatures. Samples treated between 200 and 400 °C exhibit remarkable phosphorescence in the visible range. At ≈480 and 528 nm, two distinct phosphorescent emissions occur, associated with trapped charge carriers recombinations identified by thermoluminescence (TL) and electron paramagnetic resonance spectroscopy (EPR). The structural and optical studies suggest that the activation of boric acid phosphorescence after heat treatment is correlated with the presence of defects. The afterglow results from a trapping and detrapping process, which delays the recombination at the active optical centers. Time‐dependent density functional study (DFT) of defective BOH molecules and clusters shows the emergence of near UV and blue optical transitions in absorption. These defects trigger the photoluminescence in thermally processed boric acid samples.

π-Conjugated aromatic amino molecule for interfacial modification in high-performance perovskite solar cells

Aromatic amino cations can effectively inhibit non-radiative recombination caused by defects to improve the stability and efficiency of perovskite solar cells (PSCs), and the π-conjugated benzene ring of aromatic amino exhibit better conductance compared with alkyl ammonium salts. Further optimization of such molecules requires consideration of their stability, defect passivation performance, and energy level matching between the perovskite and the transport layer. In this work, a novel π-conjugated aromatic amino molecule naphthylethyl ammonium iodide (NEAI) was applied to the interfacial modification in PSCs. Combining the density functional theory (DFT) calculations and experimental study, it can be revealed that the introduction of NEAI can not only realize the optimization of perovskite layer/HTL interface through defect passivation and more matched energy level, but also improve the moisture stability via the better hydrophobicity of modified films. Finally, the devices achieved a remarkable efficiency of 23.98 % and improved stability, retaining 90 % of the initial efficiency after 600 h placed in the dry room.

Density functional study on the mechanics, thermodynamics, and H diffusion mechanism of LiH

LiH is valued by researchers and engineers for its high-temperature resistance, high strength, high hydrogen storage capacity of up to 12.6 wt%, and high neutron absorption cross section. However, sintering and application of this material are difficult. Lithium hydride is a highly effective neutron shielding material. The neutron shielding ability of lithium hydride is directly affected by the homogeneity of the total amount and distribution of hydrogen. The heat migration of hydrogen reduces the neutron shielding effectiveness of lithium hydride. It is critical to forecast H's diffusion behavior. The diffusion coefficient is a critical metric for determining the diffusion efficiency. As a result, the stability, electronic structure, and bonding properties of LiH, LiOH, and H entering the tetrahedral interstitial sites of LiH were calculated using first principles based on density functional theory (DFT). The elastic modulus of the system was also examined to find the lowest thermal conductivity, and some thermodynamic parameters were derived by calculating LiH phonons. Finally, static calculations were used to determine the diffusion coefficient and diffusion activation energy of H in the LiH solid solution. The results revealed that the addition of H altered the characteristics of the LiH semiconductor, giving it metallic features. The lowest thermal conductivity of the LiH system was 2.9. W·m−1 K−1. The diffusion coefficient was D=(2.638×10−2cm2/s)exp(−5.811×10−3cal/KBT). The activation energy for diffusion was 0.252 eV. The diffusion coefficients, elastic constants, and lattice parameters all corresponded well with previously published results. The density functional theory was employed to study the material's system energy and electrical structure, and the microscopic migratory behavior of hydrogen was predicted theoretically. The calculation of the lowest thermal conductivity provided a reference for the screening of ceramic materials.

Electronic configuration inversion in CdIn2S4 for efficient photocatalytic hydrogen peroxide generation coupled with selective benzylamine oxidation

Vacancies engineering has sparked a huge interest in enhancing photocatalytic activity, but monovacancy simultaneously conducts as either electron or hole acceptor and redox reaction, worsening charge transfer and catalytic performance. Here, the concept of electronic inversion has been proposed through the simultaneous introduction of surface oxygen and S vacancies in CdIn2S4 (OSv-CIS). Consequently, under mild conditions, the well-designed OSv-CIS-200 demonstrated a strong rate of N-benzylidenebenzylamine production (2972.07 µmol g-1 h-1) coupled with Hydrogen peroxide (H2O2) synthesis (2362.33 µmol g-1 h-1) (PIH), which is 12.4 times higher than that of CdIn2S4. Density functional theory (DFT) simulation and characterization studies demonstrate that oxygen is introduced into the lattice on the surface of the material, reversing the charge distribution of the S vacancy and enhancing the polarity of the total charge distribution. It not only provides a huge built-in electric field (BEF) for guiding the orientation of the charge transfer, but also acts as a long-distance active site to accelerate reaction and prevent H2O2 decomposition. Our work offers a straightforward connection between the atomic defect and intrinsic properties for designing high-efficiency materials.

Unravelling the micro-mechanism of oxygen reduction reaction on Fe–N4 embedded in graphene

We conducted a first-principles density functional study on the FeN4 embedded in graphene (Fe–N4-Gra) for the Oxygen Reduction Reaction (ORR). Our study unraveled certain microscopic characteristics of the ORR process on the catalyst that are not readily observable in experiments. Firstly, we investigated the reduction preferences of the two oxygen atoms in the side-on form of O2. Our findings indicate that the 2p orbital of the Ob atom, located further from the central Fe atom, exhibits a significantly stronger overlap with the Fe 3d orbital compared to the Oa atom adjacent to Fe. This suggests a preferential reduction process occurring on the Ob atom. The Löwdin charge distribution between the two atoms further supports this observation. Secondly, we demonstrated that minimizing the orbital energy gap and enhancing the hybridization or resonance between the Fe-3d and O-2p orbitals is the origin of the activity of the Fe–N–C catalyst. We propose that future catalyst designs should consider expanding the Fe 3d or 4s orbital to overlap with the (σ2p)* orbital of O2 in order to achieve higher efficiency of 4 electron reduction process. These findings contribute to a deeper understanding of the ORR process on the Fe–N4-Gra catalyst and provide valuable insights for the development of more efficient catalysts in the future.

Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation

Supported metal clusters comprising of well-tailored low-nuclearity heteroatoms have great potentials in catalysis owing to the maximized exposure of active sites and metal synergy. However, atomically precise design of these architectures is still challenging for the lack of practical approaches. Here, we report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters. When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver high-purity hydrogen at full conversion with unexpected high activity of 0.62 molHCOOH molPt−1 s−1 and remarkable stability, significantly outperforming the previously reported catalysts. The remarkable performance is rationalized by a joint operando dual-beam Fourier transformed infrared spectroscopy and density functional theory modeling study, pointing to the Pt-Mo synergy in creating a new reaction path for consecutive HCOOH dissociations.

Supercritical water gasification of waste R410A refrigerant mixture for the resource utilization: ReaxFF reactive molecular dynamic simulation and density functional theory calculation study

Harmless treatment and resource utilization of the waste HFC refrigerants have gradually become the focus of attention. As a clean waste resource utilization technology, supercritical water gasification has good potential in converting waste refrigerant into high value-added chemicals and fuels. In this study, the supercritical water gasification mechanism of R410A, and the interaction mechanism between R125 and R32, and between H2O and R410A during the gasification process are investigated by using ReaxFF reactive molecular dynamic simulation and density functional theory calculation study. The results show that the main products obtained from the supercritical water gasification of R410A are HF, H2, CO and CO2. More than 90 % of F atoms in R410A can be mineralized to form HF molecules. H2 and CO are two important syngas products during the supercritical water gasification process. Hydrogen bonds are generated between R125 and H2O molecules and between R125 and R32 molecules, which leads to the decomposition of R125 molecules more easily, and the decomposition of R32 molecules more difficult. This work provides a possible way to convert waste HFC refrigerant mixture into high value-added chemicals and syngas.

CO oxidation over three-dimensional transition metal-doped CeO2 catalysts: A density functional calculation study

Carbon monoxide (CO) oxidation is considered one of the effective methods to address environmental pollution caused by vehicle emissions and incomplete combustion of fuels. However, designing efficient and affordable catalysts has always been a major challenge. In this study, a series of transition metals (TMs)-doped CeO2(111) catalysts were designed based on density functional theory (DFT) to catalyze the oxidation reaction of CO. According to the results, TMs-CeO2(111) catalysts exhibited excellent thermal stability and adsorption performance. In particular, Fe-CeO2(111) and Ni-CeO2(111) catalysts displayed good catalytic activity, with their respective rate-determining step energy barriers being 0.57 and 0.43 eV. Of particular note, Fe-CeO2(111) demonstrated high reaction rates at low to medium temperatures, making it a promising candidate for CO oxidation reaction catalyst. This study provides new insights into designing efficient, low-cost, and readily accessible CO oxidation catalysts, and establishes a solid foundation for the development of catalytic combustion kinetic models.

Density-functional study of electronic structure of quantum dots in high magnetic fields

Density-functional theory is used to study the electronic structure of quantum dots in magnetic field. New series of magic numbers are found for the total angular momentum of electrons. The empirical formula for the plateau width is obtained. The effect of a charged impurity on the electronic structure of a quantum dot has been studied.

Density Functional Theory and Density Functional Tight Binding Studies of Thiamine Hydrochloride Hydrates.

Thiamine hydrochloride (THCL), also known as vitamin B1, is an active pharmaceutical ingredient (API), present on the list of essential medicines developed by the WHO, which proves its importance for public health. THCL is highly hygroscopic and can occur in the form of hydrates with varying degrees of hydration, depending on the air humidity. Although experimental characterization of the THCL hydrates has been described in the literature, the questions raised in previously published works suggest that additional research and in-depth analysis of THCL dehydration behavior are still needed. Therefore, the main aim of this study was to characterize, by means of quantum chemical calculations, the behavior of thiamine hydrates and explain the previously obtained results, including changes in the NMR spectra, at the molecular level. To achieve this goal, a series of DFT (CASTEP) and DFTB (DFTB+) calculations under periodic boundary conditions have been performed, including molecular dynamics simulations and GIPAW NMR calculations. The obtained results explain the differences in the relative stability of the studied forms and changes in the spectra observed for the samples of various degrees of hydration. This work highlights the application of periodic DFT calculations in the analysis of various solid forms of APIs.

Solvent effects on Diels–Alder reaction in ionic liquids: A reaction density functional study

Extensive experimental studies have been performed on the Diels–Alder (DA) reactions in ionic liquids (ILs), which demonstrate that the IL environment can significantly influence the reaction rates and selectivity. However, the underlying microscopic mechanism remains ambiguous. In this work, the multiscale reaction density functional theory is applied to explore the effect of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) solvent on the reaction of cyclopentadiene (CP) with acrolein, methyl acrylate, or acrylonitrile. By analyzing the free energy landscape during the reaction, it is found that the polarization effect has a relatively small influence, while the solvation effect makes both the activation free energy and reaction free energy decrease. In addition, the rearrangement of local solvent structure shows that the cation spatial distribution responds more evidently to the reaction than the anion, and this indicates that the cation plays a dominant role in the solvation effect and so as to affect the reaction rates and selectivity of the DA reactions.

Spatial Heterogeneity and Strong Coupling of FeII /FeIII in anIndividual Metal-Organic Framework Nanoparticle for Efficient CO2 Photoreduction.

The synthesis and characterization of an FeII /FeIII metal-organic framework (MOF) nanocrystal with spatial heterogeneity that arises from the non-uniform distribution of different valence states is disclosed. The FeII /FeIII -Ni Prussian blue analog (PBA) delivers superior photocatalytic performance in the selective CO2 reduction reaction thanks to the strong FeII /FeIII coupling, with CO yield up to 12.27mmol g-1 h-1 and 90.6% selectivity under visible-light irradiation. Density functional theory calculation and experimental studies prove that the spatial heterogeneity of FeII /FeIII in the individual MOF nanocrystal not only directs and expedites the charge transfer within a catalyst particle but also creates the heterogeneity of catalytically-active Ni sites for efficient CO2 photoreduction. The current findings add to a growing literature of materials with compositional heterogeneity and provide a reference for future research.

Density functional theory combined thermodynamic study of the lead species adsorption on transition-metal-doped carbon materials: Intrinsic-intensified schemes

To tackle the lead (Pb) pollution in flue gas, the development of efficient adsorbents has become the desirable approach. However, the construction of adsorbent structure–activity relationship network remains challenging due to lacking the deep understanding of enhancement schemes for adsorbent modification. Here, the interaction mechanisms of transition-metal-modified carbon materials with Pb species and their intrinsic-intensified schemes were investigated by density functional theory and thermodynamics calculations, considering the effects of flue gas composition and temperature. Compared with the unmodified surface, the stability and selectivity of the modified surfaces for the adsorption of Pb species are increased dramatically and with good CO2 and HCl resistances. According to thermodynamic results, the Fe/CM and Mn/CM surfaces possess steady adsorption stability for Pb species from 100℃ to 400℃. Thereby, transition metal modification may complement carbon materials with properties such as efficiency enhancement, chlorine resistance, and wide temperature. Additionally, the Mn/CM exhibits superior properties than the Fe/CM, attributed to the lower orbital energies, stronger bonding interactions, and lower spatial resistance of the Mn and Pb species. Overall, this study provides an improved understanding for the development of efficient Pb control technology.

Shorten migration distance of ROS and accelerate electron transfer by engineering dual Lewis acid sites for synergistic activation of PMS toward rapid TC degradation

Shortening the migration distance of reactive oxygen species (ROS) and accelerating electron transfer are promising strategies for boosting the performance of catalysts to activate peroxymonosulfate (PMS). Herein, metallic Lewis acid and non-metallic Lewis acid co-doped graphitic carbon nitride (Co,B-C3N4) are successfully prepared by one-step calcination for synergistically enhanced PMS activation. Cobalt sites significantly accelerated the electron transfer process. Boron acts as an adsorption site for the contaminants, extensively reducing the distance that ROS needs to migrate. Therefore, Co,B-C3N4 demonstrates exceptional PMS activation capability, successfully degrading 99% of tetracycline (TC) in just 6 min with a rate constant reaching 0.407 min−1. Moreover, Co,B-C3N4 still displayed outstanding catalytic performance in complex water environments and long-term cycling conditions. Density Functional Theory (DFT) calculation and mechanism studies demonstrated that non-free radical path (direct electron transfer process and 1O2) plays a dominant role. This work provides a feasible strategy for designing PMS activation catalysts.

Impact of dopant X in zirconia on carbon deposition at the Nickel/X-stabilized zirconia(XSZ) surface in dry CH4 and CH4/H2O environments: First-principles density functional theory calculation and experimental study

The impact of a dopant (X; X = Y or Sc) in zirconia on carbon deposition in CH4 and CH4/H2O environments was studied experimentally and via first-principles calculations. In the CH4 environment, the apparent activation energy for the carbon deposition of Ni/ScSZ was lower than that of Ni/YSZ. SEM/EDS observation indicated the Ni/XSZ interface was key to elucidating the dopant-induced difference. To clarify the difference, the calculation presented two mechanisms: first, the vacancy in ScSZ rather than that in YSZ was readily moved to a neighboring site, thereby stabilizing the vacancy configuration after CH–CH bonding. Second, Ni was readily distorted in Ni/ScSZ at the specific vacancy configuration, thereby decreasing the forward activation energy. In CH4/H2O environment, the calculation showed larger dopant Y caused steric hindrance to prevent CH + O→CHO which was the sensitive reaction in the steam reforming. This explained lower reforming rate in Ni/YSZ than in Ni/ScSZ.

Anisotropic mechanical behavior and thermal attributes of antiferromagnetic UX2(X=P, As, Sb) materials: A density functional and elasticity theories study

In this work, we delve into the investigation of mechanical and thermal properties of antiferromagnetic UX2 (X=P, As, Sb), which are structured in the unique anti-Cu2Sb-type tetragonal form. The study’s principal findings revolve around the revelation of distinctive compressibility patterns and an observed preference for shear deformation in these compounds. These patterns are notably marked by a lower compressibility along the [100] direction than the [001] one, and the ease of shear deformation on the (100) plane compared to the (001) one. Intriguingly, our findings identify these compounds as residing on the brink of the brittle/ductile borderline as per Pugh’s ratio and Poisson’s ratio. Furthermore, the mechanical strength in these materials is significantly influenced by shear deformation, a fact indicated by the values of bulk and Young’s modulus. Among the UX2 (X=P, As, Sb) compounds, our analysis of the Debye temperature singles out UP2 as an outstanding performer due to its higher Debye temperature. A noteworthy aspect of this study is the quicker propagation of longitudinal waves over transverse waves, as indicated by the wave velocity and anisotropy calculations. We bring forth an enriched understanding of the anisotropic behavior of these materials in terms of bulk modulus, Young’s modulus, shear modulus, and Poisson’s ratio. The predicted high melting temperatures of the UX2 (X=P, As, Sb) compounds, coupled with the computed heat capacities aligning well with existing experimental data, point towards their potential applicability in high-temperature settings. Collectively, our study not only validates the methodologies employed but also brings to light fascinating new insights into the mechanical and thermal properties of these compounds, deepening our understanding of the intricate interplay between their atomic constitution and physical properties.

Some Isomers of DNNC and Radicals from Them - A DFT Treatment

In the present density functional study, some isomers of 1,3,5,5-tetranitrohexahydropyrimidine (DNNC) have been considered within the constraints of the theory and the basis set employed (UB3LYP/6-31++G(d,p)). Various geometrical, QASR and quantum chemical properties are harvested and discussed. All the considered isomers of DNNC have exothermic heat of formation and favorable Gibbs free energy of formation values at the standard states and they are stable electronically. Based on the interfrontier molecular orbital energy gap values DNNC is found to be more sensitive to impulse stimulus than the others. Also radicals obtained from DNNC isomers considered (by the homolytic rapture of C-NO2 or N-NO2 bonds) have been focus of present interest. The radicals obtained by the cleavage of geminal C-NO2 bonds have exothermic Hº and favorable Gº values at the standard states and electronically stable as their respective parent compounds. Although, some of the radicals generated by the homolytic cleavage of N-NO2 bonds are thermo chemically exothermic and stable, like the radicals mentioned above, the radicals from the isomer having all the nitro groups in vicinal configuration decomposes by eliminating one of the geminal nitro groups.

An Experimental and Density Functional Theory Simulation Study of NO Reduction Mechanisms over Fe0 Supported on Graphene with and without CO.

NO reduction over highly dispersed zerovalent iron (Fe0) supported on graphene (G), with and without the presence of CO in the reacting stream, was systematically studied using a fixed-bed reactor, and the reaction mechanism was examined with the aid of in situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations. The in situ FTIR results showed that NO adsorbed on the Fe0 site is reduced to form active surface oxygen species (O*), which is then reduced by carbon in graphene to form CO2. The presence of CO in the reacting stream helps to reduce the oxidized Fe(O) sites to regenerate Fe0 sites, making NO reduction easier. It was revealed that NO and CO2 are easily adsorbed on the active surface oxygen species (O*) to form nitrate and carbonate, inhibiting their reduction by CO and deactivating the catalyst. The DFT calculations results suggest that the role of Fe is to reduce the energy barrier of the NO adsorption and decomposition, which controls the formation of active surface oxygen species and N2. The combined FTIR and DFT results offer new insights into the possible mechanism of catalytic NO reduction over graphene loaded with Fe, with and without CO.