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DOE OSTI2015

Photoelectron spectrum of PrO{sup −}

The photoelectron (PE) spectrum of PrO{sup −} exhibits a short 835 ± 20 cm{sup −1} vibrational progression of doublets (210 ± 30 cm{sup −1} splitting) assigned to transitions from the 4f{sup 2} [{sup 3}H{sub 4}] σ{sub 6s}{sup 2} Ω = 4 anion ground state to the 4f{sup 2} [{sup 3}H{sub 4}] σ{sub 6s} Ω = 3.5 and 4.5 neutral states. This assignment is analogous to that of the recently reported PE spectrum of CeO{sup −}, though the 82 cm{sup −1} splitting between the 4f [{sup 2}F{sub 2.5}] σ{sub 6s} Ω = 2 and Ω = 3 CeO neutral states could not be resolved [Ray et al., J. Chem. Phys. 142, 064305 (2015)]. The origin of the transition to the Ω = 3.5 neutral ground state is 0.96 ± 0.01 eV, which is the adiabatic electron affinity of PrO. Density functional theory calculations on the anion and neutral molecules support the assignment. The appearance of multiple, irregularly spaced and low-intensity features observed ca. 1 eV above the ground state cannot be reconciled with low-lying electronic states of PrO that are accessible via one-electron detachment. However, neutral states correlated with the 4f{sup 2} [{sup 3}H{sub 4}] 5d superconfiguration are predicted to be approximately 1 eV above the 4f{sup 2} [{sup 3}H{sub 4}] σ{sub 6s} Ω = 3.5 neutral ground state, leading to the assignment of these features to shake-up transitions to the excited neutral states. Based on tentative hot band transition assignments, the term energy of the previously unobserved 4f{sup 2} [{sup 3}H{sub 4}] σ{sub 6s} Ω = 2.5 neutral state is determined to be 1840 ± 110 cm{sup −1}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Effect of Siloxane Ring Strain and Cation Charge Density on the Formation of Coordinately Unsaturated Metal Sites on Silica: Insights from Density Functional Theory (DFT) Studies

Amorphous silica (SiO 2 ) is commonly used as a support in heterogeneous catalysis. However, due to the structural disorder and temperature induced change of surface morphology, the structures of silica supported metal catalysts are difficult to determine. Most studies are primarily focused on understanding the interactions of different types of surface hydroxyl groups with metal ions. In comparison, the effect of siloxane ring size on the structure of silica supported metal catalysts and how it affects catalytic activity is poorly understood. Here, we have used density functional theory calculations to understand the effect of siloxane ring strain on structure and activity of different monomeric Lewis acid metal sites on silica. In particular, we have found that large siloxane rings favor strong dative bonding interaction between metal ion and surface hydroxyls, leading to the formation of high-coordinate metal sites. In comparison, metal-silanol interaction is weak in small siloxane rings, resulting in low-coordinate metal sites. The physical origin of this size dependence is associated with siloxane ring strain, and, a correlation between metal-silanol interaction energy and ring strain energy has been observed. In addition to ring strain, the strength of the metal-silanol interaction also depends on the positive charge density of the cations. In fact, a correlation also exists between metal-silanol interaction energy and charge density of several first-row transition and post-transition metals. The theoretical results are compared with the EXAFS data of monomeric Zn(II) and Ga(III) ions grafted on silica. In conclusion, the molecular level insights of how metal ion coordination on silica depends on siloxane ring strain and cation charge density will be useful in the synthesis of new catalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2026

4-Electron Oxygen Reduction Reaction (ORR) with Iron Phthalocyanine (FePc) Functionalized Nanowire Templated-3D Fuzzy Graphene (NT-3DFG)

Iron phthalocyanine (FePc) is a promising alternative to platinum-based catalysts for sustainable energy devices; however, the plane-symmetry of Fe-N 4 sites, random aggregation, and poor conductivity of FePc present major barriers for their application as oxygen reduction reaction (ORR) electrocatalysts. Here, we report the synergistic effects of FePc electrocatalysts supported by a nanowire-templated three-dimensional fuzzy graphene (FePc@NT-3DFG) substrate. The in situ functionalized oxygen groups (iFOGs) at the edge of NT-3DFG localize Fe active sites in FePc under alkaline ORR conditions. With a uniform FePc distribution through many single layers of graphene, the NT-3DFG substrates improve O 2 adsorption and catalytic activity while stabilizing the electrochemical activity during reactions. The FePc@NT-3DFG catalyst exhibits fast ORR kinetics with an extremely low Tafel slope of 28.3 ± 2.7 mV dec −1 , a higher half-wave potential of 0.911 ± 0.004 V (vs RHE), and notable long-term stability at 0.5 V (vs RHE) of 96.0 ± 0.4% retention after 30 h. Surface chemistry spectra validate electronic configuration modification of Fe at the iFOGs. Density functional theory calculations indicate that the extra layers of graphene improve oxygen adsorption. Moreover, additional exploration of other transition metal phthalocyanines supports the effects of iFOGs through the transition toward 4e − ORR. This work offers an expanded strategy for active site modification through edge-based graphene substrates for 4e − ORR.

X-ray absorption spectroscopy↗
DOE OSTI2024

Destabilizing high-capacity high entropy hydrides via earth abundant substitutions: From predictions to experimental validation

The vast chemical space of high entropy alloys (HEAs) makes trial-and-error experimental approaches for materials discovery intractable and often necessitates data-driven and/or first principles computational insights to successfully target materials with desired properties. In the context of materials discovery for hydrogen storage applications, a theoretical prediction-experimental validation approach can vastly accelerate the search for substitution strategies to destabilize high-capacity hydrides based on benchmark HEAs, e.g. TiVNbCr alloys. Here, in this study, machine learning predictions, corroborated by density functional theory calculations, predict substantial hydride destabilization with increasing substitution of earth-abundant Fe content in the (TiVNb) 75 Cr 25-x Fe x system. The as-prepared alloys crystallize in a single-phase bcc lattice for limited Fe content x < 7, while larger Fe content favors the formation of a secondary C14 Laves phase intermetallic. Short range order for alloys with x < 7 can be well described by a random distribution of atoms within the bcc lattice without lattice distortion. Hydrogen absorption experiments performed on selected alloys validate the predicted thermodynamic destabilization of the corresponding fcc hydrides and demonstrate promising lifecycle performance through reversible absorption/desorption. This demonstrates the potential of computationally expedited hydride discovery and points to further opportunities for optimizing bcc alloy ↔ fcc hydrides for practical hydrogen storage applications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Graphene in ohmic contact for both n-GaN and p-GaN

The wrinkles of single layer graphene contacted with either n-GaN or p-GaN were found both forming ohmic contacts investigated by conductive atomic force microscopy. The local I–V results show that some of the graphene wrinkles act as high-conductive channels and exhibiting ohmic behaviors compared with the flat regions with Schottky characteristics. We have studied the effects of the graphene wrinkles using density-functional-theory calculations. It is found that the standing and folded wrinkles with zigzag or armchair directions have a tendency to decrease or increase the local work function, respectively, pushing the local Fermi level towards n- or p-type GaN and thus improving the transport properties. These results can benefit recent topical researches and applications for graphene as electrode material integrated in various semiconductor devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The electronic structure changes and the origin of the enhanced optical properties in N-doped anatase TiO{sub 2}—A theoretical revisit

We have investigated the electronic structure changes and the origin of the enhanced optical properties of N-doped anatase TiO{sub 2} using first-principles density-functional theory calculations. To determine the band gap variations induced by N-doping, we developed a new approach to locate the effective valence band maximum (VBM) by characterizing the degree of localization of the N-induced states in the band structures of various N-doped TiO{sub 2} systems. Our calculations show that the band gap variations are largely affected by the local bonding structures and doping concentration of the substitutional N atoms. As the N content is up to certain level, some local bonding structures can indeed cause band gap reduction due to the formation of band-like delocalized states above the VBM of TiO{sub 2}, while other local bonding configurations may simply form localized impurity states in the band gap. Accordingly, the N-induced localized and delocalized electronic states can exist simultaneously to contribute to the enhanced optical properties of anatase TiO{sub 2}. Our computational approach also provides a new way to investigate the band gap engineering of other wide band gap semiconductor material systems.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Diselenophosphate-Induced Conversion of an Achiral [Cu 20 H 11 {S 2 P(O i Pr) 2 } 9 ] into a Chiral [Cu 20 H 11 {Se 2 P(O i Pr) 2 } 9 ] Polyhydrido Nanocluster

A polyhydrido copper nanocluster, [Cu 20 H 11 {Se 2 P(OiPr) 2 } 9 ] (2 H ), which exhibits an intrinsically chiral inorganic core of C-3 symmetry, was synthesized from achiral [Cu 20 H 11 {S 2 P(OiPr) 2 } 9 ] (1(H)) of C-3h symmetry by a ligand-exchange method. Likewise, the structure has a distorted cuboctahedral Cu-13 core, two triangular faces of which are capped along the C-3 axis, one by a Cu-6 cupola and the other by a single Cu atom. The Cu-20 framework is further stabilized by 9 diselenophosphate and 11 hydride ligands. The number of hydride, phosphorus, and selenium resonances and their splitting patterns in multinuclear NMR spectra of 2(H) indicate that the chiral Cu20H11 core retains its C-3 symmetry in solution. Moreover, the 11 hydride ligands were located by neutron diffraction experiments and shown to be capping (3)-H and interstitial (5)-H ligands (in square-pyramidal and trigonal-bipyramidal cavities), as supported by DFT calculations on [Cu 20 H 11 (Se 2 PH 2 ) 9 ] (2 H ') as a simplified model.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were explored at 300 °C under dilute oxygen concentration over anatase TiO 2 nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO 2 . The two TiO 2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia reflects that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO 2 surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C–H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD 3 OH and CD 3 OD), confirming that the cleavage of the C–H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO 2 nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO 2 is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Two-Dimensional Moiré Polaronic Electron Crystals

Two-dimensional moiré materials have emerged as the most versatile platform for realizing quantum phases of electrons. Here, we explore the stability origins of correlated states in WSe 2 /WS 2 moiré superlattices. We find that ultrafast electronic excitation leads to partial melting of the Mott states on timescales 5 times longer than predictions from the charge hopping integrals and that the melting rates are thermally activated, with activation energies of 1⁢8±3 and 1⁢3±2 meV for the one- and two-hole Mott states, respectively, suggesting significant electron-phonon coupling. A density functional theory calculation of the one-hole Mott state confirms polaron formation and yields a hole-polaron binding energy of 16 meV. These findings reveal a close interplay of electron-electron and electron-phonon interactions in stabilizing the polaronic Mott insulators at transition metal dichalcogenide moiré interfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Fiber-Shaped Fluidic Nanogenerator with High Power Density for Self-Powered Integrated Electronics

Flow-based hydrovoltaic nanogenerators have attracted particular attention as efficient and facile miniature energy-harvesting devices. However, they deliver relatively low output power density and limited power conversion efficiency owing to inefficient charge separation and poor charge accumulation. Here, we report the fabrication of high-performance, fiber-shaped fluidic nanogenerators (FFNGs) that use MoS 2 -encapsulated C fibers. The FFNGs deliver stable output voltages of 540 mV and an ultrahigh power density of 10.8 W m -2 . With experiments and density functional theory calculations, improvements in output performance can be attributed to smaller energy requirements for Na + cation adsorption, increased charge separation, and diffusion layer formations on the near surface. Moreover, self-powered, integrated devices achieve a maximum total power conversion-storage efficiency of ~11% and stable output voltage up to 3.1 V, which is applicable for use in consumer electronics devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electric field and substrate–induced modulation of spin-polarized transport in graphene nanoribbons on A3B5 semiconductors

In this work, we present the density functional theory calculations of the effect of an oriented electric field on the electronic structure and spin-polarized transport in a one dimensional (1D) zigzag graphene nanoribbon (ZGNR) channel placed on a wide bandgap semiconductor of the A3B5 type. Our calculations show that carrier mobility in the 1D semiconductor channel of the ZGNR/A3B5(0001) type is in the range from 1.7×10{sup 4} to 30.5×10{sup 4} cm{sup 2}/Vs and can be controlled by an electric field. In particular, at the critical value of the positive potential, even though hole mobility in an one-dimensional 8-ZGNR/h-BN semiconductor channel for spin down electron subsystems is equal to zero, hole mobility can be increased to 4.1×10{sup 5} cm{sup 2}/Vs for spin up electron subsystems. We found that band gap and carrier mobility in a 1D semiconductor channel of the ZGNR/A3B5(0001) type depend strongly on an external electric field. With these extraordinary properties, ZGNR/A3B5(0001) can become a promising materials for application in nanospintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Tuning patterning conditions by co-adsorption of gases: Br{sub 2} and H{sub 2} on Si(001)

We have studied the co-adsorption of Br{sub 2} and H{sub 2} on Si(001), and obtained co-adsorption energies and the surface phase diagram as a function of the chemical potential and pressure of the two gases. To do this, we have used density functional theory calculations in combination with ab initio atomistic thermodynamics. Over large ranges of bromine and hydrogen chemical potentials, the favored configuration is found to be either one with only Br atoms adsorbed on the surface, at full coverage, in a (3 × 2) pattern, or a fully H-covered surface in a (2 × 1) structure. However, we also find regions of the phase diagram where there are configurations with either only Br atoms, or Br and H atoms, arranged in a two-atom-wide checkerboard pattern with a (4 × 2) surface unit cell. Most interestingly, we find that by co-adsorbing with H{sub 2}, we bring this pattern into a region of the phase diagram corresponding to pressures that are significantly higher than those where it is observed with Br{sub 2} alone. We also find small regions of the phase diagram with several other interesting patterns.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Experimental and quantum mechanical characterization of an oxygen-bridged plutonium(IV) dimer

We report the synthesis and characterization of K 4 {[PuCl 2 (NO 3 ) 3 ] 2 (μ 2 -O)}·H 2 O, which contains the first known μ 2 -oxo bridge between two Pu IV metal centers. Adding to its uniqueness is the Pu-(μ 2 -O) bond length of 2.04 Å, which is the shortest of other analogous compounds. The Pu-(μ 2 -O)-Pu bridge is characterized by the mixing of s -, d -, and p -orbitals from Pu with the p -orbitals of O; the 5 f -orbitals do not participate in bonding. Natural bond orbital analysis indicates that Pu and O interact through one 3c-2e σ Pu-O-Pu and two 3c-2e π Pu-O-Pu bonding orbitals and that the electron density is highly polarized on the μ 2 -O. Bond topology properties analysis indicates that the Pu-(μ 2 -O) bond shares both ionic and covalent character. Quantum mechanical calculations also show that the dimer has multiconfigurational ground states, where the nonet, septet, quintet, triplet, and singlet are close in energy. This work demonstrates the interplay between experimental and computational efforts that is required to understand the chemical bonding of Pu compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Fe-N 4 O-C Nanoplates Covalently Bonding on Graphene for Efficient CO 2 Electroreduction and Zn-CO 2 Batteries

Electrochemical carbon dioxide (CO 2 ) reduction into value-added products holds great promise in moving toward carbon neutrality but remains a grand challenge due to lack of efficient electrocatalysts. Herein, the nucleophilic substitution reaction is elaborately harnessed to synthesize carbon nanoplates with a Fe-N 4 O configuration anchored onto graphene substrate (Fe-N 4 O-C/Gr) through covalent linkages. Density functional theory calculations demonstrate the unique configuration of Fe-N 4 O with one oxygen (O) atom in the axial direction not only suppresses the competing hydrogen evolution reaction, but also facilitates the desorption of *CO intermediate compared with the commonly planar single-atomic Fe sites. The Fe-N 4 O-C/Gr shows excellent performance in the electroreduction of CO 2 into carbon monoxide (CO) with an impressive Faradaic efficiency of 98.3% at -0.7 V versus reversible hydrogen electrode (RHE) and a high turnover frequency of 3511 h -1 . Furthermore, as a cathode catalyst in an aqueous zinc (Zn)-CO 2 battery, the Fe-N 4 O-C/Gr achieves a high CO Faradaic efficiency (≈91%) at a discharge current density of 3 mA cm -2 and long-term stability over 74 h. Here this work opens up a new route to simultaneously modulate the geometric and electronic structure of single-atomic catalysts toward efficient CO 2 conversion.

25 ENERGY STORAGE↗
DOE OSTI2021

Giant orbital polarization of Ni 2+ in a square planar environment

Finding large orbital polarization in Ni-based oxides has become a topic of paramount interest in recent years due to the prospect of finding superconductivity. In this study, we investigate the electronic structure of single-crystalline samples of Sr 2 CuO 3 and Ni-doped Sr 2 CuO 3 containing Cu/NiO 4 square planar units. Our polarization-dependent x-ray absorption spectroscopy experiments reveal extremely large orbital polarization (~63%) for Ni 2+ . Cluster calculations and ab initio calculations find that the giant orbital polarization arises due to the low-spin (S=0) configuration with two holes in the Ni 3d x 2 -y 2 orbital, contrary to the expected high-spin (S=1) state from Hund's first rule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Niobium substitution suppresses the superconducting critical temperature of pressurized MoB 2

A recent study has demonstrated that MoB 2 , transforming to the same structure as MgB 2 (P 6 /mmm), superconducts at temperatures above 30 K near 100 GPa [C. Pei et al., Natl. Sci. Rev. 10, nwad034 (2023)], and Nb substitution in MoB 2 stabilizes the P6/mmm structure down to ambient pressure [A. C. Hire et al., Phys. Rev. B 106, 174515 (2022)]. Here, the current work explores the high-pressure superconducting behavior of Nb-substituted MoB 2 (Nb 0.25 Mo 0.75 B 2 ). High-pressure x-ray diffraction measurements show that the sample remains in the ambient pressure P6/mmm structure to at least 160 GPa. Electrical resistivity measurements demonstrate that from an ambient pressure T c of 8K (confirmed by specific heat to be a bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure is thus significantly lower than that found in MoB 2 under pressure (30 K), revealing that Nb substitution results in a strong suppression of the superconducting critical temperature. Our calculations indeed find a reduced electron-phonon coupling in Nb 0.25 Mo 0.75 B 2 , but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Magnetism in Boron Nitride Monolayer Induced by Cobalt or Nickel Doping

We have investigated the electronic structure and magnetic properties of cobalt (Co)- or nickel (Ni)-doped hexagonal boron nitride (h-BN) monolayer using density functional theory calculations. The h-BN monolayer without any doping is a nonmagnetic insulator. Our studies show that isolated Co or Ni atom can both induce the local magnetic state in h-BN monolayer. And the impurity energy levels will be formed in the band gap. Results of our first-principle calculations reveal that isolated Co atom can result in a magnetic moment of 3.57 μ{sub B}, while the Ni atom is 0.87 μ{sub B}. And the magnetic moments mainly come from d orbitals of the doped atoms. The studies of magnetic coupling show that the two Co atoms at different distances in h-BN monolayer do not always couple ferromagnetically. When the distances are 2.504 and 6.625 Å, the spins induced by the two Co atoms will form antiferromagnetic ground states. While the two Ni atoms at different distances in the monolayer always couple ferromagnetically. So our results imply that the Ni-doped h-BN is more suitable for spintronic material than Co doping. These results are useful for spintronic application.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Role of Hydrogen-bonded Bimolecular Formic Acid–Formate Complexes for Formic Acid Decomposition on Copper: A Combined First-Principles and Microkinetic Modeling Study

Hydrogen bonding interactions alter the nanoscale reaction mechanisms of many chemistries. Yet, it remains unclear how they affect heterogeneously catalyzed decomposition of formic acid (FA), a reaction of intense interest since FA is a promising hydrogen carrier. In this work, we elucidate how hydrogen bonding affects the reaction mechanisms for FA decomposition on Cu(111) by combining first-principles density functional theory calculations to calculate reaction energetics, Latin-hypercube sampling to elucidate stable high-coverage adsorbate configurations, and coverage self-consistent mean-field microkinetic models to predict reaction kinetics. We demonstrate that hydrogen-bonded complexes of FA with formate (bimolecular FA–HCOO complexes) can play a dominant role in FA decomposition. Specifically, our first-principles calculations show that hydrogen bonding of FA with HCOO may stabilize the crucial monodentate HCOO intermediate and the transition states for HCOO decomposition, especially at low coverages. We predict that, depending on the reaction conditions, 40–80% of the reaction flux goes through pathways involving the bimolecular FA–HCOO complexes. Additionally, the active site for FA decomposition on Cu(111) involves a high coverage (~0.4 monolayers (ML)) of these complexes, which unexpectedly stabilize intermediates and transition states via van der Waals interactions. Our work provides molecular insights consistent with previous experimental observations on supported Cu/Al 2 O 3 catalysts. This paves the way toward the development of novel catalysts for FA decomposition as well as for other industrially important chemistries with intermediates capable of hydrogen bonding, such as ammonia electrooxidation and CO 2 hydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗