How spin state and oxidation number of transition metal atoms determine molecular adsorption: a first-principles case study for NH3.

Understanding how the electronic state of transition metal atoms can influence molecular adsorption on a substrate is of great importance for many applications. Choosing NH3 as a model molecule, its adsorption behavior on defected SnS2 monolayers is investigated. The number of valence electrons n is controlled by decorating the monolayer with different transition metal atoms, ranging from Sc to Zn. Density-Functional Theory based calculations show that the adsorption energy of NH3 molecules oscillates with n and shows a clear odd-even pattern. There is also a mirror symmetry of the adsorption energies for large and low electron numbers. This unique behavior is mainly governed by the oxidation state of the TM ions. We trace back the observed trends of the adsorption energy to the orbital symmetries and ligand effects which affect the interaction between the 3σ orbitals (NH3) and the 3d orbitals of the transition metals. This result unravels the role which the spin state of TM ions plays in different crystal fields for the adsorption behavior of molecules. This new understanding of the role of the electronic structure on molecular adsorption can be useful for the design of high efficiency nanodevices in areas such as sensing and photocatalysis.

Stabilization of Short- and Long-Range Magnetic Ordering through the Cooperative Effect of 1D [CuO4]∞ Chains and [CuO2X2] Quadrilateral in Quasi-1D Spin-1/2 Systems.

Quasi-1D chain antiferromagnets with reduced structural dimensionality are a rich playground for investigating novel quantum phenomena. We report the synthesis, crystal structure, and magnetism of two novel quasi-1D antiferromagnets, ß-PbCu2(TeO3)2Cl2 (I) and PbCu2(TeO3)2Br2 (II). Their magnetic frameworks are constructed via Cu-based quasi-1D [Cu(2)O4]∞ zigzag chains with square-planar [Cu(1)O2X2] (X=Cl or Br) separated among 1D chains. Specific heat measurements show λ peaks at ~9 K and ~19 K for I and II, respectively. Moreover, both broad maximums (χmax=90 K for I and 80 K for II) and small kinks (TN≈9 K for I and 19 K for II) have been observed in magnetic susceptibility measurements of I and II. Bonner-Fisher model fitting, and theoretical analyses were performed to evaluate the magnetic exchange interactions. Our experimental and theoretical results and structure-properties relationship analysis reveal the coexistence of short- and long-range magnetic ordering from the cooperative effect of 1D [CuO4]∞ chains and [CuO2X2] quadrilateral.

Activating Bi p-orbitals in Dispersed Clusters of Amorphous BiOx for Electrocatalytic Nitrogen Reduction.

Catalytic strategies based on main group metals are significantly less advanced than those of transition metal catalysis, leaving untapped areas of potentially fruitful research. We here demonstrate an effective approach for the modulation of Bi 6p energy levels during the construction of atomically dispersed clusters of amorphous BiOx . Bi oxidation state is proposed to strongly affects the nitrogen fixation activity, with the half-occupied pz orbitals of the Bi2+ ions being highly efficient toward electron injection into the inert N2 molecule. With sufficient catalytic sites to adsorb and activate N2 , the bonding between N2 and catalyst is able to be in situ identified. The catalyst shows an outstanding Faraday efficiency (≈30 %) and high yield (≈113 µg h-1 mg-1 cat ) in NH3 production, outperforming most of the existing catalysts in aqueous solution. These results lay the basis for developing the potential of p-block elements for catalysis of multi-electron reactions.

New Insight of Pyrrole-Like Nitrogen for Boosting Hydrogen Evolution Activity and Stability of Pt Single Atoms.

Single atomic Pt catalysts exhibit particularly high hydrogen evolution reaction (HER) activity compared to conventional nanomaterial-based catalysts. However, the enhanced mechanisms between Pt and their coordination environment are not understood in detail. Hence, a systematic study examining the different types of N in the support is essential to clearly demonstrate the relationship between Pt single atoms and N-doped support. Herein, three types of carbon nanotubes with varying types of N (pyridine-like N, pyrrole-like N, and quaternary N) are used as carbon support for Pt single atom atomic layer deposition. The detailed coordination environment of the Pt single atom catalyst is carefully studied by electron microscope and X-ray absorption spectra (XAS). Interestingly, with the increase of pyrrole-like N in the CNT support, the HER activity of the Pt catalyst also improves. First principle calculations results indicate that the interaction between the dyz and s orbitals of H and sp3 hybrid orbital of N should be the origin of the superior HER performance of these Pt single atom catalysts (SACs).

Minimum Tracking Linear Response Hubbard and Hund Corrected Density Functional Theory in CP2K.

We present the implementation of the Hubbard (U) and Hund (J) corrected Density Functional Theory (DFT + U + J) functionality in the Quickstep program, which is part of the CP2K suite. The tensorial and Löwdin subspace representations are implemented and compared. Full analytical DFT + U + J forces are implemented and benchmarked for the tensorial and Löwdin representations. We also present the implementation of the recently proposed minimum-tracking linear-response method that enables the U and J parameters to be calculated on first-principles basis without reference to the Kohn-Sham eigensystem. These implementations are benchmarked against recent results for different materials properties including DFT + U band gap opening in NiO, the relative stability of various polaron distributions in TiO2, the dependence of the calculated TiO2 band gap on +J corrections, and, finally, the role of the +U and +J corrections for the computed properties of a series of the hexahydrated transition metals. Our implementation provides results consistent with those already reported in the literature from comparable methods. We conclude the contribution with tests on the influence of the Löwdin orthonormalization on the occupancies, calculated parameters, and derived properties.

The Role of Water Molecules on Polaron Behavior at Rutile (110) Surface: A Constrained Density Functional Theory Study.

Understanding the behavior of a polaron in contact with water is of significant importance for many photocatalytic applications. We investigated the influence of water on the localization and transport properties of polarons at the rutile (110) surface by constrained density functional theory. An excess electron at a dry surface favors the formation of a small polaron at the subsurface Ti site, with a preferred transport direction along the [001] axis. As the surface is covered by water, the preferred spatial localization of the polarons is moved from the subsurface to the surface. When the water coverage exceeds half a monolayer, the preferred direction of polaron hopping is changed to the [110] direction toward the surface. This characteristic behavior is related to the Ti3d-orbital occupations and crystal field splitting induced by different distorted structures under water coverage. Our work describes the reduced sites that might eventually play a role in photocatalysis for rutile (110) surfaces in a water environment.

Nonadiabatic Dynamics of Polaron Hopping and Coupling with Water on Reduced TiO2.

By interplay between first-principles molecular dynamics and nonadiabatic molecular dynamics simulations based on the decoherence-induced surface-hopping approach, we investigate and quantify the mechanisms through which different electron polaron hopping regimes in the reduced anatase TiO2(101) surface influence recombination of photogenerated charge carriers, also in the presence of adsorbed water (H2O) molecules. The simulations reveal that fast hopping regimes promote ultrafast recombination of photogenerated charge-carriers. Conversely, charge recombination is delayed in the presence of slower polaron hopping and even more so if the polaron is pinned at one Ti-site, as typical following adsorption of H2O on the anatase(101) surface. These trends are related to the observed enhancement of the space and energy overlap between conduction band minimum and polaron band gap states, and the ensuing nonadiabatic couplings (NAC) strengths, during a polaronic hop. We expect these insights on the beneficial role of polaron diffusion pinning for the extended lifetime of photoexcitations in TiO2 to sustain ongoing developments of photocatalytic strategies based on this substrate.

Atomic layer deposited Pt-Ru dual-metal dimers and identifying their active sites for hydrogen evolution reaction.

Single atom catalysts exhibit particularly high catalytic activities in contrast to regular nanomaterial-based catalysts. Until recently, research has been mostly focused on single atom catalysts, and it remains a great challenge to synthesize bimetallic dimer structures. Herein, we successfully prepare high-quality one-to-one A-B bimetallic dimer structures (Pt-Ru dimers) through an atomic layer deposition (ALD) process. The Pt-Ru dimers show much higher hydrogen evolution activity (more than 50 times) and excellent stability compared to commercial Pt/C catalysts. X-ray absorption spectroscopy indicates that the Pt-Ru dimers structure model contains one Pt-Ru bonding configuration. First principle calculations reveal that the Pt-Ru dimer generates a synergy effect by modulating the electronic structure, which results in the enhanced hydrogen evolution activity. This work paves the way for the rational design of bimetallic dimers with good activity and stability, which have a great potential to be applied in various catalytic reactions.

Author Correction: Atomic layer deposited Pt-Ru dual-metal dimers and identifying their active sites for hydrogen evolution reaction.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.