Zhansheng Lu

Oxygen vacancy formation energy in Pd-doped ceria: A DFT+U study

Using the DFT+U method, i.e., first principles density functional theory calculations with the inclusion of on-site Coulomb interaction, the effects of Pd doping on the O vacancy formation energy (E(vac)) in CeO(2) has been studied. We find that E(vac) is lowered from 3.0 eV in undoped ceria to 0.6 eV in the Pd-doped compound. Much of this decrease can be attributed to emerging Pd-induced gap states above the valence band and below the empty Ce 4f states. These localized defect states involve the Pd ion and its nearest neighbors, which are also the main acceptors of the extra electrons left on reduction. The effect of the Pd dopant on the geometric structure is very modest for CeO(2) but considerable for CeO(2-x).

Interfacial properties of NM/CeO2(111) (NM = noble metal atoms or clusters of Pd, Pt and Rh): a first principles study

Results from first principles calculations present a rather clear atomic and electronic level picture of the interaction of single noble metals (NM: Pd, Pt and Rh) and the corresponding NM(4) clusters with a CeO(2)(111) surface. The most preferable adsorption sites for both the Pd and Pt adatoms are the surface O-bridge sites, while the Rh adatom prefers to stay at the O-hollow site. The Rh adatom shows much stronger interaction with the CeO(2)(111) surface than the Pd and Pt adatoms do, while the Pd adatom has the smallest adsorption energy. The dependence of the Rh/ceria interfacial properties on the value of the Hubbard U-term was tested systematically. The small clusters show stronger interaction than the corresponding single NM adatoms on the CeO(2)(111) surface. The reaction of [Formula: see text] was found for both the single NM adatoms and the small cluster adsorbate, indicating that NM adsorbates were mainly oxidized by the surface Ce ions with obvious charge transfer from NM to the CeO(2)(111) surface. The three base atoms of the small clusters that bonded with the CeO(2)(111) surface showed positive charges, while the top metal atoms of the NM(4) clusters had a small negative charge.

Structural and electronic properties of NM-doped ceria (NM = Pt, Rh): a first-principles study

The effects of noble metal (NM) dopants (NM = Pt, Rh) on the structural and electronic properties of ceria are studied using a density functional theory (DFT) method, with the inclusion of on-site Coulomb interaction (DFT + U). It is found that these NM dopants give rise to large perturbations of the atomic and electronic structures of ceria and induce metal-induced gap states at the Fermi level suitable for accommodating extra electrons, thereby lowering the reduction energy of ceria and making the NM-doped cerias more reducible than pure ceria. This mechanism for facilitating the reduction of ceria is different from that of Zr-doped ceria where the increased reducibility is largely due to the structural distortions and not to electronic modifications.

First-principles studies of BN sheets with absorbed transition metal single atoms or dimers: stabilities, electronic structures, and magnetic properties

BN sheets with absorbed transition metal (TM) single atoms, including Fe, Co, and Ni, and their dimers have been investigated by using a first-principles method within the generalized gradient approximation. All of the TM atoms studied are found to be chemically adsorbed on BN sheets. Upon adsorption, the binding energies of the Fe and Co single atoms are modest and almost independent of the adsorption sites, indicating the high mobility of the adatoms and isolated particles to be easily formed on the surface. However, Ni atoms are found to bind tightly to BN sheets and may adopt a layer-by-layer growth mode. The Fe, Co, and Ni dimers tend to lie (nearly) perpendicular to the BN plane. Due to the wide band gap of the pure BN sheet, the electronic structures of the BN sheets with TM adatoms are determined primarily by the distribution of TM electronic states around the Fermi level. Very interesting spin gapless semiconductors or half-metals can be obtained in the studied systems. The magnetism of the TM atoms is preserved well on the BN sheet, very close to that of the corresponding free atoms and often weakly dependent on the adsorption sites. The present results indicate that BN sheets with adsorbed TM atoms have potential applications in fields such as spintronics and magnetic data storage due to the special spin-polarized electronic structures and magnetic properties they possess.

First-principles and experimental study of nitrogen/sulfur co-doped carbon nanosheets as anodes for rechargeable sodium ion batteries

Heteroatom doped carbon materials have recently demonstrated an outstanding sodium storage ability and are being considered as the most promising candidate as anodes for sodium ion batteries. However, there is limited understanding of the relationship between structural and electronic properties and electrochemical storage capacity. First-principles calculations on a doped graphene cluster propose that N, S co-doping can promote the electronegativity, adsorption capacity of Na atoms and diffusion of Na+ ions on graphene sheets, especially for the sample consisting of more pyridinic-N, while excessive O atoms may alleviate these. All these features render N, S co-doped carbon as a superior anode for sodium ion batteries. Therefore, the N, S co-doped carbon nanosheets are fabricated via a simple thermal treatment method using gelatin as the carbon source and thiourea as the N and S precursor. The optimized product (mgelatin : mthiourea = 1 : 10) results in a superb cycling capacity of 300 mA h g−1 after 500 cycles, with a coulombic efficiency of ∼100%. This study provides a facile and reliable route to prepare co-doped carbon with enhanced sodium storage properties.

Pd1/BN as a promising single atom catalyst of CO oxidation: a dispersion-corrected density functional theory study

Single metal atom catalysts exhibit extraordinary activity in a large number of reactions, and some two-dimensional materials (such as graphene and h-BN) are found to be prominent supports to stabilize single metal atoms. The CO oxidation reaction on single Pd atoms supported by two-dimensional h-BN is investigated systematically by using dispersion-corrected density functional theory study. The great stability of the h-BN supported single Pd atoms is revealed, and the single Pd atom prefers to reside at boron vacancies. Three proposed mechanisms (Eley–Rideal, Langmuir–Hinshelwood, and a “new” termolecular Eley–Rideal) of the CO oxidation were investigated, and two of them (the traditional Langmuir–Hinshelwood mechanism and the new termolecular Eley–Rideal mechanism) are found to have rather small reaction barriers of 0.66 and 0.39 eV for their rate-limiting steps, respectively, which suggests that the CO oxidation could proceed at low temperature on single Pd atom doped h-BN. The current study will help to understand the various mechanisms of the CO oxidation and shed light on the design of CO oxidation catalysts, especially based on the concept of single metal atoms.

Repairing sulfur vacancies in the MoS2 monolayer by using CO, NO and NO2 molecules

As-grown transition metal dichalcogenides are usually chalcogen deficient and contain a high density of chalcogen vacancies, which are harmful to the electronic properties of these materials. Based on the first-principles calculation, in this study the repairing of the S vacancy in the MoS2 monolayer has been investigated by using CO, NO and NO2 molecules. For CO and NO, the repairing process consists of the first molecule filling the S vacancy and the removing of the extra O atom by the second molecule. However, for NO2, when the molecule fills the S vacancy, it is dissociated directly to form an O-doped MoS2 monolayer. After the repair, the C, N and O-doped MoS2 monolayers can be obtained by the adsorption of CO, NO, and NO2 molecules, respectively. And in particular, the electronic properties of these materials can be significantly improved by N and O doping. Furthermore, according to the calculated energy, the process of S vacancy repairing with CO, NO and NO2 should be easily achieved at room temperature. This study presents a promising strategy for repairing MoS2 nanosheets and improving their electronic properties, which may also apply to other transition metal dichalcogenides.

SOx on ceria from adsorbed SO2

Results from first-principles calculations present a rather clear picture of the interaction of SO(2) with unreduced and partially reduced (111) and (110) surfaces of ceria. The Ce(3+)∕Ce(4+) redox couple, together with many oxidation states of S, give rise to a multitude of SO(x) species, with oxidation states from +III to +VI. SO(2) adsorbs either as a molecule or attaches via its S-atom to one or two surface oxygens to form sulfite (SO(3)(2-)) and sulfate (SO(4)(2-)) species, forming new S-O bonds but never any S-Ce bonds. Molecular adsorption is found on the (111) surface. SO(3)(2-) structures are found on both the (111) and (110) surfaces of both stoichiometric and partially reduced ceria. SO(4)(2-) structures are observed on the (110) surface together with the formation of two reduced Ce(3+) surface cations. SO(2) can also partially heal the ceria oxygen vacancies by weakening a S-O bond, when significant electron transfer from the surface (Ce4f) into the lowest unoccupied molecular orbital of the SO(2) adsorbate takes place and oxidizes the surface Ce(3+) cations. Furthermore, we propose a mechanism that could lead to monodentate sulfate formation at the (111) surface.

Palladium nanoparticles with high energy facets as a key factor in dissociating O2 in the solvent-free selective oxidation of alcohols

Palladium (Pd) nanocatalysts with high energy facets {110} supported on flower-like hydroxyapatite (F-HAP) were successfully prepared. Based on the experimental data and theoretical calculations, it was found that the O2 dissociation on Pd {110} facets could be key to the performance of Pd nanoparticles in the solvent-free selective oxidation of alcohols.

Observation of Rotated-Oriented Attachment during the Growth of Ag2S Nanorods under Mediation of Protein

In this study, protein-conjugated Ag2S nanorods were prepared in aqueous solution, and high-resolution transmission electron microscopy (HRTEM) was used to track the whole process of the nanorod growth. Our results showed that the final products were formed via two-step oriented attachment, that is, particle-particle and rod-rod oriented. More interestingly, before oriented attachment, the nanoparticles or nanorods attached without sharing the same lattice plane; they could then rotate to a perfect array and fuse together by eliminating the two high energy surfaces. On the basis of the calculation of surface energy, two-step attachments and rotations were brought forward, and the role of protein in the forming process of nanorods was discussed.