Fast carrier diffusion via synergistic effects between lithium-ions and polarons in rutile TiO2.

Carrier mobility in titanium dioxide (TiO2) systems is a key factor for their application as energy materials, especially in solar cells and lithium-ion batteries. Studies on the diffusion of Li-ions and polarons in rutile TiO2 systems have attracted extensive attention. However, how their interaction affects the diffusion of Li-ions and electron polarons is largely unclear and related studies are relatively lacking. By using first-principles calculations, we systematically investigate the interaction between the intercalated Li-ions and electron polarons in rutile TiO2 materials. Our analysis shows that the diffusion barrier of the electron polarons decreases around the Li-ion. The interaction between the Li-ions and polarons would benefit their synergistic diffusion both in the pristine and defective rutile TiO2 systems. Our study reveals the synergistic effects between the ions and polarons, which is important for understanding the carrier properties in TiO2 systems and in further improving the performance of energy materials.

Nuclear Quantum Effects on the Charge-Density Wave Transition in NbX2 (X = S, Se).

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe2 exhibits CDW order at low temperature, while bulk NbS2 displays no CDW instability. The CDW transitions in single-layer NbS2 and NbSe2 are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS2, resulting in distinct CDW features in the NbS2 and NbSe2 systems.

Activating the Electrocatalysis of MoS2 Basal Plane for Hydrogen Evolution via Atomic Defect Configurations.

Point defects of heteroatoms and vacancies can activate the inert basal plane of molybdenum sulfide (MoS2 ) to improve its performance on catalyzing the hydrogen evolution reaction (HER). However, the synergy between heteroatoms and vacancies is still unclear. Here, a chemical vapor deposition-assisted in situ vanadium (V) doping method is used to synthesize monolayer MoS2 with abundant and tunable vacancies and V-dopants in the lattice. Ten delicate defect configurations are prepared to provide a complex system for the relationship investigation between microstructure and catalytic performance. The combination of on-chip electrochemical tests and theoretical calculations indicates that the HER performance greatly depends on the type and amount of defect configurations. The optimal configuration is that three V atoms are aggregated and accompanied by abundant sulfur vacancies, in which, H atoms directly interact with Mo and V atoms to form the most stable metal-bridge structure. The on-chip measurements also confirm that the sample with high concentrations of this type of defect configuration exhibits the best catalytic performance, indicating the efficient synergy in the optimal configuration. The revealed effects of defect configurations are expected to inspire the design and regulation of high-efficiency 2D catalysts.

A first-principles study of exciton self-trapping and electric polarization in one-dimensional organic lead halide perovskites.

Revealing the origin of self-trapped excitons is a prerequisite for further improving the photoluminescence efficiency of low-dimensional organic perovskites. Here, the microscopic formation mechanism of intrinsic self-trapped excitons in one-dimensional (1D) C4N2H14PbX4 (X = Cl, Br and I) systems is investigated, and the polarization-luminescence relationship is established. Our results show that 1D-C4N2H14PbX4 has a low electronic dimension (flat band characteristics), which facilitates the formation of intrinsic self-trapped excitons. The potential well formed by local distortion of the [PbX6] octahedron is the origin of exciton self-trapping. Combined with the electronic density of states and partial charge density, we further confirmed the existence of intrinsic self-trapping excitons in 1D-C4N2H14PbX4. In addition, we found that the breaking of the central inversion symmetry will induce electric polarization, which greatly improves the transition probability of electrons. These results could potentially offer a new direction for improving the luminescence properties of 1D organic lead halide perovskites.

Engineering of Electronic States on Co3 O4 Ultrathin Nanosheets by Cation Substitution and Anion Vacancies for Oxygen Evolution Reaction.

Due to the earth abundance and tunable electronic properties, etc., transition metal oxides (TMOs) show attractive attention in oxygen evolution reaction. O-vacancies (Vo ) play important roles in tailoring the local surface and electronic environment to lower the activation barriers. Herein, an effective strategy is shown to enhance the oxygen evolution reduction (OER) performance on Co3 O4 ultrathin nanosheets via combined cation substitution and anion vacancies. The oxygen-deficient Fe-Co-O nanosheets (3-4 nm thickness) display an overpotential of 260 mV@10 mA cm-2 and a Tafel slope of 53 mV dec-1 , outperforming those of the benchmark RuO2 in 1.0 m KOH. Further calculations demonstrate that the combined introduction of Fe cation and Vo with appropriate location and content finely tune the intermediate absorption, consequently lowering the rate-limiting activation energy from 0.82 to as low as 0.15 eV. The feasibility is also proved by oxygen-deficient Ni-Co-O nanosheets. This work not only establishes a clear atomic-level correlation between cation substitution, anion vacancies, and OER performance, but also provides valuable insights for the rational design of highly efficient catalysts for OER.

Ab initio study of the miscibility for solid hydrogen-helium mixtures at high pressure.

Understanding the behavior of H2-He binary mixtures at high pressure is of great importance. Two more recent experiments [J. Lim and C. S. Yoo, Phys. Rev. Lett. 120, 165301 (2018) and R. Turnbull et al., ibid. 121, 195702 (2018)] are in conflict, regarding the miscibility between H2 and He in solids at high pressure. On the basis of first-principles calculations combined with the structure prediction method, we investigate the miscibility for solid H2-He mixtures at pressures from 0 GPa to 200 GPa. It is found that there is no sign of miscibility and chemical reactivity in H2-He mixtures with any H:He ratio. Moreover, instead of H2-He mixtures, the calculated Raman modes of the N-H mixtures can better explain the characteristic peaks observed experimentally, which were claimed to be the H-He vibrational modes. These calculation results are more in line with the experimental findings by Turnbull et al. [Phys. Rev. Lett. 121, 195702 (2018)].

Synergy of tellurium and defects in control of activity of phosphorene for oxygen evolution and reduction reactions.

Developing low-cost and metal-free electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is desirable for renewable energy technologies. Recent experiments show that tellurium (Te) atoms can be effectively doped into black phosphorus (BP) nanosheets, and they greatly improve its OER catalytic performance. However, the specific active sites and microscopic configurations in the atomic-scale are still ambiguous. Here, we show that the doped Te atoms prefer to bond with each other to form clusters in phosphorene and they can be further stabilized by various intrinsic defects (Stone-Wales, single vacancy defects and zigzag nanoribbon). Benefiting from the reduced binding strength of O*, Te dopants and intrinsic defects synergistically boost the catalytic activity of phosphorene. The best OER catalytic activity could be realized in the cluster SW2-Te1p (Stone-Wales defect decorated by one Te atom). For ORR, the cluster Pri-Te3p (pristine phosphorene decorated by three Te atoms) exhibits optimal catalytic activity. Calculated ORR/OER potential gaps indicate that the SW2-Te3p cluster most likely acts as the efficient bifunctional catalytic site for both ORR and OER.

Doping single transition metal atom into PtTe sheet for catalyzing nitrogen reduction and hydrogen evolution reactions.

We systematically explored the catalytic performance of a novel two dimensional material PtTe sheet for nitrogen reduction reaction (NRR) and hydrogen evolution reaction (HER) by using first-principles calculation. Although pristine PtTe shows poor NRR and HER activity, doping transition metal (TM) atoms into the lattice could effectively enhance the catalytic performance. Calculations show that four TM doped structures, including W-Pt18Te17, Ru-Pt18Te17, Mo-Pt18Te17, and Cr-Pt18Te17, are promising NRR catalysts on the prerequisite of whose HER activities are effectively suppressed. Moreover, the HER performance of the PtTe sheet could also be significantly improved with introduced TM atoms. In particular, Cu, Cr, Co, Ni, Mo, Rh, Ru, and Tc atoms supported by the PtTe sheet with Te-vacancy are promising HER electrocatalysts. The improved HER performance is attributed to the reduced adsorption free energy of the H atom. Both the doped TM atoms and the adjacent Pt atoms are effective active sites. Among all considered structures, Mo-, Cr-, and Ru-Pt18Te17 sheets boost catalytic activity for both NRR and HER. This study provides new design strategies to enhance the catalytic performance of the PtTe sheet for the NRR/HER.

Bifunctional mechanism of N, P co-doped graphene for catalyzing oxygen reduction and evolution reactions.

The development of bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is highly desirable for fuel cells and rechargeable metal-air batteries. Till now, it is still challenging to achieve both efficient activities on a single commercial noble-metal catalyst. Recently, N, P co-doped graphene has shown good bifunctional evidence. However, the atomic-scale understanding of the bifunctional mechanism is still lacking. Here, we show that the N and P atoms prefer to bond with each other, forming embedded N-P clusters in graphene. The catalytic performances of the N-P clusters are sensitive to their geometries, especially the N:P ratios. The N:P ratio of ∼2 is optimal for OER, while ∼3 is optimal for ORR. Through evaluating the ORR/OER potential gaps, we found that the N-P cluster designated as NC 2PC 1 shows both the high performances of ORR and OER, responsible for the unique bifunctionality in the N, P co-doped graphene.

Ferroelectric Problem beyond the Conventional Scaling Law.

Ferroelectric (FE) size effects against the scaling law were reported recently in ultrathin group-IV monochalcogenides, and extrinsic effects (e.g., defects and lattice strains) were often resorted to. Via first-principles based finite-temperature (T) simulations, we reveal that these abnormalities are intrinsic to their unusual symmetry breaking from bulk to thin film. Changes of the electronic structures result in different order parameters characterizing the FE phase transition in bulk and in thin films, and invalidation of the scaling law. Beyond the scaling law T_{c} limit, this mechanism can help predict materials that are promising for room-T ultrathin FE devices of broad interest.

The vertical growth of MoS2 layers at the initial stage of CVD from first-principles.

Chemical vapor deposition (CVD) is the highly preferred method for mass production of transition metal dichalcogenide (TMD) layers, yet the atomic-scale knowledge is still lacking about the nucleation and growth. In this study, by using first-principles calculations, we show that, on Au(111) surface, one-dimensional (1D) MoxSy chains are first formed by coalescing of smaller feeding species and are energetically favored at the early stage of nucleation. Two-dimensional (2D) layers can be stabilized only after the number of Mo atoms exceeds ∼12. A vertical growth mode is revealed which accomplishes the structural transformation from the 1D chains to the 2D layers for the clusters while growing. The competition between intralayer and interlayer interactions is the key. These findings serve as new insights for better understanding the atomistic mechanism of the nucleation and growth of TMDs on the surface.

The collective and quantum nature of proton transfer in the cyclic water tetramer on NaCl(001).

Proton tunneling is an elementary process in the dynamics of hydrogen-bonded systems. Collective tunneling is known to exist for a long time. Atomistic investigations of this mechanism in realistic systems, however, are scarce. Using a combination of ab initio theoretical and high-resolution experimental methods, we investigate the role played by the protons on the chirality switching of a water tetramer on NaCl(001). Our scanning tunneling spectroscopies show that partial deuteration of the H2O tetramer with only one D2O leads to a significant suppression of the chirality switching rate at a cryogenic temperature (T), indicating that the chirality switches by tunneling in a concerted manner. Theoretical simulations, in the meantime, support this picture by presenting a much smaller free-energy barrier for the translational collective proton tunneling mode than other chirality switching modes at low T. During this analysis, the virial energy provides a reasonable estimator for the description of the nuclear quantum effects when a traditional thermodynamic integration method cannot be used, which could be employed in future studies of similar problems. Given the high-dimensional nature of realistic systems and the topology of the hydrogen-bonded network, collective proton tunneling may exist more ubiquitously than expected. Systems of this kind can serve as ideal platforms for studies of this mechanism, easily accessible to high-resolution experimental measurements.

Hydrogen induced contrasting modes of initial nucleations of graphene on transition metal surfaces.

Our first-principles calculations reveal that there exist contrasting modes of initial nucleations of graphene on transition metal surfaces, in which hydrogen plays the role. On Cu(100) and Cu(111) surfaces, an sp2-type network of carbons can be automatically formed with the help of hydrogen under very low carbon coverages. Thus, by tuning the chemical potential of hydrogen, both of the nucleation process and the following growth can be finely controlled. In contrast, on the Ni(111) surface, instead of hydrogen, the carbon coverage is the critical factor for the nucleation and growth. These findings serve as new insights for further improving the poor quality of the grown graphene on transition metal substrates.

Direct observation of ordered configurations of hydrogen adatoms on graphene.

Ordered configurations of hydrogen adatoms on graphene have long been proposed, calculated, and searched for. Here, we report direct observation of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. On the top side of the graphene plane, H atoms in the configurations appear to stick to carbon atoms in the same sublattice. Scanning tunneling spectroscopy measurements revealed a substantial gap in the local density of states in H-contained regions as well as in-gap states below the conduction band due to the incompleteness of H ordering. These findings can be well explained by density functional theory calculations based on double-sided H configurations. In addition, factors that may influence H ordering are discussed.

Nuclear quantum effects on the high pressure melting of dense lithium.

Using a self-developed combination of the thermodynamic integration and the ab initio path-integral molecular dynamics methods, we quantitatively studied the influence of nuclear quantum effects (NQEs) on the melting of dense lithium at 45 GPa. We find that although the NQEs significantly change the free-energies of the competing solid and liquid phases, the melting temperature (Tm) is lowered by only ∼15 K, with values obtained using both classical and quantum nuclei in close proximity to a new experiment. Besides this, a substantial narrowing of the solid/liquid free-energy differences close to Tm was observed, in alignment with a tendency that glassy states might form upon rapid cooling. This tendency was demonstrated by the dynamics of crystallization in the two-phase simulations, which helps to reconcile an important conflict between two recent experiments. This study presents a simple picture for the phase diagram of lithium under pressure. It also indicates that claims on the influence of NQEs on phase diagrams should be carefully made and the method adopted offers a robust solution for such quantitative analyses.

Improved Carrier Separation and Recombination by Ferroelectric Polarization in the CuBiP2Se6/C2N Heterostructure: A Nonadiabatic Molecular Dynamics Study.

The rapid recombination of photogenerated carriers heavily restricts the photocatalytic efficiency. Here, we propose a new strategy to improve catalytic efficiency based on the ferroelectric van der Waals heterostructure (CuBiP2Se6/C2N). Combining density functional theory and the nonadiabatic molecular dynamics (NAMD) method, we have systematically analyzed the ground-state properties and carrier dynamics images in the CuBiP2Se6/C2N heterostructure. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the photogenerated carriers separation. NAMD results demonstrate that the excited-state carrier transfer and recombination processes in the CuBiP2Se6/C2N are consistent with a type II mechanism. Meanwhile, constructing the ferroelectric heterostructure can effectively prolong the carrier lifetime, from ∼65.98 to ∼124.54 ps. Moreover, the high quantum efficiency and tunable band edge positions mean that the CuBiP2Se6/C2N heterostructure is an excellent potential candidate material for photocatalytic water splitting.

Ferroelectric Polarization and Single-Atom Catalyst Synergistically Promoting CO2 Photoreduction of CuBiP2Se6.

Further improving the activity and selectivity of photocatalytic CO2 reduction remains a challenge. Herein, we propose a new strategy for synergistically promoting photocatalytic CO2 reduction by combining two-dimensional (2D) ferroelectric polarization and single-atom catalysis. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the separation and migration of photogenerated carriers, which provides a prerequisite for enhancing the photocatalytic efficiency. In addition, the introduction of single Ag atoms can act as an electron reservoir to significantly modify the bonding configurations on the surface through proper static electron transfer, thus effectively promoting the adsorption and activation of CO2 molecules. More importantly, we found that switching the ferroelectric polarization can synergistically optimize the limiting potential as well as control the final products. This study provides a new approach for enhancing the catalytic activity and selectivity of photocatalytic CO2 reduction.

The atomic structures of carbon nitride sheets for cathode oxygen reduction catalysis.

Carbon nitride sheets are promising Pt replacement materials for cathode oxygen reduction catalysis. Using first principles calculations with a global optimization method, we search for the most stable structures of the monolayer carbon nitrides at various C:N ratios. The results show that the larger the ratio, the more energetically favorable the obtained structures, and the more preferably for the C, N atoms to assume sp(2) configurations. A volcano shape is revealed for the curve of the representative O2 adsorption energies on the sheets vs. the ratios. In the ratio range of 2.0-3.0, the sheets not only have lower formation energies than the stable graphitic-C3N4, but also can potentially catalyze the oxygen reduction as efficiently as Pt.

Understanding the Activity and Design Principle of Dual-Atom Catalysts Supported on C2N for Oxygen Reduction and Evolution Reactions: From Homonuclear to Heteronuclear.

Using large-scale ab initio calculations and taking the two-dimensional C2N monolayer as a substrate, we sampled a large combinatorial space of C2N-supported homonuclear and heteronuclear dual-atom catalysts and built a detailed view of catalytic activity and stability toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The results indicate that regulating combinations of metal pairs could widely tune the catalytic performance. Pd2-, Pt2-, and PdPt-C2N could effectively balance the adsorption strength of intermediates and achieve optimal bifunctional activity. The favorable catalytic performance could also be realized on GaPd-C2N for the ORR and PdRh-C2N for the OER, surpassing corresponding homonuclear counterparts. The thermodynamic and electrochemical stability simulations reveal that these metal pairs can be stably anchored onto the C2N matrix. Multiple-level descriptors, including Gibbs free energy, d-band center, and bonding/antibonding orbital population, are established to track the activity trend and reveal the origin of activity, indicating that catalytic activity is intrinsically governed by the d-band center of metal pairs.

Origin of Broadband Emission and Large Stokes Shift in Antimony Trisulfide.

The antimony trisulfide (Sb2S3) has been theoretically predicted to have various merits in exploiting high-performance thin-film solar cells and attracted intense attention. However, the power conversion efficiency of Sb2S3-based solar cells is yet to be satisfactory in experiments and the origin of large open circuit voltage (VOC) loss is still a controversial question. Based on first-principles calculations, we have systematically analyzed the excited state behavior and dynamics images of carriers in Sb2S3 materials. Our calculations showed that intrinsic defects like vacancy (VSb and VS) and antisites (SbS and SSb) are energetically accessible. More importantly, we found that the sulfide vacancy-bound excitons can produce a large Stokes shift of ∼0.66 eV, which could well rationalize the experimental observations like the reduction of VOC. These new findings suggest that the performance of Sb2S3-based solar cells might be largely enhanced by avoiding sulfide vacancy defects.