Computational Screening of a Single-Atom Catalyst Supported by Monolayer Nb2S2C for Oxygen Reduction Reaction.

The search for high-performance catalysts to improve the catalytic activity for an oxygen reduction reaction (ORR) is crucial for developing a proton exchange membrane fuel cell. Using the first-principles method, we have performed computational screening on a series of transition metal (TM) atoms embedded in monolayer Nb2S2C to enhance the ORR activity. Through the scaling relationship and volcano plot, our results reveal that the introduction of a single Ni or Rh atom through substitutional doping into monolayer Nb2S2C yields promising ORR catalysts with low overpotentials of 0.52 and 0.42 V, respectively. These doped atoms remain intact on the monolayer Nb2S2C even at elevated temperatures. Importantly, the catalytic activity of the Nb2S2C doped with a TM atom can be effectively correlated with an intrinsic descriptor, which can be computed based on the number of d orbital electrons and the electronegativity of TM and O atoms.

The effect of vacancy defects on the electromechanical properties of monolayer NiTe2 from first principles calculations.

The electromechanical properties of monolayer 1-T NiTe2 under charge actuation were investigated using first-principles density functional theory (DFT) calculations. Monolayer 1-T NiTe2 in its pristine form has a work area density per cycle of up to 5.38 MJ m-3 nm upon charge injection and it can generate a strain and a stress of 1.51% and 0.96 N m-1, respectively. We found that defects in the form of vacancies can be exploited to modulate the electromechanical properties of this material. The presence of Ni-vacancies can further enhance the generated stress by 22.5%. On the other hand, with Te-vacancies, it is possible to improve the work area density per cycle by at least 145% and also to enhance the induced strain from 1.51% to 2.92%. The effect of charge polarity on the contraction and expansion of monolayer 1T-NiTe2 was investigated. Due to its excellent environmental stability and good electromechanical properties, monolayer NiTe2 is considered to be a promising electrode material for electroactive polymer (EAP) based actuators.

Revisiting the Kittel's model of antiferroelectricity: phase diagrams, hysteresis loops and electrocaloric effect.

We revisit the Kittel's model of antiferroelectricity by extending the model to study the phase transitions, hysteresis loop behaviors and electrocaloric effect (ECE) of antiferroelectrics (AFEs). By considering both the first- and second-order AFEs, explicit expressions for the physical and staggered polarizations of AFEs in the stable states are derived. We also obtain the analytical solutions for describing the dielectric susceptibilities of AFEs in the AFE and paraelectric (PE) phases. Coercive fields in AFE are also derived and studied. To verify the usefulness of the Kittel's model of antiferroelectricity, we apply the model to systematically investigate the phase transitions, hysteresis loops and ECEs of PbZrO3(PZO). By adopting appropriate values of the Kittel's parameters for first-order transition, analytical and numerical results are obtained and discussed. Our results show that PZO exhibits a complex temperature (T)-electric field (E) phase diagram, consisting of the AFE, ferroelectrics, ferrielectric, PE and mixed phases. TheT-Ephase diagram is qualitatively agreed with the new AFE model that was derived based on symmetry by Tolédano and Khalyavin (2019Phys. Rev.B99024105). We found that the calculated zero-field dielectric susceptibility is qualitatively and quantitatively agreed with experimental results. We show that the polarizations and dielectric susceptibilities of PZO in heating and cooling deviate from each other, as expected for the first-order materials. Our calculated results also reveal that the ECE in PZO has an electro-heating of ΔT≈ +6.5 °C and an electro-cooling of ΔT≈ -4.0 °C, respectively, which are comparable to the experimental results.

A first-principles study of two-dimensional NbSe2H/g-ZnO van der Waals heterostructures as a water splitting photocatalyst.

Based on first-principles calculations, we propose a new two-dimensional (2D) van der Waals (vdW) heterostructure that can be used as a photocatalyst for water splitting. The heterostructure consists of vertically stacked 2D NbSe2H and graphene-like ZnO (g-ZnO). Depending on the stacking orders, we identified two configurations that have high binding energies with an energy band gap of >2.6 eV. These 2D systems form a type-II heterostructure which enables the separation of photoexcited electrons and holes. The presence of a strong electrostatic potential difference across the 2D NbSe2H and g-ZnO interface is expected to suppress the electron-hole recombination leading to an enhancement in the efficiency of the photocatalytic activity. Our study also shows that the 2D NbSe2H/g-ZnO vdW heterostructure has good thermodynamic properties for water splitting. Furthermore, the optical absorption of the 2D NbSe2H/g-ZnO vdW heterostructure extends into the visible light region. Our results suggest that the 2D NbSe2H/g-ZnO vdW heterostructure is a promising photocatalytic material for water splitting.

A first-principles comparative study of lithium, sodium, potassium and calcium storage in two-dimensional Mg2C.

Two-dimensional (2D) materials have recently emerged as potential candidates for high-capacity lithium-ion batteries anode materials because of their compelling physicochemical and structural properties. In the present study, we use first-principles calculations to investigate the performance of 2D Mg2C as anode materials for Li, Na, K and Ca-ions batteries. The calculated average open-circuit voltage are 0.37, 0.50, 0.03 and 0.06 eV vs Li, Na, K, Ca. No significant structural deformations are observed on the 2D Mg2C upon the adsorption of Li, Na, K or Ca and the metallic characteristic of the 2D Mg2C is retained. The metallic behaviour of both pristine and adsorbed Mg2C ensures the desirable electric conductivity, implying the advantages of 2D Mg2C for batteries. The Na and K atoms show an extremely high diffusivity on the 2D Mg2C with a low energy barrier of 0.08 and 0.04 eV respectively, which is about an order of magnitude smaller than that of Li atom. For the Na and K atoms, the theoretical storage capacity can reach up to 1770 mAh g-1, nearly two times that of the Li atom of 885 mAh g-1. Our study suggests that the 2D Mg2C is a promising anode material which offers a fast ion diffusion and high storage capacity.

Rich antiferroelectric phase diagram of antiferroelectric-ferroelectric superlattices: internal electric field- and interface induced phase transitions.

We propose a thermodynamic model to the study the antiferroelectric (AFE) phase transitions in antiferroelectric-ferroelectric (AFE-FE) superlattices in which the coupling at the interface between two layers is mediated by local polarizations. Phase diagram of the AFE layer in term of the degree of interfacial effect λ and temperature T involving ferrielectric (FI) and ferroelectric (FE) phases is investigated. These two phases are stabilized by the interfacial effect and internal electric field. AFE thickness L AFE versus T phase diagram is also constructed. Intermediate regions of two-phase coexistence (IM) emerge in the λ-T and L AFE-T phase diagrams, if certain interface properties λ and layer thickness L AFE criteria are met. These IM regions are metastable states, which exist as a transition state between two phases. A tricritical point locates at the boundaries across the FI, IM and FE phases is found in the L AFE-T phase diagram. Competition among the internal electric field due to the electrostatic coupling, the FE ordering arises from the interfacial effect and the antiferroelectric ordering within the AFE layer giving rises to the rich AFE phase diagram.

Empyema of the gallbladder.

There is fairly widespread belief that suppuration superimposed upon acute cholecystitis is a rather uncommon condition. Scanty information on this subject is found in the published literature. No doubt, the scarcity of data further reinforces this clinical impression. Yet, from the few studies available, a surprisingly high incidence (7-11%) is documented. We reviewed 518 patients who underwent emergency cholecystectomy and found 139 cases to have frank pus present within the gallbladder. The clinical presentation is often difficult to distinguish from that of acute cholecystitis but diagnosis is crucial to identify this potentially fatal surgical emergency. There are encouraging reports of specific ultrasonic features which can demonstrate the increased viscosity produced by the purulent and fibrinous content within the gallbladder.

Omental cryptococcoma. An unusual presentation of cryptococcosis.

A case of an omental cryptococcal granuloma, believed to be the first reported, presented as an abdominal mass in a 29-year-old man. An exploratory laparotomy revealed a soft, 8-cm lesion in the omentum. Pathologic examination showed this to be a cryptococcal granuloma containing masses of budding, encapsulated yeasts. Aggregates of eosinophils were also noted. This case is unusual in both site (as cryptococcosis tends to localize in the central nervous system) and size.

Sclerotherapy for bleeding varices.

The efficacy of paravariceal sclerotherapy for bleeding oesophageal varices was evaluated. Alcoholic and post-hepatitis liver cirrhosis were the main causes of portal hypertension. A total of 28 patients between June 1982 and December 1983 were treated by this method using a flexible endoscope. Bleeding was initially controlled in 90% of patients. However 30% of patients developed recurrent bleeding within 12 months. One year survival of the patients was 70% indicating this treatment has no effect on the long term survival of the underlying disease. The technique is discussed and its complications analysed.

Traumatic duodenal injuries.

Eleven cases of duodenal injury are reviewed. A correct preoperative diagnosis was made in only one patient. In 10 patients, the duodenum was disrupted in the second part. When duodenal injury was associated with devitalisation of tissue or there was delay in operative intervention, repair was aided by a serosal patch graft. In these high risk cases, an attempt to defunction the injured area was done by suction gastrostomy or gastrojejunostomy with vagotomy, Three patients died and in two of these, there was delay in surgical intervention.