A Novel Electrocatalyst Pd(II)@Ni3 (HITP)2 for Ultrasensitive Detection of Chloramphenicol: Experimental and Computational Investigation.

Ultrasensitive electrochemical detection on hazardous substances like antibiotics and pesticides is essential but still challenging in rapid test technology. Herein, the first electrode using highly conductive metal-organic frameworks (HCMOFs) for electrochemical detection of chloramphenicol is proposed. The design of electrocatalyst Pd(II)@Ni3 (HITP)2 with ultra-sensitivity in detection of chloramphenicol is demonstrated by loading Pd onto HCMOFs. An ultra-low limit of detection (LOD) of 0.2 nM (64.6 pg/mL) was observed for these materials, which is 1-2 orders of magnitude lower than the other reported materials for chromatographic detection. Moreover, the proposed HCMOFs showed long-time stability over 24 h. The superior detection sensitivity can be attributed to the high conductivity of Ni3 (HITP)2 , and the large Pd loading. The experimental characterizations and computational investigation determined the Pd loading mechanism in Pd(II)@Ni3 (HITP)2 , revealing PdCl2 adsorption on the abundant adsorption sites of Ni3 (HITP)2 . The proposed electrochemical sensor design using HCMOFs was demonstrated to be both effective and efficient, showing that the adoption of HCMOFs decorated with other effective electrocatalysts, which combine high conductivity and high catalytic activity, is of great advantage for ultrasensitive detection.

Prediction of sodium binding energy on 2D VS2via machine learning: a robust accompanying method to ab initio random structure searching.

In this work, we employed the back-propagation neural network (BPNN) for predicting the energetics of different sodium adsorption phases on the VS2 monolayer generated via ab initio random structure searching (AIRSS). Two key adsorption features were identified as inputs: the average Na-Na distance and a defined adsorption feature marker that indicates the number of nearest-neighbor pairs within a sodium cluster. Using the stoichiometric structure Na0.5VS2 as the test system, we first generated 50 random sensible structures via AIRSS and optimized them via density functional theory (DFT) calculations to obtain the sodium binding energy per atom. From these, 30 were utilized to train 3000 BPNNs with varying numbers of neurons and types of activation functions. The remaining 20 were employed to verify the generalization of the best identified BPNN model for the considered Na0.5VS2 system. The calculated mean absolute error for the predicted sodium binding energy per atom is smaller than 0.1 eV. This suggests that the identified BPNN model was able to predict the sodium binding energy per atom on VS2 with outstanding accuracy. Our results demonstrated that with the assistance of BPNN, it is possible to perform AIRSS with hundreds of random sensible structures without relying solely on DFT calculations. The uniqueness of this method lies on the utilization of a very large number of BPNN models to be trained by a relatively small number of structures. This is particularly very useful for large systems wherein the data come from DFT calculations, which is computationally expensive. Moreover, with the assistance of machine learning, the theoretical estimation of important metal-ion battery metrics such as specific energy capacity and open circuit voltage via AIRSS could be made more accurate and reliable.

Discovery and Facile Synthesis of a New Silicon Based Family as Efficient Hydrogen Evolution Reaction Catalysts: A Computational and Experimental Investigation of Metal Monosilicides.

A new family of transition-metal monosilicides (MSi, M = Ti, Mn, Fe, Ru, Ni, Pd, Co, and Rh) electrocatalysts with superior electrocatalytic performance of hydrogen evolution is reported, based on the computational and experimental results. It is proposed that these MSi can be synthesized within several minutes by adopting the arc-melting method. The previously reported RuSi is not only fabricated more readily but eventually explored 8 MSi that can be good hydrogen evolution reaction catalysts. Silicides then can be another promising electrocatalysts family as carbides, wherein carbon has the same electronic configuration as silicon. All explored silicides electrodes exhibited low overpotentials (34-54 mV at 10 mA cm-2 ) with Tafel slopes from 23.6 to 32.3 mV dec-1 , which are comparable to that of the commercial 20 wt% Pt/C (37 mV, 26.1 mV dec-1 ). First-principles calculations demonstrated that the superior performance can be attributed to the high catalytic reactivity per site that can even function at high hydrogen coverages (≈100%) on multiple low surface energy facets. The work sheds light on a new class of electrocatalysts for hydrogen evolution, with earth-abundant and inexpensive silicon-based compounds.

Generating large out-of-plane piezoelectric properties of atomically thin MoS2via defect engineering.

We calculated the piezoelectric properties of asymmetrically defected MoS2 using density functional theory. By creating uneven numbers of defects on either side of two-dimensional MoS2, the out-of-plane centrosymmetry of the charge distribution is clearly broken, and the out-of-plane piezoelectric response is induced. The largest out-of-plane piezoelectric response is associated with the highest defect ratio for MoS2 to be semiconducting. We calculated the critical defect density of the metal-insulator transition of the asymmetrically defected MoS2 to be 9.90 × 1014 cm-2 and chemical formula MoS1.22. The d33 of the multilayer of optimally defected MoS2 is found to be greater than those of AlN and ZnO, and in the same order of magnitude as lead zirconate titanate. All two-dimensional transition metal dichalcogenides can in principle be fabricated as piezoelectric with this approach. The required defect engineering is readily available with various types of ion irradiation or plasma treatment. By controlling the dose of the ion, the defect ratio and hence the piezoelectricity can be tuned. Such asymmetrically defected transition metal dichalcogenides can easily be integrated into two-dimensional transition metal dichalcogenide based devices, which is hard for conventional piezoelectric thin films to rival.

First-principles investigation of the hydrogen evolution reaction of transition metal phosphides CrP, MnP, FeP, CoP, and NiP.

We comprehensively investigated the hydrogen evolution reaction (HER) activity of a series of transition metal phosphides (MPs) (M = Cr, Mn, Fe, Co, and Ni) using first-principles calculations. The free energy difference was calculated for possible sites on the surface to pinpoint the reactive sites and the associated catalytic activities. We found that the chemical properties of these considered MPs are different from those of WP, including CrP which has the same electronic configuration as WP but was shown not to be a good electrocatalyst. Different reactive sites other than WP were predicted, and notably, unlike WP, phosphorus can participate/catalyze the HER on the considered MP. Among these MPs, there are more active sites on FeP, CoP, and NiP than CrP and MnP. Our electronic structure analysis suggests that the spin polarization is critical in determining the hydrogen adsorption and hence the HER performance. We further explored the HER of metal- or phosphorus-deficit MPs, as samples can be grown under different conditions. In particular, phosphorus-deficit FeP, CoP, and NiP were found to have enhanced HER performance, with either better catalytic activities or more active sites. Therefore, we proposed that controlling of these defects can be an effective approach to tune the HER catalytic ability of these MPs. It can serve as the design principle to synthesize new MP based electrocatalysts.

Multiferroic hydrogenated graphene bilayer.

We investigated the multiferroic properties of a hydrogenated graphene bilayer using first-principles calculations. The proposed material is composed of one fully hydrogenated and one semi-hydrogenated graphene monolayer. Inside the van der Waals gap, hydrogen atoms are only adsorbed on either the top or the bottom layer of graphene, thus breaking the centrosymmetry. The calculated electric polarization is 0.137 × 10-10 C m-1, with the transition barrier of switching the polarization being 393 meV per formula unit. We showed that ferroelectricity can be preserved down to atomic thickness. We also studied the domain wall energy and its migration for various domain wall densities, and our results indicate a robust polarization configuration against room temperature thermal fluctuation. As graphene is known to be able to sustain large strain, we further explored ferroelectricity tuning via strain, and found that the polarization can be effectively tuned up to 20% without perturbing the polarization switching barrier. Our results suggest a realizable multiferroic two-dimensional material using the most used two-dimensional material, graphene.

First-principles investigation of the hydrogen evolution reaction on different surfaces of pyrites MnS2, FeS2, CoS2, NiS2.

We theoretically investigated hydrogen evolution reaction (HER) on the XRD observed (100), (110), (111), and (210) surfaces of pyrite structure CoS2. The random structure searching method was employed in this work to thoroughly and less-biasedly identify the active sites for each considered surface. We calculated the free energy of hydrogen adsorption, and found that (110) and (210) surfaces are more active than the conventionally assumed (100) facet. While the lowest energy active site on the (100) and (210) surfaces is the five-coordinated transition metal site that is commonly seen in other HER catalysts, the lowest energy active site on the (110) surface is the two-coordinated S site, which is a S tetrahedron with two corners missing. Besides those lowest energy active sites, both (110) and (210) have more than one species of active site on the surface, including not fully coordinated transition metals and sulfur. We further explored the reaction for MnS2, FeS2, and NiS2, and analyzed the density of states. Our results showed both CoS2 and NiS2 (110) and (210) surfaces are catalytically reactive for HER.

High-Sulfur-Vacancy Amorphous Molybdenum Sulfide as a High Current Electrocatalyst in Hydrogen Evolution.

The remote hydrogen plasma is able to create abundant S-vacancies on amorphous molybdenum sulfide (a-MoSx ) as active sites for hydrogen evolution. The results demonstrate that the plasma-treated a-MoSx exhibits superior performance and higher stability than Pt in a proton exchange membrane based electrolyzers measurement as a proof-of-concept of industrial application.

Activating and tuning basal planes of MoO2, MoS2, and MoSe2 for hydrogen evolution reaction.

We investigated the defected two-dimensional materials MoX2 (X = O, S, Se) for hydrogen evolution reaction by first principles calculations. While the basal plane is inert for pristine MoX2, we found that the defected MoX2 can adsorb hydrogen atoms at defect sites, with appropriate adsorption energies for hydrogen evolution. By analyzing density of states and charge density, we showed that a dangling bond state slightly below the Fermi energy emerges when a defect is created. We proposed that this state is responsible for hybridizing with the hydrogen atom 1s state and hence the adsorption. Knowing the mechanism, we further considered tuning the reaction using adatoms (several first-row transition metals, B, C, N, O). We found that C and O adatoms can make defected MoX2 ideal for hydrogen evolution at higher defect levels (H coverage).

A first-principles examination of conducting monolayer 1T'-MX2 (M = Mo, W; X = S, Se, Te): promising catalysts for hydrogen evolution reaction and its enhancement by strain.

We investigated the application of 1T'-MX2 (M = Mo, W; X = S, Se, Te) 2D materials as hydrogen evolution reaction (HER) catalysts using density functional theory. Our results show that 1T'-MX2 have lower energies and are dynamically more stable than their 1T counterparts, therefore likely more relevant to previous experimental findings and applications. We found that sulfides are better catalysts, followed by selenides and tellurides. Specifically, 1T'-MoS2 and WS2 are the best HER catalysts among MX2. We proposed a mechanism, rather than the metallicity surmised previously, based on the calculated density of states. On the other hand, the effectively stretched (compressed) X site on the 1T' 2 × 1 reconstruction from 1T is shown to be more (less) active for the HER. We further exploited the application of external strain to tune and boost the HER performance. Our findings are of significance in the elucidation of previous experimental studies and exploration of potential materials for clean energy applications.

Li adsorption, hydrogen storage and dissociation using monolayer MoS2: an ab initio random structure searching approach.

Utilizing ab initio random structure searching, we investigated Li adsorption on MoS2 and hydrogen molecules on Li-decorated MoS2. In contrast to graphene, Li can be adsorbed on both sides of MoS2, with even stronger binding than on the single side. We found that high coverages of Li can be attained without Li clustering, which is essential for hydrogen storage and Li ion batteries. Moreover, regarding battery applications, Li diffusion was also found to be easy. The fully-lithiated MoS2 can then adsorb H2 with 4.4 wt%. Interestingly, our calculations revealed that hydrogen molecules can be dissociated at high Li coverage with a minimal energy barrier. We further showed that the dissociated hydrogen atom can readily diffuse on the surface, thus keeping the reaction site active. We therefore propose that Li-MoS2 could be an inexpensive alternative catalyst to noble metals in hydrogen dissociation reactions.

Towards the ionic limit of two-dimensional materials: monolayer alkaline earth and transition metal halides.

We theoretically explored new two-dimensional materials near the ionic instability (three-dimensional structures are favored), with covalent bonded systems (graphene) sitting at the opposite end of the spectrum. Accordingly, monolayer alkaline earth and transition metal halides, many of their bulk forms being layered structures, were investigated by density functional calculations. We thus predicted a new class of two-dimensional materials by performing structure relaxation, cohesive/formation energy and full phonon dispersion calculations. These materials exhibit strong ionic bonding character, as revealed by significant charge transfers. The superior charge donating/accepting abilities and the large specific area make these new materials promising for adsorption and catalytic reactions. We demonstrated adsorption and diffusion of Li on these materials, which are relevant for Li ion battery electrodes and hydrogen storage. Also the new materials with varied charge donating abilities and their nanostructures can enhance and tune catalytic reactions, such as Ziegler-Natta catalysts. Moreover, they exhibit diverse electronic properties that can be of great application interest, ranging from insulators to metals, and even spin-polarized semiconductors.

Formation of multiple-flux-quantum vortices in mesoscopic superconductors from simulations of calorimetric, magnetic, and transport properties.

Because of strong flux confinement in mesoscopic superconductors, a "giant" vortex may appear in the ground state of the system in an applied magnetic field. This multiquanta vortex can then split into individual vortices (and vice versa) as a function of, e.g., applied current, magnetic field, or temperature. Here we show that such transitions can be identified by calorimetry, as the formation or splitting of a giant vortex results in a clear jump in measured heat capacity versus external drive. We attribute this phenomenon to an abrupt change in the density of states of the quasiparticle excitations in the vortex core(s), and further link it to a sharp change of the magnetic susceptibility at the transition--proving that the formation of a giant vortex can also be detected by magnetometry.

Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions.

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi2Te3 heterojunction which combines monolayer graphene and a Bi2Te3 nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi2Te3 when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi2Te3 bulk band. The increased modulation depth of the graphene/Bi2Te3 heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi2Te3 thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi2Te3 heterojunction and demonstrates its potential for applications.

MoSx-coated NbS2 nanoflakes grown on glass carbon: an advanced electrocatalyst for the hydrogen evolution reaction.

Recent experimental and theoretical studies have demonstrated that two-dimensional (2D) transition metal dichalcogenide (TMDC) nanoflakes are one of the most promising candidates for non-noblemetal electrocatalysts for hydrogen evolution reaction (HER). However, it is still challenging to optimize their conductivity and enrich active sites for highly efficient electrochemical performance. Herein, we report a chemical vapor deposition (CVD) and thermal annealing two-step strategy to controllably synthesize hybrid electrocatalysts consisting of metallic NbS2 nanoflake backbones and a highly catalytic active MoSx nanocrystalline shell on polished commercial glass carbon (GC). In addition, the amount of MoSx in the hybrids can be easily adjusted. We first demonstrate that a small amount of MoSx significantly promotes the HER activity of 2D NbS2 nanoflakes, which is in good agreement with the density functional theory (DFT) calculation results. Moreover, the optimized MoSx@NbS2/GC electrocatalyst displays superior HER activity with overpotential of -164 mV at -10 mA cm-2, a small Tafel slope of 43.2 mV dec-1, and prominent electrochemical stability. This study provides a new path for enhancing the HER performance of 2D TMDC nanoflakes.

Metallic VS2 Monolayer Polytypes as Potential Sodium-Ion Battery Anode via ab Initio Random Structure Searching.

We systematically investigated the potential of single-layer VS2 polytypes as Na-battery anode materials via density functional theory calculations. We found that sodiation tends to inhibit the 1H-to-1T structural phase transition, in contrast to lithiation-induced transition on monolayer MoS2. Thus, VS2 can have better structural stability in the cycles of charging and discharging. Diffussion of Na atom was found to be very fast on both polytypes, with very small diffusion barriers of 0.085 eV (1H) and 0.088 eV (1T). Ab initio random structure searching was performed in order to explore stable configurations of Na on VS2. Our search found that both the V top and the hexagonal center sites are preferred adsorption sites for Na, with the 1H phase showing a relatively stronger binding. Notably, our random structures search revealed that Na clusters can form as a stacked second layer at full Na concentration, which is not reported in earlier works wherein uniform, single-layer Na adsorption phases were assumed. With reasonably high specific energy capacity (232.91 and 116.45 mAh/g for 1H and 1T phases, respectively) and open-circuit voltage (1.30 and 1.42 V for 1H and 1T phases, respectively), VS2 is a promising alternative material for Na-ion battery anodes with great structural sturdiness. Finally, we have shown the capability of the ab initio random structure searching in the assessment of potential materials for energy storage applications.

Quantum rotor in nanostructured superconductors.

Despite its apparent simplicity, the idealized model of a particle constrained to move on a circle has intriguing dynamic properties and immediate experimental relevance. While a rotor is rather easy to set up classically, the quantum regime is harder to realize and investigate. Here we demonstrate that the quantum dynamics of quasiparticles in certain classes of nanostructured superconductors can be mapped onto a quantum rotor. Furthermore, we provide a straightforward experimental procedure to convert this nanoscale superconducting rotor into a regular or inverted quantum pendulum with tunable gravitational field, inertia, and drive. We detail how these novel states can be detected via scanning tunneling spectroscopy. The proposed experiments will provide insights into quantum dynamics and quantum chaos.

Orbital ordering and Jahn-Teller distortion in Perovskite ruthenate SrRuO3.

Local density approximation plus on-site Coulomb interaction U band structure calculations reveal that SrRuO3 exhibits a half-metallic ground state with an integer spin moment of 2.0 microB/SrRuO3. An associated tilting 4dt2g orbital ordering on a Ru sublattice is observed under the on-site Coulomb interaction U in the presence of lattice distortion. This finding unravels the on-site Coulomb correlation as the driving force of the 4d orbital ordering and Jahn-Teller distortion as well as of the half-metallic ground state.