Visualizing Phase Evolution of Co2C for Efficient Fischer-Tropsch to Olefins.

Cobalt carbide (Co2C) possesses high catalytic efficiency Fischer-Tropsch synthesis (FTS), while the products selectivity appears sensitive to crystallography geometry. Since the Anderson-Schulz-Flory (ASF) distribution in FTS is broken through fabricating facetted Co2C nanocrystals, yet the underlying mechanism of Co2C crystallization remains unclarified suffering from sophisticated catalyst composition involving promoter agents. Herein, the synthesis of high-purity single-crystal nanoprisms (Co2C-p) for highly efficient FTS is reported to lower olefins. Through comprehensive microstructure analysis, e.g., high-resolution TEM, in situ TEM and electron diffraction, as well as finite element simulation of gas flow field, for the first time the full roadmap of forming catalytic active cobalt carbides is disclosed, starting from reduction of Co3O4 precursor to CoO intermediate, then carburization into Co2C-s and subsequent ripening growth into Co2C-p. This gas-induced engineering of crystal phase provides a new synthesis strategy, with many new possibilities for precise design of metal-based catalyst for diverse catalytic applications.

Elastic Properties of Low-Dimensional Single-Crystalline Dielectric Oxides through Controlled Large-Area Wrinkle Generation.

Freestanding single-crystalline SrTiO3 membranes, as high-κ dielectrics, hold significant promise as the gate dielectric in two-dimensional (2D) flexible electronics. Nevertheless, the mechanical properties of the SrTiO3 membranes, such as elasticity, remain a critical piece of the puzzle to adequately address the viability of their applications in flexible devices. Here, we report statistical analysis on plane-strain effective Young's modulus of large-area SrTiO3 membranes (5 × 5 mm2) over a series of thicknesses (from 6.5 to 32.2 nm), taking advantage of a highly efficient buckling-based method, which reveals its evident thickness-dependent behavior ranging from 46.01 to 227.17 GPa. Based on microscopic and theoretical results, we elucidate these thickness-dependent behaviors and statistical data deviation with a bilayer model, which consists of a surface layer and a bulk-like layer. The analytical results show that the ∼3.1 nm surface layer has a significant elastic softening compared to the bulk-like layer, while the extracted modulus of the bulk-like layer shows a variation of ∼40 GPa. This variation is considered as a combined contribution from oxygen deficiency presenting in SrTiO3 membranes, and the alignment between applied strain and the crystal orientation. Upon comparison of the extracted elastic properties and electrostatic control capability to those of other typical gate dielectrics, the superior performance of single-crystalline SrTiO3 membranes has been revealed in the context of flexible gate dielectrics, indicating the significant potential of their application in high-performance flexible 2D electronics.

Efficient degradation of organics by ultrasonic piezoelectric effect on CuO-BTO/AFC composite.

The recombination of photoexcited electron-hole pairs greatly limits the degradation performance of photocatalysts. Ultrasonic cavitation and internal electric field induced by the piezoelectric effect are helpful for the separation of electron-hole pairs and degradation efficiency. The activated foam carbon (AFC) owing to its high surface area is often used as the substrate to grow catalysts to provide more reactive active sites. In this work, CuO@BaTiO3(CuO@BTO) heterostructure is prepared by hydrothermal method on the surface of AFC to investigate the ultrasonic piezoelectric catalysis effect. X-ray diffraction (XRD), Raman spectroscopy, energy dispersive x-ray spectroscopy (EDS) and scanning electron microscopy (SEM) were used to analyze the structure and morphology of CuO-BTO/AFC composite. It is found that the CuO-BTO/AFC composite exhibits excellent piezo-catalytic performance for the degradation of organics promoted by ultrasonic vibration. The CuO-BTO/AFC composite can decompose methyl orange and methylene blue with degradation efficiency as high as 93.9% and 97.6% within 25 min, respectively. The mechanism of piezoelectricity enhanced ultrasound supported catalysis effect of system CuO-BTO/AFC is discussed. The formed heterojunction structure between BTO and CuO promotes the separation of positive and negative charges caused by the piezoelectric effect.

Integration of One-Dimensional (1D) Lead-Free Perovskite Microbelts onto Silicon for Ultraviolet-Visible-Near-Infrared (UV-vis-NIR) Heterojunction Photodetectors.

Lead-free perovskites are considered to be candidates for next-generation photodetectors, because of their excellent charge carrier transport properties and low toxicity. However, their application in integrated circuits is hindered by their inadequate performance and size restrictions. To aim at the development of lead-free perovskite-integrated optoelectronic devices, a CsAg2I3/silicon (CAI/Si) heterojunction is presented in this work by using a spatial confinement growth method, where the in-plane growth of CAI microbelts with high-quality single-crystal characteristics is primarily dependent on the concentration of surrounding precursor solution. The fabricated photodetectors based on the CAI/Si heterojunctions exhibit a broad-spectrum detection capability in the ultraviolet-visible-near-infrared (UV-vis-NIR) range. In addition, the photodetectors show good photoelectric detection performance, including a maximum responsivity of 48.5 mA/W and detectivity of 1.13 × 1011 Jones, respectively. Besides, the photodetectors have a rapid response of 6.5/224 µs and good air stability for over 2 months. This work contributes a new idea to design next-generation optoelectronic devices with high integration density.

The effect of fission products Xe and Cs on the thermal conductivity of the U3Si2lattice: a first-principles study.

In the past decades, uranium silicide (U3Si2) as a promising accident tolerant fuel (ATF) has drawn considerable attention in the field of nuclear physics. In comparison with traditional nuclear fuel (UO2), the U3Si2has higher thermal conductivity and uranium density, thereby resulting in lower centerline temperatures and better fuel economy. However, during the nuclear fission reaction, some unexpected fission products, such as Xe and Cs, are released and form the defective states. In this study, we explore the influence of Xe and Cs on the thermal conductivity of the U3Si2lattice from 200 to 1500 K using density functional theory calculations combined with Boltzmann transport equation. Our results reveal that the lattice and electronic thermal conductivities of defective U3Si2are reduced at a constant temperature, as compared with that of ideal system, thus resulting in a decrease of the total thermal conductivity. In the case of Cs occupation at U1 site, the total thermal conductivity (4.42 W mK-1) is decreased by ∼56% at 300 K, as compared with the value of 9.99 W mK-1for ideal system. With U1 and Si sites being occupied by Xe, the total thermal conductivities (4.45 and 6.52 W mK-1) are decreased by ∼55% and 35% at 300 K, respectively. The presented results suggest that the U3Si2has potential as a promising ATF at high temperatures.

Critical role of hydrogen for superconductivity in nickelates.

The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized by a topotactic reaction with metal hydrides1. This method changes the nickelates from the perovskite to an infinite-layer structure by deintercalation of apical oxygens1-3. Such a chemical reaction may introduce hydrogen (H), influencing the physical properties of the end materials4-9. Unfortunately, H is insensitive to most characterization techniques and is difficult to detect because of its light weight. Here, in optimally Sr doped Nd0.8Sr0.2NiO2H epitaxial films, secondary-ion mass spectroscopy shows abundant H existing in the form of Nd0.8Sr0.2NiO2Hx (x ≅ 0.2-0.5). Zero resistivity is found within a very narrow H-doping window of 0.22 ≤ x ≤ 0.28, showing unequivocally the critical role of H in superconductivity. Resonant inelastic X-ray scattering demonstrates the existence of itinerant interstitial s (IIS) orbitals originating from apical oxygen deintercalation. Density functional theory calculations show that electronegative H- occupies the apical oxygen sites annihilating IIS orbitals, reducing the IIS-Ni 3d orbital hybridization. This leads the electronic structure of H-doped Nd0.8Sr0.2NiO2Hx to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for superconductivity in epitaxial infinite-layer nickelates.

The effects of point defects on thermal-mechanical properties of BiCuOTe: a first-principles study.

Recently, BiCuOTe as a promising thermoelectric material has attracted extensive interest due to its lower thermal conductivity and higher electrical conductivity. However, little is known about the role of point defects in the growth, processing, and device degradation of this material. Moreover, the elastic properties which provide valuable information about the bonding characteristics, heat conductivity, and their anisotropic characters are investigated for effective design and characterization of new devices. Motivated by these considerations, a first-principles study about the stability of point defects and their effects on the thermal-mechanical properties of BiCuOTe was performed. The vacancies are found to be more stable than the interstitials. XO (here X occupying the O lattice site, with X = Cu, Bi or Te) are generally unfavorable among the considered point defects. Point defects generally have negative effects on elastic constants (except C66), suggesting that the resistance of defective systems to uniaxial and shear deformation is usually weaker than the that of ideal BiCuOTe. Similarly, point defects could deteriorate the ability to resist external compression. However, the introduction of point defects may improve the elastic compliances and depress the Debye temperature, which may increase the thermal expansion efficient of BiCuOTe. As compared with the ideal system, the point defects such as CuBi, TeBi, BiTe, OCu, CuTe and TeCu may generally reduce the phonon thermal conductivity. This study would provide insights into the effect of point defects on the elastic and thermal properties of BiCuOTe and has important implications in the rational design of superior thermoelectric materials.

CuO nanorod arrays by gas-phase cation exchange for efficient photoelectrochemical water splitting.

CuO has been considered a promising candidate for photoelectrochemical water splitting electrodes owing to its suitable bandgap, favorable band alignments, and earth-abundant nature. In this paper, a novel gas-phase cation exchange method was developed to synthesize CuO nanorod arrays by using ZnO nanorod arrays as the template. ZnO nanorods were fully converted to CuO nanorods with aspect ratios of 10-20 at the temperature range from 350 to 600 °C. The as-synthesized CuO nanorods exhibit a photocurrent as high as 2.42 mA cm-2 at 0 V vs. RHE (reversible hydrogen electrode) under 1.5 AM solar irradiation, demonstrating the potential as the photoelectrode for efficient photoelectrochemical water splitting. Our method provides a new approach for the rational fabrication of high-performance CuO-based nanodevices.

Strain Tunable Thermoelectric Material: Janus ZrSSe Monolayer.

Thermoelectric (TE) performance of the Janus ZrSSe monolayer under biaxial strain is systematically explored by the first-principles approach and Boltzmann transport theory. Our results show that the Janus ZrSSe monolayer has excellent chemical, dynamical, thermal, and mechanical stabilities, which provide a reliable platform for strain tuning. The electronic structure and TE transport parameters of the Janus ZrSSe monolayer can be obviously tuned by biaxial strain. Under 2% tensile strain, the optimal power factor PF of the n-type-doped Janus ZrSSe monolayer reaches 46.36 m W m-1 K-2 at 300 K. This value is higher than that of the most classical TE materials. Under 6% tensile strain, the maximum ZT values for the p-type- and n-type-doped Janus ZrSSe monolayers are 4.41 and 4.88, respectively, which are about 3.83 and 1.49 times the results of no strain, respectively. Such high TE performance can be attributed to high band degeneracy and short phonon relaxation time under strain, causing simultaneous increase of the Seebeck coefficient and suppression of the phonon thermal transport. Present work demonstrates that the Janus ZrSSe monolayer is a promising candidate as a strain-tunable TE material and stimulates further experimental synthesis.

Boosting thermoelectric performance of HfSe2 monolayer by selectivity chemical adsorption.

In this work, a strategy to boosting thermoelectric (TE) performance of 2D materials is explored. We find that, appropriate chemical adsorption of atoms can effectively increase the TE performance of HfSe2 monolayer. Our results show that the adsorption of Ni atom on HfSe2 monolayer (Ni-HfSe2) can improve the optimal power factor PF and ZT at 300 K, increased by more than ∼67% and ∼340%, respectively. The PF and ZT of Ni-HfSe2 at 300 K can reach 85.06 mW m-1 K-2 and 3.09, respectively. The detailed study reveal that the adsorption of Ni atom can induce additional conductional channels of electrons, enhance the coupling of acoustic-optical phonons and the phonon anharmonicity, resulting in an obvious increment of electrical conductivity (increased by more than ∼89%) in n-type doped system and an ultralow phonon thermal conductivity (1.17 W/mK at 300 K). The high electrical conductivity and ultralow phonon thermal conductivity results in the significant increments of PF and ZT. Our study also shows that, Ni-HfSe2 is a thermal, dynamic and mechanical stable structure, which can be employed in TE application. Our research indicates that selectivity chemical adsorption is a promising way to increase TE performance of 2D materials.

First-principles study of the stability and migration of Xe and Cs in U3Si.

In the past several years, the U3Si has been suggested as an alternative nuclear fuel for light water reactors due to its high uranium density and outstanding thermal conductivity. In order to gain fundamental insights into the behavior of fission products in U3Si, the trapping and migration behaviors of the fission products Xe and Cs in U3Si are investigated using density functional theory calculations in this work. UnderU-rich and Si-rich conditions, both the Xe and Cs atoms prefer to substitute for Si andUatoms, respectively. Besides, both Xe and Cs tend to migrate through the vacancy-mechanism. It is noticeable that Xe diffuses faster and forms Xe bubbles more easily than Cs, which is mainly caused by the weaker interaction between Xe and its surrounding atoms.

First-principle study of neutron irradiation induced performance degradation of amorphous porous silica.

Neutron irradiation induced degradation of porous silica film is studied by Molecular Dynamics and Density-Functional theory-based methods. The degradation of microscopic structure, thermal property, and optical property of porous silica film are systematically investigated. Low-energy recoil is used to simulate the neutron irradiation effect. The pair and bond angle distributions, and coordination number distributions reveal that, under neutron irradiation, the microscopic structure of porous silica film is obviously modified, and the coordination defects are induced. We find that the higher recoil energy, the more coordination defects are formed in the film. The increased defects lead to a decrease in thermal conductivity. In addition, neutron irradiation induces additional optical absorption peaks in UV region and increasement in refractive index, resulting in a noticeable reduction in light transmittance. The detailed calculation of density of states reveals that these optical absorption peaks originate from the irradiation induced defect states in band gap. Our work shows that low-energy neutron irradiation can induce obvious defect density and degrade thermal and optical properties of porous silica film, which are responsible for subsequent laser-induced damage.

Highly Sensitive and Selective Surface Acoustic Wave Ammonia Sensor Operated at Room Temperature with a Polyacrylic Acid Sensing Layer.

In this study, polyacrylic acid (PAA) films were deposited onto a quartz surface acoustic wave (SAW) resonator using a spin-coating technique for ammonia sensing operated at room temperature, and the sensing mechanisms and performance were systematically studied. The oxygen-containing functional groups on the surfaces of the PAA film make it sensitive and selective to ammonia molecules, even when tested at room temperature. The ammonia molecules adsorbed by the oxygen-containing functional groups of PAA (e.g., hydroxyl and epoxy groups) increase the membrane's stiffness, which was identified as the primary mechanism leading to the positive frequency shifts. However, mass loading due to adsorption of ammonia molecules is not a major reason as it will result in a negative frequency shifts. When the PAA coated SAW sensor was exposed to ammonia with a low concentration of 500 ppb, it showed a positive frequency shift of 225 Hz, with both good repeatability and stability, as well as a good selectivity to ammonia compared with those to C2H5OH, H2, HCl, H2S, CO, NO2, NO, and CH3COCH3.

High-κ perovskite membranes as insulators for two-dimensional transistors.

The scaling of silicon metal-oxide-semiconductor field-effect transistors has followed Moore's law for decades, but the physical thinning of silicon at sub-ten-nanometre technology nodes introduces issues such as leakage currents1. Two-dimensional (2D) layered semiconductors, with an atomic thickness that allows superior gate-field penetration, are of interest as channel materials for future transistors2,3. However, the integration of high-dielectric-constant (κ) materials with 2D materials, while scaling their capacitance equivalent thickness (CET), has proved challenging. Here we explore transferrable ultrahigh-κ single-crystalline perovskite strontium-titanium-oxide membranes as a gate dielectric for 2D field-effect transistors. Our perovskite membranes exhibit a desirable sub-one-nanometre CET with a low leakage current (less than 10-2 amperes per square centimetre at 2.5 megavolts per centimetre). We find that the van der Waals gap between strontium-titanium-oxide dielectrics and 2D semiconductors mitigates the unfavourable fringing-induced barrier-lowering effect resulting from the use of ultrahigh-κ dielectrics4. Typical short-channel transistors made of scalable molybdenum-disulfide films by chemical vapour deposition and strontium-titanium-oxide dielectrics exhibit steep subthreshold swings down to about 70 millivolts per decade and on/off current ratios up to 107, which matches the low-power specifications suggested by the latest International Roadmap for Devices and Systems5.

Ultrahigh oxygen evolution reaction activity in Au doped co-based nanosheets.

Oxygen evolution reaction (OER) has attracted enormous interest as a key process for water electrolysis over the past years. The advance of this process relies on an effective catalyst. Herein, we employed single-atom Au doped Co-based nanosheets (NSs) to theoretically and experimentally evaluate the OER activity and also the interaction between Co and Au. We reveal that Au-Co(OH)2 NSs achieved a low overpotential of 0.26 V at 10 mA cm-2. This extraordinary phenomenon presents an overall superior performance greater than state-of-the-art Co-based catalysts in a sequence of α-Co(OH)2 < Co3O4 < CoOOH < Au-Co(OH)2. With ab initio calculations and analysis in the specific Au-Co(OH)2 configuration, we reveal that OER on highly active Au-Co(OH)2 originates from lattice oxygen, which is different from the conventional adsorbate evolution scheme. Explicitly, the configuration of Au-Co(OH)2 gives rise to oxygen non-bonding (ONB) states and oxygen holes, allowing direct O-O bond formation by a couple of oxidized oxygen with oxygen holes, offering a high OER activity. This study provides new insights for elucidating the origins of activity and synthesizing efficient OER electrocatalysts.

First-principles study of fission products Xe and Cs behaviors in U3Si2.

In the past several decades, the U3Si2has received much attention for the development of accident tolerant fuel in light water reactors because of its superior thermal conductivity and higher uranium density. In this study, density functional theory calculations have been carried out to study the occupation and diffusion behaviors of fission products Xe and Cs in U3Si2. It is revealed that the occupation sites of Xe and Cs depend on the chemical environment, and both of Xe and Cs are favorable to substitute for U or Si sites. The diffusions of Xe and Cs in U3Si2are predicted to be via the vacancy mechanism and both of Xe and Cs form cluster easily. As compared with Cs, the Xe exhibits a smaller solubility, faster diffusion as well as stronger clustering tendency, which may cause larger bubble size for Xe than Cs under the same conditions in U3Si2. The differences in the diffusion behaviors between Xe and Cs mainly result from their different valence electronic configurations and different atomic radii.

First-principles study of point defects in U3Si2: effects on the mechanical and electronic properties.

In recent years, U3Si2 has been proposed as an alternative nuclear fuel material to uranium dioxide (UO2) because of its intrinsically high uranium density and thermal conductivity. However, the operation environment in the nuclear reactor is complex and extreme, such as in-pile neutron irradiation, and thus it is necessary to explore the radiation response behavior of U3Si2 and the physical properties of its damaged states. In the present study, first-principles calculations based on density functional theory were carried out to investigate the mechanical and electronic properties of defective U3Si2. Our results showed that the defect stability in U3Si2, except its interstitial defects, is dependent on its chemical environment. When vacancy, antisite or interstitial defects are introduced into U3Si2, its elastic modulus are decreased and its ductility is enhanced. Although the presence of defects in U3Si2 does not change its metallic nature and the electron distribution in its Fermi level, their effect on the partial chemical bonding interaction is significant. This study suggests that under a radiation environment, the created defects in U3Si2 remarkably affect its mechanical and electronic properties.

Nickel-Based Selenides with a Fractal Structure as an Excellent Bifunctional Electrocatalyst for Water Splitting.

Nickel-based selenides are believed to be promising non-precious metal electrocatalysts, and have been widely used for both oxygen evolution reactions (OER) and hydrogen evolution reactions (HER). Here, we control the aging time to prepare NixSey with different fractal structures as a bifunctional catalyst. An obtained sample with an aging time of 80 min shows outstanding electrocatalytic performance for hydrogen evolution reactions (HER) with an overpotential of 225 mV (η@10 mA/cm2) and for oxygen evolution reactions (OER) with an overpotential of 309 mV (η@50 mA/cm2). Moreover, to further improve catalytic activity, we doped Fe in NixSey to obtain the ternary nickel-based selenide, Fe0.2Ni0.8Se (FNSs). The HER activity of FNS increased two-fold at 10 mA/cm2, and the overpotential of OER decreased to 255 mV at 50 mA/cm2. The synthetic strategy and research results of this work have a certain reference value for other low-cost and high-efficiency transition metal catalysts for electrocatalytic water splitting.

Exceptional Photocatalytic Activities of rGO Modified (B,N) Co-Doped WO3 , Coupled with CdSe QDs for One Photon Z-Scheme System: A Joint Experimental and DFT Study.

Artificial Z-scheme, a tandem structure with two-step excitation process, has gained significant attention in energy production and environmental remediation. By effectively connecting and matching the band-gaps of two different photosystems, it is significant to utilize more photons for excellent photoactivity. Herein, a novel one-photon (same energy-two-photon) Z-scheme system is constructed between rGO modified boron-nitrogen co-doped-WO3 , and coupled CdSe quantum dots-(QDs). The coctalyst-0.5%Rhx Cr2 O3 (0.5RCr) modified amount-optimized sample 6%CdSe/1%rGO3%BN-WO3 revealed an unprecedented visible-light driven overall-water-splitting to produce ≈51 µmol h-1 g-1 H2 and 25.5 µmol h-1 g-1 O2 , and it remained unchanged for 5 runs in 30 h. This superior performance is ascribed to the one-photon Z-scheme, which simultaneously stimulates a two photocatalysts system, and enhanced charge separation as revealed by various spectroscopy techniques. The density-functional theory is further utilized to understand the origin of this performance enhancement. This work provides a feasible strategy for constructing an efficient one-photon Z-scheme for practical applications.

Promoting the Oxygen Evolution Activity of Perovskite Nickelates through Phase Engineering.

Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and cost-effectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO3 by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO3 film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO3 single crystalline film. The obtained current density of 1038 µΑ cm-2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.