Search

Gold (Au) as co-catalyst is remarkable for activating methane (CH4), especially atomically dispersed Au with maximized exposing active sites and specific electronic structure. Furthermore, singlet oxygen (1O2) typically manifests a mild redox capacity with a high selectivity to attack organic substrates. Peroxomonosulfate (PMS) favors to produce oxidative species 1O2 during the photocatalytic reactions. Thus, combining atomic Au as co-catalyst and 1O2 as oxidant is an effective strategy to selectively convert CH4. Herein, we synthesized atomically dispersed Au on WO3 (Au/WO3), where Au was in the forms of single atoms and clusters. At room temperature, such Au/WO3 exhibited enhanced photocatalytic conversion of CH4 to CH3CH3 with a selectivity, up to 94%, under visible light. The radicals-pathway mechanism of CH4 coupling has also been investigated through detection and trapping experiment of active species. Theoretical calculations further interpret the electronic structure of Au/WO3 and tip-enhanced local electric field at the Au sites for promoting CH4 conversion.

The flexoelectric effect, which manifests itself as a strain-gradient-induced electrical polarization, has triggered great interest due to its ubiquitous existence in crystalline materials without the limitation of lattice symmetry. Here, we propose a flexoelectric photodetector based on a thin-film heterostructure. This prototypical device is demonstrated by epitaxial LaFeO3 thin films grown on LaAlO3 substrates. A giant strain gradient of the order of 106/m is achieved in LaFeO3 thin films, giving rise to an obvious flexoelectric polarization and generating a significant photovoltaic effect in the LaFeO3-based heterostructures with nanosecond response under light illumination. This work not only demonstrates a novel self-powered photodetector different from the traditional interface-type structures, such as the p–n and Schottky junctions but also opens an avenue to design practical flexoelectric devices for nanoelectronics applications.

Antiferromagnetic insulators are a ubiquitous class of magnetic materials, holding the promise of low-dissipation spin-based computing devices that can display ultra-fast switching and are robust against stray fields. However, their imperviousness to magnetic fields also makes them difficult to control in a reversible and scalable manner. Here we demonstrate a novel proof-of-principle ionic approach to control the spin reorientation (Morin) transition reversibly in the common antiferromagnetic insulator α-Fe2O3 (haematite) – now an emerging spintronic material that hosts topological antiferromagnetic spin-textures and long magnon-diffusion lengths. We use a low-temperature catalytic-spillover process involving the post-growth incorporation or removal of hydrogen from α-Fe2O3 thin films. Hydrogenation drives pronounced changes in its magnetic anisotropy, Néel vector orientation and canted magnetism via electron injection and local distortions. We explain these effects with a detailed magnetic anisotropy model and first-principles calculations. Tailoring our work for future applications, we demonstrate reversible control of the room-temperature spin-state by doping/expelling hydrogen in Rh-substituted α-Fe2O3.

Distorted thermal properties Thermoelectric devices can convert waste heat into electricity, providing one path for improving energy efficiency. Jiang et al. leveraged entropy engineering to synthesize a single-phase high-entropy alloy with attractive thermoelectric properties. By increasing the number of elements in the alloy, the resulting disorder helps to stabilize against breakdown into multiple phases. The disordered and distorted crystal lattice suppresses thermal transport while maintaining the electrical properties, which boosts the heat-conversion efficiency of the material. Science , this issue p. 830

An atomically dispersed structure is attractive for electrochemically converting carbon dioxide (CO2) to fuels and feedstock due to its unique properties and activity. Most single-atom electrocatalysts are reported to reduce CO2 to carbon monoxide (CO). Herein, we develop atomically dispersed indium (In) on a nitrogen-doped carbon skeleton (In-N-C) as an efficient catalyst to produce formic acid/formate in aqueous media, reaching a turnover frequency as high as 26771 h-1 at -0.99 V relative to a reversible hydrogen electrode (RHE). Electrochemical measurements show that trace amounts of In loaded on the carbon matrix significantly improve the electrocatalytic behavior for the CO2 reduction reaction, outperforming conventional metallic In catalysts. Further experiments and density functional theory (DFT) calculations reveal that the formation of intermediate *OCHO on isolated In sites plays a pivotal role in the efficiency of the CO2-to-formate process, which has a lower energy barrier than that on metallic In.

The piezoelectric constant (d33) and converse piezoelectric coefficient (d33*) of a piezoelectric material are critically important parameters for sensors and actuators. Here, we simultaneously achieve a high d33 of 595 ± 10 pC/N and a large d33* of ∼776 ± 20 pm/V in (K,Na)NbO3 (KNN)-based ceramics, which exceed those of PZT5H and PZT4 ceramics, presenting good potential for practical piezoelectric applications. Moreover, the ceramic exhibits a relaxor-like and diffuse dielectric behavior due to the occurrence of local heterogeneity. According to the experiments and atomic resolution polarization mapping by Z-contrast imaging, hierarchical architecture of nanodomains and even smaller polar nanoregions with multiphase coexistence caused by compositional modification is the structural origin of the enhanced piezoelectric properties in this work. This work would pave a practical way to future applications of lead-free KNN-based ceramics.

Two-dimensional (2D) metallic transition-metal dichalcogenides (MTMDCs) are considered as ideal electrode materials for enhancing the device performances of 2D semiconducting transition-metal dichalcogenides, due to their similar atomic structures and complementary electronic properties. Vanadium ditelluride (VTe2) behaves as a fascinating material in MTMDCs family, presenting room-temperature ferromagnetism, charge density waves order, and topological property. However, its practical applications in universal electrode/energy-related fields remain unexplored. Herein, we achieved the direct synthesis of ultrathin, large-domain, and thickness-tunable 1T-VTe2 nanosheets on an easily available mica substrate by chemical vapor deposition (CVD). We further uncover that the CVD-derived 1T-VTe2 can serve as a high-performance electrode material thanks to its ultrahigh conductivity. Accordingly, a 6 times higher field-effect mobility (∼47.5 cm2 V-1 s-1) was achieved in 1T-VTe2-contacted monolayer MoS2 devices than that using a conventional Ti/Au electrode (∼8.1 cm2 V-1 s-1). Moreover, the CVD-synthesized 1T-VTe2 nanosheets are revealed to present excellent electrocatalytic activity for hydrogen evolution reaction. These results should propel the direct application of CVD-grown 2D MTMDCs as high-performance electrode materials in all 2D materials related devices.

The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au4Pt2 clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au4Pt2/G) with exceptional activity for electrochemical nitrogen (N2) reduction. Our mechanistic study reveals that each N2 molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N2 via an enhanced back donation of electrons to the N2 LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant., The fabrication of singly dispersed metal cluster catalysts with atomic-level control of dopants is a long-standing challenge. Here, the authors report a strategy for the synthesis of a precisely doped single cluster catalyst which shows exceptional activity for electrochemical dinitrogen reduction.

We report a simple procedure to fabricate single crystals 3D C60 having an FCC structure on silicon substrates using a vapour–solid set-up in vacuum conditions. The morphology of the deposited film can be tuned by controlling the temperature and position of the substrate. The as-fabricated samples are extensively characterised by transmission electron microscopy, scanning electron microscope, X-ray powder diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and nano-indentation, which allow us to shed light on the recrystallization process of the C60. In addition, the growth mechanism of the formation of crystalline 3D structure of the C60 film is discussed in detail. Based on the newly gained knowledge of mechanism and its unique properties, fullerene has shown huge potential as a solid lubricant on various kinds of substrates.

Simple strain enhancement Piezoelectric materials are important as sensors and transducers for applications such as ultrasonics. Liu et al. discovered nanopillar regions in a sodium-niobium-oxide film that substantially improve the piezoelectric properties (see the Perspective by Bassiri-Gharb). These nanopillar regions reverse where the cations and anions are located in the crystal structure, with a distinctive boundary in between. This difference in structure results in a strain-sensitive polarity that enhances the piezoelectric properties in a chemically simple material. Science this issue p. 292 ; see also p. 252

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronics devices. However, the synthesis of 2D magnetic crystals, especially the direct growth on SiO2/Si substrate, is just in its infancy. Here, we report a chemical vapor deposition (CVD)-based rational growth approach for the synthesis of ultrathin FeTe crystals with controlled structural and magnetic phases. By precisely optimizing the growth temperature (Tgrowth), FeTe nanoplates with either layered tetragonal or non-layered hexagonal phase can be controlled with high-quality. The two controllable phases lead to square and triangular morphologies with a thickness down to 3.6 and 2.8 nm, respectively. More importantly, transport measurements reveal that tetragonal FeTe is antiferromagnetic with a Neel temperature (TN) about 71.8 K, while hexagonal FeTe is ferromagnetic with a Curie temperature (TC) around 220 K. Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from a concomitant lattice distortion and the spin-lattice coupling. This study represents a major step forward in the CVD growth of 2D magnetic materials on SiO2/Si substrates and highlights on their potential applications in the future spintronic devices., Comment: 15 pages, 5 figures

State-of-the-art chemical sensors based on covalent organic frameworks (COFs) are restricted to the transduction mechanism relying on luminescence quenching and/or enhancement. Herein, we present an alternative methodology via a combination of in situ-grown COF films with interdigitated electrodes utilized for capacitive benzene detection. The resultant COF-based sensors exhibit highly sensitive and selective detection at room temperature toward benzene vapor over carbon dioxide, methane, and propane. Their benzene detection limit can reach 340 ppb, slightly inferior to those of the metal oxide semiconductor-based sensors, but with reduced power consumption and increased selectivity. Such a sensing behavior can be attributed to the large dielectric constant of the benzene molecule, distinctive adsorptivity of the chosen COF toward benzene, and structural distortion induced by the custom-made interaction pair, which is corroborated by sorption measurements and density functional theory (DFT) calculations. This study provides new perspectives for fabricating COF-based sensors with specific functionality targeted for selective gas detection.

In recent years, two‐dimensional (2D) ternary materials have attracted wide attention due to their novel properties which can be achieved by regulating their chemical composition with a very great degree of freedom and adjustable space. However, as for the precise synthesis of 2D ternary materials, great challenges still lie ahead that hinder their further development. In this work, we demonstrated a simple and reliable approach to synthesize 2D ternary‐layered BiOCl crystals through a microwave‐assisted space‐confined process in a short time (

Nanoscale phase mixtures in transition-metal oxides (TMOs) often render these materials susceptible to external stimuli (electric field, mechanical stress, etc.), which can lead to rich functional properties and device applications. Here, direct observation and multifield manipulation of a nanoscale mixture of brownmillerite SrFeO

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have emerged as attractive platforms in next-generation nanoelectronics and optoelectronics for reducing device sizes down to a 10 nm scale. To achieve this, the controlled synthesis of wafer-scale single-crystal TMDs with high crystallinity has been a continuous pursuit. However, previous efforts to epitaxially grow TMD films on insulating substrates (e.g., mica and sapphire) failed to eliminate the evolution of antiparallel domains and twin boundaries, leading to the formation of polycrystalline films. Herein, we report the epitaxial growth of wafer-scale single-crystal MoS2 monolayers on vicinal Au(111) thin films, as obtained by melting and resolidifying commercial Au foils. The unidirectional alignment and seamless stitching of the MoS2 domains were comprehensively demonstrated using atomic- to centimeter-scale characterization techniques. By utilizing onsite scanning tunneling microscope characterizations combined with first-principles calculations, it was revealed that the nucleation of MoS2 monolayer is dominantly guided by the steps on Au(111), which leads to highly oriented growth of MoS2 along the ⟨110⟩ step edges. This work, thereby, makes a significant step toward the practical applications of MoS2 monolayers and the large-scale integration of 2D electronics.

The ability to tune both local and global environments of a single-metal active center on a support is crucial for the development of highly robust and efficient single-atom electrocatalysts (SAECs...

Developing highly active oxygen evolution reaction (OER) catalysts with fast OER kinetics is crucial for disruptively changing the energy technology, where unlocking of the catalytic origin is the ...

To provide alternative battery technologies to lithium ion battery, multivalent metal ion batteries with their high theoretical capacities and ease of preparation have gradually gained attention from both academia and industries. In this work, we report bismuth ion battery (BIB) as a promising trivalent metal ion battery, next to the only known aluminum ion battery. Our BIB successfully demonstrates battery behavior with discharge plateaus at 0.5 and 0.2 V. Gravimetric capacity of 300 mAh g-1 at current density of 0.2 A g-1 was obtained with ca. 98% coulombic efficiency. In addition, stable cyclic life was achieved after 100 cycles at 0.3 A g-1 which further suggests its suitability as potential trivalent metal ion battery.

The breaking of multiple symmetries by periodic lattice distortion at a commensurate charge density wave (CDW) state is expected to give rise to intriguing interesting properties. However, accessing the commensurate CDW state on bulk TaS2 crystals typically requires cryogenic temperatures (77 K), which precludes practical applications. Here, we found that heteroepitaxial growth of a 2H-tantalum disulfide bilayer on a hexagonal-boron nitride (h-BN) substrate produces a robust commensurate CDW order at room temperature, characterized by a Moire superlattice of 3 × 3 TaS2 on a 4 × 4 h-BN unit cell. The CDW order is confirmed by scanning transmission electron microscopy and Raman measurements. Theoretical calculations reveal that the stabilizing energy for the CDW phase of the monolayer and bilayer 2H-TaS2-on-h-BN substrates arises primarily from interfacial electrostatic interactions and, to a lesser extent, interfacial strain. Our work shows that engineering interfacial electrostatic interactions in an ultrathin van der Waals heterostructure constitutes an effective way to enhance CDW order in two-dimensional materials.

Regulating the temperature of buildings has the highest expenditure of energy (35–40%); thus, transparent heat reflectors (THRs) have been at the forefront of passive cooling technology due to its ability to reflect near-infrared (NIR) wavelengths without compromising optical transmission. Current designs like metallic thin films and dielectric–metal–dielectric are expensive and face complications with material incompatibility, oxidation, and interdiffusion, which hamper their performance. A cost-effective THR using a single-layer coating has not yet been achieved due to the susceptibility of metals to oxidation. In this work, a layer of CuS was fabricated onto flexible polyethylene terephthalate (PET) via radio frequency magnetron sputtering (RFMS), a high throughput method known for its conformal large-area fabrication. The 150 nm-thick CuS-PET films demonstrated a visible transmission T550 nm of 64.1% and NIR reflectance of 50% that resulted in 8 °C passive cooling. Additionally, wetting the surface re...

The metallic elements with a small atomic radius can not only compensate for the intrinsic vacancies to improve the carrier mobility, but also can form interstitials to reduce lattice thermal conductivity. In this work, we investigate the contrasting roles of three different metallic elements with small atomic radius on the thermoelectric properties of n-type PbM0.01Se (M = Cu, Zn, Ni). We find that all the metallic elements can increase the carrier concentration as electron donors and improve the electrical transport properties. In addition, the lattice thermal conductivity of PbSe can be significantly decreased through the formation of interstitials by the incorporated extra metallic elements. We also find that the PbCu0.01Se sample exhibits a unique and complex behavior compared with the other samples due to the diffusion and the thermally activated oxidation of Cu+ ions. When the temperature increased to 673 K, the carrier concentration of PbCu0.01Se sample increases suddenly and continues to increase by nearly two folds, which we ascribe to the release of 3d orbital electrons in Cu+ ions. This results in a strong decoupling of the charge and heat transport in the material, which can be manifested by the overestimation of the electronic thermal conductivity when assuming strong electron–phonon coupling in Wiedemann–Franz law. A maximum ZT of ∼1.6 can be reached at 773 K and the average ZT of ∼0.96 can be achieved in PbCu0.01Se. Our results demonstrate the exceptional contrasting behavior of Cu in PbSe by comparing with Zn and Ni, and the thermoelectric performance of n-type PbSe can be enhanced through the introduction of extra metallic elements with small radius.

Grain boundaries could exhibit exceptional electronic structure and exotic properties, which are determined by a local atomic configuration and stoichiometry that differs from the bulk. However, optical and plasmonic properties at the grain boundaries in metallic oxides have rarely been discussed before. Here, we show that non-stoichiometric grain boundaries in the newly discovered metallic SrNbO3 photocatalyst show exotic electronic, optical and plasmonic phenomena in comparison to bulk. Aberration-corrected scanning transmission electron microscopy and first-principles calculations reveal that a Nb-rich grain boundary exhibits an increased carrier concentration with quasi-1D metallic conductivity, and newly induced electronic states contributing to the broad energy range of optical absorption. More importantly, dielectric function calculations reveal extended and enhanced plasmonic excitations compared with bulk SrNbO3. Our results show that non-stoichiometric grain boundaries might be utilized to control the electronic and plasmonic properties in oxide photocatalysis.

Development of efficient and durable oxygen evolution reaction (OER) catalysts has a direct impact on the water splitting efficiency and cost effectiveness. In this work, we report the successful synthesis of a new Ni(OH)2 structure, strain-stabilized Ni(OH)2 nanoribbons (NR-Ni(OH)2) two to three layers thick, with widths of 2–5 nm, via an electro-oxidation route. Conventional Ni(OH)2 usually has negligible OER activity, while NR-Ni(OH)2 shows high activity for the oxygen evolution reaction and an overpotential of 162 millivolts and furthermore exhibits long-term stability in alkaline electrolyte. The substantial reduction in the overpotential of NR-Ni(OH)2 is due to its easier OOH* adsorption by the active four-coordinated Ni edge sites. The enhanced catalytic activity of NR-Ni(OH)2 makes it an excellent candidate for industrial applications.

Hollow-structuring of active components is among the most effective strategies for improving the kinetics of oxygen electrode catalysts, benefiting from the much-improved active surface area, enhanced accessible active sites, and formation of the desired defects on the exposed surface. Integration of active hollow nanostructures with functionalized carbon nanofibers synergizes the electrochemical performance and mechanical flexibility, which are particularly of interest for all-solid-state zinc–air batteries (FASS ZABs). In the present work, we demonstrate that the electrospinning of Prussian blue analogs can give rise to hollow FeCo alloy nanoparticles assembled in nitrogen-doped carbon nanofibers (h-FeCo alloy/N-CNFs). The h-FeCo alloy/N-CNFs thus derived exhibit superior bifunctional activities for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), and impressive performance in rechargeable Zn–air batteries. When integrated into a rechargeable FASS ZAB, they exhibit a high open circuit voltage of 1.335 V and a stable discharge–charge plateau around 1.1 V to 2.0 V, together with a large voltage efficiency of 63.5%, under bending conditions.

Description, Role of charged defects in resistance fatigue of ferroelectric tunnel junctions has been revealed at atomic scale., Ferroelectric tunnel junctions (FTJs) are promising candidates for next-generation memories due to fast read/write speeds and low-power consumptions. Here, we investigate resistance fatigue of FTJs, which is performed on Pt/BaTiO3/Nb:SrTiO3 devices. By direct observations of the 5–unit cell–thick BaTiO3 barrier with high-angle annular dark-field imaging and electron energy loss spectroscopy, oxygen vacancies are found to aggregate at the Pt/BaTiO3 interface during repetitive switching, leading to a ferroelectric dead layer preventing domain nucleation and growth. Severe oxygen deficiency also makes BaTiO3 lattices energetically unfavorable and lastly induces a destruction of local perovskite structure of the barrier. Ferroelectric properties are thus degraded, which reduces barrier contrast between ON and OFF states and smears electroresistance characteristics of Pt/BaTiO3/Nb:SrTiO3 FTJs. These results reveal an atomic-scale fatigue mechanism of ultrathin ferroelectric barriers associated with the aggregation of charged defects, facilitating the design of reliable FTJs and ferroelectric nanoelectronic devices for practical applications.

Two-dimensional (2D) Fe-chalcogenides (e.g., FeS, FeSe, and FeTe, etc.) have sparked extensive interest due to their rich phase diagrams including superconductivity, magnetism, and topological state, as well as versatile applications in electronic devices and energy related fields. However, the phase-tunable synthesis and green transfer of such fascinating materials still remain challenging. Herein, we develop a temperature-mediated chemical vapor deposition (CVD) approach to grow ultrathin nonlayered hexagonal and layered tetragonal FeTe nanosheets on mica substrates, with their thicknesses down to ∼2.3 and ∼4.0 nm, respectively. Interestingly, we have observed exciting ferromagnetism with the Curie temperature approaching ∼300 K and high conductivity (∼1.96 × 105 S m-1) in 2D hexagonal FeTe. More significantly, we have designed a swift, high-efficiency, and etching-free method for the transfer of 2D FeTe nanosheets onto arbitrary substrates, and such a transfer strategy enables the cyclic utilization of growth substrates. These results should propel the further development of phase-tunable synthesis and green transfer of 2D Fe-chalcogenides, as well as their potential applications in spintronic devices.

Clean water and sanitation are the sixth goal of the United Nation’s sustainable development goals by 2030. Thus, research into expanding clean water supply has been steadily growing. Photothermal vaporization is a technique that utilizes light-to-heat conversion and proper thermal insulation to localize heat and produce steam at accelerated rates at temperatures below 100 °C. Covellite (CuS) has been investigated for this application as it possesses localized surface plasmon resonance (LSPR) in the near-infrared (IR) wavelengths, converting light into heat. However, recent studies on CuS have been limited to preparation via wet chemical methods to achieve nanoscale structures. These methods require an extra step of material transfer, leading to adhesion issues with the working substrate. In this work, we report the growth of polycrystalline CuS nanopillars using rf magnetron sputtering, a high-throughput physical vapor deposition process. By introduction of N2 gas, an etchant, in the plasma, a balance of...

To date, thermoelectric performance has been enhanced through improving the power factor and/or reducing the lattice thermal conductivity. Here, we report an effective method of boosting thermoelec...

Integrating multiple properties in a single system is crucial for the continuous developments in electronic devices. However, some physical properties are mutually exclusive in nature. Here, we report the coexistence of two seemingly mutually exclusive properties-polarity and two-dimensional conductivity-in ferroelectric Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ thin films at the LaAlO$_3$/Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ interface at room temperature. The polarity of a ~3.2 nm Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ thin film is preserved with a two-dimensional mobile carrier density of ~0.05 electron per unit cell. We show that the electronic reconstruction resulting from the competition between the built-in electric field of LaAlO$_3$ and the polarization of Ba$_{0.2}$Sr$_{0.8}$TiO$_3$ is responsible for this unusual two-dimensional conducting polar phase. The general concept of exploiting mutually exclusive properties at oxide interfaces via electronic reconstruction may be applicable to other strongly-correlated oxide interfaces, thus opening windows to new functional nanoscale materials for applications in novel nanoelectronics., Comment: Main text: 25 pages, 4 figures Supplementary note: 19 pages, 11 figures

Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (μ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (μW cm−1 K−2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZTmax) of ~1.6 at 873 K and an average ZT (ZTave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.

The electrocaloric effect in ferroelectric materials has drawn much attention due to its potential applications in integrated circuit cooling and novel cooling devices. In contrast to the widely researched positive electrocaloric effect, the negative electrocaloric effect has received much less attention due to the lack of any effective methods for significant enhancement. In this work, we fabricated PbZrO3 thin film on a Pt/Si substrate by the sol-gel method. By controlling the interface conditions between the thin film and substrate, we induced defects into the interface and stabilized a transient ferroelectric phase in the PbZrO3 thin film. The emergence of the transient ferroelectric phase postpones the antiferroelectric-ferroelectric phase transition. As a result, a negative electrocaloric effect up to -18.5 K is estimated near room temperature, the highest one ever reported in this temperature range. This result suggests a new strategy to enhance the negative electrocaloric effect and may benefit the application of PbZrO3 thin films in cooling devices.

Piezocatalysis, converting mechanical vibration into chemical energy, has emerged as a promising candidate for water-splitting technology. However, the efficiency of the hydrogen production is quite limited. We herein report well-defined 10 nm BaTiO3 nanoparticles (NPs) characterized by a large electro-mechanical coefficient which induces a high piezoelectric effect. Atomic-resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and scanning probe microscopy (SPM) suggests that piezoelectric BaTiO3 NPs display a coexistence of multiple phases with low energy barriers and polarization anisotropy which results in a high electro-mechanical coefficient. Landau free energy modeling also confirms that the greatly reduced polarization anisotropy facilitates polarization rotation. Employing the high piezoelectric properties of BaTiO3 NPs, we demonstrate an overall water-splitting process with the highest hydrogen production efficiency hitherto reported, with a H2 production rate of 655 μmol g-1 h-1 , which could rival excellent photocatalysis system. This study highlights the potential of piezoelectric catalysis for overall water splitting.

Controlled substitutional doping of two-dimensional transition-metal dichalcogenides (TMDs) is of fundamental importance for their applications in electronics and optoelectronics. However, achievin...

Extrinsically controlling the intrinsic activity and stability of two-dimensional (2D) semiconducting materials by substitutional doping is crucial for energy-related applications. However, an in s...

Enabled by the advances in aberration‐corrected scanning transmission electron microscopy (STEM), atomic‐resolution real space imaging of materials has allowed a direct structure‐property investigation. Traditional ways of quantitative data analysis suffer from low yield and poor accuracy. New ideas in the field of computer vision and machine learning have provided more momentum to harness the wealth of big data and sophisticated information in STEM data analytics, which has transformed STEM from a localized characterization technique to a macroscopic tool with intelligence. In this review article, we discuss the prime significance of defect topology and density in two‐dimensional (2D) materials, which have proved to be a powerful means to tune a wide range of properties. Subsequently, we systematically review advanced data analysis methods that have demonstrated promising prospects in analyzing STEM data, particularly for identifying structural defects, with high throughput and veracity. A unified framework for atomic structure identification is also summarized.

Defects are commonly found in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials. Such defects usually dictate the optical and electrical properties of TMDs. It is thus important ...

The development of both flexible solid-state Zn-air and Al-air batteries are challenged by the efficient and stable air cathodes with high catalytic activities in both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). In this work, we report the rational design of hollow N-doped carbon nanotube arrays embedded with confined Co nanoparticles (HCA-Co) through a facile solution-reaction and annealing process. Due to the unique integration of hollow carbon nanoarray with tiny cobalt nanoparticles, the obtained flexible HCA-Co electrode shows promising catalytic properties toward both ORR and OER that achieves a current density of 10 mA cm−2 at small overpotential of 290 mV in OER, and demonstrates an onset potential of 0.92 V in ORR. The HCA-Co can be applied as a binder-free air-cathode for flexible all-solid-state zinc-air batteries, which presents a relatively high open circuit potential (1.40 V) with better cycling stability than Pt/C based battery. The HCA-Co is also utilised as cathode for solid-state Al battery, which shows a high open circuit potential (1.966 V) with better mechanical flexibility than that of Pt/C-based battery. Such flexible electrode with excellent bifunctional catalytic properties hold great promise for the application in flexible electronics.

Gas sensing technologies for smart cities require miniaturization, cost-effectiveness, low power consumption, and outstanding sensitivity and selectivity. On-chip, tailorable capacitive sensors integrated with metal-organic framework (MOF) films are presented, in which abundant coordinatively unsaturated metal sites are available for gas detection. The in situ growth of homogeneous Mg-MOF-74 films is realized with an appropriate metal-to-ligand ratio. The resultant sensors exhibit selective detection for benzene vapor and carbon dioxide (CO2 ) at room temperature. Postsynthetic modification of Mg-MOF-74 films with ethylenediamine decreases sensitivity toward benzene but increases selectivity to CO2 . The reduced porosity and blocked open metal sites caused by amine coordination account for a deterioration in the sensing performance for benzene (by ca. 60 %). The enhanced sensitivity for CO2 (by ca. 25 %) stems from a tailored amine-CO2 interaction. This study demonstrates the feasibility of tuning gas sensing properties by adjusting MOF-analyte interactions, thereby offering new perspectives for the development of MOF-based sensors.

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. The optoelectronic functionality of 2D heterostructure devices is critically dependent on the relative rotation angle of the layers. However, the role of the relative rotation angle of the constituent layers on intralayer absorption is not clear yet. Here, we investigate MoS2/WSe2 vdW heterostructures using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy and report that momentum conservation is a critical factor in the intralayer absorption of TMDC vdW heterostructures. The evolution of the intralayer excitonic low-loss EEL spectroscopy peak broadenings as a function of the rotation angle reveals that the interlayer charge transfer rate can be about an order of magnitude faster in the aligned (or anti-aligned) case than in the misaligned cases. These results provide a deeper insight into the role of momentum conservation, one of the fundamental principles governing charge transfer dynamics in 2D vdW heterostructures.

Exploring efficient strategies to increase CO2 photoreduction performance is a key challenge in the energy conversion field. Herein, a cooperative role involving an ultrathin 2D structure and surface defects is employed to design defective Bi2MoO6 ultrathin nanosheets, to boost the CO2 photoreduction activity under water with no sacrificial agent, co-catalyst or extra photosensitizer. Bi2MoO6 ultrathin nanosheets with surface “Bi O″ vacancy pairs are grown via a template-directed strategy, as proved by STEM-ADF and positron annihilation spectroscopy. The engineered “Bi O″ vacancy pairs tune the local atomic structure, electronic structure of Bi2MoO6 and serve as charge separation centers to boost the electron-hole separation. Meanwhile, the defective ultrathin structure favors the CO2 adsorption, activation and CO desorption processes. With the merits of atomically-thin configuration and surface defects, the defective Bi2MoO6 ultrathin nanosheets display 2.55 times improved CO formation rate than their bulk counterpart under light irradiation.

Structural defects in two-dimensional transition-metal dichalcogenides can significantly modify the material properties. Previous studies have shown that chalcogen defects can be created by physical sputtering, but the energetic ions can potentially displace transition-metal atoms at the same time, leading to ambiguous results and in some cases, degradation of material quality. In this work, selective sputtering of S atoms in monolayer MoS2 without damaging the Mo sublattice is demonstrated with low-energy helium plasma treatment. Based on X-ray photoelectron spectroscopy analysis, wide-range tuning of S defect concentration is achieved by controlling the ion energy and sputtering time. Furthermore, characterization with scanning transmission electron microscopy confirms that by keeping the ion energy low, the Mo sublattice remains intact. The properties of MoS2 at different defect concentrations are also characterized. In situ device measurement shows that the flake can be tuned from a semiconducting to metallic-like behavior by introducing S defects due to the creation of mid-gap states. When the defective MoS2 is exposed to air, the S defects are soon passivated, with oxygen atoms filling the defect sites.

Proton exchange membrane fuel cell (PEMFC) is one of the most promising candidates for electrochemical energy conversion, but the requirement of expensive Pt-based catalysts on the cathodes prevent...

Development of highly efficient and durable hydrogen evolution reaction (HER) electrocatalysts has a direct impact on water splitting efficiency and cost-effectiveness. In this work, N-doped CoP2 is successfully synthesized for efficient HER in an alkaline electrolyte, which needs an overpotential of only 64 mV to drive a current density of 10 mA cm-2, with a small Tafel slope of 47.4 mV dec-1 and excellent stability for 15 h without any performance loss in 1 M KOH. This represents one of the best HER catalysts in the alkaline electrolyte so far. The successful doping of N into CoP2 is confirmed using X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and scanning transmission electron microscopy characterizations. It is revealed by first-principle calculations that the partial replacement of P with N not only facilitates electron transfer but also optimizes the Gibbs free energies of H*, H2O, and OH* adsorption on the P active sites, thus facilitating the HER process. This work highlights that anion modification of transition-metal phosphides would be an effective and feasible method to enhance their HER activities and provide new insights for the design of novel HER electrocatalysts.

We report the synthesis of a spectrum of highly crystalline molybdenum sulphide/oxide-based materials such as MoS2 (2-H), MoO2 and unique hybrid MoS2/MoO2 nanostructures through chemical vapour deposition (CVD). The as-fabricated samples were scrutinized with materials characterization techniques. Our results reveal that, by tuning growth parameters, different samples demonstrate unique morphologies, which can be unequivocally tied to their chemical composition. We also explore their potential application as electrochemical catalysts, which we found that in addition to using large specific surface area and conductive substrate to improve interlayer conductivity,chemical heterogeneity and efficient charge transport are also essential for good catalytic activity.

Fluorescent quantum rods (QRs) have received much attention recently because of their properties of polarized light emission with narrow full width at half-maximum. Therefore, QRs can be used in liquid crystal display (LCD) backlight, as an active polarizer. In order to emit light with a high degree of polarization (DOP), QRs need to be controlled and aligned along one direction on large-scaled devices. However, most of the proposed effective alignment approaches cannot be applied to a large area. In this paper, we report for the first time the contact ink-jet printing (CIJP) method to align CdSe/CdS core/shell QRs. With the ink being printed, the QRs can align along the fluid ink flow direction resulting in aligned QRs along printing direction and forming a film with a high DOP of 42%. Furthermore, CIJP can realize arbitrarily large area fabrication without reducing DOP value. It is expected that this method can be utilized to print on large-scaled substrates directly to fabricate polarized optoelectronic devices, and further applied to LCD backlights and any other applications where polarized light is needed.

Efficient photocatalysis is important for sustainable energy. Recently, an unconventional photocatalyst based on intrinsic plasmon, called intrinsic plasmonic photocatalyst (IPP), seems promising for higher efficiency in hydrogen evolution. This catalyst seems to benefit from the advantages of visible light absorption, plasmon-assisted hot carrier generation, and good catalytic stability over conventional semiconductor photocatalysts. In this work, we report the relative hydrogen evolution efficiency under visible light irradiation of a family of IPP based on alkaline earth niobates (MNbO3, where M = Ca, Sr, or Ba), with efficiency of CaNbO3 > SrNbO3 > BaNbO3. The contributions of electron–phonon coupling time constant and solar energy absorption to the hydrogen evolution efficiency are identified as key based on our comprehensive study and characterization of carrier density (1022 cm–3), plasmon absorption, carrier dynamics, and surface area. This study demonstrates a generic approach to create a family of IPPs and further validates the role of solar energy absorption by intrinsic plasmon resonance in the enhancement of photocatalytic efficiency. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version The NUS researchers acknowledge Singapore National Research Foundation under its Competitive Research Funding ’Oxide Electronics on silicon beyond Moore’ (NRF-CRP15-2015-01). X.R.W. acknowledges the support from the Nanyang Assistant Professorship grant from Nanyang Technological University, Academic Research Fund Tier 1 (RG108/17 and RG177/18) from Singapore Ministry of Education. P.Y. is supported by SSLS via NUS Core Support (C-380-003-003-001). The authors thank the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the research. The laboratory is a National Research Infrastructure under the National Research Foundation (NRF) Singapore. The authors also thank for the support from Dr. Zhongxin Chen for the photocatalytic reaction measurements. The computational work for this article was partially performed on resources of the National Supercomputing Centre, Singapore (https://www.nscc.sg).

Supported platinum (Pt) catalysts present excellent chemoselectivity for a variety of hydrogenation reactions. Minimizing the use of such expensive catalysts while retaining their catalytic activities has been one of the most challenging topics in recent years. The construction of well dispersed, stable Pt single atom catalysts both increases the density of catalytically active sites while reducing Pt usage. Here we report the synthesis of Pt single atoms trapped in carbon onion graphitic shells by arc-discharge in aqueous solution. Such graphitic shells not only limit the chemical/thermal coarsening of Pt single atoms, but also enhance the conductivity for accelerating the penetration and exchange of ions and electrons during the catalytic process. Experimental results demonstrate that the Pt single atom catalysts show superior performance and no detectable loss of activity and selectivity over 10 cycles (both remain >99%) for the hydrogenation of p-chloronitrobenzene. This novel atomically dispersed form of Pt catalyst is indicative of a new avenue towards efficient synthesis of single atom catalysts with improved performance.

Two-dimensional (2D) heterojunction held together by van der Waals or covalent interactions give high flexibility in modifying their electrocatalytic activity and ion storage performance, since the surface work function and electronic states are dependent on the topmost layer, also the overall heterostructure. The MoSe2/MoS2 heterostructure based on 2D MoSe2 thin-flake and MoS2 nanosheet was designed through heterojunction engineering and prepared by epitaxial growth process. The abundant interfaces in the MoSe2/MoS2 heterostructure not only enable more exposed active sites for electrochemical reaction, but also facilitate the charge transport due to the open porous space within the interlaced nanosheet arrays, arising from with the synergistic effect by the combination of MoS2 and MoSe2. As expected, the MoSe2/MoS2 heterostructure exhibits excellent hydrogen evolution property with a small Tafel slope of 61 mV dec−1, lower overpotential of 162 mV at 10 mA cm−2, and long-term stability. It also delivers a much boosted supercapacitance performance with a high specific capacitance of 1229.6F g−1 at 1 A g−1 and 92.8% capacitance retention after 2000 cycles. The asymmetric supercapacitor made of the MoSe2/MoS2//nitrogen-doped carbon shows a stable potential window of 1.8 V.