Bangjiao Ye

Vacancy Associates Promoting Solar-Driven Photocatalytic Activity of Ultrathin Bismuth Oxychloride Nanosheets

Crystal facet engineering of semiconductors is of growing interest and an important strategy for fine-tuning solar-driven photocatalytic activity. However, the primary factor in the exposed active facets that determines the photocatalytic property is still elusive. Herein, we have experimentally achieved high solar photocatalytic activity in ultrathin BiOCl nanosheets with almost fully exposed active {001} facets and provide some new and deep-seated insights into how the defects in the exposed active facets affect the solar-driven photocatalytic property. As the thickness of the nanosheets reduces to atomic scale, the predominant defects change from isolated defects V(Bi)‴ to triple vacancy associates V(Bi)‴V(O)••V(Bi)‴, which is unambiguously confirmed by the positron annihilation spectra. By virtue of the synergic advantages of enhanced adsorption capability, effective separation of electron–hole pairs and more reductive photoexcited electrons benefited from the V(Bi)‴V(O)••V(Bi)‴ vacancy associates, the ultrathin BiOCl nanosheets show significantly promoted solar-driven photocatalytic activity, even with extremely low photocatalyst loading. The finding of the existence of distinct defects (different from those in bulks) in ultrathin nanosheets undoubtedly leads to new possibilities for photocatalyst design using quasi-two-dimensional materials with high solar-driven photocatalytic activity.

Low Overpotential in Vacancy-Rich Ultrathin CoSe2 Nanosheets for Water Oxidation

According to Yang Shao-Horns principle, CoSe2 is a promising candidate as an efficient, affordable, and sustainable alternative electrocatalyst for the oxygen evolution reaction, owing to its well-suited electronic configuration of Co ions. However, the catalytic efficiency of pure CoSe2 is still far below what is expected, because of its poor active site exposure yield. Herein, we successfully overcome the disadvantage of insufficient active sites in bulk CoSe2 by reducing its thickness into the atomic scale rather than any additional modification (such as doping or hybridizing with graphene or noble metals). The positron annihilation spectrometry and XAFS spectra provide clear evidence that a large number of VCo″ vacancies formed in the ultrathin nanosheets. The first-principles calculations reveal that these VCo″ vacancies can serve as active sites to efficiently catalyze the oxygen evolution reaction, manifesting an OER overpotential as low as 0.32 V at 10 mA cm(-2) in pH 13 medium, which is superior to the values for its bulk counterparts as well as those for the most reported Co-based electrocatalysts. Considering the outstanding performance of the simple, unmodified ultrathin CoSe2 nanosheets as the only catalyst, further improvement of the catalytic activity is expected when various strategies of doping or hybridizing are used. These results not only demonstrate the potential of a notable, affordable, and earth-abundant water oxidation electrocatalyst based on ultrathin CoSe2 nanosheets but also open up a promising avenue into the exploration of excellent active and durable catalysts toward replacing noble metals for oxygen electrocatalysis.

High Thermoelectric and Reversible p-n-p Conduction Type Switching Integrated in Dimetal Chalcogenide

The subject of the involved phase transition in solid materials has formed not only the basis of materials technology but also the central issue of solid-state chemistry for centuries. The ability to design and control the required changes in physical properties within phase transition becomes key prerequisite for the modern functionalized materials. Herein, we have experimentally achieved the high thermoelectric performance (ZT value reaches 1.5 at 700 K) and reversible p-n-p semiconducting switching integrated in a dimetal chalcogenide, AgBiSe(2) during the continuous hexagonal-rhombohedral-cubic phase transition. The clear-cut evidences in temperature-dependent positron annihilation and Raman spectra confirmed that the p-n-p switching is derived from the bimetal atoms exchange within phase transition, whereas the full disordering of bimetal atoms after the bimetal exchange results in the high thermoelectric performance. The combination of p-n-p switching and high thermoelectric performance enables the dimetal chalcogenides perfect candidates for novel multifunctional electronic devices. The discovery of bimetal atoms exchange during the phase transition brings novel phenomena with unusual properties which definitely enrich solid-state chemistry and materials science.

Vacancy associates-rich ultrathin nanosheets for high performance and flexible nonvolatile memory device.

On the road of innovation in modern information technology, resistive switching random access memory (RRAM) has been considered to be the best potential candidate to replace the conventional Si-based technologies. In fact, the key prerequisite of high storage density and low power consumption as well as flexibility for the tangible next generation of nonvolatile memories has stimulated extensive research into RRAM. Herein, we highlight an inorganic graphene analogue, ultrathin WO3·H2O nanosheets with only 2-3 nm thickness, as a promising material to construct a high performance and flexible RRAM device. The abundant vacancy associates in the ultrathin nanosheets, revealed by the positron annihilation spectra, act not only carrier reservoir to provide carriers but also capture center to trap the actived Cu(2+) for the formation of conductive filaments, which synergistically realize the resistive switching memory with low operating voltage (+1.0 V/-1.14 V) and large resistance ON/OFF ratio (>10(5)). This ultrathin-nanosheets-based RRAM device also shows long retention time (>10(5) s), good endurance (>5000 cycles), and excellent flexibility. The finding of the existence of distinct defects in ultrathin nanosheets undoubtedly leads to an atomic level deep understanding of the underlying nature of the resistive switching behavior, which may serve as a guide to improve the performances and promote the rapid development of RRAM.

Dual Vacancies: An Effective Strategy Realizing Synergistic Optimization of Thermoelectric Property in BiCuSeO

Vacancy is a very important class of phonon scattering center to reduce thermal conductivity for the development of high efficient thermoelectric materials. However, conventional monovacancy may also act as an electron or hole acceptor, thereby modifying the electrical transport properties and even worsening the thermoelectric performance. This issue urges us to create new types of vacancies that scatter phonons effectively while not deteriorating the electrical transport. Herein, taking BiCuSeO as an example, we first reported the successful synergistic optimization of electrical and thermal parameters through Bi/Cu dual vacancies. As expected, as compared to its pristine and monovacancy samples, these dual vacancies further increase the phonon scattering, which results in an ultra low thermal conductivity of 0.37 W m(-1) K(-1) at 750 K. Most importantly, the clear-cut evidence in positron annihilation unambiguously confirms the interlayer charge transfer between these Bi/Cu dual vacancies, which results in the significant increase of electrical conductivity with relatively high Seebeck coefficient. As a result, BiCuSeO with Bi/Cu dual vacancies shows a high ZT value of 0.84 at 750 K, which is superior to that of its native sample and monovacancies-dominant counterparts. These findings undoubtedly elucidate a new strategy and direction for rational design of high performance thermoelectric materials.

Highly Efficient and Exceptionally Durable CO2 Photoreduction to Methanol over Freestanding Defective Single-Unit-Cell Bismuth Vanadate Layers

Unearthing an ideal model for disclosing the role of defect sites in solar CO2 reduction remains a great challenge. Here, freestanding gram-scale single-unit-cell o-BiVO4 layers are successfully synthesized for the first time. Positron annihilation spectrometry and X-ray fluorescence unveil their distinct vanadium vacancy concentrations. Density functional calculations reveal that the introduction of vanadium vacancies brings a new defect level and higher hole concentration near Fermi level, resulting in increased photoabsorption and superior electronic conductivity. The higher surface photovoltage intensity of single-unit-cell o-BiVO4 layers with rich vanadium vacancies ensures their higher carriers separation efficiency, further confirmed by the increased carriers lifetime from 74.5 to 143.6 ns revealed by time-resolved fluorescence emission decay spectra. As a result, single-unit-cell o-BiVO4 layers with rich vanadium vacancies exhibit a high methanol formation rate up to 398.3 μmol g-1 h-1 and an apparent quantum efficiency of 5.96% at 350 nm, much larger than that of single-unit-cell o-BiVO4 layers with poor vanadium vacancies, and also the formers catalytic activity proceeds without deactivation even after 96 h. This highly efficient and spectrally stable CO2 photoconversion performances hold great promise for practical implementation of solar fuel production.

Implementation of a quantum algorithm to solve the Bernstein-Vazirani parity problem without entanglement on an ensemble quantum computer

Bernstein and Varizani have given the first quantum algorithm to solve parity problem in which a strong violation of the classical imformation theoritic bound comes about. In this paper, we refine this algorithm with fewer resource and implement a two qubits algorithm in a single query on an ensemble quantum computer for the first time.

Electric-Field-Driven Dual Vacancies Evolution in Ultrathin Nanosheets Realizing Reversible Semiconductor to Half-Metal Transition

Fabricating a flexible room-temperature ferromagnetic resistive-switching random access memory (RRAM) device is of fundamental importance to integrate nonvolatile memory and spintronics both in theory and practice for modern information technology and has the potential to bring about revolutionary new foldable information-storage devices. Here, we show that a relatively low operating voltage (+1.4 V/-1.5 V, the corresponding electric field is around 20,000 V/cm) drives the dual vacancies evolution in ultrathin SnO2 nanosheets at room temperature, which causes the reversible transition between semiconductor and half-metal, accompanyied by an abrupt conductivity change up to 10(3) times, exhibiting room-temperature ferromagnetism in two resistance states. Positron annihilation spectroscopy and electron spin resonance results show that the Sn/O dual vacancies in the ultrathin SnO2 nanosheets evolve to isolated Sn vacancy under electric field, accounting for the switching behavior of SnO2 ultrathin nanosheets; on the other hand, the different defect types correspond to different conduction natures, realizing the transition between semiconductor and half-metal. Our result represents a crucial step to create new a information-storage device realizing the reversible transition between semiconductor and half-metal with flexibility and room-temperature ferromagnetism at low energy consumption. The as-obtained half-metal in the low-resistance state broadens the application of the device in spintronics and the semiconductor to half-metal transition on the basis of defects evolution and also opens up a new avenue for exploring random access memory mechanisms and finding new half-metals for spintronics.

Highly Efficient Photothermal Effect by Atomic-Thickness Confinement in Two-Dimensional ZrNCl Nanosheets

We report a giant photothermal effect arising from quantum confinement in two-dimensional nanomaterials. ZrNCl ultrathin nanosheets with less than four monolayers of graphene-like nanomaterial successfully generated synergetic effects of larger relaxation energy of photon-generated electrons and intensified vibration of surface bonds, offering predominantly an enhancement of the electron-phonon interaction to a maximized extent. As a result, they could generate heat flow reaching an ultrahigh value of 5.25 W/g under UV illumination with conversion efficiency up to 72%. We anticipate that enhanced electron-phonon coupling in a quantum confinement system will be a powerful tool for optimizing photothermal conversion of inorganic semiconductors.

Room-temperature ferromagnetism in Cu-implanted 6H-SiC single crystal

200 keV Cu+ ions were implanted into 6H-SiC single crystal at room temperature with fluence of 8 × 1015 cm−2. No ferromagnetism (FM)-related secondary phase was found by the results of high-resolution x-ray diffraction and x-ray photoelectron spectroscopy. Positron annihilation lifetime spectroscopy results indicated that the main defect type was silicon vacancy and the concentration of it increased after Cu implantation. The room-temperature ferromagnetism was detected by superconducting quantum interference device. First-principles calculations revealed that the magnetic moments mainly come from the 2p orbitals of C atoms and 3d orbitals of Cu dopant. The origin of the FM has been discussed in detail.