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

Porous Liquid: A Stable ZIF‑8 Colloid in Ionic Liquid with Permanent Porosity

We reported an example of metal–organic framework (MOF)-based porous liquid by dispersing ZIF-8 ({Zn­(mim)₂}, mim = 2-methylimidazole) nanocrystallites in ionic liquid (IL) of [Bpy]­[NTf₂] (<i>N</i>-butyl pyridinium bis­(trifluoromethyl sulfonyl)­imide). Two essential challenges, stable colloid formation and porosity retention, have been overcome to prepare MOF-based porous liquid. Preventing ZIF-8 nanocrystals from aggregation before dispersing is vital to form a stable ZIF-8 colloid in IL via enhancing the interaction between ZIF-8 and IL. The resultant ZIF-8–[Bpy]­[NTf₂] colloid is able to be stable over months without precipitating. [Bpy]­[NTf₂] with larger ion sizes cannot occupy pores in ZIF-8, leaving the ZIF-8 cage empty for enabling access by guest molecules. The porosity of this porous liquid system was verified by positron (e⁺) annihilation lifetime spectroscopy and I₂ adsorption in ZIF-8 in the colloid. MOF-based porous liquids could provide a new material platform for liquid-bed-based gas separations

Defect-Assisted High Anion Conductivity in Diethyldimethylammonium <i>d</i>‑Camphorsulfonate Plastic Crystal: A Size Effect of Target Ions

We demonstrate anion (chloride and fluoride ions) transfer in an organic ionic plastic crystal (OIPC), diethyldimethylammonium d-camphorsulfonate, and the role of defects in ion conduction. The phase transitions, crystalline structures, dynamics, and ion transfer mechanisms of the pure material and the doped mixtures were investigated using a combination of differential scanning calorimetry, X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and electrochemical impedance spectroscopy. The doped mixtures show minor modifications in thermal behaviors and solid phase structures to the host material. The ion mobility of the pure material in the plastic phase was assigned mainly to cations. The fluoride salt-doped mixture has drastically enhanced conductivity at all tested temperatures, and the chloride salt-doped mixture displays a strong temperature-dependent behavior. Ionic conductivity measurements suggest transfer mechanisms through crystalline phases. The pure material and doped mixtures exhibit similar defect volumes and concentrations, and both factors are phase-dependent, as determined using positron annihilation lifetime spectroscopy. The conductivity displays dependence on not only the defect volume but also the target ion volume. The relationships between the defect volume and the conductivity qualitatively follow the Cohen–Turnbull free volume model, while critical volumes were very large. For the first time in OIPCs, the size of the target ions was found to significantly influence ionic conductivity

Novel Perovskite Solar Cell Architecture Featuring Efficient Light Capture and Ultrafast Carrier Extraction

A new perovskite solar cell (PSC) structure with a functionalized interface between perovskite and a hole transport material has been proposed in this report. The short circuit current density of PSC was notably enhanced with the novel architecture (with an increase of 8.7%), and a power conversion efficiency (PCE) of 16.93% was achieved. With the increased perovskite/hole conductor interface, hysteresis suppression was observed. The advantages of this structure in light-harvesting efficiency, trap density, and carrier separation rate were proved by various characterization and analysis studies. It is noteworthy that a PCE of 14.67% was achieved with poly­(3-hexyl-thiophene), which to our knowledge is the highest performing PSC based on this material

High Thermoelectric and Reversible <i>p‑n‑p</i> 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 <i>p-n-p</i> semiconducting switching integrated in a dimetal chalcogenide, AgBiSe₂ during the continuous hexagonal–rhombohedral–cubic phase transition. The clear-cut evidences in temperature-dependent positron annihilation and Raman spectra confirmed that the <i>p-n-p</i> 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 <i>p-n-p</i> 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

Nickel Vacancies Boost Reconstruction in Nickel Hydroxide Electrocatalyst

Because the reconstruction of catalysts is generally observed during oxidation reactions, understanding the intrinsic structure-related reconstruction ability of electrocatalysts is highly desirable but challenging. Herein, a controllable hydrolysis strategy is developed to obtain nickel hydroxide electrocatalysts with controllable nickel vacancy (V_Ni) concentrations, as confirmed by advanced spectroscopic characterization. Electrochemical measurements show that the reconstruction can be promoted with the increase of V_Ni concentration to generate true active components, thereby boosting activities for both oxygen evolution reaction (OER) and urea oxidation reaction (UOR). Density functional theory calculations confirm that the increased V_Ni concentration yields decreased formation energies of the true active components during reactions. This work provides fundamental understanding of the reconstruction ability of electrocatalysts in anodic oxidation reactions from the view of intrinsic defects

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 <i>o</i>-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 <i>o</i>-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 <i>o</i>-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 <i>o</i>-BiVO₄ layers with poor vanadium vacancies, and also the former’s 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

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

Achieving High Selectivity in Photocatalytic Oxidation of Toluene on Amorphous BiOCl Nanosheets Coupled with TiO₂

The inert C(sp3)–H bond and easy overoxidation of toluene make the selective oxidation of toluene to benzaldehyde a great challenge. Herein, we present that a photocatalyst, constructed with a small amount (1 mol %) of amorphous BiOCl nanosheets assembled on TiO2 (denoted as 0.01BOC/TiO2), shows excellent performance in toluene oxidation to benzaldehyde, with 85% selectivity at 10% conversion, and the benzaldehyde formation rate is up to 1.7 mmol g–1 h–1, which is 5.6 and 3.7 times that of bare TiO2 and BOC, respectively. In addition to the charge separation function of the BOC/TiO2 heterojunction, we found that the amorphous structure of BOC endows its abundant surface oxygen vacancies (Ov), which can further promote the charge separation. Most importantly, the surface Ov of amorphous BOC can efficiently adsorb and activate O2, and amorphous BOC makes the product, benzaldehyde, easily desorb from the catalyst surface, which alleviates the further oxidation of benzaldehyde, and results in the high selectivity. This work highlights the importance of the microstructure based on heterojunctions, which is conducive to the rational design of photocatalysts with high performance in organic synthesis

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 Cu²⁺ 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 (>10⁵). This ultrathin-nanosheets-based RRAM device also shows long retention time (>10⁵ 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