Abstract
Defect engineering is a strategy that is attracting widespread attention for the possibility of modifying battery active materials in order to improve the cycling stability of the ...electrodes. However, accurate investigation and quantification of the effect of the defects on the electrochemical energy storage performance of the cell are not trivial tasks. Herein, we report the quantification of vanadium-defective clusters (i.e., up to 5.7%) in the V
2
O
3
lattice via neutron and X-ray powder diffraction measurements, positron annihilation lifetime spectroscopy, and synchrotron-based X-ray analysis. When the vanadium-defective V
2
O
3
is employed as cathode active material in an aqueous Zn coin cell configuration, capacity retention of about 81% after 30,000 cycles at 5 A g
−1
is achieved. Density functional theory calculations indicate that the vanadium-defective clusters can provide favorable sites for reversible Zn-ion storage. Moreover, the vanadium-defective clusters allow the storage of Zn ions in V
2
O
3
, which reduces the electrostatic interaction between the host material and the multivalent ions.
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.
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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 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.
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According to Yang Shao-Horn’s 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.
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The lack of high-performance and substantial supply of anion-exchange membranes is a major obstacle to future deployment of relevant electrochemical energy devices. Here, we select two isomers ...(m-terphenyl and p-terphenyl) and balance their ratio to prepare anion-exchange membranes with well-connected and uniformly-distributed ultramicropores based on robust chemical structures. The anion-exchange membranes display high ion-conducting, excellent barrier properties, and stability exceeding 8000 h at 80 °C in alkali. The assembled anion-exchange membranes present a desirable combination of performance and durability in several electrochemical energy storage devices: neutral aqueous organic redox flow batteries (energy efficiency of 77.2% at 100 mA cm
, with negligible permeation of redox-active molecules over 1100 h), water electrolysis (current density of 5.4 A cm
at 1.8 V, 90 °C, with durability over 3000 h), and fuel cells (power density of 1.61 W cm
under a catalyst loading of 0.2 mg cm
, with open-circuit voltage durability test over 1000 h). As a demonstration of upscaled production, the anion-exchange membranes achieve roll-to-roll manufacturing with a width greater than 1000 mm.
The structure and activity in CO oxidation of Ag/CeO2 employing CeO2 nanorods and nanocubes as the support have been studied. The experimental results reveal the shape-dependent interplay between ...oxygen vacancies and Ag-CeO2 interaction and their influence on the catalytic activity. Silver supported on CeO2 nanocubes with a loading of 1% shows the highest specific catalytic activity. Display omitted
► The interplay between oxygen vacancies and Ag–CeO2 interaction depends on the shape of CeO2. ► Such an interplay controls the structures and catalytic activity of Ag/CeO2 catalysts. ► CeO2 nanorods stabilize cationic Ag species. ► CeO2 nanocubes facilitate the formation of metallic Ag nanoparticles. ► The Ag nanoparticle–CeO2 interface is the active structure in catalyzing CO oxidation.
Ag/CeO2 catalysts employing CeO2 nanocubes (c-CeO2) and nanorods (r-CeO2) as the support were prepared by conventional incipient wetness impregnation followed by calcination at 500°C in air. Their structures have been characterized in detail and their catalytic activities in CO oxidation have also been tested. c-CeO2 and r-CeO2 nanocrystals exhibit different concentrations and structures of oxygen vacancies. The silver-r-CeO2 interaction is stronger than the silver-c-CeO2 interaction. Fine Ag nanoparticles form in 1%-Ag/c-CeO2 and grow in size in 3%-Ag/c-CeO2; however, positively charged Agn+ clusters dominate in 1%-Ag/r-CeO2, and fine Ag nanoparticles dominate in 3%-Ag/r-CeO2. Supported Ag nanoparticles are much more capable of creating oxygen vacancies in CeO2 than supported positively charged Agn+ clusters. More oxygen vacancies form in Ag/c-CeO2 than in Ag/r-CeO2. The average charge density of oxygen vacancies and the ratio between large oxygen vacancy clusters and small vacancies in CeO2 nanocrystals are enhanced when loaded with positively charged Agn+ clusters but reduced when loaded with Ag nanoparticles. Ag nanoparticles greatly promote the reduction and catalytic activity in CO oxidation of CeO2 nanocrystals but positively charged Agn+ clusters do not. These results demonstrate the concept that the interplay between oxygen vacancies and Ag–CeO2 interaction controls the structures of silver and CeO2 in Ag/CeO2 catalysts and thus their surface reactivity and catalytic activity, deepening the fundamental understanding of metal/CeO2 catalysts. These results also reveal that the interplay between oxygen vacancies and Ag–CeO2 interaction in Ag/CeO2 catalysts depends on the shape of CeO2 support, opening up a new strategy for the design of efficient and economic metal/CeO2 catalysts by engineering the shape of CeO2 support.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In polycrystalline bismuth telluride-based thermoelectric materials, mechanical-deformation-induced donor-like effects can introduce a high concentration of electrons to change the thermoelectric ...properties through the evolution of intrinsic point defects. However, the evolution law of these point defects during sample preparation remains elusive. Herein, we systematically investigate the evolution of intrinsic point defects in n-type Bi2Te3-based materials from the perspective of thermodynamics and kinetics, in combination with positron annihilation measurement. It is found that not only the mechanical deformation but also the sintering temperature is vital to the donor-like effect. The mechanical deformation can promote the formation of cation vacancies and facilitate the donor-like effect, and the sintering process can provide excess energy for Bi antisite atoms to surmount the diffusion potential barrier. This work provides us a better understanding of the evolution law of intrinsic point defects in Bi2Te3-based alloys and guides us to control the carrier concentration by manipulating intrinsic point defects.
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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, AgBiSe2 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.
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Poorly-crystalline ferrihydrite is ubiquitous in the environment and contributes to controlling the fate of arsenic in sediments and soils. Although there is evidence that ferrihydrite has a higher ...affinity towards As(III) relative to As(V), little is known about how and why As(III) is readily immobilized by ferrihydrite. In this study, ferrihydrite was employed to evaluate the As(III) and As(V) adsorption behavior. The properties of ferrihydrite such as morphology, pore size distribution, arsenic adsorption species, and adsorption energies of arsenic at different sites were carefully examined using TEM-EDS mapping, positron annihilation lifetime (PAL) spectroscopy, X-ray absorption spectroscopy (XAS), N2 adsorption isotherms, and theoretical calculations of density function theory (DFT). Batch adsorption experiments revealed that the maximum adsorption of As(III) (839.7 μmol g−1) on ferrihydrite, fitted by Langmuir model, was considerably larger than that of As(V) (372.4 μmol g−1). PAL characterization and pore size distribution analysis demonstrated that ferrihydrite had an abundance of vacancy cluster-like micropores, which consisted of 10–20 atom deficiencies (V10-20). The calculated hydrated ion size of As(III) species (4.22 Å) at pH 6.0 was smaller than the size of As(V) (5.90 Å). The higher As(III) adsorption to ferrihydrite was attributed to its surface adsorption sites as well as its abundant micropore adsorption sites, which are available only to As(III) due to their size matching well with the size of the ferrihydrite micropores, thus significantly contributing to greater As(III) immobilization.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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.
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