The generation of hydrogen from water using sunlight could potentially form the basis of a clean and renewable source of energy. Various water-splitting methods have been investigated previously, but the use of photocatalysts to split water into stoichiometric amounts of H2 and O2 (overall water splitting) without the use of external bias or sacrificial reagents is of particular interest because of its simplicity and potential low cost of operation. However, despite progress in the past decade, semiconductor water-splitting photocatalysts (such as (Ga1-xZnx)(N1-xOx)) do not exhibit good activity beyond 440 nm (refs 1,2,9) and water-splitting devices that can harvest visible light typically have a low solar-to-hydrogen efficiency of around 0.1%. Here we show that cobalt(II) oxide (CoO) nanoparticles can carry out overall water splitting with a solar-to-hydrogen efficiency of around 5%. The photocatalysts were synthesized from non-active CoO micropowders using two distinct methods (femtosecond laser ablation and mechanical ball milling), and the CoO nanoparticles that result can decompose pure water under visible-light irradiation without any co-catalysts or sacrificial reagents. Using electrochemical impedance spectroscopy, we show that the high photocatalytic activity of the nanoparticles arises from a significant shift in the position of the band edge of the material.

Mg rechargeable batteries (MgRBs) represent a safe and high-energy battery technology but suffer from the lack of suitable cathode materials due to the slow solid-state diffusion of the highly polarizing divalent Mg ion. Previous methods improve performance at the cost of incompatibility with anode/electrolyte and drastic decrease in volumetric energy density. Herein we report interlayer expansion as a general and effective atomic-level lattice engineering approach to transform inactive intercalation hosts into efficient Mg storage materials without introducing adverse side effects. As a proof-of-concept we have combined theory, synthesis, electrochemical measurement, and kinetic analysis to improve Mg diffusion behavior in MoS2, which is a poor Mg transporting material in its pristine form. First-principles simulations suggest that expanded interlayer spacing allows for fast Mg diffusion because of weakened Mg-host interactions. Experimentally, the expansion was realized by inserting a controlled amount of poly(ethylene oxide) into the lattice of MoS2 to increase the interlayer distance from 0.62 nm to up to 1.45 nm. The expansion boosts Mg diffusivity by 2 orders of magnitude, effectively enabling the otherwise barely active MoS2 to approach its theoretical storage capacity as well as to achieve one of the highest rate capabilities among Mg-intercalation materials. The interlayer expansion approach can be leveraged to a wide range of host materials for the storage of various ions, leading to novel intercalation chemistry and opening up new opportunities for the development of advanced materials for next-generation energy storage.

The larger ionic radius of Na ion (1.06 Å) compared with that of Li ion (0.76 Å) is a fundamental reason for the inferior diffusion kinetics of Na ion in intercalation hosts. Here we report interlayer expansion of intercalation hosts as a general strategy to facilitate the solid-state diffusion of Na ions. Based on this strategy, poly(ethylene oxide)-intercalated MoS 2 composites (PEO-MoS 2) were synthesized via a facile exfoliation-restacking method and tested as anode materials for Na-ion batteries (NIBs). The interlayer spacing of MoS 2 was increased from 0.615 nm to 1.45 nm by insertion of controlled amounts of PEO. The bilayer PEO-intercalated MoS 2 composite (PEO 2L-MoS 2) exhibits a specific capacity of 225 mAh g −1 under a current density of 50 mA g −1 , twice as high as that of commercial MoS 2 (com-MoS 2), and shows improved rate performance due to enhanced Na-ion diffusivity. The improvement in the electrochemical

We report the development of an efficient and earth-abundant catalyst for electrochemical overall water splitting. Trimetallic NiFeMo alloy is synthesized by hydrothermal deposition from inorganic precursors and subsequent low-temperature thermal annealing. A complete cell made of NiFeMo electrodes on nickel foam exhibits a low voltage of 1.45 V at 10 mA/cm 2 as a result of low overpotentials for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). High-resolution transmission electron microscopy reveals that nanometer-sized single-crystal domains of Ni, Fe, and Mo are intimately integrated at the atomic level, which enables a synergistic effect of metallic Ni, Fe, and Mo for efficient HER, while selfformed Ni−Fe−Mo (oxy)hydroxides on the surface of the NiFeMo anode become active sites for OER. Such a multimetallic alloy and its (oxy)hydroxides represent a typical HER/OER catalyst couple, and our method provides a new route to develop efficient low-cost metallic alloys for overall water splitting.

We report the synthesis and systematic Raman study of twisted bilayer graphene (tBLG) with rotation angles from below 10° to nearly 30°. Chemical vapor deposition was used to grow hexagon-shaped tBLG with a rotation angle that can be conveniently determined by relative edge misalignment. Rotation dependent G-line resonances and folded phonons were observed by selecting suitable energies of excitation lasers. The observed phonon frequencies of the tBLG superlattices agree well with our ab initio calculation.

Same-spot Raman-photoluminescence with two lasers in a diamond anvil cell under hydrostatic pressure reveals that CsPbBr 3 nanocrystals, mostly located on the edges of CsPb 2 Br 5 2D platelets, are responsible for CsPb 2 Br 5 's green emission. This sensitive non-invasive technique combining static and dynamic probes establishes a one-toone property-structure relationship and distinguishes light emission from point defects versus nano-inclusions.

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The observation of low-energy edge photoluminescence and its beneficial effect on the solar cell efficiency of Ruddlesden−Popper perovskites has unleashed an intensive research effort to reveal its origin. This effort, however, has been met with more challenges as the underlying material structure has still not been identified; new modelings and observations also do not seem to converge. Using twodimensional (2D) (BA) 2 (MA) 2 Pb 3 Br 10 as an example, we show that threedimensional (3D) MAPbBr 3 is formed due to the loss of BA on the edge. This self-formed MAPbBr 3 can explain the reported edge emission under various conditions, while the reported intriguing optoelectronic properties such as fast exciton trapping from the interior 2D perovskite, rapid exciton dissociation, and long carrier lifetime can be understood via the self-formed 2D/3D lateral perovskite heterostructure. The 3D perovskite is identified by submicron infrared spectroscopy, the emergence of X-ray diffraction (XRD) signature from freezer-milled nanometer-sized 2D perovskite, and its photoluminescence response to external hydrostatic pressure. The revelation of this edge emission mystery and the identification of a self-formed 2D/3D heterostructure provide a new approach to engineering 2D perovskites for high-performance optoelectronic devices.