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The manipulation of nano-objects through heating is an effective strategy for inducing structural modifications and therefore changing the optoelectronic properties of semiconducting materials. Despite its potential, the underlying mechanism of the structural transformations remains elusive, largely due to the challenges associated with their in situ observations. To address these issues, we synthesize temperature-sensitive CsPbBr3perovskite nanoplatelets and investigate their structural evolution at the nanoscale using in situ heating transmission electron microscopy. We observe the morphological changes that start from the self-assembly of the nanoplatelets into ribbons on a substrate. We identify several paths of merging nanoplates within ribbons that ultimately lead to the formation of nanosheets dispersed randomly on the substrate. These observations are supported by molecular dynamics simulations. We correlate the various paths for merging to the random orientation of the initial ribbons along with the ligand mobility (especially from the edges of the nanoplatelets). This leads to the preferential growth of individual nanosheets and the merging of neighboring ones. These processes enable the creation of structures with tunable emission, ranging from blue to green, all from a single material. Our real-time observations of the transformation of perovskite 2D nanocrystals reveal a route to achieve large-area nanosheets by controlling the initial orientation of the self-assembled objects with potential for large-scale applications.

The engineering of the structural and morphological properties of nanomaterials is a fundamental aspect to attain desired performance in energy storage/conversion systems and multifunctional composites. We report the synthesis of room temperature-stable metallic rutile VO2(VO2(R)) nanosheets by topochemically transforming liquid-phase exfoliated VSe2in a reductive Ar–H2atmosphere. The as-produced VO2(R) represents an example of two-dimensional (2D) nonlayered materials, whose bulk counterparts do not have a layered structure composed by layers held together by van der Waals force or electrostatic forces between charged layers and counterbalancing ions amid them. By pretreating the VSe2nanosheets by O2plasma, the resulting 2D VO2(R) nanosheets exhibit a porous morphology that increases the material specific surface area while introducing defective sites. The as-synthesized porous (holey)-VO2(R) nanosheets are investigated as metallic catalysts for the water splitting reactions in both acidic and alkaline media, reaching a maximum mass activity of 972.3 A g–1at −0.300 V vs RHE for the hydrogen evolution reaction (HER) in 0.5 M H2SO4(faradaic efficiency = 100%, overpotential for the HER at 10 mA cm–2= 0.184 V) and a mass activity (calculated for a non 100% faradaic efficiency) of 745.9 A g–1at +1.580 V vs RHE for the oxygen evolution reaction (OER) in 1 M KOH (overpotential for the OER at 10 mA cm–2= 0.209 V). By demonstrating proof-of-concept electrolyzers, our results show the possibility to synthesize special material phases through topochemical conversion of 2D materials for advanced energy-related applications.

The design of cost-effective and efficient electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is pivotal for the molecular hydrogen (H2) production from electrochemical water splitting as a future energy source. Herein, we show that the hybridization between multiple HER- and OER-active components is effective for the design and realization of bifunctional electrocatalysts for universal water splitting, i.e., in both acidic and alkaline media. Our strategy relies on the production and characterization of MoSe2holey flake:Mo2C ball hybrids supported by single-walled carbon nanotube (SWCNT) electrocatalysts. Flakes of MoSe2are produced through hydrogen peroxide (H2O2)-aided liquid phase exfoliation (LPE), which promotes both the exfoliation of the materials and the formation of nanopores in the flakes viachemical etching. The amount of H2O2in the solvent used for the exfoliation process is optimized to obtain ideal high ratio between edge and basal sites ratio, i.e., high-number of electrocatalytic sites. The hybridization of MoSe2flakes with commercial ball-like shaped Mo2C crystals facilitates the Volmer reaction, which works in both acidic and alkaline media. In addition, the electrochemical coupling between SWCNTs (as support) and MoSe2:Mo2C hybrids synergistically enhances both HER- and OER-activity of the native components, reaching high η10in acidic and alkaline media (0.049 and 0.089 V for HER in 0.5 M H2SO4and 1 M KOH, respectively; 0.197 and 0.241 V for OER in 0.5 M H2SO4and 1 M KOH, respectively). The exploitation of the synergistic effects occurring between multicomponent electrocatalysts, coupled with the production of the electrocatalysts themselves through scalable and cost-effective solution-processed manufacturing techniques, is promising to scale-up the production of H2viaefficient water splitting for the future energy portfolio.

The development of large-scale production methods of two-dimensional (2D) crystals, with on-demand control of the area and thickness, is mandatory to fulfill the potential applications of such materials for photovoltaics. Inverted bulk heterojunction (BHJ) organic solar cell (OSC), which exploits a polymer–fullerene binary blend as the active material, is one potentially important application area for 2D crystals. A large ongoing effort is indeed currently devoted to the introduction of 2D crystals in the binary blend to improve the charge transport properties. While it is expected that the nanoscale domains size of the different components of the blend will significantly impact the performance of the OSC, to date, there is no evidence of quantitative information on the interplay between 2D crystals and fullerene domains size. Here, we demonstrate that by matching the size of WSe2few-layer 2D crystals, produced by liquid-phase exfoliation, with that of the PC71BM fullerene domain in BHJ OSCs, we obtain power conversion efficiencies (PCEs) of ∼9.3%, reaching a 15% improvement with respect to standard binary devices (PCE = 8.10%), i.e., without the addition of WSe2flakes. This is the highest ever reported PCE for 2D material-based OSCs, obtained thanks to the enhanced exciton generation and exciton dissociation at the WSe2-fullerene interface and also electron extraction to the back metal contact as a consequence of a balanced charge carriers mobility. These results push forward the implementation of transition-metal dichalcogenides to boost the performance of BHJ OSCs.

An increasing number of studies have recently reported the rapid degradation of hybrid and all-inorganic lead halide perovskite nanocrystals under electron beam irradiation in the transmission electron microscope, with the formation of nanometer size, high contrast particles. The nature of these nanoparticles and the involved transformations in the perovskite nanocrystals are still a matter of debate. Herein, we have studied the effects of high energy (80/200 keV) electron irradiation on colloidal cesium lead bromide (CsPbBr3) nanocrystals with different shapes and sizes, especially 3 nm thick nanosheets, a morphology that facilitated the analysis of the various ongoing processes. Our results show that the CsPbBr3nanocrystals undergo a radiolysis process, with electron stimulated desorption of a fraction of bromine atoms and the reduction of a fraction of Pb2+ions to Pb0. Subsequently Pb0atoms diffuse and aggregate, giving rise to the high contrast particles, as previously reported by various groups. The diffusion is facilitated by both high temperature and electron beam irradiation. The early stage Pb nanoparticles are epitaxially bound to the parent CsPbBr3lattice, and evolve into nonepitaxially bound Pb crystals upon further irradiation, leading to local amorphization and consequent dismantling of the CsPbBr3lattice. The comparison among CsPbBr3nanocrystals with various shapes and sizes evidences that the damage is particularly pronounced at the corners and edges of the surface, due to a lower diffusion barrier for Pb0on the surface than inside the crystal and the presence of a larger fraction of under-coordinated atoms.

We describe two synthesis approaches to colloidal Cu3P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu3P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu3P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu3P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu–Cu3P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu–Cu3P eutectic of the Cu–P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu–Cu3P particles do not display an electrochemical behavior that would make them suitable for applications in batteries.