Thermally-induced drift of A-site cations at solid-solid interface in physically paired lead halide perovskites.

The promise of hybrid organic-inorganic halide perovskite solar cells rests on their exceptional power conversion efficiency routinely exceeding 25% in laboratory scale devices. While the migration of halide ions in perovskite thin films has been extensively investigated, the understanding of cation diffusion remains elusive. In this study, a thermal migration of A­site cations at the solid-solid interface, formed by two physically paired MAPbI3 and FAPbI3 perovskite thin films casted on FTO, is demonstrated through continuous annealing at comparably low temperature (100 °C). Diffusion of methylammonium (CH3NH3+, MA+) cations into the low­symmetry yellow δ­FAPbI3 phase triggers a transition from the yellow (δ) to black (α) phase evident in the distinctive color change and verified by shifts in absorption bands and X­ray diffraction patterns. Intermixing of the A­site cations MA+ and FA+ (CH(NH2)2+) occurred for both systems, α­MAPbI3/δ­FAPbI3 and α­MAPbI3/α­FAPbI3. The structural and compositional changes in both cases support a thermally activated ion drift unambiguously demonstrated through changes in the absorption and X-ray photoelectron spectra. Moreover, the physical contact annealing (PCA) leads to healing of defects and pinholes in α­MAPbI3 thin films, which was correlated to longer recombination lifetimes in mixed MAxFA1-xPbI3 thin films obtained after PCA and probed by ultrafast transient absorption spectroscopy.

Piezo-enhanced activation of dinitrogen for room temperature production of ammonia.

The catalytic conversion of nitrogen to ammonia remains an energy-intensive process, demanding advanced concepts for nitrogen fixation. The major obstacle of nitrogen fixation lies in the intrinsically high bond energy (941 kJ mol-1) of the N≡N molecule and the absence of a permanent dipole in N2. This kinetic barrier is addressed in this study by an efficient piezo-enhanced gold catalysis as demonstrated by the room temperature reduction of dinitrogen into ammonia. Au nanostructures were immobilized on thin film piezoelectric support of potassium sodium niobate (K0.5Na0.5NbO3, KNN) by chemical vapor deposition of a new Au(III) precursor [Me2Au(PyTFP)(H2O)]1(PyTFP = (Z)-3,3,3-trifluoro-1-(pyridin-2-yl)-prop-1-en-2-olate) that exhibited high volatility (60 °C, 10-3mbar) and clean decomposition mechanism to produce well adherent elemental gold films on KNN and Ti substrates. The gold-functionalized KNN films served as an efficient catalytic system for ammonia production with a Faradaic efficiency of 18.9% achieved upon ultrasonic actuation. Our results show that the spontaneous polarization of piezoelectric materials under external electrical fields augments the sluggish electron transfer kinetics by creating instant dipoles in adsorbed N2molecules to deliver a piezo-enhanced catalytic system promising for sustained activation of dinitrogen molecules.

Nacre-Mimetic, Mechanically Flexible, and Electrically Conductive Silk Fibroin-MXene Composite Foams as Piezoresistive Pressure Sensors.

The hierarchical nacre-like three-dimensional (3D) assembly of porous and lightweight materials is in high demand for applications such as sensors, flexible energy storage and harvesting devices, electromagnetic interference shielding, and biomedical applications. However, designing such a biomimetic hierarchical architecture is highly challenging due to the lack of experimental approaches to achieve the necessary control over the materials' microstructure on the multilength scale. Aerogels and foam-based materials have recently been developed as attractive candidates for pressure-sensing applications. However, despite recent progress, the bottleneck for these materials to achieve electrical conductivity combined with high mechanical flexibility and fast strain recovery remains. In this study, for the first time, inspired by the multiscale architecture of nacre, we fabricated a series of ultra-lightweight, flexible, electrically conductive, and relatively high-strength composite foams through hybridizing the cross-linked silk fibroin (SF) biopolymer, extracted from Bombyx mori silkworm cocoon, reinforced with two-dimensional graphene oxide (GO) and Ti3C2 MXene nanosheets. Nacre is a naturally porous material with a lightweight, mechanically robust network structure, thanks to its 3D interconnected lamella-bridge micromorphology. Inspired by this material, we assemble a cross-linked SF fibrous solution with MXene and GO nanosheets into nacre-like architecture using a bidirectional freeze-casting technique. Subsequent freeze-drying and gas-phase hydrophobization resulted in composite foams with 3D hierarchical porous architectures with a unique combination of mechanical resilience, electrical conductance, and ultra-lightness. The developed composite presented excellent performances as piezoresistive pressure-sensing devices and sorbents for oil/water separation, which indicated great potential in mechanically switchable electronics.

Influence of the choice of precursors on the synthesis of two-dimensional transition metal dichalcogenides.

The interest in transition metal dichalcogenides (TMDCs; MEy/2; M = transition metal; E = chalcogenide, y = valence of the metal) has grown exponentially across various science and engineering disciplines due to their unique structural chemistry manifested in a two-dimensional lattice that results in extraordinary electronic and transport properties desired for applications in sensors, energy storage and optoelectronic devices. Since the properties of TMDCs can be tailored by changing the stacking sequence of 2D monolayers with similar or dis-similar materials, a number of synthetic routes essentially based on the disintegration of bulk (e.g., chemical exfoliation) or the integration of atomic constituents (e.g., vapor phase growth) have been explored. Despite a large body of data available on the chemical synthesis of TMDCs, experimental strategies with high repeatability of control over film thickness, phase and compositional purity remain elusive, which calls for innovative synthetic concepts offering, for instance, self-limited growth in the z-direction and homogeneous lateral topography. This review summarizes the recent conceptual advancements in the growth of layered van der Waals TMDCs from both mixtures of metal and chalcogen sources (multi-source precursors; MSPs) and from molecular compounds containing metals and chalcogens in one starting material (single-source precursor; SSPs). The critical evaluation of the strengths, limitations and opportunities of MSP and SSP approaches is provided as a guideline for the fabrication of TMDCs from commercial and customized molecular precursors. For example, alternative synthetic pathways using tailored molecular precursors circumvent the challenges of differential nucleation and crystal growth kinetics that are invariably associated with conventional gas phase chemical vapor transport (CVT) and chemical vapor deposition (CVD) of a mixture of components. The aspects of achieving high compositional purity and alternatives to minimize competing reactions or side products are discussed in the context of efficient chemical synthesis of TMDCs. Moreover, a critical analysis of the potential opportunities and existing bottlenecks in the synthesis of TMDCs and their intrinsic properties is provided.