RESUMO
The spontaneous centrosymmetry-breaking and robust room-temperature ferroelectricity in niobium oxide dihalides spurs a flurry of explorations into its promising second-order nonlinear optical properties, and promises potential applications in nonvolatile electro-optical and optoelectronic devices. However, the ambient stability of the niobium oxide dihalides remains questionable, which overshadows their future development. In this work, the chemical degradation of NbOI2 is comprehensively investigated using combined chemical and optical microscopies in conjunction with spectroscopies. We unveil the highly anisotropic degradation kinetics of NbOI2 driven by the hydrolysis process of the unstable dangling iodine bonds dominantly on the (010) facet and progressing along the c axis. Knowing its degradation mechanism, the NbOI2 flake can then be stabilized by the hexagonal boron nitride encapsulation, which isolates the air moisture. These findings provide direct insights into the ambient instability of NbOI2, and they deliver possible solutions to circumvent this issue, which are essential for its practical integration in photonic and electronic devices.
RESUMO
Recent progress in two-dimensional ferroelectrics greatly expands the versatility and tunability in van der Waals heterostructure based electronics. However, the switching endurance issue that widely plagues conventional ferroelectrics in practical applications is hitherto unexplored for van der Waals layered ferroelectrics. Herein, we report the observation of unusual polarization fatigue behaviors in van der Waals layered CuInP2S6, which also possesses finite ionic conductivity at room temperature. The strong intertwinement of the short-range polarization switching and long-range ionic movement in conjunction with the van der Waals layered structure gives rise to unique morphological and polarization evolutions under repetitive electric cycles. With the help of concerted chemical, structural, lattice vibrational and dielectric analyses, we unravel the critical role of the synergy of ionic migration and surface oxidation on the anomalous polarization enhancement and the eventual polarization degradation. This work provides a general insight into the polarization fatigue characteristics in ionically-active van der Waals ferroelectrics and delivers potential solutions for the realization of fatigue-free capacitors.
RESUMO
Transition metal thiophosphate, CuInP2S6 (CIPS), has recently emerged as a potentially promising material for photoelectrochemical (PEC) water splitting due to its intrinsic ferroelectric polarization for spontaneous photocarrier separation. However, the poor kinetics of the hydrogen evolution reaction (HER) greatly limits its practical applications. Herein, we report self-enhancing photocatalytic behavior of a CIPS photocathode due to chemically driven oxygen incorporation by photoassisted acid oxidation. The optimal oxygen-doped CIPS demonstrates a >1 order of magnitude enhancement in the photocurrent density compared to that of pristine CIPS. Through comprehensive spectroscopic and microscopic investigations combined with theoretical calculations, we disclose that oxygen doping will lower the Fermi level position and decrease the HER barrier, which further accelerates charge separation and improves the HER activity. This work may deliver a universal and facile strategy for improving the PEC performance of other van der Waals metal thiophosphates.
