New Insights into the Electronic Structure and Photoelectrochemical Properties of Nitrogen-Doped HNb₃O₈ via a Combined in Situ Experimental and DFT Investigation

The nitrogen-doping approach has been intensively adopted to improve various properties of metal oxides, especially for adjusting the energy band structure and extending the photoresponse range of oxide photocatalysts. However, the nitrogen doping behavior is still unintelligible and complex due to the diversity of compositions and crystal structures. In this work, new insights into the electronic structure and photoelectrochemical (PEC) properties of nitrogen-doped HNb₃O₈ were presented. On the one hand, we utilized an in situ experimental strategy to ascertain the effect of nitrogen doping on the energy band and photoelectrochemical (PEC) properties of HNb₃O₈ and nitrogen-doped HNb₃O₈ (N-HNb₃O₈). Their energy band level, donor densities, and interfacial charge transfer properties were studied by Mott–Schottky plots and electrochemical impedance spectroscopy. After nitrogen doping, the conduction band position is unusually descended by 0.23 eV, the valance band position is raised by 0.51 eV, the donor density (<i>N</i>_d) is increased from 3.71 × 10²¹ to 6.46 × 10²¹ cm^–3, and interfacial charge transfer efficiency is reduced, though. On the other hand, density functional theoretical calculations were also conducted, so as to understand the electronic structures of HNb₃O₈ and N-HNb₃O₈. After nitrogen doping, the electronic structure is modified due to the upshift of the valance band edge consisting of hybrid N 2p and O 2p orbitals and the downshift of the conduction band edge consisting of the H 1s and Nb 4d orbitals. Furthermore, these insights into the behavior of nitrogen-doped semiconductors have important guiding significance toward their potential applications

Bismuth Ferrite-Based Lead-Free High-Entropy Piezoelectric Ceramics

Piezoelectric ceramics, as essential components of actuators and transducers, have captured significant attention in both industrial and scientific research. The “entropy engineering” approach has been demonstrated to achieve excellent performance in lead-based materials. In this study, the “entropy engineering” approach was employed to introduce the morphotropic phase boundary (MPB) into the bismuth ferrite (BF)-based lead-free system. By employing this strategy, a serial of novel “medium to high entropy” lead-free piezoelectric ceramics were successfully synthesized, namely (1–x)BiFeO3–x(Ba0.2Sr0.2Ca0.2Bi0.2Na0.2)TiO3 (BF–xBSCBNT, x = 0.15–0.5). Our investigation systematically examined the phase structure, domain configuration, and ferroelectric/piezoelectric properties as a function of conformational entropy. Remarkable performances with a largest strain of 0.50% at 100 kV/cm, remanent polarization ∼40.07 μC/cm2, coercive field ∼74.72 kV/cm, piezoelectric coefficient ∼80 pC/N, and d33* ∼500 pm/V were achieved in BF–0.4BSCBNT ceramics. This exceptional performance can be attributed to the presence of MPB, coexisting rhombohedral and cubic phases, along with localized nanodomains. The concept of high-entropy lead-free piezoelectric ceramics in this study provides a promising strategy for the exploration and development of the next generation of lead-free piezoelectric materials