Oxygen-Vacancy-Driven Orbital Reconstruction at the Surface of TiO2 Core-Shell Nanostructures

Oxygen Bridge Formed by Doping Nonmetal Atoms into Cationic Vacancies To Enhance the Photoelectrochemical Oxygen Evolution Reaction

To enhance photoelectrochemical (PEC) water splitting for renewable energy conversion, the conventional strategy is doping nonmetals into anionic vacancies. Compared to anionic vacancies, cationic vacancies are theoretically more effective and reliable for anchoring nonmetals owing to their larger radii and unique advantages. The current research mainly focuses on anionic vacancies, while there are few studies on cationic vacancies due to high formation energy and challenging characterizations by convenient techniques. To overcome the current limitations, nonmetallic S and P atoms are successfully doped into cationic vacancies on the TiO₂ surface for tuning local electronic structures. In contrast to the traditional strategy of reducing the bandgaps, nonmetallic atom doping into cationic vacancies facilitates efficient electronic regulation for PEC enhancement without changing the bandgap. The enhanced performance is attributed to the formation of an oxygen bridge, which can accumulate electrons from surrounding S/P atoms. Significantly, the electron-enriched oxygen bridge efficiently transfers electrons to activate reaction site Ti, which can promote the oxygen evolution reaction performance. Density functional theory calculations reveal that the decrease of reaction energy barriers and the optimization of local electron distribution are conducive to electronic transmission. This would provide a high-efficiency electronic tuning strategy for improving PEC performance.

Aliovalent Ta-Doping-Engineered Oxygen Vacancy Configurations for Ultralow-Voltage Resistive Memory Devices: A DFT-Supported Experimental Study

Alteration of transport properties of any material, especially metal oxides, by doping suitable impurities is not straightforward as it may introduce multiple defects like oxygen vacancies (V_o) in the system. It plays a decisive role in controlling the resistive switching (RS) performance of metal oxide-based memory devices. Therefore, a judicious choice of dopants and their atomic concentrations is crucial for achieving an optimum V_o configuration. Here, we show that the rational designing of RS memory devices with cationic dopants (Ta), in particular, Au/Ti_1-xTa_xO_2-δ/Pt devices, is promising for the upcoming non-volatile memory technology. Indeed, a current window of ∼10⁴ is realized at an ultralow voltage as low as 0.25 V with significant retention (∼10⁴ s) and endurance (∼10⁵ cycles) of the device by considering 1.11 at % Ta doping. The obtained device parameters are compared with those in the available literature to establish its excellent performance. Furthermore, using detailed experimental analyses and density functional theory (DFT)-based first-principles calculations, we comprehend that the meticulous presence of V_o configurations and the columnar-like dendritic structures is crucial for achieving ultralow-voltage bipolar RS characteristics. In fact, the dopant-mediated V_o interactions are found to be responsible for the enhancement in local current conduction, as evidenced from the DFT-simulated electron localization function plots, and these, in turn, augment the device performance. Overall, the present study on cationic-dopant-controlled defect engineering could pave a neoteric direction for future energy-efficient oxide memristors.

Black 3D-TiO2 Nanotube Arrays on Ti Meshes for Boosted Photoelectrochemical Water Splitting

Black 3D-TiO2 nanotube arrays are successfully fabricated on the Ti meshes through a facile electrochemical reduction method. The optimized black 3D-TiO2 nanotubes arrays yield a maximal photocurrent density of 1.6 mA/cm2 at 0.22 V vs. Ag/AgCl with Faradic efficiency of 100%, which is about four times larger than that of the pristine 3D-TiO2 NTAs (0.4 mA/cm2). Such boosted PEC water splitting activity primarily originates from the introduction of the oxygen vacancies, which results in the bandgap shrinkage of the 3D-TiO2 NTAs, boosting the utilization efficiency of visible light including the incident, reflected and/or refracted visible light captured by the 3D configuration. Moreover, the oxygen vacancies (Ti3+) can work as electron donors, which leads to the enhanced electronic conductivity and upward shift of the Fermi energy level, and thereby facilitating the transfer and separation of the photogenerated charge carrier at the semiconductor-electrolyte interface. This work offers a new opportunity to promote the PEC water splitting activity of TiO2-based photoelectrodes.