Usman Khan

As a leading candidate for next‐generation electronics, high‐performance 2D Bi ₂ O ₂ Se has garnered significant interest in the scientific community. However, the understanding of its defect mechanisms remains elusive, hindering the development of further functionalities. In this study, we utilized Raman spectroscopy and ultrafast pump–probe experiments to investigate Bi ₂ O ₂ Se thin films prepared via a solution‐based technique. By comparing carrier decay times under low and high fluences in 25 nm‐ and 115 nm‐thick films, we proposed the defect mechanisms associated with two common defects in the thicker film: Se vacancies and Se–Bi antisites, which exhibit shallow donor and deep donor behaviors, respectively. Additionally, Se–Bi antisites became prominent under higher fluences. Our insights into these defect mechanisms offer valuable guidance for defect engineering in high‐power electronics.

Ni ₈₈ Fe ₅ Mo ₇ -MMOF/NFF combines mechanical robustness with efficient oxygen evolution, achieving an overpotential of 322 mV at 2000 mA cm ⁻² and sustained operation for ∼400 h.

Efficient hydrogen production via electrochemical water splitting is vital for sustainable energy applications, with the HER in acidic media requiring highly effective catalysts. In this study, we report the synthesis of Bi₂O₂Se nanosheets through a scalable hydrothermal method, achieving exceptional catalytic performance in acidic conditions. The Bi₂O₂Se nanosheets exhibit a low overpotential of 104 mV at 10 mA cm^-2, significantly outperforming other bismuth-based HER catalysts. The superior activity is attributed to the unique structural and electronic properties of Bi₂O₂Se, which provide abundant active sites and enhance charge transfer efficiency. Electrochemical studies, including Tafel slope analysis and impedance spectroscopy, confirm rapid HER kinetics and reduced charge-transfer resistance. Additionally, the catalysts demonstrate excellent long-term stability under acidic conditions, maintaining their performance during extended electrolysis. This work highlights the potential of Bi₂O₂Se as a highly efficient and cost-effective catalyst tailored for acidic HER applications. The dual-electrode system comprising Bi₂O₂Se@CP as the cathode and RuO₂@CP as the anode demonstrated outstanding performance in overall water splitting. This system required a battery voltage of only 1.49 V to achieve a current density of 10 mA cm^-2, highlighting the superior electrocatalytic efficiency of Bi₂O₂Se in conjunction with RuO₂. By offering valuable insights into the design and optimization of bismuth-based materials, these findings pave the way for advancing sustainable hydrogen production technologies through scalable and efficient catalytic solutions.

Bi2O2Se has attracted intensive attention due to its potential in electronics, optoelectronics, as well as ferroelectric applications. Despite that, there have only been a handful of experimental studies based on ultrafast spectroscopy to elucidate the carrier dynamics in Bi2O2Se thin films, Different groups have reported various ultrafast timescales and associated mechanisms across films of different thicknesses. A comprehensive understanding in relation to thickness and fluence is still lacking. In this work, we have systematically explored the thickness-dependent Raman spectroscopy and ultrafast carrier dynamics in chemical vapor deposition (CVD)-grown Bi2O2Se thin films on mica substrate with thicknesses varying from 22.44 nm down to 4.62 nm at both low and high pump fluence regions. Combining the thickness dependence and fluence dependence of the slow decay time, we demonstrate a ferroelectric transition in the thinner (< 8 nm) Bi2O2Se films, influenced by substrate-induced compressive strain and non-equilibrium states. Moreover, this transition can be manifested under highly non-equilibrium states. Our results deepen the understanding of the interplay between the ferroelectric phase and semiconducting characteristics of Bi2O2Se thin films, providing a new route to manipulate the ferroelectric transition.

A systematic study of thickness- and fluence-dependent carrier dynamics reveals the hidden ferroelectric transition in CVD-grown Bi ₂ O ₂ Se thin films, offering potential applications in optoelectronic devices that benefit from this transition.

Industrially profitable alkaline electrolyzers are among the vital challenges in developing a feasible and renewable hydrogen economy. The widely available electrocatalysts are effective for hydrogen generation at low current density (∼10 mA cm⁻²); howbeit, enormous efforts are required to meet industrial expectations to acquire large current density (>500 mA cm⁻²) for practical water splitting applications. Herein, we demonstrate that thermal effects can regulate the electron transfer at the interface between the alkaline electrolyte and nickel nanothorn array (Ni NTA) surface, leading to extraordinary hydrogen evolution reaction (HER) at large current density. The Ni NTA electrode tested at a relatively high temperature of 60 °C manifests the ability to generate a large current density of 1000 mA cm⁻² at an overpotential of merely 140 mV, comparable to that of platinum-based electrodes and dramatically lower than that of recently reported Ni-based catalysts. The simulation result reveals that the production of H⁺ ions increases around the tip of the nanothorn structure as the temperature increases from room temperature, which greatly promotes the faster reaction kinetics and mass transfer. This work can provide opportunities for implementing earth-abundant catalysts with exceptional performance for industrial electrolyzers at large current densities.

Photocatalytic CO₂ reduction to valuable hydrocarbon solar fuel is of great significance but still challenging. Strong CO₂ enrichment ability and easily adjustable structures make metal-organic frameworks (MOFs) potential photocatalysts for CO₂ conversion. Even though pure MOFs have the potential for photoreduction of CO₂, the efficiency is still quite low due to rapid photogenerated electron–hole recombination and other drawbacks. In this work, graphene quantum dots (GQDs) were in situ encapsulated into highly stable MOFs via a solvothermal method for this challenging task. The GQDs@PCN-222 with encapsulated GQDs showed similar Powder X-ray Diffraction (PXRD) patterns to PCN-222, indicating the retained structure. The porous structure was also retained with a Brunauer–Emmett–Teller (BET) surface area of 2066 m²/g. After incorporation of GQDs, the shape of GQDs@PCN-222 particles remained, as revealed by the scanning electron microscope (SEM). As most of the GQDs were covered by thick PCN-222, it was hard to observe those GQDs using a transmission electron microscope (TEM) and a high-resolution transmission electron microscope (HRTEM) directly, the treatment of digested GQDs@PCN-222 particles by immersion in a 1 mM aqueous KOH solution can make the incorporated GQDs visible in TEM and HRTEM. The linker, deep purple porphyrins, make MOFs a highly visible light harvester up to 800 nm. The introduction of GQDs inside PCN-222 can effectively promote the spatial separation of the photogenerated electron–hole pairs during the photocatalytic process, which was proved by the transient photocurrent plot and photoluminescence emission spectra. Compared with pure PCN-222, the obtained GQDs@PCN-222 displayed dramatically enhanced CO production derived from CO₂ photoreduction with 147.8 μmol/g/h in a 10 h period under visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. This study demonstrated that the combination of GQDs and high light absorption MOFs provides a new platform for photocatalytic CO₂ reduction.