ZY

Small. 2024 Aug 21:e2403460. doi: 10.1002/smll.202403460. Online ahead of print.

ABSTRACT

In the realm of photovoltaic research, 2D transition metal carbides (MXenes) have gained significant interest due to their exceptional photoelectric capabilities. However, the instability of MXenes due to oxidation has a direct impact on their practical applications. In this work, the oxidation process of Nb₂CT_x MXene in aqueous systems is methodically simulated at the atomic level and nanosecond timescales, which elucidates the structural variations influenced by the synergistic effects of water and dissolved oxygen, predicting a transition from metal to semiconductor with 44% C atoms replaced by O atoms in Nb₂CT_x. Moreover, Nb₂CT_x with varying oxidation degrees is utilized as electron transport layers (ETLs) in perovskite solar cells (PSCs). Favorable energy level alignments with superior electron transfer capability are achieved by controlled oxidation. By further exploring the composites of Nb₂CT_x to its derivatives, the strong interaction of the nano-composites is demonstrated to be more effective for electron transport, thus the corresponding PSC achieves a better performance with long-term stability compared with the widely used ETLs like SnO₂. This work unravels the oxidation dynamics of Nb₂CT_x and provides a promising approach to designing ETL by exploiting MXenes to their derivatives for photovoltaic technologies.

PMID:39169745 | DOI:10.1002/smll.202403460

ACS Appl Mater Interfaces. 2023 Oct 30. doi: 10.1021/acsami.3c11151. Online ahead of print.

ABSTRACT

Challenges to upscaling metal halide perovskites (MHPs) include mechanical film stresses that accelerate degradation, dominate at the module scale, and can lead to delamination or fracture. In this work, we demonstrate open-air blade coating of single-step coated perovskite as a scalable method to control residual film stress after processing and introduce beneficial compression in the thin film with the use of polymer additives such as gellan gum and corn starch. The optoelectronic properties of MHP films with compression are improved with higher photoluminescence yields. MHP film stability is significantly improved under compression, under humidity, heat, and thermal cycling. By measuring the evolution of film stresses, we demonstrate for the first time that stress relaxation occurs in MHP films with tensile stress that correlates with film degradation. This discovery of a new mechanism underpinning MHP degradation shows that film stress can be used as a parameter to screen MHP devices and modules for quality control before deployment as a design for reliability criterion.

PMID:37903284 | DOI:10.1021/acsami.3c11151

Bond polarization of doped atoms and carbon and lattice defects are considered important aspects in the catalytic mechanisms of oxygen reduction reaction (ORR) on heteroatom-doped carbon catalysts. Previous work on metal-free catalysts has focused either on bond polarization or lattice defects. Here multi-heteroatom doped defect-enriched carbon nanotubes (MH-DCNTs) that combine both effects to enhance ORR activity are designed. Lattice defects in MH-DCNTs are enriched by unzipping and length-shortening of carbon nanotubes, and also by creating carbon vacancies via decomposition of doped F atoms. Electrochemical analysis using rotating disc electrode voltammetry shows that the ORR kinetic current density of MH-DCNT increases with lattice-defect density, the latter of which is verified by Raman spectroscopy, while the onset potential increases with annealing temperatures. An optimized MH-DCNT ORR catalyst exhibits a half-wave potential of 0.81 V versus reversible hydrogen electrode and limiting current density of 5.0 mA cm⁻² at an electrode rotation speed of 1600 rpm in 0.1 m KOH. Further, it is demonstrated that MH-DCNT, as a cathode catalyst layer in an anion exchange membrane fuel cell (AEMFC), delivers a peak power density of 250 mW cm⁻², which is ≈70% the performance of an AEMFC using a conventional Pt/C catalyst.

Multi-heteroatom doped, defect-enriched carbon nanotubes (MH-DCNTs) are designed to combine two catalytic mechanisms: bond polarization and lattice defects for enhanced oxygen reduction reaction (ORR) activity. ORR kinetic current of MH-DCNT correlates with lattice-defect density, while onset potential increases with synthesis temperatures. Optimized MH-DCNTs exhibit comparable ORR performances with Pt-catalyst in both half-cells and full anion exchange membrane fuel cells.

The development of high resolution, high aspect ratio metal dichalcogenide nanostructures is one of the most important issues in 2D material researchers due to the potential to exploit their properties into high performance devices. In this study, for the first time a way of fabricating metal dichalcogenide nanostructures with high resolution (<50 nm scale) and high aspect ratios (>120) by chemical vapor deposition assisted secondary sputtering phenomenon is reported. This approach can universally synthesize various types of metal dichalcogenides including MoS₂, WS₂, and SnS₂, implying the possibility for further utilization with selenides and tellurides. Also, this method can produce highly periodic complex patterns such as hole–cylinder, concentric rings, and line patterns, which are unprecedented in previous reports. The feature size and aspect ratio of the metal dichalcogenide structures can be manipulated by controlling the dimensions of the photoresist prepatterns, while the pattern resolution and layer orientation can be manipulated by controlling the thickness of the deposited metal film. It is demonstrated that nanostructures with high resolution and high aspect ratio significantly improve gas-sensing properties compared with previous metal dichalcogenide films. It is believed that the method can be a foundation for synthesizing various materials with complex patterns for future applications.

A universal way for fabricating periodic metal dichalcogenide nanostructures with high resolution (<50 nm), high aspect ratio (>120), and complex shapes is developed for the first time. Various metal dichalcogenides are demonstrated in this work, and nanostructures of MoS₂, WS₂, and SnS₂ with serpentine, line, hole–cylinder, and concentric patterns are fabricated over a large area.