In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Defect engineering of two-dimensional materials for advanced energy conversion and storage.

Single atom doping in 2D layered MoS2 from a periodic table perspective

Rechargeable aqueous zinc‐ion batteries (ZIBs) have aroused tremendous attention in energy storage system due to their high safety, eco‐friendliness, low cost, and for their good compatibility. Transition metal sulfides and selenides are considered to be promising cathodes for aqueous ZIBs owing to their unique layered structure and tunable interlayer spacing for the accelerating diffusion and reversible intercalation of hydrated Zn2+. However, their practical applications are severely impeded by some defects, such as the inferior electronic conductivity, large ion diffusion energy barrier, and bad cyclic stability. In this review, the various modification strategies including phase engineering, defect engineering, interlayer intercalation, in situ electrochemical oxidation, hybridization, doping effects, and surface modification are categorized and highlighted to improve the electrochemical properties of transition metal sulfides and selenides cathode materials, which are discussed and summarized corresponding to particular modification strategies. Finally, several key breakthrough directions such as mechanism exploration technology, electrolyte strategies, synergistic engineering, high‐capacity conversion‐type, high‐voltage cathode materials, and rocking‐chair type batteries are proposed to further push forward the development of aqueous ZIBs, to guide the design of advanced‐properties cathode materials for aqueous ZIBs.

Recent Advances of Transition Metal Sulfides/Selenides Cathodes for Aqueous Zinc‐Ion Batteries

Improving the device performances of two-dimensional semiconducting transition metal dichalcogenides: Three strategies

The rise of two-dimensional tellurium for next-generation electronics and optoelectronics

2D semiconductors are emerging as plausible candidates for next‐generation “More‐than‐Moore” nanoelectronics to tackle the scaling challenge of transistors. Wafer‐scale 2D semiconductors, such as MoS2 and WS2, have been successfully synthesized recently; nevertheless, the absence of effective doping technology fundamentally results in energy barriers and high contact resistances at the metal–semiconductor interfaces, and thus restrict their practical applications. Herein, a controllable doping strategy in centimeter‐sized monolayer MoS2 films is developed to address this critical issue and boost the device performance. The ultralow contact resistance and perfect Ohmic contact with metal electrodes are uncovered in monolayer Fe‐doped MoS2, which deliver excellent device performance featured with ultrahigh electron mobility and outstanding on/off current ratio. Impurity scattering is suppressed significantly thanks to the ultralow electron effective mass and appropriate doping site. Particularly, unidirectionally aligned monolayer Fe‐doped MoS2 domains are prepared on 2 in. commercial c‐plane sapphire, suggesting the feasibility of synthesizing wafer‐scale 2D single‐crystal semiconductors with outstanding device performance. This work presents the potential of high‐performance monolayer transistors and enables further device downscaling and extension of Moore's law.

Reducing Contact Resistance and Boosting Device Performance of Monolayer MoS2 by In Situ Fe Doping

Two-dimensional multiferroic materials have garnered broad interests attributed to their magnetoelectric properties and multifunctional applications. Multiferroic heterostructures have been realized, nevertheless, the direct coupling between ferroelectric and ferromagnetic order in a single material still remains challenging, especially for two-dimensional materials. Here, we develop a physical vapor deposition approach to synthesize two-dimensional p-doped SnSe. The local phase segregation of SnSe2 microdomains and accompanying interfacial charge transfer results in the emergence of degenerate semiconductor and metallic feature in SnSe. Intriguingly, the room-temperature ferrimagnetism has been demonstrated in two-dimensional p-doped SnSe with the Curie temperature approaching to ~337 K. Meanwhile, the ferroelectricity is maintained even under the depolarizing field introduced by SnSe2. The coexistence of ferrimagnetism and ferroelectricity in two-dimensional p-doped SnSe verifies its multiferroic feature. This work presents a significant advance for exploring the magnetoelectric coupling in two-dimensional limit and constructing high-performance logic devices to extend Moore's law. 

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Two-dimensional multiferroic material of metallic p-doped SnSe

The electrocatalytic sulfion (S2−) recycling and hydrogen (H2) evolution play indispensable roles in converting waste into sustainable energy and boosting hydrogen economy. However, the development of such promising technologies is hindered by the lack of electrocatalysts with low‐cost, high‐efficient, and robust stability. 2D VSe2 and SnSe2 hold great application potentials in the energy‐related fields yet face daunting challenges in terms of large‐scale synthesis. Herein, a gram‐scale and high‐yield strategy is designed with tetradecylamine (TDA) assistance for synthesizing 2D VSe2 and SnSe2. The cooperativity of TDA with V4+ (Sn4+) cations and its stabilization to (100) facets of products contribute to the formation of ultrathin VSe2 nanosheets with abundant wrinkles and SnSe2 nanoflowers with hierarchical structures, which deliver excellent hydrogen evolution reaction and oxygen evolution reaction performances. Significantly, a record‐breaking sulfion oxidation reaction efficiency in 2D VSe2 is uncovered, and the current density is one order of magnitude larger than that of commercial Pt/C catalyst. This work presents a significant breakthrough for scalable synthesize 2D electrocatalysts and provides a new paradigm for sustainable hydrogen generation and water purification.

Gram‐Scale Synthesized Two‐Dimensional VSe2 and SnSe2 for Ultrahigh Electrocatalytic Sulfion Recycling

Two-dimensional (2D) iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and catalysis. However, the controlled syntheses and intrinsic property explorations of such fascinating materials still remain daunting challenges, especially for 2D nonlayered Fe7S8 with mixed-valence states and high conductivity. Herein, we design a general and temperature-mediated chemical vapor deposition (CVD) approach to synthesize ultrathin and large-domain Fe7S8 nanosheets on mica substrates, with the thickness down to ∼4.4 nm (2 unit-cell). Significantly, we uncover a quadratic-dependent unsaturated magnetoresistance (MR) with out-of-plane anisotropy in 2D Fe7S8, thanks to its ultrahigh crystalline quality and high conductivity (∼2.7 × 105 S m-1 at room temperature and ∼1.7 × 106 S m-1 at 2 K). More interestingly, the CVD-synthesized 2D Fe7S8 nanosheets maintain robust environmental stability for more than 8 months. These results hereby lay solid foundations for synthesizing 2D nonlayered iron chalcogenides with mixed-valence states and exploring fascinating quantum phenomena.

Mo Cheng

Papers

Reducing Contact Resistance and Boosting Device Performance of Monolayer MoS2 by In Situ Fe Doping

Two-dimensional multiferroic material of metallic p-doped SnSe

Improving the device performances of two-dimensional semiconducting transition metal dichalcogenides: Three strategies

Gram‐Scale Synthesized Two‐Dimensional VSe2 and SnSe2 for Ultrahigh Electrocatalytic Sulfion Recycling

Controllable Synthesis Quadratic-Dependent Unsaturated Magnetoresistance of Two-Dimensional Nonlayered Fe7S8 with Robust Environmental Stability.