2D-2D heterostructure g-C3N4-based materials for photocatalytic H2 evolution: Progress and perspectives.

Photocatalytic hydrogen generation from direct water splitting is recognized as a progressive and renewable energy producer. The secret to understanding this phenomenon is discovering an efficient photocatalyst that preferably uses sunlight energy. Two-dimensional (2D) graphitic carbon nitride (g-C3N4)-based materials are promising for photocatalytic water splitting due to special characteristics such as appropriate band gap, visible light active, ultra-high specific surface area, and abundantly exposed active sites. However, the inadequate photocatalytic activity of pure 2D layered g-C3N4-based materials is a massive challenge due to the quick recombination between photogenerated holes and electrons. Creating 2D heterogeneous photocatalysts is a cost-effective strategy for clean and renewable hydrogen production on a larger scale. The 2D g-C3N4-based heterostructure with the combined merits of each 2D component, which facilitate the rapid charge separation through the heterojunction effect on photocatalyst, has been evidenced to be very effective in enhancing the photocatalytic performance. To further improve the photocatalytic efficiency, the development of novel 2D g-C3N4-based heterostructure photocatalysts is critical. This mini-review covers the fundamental concepts, recent advancements, and applications in photocatalytic hydrogen production. Furthermore, the challenges and perspectives on 2D g-C3N4-based heterostructure photocatalysts demonstrate the future direction toward sustainability.

Ultrasonic-Assisted Synthesis of N-Doped, Multicolor Carbon Dots toward Fluorescent Inks, Fluorescence Sensors, and Logic Gate Operations.

Over past decades, the multicolor carbon dots (M-CDs) have attracted enormous attentions due to their tunable photoluminescence and versatile applications. Herein, the nitrogen-doped (N-doped) M-CDs including green, chartreuse, and pink emissive CDs are successfully synthesized by ultrasonic treatment of kiwifruit juice with different additive reagents such as ethanol, ethylenediamine, and acetone. Owing to their strong fluorescence upon irradiation with 365 nm UV light, the highly water-soluble M-CDs present great potential in the anticounterfeit field as fluorescent inks. Particularly, the resulting green emission CDs (G-CDs) with excellent fluorescence and stability are applied as a label-free probe model for "on-off" detection of Fe3+. The fluorescence of G-CDs is significantly quenched by Fe3+ through static quenching. The nanoprobe demonstrates good selectivity and sensitivity toward Fe3+ with a detection limit of ~0.11 µM. Besides, the quenched fluorescence of G-CDs by Fe3+ can be recovered by the addition of PO43- or ascorbic acid (AA) into the CDs/Fe3+ system to realize the "off-on" fluorescent process. Furthermore, NOT and IMPLICATION logic gates are constructed based on the selection of Fe3+ and PO43- or AA as the inputs, which makes the G-CD-based sensors utilized as various logic gates at molecular level. Therefore, the N-doped M-CDs hold promising prospects as competitive candidates in monitoring the trace species, applications in food chemistry, anticounterfeit uses, and beyond.

MOFs Derived Hetero-ZnO/Fe2O3 Nanoflowers with Enhanced Photocatalytic Performance towards Efficient Degradation of Organic Dyes.

It is still a challenge for wastewater treatment to develop efficient yet low-cost photocatalysts on a large scale. Herein, a facile yet efficient method was devised to successfully synthesize ZnO/Fe2O3 nanoflowers (NFs) by using metal organic framework ZIF-8 as the precursor. The photocatalytic activities of the as-prepared hetero-ZnO/Fe2O3 NFs are purposefully evaluated by photocatalytic degradation of methylene blue (MB) and methyl orange (MO) under UV light irradiation. The resulting ZnO/Fe2O3 NFs display even higher photocatalytic activities than those of single-phase ZnO and Fe2O3 as a photocatalyst for the degradation of both MB ad MO. Particularly, nearly 100% MB can be photocatalytically degraded in 90 min under UV light irradiation using the hetero-NFs photocatalyst. The enhanced photocatalytic properties are probably ascribed to the synergistic contributions from the suitable band alignment of ZnO and Fe2O3, large surface area, and strong light absorption property. Radical scavenger experiments prove that the photogenerated holes, ·OH and ·O2-, play key roles in photocatalytic degradation process of organic dyes. Accordingly, the photocatalytic degradation mechanism of hetero-ZnO/Fe2O3 NFs towards dyes is tentatively proposed. The work contributes an effective way to rationally design and fabricate advanced photocatalysts with heterojunction structures for photocatalytic applications.

Formation of Nanodimensional NiCoO2 Encapsulated in Porous Nitrogen-Doped Carbon Submicrospheres from a Bimetallic (Ni, Co) Organic Framework toward Efficient Lithium Storage.

Recently, rock-salt NiCoO2 (NCO) with desirable electronic conductivity has drawn enormous interest worldwide for energy-related applications. However, the intrinsically sluggish kinetics and electrode aggregation/volumetric change/pulverization during Li-insertion/-extraction processes hugely limit its applications in Li-ion batteries (LIBs). In the contribution, we first devise a bottom-up method for scalable fabrication of the nanodimensional NCO particles encapsulated in porous nitrogen-doped carbon submicrospheres (NCS), which are derived from a bimetal (Ni, Co) metal-organic framework. The porous NCS, as a flexible conductive skeleton, can buffer distinct volume expansion as an efficient buffering phase, restrain agglomeration of nanoscaled NCO, and enhance electronic conductivity and wettability of the electrode. Benefiting from the synergistic functions between the nanodimensional NCO and porous NCS, the obtained NCO@NCS anode (∼74.5 wt % NCO) is endowed with remarkable high-rate reversible capacity (∼403.0 mAh g-1 at 1.0 A g-1) and cycling behaviors (∼371.4 mAh g-1 after being cycled for 1000 times at 1.0 A g-1) along with a high lithium diffusion coefficient and remarkable pseudocapacitive contribution. Furthermore, the NCO@NCS-based full LIBs exhibit competitive lithium-storage properties in terms of energy density (∼217.0 Wh kg-1) and cyclic stability. Furthermore, we believe that the methodology is highly promising in versatile design and construction of binary metal oxide/carbon hybrid anodes for advanced LIBs.

A two-dimensional assembly of ultrafine cobalt oxide nanocrystallites anchored on single-layer Ti3C2Tx nanosheets with enhanced lithium storage for Li-ion batteries.

Recently, Ti-based MXenes were expected to compete with graphene and other carbonaceous materials towards Li-ion batteries (LIBs) due to their two-dimensional (2D) open structure, cost efficiency, superior conductivity and low Li+ diffusion barrier. However, the relatively moderate capacity and aggregation tendency hamper their practical applications in next-generation LIBs. Herein, we explore for the first time a scalable bottom-up approach to fabricate a series of Co3O4@single-layer Ti3C2Tx (s-Ti3C2Tx) hybrids, where numerous homogeneous Co3O4 nanocrystallites (NCs), serving both as a spacer and electroactive phase, are anchored uniformly on the surface of s-Ti3C2Tx nanosheets (NSs) through the Co-O-Ti interfacial bonds. Furthermore, detailed experimental analyses clearly shed light upon the formation mechanism of the hybrid Co3O4@s-Ti3C2Tx NSs. Thanks to the structural and compositional merits, the 2D Co3O4@s-Ti3C2Tx NSs even exhibit a remarkable high-rate capacity of ∼223 mA h g-1 at an ultra-high current density of 10 A g-1, and a long-span cycle life with a high reversible capacity of 550 mA h g-1 at 1 A g-1 after 700 consecutive cycles. Corresponding density functional theory calculation further confirms that the Co-O-Ti interfacial function leads to an even higher pseudocapacitive contribution and faster lithium storage behavior due to the enhanced interfacial electron transfer.