Highly efficient photothermal effect by atomic-thickness confinement in two-dimensional ZrNCl nanosheets

Nitrogen adsorption via charge transfer on vacancies created during surfactant assisted exfoliation of TiB2

Titanium diboride (TiB₂), a layered ceramic material, comprised of titanium atoms sandwiched in between honeycomb planes of boron atoms, exhibits a promising structure to utilize the rich chemistry offered by the synergy of titanium and boron. TiB₂ has been primarily investigated and applied in its bulk form. This perspective is, however, fast evolving with a number of efforts aimed at exfoliating TiB₂. Here, we show that it is possible to delaminate TiB₂ into ultrathin, minimally functionalized nanosheets with the aid of surfactants. These nanosheets exhibit crystalline nature and their chemical analysis reveals vacant sites within the nanosheets. These vacancies facilitate the chemisorption of N₂ onto the TiB₂ nanosheets under ambient conditions without the aid of any energy, this finding was unexpected. This remarkable activity of TiB₂ nanosheets is attributed to vacancies and the Ti-B synergy, which enhance the adsorption and activation of N₂. We obtained supplemental insights into the N₂ adsorption by Density Functional Theory (DFT) studies, which reveal how charge transfer among Ti, B, and N₂ results in N₂ adsorption. The DFT studies also show that nanosheets having more vacancies result in increased adsorption when compared with nanosheets having less vacancies and bulk TiB₂.

Two-dimensional nanomaterials: synthesis and applications in photothermal catalysis

Photothermal catalysis, as one of the emerging technologies with synergistic effects of photochemistry and thermochemistry, is highly attractive in the fields of environment and energy. Two-dimensional (2D) nanomaterials have received extensive attention toward photothermal catalysis because of their ultrathin layer structures, superior physical and optical properties, and high surface areas. These merits are beneficial for shortening the transfer distance of charge carriers, improving the efficiency of solar to thermal, and providing a great opportunity for the development of photothermal chemistry. In this review, we have summarized the state-of-art advances in various 2D nanomaterials with emphasis on the driving force and relevant mechanism of photothermal catalysis, including the involved three types, namely, localized surface plasmonic resonance (LSPR), nonradiative relaxation, and thermal vibrations of molecules. Moreover, the synthesis strategies of 2D materials and their photothermal applications in carbon dioxide (CO₂) conversion, hydrogen (H₂) production, volatile organic compounds (VOCs) degradation, and water (H₂O) purification have been discussed in detail. Ultimately, the existing challenges and prospects of future development in the field are proposed. It is believed that this review will afford a great reference for the exploration of the high-efficiency 2D nanomaterials and their structure-activity relationship.

Layered β-ZrNBr Nitro-Halide as Multifunctional Photocatalyst for Water Splitting and CO2 Reduction

Developing mixed-anion semiconductors for solar fuel production has inspired extensive interest, but the nitrohalide-based photocatalyst is still in shortage. Here we report a layered nitro-halide β-ZrNBr with a narrow band gap of ca. 2.3 eV and low defect density to exhibit multifunctionalities for photocatalytic water reduction, water oxidation and CO₂ reduction under visible-light irradiation. As confirmed by the results of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the formation of anion vacancies in the nitro-halide photocatalyst was inhibited due to its relatively high formation energy. Furthermore, performance of β-ZrNBr can be effectively promoted by a simple exfoliation into nanosheets to shorten the carrier transfer distance as well as to promote charge separation. Our work extends the territory of functional photocatalysts into the nitro-halide, which opens a new avenue for fabricating efficient artificial photosynthesis.

Layer-Dependent Raman Spectroscopy and Electronic Applications of Wide-Bandgap 2D Semiconductor β-ZrNCl

In recent years, 2D layered semiconductors have received much attention for their potential in next-generation electronics and optoelectronics. Wide-bandgap 2D semiconductors are especially important in the blue and ultraviolet wavelength region, although there are very few 2D materials in this region. Here, monolayer β-type zirconium nitride chloride (β-ZrNCl) is isolated for the first time, which is an air-stable layered material with a bandgap of ≈3.0 eV in bulk. Systematical investigation of layer-dependent Raman scattering of ZrNCl from monolayer, bilayer, to bulk reveals a blueshift of its out-of-plane A_1g peak at ≈189 cm^-1 . Importantly, this A_1g peak is absent in the monolayer, suggesting that it is a fingerprint to quickly identify the monolayer and for the thickness determination of 2D ZrNCl. The back gate field-effect transistor based on few-layer ZrNCl shows a high on/off ratio of 10⁸ . These results suggest the potential of 2D β-ZrNCl for electronic applications.

Enhancement of Solar-Driven Photocatalytic Activity of BiOI Nanosheets through Predominant Exposed High Energy Facets and Vacancy Engineering

The increasing application of exposed high energy facet is an effective strategy to improve the photocatalytic performance of photocatalysts because the vacancies are beneficial to photocatalytic reaction. Vacancy dominates numerous distinct properties of semiconductor materials and thus plays a conclusive role in the photocatalysis applications. In this work, two kinds of BiOI nanomaterials with different vacancies are synthesized via a facile solvothermal method. The positron annihilation analysis shows that the thinner BiOI nanosheets possess larger-sized vacancy than BiOI nanoplates. Thus, BiOI nanosheets show the enhanced separation efficiency of electron-hole pairs and adsorption ability for contaminants under visible light. The results are also validated with the first-principle computation. Therefore, higher photocatalytic activity to the photodegradation of tetracycline is observed from the nanosheets than that obtained from BiOI nanoplates. This work not only arouses attention to vacancies, but also opens up an avenue for precision design of vacancies to prepare novel photocatalytic materials driven under solar light.

Absence of van der Waals Gap in Ternary Thorium Nitride ThNF and ThNCl

Two kinds of ternary thorium nitride compounds, ThNF and ThNCl, are synthesized. Via the refinement of X-ray diffraction patterns, the accurate crystal structure of the two compounds is solved. Although ThNF and ThNCl share a similar structure with MNX (M = Ti, Zr, Hf; X = Cl, Br) compounds, the interaction between adjacent ThNF and ThNCl layers is not a van der Waals gap. For ThNF, the strong electronegativity of F ions leads to the bonding of Th to the F both in the nearest neighbor layer and the next nearest neighbor layer, which results in the absence of a van der Waals gap between ThNF layers. However, for ThNCl, the reason for the absence of a van der Waals gap could be attributed to the large Th-Cl bond length due to the partially covalent Th-Cl bond as well as the flat ThN layer. It is the absence of van der Waals gap that results in the failure of intercalating cations into ThNF and ThNCl. Our result reveals the reason for unsuccessful intercalation in ThNF and ThNCl, thereby providing a deeper understanding for the interlayer interaction in ternary layer structures in metal nitride halides.

Learning from Solar Energy Conversion: Biointerfaces for Artificial Photosynthesis and Biological Modulation

Three seemingly distinct directions of nanomaterials research, photovoltaics, biofuel production, and biological modulation, have been sequentially developed over the past several decades. In this Mini Review, we discuss how the insights gleaned from nanomaterials-based solar energy conversion can be adapted to biointerface designs. Because of their size- and shape-dependent optical properties and excellent synthetic control, nanomaterials have shown unique technological advantages as the light absorbers or energy transducers. Biocompatible nanomaterials have also been incorporated into biological systems including biomolecules, bacteria, and eukaryotic cells for a large collection of fundamental studies and applications. For the photocatalytic biofuel production, either isolated bacterial enzymes or the entire bacteria have been hybridized with the nanomaterials, where functions from both parts are synergistically integrated. Likewise, interfacing nanomaterials with eukaryotic systems, whether in individual cells or tissues, has enabled optical modulation of cellular behavior and the construction of active cellular materials. Here we survey different approaches in which nanomaterials are used to elicit electrical or mechanical changes in single cells or cellular assemblies via photoelectrochemical or photothermal processes. We end this Mini Review with the discussion of future nongenetic modulation at the intracellular level.

Hexagonal polypyrrole nanosheets from interface driven heterogeneous hybridization and self-assembly for photothermal cancer treatment

Hexagonal-shaped polypyrrole (PPy) nanosheets were fabricated by the generation and anisotropic self-assembly of FeOOH–PPy heterostructures for photothermal cancer treatment.

Band offsets in new BN/BX (X = P, As, Sb) lateral heterostructures based on bond-orbital theory

Identifying heterostructures with tunable band alignments remains a difficult challenge. Here, based on bond-orbital theory, we propose a series of new BN/BX (X = P, As, Sb) lateral heterostructures (LHS). Our first principles calculations reveal that the LHS interlines have a substantial impact on the electronic properties. Importantly, we start with the chemical concepts, such as bond length and strength as well as orbital overlap interaction, in an attempt to thoroughly investigate the electronic properties, namely the band offset, the band gap (Eg) and the state of the energy level. We demonstrate that the newly designed BN/BX LHS have profound implications for developing advanced optoelectronics, such as high-performance light-emitting diodes and lasers. Furthermore, the new BN/BX LHS designed from the chemical viewpoint can shed new light on overcoming the enormous hurdle of ineffective and laborious material design.

Improving Photocatalytic Water Treatment through Nanocrystal Engineering: Mesoporous Nanosheet-Assembled 3D BiOCl Hierarchical Nanostructures That Induce Unprecedented Large Vacancies

Vacancy control can significantly enhance the performance of photocatalytic semiconductors for water purification. However, little is known about the mechanisms and approaches that could generate stable large vacancies. Here, we report a new mechanism to induce vacancy formation on nanocrystals for enhanced photocatalytic activity: the introduction of mesopores. We synthesized two nanosheet-assembled hierarchical 3D BiOCl mesoporous nanostructures with similar morphology and exposed facets but different nanosheet thickness. Positron annihilation analysis detected unprecedentedly large V_Bi^‴ V_O^•• V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates (as well as V_Bi^‴ V_O^•• V_Bi^‴) on BiOCl assembled from 3-6 nm nanosheets but only V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates on BiOCl assembled from thicker (10-20 nm) nanosheets. Comparison of vacancy properties with 2D ultrathin 2.7 nm nanosheets (with V_Bi^‴ V_O^•• V_Bi^‴ and V_Bi^‴) indicates that nanosheet thinness alone cannot explain the formation of such large atom vacancies. On the basis of density functional theory computations of formation energy of isolated Bi vacancy, we show that mesopores facilitate the formation of large vacancies to counterbalance thermodynamic instability caused by incompletely coordinated Bi and O atoms along the mesopore perimeters. We corroborate that the extraordinarily large V_Bi^‴ V_O^•• V_Bi^‴ V_O^•• V_Bi^‴ vacancy associates facilitate photoexcitation of electrons and prevent the recombination of electron-hole pairs, which significantly enhances photocatalytic activity. This is demonstrated by the rapid mineralization of bisphenol A (10^-5 M) with low photocatalyst loading (1 g L^-1), as well as enhanced bacterial disinfection. Improved electron-hole separation is also corroborated by enhanced photocatalytic reduction of nitrate.

DFT coupled with NEGF study of a promising two-dimensional channel material: black phosphorene-type GaTeCl

A stable three-dimensional layered GaTeCl bulk counterpart is first known from experiment since 1980s. In this study, we propose a two-dimensional GaTeCl, the band structure of which has a tendency of intrinsic direct-to-indirect band gap transitions as a result of a decrease in the layer number, while the changes in the band gap value are minor. The GaTeCl monolayer possesses a wide indirect band gap of 3.06 eV and high hole mobility of up to 4710 cm² V^-1 s^-1, which, intriguingly, can be converted into direct band-gap semiconductors under a slight tensile strain. The GaTeCl monolayer is calculated to have an ideal cleavage energy of about 32 meV per atom; therefore, the synthesis of GaTeCl monolayer through exfoliation of bulk GaTeCl is available. In this regard, we simulate a monolayer GaTeCl MOSFETs device based on first-principles method quantum transport approach. The underlying device performance could pave the way for it to be a promising candidate as a suitable FET channel material.

Layered tin monoselenide as advanced photothermal conversion materials for efficient solar energy-driven water evaporation

Solar energy-driven water evaporation lays a solid foundation for important photothermal applications such as sterilization, seawater desalination, and electricity generation. Due to the strong light-matter coupling, broad absorption wavelength range, and prominent quantum confinement effect, layered tin monoselenide (SnSe) holds a great potential to effectively harness solar irradiation and convert it to heat energy. In this study, SnSe is successfully deposited on a centimeter-scale nickel foam using a facile one-step pulsed-laser deposition approach. Importantly, the maximum evaporation rate of SnSe-coated nickel foam (SnSe@NF) reaches 0.85 kg m^-2 h^-1, which is even 21% larger than that obtained with the commercial super blue coating (0.7 kg m^-2 h^-1) under the same condition. A systematic analysis reveals that its good photothermal conversion capability is attributed to the synergetic effect of multi-scattering-induced light trapping and the optimal trade-off between light absorption and phonon emission. Finally, the SnSe@NF device is further used for seawater evaporation, demonstrating a comparable evaporation rate (0.8 kg m^-2 h^-1) to that of fresh water and good stability over many cycles of usage. In summary, the current contribution depicts a facile one-step scenario for the economical and efficient solar-enabled SnSe@NF evaporation devices. More importantly, an in-depth analysis of the photothermal conversion mechanism underneath the layered materials depicts a fundamental paradigm for the design and application of photothermal devices based on them in the future.

van der Waals Layered Materials: Opportunities and Challenges

Since graphene became available by a scotch tape technique, a vast class of two-dimensional (2D) van der Waals (vdW) layered materials has been researched intensively. What is more intriguing is that the well-known physics and chemistry of three-dimensional (3D) bulk materials are often irrelevant, revealing exotic phenomena in 2D vdW materials. By further constructing heterostructures of these materials in the planar and vertical directions, which can be easily achieved via simple exfoliation techniques, numerous quantum mechanical devices have been demonstrated for fundamental research and technological applications. It is, therefore, necessary to review the special features in 2D vdW materials and to discuss the remaining issues and challenges. Here, we review the vdW materials library, technology relevance, and specialties of vdW materials covering the vdW interaction, strong Coulomb interaction, layer dependence, dielectric screening engineering, work function modulation, phase engineering, heterostructures, stability, growth issues, and the remaining challenges.

A unique semiconductor-carbon-metal hybrid structure design as a counter electrode in dye-sensitized solar cells

The catalytic activity of counter electrodes (CEs) severely restricts the photovoltaic conversion efficiency of dye-sensitized solar cells. However, electrons trapped by bulk defects greatly reduce the catalytic activity of the CE. In this study, we report a novel In₂S₃-C-Au hybrid structure designed by simply decorating Au particles on the surface of carbon-coated hierarchical In₂S₃ flower-like architectures, which could avoid the abovementioned problems. This effect can be attributed to the unique contribution of indium sulfide, carbon, and Au from the hybrid structure, as well as to their synergy. Electrochemical measurements revealed that the hybrid structure possessed high catalytic activity and electrochemical stability for the interconversion of the redox couple I₃^-/I^-. Moreover, this superior performance can be incorporated into the dye-sensitized solar cells system. We used this hybrid structure as a counter electrode by casting it on an FTO substrate to form a film, which displayed better photovoltaic conversion efficiency (8.91%) than the commercial Pt counterpart (7.67%).

Solar-Light-Driven Pure Water Splitting with Ultrathin BiOCl Nanosheets

A suitable photocatalyst for overall water splitting has been produced by overcoming the disadvantage of the band structure in bulk BiOCl by reducing the thickness to the quantum scale. The ultrathin BiOCl nanosheets with surface/subsurface defects realized the solar-driven pure water splitting in the absence of any co-catalysts or sacrificial agent. These surface defects cannot only shift both the valence band and conduction band upwards for band-gap narrowing but also promote charge-carrier separation. The amount of defects in the outer layer surface of BiOCl results in an enhancement of carrier density and faster charge transport. First-principles calculations provide clear evidence that the formation of surface oxygen vacancies is easier for the ultrathin BiOCl nanosheets than for its thicker counterpart. These defects can serve as active sites to effectively adsorb and dissociate H2 O molecules, resulting in a significantly improved water-splitting performance.

The Hydric Effect in Inorganic Nanomaterials for Nanoelectronics and Energy Applications

Protons, as one of the world's smallest ions, are able to trigger the charge effect without obvious lattice expansion inside inorganic materials, offering a unique and important test-bed for controlling their diverse functionalities. Arising from the high chemical reactivity of hydrogen (easily losing an electron) with various main group anions (easily accepting a proton), the hydric effect provides a convenient and environmentally benign route to bring about fascinating new physicochemical properties, as well as to create new inorganic structures based on the "old lattice" without dramatically destroying the pristine structure, covering most inorganic materials. Moreover, hydrogen atoms tend to bond with anions or to produce intrinsic defects, both of which are expected to inject extra electrons into lattice framework, promising advances in control of bandgap, spin behavior, and carrier concentration, which determine functionality for wide applications. In this review article, recently developed effective hydric strategies are highlighted, which include the conventional hydric reaction under high temperature or room temperature, proton irradiation or hydrogen plasma treatment, and gate-electrolyte-driven adsorption or doping. The diverse physicochemical properties brought by the hydric effect via modulation of the intrinsic electronic structure are also summarized, finding wide applications in nanoelectronics, energy applications, and catalysis.