Preparation of 2D graphitic carbon nitride nanosheets by a green exfoliation approach and the enhanced photocatalytic performance

In this paper, we reported a new and environment-friendly strategy to exfoliate graphitic carbon nitride (CN) in hot water to obtain ultrathin CN nanosheets (CNNS). By thermal treating, water molecules were intercalated into the interlayer space of bulky CN and further hydrolyzed the bridge-linked N groups between tri-s-triazine units, thereby cutting the CN layers into CNNS. Due to the negative charges on surface (Zeta potential was ?13 mV at pH 7), the CNNS colloids were extremely stable. The physicochemical characterization indicated that the as-prepared CNNS had a typical 2D morphology with a ~1.2 nm thickness and numerous –OH groups on surface. Moreover, the high charge separation and transport ability were achieved in CNNS because of the retaining of conjugated CN system. Compared to the bulk CN, the as-prepared ultrathin CNNS exhibited an enhanced photocatalytic performance (four times higher than that of the bulk CN) for degradation of organic dye under visible light irradiation. Additionally, the superior reusability and the excellent generality of CNNS for decomposing other pollutants were also demonstrated. Finally, we proposed a possible mechanism of CNNS based on the examined band structure and the main active species determined by quenching of various active species.     

Drastic Enhancement of Photocatalytic Activities over Phosphoric Acid Protonated Porous g‐C3N4 Nanosheets under Visible Light

A simple method is developed to fabricate protonated porous graphitic carbon nitride nanosheets (P‐PCNNS) by protonation–exfoliation of bulk graphitic carbon nitride (BCN) with phosphoric acid (H₃PO₄). The H₃PO₄ treatment not only helps to exfoliate the BCN into 2D ultrathin nanosheets with abundant micro‐ and mesopores, endowing P‐PCNNS with more exposed active catalytic sites and cross‐plane diffusion channels to facilitate the mass and charge transport, but also induces the protonation of carbon nitride polymer, leading to the moderate removal of the impurities of carbon species in BCN for the optimization of the aromatic π‐conjugated system for better charge separation without changing its chemical structure. As a result, the P‐PCNNS show much higher photocatalytic performance for hydrogen evolution and CO₂ conversion than bare BCN and graphitic carbon nitride nanosheets.     

Controllable preparation of graphitic carbon nitride nanosheets via confined interlayer nanospace of layered clays

This work described the size-controllable preparation of novel graphitic carbon nitride nanosheets using the interlayer space of smectite clays as a confined nanoreactor through cyanamide condensation. The prepared solids were characterized by X-ray diffraction, FT-IR, SEM and TEM. Results revealed that when ammonium-saponite was used, graphitic carbon nitride with an average crystal size of about 4.8 nm, the particle size of 50-100 nm and a C/N = 1.931 was obtained. When montmorillonite was used, graphitic carbon nitride with a crystal size of approximate 6.0 nm, the particle size of 100-150 nm and a C/N = 0.959 was yielded. The space-confined preparation of carbon nitride using layered clay minerals not only changes the texture of the resultant carbon nitride, but also alters the composition of products owing to the change of the whole reaction scheme to yield some new species. Such controllability over the size can be ascribed to the difference of the interlayer space and the cation exchange capacity of saponite and montmorillonite.     

One-pot synthesis of array-like sulfur-doped carbon nitride with covalently crosslinked ultrathin MoS2 cocatalyst for drastically enhanced photocatalytic hydrogen evolution

Constructing noble-metal-free loaded catalyst with high-efficiency photocatalytic activity by a simple and scalable method is of profound significance for fundamental research and practical application.Herein,a simple one-pot method was used to synthesize novel samples of array-like sulfur-doped graphitic carbon nitride(SCN)nanosheets with ultrathin MoS2 loading(MS/SCN-x％).The ultrathin MoS2 cocatalyst was evenly distributed on the surface of SCN and was linked to the main catalyst by covalent chemical bonds.Benefited from the multiple advantages of the array-like porous nanosheets structure with rich exposed surface,covalent cross-linking structure,and enhanced visible light absorption,the MS/SCN-2.5％composites drastically improve hydrogen evolution performance,which is superior to orig-inal MoS2 nanosheet modified by two-step mixing method,and also rivals with Pt/SCN.The designing strategy of photocatalyst modified by noble-metal-free cocatalyst with covalent bond structure provides fascinating insights into enhanced photocatalytic hydrogen evolution.     

Preparation and characterization of morph-genetic aluminum nitride/carbon composites from filter paper

Morph-genetic aluminum nitride/carbon composites with cablelike structure were prepared from filter paper template through the surface sol-gel process and carbothermal nitridation reaction. The resulting materials have a hierarchical structure originating from the morphology of cellulose paper. The aluminum nitride/carbon composites have the core-shell microstructure, the core is graphitic carbon, and the shell is aluminum nitride nanocoating formed by carbothermal nitridation reduction of alumina with the interfacial carbon in nitrogen atmosphere. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and transmission electron microscope were employed to characterize the structural morphology and phase compositions of the final products.     

新型二维硅碳氮纳米片的制备及其电化学性能

本文采用高温煅烧方法,制备了新型硅碳氮(SiCN)纳米材料,利用场发射扫描电镜(FESEM)、X射线光电子能谱(XPS)等手段对该材料进行了一系列的结构和物相表征,发现该材料是具有微孔结构的层状材料,且其比表面积达到了420m2/g。通过循环伏安(CV),恒流电流充放电(GCD)和交流阻抗(EIS)等检测手段研究了该材料的超级电容器性能,结果表明:该电极材料在酸性电解液中,表现出优异的比容量和循环稳定性能。本文设计并提出的这种新颖简单的SiCN材料的制备方法也可应用于其他能量存储材料的开发与研究。     

Flexible Co–Mo–N/Au Electrodes with a Hierarchical Nanoporous Architecture as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Designing highly active and robust electrocatalysts for oxygen evolution reaction (OER) is crucial for many renewable energy storage and conversion devices. Here, self-supported monolithic hybrid electrodes that are composed of bimetallic cobalt–molybdenum nitride nanosheets vertically aligned on 3D and bicontinuous nanoporous gold (NP Au/CoMoN_x) are reported as highly efficient electrocatalysts to boost the sluggish reaction kinetics of water oxidation in alkaline media. By virtue of the constituent CoMoN_x nanosheets having large accessible CoMoO_x surface with remarkably enhanced electrocatalytic activity and the nanoporous Au skeleton facilitating electron transfer and mass transport, the NP Au/CoMoN_x electrode exhibits superior OER electrocatalysis in 1 m KOH, with low onset overpotential (166 mV) and Tafel slope (46 mV dec⁻¹). It only takes a low overpotential of 370 mV to reach ultrahigh current density of 1156 mA cm⁻², ≈140-fold higher than free CoMoN_x nanosheets. The electrocatalytic performance makes it an attractive candidate as the OER catalyst in the water electrolysis.     

Graphene quantum dots-assisted exfoliation of graphitic carbon nitride to prepare metal-free zero-dimensional/two-dimensional composite photocatalysts

The high aspect-ratio morphology of two-dimensional (2D) nanostructures endues them with distinct advantages for photocatalytic or photoelectrical applications. Although various attempts have been devoted to the liquid exfoliation of graphitic carbon nitride (g-C₃N₄) to obtain ultrathin nanosheets (CNNSs), the high exfoliation efficiency, well preservation of in-planar structure and facile operation cannot be simultaneously realized. Furthermore, functionalization of CNNSs is highly desired to promote the capability of photoabsorption, charge separation and transfer. Herein, we one-step prepared well-dispersed graphene quantum dots (GQDs)-modified CNNSs (GQDs/CNNSs) colloids via a facile and efficient GQDs-assisted exfoliation approach in a normal ultrasonic water bath. The exfoliation procedure was optimized by tuning the dopant in GQDs, ultrasonic time and GQDs dosage. The obtained colloidal GQDs/CNNSs show a typical 2D morphology with lateral size of several 100 nm and ultrathin thickness of 1.5–1.8 nm. What is more, we can tailor the semiconductive behavior of GQDs by heteroatom doping and achieve a p–n-type P-doped GQDs-modified CNNSs colloids. This p–n GQDs/CNNSs material presents the enhanced separation efficiency of photoexcited carriers and photocatalytic activity in comparison with bulky g-C₃N₄ (CN) and other CNNSs materials from acid or alkali exfoliation.     

A Novel Heterostructure Based on RuMo Nanoalloys and N-doped Carbon as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N-doped carbon nanosheets is designed and synthesized. In this protocol, metal-containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as-made RuMo-nanoalloys-embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec⁻¹), and a high turnover frequency (3.57 H₂ s⁻¹) in alkaline media, outperforming current Ru-based electrocatalysts. First-principle calculations based on typical 2D N-doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon–metal heterostructures for highly efficient electrocatalysis.     

Sulfur‐Modified Graphitic Carbon Nitride Nanostructures as an Efficient Electrocatalyst for Water Oxidation

There is an urgent need to develop metal‐free, low cost, durable, and highly efficient catalysts for industrially important oxygen evolution reactions. Inspired by natural geodes, unique melamine nanogeodes are successfully synthesized using hydrothermal process. Sulfur‐modified graphitic carbon nitride (S‐modified g‐CN _x ) electrocatalysts are obtained by annealing these melamine nanogeodes in situ with sulfur. The sulfur modification in the g‐CN _x structure leads to excellent oxygen evolution reaction activity by lowering the overpotential. Compared with the previously reported nonmetallic systems and well‐established metallic catalysts, the S‐modified g‐CN _x nanostructures show superior performance, requiring a lower overpotential (290 mV) to achieve a current density of 10 mA cm^?2 and a Tafel slope of 120 mV dec^?1 with long‐term durability of 91.2% retention for 18 h. These inexpensive, environmentally friendly, and easy‐to‐synthesize catalysts with extraordinary performance will have a high impact in the field of oxygen evolution reaction electrocatalysis.     

Orientation controlled preparation of nanoporous carbon nitride fibers and related composite for gas sensing under ambient conditions

Creating pores in suprastructures of two-dimensional (2D) materials while controlling the orientation of the 2D building blocks is important in achieving large specific surface areas and tuning the anisotropic properties of the obtained functional hierarchical structures.In this contribution,we report that arranging graphitic carbon nitride (g-C3N4) nanosheets into one-dimensional (1D) architectures with controlled orientation has been achieved by using 1D oriented melem hydrate fibers as the synthetic precursor via a polycondensation process,during which the removal of water molecules and release of ammonia gas led to the creation of pores without destroying the 1D morphology of the oriented structures.The resulting porous g-C3N4 fibers with both meso-and micro-sized pores and largely exposed edges exhibited good sensing sensitivity and selectivity towards NO2.The sensing performance was further improved by hybridization of the porous fibers with Au nanoparticles (Au NPs),leading to a detection limit of 60 ppb under ambient conditions.Our results suggest that the highly porous g-C3N4 fibers and the related hybrid structures with largely exposed graphitic layer edges are excellent sensing platforms and may also show promise in other electronic and electrochemical applications.     

Effective mechanical properties of hexagonal boron nitride nanosheets

We propose an analytical formulation to extract from energy equivalence principles the equivalent thickness and in-plane mechanical properties (tensile and shear rigidity, and Poisson's ratio) of hexagonal boron nitride (h-BN) nanosheets. The model developed provides not only very good agreement with existing data available in the open literature from experimental, density functional theory (DFT) and molecular dynamics (MD) simulations, but also highlights the specific deformation mechanisms existing in boron nitride sheets, and their difference with carbon-based graphitic systems.     

Proton‐Functionalized Two‐Dimensional Graphitic Carbon Nitride Nanosheet: An Excellent Metal‐/Label‐Free Biosensing Platform

Ultrathin graphitic carbon nitride (g‐C₃N₄) nanosheets, due to their interesting two‐dimensional graphene‐like structure and unique physicochemical properties, have attracted great research attention recently. Here, a new approachis developed to prepare, for the first time, proton‐functionalized ultrathin g‐C₃N₄ nanosheets by sonication‐exfoliation of bulk g‐C₃N₄ under an acid condition. This method not only reduces the exfoliation time from more than 10 h to 2 h, but also endows the nanosheets with positive charges. Besides retaining the properties of g‐C₃N₄, the obtained nanosheets with the thickness of 2–4 nm (i.e., 6–12 atomic monolayers) also exhibit large specific surface area of 305 m² g^?1, enhanced fluorescence intensity, and excellent water dispersion stability due to their surface protonation and ultrathin morphology. The well‐dispersed protonated g‐C₃N₄ nanosheets are able to interact with negatively charged heparin, which results in the quenching of g‐C₃N₄ fluorescence. A highly sensitive and highly selective heparin sensing platform based on protonated g‐C₃N₄ nanosheets is established. This metal‐free and fluorophore label‐free system can reach the lowest heparin detection limit of 18 ng mL^?1.     

Growth of Highly Active Amorphous RuCu Nanosheets on Cu Nanotubes for the Hydrogen Evolution Reaction in Wide pH Values

Rational design of low‐cost, highly efficient, and stable electrocatalysts for the hydrogen evolution reaction (HER) has attracted wide attention. Herein, 3D RuCu nanocrystals (NCs) are successfully synthesized by a facile wet chemistry method, in which amorphous RuCu nanosheets are directly grown on crystalline Cu nanotubes (NTs). Importantly, the obtained 3D RuCu NCs only need 18 and 73 mV to deliver the current density of 10 mA cm^?2 for HER in alkaline and neutral media, respectively. Density functional theory calculations and experiments reveal that the Ru sites on the surface of amorphous nanosheets are the highly active centers for HER. Moreover, this catalyst can expose more surface area for water splitting compared to pure nanosheets because the unique 3D structure can effectively prevent the aggregation of nanosheets. Meanwhile, the interface between amorphous nanosheets and crystalline NTs is essential to boost the HER performance because the amorphous phase with many unsaturated bonds can facilitate adsorption of reactants and crystalline Cu with superior conductivity can promote the transfer of electrons. This work provides a facile method to prepare an original 3D Ru‐based electrocatalyst with highly active HER performance in wide pH values.     

Chlorine doped graphitic carbon nitride nanorings as an efficient photoresponsive catalyst for water oxidation and organic decomposition

Rationally engineering the microstructure and electronic structure of catalysts to induce high activity for versatile applications remains a challenge. Herein, chlorine doped graphitic carbon nitride (Cl-doped g-C₃N₄) nanorings have been designed as a superior photocatalyst for pollutant degradation and oxygen evolution reaction (OER). Remarkably, Cl-doped g-C₃N₄ nanorings display enhanced OER performance with a small overpotential of approximately 290 mV at current density of 10 mA cm⁻² and Tafel slope of 83 mV dec^-1, possessing comparable OER activity to precious metal oxides RuO₂ and IrO₂/C. The excellent catalytic performance of Cl-doped g-C₃N₄ nanorings originates from the strong oxidation capability, abundant active sites exposed and efficient charge transfer. More importantly, visible light irradiation gives rise to a prominent improvement of the OER performance, reducing the OER overpotential and Tafel slope by 140 mV and 28 mV dec^-1, respectively, demonstrating the striking photo-responsive OER activity of Cl-doped g-C₃N₄ nanorings. The great photo-induced improvement in OER activity would be related to the efficient charge transfer and the OH radicals arising spontaneously on CN-Cl₁₀₀ catalyst upon light irradiation. This work establishes Cl-doped g-C₃N₄ nanorings as a highly competitive metal-free candidate for photoelectrochemical energy conversion and environmental cleaning application.     

Constructing Built‐in Electric Field in Ultrathin Graphitic Carbon Nitride Nanosheets by N and O Codoping for Enhanced Photocatalytic Hydrogen Evolution Activity

Codoping of N and O in ultrathin graphitic carbon nitride (g‐C₃N₄) nanosheets leads to an inner electric field. This field restrains the recombination of photogenerated carriers and, thus, enhances hydrogen evolution. The layered structure of codoped g‐C₃N₄ nanosheets (N‐O‐CNNS) not only provides abundant sites of contact with the reaction medium, but also decreases the distance over which the photogenerated electron–hole pairs are transported to the reaction interface. Quantum confinement in the ultrathin structure results in an increased bandgap and makes the photocatalytic reaction more favorable than bulk g‐C₃N₄. Under visible light irradiation, N‐O‐CNNS with 3 wt% Pt achieves a hydrogen evolution rate of 9.2 mmol g^?1 h^?1 and a value of 46.9 mmol g^?1 h^?1 under AM1.5 with 5 wt% Pt. Thus, this work paves the way for designing efficient nanostructures with increased separation/transfer efficiency of photogenerated carriers and, hence, increased photocatalytic activities.     

Type‐I Transition Metal Dichalcogenides Lateral Homojunctions: Layer Thickness and External Electric Field Effects

Transition metal dichalcogenide (TMD) heterostructures have been widely explored due to the formation of type‐II band alignment and interlayer exciton. However, the studies of type‐I TMD heterostructures are still lacking, which limit their applications in luminescence devices. Here, the 1L/nL MX₂ (n = 2, 3, 4; M = Mo, W; X = S, Se) lateral homojunction based on the layer‐dependent band gaps of TMD nanosheets is theoretically simulated. The studies show that the TMD homojunction presents with high thermal stability and type‐I band alignment. The band offset and quantum confinement of carriers can be easily tuned by controlling the thickness of the multilayer region. Moreover, the electric field can decrease the band gaps of 1L/3L and 1L/4L homojunctions linearly. Interestingly, for the 1L/2L MX₂ homojunction, the gap value is robust to the weak electric field, while it drops sharply under a strong electric field. This study sheds light on the physical pictures in the TMD lateral homojunction, and provides a practicable and general approach to engineer a type‐I homojunction based 2D semiconductor materials.     

铂修饰二氧化钛纳米片的制备及其光电催化析氢反应研究

利用静电吸附作用在二氧化钛纳米片上负载铂原子制备了两种不同形态的铂催化剂。SEM、XRD、TEM测试结果表明, 改变铂负载量可以调控铂的形貌结构。在低Pt负载(0.2wt%)下, 铂原子主要是半径约2 nm的纳米簇, 当Pt负载量增加到1wt%时, 铂原子在二氧化钛纳米片上堆积成纳米颗粒。调控Pt负载量和纳米结构, 可以显著提高二氧化钛纳米片催化析氢反应的活性。在AM1.5太阳光照射下, 两种催化剂的塔菲尔斜率都小于100 mV/dec, 分别为56和90 mV/dec。与TiO₂-Pt1%催化剂相比, TiO₂-Pt0.2%具有更理想的金属-半导体界面, 有利于光生电子迁移至铂纳米簇表面, 因而具有更高的催化活性。本实验为研究更加高效的铂催化剂和其他光电催化剂提供了新的途径。     

Facile Solid-State Synthesis Route to Metal Nitride Nanoparticles

By a facile and efficient solid-state reaction route using an organic reagent cyanamide （CN2H2） as a precursor with another one being metal oxides, we successfully synthesized seven technologically important metal nJtrJdes including cubic VN, CrN, NbN, hexagonal GaN, AIN, BN, and WN at moderate temperatures. The experimental results show that cyanamide （CN2H2） is a powerfully reducing and nitridizing reagent and the metal oxides are completely converted into the corresponding nitride nanoparticles at lower temperatures than that reported in the conventional methods. It is found that EN2H2 can exhibit some interesting condensation processes, and the final products, highly active carbon nitride species, play a crucial role in the reducing and nitridizing processes.     

Tuning the Activity of Carbon for Electrocatalytic Hydrogen Evolution via an Iridium‐Cobalt Alloy Core Encapsulated in Nitrogen‐Doped Carbon Cages

Graphene, a 2D material consisting of a single layer of sp²‐hybridized carbon, exhibits inert activity as an electrocatalyst, while the incorporation of heteroatoms (such as N) into the framework can tune its electronic properties. Because of the different electronegativity between N and C atoms, electrons will transfer from C to N in N‐doped graphene nanosheets, changing inert C atoms adjacent to the N‐dopants into active sites. Notwithstanding the achieved progress, its intrinsic activity in acidic media is still far from Pt/C. Here, a facile annealing strategy is adopted for Ir‐doped metal‐organic frameworks to synthesize IrCo nanoalloys encapsulated in N‐doped graphene layers. The highly active electrocatalyst, with remarkably reduced Ir loading (1.56 wt%), achieves an ultralow Tafel slope of 23 mV dec^?1 and an overpotential of only 24 mV at a current density of 10 mA cm^?2 in 0.5 m sulfuric acid solution. Such superior performance is even superior to the noble‐metal catalyst Pt. Surface structural and computational studies reveal that the superior behavior originates from the decreased ΔG_H* for HER induced by the electrons transferred from the alloy core to the graphene layers, which is beneficial for enhancing C? H binding.