The research in two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs) and black phosphorus, has been further flourished with the recent emergence of heterostructures composed of dissimilar 2D materials. The interfacing/coupling between different constituent components in a heterostructure has given rise to interesting phenomena and useful properties. For example, depending on the type of 2D materials, the distance and the kind of bonding between them, as well as the crystalline property of the hetero-interface, the interface may provide charge traps, exciton recombination centers, or bridges for effective charge/energy transfer. It has also been found that the spatial arrangement in addition to the composition of the constituents is an important factor influencing the overall properties of the heterostructures. Although many methods, such as dry transfer and vapor-phased growth are able to yield heterostructures from pristine or highly crystalline 2D crystals with spatial control, such as vertical heterostructures and lateral heterostructures, these methods are generally not scalable, which has restricted the use of the obtained heterostructures mostly to fundamental studies. The solution-phased synthesis methods, such as solvothermal/hydrothermal synthesis, electrochemical deposition and hot-injection method, may be more suitable for mass production of functional heterostructures despite the relatively low product quality. In the past couple of years, a diverse kinds of hetero/hybrid structures of 2D materials have been prepared successfully in wet-chemical processes. However, precise control over the geometric arrangement of the constituent components has been challenging in solution. Currently, four types of heterostructures including 2D crystals grown on a larger 2D template, vertical heterostructures, lateral heterostructures, and core-shell heterostructures have been prepared in solution. For the first type, flexible 2D nanosheets such as graphene and monolayer TMDs are used as synthesis templates to support the nucleation and growth of other 2D crystals. For vertical heterostructures, relatively rigid nanoplates are used to allow continuous deposition of 2D layers of other materials to form sandwich-like structures. The formation of lateral heterostructures requires edge growth on existing 2D materials without basal deposition, and therefore other methods such as cation exchange can be used as alternative routes. The preparation of core-shell 2D heterostructures generally involves both epitaxial edge growth and basal deposition and has been realized in both metallic and semiconductor structures. In this review, these kinds of heterostructures based on 2D materials will be discussed in terms of their synthesis methods, properties and possible applications. In addition, we will discuss the challenges and possible opportunities in this research direction. 相似文献
Photocatalytic reduction of CO2 to hydrocarbon compounds is a promising method for addressing energy shortages and environmental pollution. Considerable efforts have been devoted to exploring valid strategies to enhance photocatalytic efficiency. Among various modification methods, the hybridization of different photocatalysts is effective for addressing the shortcomings of a single photocatalyst and enhancing its CO2 reduction performance. In addition, metal-free materials such as g-C3N4 and black phosphorus (BP) are attractive because of their unique structures and electronic properties. Many experimental results have verified the superior photocatalytic activity of a BP/g-C3N4 composite. However, theoretical understanding of the intrinsic mechanism of the activity enhancement is still lacking. Herein, the geometric structures, optical absorption, electronic properties, and CO2 reduction reaction processes of 2D/2D BP/g-C3N4 composite models are investigated using density functional theory calculations. The composite model consists of a monolayer of BP and a tri-s-triazine-based monolayer of g-C3N4. Based on the calculated work function, it is inferred that electrons transfer from g-C3N4 to BP owing to the higher Fermi level of g-C3N4 compared with that of BP. Furthermore, the charge density difference suggests the formation of a built-in electric field at the interface, which is conducive to the separation of photogenerated electron-hole pairs. The optical absorption coefficient demonstrates that the light absorption of the composite is significantly higher than that of its single-component counterpart. Integrated analysis of the band edge potential and interfacial electronic interaction indicates that the migration of photogenerated charge carriers in the BP/g-C3N4 hybrid follows the S-scheme photocatalytic mechanism. Under visible-light irradiation, the photogenerated electrons on BP recombine with the photogenerated holes on g-C3N4, leaving photogenerated electrons and holes in the conduction band of g-C3N4 and the valence band of BP, respectively. Compared with pristine g-C3N4, this S-scheme heterojunction allows efficient separation of photogenerated charge carriers while effectively preserving strong redox abilities. Additionally, the possible reaction path for CO2 reduction on g-C3N4 and BP/g-C3N4 is discussed by computing the free energy of each step. It was found that CO2 reduction on the composite occurs most readily on the g-C3N4 side. The reaction path on the composite is different from that on g-C3N4. The heterojunction reduces the maximum energy barrier for CO2 reduction from 1.48 to 1.22 eV, following the optimal reaction path. Consequently, the BP/g-C3N4 heterojunction is theoretically proven to be an excellent CO2 reduction photocatalyst. This work is helpful for understanding the effect of BP modification on the photocatalytic activity of g-C3N4. It also provides a theoretical basis for the design of other high-performance CO2 reduction photocatalysts. 相似文献
The effect of caffeine on the phase behavior of aqueous dispersion of cubic phase was studied. The morphologies of the dispersed particles were observed by means of cryo-transmission electron microscopy (TEM) and the sizes of the dispersed particles were determined by dynamic light scattering (DLS). It was found that the addition of caffeine could lead to the phase transition from cubic phase to vesicle, and this might be due to the “salt out” effect of caffeine on the monoglyceride molecules in aqueous phase. 相似文献
Block copolymer nanopaticles were prepared from the mixture solutions containing good/poor solvents by a simple evaporation process. The block copolymers formed disorder, unidirectionally stacked lamellar, and onion‐like structures in nanoparticles depending on preparation temperatures. Thermal annealing induced the disorder‐order phase transition and order‐order phase transformation in the block copolymer nanoparticles, even though the annealing temperature is lower than the of one polymer segment. The unusual thermal behaviors suggest that the glass transition temperature of the block copolymer is decreased by the effect of nanoparticle, whose surface areas are larger than their volumes.
Although vaterite is the least stable anhydrous calcium carbonate polymorph, it is formed as a metastable phase in some normal
and pathological biomineralisation processes. In this work, thermodynamic aspects of the vaterite-calcite phase transition
were comprehensively studied. Vaterite samples were prepared by different methods and characterised for the composition, crystal
structure, specific surface and grain size. All products were identified to be pure vaterite by careful X-ray diffraction
measurements. The enthalpy and Gibbs energy of transition were determined by precise calorimetric and potentiometric measurements.
The reliability of the thermodynamic data for the vaterite-calcite phase transition derived from this work was shown by the
use of different calorimetric methods to determine the enthalpy of transition and the independent measurements of heat capacity
and entropy of vaterite. Our recommended values are ΔtrsG*=−2.9±0.2 kJ mol−1 , Δ trsH *=−3.4±0.2 kJ mol−1 and Δ trsS *=−1.7±0.9 J K−1 mol−1 , where the uncertainties are given as twice the standard deviations.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
