Metal–organic frameworks (MOFs) are often synthesized using various additives to modulate the crystallization. Here, we report the direct imaging of the crystal surface of MOF MIL-101 synthesized ...with different additives, using low-dose high-resolution transmission electron microscopy (HRTEM), and identify three distinct surface structures, at subunit cell resolution. We find that the mesoporous cages at the outermost surface of MIL-101 can be opened up by vacuum heating treatment at different temperatures, depending on the MIL-101 samples. We monitor the structural evolution of MIL-101 upon vacuum heating, using in situ X-ray diffraction, and find the results to be in good agreement with HRTEM observations, which leads us to speculate that additives have an influence not only on the surface structure but also on the stability of framework. In addition, we observe solid–solid phase transformation from MIL-101 to MIL-53 taking place in the sample synthesized with hydrofluoric acid.
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the ...electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices.
Rational design and synthesis of heterostructures based on transition metal dichalcogenides (TMDs) have attracted increasing interests because of their promising applications in electronics, ...catalysis, etc. However, the construction of epitaxial heterostructures with an interface at the edges of TMD nanosheets (NSs) still remains a great challenge. Here, we report a strategy for controlled synthesis of a new type of heterostructure in which TMD NSs, including MoS2 and MoSe2, vertically grow along the longitudinal direction of one-dimensional (1D) Cu2–x S nanowires (NWs) in an epitaxial manner. The obtained Cu2–x S-TMD heterostructures with tunable loading amount and lateral size of TMD NSs are achieved by the consecutive growth of TMD NSs on Cu2–x S NWs through gradual injection of chalcogen precursors. After cation exchange of Cu in Cu2–x S-TMD heterostructures with Cd, the obtained CdS–MoS2 heterostructures retained their original architectures. Compared to the pure CdS NWs, the CdS–MoS2 heterostructures with 7.7 wt % loading of MoS2 NSs exhibit the best performance in the photocatalytic hydrogen evolution reaction with a H2 production rate up to 4647 μmol·h–1·g–1, about 58 times that catalyzed with pure CdS NWs. Our synthetic strategy opens up a new way for the controlled synthesis of TMD-based heterostructures, which could have various promising applications.
Conventionally, the phase transformation of inorganic nanocrystals is realized under extreme conditions (for example, high temperature or high pressure). Here we report the complete phase ...transformation of Au square sheets (AuSSs) from hexagonal close-packed (hcp) to face-centered cubic (fcc) structures at ambient conditions via surface ligand exchange, resulting in the formation of (100)f-oriented fcc AuSSs. Importantly, the phase transformation can also be realized through the coating of a thin metal film (for example, Ag) on hcp AuSSs. Depending on the surfactants used during the metal coating process, two transformation pathways are observed, leading to the formation of (100)f-oriented fcc Au@Ag core-shell square sheets and (110)h/(101)f-oriented hcp/fcc mixed Au@Ag nanosheets. Furthermore, monochromated electron energy loss spectroscopy reveals the strong surface plasmon resonance absorption of fcc AuSS and Au@Ag square sheet in the infrared region. Our findings may offer a new route for the crystal-phase and shape-controlled synthesis of inorganic nanocrystals.
The synthesis of ultrathin face‐centered‐cubic (fcc) Au@Pt rhombic nanoplates is reported through the epitaxial growth of Pt on hexagonal‐close‐packed (hcp) Au square sheets (AuSSs). The Pt‐layer ...growth results in a hcp‐to‐fcc phase transformation of the AuSSs under ambient conditions. Interestingly, the obtained fcc Au@Pt rhombic nanoplates demonstrate a unique (101)f orientation with the same atomic arrangement extending from the Au core to the Pt shell. Importantly, this method can be extended to the epitaxial growth of Pd on hcp AuSSs, resulting in the unprecedented formation of fcc Au@Pd rhombic nanoplates with (101)f orientation. Additionally, a small amount of fcc (100)f‐oriented Au@Pt and Au@Pd square nanoplates are obtained with the Au@Pt and Au@Pd rhombic nanoplates, respectively. We believe that these findings will shed new light on the synthesis of novel noble bimetallic nanostructures.
Phase change: Ultrathin Au@Pt and Au@Pd core–shell nanoplates were prepared from Au square sheets. A phase transformation from hexagonal close‐packed (hcp) to face‐centered cubic (fcc) is observed upon coating the hcp Au square sheets with Pt or Pd under ambient conditions. The prepared fcc Au@Pt and Au@Pd rhombic nanoplates demonstrate unique (101)f orientation (picture shows a typical fcc Au@Pt rhombic nanoplate).
Open and friendly: The smallest member of the rht‐type metal–organic frameworks (MOFs, see picture) constructed by a hexacarboxylate ligand with a nitrogen‐rich imino triazine backbone shows a ...significantly enhanced gas binding affinity relative to all other isoreticular rht‐type MOFs. The high adsorption capacity and remarkable selectivity of CO2 are attributed to the high density of open metal and Lewis basic sites in the framework.
Metal nanomaterials normally adopt the same crystal structure as their bulk counterparts. Herein, for the first time, the unusual 4H hexagonal Ir, Rh, Os, Ru and Cu nanostructures have been ...synthesized on 4H Au nanoribbons (NRBs)
solution-phase epitaxial growth under ambient conditions. Interestingly, the 4H Au NRBs undergo partial phase transformation from 4H to face-centered cubic (fcc) structures after the metal coating. As a result, a series of polytypic 4H/fcc bimetallic Au@M (M = Ir, Rh, Os, Ru and Cu) core-shell NRBs has been obtained. We believe that the rational crystal structure-controlled synthesis of metal nanomaterials will bring new opportunities for exploring their phase-dependent physicochemical properties and promising applications.
Abstract
The defects in metal-organic frameworks (MOFs) can dramatically alter their pore structure and chemical properties. However, it has been a great challenge to characterize the molecular ...structure of defects, especially when the defects are distributed irregularly in the lattice. In this work, we applied a characterization strategy based on solid-state nuclear magnetic resonance (NMR) to assess the chemistry of defects. This strategy takes advantage of the coordination-sensitive phosphorus probe molecules, e.g., trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO), that can distinguish the subtle differences in the acidity of defects. A variety of local chemical environments have been identified in defective and ideal MOF lattices. The geometric dimension of defects can also be evaluated by using the homologs of probe molecules with different sizes. In addition, our method provides a reliable way to quantify the density of defect sites, which comes together with the molecular details of local pore environments. The comprehensive solid-state NMR strategy can be of great value for a better understanding of MOF structures and for guiding the design of MOFs with desired catalytic or adsorption properties.
Abstract
Riemann surfaces inspired chemists to design and synthesize such multidimensional curved carbon architectures. It has been predicted that carbon nanosolenoid materials with Riemann surfaces ...have unique structures and novel physical properties. Here we report the first synthesis of a nitrogen-doped carbon nanosolenoid (
N-CNS
) using bottom-up approach with a well-defined structure.
N-CNS
was obtained by a rational Suzuki polymerization, followed by oxidative cyclodehydrogenation. The successful synthesis of
N-CNS
was fully characterized by GPC, FTIR, solid-state
13
C NMR and Raman techniques. The intrinsic single-strand molecular structures of
N-CNS
helices can be clearly resolved using low-dose integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) technique. Possessing unique structural and physical properties, this long π-extended polymer
N-CNS
can provide new insight towards bottom-up syntheses of curved nanoribbons and potential applications as a metal-free photocatalyst for visible-light-driven H
2
evolution and highly efficient photocatalyst for photoredox organic transformations.
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