Day, (2010) Modelling organic crystal structures using distributed multipole and polarizability-based model intermolecular potentials. Physical
Graphitic carbon nitride compounds were prepared by thermal treatment of C−N−H precursor mixtures (melamine C 3 N 6 H 9 , dicyandiamide C 2 N 4 H 4 ). This led to solids based on polymerized heptazine or triazine ring units linked by −N or −NH− groups. The H content decreased, and the C/ N ratio varied between 0.59 and 0.70 with preparation temperatures between 550 and 650 °C due to increased layer condensation. The UV−vis spectra exhibited a strong π−π* transition near 400 nm with a semiconductor-like band edge extending into the visible range. Samples synthesized at 600−650 °C showed an additional absorption near 500 nm that is assigned to n−π* electronic transitions involving the N lone pairs. These are forbidden for planar symmetric s-triazine or heptazine structures but become allowed as increased condensation causes distortion of the polymeric units. Photocatalysis studies showed there was no correlation between the increased visible absorption due to this feature and H 2 evolution from methanol used for the anodic reaction. In the absence of any cocatalyst the sample synthesized at 550 °C showed 1.5 μmol h −1 H 2 evolution with UV−vis irradiation, but this dropped to ∼0.23 μmol h −1 when the UV spectrum was blocked. Use of a Pt cocatalyst was required to observe H 2 evolution from the other samples. Using a more powerful (300 W) lamp led to higher H 2 production rates (31.5 μmol h −1 ) with visible illumination. We suggest the distorted N sites caused by increased polymerization result in electron/hole traps that counter the photocatalytic efficiency. Issues concerning sample porosity are also present. Photocatalytic O 2 evolution was determined for RuO 2 -coated samples using the 300 W lamp with aqueous AgNO 3 solution as the sacrificial agent. The materials all showed production rates ∼9 μmol h −1 . A highly crystalline compound containing polytriazine structural units ((C 3 N 3 ) 2 (NH) 3 •LiCl) prepared in this study did not show measurable photocatalytic activity.
The structure and electronic properties of single-layered carbon nitride graphenes are examined computationally with hybrid-exchange functionals in periodic density functional theory calculations. Unlike pure carbon graphene that provides a metallic nanomaterial, carbon nitride graphenes form semiconductors with band gaps ranging up to 5 eV. The band gap is sensitive to external perturbations that can be introduced chemically by adatom adsorption or physically by constraining the lattice parameter. Carbon nitride graphenes could possibly pave the way for a new range of smaller and faster transistors, as well as have useful sensing and actuating properties.
Porous metal-organic frameworks (MOFs) display a tremendous range of crystal structures, [1] rich host-guest chemistry, and potential for major impact in adsorption and separation technologies [2] and catalysis. [3] A growing sub-class of "soft" MOFs behave in a remarkable guest-responsive fashion upon gas or solvent adsorption/desorption and have gained considerable attention, exhibiting a wide range of structural transitions [4,5] and the topic has been the subject of a recent review.[6] Here, we focus on the experimentally well-documented MIL-53(Al) material [6] known for its reversible switching between a large pore (lp) and a narrow pore (np) form upon gas or solvent adsorption. Interestingly, Liu et al. [7] have recently reported the occurrence of the lp to np transition without a guest molecule, establishing the intrinsic bistable behavior of the MIL-53(Al) host. While previous simulation studies [8] have examined the role of guest molecules in the lp to np transition, the driving force for the formation of the guest-free np structure has yet to be elucidated. More generally, the question of the origin of the intrinsic bistability of this topical MOF remains open. Here we show that dispersive interactions cause the np structure to stabilize at low temperature and entropy drives the structural transition to the lp phase. Figure 1 depicts the np and lp MIL-53(Al) structures: between 325 K and 375 K a marked transition occurs where the unit cell volume nearly doubles from 864 3 in the np structure to 1419 3 in the lp structure. It has been suggested that the driving force for this transition is provided by low energy phonon modes [7] and we speculate that dispersion interactions, potentially emanating from p-p stacking of the phenyl ligands may be important. Density functional theory (DFT) offers the possibility of probing the structure-energy relationship. However, previously reported DFT [7] and forcefield [8] studies have been unable to stabilize the np structure. The np structure opens up on relaxation to give the lp form. A suspicion is that dispersive interactions, which are absent from "standard" GGA functionals used for DFT (e.g. leading to the exfoliation of graphite into unconnected sheets), may be key to understanding the bistability of this material. Here, we examine two complementary approaches to explore the role of dispersive effects within the DFT regime: 1) An efficient non-local functional, [9] vdW-DF, where dispersion is calculated self-consistently and depends on the unique electron density on each atom. 2) Dispersive interactions that depend on empirical ÀC 6 r À6 terms, the DFT-D method [10] and its solid-specific reparameterization.[11] Further details of the methods and settings used in this work are presented in the Supporting Information.In Table 1, we present a summary of our results. In keeping with previous work, with a standard GGA functional the np structure yields no local minimum and the structure opens up to give the lp form. Furthermore, when the atomic
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