RESUMO
The large inherent flexibility and highly modular nature of metal-organic frameworks (MOFs) make them ideal candidates for the study of negative thermal expansion (NTE). Among diverse organic ligands, the biphenyl unit, which can unrestrictedly rotate along its C-C single bond, can largely enhance the structural flexibility. Herein, we explored the thermal expansion behaviors of four indium biphenyl tetracarboxylates (BPTCs). Owing to the different dihedral angles of BPTC ligands and coordination mode of In3+, they show distinct topologies: InOF-1 (nti), InOF-2 (unc), InOF-12 (pts) and InOF-13 (nou). Intriguingly, it is found that the thermal expansion is highly dependent on the specific topology. The MOFs featuring mononuclear nodes show normal positive thermal expansion (PTE), and the magnitudes of coefficients follow the trend of InOF-2 < InOF-12 < InOF-13, inversely related to averaged molecular volumes. In contrast, the InOF-1, composed of a 1D chain of corner-shared InO6 octahedrons, shows pronounced NTE. Detailed high-resolution synchrotron powder X-ray diffraction and lattice dynamic analyses shed light on the fact that NTE in the InOF-1 is a synergy effect of the spring-like distortion of the inorganic 1D helical chain and twisting of the BPTC ligands. The present work shows how the topological arrangement of building blocks governs the thermal expansion behaviors.
RESUMO
Semiconductor photocatalytic water splitting is a green way to convert solar energy into chemical energy, but the recombination of electron and hole pairs and the low utilization of sunlight restrict the development of photocatalytic technology. By comparing the morphologies and hydrogen production properties of different proportions of solid solutions (CdxZn1-xS), one-dimensional (1D) Cd0.9Zn0.1S nanorods (NRs) with the best photocatalytic properties are obtained. In addition, 1D W18O49 nanowires are assembled on the surface of 1D Cd0.9Zn0.1S NRs to construct a novel 1D/1D step-scheme (S-scheme) W18O49/Cd0.9Zn0.1S heterojunction photocatalyst. The W18O49/Cd0.9Zn0.1S heterojunction expands the optical absorption capacity of Cd0.9Zn0.1S NRs to provide more energy for the photoexcitation of electrons. The optimal hydrogen production rate of W18O49/Cd0.9Zn0.1S NRs with W18O49 content of 9 wt% is as high as 66.3 mmol·h-1·g-1, which is 5.7 times and 1.6 times higher than that of Cd0.9Zn0.1S NRs and 1 wt% Pt/Cd0.9Zn0.1S NRs. The apparent quantum efficiency (AQE) of 9 wt% W18O49/Cd0.9Zn0.1S reaches 56.0 % and 25.9 % under light wavelength irradiation at 370 and 456 nm, respectively. After the 20 h cycle stability test, the activity of photocatalytic hydrogen evolution does not decrease, due that the severe photo-corrosion of Cd0.9Zn0.1S NRs is efficiently inhibited. This work not only provides a simple and controllable synthesis method for the preparation of heterojunction structure, but also opens up a new way to improve the hydrogen evolution activity and stability of sulfur compounds.