Synthesis and crystal growth

Abstract:

Recent theoretical proposals have argued that cobaltates with edge-sharing octahedral coordination can have significant bond-dependent exchange couplings thus offering a platform in 3d ions for such physics beyond the much-explored realizations in 4d and 5d materials. Here we present high-resolution inelastic neutron scattering data within the magnetically ordered phase of the stacked honeycomb magnet CoTiO3 revealing the presence of a finite energy gap and demonstrate that this implies the presence of bond-dependent anisotropic couplings. We also show through an extensive theoretical analysis that the gap further implies the existence of a quantum order-by-disorder mechanism that, in this material, crucially involves virtual crystal field fluctuations. Our data also provide an experimental observation of a universal winding of the scattering intensity in angular scans around linear band-touching points for both magnons and dispersive spin-orbit excitons, which is directly related to the non-trivial topology of the quasiparticle wavefunction in momentum space near nodal points. The deposited data package contains neutron powder diffraction and single crystal inelastic neutron scattering data to probe the magnetic ordering and dynamics. The zip archive contains the data in the format of multi-column ASCII files, a README.txt that explains the content of all the files and matlab code to plot the data in all the files, together with relevant RGB colourmap.

Abstract:

Sustainable hydrogen production is attracting increasing attention and visible-light-driven water splitting is considered as one of the most promising approaches for hydrogen evolution and solar energy storage. Different materials have been screened at mild conditions in recent decades and 2-dimensional (2D) layered materials are considered good candidates for the photocatalytic water splitting reaction. 2D single layer MoS2 has shown its potential in various catalytic systems, and has also been used in photocatalytic water splitting reaction recently. However, current studies of MoS2 monolayers give low intrinsic activity, preventing it from practical applications. This is attributed to the rapid recombination of the photo-excited charge carriers at room temperature, resulting in poor quantum efficiency (QE). Herein, a state-of-the-art strategy to prolong the exciton lifetimes is reported, which is achieved by combining the 2D MoS2 nanosheets with solid state polar-faceted supports. The charge separation process can be facilitated by the strong local polarisation introduced by the polar-faceted supports, and tuned by changing the supports with different surface polarities. Polar oxide surface is the exposure of oxygen-terminated high energetic facet, which is known to give a net dipole moment perpendicular to its surface. Such variation in the surface properties of the support to the above metal would lead to a difference in metal-support interaction(s). The resulting composite structures show great enhancement toward the visible-light-driven photocatalytic water splitting reaction, giving hydrogen and oxygen evolution in a stoichiometric 2:1 ratio at elevated temperatures from pure water. Photocatalytic performances are improved by the prolonged exciton lifetimes and exceptional hydrogen evolution activity of 2977 μmol g−1 h−1 with impressive QEs are obtained over Ru-doped MoS2 nanosheets on polar ceria support, which is among the best of the reported results of similar catalytic systems to date. More excitingly, the linear relationship between the exciton lifetimes and strength of the local polarisation is also observed, indicating that the rational design of photocatalysts can be simply achieved via engineering their local polarisation by incorporation of polar-faceted materials.