DFT studies reveal a hierarchy of water aggregates including dimers, quasi-1D ladders and a novel honeycomb-double monolayer.

Single-atom catalysts (SACs) have emerged as a new class of materials for the development of active and selective catalysts. These materials are commonly based on anchoring a noble transition metal to some kind of carrier. In the present work, we demonstrate that MXenes─two-dimensional materials with application in energy storage and conversion─spontaneously form SAC-like sites under anodic polarization conditions, using the applied electrode potential as a probe to form catalytically active surface sites reminiscent of a SAC-like structure. Combining ab initio molecular dynamics simulations and electronic structure calculations in the density functional theory framework, we demonstrate that only the SAC-like sites rather than the basal planes of MXenes are highly active and selective for the oxygen evolution or chlorine evolution reactions, respectively. Our findings may simplify synthetic routes toward the formation of active and selective SAC-like sites and could pave the way for the development of smart materials by incorporating fundamental principles from nature into material discovery: while the pristine form of the material is inactive, the application of an electrode potential activates the material by the formation of active and selective single-atom centers.

Herein, we report nanosecond, single-pulse laser post-processing (PLPP) in a liquid flat jet with precise control of the applied laser intensity to tune structure, defect sites, and the oxygen evolution reaction (OER) activity of mesostructured Co₃O₄. High-resolution X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) are consistent with the formation of cobalt vacancies at tetrahedral sites and an increase in the lattice parameter of Co₃O₄ after the laser treatment. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) further reveal increased disorder in the structure and a slight decrease in the average oxidation state of the cobalt oxide. Molecular dynamics simulation confirms the surface restructuring upon laser post-treatment on Co₃O₄. Importantly, the defect-induced PLPP was shown to lower the charge transfer resistance and boost the oxygen evolution activity of Co₃O₄. For the optimized sample, a 2-fold increment of current density at 1.7 V vs RHE is obtained and the overpotential at 10 mA/cm² decreases remarkably from 405 to 357 mV compared to pristine Co₃O₄. Post-mortem characterization reveals that the material retains its activity, morphology, and phase structure after a prolonged stability test.

One-dimensional tungsten disulfide (WS₂) single-walled nanotubes (NTs) with either achiral, i.e., armchair (n, n) and zigzag-type (n, 0), or chiral (2n, n) configuration with diameters d_NT > 1.9 nm have been found to be suitable for photocatalytic applications, since their band gaps correspond to the frequency range of visible light between red and violet (1.5 eV < Δε_gap < 2.6 eV). We have simulated the electronic structure of nanotubes with diameters up to 12.0 nm. The calculated top of the valence band and the bottom of the conduction band (ε_VB and ε_CB, respectively) have been properly aligned relatively to the oxidation (εO₂/H₂O) and reduction (εH₂/H₂O) potentials of water. Very narrow nanotubes (0.5 < d_NT < 1.9 nm) are unsuitable for water splitting because the condition ε_VB < εO₂/H₂O < εH₂/H₂O < ε_CB does not hold. For nanotubes with d_NT > 1.9 nm, the condition ε_VB < εO₂/H₂O < εH₂/H₂O < ε_CB is fulfilled. The values of ε_VB and ε_CB have been found to depend only on the diameter and not on the chirality index of the nanotube. The reported structural and electronic properties have been obtained from either hybrid density functional theory and Hartree–Fock linear combination of atomic orbitals calculations (using the HSE06 functional) or the linear augmented cylindrical waves density functional theory method. In addition to single-walled NTs, we have investigated a number of achiral double-walled (m, m)@(n, n) and (m, 0)@(n, 0) as well as triple-walled (l, l)@(m, m)@(n, n) and (l, 0)@(m, 0)@(n, 0) nanotubes. All multiwalled nanotubes show a common dependence of their band gap on the diameter of the inner nanotube, independent of chirality index and number of walls. This behavior of WS₂ NTs allows the exploitation of the entire range of the visible spectrum by suitably tuning the band gap.

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (Co${}_3$O${}_4$). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O $1s$ to O $2p$ states hybridized with the unoccupied $3d$ states of cobalt atoms. Also, since Co${}_3$O${}_4$ contains two types of Co atoms, i.e., Co${}^{3+}$ and Co${}^{2+}$, we find that contribution of Co${}^{2+}$ ions to the pre-edge peak is solely due to single spin-polarized $t_{2\mathrm{g}}$ orbitals ($d_{xz}$, $d_{yz}$, and $d_{xy}$) while that of Co${}^{3+}$ ions is due to spin-up and spin-down polarized $e_{\mathrm{g}}$ orbitals ($d_{x^2-y^2}$ and $d_{z^2}$). Furthermore, we deduce the magnetic moments on the Co${}^{3+}$ and Co${}^{2+}$ to be zero and 3.00 ${\mu }_B$ respectively. This is consistent with an earlier experimental study which found that the magnetic structure of Co${}_3$O${}_4$ consists of antiferromagnetically ordered Co${}^{2+}$ spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.