Vein-Like Ni-BTC@Ni3S4 with Sulfur Vacancy and Ni3+ Fabricated In Situ Etching Vulcanization Strategy for an Electrochemical Sensor of Dopamine

In this study, a novel Ni-BTC@Ni₃S₄ composite was fabricated by solvothermal reaction using an in situ etching vulcanization strategy and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and Brunauer-Emmett-Teller (BET) analyses. The existence of a sulfur vacancy and Ni³⁺ in the as-prepared vein-like Ni-BTC@Ni₃S₄ greatly promoted the electrochemical sensing activity of the materials. Herein, a simple electrochemical sensor (Ni-BTC@Ni₃S₄/CPE) has been fabricated and used for the detection of dopamine (DA). The current signal of the Ni-BTC@Ni₃S₄/CPE-modified electrode was linear with the concentration of DA in the range of 0.05-750 μM (R² = 0.9995) with a sensitivity of 560.27 μA·mM^-1·cm^-2 and a detection limit of 0.016 μM. At the same time, the sensor has good stability and anti-interference ability. This study could provide a new idea and strategy for the structural regulation of composite electrode-modified materials and sensitive sensing detection of small biological molecules.

Engineering the Near-Surface Structure of WO3 by an Amorphous Layer with Trivalent Ni and Self-Adapting Oxygen Vacancies for Efficient Photocatalytic and Photoelectrochemical Acidic Oxygen Evolution Reaction

Exploiting an effective strategy to tailor the construction, composition, and local electronic structure of the photocatalyst surface is pivotal to photocatalytic activity, but remains challenging. Transition metal elements can boost the oxygen evolution reaction activity especially one like Ni in high oxidation states, whereas it is uneasy to prepare Ni³⁺ under mild conditions or play to their strengths in acidic conditions. In this article, we report a facile "etch and dope" synthesis of Ni³⁺-doped WO₃ nanosheets with oxygen vacancies. Through detailed experimental and theoretical studies, it is established that the abundant oxygen vacancies and the doped Ni³⁺ ions in the near-surface amorphous layer can synergistically optimize the surface electronic structure of WO₃ and the adsorption and desorption of intermediates. Impressively, the etched WO₃ nanosheets coupled with Ni³⁺ offer a greatly promoted photocatalytic performance of 1.78 mmol g^-1 h^-1, and the photoanode achieves a photocurrent density of 2.11 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (V_RHE). This work provides a new inspiration for rational manufacture of defects and high-valence metal ions in catalysts for photocatalytic and photoelectrochemical reactions.

Theoretical studies of the g factors and local structures of the Ni3+ centers in Na2 Zn(SO4 )2 ·4H2 O and K2 Zn(SO4 )2 ·6H2 O crystals

The local structures and the g factors g_i (i = x, y, z) for Ni³⁺ centers in Na₂ Zn(SO₄ )₂ ·4H₂ O (DPPH) and K₂ Zn(SO₄ )₂ ·6H₂ O (PHZS) crystals are theoretically studied by using the perturbation formulas of the g factors for a 3d⁷ ion with low spin (S = 1/2) in orthorhombically compressed octahedra. In these formulas, the contributions to g factors from both the spin-orbit coupling interactions of the central ion and ligands are taken into account, and the required crystal-field parameters are estimated from the superposition model and the local geometry of the systems. Based on the calculations, the Ni-O bonds are found to suffer the axial compression δz (or Δz) of about 0.111 Å (or 0.036 Å) along the z-axis for Ni³⁺ centers in DPPH (or PHZS) crystals. Meanwhile, the Ni-O bonds may experience additional planar bond length variation δx (≈0.015 Å) along x- and y-axes for the orthorhombic Ni³⁺ center in DPPH. The theoretical g factors agree well with the experimental data. The obtained local structural parameters for both Ni³⁺ centers are discussed.

Studies on the local structures for the trigonal Ni3+ centers in cathode materials LiAly Co1-y O2 (y = 0, 0.1, 0.5, and 0.8)

The local angular distortions Δθ are theoretically studied for the various Ni³⁺ centers in LiAl_y Co_1-y O₂ at different Al concentrations (y = 0, 0.1, 0.5, and 0.8) based on the perturbation calculations of electron paramagnetic resonance g factors for a trigonally distorted octahedral 3d⁷ cluster with low spin (S = 1/2). Due to the Jahn-Teller effect, the [NiO₆ ]^9- clusters are found to experience the local angular distortions (Δθ ≈ 5°-9°) along the C₃ axis. The variation trend of Δθ with y is in accordance with that of anisotropy (Δg = g_||  - g_⊥ ). As the substitutions can weaken bond strengths between transition metal and oxygen and the structural stability plays an important role in cathode performances, detailed investigations on the structural properties of the cathode materials LiAl_y Co_1-y O₂ can be practically helpful to understand the performances of these materials. The oxy-redox properties of LiAl_y Co_1-y O₂ systems are comprehensible in the framework of Ni³⁺ /Ni⁴⁺ couples, and the trigonally compressed octahedral [NiO₆ ]^9- clusters are applicable to the clarification of the electrochemical properties of lithium nickel oxide batteries. It appears that LiAl_0.8 Co_0.2 O₂ with the largest Al concentration may correspond to the smallest distortion among the mixing systems.