Phosphorus-Induced One-Step Synthesis of NiCo2S4 Electrode Material for Efficient Hydrazine-Assisted Hydrogen Production.

Rational control of the reaction parameters is highly important for synthesizing active electrocatalysts. NiCo2S4 is an excellent spinel-based electrocatalyst that is usually prepared through a two-step synthesis. Herein, a one-step hydrothermal route is reported to synthesize P-incorporated NiCo2S4. We discovered that the inclusion of P caused formation of the NiCo2S4 phase in a single step. Computational studies were performed to comprehend the mechanism of phase formation and to examine the energetics of lattice formation. Upon incorporation of the optimum amount of P, the stability of the NiCo2S4 lattice was found to increase steadily. In addition, the Bader charges on both the metal atoms Co and Ni in NiCo2S4 and P-incorporated NiCo2S4 were compared. The results show that replacing S with the optimal amount of P leads to a reduction in charge on both metal atoms, which can contribute to a more stable lattice formation. Further, the electrochemical performance of the as-synthesized materials was evaluated. Among the as-synthesized nickel cobalt sulfides, P-incorporated NiCo2S4 exhibits excellent activity toward hydrazine oxidation with an onset potential of 0.15 V vs RHE without the assistance of electrochemically active substrates like Ni or Co foam. In addition to the facile synthesis method, P-incorporated NiCo2S4 requires a very low cell voltage of 0.24 V to attain a current density of 10 mA cm-2 for hydrazine-assisted hydrogen production in a two-electrode cell. The free energy profile of the stepwise HzOR has been investigated in detail. The computational results suggested that HzOR on P-incorporated NiCo2S4 was more feasible than HzOR on NiCo2S4, and these findings corroborate the experimental evidence.

Ligand-Tuned Energetics for the Selective Synthesis of Ni2P and Ni12P5 Possessing Bifunctional Electrocatalytic Activity toward Hydrogen Evolution and Hydrazine Oxidation Reactions.

The occurrence of many phases and stoichiometries of nickel phosphides calls for the development of synthetic levers to selectively produce phases with purity. Herein, thiol (-SH) and carboxylate (-COO-) functional groups in ligands were found to effectively tune the energetics of nickel phosphide phases during hydrothermal synthesis. The initial kinetic product Ni2P transforms into thermodynamically stable Ni12P5 at longer reaction times. The binding of carboxylate onto Ni2P promotes this phase transformation to produce pure-phase Ni12P5 within 5 h compared to previous reports (∼48 h). Thiol-containing ligands inhibit this transformation process by providing higher stability to the Ni2P phase. Cysteine-capped Ni2P showed excellent geometric and intrinsic electrocatalytic activity toward both hydrogen evolution and hydrazine oxidation reactions under alkaline conditions. This bifunctional electrocatalytic nature enables cysteine-capped Ni2P to promote hydrazine-assisted hydrogen generation that requires lower energy (0.46 V to achieve 10 mA/cmgeo2) compared to the conventional overall water splitting (1.81 V to achieve 10 mA/cmgeo2) for hydrogen generation.

Prudent electrochemical pretreatment to promote the OER by catalytically inert "Iron incorporated metallic Ni nanowires" synthesized via the "non-classical" growth mechanism.

This study provides new insight towards the non-classical "amorphous to crystalline" growth mechanism for metal nanowire synthesis and reports an electrochemical strategy to activate inactive materials into efficient electrocatalysts for the OER. Despite considerable research on transition metal oxides/hydroxides, especially NiFe based hydroxides as OER electrocatalysts, poor conductivity of these materials plagues their catalytic efficiency. In contrast, lack of catalytic centers hinders the OER performance of conductive metals. Herein, we devised a suitable precondition strategy to transform only the surface of conductive metallic Ni nanowires into active catalytic centers. The resulting material with intimate contact between the electrically conductive core and electrocatalytically active surface showed promising "specific" and "geometric" electrocatalytic activity towards the alkaline OER at different pH. Upon iron incorporation, the Fe centers incorporated at the surface as well as in the bulk of the nanowires were found to further boost the OER activity of these materials. A one-pot strategy was adopted to produce iron free/incorporated Ni nanowires covered with nano-spikes. Growth analysis revealed a unique "non-classical amorphous-to-crystalline transformation" to be responsible for the formation of metallic nanowires.

Ligand sensitized strong luminescence from Eu3+-doped LiYF4 nanocrystals: a photon down-shifting strategy to increase solar-to-current conversion efficiency.

Lanthanide (Ln)-doped nanocrystals generally display low luminescence quantum efficiency due to forbidden nature of the 4f-4f transition besides possessing low absorption cross sections (∼10 M-1 cm-1). Considering the demand for these materials, particularly for light emission and bioimaging applications, it is very important to improve their quantum efficiency. This work demonstrates a strategy to enhance Si solar cell efficiency via sensitization of Eu3+ ions luminescence from colloidal nanocrystals. We have for the first time developed a simple ligand exchange approach to attach 4,4,4-trifluoro-1-phenyl-1,3 butanedione (TPB) to the surface of Eu3+-doped LiYF4 nanocrystals (NCs). Owing to the good overlap between the emission of the TPB ligands and the energy levels of Eu3+ ions, an efficient energy transfer takes place from the ligand to Eu3+ ions upon ultraviolet (UV) excitation of the ligand, leading to intense red emission. The sensitization of Eu3+ ions greatly enhanced the quantum yield of Eu3+ ions (∼31%) compared to the ∼5% obtained via direct excitation of Eu3+ ions (λexi = 394 nm) in Eu3+-doped LiYF4 NCs. A device was fabricated by embedding the nanocrystals on a Si solar cell to capture the UV photons and convert them into visible ones, which subsequently creates charge carriers inside the cell. Upon exposure to UV light, the nanocrystal embedded Si solar cell shows overall enhancement in the photocurrent upon excitation under UV radiation.

Enhanced visible and near infrared emissions via Ce(3+) to Ln(3+) energy transfer in Ln(3+)-doped CeF3 nanocrystals (Ln = Nd and Sm).

We report the enhancement of both visible and near infrared (NIR) emissions from Nd(3+) ions via Ce(3+) sensitization in colloidal nanocrystals for the first time. This is achieved in citrate capped Nd(3+)-doped CeF3 nanocrystals under ultraviolet (UV) irradiation (λex = 282 nm). The lasing transition ((4)F3/2 → (4)I11/2) at 1064 nm from Nd(3+)-doped CeF3 nanocrystals has much higher emission intensity via Ce(3+) ion sensitization compared to the direct excitation of Nd(3+) ions. The nanocrystals were prepared using a simple microwave irradiation route. Moreover, the study has been extended to Sm(3+)-doped CeF3 nanocrystals which show strong characteristic emissions of Sm(3+) ions via energy transfer from Ce(3+) ions. The energy transfer mechanism from Ce(3+) to Nd(3+) and Sm(3+) ions is proposed.