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- 2025| ElsevierThe use of surfactants in catalyst synthesis is essential for controlling particle dimensions but often hinders catalytic performance by blocking active sites. To overcome this challenge, this study investigates a solvent-free solid-state pyrolysis approach as an alternative to colloidal synthesis with surfactants. Nickel selenobenzoate was employed as a metal-organic precursor to synthesize NiSe2 nanosheets, aiming to avoid surfactants while preserving active surface sites for catalysis. However, results indicate that merely eliminating surfactants in solvent-free synthesis is insufficient, as the decomposition of the metal-organic precursor leads to an amorphous carbonaceous residue, which negatively impacts electrochemical performance. To understand these effects, NiSe2 nanosheets synthesized via solid-state pyrolysis were compared with those produced using a conventional oleylamine-assisted colloidal approach. Electrochemical tests revealed that oleylamine-capped NiSe2 exhibited superior performance in both supercapacitance and hydrogen/oxygen evolution reactions (HER/OER), highlighting the adverse effects of carbon residue in the solid-state route. Further optimization of solid-state pyrolysis enabled controlled surface and structural modifications, leading to the formation of a NiSe2/Ni0.85Se heterostructure via gradual selenium loss, which improved HER activity. At higher temperatures, further Se loss resulted in a phase transformation from NiSe2 to Ni0.85Se while preserving the nanosheet morphology. Notably, this Ni0.85Se phase exhibited the best HER and OER performance, attributed to enhanced conductivity and the partial conversion of residual carbon into crystalline graphitic carbon at elevated temperatures. These findings underscore the need for careful precursor and method selection in solvent-free synthesis to optimize catalytic material performance, offering valuable insights for the design of next-generation electrocatalysts.
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- 2025| Royal Society of Che...To overcome the potential issue of active site blockage by surfactants in colloidal synthesis, alternative synthetic approaches must be explored. In this study, we investigated both solvent-free and colloidal thermolysis routes to synthesize nickel sulfides (NiS and Ni3S2) using sulfur-based Ni complexes, [Ni(S2CO(C2H5))2] (Ni-Xan) and [Ni(S2CN(C2H5)2)2] (Ni-DTC) as precursors. The solvent-free decomposition of these complexes produced ligand-free NiS and Ni3S2 in the absence or presence of triphenylphosphine (TPP), respectively. In contrast, colloidal thermolysis in oleylamine (OLA) led to phase-selective nickel sulfide formation (NiS and Ni3S2), with TPP facilitating desulfurization. The electrochemical performance of the synthesized materials was evaluated in water splitting and supercapacitance applications. Among the tested materials, NiS synthesized from Ni-Xan in OLA exhibited the highest specific capacitance (809.2 F g−1 at 1 A g−1) and energy density (34.9 Wh kg−1), while NiS derived from Ni-DTC in OLA achieved the highest power density (281.7 Wh kg−1). Additionally, the Ni3S2 electrode obtained via the colloidal route demonstrated superior HER performance, requiring only 197 mV (Tafel slope: 159 mV dec−1) to reach a current density of 10 mA cm−2. These findings underscore that simply eliminating surfactants and adopting a solvent-free method is not inherently sufficient to achieve high electrochemical performance. This study provides insights into the limitations of solvent-free synthesis and outlines potential prerequisites that may guide future optimization for improved electrochemical performance.
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- Nickel phosphide exists in various compositions, and the synthesis of pure-phase nickel phosphides is of immense interest due to their wide scale applications in different electrocatalytic reactions. We report the facile synthesis of nickel phosphides and rare transition metal-induced phase transformations within this system. Phase selective synthesis of pure Ni2PorNi5P4 was achieved by decomposition of nickel acetate tetrahydrate Ni(AC)2$4H2O in optimized mixed solvent systems, i.e., in tri-octylphosphine oxide (TOPO)/tri-n-octylphosphine (TOP) or hexadecylamine (HDA)/TOP, respectively, by hot injection route. The doping of 5% Cu or Mn in either of the nickel phosphide phases yielded a mixture of phases (Ni2P/ Ni5P4). However, increasing the Cu or Mn content to 10% resulted in the complete transformation of phase, i.e., from Ni2P to pure Ni5P4 and vice versa. Lattice stress and size of incorporated dopants, as well as the nature of surfactants employed, were discussed as probable causes of these rare phase transformations. Moreover, in order to establish structure–activity relationship, we studied the comparative effect of transition metal dopants in both nickel rich and nickel deficient phases. Therefore, initially formed and transformed phosphides were investigated as electrocatalysts for overall water splitting and supercapacitance. NiP-5 (Ni2P formed on 10% Cu doping of Ni5P4) delivered a current density of 10 mA cm2 with the lowest overpotential of 146 mV among all samples for HER while NiP-3 (Ni5P4 formed from Ni2P on 10% Cu doping) similarly required the least overpotential of 276 mV for the OER at the same current density. NiP-2 (pristine Ni5P4) had the highest calculated specific capacitance of 1325 F g 1 at 2 A g 1. These phase transformations resulted in better catalytic activity and stability as well as reaction kinetics indicating suitability in practical water splitting technologies.
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- Herein, the synthesis of three nickel(II) dithiophosphonate complexes of the type [Ni{S2P(OR)(4-C6H4OMe)}2] [R=H (1), C3H7 (2)] and [Ni{S2P(OR)(4-C6H4OEt}2] [R=(C6H5)2CH (3)] is described; their structures were confirmed by single-crystal X-ray studies. These complexes were subjected to surfactant/solvent reactions at 300 °C for one hour as flexible molecular precursors to prepare either nickel sulfide or nickel phosphide particles. The decomposition of complex 2 in tri-octylphosphine oxide/1-octadecene (TOPO/ODE), TOPO/tri-n-octylphosphine (TOP), hexadecylamine (HDA)/TOP, and HDA/ODE yielded hexagonal NiS, Ni2P, Ni5P4, and rhombohedral NiS, respectively. Similarly, the decomposition of complex 1 in TOPO/TOP and HDA/TOP yielded hexagonal Ni2P and Ni5P4, respectively, and that of complex 3 in similar solvents led to hexagonal Ni5P4, with TOP as the likely phosphorus provider. Hexagonal NiS was prepared from the solvent-less decomposition of complexes 1 and 2 at 400 °C. NiS (rhom) had the best specific supercapacitance of 2304 F g−1 at a scan rate of 2 mV s−1 followed by 1672 F g−1 of Ni2P (hex). Similarly, NiS (rhom) and Ni2P (hex) showed the highest power and energy densities of 7.4 kW kg−1 and 54.16 W kg−1 as well as 6.3 kW kg−1 and 44.7 W kg−1, respectively. Ni5P4 (hex) had the lowest recorded overpotential of 350 mV at a current density of 50 mA cm−2 among the samples tested for the oxygen evolution reaction (OER). NiS (hex) and Ni5P4 (hex) had the lowest overpotentials of 231 and 235 mV to achieve a current density of 50 mA cm−2, respectively, in hydrogen evolution reaction (HER) examinations.
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- Increasing demand for sustainable energy has boosted the exploration of inexpensive and efficient catalysts. Transition metal sulfides have been proven as efficient electrocatalysts for energy storage or energy generation applications. Herein, cubic phase α-MnS and transition metal (Cu2+, Fe3+, and Ni2+) doped MnS nanoparticles were synthesized via the hot injection method from their piperazinyl dithiocarbamate complexes, respectively. The morphology of pristine and TM-doped MnS nanoparticles was studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analysis, while optical and structural properties were studied using UV–visible spectroscopy and powder X-ray diffraction (p-XRD), respectively. p-XRD analysis confirmed the successful incorporation of dopants into MnS lattice structure and suitability of heterocyclic dithiocarbamate complexes for phase/composition controlled synthesis of nanomaterials. The effect of doping on electrocatalytic properties was also investigated. The MnS-based electrodes doped with Ni and Fe presented satisfactory specific capacitances of 840 and 900 F/g at 2 mV/s scan rate. In addition, the testing for electrocatalysis for the water-splitting process demonstrated that Ni–MnS had a superior performance for HER with a η of 132 mV at 10 mA/cm2 and Tafel slope of 44 mV/dec. On the other hand, Fe–MnS showed better performance towards OER with a η of 280 mV at 10 mA/cm2 and a Tafel slope of 60 mV/dec.
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