Sol-Gel Synthesis and Enhanced Catalytic Performance of M_0.2Ni_0.8MoO₄ (M = Cu²⁺ or Zn²⁺) for Hydrogen Evolution Reaction and Photocatalytic Degradation of Methylene Blue Dye

In this study, Cu_0.2Ni_0.8MoO₄ and Zn_0.2Ni_0.8MoO₄ materials were synthesized using sol-gel hydrothermal technology with polyvinyl alcohol as a surfactant. X-ray diffraction (XRD), ATR-FT-IR spectrometer, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS) and Brunauer–Emmett–Teller (BET) analysis were used to characterize the materials. XRD analysis confirmed a monoclinic α-NiMoO₄ phase upon Zn²⁺ incorporation, while Cu²⁺ doping induced a phase transition to β-NiMoO₄. Zn²⁺ doping enhanced crystallinity, whereas Cu²⁺ doping reduced it by 28%. FESEM revealed foamy particle morphology and agglomerated nanospheres for both doped samples. Cu_0.2Ni_0.8MoO₄ exhibits a higher surface area (111.38 m²/g) and larger pore volume (0.202 cm³/g) compared to Zn_0.2Ni_0.8MoO₄ (36.02 m²/g, 12.61 Å), highlighting superior textural properties. The XPS spectra of the Cu_0.2Ni_0.8MoO₄ sample show increased oxygen vacancies and the presence of both Mo⁶⁺ and Mo⁵⁺ states, which enhance the material’s electronic structure and redox properties, potentially improving its catalytic performance. Cu_0.2Ni_0.8MoO₄ and Zn_0.2Ni_0.8MoO₄ were synthesized and evaluated as cathodes for the hydrogen evolution reaction (HER) in alkaline media. Polarization techniques and electrochemical impedance spectroscopy (EIS) demonstrated that Cu_0.2Ni_0.8MoO₄ exhibited superior HER performance, with an overpotential of 302 mV at 10 mA cm^-2, compared to 445 mV for Zn_0.2Ni_0.8MoO₄. The Cu²⁺-doped electrode showed the lowest charge transfer resistance, indicating enhanced catalytic activity. EIS analysis and an equivalent circuit model provided insight into the electrode/electrolyte interface, confirming Cu_0.2Ni_0.8MoO₄ as a promising HER catalyst. Cu_0.2Ni_0.8MoO₄ was synthesized and exhibited outstanding photocatalytic and electrocatalytic properties. It achieved 94.8% degradation of methylene blue (MB) dye under UV light in 2 hours, following a pseudo-first-order kinetic model with a rate constant of 0.0197 min^-1. Scavenger studies confirmed the roles of OH*, holes, and O₂^*- radicals in the degradation process. As an electrode for the hydrogen evolution reaction (HER), it demonstrated superior activity, with low overpotential and charge transfer resistance in alkaline media. These results highlight Cu²⁺-doped α-NiMoO₄ as a highly versatile material for environmental remediation and sustainable hydrogen production, making it a promising candidate for energy and environmental applications.