Bandgap Modulation of Hydrothermally Synthesized CZTS Nanoparticles through Ni Incorporation

Kesterite (Cu₂ZnSnS₄) has gained significant attention in optoelectronic materials research for future solar energy applications due to its composition of earth-abundant elements, nontoxic nature, and cost-effectiveness as a p-type semiconductor material. This study investigates the effect of Ni doping on Cu₂Zn_1−xSnS₄Ni_x nanoparticles (NPs) synthesized via the hydrothermal method, with varying Ni concentrations (x = 0, 0.005, 0.01, 0.05, 0.1). x-ray diffraction (XRD) and Raman spectroscopy confirm the retention of the kesterite structure with high crystallinity, while x-ray photoelectron spectroscopy (XPS) verifies the incorporation of Ni²⁺ ions into the Cu₂ZnSnS₄ (CZTS) lattice. The oxidation states of the Ni⁺² metal dopants are clear from the XPS analysis. Field-emission scanning electron microscopy (FESEM) and energy-dispersive x-ray (EDX) analysis reveal changes in particle morphology and elemental distribution due to Ni doping, with FESEM images showing that the particle size of CZTS NPs ranges from 100 nm to 150 nm as the Ni concentration increases. High-resolution transmission electron microscopy (HRTEM) shows clear lattice fringes of both pristine and Ni-doped samples, confirming the crystallinity and highlighting minor distortions in the lattice due to Ni incorporation, which introduces lattice strain. Ultraviolet–visible–near-infrared (UV-Vis-NIR) spectroscopy shows a significant reduction in the optical bandgap from 1.50 eV for pristine CZTS to 1.38 eV for Ni-doped samples, highlighting the importance of bandgap tailoring to optimize CZTS NPs for enhanced solar energy absorption.