The use of oxide photocatalysts for removing pollutants from the environment has always been regarded as a promising solution and has attracted considerable attention. The current study investigates the bandgap engineering of (CeGdSmYZr)O2 high-entropy oxide nanoparticles synthesized through a simple solution combustion technique. Different fuel-to-oxidizer ratios such as 0.6, 0.8, 1, 1.2, and 1.4 have been used to synthesize high-entropy (CeGdSmYZr)O2 oxide nanoparticles. Various characterization techniques including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV–visible spectroscopy, surface area measurements, and photoluminescent spectroscopy (PL) were used to investigate the structural, morphological, optical, and luminescent properties. XRD results confirm the formation of single-phase fluorite oxide while the fuel-to-oxidizer ratio was found to have an impact on the bandgap and crystallite size. This tailoring of bandgap has been shown to reflect in the photocatalytic reduction of toxic Cr(VI). The results showed that the photocatalytic reduction of Cr(VI) was significantly enhanced with an increase in the fuel-to-oxidizer ratio and a Cr(VI) reduction of 99.14 % was achieved involving 2 ml of formic acid. As a result, the findings of this study could provide insights into the development of new high-entropy oxide photocatalysts for the efficient reduction of toxic Cr(VI). © 2024 The Society of Powder Technology Japan