Among visible-light semiconductors for Photoelectrochemical (PEC) water splitting, bismuth vanadate (BiVO₄) has emerged as a leading candidate due to its suitable bandgap (~2.4 eV), relative abundance, and good photochemical stability. Nevertheless, its performance is hindered by short carrier diffusion lengths, sluggish oxygen evolution kinetics, and severe bulk recombination [1]. To address these intrinsic limitations, recent strategies have focused on heterojunction engineering and, more recently, on entropy-driven material design as a means to modulate electronic structure and interfacial reactivity [2,3].

In this work, we establish a platform for the rational integration of BiVO₄ with multicationic high-entropy oxide (HEO) architectures through entropy- and interface-engineered design to construct tailored heterojunctions capable of enhancing charge separation, catalytic activity and long-term PEC durability. HEO offer unprecedented compositional flexibility and entropy-stabilized phase formation [4], while their locally disordered environments can tune electronic properties, increase defect tolerance, and provide catalytically active sites [5]. To deepen the understanding of entropic effects on PEC behavior, we explore two complementary approaches. First, we examine the one-pot synthesis of high-entropy bismuth vanadates incorporating five distinct cations at low concentrations (<2%) substituting either Bi or V sites. This controlled substitution aims to preserve the optical absorption properties of BiVO₄ while introducing catalytic functionality and mitigating recombination pathways. Second, we compare this bulk entropy-driven strategy with a heterostructure approach, in which a conventional BiVO₄ layer is coupled to an externally deposited HEO overlayer. To fabricate these BiVO₄/HEO heterojunctions, we employ a solution-based thin-film process that ensures compositional precision and scalability. By tuning precursor concentration, spin-coating cycles, and thermal profiles, we obtain dense, adherent HEO films with controlled microstructure, while maintaining compatibility with low-cost, large-area manufacturing [6]. Through complementary advanced characterization, we confirm the formation of single-phase high-entropy layers and intimate interfacial contact with BiVO₄, modulated surface states, multivalent active sites within the HEO component, and  band offsets. Overall, this study positions HEO heterojunctions as a versatile and scalable strategy for cost-effective solar-to-hydrogen technologies.