Effect of charge carrier density on the photocatalytic performance of CoFe₂O₄/Cu₂O n-p heterojunctions under visible light

The depletion of fossil fuels and the advancement of climate change have driven the search for sustainable alternatives, such as the production of hydrogen via photocatalytic water splitting. This study examines the impact of charge carrier density on CoFe₂O₄/Cu₂O n-p heterojunctions; this factor is rarely studied and provides an insight into their photocatalytic behavior. Additionally, this study introduces a new heterojunction for hydrogen generation. The CoFe₂O₄ and Cu₂O were synthesized using the Pechini method and the Benedict reaction, respectively. Heterojunctions with mass ratios of 25/75, 50/50 and 75/25% (designed as C2575, C5050 and C7525 respectively) were prepared through impregnation, sonication, and thermal treatment under inert conditions. Materials were characterized by means of TGA, XRD, UV-Vis, BET, SEM, TEM, HRTEM, XPS, UPS, Hall effect, EIS and Photoluminescence to evaluate their structural, surface, and optoelectronic properties. Then, photocatalytic hydrogen production was measured by gas chromatography during 8 hours taking samples each hour and a final sample at 24 h. The heterojunctions exhibited higher photocatalytic activity in comparison with the pristine materials, achieving a hydrogen production up to 60 and 147 times higher after 8 and 24 hours respectively and an average reaction rate increased by 60-fold. The C7525 heterojunction exhibited the highest activity attributed to an optimal CoFe₂O₄/Cu₂O ratio and heterojunctions exhibited a direct correlation between hydrogen production and charge carrier density. The material with the highest carrier density achieved the highest hydrogen yield. This correlation is attributed to more efficient charge separation, better absorption of visible light, and lower electron-hole pair recombination rates, which were verified by Hall Effect, XPS, EIS, UV-Vis, and PL results. These findings highlight the potential of CoFe₂O₄/Cu₂O heterojunctions and demonstrate that tuning charge carrier density is a promising strategy for improving the efficiency of photocatalytic hydrogen production.