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[Display omitted] • Cu-HEO heterostructures were constructed by using Cu-containing HEO precursors. • Effects of Cu adding methods on Cu NPs size and activity of Cu-HEO were investigated. • The diffusion hysteresis effect of HEO was favor for the formation smaller Cu NPs. • Cu 0.5 -HEO shows superb photocatalytic activity and CH 4 selectivity for CO 2 reduction. The construction of metal–semiconductor heterojunction has been proven to be an effective way to improve the separation efficiency of photogenerated electrons and holes, thus enhancing photocatalytic activity of semiconductor with narrow band gap. Here, we construct a Cu-(Ga 0.2 Cr 0.2 Mn 0.2 Ni 0.2 Zn 0.2) 3 O 4 high entropy oxide (HEO) heterojunction by using Cu-containing HEO precursor, and the effect of Cu addition method on the photocatalytic performance was investigated. The results show that the addition method of Cu has a great influence on the size of Cu nanoparticles (NPs), heterojunction amount and photocatalytic activity of Cu-HEO catalysts. Cu-HEO samples prepared by direct introduction method possess much smaller Cu particles due to the uniform distribution and stronger diffusion hysteresis effect of Cu in HEO, and exhibit superior photocatalytic activity and CH 4 selectivity for CO 2 reduction. With the increase of Cu loading, the photocatalytic activity of Cu-HEO samples firstly increases and then decreases. Among these photocatalysts, Cu 0.5 -HEO obtained by the direct introduction method displays the highest CO 2 reduction activity, and the yields of CO and CH 4 reach to 5.66 and 33.84 µmol/h/g cat , respectively. This work innovatively uses HEO precursors with unique properties to construct metal-HEO heterojunctions with smaller metal particles and more active sites. [ABSTRACT FROM AUTHOR]