  Graphene is a light material for long-distance spin transport due to its low
spin-orbit coupling, which at the same time is the main drawback to exhibit a
sizeable spin Hall effect. Decoration by light atoms has been predicted to
enhance the spin Hall angle in graphene while retaining a long spin diffusion
length. Here, we combine a light metal oxide (oxidized Cu) with graphene to
induce the spin Hall effect. Its efficiency, given by the product of the spin
Hall angle and the spin diffusion length, can be tuned with the Fermi level
position, exhibiting a maximum (1.8 $\pm$ 0.6 nm at 100 K) around the charge
neutrality point. This all-light-element heterostructure shows a larger
efficiency than conventional spin Hall materials. The gate-tunable spin Hall
effect is observed up to room temperature. Our experimental demonstration
provides an efficient spin-to-charge conversion system free from heavy metals
and compatible with large-scale fabrication.
