  The mineral sphalerite (ZnS) is a typical constituent at the periphery of
submarine hydrothermal deposits on Earth. It has been frequently suggested to
have played an important role in the prebiotic chemistry due to its prominent
photocatalytic activity. Nevertheless, the need for {\lambda} < 344 nm UV
radiation, which accounts for a very minor part of the energy range of the
incoming solar spectrum, limits the application of this semiconductor. In this
paper we employed a simple co-precipitation method for the fabrication of Fe,
Ni, and Cu-doped ZnS colloids and investigated their activities in the
photocatalyzed reduction of fumaric acid. The results show that the
photocatalytic activity of pristine ZnS is almost identical with that of 0.1
atom% Fe-doped ZnS, but decreases by doping 0.1 atom% Ni. However, it can be
significantly enhanced by doping Cu because this dopant makes the optical
absorption edges of ZnS shift from UV band to longer wavelengths. The optimal
doping concentration was found to be 0.3 atom%. Even under {\lambda} > 450 nm
light irradiation, the photocatalyst Zn1-xCuxS can drive the reduction of
fumaric acid to produce succinic acid. Given the existence of this doped
semiconductor in the hydrothermal vents on early Earth and its capability to
utilize both UV and visible light, ZnS might have participated more efficiently
than ever estimated in the prebiotic chemical evolution.
