Simple
solution combustion synthesis was adopted to synthesize
ZnO–ZnS (ZSx) nanocomposites using zinc nitrate
as an oxidant and a mixture of urea and thiourea as a fuel. A large
thiourea/urea ratio leads to more ZnS in ZSx with
heterojunctions between ZnS and ZnO and throughout the bulk; tunable
ZnS crystallite size and textural properties are an added advantage.
The amount of ZnS in ZSx can be varied by simply
changing the thiourea content. Although ZnO and ZnS are wide band
gap semiconductors, ZSx exhibits visible light absorption,
at least up to 525 nm. This demonstrates an effective reduction of
the optical band gap and substantial changes in its electronic structure.
Raman spectroscopy, transmission electron microscopy, X-ray photoelectron
spectroscopy, and secondary-ion mass spectrometry results show features
due to ZnO and ZnS and confirm the composite nature with heterojunctions.
The above mentioned observations demonstrate the multifunctional nature
of ZSx. Bare ZSx exhibits a promising
sunlight-driven photocatalytic activity for complete mineralization
of endocrine disruptors such as 2,4-dichlorophenol and endosulphan.
ZSx also exhibits photocurrent generation at no applied
bias. Dye-sensitized solar cell performance evaluation with ZSx shows up to 4% efficiency and 48% incident photon conversion
efficiency. Heterojunctions observed between ZnO and ZnS nanocrystallites
in high-resolution transmission electron microscopy suggest the reason
for effective separation of electron–hole pairs and their utilization.