Developing
an efficient photocatalyst for concurrent hydrogen production
and environmental remediation by using solar energy is a challenge.
Defect engineering, although it offers a strategical promise to enhance
the photocatalytic performance, has limitations that come from the
ambiguity surrounding its role. In the current work, a comprehensive
study on defects in promoting the charge transfer, band edge modulation,
and surface reaction was carried out. The excess electrons springing
from defects act like donor states and cause band bending at the junction
interface. Characterization techniques such as X-ray photoelectron
spectroscopy, ultraviolet photoelectron spectroscopy, electron spin
resonance, and photoluminescence were employed to investigate defect
functionality, and its ultimate effect on photocatalytic performance
was studied by simultaneous H2 production and methylene
blue degradation. The role of graphene in optoelectronics and defect
formation in the composite catalysts was explored. In addition, efforts
have been made to unveil the reaction pathway for hydrogen evolution
reaction and oxygen evolution reaction where excess defect density
greatly hampered the quantum yield of the process. Results suggest
that maintaining optimal defect concentration aborts the undesired
thermodynamically favored back reactions. The conduction band and
valence band values of the catalysts indicate that the photocatalytic
mechanism was dominated by the electron pathway. Graphene acted as
an effective electron sink when its concentration was around 2.5–3%.
The superior activity of TiO2-ZnS-rGO was attributed to
the narrow bandgap, rapid separation of photo-excited charge carriers,
and favorable conduction band position for photocatalytic reactions.
This work may assist in exploring the fundamental role of defects
in driving the photocatalytic reactions and improve the selectivity
in heterogeneous photocatalysis.