Abstract
The photocatalytic water splitting activity of nanocomposite photocatalysts of TiO
2
with CoO
x
was studied under UV and visible light, and the catalysts were characterized by XRD, XPS, and UV-vis techniques. The presence of CoO
x
enhances the hydrogen production activity of TiO
2
by five times at an optimal loading of .2 wt. %. To investigate the role of CoO
x
, the photocatalytic activity was also studied under visible light and with different amounts of sacrificial agent. Our results indicate that the increasing activity was not due to increasing absorption of the visible light but most likely due to the role of CoO
x
nanoparticles as hole scavengers at the interface with TiO
2
. XPS Co2p analyses of CoO/TiO
2
showed a considerable decrease in their signal after prolonged reaction time (44 h) when compared to that of the fresh catalyst. Because part of Co
2+
cations is dissolved in solution, in neutral or acidic pH, the possible increase in the reaction rate upon their addition to TiO
2
under UV excitation was investigated. No change in the reaction rate was observed upon, on purpose, addition Co
2+
cations to TiO
2
under UV excitation. Thus, one may rule out the reduction of Co
2+
to Co
0
with excited electrons within TiO
2
. In order to further increase the reaction rate, we have synthesized and tested a hybrid system composed of CoO and Pd nanoparticles (Pd wt. % = 0.1, 0.3, 0.5, and 1 wt. %) where 0.3 wt. % Pd - 2 wt. % CoO/TiO
2
showed the highest rate.