Photocatalytic Water Oxidation Using Wide-Gap Semiconductor Modified with First-Low Transition-Metal Oxides Nanoparticles Under Visible Light
Authors: Okazaki Megumi , Maeda Kazuhiko 
Published: February 2020

Water splitting using a heterogeneous photocatalyst has recently attracted attention to get hydrogen from water and sunlight energy. TiO2 is known as one of the best photocatalysts for water splitting. However, metal oxide semiconductor photocatalysts generally have large band gap, so that they absorb only UV light. Recently, our group reported a new water oxidation photocatalyst that consists of TiO2 and nanoparticulate cobalt hydroxide (or oxide). In this system, electron transfer from cobalt nanoparticle to the conduction band of TiO2 is believed to proceed, enabling to absorb wide range visible light up to 850 nm for water oxidation.

In this work, we employed other first-low transition-metal oxides (MO
x
, M = Mn, Fe, Ni, Cu) for the surface modifiers alternative to CoO
x
. Since the electronic interaction between cobalt on surface and titanium in support should be important in the CoO
x
/TiO2 system, it is expected that similar effects may occur on other first-row transition-metal oxides and work as water oxidation photocatalyst. As a result, it was revealed that MO
x
/TiO2 samples absorb a wide range of visible light. However, all of the tested MO
x
/TiO2 except NiO
x
/TiO2 did not show photocatalytic activity for water oxidation. Furthermore, the activity of NiO
x
/TiO2 was much lower than that of CoO
x
/TiO2. To clarify the reason of photocatalytic inactiveness despite of their visible light absorption capability, we investigated the catalytic activities of surface MO
x
nanoparticles for water oxidation under the conditions where the contribution of light absorption by MO
x
/TiO2 to the reaction could be neglected. It was confirmed that the CoO
x
/TiO2 performed the best catalytic activity among the MO
x
/TiO2 samples. This result strongly suggests that not only the visible light absorption capability but also the intrinsic catalytic activity of MO
x
for water oxidation is important to obtaining high photocatalytic activity in MO
x
/TiO2 system.

Further investigation for relationship between surface CoO
x
nanoparticles and a support metal oxide semiconductor was conducted using SrTiO3 that possessed different particle sizes. As the conduction band minimum of SrTiO3 is more negative than that of TiO2, one may expect higher reduction ability of the injected electrons when sensitized by CoO
x
nanoparticles. The size of the loaded CoO
x
depended on the SrTiO3 support size, even though the CoO
x
modification was conducted under the same conditions. CoO
x
particles easily underwent aggregation on larger size SrTiO3, while highly dispersed CoO
x
was confirmed on smaller SrTiO3 support. The size growth of CoO
x
could increase the visible light absorption capability, which lead to higher photocatalytic activity. However, CoO
x
agglomerates could diminish the active sites for water oxidation reaction, which contributed to lower photocatalytic activity. Therefore, this result suggests that both the morphology of CoO
x
and the visible light absorption capability could be controlled by an appropriate choice of support material so as to maximize the photocatalytic activity.

Figure 1