Method makes the difference

The photocatalytic performance of copper oxide (Cu₂O) films for water splitting is strongly dependent on the fabrication method used for their preparation, according to a study by Andre Baranov and D. S. Zimbovskii at Moscow State University¹.

Making highly active photocatalysts requires an understanding of the optimal combination of materials as well as the techniques used to form the heterostructures. However, comparing catalytic activities from different studies is not straightforward because of the many variables in their fabrication and in the electrochemical cells used to probe their activities. Now, a study in the journal Inorganic Materials reports the relationship between the method of fabricating Cu₂O films and the photocatalytic activity of the samples.

“We chose Cu₂O oxide because it’s cheap, environmentally safe, and its energy bandgap is ideal for light absorption,” says Baranov. “In addition, and essential for our investigation, Cu₂O layers can be synthesised by a variety of routes.”

Four different synthetic techniques to form Cu₂O heterostructures were compared by Zimbovskii and Baranov. Three of the techniques involved an oxidation process to form Cu₂O layers on copper foil; chemical, electrochemical or hydrothermal oxidation. The fourth technique involved electrodepositing Cu₂O layers from solution on to a fluorine-doped tin oxide (FTO) surface.

For the electrochemical technique, a two-electrode cell was used to oxidize a copper anode, which was then thermally reduced in an inert atmosphere, resulting in the formation of Cu₂O heterostructure consisting of a film and surface rods (Fig. 1). Chemical oxidation was performed using ammonium persulfate, and resulted in the same rod-like heterostructure, whereas hydrothermal oxidation and electrodeposition resulted in smooth copper oxide films.

“The electrochemical and chemical methods achieve good contact at the interface between the Cu₂O and Cu substrate,” explains Baranov. “Furthermore, the morphologically complex rods create a larger surface area than that created using our hydrothermal and electrodeposition routes.”

The photocatalytic activity of the different Cu₂O layers varied considerably. The layer with the highest activity of 1.6 mA cm^-2 was the one prepared by electrochemical oxidation. The film with the lowest activity, of 0.4 mA cm^-2, was the electrodeposited Cu₂O.

A key disadvantage of Cu₂O photocatalysts is their degradation. However, this can be prevented to some extent by including another layer, such as graphene or titanium dioxide. “In the future, we envisage combining other oxides together with Cu₂O, for example ZnO, to enable better charge separation and protection,” says Baranov.