Functionalized metal oxide nanostructures as catalysts for solar water splitting

The splitting of water by solar energy to produce hydrogen and oxygen is believed to be an ideal approach to solving the growing global demand for environmentally friendly energy.^{1, 2} Semiconductor materials that act as photoelectrochemical (PEC) catalysts in this process play the most important role in determining its efficiency. Owing to their high stability, simple synthesis, nontoxicity, and low cost, metal oxides such as zinc oxide (ZnO) and α-ferric oxide (α-Fe₂O₃)—especially those with a 1D charge transport pathway—are considered potential PEC catalysts.^3,4 However, some drawbacks have limited their application in solar water splitting. For example, ZnO has poor optical absorption ability, whereas in α-Fe₂O₃ the mobility of charge carriers is restricted and extensive recombination of electrons and holes occurs on its surface.

A promising way of modifying the optoelectronic properties of 1D metal oxides to considerably improve their PEC performance is by functionalizing them with heterojunction structures, where two semiconductors with different band gaps are brought into contact. With this in mind, we have fabricated 1D heterojunction structures based on both ZnO and α-Fe₂O₃.

Functionalizing ZnO nanorods with quantum dots is an effective way to activate ZnO in visible light. We used a combination of chemical etching of ZnO nanorods and simultaneous chemical deposition of cadmium selenide (CdSe) to prepare a CdSe/ZnO heterojunction with a double-layered tubular structure for PEC water splitting.³ Figure 1 illustrates the mechanism of the water-splitting process based on band alignment. In our CdSe/ZnO tubular heterojunction, electrons induced by visible light in CdSe can be efficiently injected into ZnO nanotubes via its type II band structure and then transferred to a fluorine-doped tin oxide substrate along single-crystal ZnO nanotubes. Furthermore, the chemical etching process increased the exposed surfaces of {1010} planes, which possess higher photocatalytic activity. Thanks to these improvements—enhanced optical absorption ability, greater number of exposed {1010} planes, larger specific surface area, and superior charge separation efficiency—our CdSe/ZnO tubular nanotube arrays displayed a photocurrent density of 2.5mA/cm² at 0V versus silver/silver chloride (Ag/AgCl), which was 12 times higher than that of bare ZnO nanorod arrays.

Figure 1. Operating mechanism of cadmium selenide (CdSe)/zinc oxide (ZnO) double-layered nanotube arrays in solar water splitting. CB: Conduction band. FTO: Fluorine-doped tin oxide. Na₂S: Sodium sulfide. NHE: Normal hydrogen electrode. Ox: Oxidation. Pt: Platinum. Red: Reduction. Uv: Ultraviolet. VB: Valence band. Vis: Visible light.

Regarding α-Fe₂O₃, many efforts have been made to improve the material's charge carrier mobility by doping it with foreign elements.⁵ However, the problem of surface recombination also needs to be addressed for α-Fe₂O₃ to be applied in solar water splitting. By decorating α-Fe₂O₃ nanorods with a titanium dioxide overlayer using plasma-enhanced chemical vapor deposition, we demonstrated that the surface recombination centers of α-Fe₂O₃ can be efficiently passivated, which increased its PEC performance fivefold.⁶

Recently, we fabricated α-Fe₂O₃/silver ferric oxide (Ag_x Fe_2−xO₃) core/shell nanorod arrays as photoanodes for solar water splitting via an all-solution process.⁷ Mott–Schottky and x-ray absorption near-edge structure (XANES) analysis indicated that surface doping with silver can not only increase the concentration of charge carriers near the surface but also contribute to oxygen electrocatalysis, which reduces the accumulation and recombination of holes on the surface. Furthermore, in situ XANES revealed that oxygen-related holes dominate the oxygen evolution process in α-Fe₂O₃/Ag_xFe_2−xO₃ core/shell nanorod arrays and the Ag_xFe_2−xO₃ overlayer could accelerate the reaction kinetics of surface oxidation, synergistically contributing to improved PEC performance: see Figure 2. As a result, α-Fe₂O₃/Ag_xFe_2−xO₃ core/shell nanorod films demonstrated much higher PEC performance than pristine α-Fe₂O₃ nanorod films, especially in the visible-light region. The incident photon-to-current efficiency at 400nm increased from 2.2% to 8.4% at 1.23V versus a reversible hydrogen electrode.

Figure 2. Process of oxidation of water using core/shell-structured α-ferric oxide (α-Fe₂O₃)/silver ferric oxide (Ag_xFe_2-xO₃) nanorod arrays. e^-: Electron. h⁺: Hole. S₁, S₂, and S₃: First, second, and third stages in process.

In summary, we have proved that functionalizing metal oxide nanostructures to form heterojunctions is an efficient strategy for modifying the optoelectronic properties and PEC performance of both wide-band-gap (ZnO) and narrow-band-gap (α-Fe₂O₃) semiconductor materials. However, detailed investigation is still needed to prevent recombination centers that are caused by a mismatch of the crystal phases or components at the interface of heterojunctions.