(419d) Photoelectrochemistry of GaxZn1-XOyN1-Y/TiO2 Heterostructures

Photoelectrochemical water splitting represents one method for producing hydrogen without also generating greenhouse gases. Feasibility of this method, however, relies on the use of photoactive materials that are stable long-term and able to utilize the visible light spectrum effectively. While metal oxides like TiO₂ and SrTiO3 are somewhat effective at splitting water, they only absorb in the ultraviolet region, limiting their utility. Some visible light absorbers, such as Fe₂O₃ and BiVO₄, are unable to split water without an externally applied bias due to unfavorable band positions. Others, like CdS, suffer from photocorrosion and quickly destroy themselves.

Metal oxynitrides have exhibited promise as a material that is stable, has band positions that allow for overall water splitting without external bias, and absorb light in the visible region. The primary limiting factor for these materials is the typically multi-day synthesis required to make them. We previously developed a combustion synthesis method that produces gram-scale amounts of Ga_xZn_1-xO_yN_1-y with essentially the same physical and photoelectrochemical properties of Ga_xZn_1-xO_yN_1-y produced by high-temperature ammonolysis. The as-synthesized product, however, exhibited relatively poor photoelectrochemical performance, although both hydrogen and oxygen gas were measured by GC, and photocurrent was observed in a 2-electrode system without external bias.

In this talk, we discuss the formation of a Ga_xZn_1-xO_yN_1-y/TiO₂ heterostructure to try and enhance charge separation, a primary issue we previously identified as limiting photoelectrochemical efficiency of this material, as well as how the favorable band alignment affects photoelectrochemical performance. We synthesized both physical heterostructures (separate painted layers) as well as core-shell style heterostructures. Samples were characterized by UV-Vis spectroscopy, XRD, XPS, and imaging as appropriate. Photoelectrochemical measurements were conducted to evaluate potential and actual performance. We found that while the somewhat large particle size of the oxynitride still presents issues, charge separation is enhanced by the presence of the TiO₂, increasing overall efficiency. Finally, we identify here “chokepoints” that must be further addressed, including the aforemention particle sizing.