Synthesis and characterization of (Ga1−xZnx)(N1−xOx) nanocrystals with a wide range of compositions

Abstract

We describe the synthesis and characterization of wurtzite (Ga_1−xZn_x)(N_1−xO_x) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga_1−xZn_x)(N_1−xO_x), a solid solution of GaN and ZnO, is an intriguing material because it exhibits composition-dependent visible absorption even though the parent semiconductors absorb in the UV. When functionalized with co-catalysts, (Ga_1−xZn_x)(N_1−xO_x) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga_1−xZn_x)(N_1−xO_x) nanocrystals to understand how they form by nitridation of ZnO and ZnGa₂O₄ nanocrystalline precursors. We find that the ZnO precursor is critical for the formation of crystalline (Ga_1−xZn_x)(N_1−xO_x) at 650 °C, consistent with a mechanism in which wurtzite (Ga_1−xZn_x)(N_1−xO_x) nucleates topotactically on wurtzite ZnO at an interface with ZnGa₂O₄. Using this information, we expand the range of compositions from previously reported 0.30 ≤ x ≤ 0.87 to include the low-x and high-x ends of the range. The resulting compositions, 0.06 ≤ x ≤ 0.98, constitute the widest range of (Ga_1−xZn_x)(N_1−xO_x) compositions obtained by one synthetic method. We then examine how the band gap depends on sample composition and find a minimum of 2.25 eV at x = 0.87, corresponding to a maximum possible solar-to-H₂ power conversion efficiency of 12%. Finally, we examine the photoelectrochemical (PEC) oxidation behavior of thick films of (Ga_1−xZn_x)(N_1−xO_x) nanocrystals with x = 0.40, 0.52, and 0.87 under visible illumination. (Ga_1−xZn_x)(N_1−xO_x) nanocrystals with x = 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga_1−xZn_x)(N_1−xO_x) of the same composition. The highest photocurrents are observed at x = 0.52, even though x = 0.87 absorbs more visible light, illustrating that the observed photocurrents are a result of an interplay of multiple parameters which remain to be elucidated. This set of characterizations provides information useful for future studies of composition-dependent PEC properties of nanoscale (Ga_1−xZn_x)(N_1−xO_x).