Bulk oxy(nitride) (Ga(1-x)Zn(x))(N(1-x)O(x)) is a promising photocatalyst for water splitting under visible illumination. To realize its solar harvesting potential, it is desirable to minimize its band gap through synthetic control of the value of x. Furthermore, improved photochemical quantum yields may be achievable with nanocrystalline forms of this material. We report the synthesis, structural, and optical characterization of nanocrystals of (Ga(1-x)Zn(x))(N(1-x)O(x)) with the values of x tunable from 0.30 to 0.87. Band gaps decreased from 2.7 to 2.2 eV over this composition range, which corresponded to a 260% increase in the fraction of solar photons that could be absorbed by the material. We achieved nanoscale morphology and compositional control by employing mixtures of ZnGa(2)O(4) and ZnO nanocrystals as synthetic precursors that could be converted to (Ga(1-x)Zn(x))(N(1-x)O(x)) under NH(3). The high quality of the resulting nanocrystals is encouraging for achieving photochemical water-splitting rates that are competitive with internal carrier recombination pathways.
We describe the synthesis and characterization of wurtzite (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga 1Àx Zn x )(N 1Àx O 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 1Àx Zn x )(N 1Àx O x ) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals to understand how they form by nitridation of ZnO and ZnGa 2 O 4 nanocrystalline precursors. We find that the ZnO precursor is critical for the formation of crystalline (Ga 1Àx Zn x )(N 1Àx O x ) at 650 C, consistent with a mechanism in which wurtzitenucleates topotactically on wurtzite ZnO at an interface with ZnGa 2 O 4 . 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 1Àx Zn x )(N 1Àx O 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 ¼ 0.87, corresponding to a maximum possible solar-to-H 2 power conversion efficiency of 12%. Finally, we examine the photoelectrochemical (PEC) oxidation behavior of thick films of (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals with x ¼ 0.40, 0.52, and 0.87 under visible illumination. (Ga 1Àx Zn x )(N 1Àx O x ) nanocrystals with x ¼ 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga 1Àx Zn x )(N 1Àx O x ) of the same composition. The highest photocurrents are observed at x ¼ 0.52, even though x ¼ 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 1Àx Zn x )(N 1Àx O x ). † Electronic supplementary information (ESI) available: Fitting of XRD patterns in Fig. 2; TEM images of the samples from Fig. 2; XRD patterns, elemental analysis by ICP-OES, and diffuse reectance spectra of the products from nitridation of the starting mixture with x ¼ 0.78 with varying nitridation time; TEM images of the nitrided products of x ¼ 0.06, 0.24, 0.91, and 0.98; XPS spectra of Zn2p 3/2 , O1s, Ga2p 3/2 , and N1s in samples with several compositions; determination of band gap as a function of composition. See
Solid-state chemical transformations at the nanoscale can be a powerful tool for achieving compositional complexity in nanomaterials. It is desirable to understand the mechanisms of such reactions and characterize the local-level composition of the resulting materials. Here, we examine how reaction temperature controls the elemental distribution in (GaZn)(NO) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGaO NCs. (GaZn)(NO) is a visible-light absorbing semiconductor that is of interest for applications in solar photochemistry. We couple elemental mapping using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope (STEM-EDS) with colocation analysis to study the elemental distribution and the degree of homogeneity in the (GaZn)(NO) samples synthesized at temperatures ranging from 650 to 900 °C with varying ensemble compositions (i.e., x values). Over this range of temperatures, the elemental distribution ranges from highly heterogeneous at 650 °C, consisting of a mixture of larger particles with Ga and N enrichment near the surface and very small NCs, to uniform particles with evenly distributed constituent elements for most compositions at 800 °C and above. We propose a mechanism for the formation of the (GaZn)(NO) NCs in the solid state that involves phase transformation of cubic spinel ZnGaO to wurtzite (GaZn)(NO) and diffusion of the elements along with nitrogen incorporation. The temperature-dependence of nitrogen incorporation, bulk diffusion, and vacancy-assisted diffusion processes determines the elemental distribution at each synthesis temperature. Finally, we discuss how the visible band gap of (GaZn)(NO) NCs varies with composition and elemental distribution.
(Ga(1-x)Zn(x))(N(1-x)O(x)) is a visible absorber of interest for solar fuel generation. We present a first report of soluble (Ga(1-x)Zn(x))(N(1-x)O(x)) nanocrystals (NCs) and their excited-state dynamics over the time window of 10(-13)-10(-4) s. Using transient absorption spectroscopy, we find that excited-state decay in (Ga0.27Zn0.73)(N0.27O0.73) NCs has both a short (<100 ps) and a long-lived component, with a long overall average lifetime of ∼30 μs. We also find that the strength of the visible absorption is comparable to that of direct band gap semiconductors such as GaAs. We discuss how these results may relate to the origin of visible absorption in (Ga(1-x)Zn(x))(N(1-x)O(x)) and its use in solar fuel generation.
Co(Al1−xGax)2O4 spinels synthesized from molecular precursors exhibit low energy (<2.5 eV) ligand–field transitions that contribute between 46 and 72% of the photocatalytic activity.
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