Strong Visible Absorption and Broad Time Scale Excited-State Relaxation in (Ga 1-x Zn x )(N 1-x O x ) Nanocrystals Chi-Hung Chuang, † Ying-Gang Lu, † Kyureon Lee, † Jim Ciston, ‡ and Gordana Dukovic* ,† † Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States ‡ National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States * S Supporting Information ABSTRACT: (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 (Ga 0.27 Zn 0.73 )(N 0.27 O 0.73 ) 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. C onverting and storing solar energy in chemical bonds is a desirable approach to renewable energy available on demand. 1 Solar fuels can be generated photoelectrochemically using semiconductors that harvest sunlight and allow photo- excited electrons to reduce H + or CO 2 and holes to oxidize water. 2 One of the major challenges in the quest for solar fuels is finding semiconductors that absorb visible light, have appropriate band edge energies for the reduction and oxidation half- reactions, and are resistant to photo-oxidation. 2,3 Furthermore, a semiconductor that satisfies these requirements should have a sufficiently long excited state lifetime so that the photochemical pathways can compete with energy-wasting relaxation. The oxynitride (Ga 1-x Zn x )(N 1-x O x ) has intriguing optical properties relevant to solar fuel generation. This solid solution of GaN and ZnO absorbs visible light, with the band gap determined by the value of x, even though both of the constituent semiconductors have band gaps >3 eV. 4 When functionalized with a H + reduction co-catalyst, bulk (Ga 1-x Zn x )- (N 1-x O x ) is capable of overall water-splitting under visible excitation. 4 Moreover, this material is stable for months under water-splitting conditions. 5 The origin of the composition- dependent visible absorption is not well understood. Proposed explanations include valence-band edge upshift due to mixing of ZnO and GaN orbitals, impurity level absorption, and interfacial absorption. 6-13 This is a challenging question in part because electronic structure depends on compositional disorder (i.e., atomic-level connectivity of the four elements), which may depend on synthesis temperature and is difficult to measure experimentally. 8,10,11 We recently synthesized single-crystalline nanoparticles of (Ga 1-x Zn x )(N 1-x O x ) with a broad composition range (0.3 < x < 0.87) and absorption onsets that range from 2.7 eV for x = 0.30 to 2.2 eV for x = 0.87. 14 A band gap of 2.2 eV corresponds to a maximum solar-to-H 2 conversion efficiency that approaches a highly desirable 15%, assuming a quantum efficiency of 100%. 3 However, the highest reported apparent water-splitting quantum efficiency achieved with (Ga 1-x Zn x )(N 1-x O x ) is under 20%. 15 The relaxation dynamics of the photoexcited carriers in this material are not well understood. 13 Thus, it is not clear whether the low water-splitting quantum efficiency is a consequence of short carrier lifetimes inherent to this semiconductor or a reflection of materials properties, such as crystallinity and defects, that could be controlled via synthesis and processing. Here, we report the measurement of excited-state dynamics in (Ga 1-x Zn x )(N 1-x O x ) nanocrystals (NCs) with x = 0.73 using transient absorption (TA) spectroscopy over a wide time window (10 -13 -10 -4 s). (Ga 0.27 Zn 0.73 )(N 0.27 O 0.73 ) was chosen because ZnO-rich (Ga 1-x Zn x )(N 1-x O x ) compositions have smaller band gaps and are therefore more interesting for solar fuel generation. 14-16 To enable the TA studies, we first solubilized (Ga 0.27 Zn 0.73 )(N 0.27 O 0.73 ) NCs in toluene using a long-chain organosilane. We directly measured the molar absorptivity in the visible and found it to be in the range of direct band gap absorption in semiconductors such as GaAs. TA spectra of solubilized (Ga 0.27 Zn 0.73 )(N 0.27 O 0.73 ) NCs had two main features: a UV bleach centered at 365 nm and a broad visible bleach centered at 425 nm. Both features had similar decay kinetics at early times (100 fs-3 ns), suggesting that they have similar electronic character. The decay of the visible bleach had both a fast (<50 ps) component and a long-lived component, with a very long average lifetime of ∼30 μs. Along with the strong visible absorption, TA data are consistent with the theoretical predictions that visible absorption in ZnO-rich (Ga 1-x Zn x )- (N 1-x O x ) originates from a transition between a valence band that arises from intermixing of ZnO and GaN and a conduction band that contains mostly Zn and O orbitals. 6-8 We conclude with a discussion of how the optical properties reported here relate to potential applications of (Ga 1-x Zn x )(N 1-x O x ) in solar fuel generation. The synthesis method that we developed for (Ga 1-x Zn x )- (N 1-x O x ) NCs produces a powder of insoluble particles with no surface-capping ligands. 14 Absorption spectra of such particles Received: March 3, 2015 Published: May 2, 2015 Communication pubs.acs.org/JACS © 2015 American Chemical Society 6452 DOI: 10.1021/jacs.5b02077 J. Am. Chem. Soc. 2015, 137, 6452-6455