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We describe the synthesis and characterization of wurtzite (Ga1-xZnx)(N1-xOx) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga1-xZnx)(N1-xOx), 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, (Ga1-xZnx)(N1-xOx) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) at 650 °C, consistent with a mechanism in which wurtzite (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx) nanocrystals with x = 0.40, 0.52, and 0.87 under visible illumination. (Ga1-xZnx)(N1-xOx) nanocrystals with x = 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga1-xZnx)(N1-xOx) 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 (Ga1-xZnx)(N1-xOx).

We describe the synthesis and characterization of wurtzite (Ga1−xZnx)(N1−xOx) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga1−xZnx)(N1−xOx), 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, (Ga1−xZnx)(N1−xOx) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga1−xZnx)(N1−xOx) 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 (Ga1−xZnx)(N1−xOx) at 650 °C, consistent with a mechanism in which wurtzite (Ga1−xZnx)(N1−xOx) 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 (Ga1−xZnx)(N1−xOx) 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 (Ga1−xZnx)(N1−xOx) nanocrystals with x = 0.40, 0.52, and 0.87 under visible illumination. (Ga1−xZnx)(N1−xOx) nanocrystals with x = 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga1−xZnx)(N1−xOx) 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 (Ga1−xZnx)(N1−xOx). [ABSTRACT FROM AUTHOR]

Results of our study on ultrafast electron transfer (eT) dynamics from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated within octa acid (OA) capsules as electron donors to methyl viologen dissolved in water as acceptor are presented. Upon photoexcitation, coumarin inside the OA capsule transfers an electron to the acceptor electrostatically attached to the capsule leading to a long-lived radical-ion pair separated by the OA capsular wall. This charge-separated state returns to the neutral ground state via back electron transfer on the nanosecond time scale. This system allows for ultrafast electron transfer processes through a molecular wall from the apolar capsular interior to the highly polar (aqueous) environment on the femtosecond time scale. Employing femtosecond transient absorption spectroscopy, distinct rates of both forward (1-25 ps) and backward eT (700-1200 ps) processes were measured. Further understanding of the energetics is provided using Rehm-Weller analysis for the investigated photoinduced eT reactions. The results provide the rates of the eT across a molecular wall, akin to an isotropic solution, depending on the standard free energy of the reaction. The insights from this work could be utilized in the future design of efficient electron transfer processes across interfaces separating apolar and polar environments.

Developing type-II heterostructures with a spatial separation of photoexcited electrons and holes is a useful route to promote photocatalytic hydrogen generation. However, few investigations on the charge transfer process across the heterojunction have been carried out, which can allow us to uncover the reaction mechanism. Herein, CdSe quantum dots (QDs) and TiO 2 nanocrystals were synthesized and combined in water yielding CdSe/TiO 2 type II heterostructures. It was found that mercaptopropionic acid as bifunctional molecules could bind with CdSe and TiO 2 to form a cross-linked morphology. The charge carrier dynamics of bare CdSe and CdSe/TiO 2 were detected using femtosecond transient absorption spectroscopy. In the presence of TiO 2 , the average exciton lifetime of CdSe QDs was apparently decreased, owing to the electron transfer from photoexcited CdSe to TiO 2 . Particularly, the electron-transfer rate from small CdSe QDs (3.0 nm) was much faster than that from big CdSe QDs (4.2 nm). The improved photocatalytic hydrogen generation was observed for CdSe/TiO 2 compared to bare CdSe QDs. The enhancement factor for small CdSe QDs was higher than that for big CdSe QDs, which was in good agreement with the electron-transfer rates. This result indicated that the electron transfer between CdSe and TiO 2 played an important role in photocatalytic hydrogen generation on CdSe/TiO 2 type-II heterostructure. Our study provides a fundamental guidance to construct efficient heterostructured photocatalysts by delicate control of the band alignment.