Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wave function overlap between the electron and hole. However, due to fast and efficient hole trapping and nonuniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS “rod” or “bulb” regions of nonuniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the “dot” material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron–hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone.Keywords: heterostructures; hole trapping; Nanocrystals; nanorods; photophysics; ultrafast spectroscopy;

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 (Ga1–xZnx)(N1–xOx) nanocrystals (NCs) synthesized via the solid-state nitridation of a mixture of nanoscale ZnO and ZnGa2O4 NCs. (Ga1–xZnx)(N1–xOx) 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 (Ga1–xZnx)(N1–xOx) 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 (Ga1–xZnx)(N1–xOx) NCs in the solid state that involves phase transformation of cubic spinel ZnGa2O4 to wurtzite (Ga1–xZnx)(N1–xOx) 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 (Ga1–xZnx)(N1–xOx) NCs varies with composition and elemental distribution.Keywords: colocation; elemental correlation; nanocrystals; oxynitride; solid state; STEM-EDS;

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).

(Ga1–xZnx)(N1–xOx) is a visible absorber of interest for solar fuel generation. We present a first report of soluble (Ga1–xZnx)(N1–xOx) 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 (Ga1–xZnx)(N1–xOx) and its use in solar fuel generation.

Cobalt mixed-metal spinel oxides, Co(Al1−xGax)2O4, have been predicted to exhibit promising properties as photocatalysts for solar energy conversion. In this work, Co(Al1−xGax)2O4 were synthesized with a range of 0 ≤ x ≤ 1 via both single-source and multi-source routes. Single-source molecular precursors, [Co{M(OtBu)4}2] (M = Al or Ga), were decomposed at 300 °C to form amorphous oxides. Multi-source precursors, stoichiometric mixtures of metal acetylacetonate (acac) complexes, were used to form nanocrystalline spinel materials. Both were subsequently converted to bulk spinel products by annealing at 1000 °C. The properties of materials fabricated from the single-source and multi-source synthetic routes were compared by analysing data from X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis spectrophotometry, inductively coupled plasma-optical emission spectroscopy, and gas sorption measurements. The X-ray diffraction data of the materials showed ideal solid solution behavior that followed Vegard's law for both routes, with the multi-source route giving more crystalline bulk material than the single-source route. UV-vis absorbance data revealed that the absorption onset energies of Co(Al1−xGax)2O4 decreased monotonically with increasing x (from 1.84 eV for x = 0 to 1.76 eV for x = 1 from the single-source method; 1.75 eV for x = 0 to 1.70 eV for x = 1 from the multi-source method). The photocatalytic activities of the spinel oxides were evaluated via the photodegradation of methyl orange and phenol, which showed that the photoactivity of Co(Al0.5Ga0.5)2O4 was dependent on both pH and substrate. Remarkably, under appropriate substrate binding conditions (pH 3 with methyl orange), low energy (<2.5 eV) ligand–field transitions contributed between 46 and 72% of the photoactivity of Co(Al0.5Ga0.5)2O4 prepared from the multi-source route.

Electron transfer from photoexcited CdS nanorods to [FeFe]-hydrogenase is a critical step in photochemical H2 production by CdS–hydrogenase complexes. By accounting for the distributions in the numbers of electron traps and enzymes adsorbed, we determine rate constants and quantum efficiencies for electron transfer from transient absorption measurements.

The ligands that passivate the surfaces of semiconductor nanocrystals play an important role in excited state relaxation and charge transfer. Replacement of native long-chain organic ligands with chalcogenides has been shown to improve charge transfer in nanocrystal-based devices. In this report, we examine how surface-capping with S2–, Se2–, and Te2– impacts photoexcited state relaxation in CdSe quantum dots (QDs). We use transient absorption spectroscopy with state-specific pumping to reveal the kinetics of electron and hole cooling, band edge electron relaxation, hole trapping, and trapped hole relaxation, all as a function of surface-capping ligand. We find that carrier cooling is not strongly dependent on the ligand. In contrast, band edge relaxation exhibits strong ligand dependence, with enhanced electron trapping in chalcogenide-capped QDs. This effect is the weakest with the S2– ligand, but is very strong with Se2– and Te2–, such that the average band edge electron lifetimes for QDs capped with those ligands are under 100 ps. We conclude that, unlike the case of S2–, improvements in electron transfer rates with Se2– and Te2– ligands may be overshadowed by the extreme electron lifetime shortening that may lead to low quantum yields of electron transfer.

This Article describes the electron transfer (ET) kinetics in complexes of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence of an electron donor, these complexes produce H2 photochemically with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI play a critical role in the overall photochemical reactivity, as the quantum efficiency of ET defines the upper limit on the quantum yield of H2 generation. We investigated the competitiveness of ET with the electron relaxation pathways in CdS NRs by directly measuring the rate and quantum efficiency of ET from photoexcited CdS NRs to CaI using transient absorption spectroscopy. This technique is uniquely suited to decouple CdS→CaI ET from the processes occurring in the enzyme during H2 production. We found that the ET rate constant (kET) and the electron relaxation rate constant in CdS NRs (kCdS) were comparable, with values of 107 s–1, resulting in a quantum efficiency of ET of 42% for complexes with the average CaI:CdS NR molar ratio of 1:1. Given the direct competition between the two processes that occur with similar rates, we propose that gains in efficiencies of H2 production could be achieved by increasing kET and/or decreasing kCdS through structural modifications of the nanocrystals. When catalytically inactive forms of CaI were used in CdS–CaI complexes, ET behavior was akin to that observed with active CaI, demonstrating that electron injection occurs at a distal iron–sulfur cluster and is followed by transport through a series of accessory iron–sulfur clusters to the active site of CaI. Using insights from this time-resolved spectroscopic study, we discuss the intricate kinetic pathways involved in photochemical H2 generation in CdS–CaI complexes, and we examine how the relationship between the electron injection rate and the other kinetic processes relates to the overall H2 production efficiency.

Chalcogenide ligands (S2–, Se2–, Te2–) are attractive candidates for passivation of surfaces of colloidal quantum dots (QDs) because they can enhance interparticle or particle–adsorbate electronic coupling. Devices made with QDs in which insulating long-chain aliphatic ligands were replaced with chalcogenide ligands have exhibited improved charge transfer and transport characteristics. While these ligands enable promising device performance, their impact on the electronic structure of the QDs that they passivate is not understood. In this work, we describe significant (up to 250 meV) changes in band gap energies of CdTe QDs that occur when native aliphatic ligands are replaced with chalcogenides. These changes are dependent on the ligand and the particle size. To explain the observed changes in band gap energies, we used the single band effective mass approximation to model the ligand layer as a thin shell of Cd-chalcogenide formed by the bonding of chalcogenide ligands to partially coordinated Cd surface atoms. The model correctly predicted the observed trends in CdTe QD band gap energies. The model also predicts that electrons and holes in chalcogenide-capped QDs can be significantly delocalized outside the core/shell structure, enhancing electronic coupling between QDs and adjacent species. Our work provides a simple description of the electronic structure of chalcogenide-capped QDs and may prove useful for the design of QD-based devices.

We describe the charge transfer interactions between photoexcited CdS nanorods and mononuclear water oxidation catalysts derived from the [Ru(bpy)(tpy)Cl]+ parent structure. Upon excitation, hole transfer from CdS oxidizes the catalyst (Ru2+ → Ru3+) on a 100 ps to 1 ns timescale. This is followed by 10–100 ns electron transfer (ET) that reduces the Ru3+ center. The relatively slow ET dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is necessary for water oxidation.

We describe the role of phosphonic acids in the synthesis of anisotropic colloidal ZnO nanocrystals (nanorods) and, specifically, the discovery of an insoluble layered Zn–phosphonate intermediate. This compound is formed by the reaction of the molecular Zn precursor Zn(OAc)2 with phosphonic acids, and it acts as a heterogeneous Zn source during nanocrystal formation. Layered metal phosphonates have been studied extensively but have not been described in the context of nanocrystal synthesis. Layered Zn–octadecylphosphonate can be used as a sole precursor to obtain isotropic, soluble ZnO nanocrystals. However, for anisotropic rod-like shapes both the heterogeneous and the homogeneous (Zn(OAc)2) sources of Zn are necessary. The existence of a heterogeneous metal source described here is in contrast to the mechanisms of particle nucleation and growth from homogeneous molecular precursors often used to describe nanocrystal formation. Since many metals form layered phosphonates, our findings have implications for synthesis of nanocrystals of other semiconductors.Keywords: anisotropic growth; colloidal nanocrystals; metal phosphonates; nanocrystal synthesis; zinc oxide;

Bulk oxy(nitride) (Ga1–xZnx)(N1–xOx) 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 (Ga1–xZnx)(N1–xOx) 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 ZnGa2O4 and ZnO nanocrystals as synthetic precursors that could be converted to (Ga1–xZnx)(N1–xOx) under NH3. The high quality of the resulting nanocrystals is encouraging for achieving photochemical water-splitting rates that are competitive with internal carrier recombination pathways.

We have developed complexes of CdS nanorods capped with 3-mercaptopropionic acid (MPA) and Clostridium acetobutylicum [FeFe]-hydrogenase I (CaI) that photocatalyze reduction of H+ to H2 at a CaI turnover frequency of 380–900 s–1 and photon conversion efficiencies of up to 20% under illumination at 405 nm. In this paper, we focus on the compositional and mechanistic aspects of CdS:CaI complexes that control the photochemical conversion of solar energy into H2. Self-assembly of CdS with CaI was driven by electrostatics, demonstrated as the inhibition of ferredoxin-mediated H2 evolution by CaI. Production of H2 by CdS:CaI was observed only under illumination and only in the presence of a sacrificial donor. We explored the effects of the CdS:CaI molar ratio, sacrificial donor concentration, and light intensity on photocatalytic H2 production, which were interpreted on the basis of contributions to electron transfer, hole transfer, or rate of photon absorption, respectively. Each parameter was found to have pronounced effects on the CdS:CaI photocatalytic activity. Specifically, we found that under 405 nm light at an intensity equivalent to total AM 1.5 solar flux, H2 production was limited by the rate of photon absorption (∼1 ms–1) and not by the turnover of CaI. Complexes were capable of H2 production for up to 4 h with a total turnover number of 106 before photocatalytic activity was lost. This loss correlated with inactivation of CaI, resulting from the photo-oxidation of the CdS capping ligand MPA.

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).

Cobalt mixed-metal spinel oxides, Co(Al1−xGax)2O4, have been predicted to exhibit promising properties as photocatalysts for solar energy conversion. In this work, Co(Al1−xGax)2O4 were synthesized with a range of 0 ≤ x ≤ 1 via both single-source and multi-source routes. Single-source molecular precursors, [Co{M(OtBu)4}2] (M = Al or Ga), were decomposed at 300 °C to form amorphous oxides. Multi-source precursors, stoichiometric mixtures of metal acetylacetonate (acac) complexes, were used to form nanocrystalline spinel materials. Both were subsequently converted to bulk spinel products by annealing at 1000 °C. The properties of materials fabricated from the single-source and multi-source synthetic routes were compared by analysing data from X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis spectrophotometry, inductively coupled plasma-optical emission spectroscopy, and gas sorption measurements. The X-ray diffraction data of the materials showed ideal solid solution behavior that followed Vegard's law for both routes, with the multi-source route giving more crystalline bulk material than the single-source route. UV-vis absorbance data revealed that the absorption onset energies of Co(Al1−xGax)2O4 decreased monotonically with increasing x (from 1.84 eV for x = 0 to 1.76 eV for x = 1 from the single-source method; 1.75 eV for x = 0 to 1.70 eV for x = 1 from the multi-source method). The photocatalytic activities of the spinel oxides were evaluated via the photodegradation of methyl orange and phenol, which showed that the photoactivity of Co(Al0.5Ga0.5)2O4 was dependent on both pH and substrate. Remarkably, under appropriate substrate binding conditions (pH 3 with methyl orange), low energy (<2.5 eV) ligand–field transitions contributed between 46 and 72% of the photoactivity of Co(Al0.5Ga0.5)2O4 prepared from the multi-source route.

Electron transfer from photoexcited CdS nanorods to [FeFe]-hydrogenase is a critical step in photochemical H2 production by CdS–hydrogenase complexes. By accounting for the distributions in the numbers of electron traps and enzymes adsorbed, we determine rate constants and quantum efficiencies for electron transfer from transient absorption measurements.