Interface-modulated kinetic differentials in electron and hole transfer rates as a key design principle for redox photocatalysis by Sb2VO5/QD heterostructures.

The efficient conversion of solar energy to chemical energy represents a critical bottleneck to the energy transition. Photocatalytic splitting of water to generate solar fuels is a promising solution. Semiconductor quantum dots (QDs) are prime candidates for light-harvesting components of photocatalytic heterostructures, given their size-dependent photophysical properties and band-edge energies. A promising series of heterostructured photocatalysts interface QDs with transition-metal oxides which embed midgap electronic states derived from the stereochemically active electron lone pairs of p-block cations. Here, we examine the thermodynamic driving forces and dynamics of charge separation in Sb2VO5/CdSe QD heterostructures, wherein a high density of Sb 5s2-derived midgap states are prospective acceptors for photogenerated holes. Hard-x-ray valence band photoemission spectroscopy measurements of Sb2VO5/CdSe QD heterostructures were used to deduce thermodynamic driving forces for charge separation. Interfacial charge transfer dynamics in the heterostructures were examined as a function of the mode of interfacial connectivity, contrasting heterostructures with direct interfaces assembled by successive ion layer adsorption and reaction (SILAR) and interfaces comprising molecular bridges assembled by linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate ultrafast (<2 ps) electron and hole transfer in SILAR-derived heterostructures, whereas LAA-derived heterostructures show orders of magnitude differentials in the kinetics of hole (<100 ps) and electron (∼1 ns) transfer. The interface-modulated kinetic differentials in electron and hole transfer rates underpin the more effective charge separation, reduced charge recombination, and greater photocatalytic efficiency observed for the LAA-derived Sb2VO5/CdSe QD heterostructures.

Influence of donor-to-acceptor ratio on excited-state electron transfer within covalently tethered CdSe/CdTe quantum dot colloidal heterostructures.

Carbodiimide-mediated coupling chemistry was used to synthesize heterostructures of CdSe and CdTe quantum dots (QDs) with varying ratios of electron-donating CdTe QDs and electron-accepting CdSe QDs. Heterostructures were assembled via the formation of amide bonds between the terminal functional groups of CdTe-adsorbed 4-aminothiophenol (4-ATP) ligands and CdSe-adsorbed N-hydroxysuccinimide (NHS) ligands. The number of charge acceptors on the surfaces of QDs can greatly influence the rate constant of excited-state charge transfer with QDs capable of accommodating far more acceptors than molecular chromophores. We report here on excited-state electron transfer within heterostructure-forming mixtures of 4-ATP-capped CdTe and NHS-capped CdSe QDs with varying molar ratios of CdTe to CdSe. Photophysical properties and charge transfer were characterized using UV-vis absorption, steady-state emission, and time-resolved emission spectroscopy. As the relative concentration of electron-accepting CdSe QDs within mixtures of 4-ATP-capped CdTe and NHS-capped CdSe QDs increased, the rate and efficiency of electron transfer increased by 100-fold and 7.4-fold, respectively, as evidenced by dynamic quenching of band-edge emission from CdTe QDs. In contrast, for non-interacting mixtures of thiophenol capped CdTe QDs and NHS-capped CdSe QDs, which served as control samples, photophysical properties of the constituent QDs were unperturbed and excited-state charge transfer between the QDs was negligible. Our results reveal that carbodiimide-mediated coupling chemistry can be used to control the relative number of donor and acceptor QDs within heterostructures, which, in turn, enables fine-tuning of charge-transfer dynamics and yields. These amide-bridged dual-QD heterostructures are, thus, intriguing for light harvesting, charge transfer, and photocatalysis.

The Middle Road Less Taken: Electronic-Structure-Inspired Design of Hybrid Photocatalytic Platforms for Solar Fuel Generation.

The development of efficient solar energy conversion to augment other renewable energy approaches is one of the grand challenges of our time. Water splitting, or the disproportionation of H2O into energy-dense fuels, H2 and O2, is undoubtedly a promising strategy. Solar water splitting involves the concerted transfer of four electrons and four protons, which requires the synergistic operation of solar light harvesting, charge separation, mass and charge transport, and redox catalysis processes. It is unlikely that individual materials can mediate the entire sequence of charge and mass transport as well as energy conversion processes necessary for photocatalytic water splitting. An alternative approach, emulating the functioning of photosynthetic systems, involves the utilization of hybrid systems wherein different components perform the various functions required for solar water splitting. The design of such hybrid systems requires the multiple components to operate in lockstep with optimal thermodynamic driving forces and interfacial charge transfer kinetics. This Account describes a new class of nanoscale heterostructures comprising M xV2O5 nanowires, where M is a p-block cation with a ( n - 1) d10 ns2 np0 electronic configuration characterized by a stereoactive lone pair of electrons and x is its stoichiometry, interfaced with II-VI semiconductor quantum dots (QDs). Photocatalytic water splitting involves the transfer of excited-state holes from QDs to mid-gap states (derived from the stereoactive lone pairs of p-block cations) of nanowires, hole transport through nanowires, the reduction of protons at a QD-immobilized catalyst, and water oxidation at an anode. The M xV2O5/QD architectures provide a vast design space for evolutionary optimization of function with considerable tunability of composition and structure of the individual components as well as of the interfacial structure, thereby facilitating programmability of absorption spectra, energetic offsets, and charge-transfer reactivity. The available design space spans choice of the p-block cation M, its stoichiometry x, the composition and size of various QDs, and the nature of the nanowire/QD interface. This multivariate parameter space has been navigated by integrating first-principles modeling, diversified synthesis, spectroscopic measurements, and catalytic evaluation to facilitate the rational design of several generations of heterostructures and the systematic improvement of their photocatalytic performance. The electronic structures of the target heterostructures are predicted by DFT calculations in light of the revised lone pair model of stereoactive structural distortions and evaluated by hard X-ray photoelectron spectroscopy such as to systematically tune the interfacial band offsets. Central to this approach is the development of a topochemical "etch-a-sketch" intercalation approach that allows for facile installation of p-block cations in metastable polymorphs of V2O5. The interfacial charge transfer kinetics of M xV2O5/QD heterostructures is further evaluated by transient absorption spectroscopy to measure excited-state charge-transfer dynamics and is found to depend sensitively on interfacial structure and the thermodynamic driving forces in accordance with Marcus theory. The integration of theory and experiment has allowed for the design of viable photocatalytic architectures exemplified by the exceptional catalytic performance of ß-Pb xV2O5/CdX (X= S, Se) architectures, which has subsequently been elaborated to other lone-pair M xV2O5 compounds, demonstrating the effective exploitation of the opportunities for programmability available in the design space.

Type-II heterostructures of α-V2O5 nanowires interfaced with cadmium chalcogenide quantum dots: Programmable energetic offsets, ultrafast charge transfer, and photocatalytic hydrogen evolution.

We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V2O5 nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V2O5/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V2O5 NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10-12-10-11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10-6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V2O5/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H2 with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V2O5 NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V2O5/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.

Hole Extraction by Design in Photocatalytic Architectures Interfacing CdSe Quantum Dots with Topochemically Stabilized Tin Vanadium Oxide.

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable ß-Sn0.23V2O5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and ß-Sn0.23V2O5/CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The ß-Sn0.23V2O5/CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.

Implantable Tin Porphyrin-PEG Hydrogels with pH-Responsive Fluorescence.

Tetracarboxy porphyrins can be polymerized with polyethylene glycol (PEG) diamines to generate hydrogels with intense, near-infrared, and transdermal fluorescence following subcutaneous implantation. Here, we show that the high density porphyrins of the preformed polymer can be chelated with tin via simple incubation. Tin porphyrin hydrogels exhibited increasing emission intensities, ratios, and lifetimes from pH 1 to 10. Tin porphyrin hydrogel emission was strongly reversible and pH responsiveness was observed in the physiological range between pH 6 and pH 8. pH-sensitive emission was detected via noninvasive transdermal fluorescence imaging in vivo following subcutaneous implantation in mice.

Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2.

Linear aminoalkanoic acids (AAAs) and mercaptoalkanoic acids (MAAs) were characterized as bifunctional ligands to tether CdSe QDs to nanocrystalline TiO2 thin films and to mediate excited-state electron transfer (ET) from the QDs to TiO2 nanoparticles. The adsorption of 12-aminododecanoic acid (ADA) and 12-mercaptododecanoic acid (ADA) to TiO2 followed the Langmuir adsorption isotherm. Surface adduct formation constants (Kad) were ∼10(4) M(-1); saturation amounts of the ligands per projected surface area of TiO2 (Γ0) were ∼10(-7) mol cm(-2). Both Kad and Γ0 differed by 20% or less for the two linkers. CdSe QDs adhered to ADA- and MDA-functionalized TiO2 films; data were well modeled by the Langmuir adsorption isotherm and Langmuir kinetics. For ADA- and MDA-mediated assembly values of Kad were (1.8 ± 0.4) × 10(6) and (2.4 ± 0.4) × 10(6) M(-1), values of Γ0 were (1.6 ± 0.3) × 10(-9) and (1.2 ± 0.1) × 10(-9) mol cm(-2), and rate constants were (14 ± 5) and (60 ± 20) M(-1) s(-1), respectively. Thus, the thermodynamics and kinetics of linker-assisted assembly were slightly more favorable for MDA than for ADA. Steady-state and time-resolved emission spectroscopy revealed that electrons were transferred from both band-edge and surface states of CdSe QDs to TiO2 with rate constants (ket) of ∼10(7) s(-1). ET was approximately twice as fast through thiol-bearing linker 4-mercaptobutyric acid (MBA) as through amine-bearing linker 4-aminobutyric acid (ABA). Photoexcited QDs transferred holes to adsorbed MBA. In contrast, ABA did not scavenge photogenerated holes from CdSe QDs, which maximized the separation of charges following ET. Additionally, ABA shifted electron-trapping surface states to higher energies, minimizing the loss of potential energy of electrons prior to ET. These trade-offs involving the kinetics and thermodynamics of linker-assisted assembly; the driving force, rate constant, and efficiency of ET; and the extent of photoinduced charge separation can inform the selection bifunctional ligands to tether QDs to surfaces.

Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance.

A library of six selenorhodamine dyes (4-Se-9-Se) were synthesized, characterized, and evaluated as photosensitizers of TiO2 in dye-sensitized solar cells (DSSCs). The dyes were constructed around either a bis(julolidyl)- or bis(half-julolidyl)-modified selenoxanthylium core functionalized at the 9-position with a thienyl group bearing a carboxylic, hydroxamic, or phosphonic acid for attachment to TiO2. Absorption bands of solvated dyes 4-Se-9-Se were red-shifted relative to the dimethylamino analogues. The dyes adsorbed to TiO2 as mixtures of monomeric and H-aggregated dyes, which exhibited broadened absorption spectra and increased light-harvesting efficiencies relative to the solvated monomeric dyes. Carboxylic acid-bearing dyes 4-Se and 7-Se initially exhibited the highest incident photon-to-current efficiencies (IPCEs) of 65-80% under monochromatic illumination, but the dyes desorbed rapidly from TiO2 into solutions of HCl (0.1 M) in a CH3CN:H2O mixed solvent (120:1 v:v). The hydroxamic acid- and phosphonic acid-bearing dyes 5-Se, 6-Se, 8-Se, and 9-Se exhibited lower IPCEs (49-65%) immediately after preparation of DSSCs; however, the dyes were vastly more inert on TiO2, and IPCEs decreased only minimally with successive measurements under constant illumination. Power-conversion efficiencies (PCEs) of the selenorhodamine-derived DSSCs were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given anchoring group, the bis(half-julolidyl) dyes exhibited higher open-circuit photovoltages and PCEs than the corresponding bis(julolidyl) dyes. The hydroxamic acid- and phosphonic acid-bearing dyes are intriguing photosensitizers of TiO2 in light of their aggregation-induced spectral broadening, high monochromatic IPCEs, and relative inertness to desorption into acidic media.

Photoinduced electron transfer from quantum dots to TiO2: elucidating the involvement of excitonic and surface states.

Colloidal semiconductor quantum dots (QDs) exhibit excitonic and surface states, both of which may participate in charge-transfer processes relevant to solar energy conversion. To explore this inherent complexity of the charge-transfer mechanisms of QDs, we used steady-state and time-resolved emission measurements to characterize excited-state electron transfer (ET) from core-only CdSe QDs and core/shell CdSe/ZnS QDs to TiO2 nanoparticles (NPs). Core-only QDs transferred electrons from both excitonic and surface states to TiO2 with rate constants of ET (ket) of approximately (1-3) × 10(8) s(-1) and (4-7) × 10(7) s(-1), respectively. Efficiencies of ET (ηet) from excitonic and surface states were approximately 71-82% and 64-76%, respectively. Thus, trapping of electrons lowered their potential energy but did not greatly affect the efficiency of their transfer to TiO2. Photogenerated holes were transferred from core-only CdSe QDs to adsorbed 3-mercaptopropionic acid (MPA), which linked the QDs to TiO2. We characterized core/shell CdSe/ZnS QDs as alternatives to core-only QDs. The ZnS shell eliminated the undesirable trapping of electrons and transfer of photogenerated holes to MPA. We measured ket of approximately (1-3) × 10(8) s(-1) and ηet of approximately 66-85% for ET from excitonic states of core/shell CdSe/ZnS QDs to TiO2 NPs. The insensitivity of ket to the presence of the ZnS shell may have arisen from increased cross-linking of core/shell QDs to TiO2. Our results highlight the involvement of surface states in excited-state ET processes of core-only QDs and, for the heterostructures reported here, the improved performance of core/shell CdSe/ZnS QDs relative to core-only CdSe QDs.

From seconds to femtoseconds: solar hydrogen production and transient absorption of chalcogenorhodamine dyes.

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO2 catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophore's triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO2. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production.

Linker-assisted attachment of CdSe quantum dots to TiO2: Time- and concentration-dependent adsorption, agglomeration, and sensitized photocurrent.

We have characterized the concentration and time dependences of the attachment of colloidal CdSe quantum dots (QDs) to 16-mercaptohexadanoic acid (MHDA)-functionalized nanocrystalline TiO2 thin films. The amount of QDs and the extent of their agglomeration on MHDA-functionalized TiO2 films were characterized by transmission- and reflectance-mode UV/vis absorption spectroscopy and scanning electron microscopy. Optically transparent films with spatially homogeneous coloration and minimal agglomeration of QDs were prepared from 2.2 and 5.0 µM toluene dispersions of QDs at short reaction times (<5 h). In contrast, prolonged exposure of MHDA-functionalized TiO2 films to 22 µM dispersions of QDs yielded relatively opaque QD-functionalized films with spatially inhomogeneous coloration and substantial agglomeration of QDs. Agglomeration of QDs decreased the absorbed photon-to-current efficiencies of QD-sensitized solar cells (QDSSCs) by almost 3-fold. These results highlight the profound influence of agglomeration on the optical properties and interfacial electron-transfer reactivity of QD-functionalized TiO2 films prepared by in situ linker-assisted assembly as well as the photoelectrochemical performance of QDSSCs incorporating such films.

Relating structure and photoelectrochemical properties: electron injection by structurally and theoretically characterized transition metal-doped phenanthroline-polyoxotitanate nanoparticles.

Whereas a large number of sensitized polyoxotitanate clusters have been reported, information on the electrochemical properties of the fully structurally defined nanoparticles is not available. Bridging of this gap will allow a systematic analysis of the relation between sensitizer-cluster binding geometry, electronic structure and electron injection properties. Ti17O28(O(i)Pr)16(Fe(II)Phen)2 is a member of a doubly-doped series of nanoclusters in which the phenanthroline is attached to the surface-located transition metal atom. The visible spectrum of a dichloromethane solution of the studied sample shows a series of absorption bands in the 400-900 nm region. Theoretical DOS and TDDFT calculations indicate that the bands in increasing wavelength order correspond essentially to metal-to-core charge transfer (MCCT) at ∼460 nm, metal-to-ligand charge transfer (MLCT) at ∼520 nm and d-d metal-atom transitions. Exposure of a thin layer of the sample to light in a photoelectrochemical cell produces an electric current in the 400 to ∼640 nm region. The fit of the wavelength range of the electron injection with the results of the calculations suggests that charge injection into the FTO anode occurs both from the TiO cluster and from the phenanthroline ligand. Injection from the phenanthroline via the cluster orbitals is ruled out by the lower energy of the phenanthroline orbitals.

Effect of darapladib on major coronary events after an acute coronary syndrome: the SOLID-TIMI 52 randomized clinical trial.

IMPORTANCE: Lipoprotein-associated phospholipase A2 (Lp-PLA2) has been hypothesized to be involved in atherogenesis through pathways related to inflammation. Darapladib is an oral, selective inhibitor of the Lp-PLA2 enzyme. OBJECTIVE: To evaluate the efficacy and safety of darapladib in patients after an acute coronary syndrome (ACS) event. DESIGN, SETTING, AND PARTICIPANTS: SOLID-TIMI 52 was a multinational, double-blind, placebo-controlled trial that randomized 13,026 participants within 30 days of hospitalization with an ACS (non-ST-elevation or ST-elevation myocardial infarction [MI]) at 868 sites in 36 countries. INTERVENTIONS: Patients were randomized to either once-daily darapladib (160 mg) or placebo on a background of guideline-recommended therapy. Patients were followed up for a median of 2.5 years between December 7, 2009, and December 6, 2013. MAIN OUTCOMES AND MEASURES: The primary end point (major coronary events) was the composite of coronary heart disease (CHD) death, MI, or urgent coronary revascularization for myocardial ischemia. Kaplan-Meier event rates are reported at 3 years. RESULTS: During a median duration of 2.5 years, the primary end point occurred in 903 patients in the darapladib group and 910 in the placebo group (16.3% vs 15.6% at 3 years; hazard ratio [HR], 1.00 [95% CI, 0.91-1.09]; P = .93). The composite of cardiovascular death, MI, or stroke occurred in 824 in the darapladib group and 838 in the placebo group (15.0% vs 15.0% at 3 years; HR, 0.99 [95% CI, 0.90-1.09]; P = .78). There were no differences between the treatment groups for additional secondary end points, for individual components of the primary end point, or in all-cause mortality (371 events in the darapladib group and 395 in the placebo group [7.3% vs 7.1% at 3 years; HR, 0.94 [95% CI, 0.82-1.08]; P = .40). Patients were more likely to report an odor-related concern in the darapladib group vs the placebo group (11.5% vs 2.5%) and also more likely to report diarrhea (10.6% vs 5.6%). CONCLUSIONS AND RELEVANCE: In patients who experienced an ACS event, direct inhibition of Lp-PLA2 with darapladib added to optimal medical therapy and initiated within 30 days of hospitalization did not reduce the risk of major coronary events. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01000727.

Influence of dispersion forces and ordering on the compositions of mixed monolayers of alkanoic acids on nanocrystalline TiO2 films.

Lateral dispersion forces induce the ordering of n-alkanoic acids on nanocrystalline TiO2 films and cause the compositions of mixed monolayers to change. The equilibrium formation of single-component monolayers of n-alkanoic acids and 6-bromohexanoic acid (Br6A) on TiO2 was well-modeled by the Langmuir adsorption isotherm. Surface adduct formation constants were 10(3)-10(4) M(-1), and saturation amounts of adsorbates per projected surface area of TiO2 were on the order of 10(-7) mol cm(-2). The adsorption of n-heneicosanoic acid (21A) followed Langmuir kinetics, whereas the net rates of adsorption of shorter n-alkanoic acids and Br6A were slower than predicted by simple Langmuir kinetics, suggesting that desorption was non-negligible. At high surface coverages, n-alkanoic acids with 14 or more methylene groups formed ordered, crystalline monolayers, as evidenced by shifts of asymmetric and symmetric CH2 stretching bands in IR spectra. The extent of ordering was similar to that of self-assembled monolayers of alkanethiols on gold. The formation of ordered monolayers was well-modeled by an idealized mechanism, in which adsorption and desorption followed Langmuir kinetics and ordering was first-order with respect to the fractional surface coverage of adsorbates. Dispersion forces and ordering affected the compositions of mixed monolayers of 21A and Br6A on TiO2 films that remained in contact with mixed coadsorption solutions. When the fractional surface coverage of 21A was sufficiently high to induce ordering, it displaced Br6A from TiO2. We propose that these compositional changes were driven by the stabilization of 21A via cohesive lateral dispersion forces. Our results reveal that mixed monolayers on nanocrystalline TiO2 films are dynamic and that noncovalent intermolecular interactions can profoundly influence their compositions and properties.

Synthesis and photoelectrochemical performance of chalcogenopyrylium monomethine dyes bearing phosphonate/phosphonic acid substituents.

Chalcogenopyrylium monomethine dyes were prepared via condensation of a 4-methylchalcogenopyrylium compound with a chalcogenopyran-4-one bearing a 4-(diethoxyphosphoryl)phenyl substituent (or the phosphonic acid derivative). The dyes have absorbance maxima of 603-697 nm in the window where the solar spectrum is most intense. The dyes formed H-aggregates on TiO2, increasing the light-harvesting efficiency of the dyes. Shortcircuit photocurrent action spectra were acquired to evaluate the influence of dye structure on the photoelectrochemical performance.

Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution.

We used linker-assisted assembly (LAA) to tether CdS quantum dots (QDs) to MoS2 nanosheets via L-cysteine (cys) or mercaptoalkanoic acids (MAAs) of varying lengths, yielding ligand-bridged CdS/MoS2 heterostructures for redox photocatalysis. LAA afforded precise control over the light-harvesting properties of QDs within heterostructures. Photoexcited CdS QDs transferred electrons to molecularly linked MoS2 nanosheets from both band-edge and trap states; the electron-transfer dynamics was tunable with the properties of bridging ligands. Rate constants of electron transfer, estimated from time-correlated single photon counting (TCSPC) measurements, ranged from (9.8 ± 3.8) × 106 s-1 for the extraction of electrons from trap states within heterostructures incorporating the longest MAAs to >5 × 109 s-1 for the extraction of electrons from band-edge or trap states in heterostructures with cys or 3-mercaptopropionic acid (3MPA) linkers. Ultrafast transient absorption measurements revealed that electrons were transferred within 0.5-2 ps or less for CdS-cys-MoS2 and CdS-3MPA-MoS2 heterostructures, corresponding to rate constants ≥5 × 109 s-1. Photoinduced CdS-to-MoS2 electron transfer could be exploited in photocatalytic hydrogen evolution reaction (HER) via the reduction of H+ to H2 in concert with the oxidation of lactic acid. CdS-L-MoS2-functionalized FTO electrodes promoted HER under oxidative conditions wherein H2 was evolved at a Pt counter electrode with Faradaic efficiencies of 90% or higher and under reductive conditions wherein H2 was evolved at the CdS-L-MoS2-heterostructure-functionalized working electrode with Faradaic efficiencies of 25-40%. Dispersed CdS-L-MoS2 heterostructures promoted photocatalytic HER (15.1 µmol h-1) under white-light illumination, whereas free cys-capped CdS QDs produced threefold less H2 and unfunctionalized MoS2 nanosheets produced no measurable H2. Charge separation across the CdS/MoS2 interface is thus pivotal for redox photocatalysis. Our results reveal that LAA affords tunability of the properties of constituent CdS QDs and MoS2 nanosheets and precise, programmable, ligand-dependent control over the assembly, interfacial structure, charge-transfer dynamics, and photocatalytic reactivity of CdS-L-MoS2 heterostructures.

Influence of solvation and the persistence of adsorbed linkers on the attachment of CdSe quantum dots to TiO2 via linker-assisted assembly.

CdSe quantum dots (QDs) were attached to the surfaces of nanocrystalline TiO(2) films functionalized with 16-mercaptohexadecanoic acid (MHDA). The solvent from which MHDA was adsorbed to TiO(2) determined the amount of adsorbed MHDA, the extent of ordering within monolayers, and the lability of MHDA-TiO(2) interactions, which in turn dictated the quality of QD-functionalized TiO(2) films. When MHDA was adsorbed to TiO(2) from dichloromethane, toluene, or heptane, it desorbed into toluene dispersions of CdSe QDs, causing the flocculation of QDs and the formation of opaque, nonuniform QD-functionalized TiO(2) films overcoated with thick (0.1-0.5 µm) layers of agglomerated CdSe. When MHDA was adsorbed to TiO(2) from tetrahydrofuran, it persisted upon exposure to toluene dispersions of QDs. The resulting QD-functionalized TiO(2) films were transparent with a uniform loading of QDs and without an agglomerated overlayer. Control experiments revealed that flocculation and the formation of low-quality films were correlated with the presence of MHDA in dispersions of QDs; we propose that MHDA adsorbed to CdSe QDs and decreased their dispersibility in nonpolar solvents. The susceptibility of QDs to MHDA-induced flocculation and agglomeration increased with postsynthesis purification. Our results highlight the influence of solvation on the persistence of linker-substrate interactions, which is of central importance in optimizing the linker-assisted assembly of interfaces.

Influence of surface-attachment functionality on the aggregation, persistence, and electron-transfer reactivity of chalcogenorhodamine dyes on TiO2.

Chalcogenorhodamine dyes bearing phosphonic acids and carboxylic acids were compared as sensitizers of nanocrystalline TiO(2) in dye-sensitized solar cells (DSSCs). The dyes were constructed around a 3,6-bis(dimethylamino)chalcogenoxanthylium core and varied in the 9 substituent: 5-carboxythien-2-yl in dyes 1-E (E = O, Se), 4-carboxyphenyl in dyes 2-E (E = O, S), 5-phosphonothien-2-yl in dyes 3-E (E = O, Se), and 4-phosphonophenyl in dyes 4-E (E = O, Se). All dyes adsorbed to TiO(2) as mixtures of H aggregates and monomers, which exhibited broadened absorption spectra relative to those of purely amorphous monolayers. Surface coverages of dyes and the extent of H aggregation varied minimally with the surface-attachment functionality, the structure of the 9-aryl group, and the identity of the chalcogen heteroatom. Carboxylic acid-functionalized dyes 1-E and 2-E desorbed rapidly and completely from TiO(2) into acidified CH(3)CN, but phosphonic acid-functionalized dyes 3-E and 4-E persisted on TiO(2) for days. Short-circuit photocurrent action spectra of DSSCs corresponded closely to the absorptance spectra of dye-functionalized films; thus, H aggregation did not decrease the electron-injection yield or charge-collection efficiency. Maximum monochromatic incident photon-to-current efficiencies (IPCEs) of DSSCs ranged from 53 to 95% and were slightly higher for carboxylic acid-functionalized dyes 1-E and 2-E. Power-conversion efficiencies of DSSCs under white-light illumination were low (<1%), suggesting that dye regeneration was inefficient at high light intensities. The photoelectrochemical performance (under monochromatic or white-light illumination) of 1-E and 2-E decayed significantly within 20-80 min of the assembly of DSSCs, primarily because of the desorption of the dyes. In contrast, the performance of phosphonic acid-functionalized dyes remained stable or improved slightly on similar timescales. Thus, replacing carboxylic acids with phosphonic acids increased the inertness of chalcogenorhodamine-TiO(2) interfaces without greatly impacting the aggregation of dyes or the interfacial electron-transfer reactivity.

Differences in soil mobility and degradability between water-dispersible CdSe and CdSe/ZnS quantum dots.

The relative leaching potential and degradation of water-dispersible CdSe and CdSe/ZnS quantum dots (QDs) were evaluated using small-scale soil columns. The potential of QDs to release toxic Cd(2+) and/or Se(2-)/SeO(3)(2-) ions upon degradation is of environmental concern and warrants investigation. Both classes of QDs exhibited limited soil mobility in CaCl(2), with more than 70% of the total Cd and Se species from QDs retained in the top soil after passing 10 column volumes of solution through the soil column. However, mobilization of Cd- and Se-species was observed when EDTA was used as the leaching solution. Approximately 98% of the total Cd(2+) loaded leached out from the Cd(2+)-spiked soil, while only 30% and 60% leached out from the CdSe and CdSe/ZnS QD-spiked soils, respectively. Soil column profiles and analysis of leachates suggest that intact QDs leached through the soil. Longer incubation (15 days) in soil prior to leaching indicated some degradation and/or surface modification of both QDs. These results suggest that chelating agents in the environment can enhance the soil mobility of intact and degraded QDs. It is apparent that QDs in soil, including the polymer-coated CdSe/ZnS QDs that are generally assumed to possess a higher degree of environmental stability, can undergo chemical transformations, which subsequently dictate their overall mobility.

Study on the effects of humic and fulvic acids on quantum dot nanoparticles using capillary electrophoresis with laser-induced fluorescence detection.

The increasing production and use of quantum dot (QD) nanoparticles have caused concerns on the possibility of contaminating the aquatic and terrestrial ecosystems with wastes that may contain QDs. Therefore, studies on the behavior of QDs upon interaction with components of the natural environment have become of interest. This study investigated the fluorescence and electrophoretic mobility of carboxylic or amine polyethylene glycol (PEG)-functionalized CdSe/ZnS QDs in the presence of two aquatic humic substances (HS), Suwannee River humic and fulvic acids, using capillary electrophoresis with laser-induced fluorescence detection. Results showed initial enhancement in fluorescence of QDs at the onset of the interaction with HS, followed by fluorescence quenching at longer exposure with HS (>30 min). It was also observed that the electrophoretic mobility of QDs increases with increasing concentration of HS, suggesting an increase in the ratio in charge to hydrodynamic size of the nanoparticles. To determine if the QDs degraded upon interaction with HS, the QD-HS mixtures were dialyzed to separate free Cd2+ from intact QDs, followed by analysis of the solutions using inductively coupled plasma-mass spectrometry. Results suggested that degradation of QDs in the presence of HS did not occur within the period of incubation.