Session III

Monday, May 22nd

Session III


Superior Hole Transfer Performance in Chloride Treated CdTe Quantum Dots Covalently Attached to Molecular Acceptors

Saurabh Chauhan and David F. Watson

 University at Buffalo, Department of Chemistry

Hole transfer from photoexcited chloride treated CdTe quantum dots (QDs) covalently linked to molecular hole acceptor 6-ferrocenylhexanethiol (FcC6SH) was studied. Chloride treatment of CdTe QDs resulted in effective passivation of surface trap states of CdTe QDs indicated by high fluorescence quantum yield (QY) and nearly mono-exponential decay behavior. Steady state and time resolved emission measurements on mixed dispersions containing chloride treated CdTe QDs and FcC6SH revealed that holes were transferred from photoexcited CdTe QDs to FcC6SH with efficiency of hole transfer (ηHT) of (98.7±0.3)% and overall rate constant of hole transfer (kHT) of approximately (3.0±0.4)×109 s-1. On the contrary, untreated CdTe QDs transferred holes with ηHT of (57.0±3.0)% with kHT of (6.5±0.1) )×107 s-1 on addition of same concentration of FcC6SH. 1H NMR measurements on mixed dispersions allowed facile quantification of bound FcC6SH to the surface of CdTe QDs. The chloride treated CdTe QDs showed at least 10 times higher coverage of FcC6SH as compared to untreated as-synthesized CdTe QDs. The higher loading of FcC6SH was attributed to efficient ligand exchange of FcC6SH with labile chlorides at the surface of chloride treated CdTe QDs. The higher loading of FcC6SH on chloride treated CdTe QDs provided multiple hole transfer pathways and resulted in superior hole transfer performance of chloride treated CdTe QDs versus untreated CdTe QDs. kHT increased linearly with surface coverage of FcC6SH on the surface of chloride treated CdTe QDs at low concentration of FcC6SH indicating the additive nature of hole transfer pathways; however, at high surface coverages of FcC6SH, kHT levelled off.  Finally, titration of hexanethiol (HT) with chloride treated CdTe QDs did not show any emission quenching precluding the possibility of deleterious hole trapping by thiolates at the surface of chloride treated CdTe QDs. These results show the versatility of chloride treatment, which not only effectively passivated the surface of CdTe QDs but also enabled highly efficient hole transfer.


Excited-State Charge Transfer in V2O5/Quantum Dot Type II Heterostructures

Aaron Sheng and David F. Watson

University at Buffalo, Department of Chemistry

Efficient charge separation following photoexcitation is the key to harness solar energy efficiently by semiconductor heterostructures. Cadmium chalcogenide quantum dots (QDs) interfaced with V2O5 nanowires provide a promising system for solar energy conversion or photocatalysis. The type II interface between QDs and V2O5 promotes spatial separation of charge carriers in different components of the heterostructure (electrons in V2O5 and holes in QDs).  We prepared V2O5/CdSe heterostructures via linker-assisted assembly (LAA) and successive ionic layer adsorption and reaction (SILAR) and studied charge transfer dynamics in these heterostructures. Attachment of CdSe QDs onto V2O5 NWs was confirmed by transmission electron microscopy (TEM) and Raman spectroscopy. Charge transfer dynamics in CdSe/V2O5 heterostructures was studied by transient absorption spectroscopy. TA spectrum of QD/V2O5 heterostructures revealed a new induced absorption band when CdSe QD were present on the nanowire as opposed to V2O5 NW alone. This new induced absorption band was attributed to the charge separated state where electrons were localized in the CB of V2O5 NWs and holes in VB of CdSe QDs. Ultrafast TA measurements revealed that the charge separation occurred within picoseconds for SILAR and LAA prepared heterostructures. The charge separated state decayed on microsecond timescales with lifetimes of 1 us and 2 us for SILAR- and LAA- prepared heterostructures, respectively. Thus, SILAR assembled heterostructure showed faster charge separation compared to LAA assembled heterostructure, as well as faster recombination rate, which is consistent with the much closer contact between QDs and V2O5 in SILAR prepared heterostructures as compared to LAA prepared structures.


Quantum-Dot Sensitized Solar Cells: The Search for a More Stable and Persistent Molecular Linker

Zachery Schmidt and David F. Watson

University at Buffalo, Department of Chemistry

This presentation will focus on the effects of using different surface anchoring groups on a CdSe quantum dot sensitized solar cell (QDSSC) prepared by linker-assisted assembly. In linker-assisted assembly, bifunctional ligands tether nanoparticles to substrates. Most common QDSSCs use carboxylic anchoring groups to attach QDs to TiO2. This type of linker has shown good electron transfer but poor stability and persistence on the surface. To resolve this issue two new linkers, one with a phosphonic anchoring group (2-mercaptoethylphosphonic acid (PA) and the other with a hydroxamic anchoring group (3-mercaptopropylhydroxamic acid (HA), have been synthesized. PA and HA have shown improved stability and persistence compared to carboxylic anchoring groups such as mercaptopropionic acid (MPA). Phosphonic and hydroxamic anchoring groups have shown to improve stability for other solar cell systems, such as dyes, porphyrins, and squaraines, with mixed results concerning quantum efficiencies when compared to carboxylic acids. PA and HA have both been found to be more stable than MPA in QDSSCs. Results of time-resolved spectroscopic experiments to characterize excited-state electron transfer from CdSe QDs, via the molecular linkers, to TiO2, will be presented.


A study on concentration dependent charge-discharge characteristics of non-aqueous redox flow battery systems utilizing common organic electrolytes

Anjula Kosswattaarachchi and Timothy R. Cook

University at Buffalo, Department of Chemistry

Non-aqueous redox flow batteries (NARFBs) are promising as high capacity energy storage devices. However, the current performance, and energy densities reported for NARFBs are far behind their potential. This is largely owing to low redox active species concentration utilized in these devices. Although, there has been great improvement in attempts to increase the concentration of redox active materials employed, charge-discharge experiments reported in majority of NARFB literature were performed at dilute concentrations of active species. It is possible that, at high concentrations, major changes in electrochemical behavior of non-aqueous electrolytes take place due to changes in solvation structure, which in turn would affect the overall cell performance. Accordingly, we studied a series of NARFB systems utilizing ferrocene/ TEMPO as posolyte, and cobaltocenium hexafluorophosphate/N-methylphthalimide as negolyte, to investigate the effect of concentration on charge-discharge profiles. Cycling studies were performed with four combinations of the above mentioned catholyte, and anolyte materials, at dilute concentrations, and at the highest possible concentrations. UV-Vis spectroscopy was used to determine the maximum solubilities of the selected common organic redox active species. Electrochemical characterizations included cyclic voltammetry, electrochemical impedance spectroscopy and charge-discharge cycling. 


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

Natalia Rivera-Gonzalez, Saurabh Chauhan and David F. Watson

University at Buffalo, Department of Chemistry

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 excitedstate 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 104 M1; saturation amounts of the ligands per projected surface area of TiO2 (Γ0) were 107 mol cm2. 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) × 106 and (2.4 ± 0.4) × 106 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 107 s1. 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 electrontrapping surface states to higher energies, minimizing the loss of potential energy of electrons prior to ET. These tradeoffs 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.


Novel rhenium(I) phosphazane complexes with applications towards CO2 reduction catalysis and coordination-driven self-assembly

Matthew Crawley and Timothy R. Cook

University at Buffalo, Department of Chemistry

A wide range of rhenium(I)-based CO2 reduction catalysts have been studied and reported in the literature to date, however, none feature phosphazane ligands, which exhibit far greater tunability both in terms of electronic structure and steric demand, in comparison to other supporting ligands currently in use. To the same end, coordination-driven self-assembly is also dominated by the use of polypyridyl donor building blocks in the synthesis of metallacycle and metallacage architectures. Our work looks to expand the library of ligands currently utilized in coordination-driven self-assembly in terms of both angularity and donor atom. This work’s aim is to bridge coordination-driven self-assembly with small molecule activation, specifically, CO2 reduction catalysis through the synthesis of electrochemically-active polynuclear metal-organic polyhedra (MOP) architectures featuring functionalized donor and acceptor species.  A series of Re(I) chelating-phosphazane (PNP) complexes supported by polypyridyl and carbonyl ligands have been designed, synthesized, and in addition to routine characterization, their structural and electrochemical properties have been examined via single-crystal X-ray diffraction studies and cyclic voltammetry. Initial finding show that the monomeric Re(dppa)(bpy)(CO)2OTf (dppa = bis(diphenylphosphino)amine, bpy = 2,2ʹ-bipyridine) complex exhibits promising electrochemistry towards the ultimate goal of catalyzed CO2 reduction via self-assembled MOP.