RESUMO
Photocatalytic CO2 reduction offers a promising strategy to produce hydrocarbons without reliance on fossil fuels. Visible light-absorbing colloidal nanomaterials composed of earth-abundant metals suspended in aqueous media are particularly attractive owing to their low-cost, ease of separation, and highly modifiable surfaces. The current study explores such a system by employing water-soluble ZnSe quantum dots and a Co-based molecular catalyst. Water solubilization of the quantum dots is achieved with either carboxylate (3-mercaptopropionic acid) or ammonium (2-aminoethanethiol) functionalized ligands to produce nanoparticles with either negatively or positively-charged surfaces. Photocatalysis experiments are performed to compare the effectiveness of these two surface functionalization strategies on CO2 reduction and ultrafast spectroscopy is used to reveal the underlying photoexcited charge dynamics. We find that the positively-charged quantum dots can support sub-picosecond electron transfer to the carboxylate-based molecular catalyst and also produce >30% selectivity for CO and >170 mmolCO gZnSe-1. However, aggregation reduces activity in approximately one day. In contrast, the negatively-charged quantum dots exhibit >10 ps electron transfer and substantially lower CO selectivity, but they are colloidally stable for days. These results highlight the importance of the quantum dot-catalyst interaction for CO2 reduction. Furthermore, multi-dentate catalyst molecules create a trade-off between photocatalytic efficiency from strong interactions and deleterious aggregation of quantum dot-catalyst assemblies.
RESUMO
The compound [5,10,15,20-tetrakis(4-fluoro-2,6-dimethylphenyl)porphyrinato]platinum(II), [Pt(C52H40F4N4)] or Pt(II)TFP, has been synthesized and structurally characterized by single-crystal X-ray crystallography. The Pt porphyrin exhibits a long-lived phosphorescent excited state (τ0 = 66â µs), which has been characterized by transient absorption and emission spectroscopy. The phosphorescence is extremely sensitive to oxygen, as reflected by a quenching rate constant of 5.0 × 108â M-1â s-1, and as measured by Stern-Volmer quenching analysis.
RESUMO
It is critical to find methods to control the thermodynamic driving force for photoexcited charge transfer from quantum dots (QDs) and explore how this affects charge transfer rates, since the efficiency of QD-based photovoltaic and photocatalysis technologies depends on both this rate and the associated energetic losses. In this work, we introduce a single-pot shell growth and Cu-catalyzed cation exchange method to synthesize CdxZn1-xSe/CdyZn1-yS QDs with tunable driving forces for electron transfer. Functionalizing them with two molecular electron acceptorsânaphthalenediimide (NDI) and anthraquinone (AQ)âallowed us to probe nearly 1 eV of driving forces. For AQ, at lower driving forces, we find that higher Zn content results in a 130-fold increase of electron transfer rate constants. However, at higher driving forces electron transfer dynamics are unaltered. The data are understood using an Auger-assisted electron transfer model and analyzed with computational work to determine approximate binding geometries of these electron acceptors. Our work provides a method to tune QD reducing power and produces useful metrics for optimizing QD charge transfer systems that maximize rates of electron transfer while minimizing energetic losses.
RESUMO
Computational models have been developed that can accurately predict the electronic structure and thus optical properties of a variety of quantum dot (QD) materials. However, the application of these models to core/shell and other heterostructured QDs has received less experimental corroboration owing to the difficulty in systematically synthesizing and characterizing large ranges of geometries. In the current work, we synthesized a library of core/shell CdSe/CdS QDs with varying core sizes and shell thicknesses, and have characterized their radiative recombination rates. We find that the core size has only a modest effect on the radiative recombination rates, far less than is predicted by conventional effective mass models. In order to theoretically describe the experimental data, we performed an empirical modification of an effective mass model. We find that the conduction band offset between CdSe and CdS must be empirically altered based on QD core size in order to match our experimental data. This is hypothesized to be a result of reduced interfacial strain in core/shell QDs with smaller cores. The resultant relationship between conduction band offset and core size is used to create a predictive map of radiative lifetime as a function of core size and shell thickness. This map will be useful to researchers implementing CdSe/CdS core/shell QDs for a variety of applications since it can provide geometry specific excited state lifetimes.
