Photocharging of Colloidal CdS Nanocrystals

Abstract

The generation of reduced semiconductor nanocrystals is of interest for a variety of optoelectronic applications. In comparison to other nanocrystalline materials, little work has been reported on reduction of CdS nanocrystals, which are particularly interesting for solar photochemistry applications. Most nanocrystal reduction strategies require electron donors that reduce ground state or photoexcited nanocrystals. In this work, we report the discovery of photocharging of CdS nanocrystals under continuous wave illumination with no added reductants. The long-lived reduced states form under illumination, saturate at high concentrations, and recover over time scales of minutes when illumination stops. This process occurs in CdS nanocrystals of different sizes, morphologies, organic surface capping ligands, and multiple solvents but not in CdSe nanocrystals. We propose a charging mechanism in which the photoexcited holes oxidize surface-capping ligands, which then dissociate from the nanocrystal surface. We contrast this ligand-mediated process with solvent-mediated photoreduction that occurs in CdS nanocrystals with polar ligands, which requires hole scavengers.

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Recombination

“…After oxygen exposure, partial recovery of the neutral PL decay kinetics is observed, due to partial scavenging of electrons from doped CdS states, while surface-modification of the nanocrystals may prevent full recovery of the initial PL dynamics. Our PL data are fully consistent with reported indications of Auger recombination in photodoped QDs, 71,113,115 demonstrating the occurrence of this mechanism under our conditions. Potential role of DMPU solvent.…”

“…114 Spectral changes at higher-energy transitions were also visible, as consistent with literature reports of doped CdS QDs. 115 These photodoped QDs displayed similar spectral changes to those prepared using the established chemical reduction with Na/biphenyl (Supporting Information Figure S8). 116 The anionic QDs were stable under nitrogen and were quenched by introduction of oxygen (air).…”

“…113 Analysis of time-resolved PL decay of QDs before and after photodoping (Figure 6B) shows multiexpoential decay in all QD samples, consistent with other reports of CdS QDs (Supporting Information Figure S13). 115 Before photodoping, the QD sample exhibited decay components of τ ~ 16 ns, assigned to radiative decay lifetimes measured in similar QDs, as well as faster decay components of τ ~ 1 ns. After photodoping, the τ ~ 16 ns component is nearly eliminated, resulting in an approximate threefold decrease in the average PL lifetime of the nanocrystals (see Supporting Information Figure S13 for fitting data).…”

“…We also observed reversible photodoping of QDs in the absence of reductant, consistent with recent findings that CdS QDs can become photodoped through oxidation of their capping ligands or surface-bound water molecules without added reductants. 115,117 While this phenomenon can be used to generate populations of reduced QDs for spectroscopic study, added chemical reductants are required for catalytic transformations. No reduction of 1a to 2a occurs in the absence of terminal reductant (amine or formate) because these reductant-free photodoping pathways cannot provide enough electrons to produce a measurable amount of product in the catalytic reaction (Table 1, entry 9).…”

“…38 Absorbance tails in the visible spectrum have also been observed following chemical doping of CdSe QDs by Na/biphenyl, ascribed to the broadening and red shifting of the excitonic features by the doped electrons and excitations of electrons in surface trap states. 115,118 Light scattering due to QD aggregation may contribute to the broad feature, however both observed spectral changes were fully reversible after exposure to oxygen (Figure 4B, green trace), strongly suggesting that they arise from the presence of injected electrons residing in the CB (the exciton bleaching) as well as newly formed surface states (the additional broad features). 103,118 To confirm the origin of these spectral changes within the catalytic reaction, we undertook spectroelectrochemistry studies, as previously employed to study CdSe nanocrystals 67,119,120 and deeply reducing photocatalytic systems.…”