Waveguide-Based Spectroelectrochemical Characterization of Band Edge Energies in Submonolayers of CdSe Quantum Dots Tethered to Indium–Tin Oxide Electrodes

Abstract

We present here high sensitivity attenuated total reflectance (ATR) spectroelectrochemical studies of electron injection (reduction) into surface-tethered, submonolayer to monolayer coverages of CdSe quantum dots (QDs) linked to indium–tin oxide (ITO) electrodes using a strong X-type bifunctional phosphonic acid (PA) surface linker, octanediphosphonic acid (ODiPA). Estimates of conduction band energies (E CB) were obtained from the onset of absorbance bleaching as a function of QD diameter (3.2–6.4 nm) and as a function of the supporting electrolyte (LiClO4) concentration. For CdSe QDs created from combinations of moderately strong stearic acid, hexadecylamine, trioctylphosphine oxide, and trioctylphosphine ligands, surface-tethering was accompanied by decreases in QD diameter and loss of up to 25% volume for the largest QDs. For QDs prepared with PA ligands, followed by aggressive (3×) pyridine exchange to produce QDs with weak capping ligands, no size reduction was observed as a result of adsorption to the ODiPA/ITO surface. For both types of tethered CdSe QDs, significant stabilization of the reduction product of the surface-tethered QD was observed with ca. 700 meV lowering of E CB relative to estimates of E CB obtained from our recent in vacuuo UV-photoemission studies of bare CdSe QDs tethered to Au surfaces. A sizeable fraction of that stabilization is proposed to arise from the tethering of these asymmetric QDs to a complex, high dielectric constant interface region. At least 200 meV of the stabilization arises from concentration-dependent charge screening by the solution counter ion (Li+), with no evidence for the incorporation of Li+ as a result of the electron injection process. The overall stabilization in the reduced form of these tethered QDs is larger than seen for previous spectroelectrochemical studies of QD reduction, in solution, tethered at higher coverages to transparent electrodes, or as electrophoretically deposited multilayer QD thin films. This waveguide ATR spectroelectrochemical approach to estimating energetics for QDs tethered to semiconductor or oxide substrates at low surface coverages is likely to be relevant for a wide array of energy conversion and energy storage processes using nanomaterials and may be especially useful for studying the effects of surface coverage, type of surface linker, contacting solvent/electrolytes, and adsorbed molecular reactants.

Spectroelectrochemical Characterization of Energetics in Type I vs Quasi-Type II Heterojunctions in CdSe@CdS Nanorod Films

“…Previous studies of the reduction of CdSe QDs, as thin films of near monolayer coverage and greater, have shown that the electron injection process is tightly coupled to the uptake of a positive counterion (e.g., Li + ) near the reduced nanocrystal to help to stabilize those QDs that have been reduced and bear excess negative charge. 28,55 Houtepen and co-workers showed that the size and physical proximity of the counterion can strongly impact the reduction potential of the QD, 55 whereas our own recent work has shown that when using Li + as the counterion, there is a logarithmic dependence of bleaching potential and apparent E CB on the concentration of the counterion. 28 We posit that for these multilayer NR films, creation of the pathways needed to achieve both electrochemical equilibration of the NRs with the ITO substrate and intercalation of the counterions needed to maintain electroneutrality is hindered by the presence of high concentrations of excess ligands that accompany NR film processing.…”

“…Absorbance difference spectra were recorded using a potential step method over a wavelength range of 200−1000 nm. 28 The potential was stepped to −0.700 V vs Ag/AgNO 3 where no electron injection into the NR is expected, and the first blank spectrum was acquired after a waiting period of 10 s. After another 10 s waiting period, the first sample spectrum was subsequently acquired at −0.700 V. The process was repeated: A second blank spectrum was acquired at −0.700 V, and then the second sample spectrum was acquired at a potential shifted by −0.020 V relative to the first sample spectrum. Sample spectra were acquired over the range of −0.700 to −1.500 V at 20 mV intervals, and each was referenced to the −0.700 V blank spectrum that was acquired immediately prior.…”

“…As confirmed in the XPS studies shown below, UV-ozone treatment after each deposition cycle was required to degrade these excess ligands to remove barriers to charge injection into the NR and close association of the Li + counterions needed to maintain electroneutrality. 28 3.2. TEM and Spectral Characterization of CdSe@CdS NRs in Solution.…”

“…28−30 The onset potentials for bleaching of the lowest energy excitonic absorbance peaks can be used to estimate the one-electron reduction potentials, and E CB , for monolayer-tethered ligand-capped CdSe quantum dots. 28−30 Using this approach, we recently showed that QD sizedependent estimates of E CB were significantly farther from vacuum than estimates of E CB for nearly ligand-free QDs arising from photoemission experiments in a vacuum [28][29][30]39 We also recently showed that the spectroelectrochemical approach can be used to track energetics of CdSe NRs that have been decorated with Au tips and subsequently compared with estimates from photoemission spectroscopies. 30 In this contribution, we show that spectroelectrochemically monitored absorbance bleaching accompanying electron injection into multilayer CdSe@CdS NRs on ITO electrodes can be used to estimate conduction band energies for both the CdSe seed and the CdS rod, uniquely differentiating Type I versus quasi-Type II heterojunction formation.…”

“…Electrochemical and spectroelectrochemical characterization methods offer alternatives to estimation of band edge energies, with typically better energy resolution than most spectroscopic probes, either as nanocrystal solutions or for tethered monolayer or solution-deposited multilayer films. ,,,− Voltammetric methods have been used to measure onset or peak potentials for reduction (electron injection into the nanocrystal) or oxidation (hole injection into the nanocrystal) from which estimates for either E CB or E VB are obtained if the electrochemical potentials can be calibrated to vacuum scales, typically using reference redox couples such as ferrocene/ferricenium (Fc/Fc + ). , These voltammetric methods, however, often require multilayer nanocrystalline films or concentrated nanoparticle solutions to ensure that Faradaic currents are significantly larger than capacitive (ion displacement) currents.…”