Electrochemistry of CdS Nanoparticles: A Correlation between Optical and Electrochemical Band Gaps

This communication reports novel electrochemistry of cadmium sulfide quantum dots (Q-CdS) in N,N′-dimethylformamide (DMF). During the past two decades, semiconductor nanoparticles or quantum dots (QDs), in particular those of the cadmium chalcogenides, have attracted considerable attention due to their tunable electronic properties as a function of sizesthe so-called quantum size effect. Recently, TOPO (tri-n-octylphosphine oxide)-capped Q-CdSe has been used for stimulated light emission where the laser frequency was tuned through simple variation of particle size.1 The photoelectrochemical2 and photophysical3 properties of these particles in their size-quantized states have been wellstudied. However, there have been relatively few reports on the electrochemical properties of semiconductor QDs.4,5 Using the “particle in a box” model, Brus6 has predicted a dependence of redox potential on particle size for Q-CdS. However, to our knowledge, this model has not been tested by actual electrochemical measurements of QDs in solution, largely because of the limited solvent window of many solvent/electrolyte systems and the instability of the particles. Here this is considered, and we show a direct correlation between the electrochemical band gap and the electronic spectra of CdS nanoparticles in DMF. Thioglycerol-capped, Q-CdS was chosen for study because stable monodisperse particles, which are readily soluble in DMF can be prepared relatively easily. We followed the method reported by Weller et al.7 and as-prepared particles were sizeselected8 to obtain monodisperse (<10%) fractions. The final four fractions (six to ten, denoted I-IV hereafter) were chosen for further study, and these were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV-vis absorbance spectroscopy (Supporting Information). For the electrochemical measurements, CdS particle solutions were prepared in DMF with tetrahexylammonium perchlorate (THAP) as supporting electrolyte. All measurements were carried out in a helium-filled drybox, and the electrochemical properties of the dispersions were investigated by cyclic voltammetry (CV) at both Pt (A ) 0.062 cm2 ) and Au electrodes (A ) 0.3 cm2). A threeelectrode configuration was used where a Pt coil and a silver wire served as the counter and quasi-reference electrodes, respectively. The cell potential was normalized to NHE using the Fc/Fc+ couple (with the potential of this couple taken as 0.47V vs NHE). The solubility of the particles in DMF was found to be inversely proportional to their size, and the final fraction (IV) (2.4 ( 0.3 nm diameter) was the most soluble. A typical CV for IV at the Pt electrode is given in Figure 1 where it is compared with the response for the supporting electrolyte alone (Figure 1a). Increasing the amount of Q-CdS added resulted in increases in all peak currents without significant shifts in peak potentials. An identical CV was obtained with a Au electrode. Clear oxidation and reduction peaks are apparent at -2.15 V (A1) and 0.80 V (C1), respectively. The additional peaks only appear on scan reversal after traversing either A1 or C1 (Figure 1b). To show that the CV response is due to redox reactions of a solution species rather than an adsorbed film, the dependence of peak current and potential on scan rate (ν) from 10 to 500 mVs-1 was also investigated (Figure 2a). The linear fit of peak current versus ν1/2 for peaks A1 and C1 up to 100 mV s-1 indicates diffusion of solution Q-CdS to the electrode surface. The peak position shifted with increasing ν, indicating kinetic effects. The current response for the other peaks was neither clearly proportional to ν nor ν1/2. The response was stable on repetitive scanning and for several days with no evidence of fouling on either electrode surface The peak-to-peak separation between A1 and C1 is 2.96 V, a value comparable to the 3.23 eV (1s-1s transition) calculated from the electronic spectra (Supporting Information, Figure S1). Thus, these oxidation and reduction peaks can be correlated (1) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (2) (a) Hickey, S. G.; Riley, D. J. J. Phys. Chem. B. 1999, 103, 4599, (b) Hickey, S. J.; Riley, D. J.; Tull, E. J. J. Phys. Chem. B 2000, 104, 7623. (c) Torimoto, T.; Nagakubo, S.; Nishizawa, M.; Yoneyama, H. Langmuir 1998, 14, 7077. (d) Boxall, C.; John Albery, W. Phys. Chem. Chem. Phys. 2000, 2, 3651. (e) Boxall, C.; John Albery, W. Phys. Chem. Chem. Phys. 2000, 2, 3641. (3) (a) Burda, C.; Link, S.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 10775. (b) Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 1783. (c) Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A. J. Phys. Chem. A 1998, 102, 5652. (4) Hoyer, P.; Weller, H. Chem. Phys. Lett. 1994, 221, 379. (5) Chen, S.; Truax, L. A.; Sommers, J. M. Chem. Mater. 2000, 12, 3864. (6) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (7) (a) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmueller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (b) Chemseddine, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 636. (8) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. Figure 1. (a) CV response in the absence and presence of thioglycolcapped CdS Q-particles (1 mg/mL of fraction IV) at a Pt electrode. Sweep rate ) 50 mV s-1 and [THAP] ) 0.05 M. (b) Variation of the initial scan direction for IV illustrating that peaks A2, A3, and C3 are related to C1 and A1; sweep rate ) 10 mV s-1.