Size-Dependent Electrochemical Behavior of Thiol-Capped CdTe Nanocrystals in Aqueous Solution Sergey K. Poznyak, ² Nikolai P. Osipovich, ² Alexey Shavel, ‡ Dmitri V. Talapin, ‡ Mingyuan Gao, § Alexander Eychmu 1 ller, ‡ and Nikolai Gaponik* ,‡ Physico-Chemical Research Institute, Belarussian State UniVersity, 220050 Minsk, Belarus, Institute of Physical Chemistry, UniVersity of Hamburg, Grindelallee 117, D-20146 Hamburg, Germany, and Key Laboratory of Colloid, Interface Science and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Science, Zhong Guan Cun, Bei Yi Jie 2, Beijing 100080, People’s Republic of China ReceiVed: August 31, 2004; In Final Form: October 19, 2004 Electrochemical studies of thiol-capped CdTe nanocrystals in aqueous solution have demonstrated several distinct oxidation and reduction peaks in the voltammograms, with the peak positions being dependent on the size of the nanocrystals. While the size dependence of the reduction and one of the oxidation potentials can be attributed to altering the energetic band positions owing to the quantum size effect, an extraordinary behavior was found for the oxidation peak observed at less positive potentials. In contrast to a prediction based on the quantum size effect, this peak moves to more negative potentials as the nanocrystals’ size decreases. Moreover, the contribution of the charge associated with this peak compared to the total charge passed during the nanocrystal oxidation correlates well with the photoluminescence (PL) efficiency of individual fractions of the CdTe nanocrystals. These experimental observations allow a peak to be assigned to the oxidation of Te-related surface traps. The intra-band-gap energy level assigned to these Te-related trap states shifts toward the top of the valence band as the nanocrystal size increases, thus allowing the higher photostability of the larger nanocrystals to be explained. At a certain nanocrystal size, the trap level can even move out of the band gap. 1. Introduction The unique size-dependent optical properties of colloidal semiconductor nanocrystals (NCs) have been the subject of considerable interest in the past two decades. 1,2 Strongly luminescing II-VI semiconductor NCs have found potential applications in biological imaging and labeling, 3-7 photovol- taics, 8 electroluminescence devices, 9-13 and so forth. Struc- tural, 14 photophysical, 1,14-16 photochemical, 17,18 and photoelectro- chemical 19-22 properties of the semiconductor NCs have been intensively studied; however, considerably less attention has been given to their electrochemical properties. 19,23-26 Several reports have been devoted to the study of a correlation between the optical band gap of the NCs and the band gap estimated from the oxidation and reduction peak positions in cyclic voltammograms. 24,25 These studies have been performed in nonaqueous media, and the results obtained are somewhat contradictory. Whereas Kucur et al. 25 found good agreement between electrochemical and optical E g values for CdSe NCs with different sizes, Bard and co-workers 24 revealed that the electrochemical band gaps were smaller than the optical gaps for CdS NCs. In their recent work, Bard et al. found that the electrochemical band gap (∼2.1 eV) between the first anodic and cathodic peaks was close to the optical E g value (2 eV) for TOPO-capped CdTe nanoparticles. 27 The authors of the elec- trochemical studies noted that upon changing the particle size the reduction and oxidation peaks in the voltammograms were shifted in the direction predicted by theory. 19,24,25,27 Water soluble thiol-capped CdTe NCs have been demon- strated to be one of the most robust and highly fluorescent nanoparticle materials directly synthesized in an aqueous medium. 28 They can be successfully incorporated into ultrathin polymer films using the layer-by-layer assembly method. 11,29 Different color emissions can be electrically generated upon applying an external bias when the film is sandwiched between two conducting electrodes. Such electroluminescence devices have shown strong size-dependent electro-optical properties, such as electroluminescence efficiency and electrical conductiv- ity. 11 In addition, the CdTe NCs also demonstrate a distinct size- dependent photostability. 28 Even taking into account the quan- tum size effect, it is still difficult to understand some of these size-dependent behaviors of the NCs. For nanoparticle materials, where the surface-to-volume ratio is considerably large, the state of the surface becomes important in determining many of the nanocrystal properties, for example, trapping of charge carriers and emission. Since the energy levels of surface traps are difficult to predict, a search for new approaches to their investigation is of current interest. Here, we report the results of electrochemical studies on CdTe NCs with different sizes and (or) different optical properties by the use of cyclic voltammetry (CV). The motivation for this research was to investigate correlations between the electro- chemical properties of CdTe NCs with different sizes and their optical properties and stability. The establishment of such cor- relations in aqueous buffer solutions is of special interest for bioapplications of NCs. Since the electrochemical degradation of nanoparticles should start from the particle surface, the CV method might also provide useful information on the surface of NCs. 2. Experimental Section CdTe NCs stabilized by thioglycolic (TGA) or 3-mercapto- propionic (MPA) acids were synthesized according to the * To whom correspondence should be addressed. E-mail: gaponik@chemie.uni-hamburg.de. ² Belarussian State University. ‡ University of Hamburg. § Chinese Academy of Science. 1094 J. Phys. Chem. B 2005, 109, 1094-1100 10.1021/jp0460801 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/04/2005