Size, surface, and compositional tuning of the electronic and photocatalytic properties of II-VI quantum-size semiconductor nanocrystals

Phosphatidylcholine vesicles provide reaction compartments for synthesis of size-quantized CdS nanocrystals of dimension predicted to within {dollar}pm{dollar}2 A based on initial encapsulated {dollar}rm CdClsb2{dollar} concentration and vesicle diameter. Vesicle formation by detergent dialysis of phosphatidylcholine/hexylglucoside mixed micelles yields highly monodisperse lipid capsules within which monodisperse CdS nanoparticles are precipitated with sulfide. Size-quantized CdS nanocrystals, with diameters ranging from 20 to 60 A, have been produced with typical standard deviations about the mean diameter of {dollar}pm{dollar}8% as measured by transmission electron microscopy. By including {dollar}rm ZnClsb2{dollar} or {dollar}rm HgClsb2{dollar} in the dialyzate prior to vesicle formation, quantum-sized {dollar}rm Znsb{lcub}y{rcub}Cdsb{lcub}1-y{rcub}S{dollar} or {dollar}rm Hgsb{lcub}y{rcub}Cdsb{lcub}1-y{rcub}S{dollar} nanocrystal alloys with controlled stoichiometry are generated. Spectrophotometric and spectrofluorimetric measurements are consistent with highly crystalline, monodisperse particles with few core or surface defects. The alloyed nanocrystal spectra shift consistently with composition indicating a high degree of compositional control. Measured exciton energies for CdS show excellent agreement with data in the literature. The empirical pseudopotential model presented by Ramakrishna and Friesner for a cubic CdS lattice, correcting for experimentally measured lattice contractions, best fits the data. Size-quantized CdS nanocrystals serve as photocatalysts for nitrate reduction at neutral pH under conditions that mimic illumination by sunlight with overall product quantum yields of up to 4% for {dollar}{lcub}sim{rcub}20{dollar} A, amine-terminated particles. Due to the effects of quantum confinement on electron and hole redox potentials, photocatalyzed nitrate reduction rates depend strongly on the particle size, and the fastest reduction rates are observed with the smallest nanocrystals. Using a Tafel plot and the empirical pseudopotential model to estimate electron redox potentials, the apparent electron transfer coefficient and the apparent standard rate constant is estimated at 0.23 and {dollar}4.0times 10sp{lcub}-12{rcub}{dollar} cm/sec, respectively, for amine-terminated particles. Nitrate adsorption is important in this system and the effect on photoreduction rates is described well by a Langmuir-Hinschelwood expression. Nitrate reduction rates are reduced two-fold or more on negatively charged, carboxy-terminated nanocrystals that electrostatically repel nitrate. Reaction rates are additionally influenced by competetive chloride adsorption and surface charge modification due to solution pH.