QUANTUM CONFINEMENT EFFECTS ENABLE PHOTOCATALYZED NITRATE REDUCTION AT NEUTRAL PH USING CDS NANOCRYSTALS

Size-quantized CdS nanocrystals serve as photocatalysts for nitrate re duction at neutral DH under conditions that mimic illumination by sunl ight with overall product quantum yields of up to 4% for similar to 20 Angstrom, amine-terminated particles. Due to the effects of quantum c onfinement on electron and hole redox potentials, photocatalyzed nitra te reduction rates depend strongly on the apparent particle size, and the fastest reduction rates are observed with the smallest nanocrystal s which have the highest exciton energy, Using a Tafel plot and the em pirical pseudopotential model to estimate electron redox potentials, t he apparent electron transfer coefficient and the apparent standard ra te constant are estimated at 0,23 and 4.0 x 10(-12) cm/s, respectively , for amine-terminated particles. The apparent values for these consta nts indicate sluggish kinetics and the probable influence of adsorptio n and double-layer effects on the observed reaction rate. The effect o f nitrate adsorption on photoreduction rates is described well by a La ngmuir-Hinschelwood expression, Nitrate reduction rates are reduced 2- fold or more on negatively charged, carboxy-terminated nanocrystals th at electrostatically repel nitrate. Chloride competes with nitrate for access to particle surfaces, and reduced photoreduction rates are obs erved for both amine- and carboxy-terminated particles with increased NaCl concentration. The rate of photocatalyzed nitrate reduction on th e amine-capped particles goes through a minimum at about pH 6.5, where as the efficiency of nitrate reduction for the carboxy-terminated syst em decreases monotonically with increasing pH. in the absence of an el ectron donor ether than water, rapid photocorrosion is observed, there fore, formate is used as the sacrificial electron donor in this study.