Adsorption of alkanolamines onto semiconductor surfaces: Cadmium selenide photoluminescence as a probe of binding and film reactivity toward carbon dioxide

Films of ethanolamine, 3-aminopropanol, and 4-aminobutanol, 1-3, serve as transducers for the detection of CO₂ when deposited onto emissive CdSe substrates: the band-gap photoluminescence (PL) intensity of the semiconductor is reversibly quenched by exposure to CO₂, while uncoated CdSe samples display no response relative to a nitrogen reference ambient. Infrared (IR) spectra obtained with films of these alkanolamines and related alcohols and amines, 4-13, are consistent with selective reaction of the amine functionality in these films to form carbamates. The direction of the PL response corresponds to an enhancement of Lewis acidity accompanying the reaction with CO₂, and use of a dead-layer model permits an estimation of the adduct-induced expansion of the depletion width as being roughly a few hundred Angstroms. Both IR and PL data reveal that the film response to CO₂ begins at ∼0.1 atm and saturates by ∼0.3 atm, and their similarity indicates that the partition coefficient for binding CO₂ to the bulk film compared to the CdSe-film interface is roughly unity. Adsorption of 1-3 onto CdSe substrates from N₂-saturated THF solution causes reversible enhancement of the semiconductor's PL intensity, consistent with a Lewis-basic interaction with the surface. The PL changes can be fit to a dead-layer model and correspond to an adduct-induced contraction of the depletion width of ∼200 Å for 1 and 3 but ∼600 Å for 2. The solution concentration dependence of PL changes for 1-3 could be well fit to either single-site or multisite Langmuir adsorption isotherm models, yielding modest binding constants of ∼10² M^-1. Steric and electronic factors contributing to these effects are discussed based on the film and solution PL responses of the other amines and alcohols investigated. © 1999 American Chemical Society.