Bilayer Lipid Membrane-Semiconductor Junctions. Spectroscopic and Electrochemical Characterizations and Photoelectron Transfer

Three different systems of glyceryl monooleate (GMO) bilayer lipid membrane (BLM) supported semiconductor particles have been prepared and characterized. A single composition of particulate semiconductor deposited on only one side of the BLM constituted System A, two different compositions of particulate semiconductors sequentially deposited on the same side of the BLM represented System B, and two different compositions of particulate semiconductors deposited on the opposite sides of the BLM made up System C. Effective refractive indices and optical thicknesses of GMO-BLM-supported In₂S₃ and ZnS particles (System A), determined by Brewster-angle and reflection measurements, allowed the assessment of the maximum sizes and the volume fractions of semiconductor particles to be on the order of 1200 Å and 0.3, respectively. Since semiconductor particles are highly porous structures, only the first layer of particulates penetrated into the BLM and were considered in the proposed equivalent circuit and band models. The presence of semiconductors on the BLM surface has been established by voltage-dependent capacitance measurements, absorption spectroscopy, and optical microscopy. Subsequent to the injection of H₂S, the first observable change was the appearance of fairly uniform white dots on the black film. These dots rapidly moved around and grew in size, forming islands which then merged with themselves and with a second generation of dots, which ultimately led to a continuous film which continued to grow in thickness. Cyclic voltammetry established the current rectifying behavior for the semiconductor-particle-coated BLMs. CdS, ZnS, and In₂S₃ (System A) formed an n-type, while Cu_2-(x+y)S (System A) behaved like a p-type, electrolyte-semiconductor (ES) junction. Semiconductor-semiconductor heterojunction (SS') formation was established for System C. Transfer of conduction-band electrons to dissolved oxygen (for the n-type ES junction) and across the membrane was considered to be responsible for the observed dark currents. Steady-state illumination of a CdS-containing BLM resulted in the prompt development of -150 to -200 mV (cis side negative) potential difference in an open circuit across the GMO BLM. This initial photovoltage, V ₁, quickly decayed to a steady value, V _s (- 100 to -150 mV). When the illumination was turned off, the potential difference across the GMO BLM decreased to its dark value in 3-4 minutes.