Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules

Abstract In semiconductor particles of nanometer size, a gradual transition from solid‐state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q‐particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q‐particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance‐specific quantity in macrocrystalline materials, increases by several electron volts in Q‐particles with decreasing particle size. In Q‐particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico‐chemical properties of the particle. Q‐particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q‐particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q‐particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q‐particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q‐particles, as well as with the spectroscopic, electrochemical, and electron‐microscopic investigation of these particles.