Potential-pH Diagrams for Oxidation-State Control of Nanoparticles Synthesized via Chemical Reduction

In the past, many synthesis methods of nanoparticles have been reported, but the synthesis processes have not been well discussed from the viewpoint of thermodynamics. In this chapter, a general concept using potential-pH diagrams is described for oxidation-state control of nanoparticles synthesized via chemical reduction (also called electroless deposition or liquid-phase reduction). By comparing kinetically determined mixed potential measured in reaction solution and thermodynamically drawn potential diagrams, e.g., potential–pH diagram, it is possible to know “what chemical species is stable in the reaction solution?.” It is predicted from potential-pH diagrams that nanoparticles in different oxidation states can be selectively synthesized by controlling the mixed potential. This concept is verified by selectively synthesizing Cu and Cu2O nanoparticles from CuO aqueous suspension via chemical reduction using the concept as an example. The advantage of this chemical reduction method is that abundant nanoparticles can be obtained for a short time by a simple operation. An extremely small activity of Cu2+ aquo ion is achieved by using insoluble CuO powder as a Cu(II) ionic source, which is a key for the synthesis of nanosized particles. The dependency of mixed potential on pH and temperature is discussed in the verification process. This chapter is written based on the result of the authors’ paper (Yagi et al., 2009) with further detailed information on practical thermodynamic calculation and drawing procedure of potential-pH diagrams.