Fermi Level of Surface States in TiO2 Nanoparticles

We analyze the nonequilibrium steady-state statistics of electrons in nanoparticulate semiconductors containing a variety of electronic states in relation to applications in dye-sensitized solar cells and photocatalysis. The electrons trapped in surface states do not generally equilibrate to the free electrons' Fermi level, E Fn . We show that a distinct Fermi level for surface states, E Fs, can be defined consistently with Fermi−Dirac statistics, determining the surface states' occupancy far from equilibrium. The difference Δ = E Fn − E Fs depends on the rate constants for charge transfer and detrapping and can reach several hundred millielectronvolts. The E Fs becomes a key concept for the analysis of charge transfer and recombination kinetics in nanoparticulate semiconductors.