Most of the samples of naturally occurring specular hematite (Québec, Canada) containing 0.6 mass % Ti were found to be degenerate semiconductors due to oxygen deficiency in these samples and yielded negligible photocurrents (iph) in a photoelectrochemical (PEC) cell. When heated in air to 900 °C for 1 h or more after polishing, these samples developed substantial photocurrents as photoanodes in a PEC cell. The PEC, solid-state, and optical properties have been investigated by current-voltage and impedance measurements and by photocurrent spectroscopy. Information has been derived on the trapping centers and surface states with energies within the band-gap region. Inherent bulk states at 1.5 eV optical (~0.5-0.7 eV below the conduction band edge) are responsible for trapping of the majority carriers and for electron-hole recombination and surface states. The trapping and detrapping effects due to surface states could be eliminated and ipb greatly enhanced by filling the surface states by pyrogallol adsorption. The deep impurity/donor levels are also responsible for the trapping and recombination effects and consequently the low photon conversion efficiencies. Methods of improving the efficiencies have been suggested.
The dissociation of surface hydroxyl groups of crystalline SiO?, ZrOz, and Tho?, in aqueous suspensions, has been studied as a function of pH, a t different ionic strengths of I 10 specific adsorption of I<+ predominates. T h o ? appears to exhibit a greater tendency for the specific adsorption of I<+.'The double lajrer a t an oxide-solution interface can be investigated by studying the equilibrium distribution of the potential-determining ions (M+ and 01-I-) between the interface and the bulk solution, as a function of pH, and ionic strength. The experinlental and thermodynamic principles underlying this method are ~vell established in the earlier \\-orla of others on AgI (1,2), AgZS (3,4), and Fe203 ( 5 ) . The present ~vork, using crystalline samples of quartz, ZrOs, and Tho2 of low and known surface areas, is based on similar principles but uses different techniques so as to ~nininiize any secondary effects that may arise fro111 slime formation and solubility of the material. The work is primarily aimed a t deriving information on the nature of interactions a t the interface from the double layer characteristics. The thermodynamic principles from this point of view are outlined below for the purpose of the discussion to follo\v.The origin of charge (k) a t an amphoteric oxide-solution interface may be attributed to the basic or acidic dissociation of the nletal hydroxide groups on the surface. T h e equilibria are
M+ \h[/OHOH-\
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