Effect of Copper Precursor on the Stabilization of Titania Phases, and the Optical Properties of Cu/TiO₂ Prepared with the Sol−Gel Technique

In the sol−gel Cu/TiO 2 system, nanocrystalline brookite, which was stable even after annealing the sample at 400 °C, was generated in high concentrations. This was achieved with copper chloride as copper precursor. The respective samples had a light absorption band at 562 nm that corresponded to an energy bandgap of 2.30 eV, which is smaller than the values reported for titania. When copper sulfate was the precursor, brookite was absent, but nanocrystalline anatase, the main phase, was stabilized even at 800 °C by sulfate ions. After annealing the samples at low temperatures, copper was well dispersed for 1 wt % copper, but part of it segregated forming copper compounds when the copper concentration was 10 wt %. By annealing the samples at 800 °C, copper oxidized to CuO; except in the 10 wt %-copper sample prepared with copper sulfate, where it formed antlerite.