STM study of rhodium deposition on the TiO2(110)-(1x2) surface

The structure of rhodium deposited on a well-ordered TiO2(110)-(1 x 2) surface in the submonolayer region and the effect of annealing were studied by scanning tunneling microscopy and Auger electron spectrometry. Deposition of Rh at low coverage gave nanosize (diameter of 1-3 nm) bumpy structures as a result of 3D particle formation. This process roughens the surface considerably. No further radical change in the surface morphology was observed at 0.10-1.0 ML (ML=monolayer). Following annealing of the Rh/TiO2(110)-(1 x 2) system, three different processes can be distinguished: (i) the encapsulation of Rh particles in the temperature range 500-700 K as indicated by the decrease of the relative AES signal of Rh by 40%. This process is not accompanied by appreciable changes in STM images at higher Rh coverages, but results in an increase of the volume of outrising structures at very low Rh deposition (0.01 ML). (ii) An increase in the size of nanoparticles between 700 and 900 K, indicating their coalescence. The extent of this process depended on the amount of the deposited Rh. (iii) The separation of the 3-5 nm diameter and 3-5 atomic-layers thick Rh crystallites (with their (111) plane parallel to the substrate) from titania above 1100 K, which again exhibits a well-ordered (1 x 2) terrace structure. The evaluation of STM images at different Rh coverages led to the conclusion that this latter process is probably due to the de-encapsulation of the encapsulated Rh crystallites.