DISPERSION AND DISTRIBUTION OF TITANIUM SPECIES BOUND TO SILICA FROM TICL4

The paper examines the effects of the porous high surface area silica support and the reaction temperature (175-550-degrees-C) on the disper sion and distribution of the titanium species formed during a single s aturating reaction cycle of TiCl4, used in preparing Ti/silica samples by atomic layer epitaxy (ALE). Samples were analyzed by chemical etch ing, X-ray powder diffraction (XRD), scanning and transmission electro n microscopy (SEM, TEM), and X-ray photoelectron spectroscopy (XPS). T he reactions were highly surface-controlled, i.e. controlled by the OH groups of silica, which determined the numbers of titanium atoms boun d. Only amorphous titanium species was formed at lower reaction temper atures (< 300-degrees-C) and the amount decreased with increase in the reaction temperature. At temperatures above 300-degrees-C an increasi ng number of OH groups of silica most probably were directly chlorinat ed by TiCl4 leading to volatile Ti(OH)4-xClx (x = 1-3) species. This s pecies then decomposed to TiO2. At 390-degrees-C and below, pure anata se was formed, whereas at 550-degrees-C the reaction led exclusively t o rutile. The observed TiO2 Particles were platelike and evenly distri buted throughout the silica particles. The largest particles are propo sed to be formed in the largest silica pores with the highest density of OH groups and the smallest in the micropores. The silica pores were not blocked by TiO2 Particles, even though the diameter of most of th e particles was about 500 nm. The XPS studies of the external silica s urfaces revealed changes in Ti2p binding energy with increased reactio n temperature. This was caused by size effects in XPS due to crystalli zation. In amorphous Ti/silica samples a decrease in binding energy wi th preheat temperature of silica was also observed. In addition, the X PS atomic Ti/Si ratios gave information on the distribution of titaniu m species within the silica particles and agreed with the observed pla telike habit of the TiO2 particles.