ELECTRONIC PROPERTIES OF TIO2 DERIVED NANOBELTS

D. CADAVID; R.F. EGERTON; M. MALAC; M.S. MORENO

Buenos Aires

Congreso; 10th Inter-American Congress of Electron Microscopy 2009, CIASEM; 2009

TiO2-based nanostructures are considered good candidates for a number of applications innumerous fields as environmental decontamination, photocatalysis, electrocatalytic storage,solar cells, antibacterial agent, etc [1-3]. These applications are based on their electronicproperties and high surface area which need to be characterized. Titania derived nanobelts aresensitive to electron beam irradiation requiring to assess the conditions under which theelectronic properties can be studied. Investigation of electron irradiation damage is also ofinterest.In this work the titania-based nanobelts were obtained by hydrothermal treatment of TiO2 inconcentrated NaOH aqueous solution at temperatures above 150ºC. Their morphological andelectronic properties were studied by means of EELS and spatially resolved EELS (SREELS).We have studied the radiation damage by evaluating the loss of long range order using parallelnano beam diffraction (NBD) and followed the loss of short range order by EELS. A HitachiHF3300 microscope was used at incident energies of 100 kV and 300 kV and room temperature.Irradiation series typically took over 30 to 40 minutes (approximate dose of 1000 C/cm2).Figure 1 (left) shows that the obtained nanobelts has lengths of several microns, their widthbeing typically below 100 nm. Thickness mapping (right) shows that the belts have a highwidth/thickness ratio typically higher than 5.Changes in the low-loss EEL spectra are shown in Figure 2 as a function of irradiation dose.Progressive broadening of some peaks marked with arrows (at 14 eV and 19 eV) is apparent. Noother changes were observed at higher doses. Thickness mapping (not shown) revealed changesin the irradiated areas, confirming that the damage is accompanied by mass loss.At 300 kV the damage mechanism involves a two steps sequence with a fast initial processfollowed by a slow process with measured cross sections of about 100 barn and 15 barnrespectively. At 100 kV the damage proceed through a one-step mechanism with a cross sectionof 35 barn. That is, there seems to be a threshold for the fast process.Existence of a threshold suggests that both process could be attributed to displacement byknock-on, but of different elements. This interpretation seem to be supported by the measuredcross sections. The fast process seems to correspond to Ti displacement (at surface or in bulk)with a threshold above 100 keV. The slow process may correspond to oxygen or hydrogendisplacement with threshold below 100 keV and smaller cross section. However the slowprocess could be due to radiolysis. A temperature dependent study will provide furtherevidence.Figure 3 (left) shows a SREEL spectrum. Surface and bulk states can be identified (right). Adisplacement in the peak position can be appreciated.For the study of the bulk electronic properties and identification of EELS features we havecarried out real space multiple scattering calculations using the ab-initio program FEFF8.2 forthe stoichiometric material. Our results shows that the material is a wide band-gapsemiconductor. The calculated densities of states suggest a covalent nature.Our results show that the nanobelts damages at relatively low dose at 300 kV. The combinationof different techniques (SREELS, NBD, thickness mapping) indicates that the damage isaccompanied of mass loss. Calculations suggest that replacing H by Na does not have mucheffect on the Oxygen fine structure.