Interaction between photocatalyst surface nanostructures and ultraviolet light by apertureless scanning nearfield optical microscopy

点击翻译按钮获取中文摘要

Ultraviolet (UV) irradiation on titanium dioxide surface gives apparent structural changes as observed by scanning tunneling microscopy (STM), which are the consequence of electronic excitation of the surface by UV light. Here local photoresponse of a rutile TiO_2(110) surface is visualized at atomic scale by STM. Almost all the areas on the (110) surface responded to the illumination of 325-nm ultra-violet (UV) light, in accordance with the minimum band-gap (3.2eV) electron transition, with steps and (1×2) ridge areas exhibiting higher response intensities. When the same surface is illuminated with blue light at 442nm, which is well below the minimum band-gap transition energy, most of the surface areas were dormant, but certain areas at step regions and (1×2) ridge structures responded to the blue light. These sites are associated with oxygen deficiency on the surface, which may give a clue to the possibility of surface engineering of titania photocatalyst to be employed under visible light.We also conducted a comprehensive Kelvin probe force microscopy (KPFM) study on the "classical" organic solar cell system consisting of MDMO-PPV:PCBM blends. The KPFM method yields detailed information of topography and local work function at nanometer scale. Experiments were performed either in dark or under laser illumination at 442nm or 675nm to extract the locally resolved surface photovoltage (SPV). We identified distinct differences in the energetics on the surface of chlorobenzene and toluene cast blend films. Together with high resolution scanning electron microscopy (SEM) experiments we were able to interpret the KPFM results and to draw some conclusions for the electron transport towards the cathode in the solar cell configuration. The results suggest a blocking behavior of the toluene cast film surfaces against electron propagation towards the cathode, usually evaporated on top of the films in organic solar cells.