Chemical Modification Strategies to Tune the Surface Band Bending and Electron Density of Square SnO ₂ Nanotubes

SnO 2 is a wide-band-gap semiconductor whose surfaces are characterized by the presence of a 2D electron gas that forms inside a potential well created by the downward bending of its electronic bands in response to donor-like surface states. This makes this material environmentally sensitive as electrophilic molecules, such as O 2 and H 2 O, are attracted to its electron-rich surfaces. This is advantageous in certain gas sensing and catalytic applications but is problematic in other uses of SnO 2 such as UV photodetectors, transparent electronic devices, and solar cells. Therefore, the ability to tune the band bending and electron density at SnO 2 surfaces is important for optimizing its use for different applications. In this work, we investigate the ability of a range of surface modification procedures such as the electrografting of nitrophenyl layers, covalent binding of different phosphonic acid molecules, and sulfurization to alter the surface band bending of perfectly square SnO 2 nanotubes. These nanostructures provide an interesting test case as they combine a very strong 2D electron gas with high crystal quality and large surface-to-volume ratios mainly involving the SnO 2 (110) surface. We use synchrotron X-ray photoelectron spectroscopy to show that, under certain conditions, electrografted NP layers and Na 2 S sulfur treatment can be used to almost completely remove the downward band bending of square SnO 2 nanotubes and create SnO 2 (110) surfaces with almost flat electronic bands. Although all the phosphonic acid molecules studied were able to covalently bind to the SnO 2 nanotubes, they produced negligible changes in band bending, with the exception of octadecylphosphonic acid that produced a significant increase in downward band bending. These findings are likely to provide useful addition tools for the bidirectional tuning of SnO 2 surfaces for different applications.