New reaction pathway discovered

A novel chemical reaction pathway on titanium dioxide (TiO2), a useful photocatalytic material, has been successfully demonstrated by manipulating an atomic defect using the probe of a scanning tunneling microscope (STM). The new reaction mechanism is based around an applied electric field, which reduces the width of the reaction barrier, allowing hydrogen atoms to tunnel away from the surface.

As the action could lead to the manipulation of the atomic-scale transport channels of hydrogen, there could be many applications for such a mechanism, particularly in future approaches to hydrogen storage (or transport), with hydrogen is being seen as a clean and renewable alternative to the use of hydrocarbons, and also in the design of nanoscale switching devices.

“The new reaction pathway could be exploited in nanoscale switching devices and hydrogen storage technology."Taketoshi Minato

The study, published in ACS Nano [Minato et al. ACS Nano (2015) DOI: 10.1021/acsnano.5b01607], by a team, from Tohoku University, RIKEN, the University of Tokyo, Chiba University and University College London, used STM to directly visualize single hydrogen ions, a common atomic defect on TiO2. The approach allowed the surface structure of a solid surface to be observed on an atomic scale, achieved by scanning a sharp probe across the surface and then monitoring the tunneling current.

They managed to desorb individual hydrogen ions from the surface by using the STM probe to apply electrical pulses to the hydrogen. In addition to injecting electrons into the sample, the pulse produces an electric field that, rather than facilitating desorption by reducing the barrier height, causes a reduction in its width, which, when coupled with the electron excitation induced by the STM tip, leads to the tunneling desorption of the hydrogen.

The team had previously explored the reaction mechanism of single molecules on metals by using STM, and realized the technique could be applied for the manipulation of defects on TiO2. As lead author Taketoshi Minato said, “The new reaction pathway could be exploited in nanoscale switching devices and hydrogen storage technology. For instance, electric fields could be used to extract hydrogen from a TiO2-based storage device”.

The approach could be applied to the manipulation of other defects, such as hydrogen defects on other oxides, which this reaction pathway could be valid. However, for the mechanism to also facilitate hydrogen storage its applicability for a macro-scale reaction needs to be investigated; and to establish its potential as a switching device, the performance of the reaction in terms of factors such as speed, conversion efficiency and cyclability should be assessed.