AFM Materials Journal Club

Image Surface and Interfacial Photogenerated Charge Carriers

by Ruotian Chen, Fengtao Fan, Thomas Dittrich and Can Li

Chem. Sov. Rev., 2018, 47, 8238

In solar devices, solar photons interact with the semiconductor, which promotes electrons to the conduction band and creates bounded electron-hole pairs. The bend band structure near the surface facilitates the separation of these pairs into free photocarriers that are subsequently moved to the external circuit to do electrical work (e.g., photovoltaics) or to the catalytic charge accumulation centers for chemical reactions (e.g., solar fuels devices). Understanding the dynamics of the light-induced surface/interfacial charge transfer is extremely important for designing efficient devices.

 In 2015, the authors developed spatially resolved surface photovoltage spectroscopy (SRSPS) on an open-stage AFM platform [Angew. Chem. Int. Ed. 2015, 54, 9111]. SRSPS integrates KPFM with modulated external illumination systems, which enables the nm-scale measurement of light-induced surface potential changes. SRSPS was first used to image highly anisotropic photogenerated charge separations on different facets of a single BiVO4 particle. This showed that the surface photovoltage signal intensity on the {011} facet was 70 times stronger than that on the {010} facets. Following that, in the past 4 years, the authors have applied SRSPS to a variety of systems including plasmonic photocatalysts [JACS, 2017, 139, 11771], BiVO4 with cocatalysts (MnOx), dual cocatalysts (MnOx/Pt) [Nano Lett.2017 17, 6735], Cu2O photocatalytst [Nat. Energy,  2018, 3, 655 ; Nano Lett. 2019,19, 426], and etc. The review article featured here highlights the principles and development of SRSPS and reviews key applications examples where insights from SRSPS have provided guidelines for device designs. Ongoing work is extending the technique towards in situ or in operando experiments with photocatalysts and using an insulated SECM probe in a  hotoelectrochemical environment [Nat. Energy 2018, 3, 46].