Photocurrent Response of Surface-Functionalized Metal Oxides with Well-Matched Energy Levels: From Nothing to Something

In recent years, an enormous amount of research has been devoted to the study of photosensitive materials from both fundamental and practical viewpoints, due to their wide applications in photocatalytic1–3 and optoelectronic devices,4, 5 ultraviolet (UV) photodetectors,6–9 photoswitch microdevices,10, 11 light‐emitting diodes,12, 13 photovoltaic devices,14–16 and photoelectrochemical cells.17 Metal oxides, such as ZnO, TiO2, SnO2, and NiO have been the most investigated photosensitive materials.3, 6–8, 18–21 To enhance and take full advantage of their photosensitivity, functionalizing their surface with a polymer that has a high light absorption ability has become one of the widely used methods.1–12, 22–24 For example, Z. L. Wang et al. reported that the UV photocurrent of a ZnO nanobelt‐based sensor was enhanced by close to five orders of magnitude after functionalizing its surface with polystyrene sulfate which has a high UV absorption ability.25 T. Sasaki et al. reported the assembly of a TiO2 nanoparticle film with poly(3,4‐ethylenedioxythiophene) and poly(4‐styrene sulfonate) (PEDOT‐PSS) through layer‐by‐layer fabrication in the nanometer scale. The electric conductivity of the TiO2 composite films could be tuned by UV and visible (Vis) light.22 Thus, sunlight or photon energy can be used and transformed to electrical energy by UV‐photosensitive metal oxides after their surfaces have been functionalized with a dye that has a high Vis absorption ability. To date, most of the dye‐sensitized solar cells are based on the surface functionalization of UV‐photosensitive metal oxides by dyes.26–28 However, to the best of our knowledge, all of the reports on surface functionalization enhanced only the UV photosensitivity of the metal oxide. In other words, this method has been used exclusively to enhance the UV photocurrent in metal oxides that already have UV‐photosensitive properties, but not to induce UV photocurrent in metal oxides that have no UV‐photosensitive properties. In fact, to the best of our knowledge, there are no surface‐functionalizing reports on inducing UV or Vis photocurrent in metal oxides that have no UV‐ or Vis‐photosensitive properties. No longer silent: A surface‐functionalization method with well‐matched energy levels is presented to induce UV or Vis photoresponse in semiconducting materials (see picture) that provide only weak photoresponse prior to the treatment or are completely silent. UV and Vis photoresponses of the metal oxides