Abstract
Most solar-energy conversion applications are based on trapping and transferring photoinduced electrons on oxide semiconductor nanoparticles, such as titanium dioxide, and broad UV-vis absorption (400~800 nm) and monotonic IR absorption (1100~3000 cm−1) signals have long been considered signatures of the electron-trapping state on titanium dioxide nanoparticles. Here we show that, under proton-free conditions and using iodide ions in acetonitrile as the hole scavenger, the intrinsic electron-trapping feature of titanium dioxide nanoparticles does not exhibit the characteristic UV-vis absorption and infrared absorption signatures. Further electron spin resonance studies identify the proton-free electron-trapping state as the lattice octahedral Ti6c3+ species, differing from the traditional proton-participating surface tetrahedral Ti4c3+ species. Synchronized radiation ultraviolet photoelectron spectroscopy results also show that the internal electron-trapping state without protons has a larger Ti3d binding energy (1.8 eV) than the blue electron-trapping state (1.3 eV) that forms when protons participate and thus shows different electron transfer abilities.
Highlights
Most solar-energy conversion applications are based on trapping and transferring photoinduced electrons on oxide semiconductor nanoparticles, such as titanium dioxide, and broad UV-vis absorption (400~800 nm) and monotonic IR absorption (1100~3000 cm−1) signals have long been considered signatures of the electron-trapping state on titanium dioxide nanoparticles
The electron spin resonance (ESR) (4 K) studies indicated that these proton-free trapped electrons (I− hole scavenger) almost all entered the bulk of the TiO2 nanoparticles as the interstitial six-coordinated Ti6c3+ species, which is distinct from the common protonparticipating electron-trapping state of the surface four-coordinated Ti4c3+ species
Result shows that the signature absorption of electrontrapping state around 700 nm can only be observed on the preilluminated TiO2 nanoparticles from methanol solution, which is consistent with the optical-fiber UV-vis absorption spectra
Summary
Most solar-energy conversion applications are based on trapping and transferring photoinduced electrons on oxide semiconductor nanoparticles, such as titanium dioxide, and broad UV-vis absorption (400~800 nm) and monotonic IR absorption (1100~3000 cm−1) signals have long been considered signatures of the electron-trapping state on titanium dioxide nanoparticles. UV-vis and IR absorption with a distinct blue color has long been used to recognize and confirm surface Ti3+ species as the electron-trapping states on TiO2 nanoparticles, as further evidenced by low-temperature (77 and 4 K) electron spin resonance (ESR) results[13], and is denoted as the blue state[22,23] These bluestate trapped electrons have been quantitatively titrated by Fe3+-1,10-phenanthroline reagent or TEMPO·/tBu3ArO· radicals[16,19]. We report the photoinduced electron-trapping feature of TiO2 nanoparticles with iodide ions as a hole scavenger under strictly proton-free conditions This feature is completely different from the common blue state observed under protonparticipating conditions and does not exhibit the characteristic UV-vis (400–800 nm) and IR absorption (1100–3000 cm−1).
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