Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles

Abstract

Titanium dioxide (TiO 2 ) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO 2 is well understood. However, surface structure of nanoparticulate TiO 2 , which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defects structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO 2 nanoparticles. We report the synthesis and characterization of ~ 4 nm TiO 2 anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO 2 nanoparticles without affecting parameters such as size, shape, core crystallinity and solubility.The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO 2 defect states on photoinduced electron transfer dynamics.3

Defeating Loss Mechanisms in 1D TiO2‐Based Hybrid Solar Cells

“…Absorption spectra of SQ2 on the three different TiO 2 surfaces show a change in the aggregation of dye molecules depending on annealing treatment, leading to an enhanced absorption of higher energies in Sample 600/450 (Figure S6, Supporting Information). One possible reason for the increase in light absorption for Sample 600/450 is due to an increased density of defect states between the annealed rutile and anatase layer, as discussed by Llansola‐Portoles et al These defect states could expedite the reduced PL lifetimes discussed above. While it is unclear why they are not seen for Sample 450/450, it is believed that Sample 450/600 causes the rearrangement of the TiCl 4 ‐induced layer for the removal of these states.…”

PbTiO3 as Electron‐Selective Layer for High‐Efficiency Perovskite Solar Cells: Enhanced Electron Extraction via Tunable Ferroelectric Polarization

“…The increased J sc is mainly due to the effective charge extraction from CH 3 NH 3 PbI 3 . It shows that the ferroelectric polarization is also responsible for the carrier separation and charge transfer even with the 400 nm thickness of PTO (much thicker than an ≈60 nm of the commonly used TiO 2 ). After negative poling, the J sc decreases gradually with the backward voltage increase.…”

“…Many other types of defects such as oxygen vacancies are present at the TiO 2 /perovskite interface due to the high‐energy bond configurations of TiO 2 surface, which can act as trap states and affect photogenerated carrier extraction and exciton separation, resulting in high recombination rates . The deposition of a passivation layer on the TiO 2 can effectively passivate these trap states, just as the solid‐state ionic liquids can reduce the electron trap‐state density of perovskite absorber surface .…”

Low Temperature Synthesis of Large‐Size Anatase TiO2 Nanosheets with Enhanced Photocatalytic Activities

“…However, TiO 2 has a relatively small exciton radius (≈1 nm), [28] and thus, for TiO 2 nanoparticles with diameter larger than 1 nm, no appreciable quantum confinement effect, thus no pronounced energy-gap-widening, should be observed. [29] Therefore, the E g value of NP should be calibrated to 3.2 eV with -0.24 eV. The E g value of NS should be calibrated with −0.24 eV too.…”

“…This may lead to shifting of the position of absorption peaks, and thus the optical gap energy deviate from the standard 3.2 eV. However, TiO 2 has a relatively small exciton radius (≈1 nm), and thus, for TiO 2 nanoparticles with diameter larger than 1 nm, no appreciable quantum confinement effect, thus no pronounced energy‐gap‐widening, should be observed . Therefore, the E g value of NP should be calibrated to 3.2 eV with –0.24 eV.…”