On the Importance of Nanoparticle Necks and Carbon Impurities for Charge Trapping in TiO₂

Abstract

Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

Engineering Defects and Interfaces of Atomic Layer-Deposited TiOx-Protective Coatings for Efficient III–V Semiconductor Photocathodes

“…Beyond native point defects such as oxygen vacancies and Ti 3+ , impurity atoms can be incorporated in ALD films due to incomplete removal of the ligands from metalorganic precursors. In particular, we investigated the presence of C from TTIP and TDMAT, as well as N from TDMAT, both of which can introduce electronically active states above the valence band edge. , However, XPS measurements show no indication of the presence of N in TDMAT-derived films (Figure S6), which is consistent with prior findings for films grown at 200 °C that show no or negligible amounts of nitrogen. , As is typically the case, XPS measurements of the C 1s core level region are complicated by the presence of adventitious carbon. Nevertheless, the resulting spectra can be deconvoluted into four spectral components associated with C–H and C–C (284.8 eV), alcohols and ethers (286.3 eV), ketones (287.8 eV), and esters or acid groups (288.8 eV), as shown in Figure S7.…”

“…In addition, a second electronic level at ∼1.0 eV above the VB edge is observed. This feature is consistent with the presence of compensating acceptor states, such as those arising from C impurities in TiO x , which can serve to reduce the concentration of deep Ti 3+ states. , Indeed, the assignment of this photoemission band to compensating C acceptor states is consistent with the increased C 1s signal at 284.8 eV and decreased gap-state emission at a binding energy of 0.7 eV (approximately 1.1 eV below the CB edge), as well as the reduced sub-band-gap optical absorption and higher resistivity for this sample compared to the TDMAT/H 2 O sample.…”

Orange peel biochar/clay/titania composites: low cost, high performance, and easy-to-reuse photocatalysts for the degradation of tetracycline in water

“…67,68 However, if those traps are too deep, they can act as recombination centers if located within the bulk or innermost part of the nanoparticles thereby reducing degradation activity. 68,69 This could explain, why the degradation efficiency of C300KT is the lowest (see below for details).…”

Defect-Mediated Energy States in Brookite Nanorods: Implications for Photochemical Applications under Ultraviolet and Visible Light

“…In the context of TiO 2 , symmetrical EPR signals with g factors close to the free spin value g = 2.0023 are often assigned, without significant proofs, to ill-defined oxygen vacancy centers. Recently, these assignments and the origin of such EPR signals have been reconsidered by Diwald and co-workers and associated with carbon-containing impurities at particles necks. During the TiO 2 nanoparticle synthesis, irrespective of whether performed in the gas phase or in solution, carbon impurities can be easily incorporated in the matrix, leading to substitutional (C O ) defects under oxygen-poor conditions .…”