Aqueous Solution Processing of F-Doped SnO₂ Transparent Conducting Oxide Films Using a Reactive Tin(II) Hydroxide Nitrate Nanoscale Cluster

Abstract

Solution deposition is a scalable method to fabricate transparent conducting and semiconducting oxide films that could enable low-cost large-area optoelectronic devices, including solar cells and electrochromic windows. However, high temperatures (>500 °C) are typically required to remove excess counterions and ligands from solution-deposited films. We report the synthesis of reactive Fmodified tin(II) hydroxide nitrate nanoscale cluster precursors from the controlled dissolution of SnO and SnF 2 in minimal nitric acid (∼1.6 mol HNO 3 per mol Sn). After spin-casting to form films, heating at temperatures < 100 °C consumes the nitrate counterions likely by oxidation of the tin(II) precursor to form amorphous F:SnO 2 . The optical, electrical, structural, and morphological properties of F-doped and undoped SnO 2 thin films are reported as a function of annealing temperature on both glass and polyimide (Kapton) substrates. Electron microscopy and X-ray reflectivity results show that the films are uniform and crack-free with <10% thickness reduction after annealing at temperatures up to 450 °C. X-ray diffraction shows the formation of crystalline SnO 2 at >300 °C. Quartzcrystal microbalance, X-ray photoelectron spectroscopy, and infrared spectroscopy show near-complete removal of nitrate counterions and hydroxide/water by 200 °C, leading to an approximate 30% mass loss from the as-deposited film. Secondary-ion mass spectrometry shows that the F concentration in the annealed films scales with the concentration in the precursor solution. The F-doped films are conductive when annealed at ≥250 °C in H 2 /N 2 gas and at ≥350 °C in air. The lowest electrical resistivity (ρ) of 1.5 × 10 −4 Ω•m was obtained from 10 atom % F-doped SnO 2 films annealed at 600 °C in air. These films had a Hall mobility of 4.2 cm 2 V −1 s −1 and a carrier concentration of 9.5 × 10 19 cm −3 . Films deposited directly on polyimide sheets were crack-free after annealing at temperatures below 350 °C. The films exhibited ρ ≈ 10 −2 Ω•m after H 2 /N 2 annealing and were stable to more than 200 bending cycles at angles up to 90°, demonstrating flexibility.

Highly efficient electrochemical degradation of perfluorooctanoic acid (PFOA) by F-doped Ti/SnO2 electrode

“…However, interstitial halogen atoms would increase the lattice disorder remarkably and thus interrupt crystal grown of SnO 2 , resulting in the presence of inside stress [24,29]. It is easily to generate uneven surface of SnO 2 coating, which finally limits electrode longevity.…”

“…But, further increasing F doping amount from 0.25 to 5.0, the rate constant took on a decrease from 1.24 × 10 -1 min -1 to 4.69 × 10 -2 min -1 . The decrease could be ascribed to that there is a solubility limit of Fˉ in SnO 2 lattice, beyond which the excess Fˉ do not occupy the proper substitutional lattice sites to contribute to free electrons, but rather go to interstitial sites, leading to a decrease of free electron [29,42]. It would restrain the electrolysis of PFOA.…”

“…Moreover, the presence of interstitial Fˉ would also increase the lattice disorder remarkably [22,29,43], decreasing the chemical stability of electrode. In fact, many reports about F-doped SnO 2 film as TCO have used a range of analytical tools to verify that proper Fˉ doping in the enormous O vacancies of SnO 2 lattice increased carrier concentrations and thus decreased the sheet resistance [22,26,44]; however, excess F doping, residing in interstitial sites at the grain boundary region or in dopant clusters, could hinder the electron transport through the grain boundary and increase carrier scattering, thus causing the increase of the resistivity [29,32,42,43].…”

“…In fact, many reports about F-doped SnO 2 film as TCO have used a range of analytical tools to verify that proper Fˉ doping in the enormous O vacancies of SnO 2 lattice increased carrier concentrations and thus decreased the sheet resistance [22,26,44]; however, excess F doping, residing in interstitial sites at the grain boundary region or in dopant clusters, could hinder the electron transport through the grain boundary and increase carrier scattering, thus causing the increase of the resistivity [29,32,42,43]. Table 1 Calcination temperature affects the crystal structure and morphology of metallic oxides; thus it is closely related to the electrooxidation reactivity of SnO 2 electrode.…”

Synthesis of Polyvinylpyrrolidone-Stabilized Nonstoichiometric SnO2 Nanosheets with Exposed {101} Facets and Sn(II) Self-Doping as Anode Materials for Li-Ion Batteries

“…Tin dioxide (SnO 2 ) is an important n-type semiconducting metal oxide with coexistence of conductivity and transparency owing to its natural nonstiochiometry such as tin interstitial and oxygen vacancies, [1] which has been widely used in transparent conducting films, [2,3] gas sensors, [4][5][6] lithium ion batteries, [7][8][9][10] and solar cells. [11][12][13] Thus, synthesis of SnO 2 nanostructures with well-defined morphologies, highly reactive surfaces and tunable nonstoichiometric defects have attracted intense research interests due to their shape, size, surface and composition dependent properties.…”