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Visible Light Photocatalytic Activity in AACVD-Prepared N-modified TiO2 Thin Films
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-2672-0041
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2014 (English)In: Chemical Vapor Deposition, ISSN 0948-1907, E-ISSN 1521-3862, Vol. 20, no 1-3, 91-97 p.Article in journal (Refereed) Published
Abstract [en]

Nitrogen-modified TiO2 thin films are obtained, for the first time, from aerosol-assisted (AA)CVD-prepared samples via a post-treatment method involving immersion in liquid ammonia to achieve nitrogen-modified TiO2 and visible-light photo-activity. The resulting modified and unmodified TiO2 films are characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution (HR)TEM, energy dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAED), UV-vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). This shows that the films are approximate to 200nm thick and contain anisotropic crystals of anatase TiO2. XPS shows that the nitrogen is successfully added to the surface of the film interstitially at 0.7 at.-%, but is only present to a film depth of 50nm. The nitrogen doping causes a red shift in the absorption band and a band gap narrowing of approximate to 0.1eV. The surface-bound nitrogen results from the post-treatment method of doping where the films are soaked in liquid ammonia before annealing. The photocatalytic efficiencies of the films under visible light (>385nm) are evaluated by measuring formaldehyde formation from the probe molecule tris(hydroxymethyl)aminomethane (Tris). Hydrogen abstraction from Tris, obtained from, e.g., photocatalytically produced OH radicals, leads to formaldehyde formation which is then detected through a modified version of the Hantzsch reaction. The results show that the N-modified film possess remarkable photocatalytic properties with an apparent photochemical quantum yield of approximate to 8%. Nitrogen-modified TiO2 thin films are obtained from aerosol-assisted (AA)CVD-prepared samples via a post-treatment method of immersion in liquid ammonia and calcining at 500 degrees C. The films are characterized and shown to have visible light photocatalytic activity. Visible light photoactivity is shown by measuring formaldehyde formation from the probe molecule tris(hydroxymethyl)aminomethane.

Place, publisher, year, edition, pages
2014. Vol. 20, no 1-3, 91-97 p.
Keyword [en]
AACVD, N-modified TiO2, Photocatalysis, Tris, Visible light
National Category
Chemical Sciences
URN: urn:nbn:se:kth:diva-134317DOI: 10.1002/cvde.201307086ISI: 000332380300012ScopusID: 2-s2.0-84901688668OAI: diva2:665864

QC 20140625. Updated from manuscript to article in journal.

Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2014-06-25Bibliographically approved
In thesis
1. Heterogeneous TiO2 Photocatalysis: Fundamental Chemical Aspects and Effects of Solid Phase Alterations
Open this publication in new window or tab >>Heterogeneous TiO2 Photocatalysis: Fundamental Chemical Aspects and Effects of Solid Phase Alterations
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Heterogeneous photocatalysis on TiO2 is an emerging green technology for water disinfection. The rationale for this technology is based on in-situ generation of highly reactive transitory species for degradation of organic and inorganic pollutants as well as microorganisms. Recent research has concentrated on improving the efficiency of the photocatalytic process, however, some fundamental information on the mechanistic aspects and rate limiting properties still remain elusive. 

   The focus of this thesis has been to identify the primary oxidant in heterogeneous TiO2 photocatalysis and to create prerequisites for further evaluation of how selected internal (material specific) and external (system specific) alterations influence the photocatalytic activity. Furthermore, an attempt to induce visible light activity to a modified TiO2 film was also made.

   Production of H2O2 was used to probe the existence of the hydroxyl radical as the primary oxidizing species in aqueous TiO2 photocatalysis. The only possible pathway to produce H2O2 in an oxygen free environment is through hydroxyl radical recombination. A significant amount of H2O2 could be detected in deoxygenated solutions confirming the existence of hydroxyl radicals. To further elucidate the origin of the H2O2, experiments with the hydroxyl radical scavenger Tris(hydroxymethyl)aminomethane (Tris) were performed. The results further support the hypothesis that the hydroxyl radical is the primary oxidant in TiO2 photocatalysis.

   Tris was evaluated as a probe in aqueous photocatalysis. Hydrogen abstracting species such as hydroxyl radicals are able to abstract hydrogen atoms from Tris, which leads to formation of formaldehyde. Formaldehyde was detected and quantified by a modified version of the Hantzsch reaction. This route to probe the photocatalytic efficiency allows for assessment of the maximum photocatalytic efficiency with high accuracy and sensitivity and was further used to study how selected solid phase alterations and dissolved electron acceptors affect the photocatalytic efficiency. The results showed that the surface area of immobilized photocatalysts affects the efficiency and a high surface area is advantageous for photocatalysis. It was also shown that TiO2 enhanced with Ag nanoparticles significantly increases photocatalytic activity. This is explained partly by an increased O2 adsorption and reduction process on the Ag enhanced TiO2 compared to pure TiO2 and partly as a Schottky barrier formation at the metal-semiconductor interface. These processes lead to a prolonged charge separation in the photocatalyst, which is advantageous for the efficiency. Moreover, the effect of the external, dissolved electron acceptors H2O2 and O2 were also evaluated by Tris. The results showed an increased photocatalytic activity upon addition of the electron acceptors. It was also shown that the adsorption affinity of a reactant to the photocatalyst is rate controlling and governs the kinetics.

   An attempt to induce visible light activity into a TiO2 film was also made by a post-treatment in liquid NH3. The slightly narrowed bandgap of the resulting film caused a red-shift in the absorption band and the film showed visible light activity under illumination by white light with a cut-off filter at 385 nm.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. v, 77 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2013:48
National Category
Physical Chemistry
urn:nbn:se:kth:diva-134003 (URN)978-91-7501-930-7 (ISBN)
Public defence
2013-12-12, E3, Osquarsbacke 14, KTH, Stockholm, 10:00 (English)

QC 20131121

Available from: 2013-11-21 Created: 2013-11-14 Last updated: 2013-11-21Bibliographically approved

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