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Heterogeneous TiO2 Photocatalysis: Fundamental Chemical Aspects and Effects of Solid Phase Alterations
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Applied Physical Chemistry.ORCID iD: 0000-0003-2672-0041
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.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:48
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-134003ISBN: 978-91-7501-930-7 (print)OAI: oai:DiVA.org:kth-134003DiVA: diva2:665632
Public defence
2013-12-12, E3, Osquarsbacke 14, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131121

Available from: 2013-11-21 Created: 2013-11-14 Last updated: 2013-11-21Bibliographically approved
List of papers
1. Tris(hydroxymethyl)aminomethane as a probe in heterogeneous TiO 2 photocatalysis
Open this publication in new window or tab >>Tris(hydroxymethyl)aminomethane as a probe in heterogeneous TiO 2 photocatalysis
2012 (English)In: Journal of AOTS. Advanced Oxidation Technologies, ISSN 1203-8407, Vol. 15, no 2, 392-398 p.Article in journal (Refereed) Published
Abstract [en]

In this work we present a new route to probe the efficiency of aqueous TiO 2 photocatalysis using Tris(hydroxymethyl)aminomethane (Tris). Hydrogen abstracting species such as hydroxyl radicals react with Tris through hydrogen abstraction, which leads to the formation of formaldehyde. Formaldehyde was subsequently detected and quantified spectrophotometrically at 368 nm using a modified version of the Hantzsch reaction. The effect of Tris concentration on the photocatalytic production of formaldehyde was studied in the range 0.1-500 mM Tris. A strong concentration dependence was observed below 100 mM, which indicates that not all photo-produced (hydroxyl) radicals are scavenged at Tris concentrations below 100 mM. Therefore, in order to assess the maximum efficiency of the photocatalytic system, Tris concentrations above 100 mM are required. To evaluate the effect of O 2 on the formaldehyde yield in the reaction between Tris and the hydroxyl radical, a gamma radiolysis study was performed where Tris solutions saturated with O 2 and N 2, respectively, were irradiated in parallel and the yield of formaldehyde was compared. These results showed that O2 does not influence the formaldehyde yield.

Keyword
Relative Photonic Efficiencies, Rutile Phase-Transition, Electron-Spin-Resonance, Titanium-Dioxide, Semiconductor Photocatalysis, Oxidative-Degradation, Aqueous Suspensions, Hydroxyl Radicals, Quantum Yields, Adsorption
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-101596 (URN)000306963800019 ()2-s2.0-84864538063 (Scopus ID)
Note

QC 20120830

Available from: 2012-08-30 Created: 2012-08-30 Last updated: 2017-12-07Bibliographically approved
2. Effects of O2 and H2O2 on TiO2 photocatalytic efficiency quantified by formaldehyde formation from tris(hydroxymethyl)aminomethane
Open this publication in new window or tab >>Effects of O2 and H2O2 on TiO2 photocatalytic efficiency quantified by formaldehyde formation from tris(hydroxymethyl)aminomethane
2013 (English)In: Journal of AOTS. Advanced Oxidation Technologies, ISSN 1203-8407, Vol. 16, no 1, 16-22 p.Article in journal (Refereed) Published
Abstract [en]

The impact of O2 and H2O2 on the efficiency of TiO2 photocatalysis has been studied in this work. Tris(hydroxymethyl)aminomethane (Tris) was used to probe the efficiency. Upon hydrogen abstraction by e.g. hydroxyl radicals from Tris, formaldehyde is formed. This product was detected and quantified using a modified version of the Hantzsch method. A significant increase in the formaldehyde production rate was observed upon addition of O2 or H2O2. It was also found that O2 and H2O2 are equally effective in scavenging the photo-excited electron, which is probably a result of their similar adsorption properties. A strong concentration dependence, independent of O2 or H2O2 content, was found at low Tris concentrations (<100 mM). Adsorption studies of H2O 2 onto the TiO2 surface were performed in order to explore the rate controlling reactions. The results show that H2O 2, having a stronger adsorption affinity than Tris towards the photocatalytic surface, govern the kinetics, but only until a monolayer is built up.

Keyword
Binary Metal-Oxides, Titanium-Dioxide, Heterogeneous Photocatalysis, Hydrogen-Peroxide, Solid-Solutions, Degradation, Suspensions, Adsorption, Kinetics, Surface
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-117784 (URN)000314536000002 ()2-s2.0-84872537118 (Scopus ID)
Note

QC 20130205

Available from: 2013-02-05 Created: 2013-02-05 Last updated: 2017-12-06Bibliographically approved
3. Improved Texturing and Photocatalytic Efficiency in TiO2 Films Grown Using Aerosol-Assisted CVD and Atmospheric Pressure CVD
Open this publication in new window or tab >>Improved Texturing and Photocatalytic Efficiency in TiO2 Films Grown Using Aerosol-Assisted CVD and Atmospheric Pressure CVD
2013 (English)In: Chemical Vapor Deposition, ISSN 0948-1907, E-ISSN 1521-3862, Vol. 19, no 10-12, 355-362 p.Article in journal (Refereed) Published
Abstract [en]

Four different TiO2 films are formed on glass at 500 degrees C by aerosol-assisted (AA)CVD, atmospheric pressure (AP)CVD, AACVD followed by APCVD, and APCVD followed by AACVD. The APCVD films are formed from reaction of TiCl4(g) whilst the AACVD films are made by decomposing Ti[OCH(CH3)(2)](4) contained in an aerosol. The film composition is studied using X-ray photoelectron spectroscopy (XPS) to ascertain the purity of the films, and no Cl traces can be found on any of the surfaces. The use of different combinations of CVD gives rise to significant changes in microstructure and preferred orientations. X-ray diffraction (XRD) patterns and Raman spectroscopy (RS) confirm that TiO2 in the anatase form is the dominant phase on all samples. All films show superhydrophilicity after 50min of black-light irradiation. The photocatalytic efficiencies of the films are assessed qualitatively by an ink test based on Resazurin, and quantitatively studied by measuring formaldehyde formation from tris(hydroxymethyl)aminomethane (Tris). Both methods show that the AACVD film and the film seeded by APCVD are the most photocatalytically efficient ones, both having an apparent quantum yield (AQY) of around 4.2%, while the APCVD film and the film seeded by AACVD have an AQY of 0.8% and 1.5%, respectively. The changes in photocatalytic activity are explained in part by changes in film microstructure and the accessible surface area.

Keyword
Photocatalytic activity, Resazurin, Seeded TiO2 films, Tris
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-134313 (URN)10.1002/cvde.201307067 (DOI)000328452800007 ()2-s2.0-84894557680 (Scopus ID)
Note

QC 20140114

Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2017-12-06Bibliographically approved
4. Silver enhanced TiO2 thin films: photocatalytic characterization using aqueous solutions of tris(hydroxymethyl)aminomethane
Open this publication in new window or tab >>Silver enhanced TiO2 thin films: photocatalytic characterization using aqueous solutions of tris(hydroxymethyl)aminomethane
Show others...
2014 (English)In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 43, no 1, 344-351 p.Article in journal (Refereed) Published
Abstract [en]

The photocatalytic activity in aqueous solutions of TiO2 and Ag enhanced TiO2 sol-gel produced films was characterized using tris(hydroxymethyl)aminomethane (Tris) under black light (365 nm) and the observed differences in efficiency were further investigated by O-2 adsorption studies using the same probe. Hydrogen abstracting species, such as hydroxyl radicals formed upon photocatalysis, are able to abstract hydrogen from Tris. This reaction leads to the formation of formaldehyde which was detected and quantified through a modified version of the Hantzsch reaction. It was found that the Ag enhanced TiO2 film increased the apparent quantum yield from 7% to 12%, partly as a result of a Schottky barrier formation at the metal-semiconductor interface and partly as the sensitizing effect of Ag nanoparticles extends the visible light absorption, which through electron transfer processes enable an efficient charge separation in the TiO2 by attracting acceptor species more efficiently than pure TiO2. The O-2 adsorption studies in this paper showed that the Ag enhanced TiO2 film has a stronger adsorption affinity than pure TiO2 towards O-2, which make the reduction of O-2 more efficient with a subsequent enhanced electron-hole lifetime. It was also found that the Ag enhanced TiO2 film had a poorer adsorption affinity for Tris than the pure TiO2 film, which is a consequence of fewer available surface adsorption sites due to the Ag coverage at 64% which agrees well with the obtained adsorption equilibrium constants (K-LH(TiO2) = 615 M-1 and KLH(Ag-TiO2) = 320 M-1).

Keyword
Adsorption affinity, Adsorption equilibrium constants, Electron transfer process, Metal semiconductor interface, Photocatalytic activities, Sensitizing effects, Trishydroxymethylaminomethane, Visible light absorption
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-134314 (URN)10.1039/C3DT52270A (DOI)000327665200044 ()2-s2.0-84889678813 (Scopus ID)
Note

QC 20140109

Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2017-12-06Bibliographically approved
5. Comment on the Use of Phenols as Probes for the Kinetics of Heterogeneous Photocatalysis
Open this publication in new window or tab >>Comment on the Use of Phenols as Probes for the Kinetics of Heterogeneous Photocatalysis
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-134315 (URN)
Note

QS 2013

Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2013-11-21Bibliographically approved
6. Formation of H2O2 in TiO2 Photocatalysis of Oxygenated and Deoxygenated Aqueous Systems: A Probe for Photocatalytically Produced Hydroxyl Radicals
Open this publication in new window or tab >>Formation of H2O2 in TiO2 Photocatalysis of Oxygenated and Deoxygenated Aqueous Systems: A Probe for Photocatalytically Produced Hydroxyl Radicals
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 19, 10083-10087 p.Article in journal (Refereed) Published
Abstract [en]

The formation of H2O2 in oxygenated and deoxygenated aqueous solutions using immobilized TiO2 illuminated by black light (365 nm) was studied to verify the presence of hydroxyl radicals in TiO2 photocatalysis. In oxygen containing systems, formation of H2O2 proceeds through reduction of molecular oxygen by conduction band electrons or by recombination of hydroxyl radicals. In oxygen free solutions recombination of hydroxyl radicals constitutes the only pathway to H2O2 formation. Detection of H2O2 in absence of oxygen therefore serves as an indicator for hydroxyl radical formation. The H2O2 concentration was determined using the Ghormley triiodide method. It was found that a significant amount of H2O2 was produced in the deoxygenated aqueous solutions supporting the hypothesis of hydroxyl radical production in photocatalysis. To further elucidate the origin of the H2O2, experiments using the radical scavenger tris(hydroxymethyL)aminomethane (Tris) were conducted. The results showed that the H2O2 concentration increased in the oxygenated system as Tris protects the H2O2 from decomposition by hydroxyl radicals. In the deoxygenated system, no H2O2 could be detected due to hydroxyl radical scavenging by Tris, which prevents H2O2 formation. The results presented support the hypothesis that the hydroxyl radical is the primary oxidant in aqueous TiO2 photocatalysis.

Keyword
Electron-Spin-Resonance, Heterogeneous Photocatalysis, Hydrogen-Peroxide, Thin-Films, Degradation, Water, Decomposition, Tris(Hydroxymethyl)Aminomethane, Mechanism, Surface
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-134316 (URN)10.1021/jp500315u (DOI)000336198900026 ()2-s2.0-84900795141 (Scopus ID)
Note

QC 20140624. Updated from manuscript to article in journal.

Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2017-12-06Bibliographically approved
7. Visible Light Photocatalytic Activity in AACVD-Prepared N-modified TiO2 Thin Films
Open this publication in new window or tab >>Visible Light Photocatalytic Activity in AACVD-Prepared N-modified TiO2 Thin Films
Show others...
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.

Keyword
AACVD, N-modified TiO2, Photocatalysis, Tris, Visible light
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-134317 (URN)10.1002/cvde.201307086 (DOI)000332380300012 ()2-s2.0-84901688668 (Scopus ID)
Note

QC 20140625. Updated from manuscript to article in journal.

Available from: 2013-11-21 Created: 2013-11-21 Last updated: 2017-12-06Bibliographically approved

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