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Nanostructured Metal Oxide Semiconductors for Functional Applications
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-2099-7605
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is about nanostructured metal oxides, their properties, and some of their applications. Semiconducting metal oxides like TiO2, ZnO,and SnO2 have a wide band gap, which means they absorb UV light andgenerate electron-hole pairs. These charge carriers can be harnessed andused for a variety of purposes, such as electricity generation in solar cells,hydrogen production by means of photolysis and electrolysis, andenvironmental remediation by mineralizing pollutants in photocatalyticreactions. However, they are typically not very efficient when comparedwith e.g. noble metals in catalysis or silicon in solar cells , and so a widevariety of strategies have been employed to remedy their weaknesses.Such strategies include structuring the materials at the nanoscale, and thefabrication of composite materials and heterostructures.In this work, some advanced hybrid materials have been studied,composed of metal oxide and various additives, such as reduced grapheneoxide (rGO), other metal oxides, and flavonoids. The materials have beenextensively characterized in order to determine how these additives affectthe processes going on in some of the mentioned applications. Thestudied systems include rGO-ZnO, SnO2-ZnO, Rh-TiO2, MoO2, SiO2coupled to 3-hydroxyflavone and 7-hydroxyflavone, and 3-hydroxyflavone-TiO2.Particles of ZnO-encapsulated rGO exhibited a good photocatalyticactivity towards the degradation of rhodamine B and phenol, but it wasfound that the main determinant of the performance was the quality ofthe semiconductor component as opposed to any favorable interactionsbetween ZnO and rGO. However, the incorporation of rGO could stillalmost double the observed performance, which was attributed to apassivation of the defects in the metal oxide host, as well as a beneficialimpact of electrochemical properties such as charge transfer resistance anddouble-layer capacitance of the resulting material.Core-shell nanoparticles consisting of a SnO2 core and a ZnO shell weresuccessfully synthesized and employed as photocatalyst and as photoanodein dye-sensitized solar cells (DSSCs). The ZnO shell improved theperformance in both photocatalysis and DSSCs by nearly a factor of two,due to a combination of the favorable properties of the two metal oxides ,and the formation of a heterojunction in the interface between them.Rhodium as an additive to TiO2 nanocrystals proved to effectivelyimprove the response in gas sensing experiments. The rhodium exhibitedcomplex speciation, however, being distributed as a homogeneouscoating of Rh(III) as well as nanocrystals of elemental rhodium,highlighting the need for deep characterization in this class of materials.Metallic MoO2 nanocrystals were synthesized and tested in photocatalysis.Due to their electronic nature, they cannot support photocatalysisaccording to the traditional reaction scheme, because metals cannotgenerate electron-hole pairs. However, they still exhibited significantphotocatalytic activity towards methylene blue, rhodamine B, andparacetamol. This was attributed to a direct sensitization mechanismwhere the dye is photoexcited and undergoes electron transfer, madepossible due to the comparatively low work function in MoO2. This alsoenables it to assist in the degradation of non-absorbing molecules in thesolution. 3-hydroxyflavone (3HF) and 7-hydroxyflavone (7HF) were combinedwith MCM-41 silica nanoparticles via a post-doping procedure, and theirphotophysics characterized by steady-state and time-resolvedspectroscopic techniques. Both flavonoid-coated nanoparticles turned outto be highly fluorescent and stable when exposed to air at roomtemperature, showing that organic fluorophore-based solid-state emitterscan be obtained by simple methods. Furthermore, 3HF was coupled toTiO2 nanoparticles with a similarly simple adsorption procedure. In thiscase the result was a chemisorption of the flavonoid, which appears to bevery similar to a chelation of the metal ions in the metal oxide substrate.The fluorescence in the resulting materials is nearly completely quenched,but when a nanometer-thin layer of Al2O3 is applied on the TiO2, it isinstead strongly enhanced. This work therefore represents a rather noveland facile way to produce flavonoid-metal complexes.

Place, publisher, year, edition, pages
Luleå University of Technology, 2021.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Nano Technology Other Physics Topics
Research subject
Experimental Physics
Identifiers
URN: urn:nbn:se:ltu:diva-83181ISBN: 978-91-7790-773-2 (print)ISBN: 978-91-7790-774-9 (electronic)OAI: oai:DiVA.org:ltu-83181DiVA, id: diva2:1534419
Public defence
2021-06-01, E632, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2021-03-05 Created: 2021-03-05 Last updated: 2022-06-30Bibliographically approved
List of papers
1. Thermal Defect Modulation and Functional Performance: A Case Study on ZnO–rGO Nanocomposites
Open this publication in new window or tab >>Thermal Defect Modulation and Functional Performance: A Case Study on ZnO–rGO Nanocomposites
2019 (English)In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 256, no 12, article id 1900239Article in journal (Refereed) Published
Abstract [en]

Herein, a reduced graphene oxide–zinc oxide (rGO–ZnO) hybrid nanocomposite (1 wt% rGO) is synthesized and heat treated at different temperatures, aimed at modulating the intrinsic bulk/surface defects naturally present in nano‐ZnO. The correlation of both the dispersion of rGO within the metal oxide scaffold and the defects present on the semiconductor crystalline lattice with the photocatalytic performance toward the degradation of a molecular dye in water is investigated and discussed. It is shown that several processes compete to determine the catalytic skill of the nanocomposite, which can be enhanced by a simple thermal treatment at moderate temperatures.

Place, publisher, year, edition, pages
John Wiley & Sons, 2019
Keywords
crystalline defects, hybrid nanocomposites, photocatalysis, photoluminescence, reduced graphene oxide, thermal treatment, zinc oxide
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-76877 (URN)10.1002/pssb.201900239 (DOI)000495784800001 ()2-s2.0-85075015324 (Scopus ID)
Note

Validerad;2020;Nivå 2;2019-12-16 (johcin)

Available from: 2019-11-27 Created: 2019-11-27 Last updated: 2023-09-04Bibliographically approved
2. Electron spectroscopies of 3-hydroxyflavone and 7-hydroxyflavone in MCM-41 silica nanoparticles and in acetonitrile solutions. Experimental data and DFT/TD-DFT calculations
Open this publication in new window or tab >>Electron spectroscopies of 3-hydroxyflavone and 7-hydroxyflavone in MCM-41 silica nanoparticles and in acetonitrile solutions. Experimental data and DFT/TD-DFT calculations
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2021 (English)In: Data in Brief, E-ISSN 2352-3409, Vol. 34, article id 106630Article in journal (Refereed) Published
Abstract [en]

The data presented here concern the photophysical characterization of luminescent MCM-41 nanoparticles doped with 3-hydroxyflavone and 7-hydroxyflavone, two fluorescent flavonoids. UV-Vis and fluorescence spectra obtained on freshly-prepared samples and aged (2 months exposed to air) samples are shown. The effect of light exposure is also studied. In parallel, experiments have been carried out in acetonitrile solutions of the two flavonoids as a term of comparison. Time-dependent density functional theory calculations have also been used to simulate UV-Vis and emission spectra of different species for both flavonoids (neutral molecule, tautomers, cationic and anionic forms), taking into account the effect of the surrounding medium (solvent). Density functional theory calculations of vibrational spectra (IR, Raman) of neutral and tautomeric species of 3HF and 7HF are also provided.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
3-hydroxyflavone, 7-hydroxyflavone, Excited-state intramolecular proton transfer, Fluorescent nanoparticles, MCM-41, Photostability of flavonoids, TD-DFT calculations on absorption and fluorescence spectra of flavonoids
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-82323 (URN)10.1016/j.dib.2020.106630 (DOI)000617525400020 ()33409341 (PubMedID)2-s2.0-85098751253 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationLuleå University of TechnologyThe Kempe FoundationsVinnova, 2015-01513
Note

Godkänd;2021;Nivå 0;2021-01-12 (alebob);

Finansiär: Labex Matisse

Available from: 2021-01-12 Created: 2021-01-12 Last updated: 2023-10-28Bibliographically approved
3. Fluorescent silica MCM-41 nanoparticles based on flavonoids: Direct post-doping encapsulation and spectral characterization
Open this publication in new window or tab >>Fluorescent silica MCM-41 nanoparticles based on flavonoids: Direct post-doping encapsulation and spectral characterization
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2021 (English)In: Dyes and pigments, ISSN 0143-7208, E-ISSN 1873-3743, Vol. 185, Part A, article id 108870Article in journal (Refereed) Published
Abstract [en]

Flavones and flavonols are naturally-occurring organic molecules with interesting biological, chemical and photophysical properties. In recent years their interaction with silica surfaces has received increasing attention. In this work, the flavonol 3-hydroxyflavone (3HF) and the flavone 7-hydroxyflavone (7HF) have been encapsulated in MCM-41 mesoporous silica nanoparticles (NP) via a post-doping procedure, and their photophysics characterized by both steady state and time-resolved spectroscopic techniques. Both flavonoid-doped NPs resulted to be highly fluorescent, even after two months of exposure to air at room temperature. UV light irradiation results in a moderate decrease of the fluorescence quantum yield. Complementary UV-Vis and fluorescence experiments of 3HF and 7HF in solutions and TD-DFT calculations to simulate absorption and emission spectra have been carried out in order to better rationalize the exact nature of the emitting species. Whereas for 3HF-doped NPs the tautomer emission in the green predominates, the fluorescence of 7HF-doped NPs is likely to arise from the cationic or the phototautomeric form of the flavonoid. The results show that organic fluorophore-based fluorescent silica NPs can be easily obtained by a post-doping procedure and represent a first step towards the development of a simple strategy for the encapsulation in MCM-41 NPs of flavonoids and other organic molecules.

Place, publisher, year, edition, pages
Elsevier, 2021
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-80873 (URN)10.1016/j.dyepig.2020.108870 (DOI)000600372200007 ()2-s2.0-85091964068 (Scopus ID)
Note

Godkänd;2020;Nivå 0;2020-12-02 (johcin);Konferensartikel i tidskrift

Available from: 2020-09-22 Created: 2020-09-22 Last updated: 2023-10-28Bibliographically approved
4. Reduced graphene oxide-ZnO hybrid composites as photocatalysts: The role of nature of the molecular target in catalytic performance
Open this publication in new window or tab >>Reduced graphene oxide-ZnO hybrid composites as photocatalysts: The role of nature of the molecular target in catalytic performance
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2021 (English)In: Ceramics International, ISSN 0272-8842, E-ISSN 1873-3956, Vol. 47, no 14, p. 19346-19355Article in journal (Refereed) Published
Abstract [en]

Spurred by controversial literature findings, we enwrapped reduced graphene oxide (rGO) in ZnO hierarchical microstructures (rGO loadings spanning from 0.01 to 2 wt%) using an in situ synthetic procedure. The obtained hybrid composites were carefully characterized, aiming at shining light on the possible role of rGO on the claimed increased performance as photocatalysts. Several characterization tools were exploited to unveil the effect exerted by rGO, including steady state and time resolved photoluminescence, electron microscopies and electrochemical techniques, in order to evaluate the physical, optical and electrical features involved in determining the catalytic degradation of rhodamine B and phenol in water.

Several properties of native ZnO structures were found changed upon the rGO enwrapping (including optical absorbance, concentration of native defects in the ZnO matrix and double-layer capacitance), which are all involved in determining the photocatalytic performance of the hybrid composites. The findings discussed in the present work highlight the high complexity of the field of application of graphene-derivatives as supporters of semiconducting metal oxides functionality, which has to be analyzed through a multi-parametric approach.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
Reduced graphene oxide, Zinc oxide, Photoluminescence, Crystalline defects, Photocatalysis
National Category
Condensed Matter Physics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-83804 (URN)10.1016/j.ceramint.2021.03.271 (DOI)000663659900003 ()2-s2.0-85103703343 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationThe Kempe FoundationsVinnova, 2015-01513Carl Tryggers foundation , CTS 19:70EU, Horizon 2020Luleå University of Technology
Note

Validerad;2021;Nivå 2;2021-06-23 (beamah)

Available from: 2021-04-19 Created: 2021-04-19 Last updated: 2022-06-30Bibliographically approved
5. Rhodium as efficient additive for boosting acetone sensing by TiO2 nanocrystals: Beyond the classical view of noble metal additives
Open this publication in new window or tab >>Rhodium as efficient additive for boosting acetone sensing by TiO2 nanocrystals: Beyond the classical view of noble metal additives
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2020 (English)In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 319, article id 128338Article in journal (Refereed) Published
Abstract [en]

Anatase TiO2 nanocrystals were prepared by solvothermal synthesis and modified by in- situ generated Rh nanoparticles, with a starting nominal Rh:Ti atomic concentration of 0.01 and 0.05. After heat-treatment at 400 °C the TiO2 host was still in the anatase crystallographic phase, embedding Rh nanoparticles homogeneously distributed and whose surface had been oxidized to Rh2O3, as established by X-ray diffraction, Transmission Electron Microscopy and X-ray Photoelectron spectroscopy. Moreover, Rh seemed also homogeneously distributed in elemental form or as Rh2O3 nanoclusters. The acetone sensing properties of the resulting materials were enhanced by Rh addition, featuring a response increase of one order of magnitude at the best operating temperature of 300 °C. Moreover, Rh addition enlarged the detection range down to 10 ppm whereas pure TiO2 was not able of giving an appreciable response already at a concentration as high as 50 ppm. From the sensing data, the enhancement of the sensor response was attributed to the finely dispersed Rh species and not to the oxidized Rh nanocrystals.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
TiO2 nanocrystals, Acetone sensing, Rhodium, Noble metal additives, Solvothermal synthesis
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-79024 (URN)10.1016/j.snb.2020.128338 (DOI)000539981500011 ()2-s2.0-85085289426 (Scopus ID)
Note

Validerad;2020;Nivå 2;2020-06-09 (alebob)

Available from: 2020-05-27 Created: 2020-05-27 Last updated: 2023-09-04Bibliographically approved
6. Visible Light Photodegradation of Dyes and Paracetamol by Direct Sensitization Mechanism onto Metallic MoO2 Nanocrystals
Open this publication in new window or tab >>Visible Light Photodegradation of Dyes and Paracetamol by Direct Sensitization Mechanism onto Metallic MoO2 Nanocrystals
Show others...
2021 (English)In: Journal of Photochemistry and Photobiology A: Chemistry, ISSN 1010-6030, E-ISSN 1873-2666, Vol. 413, article id 113258Article in journal (Refereed) Published
Abstract [en]

MoO2 nanocrystals were prepared by solvothermal treatment of a Mo chloromethoxide at 250 °C in oleic acid. The monoclinic MoO2 phase, with a mean crystallite size of 29 nm, formed through reduction of molybdenum bronzes. The as-prepared MoO2 nanocrystals were free from organics, allowing their use in photodegradation tests of organic pollutants (methylene blue, rhodamine B, paracetamol), without any preliminary purification treatment of the nanocrystals. It was found that MoO2 was an efficient adsorbent of methylene blue (43 mg g-1 for 1.5 × 10−4 M concentration) in the dark and an efficient photodegradation catalyst under visible light (all methylene blue removed from the solution after 240 min). From the analysis of the combined photodegradation tests of rhodamine B and paracetamol, it was clarified that direct sensitization was responsible for photodegradation. This finding was related to the work function value of metallic MoO2, placed at more negative values if compared with other metallic materials.

Place, publisher, year, edition, pages
Elsevier, 2021
Keywords
photodegradation, dye sensitization, direct sensitization mechanism, methylene blue, MoO2, solvothermal synthesis
National Category
Materials Chemistry
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-83178 (URN)10.1016/j.jphotochem.2021.113258 (DOI)000640902200004 ()2-s2.0-85103092413 (Scopus ID)
Funder
European Regional Development Fund (ERDF), CUP: B61G18000070009
Note

Validerad;2021;Nivå 2;2021-04-08 (alebob);

Finansiär: MIUR-PON TARANTO (ARS01_00637)

Available from: 2021-03-05 Created: 2021-03-05 Last updated: 2021-06-01Bibliographically approved
7. Semiconducting metal oxides empowered by graphene and its derivatives: Progresses and critical perspective on selected functional applications
Open this publication in new window or tab >>Semiconducting metal oxides empowered by graphene and its derivatives: Progresses and critical perspective on selected functional applications
Show others...
2020 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 128, no 18, article id 180905Article, review/survey (Refereed) Published
Abstract [en]

This Perspective presents and discusses the most recent advancements in the field of exploitation of hybrid nanostructured composites consisting of semiconducting metal oxides and graphene and its derivatives (graphene oxide, reduced graphene oxide, graphene quantum dots, and carbon nanotubes) in specific fields of applications, namely, photovoltaics, water splitting, photocatalysis, and supercapacitors. These hybrid materials have received remarkable attention over the last decade thanks to claimed outstanding functional optoelectronic properties, especially as for (photogenerated) charge carriers storage and transport, allowing the promotion of useful reactions and enhancement of the efficiency of several processes based on charge exchange. In situ and ex situ synthetic strategies have been applied in order to optimize the contact between the two partners and efforts have as well been devoted to investigate the best amount of carbon material to insert in the semiconductor scaffold. We provide the reader with an overview of the research carried out in the last decade, together with a critical analysis of the claimed benefits provided by the carbon materials, also highlighting the current questions waiting for the scientific community to provide an answer to.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2020
National Category
Other Physics Topics
Research subject
Experimental Physics
Identifiers
urn:nbn:se:ltu:diva-81656 (URN)10.1063/5.0021826 (DOI)000594430300001 ()2-s2.0-85096242612 (Scopus ID)
Note

Validerad;2020;Nivå 2;2020-12-02 (johcin)

Available from: 2020-11-27 Created: 2020-11-27 Last updated: 2022-06-30Bibliographically approved
8. Chemisorption of 3-hydroxyflavone Onto TiO2 Nanoparticles: Optical and Charge Transfer Properties
Open this publication in new window or tab >>Chemisorption of 3-hydroxyflavone Onto TiO2 Nanoparticles: Optical and Charge Transfer Properties
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

3-hydroxyflavone (3HF) is chemisorbed on TiO2 nanoparticle films with an Al2O3 interlayer through a facile dip coating process. The films are characterized with UV-Vis and IR absorption spectroscopy, fluorescence spectroscopy, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The 3HF adduct is shown to bond to the substrate through the carbonyl group, resulting in a bathochromic shift in its absorption spectra similar to that observed in 3HF-metal chelates. The emission spectrum is quenched in the case of adsorption on TiO2, but strongly enhanced in the case of Al2O3, with a quantum yield (9.3%) exceeding that of uncoordinated 3HF in acetonitrile.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:ltu:diva-83440 (URN)
Available from: 2021-03-29 Created: 2021-03-29 Last updated: 2021-04-06

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