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Reduced graphene oxide-ZnO hybrid composites as photocatalysts: The role of nature of the molecular target in catalytic performance
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-2099-7605
National Research Council, Institute for Microelectronics and Microsystems, Via Piero Gobetti 101, 40129, Bologna, Italy. Department of Chemistry “G. Ciamician”, University of Bologna, Via Francesco Selmi 2, 40126, Bologna, Italy.
National Research Council, Institute for Microelectronics and Microsystems, Via Piero Gobetti 101, 40129, Bologna, Italy.
National Research Council, Institute for Microelectronics and Microsystems, Via Piero Gobetti 101, 40129, Bologna, Italy.
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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. Vol. 47, no 14, p. 19346-19355
Keywords [en]
Reduced graphene oxide, Zinc oxide, Photoluminescence, Crystalline defects, Photocatalysis
National Category
Condensed Matter Physics
Research subject
Experimental Physics
Identifiers
URN: urn:nbn:se:ltu:diva-83804DOI: 10.1016/j.ceramint.2021.03.271ISI: 000663659900003Scopus ID: 2-s2.0-85103703343OAI: oai:DiVA.org:ltu-83804DiVA, id: diva2:1545376
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
In thesis
1. Nanostructured Metal Oxide Semiconductors for Functional Applications
Open this publication in new window or tab >>Nanostructured Metal Oxide Semiconductors for Functional Applications
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:nbn:se:ltu:diva-83181 (URN)978-91-7790-773-2 (ISBN)978-91-7790-774-9 (ISBN)
Public defence
2021-06-01, E632, Luleå, 09:00 (English)
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Supervisors
Available from: 2021-03-05 Created: 2021-03-05 Last updated: 2022-06-30Bibliographically approved

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