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Development of Graphitic Carbon Nitride based Semiconductor Photocatalysts for Organic Pollutant Degradation
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

As a potential solution to the global energy and environmental pollution, design and synthesis of artificial photocatalysts with high activities have attracted increasing scientific interests worldwide. In recent years, the graphitic carbon nitride (g-C3N4) has shown new possible applications in the photocatalytic field due to its unique properties. However, the photocatalytic efficiency of the pristine g-C3N4 is greatly limited by the high recombination rate of the photo-induced electron-hole pairs. In this thesis, the aim is to design and fabricate efficient g-C3N4 based photocatalysts with enhanced photocatalytic activities under a visible light irradiation.

In order to achieve this goal, two strategies have been employed in the present thesis. First, the as-obtained g-C3N4 was used as the host material to construct staggered-aligned composite photocatalysts by selecting semiconductors with suitable band positions. By this method, three kinds of g-C3N4-based composite photocatalysts such as g-C3N4/ZnS nanocage, g-C3N4/m-Ag2Mo2O7 and g-C3N4/MIL-88A were successfully fabricated. Second, the microstructure of the g-C3N4 was modified by the H2O2-treatment at an elevated temperature and ambient pressure.

In this study, the g-C3N4 was prepared by a simple pyrolysis of urea. As for all the as-synthesized phtocatalysts, the structures, morphologies and the optical properties were carefully characterized by the following techniques: XRD, SEM, TEM, FT-IR and DRS. Also, the band edge positions of m-Ag2Mo2O7 and MIL-88A were studied by the Mott-Schottky methods. Thereafter, the photocatalytic activities were evaluated by using a solution of rhodamine B (RhB) as a target pollutant for the photodegradation experiments performed under a visible light irradiation. The results showed that all the aforementioned g-C3N4-based photocatalysts exhibited enhanced photocatalytic activities in comparison with the pristine g-C3N4. For the case of the g-C3N4-based composite photocatalysts, the enhancement factor over the pristine g-C3N4 can achieve values ranging from 2.6 to 3.4. As for the H2O2-treated g-C3N4, the degradation rate constant can be 4.6 times higher than that of the pristine g-C3N4.

To understand the key factors in new materials design, we also devote a lot of efforts to elucidate the basic mechanisms during the photocatalytic degradation of organic pollutant. Based on the results of the active species trapping (AST) experiments, the main active species in each photocatalytic system were determined. In the g-C3N4/m-Ag2Mo2O7 and the g-C3N4/MIL-88A system, three kinds of active species of ·O2-, h+ and ·OH were found to be involved in the photocatalytic reaction. Among them, the ·O2- and h+ were the main active species. In the g-C3N4/ZnS and H2O2-treated g-C3N4 photocatalytic systems, the main active species was determined as the ·O2-. The reaction pathways of these active species were also demonstrated by comparing the band edge positions with the potentials of the redox couple. In addition, the relationship between the active species and the photocatalytic behaviors of N-de-ethylation and conjugated structure cleavage were studied. Finally, possible mechanisms to explain the enhanced photocatalytic activities were proposed for each photocatalytic system.

The results in this thesis clearly confirm that the photocatalytic activity of the g-C3N4 based photocatalyst can efficiently be enhanced by constructions of staggered-aligned composites and by modification of the microstructure of the g-C3N4. The enhanced photocatalytic performance can mainly be ascribed to the efficient separation of the photo-induced electron-hole pairs and the increase of the active sites for the photocatalytic reaction.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , ix, 50 p.
Keyword [en]
graphitic carbon nitride, visible light-driven photocatalyst, staggered band alignment, electron-hole pair separation, band edge position
National Category
Natural Sciences Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-173216ISBN: 978-91-7595-675-6 (print)OAI: oai:DiVA.org:kth-173216DiVA: diva2:852252
Public defence
2015-09-30, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150909

Available from: 2015-09-09 Created: 2015-09-07 Last updated: 2015-09-09Bibliographically approved
List of papers
1. Fabrication of novel g-C3N4/nanocage ZnS composites with enhanced photocatalytic activities under visible light irradiation
Open this publication in new window or tab >>Fabrication of novel g-C3N4/nanocage ZnS composites with enhanced photocatalytic activities under visible light irradiation
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2014 (English)In: CrystEngComm, ISSN 1466-8033, E-ISSN 1466-8033, Vol. 16, no 21, 4485-4492 p.Article in journal (Refereed) Published
Abstract [en]

In order to develop efficient visible light-driven photocatalysts for environmental applications, novel g-C3N4/nanocage ZnS composites have been successfully fabricated via an anion exchange route using ZIF-8 as a self-sacrificing template. The as-prepared g-C3N4/ZnS composites were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy and photoluminescence. In addition, the photocatalytic oxidation of organic contaminants in aqueous solution was investigated with the g-C3N4/ZnS composite system under visible light irradiation by using rhodamine B as a model compound. The results indicate that the g-C3N4/ZnS composite photocatalysts show higher photocatalytic activity than the single component (pure g-C3N4 or ZnS). More specifically, the optimum photocatalytic activity of the g-C3N4/ZnS composite was achieved with a weight ratio of 6 : 1, which is as high as 37.8 and 2.8 times those of individual ZnS and g-C3N4 under visible light irradiation. The enhanced photocatalytic activity of g-C3N4/ZnS is mainly attributed to an efficient electron-hole separation at the interface of the two semiconductors, as determined by photoluminescence spectroscopy. Moreover, it is observed that. O-2 is the main active species in the photocatalytic oxidation of RhB solution using a g-C3N4/nanocage ZnS composite.

Keyword
Graphitic Carbon Nitride, Hydrogen Evolution, Heterojunctions, Photodegradation, Nanosheets, Nanowires, Mechanism, Water
National Category
Other Chemistry Topics
Identifiers
urn:nbn:se:kth:diva-146581 (URN)10.1039/c4ce00107a (DOI)000335923800020 ()2-s2.0-84899806251 (Scopus ID)
Note

QC 20140613

Available from: 2014-06-13 Created: 2014-06-12 Last updated: 2017-12-05Bibliographically approved
2. Visible light-driven g-C3N4/m-Ag2Mo2O7 composite photocatalysts: synthesis, enhanced activity and photocatalytic mechanism
Open this publication in new window or tab >>Visible light-driven g-C3N4/m-Ag2Mo2O7 composite photocatalysts: synthesis, enhanced activity and photocatalytic mechanism
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2014 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 92, 51008-51015 p.Article in journal (Refereed) Published
Abstract [en]

The g-C3N4/m-Ag2Mo2O7 composite photocatalysts with well-aligned band structures are successfully fabricated by a simple two-step method with different mass contents of m-Ag2Mo2O7. The as-prepared samples are evaluated as photocatalysts toward rhodamine B (RhB) degradation in aqueous solution under visible light irradiation (lambda > 420 nm). The results demonstrate that the photocatalytic activities of the composites are strongly influenced by the weight ratio of g-C3N4 to m-Ag2Mo2O7. When it is 6 : 1, the composite exhibits the highest photocatalytic efficiency. More specifically, this value is as high as 3.4 and 6.1 times that of pure g-C3N4 and P25 respectively. In order to investigate the mechanism causing the enhanced photocatalytic degradation, the band structures are determined by UV-vis diffuse reflection spectroscopy and the Mott-Schottky technique. Moreover, the reactive radicals involved in the photocatalytic process are examined in detail via active species trapping (AST) experiments. The improved photocatalytic activities can be attributed to the efficient separation of the photo-induced charge carriers and the strong redox capacities benefit from the synergetic effect between g-C3N4 and m-Ag2Mo2O7.

Keyword
Composite photocatalysts, Photo-catalytic, Visible light
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-157253 (URN)10.1039/c4ra09224d (DOI)000344325400092 ()2-s2.0-84908209962 (Scopus ID)
Note

QC 20141208. QC 20160226

Available from: 2014-12-08 Created: 2014-12-08 Last updated: 2017-12-05Bibliographically approved
3. Construction of heterostructuredg-C3N4/MIL-88A with enhanced photocatalytic activity under visible-light irradiation
Open this publication in new window or tab >>Construction of heterostructuredg-C3N4/MIL-88A with enhanced photocatalytic activity under visible-light irradiation
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-173318 (URN)
Note

QS 2015

Available from: 2015-09-09 Created: 2015-09-09 Last updated: 2015-09-09Bibliographically approved
4. Investigation of the photocatalytic mechanism of the H2O2-treated g-C3N4 derived from urea with superior visible-light photocatalytic activity
Open this publication in new window or tab >>Investigation of the photocatalytic mechanism of the H2O2-treated g-C3N4 derived from urea with superior visible-light photocatalytic activity
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-173319 (URN)
Note

QS 2015

Available from: 2015-09-09 Created: 2015-09-09 Last updated: 2015-09-09Bibliographically approved
5. A study of the photocatalytic N-de-ethylation and chromophore cleavage of RhB over four different g-C3N4 based phtocatalysts
Open this publication in new window or tab >>A study of the photocatalytic N-de-ethylation and chromophore cleavage of RhB over four different g-C3N4 based phtocatalysts
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-173320 (URN)
Note

QS 2015

Available from: 2015-09-09 Created: 2015-09-09 Last updated: 2015-09-09Bibliographically approved

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