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Antireflective coatings of ZnO quantum dots and their photocatalytic activity
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2012 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 2, no 27, 10298-10305 p.Article in journal (Refereed) Published
Abstract [en]

Thin films of ZnO quantum dots of different sizes have been deposited on conducting glass substrates. The films are transparent and work as antireflective coatings in the visible region. The negative absorption reaches down to -0.25 which represent a 77% increase in the transmitted light. Over a large part of the visible spectrum the increased transmittance is over 25%, and we demonstrate this to be a thin film effect. Under simulated solar illumination these films show a relatively high photocatalytic activity towards decomposition of methylene blue. The rate of photodecomposition depends on particle size and the smallest particles, which are less than 4 nm in diameter, show the highest quantum efficiency. We find the overall efficiency to be in the same order of magnitude to what's reported for commercial photocatalytic products like Degussa P25 and Pilkinton Active™, and maybe even somewhat better. We also demonstrate an increased hydrophilicity for the films under UV radiation. The photocatalytic oxidation of water into oxygen as a function of applied bias was measured in a three electrode system. The overall efficiency is small due to the high band gap but the internal quantum efficiency reaches over 10%.

Place, publisher, year, edition, pages
2012. Vol. 2, no 27, 10298-10305 p.
Keyword [en]
Anti reflective coatings, Applied bias, Conducting glass, Degussa P25, Different sizes, Film effects, Internal quantum efficiency, Large parts, Methylene Blue, Overall efficiency, Photo-catalytic, Photo-decomposition, Photocatalytic activities, Photocatalytic oxidations, Solar illumination, Three electrode-system, Transmitted light, Visible region, Visible spectra, ZnO quantum dots, Aromatic compounds, Coatings, Photocatalysis, Quantum efficiency, Semiconductor quantum dots, Substrates, Thin films, Ultraviolet radiation, Zinc oxide
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-184913DOI: 10.1039/c2ra21566gISI: 000312138400028OAI: oai:DiVA.org:uu-184913DiVA: diva2:570577
Available from: 2012-11-19 Created: 2012-11-15 Last updated: 2017-12-07Bibliographically approved
In thesis
1. ZnO Quantum Dots: Size Dependent Optical, Vibrational and Photoelectrochemical Properties
Open this publication in new window or tab >>ZnO Quantum Dots: Size Dependent Optical, Vibrational and Photoelectrochemical Properties
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is devoted to quantum dots of ZnO in the size regime 2.5-10 nm.

The focus is directed towards their size dependent properties with special

emphasis on the optical, vibrational and photocatalytic properties. The particles

were synthesized by hydrolysis in alkaline zinc acetate solution.

Analysis and characterisation were performed on both growing particles in

solution as well as on thin particle films. The main characterisation techniques

were UV-vis, florescence, XRD, Raman, electrochemical- and photoelectrochemical

methods.

Based on a large set of XRD and UV-vis measurements an empirical relation

between the band gap and the particle size were developed. This enabled

size resolved measurements of the visible fluorescence on growing particles

in solution. We report evidence of mobile trap states responsible for the fluorescence

and discuss a possible surface dependent mechanism for this.

The quantum confinement of the phonon modes were investigated with

Raman spectroscopy and molecular dynamic simulations. We report on a

size depended suppression of the phonon modes and the contribution from

surface effects.

The absolute position of the conduction band edge was determined as a

function of particle size with different photoelectrochemical methods that we

describe in detail. We demonstrate that most of the size dependent shift in

the band gap occurs by a change in the position of the conduction band. We

also show that the parabolic band approximation is valid in a region of

slightly less than 0.1 eV from the conduction band edge, and this even under

an external electric field. An interesting electroabsorption phenomenon

where the absorption locally increases under an applied potential are described

and explained in terms of the quantum confined Stark effect and the

Franz-Keldysh effect.

The produced films of the particles show antireflective properties on conducting

glass substrates, which could be interesting from a technological

perspective. The films are demonstrated to show photocatalytic activity with

respect to degradation of organic dyes and for solar water splitting.

Place, publisher, year, edition, pages
T.J Jacobsson production, 2012. 195 p.
Keyword
ZnO, nanoparticels, quantum dots
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-193235 (URN)
Supervisors
Available from: 2013-01-30 Created: 2013-01-29 Last updated: 2013-01-30Bibliographically approved
2. Highly Efficient CIGS Based Devices for Solar Hydrogen Production and Size Dependent Properties of ZnO Quantum Dots
Open this publication in new window or tab >>Highly Efficient CIGS Based Devices for Solar Hydrogen Production and Size Dependent Properties of ZnO Quantum Dots
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Materials and device concepts for renewable solar hydrogen production, and size dependent properties of ZnO quantum dots are the two main themes of this thesis.

ZnO particles with diameters less than 10 nm, which are small enough for electronic quantum confinement, were synthesized by hydrolysis in alkaline zinc acetate solutions. Properties investigated include: the band gap - particle size relation, phonon quantum confinement, visible and UV-fluorescence as well as photocatalytic performance. In order to determine the absolute energetic position of the band edges and the position of trap levels involved in the visible fluorescence, methods based on combining linear sweep voltammetry and optical measurements were developed.

The large band gap of ZnO prevents absorption of visible light, and in order to construct devices capable of utilizing a larger part of the solar spectrum, other materials were also investigated, like hematite , Fe2O3, and CIGS, CuIn1-xGaxSe2.

The optical properties of hematite were investigated as a function of film thickness on films deposited by ALD. For films thinner than 20 nm, a blue shift was observed for both the absorption maximum, the indirect band gap as well as for the direct transitions. The probability for the indirect transition decreased substantially for thinner films due to a suppressed photon/phonon coupling. These effects decrease the visible absorption for films thin enough for effective charge transport in photocatalytic applications.

CIGS was demonstrated to be a highly interesting material for solar hydrogen production. CIGS based photocathodes demonstrated high photocurrents for the hydrogen evolution half reaction. The electrode stability was problematic, but was solved by introducing a modular approach based on spatial separation of the basic functionalities in the device. To construct devices capable of driving the full reaction, the possibility to use cells interconnected in series as an alternative to tandem devices were investigated. A stable, monolithic device based on three CIGS cells interconnected in series, reaching beyond 10 % STH-efficiency, was finally demonstrated. With experimental support from the CIGS-devices, the entire process of solar hydrogen production was reviewed with respect to the underlying physical processes, with special focus on the similarities and differences between various device concepts.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 155 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1134
Keyword
ZnO, Nanoparticles, Quanum Dots, Size Dependent Properties, Hematite, CIGS, Solar Water Splitting, Hydrogen Production, PEC, Photoelectrochemical cells, PV-electrolysis
National Category
Inorganic Chemistry Physical Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-221260 (URN)978-91-554-8918-2 (ISBN)
Public defence
2014-05-23, Häggsalen, Ångström Laboratory, Lägehydsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2014-04-24 Created: 2014-03-27 Last updated: 2014-05-27Bibliographically approved

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Jacobsson, JesperEdvinsson, Tomas

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