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ZnO Quantum Dots: Size Dependent Optical, Vibrational and Photoelectrochemical Properties
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
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 [en]
ZnO, nanoparticels, quantum dots
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-193235OAI: oai:DiVA.org:uu-193235DiVA: diva2:601526
Supervisors
Available from: 2013-01-30 Created: 2013-01-29 Last updated: 2013-01-30Bibliographically approved
List of papers
1. Antireflective coatings of ZnO quantum dots and their photocatalytic activity
Open this publication in new window or tab >>Antireflective coatings of ZnO quantum dots and their photocatalytic activity
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%.

Keyword
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:nbn:se:uu:diva-184913 (URN)10.1039/c2ra21566g (DOI)000312138400028 ()
Available from: 2012-11-19 Created: 2012-11-15 Last updated: 2017-12-07Bibliographically approved
2. Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots: Size and Band Gap Correlation and Evidence of Mobile Trap States
Open this publication in new window or tab >>Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots: Size and Band Gap Correlation and Evidence of Mobile Trap States
2011 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 50, no 19, 9578-9586 p.Article in journal (Refereed) Published
Abstract [en]

ZnO nanoparticles constitute a convenient model system for fundamental studies with many possible technical applications in, for example, sensors and the field of catalysis and optoelectronics. A large set of ZnO quantum dots in the size range 2.5-7 nm have been synthesized and analyzed in detail. Time resolved in situ UV-vis absorption measurements were used to monitor the growth of these particles in solution by correlating the optical band gap to particle size given from X-ray diffraction (XRD) measurements. The particles formed were isotropic in shape, but small initial deviations gave indications of a transition from thermodynamic to kinetically controlled growth for particles around 4 nm in diameter. On the basis of this, the behavior and mechanisms for the particle growth are discussed. The fluorescence dependence on particle size was investigated by combining fluorescence and UV-vis measurements on growing particles. This revealed that the positions of the fluorescence trap states are mobile toward the conduction- and valence band. A broadening of the trap states was also found, and a surface dependent mechanism of the trap state shift and broadening is proposed.

National Category
Chemical Sciences Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-161945 (URN)10.1021/ic201327n (DOI)000295115000047 ()
Available from: 2011-11-23 Created: 2011-11-21 Last updated: 2017-12-08Bibliographically approved
3. Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
Open this publication in new window or tab >>Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 29, 15692-15701 p.Article in journal (Refereed) Published
Abstract [en]

The absolute position of the conduction and the valence band edges of ZnO quantum dots (Qdots) has been determined as a function of particle size with potential dependent absorption spectroscopy. The absolute position of the band edges are vital for which catalytic reactions that can occur at the surface. They are also crucial parameters for charge injection and extraction in nanoparticular solar cells and other optoelectronic devices based on nanoparticles. The position of the conduction band edge was determined by potentiostatic population of the conduction band states and monitoring the resulting increase in the optical band gap. This was performed for ZnO particles in the quantum confined region with diameters ranging between 4 and 9 nm. The particles were deposited into thin films giving an ensemble of particles for which the analysis could be performed. The relevant equations were derived and their validity in terms of applied potential and kinetic considerations was quantified. We find that essentially all of the quantum size effect of increased band gap is occurring by a shift of the conduction band edge. The extent of the validity of the parabolic approximation, which is one of the assumptions in the analysis, is investigated, both experimentally and with density functional theory calculations of bulk ZnO Here, we find that the parabolic approximation only is valid in an energy range of slightly less than 0.1 eV from the conduction band edge but in that regime constitutes an excellent approximation. We also demonstrate that the validity of the parabolic approximation follows a rising Fermi level into the conduction band energy levels.

National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-179918 (URN)10.1021/jp302220w (DOI)000306725200061 ()
Available from: 2012-08-27 Created: 2012-08-27 Last updated: 2017-12-07Bibliographically approved
4. Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
Open this publication in new window or tab >>Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 12, 6893-6901 p.Article in journal (Refereed) Published
Abstract [en]

The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E-2 Raman active optical phonon at 436 cm(-1) is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260-340 attributed to surface vibrations. The E-1-LO mode at 585 cm(-1) is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 angstrom) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode. 

National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-129300 (URN)10.1021/jp300985k (DOI)000302051100015 ()
Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved

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