Optical properties of growing ZnO QDs in suspension and their effect on PC activity

Taha Ahmed

June 12, 2017

Introduction

Simple suspensions of ZnO quantum dots

ZnO is cheap, non-toxic, and easy to make nanoparticles of. In the QD regime, reactivity increases significantly.

I would like to present an investigation into ZnO QD suspensions as an efficient and environmentally-neutral photocatalyst of distributed pollutants.

Wurtzite ZnO offers advantages over anatase TiO2

  • direct band gap (higher absorption coefficient)
  • higher charge carrier mobility
  • more versatile for synthesis of low-dimensional structures

Synthesis

Just mix Zn(OAc)2 and alkali

Precursor solutions in EtOH:

M/g mol-1 n/mmol V/mL m/g C/mol L-1
Zn(Ac)2·2H2O 219.50 5.0 50 1.0975 0.10
LiOH·H2O 41.96 7.0 50 0.2937 0.14
  • Zinc acetate was dissolved in boiling EtOH.
  • Lithium hydroxide was gently dissolved in EtOH at 45°C.

Spanhel and Anderson, JACS (1991). DOI: 10.1021/ja00008a004.
Meulenkamp, J Phys Chem B (1998). DOI: 10.1021/jp980730h
Jacobsson and Edvinsson, Inorg Chem (2011). DOI: 10.1021/ic201327n

  • Building on previous workers, we have developed a simple and reliable synthesis of ZnO QD suspensions
  • Zinc oxide nanoparticle growth starts when the two solutions are mixed,t = 0, and can then be monitored in-situ using UV/Vis spectrophotometry.

Spanhel and Anderson, JACS (1991). DOI: 10.1021/ja00008a004.
Meulenkamp, J Phys Chem B (1998). DOI: 10.1021/jp980730h
Jacobsson and Edvinsson, Inorg Chem (2011). DOI: 10.1021/ic201327n

A few challenges encountered during synthesis

  • The LiOH stock contains approx. 1% carbonate, and solubility of Li2CO3 decreases with temperature
  • CO2 (from air) will dissolve into solvent, forming even more carbonate – to counteract this I used gentle stirring (took forever) and worked under N2 overpressure (worked well).
  • None of the precursor solutions are stable - don’t delay mixing!

Results

Tracking band gap during PC

Two important spectral features

  • ZnO band edge around 350 nm (depends on QD size)
  • Absorbance from methylene blue (MB) dye at 665 nm

Tracking the band gap optically (QD size)

Growing QD: moving band edge

Assuming a spherical cow…

Previous work in our group showed that particle diameter can be calculated from the band gap using the empirical relation

\[ E_\text{g} = 3.30 + \frac{0.293}{d} + \frac{3.94}{d^2} \]

which can be re-arranged into

\[ d = \frac{2 \times 3.94}{-0.293 + \sqrt{0.293^2 - 4 \times 3.94 \times (3.30 - E_\text{g})}} \]

Jacobsson and Edvinsson, Inorg Chem (2011). DOI: 10.1021/ic201327n

Determine the band gap: Tauc analysis

Growing QDs: optical band gap vs time

QD growth over time

Time/min Particle diameter/nm
1 0
10 2
100 4
1000 6
10000 8
100000 10

As nanoparticles grow, reactants are consumed. Growth steadily slows down.

\[ d = 1.9998 \times \log(t) - 0.0232 \]

Photocatalytic degradation

PC degradation occurs while the QDs are growing, meaning

  • reactivity of catalyst decreases, but also
  • ZnO will eventually coalesce into aggregates and sediment harmlessly.

  • UV photons have enough energy to break covalent bonds in molecules, but most organic compounds require a catalyst to overcome activation barriers
  • A wide band gap semiconductor such as ZnO produces highly oxidising holes in the VB upon UV illumination that will form reactive hydroxyl radicals in water
  • these species (and others) act as strong, non-selective oxidants and usually lead to complete mineralisation of the undesired compounds.

Solar simulator: Xe arc lamp, AM1.5G filter. Distance from lamp determines Suns.

A note on toxicity

  • ZnO (bulk) is not considered toxic to humans or animals,
  • nanoparticles are generally considered toxic due to their size (irrespective of composition).

These experiments suggest that we can have the cake and eat it too:

  • nanoparticles give us increased reactivity,
  • followed by agglomeration that causes the oxide catalyst to sediment!

Conclusion

ZnO QD suspension effectively photocatalyses degradation of MB

  • under AM1.5G illumination
  • in water, and in water/ethanol mixture
  • during photodegradation the ZnO QDs aggregate with each other or with degradation by-products (loss of band edge)
  • solution-based synthesis of semiconductor QDs can give very high level of control over QD size (optoelectronic properties)

Possible future work

  • Investigate varying the QD growth velocity (concentration, molar ratio)
  • QD size distributions (by dynamic light scattering)
  • …

Thanks to

  • My PhD advisors Tomas Edvinsson and Jiefang Zhu, Uppsala university.

Thank you for your attention!

https://chepec.se

Appendices

Self-degradation of MB under 1 Sun

Self-degradation of MB under 1 Sun

Suitability of methylene blue

  • Absorbs mainly around 660 nm
  • Does not absorb where ZnO band edge occurs
  • Strong absorption coefficient

QD growth