Quantum dot photodegradation due to CdSe-ZnO charge
transfer: transient absorption study
Supplemental information
K. Žídek, K. Zheng, P. Chábera, M. Abdellah, T. Pulleritsa)
Department of Chemical Physics, Lund University, Box 124, 22100, Lund, Sweden
Materials
All chemicals used in the experiment were purchased from Aldrich without further
purification. Cadmium oxide (99.5%), selenium (>99.5%), oleic acid (OA, >99%), and
trioctylphospine (TOP, >90%) were used to prepare CdSe quantum dots (QDs). 2mercaptoproponic acid (2-MPA, >95%) and tetramethylammonium hydroxide (TMAOH, 25%
wt. in methanol) were used to exchange the ligand of QDs for anchoring to the nanowires
(NWs) surface. Solvents used in the QD synthesis and ligand exchange included 1octadecane (ODE, 90%), toluene (anhydrous, 99.8%), methanol (anhydrous, 99.8%), acetone
(HPLC, >99.8%), ethanol (99.7%). Zinc acetate (>99%), ethylamine (>99.5%) and 2methoxyethanol (anhydrous, 99.8%) were used to prepare ZnO seed layer for nanowires
synthesis, and the precursors for NWs growth included zinc nitrate hexahydrate (>99%) and
hexamethylenetetramine (HMTA, >99%).
ZnO nanowires array synthesis
The synthesis of ZnO NWs followed the wide-know hydrothermal method.1 50 mM
solution of zinc acetate dihydrate (Zn(CH3COO)2∙2H2O) and ethanolamine in 2a)
Author to whom correspondence should be addressed. Electronic mail: Tonu.Pullerits@chemphys.lu.se.
1
methoxyethanol was spin-coated onto a fused silica substrate for 3 times and then sintered at
450 °C for 30 min in ambient to form a thin “seed layer” primarily. The hydrothermal growth
of highly oriented ZnO NWs on the seeded substrate was carried out at 92.5 °C for 4 hours in
aqueous solution containing 20 mM HMTA and zinc nitrate. After the growth, the sample
was carefully rinsed in de-ionized water and dried at 100 oC in air.
CdSe QDs synthesis
Synthesis of oleic acid capped CdSe QDs followed the method used by Bullen et al.2 In a
three-neck flask, a 70 ml octadecene ODE solution containing 5.54 g OA and 0.51 g CdO
was first heated to 180 °C to form a clear solution under N2 atmosphere; 1.3 mmol Se powder
and 0.5 g TOP were dissolved in 10 ml ODE via 1 h stirring for from TOP-Se solution. Then
the TOP-Se solution was quickly injected in the flask after the Cd2+ precursor solution was
heated up to 240 °C. After 2 minutes reaction, the flask was removed from the heater and
cooling down to room temperature. Finally, methanol and acetone were added to precipitate
QDs, which were then dissolved in toluene.
Ligand exchange
The CdSe QDs for sensitization were prepared by exchanging the capping agent of asobtained OA-capped QDs with 2-MPA. 1 ml of OA-capped QDs with the concentration of
0.4 mM were mixed with 0.1 ml 2-MPA and 1ml acetone by stirring for 30 min. Then 0.5 ml
ethanol was added to the mixture with another 15 min stirring. The reactants were centrifuged
at 3500 rpm for 5 min with the supernatant discarded. After drying in N2 gas, the resulting
precipitates were dissolved in ethanol and then TMAOH in methanol was added dropwise, so
that a totally clear solution containing linker-capped CdSe QDs was formed.
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Sensitization of CdSe QDs on ZnO nanowires
For sensitization, the as-obtained ZnO NWs layers were first heated at 70 oC for 30 min
and then directly immersed into CdSe QDs ethanol solution. The sensitization was kept in
dark for 2h. Afterwards, the films were washed thoroughly with ethanol and dried in N2
atmosphere.
REFERENCES
1
C.R. Bullen, and P. Mulvaney, Nano Lett. 4, 2303 (2004).
2
L.E. Greene, B.D. Yuhas, M. Law, D. Zitoun, P. Yang, Inorg. Chem. 45, 7535 (2006).
3