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Using the acid concentration and chain length as a tuning strategy in the hot injection synthesis, an experimental and theoretical analysis

Sofie Abé (UGent) , Richard Capek (UGent) , Bram De Geyter (UGent) and Zeger Hens (UGent)
Author
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Abstract
In many hot injection syntheses for colloidal nanoparticles, size control is achieved by stopping the reaction during growth. This gives access to highly monodisperse samples of nanoparticles, but often implies a reduction of the synthesis yield. Here, we discuss how the concentration of excess carboxylic acid and the acid chain length affects the mean size of an ensemble of colloidal nanoparticles at the end of the reaction driven growth regime (postfocused diameter) and to what extent adjusting it can be used for size tuning. We show that for a particular CdSe quantum dot (QD) synthesis, an increase in this carboxylic acid concentration or decrease in the chain length leads to larger postfocused diameters of the QDs and vice versa while not having any effect on the formation rate of CdSe. Furthermore, we observed a decrease in the number of QDs that are formed with an increasing amount of acid or reduced acid chain length, indicating that the change in postfocused diameter originates from a distribution of the monomer over a different number of particles. These experimental results are compared to a comprehensive computational model of nucleation and growth we developed earlier1 and which shows good agreement with the abovementioned CdSe QD synthesis. We find that our observations can be explained theoretically by a variation of the solubility of the monomer as this directly affects the balance between nucleation and growth and therefore changes the amount of QDs that are formed during a hot injection synthesis. This interpretation helps in understanding literature results on other QD syntheses2-4 suggesting that our discussion goes beyond this particular CdSe synthesis. When considering size tuning, our experiments and examples in literature indicate that the postfocused size can be tuned by tuning the monomer solubility, however, the approach has limited use for reaction upscaling or automation. The important disadvantage is the increase in size dispersion for high carboxylic acid concentrations and short chain lengths, which is of such magnitude that the synthesized QDs are not useful for high end applications. This makes size tuning of the postfocused diameter by adjusting the acid concentration or chain length not a viable strategy, opposed to our previously reported tuning by the reaction rate1. references: 1. Abe, S.; Čapek, R. K.; De Geyter, B.; Hens, Z., Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application, ACS Nano, 2012, 6 (1), 42–53. 2. Yu, W. W.; Peng, X. G., Formation of high-quality CdS and other II-VI semiconductor nanocrystals in noncoordinating solvents: Tunable reactivity of monomers. Angewandte Chemie-International Edition 2002, 41 (13), 2368-2371. 3. Bullen, C. R.; Mulvaney, P., Nucleation and growth kinetics of CdSe nanocrystals in octadecene. Nano Letters 2004, 4 (12), 2303-2307. 4. Owen, J. S.; Chan, E. M.; Liu, H.; Alivisatos, A. P., Precursor Conversion Kinetics and the Nucleation of Cadmium Selenide Nanocrystals. Journal of the American Chemical Society 2010, 132 (51), 18206-18213.
Keywords
nanomaterials, colloids, reaction mechanism, quantum dots, kinetics

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Chicago
Abé, Sofie, Richard Capek, Bram De Geyter, and Zeger Hens. 2012. “Using the Acid Concentration and Chain Length as a Tuning Strategy in the Hot Injection Synthesis, an Experimental and Theoretical Analysis.” In NaNax 5, Abstracts.
APA
Abé, S., Capek, R., De Geyter, B., & Hens, Z. (2012). Using the acid concentration and chain length as a tuning strategy in the hot injection synthesis, an experimental and theoretical analysis. NaNax 5, Abstracts. Presented at the 5th Conference on Nanoscience with Nanocrystals (NaNax 5).
Vancouver
1.
Abé S, Capek R, De Geyter B, Hens Z. Using the acid concentration and chain length as a tuning strategy in the hot injection synthesis, an experimental and theoretical analysis. NaNax 5, Abstracts. 2012.
MLA
Abé, Sofie, Richard Capek, Bram De Geyter, et al. “Using the Acid Concentration and Chain Length as a Tuning Strategy in the Hot Injection Synthesis, an Experimental and Theoretical Analysis.” NaNax 5, Abstracts. 2012. Print.
@inproceedings{3203363,
  abstract     = {In many hot injection syntheses for colloidal nanoparticles, size control is achieved by stopping the reaction during growth. This gives access to highly monodisperse samples of nanoparticles, but often implies a reduction of the synthesis yield. Here, we discuss how the concentration of excess carboxylic acid and the acid chain length affects the mean size of an ensemble of colloidal nanoparticles at the end of the reaction driven growth regime (postfocused diameter) and to what extent adjusting it can be used for size tuning. We show that for a particular CdSe quantum dot (QD) synthesis, an increase in this carboxylic acid concentration or decrease in the chain length leads to larger postfocused diameters of the QDs and vice versa while not having any effect on the formation rate of CdSe. Furthermore, we observed a decrease in the number of QDs that are formed with an increasing amount of acid or reduced acid chain length, indicating that the change in postfocused diameter originates from a distribution of the monomer over a different number of particles.
These experimental results are compared to a comprehensive computational model of nucleation and growth we developed earlier1 and which shows good agreement with the abovementioned CdSe QD synthesis. We find that our observations can be explained theoretically by a variation of the solubility of the monomer as this directly affects the balance between nucleation and growth and therefore changes the amount of QDs that are formed during a hot injection synthesis. This interpretation helps in understanding literature results on other QD syntheses2-4 suggesting that our discussion goes beyond this particular CdSe synthesis. When considering size tuning, our experiments and examples in literature indicate that the postfocused size can be tuned by tuning the monomer solubility, however, the approach has limited use for reaction upscaling or automation. The important disadvantage is the increase in size dispersion for high carboxylic acid concentrations and short chain lengths, which is of such magnitude that the synthesized QDs are not useful for high end applications. This makes size tuning of the postfocused diameter by adjusting the acid concentration or chain length not a viable strategy, opposed to our previously reported tuning by the reaction rate1.
references:
1. Abe, S.; \v{C}apek, R. K.; De Geyter, B.; Hens, Z., Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application, ACS Nano, 2012, 6 (1), 42--53.
2. Yu, W. W.; Peng, X. G., Formation of high-quality CdS and other II-VI semiconductor nanocrystals in noncoordinating solvents: Tunable reactivity of monomers. Angewandte Chemie-International Edition 2002, 41 (13), 2368-2371.
3. Bullen, C. R.; Mulvaney, P., Nucleation and growth kinetics of CdSe nanocrystals in octadecene. Nano Letters 2004, 4 (12), 2303-2307.
4. Owen, J. S.; Chan, E. M.; Liu, H.; Alivisatos, A. P., Precursor Conversion Kinetics and the Nucleation of Cadmium Selenide Nanocrystals. Journal of the American Chemical Society 2010, 132 (51), 18206-18213.},
  author       = {Ab{\'e}, Sofie and Capek, Richard and De Geyter, Bram and Hens, Zeger},
  booktitle    = {NaNax 5, Abstracts},
  keyword      = {nanomaterials,colloids,reaction mechanism,quantum dots,kinetics},
  language     = {eng},
  location     = {Fuengirola, Spain},
  title        = {Using the acid concentration and chain length as a tuning strategy in the hot injection synthesis, an experimental and theoretical analysis},
  year         = {2012},
}