Room-temperature sol–gel synthesis of organic ligand-capped ZnO nanoparticles
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Powders of zinc oxide nanoparticles with individual particle sizes below 10 nm in diameter are readily produced in base-induced sol–gel processes from ethanolic solutions of zinc acetate dihydrate. These particles are covered with acetate molecules and without further stabilization, they grow when stored as a powder. Here, we present three organic ligands, which reproducibly stabilize individual particle sizes <5 nm within the agglomerated powders for extended periods of time, up to months. Citric acid and 1,5-diphenyl-1,3,5-pentanetrione result in average diameters of 3 nm, whereas dimethyl-L-tartrate stabilizes 2.1 nm. X-ray diffraction and pair distribution function analysis were used to investigate the structural properties of the particles. TEM data confirm the individual particle size and crystallinity and show that the particles are agglomerated without structural coherence. Besides the introduction of these novel ligands for ZnO nanoparticles, we investigated, in particular, the influence of each synthesis step onto the final nanoparticle size in the powder. Previous studies often reported the employed synthesis parameters, but did not motivate the reasoning for their choice based on detailed experimental observations. Herein, we regard separately the steps of (i) the synthesis of the colloids, (ii) their precipitation, and (iii) the drying of the resulting gel to understand the role of the ligands therein. ZnO particles only covered with acetate grow to 5 nm during the drying process, whereas particles with any of the additional ligands retain their colloidal size of 2–3 nm. This clearly shows the efficient binding and effect of the presented ligands.
KeywordsZnO Nanoparticle Sol–gel synthesis Organic ligand Room temperature synthesis
We want to acknowledge a scholarship of the Friedrich-Alexander-University Erlangen-Nürnberg for financial support and the BMBF under grant numbers 05K10WEB and 05K13WE2 for travel expenses. Beamtime at Argonne National Laboratory and the European Synchrotron Radiation Facility is gratefully acknowledged. We thank K. Chapman and K. Beyer from beamline 11-ID-B, APS for support during our beamtime as well as J. Hudspeth and S. A. J. Kimber from beamline ID-15-B, ESRF. We acknowledge H. Brückner and A. Windmüller for synthesis of nanoparticle powders for these experiments as well as J. Harz, N. Pfeiffer, and M. Fischer for optimization of the synthesis recipes. K. Götz is acknowledged for his help during beamtime. We further thank C. Damm and A. Windmüller for carrying out the TGA measurements.
Conflict of interest
The authors declare no competing financial interests.
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