Analysis of Optical Absorbance Spectra for the Determination of ZnO Nanoparticle Size Distribution, Solubility, and Surface Energy Doris Segets, Johannes Gradl, Robin Klupp Taylor, Vassil Vassilev, and Wolfgang Peukert* Institute of Particle Technology, Friedrich-Alexander-University Erlangen-Nuremberg, Cauerstr. 4, 91058 Erlangen, Germany T he synthesis and characterization of ZnO nanoparticles has found wide- spread interest during the past 10 years due to their unique electro-optical properties. 1-3 For instance, ZnO nanoparti- cles were recently functionalized with charge transfer complexes, such as porphy- rins, and the charge transfer for the dye molecules and the nanoparticles was stud- ied in detail. 4 However, the reproducible, noninvasive determination of small particle sizes in the sub-10 nm range remains an im- portant and challenging task. In most cases, the quantum size effect (e.g., the blue shift of the absorption edge of semiconductor nanoparticles 5 with decreasing particle size) is used, for example, to determine slow rip- ening kinetics 6-8 or to investigate nanopar- ticle growth. 9,10 In several cases, the particle size is determined from the effective mass approximation (EMA) derived by Brus. 11 Since this approach only allows a rough es- timation of the real particle size, Viswanatha et al. 12 showed that the EMA breaks down for particle sizes below 5 nm. Furthermore, the calculation of a mean particle diam- eter taking the onset of an absorption edge only provides a rough estimation and will never give the whole PSD. In con- trast, Pesika et al. 13 developed a model to determine PSDs using the slope be- tween the onset and the maximum of the investigated spectrum based on the EMA. This description is more accurate than just considering one single wavelength, but information is lost by neglecting ab- sorbance information at wavelengths shorter than the peak wavelength. Never- theless, to the best of our knowledge, there are no further approaches in the lit- erature that provide access to a PSD from optical absorbance. In this paper, we in- troduce a new model for calculating PSDs from absorbance spectra using informa- tion from the whole spectrum. By means of a tight binding model (TBM) from Viswanatha, we correlate the measure- ment wavelengths with different particle sizes. Furthermore, our considerations are based on the optical properties of bulk ZnO. 14,15 We validate our model and compare the results with other measure- ment techniques to determine PSDs, in- cluding dynamic light scattering (DLS) and transmission electron microscopy (TEM). In a further step, our model has been extended to the calculation of solid concentration. Finally, the determination of temperature-dependent ripening rates enables the calculation of the activation energy of ripening and the determination of widely unknown solubility data of ZnO in ethanol. Therefore, mean solubilities and macroscopic surface energies are cal- culated without assuming any thermody- namic materials data. *Address correspondence to w.peukert@lfg.uni-erlangen.de. Received for review March 6, 2009 and accepted May 20, 2009. Published online June 9, 2009. 10.1021/nn900223b CCC: $40.75 © 2009 American Chemical Society ABSTRACT We present a model to calculate particle size distributions (PSDs) of colloidal ZnO nanoparticles from their absorbance spectra. Using literature values for the optical properties of bulk ZnO and correlating the measurement wavelengths in the UVvisible regime with distinct particle sizes by a tight binding model (TBM), an algorithm deconvolutes the absorbance spectra into contributions from size fractions. We find an excellent agreement between size distributions determined from TEM images and the calculated PSDs. For further validation, bimodal PSDs have been investigated and an approach to determine not only particle size but also solid concentration is introduced. We will show the applicability of our model by the determination of temperature-dependent ripening rates, which enables the calculation of solubilities, surface tensions, and the activation enthalpy of ripening. In principle, our methodology is applicable to different semiconductor nanoparticles in various solvents as long as their bulk properties are known and scattering is negligible. KEYWORDS: ZnO nanoparticles · absorbance · particle size distribution · ripening rates · solubility ARTICLE www.acsnano.org VOL. 3 ▪ NO. 7 ▪ 1703–1710 ▪ 2009 1703 Downloaded by UNIVERSITAET ERLANGEN on September 21, 2009 | http://pubs.acs.org Publication Date (Web): June 9, 2009 | doi: 10.1021/nn900223b