In Situ Study on the Evolution of Multimodal ... - ACS Publications

Subscriber access provided by United Arab Emirates University | Libraries Deanship

Article

In Situ Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities Torben Schindler, Johannes Walter, Wolfgang Peukert, Doris Segets, and Tobias Unruh J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b08005 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 20, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

In Situ Study on the Evolution of Multimodal Particle Size Distributions of ZnO Quantum Dots: Some General Rules for the Occurrence of Multimodalities Torben Schindlerx, Johannes Walter†, Wolfgang Peukert†, Doris Segets†, Tobias Unruhx x

Chair of Crystallography and Structural Physics, Friedrich-Alexander-Universität ErlangenNürnberg (FAU), Staudtstr. 3, 91058 Erlangen, Germany. † Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Cauerstr. 4, 91058 Erlangen, Germany.

Corresponding authors: Prof. Dr. Tobias Unruh Staudtstr. 3 91058 Erlangen Email: [email protected] Dr. Doris Segets Cauerstr. 4 91058 Erlangen Email: [email protected]

Abstract: Properties of small semiconductor nanoparticles (NPs) are strongly governed by their size. Precise characterization is a key requirement for tailored dispersities and thus for high-quality devices. Results of a careful analysis of particle size distributions (PSDs) of ZnO are presented combining advantages of UV/Vis absorption spectroscopy, analytical ultracentrifugation and small angle X-ray scattering (SAXS). Our study reveals that careful cross-validation of these different methods is mandatory to end up with reliable resolution. PSDs of ZnO NPs are multimodal on a size range of 2 to 8 nm, a finding that is not yet sufficiently addressed. In the second part of our work the evolution of PSDs was studied using in situ SAXS. General principles for the appearance of multimodalities covering a temperature range between 15 °C and 45 °C were found which are solely determined by the ageing state indicated by the size of medium-sized fraction. Whenever this fraction exceeds a critical size, a new multimodality is identified, independent of the particular time-temperature combination. A fraction of larger particles aggregates first before a fraction of smaller particles is detected. Fixed multimodalities have not yet been addressed adequately and could only be evidenced due to careful size analysis.

Keywords: ZnO quantum dots, small angle X-ray scattering, analytical ultracentrifugation, multimodal particle size distribution, ripening 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

distribution with three spherical main particle populations between 2 nm and 10 nm.19 From their SAXS data, they assumed coagulation of the primary particles to form larger aggregates to be the only reason for this multimodality. SAXS is a powerful method to detect in situ size and shape of NPs in suspension, which was already shown for e.g. Au, Ag, SiO2, CdS(e) and PbS.20-24 However, for complex particle systems with bi- or even multimodal size distributions, a combination of SAXS with complementary techniques is needed to achieve comprehensive results because a reliable model is required for the SAXS analysis. These complementary methods should also be in situ techniques in order to avoid artifacts from sample preparation which does not hold for e.g. transmission electron microscopy (TEM). In this work we used UV/Vis absorption spectroscopy and analytical ultracentrifugation (AUC) in addition to our SAXS measurements to find a suitable model for the scattering analysis to determine the PSD of the NPs. For the characterization by UV/Vis spectroscopy, the size dependent absorption behavior of small (ZnO) semiconductor NPs (usually known as quantum size effect) is used, whereas during characterization by AUC the sedimentation of the particles in an externally applied centrifugal field is followed. In brief, via the absorbance measurements the semiconducting particle core is detected whereas AUC provides a sedimentation equivalent diameter including the solid particle core and the ligand shell, which becomes important for small QDs.25, 26 SAXS is sensitive on regions of high material density (high scattering length density) and thus detects the solid core of ZnO particles. To achieve a detailed picture of the PSD, the same sample was investigated using the three techniques and the results were carefully analyzed. We found a multimodal size distribution already after 120 min of ageing at 35 °C which evolved for longer ripening times. Besides the main particle fraction, a small percentage of agglomerates was detected (about 10 vol%) and furthermore a not yet reported fraction of smaller ZnO particles was found (about 10 vol%).

Introduction: ZnO semiconductor nanoparticles (NPs) exhibit intriguing electro-optical properties which favor their application in a variety of electronic devices such as transistors, thin film solar cells and light emitting diodes.1-5 The reason for these properties is the quantum size effect which allows to tailor the physical properties of materials by adjusting size and shape of the NPs, respectively. Many different routes to synthesize ZnO NPs are described in the literature. Among these are chemical vapor deposition (CVD), laser ablation and solution based routes.6-8 The advantage of solution processed quantum dots (QDs) is their economically priced and highly reproducible production. Furthermore, size and shape of the NPs can be varied by applying different synthesis conditions and/or postpreparation modification.9 In the case of the synthesis of ZnO NPs, sol-gel based processes using ZnAc2 x 2H2O in an alkaline alcoholic solution have been studied in detail8, 10-12 and different shapes have been reported.13-15 In general, exact in situ characterization methods with good temporal resolution are seen to be indispensable for a targeted production of particles with desired properties. They give access to nucleation, growth, and aging processes with their underlying kinetics. The experimental detection of the temporal evolution of the particle size distribution (PSD) during the whole particle formation process is a prerequisite for a deeper insight into growth and ripening mechanisms. Studies using different techniques revealed the occurrence of different multi-step mechanisms. However, these studies mainly focus on the determination of mean particle sizes and their dependence on time and temperature using often ex situ techniques like electron microscopy.16-18 Studies in which the PSD is described in detail in situ are rare, although Tokumoto et al. performed already in 1999 a comprehensive in situ small angle X-ray scattering (SAXS) study on the PSD of ZnO NPs. They found – however without cross-validation by any other independent method – a multimodal size 2

ACS Paragon Plus Environment

Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


when sedimentation coefficients shall be translated to core particle sizes. Combining both, UV/Vis and AUC, as a starting point for the interpretation of SAXS measurements, we get reliable information on aspect ratios, the particle size range and the PSD shape to establish an accurate model. Well crossvalidated models provide unique insights to particle formation and the dispersity evolution on a length scale of a few nanometers. This opens a completely new field with respect to the mechanistic understanding of particle formation.

After having established a profound model for the evaluation of SAXS measurements on ZnO QDs, we applied it in the second part of this paper to analyze the evolution of the PSD during the aging process by in situ SAXS. We characterized the ZnO QD ageing at different temperatures between 15 °C and 45 °C, whereby special emphasis was put on the determination of the onsets of multimodalities found in the analyzed PSDs. Some general principles for the appearance of those multimodalities were identified which are determined by the evolution – or ageing state – of the medium sized particles. This closer analysis of these discrete features beyond the main fraction could only be achieved by the careful combination of different analytical methods. We believe that this analysis will shed light on the understanding of the complex interplay between Ostwald ripening and aggregation of discrete populations in the near future.

Materials and Methods: Particle synthesis: Zinc acetate dihydrate (ZnAc2 x 2H2O, z.A., VWR Germany), lithium hydroxide (LiOH, 98 %, VWR Germany) and absolute ethanol (EtOH, 99.98 %, VWR Germany) were used without any further purification. The zinc oxide NPs were prepared based on the routine developed by Spanhel8 and Meulenkamp.10 2.195 g zinc acetate dihydrate (ZnAc2 x 2H2O, 0.01 mol) were solved in 100 ml ethanol and refluxed at 80 °C for 180 min. Meanwhile, 100 ml ethanolic solution of lithium hydroxide (LiOH, 0.2395 g, 0.01 mol) was prepared. After reflux of the zinc precursor solution and its subsequent cooling to room temperature the two solutions were mixed and ZnO NPs formed. Immediately after mixing, the suspension was stored in an incubator (LAB-Therm series, Kuhner, Switzerland) without shaking at 35 °C. After ripening for 120 min (sample A), 180 min (sample B), 240 min (sample C), and 360 min (sample D), respectively, small samples were taken from the solution and cooled to -10 °C to prevent further ageing of the QDs before they were analyzed by means of UV/Vis, AUC, and SAXS, respectively. The samples used for the in situ SAXS measurements were prepared using the same routine and filled into thermostated quartz capillaries of 1 mm inner diameter directly after mixing of the two ethanolic solutions.

Comparison of analytical methods A detailed comparison of the different analytical methods and their specific advantages and disadvantages together with the specific physical properties that are used for the detection is given in the supporting information (SI1). The first two techniques (UV/Vis and AUC) are used to get an accurate insight into the size and shape of the PSDs, whereas the latter (SAXS) is of major importance for the further interpretation of our measurement data with high temporal resolution. In brief, putting all advantages and disadvantages together, we believe that the combination of UV/Vis, AUC and SAXS is an excellent way to get maximum insights to the evolution of a PSD. UV/Vis enables a fast snapshot of the PSD under investigation and usually provides a very good assignment of the particle size range. AUC provides outstanding accuracy in terms of the particle size, in case information regarding the morphology of the shell does exist, as well as high resolution regarding the shape of the PSD. However, it cannot be applied in situ to monitor the ageing process of ZnO and requires prior knowledge

UV/Vis absorption spectroscopy: 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 19

we used an approach, which is based on the method of Lees et al.25 Our procedure will be presented in the following. The basic dependencies of the sedimentation coefficient are given by Svedberg’s equation:29

Absorbance spectra were recorded in the wavelength range between 200 nm and 500 nm using a Cary 100 Scan UV/Visible spectrophotometer (Varian Deutschland GmbH, Germany) in a quartz glass cuvette of 0.2 mm optical path length (Hellma Analytics, Germany). The optical path length was chosen to avoid dilution of the samples prior the measurement.

=

1 − 1 − = 3

where s denotes the sedimentation coefficient, m the mass of the particle, ν the partial specific volume of the particle which is the inverse of the solvated density, ρsolvent the density and η the viscosity of the solvent and xh the hydrodynamic diameter of the particle. The mass as well as the partial specific volume of the particle is defined by the ratio of the core and the shell:

Analytical Ultra Centrifugation: A user modified preparative ultracentrifuge, type Optima L-90K from Beckman Coulter, with an integrated UV/Vis detector was used to acquire the experimental data.27 Titanium centerpieces, path length 12 mm, were used for all experiments. Sedimentation velocity data has been acquired with a radial step size of 50 µm at 40 krpm and 10 °C. The samples were filled within the cooled cell and were immediately placed in the cooled rotor to reduce the time for temperature equilibration and to prevent the sample from further ripening during the experiment as far as possible. All samples were measured simultaneously to minimize time of storage. The hydrodynamic and core sizes of the ZnO NPs that are needed for the comparison with SAXS and UV/Vis data have to be calculated based on the sedimentation coefficient distributions obtained by the AUC analysis. For this, a 2-dimensional evaluation was checked but not applied due to particularities of the present data. Since the small ZnO NPs may tend to further ripen, the measurements were performed at a high rotor speed to keep the experimental time short. This reduced the spreading of the sedimentation boundary by diffusion and increased the resolution in the sedimentation coefficient. However, this procedure made the data also unsuitable for a reliable 2-dimensional analysis because it needs the sedimentation and diffusion information to be represented in the data in a sufficient manner. Due to the fact that we have sufficient information regarding the shell morphology provided by other techniques such as TGA,28 we did not apply 2-dimensional analysis. Instead,

1 −

+ 1 − − 3 = 0

Assuming that the shell morphology is independent of the particle size for the considered size range in good approximation, the hydrodynamic particle diameter including shell as well as the mass of the shell can be approximated. Those are a function of the core size, the surface coverage (SC), the solvent density and the density as well as length of the ligand: = + 2!"#$% = &

= '() ∙ "#$% + 1 − () ∙ + ∙ . / 0 ∙ , . − 6 This requires the assumption that associated solvent molecules are located in between the ligand molecules but do not contribute significantly to the hydrodynamic diameter.25 Moreover, a spherical shape has to be assumed for the NPs, which is true in good approximation for these small ZnO QDs.30 The accuracy of this approach depends on the chosen material and solvent parameters. For the acetate stabilized ZnO NPs of our study, the 4


Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ambient conditions and located about 50 mm behind the second slit and 30 mm in front of the 2.5 m long detector tube. The whole X-ray path of the instrument is evacuated except a length of about 50 mm around the sample. For detection, a water-cooled and vacuum tight Pilatus3 300K detector with 500 Hz readout electronics is used (Dectris AG, Baden, Switzerland). The detector can be moved by a high-precision 3D positioning system inside the detector tube and allows for sample-detectordistances (SDDs) between 100 mm and 2100 mm. Mica sheets of 10 µm thickness are used as beam windows before and behind the sample. A photograph of the instrument with the sample environment is displayed in SI2. For all measurements of the present study, the SDD was set to 480 mm resulting in a Q-range between 0.3 nm-1 and 5.0 nm-1. This scale was calibrated using a silver behenate standard (Rose Chemicals Ltd.) with a d-spacing of 5.838 nm.33 The samples were filled into quartz capillaries with a mean diameter of 1 mm and a wall thickness of 10 µm (Hilgenberg GmbH, Malsfeld, Germany). The glass capillaries were mounted in a temperature controlled capillary holder which was kept at a temperature of 10 °C for the samples aged for 120 min (sample A), 180 min (sample B), 240 min (sample C) and 360 min (sample D), respectively. For in situ measurements the same ansatz was used but for these experiments the temperature was set to the respective temperature between 15 °C and 45 °C. The X-ray beam transmission of the sample was measured for 0.1 s with the Pilatus detector but no beam stop in the beam. The ratio of the integrated primary beam intensities with and without the sample in the beam, respectively, was used to calculate the transmission of the sample. For calibration of the absolute scale of the differential scattering cross section, a glassy carbon standard that was kindly provided by the 15ID-D USAXS beamline at the Advanced Photon Source, Argonne, IL, USA was used.34

morphology of the shell has been investigated in a previous work.28, 31 For the acetate molecules, a SC of ~50 % and a length of 0.47 nm were used for the calculations. The density and the viscosity of the solvent at 10 °C were measured using the densitometer DMA5000 in combination with the viscometer Lovis 2000 (both from Anton Paar, Graz, Austria). The density was 797.93 kg m-3 and the viscosity was 1.445 mPa s. For ZnO the density of the bulk material which is 5610 kg m-3 and for the acetate molecules the density of acetic acid (1050 kg m³) was used. Datasets were evaluated using the c(s)-analysis in Sedfit, Version 14.6e.32 A wavelength of 270 nm was chosen for all evaluations, since higher wavelengths would result in a size dependent extinction coefficient of the semiconductor material. The partial specific volume that is needed for the data evaluation and the correction for diffusional broadening of the sedimentation boundary was set to 0.25 – 0.30 cm3 g-1 depending on the mean particle size of the sample. The effect of the partial specific volume was checked by varying the partial specific volume during data evaluation. No significant influence on the obtained sedimentation coefficient distribution was found. Data were fitted with a resolution of 100 points and a second derivative regularization using a confidence level (F-ratio) of 0.9. Small Angle X-ray Scattering instrument: For the SAXS measurements a unique and highly customized instrument was used which has been developed by SAXSLab (Skovlunde, Denmark) in close cooperation with our group. The microfocus 30 W Genix X-ray source with a low divergence optics (Xenocs, Grenoble) provides a collimated Cu-Kα X-ray beam (λ = 0.154 nm) with a flux of about 3 x 108 photons per second. The beam is collimated by two sets of double slit systems which are adjusted for a beam size at the sample position of 0.4 x 0.4 mm2. The blades of the last slit are made of Si single crystals to minimize slit scattering (‘scatterless slits’). The sample position used for the presented studies is at

Small Angle X-ray Scattering data evaluation: 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Small angle scattering is well-suited to determine size and shape of NPs with nm resolution.35 In contrast to the absorbance properties of ZnO NPs that are related to the semiconducting part of the particle core, the scattering signal I(Q) which is detected by SAXS is directly linked to the distribution of the electron density ρ(r) within the sample:

also show the negligible differences between core-shell and core-only model are presented elsewhere.36 SAXS measurements were performed for samples of the same suspension that were isolated and analyzed after 120 min (sample A), 180 min (sample B), 240 min (sample C) and 360 min (sample D) of ageing at 35 °C. The temperature during the measurement was set to 10 °C to prevent the NPs from further ripening. Each sample was measured for 60 min and during this time 60 detector images were recorded. The 2D images were reduced to 1D profiles by an azimuthal averaging using the program fit2dcorr37 which is a C++-extension program for Fit2D.38 Only small and nonsystematic deviations within the statistical error were observed when comparing the 60 1D profiles of each measurement. This demonstrates that no significant ripening occurred during the measurement. Consequently, the data of each measurement could be averaged to get better statistics. As reference pure ethanol was measured at 10 °C. After transmission correction which was done for all measurements separately, the data of the ethanol measurement was subtracted from the measurement data of the two samples. The analysis of the kinetic in situ measurements was performed similarly, however, the temperature during the measurement was kept constant at the desired temperature and the samples were measured repetitively for 10 minutes. For further data analysis the fit program SASfit39 was used. In the small angle approximation the scattered intensity of particles (eq. 5) can be rewritten as:

12∝34 56 77 − 897; :

where:

= 34=

Page 6 of 19

4 2 sinD = E @ 9

denotes the modulus of the scattering vector 2 = FG − F" . ki and kf are the wave vectors of the incident and scattered waves, respectively, P(u) is the Patterson function and FT denotes the Fourier transform with respect to u. As we assume elastic scattering only, we get HFG H = JK |F" | = with λ denoting the wavelength of the L X-rays. d represents a measure of Q in terms of a length scale in direct space. For a diluted dispersion of compact particles such as ZnO QDs, I(Q) is in good approximation proportional to the intensity weighted scattering of many single particles within the scattering volume. For small Q-values the ZnO NPs can be well approximated by small particles with constant scattering length density. The particles are surrounded by the dispersion medium which can be described by a constant scattering length density for low Q-values. Thus, the presented model is well-suited for the description of the ZnO dispersions studied here when respecting an adequate form factor for the shape and appropriate distribution functions for the size of the ZnO NPs. The scattering contrast of the stabilizer shell with respect to the pure solvent is rather low. Thus, structural information on the stabilizer shell cannot be extracted from SAXS experiments simplifying the chosen model from spherical core-shell particles to spherical particles. Successful SANS experiments which

1> = MJ ∙ (>

∙ 6 N ∙ O ∙ PQJ

∙ RS>, QUJ 9Q V

with M = − being the scattering length contrast between the ZnO NPs ( ) and the solvent (EtOH , ). (> is the structure factor, N is the total number of 6


Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


particles in the scattering volume, O is the particle size distribution function, PQ is the particle volume and S>, Q is the particle form factor. In the case of diluted and thus isolated spherical particles, the structure factor becomes equal to one: (> = 1 . The form factor for spherical particles is given by: =

O J _− ln _ aa 1 ; b O = ∙ ]^ 5 2\ ∙ O 2\ J

Where \ denotes the standard deviation and the mean particle size. For multimodal PSDs each particle size fraction was fitted with a lognormal size distribution.

S>, Q

3'WX>Q − >Q ∙ YZ>Q+ [ >Q.

Data analysis: UV/Vis - results: The UV/Vis absorbance spectra for the sample ripened for 120 min (sample A), 180 min (sample B), 240 min (sample C), and 360 min (sample D) are displayed in Figure 1. Due to the elevated temperature of 35 °C and the comparatively long ageing times, all spectra exhibit broad absorbance bands around 340 nm (3.65 eV). Comparing spectra of samples A and D with each other it becomes clear that a maximum red-shift of about 4 nm (~0.04 eV, analyzed at half peak maximum position) is observed. This indicates that the ripening process is not yet finished and the increase of the mean particle size is still ongoing. The PSD can be extracted from the UV/Vis absorbance spectra by an elaborated data analysis described elsewhere.40-42 Thereby a well-established TBM of Viswanatha et al. for ZnO NPs was used to describe the correlation between size of particles and band gap energy.43 In fact, the difference of the actual band gap of the sample to the bulk band gap is needed, where the first is tested by UV/Vis, while the latter needs to be known. In our analysis we used 3.4 eV. Noteworthy, values in the range from 3.1 eV to 3.44 eV are reported in the literature.44, 45 The resulting PSDs are displayed in Figure 1b. It becomes clear that all distributions after 180 min of ageing are bimodal as a small shoulder at a somehow constant position around 2.8 nm is observed for the smaller size fraction (indicated by a black arrow). Maxima between 3.2 nm and 3.5 nm are monitored for the larger size fraction

For the polydispersity a lognormal distribution was assumed:

Figure 1. a) Absorbance spectra of ZnO NPs after 120 min (sample A, black solid line), 180 min (sample B, blue dashed line), 240 min (sample C, green dashed-dotted line) and 360 min (sample D, red dotted line) of ageing and b) the corresponding PSDs. The black arrow indicates the shoulder in the PSD curve observed for all samples. 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for samples A-D, respectively. This bimodality was already observed in previous studies, however, as ex situ TEM studies did not provide sufficiently well statistics for a proper confirmation it was not investigated further.42 Additionally, during the analysis of the absorbance data, the distribution of smaller sized particles is expected to become less well resolved than the distribution of larger particles. One reason is that the bulk properties used for the size determination become less adequate for smaller QDs for which a more pronounced discretization of the band gap structure is expected (strong quantum confinement). Furthermore, it is hard to detect the signal of the strongly blue-shifted absorption edge of very small particles besides a strong signal caused by the major fraction of larger particles. The combination of absorption spectroscopy with classification in a centrifugal field overcomes these difficulties as will be shown in the next section.

Page 8 of 19

to eqs 1-4 assuming spherical NPs as previously described. The difference in the peak positions of the smallest and the medium size fraction, which is about 1 nm, is similar to the results found by Cölfen et al. on ZnO NPs.46 However, since the absolute positions of all peaks and the distribution width of the different fractions shift with time, magic states as proposed by those authors seem to be unlikely. AUC studies on ZnO NPs obtained using tetramethylammonium hydroxide (TMAOH) at a slightly higher Zn2+:OH- ratio (1:1.6 in contrast to 1:1 as applied with LiOH in this study) as hydroxide source, are reported by Wood et al.47 They confirmed the continuous shift of the PSD towards larger particle sizes but did not find any multimodality. This might be explained either

AUC - results: Since the UV/Vis analysis provided a good estimate of the mean particle size with an additional fraction of small particles, AUC measurements were performed to increase the resolution. The sedimentation coefficient distributions obtained from sedimentation experiments of the samples are displayed in Figure 2a. For all samples highly resolved sedimentation coefficient distributions could be obtained and three distinct main species are clearly recognized after 180 min. It becomes clear that the peak of each species is shifted to increased sedimentation coefficients, whereas the concentration of the larger species (with respect to the main fraction) increases and the concentration of the small species (with respect to the main fraction) reduces with time. The knowledge of the multimodality of the distributions, which could not be sufficiently resolved by UV/Vis beforehand, is of high importance and will be of benefit for the subsequent SAXS analysis. Based on the sedimentation coefficient data the core size distributions were calculated according

Figure 2. Normalized sedimentation coefficient (a) and core particle size (b) distributions of sample A (black solid line), sample B (blue dashed line), sample C (green dashed-dotted line) and sample D (red dotted line). 8


Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


by less resolution achieved during their AUC experiments or by the different chemical structure of the hydroxide source and/or the increased hydroxide concentration (with respect to the Zn2+ concentration). Nevertheless, peak broadening was also observed for longer ageing times, which is consistent to our results.

largest fraction by SAXS is not possible. From Figure 3b and Table 1 in which the results for samples A and D are summarized, it can be extracted that the PSD of sample A consists of three particle populations with the smallest particle size fraction having a mean particle diameter of 2.1 nm incorporating about 11.3% of the volume of ZnO. The second population has a mean particle diameter of 3.5 nm and has the largest contribution to the overall detected amount of ZnO (75.4vol-%). An additional amount of about 13.3 vol-% of the ZnO is incorporated within the particle fraction with the largest size exceeding 5 nm. Noteworthy, when comparing the particle numbers instead of the

SAXS - results: The SAXS patterns of the samples A-D are displayed in Figure 3a. Both, the increase of the intensity at small Q-values and the shift of the intensity at about Q = 2.3 nm-1 (black arrows) towards smaller Q-values indicate slow particle growth over time. For a quantitative determination of the shape and the PSD of the NPs, the scattering curves were analyzed on the basis of different models. Monomodal size distributions of spherical, rodand disc-like shapes like they have already been used for e.g. SiO222 could not reproduce the experimental data sufficiently. In these cases slight deviations of the fit from the measured data could be observed in the low and high Qrange, respectively. Similar results were achieved for monomodal PSDs with pronounced anisometric particle shapes. This strongly indicates that although the PSD of the NPs is expected to be quite narrow, the interpretation of the data by a monomodal size distribution is not possible. Furthermore, the AUC analysis already indicated that the PSDs should consist of (i) an intermediate, medium sized fraction that contains the largest amount of particles in terms of their mass, (ii) an additional, single fraction of smaller particles especially after 180 min of ageing and (iii) a small volume fraction of larger particles. Applying multimodal size distributions to the SAXS data, it turned out that a minimum of three polydisperse particle populations, with the shape of the particles being spherical or only slightly differing from spheres, was necessary to model the SAXS data with satisfying R² (cf. Figure S3). Noteworthy, this could be further confirmed by using a Monte Carlo method (cf. Figure S4).48 In principle, this fits very well to the results obtained by AUC when we consider that a resolution of the multimodality of the

Figure 3. a) Corrected SAXS data of sample A (black solid line), sample B (blue dashed line), sample C (green dashed-dotted line) and sample D (red dotted line). b) PSDs as obtained from the fit for samples A-D. The black arrows indicate general changes of the curve shapes with increasing ageing time. 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

Table 1. Parameters of the trimodal particle populations of the ZnO NPs found by SAXS. Sample A – 120 min @ 35 °C Mean particle diameter / nm σ (cf. eq. 9) /% Relative ZnO volume normalized to sample A / vol-%

Sample D – 360 min @ 35 °C

2.1

3.5

6.6

1.5

3.6

6.9

26

14

16

15

13

20

11.3

75.4

13.3

10.0

73.3

19.5

100

volume, the fraction of the largest particles represents only 1.6% of the total particle number, whereas the smallest population contains more than 40% of all particles. The relative ZnO volume obtained from the analysis of the SAXS pattern of sample D shows a similar distribution. Only the volume fraction

102.8 of large particles is increased from 13.3 to 19.5vol-%. A small shift towards larger particle sizes is seen for the main and the larger fraction with increasing ageing time. The overall detected volume has slightly increased which indicates an ongoing transfer of ZnO from solution to the solid phase. Thus, although SAXS has limitations with respect to the resolution of larger particles, the existence of three main contributions could be clearly identified for all samples. We find a nearly perfect agreement (R² = 0.997) when fitting both datasets with a trimodal PSD that allows us to detect small changes of the dispersion between 120 min (sample A) and 360 min of ageing at 35 °C (sample D). Comparison of the analytical results A direct comparison of the PSDs obtained by the three different methods, exemplarily shown for samples A and D, is illustrated in Figure 4. Regarding sample A, an excellent agreement is found for UV/Vis, SAXS and AUC data. The location of the peak maximum differs by less than 2 Ångström for the three different methods. The volume weighted mean particle sizes of and UV/Vis AUC (x1,3 = 3.62 nm) (x1,3 = 3.76 nm) are also nearly identical. In comparison, SAXS analysis (x1,3 = 4.07 nm) provided a by 0.45 nm larger weight average mean core size. Presumably this is due to the increased width of the distribution and slight overestimation of the largest species of agglomerates caused by the limited resolution in the small Q-regime. The results obtained for sample D also provide a reasonable agreement between the three

Figure 4. Direct comparison of PSDs derived from SAXS (solid lines), AUC (dashed lines) and UV/Vis (dotted lines) for ZnO NPs aged for a) 120 min (sample A) and b) 360 min (sample D) at 35 °C. 10


Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fundamentally different techniques. In comparison to UV/Vis and SAXS, AUC analysis shows a much better resolution of the PSD, which can however only be achieved by dilution of the sample. For AUC and UV/Vis a very good agreement regarding the width of the PSD is obtained. The lower and upper limit of the PSD found by hydrodynamic evaluation of the AUC data was further confirmed by a combined spectrum analysis approach. For this we used a technique presented previously to extract the extinction spectra of the smaller and the main species.49 The extracted spectra were then independently analyzed by the UV/Vis algorithm to obtain the PSDs. Thus, AUC is used to “classify” the sample prior UV-analysis. Even though slight variations are to be expected due to limited resolution of the spectrum extraction for small NPs, the lower (~ 2.9 nm) and upper limit (~ 6 nm) fit well to the results obtained by the hydrodynamic AUC analysis shown in Figure 4b. More information regarding the applied evaluation is available in the supporting information, Figures S5-S7. The difference in the mean core size of AUC (4.42 nm) compared to UV/Vis (4.09 nm) and SAXS (4.42 nm) is in the same range as for sample A. UV/Vis slightly underestimates the mean core size. In contrast, an excellent agreement for the mean core size is found by AUC and SAXS. However, the SAXS results show again a larger width of the distribution. So far, it is not clear why this is the case and it has to be investigated in further studies. Even though the reason for the increased width of the SAXS derived PSD is not yet allegeable for this study, the good consistency of the data analyses demonstrates that the chosen SAXS model is able to describe the shape of the PSDs. In contrast to AUC, no long measurements have to be conducted as acquisition times can be reduced to the order of minutes for a laboratory instrument and no dilution of the sample is needed. This allows in situ analysis of multimodality with high precision in the native ZnO dispersion during long-term ageing.

Evolution of PSD analyzed by SAXS In the next step, studies on the evolution of the larger aggregates and especially the appearance of the – until now unknown – fraction of smallest particles with time were performed. From the data discussed so far, the exact appearance of agglomerates and smallest particles with time could not be deduced. Only SAXS provides the possibility of in situ measurements (not possible by AUC) in combination with adequate description of the smaller particles as well as certain access to the agglomerates (not possible by UV/Vis) in addition to the main fraction. Therefore, in the second part of our work in situ SAXS measurements were performed to monitor the dynamic changes of the evolving dispersity of ZnO NPs between 15°C and 45°C in more detail. The SAXS patterns of our in situ measurements recorded during the ageing of ZnO dispersions are exemplarily shown for 20 °C and 35 °C in Figure 5a and b, respectively. At both temperatures the scattered intensity shifts towards smaller Q-values and increases in intensity with time as it is indicated by black arrows. As mentioned, this already reveals an ongoing increase of the mean particle diameter. A detailed analysis of the SAXS data shows that for the ripening at 20 °C a single particle fraction is sufficient to obtain a very good agreement (R²=0.995) of the fit with the measurement data during the first 240 min after synthesis (fraction #1). After 240 min, an additional second fraction with larger particle sizes has to be used to fit the data (fraction #2, R² = 0.994) and after more than 360 min a third fraction of smaller particles is necessary (fraction #3, R² = 0.996). In addition to this necessity of additional fractions, the particle size of the main fraction #1 continuously increases from about 1.7 nm after 10 min to 3.3 nm after 840 min of ageing. In comparison, when ageing is performed at 35 °C a much faster growth and earlier formation of a multimodal distribution is 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

observed. The first SAXS pattern can be fitted using a monomodal size distribution (fraction #1, R² = 0.994) but already after 40 min a fraction of larger particles (fraction #2, R² = 0.997) can be distinguished from the main fraction and after 100 min a third fraction of small particles arises (fraction #3, R² = 0.996). Again, the particle size of fraction #1 shifts with increasing ripening time continuously from 1.9 nm in the beginning to 4.0 nm after 720 min. For a better understanding of the appearance of the additional fractions during ageing, results of all investigated temperatures between 15 °C and 45 °C were analyzed as summarized in Table 2. Special emphasis was given to the point of appearance of additional fractions #2 and #3, not only with respect to the absolute time but also with respect to the actual size of fraction #1

Page 12 of 19

when those multimodalities appeared. Interestingly, the sample ripened at 15 °C did not show the formation of multimodalities, most probable due to the slow ripening process at 15°C, which does not result in the formation of additional fractions (modal at the end of the experiment after 360 min was 2.65 nm). Besides the continuous shift in particle size of fraction #1, we can clearly detect two further particle fractions for all samples (except for 15 °C) at the end of our observation time by SAXS. Always the agglomerate fraction #2 arises prior to fraction #3, whereas the latter was always found slightly overlapping with the main fraction with smaller mean sizes of about 2 nm. As this fraction was also seen by UV/Vis, the NPs within this fraction seem to have semiconducting properties like the main

Figure 5. Transmission and background corrected SAXS data in dependence of time at a) 20 °C (blue) and b) 35 °C (red), respectively, together with the corresponding PSDs derived from the SAXS data for c) 20 °C (blue) and d) 35 °C (red); the different particle size fractions are indicated with #1, #2 and #3 in the order of their appearance with time as well as highlighted by arrows. 12 ACS Paragon Plus Environment

Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fraction. Thus, in contrast to other PSD studies of ZnO in the literature19 we find beneath larger agglomerates a third particle fraction with smaller particle sizes than the main fraction. Clearly, this cannot be explained by agglomeration. In a very recent study by Abécassis and coworkers a similar evolution of the PSD for CdSe nanoparticles was found.50 After the synthesis the PSD was monodisperse, but during the ripening two particle size fractions could be distinguished using SAXS involving also a portion of smaller particles compared to the main fraction. Already knowing from a previous study51 that the main fraction follows some general rules and can be described by self-similar PSDs, we decided to analyze the particle formation in more detail independent of time and temperature. First of all the volume contributions of the different particle size fractions i were analyzed. They are calculated by the mean volume of the found ZnO NPs (Pc = 4/3Oc. ), where Oc is the mean radius of fraction j, times the particle number of this fraction ( Nc ), divided by the total amount of ZnO in the first measurement (P ,e ): P ,c =

:fgf,h :fgf,i

=

jh ∙k/.hl K :mno,i

jp ∙k/.hl K

= ∑l j s

l i,r ∙k/.i,r K

60 min (appearance of fraction #2, see differently colored background in Figure 6a), the ZnO volume of fraction #1 (blue squares) continuously decreases whereas more and more solid material is found in fraction #2 (brown circles). After 90 min fraction #3 appears (green triangles) leading to a further reduction of ZnO volume in fraction #1. The latter incorporates after its detection about 10% of the total volume of ZnO. To illustrate the size evolution independent of time and temperature we used the modal size of #1 (which is defined as the particle size of the peak maximum) and firstly solely plotted the number of size fractions needed for a successful SAXS fit which is depicted in Figure 6b. Interestingly, it becomes clear that the - at first glance stochastic (cf. Table 2) - appearance of additional fractions can be perfectly calibrated against the size evolution of the particles appearing in fraction #1. Independent of the applied temperatures, all PSDs evolve similar. Thus, independent of the applied temperature, the formation of fraction #2 (agglomerates) occurs as soon as the modal particle size of fraction #1 exceeds about 2.75 ± 0.07 nm. The same holds for fraction #3 (small particles) which is identified for modal sizes of fraction #1 above 3.11 ± 0.16 nm (see differently colored background in Figure 6). This already indicates that the evolution of multimodalities follows some general rules which are independent of the applied time/temperature combination. Noteworthy, the evolution of the medium sized main fraction at the same synthesis conditions was already analyzed by UV/Vis very well and successfully modeled by population balance equations (PBEs) assuming Ostwald ripening.51

(10)

The resulting volume fraction analysis is exemplarily illustrated for 35 °C in Figure 6a (for 20 °C see Figure S8). It becomes clear that the total volume of ZnO in the solid phase (black diamonds) stays almost constant during the ageing process. However, a pronounced reorganization of solid material within the different fractions is observed. As soon as the first agglomerates are detected by SAXS after

Table 2. Occurrence of different particle fractions as observed by SAXS for different temperatures. Ageing temperature / °C First occurrence of #2 / min Size of #1 / nm First occurrence of #3 / min Size of #1 / nm Size of #1 after 360 min / nm

15 2.65

20 240 2.78 360 3.05 3.05

30 50 2.75 130 3.03 3.25

13 ACS Paragon Plus Environment

35 40 2.68 100 3.14 3.48

40 30 2.82 100 3.27 3.92

45 20 2.78 30 3.05 4.02


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Moreover, the numeric studies revealed that the PSD converges to a self-preserving stable shape. The present results imply that additional mechanisms take place which finally lead to the – small but existing – fractions of discrete agglomerates and smaller particles. From the SAXS data we cannot distinguish if this fraction is newly formed during the ageing process or if it exists from the beginning due to the fact that the sizes of fractions #1 and #3 are very similar when appearance of small particles is noted.

Page 14 of 19

Unfortunately, due to the pronounced ripening of smaller ZnO particles, AUC studies of smaller particles than those already discussed herein are not possible for the time being. In future, those could shed light on the mechanism and help to unambiguously distinguish between serial nucleation and growth steps, ripening and agglomeration. The reason for the large-particle fraction #2 can be already explained by agglomeration whereas the mechanism for the formation of the smallparticle fraction #3 must be clarified in further studies. Our results clearly demonstrate the strength and necessity of combined approaches like UV/Vis, AUC and SAXS for careful PSD analysis and open many new questions whose answers may be found by sophisticated in situ studies. A careful analysis of PSDs provides detailed information on the ageing process of ZnO QDs which can be measured non-destructively and without changing the sample environment. Our experiments unambiguously reveal a multimodality which was for a long time rather seldom addressed in the literature, where most often only the mean particle size of a colloidal solution is tested. However, our findings are supported by reports in literature that claim more complicated growth mechanisms than simple Ostwald ripening.11, 12, 53 We believe that the consideration of multimodalities will become more and more important in the near future as they are mandatory for understanding any kind of particle formation mechanism. Hence, concepts for the analyzation of PSDs below 10 nm with Ångström resolution are strongly needed.

Figure 6. a) Scheme of the evolution of the ZnO volume within the different particle fractions for ageing at 35°C. b) Scheme of the size evolution independent of the chosen time and temperature conditions. The number of fractions for successful SAXS fits is plotted against the size evolution of the mode (particle size of peak maximum) of fraction #1.

Conclusion We demonstrated that a combined approach of three different techniques, namely UV/Vis absorbance spectroscopy (UV/Vis), analytical ultracentrifugation (AUC) and small angle X-ray scattering (SAXS) has to be used to precisely determine the underlying particle size distributions (PSDs) of ZnO quantum dots (QDs). UV/Vis has a low resolution when

Thus, multi-step mechanisms as they are described in the literature in which the formation and growth of ZnO occurs in different ripening regimes nicely fits to our results.16-18, 52 14


Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the near future. Thus, complementary strategies like ours for precise PSD analysis need to be developed.

approaching the bulk band gap. The method is restricted to the semiconducting particle core but the particle size range can be positioned precisely (in addition to the high availability in every lab and the possibility of fast in situ measurements). For AUC the translation of sedimentation coefficients to particle diameters and the missing possibility to measure in situ to perform kinetic studies are drawbacks, but resolution and accuracy of this experimental technique are outstanding. However, due to comparably long measurement times, highly time-resolved kinetic studies are not possible. SAXS has the challenge of finding an adequate model with a limited number of degrees of freedom (concentration, size, shape). However, having once established a carefully crossvalidated model, detailed information about particle shape, size and polydispersity can be extracted in situ. The application of all three techniques to the same samples originating from the same synthesis batch was overcoming the drawbacks of the single techniques and combining their advantages to achieve reliable PSDs. With this information we could extract a profound and well-validated model for our SAXS analysis that was used to monitor the evolution of the ZnO PSD aged at temperatures between 15 °C and 45 °C in situ. Our study revealed that for all temperatures the PSD after the synthesis is monomodal. However, with ongoing ageing additional particle populations are observed which are not forming arbitrarily but their appearance and evolution seems to follow fixed rules. The formation of additional particle fractions can be detected for the different temperatures at nearly identical modal particle sizes of the main fraction. Although an unambiguous explanation of the exact superimposed mechanisms was not accessible from the existing dataset, it can already be stated that the evolution of the agglomerates and the smaller sized fraction occurs always at a certain time-temperature combination. We believe that the consideration of multimodalities is not only an important finding for ZnO NPs but will become relevant for other material systems in

Acknowledgements: The authors would like to acknowledge the funding of the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence Engineering of Advanced Materials, projects PE 427/28-1 and 25-2 and within the GRK 1896. Supporting Information Available: Detailed comparison of the analytical methods, photograph of the SAXS instrument, SAXS PSD with single fractions, PSD of a Monte Carlo SAXS analysis, further information for the AUC analysis and evolution of ZnO volume at 20°C. This material is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interest. Literature: 1. Spanhel, L., Colloidal ZnO Nanostructures and Functional Coatings: A Survey. J Sol-Gel Sci Technol 2006, 39, 7-24. 2. Qian, L.; Zheng, Y.; Xue, J. G.; Holloway, P. H., Stable and Efficient Quantum-dot Light-Emitting Diodes Based on Solution-Processed Multilayer Structures. Nat. Photonics 2011, 5, 543-548. 3. Liu, C. H.; Zapien, J. A.; Yao, Y.; Meng, X. M.; Lee, C. S.; Fan, S. S.; Lifshitz, Y.; Lee, S. T., HighDensity, Ordered Ultraviolet Light-Emitting ZnO Nanowire Arrays. Adv. Mater. 2003, 15, 838-841. 4. Hirschmann, J.; Faber, H.; Halik, M., Concept of a Thin Film Memory Transistor Based on ZnO Nanoparticles Insulated by a Ligand Shell. Nanoscale 2012, 4, 444-7. 5. Pillai, S. C.; Kelly, J. M.; McCormack, D. E.; Ramesh, R., Self-assembled Arrays of ZnO Nanoparticles and Their Application as Varistor Materials. J. Mater. Chem. 2004, 14, 1572-1578.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6. Ogawa, M. F.; Natsume, Y.; Hirayama, T.; Sakata, H., Preparation and Electrical-Properties of Undoped Zinc-Oxide Films by Cvd. J. Mater. Sci. Lett. 1990, 9, 1351-1353. 7. Narasimhan, K. L.; Pai, S. P.; Palkar, V. R.; Pinto, R., High Quality Zinc Oxide Films by Pulsed Laser Ablation. Thin Solid Films 1997, 295, 104-106. 8. Spanhel, L.; Anderson, M. A., Semiconductor Clusters in the Sol-Gel Process: Quantized Aggregation, Gelation, and Crystal Growth in Concentrated Zinc Oxide Colloids. J. Am. Chem. Soc. 1991, 113, 2826-2833. 9. Oskam, G.; Hu, Z. S.; Penn, R. L.; Pesika, N.; Searson, P. C., Coarsening of Metal Oxide Nanoparticles. Phys. Rev. E 2002, 66, 011403. 10. Meulenkamp, E. A., Synthesis and Growth of ZnO Nanoparticles. J. Phys. Chem. B 1998, 102, 5566-5572. 11. Santra, P. K.; Mukherjee, S.; Sarma, D. D., Growth Kinetics of ZnO Nanocrystals in the Presence of a Base: Effect of the Size of the Alkali Cation. J. Phys. Chem. C 2010, 114, 22113-22118. 12. Viswanatha, R.; Santra, P. K.; Dasgupta, C.; Sarma, D. D., Growth Mechanism of Nanocrystals in Solution: ZnO, a Case Study. Phys. Rev. Lett. 2007, 98, 255501. 13. Guo, L.; Yang, S. H.; Yang, C. L.; Yu, P.; Wang, J. N.; Ge, W. K.; Wong, G. K. L., Highly Monodisperse Polymer-Capped ZnO Nanoparticles: Preparation and Optical Properties. Applied Physics Letters 2000, 76, 2901-2903. 14. Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y.; Neumark, G. F.; O'Brien, S., Zinc Oxide Quantum Rods. J. Am. Chem. Soc. 2004, 126, 62067. 15. Leidinger, P.; Dingenouts, N.; Popescu, R.; Gerthsen, D.; Feldmann, C., ZnO Nanocontainers: Structural Study and Controlled Release. J. Mater. Chem. 2012, 22, 14551-14558. 16. Vega-Poot, A. G.; Rodriguez-Gattorno, G.; Soberanis-Dominguez, O. E.; Patino-Diaz, R. T.; Espinosa-Pesqueira, M.; Oskam, G., The Nucleation Kinetics of ZnO Nanoparticles from ZnCl2 in Ethanol Solutions. Nanoscale 2010, 2, 2710-2717. 17. Layek, A.; Mishra, G.; Sharma, A.; Spasova, M.; Dhar, S.; Chowdhury, A.; Bandyopadhyaya, R., A Generalized Three-Stage Mechanism of ZnO Nanoparticle Formation in Homogeneous Liquid Medium. J. Phys. Chem. C 2012, 116, 24757-24769.

Page 16 of 19

18. Liang, M. K.; Limo, M. J.; Sola-Rabada, A.; Roe, M. J.; Perry, C. C., New Insights into the Mechanism of ZnO Formation from Aqueous Solutions of Zinc Acetate and Zinc Nitrate. Chem. Mater. 2014, 26, 4119-4129. 19. Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V.; Craievich, A. F., SAXS Study of the Kinetics of Formation of ZnO Colloidal Suspensions. J. NonCryst. Solids. 1999, 247, 176-182. 20. Abecassis, B.; Testard, F.; Spalla, O.; Barboux, P., Probing In Situ the Nucleation and Growth of Gold Nanoparticles by Small-angle X-ray Scattering. Nano Lett. 2007, 7, 1723-7. 21. Harada, M.; Katagiri, E., Mechanism of Silver Particle Formation During Photoreduction Using in Situ Time-Resolved SAXS Analysis. Langmuir 2010, 26, 17896-905. 22. Fouilloux, S.; Tache, O.; Spalla, O.; Thill, A., Nucleation of Silica Nanoparticles Measured in Situ During Controlled Supersaturation Increase. Restructuring Toward a Monodisperse Nonspherical Shape. Langmuir 2011, 27, 12304-11. 23. Meneau, F.; Cristol, S.; Sankar, G.; Dolbnya, I. P.; Bras, W.; Catlow, C. R. A.; Thomas, J. M.; Greaves, G. N., In Situ Study of the Formation of CdS Nanoparticles by Small-Angle X-ray Scattering. J. Appl. Crystallogr. 2003, 36, 718-721. 24. Koupanou, E.; Ahualli, S.; Glatter, O.; Delgado, A.; Krumeich, F.; Leontidis, E., Stabilization of Lead Sulfide Nanoparticles by Polyamines in Aqueous Solutions. A Structural Study of the Dispersions. Langmuir 2010, 26, 16909-20. 25. Lees, E. E.; Gunzburg, M. J.; Nguyen, T. L.; Howlett, G. J.; Rothacker, J.; Nice, E. C.; Clayton, A. H.; Mulvaney, P., Experimental Determination of Quantum Dot Size Distributions, Ligand Packing Densities, and Bioconjugation Using Analytical Ultracentrifugation. Nano Lett. 2008, 8, 2883-90. 26. Nontapot, K.; Rastogi, V.; Fagan, J. A.; Reipa, V., Size and Density Measurement of Core-shell Si Nanoparticles by Analytical Ultracentrifugation. Nanotechnology 2013, 24, 155701. 27. Walter, J.; Löhr, K.; Karabudak, E.; Reis, W.; Mikhael, J.; Peukert, W.; Wohlleben, W.; Cölfen, H., Multidimensional Analysis of Nanoparticles with Highly Disperse Properties Using Multiwavelength Analytical Ultracentrifugation. ACS Nano 2014, 8, 8871-8886.


Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28. Marczak, R.; Segets, D.; Voigt, M.; Peukert, W., Optimum Between Purification and Colloidal Stability of ZnO Nanoparticles. Adv. Powder Technol. 2010, 21, 41-49. 29. Svedberg, T.; Rinde, H., The Determination of the Distribution of Size of Particles in Disperse Systems. J. Am. Chem. Soc. 1923, 45, 943-954. 30. Biswas, K.; Varghese, N.; Rao, C. N. R., Growth Kinetics of Nanocrystals and Nanorods by Employing Small-angle X-ray Scattering (SAXS) and Other Techniques. J. Mater. Sci. Technol. 2008, 24, 615-627. 31. Segets, D.; Marczak, R.; Schafer, S.; Paula, C.; Gnichwitz, J. F.; Hirsch, A.; Peukert, W., Experimental and Theoretical Studies of the Colloidal Stability of Nanoparticles - a General Interpretation Based on Stability Maps. ACS Nano 2011, 5, 4658-69. 32. Schuck, P., Diffusion-deconvoluted Sedimentation Coefficient Distribution for the Analysis of Interacting and Non-interacting Protein Mixtures. In Analytical Ultracentrifugation. Techniques and Methods, Scott, D. J.; Harding, S. E.; Rowe, A. J., Eds. The Royal Society of Chemistry: Cambridge, 2005; pp 26-49. 33. Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y., X-Ray-Powder Diffraction Analysis of Silver Behenate, a Possible Low-Angle Diffraction Standard. J. Appl. Crystallogr. 1993, 26, 180-184. 34. Zhang, F.; Ilavsky, J.; Long, G. G.; Quintana, J. P. G.; Allen, A. J.; Jemian, P. R., Glassy Carbon as an Absolute Intensity Calibration Standard for SmallAngle Scattering. Metall. Mater. Trans. A 2010, 41A, 1151-1158. 35. Unruh, T., Interpretation of Small-Angle Xray Scattering Patterns of Crystalline Triglyceride Nanoparticles in Dispersion. J. Appl. Crystallogr. 2007, 40, 1008-1018. 36. Schindler, T.; Schmiele, M.; Schmutzler, T.; Kassar, T.; Segets, D.; Peukert, W.; Radulescu, A.; Kriele, A.; Gilles, R.; Unruh, T., A Combined SAXS/SANS Study for the in Situ Characterization of Ligand Shells on Small Nanoparticles: The Case of ZnO. Langmuir 2015, 31, 10130-6. 37. https://sourceforge.net/projects/fit2dcorr/. 38. Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D., TwoDimensional Detector Software: From Real Detector

to Idealised Image or Two-Theta Scan. High Pressure Res. 1996, 14, 235-248. http://sourceforge.net/projects/sasfit/. 39. 40. Segets, D.; Komada, S.; Butz, B.; Spiecker, E.; Mori, Y.; Peukert, W., Quantitative Evaluation of Size Selective Precipitation of Mn-doped ZnS Quantum Dots by Size Distributions Calculated from UV/Vis Absorbance Spectra. J. Nanopart. Res. 2013, 15, 1-13. 41. Segets, D.; Lucas, J. M.; Klupp Taylor, R. N.; Scheele, M.; Zheng, H.; Alivisatos, A. P.; Peukert, W., Determination of the Quantum Dot Band Gap Dependence on Particle Size from Optical Absorbance and Transmission Electron Microscopy Measurements. ACS Nano 2012, 6, 9021-32. 42. Segets, D.; Tomalino, L. M.; Gradl, J.; Peukert, W., Real-Time Monitoring of the Nucleation and Growth of ZnO Nanoparticles Using an Optical Hyper-Rayleigh Scattering Method. J. Phys. Chem. C 2009, 113, 11995-12001. 43. Viswanatha, R.; Sapra, S.; Satpati, B.; Satyam, P. V.; Dev, B. N.; Sarma, D. D., Understanding the Quantum Size Effects in ZnO Nanocrystals. J. Mater. Chem. 2004, 14, 661-668. 44. Srikant, V.; Clarke, D. R., On the Optical Band Gap of Zinc Oxide. J. Appl. Phys. 1998, 83, 5447-5451. 45. Coleman, V. A.; Jagadish, C., Chapter 1 Basic Properties and Applications of ZnO. In Zinc Oxide Bulk, Thin Films and Nanostructures, Jagadish, C.; Pearton, S., Eds. Elsevier Science Ltd: Oxford, 2006; pp 1-20. 46. Cölfen, H.; Pauck, T., Determination of Particle Size Distributions with Angström Resolution. Colloid Polym. Sci. 1997, 275, 175-180. 47. Wood, A.; Giersig, M.; Hilgendorff, M.; VilasCampos, A.; Liz-Marzan, L. M.; Mulvaney, P., Size Effects in ZnO: The Cluster to Quantum Dot Transition. Aust. J. Chem. 2003, 56, 1051-1057. 48. Pauw, B. R.; Pedersen, J. S.; Tardif, S.; Takata, M.; Iversen, B. B., Improvements and Considerations for Size Distribution Retrieval from Small-angle Scattering Data by Monte Carlo Methods. J. Appl. Crystallogr. 2013, 46, 365-371. 49. Walter, J.; Sherwood, P. J.; Lin, W.; Segets, D.; Stafford, W. F.; Peukert, W., Simultaneous Analysis of Hydrodynamic and Optical Properties Using Analytical Ultracentrifugation Equipped with



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Multiwavelength Detection. Anal. Chem. 2015, 87, 3396-403. 50. Abecassis, B.; Bouet, C.; Garnero, C.; Constantin, D.; Lequeux, N.; Ithurria, S.; Dubertret, B.; Pauw, B. R.; Pontoni, D., Real-Time In Situ Probing of High-temperature Quantum Dots Solution Synthesis. Nano Lett. 2015, 15, 2620-6. 51. Haderlein, M.; Segets, D.; Groschel, M.; Pflug, L.; Leugering, G.; Peukert, W., FIMOR: An Efficient Simulation for ZnO Quantum Dot Ripening Applied to the Optimization of Nanoparticle Synthesis. Chem. Eng. J. 2015, 260, 706-715. 52. Caetano, B. L.; Santilli, C. V.; Meneau, F.; Briois, V.; Pulcinelli, S. H., In Situ and Simultaneous UV-vis/SAXS and UV-vis/XAFS Time-Resolved Monitoring of ZnO Quantum Dots Formation and Growth. J Phys Chem C 2011, 115, 4404-4412. 53. Viswanatha, R.; Amenitsch, H.; Sarma, D. D., Growth Kinetics of ZnO nanocrystals: A few Surprises. J. Am. Chem. Soc. 2007, 129, 4470-4475.


Page 18 of 19

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents:


In Situ Study on the Evolution of Multimodal ... - ACS Publications

Recommend Documents