SANS Study for the in Situ Characterization of

Article pubs.acs.org/Langmuir

A Combined SAXS/SANS Study for the in Situ Characterization of Ligand Shells on Small Nanoparticles: The Case of ZnO T. Schindler,† M. Schmiele,† T. Schmutzler,† T. Kassar,† D. Segets,‡ W. Peukert,‡ A. Radulescu,§ A. Kriele,∥ R. Gilles,⊥ and T. Unruh*

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 10, 2015 | http://pubs.acs.org Publication Date (Web): September 10, 2015 | doi: 10.1021/acs.langmuir.5b02198

†

Chair of Crystallography and Structural Physics, Friedrich-Alexander-Universät Erlangen-Nürnberg, Staudtstraße 3, 91058 Erlangen, Germany ‡ Institute of Particle Technology, Friedrich-Alexander-Universät Erlangen-Nürnberg, Cauerstraße 4, 91058 Erlangen, Germany § Forschungszentrum Jülich GmbH, Jülich Centre for Neutron Science (JCNS), Outstation at MLZ, 85747 Garching, Germany ∥ Helmholtz Zentrum Geesthacht, Max-Plank-Straße 1, 21502 Geesthacht, Germany ⊥ Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, 85747 Garching, Germany S Supporting Information *

ABSTRACT: ZnO nanoparticles (NPs) have great potential for their use in, e.g., thin film solar cells due to their electro-optical properties adjustable on the nanoscale. Therefore, the production of well-defined NPs is of major interest. For a targeted production process, the knowledge of the stabilization layer of the NPs during and after their formation is of particular importance. For the study of the stabilizer layer of ZnO NPs prepared in a wet chemical synthesis from zinc acetate, only ex situ studies have been performed so far. An acetate layer bound to the surface of the dried NPs was found; however, an in situ study which addresses the stabilizing layer surrounding the NPs in a native dispersion was missing. By the combination of small angle scattering with neutrons and X-rays (SANS and SAXS) for the same sample, we are now able to observe the acetate shell in situ for the first time. In addition, the changes of this shell could be followed during the ripening process for different temperatures. With increasing size of the ZnO core (dcore) the surrounding shell (dshell) becomes larger, and the acetate concentration within the shell is reduced. For all samples, the shell thickness was found to be larger than the maximum extension of an acetate molecule with acetate concentrations within the shell below 50 vol %. Thus, there is not a monolayer of acetate molecules that covers the NPs but rather a swollen shell of acetate ions. This shell is assumed to hinder the growth of the NPs to larger macrostructures. In addition, we found that the partition coefficient μ between acetate in the shell surrounding the NPs and the total amount of acetate in the solution is about 10% which is in good agreement with ex situ data determined by thermogravimetric analysis.

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INTRODUCTION ZnO semiconductor nanoparticles (NPs) are of high interest for a variety of applications due to their promising electrooptical properties which can be adjusted at the nanoscale due to the quantum size effect.1−5 This makes them highly potential candidates for integration into technological devices such as electronic circuits and thin film solar cells.1−7 For this purpose, stable and well-defined NPs need to be prepared. In this context, the sol−gel process is a versatile approach as the NPs can easily be formed and their size and shape can be controlled by applying different synthetic conditions.8−17 Knowledge about oxidic interfaces is limited, although Lin et al. recently published a detailed study of ligand exchange reactions.16 However, the structure of a ligand shell around a colloidal surface is not yet understood. A standard routine for the preparation of ZnO NPs in solution was introduced by Spanhel and Meulenkamp.18,19 This synthesis implies the formation of a Zn4O6+·6Ac− precursor molecule in ethanolic solution, where the central oxygen is © XXXX American Chemical Society

surrounded tetragonally by four zinc atoms, stabilized by six acetate units on the edges of the tetragonal pyramid.11 The formation of ZnO occurs rapidly upon the addition of an ethanolic alkaline solution to this precursor solution. During a temperature-dependent ripening process, the formed NPs grow in size, but stay in the size regime of a few nanometers.10 The reason for this limited growth might be due to the stabilization of the ZnO NP by the acetate anions in the solution, which can bind to the NP surface. Sakohara et al. analyzed dried ZnO NP powder by Fourier transform infrared spectroscopy (FTIR) and could identify surface-bound acetate on the ZnO NP due to a shift of the C−O stretch vibration of the acetate.20 Further studies using photoluminescence and (micro-) Raman spectroscopy found similar results for dried NPs in which the acetate is bound to the NP surface.21 From Received: June 15, 2015 Revised: August 25, 2015

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DOI: 10.1021/acs.langmuir.5b02198 Langmuir XXXX, XXX, XXX−XXX

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using zinc acetate dihydrate, the anhydrate was used, and the respective amount of D2O was added in order to minimize the background in the SANS measurements due to the incoherent scattering of H2O. A standard sample was prepared by adding 97.75 mg zinc acetate (ZnAc2, 0.5 mmol) to 5 mL of EtOD and additional 18 μL of D2O (1 mmol). The dispersion was refluxed at 80 °C for 3 h, which leads to a clear precursor solution. Meanwhile, 5 mL of an ethanolic lithium hydroxide solution (LiOH, 0.012 g, 0.5 mmol) was prepared. For contrast matched samples, the volumetric ratio of EtOD to EtOH used for the LiOH solution was 1.84:1 so that the matching ratio of the sample after mixing with the fully deuterated precursor solution was 3.67:1 (for details, see Table 1). At this ratio, the SLD of

the weight loss of dried NPs as observed by thermogravimetric analysis (TGA), Marczak et al. calculated the average number of acetate molecules bound to a single 5 nm sized ZnO NP to be about 700 (∼9 molecules/nm2).22 However, all these studies were conducted on dried ZnO NPs; an in situ study is still missing. Standard in situ techniques for the characterization of NPs like absorption spectroscopy (UV/vis) and small-angle X-ray scattering (SAXS) are sensitive to the ZnO core only, as the organic stabilizing shell surrounding the NPs is hardly probed by these techniques. Photon correlation spectroscopy (PCS) can detect the hydrodynamic radius of particles including their shell in solution, however, the different influences of shell and core can only be discriminated for known core sizes in model systems with large stabilizer molecules as it has been demonstrated recently.23 In contrast to these methods, small angle neutron scattering (SANS) probes the organic stabilizer in the dispersion due to the high sensitivity of neutrons to hydrogen. To reduce incoherent scattering and to enlarge the contrast of the protonated organic moieties, it is convenient to use deuterated solvents. The ZnO core, which dominates the scattering signal using small-angle X-ray scattering (SAXS) has only an intermediate scattering length density (SLD) for neutrons, which is in between the SLDs of the protonated and the deuterated solvent. By using the correct mixture of deuterated and protonated ethanol, the solvent-SLD can be matched to that of the ZnO core. In this case, no coherent small angle scattering signal of the NP core will be detected, and the main SANS signal will arise from the distribution of the acetate molecules in the solution. This technique using differential scatter of hydrogen versus deuterium is called contrast matching, which provides a powerful tool in SANS. Thus, in contrast to X-ray scattering, heterogeneous distributions of organic moieties can nicely be resolved by SANS which allows one to probe the stabilizing acetate layer. Thus, the combination of SANS and SAXS is well-suited to detect small organic moieties within the interface between ZnO NPs and the continuous phase of a native dispersion in situ during ripening. A reliable model for the analysis of SAXS data could be established by a detailed study combining SAXS, analytical ultracentrifugation (AUC) and UV/vis spectroscopy, which will be published elsewhere.24 In this work, a combined SAXS/SANS study allows for the first time determining the size distribution of the cores of the NPs and the distribution of the stabilizer molecules simultaneously in the native solution. A similar approach has already been tested very successfully for suspensions of organic nanocrystals stabilized by phospholipids in the presence of costabilizers such as sodium glycocholate.25−29 To transfer this approach to semiconducting NPs, a sample of ZnO NPs prepared in a standard synthesis produced large enough quantaties to be divided in two parts and measured simultaneously by SANS and SAXS.

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Table 1. Overview of the Neutron Scattering Length Densities of the Used Materials and Solutions substance/solution EtOD EtOH ZnO Ac LiOH precursor-solution LiOH-solution (matched) LiOH-solution (deuterated) ZnO-solution (matched) ZnO-solution (deuterated)

density/(g/ cm3)

scattering length density/1010 cm−2

0.9 0.79 5.61 1.05 1.46 0.9 0.86 0.9

6.15 −0.34 4.77 1.47 0.06 6.11 3.34 6.05

0.88 0.9

4.73 6.08

the solvent equals the SLD of the ZnO core, and thus the contrast of the core vanishes (contrast matching). For the nonmatched samples, LiOH was dispersed in pure EtOD. After cooling the precursor solution to room temperature, it was rapidly mixed with the respective LiOH solution to form the ZnO NPs. The nanosuspension was divided in two parts and stored at 20 and 50 °C, respectively. For the measurements, small samples were taken from the suspension directly after mixing (sample A), after storage at 20 °C, and storing times of 3 h (sample B) and 24 h (sample C), while the samples from the suspensions stored at 50 °C were taken after 1 h (sample D) and 3 h (sample E). The samples were measured by SAXS and SANS at a temperature of 10 °C to prevent further ripening during the measurements. SANS Instrument. The SANS measurements were performed at the KWS-2 instrument at the Jülich Center for Neutron Science (JCNS) at the Heinz Maier-Leibnitz Zentrum (MLZ) at the FRM II in Garching, Germany.30 The beam dimensions were set to 8 × 8 mm2 at the sample position, and the detector was placed at a sample detector distance (SDD) of 1.77 m with a collimation length of 8 m. The velocity selector provided a wavelength of λ = 4.967 Å with a spread of Δλ/λ = 20%. By this setting, a Q-range between 0.3 nm−1 and 3 nm−1 could be accessed which was chosen to overlap with the Q-range of the SAXS measurements. The samples were filled into Hellma QX 404 quartz cuvettes (Hellma GmbH, Müllheim, Germany) with a sample thickness of 1 mm and placed in a multiposition copper sample holder provided at the instrument. The temperature was controlled by an external water bath and set to 10 °C for all measurements. To prevent the condensation of water on the surface of the cuvettes a constant flow of dried air was directed to the cells. Each sample was measured for 1 h to achieve reasonable statistics. For data reduction the QTIKWS program was used.31 The recorded data were corrected for background in the neutron guide hall with a boron carbide sample, for transmission using a measurement at a detector distance and a collimation length of 8 m each and for detector sensitivity determined by a measurement of polymethylmethacrylat (Plexiglas), which also allowed for the calibration to the absolute scale of the differential scattering cross section. For all samples, the respective solvent (deuterated or mixture of deuterated and

MATERIALS AND METHODS

Particle Synthesis. Anhydrous zinc acetate (ZnAc2, 99.9%, VWR Germany), lithium hydroxide (LiOH, 98%, VWR Germany), deuterated water (D2O, 99.9%, Eurisotop), absolute ethanol (EtOH, 99.98%, VWR Germany), and deuterated ethanol (d6-EtOH (EtOD), 99%, Eurisotop) were used without any further purification. The ZnO NPs were prepared according to a modified route based on the routine developed by Spanhel and Meulenkamp.17−19 However, instead of B


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(PSD) function, V (R) the particle volume and F(Q,R) the particle form factor. For diluted and thus isolated spherical particles, the structure factor becomes equal to one: S(Q) = 1. The form factor for spherical particles is given by38

protonated ethanol) was measured as reference and subtracted from the corrected data. SAXS Instrument and Data Reduction. SAXS images were collected using a Kratky-type SAXS instrument (S3-MICROpix, Hecus XRay Systems GmbH, Graz, Austria) provided by the Material Science lab (operated in collaboration with TU München and Helmholtz Zentrum Geesthacht at MLZ). The instrument is equipped with a 50 W X-ray generator and a microfocus X-ray tube with a Cu target. A FOX 3D multilayer mirror optics (Xenocs, Sassenage, France) extracts the Cu Kα1,2 radiation with an average wavelength of about 1.5418 Å and focuses the primary beam onto the detector. The Kratky block collimator (equipped with 200 and 1000 μm slits in the vertical and horizontal directions, respectively) provides a beam size of about 0.2 × 0.25 mm2 at the sample position and a flux of about 107 photons per second. The beam path in the camera housing was completely evacuated (∼2 mbar) to reduce air scattering. 2D SAXS patterns were recorded with a Pilatus 100 K detector (Dectris AG, Baden, Switzerland). The suspensions were contained in a home-built capillary sample holder. The quartz capillary (Hilgenberg GmbH, Malsfeld, Germany) had a mean diameter of 1 mm and a wall thickness of 10 μm according to the manufacturer’s specification. The samples and the ethanolic background were measured in the same capillary, which allows for an accurate background subtraction. The temperature of the sample stage was set to 10 °C for all measurements to prevent the NPs from further ripening. Each sample was measured for 1 h similar to the SANS measurements. The sample−detector distance was 289.5 mm, and the 2θ scale of the camera was calibrated using a silver behenate standard (Eastman Kodak Co.).32 The accessible Q-range was between 0.5 nm−1 and 5 nm−1. For data reduction the program FIT2DCORR33 was used which is a C++-extension program for FIT2D34 using LIBTIFF.35 Transmission of the sample was measured for 0.1 s with the Pilatus detector and the tungsten beamstop being removed. The ratio of the integrated primary beam intensity with and without the sample in the beam was used to calculate the transmission. Calibration of the scattering intensity on an absolute scale was done using a glassy carbon sample,36 kindly provided by the 15ID-D USAXS beamline at the Advanced Photon Source, Argonne, IL, USA. Using the calibration factor obtained from the glassy carbon measurement, the transmission for each sample and the thickness of the capillary, the scattering curves were put on an absolute scale37 and the scattering pattern of the ethanol background was subsequently subtracted. Analysis of SAXS and SANS data. While X-rays are predominantly scattered by the electrons of a sample, neutrons are mainly scattered by the nuclei. In both cases, the scattered intensity I(Q) can be expressed in terms of the scattering length density ρ(r) of the respective materials for X-rays and neutrons, respectively:

∫V ρ(r)ρ(r − u)d r) = FT(P(u))

I(Q) = FT(

Q=

4π 2π sin(θ ) = λ d

F(Q , R ) =

I(Q , R , ΔR , Δρ1 , Δρ2 ) = [F(Q , (R + ΔR ))·Δρ2 ·V (R + ΔR ) − F(Q , R )· (Δρ2 − Δρ1)· V (R )] 2

(5)

with Δρ1/2 = ρcore/shell − ρsol being the SLD contrast of the core and the shell to the dispersion medium, respectively. The polydispersity of the particle radius was respected assuming a log-normal distribution of the form39

f (R ) =

⎛ ⎛ ⎞2 ⎞ R ⎟ ⎟ ⎜ ⎜ln r ⎝ mean ⎠ 1 ⎟ · exp⎜− ⎜ ⎟ 2π σR 2σ 2 ⎜ ⎟ ⎝ ⎠

( )

(6)

with the mean particle size rmean and its standard deviation σ. The particle concentration of the dispersions studied here is about 1015 particles per milliliters, resulting in a mean average distance of the particles of about 13 nm. As our data analysis does not give evidence for a significant contribution of a structure factor to the scattering data, the structure factor of the particle dispersions was regarded to be essentially equal to unity in the whole examined Q-range. This result was also found in a careful examination of the PSD of ZnO for the combination of SAXS with analytical ultracentrifugation (AUC) and UVVis spectroscopy.24 Furthermore, ZnO NPs can be regarded as small particles with constant SLD in the low Q-range studied here, which also holds for the dispersion medium.22,40 In the following, the SLD of the stabilizing shell around the particles is assumed to be constant and originates from a mixture of ethanol and acetate anions, which allows one to distinguish an acetate monolayer, a diffuse acetate shell around a particle, and no acetate shell at all. This shell describes the native stabilizing layer of the NPs and how far it extends into the dispersion medium. In contrast, shells observed by, e.g., PCS and AUC are mainly determined by hydrodynamic effects.41 For the data analysis, a trimodal PSD was assumed according to recent findings for corresponding nanoparticle dispersions as presented elsewhere.24 It was shown by the use of three different experimental techniques that beneath the main fraction of medium sized particles, a small fraction of larger particles and a small fraction of smaller NPs exist. As the influence of the two further fractions is small, similar shell parameters were used for all three fractions. The scattering pattern is dominated by the medium-sized fraction, and thus the shell parameters are also dominated by the medium-sized fraction and were evaluated with respect to the medium-sized fraction. SAXS and SANS Data Analysis. The structural model for the ZnO nanosuspensions described above was fitted simultaneously to the reduced SAXS and SANS data using the program SASfit.39 The data could be reproduced perfectly by the fits within the error limits caused by limited counting statistics. For each sample with its combined SAXS/SANS data sets, the following fit parameters could be deduced: the total number of particles in the dispersion N, the mean particle radius rmean, the standard deviation for the dominant particle size fraction σ, and the SLD of the stabilizing shell of these particles. As mentioned above and described elsewhere, the particle size distribution is trimodal.24 However, due to the small amount of the small- and large-sized particle fractions, their influence on the scattering pattern is low and mainly influences the small and large Q-regime of our measurements. Thus, the SAXS/SANS measurements are most sensitive toward the fraction of medium sized particles,

(1)

(2)

∫ N ·f (R)·V (R)2 ·[F(Q , R)]2 dR

(4)

The scattered intensity of a single homogeneous spherical particle of radius R surrounded by a homogeneous shell of thickness ΔR and dispersed in a homogeneous medium can be calculated according to39

Q is the modulus of the scattering vector Q = kf − ki, where 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 |kf| = |ki| = 2π/λ, with λ denoting the wavelength of the Xrays and neutrons, respectively. d represents a measure of Q in terms of a length scale in direct space. In the small angle approximation the scattered intensity of particles eq 1 can be rewritten as38 I(Q ) = (Δρ)2 · S(Q )·

3(sin(QR ) − QR· cos(QR )) (QR )3

(3)

with Δρ = ρcore − ρsol being the SLD contrast between the ZnO NPs (ρcore) and the solvent (ethanol, ρsol). S(Q) denotes the structure factor, N the number of particles, f(R) the particle size distribution C


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Langmuir representing the main volume fraction of the ZnO NPs in the dispersions (>75 vol %) which will be described elsewhere.24 The size and shell characteristics of this fraction can be resolved in detail, while for the other two fractions, for simplification, a monolayer of acetate was assumed, which had only a negligible effect on the fit quality.

least not all of the acetate anions are closely bound to the surface of the particles, but they are in close proximity to it. The SAXS and SANS data for samples B and D, respectively, are displayed in Figure 2. Again, all fit-curves are in good

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RESULTS AND DISCUSSION The SAXS and SANS data for sample A with the respective curves of the simultaneous fit are displayed in Figure 1 (in

Figure 2. SAXS and SANS data and corresponding fit curves of simultaneous fits for (a) sample B and (b) sample D. Figure 1. SAXS and SANS data for sample A dispersed in (a) pure EtOD and (b) in the EtOH/EtOD mixture. The solid lines represent the best simultaneous fits to the data, and the dotted lines represent corresponding fits when neglecting a stabilizing shell surrounding the ZnO NPs. The schemes on the right side depict the contrast for neutrons between solvent, shell, and core for EtOD and for the matching EtOD/EtOH mixture, respectively.

agreement with both data sets. The small maximum at Q ∼ 1.5 nm−1 in the SANS pattern was not seen in further SANS measurements and is thus ascribed to statistical fluctuations. A summary of the parameters for the best fits to the experimental SAXS/SANS data sets is given in Table 1. From these fits it can be concluded that, with increasing core diameter, the shell thickness increases, too, while the concentration of acetate anions within the shell decreases. This effect might be directly linked to the particle size due to a higher reactivity of smaller NPs or to another ripening effect such as a particular modification of the surface of the NPs. A temperature effect can be excluded because all samples were measured at the same temperature (10 °C) to prevent them from further ripening. A closer look to the parameters of the acetate shell reveals that the detected scattering contrast between the stabilizing shell of the ZnO NPs and the dispersion medium is lower than expected for a closely packed acetate layer. From the actual contrast values of the stabilizing shell, the volume of acetate within the shell can easily be calculated, as it is just the fraction of the detected contrast between the shell and the solvent to the contrast of a pure acetate layer to the solvent times the volume of the shell (cf. Table 2, column 4):

Figure S1 the SAXS/SANS pattern of all samples can be seen in complete extension). The perfect agreement of the fit with the data is obvious. However, when neglecting the existence of a stabilizing shell (dotted lines in Figure 1), the fits become worse and especially the neutron data can by no means be reproduced by the scattering model. Thus, it can unambiguously be concluded that an acetate enriched stabilizing shell surrounds the dispersed ZnO NPs. The same result was achieved for samples in which the scattering density of the dispersion medium was matched to the scattering density of ZnO by a EtOD/EtOH mass ration of 3.67:1 (cf. Figure 1B). Here, no scattering contribution of the ZnO core of the particles is observed, and thus the SANS signal arises from the acetate shell only. This highlights the enrichment of acetate anions in the vicinity of the NPs. In the case of a homogeneous distribution of the acetate anions, no coherent scattering signal would be expected within the detected Q-range. The thickness of the acetate layer was determined to be 0.7 nm for sample A, which is slightly larger than the diameter of an acetate molecule (0.47 nm).40 Thus, at

V (Ac) = V (shell)·

ρ(shell) ρ(Ac pure)

(7)

where the shell volume V(shell) is D


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Table 2. Parameters of the ZnO NPs and Their Shell As Determined from the Simultaneous Fits (cf. Figure 1 and 2)a rmean / nm

σ/ -

dshell / nm

Δρ(shell)/ Δρ(Acpure)

Vshell / nm3

VAc / nm3

n(Ac)shell / -

n(ZnO)NP / -

μ/-

1.3 1.6 1.8 1.8 1.9 2.5

0.16 0.15 0.16 0.16 0.15 -

0.7 0.8 0.9 1.1 1.3 -

38% 39% 38% 22% 17% -

24.3 39.2 69.5 74.2 109 -

7.4 15 20 15 18 -

97 161 214 160 194 719

382 686 1007 1008 1208 2700

12.7% 11.7% 10.6% 7.9% 8.0% 13.3%

sample A (after mixture) sample B (3h at 20 °C) sample C (24h at 20 °C) sample D (1h at 50 °C) sample E (3h at 50 °C) Marczak (2010)

a rmean is the mean radius of the NPs, dshell is the thickness of the acetate layer, Δρ(shell) is the contrast of the shell, Δρ(Acpure) is the contrast of a pure acetate layer to the solvent, Vshell/Vac is the volume of the shell and of the acetate within the shell, respectively, n(Ac)shell and n(ZnO)NP are the numbers of acetate molecules or ZnO units, respectively, and μ is the partition coefficient between acetate in the solution and within the shell.


V (shell) =

4π 4π (rmean + dshell)3 − rmean 3 3 3

The values of μ are listed in Table 2 for the different samples. It is observed that about 10% of the total amount of acetate is located close to the ZnO NPs. However, a small decrease of μ for longer ripening times and for higher temperatures is observed. This indicates a change in the partition coefficient for larger particles for which the total surface reduces. It has to be stated that from our study we cannot directly distinguish acetate molecules chemically bound to the surface of the NP and unbound acetate molecules located in the vicinity of the NP. However, we believe that this question can be addressed by combining our approach with further spectroscopic techniques. Nevertheless, a quantitative in situ detection of the acetate shell surrounding the ZnO NPs in the native dispersion could be achieved here. Comparing these results with the ex situ data of Marczak et al., a good agreement is found.22 For ZnO NPs with a mean diameter of 5 nm, a number of n(Ac)shell = 719 acetate molecules in the stabilizer shell has been reported. This corresponds to a partition coefficient of μ = 13.3% (cf. Table 2). The slightly higher value found for these NPs compared to about 10% found in our study might be explained by deuteration effects for our samples or by residual acetate ions, which condensate on the NPs and could not be removed by the washing and drying step applied for the ex situ measurements. By the SAXS/SANS approach, we cannot distinguish between acetate and lithium in the shell. Thus, a possible model is also a diffuse double layer, in which the acetate is close to the ZnO core and then a second layer of lithium counterions extends into the dispersion medium. However, the amount of lithium should be rather small, as the amount of acetate in the shell found in this analysis matches nicely to the results found by the ex situ approach. However, beyond this comparison, we do not have a clear indication for one or the other model. Future studies on ZnO during washing and functionalization and ligand exchange reactions will shed light on this issue as well. In contrast to ex situ measurements, it has to be stated that it is only the combination of time-resolved in situ SAXS and SANS measurements, which allows one to monitor the structural changes of the stabilizing layer surrounding the ZnO NPs during the ripening process of the dispersion, which is also depicted in Figure 3. Finally, for an enhanced reliability of the correlation of the achieved data, a combined SAXS/ SANS instrument allowing for the observation of the scattering data of the same sample volume at the same time would be highly desirable. To the best of our knowledge, this is the first in situ observation of the acetate containing stabilizing layer around ZnO NPs and their evolution upon particle ripening in their native solution.

(8)

The volume of the acetate molecules is found to be clearly below 50% for all samples, meaning that less than half of the volume within the shell is occupied by acetate molecules, whereas the rest is occupied by solvent molecules (ethanol). Thus, it is not a densely packed monolayer of acetate anions that covers the NPs, but rather an acetate-enriched shell that stabilizes the NPs most probable by electrostatic repulsion. Finally, the average number of acetate molecules surrounding the NPs can be determined from the respective volume of acetate within the shell and the volume of a single acetate molecule, which is calculated from its density and equals approximately vm(Ac) = 0.089 nm3. The number of acetate anions within the shell is about 100 acetate molecules for the ZnO NPs after synthesis (diameter d = 2.6 nm, ∼ 4.6 molecules/nm2) and increases to about 200 for the larger sized particles after ripening (d = 3.6 nm, ∼ 5.3 molecules/nm2; cf. Table 2, column 8). From these numbers the partition coefficient μ defined as the ratio of the number of acetate molecules within the stabilizing layer of the ZnO NPs, n(Ac)shell, to the total number of acetate molecules, n(Ac)max, can be calculated. The number of ZnO units per NP, n(ZnO), is determined using the volume of a ZnO particle, V(ZnO), and the molecular volume of one ZnO unit vm(ZnO) = 0.024 nm3 which is calculated from the bulk density of ZnO (5.61 g/cm3): 4π

n(ZnO) =

rmean 3 V (ZnO) = 3 vm(ZnO) vm(ZnO)

(9)

Accordingly, the number of acetate molecules within the vicinity of the ZnO NP, n(Ac)shell, can be calculated as n(Ac)shell

V (shell) = = vm(Ac)

4π (r 3 mean

+ dshell)3 −

4π r 3 3 mean

vm(Ac) (10)

As the synthesis starts from ZnAc2, the overall ratio of acetate to zinc is 2:1, and thus the amount of acetate in the vicinity of the ZnO cores, n(Ac)max, can in maximum be twice the number of zinc atoms in the NP, which is also twice the number of ZnO units in the NP n(ZnO): n(Ac)max = 2 ·n(ZnO)

(11)

The partition coefficient μ can finally be determined as

μ=

n(Ac)shell n(Ac)max

(12) E


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Scanning Probes. This work is based on experiments performed at the KWS 2 instrument operated by JCNS at Heinz MaierLeibnitz Zentrum (MLZ), Garching, Germany. We thank Wei Lin and Johannes Walter for helpful discussions.

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Figure 3. Scheme of the stabilizing layer of ZnO NPs after synthesis (left side) and after further ripening (right side). The net charge of the acetate anions in the stabilizing layer increases upon ripening.


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CONCLUSION In this study we successfully characterized the acetate stabilizing layer of ZnO NPs in their native dispersed state by a combination of state-of-the-art SAXS and SANS experiments. By SAXS the size distribution of the spherical ZnO core particles could be determined, while the SANS data allowed for the structural characterization of the acetate layer surrounding the NPs. By this combined SAXS/SANS approach, we could demonstrate the existence of an enhanced acetate anion concentration within a thin shell surrounding the ZnO NPs, and the acetate distribution could be determined quantitatively. The acetate shell is not built up by a single closely packed monolayer of acetate molecules but is rather a swollen layer of acetate anions and solvent molecules. Less than half of the shell does consist of acetate anions, and the shell thickness is larger than the diameter of a single acetate anion. Upon ripening, the shell becomes larger, and the acetate concentration within the shell is reduced. The partition coefficient μ between the acetate within the shell and the total amount of acetate is about 10%, which is in good agreement with ex situ results. For longer ripening times and higher ripening temperatures, μ slightly decreases. The combination of in situ SAXS and SANS measurements for the study of the ligand shell of organic NPs introduced by our group could successfully be transferred to semiconducting NPs and will be of importance for further material systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02198. Representation of the SAXS/SANS pattern with their full extension, plot with fit results for different polydispersities and TEM image of the ZnO nanoparticles (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Address: Staudtstraße 3, 91058 Erlangen, Germany. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the funding of the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence Engineering of Advanced Materials (EAM) and GRK 1896 In-Situ Microscopy with Electrons, X-rays and F


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SANS Study for the in Situ Characterization of

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