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Revisiting the ZnO Q-Dot Formation Toward an Integrated Growth Model: From Coupled Time Resolved UV-Vis/SAXS/XAS Data to Multivariate Analysis Bruno Leonardo Caetano, Valérie Briois, Sandra Helena Pulcinelli, Florian Meneau, and Celso V. Santilli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10062 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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The Journal of Physical Chemistry

Revisiting the ZnO Q-dot Formation Toward an Integrated Growth Model: From Coupled Time Resolved UV-Vis/SAXS/XAS Data to Multivariate Analysis Bruno L. Caetanoa, b*, Valérie Brioisb, Sandra H. Pulcinellia, Florian Meneaub, c and Celso V. Santillia a

Instituto de Química, UNESP, Rua Professor Francisco Degni, s/n°,14800-900 Araraquara,

SP, Brazil b

Synchrotron SOLEIL, L’Orme des Merisiers, BP48, Saint Aubin, 91192 Gif-sur Yvette, France

c

Laboratório Nacional de Luz Síncrotron, CEP 13083-970, Caixa Postal 6192, Campinas, São

Paulo, Brazil

ACS Paragon Plus Environment

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ABSTRACT: Herein we discuss in depth the mechanisms of growth of ZnO quantum dots prepared from zinc oxy-acetate ethanolic solutions by LiOH addition. The nucleation, growth and aggregation of ZnO nanocrystals were unambiguously unravelled and associated to defined families of quantum dots and well described from two kinetic models: coalescence and Ostwald ripening. The in situ monitoring by X-ray absorption spectroscopy combined with UV-vis spectroscopy allows the determination of the time evolution of the proportions of each family of quantum dots of characteristic sizes from a multivariate data analysis. The aggregation index was calculated from combined UV-vis absorption and small-angle X-ray scattering measurements. The growth of single quantum dot nucleus results from coalescence by pure oriented attachment between themselves, then aggregation occurs through the welding of adjacent coalesced nanocrystals. At the advanced stage the quantum dot coarsening follows the Ostwald ripening mechanism. From the higher oriented attachment efficiency observed here as compared with earlier reported studies using NaOH and KOH, we propose a unified mechanism to describe the coalescence and coarsening of ZnO nanocrystals based on the shielding caused by the adsorption of the alkali solvated cations around the nanocrystals. This latter shielding effect has been also proved to be very efficient for the preparation of quantum dot powder with controlled size as demonstrated by X-ray diffraction and transmission electron microscopy.


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1. INTRODUCTION Semiconductor quantum dots (Q-dots) have attracted wide interest during the past decades due to their unique optical properties and numerous applications in optoelectronic devices, biological labels and photocatalysis.1,

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Among the II-VI semiconductor Q-dots, zinc oxide (ZnO) has

become the most widely studied metal oxide material. ZnO presents a direct optical gap around 3.4 eV and an exciton binding energy of ∼60 meV. In addition, it is possible to vary its photoluminescence properties via the adjustment of its morphology (size and shape) using easy soft-chemistry synthesis routes.3 In this way, it has been explored the application of ZnO nanoparticles as UV light emitting diodes, safe and cheap fluorescent labels for anti-fake labelling in the security safeguarding for various merchandise

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and in nanopharmaceutical

products for imaging.4 Other well-known applications of ZnO based on the related Q-dot properties can be found as catalysts5, solar cells6, gas-sensor7 and UV-blocker in suncreens.8 Moreover, comparing with other II- VI semiconductor metals such as CdSe and CdTe, ZnO is an environmentally friendly oxide, inexpensive luminescent material9 and with a tunable fluorescence-range being wider in principle.2, 10, 11 A typical approach to prepare colloidal ZnO Q-dots is the sol–gel method via hydrolysis of a zinc salt dissolved in alcohol.12, 13 Although the number of reports on the sol-gel synthesis of ZnO nanoparticles using the alcoholic solution is high

12, 14, 15

, the limited understanding of the

mechanisms of precursor reactions leading to the nuclei formation and of subsequent growth hampers the tailoring of the size and shape of nanocrystalline particles in a predictable way. The vast majority of former studies consider that colloidal nanoparticles growth, for example, CdSe 16

, ZnO

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, TiO218 and CdS19, follows the Ostwald Ripening (OR) mechanism. This coarsening

process, which is based on the diffusion-limited accretion of metallic species to the surface of


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larger nanoparticles at the expense of dissolution of smaller ones, was mathematically developed by Lifshitz, Slyozov and Wagner, becoming known as LSW model.20,

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The coalescence is

another significant mechanism of nanoparticles growth, which emerged from transmission electron microscopy observation.22-24 In this mechanism, the coalescence of dispersed nanocrystals results from the formation of coherent grain-grain interfaces as a consequence of aggregation followed by rotation among neighboring grains till the elimination of common grain boundaries. This mechanism is commonly known as Oriented Attachment (OA) induced by nanocrystal rotation. Finally, random aggregation process leading to the growth of hierarchical or fractal structure was also proposed for ZnO growth in earlier small-angle X-ray scattering (SAXS) studies.15, 25 Nucleation processes often occur on a relatively short time scale and thus are a challenge for direct time-resolved structural characterization of nuclei. This characteristic makes poorly reliable a full description of the mechanism and kinetics of nanocrystal nucleation and growth. Recently, the time resolution provided by SAXS and X-ray Absorption Spectroscopy (XAS), available at a third-generation synchrotron radiation facility, combined with conventional UltraViolet−visible (UV-vis) spectroscopy has emerged as a powerful and well-adapted approach for in situ reaction monitoring, allowing the characterization of complex mixtures including colloids.25,

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Additionally, the use of multivariate data resolution methods for analysing

overlapped spectral data as those measured by UV-vis spectroscopy during nucleation and growth of various Q-dots has been reported as a powerful method for unravelling the time evolution of the successive formed species and for enabling the determination of spectra of those isolated species. 27-29


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This paper is the last of a trilogy dedicated to unravel the universal role of the alkaline cations on the formation and growth of ZnO nanocrystals under low nominal hydrolysis ratio [MOH]/[Zn] = 0.5 (M = Li, Na and K) from ethanolic solutions of zinc oxy-acetate based precursors. Following the previous time resolved methodology applied to study the ZnO nanoparticles formation induced by KOH25 and NaOH30 this paper focuses on the ZnO nanocrystal growth under LiOH addition to zinc oxy-acetate ethanolic solution. From simultaneous measurements by UV-vis combined with XAS and UV-vis combined with SAXS we discuss in-depth the stepped temporal evolution of processes governing the ZnO Q-dot growth and demonstrate their universal control caused by the shielding of the alkali cation at the nanocrystallite surface. This shielding effect, already proposed by Sarma et al.

17, 31

for high

[MOH]/[Zn] ratios, is herein validated for hydrolysis ratios lower than 1. From transmission electron microscopy (TEM and HRTEM) and X-ray diffraction (XRD) results we show that the passivating layer associated to the presence of alkali counter-ions around the nanocrystals stays active in the preservation of the Q-dot size in the powder extracted from the colloidal suspensions. 2. EXPERIMENTAL SECTION 2.1 Sample Preparation ZnO colloidal suspensions were prepared following the method proposed by Spanhel and Anderson.13 The Zn4OAc6 precursor was first prepared by refluxing an absolute ethanol solution containing 0.05 mol·L−1 zinc acetate dihydrate ZnAc2·2H2O for 3 h at 80°C.32 The thus obtained transparent solution was stored at ~4°C to prevent any further condensation process. Hydrolysis and condensation reactions were carried out by adding LiOH absolute ethanol solution (0.5


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mol·L-1) to the precursor solution thermostated at 40ºC with a nominal molar ratio of [LiOH]/[Zn] = 0.5. 2.2 Quick-EXAFS and UV-vis monitoring For the in situ combined UV-vis/XAS measurements, freshly prepared precursor solutions were introduced into a specially designed liquid cell

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with an X-ray path of about 6 mm. An

immersion probe with optical path of 2 mm was inserted from the top of the cell into the solution in order to measure the UV-vis spectrum with a Cary 50 spectrometer (Varian). The reaction solution, thermostated at 40 °C, was stirred using a magnetic rod inside the cell in order (i) to provide a homogeneous reaction medium without temperature or solute concentration gradient and (ii) to avoid particle settling. The monitoring was carried out twice using two operation modes available at the SAMBA beamline (SOLEIL synchrotron): quick-EXAFS mode at the early stage of nucleation and growth of ZnO Q-dots (time < 20 min) and a slower step-by-step acquisition mode at the advanced reaction stages (20 min < time < 520 min).33 Details about the beamline set-up used for Quick-EXAFS measurements and for step-by-step XAS measurements were given in references25, 30 , respectively. We checked that the UV-vis data recorded during both reactions share common spectra (in wavelength position and absorbance intensity) on the common period of reactions (for 0 < time < 20 min) as presented in Figure S1 shown in Supplementary Information allowing to concatenate the XAS and UV-vis data provided by both time-resolved measurements. XAS spectra of size calibrated ZnO Q-dots (Section 2.5) were recorded as references in both acquisition modes and used as one of the main components for linear combinations (LC) simulations of the normalized EXAFS (Extended X-ray Absorption Fine Structure) spectra of


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colloidal suspensions. The second component used for the LC analysis is the initial zinc oxyacetate precursor solution for which the XAS spectrum has been recorded before the addition of LiOH solution. The analysis of the XAS data was carried out using the software package Athena34 and fully detailed in. 25, 30 2.3 Multivariate Curve Regression with Alternating Least Square Fittings (MCR-ALS) The use of the MCR-ALS method for the analysis of evolving UV-vis spectra measured during nucleation and growth of various Q-dots has been recently reported in the literature.27-29 The basic assumption for applying MCR-ALS method is the inner linear structure of the data set which is fully verified for spectroscopic techniques following the Beer-Lambert law as UV-vis spectroscopy. In that case, the experimental data are first represented in a matrix form, called D(Nsnapshots, Nwavelength), in which the Nsnapshots rows are the UV-vis spectra recorded at the monitored wavelengths (Nwavelength) during the experiment. Taking into account the linear structure of the data set, a bilinear decomposition of the matrix D can be done into the matrix containing pure concentration profiles C (Nsnapshots, k) and the matrix containing pure UV-vis spectra ST(k, Nwavelength) of the k species of the unknown mixtures according to relation (1): D= C ST +E

(1)

where the matrix E(Nsnapshots, Nwavelength) contains the residual variation of the data (typically the experimental noise). The superscript T means the transpose of matrix S, where pure UV-vis spectra are columns. Matrices C and ST are responsible for the observed data variance. MCR techniques do not require a priori information concerning the data for resolving C and ST, except an estimation of the number of pure components in the system i.e. the determination of the k rank characterizing matrices C and ST. The rank analysis of the data matrices is usually obtained


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through Singular Value Decomposition (SVD) method which describes the data matrix into orthogonal vectors. Once the number of species is estimated, ALS optimization can start using initial estimates of either the C or the ST matrix. Furthermore, the MCR-ALS method imposes C and ST to follow physically and chemically meaningful constraints like non-negativity of UV-vis absorbance and concentration, unimodality (profiles without double peaks) and closure (the concentrations of all the components is equal to a constant value) for instance. SVD and MCRALS analysis were achieved herein by using the mcr_main Matlab graphical-user interface written by Tauler and coworkers.35 It is noteworthy that MCR-ALS analysis for the study of the growth of colloidal zinc oxide nanoparticles has been recently reported using the same tools29, but the closure constraint was not applied because of the lack of information about the stoichiometry of the evolving species. In our case, the complementary information obtained by XAS allows us to determine the respective proportions of unreacted zinc oxyacetate precursors and of ZnO Q-dots (with no size discrimination). This information about respective proportions of precursors and ZnO Q-dots will be further used during the MCR-ALS minimization performed on the UV-vis data simultaneously recorded with XAS. 2.4 In Situ and Simultaneous SAXS/UV-Vis Monitoring The SAXS experiments were performed on the SWING beamline at the SOLEIL synchrotron source using an AVIEX CCD camera, placed in a vacuum detection tunnel. The reaction was conducted into a 3 neck flask kept on a magnetic stirrer hot plate. In one of the mouths of the balloon, an immersion UV-vis probe was placed directly in the solution. A peristaltic pump coupled to a hose immersed in the reaction medium was used to circulate continuously the reaction solution in the capillary sample holder used for time resolved SAXS measurements. The sample to detector distance was 2.468 m and a beam stop of 3 mm (vertical size) with a


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photodiode inserted in its centre enabled to measure the transmitted intensity. The beamline energy was set to 7.7 keV, the resulting scattering vector, q, ranged from 0.03 to 3.8 nm-1. The parasitic signal resulting from the scattering of the capillary filled with ethanol was subtracted from the total scattering intensity. The resulting curves were normalized to take into account the effects related to the detector sensitivity and sample transmission. UV-vis absorption spectra were recorded simultaneously with SAXS data using the same spectrometer than for combination with XAS (Section 2.2). The size of ZnO Q-dot was determined from the absorption spectra using the effective mass model derived by Brus36 as fully explained in ref.37 2.5 ZnO Q-dot powder Powdered ZnO nanoparticles with different Q-dot sizes were isolated from the colloidal suspension after different reaction times (5, 35, 60, 130, 420 and 4530 min) following the procedure described by Meulenkamp.38 A few mL of colloidal suspension was removed from the reaction bath and the ZnO Q-dots were separated by the addition of heptane as a non-solvent. The supernatant solution was discarded and the decanted solid was collected, dried and repeatedly washed in order to remove the unreacted precursor and the side reaction products. Then the powders were characterized by X-ray diffraction (XRD) with a D2-phaser diffractometer from BRUKER equipped with 1-D detector (Lynxeye) using the CuKα radiation monochromatized by a curved graphite single crystal. The Q-dot shape and size of the powder extracted after 130 min of reaction was analyzed by high-resolution transmission electron microscopy (HRTEM; JEOL 3010) operating at 300 kV at the Brazilian Nanotechnology National Laboratory (LNNano). The sample was prepared by dropping an isopropanolic suspension of the powder onto amorphous carbon films supported on nickel grids. Particle size analysis was performed using the Image J software.39 We were unable to analyze the samples at


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the early stages of reaction due to the high instability of the so-small particles under electron beam leading to particle aggregation.

3. RESULTS AND DISCUSSION 3.1 Q-dot nucleation and growth monitored by XAS and UV-vis The growth of ZnO Q-dots was first monitored by XAS combined with UV-vis spectroscopy in order to identify the zinc-based species involved during hydrolysis-condensation reactions in alcoholic solution and their evolution as a function of reaction time. Figure 1 displays the time evolution of selected Fourier Transforms (FT) of the EXAFS spectra recorded by quick-EXAFS and step-by-step XAS. The FT presented herein are not corrected from phase shift, then the position of the different contributions of neighbors appears at slightly shorter distances than the crystallographic distances of crystalline ZnO. The first peak in the FT spectra is related to the first coordination sphere of oxygen atoms around the zinc atom, and the second is mainly related to the Zn-Zn contribution. The intensity of the peak related to the oxygen first neighbor contribution remains essentially constant in position and intensity, which is consistent with the tetrahedral oxygen coordination of both the tetrameric precursor40 and ZnO structures. An increasing intensity of the second intense peak in the FT corresponding to the Zn-Zn contribution at the crystallographic distance of 3.23 Å is observed as the time is increasing. The information provided by the local order XAS technique clearly evidences a direct transformation of the zinc oxy-acetate based species into ZnO Q-dots, with no other products resulting from the presence of water (as reaction media41 or produced by esterification42).


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Figure 1. Fourier Transforms (not phase corrected) of the EXAFS spectra recorded during the growth of the ZnO Q-dots by quick-EXAFS (first 20 min) and step-by-step XAS displayed as a function of time. Namely, satisfactory linear combinations (as shown in supplementary information in Figure S2) reproduce the time-resolved data considering only the EXAFS spectrum of the precursor solution and the ones of ZnO Q-dots extracted from the solution. The conversion of precursor species into ZnO Q-dots occurs very quickly since their presence is verified immediately after the addition of LiOH and about 15% of precursors is consumed within the first ~ 5 min of reaction. After this period, a quasi-steady state chemical equilibrium is achieved with further consumption of precursors which does not exceed 10 % after 520 min of reaction.


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Concomitantly, the intensity of the Zn-Zn contribution on the FT (Figure 1) still increases evidencing the growth of ZnO Q-dots. It is worth to note that the concentration profiles of precursors and ZnO Q-dots determined by quick-EXAFS and step-by-step XAS are in very good agreement (Figure S3). Together with the superimposable UV-vis spectra reported at common times of reaction for both data set (Figure S1), this feature observed for concentration profiles is indicative of good reproducible reaction conditions and gives reliability to the use of a unique concentration profile as a function of time for precursors obtained by the concatenation of the data finely determined over the first 20 min of reaction by quick-EXAFS and those determined at longer reaction time by step-by-step XAS. Figure 2 shows the 3D-stacked UV-vis spectra plotted as a function of time during the reactions monitored by quick-EXAFS and step-by-step XAS. The evolutions observed by UVvis are well in line with the aforementioned XAS evolutions related to the initial increase of the quantity of ZnO Q-dots then followed by their nearly invariance over time (Figure S2) and increase of Zn-Zn FT contributions (Figure 1) interpreted as a growth of ZnO Q-dots. An increase of the absorbance is observed immediately after the addition of LiOH into the precursor solution related to the prompt formation of ZnO nanocrystals.25 Well-defined excitonic peaks significantly blue-shifted compared to the bulk ZnO crystal value, indicating that the nanoparticles are in the quantum regime17, 31, 43, 44, are observed at the early stage of reaction (t 50 min, continuous line, reliability factor R = 0.97). This indicates that diffusion-limited coarsening caused by the dissolution/reprecipitation process dominates the kinetic of the ZnO Qdot growth at the advanced time period. This finding is in agreement with studies carried out by several groups describing the growth mechanism of ZnO nanoparticles by an OR process. 14, 43, 45 Considering the remarkable deviation of the LSW linear behavior verified at the early stage (t < 50 min) the equation of the OA model (Eq. 3) was applied to fit the experimental evolution of Q-dot radii for 5min < t < 30 min. The best fit achieved from nonlinear least-squares procedure for this time period (continuous line, R= 0.94) shows a good agreement between experimental results and the OA coalescence model (Eq. 3) limited to single coalescence step between the primary nanocrystals. Both models fail to describe the experimental behaviors during the nucleation step (t < 5 min) and secondary precipitation step verified at 30 min < t < 50 min. As discussed in the section 3.4, the chemical equilibrium condition is not observed during these time periods.


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Figure 4. Time evolution of the cube of ZnO Q-dot radius calculated from the UV-vis data collected simultaneously with the XAS (black circles) and SAXS (red circles). They follow a hyperbolic function, Eq. (3), (continuous line) at the early stage (first 30 min) of the reaction corresponding to an oriented attachment mechanism, while both data sets display a linear dependency corresponding to LSW coarsening kinetic, Eq.(2), (continuous line) at advanced reaction times (t > 50min). The dashed lines highlight the deviation between the experimental curves and the theoretical functions described by equations (2) and (3). 3.3 Aggregation contribution Figure 5(a) presents a three-dimensional log-log plot of the SAXS curves as a function of the reaction time that was measured simultaneously with the UV-vis spectra. For clarity, we


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extracted eight profiles of SAXS data corresponding to specific times from the start to the end of the reaction and plotted together in Figure 5(b).

1

0.1

1

Time (min)

(b)

10

120 40 10 1

I(q)∝ q-α

1

4.0 3.6

Intensity (arb.units)

1 -1 q (nm )

Ti

0.1

m

e

(m

in )

120

Intensity (arb.units)

10

(a)

slope

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0.1

3.2 0

20

40

60

80 100 120

Time (min)

0.1

1

q (nm -1)

Figure 5: a) 3D log-log plot of the SAXS curves as a function of the reaction time that was measured simultaneously with the UV-vis spectra. b) SAXS patterns corresponding to specific times from the start to the end of the reaction. The insert displays the time evolution of the slope (α) of the linear regime corresponding to the asymptotic power law behavior. The shape of SAXS curves is characteristic of a dilute system of particles, thus the curves can be analyzed assuming a two-electron density model consisting of a dilute set of isolated particles or aggregates embedded in a homogeneous liquid solution.50 Irrespective of the reaction time all SAXS curves present an asymptotic linear trend at high q-range (q > 2 nm-1). The insert of


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Figure 5(b) shows the time evolution of the slope at high q-range (q> 2 nm-1). Between the start of the reaction up to 30 min we observe a change in this slope value from -3 to -4, remaining constant until the end of the reaction. The -3 slope indicates the presence of nano-heterogeneities inside the scatters, suggesting the existence of nanovoids inside the aggregates of ZnO nanoparticles. Otherwise, the -4 slope, which satisfies the classic Porod law (I(q) α q-4), implies the presence of a two-phase system with sharp interface and the absence of significant amounts of small pores inside the scattering object. Thus the decrease of the slope between -3 to -4 may be due to densification, i.e. elimination of nanovoids probably caused by nanocrystal-rotation involved in Q-dot coalescence driven by the OA mechanism. At low q-values a gradual increase of the scattering intensity with the reaction time is observed. The plateau, i.e., the so-called Guinier region,50 is characteristic of the scattering of a dilute set of non-interacting particles. This feature shifts to lower q values as the reaction time advances, indicating that the individual nanoparticles either increase in size or aggregate, forming larger structures. From the Guinier region it is possible to calculate the radius of gyration of particles or aggregates, Rg, and consequently, the spherical radius, RSAXS (RSAXS= ହ

ට ܴ݃) ଷ

50

. The access to RUV-vis, which is only sensitive to the Q-dot size, recorded

simultaneously with RSAXS, sensitive to individual and aggregated nanocrystals, is essential for emphasizing the aggregation time period. We define hereafter the aggregation index as the ratio RSAXS/RUV-vis. The values of RSAXS, RUV-vis, and RSAXS/RUV-Vis are gathered in Table S1. 3.4 Towards a unified growth mechanism Figure 6 shows a comparison between the time evolutions of the average spherical radii RSAXS, the average Q-dot radii called RUV-vis and the ratio RSAXS/RUV-vis extracted from the combined


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SAXS/UV-vis experiments together with the concentration profiles of precursors and Q-dots determined from the MCR-ALS analysis of the combined XAS/UV-vis measurements.

Figure 6: Time evolution of the Q-dot components concentration profiles as well as its size over the first 120 min of reaction. The continuous lines display the concentration profiles of the as-determined Q-dot components (QD1, QD2, QD3, QD4, and QD5), while the dash lines the percentage of precursor and the sum of the contributions QD3 to QD5. The point connected lines allows the comparison between the time evolutions of the average spherical radii RSAXS, the Qdot radii RUV-vis, and the ratio RSAXS/RUV-vis also called the aggregation index. A schematic representation of the main stages involved in the ZnO Q-dot growth mechanism is presented at the bottom of the figure.


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Thanks to the combination of both techniques leading to unambiguous correlated results and chemometric analysis, it is possible to understand more clearly the steps of the mechanisms of formation and growth of ZnO Q-dots. The first important observation is that regardless the technique used in determining the radius both RSAXS, and RUV-vis always increase with increasing reaction time, but at different ratios RSAXS/RUV-vis. This finding associated to the evolution of precursor and ZnO Q-dot speciation allows to describe the formation and growth of ZnO Q-dots as a stepped kinetic process composed basically of four main stages schematized at the bottom of Figure 6. After the nucleation and growth period (stage 1, t < 5 min) leading to the formation of QD1 (see Section 3.1), the dependence of the average cube Q-dot radii (Figure 4) clearly evidences the dominance of the OA coalescence mechanism until t = 30 min, called as stage 2. The ratio between RSAXS and RUV-vis shown in Figure 6 points out that this OA mechanism is composed of two steps. At the initial step (5 min < t < 15 min) the aggregation index RSAXS/RUV-vis is close to one, indicating that the Q-dot measured by UV-vis and the particles aggregates measured by SAXS have similar size and similar growth rate. This step is coincident with the decrease of QD1 nucleus concentration at the benefit of the formation of QD2 when the equilibrium between precursors and Q-dots reaches a steady state regime, as shown by the quasi invariance of the proportion of precursors in solution. This behavior reveals the high efficiency of formation of a single Q-dot, QD2, from OA between neighboring Q-dots QD1. Between (15 min < t < 30 min) the RSAXS grows faster than the average RUV-vis, leading to a continuous increase of the aggregation index from ≈1 to 1.3. This behavior is concomitant with a secondary consumption of precursors and the dominant amount of QD3.


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At stage 3 (30 min < t < 50 min) we propose that a secondary precipitation of precursors occurs at the neck existing between two neighboring nanocrystals. This results from the effect of the localized decrease of solubility caused by the concave curvature of the necks as foreseen by the Ostwald-Freundlich equation.48 The accumulation of precursors at the neck induces the welding of nanocrystals hindering the further rotation of crystals to form single Q-dots. QD3 is therefore described as an aggregate of welded nanocrystals. This finding is well in line with the shape of pure spectra reported for QD1, QD2 and QD3 determined from the MCR-ALS analysis (Figure 3): QD1 and QD2 are single Q-dots resulting from the nucleation and OA process, respectively and then their UV-vis absorption spectra display well-defined excitonic peaks, whereas such feature is vanishing for QD3 spectrum as a consequence of aggregation between single Q-dots, QD1 and QD2. At stage 4 (t > 50 min), the RSAXS/RUV-vis ≈1.4 value stays nearly invariant indicating that no further aggregation growth occur during the Ostwald-ripening (OR) pointed out previously by the linear dependence with time of the cube of RUV-vis (Figure 4). The evolution of the proportion of Q-dots in presence at this stage shows that the formation of QD4 results from the dissolution of QD2 and reprecipitation at the surface of QD3 and the one of QD5 results from dissolution of QD3 and reprecipitation at the surface of QD4. It is noteworthy that the RSAXS/RUV-vis index plotted on Figure 6 is amazingly superimposable with the sum of concentration profiles of QD3 to QD5. For t < 30 min, only QD3 contributes to the sum indicating that the deviation of OA growth is due to the formation of Q-dots aggregates. For t > 30 min, the increase of the proportion of QD4 and QD5 Q-dots resulting from Ostwald ripening process leads to a significant contribution of the invariance of aggregation index around ≈1.4. The fine description of the contribution of the different processes involved during the


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growth of ZnO Q-dots has been achieved for the first time with the unique conjunction of complementary information coming from three independent techniques and multivariate data analysis. 3.5 Universality of unified growth mechanism Within the same time resolution than the one reported herein, the pure oriented attachment mechanism was not isolated from our previous studies of ZnO Q-dots growth using KOH or NaOH as alkaline base catalysts.

25, 30

Indeed, it was observed, just after nucleation-growth, for

KOH and NaOH an initial aggregation index higher than one (see Table S2). The aggregation index order, RSAXS/RUV-vis ≈ 1, 1.3 and 1.5, follows the same sequence of the increase of the base strength, i.e. LiOH, NaOH and KOH, respectively. The comparison (Table S2) with the aggregation index reported for studies of ZnO Q-dots prepared with KOH or NaOH at the end of coalescence process indicated as well that the aggregation trend follows the base strength sequence since RSAXS/RUV-Vis ≈ 1.4, 1.8 and 3.1 for LiOH, NaOH and KOH, respectively. Moreover, the same order was verified for the ZnO Q-dot radius at this stage, RUV-Vis ≈ 2.1, 2.0 and 2.6 for LiOH, NaOH and KOH, respectively (Table S2). Despite differences in the synthesis conditions, the above findings are qualitatively similar to the trends earlier reported for the growth of ZnO nanocrystals prepared with more than 10 times larger [MOH]/[Zn] ratio (M= Li, Na and K) as respect to the one used herein. In this study, Sarma et al.17, 31 have shown that there is a critical concentration of base, depending on the used alkali base, beyond which the coarsening of ZnO nanoparticles is inhibited. They explained the dependency of this critical concentration with the nature of alkali base from the antagonist role taken by two independent factors, the size of the solvated basic cation (RLi+>RNa+>RK+) and the


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dissociation constant of the base (KDLiOH < KDNaOH < KDKOH) Both concurrent factors determine the formation of a passivating layer of alkali cations around the negatively charged growing ZnO nanocrystals. Moreover, it was verified that the concentration of the cation, CM+ required to form a shell around the ZnO nanoparticle is proportional to the ratio of the surface area of the Q-dot and the cross-sectional area of the cation, that is, CM+ = 4πR2Qdot/πR2M+. Thus, for a given size of Zn Q-dots, different concentrations of each one of the three bases will be required to stop the growth due to the different sizes of the solvated cations, (Li+, Na+, K+). This is clearly reflected in the fact that at the end of the coalescence stage we find a RUV-Vis ≈ 2.1, 2.0 and 2.6 nm for LiOH, NaOH, and KOH, respectively (Table S2). Obviously in the presence of a so-low base concentration, like the one used herein ([LiOH]/[Zn] = 0.5), the solvated lithium cation forms an incomplete shell around the ZnO Q-dot, insufficient to inhibit the OA between the nanocrystals. Due to the difference in hydrodynamic size of solvated cation changing Li by K, the steric hindrance caused by solvated cation shell at the surface of the same size of Q-dot and for the same base concentration is higher for Li than K. As schematically presented in both Figure S10 and graphical abstract, the OA coalescence between two nanocrystals induces the exudation of Li+ from the nanocrystals boundary area and its redistribution at the surface of the growing nanocrystal. This causes a gradual filling of the shell of Li+ counter-ions at the surface of the coalesced nanocrystal, inhibiting further coalescence. Due to the higher hydrodynamic size of solvated Li cations compared to the one of K (see Figure S10 or graphical abstract), the complete filling of the protective layer of counter-ions at the surface of the same size of Q-dot will be achieved with almost three times lower amount of solvated Li+ cations than K+. Our experimental results obtained with under stoichiometric concentration of alkali bases, in addition to the ones earlier reported using an excess of the alkali base 17, 31 establish this phenomenon as a


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general mechanism in which the adsorbed alkali cations provide a shield around the ZnO nanocrystal inhibiting coalescence and hindering coarsening. 3.6 Powdered Q-dot This section intends to demonstrate that the proposed mechanism of nanocrystal growth inhibition by alkali cations passivating layer remains active during the conversion of colloids into dried powders. In order to preserve the counter-ion shielding effect it is essential to maintain the Li+ cation in the medium during the extraction of ZnO nanocrystal from the liquid colloidal suspension. Indeed it is well known that the water addition to colloids or the water washing of the precipitate causes high instability of ZnO sol leading to flocculation, nanocrystal growth and dissolution-reprecipitation of hydroxylated compounds like zinc hydroxide double salts.

42, 51, 52

Taking into account our findings, all these transformations induced by water can be explained by the destabilization created by desorption of the counter-ions from the surface of ZnO nanoparticles. Moreover, the extraction of ZnO nanocrystal by evaporation of alcohol under heating is also deleterious because the increase of temperature favors the esterification reactions that lead to the production of considerable quantity of water.42, 53 Several extraction processes allow

for

overcoming

such

limitations,

like

freeze-drying,

ultracentrifugation

and

electrophoresis, however, in this paper we consider an easier process based on the decantation of the ZnO nanocrystal induced by the addition of a non-solvent, as described in section 2.3. The size preservation of the ZnO Q-dot extracted from the colloidal suspension at the end (130 min) of the monitoring by UV-Vis and SAXS is verified by the TEM and HRTEM images displayed in Figure 7(a) and 7(b), respectively. A high crystallinity is evidenced in the highresolution HRTEM image shown in Fig. 7(b) where the distance between the parallel lattices


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(0.26 nm) corresponds to the d-spacing value of the (002) planes. This finding indicates that the observed oriented attachment between both nanocrystals occurs along the c-axis, which is a typical preferential growth direction for the ZnO wurtzite structure.54 This image confirms that the coalescence occurs when two nanocrystals assume the same crystallographic orientation, resulting in a single crystalline particle. The size of primary nanocrystal is ≈2.6 nm and of the coalesced cluster in the growth direction is 5.2 nm, which is in reasonable agreement with the average particle size (4.0 ±1.1 nm) found by TEM.

Figure 7. a) TEM and b) HRTEM images of the ZnO Q-dot extracted from the colloidal suspension after 130 min of reaction. To get more insight into the formation of the larger Q-dot by OA we have analyzed XRD patterns of ZnO powders extracted at different reaction times from the colloidal system. A typical evolution of the diffraction patterns is displayed in Figure 8 (a). Except for the powder extracted after 5 min, all the other samples show the diffraction peaks of the ZnO hexagonal


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phase with wurtzite structure (space group P63mc or C46v). The diffraction peaks corresponding to (100), (002) and (101) planes are broadened and convoluted as expected for nanocrystalline powder. As the reaction time increases from 5 to 60 min the peaks become narrower and more intense, indicating the crystallite growth. In order to determine crystallite size from the diffraction peak profile a deconvolution was performed by using a pseudo-Voigt function55, a typical example is displayed in the supplementary information (Fig. S11). Assuming that the peak broadening is essentially due to the size effects, the width of the [110] and [001] Bragg peaks was used to calculate the average size from the classical Scherrer formula

56

: D= 0.89.λ/β.cosθ, where λ, θ and β are the X-ray

wavelength (1.5405 Å), the Bragg diffraction angle and the line width at half maximum, respectively. Figure 8(b) shows the evolution of cube average crystalline size in both [002] and [110] directions as a function of the reaction times, showing that a quasi-steady state is reached after 2 h. For this reaction time the average size along [002] (D = 6.6 nm) is the double of the [110] (D = 3.2 nm) confirming again that the preferential growth along the c-axis occurs by the OA of the quite isometric pristine crystallite of ≈ 3.2 nm size, forming dimers. Another evidence of the single coalescence step of pristine nanocrystal particles is the good agreement between experimental data and the hyperbolic function (Eq. 3) displayed by the continuous line in Fig 8 (b). Furthermore, the quite good agreement between the crystallite size of 2 h extracted powder and the UV-Vis Q-dot size (4.3 nm) and average spherical size determined by SAXS (5.8 nm) confirms again the absence of crystallite growth during the extraction of ZnO nanoparticles from the colloidal system.


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Figure 8. Evolution of a) the X-ray diffraction pattern of ZnO powders extracted from the liquid colloidal suspension after different reaction times. b) Time evolution of cube of crystallite size calculated for the [002] and [110] directions. Continuous line is the Eq (3) fitting of crystallite size along [002] CONCLUSIONS The chemometric analysis of absorption UV-vis data constrained with the precursor concentration determined by XAS and discussed in light of nanoscopic characteristics determined by SAXS was a major contribution for the in-depth understanding of the growth process of ZnO Q-dots. The full time evolution of the UV-vis absorption spectra recorded during the zinc oxy-acetate ethanolic solution hydrolysis and condensation induced by LiOH at low


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nominal hydrolysis ratio ([OH/[Zn] = 0.5) can be described by the evolving proportions of a few Q-dots species with characteristic sizes as determined by MCR-ALS. After nucleation induced by the supersaturation created by LiOH addition, the structural evolution of ZnO Q-dot, under quasi steady state chemical equilibrium, follows three different kinetic models: i) At 5 < t < 30 min, the Q-dot size growth found from the UV-vis results and SAXS monitoring is in good agreement with the mechanism of coalescence by oriented attachment limited to a single aggregation step. This process is mainly responsible for the growth of the ZnO nucleus. ii) At 30 < t < 50 min, the welding of nanocrystals is observed caused by a secondary precipitation of precursors at the neck formed between aggregated nanocrystals hindering the further rotation of nanocrystals to form single Q-dots. iii) At t > 50 min, the Q-dot size growth obeys satisfactorily to the Ostwald ripening mechanism involving successive families of Q-dots. The aggregation index calculated from SAXS and UV-vis results reveals a high efficiency of the oriented attachment between the neighboring Q-dots during the first 15 min of reaction, leading to the fully conversion of nanocrystal aggregates into a single Q-dot. The comparison with earlier reports of ZnO Q-dot synthesized using KOH and NaOH indicates that the oriented attachment efficiency decreases in the same sequence of the radii of the solvated basic cation (RLi+>RNa+>RK+). These findings allow us to propose a universal mechanism in which the


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adsorbed alkali cations provide a shield around the ZnO nanocrystal inhibiting coalescence continuity and hindering coarsening. The XRD and TEM results on powders extracted from the colloidal suspensions by addition of a nonsolvent evidence that the mechanism of ZnO nanocrystal growth inhibition by alkali shielding remains active during the powder isolation. ASSOCIATED CONTENT Supporting Information Common UV-vis spectra measured during quick-EXAFS and step-by-step XAS monitoring, linear combination fitting of EXAFS spectra, precursor and ZnO Q-dots profiles determined from XAS, SVD analysis of concatenated UV-vis data, concentration profiles and reconstructed UV-Vis data from MCR-ALS, deconvolution of diffraction peak of ZnO Q-dot powder. The values of aggregation index, RUV-vis and RSAXS measured in function of the reaction are presented in Table S1. Aggregation index and RUV-vis radii for ZnO Q-dots growth from LiOH, NaOK and KOH addition are reported in Table S2. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION

Corresponding Author * [email protected] and [email protected]

Author Contributions B.L.C is the main author contributing in all steps. F.M. planned and discussed the SAXS experiments. V.B. planned and discussed the XAS experiments and analyzed by MCR-ALS the UV-Vis data. C.V.S (post-doctoral supervisor of B.L.C) suggested the experiments, discussed the


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results, proposed the integrated growth model. The manuscript was jointly written, read and commented by all authors Notes The authors declare no conflicting financial interests.

ACKNOWLEDGMENT The authors want to thank SOLEIL for providing beamtime at the SAMBA and SWING beamlines and chemistry laboratories, to Dr Marina Magnani (IQ-UNESP) and to LNNano (CNPEM) for the TEM images. We also thank the financial support from CNPq, FAPESP and CAPES/COFECUB cooperation program. This work was partially supported by the Brazilian National Council for Scientific and Technological Development (CNPq), the Foundation for Research Support of the State of São Paulo (FAPESP) and the CAPES/COFECUB-cooperation programs. REFERENCES (1) Li, S.; Toprak, M. S.; Jo, Y. S.; Dobson, J.; Kim, D. K.; Muhammed, M. Bulk Synthesis of Transparent and Homogeneous Polymeric Hybrid Materials with ZnO Quantum Dots and PMMA. Adv. Mater. 2007, 19, 4347-4352. (2) Xu, X.; Xu, C.; Wang, X.; Lin, Y.; Dai, J.; Hu. Control mechanism behind broad fluorescence from violet to orange in ZnO quantum dots. J. Cryst. Eng. Comm. 2013, 15, 977981. (3) Lizandara-Pueyo, C.; van den Berg, M. W. E.; De Toni, A.; Goes, T.; Polarz, S. Nucleation and Growth of ZnO in Organic Solvents - an in Situ Study. J. Am. Chem. Soc. 2008, 130, 16601-16610. (4) Yuan, Q.; Hein, S.; Misra, R. D. K. New generation of chitosan-encapsulated ZnO quantum dots loaded with drug: Synthesis, characterization and in vitro drug delivery response. Acta Biomater. 2010, 6, 2732-2739. (5) Kuo, T.-J.; Lin, C.-N.; Kuo, C.-L.; Huang, M. H. Growth of Ultralong ZnO Nanowires on Silicon Substrates by Vapor Transport and Their Use as Recyclable Photocatalysts. Chem. Mater. 2007, 19, 5143-5147.


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Figure 1. Fourier Transforms (not phase corrected) of the EXAFS spectra recorded during the growth of the ZnO Q-dots by quick-EXAFS (first 20 min) and step-by-step XAS displayed as a function of time.


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Figure 2. 3D-stacked UV-vis spectra recorded in situ, (simultaneously with the quick-EXAFS (first 20 min) and step-by-step XAS experiments), during the formation of the ZnO Q-dots.



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Figure 3. UV-vis pure spectra of ZnO Q-dots extracted from the chemometric analysis. The six components are labelled QD1 to QD6 according to their appearance order in the concentration profile plot. The ZnO Qdot radius corresponding to each spectrum is displayed in the legend of the curves.


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Figure 4. Time evolution of the cube of ZnO Q-dot radius calculated from the UV-vis data collected simultaneously with the XAS (black circles) and SAXS (red circles). They follow a hyperbolic function, Eq. (3), (continuous line) at the early stage (first 30 min) of the reaction corresponding to an oriented attachment mechanism, while both data sets display a linear dependency corresponding to LSW coarsening kinetic, Eq.(2), (continuous line) at advanced reaction times (t > 50min). The dashed lines highlight the deviation between the experimental curves and the theoretical functions described by equations (2) and (3).



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Figure 5. a) 3D log-log plot of the SAXS curves as a function of the reaction time that was measured simultaneously with the UV-vis spectra. b) SAXS patterns corresponding to specific times from the start to the end of the reaction. The insert displays the time evolution of the slope (α) of the linear regime corresponding to the asymptotic power law behavior.


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Figure 6. Time evolution of the Q-dot components concentration profiles as well as its size over the first 120 min of reaction. The continuous lines display the concentration profiles of the as-determined Q-dot components (QD1, QD2, QD3, QD4, and QD5), while the dash lines the percentage of precursor and the sum of the contributions QD3 to QD5. The point connected lines allows the comparison between the time evolutions of the average spherical radii RSAXS, the Q-dot radii RUV-vis, and the ratio RSAXS/RUV-vis also called the aggregation index. A schematic representation of the main stages involved in the ZnO Q-dot growth mechanism is presented at the bottom of the figure.



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Figure 7. a) TEM and b) HRTEM images of the ZnO Q-dot extracted from the colloidal suspension after 130 min of reaction.


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Figure 8. Evolution of a) the X-ray diffraction pattern of ZnO powders extracted from the liquid colloidal suspension after different reaction times. b) Time evolution of cube of crystallite size calculated for the [002] and [110] directions. Continuous line is the Eq (3) fitting of crystallite size along [002]


XAS ... - ACS Publications

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