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In Situ and Simultaneous UV-vis/SAXS and UV-vis/XAFS Time-Resolved Monitoring of ZnO Quantum Dots Formation and Growth Bruno L. Caetano,†,‡ Celso V. Santilli,† Florian Meneau,‡ Valerie Briois,‡ and Sandra H. Pulcinelli*,† † ‡
Instituto de Química, UNESP, Rua Professor Francisco Degni, s/n°,14800-900 Araraquara, SP, Brazil Synchrotron SOLEIL, L’Orme des Merisiers, BP48, Saint Aubin, 91192 Gif-sur Yvette, France
bS Supporting Information ABSTRACT: The processes of formation, aggregation, and growth of ZnO quantum dots (Qdot) induced by the addition of KOH solution ([OH]/[Zn] = 0.5) to the zinc oxy-acetate ethanolic solution at 40 °C were examined in depth by in situ and simultaneous time-resolved monitoring of UV-vis absorption spectra combined with small-angle X-ray scattering (SAXS) and with X-ray absorption fine structure (XAFS) measurements. XAFS results recorded at the Zn K-edge evidence the presence of both the zinc oxy-acetate precursor and the ZnO nanocrystals. Each XAFS spectrum could be described by a linear combination of each reference spectra, the composition evolving from ∼47% to ∼82% of ZnO from the beginning to the end of time monitoring. The time evolution of the UV-vis spectra shows a pronounced red shift of the excitonic peak during the first 100 min of reaction, characterizing the ZnO Qdot growth. The SAXS curves obtained simultaneously with the UV-vis spectra follow the Porod law at the large q-range, indicating the presence of well-defined smooth surface particles. At the intermediate q-range, the SAXS profiles evolve from a Guinier pattern, typical of diluted sets of aggregates, to a power law dependence characteristic of fractal structures. For reaction times higher than 400 min, the slope of the fractal region of SAXS curves reaches a value characteristic of diffusion controlled cluster-cluster aggregation process. The results of this in situ and combined techniques investigation show that the kinetic of formation of colloidal ZnO nanocrystal is a step process composed of four main stages: (i) ZnO Qdot nucleation and growth; (ii) growth of compact ZnO Qdot aggregates; (iii) growth of fractal aggregates; and (iv) secondary nucleation and fractal aggregates growth.
1. INTRODUCTION Nanocrystal semiconductor technology emerged in the early 1980s when Efros1 and Brus2 independently observed striking differences of color for solutions made from the same colloidal species. The origin of this unusual optoelectronic property is the quantum size (Qsize) effect observed when the exciton Bohr size is larger than the semiconductor nanocrystal size, the so-called “quantum dots” (Qdot). In the Qdot, the optical band gap and the energy of the emitted light can be tuned by changing the nanocrystal size. Typical semiconductors presenting this striking property are cadmium selenium and cadmium tellurium alloys, zinc sulfide, and zinc oxide.1-4 Regarding ZnO, bulk wurtzite is an n-type semiconductor with a wide band gap of 3.37 eV and exciton Bohr size of about 6 nm. Thus, during ZnO nanocrystal growth up to 6 nm, the wavelength of the absorption edge will experience a red shift to the bulk value located around 370 nm.2 However, one of the great challenges for practical applications of this Qsize effect is related to the ability to synthesize nanocrystals of the required Qdot size and with precise control of size dispersity.5-10 The growing research for obtaining such r 2011 American Chemical Society
nanocrystals largely uses the solution synthesis,9-11 due to its ease of implementation and high degree of flexibility. A tremendous amount of activity on ZnO nanoparticle solution synthesis has emerged11 with the development of zinc acetate (ZnAc2 3 2H2O where Ac = CH3COO) sol-gel route proposed by Spanhel and Anderson.12 This route involves two main steps: (i) the heating of the zinc acetate alcoholic solution above 80 °C leads to the conversion of ZnAc2 3 2H2O into tetrameric oxy-acetate Zn4O(Ac)6 precursor and to the liberation of water and acetate molecules;13 (ii) then, the polycondensation reaction of this precursor and subsequent formation of zinc oxide nanoparticles is induced by addition of a base or water.14-16 The particle size of the initial sol can be tuned from 3 to 6 nm by aging or by partial evaporation of alcohol. However, the most crucial point of this route is the high sensitivity of the Zn4O(Ac)6 precursor and ZnO nanoparticles to water, Received: October 6, 2010 Revised: January 11, 2011 Published: March 01, 2011 4404
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The Journal of Physical Chemistry C strongly influencing the nanocrystal size.15-20 Moreover, the side reaction involving alcohol esterification gives rise to a continuous water release, which favors the concurrent formation of zinc hydroxy-acetate Zn5(OH)8(Ac)2(H2O)2, a lamellar compound also called zinc hydroxy double salt (Zn-HDS).18-20 The formation of larger Zn-HDS particles from the dissolution of ZnO nanoparticles and subsequent reaction between hydrolyzed zinc species and zinc oxy-acetate was unambiguously evidenced by some of us.20 Despite the abundant literature dealing with the coarsening process of ZnO nanoparticles in solution, the current mechanistic views are quite heterogeneous. The diffusion-limited Ostwald ripening mechanism was first proposed to describe the ZnO Qdot growth.21-23 According to the Ostwald-Freundlich equation, the small particles undergo dissolution and the dissolved atoms reprecipitate at the surface of larger particles. In this mechanistic view, particles smaller than the average size disappear, while larger ones become larger and larger. A different view of ZnO nanocrystal growth, which emerged from transmission electron microscopy results, is the grain coalescence induced by grain reorientation.24-27 According to this model, grain rotation among neighboring grains results in a coherent grain-grain interface, which leads to the coalescence of neighboring grains via the elimination of common grain boundaries. 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.15,28 The aim of this work is to give experimental evidence of the different steps involved in the process of ZnO nanoparticle formation and growth under synthesis conditions close to that proposed early by Spanhel and Anderson,12 i.e., [OH]/[Zn] = 0.5, T = 40 °C. To reach this goal, we examine in depth the structural evolution of zinc species in solution and in condensed phase by in situ and simultaneous time-resolved monitoring of UV-vis absorption combined with small-angle X-ray scattering (SAXS) and with X-ray absorption fine structure (XAFS).
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. ZnO colloidal suspensions were prepared following the method proposed by Spanhel and Anderson.12 The Zn4OAc6 tetrameric precursor was first prepared by refluxing an absolute ethanol solution containing 0.05 mol L-1 zinc acetate dihydrate ZnAc2 3 2H2O for 3 h at 80 °C. The thus-obtained transparent precursor solution was stored at ∼4 °C to prevent any further condensation process. Hydrolysis and condensation reactions were carried out in a liquid cell by adding 1.25 mL of KOH absolute ethanol solution (0.5 mol L-1) to 23.75 mL of the precursor solution thermostated at 40 °C. This gave rise to a nominal molar ratio of [KOH]/[Zn] = 0.5. In preliminary experiments, ZnO Qdots with a calibrated size were prepared according to the same procedure. The extracted powdered Qdots were then used as references for the analysis of the XAFS data presented herein. A few mL of colloidal suspension were removed from the reaction bath at different times, during the Qdot growth as monitored by UV-vis spectroscopy. ZnO Qdots were then separated from the colloidal suspension by addition of heptane, an organic “nonsolvent”, following the procedure previously described by Meulenkamp.17 The supernatant solution was discarded and the collected solid precipitate was dried and repeatedly washed in order to remove the unreacted precursor and the side reaction products. Then the
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powders were ground, pressed into pellets, and characterized by XAFS. The Qdot sizes associated to these so-called “calibrated ZnO Qdots” were determined from the UV-vis measurements and correlated to the XAFS data concerning the Zn-Zn contribution as second neighbors. 2.2. In Situ Synthesis Cell. For the in situ combined UVvis/XAFS and UV-vis/SAXS measurements, freshly prepared precursor solutions were introduced into a specially designed liquid cell,29 which consists of a KelF holder with X-ray transparent windows for the X-rays to enter and exit. The thickness of the cell was adjusted to an optical path of about 6 mm for XAFS and 2 mm for SAXS measurements. Moreover, an immersion probe was inserted from the top of the cell into the solution in order to measure the UV-vis spectrum. 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. 2.3. UV-vis Spectroscopy. UV-vis absorption spectra were recorded simultaneously with SAXS and XAFS data by using a Varian Cary 50 UV-vis spectrophotometer equipped with a fiber-optic coupler, connected to an immersion probe with an optical path of 2 mm.30 The spectra were recorded between 240 and 540 nm, with a wavelength step of 1 nm, an average time of 0.1 s per point and a time framing of 1 min. The size of ZnO Qdots was determined from the absorption spectra using the effective mass model derived by Brus2 as fully explained in ref 20. In order to be sensitive to the formation of a small amount of ZnO nanoparticles, the UV-vis spectra were corrected from the absorption spectrum of the zinc oxy-acetate precursor solution, which contributes significantly at short wavelengths. 2.4. X-ray Absorption Spectroscopy. X-ray absorption data were collected on the SAMBA beamline31 at the Soleil synchrotron source using the fixed exit sagitally focusing Si(111) double crystal monochromator. The grazing incidence of the white and monochromatic beams on both Pd-coated collimating and focusing mirrors was set at 4 mrad, ensuring an efficient harmonic rejection. The beam size at the sample position was about 2 mm (H) 0.5 mm (V). Data were recorded in transmission by using ionization chambers filled with a N2/He gas mixture. Data were collected in the so-called fast scanning mode, which enabled us to reduce the collection time to about 210 s for a whole XAFS spectrum. XAFS spectra of size calibrated ZnO Qdots were recorded as references and used as one of the main components for simulating by Linear Combinations (LC) the normalized XAFS spectra of colloidal suspensions. The second component used for the LC analysis is the initial zinc oxy-acetate precursor solution for which the XAFS spectrum has been recorded before the addition of KOH solution. The analysis of the X-ray absorption data was carried out using the software package Athena.32 The maximum of the first derivative of the zinc reference foil recorded simultaneously with the data was used to calibrate each data set, so that the threshold energy, E0, was not taken into account as additional parameter for the LC fits. Prior to the LC fits, a linear background was fitted to the pre-edge region and subtracted from the spectra, which were normalized in a consistent way. LC procedure was carried out on the normalized XAFS data over the -50 to 800 eV energy range with respect to the energy threshold (E0 = 9662 eV). A postedge background using the Autobk was used to isolate the XAFS oscillations χ(k). Then the XAFS data were Fourier transformed using a k3weighted Kaiser-Bessel window. 4405
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2.5. Small Angle X-ray Scattering. The SAXS experiments were performed on the SWING beamline at the SOLEIL synchrotron source. The SAXS measurements were recorded using an AVIEX CCD camera, placed in a vacuum detection tunnel. The sample to detector distance was fixed at 2.3 m and a beamstop of 3 mm (vertical size) with a photodiode inserted in its center enabled measurement of the transmitted intensity. The beam size at the sample position was about 50 300 μm (V H). The beamline energy was set to 7.7 keV, the resulting scattering vector, q, ranged from 0.03 to 2.8 nm-1. The parasitic scattering from the cell and air 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. The analysis of the SAXS data was carried out using the software package SASFIT.33 2.5.1. SAXS Method34. Experimental SAXS curves were analyzed assuming a two-electron density model consisting of a dilute set of isolated particles or aggregates embedded in a homogeneous liquid solution. Under such assumptions, the scattered intensity I(q) in the low-q region (q 3 Rg ≈ 1) can be approximated by the following: ! -Rg2 q2 IðqÞ ¼ Ið0Þexp ð1Þ 3
where I(0) is a scaling factor and Rg is the Guinier radius or radius of gyration of scatters (particles or aggregates). At intermediate and high q-ranges, the SAXS intensity produced by a two-phase system decays as a power law: IðqÞ µ qR
ð2Þ
with R ¼ - 2D þ Ds
ð3Þ
where D is the mass fractal dimensionality (0 e D e 3) and Ds the surface fractal dimensionality (2 e Ds e 3). For uniform (nonfractal) three-dimensional object D = 3, Ds = 2, and R = -4, so that eq 2 is reduced to the classical Porod law obeyed by systems with sharp boundaries. For a mass fractal object, D = Ds and R = -D. In this case, the mass fractal dimensionality is obtained directly from the slope of the log I(q) versus log q plot at the intermediate q-range corresponding to 1/q values between the primary particles radius and the fractal aggregates radius. Furthermore, a second linear regime with a slope R = -4 is attained at high q-range corresponding to 1/q values lower than the size of the primary particle. In the case of solutions of isolated fractal aggregates (diluted solution), the average gyration radius of aggregate (Rg) can be evaluated from the Guinier law (eq 1). Since I(0) is proportional to the mass of the aggregates, M, and Rg corresponds to the mass density radius, the following relation holds: Ið0Þ µ Rg D
Figure 1. (a) Time evolution of the XAFS spectra and (b) the corresponding Fourier transformed XAFS signals.
ð4Þ
So that, a log-log plot of I(0) as a function of Rg is expected to be linear allowing to determine the fractal dimensionality, D.
3. RESULTS 3.1. Time Evolution of Zinc Species from in Situ XAFS. Figure 1(a),(b) shows the time evolution of the XAFS
spectra and of their corresponding Fourier Transforms (FT). Any
phase different from the precursor or from the ZnO nanocrystals is evidenced in the XAFS spectra. 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 precursor and ZnO structures. An increasing intensity of the second intense peak in the FT corresponding to the Zn-Zn contribution at 3.23 Å is observed. This can be either a consequence of an increase of ZnO/precursor proportion and/or due to an increase of the size of the ZnO Qdots.20 Actually, the discrimination between both effects is not straightforward using the results of the fits of the XAFS data alone. This is why we combined UV-vis and XAFS techniques. Indeed, UV-vis spectroscopy enables us to gain information about the size of the ZnO Qdots and about their relative number in solution at any time of the reaction. ZnO aliquots were recovered from the solution (as described in Section 2.1) at characteristic times, 3, 40, and 400 min, their corresponding UV-vis spectra being collected in situ in solution. In a second step, XAFS spectra were measured at the solid state on the recovered and washed ZnO. Both UV-vis and FT of the XAFS data for these aliquots are displayed in the Supporting Information (Figure S1). The dependency of XAFS structural parameters concerning the Zn-Zn contribution and the radius of ZnO Qdot found from the corresponding UV-vis spectra are summarized in Table I. This correlation between Qdot size and Zn-Zn local structure enables us to simulate normalized XAFS spectra of colloidal suspensions by the LC of spectra, corresponding to the precursor and to the ZnO Qdots. In other words, we started the LC using 4406
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Table I. XAFS Structural Parameters for the Second Zinc Coordination Sphere of ZnO Qdot Particles Extracted from the Reaction Solution after Different Time Periods and Average Qdot Radius Determined from UV-vis Spectroscopic Resultsa samples
time (min)
NZn-Zn
σ2 (Å2)
RUV-vis (nm)
aliquot_1
03
6.7 ( 0.6
0.016
1.7
aliquot_2 aliquot_3
40 400
8.7 ( 0.7 11.5 ( 2.5
0.015 0.016
2.0 2.4
NZn-Zn and σ are the coordination number and the Debye Waller factor related to Zinc neighbour shell at 3.23 Å, respectively.
a
Figure 2. Time evolution of the zinc species proportion determined from a linear combination of XAFS spectra of the zinc oxy-acetate precursor and ZnO nanoparticles. Additionally, the in situ measured UV-vis absorbance at 520 nm is displayed as a continuous line (see Figure 3).
normalized XAFS spectra of the precursor and of ZnO Qdot powder early extracted from the reaction bath (aliquot 1). When the size of Qdots growing with time in solution was larger than the size of aliquot 1, we used for the LC analysis, the normalized XAFS spectra of aliquot 2 and of precursor and finally at the advanced stage we used the third aliquot instead of aliquot 2. This method allows the determination of the proportion of each Zn(II) species as a function of time. These results are shown in Figure 2, which combines the time evolution of the fractions of precursor and ZnO Qdots with the time evolution of the UV-vis absorbance recorded at 520 nm (discussed in the following section). It is first important to point out that during the first 4 min of reaction, about half of the precursors are consumed to form Qdot. Then, three regimes are evidenced: at the early stage (5 min < t < 30 min), a very small increase of the ZnO fraction, at the expenses of the precursor one is observed and then followed by a plateau, characteristic of a steady-state chemical equilibrium regime (30 min < t < 400 min). At the advanced stage of the reaction (t > 400 min), the increase of both the fraction of ZnO and absorbance in the UV-vis demonstrates a secondary precipitation process coupled to the growth of large particles or aggregates. 3.2. Time Evolution of Qdot from in Situ UV-vis. The nucleation and growth of the ZnO presented in Figure 1 has been simultaneously monitored by in situ UV-vis spectroscopy. Figure 3(a),(b) shows the 3D-stacked UV-vis spectra plotted as a function of time and a selection of UV-vis spectra at different reaction times, respectively. As highlighted in Figure 3(b), the absorption onset immediately after the reactants mixing is at about 335 nm, which is attributed to the first
Figure 3. (a) Stacked UV-vis spectra recorded in situ during the formation of ZnO nanoparticles and (b) a selected few of these spectra.
excitonic state of ZnO Qdot. The observed first exciton state energy (3.73 eV) is significantly enhanced compared to the free exciton energy in the bulk ZnO (3.36 eV)11 due to the quantum confinement effect. Thus, the continuous red-shift of the onset of the excitonic peak to 370 nm is characteristic of the increase of ZnO Qdot size. Moreover, the intensity of the excitonic peak is almost time independent suggesting that the total amount of Zn atoms forming the Qdot stays almost invariant during the time frame considered in Figure 3(b). For time period shorter than 500 min, the absorbance baseline at wavelength higher than 400 nm is close to zero, indicating a negligible light scattering by large objects. By opposition, a pronounced increase in absorbance at wavelength larger than 400 nm is observed for the time period longer than 500 min, evidencing the increasing contribution of large particles or aggregates. In fact, the comparison between the time evolutions of the UV-vis absorbance recorded at 520 nm and the proportion of zinc species (Figure 2) suggests that the growth of these large objects is coupled with a secondary nucleation of ZnO. Using the effective mass model,2 the average Qdot radius can be obtained from the cutoff wavelength determined by the intersection of the tangent of the excitonic peak threshold with the wavelength axis after the removal of the absorbance tail associated to the scattering effect.16 Figure 4 shows the time dependence of the average ZnO Qdot radius. In agreement with early reports on ZnO nanoparticle growth,14 a stepped feature is verified for the time evolution of the average Qdot radius. At the early stage (t < 30 min), a rapid growth of Qdot is observed from ∼1.8 nm to ∼2.3 nm, then the growth rate decelerates continually, being effectively arrested at ∼140 min with a radius of about 2.6 nm. Finally, a second growing stage, which begins after 280 min, occurs in the same time frame of the secondary nucleation processes evidenced in Figure 2. 4407
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Figure 4. Time evolution of the average ZnO Qdot radius determined from UV-vis spectra. The insert displays the cube of the Qdot radius as a function of time. The continuous line is the best fit of the experimental points with eq 5.
The insert of Figure 4 displays the cube of the Qdot radius as a function of time. It is clear that the Lifshitz, Slyozov, and Wagner35,36 equation (R(t)3- R(0)3 = k 3 t) which describes the Ostwald ripening mechanism cannot fit the experimental points, since it should display a linear behavior. In fact, the best fit for the time evolution of the cube of the average Qdot radius was achieved by using a hyperbolic function y = c 3 x/(1 þ c 3 x), continuous line in the R(t)3 vs t plot. This behavior is discussed in detail in Section 4.2. 3.3. Time Evolution of the Nanostructure from in Situ SAXS. Figure 5(a) presents a three-dimensional stacked loglog plot of the SAXS curves as a function of time. SAXS measurements were also carried out simultaneously with the UV-vis ones. These UV-vis data display the same time evolution than those shown in Figure 3. For clarity, we extracted six profiles of SAXS data corresponding to specific times from the start to the end of the reaction and plotted them in Figure 5(b). Irrespective of the reaction time all SAXS curves present an asymptotic linear trend at high q-range (q > 0.7 nm-1) with a slope R = -4. As the reaction time increases, the extension of this regime increases and the linear behavior with R = -4 becomes more evident. This behavior, which satisfies the classic Porod law (I(q) µ 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. In other words, the primary scatters are nonfractal objects. Moreover, a gradual increase in the scattering intensity in the low q-region is observed over the whole kinetic. The plateau observed at low q-values, also called Guinier region,34 is characteristic of the scattering of a dilute set of noninteracting particles. This feature shifts to lower q values as the reaction time advances, indicating that either the individual nanoparticles increases in size or that the nanoparticles aggregate and therefore form larger structures. After 350 min of reaction, suddenly the scattering intensity corresponding to the plateau regime tremendously increases, the Guinier region shifts to very small q-values and finally vanishes. As a consequence, a second linear region is clearly observed in the low q-region of the scattering profiles, after 550 min of reaction. Due to the decade range of linearity and to the low slope value -1.76, we assume that this regime is associated to the presence of a mass fractal structure (see eq 3). A comparison between the time evolution of average spherical radii RSAXS calculated from Rg value (RSAXS = (5/3)1/2Rg)
Figure 5. (a) Stacked SAXS curves recorded in situ during the formation of ZnO nanoparticles and (b) selected in situ SAXS profiles measured at the indicated reaction time (min).
and the Qdot radii (RUV-vis) is displayed in Figure 6. This figure displays also the ratio RSAXS/RUV-vis evidencing that the difference between these average sizes is minimal at ∼5 min (see insert) and increases continuously after this initial time period. This finding indicates that the particles probed by SAXS are in fact aggregates composed of a few ZnO Qdots. The increase of RSAXS/RUV-vis with the time evidence that the number of Qdot forming the aggregates increases as well. Furthermore, a near linear growth of the aggregate radius is evidenced from the time frame between 30 and 250 min. Complementary information concerning the Qdot aggregation process and about the aggregate structure can be obtained by the dependency between Rg and I(0) displayed in Figure 7. This log-log plot exhibits two well-defined linear ranges demonstrating the validity of eq 4 over a large time period. The first linear regime, with the experimental points measured between 5 and 30 min, presents a straight line of slope equal to 3.0. This implies that the mass and the radius of gyration of the Qdot aggregates are related by a power law (eq 4) with exponent 3.0, as expected for uniform Euclidian objects like as dispersions of dense colloidal particles or nonfractal aggregates. The second straight line, for reaction time above 100 min, presents a slope of 1.6, which is close to the D value observed in Figure 5 for the SAXS curve measured after 600 min. This suggests that the same mechanism controls the aggregation growth in the time frame between 100 and 600 min. The D value of 1.76 is characteristic of a fractal structure formed by the diffusion controlled clustercluster aggregation mechanism,37 indicating that after 100 min of reaction the aggregation of dense colloidal particles or nonfractal aggregates results in fractal objects. 4408
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Figure 6. Comparison of the time evolution of the ZnO Qdot radius (RUV-vis) with the spherical nanoparticle radius determined by SAXS (RSAXS) and the corresponding RSAXS/RUV-vis ratio.
Figure 7. A log-log plot of I(0) vs Rg. The slopes in the two linear ranges are indicated.
4. DISCUSSION The results of in situ and combined monitoring by XAFS/ UV-vis and SAXS/UV-vis show that the formation of colloidal ZnO nanoparticles follows a stepped kinetic composed of four main stages schematized in Figure 8. The consequences of each stage on the evolution of the ZnO colloidal nanostructure are discussed in the following section. 4.1. First Stage: Nucleation (t < 5 min). During the first five minutes of reaction, the fraction of ZnO calculated from LC of XAFS data tremendously increases, while the average radius of Qdot increases from 1.8 to 2.1 nm. By contrast, the calculation of the Guinier radius Rg shows that at time t0, the first detectable size of the ZnO nanoparticles has already reached 2.0 nm and remains essentially constant until 5 min of reaction. We have also noted that the intensity I(0) of the SAXS increases during this time period (Figure 7). This invariance of Rg associated with the increase of I(0) indicates that the number of ZnO particles increases, confirming the nucleation process. This set of results indicates that the equilibrium between ZnO Qdots and precursor species is not established, favoring the nucleation and growth of new ZnO particles. We also observe a slight increase of the pH during the first 5 min due to the continuation of the dissolution of KOH salt. In fact, the presence of a few residual amounts of undissolved KOH is responsible for the small SAXS features at the low q-range, vanishing within 5 min (Figure 5). The release of OH- by the continuous dissolution of KOH maintains the
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solution supersaturation giving rise to the necessary conditions for the slow nucleation and growth of ZnO nanoparticles. Unfortunately, due to the experimental time acquisition, 210 s per spectra for the XAFS and 30 s per frame for the SAXS, too little information was collected in that period of time to be able to give a quantitative analysis of this initial nucleation process. 4.2. Second Stage: Growth of Compact Aggregates (5 min < t < 100 min). In the region of time 5 min < t < 100 min, an increase of the Qdot radius as well as a slight increase in the fraction of ZnO formed, from XAFS, was observed. This region refers to a little progression of the nucleation and growth, where ZnO nuclei are formed and simultaneously ZnO Qdots grow. In this period, the difference between the Qdot radius and the Rg increases continuously, indicating the progressive aggregation of Qdots. Furthermore, the power law dependency between I(0) and Rg with an exponent D = 3.0 ((4) and Figure 7) coupled to the asymptotic Porod regime at high q-range showing slope R = -4 are complementary evidence of the presence of compact Qdots aggregates (nonfractal three-dimensional objects, D = 3) with sharp interface (Ds = 2), see eq 3. Despite the value of D = 3.0 expected for the growth of fractal aggregates from the mechanism of reaction-limited monomer cluster aggregation (RLMCA),38 the narrow increase of Rg, from 2 to 3 nm, involved in this step is inconsistent with the growth of mass fractal structures. It is noteworthy to stress that the evolution of the Qdot radii clearly shows that no significant growth occurred and that the linear dependency of the cube of the Qdot radii with the time (insert Figure 4) expected for diffusion-limited coarsening is not verified. This indicates that Ostwald ripening is not an important mechanism for ZnO Qdot growth in spite of the solubility of this oxide. In fact, the presence of high amount of precursors in solution (Figure 2) can minimize the solubilization of bulk ZnO and effectively suppress the coarsening of particle by dissolution/ reprecipitation process. Hence, it can be expected that the minimum RSAXS/RUV-vis value observed at the beginning of this time period is a consequence of the coalescence between ZnO Qdots. The coalescence of nanocrystalline particles involves the formation of coherent grain-grain interface, which leads to the elimination of common boundaries among neighboring grains. This attachment of nanocrystals with the same crystallographic orientation can be attained either by oriented collisions or by the realignment of particles in contact. The kinetic of the oriented attachment (OA) process limited by the grain-rotation for a single coalescence step of pristine nanoparticles (i.e., particles that have not participated in OA growth) is described by the hyperbolic relation:39,40 R 3 ¼ c 3 t=ð1 þ c 3 tÞ
ð5Þ
where c is a constant depending on both the concentration of nanoparticles and the suspension viscosity. This equation of the OA model was applied to fit the experimental kinetic data of the cube of the average ZnO Qdot radii. The result of the nonlinear least-squares fitting procedure is displayed as continuous line in the insert of Figure 4. The agreement between the model and the experimental results is surprisingly good, also describing the growth saturation step observed near 100 min, even though the model takes into account only OA growth based on a single coalescence step (2A = B). This feature implies that the OA is limited to the coalescence between the primary particles. In fact, due to 4409
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Figure 8. Schematic evolution of colloidal system as derived from the time evolution of the fractions of precursor and zinc oxide (XAFS), the Qdots radius RUV-vis and the aggregate of gyration radius Rg (SAXS).
reduction of particle mobility and steric hindrance effects resulting from particle growth, higher orders of coalescence are harder to be achieved. As a consequence, the number of Qdots forming the aggregates progressively increases as evidenced in Figure 6. 4.3. Third Stage: Growth of Fractal Aggregates (100 min < t < 350 min). The period of time 100 min < t < 350 min corresponds to a steady-state chemical equilibrium zone, where no fresh ZnO nuclei are formed (Figure 2) and a negligible increase in Qdot size is evidenced by UV-vis (Figures 4 and 6). Furthermore, the increasing difference between the Qdot radius and RSAXS (Figure 6) implies that the aggregation process has an increased relevance in this time period. Similar to the previous step, in order to identify the aggregate growth process occurring in this time period, the exponent of eq 4 was calculated from the slope of the straight line verified in the log-log plot of I(0) vs Rg, (Figure 7). The obtained D ≈ 1.6 value clearly indicates the growth of small fractal aggregates. This value observed for Rg > 4.4 nm is lower than the fractal dimensionality D of objects growing by the mechanism of diffusion-limited cluster-cluster aggregation (DLCCA). Sander37 reported a value of D = 1.75, meanwhile Lach-Hab et al.41 calculated a value of D = 1.80 for dilute systems. This value was obtained by studying the radial distribution functions of simulated clusters in the flocculation regime before the gel is formed. A value D = 1.80 and 1.78 was also obtained by simulating the DLCCA by the hierarchical and the polydisperse method.42 The deviation between the theoretical D ≈ 1.75-1.80 expected for the DLCCA mechanism and the experimental value (D ≈1.6) found from the slope of the straight line of Figure 7 can result from the strong effect of polydispersion on both the I(0) and the average value of the Guinier gyration radius that weight much more the large objects. In fact, we observed that all the SAXS curves measured in this time frame (100 min < t < 350 min), present a single crossover point at qiso = 1.1 nm-1, characterizing an isosbestic point (Figure 5). The existence of a single isosbestic point implies that
Figure 9. Fraction of compact and fractal aggregates obtained by linear combination of SAXS data between 100 and 350 min.
a single equilibrium describes the nanostructural transformation occurring from 100 to 350 min. For example, it can correspond to the equilibrium between the compact (nonfractal) aggregate (initial state at 100 min) and the incipient fractal aggregate with D ≈ 1.6 (final state at 350 min). In this case, the scattering object at the initial state is defined by D = 3.0 and Rg ≈ 4.4 nm, while the final state is characterized by D ≈ 1.6 and Rg ≈ 13.6 nm. In this case, Rg corresponds to the average gyration radius of the isolated (no-correlated) fractal aggregate. These two populations of aggregates remain throughout the 100 to 350 min period, one growing in number at the expenses of the other one. This isosbestic point enables the use of the SAXS profiles at 100 and 350 min as references during that period of transformation. Using these two SAXS profiles, we performed a linear combination of the SAXS data measured in between 100 and 350 min and extracted the fractions of the two populations present in solutions versus the time as shown in Figure 9. This striking property confirms the above-mentioned process of clustercluster aggregation, occurring between 100 and 350 min. 4410
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For the kinetic analysis of this aggregation process, we assumed that the so-obtained fraction of each two aggregate families is proportional to the concentration of the given species. Scaling arguments and numerical simulations indicate that kinetics of aggregation processes involving collision between small and large aggregates is described by an exponential function.43 According to this model, the concentration of smaller aggregates (Cs(t)) decreases exponentially with time (eq 6), while the concentration of large (Cl(t)) aggregates increases exponentially (eq 7): Cs ðtÞ ¼ A expð-ktÞ
ð6Þ
Cl ðtÞ ¼ Bð1 - expð-ktÞÞ
ð7Þ
The results of the fitting procedure for eqs 6 and 7 with an apparent constant rate k = 0.87 s-1 are displayed by the continuous line in Figure 9. The excellent agreement between the experimental results and the prediction of kinetic theory confirms the diffusion control of the ZnO aggregation processes. Finally, it is noteworthy to stress that the near linear growth of the average gyration radius of aggregates observed in this time period (Figure 6) is also in excellent agreement with what it is expected from theoretical and experimental studies concerning the DLCCA41-43 mechanism. 4.4. Fourth Stage: Secondary Nucleation and Fractal Aggregates Growth. In this last stage, the conversion from compact to fractal aggregates, mentioned above, is disturbed by the displacement of the chemical equilibrium of the surrounding solution. This feature is evidenced by a secondary formation of ZnO and the proportional decrease of zinc oxy-acetate precursor in solution, as revealed by XAFS monitoring (Figure 2). The change from the steady-state chemical equilibrium to a chemically “unstable” condition is mostly due to the change of water concentration. Indeed, the formation of ZnO by controlled H2O addition or by side reactions leading to H2O production is well reported in literature.18-22 For instance, the esterification reaction can produce water to the point where a critical concentration of water is reached, breaking the previous observed chemical equilibrium. The increase of [H2O]/[Zn] ratio favors the hydrolysis of zinc oxy-acetate precursor. Under basic conditions, the deprotonation of hydrolyzed species proceeds by oxolation reactions forming “Zn-O-Zn” bridges, which lead to the formation of ZnO nanoparticles. The hydrolysis reaction is expected to proceed considerably more slowly for [H2O]/[Zn] < 2.22 Indeed, under the experimental conditions studied herein, it took more than 6 h to induce the secondary nucleation and growth the ZnO nanoparticles (Figure 2). During this secondary nucleation process, the scattering intensity increases dramatically and the Guinier region shifts toward the small q-region to finally vanish: the scattering profiles now display two linear regions (Figure 5). For q higher than 0.7 nm-1, the Porod law (I(q) µ q-4) holds, attesting the smoothness of the nanoparticle surface, while for q smaller than 0.7 nm-1 the slope is close to ∼1.76, characterizing a fractal structure. It is noteworthy to stress that all scattering curves measured at this last stage present a crossover point between the Porod and the fractal regime localized at q ≈ 0.9 nm-1. This finding indicates that the size of the structural unit of fractal aggregate is essentially time invariant. Furthermore, irrespective of the increasing extent of the straight line corresponding to the fractal regime, the slope is timeinvariant. This feature indicates that the mechanism of aggregation growth is not affected by the formation of an increasing amount of
ZnO particles. These simultaneous events coupled to the experimental fractal dimensionality of 1.76 are consistent with the clustercluster aggregation growth involving a polydisperse aggregation model.42,43
5. CONCLUSIONS The hydrolysis-condensation of zinc oxy-acetate precursors in ethanol solution induced by KOH addition, leading to the formation of ZnO nanoparticles was investigated by a combination of in situ time-resolved XAFS/UV-vis and SAXS/UV-vis measurements. It has been shown quantitatively that the formation of ZnO colloids at 40 °C involves the following stepped temporal evolution: (i) at a very early stage the nucleation and growth of ZnO Qdots occur at the expense of the zinc oxyacetate consumption. Due to the slow dissolution of KOH in supersaturated solution, this step is achieved in 5 min leading to a conversion of about 52% of zinc precursor in ZnO; (ii) An intermediate stage is characterized by the growth of compact aggregates of ZnO Qdots and subsequent coalescence of primary particles. The analysis of the ZnO Qdot growth kinetic is in agreement with that expected by the oriented attachment induced by grain-rotation model; (iii) At the subsequent stage the growth of fractal aggregates occurs in a steady-state chemical equilibrium condition. This step is characterized by a single equilibrium between the compact aggregates (D ≈ 3) and the fractal aggregates (D ≈ 1.6). These findings are consistent with the mechanism of diffusion-limited cluster-cluster aggregation. (iv) At the advanced stage, the steady-state chemical equilibrium is disturbed leading to a secondary nucleation and growth of ZnO nanoparticles and a continuous growth of fractal aggregates. These features are consistent with the cluster-cluster aggregation growth involving a polydisperse aggregation model. ’ ASSOCIATED CONTENT
bS
Supporting Information. The quantitative analysis of XAFS as a function of the ZnO Qdot (Figure S1 (a) and (b)). This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ55 16 33019638. Fax: þ55 16 33019692. E-mail:
[email protected] (S.H.P.);
[email protected] (B.L.C.).
’ ACKNOWLEDGMENT The authors want to thank SOLEIL for providing beamtime at the SAMBA and SWING beamlines and for financial support of the experiments. This work was also partially supported by the FAPESP and CAPES/COFECUB cooperation program (Ph 564/07). ’ REFERENCES (1) Efros, Al. L.; Efros, A. L. Sov. Phys. Semicond. 1982, 16, 772–775. (2) Brus, L. E. J. Chem. Phys. 1984, 80, 4403–4407. (3) Nirmal, M.; Norris, D. J.; Kunos, M.; Bawendi, M. G.; Efros, Al. L.; Rosen, M. Phys. Rev. Lett. 1995, 75, 3728–3731. (4) Jiang, W.; Singhal, A.; Zheng, J.; Wang, C.; Chan, W. C. W. Chem. Mater. 2006, 18, 4845–4854. 4411
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The Journal of Physical Chemistry C (5) Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125, 13205–13218. (6) Kittilstved, K. R.; Liu, W. K.; Gamelin, D. R. Nat. Mater. 2006, 5, 291–297. (7) Tan, S. T.; Chen, B. J.; Sun, X. W.; Fan, W. J. J. Appl. Phys. 2005, 98, 013505-1–013505-5. (8) Andelman, T.; Gong, Y. Y.; Polking, M.; Yin, M.; Kuskovsky, I.; Neumark, G.; O’Brien, S. J. Phys. Chem B 2005, 109, 14314–14318. (9) Lin, K. F.; Cheng, H. M.; Hsu, H. C.; Lin, L. J.; Hsieh, W. F. Chem. Phys. Lett. 2005, 409, 208–211. (10) Chen, Y.; Kim, M.; Lian, G.; Johnson, M. B.; Peng, X. J. Am. Chem. Soc. 2005, 127, 13331–13337. (11) Spanhel, L. J. Sol-Gel Sci. Technol. 2006, 39, 7–24. (12) Spanhel, L.; Anderson, M .A. J. Am. Chem. Soc. 1991, 113, 2826–2833. (13) Tokumoto, M. S.; Briois, V.; Santilli, C. V.; Pulcinelli, S. H. J. Sol-Gel Sci. Technol. 2003, 26, 547–551. (14) Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V.; Craievich, A. F. J. Non-Cryst. Solids 1999, 247, 176–182. (15) Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V.; Briois, V. J. Phys. Chem. B 2003, 107, 568–574. (16) Briois, V.; Giorgetti, Ch.; Dartyge, E.; Baudelet, F.; Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V. J. Sol-Gel Sci. Technol. 2006, 39, 25–36. (17) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566–5572. (18) Hosono, E.; Fujihara, S.; Kimura, T.; Imai, H. J Sol-Gel Sci. Technol. 2004, 229, 71–79. (19) Wang, H.; Xie, Ch.; Zeng, D. J. Cryst. Growth 2005, 277, 372–377. (20) Briois, V.; Giorgetti, Ch.; Baudelet, F.; Blanchandin, S.; Tokumoto, M. S.; Pulcinelli, S. H.; Santilli, C. V. J. Phys. Chem. C 2007, 111, 3253–3258. (21) Pesika, N. S.; Stebe, K. J.; Searson, P. C. J Phys. Chem B 2003, 107, 10412–10415. (22) Hu, Z.; Ramírez, D. J. E.; Cervera, B. E. H.; Oskam, G.; Searson, P. C. J. Phys. Chem. B 2005, 109, 11209–11214. (23) Wood, A.; Giersig, M.; Hilgendorff, M.; Vilas-Campos, A.; LizMarzan, L. M.; Mulvaney, P. Aust. J. Chem. 2003, 56, 1051–1057. (24) Pacholski, C.; Kornowski, A.; Weller, H. Angew Chem. Int. 2002, 41, 1188–1191. (25) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H.; Wang, R. M. J. Phys. Chem. B 2005, 109, 87866–8790. (26) Liu, J.; Huang, X.; Sulieman, K. M.; Sun, F.; He, X. J. Phys. Chem. B 2006, 110, 10612–10618. (27) Oskam, G.; Hu, Z.; Penn, R. L.; Pesika, N. S.; Searson, P. C. Phys. Rev. E 2002, 66, 011403-1–011403-4. (28) Lorenz, C.; Emmerling, A.; Fricke, J.; Schmidt, T.; Hilgendorff, M.; Spanhel, L. J. Non-Cryst. Solids 1998, 238, 1–5. (29) St€otzel, J.; L€utzenkirchen-Hecht, D.; Frahm, R.; Santilli, C. V.; Pulcinelli, S. H.; Kaminski, R. C.; Fonda, E.; Villain, F.; Briois, V. J. Phys. Chem. C 2010, 11, 6228–6236. (30) Briois, V.; Belin, S.; Villain, F.; Bouamrane, F.; Lucas, H.; Lescouezec, R.; Julve, M.; Verdaguer, M.; Tokumoto, M. S.; Santilli, C. V.; Pulcinelli, S. H.; Carrier, X.; Krafft, J. M.; Jubin, C.; Che, M. Phys. Scr. 2005, T115, 38–44. (31) Belin, S.; Briois, V.; Traverse, A.; Idir, M.; Moreno, T. Phys. Scr. 2005, 115, 980–983. (32) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537–541. (33) Kohlbrecher, J.; Bressler, I. SASfit 0.92.2; Paul Scherrer Institute: Villigen, 2007. (34) Craievich, A. F. In Handbook of Sol-Gel Science and Technology; Sakka, A., Almeida, R., Eds.; Kluwer Publishers: Norwell, MA, 2005; Vol. 2, pp 161-189. (35) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35–50. (36) Wagner, C. Z. Elecktrochem. 1961, 65, 581–591. (37) Sander, L. M. Contemp. Phys. 2000, 41, 203–218.
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(38) Keefer, K. D. In Better Ceramics Through Chemistry II; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; MRS: Pittsburgh, Pa, 1986; pp 295-304. (39) Lee, E. J. H.; Ribeiro, C.; Longo, E.; Leite, E. R. Chem. Phys. 2006, 328, 229–235. (40) Lee, E. J. H.; Ribeiro, C.; Longo, E.; Leite, E. R. J. Phys. Chem. B 2005, 109, 20842–20846. (41) Lach-Hab, M.; Gonzalez, A. E.; Blaistein-Barojas, E. Phys. Rev. E 1996, 54, 5456–5462. (42) Meakin, P. Annu. Rev. Phys. Chem. 1988, 39, 237–267. (43) Kang, K.; Redner, S.; Meakin, P. Phys. Rev. A 1996, 33, 1171–1182.
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