Nucleation and growth of ZnO in organic solvents - An in situ study

Article abstract of DOI:10.1021/ja804071h

ZnO is a metal oxide material which possesses versatile properties and applications. Therefore, the target-oriented preparation of ZnO has become a major issue. Many preparation techniques involve bottom-up methods from precursor solutions. In the current contribution, a special precursor system is described that enables a fine-control of kinetic parameters for the nucleation and growth of ZnO in various organic solvents. A large variety of analytical techniques could be applied in an in situ fashion to probe for the ZnO formation at all times and all length scales. Among the analytical techniques are UV/vis, Raman, Fluorescence, X-ray absorption, ¹H NMR-spectroscopy, dynamic light-scattering, and TEM. Three different regimes for nucleation and growth with different characteristics could be identified. Furthermore, the effect of different parameters on the resulting ZnO particle size was investigated.

Full text of DOI:10.1021/ja804071h

Published on Web 11/14/2008
Nucleation and Growth of ZnO in Organic Solvents - an in
Situ Study
C. Lizandara-Pueyo,† M. W. E. van den Berg,‡ A. De Toni,‡ T. Goes,† and
S. Polarz*,†
Department of Chemistry, UniVersity of Konstanz, 78457 Konstanz, Germany, and
Ruhr-UniVersity Bochum, UniVersitaestrausse 150, 44780 Bochum, Germany
Received June 6, 2008; E-mail: sebastian.polarz@uni-konstanz.de
Abstract: ZnO is a metal oxide material which possesses versatile properties and applications. Therefore,
the target-oriented preparation of ZnO has become a major issue. Many preparation techniques involve
bottom-up methods from precursor solutions. In the current contribution, a special precursor system is
described that enables a fine-control of kinetic parameters for the nucleation and growth of ZnO in various
organic solvents. A large variety of analytical techniques could be applied in an in situ fashion to probe for
the ZnO formation at all times and all length scales. Among the analytical techniques are UV/vis, Raman,
Fluorescence, X-ray absorption, 1H NMR-spectroscopy, dynamic light-scattering, and TEM. Three different
regimes for nucleation and growth with different characteristics could be identified. Furthermore, the effect
of different parameters on the resulting ZnO particle size was investigated.
displays, as a high-efficient green phosphor,5 as an UV-
photodetector material,6 as a gas-sensor,7,8 or varistor.9 Apart
Introduction
Zinc oxide (ZnO) has become the most widely studied metal
oxide material. The tremendous interest is provoked by the
multifunctional character of ZnO, the option to vary its
properties via the adjustment of morphology (size and shape),
and the ease of synthesis of even very complex ZnO materials.
It crystallizes in the Wurtzite structure. The P63mc crystal
structure is unique due to its noncentrosymmetrical elemental
cell possessing a hexad axis of symmetry. P63mc is a member
of the polar crystal class. For such crystals particular surfaces
exist corresponding to lattice planes containing only one type
of atoms, either Zn or O. Thus, a ZnO crystal terminated by
polar surfaces like the unreconstructed (0001)-surface (Zn2+
from the mentioned, upcoming applications it is also important
in large-scale products like in catalysis, respectively methanol
synthesis over Cu/ZnO catalysts,10-12 or in the rubber industry,
or as an UV-blocker in sun-lotions.13 A more comprehensive
description of zinc oxide can be found in one of the recent,
excellent review articles.14
Due to its importance and the potential of zinc oxide, it is
obvious that enormous efforts have been made to explore
expedient synthetic ways to ZnO with different particle size,
various particle shapes and tailor-made hierarchical organization.
(5) Darici, Y.; Holloway, P. H.; Sebastian, J.; Trottier, T.; Jones, S.;
Rodriquez, J. J. Vac. Sci. Technol. 1999, 17, 692.
terminated) and the (0001)-surface (O2- terminated) exhibits a
j
(6) Monroy, E.; Omnes, F.; Calle, F. Semicond. Sci. Technol. 2003, 18,
R33.
dipole moment along the crystallographic [0001] direction. As
a consequence, ZnO is both pyroelectric and piezoelectric which
represents the basis for applications as electromechanical or
thermoelectrical coupling devices.1 ZnO is a II-VI wide-gap
semiconductor with a direct gap around 3.4 eV and an exciton
binding energy of ∼60 meV. This makes ZnO a promising
material for UV light-emitting-diodes (LEDs) and lasers.2,3 In
addition, ZnO is also used in solar cells,4 fieldsemission
(7) Polarz, S.; Roy, A.; Lehmann, M.; Driess, M.; Kruis, F. E.; Hoffmann,
A.; Zimmer, P. AdV. Funct. Mater. 2007, 17, 1385.
(8) Lin, H. M.; Tzeng, S. J.; Hsiau, P. J.; Tsai, W. L. Nanostruc. Mater.
1998, 10, 465.
(9) Clarke, D. R. J. Am. Cer. Soc. 1999, 82, 485.
(10) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl.
Catal., A 1999, 179, 21.
(11) Polarz, S.; Strunk, J.; Ischenko, V.; Van den Berg, M.; Hinrichsen,
O.; Muhler, M.; Driess, M. Angew. Chem. 2006, 118, 3031.
(12) Polarz, S.; Neues, F.; Van den Berg, M.; Gru¨nert, W.; Khodeir, L.
J. Am. Chem. Soc. 2005, 127, 12028.
† Konstanz University.
(13) Fan, J.; Freer, R. J. Appl. Phys. 1995, 77, 4795.
(14) (a) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829. (b) Ozgur,
U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.;
Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98. (c)
Klingshirn, C. Phys. Status Solidi B 2007, 244, 3027. (d) Klingshirn,
C. ChemPhysChem 2007, 8, 782. (e) Woell, C. Prog. Surf. Sci. 2007,
82, 55.
‡ Ruhr-University Bochum.
(1) (a) Look, D. C. Mater. Sci. Eng., B 2001, 80, 383. (b) Hickerne, IEEE
Trans. MicrowaVe Theory Tech. 1969, MT17, 957.
(2) (a) Look, D. C.; Claftin, B. Phys. Status Solidi B 2004, 241, 624. (b)
Look, D. C.; Reynolds, D. C.; Litton, C. W.; Jones, R. L.; Eason,
D. B.; Cantwell, G. Appl. Phys. Lett. 2002, 81, 1830.
(3) Meyer, B. K.; Alves, H.; Hofmann, D. M.; Kriegseis, W.; Forster,
D.; Bertram, F.; Christen, J.; Hoffmann, A.; Strassburg, M.; Dworzak,
M.; Haboeck, U.; Rodina, A. V. Phys. Status Solidi B 2004, 241, 231.
(4) (a) Martinez, M. A.; Herrero, J.; Gutierrez, M. T. Sol. Energy Mater.
Sol. Cells 1997, 45, 75. (b) Anderson, N. A.; Ai, X.; Lian, T. Q. J.
Phys. Chem. B 2003, 107, 14414. (c) Keis, K.; Lindgren, J.; Lindquist,
S. E.; Hagfeldt, A. Langmuir 2000, 16, 4688.
(15) (a) Shiloh, M.; Gutman, J. J. Cryst. Growth 1971, 11, 105. (b) Hassani,
S. S.; Lusson, A.; Tromson-Carli, A.; Ballutaud, D.; Didier, G.;
Triboulet, R. J. Cryst. Growth 1999, 207, 30. (c) Sekiguchi, T.; Haga,
K.; Inaba, K. J. Cryst. Growth 2000, 214, 68. (d) Ataev, B. M.;
Bagamadova, A. M.; Djabrailov, A. M.; Mamedov, V. V.; Rabadanov,
R. A. Thin Solid Films 1995, 260, 19. (e) Wu, J. J.; Liu, S. C. AdV.
Mater. 2002, 14, 215.
9
10.1021/ja804071h CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 16601–16610 16601

A R T I C L E S
Lizandara-Pueyo et al.
ZnO can be prepared from the vapor phase using different
methods including chemical vapor deposition,15 or physical
methods like RF magnetron sputtering, molecular-beam epitaxy
or pulsed-laser deposition.3,16 Despite that the mentioned
methods furnish ZnO with very defined and interesting proper-
ties, their big disadvantage is regarding large-scale production.
Larger amounts at significantly less technical effort can be
provided using bottom-up techniques like precipitation/ crystal-
lization from the liquid phase including melts.17 The predomi-
nant precursor-type for zinc oxide are Zn2+ salts combined with
relatively primitive anions, mainly nitrate and acetate. The
lowering of the pH value leads to the transient formation of
zinc hydroxide which then transforms into ZnO by dehydration.
As a consequence most studies about the formation of ZnO from
solution have focused on water and other very polar solvents.18,19
Good precursors for the generation of ZnO in organic solvents
are organometallic zinc complexes. A nice study was published
by Chaudret et al.20 The authors were able to prepare mono-
disperse ZnO colloids from the controlled oxidation of dieth-
ylzinc (ZnEt2) dissolved in tetrahydrofurane (THF). However,
to the best of our knowledge there exists currently no mecha-
nistic report about the nucleation and growth of ZnO in organic
solvents. On the one hand, due to the excellent relevance of
zinc oxide it is also very important to acquire a detailed
knowledge about the formation of ZnO in nonaqueous systems.
On the other hand, the free excess surface energy γLTS between
the liquid and the forming solid plays a dominant role in all
equations describing nucleation and growth. γLTS is directly
influenced by the choice of the solvent. For instance, the free
enthalpy for nucleation ∆GN contains one surface and one
volume term:
volume term in eq 1 is only important at small particle sizes
due to its r2 dependence in comparison to the r3 dependence of
the volume term. Therefore, γLTS is very important during
nucleation and initial growth. γLTS can be varied if the nucleation
experiments are accomplished in diverse solvents.
We present here a mechanistic study on the nucleation and
growth of ZnO in organic, aprotic solvents. A special precursor
system is employed which allows to perform a fine-tuning of
nucleation and growth and, thus, permits to investigate the
ongoing processes by in situ methods.
Results and Discussion
Setting up the System. It was already mentioned that previous
studies on ZnO formation from homogeneous solution have been
performed on polar, protic solvents, most frequently on water.18,19
Zn2+-salts are of only limited use as far as apolar, aprotic
solvents are concerned because their limited solubility in such
media would not allow to reach a sufficiently high supersatu-
ration level. The bis-alkylzinc compounds utilized by Chaudret
et al. might be an alternative. However, the disadvantage of
ZnR2 is that they are difficult to handle due to the pronounced
reactivity.20 It would be very difficult to perform the nucleation
and growth experiments under controlled conditions using bis-
alkyzinc compounds as precursors for ZnO.
Our group introduced a different organometallic precursor
system to ZnO materials chemistry recently: Alkylzincalkoxides
with heterocubane architecture.7,11,12,23-25 A large variety of
the tetrameric compounds [RZnOR′]4 can be prepared from the
reaction of ZnR2 with the respective alcohol in high yield (see
experimental part). The molecular zinc-oxo heterocubanes
possess many advantages: They are highly soluble and stable
in most aprotic and dry organic solvents. Due to their organo-
metallic character they react to ZnO either by thermal reaction
or the reaction with water.12 It has to be stressed that in the
latter case water does not the play the role of a solvent, it is a
reactant. The reactivity of the [RZnOR′]4 compounds is much
lower than the reactivity of the pure bis-alkylzinc compounds
mentioned before, and it can be adjusted via the modification
of the organic rests attached to the heterocubane core. Last but
not least, the ‘Zn4O4′ heterocubane core can be regarded as a
preformed embryo for ZnO. The structure of [MeZnOisoPr]4 for
instance (determined from single-crystal analysis)25 is plotted
in Figure 1a in such a way that the central “Zn4O4” unit becomes
visible. Furthermore, the tetrahedral coordination of the Zn-
centers is highlighted which is also the coordination mode of
zinc in ZnO. However, some differences between the precursor
and ZnO-Wurtzite are also evident. The zinc-centered tetrahedra
are linked to each other via edges for the molecular precursor
and via angles in the Wurtzite crystal structure.
4
3
∆GN ) 4πr2γLTS + πr3∆Gbulk
(1)
with ∆Gbulk being the free enthalpy of the bulk:
-RT ln S
∆Gbulk
)
(2)
Vm
with Vm being the molar volume of the bulk crystal, and S being
the supersaturation.21,22 The so-called supersaturation S is the
quotient of the actual concentration and the concentration of
the respective species at equilibrium conditions. Therefore, S
indicates how far away from equilibrium the system is. The
(16) (a) Sun, X. W.; Kwok, H. S. J. Appl. Phys. 1999, 86, 408. (b) Bagnall,
D. M.; Chen, Y. F.; Shen, M. Y.; Zhu, Z.; Goto, T.; Yao, T. J. Cryst.
Growth 1998, 185, 605. (c) Fujimura, N.; Nishihara, T.; Goto, S.;
Xu, J. F.; Ito, T. J. Cryst. Growth 1993, 130, 269.
(17) (a) Sekiguchi, T.; Miyashita, S.; Obara, K.; Shishido, T.; Sakagami,
N. J. Cryst. Growth 2000, 214, 72. (b) Reynolds, D. C.; Litton, C. W.;
Look, D. C.; Hoelscher, J. E.; Claflin, B.; Collins, T. C.; Nause, J.;
Nemeth, B. J. Appl. Phys. 2004, 95, 4802. (c) Ohyama, M.; Kozuka,
H.; Yoko, T. Thin Solid Films 1997, 306, 78. (d) Meulenkamp, E. A.
J. Phys. Chem. B 1998, 102, 5566.
(21) Mullin, J. W. Crystallization, 3rd ed.; Oxford University Press: Oxford,
1997.
(22) Park, J.; Soon, J. J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem.,
Int. Ed. 2007, 46, 4630.
(18) (a) Wong, E. M.; Bonevich, J. E.; Searson, P. C. J. Phys. Chem. B
1998, 102, 7770. (b) Viswanatha, R.; Sapra, S.; Satpati, B.; Satyam,
P. V.; Dev, B. N.; Sarma, D. D. J. Mater. Chem. 2004, 14, 661.
(19) (a) Turgeman, R.; Tirosh, S.; Gedanken, A. Chem.-Eur. J. 2004, 10,
1845. (b) Uthirakumar, P.; Lee, Y. S.; Suh, E. K.; Hong, C. H. Phys.
Lett. A 2006, 359, 223. (c) Viswanatha, R.; Amenitsch, H.; Sarma,
D. D. J. Am. Chem. Soc. 2007, 129, 4470. (d) Viswanatha, R.; Santra,
P. K.; Dasgupta, C.; Sarma, D. D. Phys. ReV. Lett. 2007, 98.
(20) (a) Monge, M.; Kahn, M. L.; Maisonnat, A.; Chaudret, B. Angew.
Chem., Int. Ed. 2003, 42, 5321. (b) Kahn, M. L.; Monge, M.; Colliere,
V.; Senocq, F.; Maisonnat, A.; Chaudret, B. AdV. Funct. Mater 2005,
15, 458. (c) Kahn, M. L.; Monge, M.; Snoeck, E.; Maisonnat, A.;
Chaudret, B. Small 2005, 1, 221. (d) Kahn, M. L.; Cardinal, T.;
Bousquet, B.; Monge, M.; Jubera, V.; Chaudret, B. ChemPhysChem
2006, 7, 2392.
(23) (a) Polarz, S.; Orlov, A.; Schu¨th, F.; Lu, A. H. Chem.-Eur. J. 2007,
13, 592. (b) Orlov, A.; Roy, A.; Lehmann, M.; Driess, M.; Polarz, S.
J. Am. Chem. Soc. 2007, 129, 371. (c) Polarz, S.; Roy, A.; Merz, M.;
Halm, S.; Schro¨der, D.; Scheider, L.; Bacher, G.; Kruis, F. E.; Driess,
M. Small 2005, 1, 540. (d) Polarz, S.; Orlov, A.; Van den Berg, M.;
Driess, M. Angew. Chem., Int. Ed. 2005, 44, 7892. (e) Roy, A.; Polarz,
S.; Rabe, S.; Rellinghaus, B.; Zahres, H.; Kruis, F. E.; Driess, M.
Chem.-Eur. J. 2004, 10, 1565.
(24) Van den Berg, M.; Polarz, S.; Tkachenko, O. P.; Klementiev, K. V.;
Bandyopadhyay, M.; Khodeir, L.; Gies, H.; Muhler, M.; Gru¨nert, W.
J. Catal. 2006, 241, 446.
(25) Ischenko, V.; Polarz, S.; Grote, D.; Stavarache, V.; Fink, K.; Driess,
M. AdV. Funct. Mater. 2005, 15, 1945.
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Nucleation and Growth of ZnO in Organic Solvents
A R T I C L E S
Figure 1. Generation of ZnO with Wurtzite crystal structure from the reaction of the organometallic zinc-oxo heterocubanes precursors with water is
imaged. The molecular structure of the heterocubane [MeZnOisoPr]4 (H-atoms omitted) is shown exemplarily for the precursor class. Zinc atoms, black;
oxygen atoms, red; carbon atoms, gray. The “Zn4O4” core is highlighted by the thick bonds. Furthermore, the tetrahedral coordination of zinc is indicated
by the blue, translucent coordination polyeders. The vertices of each tetrahedron are given by three oxygen atom and one -CH3-group.
Figure 2. (a) UV-vis absorption spectra measured in transmission modus for the reaction of [MeZnOisoPr]4 with water in THF as a solvent (c ) 8·10-4
mol/L; T ) rt). (b) Absorption values as a function of time measured at a fixed wavelength of λ ) 350 nm for different heterocubanes [MeZnOR]4. (c)
Reaction rate as a function of time.
The formation of ZnO can be followed by optical spectros-
copy in the UV/vis region, absorption and fluorescence mea-
surements, by dynamic light scattering (DLS), and powder X-ray
diffraction (PXRD). Different heterocubanes [MeZnOR′]4 with
R′ ) CH3 ≈ Me, CH2CH3 ≈ Et, CH2CH2CH3 ≈ Pr, CH(CH3)2
was added using a cuvette as a reaction vessel (see Figure 2a).
UV/vis spectra were recorded under constant stirring. With time
the evolution of an absorption edge can be seen. Because all
other products of the reaction between the heterocubanes and
water do not possess an absorption band above λ ) 250 nm,
the absorption band can be correlated to the appearance of zinc
oxide. A significant blue-shift of the adsorption edge to λ ≈
325-360 nm in comparison to bulk-ZnO (λgap ≈ 380 nm) is
found which can be explained by a quantum-size effect caused
≈
isoPr, CH2CH2CH2CH3 ≈ Bu, C(CH3)3 ≈ tertBu have been
synthesized (see Experimental Section and Supporting Informa-
tion S-1). Solutions in tetrahydrofurane (THF) were prepared
and an equimolar amount of water with respect to ZnMe-groups
9
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A R T I C L E S
Lizandara-Pueyo et al.
Figure 3. Temperature dependency (a; T ) 10 °C f dotted line; T ) 25 °C f dashed line; T ) 33 °C f solid line; c ) 8·10-4 mol/L) and concentration
dependency (b; c ) 8·10-4 mol/L f black line; c ) 1.2·10-3 mol/L f gray line) of the reaction of [MeZnOtertBu]4 with water.
by very small ZnO particles18,26 The UV-vis data, respectively
the absorption as a function of time A(t) should give valuable
information about the formation of ZnO (eq 3).
nometallic precursors have been used for sol-gel processes very
rarely which makes it very difficult to relate previous findings
to the study presented here. However, it is well established for
the hydrolysis of transition metal alkoxides containing ad-
ditional, stabilizing ligands that the less electronegative ligands
are cleaved first and then the more electronegative ligands.
Regarding our system, the alkyl zinc alkoxides, it can be
expected that the CH3 ligand is less electronegative than the
OR ligand. Therefore, a chemically reasonable suggestion is
that water will react rapidly with the Zn+-CH3- group leaving
Zn-OH behind. Zn-OH groups will polycondensate to
Zn-O-Zn entities accompanied by water elimination (for better
illustration, the discussed mechanism is shown in the Supporting
Information S-2). 1H NMR spectroscopy is a suitable technique
to check the latter hypothesis because the current process is
accomplished in the homogeneous phase. In-situ 1H NMR
measurements recorded in deuterated THF are shown in Figure
4a. Signals for residual, nondeuterated THF (δ ) 3.58, 1.72
ppm), for the heterocubane precursor (δ ) -0.47, 1.35 ppm),
for the products of the reaction methane and tert-butanol (δ )
0.14, 1.13, 3.66 ppm) and for an intermediate species (δ )
-0.91 ppm) can be observed. Additional information was
acquired in the following way: First, each of the above-
mentioned signals was integrated as a function of time. Then,
a normalization procedure was performed. The signals corre-
sponding to the precursors were set to Inorm ) 100% at t ) 0
min and the signals corresponding to products were set to Inorm
) 100% at t ) 60 min. This procedure allows a comparison of
consumption and emergence of different species to each other
(see Figure 4b).
dcZnO(t)
A(t) f V )
) kges · C[MeZnOR] (t)n1 · CH O(t)n2 (3)
4 2
dt
with kges being the reaction rate constant and n1/2 being the
reaction orders. Therefore, kinetic data were recorded at a fixed
wavelength λ ) 325nm for all the heterocubanes mentioned
(Figure 2a). The result is shown in Figure 2b. The organic group
shielding the “Zn4O4” core has a significant impact on the rate
of ZnO formation. The [MeZnOMe]4 heterocubane reacts very
rapidly. The reaction is finished after 15min. The more shielded
the inorganic core of the precursor is the slower becomes the
process. The reaction of [MeZnOtertBu]4 is not completely
finished even after 100 min. According to eq 3 two further
possibilities exist to alter the rate of the reaction. These are to
change the reaction rate constant or the concentration of at least
one precursor (Figure 3b). A change in temperature (Figure 3a)
alters the reaction rate constant according to Arrhenius law (eq
4).
EA,ges
kges ) k0gesexp -
(4)
(
)
RT
with EA,ges being the activation barrier. The presented data
indicate that the chosen system allows for the fine-tuning of
kinetic factors in such a way that a large variety of analytical
techniques can be applied for a detailed in situ investigation of
homogeneous nucleation and growth of ZnO from the mentioned
molecular precursors. A particularly slow reaction can be
realized for [MeZnOtertBu]4 as a precursors at low temperature
and at low concentration.
1
The findings from H NMR are supported by FT-Raman
spectra recorded for the reaction of [MeZnOtertBu]4 with water
under similar conditions (see Supporting Information S-3).
Raman spectroscopy can also be applied as an in situ technique
in the current case because all heterocubanes precursors possess
well defined Raman spectra (see Supporting Information S-4).
Furthermore, FT-Raman is also fast enough in comparison to
the relevant time scale (a spectrum of sufficient quality can be
recorded within 15 s), and water and hydroxo-groups are not
visible in Raman. The FT-Raman spectrum of [MeZnOtertBu]4
is shown in Supporting Information S-4. In the accessible
spectral window in THF (800-200 cm-1) which was used as a
In situ Investigation of Homogeneous ZnO Nucleation and
Growth. It is necessary for the nucleation of ZnO that a
sufficiently high supersaturation builds up,22 originating from
the reaction of the molecular precursor with water during the
initiation period of the entire process. At this point it is valuable
to see the experiments discussed in the current paper also in
the context of knowledge reported in sol-gel process science.27
Unlike to silica and organosilica materials, there are only limited
mechanistic studies on the formation of transition metal oxides
via the sol-gel process.28 Much of the attention has focused
on pure metal alkoxide systems as sol-gel precursors, eventu-
ally stabilized by nucleophilic, oxygen donor ligands.29 Orga-
(28) (a) Livage, J.; Sanchez, C.; Henry, M.; Doeuff, S. Solid State Ionics
1989, 32-33, 633. (b) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid
State Chem. 1988, 18, 259.
(26) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183.
(27) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and
Chemistry of Sol-Gel Processing, 1st ed.; Academic Press Inc: London,
1990.
(29) (a) Mehrotra, R. C.; Singh, A. Prog. Inorg. Chem. 1997, 46, 239. (b)
Hubert-Pfalzgraf, L. G. J. Mater. Chem. 2004, 14, 3113.
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Nucleation and Growth of ZnO in Organic Solvents
A R T I C L E S
1
Figure 4. (a) In situ H NMR spectra of the reaction of [MeZnOtertBu]4 with water in deuterated THF as as a solvent. (b) Integrated, normalized intensity
of the signals corresponding to important species in the reaction plotted as a function time. (c) Proposed mechanism. For better visibility all groups except
the ones directly reacting are shown as edges in the heterocubane structure.
solvent, this particular precursor exhibits five characteristic
bands. Comparison to the Raman spectra of the alternative
[MeZnOR]4 compounds reveals that the bands at Vj1 ) 220 cm-1,
and Vj2 ) 547 cm-1 correlate to the “Zn4O4” core because they
are present in all precursors. The bands at Vj3 ) 480 cm-1 and
Vj4 ) 771 cm-1 are specific for the OtertBu-group, and the band
at Vj5 ) 383 cm-1 correlates to a “ZnO-C-C” entity. Expect-
edly, the bands Vj1 vanish with time and a new band at Vj6 )
750 cm-1 appears right at the beginning of the reaction. This
new bands originates from the free alcohol tert-butanol. Interest-
ingly, the band at Vj5 ) 383 cm-1 does not vanish completely.
This might indicate that some OtertBu groups remain bound on
the surface of the ZnO-particles.
the cleavage of the Zn-OR bond might be the first step of the
reaction, our data appear to be most consistent with a nucleo-
philic attack of water at the Lewis-acidic Zn2+ center (Figure
4c) forming a penta-coordinated species. The released proton
attacks the alkoxide group and induces the formation of the
alcohol. Further, it is reasonable to assume that the “Zn4O4”
oxocluster now possesses one OH corner instead of OtertBu. The
latter intermediate is unstable against the reaction with a second
intermediate. Two Zn-O-Zn bonds are formed in the next step
and methane is released. The discussed mechanism shown in
Figure 4c is also consistent with the observation that the alkoxide
group has a profound impact on the kinetic of ZnO formation
shown in Figure 2b. To derive the dependence of the reaction
rate with time it is necessary to extract quantitative information
from the UV-vis data. Using Lambert-Beers law (eq 5) it
should be possible to transform the measured adsorption values
A(t) into the corresponding concentration values for ZnO in
solution (cZnO(t)).
Summarizing the results from both analytical methods, one
can say: Unlike to the expectations from the literature on sol-gel
processes, the Zn-CH3 function is not consumed prior to the
alkoxide group (Figure 4b). The free alcohol is formed already
at the beginning of the reaction, although it is known for various
alkoxide sol-gel precursors that the steric demand of the tert-
butyl group stabilizes the metal-oxygen bond. The emergence
of methane occurs later than expected and is coupled to the
degradation of the intermediate species (Figure 4b). The
mechanism that was proposed before (see also Supporting
Information S-2) seems to be incorrect because it requires the
immediate consumption of ROZn-CH3 accompanied by the
A(t)λ)325nm ) ελ)325nm · cZnO(t) · d
(5)
with ελ)
being the extinction coefficient at λ ) 325nm
325nm
and d being the inner extension of the cuvette ) 10.000 mm.
Obviously ελ) is required first. Therefore, ελ) was
325nm
325nm
determined from an independent experiment. A reference system
containing ZnO colloids in a similar size range was prepared
and used to acquire a calibration curve for the extinction
coefficient of ZnO in homogeneous solution (see Supporting
Information S-5). The preparation of the colloidal ZnO solutions
is behind the scope of the current manuscript and will be
presented elsewhere. The extinction coefficient was determined
1
immediate formation of CH4. The occurrence of the H NMR
signal at low field for the intermediate species indicates that
still some sort of [XOZn-CH3]x entity is present. Furthermore,
the intermediate species is formed simultaneously to the release
of the free alcohol. Although we can not rule out for sure that
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16605

A R T I C L E S
Lizandara-Pueyo et al.
Figure 5. (a) Time-dependent development of the band gap energy of ZnO formed from the reaction of [MeZnOtertBu]4 with water (c ) 8·10-4 mol/L, T
) 25 °C), and the ZnO particle size (gray points) determined from Egap. Fluorescence spectra monitoring the same process are shown in (b).
to ελ) 325nm ) 6.6859·104 L·mol-1 ·m-1. The first derivative of
A(t) is the reaction rate V (eq 3). The plot of V against time
reveals similarities in the kinetic behavior independent from the
type of heterocubane used. Four phases can be identified. In
the first phase, the initiation period, very limited ZnO is formed.
During this phase the molecular reactions discussed in the
previous chapter are progressing until a sufficiently high
supersaturation is reached. Then, the ZnO formation rate speeds
up until a maximum is reached (phase II). At the short
wavelength (λ ) 325 nm) used, the formation of even very small
ZnO particles can be observed. Therefore, phase II can be
attributed to the nucleation process. The remaining ZnO-
embryos (≈ molecular precursors) are consumed during phase
III. Surprisingly, for a short moment the ZnO formation rate
appears to increase again in phase IV. However, this is a virtual
increase because a visual inspection of the cuvette indicates that
the solution becomes first turbid and then flocculates in phase
IV. The second maximum in Figure 2c can be explained by the
enhanced scattering of the solution containing dispersed, reflect-
ing particles. Because the experiments were performed in
transmission modus, less light reaches the detector. When
flocculation occurs and particles begin to sediment, more light
is transmitted and the absorption decreases again. The floccula-
tion is reasonable because we have not used any stabilization
agent during the experiments.
measure for particle sizes below Dp ≈ 4 nm due to the significant
blue-shift in this region. Therefore, UV/vis spectroscopy is a
suitable technique to investigate the nucleation process in the
early stages of the process. The values for Egap and Dp calculated
from the EM model are shown in Figure 5a for different times
during the formation of ZnO. During the first 20 min of the
reaction ZnO particles with Dp ) 1.5-2 nm are found, which
we attribute to the critical nuclei. Then, the particle size rises
constantly up to Dp ≈ 8 nm. The conclusions made from UV/
vis spectroscopy are furthermore supported by in situ fluores-
cence spectroscopy shown in Figure 5b. The fluorescence signal
at λ ) 350 nm vanishes in the beginning of the reaction and
can therefore be associated to the heterocubane [MeZnOtertBu]4.
A new signal forms at λ ) 367 nm and then decays again (see
Supporting Information S-7). Therefore, the fluorescence signal
at λ ) 367 nm should be connected to the formation of the
critical nuclei. Finally, a third fluorescence signal with steadily
increasing intensity forms at λ ≈ 377 nm which can be attributed
to larger ZnO particles. It should also be noted that a fourth
fluorescence signal, the so-called defect-luminescence was
detected at λ ) 550 nm.25
So far no information about the internal structure of the
critical nuclei (CN) was accessible. Because their size is below
2 nm the application of PXRD is not an option for further
investigation of the CN. On the other hand, X-ray absorption
spectroscopy (XAS) enables to probe very small particles. The
X-ray Absorption Near Edge Structure (XANES) of the Zn
K-edge probe unoccupied p-like states of the first nearest
neighbors and are thus highly sensitive to both the ligand type,
the symmetry surrounding the central atom, and the observed
changes in the first-shell environment of the central zinc atom.
Based on the knowledge about our system and the kinetic factors
the experimental parameters were adjusted in such a way that
XAS data could be acquired (see Experimental Section) during
the period when nucleation takes place (t1), when growth takes
place (t2) and when the process is nearly finished (t3). The
XANES of the progressing reaction, compared to bulk ZnO as
a reference is shown in Figure 6a and the EXAFS in Figure 6b.
The XANES of bulk ZnO is characterized by five features
E1;shoulder ) 9663 eV, E2 ) 9668 eV, E3 ) 9679 eV, E4 ) 9714
eV and E5 ) 9738 eV. The unsplit peak E2 is the main and
most intense shape resonance and constitutes the typical
whiteline feature.32 Peak E3 is typical of Zn2+ in a tetrahedral
The absorption spectra shown in Figure 2a contain further
information. According to the effective mass (EM) model
introduced by Brus the band gap of semiconductor nanoparticles
depends on crystal size Dp like described in eq 6:26,30
p2π2
2D2p
1.8e2
4π
Egap(Dp) ) E0gap + k
-
- k′
(6)
0Dp
with E0gap being the band gap of bulk ZnO ) 5.39·10-19 J (3.37
eV), ε0 being permittivity of free space, ε being the relative
permittivity ) 3.7, e being the electric charge, k ) 7.01·1030
ms-1, and k′ ) 6.81·1021 J.31 To verify that the effective mass
model is applicable to the samples described in the current study,
different ZnO materials were prepared as reference samples,
the band gap was determined by UV/vis measurements and the
crystal size was determined from PXRD data. Then, the results
were compared to the EM-model, revealing a good agreement
(see Supporting Information S-6). It is seen that the derivation
from the band gap of bulk ZnO represents a very sensitive
(30) (a) Brus, L. E. J. Chem. Phys. 1983, 79, 5566. (b) Brus, L. E. J. Chem.
Phys. 1984, 80, 4403. (c) Rosetti, R.; Ellison, J. L.; Gibson, J. M.;
Brus, L. E. J. Chem. Phys. 1984, 80, 4464.
(32) Larcheri, S.; Armellini, C.; Rocca, F.; Kuzmin, A.; Kalendarev, A.;
Dalba, G.; Graziola, R.; Purans, J.; Pailharey, D.; Jandard, F.
Superlattices Microstruct. 2006, 39, 267.
(31) Brus, L. E. J. Nanostruct. Mater. 1992, 1, 71.
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16606 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Nucleation and Growth of ZnO in Organic Solvents
A R T I C L E S
Figure 6. (a) Zn-K XANES and (b) EXAFS for the reaction of [MeZnOtertBu]4 with water. The nucleation period (black), the growth period (red) and at
the end of the reaction (blue). Bulk ZnO as a reference is shown in dark gray (bottom graph).
environment, as studies on solvated zinc clusters have shown.33
The two distinct features E4 and E5 are EXAFS-like in nature,
but are highly dependent on the first coordination shell. The
distance between features E4 and E5 is indicative (in an empirical
sense) for the metal-ligand distance.34 The XANES recorded
during the nucleation period (t1) is markedly different to the
reference sample (E1(t1) ) 9663 eV; E2(t1) ) 9671 eV; E3(t1)
) 9685 eV; E4(t1) ) 9722 eV; E5(t1) ) missing). The signal at
E1 which is indicative for an anisotropic coordination shell
around Zn2+ centers possesses pronounced intensity.33 The
assumption of an asymmetric, tetrahedral coordination is reason-
able because due to the restricted size of the critical nuclei a
significant amount of atoms are automatically located at the
surface. Then, these zinc atoms are centers which are bound to
OH or alkoxide groups (which have been found be FT-Raman
spectroscopy; see above) at the surface and Zn-O(Zn) in
direction of the volume of the nucleus. The presence of surface
groups is supported by EXAFS (Figure 6b). An additional
shoulder appears in the first coordination shell around the zinc
atoms at a distance of 1.1 Å. It is known for semiconductor not
that localized variations in electronic properties are induced by
the presence of an interface, and that one has to expect
significant surface reconstructions. Such surface reconstructions
result frequently in the shortening of bonding distances between
the atoms in the first and second surface layers. Furthermore, it
is seen that the distances for shells around the central Zn center
is comparable to the pattern found for bulk ZnO. As a
consequence, it seems that the critical nuclei already have
Wurtzite structure. With time the intensity of the peak E1
decreases until at t3 the entire XANES (Figure 6a) is very close
to bulk ZnO. The EXAFS (Figure 6b) for t2 is a superposition
of the features of t1 and t3 indicating that grown nanoparticles
and still some critical nuclei are present. Close to the end of
the reaction the Zn-O shell consists of merely “framework”
zinc-to-oxygen contributions, and as a result the Zn-O peak
becomes narrower and increases in amplitude. The final stage
is characterized by an even stronger increase of the Zn-O
amplitude, whereas the Zn-Zn shell has shifted to 2.55 Å. In
comparison, the bulk zinc oxide sample has Zn-Zn shell at
2.87 Å. The difference can be explained by the nanoparticulate
character of the zinc oxide phase which is in good agreement
to the literature.24
The formation of particles can also be followed by dynamic
light scattering (DLS) measurements. The results are shown in
Figure 7a. Until t ≈ 20 min there is no formation of particles
which is in good agreement with the length of the initiation
period deduced from UV-vis measurements. It should be noted
that it is not possible to detect the critical nuclei (Dp > 2nm)
with our DLS equipment. Then, the formation of ZnO particles
≈ 4 nm in size occurs which further grow to ≈ 8.4 nm which
is also in good agreement with the particle sizes deduced from
in situ UV-vis measurements (Figure 5a). The 8.4 nm large
particles are present almost until the end of the reaction (t )
85 min), however, they vanish finally. Simultaneously to the
primary growth process there is secondary growth as soon as
the primary particles are formed. Larger aggregates are found,
and the size of these aggregates increases steadily until 190 nm
at t ) 105 min. The solution has become turbid at this point
and precipitation of ZnO occurs at t > 110 min. Simultaneous
to the DLS measurement samples were taken from the solution,
one drop was placed on a carbon coated copper carbon grid
and the grid was immediately dried in a desiccators by applying
oil-pump vacuum. Then, the samples were analyzed with
transmission electron microscopy (TEM) shown in Figure 7 and
Supporting Information S-6. After t ) 30 min some ZnO
particles have formed. The aggregation state is rather low and
(33) Takahashi, M.; Tanida, H.; Kawauchi, S.; Harada, M.; Watanabe, I.
J. Synchroton Rad. 1999, 6, 278.
(34) Zhao, W.; Chu, W. S.; Yang, F. F.; Yu, M. J.; Chen, D. L.; Guo,
X. Y.; Zhou, D. W.; Shi, N.; Marcelii, A.; Niu, L. W.; Teng, M. K.;
Gong, W. M.; Benfatto, M.; Wu, Z. Y. Nucl. Instrum. Methods Phys.
Res. A 2007, 580, 451.
9
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A R T I C L E S
Lizandara-Pueyo et al.
Figure 7. (a) Particle sizes determined from DLS measurements as a function of time for the reaction of [MeZnOtertBu]4 (c ) 3·10-3 mol/L; T ) 10 °C)
with water. (b) TEM micrographs of samples taken at different stages in the growth process. t ) 30 min (b), t ) 43 min (c), t ) 66 min (d), t ) 78 min
(e).
Figure 8. (a) Particle size determined from PXRD measurements depending on the concentration of [MeZnOtertBu]4. (b) Effect of an excess of water on
ZnO growth for the two regimes; regime II (0), regime III (O).
a high number of very small particles in the size-range Dp ≈ 4
nm are present.
collected and the diffractograms were evaluated using a refined
Warren-Averbach examination (see Supporting Information S-6)
which is much more precise than the analysis of the width of
singe reflexes using the Scherrer method.
At t ) 43 min growth has occurred and only particles with
Dp ) 7-8 nm can be seen. The aggregation degree is still
relatively low. In the following the size of the primary particles
changes not significantly but the agglomeration is strongly
enhanced, until at t ) 78 min only large agglomerates and no
free particles can be seen.
Dependence of ZnO Formation on External Parameters. It
was shown in the previous section that flocculation effectively
interrupts the growth process of the single ZnO particles. The
investigation of the primary particles size in the precipitated
ZnO materials by PXRD allows the investigation of the effects
of different parameters (concentration of reactands, temperature
and solvent) on growth processes. In order to obtain reliable
information about the particle size after growth PXRD data were
First experiments were performed at different concentrations
of [MeZnOtertBu]4 and corresponding amounts of H2O. The size
of the primary particles was determined from PXRD analysis
of the precipitates formed during phase IV (compare to Figure
2c). It is seen that there is no simple relation between the
concentration of the reactands and the ZnO particle size (Figure
8). Three regimes can be identified. At very low reactant
concentration the particle size increases slightly when the
concentration is elevated. After nucleation there is not enough
heterocubane left to lead to significant growth. Surprisingly, the
ZnO particle size decreases again although the precursor
concentration is now higher in regime II. This unusual behavior
9
16608 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Nucleation and Growth of ZnO in Organic Solvents
A R T I C L E S
Figure 9. (a) Dependence of the ZnO particle size from the temperature applied for the reaction of [MeZnOtertBu]4 with water, and the effect of different
solvents.
can be explained if one considers the number of critical nuclei
N(r*) and its dependence on concentration (eq 7):
Finally, the effect of different solvents on the ZnO particle
size was investigated. The result (see Figure 9b) is rather
surprising. The ZnO particle size seems to depend on the polarity
of the solvent expressed as the dielectric constant ε. It seems
that the ZnO grows more in an apolar solvent like in n-pentane
(Dp ) 21.5 nm) in comparison to a more polar solvent like THF
(Dp ) 7.7 nm). The explanation for this unusual behavior is
probably again the number of critical nuclei formed. N(r*)
depends strongly on γZnO-Solv (see eq 7). A solvent of low polarity
cannot interact so well with the forming, highly polar ZnO
crystal. γZnO-Solv will be high for nonpolar solvents, and according
to eq 8 N(r*) will also be small. Then, the increase of particle
size in nonpolar solvents can be explained by the nucleation-
dominated regime situation (see above). Although dichlo-
romethane possesses a very high polarity, the resulting particle
size is slightly higher compared to THF as a solvent. This
indicates that more factors than only polarity are important, for
instance the ability of a solvent to interact with the growing
ZnO nuclei by coordinative bonds. CH2Cl2 is in contrast to THF
a noncoordinating solvent. It cannot stabilize the ZnO surfaces
by a specific Lewis-base-acid interaction with low coordinated
16πγ3ZnO-solvV2m
G(r*)
RT
3(RT ln S)2
N(r*) ) C · exp -
) C · exp -
(
)
(
)
kT
(7)
with G(r*) being the free enthalpy of the critical nuclei,
γZnO-Solv being the interfacial tension between ZnO and the
solvent, Vm being the molar volume, and S being the super-
saturation. A higher concentration of reactands leads to a faster
reaction (compare to eq 3). This in turn means that a higher
supersaturation is reached in less time (see Figure 3b), respec-
tively the supersaturation will become high. At higher values
for S a larger number of critical nuclei are formed and among
those the remaining amount of ZnO “hidden in the remaining
precursor” is shared during growth. Regime II can therefore be
characterized as the nucleation dominated regime. As a result,
because more and more particles are formed the particle size
decreases in regime II until sufficient precursor is present to
feed the growth process. Then, in regime III which might be
called the growth-dominated regime, the number of critical
nuclei formed is less important. Significant growth takes place
and the particle size increases again. The two regimes II and
III react very different to the presence of an excess of water
(Figure 8b). An excess of water will also lead to an enhancement
of the reaction rate (see eq 3). In regime II the latter leads to
more critical nuclei formed and according to the same arguments
mentioned above to a slight decrease in ZnO particle size. On
the other hand, if there is plenty of ZnO precursor present
(regime III) the increase in reaction speed also by water induces
a significant growth of the particles.
Zn2+ centers. Therefore, γZnO-CH Cl will be larger than γZnO-THF
,
the number of critical nuclei wi2ll 2be smaller, and the particles
will grow larger.
Conclusion
Organometallic methylzinc alkoxide heterocubanes were
investigated as versatile precursors for the synthesis of ZnO in
organic solvents. It could be shown that nanocrystalline ZnO
particles could be formed even at temperatures below room
temperature with water as a reactant. Furthermore, it was
demonstrated that the reactivity of the precursors can be tuned
in such a way that the entire process becomes slow enough to
use standard analytical tools for the in situ investigation of ZnO
nucleation and growth. Several interesting findings are reported.
The strong impact of the alkoxide group was clarified by the
mechanism of the reaction of H2O with [MeZnOR]4. Unexpect-
edly, the alkoxide group becomes cleaved prior to the Zn-CH3
group. The occurrence of a relatively stable intermediate in
which OH represents one edge of the “Zn4O4” core was
postulated. The consumption of the intermediate leads to the
formation of new Zn-O-Zn bonds, resulting in a raise of
supersaturation. Critical nuclei are formed which are of the order
of ∼1.5nm in size and have Wurtzite structure. The conditions
under which the following growth of ZnO occurs are dictated
mainly by the precursor concentration. At “medium” concentra-
A complex behavior is also seen when the reaction is
conducted at different temperatures (Figure 9a). Similarly to
the concentration dependence there are two regimes seen. In
regime II′ the particle size drops with increasing temperature
due to comparable arguments mentioned above: With increasing
temperature, the reaction speeds up (see Figure 3a), and as a
result the same amount of ZnO becomes distributed among a
larger number of particles. The decrease of particle sizes with
decreasing temperature however must be connected to a different
effect (regime I′). At the current point we are not absolutely
sure about the factors playing a role here. However, we suppose
that the situation is similar to regime I (Figure 8a). The reaction
is so slow at low temperatures that although less critical nuclei
are formed they are not sufficiently fed to ensure significant
growth.
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A R T I C L E S
Lizandara-Pueyo et al.
absorption edges of Zn at 9659.0 eV was measured at beamline
ID26 (ESRF, Grenoble (F)). The spectra µ(k) were measured in
transmission mode using photodiodes. The energy was calibrated
immediately prior to the measurement using a zinc metal foil. Prior
to measurement, the reaction was initiated by adding solutions of
Zn-cubane in dry THF and H2O in THF under vigorous stirring.
An aliquot of the reaction mixture was transferred after a few
seconds to the measurement cell (see Supporting Information S-9),
which was then placed in the beam. In order to slow the reaction
sufficiently for its progress to be monitored by XAS-measurement,
the cell was actively cooled to a constant temperature of 5 °C.
XANES data were taken continuously during the ongoing reaction,
whereas the full EXAFS were only measured at the initial,
intermediate and final stage of the reaction (denoted t1, t2 and t3,
respectively). Data treatment was carried out using the software
package VIPER.35 For background subtraction a Victoreen poly-
nomial was fitted to the pre-edge region. A smooth atomic
background µ0(k), was evaluated using smoothed cubic splines. The
radial distribution function FT[k2ꢀ(k)] was obtained by Fourier
transformation of the k2-weighted experimental function ꢀ(k) ) (µ(k)
- µ0(k))/µ0(k) multiplied by a Bessel window.
tions the growth is influenced mainly by the number of critical
nuclei which emerge during the initial period. Interestingly, in
this nucleation dominated regime the choice of solvent is most
important. At otherwise constant conditions the largest particle
were found in solvents least compatible with ZnO and its
surfaces. The situation changes when the concentration is either
lowered or increased significantly. For very high concentration
of heterocubanes, although due to the rapid reaction a high
number of critical nuclei are present large particles form. In
this growth dominated regime the particles grow proportional
to heterocubane concentration. At very low precursor concentra-
tions, despite the limited number of critical nuclei, there are
not enough ZnO embryos left for further growth. Therefore, in
this third regime particle size increases as well with increasing
concentration.
Experimental Section
All starting compounds were received from Aldrich, were purified
and carefully dried prior to use. All reactions were performed under
strict exclusion of air and humidity using Schlenck technique.
Preparation of ZnMe2. ZnCl2 is dried via the treatment with
SOCl2 under reflux. Remaining SOCl2 is removed in vacuum. A
solution of CH3MgI (1.25 mol) in n-Bu2O is slowly added to the
ZnCl2 (0.57 mol) under vigorous stirring. The suspension is heated
for 60 h at 60 °C under reflux. The ZnMe2 (bp ) 43 °C) is collected
by distillation in the temperature range 130-160 °C; 40.8 g ZnMe2
is obtained in 75% yield.
Preparation of the Precursors [MeZnOR]4. ZnMe2 is diluted
with freshly dried and distilled toluene. The solution is cooled to
-78 °C. An equimolar amount of alcohol in toluene is added
dropwise. The solution is warmed to RT and stirred for 12 h. The
toluene is removed in vacuum. The resulting white solid is taken
up in dry hexane. Residual solid products are separated by Schlenck
centrifugation. The pentane is removed from the solution by vacuum
drying.
Analytic Techniques. NMR-spectra were acquired on a Varian
Unity INOVA 400 spectrometer. Raman spectra were performed
on a Perkin-Elmer Raman station 400 Raman Spectrometer. IR
spectra were performed on a Perkin-Elmer Spectrum 100 FT-IR
spectrometer using a Perking Elmer Universal ATR sampling
accessory. X-Ray diffraction was performed on a Bruker AXS D8
Advance diffractometer using CukR1 radiation and a Bruker AXS
Sol-X solid state energy dispersive detector position. The UV/vis
measurements were done on a Varian Cary 100 scan UV/vis
spectrophotometer. The DLS experiment was performed on a
Viscotek 802DLS. TEM images were acquired on a Zeiss Libra
120 at 120kv acceleration voltage. The TEM-samples were prepared
by dropping a drop of the reaction solution on a a carrier covered
with a holy carbon foil (Plano company, S160A3). Fluorescence
spectra were measured on a Varian Carey Eclipse instrument. The
Acknowledgment. In memory of Dr. B. Jahn. Grillo-Werke
AG/Grillo Zinkoxid GmbH is most gratefully acknowledged for
financial support. The Deutsche Forschungsmeinschaft (DFG) is
acknowledged for funding (Project SP780/4-1: Collaborative re-
search center SFB558). We acknowledge the European Synchrotron
Radiation Facility for provision of synchrotron radiation facilities,
and we thank Dr. Marcin Sikora and Dr. Pieter Glatzel and for
assistance in using beamline ID26. We also gratefully thank Dr.
O. Tkachenko for her relentless support of our work. We thank A.
Kuschel for helping us with TEM measurements and U. Haunz for
help with the in situ NMR measurements. Prof. Dr. H. Brintzinger
is acknowledged for valuable discussions.
1
Supporting Information Available: S-1: H NMR data for
the prepared heterocubanes; S-2: Scheme of the excluded,
molecular mechanism; S-3: Time-dependent FT-Raman spectra;
S-4: Raman spectra of the used heterocubanes; S-5: Determi-
nation of the extinction coefficient for ZnO in homogeneous
solution; S-6: Band-gap versus particle size; S-7: Time depend-
ent fluorescence spectra; S-8: TEM images recorded during
nucleation and growth; S-9: XAS sample cell. This information
is available free of charge via the Internet at http://pubs.acs.org.
JA804071H
(35) Klementiev, K. V. VIPER for Windows; 2005.
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16610 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008