One-step controllable synthesis for high-quality ultrafine metal oxide semiconductor nanocrystals via a separated two-phase hydrolysis reaction

Article abstract of DOI:10.1021/ja0778702

A one-step synthesis method is described to prepare high-quality ultrafine inorganic semiconductor nanocrystals via a two-phase interface hydrolysis reaction under hydrothermal conditions. With the synthesis of ZrO₂ quantum dots as an example, we show that the prepared nanocrystals have good monodispersity and high crystallinity, as well as other related superior properties, e.g., strong photoluminescence and excellent photocatalytic activities. Also the crystal size can be conveniently adjusted in the range below 10 nm through controlling the reaction temperature. Besides that, this method also shows other distinct advantages compared with other methods reported previously. First, the preparation process is simple and cheap and does not contain any complicated posttreatment procedure. Second, products (without coating) can be collected from the organic phase which effectively avoids grain aggregation induced by the capillary concentration in the water environment. Third, the production yield is very high (almost 100%) and the organic and water phases after reaction can be easily recycled for next reaction. Therefore, it provides a promising strategy for the large-scale industrial production of different kinds of high-quality inorganic nanocrystals.

Full text of DOI:10.1021/ja0778702

Published on Web 02/07/2008
One-Step Controllable Synthesis for High-Quality Ultrafine
Metal Oxide Semiconductor Nanocrystals via a Separated
Two-Phase Hydrolysis Reaction
Kangjian Tang,†,‡ Jianan Zhang,| Wenfu Yan,| Zhonghua Li,| Yangdong Wang,‡
Weimin Yang,‡ Zaiku Xie,‡ Taolei Sun,*,†,§ and Harald Fuchs†,§
Physikalisches Institut, Muenster UniVersity, Muenster 48149, Germany, Center for
Nanotechnology, Muenster 48151, Germany, Shanghai Research Institute of Petrochemical
Technology, SINOPEC, Shanghai 201208, P. R. China, and State Key Laboratory of Inorganic
Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012
P.R. China
Received October 31, 2007; E-mail: Sunt@uni-mienster.de
Abstract: A one-step synthesis method is described to prepare high-quality ultrafine inorganic semiconductor
nanocrystals via a two-phase interface hydrolysis reaction under hydrothermal conditions. With the synthesis
of ZrO2 quantum dots as an example, we show that the prepared nanocrystals have good monodispersity
and high crystallinity, as well as other related superior properties, e.g., strong photoluminescence and
excellent photocatalytic activities. Also the crystal size can be conveniently adjusted in the range below 10
nm through controlling the reaction temperature. Besides that, this method also shows other distinct
advantages compared with other methods reported previously. First, the preparation process is simple
and cheap and does not contain any complicated posttreatment procedure. Second, products (without
coating) can be collected from the organic phase which effectively avoids grain aggregation induced by
the capillary concentration in the water environment. Third, the production yield is very high (almost 100%)
and the organic and water phases after reaction can be easily recycled for next reaction. Therefore, it
provides a promising strategy for the large-scale industrial production of different kinds of high-quality
inorganic nanocrystals.
developed to fabricate these kinds of materials,13 such as the
Introduction
Inorganic semiconductor nanocrystals1-3 have attracted great
interest in recent decades due to their importance in a wide
variety of applications including photocatalysis,4-6 electric
devices,7 light-emitting devices,8,9 solar cells,10 bioimaging,
biosensors,11,12 etc. Although numerous methods have been
sol technique,14,15 micelles or emulsion method,16 sol-gel
process,17,18 hydrothermal synthesis,19,20 pyrolysis,21,22 chemical
vapor deposition,23 etc., they usually suffer from complicated
preparation and posttreatment procedures or problems of ag-
gregation or poor monodispersity, which greatly influence their
properties and restrict their large-scale production and successful
applications in industry. Simple methods that are suitable for
industrial production to prepare monodispersed semiconductor
nanocrystals with tunable size (especially under 10 nm) still
remain a big challenge.
† Muenster University.
‡ SINOPEC.
§ Center for Nanotechnology.
| Jilin University.
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J. AM. CHEM. SOC. 2008, 130, 2676-2680
10.1021/ja0778702 CCC: $40.75 © 2008 American Chemical Society

Metal Oxide Semiconductor Nanocrystals
A R T I C L E S
Scheme 1. Diagram for the Preparation Process with ZrO2 as an
Examplea
a Key: (1) Evaporation and diffusion of two phases in the autoclave is
shown. (2) Hydrolysis reaction occurs at the interface area of two phases.
(3) Shown are zro2 qdots formation and crystallization by the hydrolysis
and hydrothermal reactions, which will mainly deposit into the organic
phase. (4) After the separation of solid products, the organic phase could
be easily recycled for the next reaction by the fractionation method.
In this contribution, we report a novel simple one-step
controllable preparation method for high-quality ultrafine metal
oxide semiconductor nanocrystals via the two-phase hydrolysis
reaction under hydrothermal conditions. With an example of
sub-10 nm ZrO2 quantum dots (qdots), we show that the
products have good monodispersity and high crystallinity, as
well as other related superior properties, e.g., strong photolu-
minescence, excellent photocatalytic activities, etc. More in-
terestingly, the crystal size can be conveniently adjusted in the
range below 10 nm through controlling the reaction temperature.
Besides that, this method also shows other distinct advantages
compared with other methods reported previously. First, the
preparation process is simple and cheap and does not contain
any complicated posttreatment procedure. Second, products
(without coating) can be collected from the organic phase which
effectively avoids grain aggregation induced by the capillary
concentration in water environment. Third, the production yield
is very high (almost 100%) and the organic and water phases
after reaction can be easily recycled for the next reaction. It
would be especially beneficial for industrial production and help
to largely reduce production costs. Therefore, it provides a
promising strategy for large-scale industrial production of
different kinds of high-quality inorganic nanocrystals, provided
that an appropriate two-phase reaction can be found.
Figure 1. (a) Low-resolution TEM and (b) corresponding high-resolution
TEM images of ZrO2 qdots prepared at 160 °C. Further magnified images
show the coexistence of the (c) tetragonal and (d) monoclinic ZrO2 qdots
in the samples.
same time, the hydrothermal conditions also provides a stable
environment for further crystallization.
The tiny toluene droplets can act as nanoreactors with uniform
sizes which confine the further growth of the nanocrystals. It is
a dynamic process that the droplet sizes and the collisions
between different droplets largely depend on the reaction
temperature. Therefore, it provides a convenient approach to
control the nanocrystal size through adjusting the reaction
temperature. The nanocrystals will deposit in the organic phase
because toluene has a higher boiling point than water and thus
the interface may locate at the organic phase side. The separation
of the water and organic phases can efficiently prevent further
growth of the nanocrystals during the remaining hydrothermal
period because the hydrolysis reaction can only occur at the
interface area, which is especially helpful to prepare the
monodispersed ultrafine (e.g., <10 nm) nanocrystals.
Product Characterizations. The ZrO2 samples collected
from the organic phase are all fine white powders with a yield
higher than 99.9% (only very small amount (less than 0.1%) of
1
product can be found in water phase). H NMR (Supporting
Results and Discussion
Information Figure S1) study reveals that there is no unreacted
TBZ and only an amount of n-butanol (the resultant of the
hydrolysis reaction) exists in the organic phase, indicating that
the reaction is very complete. And the chemical analysis on
the organic and water phases further shows that there is only a
trace amount of water in the organic phase and negligible toluene
in the water phase. The above obsevations show that the
resultant solutions can be easily recycled for the next reaction,
and thus, this method is environmental friendly and has great
potential for large-scale production in industry.
Figure 1a shows the typical transmission electron microscope
(TEM) image for a product prepared with hydrothermal tem-
perature of 160 °C. The product shows excellent dispersity
although there is no organic coating (as the reaction system only
contains the reactant, water, and toluene) on the particle surface.
This is much different from the phenomena observed for other
methods to prepare the uncoated inorganic nanocrystals, in
which serious aggregation will usually be observed due to the
The essence of our method is the combination of the interface
hydrolysis and nucleation process in the confined nanoscale
organic matrix and the crystallization process under hydrother-
mal conditions. Scheme 1 shows the preparation process of ZrO2
qdots as an example. The water phase and the organic phase
(toluene, bp 110 °C) containing a metal alkyl oxide precursor
(Zr(OC4H9)4 (TBZ)) were placed separately in the same Teflon-
lined hydrothermal chamber. Under hydrothermal temperatures
(100-240 °C), the water and organic phases are much different
from those at normal conditions because of the autogenous high
pressure when the temperature exceeds the boiling points of
water and toluene. It is a nonliquid and nongas state, and the
two phases may exist as tiny droplets that diffuse gradually to
the other side to form an interface. The hydrolysis reaction
between TBZ and water will occur immediately when the water
and toluene droplets meet and collide with each other, and the
crystal nucleus will form simultaneously in this way. At the
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J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2677

A R T I C L E S
Tang et al.
Table 1. Average Sizes of the TiO2 Nanocrystals Prepared at
Different Temperatures
D([hkl]a/nm
temp/
°
C
[101]
[200]
[105]
[211]
av size/nm
100
120
140
180
220
7.0
10.9
14.2
15.5
20.9
5.9
8.0
13.9
15.7
22.6
8.0
10.0
13.9
17.3
24.3
6.0
7.0
12.2
15.5
20.7
6.7
8.9
13.5
16.0
22.1
a D is the size in different crystal dimensions [hkl] that calculated from
the profile analysis of the corresponding powder XRD peaks (see Supporting
Information).
Figure 2. (a) Size distribution function of ZrO2 qdots that calculated from
the SAXS data. (b) Relationship between the particle size (maximum
probability of diameter) and the hydrothermal temperature.
capillary condensation induced by water evaporation. This
reveals that the collection in a suitable organic phase may serve
as an efficient strategy to solve such a kind of problem. The
corresponding high-resolution TEM image (Figure 1b) shows
that the samples are well crystallized round qdots with good
monodispersity and particle size of ∼4.4 nm, the crystal lattice
of which can be clearly observed in further magnified images
(Figure 1c,d). Meanwhile, the orthogonal lattice (D1 ≈ 0.296
nm, Figure 1c) and the parallel lattice (D2 ≈ 0.318 nm, Figure
1d) for different particles can be indexed to the (111) plane of
the tetragonal and the (1h11) plane of the monoclinic ZrO2
structure, respectively. This implies the coexistence of the
tetragonal and monoclinic ZrO2 qdots in the samples.
The particle sizes of the ZrO2 qdots prepared under different
hydrothermal temperatures were further investigated by the
small-angle X-ray scattering (SAXS) technique.26 The curves
of the distribution function (Figure 2a) that calculated from the
SAXS patterns (see Supporting Information) show a good
monodispersity for all the samples that were prepared at different
temperatures, and their average sizes increase monotonously
from ∼1.0 to ∼6.1 nm when the temperature increases from
110 to 240 °C (Figure 2b).
This indicates an excellent controllability over the grain size
of the nanocrystals through adjusting the reaction temperature,
which may be due to the thermodynamic reasons of the
hydrolysis reaction. To confirm this effect, we used the same
method to prepare TiO2 nanocrystals (precursor: Ti(OC4H9)4),
which show good crystallinity and monodispersity similar to
those of the ZrO2 material reported above. The grain size also
increases regularly from ∼6.7 to ∼22.1 nm (Table 1) when the
reaction temperature changes from 100 to 220 °C. As indicated
by the X-ray line profile analysis (Supporting Information Figure
S2), the as-prepared TiO2 nanocrystals have approximate sizes
Figure 3. XRD patterns for ZrO2 qdots that were prepared at different
temperatures: (a) 100 °C; (b) 120 °C; (c) 140 °C; (d) 160 °C; (e) 180 °C;
(f) 200 °C. Red lines: characteristic peaks for the tetragonal phase. Blue
lines: characteristic peaks for the monoclinic phase.
at different dimensions (e.g., [101], [200], [105], [211]; see
Table 1), indicating a proximately spherical morphology of the
nanocrystals.
The crystallinity is further investigated by powder X-ray
diffraction (XRD) measurement (Figure 3), in which charac-
teristic peaks of monoclinic ZrO2 can all be found in the XRD
patterns for different samples. At the same time, the broadening
and the overlapping of the 2θ peaks after 30° also cover all the
typical peaks of the tetragonal phase. Although the tetragonal
phase is thermodynamically unstable at room temperature, and
the corresponding bulk material can only exist at a temperature
higher than 1170 °C, Garvie27 theoretically predicted that pure
tetragonal ZrO2 can be stable at room temperature when the
particle size is less than 30 nm.28 Therefore, these data also
support the existence of the tetragonal ZrO2 qdots. It can also
be observed that the products prepared at 100 °C show very
broad XRD peaks, which may be because of the very small
particle size (the nanocrystals can only be called as quasi-
crystal), and the peaks become much sharper with the increase
of the preparation temperature, indicating an crystallinity
increase with the increase of the crystal size. Further analysis
of the XRD patterns indicates that the samples prepared at
100 °C (Figure 3a) and 110 °C are mainly composed of
(27) Garvie, R. C. J. Phys. Chem. 1978, 82, 218-224.
(28) Xie, S.; Iglesia, E.; Bell, A. T. Chem. Mater. 2000, 12, 2442-2447.
(26) Schomaker, V.; Glauber, R. Nature 1952, 290-291.
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Metal Oxide Semiconductor Nanocrystals
A R T I C L E S
Figure 5. Photocatalytic properties of the ZrO2 qdots with different particle
sizes: (O) hydrogen output efficiency; (0) specific reaction rate considering
the specific surface area of the nanocrystals.
technology,31 etc. Moreover, as an n-type semiconductor, it is
also widely used as photocatalyst in photochemical reactions,
such as photocatalytic oxidation of propene, photoreduction of
carbon dioxide, etc.,33,34 and the nanosized particles are reported
to possess excellent properties because of the large specific
surface area and other effects.35,36 When the size decreases to
a value that is comparable to the Bohr exciton radius of the
material, e.g., 10 nm or less, it will exhibit some unique
properties because of the nanosize effect, such as photolumi-
nescence, etc. To check the performance of the as-prepared ZrO2
qdots, the luminescent and photocatalytic properties were further
investigated. As shown in the excitation spectra (Figure 4a),
the absorption peaks for the ZrO2 qdots exhibit obvious red-
shift from ∼278.0-284.0 to ∼288.5 nm when the size of the
qdots increases from ∼ 1.0 nm (prepared at T ) 110 °C) to
∼6.1 nm (T ) 240 °C), indicating a distinct quantum size effect.
The emission spectra (Figure 4b) show four emission peaks in
the blue light region for all the samples. As a result, strong
blue light emission (inset of Figure 4b) when samples are
irradiated by a UV-light lamp (365 nm) can be clearly observed
even at daytime. These features are related to the near band-
edge transitions and a midgap trap state that results from
incomplete surface passivation such as surface defects and
oxygen vacancies,15,37 and the change of the corresponding band
gap with the decrease of the nanocrystal size.
Significantly, the as-prepared ZrO2 qdots also show excellent
and size-dependent photocatalytic activity. Figure 5 shows the
relationship between the particle size and the photocatalytic
efficiency of the ZrO2 qdots (as denoted by the hydrogen output
in the photocatalytic hydrogen production experiment). It can
be observed that the photocatalytic efficiency increases first
with the increase of crystal size and reaches a maximum value
of 1676 µmol‚g-1‚h-1 at the size of ∼5.5 nm (prepared at
210 °C) and then decreases dramatically when the size increases
further. Because the specific surface area (SSA) is an important
factor influencing the photocatalytic efficiency, we measured
the SSA values of the samples prepared at different tempera-
tures. As shown in Table 2, the SSA values increase significantly
Figure 4. Fluorescence spectra of the ZrO2 qdots prepared at different
temperatures (T) and dispersed in chloroform: (a) excitation spectra, in
which an obvious red-shift of the absorption peaks can be observed; (b)
emission spectra (excitation wavelength: 285 nm). Four main peaks located
at 331, 344, 401, and 423 nm in the blue-light region can be observed.
Inset: blue-light emission of ZrO2 qdots dispersed in chloroform (recorded
by a CCD camera in a dark room, irradiated by a black-light lamp (365
nm)) (left tube, with ZrO2 qdots; right tube, without ZrO2 qdots).
tetragonal ZrO2 nanocrystals, and there is almost no monoclinic
phase because of the lack of the corresponding characteristic
peaks before 30°. However, the characteristic peaks of the
monoclinic phase begin to appear at 120 °C and their relative
intensities increase with the increase of preparation temperature,
indicating the appearance and the content increase of the
monoclinic nanocrystals.29,30 For samples prepared at temper-
atures higher than 200 °C (Figure 3f), the monoclinic phase
becomes the main component and the content of the tetragonal
phase is very small, which is consistent with the high-resolution
TEM observation. This indicates an interesting nanosize-
dependent phase transition from tetragonal to monoclinic
nanocrystals at low temperatures, which will be examined in
detail in further research.
(31) Cahn, R. W. Nature 1988, 332, 112-113.
Photoluminescent and Photocatalytic Activities of the
(32) Corma, A. Chem. ReV. 1995, 95, 559-614.
(33) Witko, M. J. Mol. Catal. 1991, 70, 277-333.
(34) Wang, C.-M.; Fan, K.-N.; Liu, Z.-P. J. Am. Chem. Soc. 2007, 129, 2642-
2647.
30
Samples. ZrO2 is a classic material that has been widely
applied in many industrial fields, such as advanced ceramics
(35) Edelstein, A. S.; Cammaratra, R. C. Nanomaterials: Synthesis, Properties
and Applications; Institute of Physics Publishing: London, 1998.
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A R T I C L E S
Tang et al.
Table 2. Average Particle Size and Specific Surface Area (BET)
of the ZrO2 Nanocrystals Prepared at Different Temperatures
interface reaction, which efficiently prevents the further growth
of the crystals, we show that this method is very suitable to
prepare monodispersed, highly crystallized, ultrafine (typically
10 nm or less) nanocrystals. The collection in the organic phase
can efficiently prevent nanocrystal aggregation, although there
is no organic coating on their surface. The size of the nano-
crystals can be conveniently adjusted through controlling the
reaction temperature. Moreover, other advantages, e.g., simple
and low cost, high production yield, highly recyclable reaction
solutions, etc., make them easy to be industrialized. It is also
considered that this method can be easily extended to the
synthesis of other high-quality inorganic nanocrystals, e.g.,
metals,39 metal sulfides, metal selenides,40 etc., if only suitable
interface reactions could be found.
temp/
°C
param
110
130
150
170
190
210
240
av particle size/nm 1.3
2.1
3.4
4.3
4.9
5.5
6.1
specific surf area/ 273.3 209.1 177.3 182.8 153.8 168.3 124.1
m2 g-1
with the decrease of the reaction temperatures.38 The specific
reaction rate of the photocatalytic reaction after considering the
SSA values is also included in Figure 5. It can be observed
that 5.5 nm also shows the highest specific reaction rate, which
indicates that the crystallinity is another important factor
influencing the photocatalytic efficiency of ZrO2 nanocrystals.
To make a comparison, we further investigated the photocata-
lytic activity of the commercial ZrO2 (Beijing Chemical Plant,
Beijing, China) and TiO2 (p25, Degussa) powders, which show
Acknowledgment. We thank the partially financial support
from the SINOPEC and the Federal Ministry of Education and
Research of Germany (BMBF).
hydrogen output efficiencies of about 673 and 300 µmol‚g-1h-1
,
respectively. This indicates that the as-prepared ZrO2 qdots have
excellent photocatalytic activity that are much superior to that
of the the commercial products.
Supporting Information Available: Experimental details, 1-
HNMR analysis of the organic phase after reaction, calculation
method for the size distribution function of ZrO2 qdots by SAXS
analysis, and XRD patterns of the TiO2 nanocrystals. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Conclusions
In summary, a simple one-step synthesis method is described
to fabricate high-quality metal oxide semiconductor nanocrystals
via a two-phase interface hydrolysis reaction under hydrothermal
conditions. Due to the confined nucleation and the hydrothermal
crystallization processes, as well as the advantages of the
JA0778702
(39) Jana, N. R.; Peng, X. J. Am. Chem. Soc. 2003, 125, 14280-14281.
(40) Pradha, N.; Xu, H.; Peng, X. Nano Lett. 2006, 6, 720-724.
(38) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171-1210.
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2680 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008