Synthesis and thermal decomposition of Zn(tda)H2O [tda = S(CH2COO)22-]

Article abstract of DOI:10.1021/ic061531c

A novel two-dimensional coordination polymer Zn(tda)H₂O [tda = S(CH₂COO)₂^2-] was synthesized under hydrothermal conditions. The compound crystallized in monoclinic space group P2₁ with a = 16.4154(17) A, b = 5.2133(6) A, c = 16.4210(17) A, β = 114.165(2)°, V = 1282.1(2) A³, and Z = 8. The structure features two-dimensional, noncentrosymmetric networks with a pseudohexagonal network of Zn²⁺ coordinated by tda and water molecules. Zn(tda)H₂O decomposed at T > 300°C to form a ZnO sponge with a surface area ～ 40 m ²/g, which makes it an attractive precursor for nanoporous ZnO.

Full text of DOI:10.1021/ic061531c

Inorg. Chem. 2006, 45, 9634−9636
Synthesis and Thermal Decomposition of Zn(tda)H2O [tda
)
2-
S(CH2COO)2
]
Ming-Cheng Wu and Chi-Shen Lee*
Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung
UniVersity, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan
Received August 14, 2006
A novel two-dimensional coordination polymer Zn(tda)H2O [tda
)
coordination polymers containing rare-earth cations La3+ and
Pr3+ are reported.7 We focused on the synthesis of a
coordination polymer using tda as a bridging ligand to
coordinate with transition metals and found that, under
hydrothermal conditions, tda and Zn2+ link together to form
a new coordination polymer Zn(tda)H2O. The structure
comprises Zn2+ cations interconnected with pentadentate tda
linkers. Zn(tda)H2O decomposes to form a notable spongelike
ZnO for T > 300 °C, which makes it an attractive solid
precursor for nanoporous ZnO. Although the latter material
can be prepared by various methods, such as solution
combustion,8 thermodecomposition,9 and sol-gel methods,10
these processes generally need an organometallic precursor11
or multiple steps to yield this product. The method presented
here requires only one solid precursor, and nanoporous ZnO
is obtained directly from thermal decomposition. Here we
report the detailed synthesis, structural characterization, and
thermal decomposition of Zn(tda)H2O.
Zn(tda)H2O was synthesized hydrothermally from reaction
mixtures containing Zn, tda, and H2O at 180 °C over 2 days.
The product exhibits needle-shaped, transparent crystals with
a yield near 90% (according to Zn). The compound crystal-
lizes in a new structural type with monoclinic space group
P21 and eight formula units per unit cell. Single-crystal X-ray
analysis revealed a novel two-dimensional (2D) network; its
three-dimensional (3D) stacking is noncentrosymmetric. The
acentric structural feature of Zn(tda)H2O is confirmed with
measurements of second-harmonic generation (SHG). The
3D structure of Zn(tda)H2O is shown in Figure 1a. There
are four independent Zn2+ positions in a unit cell; each Zn2+
is surrounded with one S and five O atoms in a distorted
2
S(CH2COO)2 -] was synthesized under hydrothermal conditions.
The compound crystallized in monoclinic space group P21 with a
)
16.4154(17) Å, b 16.4210(17) Å, â )
)
5.2133(6) Å, c
)
114.165(2) , V ) 8. The structure features
°
)
1282.1(2) Å3, and Z
two-dimensional, noncentrosymmetric networks with a pseudohex-
+
agonal network of Zn2 coordinated by tda and water molecules.
Zn(tda)H2O decomposed at T > 300
°C to form a ZnO sponge
with a surface area
∼
40 m2/g, which makes it an attractive
precursor for nanoporous ZnO.
With polydentate ligands and transition metals as building
units, coordination polymers with desired structural motifs
and properties can be synthesized;1 these can serve as
functional materials, such as materials to store hydrogen,2
catalysts,3 and nonlinear optical materials.4 A polycarboxylate
compound is one such multidentate ligand used to synthesize
coordination polymers. For ligands containing two carboxy-
late groups, much work has focused on coordination poly-
mers containing iminodiacetate [ida ) HN(CH2COO)22-] or
oxydiacetate [oda ) O(CH2COO)22-] ions with various metal
cations.5 Coordination polymers containing thiodiacetate
ligands [tda ) S(CH2COO)22-] are rare; only a few metal
complexes such as M(tda)(H2O)4 (M ) Ni, Zn)6 and
* To whom correspondence should be addressed. E-mail: chishen@
mail.nctu.edu.tw.
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9634 Inorganic Chemistry, Vol. 45, No. 24, 2006
10.1021/ic061531c CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/28/2006

COMMUNICATION
Figure 1. (a) 3D structure of Zn(tda)H2O projected along [010]. Color
code: Zn, red; C, black; O, blue; S, yellow. H atoms are omitted for clarity.
(b) 2D structure of Zn(tda)H2O projected along [101h].
octahedral geometry. The S and O atoms at the axial
positions are from the tda ligand and H2O molecule; the other
four O atoms at equatorial positions are from three separate
tda molecules [dZn-S ) 2.523(1) Å; dZn-O ) 2.105(4) Å].
There are four independent tda molecules in a unit cell; for
each tda ligand, the S and O atoms bind to three Zn2+ ions;
one S and two O atoms at the equatorial position bind to
one Zn2+ ion, and another two O atoms coordinate to two
adjacent Zn2+ ions. The binding between Zn2+ and tda forms
a corrugated 2D network that lies in a plane perpendicular
to [101h] (Figure 1b). The 2D sheets are stacked together
along the [101h] direction and are held together by van der
Waals interactions. This uncommon structure, containing
Zn-S bonds pointing in the opposite direction in adjacent
metal centers, is closely related to Cu(oda)(H2O)0.5,12 but
Zn(tda)H2O forms an undulating pseudohexagonal 2D net-
work instead of a pseudorectangular 2D framework for
Cu(oda)(H2O)0.5.
Zn(tda)H2O is stable in air and water. Thermogravimetric
analysis (TGA) shows that Zn(tda)H2O loses H2O (12.1%
mass loss, expected 12.3%) in the temperature range 250-
260 °C to transform to a semitransparent, dehydrated form
of Zn(tda). With continued heating, the compound decom-
poses to form a yellowish foam structure for T > 300 °C.
The TGA experiment was terminated because of a large
expansion of product volume. According to powder X-ray
diffraction (XRD) and energy-dispersive spectrometry (EDS)
analysis, a solid solution of ZnO/ZnS was observed from a
low-temperature product (T < 500 °C) because the Zn2+
cation in Zn(tda)H2O is coordinated to both S (×1) and O
(×5) atoms. Pure ZnO is prepared upon thermal decomposi-
tion of Zn(tda)H2O at T > 500 °C, as confirmed by powder
XRD and EDS analysis.
Figure 2. Temperature-dependent synchrotron powder diffraction patterns
of Zn(tda)H2O. Diffraction features marked with asterisks are for ZnO.
Figure 3. (a) SEM image of ZnO from thermal treatment at T > 550 °C.
The photograph includes magnified pictures showing the fine structure of
ZnO flakes. (b) Excitation, emission, and diffuse-reflectance spectra of
porous ZnO at 25 °C.
been indexed in a model of monoclinic cell a ) 16.38(2) Å,
b ) 5.223(4) Å, c ) 16.48(2) Å, â ) 114.0(1)°, and V )
1288 (4) Å3, similar to the Zn(tda)H2O pattern except that
[202h] and [404h] reflections are absent. These results indicate
that the structure of Zn(tda)H2O remains intact when all water
molecules are removed to form dehydrated Zn(tda). When
the dehydrated material was exposed to water, the structure
collapsed and converted into a molecular form of Zn(tda)-
(H2O)4,6 indicative of an irreversible process. Zn(tda) began
to decompose for T > 300 °C. The diffraction pattern for T
) 500 °C indicated a poorly crystalline nature, with broad
reflections corresponding to ZnO. According to the Scherrer
equation, the average size of the ZnO particles is ∼10 nm,
confirming the presence of a nanocrystalline ZnO phase.
The resulting material reveals a porous and spongelike
structure, which was examined with a scanning electron
microscope (SEM). Figure 3a shows porous ZnO with an
extensive connection of thick flakes containing large pores
of average diameter ∼10 µm. The fine structure of a single
flake consists of countless ZnO crystals of average size ∼10
nm, which is consistent with the result from powder
diffraction tests (inset of Figure 3a). A reason for the
formation of ZnO foam might be the major alteration of
coordination modes from an octahedral to a tetrahedral
environment for the Zn2+ cation. The analogous compound
Ni(tda)H2O was synthesized with the same structure as
Zn(tda)H2O, but thermal decomposition of Ni(tda)H2O
yielded microcrystalline NiS/NiO because coordination of
Ni2+ in both compounds is the same: six-coordinate.
The thermal decomposition of Zn(tda)H2O was investi-
gated on a powder sample with synchrotron powder XRD
for various temperatures in the range of 25-500 °C (NSRRC
BL01C2 line, λ ) 0.775 Å); the results appear in Figure 2.
Near 25 °C, the powder pattern exhibits complicated features
with diffraction signals contributed by water molecules in
[202h] and [404h] diffractions. During dehydration (25 °C <
T < 300 °C), the intensities of [202h] and [404h] decreased
gradually from 25 to 300 °C, indicating that H2O molecules
have been released. The powder pattern at T ) 300 °C has
(12) Whitlow, S. H.; Davey, G. J. Chem. Soc., Dalton Trans. 1975, 1228.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9635

COMMUNICATION
Figure 3b shows the photoluminescence (PL) of porous
ZnO at 25 °C recorded with an excitation energy of 3.42
eV. A strong UV emission was observed at EMAX ) 2.49
eV, which might arise from the recombination of electrons
in singly occupied oxygen vacancies with photoexcited holes
due to an irregular morphology with crystal defects in these
porous materials (Figure 3b).13 The green light is readily
visible with the naked eye when excited with a laboratory
UV lamp. For samples collected at varied decomposition
temperatures, the intensity of the green emission at E ∼ 2.49
eV increased significantly as porous ZnO formed (Figure
S2 in the Supporting Information), indicative of a large
concentration of oxygen vacancies in porous ZnO as
synthesized. The UV-visible diffuse-reflectance spectrum
shows an onset energy similar to that of the PL excitation
spectrum (Figure 3b). The N2 Brunauer-Emmett-Teller
surface area of a porous ZnO sample was 39.8 m2/g,
consistent with the porous structure according to SEM
measurements.
Zn(tda)H2O can serve as a single solid precursor for the
preparation of nanoporous ZnO. The obtained nanoporous
ZnO possesses a large surface area with many pores and
walls, beneficial for applications such as a catalyst and a
gas sensor. The same idea is applicable to the preparation
of new nanoporous materials with a coordination polymer
as a precursor. In work in progress, we are employing this
precursor to prepare a ZnO thin film for photovoltaic
experiments.
Acknowledgment. We thank Dr. H. S. Sheu at National
Synchrotron Radiation Research Center for technical as-
sistance with the XRD experiment, Prof. T. M. Chen for
measurements of PL and UV diffuse reflectance, and Prof.
Eric W. G. Diau for SHG measurements. The National
Science Council (Grants NSC94-2120-M-009-014 and NSC93-
2113-M-009-007) and Institute of Nuclear Energy Research,
Atomic Energy Council, Taiwan (Grant NL940251), sup-
ported this research.
In summary, a new coordination polymer, Zn(tda)H2O,
with a novel 2D sheet structure has been synthesized.
Supporting Information Available: CIF file of Zn(tda)H2O,
UV and PL spectra, a TGA curve, SEM images, and details of
experiments. This material is available free of charge via Internet
at http://pubs.acs.org.
(13) (a) Shi, S. L.; Li, G. Q.; Xu, S. J.; Zhao, Y.; Chen, G. H. J. Phys.
Chem. B 2006, 110, 10475. (b) O¨ zgu¨r, U¨ .; Alivov, Y. I.; Liu, C.; Teke,
A.; Reshchikov, M. A.; Dogˇan, S.; Avrutin, V.; Cho, S.-J.; Morkoc¸d,
H. J. Appl. Phys. 2005, 98, 041301.
IC061531C
9636 Inorganic Chemistry, Vol. 45, No. 24, 2006