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www.sciencemag.org/cgi/content/full/317/5837/490/DC1
Supporting Online Material for
Porous Semiconducting Gels and Aerogels from Chalcogenide Clusters
Santanu Bag, Pantelis N. Trikalitis, Peter J. Chupas, Gerasimos S. Armatas, Mercouri G. Kanatzidis*
*To whom correspondence should be addressed. E-mail: [email protected]
Published 27 July 2007, Science 317, 490 (2007)
DOI: 10.1126/science.1142535
This PDF file includes: Materials and Methods
Figs. S1 to S6
References
MATERIALS AND METHODS
Synthesis of chalcogenide cluster. (TMA)4Ge4S10 (S1), (TMA)4Ge4Se10 (S2),
(TEA)4Sn4Se10 (S3), K4Sn2Se6 (S4), Na4SnS4.14H2O (S5), K4SnSe4 (S6) (where TMA =
tetramethyl ammonium and TEA = tetraethyl ammonium) were used as starting materials
for the cluster building blocks and K2PtCl4 was used as the Pt2+ source. All selenide
compounds were handled inside a glove box under nitrogen atmosphere.
General procedure for chalcogel synthesis. In a typical preparation, 0.09 g (0.1 mmol)
of (TMA)4Ge4S10 was dissolved in 3 ml of water in a vial. In another vial, 0.08 g (0.2
mmol) of K2PtCl4 was dissolved in 2 ml of water. These two solutions were mixed and
poured onto a plastic petridish which was then left undisturbed with cover for a couple of
days during gelation. At this point, the original pink brown solution turned to a dark pink
brown gel. Ethanol was added to the petridish to age the gel. During this aging, the
hydrogel got sufficient mechanical stability to be washed with water which removed all
the soluble by-products from the gel. After several washings with water, ethanol was
added and decanted 4 to 5 times over a period of 2 days. The wet gel was cut into small
pieces with a razor blade and stacked into a critical point drying bracket, which was
subsequently placed into a critical point drying chamber for supercritical drying. After
supercritical drying, aerogels were obtained in high yield (>90%). The same procedure
was also applied to the other chalcogel systems. The only exception was the total volume
of added water (for (TMA)4Ge4Se10 and K4SnSe4 8 ml each, for (TEA)4Sn4Se10 and
K4Sn2Se6 10 ml each and for Na4SnS4.14H2O 4 ml). Critical point drying of the
chalcogels was done in Bal-Tec CPD 030 (Balzers) instrument. The supercritical drying
was performed by exchanging ethanol with liquid carbon dioxide 7-8 times over a period
of 4 hours at 10°C and then raising the temperature to 40°C. After supercritical drying for
about 15 minutes, the gas was slowly vented out.
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Heavy metal adsorption experiment. A stock solution containing 645 ppm Hg2+ was
prepared by accurately measuring the required amount of HgCl2 and dissolving it in
deionized water. The other stock solutions of varying Hg2+ concentrations (up to 92 ppm)
were prepared by diluting the 645 ppm stock solution. To assure the correctness of the
concentrations of the stock solutions, they were analyzed by inductively coupled plasma
atomic emission/optical emission spectroscopy (ICP-AES/OES). For heavy metal ion
adsorption experiment, 10 ml of the stock solution was accurately taken out, added to 10
mg of the chalcogel sample, stirred for 24 hours at room temperature, centrifuged and
then the supernatant solution was analyzed for the final metal ion concentration.
Stock solution of Zn2+ was made by dissolving the Zn(CH3COO)2 salt.
Characterization
Pair distribution function analysis. Diffraction experiments were performed at the
Advanced Photon Source (APS) located at Argonne National Laboratory, Argonne,
Illinois (USA) using the high energy x-rays available at 11-ID-B. The detector was
mounted orthogonal to the beam path, and was centered on the beam. A Mar345 Image
plate was used to collect data on Chalcogel-1 while an amorphous-Si area detector
produced by GE Healthcare was used to collect data on Chalcogel-2. The sample to
detector distance and tilt of the detector relative to the beam were refined using a LaB6
calibrant within the Fit-2D software (S7) with the wavelength of the incident x-rays
calibrated. For data collection, an x-ray energy of 58.2 keV (λ = 0.2128 Å) was used to
record diffraction patterns to high values of momentum transfer while eliminating
fluorescence from the sample. The two dimensional images were integrated within Fit 2D
to obtain the one dimensional powder diffraction pattern, masking areas obscured by the
beam stop arm (S7).
The PDFs, G(r) = 4πr[ρ(r)−ρo] where ρ(r) and ρo are the instantaneous and average
densities, were extracted using PDFgetX2 (S8, S9), subtracting the contributions from the
sample environment and background to the measured diffraction intensities. Corrections
for multiple scattering, x-ray polarization, sample absorption, and Compton scattering
were then applied to obtain the structure function S(Q) (S8, S9). Direct Fourier transform
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of the reduced structure function F(Q) = Q[S(Q) − 1] yielded G(r), the pair distribution
function.
Nitrogen physisorption measurement. Nitrogen adsorption and desorption isotherms
were measured at 77 K on a Micromeritics ASAP 2010 system. For each measurement,
about 200 mg of samples were taken. Before measurement, samples were degassed at 348
K under vaccum (<10-4 mbar) for overnight.
XPS analysis. X-ray photoelectron spectroscopy was acquired on a Perkin Elmer Phi
5400 ESCA system equipped with a Magnesium Kα x-ray source. Samples were analysed
at pressures between 10-9 and 10-8 torr with a pass energy of 29.35 eV and a take-off
angle of 45°. All peaks were referred to the signature C1s peak for adventitious carbon at
284.6 eV.
NMR analysis. Nuclear magnetic resonance spectrum was recorded on a Varian
UnityPlus-500 NMR spectrometer equipped with a 5-mm broad band probe over the
frequency range 50-220 MHz and was obtained without locking. The spectrometer
frequency was 95.367 MHz for 77Se. Typical measurement conditions were: spectral
width of 39024.4 Hz, a 90° pulse angle, acquisition time of 0.5 sec with data point
resolution of 1.0 Hz/pt and 0.2 relaxation delay between scans. The pulse width setting
was 6.0 µs. Exponential multiplication of free induction decay of 5 Hz was used. The 77Se nucleus was externally referenced to neat sample of (CH3)2Se at 25°C.
ESI MS analysis. Electrospray ionization mass spectra were obtained in a Q-Tof
UltimaTM API (micromass). Sample was introduced by flow injection method at the rate
of 0.25 ml/min. 35 volt, 2.5 kV, 9 kV and 2.4 kV were the respective cone, capillary,
TOF and MCP voltages during measurement. Respective source and desolvation
temperature of 100°C and 250°C were used during analysis.
Infrared spectroscopy. FT-IR spectra were recorded on a Nicolet 750 Magna-IR series
II spectrometer with 2 cm-1 resolution.
EDS analysis. Semiquantitative microprobe analyses were performed on a JEOL JSM-
6400 scanning electron microscope (SEM) equipped with a Noran energy-dispersive x-
ray detector. Data acquisition was performed several times in different areas of the
samples using an accelerating voltage of 25 kV and 60-s accumulation time.
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TEM images. TEM samples were prepared by suspending the aerogel sample in ether
and then casting on holey carbon coated Cu grid. High-resolution transmission electron
micrograph (TEM) was obtained with a JEOL 2200FS instrument (field emission)
operating at 200 kV.
Powder x-ray diffraction measurement. PXRD data were collected overnight on an
Inel CPS 120 powder diffractometer (40 kV, 20 mA) with a graphite monochromatized
Cu Kα radiation in asymmetric reflection mode.
Optical band-gap measurement. UV-Vis diffuse reflectance spectra were recorded at
room temperature with a Shimadzu model UV-3101PC double-beam, double
monochromator spectrometer in the wavelength range 200-2,500 nm, using powder
BaSO4 as a 100% reflectance standard. Reflectance data were converted to adsorption
(α/S) data according to the Kubelka-Munk equation: (α/S) = (1-R)2/(2R), where R is the
reflectance and α, S are the adsorption and scattering coefficients, respectively.
Thermogravimetric analysis. TGA data were obtained with a Shimadzu TGA-50
thermal analyzer at a heating rate of 10°C/min under nitrogen flow.
Inductively Coupled Plasma-Atomic Emission (Optical Emission)/ Mass
Spectroscopy [ICP-AES(OES)/MS] analysis. Accurate determinations of Hg2+ and Zn2+
concentrations were performed by ICP-AES using VISTA MPX CCD SIMILTANEOUS
ICP-OES instrument. Standards of the ions of interest (Hg2+ and Zn2+) were prepared by
diluting commercial (Aldrich or GFS chemicals) 1000 ppm ICP-standards of these ions.
Ten calibration standards from 0.5 ppm to 9 ppm were made. The calibration was linear
with errors around 1%. The samples were also diluted before the measurements, so that
their concentrations can fall within the range of calibration. The ICP-AES intensity was
the result of three (30 seconds) exposures. For each sample, three readings of the ICP-
AES intensity were recorded and averaged. The standards were reanalyzed after analysis
of the samples. The distribution coefficient Kd, used for the determination of the affinity
of compounds for Hg2+ is given by the equation m
CCCVK ff
d]/)[( 0 −= where C0 and Cf
are the initial and equilibrium concentrations of Hg2+ (ppm), V is the volume (ml) of the
testing solution and m is the amount (g) of the chalcogel sample used in the experiment.
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The equilibrium Hg2+ concentrations after adsorption was usually found less than 0.1
ppm which is the detection limit of ICP-AES. Quadrupole ICP-MS is capable of
identifying elements from ppt-ppb levels. To accurately determine the amount of Hg2+ a
computer-controlled Thermo Elemental (Waltham, MA) PQ ExCell Inductively Coupled
Plasma Mass Spectrometer (ICP-MS) with a quadrupole setup was used. Isotopes 199Hg, 200Hg, 202Hg were analyzed. Ten standards of Hg2+ in the range of 1-40 ppb were
prepared by diluting a commercial (Aldrich) ~1000 ppm Hg2+ solution. All samples
(including standards) were prepared in a 3% aqua regia solution with 5ppb 115In internal
standard in order to correct for instrumental drift and matrix effects during analysis. To
help stabilization of Hg2+ in solution and to avoid contamination of the plasma by trace
mercury amounts, solution of Au (of about 10 times higher concentration than Hg) was
added to the standards and Hg-containing samples. In addition, aqua regia seem to be
more effective to stabilize the Hg2+ being in ppb levels than the nitric acid. Specifically,
standard solutions acidified with aqua regia retain Hg2+ in the solution for more than two
months. In contrast, Hg2+ standard solutions acidified with nitric acid lost its Hg content
after one month.
Heavy metal adsorption
Chalcogels described have the potential to remove almost the entire content of Hg from
highly contaminated aqueous solutions; Chalcogel-1 reduced 645 ppm of metal ions of a
HgCl2 solution to 0.04 ppm. Given that the saturation point of Hg removal is not
achieved at this concentration, the highest possible capacity would be enormous. The
high KdHg (0.92×107 to 1.61×107 ml/g) value of Chalcogel-1 exceeds those reported for
commercial resins (1.80×104 to 5.10×105 ml/g) (S10), silane chelating fibers (3.00×105 to
3.80×106 ml/g) (S11) and they are comparable with the KdHg values for mesoporous thiol-
functionalized silicates (3.40×105 to 1.01×108 ml/g)(S12, S13).
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SUPPORTING FIGURES AND TEXT
Snapshots of gel formation and pore size distribution plot
Fig. S1. Snapshots of gel (Chalcogel-1) formation when (A) precursor solutions were just mixed, (B) hydrogel was formed, (C) vial in upside down position showing no supernatant liquid. Pore size distribution plots of (D) Chalcogel-1 and (E) Chalcogel-2 calculated from desorption isotherm by the BJH method.
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X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) was used to verify the oxidation states and
possible coordination environments of the elements present in the aerogels. The XPS
spectrum of Chalcogel-1 (Fig. S2, A) showed Ge 3d peak with binding energy of 32.1 eV
which is slightly higher than that in starting (TMA)4Ge4S10 (31.4 eV). This is probably
due to drift of negative charge on terminal S atoms towards the linking Pt2+ center after
binding. This is consistent with the observed decrease in binding energy of the platinum
4f peak. The Pt 4f peak obtained (72.2 eV) in Chalcogel-1 is slightly less than that in
K2PtCl4 (72.8 eV) (Fig. S2, B). This also supports the fact that after the metathesis
reaction the electronegative chloride ions are replaced by less electronegative terminal
sulfur atoms of [Ge4S10]4- resulting slight decrease in oxidation state of platinum (II).
However, no platinum 4f peak at around 71.1 eV is observed, excluding the possibility of
having Pt nanoparticles in these systems. The XPS of the Sn containing chalcogels also
showed increase of Sn 3d binding energy peak relative to that in the starting materials
and decrease of Pt 4f binding energy. For example, the Sn 3d peak in Na4SnS4·14H2O has
binding energy of 485.5 eV whereas that in Chalcogel-6 is 487.2 eV and Pt 4f peak is at
72.4 eV (Fig. S2, C and D).
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Fig. S2. X-ray photoelectron spectra. (A) XPS for germanium and (B) platinum in Chalcogel-1; (C) tin and (D) platinum in Chalcogel-6. Comparisons were made with their respective starting materials. The peak positions are from the curve fitting the experimental data after Shirley background (green open circles) removal.
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77Se NMR spectrum of (TMA)4Ge4Se10 in water during gelation and ESI MS
spectrum of Chalcogel-1
Further support for the integrity of the starting clusters in the final aerogels, is provided
by NMR and electrospray ionisation mass spectroscopy (ESI MS). As solution phase 77Se NMR data can give good indication of stability of clusters in solution (S14), we
investigated the fate of the [Ge4Se10]4- cluster in water after the addition of Pt2+ salt. Since
clear aqueous solution of [Ge4Se10]4- and Pt2+ mixture takes hours (overnight) to become
a hard gel in the NMR tube, we performed spectral acquisition in that time interval. From
the 77Se NMR spectra (Fig. S3), it is clear that no other species (e.g.[Ge2Se6]4-/ [Ge2Se7]4-
or [GeSe4]4-) is involved in the self assembly process towards the formation of the
hydrogel. When the terminal Se atoms are coordinated to Pt, the resonance is shifted
presumably due to binding with Pt2+. The two distinct peaks of the precursor [Ge4Se10]4-
at 73.2 and 205.3 ppm, (inset of Fig. S3) corresponding to terminal and bridging Se
atoms, shift to 78.7 ppm and 212.5 ppm respectively (peak ratio remains ~ 1:1.5).
SeGe
Se
SeSe
Se
GeSe
Ge
SeGe Se
Se
Se
Sebr
Set
Fig. S3. 77Se NMR spectrum of (TMA)4Ge4Se10 in water during gelation in the presence of Pt2+. Inset: 77Se NMR spectrum of (TMA)4Ge4Se10 in water.
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Because a similar NMR study with [Ge4S10]4- is not possible (due to the NMR inactive
nuclei), we sought to apply electrospray ionisation mass spectroscopy (ESI MS) to probe
any interactions between the Pt2+ and [Ge4S10]4- cluster. From the negative ionisation
mode (Fig. S4), we observed peaks at m/z 825.7 and 860.9 and isotopic patterns of which
suggest [Pt(H3O)(Ge4S10)]1- and [Pt(H3O)(HCl)(Ge4S10)]1- respectively. Other peaks at
m/z 614.5, 687.6, 725.5 and 760.6 could be matched to the isotopic distribution patterns
of [(3H)(Ge4S10)]1-, [(TMA)(2H)(Ge4S10)]1-, [(TMA)(K) (H)(Ge4S10)]1- and
[(2TMA)(H)(Ge4S10)]1- respectively (TMA: tetramethylammonium cation). These data
clearly indicate that the adamantane cluster retains its structure and binds to platinum.
Fig. S4. ESI MS spectrum of Chalcogel-1.Data was acquired in the negative ionization mode. The insets show the observed peaks (black) and their simulated patterns (red).
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Far IR spectrum of Chalcogel-1
Comparison of vibrational frequencies of free [Ge4S10]4- with those of Chalcogel-1 in the
far IR region (150-500 cm-1) shows characteristic peaks in the same range as in the free
adamantane cluster (S15, S16). However, due to different binding environments and
amorphous nature of aerogel the peaks are broader (Fig. S5). Peaks due to terminal Ge-St
stretching frequencies between 340 and 480 cm-1 are shifted to lower frequency region
due to weakening of Ge-S bond upon binding with platinum.
450 400 350 300 250 200
Wavenumber (cm-1)
Fig. S5. Comparison of Far IR spectrum of Chalcogel-1 (black dotted line) with that of starting material (TMA)4Ge4S10 (red solid line). Thermogravimetric analysis data The thermal stability of the aerogels was investigated with thermogravimetric analysis
(TGA) and pyrolysis mass spectroscopy (MS). Critical point dried Chalcogel-1 began to
lose weight above 180˚C. The loss was gradual up to 600˚C and then became rapid. From
room temperature to 180°C, a small 2% weight loss is due to physisorbed or chemisorbed
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water or ethanol (Fig. S6). An observed 13% weight loss from 180°C to 440°C
temperature range is accounted for by the loss of four S atoms. The residue at this
temperature is still amorphous. For Chalcogel-1 and Chalcogel-2 again start loosing
weight from 540°C and the residue left at 650°C is amorphous with partially crystalline
‘platinum germanium sulfide or selenide’ (PtGeSe) (S17). Other chalcogels showed
similar thermal behavior.
100 200 300 400 500 600 700 80060
65
70
75
80
85
90
95
100
105
Wei
ght l
oss
(%)
Temperature (°C)
Fig. S6. TGA curves of Chalcogel-1 (black line) and Chalcogel-2 (red line) under nitrogen flow. The temperatures of weight loss are indicated in the corresponding differential thermogravimetric (DTG) curves (dotted line).
References S1. C. L. Bowes et al., Chem. Ber. 129, 283 (1996). S2. H. Ahari et al., J. Chem. Soc., Dalton Trans., 2023 (1998). S3. P. N. Trikalitis, K. K. Rangan, M. G. Kanatzidis, J. Am. Chem. Soc. 124, 2604 (2002). S4. B. Eisenmann, J. Hansa, Z. Kristallogr. 203, 299 (1993). S5. V. W. Schiwy, S. Pohl, B. Krebs, Z. Anorg. Allg. Chem. 402, 77 (1973). S6. K. O. Klepp, Z. Naturforsch., B: Chem. Sci. 47, 411 (1992). S7. A. P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch, D. Häusermann, High Press. Res. 14, 235 (1996). S8. T. Egami, S. J. L. Billinge, Underneath the Bragg Peaks: Structural Analysis of Complex Materials (Pergamon Press, Amsterdam, 2003). S9. X. Qiu, J. W. Thompson, S. J. L. Billinge, J. Appl. Cryst. 37, 678 (2004). S10. X. Chen, X. Feng, J. Liu, G. E. Fryxell, M. Gong, Sep. Sci. Technol. 34, 1121 (1999).
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S11. C. Liu, Y. Huang, N. Naismith, J. Economy, J. Talbott, Environ. Sci. Technol. 37, 4261 (2003). S12. X. Feng et al., Science 276, 923 (1997). S13. J. Liu et al., Adv. Mater. 10, 161 (1998). S14. J. Campbell et al., Inorg. Chem. 34, 6265 (1995). S15. A. Müller, B. N. Cyvin, S. J. Cyvin, S. Pohl, B. Krebs, Spectrochim. Acta 32A, 67 (1976). S16. B. Krebs, S. Pohl, Z. Naturforsch. 26b, 853 (1971). S17. S. Abrahams, J. L. Bernstein, Acta Crystallographica B 33, 301 (1977).
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