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: m-kanatzidis@northwestern.edu

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).

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