Article pubs.acs.org/cm
High-Surface-Area Antimony Sulfide Chalcogels Kota Surya Subrahmanyam,† Christos D. Malliakas,† Saiful M. Islam,† Debajit Sarma,† Jinsong Wu,‡ and Mercouri G. Kanatzidis*,† †
Department of Chemistry, 2145 Sheridan Road, Northwestern University Evanston, Illinois 60208, United States Department of Materials Science and Engineering, NUANCE Center, Northwestern University, Evanston, Illinois 60208, United States
‡
S Supporting Information *
ABSTRACT: Chalcogels are a new class of aerogel materials with diverse properties relevant to catalysis, ion-exchange, and gas adsorption. We report the synthesis of high-surface-area antimony sulfide chalcogels through the sol−gel process followed by supercritical drying. Four different synthetic routes were employed: (1) hydrolysis of sodium thioantimonite (Na3SbS3); (2) ligand metathesis between Sb3+ metal linker and SbS33− anion; (3) reaction of Sb2S3 with Na2S·9H2O; and (4) reaction of Sb2S3 with KOH. All these reactions enable the formation of antimony sulfide gels. The aerogels derived after supercritical drying exhibit high porosity with Brunauer− Emmett−Teller (BET) surface areas up to 300 m2 g−1. The oxidation state of antimony in these chalcogels has been assigned by X-ray photoelectron spectroscopy (XPS) to be +3. Pair distribution function analysis suggests that the local environment around the Sb atoms is very similar to that of crystalline Sb2S3. All the antimony sulfide chalcogels possess the band gap of ∼1.75 eV, and they are thermally stable even up to 600 °C.
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INTRODUCTION Aerogels are a unique class of porous materials with low density, high internal surface area, and distinctive morphological and chemical properties.1,2 They are mostly empty space and are composed of interconnected nanosized building units. In general, aerogels are derived from gels utilizing specific drying techniques such as supercritical drying or freeze-drying, where the liquid component of the gel is replaced by air. Owing to their unique physical and chemical properties, conventional aerogels have drawn an immense interest in material science having a wide range of applications in catalysis,3 thermal insulation,4 sensors,5,6 cosmic dust collection,7 and environmental remediation.8,9 Historically aerogels have metal oxides such as silica,10 titania,1 other main group and transition-metal oxides,11 or organic aerogels.12 The oxide-based gels are prepared by hydrolysis followed by condensation (sol−gel) routes where nanoparticles obtained from their molecular precursors were aggregated to form the wet gel networks.13 Organic gels were first synthesized through covalent crosslinkage of the polymeric clusters, which were obtained by the polycondensation of resorcinol with formaldehyde12,14 and extended to other chemical compositions including polyurethane (PU),15,16 polyimide (PI),17 poly(vinyl alcohol) (PVA),18 and poly(vinyl chloride).19 Subsequent to these studies, a new class of aerogels has emerged based on sulfides, selenides, and tellurides, which possess very different properties from the conventional aerogels.20 Different routes like thiolysis,21,22 nanoparticle condensation,23,24 and metathesis25 © 2016 American Chemical Society
have been reported to prepare chalcogenide aerogels. Gels based on nanocrystals of LaSx, WSx, ZnS, GeSx, CdS, CdSe, PbS have been prepared employing thiolysis and nanoparticle condensation routes.20 Utilizing a freeze-drying technique, monolithic gels from WS2, MoS2, and MoS2/graphene have been shown to form. 26 Another route giving gels of chalcogenide networks involves the coordinative reactions of chalcogenide anions with metal cations, and the resultant gels are referred to as “chalcogels”. The metathesis technique uses anionic building blocks [MQ4]4−, [M2Q6]4−, and [M4Q10]4− (M = Ge, Sn; Q = S, Se),25,27 [MQ4]2− and [M3Q13]2− (M = Mo; Q = S, Se), [MQ3]3− (M = Sb, AS; Q = S),25,27−30 including linear polysulfide ligands Sx2− (x = 3, 4, and 5)31,32 with different metal linkers Pt, Ni, Co, Sb, Zn, Pb, and Bi33 (as well as biomimetic clusters34−36). Another approach is an oxidative coupling process to synthesize molybdenum-sulfide (MoSx) chalcogel.37 The obtained MoSx aerogel was very effective as an adsorbent for iodine and mercury. The chalcogels have exhibited remarkable properties such as efficient hydrodesulfurization (HDS) catalysis,28 electrocatalysis,38 highly selective gas adsorption,29 massive uptake of iodine39,40 and mercury,32 and very selective heavy metal removal from aqueous solutions.25,41 FeS- and FeMoS-based biomimetic chalcogels are shown to be effective as photoReceived: July 16, 2016 Revised: September 23, 2016 Published: September 23, 2016 7744
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Chemistry of Materials catalysts in H2 production and N2 conversion to NH3.42,43 In addition to the above-discussed repertoire, preparation of aerogels from one-dimensional (1D) nanowires, nanotubes and two-dimensional (2D) nanosheets were documented.44,45 Crystalline antimony sulfide (Sb2S3) is important because of potential applications in many areas such as optoelectronics, solar cells, and thermoelectric cooling technologies. Here we report new binary antimony sulfide chalcogels with high surface area prepared using four different synthesis methods. The highly porous antimony sulfide (SbSx) chalcogels can be prepared by (1) hydrolysis of Na3SbS3; (2) metathesis between Sb3+ and SbS33−; (3) reaction of Sb2S3 with Na2S·9H2O; and (4) reaction of Sb2S3 with KOH. Supercritical drying of the formed gels supplants the liquid in the gel pores with CO2 gas while retaining the gel’s pore structure. The new chalcogels possess high surface area with pores ranging from meso to macro scale. X-ray photoelectron spectroscopy (XPS) confirms that the oxidation state of Sb is 3+. Pair distribution function (PDF) analysis46,47 suggests that the local and medium range structure of SbSx chalcogels have a lot of similarities between the structures of “Sb2S3” isolated clusters and Sb2S3 finite sheets.
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EXPERIMENTAL SECTION Figure 1. Photographs of (a) SbS-I, (b) SbS-II, (c) SbS-III, and (d) SbS-IV wet gels. Electron micrographs of SbS-I aerogel: (e) SEM micrograph, (f) low- and (g) high-resolution TEM micrographs, and (h) HAADF-STEM image. Inset in (f) represents the selected area diffraction pattern from the SbS-I chalcogel indicating diffuse scattering and lack of crystalline character in the chalcogel network.
Synthesis of Chalcogels. We prepared the antimony sulfide chalcogels employing four different but straightforward routes, and we denote the respective samples here as SbS-I, SbS-II, SbS-III, and SbSIV. The reactions to form SbS-I and SbS-II chalcogels conducted in a nitrogen-filled glovebox, whereas all the reactions to prepare SbS-III and SbS-IV were carried out at ambient atmosphere. The SbS-I chalcogel was obtained by hydrolysis of Na3SbS3 in the presence of formamide at room temperature. Na3SbS3 itself was prepared by heating a stoichiometric mixture of Na2S, Sb, and S for 0.5 h at 700 °C in an evacuated sealed quartz tube.48 To form the SbS-I gel, 140 mg of Na3SbS3 was dissolved in a mixture of formamide and water (2 mL + 2 mL), and the resultant solution was left undisturbed for overnight at room temperature. The solution slowly coagulated and resulted into a strong orange-red colored monolithic gel. In a metathesis route, to form the SbS-II gel, 0.2 mmol of K2(SbC4H2O6)2·3H2O dissolved in 2 mL of formamide was slowly added to 0.4 mmol of Na3SbS3 in 4 mL of formamide (FM), and the resultant product was left standing for a week. The SbS-III chalcogel was synthesized by reacting antimony sulfide (Sb2S3) powder with Na2S·9H2O in the presence of water followed by addition of formamide. In a typical reaction, 0.3 g of Na2S·9H2O was added to 140 mg of Sb2S3 and mechanically mixed; then 3 mL of water was added to the mixture, and the mixture was stirred for 5 h. Finally, 3 mL of formamide and 3 mL of water were added to the resultant product to form the gel. The SbS-IV chalcogel was prepared similarly to SbS-III by reacting antimony sulfide (Sb2S3) powder with KOH in the presence of water followed by addition of formamide. To obtain the gel, 187.5 mg of KOH was added to 200 mg of Sb2S3 and mechanically mixed. The mixture was added with 3 mL of water and stirred for 30 min. To the resultant product, 3 mL of formamide and 3 mL of water were added. In both the cases of SbS-III and SbS-IV, the gel formation occurred in 5 h. In Figure 1, we show the photographs of SbS-I, SbS-II, SbS-III, and SbS-IV wet gels. The chemical equations for the formation of SbS-I, SbS-II, SbS-III, and SbS-IV and reaction types are listed below.
SbS−III: Na 2S + Sb2S3 → Na 0.1Sb2S3 + Na 2S (acid base reaction followed by hydrolysis) SbS−IV: KOH + Sb2S3 → K 0.2Sb2S3 + K 2S (acid base reaction followed by hydrolysis) (4) The obtained orange-red colored SbSx chalcogels were washed thoroughly with water and ethanol mixture (1:1 by volume) followed by ethanol for 8 days. In this process, the byproducts and any soluble unreacted precursors with the chalcogels were washed away with water and ethanol twice a day for 4 days and finally washed with ethanol twice a day for 4 more days. During washing, soaked solvents were exchanged with fresh solvent twice a day. After completion of this washing process, the entire gel framework was filled with ethanol. To obtain the aerogel, these monolithic gels were cut into pieces and dried super critically at 42 °C and a pressure of 9.65 × 106 Pa (1400 psi) employing a Autosamdri −815B (Tousimis) instrument. This process replaces ethanol from the pores of the gel with air keeping the solid network intact. In order to obtain xerogels, wet gels were dried under vacuum at 50 °C. Dried gels were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), thermogravimetric analysis (TGA), pair distribution function (PDF) analysis, and nitrogen adsorption− desorption isotherm measurements.
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SbS−I: Na3SbS3 + H 2O → Na 0.3Sb2S3 + Na 2S + H 2O (hydrolysis)
(3)
RESULTS AND DISCUSSION The aero and xero versions of the SbSx gels look orange red in color while all the aerogels are fluffy in nature. To verify the morphology and composition of the SbSx chalcogels, SEM and elemental energy-dispersive spectroscopy (EDS) analyses were
(1)
SbS−II: K 2(SbC4H 2O6 )2 3H 2O + Na3SbS3 → K 0.15Na 0.3Sb2S2.5 + K 2(Na3C4 H 2O6 )2 3H 2O (metathesis) (2) 7745
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Chemistry of Materials performed. In Figures 1e, S1a, 2a, and 2c, we depict the typical SEM micrographs of SbSx aerogels. From SEM micrographs, all the SbSx aerogels appear squishy, whereas xerogels look firm (See Figures S2−S5). The elemental energy-dispersive spectroscopy (EDS) data collected from multiple sample locations showed mainly the presence of Sb and S along with of residual Na in SbS-I and SbS-III, Na and K in SbS-II, and K in SbS-IV (Figures S2−S5 in Supporting Information). From EDS analyses, the average elemental compositions of SbS-I, SbS-II, SbS-III, and SbS-IV can be written as Na 0.3 Sb 2 S 3 , K0.15Na0.3Sb2S2.5, Na0.1Sb2S3, and K0.2Sb2S3, respectively. The observed residual sodium or potassium in SbSx chalcogels is in fact part of the material and neutralizes the residual anionic charge in the network. The microstructure of chalcogels were investigated utilizing both the TEM and STEM imaging, and the representative micrographs are shown in Figures 1 and 2.
Figure 3. (a) Powder X-ray diffraction pattern, (b) N2 adsorption and desorption isotherms at 77K, (c) solid-state UV−vis/NIR optical absorption spectrum (converted from reflectance), and (d) TGA trace of the SbS-I aerogel.
242 m2/g, respectively. The observed surface areas are equivalent to silica surface areas in the range of 1400−1670 m2/g, and they are comparable to porous silica aerogels. The hysteresis loops observed at high pressure regime were obtained due to (inset in Figure S7b) pore blocking (percolation) effects.49 SbS-I, II, III, and IV have the BJH adsorption and desorption average pore diameters of 15 and 12 nm, 12 and 10 nm, 13 and 14 nm, and 14 and 16 nm, respectively. Vacuum drying of the SbSx wet gels produced xerogels with significantly lower surface areas in the range of 20−90 m2/g. The UV−vis/NIR spectra of SbSx chalcogels reveal that they exhibit a band gap of 1.75 eV absorbing light in the visible region (Figures 3c, S6c, S7c, and S8c). The XPS data (Figure 4) indicated the presence of antimony, sulfur, and traces of alkali metals. The doublets at 529.15 (538.51), 529.8(539.14), 529.68(539.03), and 529.53(538.87)
Figure 2. SEM micrographs of (a) SbS-III and (c) SbS-IV aerogels. TEM micrographs of (b) SbS-III and (d) SbS-IV aerogels. Insets in (b) and (d) show the selected area diffraction (SAD) patterns from the corresponding chalcogels.
The combined high-resolution TEM and STEM analyses suggest that SbS-I and SbS-II chalcogels (see Figure 1f−h and Figure S1b−d) show a porous network with pores in the range of meso (2−50 nm) and macro (>50 nm) region, while SbS-III and SbS-IV show bundles of nanowires (Figure 2b,d). The high-angle annular dark-field (HAADF)-STEM images of SbS-I and SbS-II chalcogels (Figure 1h and Figure S1d) show Z-contrast where dark regions represent pores. The pores, which are in the range of 50−100 nm, are shown by the red arrowheads, whereas the smaller pores (having dark contrast) are represented by green arrowheads. The selected area electron diffraction (SAED) patterns on all the SbSx chalcogels showed diffuse rings, revealing their amorphous nature. Representative PXRD patterns from all the SbSx samples are shown in Figures 3a, S6a, S7a, and S8a. All the chalcogels mainly show the broad features centering around 30° 2θ, indicating that the gels are amorphous. The porosity of the aerogels was evaluated by using Brunauer−Emmett−Teller (BET) model and N2 adsorption data at 77 K. In Figures 3b, S6b, S7b, and S8b, we show the N2 adsorption−desorption isotherms of SbSx chalcogels. All chalcogels exhibit combined characteristics of type I and type-II adsorption isotherms with hysteresis at the highpressure regime. The aerogels of SbS-I, SbS-II, SbS-III, and SbS-IV exhibit high specific surface areas of 290, 257, 201, and
Figure 4. (a) X-ray photoelectron spectra of the SbS-I, SbS-II, SbS-III, and SbS-IV. Inset along the SbS-II represents the K 2p photoelectron spectra. 7746
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Chemistry of Materials
ordering. The simulated PDF of the finite Sb2S3 layer with a size of 20 Å does not agree very well with the experimental data in the range above 6 Å, suggesting that the medium range structure in the gels is different from the ideal structure of crystalline Sb2S3.
eV can be assigned as Sb 3d5/2(3d3/2) energies in SbS-I, SbS-II, SbS-III, and SbS-IV respectively.50 These energies predominantly denote +3 oxidation states of antimony.51,52 The sharp splitting of the d orbital of antimony originates from the strong spin−orbit coupling. Apart from this, for SbS-I, SbS-II, SbS-III, and SbS-IV, the deconvoluted binding energies of 161.10(162.23), 161.47(162.62), 161.46(162.70), and 161.12(162.29) eV observed, respectively. These values are the characteristics of 2p3/2(2p1/2) energies of S2− ions.53−55 To analyze the thermal stability, SbSx chalcogels were heated under nitrogen atmosphere from room temperature to 600 °C with a heating rate of 10 °C/min. Figures 3d, S6d, S7d, and S8d represent the TGA traces of SbSx chalcogels. SbS-I and SbS-II chalcogels show minor weight loss around 200 °C and exhibit no weight loss up to 600 °C. The minor loss around 200 °C is due to evaporation of physisorbed solvent molecules left from the synthesis of the gels. SbS-IV chalcogel shows a minor loss after 200 °C might be due to release of residual sulfur from the network, whereas SbS-III exhibits weight loss up to 15 wt % at 350 °C along with the minor loss due to evaporation of residual sulfur around 200 °C. The synthesized SbSx gels showed broad diffuse diffraction peaks, which reflect the lack of long-range periodicity in the structure, Figures 3a, S6a, S7a, and S8a. We used the Pair Distribution Function technique46,47 in order to get insights into the local structure of these chalcogels. The experimental PDF plot of a representative SbSx sample is shown in Figure 5.
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CONCLUSIONS High-surface-area antimony sulfide chalcogels can be synthesized via several simple and novel routes. The conventional metathesis approach formed the gel in 1 week, whereas hydrolysis of sodium antimony sulfide and the reaction of the antimony sulfide with sodium sulfide or potassium hydroxide resulted in stable gels in just a few hours. The chalcogels obtained from the hydrolysis of sodium antimony sulfide and metathesis procedures contain spongy porous networks, while the gels obtained from the other procedures consist of fiber bundles. The antimony in all chalcogels is in the +3 oxidation state, and the structure of the building blocks in the amorphous network is similar to within a 15 Å radius to the local structure of crystalline Sb2S3. The chalcogels exhibit high BET surface areas with equivalent silica surface areas close to 1700 m2/g. Because of the high polarizability of the sulfide surfaces, the SbSx aerogels may have uses in gas separation. Given the high surface area and soft nature of these materials, investigation of their capability to capture iodine and noble gases would be worthwhile.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02913. Characterization of SbSx chalcogels (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS These studies were supported primarily by a NEUP grant from the Department of Energy, Office of Nuclear Energy. K.S.S. acknowledges the Indo−U.S. Science and Technology Forum (IUSSTF) for a postdoctoral fellowship. M.S.I. thanks the MRSEC program (NSF DMR-1121262) at the Materials Research Center. SEM, TEM, STEM and XPS were performed at the EPIC facility of the NUANCE Center at Northwestern University. The NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, the Keck Foundation, the State of Illinois, and Northwestern University. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Figure 5. Comparison of experimental pair distribution function plot of SbS−I with the simulated plots of “Sb2S3” fragment and Sb2S3 nanolayer.
All chalcogels obtained by the different synthetic methods described above produce similar PDF plots (Figure S9), suggesting that the local structure in these random networks of SbSx is the same regardless of the synthetic procedure used. We compared the experimental PDF of SbSx against two different models that were derived from the structure of crystalline Sb2S3. The first model is composed of Sb2S3-derived clusters that contain five Sb atoms, and the second model is a finite layer of Sb2S3 atoms (insets in Figure 5). To within a radius of 20 Å, the experimental PDFs of SbSx show many similar features when compared against the simulated PDF plots of the Sb2S3 clusters and layers of Sb2S3, Figure 5. The first peak at around 2.7 Å corresponds to Sb−S bonds, and the peak at ∼3.9 Å corresponds to S−S and Sb−Sb second-neighbor interactions. The experimental PDF shows correlations at least up to 15 Å indicating that there is a large degree of medium range
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