Antimony Chalcogenide van der Waals Nanostructures for

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Antimony Chalcogenide van der Waals Nanostructures for Energy Conversion and Storage Rui Gusmão,† Zdeneǩ Sofer,† Jan Luxa,† and Martin Pumera*,†,‡,§ †

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Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, Prague 6 166 28, Czech Republic ‡ Future Energy and Innovation Laboratory, Central European Institute of Technology, Brno University of Technology, Purkyňova 656/123, Brno CZ-616 00, Czech Republic § Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea S Supporting Information *

ABSTRACT: Among van der Waals nanomaterials, topological insulators such as antimony chalcogenides (Sb2X3, X = S, Se, Te) have interesting thermoelectric and optical properties. Expressly, Sb2S3 is regarded as a favorable anode material for batteries owing to its predicted specific capacity. Both Sb2S3 and Sb2Se3 have an isomorphous tubular one-dimensional (1D) crystal structure, whereas Sb2Te3 has a two-dimensional (2D) layered structure. The synthesized bulk crystals of Sb2X3 were submitted to liquid-phase shear force exfoliation. The 1D and 2D Sb2X3 undergo downsizing processes, also complemented by delamination, to submicron sheets and with an average thickness down to 37 nm. The inherent electrochemical and heterogeneous electron transfer properties of the materials were first studied. Exfoliated Sb2S3 had the best performance for the hydrogen evolution reaction in a wide pH range, with improvements in the overpotential of up to 500 mV, with respect to the starting material. Specifically, in acidic media, exfoliated Sb2S3 also has a good stability for multiple cycles and continuous operation. Likewise, exfoliated Sb2S3 had the highest gravimetric capacitive behavior in alkaline solution. KEYWORDS: layered materials, topological insulator, shear-force exfoliation, water splitting, electrochemical capacitor

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INTRODUCTION An upsurge of innovative flat nanomaterials has occurred in the wake of graphene. Two-dimensional (2D) layered materials are (re)introduced regularly, such as transition metal dichalcogenides (TMDs, e.g., MoS2),1,2 metal phosphorus chalcogenides (MPX3, X = S, Se),3,4 and pnictogens5,6 to name some of the most prominent currently. The layered materials have attracted extensive attention, mainly due to their multipurpose uses in energy applications.7−9 Pnictogen chalcogenides are versatile semiconductor nanomaterials that fall outside the 2D TMD category because pnictogens are in the p-block. To the extent of our knowledge, pnictogen chalcogenides have only been conveyed for the heavier pnictogens, Sb and Bi.10−13 These are members of a larger group, designated as topological insulators.14 Some of these are naturally occurring, such as antimonite (Sb2S3) and antimonselite (Sb2Se3), having an isomorphous tubular onedimensional (1D) crystal structure, with (Sb4X6)n ribbons held together by van der Waals (vdW) forces, shown in Figure 1A1 and B1.15 Although the bulk Sb2S3 crystal has a tubular 1D crystal structure, it has been recently reported that the parallel zigzag sheets along the b-axis make it feasible to fabricate 2D Sb2S3.16 In contrast, Sb2Te3 has a 2D layered crystal structure © XXXX American Chemical Society

to begin with (Figure 1C1), being synthesized by the reaction of the individual elements at high temperature.11 The unit cell consists of three quintuple layers (QL), with each individual QL having atoms covalently bonded in the following atomic arrangement: Te−Sb−Te−Sb−Te (Figure 1C1). In between QL layers, weak vdW interactions stack and hold them together, characteristics that promote cleavage and the pliable character of Sb2Te3. The naturally occurring Sb2S3 and Sb2Se3 are currently used in hybrid solar cells.17−19 The 1D Sb2S3 has also found applications in energy storage20−24 due to its reversible theoretical capacity of 946 mAh g−1,21 which is considerably higher compared to that of Sb-based anode materials (660 mAh g−1), since it is able to accommodate 12 moles of Li+ or Na+ per Sb2S3 mole.16 Electrochemical double-layer capacitors or supercapacitors also play an imperative role in electrochemical energy storage systems.9 Nowadays, this is a dynamic area of research, hoping to build up sustainable power resources for an assortment of utilizations.25 In the case of the heavier 2D Sb2Te3, it is a vital component for phase-change Received: August 4, 2019

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DOI: 10.1021/acssuschemeng.9b04415 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Representation of the unit cell crystal structure of the different Sb2X3. The Sb cations are represented by orange spheres, and the chalcogenides are S, red (A1); Se, green (B1); and Te, blue (C1). Sb2X3 characterization: scanning electron microscopy (SEM) micrographs of assynthesized Sb2S3 (A2), Sb2Se3 (B2), and Sb2Te3 (C2). Scale bars represent 2 μm. X-ray diffraction (XRD) patterns of starting Sb2S3 (A3), Sb2Se3 (B3), and Sb2Te3 (C3) crystals.

form water, which is accompanied by generation of an electrical potential. In this work, the antimony chalcogenides (Sb2X3, X = S, Se, Te) were synthesized, submitted to shear force exfoliation, and surveyed for their structural properties. The 1D and 2D Sb2X3 undergo downsizing processes, also complemented by delamination, to submicron sheets and with thicknesses up to 60 nm. The feasibility of this liquid-phase shear exfoliation in aqueous solvent as a scalable production of nanosheets has been previously proven for other 2D layered materials.37 The materials are tested in energy conversion and storage applications. The best performance of the Sb2X3 was obtained for exfoliated Sb2S3, which has the lowest overpotential and better stability registered for the cathodic water-splitting reaction, in the hydrogen evolution reaction. Furthermore, the exfoliated Sb2S3 has the highest gravimetric capacitive behavior in alkaline solution.

data-storage materials and has lately captured interest due to its topologically insulating nature.26,27 Overall, these materials possess interesting thermoelectric,19,26−31 optical,18 and electrical properties,10,27,29,32 which can be enhanced by downsizing or delamination processes. Electrochemical water splitting is an important energy conversion strategy, which consists of the half-reactions, hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode. The theoretical potential needed to initiate water splitting is 1.23 V. However, because of kinetics effects, an additional potential, or overpotential, is needed.33 Consequently, efforts have been dedicated to formulate catalysts to minimize the overpotential for water splitting.34 While the HER process is more favorable in acidic solutions, the full water splitting is customarily carried out in an alkaline electrolyte. Currently, costly noble metals such as Pt have the highest effectiveness for HER, whereas the lowermost overpotentials for OER are attained with the also expensive IrO2 and RuO2.35 Moreover, Pt kinetics and corrosion stability have to be improved significantly, especially under automotive drive cycles. OER denotes a vital anodic semireaction in water splitting, with uses in electrolyzers and rechargeable metal−air batteries.34 However, experimentally, it is generally used to apply a higher potential than this to promote the electrochemical OER, which results in the consumption of excess energy and decreases conversion efficiency. It is of extraordinary significance to attain materials that are able to perform as efficient catalysts in a wide pH range, thus matching the requirements of electrolysis devices involving proton-exchange membranes (acidic condition) and current alkaline electrolysis equipment. The oxygen reduction reaction (ORR) is another cathodic reaction involved in fuel cell technologies.36 In the ORR, molecular oxygen is electrochemically reduced by four protons and electrons to

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RESULTS AND DISCUSSION The synthesized Sb2X3 consist of wide crystals of metallic luster (Figure S1A). The lighter Sb2S3 and Sb2Se3 are isomorphous, crystallizing in an orthorhombic crystal structure (Pnma space group, Figure 1A1 and B1). Their 1D nanostructures consist of immeasurable zigzag ribbons (Sb4X6)n held together by weak vdW forces, enabling cleavage along the b-axis direction.16 In contrast, the hexagonal Sb2Te3 is a 2D layered material of a rhombohedral structure and R3m symmetry (Figure 1C1). Synthesized crystals (Sb2X3-start) were observed by scanning electron microscopy (SEM) as shown in Figure 1A2−C2, revealing their different intrinsic nature. While the 1D Sb2S3-start and Sb2Se3-start exhibit massive flakes together with irregular and heterogeneous size grains, the 2D Sb2Te3-start has defined platy-shaped crystals of longitudinal corrugations (additional images shown in Figure B


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Figure 2. TEM and high-resolution TEM (HR-TEM) micrographs of shear-exfoliated Sb2S3 (A1 and A2), Sb2Se3 (B1 and B2), and Sb2Te3 (C1 and C2) nanosheets. Scale bars represent 200 nm and 5 nm, respectively. Inserted image shows corresponding SAED.

Figure 3. Characterization of Sb2S3-exf (A1, B1), Sb2Se3-exf (A2, B2), and Sb2Te3-exf (A3, B3). High-resolution XPS spectra and peak deconvolution of Sb 3d core level (A), coincidental with O 1s core level, and the corresponding chalcogen (B): S 2p (B1), Se 3d (B2), and Te 3d (B3).

S2), which were already observable at low magnification under the optical microscope (Figure S1A). Energy-dispersive X-ray spectroscopy (EDS) and the mapping of elements in Figure S3 show the distribution of the main elements expected for each corresponding Sb2X3start, Sb, S, Se, and Te. EDS spectra of the Sb2X3-start are shown in Figure S4 for which the derived results of atom % point to atomic ratios of 1.4, 1.5, and 1.4 for Sb2S3-start, Sb2Se3-start, and Sb2Te3-start, respectively (Table S1). The phase purity of Sb2X3-start synthesized crystals was controlled by X-ray diffraction (XRD) as shown in Figure 1A3−C3. All samples were single-phase without any reaction or oxidation byproduct. Sb2Te3 shows preferential orientation due to its layer character and preferential in-plane cleavage of layers held by weak vdW forces.

The antimony chalcogenide crystals were mechanically ground and subsequently subjected to shear force exfoliation in aqueous media in the presence of surfactant for 1 h as shown in Figure S1B (exfoliated denoted Sb2X3-exf). After separation of poorly exfoliated material by iterative centrifugations, the processed materials are obtained as fine powders of gray shades as shown in Figure S1C. The recovered yield of 1D Sb2X3-exf and 2D Sb2Te3-exf, from the top 75% of the suspensions, was ca. 10%, in agreement with yields reported using this same method of exfoliation of 2D layered materials.37 The morphologies of the processed Sb2X3 were surveyed by transmission electron microscopy (TEM) images, in which Sb2Te3-exf exhibits well-defined few-layered and laminate-like nanosheets, with sizes of a few hundred nanometers as shown in Figure 2A1−C1. EDS element maps of the processed nanosheets indicate that Sb2X3 preserve a uniform distribution C


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Figure 4. Cyclic voltammograms of inherent electrochemistry for Sb2X3-start and Sb2X3-exf (A−C). Conditions: 0.1 M phosphate-buffered saline (PBS), pH 7.2. Cyclic voltammograms of GCbare and GC modified by each Sb2X3-start and Sb2X3-exf (D). Conditions: 0.1 M KCl, 2 mM ferro/ ferricyanide. Corresponding average ΔEp of the individual materials (E). All CVs were performed at a scan rate (ν) of 100 mV s−1.

Figure S7 shows the X-ray photoelectron spectroscopy (XPS) survey spectra of the Sb2X3-start crystals and Sb2X3-exf. The spectra confirm the presence of the expected elements for each material, thus indicating that the processing method does not cause extensive degradation of the material or introduce undesirable contaminations. Furthermore, the presence of a C 1s signal can be noticed in all materials, which is due to the use of underlying carbon tape. It is noticeable that processed materials do not have a detectable Na 1s signal, which suggests the effective removal of excess surfactant. From the atomic percentage of elements shown in Table S2, the Sb/X atomic ratio for the starting materials was offset from the expected value due to the overlapping of O 1s and Sb 3d regions, causing an overestimation of the pnictogen element. Nevertheless, EDS results confirmed the expected ratios for the Sb2X3-start. The XPS spectra of Sb 3d and the respective chalcogenide (S 2p, Se 3d, and Te 3d) core levels, and deconvolution of Sb2X3-exf are shown in Figure 3. Sb2Te3-exf have signals that are shifted to higher binding energies, with additional signals assigned to the presence antimony oxide, Sb2O3.31 In the case of Sb2Te3-exf, peak deconvolution indicates the presence of an additional oxide, Sb2O5, as shown in Figure 3A3. The formation of the oxide species of Sb2Te3-exf is also demonstrated by the Te 3d core-level deconvolution (Figure 3B3). It is feasible that a higher content of Te oxides was formed, including TeO2 and TeO3, due to tellurides being more susceptible to surface oxidation than

of the anticipated elements originated from their corresponding starting materials, with noticeable and intense colors for Sb and X (X = S, Se, Te) due to their relatively higher content in the exfoliated samples. The homogeneous distribution of elements is clearly visible on EDS elemental distribution mapping (Figure S5B−D). The hydrodynamic radii of Sb2X3-exf were measured by dynamic light scattering (DLS). The size distribution profiles show that the Sb2X3-exf undergo predominantly a downsizing process. The Sb2Te3-exf nanosheets exhibited a distribution range of 150−850 nm, Sb2S3-exf yielded fragments spanning within the range of 450−700 nm, and Sb2Se3-exf had predominantly smaller fragments at 50−500 nm as shown in Figure S5A. The thickness distribution profiles of the Sb2X3-exf nanosheets were estimated by an optical profilometer, pointing to an average of 51.5 ± 4.7 nm for Sb2S3-exf, 60.7 ± 10.6 nm for Sb2Se3-exf, and 36.5 ± 11.6 nm for Sb2Te3-exf (shown in Figure S6). The morphology was further characterized by TEM/HRTEM microscopy using the exfoliated chalcogenides (Figure 2). The morphology of exfoliated materials clearly reflects the 1D character of Sb2S3 and Sb2Se3 and the 2D structural characteristic of Sb2Te3. The high-resolution TEM images and corresponding SAED diffraction patterns reveal the orthorhombic structure of Sb2S3 and Sb2Se3 and hexagonal structure of Sb2Te3 (Figure 2A2−C2). D


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Figure 5. Stability tests of Sb2X3-exf HER performance by LSV polarization curves before and after 100 CVs in 0.5 M H2SO4 (A), 3.5 wt % NaCl (B), 0.1 M PBS (C), and 1.0 M KOH (D) with a ν of 5 mV s−1. Long-term stability HER test for Sb2S3-exf for 2 h by chronopotentiometry at a fixed current density of 10 mA cm−2 in 0.5 M H2SO4 and 1.0 M KOH (E).

other chalcogens.38 Likewise, for the electrochemical exfoliation of the analogous topological insulators Bi2X3, a higher percentage of oxidation states was also observed for Bi2Te3 in comparison with Bi2Se3.33,40 Prior to applying these materials to electrocatalytic reactions, it is crucial to build knowledge on their fundamental electrochemical properties. As previously stated, Sb2 X 3 materials are constituted by Sb in the +3 oxidation state and the chalcogenide (S, Se, Te) in the −2 oxidation state. Thus, as known for other materials such as TMDs39 and Bi2X3 (Se, Te),40 it is expectable to electrochemically oxidize or reduce Sb2X3. This is also designated as inherent electrochemistry, which is greatly affected by the chalcogen element. The inherent electrochemistry of the Sb 2X 3 materials was investigated by cyclic voltammetry (CV). Figure 4A−C shows the overlaid anodic scanned CVs of modified glassy carbon (GC) electrodes with individual Sb2X3-start and the corresponding Sb2X3 exfoliated. The Sb2S3- and Sb2Se3modified electrodes have a common oxidation peak in the anodic branch for potentials higher than +1.0 V, which is displaced to lower oxidative potentials for Sb2Te3. Likewise, the common oxidation process for all materials at ca. −0.5 V also suffers from some potential shifts depending on the chalcogenide. On the other hand, the coincidental reduction of Sb2X3 shifts to less negative potentials for the heavier chalcogenides. Exfoliation or downsizing caused changes in

the electrochemistry of Sb2S3 and Sb2Se3, which are not so pronounced for Sb2Te3. The redox signals to the oxidation and reduction potentials described are also consistent with the oxidation of the Sb 2D layered material31 and reduction of Sb3+ films.41 The electrochemical performance of Sb2X3 materials in the presence of ferro/ferricyanide, [Fe(CN)6]3−/4−, an innersphere redox probe (Figure 4D), was studied by CVs to evaluate heterogeneous electron transfer (HET). The potential peak-to-peak separation (ΔEp) is inversely proportional to the efficacy of HET at the modified electrode surfaces. The unmodified GC electrode has an average ΔEp of 155 mV (Figure 4E). Sb2S3 and Sb2Te3 have lower ΔEp (103 and 76 mV, respectively), which translate to improved HET processes with respect to GCbare and with the processing of the materials causing limited impact on their HET performance. Sb2Te3 had a post-ferrocyanide oxidation peak, which is due to the inherent oxidation process of these materials at +0.5 V (Figure 4C). Contrasts were more evident for Sb2Se3 materials, with the downsizing process accentuating the characteristics of the topological insulator of Sb2Se3-exf. This is a phenomenon also observed when the processed material exfoliation is accompanied by some level of oxidation.37 The HET rate constants (kobs0) are then calculated based on the classical Nicholson approach,42 which implicate that larger separations between the anodic and cathodic peaks result in E


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Figure 6. (A) CVs of the GC modified with the starting and exfoliated Sb2X3 at ν of 100 mV s−1. (B) Galvanostatic charge−discharge curves at 0.05 A g−1 current density. (C) Gravimetric capacitance at current densities 0.2−0.02 A g−1. All electrochemical curves are recorded in 5.0 M KOH aqueous media.

slower HET rates. A summary of all ΔEp and corresponding kobs0 values is shown in Table S3. The observed HET trends reveal that Sb2X3-start have higher kobs0 values than the corresponding Sb2X3-exf. One possible explanation for the lower kobs0 values of Sb2X3-exf is usually attributed to the fact that exfoliation processes are often accompanied by partial oxidation of the processed materials. Significant amounts of chalcogenide and antimony oxides have been detected in the XPS analysis of Sb2X3-exf (Figure 3). The surface oxides on the Sb2X3-exf may experience electrostatic repulsion from the negatively charged [Fe(CN)6]3−/4− redox probes, which hindered the HET.43 Energy storage and conversion applications of Sb2X3 were assessed by capacitive charge density and by analysis of electrocatalytic activity for the water-splitting reactions. The oxygen evolution reaction (OER) anodization voltammograms in 1.0 M KOH alkaline media displayed in Figure S8A show that the preeminent catalytic response is obtained for Sb2S3exf, averaging an overpotential of 1.03 V (Figure S8B). Sb2Se3 and Sb2Te3 displayed a higher OER overpotential than that of the GCbare electrode; however, there was a significant lowering of the overpotential by 0.15 V in Sb2Te3-exf when compared with Sb2Te3-start (Figure S8B). It must be remarked that the registered OER overpotentials for Sb2X3 are nonetheless high considering the performance of the benchmark catalyst IrO2 (shown in Figure S8). The oxygen reduction reaction (ORR) response of Sb2X3 was also tested, as shown in Figure S8C,D. As depicted in Figure S8C, the linear sweep voltammetry (LSV) shows a welldefined peak at ca. 0.6 V vs RHE in air-saturated 1.0 M KOH, which is absent in purged media (Figure S8E), indicating that the peaks obtained are due to reduction of oxygen. Of the different Sb2X3, the ORR half-peaks were registered at slightly more positive potentials than the GCbare electrode, which translates into their response not being significantly catalyzed and no measurable improvement between the exfoliated and starting counterparts. To compare the hydrogen evolution reaction (HER) catalytic activities of the materials, we shall consider the overpotential that the electrocatalyst requires to yield a current density of 10 mA cm−2, which corresponds to the current density expected for a 12.3% efficient solar water-splitting device.33,44 Two methods are usually considered to elucidate the electrocatalytic stability of a HER catalyst.33,44,45 One route is to conduct the recycling experiment by performing multiple CVs or LSVs. In this case, we present the HER polarization curves of the Sb2X3-exf materials after 100 cycles. The HER polarization curves before and after CV of Sb2X3exf in the different media (H2SO4, 3.5 wt % NaCl, PBS, and

KOH) are shown in Figure 5. The polarization curves of Sb2X3-start in the same media are shown in Figure S9, which also includes the HER curves of the Pt/C-modified GC electrode. In acidic media, 0.5 M H2SO4 in Figure 5A, the LSV before 100 CVs of Sb2Te3-exf has a pronounced reduction peak at ca. −0.88 V, which occurs before true HER. This peak is absent after 100 CVs; thus, the overpotential to be considered for comparison purposes should be 1.1 V. In the acidic media, the lowest overpotential of ca. 0.93 V was registered for Sb2S3-exf in the first polarization scan. With respect to the stability of Sb2X3-exf after multiple cycles, unfavorable shifts were observed, although Sb2S3-exf held the lowest overpotential between the materials. For the other media, Sb2S3-exf yielded the lowest overpotentials in a wide pH range. For saline 3.5 wt % NaCl media (Figure 5B), the materials registered higher overpotentials, and in neutral media (Figure 5C), Sb2S3-exf had the most unfavorable shift in overpotential after multiple cycles. Overall, the exfoliation process of the Sb2X3 results in improvements in the stability and the overpotentials at 10 mA cm−2 up to 480 mV, when comparing Sb2S3-start and Sb2S3-exf. Furthermore, for the Sb2X3-exf, the order of improved performance and stability follows the trend Sb2S3-exf > Sb2Se3-exf > Sb2Te3-exf. In parallel with 2D TMDs, the chalcogenide sites at the layer edges have been identified as highly active and catalytic sites.2,46 In a more critical assessment of Sb2X3-exf performance in the different media, even though Sb2X3-exf has the lowest overpotential, its values can be considered high when compared with some values reported for 2D exfoliated TMDs.33 Tafel plots are commonly used to estimate the rate-limiting step mechanism in HER (further mechanistic details are given in Supporting Information (SI)). From the Tafel equation, it is possible to obtain two important parameters: the Tafel slope (b) and the exchange current density (j0). The Tafel slope (b) is related to the catalytic mechanism of the electrode reaction (more information is given in SI), whereas j0 describes the intrinsic catalytic activity of the electrode material under equilibrium conditions.33,44,45 A catalytic material having a high j0 and a small Tafel slope (b) is thus appropriate. The HER data of Sb2X3 materials is summarized in Table S4. As expected, Pt/C had the lowest Tafel slope (not shown).47 Among the Sb2X3 materials, the Sb2X3-exf had lower Tafel slopes and higher j0 than the corresponding starting material, which means that the processed materials require a reduced amount of overpotential to increase the catalytic current by an order of magnitude.48 As observed for other 2D layered materials, the HER of the exfoliated materials was favored when compared with the starting materials.2 On the basis of F


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taken into consideration prior to exploring their applications in electrochemical energy conversion and catalysis. Among the exfoliated materials, Sb2S3-exf had the best performance for HER in a wide pH range, with the lowest overpotential and improvements up to 500 mV relative to the Sb2S3-start. Sb2S3exf had a better stability in acidic media after multiple cycles and also registered a lower overpotential required to reach a current density of 10 mA cm−2, when compared to alkaline media. It should be mentioned that, although Sb2X3-exf have the lowest overpotential, these values can nevertheless be considered high when compared with 2D exfoliated TMDs. Sb2X3 were also evaluated for their capacitive properties. Sb2S3-exf had the highest capacitive values as determined by charge/discharge curves at different current densities. The contrasts observed between starting and exfoliated Sb2X3 can be related to a higher surface−volume ratio and created active edge sites. These results can have profound repercussions in future applications of vdW 1D and 2D nanostructure exploration in energy-related fields.

the considerations of the Tafel plots (detailed in SI), for the majority of the Sb2X3-exf in the different media, the Tafel slopes point to the rate-determining step (RDS) being the Volmer step (120 mV dec−1). The Tafel slope of Sb2S3-exf in acidic media (60 mV dec−1) points to an RDS that may involve the spillover step. Another strategy to study the electrocatalytic activity of the materials is to measure the current variation with time, by chronoamperometry or chronopotentiometry. For this measurement, shown in Figure 5E, Sb2X3-exf was selected with a current density of 10 mA cm−2 for a long period of time (2 h) in the most acidic and alkaline media. As expected, the overpotential required to maintain the applied current density was lower for acidic media. In addition, the bubble effect is also an issue that cannot be ignored. During water splitting, gas bubbles are generated and accumulated on the electrode surface; simultaneously, some do not leave the electrode immediately. This can lead to the loss of the effective active area and thus the increase of the reaction overpotential. Finally, we investigated the energy storage application of the Sb2X3 nanostructures in purged aqueous 5.0 M KOH, to test them as electrochemical capacitors. Figure 6A compares the CVs of the different materials at a scan rate (ν) of 100 mV s−1. A rectangular voltammogram is not detected, with some small waves due to the inherent redox reactions on the Sb2X3 surfaces; thus, these materials display pseudocapacitive behavior. The CVs of the different materials for ν from 10 to 1000 mV s−1 are shown in Figure S10. To have more rigorous capacitance values of the Sb2X3-exf, galvanostatic charge/discharge curves were performed. Figure 6B demonstrates the typical charge/discharge curves of the Sb2X3 nanostructures at a current density of 0.05 A g−1. Sb2Te3-exf displayed triangular charge/discharge cycles. For Sb2S3-exf and Sb2Se3-exf, a lengthened discharging occurred at 0.02 and 0.05 A g−1 current densities, respectively. This is feasible to occur as additional pores of processed materials were accessed by the electrolyte. The corresponding gravimetric capacitances were calculated from the galvanostatic discharge curves. At all current densities, the material with the highest capacitive value was Sb2S3-exf, which had the longest discharge time. Figure 6C summarizes the capacitance values for Sb2X3 at different current densities 0.02−0.2 A g−1, indicating that the downsizing of the 1D material had advantageous effects on its gravimetric capacitance compared with Sb2Se3-exf and Sb2Se3start, with increments up to 2-fold. Notwithstanding, the values reported here for Sb2X3-exf are lower than those reported for exfoliated 2D layered transition metal dichalcogenides such as MoS2, WSe2, or TiS2.49−51

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Reagents. Antimony, sulfur, selenium, and tellurium granules of 5N purity were acquired from STREM. Potassium hexacyanoferrate(II) trihydrate, potassium hydroxide, dipotassium hydrogenphosphate, monopotassium phosphate, potassium chloride, sulfuric acid, cholalic acid sodium salt, N,N-dimethylformamide, iridium(IV) oxide hydrate, and platinum on graphitize carbon (20% loading) were purchased from Sigma Aldrich. Synthesis. Antimony chalcogenides were synthesized by the direct reaction of antimony and chalcogenide in a quartz glass ampoule under high vacuum at 700 °C at a heating rate of 5 °C min−1 and a cooling rate of 1 °C min−1. Full details are given in SI. Material Processing. Following a procedure previously optimized for other 2D layered materials,37 the starting material was submitted to shear force dispersion using an immersion blender, as displayed in Figure S1. The production yield of the recovered processed material was ca. 10%. Further details are given in SI. Characterization. A scanning electron microscope (JEOL 7600F, Japan) was used for taking SEM micrographs. The XPS spectra were attained with an X-ray photoelectron Phoibos 100 MCD-5 spectrometer (SPECS, Germany). TEM images were obtained using an EFTEM Joel 2200 FS microscope. The DLS measurements were done with a Zetasizer Nano ZS (Malvern, England). XRD data were acquired at room temperature with Bruker D8 Discoverer (Bruker, Germany). The heights of Sb2Te3-exf nanosheets were obtained by an optical profilometer (Sensofar, Spain). Further details are given in SI. Electrochemistry. Electrochemical measurements were done with Autolab PGSTAT204 (Eco Chemie, Utrecht, The Netherlands) at room temperature, using a three-electrode system. GC (ϕ = 3 mm) was used as a working electrode, a platinum disk as a counter electrode (ϕ = 2 mm), and Ag/AgCl as the reference electrode. For water-splitting catalytic reactions, a second bare GC was used as the counter electrode. All electrodes were acquired from CH Instruments (TX). Materials were suspended in dimethylformamide at 5 mg mL−1 and sonicated for 15 min (FB11203, 80 KHz, T < 15 °C). Additionally, GC surfaces were polished with alumina slurry and rinsed thoroughly. Each GC was modified by dropping 2 μL of each suspended material onto its surface. The suspension medium was evaporated at room temperature. Further details can be found in SI.

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CONCLUSIONS Antimony chalcogenide (Sb2X3) vdW nanostructures were synthesized and subjected to high-energy shear force mixing in aqueous media. The characterization of Sb2X3-exf reveals that the 1D layered materials undergo a downsizing process, whereas for the 2D Sb2Te3-exf, this is accompanied by exfoliation. Sb2Te3-exf nanosheets have a wide range of distribution of lateral size, reaching 850 nm and an average sheet thickness of 36.5 nm, and are the materials with a higher degree of oxidation as inferred from Sb 3d and Te 3d core levels. Exfoliated Sb2X3 yielded interesting electrochemical performance, having characteristic redox behavior, which shows dependence on the chalcogen element, a factor that should be

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b04415. G


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Photographs of the workflow; additional SEM and TEM images with EDS mapping of elements; lateral size and thickness measurements; XPS survey spectra; LSV polarization curves for OER and ORR measurements; HER Tafel plots; CVs of Sb2X3 for capacitance experiments and equations; further experimental details (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.P. acknowledges the financial support of Grant Agency of the Czech Republic (EXPRO: 19-26896X). Z.S. was supported by Czech Science Foundation (GACR No. 19-26910X) and by the financial support of the Neuron Foundation. R.G. acknowledges the European Structural and Investment Funds, CHEMFELLS II no. CZ.02.2.69/0.0/0.0/18_070/ 0010465.

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Antimony Chalcogenide van der Waals Nanostructures for

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