Shape Control of Ternary Sulfide Nanocrystals - ACS Publications


Dec 14, 2017 - Shape Control of Ternary Sulfide Nanocrystals. Yanyan Xu,. †,∥. Long Yuan,. †,∥. Renguo Xie,. ‡. Lei Wang,. §. Qingshuang Li...
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Shape Control of Ternary Sulfide Nanocrystals Yanyan Xu,†,∥ Long Yuan,†,∥ Renguo Xie,‡ Lei Wang,§ Qingshuang Liang,† Zhibin Geng,† Huanhuan Liu,† and Keke Huang*,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ College of Chemistry, Jilin University, Changchun 130012, P. R. China § Key Laboratory of Eco-chemical Engineering, Ministry of Education, Inorganic Synthesis and Applied Chemistry, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China S Supporting Information *

ABSTRACT: Synthesis of semiconductor nanocrystals with a definite shape is the foundation of their anisotropy properties investigation; however, it is more challenging in ternary metal sulfides than that of noble metal and binary sulfides. In this paper, we report a solvent polarity control strategy to prepare a family of ternary sulfide (Ag3SbS3) semiconductor nanocrystals with tunable polyhedral shapes. The crystal growth speed along different directions was confined by the capping effect of the polarity of solvents that was defined by reaction temperature. Crystal shape of Ag3SbS3 nanocrystals could be tailored as a sphere, hexagonal plate, and prism. A shape-controllable growth mechanism was analyzed based on the Bravais−Friedel− Donnay−Harker theory by taking crystal structure characteristics and the polarity of solvents into consideration. The semiconductor nanocrystals show a near value of the band gaps for different shaped samples and facetdependent photocatalytic water-splitting activities, which may result from the discrimination of the terminal surface structure and binding energy of Sb and S for the three different shaped nanocrystals. Thus, we provide a new crystal shape tunable strategy for ternary sulfide nanocrystal synthesis, which is important for optimizing properties and applications of sulfide semiconductor nanocrystals.



INTRODUCTION Synthesis of polyhedral semiconductor nanocrystals with controllable exposure of crystal facets is important for discovering new materials with improved properties. The tunable methods have been developed for many famous families of functional materials, such as noble metal nanocrystals,1 binary oxides,2 fluorides,3 as well as perovskite structure ternary oxides.4,5 There are also many researches on the synthesis and shape control over metal sulfide nanocrystals, such as ZnS,6 PbS,7,8 CdS,9 CuS,10 and Cu2−xS.11,12 However, it is more challenging to synthesize and control the polyhedral shape of ternary sulfides. For ternary chalcogenides, only Cu2SnSe313,14 was found with shape-tunable growth and shape-dependent properties. Although Ag8SnS6 has been reported on its shapecontrolling synthesis, only a tetrahedral submicropyramid was obtained, and another morphology of assembled particles without definite facets.15 The shape and crystal facet controllable synthesis of ternary sulfide semiconductor nanocrystals is still a challenging issue in their synthetic chemistry. Ag3SbS3 is an important narrow band-gap semiconductor, which shows potential applications as piezoelectric16 and nonlinear optical material,17 Ag+ ionic conductor,18,19 as well as solar absorber.20−22 The structure of Ag3SbS3 was first determined based on mineral.23 Then, single crystal24,25 and powder26 © XXXX American Chemical Society

forms of Ag3SbS3 samples have been synthesized with many methods.27,28 However, no studies have been focused on the shapecontrollable growth and facet-dependent properties of this material. Recently, we developed a crystal facet controllable growth method in a series of ternary metal oxides with a perovskite structure, such as rare-earth ferrites,29 chromites,30 and manganites.31 The shape of nanocrystals in the synthesis is usually dependent on the effect of surfactants,32 as well as dopants of other metals.33 The shape tuning effect is often ascribed to the capping function of surfactants; however, the structure of the exposed facet is often omitted in discussing the formation of new facets. In this paper, we report a shape-controllable synthesis method of Ag3SbS3 nanocrystals by tuning the polarity of solvents. Oleylamine and oleic acid play different roles on the crystal facet growth of Ag3SbS3 crystals due to the binding effect of their terminal polar groups with different crystal planes. Crystal shape of Ag3SbS3 could be tunable with reaction temperature and the relative ratio of reactants. The crystals show excellent performance of shapedependent photocatalytic water-splitting properties at room temperature. Optoelectronic property study of the as-prepared Received: September 22, 2017 Revised: November 12, 2017 Published: December 14, 2017 A

DOI: 10.1021/acs.cgd.7b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Characterization. The crystal structures of the as-prepared products were investigated by powder X-ray diffraction (XRD, D/MAX2550, Rigaku, Japan) with graphite monochromic Cu Kα radiation. The 2θ range was 10−80°, and the scan speed was 3°/min. The microstructures were characterized by transmission electron microscopy (TEM, Tecnai G2S-Twin F20, FEI, Holland) and the scanning electron microscopy (SEM, Helios NanoLab 600i, FEI, Holland). Fourier transform infrared (FTIR) spectra were acquired in the transmission mode using a Bruker Optics-IFS-66 V/S spectrometer in the wavenumber range of 500−4000 cm−1. FTIR spectra of oleic acid, oleylamine, and octadecylene were collected by drop-casting the solvent chemicals into the KBr tablets. FTIR spectra of the as-synthesized and ligand-exchanged Ag3SbS3 nanocrystals were obtained by measuring the samples mixed KBr tablets. UV−vis absorption spectra were rescored to characterize the optical absorption properties of the powder samples, and the absorption data were calculated to the band gaps of the semiconductor materials. Band gaps of the as-prepared Ag3SbS3 samples were calculated from absorption data. X-ray photoelectron spectra (XPS) were acquired to analysis the surface composition of materials with an ESCALAB 250Xi electron energy spectrometer from Thermo company, and Al Kα (1486.6 eV) served as the X-ray excitation source.

Ag3SbS3 indicates that it could also be an absorber for photovoltaic applications. The crystal facet tunable synthesis method and mechanism study in the Ag3SbS3 nanocrystal may open an avenue for ternary sulfide semiconductor nanocrystal shape design and shape-dependent applications.



EXPERIMENTAL SECTION

Chemicals. Silver nitrate (AgNO3, 99.8%) and antimony-(III) trichloride (SbCl3, 99.0%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Sublimed sulfur (S, 99.5%), oleylamine (OLA, 90%), and oleic acid (OA, 90%) were obtained from Aladdin. 1-Octa-decene (ODE, 90%) was purchased from Alfa Aesar Co., Inc. Hexane (99.7%, A.R.), ethanol (95.5%, A.R.), and sodium sulfide anhydrous (Na2SO3, 97%) were obtained from Beijing Chemical Industrial Group Co., Ltd. Sodium sulfide nonahydrate (Na2S·9H2O, 98%) was from Xilong Chemical Co., Ltd. All chemicals were directly used without any further purification. Synthesis of Ag3SbS3 Nanospheres. To prepare spherical Ag3SbS3 nanocrystals, 1 mmol of AgNO3, 1 mmol of SbCl3, and 2 mmol of sulfur powder were dissolved into the 1 mL of oleylamine, 1 mL of oleic acid, and 8 mL of octadecene in a three-neck round-bottom flask, and degassed under vacuum through magnetic stirring at 110 °C for 30 min. Then, the temperature was gradually heated to 170 °C, and the mixture was magnetically stirred for 1 h under an argon atmosphere. Afterward, the solution was cooled down to room temperature, and the products were transferred into the centrifuge tube (capacity of 50 mL). Hexane and ethanol were used to wash the solution according to the approximate proportion of 1:1. The mixture was centrifuged at 8000 rpm for 5 min, and the precipitates were separated. Then, the colloidal nanocyrstals were washed using the same procedure. Synthesis of Ag3SbS3 Hexagonal Plates. The experiment was carried out using a standard Schlenk line technique. In a typical synthesis, nonstochiometric amounts of AgNO3 (1 mmol), SbCl3 (1 mmol), and sulfur powder (2 mmol) were dissolved into 8 mL of octadecene with the presence of 1 mL of oleylamine and 1 mL of oleic acid in a 100 mL three-neck flask under vacuum through magnetic stirring at 50 °C for 5 min. The solution was maintained at 110 °C under vacuum and an inert atmosphere for 30 min in order to discard the low boiling point impurities. Afterward, the mixture was gradually heated up to 190 °C for 60 min. In the reaction, the color of the solution changed from brown to orange. Then, the colloidal solution was maturely cooled down to room temperature, and transferred into the 50 mL centrifuge tube. The hexane was added to disperse the nanocyrstals, and the ethanol was added to deposit the samples, which was centrifuged at 8000 rpm for 5 min, and the upper solution was removed. The precipitant was further washed by repeating the aforementioned steps three times. Finally, the samples were vacuum-dried for the following test. Synthesis of Ag3SbS3 Hexagonal Prisms. The above-mentioned experimental process was also followed for the synthesis of plate-shaped and hexagonal prism Ag3SbS3 nanocrystals. The reaction temperature was set at 220 °C, and the reaction time was adjusted to 2 min. Synthesis conditions of reaction temperature, time, and the relative amount of reactants were also changed to study the crystal growth tailoring factors on their shapes (Tables S1 and S2). Ligand Exchange. The as-synthesized Ag3SbS3 nanocrystals were coated with organic ligand that should be washed by solvent exchange reactions to achieve a good dispersing effect in water for studying their intrinsic property. First, the exchange solution was prepared by dissolving 0.836 g of Na2S·9H2O into the formamide (20 mL) solvent. Second, 0.05 g of as-synthesized Ag3SbS3 nanocrystals was dispersed into 50 mL of hexane in a glass beaker; then, 10 mL of Na2S formamide solution was added with continuous magnetic stirring for 3 h at room temperature. A stratification phenomenon of hexane (top) and formamide (bottom) was clearly observed in the mixed solution with a sign of the color of hexane phase changing from yellow to colorless. Third, Ag3SbS3 samples after the ligand-exchanged reaction were obtained by centrifuge washing in hexane solvent at 8000 rpm for 5 min for three times, followed by 10 mL of acetone centrifuged washing once. The resulting products were dried under vacuum for 20 min.



RESULTS AND DISCUSSIONS Sample composition, crystal morphology, and size distribution of the as-synthesized Ag3SbS3 nanocrystals are shown in Figure 1. Sample composition of as-prepared Ag3SbS3 nanocrystals was determined by characteristic X-ray energy-dispersive spectroscopy (EDS) in Figure 1a−c. The quantitative analysis results confirmed the presence of silver, antimony, and sulfur elements in as-prepared products. The Ag/Sb/S atomic ratio of the final samples is 30.94:10.94:28.74, 38.16:12.38:34.76, and 37.69:11.38:33.40, respectively, which is close to the nominal chemical stoichiometric ratio of 3:1:3, indicating the formation of Ag3SbS3 nanomaterials. Crystal shapes of as-prepared Ag3SbS3 are spherical, hexagonal plates, and hexagonal prisms at the reaction temperature of 170, 190, and 220 °C, respectively (Figure 1d−f). Crystal shapes are uniform for all the three samples in each reactive condition. Obviously, the shape of products is controllable by tuning the reaction temperature. The size distribution histograms of the as-synthesized samples are shown in Figure 1g−i. The average diameter of a sphere is 125 nm. The average thickness and diameter for hexagonal plates are 34 and 197 nm, respectively. The average length and diameter of hexagonal prisms are 342 and 196 nm, respectively. Particle sizes are getting larger with the increasing of reaction temperature accordingly, which means the crystals are prone to growing into large polymorphs at high temperature. Figure 2 shows powder X-ray diffraction patterns of the as-obtained Ag3SbS3 samples. The diffraction peaks match well with the standard pattern of the hexagonal structure of Ag3SbS3 (pyrargyrite phase with JCPDS card No. 74-1875). The space group of Ag3SbS3 is R3c (No. 161), with the calculated lattice parameters of a = b = 11.04 Å and c = 8.71 Å. Characteristic peaks of bulk Ag3SbS3 are shown in the XRD pattern. No distinguishable diffraction peaks of other phases such as Ag2S, AgSbS2, and Sb2S3 could be observed in the XRD patterns of these samples, suggesting that the as-synthesized nanocrystals are phase-pure. TEM and HRTEM images of as-prepared Ag3SbS3 samples are shown in Figure 3. The as-prepared samples display a nearly uniform shape with narrow size distribution. A standard deviation of 5−10% was obtained without postpreparation fractionation or size sorting. Figure 3d−f shows HRTEM and SAED patterns of three Ag3SbS3 samples. TEM images of these nanocrystals reveal high crystallinity with continuous lattice fringes for the B

DOI: 10.1021/acs.cgd.7b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. Composition of EDS spectra, SEM images, and particle size distribution histograms of as-synthesized Ag3SbS3 nanocrystals at different reaction temperatures and times: (a, d, g) 170 °C (1 h); (b, e, h) 190 °C (1 h); (c, f, i) 220 °C (2 min), respectively.

Figure 2. XRD patterns of as-prepared samples obtained at various reaction temperatures: (a) 170 °C; (b) 190 °C; and (c) 220 °C. The diffraction pattern of hexagonal phase (JCPDS No. 74-1875) is shown at the bottom for comparison.

Figure 3. Typical TEM (a−c) and HRETM (d−f) images of the as-prepared Ag3SbS3products with different reaction temperatures: (a, d) sphere; (b, e) plate; (c, f) prism. The corresponding FFT patterns are shown, respectively, in the insets of HRTEM images.

whole particle. The interplanar spacings of the spherical Ag3SbS3 nanocrystals lattice fringe are 3.18 and 5.52 Å, respectively, which are consistent with the (030) and (12̅ 0) crystal planes, in agreement with the XRD results. HRTEM results of Ag3SbS3 plates and prisms show typical (030) and (012) crystal plane distances of 3.18 and 3.96 Å, respectively. The fast Fourier transform (FFT) pattern of the HRTEM image is shown in the inset of Figure 3d−f. The corresponding pattern confirms that the samples are single crystals

with a perfect hexagonal spot pattern, which are attributed to the (012) and (300) under the incident ⟨001⟩ direction. A well-defined crystal shape of Ag3SbS3 nanoparticles has been obtained, and the shape can be tunable with only changing the reaction temperature according to SEM and TEM results. According to Bravais−Friedel−Donnay−Harker theory, the crystal facet with the largest distance value will be kept in the terminated surface of crystals. In the crystal lattice of pyrargyrite Ag3SbS3, C

DOI: 10.1021/acs.cgd.7b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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the most stable crystal facets are {12̅ 0} and {012} planes. The crystal structure of Ag3SbS3 can be indexed to the hexagonal system, which is exchangeable to each other.34 The facets of (1̅20), (110), (21̅0), (12̅0), (1̅1̅0), and (2̅10) are crystallographically equivalent in the hexagonal system, which indicates the same structure of these facets, so does that of (012), (1̅02), (112̅ ), (011)̅ , (111̅ )̅ , and (10̅ 1)̅ facets (Figure 4).

Therefore, colloidal Ag3SbS3 nanocrystals were shown to adopt a nearly spherical morphology at early growth stages (170 °C). Hexagonal prism crystals are the main product at 220 °C for the same reaction system, which indicates the crystal growth speed perpendicular to the c-axis is getting slower when the reaction temperature is increased. This result indicates higher stabilization energy of −COOH to Sb−S bonds than that of −NH2 to S atoms. A similar capping effect of oleic acid molecules has also been reported in the ultrathin PbS nanosheet preparation.36 Thus, the capping effect is similar at low temperature for these two molecules and is getting more different at elevating temperatures due to the electrostatic interactions at the surface of each kind of facets of Ag3SbS3 crystals. Besides the impact of reaction temperature, other factors such as reaction time and reactant ratio are also considered in Ag3SbS3 nanocrystal shape tunable synthesis. The synthetic conditions for tunable growth of Ag3SbS3 samples are shown in Tables S1 and S2. SEM graphs of the as-prepared products are shown in Figure S1 for the reaction temperatures of 170, 180, and 190 °C for 1 h with the same reactants and solvents. Sphere shaped products were obtained at 170 °C (Figure S1a), while they are getting well-defined shapes when the temperature reaches 180 and 190 °C (Figure S1b,c). This result indicates that the reaction temperature affects the crystal growth speed for different directions. By maintaining the reaction temperature at 190 °C, crystals of Ag3SbS3 grow from hexagonal plates (Figure S1e) for 0.5 h to thick hexagonal blocks (Figure S1d) for 1.5 h. This means the growth process of Ag3SbS3 crystal mainly propagates along the z-axis. When the reaction temperature is over 200 °C, Ag3SbS3 crystals formed within 2 min, although with poor uniformity and phase purity (Figure S1g). Further increasing the reaction temperature to 220 and 240 °C, higher length to diameter prisms were obtained (Figure S1h,i). These results indicate that the reaction temperature determined crystallization processes of Ag3SbS3 nanocrystals. Reactant amounts also play important roles on crystal facet growth of Ag3SbS3 nanocrystals. For example, hexagonal (Figure S2a,c,f,g), trigonal plate (Figure S2b), tetrahedral (Figure S2e), and prism shape (Figure S2i) crystals could be prepared by tuning the reactant amounts of Ag, Sb, and S, respectively. Due to the organic ligands (such as OAm and OA) attached on the surface of nanocrystals, ligands exchanged experiment was carried out to remove the surface organic coatings.37 The phenomenon that the yellow nanocrystals transfer from hexane to FA could be observed and the final samples became black (Figure 5a). FTIR spectra of the original organic ligands and nanocrystals before and after ligand exchange are shown in Figure 5b. Transmission peaks at the high-frequency region (3000−2800 cm−1) and the lower frequency region (700−1500 cm−1) belong to C-H stretching vibrations. N-H stretching bands are located in the region (3500−3300 cm−1), N-H bending bands are located in the region (1600−1500 cm−1).38 2926, 2852, and 1486 cm−1 are coincident with the FTIR spectrum of OAm. 1713 and 1486 cm−1 are matching with the features of OA.39 It is obvious that the transfer of samples from hexane to FA leads to the disappearance of the bands at 3450, 1713, 2926, and 2852 cm−1 corresponding to the N-H, CO, C-H stretching in the organic ligands, respectively. These results indicate that the organic ligands have been removed after the exchange processes. To evaluate the optical properties of Ag3SbS3, UV−vis−NIR absorption spectra of as prepared three samples are shown in Figure 6. It was found that these samples have strong absorption in the visible region. In addition, the band edge of absorption spectra is almost the same, and no difference is observable from

Figure 4. Crystal shape of Ag3SbS3 enclosed by different crystal facet surface area: (a) hexagonal plates and (b) hexagonal prisms. Surface structure of Ag3SbS3 crystals view (c) along and (d) perpendicular to c-axis. Crystal facet structure of (e) (1̅20) and (f) (012) facets. Sb, green balls; Ag, blue balls; S, red balls, respectively.

Crystal of Ag3SbS3 with either hexagonal prism shape or plate shape is dodecahedron with 6 trapezoids (i.e., {1̅20} facets) that share toplines and baselines with an angle of 120° and 3 edgeshared rhombuses (i.e., {012} facets) in each side of the c-axis direction. Structures of {1̅20} and {012} facets are schematically shown in Figure 4(e,f), respectively. In the Ag3SbS3 lattice, each Sb atom is bonded to three S atoms, forming tetragonal groups, that are bonding to Ag atoms and forming a three-dimensional structure. Structure of {1̅20} facets is composed by in-plane Sb−S bonds with one off-plane pointed-out S atom and the other one inside of the lattice. These facets could be stabilized by oleic acid molecules because of the dipole interaction of −COOH groups and out-plane Sb−S bonds. Crystal facets of {012} facets, however, are composed of three pointed-out S atoms, which may be stabilized by oleylamine molecules with −NH2 groups.35 These two molecules worked as capping agents in the synthesis of Ag3SbS3 nanocrystals. The crystal shape of Ag3SbS3 is mainly determined by these two solvent molecules in the nucleation and crystal growth process. It seems that the dissociation of −NH2 groups from {012} facets is much easier than that of −COOH groups from {1̅20} facets when the reaction temperature increased from 190 to 220 °C. Crystal growing speed for each possible direction seems identical below 190 °C, as can be seen from the shape of plate Ag3SbS3 nanoparticles from Figure 1d,e and Figure 3a,b, which means a capping effect of oleic acid and oleylamine. D

DOI: 10.1021/acs.cgd.7b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 7. Current−potential (I−V) curve of the Ag3SbS3 (the samples were prepared at 190 °C) nanocrystal film tested in dark and upon illumination (100 mW cm−2). The Ag-Ag3SbS3-ITO structure for the I−V measurements (the lower right side).

Figure 5. (a) Orange-colored solution of Ag3SbS3 nanocrystals tranfers from hexane to formamide (FA) upon exchange of original organic surface ligands with S2−, and the color of solution turned black. (b) FTIR spectra of oleic acid, oleylamine, octadecylene, Ag3SbS3 nanocrystals before exchanging ligand, and Ag3SbS3 nanocrystals after exchanging ligand.

Figure 8. Photocatalytic H2 evolution catalyzed by different shapes of Ag3SbS3 nanocrystals. Lines are linear fitting of the amount of the mole of H2 dependent on illumination time. Sample named Raw is the as-synthesized nanoplate shaped Ag3SbS3 without exchange of its surface ligands.

the contour profile of the spectra. This result suggests that the quantum size effect is not obvious due to their large size. Further inspection of the band gap of samples was carried out according to absorption spectra by using Tauc plots, i.e., the plot of (αhv)n versus hv (where α, h, and v, respectively, represent the absorption coefficient, Planck’s constant, and frequency). Previous studies reveal the Ag3SbS3 is a direct band-gap material. Thus, the value of n should be set as 2 for the semiconductor materials with a direct band gap. By making the tangent line, the calculation results of the band gaps of the as-prepared samples are displayed in the Figure 6 (insets). The energy band gaps of Ag3SbS3 samples were 1.95, 1.93 and 2.08 eV, respectively. According to previous work, the energy band gap of Ag3SbS3 nanoparticles is estimated to be ∼1.5−1.7 eV22, which is smaller than our results. The energy band gap of Ag3SbS3 are dependent on the particle size of nanocrystals. To assess the suitability of Ag3SbS3 as an optically active component, a photoresponsive device with a configuration of

Figure 6. UV−vis optical absorption spectrum of Ag3SbS3 NCs with various shapes: (a) sphere; (b) hexagonal plate; (c) hexagonal prism. The insets show the corresponding plots of (αhv)2vs (hv) photon energy for the samples. E

DOI: 10.1021/acs.cgd.7b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 9. XPS spectra of Ag3SbS3 samples with (a, d, g) sphere, (b, e, h) hexagonal plate, and (c, f, i) hexagonal prism of Ag, Sb, and S, respectively. Binding energy curves of all elements are fits to the data (black open circles) using mixed Gaussian−Lorentzian functions, and olive color curves represent the sum of the peak fits. Peak positions are marked with the same color of simulated peak lines. The Shirley method was used to subtract backgrounds.

as-prepared film. Figure 7 shows the current−voltage curves for the device in the dark as well as under AM1.5 illumination. The photocurrents were increased with increasing bias. Linear current versus voltage was observed, indicating the Ohmic contact to the structure. The reason for this phenomenon may be

ITO/NCs/metal was fabricated to validate the optoelectronic properties of the as-synthesized Ag3SbS3 prism shaped crystals (Figure 7). The film was fabricated on a quartz substrate via spin-casting using a toluene solution of Ag3SbS3 nanocrystals. The patterned silver electrode was then deposited on the F

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Peaks at 531−532 eV correspond to the small portion of oxidized SbOx43 and adsorbed oxygen44 at the surface of samples. The high-resolution spectra of S 2p show two peaks for S 2p3/2 and S 2p1/2 at 160.95−161.5 eV and 162.05−162.6 eV that were attributed to a single doublet from S−Sb bonds.45 Plate Ag3SbS3 shows the highest binding energy of Ag and S and the lowest binding energy of Sb, which means Sb in plate shaped crystals is saturated coordination. This result is in accordance with the surface structure of {012} facets (Figure 4f). The binding energy of Sb in prism Ag3SbS3 samples is lower than that of the plate sample, which indicates that the Sb atoms in the prism crystal are partially coordinatively stabilized by −S ligands at the surface. Thus, the binding energy analysis of different shaped Ag3SbS3 samples also confirms the polar characteristic of {1̅20} facets with high hydrogen generation efficiency.

that the electrons in the valence band are excited to the conduction band under light conditions and the holes produce in the film. The hole−electron pairs are generated on the surface of the Ag3SbS3 thin film under illumination, thereby enhancing the conductivity of the film. The thin film tested in ambient condition did not show detectable decrease in photocurrent during the initial cycling. These preliminary results on the photoresponse of Ag3SbS3 nanocrystal films are competitive with the dynamics observed for thin films of other well-established nanocrystalline materials. Additional optimization in film thickness, surface chemistry, and the shapes of materials is expected to result in further improvements to the photocurrents. The result indicates that as-prepared Ag3SbS3 nanocrystals have potential application in photovoltaics. The faceted Ag3SbS3 samples were tested for the photoreduction of water in the presence of an electron donor under ambient conditions. Photocatalytic water-splitting property for the Ag3SbS3 nanocrystals with three shapes is shown in Figure 8. Lines are linear fitting of the H2 amount with increasing catalytic duration for each shape of catalyst. The as-synthesized Ag3SbS3 nanocrystals are capped with organic ligands that show no photocatalytic activity. The performance of spherical Ag3SbS3 particles shows the highest H2 evolution rates (2.77 μmol H2/min for 1 mmol of spherical samples), which is understandable due to the smallest particle size and poorest crystallinity that crystallized at 170 °C. Thus, although block shaped Ag3SbS3 shows a near value of energy band gap (Figure 5a,b), the H2 evolution rate is much lower. In comparison with the catalytic behavior of hexagonal prism and plate, a clearly higher H2 evolution rate could be found in the prism shaped catalysts, i.e., 1.065 and 0.7288 μmol H2/min for the prism and block, respectively. This result indicates that the prism exposed more active surface than that of plates in photocatalytic water-splitting processes. The nanocrystals are not stable and decomposed into Ag2S phase after the photocatalytic reactions (Figure S3). From Figures 1 and 3, it can be concluded that the prism shaped crystals are grown with a much larger size than that of plate shaped crystals. Theoretically, the smaller size the catalysts are, the higher catalytic properties they will show. Here, large sized prism crystals show nearly 50% higher catalytic speed for H2 evolution than the small sized plates, which means the shape of crystals, i.e., the exposed crystal facets, plays a crucial role in the water-splitting process. As shown in Figure 4, prism shaped crystals show more surface percentage of {1̅20} facets, which is composed by a planar Sb−S layer with offplane pointed Sb−S bonds. Thus, there is a polarity between the off-plane Sb−S bonds, which may provide strong affinity to the polar H2O molecules. The plate shaped Ag3SbS3 crystals, however, are exposed with the largest percentage of {012} facets, which extruded three Sb−S bonds of the crystal plane. This configuration of SbS3 groups offers a nearly nonpolar interaction between the crystal plane surface and adsorbed water molecules, which is detrimental to water-splitting reactions. High-resolution XPS spectra of Ag 3d, Sb 3d, and S 2p for the as-prepared three shapes of Ag3SbS3 samples are obtained using C 1s as the reference at 284.6 eV (Figure 9). Peaks of Ag 3d5/2 and Ag 3d3/2 in as-prepared Ag3SbS3 samples with different shapes are deconvoluted into two peaks yielding two doublets with the binding energy peaks at 367.5−368.05 eV (Ag 3d5/2) and 368.35−368.55 eV (Ag 3d5/2), 373.5−374.1 eV (Ag 3d3/2) and 374.4−374.65 eV (Ag 3d3/2), respectively.40 Binding energy peaks of Sb 3d were located at 530 and 539.5 eV for Sb 3d5/2 and Sb 3d3/2, respectively,41 which are a little higher than that of Sb2S3,42 indicating a strong bonding of Sb−S in Ag3SbS3 samples.



CONCLUSIONS In summary, Ag3SbS3 nanocrystals with tunable shapes were synthesized by tuning the polarity of solvents in a one-pot reaction. A crystal shape-controllable growth mechanism was discussed based on the polarity difference of oleylamine and oleic acid, which were tunable by the reaction temperature. The as-synthesized Ag3SbS3 nanocrystals show a shape-dependent band gap and facet-dependent photocatalytic water-splitting properties. Optoelectronic properties of Ag3SbS3 demonstrated their suitability as optically active absorber materials for photovoltaic application. This work not only provides a strategy for ternary sulfide crystal facet and shape tailoring growth but also facilitates the perspective of new materials design toward facet-dependent physical/chemical applications.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01355. Photocurrent device and measurement details; photocatalytic water-splitting properties measurement; the synthesis condition and SEM graphs of other Ag3SbS3 samples synthesized at other temperatures, reactant ratio; XRD patterns of the Ag3SbS3 crystals before and after photocatalytic experiment (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] ORCID

Long Yuan: 0000-0002-3047-0295 Lei Wang: 0000-0001-7275-4846 Keke Huang: 0000-0002-8995-2176 Author Contributions ∥

Y.X. and L.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grants 21427802 and 21671076). REFERENCES

(1) Quan, Z.; Wang, Y.; Fang, J. High-Index Faceted Noble Metal Nanocrystals. Acc. Chem. Res. 2013, 46, 191−202.

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Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.7b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX