DOI: 10.1002/chem.201501000
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& Nanocrystals
Monodisperse AgSbS2 Nanocrystals: Size-Control Strategy, LargeScale Synthesis, and Photoelectrochemistry Bin Zhou,[a, b] Mingrun Li,[a] Yihui Wu,[a, b] Chi Yang,[a] Wen-Hua Zhang,*[a] and Can Li*[a] Abstract: We report an efficient approach to the synthesis of AgSbS2 nanocrystals (NCs) by colloidal chemistry. The size of the AgSbS2 NCs can be tuned from 5.3 to 58.3 nm with narrow size distributions by selection of appropriate precursors and fine control of the experimental conditions. Over 15 g of high-quality AgSbS2 NCs can be obtained from one single reaction, indicative of the up-scalability of the present synthesis. The resulting NCs display strong absorptions in
Introduction Colloidal semiconductor nanocrystals (NCs) have received considerable attention owing to their novel size- and shape-dependent properties and application in photovoltaic systems,[1] light-emitting diodes,[2] photocatalysis,[3] thermoelectrics,[4] biolabeling,[5] and so on. Motivated by these great application potentials, recent researches have been directed toward the synthesis of semiconductor colloid NCs with feature of solution compatible processing, which permits a variety of optoelectronic devices to be fabricated through a low-cost solution-phase process such as spin-casting, dip-coating, spraycoating, and roll-to-roll printing.[1a, 6] The design and synthesis of semiconductor colloidal NCs, which were previously dominated by binary semiconductors (e.g., CdSe,[7] CdS,[8] PbS,[9] Ag2S[10]), have recently expanded to explore more complex multicomponent semiconductor NCs (MCS NCs). Among them, Cu-base MCS NCs have drawn a great deal of attention due to their optimal band gaps, large absorption coefficients, and environmental friendliness, whereas Ag-based MCS NCs have been largely overlooked, despite of their technologically applicable properties as Cu-based ones have exhibited.[11]
[a] B. Zhou, M. Li, Y. Wu, C. Yang, Prof. W.-H. Zhang, Prof. C. Li State Key Laboratory of Catalysis Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian National Laboratory for Clean Energy Dalian 116023 (P.R. China) E-mail: [email protected] [email protected] [b] B. Zhou, Y. Wu University of Chinese Academy of Sciences Beijing 100049 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501000. Chem. Eur. J. 2015, 21, 11143 – 11151
the visible-to-NIR range and exceptional air stability. The photoelectrochemical measurements indicate that, although the pristine AgSbS2 NC electrodes generate a cathodic photocurrent with a relatively small photocurrent density and poor stability, both of them can be significantly improved subject to CdS surface modification, showing promise in solar energy conversion applications.
To date, researches on Ag-based MCS NCs are mainly limited to the I-III-VI2 systems. Torimoto and co-workers synthesized a number of Ag-based I-III-VI2 NCs (e.g., AgInS2,[12] ZnSAgInS2,[13] AgInS2-AgGaS2,[14] and ZnSe-AgInSe2 solid solution[15]) by thermal decomposition of a metal ion–diethyldithiocarbamate complex in primary amines, and Han and co-workers achieved the colloidal synthesis of AgGaS2 NCs through a similar way.[16] Vittal and co-workers obtained AgInSe2 nanorods through a thermal decomposition of a single-source precursor.[17] Allen and co-workers presented a synthetic method based on an anion exchange reaction to prepare air-stable, near-infrared-emitting AgInSe2 NCs.[18] In contrast, very few of bismuth-containing Ag-based I-V-VI2 NCs have been explored.[4a, 19] Moreover, no study on the colloidal synthesis of antimony-containing Ag-based I-V-VI2 NCs has been reported up to date, despite of the fact that their bulk counterparts hold great application promise in thermoelectrics and photovoltaics.[20] It is essential to develop a synthetic method that enables the convenient production of phase-pure antimonycontaining Ag-based I-V-VI2 NCs to gain insight into their fundamental properties and further explore their potential applications. The development of facile, reproducible synthetic strategies to NCs with a uniform size, a well-defined shape, and a stoichiometric composition is extremely important for further fundamental studies and practical applications.[21] Thanks to the previous efforts, the synthesis of NCs with fine size control has been achieved in a number of noble metal NCs (e.g., Au, Ag, and Pt) and binary semiconductor NCs (e.g., PbS, CdS, or CdSe).[8, 9, 22] However, very few attempts have been made in the size-control synthesis of MCS NCs.[23] This is probably due to the significant difficulty in balancing the reactivity of each precursor for a multicomponent system,[24] let alone to achieve a precise control over their sizes.
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Full Paper Here, we present the first colloidal synthesis of the cubic AgSbS2 (miargyrite) NCs as an example to show the fine size control of a multicomponent system. Through careful selection of the metal and sulfide precursors and manipulation of the reaction conditions, we obtained high-quality AgSbS2 NCs with narrow size distributions in an exceptional large size range (5.3–58.3 nm). Moreover, particular care was taken in designing a scalable synthetic process because the ability to scale up is pivotal for the final applications. Over 15 g of high-quality AgSbS2 NCs have been obtained, in one single reaction, by using element sulfur as precursor and increasing the concentration and total volume of the reaction mixture. The AgSbS2 NCs exhibit exceptional air stability, evidenced by unchanged XRD patterns after storage under ambient conditions for eight months, and display strong absorptions in the visible-to-NIR range. Photoelectrodes made of AgSbS2 NCs were fabricated to study the photoelectrochemical properties, and a long-term stability of the AgSbS2 electrodes was achieved with CdS surface modification.
Results and Discussion Synthesis and characterization of the AgSbS2 nanocrystals
s a), b), c), d), and e) exhibit an average diameter of (10.0 1.1) (s = 10.9), (20.5 1.5) (s = 7.5), (31.8 2.0) (s = 6.1), (47.7 2.8) (s = 5.9), and (58.3 3.6) nm (s = 6.2 %), respectively. Each distribution was collected from 250 individual NCs and subsequently used to calculate the average size and the standard deviation of the distributions. All of the as-synthesized NCs displayed a spherical geometry with standard deviations around or less than 10 %. Figure 1 f presents the size distribution histograms of the AgSbS2 NCs gathered from Figures 1 a–e, revealing a narrow size distribution of the present NCs. In many practical applications, it is desirable to employ monodisperse colloidal NCs.[9] Fortunately, we can not only achieve the fine control over the particle size in a large range, but also realize a high monodispersity for the AgSbS2 NCs by manipulating the reaction parameters. Figures 1 g–i show that more than 105 of typical AgSbS2 NCs with highly uniform size were displayed in a 7.3 mm Õ 7.3 mm region, highlighting the uniformity of the present AgSbS2 NCs. The inset of Figure 1 i shows a high-resolution TEM image of a single AgSbS2 NC. The lattice spacing of 3.28 æ is in accordance with the (111) interplanar distance of cubic AgSbS2. The selected area electron diffraction (SAED) patterns (Figure S1 in the Supporting Information) of the AgSbS2 NCs clearly show diffraction rings that match well with the power X-ray diffraction patterns (XRD). These results indicate the successful preparation of sizetunable, well-crystalline AgSbS2 NCs.
The synthesis of AgSbS2 colloidal NCs was performed under an inert atmosphere by a simple non-injection synthetic approach. Briefly, in the synthesis of 5.3–20.5 nm AgSbS2 NCs, AgNO3 (0.25 mmol), SbCl3 (0.25 mmol), bis(trimethylsilyl)sulfide (TMS) (0.5 mmol), oleylamine (OLA), and oleic acid (OA) were mixed in a reaction tube, then the tube was inserted into a preheated oil bath at 180– 220 8C with the reaction lasting for 1.5-5 min. For the synthesis of 20.2–58.3 nm AgSbS2 NCs, AgNO3 (0.25 mmol), Sb(dedtc)3 (dedtc = diethyldithiocarbamate), (0.25 mmol), TMS (0–0.5 mmol), S (0–2 mmol), and OLA were mixed in a reaction tube, and the reaction was proceeded in a preheated oil bath at 160220 8C for 1.5–12 min. The asprepared nanocrystals were then washed several times with hexane and ethanol to obtain AgSbS2 nanocrystal products. Full experimental details can be found in the Experimental Section. Figure 1. TEM images of AgSbS2 NCs with different sizes: a) (10.0 1.1) (s = 10.9), b) (20.5 1.5) (s = 7.5), The representative transmis- c) (31.8 2.0) (s = 6.1), d) (47.7 2.8) (s = 5.9), and e) (58.3 3.6) nm (s = 6.2 %). Part f) shows the size distribution of the AgSbS2 NCs from Figures 1 a–e. Each distribution was collected from 250 individual NCs and subsequently sion electron microscope (TEM) used to calculate the average size and the standard deviation of the distributions. Parts g)–i) show the TEM images of the AgSbS2 NCs in images with different magnifications, highlighting the uniformity of the NCs. The inset of i) shows the highFigure 1 show that the sample- resolution (HR) TEM image of a typical AgSbS2 NC. Scale bars = 5 nm. Chem. Eur. J. 2015, 21, 11143 – 11151
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Full Paper Power X-ray diffraction (XRD) analysis was employed to investigate the crystallographic structure of the AgSbS2 NCs. Figure 2 c shows the XRD patterns of different-sized AgSbS2 NCs. All diffraction peaks match well with the cubic structured AgSbS2 (JSPDS No. 17-0456). Surprisingly, the cubic AgSbS2 (b-miargyrite, Figure 2 b) is a high-temperature polymorph,
Figure 3. a) SEM image of the 58.3 nm AgSbS2 NCs. b) Energy-dispersive X-ray spectroscopy (EDS) analysis of the AgSbS2 NCs performed on the SEM image area shown on the left.
Figure 2. Crystal structure of a) monoclinic AgSbS2 (a-miargyrite) and b) cubic AgSbS2 (b-miargyrite). c) XRD patterns of the different sized AgSbS2 NCs.
Figure 4. a) UV/Vis-NIR absorption spectra of the AgSbS2 NCs with varied diameters, b) Tauc plots for the corresponding absorbance curves.
whereas the thermodynamically stable structure is the monoclinic AgSbS2 (a-miargyrite, Figure 2 a). The b-miargyrite was usually formed by thermal-induced transformation of a-miargyrite crystals (the phase transformation from a-miargyrite to b-miargyrite occurs at 380 8C for bulk materials).[25] In contrast, our b-miargyrite NCs were synthesized at significantly lower temperature (180 8C). It has been reported that, in some cases of colloidal synthesis, metastable NCs can be stabilized at relatively low temperature by controlling the crystal size, the coordination environment, and the growth kinetics owing to the surface energy and surface stress.[26] A similar case should occur for the present cubic AgSbS2 NCs. A scanning electron microscope (SEM) image of a drop-cast film of the 58.3 nm AgSbS2 NCs is shown in Figure 3 a, presenting the high uniformity and the spherical morphology of the AgSbS2 NCs in this study. The corresponding energy-dispersive X-ray spectroscopy (EDS) results confirmed the presence of silver, antimony, and sulfur. The quantitative analysis result (Figure 3 b inset) discloses that the Ag/Sb/S atomic ratio of these NCs is 25.6:24.1:50.3, which is very close to the stoichiometric ratio of 1:1:2 for ideal AgSbS2 materials. To evaluate the size dependence of the optical properties, UV/Vis-NIR absorption spectra of AgSbS2 NC films with different sizes were investigated. All of the spectra were normalized at 1200 nm for clarity, as shown in Figure 4 a. It can be seen that all of the NCs, regardless of their sizes, have a strong absorption in the whole visible to near-infrared range. The onsets
of the optical absorption spectra were gradually blue shifted upon decreasing the particle size of the AgSbS2 NCs. Tauc plots in the form of (ahn)n (n = 2 for direct band-gap semiconductors, n = 1/2 for indirect band-gap semiconductors) were performed to analyze the optical band gaps of the AgSbS2 NCs (displayed in Figure 4 b). The most linear spectra were observed for n = 2, indicating that the present AgSbS2 NCs exhibit direct band gaps. Figure 4 b further shows the estimated band gaps of 1.57, 1.47, 1.34, and 1.32 eV for the AgSbS2 NCs with average diameters of 5.3, 10.0, 20.5, and 31.8 nm, respectively, agreeing reasonably well with the band gap of cubic AgSbS2 bulk materials at 1.23–1.44 eV.[25, 27] The optical property of the semiconductor NCs can be influenced by the surface properties and the quantum size effect. Considering the fact that the present NCs were synthesized under similar conditions, the surface properties of the NCs should exert similar influence on their optical absorption. In contrast, the particle sizes of these AgSbS2 NCs change significantly. The variation in the band gaps of the present AgSbS2 NCs could hence be attributed to the quantum size quantum effect. These results demonstrate that the AgSbS2 NCs with an average size of 5.3 and 10.0 nm are small enough to show the quantum size effect.
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Size-control strategy The size-control colloidal synthesis of phase pure, nearly monodisperse AgSbS2 NCs is achievable by suitable balancing the
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Full Paper control factors, in particular the reagent choice, the nucleation, and the growth temperatures. To successfully develop a synthetic strategy to size-tunable AgSbS2 NCs, initial exploration was made by testing a variety of ligands and precursors. We found that the combination of amines and carboxylic acids is a good choice for balancing the reactivity of the precursors. AgNO3 can be dissolved quite well in OLA, but not in OA. On the contrary, SbCl3 has very low affinity to OLA, although it can be well dissolved in OA. These phenomena can be qualitatively understood in terms of the hard–soft acid–base theory: as an acid, the monovalent Ag + ion is softer than the trivalent Sb3 + ion, whereas OLA is softer than OA. Thus, OLA binds strongly to the softer Ag + ion, whereas the OA binds stronger to the harder Sb3 + ion. The combination of OLA and OA was found to be critical to achieve monodisperse AgSbS2 NCs. The absence of either OLA or OA results in large precipitates and impurities due to the poor solubility of the corresponding cationic precursors (Figures 5 a and i). Furthermore, we found that the volume ratio of OLA to OA also played an important role in manipulating the reactivity of the cationic precursors. When using excess OLA (OLA/OA volume ratios 4:4), highly uniform AgSbS2 NCs were obtained. In contrast, only large polydisperse NCs or large precipitates were obtained if excess of OA was used. The average size of the AgSbS2 NCs can be tuned from 13.2 to 20.5 nm by varying the OLA/OA volume ratios in the range of 4:4 to 7.25:0.25. The corresponding powder XRD patterns confirmed that all of the samples are pure AgSbS2, except for sample i), which contains Sb2S3 impurities (Figure S2 in the Supporting Information). These results indicate that a fine balance of the reactivity of two cationic precursors can be achieved with the combination of OLA and OA. To gain insight into the formation process of the AgSbS2 NCs, the NC products at different reaction stages were moni-
Figure 5. TEM images of AgSbS2 NCs prepared with varying OLA/OA volume ratios: a) 8:0, b) 7.75:0.25, c) 7:1, d) 6:2, e) 5:3, f) 4:4, g) 3:5, h) 2:6, and i) 0:8. Scale bars = 200 nm. Chem. Eur. J. 2015, 21, 11143 – 11151
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Figure 6. TEM images of AgSbS2 NCs prepared with varying growth time: a) 1.5, b) 2.5, and c) 5 min and varying temperature: d) 180, e) 200, or f) 220 8C. Scale bars = 100 nm.
tored by XRD (Figure S3 in the Supporting Information) and TEM (Figures 6 a–c). The XRD results reveal that the cubic AgSbS2 was formed within the first 1.5 min. The TEM images are employed to record the shape evolution process. At first, tiny irregular rod-shaped NCs were formed at 1.5 min, and evolved into spherical polydisperse NCs within 2.5 min. When the reaction proceeds to 5 min, the NCs become larger and uniform. These observations are in good accordance with the growth mechanism of spherical Au NCs proposed by Peng and co-workers.[28] The formation of spherical NCs includes three steps: nucleation, random attachment, and intra-particle ripening. Irregular rod-shaped NCs were formed at the beginning due to the rate difference between the nucleation and the random attachment process. These irregular rod-shaped NCs turned to uniform spherical NCs through a slow intra-particle ripening process to minimize the total surface energy of the NCs. The size tunability of the AgSbS2 NCs can be further achieved by adjusting the reaction temperatures. Figures 6 d–f show that the sizes of the AgSbS2 NCs can be finely tailored from approximately 5.3 nm (up) to around 13.2 nm by increasing the reaction temperature from 180 to 220 8C. Further reducing the reaction temperature cannot afford for the growth of the AgSbS2 NCs in this study. Similar scenario where a higher reaction temperature results in larger nanoparticles have been often observed in colloidal NC synthesis.[29] It is known that increasing the reaction temperature could promote the growth rate at the nucleation stage, and the preformed nuclei would grow very fast at a high temperature, resulting in a rapid reduction in the supersaturation at the nucleation stage. In such instance, the resultant suppression in the nucleation rate would increase the overall size of the NCs. Uniform large-sized AgSbS2 NCs can be prepared by using Sb[S2CN(C2H5)2]3 (Sb(dedtc)3) as Sb precursor in the reaction. Sb(dedtc)3 could not only act as an antimony precursor but also act as a sulfur precursor. Thus, we have attempted to prepare AgSbS2 NCs by using a mixture of Sb(dedtc)3 and OLA– AgNO3, without adding another sulfur precursor. However, only large irregular-shaped crystals were obtained, as shown in
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Full Paper Figure S4 in the Supporting Information. The corresponding XRD results reveal that the products are mixtures of AgSbS2 and Ag3SbS3. By introducing the product of element sulfur dissolved in OLA (OLA-S) into the reaction mixture, the NCs obtained exhibit improved uniformity. However, the Ag3SbS3 impurities can still be observed in the XRD patterns. We hence introduced the highly reactive sulfur precursor TMS into the reaction mixture, resulting in highly uniform AgSbS2 NCs without any impurities. It is interesting to note that the combination of TMS and OLA-S has also resulted in pure AgSbS2 NCs and that the size of the AgSbS2 NCs can be tuned from 20.0 to 47.8 nm by increasing the dosage of OLA-S from 0 to 2 mmol (Figure 7 a–c), whereas the TMS dosage was fixed at 0.5 mmol. Moreover, the size of the monodisperse AgSbS2 NCs can be further enlarged from 47.8 up to about 58.3 nm by increasing the growth time from 5 to 12 min (Figures 7 d and e). Therefore, we have realized the tailoring of the AgSbS2 NCs in an exceptional size ranged from 5.3 to 58.3 nm.
Figure 7. TEM images of products prepared by using Sb(dedtc)3 as antimony precursor with varying OLAS dosage: a) 0, b) 0.5, or c) 2 mmol, and varying reaction time: d) 1.5, e) 5, or f) 12 min. Scale bars = 200 nm.
scale up in a single reaction is pivotal for further applications. TMS is an ideal choice as sulfur source in colloidal nanocrystal synthesis due to its high reactivity. However, the high cost, the high volatility, and the toxicity of TMS makes it unfavorable as a sulfur source for a large-scale synthesis. In view of cost and operative convenience, the product of element sulfur dissolved in OLA (OLA-S) is an excellent choice as a sulfur source for up-scaling synthesis of the AgSbS2 NCs. Therefore, we have conducted the experiment to replace TMS with OLA-S and using SbCl3 as Sb precursor in the synthesis of AgSbS2 NCs. It was found that the initial mole ratio of AgNO3/OLA-S strongly influenced the phase and morphology of the resulting NCs. For mole ratios within the range 1:2 to 1:4, pure cubic AgSbS2 NCs with high uniformity are formed, as shown in Figures 8 a and b and Figure S7 in the Supporting Information. A Large excess of sulfur precursor, however, results in the formation of plate-shaped impurities (Figures 8 c and d and Figure S7 in the Supporting Information). To achieve large-scale synthesis, it is necessary to carefully tune the overall concentration (designated by the concentration of the “AgSbS2 molecule”) of the reaction mixtures. We first fixed the Ag/Sb/S molar ratio at 1:1:2 to adjust the overall concentration of the reaction solution, and found that the average size of the resulting AgSbS2 NCs decreased as the overall concentration increased within the concentration ranges between 0.031 and 0.141 mmol mL¢1 (Figures 8 e–g). By increasing the overall concentration to as high as 0.217 mmol mL¢1, the reaction system became very viscous, and the magnetic stir bar almost stops stirring. In such case, mixtures of nanorods and nanoparticles were obtained owing to the poor diffusion during the nucleation and growth stage (Figure 8 h). Along the above lines, we conducted the up-scaling synthesis of AgSbS2 NCs with an AgNO3/OLAS ratio of 1:2 and an overall concentration of 0.122 mmol mL¢1. In a typical synthesis, 55 mmol of AgNO3, 55 mmol of SbCl3, 110 mmol of S, 110 mL of OA, and 340 mL of OLA were involved in the reac-
Large-scale synthesis Most of the colloidal syntheses are usually developed with low outputs, typically ranging from several tens to several hundreds of milligrams for one reaction, which is insufficient for most practical applications.[30] In many cases, several or dozens of batches have to be conducted to provide a sufficient amount of material, which is a laborious task. Moreover, possible variations in the chemical composition or the size of the NCs obtained from different batches may also become an unsolvable problem.[31] Thus, the ability to Chem. Eur. J. 2015, 21, 11143 – 11151
Figure 8. TEM images of AgSbS2 NCs prepared with varying AgNO3/OLA-S precursor ratios: a) 1:2, b) 1:4, c) 1:8, or d) 1:16, and a varying overall concentration (designated by the concentration of the “AgSbS2 molecule”): e) 0.031, f) 0.076, g) 0.122, or h) 0.217 mmol mL¢1. Scale bars = 200 nm.
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Figure 9. Photographs of a) the reaction flaks and b,c) the obtained AgSbS2 NCs powder. d) TEM image and e) powder XRD pattern of the scale-up synthesized AgSbS2 NCs.
tion, resulting in as much as 15.59 g (the yield was 96 %) of AgSbS2 NCs in a single batch synthesis (Figures 9 a–c). The obtained AgSbS2 NCs displayed spherical morphologies with an average diameter of 14 nm (Figure 9 d), and the powder XRD pattern (Figure 9 e) of the AgSbS2 NCs matched well with the cubic AgSbS2, confirming the purity of the large-scale synthesized AgSbS2 NCs. Air-stability and photoelectrochemical properties One of the advantages of semiconductor colloidal NCs is their ability in fabricating the active layers of optoelectronic devices through low-cost solution processes, such as spray-coating,
spin coating, or roll-to-roll printing.[6] Thus, air stability is a desired property although certainly not a requirement. The air stability of the AgSbS2 NCs was tested by XRD characterization. No difference in the XRD patterns can be detected (Figure 10 a) between the justly synthesized sample and the sample, which has been stored under ambient condition for eight months, indicating the superior stability of the AgSbS2 NCs under ambient condition. To test the photoresponse of the AgSbS2 NCs, thin films of the 20 nm AgSbS2 NCs were fabricated on Mo-coated sodalime glass substrates by spraying AgSbS2 NC inks onto the hot substrates. The thicknesses of the obtained films were around 600 nm. The detailed experimental conditions are given in the Experimental Section. CdS layers with a thickness of about 60 nm were deposited on the surface of the as-prepared AgSbS2 NC films by chemical bath deposition (CBD). Photoelectrochemical (PEC) measurements were performed by using a typical three-electrode configuration. The as-prepared NC electrodes, a Pt plate, and the saturated calomel electrode (SCE) were used as the working electrode, the counter electrode, and the reference electrode, respectively. All PEC measurements were conducted under AM 1.5 G simulated sunlight (100 mW cm¢2) by using a 0.5 m Na2HPO4 solution (pH adjusted to 7.3 by NaOH addition) as the electrolyte. The potentials in each measurement were converted into the values against the reversible hydrogen electrode (RHE) by the Nernst equation (ERHE = ESCE+0.059 pH+ +0.245). Figure 10 b shows the current–voltage curves under pulsed AM 1.5 G (100 mW cm¢2) illumination for bare AgSbS2 NC films (black solid line) and CdS-modified AgSbS2 NC films (gray solid line). The bare AgSbS2 NC film generates a cathodic photocur-
Figure 10. a) Powder XRD patterns of freshly prepared AgSbS2 NCs and the same sample after eight month storage in air. b) Current–potential curves for bare and CdS-modified AgSbS2 NC electrodes (RHE = reversible hydrogen electrode). c) Current–time curves for bare and CdS-modified AgSbS2 NC electrodes at an applied potential of 0 VRHE, with a 20 s ON/OFF illumination cycle. Chem. Eur. J. 2015, 21, 11143 – 11151
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Full Paper rent under illumination and exhibits p-type conductivity. The photocurrent density and the onset potential are 60 mA cm¢2 at 0 VRHE and + 0.5 VRHE, respectively. For the CdS-modified AgSbS2 NC film, the photocurrent density was increased significantly to 300 mA cm¢2 at 0 VRHE, which is five times of magnitude of that for the bare AgSbS2 NC film. Moreover, the onset potential is also slightly increased to a value of + 0.57 VRHE. This could be ascribed to the formation of p–n junctions between AgSbS2 and CdS, leading to band-bending at the solid–electrolyte interface and broadening of the depletion layer thickness, which is favorable for the charge separation. Similar results have been reported on the deposition of n-type CdS onto p-type Cu(In,Ga)(S,Se)2.[32] Photocorrosion of the electrodes is one of the major challenges for PEC hydrogen production. Ag-based materials and other potential photocathodes (e.g., Si, Cu2O) usually suffer from this problem.[33] The degradation could be suppressed by appropriate surface modification, such as TiO2 and CdS layers.[34] To examine the stability of the bare AgSbS2 NC electrode and the CdS-modified AgSbS2 NC electrode, the time curves for the AgSbS2 NC electrode and the CdS-modified AgSbS2 NC electrode were measured at an applied potential of 0 VRHE, with a 20 s ON/OFF illumination cycle for over one hour (Figure 10 c). Furthermore, a consecutive illumination stability test for more than 2 h were also conducted (Figure S8 in the Supporting Information). All measurements were performed under AM 1.5 G (100 mW cm¢2) illumination, in a phosphate buffer (pH 7.3) to avoid any change in pH. The bare AgSbS2 NC electrode shows a rapid decay of the photocurrent during the stability test, whereas the CdS-modified AgSbS2 NC electrode shows very significant improvements in both the photocurrent density and the stability. These preliminary results on the photoresponse and stability tests of the CdS-modified AgSbS2 NC films are very encouraging and are competitive with the photoelectrochemical performances of other established NC materials.[11f,h–j, 26d, 35]
Conclusion In summary, the first synthesis of monodisperse and wellcrystallized AgSbS2 NCs has been achieved through colloidal chemistry. By combination of careful control over the OLA/OA ratio and the reaction temperature, high quality AgSbS2 NCs with sizes between 5.3 to 20 nm were obtained by using SbCl3 and AgNO3 as metal source. The sizes of the AgSbS2 NCs can be further enlarged from 20.5 to 58.3 nm by using Sb(dedtc)3 as Sb precursor with an appropriate dosage of OLA-S and growth time. The as-obtained AgSbS2 NCs show strong absorption in the visible-to-NIR range and high air stability under ambient conditions. Additionally, the large-scale synthesis of AgSbS2 NCs with 15 g of the product in one single reaction has been realized by using element sulfur as precursor and optimizing the concentration and volume of the reaction mixture. Finally, photoelectrochemical measurements show the significantly improved stability of the CdS-modified AgSbS2 NC film with competitive photocurrent density. These results indicate Chem. Eur. J. 2015, 21, 11143 – 11151
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the promise of applying the obtained NCs in photoelectronics or photocatalysis.
Experimental Section Materials: Silver nitrate (AgNO3, 99.9 %), antimony(III) chloride (SbCl3, 99.999 %), bis(trimethylsilyl)sulfide (TMS, 98 %), 1-dodecylamine (1-DDA, 98 %), octanoic acid (OTA, 98 %), and 1-octadecene (ODE, 90 %) were purchased from Alfa Aesar. Antimony(III) acetate (SbAc3, 99.99 %), sulfur powder (S, 99.99 %), oleylamine (OLA, 70 %), oleic acid (OA, 90 %), and 1-dodecanethiol (1-DDT, 98 + %) were purchased from Sigma Aldrich. Sodium dihydrogen phosphate dehydrate (NaH2PO4·2 H2O, 98 %), sodium hydroxide (NaOH, 96 %), toluene (99.5 %), hexane (97 %), and ethanol (99.7 %) were obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium diethyldithiocarbamate trihydrate (Na(dedtc)·3 H2O, 99 %) was delivered from Aladdin. All chemicals were used directly without any further purification except for OLA and OA, which were degassed for 5 h at 110 8C. Synthesis of Sb(dedtc)3 : SbAc3 (20 g, 67 mmol) in anhydrous ethanol (200 mL) and Na(dedtc)·3 H2O (50 g, 222 mmol) in anhydrous ethanol (500 mL) were mixed together with stirring, resulting in a yellow suspension. After stirring for two hour, the resulting yellow precipitate was filtered and washed with anhydrous ethanol for more than three times. The obtained filter cake was dried under vacuum at 60 8C, light-yellow powders were obtained. AgNO3, SbCl3, and S stock solutions: The stock solution of OLAAgNO3 (0.1 m) and OLA-S (1 m) were prepared by dissolving AgNO3 (1.699 g, 10 mmol) and S (1.924 g, 60 mmol) in OLA (100 and 60 mL, respectively). The 1 m OA-SbCl3 stock solution was prepared by dissolving SbCl3 (2.281 g, 10 mmol) in OA (10 mL). All of these stock solutions were prepared under stirring on a 50 8C hotplate for more than 8 h in a nitrogen-filled glovebox. Synthesis of small-sized AgSbS2 NCs (5.3–20.5 nm): In a typical synthesis, a 0.1 m OLA-AgNO3 solution (2.5 mL), a 1 m OA-SbCl3 solution (250 mL), and TMS (107 mL, 0.5 mmol) were loaded sequentially into a reaction tube inside a nitrogen-filled glovebox. To adjust the overall OLA/OA ratio, specific amounts of OLA and OA were added into the reaction tube. The total volume of the reaction mixture was fixed at 8 mL. The reaction tube was inserted into a preheated oil bath at 220 8C under a nitrogen atmosphere, and the reaction was allowed to proceed for 5 min with continuous stirring. Then, the reaction tube was rapidly cooled down to room temperature with a cold water bath. Afterward, the as-prepared NCs were precipitated with anhydrous ethanol by centrifugation. The isolated NCs were then re-dispersed in hexane and re-precipitated with anhydrous ethanol. The purification process was carried out twice and the purified NCs were re-dispersed in toluene for further characterization. The size of the AgSbS2 NCs can be tuned by adjusting the OLA/OA ratio from 7.25:0.25 to 4:4. When the ratio of OLA/OA was fixed at 4:4, the size of the AgSbS2 NCs can also be tuned by varying the reaction temperature from 180 to 220 8C. Synthesis of large-sized AgSbS2 NCs (23.9–58.3 nm): In a typical synthesis, Sb(dedtc)3 (0.1417 g, 0.25 mmol), a 0.1 m OLA-AgNO3 solution (2.5 mL), TMS (107 mL, 0.5 mmol), a 1 m OLA-S solution (2 mL), and OLA (3.5 mL) were loaded into a reaction tube inside a nitrogen-filled glovebox. The mixture was then inserted into an oil bath preheated at 220 8C under a nitrogen atmosphere and maintained at this temperature for 5 min with continuous stirring. Afterward, the colloidal solution was rapidly cooled to room temperature with a water bath and purified twice adopting the
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Full Paper same procedure described before. The size of the AgSbS2 NCs can be tuned by varying the amount of OLA-S from 0 to 2 mL and by adjusting the growth time from 5 to 12 min. Synthesis of AgSbS2 NCs by using OLA-S as a sulfur source: Typically, a 0.1 m OLA-AgNO3 solution (2.5 mL), a 1 m OA-SbCl3 solution (250 mL), a 1 m OLA-S solution (500 mL), and OLA (4.75 mL) were loaded into a reaction tube inside a nitrogen-filled glovebox. The mixture was then inserted into an oil bath preheated at 220 8C under a nitrogen atmosphere and maintained at this temperature for 5 min with continuous stirring. Subsequently, the colloidal solution was rapidly cooled to room temperature with a water bath and purified twice adopting the same procedure described before. To achieve the large-scale synthesis of the AgSbS2 NCs shown in the next part, the reaction parameters were optimized by varying the Ag/S mole ratio from 1:2 to 1:16 and the overall concentration of the reaction mixture from 0.031 to 0.217 mmol mL¢1. Large-scale synthesis of AgSbS2 NCs: A 0.2 m OLA-AgNO3 solution (275 mL), a 1 m OA-SbCl3 solution (55 mL), a 1 m OLA-S solution (110 mL). and OLA (10 mL) were loaded into a large flask with a volume of 1 L. The flask was then inserted into a preheated oil bath at 220 8C, and the reaction was allowed to proceed for 12 min with continuous stirring. Afterward, the as-prepared NCs were precipitated with anhydrous ethanol by centrifugation. The isolated NCs were then re-dispersed in hexane and re-precipitated with anhydrous ethanol. The purification process was carried out twice and the purified NCs were dried under vacuum at 60 8C for 8 h. Characterization: The low-resolution TEM images were obtained on a Hitachi HT 7700 microscope, operating at 120 kV. High-resolution TEM images were taken on a FEI TECNAI F30 S-Twin microscope with an accelerating voltage of 300 kV. SEM images and element analysis were conducted on a FEI Quanta 200F scanning electron microscope equipped with an energy dispersive spectroscopy (EDS) detector. X-ray diffraction (XRD) analyses were performed on a Rigaku RINT D/Max-2500 powder diffraction system by using CuKa radiation source (l = 1.54 æ) operating at 40 kV and 200 mA. UV/Vis-NIR absorption spectra were measured by using a Cary 5000 spectrophotometer. The PEC test was conducted in a three electrode system with a potentiostat (Iviumstat, Ivium Technologies) under simulated AM 1.5 G solar-light irradiation (100 mW cm¢2, Newport Sol 3A, Class AAA Solar simulator). Fabrication of electrodes from AgSbS2 NC inks: The purified AgSbS2 NCs were dispersed in toluene with a concentration of 0.1 mmol mL¢1. Mo-coated soda-lime glasses were heated to 120 8C on a hotplate. Then AgSbS2 NC inks were spraying onto the Mocoated soda-lime glasses, resulting in uniform AgSbS2 NC films with a thickness of approximately 600 nm. Surface modification of the AgSbS2 NC films with CdS layers was performed by chemical bath deposition (CBD) by using a variation on a literature method.[32a] Prior to CdS deposition, the AgSbS2 NC films were immersed in a solution containing 7.5 mm CdAc2 and 2 m NH4OH for 10 min at 65 8C. Afterward, CBD of CdS layers was carried out by immersing the pre-treated AgSbS2 NC films in an aqueous solution containing 7.5 mm CdAc2, 0.375 m SC(NH2)2, and 2 m NH4OH at 65 8C for 5.5 min, resulting in the deposition of a CdS layer with a thickness of approximately 60 nm. After the deposition, the electrodes were rinsed with distilled water, followed by annealing in air at 200 8C for 40 min. Photoelectrochemical measurements: PEC measurements were conducted by using a normal three-electrode system, by using a prepared electrode, a Pt plate, and a saturated calomel electrode (SCE) as working electrode, counter electrode, and reference electrode, respectively. All of the PEC measurements were conducted under AM 1.5 G simulated sunlight (100 mW cm¢2) by using a 0.5 m Chem. Eur. J. 2015, 21, 11143 – 11151
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Na2HPO4 solution (pH adjusted to 7.3 by NaOH addition) as the electrolyte. The use of phosphate buffer solution as the electrolyte can avoid pH changes during the measurement. The potentials in each measurement were converted into values against the reversible hydrogen electrode (RHE) by the Nernst equation (ERHE = ESCE+0.059 pH+ +0.245). The photocurrent was measured by linear sweep voltammetry with a scan rate of 2 mV s¢1.
Acknowledgements We gratefully acknowledge financial support from the “Hundred Talents Program” of the Chinese Academy of Sciences, the National Science Foundation of China (nos. 20873141). Keywords: nanoparticles · photoelectrochemistry semiconductors · silver antimony sulfide
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Received: March 13, 2015 Published online on June 19, 2015
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Full Paper
& Nanocrystals
Monodisperse AgSbS2 Nanocrystals: Size-Control Strategy, LargeScale Synthesis, and Photoelectrochemistry Bin Zhou,[a, b] Mingrun Li,[a] Yihui Wu,[a, b] Chi Yang,[a] Wen-Hua Zhang,*[a] and Can Li*[a] Abstract: We report an efficient approach to the synthesis of AgSbS2 nanocrystals (NCs) by colloidal chemistry. The size of the AgSbS2 NCs can be tuned from 5.3 to 58.3 nm with narrow size distributions by selection of appropriate precursors and fine control of the experimental conditions. Over 15 g of high-quality AgSbS2 NCs can be obtained from one single reaction, indicative of the up-scalability of the present synthesis. The resulting NCs display strong absorptions in
Introduction Colloidal semiconductor nanocrystals (NCs) have received considerable attention owing to their novel size- and shape-dependent properties and application in photovoltaic systems,[1] light-emitting diodes,[2] photocatalysis,[3] thermoelectrics,[4] biolabeling,[5] and so on. Motivated by these great application potentials, recent researches have been directed toward the synthesis of semiconductor colloid NCs with feature of solution compatible processing, which permits a variety of optoelectronic devices to be fabricated through a low-cost solution-phase process such as spin-casting, dip-coating, spraycoating, and roll-to-roll printing.[1a, 6] The design and synthesis of semiconductor colloidal NCs, which were previously dominated by binary semiconductors (e.g., CdSe,[7] CdS,[8] PbS,[9] Ag2S[10]), have recently expanded to explore more complex multicomponent semiconductor NCs (MCS NCs). Among them, Cu-base MCS NCs have drawn a great deal of attention due to their optimal band gaps, large absorption coefficients, and environmental friendliness, whereas Ag-based MCS NCs have been largely overlooked, despite of their technologically applicable properties as Cu-based ones have exhibited.[11]
[a] B. Zhou, M. Li, Y. Wu, C. Yang, Prof. W.-H. Zhang, Prof. C. Li State Key Laboratory of Catalysis Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian National Laboratory for Clean Energy Dalian 116023 (P.R. China) E-mail: [email protected] [email protected] [b] B. Zhou, Y. Wu University of Chinese Academy of Sciences Beijing 100049 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501000. Chem. Eur. J. 2015, 21, 11143 – 11151
the visible-to-NIR range and exceptional air stability. The photoelectrochemical measurements indicate that, although the pristine AgSbS2 NC electrodes generate a cathodic photocurrent with a relatively small photocurrent density and poor stability, both of them can be significantly improved subject to CdS surface modification, showing promise in solar energy conversion applications.
To date, researches on Ag-based MCS NCs are mainly limited to the I-III-VI2 systems. Torimoto and co-workers synthesized a number of Ag-based I-III-VI2 NCs (e.g., AgInS2,[12] ZnSAgInS2,[13] AgInS2-AgGaS2,[14] and ZnSe-AgInSe2 solid solution[15]) by thermal decomposition of a metal ion–diethyldithiocarbamate complex in primary amines, and Han and co-workers achieved the colloidal synthesis of AgGaS2 NCs through a similar way.[16] Vittal and co-workers obtained AgInSe2 nanorods through a thermal decomposition of a single-source precursor.[17] Allen and co-workers presented a synthetic method based on an anion exchange reaction to prepare air-stable, near-infrared-emitting AgInSe2 NCs.[18] In contrast, very few of bismuth-containing Ag-based I-V-VI2 NCs have been explored.[4a, 19] Moreover, no study on the colloidal synthesis of antimony-containing Ag-based I-V-VI2 NCs has been reported up to date, despite of the fact that their bulk counterparts hold great application promise in thermoelectrics and photovoltaics.[20] It is essential to develop a synthetic method that enables the convenient production of phase-pure antimonycontaining Ag-based I-V-VI2 NCs to gain insight into their fundamental properties and further explore their potential applications. The development of facile, reproducible synthetic strategies to NCs with a uniform size, a well-defined shape, and a stoichiometric composition is extremely important for further fundamental studies and practical applications.[21] Thanks to the previous efforts, the synthesis of NCs with fine size control has been achieved in a number of noble metal NCs (e.g., Au, Ag, and Pt) and binary semiconductor NCs (e.g., PbS, CdS, or CdSe).[8, 9, 22] However, very few attempts have been made in the size-control synthesis of MCS NCs.[23] This is probably due to the significant difficulty in balancing the reactivity of each precursor for a multicomponent system,[24] let alone to achieve a precise control over their sizes.
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Full Paper Here, we present the first colloidal synthesis of the cubic AgSbS2 (miargyrite) NCs as an example to show the fine size control of a multicomponent system. Through careful selection of the metal and sulfide precursors and manipulation of the reaction conditions, we obtained high-quality AgSbS2 NCs with narrow size distributions in an exceptional large size range (5.3–58.3 nm). Moreover, particular care was taken in designing a scalable synthetic process because the ability to scale up is pivotal for the final applications. Over 15 g of high-quality AgSbS2 NCs have been obtained, in one single reaction, by using element sulfur as precursor and increasing the concentration and total volume of the reaction mixture. The AgSbS2 NCs exhibit exceptional air stability, evidenced by unchanged XRD patterns after storage under ambient conditions for eight months, and display strong absorptions in the visible-to-NIR range. Photoelectrodes made of AgSbS2 NCs were fabricated to study the photoelectrochemical properties, and a long-term stability of the AgSbS2 electrodes was achieved with CdS surface modification.
Results and Discussion Synthesis and characterization of the AgSbS2 nanocrystals
s a), b), c), d), and e) exhibit an average diameter of (10.0 1.1) (s = 10.9), (20.5 1.5) (s = 7.5), (31.8 2.0) (s = 6.1), (47.7 2.8) (s = 5.9), and (58.3 3.6) nm (s = 6.2 %), respectively. Each distribution was collected from 250 individual NCs and subsequently used to calculate the average size and the standard deviation of the distributions. All of the as-synthesized NCs displayed a spherical geometry with standard deviations around or less than 10 %. Figure 1 f presents the size distribution histograms of the AgSbS2 NCs gathered from Figures 1 a–e, revealing a narrow size distribution of the present NCs. In many practical applications, it is desirable to employ monodisperse colloidal NCs.[9] Fortunately, we can not only achieve the fine control over the particle size in a large range, but also realize a high monodispersity for the AgSbS2 NCs by manipulating the reaction parameters. Figures 1 g–i show that more than 105 of typical AgSbS2 NCs with highly uniform size were displayed in a 7.3 mm Õ 7.3 mm region, highlighting the uniformity of the present AgSbS2 NCs. The inset of Figure 1 i shows a high-resolution TEM image of a single AgSbS2 NC. The lattice spacing of 3.28 æ is in accordance with the (111) interplanar distance of cubic AgSbS2. The selected area electron diffraction (SAED) patterns (Figure S1 in the Supporting Information) of the AgSbS2 NCs clearly show diffraction rings that match well with the power X-ray diffraction patterns (XRD). These results indicate the successful preparation of sizetunable, well-crystalline AgSbS2 NCs.
The synthesis of AgSbS2 colloidal NCs was performed under an inert atmosphere by a simple non-injection synthetic approach. Briefly, in the synthesis of 5.3–20.5 nm AgSbS2 NCs, AgNO3 (0.25 mmol), SbCl3 (0.25 mmol), bis(trimethylsilyl)sulfide (TMS) (0.5 mmol), oleylamine (OLA), and oleic acid (OA) were mixed in a reaction tube, then the tube was inserted into a preheated oil bath at 180– 220 8C with the reaction lasting for 1.5-5 min. For the synthesis of 20.2–58.3 nm AgSbS2 NCs, AgNO3 (0.25 mmol), Sb(dedtc)3 (dedtc = diethyldithiocarbamate), (0.25 mmol), TMS (0–0.5 mmol), S (0–2 mmol), and OLA were mixed in a reaction tube, and the reaction was proceeded in a preheated oil bath at 160220 8C for 1.5–12 min. The asprepared nanocrystals were then washed several times with hexane and ethanol to obtain AgSbS2 nanocrystal products. Full experimental details can be found in the Experimental Section. Figure 1. TEM images of AgSbS2 NCs with different sizes: a) (10.0 1.1) (s = 10.9), b) (20.5 1.5) (s = 7.5), The representative transmis- c) (31.8 2.0) (s = 6.1), d) (47.7 2.8) (s = 5.9), and e) (58.3 3.6) nm (s = 6.2 %). Part f) shows the size distribution of the AgSbS2 NCs from Figures 1 a–e. Each distribution was collected from 250 individual NCs and subsequently sion electron microscope (TEM) used to calculate the average size and the standard deviation of the distributions. Parts g)–i) show the TEM images of the AgSbS2 NCs in images with different magnifications, highlighting the uniformity of the NCs. The inset of i) shows the highFigure 1 show that the sample- resolution (HR) TEM image of a typical AgSbS2 NC. Scale bars = 5 nm. Chem. Eur. J. 2015, 21, 11143 – 11151
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Full Paper Power X-ray diffraction (XRD) analysis was employed to investigate the crystallographic structure of the AgSbS2 NCs. Figure 2 c shows the XRD patterns of different-sized AgSbS2 NCs. All diffraction peaks match well with the cubic structured AgSbS2 (JSPDS No. 17-0456). Surprisingly, the cubic AgSbS2 (b-miargyrite, Figure 2 b) is a high-temperature polymorph,
Figure 3. a) SEM image of the 58.3 nm AgSbS2 NCs. b) Energy-dispersive X-ray spectroscopy (EDS) analysis of the AgSbS2 NCs performed on the SEM image area shown on the left.
Figure 2. Crystal structure of a) monoclinic AgSbS2 (a-miargyrite) and b) cubic AgSbS2 (b-miargyrite). c) XRD patterns of the different sized AgSbS2 NCs.
Figure 4. a) UV/Vis-NIR absorption spectra of the AgSbS2 NCs with varied diameters, b) Tauc plots for the corresponding absorbance curves.
whereas the thermodynamically stable structure is the monoclinic AgSbS2 (a-miargyrite, Figure 2 a). The b-miargyrite was usually formed by thermal-induced transformation of a-miargyrite crystals (the phase transformation from a-miargyrite to b-miargyrite occurs at 380 8C for bulk materials).[25] In contrast, our b-miargyrite NCs were synthesized at significantly lower temperature (180 8C). It has been reported that, in some cases of colloidal synthesis, metastable NCs can be stabilized at relatively low temperature by controlling the crystal size, the coordination environment, and the growth kinetics owing to the surface energy and surface stress.[26] A similar case should occur for the present cubic AgSbS2 NCs. A scanning electron microscope (SEM) image of a drop-cast film of the 58.3 nm AgSbS2 NCs is shown in Figure 3 a, presenting the high uniformity and the spherical morphology of the AgSbS2 NCs in this study. The corresponding energy-dispersive X-ray spectroscopy (EDS) results confirmed the presence of silver, antimony, and sulfur. The quantitative analysis result (Figure 3 b inset) discloses that the Ag/Sb/S atomic ratio of these NCs is 25.6:24.1:50.3, which is very close to the stoichiometric ratio of 1:1:2 for ideal AgSbS2 materials. To evaluate the size dependence of the optical properties, UV/Vis-NIR absorption spectra of AgSbS2 NC films with different sizes were investigated. All of the spectra were normalized at 1200 nm for clarity, as shown in Figure 4 a. It can be seen that all of the NCs, regardless of their sizes, have a strong absorption in the whole visible to near-infrared range. The onsets
of the optical absorption spectra were gradually blue shifted upon decreasing the particle size of the AgSbS2 NCs. Tauc plots in the form of (ahn)n (n = 2 for direct band-gap semiconductors, n = 1/2 for indirect band-gap semiconductors) were performed to analyze the optical band gaps of the AgSbS2 NCs (displayed in Figure 4 b). The most linear spectra were observed for n = 2, indicating that the present AgSbS2 NCs exhibit direct band gaps. Figure 4 b further shows the estimated band gaps of 1.57, 1.47, 1.34, and 1.32 eV for the AgSbS2 NCs with average diameters of 5.3, 10.0, 20.5, and 31.8 nm, respectively, agreeing reasonably well with the band gap of cubic AgSbS2 bulk materials at 1.23–1.44 eV.[25, 27] The optical property of the semiconductor NCs can be influenced by the surface properties and the quantum size effect. Considering the fact that the present NCs were synthesized under similar conditions, the surface properties of the NCs should exert similar influence on their optical absorption. In contrast, the particle sizes of these AgSbS2 NCs change significantly. The variation in the band gaps of the present AgSbS2 NCs could hence be attributed to the quantum size quantum effect. These results demonstrate that the AgSbS2 NCs with an average size of 5.3 and 10.0 nm are small enough to show the quantum size effect.
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Size-control strategy The size-control colloidal synthesis of phase pure, nearly monodisperse AgSbS2 NCs is achievable by suitable balancing the
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Full Paper control factors, in particular the reagent choice, the nucleation, and the growth temperatures. To successfully develop a synthetic strategy to size-tunable AgSbS2 NCs, initial exploration was made by testing a variety of ligands and precursors. We found that the combination of amines and carboxylic acids is a good choice for balancing the reactivity of the precursors. AgNO3 can be dissolved quite well in OLA, but not in OA. On the contrary, SbCl3 has very low affinity to OLA, although it can be well dissolved in OA. These phenomena can be qualitatively understood in terms of the hard–soft acid–base theory: as an acid, the monovalent Ag + ion is softer than the trivalent Sb3 + ion, whereas OLA is softer than OA. Thus, OLA binds strongly to the softer Ag + ion, whereas the OA binds stronger to the harder Sb3 + ion. The combination of OLA and OA was found to be critical to achieve monodisperse AgSbS2 NCs. The absence of either OLA or OA results in large precipitates and impurities due to the poor solubility of the corresponding cationic precursors (Figures 5 a and i). Furthermore, we found that the volume ratio of OLA to OA also played an important role in manipulating the reactivity of the cationic precursors. When using excess OLA (OLA/OA volume ratios 4:4), highly uniform AgSbS2 NCs were obtained. In contrast, only large polydisperse NCs or large precipitates were obtained if excess of OA was used. The average size of the AgSbS2 NCs can be tuned from 13.2 to 20.5 nm by varying the OLA/OA volume ratios in the range of 4:4 to 7.25:0.25. The corresponding powder XRD patterns confirmed that all of the samples are pure AgSbS2, except for sample i), which contains Sb2S3 impurities (Figure S2 in the Supporting Information). These results indicate that a fine balance of the reactivity of two cationic precursors can be achieved with the combination of OLA and OA. To gain insight into the formation process of the AgSbS2 NCs, the NC products at different reaction stages were moni-
Figure 5. TEM images of AgSbS2 NCs prepared with varying OLA/OA volume ratios: a) 8:0, b) 7.75:0.25, c) 7:1, d) 6:2, e) 5:3, f) 4:4, g) 3:5, h) 2:6, and i) 0:8. Scale bars = 200 nm. Chem. Eur. J. 2015, 21, 11143 – 11151
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Figure 6. TEM images of AgSbS2 NCs prepared with varying growth time: a) 1.5, b) 2.5, and c) 5 min and varying temperature: d) 180, e) 200, or f) 220 8C. Scale bars = 100 nm.
tored by XRD (Figure S3 in the Supporting Information) and TEM (Figures 6 a–c). The XRD results reveal that the cubic AgSbS2 was formed within the first 1.5 min. The TEM images are employed to record the shape evolution process. At first, tiny irregular rod-shaped NCs were formed at 1.5 min, and evolved into spherical polydisperse NCs within 2.5 min. When the reaction proceeds to 5 min, the NCs become larger and uniform. These observations are in good accordance with the growth mechanism of spherical Au NCs proposed by Peng and co-workers.[28] The formation of spherical NCs includes three steps: nucleation, random attachment, and intra-particle ripening. Irregular rod-shaped NCs were formed at the beginning due to the rate difference between the nucleation and the random attachment process. These irregular rod-shaped NCs turned to uniform spherical NCs through a slow intra-particle ripening process to minimize the total surface energy of the NCs. The size tunability of the AgSbS2 NCs can be further achieved by adjusting the reaction temperatures. Figures 6 d–f show that the sizes of the AgSbS2 NCs can be finely tailored from approximately 5.3 nm (up) to around 13.2 nm by increasing the reaction temperature from 180 to 220 8C. Further reducing the reaction temperature cannot afford for the growth of the AgSbS2 NCs in this study. Similar scenario where a higher reaction temperature results in larger nanoparticles have been often observed in colloidal NC synthesis.[29] It is known that increasing the reaction temperature could promote the growth rate at the nucleation stage, and the preformed nuclei would grow very fast at a high temperature, resulting in a rapid reduction in the supersaturation at the nucleation stage. In such instance, the resultant suppression in the nucleation rate would increase the overall size of the NCs. Uniform large-sized AgSbS2 NCs can be prepared by using Sb[S2CN(C2H5)2]3 (Sb(dedtc)3) as Sb precursor in the reaction. Sb(dedtc)3 could not only act as an antimony precursor but also act as a sulfur precursor. Thus, we have attempted to prepare AgSbS2 NCs by using a mixture of Sb(dedtc)3 and OLA– AgNO3, without adding another sulfur precursor. However, only large irregular-shaped crystals were obtained, as shown in
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Full Paper Figure S4 in the Supporting Information. The corresponding XRD results reveal that the products are mixtures of AgSbS2 and Ag3SbS3. By introducing the product of element sulfur dissolved in OLA (OLA-S) into the reaction mixture, the NCs obtained exhibit improved uniformity. However, the Ag3SbS3 impurities can still be observed in the XRD patterns. We hence introduced the highly reactive sulfur precursor TMS into the reaction mixture, resulting in highly uniform AgSbS2 NCs without any impurities. It is interesting to note that the combination of TMS and OLA-S has also resulted in pure AgSbS2 NCs and that the size of the AgSbS2 NCs can be tuned from 20.0 to 47.8 nm by increasing the dosage of OLA-S from 0 to 2 mmol (Figure 7 a–c), whereas the TMS dosage was fixed at 0.5 mmol. Moreover, the size of the monodisperse AgSbS2 NCs can be further enlarged from 47.8 up to about 58.3 nm by increasing the growth time from 5 to 12 min (Figures 7 d and e). Therefore, we have realized the tailoring of the AgSbS2 NCs in an exceptional size ranged from 5.3 to 58.3 nm.
Figure 7. TEM images of products prepared by using Sb(dedtc)3 as antimony precursor with varying OLAS dosage: a) 0, b) 0.5, or c) 2 mmol, and varying reaction time: d) 1.5, e) 5, or f) 12 min. Scale bars = 200 nm.
scale up in a single reaction is pivotal for further applications. TMS is an ideal choice as sulfur source in colloidal nanocrystal synthesis due to its high reactivity. However, the high cost, the high volatility, and the toxicity of TMS makes it unfavorable as a sulfur source for a large-scale synthesis. In view of cost and operative convenience, the product of element sulfur dissolved in OLA (OLA-S) is an excellent choice as a sulfur source for up-scaling synthesis of the AgSbS2 NCs. Therefore, we have conducted the experiment to replace TMS with OLA-S and using SbCl3 as Sb precursor in the synthesis of AgSbS2 NCs. It was found that the initial mole ratio of AgNO3/OLA-S strongly influenced the phase and morphology of the resulting NCs. For mole ratios within the range 1:2 to 1:4, pure cubic AgSbS2 NCs with high uniformity are formed, as shown in Figures 8 a and b and Figure S7 in the Supporting Information. A Large excess of sulfur precursor, however, results in the formation of plate-shaped impurities (Figures 8 c and d and Figure S7 in the Supporting Information). To achieve large-scale synthesis, it is necessary to carefully tune the overall concentration (designated by the concentration of the “AgSbS2 molecule”) of the reaction mixtures. We first fixed the Ag/Sb/S molar ratio at 1:1:2 to adjust the overall concentration of the reaction solution, and found that the average size of the resulting AgSbS2 NCs decreased as the overall concentration increased within the concentration ranges between 0.031 and 0.141 mmol mL¢1 (Figures 8 e–g). By increasing the overall concentration to as high as 0.217 mmol mL¢1, the reaction system became very viscous, and the magnetic stir bar almost stops stirring. In such case, mixtures of nanorods and nanoparticles were obtained owing to the poor diffusion during the nucleation and growth stage (Figure 8 h). Along the above lines, we conducted the up-scaling synthesis of AgSbS2 NCs with an AgNO3/OLAS ratio of 1:2 and an overall concentration of 0.122 mmol mL¢1. In a typical synthesis, 55 mmol of AgNO3, 55 mmol of SbCl3, 110 mmol of S, 110 mL of OA, and 340 mL of OLA were involved in the reac-
Large-scale synthesis Most of the colloidal syntheses are usually developed with low outputs, typically ranging from several tens to several hundreds of milligrams for one reaction, which is insufficient for most practical applications.[30] In many cases, several or dozens of batches have to be conducted to provide a sufficient amount of material, which is a laborious task. Moreover, possible variations in the chemical composition or the size of the NCs obtained from different batches may also become an unsolvable problem.[31] Thus, the ability to Chem. Eur. J. 2015, 21, 11143 – 11151
Figure 8. TEM images of AgSbS2 NCs prepared with varying AgNO3/OLA-S precursor ratios: a) 1:2, b) 1:4, c) 1:8, or d) 1:16, and a varying overall concentration (designated by the concentration of the “AgSbS2 molecule”): e) 0.031, f) 0.076, g) 0.122, or h) 0.217 mmol mL¢1. Scale bars = 200 nm.
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Figure 9. Photographs of a) the reaction flaks and b,c) the obtained AgSbS2 NCs powder. d) TEM image and e) powder XRD pattern of the scale-up synthesized AgSbS2 NCs.
tion, resulting in as much as 15.59 g (the yield was 96 %) of AgSbS2 NCs in a single batch synthesis (Figures 9 a–c). The obtained AgSbS2 NCs displayed spherical morphologies with an average diameter of 14 nm (Figure 9 d), and the powder XRD pattern (Figure 9 e) of the AgSbS2 NCs matched well with the cubic AgSbS2, confirming the purity of the large-scale synthesized AgSbS2 NCs. Air-stability and photoelectrochemical properties One of the advantages of semiconductor colloidal NCs is their ability in fabricating the active layers of optoelectronic devices through low-cost solution processes, such as spray-coating,
spin coating, or roll-to-roll printing.[6] Thus, air stability is a desired property although certainly not a requirement. The air stability of the AgSbS2 NCs was tested by XRD characterization. No difference in the XRD patterns can be detected (Figure 10 a) between the justly synthesized sample and the sample, which has been stored under ambient condition for eight months, indicating the superior stability of the AgSbS2 NCs under ambient condition. To test the photoresponse of the AgSbS2 NCs, thin films of the 20 nm AgSbS2 NCs were fabricated on Mo-coated sodalime glass substrates by spraying AgSbS2 NC inks onto the hot substrates. The thicknesses of the obtained films were around 600 nm. The detailed experimental conditions are given in the Experimental Section. CdS layers with a thickness of about 60 nm were deposited on the surface of the as-prepared AgSbS2 NC films by chemical bath deposition (CBD). Photoelectrochemical (PEC) measurements were performed by using a typical three-electrode configuration. The as-prepared NC electrodes, a Pt plate, and the saturated calomel electrode (SCE) were used as the working electrode, the counter electrode, and the reference electrode, respectively. All PEC measurements were conducted under AM 1.5 G simulated sunlight (100 mW cm¢2) by using a 0.5 m Na2HPO4 solution (pH adjusted to 7.3 by NaOH addition) as the electrolyte. The potentials in each measurement were converted into the values against the reversible hydrogen electrode (RHE) by the Nernst equation (ERHE = ESCE+0.059 pH+ +0.245). Figure 10 b shows the current–voltage curves under pulsed AM 1.5 G (100 mW cm¢2) illumination for bare AgSbS2 NC films (black solid line) and CdS-modified AgSbS2 NC films (gray solid line). The bare AgSbS2 NC film generates a cathodic photocur-
Figure 10. a) Powder XRD patterns of freshly prepared AgSbS2 NCs and the same sample after eight month storage in air. b) Current–potential curves for bare and CdS-modified AgSbS2 NC electrodes (RHE = reversible hydrogen electrode). c) Current–time curves for bare and CdS-modified AgSbS2 NC electrodes at an applied potential of 0 VRHE, with a 20 s ON/OFF illumination cycle. Chem. Eur. J. 2015, 21, 11143 – 11151
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Full Paper rent under illumination and exhibits p-type conductivity. The photocurrent density and the onset potential are 60 mA cm¢2 at 0 VRHE and + 0.5 VRHE, respectively. For the CdS-modified AgSbS2 NC film, the photocurrent density was increased significantly to 300 mA cm¢2 at 0 VRHE, which is five times of magnitude of that for the bare AgSbS2 NC film. Moreover, the onset potential is also slightly increased to a value of + 0.57 VRHE. This could be ascribed to the formation of p–n junctions between AgSbS2 and CdS, leading to band-bending at the solid–electrolyte interface and broadening of the depletion layer thickness, which is favorable for the charge separation. Similar results have been reported on the deposition of n-type CdS onto p-type Cu(In,Ga)(S,Se)2.[32] Photocorrosion of the electrodes is one of the major challenges for PEC hydrogen production. Ag-based materials and other potential photocathodes (e.g., Si, Cu2O) usually suffer from this problem.[33] The degradation could be suppressed by appropriate surface modification, such as TiO2 and CdS layers.[34] To examine the stability of the bare AgSbS2 NC electrode and the CdS-modified AgSbS2 NC electrode, the time curves for the AgSbS2 NC electrode and the CdS-modified AgSbS2 NC electrode were measured at an applied potential of 0 VRHE, with a 20 s ON/OFF illumination cycle for over one hour (Figure 10 c). Furthermore, a consecutive illumination stability test for more than 2 h were also conducted (Figure S8 in the Supporting Information). All measurements were performed under AM 1.5 G (100 mW cm¢2) illumination, in a phosphate buffer (pH 7.3) to avoid any change in pH. The bare AgSbS2 NC electrode shows a rapid decay of the photocurrent during the stability test, whereas the CdS-modified AgSbS2 NC electrode shows very significant improvements in both the photocurrent density and the stability. These preliminary results on the photoresponse and stability tests of the CdS-modified AgSbS2 NC films are very encouraging and are competitive with the photoelectrochemical performances of other established NC materials.[11f,h–j, 26d, 35]
Conclusion In summary, the first synthesis of monodisperse and wellcrystallized AgSbS2 NCs has been achieved through colloidal chemistry. By combination of careful control over the OLA/OA ratio and the reaction temperature, high quality AgSbS2 NCs with sizes between 5.3 to 20 nm were obtained by using SbCl3 and AgNO3 as metal source. The sizes of the AgSbS2 NCs can be further enlarged from 20.5 to 58.3 nm by using Sb(dedtc)3 as Sb precursor with an appropriate dosage of OLA-S and growth time. The as-obtained AgSbS2 NCs show strong absorption in the visible-to-NIR range and high air stability under ambient conditions. Additionally, the large-scale synthesis of AgSbS2 NCs with 15 g of the product in one single reaction has been realized by using element sulfur as precursor and optimizing the concentration and volume of the reaction mixture. Finally, photoelectrochemical measurements show the significantly improved stability of the CdS-modified AgSbS2 NC film with competitive photocurrent density. These results indicate Chem. Eur. J. 2015, 21, 11143 – 11151
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the promise of applying the obtained NCs in photoelectronics or photocatalysis.
Experimental Section Materials: Silver nitrate (AgNO3, 99.9 %), antimony(III) chloride (SbCl3, 99.999 %), bis(trimethylsilyl)sulfide (TMS, 98 %), 1-dodecylamine (1-DDA, 98 %), octanoic acid (OTA, 98 %), and 1-octadecene (ODE, 90 %) were purchased from Alfa Aesar. Antimony(III) acetate (SbAc3, 99.99 %), sulfur powder (S, 99.99 %), oleylamine (OLA, 70 %), oleic acid (OA, 90 %), and 1-dodecanethiol (1-DDT, 98 + %) were purchased from Sigma Aldrich. Sodium dihydrogen phosphate dehydrate (NaH2PO4·2 H2O, 98 %), sodium hydroxide (NaOH, 96 %), toluene (99.5 %), hexane (97 %), and ethanol (99.7 %) were obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium diethyldithiocarbamate trihydrate (Na(dedtc)·3 H2O, 99 %) was delivered from Aladdin. All chemicals were used directly without any further purification except for OLA and OA, which were degassed for 5 h at 110 8C. Synthesis of Sb(dedtc)3 : SbAc3 (20 g, 67 mmol) in anhydrous ethanol (200 mL) and Na(dedtc)·3 H2O (50 g, 222 mmol) in anhydrous ethanol (500 mL) were mixed together with stirring, resulting in a yellow suspension. After stirring for two hour, the resulting yellow precipitate was filtered and washed with anhydrous ethanol for more than three times. The obtained filter cake was dried under vacuum at 60 8C, light-yellow powders were obtained. AgNO3, SbCl3, and S stock solutions: The stock solution of OLAAgNO3 (0.1 m) and OLA-S (1 m) were prepared by dissolving AgNO3 (1.699 g, 10 mmol) and S (1.924 g, 60 mmol) in OLA (100 and 60 mL, respectively). The 1 m OA-SbCl3 stock solution was prepared by dissolving SbCl3 (2.281 g, 10 mmol) in OA (10 mL). All of these stock solutions were prepared under stirring on a 50 8C hotplate for more than 8 h in a nitrogen-filled glovebox. Synthesis of small-sized AgSbS2 NCs (5.3–20.5 nm): In a typical synthesis, a 0.1 m OLA-AgNO3 solution (2.5 mL), a 1 m OA-SbCl3 solution (250 mL), and TMS (107 mL, 0.5 mmol) were loaded sequentially into a reaction tube inside a nitrogen-filled glovebox. To adjust the overall OLA/OA ratio, specific amounts of OLA and OA were added into the reaction tube. The total volume of the reaction mixture was fixed at 8 mL. The reaction tube was inserted into a preheated oil bath at 220 8C under a nitrogen atmosphere, and the reaction was allowed to proceed for 5 min with continuous stirring. Then, the reaction tube was rapidly cooled down to room temperature with a cold water bath. Afterward, the as-prepared NCs were precipitated with anhydrous ethanol by centrifugation. The isolated NCs were then re-dispersed in hexane and re-precipitated with anhydrous ethanol. The purification process was carried out twice and the purified NCs were re-dispersed in toluene for further characterization. The size of the AgSbS2 NCs can be tuned by adjusting the OLA/OA ratio from 7.25:0.25 to 4:4. When the ratio of OLA/OA was fixed at 4:4, the size of the AgSbS2 NCs can also be tuned by varying the reaction temperature from 180 to 220 8C. Synthesis of large-sized AgSbS2 NCs (23.9–58.3 nm): In a typical synthesis, Sb(dedtc)3 (0.1417 g, 0.25 mmol), a 0.1 m OLA-AgNO3 solution (2.5 mL), TMS (107 mL, 0.5 mmol), a 1 m OLA-S solution (2 mL), and OLA (3.5 mL) were loaded into a reaction tube inside a nitrogen-filled glovebox. The mixture was then inserted into an oil bath preheated at 220 8C under a nitrogen atmosphere and maintained at this temperature for 5 min with continuous stirring. Afterward, the colloidal solution was rapidly cooled to room temperature with a water bath and purified twice adopting the
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Full Paper same procedure described before. The size of the AgSbS2 NCs can be tuned by varying the amount of OLA-S from 0 to 2 mL and by adjusting the growth time from 5 to 12 min. Synthesis of AgSbS2 NCs by using OLA-S as a sulfur source: Typically, a 0.1 m OLA-AgNO3 solution (2.5 mL), a 1 m OA-SbCl3 solution (250 mL), a 1 m OLA-S solution (500 mL), and OLA (4.75 mL) were loaded into a reaction tube inside a nitrogen-filled glovebox. The mixture was then inserted into an oil bath preheated at 220 8C under a nitrogen atmosphere and maintained at this temperature for 5 min with continuous stirring. Subsequently, the colloidal solution was rapidly cooled to room temperature with a water bath and purified twice adopting the same procedure described before. To achieve the large-scale synthesis of the AgSbS2 NCs shown in the next part, the reaction parameters were optimized by varying the Ag/S mole ratio from 1:2 to 1:16 and the overall concentration of the reaction mixture from 0.031 to 0.217 mmol mL¢1. Large-scale synthesis of AgSbS2 NCs: A 0.2 m OLA-AgNO3 solution (275 mL), a 1 m OA-SbCl3 solution (55 mL), a 1 m OLA-S solution (110 mL). and OLA (10 mL) were loaded into a large flask with a volume of 1 L. The flask was then inserted into a preheated oil bath at 220 8C, and the reaction was allowed to proceed for 12 min with continuous stirring. Afterward, the as-prepared NCs were precipitated with anhydrous ethanol by centrifugation. The isolated NCs were then re-dispersed in hexane and re-precipitated with anhydrous ethanol. The purification process was carried out twice and the purified NCs were dried under vacuum at 60 8C for 8 h. Characterization: The low-resolution TEM images were obtained on a Hitachi HT 7700 microscope, operating at 120 kV. High-resolution TEM images were taken on a FEI TECNAI F30 S-Twin microscope with an accelerating voltage of 300 kV. SEM images and element analysis were conducted on a FEI Quanta 200F scanning electron microscope equipped with an energy dispersive spectroscopy (EDS) detector. X-ray diffraction (XRD) analyses were performed on a Rigaku RINT D/Max-2500 powder diffraction system by using CuKa radiation source (l = 1.54 æ) operating at 40 kV and 200 mA. UV/Vis-NIR absorption spectra were measured by using a Cary 5000 spectrophotometer. The PEC test was conducted in a three electrode system with a potentiostat (Iviumstat, Ivium Technologies) under simulated AM 1.5 G solar-light irradiation (100 mW cm¢2, Newport Sol 3A, Class AAA Solar simulator). Fabrication of electrodes from AgSbS2 NC inks: The purified AgSbS2 NCs were dispersed in toluene with a concentration of 0.1 mmol mL¢1. Mo-coated soda-lime glasses were heated to 120 8C on a hotplate. Then AgSbS2 NC inks were spraying onto the Mocoated soda-lime glasses, resulting in uniform AgSbS2 NC films with a thickness of approximately 600 nm. Surface modification of the AgSbS2 NC films with CdS layers was performed by chemical bath deposition (CBD) by using a variation on a literature method.[32a] Prior to CdS deposition, the AgSbS2 NC films were immersed in a solution containing 7.5 mm CdAc2 and 2 m NH4OH for 10 min at 65 8C. Afterward, CBD of CdS layers was carried out by immersing the pre-treated AgSbS2 NC films in an aqueous solution containing 7.5 mm CdAc2, 0.375 m SC(NH2)2, and 2 m NH4OH at 65 8C for 5.5 min, resulting in the deposition of a CdS layer with a thickness of approximately 60 nm. After the deposition, the electrodes were rinsed with distilled water, followed by annealing in air at 200 8C for 40 min. Photoelectrochemical measurements: PEC measurements were conducted by using a normal three-electrode system, by using a prepared electrode, a Pt plate, and a saturated calomel electrode (SCE) as working electrode, counter electrode, and reference electrode, respectively. All of the PEC measurements were conducted under AM 1.5 G simulated sunlight (100 mW cm¢2) by using a 0.5 m Chem. Eur. J. 2015, 21, 11143 – 11151
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Na2HPO4 solution (pH adjusted to 7.3 by NaOH addition) as the electrolyte. The use of phosphate buffer solution as the electrolyte can avoid pH changes during the measurement. The potentials in each measurement were converted into values against the reversible hydrogen electrode (RHE) by the Nernst equation (ERHE = ESCE+0.059 pH+ +0.245). The photocurrent was measured by linear sweep voltammetry with a scan rate of 2 mV s¢1.
Acknowledgements We gratefully acknowledge financial support from the “Hundred Talents Program” of the Chinese Academy of Sciences, the National Science Foundation of China (nos. 20873141). Keywords: nanoparticles · photoelectrochemistry semiconductors · silver antimony sulfide
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Received: March 13, 2015 Published online on June 19, 2015
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