Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Natural Sugar: A Green Assistance To Efficiently Exfoliate Inorganic Layered Nanomaterials Kai Chen,† Wentao Zhang,† Xinjie Pan,‡ Lunjie Huang,† Jing Wang,† Qingfeng Yang,† Na Hu,§ Yourui Suo,§ Daohong Zhang,† and Jianlong Wang*,† †
College of Food Science and Engineering and ‡College of Chemistry and Pharmacy, Northwest A&F University, Yangling, 712100 Shaanxi, P. R. China § Qinghai Key Laboratory of Qinghai−Tibet Plateau Biological Resources, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, 810008 Qinghai, P. R. China S Supporting Information *
ABSTRACT: We have demonstrated that natural sugars can efficiently exfoliate inorganic layered nanomaterials with direct stirring. The representative transition-metal dichalcogenides (MoS2 and WS2), transition-metal oxide (MoO3), and graphene were explored, and the formation of ultrathin nanosheets was verified. Glucose and MoS2 selected each other as the perfect partner with superior exfoliation and excellent properties. The obtained inorganic layered nanosheets possess favorable stability and dispersity, which renders it suitable for direct homogeneous liquid applications, such as catalytic activities and sensors. With a high-throughput and green process, the sugar-assisted method may offer new ideas for inorganic layered nanomaterials synthesis and applications in a more ecofriendly way.
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INTRODUCTION The study of two-dimensional (2D) nanomaterials such as graphene and transition-metal dichalcogenides (TMDs) has become one of the most attractive areas of materials science because of their potential in a range of applications.1−3 When exfoliated into one or a limited number of layers, 2D materials were demonstrated unique electronic, optical, mechanical, and chemical characteristics, and a surging interest extending into many advanced fields has occurred.4−7 For example, fewlayered MoS2 and WS2 have been demonstrated as efficient photocatalysts for hydrogen evolution reaction and organic compound degradation because of their large surface area and suitable band structure.8,9 Moreover, although strong interlayer van der Waals interactions result in very poor solubility of these semplice-layered materials, it is desirable for the aqueous dispersion of functional MoS2 with low cytotoxicity and abundant modification sites, which renders it suitable for direct homogeneous liquid applications, such as catalytic activities and sensors.10,11 Driven by the increasing demands of ultrathin-layered nanomaterials in several fields, great efforts have been devoted © XXXX American Chemical Society
to developing various methods for preparing layered nanosheets, including intercalation-driven exfoliation, liquid-phase sonication exfoliation, electrochemical exfoliation, and plasma or laser etching.12−17 However, most of the above methodologies are time-consuming, extremely sensitive to environmental conditions, and limited to small-scale synthesis.18,19 Recently, mechanochemistry has attracted much attention in nanomaterials synthesis, which opens up ecologically and economically sustainable routes for the preparation of advanced functional materials through the simple mixing of all reagents.20−23 In order to serve high-quality synthesis, different assisted media were used to exfoliate inorganic layered materials mechanically, including surfactant solutions, ionic liquids, suitable organic solvents, etc.2,24,25 In spite of their successes in nanomaterial exfoliation, the high-cost of these reagents, together with certain toxicity, cancels out the intrinsic superiority of mechanical exfoliation. Moreover, these media are not compatible with all layered materials and require Received: February 27, 2018
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DOI: 10.1021/acs.inorgchem.8b00525 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 1. Facile Exfoliation of Inorganic Layered Nanosheets by Sugar-Assisted Stirring
redispersed in deionized water and centrifuged 10 min at 3000 rpm to remove the bulk materials. Applications. The FL quenching ability was estimated by adding different concentrations of layered MoS2 dispersions (100 μL) to 900 μL of a carbon quantum dots (CQDs) solution (4 ng mL−1). After the solution was allowed to incubate several minutes, FL emission of the solution was performed. Photocatalytic degradation of MB was carried out in a 20 mL vial containing 1 mL of a MB solution (2 × 10−3 mol L−1), 1 mL of a fresh NaOH solution (0.01 mol L−1), and a suitable amount (0.5 mg) of MoS2 nanosheets as the catalyst. The visible light was provided by a 300 W xenon lamp (Perfectlight Technology, Beijing) under illumination of AM 1.5 G (100 mW cm−2) as the irradiation source. The quantity of MB in the reaction solution was determined at λmax = 663 nm using a UV−vis spectrophotometer.
expensive equipment and a harsh match. Consequently, it remains highly challenging to develop an exfoliation method for the universal production of ultrathin 2D nanosheets in an efficient and green way. In this study, great progress in using naturally abundant sugars to assist in the exfoliation of layered nanomaterials was achieved. Native sugars are inexpensive, have wide accessibility and excellent biocompatibility, and may be the ecofriendly reagents to assist the processing of 2D materials exfoliation. The representative TMDs (MoS2 and WS2), transition-metal oxide (TMO) (MoO3), and graphene were explored. In particular, we discovered a unique exfoliation match of glucose and MoS2, which gains superior exfoliation efficiency. In the simplest case, this can be achieved by direct sonication or even mechanical stirring with the assistance of sugars.
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RESULTS AND DISCUSSION Sugars are the most abundant biomolecule in nature and are produced from a wide variety of sources. In their chemical structure, they possess plenty of hydroxy groups, which are expected to offer a powerful ability to exfoliate layered nanomaterials.26 In our experiment, a sugar-assisted stirring method was used to exfoliate inorganic layered materials, as shown in Scheme 1. A normal magnetic stirrer fitted with 30 W was applied for stirring the exfoliation. The input power was so low that decreasing the stirring speed was not enough to support the exfoliation. Thus, we used the maximum rotation rate of 2000 rpm in subsequent researches. The 3 cm magnetic rotor was used to drive the reaction, and a 100 mL beaker was utilized as the reactor. In order to compare the efficiencies of various sugars, different sugars (glucose, fructose, and arabinose) were alternatives for exfoliating MoS2 in the same way, as shown in Figure 1a. We evaluated the efficiencies of different sugars by UV−vis absorption spectroscopy. First, the exfoliation ability of glucose for MoS2 was demonstrated and showed excellent performance. In spite of having a chemical structure different from that of glucose, fructose could also aid in the exfoliation of MoS2 yet with weak exfoliation efficiency, which results from the discriminative ketone group in fructose rather than the aldehyde group in glucose. Meanwhile, we researched the capacity of arabinose, an aldopentose containing five carbon atoms including an aldehyde functional group, by employing the same addition volume. This showed the lowest yielding ability in the three sugars because fewer carbon atoms contribute to a smaller proportion of hydroxy groups in the sugar.27 In fact, monosacchrides will form cyclic structures so that the functional group (an aldehyde or a ketone group), and the number of carbon atoms could lead to the differentmembered heterocyclic skeleton corresponding to the different distribution of hydroxyl in the ring, which may be the most important effect on the exfoliation efficiency. More importantly,
EXPERIMENTAL SECTION
Materials and Methods. MoS2, WS2, MoO3 (bulk particle size WS2 > MoO3 > graphene. With the same glucose concentration (0.55 g mL−1), MoS2 achieved maximum exfoliation, more efficient than other materials. Following this convenient method for cleaving bulk into nanosheets, the sugar-assisted method may offer new ideas for inorganic layered nanomaterials synthesis and application in a more ecofriendly way. To further ascertain the nature of the dispersed material, we performed TEM, with typical images shown in Figure 2a−d. It clearly shows that all four materials possess decent nanoscale morphology and high dispersivity rather than thick sheets, which are in great agreement with what is normally observed in inorganic layered nanomaterials. Furthermore, AFM was performed to measure the thickness of the individual exfoliated nanosheets, and the images are shown in Figure 2e−h. Typically, upon centrifugation at 3000 rpm, the largest portion of nanosheets exhibited a layered structure. From Figure 2i−l, the thicknesses of MoS2, WS2, MoO3, and graphene were measured to be ≈1.0, 1.5, 1.3, and 0.7 nm, respectively, closing to one layer or two layers of nanosheets. The results of MoS2, MoO3, and graphene nanosheets correspond to a one-layer nanosheet.28,29 The thickness of WS2 is consistent with a twolayer nanosheet.30,31 In fact, the thickness of pure single-layered MoS2 nanosheets is 0.9 nm, and it is 0.34 nm for graphene. According to our results, a small increase in the thickness compared to that of the pure nanosheets can be observed as a result of surface functionalization with glucose.32 On the basis of these results, it can be seen that the nanosheets are reasonably thin with the assistance of glucose in moderate
Figure 1. (a) Absorptions at ≈679 nm of exfoliated MoS2 assisted by glucose, fructose, and arabinose. Inset: Photographs of MoS2 exfoliated by honey and syrup in food grade. (b) SEM images of exfoliated MoS2, WS2, MoO3, and graphene. (c) Stirring-exfoliated dispersions of (left to right) MoS2, WS2, MoO3, and graphene.
with all of these representative sugars, the obtained MoS2 presented homogeneous dispersion and thinner nanosheets. Moreover, the glucose syrup and native honey were utilized to exfoliate bulk MoS2 in shearing and are comparable to pure sugars (Figure 1a, inset). In view of the superior performance of glucose, in further studies glucose was selected to assist layered materials exfoliation. After stirring of bulk MoS2 in a glucose solution followed by centrifugation to remove redundant glucose and unexfoliated MoS2, a homogeneous and dark-green dispersion was obtained. The uniform morphologies of the layered MoS2 were recorded by SEM (Figure 1b). In spite of having different binding and surface energies from MoS2, other 2D materials (e.g., graphene,
Figure 2. TEM images of exfoliated (a) MoS2, (b) WS2, (c) MoO3, and (d) graphene. AFM images of exfoliated (e) MoS2, (f) WS2, (g) MoO3, and (h) graphene and (i−l) corresponding height profiles across the nanosheets. C
DOI: 10.1021/acs.inorgchem.8b00525 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. (a) UV−vis spectrum of MoS2 nanosheets. Inset: photograph of MoS2 dispersion in water. (b) Absorption of exfoliated MoS2 stirred at different stirring times and glucose concentrations.
Figure 4. (a) Raman spectra of natural MoS2 and MoS2 nanosheets. (b) HRTEM image of MoS2 nanosheets. Inset: HRTEM image showing the lattices of MoS2. (c) FT-IR spectra for glucose, MoS2. and MoS2 nanosheets. (d) TGA−mass loss curves of glucose, MoS2, and MoS2 nanosheets.
getting access to stirring and sonication, the maximal glucose concentration should be limited to 0.7 g mL−1. Compared to other methods with limited ingredients, a large amount of glucose ensures abundant contact and, therefore, more sufficient interaction between MoS2 and glucose. At the same time, in the case of a low-energy input with stirring, such a superior stripping result further illustrates the excellent ability of glucose in facilitating a layered MoS2 fabrication process. These advantages distinguish the present methodology from previously reported methods, and this technique is demonstrated to be efficient and green for the production of MoS2 nanosheets with advanced structural properties, avoiding the use of expensive solvents and special equipment. More characterizations were conducted to further affirm the thin-layer structure of the MoS2 nanosheets. Raman spectroscopy was applied to distinguish between the bulk MoS2 and exfoliated MoS2 nanosheets, and the spectra are shown in Figure 4a. The difference between E12g and A1g can be considered to be an indicator of the thickness of the sheets. The Raman peaks of MoS2 shifted when the bulk material was exfoliated to the MoS2 nanosheets. Compared to the bulk MoS2, a red shift of the peak of E12g to 378.8 cm−1 could be observed and A1g shows a small blue shift, which leads to a decrease in the position difference (Δ) from 26.9 to 25.2 cm−1
stirring. With the certitude of formation of ultrathin nanosheets, it further demonstrated the advancement of our method in terms of green and facile synthesis of inorganic layered nanomaterials. In this case, glucose was selected as the perfect partner for the efficient exfoliation of MoS2. Thus, more detailed characterizations were performed to research the property of MoS2 assisted by glucose. UV−vis absorption spectroscopy was employed to evaluate the exfoliated MoS2. From Figure 3a, characteristic absorption bands of MoS2 located at 679, 619, 472, and 406 nm are observed, which are consistent with thin MoS2 nanosheets acquired by a liquid method.33 In contrast to the negligible absorption of bulk MoS2, the progressively pronounced absorption peak at ≈679 nm may be an indicator. Different stirring times of MoS2, ranging from 0 to 48 h, were collected to evaluate the exfoliated degree under the same handling process. Along with the time growth, MoS2 was exfoliated with increasing yield. In addition to the stirring time, the exfoliation efficiency also depends on the initial glucose concentration (Figure 3b). We find that it is effective for improving the exfoliation efficiency through an increase in the glucose concentration. When the concentration of glucose increased to 0.55 g mL−1, it achieved a maximum absorption. What is more, to prevent the liquid and flexible states from D
DOI: 10.1021/acs.inorgchem.8b00525 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. (a) CQDs FL quenching by different contents of MoS2 nanosheets. (b) UV−vis spectra of the photodegradation of MB with different time intervals using MoS2 nanosheets and bulk MoS2 as catalysts. Inset: photographs showing the visually recognizable change of the FL quenching and photocatalysis reaction system, accordingly.
between E12g and A1g.34 This result corresponds well with previous reports, thus demonstrating that thinner layers of exfoliated MoS2 can be obtained from the bulk by this stirring method, which was expected for highly dispersed and homogeneous systems. Moreover, HRTEM further demonstrates the ultrathin sheets of exfoliated MoS2 (Figure 4b). The minimum MoS2 layer in region 1 reaches the one-layer nanosheets, and the thicknesses in regions 2 and 3 correspond to two-layer and multilayer lamellae, respectively. Importantly, the nanosheets are thin enough to bend, which means there is the potential to make a roll or a tube. In addition, the inset of Figure 4b shows that the nanosheet has clear lattice fringes with d = 0.27 nm, which matches the (100) plane of MoS2. In order to confirm the presence and amount of glucose on the surface of MoS2 nanosheets, FT-IR and TGA were performed. FT-IR spectra of MoS 2, glucose, and MoS2 nanosheets were measured in the range from 4000 to 400 cm−1. As shown in Figure 4c, MoS2 nanosheets displayed absorption very similar to that of native glucose. The peaks at 1410 and 2940 cm−1 result from −CH2 and −CH bending, respectively, while a group of overlapping peaks between 1180 and 953 cm−1 is attributed to the stretching vibrations of C−C and C−O and the bending of C−H.35 The results manifest the coexistence of glucose and MoS2, suggesting the production of a glucose/MoS2 composite. To further expound the combination, the thermal behavior of a MoS2 nanosheets, bulk MoS2, and pure glucose composite was analyzed using TGA at a rate of 10 °C min−1 in a nitrogen atmosphere. As shown in Figure 4d, native MoS2 exhibits negligible weight loss in the temperature range of 25−600 °C. For pure glucose, two weight losses are observed in the TGA curve. The first weight loss stage before 100 °C is due to evaporation of the water and moisture content, while the second weight loss stage, which occurs over 220 °C, can be assigned to the degradation of glucose molecules. Upon combination with glucose, the TGA curve of the MoS2 composite shows an obvious weight loss over 220 °C, which is consistent with that of pure glucose.36 The TGA results further confirmed the interaction between glucose and MoS2 and demonstrated that this high content of glucose endows MoS2 nanosheets with high water dispersion and abundant active sites. The colloidal dispersion showed negligible UV−vis absorbance loss and was stable in water for 20 days, indicating favorable stability and dispersity in water (Figure S1). The concentration of MoS2 nanosheets was determined to be 0.479 mg mL−1 with stirring, implying that this method is a large-scale and high-yield process. Following this convenient method for cleaving the bulk into nanosheets, the above materials were
readily exfoliated as well by sonication in a glucose suspension, which further proved the universality of sugars in 2D materials exfoliation. The inorganic layered nanosheets synthesized by our method possess favorable stability and dispersity, which renders them suitable for direct homogeneous liquid systems. What is more, with their large intrinsic large surface areas, layered nanomaterials show prior promise for the construction of biosensors. Few-layered MoS2, WS2, MoO3, and graphene have been demonstrated as efficient photocatalysts for hydrogen evolution reaction and organic compound degradation because of their suitable band gaps. As a verification of the superior properties of our exfoliated MoS2, confirmatory experiments were conducted to illuminate its potential in practical applications. As a normal quencher, MoS2 could efficiently quench FL as a result of the FL resonance energy transfer.37 With the addition of different concentrations of MoS2 nanosheet suspensions, the FL intensity of CQDs underwent a declining process.38,39 As shown in Figure 5a, the eventual quenching efficiency by asprepared MoS2 nanosheets achieved 93% when the ultimate concentration of MoS2 was 45 μg mL−1. Besides the quenching behavior, the photoresponsive application of MoS2 nanosheets was further demonstrated. The photocatalytic performance of MoS2 nanosheets in degrading an organic dye such as MB was demonstrated under irradiation of sunlight for 15 min.40,41 The blue color of MB rapidly faded under irradiation of sunlight for 15 min (Figure 5b). In comparison, MoS2 nanosheets efficiently bleached MB, while nonexfoliated MoS2 only partially bleached MB under sunlight. The excellent performances of the obtained MoS2 nanosheets in FL quenching and photocatalysis further confirm our expectations that MoS2 nanosheets possess excellent properties and are alternatives for multiple applications.
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CONCLUSION In summary, we demonstrate the potential that sugars can efficiently assist in the exfoliation of inorganic layered nanomaterials with direct stirring. Detailed characterization through SEM, TEM, and AFM confirmed the efficient exfoliation and formation of ultrathin MoS2, WS2, MoO3, and graphene nanosheets. Importantly, glucose and MoS2 selected each other as the perfect partner with superior exfoliation. Further characterization by Raman, HRTEM, FT-IR, and TGA verifies the combination of glucose and MoS2 and their excellent properties in photocatalysis and fluoresence quenching. As a high-throughput and soft process, a sugar-assisted method may offer a new direction for other 2D materials synthesis and application. E
DOI: 10.1021/acs.inorgchem.8b00525 Inorg. Chem. XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00525. Stability of exfoliated MoS2, SEM images of MoS2 nanosheets exfoliated by other sugars, and a comparison of different exfoliation methods (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jianlong Wang: 0000-0002-2879-9489 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 21675127) and Development Project of Qinghai Key Laboratory (Project 2017-ZJY10).
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DOI: 10.1021/acs.inorgchem.8b00525 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b00525 Inorg. Chem. XXXX, XXX, XXX−XXX
