Mesoporous Magnesium Oxide Nanosheet Electrocatalysts for the

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Mesoporous Magnesium Oxide Nanosheet Electrocatalysts for the Detection of Lead(II) Sen Liu,† Ziying Wang,† Tianyi Han,† Teng Fei,†,‡ and Tong Zhang*,† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China ‡ State Key Laboratory of Transducer Technology, Shanghai 200050, P. R. China

ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 31.40.210.78 on 05/16/19. For personal use only.

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ABSTRACT: Ultrathin two-dimensional (2D) magnesium oxide nanosheets (MgO-NSs) with mesoporous structure (Brunauer−Emmett−Teller surface area of 158 m2·g−1 and a pore size of 2−3 nm) are proposed as excellent electrocatalysts for the detection of lead(II). A sugar-blowinginduced confined synthesis method was developed to synthesize MgO-NSs by one-step calcination of a Mg(NO3)2−glucose mixture. Impressively, MgOs-NSs display excellent sensing performances for the electrochemical detection of lead(II) with a detection limit of 0.16 nM and a linear range of 20−150 nM (R2 = 0.996). This work sheds light on the preparation of ultrathin 2D mesoporous nanosheets for sensing applications. KEYWORDS: magnesium oxide, ultrathin, mesoporous, electrocatalysts, lead(II)

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oxide nanosheets (MgO-NSs). As illustrated in Figure 1, Mg(NO3)2 and glucose were dissolved in water to form a

ltrathin two-dimensional (2D) nanomaterials have attracted considerable research attention because of their unique physical/chemical properties and promising applications for electrocatalysis, photocatalysis, energy storage, electronic devices, and sensors.1−5 A series of ultrathin 2D nanomaterials, including graphene, black phosphorus, metal oxides, metal carbides, and metal chalcogenides, have been fabricated by fluid exfoliation,6 hydrothermal synthesis,7 a wetchemical method,8 and chemical vapor deposition.9 However, the close-packed basal planes of these ultrathin 2D nanomaterials remarkably hamper their practical applications. Recently, the engineering of mesopores in ultrathin 2D nanosheets has been proven as an effective strategy to alleviate strong aggregation, which has also emerged as the inevitable route to achieving nanomaterials with surface-enriched active sites.10 Meanwhile, the introduction of mesopores into ultrathin 2D nanosheets would provide high specific surface areas and interpenetrated channels. To date, various ultrathin 2D nanosheets with mesopores or micropores have been prepared by well-established strategies, such as intralayered Ostwald ripening,11 supermolecular assembly,12 graphene oxide templating,13,14 chemical exfoliation,15 a wet-chemical method,16 and surfactant-assisted synthesis.17 However, these strategies suffer from the shortcomings of rigorous and complicated experimental conditions, low product yield, and the need for special layered porous materials as precursors. Therefore, it is highly desirable to develop a facile and highly effective approach to preparing ultrathin 2D nanosheets possessing mesoporous structure with high yield. Herein, a sugar-blowing-induced confined synthesis method was proposed to prepare ultrathin 2D mesoporous magnesium © XXXX American Chemical Society

Figure 1. Synthesis and formation process for ultrathin 2D mesoporous MgO-NSs.

transparent solution. The subsequent evaporation process (100 °C, 12 h) yields a gel-like black solid. A further pyrolysis process by the direct calcination of a Mg(NO3)2−glucose mixture in an air atmosphere (550 °C, 3 h) resulted in ultrathin 2D mesoporous MgO-NSs. Although sugar-blowing methods were reported for the preparation of carbon-based materials,18,19 such method is seldom used to synthesize ultrathin 2D inorganic materials. Unlike the previous methods using NH4Cl as the blowing agent,20,21 the nitrates in inorganic salts were used as blowing agents directly. Using nitrates Received: April 2, 2019 Accepted: May 14, 2019 Published: May 14, 2019 A

DOI: 10.1021/acsanm.9b00600 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

attributed to the special structure of the polymer bubbles formed by molten syrup comprised of nanosheets connected to each other. Upon dispersion of MgO-NSs into water, a homogeneously dispersed suspension displaying the Tyndall effect was observed (inset of Figure 2b). The corresponding transmission electron microscopy (TEM) image further confirms the formation of nanosheets (Figure 2b). The highmagnification TEM image of MgO-NSs (Figure 2c) reveals that the nanosheets contain numerous pores, suggesting the formation of porous structure in MgO-NSs. The highresolution TEM (HR-TEM) image demonstrates the formation of a crystalline nature of MgO-NSs, and the interplanar distance of such crystals was measured to be 0.21 nm, corresponding to the d spacing of the (200) plane of MgO.22 Obvious pores of about 2−3 nm according to the aggregation of MgONPs were also observed, indicating that the formation of mesoporous structure stemmed from the accumulation of MgONPs. As shown in Figures 2c and S2a, the edges of the nanosheets overlapped each other, where the overlapped region may be comprised of double layers of nanosheets. From the HR-TEM image of the nanosheet overlap, the thickness of the single nanosheets was estimated to be 2.0−2.5 nm (Figure S2b). The selected-area electron diffraction (SAED) of MgONSs reveals only five obvious diffraction cycles (Figure S3). Compared to the SAED image for the MgO crystals with high crystallinity, several diffraction cycles are missing,23 further confirming the formation of MgO-based materials with structure defects or impurity. Additionally, MgO-MP samples exhibit the typical morphology of microscale plates (Figure S4). The powder X-ray diffraction (XRD) patterns indicate that both MgO-NSs and MgO-MPs show five diffraction peaks at 36.9°, 42.8°, 62.1°, 74.7°, and 78.8°, associated with the (111), (200), (220), (311), and (222) planes of MgO (JCPDS 450946; Figures 3a and S5).24 The decreasing intensity of the diffraction peaks and the widening full width at half-maximum (fwhm) for MgO-NSs suggest the formation of ultrathin or mesoporous structures. Additionally, some other weak diffraction peaks besides MgO crystals were observed on the MgO-NS samples. After calcination at 550 °C in air again, these additional diffraction peaks disappeared, indicating the formation of carbon-based impurities in MgO-NSs (Figure S6). The N2 sorption isotherm of MgO-NSs shows the type IV isotherm with an H2-type hysteresis loop in the relative pressure region (P/P0) of 0.2−0.8 (Figure 3b), indicating the formation of mesoporous structure. No obvious N2 uptake was observed for MgO-NSs at low relative pressure, suggesting the absence of microporous structure. 25 To examine the importance of glucose for the formation of mesoporous structure, the MgO-MPs were also investigated by N2 sorption, and the corresponding N2 sorption isotherm and pore-size distribution curve are shown in Figure S7. The Brunauer− Emmett−Teller surface area and pore volume of the MgO-NSs were 158 m2·g−1 and 0.25 cm3·g−1, which were larger than those of the MgO-MPs (7 m2·g−1 and 0.03 cm3·g−1) (Table S1). The pore-size distribution curve of the MgO-NSs (Figure 3c) measured by application of the Barrett−Joyner−Halenda (BJH) method from the adsorption branch of the isotherm clearly displays a centered pore size at about 2−3 nm. It should be point out that the mesoporous structure was formed in the basal planes of MgO-NSs by the aggregation of MgO-NPs, which is different from the porous structure of conventional

instead of NH4Cl as the blowing agent not only leads to a decrease in cost and the release of a low content of pollutant gases but also avoids the formation of nitrogen- or chlorinecontaining impurities. The possible mechanism for this sugarblowing confined synthesis method for the preparation of MgO-NSs was proposed as follows: In the heating process, a molten syrup was gradually polymerized into diverse sugarderived polymers (mainly melanoidin and its analogues). Then, the decomposition of nitrates leads to the release of gases, blowing molten syrup into bubbles. In the meantime, magnesium(II) was confined in the walls of the polymer bubbles. Upon further heating, the walls of the polymer bubbles were thinned by the bubble volume dilatation forced by released gases. While the mixture was heated at 550 °C in air, most of the sugar-derived polymers were removed and MgO crystals formed. Crystallization of MgO was carried out in the confined region between the carbon-based templates, leading to the formation of ultrathin nanosheets. By careful tuning of the ratio of glucose to magnesium(II), a mesoporous structure comprised of MgO nanoparticles was also achieved at the same time. An aerogel-like white solid was obtained after calcination of a glucose−Mg(NO3)2 mixture (Figure S1a), and the volume of the products was several times larger than that of a glucose− Mg(NO3)2 mixture (Figure S1b), black powder before calcination (Figure S1c), and magnesium oxide (MgO-MP) samples by calcination of Mg(NO3)2 in the absence of glucose (Figure S1d). Additionally, the MgO-NSs possess superlight properties and easily float into an air environment by a slight breath or minor action, which is similar to the commercial fumed silica. Just about 0.196 g of MgO-NSs could completely fill a 150 mL quartz breaker (Figure S1a). In contrast, MgOMPs with the same weight cannot cover the bottom of the quartz breaker (Figure S1d). These observations could be attributed to the unique properties of MgO-NSs, including the ultrathin 2D and mesoporous structures. The scanning electron microscopy (SEM) image of MgONSs (Figure 2a) reveals that the samples consist of uniform nanosheets, which were grown on the several-microsized plate. This structure is the typical morphology of three-dimensional (3D) carbon-based materials prepared by the sugar-blowing method.18−21 The formation of such structures could be

Figure 2. (a) SEM, (b and c) TEM, and (d) HR-TEM images of MgO-NSs. The inset of part b is the homogeneously dispersed suspension of MgO-NSs displaying the Tyndall effect, and the inset of part d is the HR-TEM image of the typical MgO crystals in MgO-NSs. B

DOI: 10.1021/acsanm.9b00600 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 3. (a) XRD pattern, (b) N2 sorption isotherm, (c) BJH pore-size distribution curve, (d) XPS spectra, and (e) Mg 2p, (f) O 1s, and (g) C 1s NMR spectra of MgO-NSs.

Figure 4. (a) SWASV curves of a bare GCE and MgO-NSs/GCE and MgO-NPs/GCE in 0.1 M NaAc-HAc (pH 5.0) containing 20 nM lead(II). (b) SWASV curves of MgO-NSs/GCE at different concentrations of lead(II) of 10, 20, 40, 60, and 150 nM. (c) Calibration curve of MgO-NPs/ GCE toward lead(II). (d) Electrochemical responses for MgO-NSs/GCE to different metal ions including lead(II), cadmium(II), chromium(III), mercury(II), and zinc(II).

carbon element in the final samples could be deconvoluted into two peaks at 284.5 and 285.6 eV (Figure 3g), associated with the C−C and C−O bonds in a carbon-based impurity derived from the residual glucose.26 The XPS results reveal that the content of the carbon element in MgO-NSs is about 12.28 atom %. The advantage of the present strategy is the high product yield, with nearly 100% magnesium(II) converted into MgO crystals, which is much higher than that of most

2D nanomaterials obtained by the assembly of various nanosheets. X-ray photoelectron spectroscopy (XPS) results confirm that MgO-NSs are composed of the magnesium, oxygen, and carbon elements (Figure 3d). The Mg−O band in the Mg 2p spectrum was observed at 49.76 eV (Figure 3e), and the O− Mg band in the O 1s spectrum was observed at 529.81 eV (Figure 3f), indicating the formation of MgO materials. The C

DOI: 10.1021/acsanm.9b00600 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Typical SWASV responses of MgO-NSs/GCE for lead(II) concentrations from 20 to 150 nM are shown in Figure 4b. Well-defined stripping peaks attributed to the oxidation of lead(0) were observed at about −0.50 V, and the intensity of the stripping peaks gradually enhanced with increasing lead(II) concentrations. The corresponding calibration curve (Figure 4c) displays the linear range of a lead(II) sensor from 20 to 150 nM (R2 = 0.996). The detection limit of the present lead(II) sensor is estimated to be 0.16 nM. The selectivity of the lead(II) sensor was examined in the presence of common heavy-metal-ion interferences, including cadmium(II), chromium(III), mercury(II), and zinc(II). Figure 4d shows the electrochemical responses of the lead(II) sensors toward interfering ions several-fold more concentrated than lead(II). Although the concentration of the interfering metal ions (20 μM) is 1000-fold more concentrated compared to lead(II) (20 nM), the responses of all of these interfering ions are about 0.79−0.97, which is lower than that of 20 nM lead(II) (1.2). By an increase in lead(II) to 150 nM, the difference in the responses between the lead(II) and interfering ions becomes more obvious, indicating good selectivity of the present MgONSs-based lead(II) sensors. The reproducibility was estimated from the response to 20 nM lead(II) at five MgO-NSs/GCEs prepared under the same conditions, and a relative standard deviation (RSD) of 5.8% was acquired. To determine the longterm stability of the sensor system, the MgO-NSs/GCE was stored in air under ambient conditions when not in use. It retained 91% of its initial current response to lead(II) after 2 weeks of storage. The repeatability of the MgO-NSs-based lead(II) sensor over eight measurements was 4.6%, calculated from the RSD. Additionally, the present MgO-NSs-based lead(II) sensor could be used for the detection of lead(II) in a real water sample (see the details in Figure S15 and Note 6). The figures of merit for the MgO-NSs-based lead(II) sensors are shown with aspects including (a) a low detection limit of 0.16 nM, (b) an analytical sensitivity of 7.7 μA·μM−1, (c) a linear detection range from 20 to 150 nM, (d) a reproducibility with a RSD of 5.8% (for n = 5), (e) a long-time stabiliy of 91% after 2 weeks of storage, and (f) a repeatability with a RSD of 4.6% (for n = 8). According to the results of the SAED patterns, the impurities and structural defects are formed MgO-NS materials, which are beneficial for enhanced sensing performance. The structure defects provide more active sites for the adsorption of lead(II) during the adsorption and preconcentration processes. The formation of carbon-based impurities could increase the electron-transfer rate during the sensing process. As shown in Table S2, the MgO-NSs-based lead(II) sensors thus fabricated exhibit obvious advantages compared with the previously reported electrochemcial lead(II) sensors (see the details in Note 7). By a comparison of with the previous MgO-based materials, the present work exhibits obvious novelty and advances (see the details in Note 8). In summary, uniform and ultrathin 2D MgO-NSs with mesoporous structure were prepared through a sugar-blowinginduced confined synthesis method. The MgO-NSs-based lead(II) sensors exhibited a wide linear range, a low detection limit, and good selectivity. We hope this synthetic procedure will facilitate the fabrication of other ultrathin 2D mesoporous nanomaterials, so that the potential applications of ultrathin 2D mesoporous nanomaterials can be explored in more areas.

previously reported strategies for ultrathin 2D nanomaterials. The photograph of the MgO-NSs prepared by calcinations of 2.50 g of Mg(NO3)2·6H2O and 3.60 g of glucose·H2O further indicates the preparation of MgO-NSs on a large scale (Figure S8). The effect of the glucose amount on the structure of MgONSs was further investigated. The SEM images of MgO-NSs0.9 (prepared in the presence of 0.90 g of glucose) reveal 3D porous structure composed of microscale slices (Figure S9a,b). However, MgO-NSs-0.9 exhibits low uptake of N2 with increasing relative pressure and wide pore-size distribution (Figure S10a,b). By further increasing the glucose amount to 2.70 and 3.60 g, both MgO-NSs-2.7 and MgO-NSs-3.6 exhibit 3D porous structure consisting of microscale plates and poor mesoporous structure (Figures S9c−f and S10a,b). As shown in Table S1, MgO-NSs prepared in the presence of 1.80 g of glucose·H2O exhibit larger specific surface area and mesoporous pore volume than those of other MgO-NSs samples and MgO-MPs. The formation of mesoporous structure in MgO-NSs is attributed to the fact that crytallization of MgONPs was confined by sugar-derived polymers, and the poor mesoprous structure was obtained with low (0.9 g) or high content (2.7 and 3.6 g) of glucose (see the details in Note 1). Furthermore, the XPS results suggest that no obvious differences in the surface structure and chemical valence states of the magnesium and oxygen elements were observed for MgO-based samples beyond MgO-NSs (see the details in Figure S11 and Note 2). Motivated by the intriguing ultrathin 2D and mesoporous structures of MgO-NSs, we therefore investigated their electrocatalytic performances by choosing the detection of lead(II) as a model reaction. Figure 4a displays the typical square-wave anodic stripping voltammetry (SWASV) curves of a bare glassy carbon electrode (GCE) and MgO-NS- and MgO-MP-modified GCE (MgO-NSs/GCE and MgO-MPs/ GCE) in a 0.1 M NaAc-HAc (pH 5.0) solution containing 20 nM lead(II). No obvious peak was obtained for 20 nM lead(II) at a bare GCE after closed-circuit accumulation for 900 s (black line). Notably, a strong stripping peak at −0.50 V was observed for MgO-NSs/GCE attributed to the oxidation of lead(0) to lead(II),27,28 indicating good electrocatalytic activity for the detection of lead(II). Although MgO-MPs/GCE also exhibits a stripping peak associated with the oxidation of lead(0) at −0.50 V (blue line), the intensity is lower than that of MgO-NSs/GCE, suggesting that the unique properties of the ultrathin 2D and mesoporous structures are essential for improvement of the electrocatalytic activity. The oxidation potential for lead(0) to lead(II) for MgO-NSs is observed at −0.50 V, which is higher than that of graphene-modified carbon nanosheets (−0.60 V).27 This phenomenon may be attributed to the different adsorption sites in MgO-NSs (oxygen-containing bonds) and graphene-modified carbon nanosheets (carbon-containing bonds). Additionally, the pH value for lead(II) sensing in this work (5.0) is also slightly lower that in that work (5.5). By tuning of the pH values, this sensor exhibits the biggest oxidation current at the pH value of 5.0, as shown (see the details in Figure S12 and Note 3). Additionally, the overall sensing mechanism for the detection of lead(II) by MgO-NSs is also proposed (see the details in Note 4). As shown in Figure S13, among all of the MgO-NSs samples, MgO-NSs-1.8 exhibits the best sensing performance (see the details in Note 5). D

DOI: 10.1021/acsanm.9b00600 ACS Appl. Nano Mater. 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/acsanm.9b00600.

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Experimental section, photographs of the various solid samples, TEM, SAED, and SEM images, XRD patterns, N2 sorption isotherm, BJH pore-size distribution curves, NMR spectra, SWASV curves, parameters of the MgObased materials prepared in the present work, and a comparison of the sensing performances (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sen Liu: 0000-0001-7880-3223 Teng Fei: 0000-0002-6571-8270 Tong Zhang: 0000-0002-2690-859X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially supported by National Natural Science Foundation of China (Grant 61671218). REFERENCES

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