Inflating Strategy To Form Ultrathin Hollow MnO2 Nanoballoons Juanjuan Shang,† Beibei Xie,† Ya Li,† Xin Wei,† Na Du,† Haiping Li,‡ Wanguo Hou,† and Renjie Zhang*,†,‡,§ †
Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education of the P. R. China, ‡National Engineering Technology Research Center for Colloidal Materials, and §Key Laboratory of Special Functional Aggregated Materials of the Ministry of Education of the P. R. China, Shandong University, Jinan 250199, P. R. China S Supporting Information *
ABSTRACT: Ultrathin MnO 2 hollow nanoballoons (UMHNBs) have a large ratio of interfacial to total atoms, corresponding to expected improved performance. However, their synthesis is a challenge due to difficulty in controlling the concentration of the unit cells. Herein, we describe a strategy to synthesize dry intact UMHNBs through a one-step synthesis by inflating MnO2 (reduced from KMnO4) with CO2 (oxidized from single-layer graphene oxide nanosheets) followed by instant freezedrying. UMHNBs are 30−500 nm in diameter with a shell thickness of 3.7 nm, packing with laminar [MnO6] unit cells in the form of δ-MnO2. UMHNBs show efficient catalytic activity for decomposing the organic dye methylene blue (MB), 15 times the biggest reported value, and have long-term catalytic efficacy and durability. The described strategy in this paper makes use of graphene nanosheets to assemble durable ultrathin hollow nanoballoons. KEYWORDS: manganese oxide, self-assembly, catalytic activity, organic dye, decomposition been used to “leaven” compact graphene structures to porous foams.21 KMnO4 was originally used by Hummers to oxidize and tear graphite flakes into GO.22 CO2 produced during the oxidation of GO should be capable of inflating the reduced product of KMnO4, MnO2, to UMHNBs. In this work, we confirm the formation of UMHNBs via a one-step in situ redox reaction between GO and KMnO4. UMHNBs with large ratios of interfacial to total unit cells show efficient catalytic performance. Besides the schematic formation of UMHNBs and their efficient catalysis for decomposition of organic molecules, we also address the essential drying process to obtain dry intact inflated UMHNBs, which avoids collapse of the ultrathin shell induced by capillary force.
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nO2 has been widely used in catalysis, supercapacitors, water treatment, and molecular sieves owing to its unique physicochemical properties.1−4 The performance of MnO2 can be regulated by controlling its morphology (shape and size).5 Thus far, one-dimensional (1D) nanorods, nanobelts, nanotubes,6−8 2D nanosheets,9 3D hollow spheres, and urchins10 have been reported. Regarding catalytic performance, MnO2 hollow microspheres of 1 μm in diameter and 0.1 μm in shell thickness possess better catalytic activity than MnO2 nanowires.11 Ultrathin hollow nanospheres of gold or silica nanospheres with shell thicknesses of several nanometers are revealed to possess improved properties.12,13 If ultrathin MnO2 hollow nanoballoons (UMHNBs) of tens to hundreds of nanometers in diameter and several nanometers in shell thickness can be synthesized, catalytic performance should be improved further, considering the larger ratio of interfacial to total unit cells. Until now, the diameter of reported MnO2 hollow spheres was in the range of several hundreds of nanometers to several micrometers. For example, the shell thickness of MnO2 hollow spheres with a diameter of 4−6, 3−4, 2−3, and 1 μm was 1.5,14 0.5,15 0.216 and 0.3 μm,17 respectively. Using a template method, the shell thickness of MnO2 hollow spheres with a diameter of 5, 3, 0.5, and 0.3 μm was 0.1,3 1,18 0.1 μm,19 and 70 nm,20 respectively. Gaseous species such as H2O and CO2 produced during the reduction of graphene oxide (GO) to reduced GO (rGO) have © 2016 American Chemical Society
RESULTS AND DISCUSSION Morphology and Structure of UMHNBs. The MnO2 from KMnO4 (reduced by GO) appears as hollow nanoballoons as observed by TEM and HRTEM (Figure 1a and Supporting Information, Figure S1a,b) when the MnO2 suspensions in water are instantly frozen followed by freezedrying. As revealed by TEM, SEM, and AFM results, the diameter of the nanoballoons is in the 30−500 nm region Received: February 17, 2016 Accepted: May 17, 2016 Published: May 17, 2016 5916
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Figure 1. Morphology and diameter of MnO2 dried by instant freezing and by air: (a) TEM image; (b) SEM image; (c) AFM height image and section analysis of UMHNBs prepared from KMnO4 (reduced by GO) in a one-step synthesis (UMHNBs on silicon for SEM and AFM and on a copper grid for TEM were instantly frozen by liquid nitrogen followed by freeze-drying); (d) TEM image; (e) SEM image; (f) AFM height image and section analysis of air-dried UMHNBs by air.
Figure 2. Spectroscopic characteristics of UMHNBs: (a) XRD pattern, (b) Mn 2p, (c) O 1s XPS, (d) UV−vis, (e) FTIR, and (f) Raman spectra of UMHNBs.
(Figure 1a−c) due to lateral size difference of GO nanosheets. The elemental mappings (Supporting Information, Figure S1d−f) show that Mn and O elements are coexistent and homogeneously distributed in nanoballoons. UMHNBs have good structural stability, which can keep the morphology of nanoballoons in aqueous solution for more than one year. Hollow nanoballoons with ultrathin shells tend to flatten after air-drying, similar to extensively observed hollow microcapsules.23,24 The air-dried UMHNBs collapse, as observed by TEM (Figure 1d) and SEM (Figure 1e), with an average shell thickness of 3.7 nm as analyzed by AFM (Figure 1f). Two reasons that UMHNBs flatten to nanosheets are as follows: (1) the capillary force from water pulls the top part shell to the bottom shell adhered to the solid surface during evaporation of water between UMHNBs; (2) the shell is too thin to prevent the morphology change. The instantly frozen water exists as ice. During the freeze-drying process, sublimated water does not produce capillary force; consequently, MnO2 exists as inflated hollow nanoballoons. In other words, UMHNBs instantly frozen by liquid nitrogen followed by freeze-drying keep the same original hollow morphology as that in solution. This might be the main reason why others have not reported MnO2 hollow nanoballoons using the same synthesis method; another reason might be that the size of GO nanosheets they used is big, on the micrometer scale.25 The instant freezing technique should be universal for allowing a dry, hollow structure with an ultrathin shell to retain its original morphology. δ-MnO2 exists in UMHNBs, as confirmed by the sharp XRD peak at 2θ = 12.3° (Figure 2a) corresponding to a layer spacing of 0.72 nm (similar to the reported value of 0.73 nm25) between the (001) lattice layers of δ-MnO2. Combining the layer spacing of 0.72 nm and the shell thickness of 3.7 nm, the shell of UMHNBs is calculated to be formed by five layers of [MnO6] unit cells. The mean thickness of δ-MnO2 microcrystals in UMHNBs in the direction perpendicular to (001) is consistent (ca. 5.3 nm), which is calculated by using Scherrer’s expression,
0.89λ /β cos θ
where λ is the wavelength of the X-ray, β is the full width at half-maximum (fwhm, radian), and θ is the diffraction angle (radian). UMHNBs show XPS peaks of Mn 2p1/2 at 654.5 eV and Mn 2p3/2 at 642.4 eV (Figure 2b and Supporting Information, Figure S2), corresponding to 12.1 eV spininduced energy separation for Mn4+2p electrons.26 The sharp O 1s peak at 530.1 eV (Figure 2c and Supporting Information, Figure S2) corresponds to the oxygen in the [MnO6] unit cell. The O 1s shoulder peak at about 533.0 eV is from the O−H groups including the Mn−O−H.5 The GO nanosheets we used are 30−500 nm wide (Supporting Information, Figure S3a,b) and 0.9 nm thick (Supporting Information, Figure S3c), corresponding to a single layer of carbon lattice.27 The size distribution of GO is consistent with that of UMHNBs. UMHNBs consist of MnO2 with a neglected GO residue, as indicated by the absence of (i) the characteristic UV absorption peak of GO at 226 nm (Supporting Information, Figure S4a), (ii) the typical stretching vibration bands of GO for CO at 1728 cm−1 and for CC at 1621 cm−1 (Supporting Information, Figure S4b) in the FTIR spectra, and (iii) the G band at 1600 cm−1 and the D band at 1354 cm−1 (Supporting Information, Figure S4c) of GO in the Raman spectra of UMHNBs. The zero point of charges of δ-MnO2 is 2.3,28 so the O−H groups binding on UMHNBs at pH 6.4−6.7 are deprotonated, making UMHNBs negatively charged29−31 (which is related to electrostatic adsorption of oppositely charged organic dyes to be discussed later), corresponding to the measured ζ potential value of −33.7 mV. Other basic spectroscopic characteristics of UMHNBs are as follows. UMHNBs show a broad UV−vis absorption peak in the 250−500 nm region (Figure 2d). The maximum absorption is at 366 nm, corresponding to the d−d transition of Mn4+ cations in the [MnO6] octahedra unit cells.32 The vibration absorption peaks of Mn−O bonds of UMHNBs 5917
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a A UMHNB packing with five layers of [MnO6] unit cells, formed from reduced MnO4− anions accompanying with the oxidation of a GO nanosheet to CO2. To make MnO4− anions, CC bonds, and [MnO6] unit cells legible, not all but only a few reaction sites are illustrated in steps a−d; the consumed parts of GO are not removed.
appear at 645, 574, 510, and 449 cm−1 (Figure 2e).33 The stretching and bending vibration absorption peaks of the O−H groups binding on UMHNBs34 appear at 3495 and 1624 cm−1, respectively.35 UMHNBs show the characteristic Raman shifts of [MnO6] octahedra at 498, 570, and 647 cm−1, and that of the Mn−O−H at 1210 cm−1 (Figure 2f).36,37 UMHNBs have high ratio of interfacial to total [MnO6] unit cells, which is equal to the number of interfacial [MnO6] unit cells divided by the number of total [MnO6] unit cells. Although it cannot be directly measured, it can be understood concerning specific surface area, first evaluated on the basis of the AFM, TEM, and XRD results. Taking a UMHNB of 70 nm in radius and 3.7 nm in shell thickness as example, the specific surface area of UMHNB is calculated to be 137 m2/g, which is inversely proportional to radius. The measurement of BET specific surface area (228.2 m2/g) (Supporting Information, Figure S5a) by N2 adsorption/desorption is consistent with and larger than the calculated result due to possible penetration and adsorption in the inner wall of UMHNBs. The BET specific surface area of UMHNBs is also larger than those of reported MnO2 nanosheets (157 m2/g)25 and MnO2 hollow nanospheres (167 m2/g).19 The corresponding BJH average pore size of UMHNBs is 115 nm (Supporting Information, Figure S5b). The high ratio of interfacial to total [MnO6] unit cells should lead to expected improved catalytic efficiency, since it makes more unit cells of UMHNBs exposed to surrounding molecules favorable for highly efficient utilization of catalyst. Formation Mechanism of UMHNBs. The formation mechanism of UMHNBs is worthy of discussion. The reaction equation can be expressed by
with the reduction of KMnO4, the carbon atoms of GO between the two δ-MnO2 nanosheets are oxidized to CO2, which allows the nanosheets to curl. UMHNBs are then formed driven by minimization of surface energy. Extra [MnO6] unit cells can diffuse and grow on the hollow nanoballoons to form an ultrathin shell of five-layer [MnO6] unit cells in the form of δ-MnO2. In other words, the internal CO2 from GO inflates two layers of adjacent MnO2 nanosheets (originally separated by a GO nanosheet) to hollow nanoballoons, similar to the “leavening” effect observed by Niu and co-workers, who used gaseous species such as H2O and CO2 to inflate compact graphene layers to porous foams.21 The shell thickness stops increasing after the carbon atoms in GO are nearly fully oxidized, since the concentration of [MnO6] unit cells reduced from MnO4− anions does not change. The diameter of UMHNBs is dependent on the lateral size of GO, so the size distribution of UMHNBs is consistent with that of GO. Taking a GO nanosheet 100 nm in width as example, it has carbon atoms of approximately 105, producing 107 nm3 of CO2 after total oxidation, much larger than the volume of a UMHNB of 70 nm in diameter, 106 nm3, which needs only about 104 carbon atoms in order to be oxidized to CO2 for inflation. Finally, H2O infiltrates and absorbs dissolved CO2. A brief formation mechanism of UMHNBs is illustrated in Scheme 1. Catalytic Performance of UMHNBs. UMHNBs synthesized in our one-step process are hollow and intact. Although MnO2 hollow microspheres have been previously observed, UMHNBs we obtained in this paper are much smaller in diameter and much thinner in shell. Therefore, they should possess superior catalytic performance, which can be evaluated through degradation degree of an organic dye methylene blue (MB) oxidized by H2O2 catalyzed by UMHNBs. H2O2 and UMHNBs play the roles of oxidant and catalyst, respectively. The pH of the (MB + UMHNBs + H2O2) solution was ca. 6.4−6.7 during the catalytic reaction. MB degrades very quickly over time (Figure 3a,b), with a degradation degree of up to 96.1% and a weight ratio of degraded MB to UMHNBs of 7.69 mg/mg at 100 min, which is far more than those reported values, such as 0.11 mg/mg for Au@TiO2,40 0.25 mg/mg for graphene/MnO2 hybrids,41 and 0.5 mg/mg for manganese oxide loaded hollow silica particles,42 confirming the excellent catalytic activity of UMHNBs. The highest degradation efficiency of MB is 96.1% when the H2O2 and UMHNBs coexisted in the catalytic reaction, while
GO + KMnO4 → MnO2 + K+ + CO2
During the redox reaction between GO and KMnO4, CC bonds of GO binding two O atoms of MnO4− anions are first oxidized to vicinal diols and further oxidized to diones accompanying with the breakage of corresponding C−C bonds.38 The MnO4− anions are so reduced to form [MnO6] unit cells of MnO2.25 CC bonds on the planar GO skeleton helps to orient [MnO6] unit cells parallel on the two surfaces of each GO plane in good order. Driven by crystallization energy, [MnO6] unit cells form δ-MnO2 nanosheets above and below the original GO surface.25 It is worth noting that the CO2 from the oxidation of carbon atoms in GO by KMnO425,39 is responsible for the hollow balloon morphology of MnO2. Along 5918
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OH and O2•− radicals, respectively.47,48 The degradation efficiency of MB obviously decreases in the existence of TBA or BQ, and the degradation efficiency of MB with TBA is lower than that with BQ (Supporting Information, Figure S6d). Thus, the •OH radicals are more active than O2•− radicals in degrading MB.
CONCLUSIONS A one-step synthesis strategy of UMHNBs with diameters ranging from 30 to 500 nm with a shell consisting of [MnO6] unit cells in the form of δ-MnO2 is proposed by making good use of KMnO4 and the small size of GO nanosheets as precursors and adopting instant freeze-drying. UMHNBs exhibit a highly catalytic performance to degrade MB in the presence of H2O2, showing a large mass ratio of degraded dye to catalyst, 15 times that of the largest reported value. The high catalytic activity of UMHNBs is attributed to their high ratio of interfacial to total unit cells, corresponding to highly efficient utilization of catalyst. UMHNBs have long-term catalytic efficacy and durability. The strategy in this work is of great importance in controlled assembly of ultrathin hollow nanomaterials and in applications like water treatment for decomposing organic pollutants.
Figure 3. Catalytic performance of UMHNBs in the presence of H2O2 and schematic catalytic degradation of MB molecules: (a) absorption spectra of MB (400.0 mg/L) over time in the presence of UMHNBs and H2O2; (b) degradation of MB over time under different conditions, including controlled experiments of MB + UMHNBs + H2O, MB + H2O2 + H2O; (c) electrostatic adsorption of MB on a UMHNB with large ratio of interfacial to total [MnO6] unit cells and catalytic decomposition of H2O2 by UMHNB to form radicals enable effective degradation of MB.
METHODS Synthesis of UMHNBs. The mixture solution of KMnO4 (5.0 mL, 16.0 mg/mL) and GO (6.0 mL, 1.6 mg/mL) was kept at 60 °C in a water bath for 15 h under continuous stirring and then repeatedly centrifuged (142413 g, 30 min) and washed with water until total removal of the residual KMnO4. Preparation of Dry Intact Inflated UMHNBs by Instant Freeze-Drying. Aqueous suspensions of UMHNBs were instantly frozen by liquid nitrogen followed by freeze−drying to maintain their original morphology as in solution.
the contrast experiments show that the degradation efficiency of MB without UMHNBs or H2O2 is 18.3% or 6.9%, respectively (Supporting Information, Figure S6a,b), indicating that UMHNBs play the main role in the fast degradation of MB. The degradation degree of MB does not show noticeable changes after five cycles when UMHNBs are used as catalyst. The morphology and size of UMHNBs (Supporting Information, Figure S6c) are similar to those of starting ones, indicating that UMHNBs have long-term catalytic efficacy and durability. Catalytic Mechanism of UMHNBs. Since degradation of dye molecules occurs on or near the catalyst surface,43 clearly UMHNBs with a large ratio of interfacial to total [MnO6] unit cells expose more active [MnO6] to the surrounding H2O2 and MB, resulting in efficient catalysis degradation of MB. A brief catalytic mechanism of UMHNBs to degrade MB can be described as follows: first, the negatively charged UMHNBs efficiently accumulate the positively charged MB molecules44 on their surface through electrostatic interaction. At the same time, UMHNBs rapidly catalyze neighboring H2O2 to radicals including •OH and O2•− (or HOO•).45 Thus, the local high concentrated radicals effectively degrade the accumulated MB46 to CO2 and H2O,41 as illustrated in Figure 3c. The catalytic capacity of UMHNBs varies with the concentration of MB. When the concentrations of MB solution are 57.1 and 16.0 mg/L, the corresponding weight ratios of degraded MB to UMHNBs are 3.4 and 6.1 mg/mg, respectively, which can be due to that the occupied absorption sites on UMHNBs by MB decrease the formation of radicals on UMHNBs from H2O2. The HOO• radicals are difficult to be experimentally characterized, which decompose immediately to O2•− radicals and H+. The existence of radicals of •OH and O2•− is confirmed by a control experiment in which tert-butyl alcohol (TBA) and p-benzoquinone (BQ) are scavengers of
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01229. Additional SEM image, TEM image, and AFM height image and section analysis, UV−vis, FTIR, and Raman spectra of GO, elemental mappings, TEM and HRTEM images, SAED, XPS spectrum, N2 adsorption/desorption isotherms, corresponding pore size distribution, contrast catalytic experiments, and cycling stability of UMHNBs (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions
R.Z. conceived the project. J.S. carried out main experimental works, data analysis, and manuscript composition. B.X., Y.L., and X.W. participated in measurements. N.D., H.L., and W.H. participated in discussion. R.Z. supervised each step. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (Nos. 21273135, 21573133, and 21403128), Shandong Provincial Natural Science Foundation, China 5919
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DOI: 10.1021/acsnano.6b01229 ACS Nano 2016, 10, 5916−5921
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DOI: 10.1021/acsnano.6b01229 ACS Nano 2016, 10, 5916−5921
