Synthesis of Hierarchical Hollow MnO2 Microspheres and Potential

(3) Therefore, it is still desirable to develop facile and effective synthesis routes for self-assembling ... To the best of our knowledge, this kind ...
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Synthesis of Hierarchical Hollow MnO2 Microspheres and Potential Application in Abatement of VOCs Dongyan Li,‡,† Xiaofeng Wu,† and Yunfa Chen†,* †

State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Hierarchical hollow MnO2 microspheres have been synthesized by a facile hydrothermal method based on the decomposition of KMnO4 precursor in nitric acid solution in the presence of Ce3+ ions. The hierarchical hollow microspheres consisted of discuslike nanoplatelets and nanorods. The Brunauer−Emmet−Teller (BET) specific surface area and the pore volume of the hierarchical hollow microspheres are 29.2 m2 g−1 and 0.30 cm3 g−1, respectively. A possible formation mechanism of hierarchical hollow microspheres is proposed. Ce3+ ions play a crucial role in controlling the morphology and crystalline structure of MnO2. The concentration of Ce3+ ions is a key factor for the formation of the hierarchical hollow microspheres. The as-prepared hierarchical hollow MnO2 microspheres exhibit high catalytic ability for the oxidation of benzene.

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supercapacitor.22 Until now, hierarchical hollow structures of MnO2 consisting of 1D nanorods,23 1D nanotubes,24,25 or 2D nanosheets26,27 have been obtained. However, almost all of the reported hierarchical hollow structures of MnO2 are composed of only one kind of morphological or dimensional nanostructure. Hence, it is significant to develop a simple method for the fabrication of the hierarchical hollow MnO2 consisting of different morphological or dimensional nanocrystals for the extension of its properties and potential applications. Recently, researchers reported that some univalent ions (K+, NH4+, and H+) can influence the crystal structure of MnO2,28 bivalent ions (Cu2+) can influence the morphology of the product,27 and tervalent ions (Fe3+, Al3+) have a great effect on the morphology and crystalline structure of MnO2 in different reaction systems.24,29 These results showed that the morphology or the crystalline of MnO2 can be controlled by adjusting the category or valence of ions. Herein, we report a simple hydrothermal approach for preparing hierarchical hollow MnO2 microspheres consisting of nanorods and discuslike nanoplatelets with the facilitation of Ce3+ ions. The concentration of Ce3+ ions is the determining factor for the formation of the hierarchical hollow structure. To the best of our knowledge, this kind of hierarchical hollow MnO2 microspheres consisting of 1D nanorods and 2D discuslike nanoplatelets has not been reported previously. Moreover, the as-prepared hierarchical hollow MnO2 microspheres exhibit much higher catalytic ability

INTRODUCTION In recent years, hollow structures have received significant attention because of their higher specific surface area, lower density, larger cavity volume, better permeation, and widespread applications in catalysis, drug deliveries, cells, sensors, contaminated waste treatment, and so on.1−9 Among different kinds of hollow structures, hierarchical hollow structures are drawing much intense interest because hollow structures with different dimensional shells not only exhibit their inherent properties of hollow structure but also, more importantly, possess new physicochemical properties and wide potential applications endowed by their unique shell architectures.10 To date, numerous efforts have been made to design and synthesize the hierarchical hollow structures with different morphology in shells. Series of hierarchical hollow inorganic compounds assembled by nanoparticles,11,12 nanobubbles,13 nanorods,14−16 nanotubes,17 nanobelts, or nanosheets18,19 have been synthesized by template or template-free methods. The conventional template method often requires removal of the template after synthesis, which may damage the structural integrity of the final product. Compared with the template method, the template-free method based on self-assembly is facile and relatively environment-friendly.3 Therefore, it is still desirable to develop facile and effective synthesis routes for selfassembling micro- or nanostructures into hierarchical hollow structures with desired chemical composition and controlled morphology. As a type of important functional inorganic material, manganese oxide has attracted great interest due to its distinctive physical and chemical properties, which lead to its application in catalysis,20,21 adsorption,8 and electrochemical © XXXX American Chemical Society

Received: December 26, 2012 Revised: April 10, 2013

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than that of commercial MnO2 in the catalytic combustion of benzene.

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EXPERIMENTAL SECTION Synthesis of Hierarchical Hollow MnO2 Microspheres. All chemical reagents were analytical grade and used as-received without further purification. The hierarchical hollow MnO2 microspheres were obtained by a facile hydrothermal method. In a typical procedure, 0.006 mol of KMnO4 was dissolved in 70 mL of H2O, followed by addition of 4 mL of concentrated HNO3 (16 mol/L) and 0.0017 mol of Ce(NO3)3·6H2O under magnetic stirring. The mixture was transferred into a Teflonlined autoclave (100 mL) and maintained at 140 °C for 3 h. After the autoclave was cooled down to room temperature, the precipitates were collected by centrifugation, washed with distilled water and ethanol for several times, and then dried at 80 °C for overnight. Characterization. The crystal structure was recorded on Xray diffraction (XRD) (Philips X′Pert PRO MPD) with Cu Kα as the radiation source. The morphology was observed by fieldemission scanning electron microscopy (FESEM, JSM-6700F) and transmission electron microscopy (TEM, JEOL JEM-2010) equipped with an energy dispersive X-ray spectroscopy (EDS) analysis system for determination of the chemical composition of the samples. High-angle annular dark-field scanning transmission electron microscopy (HAADF−STEM) was performed in Tecnai G2 F20 U-TWIN. X-ray photoelectron spectroscopy (XPS) was obtained with ESCALab220i-XL. The contents of Ce in all samples were obtained by inductively coupled plasma (ICP, VISTA-MPX). The nitrogen adsorption−desorption isotherm was analyzed in an automatic adsorption system (Autosorb-1, Quantachrome) at 77 K. The catalytic oxidation of benzene was performed in a quartz tubular fixed bed reactor (internal diameter (i.d.) = 7 mm) at atmospheric pressure, with the catalyst (40−60 mesh) loading of 0.1 g between two quartz wool plugs. The catalysts were obtained by calcinating the samples at 573 K for 2 h in static air. The reaction mixture consisting of air/benzene (AGA, 1000 ppm of benzene) flowed at a rate of 100 mL/min (weight hourly space velocity (WHSV), 60 000 mL g−1 h−1). The reactions were carried out in the range 373−623 K with intervals of 20 K. The products of the catalytic reaction were analyzed by an online gas chromatography (Agilent 6890) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID).

Figure 1. XRD patterns of the as-synthesized hierarchical hollow MnO2 microspheres with addition of 0.0017 mol of Ce3+ ions in the reaction system.

Figure 2. EDS spectrum of the as-synthesized hierarchical hollow MnO2 microspheres with addition of 0.0017 mol of Ce3+ ions in the reaction system.

STEM were carried out. Cerium 3d photoemission spectra and HAADF−STEM images of the hierarchical hollow MnO2 microspheres are shown in Figure S1 of the Supporting Information and Figure 3, respectively. The values of binding energy at 903.5 and 885.8 eV are assigned to Ce4+ and the values of binding energy at 898.2 and 881.8 eV are attributed to Ce3+ (Figure S1 of the Supporting Information).30 The results indicate that both Ce3+ and Ce4+ oxidation states coexist in the sample. The presence of Ce4+ is due to the partial oxidation of

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RESULTS AND DISCUSSION The XRD pattern of as-synthesized hierarchical hollow MnO2 microspheres is shown in Figure 1. The major diffraction peaks in the XRD pattern can be indexed to the orthorhombic γMnO2 with lattice contents of a = 6.3600 Å and c = 4.0900 Å (JCPDS Card No. 14-0644). Figure 2 shows the EDS spectrum of as-obtained MnO2 microspheres, in which the signals for Mn and O with the atomic ratio of approximate 1:2 are obviously observed indicating that the chemical composition of the product is MnO2. Cu and C come from the carbon-coated copper grids, which are used as the support for the characterizations of the samples. Al might come from the sample desk. Besides, very small amounts of Ce are detected, which indicate that Ce exist in this hierarchical hollow MnO2 structure. To further investigate the state of cerium in the hierarchical hollow MnO2 microspheres, XPS and HAADF−

Figure 3. HAADF−STEM images of the as-synthesized hierarchical hollow MnO2 microspheres with addition of 0.0017 mol of Ce3+ ions in the reaction system. B

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thickness of the shell is ∼500 nm and the diameter of the inner cavity is ∼4 μm. The high-resolution TEM (HRTEM) images of one of the nanorods and discuslike nanoplatelets are presented in parts e and f of Figure 4, respectively. The fringe distance of the nanorod is calculated to be 0.26 nm, which agrees well with the spacing between the [031] planes of γMnO2. The crystal planes of the discuslike nanoplatelet have lattice spacing of ∼0.26 and 0.14 nm corresponding to [031] and [421] planes of γ-MnO2, respectively. The pore structure of hierarchical hollow microspheres was characterized by nitrogen adsorption and desorption measurements. Figure 5 presents the nitrogen adsorption−desorption

Ce3+ in oxidative medium. The elemental map of hierarchical structures (Figure 3) indicates that cerium is uniformly distributed in the sample. Compared with the figures of the elemental maps of manganese and oxygen, the figure of the elemental map of cerium approximately correlates with those of manganese and oxygen, which confirms that cerium ions are incorporated into the MnO2 structure. There are two possible positions for cerium in the MnO2 structure: framework or tunnel sites. It is almost impossible for the replacement of manganese by cerium in the framework because the ionic radius of cerium ions (Ce4+, 0.087 nm; Ce3+, 0.102 nm) are much larger than that of the manganese ions (Mn4+, 0.053 nm). The real lattice of γ-MnO2 includes irregular [1 × 1] and [1 × 2] tunnels, and even larger tunnels (e.g., [2 × 2]),31 for which the size is [0.46 nm × 0.46 nm] an d is large enough for the accommodation of cerium ions. Hence, we can conclude that cerium ions are incorporated into the tunnel sites. SEM and TEM images of MnO2 microspheres are shown in Figure 4. The low-magnification SEM image reveals that the

Figure 5. Nitrogen adsorption−desorption isotherms and pore size distribution (inset) of the as-synthesized hierarchical hollow MnO2 microspheres with addition of 0.0017 mol of Ce3+ ions in the reaction system.

isotherms and pore size distribution (inset plots) of the hollow microspheres. A hysteresis loop is observed in the nitrogen adsorption−desorption isotherms, which reveals the presence of mesopores in MnO 2 microspheres. The pore size distribution is calculated from desorption branch by the Barrett−Joyner−Halenda (BJH) method indicating that the average pore size is ∼2 nm. The BET specific surface area and the pore volume of the hierarchical hollow microspheres are 29.2 m2 g−1 and 0.30 cm3 g−1, respectively. To understand the formation process of the hierarchical hollow structure, a series of time-dependent experiments were carried out at 140 °C by keeping other reaction parameters constant. Figure 6 shows SEM (left) and TEM (right) images of the samples obtained at different reaction time. After 30 min of reaction, coherent spheres with diameter ∼1 μm are formed (part a of Figure 6). The surfaces of the spheres are composed of wormlike nanorods with width of ∼20 nm and length of ∼50 nm, respectively. TEM image of one of the spheres shows that the whole sphere has uniform contrast, which reveals that the sphere is solid (part b of Figure 6). The surfaces of the spheres become rough and are strewn with scrolling nanosheets (part c of Figure 6). The TEM image of one of the spheres presents that the thickness of sheet layer is ∼100 nm (part d of Figure 6). The contrast between pale edge and dark core indicates the density variation between the nanosheets and the solid core. After reacting for 1.5 h, scrolling nanosheets grow up and nanoplatelets grow from the edge of scrolling nanosheets (parts e and f of Figure 6). As the reaction time is prolonged, the core of the microsphere gradually dissolves and the shell of the microsphere gradually builds up. The particles from the dissolving core provide the materials for the growth of the shell, which is the process of redissolution−reprecipitation.

Figure 4. SEM (a−c) and TEM (d−f) images of the as-synthesized hierarchical hollow MnO2 microspheres with addition of 0.0017 mol of Ce3+ ions in the reaction system.

product is composed of hierarchical hollow microspheres with diameter range from 3−4 μm (part a of Figure 4). The highmagnification SEM image of an individual broken microsphere shown in part b of Figure 4 confirms the hollow structure of the product. The external surface of one of the microspheres in part c of Figure 4 shows that the surface of the hollow microsphere is commonly assembled by closely packed discuslike nanoplatelets and nanorods. The diameter of discuslike nanoplatelets ranges from 100 to 500 nm and the thickness of the edge of the discus is ∼30 nm. The width and length of the nanorods are ∼100 nm and ∼1 μm, respectively. The detailed morphology and structure of hierarchical hollow microspheres are further investigated by TEM. The TEM image of an individual broken microsphere is shown in part d of Figure 4, in which the hollow structure could be easily identified by the contrast between the dark edge and the pale center. The C

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sample is gradual enhanced and changed into γ-MnO2 phase. When the reaction time is 2 h, γ-MnO2 phase is obtained as dominant product. To investigate the effects of Ce3+ ions on the morphology and crystalline structure, a series of experiments were carried out with different concentrations of Ce3+ ions in the reaction system while keeping other reaction parameters constant. The SEM images and XRD patterns of the samples obtained with different concentrations of Ce3+ ions are shown in Figures 8

Figure 8. SEM of samples obtained with different concentrations of Ce3+ ions: (a) 0 mol/L, (b) 0.013 mol/L, (c) 0.033 mol/L.

Figure 6. SEM and TEM of the samples obtained with different reaction time with addition of 0.0017 mol of Ce3+ ions in the reaction system: (a) (b) 0.5 h, (c) (d) 1.0 h, (e) (f) 1.5 h, (g) (h) 2.0 h.

When the reaction time reaches 2 h, the surfaces of the spheres are composed of nanoplatelets and nanorods (part g of Figure 6). The distinct color contrast between the black edge and the bright center indicates that hollow structure has been formed (part d of Figure 6). When the reaction time is up to 3 h, some hollow spheres tend to break (part a of Figure 4). The XRD patterns of samples obtained with different reaction time are shown in Figure 7. When the reaction time is 30 min, an amorphous MnO2 is formed. As the increase of the hydrothermal treatment time, the degree of the crystallinity of the

Figure 9. XRD patterns of samples obtained with different concentrations of Ce3+ ions: (a) 0 mol/L, (b) 0.013 mol/L, (c) 0.033 mol/L.

and 9, respectively. When there is no Ce3+ ions in the reaction system, flowerlike solid spheres assembled by interlocked nanosheets with diameter in the range of 3−5 μm are obtained (part a of Figure 8). When 0.013 mol/L Ce3+ ions are present in the reaction mixture, separated nanoplatelets and nanorods are found (part b of Figure 8). The reason of the formation of nanorods is that the addition of Ce3+ ions increases the chemical potential of the solution, which contributes to the growth of 1D nanostructures. Moreover, the presence of the Ce3+ ions increases the ionic strength of the reaction system, which will reduce the reaction rate according to the primary salt effect and increases the rate of aggregate nucleation. The balance between the rate of particles formation and the rate of aggregation determine the morphology of the final product. Comparing with the reaction system without Ce3+ ions, microshperes composed of interlocked nanosheets are found at 1 h reaction time in the reaction system in the presence of 0.025 mol/L (0.0017 mol) Ce3+ ions (part c of Figure 6). With the progress of reaction, new dispersed nanosheets and nanorods grow up at the edge of the interlocked nanosheets. The reaction time for forming miscoshperes composed by interlocked nanosheets is shorter than that of the reaction system without Ce3+ ions and the morphology of the product

Figure 7. XRD patterns of samples obtained with different reaction time with addition of 0.0017 mol of Ce3+ ions in the reaction system: (a) 0.5 h, (b) 1.0 h, (c) 1.5 h, (d) 2.0 h. D

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reaction temperature on the catalysts are shown in Figure 11, which suggests that the catalytic activities of the samples

continues to transform into hollow structure (part h of Figure 6). When the concentration of Ce3+ ions is up to 0.033 mol/L, the morphology of the production is still hierarchical hollow sphere, but the hollow structure tends to collapse (part c of Figure 8). The XRD pattern of the sample obtained without Ce 3+ ions can be assigned to a tetragonal phase of K0.27MnO2(H2O)0.54 with lattice contents a = 2.849 Å and c = 21.53 Å (JCPDS Card No. JPCDS 86-0666) (part a of Figure 9). When the concentration of Ce3+ ions is increased from 0.013 mol/L to 0.033 mol/L, the crystalline structures of MnO2 changes from K0.27MnO2(H2O)0.54 to γ-MnO2 (JCPDS Card No. 14-0644) (parts b and c of Figure 9). The results clearly show that the presence of Ce3+ ions leads to the change of crystalline structure of MnO2 indicating that Ce3+ ions have remarkable effects on the crystalline structure and morphology of the product and the concentration of Ce3+ ions is a key parameter for the formation of the hierarchical hollow MnO2 microspheres. On the basis of above experimental results, a possible growth mechanism is illustrated in Figure 10. In the initial reaction

Figure 11. Activities of the samples in the oxidation of benzene.

obtained in our experiments are clearly higher than commercial MnO2 and the catalytic activities of the samples (S1, S2, S3) doped with cerium are higher than the sample without cerium. Ceria can act as an oxygen buffer by storing and releasing oxygen under controlled conditions. Therefore, the presence of cerium can improve the oxygen storage capacity of the catalyst through the transformation between Ce4+ and Ce3+, which can promote catalytic ability.30,41 The sample S2 has the highest catalytic ability among all catalysts. The T50 and T90 of sample S2 are ∼220 and ∼252 °C, respectively. When the reaction temperature reached to 340 °C, a complete oxidation of benzene has been achieved for sample S2 and the products of catalytic reaction are only CO2 and H2O. To better understand the factor during the catalytic process, the cerium content in all samples of MnO2 doped with cerium are analyzed by ICP. The results of cerium contents are summarized in Table S1 of the Supporting Information. The results indicate that the cerium content of the products is increased with the increase of concentration of Ce ions in the reaction system. The cerium contents of the samples decrease in the order of S3 > S2 > S1 > S0. The catalytic abilities of the samples doped with cerium are higher than that of the sample without cerium. However, the order of the cerium content of the samples is not fully compliant with the result of catalytic tests. We deduce that the cerium content is not the key factor for the catalytic ability of samples. The XPS spectra of Ce 3d of all samples are shown in Figure S1 of the Supporting Information, and the values of Ce3+/Ce4+ are also summarized in Table S1 of the Supporting Information. The value of Ce3+/Ce4+ of S2 is equal to that of S3 and the value of Ce3+/Ce4+ of S1 is the smallest. It is reported that the increase of the content of Ce3+ can improve the lattice oxygen defects over the catalyst surface.42 Furthermore, the larger ratio of Ce3+/Ce can create a charge imbalance, the vacancies, and unsaturated chemical bonds on the catalyst surface to promote the catalytic oxidation.42−44 Hence, the higher values of Ce3+/Ce4+ of S1 and S2 may be one of the main reasons accounting for their high catalytic activity. The chemical environment of oxygen in metal oxide catalysts played an important role in their catalytic properties.45 The state of the O on the surface of catalyst also was examined. The O 1s spectra in all samples are presented in Figure S2 of the Supporting Information. The value of binding energy at 529.6− 529.8 eV is ascribed to the lattice oxygen (denoted as Oα), whereas the value of binding energy at 531−531.2 eV could be assigned to defective oxides or surface-adsorbed oxygen

Figure 10. Schematic illustration for the formation of hierarchical hollow MnO2 microspheres.

stage, small MnO2 nanoparticles are first produced through KMnO4 decomposition. MnO2 nanoparticles tend to aggregate to form amorphous solid microspheres by growth mechanism of oriented attachment for the minimization of the overall energy of the system (step 1).32−34 The rates of formation and aggregation of the MnO2 nanoparticles are influenced by the concentration of Ce3+ ions. As the reaction proceeded, the scrolling nanosheets grow from the surface of the spheres. These metastable spheres are stabilized by the outer layer of nanosheets (step 2).10 The newly formed nanosheets are looser and larger than the nanobuilding blocks of the interior part. So, the surface energy of the inner core is higher than that of the outer surface.35 Thus, the core region dissolved gradually concomitant with the Ostwald ripening process, which provide the source material for the formation of the shell.10,29,31−37 With the increase of reaction time, the discuslike nanoplatelets and nanorods grow up and the interior cavity is gradually formed (step 3). The addition of Ce3+ ions increases the chemical potential of the reaction system, which contributes to the growth of 1D nanostructures. As the reaction proceeds, the discs and nanorods grow larger at the expense of dissolving core materials. In the end, the inner core is completely consumed and the hierarchical hollow structures are formed (step 4). Manganese oxides are efficient materials for catalytic elimination of volatile organic compounds (VOCs).38−40 Herein, the as-prepared hierarchical hollow microspheres assembled by discuslike nanoplatelets and nanorods (denoted as S2) were used as catalyst to investigate their application in catalytic oxidation of benzene. As comparative experiments, the catalytic properties of commercial MnO2, the flowerlike microspheres (S0), the separated nanorods and nanoplatelets (S1), and the hierarchical hollow microspheres (S3) are evaluated. The curves of benzene conversion as a function of E

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(denoted as Oβ).45−47 The peak at binding energy of 533 eV is associated with adsorbed molecular water. The peak deconvolutions are only performed for the Oα and Oβ because the peak signal of adsorbed water is too weak. The relative percentages of Oβ and Oα + Oβ are quantified based on the area ratios of Oα and Oβ peaks. The proportion of Oβ /(Oα + Oβ) are in the order: S2 (41.97%) > S3 (35.09%) > S1 (30.85%) > S0 (27.60%), which corresponds to the result of catalytic tests. The results of O 1s spectra indicate that the present of Ce in the MnO2 structure results in more defective oxides or surfaceadsorbed oxygen on the surface. Furthermore, more Ce3+ led to more amounts of defective oxides or surface-adsorbed oxygen. Defective oxides or surface-adsorbed oxygen can enhance the mobility of lattice oxygen to the surface.47,48 Hence, the proportion of Oβ/(Oα + Oβ) in the sample S2 might be another main reason for the enhancement of the catalytic activity.

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CONCLUSIONS In summary, we have developed a facial method for the fabrication of hierarchical hollow MnO2 microspheres consisting of discuslike discs and nanorods in the presence of Ce3+ ions. Ce3+ ions play a crucial role in controlling the morphology and crystalline structure of MnO2. The concentration of Ce3+ ions is a key factor for the formation of hierarchical hollow MnO2 microspheres. It is concluded that the oriented attachment and subsequent Ostwald ripening process is responsible for the formation of the hierarchical hollow structures. The hierarchical hollow MnO2 microspheres show a higher catalytic ability for catalytic oxidation of benzene.

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ASSOCIATED CONTENT

S Supporting Information *

ICP results and XPS spectra for the characterization of the hollow structures in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (86)-(10)-82544896; fax: (86)-(10)-62542803; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by National High Technology Research and Development Program 863 of China, No. 2010AA 064903, Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB0505000, National Natural Science Foundation of China (NSFC) No. 51002154.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp312745n | J. Phys. Chem. C XXXX, XXX, XXX−XXX