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Synthesis of hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods for catalytic degradation of methylene blue
Catalysis Communications 46 (2014) 174–178
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis of hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods for catalytic degradation of methylene blue Wenshu Yang a,b, Jinhui Hao a,b, Zhe Zhang a, Baoping Lu a,b, Bailin Zhang a,⁎, Jilin Tang a,⁎⁎ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 16 October 2013 Received in revised form 9 December 2013 Accepted 17 December 2013 Available online 21 December 2013 Keywords: Hybrid materials Hierarchical structures Methylene blue Catalytic
a b s t r a c t A facile two-step method was developed for a large-scale growth of hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods (MC hybrid materials) as an efficient catalyst for water treatment. The synthesis involved a one-step hydrothermal process to prepare MnOOH nanorods and subsequently a simple solution method using hydrothermally synthesized MnOOH nanorods as both the template and Mn source to obtain MnCo2O4.5/MnOOH (MC) hybrid materials. The as-prepared MC hybrid materials with hierarchical structures could provide more active sites for catalytic degradation of methylene blue. These results indicate that the designed MC hybrid materials exhibit a promising capability for the degradation of dyes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the coming decade, water pollution is a formidable problem because of rapid population growth and extended industrial development. Dyes, as main pollutants in industrial sewage, are harmful for the living organisms in the biosphere. Different methods have been developed for water treatment [1,2]. The catalytic degradation is considered as one of the suitable treatment methods for the dye degradation due to its ease of operation and the availability of wide range of catalysts. Transition metal oxides are known to catalyze the reduction of H2O2 at low concentration with remarkable sensitivity [3–5]. Therefore, they could be considered as catalysts for the degradation of colored dye in the presence of H2O2. However, for transition metal oxides, the practical applications are largely hindered due to the relatively low surface areas and adsorption capacity. As a result, developing transition metal oxides with new morphologies and structures for enhanced properties has attracted great research interests. The development of nanostructure materials, especially metal oxides, will undoubtedly provide a promising solution to enhance the catalysis performance because of their high surface area [6–8]. Among various nanostructured materials, hierarchical materials with a high surface-to-bulk ratio, which can contact with more reactants and provide more reaction active sites, have been demonstrated as a promising candidate for the above-mentioned applications, as well as other applications [9–11]. On the other hand, hybrid structures combined with different materials have drawn immense attention due to their multifunctional properties and a wide range of ⁎ Corresponding author. Tel./fax: +86 431 85262430. ⁎⁎ Corresponding author. Tel./fax: +86 431 85262734. E-mail addresses: [email protected] (B. Zhang), [email protected] (J. Tang). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.12.018
applications [12–16]. Hence, it will be of great significance to develop effective and facile methods to synthesis hierarchical hybrid materials as a high-efficiency nanocatalyst. Integration of transition metal oxides and hydroxides into hybrid materials with core–shell structure [17–19] can enhance the overall physical and chemical properties of materials. However, in the case of core–shells, the core performance is weak due to the thick shell, and the core rarely participates in the reaction in the synthesis of shells. Although there are a few reports [17–19] on transition metal oxide/metal hydroxide hybrid nanostructures, it is rarely seen that the hydroxyl metal oxides are not only used as the template, but also participate in the synthesis of transition metal oxide. In this paper, we proposed a facile two-step method to synthesize hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods (MC) using MnOOH as template. The MnOOH nanorods are not only used as the template, but also participate in the synthesis of MnCo2O4.5. The as-synthesized hybrid materials exhibited remarkable catalytic activity for degradation of methylene blue (MB). 2. Experimental 2.1. Synthesis of catalyst The MC hybrid materials were prepared by a simple two-step process, which can be easily scaled up. Firstly, MnOOH nanorods were synthesized by a simple one-step hydrothermal method according to a literature method [20]. Briefly, 15.8 mg KMnO4 and 8 mg Poly(Nvinyl-2-pyrrolidone) (PVP) were dispersed into 15 mL Milli-Q water, followed by addition of 25.4 mg MnSO4·H2O under stirring. The mixture was stirred for 10 min, and then 2 mL ethylene glycol was added
W. Yang et al. / Catalysis Communications 46 (2014) 174–178
by drop. After stirring for 10 min, the reaction solution was transferred into a 20 mL Teflon-line stainless steel autoclave and heated at 140 °C for 24 h. The product was filtered, centrifuged, washed several times and dispersed into 5 mL Milli-Q water. Secondly, the as-obtained MnOOH nanorods (0.5 mL) were diluted with 20 mL Milli-Q water in a glass container and subjected to ultrasound for 10 min. Then, 0.1 mmol CoCl2·6H2O and 0.6 mmol urea were added and sonicated for 1 min to form a homogeneous solution. The resulting mixture was sealed and maintained at 80 °C for 24 h. After cooled to room temperature, the product was harvested, and washed with ethanol several times, and dried at 60 °C for 12 h. Other experimental details were listed in the Supporting information. 3. Results and discussion 3.1. Catalyst characterization MnOOH nanorods were synthesized by a simple one-step hydrothermal method. Field emission scanning electron microscopy (FESEM) was firstly used to examine the morphologies of nanorods. It is clearly observed that the as-prepared MnOOH nanorods have a good dispersity (Fig. 1A). The diameters of the individual MnOOH nanorods are in the range of 30–70 nm, and their lengths are ~1 μm. These MnOOH nanorods served as the template to support the subsequent growth of hierarchical MC hybrid materials by reacting with CoCl2 and urea at 80 °C for 24 h. The SEM image (Fig. 1B) indicates that MnOOH nanorods are encased by the MnCo2O4.5 nanosheets. MnCo2O4.5 nanosheets with random orientations are grown uniformly on the surface of MnOOH nanorods forming a hierarchical structure. The structure of MC hybrid materials was further investigated by transmission electron microscopy (TEM) (Fig. 1C). It can be clearly seen that, except for some MnCo2O4.5 nanosheets with a high density distribution on the surface of the nanorods, other several MnCo2O4.5 nanosheet aggregates (marked with red circle in Fig. 1C) are observed between external nanosheets and the internal template. These nanosheet aggregates were gradually formed during reaction (Fig. S3 and Supplementary material). This hierarchical structure is beneficial to the catalytic performance,
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because it can increase the specific surface area of catalysts. Highresolution TEM examination of such nanosheets (Fig. 1D) reveals that the lattice fringes correspond to an interplanar distance of 0.245 nm, which can be attributed to the (311) plane of the MnCo2O4.5 phase. X-ray diffraction (XRD) analysis was carried out to identify the crystal phases of MnOOH and MC hybrid materials. The XRD pattern of MnOOH nanorods (Fig. 2A, trace 1) exhibits the characteristic diffractions of the monoclinic phase (JCPDS Card No. 41-1379). The impurity is not observed, which indicates that the product is pure MnOOH phase. After a solution reaction, the characteristic peaks of MnOOH disappear incompletely (26.2°) and some new peaks appear (Fig. 2A, trace 2). The new peaks can be indexed as the cubic spinel phase of MnCo2O4.5 (JCPDS Card No. 32-297). The narrow diffraction peaks imply the good crystallinity of the as-prepared MC hybrid materials. X-ray photoelectron spectroscopy (XPS) and SEM mapping analysis were further utilized to probe Mn, Co and O elements of the hybrid materials. Fig. S1A presents the detailed Mn2p spectrum of MnOOH. The Mn2p spectrum exhibits two major peaks with binding energy values at 652.2 and 640.7 eV, corresponding to the Mn2p1/2 and Mn2p3/2 peaks. In the case of MnOOH, Mn3+ is the only apparent species. The detailed Mn2p3/2 spectrum of MC hybrid material is shown in Fig. 2B. It is can be assumed that the intensity decrease of the Mn2p XPS peak is due to lower manganese content in MnCo2O4.5 on the surface of MC hybrid materials. The binding energy (BE) value of Mn2p3/2 and full width of half maximum (FWHM) are 641.8 eV and 4.3 eV, respectively. The Mn2p3/2 peak component occurs at a BE value which is intermediate between the literature [21,22] ranges pertaining to Mn3 + and Mn4 +. So the Mn 2p3/2 peak is consistent with the occurrence of Mn2+ and Mn4+ species [23–25]. Fig. S1B displays the XPS spectrum of Co2p which shows two major peaks with binding energy values at 781.3 and 797.4 eV, corresponding to the Co2p3/2 and Co2p1/2 spin– orbit peaks, respectively. By using a Gaussian fitting method, the Co2p3/2 emission spectrum (Fig. 2C) is best fitted with two spin–orbit doublets, characteristic of Co2+ and Co3+, and without shakeup satellite. The atomic ratio of Co2 +/Co3 + is close to 1.2. These data show that the surface of the MC hybrid materials has a composition containing Co2+, Co3+, Mn2+, and Mn4+, Where the atomic ratio of Co/Mn is estimated to be around 3.5 by calculating the XPS peak areas. The XPS
Fig. 1. SEM images of MnOOH nanorods (A) and MC hybrid materials (B). TEM image of MC hybrid materials (C). HRTEM image of the external MnCo2O4.5 nanocrystal (D).
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Fig. 2. XRD patterns of MnOOH nanorods (A, trace 1) and MC hybrid materials (A, trace 2). XPS spectra of Mn2p3/2 (B), Co2p3/2 (C) and O1s (D) in MC hybrid materials.
spectrum of O1s (Fig. 2D) consists of two peaks, which correspond to lattice oxygen and adsorption oxygen on the sample surface. The peak at 530.8 eV corresponds to the lattice oxygen species (O2−, O−), which reflect the redox behavior of the metal. The peak at 532.5 eV cor2− responds to the adsorption oxygen species (O− 2 , O2 ), whose content reflects the concentration of oxygen vacancy in the compound. By trapping electrons, adsorption oxygen becomes the active center for the oxidation, which leads to the formation of O− 2 . Furthermore, elemental mapping analysis by SEM–EDS (Fig. S2) further confirms the presence of Co and Mn elements.
In addition, the surface area and porosity of MnOOH nanorods and MC hybrid materials were studied using nitrogen isotherm adsorption measurement (Fig. 3). MC hybrid materials show typical type-IV sorption isotherms, giving a hysteresis loop at a relative pressure of 0.4 b p/p0 b 1.0 as seen in Fig. 3B. This result suggests that MC hybrid materials are porous materials with an average pore size of 12.14 nm, which is different from that of MnOOH nanorods (Fig. 3A). Correspondingly, the Brunauer–Emmett–Teller (BET) surface area of the MC hybrid materials (48.28 m2 g−1) is much higher than that of MnOOH nanorods (15.56 m2 g−1).
Fig. 3. N2-sorption isotherm of MnOOH nanorods (A) and MC hybrid materials (B) (Inset: BJH pore size distribution).
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Moreover, the effect of temperature on the degradation reaction was explored (Fig. 5B). As the temperature increased from 50 to 80 °C, the degradation rate of MB by MC hybrid materials was increased about 10 times. On the basis of the Arrhenius equation, the apparent activation energy of MC hybrid materials was estimated to be 58.01 kJ/mol, which is lower than that of MnCo2O4 nanomaterials (72.24 kJ/mol). The value of Ea depicts that MC hybrid materials possess a higher activity than that of MnCo2O4. Combining the results of nitrogen isotherm adsorption measurement, these results indicate that the performance of MC hybrid materials is from the formation of MnCo site and the hierarchical structure. 4. Conclusion
Fig. 4. UV–Vis absorption spectra of a mixture of MB (62.5 mg L−1, 16 mL), MC hybrid materials (0.004 g), and H2O2 (4 mL, 30 wt.%) after heating at 80 °C for different time intervals of 0, 5, 10, 15, 20, 25 and 30 min.
3.2. Catalytic activity Herein, we investigated the application of as-obtained MC hybrid materials in the degradation of MB. The MC hybrid materials catalyzed the decomposition of H2O2 to generate free radical species, HO•, HOO•, or O•− 2 . These free radical species further degraded the MB molecules [26]. A typical UV–Vis curve and the degradation performance of MB by MC hybrid materials are shown in Fig. 4. The spectrum at t = 0 is recorded from the starting solution of MB with a concentration of 10 mg L−1 (without H2O2 and catalyst). Four characteristic peaks (245, 292,614 and 665 nm) observed are consistent with those reported previously [27]. As soon as H2O2 and MC hybrid materials were added, the color of the mixture turned from blue to gray quickly and the intensities of MB absorption peaks decreased quickly within only 10 min (Fig. 4). The MB bands at 292 and 245 nm are masked by the strong absorption of hydrogen peroxide in the range 185–300 nm. The degradation efficiency of MB molecules is calculated by (I0 − I) / I0, where I0 is the absorbance at 665 nm at t = 0 and I is the absorbance at a given reaction time. In the absence of catalyst or H2O2, no obvious dye degradation is observed after reaction for 1 h (curves d and e in Fig. 5A). In the presence of MC hybrid materials and H2O2 at 80 °C for 15 min, more than 90% of MB is degraded as a result of the catalyzed reaction of catalyst and H2O2 (curve a in Fig. 5A), which is more effective than that of MnOOH nanorods (curve c in Fig. 5A) and MnCo2O4 nanomaterials (curve b in Fig. 5A). These results indicate that MC hybrid materials can efficiently catalyze the degradation of MB.
In this paper, the synthesis of MnCo2O4.5/MnOOH hybrid materials through a facile two-step method was investigated in details. The synthesis involved a one-step hydrothermal process to prepare MnOOH nanorods and subsequently a simple solution method to obtain MC hybrid materials. In the synthesis process, MnOOH nanorods are used as both the template and Mn source. Moreover, the hierarchical structures may have a high specific surface area that can provide more active sites. The MC hybrid materials are efficient for the catalytic degradation of MB in the presence of H2O2. The results suggest that the MC hybrid materials could be a promising material for a wide range of potential applications in water treatment. Acknowledgments This work was supported by the National Natural Science Foundation (No. 21375122). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2013.12.018. References [1] V.K. Gupta, S. Agarwal, T.A. Saleh, J. Hazard. Mater. 185 (2011) 17. [2] S. Zor, B. Yazici, M. Erbil, H. Galip, Water Res. 32 (1998) 579. [3] T.R. Hinklin, J. Azurdia, M. Kim, J.C. Marchal, S. Kumar, R.M. Laine, Adv. Mater. 20 (2008) 1373. [4] T.J. Yoon, J.S. Kim, B.G. Kim, K.N. Yu, M.H. Cho, J.K. Lee, Angew. Chem. Int. Ed. 44 (2005) 1068. [5] J.F. Marco, J.R. Gancedo, M. Gracia, J.L. Gautier, E.I. Rios, H.M. Palmer, C. Greaves, F.J. Berry, J. Mater. Chem. 11 (2001) 3087. [6] Z.R. Tang, F. Li, Y. Zhang, X. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 7880. [7] M.L. Chen, J.S. Bae, H.S. Yoon, C.S. Lim, W.C. Oh, Bull. Korean Chem. Soc. 32 (2011) 815. [8] A. Giri, N. Goswami, M. Pal, M.T.Z. Myint, S. Al Harthi, A. Singha, B. Ghosh, J. Dutta, S.K. Pal, J. Mater. Chem. C. 1 (2013) 1885.
Fig. 5. Time profiles (A) of MB degradation in MC hybrid material + MB + H2O2 (a), MnCo2O4 + MB + H2O2 (b), MnOOH nanorod + MB + H2O2 (c), H2O2 + MB (d) and MC hybrid material + MB (e) systems. Time profiles (B) of MB degradation at 80 °C (a), 65 °C (b) and 50 °C (c) in MC hybrid material + MB + H2O2 system.
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[9] J. Hao, W. Yang, Z. Zhang, S. Pan, B. Lu, X. Ke, B. Zhang, J. Tang, Nanoscale 5 (2013) 3078. [10] J. Liu, G. Zhang, CrystEngComm 15 (2013) 382. [11] Q.P. Luo, X.Y. Yu, B.X. Lei, H.Y. Chen, D.B. Kuang, C.Y. Su, J. Phys. Chem. C 116 (2012) 8111. [12] I. Lee, J.B. Joo, Y. Yin, F. Zaera, Angew. Chem. Int. Ed. 50 (2011) 10208. [13] T. Kim, E. Momin, J. Choi, K. Yuan, H. Zaidi, J. Kim, M. Park, N. Lee, M.T. McMahon, A. Quinones-Hinojosa, J.W.M. Bulte, T. Hyeon, A.A. Gilad, J. Am. Chem. Soc. 133 (2011) 2955. [14] M. Montazeri, M. Fickenscher, L.M. Smith, H.E. Jackson, J. Yarrison-Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish, Y. Guo, J. Zou, M.-E. Pistol, C.E. Pryor, Nano Lett. 10 (2010) 880. [15] L. Yu, G. Zhang, C. Yuan, X.W. Lou, Chem. Commun. 49 (2013) 137. [16] M.J. Becker, W. Xia, J.P. Tessonnier, R. Blume, L. Yao, R. Schloegl, M. Muhler, Carbon 49 (2011) 5253.
[17] X. Xia, J. Tu, Y. Zhang, J. Chen, X. Wang, C. Gu, C. Guan, J. Luo, H.J. Fan, Chem. Mater. 24 (2012) 3793. [18] J.H. Zhong, A.L. Wang, G.R. Li, J.W. Wang, Y.N. Ou, Y.X. Tong, J. Mater. Chem. 22 (2012) 5656. [19] D. Pan, H. Zhang, T. Fan, J. Chen, X. Duan, Chem. Commun. 47 (2011) 908. [20] L.L. Zhang, X.B. Zhang, Z.L. Wang, J.J. Xu, D. Xu, L.M. Wang, Chem. Commun. 48 (2012) 7598. [21] H.W. Nesbitt, D. Banerjee, Am. Mineral. 83 (1998) 305. [22] S. Ardizzone, C.L. Bianchi, D. Tirelli, Colloids Surf A 134 (1998) 305. [23] S.P. Kowalczyk, L. Ley, F.R. McFeely, D.A. Shirley, Phys. Rev. B 11 (1975) 1721. [24] C.S. Fadley, D.A. Shirley, Phys. Rev. A 2 (1970) 1109. [25] J.C. Carver, T.A. Carlson, Gk Schweitz, J. Chem. Phys. 57 (1972) 973. [26] X. Peng, I. Ichinose, Nanotechnology 22 (2011). [27] T.Y. Zhang, T. Oyama, A. Aoshima, H. Hidaka, J.C. Zhao, N. Serpone, J. Photochem. Photobiol. A 140 (2001) 163.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis of hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods for catalytic degradation of methylene blue Wenshu Yang a,b, Jinhui Hao a,b, Zhe Zhang a, Baoping Lu a,b, Bailin Zhang a,⁎, Jilin Tang a,⁎⁎ a b
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 16 October 2013 Received in revised form 9 December 2013 Accepted 17 December 2013 Available online 21 December 2013 Keywords: Hybrid materials Hierarchical structures Methylene blue Catalytic
a b s t r a c t A facile two-step method was developed for a large-scale growth of hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods (MC hybrid materials) as an efficient catalyst for water treatment. The synthesis involved a one-step hydrothermal process to prepare MnOOH nanorods and subsequently a simple solution method using hydrothermally synthesized MnOOH nanorods as both the template and Mn source to obtain MnCo2O4.5/MnOOH (MC) hybrid materials. The as-prepared MC hybrid materials with hierarchical structures could provide more active sites for catalytic degradation of methylene blue. These results indicate that the designed MC hybrid materials exhibit a promising capability for the degradation of dyes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the coming decade, water pollution is a formidable problem because of rapid population growth and extended industrial development. Dyes, as main pollutants in industrial sewage, are harmful for the living organisms in the biosphere. Different methods have been developed for water treatment [1,2]. The catalytic degradation is considered as one of the suitable treatment methods for the dye degradation due to its ease of operation and the availability of wide range of catalysts. Transition metal oxides are known to catalyze the reduction of H2O2 at low concentration with remarkable sensitivity [3–5]. Therefore, they could be considered as catalysts for the degradation of colored dye in the presence of H2O2. However, for transition metal oxides, the practical applications are largely hindered due to the relatively low surface areas and adsorption capacity. As a result, developing transition metal oxides with new morphologies and structures for enhanced properties has attracted great research interests. The development of nanostructure materials, especially metal oxides, will undoubtedly provide a promising solution to enhance the catalysis performance because of their high surface area [6–8]. Among various nanostructured materials, hierarchical materials with a high surface-to-bulk ratio, which can contact with more reactants and provide more reaction active sites, have been demonstrated as a promising candidate for the above-mentioned applications, as well as other applications [9–11]. On the other hand, hybrid structures combined with different materials have drawn immense attention due to their multifunctional properties and a wide range of ⁎ Corresponding author. Tel./fax: +86 431 85262430. ⁎⁎ Corresponding author. Tel./fax: +86 431 85262734. E-mail addresses: [email protected] (B. Zhang), [email protected] (J. Tang). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.12.018
applications [12–16]. Hence, it will be of great significance to develop effective and facile methods to synthesis hierarchical hybrid materials as a high-efficiency nanocatalyst. Integration of transition metal oxides and hydroxides into hybrid materials with core–shell structure [17–19] can enhance the overall physical and chemical properties of materials. However, in the case of core–shells, the core performance is weak due to the thick shell, and the core rarely participates in the reaction in the synthesis of shells. Although there are a few reports [17–19] on transition metal oxide/metal hydroxide hybrid nanostructures, it is rarely seen that the hydroxyl metal oxides are not only used as the template, but also participate in the synthesis of transition metal oxide. In this paper, we proposed a facile two-step method to synthesize hierarchical MnCo2O4.5 nanostructure modified MnOOH nanorods (MC) using MnOOH as template. The MnOOH nanorods are not only used as the template, but also participate in the synthesis of MnCo2O4.5. The as-synthesized hybrid materials exhibited remarkable catalytic activity for degradation of methylene blue (MB). 2. Experimental 2.1. Synthesis of catalyst The MC hybrid materials were prepared by a simple two-step process, which can be easily scaled up. Firstly, MnOOH nanorods were synthesized by a simple one-step hydrothermal method according to a literature method [20]. Briefly, 15.8 mg KMnO4 and 8 mg Poly(Nvinyl-2-pyrrolidone) (PVP) were dispersed into 15 mL Milli-Q water, followed by addition of 25.4 mg MnSO4·H2O under stirring. The mixture was stirred for 10 min, and then 2 mL ethylene glycol was added
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by drop. After stirring for 10 min, the reaction solution was transferred into a 20 mL Teflon-line stainless steel autoclave and heated at 140 °C for 24 h. The product was filtered, centrifuged, washed several times and dispersed into 5 mL Milli-Q water. Secondly, the as-obtained MnOOH nanorods (0.5 mL) were diluted with 20 mL Milli-Q water in a glass container and subjected to ultrasound for 10 min. Then, 0.1 mmol CoCl2·6H2O and 0.6 mmol urea were added and sonicated for 1 min to form a homogeneous solution. The resulting mixture was sealed and maintained at 80 °C for 24 h. After cooled to room temperature, the product was harvested, and washed with ethanol several times, and dried at 60 °C for 12 h. Other experimental details were listed in the Supporting information. 3. Results and discussion 3.1. Catalyst characterization MnOOH nanorods were synthesized by a simple one-step hydrothermal method. Field emission scanning electron microscopy (FESEM) was firstly used to examine the morphologies of nanorods. It is clearly observed that the as-prepared MnOOH nanorods have a good dispersity (Fig. 1A). The diameters of the individual MnOOH nanorods are in the range of 30–70 nm, and their lengths are ~1 μm. These MnOOH nanorods served as the template to support the subsequent growth of hierarchical MC hybrid materials by reacting with CoCl2 and urea at 80 °C for 24 h. The SEM image (Fig. 1B) indicates that MnOOH nanorods are encased by the MnCo2O4.5 nanosheets. MnCo2O4.5 nanosheets with random orientations are grown uniformly on the surface of MnOOH nanorods forming a hierarchical structure. The structure of MC hybrid materials was further investigated by transmission electron microscopy (TEM) (Fig. 1C). It can be clearly seen that, except for some MnCo2O4.5 nanosheets with a high density distribution on the surface of the nanorods, other several MnCo2O4.5 nanosheet aggregates (marked with red circle in Fig. 1C) are observed between external nanosheets and the internal template. These nanosheet aggregates were gradually formed during reaction (Fig. S3 and Supplementary material). This hierarchical structure is beneficial to the catalytic performance,
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because it can increase the specific surface area of catalysts. Highresolution TEM examination of such nanosheets (Fig. 1D) reveals that the lattice fringes correspond to an interplanar distance of 0.245 nm, which can be attributed to the (311) plane of the MnCo2O4.5 phase. X-ray diffraction (XRD) analysis was carried out to identify the crystal phases of MnOOH and MC hybrid materials. The XRD pattern of MnOOH nanorods (Fig. 2A, trace 1) exhibits the characteristic diffractions of the monoclinic phase (JCPDS Card No. 41-1379). The impurity is not observed, which indicates that the product is pure MnOOH phase. After a solution reaction, the characteristic peaks of MnOOH disappear incompletely (26.2°) and some new peaks appear (Fig. 2A, trace 2). The new peaks can be indexed as the cubic spinel phase of MnCo2O4.5 (JCPDS Card No. 32-297). The narrow diffraction peaks imply the good crystallinity of the as-prepared MC hybrid materials. X-ray photoelectron spectroscopy (XPS) and SEM mapping analysis were further utilized to probe Mn, Co and O elements of the hybrid materials. Fig. S1A presents the detailed Mn2p spectrum of MnOOH. The Mn2p spectrum exhibits two major peaks with binding energy values at 652.2 and 640.7 eV, corresponding to the Mn2p1/2 and Mn2p3/2 peaks. In the case of MnOOH, Mn3+ is the only apparent species. The detailed Mn2p3/2 spectrum of MC hybrid material is shown in Fig. 2B. It is can be assumed that the intensity decrease of the Mn2p XPS peak is due to lower manganese content in MnCo2O4.5 on the surface of MC hybrid materials. The binding energy (BE) value of Mn2p3/2 and full width of half maximum (FWHM) are 641.8 eV and 4.3 eV, respectively. The Mn2p3/2 peak component occurs at a BE value which is intermediate between the literature [21,22] ranges pertaining to Mn3 + and Mn4 +. So the Mn 2p3/2 peak is consistent with the occurrence of Mn2+ and Mn4+ species [23–25]. Fig. S1B displays the XPS spectrum of Co2p which shows two major peaks with binding energy values at 781.3 and 797.4 eV, corresponding to the Co2p3/2 and Co2p1/2 spin– orbit peaks, respectively. By using a Gaussian fitting method, the Co2p3/2 emission spectrum (Fig. 2C) is best fitted with two spin–orbit doublets, characteristic of Co2+ and Co3+, and without shakeup satellite. The atomic ratio of Co2 +/Co3 + is close to 1.2. These data show that the surface of the MC hybrid materials has a composition containing Co2+, Co3+, Mn2+, and Mn4+, Where the atomic ratio of Co/Mn is estimated to be around 3.5 by calculating the XPS peak areas. The XPS
Fig. 1. SEM images of MnOOH nanorods (A) and MC hybrid materials (B). TEM image of MC hybrid materials (C). HRTEM image of the external MnCo2O4.5 nanocrystal (D).
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Fig. 2. XRD patterns of MnOOH nanorods (A, trace 1) and MC hybrid materials (A, trace 2). XPS spectra of Mn2p3/2 (B), Co2p3/2 (C) and O1s (D) in MC hybrid materials.
spectrum of O1s (Fig. 2D) consists of two peaks, which correspond to lattice oxygen and adsorption oxygen on the sample surface. The peak at 530.8 eV corresponds to the lattice oxygen species (O2−, O−), which reflect the redox behavior of the metal. The peak at 532.5 eV cor2− responds to the adsorption oxygen species (O− 2 , O2 ), whose content reflects the concentration of oxygen vacancy in the compound. By trapping electrons, adsorption oxygen becomes the active center for the oxidation, which leads to the formation of O− 2 . Furthermore, elemental mapping analysis by SEM–EDS (Fig. S2) further confirms the presence of Co and Mn elements.
In addition, the surface area and porosity of MnOOH nanorods and MC hybrid materials were studied using nitrogen isotherm adsorption measurement (Fig. 3). MC hybrid materials show typical type-IV sorption isotherms, giving a hysteresis loop at a relative pressure of 0.4 b p/p0 b 1.0 as seen in Fig. 3B. This result suggests that MC hybrid materials are porous materials with an average pore size of 12.14 nm, which is different from that of MnOOH nanorods (Fig. 3A). Correspondingly, the Brunauer–Emmett–Teller (BET) surface area of the MC hybrid materials (48.28 m2 g−1) is much higher than that of MnOOH nanorods (15.56 m2 g−1).
Fig. 3. N2-sorption isotherm of MnOOH nanorods (A) and MC hybrid materials (B) (Inset: BJH pore size distribution).
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Moreover, the effect of temperature on the degradation reaction was explored (Fig. 5B). As the temperature increased from 50 to 80 °C, the degradation rate of MB by MC hybrid materials was increased about 10 times. On the basis of the Arrhenius equation, the apparent activation energy of MC hybrid materials was estimated to be 58.01 kJ/mol, which is lower than that of MnCo2O4 nanomaterials (72.24 kJ/mol). The value of Ea depicts that MC hybrid materials possess a higher activity than that of MnCo2O4. Combining the results of nitrogen isotherm adsorption measurement, these results indicate that the performance of MC hybrid materials is from the formation of MnCo site and the hierarchical structure. 4. Conclusion
Fig. 4. UV–Vis absorption spectra of a mixture of MB (62.5 mg L−1, 16 mL), MC hybrid materials (0.004 g), and H2O2 (4 mL, 30 wt.%) after heating at 80 °C for different time intervals of 0, 5, 10, 15, 20, 25 and 30 min.
3.2. Catalytic activity Herein, we investigated the application of as-obtained MC hybrid materials in the degradation of MB. The MC hybrid materials catalyzed the decomposition of H2O2 to generate free radical species, HO•, HOO•, or O•− 2 . These free radical species further degraded the MB molecules [26]. A typical UV–Vis curve and the degradation performance of MB by MC hybrid materials are shown in Fig. 4. The spectrum at t = 0 is recorded from the starting solution of MB with a concentration of 10 mg L−1 (without H2O2 and catalyst). Four characteristic peaks (245, 292,614 and 665 nm) observed are consistent with those reported previously [27]. As soon as H2O2 and MC hybrid materials were added, the color of the mixture turned from blue to gray quickly and the intensities of MB absorption peaks decreased quickly within only 10 min (Fig. 4). The MB bands at 292 and 245 nm are masked by the strong absorption of hydrogen peroxide in the range 185–300 nm. The degradation efficiency of MB molecules is calculated by (I0 − I) / I0, where I0 is the absorbance at 665 nm at t = 0 and I is the absorbance at a given reaction time. In the absence of catalyst or H2O2, no obvious dye degradation is observed after reaction for 1 h (curves d and e in Fig. 5A). In the presence of MC hybrid materials and H2O2 at 80 °C for 15 min, more than 90% of MB is degraded as a result of the catalyzed reaction of catalyst and H2O2 (curve a in Fig. 5A), which is more effective than that of MnOOH nanorods (curve c in Fig. 5A) and MnCo2O4 nanomaterials (curve b in Fig. 5A). These results indicate that MC hybrid materials can efficiently catalyze the degradation of MB.
In this paper, the synthesis of MnCo2O4.5/MnOOH hybrid materials through a facile two-step method was investigated in details. The synthesis involved a one-step hydrothermal process to prepare MnOOH nanorods and subsequently a simple solution method to obtain MC hybrid materials. In the synthesis process, MnOOH nanorods are used as both the template and Mn source. Moreover, the hierarchical structures may have a high specific surface area that can provide more active sites. The MC hybrid materials are efficient for the catalytic degradation of MB in the presence of H2O2. The results suggest that the MC hybrid materials could be a promising material for a wide range of potential applications in water treatment. Acknowledgments This work was supported by the National Natural Science Foundation (No. 21375122). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2013.12.018. References [1] V.K. Gupta, S. Agarwal, T.A. Saleh, J. Hazard. Mater. 185 (2011) 17. [2] S. Zor, B. Yazici, M. Erbil, H. Galip, Water Res. 32 (1998) 579. [3] T.R. Hinklin, J. Azurdia, M. Kim, J.C. Marchal, S. Kumar, R.M. Laine, Adv. Mater. 20 (2008) 1373. [4] T.J. Yoon, J.S. Kim, B.G. Kim, K.N. Yu, M.H. Cho, J.K. Lee, Angew. Chem. Int. Ed. 44 (2005) 1068. [5] J.F. Marco, J.R. Gancedo, M. Gracia, J.L. Gautier, E.I. Rios, H.M. Palmer, C. Greaves, F.J. Berry, J. Mater. Chem. 11 (2001) 3087. [6] Z.R. Tang, F. Li, Y. Zhang, X. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 7880. [7] M.L. Chen, J.S. Bae, H.S. Yoon, C.S. Lim, W.C. Oh, Bull. Korean Chem. Soc. 32 (2011) 815. [8] A. Giri, N. Goswami, M. Pal, M.T.Z. Myint, S. Al Harthi, A. Singha, B. Ghosh, J. Dutta, S.K. Pal, J. Mater. Chem. C. 1 (2013) 1885.
Fig. 5. Time profiles (A) of MB degradation in MC hybrid material + MB + H2O2 (a), MnCo2O4 + MB + H2O2 (b), MnOOH nanorod + MB + H2O2 (c), H2O2 + MB (d) and MC hybrid material + MB (e) systems. Time profiles (B) of MB degradation at 80 °C (a), 65 °C (b) and 50 °C (c) in MC hybrid material + MB + H2O2 system.
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