The effect of pH and anions on the anisotropic growth of MnO2

The effect of pH and anions on the anisotropic growth of MnO2

Materials Research Bulletin 47 (2012) 3377–3382 Contents lists available at SciVerse ScienceDirect Materials Research Bulletin journal homepage: www...

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Materials Research Bulletin 47 (2012) 3377–3382

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin journal homepage:

The effect of pH and anions on the anisotropic growth of MnO2 Zhong-Hai Ji a, Bin Dong a,c,*, Yun-Qi Liu a,b, Chen-Guang Liu a,b,* a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, PR China Key Laboratory of Catalysis, China National Petroleum Corp. (CNPC), China University of Petroleum (East China), Qingdao 266555, PR China c College of Science, China University of Petroleum (East China), Qingdao 266555, PR China b



Article history: Received 19 December 2011 Received in revised form 29 June 2012 Accepted 28 July 2012 Available online 3 August 2012

MnO2 with different morphologies was controllably synthesized. The co-effect of some anions (SO42 , NO3 , PO43 and Cl ) and pH on the hydrothermal crystallization of MnO2 was also investigated systematically. The final products were characterized by XRD, TEM and SEM. Based on our experimental results, we propose that the coarsening of MnO2 can be impeded by anions. While when pH is dominant, this dissolution procedure can be accelerated and MnO2 nanorod formed through dissolution– recrystallization process. The present work may provide a new sight into the anisotropic growth of MnO2. ß 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds A. Oxides B. Chemical synthesis B. Crystal growth

1. Introduction There has been intensive research on manganese oxides due to their extensive applications in catalysts, molecular-sieves, ionexchangers, energy storage and so on [1–4]. Previous researches have shown that the morphology, dimensionality, size and crystallographic form have a great influence on the performance of MnO2 in its application [5], which underlines the importance of morphological and structural control of MnO2 nanostructures. Whereas, because of the complexity of reaction systems and little data about the effect of reaction conditions on the growth behavior of crystallites, controllable synthesis of nanostructures is still a great challenge. Currently, great efforts have been devoted to the study on the co-effect of pH (or the concentration of H+) and inorganic cations on the morphologies and structures of MnO2. For example, Shen et al. [6] have successfully fabricated manganese oxides with different tunnel structures by altering pH to adjust the sizes of the hydrated templates. Huang et al. [7] have found the competition mechanism between H+ and cations (such as K+, NH4+) in the formation of a-and b-MnO2. However, to the best of our

* Corresponding authors at: State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, PR China. Tel.: +86 532 86981716; fax: +86 532 86981787. E-mail addresses: [email protected] (B. Dong), [email protected] (C.-G. Liu). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.

knowledge, there is still no report on the co-effect of pH and anions on the morphology and structures of MnO2. In addition, the effect of pH is generally imposed on the final products generally via dissolution–recrystallization process [8,9], while Gao et al. [10] believed that anions exert their effects on the growth of onedimensional (1D) MnO2 by impeding the curling process of layer structures. Therefore, we anticipate that the study of the co-effect of pH and anions on the growth of MnO2 under hydrothermal conditions would be of tremendous interest in the interpretation of formation mechanism of 1D MnO2, which, in turn, may be significative to the synthesis of nanostructures on design. Herein, systematic experiments were developed to investigate the definite role of pH and anions in the formation of 1D MnO2 and their effects on the morphology. 2. Experimental 2.1. Sample preparation All chemicals were of analytical grade and were used as received without further purification. Deionized water was used throughout. In a typical procedure, 4 mmol KMnO4 and an equal amount of (NH4)2SO4 were dissolved into 80 ml deionized water under magnetic stirring. After stirring for 15 min, this solution was then transferred to a 100 ml Teflon-lined autoclave, sealed and maintained at 140 8C for 12 h. After the autoclave cooling to room temperature naturally, the resulting product was collected, washed with deionized water and absolute alcohol several times,


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Fig. 1. XRD pattern of MnO2 materials synthesized: (a) with pH adjusted to about 5 by HCl and (b) without any acids.

and then dried at 80 8C for 10 h. A parallel experiment with pH adjusted to c.a. 5.0 by HCl was also conducted. 2.2. Characterization X-ray diffraction (XRD) was carried out on Panalytical X’ Pert Pro MPD diffractometer. Fourier transformation infrared (FT-IR) spectrum was performed on a Newus (Nicolet, USA) FTIR spectrometer using a KBr pellet technique. SEM images were recorded on a Hitachi S-4800 scanning electron microscope. Transmission electron microscope (TEM), high resolution TEM (HRTEM), and the selected area electron diffraction (SAED) patterns were taken on a JEOL JEM-2100 UHR transmission electron microscope. 3. Results and discussion 3.1. Characterization of as-obtained products The crystal structures and phase compositions of samples were characterized by XRD. As shown in Fig. 1, all the diffraction patterns can be readily indexed to a-MnO2 (JCPDS 44-0141). No peaks are observed for the impurities, indicating the high purity of the final products. Fig. 1a shows that all diffraction peaks get much sharper when the pH was adjusted to 5 by adding certain amounts of HCl, which indicates that the crystallinity of the MnO2 hydrothermally prepared could be improved by decreasing pH. It can be seen from Fig. 1b that the XRD pattern of samples prepared in neutral medium shows a slight shift to the left compared to that presented in Fig. 1a. We propose that this could be induced by intercalation of inorganic cations in the tunnel structure of MnO2. The resultant products prepared without any acids were also characterized by FT-IR (Fig. 2). The bands observed at 3429 and 1634 cm 1 corresponded to the stretching vibration and the bending vibration of O–H group of adsorbed water, respectively [11]. The bands located at ca. 718, 525 and 475 cm 1 can be ascribed to the Mn–O vibrations of [MnO6] octahedron [12]. Notably, the band at 1402 cm 1 indicates the existence of NH4+ in the as-synthesized samples [12], which may explain the slight changes of lattice constant detected by XRD. Fig. 3a shows that clew-like particles coexisting with nanorods were obtained for samples which were fabricated in the absence of

Fig. 2. Infrared spectra of samples prepared without any acids.

HCl. A magnified view of one of the nanospheres reveals that these particles are constructed by sheet-like structures with worm-like structures growing along the edge (Fig. 3b). It is obvious that some of them have grown into nanorods (Fig. 3b). Fig. 3c and d shows that only nanowires with diameters 40–200 nm and lengths up to 4.5 mm formed after adjusting the pH to 5 by HCl. 3.2. Effect of anions and pH Dissolution–recrystallization process which is acidity dependent has been reported in some papers as the formation mechanism of 1D structure [8,9,13–15,23]. While a distinct theory that 1D structure should be formed through the curling process of layer precursors has also been put forward [16–18]. Recently, Gao et al. [10,19] have found that MnO2 nanorods were more likely to be obtained in the presence of anions with smaller radius rather than larger anions, such as SO42 , PO43 , and NO3 , which may prohibit the curling process of lamellar structures. To identify the key role in formation of 1D MnO2 in the present work, a series of parallel experiments was carried out by altering the type of acid with the same pH value. As seen in Fig. 4a and b, pure a-MnO2 was obtained with the introduction of HNO3 or H2SO4. When H3PO4 was added into the system, birnessite-type d-MnO2 was fabricated (Fig. 4c). Weak and wide peaks indicate the poor crystallinity of as-obtained samples. We anticipated that PO43 might hamper the crystallization process of MnOX units. Wang and Li [16] pointed out that a moderate concentration of K+ or NH4+ is necessary to stabilize the lamellar structure of d-MnO2. Notably, the formation of dMnO2 in this paper seemed to be related to the existence of PO43 . Similar phenomenon was also detected by Zhou et al. [20], but how anions like PO43 influence the octahedra framework is still unknown. Fig. 5 shows the morphology of the resulting samples obtained under various synthetic conditions. It is discernible that nanorods were produced when HNO3 and H2SO4 were introduced respectively (Fig. 5a and c). Fig. 5b depicts HRTEM image of a single rod obtained in the system containing HNO3. The lattice spacing is about 0.69 nm, which corresponds to the (1 1 0) plane of a-MnO2 (JCPDS 44-0141). The inset of the image shows the SAED pattern taken from the rod, which suggests the nanorod is single-crystalline. The polycrystalline electron diffraction rings are observed in the SAED pattern (inset in Fig. 5d) taken from an individual nanorod, implying that the nanorod

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Fig. 3. SEM images of MnO2 materials synthesized (a and b) in neutral systems, (c and d) at pH = 5 adjusted by HCl.

synthesized in the system containing H2SO4 is unstable under electron irradiation. Since SO42 and NO3 have been found to prevent the growth of MnO2 nanorods [10,19], pH is believed to play a crucial role in the formation of MnO2 nanorods based on the

above results. While in the case of H3PO4, d-MnO2 nanoflower with diameter from 2 to 5 mm was obtained as shown in Fig. 5e, indicating the structure of MnO2 can be greatly influenced by the type of anions. Although there was certain concentration of SO42 in solution, according to previous research that PO43 favors the formation of MnO2 nanoflowers and only tiny fragments were prepared when SO42 and PO43 coexisted in solution [10,19,20], the existence of PO43 is deemed the principal factor that deduced the formation of MnO2 nanoflowers. Fig. 5f and g shows that the surface of particles was quite similar to that obtained by reaction between KMnO4 and (NH4)2SO4 in neutral medium. We anticipated that the morphological variation in the H3PO4 medium along with the decrease of pH was analogous to that in the HCl medium except that the formation of nanorods in the presence of H3PO4 requires a lower pH value. To verify this, the experiment with the pH value adjusted to about 2 by H3PO4 was carried out. Fig. 6a shows that nanorods of MnO2 with the diameter of 15–30 nm and the lengths up to 1 mm were produced. Some fragments were also clearly observed in Fig. 6b. The corresponding XRD investigation indicates that as-obtained samples can be indexed to pure a-MnO2 (Fig. 7). 3.3. Plausible mechanism

Fig. 4. XRD patterns of nanoproducts synthesized with pH adjusted to 5 by (a) HNO3, (b) H2SO4, and (c) H3PO4, respectively.

Given the higher impact of pH on formation of MnO2 nanorods and the absence of evidence of curling lamellar precursors by SEM


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Fig. 5. SEM and TEM images of the as prepared products in the presence of H2SO4 (a and b), HNO3 (c and d), and H3PO4 (e, f and g), respectively.

Fig. 6. SEM and TEM images of products obtained with pH adjusted to about 2 by H3PO4.

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Fig. 7. XRD pattern of MnO2 materials synthesized at the pH value of 2 with addition of H3PO4.


Because protuberances in the surface could provide highenergy sites for crystal growth [23], these atoms were transferred to the surfaces of the nanospheres and adsorbed onto the edges of the sheets prior to other places. Due to the one-dimensional growth habit of manganese dioxide crystals, these atoms that adsorbed on particles grew into worm-like structures. Such worm-like structures finally developed into nanorods with the continuous dissolution of sphere-like nanoparticles. Nevertheless, MnO2 with 1D structure was not always obtained with the decrease of pH, for instance, in the case of H3PO4. We assume that this is in agreement with theory that adsorption of anions can suppress the dissolution stage [24,25]. Such suppressive effect can be overcome by the deliberate adding of H+, which boosts the solubility of Mn species and eventually promotes the growth stage of MnO2 nanorods. Among the anions investigated above, PO43 has the strongest inhibiting effect on the dissolution of Mn species, and thus only at a relatively lower pH can MnO2 nanorods or nanowires be fabricated (see Fig. 8).

4. Conclusions observation, we propose that nucleation–dissolution–recrystallization mechanism is responsible for the growth of 1D MnO2 [8]. Although the investigation on morphological and structural evolution was not carried out in our research, the growth process of 1D MnO2 can be still proposed based on our experimental results and previous reports [21,22]. First, the MOx units produced from the redox reaction between MnO4 and NH4+ formed the layer structure. Under hydrothermal conditions, these lamellar structures were not thermodynamically stable and assembled into sphere-like particles. Then, the sphere-like nanoparticles gradually dissolved in the solution and generated free MnO2 atoms.

In summary, MnO2 with different morphologies was controllably synthesized, and the co-effect of some anions (SO42 , NO3 , PO43 and Cl ) and pH on the hydrothermal crystallization of MnO2 was investigated systematically. We believe that the growth of 1D MnO2 proceeds by the dissolution–recrystallization mechanism and an opposite effect should exist between anions (SO42 , NO3 , PO43 and Cl ) and pH in the growth of 1D MnO2. When the pH effect is dominant, 1D MnO2 is favored. While when inhibiting effect on the dissolution of Mn species by adsorption of anions prevails, sphere-like particles are more likely to be obtained.

Fig. 8. Schematic illustration of growth process of 1D MnO2.


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Acknowledgments This work was financially supported by the Major State Basic Research Development Program of China (973 Program, 2010CB226905), the Fundamental Research Funds for the Central Universities (10CX04019A) and the National Natural Science Foundation of China (No. 21006128). References [1] [2] [3] [4] [5] [6]

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