Formation of Na0.44MnO2 nanowires via stress-induced splitting of birnessite nanosheets

Abstract

High aspect ratio Na 0.44 MnO 2 nanowires with a complex one-dimensional (1 D) tunnel structure have been synthesized. We found that the reaction went through layered birnessite nanosheet intermediates, and that their conversion to the fi nal product involved splitting of the nanosheets into nanowires. Based on our observations, a stress-induced splitting mechanism for conversion of birnessite nanosheets to Na 0.44 MnO 2 nanowires is proposed. The fi nal and intermediate phases show topotaxy with 001 f // 020 b or 110 b where f represents the fi nal Na 0.44 MnO 2 phase and b the intermediate birnessite phase. As a result of their high surface areas, the nanowires are effi cient catalysts for the oxidation of pinacyanol chloride dye. KEYWORDSBirnessite, manganese oxide, nanowire, nanosheet, stress, conversion Manganese oxide nanowires with layered or tunnel structures are attractive for applications in batteries, separation and catalysis due to their open-framework structures and interesting redox properties [1 9]. There have been several studies of their synthesis in the literature: for example, MnO 2 nanorods were synthesized through a carefully controlled hydrothermal reaction by oxidizing MnSO 4 with (NH 4 ) 2 S 2 O 8 or KMnO 4 [10,11]. Nanowires with cryptomelane [12], romanechite [6,13], and RUB 7 [13 15] structures have also been produced in aqueous solution. Most of these approaches involved a birnessite phase observed as an intermediate [11] or used as precursor [6, 13 15]. Birnessite possesses a layered structure formed of edge-sharing MnO 6 octahedra with Na + cations and H 2 O molecules filling the interlayer space ( Fig. 1 (a)). Conversion of birnessite has been used as a general strategy to obtain a variety of tunnel structures [1]. However, the underlying mechanism remains unclear. Recently, the curling or rolling up of birnessite layers as a result of weakened interlayer interactions during hydrothermal treatment was proposed [11]. While this mechanism can explain tube formation from exfoliated molecular sheets, it cannot rationalize the formation of non-hollow manganese oxide nanowires.Herein we report a stress-induced splitting mechanism that transforms birnessite nanosheets into Na 0.44 MnO 2 nanowires. Na 0.44 MnO 2 has a complex tunnel structure. It consists of columns of edge-sharing MnO 5 square pyramids and sheets of edge-sharing MnO 6 octahedra extending parallel to the c-axis (Fig. 1 (b)). They are connected by cornerNano Res (2009) sharing to form two types of tunnels: large S-shaped, and six-sided tunnels, both occupied by Na + cations.In our study, Na 0.44 MnO 2 nanowires were synthesized by a hydrothermal method. Their X-ray diffraction (XRD) pattern shows diffraction peaks in good agreement with the standard pattern (PDF No. 27 0750), except for some variation in peak intensities due to nanowire orientation ( Fig. 2(a)). An scanning electron microscopy (SEM) image reveals that they are pure nanowires with average length over 10 μm and diameter smaller than 100 nm (Fig. 2(b)...

Shape-controlled synthesis of Mn3O4 nanocrystals and their catalysis of the degradation of methylene blue

“…1(a)) indicated that the shape of the resulting nanocrystals was almost octahedral, but they are tetragonally distorted due to the Jahn-Teller effect on Mn 3+ ions [29]. Edge-shared [MnO 6 ] octahedral units [8,30,31] originate from the reaction at the nonbridging hydroxyl groups of adjacent micellar particle surfaces and form a chemical bond. Their construction results in agglomeration of most of the nanocrystals.…”

Structural-controlled synthesis of manganese oxide nanostructures and their electrochemical properties

“…in a basic or dilute acidic medium or by electrochemical conversion from Mn 3 O 4 thin films [21,[24][25][26][27]. In particular, hydrothermal treatment of Mn 2 O 3 or Mn 3 O 4 powders in NaOH solution leads to the production of Na-bir nanobelts [26], or tunnel structure Na 0.44 MnO 2 nanowires [27].…”

“…A second hydrothermal treatment was adopted to investigate the stability of birnessite nanowires in NaOH solution [27]. The birnessite nanowires were mixed again with 20 mL of 6 M NaOH solution and heated at 180 • C for another 72 h. The XRD profile (Fig.…”

Flux synthesis and growth mechanism of Na0.5MnO2 whiskers

“…The FFT pattern of the nanodisc shows six-fold symmetry, whereas the whisker shows rectangular symmetry. These indicate that the nanodiscs and whiskers are Na-birnessite and Na 0.5 MnO 2 , respectively [14]. In Fig.…”

“…Na 0.44 MnO 2 was early synthesized via high temperature solid state reaction of stoichiometric Na 2 CO 3 and Mn 2 O 3 [9,12,13]. Recently, Na 0.44 MnO 2 nanowires with high aspect ratio have been synthesized using hydrothermal method [7,14]. The size of the crystals obtained by the above methods is small with diameters below 500 nm.…”