Polyvinyl Pyrrolidone-Assisted Synthesis of Crystalline Manganese Vanadate Microtubes

Manganese vanadate microtubes have been synthesized by a facile polyvinyl pyrrolidone-assisted hydrothermal route. X-ray diffraction pattern confirms that the microtubes are composed of monoclinic MnV 2 O 6 , tetragonal V 2 O 5 and orthorhombic MnO 2 phases. The outer diameter and inner diameter of the microtubes are about 300 nm-3 μm and 200 nm-1 μm, respectively. The tube wall thickness of the microtubes is about 50 nm-1 μm. The possible formation process of the manganese vanadate microtubes has been proposed as a polyvinyl pyrrolidone-assisted growth mechanism.


Introduction
Recently, great research interest has been devoted to functional materials with solid rod-shaped structures and hollow tubular structures owing to their distinctive physical, chemical properties and potential application in the nanoscale devices 1,2 .Efforts have also been made to synthesize rod-shaped and tubular vanadate structures because of their potential applications in the fields of lithium batteries, sensors and photocatalysis 3 .Different rod-shaped and tubular vanadate structures, such as LiV 3 O 8 nanorods 4 , silver vanadate nanorods 5,6 , cerium vanadate nanorod arrays 7 , FeVO 4 nanorods 8 and bismuth vanadate nanotubes 9 have been synthesized by hydrothermal route, nanoporous anodic aluminum oxide template via sol-gel method and wet chemical process.
Manganese (Mn) vanadate, as a kind of important transition metal vanadate, has been researched extensively for lithium ion rechargeable batteries 10,11 .Mn vanadate powders have been prepared by solid state reaction process 12 , solution method 13,14 .Tubular Mn vanadate instead of bulk Mn vanadate may show novel physical and chemical properties due to special tubular structure for efficient electron transport and confinement effect.Therefore, it is important to synthesize crystalline tubular Mn vanadate by a facile route for the research of novel physical and chemical properties.
Functional materials with special morphologies, such as alumina nanotubes 15 , AlOOH nanotubes 16 , ZnO nanowires/nanorods 17,18 and copper nanowires 19 have been synthesized by a facile hydrothermal route using different surfactants.Surfactants can be used as versatile soft templates for the formation of functional materials with different morphologies.Polyvinyl pyrrolidone (PVP) is a kind of important surfactant which can assist the growth of functional materials with different morphologies [20][21][22] .In the paper, crystalline Mn vanadate microtubes have been successfully synthesized via a facile PVP-assisted hydrothermal route using sodium orthovanadate (Na 3 VO 4 ) and Mn acetate (Mn(CH 3 COO) 2 •4H 2 O) as the raw materials, PVP as the surfactant.The possible growth process of the Mn vanadate microtubes has been discussed.

Experimental
Na 3 VO 4 (AR grade, purity: ≥99.9%) and PVP (AR grade) were purchased from Aladdin Reagent Co., Ltd. of China.Mn(CH 3 COO) 2 •4H 2 O (AR grade, purity: ≥99.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd. of China.In a typical procedure, Na 3 VO 4 , Mn(CH 3 COO) 2 •4H 2 O and PVP were dissolved in 60 mL distilled water.Then, the mixture was placed into a 100 mL autoclave with a Teflon liner.The autoclave was maintained at 180 °C for 24 hours.Subsequently the autoclave was cooled naturally in air.The resulting black precipitates were filtered, washed with distilled water for several times and dried at 60 °C in air.Finally, the black Mn vanadate powders were gained.
The synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), infrared spectroscopy (IR) and photoluminescence (PL) spectrum.XRD pattern was carried out on a Bruker AXS D8 X-ray diffractometer with Cu-Kα radiation (λ = 1.5406Å).The products were scanned at a scanning rate of 0.05 °/s in the 2θ range of 10° ~ 80°.SEM observation was performed using nova nanoSEM FEI 430 SEM.TEM and HRTEM observations were performed using JEOL JEM-2100 TEM with a GATAN digital photography system.IR spectroscopy (Perkin Elmer PE, WQF-410 spectrometer) was used at room temperature in the range of 400-4000 cm -1 .PL measurement was carried out at room temperature using 212 nm as the excitation wavelength with a luminescence spectrometer (Cary Eclipse) in the range of 350-600 nm.

Results and Discussion
Figure 1a displays the SEM image of the Mn vanadate products grown from 180 °C for 24 hours using PVP as the surfactant.It is observed that the Mn vanadate products are composed of a large quantity of tubular structures with the length of about several dozens of micrometers.Some microtubes can be seen obviously which is designated by white arrows.No other structures are observed besides the tubular structures.The outer diameter and inner diameter of the Mn vanadate microtubes are about 300 nm to 3 µm and 200 nm to 1 µm, respectively.The tube wall thickness of the microtubes is about 50 nm to 1 µm.The cracked microtube also shows the curved structure (designated by black arrow).The morphology of the Mn vanadate microtubes is similar to that of ZnO microtubes [23][24][25] and BiVO 4 microtubes 26 .The results indicate that the PVP-assisted hydrothermal route is an effective method for preparing Mn vanadate microtubes.To analyze the role of the PVP on the formation of Mn vanadate microtubes, the experiment under same synthesis conditions without PVP was performed.The SEM image of the products is shown in Figure 1b.It is interesting that only irregular particles with the sub-microscale size are obtained.The irregular particles are very different from those obtained from the same hydrothermal conditions using PVP as the surfactant.Generally, rod-like Mn vanadate containing crystal water can easily obtained by hydrothermal method.For example, Inagaki et al. 27 and Morishita et al. 12 reported the synthesis of rod-shaped MnV 2 O 6 using Mn(CH 3 COO) 2 and V 2 O 5 with a metal ion concentration of 0.01-1.0mol.L -1 at 135-200 °C under hydrothermal conditions.However, only irregular Mn vanadate particles are obtained without PVP.In our experiment, Na 3 VO 4 is used as the V raw material instead of V 2 O 5 and PVP is used as the surfactant.PVP and Na 3 VO 4 are considered to have the essential roles on the formation of the Mn vanadate microtubes under present hydrothermal conditions.
The composition of the microtubes has been analyzed using energy dispersive spectrometer (EDS) equipped in the FESEM.Figure 2b is the EDS spectrum of the microtubes corresponding to the white square in Figure 2a.It is clear that the Mn vanadate microtubes are composed of Mn, V and O.
The phase of the Mn vanadate microtubes is identified by XRD which is shown in Figure 3a.Most diffraction peaks can be indexed to monoclinic MnV The phase is totally different from that obtained using PVP as the surfactant.The result shows that the PVP induces the phase transformation of the products from irregular particles to microtubes.
More structure formation of the Mn vanadate microtubes can be provided by TEM observations.Figure 4 is the TEM and HRTEM images of the Mn vanadate microtubes.From the general TEM image of the Mn vanadate microtubes (Figure 4a), the Mn vanadate microtubes with smooth surface exhibit the diameter of less than 3 µm and length of several dozens of micrometers.The morphology and size of the Mn vanadate microtubes are similar to those observed by SEM observation.Obviously, TEM image at the tip of the Mn vanadate microtube is shown in Figure 4b, c  the tubular structure.The inner diameter and outer diameter are about 300 nm and 500 nm, respectively.The tube wall thickness of the microtube is about 50 nm.HRTEM analysis on the Mn vanadate microtubes may provide more structural information which may help to analyze the crystalline structure.However, the thickness of the Mn vanadate microtubes prevents the HRTEM observation.So only HRTEM image of the microtubes at the tip is measured which is shown in Figure 4d.The HRTEM image obviously shows that the microtubes have good crystalline structure.
The IR spectrum at 400-4000 cm -1 of the Mn vanadate microtubes obtained from 180 °C for 24 hours is shown in Figure 5.The broad absorption bands at 2800-3800 cm -1 with the absorption peaks of 3494 and 3423 cm -1 are the characteristic stretching vibration of −OH originated from water.The absorption peaks at 1647, 1463 and 1294 cm -1  2013; 16 (1) are assigned to the C=O stretching, CH 2 bending and C-N stretching vibration, respectively in the PVP 28 .The absorption peak at 713 cm -1 contributes to the Mn-O vibration of the products 29 .Yamaguchi et al. 30 reported that YVO 4 synthesized by the sol-gel procedure showed vibrations of V-O bonding at 870, 820 and 430 cm -1 .IR spectrum of LaVO 4 also exhibited the vibration of V-O bonding at 432 cm -1 reported by Manca and Baran 31 .The absorption peaks located at 883, 812 and 426 cm -1 are very close to those reported by above literatures.Therefore, these absorption peaks at 883, 812 and 426 cm -1 are assigned to the vibration of V-O bonding.
The room temperature PL spectrum of the Mn vanadate microtubes (Figure 6) shows violet and blue light emission centered at 425 nm and 492 nm, respectively.A broad band emission from 400 to 700 nm was observed from PL spectrum of metavanadates AVO 3 (A=K, Rb and Cs) 32 .Broad band emission spectrum between 400 and 800 nm from the M 2 V 2 O 7 (M=Ca, Sr and Ba) was also reported by Nakajima et al. 33 The origin of the luminescence of M 2 V 2 O 7 phosphors may be the charge transfer transition from the oxygen 2p to vanadium 3d orbitals in the VO 4 tetrahedra 34 .The violet and blue light emission centered at 425 nm and 492 nm of the Mn vanadate microtubes are very similar to those of the above literatures.The Mn vanadate microtubes are mainly composed of monoclinic MnV 2 O 6 and V 2 O 5 phases besides MnO 2 phase.Therefore, the PL spectrum of the Mn vanadate microtubes is   synthesized by adding PVP under hydrothermal conditions.Therefore, PVP is considered to be a structure-directing agent for the growth of the Mn vanadate microtubes.PVP is a long chain polymer with each pyrrolidone unit chemically bonded to a polyethylene main chain 47 forming PVP micelles.The PVP micelles are filled with polyethylene chains and entrapped water which may have a good stability to solubilize nanoparticles.Under the hydrothermal conditions, MnV 2 O 6 nanoparticles are generated and crystallize to form MnV 2 O 6 nuclea.Hence MnV 2 O 6 nanoparticles can exist either in water or in the PVP micelles.PVP which is used as a structure-directing agent plays a crucial role in controlling the distribution of MnV 2 O 6 nanoparticles in the hydrothermal solution and leads to the formation of Mn vanadate microtubes following a polyol-assisted formation mechanism 48,49 .Only irregular particles are generated without PVP.However, a plenty of micelles and Mn vanadate nanoparticles are filled in the PVP micelles.PVP micelles alter the surface energies of the Mn vanadate surfaces to promote the selective anisotropic growth of crystals leading to the formation of Mn vanadate microtubes.

Conclusions
In summary, novel crystalline Mn vanadate microtubes have been synthesized via a facile hydrothermal route in the presence of PVP.The Mn vanadate microtubes with the length of about several dozens of micrometers are composed of monoclinic MnV 2 O 6 , tetragonal V 2 O 5 and orthorhombic MnO 2 phases.The outer diameter and inner diameter of the Mn vanadate microtubes are about 300 nm-3 µm and 200 nm to 1 µm, respectively.The tube wall thickness of the microtubes is about 50 nm to 1 µm.PVP-assisted hydrothermal route is potentially extendable to other inorganic tubular materials.considered to be originated from V-O bonding.The emission centered at 425 nm is also considered to be the overtones of the excitation wave, which is similar to that reported by McGinley et al. 35 However, the PL spectrum is very noisy.The samples present OH groups in the structure of material.This may be principal effect that luminescence is being suppressed.In addition, No PL peaks are observed from MnO 2 .So MnO 2 in the Mn vanadate microtubes is also considered to contribute to the suppression of the luminescence.
The analysis on the formation mechanism of Mn vanadate microtubes is very important to understand the synthetic methods for the formation of tubular structure.Several models, such as curving followed by seaming of molecular layers 36 , helical nanobelt-twist-join-growth process 37 and rolling mechanism for the conversion from nanosheets to nanotubes 38 have been proposed which can not explain the formation process of the Mn vanadate microtubes.Recently, Mo et al. 39 reported the synthesis of β-Mn 2 V 2 O 7 microtubes with a length of 15-25 µm, 2.5-3.5 µm external diameter and 0.4 µm wall thickness, as well as β-Mn 2 V 2 O 7 hollow microspheres in a suitable molar ratio of NH 3 VO 3 and MnCO 3 powders via a hydrothermal process without surfactants.They contribute to the tubular morphology caused by layered oxide structure.However, different from the raw materials of NH 3 VO 3 and MnCO 3 reported by Liu et al. 37 , Na 3 VO 4 , Mn(CH 3 COO) 2 and PVP are used as the raw materials in our experiments.Therefore, the formation of the Mn vanadate microtubes may take a different formation process.In fact, crystalline MnV 2 O 6 powders with irregular particles and rod-like particles have been synthesized via a hydrothermal process using Mn(CH 3 COO) 2 and V 2 O 5 as the raw materials without surfactants 27,40 .It is well known that surfactant-assisted reaction to control the nucleation and growth is a simple and effective method.The surfactant molecules can modulate the kinetics of the crystal growth and determine the subsequent morphology of the product 20,[41][42][43] .
In recent years, some research groups [44][45][46] reported that the surfactants could alter the surface energy of various crystallographic surfaces to promote selective anisotropic growth of crystals.Only Mn vanadate microtubes can be 2 O 6 phase (JCPDS card, No. 40-0165).Some diffraction peaks of tetragonal V 2 O 5 (JCPDS card, No. 45-1074) and orthorhombic MnO 2 phase (JCPDS card, No. 39-0375) are also indexed besides monoclinic MnV 2 O 6 phase.The V 2 O 5 and MnO 2 phases may originate from the residue decomposed from Na 3 VO 4 and Mn(CH 3 COO) 2 .The XRD result shows that the Mn vanadate microtubes are composed of monoclinic MnV 2 O 6 , tetragonal V 2 O 5 and orthorhombic MnO 2 phases.The XRD pattern of the irregular particles obtained without PVP (Figure 3b) shows that the irregular particles are composed of orthorhombic MnV 2 O 5 phase (JCPDS card, No. 51-0203).

exhibitingFigure 1 .
Figure 1.(a) SEM image of the Mn vanadate microtubes obtained from 180 °C for 24 hours using PVP as the surfactant, (b) SEM image of the samples obtained from 180 °C for 24 hours without PVP.

Figure 3 .
Figure 3. (a) XRD pattern of the Mn vanadate microtubes obtained from 180 °C for 24 hours using PVP as the surfactant, (b) XRD pattern of the samples obtained from 180 °C for 24 hours without PVP.

Figure 2 .
Figure 2. (a) SEM image of the Mn vanadate microtubes, (b) EDS spectrum of the microtubes corresponding to the white square in Figure 2a.

Figure 4 .
Figure 4. (a) General TEM image of the Mn vanadate microtubes, (b) and (c) TEM image of single Mn vanadate microtube with different magnifications showing the typical tubular structure, (d) HRTEM image of the Mn vanadate microtube.

Figure 6 .
Figure 6.PL spectrum of the Mn vanadate microtubes and MnO 2 .