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2021 : Volume 1, Issue 1

One - Step Green Synthesis of V2O5 and VO2 Using Extract of Orange Peel: Effect of Calcination Atmosphere

Author(s) : Hanaa M. Abuzeid 1 , Aisha M. Moustafa 2 and Ahmed M Hashem 1

1 Department of Inorganic Chemistry , National Research Centre , Egypt

2 Department of Solid State Physics , National Research Centre , Egypt

Glob J Chem Sci

Article Type : Research Article

DOI : https://doi.org/10.53996/2769-6170.gjcs.1000106

Abstract

In this work two vanadium oxides (V2O5 and VO2) were prepared by green synthesis method using ammonium vanadate (NH4VO3) and extract of orange peel. V2O5 and VO2 have various applications in lithium-ion battery, sensors, transistors, photo catalysis, supercapacitors and electrochromic devices. The effect of calcination atmosphere is obvious as calcination the precursor of (NH4VO3) and extract of orange peel in air at 450? yielded V2O5 while calcination in vacuum at the same temperature gave the reduced form VO2 as estimated by X-ray powder diffraction (XRD). These two different phases of vanadium oxides have different morphologies. Investigation by transmission electron microscope revealed that V2O5 oxide which synthesized in air at 450? showed big particles with laminar shape. The particles look arranged layer over layer with sizes exceed 200 nm. While images of VO2 prepared in vacuum at the same temperature showed different shape and size for their particles. The neutral atmosphere provided by vacuum not only gives reduced form of vanadium oxide (VO2) but also minimizes and changes their particles size and shape, respectively. Unidentified shape of nano sized particles were observed for this reduced oxide. These particles have sizes less than 100 nm and much lower than those observed for V2O5 particles.

Keywords

V2O5; VO2; Green synthesis; Laminar morphology

Introduction

Nano-sized materials especially transition metal oxides (TMOs) have great scientific interests due to their various application in lithium-ion battery, sensors, transistors, photocatalysis, supercapacitors and electrochromic devices [1,2]. Among these TMOs, vanadium compounds have been widely used in various applications. These compounds have good activity, noticeable thermal resistance, accessibility and high stability in the catalytic processes and can be synthesized easily [3]. Vanadium as an element can exist in different stable valence states and hence can form various oxides e.g. VO, V2O3, VO2 and V2O5 [4,5]. Due to their novel physical and chemical properties, vanadium pentoxide (V2O5) and vanadium dioxide (VO2) have received much attention in materials science, electrical engineering and physics communities [6,7]. The most stable oxide is vanadium pentoxide (V2O5) [8-10]. V2O5 is an interesting material as it has unique layered structure with rather good electrical and optical properties. At room temperature V2O5 has an electronic conductivity of order 0.5 Scm-1 and large optical energy gap in the visible region of 2.5 eV [11,12]. In addition, V2O5 is abundant, cost effective, rather environmental benign material, thermal stable oxide and has good electrochemical performance as cathode as it can guest cations and has excellent interaction for ions or molecules [13-15]. Bulk V2O5 suffers from low electrical conductivity and significant volume expansion upon cycling as cathode in lithium batteries [16]. To alleviate such drawback of this bulk oxide, various methods have been developed to synthesize nano sized V2O5 [17-19]. On contrary to bulk V2O5, Nano sized one could provide large surface area with more available active sites which improved its electrical conductivity [20]. V2O5 can be synthesized in various morphologies e.g., Nano ribbons, nano sheets, nanowires, nanorods, nano-urchins, nanotubes, and nanospikes [21-23]. VO2 has at least six polymorphs two of them are thermodynamically stable phases e.g., rutile VO2(R) (P42/mnm) and monoclinic VO2(M) (P21/c) [24-26]. Other metastable four phases are tetragonal VO2(A) (P42/ncm) [27], monoclinic VO2(B) (C2/m) [28], paramontroseite VO2(P) (Pbnm) [29], and VO2(D) (P2/c) [30], respectively. This VO2 oxide can be used in many applications in switches for computing [31], thermochromic windows [32] and sensor applications [33]. Synthesis techniques, annealing atmosphere, pressure, phase, vanadium concentration, pH, and fabrication method can affect the structural properties, optical, electrical and the oxidation states of vanadium [34]. V2O5 or VO2 can be synthesized by different methods by controlling the valence of vanadium by using annealing atmospheres of air or vacuum, respectively [35]. The two oxides of vanadium, V2O5 and VO2 were prepared traditionally by different chemical techniques e.g., sol-gel method [36,37], hydrothermal method [38,39] and chemical vapor deposition [40,41]. These methods are not scalable due to its high cost, restrict conditions and low yield. So, development of low cost, facile and scalable synthesis methods is still a great challenge [42]. On the contrary to the traditional synthesis methods, a green synthesis does not require high temperature, grinding steps as the products can be formed directly in the aqueous medium and sometimes required just drying using low temperature without consuming high energy [43]. Green synthesis methods for large scale production using extracts of plants attracted great attention as eco-friendly, facile, cost-effective and non- harmful technique [44-47]. These methods do not require environmentally hazardous synthetic reducing agents. Also, these methods do not require external capping and stabilizing agents as the extract of plant has dual roles as reducing and capping agent. This ability enables the large-scale production of nanoparticles within short time and consumed less energy [48]. Waste of orange peel has high reductive compounds, such as aldehydes, citrates and phenolic and flavonoid groups [49]. In addition, this waste contains ascorbic acid and citric acid which are used as stabilizing (capping) and reducing agents in synthesis of nanoparticle [49]. Green synthesis methods consumed the huge amount of waste produced from industrial factories especially orange peel which used to synthesize MnO2 nanoparticles [50,51] Here in this work, we use extract of orange peel to synthesize two kinds of vanadium oxides within one step green synthesis method. Besides using this green synthesis method, we study the effect of calcination atmosphere on the synthesis of these oxides. Ambient or air atmosphere yields V2O5, while neutral or reductive atmosphere yields VO2. These two oxides have different phases and different morphologies as detected from investigation by X-ray diffraction (XRD, Rietveld refinement of XRD data and transmission electron microscope (TEM).

Experimental

V2O5 can be prepared easily by thermal decomposition of ammonium vanadate (NH4VO3) in the air atmosphere. Also, VO2 could be synthesized by the same thermal decomposition but in an inert atmosphere. Usually, thermal decomposition gives rather big particles of prepared oxides either V2O5 or VO2 with less homogeneity. So simple modification was introduced to give nano sized V2O5 or VO2 using the same ammonium vanadate (NH4VO3) as described below. In this study vanadium oxides (V2O5 and VO3) were prepared by sol-gel green synthesis method using extract of orange peel as a chelating agent for ammonium vanadate (NH4VO3). The extract of orange peel was obtained from boiling the cleaned slices of the peel for 10 minutes at 100? and then filtrate the solution to obtain the clear extract. 4.2 g of ammonium vanadate was dissolved in 100 ml water at 60°C followed by the addition of the prepared extract of orange peel drop by drop with stirring. The gel was formed through gentle heating with change in color from white to dark green. The dried dark green xerogel (precursor) was divided into two portions, one part was calcined in air and the other was calcined under vacuum at 450°C for 5 hrs. As we previously stated we aim to benefit from the huge quantity of orange peel to prepare different polymorphs of vanadium oxides by facile and cost-effective method. This will reflect positively on the environment by using the waste of orange peel to prepare these kinds of oxides which have various applications by this economical method. The crystal structure of the final product was determined by XRD using Philips expert apparatus equipped with a CuKα X-ray source (λ= 1.54056 Å) in the 2θ range of 10-80°C. Further Rietveld refinement was carried out for the obtained XRD data using a full-proof standard analytical method that is originally developed to refine the crystal structure. The as-prepared samples were subjected to investigation by transmission electron microscopy (TEM, JEM-2100, JEOL, Ltd).

Results and discussion

Calcination atmosphere has an effect on the structure and particle size of the prepared oxides. Ambient or air atmosphere yielded V2O5 oxide while calcination under neutral atmosphere yielded suboxide of vanadium (VO2). Also, the size and shape of particles are altered and affected by the using calcination atmosphere as we will discuss on the following sections. The diffraction patterns analyzed with expert High Score Plus program software of the prepared vanadium oxides synthesized at 450°C in air and under vacuum are shown in figure (1). As observed the sample calcined in air at 450°C has sharp peaks (high intensity and small width) which express its large crystallite size. By inspecting the diffraction pattern of this sample, it is indexed as pure single phase V2O5 (Shcherbinaite) with orthorhombic structure. The characteristic peaks of this oxide are located at 2? positions 15.382, 20.281, 21.713, 26.158, 31.013 and 31.153. These peaks are corresponding to the reflections 200, 001, 101, 110 301 and 400 planes, respectively and matched with the ICDD card no. 01-074-4605 without additional peaks related to any other phases. When we try to scrutinize the X-Ray diffractogram of the sample heat treated in vacuum at 450°C, other three phases were detected. The first phase is the monoclinic VO2 (major) matched with the ICDD card no. 98-00-0199. The second one is a minor tetragonal phase of Rutile structure of VO2 matched with the ICDD card no. 98-002-4926. The last one is a minor with stoichiometry VO which matched with the ICDD card no. 96-900-8767. It is clear that this sample suffers from polymorphism and different valences of vanadium oxide, because of that, the heating atmosphere affecting the oxidation state. Rietveld refinement method was used to confirm the formation of these phases and quantitative phase analysis. Rietveld method is the best method to study the crystal structure of these compounds. This method is a standard analytical method that is originally developed to refine the crystal structure. This refinement is obtained through a model which describes the scattering intensity from polycrystalline sample at each point in XRD. So, the Rietveld method is capable of extracting detailed information form the XRD data. Moreover, it is the best method for quantitative phase analysis, accurate unit cell dimensions, internal microstructure (crystallite size and micro strain). A closer look on the XRD shown in (Figure 1), it is appeared that the sample heated in the air atmosphere has higher peak intensities so we expect that the fitting could be done more accurately. However, the other sample heated under vacuum at the same temperature suffers from low intensity peaks and the background noise. This sample contains more than one phase. These properties make the Rietveld refinement calculations much more defy. At the commencement the instrumental constants were determined using standard Quartz sample. To refine the crystal structure of the first compound heated in air, the space group was Pmmn and the vanadium atom occupies the 4f position, the oxygen atoms occupy the 4f and 2a positions. The obtained refined parameters for this sample heated in air are given in. The quality of the refinement detected from the low value of goodness of-fit (c2)parameter as it close to one as shown in (Table 1).


Figure 1: X-Ray diffraction patterns of vanadium oxides samples prepared in (a) air, (b) vacuum at 450°C.

Property

V2O5

x

y

z

O1 4f

0.1049

0.25

0.5322

O2 4f

0.0718

0.25

-0.0012

O3 2a

0.25

0.25

-0.0127

V 4f

0.1008

0.25

0.8945

S.G.

Pmmn

a

11.5114(1)

b

3.56327(4)

c

4.37510(4)

?=?-?

90

Volume

179.459(3) Å3

Calculated Density

3.36568 g/cm3

Microstrain

0.4093(3)

Crystallite size nm

287(2)

Rwp

42.8

Rexp

38.5

c2

1.235

(V – O1)

1.58580(1)

(V – O1)

2.79040(3)Å

(V – O2)

2.03859(2)

2 x (V – O2)

1.87179(1)

V-O3

1.76484(2)


Table 1: Refined parameters of sample prepared in air at 450°C (V2O5). Goodness of fit index (c2)=(Rwp/Rexpected)2.

V2O5 crystallizes in primitive orthorhombic unit cell related to the space group Pmmn no (59). The unit cell contains two formula units, the refined values of lattice constants are a=11.5114(2) Å, b= 3.5633(1) Å and c = 4.3751(1) Å, that is in assent with results of many authors [52, 53]. The orthorhombic distortion is given by (a-b)/(a+b) was found to be 0.5272. The micro strain was found to be=0.409(8), while the crystallite size = 287(1) nm as expected due to the high intensity and sharp peaks as shown in (Figure 2).

 

Figure 2: Rietveld refinement profile fitting for the V2O5 sample prepared in air at 450°C.

In the crystal structure of Pmmn of V2O5, vanadium atom is a six-fold coordinated and forming octahedron with different oxygen atoms that have different types of bonding. The vanadium pentoxide is a layered compound, which bonded habitually by weak van der Waals (vdW) interactions. This weak interaction between V2O5 layers is easy to schism at the (001) plane [54, 55]. Moreover, vanadium atom has robustly correlated d electrons in its valence shell. (Figure 3a) shows a view along b axis of V2O5 Pmmn which composed of distorted VO6 octahedra structures. The vanadyl oxygen atom O1 (apical) and the VO6 octahedra linked from the brims by series of oxygen atoms (O2) and from the corners by bridging oxygen atoms (O3). The atomic positions and the crystal structure parameters are listed in table 1. (Figure 3b) shows the coordination polyhedral of O around which partake brinks to form tortuous double chains along [001] and are cross-linked along [100] through partake corners, thus forming slabs in the zz plane.


Figure 3a: A view along b axis of V2O5.


Figure 3b: A view along c axis of V2O5.

It is well known that, vanadium dioxide (VO2) exhibited the metal to insulator phase transition due to different phases with different crystal structures. So, the second sample heat treated under vacuum at 450°C contains different phases. (Figure 4) represents the Rietveld refinement fitting of the VO2 which has three phases as observed from the qualitative phase analysis. The obtained results of Rietveld refinement of VO2 are tabulated in (Table 2). From this table it is clear that c2 = 1.064 is close to unit which proves the quality of the refinement. From this table it is clear also that the quantitative phase analysis revealed the percentage of the two polymorphs of VO2. These percentages recorded for monoclinic insulator phase and (tetragonal) rutile metallic phase is 69.2% and 18.1%, respectively and the percentage of VO was 12.7%.


Figure 4: Rietveld refinement profile fitting for the sample prepared in vacuum at 450°C.

The obtained major monoclinic phase with space group C2/m (No.14), and refined unit cell parameters a = 12.0582(3)Å, b = 3.6893(1)Å, c = 6.4187(3)Å, ?° = 106.961(5)Å as given in table 2 are in agreement with the results of M. Saini et al and H. Qiu et al [56, 57]. Phase transition from the highly symmetrical quadrilateral structure, to the lower symmetry monoclinic structure was observed by W. Chen et al [58] during the synthesis of vanadium dioxide nanorods by hydrothermal method. Two differing V–V bond lengths are formed, 0.312 nm and 0.265 nm.

Property

Polymorph 1 (Monoclinic) VO2

Polymorph 2 (Rutil) (Tetragonal)VO2

VO

Weight Percentage%

69.2

18.1

12.7

a Å

12.0582(3)

4.5272(2)

4.055(1)

b Å

3.6893(1)

4.5272(2)

4.055(1)

C Å

6.4187(3)

2.8793(2)

4.055(1)

?°

106.961(5)

 

 

Volume Å3

273.130(2)

59.013(5)

66.676(4)

Crystallite size nm

79(3)

653(8)

76(1)

Microstrain

0.73(2)

0.98(4)

0.59(2)

Rwp

56.2

Rexp

54.4

c2

1.068


Table 2: Refined parameters of sample prepared in vacuum at 450°C (VO2)

Refinement results of the first minor polymorph of VO2 (tetragonal) rutile metallic phase (R) are tabulated in table 2. Rutile VO2 with a space group of P42/mnm has refined unit cell parameters a = b = 4.5324(2) Å, c =2.7825(2) Å which agreed with results of Z. Shao et al [59]. In VO2 (R) crystal, the tetravalent vanadium ions reside at both of body centers of the tetragonal structure, and each V4+ ion besetment with 6O2- ions comprise an octahedral VO6 unit. The nearest V-V atoms distance is equal to 0.287 nm in the z-axis direction. Unique physical properties of VO2 by reason of its polymorphism due to the rearrangement of atoms inside the unit cell encourage M. A Rodriguez et al [60] to study the transformation temperature of insulator monoclinic to the metallic tetragonal rutile phase using In-Situ high temperature XRD and they obtained pure tetragonal VO2 Rutile single phase at 70°C. The third phase in the second sample was the low valence VO phase which crystallized in the rock salt structure, space group Fm-3m and the refined cubic unit cell parameter was a=4.055(1) Å. From the refined parameters of the second sample tabulated in table 2 it is clear that the micros train values of the three phases were, 0.73(2), 0.98(4) and 0.59(2), respectively which indicated that the tetragonal rutile metallic phase has a large number of defects due to the transformation from high symmetry to lower symmetry phase (Figures 5 and 6).

 

Figure 5: TEM images at two magnifications for V2O5 sample prepared in air at 450°C.