Synthesis and characterization of reduced graphene oxide–V2O5 nanocomposite for enhanced photocatalytic activity under different types of irradiation

Accepted Manuscript Synthesis and characterization of reduced graphene oxide–V2O5 nanocomposite for enhanced photocatalytic activity under different types of irradiation Elaheh Aawani, Nafiseh Memarian, Hamid Rezagholipour Dizaji PII:

S0022-3697(18)31635-4

DOI:

10.1016/j.jpcs.2018.09.028

Reference:

PCS 8741

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 17 June 2018 Revised Date:

19 September 2018

Accepted Date: 20 September 2018

Please cite this article as: E. Aawani, N. Memarian, H.R. Dizaji, Synthesis and characterization of reduced graphene oxide–V2O5 nanocomposite for enhanced photocatalytic activity under different types of irradiation, Journal of Physics and Chemistry of Solids (2018), doi: https://doi.org/10.1016/ j.jpcs.2018.09.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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hν ν ≥ Eg

e‒ e‒ e‒ e‒

Reduction •

‒

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Conduction band

Dye

Graphene-V2O5 nanocomposite Eg = 1.6 eV



+•OH Degraded

Dye

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products

products

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h+ h+ h+ h+

Oxidation

Degraded

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Valence band

ACCEPTED MANUSCRIPT Synthesis and characterization of reduced graphene oxide–V2O5 nanocomposite for enhanced photocatalytic activity under different types of irradiation

Elaheh Aawania, Nafiseh Memariana, Hamid Rezagholipour Dizajia,*

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Faculty of Physics, Semnan University, 35195-363, Semnan, Iran

[email protected], [email protected], [email protected]*

Strong photoluminescence quenching in the visible range of the spectrum for

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•

reduced graphene oxide (rGO)-V2O5 nanocomposite compared with V2O5 nanorods •

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was observed.

Photoluminescence quenching was found to enhance charge separation and promote the photocatalytic activity of the rGO-V2O5 nanocomposite.

•

The degradation of methylene blue solution was investigated under different types of irradiation with rGO-V2O5 nanocomposite as a catalyst.

The rGO-V2O5 nanocomposite exhibited sufficient photocatalytic activity for

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•

degradation of methylene blue solution under mercury-lamp, visible-light, and UV-

Abstract

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light irradiation.

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Photocatalytic degradation of methylene blue with use of reduced graphene oxide (rGO)V2O5 nanocomposites under mercury-lamp, visible-light, and UV-light irradiation is investigated. Graphene oxide (GO) synthesized by a modified Hummer’s method was reduced to rGO by chemical reduction with hydrazine hydrate. In addition, V2O5 nanorods and rGO-V2O5 nanocomposite were synthesized by a hydrothermal method. The samples prepared were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM),

Raman

spectroscopy,

UV-vis

diffuse reflectance spectra,

photoluminescence, and UV-vis absorption analysis. XRD analysis of rGO-V2O5 nanocomposite indicated characteristic peaks of GO and V2O5 nanorods. Different nanostructure morphologies of the samples were revealed by FE-SEM. Raman spectroscopy 1

ACCEPTED MANUSCRIPT showed the coexistence of rGO characteristic peaks and different vibrational modes of V2O5. The band gap of V2O5 nanorods (2.26 eV) reduced after graphene decoration to 1.60 eV for rGO-V2O5 nanocomposite. Moreover, unlike V2O5 nanorods, strong photoluminescence quenching in the visible range of the spectrum for rGO-V2O5 nanocomposite was observed. Detailed UV-vis absorption spectroscopy analysis revealed gradual degradation of methylene

degradation

efficiency

for

decolorizing

methylene

blue

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blue with increasing time under illumination with different irradiation sources. The maximum solution

by

rGO-V2O5

nanocomposites is about 85% after 255 min of illumination with a mercury lamp.

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Keywords: V2O5 nanorods, Reduced graphene oxide‒V2O5 nanocomposites, Photocatalytic

1.

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activity, Methylene blue degradation

Introduction

Today, excessive energy consumption leads to energy constraint and environmental pollution, which are considered to be the main global problems faced by humans. Increasing the level of pollution can cause irreversible damage to the environment and human life. The main source

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of water pollution, which in turn causes soil contamination, is industrial wastewater. Pollutants and industrial wastewater enter the underground water network after contaminating soil and surface water. One method of wastewater treatment is photocatalytic degradation that

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eliminates hazardous heavy contaminants [1, 2]. It has many advantages, such as low cost, full mineralization, and a low-temperature process [3]. In addition to its capability of eliminating hazardous contaminants, some other photocatalytic properties recently attracted

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the attention of researchers, such as hydrogen generation and water splitting [4-6]. The best photocatalytic materials, which have an extended surface area and recyclability, are nanosized metal oxides and metal oxide nanocomposites. Graphene, one of the basic structures of carbon allotropes (nanotubes, fullerenes, diamonds), is a single layer of carbon atoms, arranged in a two-dimensional honeycomb configuration. Carbon atoms are bonded in graphene by sp2 hybridization [7]. The band gap of graphene is zero, and the susceptibility of graphene to oxidative environments is its main disadvantage when used as a catalyst alone. Graphene has been synthesized in various ways, including

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ACCEPTED MANUSCRIPT electrochemical exfoliation [8], mechanical exfoliation [9], epitaxial growth [10], chemical vapor deposition [11], and Hummer’s method [12, 13]. Graphene-based nanocomposites, which are also called hybrid or carbon-decorated materials, have a variety of applications [14-17]. One recent application of graphene nanocomposites is as photocatalysts. Photocatalytic activity has been increased by

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combination of metal oxides with carbon-based materials [18-20]. Compared with pure metal oxide, a composite of graphene‒metal oxide has reasonable photocatalytic activity. Because of these advantages, vanadium pentoxide (V2O5) has been intensively investigated as a super candidate among different transition metal oxides: it is cost-effective, has several oxidation

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states, has high-availability resources, has a layer-by-layer structure, and has high energy density [21, 22]. V2O5-based materials have gained significant applications in different fields,

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such as lithium-ion batteries [23, 24], field-effect transistors [25, 26], gas sensors [27], hydrogen generation [28], and supercapacitors [29, 30]. Because of the low band gap of V2O5 (2.2 eV), a composite of graphene nanosheets with V2O5 has good photocatalytic activity. Vanadium oxides exhibit different nanostructures, such as nanorods, nanourchins, nanorods, nanotubes, and nanospikes [31-34]. V2O5 nanoparticles have been synthesized chemically by different methods, such as a sol-gel method [35], electrospinning [36], a hydrothermal

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method [37, 38], chemical vapor deposition [39], flame spray pyrolysis [40], and precipitation [41]. Control of the resistivity of V2O5 has been done by investigation of activated carbon and carbon nanotubes [42].

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In the present work, graphene oxide was prepared by a modified Hummer's method. In addition, V2O5 nanorods produced by a hydrothermal method were used for the preparation of a nanocomposite material. The photocatalytic activity of rGO-V2O5 nanocomposites

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produced by the hydrothermal method was investigated with methylene blue (MB) dye under UV-light, visible-light, and mercury-lamp irradiation.

2. 2.1.

Experimental

Preparation of graphene oxide powder by a modified Hummer’s method

Graphene oxide (GO) was prepared from graphite powder according to a modified Hummer’s method [9]. In this procedure, 69 mL concentrated H2SO4 (Merck, 98%) was poured into a mixture of 3.0 g graphite flakes (Merck, 99.99%) and 1.5 g NaNO3 (Merck, 99.99%), and the solution was stirred for 15 min. Then an ice bath was used to reduce the temperature of the 3

ACCEPTED MANUSCRIPT mixture to 0 °C. In the next step, 9 g KMnO4 (Merck, 99.5 %) was slowly added to the solution to not allow the reaction temperature to exceed 20 °C. The solution was heated to 35 °C and stirred for 7 h. Then 9.0 g KMnO4 was added to the solution, which was heated at 35 °C and stirred for 12 h. The temperature of the mixture was then reduced to 25 °C (room temperature). The solution obtained was added to cold water (400 mL) containing 7 mL H2O2

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(Merck, 30%). After air cooling, the GO suspension was centrifuged (3800 rpm for 12 min) and washed several times with 200 mL of 30% HCl solution (Merck, 37%) and, repeatedly, with 200 mL deionized water and 200 mL ethanol separately. Finally, the GO solution was filtered through a polytetrafluoroethylene (PTFE) membrane filter with 0.2 µm pore size. The

Conversion of GO to rGO

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2.2.

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GO products were dried for 12 h in an oven at 60 °C.

To produce rGO nanosheets, GO was reduced chemically with use of hydrazine hydrate as a reductant. A suspension containing 200 mg GO was dispersed in 100 mL deionized water, which was then ultrasonicated for 30 min. In the next step, 1 mL hydrazine hydrate (Merck, 99.99%) was added to the GO solution. The solution was then heated to 70 °C in a water bath

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with stirring for 80 min continually. The solution obtained was filtered through a PTFE membrane filter and washed three times with deionized water and ethanol to eliminate the

2.3.

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residual adsorption impurities. The rGO nanosheets were dried in an oven for 12 h.

Fabrication of V2O5 nanorods

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For the synthesis of V2O5 nanorods, 0.1 M ammonium metavanadate (NH4VO3; Merck, 99%) was dissolved in 70 mL deionized water, and the solution obtained turned light yellow. The pH of the solution was reduced to approximately 2 by addition of dilute sulfuric acid (1:4 v/v H2SO4/H2O) drop by drop with stirring. The resultant solution was transferred into a Teflonlined stainless-steel autoclave and kept at 150 °C for 18 h and then allowed to cool to room temperature naturally. The precipitates were centrifuged (3000 rpm for 10 min) and washed with deionized water and absolute ethanol repeatedly, and then the powder was collected with a PTFE membrane. The powder was dried at 80 °C for 24 h. The as-prepared sample was heat-treated at 300, 400, and 500 °C for 90 min in a furnace.

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ACCEPTED MANUSCRIPT 2.4.

Synthesis of rGO-V2O5 nanocomposite by a hydrothermal method

We prepared rGO-V2O5 nanocomposite by a simple hydrothermal method. Typically, rGO prepared by the modified Hummer’s method (0.21 g) was suspended in 20 mL deionized water. The suspension was then ultrasonicated for 1 h and stirred well. Then V2O5 nanorods (0.42 g) fabricated by the hydrothermal method were calcined at 400 °C for 90 min and

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suspended in 50 mL deionized water, and the suspension was then ultrasonicated for 1 h. The resultant V2O5 and rGO solutions were mixed together, and then 5 mL of 30% H2O2 was added to this mixture, and the resultant mixture was stirred for 2 h. After stirring, the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and maintained at 150 °C

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for 15 h in an oven. Then the autoclave was allowed to naturally cool to room temperature inside the oven. Precipitates were collected with a PTFE membrane after centrifugation (3500

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rpm for 10 min), washed with deionized water and ethanol, and then dried at 60 °C for 16 h in the oven.

2.5.

Photocatalytic activity of rGO‒V2O5 nanocomposite

The photocatalytic activity of rGO-V2O5 nanocomposite was investigated by photocatalytic

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degradation of MB, a dye normally resistant against biodegradation. The photocatalytic experiments were performed under visible-light (three 40 W visible lamps, wavelength of 450–700 nm), UV-light (three 8 W UV lamps, central wavelength of 360 nm), and mercurylamp (50 W, wavelength of 400 nm) irradiation. The photocatalytic decomposition of MB

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solution (50 ppm) by rGO-V2O5 nanocomposite as a photocatalyst (0.01 g) was performed in a 100 mL beaker at 30 °C with stirring and irradiation by a 50 W mercury lamp, three 40 W

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lamps as the visible-light source, and three 8 W lamps as the UV-light source. Before illumination, the mixture of 50 mL MB and 10 mg rGO-V2O5 nanocomposite was stirred in the dark for 30 min to ensure adsorption-desorption equilibrium of the MB on the surface of the catalyst. Then the suspension was irradiated by mercury-lamp, visible-light, and UV-light sources for different times. All the experiments were performed in similar conditions at a constant temperature of 30 °C. About 3 mL of dye solution was removed at the required time and the absorbance of the remaining MB with photocatalyst solution was measured with a UV-vis spectrophotometer.

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Material characterization

The crystalline structure of the as-prepared specimens was determined by X-ray diffraction (XRD) analysis (Bruker, D8 Advance). The morphology of the samples was investigated by field-emission scanning electron microscopy (FE-SEM; Zeiss, Sigma VP-500). Raman spectroscopy was used to determine the quality of GO, rGO, and rGO-V2O5 nanocomposite

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by use of a Teksan Takram P50C0R10 Raman spectrometer, with an excitation laser source of 532 nm. UV-vis diffuse reflectance spectra (DRS) and photoluminescence (PL) spectra of the samples were recorded with an Avantes AvaSpec 3648 spectrophotometer (with Light source: DHs Deuterium-Halogen for DRS analysis and LED laser (405 nm) as the excitation

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source with an optical filter for PL spectra). UV-vis absorption spectroscopy (PerkinElmer, Lambda 25) of V2O5 nanorods and rGO-V2O5 nanocomposite was performed to compare

4.

Results and discussion

4.1.

XRD analysis

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photocatalytic properties of the samples under different irradiation conditions.

Fig. 1 shows the XRD patterns of V2O5 nanorods calcined at 300, 400, and 500 °C for 90

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min. As seen from the XRD pattern of the as-prepared V2O5 powder (Fig. 1a), peaks due to impurity appear at 2θ values of 9.24°, 11.42°, 22.80°, 40.80°, 57.50°, and 63.51°. Moreover, one may notice that the pure V2O5 phase has not completely formed. From Fig. 1b it is observed that the V2O5 crystalline phase began to appear when the powder was treated at 300

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°C, but a weak peak due to impurity can still be seen at 2θ = 9°. As is clear from Fig. 1b, the intensity of the XRD peaks increased a little on increase of the heat-treatment temperature to

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400 and 500 °C. This increase is attributed to a higher degree of crystallinity of the specimen as a result of the increased calcination temperature, which in turn leads to an increase of grain size. From comparison of the XRD patterns shown in Fig. 1, the specimen calcined at 300 °C is impure. On the other hand, in the case of the specimen calcined at 500 °C, most of the nanorods were converted to nanosheets, and hence the specimen calcined at 400 °C was selected as the best one for preparing the nanocomposite in the present work. After calcination, all samples had an orthorhombic structure with space group Pmmn, matching ICDD card number 01-085-0601.

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ACCEPTED MANUSCRIPT The average crystallite size of V2O5 nanorods was computed with the Debye-Scherrer equation [43‒45]: D=



,



(1)

where β is the full width at half maximum, λ is the wavelength of X-rays (1.5418 Å), K is the

corresponding Bragg angle at the center of the peak.

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Scherrer constant (usually K = 0.9), which is dependent on the crystallite shape, and θ is the

The average crystallite size obtained with Eq. (1) for the samples after calcination at 300,

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400, and 500 °C was 22.8, 29.4, and 29.5 nm respectively.

Fig. 2 shows the XRD patterns of GO prepared by Hummer’s method (HGO), rGO, V2O5 after calcination at 400 °C, and rGO-V2O5 nanocomposite. The XRD pattern of rGO-V2O5

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nanocomposite contains characteristic peaks of rGO and V2O5 nanorods. The peaks related to rGO are indicated by an asterisk in Fig. 2. The extra peaks observed in this sample are due to partially reduced graphene and carbon hexagonal structure, which in turn is a result of nonseparation of graphite sheets in the sample [3, 31].

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4.2. FE-SEM morphological analysis

The morphologies of the samples were studied by FE-SEM. Fig. 3 shows the FE-SEM images of HGO (Fig. 3a), rGO nanosheets (Fig. 3b), V2O5 powder before and after

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calcination at 300, 400, and 500 °C for 90 min (Fig. 3c–f), and rGO-V2O5 nanocomposite synthesized by the hydrothermal method (Fig 3g, h). As seen in Fig. 3b, two-dimensional

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wrinkled rGO nanosheets several micrometers in length are obtained. In addition, Fig. 3b confirms the formation of aggregated and crumpled nanosheets of rGO that stacked together and formed a disordered solid. As shown in Fig. 3c, the as-prepared powder of V2O5 before calcination is in the form of nanorods. On heating at 300 °C (Fig. 3d), the nanorods stick together, and further increase of the temperature leads to more sticking of nanorods to each other (Fig. 3e) and the morphology of the powder becomes a nanosheet after heating at 500 °C (Fig. 3f). The average diameter and length of the V2O5 nanorods before and after calcination are indicated in Fig. 3c–f. The average size of the nanostructures obtained by FESEM is much greater than the crystallite size calculated from XRD spectra because the structures observed in the FE-SEM images usually consist of different areas with various

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ACCEPTED MANUSCRIPT orientations, leading to different crystallite sizes. Fig. 3g and h clearly show the attachment of V2O5 nanorods on rGO nanosheets. It is observed that the rGO nanosheets are decorated with V2O5 nanorods. In the image in Fig. 3g, which was obtained by the back-scattered-reflection method, V2O5 nanorods and rGO nanosheets are indicated by circles, where the latter regions are dark, while the former ones are light. The energy-dispersive spectroscopy graph

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corresponding to the nanocomposite specimen is shown in the inset in Fig. 3h, which shows that the real ratio of rGO to V2O5 remains the same as their initial ratio (1:2) after the hydrothermal process.

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4.3. Raman spectroscopy

To determine the structural quality of HGO, rGO, and nanocomposite samples, Raman

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spectroscopy was performed at room temperature; the results are presented in Fig. 4. As can be seen, the G peak and the D peak, which are characteristic peaks of carbon-based materials, are present in the spectra. The G peak (about 1580 cm‒1) corresponds to the in-plane bond vibration of sp2-hybridized carbon. The D peak (around 1350 cm‒1) is related to the breathing mode of aromatic rings due to the defect in the sample [46]. The ID/IG ratio is often used to

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estimate the sp2 domain size of graphite-based materials. Here ID/IG is slightly increased in the rGO sample with respect to the HGO sample (1.08 and 0.93, respectively). It is generally believed that the oxygen atoms are removed followed by the formation of double bonds during the reduction process [47], which is contrary to the present observation. On the other

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hand, ID/IG decreased to 0.92 for rGO-V2O5 nanocomposite. This could be due to a restoration of graphitic structure of rGO nanosheets and increase of the average size of sp2

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carbon domains during the hydrothermal process [48]. This ratio is much smaller than that reported for rGO-V2O5 nanocomposite (~1.84) [3, 49]. Moreover, the Raman peaks related to V2O5 can be seen in the rGO-V2O5 nanocomposite spectrum. The peak centered at 268 cm‒1 is attributed to the bending vibration of O3–V–O2 (B2g vibrational mode). The peak centered at 514 cm‒1 belongs to the stretching vibration of V–O2 (Ag vibrational mode). The peak at 687 cm‒1 is due to the stretching vibration of V–O1 (B2g and B3g vibrational modes) [49].

4.4. UV-vis DRS analysis

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ACCEPTED MANUSCRIPT The UV-vis DRS of V2O5 powders (calcined at different temperatures) and rGO-V2O5 nanocomposite indicate high absorption in the UV region extending to the visible region (Fig. 5a) as a result of transition from the O 2p valence band to the empty V 3d orbitals in the conduction band [50]. The band gap (Eg) of the V2O5 and rGO-V2O5 nanocomposite powders can be estimated by several methods [51‒53]. Here the optical band gap of the samples is

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calculated by use of Tauc’s equation [54‒57]:

αhυ = Α(hυ−Eg)n/2,

(2)

where α, h, υ, A, and Eg are the absorption coefficient, Planck’s constant, the light frequency,

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a constant value, and the band gap, respectively, and n depends on the characteristics of the transitions in a semiconductor (for an indirect transition n = 1, and for a direct transition n = 4). Plots of (αhυ)2 versus hυ of V2O5 nanorods and rGO-V2O5 nanocomposites are presented

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in Fig. 5b. The Eg values were obtained by extrapolation of the linear portion of the curves to the x axis ((αhυ)2=0).

As observed from Fig. 5a, the absorption edge of the V2O5 nanorod samples calcined at different temperatures falls nearly at the same wavelength with almost the same band gap (2.26 eV). This may be due to the V2O5 phase formation of the calcined specimens. The band

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gap obtained for the rGO-V2O5 nanocomposites is 1.60 eV. Although Fig. 5a indicates higher absorbance of nanocomposite compared with V2O5 nanorods, Fig. 5b indicates the reverse for their band gaps. The latter may be because of the shift of the absorption edge to higher wavelengths for the nanocomposite and hence lowering of the band gap. The values obtained

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are in good agreement with the data reported in the literature [58].

4.5. PL analysis

To evaluate more precisely the optical properties of the samples, their PL spectra were recorded at room temperature, and are shown in Fig. 6. It is clear from Fig. 6 that the PL peak of V2O5 nanorods, covering a broad range between 400 and 800 nm, is in good agreement with calculated band gap from DRS. The peak observed at 570 nm is attributed to the recombination of electron-hole pairs from the V 3d split-off conduction band to the top of the O 2p valence band. The peak observed at 710 nm in the PL spectrum of rGO-V2O5 nanocomposite may be due to an extrinsic transition occurring as a result of the lowest defect donor band to the O 2p valence band recombination [46, 59, 60]. Another observed peak, at 9

ACCEPTED MANUSCRIPT 545 nm, in the PL spectrum of rGO-V2O5 nanocomposite is related to electron transfer from the conduction band of V2O5 nanorods to graphene sheets [61]. The PL peak positions for V2O5 nanorods are in good agreement with the reported data [59-62]. Recently, Pan and Wei [62] fabricated V2O5 nanorods on silicon substrates by a thermal evaporation technique. Since PL is due to electron-hole recombination and the emission of photons, strong PL

radiative recombination of electrons and holes.

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4.6. Photocatalytic performance of the specimens

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quenching of the spectrum for rGO-V2O5 nanocomposite implies a lower efficiency of

The photocatalytic degradation of MB at a concentration of 50 ppm in aqueous solution was performed with V2O5 nanorods and rGO-V2O5 nanocomposite (0.01 g) as photocatalysts

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under ordinary mercury-lamp, visible-light, and UV-light irradiation (Fig. 7). Three 40 W fluorescent lamps, three 8 W UV lamps, and a 50 W mercury lamp were used as the light sources, maintained 15 cm above the MB solution. Fig. 7a shows the degradation of MB with V2O5 nanorods under mercury-lamp irradiation. As shown, MB with V2O5 nanorods as a catalyst has very low degradation. The UV-vis spectra of MB solution revealed degradation

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of MB with increase of time. The height of the absorption peak of MB at 665 nm continuously declines with increasing irradiation time, along with a variation of the color of the solution from deep blue to bright blue. Fig. 7b–d clearly shows the degradation of MB with rGO-V2O5 nanocomposite under mercury-lamp, visible-light, and UV-light irradiation

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for different times.

The results indicates that MB with rGO-V2O5 nanocomposite solution irradiated by a

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mercury lamp for 255 min degraded significantly compared with that irradiated by visible light for the same time.

The increase in photocatalytic activity observed in Fig. 7b–d may be attributed to both the lower band gap of rGO-V2O5 nanocomposite compared with V2O5 nanorods and PL quenching. The latter occurs when the generated charge carriers participate in the dye photodegradation process rather than in the radiative recombination one. The degradation percent (ED) of MB was evaluated by the following equation: ED= (A0‒A)/A0 × 100,

(3)

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ACCEPTED MANUSCRIPT where A0 is initial absorbance of MB solution and A is the absorbance of MB solution after degradation. Fig. 8 shows the degree of degradation of MB solution obtained with V2O5 nanorods and rGO-V2O5 nanocomposite after irradiation under a mercury lamp, visible light, and UV light for different times. Maximum degradation (85% in 255 min) was obtained with mercury-

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lamp irradiation.

The Langmuir-Hinshelwood model [63] has been used to calculate the kinetics of reactions between dye molecules and photocatalysts. Using this model for photocatalytic degradation, one may determine the correlation between the apparent first-order reaction rate constant and

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the initial concentration of the organic substance [64]. Fig. 9 demonstrates that MB degradation follows the pseudo-first-order kinetics given by Eq. (4):

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‒(dC/dt) = KappC,

(4)

where Kapp is the apparent reaction rate constant and t is the reaction time. Integrating this equation (with the restriction C = C0 at t = 0, with C0 being the initial concentration of the solution in the dark) results in the following equation:

(5)

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‒ln(Ct/C0) = Kappt,

where C0 is the initial concentration of MB and Ct is the concentration of MB at reaction time t. The slope of the linear plot gives the apparent first-order reaction rate constant (Kapp) [65]. The calculated apparent reaction rate constants are presented in Fig. 10. As is clear from Fig.

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10, the highest Kapp (0.00784 min-1) belongs to the sample irradiated by the mercury lamp.

4.7 Photocatalysis mechanism The photoreaction mechanism is illustrated in Fig. 11. When an aqueous solution containing photocatalyst and contaminant molecules (here MB) is irradiated by UV or visible light, electron-hole pairs are generated (Eq. (6)). These photogenerated electrons and holes are strong reducing and oxidizing agents, respectively. The generated holes in the valence band react with H2O molecules, resulting in the formation of hydroxyl radical (•OH) as given by Eq. (7) and demonstrated in Fig. 11. The excited electrons in the conduction band react with O2 molecules to form superoxide radical ion (•

) (Eq. (8)). Finally, the hydroxyl radicals,

which are able to oxidize and mineralize the organic molecules, react with MB molecules, 11

ACCEPTED MANUSCRIPT which results in the production of different species, such as carbon dioxide, water, and other decomposition products at extremely low concentrations (Eq. (9)) [66‒69]. rGO-V2O5 nanocomposite + hν → rGO-V2O5 + (ℎ+ + rGO-V2O5 nanocomposite (ℎ+ ) +

→ rGO-V2O5 nanocomposite + •OH

) + O2 → rGO-V2O5 nanocomposite + •O2−

MB + •OH + O2 → CO2 + H2O + other decomposition products

Conclusions

(7) (8) (9)

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5.

(6)

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rGO-V2O5 nanocomposite (

)

We prepared rGO-V2O5 nanocomposite by a hydrothermal method. The V2O5 nanorods and

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GO required for that purpose were prepared by a hydrothermal method in acidic conditions and with use of a modified Hummer’s method, respectively. XRD analysis confirmed the coexistence of rGO and V2O5 phases without any impurity or any other phases. In the FESEM images, well-distributed rGO nanosheets and V2O5 nanorods were observed. Raman spectroscopy showed the ID/IG ratio decreased for rGO-V2O5 nanocomposite, which is attributed to the restoration of the graphitic structure of rGO nanosheets during the

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hydrothermal process. PL quenching was found to enhance charge separation and promote the photocatalytic activity of rGO-V2O5 nanocomposite. The degradation of MB solution was investigated under different irradiation sources—namely, mercury lamp, visible light, and

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UV light—with rGO-V2O5 nanocomposite as a catalyst. The results revealed that the rGOV2O5 nanocomposite exhibited appropriate photocatalytic activity needed for degradation of MB solution when irradiated by those light sources. A photodegradation efficiency of 85%

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was obtained for decolorization of MB solution by rGO-V2O5 under mercury-lamp irradiation for 255 min.

References [1] C.A. Nascimento, A.C. Teixeira, R. Guardani, F.H. Quina, O. Chiavone-Filho, A.M. Braun, Industrial wastewater treatment by photochemical processes based on solar energy, J. Sol. Energy Eng. 129 (2007) 45-52.

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ACCEPTED MANUSCRIPT [2] O. Legrini, E. Oliveros, A. M. Braun, Photochemical processes for water treatment. Chem Rev. 93 (1993) 671-698. [3] M. Shanmugam, A. Alsalme, A. Alghamdi, R. Jayavel, Enhanced photocatalytic performance of graphene-V2O5 nanocomposite in the degradation of methylene blue dye

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under direct sunlight, ACS Appl. Mater. Interfaces 7 (2015) 14905−14911. [4] J. Zhang, L. Wang, X. Liu, X. Li, W. Huang, High-performance CdS–ZnS core–shell nanorod array photoelectrode for photoelectrochemical hydrogen generation, J. Mater. Chem. A 3 (2015) 535‒541.

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[5] J. Zhang, Q. Zhang, L. Wang, X. Li, W. Huang, Interface induce growth of intermediate layer for bandgap engineering insights into photoelectrochemical water splitting, Sci. Rep. 6

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(2016) 27241.

[6] J. Zhang, A. Chen, L. Wang, X. Li, W. Huang, Striving toward visible light photocatalytic water splitting based on natural silicate clay mineral: the interface modification of attapulgite at the atomic-molecular level, ACS Sustain. Chem. Eng. 4 (2016) 4601‒4607 [7] K.S. Novoselov, A.K. Geim, S.V. Morozov, D.A. Jiang, Y. Zhang, S.V. Dubonos, I.V.

(2004) 666–669.

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Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films. Science 306

[8] E. Aawani, H. Rezagholipour Dizaji, N. Memarian, Exfoliation of graphite in various electrolytes and preparation of graphene films via electrochemical approach, Opt. Quantum

EP

Electron. 50 (2018) 129.

[9] M. Yi, Z. Shen, A review on mechanical exfoliation for the scalable production of

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graphene, J. Mater. Chem. A, 3 (2015) 11700-11715. [10] W.A. De Heer, C. Berger, X. Wu, P.N. First, E. H. Conrad, X. Li, T. Li, M. Sprinkle, J. Hass, M.L. Sadowski, M. Potemski, Epitaxial graphene. Solid State Commun. 143 (2007) 92–100.

[11] Y.I. Zhang, L. Zhang, C. Zhou, Review of chemical vapor deposition of graphene and related applications. Acc. Chem. Res. 46 (2013) 2329–2339. [12] K. Parvez, S. Yang, X. Feng, K. Müllen, Exfoliation of graphene via wet chemical routes, Synth. Met. 210 (2015) 123–132.

13

ACCEPTED MANUSCRIPT [13] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide. ACS Nano 4 (2010) 4806–4814. [14] J. Zhang, Q. Wang, L. Wang, X. Li, W. Huang, Layer-controllable WS2-reduced graphene oxide hybrid nanosheets with high electrocatalytic activity for hydrogen evolution,

RI PT

Nanoscale 7 (2015) 10391‒10397.

[15] J. Zhang, R. Cui, X. Li, X. Liu, W. Huang, A nanohybrids coupling of NiPS3 canoparticles and defective graphene as a pH-universal electrocatalysts for efficient hydrogen

SC

evolution, J. Mater. Chem. A 5 (2017) 23536‒23542.

[16] R. Chen, Y. Song, Z. Wang, Y. Gao, Y. Sheng, Z. Shu, J. Zhang, X. Li, Porous nickel disulfide/reduced graphene oxide nanohybrids with improved electrocatalytic performance

M AN U

for hydrogen evolution, Catal. Commun. 85 (2016) 26‒29.

[17] Y. Cui, R. Zhang, J. Zhang, Z. Wang, H. Xue, W. Mao, W. Huang, Highly active and stable electrocatalytic hydrogen evolution catalyzed by nickel, iron doped cobalt disulfide@reduced graphene oxide nanohybrid electrocatalysts, Mater. Today Energy 7 (2018) 44‒50.

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[18] F. Haque, T. Daeneke, K. Kalantar-zadeh, J.Z. Ou, Two-dimensional transition metal oxide and chalcogenide-based photocatalysts, Nano Micro Lett. 10 (2018) 23. [19] R.K. Upadhyay, N. Soin, S.S. Roy, Role of graphene/metal oxide composites as

3823-3851.

EP

photocatalysts, adsorbents and disinfectants in water treatment, a review, RSC Adv. 4 (2014)

AC C

[20] J. Zhang, Z. Xiong, X.S. Zhao, Graphene–metal–oxide composites for the degradation of dyes under visible light irradiation, J. Mater. Chem. 21 (2011) 3634-3640. [21] D.H. Nagaraju, Q. Wang, P. Beaujuge, H.N. Alshareef, Two-dimensional heterostructures of V2O5 and reduced graphene oxide as electrodes for high energy density asymmetric supercapacitors, J. Mater. Chem. A. 2 (2014) 17146-17152. [22] V. Aravindan, Y.L Cheah, W.F Mak, G. Wee, B.V. Chowdari, S. Madhavi, Fabrication of high energy‐density hybrid supercapacitors using electrospun V2O5 nanofibers with a self‐ supported carbon nanotube network, ChemPlusChem. 77 (2012) 570-575.

14

ACCEPTED MANUSCRIPT [23] F. Maroni, A. Birrozzi, G. Carbonari, F. Croce, R. Tossici, S. Passerini, F. Nobili, Graphene/V2O5 cryogel composite as a high-energy cathode material for lithium-ion batteries, ChemElectroChem. 4 (2017) 613–619. [24] F. Huguenin, A.R. Martins, R.M. Torresi, Nanocomposites from V2O5 and lithium-ion

RI PT

batteries, Nanoenergy (2018) 223-249. [25] M. Minagawa, S. Kitamura, A. Baba, K. Shinbo, K. Kato, F. Kaneko, Fabrication and characteristics of field-effect transistors with vanadium pentoxide and copper phthalocyanine multilayers, Appl. Phys. Express 2 (2009) 071502.

SC

[26] G.T. Kim, J. Muster, V. Krstic, J.G. Park, Y.W. Park, S. Roth, M. Burghard, Field-effect transistor made of individual V2O5 nanofibers. Appl. Phys. Lett. 76 (2000) 1875–1877.

M AN U

[27] J.F. Liu, X. Wang, Q. Peng, Y.A. Li, Vanadium pentoxide nanobelts: highly selective and stable ethanol sensor materials, Adv. Mater. 17 (2005) 764‒767. [28] S. Kajita, T. Yoshida, N. Ohno, Y. Ichino, N. Yoshida, Fabrication of photocatalytically active vanadium oxide nanostructures via plasma route, J. Phys. D Appl. Phys. 51 (2018) 215201.

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[29] M. Jayalakshmi, M. Mohan Rao, N. Venugopal, Kwang-Bum Kim, Hydrothermal synthesis of SnO2–V2O5 mixed oxide and electrochemical screening of carbon nano-tubes (CNT), V2O5, V2O5–CNT, and SnO2–V2O5–CNT electrodes for supercapacitor applications,

EP

J. Power Sources. 166 (2007) 578–583.

[30] Y. Wang, H.J. Zhang, W.X. Lim, J.Y. Lin, C. C. Wong, Designed strategy to fabricate a patterned V2O5 nanobelt array as a superior electrode for Li-ion batteries, J. Mater. Chem. 21

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(2011) 2362–2368.

[31] M. Lee, S.K. Balasingam, H.Y. Jeong, W.G. Hong, B.H. Kim, Y. Jun, One-step hydrothermal synthesis of graphene decorated V2O5 nanobelts for enhanced electrochemical energy storage. Sci. Rep. 5 (2015) 8151. [32] Jacques Livage, Hydrothermal synthesis of nanostructured vanadium oxides, Materials, 3 (2010) 4175-4195. [33] W. Avansi Jr., C. Ribeiro, E.R. Leite, V.R. Mastelaro, Vanadium pentoxide nanostructures: an effective control of morphology and crystal structure in hydrothermal conditions, Cryst. Growth Des. 9 (2009) 3626-3631. 15

ACCEPTED MANUSCRIPT [34] C. O'Dwyer, D. Navas, V. Lavayen, E. Benavente, M.A. Santa Ana, G. González, S.B. Newcomb, C.M. Sotomayor Torres, Nano-urchin:  the formation and structure of highdensity spherical clusters of vanadium oxide nanotubes, Chem. Mater. 18 (2006) 3016–3022. [35] K. Ishii, Y. Kimura, T. Yamazaki, Y. Oaki, H. Imai, Evolution analysis of V2O5·nH2O gels for preparation of xerogels having a high specific surface area and their replicas, RSC

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Adv. 7 (2017) 35711–35716.

[36] R. Berenguer, M.O. Guerrero-Pérez, I. Guzmán, J. Rodríguez-Mirasol, T. Cordero, Synthesis of vanadium oxide nanofibers with variable crystallinity and V5+/V4+ ratios, ACS

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Omega 2 (2017) 7739−7745.

[37] Y. Wang, H.J. Zhang, K.W. Siah, C.C. Wong, J. Lin, A. Borgna, One pot synthesis of self-assembled V2O5 nanobelt membrane via capsule-like hydrated precursor as improved

M AN U

cathode for Li-ion battery, J. Mater. Chem. 21 (2011) 10336–10341.

[38] T. Chirayil, P.Y. Zavalij, M.S. Whittingham, Hydrothermal synthesis of vanadium oxides, Chem. Mater. 10 (1998) 2629-2640.

[39] D. Vernardou, M.E. Pemble, D.W. Sheel, The growth of thermochromic VO2 films on glass by atmospheric-pressure CVD: A comparative study of precursors, CVD methodology,

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and substrates, Chem. Vap. Depos, 12 (2006) 263-274. [40] W.Y. Teoh, A perspective on the flame spray synthesis of photocatalyst nanoparticles,

EP

Materials. 6 (2013) 3194-3212.

[41] S.H. Ng, S.Y. Chew, J. Wang, D. Wexler, Y. Tournayre, K. Konstantinov, H.K. Liu, Synthesis and electrochemical properties of V2O5 nanostructures prepared via a precipitation

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process for lithium-ion battery cathodes. J. Power Sources 174 (2007) 1032–1035. [42] M. Zeiger, T. Ariyanto, B. Krüner, N.J. Peter, S. Fleischmann, B.J. Etzold, V. Presser, Vanadium pentoxide/carbide-derived carbon core–shell hybrid particles for high performance electrochemical energy storage, J. Mater. Chem. A, 4 (2016) 18899–18909. [43] H. Wang, L.F. Cui, Y. Yang, H. Sanchez Casalongue, J.T. Robinson, Y. Liang, Y. Cui, H. Dai, Mn3O4−graphene hybrid as a high-capacity anode material for lithium ion batteries, J. Am. Chem. Soc. 132 (2010) 13978‒13980. [44] U. Holzwarth, N. Gibson, The Scherrer equation versus the Debye–Scherrer equation, Nat. Nanotechnol. 6 (2011) 534. 16

ACCEPTED MANUSCRIPT [45] M.L. Ronquillo, P.S. Jacinto, P. Ovalle, L.R. Vázquez, E.C. Martínez, E. Marinero, V. Garibay. Synthesis and structural characterization of monocrystalline α-V2O5 nanowires, Mater. Sci. Appl., 7 (2016) 484-495. [46] M. Beaula Ruby Kamalam, B.K. Balachander, K. Sethuraman, Solvothermal synthesis and characterization of reduced graphene oxide/vanadium pentoxide hybrid nanostructures,

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Mater. Today Proc. 3 (2016) 2132–2140.

[47] H.J. Shin, K.K. Kim, A. Benayad, S.M. Yoon, H.K. Park, I.S. Jung, M.H. Jin, H.K. Jeong, J.M. Kim, J.Y. Choi, Y.H. Lee, Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance, Adv. Funct. Mater. 19 (2009) 1987–

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1992.

[48] S. Pei, J. Zhao, J. Du, W. Ren, H.M. Cheng, Direct reduction of graphene oxide films

M AN U

into highly conductive and flexible graphene films by hydrohalic acids, Carbon 48 (2010) 4466 –4474.

[49] Y. Liu, Y. Wang, Y. Zhang, S. Liang, A. Pan, Controllable preparation of V2O5/graphene nanocomposites as cathode materials for lithium-ion batteries, Nanoscale Res. Lett. 11 (2016) 549

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[50] Y. Li, J.L. Kuang, Y. Lu, W.B. Cao, Facile Synthesis, Characterization of flower-like vanadium pentoxide powders and their photocatalytic behavior, Acta Metall. Sin. Engl. Lett. 30 (2017) 1017–1026.

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[51] M. Dressel, G. Grüner, Electrodynamics of Solids: Optical Properties of Electrons in Matter Cambridge University Press (2002)

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[52] M. Fox, Optical Properties of Solids, Oxford University Press (2001). [53] D. Dasgupta, F. Demichelis, C. F. Pirri, A. Tagliaferro, π bands and gap states from optical absorption and electron-spin-resonance studies on amorphous carbon and amorphous hydrogenated carbon films, Phys Rev. B 43 (1991) 2131. [54] X. Xiao, C. Liu, R. Hu, X. Zuo, J. Nan, L. Li, L. Wang, Oxygen-rich bismuth oxyhalides: generalized one-pot synthesis, band structures and visible-light photocatalytic properties. J. Mater. Chem. 22 (2012) 22840. [55] M. Ye, D. Zheng, M. Lv, C. Chen, C. Lin, Z. Lin, Hierarchically structured nanotubes for highly efficient dye‐sensitized solar cells, Adv. Mater. 25 (2013) 3039‒3044. 17

ACCEPTED MANUSCRIPT [56] M.V. Narayana, S.N. Jammalamadaka, Tuning optical properties of graphene oxide under compressive strain using wet ball milling method, Graphene 5 (2016) 73‒80. [57] F.M. Hassan, V. Chabot, J. Li, B.K. Kim, L. Ricardez-Sandoval, A. Yu, Pyrrolicstructure enriched nitrogen doped graphene for highly efficient next generation

RI PT

supercapacitors, J. Mater. Chem. A 1 (2013) 2904‒2912. [58] F.K. Butt, C. Cao, F. Idrees, M. Tahir, R. Hussain, A.Z. Alshemary, Fabrication of V2O5 super long nanobelts: optical, in situ electrical and field emission properties, New J. Chem. 39 (2015) 5197‒5202.

vanadium oxides. Chem. Mater. 18 (2006) 2787–2804.

SC

[59] Y. Wang, G. Cao, Synthesis and enhanced intercalation properties of nanostructured

M AN U

[60] S. Atzkern, S.V. Borisenko, M. Knupfer, M.S. Golden, J. Fink, A.N. Yaresko, V.N. Antonov, M. Klemm, S. Horn, Valence band excitations in V2O5. Phys. Rev. B 61 (2000) 12792–12798.

[61] K. Siwińska-Stefańska, A. Kubiaka, A. Piasecki, J. Goscianska, G. Nowaczyk, S. Jurga, T. Jesionowski, TiO2-ZnO binary oxide systems: comprehensive characterization and tests of

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photocatalytic activity, Materials. 11 (2018) 841.

[62] K.Y. Pan, D.H. Wei, Optoelectronic and electrochemical properties of vanadium pentoxide nanowires synthesized by vapor-solid process, Nanomaterials 6 (2016) 140.

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[63] T. Ghosh, K. Ullah, V. Nikam, C.Y. Park, Z.D. Meng, W.C. Oh, The characteristic study and sonocatalytic performance of CdSe–graphene as catalyst in the degradation of azo dyes

AC C

in aqueous solution under dark conditions, Ultrason. Sonochem. 20 (2013) 768–776. [64] A. Arabi, M Fazli, M.H. Ehsani, Synthesis and characterization of calcium-doped lanthanum manganite nanowires as a photocatalyst for degradation of methylene blue solution under visible light irradiation, Bull. Mater. Sci. 41 (2018) 77. [65] A. Arabi, M Fazli, M.H. Ehsani, Tuning the morphology and photocatalytic activity of La0.7Ca0.3MnO3 nanorods via different mineralizer-assisted hydrothermal syntheses, Mater. Res. Bull. 90 (2017) 205–211. [66] M.A. Behnajady, N. Modirshahla, R. Hamzavi, Kinetic study on photocatalytic degradation of C.I. Acid Yellow 23 by ZnO photocatalyst. J. Hazard. Mater. 133 (2006) 226– 232. 18

ACCEPTED MANUSCRIPT [67] S. Wang, J.H. Yun, B. Luo, T. Butburee, P. Peerakiatkhajohn, S. Thaweesak, M. Xiao, L. Wang, Recent progress on visible light responsive heterojunctions for photocatalytic applications, J. Mater. Sci. Technol. 33 (2017) 1‒22. [68] N. Soltani, E. Saion, M.Z. Hussein, M. Erfani, A. Abedini, G. Bahmanrokh, M. Navasery, P. Vaziri, Visible light-induced degradation of methylene blue in the presence of

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photocatalytic ZnS and CdS nanoparticles, Int. J. Mol. Sci. 13 (2012) 12242‒12258.

[69] X. Xu, R. Lu, X. Zhao, S. Xu, X. Lei, F. Zhang, D.G. Evans, Fabrication and photocatalytic performance of a ZnxCd1−xS solid solution prepared by sulfuration of a single

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layered double hydroxide precursor, Appl. Catal. B 102 (2011) 147‒156.

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Fig. 1. X-ray diffraction patterns of as-prepared V2O5 nanorods before calcination (a) and after calcination at temperatures of (b) 300 °C, (c) 400 °C, and (d) 500 °C for 90 min. Fig. 2. X-ray diffraction spectra of pristine graphene oxide prepared by Hummer’s method (HGO), reduced graphene oxide (rGO), pristine V2O5, and rGO-V2O5 nanocomposite. Asterisks indicate peaks due to rGO. Pound signs indicate peaks due to V2O5 nanorods.

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Fig. 3. Field-emission scanning electron microcopy images of (a) graphene oxide prepared by Hummer’s method, (b) reduced graphene oxide (rGO), V2O5 nanopowder before calcination (c) and after calcination at (d) 300 °C, (e) 400 °C, and (f) 500 °C for 90 min, and (g, h) rGOV2O5 nanocomposite synthesized by a hydrothermal method, with its energy-dispersive

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Fig. 6. Photoluminescence spectra of V2O5 nanorods and reduced graphene oxide (rGO)V2O5 nanocomposite. Fig. 7. Absorption spectra of methylene blue solutions (50 ppm) for (a) V2O5 nanorods and (b–d) reduced graphene oxide–V2O5 nanocomposite as a photocatalyst under different types of irradiation for different times. 19

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oxide–V2O5 nanophotocatalyst for mercury-lamp, visible-light, and UV-light irradiation. Fig 10. Apparent reaction rate constants of methylene blue degradation for different irradiation sources.

Fig. 11. Charge transfer and photodegradation of methylene blue mechanism with reduced

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Fig. 1. X-ray diffraction patterns of as-prepared V2O5 nanorods before calcination (a) and after calcination at temperatures of (b) 300 °C, (c) 400 °C, and (d) 500 °C for 90 min.

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Fig. 2. X-ray diffraction spectra of pristine graphene oxide prepared by Hummer’s method (HGO), reduced graphene oxide (rGO), pristine V2O5, and rGO-V2O5 nanocomposite. Asterisks indicate peaks due to rGO. Pound signs indicate peaks due to V2O5 nanorods.

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Fig. 4. Raman spectra of graphene oxide prepared by Hummer’s method (HGO), reduced graphene oxide (rGO) nanosheets, and rGO-V2O5 nanocomposite.

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Fig. 5. (a) Diffuse reflectance spectra of V2O5 nanorod powders thermally treated at 400 °C, and (b) (αhυ)2 versus hυ for V2O5 nanorods and reduced graphene oxide (rGO)-V2O5 nanocomposites.

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Fig. 6. Photoluminescence spectra of V2O5 nanorods and reduced graphene oxide (rGO)-V2O5 nanocomposite.

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Fig. 7. Absorption spectra of methylene blue solutions (50 ppm) for (a) V2O5 nanorods and (b–d) reduced graphene oxide–V2O5 nanocomposite as a photocatalyst under different types of irradiation for different times.

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Fig. 8. Degradation percent of methylene blue in the presence of 0.01 g of the reduced graphene oxide–V2O5 photocatalyst under mercury-lamp, visible-light, and UV-light irradiation.

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Fig. 9. Pseudo-first order kinetics plots of methylene blue degradation over reduced graphene oxide–V2O5 nanophotocatalyst for mercury-lamp, visible-light, and UV-light irradiation.

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Fig 10. Apparent reaction rate constants of methylene blue degradation for different irradiation sources.

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Fig. 11. Charge transfer and photodegradation of methylene blue mechanism with reduced graphene oxide–V2O5 nanocomposite.