All of the chemical reagents were analytical grade. They were purchased from Acros Organics and used without further purification. The detailed for the synthesis was as follows. GO nanosheets are synthesized from pure graphite which is treated with KMnO₄ and H₂SO₄ according to the modified Hummer’s method [18] . The obtained GO was used as the starting precursor for the synthesis of chemically reduced graphene oxide (rGO) by hydrazine hydrate (N₂H₄-2H₂O) at 40˚C with stirring for 6 h [19] [20] .

VO-NPs were synthesized using a hydrothermal procedure. In a typical synthesis addition of 0.75 ml of sulphuric acid, H₂SO₄, into an aqueous suspension of 0.45 g of V₂O₅ was followed by the addition of N₂H₄-2H₂O. After stirring thoroughly, the solution’s color changed from yellow (V⁵⁺ valence state) to blue, indicating the reduction of V⁵⁺ to yield V⁴⁺ ions in solution. The pH of the resultant strongly acidic blue VO²⁺ solution was then adjusted to pH ≈ 5 by adding NaOH solution. Heat could also be applied to the solution while adding NaOH solution. A gray to brown precipitate was formed during the addition of NaOH solution. The hydrothermal synthesis was carried out in a Teflon-lined autoclave at 230˚C for about 48 h. In this step, the synthesized VO-NPs had nanobelts like shape.

The same steps were followed to prepare VO-NPs/Graphene nanocomposites, the appropriate amount (1:1 in molar ratio) by adding graphene solution to the prepared solution before running the hydrothermal synthesis. All samples were filtered and washed with water and ethanol to remove the organics residues and then dried at 60˚C for few hours.

In this study, XRD patterns were obtained from a Rigaku Smart Lab system using Cu Ka (λ = 1.54178 Å), transmission electron microscopy (TEM) is employed to observe the morphology of rGO, VO-NPs and rGO/ VO-NPs nanocomposites, ATR-FTIR spectra were obtained using a Broker Vertex 70 with ATR system, Raman spectra were obtained using a Bruker Multi Ram with and excitation wavelength of 1064 nm, Uv-Vis (reflectance/absorbance) spectra were carried out using PerkinElmer LAMBDA 1050 UV/Vis/NIR spectrometer along with 150 mm integrating sphere (PerkinElmer, Inc., Shelton, CT USA). Finally, conductivity was measured with a Picoammeter/Voltage Source-Keithley Model 6487 system.

The structural information for rGO, VO-NPs and rGO/VO-NPs are shown in (Figure 1). The XRD pattern of rGO (Figure 1(a)) exhibited a broad peak at 2θ = 23.5˚; compared with characteristic peak of GO at 2θ = 11˚ in our previous work confirming that GO was completely reduced by hydrazine solution [18] . The diffraction peak at about 2θ = 43˚ is associated to the (100) plane of the hexagonal carbon structure [21] . Figure 1(b) shows the XRD pattern of the as-prepared vanadium oxide nanoparticles. This pattern revealed the existence of the two phases of vanadium dioxide (VO₂) and tetravanadium septoxide (V₄O₇). For VO₂strong diffraction peaks were observed at Bragg angles 20.16˚, 33.03˚, 36.35˚, 41.65˚, 43.97˚, 46.59˚, 49.79˚, 61.29˚ and 81.79˚ are assigned to the (110) () () () (012) () () (310) and) planes; this series can be indexed to the monoclinic (b) crystalline phase (Space group 14: P121/c¹) of VO₂ with lattice constants a = 5.849727 Å, b = 4.600944 Å, c = 5.474888 Å and β = 122.599998 (ICDD card N˚ 9009089). Furthermore, the diffraction peaks at Bragg angles 24.61˚, 25.53˚, 30.98˚, 33.03˚, 34.63˚, 36.35˚, 41.65˚, 43.97˚, 46.59˚, 49.79˚, 61.29˚ and 81.79˚, which they could be assigned to the (102) () () () () () () () () (115) () and () planes, which can be readily indexed to the triclinic crystalline phase (Space group 2:A⁻¹) of V₄O₇ with the lattice constants a = 5.456640 Å, b = 6.941393 Å and c = 12.139513 Å (α = 95.099998, β = 122.599998, γ = 109.250000) (ICDD card N˚ 1008024), revealing that the V⁵⁺ ions in V₂O₅ have been reduced to V⁴⁺ ions by the hydrazine in the reaction.

In the XRD patterns of rGO/VO-NPs nanocomposites (Figure 1(c)) shows the existence of three phases, vanadium dioxide (VO₂), Hexavanadium tridecaoxide (V₆O₁₃) and Duttonite (H₂VO₃). The diffraction peaks of VO₂were observed at 20.08˚, 33.88˚, 36.25˚, 40.82˚, 49.30˚, 64.36˚, 81.70˚; this peaks could be assigned to the (110) (130) (021) (121) (211) (002) and (171) planes, and they can be perfectly indexed to the orthorhombic crystalline phase (Space group 62:Pbnm) of VO₂ with the lattice constants a = 5.0077 Å, b = 9.3563 Å and c = 2.8964 Å (α = 90.0000, β = 90.0000, γ = 90.0000); this is in agreement with the reported values (ICDD card N˚ 9000071). In the case of V₆O₁₃ diffraction peaks were observed at 26.94˚, 29.78˚, 33.88˚, 36.25˚, 44˚, 45.72˚, 49.30˚, 51.49˚, 58.20˚, 64.36˚, 81.70˚ which could be assigned to the (300) () (302) () (410) () (020) (502) (321) (224) and () planes; this can be readily indexed to the monoclinic (b) crystalline phase (Space group 7: P1c¹) of V₆O₁₃ with the lattice constants a = 10.0764 Å, b = 3.7161 Å and c = 11.9817 Å (α = 90.0000, β = 100.9140, γ = 90.0000) (ICDD card N˚ 2104009). For H₂VO₃ the diffraction peaks were observed at 20.08˚, 24.55˚, 26.94˚, 29.78˚, 33.88˚, 36.25˚, 40.82˚, 45.72˚, 49.30˚, 51.49˚, 58.20˚, 81.70˚ which could be assigned to the (200) (110) (011) (002) (21-1) (20-2) (400) (020) (31-2) (40-2) (321) and (620) planes, which can be readily indexed to the monoclinic(b) crystalline phase (Space group 15:I12/c1) of H₂VO₃ with calculated lattice constants a = 8.8268 Å, b = 3.9620 Å and c = 5.9781 Å and β = 90.6700, which corresponds to H₂VO₃; this is compatible with the standard value of ICDD card N˚ 9011014 already described in the literature.

The effects of rGO sheets on the morphologies of vanadium oxides nanoparticles have been investigated. Figure 2 presents typical TEM images of rGO, VO-NPs and the resulting nanocomposites. Figure 2(a) shows TEM images of rGO which revealed a wide and flat individual graphene sheets with a few folds along their length

without any discernible roughness. In fact, this confirms that graphene sheets were fully exfoliated. Figure 2(b) shows a typical panoramic TEM image of the VO-NPs film reveals that the vanadium oxides nanoparticles are made of the homogenous phase with uniform particles which display nanobelts morphology. The nanobelts with very smooth and flat surfaces have a typical length of up to 1 μm and a width of about 9 - 14 nm. Figure 2(c) shows an important modification on the morphology of these nanostructures were prepared in the presence of graphene nanosheets. The growth of VO-NPs, in this case, leads to a rod-like shape with very smooth and flat surfaces and a typical length of up to 100 nm and a width of about 9 nm. These results support the spectroscopic investigations and confirm the effect of graphene sheets on the growth of vanadium oxide nanostructures.

ATR-FTIR spectroscopy is one of the powerful tools for detecting the species on the nanostructures surface. Figure 3 provides the infrared spectra of rGO, VO-NPs and rGO/VO-NPs nanocomposites. In Figure 3(a), the spectrum of rGO has two peaks: the first at 1630 cm⁻¹ originated from in plane C=C bands and the skeletal vibration of the graphene sheets and the second peak located at 1770 cm⁻¹ may be due to the C=O stretching of the remaining carboxylic groups placed at edges of rGO or CO₂ [18] . As we can see VO-NPs (Figure 3(b)) infrared spectrum has multiple bands, the two ones at 617 and 670 cm⁻¹ are associated to the characteristic V-O-V octahedral bending modes [22] . Moreover, the peak located at 893 cm⁻¹ is attributed to the coupled vibration V=O and V-O-V [23] . The stretching vibration of the V=O (vanadyl) bond is observed at 990 cm⁻¹ while the band located at 1070 cm⁻¹ is assigned to the oxidation state from V⁵⁺ to V⁴⁺ [23] .The ATR-FTIR spectrum of the nano-

composites prepared from rGO and VO-NPs is shown in Figure 3(c).The vibration modes of vanadium oxide are generally identified in the region below 1000 cm⁻¹. The vibrational bands observed at 668 cm⁻¹ and 750 cm⁻¹ regions can be attributed to (V-O-V) as reported for vanadium oxide [24] .The characteristic peak at 910 cm⁻¹ corresponds to V⁴⁺ terminal oxygen bonds (V⁴⁺=O) and the peak at 1005 cm⁻¹ which corresponds to V⁵⁺ terminal oxygen bonds (V⁵⁺=O) becomes stronger, [22] -[25] . In addition, the spectrum contain sharp band at 1057 cm⁻¹ corresponding to C-O mode [26] , and two overlapping bands located at 1285 and 1350 cm⁻¹ are assigned as vibrations corresponding to the C-C-C angle and epoxides C-O-C, respectively [26] [27] . This may be due to the functionalization with oxygen functional groups onto the surface of rGO indicating the re-oxygenation of rGO nanosheets after the coating with vanadium oxidenanostructures. This may lead to the nucleation of VO-NPs on the surface of graphene layers which will affect the growth of these metal oxide nanoparticles. The spectrum revealed also absorption bands at around 2940 and 3126 cm⁻¹ assigned to the stretching and bending modes of the CH₂ and N-H vibrations that exist in the rGO after reduction with hydrazine while the peak observed at 3360 cm⁻¹ can be attributed to the O-H stretching.

Raman spectroscopy is a powerful and nondestructive tool to characterize carbonaceous materials because of their high Raman intensities, further information on the structure of rGO, VO-NPs and rGO/VO-NPs nanocomposites were obtained from the associated Raman spectra. As shown in Figure 4(a), the rGO Raman spectrum is characterized by the G band located at 1600 cm⁻¹ corresponding to the first-order scattering of the E_2g phonons and the D band around 1330 cm⁻¹ arising from the breathing mode of k-point phonons of A_1g symmetry [18] . In addition G band represent the in-plane stretching of ordered sp² bonded carbon atoms while D band is ascribed to defects such as the density of impurity, disordered carbon and oxygen-containing functional groups on graphene [28] [29] . Besides this, rGO spectrum exhibit another broadband around 2600 - 2800 cm⁻¹ corresponding to 2D band which is the result of a two-phonon lattice vibrational process and is often referred to the rGO number of layers [29] . The spectrum of VO-NPs displays a series of Raman band in the range of 200 - 1000 cm⁻¹ (Figure 4(b)), which is due to the various vibrations of V-O type. In fact, the Raman spectrum shows the existence of a band at 287 cm⁻¹ which could be attributed to V-O-V in the bending mode [30] . The two peaks located at 339 and 618 cm⁻¹ are associated to the vibrational modes B_g and A_g respectively [31] . The peaks of weaker intensity which could be attributed to V-O-V bridging modes were observed at 407 cm⁻¹ and 485 cm⁻¹ [30] -[32] . The peak at 524 cm⁻¹ is attributed to the triply coordinated oxygen (V₃-O) stretching mode of edge-shared oxygen in common with three pyramids [33] [34] . In addition, the peak at 688 cm⁻¹ corresponds to

the doubly coordinated oxygen (V₂-O) stretching mode, which is due to corner-shared oxygen common to two pyramids [33] [34] . Furthermore, the peak at 982 cm⁻¹ could be attributed to V=O stretching vibrations [30] which are in agreement with the ATR-FTIR. The Raman spectrum of the nanocomposites prepared from rGO and VO-NPs is shown in Figure 4(c). As we can see, the series of the intensity of Raman band in the range of 200 - 1000 cm⁻¹ becomes more intense, which indicated a clear composite interaction between vanadium oxide nanobelts and rGO nanosheets. Below 1000 cm⁻¹, the vibrational Raman active mode of rGO/VO-NPs can be described in terms of vanadium-oxygen stretching modes, vanadium-oxygen-vanadium bending vibrations, and translational modes [35] . The new two peaks appear at 818 and 907 cm⁻¹ are assigned to the B_g vibrational mode and the presence of hydrated VO-NPs nanorods. In addition, the two peaks located at 998 and 1096 cm⁻¹ becomes very intense compared with vanadium oxide nanobelts peaks, these results may be due to the change of size and or shape of VO-NPs which confirms the TEM observations. In addition, the area ratio of sp³ to sp² for rGO and rGO/VO-NPs is 1.38 and 1.13, respectively. The intensity ratio I_D/I_G which corresponding to the sp³/sp² hybridization ratio decreased to 1.13 compared with that of rGO (1.38). This change can be explained by the increase of graphitic domain created in rGO/VO-NPs nanocomposites. Therefore, the Raman results are consistent with the formation of rGO/VO-NPs nanocomposites.

Figure 5 shows the absorbance spectra of rGO, VO-NPs and rGO/VO-NPs nanocomposites in the wavelength range 200 - 1000 nm. The spectrum of reduced graphene oxide (Figure 5(a)) has an absorption peak at 270 nm. This absorption peak is attributed to n−π* transition of C-O bonds now embedded by exfoliation and intercalation on the graphene [18] . Compared with graphene oxide, the red shift from 230 to 270 nm is due to the change in the electronic configuration after the reduction process from GO to rGO [18] . Generally, the GO absorbance spectrum is known by the presence of three principal peaks, the main absorbance peak attributed to the π−π* transitions of C-C occurs at around 230 nm, the broad absorption spectra indicating a well-defined band-edge in the Uv-Vis-NIR energy range and a shoulder around 320 nm may be attributed to π−π* transitions of C-O. The transformation of GO in our sample is confirmed by the disappearance of C-C band centered at around 230 nm and its shifting to 270 nm upon reduction likely due to the decrease in the concentration of carboxyl groups indicating that the electronic conjugation within the reduced graphene sheets was revived upon reduction of graphene oxide [18] . The spectrum of VO-NPs (Figure 5(b)) has two bands located at around 253 nm and 370 nm, the first should be assigned to charge transfer transition, involving oxygen and vanadium (IV) in tetrahedral coordination, present in isolated species [36] [37] . The second broad band at 370 nm is attributed to octahedral

V⁴⁺ species [36] . Moreover, the Uv-Vis spectrum of rGO/VO-NPs nanocomposites shown in (Figure 5(c)) presents the existence of four bands. The intense one is located at 273 nm can be attributed to the absorption of graphene layers. The second one is due to the absorption of VO-NPs. Moreover, the band at 490 nm can be attributed to the electron transfer from oxygen atoms to vanadium in octahedral coordination [37] . Indeed, the origin of this change of spectral band position was suggested to be the contribution of a quantum size effect in the VO-NPs [37] [38] . Finally, the band at 690 nm is attributed to d-d transition of V⁴⁺ [39] .

The reflectance spectra of VO-NPs and the rGO/VO-NPs nanocomposites films on glass substrates measured in the wavelength range 200 - 1200 nm are shown in Figure 6. Reflectivity change was observed distinctly in the wavelength region 200 - 400 nm and slightly above 400 nm. The reflectance of the VO-NPs films has exceeded that of the rGO/VO-NPs nanocomposites films in the Uv-Vis-NIR region. The VO-NPs films switches from high reflectance (78%) to lower reflectance (49%) after the introduction of rGO, clearly showing the strong effect of rGO layers on the growth of vanadium oxide nanostructures which leads to a change in the form, the shape, and the phase. Hence, the reduced graphene oxide has an important effect on the stoichiometry and crystallinity of VO-NPs in the composite which have an influence on the optical properties.

Figure 7 shows a comparison of the I-V characteristics of VO-NPs and the rGO/VO-NPs nanocomposites. The virtually linear I-V curve obtained from VO-NPS film presents the ohmic behavior of the contact. At a negative voltage, the value of the current at which conductivity starts its linear behavior is −120 (μA). With an increase of the applied voltage, the increment in the curent reduces the occupancy of defect states, which is due to a large number of charge carriers available for conduction. For the rGO/VO-NPs nanocomposites films we observed that the conductivity reduced compared with VO-NPs films which is due to an increase in the mobility of the charge carriers after the introduction of graphene sheets, also these results can be due to the modification on the morphology and VO-NPs structure from nanobelts to nanorods which was confirmed by the TEM observations. Furthermore, the introduction of rGO sheets showing in one hand a large number of charge carriers available for conduction and in the other hand that the number of electronic defects decreases in the rGO/VO-NPs nanocoposites so that the mobility increases. However, these results show that the principal reason for a good electrical conductivity of rGO/VO-NPs nanocomposites films is decreasing the number of electronic defects in a linear

way. Hence, graphene can provide the conductive pathway to improve the electrical conductivity of vanadium oxides nanoparticles.