Graphene Oxide Concentration Effect on the Optoelectronic Properties of ZnO/GO Nanocomposites

In this work, the effects of graphene oxide (GO) concentrations (1.5 wt.%, 2.5 wt.%, and 5 wt.%) on the structural, morphological, optical, and luminescence properties of zinc oxide nanorods (ZnO NRs)/GO nanocomposites, synthesized by a facile hydrothermal process, were investigated. X-ray diffraction (XRD) patterns of NRs revealed the hexagonal wurtzite structure for all composites with an average coherence length of about 40–60 nm. A scanning electron microscopy (SEM) study confirmed the presence of transparent and wrinkled, dense GO nanosheets among flower-like ZnO nanorods, depending on the GO amounts used in preparation. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–Vis) absorption spectroscopy, and photoluminescence (PL) measurements revealed the impact of GO concentration on the optical and luminescence properties of ZnO NRs/GO nanocomposites. The energy band gap of the ZnO nanorods was independent of GO concentration. Photoluminescence spectra of nanocomposites showed a significant decrease in the intensities in the visible light range and red shifted suggesting a charge transfer process. The nanocomposites’ chromaticity coordinates for CIE 1931 color space were estimated to be (0.33, 0.34), close to pure white ones. The obtained results highlight the possibility of using these nanocomposites to achieve good performance and suitability for optoelectronic applications.


Characterization
The crystal structure of the composites was determined with an X-ray diffractometer (Rigaku Smart Lab system) using CuKα radiation (λ = 1.54178 Å). A field-emission scanning electron microscope (FESEM) JEOL Model JSM 6390F, (JEOL USA, Inc., Peabody, MA, USA) working in high and low vacuum from 0.5 to 30 kV accelerating voltage, equipped with an LaB6 cathode, InLens, and SE2 detectors, and an energy-dispersive X-ray spectrometer (EDX) (Bruker Nano GmbH, Berlin, Germany) were used to characterize the surface morphology of the samples. Raman and FTIR spectroscopic studies were performed in order to analyze the chemical composition in the nanocomposites employing a BRUKER-RFS27 FT-Raman spectrometer (Bruker Optik GmbH, Bremen, Germany) and a PerkinElmer FTIR spectrometer (PerkinElmer, Inc., MA, USA) respectively. A PerkinElmer LAMBDA 45 UV/Vis/NIR spectrometer (PerkinElmer, Inc., MA, USA) was used for measuring the specular transmission. The steady state PL measurements were done using 325 nm UV excitation at room temperature using an Edinburgh FL 920 photoluminescence spectrometer (Livingston, UK) with double monochromators and a 450 W Xe lamp as the excitation source.

Characterization
The crystal structure of the composites was determined with an X-ray diffractometer (Rigaku Smart Lab system) using CuKα radiation (λ = 1.54178 Å). A field-emission scanning electron microscope (FESEM) JEOL Model JSM 6390F, (JEOL USA, Inc., Peabody, MA, USA) working in high and low vacuum from 0.5 to 30 kV accelerating voltage, equipped with an LaB6 cathode, InLens, and SE2 detectors, and an energy-dispersive X-ray spectrometer (EDX) (Bruker Nano GmbH, Berlin, Germany) were used to characterize the surface morphology of the samples. Raman and FTIR spectroscopic studies were performed in order to analyze the chemical composition in the nanocomposites employing a BRUKER-RFS27 FT-Raman spectrometer (Bruker Optik GmbH, Bremen, Germany) and a PerkinElmer FTIR spectrometer (PerkinElmer, Inc., MA, USA) respectively. A PerkinElmer LAMBDA 45 UV/Vis/NIR spectrometer (PerkinElmer, Inc., MA, USA) was used for measuring the specular transmission. The steady state PL measurements were done using 325 nm UV excitation at room temperature using an Edinburgh FL 920 photoluminescence spectrometer (Livingston, UK) with double monochromators and a 450 W Xe lamp as the excitation source.

XRD Analysis
The XRD analysis was carried out to investigate the crystal structure and phase composition of the ZnO nanorods and ZnO NRs/GO nanocomposites with different concentrations of GO ( Figure 2 [21,22]. The average crystallite size of ZnO NRs was calculated using the Debye-Scherrer equation of the (101), (002), (101) diffraction planes. The average size of ZnO nanorods within nanocomposites is slightly smaller for increasing GO concentration in comparison with that of ZnO NRs; this might be due to the cleavage of some bigger nanorods during the hydrothermal process [23]. Furthermore, the intensity of the peaks corresponding to ZnO NRs decreased from ZnO NRs to ZnO NRs/GO nanocomposites, which is obviously visible from the XRD pattern and may be due to the functionalization of GO by ZnO nanorods and the decreases in the crystallite size of ZnO.

XRD Analysis
The XRD analysis was carried out to investigate the crystal structure and phase composition of the ZnO nanorods and ZnO NRs/GO nanocomposites with different concentrations of GO ( Figure  2) [21,22]. The average crystallite size of ZnO NRs was calculated using the Debye-Scherrer equation of the (101), (002), (101) diffraction planes. The average size of ZnO nanorods within nanocomposites is slightly smaller for increasing GO concentration in comparison with that of ZnO NRs; this might be due to the cleavage of some bigger nanorods during the hydrothermal process [23]. Furthermore, the intensity of the peaks corresponding to ZnO NRs decreased from ZnO NRs to ZnO NRs/GO nanocomposites, which is obviously visible from the XRD pattern and may be due to the functionalization of GO by ZnO nanorods and the decreases in the crystallite size of ZnO. The variation of lattice parameters of ZnO NRs with different GO concentrations can be calculated by means of a hexagonal lattice relation [24] as follows:  The variation of lattice parameters of ZnO NRs with different GO concentrations can be calculated by means of a hexagonal lattice relation [24] as follows: where d is the interplanar distance and h, k, and l denote Miller indices; a and c refer to the lattice constants of the hexagonal structure. The average crystallite size (D), or more precisely the coherent lengths along a crystallographic direction, of ZnO NRs can be calculated using the Debye-Scherrer equation [25] as follows: where k is a dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at half height of the maximum intensity (FWHM), and θ is the Bragg angle. The d-spacing values for ZnO nanoparticles were calculated using Bragg's law (Equation (3)): where n is a positive integer (equal to 1), λ is the wavelength of the incident X-ray beam (λ = 1.54178 Å), and θ is the angular position of the hkl reflection. The values of lattice constants and crystallite sizes were determined accordingly and are summarized in Table 1. This 101 interplanar spacing was found to slightly decrease, because of a smaller number of oxygen-containing groups produced by the one-pot hydrothermal treatment during ZnO NRs/GO growth [26,27]. Obviously, GO layers are roughly transparent and, as their amount increases, the layers get thicker because of their tendency to be aggregated by van der Waals forces [28] (Figure 3d). It should also be noted that SEM micrographs show similar morphological features for different ZnO amounts in nanocomposites, depending on the initial GO concentration. By increasing the GO concentration from GO1 = 1.5 wt.% to GO3 = 5 wt.%, the density of the ZnO nanorods decreased. In addition, SEM images of nanocomposites revealed few-layered graphene oxide sheets, no folds with a flat surface, which indicates a high quality of GO. These results indicate the formation of ZnO/GO nanocomposites. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 18 presence of several GO nanosheets among flower-shaped ZnO nanostructures. Obviously, GO layers are roughly transparent and, as their amount increases, the layers get thicker because of their tendency to be aggregated by van der Waals forces [28] (Figure 3d). It should also be noted that SEM micrographs show similar morphological features for different ZnO amounts in nanocomposites, depending on the initial GO concentration. By increasing the GO concentration from GO1 = 1.5 wt.% to GO3 = 5 wt.%, the density of the ZnO nanorods decreased. In addition, SEM images of nanocomposites revealed few-layered graphene oxide sheets, no folds with a flat surface, which indicates a high quality of GO. These results indicate the formation of ZnO/GO nanocomposites.   Table 1 summarizes the size information for all samples based on SEM characterization and XRD measurements.   Table 1 summarizes the size information for all samples based on SEM characterization and XRD measurements.

SEM and EDX Studies
The atomic composition of ZnO NRs ( Figure 5) is confirmed by EDX analysis. For ZnO NRs/GO nanocomposites, the presence of Zn, O, and C atoms is revealed. The EDX results are in good agreement with the expected chemical composition, which clearly indicates that ZnO/GO nanocomposites were successfully synthesized.   The atomic composition of ZnO NRs ( Figure 5) is confirmed by EDX analysis. For ZnO NRs/GO nanocomposites, the presence of Zn, O, and C atoms is revealed. The EDX results are in good agreement with the expected chemical composition, which clearly indicates that ZnO/GO nanocomposites were successfully synthesized.

FTIR and Raman Analysis
FTIR spectroscopy was used to investigate the surface functional groups in GO layers, ZnO NRs, and ZnO NRs/GO nanocomposites. Figure 6 indicates the presence of eight absorption bands in the FTIR spectrum of GO. The absorption peaks located at 3541-3367 cm −1 can be ascribed to O-H group stretching vibration, while the peaks between 2907 and 2232 cm −1 correspond to the asymmetric and symmetric stretching vibrations of C-H [28,29]. The bands observed at 1726 cm −1 , 1615 cm −1 , and 1416 cm −1 are attributed to C=O, C=C, and C-O-C stretching vibrations, respectively [30]. Additionally, the band located at 1222 cm −1 can be ascribed to C-OH stretching vibrations [31]. The FTIR spectrum of ZnO NRs comprises several absorption bands (blue curve). The absorption peak at 828-540 cm −1 agrees with Zn-O stretching vibration. The absorption peak at 1227 cm −1 can be attributed to C-OH stretching vibrations [31]. The peaks around 2905 cm −1 are attributable to C-H bonds. The absorption band located at 3421 cm −1 corresponds to the hydroxyl group [32]. The FTIR spectra of the ZnO NRs/GO nanocomposites exhibit a number of absorption bands lying in the range from 400 to 4000 cm −1 . The peaks at 549 and 849 cm −1 can be ascribed to Zn-O stretching vibration, thus confirming the presence of ZnO in nanocomposites [33]. Furthermore, the peak observed around 1266 cm −1 can be assigned to C-OH stretching vibration [31], that shifted from 1222 cm −1 in the case of GO due to composite formation. The absorption peaks at 2949 and 2289 cm −1 are attributable to the C-H and O=C=O stretching vibrations, respectively [26], while the absorption band around 3411-3610 cm −1 is owing to the O-H stretching vibration. It is recognized that any change in the position and intensity of FTIR peaks of ZnO NRs/GO nanocomposites with respect to those of GO reflects the contribution of functional groups of GO/ZnO nanorods [17].

FTIR and Raman Analysis
FTIR spectroscopy was used to investigate the surface functional groups in GO layers, ZnO NRs, and ZnO NRs/GO nanocomposites. Figure 6 indicates the presence of eight absorption bands in the FTIR spectrum of GO. The absorption peaks located at 3541-3367 cm −1 can be ascribed to O-H group stretching vibration, while the peaks between 2907 and 2232 cm −1 correspond to the asymmetric and symmetric stretching vibrations of C-H [28,29]. The bands observed at 1726 cm −1 , 1615 cm −1 , and 1416 cm −1 are attributed to C=O, C=C, and C-O-C stretching vibrations, respectively [30]. Additionally, the band located at 1222 cm −1 can be ascribed to C-OH stretching vibrations [31]. The FTIR spectrum of ZnO NRs comprises several absorption bands (blue curve). The absorption peak at 828-540 cm −1 agrees with Zn-O stretching vibration. The absorption peak at 1227 cm −1 can be attributed to C-OH stretching vibrations [31]. The peaks around 2905 cm −1 are attributable to C-H bonds. The absorption band located at 3421 cm −1 corresponds to the hydroxyl group [32]. The FTIR spectra of the ZnO NRs/GO nanocomposites exhibit a number of absorption bands lying in the range from 400 to 4000 cm −1 . The peaks at 549 and 849 cm −1 can be ascribed to Zn-O stretching vibration, thus confirming the presence of ZnO in nanocomposites [33]. Furthermore, the peak observed around 1266 cm −1 can be assigned to C-OH stretching vibration [31], that shifted from 1222 cm −1 in the case of GO due to composite formation. The absorption peaks at 2949 and 2289 cm −1 are attributable to the C-H and O=C=O stretching vibrations, respectively [26], while the absorption band around 3411-3610 cm −1 is owing to the O-H stretching vibration. It is recognized that any change in the position and intensity of FTIR peaks of ZnO NRs/GO nanocomposites with respect to those of GO reflects the contribution of functional groups of GO/ZnO nanorods [17].
Raman spectroscopy is a powerful characterization technique, particularly useful for further analysis of the structural properties of ZnO and related composites. The GO spectrum (Figure 7a), exhibits two sharp peaks at 1344 cm −1 and 1599 cm −1 , labeled as D and G bands, respectively [34]. The D band is related to a structural disorder of graphene, whereas the G band is assigned to the sp 2 hybridized carbon atoms (C=C stretching), hence confirming the presence of carbon in nanocomposites [34]. Figure 7b shows the Raman spectrum of ZnO nanorods. The peaks around 325 cm −1 and 436 cm −1 correspond to the polar and E 2 (high) vibrational modes of the ZnO structure, respectively, confirming the hexagonal wurtzite phase of ZnO nanorods [35]. The peak at~569 cm −1 is due to the A 1 (LO) mode of ZnO [36], whereas the peak found at 1100 cm −1 corresponds to the phonon scattering phenomenon [37].
For the ZnO/GO nanocomposites (Figure 7c), all three Raman spectra exhibited identical spectral characteristics, with six leading Raman-active regions, i.e., four regions for the ZnO NRs and the other two regions associated with the GO (D and G bands), which confirms the successful formation of nanocomposites. The only difference was the peaks' intensity that increased with increasing GO concentration (Figure 7d). Moreover, in the case of the nanocomposite, the D and G bands are shifted to lower wavenumbers, 1334 cm −1 and 1594 cm −1 , compared to GO, due to the hybridization of ZnO.
Furthermore, the peak positions of the D and G bands of GO and ZnO NRs/GO nanocomposites were nearly identical, whereas the intensity ratio of the D and G bands (I D /I G ) changed (Figure 7e): I D /I G = 0.92, 1.00, 1.03, and 1.05 for GO, ZnO NRs/GO1, ZnO NRs/GO2, and ZnO NRs/GO3, respectively. It is well known that the intensity peak ratio of the D and G bands indicates the degree of structural defect concentration in graphitic materials [38]. The increase in the I D /I G ratio for ZnO NRs/GO nanocomposites attests that the defect concentration in ZnO NRs/GO is higher than in GO, due to the effect of the hydrothermal treatment on the formation of both ZnO and GO. Raman spectroscopy is a powerful characterization technique, particularly useful for further analysis of the structural properties of ZnO and related composites. The GO spectrum (Figure 7a), exhibits two sharp peaks at 1344 cm −1 and 1599 cm −1 , labeled as D and G bands, respectively [34]. The D band is related to a structural disorder of graphene, whereas the G band is assigned to the sp 2 hybridized carbon atoms (C=C stretching), hence confirming the presence of carbon in ID/IG = 0.92, 1.00, 1.03, and 1.05 for GO, ZnO NRs/GO1, ZnO NRs/GO2, and ZnO NRs/GO3, respectively. It is well known that the intensity peak ratio of the D and G bands indicates the degree of structural defect concentration in graphitic materials [38]. The increase in the ID/IG ratio for ZnO NRs/GO nanocomposites attests that the defect concentration in ZnO NRs/GO is higher than in GO, due to the effect of the hydrothermal treatment on the formation of both ZnO and GO.

UV-Vis Absorption Spectroscopy
The UV-Vis absorption spectra of the ZnO NRs and ZnO NRs/GO nanocomposites with various concentrations of GO are shown in Figure 8. The strong absorption peaks at 366-370 nm, characteristic for all spectra, can be related to the electron transitions from the valence band to the conduction band (O2p-Zn3d) [38]. No apparent absorption was observed in the visible region. In general, the absorption spectrum of GO exhibits two main particularities, namely the absorption peak at around 230 nm, which is attributed to the π-π* transition, and the absorption peak at around 300 nm, due to the n-π* transition of aromatic C-C bonds [39]. As can be shown, the absorbance of the

UV-Vis Absorption Spectroscopy
The UV-Vis absorption spectra of the ZnO NRs and ZnO NRs/GO nanocomposites with various concentrations of GO are shown in Figure 8. The strong absorption peaks at 366-370 nm, characteristic for all spectra, can be related to the electron transitions from the valence band to the conduction band (O 2p -Zn 3d ) [38]. No apparent absorption was observed in the visible region. In general, the absorption spectrum of GO exhibits two main particularities, namely the absorption peak at around 230 nm, which is attributed to the π-π* transition, and the absorption peak at around 300 nm, due to the n-π* transition of aromatic C-C bonds [39]. As can be shown, the absorbance of the ZnO NRs/GO1 increases compared to that of the ZnO NRs due to the absorption contribution of GO. Moreover, when compared to ZnO/GO nanocomposites, the absorption edges are gradually redshifted for ZnO NR/GO2 and ZnO NRs/GO3 nanocomposites, due to the interaction of ZnO NRs with GO.
The optical band gap of the composites was evaluated by a Tauc plot, based on the following relation [40]: where hν is the photon energy, α is the absorption coefficient, A is an energy independent constant, and E g is the band gap. Figure 9 shows the variation of (αhν) 2 versus hν in the fundamental absorption region. The inferred band gap values of the ZnO NRs nanocomposites are around 3.10 eV.

Photoluminescence Spectroscopy
The PL spectra of ZnO NRs and ZnO NRs/GO nanocomposites, recorded with a 325 nm excitation wavelength, are shown in Figure 10. As can be easily ascertained, the samples display three spectrally distinct PL bands-the low intensity band with maximum at 387 nm (UV emission), the

Photoluminescence Spectroscopy
The PL spectra of ZnO NRs and ZnO NRs/GO nanocomposites, recorded with a 325 nm excitation wavelength, are shown in Figure 10. As can be easily ascertained, the samples display three spectrally distinct PL bands-the low intensity band with maximum at 387 nm (UV emission), the low intensity band centered at 425 nm (blue emission), as well as the high intensity and sharp band peaked at 575 nm (green emission). The near-band-edge (NBE) UV emission is due to free ZnO exciton recombination [16]. The blue emission at 425 nm is attributed to the electron transition from a shallow donor level of zinc interstitials (Zn i ) to the top of the valence band (VB) [41]. The green emission peak observed around 565 nm is due to the transitions from the oxygen vacancy (VO) level of ZnO NRs [42]. Moreover, as shown by PL spectra, the luminescence is substantially quenched after the addition of GO, due to the interaction between excited ZnO NRs and GO. As GO concentration is increased, the PL intensity is decreased, which is caused by the interfacial charge transfer between ZnO NRs and GO layers [43,44]. For a more detailed analysis of emission bands, Figure 11a-d shows the Gaussian fitting of the PL spectra of ZnO NRs and ZnO NRs/GO nanocomposites. In each case, three PL peaks were fitted for the broad visible emission band (in the range of 450-650 nm) and one peak for the UV emission peaked at around 382-388 nm. Apparently, samples exhibit luminescence bands at 517-521 nm that can be related to the emission from defect states due to the presence of oxygen functional groups [45]. Two others peaks at 565-570 nm and 618-624 nm were associated with optical transitions from the VO level and extended oxygen vacancy states to the valance band, respectively [42,46]. It can be also ascertained that when GO's concentrations increased, emission bands of ZnO NRs/GO exhibited a redshift as compared to those of ZnO NRs. This may be attributed to the interaction between the surface defects of ZnO and the π-electron cloud of the graphene [47]. Studies regarding the theoretical evaluation of optoelectronic properties of 2D materials were published previously. It was demonstrated that localization of electronic states plays a key role in determining the properties of the considered systems [48]. For a more detailed analysis of emission bands, Figure 11a-d shows the Gaussian fitting of the PL spectra of ZnO NRs and ZnO NRs/GO nanocomposites. In each case, three PL peaks were fitted for the broad visible emission band (in the range of 450-650 nm) and one peak for the UV emission peaked at around 382-388 nm. Apparently, samples exhibit luminescence bands at 517-521 nm that can be related to the emission from defect states due to the presence of oxygen functional groups [45]. Two others peaks at 565-570 nm and 618-624 nm were associated with optical transitions from the VO level and extended oxygen vacancy states to the valance band, respectively [42,46]. It can be also ascertained that when GO's concentrations increased, emission bands of ZnO NRs/GO exhibited a redshift as compared to those of ZnO NRs. This may be attributed to the interaction between the surface defects of ZnO and the π-electron cloud of the graphene [47]. Studies regarding the theoretical evaluation of optoelectronic properties of 2D materials were published previously. It was demonstrated that localization of electronic states plays a key role in determining the properties of the considered systems [48].
The PL intensity of the UV region is increased; however, in the visible region it is decreased by increasing the GO concentration (inset of Figure 10). The I vis /I UV ratio represents one of the important factors for comparing optical characteristics. When comparing the optical properties of nanocomposites, the I UV /I vis ratio of ZnO NRs/GO2 means fewer structural defects and a better crystalline quality than the other nanocomposites (ZnO NRs/GO1, ZnO NRs/GO3) (Figure 12a). The PL intensity of the UV region is increased; however, in the visible region it is decreased by increasing the GO concentration (inset of Figure 10). The Ivis/IUV ratio represents one of the important factors for comparing optical characteristics. When comparing the optical properties of nanocomposites, the IUV/Ivis ratio of ZnO NRs/GO2 means fewer structural defects and a better crystalline quality than the other nanocomposites (ZnO NRs/GO1, ZnO NRs/GO3) (Figure 12a).
However, the emissions have been also characterized by calculating chromaticity color coordinates, using the CIE 1931 chromaticity diagram (Figure 12b). The color coordinates of ZnO NRs were estimated to be about (0.39, 0.44), corresponding to green-orange emission. Clearly, the emission from ZnO nanorods does not belong to the white light region. The obtained color coordinates for ZnO NRs/GO1, ZnO NRs/GO2, and ZnO NRs/GO3 were found to be (0.36, 0.39), (0.35, 0.37), and (0.33, 0.34), respectively, and display a shift towards the white light coordinates (0.33, 0.33) with the increase in GO concentration. We believe that our findings are likely to pave the way for realizing white light emission by means of ZnO/GO nanocomposites. The obtained results could be even used for commercial LED application.

Conclusions
The nanocomposites were successfully prepared via the hydrothermal process. The XRD measurements indicated the formation of the hexagonal wurtzite structure of ZnO. The SEM and EDX studies revealed the formation of GO layers and flower-shaped ZnO NRs. A decreasing grain However, the emissions have been also characterized by calculating chromaticity color coordinates, using the CIE 1931 chromaticity diagram (Figure 12b). The color coordinates of ZnO NRs were estimated to be about (0.39, 0.44), corresponding to green-orange emission. Clearly, the emission from ZnO nanorods does not belong to the white light region. The obtained color coordinates for ZnO NRs/GO1, ZnO NRs/GO2, and ZnO NRs/GO3 were found to be (0.36, 0.39), (0.35, 0.37), and (0.33, 0.34), respectively, and display a shift towards the white light coordinates (0.33, 0.33) with the increase in GO concentration. We believe that our findings are likely to pave the way for realizing white light emission by means of ZnO/GO nanocomposites. The obtained results could be even used for commercial LED application.

Conclusions
The nanocomposites were successfully prepared via the hydrothermal process. The XRD measurements indicated the formation of the hexagonal wurtzite structure of ZnO. The SEM and EDX studies revealed the formation of GO layers and flower-shaped ZnO NRs. A decreasing grain size tendency together with increasing amount of GO was observed. The UV-Vis spectra revealed that the nanocomposites exhibited redshifted absorption peaks compared to pure ZnO NRs, with the strongest absorption around 370 nm. The PL spectra of the nanocomposites showed a strong near-band-edge UV emission at 350-400 nm and large visible emission bands at 500-650 nm. The obtained results also indicated the impact of GO concentration on the optical properties of the ZnO NRs. Moreover, the color coordinates of ZnO NRs/GO nanocomposites are close to the white light coordinate (0.33, 0.33).