UV-Assisted Photocatalytic Synthesis of ZnO-Reduced Graphene Oxide Nanocomposites with Enhanced Photocatalytic Performance in Degradation of Methylene Blue

Natural graphite powder (<45 μm, ≥ 99.99%), concentrated sulfuric acid (H₂SO₄, 98.08%), Sodium Nitrate (NaNO₃), hydrogen peroxide (H₂O₂, 30%), potassium permanganate(KMnO₄) and ethanol (∼99.5%), all were obtained from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Commercially available particulate ZnO nanoparticles (30 nm ± 10 nm) 99.9% metal basis were purchased from Aladdin Industrial Corporation Shanghai China. All of the material were used as received without further purification. All solutions were prepared using Milli-Q water (>18.2 MΩ cm resistivity) the solvent.

GO was obtained with ultrasonic exfoliation of graphite oxide, Which was prepared by Hummers `method,²³ in an ultrasonic bath (FRQ-1006HT,300W,Front Ultrasonic). Typically,5 g Natural graphite powder (45 μm, 99%), and 2.5 g Sodium Nitrate (NaNO₃), were added to 115 ml of concentrated sulfuric acid (H₂SO₄, 98.08%), in a round bottom flask. And stirred in the ice bath for 30 min at a temperature of 5°C Then 15 g of KMnO4 was mixed into the suspension .The flask was placed in water-bath and was maintained at a temperature of 35°C for 30 min, Over the next 30 min, Water ultra-purified (UP) filtered resistivity (18 MΩ cm) was used throughout the experiment. 230 ml of water was added drop by drop with the help of burrete into the suspension, and the obtained suspension was heated to 98°C. After 15 min, the as prepared suspension was then further diluted to approximately 420 ml and treated by 50 ml 30 wt% hydrogen peroxide. The graphite oxide was obtained after centrifugation, washing, and drying. The GO was obtained by ultrasonic exfoliation of the as-prepared graphite oxide.

The ZnO-RGO nanocomposites have been synthesized via a simple superficial UV-assisted photocatalytic approach as illustrated in Figure 10. The comprehensive synthesis procedure was as follows: A certain amount of 1.8 mg/ml of GO prepared by modified Hummers method, and 1 g ZnO nanoparticles (commercially prepared 30 nm) were dispersed in 60ml ethanol with the assistance of ultrasonication and sonicated for 30 min to form a homogeneous ZnO/ GO suspension. UV-assisted photocatalytic reduction of GO by ZnO nanoparticles in ethanol was carried out in a 100 ml cylindrical quartz glass vessel using a 500 W high- pressure Hg lamp with the main wavelength at 365 nm for about 7 h under continuous magnetic stirring. After UV irradiation, it was noticeably found that the color of the suspension had changed into grayish-black, demonstrating the successful chemical reduction of GO sheets) it should be noticed that no further reduction of GO under UV irradiation for longer time is detected by Fourier transform infrared spectroscopy (FTIR) measurement which is generally used to illustrate the degree of converting GO to RGO. The as-synthesized ZnO/RGO nanocomposites with 2.5, 2, 1.8, 1.6 wt% RGO, named as ZG-1, ZG-2, ZG-3, ZG-4 respectively1were centrifuged rinsed with deionized water, and vacuum- dried at 60°C for 24 h.

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X-ray diffraction (XRD) pattern was recorded with a Bruker D8 Advance X-ray Diffractometer using Cu Kα (λ = 0.1549 nm) radiation in the diffraction angle range of 10 to 70° under a scan rate of 0.01° s⁻¹ with an accelerating voltage of 40 kV. X-ray photoelectron spectroscopy (XPS) measurements were taken by using a spectrometer (Thermo Fisher Scientific, ESCALAB 250) with a monochromic Al Kά source at 1486.7 eV, at a voltage of 15 kV and an emission current of 10 mA.). Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet 370FT-IR spectrometer using pressed KBr pellet Japan). Raman spectra were collected using an INVIA spectrophotometer (Renishaw, UK, with an excitation of 541.5 nm laser light. The light absorption properties of the sample were evaluated over the spectral region of 200–800 nm by using a UV-visible spectrophotometer (UV-1240, SHI- MADZU, Japan). Morphological analysis was performed with using Scanning electron microscopy (SEM; Hitachi S-4500), transmission electron microscopy (TEM; JEOL 2010F), and HRTEM (TENICA G2 measurements were performed at room F30 S-TWIN) All the temperature

The photocatalytic test was performed by the photoreactor deliberated with an internal light source surrounded with a quartz jacket (500W - pressure mercury lamp through core emission wavelength of 310–400 nm). 0.05 g of the sample was dispersed in dye solution (100 ml, 10mg/l). The integrated light source striking the sample. The mixed suspension was magnetically stirred for 0.5h in darkness to reach an adsorption-desorption equilibrium between the organic molecules and the catalyst surface before exposure to irradiation. The degradation of dyes was determined by UV−visible spectrophotometer (UV-1240, SHI-MADZU, Japan). At given interludes of illumination, the samples of the reaction solution were taken out and examined.

The graphene oxide upon exfoliation carries appropriate functional groups (e.g., epoxides, hydroxyls, carboxylic acids).In the present study, we prepared a mixture of graphene oxide and ZnO in the ethanol- water system and subjected it to steady state UV Irradiation. The change in the absorption spectrum after 30 min period of UV-irradiation is shown in Figure 1A. The samples were placed in the photoreactor deliberated with an internal light source surrounded by a quartz jacket while nitrogen was bubbled through the solution. The samples were periodically examined for changes in absorption. The change in color from light brown to grayish black can be seen as the reduction of GO proceeds in Figure 1B. This color change has previously been suggested as a partial restoration of the π network within the carbon structure and has been witnessed through the chemical reduction of the GO sheets.²⁴ The photocatalytic property of semiconductor ZnO is well researched. When ZnO was excluded during UV-irradiation from the suspension no significant changes in the absorption could be seen. This observation establishes the facilitating role of ZnO photocatalyst1under UV-light in the reduction of GO in polar solvents.²⁵ Upon UV irradiation of a deaerated suspension of ZnO and GO in ethanol- water system, the holes are scavenging to produce ethoxy radicals one observes charge separation, thus leaving the electrons to accumulate within a ZnO nanoparticle equation(a).The accumulated electrons serve to interact with the graphene oxide sheets in order to reduce it partially equation (b).²⁶

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The powder X-ray diffraction patterns of GO, ZnO and 2.5 wt% of ZnO/rGO are shown in Figure 2. The peaks of scattering angles 2θ values located at ca... 31.7°, 34.3°,36.3°, 47.6°, 56.5°, 63.0°, 66.5°,67.9°, 69.1°, can be indexed to [1 0 0] [,0 0 2], [1 0 1], [1 0 2], [1 1 0], [1 0 3], [2 0 0], [11 2]and [2 0 1] Facet crystal planes of hexagonal wurtzite ZnO (JCPDS No. 36–1451), respectively.^27,28 the most intensive sharp peak (001) centered in around 2θ = 10.6°in the diffractogram of GO is due to the presence of oxygen carrying groups on the GO-surface. This peak was considerably decreased in ZnO/RGO nanocomposite.²⁹ The nanocomposite ZG-1 and ZnO have similar XRD patterns. It may be because the regular layer stacking of GO was destroyed due to the crystal growth of ZnO between the interlayers of GO, leading to the diminishing of the diffraction peaks for GO. Therefore, XRD results propose that the ZnO nanoparticle might become attached to those GO nanosheets. This confirmed that the sample was crystalline and the presence of RGO did not result in variations in special orientations of ZnO or development of new crystal orientations.³⁰ However, peaks associated only with ZnO are observed in nanocomposites and no peak was assigned to GO, indicating that it was reduced to graphene become attached to ZnO during the photocatalytic process.¹¹ One is the low amount and relatively low diffraction intensity of RGO in the composites of RGO/ZnO .The other is probably due to the disappearance of the layer-stacking regularity after reduction of GO.³¹

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FTIR spectra of GO and ZG-1 are illustrated in Figure 3. The peak observed at 3428 cm¹ assigned to OH (hydroxyl) group of absorbed water. The stretching, vibration at 1723 cm⁻¹ represents the presence of C = O (carbonyl) group. The peaks at 1380, 1220 and 1077 cm⁻¹ correspond to the C-OH, C-O-C, and C-O (alkoxy) vibrations, respectively.³² The peak at 470 cm⁻¹ of RGO/ZnO nanocomposite is similar to that of pure ZnO which represent the stretching vibration of ZnO.³³ It can be observed that the intensities of absorption bands of oxygen-containing functional groups such as C-O (1723 cm⁻¹), C OH (1380 cm⁻¹) and C- O (1220 cm⁻¹) were dramatically reduced. However, it can be seen that the spectrum at 3400 cm⁻¹, retains a broad absorption spectrum, which was attributed to the residual O-H groups of the RGO. These results implied that the GO in nanocomposite accompanied by a partial reduction to graphene through the photocatalytic treatment.³⁴

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X-ray photoelectron spectroscopy (XPS), is a surface analysis technique indicating physical attachment of the ZnO nanoparticles to the graphene oxide sheets. Figure 4a shows the XPS survey spectrum and the inset show the high-resolution spectrum of Zn 2p of the composite. Survey spectrum confirms the presence of the Zn, O, and C .the peaks at1021.3 eV and 1045.5 eV positions correspond Zn (2p1/2) Zn (2p3/2) respectively. The separation of peak energy is 23.2eV confirms the + 2 oxidation state of zinc.³⁵ Figure 4b shows the deconvoluted O1s spectrum indicating two types of oxygen were present having the binding energies at 530.22 eV (O-C) and 531.3 eV which were associated with ZnO. Figure 4c C1s peak in the XPS spectrum shows the presence of four types of carbon bonds, that is the non-oxygenated ring Carbon at a binding energy (284.5eV)including C-C,C = C, and C-H).while the other two at higher binding energies are assigned to the carbonyl carbon in1C = O(287.8eV) and the carboxylate carbon in C-O-C,C-OH(289.0),³⁶ indicating a remarkable degree of oxidation for the GO nanosheets. In fact, these oxygenated species on GO provides the active sites for the deposition of ZnO nanoparticles. Figures 4c, 4d for comparison with GO (C1s) the intensity of carbonyl groups is dramatically decreased in composite. On the basis of XPS results, the effective reduction of ZnO-GO through photocatalysis can be concluded.³⁷

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To unveil the morphology of GO, ZnO, ZnO/RGO we have comparatively characterized the relevant samples by a series of joint techniques (SEM), (TEM) and (HRTEM). SEM micrographs Figure 5a, 5b showed a highly agglomerated state of nanoparticles with fluffy graphene sheet morphology shows that zinc oxide nanoparticles are decorated on the surface of reduced graphene oxide via UV- assisted photocatalytic reduction. In addition, the statistical size distribution of ZnO nanoparticles is in the range of 80–100 nm for ZG-1.³⁸ The TEM image of the sample Figures 5c, 5d shows the zinc oxide nanoparticles distribution.³⁹ These images confirm our ability to achieve fairly uniform dispersion of zinc nanoparticles on graphene sheet the interaction of - OH and- COOH functional groups of GO with the

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Semiconductors facilitate this attachment, the functional groups are reduced during the photocatalytic process. And thereby the unpaired pi electrons are more available to bind to the surface atoms of semiconductors .²⁷ Furthermore, the high-resolution TEM (HR-TEM) images of ZnO/RGO Figure 5e, display the clear lattice fringe indicate high degree crystallinity, which is notarized to be ZnO phase by the interplanar spacing value (0.28 nm for ZnO (100) facet), Supportive with the XRD results.⁴⁰ The identified reduced graphene oxide layers in the edge area of ZnO/RGO indicate the intimate interface contact between ZnO and reduced graphene oxide sheet, makes possible the electronic interaction between ZnO and graphene, and improves the charge separation and the photocatalytic activity.⁶ Raman spectroscopy is a reliable technique for analyzing the electronic structure of chemical compounds and is widely applied to distinguish the crystalline order/disorder and lattice defect in carbon- based material. The Raman spectra of GO and ZnO/RGO is characterized by two broad bands. The G band corresponds to the first-order scattering of the E_2g mode observed for sp² carbon domains and is, therefore, prominent in all graphite-based material, usually arise at ca.1585 cm⁻¹.whereas the D band is attributed to a breathing mode of κ-point phonons of A_1g symmetry, which is a common feature of sp³ defects in carbon and usually, can be associated with the structural defects arise at 1330 cm⁻¹as shown in Figure 6.¹⁰ In general, the intensity ratio of the D and G band (I_D/I_G) is an important tool to measure the degree of disorder and the average size of the sp² domains in carbon-based materials. The D band shifted from 1353 to 1347 cm⁻¹.Whereas the G band shifted from 1594 to 1587 cm⁻¹, indicating graphite oxide has been reduced to GO. The D/G ratio confirms the degree of functionalization.⁴⁰ According to D/G intensity ratio of ZG-1 increased to 1.1 compared to that of GO which is 0.86.which is usually observed in reduced GO, indicating the successful transformation from GO to RGO in the composite.¹²

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The photosensitive properties of the samples were explored by UV−vis spectroscopy. The room temperature absorption spectra of pure ZnO and that of nanocomposites ZG-1, ZG-2, ZG-3, and ZG-4, respectively are shown in Figure 7. The relative intensity of the ZnO peak (∼374 nm) increases with GO loading in the nanocomposites. The sharp characteristic absorption peak at 374 nm indicates the presence of good crystalline impurity suppressed ZnO nanostructures. It is observed that and the absorbance of ZnO–RGO composite increases even in the visible light region with the increase of RGO content, which is similar to those reported in the hybridization of TiO2 with graphite-like carbon layers or RGO. Therefore, the presence of RGO in ZnO can increase the light absorption intensity and range, which is beneficial to the photocatalytic performance.²⁰ Compared with ZnO, the ZnO/RGO nanocomposite materials show red-shift of the surface Plasmon absorption, which is due to strong interfacial hybridization between ZnO and the neighboring RGO. The redshift indicates that the bandgap energy of nanocomposite materials was less than that of pure ZnO.⁴¹ The bandgap values of the synthesized catalysts and ZnO were evaluated using Tauc plot established on UV-vis diffuse reflectance spectra and their corresponding plots are shown in Figure 8. The bandgap values were calculated using Tauc plot based on Kubelka–Munk equation. A Tauc plot was obtained by plotting (F(R) *hv)^1/2 versus (hv) to the energy axis where R is the reflectance, the intercept of the plot between (F(R)*E) vs E gives the bandgap energy.^42,43

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Band gap can be originated by passing a tangential line from side to side the edge of the curve to the x-axis. The estimated optical bandgap value ZnO/RGO nanocomposites is small when compared with ZnO catalysts are tabulated in Table I.⁴⁴

The photocatalytic activity of ZnO and of the as-obtained ZnO/RGO nanocomposites with different graphene content ZG-1, ZG-2, ZG-3and ZG-4 was measured in term of MB degradation under visible light irradiation. Time course, degradation curves for MB were shown in Figure 9a. The change in the absorption peak of MB aqueous solution at 664nm showed the change of its concentration in the presence of ZnO catalyst Figure 9b. There were four different MB species found in aqueous solution the monomer (MB+), the protonated monomer (MBH⁺²), the dimer ((MB⁺²) and the trimer ((MB⁺³) the monomer adsorbed at 664 nm, by increasing the dye concentration dimers were formed, which produced bands at 605 nm a further increase of the concentration led to trimerization lead to the formation of a band which was noted at ∼580 nm. The concentration of MB has been reduced with increasing visible light treatment time.⁴⁵ It was confirmed From Figure 9a that the photodegradation of MB has taken place in the presence of photocatalyst. Moreover, the concentration of MB was hardly reduced under UV light irradiation in the absence of the photocatalyst, it was also observed that there was very less discoloration by the pure ZnO as shown in Figure 9b, due to its larger bandgap (3.37 eV). After 120 min, the degradation efficiency was calculated to be (∼ 68%).⁴⁶ Among the as- prepared samples, ZG-1 shows higher degradation efficiency increased to (∼ 80%) within a short period of irradiation 120 min as shown in (Figure 9c), which was ascribed to the bandgap narrowing of the composite material in comparison to the pure ZnO.⁴⁷ The efficiency of MB photodegradation followed the order ZG-1- > ZG-2 > ZG-3 > ZnO ZG-4 > after 120min of reaction, the degradation efficiencies of MB were approximately 80%, 73%, 71%, 67% and 68%, and for ZG-1, ZG-2, ZG-3, ZG-4 and pure ZnO respectively Figure 9d. It is well known that the photocatalytic degradation of organic dyes follows pseudo first-order kinetics in this system, it can be explained by a Langmuir-Hinshelwood model.⁴⁸ The obvious reaction rate constants (k) for the degradation of MB can be derived from the linear slope of the relationship between ln(C/C°) and t, where C° and C are the concentrations of initial solution and after t (min) of irradiation, and follow the order: ZG-1 (0.04 min⁻¹) >ZG-2 (0.01 min⁻¹) >ZG-3 (0.008 min⁻¹) >ZG-4 (0.006 min⁻¹) >ZnO (0.004min⁻¹) respectively.⁴⁷ Based on the slope of the linear fitted graph, the calculated k values show that the ZG-1 nanocomposite has a high degradation rate than pure ZnO,The degradation rate (D %) had a mathematical expression as follows. D% = C°- C/ C°x100 Where C° and C are the concentrations of initial solution and after t (min) of irradiation.⁴⁹

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Finally, on the basis of the above results, a recommended reaction mechanism for photocatalytic degradation of MB in aqueous solution over the ZnO/RGO nanocomposite has been proposed, as pictorially illustrated in Figure 10. The strong coupling between ZnO and graphene, which supports interfacial charge transfer and impedes electron–hole recombination. The degradation of methylene blue under visible light by ZnO/RGO nanocomposite revealed a redshift of the band edge, and a significant reduction of the bandgap (2.93 eV). At the same time, the composite shows higher surface area Table I as compared to pure ZnO.⁵⁰ Under UV light irradiation a high energy photon excites an electron from the valence band to the conduction band of ZnO, photogenerated electrons in ZnO move freely toward the surface of the graphene sheet which efficiently separates them and minimizes the charge recombination processes, thus enhancing the photocatalytic performances. The () captured by RGO reacted with O₂ to form O₂^·− while the holes in ZnO generate 2OH radicals. As highly reactive species, OH· and O· radicals oxidize the MB dye into the water, carbon dioxide and inorganic salts. The decrease in photocatalytic activity at the higher graphene loading amount was credited to the increased absorbance and scattering of photons through excess graphene in the photosystem and. The mechanism for the photocatalytic degradation of MB in our experiment was suggested as follows.¹²