One-pot sol–gel synthesis of reduced graphene oxide uniformly decorated zinc oxide nanoparticles in starch environment for highly efficient photodegradation of Methylene Blue

Abstract

ZnO NPs + reduced graphene oxide (rGO) nanocomposites were synthesized using a sol–gel method with starch as the polymerisation agent. Long-chain starch compounds were used to terminate the growth of the ZnO NPs on rGO and stabilise them. The resulting products were annealed at 350 °C to remove the starch and produce a reduced graphene oxide (rGO) sheet in one-pot without any post-annealing processes. Microscopic studies showed that the NPs were dispersed on the rGO sheet. They had a spherical shape and a size of approximately 25 ± 10 nm. In addition, these studies revealed that the NPs were single crystals. The X-ray diffraction pattern of the NPs indicated a hexagonal (wurtzite) structure. The results of Fourier transform infrared spectrum analysis (FTIR) revealed that the GO sheet was transformed into rGO via the sol–gel method in the starch environment. The results of photoluminescence spectroscopy demonstrated that the incorporation of reduced graphene oxide (rGO) sheets with ZnO NPs suppressed the electron–hole recombination of the composite. Therefore, a significant enhancement in the photocatalytic degradation of methylene blue (MB) was observed with the ZnO NPs + rGO nanocomposite compared to the bare ZnO nanoparticles.

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1. Introduction

Today, organic dyes and their waste products have become some of the most serious sources of water pollution. Such organic dyes pass through traditional wastewater treatment plants and remain in the water because of their high stability against light, temperature, chemical, and microbial treatments.^1–3 Many semiconductors can be used in the photodegradation of recalcitrant organic pollutants.^4–7 According to the latest reports, zinc oxide (ZnO) was one of the first discovered and non-toxic II–VI semiconductors with a wide direct band gap (3.3 eV); this semiconductor is extensively used in photonics,⁸ crystals,⁹ photocatalysts,^10,11 sensors,^12,13 light-emitting diodes (LEDs),^14,15 and electroluminescent materials (ELM)¹⁶ and has been given attention because of its large band gap (3.3 eV) and other beneficial characteristics. In addition, the technological significance of ZnO nanostructures with quasi-one-dimensional structures,¹⁷ which have diameters in the range of tens to hundreds of nanometres, makes them interesting from a scientific point of view. In this size range, they are expected to possess interesting physical properties and pronounced couplings that are quite different from their bulk counterparts.¹⁸ Therefore, ZnO can be used as a photocatalyst material to remove organic dyes from wastewater with high efficiency. A photocatalyst is also called a photochemical catalyst, and its performance is similar to that of chlorophyll in photosynthesis, which is a natural phenomenon. In a photocatalytic system, a photoinduced molecular transformation or reaction occurs at the surface of the catalyst. The fundamental mechanism of a photocatalytic reaction is the generation of an electron–hole pair, which can be described as follows: when a photocatalyst is illuminated by light with more energy than its band gap energy, electron–hole pairs diffuse out to the surface of the photocatalyst and participate in chemical reactions with the electron donors and acceptors. These free electrons and holes transform the surrounding oxygen or water molecules into hydroxyuracil (HOU)¹⁹ free radicals with super-strong oxidation characteristics. Meanwhile, this super-strong oxidation of HOU free radicals generated at the surfaces of ZnO nanoparticles make HOU free radicals harmful to human beings when used in cosmetics. Therefore, it is necessary to modify the application of ZnO nanoparticles as a photocatalyst material to provide good UV protection.

Graphene, a one-atom-thick sheet of sp²-bonded carbon atoms with high electrical conductivity, large surface-to-volume ratios, and excellent chemical tolerance, is a good matrix for nanocomposites. Therefore, semiconductor-decorated graphene composites have been the focus of research in recent years because of their multifunctional abilities.^20–22 The charge-transferring, magnetic and electronic interactions between graphene sheets and the attached semiconductor nanostructures can improve their performance in various applications.^23–25 In a semiconductor, electrons can be moved from the valence band to the conduction band by photons with energies (hν) that are equal to or greater than the band gap energy of the semiconductor, resulting in the generation of electron–hole pairs. These electron–hole pairs play an important role in the photocatalytic degradation of pollutants and solar energy conversion. However, the photoexcited electrons in the conduction band and holes in the valence band are unstable and can easily recombine, which decreases their photocatalytic efficiency. The photoexcited electrons from the conduction band can be accepted by the graphene sheets incorporated into the semiconductor and suppress the recombination of electrons and holes. Moreover, graphene nanosheets assist in the growth and dispersion of nanoparticles on the semiconductor surface, which prevents the aggregation of the nanoparticles and produces a higher surface area for the photocatalyst.^26,27

We believe that the sol–gel method is one of the best methods to obtain a uniform distribution of semiconductor nanomaterials. In addition, using a suitable polymer agent, such as starch, can improve the quality of the final product. Therefore, in this study, a simple sol–gel method was used to synthesize ZnO nanoparticles with a narrow size distribution, which were decorated on a reduced graphene oxide (rGO) sheet in a starch environment. Starch was used as a polymerisation agent, and it served as the terminator for the growth of the ZnO NPs because it expanded during the calcination process, which prevented the particles from coming together easily. In addition, one of the merits of this method is that the starch and the ZnO nanoparticle products are environmentally friendly. The other merit is that the starch reduces GO under mild conditions,²⁸ and it simultaneously plays an important role as a capping agent in stabilising the as-prepared graphene. Then, the resulting products were used as a photocatalyst to remove Methylene Blue (MB), which is a dye material. To the best of our knowledge, there have been no reports on one-pot sol–gel syntheses of reduced graphene oxide uniformly decorated with zinc oxide nanoparticles in a starch environment. Here, we report for the first time a fast, one-step, cost-effective, and environmentally friendly synthesis of rGO uniformly decorated with hierarchical ZnO NPs using a one-pot sol–gel method in a starch environment.

2. Experimental

2.1 Materials

Graphite flakes (code no. 3061) were purchased from Ashbury Inc. (NJ, USA). Sulphuric acid (H₂SO₄, 98%), potassium permanganate (KMnO₄, 99.9%), hydrogen peroxide (H₂O₂, 30%), hydrochloric acid (HCl, 37%), and sodium hydroxide (NaOH, 99.99%) were purchased from Merck. Zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O) was purchased from Systerm, Malaysia. Starch was obtained from Sigma-Aldrich (St. Louis, MO). Distilled water was used throughout the sample preparation.

2.2 Preparation of exfoliated graphite GO

Exfoliated graphite oxide was prepared based on the modified Hummer's method.²⁹ Typically, graphite flakes were oxidised by mixing H₂SO₄ and H₃PO₄ with a ratio 4 : 1 (v/v) at room temperature. The graphite and potassium permanganate were added slowly to the above mixture solution. Then, the mixtures were stirred for three days to complete the oxidation of the graphite. After that, hydrogen peroxide was added to stop the reaction. The mixture was sonicated and washed with HCl and water several times until the pH became neutral. During the washing and sonication process, the graphite oxide was exfoliated to GONs. The product was dried in a vacuum oven overnight at 60 °C. The resulting product was a loose brown powder with a hydrophilic nature.

2.3 Preparation of ZnO NPs + rGO composite

ZnO NPs were synthesized and decorated with the resulting GO sheets via a sol–gel method in a starch environment. In this synthesis, analytical grade zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O), starch and distilled water were used as the starting materials. First, a starch solution was prepared by adding starch at 22 wt%/v to 50 ml of distilled water at 60 °C. The zinc nitrate (4.46 g) was dissolved separately in a minimal amount of distilled water at room temperature, and a solution of GO with a 1.7 wt%/v concentration was added to this zinc nitrate solution. Finally, the resulting solution was added to the starch solution. After this, the compound solutions were stirred and heated at 80 °C until a gel with a dark-brown colour was obtained. The gel was calcined at 350 °C for 1 h at a heating rate of 2 °C min⁻¹ to obtain the nanocomposite. Eventually, the product, ZnO NPs + rGO, was obtained using centrifugation, washed with distilled water and ethanol several times to remove the excess polymer and ions, and then dried at 60 °C for 24 h in a vacuum oven. To optimise the experimental conditions for the preparation of pure ZnO NPs and ZnO NPs + rGO, numerous samples with different parameters were synthesized, as listed in Table 1. The experimental mechanisms discussed above are summarised in Fig. 1.

Table 1 Experimental conditions for the preparation of ZnO NPs and ZnO NPs + rGO

Sample	Temperature (°C)	Time (h)	Rate (°C min^−1)	GO (wt%/v)	Starch (wt%/v)
A	300	2	2	1.7	22
B	350	2	2	1.7	30
C	350	2	2	1.7	12
D	350	2	2	1.7	22
F	350	1	2	1.7	22
G	350	3	2	1.7	22
H	350	2	1	1.7	22
I	350	2	3	1.7	22
J	350	1	2	3.3	22
K	350	1	2	0.9	22
L	350	1	2	0	22
M	400	2	2	1.7	22

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Fig. 1 Schematic illustration of formation mechanism for ZnO NPs + rGO composite via sol–gel method.

2.4 Preparation of photocatalytic degradation samples

The photocatalytic performance of the as-prepared samples was evaluated using the photocatalytic degradation of Methylene Blue (MB) under UV light irradiation. Here, 10 mg of the resulting material was dispersed in 30 ml of the MB aqueous solution (10 mg l⁻¹). The mixed suspension was magnetically stirred for 1 h in the dark to reach an adsorption–desorption equilibrium. Under ambient conditions and stirring, the mixed suspension was exposed to UV irradiation produced by a 500 W high-pressure Hg lamp with the main wave crest at 365 nm for different times (1 to 6 h). At certain time intervals, 2.5 ml of the mixed suspension was extracted and centrifuged to remove the photocatalyst. The degradation process was monitored by measuring the absorption of MB in the filtrate at 664 nm using a UV-vis absorption spectrometer.

2.5 Characterisation

The resulting powder was characterised using several tools to check its quality. The crystal phase, morphology, and microstructure of the product were characterised using X-ray powder diffraction, XRD (Philips, X'pert, system using CuKα radiation), field emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 400 operated at 10.0 kV), a Raman spectrometer (Renishaw inVia Raman^'s microscope using laser excitation at λ = 514 nm), a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F), and Fourier transform infrared spectrometry (FTIR, Perkin-Elmer System 2000 series spectrophotometer (USA) using the KBr method). UV-visible spectroscopy (Thermo Scientific Evolution) was applied to determine the optical properties. In addition, iSolution software was used to measure the particle size, as shown in Fig. 3(a).

3. Results and discussion

3.1 Morphology

Fig. 2 shows TEM images of the ZnO NPs that were decorated on the rGO sheet. The TEM images show that the ZnO NPs had spherical shapes. The TEM images at different magnifications (1 μm to 2 nm) reveal that the ZnO NPs were dispersed on the rGO (Fig. 2(a)–(f)). In addition, these images show that the average particle size was approximately 25 ± 10 nm. The inset of Fig. 2(f) shows an HRTEM image of a single nanoparticle. As observed, the nanoparticle is a single crystal with a high crystal quality, and there are no defects from stacking faults. Furthermore, the HRTEM image shows that the lattice distance is approximately 0.27 nm, which is consistent with the distance along the c-axis of a bulk wurtzite ZnO crystal. Therefore, based on the HRTEM image, the nanoparticles were grown in the [001] direction without any defects. Finally, Fig. 3(a) and (b) shows size histograms of the ZnO NPs below the relative TEM images. These histograms indicate that the main particle sizes of the ZnO NPs calcined at a temperature of 350 °C were approximately 25 ± 10 nm. The TEM and size distribution results confirm that a narrow size distribution can be obtained for ZnO NPs prepared in a starch environment and calcined at a temperature of 350 °C. Meanwhile, we used the iSolution software to calculate the particle sizes.

Fig. 2 TEM images low-magnification; (a), (b) high-magnification; (c)–(e) HRTEM; (f) images of ZnO NPs + rGO.

Fig. 3 Size distribution diagrams of ZnO NPs + rGO by histogram curve.

3.2 Crystalline structure

The XRD patterns of the resulting products are shown in Fig. 4. The XRD pattern of GO indicates an intense and sharp diffraction peak at 2θ = 10.6°, attributed to the (001) lattice plane corresponding to a d-spacing of 0.83 nm, which is consistent with the lamellar structure of GO. In addition, Fig. 4 shows the XRD pattern of the ZnO NPs that were decorated on the rGO sheet. All of the detectable peaks can be indexed to the ZnO wurtzite structure (JCPDS card no. 00-036-1451). There are no peaks from GO or other impurities in the XRD pattern of the ZnO NPs. This result could be because of the transformation of GO to rGO; the rGO peaks do not appear here because of the strong peak for the ZnO NPs in the XRD pattern. Furthermore, this pattern shows the XRD results for the ZnO NPs. The ZnO NPs, which were synthesized under the same conditions in a starch environment, do not show crystalline behaviour. Thus, the GO sheets can use a reduced calcination temperature to form ZnO crystals in starch environments. Further characterisations will also confirm these results.

Fig. 4 XRD patterns of the GO sheet, ZnO NPs, and ZnO NPs + rGO nanocomposite.

3.3 Chemical composition

Fig. 5 shows the FTIR spectra of the pristine GO, starch powder, ZnO NPs, and ZnO NPs + rGO nanocomposites. In the FTIR spectrum of GO, the broad peak centred at 3190 cm⁻¹ is attributed to the O–H stretching vibrations, and the peaks at 1731, 1625, 1183, and 1040 cm⁻¹ are assigned to the C=O stretching, sp²-hybridised C=C group, O–H deformation, C–OH stretching, and C–O stretching, respectively.³⁰ In contrast, the peaks at 1731, 1183, and 1040 cm⁻¹ are missing from the FTIR spectra of the ZnO NPs + rGO nanocomposites, which indicate the reduction of GO and its transformation to rGO.^31,32 In addition, the FTIR spectrum of starch powder is shown in Fig. 5. The peaks of the starch are dramatically smaller in the FTIR spectra of the ZnO NPs + rGO nanocomposites. In fact, the annealing process at 350 °C caused the transformation of GO to rGO and removed the starch, which is in good agreement with the XRD results. In addition, the FTIR spectrum of the ZnO NPs + rGO shows a peak at 437 cm⁻¹. The band at 437 cm⁻¹ corresponds to the E₂ mode of hexagonal ZnO (Raman active).³³ Therefore, the resulting nanocomposite consisted of ZnO NPs decorated on an rGO sheet. A weak peak appears at 437 cm⁻¹ for the pure ZnO NPs. Therefore, the FTIR results also show that pure ZnO NPs cannot form at 350 °C in a starch environment.

Fig. 5 FTIR spectra of the GO sheet, starch powder, ZnO NPs, and ZnO NPs + rGO nanocomposites.

3.4 Optical properties

Fig. 6 shows the Raman spectrum of the ZnO NPs + rGO nanocomposite. The graphene obtained from the chemical reduction of GO exhibits two characteristic main peaks: the D band at ∼1365 cm⁻¹, which arises from a breathing mode of κ-point photons with A_1g symmetry, and the G band at ∼1610 cm⁻¹, which arises from the first-order scattering of the E_2g mode phonons of the sp²-bonded carbon atoms.³⁴ The D and G band positions and intensity ratios of I(D)/I(G) for the GO and ZnO NPs + rGO composites, prepared using the sol–gel method, are summarised in Table 2. In comparison to the pristine GO, the Raman spectrum of the ZnO NPs + rGO nanocomposite shows that the D and G bands shifted to lower wave numbers at ∼1357 and ∼1600 cm⁻¹, respectively, because of the reduction process for the GO, which can be supported by starch as the reducing, capping, and stabilising agent.³⁵ In addition to the peaks associated with the D and G bands of graphene, the Raman spectrum of the ZnO NPs + rGO nanocomposite shows a sharp and narrow peak at 437 cm⁻¹ corresponding to the E₂ (high) mode of the Raman active mode, a characteristic peak for the wurtzite hexagonal phase of ZnO. The Raman results confirmed that the ZnO NPs + rGO nanocomposite was composed of graphene nanosheets and pure ZnO.

Fig. 6 Raman spectrum of the ZnO NPs + rGO nanocomposites and GO.

Table 2 D and G peak positions and intensity ratios of I(D)/I(G) (obtained by Raman analysis) of GO and ZnO NPs + rGO composites prepared sol–gel method

	GO	ZnO NPs + rGO
D band (cm^−1)	1365	1357
G band (cm^−1)	1610	1600
I(D)/I(G)	0.81	1.13

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The UV-vis absorption spectra of the ZnO NPs and ZnO NPs + rGO at room temperature are shown in Fig. 7. These spectra reveal a characteristic absorption peak for ZnO at a wavelength of ∼360 nm for ZnO NPs and ∼375 nm for the ZnO NPs + rGO samples due to the electron transitions from the valence band to the conduction band (O_2p → Zn_3d), which can be assigned to the intrinsic band-gap absorption of ZnO.³⁶ Furthermore, it is observed that the sharp characteristic absorption peak at 365 nm indicates the existence of good crystalline and impurity suppressed ZnO NPs.^37,38 As observed, the small redshift (∼15 nm) of the absorption edge compared to pure ZnO should be attributed to the chemical bonding between ZnO and rGO, which is similar to the result in the case of ZnO NPs + rGO composite materials.^38,39 However, it is observed that the absorbance of the ZnO NPs + rGO composite increases in comparison to the absorbance of the ZnO NPs. This increase in absorbance may be due to the absorption contribution from rGO, the increase in the surface electric charge of the oxides, and the modification of the fundamental process of electron–hole pair formation during irradiation.⁴⁰ Therefore, the presence of rGO in the ZnO can increase the light absorption, which is beneficial to the photocatalytic performance.

Fig. 7 UV-vis spectra of the pure ZnO NPs and ZnO NPs + rGO nanocomposites.

4. Photocatalytic measurements

4.1 MB degradation

Fig. 8 illustrates the optical absorption spectra of the MB aqueous solution with 10 mg of the as-prepared ZnO NPs + rGO composite after exposure to UV-vis light irradiation for different intervals of times. The intensity of the absorption peak of the MB at 663 nm decreases with an increase in the irradiation time, which indicates that the MB molecules are degraded by the catalysis.

Fig. 8 The UV-vis absorbance of MB over time during photocatalytic degradation under UV-vis light irradiation using ZnO NPs + rGO.

Further experiments were performed to compare the effect of graphene oxide on the catalytic activity of the as-prepared ZnO NPs (sample L), and the results are shown in Fig. 9. The samples K and J with concentrations of 0.9 and 3.3 GO (wt%/v), respectively, and F, which is the same ZnO NPs + rGO with a concentration of 1.7 GO (wt%/v) of the composite, show significant improvements and higher efficiencies in the photodegradation of MB compared to the pure ZnO NPs. The efficiency for bare ZnO NPs is 37%, and nearly 63% of the primary dye still remained in the solution for pure ZnO NPs. For the K and J composites over the same time interval as the ZnO NPs, the efficiency has increased to 86% and 88% for sample K and sample J, respectively, and reached a maximum value of 92.5% for ZnO NPs + rGO (sample F) (Table 3). In general, the degradation efficiency of MB dye was calculated using the following expression:⁴¹

where C₀ is the absorbance of MB in dark and C_t is the absorbance of MB under light irradiation conditions at time t minutes.

Fig. 9 Degradation rate of MB at different intervals with and without catalyst.

Table 3 Photocatalytic degradation percent of MB on samples

Sample	GO (wt%/v)	Degradation efficiency (%)
ZnO NPs (sample L)	0	37%
Sample K	0.9	86%
Sample J	3.3	88%
ZnO NPs + rGO (sample F)	1.7	92.5%

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The photocatalytic rate constant for the MB degradation (k) was determined from the first-order plot using the following equation:⁴²

Here, C₀ is the initial absorbance, C_t is the absorbance after time t, and k is the first-order rate constant. Fig. 10 shows this plot for the ZnO NPs and ZnO NPs + rGO nanocomposites. The calculated rate constants for the ZnO NPs and ZnO NPs + rGO nanocomposites were 0.011 and 0.023 min⁻¹, respectively.

Fig. 10 Kinetic study of photocatalytic MB degradation using ZnO NPs and ZnO NPs + rGO.

The stability test of photocatalytic degradation of MB by ZnO NPs + rGO under UV-vis light irradiation was performed and illustrated in Fig. 11. The photocatalytic measurement was performed with five consecutive cycles, each for 120 min. There was no significant decrease in the photodegradation efficiency, indicating perfect reuse and performance of the ZnO NPs + rGO composite. In addition, for stability of the nanocatalyst, the nanocomposites were characterised using XRD and FTIR after five treatments (600 min). These results are shown in Fig. 12(a) and (b). No difference between these results and the first characterisations that were performed before treatment could be observed. Therefore, it can be understood that the nanocomposites will be stable in crystal structure, phase and chemical composition after photocatalyst treatment, which is beneficial to the photocatalytic performance.

Fig. 11 Photo-stability of ZnO NPs + rGO by investigating its photocatalytic activity under UV light irradiation with five times of cycling uses.

Fig. 12 XRD pattern (a), FTIR spectra (b) of ZnO NPs + rGO after five cycles.

4.2 Photodegradation reaction mechanisms

In addition, Fig. 13 shows that the conduction band and valence band for ZnO are −4.05 eV and −7.25 eV (vs. vacuum), respectively.³⁷ The work function of rGO is −4.8 eV.⁴¹ Therefore, under UV-light irradiation, electron–hole pairs are generated within ZnO, and these photoinduced electrons are easily transferred from the ZnO conduction band to rGO sheet via a percolation mechanism and then scavenged by dissolved oxygen, which causes electron–hole separation. In contrast, the holes can react with either adsorbed water or surface hydroxyls to form hydroxyl radicals. Consequently, the adsorption equilibrium is destroyed, and more MB molecules could move from the solution to the interface and, thus, decompose to CO₂, H₂O, and other minerals via a redox reaction. In general, from the thermodynamic point of view, if the conduction band was more negative than the O₂/O₂⁻ couples, the photogenerated electrons could reduce O₂ to produce O₂⁻. Meanwhile, if the position of the valence band was more positive than the OH⁻/˙OH couples, the photogenerated holes could oxidize OH⁻ or H₂O to form ˙OH. Some semiconductors, including TiO₂, ZnO, SnO₂, SrTiO₃, BaTiO₃ and NiO, corresponded to the case considered above.^6,43–47 Accordingly, when a semiconductor is illuminated with photons, electrons in the valance band of the semiconductor are excited into the conduction band, resulting in the generation of electron–hole pairs. These electron–hole pairs either recombine or migrate to the surface of the photocatalyst to initiate a series of photocatalytic reactions and produce hydroxyl radicals, ˙OH and superoxide radicals, ˙O₂⁻ in water, resulting in the degradation of organic pollutants. It has been found that ˙OH is a major contributor to the photocatalytic degradation of the dye. Therefore, the photodegradation reaction mechanisms of MB under UV-vis light irradiation are summarised by the following equations:

ZnO + hν → ZnO(e⁻ + h⁺) (3)

e⁻ + rGO → rGO(e⁻) (4)

rGO(e⁻) + O₂ → rGO + ˙O₂⁻ (5)

h⁺ + OH⁻ → ˙OH (6)

˙OH + MB → degradation (CO₂·H₂O)^6,46,48 (7)

Fig. 13 The energy level diagram for ZnO NPs + rGO.

Therefore, rGO can effectively improve the charge separation and suppress the recombination of excited carriers, indicating the higher photocatalytic activity of the ZnO NPs + rGO. To further confirm the above assumption, photoluminescence (PL) spectra of the as-prepared samples were investigated as shown in Fig. 14. The two typically sharp and broad peaks of pure ZnO nanoparticles can be found at 364 and 538 nm,⁴⁹ corresponding to the near band edge (NBE) emission and deep level emission (DLE), respectively. The NBE emission originates from the recombination of free excitons in the near band edge of the wide band gap ZnO nanoparticles, and the DLE emission is assigned to various intraband defects in the crystal, such as zinc vacancies, interstitial zinc, oxygen vacancies, interstitial oxygen, and antisite oxygen.⁵⁰ The UV emission is also called near-band-edge (NBE) emission because of the recombination of free excitons through an exciton–exciton collision process. It has been suggested that the green band emission (deep level emission (DLE)) corresponds to a singly ionised oxygen vacancy in ZnO and results from the recombination of a photogenerated hole with the singly ionised charge state of this defect.²⁷ In addition, compared to the pure ZnO nanoparticles and ZnO NPs + rGO, the PL spectra are approximately similar to the pure ZnO but are quenched in the ZnO NPs + rGO. A significant fluorescence quenching of ZnO NPs can be observed after coupling ZnO NPs with rGO due to the interactions of the ZnO NPs surfaces with rGO, which illustrates that the electron–hole pairs in the excited ZnO NPs could be efficiently separated, and efficient transfer of photoinduced electrons between ZnO NPs and rGO could occur. In fact, the large surface area of the rGO causes the ZnO NPs to disperse. Therefore, the dispersed NPs absorb light and generate more electron–hole pairs to remove dye molecules.

Fig. 14 PL spectra of the pure ZnO NPs, ZnO NPs + rGO.

5. Conclusions

ZnO NPs + rGO were synthesized using the sol–gel method in a starch environment. TEM images showed that the ZnO NPs were decorated and dispersed on the rGO. An HRTEM image of the NPs revealed that the ZnO NPs were single crystals without any defects. The XRD pattern of the ZnO NPs + rGO indicated a hexagonal phase of the product obtained. The FTIR results showed that the annealing process removed the starch environment and formed the ZnO structure. In addition, the FTIR showed that the GO was transformed into rGO by the annealing process in a starch environment. The photocatalyst activity showed the high MB removal efficiency of the ZnO NPs + rGO in comparison to the ZnO NPs. This method can be used for the large-scale removal of pollutants from wastewater. Generally, it was established that the reduced graphene oxide sheets played important roles in enhancing the photocatalytic efficiency of the ZnO NPs + rGO nanocomposite compared to the bare ZnO NPs: (1) the prevention of ZnO NPs + rGO agglomeration, leading to the growth of small nanoparticles on the surfaces, (2) the increasing adsorption of MB molecules, and (3) the suppression of electron–hole recombination.

Acknowledgements

Majid Azarang gratefully acknowledges the support provided by the University of Malaya for this research work through research Grant no. PG058-2012B and acknowledges the University of Sistan and Baluchestan, Zahedan, Iran. In addition, Ahmad Shuhaimi acknowledges Grants UM.C/625/1/HIR/MOHE/SC/06 (High Impact Research Grant-HIR), FP009-2013A (Fundamental research Grant Scheme-FRGS), and RG141-11AFR & RP007B-13AFR (University of Malay Research Grant-UMRG).

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