Enhanced Photocatalytic Activity of Cu 2 O/ZnO/GO Nanocomposites on the Methylene Blue Degradation

: This study synthesized nanocomposite photocatalyst materials from a mixture of Cu 2 O nanoparticles, ZnO nanoparticles, and graphene oxide (GO) through coprecipitation and hydrothermal methods. This study aims to determine the optimum composition of Cu 2 O/ZnO/GO nanocomposites in degrading methylene blue. The nanocomposite was synthesized in two steps: 1 the synthesis of Cu 2 O and ZnO nanoparticles through the coprecipitation method and the preparation of GO through the modified Hummer method.


Introduction:
The higher activity of the textile industry's development indirectly affects the degradation of environmental quality, especially the generating of polluted water during industrial processes 1 .Most textile industries rely heavily on organic dyes in their dyeing processes 2 .This is extremely harmful if dye waste is released directly into a river without being treated, as the waste will become environmentally toxic and difficult to degrade naturally 3,4 .Another issue is the increasing demand for clean water, resulting in large amounts of wastewater.As a result, wastewater treatment has become one of the decade's biggest challenges 5 .
Generally, textile industry waste contains many azo dyes, compounds with benzene rings 6 .These dyes can be classified as reactive, acid, direct, basic, mordant, disperse, pigment, vat, anionic and ingrain, sulfur, solvent, and disperse dye 7 .Several purification procedures, such as adsorption, coagulation, and advanced oxidation, have been used to reduce the effluent, which contains a variety of dyes in significant amounts, considering the dye's stability and potential toxicity in water 8 .Advanced oxidation is a promising technique because it can transform chemical structures in various contaminated wastewater via oxidation processes to become hydroxyl radicals 9 .
Compared to other advanced oxidation processes such as ozonation, fenton, and adsorption, heterogeneous photocatalysis is more cost-effective to produce a rapid oxidative reaction, free of toxic compounds, and environmentally benign 10 .Waste treatment methods via photocatalytic degradation can effectively lower organic dye levels.Among the various types of photocatalysis, many researchers have focused their attention on semiconductor materials such as titanium dioxide (TiO2), zinc oxide (ZnO), and copper oxide (Cu2O) 11 .Among these semiconductor materials, the photocatalytic degradation capability of Cu2O is of significant interest, as Cu2O is an environmentally friendly ptype semiconductor material with a band gap energy (Eg) of approximately 2.0-2.2eV, implying that the degradation activity is relatively high 12 .In addition, Cu2O photocatalysis can be carried out with the assistance of sunlight 13 .However, like other semiconductor materials, pure Cu2O exhibits a poor photocatalytic degradation capacity for organic dyes, such as Methylene Blue (MB) 14 .The degradation of MB via photocatalysis is based on the fast recombination of electron-hole pairs upon light absorption 15 .Thus, photocatalytic degradation by Cu2O primarily aims to separate the electrons and holes generated by light absorption.
An alternative method for increasing the efficiency of photocatalytic degradation by Cu2O is by incorporating additional materials as electron acceptors 16 .Numerous materials, including n-type semiconductors (ZnO and TiO2), the conductive polymer polyaniline (PANI), graphene, and graphene oxide (GO), have been utilized to synthesize photocatalysts based on Cu2O composites 17,18 .Regarding this, ZnO is very promising as a supporting material for Cu2O composites because ZnO has a band gap of 3.2 eV, a binding energy of 60 MeV, and an efficient electron-hole separation 19 .Besides, GO is a semiconductor material (sp 2 hybridization structure) that exhibits chromophoric properties, allowing it to absorb free electrons rapidly 20 .Therefore, the electron generated by Cu2O can easily be transported to GO, avoiding the recombination of electron and hole pairs 21 .
Furthermore, since the GO surface comprises chemical bonds with oxygen, it is well suited for the adsorption of organic dyes, enhancing the composite's photocatalytic degradation efficiency.Different investigations have used a combination of Cu2O photocatalysts, ZnO, and GO for the degradation of dye materials, as demonstrated in Zou et al. 22 , Ma et al. 23 , and Huang et al. 24 .Thus, based on the advantages of each material, ZnO and GO seem to be very promising as supporting materials for enhancing the photocatalytic degradation efficiency of Cu2O-based nanocomposites against methylene blue.This work is significant in providing an environmentallyfriendly and rapid method as an alternative to reduce dye effluents, such as methylene blue.This research further furnishes the most recent discoveries of the potency of employing this nanocomposite with a considerable decomposition rate.This study aims to determine the most effective composition of Cu2O/ZnO/GO nanocomposites for the degradation of methylene blue as an elaboration for our previous work 25 .

Materials
The materials used in this study were natural graphite, polyvinylpyrrolidone (

Instrumentation
X-ray diffraction (XRD, Shimadzu Analytical Type 600) was used to determine physical properties, notably the crystal structure.Fieldemission scanning electron microscope (FESEM) was employed to assess surface morphology and elemental content.
Brunauer-Emmett-Teller analyzer (BET Sorption Analyzer NOVA 1000) was used to analyze specific surface area, pore volume, and pore size.In addition, fourier transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum One) was used to determine functional groups of materials.Ultraviolet-visible spectroscopy (UV-Vis, Perkin Elmer) was used to determine the optical properties, including the absorbance value and energy gap.Meanwhile, photoluminescence spectroscopy was used to assess photocatalytic performance, specifically photocatalytic degradation activity.

Procedure Preparation of Cu2O Nanoparticles:
Cu2O nanoparticles were synthesized through the coprecipitation method using Cu(NO3)2.3H2Oprecursor as a source for Cu 2+ ions.The procedure followed Regmi et al. 26 with some modifications.Five grams of Cu(NO3)2.3H2Owere dissolved in distilled water.Then, 0.2 g of PVP was dissolved in EG at 40 °C.The Cu(NO3)2.3H2Osolution was then added to the PVP/EG solution and stirred for 30 minutes at 180 °C.After homogenization, the solution was centrifuged and rinsed three times using distilled water and ethanol.After that, the material was dried for 12 hours at 60 °C to obtain Cu2O nanoparticle powder.

Preparation of ZnO Nanoparticles:
ZnO nanoparticles were synthesized by dissolving 3 mmol of Zn(NO3)2.6H2O in 50 mL of distilled water.Then, 6 mmol of C6H5Na3O7.2H2O(sodium citrate) was gently added to the solution while stirring until it turned transparent.Sodium citrate was added to increase the interlayer spacing of ZnO nanoparticles along the c-axis 27 .Then, up to 10 mL of 0.1 M NaOH was added dropwise under stirring for 60 minutes at room temperature.The precipitate was then centrifuged and neutralized with distilled water and ethanol to get pure ZnO nanoparticles.After washing, the precipitate was dried in an oven at 60 °C.

Preparation of GO:
The modified Hummer method was applied to prepare GO 28 .Two grams of graphite powder were dissolved in 98 mL of 98% H2SO4, and 4 g of NaNO3 were added after 1 hour of stirring.After 2 hours of stirring, 8 grams of KMnO4 was gradually added to the mixture.The stirring was stopped after 24 hours.The mixture was then washed using distilled water, and the remaining KMnO4 was removed using H2O2.After that, the mixture was centrifuged and washed with HCl and distilled water to neutralize pH and remove any remaining SO 2-ions.The mixture was then dried at 110 °C for 12 hours to obtain graphite oxide sheets.40 mg of graphite oxide was dissolved in distilled water and then ultrasonifying for 90 minutes at 50/60 Hz.Finally, graphite oxide was reduced by adding 0.8 g of Zn metal and 35% HCl and stirring for 1 hour to obtain GO.

Photocatalytic Testing:
To obtain the optimal composition of Cu2O/ZnO/GO nanocomposites, the ratio of Cu2O:ZnO nanoparticles was varied using the hydrothermal method in 1:1, 1:2, and 2:1.After determining the optimal ratio (based on our previous study 25 , it was 1:2), the GO composition used in the nanocomposite was determined by varying into 1%, 3%, 5%, and 10%.Photocatalytic testing was conducted using UV light from a halogen lamp.The optimal wavelength of 664 nm was used for this procedure.The photocatalytic reaction was initiated by adding Cu2O/ZnO/GO nanocomposites to 40 mL of 5 mg L -1 MB solution.The mixture was sonicated and then stirred in the darkness.The solution was exposed to UV light for 120 minutes while constantly stirred.Each solution was taken in 4 mL increments of 10, 30, 60, 90, and 120 minutes and then centrifuged.The absorbance of the supernatant was then measured to evaluate the reaction rate of the methylene blue degradation.

Results and Discussion: XRD Characterization
XRD analysis examined the X-ray diffraction pattern formed by each Cu2O/ZnO/GO nanocomposite sample after GO doping.As illustrated in Fig. 1, the difference in peaks from XRD data for Cu2O/ZnO/GO nanocomposites (Cu2O:ZnO = 1:2) with 1%, 3%, 5%, and 10% GO additions is almost non-significant.The size of the distributed particles in the Cu2O/ZnO/GO nanocomposites can be determined using the diffractogram by calculating the FWHM (full width at half maximum) from the pattern peak using the Scherrer equation approach (Equation 1) 29 .The crystallization size (D) of ZnO nanoparticles in Cu2O/ZnO/GO nanocomposites at the highest diffraction pattern peak was calculated, and the results are shown in Table 1.Furthermore, since other peaks excluded from Table 1 were considered as ZnO with low intensities, the corresponding data were not provided.

𝐷 =
λ βcosθ 1 where D is the crystallite size (nm), k is the shape factor (0.9), β is full width at half maximum of peak (FWHM), λ is X-ray wavelength (0.154 nm), and θ is diffraction angle (radians).According to the findings, increasing the amount of GO in the nanocomposite causes a growth in the size of the ZnO crystals, which comes out to 24.26, 27.29, 31.19 and 31.19 nm for 1, 3, 5, and 10% of GO, respectively.Hence, it is reasonable to propose that the rGO sheets play a role in the growth of nanocrystals and act as nucleation centers and templates.This can cause the nanocrystals to grow on specific sites and directions, ultimately changing the material's morphology and close contact between CuO and ZnO nanoparticles 30 .This occurred because of the mutual attraction between Cu + and GO on ZnO.Despite this, the crystal size of the sample with 5% GO and that with 10% GO shows the same value.This could be because the amount of GO that can occupy Cu2O/ZnO has reached its max amount, meaning that adding more GO will not significantly impact the crystal's size.In addition, the larger crystal size can potentially speed up the charge carrier process that results from the absorption of photon energy to carry out photocatalytic activity optimally 31 , thereby increasing the ability to adsorb methylene blue compounds.

FESEM Morphology
FESEM analysis was performed on Cu2O/ZnO/GO nanocomposites with GO additions of 1%, 3%, 5%, and 10%.This analysis aimed to evaluate the surface morphology of nanocomposites, as presented in Fig. 2. The results of the FESEM examination yielded an image of reduced graphene oxide nanosheets with a layered structure 25 .According to morphology analysis, GO appears to be composed of thinner sheets that are accumulating due to the chemical exfoliation of graphite oxide.The surface of the Cu2O/ZnO nanoparticles is well decorated with GO nanosheets, as demonstrated by the SEM images prominently and clearly.In the meantime, Cu2O nanoparticles are still present in nanocubes attached to the surface of ZnO and scattered throughout the surface.In addition to this, ZnO nanoparticles have needle shapes and sizes that are not consistent.In supporting this, XRD characterization found that incorporating GO into the nanoparticles resulted in a lower texture coefficient for ZnO (10% GO).This could be attributed to the electrochemical growth of ZnO nanocrystals onto the prominent GO sheets 32 .

BET Analysis
The BET method determined the photocatalyst's surface area, average pore radius, and pore volume.In this study, the specific surface area refers to the region of active absorption on the surface of the Cu2O/ZnO/GO nanocomposites photocatalyst when nitrogen gas is applied.The larger the active absorption area of the nanocomposite, the higher the contact between the reactants and the catalyst surface, enhancing the rate of MB degradation.Along with the nanocomposite's specific surface area, the pore size and volume can support MB molecules in entering and attaching to the surface of the Cu2O/ZnO/GO nanocomposite catalytic pores.Table 2 shows the BET characterization.Because of the incorporation of GO doping, the specific surface area of the Cu2O/ZnO/GO photocatalyst increased from 26.291 m 2 g -1 (1% GO) to 35.874 m 2 g -1 (10% GO).In conjunction with the incorporation of GO doping, the same trend can be seen in the average pore size.The pore volume, on the other hand, goes from 0.103 cm 3 g -1 (1% GO) to 0.092 cm 3 g -1 (10% GO), a significant decrease.This is because of the cross-linking between the Cu2O/ZnO and hexagonal GO groups.This cross-linking creates a relatively covalent solid bond on the pore surface in a sheet, effectively covering large pores with an active catalyst site.In addition, these results are also supported by Tantubay et al. 33 , who stated that adding rGO in CuO/ZnO nanoparticles significantly improved the specific surface area.Consequently, it can be inferred that the GO composition variations in this research impacted the nanocomposite's capability to oxidize the MB compound to the most significant potential extent.

FTIR Characterization
The addition of GO dopant variations to Cu2O/ZnO/GO nanocomposites showed no effect on the functional groups, as observed by FTIR.The GO dopant in the nanocomposite is a stabilizer for forming Cu2O and ZnO nanoparticles and a component for filling large cavities generated during the process.Apart from functioning as a filler, GO has no impact on the chemical structure of the Cu2O/ZnO/GO nanocomposites in any way.This is consistent with the results of the FTIR characterization of Cu2O/ZnO/GO nanocomposites presented in Fig. 3, specifically at wavenumbers around 1390 and 1590 cm -1 .At wavenumber 1390 cm -1 , there are observed bending vibrations of the Zn-O bond 34 .In addition, the band at 1590 cm -1 can be attributed to the presence of the symmetric stretching of the Cu-O bond 35 .

Degradation of MB
The photocatalytic degradation of MB by Cu2O/ZnO/GO nanocomposites was monitored using a UV-Vis spectrometer, as resulted in Fig. 4.
At first sight, it appeared as though the absorbance value decreased over time, but the trend was not constant.Table 3 shows the absorbance values more accurately.Determining the absorbance value for the Cu2O/ZnO/GO treatment with the 10% GO concentration proved challenging.On the surface of the Cu2O/ZnO/GO nanocomposites, there may have been material defects that prevented the photocatalyst from functioning as it should have.When the nanocomposite was exposed to radiation from a halogen lamp, this resulted in the formation of many holes as well as electron pairs within the material.Some electron and hole pairs cannot pair successfully because they cannot move to the particle surface and interact with the MB.This prevents those electron and hole pairs from successfully pairing.

Photoluminescence Analysis
The photoluminescence test measured the photocatalyst's ability to degrade MB compounds by photocatalysis.The addition of doping material in GO has a varied influence on the degradation process for each composition variation of the nanocomposites, as illustrated in Fig. 5.The degradation percentages based on these data were 82.0%, 86.0%, 91.4%, and 79.3% for Cu2O/ZnO/GO 1%, 3%, 5%, and 10%, respectively.The introduction of GO into the crystal structure causes an increase in the number of defect sites, reducing the photocatalyst material's capacity to absorb light.This is demonstrated by the results of photoluminescence testing, which are depicted in Fig. 5.These tests showed that 120 minutes of exposure to UV light resulted in the highest MB degrading efficiency of 91.4% for Cu2O/ZnO/GO 5% nanocomposite, with the changes of C/Co from 1 being 0.086.This is because the valence band contains an extra band that acts as the tail states.This causes the excitation of electrons to occur more slowly at lower photon energy levels, reducing the band gap's energy value (Eg).An energy band gap of 1.85 eV has been found in the synthesized nanocomposite.The photocatalytic process will be more effective if the material used as the photocatalyst has a lower band gap energy.This will allow for more significant photon and electron excitation absorption.In addition, because GO has a large active surface area on Cu2O/ZnO/GO nanocomposites, these nanocomposites have an increased ability to conduct electricity.During the photocatalytic process, electrons in the valence band became excited, which resulted in the formation of holes.The photogenerated holes will then directly oxidize methylene blue into reactive intermediates, or they will react with hydroxyl ion (OH -) from water, resulting in a highly oxidizing agent of hydroxyl radicals (OH•), which will complete the photocatalytic reaction.Both reactions are possible 36 .
The photodegradation process initiated by the nanocomposites in this study commenced with the absorption of photons by Cu2O and ZnO.In sequence, there is a reduction of oxygen, followed by the neutralization of OH-ions by positive holes to form OH•, neutralization of oxygen by protons, then leading to the formation of hydrogen peroxide, and finally, the decomposition of hydrogen peroxide.MB dye degradation can occur through three distinct mechanisms: oxidative decomposition of the organic matter catalyzed by hydroxyl radicals (OH•), direct oxidation by hole-electron pairs, and direct reduction through electron-ion interactions as oxidation with anionic radical superoxide species.

Conclusion:
In this study, the optimum percentage of GO dopants added in a Cu2O/ZnO/GO nanocomposite (Cu2O:ZnO = 1:2) for MB degradation was 10%.According to the results of BET analysis, this nanocomposite's specific surface area, pore radius, and pore volume were 35.874 m 2 g -1 , 1.9073 nm, and 0.092 cm 3 g -1 , respectively.Furthermore, the photocatalysis process degraded MB up to 82.0%, 86.0%, 91.4%, and 79.3% using the Cu2O/ZnO nanocomposites (Cu2O:ZnO = 1:2) with GO of 1%, 3%, 5%, and 10%, respectively.This study presents insights for potential future studies to examine the efficacy of this nanocomposite in the degradation of other dyes.Additionally, other researchers may also evaluate the selectivity of this material for a variety of dyes.

Table 3 . Absorbance values obtained during photocatalysis of MB using Cu2O/ZnO/GO nanocomposites.
ZnO was 1:2, wavelength 664 nm, the volume of MB was 40 mL, and each increment was 4 mL.