Visible-Light-driven Photocatalytic Properties of Copper(I) Oxide (Cu 2 O) and Its Graphene-based Nanocomposites

In this study, an improved process was proposed for the synthesis of structure-controlled Cu2O nanoparticles, using a simplified wet chemical method at room temperature. A chemical solution route was established to synthesize Cu2O crystals with various sizes and morphologies. The structure, morphology, and optical properties of Cu2O nanoparticles were analyzed by X-ray diffraction, SEM (scanning electron microscope), and UV-Vis spectroscopy. By adjusting the aqueous mixture solutions of NaOH and NH2OH•HCl, the synthesis of Cu2O crystals with different morphology and size could be realized. Strangely, it was found that the change in the ratio of de-ionized water and NaOH aqueous solution led to the synthesis of Cu2O crystals of different sizes, while the morphology of Cu2O crystals was not affected. The synthesized Cu2O crystal samples were used as photocatalysts for methyl orange (MO) dye decomposition, as a model molecule, to evaluate the photocatalytic activities. However, under 200 watts of a visible light source, there are four samples with and without graphene-based nanocomposite of Cu2O NPs. The results showed that, compared with roughly spherical, irregular but thick plates, brick and small granule spheres shaped Cu2O nanoparticles provided better activity. The Cu2O sample with irregular but thick platelet-like shapes, having an average particle size of 0.53 µm, exhibited excellent photocatalytic activity (99.08% degradation). In addition, by reducing the size of Cu2O particles and preparing their graphene composition, one can fabricate a sample (Cu2-Cu2Gr) with the highest efficiency which has significantly better photocatalytic activity in comparison to the others. This work represents an innovative strategy for pre-the-case production of nanomaterials with shapes and sizes, that is, Cu2O crystals, with excellent photocatalytic activity through compositing with graphene.


Introduction
In the past few years, mastering the tunable structure of inorganic nano-materials to find new properties has become one of the main goals of nanoscience and technology 1 . Such types of nanomaterials have size-dependent optical, electronic, magnetic, chemical, and mechanical properties that are not available in their bulky counterparts 2 . Due to its special characteristics, including its capacity to accelerate chemical processes in the presence of light, and its ban gap being in the visible spectrum, having a shortfall in electrons and holes, and having high stability, Cu2O is a very alluring metal oxide semiconductor 3 . As an important type of semiconductor with a band gap of 2.17 eV 4 , Cu2O has been widely used in catalysis 5 , magnetic storage media 6 , solar cells 7 , dyes photodegradation 8 , lithium-ion battery electrodes 9 , and gas sensors 10 . In fact, the size, shape, and form of the nanoparticles have a significant impact on their attributes and activities 11 . Among all metal oxides, copper oxide nanomaterials have received more attention due to their unique properties 12 . Although there are several methods to synthesize Cu2O nanostructures 13 , the chemical reduction method is a more convenient way to synthesize copper nanoparticles, by changing experimental parameters such as temperature, concentration, pH, etc. 14 . This method not only produces a large number of nanoproducts in a short period but also leads to a highquality structure which enhances the nano-product performance 15 . The main disadvantage of the synthesis of Cu2O nanoparticles by the reduction method is the stability of the acquired nanoparticles 16 . When prepared in an aqueous medium, the newly formed Cu2O nanoparticles will be oxidized under ambient conditions regardless of the nature of the reducing agent used 17 . As aforementioned, it is well known that the shape and size of inorganic materials have a great influence on their physicochemical properties. The synthesis of Cu2O nanostructures is becoming more and more attractive due to the relatively easy preparation, low cost, and wide range of potential applications 18 . In recent years, scientists have made a lot of efforts in the synthesis of Cu2O nanomaterials with different shapes, such as nanorods 19 , nanowires, nanobelts 20 , octahedrons 3 , etc. However, how to effectively achieve the controlled synthesis of materials with the required shape and size remains a huge challenge. Emphasizing the use of Cu2O nanoparticles, it is possible to have more understanding of the photocatalytic properties of Cu2O, which can be provided by consuming these nanocrystals with a certain shape 5,21 .
Graphene is an advantageous substance with sp 2 -hybrid carbon, which has also been introduced into science, medicine, and engineering circles 22 . In 2004, this visual material was produced for the first time during the electro-exfoliation process. In addition to adopting carbon doping for preparation of materials with smaller bandgap, composition formation with graphene nanosheets also increase the light absorption rate of semiconductor-graphenebased NPs. Graphene used as a photosensitizer further expands the fascination with visible light and also increases the photolysis of terminal semiconductor contaminants 23 . The effective compensation of photogenerated electron-hole pairs can reduce energy-ineffective electron-hole recombination 24 . The liquefied mixture of CuCl2, SDS, NaOH, and NH2OH•HCl is arranged to prepare these nanosubstances at room temperature. Cu2O rhombohedral dodecahedrons show exceptionally high-quality photocatalytic activity.
In this study, the synthesis and photocatalytic activity of Cu2O and Cu2O/graphene were scrutinized. In graphene-based composites, due to the adsorption of dyes on the graphene surface and the easy transfer of electrons to the metal oxide conduction band, the photocatalytic activity is significantly enhanced in the graphene-based Cu2O heterojunction composite. However, despite its huge potential, the photocatalytic efficiency of pristine graphene is still very fast due to the rapid recombination of photogenerated electron-hole pairs in single-phase semiconductors. Therefore, the composition of Cu2O can lead to the preparation of a more outstanding photocatalyst. As far as we know, no early reports have been made on the comparison of the photocatalytic activity of Cu2O and its composition with graphene. Therefore, this research was designed to afford such a comparison.

Instrumentation
The phase purity and crystal structure of the prepared samples were examined by X-ray diffraction (XRD) recorded on a D8 Advance X-ray diffraction (Bruker, Germany) with a Cu anode as an X-ray source, employing a scanning rate of 0.02ᵒ /second in the 2θ range of 20ᵒ to 80ᵒ. The machine was operated at 40 kV and 35 mA with a radiation source of Cu Kα λ = 0.15418 nm. Scanning electron micrograph (SEM) images were obtained using a field emission SEM (FESEM) instrument (Hitachi S-4800 II, Japan) and TEM was used to analyze molecule levels for rGO samples using Philips EM120 TEM instrument operating at an accelerating voltage of 80 kV. The UV-Vis absorbance spectroscopy apparatus (Cecil Instruments -Cecil 7400 Double Beam UV/VIS Spectrophotometer, Aquarius Company with wavelength 190 nm-1100 nm).

Synthesis of Cu2O Nanostructures
Cu2O nanocrystals with different structures are mostly prepared by wet-chemical, electrodeposition, and solvothermal production methods 25 . However, in this project, the wetchemical production method was applied to producing Cu2O nanocrystals to obtain tunable structures. Firstly, for the synthesis of Cu2O nanocrystals, stock solutions of all chemicals were prepared before mixing the given ingredients. For this purpose, all-glass vials were washed amid detergent, and standard de-ionized water and finally rinsed through acetone. As the amounts of exploited regents have been given in Table 1. Firstly, for the production of Cu2O nanocrystals with dissimilar morphologies, 9.0 mL, 8.95 mL, 8.90 mL, and 8.85 mL of de-ionized water were added to four glass beakers labelled with 1, 2, 3, and 4, respectively. Afterwards, 0.1 mL of 0.1 M CuCl2 solution was added to each of the four different labelled beakers. The reaction mixture was stirred continuously at room temperature for 3 minutes to make a homogeneous solution. After stirring for about 5 minutes, 0.20 mL, 0.25 mL, 0.30 mL, and 0.35 mL of 1.0 M NaOH (as given in Table 1. were poured into different glass beakers of 1, 2, 3, and 4, respectively. The beakers were stirred again for another 5 minutes. After that, 89 mg of SDS were added to the labelled beakers one by one, and further continuous stirring was continued at room temperature until the powder was completely dissolved. When SDS was completely dissolved in all four beakers, 0.70 mL of 0.2 M NH2OH·HCl was mixed with all these solutions and stirred at room temperature for 3 minutes. After shaking the solutions, the four samples were immediately covered with high-quality aluminum foil and kept in the cabinet for 2 hours to obtain the required Cu2O nanocrystal products with various morphologies. The total volume of the solution in each beaker was 10 mL, and its colour changed from blue to orange and then to pale yellow during the process. Various Cu2O nanoparticles with different structures were prepared by using different chemical solutions containing pre-specified concentrations of reagent (Fig 1), summarized in Table 1.

Preparation of Cu2O/rGO Nanocomposites
Various nanocomposites were constructed with a Cu2O/rGO ratio of 6:3 (w/w). For these syntheses, 90 mg of Cu2O and 45 mg of rGO were poured in two different beakers containing 50 ml of doubly distilled water and sonicated for 30 minutes. The homogenized suspensions of rGO and Cu2O were mixed and further agitated with ultrasonic wave for 60 minutes. Eventually, the acquired suspensions of Cu2O/rGO were oven-dried and exploited for photocatalytic system.

Evaluation of Photocatalytic Performance under Visible Light Irradiation
The photocatalytic activity of the prepared Cu2O samples on the degradation of methyl orange (MO) dye solution was evaluated under visible light irradiation. A 200-watt visible lamp was utilized as the light source to provide visible light illumination. All experiments were performed in air at room temperature. Cu2O/rGO composite was constructed with a Cu2O/rGO ratio of 6:3 (w/w). In each experiment, 10 mg of photocatalyst powder (bare Cu2O or Cu2O/rGO composite) was added to 50 mL of MO solution (0.03 mM). Before irradiation, the suspension was magnetically stirred in the dark for 30 minutes to allow MO to reach adsorptiondesorption equilibrium on the catalyst surface and then exposed to visible light irradiation. While stirring the sample under visible light, 3 mL of the suspension was collected every 10 minutes and filtered to remove catalyst nanoparticles. The MO solution without a catalyst was then analyzed using a UV-Vis spectrophotometer.

Structure and Morphology
To check the structure and size of the nanocrystals in these four Cu2O samples, X-ray diffraction of the deposit was recorded for each sample. Fig 2 shows the obtained X-ray diffraction arrangements which display that the peak positions at 2θ (degree) angles are equal to (110), (111), (200), (211), (221), (220), (310), (3111) and (222) lattice plane. All these peaks originated from copper(I) NPs which were fabricated with different H2O: NaOH ratios. Due to the change in the ratio of H2O: NaOH, the (111) peak intensity of samples Cu1 to Cu3 increases, while the (200) peak intensity decreases. Since the elongated nanostructures show more intense (111) planes 26 , increases in (111) peak intensity from samples Cu1 to Cu3 and the simultaneous decrease in (200) peak intensity reveals that from samples Cu1 to Cu3 the dimensions of Cu2O nanoparticles becomes greater. Nevertheless, in sample Cu4 again the peak intensity of the (111) plane decreases, as the peak intensities in Fig 2. exhibited.

Figure 1. The XRD sketches of rGO and four Cu2O NPs
The peak intensity of the prepared Cu2O samples with different concentrations of deionized water and NaOH decreased significantly from sample Cu1 to sample Cu2. This means that when the meditation of NaOH increases, the peak intensity in the XRD decreases steadily, and the (211) diffraction peak in the XRD pattern almost disappears. Therefore, the deposited samples with different amounts of NaOH showed dramatic variations in the crystallinity and structure of Cu2O 27 . The results showed that NaOH has an extraordinary effect on the surface morphology of Cu2O samples, which indicates that the shape of the interface is affected by the concentration of NaOH in this process. This reflection, which is different from the transition, is caused by the consistency of the particle shape. Due to the nanosize effect, some peak widths are observed. Since the peak intensity of XRD becomes irrelevant to the size of the NP of 150 nm or more, the sample Cu1 exhibits sharp (111) peak and (200) peak intensities due to the high single-crystallization rate. Compared with all other samples, the intensity of the (111) peak is higher in Cu2. This indicates that compared with the (111) peak of other samples, 0.25mL NaOH solution is more suitable for the highintensity growth on the (111) surface. However, the intensity of the (111) peak in samples Cu2 and Cu3 are similar, but in Cu1, the (220) peak is more prominent, which indicates that the surface morphology of the nanostructure has undergone tremendous changes. Compared with the sample Cu2/Cu3 with the same growth but lower intensity, sample Cu1 has fewer planes (111). Similar to Cu1, sample Cu4 has the highest intensity in the (311) peak compared with all other samples. The (110)  In this equation, "D" is crystallite size, "K" is Sherrer constant and equal to 0.9, "λ" is the wavelength of X-rays used during the experiment and corresponded to CuKα 1.5406 Å, "θ" is Bragg's angle while "β" is full width at half maxima (FWHM) 28 . The results obtained from the Scherrer equation and the average crystallite sizes calculated by the method discussed below are summarized as follows in Table 2.
The reduction in the size of the nanocomposite crystallites shows a larger surface-tovolume ratio. When sunlight is irradiated, more lightexcited electron-hole pairs are generated and their photocatalytic activity is enhanced. Measuring the dislocation density due to crystal defects was performed by using the formula δ =1/D 2 28 , and the calculated values are listed in Table 2. The dislocation density (δ) increases as the crystallite size decreases, and vice versa. Significant changes were found through the strains in the four Cu2O, which changed from tensile strains in the nanoparticles to compressive strains. This change in microstrain may be due to the changes in the structure, crystallite size, and morphology, given in Table 2. Therefore, in all four samples, the intensity of the (111) surface is more prominent than that of the Cu2 sample, which has more photocatalytic activity and, the result is given in Table 3. With 90.01% photodegradation of MO (Table 3), it proved that is the best photocatalyst.

SEM Analysis
It is observed from the SEM image that these four samples show various structures. The Cu1 sample is roughly spherical with an average elongation of 2.77 μm. Sample Cu2 has an irregular but thick plate-like shape with an average particle size of 0.53 µm. Cu3 looks like a brick with a maximum average particle size of 4.22 µm, which shows more elongation in the nanocrystalline structure. Among the Cu4 nanoparticles, they are small particles that appear in clusters. The average particle size cannot be measured, but the estimated size is less than 100 nm. The Cu4 sample has a powder structure composed of small crystal grains, and the structure is agglomerated due to improper/fine pulverization of the dried sample. There is elongation from Cu2 to Cu3, and it is confirmed by XRD that the (111) peak intensity increases but the (200) peak intensity decreases.  For achieving, better performance of Cu nanoparticles used rGO and rGO have a humidity sensor and a large surface area to volume ratio (displayed in TEM image; Fig 4), high vacancy density, and hydrophilic functional group on the surface of rGO, which creates new water molecules, that are easily adsorbed on the material surface and thus improves the response of the sensor. Furthermore, the special structure of rGO/Cu allows the charge to transfer from water to Cu nanoparticles, thereby increasing conductivity.

Optical Properties
The spectral analysis of Cu2O nanoparticles is carried out by using UV-Vis absorption spectra with a wavelength of 190 nm-1100 nm. Fig 5 shows the results of the UV-Vis absorption spectra of samples Cu1, Cu2, Cu3, and Cu4. It was observed that the ratio of DI-H2O: NaOH in the reaction solution strongly affected the grain size of the nanoparticles (shown in XRD patterns; Fig 2 and the morphological size of the nanoparticles (shown in SEM micrographs; Fig 3 . One can calculate the band gap of the Cu2O products by using the converted Kubelka-Munk function versus the energy of the excitation light 29 . The peak positions of samples Cu1, Cu2, Cu3, and Cu4 appear at 655 nm, 517 nm, 600 nm, and 511 nm, and the measured energy gaps are in the range of 2.12 eV to 2.24 eV, given in Fig  6. Hence, it is revealed that the change in the ratio of DI-H2O:NaOH in the reaction solution causes a significant change in the peak position, that is, it causes the color shift of the absorption peak.  As can be seen in Fig 5, absorption spectrum of rGO showed that absorption peak of π to π* electronic transition of aromatic carbon appeared at wavelength lower than 300 nm (272 nm). Fig. 6 also shows that rGO a band gap of 1.91 eV which is lower than those of Cu2O samples. Synthesizing composite with metal oxides semiconductors and rGO can lead to noticeable increase in efficiency of semiconductors. Such results are related to the high surface area of rGO for maximum adsorption and lesser electron-hole recombination which is a result of the higher conductivity offered by rGO sheets.
For all four Cu2O samples, the absorbance gradually increased with the maximum peak in the visible light region, and the absorbance peaks of samples Cu2 and Cu3 suddenly decreased at about 600 nm. Then, these peaks reach a maximum value above 600 nm in the visible light region, and they also move to the UV region. Therefore, the size of Cu2O nanoparticles increases with the increase in elongation, and the absorbance peak also increases but becomes straighter and shifts to the blue region of the spectrum. The shift of the absorbance peak from the higher (red) to the lower (blue) region indicates that the size of the nanoparticles is reduced. Therefore, the amount of NaOH strongly affects the size of the nanoparticles.
The added amount of NaOH affects the UV-Vis absorption spectrum of Cu2O nanocrystals in different ways. The peak in the visible light region gradually increases and moves into the ultraviolet region. As a result, the experiment proves that the redshift is indulged in the blue region, which means that most of the cubic crystal has moved into the octahedron, and the lessons learned from this figure are retained, that is, the absorption of Cu2O crystal shifts to blue and its elongation decreases/size dramatic increase. Therefore, XRD crystal size analysis and SEM size analysis confirmed the sizes of the four samples. These crystalline sizes are given in Table 2.

Light-driven Photocatalytic Degradation of Bare Cu2O NPs and Graphene-Based Composites of Cu2O NPs
In the procedure of photocatalytic activity, Cu2O has been used as a photocatalyst for the production of H2 and usual pollutants degradation, under visible light exposure 30 . Previous analyses have revealed that the photocatalytic performance of Cu2O is a high-class evaluation of morphology 31 . The photocatalytic performance of different bare CuO NPs and their composites with rGO was evaluated by the photodegradation of methylene orange under visible light irradiation. The first operation of the added amount of nanocatalyst in the degradation process is discussed 32 . When exposed to visible light, Cu2O semiconductor oxide can be excited to generate electrons and most forms of holes 21 . These electrons and holes can initiate a chain of photodegradation processes. The pores in the valence band can oxidize the hydroxide ions adsorbed on the outside of the catalyst, thereby generating hydroxide ions that play an important role in photodegradation. On the contrary, the photogenerated electrons conducted by Cu from holes can be captured by absorbed O2, and O2 is initially generated (H2O2) and OH2 or O2 -33 . The O2can additionally interact with H2O2 which simplifies the return of OH2•OH and infield contributing to the photodegradation procedure of methyl orange 34 . In conclusion, methyl orange (MO) can be oxidized into intermediates. It can be absorbed from the surface of Cu2O nanoparticles.  Fig 8D shows the degradation process of sample Cu4 and composite material sample Cu4rGO with MO solution. The exposed sample time is 60 minutes, and the composite material sample's 50 minutes is the second-largest degradation time in the photocatalysis experiment. All four bare samples and composites of grapheme are examined bit by bit and upgraded with time increasing in a dark box with a hanging 200 Watt visible bulb. The reactant progress has been examined, and Cu2 was observed with higher absorbance than that of the illustrative calculation of other Cu2O NPs, which might survive recognized to (111) surfaces and are generally little sized. The photocatalytic decolonization of the methylene orange solution had been evaluated to completion at room temperature. One of them was the created sample, which contained reduced graphene powder and Cu2O in a 2:1 ratio. Half of the  In the photocatalytic process, it was observed that the sample Cu2 has the highest adsorption capacity than other samples Cu3, Cu1, and Cu4. That might be due to the nanoparticles size of sample Cu2 which is smaller than other samples. The affiliation stuck between irradiation time along with photocatalytic degradation ratio of MO is exhibited in Fig 9. The photocatalytic degradation ratio is deliberate via using a first-order kinetic Equation  minutes, which is 92.15%; for Cu2rGO, the high efficiency is observed at 50 minutes, which is 99.08%, showing the largest degradation, compared with other studied photocatalysts. This may be due to their small sizes which were observed from SEM, compared to the other three samples. Graphite-like materials (graphene) can increase the surface charge of Cu2O in the photocatalyst and inhibit the recombination of electron-hole pairs. The Cu3rGO observed time for reaching the highest efficiency is 50 minutes, and the observed efficiency is 89.35%, which is due to the 10% increase of reduced graphene over bare Cu1. Similarly, the degradation process of Cu4rGO was repeated, the observed time was 50 minutes, and the observed degradation efficiency was 96.9%. It had the second smallest size, after Cu2rGO, and it also had the second degradation efficiency, compared with other samples.  The decrease in photocatalytic efficiency in Cu3 may be attributed to the increase in the ratio of NaOH and the collection of Cu2O particles. This is because the particle size has increased to micrometres, as mentioned earlier in Table 3.

Conclusion
A simplified wet chemical method was proposed to control structure during the synthesis of copper oxide (Cu2O) nanoparticles (NPs). This can be achieved by adjusting the quantity of the ratio of deionized water and sodium hydroxide in the mixed aqueous solution. The crystal sizes of Cu2O NPs required for the measurement are Cu1/13.006 nm, Cu2/11.66 nm, Cu3/14.85 nm, and Cu4/12.35 nm, respectively. The structure and optical properties of copper(I) oxide are obtained by X-ray diffraction and UV-Vis spectroscopy. Methyl orange (MO) dye was selected as the probe molecule to evaluate the photocatalytic activity of four exposed Cu2O NPs and reduced graphene samples just used under 200 watts of visible light. Experiments showed that the use of a composite material of bare and reduced graphene can more effectively degrade MO in the mixed aqueous solution. The photocatalytic decolourization of MO in an aqueous Cu2O suspension based on bare and reduced graphene composites was carried out by UV-VIS spectroscopy to study the photocatalytic behaviour of Cu2O NPs with various crystal sizes. Under visible light, the absorption of all bare copper(I) nanoparticles was observed, and the decolourization efficiency of the MO solution reached by the bare Cu2O NP was 82.35%, 90.01%, 78.26%, 86.35%, and 81.80%, respectively. However, it was observed that the absorption of all reduced graphene alloy-based copper(I) oxides reduced the time, and the highest efficiency reached 92.15%, 99.08%, 89.35%, 96.89%, and 91.9%, respectively. These results showed that the efficiency of the sample number (Cu2-Cu2rGO) has a higher performance both in the case of bare and rGO composites, but the sample (Cu3-Cu3rGO) has a lower absorption rate than the other samples. The photocatalytic results showed that copper(I) NPs produced with 0.25 mL NaOH (1.0 M) were the best sample during preparation. Because it has good light harvesting ability, and both in the bare sample and in the composite forms it has the highest MO solution decolourization efficiency, respectively in 70 minutes and 50 minutes. Therefore, the composition of Cu2O/graphene photocatalyst may be a potential choice for using solar energy to purify wastewater and remove pollutants (especially emerging pollutants). -Authors sign on ethical consideration's approval.
-Ethical Clearance: Not applicable; there was no need for the project to be approved by any ethical committee, since it was not engaged with human or animals.