Sonochemical Synthesis, Characterization, and Photocatalytic Performance Evaluation of Au/ZnO Nanocomposite for Methyl Orange Degradation

: This study examines the impact of gold (Au) incorporation on zinc oxide (ZnO) nanoparticles. Both pure ZnO and Au/ZnO nanocomposite have been synthesized using a unique and environmentally friendly sonochemical approach. The as-synthesized Au/ZnO nanocomposite was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet-visible (UV-vis) spectroscopic techniques. Under UV-visible irradiation, the photocatalytic effectiveness of ZnO nanoparticles and Au/ZnO nanocomposites for the degradation of dye was examined. The results demonstrated that the nanocomposite has greater photocatalytic activity than ZnO nanoparticles. This is due to the effective electron transfer from ZnO to Au prolonging the lifetime of photogenerated holes, which play the most important role in the dye degradation process.


Introduction
Pollution in the air and water is primarily caused by chemicals generated by different industries.Organic dyes such as methylene blue (MB), methylene orange (MO), and rhodamine blue are the most common sources of hazardous coloring agents.The presence of these contaminants in groundwater and surface water is harmful to both the ecosystem and living creatures (Hossain, Furusawa, & Sato, 2023).Rapid industrialization is the major source of pollutants discharged through effluents.Colorants such as organic dyes have become more prevalent in water bodies due to their use in the cosmetics, food, textile, and paper industries (Janani et al., 2021).
Photocatalysis is regarded as one of the most effective and adaptable approaches for the elimination of organic contaminants from wastewater.As semiconductor materials possess special photocatalytic activity, they are considered prominent catalysts for water purification under light irradiation (Liu et al., 2019).Using semiconductor materials for the detoxification of contaminants offers several benefits over conventional treatment techniques by decomposing toxic organic dyes into environment-friendly products with no waste generation (Lam, Choong, et al., 2022).
ZnO is a technologically essential semiconductor and is believed to be a competing photocatalyst among many other metal oxide semiconductors due to its high light sensitivity, large area-to-volume ratio, wide band gap (3.37 eV), low cost, and good chemical stability (Lam, Sin, et al., 2022;Mei, Zhou, Lu, & Cheng, 2020;Sivakumar, Gajendiran, & Jayavel, 2020).To be more precise, the high surface reactivity of zinc oxide tends to create a significant number of defect sites from oxygen nonstoichiometric reactions, which is one of the reasons why zinc oxide is a better electrocatalyst than other metallic nanoparticles (Shaymardanov, Rustamova, Jalolov, & Urolov, 2023).
The noble metal gold has been used in the production of ZnO-metal composite due to its strong electron attraction behavior and ability to build a large Schottky barrier between semiconductors and metals (Bora, Kyaw, Sarkar, Pal, & Dutta, 2011).Au has the benefit of not corroding in photocatalytic processes, and gold doping is a handy approach to regulate the flow of photogenerated charge carriers under UV irradiation, which helps to prevent electron-hole recombination (Long et al., 2011).
Due to localized "hot spots" created by bubble collapse, the sound's diffused energy is condensed into chemically usable energy with temperatures up to 5000 °C, pressures up to 1000 atm, and a lifespan of less than a millisecond (Bhangu & Ashokkumar, 2016;Z. Li et al., 2021).
Here, we report the synthesis of Au/ZnO nanocomposite using a sonochemical method and investigate the influence of gold inclusion on the photocatalytic performance of ZnO.

Materials
All the chemicals employed were of analytical grade and did not undergo any further purification.The chemicals used in this experiment were zinc nitrate hexahydrate (Sigma Aldrich, reagent grade 98%), ethanol (Merck, Germany), ammonium hydroxide (Merck, Germany), sodium citrate dibasic dehydrate (Sigma Aldrich, USA), and gold (III) chloride trihydrate (Sigma Aldrich, USA).

Synthesis of Au/ZnO Nanocomposite
0.2 M aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2.6H2O))was incorporated into a mixture consisting of 200 ml of ethanol and 100 ml of water under magnetic stirring.After 10 min, 8.16 ml of NH4OH was added to the mixture.The mixture was ultrasonically irradiated at room temperature for 4 h, and then centrifuged at 7000 rpm for 10 min.After being rinsed with ethanol three times, the residue was dried at 120 °C for 24 h.To coat the exterior of ZnO NPs with gold, 0.3 g of ZnO NP was sonicated for 10 min while being submerged in a 40 ml aqueous solution of 0.01 mmol HAuCl4.The solution was occasionally sonicated to minimize agglomeration during stirring and brought to the boiling point.Next, 10 ml of 0.01 M sodium citrate solution was added.The hue went from white to gray.Then, the solution was agitated for 2 h at room temperature so that the Au could thoroughly modify the ZnO nanoparticles.To prevent the solution from clumping up too much while being stirred, it was occasionally sonicated.
After centrifuging the resulting powder at 8500 rpm for 10 min, it was rinsed three times with deionized water and absolute ethanol before being dried in a vacuum oven at 80 °C for 24 h.The dried nanocomposites were crushed into fine powders and calcined at 500 °C in a muffle furnace.The calcined samples were kept under an inert atmosphere until they were ready for use.

Characterization
The XRD spectra were recorded with a powder diffractometer (Ultima IV, Rigaku Corporation, Japan).A field emission scanning electron microscope (FE-SEM) (Hitachi Ltd., Japan) was used to study the morphological structure of the samples.The FE-SEM observations were conducted at a voltage of 5 kV and a current of 10 μA.Carbon tape was used to hold the powder prior to observation.Next, platinum was coated onto samples using a Pt-Pd ion coater (Hitachi, E-1030, Japan.A UV-visible spectrophotometer (SHIMADZU UV-1650pc, Japan) was used to record the dye concentration.

Photocatalytic Experiments
In a 100 ml quartz beaker, 75 ml of 5×10 -4 M methyl orange solution and 0.25 g of catalyst were allowed to reach equilibrium while being agitated for 30 min in the dark.Experiments involving photocatalytic degradation were conducted under ambient conditions.For the photocatalytic stability study, pure ZnO and Au/ZnO samples were used.3 ml of the solution was taken out at various time intervals and centrifuged to remove the catalyst from the solution.The spectra were recorded using a Shimadzu UV-visible spectrophotometer (UV-1650pc).Degradation (or elimination) % was determined according to the following equation: where A0 is the absorbance of the MO dye in solution at the preliminary stage and A is the absorbance after a certain time (t).

Discussion
In XRD analysis, crystal quality was assessed for ZnO and Au/ZnO composite.The powdered XRD pattern for the synthesized compound is shown in Figure 1.The crystallite size (D) of the samples was determined by the X-ray line broadening method using the Scherrer equation (Viter et al., 2015).
where λ = 0.154 nm (X-ray wavelength), θ is the diffraction angle and β is the full width at half maximum (FWHM).Initial morphological analysis of ZnO and Au/ZnO samples was conducted using FE-SEM.It can be seen from the SEM micrograph of ZnO (Figure 2a) that some spheres are aggregated and linked to one another, and their surfaces are not smooth.On the contrary, smooth spherical spheres were observed in Au/ZnO samples indicating the coating of Au on the ZnO surface (Figure 2b-2d).Energy-dispersive X-ray (EDS) spectral peaks verify the existence of Zn, O, and Au (Figure 3).
The presence of distinct peaks suggests that the specimen is composed of Au and ZnO (M.-K. Lee & Tu, 2008).
Under UV irradiation, methyl orange (MO) was selected as a model pollutant for evaluating the photocatalytic activity of ZnO and Au/ZnO.A graph of the degradation percentages of MO using these photocatalysts is depicted in  To efficiently separate photoelectron (e -) and hole (h + ) pairs, it is commonly assumed that excited photoelectrons from the ZnO conduction band may swiftly and readily move to the AuNPs surface.With Au acting as an electron sink, photogenerated electron/hole pairs on ZnO surfaces are allowed to spread more evenly, thereby reducing the recombination of charges.(Verma, Tirumala Rao, Singh, & Kaul, 2021).
The rate constants were calculated according to the following equation (Vu et al., 2021): where k is the pseudo-first-order rate constant; C0 and Ct are the concentrations of dye at initial (t = 0) and time t (min), respectively.
It was found that the rate constant of Au/ZnO is considerably higher than ZnO NPs (Table 2).The following straightforward process, outlined in Equations (1-7), may be used to explain the degradation of MO by ZnO and Au/ZnO composite.
OH･ + MO → colorless product (8) O2 − + MO → colorless product (10) It may entail the direct interaction of the dye with photogenerated holes in a method comparable to the photo-Kolbe reaction or oxidation via sequential assaults by hydroxyl radicals or superoxide species.It has been shown that the presence of the hydroxyl radical, a particularly potent non-selective oxidant, may result in the partial or total oxidation of a variety of organic compounds.After being excited by UV light, ZnO contributes to the dissociation of water molecules into positively charged ions (H + ) and negatively charged ions (OH -).The very unstable hydroxyl ions are believed to be responsible for the decomposition of the dye.It is done by repeatedly assaulting the N=N link and, as a result, gradually separating the linear chains from the rings.In the presence of dissolved oxygen, the hydroxyl group forms a new bridging with the unattached N=N in the ring opening to form NOOH.This occurs when the hydroxyl group is exposed to oxygen.
Au acts as an electron sink (receptor) (Peychev & Vasileva, 2021).During the light excitation, an electron created by the ZnO is temporarily held on the Au surface.To keep the charge/electron balance in the ZnO, the electron is transferred back to the ZnO.

Conclusion
In this study, the sonochemical synthesis method was used to synthesize Au/ZnO nanocomposite.ZnO and Au's structural patterns were revealed by XRD spectra of Au/ZnO particles.Homogeneous distribution of the Au was observed in the nanocomposite samples.In the presence of ultraviolet light irradiation, MO aqueous solution was studied for photocatalysis.Au/ZnO shows greater photocatalytic activity as indicated by the percentage of degradation and reaction rate as well.

Figure 1 .
Figure 1.XRD Patterns of (a) ZnO Nanoparticle and (b) Au/ZnO Nanocomposite after Heat Treatment at 500 o C. * Indicates the Peaks Originated Due to Au Inclusion in the Au/ZnO Sample

Figure 4 .
It shows that the decomposition of MO by Au/ZnO is much quicker than by uncoated ZnO, demonstrating the crucial role of Au as a photocatalyst in this process.

Table 1
displays the typical crystalline sizes of the ZnO and Au/ZnO specimens.