Adsorption and sonocatalytic performance of magnetite ZnO/CuO with NGP variation

A series of Fe3O4/ZnO/CuO/nanographene platelets (Fe3O4/ZnO/CuO/NGP) nanocomposites with various NGP weight percents were studied as catalysts for methylene blue removal under adsorption followed by sonocatalytic process. Weight percents (wt.%) of NGP in the nanocomposites were varied (5, 10, and 15 wt.%). The physicochemical properties of the samples were characterized using X-Ray diffraction (XRD), ultraviolet-visible (UV-VIS) spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis, and a vibrating sample magnetometer (VSM). The heterogeneous structure of all samples consisted of the cubic spinel structure of Fe3O4, hexagonal wurtzite structure of ZnO, monoclinic structure of CuO, and graphite-like structure of NGP. With increasing NGP weight percent, sample surface area increased from 14 m2/g to 23 m2/g. Adsorption and sonocatalytic activity were examined on degradation of methylene blue in alkaline conditions. The results show that the adsorption ability of samples increased with increasing NGP weight percent. However, in the sonocatalytic process, Fe3O4/ZnO/CuO/NGP with 10 wt.% NGP exhibited the maximum degradation. The effect of addition different radical scavenger was also examined to understand the sonocatalytic mechanism.


Introduction
Organic dyes used in industrial processes contaminate the environment and its surroundings [1][2]. For that reason, an effective method is needed to manage dye waste water before it is disposed of in the environment. Because it is cheap and easy, adsorption is the most commonly used method of organic dye removal [3][4].
The magnetic material is one of the potential adsorbent in adsorption process for organic dye removal. It has not only the good adsorption capacity but could make the separation process of material from organic dye solution became easier [5]. Among other magnetic material, Fe3O4 is the promising material because it has high adsorption capacity and unique magnetic properties. [6].
Aside from the adsorption process, advanced oxidation processes (AOPs) have been used for the removal of organic dye, as their observed degradation ability is better than adsorption process and could mineralize hazardous substances into simple molecules [7]. The AOPs method recently used is photocatalytic. However photocatalytic process has limitation when applied to degrade the high concentrate organic dye due to limited light penetration [8]. It is thought that ultrasonic waves may resolve that issue because of their high penetration of any liquid medium [8][9]. Applying ultrasonic waves in water causes acoustic cavitation, consisting of processes of nucleation, rapid growth, and Applying ultrasonic waves in water causes acoustic cavitation, consisting of processes of nucleation, rapid growth, and implosion, involving bubble collapse. This collapse produces high-temperature (5000 K) and high-pressure (1000 atm) "hot spots," followed by light emission [10][11], commonly known as sonoluminescence that could interacts with the catalyst to form electron-hole pairs that degrade the organic dye [12].
The semiconductor zinc oxide (ZnO) is commonly used in sonocatalytic process because it is chemically stable, cheap, and eco-friendly [13]. However, sonocatalytic activity of single ZnO is limited by the high rate of recombination of electrons and holes [14]. Combining ZnO with CuO and Fe3O4 is believed to hamper electron-hole combination, therefore increasing the efficiency of organic dye removal [15][16]. Because of its large surface area and good electron transport, the addition of graphene is also believed to increase adsorption capacity and sonocatalytic activity [17]. In our previous works, we have investigated the sonocatalytic activity using NGP materials on Fe3O4/ZnO/CuO nanocomposites [18]. However, the influence of NGP loading as well as adsorption performance of Fe3O4/ZnO/CuO/NGP are also need to be investigate. Therefore, in this research we try to investigate the influence of NGP loading in the adsorption ability and sonocatalytic performance of Fe3O4/ZnO/CuO/NGP nanocomposites.

Materials and methods
Nanoparticles of CuO and Fe3O4 and nanocomposite of Fe3O4/ZnO/CuO with molar ratio of Fe3O4:ZnO:CuO is 1:1:5 were synthesized as described in our previous study [19]. In synthesizing Fe3O4/ZnO/CuO/NGP, nanographene platelets (NGP) were purchased from Angstron Materials. In the typical process NGP were dispersed into a mixture of water and ethanol, using an ultrasonic bath for 2 hours. The synthesized 2 g of Fe3O4/ZnO/CuO was then added to the solution and stirred with a magnetic stirrer for an hour to produce the homogeneous suspension. That suspension was then heated at 120°C for 3 hours, and the precipitate obtained by centrifugation was then dried at 70°C for 12 hours. For present purposes, the amount of NGP was varied at 5 wt.%, 10 wt.%, and 15 wt.%, respectively denoted as FZC, FZC-5 wt.% NGP, FZC-10 wt.% NGP, and FZC-15 wt.% NGP.
Adsorption and sonocatalytic process were analyzed using methylene blue as a model of the organic pollutant. For present purposes, concentration of methylene blue was 20 mg/L, and catalyst dosage was 0.3 g/L. Typically, the methylene blue solution was poured into a 100 mL beaker; Fe3O4/ZnO/CuO/NGP nanocomposite was then added to the solution and stirred using a magnetic stirrer. Adsorption of methylene blue was performed in dark conditions for 4 hours. Meanwhile, the sonocatalytic process was analyzed using an ultrasonic bath operating at a frequency of 40 kHz and 150 W. At specified intervals, the methylene blue solution was taken and analyzed for color degradation by UV-Vis spectroscopy, using the following equation: Decolorization = C C where Ct and Co are the concentrations of methylene blue at time t and in the initial condition (mg/L). The adsorption capacity of the samples was analyzed using the equation where qe is the equilibrium adsorption capacity (mg/g), V is the volume of solution (mL), and m is the amount of catalyst used (g).  Figure 1a shows the results of XRD measurement for Fe3O4/ZnO/CuO/NGP nanocomposite variations of 5, 10 and 15 wt.% (respectively denoted as FZC-5 wt.% NGP, FZC-10 wt.% NGP, and FZC-15 wt.% NGP). For comparison, the XRD spectra of Fe3O4/ZnO/CuO nanocomposite (FZC) were also plotted. These spectra show the cubic spinel crystal structure of Fe3O4, hexagonal wurtzite structure of ZnO, and monoclinic structure of CuO. The presence of NGP was confirmed at a value of 2θ = 26°, indicating the existence of a graphitic-like crystal structure. Lattice parameter values of Fe3O4, ZnO, CuO, and NGP were analyzed using the Rietveld refinement method. As seen in table 1, the lattice parameter values of Fe3O4, ZnO, CuO, and NGP did not change significantly, indicating that the formation of Fe3O4/ZnO/CuO/NGP nanocomposite does not alter the crystal structure of each material.

Results and discussion
FT-IR measurement of FZC-10 wt.% NGP nanocomposite is shown in figure 1b and the results for FT-IR of NGP and FZC nanocomposite are also shown. NGP's vibration mode at wavenumbers 1050 and 1621 cm -1 indicates vibration mode C-C and C=O, while vibration mode of OH was detected at wavenumber 3400 cm -1 [20]. The FCZ nanocomposite material's vibration mode at wavenumbers 400 to 800 cm -1 indicates Cu-O, Fe-O and Zn-O vibration [21][22][23]. For the FZC-10 wt.% NGP nanocomposite, all vibration modes for FZC as well as NGP could be detected, indicating the presence of Fe3O4/ZnO/CuO nanocomposite and NGP in the sample.  UV-Vis diffuse reflectance measurements for the samples (FZC, FZC-5 wt.% NGP, FZC-10 wt.% NGP and FZC-15 wt.% NGP) are shown in figure 2a, representing samples' reflectance ability. The results show that sample reflectance ability gradually decreased in the visible light range with incorporation of 5, 10, and 15 wt.% NGP. The value of the energy gap was analyzed using the Kubelka-Munk equation [24], extrapolating the F(R) 2 curve to the energy value in F(R) 2 = 0. The results confirm that addition of NGP reduced the energy gap value from 2.53 eV to 2.32 eV.
VSM measurements for FZC and FZC-10 wt.% NGP nanocomposites are shown in figure 2b, which also shows the VSM measurement of Fe3O4 nanoparticles for comparison. The results show that magnetization of FZC-10 wt.% NGP nanocomposites is lower than that of Fe3O4 nanoparticles but higher than the magnetic saturation value of FZC-10 nanocomposites. The high magnetization value is very potential for reusability of catalyst by using magnetic separation technique. The inset in figure 2b shows the separation of FZC-10 wt.% NGP nanocomposites material from degreded methylene blue solution using an external magnet, showing that the FZC-10 wt.% NGP nanocomposite can be easily attracted by an external magnetic.
The The sonocatalytic activity of all samples is shown in figure 3b. The results indicate that the addition of NGP to Fe3O4/ZnO/CuO nanocomposite can increase the degradation efficiency of methylene blue through sonocatalytic process, but only up to 10 wt.% NGP. Addition of 15 wt.% of NGP reduced the efficiency of methylene blue degradation through sonocatalytic process. It is probably due to the higher NGP loading reduce the active sites on the sonocatalytic process. The catalyst stability result detailed in figure 4a shows that repeated (four times) usage of Fe3O4/ZnO/CuO/NGP nanocomposite reduced degradation efficiency only until 92%, indicating the good stability of the Fe3O4/ZnO/CuO/NGP sample. On the other hand, separation using an external magnet (inset figure 2b) minimized the occurrence of catalyst weight loss.
To understand the sonocatalytic mechanism, various scavengers were added. There are three type of scavenger: (1) sodium sulfate as electron scavenger, (2) ammonium oxalate as hole scavenger, and (3) tert-butyl alcohol as hydroxyl radical scavenger. The results (figure 4b) indicate that the addition of electron, hole, and hydroxyl radical scavengers reduce sonocatalytic performance. The addition of a scavenger could trap active species involved in the sonocatalytic process and it cannot react in methylene blue degradation therefore the sonocatalytic performance decrease. The addition of hole and OH radical