One Pot and Facile Preparation of Pure ZnO and Cudoped Au-ZnO Nano-particles : Photocatalytic Properties

In this present work, a new synthesis process, named hydrothermal method, of hybrid Au-ZnO nanoparticles (Au-Zn 1-x Cu x O; where 0<x<1%) was presented and discussed in detail. Nanocrystals of copper doped zinc oxide in the presence of diethylene glycol (DEG) as solvent were synthesized using the One Pot and Facile new proposed synthesis process. The photocatalytic activities of the synthesized nanoparticles were tested and analyzed for the degradation of methylene blue (MB) under white light illumination. The performances of the synthesized Au-ZnO nanoparticles showed a pronounced enhancement compared to either pure ZnO nanoparticles or to undoped Au-ZnO nanocomposites. Various key parameters such a photocatalyst loading, the MB concentration, and type current gas have been systematically investigated on the catalytic activities of the as-prepared.


Introduction
During the last few years, organic contaminants have attracted great interest from the scienti c and industrial community in all over the world as major pollutants, which noteworthy contributing to environmental degradation, especially water and the atmosphere.
To the best of our knowledge, because of they contain many dyes, the discharged wastes are considered to be poisonous to microorganisms, aquatic life, human beings, and so on [1]. In this regard, hybrid Metal (Au, Ag, Cu)/semi conductor (ZnO, TiO 2 , ZnS) nanomaterial samples have received a great deal of attention [2][3][4][5] due to the synergetic interaction between the metal and semiconductor components [6,7].
Practically, among the wide variety of the hybrid samples based on metal-semiconductors nanomaterials, hybrid nanoparticles based on Au-ZnO have received considerable attention from academia as well as industry [8][9][10][11][12]. On the other hand, hybrid nanoparticles based on Au-ZnO are crucial samples for research and development of numerous applications such as solar energy conversion [13], biological detection [14,15] sensing elds [16][17][18], photo-catalysis eld [19][20][21] and so on, due to its impressive properties. Consequently, a plethora of research groups have investigated the catalytic effect of Au-ZnO, which are synthesized using various methods like chemical vapor deposition [22], electrodeposition synthesis [23], wet-chemical synthesis [24], coprecipitation method [25], hydrothermal method [26] and Polyol method [27]. Bifunctional nanoparticle Au-ZnO has several advantages: low toxicity, biocompatibility and high chemical stability. The photocatalytic degradation test under UV irradiation of rhodamine B (RhB) deposited on Au-ZnO NPs shows that these NPs exhibit good catalytic performances compared to that of pure ZnO [28] with an increase in the degradation e ciency of a factor 10. This is attributed to charge transfer between the Au and ZnO [29]. Hang Yu and al. [30] prepared Au-ZnO nanocomposites and ZnO nanowires by a simple chemical method and found that the Au-ZnO hybrids posses higher photocatalytic activity for degradation of benzene; Under UV irradiation and visible light, the degradation percentages reach 56.0% and 33.7% respectively. Yuanzhi Chen and al [31] synthesized Au-ZnO nano owers, nanomultipods and nanopyramids by a one-pot method. In comparison with pure ZnO nanocrystals, hybrid Au-ZnO nanoparticles exhibit very high photocatalytic activity during the degradation of Rhodamine B. The photocatalytic properties of hybrid nanowires Au-ZnO synthesized bye Xue Zhao and al. [32] showed a photodegradation of dye methyl orange (MO) increased by a factor 3.
In this work, we report on the one pot and facile synthesis of pure ZnO and hybrid Au-ZnO nanoparticles (Au-Zn 1-x Cu x O; where 0<x<1%) formed by hydrothermal method in diethylene-glycol (DEG) as a solvent, by the sole use of gold, zinc acetate and copper chloride precursors without the adding any other reagents.

Photo degradation experiments
The photocatalytic activities of pure ZnO, pure Au-ZnO and Cu doped Au-ZnO (Au-Zn 1-x Cu x O; where 0<x<1%) nanoparticles were evaluated by the degradation of MB under solar light irradiation [36,37]. In each test, the evaluation of the photo-degradation performance of MB was prepared as follows: 30 mg of the nanoparticles was dissolved in 0.1 L solution of methylene blue (MB) (3 mg / 0.1 L) at pH = 6. Using UV-Visible Spectrophotometer Shimadzu 1650PC, the concentration of (MB) in each degraded sample was determined. The percentage of degradation of (MB) was determined using the following formula: Where C 0 is the initial concentration of MB and Ct is the concentration at time t.  [38]. The lines are particularly wide re ecting the nanometric character of the Au-Zn 1-x Cu x O nanoparticles produced. The diffractograms have shown that the nanoparticles thus produced exhibit very high crystallinity without any heat treatment. We also note that, the X-ray difractions do not show any additional peak corresponding to the oxides mixed CuO, Cu2O, Cu-Zn or to other phases containing copper. Obviously, this result is due to the small amount of Copper incorporated in the materials. Tab.1. shows, the average size of the ZnO crystallites, determined using the Debye-Scherrer formula [39]. The lattice parameters of the synthesized were re ned by the Rietveld method [40,41].The variation of lattice parameters a and c as function Cu content (0<x<1%) with the molar substitution of Zn 2+ , is shown in table 1. It is seen that, the lattice parameters decrease with increasing concentration of copper. Doping makes the ZnO volume slightly smaller, the radius of the  (Fig. 3b) shows the existence of copper. This result thus con rms the substitution of Zn 2+ by Cu 2+ ion. The presence of Ni results from the grid used for the TEM/EDX experiments.

Results
The absorption spectra of the different samples Au-Zn 1-x Cu x O show two absorption bands (Fig. 4a). The ZnO nanoparticles present a maximum absorption around 360 nm. A second broad absorption band centered at about 548 nm, which is assigned to the surface plasmon resonance of the gold nanoparticles (Fig. 4a). Using Tauc's plot method [43], the band gap of the as-synthesized can be calculated. Figure 4.b shows the so obtained bands gaps, they are respectively 3.17, 3.08 and 2.53 eV for x = 0, 0.1 and 1%. The blue shift in the band gap width with the increase in the copper doping content is certainly due to the decrease in the size and lattice parameters of the ZnO nanocrystals, obtained by measurement of X-ray diffraction (Tab. 1) [44].

Degradation of M.B
The catalytic tests of Au-Zn 1-x Cu x O nanoparticles were evaluated for the degradation of MB in aqueous solutions exposed to sunlight. Figure 5a shows the UV-vis spectra of MB containing 0.1 g of suspended Au-ZnO at various irradiation times. The material exhibits an intense absorption peak centered on 663 nm which indicates the intial concentration of MB in the absence of any catalysts. It can clearly be seen that the maximum absorption of MB decreased considerably with increasing irradiation time. To compare the percentage of degradation of methylene blue according to the addition of different photocatalysts, the M.B degradation process was investigated by the reaction kinectics. The given equation has been used to t the experimental data: Where k corresponds to the degradation constant, C 0 and C t are the concentration and MB and t denotes the reaction time. The MB is not degradable at room temperature after 12 h under sun light, explicating that in the experimental conditions, we can neglect the photolysis of the MB molecule. Therefore, without the addition of the Au-Zn 1-x Cu x O nanocomposies,the solution exhibits an intense blue color, which gradually decreases as a function of the irradiation time until it becomes colorless, in the presence of the Au-ZnO nanoparticles (Fig. 5a). The percentage of degradation is of the order of 62%.
The result of photocatalytic degradation of MB under solar light illumination in aqueous solution suggested that Cu doped Au-ZnO exhibited higher photodegradation towards MB than Au-ZnO nanoparticles. In order to compare the effect of copper doping for the Au-Zn 1-x Cu x O (0<x<1%) hybrids on the percentage degradation of MB, all the tests were examined under the same conditions while keeping the other parameters unchanged. Figure 5b shows the C t / C 0 versus time plot for different catalysts. We nd that the catalytic performance of the photocatalysts is proportional with the increase in the level of copper incorporated in the zin oxide material. The MB degradation e ciency increased from 62 % to 92% for Au-ZnO and Cu-doped Au-ZnO (1%) [36]. The following mechanism can explain the important result obtained for the degradation of MB in the presence of the Cu doped Au-ZnO catalyst (Fig. 6). ZnO can generate holes in the valence band (VB) and electrons in the conduction band (CB) under solar irradiation. The photoinduced electrons in the CB of ZnO can transfer to Au since the empty conduction band energy levels of Au lie below the CB of ZnO [33,36,45]. The inhibition of recombination of photoinduced electrons and holes is signi cant due to the formation of the Schottky barrier at the Au-ZnO interface. Therefore, the metal domain can function as an electron sink to achieve enhanced separation of charge    Absorption spectra (a) and Tauc plot for the absorption spectra (b) of pure Au-ZnO and Cu doped Au-ZnO nanoparticles.