Removal of Methyl Tertiary-Butyl Ether via ZnO-AgCl Nanocomposite Photocatalyst

ZnO-AgCl nanocomposites with different mass ratios were synthesized via a one-pot wet chemical method. The synthesized nanocomposites were characterized by SEM, XRD and photoluminescence spectroscopy. The ZnO-AgCl nanocomposites exhibited a significant enhancement of photocatalytic activity towards degradation of methyl tertiary butyl ether in aqueous solution due to the effective carrier separation performance. In addition, the proposed ZnO-AgCl also exhibited an excellent photostability.


Introduction
Methyl tertiary-butyl ether (MTBE) has received a very high attention over the last decade due to its widespread detection in indoor environments 1,2 .MTBE is used as an additive of oxygen added to gasoline in order to elevate the octane number, combustion improvement, and reduce CO 2 production.However, MTBE has been recognized as a prevalent and persistent groundwater and surface water pollutant.Acute toxicity levels of MTBE for rodent LD 50 and lethal air concentrations for rats (LC 50 ) were estimated to be 3.8-3.9g/kg and 65-126 g/m 3 , respectively 3,4 .However, the presence of MTBE in water is mainly responsible for taste and odor related problems.Moreover, the International Agency of Research on Cancer (IARC) and the U.S. Environmental Protection Agency (EPA) classified MTBE as a health risk threat in 2000 5 .Therefore, removal of MTBE from water is an important issue.
So far, many methods have been studied for removing MTBE.Such as adsorption 6 , air stripping 7 , photocatalysis 8 , ozone treatment 9 , Fenton process 10, high energy electron beam irradiation 11 , cavitation 12 , biodegradation 13 and electrochemical oxidation 14 .Among them, designing suitable photocatalyst for MTBE degradation received a considerable attention due to its green approach, free sun energy and easy to scale-up.ZnO is a well-known n-type semiconductor with band gap energy of 3.37 eV, hence is considered as one of the best photocatalysts to deal with organic pollutant [15][16][17][18][19][20][21] .However, the photocatalytic activity of ZnO is still restricted by fast recombination of the photogenerated electron-hole pairs [22][23][24] .Many efforts have been made for extending the absorption range of ZnO, such as elements doping and metal-semiconductor heterostructure formation.Silver halides are widely recognized as photosensitive materials [25][26][27][28][29][30][31][32][33][34][35] .Recently, research showed that the Ag-AgCl supported metal oxide semiconductor has enhanced photocatalytic activity under visible or simulated solar light 36,37 .Therefore, combining ZnO with silver halide is expected as an ideal photocatalyst for MTBE removal.Herein, we proposed a simple one-pot hydrothermal method for ZnO-AgCl nanocomposite formation.The photocatalytic activity of prepared ZnO-AgCl nanocomposite was tested.To the best of our knowledge, this is the first report of using ZnO-AgCl nanocomposite for MTBE photodegradation.

Preparation of ZnO-AgCl nanocomposite
Certain amount of zinc nitrate hexahydrate was dissolved in 50 mL of water.Then, 5 mL of ammonium hydroxide was added into the solution for 15 min stirring.Afterward, a certain amount of AgNO 3 solution (50 mM) was slowly added.After 30 min stirring, 5 mL of NaCl (0.5 M) was slowly added into the formed suspension.Then the suspension was refluxed at 110°C for 30 min.The precipitate was collected by centrifugation followed with water wash.The final product was obtained by calcination of the precipitate in a furnace at 150°C for 1 h (denoted as ZnO-AgCl-1, ZnO-AgCl-2 and ZnO-AgCl-3 for the mole ratios of Zn/Ag set as 3:1, 2:1 and 1:1, respectively).

Characterizations
The morphology and structure of the prepared samples were characterized using a field emission scanning electron microscope (FESEM, ZEISS SUPRA 40VP, Germany) and an X-ray diffractometer (D8 -Advance XRD, Bruker, Germany) with Cu Kα radiation, respectively.The Photoluminescence (PL) emission curves were obtained by a Fluorescence spectrophotometer (HORIBA Jobin Yvon Fluorolog-3-P, Japan) In a typical photodegradation experiment, 40 mg of photocatalyst was added into MTBE solution and kept under dark conditions for 30 min.After light irradiation, 2 mL of suspension was then taken out a certain period and the photocatalyst was separated by centrifugation.The concentration of MTBE was detected by GC-FID analysis 38 .For comparison purpose, nano-size ZnO was synthesized using a similar method for ZnO-AgCl preparation except adding AgNO 3 .P25 (commercial TiO 2 photocatalyst, containing both anatase and rutile phase of TiO 2 ) was also purchased and used as control group.

Characterization of ZnO-AgCl nanocomposite
Fig. 1 shows the SEM images of ZnO-AgCl-1 and ZnO-AgCl-3.As can be seen, the ZnO particles present a rod structure in the ZnO-AgCl nanocomposite with length varying from 30-200 nm.AgCl sphere are attached to the ZnO particles with a diameter of 180 nm.The attachment of the ZnO rods on the AgCl surface can be clearly absorbed.
The formation of ZnO and AgCl nanoparticles should be independent during the chemical reduction process.However, the high temperature reflux condition could produce diffusion-limited-aggregation process, which involves cluster growth by the adhesion of a particle to a seed on contact and growing surface after the reduction 35,39 .It also can be seen that the size of ZnO particles slightly decrease after the mole ratio of Ag increases in the preparation stage.The morphology of ZnO-AgCl-2 (not present) is similar to that of ZnO-AgCl-1 while the ZnO-AgCl-3 shows a much higher content of AgCl particles than that of ZnO-AgCl-1.   the AgCl ratio increasing, suggesting the increase in AgCl content in the nanocomposite.
Fig. 2B shows the PL spectra of ZnO-AgCl-1, ZnO-AgCl-2 and ZnO-AgCl-3.Three main emission bands are observed in the ZnO, which can be attributed to the recombination of photogenerated electron-hole pairs, impurities and structural defects, and oxygen/zinc interstitials, for peaks located at 433 nm, 474 nm and 522 nm, respectively 40,41 .The PL intensities decreasing along with the content of AgCl increasing, synergistic effect between ZnO and AgCl nanoparticles, which effectively reduces the electron-hole pairs recombination and facilitates the charge carrier separation.Among all these samples, ZnO-AgCl-3 displays the lowest PL intensity, which may lead to the highest photocatalytic activity.

Photodegradation of MTBE by ZnO-AgCl nanocomposites
The photocatalytic activity of as-prepared ZnO-AgCl nanocomposites was investigated by photodegradation of MTBE.Fig. 3A shows the photodegradation profile using 40 mg ZnO-AgCl-1, ZnO-AgCl-2 and ZnO-AgCl-3 without light irradiation.Where C0 and C are the initial concentration of MTBE and concentration after irradiation, respectively.It can be seen that the concentration of MTBE did not show a significant decreasing.The slight decreasing can be attributed to the adsorption of MTBE by nanocomposite.Therefore, the results indicate ZnO-AgCl nanocomposite show no photocatalytic activity towards decomposition of MTBE without light irradiation.
We further studied the photocatalytic properties of ZnO-AgCl nanocomposite.Fig. 3B shows the photodegradation profile using 40 mg ZnO-AgCl-1, ZnO-AgCl-2 and ZnO-AgCl-3 under visible light irradiation.It can be clearly seen that three photocatalysts show positive degradation performance toward MTBE.After 120 min visible irradiation, ZnO-AgCl-1, ZnO-AgCl-2 and ZnO-AgCl-3 nanocomposites can remove 62.1%, 92.3% and 77.11% of MTBE from water, respectively.The results did not match to our prediction from PL results.The photocatalytic activity of the ZnO-AgCl-2 showed a higher performance than that of the ZnO-AgCl-3, which could ascribe the sufficient amount of ZnO in the nanocomposite.Therefore, the ratio between ZnO and AgCl is very important factor in the photodegradation performance of ZnO-AgCl nanocomposite.The possible mechanistic pathway of the catalysts for photocatalytic degradation of MTBE is proposed as follows: under light irradiation, photogenerated electron-hole pairs (e cb − -h vb + ) were formed in ZnO rods.Then, the h vb + could react with AgCl to form Ag + and Cl 0 .A part of h vb + generated by ZnO reacted with OH − to form OH. The Cl 0 and OH could react with the MTBE pollutant.Therefore, although the ZnO-AgCl-3 showed a best electron-hole pairs recombination suppression performance, the lack of ZnO to produce the h vb + restricted its photocatalytic activity.In order to evaluate the enhancement of nanocomposite, we also compared ZnO-AgCl-2 with nano-ZnO and commercial photocatalyst P25 (TiO 2 , rutile: anatase = 85: 15).Fig. 3C depicts the removal rate of three photocatalysts using the same conditions.As can be seen that, the removal efficiency order is ZnO-AgCl-2>P25>nano-ZnO.Therefore, our proposed ZnO-AgCl nanocomposite exhibits a clear enhancement towards photodegradation of MTBE.
Besides the visible light irradiation, we also tested the photodegradation performance of ZnO, ZnO-AgCl-2 nanocomposites and P25 under UV light irradiation.Fig. 3D displays the removal rate of MTBE using different photocatalysts after 60 mins UV light irradiation.It can be seen that the ZnO-AgCl-2 still exhibits best performance among the photocatalysts, indicating the ZnO-AgCl nanocomposite can be used as a photocatalyst under both UV light and visible light conditions.
The effects of photocatalyst loading were investigated at 10 mg, 20 mg, 30 mg, 40 mg, 50 mg and 60 mg of ZnO-AgCl-2 nanocomposite.As shown in Fig. 4A, all experiments showed clear degradation of MTBE.When insufficient amount of ZnO-AgCl-2 nanocomposite was added into MTBE solution, the low removal rate could be the result of limited production of OH -caused by the inadequate conversion from light energy to chemical energy 42 .This can be evidenced by the enhancement of removal rate after increase in amount of ZnO-AgCl-2 nanocomposite.On the other hand, at a high concentration of ZnO-AgCl-2 nanocomposite, aggregation of photocatalyst is causing a decrease in the number of surface active sites and an increase in opacity and light scattering leads to a decrease in the transmission of irradiation through the sample 43 .Therefore, 40 mg ZnO-AgCl-2 nanocomposite was used in this study.
The effect of pH on the photodegradation was also studied.As shown in Fig. 4B, the pH condition can apply a significant role in the photodegradation results due to the photocatalytic destruction mechanism in different pHs has different activation energies 44 .At acid condition, the photocatalyst is protonated and became positively charged, which is more favorable for the electron to move from the valance band to the conduction band of the semiconductor particle.Similar phenomenon was observed by other researchers [45][46] .Therefore, pH 3 is selected for optimum condition.
Fig. 4C shows the effect of initial concentration of MTBE on the photodegradation process.It can be seen that the higher initial MTBE concentration could lead to low final removal rate.This is caused not simply by the increase in amount of target molecules but the results are also influenced by suppression of OH ─ radicals formation as a consequence of poisoning the catalyst surface by pollutant ions 47 .
The stability of photocatalyst in photocatalytic reaction is an important factor in the practical applications.Therefore, the reusability of the ZnO-AgCl-2 nanocomposite was tested by 5 photodegradation cycles.As shown in Fig. 4D, the ZnO-AgCl-2 nanocomposite remain more than 80% of photodegradation performance in the 5th cycle of photodegradation, indicating that the proposed photocatalyst owing an excellent stability.

Conclusion
In this study, a facile one-pot hydrothermal approach has been used to synthesize ZnO-AgCl nanocomposites, and their corresponding photocatalytic performance under visible irradiation was investigated.The results on MTBE photodegradation indicated that the as-prepared ZnO-AgCl nanocomposites have a superior photocatalytic activity than bare ZnO and P25.Additionally, the reusability test showed that the ZnO-AgCl photocatalyst remained more than 80% photocatalytic activity in the fifth cycle, which made it high promising for practical applications such as waste water purification.