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Synthesis of Ag–ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue
Accepted Manuscript Synthesis of Ag-ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue M. Arab Chamjangali, G. Bagherian, A. Javid, S. Boroumand, N. Farzaneh PII: DOI: Reference:
S1386-1425(15)00680-0 http://dx.doi.org/10.1016/j.saa.2015.05.067 SAA 13735
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
10 December 2014 12 May 2015 23 May 2015
Please cite this article as: M. Arab Chamjangali, G. Bagherian, A. Javid, S. Boroumand, N. Farzaneh, Synthesis of Ag-ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.05.067
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Synthesis of Ag-ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue M. Arab Chamjangali*a, G. Bagheriana, A. Javidb, S. Boroumanda, N. Farzaneha a
b
College of Chemistry, Shahrood University, Shahrood, P.O. Box 36155-316, Iran.
School of public health, Shahroud University of Medical Sciences, Shahroud, Iran
Abstract In this study, the photo-decolorization of a mixture of methylene blue (MB) and methyl orange (MO) was investigated using Ag-ZnO multipods. The photo-catalyst used, ZnO multipods, was successfully synthesized. The surface of ZnO microstructure was modified by deposition of different amounts of Ag nanoparticles (Ag NPs) using the photo-reduction method. The as-prepared samples were characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis and atomic absorption spectroscopy. The photo-catalytic efficiency of Ag-ZnO is mainly controlled by the amount of Ag NPs deposited on the ZnO surface. The results obtained suggest that Ag-ZnO containing 6.5% Ag NPs, has the highest photo-catalytic performance in the simultaneous photo-degradation of dyes at a shorter time. Keywords: Simultaneous; Photo-catalytic degradation; Photo-reduction; Ag-ZnO; Methyl orange; Methylene blue.
*
Corresponding author: Tel.: +98-23-32395441; E-mail: [email protected]
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1. Introduction Synthetic dyes are present in our daily life, and their usage is continuously growing. Dyes have been widely used as colorants in different industries such as textile, paper, pharmaceutical, food, and cosmetic [1]. Additionally, dyes are considered as hazardous substances with recalcitrant structures against convectional wastewater treatments. Up to the present time, a lot of papers have been published in the area of dye treatments in wastewaters. Several traditional methods such as coagulation [2], foam flotation [3], adsorption in waste materials [4], membranes [5], adsorption using activated carbon [6], and combined coagulation/carbon adsorption [7] have been used for removal of dyes from wastewaters. Most of these methods are non-destructive, and generate secondary toxic wastes or concentrated pollutant-containing phase. Currently, advanced oxidation processes (AOPs) such as photo-catalytic degradation, Fenton/photo-Fenton, and ozonation have been identified as powerful methods for wastewater purification without generation of secondary toxic materials [8, 9]. Among the different AOPs, the photo-assisted catalytic decomposition of organic pollutants, employing semi-conductors as photo-catalysts, has been a promising method for the degradation of pollutants in wastewater. TiO2 and ZnO are the most favorable photo-catalysts used for the photo-assisted catalytic decomposition of organic contaminants. Both materials exhibit very similar band gaps (ZnO, 3.37 eV and TiO2, 3.2 eV) and conduction band edge positions [10]. ZnO is a multi-functional semi-conductor with wide band gap energy and a high excitation binding energy of 60 mV, and has been generally considered as a photo-catalyst due to its nontoxic semi-conductor property, low cost, and different morphology [8, 11]. Since the photo-catalytic reaction takes place at the interface between the photo-catalyst and dye, the photo-catalytic activity of the photo-catalyst depends upon the growth manner of the crystal, and thus the morphology and dimensionality of
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ZnO play an important role in the photo-catalytic process [12, 13]. Therefore, synthesis and application of 3D ZnO nanostructures with different morphologies are now important to answer the demand for exploring the potentials of ZnO. Some recent efforts have been focused on the preparation of ZnO nanostructures with various morphologies including the tower-like [14], dumbbell-like [15], nut-like [16], and flower-like [17,18] ones. Among the 3D morphologies of ZnO nanostructures, multiple rods (multipods) have high photo-catalytic performance owing to their branched structure that makes a large area surface for the adsorption dye. Furthermore, the full tips in the branched structures reduce their aggregation [19]. The modification of semi-conductor surfaces by noble metal deposition can be used to improve the photo-catalytic activity of semi-conductors [20-23]. By depositing noble metal nanoparticles (NPs) on semi-conductors, the photo-generated electrons in the conduction band of a semi-conductor can be transferred to metal deposits due to the Schottky barrier formed at the metal–semi-conductor interface [24, 25], while the holes can remain on the semi-conductor surface. Thus the noble metal can act as an electron sink, facilitating the charge separation and improving the photo-catalytic efficiency. Among the noble metals, Ag NPs have been widely used for modification of the ZnO surface for enhancing the photo-catalytic activity of ZnO in the degradation or decolorization of organic dyes [26-35]. Although there are reports about the synthesis of Ag-ZnO composite, it is still a challenge to explore the facile methods to synthesize other novel and uniform Ag-ZnO nanostructures. An effective, clean, and cheap method for loading the metals Ag, Pd, and Au on the surface of the photo-catalyst is the photo-reduction of the metal ions. The lack of the toxic reducing agents and organic solvents in the reduction of metal ions materials by photo-irradiation are the main advantages of this procedure [36]. In this method, the electrons generated on the
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surface of the photo-catalyst, under UV irradiation process, reduce the metal ions present in solution and surface of photo-catalyst. Synthetic azo dyes are extensively used in the industries. These compounds have one or more azo groups (N=N) in their chemical structures as dye centers. Also azo dyes contain functional groups such as –NH2, –OH, –CH3, and –SO3, which are responsible for the fixation of these dyes to fibers [37]. Methyl orange (MO), also known as C.I. Acid Orange 52, is a model of anionic azo dyes, which is an intensely colored compound and is used in dyeing and printing textiles. Thiazine dyes are a group of basic dyes whose molecules contain the thiazine heterocyclic. Among the thiazine dyes, methylene blue (MB) is of the greatest industrial significance, and is employed as a dye for temporary hair colorants, cotton, wool, leather, and paper. MB can cause eye burn in human and aquatic animals. It can also cause irritation to the stomach with indications of nausea, vomiting, and diarrhea [38]. Therefore, it becomes necessary to remove these dyes from industrial effluents to produce a safe and clean aqueous environment. Although many researchers have studied the treatment of wastewater containing just one dye, there are very limited publications on the photo-degradation of mixture of dyes [39-47]. In these publications, the photo-degradation or decolorization of binary or ternary mixtures of dyes have been studied using different photo-catalysts such as TiO2 [39-42], Ag+-doped TiO2 [43], modified Ti-MCM-41 [44], and CuO/nano-zeolite X [45] or by the Photo-Fenton process [46, 47]. According to our literature survey, the use of Ag-ZnO nanostructures with multiple rod (multipod) morphology in the simultaneous photo-degradation of MO and MB has not been reported in the scientific literatures. Since the photo-catalytic degradation of dye mixtures has so far been rarely reported, the present work reports an experimental investigation on the photocatalytic degradation of mixtures of MO (an anionic azo dye) and MB (a cationic thiazine dye),
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as a mixed contaminants model of textile wastewater using Ag-ZnO with a known multipods morphology. The influence of various operational parameters affecting the photo-catalytic degradation of dye mixtures was systematically studied to obtain the optimum conditions for the highest degradation efficiency of the dye mixtures.
2. Experimental 2.1. Chemicals All the chemicals used were of reagent grade or higher purity. The chemicals including silver nitrate, zinc acetate dehydrate, sodium hydroxide, potassium nitrate, hydrochloric acid, sulfuric acid and potassium hydroxide were purchased from Merck. MO and MB were purchased from Aldrich. Deionized (DI) water was used throughout this study. 2.2. Apparatus Scanning electron microscopy (SEM) observations were performed by a Hitachi S-4160 electron microscope in order to determine the morphology of the samples. X-ray diffraction (XRD) patterns and crystal phase of the samples were recorded by a Burker AXS (Model B8Advance) diffractometer with Cu Kα radiation (λ = 0.15418 nm). A Shimadzu Model AA-670 flame atomic absorption spectrometer with an air–acetylene flame equipped with a silver hollow cathode lamp was used for quantification of the silver content under the manufacturer recommended conditions. The UV-visible spectra were recorded double beam Ray Leigh UV2610 UV-visible spectrophotometer using a pair of 1.0 cm quartz cell. The pH measurements and adjustments were carried out using a Metrohm 744 pH-meter equipped with a combined glass electrode. A Hettich centrifuge Model Rotine 380 was used for separation of precipitates from solution. UV-irradiation was provided by a high pressure mercury lamp (Philips-400 W).
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2.3. Synthesis of ZnO multipods The ZnO multipod was synthesized according to the method reported by Chen and coworkers [19]. Specifically, KOH (2.20 g) and Zn(CH3COO)2. 2H2O (1.10 g) were dissolved in 10.0 mL of distilled water in two beakers and then the aqueous solution of zinc salt was dropped into a solution of KOH under stirring. At the end, the transparent solution obtained was placed in an oven, with the temperature set at 70 oC. After 10 h, the resulting white precipitate was collected by centrifugation, washed with DI water for several times, and dried in vacuum.
2.4. Synthesis of Ag-ZnO multipods Synthesis and doping of Ag NPs on the surface of photo-catalyst were done according to the method reported for the synthesis of flower-like Ag-ZnO nanostructure reported by Arab Chamjangali and co-workers [48]. In brief, 0.2000 g of as-prepared ZnO multipods was transferred into a 250-mL beaker and dispersed in 30 mL of water. In order to eliminate any impurities on the surface of ZnO, the dispersed solution was irradiated by a 400 W high pressure mercury lamp for 10 min. Then different amounts of AgNO3 (0.03-1.2 mmol) were added and the solution was diluted to about 100 mL. The solution was vigorously stirred in the dark for 15 min to establish desorption/adsorption equilibrium of silver ions. For removal of any O2 adsorbed on Ag-ZnO or dissolved in the solution, the suspension was purged with pure N2 gas for 5 min. After that the solution was irradiated by a 400 W UV Hg lamp with continuous nitrogen purging for 40 min. The precipitate obtained was separated from the solution by centrifugation and washed with distilled water and ethanol repeatedly to remove the residual Ag+, and next dried at 80 oC for 4 h. The Ag-ZnO samples with different amounts of loaded Ag NPs were prepared by applying different amounts of AgNO3 as loading solutions with (Table 1).
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2.5. Photo-catalytic studies The photo-catalytic capabilities of as-synthesized ZnO and Ag-ZnO with multipods structure were investigated using a binary mixture of MO and MB as the target compounds in a home-made photo-reactor equipped with 400 W Hg lamp. The distance between the solution surface and the light source was about 12 cm. In a typical experiment, a specified amount of photo-catalyst (40 mg) was added into 100.0 mL of a mixture containing MO (10 mg L-1) and MB (5 mg L-1) with an adjusted pH of 6.5. Before irradiation, the suspension was stirred magnetically in the dark for 20 min to reach the adsorption/desorption equilibrium. Afterwards, the mixture was exposed to UV irradiation with magnetic stirring. Then the residual concentrations of both dyes in the mixture were estimated using the measured absorbance at maximum absorption bands of the dyes (272 and 464 nm for MO; 290 and 664 nm for MB).
3. Result and discussion 3.1. Characterization of photo-catalyst 3.1.1. XRD analysis The XRD analysis was employed to identify the phase structure and the purity of the products. For this purpose, the XRD patterns for pure ZnO multipods and selected Ag-ZnO samples (S2, S4 and S5) were recorded. The sharp peaks in the XRD patterns (Fig. 1) for bare ZnO and Ag-ZnO samples indicate a good crystallinity in the synthesized multipods nanostructures. The 2θ diffraction peaks at 31°, 34°, and 35.5° ( Fig. 1a) corresponded to (100), (002), and (101) planes of the Wurtzite structure of ZnO (JCPDS NO. 361451). Also, in Fig. 1bd, peaks at 37.5°, 43.4°, and 64.8° correspond to the (111), (200), and (220) crystal planes of the cubic lattice structure of Ag, which are in agreement with the standard Ag pattern (JCPDS NO.
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04-0783). The corresponding lattice parameters for Ag-ZnO samples are very close to such parameters for the bare ZnO and standard ZnO structure. This confirms that the silver atoms are not incorporated into the ZnO lattice and are loaded onto the ZnO surface. The particle size of the samples was calculated using the Scherrer's formula. According to the calculations, the average crystal size of ZnO in the bare ZnO and Ag-ZnO samples was nearly constant and about 23 nm. Furthermore, the average of crystal size of Ag NPs was 18 nm in the studied samples. The comparison of the XRD patterns and average size of ZnO before and after modification with Ag NPs confirms that the crystal structure of ZnO is intact during modification process. The intensity ratio of the diffraction peak corresponds to the (111) plane of metallic Ag to the diffraction peak of the (101) plane of ZnO (
I 111 ) for each Ag-ZnO samples, prepared by using I 101
different concentrations of AgNO3 in loading solution, was calculated using their XRD patterns (Fig. 1b-d). The calculated values were about 0.19, 0.28 and 0.41 for S2, S4 and S5 samples, respectively. It is clear that by increasing in AgNO3 amount, the values for the (
I 111 ) ratio I 101
increases and this confirms that the loaded amount of Ag NPs increases with increasing in the initial concentration of AgNO3 in loading solution. These results are in accordance with those obtained by AAS (Table 1). Furthermore, the calculated crystal size of Ag NPs was approximately constant for all studied Ag-ZnO samples with an average size of 18 nm. This confirms that the amount of loaded Ag NPs has not considerable effect on the Ag NPs size. 3.1.2. SEM analysis Fig. 2 displays the SEM images of ZnO and Ag-ZnO, confirming the 3-D multipod structures of the two samples. SEM characterizations reveal that the microstructures are assembled by spherical multipods with the rod shape branches of building blocks. The typical 8
length of a branch (measured from bottom to tip) is 2.5 µm. It can be seen that the surface morphology of ZnO is homogeneous and relatively smooth, which suggests the high dispersion of Ag NPs on the surface of ZnO. In addition, the microstructure of Ag-ZnO is almost maintained, and does not change during photo-deposition of Ag NPs.
3.1.3. pH of zero point charge (ZPC) The pH of zero point charge for ZnO and Ag-ZnO photo-catalysts was determined by salt addition method [49, 50]. In this study, 0.0400 g portions of the samples (ZnO or Ag-ZnO) were added to 100 mL of 0.10 M potassium nitrate solutions. The initial pH of each suspension (pHi) was adjusted in the range of 4.0-9.0 by addition of HCl (1.0 M) or NaOH (1.0 M). The mixture was kept in an end-to-end water shaker bath for 18 h and after that the final pH value of each suspension (pHf) was recorded by using a pH–meter. The ZPC of samples were determined by plotting ∆pH (pHf-pHi) versus initial pH values (Fig. 3). By evaluation the intersections of the plots with initial pH axis, the pH of ZPC was obtained around 8.0 for both ZnO and Ag-ZnO suspensions.
3.1.4. UV-vis spectra of ZnO and Ag-ZnO Fig. 4. shows the absorption spectra of the ZnO and Ag-ZnO nanostructures. The presence of absorbance maxima at about 370 nm for both ZnO and Ag-ZnO clearly exhibits that the materials are photoactive. In addition, a significant decrease in absorbance intensity of ZnO peak at 370 nm for Ag-ZnO indicates the presence of silver on ZnO surface [51]. The direct band gap of semiconductors is calculated using the following equation [52]:
ߙ=
(ఔିாಳೠೖ )మ ఔ
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Where α is the absorption coefficient, C is a constant, hν is the photon energy and ܧ௨ is the band gap energy. From Tauc plots, (αhν) 2 as a function of photon energy (hν), the band gap energies of 2.9 and 2.8 eV were found for ZnO and Ag-ZnO samples, respectively. These values are slightly lower than that of bulk ZnO (3.37 eV) but are in accordance with range reported (2.85-3.06) for ZnO nanostructures with different crystallite size and morphology [53].
3.2. Absorption spectra of dyes mixture In the multi-dye photo-degradation system, the quantification of the interference of one dye with others caused by overlapping of absorption spectra or any interactions is very important. Therefore the UV-vis absorption spectra of the two dyes alone and in their mixture were recorded (Fig. 5). It is obvious that λmax and maximum absorbance values of both MO and MB do not change on mixing. Thus the degradation/decolorization of MO and MB in the mixture solution can be studied separately at their corresponding λmax.
3.3. Photo-catalytic activity of Ag-ZnO The effect of UV irradiation time on the absorption spectra of dyes mixture in the presence of Ag-ZnO was investigated. In this study a mixture of two dyes (pH = 6.5) containing MO (10.0 mg L-1), MB (5.0 mg L-1), and 400.0 mg L-1 of Ag-ZnO (S5) was prepared and the absorption spectra of mixture was recorded at different time intervals. Fig. 6 shows the time-dependent UVvisible spectrum of MO and MB mixture. It can be seen that the maximum absorbance peaks of the two dyes disappear completely after 40 min. The decrease in the absorption band intensities of the two dyes and lack of formation of a new peak indicates that the dyes have been degraded entirely by UV irradiation in the presence of the Ag-ZnO multipods photo-catalyst. It is obvious
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that the rate of degradation of MO, with resistant structure, is slower than that for MB. The absorbance of visible bands represents the color of dye solutions and its decrease was used to monitor the decolorization of dyes. The absorbance of dyes in UV region (about 270 nm) is due to the aromatic content of dues and its decrease was used to monitor the degradation of aromatic structure of the dyes mixture [48]. The photo-catalytic activities of the synthesized ZnO and Ag-ZnO were evaluated by decolorization of the mixture of MO and MB under different conditions (Fig. 7). The results obtained show that concentration of the mixture of dyes changes a little after irradiation for 60 min in the absence of the photo-catalyst, revealing that the photolysis can be neglected compared with the photo-catalysis. On the other hand, in darkness (no UV light irradiation), concentration of the two dyes was almost constant and did not change even after 60 min. This indicates that adsorption of the two dyes on the surface of photo-catalyst reaches an equilibrium condition after 20 min. Thus in all experiments, the suspension containing the mixture of dyes and photocatalysts was continuously stirred for 20 min in the dark. As shown in Fig. 7, the loading of silver NPs can significantly increase the photo-catalytic performance.
3.4. Optimization of photo-catalytic conditions The decrease in absorbance value of the samples at λ max of the dyes after irradiation in a certain time interval shows the rate of decolorization and therefore the photo-degradation efficiency of the dyes. The decolorization and degradation efficiency were calculated as follow: E (%) =
C 0 − Ct × 100 C0
(1)
where C0 is the initial concentration of MO and MB and C is the concentration of MO and MB in the mixture after irradiation at the selected time intervals. 11
In order to obtain maximum degradation/decolorization efficiency, the pH, effect of AgNO3 concentration on the deposition solution, amount of catalyst, and concentration of dyes were studied and optimized using one-at- a time optimization procedure. All experiments were conducted in triplicates, and the results were shown in the mean values.
3.4.1. Influence of pH The wastewater released from the industry has a wide range value of pH. Hence, pH of the solution is one of the important operational variables affecting on the photo-catalytic treatment of wastewaters. In the study of pH effect, H2SO4 and NaOH were used to adjust pH of MO and MB mixture. The binary solutions of MO (10.0 mg L-1) and MB (5.0 mg L-1) containing 300 mg L-1 of Ag-ZnO (S5) were prepared by mixing known amounts of dyes aqueous solutions and adjusting the solutions pH in the range of 4.0-10.0. Then the photo-catalytic procedure was carried out. Fig. 8 shows the decolorization efficiency of the mixed dyes as a function of pH. As seen, the highest decolorization efficiency occurs at the pH of 6.5.
3.4.2. Influence of catalyst concentration Photo-catalyst concentration is another critical parameter affecting to the photo-catalytic activity. In order to avoid the usage of excess photo-catalyst, it is essential to find out the optimum photo-catalyst concentration to reach a high photo-catalytic efficiency. For this purpose, the effect of various amounts of the photo-catalyst was studied by employing different concentrations of S5 sample in the range of 200-500 mg L-1 at pH 6.5. The results obtained (Fig. 9) show that with increase in the catalyst amount from 200 mg L-1 to 400 mg L-1, the decolorization efficiency in the reaction mixture increase 32% and 23% for MO and MB, respectively. Then the increment catalyst amount from 400 mg L-1 to 500 mg L-1 had a negative 12
effect on the photo-catalytic performance. It is well-known that the increase in the amount of photo-catalyst increases the number of active sites on the photo-catalyst surface, causing an increase in the number of photons absorbed and also the number of the dye molecules adsorbed [54, 55]. On the other hand, due to the agglomeration of catalyst particles at high catalyst concentration, a part of the photo-catalyst surface becoming unavailable for photon absorption and the photo-catalytic activity decreases [56, 48]. The results obtained show that an optimum amount of catalyst for decolorization of the mixture of dyes is 400 mg L-1.
3.4.3. Influence of initial dye concentration The effect of initial concentration of MO and MB in their mixture on the photo-catalytic efficiency was tested by varying the initial concentration of the two dyes from 5.0-20.0 mg L-1 for MO and 2.5-10.0 mg L-1 for MB at the obtained optimum conditions. The results obtained are given in Fig. 10. It is obvious that the simultaneous decolorization efficiency decreases with increase in the initial concentration of the two dyes in solution. This occurrence is due to this fact that the high concentration of dyes hinders that the photons reach into the surface of the photocatalyst and absorption of photons by the catalyst decreases and the photo-catalytic efficiency reduces. Besides, at high dye concentrations the dye molecules are adsorbed on the surface of Ag-ZnO and the active sites for the generation of hydroxyl radicals reduce [48, 57]. However, the further studies were carried out in a mixture of 10.0 mg L-1 of MO and 5.0 mg L-1.
3.4.5. Influence of amount of Ag loading The relation between the amount of doped Ag and decolorization efficiency is shown in Fig 11. For this purpose, Ag-ZnO samples with different amount of loaded Ag NPs were 13
synthesized. As shown in Table 1, the weight percent of Ag in these samples increases with increase in the initial concentration of AgNO3. On the other hand, the photo-catalytic activity of ZnO NPs can be greatly improved by modifying of ZnO with an appropriate amount of Ag NPs. The photo-catalytic activates of 400 mg L-1 of these catalysts in the decolorization of 100 mL mixture of the two dyes (10.0 mg L-1 of MO and 5.0 mg L-1 of MB) were measured. As it can be seen (Fig. 11), the decolorization efficiency of Ag-ZnO increases with increase in the amount of doped Ag NPs up to 6.5% (sample S2) and then decreases with a future increase in the amount of doped Ag NPs. The percentage decolorization varied from 77.3% to 86% for MO, and 89.6 to 93.66 for MB in different concentrations of AgNO3 solution. However, when the amount of deposited Ag NPs is 6.5 %Wt (0.1 mmol AgNO3), the photo-catalytic activity reaches to its maximum value. As said, the loading of Ag NPs on the surface of photo-catalyst improved the photo-catalytic activity, which is mainly due to the better separation of photo-induced charges (electron and hole) in the photo-catalyst. The decrease in the photo-catalytic activity at deposited Ag NPs amounts greater than the optimum value is related to this note that at larger amounts of Ag NPs, the noble metal acts as a recombination center for the photo-generated electrons and holes. Furthermore, higher amounts of loaded Ag NPs will cover large area of the photo-catalyst surface and act as a barrier in the reaching the photons onto the surface of ZnO. Subsequently the photo-catalytic efficiency decreases at higher amounts of loaded Ag NPs [48].
4. Conclusions Ag-ZnO multipods with different Ag loadings were successfully prepared through photoreductive deposition as a clean, effective, and fast route. The prepared photo-catalysts were characterized by XRD, SEM, UV-vis spectrophotometry, and AAS. These Ag-ZnO multipods have potential applications in degradation/decolorization of MO and MB simultaneously. This is 14
the first report on the simultaneous degradation/decolorization of MO and MB. The decolorization extent of the dyes was obviously affected by the solution pH, amount of the catalyst, initial dye concentrations, and Ag doping. The required time for an entire decolorization of simultaneous photo-degradation of dyes using Ag-ZnO catalysts is 40 min. The present study opens a new way towards the synthesis of Ag-ZnO multipods with high photo-catalytic performance for photo-degradation/decolorization of a mixture of the dyes and other organic pollutants in the aqueous solution at a short time.
Acknowledgement The authors are thankful to the Shahrood University Research Council for supporting this work.
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Legends for Figures and Tables Fig. 1. XRD patterns of samples synthesized: a) pure ZnO, b) Ag-ZnO (S2), c) Ag-ZnO (S4) and d) Ag-ZnO (S5). Fig. 2. SEM images of synthesized samples: (a and b) pure ZnO, (c) Ag-ZnO (S4). Fig. 3. Determination of pHZPC of the ZnO and Ag-ZnO (S2) Fig. 4. UV-Vis spectra for ZnO and Ag-ZnO (S2). All powder samples were dispersed in ethanol and sonoicated for 30 min. Fig. 5. UV-visile absorbance spectra of MO (10.0 mg L-1), MB (5.0 mg L-1), and a mixture of MO (10.0 mg L-1) and MB (5.0 mg L-1). Fig. 6. UV-visible absorbance spectra for the mixture of MO and MB solution in the presence of Ag-ZnO after its exposure to UV light for different times. Experimental conditions: 100 mL mixture of MO and MB with concentrations of 10.0 mg L-1 and 5 mg L-1, respectively; Ag-ZnO (S2): 400 mg L-1 and pH: 6.5. Fig. 7. Decolorization rate of MO (a) and Decolorization rate of MB (b) in their mixture. Experimental conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5 mg L-1, respectively; Ag-ZnO (S2): 400 mg L-1 and pH: 6.5. Fig. 8. Effect of pH on the decolorization of a mixture of two dyes using Ag-ZnO under the UV irradiation time of 40 minutes. Experimental conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5 mg L-1, respectively; Ag-ZnO (S5): 300 mg L-1, and pH range: 4.0-10.0 Fig. 9. Influence of the catalyst concentration on the photo-catalytic decolorization of the mixture of two dyes using Ag-ZnO (S5) under UV irradiation time for 40 minutes. Experimental
20
conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5.0 mg L-1, respectively and pH 6.5. Fig. 10. Effect of initial dye concentration on the decolorization efficiency of MO (a) and MB (b) in the mixture of two dyes using Ag-ZnO. Experimental conditions: 100 mL mixture of MO and MB with known concentration; Ag-ZnO (S5): 400 mg L-1 and pH 6.5. Fig. 11. Influence of dopant concentration on the photo-catalytic activity of ZnO under UV irradiation time for 40 minutes. Experimental conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5.0 mg L-1, respectively; synthesized photo-catalyst : 40.0 mg, and pH 6.5. Table 1. Samples synthesized and synthesis conditions.
21
Table 1
Sample
AgNO3 (mmol)
Irradiation Time (min)
Wt % Ag from AAS results
S1
0.03
40
1.8
S2
0.1
40
6.5
S3
0.2
40
8.3
S4
0.45
40
10.5
S5
0.9
40
10.75
S6
1.2
40
11.3
22
23 110
201
103 220 200 112
102
200
111
100 002 101
201
200 112
103
102
110
100 002
a
b Ag ()
101
24 103 220 200 112 201
102
110
200
111
100 002 101
201
110 103 220 200 112
102
200
111
100 002 101
c Ag ()
d Ag ()
Fig.2. a
25
Fig.2.b
26
Fig.2.c
27
4 Ag-ZnO (S2) Pure ZnO 3
∆ pH
2
1
0 3 -1
4
5
6
7
pH
-2
Fig.3.
28
8
9
10
1.4 Pure ZnO Ag-ZnO (S2)
Absorbance (a.u.)
1.2
1.0
0.8
0.6
0.4 200
300
400
500
600
Wavelength(nm) Fig. 4.
29
700
800
900
1.0
Absorbance (a. u.)
0.8
MB MO Mixture of MO and MB
0.6
0.4
0.2
0.0 200
300
400
500
Wavelength (nm) Fig. 5.
30
600
700
800
1.0
0.0 min UV
Absorbance (a.u)
0.8
0.6
0.4
0.2
40 min UV 0.0 200
300
400
500
Wavelength (nm)
Fig. 6.
31
600
700
800
1.0 UV
Dark
0.8
C/C0
0.6
0.4
Adsoption Only UV UV+ZnO UV+Ag-ZnO
0.2
0.0 0
10
20
30
40
Time (min) Fig. 7. a
32
50
60
70
1.0
Dark
UV
0.8
C/C0
0.6
0.4
Adsorption Only UV UV+ZnO UV+Ag-ZnO
0.2
0.0 0
10
20
30
40
Time (min) Fig. 7. b
33
50
60
70
110
100
Decolorization of MB Decolorization of MO
E%
90
80
70
60 3
4
5
6
7
pH
Fig. 8.
34
8
9
10
11
100 Decolorization of MO Decolorization of MB 80
E%
60
40
20
0 200
300
400
500 -1
Amount of photocatalyst (mg Lit )
Fig. 9.
35
100
(a) 80
E%
60
40 5ppm MO+2.5ppm MB 10ppm MO+5ppm MB 15ppm MO+7.5ppm MB 20ppm MO+10ppm MB
20
0 0
10
20
30
40
Time (min)
Fig. 10. a
36
50
60
70
100
80
(b)
E%
60
40
5ppm MO+2.5ppm MB 10ppm MO+5ppm MB 15ppm MO+7.5ppm MB 20ppm MO+10ppm MB
20
0 0
10
20
30
40
Time (min)
Fig. 10. b
37
50
60
70
Fig. 11.
38
39
>Ag-ZnO multipodes were synthesized via a solution route/photo-reduction process. > AgZnO multipodes have shown high efficiency in simultaneous MO and MB degradation.> Influencing factors of simultaneous degradation are discussed.> The photocatalytic activity of Ag-ZnO was constant after seven times recycle using.
40
S1386-1425(15)00680-0 http://dx.doi.org/10.1016/j.saa.2015.05.067 SAA 13735
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
10 December 2014 12 May 2015 23 May 2015
Please cite this article as: M. Arab Chamjangali, G. Bagherian, A. Javid, S. Boroumand, N. Farzaneh, Synthesis of Ag-ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.05.067
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis of Ag-ZnO with multiple rods (multipods) morphology and its application in the simultaneous photo-catalytic degradation of methyl orange and methylene blue M. Arab Chamjangali*a, G. Bagheriana, A. Javidb, S. Boroumanda, N. Farzaneha a
b
College of Chemistry, Shahrood University, Shahrood, P.O. Box 36155-316, Iran.
School of public health, Shahroud University of Medical Sciences, Shahroud, Iran
Abstract In this study, the photo-decolorization of a mixture of methylene blue (MB) and methyl orange (MO) was investigated using Ag-ZnO multipods. The photo-catalyst used, ZnO multipods, was successfully synthesized. The surface of ZnO microstructure was modified by deposition of different amounts of Ag nanoparticles (Ag NPs) using the photo-reduction method. The as-prepared samples were characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis and atomic absorption spectroscopy. The photo-catalytic efficiency of Ag-ZnO is mainly controlled by the amount of Ag NPs deposited on the ZnO surface. The results obtained suggest that Ag-ZnO containing 6.5% Ag NPs, has the highest photo-catalytic performance in the simultaneous photo-degradation of dyes at a shorter time. Keywords: Simultaneous; Photo-catalytic degradation; Photo-reduction; Ag-ZnO; Methyl orange; Methylene blue.
*
Corresponding author: Tel.: +98-23-32395441; E-mail: [email protected]
1
1. Introduction Synthetic dyes are present in our daily life, and their usage is continuously growing. Dyes have been widely used as colorants in different industries such as textile, paper, pharmaceutical, food, and cosmetic [1]. Additionally, dyes are considered as hazardous substances with recalcitrant structures against convectional wastewater treatments. Up to the present time, a lot of papers have been published in the area of dye treatments in wastewaters. Several traditional methods such as coagulation [2], foam flotation [3], adsorption in waste materials [4], membranes [5], adsorption using activated carbon [6], and combined coagulation/carbon adsorption [7] have been used for removal of dyes from wastewaters. Most of these methods are non-destructive, and generate secondary toxic wastes or concentrated pollutant-containing phase. Currently, advanced oxidation processes (AOPs) such as photo-catalytic degradation, Fenton/photo-Fenton, and ozonation have been identified as powerful methods for wastewater purification without generation of secondary toxic materials [8, 9]. Among the different AOPs, the photo-assisted catalytic decomposition of organic pollutants, employing semi-conductors as photo-catalysts, has been a promising method for the degradation of pollutants in wastewater. TiO2 and ZnO are the most favorable photo-catalysts used for the photo-assisted catalytic decomposition of organic contaminants. Both materials exhibit very similar band gaps (ZnO, 3.37 eV and TiO2, 3.2 eV) and conduction band edge positions [10]. ZnO is a multi-functional semi-conductor with wide band gap energy and a high excitation binding energy of 60 mV, and has been generally considered as a photo-catalyst due to its nontoxic semi-conductor property, low cost, and different morphology [8, 11]. Since the photo-catalytic reaction takes place at the interface between the photo-catalyst and dye, the photo-catalytic activity of the photo-catalyst depends upon the growth manner of the crystal, and thus the morphology and dimensionality of
2
ZnO play an important role in the photo-catalytic process [12, 13]. Therefore, synthesis and application of 3D ZnO nanostructures with different morphologies are now important to answer the demand for exploring the potentials of ZnO. Some recent efforts have been focused on the preparation of ZnO nanostructures with various morphologies including the tower-like [14], dumbbell-like [15], nut-like [16], and flower-like [17,18] ones. Among the 3D morphologies of ZnO nanostructures, multiple rods (multipods) have high photo-catalytic performance owing to their branched structure that makes a large area surface for the adsorption dye. Furthermore, the full tips in the branched structures reduce their aggregation [19]. The modification of semi-conductor surfaces by noble metal deposition can be used to improve the photo-catalytic activity of semi-conductors [20-23]. By depositing noble metal nanoparticles (NPs) on semi-conductors, the photo-generated electrons in the conduction band of a semi-conductor can be transferred to metal deposits due to the Schottky barrier formed at the metal–semi-conductor interface [24, 25], while the holes can remain on the semi-conductor surface. Thus the noble metal can act as an electron sink, facilitating the charge separation and improving the photo-catalytic efficiency. Among the noble metals, Ag NPs have been widely used for modification of the ZnO surface for enhancing the photo-catalytic activity of ZnO in the degradation or decolorization of organic dyes [26-35]. Although there are reports about the synthesis of Ag-ZnO composite, it is still a challenge to explore the facile methods to synthesize other novel and uniform Ag-ZnO nanostructures. An effective, clean, and cheap method for loading the metals Ag, Pd, and Au on the surface of the photo-catalyst is the photo-reduction of the metal ions. The lack of the toxic reducing agents and organic solvents in the reduction of metal ions materials by photo-irradiation are the main advantages of this procedure [36]. In this method, the electrons generated on the
3
surface of the photo-catalyst, under UV irradiation process, reduce the metal ions present in solution and surface of photo-catalyst. Synthetic azo dyes are extensively used in the industries. These compounds have one or more azo groups (N=N) in their chemical structures as dye centers. Also azo dyes contain functional groups such as –NH2, –OH, –CH3, and –SO3, which are responsible for the fixation of these dyes to fibers [37]. Methyl orange (MO), also known as C.I. Acid Orange 52, is a model of anionic azo dyes, which is an intensely colored compound and is used in dyeing and printing textiles. Thiazine dyes are a group of basic dyes whose molecules contain the thiazine heterocyclic. Among the thiazine dyes, methylene blue (MB) is of the greatest industrial significance, and is employed as a dye for temporary hair colorants, cotton, wool, leather, and paper. MB can cause eye burn in human and aquatic animals. It can also cause irritation to the stomach with indications of nausea, vomiting, and diarrhea [38]. Therefore, it becomes necessary to remove these dyes from industrial effluents to produce a safe and clean aqueous environment. Although many researchers have studied the treatment of wastewater containing just one dye, there are very limited publications on the photo-degradation of mixture of dyes [39-47]. In these publications, the photo-degradation or decolorization of binary or ternary mixtures of dyes have been studied using different photo-catalysts such as TiO2 [39-42], Ag+-doped TiO2 [43], modified Ti-MCM-41 [44], and CuO/nano-zeolite X [45] or by the Photo-Fenton process [46, 47]. According to our literature survey, the use of Ag-ZnO nanostructures with multiple rod (multipod) morphology in the simultaneous photo-degradation of MO and MB has not been reported in the scientific literatures. Since the photo-catalytic degradation of dye mixtures has so far been rarely reported, the present work reports an experimental investigation on the photocatalytic degradation of mixtures of MO (an anionic azo dye) and MB (a cationic thiazine dye),
4
as a mixed contaminants model of textile wastewater using Ag-ZnO with a known multipods morphology. The influence of various operational parameters affecting the photo-catalytic degradation of dye mixtures was systematically studied to obtain the optimum conditions for the highest degradation efficiency of the dye mixtures.
2. Experimental 2.1. Chemicals All the chemicals used were of reagent grade or higher purity. The chemicals including silver nitrate, zinc acetate dehydrate, sodium hydroxide, potassium nitrate, hydrochloric acid, sulfuric acid and potassium hydroxide were purchased from Merck. MO and MB were purchased from Aldrich. Deionized (DI) water was used throughout this study. 2.2. Apparatus Scanning electron microscopy (SEM) observations were performed by a Hitachi S-4160 electron microscope in order to determine the morphology of the samples. X-ray diffraction (XRD) patterns and crystal phase of the samples were recorded by a Burker AXS (Model B8Advance) diffractometer with Cu Kα radiation (λ = 0.15418 nm). A Shimadzu Model AA-670 flame atomic absorption spectrometer with an air–acetylene flame equipped with a silver hollow cathode lamp was used for quantification of the silver content under the manufacturer recommended conditions. The UV-visible spectra were recorded double beam Ray Leigh UV2610 UV-visible spectrophotometer using a pair of 1.0 cm quartz cell. The pH measurements and adjustments were carried out using a Metrohm 744 pH-meter equipped with a combined glass electrode. A Hettich centrifuge Model Rotine 380 was used for separation of precipitates from solution. UV-irradiation was provided by a high pressure mercury lamp (Philips-400 W).
5
2.3. Synthesis of ZnO multipods The ZnO multipod was synthesized according to the method reported by Chen and coworkers [19]. Specifically, KOH (2.20 g) and Zn(CH3COO)2. 2H2O (1.10 g) were dissolved in 10.0 mL of distilled water in two beakers and then the aqueous solution of zinc salt was dropped into a solution of KOH under stirring. At the end, the transparent solution obtained was placed in an oven, with the temperature set at 70 oC. After 10 h, the resulting white precipitate was collected by centrifugation, washed with DI water for several times, and dried in vacuum.
2.4. Synthesis of Ag-ZnO multipods Synthesis and doping of Ag NPs on the surface of photo-catalyst were done according to the method reported for the synthesis of flower-like Ag-ZnO nanostructure reported by Arab Chamjangali and co-workers [48]. In brief, 0.2000 g of as-prepared ZnO multipods was transferred into a 250-mL beaker and dispersed in 30 mL of water. In order to eliminate any impurities on the surface of ZnO, the dispersed solution was irradiated by a 400 W high pressure mercury lamp for 10 min. Then different amounts of AgNO3 (0.03-1.2 mmol) were added and the solution was diluted to about 100 mL. The solution was vigorously stirred in the dark for 15 min to establish desorption/adsorption equilibrium of silver ions. For removal of any O2 adsorbed on Ag-ZnO or dissolved in the solution, the suspension was purged with pure N2 gas for 5 min. After that the solution was irradiated by a 400 W UV Hg lamp with continuous nitrogen purging for 40 min. The precipitate obtained was separated from the solution by centrifugation and washed with distilled water and ethanol repeatedly to remove the residual Ag+, and next dried at 80 oC for 4 h. The Ag-ZnO samples with different amounts of loaded Ag NPs were prepared by applying different amounts of AgNO3 as loading solutions with (Table 1).
6
2.5. Photo-catalytic studies The photo-catalytic capabilities of as-synthesized ZnO and Ag-ZnO with multipods structure were investigated using a binary mixture of MO and MB as the target compounds in a home-made photo-reactor equipped with 400 W Hg lamp. The distance between the solution surface and the light source was about 12 cm. In a typical experiment, a specified amount of photo-catalyst (40 mg) was added into 100.0 mL of a mixture containing MO (10 mg L-1) and MB (5 mg L-1) with an adjusted pH of 6.5. Before irradiation, the suspension was stirred magnetically in the dark for 20 min to reach the adsorption/desorption equilibrium. Afterwards, the mixture was exposed to UV irradiation with magnetic stirring. Then the residual concentrations of both dyes in the mixture were estimated using the measured absorbance at maximum absorption bands of the dyes (272 and 464 nm for MO; 290 and 664 nm for MB).
3. Result and discussion 3.1. Characterization of photo-catalyst 3.1.1. XRD analysis The XRD analysis was employed to identify the phase structure and the purity of the products. For this purpose, the XRD patterns for pure ZnO multipods and selected Ag-ZnO samples (S2, S4 and S5) were recorded. The sharp peaks in the XRD patterns (Fig. 1) for bare ZnO and Ag-ZnO samples indicate a good crystallinity in the synthesized multipods nanostructures. The 2θ diffraction peaks at 31°, 34°, and 35.5° ( Fig. 1a) corresponded to (100), (002), and (101) planes of the Wurtzite structure of ZnO (JCPDS NO. 361451). Also, in Fig. 1bd, peaks at 37.5°, 43.4°, and 64.8° correspond to the (111), (200), and (220) crystal planes of the cubic lattice structure of Ag, which are in agreement with the standard Ag pattern (JCPDS NO.
7
04-0783). The corresponding lattice parameters for Ag-ZnO samples are very close to such parameters for the bare ZnO and standard ZnO structure. This confirms that the silver atoms are not incorporated into the ZnO lattice and are loaded onto the ZnO surface. The particle size of the samples was calculated using the Scherrer's formula. According to the calculations, the average crystal size of ZnO in the bare ZnO and Ag-ZnO samples was nearly constant and about 23 nm. Furthermore, the average of crystal size of Ag NPs was 18 nm in the studied samples. The comparison of the XRD patterns and average size of ZnO before and after modification with Ag NPs confirms that the crystal structure of ZnO is intact during modification process. The intensity ratio of the diffraction peak corresponds to the (111) plane of metallic Ag to the diffraction peak of the (101) plane of ZnO (
I 111 ) for each Ag-ZnO samples, prepared by using I 101
different concentrations of AgNO3 in loading solution, was calculated using their XRD patterns (Fig. 1b-d). The calculated values were about 0.19, 0.28 and 0.41 for S2, S4 and S5 samples, respectively. It is clear that by increasing in AgNO3 amount, the values for the (
I 111 ) ratio I 101
increases and this confirms that the loaded amount of Ag NPs increases with increasing in the initial concentration of AgNO3 in loading solution. These results are in accordance with those obtained by AAS (Table 1). Furthermore, the calculated crystal size of Ag NPs was approximately constant for all studied Ag-ZnO samples with an average size of 18 nm. This confirms that the amount of loaded Ag NPs has not considerable effect on the Ag NPs size. 3.1.2. SEM analysis Fig. 2 displays the SEM images of ZnO and Ag-ZnO, confirming the 3-D multipod structures of the two samples. SEM characterizations reveal that the microstructures are assembled by spherical multipods with the rod shape branches of building blocks. The typical 8
length of a branch (measured from bottom to tip) is 2.5 µm. It can be seen that the surface morphology of ZnO is homogeneous and relatively smooth, which suggests the high dispersion of Ag NPs on the surface of ZnO. In addition, the microstructure of Ag-ZnO is almost maintained, and does not change during photo-deposition of Ag NPs.
3.1.3. pH of zero point charge (ZPC) The pH of zero point charge for ZnO and Ag-ZnO photo-catalysts was determined by salt addition method [49, 50]. In this study, 0.0400 g portions of the samples (ZnO or Ag-ZnO) were added to 100 mL of 0.10 M potassium nitrate solutions. The initial pH of each suspension (pHi) was adjusted in the range of 4.0-9.0 by addition of HCl (1.0 M) or NaOH (1.0 M). The mixture was kept in an end-to-end water shaker bath for 18 h and after that the final pH value of each suspension (pHf) was recorded by using a pH–meter. The ZPC of samples were determined by plotting ∆pH (pHf-pHi) versus initial pH values (Fig. 3). By evaluation the intersections of the plots with initial pH axis, the pH of ZPC was obtained around 8.0 for both ZnO and Ag-ZnO suspensions.
3.1.4. UV-vis spectra of ZnO and Ag-ZnO Fig. 4. shows the absorption spectra of the ZnO and Ag-ZnO nanostructures. The presence of absorbance maxima at about 370 nm for both ZnO and Ag-ZnO clearly exhibits that the materials are photoactive. In addition, a significant decrease in absorbance intensity of ZnO peak at 370 nm for Ag-ZnO indicates the presence of silver on ZnO surface [51]. The direct band gap of semiconductors is calculated using the following equation [52]:
ߙ=
(ఔିாಳೠೖ )మ ఔ
9
Where α is the absorption coefficient, C is a constant, hν is the photon energy and ܧ௨ is the band gap energy. From Tauc plots, (αhν) 2 as a function of photon energy (hν), the band gap energies of 2.9 and 2.8 eV were found for ZnO and Ag-ZnO samples, respectively. These values are slightly lower than that of bulk ZnO (3.37 eV) but are in accordance with range reported (2.85-3.06) for ZnO nanostructures with different crystallite size and morphology [53].
3.2. Absorption spectra of dyes mixture In the multi-dye photo-degradation system, the quantification of the interference of one dye with others caused by overlapping of absorption spectra or any interactions is very important. Therefore the UV-vis absorption spectra of the two dyes alone and in their mixture were recorded (Fig. 5). It is obvious that λmax and maximum absorbance values of both MO and MB do not change on mixing. Thus the degradation/decolorization of MO and MB in the mixture solution can be studied separately at their corresponding λmax.
3.3. Photo-catalytic activity of Ag-ZnO The effect of UV irradiation time on the absorption spectra of dyes mixture in the presence of Ag-ZnO was investigated. In this study a mixture of two dyes (pH = 6.5) containing MO (10.0 mg L-1), MB (5.0 mg L-1), and 400.0 mg L-1 of Ag-ZnO (S5) was prepared and the absorption spectra of mixture was recorded at different time intervals. Fig. 6 shows the time-dependent UVvisible spectrum of MO and MB mixture. It can be seen that the maximum absorbance peaks of the two dyes disappear completely after 40 min. The decrease in the absorption band intensities of the two dyes and lack of formation of a new peak indicates that the dyes have been degraded entirely by UV irradiation in the presence of the Ag-ZnO multipods photo-catalyst. It is obvious
10
that the rate of degradation of MO, with resistant structure, is slower than that for MB. The absorbance of visible bands represents the color of dye solutions and its decrease was used to monitor the decolorization of dyes. The absorbance of dyes in UV region (about 270 nm) is due to the aromatic content of dues and its decrease was used to monitor the degradation of aromatic structure of the dyes mixture [48]. The photo-catalytic activities of the synthesized ZnO and Ag-ZnO were evaluated by decolorization of the mixture of MO and MB under different conditions (Fig. 7). The results obtained show that concentration of the mixture of dyes changes a little after irradiation for 60 min in the absence of the photo-catalyst, revealing that the photolysis can be neglected compared with the photo-catalysis. On the other hand, in darkness (no UV light irradiation), concentration of the two dyes was almost constant and did not change even after 60 min. This indicates that adsorption of the two dyes on the surface of photo-catalyst reaches an equilibrium condition after 20 min. Thus in all experiments, the suspension containing the mixture of dyes and photocatalysts was continuously stirred for 20 min in the dark. As shown in Fig. 7, the loading of silver NPs can significantly increase the photo-catalytic performance.
3.4. Optimization of photo-catalytic conditions The decrease in absorbance value of the samples at λ max of the dyes after irradiation in a certain time interval shows the rate of decolorization and therefore the photo-degradation efficiency of the dyes. The decolorization and degradation efficiency were calculated as follow: E (%) =
C 0 − Ct × 100 C0
(1)
where C0 is the initial concentration of MO and MB and C is the concentration of MO and MB in the mixture after irradiation at the selected time intervals. 11
In order to obtain maximum degradation/decolorization efficiency, the pH, effect of AgNO3 concentration on the deposition solution, amount of catalyst, and concentration of dyes were studied and optimized using one-at- a time optimization procedure. All experiments were conducted in triplicates, and the results were shown in the mean values.
3.4.1. Influence of pH The wastewater released from the industry has a wide range value of pH. Hence, pH of the solution is one of the important operational variables affecting on the photo-catalytic treatment of wastewaters. In the study of pH effect, H2SO4 and NaOH were used to adjust pH of MO and MB mixture. The binary solutions of MO (10.0 mg L-1) and MB (5.0 mg L-1) containing 300 mg L-1 of Ag-ZnO (S5) were prepared by mixing known amounts of dyes aqueous solutions and adjusting the solutions pH in the range of 4.0-10.0. Then the photo-catalytic procedure was carried out. Fig. 8 shows the decolorization efficiency of the mixed dyes as a function of pH. As seen, the highest decolorization efficiency occurs at the pH of 6.5.
3.4.2. Influence of catalyst concentration Photo-catalyst concentration is another critical parameter affecting to the photo-catalytic activity. In order to avoid the usage of excess photo-catalyst, it is essential to find out the optimum photo-catalyst concentration to reach a high photo-catalytic efficiency. For this purpose, the effect of various amounts of the photo-catalyst was studied by employing different concentrations of S5 sample in the range of 200-500 mg L-1 at pH 6.5. The results obtained (Fig. 9) show that with increase in the catalyst amount from 200 mg L-1 to 400 mg L-1, the decolorization efficiency in the reaction mixture increase 32% and 23% for MO and MB, respectively. Then the increment catalyst amount from 400 mg L-1 to 500 mg L-1 had a negative 12
effect on the photo-catalytic performance. It is well-known that the increase in the amount of photo-catalyst increases the number of active sites on the photo-catalyst surface, causing an increase in the number of photons absorbed and also the number of the dye molecules adsorbed [54, 55]. On the other hand, due to the agglomeration of catalyst particles at high catalyst concentration, a part of the photo-catalyst surface becoming unavailable for photon absorption and the photo-catalytic activity decreases [56, 48]. The results obtained show that an optimum amount of catalyst for decolorization of the mixture of dyes is 400 mg L-1.
3.4.3. Influence of initial dye concentration The effect of initial concentration of MO and MB in their mixture on the photo-catalytic efficiency was tested by varying the initial concentration of the two dyes from 5.0-20.0 mg L-1 for MO and 2.5-10.0 mg L-1 for MB at the obtained optimum conditions. The results obtained are given in Fig. 10. It is obvious that the simultaneous decolorization efficiency decreases with increase in the initial concentration of the two dyes in solution. This occurrence is due to this fact that the high concentration of dyes hinders that the photons reach into the surface of the photocatalyst and absorption of photons by the catalyst decreases and the photo-catalytic efficiency reduces. Besides, at high dye concentrations the dye molecules are adsorbed on the surface of Ag-ZnO and the active sites for the generation of hydroxyl radicals reduce [48, 57]. However, the further studies were carried out in a mixture of 10.0 mg L-1 of MO and 5.0 mg L-1.
3.4.5. Influence of amount of Ag loading The relation between the amount of doped Ag and decolorization efficiency is shown in Fig 11. For this purpose, Ag-ZnO samples with different amount of loaded Ag NPs were 13
synthesized. As shown in Table 1, the weight percent of Ag in these samples increases with increase in the initial concentration of AgNO3. On the other hand, the photo-catalytic activity of ZnO NPs can be greatly improved by modifying of ZnO with an appropriate amount of Ag NPs. The photo-catalytic activates of 400 mg L-1 of these catalysts in the decolorization of 100 mL mixture of the two dyes (10.0 mg L-1 of MO and 5.0 mg L-1 of MB) were measured. As it can be seen (Fig. 11), the decolorization efficiency of Ag-ZnO increases with increase in the amount of doped Ag NPs up to 6.5% (sample S2) and then decreases with a future increase in the amount of doped Ag NPs. The percentage decolorization varied from 77.3% to 86% for MO, and 89.6 to 93.66 for MB in different concentrations of AgNO3 solution. However, when the amount of deposited Ag NPs is 6.5 %Wt (0.1 mmol AgNO3), the photo-catalytic activity reaches to its maximum value. As said, the loading of Ag NPs on the surface of photo-catalyst improved the photo-catalytic activity, which is mainly due to the better separation of photo-induced charges (electron and hole) in the photo-catalyst. The decrease in the photo-catalytic activity at deposited Ag NPs amounts greater than the optimum value is related to this note that at larger amounts of Ag NPs, the noble metal acts as a recombination center for the photo-generated electrons and holes. Furthermore, higher amounts of loaded Ag NPs will cover large area of the photo-catalyst surface and act as a barrier in the reaching the photons onto the surface of ZnO. Subsequently the photo-catalytic efficiency decreases at higher amounts of loaded Ag NPs [48].
4. Conclusions Ag-ZnO multipods with different Ag loadings were successfully prepared through photoreductive deposition as a clean, effective, and fast route. The prepared photo-catalysts were characterized by XRD, SEM, UV-vis spectrophotometry, and AAS. These Ag-ZnO multipods have potential applications in degradation/decolorization of MO and MB simultaneously. This is 14
the first report on the simultaneous degradation/decolorization of MO and MB. The decolorization extent of the dyes was obviously affected by the solution pH, amount of the catalyst, initial dye concentrations, and Ag doping. The required time for an entire decolorization of simultaneous photo-degradation of dyes using Ag-ZnO catalysts is 40 min. The present study opens a new way towards the synthesis of Ag-ZnO multipods with high photo-catalytic performance for photo-degradation/decolorization of a mixture of the dyes and other organic pollutants in the aqueous solution at a short time.
Acknowledgement The authors are thankful to the Shahrood University Research Council for supporting this work.
15
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19
Legends for Figures and Tables Fig. 1. XRD patterns of samples synthesized: a) pure ZnO, b) Ag-ZnO (S2), c) Ag-ZnO (S4) and d) Ag-ZnO (S5). Fig. 2. SEM images of synthesized samples: (a and b) pure ZnO, (c) Ag-ZnO (S4). Fig. 3. Determination of pHZPC of the ZnO and Ag-ZnO (S2) Fig. 4. UV-Vis spectra for ZnO and Ag-ZnO (S2). All powder samples were dispersed in ethanol and sonoicated for 30 min. Fig. 5. UV-visile absorbance spectra of MO (10.0 mg L-1), MB (5.0 mg L-1), and a mixture of MO (10.0 mg L-1) and MB (5.0 mg L-1). Fig. 6. UV-visible absorbance spectra for the mixture of MO and MB solution in the presence of Ag-ZnO after its exposure to UV light for different times. Experimental conditions: 100 mL mixture of MO and MB with concentrations of 10.0 mg L-1 and 5 mg L-1, respectively; Ag-ZnO (S2): 400 mg L-1 and pH: 6.5. Fig. 7. Decolorization rate of MO (a) and Decolorization rate of MB (b) in their mixture. Experimental conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5 mg L-1, respectively; Ag-ZnO (S2): 400 mg L-1 and pH: 6.5. Fig. 8. Effect of pH on the decolorization of a mixture of two dyes using Ag-ZnO under the UV irradiation time of 40 minutes. Experimental conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5 mg L-1, respectively; Ag-ZnO (S5): 300 mg L-1, and pH range: 4.0-10.0 Fig. 9. Influence of the catalyst concentration on the photo-catalytic decolorization of the mixture of two dyes using Ag-ZnO (S5) under UV irradiation time for 40 minutes. Experimental
20
conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5.0 mg L-1, respectively and pH 6.5. Fig. 10. Effect of initial dye concentration on the decolorization efficiency of MO (a) and MB (b) in the mixture of two dyes using Ag-ZnO. Experimental conditions: 100 mL mixture of MO and MB with known concentration; Ag-ZnO (S5): 400 mg L-1 and pH 6.5. Fig. 11. Influence of dopant concentration on the photo-catalytic activity of ZnO under UV irradiation time for 40 minutes. Experimental conditions: 100 mL mixture of MO and MB with concentration of 10.0 mg L-1 and 5.0 mg L-1, respectively; synthesized photo-catalyst : 40.0 mg, and pH 6.5. Table 1. Samples synthesized and synthesis conditions.
21
Table 1
Sample
AgNO3 (mmol)
Irradiation Time (min)
Wt % Ag from AAS results
S1
0.03
40
1.8
S2
0.1
40
6.5
S3
0.2
40
8.3
S4
0.45
40
10.5
S5
0.9
40
10.75
S6
1.2
40
11.3
22
23 110
201
103 220 200 112
102
200
111
100 002 101
201
200 112
103
102
110
100 002
a
b Ag ()
101
24 103 220 200 112 201
102
110
200
111
100 002 101
201
110 103 220 200 112
102
200
111
100 002 101
c Ag ()
d Ag ()
Fig.2. a
25
Fig.2.b
26
Fig.2.c
27
4 Ag-ZnO (S2) Pure ZnO 3
∆ pH
2
1
0 3 -1
4
5
6
7
pH
-2
Fig.3.
28
8
9
10
1.4 Pure ZnO Ag-ZnO (S2)
Absorbance (a.u.)
1.2
1.0
0.8
0.6
0.4 200
300
400
500
600
Wavelength(nm) Fig. 4.
29
700
800
900
1.0
Absorbance (a. u.)
0.8
MB MO Mixture of MO and MB
0.6
0.4
0.2
0.0 200
300
400
500
Wavelength (nm) Fig. 5.
30
600
700
800
1.0
0.0 min UV
Absorbance (a.u)
0.8
0.6
0.4
0.2
40 min UV 0.0 200
300
400
500
Wavelength (nm)
Fig. 6.
31
600
700
800
1.0 UV
Dark
0.8
C/C0
0.6
0.4
Adsoption Only UV UV+ZnO UV+Ag-ZnO
0.2
0.0 0
10
20
30
40
Time (min) Fig. 7. a
32
50
60
70
1.0
Dark
UV
0.8
C/C0
0.6
0.4
Adsorption Only UV UV+ZnO UV+Ag-ZnO
0.2
0.0 0
10
20
30
40
Time (min) Fig. 7. b
33
50
60
70
110
100
Decolorization of MB Decolorization of MO
E%
90
80
70
60 3
4
5
6
7
pH
Fig. 8.
34
8
9
10
11
100 Decolorization of MO Decolorization of MB 80
E%
60
40
20
0 200
300
400
500 -1
Amount of photocatalyst (mg Lit )
Fig. 9.
35
100
(a) 80
E%
60
40 5ppm MO+2.5ppm MB 10ppm MO+5ppm MB 15ppm MO+7.5ppm MB 20ppm MO+10ppm MB
20
0 0
10
20
30
40
Time (min)
Fig. 10. a
36
50
60
70
100
80
(b)
E%
60
40
5ppm MO+2.5ppm MB 10ppm MO+5ppm MB 15ppm MO+7.5ppm MB 20ppm MO+10ppm MB
20
0 0
10
20
30
40
Time (min)
Fig. 10. b
37
50
60
70
Fig. 11.
38
39
>Ag-ZnO multipodes were synthesized via a solution route/photo-reduction process. > AgZnO multipodes have shown high efficiency in simultaneous MO and MB degradation.> Influencing factors of simultaneous degradation are discussed.> The photocatalytic activity of Ag-ZnO was constant after seven times recycle using.
40