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Photocatalytic properties of [email protected] ZnO core-shell nanocomposite
Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 71–76
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic properties of Ag@Ag-doped ZnO core-shell nanocomposite ⁎
T
Hamid Reza Yousefi , Babak Hashemi Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Pechini sol-gel Core-shell ZnO Photocatalytic activity Methylene blue
In this study, pristine ZnO nanoparticles (NPs), Ag@ZnO core-shell (CS) nanocomposites and Ag@ Ag-doped ZnO CS nanocomposites were synthesized using Pechini sol-gel method. The synthesized samples were characterized by X-ray diffraction (XRD), Ultraviolet-Visible (UV–vis) spectroscopy, Photoluminescence (PL) test, Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Photocatalytic activity of the samples was tested by studying the oxidation of methylene blue (MB) solution. The enhanced photocatalytic activity of Ag@ Ag-doped ZnO CS nanocomposites was attributed to the efficient suppression of electron-hole recombination in ZnO shell due to the strong electron scavenging action of Ag NPs.
1. Introduction Industrial wastes are of the main sources of water and environmental pollutions. Textile industry as an instance, produces a large amount of harmful dye effluent [1]. These toxic dyes are resistant to degradation and are considered highly dangerous to the aquatic organisms and mankind. The textile dye, not only changes the water appearance, but also decreases the water oxygen content and can cause carcinogen and mutagen [2]. Different biological and physico-chemical treatments are generally used in wastewater treatment. However, due to complex chemical/molecular structures of dyes and their toxicity, absolute removal of dyes from the effluent is difficult if not impossible, using traditional approaches [3]. Therefore, new wastewater treatment strategies such as advanced oxidation processes of organic materials in aqueous solutions are currently being used for this purpose [4]. Photocatalytic degradation is an oxidation process in which metal oxide semiconductors are used for degradation of dyes by photocatalytic reactions. ZnO with a band gap of 3.37 eV, exciton binding energy of 60 meV, high chemical and physical stability and non-toxicity nature is a popular metal oxide for photocatalytic applications [5,6]. Even though ZnO have relatively good photocatalytic efficiency, but rapid recombination of photo-generated charge carriers limits the industrial use of ZnO as a photo-catalyst [7]. Photocatalytic activity of metal oxide semiconductors such as ZnO can be enhanced by noble metals doping such as Au [8], Ag [9], Pt [10] or Pd [11]. However, surface modifications is preferable than bulk modification [12]. In some researches core-shell (CS) structures have been used for improving of photocatalytic activity of ZnO. Liu et al. [13], synthesized
⁎
novel worm-like Ag@ZnO CS hetero-structural composites for decomposition of Rhodamine B (RhB). They reported that the Ag@ZnO composites exhibit higher photocatalytic activity compared to pristine ZnO NPs. In another study, Zhai et al [14] synthesized Ag@ZnO CS nanostructures and the PL results showed obvious increase of UV emission. Das and co-workers [15], reported successful disinfection of Gram-negative bacterium Vibrio cholerae 569B in aqueous matrix by solar photo-catalysis mediated by Ag@ZnO CS nanocomposites. In present study, Ag@ Ag-doped ZnO CS nanocomposites were synthesized by a facile two-step method to investigate synergic effects of doping and core-shell structure on photocatalytic activity of ZnO. Following the previous literature and to synthesis the samples, Pechini Sol-Gel process was used [16]. This technique has not been reported for synthesis of Ag@ Ag-doped ZnO CS nanocomposites so far. The samples were well characterized by XRD, FE-SEM, TEM, UV–vis and Photoluminescence (PL) spectroscopies and photocatalytic activity of them was studied by degradation of methylene blue (MB) in water under UV radiation. 2. Experimentation 2.1. Chemicals Analytical grades silver nitrate (AgNO3), sodium borohydride (NaBH4), poly vinyl pyrrolidone (PVP), ethylene glycol (EG), zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and citric acid (C6H8O7) were used as starting materials. All starting materials are used without further purifications. Distilled (DI) water is used as solvent in all
Corresponding author. E-mail addresses: yousefi[email protected] (H.R. Yousefi), [email protected] (B. Hashemi).
https://doi.org/10.1016/j.jphotochem.2019.02.008 Received 21 September 2018; Received in revised form 22 December 2018; Accepted 6 February 2019 Available online 14 February 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 71–76
H.R. Yousefi and B. Hashemi
experiments. Analytical grade Methylene blue (FW: 319.86 g/mol, kmax = 662 nm, C.I. Classification Number: 52015.) was used in the photocatalytic studies. Also TiO2 commertial (p25) was used as a conventional photocatalyst material in order to compare photocatalyctic properties of samples with it. 2.2. Synthesis of Ag colloids Creighton's method has been exploited for synthesizing of Ag colloids [17,18]. For this purpose, three separate solutions were prepared by dissolving of PVP, AgNO3 and NaBH4 in DI water under magnetic stirring. In the next step, PVP solution (1 mM) was added to AgNO3 solution (10 mM) in a molar ratio of 1:10. NaBH4 solution (12 mM) then was added to Ag-PVP solution dropwise under magnetic stirring and color of solution was changed to pale yellow which indicated formation of ultrafine silver NPs.
Fig. 1. XRD patterns of synthesized sample in this study.
2.3. Synthesis of Ag@ Ag-doped ZnO CS samples
Eq. (1):
Synthesis of Ag@Ag-doped ZnO CS nanocomposites were carried out according to Pechini process [16]. For this purpose, initially two separate solutions of citric acid with concentration of 20 mM (as chelating agents of metal ions) and zinc acetate dihydrate with concentration of 10 mM were prepared by dissolving of appropriate precursors in DI water. Then citric acid solution was then added to zinc acetate dihydrate solution under magnetic stirring at 70 °C for 1 h. Afterwards, silver nitrate with weight ratios of 0, 1, 3 and 5 relative to zinc acetate, was added to the solution. Finally, for synthesis of Ag@Agdoped ZnO CS nanocomposites, Ag colloid was added dropwise to Agdoped ZnO solutions under magnetic stirring in a weight ratio of Ag to ZnO 1 at 50 °C. Following this treatment, ethylene glycol (as a cross4.3 linking agent) was added to resultant solutions with weight ratio of 0.66 relative to citric acid and stirred magnetically at 70 °C for 2 h. For the purpose of polymerization of sol, temperature was increased to 120 °C and was refluxed for 2 h. Once the exposure was completed, powders were calcined at 500 °C for 5 h at a heating rate of 10 °C/min in a muffle furnace and the C–S powders were named CS, CSD1, CSD3 and CSD5, respectively. Pristine ZnO NPs were synthesized using Pechini process without addition Ag and the samples was named ZnO.
η = 1 − c /c0
(1)
Where, η is degradation rate, C0 is the initial concentration of MB and C is the final concentration of MB in solution in the presence of catalyst under UV light. For measuring the absorbance intensity of solutions, 5 ml of the solutions were sampled from the system and centrifuged (12,000 rpm) to separate the C–S powder samples and then the absorbance intensity was measured using UV–vis spectroscopy. This process was repeated three times. 3. Results and discussion Fig. 1 shows XRD patterns of synthesized samples in this study. The peaks located at 2θ = 31.87, 34.50, 36.33, 47.60, 56.65, 62.96, 66.45, 67.99, 69.19, 72.64, 77.00, 82.21 and 89.65 can be assigned to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (022), (014) and (023) planes of crystalline ZnO with hexagonal crystal structure (JCPDS Card No. 80-74), respectively. Also the peaks located at 2θ values of 38.31°, 44.39°, 64.5° 77.54°, and 81.62° can be indexed as the (111), (200), (220), (311), and (222) crystal planes of the facecentered-cubic phase of silver (JCPDS Card No. 82-0718). No other phase or compound than ZnO and Ag were observed in the samples. In CS sample, there wasn’t any peak shift relative to pure ZnO, which indicated successful coating of ZnO over Ag NPs. With addition of Ag as dopant to Ag@ZnO samples, XRD peaks were shifted relative to CS and Ag peaks were increased in intensity. This demonstrated that according to solubility of ZnO, some amount of Ag was dissolved in ZnO lattice and the excess Ag was added to The core or precipitated as Ag phase at the surface of CS samples [19,20]. Fig. 2 illustrates SEM-FESEM and TEM images of Ag NPs, ZnO NPs, and CS samples. It shows that, synthesized samples have spherical morphology and CS sample has smaller particle size than ZnO NPs. This is due to the fact that Ag particles in the sol-gel solution act as nucleation sites for ZnO nucleation and ZnO is formed on the surface of Ag particles [21]. TEM image of CS sample or Ag @ZnO NPs (Fig. 2 (c)) shows that some Ag NPs are nearly coated by a thin shell of ZnO however it is seen some Ag NPs particles which don’t completely cover by ZnO layer or don’t have any ZnO layer. This behavior has been seen in other researches [15,17] UV-Vis adsorption spectra of the samples are depicted in Fig. 3. This figure reveals a sharp peak at 398 nm for Ag NPs. This, is due to surface Plasmon resonance (SPR) of spherical Ag NPs [22]. But it isn’t seen in CS samples because the surface layer of Ag NPs restrict this effect. For ZnO sample at UV region, a peak can be observed at 370 nm due to electrons transfer from valance band to conduction band of ZnO. By doping of Ag in CS samples, this peak slightly shifted to higher wavelength due to the decrease in ZnO band gap.
2.4. Characterization of samples Phase analysis and crystallinity of synthesized CS samples were examined by X-ray diffraction (XRD-Hitachi, S 570, Japan) using CuKα radiation (λ = 1.504 Å). Morphological features were studied using Field emission scanning electron microscopy (FE-SEM, Zeiss Germany) and transmission electron microscopy (TEM- JEM-2100 F, JEOL Inc., Japan). UV–vis spectroscopy (Thermo, USA950) is exploited to study the optical properties of CS samples. Room-temperature PL measurements were carried out with a spectrometer using a He-Cd arc lamp as excitation source with excitation wavelength of 325 nm. 2.5. Photo-catalysts tests In order to evaluate photocatalytic properties of synthesized CS samples, degradation of methylene blue (MB) under UV radiation was examined. For this purpose, ISI10678-2010 standard was used. Solutions containing 200 mg of each CS sample and 100 ml of MB solution (with concentration of 5 ppm) were prepared and then magnetically stirred for 30 min under dark conditions to establish the absorption-desorption equilibrium between MB and photo-catalyst CS samples. Prepared solutions were then exposed to UV light at room temperature. Two 8 W UV lamps of A type (λmax = 365 nm) were used as UV light sources. In order to keep the homogeneity of prepared solutions during the photocatalytic tests, the solutions were magnetically stirred. Degradation rate of MB in solutions was calculated according to 72
Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 71–76
H.R. Yousefi and B. Hashemi
Fig. 2. FE-SEM micrograph of (a) Ag colloids, (b) ZnO NPs sample and (c) TEM micrograph of CS sample (d) another TEM image of the CS sample.
Fig. 4. PL spectra of different samples.
Fig. 3. UV–vis adsorption spectra of synthesized samples.
addition to the slight peaks shifts to the left. This indicates that electron-hole recombination has been decreased by Ag doping to ZnO shell the core-shell samples [18–20]. Degradation rate of MB solution in the presence of different samples is illustrated in Fig. 5. As Fig. 5 shows, CS sample has a lower photocatalytic activity relative to ZnO and TiO2 samples, due to lower catalytic activity in the UV region. It can be seen that doping of Ag to ZnO shell of core-shell samples will result in significant degradation of MB relative to ZnO and TiO2 samples, and CSD3 sample consequently has the highest degradation rate. For better understanding of the photocatalytic efficiency of the samples, the kinetic analysis of MB degradation was performed as follow. It is generally assumed that reaction kinetics can be described in
Fig. 4 shows PL spectra of the samples. Two PL peaks centered at 378 and 485 nm can be observed in UV and visible regions respectively. Emission peak in UV region is mainly due to recombination of free excitons during the exciton-exciton interaction, which is so-called near band emission (NBE). Emission in visible region is due to presence of impurities and structural defects such as oxygen vacancies and Zn interstitials in ZnO lattice structure, which is known as deep level emission. Photocatalytic activity is related to life time of excitons (electronsholes) excited by UV–vis light. As a result, photocatalytic activity enhancement strongly depends on prevention of electron-hole recombination. According to Fig. 4 when Ag dopant is added to CS samples, the PL peak intensity decreased in the UV and visible region in 73
Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 71–76
H.R. Yousefi and B. Hashemi
Table 1 Degradation parameter of MB by different samples under UV radiation. R2
Kapp(min−1)
Samples
0.98548 0.9482 0.99802 0.99349 0.98866 0.9468
0.00972 0.00626 0.02549 0.04669 0.03782 0.0158
ZnO CS CSD1 CSD3 CSD5 TiO2
Fig. 5. Effect of different samples on the photo-degradation of MB solution with time.
terms of Langmuir-Hinshelwood model [21]:
ZnO + UV(h ϑ) → ZnO(e−cb + h+vb)
(2)
Where, dc/dt is the degradation rate of MB, κ is the photo-catalysis rate constant, C is the concentration of MB and K is the equilibrium adsorption coefficient of MB. For small values of C, KC is negligible and Eq. (3) can be described as a first-order kinetics model:
ln c / c0 = κKt = κ app t
Fig. 7. UV–vis adsorption of MB solution containing CSD3 sample under UV light irradiation for diffetent times.
(3)
Where, C0 is the initial concentration of MB in solution and κapp is the apparent rate constant. Fig. 6 shows the kinetic curves or ln C0/C verse time for MB degradation by different samples at different conditions. It suggests a good linear relationship between ln C0/C and the reaction time exist. From Fig. 6, the apparent rate constant values (κapp) were calculated and listed in Table 1. The results show that the CSD3 sample has the largest κapp or the best photocatalyst activity relative to other samples. Fig. 7 presents adsorption spectra of MB solution containing CSD3 sample after UV light radiation at different time intervals. It can be noticed that MB solution shows a sharp peak at 664 nm which gradually decreases by increasing radiation time and finally disappears after 90 min. On the other hand, MB degradation was completely done after
90 min in the presence of CSD3 sample. A good photo-catalyst must have high stability and reusability. For this purpose, recycled experiments for the photo-degradation of MB solution by recycled CSD3 sample were done for three times. During these tests, experimental parameters were the same and the CSD3 powder sample was re-collected by centrifugation from MB solution after each photo degradation test and then re-dispersed in the MB solution for the next cycle. As it is shown in Fig. 8, the degradation rate (Eq. (1)) after the first cycle was 98.6% which changed to 97.3% after three cycles. This result demonstrates outstanding photo-stability of CSD3 sample. Fig. 9 Shows photocatalytic mechanism of Ag doped CS samples. When Ag was added to CS samples, according to solubility limit of ZnO, some amount of Ag dissolves in the lattice of ZnO and the remains precipitates on the surface of ZnO. When two materials with different work function are in intimate contact, electrons will transfer from the
Fig. 6. The corresponding kinetic analysis associated with a first-order reaction for. MB degradation by different samples.
Fig. 8. The cyclic stability of the CSD3 powder for the photo-degradation of MB solution under UV light radiation. 74
Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 71–76
H.R. Yousefi and B. Hashemi
MB + •OH → Degradation product
(10)
MB+•O2→ Degradation product
(11)
According to described mechanism, since the amount of Ag dopant is in the range or slightly higher than solubility limit of ZnO in CSD3 sample, Ag particles could not precipitate or a few particles precipitate on the surface of ZnO and recombination rate in this sample is low. In CSD5 sample, due to high amount of Ag dopant, it may be a continuous Ag layer covers the surface of ZnO and it restricts the UV light to reach the surface of ZnO also deposition of Ag on the Ag@Ag-doped ZnO surface will obliviously lead to agglomeration of particles and may lead to catalyst deactivation. Therefore, CSD3 sample has the highest photocatalyst activity relative to other samples. 4. Conclusion ZnO NPs and Ag@ Ag-doped ZnO CS NPs were synthesized by Pechini Sol-Gel process. Their photocatalytic performance towards MB under UV light was a good method for comparison of photocatalytic activity of the samples. It is revealed that Ag doped CS samples have a better activity relative to ZnO, TiO2 and CS samples. This is due to lower electron-hole recombination rate of these samples under UV radiation. With addition of an optimum amount of Ag to ZnO shell layer of CS samples, the best photocatalytic activity can be obtained. Acknowledgement Authors would like to acknowledge partial support of the Iran Nanotechnology Innovation Council. References
Fig. 9. Suggested behaviour of doped core shell (a) without UV light (b) Under UV light radiation.
[1] S.-A. Ong, E. Toorisaka, M. Hirata, T. Hano, Biodegradation of redox dye Methylene Blue by up-flow anaerobic sludge blanket reactor, J. Hazard. Mater. 124 (1) (2005) 88–94. [2] X. Liu, Y. Yang, X. Shi, K. Li, Fast photocatalytic degradation of methylene blue dye using a low-power diode laser, J. Hazard. Mater. 283 (2015) 267–275. [3] N. Daud, B. Hameed, Decolorization of acid red 1 by Fenton-like process using rice husk ash-based catalyst, J. Hazard. Mater. 176 (1) (2010) 938–944. [4] S. Silvestri, E.L. Foletto, Preparation and characterization of Fe 2 O 3/TiO 2/clay plates and their use as photocatalysts, Ceram. Int. 43 (16) (2017) 14057–14062. [5] R. Fagan, D.E. McCormack, D.D. Dionysiou, S.C. Pillai, A review of solar and visible light active TiO 2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern, Mater. Sci. Semicond. Process. 42 (2016) 2–14. [6] J. Yu, T. Ma, S. Liu, Enhanced photocatalytic activity of mesoporous TiO 2 aggregates by embedding carbon nanotubes as electron-transfer channel, J. Chem. Soc. Faraday Trans. 13 (8) (2011) 3491–3501. [7] A. Meng, X. Li, X. Wang, Z. Li, Preparation, photocatalytic properties and mechanism of Fe or N-doped Ag/ZnO nanocomposites, Ceram. Int. 40 (7) (2014) 9303–9309. [8] F. Lin, et al., Intermediate selectivity in the oxidation of phenols using plasmonic Au/ZnO photocatalysts, Nanoscale 9 (27) (2017) 9359–9364. [9] C. Jaramillo-Páez, J. Navío, M. Hidalgo, M. Macías, High UV-photocatalytic activity of ZnO and Ag/ZnO synthesized by a facile method, Catal. Today 284 (2017) 121–128. [10] T.R. Sobahi, M.Y. Abdelaal, R. Mohamed, M. Mokhtar, Photocatalytic degradation of methylene blue dye in water using Pt/ZnO-MWCNT under visible light, Nanosci. Nanotechnol. Lett. 9 (2) (2017) 144–150. [11] Z.S. Seddigi, et al., The efficient photocatalytic degradation of methyl tert‐butyl ether under Pd/ZnO and visible light irradiation, Photochem. Photobiol. 91 (2) (2015) 265–271. [12] B. Rajbongshi, A. Ramchiary, B. Jha, S. Samdarshi, Synthesis and characterization of plasmonic visible active Ag/ZnO photocatalyst, J. Mater. Sci. Mater. Electron. 25 (7) (2014) 2969–2973. [13] H. Liu, et al., Worm-like Ag/ZnO core–shell heterostructural composites: fabrication, characterization, and photocatalysis, J. Phys. Chem. C 116 (30) (2012) 16182–16190. [14] H. Zhai, et al., Facile one-step synthesis and photoluminescence properties of Ag–ZnO core–shell structure, J. Alloys. Compd. 600 (2014) 146–150. [15] S. Das, S. Sinha, M. Suar, S.-I. Yun, A. Mishra, S.K. Tripathy, Solar-photocatalytic disinfection of Vibrio cholerae by using Ag@ ZnO core–shell structure nanocomposites, J. Photochem. Photobiol. B, Biol. 142 (2015) 68–76. [16] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Recent advances in the liquidphase syntheses of inorganic nanoparticles, Chem. Rev. 104 (9) (2004) 3893–3946.
materials with lower work function to the material with higher work function in order to balance Fermi levels in both materials (a new Fermi level is produced). Since the work function of Ag (4.2 eV) is lower than that of ZnO (5.2 eV), electrons will transfer from Ag to ZnO. Decrease of free electrons in Ag NPs will cause formation of oxidation radicals such as •OH on the surface of Ag and anionic radicals (%O2) on the surface of ZnO as follows [22]. + hcb + OH− •OH
(4)
− ecb + O2 → •O2
(5)
When doped C–S sample is exposed to UV light or photons with energies of equal or larger than energy band gap of ZnO, electrons in valence band are excited to conduction band and holes are formed in valence band. Since the lowest conduction band energy level in ZnO is higher than newly formed Fermi level energy, exited electrons are transferred from ZnO to Ag and consequently recombination of electron-holes in ZnO is delayed. Therefore, the degradation rate is increased in the doped CS samples. With decrease in recombination rate, the electrons in conduction band react with oxygen molecules and form oxygen anionic radicals (%O2) on the surface of ZnO. These radicals react with H2O and produce %OH radicals, which eventually degrade organic pollutions. The equations showing the photocatalytic degradation of dye molecules are as follow [23,24]:
ZnO + UV(h ϑ) → ZnO(e−cb + h+νb)
(6)
− ecb + O2 → •O2
(7)
Ag + +
+ ecb
→ Ag
+ h vb + OH− → •OH
(8) (9) 75
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H.R. Yousefi and B. Hashemi [17] Y. Zhao, et al., The structure, morphology, and the metal-enhanced fluorescence of nano-Ag/ZnO core–shell structure, Appl. Nanosci. 5 (5) (2015) 521–525. [18] M.S. Hameed, J.J. Princice, N.R. Babu, A. Arunachalam, Effect of silver doping on optical properties of nanoflower ZnO thin films prepared by spray pyrolysis technique, J. Mater. Sci. Mater. Electron. 28 (12) (2017) 8675–8683. [19] M. Koleva, A.O. Dikovska, N. Nedyalkov, P. Atanasov, I. Bliznakova, Enhancement of ZnO photoluminescence by laser nanostructuring of Ag underlayer, Appl. Surf. Sci. 258 (23) (2012) 9181–9185. [20] C. Ren, et al., Synthesis of Ag/ZnO nanorods array with enhanced photocatalytic performance, J. Hazard. Mater. 182 (1-3) (2010) 123–129.
[21] K.-S. Chou, C.-Y. Ren, Synthesis of nanosized silver particles by chemical reduction method, Mater. Chem. Phys. 64 (3) (2000) 241–246. [22] M.G. Guzmán, J. Dille, S. Godet, Synthesis of silver nanoparticles by chemical reduction method and their antibacterial activity, Int J Chem Biomol Eng 2 (3) (2009) 104–111. [23] R.S. Zeferino, M.B. Flores, U. Pal, Photoluminescence and Raman scattering in Agdoped ZnO nanoparticles, J. Appl. Phys. 109 (1) (2011) p. 014308. [24] C. Karunakaran, V. Rajeswari, P. Gomathisankar, Optical, electrical, photocatalytic, and bactericidal properties of microwave synthesized nanocrystalline Ag–ZnO and ZnO, Solid State Sci. 13 (5) (2011) 923–928.
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Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic properties of Ag@Ag-doped ZnO core-shell nanocomposite ⁎
T
Hamid Reza Yousefi , Babak Hashemi Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Pechini sol-gel Core-shell ZnO Photocatalytic activity Methylene blue
In this study, pristine ZnO nanoparticles (NPs), Ag@ZnO core-shell (CS) nanocomposites and Ag@ Ag-doped ZnO CS nanocomposites were synthesized using Pechini sol-gel method. The synthesized samples were characterized by X-ray diffraction (XRD), Ultraviolet-Visible (UV–vis) spectroscopy, Photoluminescence (PL) test, Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Photocatalytic activity of the samples was tested by studying the oxidation of methylene blue (MB) solution. The enhanced photocatalytic activity of Ag@ Ag-doped ZnO CS nanocomposites was attributed to the efficient suppression of electron-hole recombination in ZnO shell due to the strong electron scavenging action of Ag NPs.
1. Introduction Industrial wastes are of the main sources of water and environmental pollutions. Textile industry as an instance, produces a large amount of harmful dye effluent [1]. These toxic dyes are resistant to degradation and are considered highly dangerous to the aquatic organisms and mankind. The textile dye, not only changes the water appearance, but also decreases the water oxygen content and can cause carcinogen and mutagen [2]. Different biological and physico-chemical treatments are generally used in wastewater treatment. However, due to complex chemical/molecular structures of dyes and their toxicity, absolute removal of dyes from the effluent is difficult if not impossible, using traditional approaches [3]. Therefore, new wastewater treatment strategies such as advanced oxidation processes of organic materials in aqueous solutions are currently being used for this purpose [4]. Photocatalytic degradation is an oxidation process in which metal oxide semiconductors are used for degradation of dyes by photocatalytic reactions. ZnO with a band gap of 3.37 eV, exciton binding energy of 60 meV, high chemical and physical stability and non-toxicity nature is a popular metal oxide for photocatalytic applications [5,6]. Even though ZnO have relatively good photocatalytic efficiency, but rapid recombination of photo-generated charge carriers limits the industrial use of ZnO as a photo-catalyst [7]. Photocatalytic activity of metal oxide semiconductors such as ZnO can be enhanced by noble metals doping such as Au [8], Ag [9], Pt [10] or Pd [11]. However, surface modifications is preferable than bulk modification [12]. In some researches core-shell (CS) structures have been used for improving of photocatalytic activity of ZnO. Liu et al. [13], synthesized
⁎
novel worm-like Ag@ZnO CS hetero-structural composites for decomposition of Rhodamine B (RhB). They reported that the Ag@ZnO composites exhibit higher photocatalytic activity compared to pristine ZnO NPs. In another study, Zhai et al [14] synthesized Ag@ZnO CS nanostructures and the PL results showed obvious increase of UV emission. Das and co-workers [15], reported successful disinfection of Gram-negative bacterium Vibrio cholerae 569B in aqueous matrix by solar photo-catalysis mediated by Ag@ZnO CS nanocomposites. In present study, Ag@ Ag-doped ZnO CS nanocomposites were synthesized by a facile two-step method to investigate synergic effects of doping and core-shell structure on photocatalytic activity of ZnO. Following the previous literature and to synthesis the samples, Pechini Sol-Gel process was used [16]. This technique has not been reported for synthesis of Ag@ Ag-doped ZnO CS nanocomposites so far. The samples were well characterized by XRD, FE-SEM, TEM, UV–vis and Photoluminescence (PL) spectroscopies and photocatalytic activity of them was studied by degradation of methylene blue (MB) in water under UV radiation. 2. Experimentation 2.1. Chemicals Analytical grades silver nitrate (AgNO3), sodium borohydride (NaBH4), poly vinyl pyrrolidone (PVP), ethylene glycol (EG), zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and citric acid (C6H8O7) were used as starting materials. All starting materials are used without further purifications. Distilled (DI) water is used as solvent in all
Corresponding author. E-mail addresses: yousefi[email protected] (H.R. Yousefi), [email protected] (B. Hashemi).
https://doi.org/10.1016/j.jphotochem.2019.02.008 Received 21 September 2018; Received in revised form 22 December 2018; Accepted 6 February 2019 Available online 14 February 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 71–76
H.R. Yousefi and B. Hashemi
experiments. Analytical grade Methylene blue (FW: 319.86 g/mol, kmax = 662 nm, C.I. Classification Number: 52015.) was used in the photocatalytic studies. Also TiO2 commertial (p25) was used as a conventional photocatalyst material in order to compare photocatalyctic properties of samples with it. 2.2. Synthesis of Ag colloids Creighton's method has been exploited for synthesizing of Ag colloids [17,18]. For this purpose, three separate solutions were prepared by dissolving of PVP, AgNO3 and NaBH4 in DI water under magnetic stirring. In the next step, PVP solution (1 mM) was added to AgNO3 solution (10 mM) in a molar ratio of 1:10. NaBH4 solution (12 mM) then was added to Ag-PVP solution dropwise under magnetic stirring and color of solution was changed to pale yellow which indicated formation of ultrafine silver NPs.
Fig. 1. XRD patterns of synthesized sample in this study.
2.3. Synthesis of Ag@ Ag-doped ZnO CS samples
Eq. (1):
Synthesis of Ag@Ag-doped ZnO CS nanocomposites were carried out according to Pechini process [16]. For this purpose, initially two separate solutions of citric acid with concentration of 20 mM (as chelating agents of metal ions) and zinc acetate dihydrate with concentration of 10 mM were prepared by dissolving of appropriate precursors in DI water. Then citric acid solution was then added to zinc acetate dihydrate solution under magnetic stirring at 70 °C for 1 h. Afterwards, silver nitrate with weight ratios of 0, 1, 3 and 5 relative to zinc acetate, was added to the solution. Finally, for synthesis of Ag@Agdoped ZnO CS nanocomposites, Ag colloid was added dropwise to Agdoped ZnO solutions under magnetic stirring in a weight ratio of Ag to ZnO 1 at 50 °C. Following this treatment, ethylene glycol (as a cross4.3 linking agent) was added to resultant solutions with weight ratio of 0.66 relative to citric acid and stirred magnetically at 70 °C for 2 h. For the purpose of polymerization of sol, temperature was increased to 120 °C and was refluxed for 2 h. Once the exposure was completed, powders were calcined at 500 °C for 5 h at a heating rate of 10 °C/min in a muffle furnace and the C–S powders were named CS, CSD1, CSD3 and CSD5, respectively. Pristine ZnO NPs were synthesized using Pechini process without addition Ag and the samples was named ZnO.
η = 1 − c /c0
(1)
Where, η is degradation rate, C0 is the initial concentration of MB and C is the final concentration of MB in solution in the presence of catalyst under UV light. For measuring the absorbance intensity of solutions, 5 ml of the solutions were sampled from the system and centrifuged (12,000 rpm) to separate the C–S powder samples and then the absorbance intensity was measured using UV–vis spectroscopy. This process was repeated three times. 3. Results and discussion Fig. 1 shows XRD patterns of synthesized samples in this study. The peaks located at 2θ = 31.87, 34.50, 36.33, 47.60, 56.65, 62.96, 66.45, 67.99, 69.19, 72.64, 77.00, 82.21 and 89.65 can be assigned to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (022), (014) and (023) planes of crystalline ZnO with hexagonal crystal structure (JCPDS Card No. 80-74), respectively. Also the peaks located at 2θ values of 38.31°, 44.39°, 64.5° 77.54°, and 81.62° can be indexed as the (111), (200), (220), (311), and (222) crystal planes of the facecentered-cubic phase of silver (JCPDS Card No. 82-0718). No other phase or compound than ZnO and Ag were observed in the samples. In CS sample, there wasn’t any peak shift relative to pure ZnO, which indicated successful coating of ZnO over Ag NPs. With addition of Ag as dopant to Ag@ZnO samples, XRD peaks were shifted relative to CS and Ag peaks were increased in intensity. This demonstrated that according to solubility of ZnO, some amount of Ag was dissolved in ZnO lattice and the excess Ag was added to The core or precipitated as Ag phase at the surface of CS samples [19,20]. Fig. 2 illustrates SEM-FESEM and TEM images of Ag NPs, ZnO NPs, and CS samples. It shows that, synthesized samples have spherical morphology and CS sample has smaller particle size than ZnO NPs. This is due to the fact that Ag particles in the sol-gel solution act as nucleation sites for ZnO nucleation and ZnO is formed on the surface of Ag particles [21]. TEM image of CS sample or Ag @ZnO NPs (Fig. 2 (c)) shows that some Ag NPs are nearly coated by a thin shell of ZnO however it is seen some Ag NPs particles which don’t completely cover by ZnO layer or don’t have any ZnO layer. This behavior has been seen in other researches [15,17] UV-Vis adsorption spectra of the samples are depicted in Fig. 3. This figure reveals a sharp peak at 398 nm for Ag NPs. This, is due to surface Plasmon resonance (SPR) of spherical Ag NPs [22]. But it isn’t seen in CS samples because the surface layer of Ag NPs restrict this effect. For ZnO sample at UV region, a peak can be observed at 370 nm due to electrons transfer from valance band to conduction band of ZnO. By doping of Ag in CS samples, this peak slightly shifted to higher wavelength due to the decrease in ZnO band gap.
2.4. Characterization of samples Phase analysis and crystallinity of synthesized CS samples were examined by X-ray diffraction (XRD-Hitachi, S 570, Japan) using CuKα radiation (λ = 1.504 Å). Morphological features were studied using Field emission scanning electron microscopy (FE-SEM, Zeiss Germany) and transmission electron microscopy (TEM- JEM-2100 F, JEOL Inc., Japan). UV–vis spectroscopy (Thermo, USA950) is exploited to study the optical properties of CS samples. Room-temperature PL measurements were carried out with a spectrometer using a He-Cd arc lamp as excitation source with excitation wavelength of 325 nm. 2.5. Photo-catalysts tests In order to evaluate photocatalytic properties of synthesized CS samples, degradation of methylene blue (MB) under UV radiation was examined. For this purpose, ISI10678-2010 standard was used. Solutions containing 200 mg of each CS sample and 100 ml of MB solution (with concentration of 5 ppm) were prepared and then magnetically stirred for 30 min under dark conditions to establish the absorption-desorption equilibrium between MB and photo-catalyst CS samples. Prepared solutions were then exposed to UV light at room temperature. Two 8 W UV lamps of A type (λmax = 365 nm) were used as UV light sources. In order to keep the homogeneity of prepared solutions during the photocatalytic tests, the solutions were magnetically stirred. Degradation rate of MB in solutions was calculated according to 72
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Fig. 2. FE-SEM micrograph of (a) Ag colloids, (b) ZnO NPs sample and (c) TEM micrograph of CS sample (d) another TEM image of the CS sample.
Fig. 4. PL spectra of different samples.
Fig. 3. UV–vis adsorption spectra of synthesized samples.
addition to the slight peaks shifts to the left. This indicates that electron-hole recombination has been decreased by Ag doping to ZnO shell the core-shell samples [18–20]. Degradation rate of MB solution in the presence of different samples is illustrated in Fig. 5. As Fig. 5 shows, CS sample has a lower photocatalytic activity relative to ZnO and TiO2 samples, due to lower catalytic activity in the UV region. It can be seen that doping of Ag to ZnO shell of core-shell samples will result in significant degradation of MB relative to ZnO and TiO2 samples, and CSD3 sample consequently has the highest degradation rate. For better understanding of the photocatalytic efficiency of the samples, the kinetic analysis of MB degradation was performed as follow. It is generally assumed that reaction kinetics can be described in
Fig. 4 shows PL spectra of the samples. Two PL peaks centered at 378 and 485 nm can be observed in UV and visible regions respectively. Emission peak in UV region is mainly due to recombination of free excitons during the exciton-exciton interaction, which is so-called near band emission (NBE). Emission in visible region is due to presence of impurities and structural defects such as oxygen vacancies and Zn interstitials in ZnO lattice structure, which is known as deep level emission. Photocatalytic activity is related to life time of excitons (electronsholes) excited by UV–vis light. As a result, photocatalytic activity enhancement strongly depends on prevention of electron-hole recombination. According to Fig. 4 when Ag dopant is added to CS samples, the PL peak intensity decreased in the UV and visible region in 73
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Table 1 Degradation parameter of MB by different samples under UV radiation. R2
Kapp(min−1)
Samples
0.98548 0.9482 0.99802 0.99349 0.98866 0.9468
0.00972 0.00626 0.02549 0.04669 0.03782 0.0158
ZnO CS CSD1 CSD3 CSD5 TiO2
Fig. 5. Effect of different samples on the photo-degradation of MB solution with time.
terms of Langmuir-Hinshelwood model [21]:
ZnO + UV(h ϑ) → ZnO(e−cb + h+vb)
(2)
Where, dc/dt is the degradation rate of MB, κ is the photo-catalysis rate constant, C is the concentration of MB and K is the equilibrium adsorption coefficient of MB. For small values of C, KC is negligible and Eq. (3) can be described as a first-order kinetics model:
ln c / c0 = κKt = κ app t
Fig. 7. UV–vis adsorption of MB solution containing CSD3 sample under UV light irradiation for diffetent times.
(3)
Where, C0 is the initial concentration of MB in solution and κapp is the apparent rate constant. Fig. 6 shows the kinetic curves or ln C0/C verse time for MB degradation by different samples at different conditions. It suggests a good linear relationship between ln C0/C and the reaction time exist. From Fig. 6, the apparent rate constant values (κapp) were calculated and listed in Table 1. The results show that the CSD3 sample has the largest κapp or the best photocatalyst activity relative to other samples. Fig. 7 presents adsorption spectra of MB solution containing CSD3 sample after UV light radiation at different time intervals. It can be noticed that MB solution shows a sharp peak at 664 nm which gradually decreases by increasing radiation time and finally disappears after 90 min. On the other hand, MB degradation was completely done after
90 min in the presence of CSD3 sample. A good photo-catalyst must have high stability and reusability. For this purpose, recycled experiments for the photo-degradation of MB solution by recycled CSD3 sample were done for three times. During these tests, experimental parameters were the same and the CSD3 powder sample was re-collected by centrifugation from MB solution after each photo degradation test and then re-dispersed in the MB solution for the next cycle. As it is shown in Fig. 8, the degradation rate (Eq. (1)) after the first cycle was 98.6% which changed to 97.3% after three cycles. This result demonstrates outstanding photo-stability of CSD3 sample. Fig. 9 Shows photocatalytic mechanism of Ag doped CS samples. When Ag was added to CS samples, according to solubility limit of ZnO, some amount of Ag dissolves in the lattice of ZnO and the remains precipitates on the surface of ZnO. When two materials with different work function are in intimate contact, electrons will transfer from the
Fig. 6. The corresponding kinetic analysis associated with a first-order reaction for. MB degradation by different samples.
Fig. 8. The cyclic stability of the CSD3 powder for the photo-degradation of MB solution under UV light radiation. 74
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MB + •OH → Degradation product
(10)
MB+•O2→ Degradation product
(11)
According to described mechanism, since the amount of Ag dopant is in the range or slightly higher than solubility limit of ZnO in CSD3 sample, Ag particles could not precipitate or a few particles precipitate on the surface of ZnO and recombination rate in this sample is low. In CSD5 sample, due to high amount of Ag dopant, it may be a continuous Ag layer covers the surface of ZnO and it restricts the UV light to reach the surface of ZnO also deposition of Ag on the Ag@Ag-doped ZnO surface will obliviously lead to agglomeration of particles and may lead to catalyst deactivation. Therefore, CSD3 sample has the highest photocatalyst activity relative to other samples. 4. Conclusion ZnO NPs and Ag@ Ag-doped ZnO CS NPs were synthesized by Pechini Sol-Gel process. Their photocatalytic performance towards MB under UV light was a good method for comparison of photocatalytic activity of the samples. It is revealed that Ag doped CS samples have a better activity relative to ZnO, TiO2 and CS samples. This is due to lower electron-hole recombination rate of these samples under UV radiation. With addition of an optimum amount of Ag to ZnO shell layer of CS samples, the best photocatalytic activity can be obtained. Acknowledgement Authors would like to acknowledge partial support of the Iran Nanotechnology Innovation Council. References
Fig. 9. Suggested behaviour of doped core shell (a) without UV light (b) Under UV light radiation.
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materials with lower work function to the material with higher work function in order to balance Fermi levels in both materials (a new Fermi level is produced). Since the work function of Ag (4.2 eV) is lower than that of ZnO (5.2 eV), electrons will transfer from Ag to ZnO. Decrease of free electrons in Ag NPs will cause formation of oxidation radicals such as •OH on the surface of Ag and anionic radicals (%O2) on the surface of ZnO as follows [22]. + hcb + OH− •OH
(4)
− ecb + O2 → •O2
(5)
When doped C–S sample is exposed to UV light or photons with energies of equal or larger than energy band gap of ZnO, electrons in valence band are excited to conduction band and holes are formed in valence band. Since the lowest conduction band energy level in ZnO is higher than newly formed Fermi level energy, exited electrons are transferred from ZnO to Ag and consequently recombination of electron-holes in ZnO is delayed. Therefore, the degradation rate is increased in the doped CS samples. With decrease in recombination rate, the electrons in conduction band react with oxygen molecules and form oxygen anionic radicals (%O2) on the surface of ZnO. These radicals react with H2O and produce %OH radicals, which eventually degrade organic pollutions. The equations showing the photocatalytic degradation of dye molecules are as follow [23,24]:
ZnO + UV(h ϑ) → ZnO(e−cb + h+νb)
(6)
− ecb + O2 → •O2
(7)
Ag + +
+ ecb
→ Ag
+ h vb + OH− → •OH
(8) (9) 75
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