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Cerium based photocatalysts for the degradation of acridine orange in visible light
Journal of Molecular Liquids 241 (2017) 20–26
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Cerium based photocatalysts for the degradation of acridine orange in visible light Tofail Arshad a, Shahid Ali Khan b,c, M. Faisal d, Zarbad Shah a, Kalsoom Akhtar e, Abdullah M. Asiri b,c, Adel A. Ismail f, Basma G. Alhogbi c, Sher Bahadar Khan b,c,⁎ a
Department of Chemistry, Bacha Khan University, Charsadda, Khyber Pukhtunkhwa, Pakistan Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia d Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia e Chemistry Department, Ewha Womans University, Seoul, Republic of Korea f Nanostructured Materials and Nanotechnology Division, Advanced Materials Department, Central Metallurgical R & D Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt b c
a r t i c l e
i n f o
Article history: Received 19 January 2017 Received in revised form 13 April 2017 Accepted 16 May 2017 Available online 24 May 2017 Keywords: Ce-Cd and Ce-Sr oxides Acridine orange dye Visible light FESEM DRS Band gap
a b s t r a c t In the present study, Ce based catalysts were synthesized for the degradation of acridine orange (AO). The Ce-Sr and Ce-Cd oxides were synthesized through sol-gel method and applied for the degradation of acridine orange dye (AO) under visible light influence for environmental remediation. 0.03 mM AO was prepared and 1 g/L of the Ce-Cd and Ce-Sr oxides were applied for degradation of AO. The structure and morphology of the synthesized materials were examined through field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), fourier transform infrared spectroscopy (FT-IR) and the HOMO and LUMO energy differences were determined through diffuse reflectance spectroscopy (DRS). Both the catalysts were grown in sheet morphology; and the average size of each sheet in Ce-Cd oxide is approximately equal to 790 nm while its average particle size is approximately equal to 20–25 nm, but Ce-Sr oxide average sheet size and particle size were smaller than Ce-Cd oxide. The band gap 2.65 and 2.7 eV for a Ce-Sr and Ce-Cd oxides signifies the high catalytic performance under visible light. The photocatalytic performances of Ce-Cd and Ce-Sr were evaluated against AO dye which resulted 90.2 and 86.6% degradation in 3 h under visible light. These studies showed the high efficiency of Ce based catalysts for environmental remediation. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Waste matter and effluents produced by different industries, contains different dyes and pollutants which have created major problems globally. These dyes are harmful for environment and have carcinogenic effects [1,2]. These dyes make water colorful and form foam like layer on the surface of water, which blocks the availability of oxygen for aquatic flora and fauna [3–6]. According to a survey, there are about 106 available dyes which are using by different industries and about 10–20% of these dyes are lost as waste materials in water streams. The main contributing sources for producing such types of wastes and heavy metals are printing, paint, leather and textile industries [7,8]. Usage of metal oxides for removal of heavy metals and toxic pollutants have attained a valuable attention. Metal oxides such as Fe2O3 and CeO2 are used for decomposition of water and also perform the mineralization of pollutants. CeO2 is a semiconductor that can absorb light in UV and visible ⁎ Corresponding author at: Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia. E-mail address: [email protected] (S.B. Khan).
http://dx.doi.org/10.1016/j.molliq.2017.05.079 0167-7322/© 2017 Elsevier B.V. All rights reserved.
range [9]. CeO2 is used for degradation of organic pollutants but it has many other applications such as polishing material, Ultraviolet blocking material, additives in ceramics and optical materials. Thus metal oxides play a vital role in environment cleaning and maintenance [10]. Cerium is mainly used as electronic promoter, and used industrially as three way catalyst as an oxygen buffer. Cerium is largely used as heterogeneous catalyst in many industrial applications [11–14]. Pollution is a major threat for the human civilization and population. Water pollution has adverse effects on all living things and has become the main concern and issue of treatment and detoxification [15,16]. Rose Bengal is a hazardous pollutant causing scaling reddening and irritation when it comes in contact with skin. It is water soluble dye and is used in medical diagnosis such as liver function testing and in cornea conjunctivitis [17–19]. Acridine orange dye is a common hazardous pollutant showing main role in water pollution.The remedy of such hazardous organic pollutant is an essential and most urgent requirement [20]. Many attempts and methods have been used for the effective detoxification such as coagulation, sedimentation, filtration, adsorption, biodegradation, but photocatalytic degradation is most effective and valuable for the removal of
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such hazardous dyes. Photocatalytic methods are less costly than other methods and comparably good because the other methods produce byproducts which requires further treatment [21–25]. Heterogeneous photocatalysis decomposes such harmful organic pollutants to water and carbon dioxide and make them eco-friendly. In recent studies, TiO2 and ZnO have been the most broadly used photocatalysts, and effectiveness of these two catalysts have been reported extensively. TiO2 have many properties such as low cost, powerful oxidative agent, nontoxic qualities and also showing absorption of visible light to some extent. The absorption of visible light is not high but only utilizes 5% of solar energy. The previous studies reveals that a lot of organic pollutants such as hydrocarbons, surfactants, herbicides, pesticides and insecticides can be effectively photo mineralized by TiO2 [26–31]. TiO2 was first used for the splitting of H2O into oxygen and hydrogen in 1970. In photocatalytic degradation, organic pollutants are attacked by hydroxyl radical and oxide radical generated on TiO2 surface by the reduction of dissolved oxygen in solution. TiO2 is known as semiconducting metal oxide with numerous properties and with band gap of 3.2 eV. TiO2 is the most used semiconductor and produces highly oxidant hydroxyl radicals for the removal of pollutants from environment effectively [32–38]. Photocatalytic degradation of organic dyes is carried out using semiconductors as catalysts [39]. The semiconductor is irradiated with a photon of sufficient energy which promotes electrons from valence band to conduction band. The electrons leave a vacant space leading to electron hole pairs. Highly reactive hydoxyl radicals are produced as a result of oxidation/reduction reactions. Dyes are soluble in water and colorize it in traces amount. This color cannot be removed easily by simple aerobic biological treatment. Many synthetic dyes are resistant to aerobic biological treatment and thus causes high COD
21
(chemical oxygen demand) of water and create many environmental consequences [40–42]. The release of dyes in waste water leads to many chemical reactions such as hydrolysis, complex formation and oxidation. The corresponding reactions generates byproducts which are more dangerous than the primary pollutants [43]. The previous studies shows that ZnO is more efficient than TiO2 in UV light photocatalytic degradation for a number of organic pollutants [44]. The major photocatalysts used for the effective remedy of organic pollutants are TiO2, WO3, TiO3, Fe2O3, ZnO and ZnS. These photocatalysts are less expensive, nontoxic, high selective and in stable condition during reactions. These photocatalysts have also sufficient energies of their band gap for activating degradation reactions [45–48]. An inorganic nanoparticle such as CeO2 has attained a considerable attention due to photochemical properties. CeO2 Photocatalyst has shown a valuable output in the fields of environmental remediation, fuel cells, sensors, oxygen storage capacitors, solar energy conversion and photochemical processes. It has a wide band gap (3.4 eV) which allows using it as an electronic material for logic operations. CeO2 and cerium based doped material has several fascinating catalytic activity due to its low cost, plentiful existence and versatile catalytic nature. The aim of this study is to find a semiconductor or catalyst that will show good efficiency in visible light for the degradation of pollutants. For instance, Ce based photocatalysts, such as Ce-Cd and Ce-Sr oxides have shown an effective results for the degradation of pollutants. The present study shows degradation phenomenon of AO dye under visible light with Ce-Cd and Ce-Sr oxides. The corresponding catalysts degraded AO dyes under the influence of visible light irradiation in high percentage. The synthesized Ce-Cd and Ce-Sr oxides have low band gap energy. The doped Ce based photocatalysts exists in the two valence
Fig. 1. FESEM images of Ce-Cd oxides (a,b) and Ce-Sr oxides (c,d).
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states Ce3+ and Ce4+, which can act as the electron acceptor to improve the separation efficiency of the photogenerated electron-hole pairs. Moreover, the absorption of the visible light extended more apparently than that of other used photocatalysts. The photodegradation yield of Ce-Cd and Ce-Sr oxides were much higher than other available commercial photocatalysts. The doped Ce based photocatalysts were chosen because these have interesting economic and physiochemical properties with comparison to other metal oxides. 2. Experimental 2.1. Chemicals All the Chemicals such as NaOH, and Cd(NO3)2.4H2O, Sr(NO3)2, Ce(NO3)2.6H2O were purchased from Sigma-Aldrich, while acridine orange dye was purchased from BDH. 2.2. Synthesis of Ce-Cd and Ce-Sr oxides The Ce-Cd and Ce-Sr oxides were prepared through sol-gel method. The Cd(NO3)2.4H2O and Ce(NO3)2.6H2O salts were well mixed in equimolar ratio in 250 mL beaker and stirred at room temperature. After that, 0.1 M NaOH was added to the resultant solution in drop wise fashion till the pH reaches 9. The pH of the solution was carefully monitored through pH meter. After the addition of base, the reaction mixture was heated at 60 °C on hot plate for 24 h with constant stirring. On completion of the reaction, the supernatant was drained from the reaction by carefully separating the precipitate. The resultant precipitate was washed with C2H5OH:H2O mixture several times. The precipitate was
then kept at 60 °C for drying in an oven for overnight and then stored in clean tubes. The same methodology was employed for Ce-Sr oxide [5]. 2.3. Photocatalytic methodology For the photocatalytic reaction, 0.03 mM stock solution of acridine orange (AO) was prepared in a beaker. During the experiment, 1 g/L of the Ce-Cd and Ce-Sr oxides were used for the degradation of AO. In this typical experiment, 85 mg of the respective catalysts were used for the degradation of 85 mL of 0.03 M AO dye. The 250 W and 400 W visible lights lamps ware used for Ce-Cd and Ce-Sr respectively. The decrease in concentration of the AO was monitored through UV–Vis spectrophotometer by taking 4 mL of dye solution from reaction mixture at regular intervals. Initially the reaction mixtures were placed in dark for 30 min, so that loss of the dye can be taken into account due to adsorption phenomenon and then transferred the reaction mixture under visible light for degradation. The following equation was used for the adsorption and photocatalytic degradation of AO dye,
R:E:ð%Þ ¼
C−Co A−Ao 100 ¼ 100 C A
where C (mg/L) represents the initial concentration of dyes; Co (mg/L) is the final concentration at time t (min). Ao is the UV–Vis absorption of same solution at time t (min) and A is the UV–Vis absorption of the original solution [49].
Fig. 2. EDS spectrum of Ce-Cd oxides (a,b) and Ce-Sr oxides (c,d).
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Fig. 3. XRD (a) and FTIR (b) spectrum of Ce-Cd and Ce-Sr oxides.
3. Results and discussion 3.1. Physiochemical characterization The FESEM images suggested that Ce-Cd and Ce-Sr oxides were grown in the form of sheets. These sheets are formed by the aggregation of small particles. The average size of each sheet in Ce-Cd oxide is approximately equal to 790 nm while its average particle size is approximately equal to 25 nm. Similarly, Ce-Sr oxide is also in sheet morphology, but the sheet and the particle size are smaller than Ce-Cd oxide as shown in Fig. 1.
The EDS spectrum of Ce-Cd oxides showed that it is made of Ce, Cd and O elements as shown in the inset of Fig. 1a, b. The EDS analysis indicated the highest weight percentage of oxygen followed by Cd and then Ce in Ce-Cd oxides as shown in the inset of Fig. 2b. Similarly, the Sr element is in the highest weight percentage followed by O and Ce in Ce-Sr oxides catalyst as shown in the inset of Fig. 2d. The XRD spectrum of the respective catalysts is shown in Fig. 3a. The CeO2 peaks were appeared around 2θ = 28.2 (111), 33.2 (200), 47.6 (220), 56.2 (311), 58.2 (222) and 69.2 (400) showing the face centered cubic phase of CeO2 in the synthesized catalysts as shown in Fig. 3a. The remaining peaks are due to cadmium and strontium oxide while some
Fig. 4. (a,b) Diffuse reflectance spectrum of Ce-Cd oxides and (Fig.c,d) Diffuse reflectance spectrum of Ce-Sr oxides.
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Table 1 Comparison of the present work with the literature. S. No
Adsorbent
Dye type
pH/temp
Time/minutes
Yield
References
1 2 3 4 5
Ce-Ag-ZnO Fe3O4/CeO2 Mn/Ce oxides CeO2 Ce-Cd Oxides Ce-Sr Oxides
Reactive Red 120 Dye 4-Chlorophenol Phenol Methylene blue Acridine orange Acridine orange
7 3 80 °C 25 °C Ambient Ambient
20 min. 120 min. 120 min. 125 min. 180 min. 180 min.
93% 66% Complete Removal 97% 90% 86%
[51] [52] [53] [9] Present Work Present Work
peaks were originating from the precursors in the respective catalysts. The IR (ATR) spectrum of both catalysts are shown in Fig. 3b. The OH stretching vibration exhibited at 3332 cm− 1 in both catalysts while the OH bending vibration are displayed at 1628 cm−1 due to the presence of water vapors [50]. The peak at 1382 cm−1 is due to the presence anions in the synthesized catalyst, while the M-O-M and M = of NO−1 3 O absorptions were appeared at 460–863 nm as shown in 3b. The effective photo degradation of dyes by photocatalysts depends on the in-resonance effect of light and less rate of recombination of electron hole pair. The generation of free radicles requires specific band gap energy as compared to incident photons [11]. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy difference was measured through solid state diffuse reflectance spectrum (DRS). The DRS spectra indicated a measured band gap at 2.65 and 2.7 eV for Ce-Sr and Ce-Cd oxides which reflects their high efficiency under visible light influence as depicted in Fig. 4a–d. The comparison of the present work with the reported literature is shown in the table-1 (Table 1).
The change in absorbance on UV–Vis spectrum indicated the rate of degradation of AO dye. The absorption spectrum indicates well-defined peak at 495 nm. The spectrum clearly displayed the absorption intensity of AO dye decreased with contact time. The fact was further confirmed through decrease in concentration of UV–Vis light as shown in Fig. 5a and b. The decrease in concentration (C/Co) of AO and % degradation of AO under visible light irradiation with Ce-Sr oxide (Fig. 6a and b) and Ce-Cd oxide (Fig. 6b and d) were shown respectively. The Ce-Cd and Ce-Sr oxides showed 90.2 and 86.5% degradation after 180 min. The Ce-Cd oxide showed good performance than Ce-Sr oxide, however with increased irradiation time, the % degradation was also increased. For instance, the % degradation increases from 86.5 to 94.6, when irradiation time was increased from 180 to 240 min by using Ce-Sr oxide. This showed that by increasing the irradiation time, the dyes will completely degraded and thus the water purification will be increases.
4. Possible mechanism of the reaction 3.2. Photocatalytic degradation of dye The Ce-Cd and Ce-Sr oxides catalysts were evaluated for photocatalytic degradation of AO dye under visible light exposure. Initially the AO dye was placed under visible light without catalyst which showed no degradation. This showed that light has itself no role in the degradation and de-coloration of AO. In this experiment, 1 g/L of Ce-Cd and Ce-Sr oxides were used for the degradation of 0.03 mM solution of AO. Prior to visible light exposure, the reaction mixture was kept in dark for half an hour to know the loss of dye due to adsorption. After that, the reaction mixture was kept under visible light irradiation with constant stirring. During the reaction progress, 4 mL of the aliquots were separated through pipette after each 30 min and measured in UV–Vis spectrophotometers. During the first 30 min, the reaction mixture was kept in dark which showed a small decrease in the concentration of AO. This showed that the catalyst was not working well in dark condition as shown in Fig. 5a, b.
When light of appropriate frequency strikes the surface of catalysts, such as Ce-Cd and Ce-Sr oxides, it excite an electron from the valance band of catalyst to its conduction band leaving a positive hole in the valance band, thus generates an e−, h+ pair system. During resonance the excited electrons in the conduction band recombine to reduce the photolytic activity. Electrons are excited from the valance band of Ce to the conduction band of Sr and Cd thus preventing it from recombination and leaving a positive hole in the valance band as shown in Fig. 7. The doping is successful in preventing the recombination and leaving the electrons in the excited state for the degradation processes. The electron excited in the conduction band reacts with oxygen to produce superoxide free radical anions (O2•−) and the hole reacts with water molecule produces hydroxyl free radical (OH•). It might be possible that both e− and positive hole (h+) transferred to the surface defects of the catalysts, where they make OH• and O2•− [4,50]. In the photo excitation state of the catalysts, hydroxyl free radicals and superoxide free radicals
Fig. 5. (a) UV–Vis spectrum of acridine orange dye with Ce-Cd oxides (b) UV–Vis spectrum of acridine orange dye with Ce-Sr oxides showing the gradual decrease in concentration with time.
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Fig. 6. (a,b) Decrease in concentration and % degradation of acridine orange dye with Cd-Ce oxide (c,d) Decrease in concentration and % degradation of acridine orange dye with Sr-Ce oxide.
are the possible redox agent that can easily oxidize the organic pollutants (AO) to H2O and CO2. 5. Conclusion This study reported the effectiveness of Ce based photocatalysts for the degradation of AO dyes under visible light exposure. The synthesis of Ce based photocatalysts was confirmed by FESEM, XRD, EDS, FT-IR and DRS. These catalysts showed small band gaps which are 2.7 and 2.65 for Ce-Cd and Ce-Sr oxides respectively. The band gap showed the high efficiency of both catalysts in visible light. Ce-Sr and Ce-Cd oxides degraded the AO approximately to 86.6 and 90.0% respectively which showed the strongest catalytic performance of both catalysts. The proposed approach has given a novel rout for the degradation of pollutants and can be used for different visible light applications.
Fig. 7. Schematic sketch of photo degradation of acidine orange dye by taking the Ce-Cd oxide.
Conflict of interest The Authors confirm that the content of this manuscript has no conflict of interest.
Acknowledgment The authors highly acknowledge Center of Excellence for Advanced Materials Research King Abdul Aziz University Jeddah Saudi Arabia and Department of Chemistry Bacha Khan University Charsadda Khyber Pakhtunkhwa Pakistan for providing research facility and collaboration. References [1] A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, J. Colloid Interface Sci. 337 (2009) 345–354. [2] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 340 (2009) 16–26. [3] S.A. Khan, S.B. Khan, T. Kamal, M. Yasir, A.M. Asiri, Int. J. Biol. Macromol. 91 (2016) 744–751. [4] S.A. Khan, S.B. Khan, A.M. Asiri, RSC Adv. 6 (2016) 83196–83208. [5] S.A. Khan, S.B. Khan, A.M. Asiri, Sci. Rep. 6 (2016) 35107. [6] R. Saravanan, S. Karthikeyan, V.K. Gupta, G. Sekaran, V. Narayanan, A. Stephen, Mater. Sci. Eng. C 33 (2013) 91–98. [7] H. Soni, J.N. Kumar, K. Patel, R.N. Kumar, Desalin. Water Treat. 57 (2016) 6355–6364. [8] R. Saravanan, S. Karthikeyan, V.K. Gupta, E. Thirumal, P. Thangadurai, V. Narayanan, A. Stephen, Mater. Sci. Eng. C 33 (2013) 2235–2244. [9] H.R. Pouretedal, A. Kadkhodaie, Chin. J. Catal. 31 (2010) 1328–1334. [10] P. Trogadas, J. Parrondo, V. Ramani, Electrochem. Solid-State Lett. 11 (2008) B113–B116. [11] S.A. Khan, S.B. Khan, A.M. Asiri, J. Mater. Sci. Mater. Electron. 27 (2016) 5294–5302. [12] M. Mansoob Khan, S.A. Ansari, M. Omaish Ansari, B.K. Min, J. Lee, M. Hwan Cho, J. Phys. Chem. C 118 (2014) 9477–9484. [13] M. Mansoob Khan, S.A. Ansari, D. Pradhan, D. Hung Han, J. Lee, M. Hwan Cho, Ind. Eng. Chem. Res. 53 (23) (2014) 9754–9763. [14] S. Ahmed, T. Kamal, S.A. Khan, Y. Anwar, M.T. Saeed, A.M. Asiri, S.B. Khan, Curr. Nanosci. 12 (5) (2016) 569–575.
26
T. Arshad et al. / Journal of Molecular Liquids 241 (2017) 20–26
[15] S.A. Khan, S.B. Khan, T. Kamal, A.M. Asiri, K. Akhtar, Recent Pat. Nanotechnol. (2016) 181–188. [16] D. Balarak, J. Jaafari, G. Hassani, Y. Mahdavi, I. Tyagi, S. Agarwal, V.K. Gupta, Colloid Interf. Sci. Commun. 7 (2015) 16–19. [17] V.K. Gupta, S. Agarwal, T.A. Saleh, J. Hazard. Mater. 185 (2011) 17–23. [18] V.K. Gupta, A. Mittal, D. Jharec, J. Mittal, RSC Adv. 2 (2012) 8381–8389. [19] V.K. Gupta, A. Nayak, Chem. Eng. J. 180 (2012) 81–90. [20] H. Khani, M.K. Rofouei, P. Arab, V.K. Gupta, Z. Vafaei, J. Hazard. Mater. 183 (2010) 402–409. [21] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface Sci. 342 (2010) 518–527. [22] D. Swami, P. Pandit, Nat. Environ. Pollut. Technol. 12 (2013) 517. [23] M. Chauhan, R. Kumar, A. Umar, S. Chauhan, G. Kumar, M. Faisal, S. Hwang, A. AlHajry, J. Nanosci. Nanotechnol. 11 (2011) 4061–4066. [24] S. Karthikeyan, V.K. Gupta, R. Boopathy, A. Titus, G. Sekaran, J. Mol. Liq. 173 (2012) 153–163. [25] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 344 (2010) 497–507. [26] N. Mohammadi, H. Khani, V.K. Gupta, E. Amereh, S. Agarwal, J. Colloid Interface Sci. 362 (2011) 457–462. [27] A. Jamal, M.M. Rahman, M. Faisal, S.B. Khan, Mater. Sci. Appl. 2 (2011) 676. [28] M. Rauf, S.S. Ashraf, J. Hazard. Mater. 166 (2009) 6–16. [29] C.-S. Lu, C.-C. Chen, L.-K. Huang, P.-A. Tsai, H.-F. Lai, Catalysts 3 (2013) 501–516. [30] Ö. Kerkez-Kuyumcu, E. Kibar, K. Dayıoğlu, F. Gedik, A.N. Akın, Ş. Özkara-Aydınoğlu, J. Photochem. Photobiol. A Chem. 311 (2015) 176–185. [31] T.A. Saleh, V.K. Gupta, J. Colloid Interface Sci. 371 (2012) 101–106. [32] R. Saravanan, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 181 (2013) 133–141. [33] S.A.B. Asif, S.B. Khan, A.M. Asiri, Nanoscale Res. Lett. 10 (2015) 1–10. [34] V.K. Gupta, R. Jain, A. Nayak, S. Agarwal, M. Shrivastava, Mater. Sci. Eng. C 31 (2011) 1062–1067.
[35] V.K. Gupta, R. Jain, A. Mittal, T.A. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Mater. Sci. Eng. C 32 (2012) 12–17. [36] R. Saravanan, V.K. Gupta, T. Prakash, V. Narayanan, A. Stephen, J. Mol. Liq. 178 (2013) 88–93. [37] R. Saravanan, T. Prakash, V.K. Gupta, A. Stephen, J. Mol. Liq. 193 (2014) 160–165. [38] R. Saravanan, F. Gracia, Mohammad Mansoob Khan, V. Poornima, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 209 (2015) 374–380. [39] M.M. Khan, S.A. Ansari, D. Pradhan, M. Omaish Ansari, D. Hung Han, J. Lee, M. Hwan Cho, J. Mater. Chem. A 2 (2014) 637–644. [40] C. Sajan, B. Basavalingu, S. Ananda, K. Byrappa, J. Geol. Soc. India 77 (2011) 82–88. [41] J. Zhang, Z. Xiong, X. Zhao, J. Mater. Chem. 21 (2011) 3634–3640. [42] T.A. Saleh, V.K. Gupta, J. Colloid Interface Sci. 362 (2011) 337–344. [43] R. Saravanan, E. Thirumal, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 177 (2013) 394–401. [44] H. Zhang, Y. Zhang, D. Zhang, Color. Technol. 123 (2007) 101–105. [45] S. Dalhatou, C. Pétrier, S. Laminsi, S. Baup, Int. J. Environ. Sci. Technol. 12 (2015) 35–44. [46] S.B. Gajbhiye, Int. J. Mod. Eng. Res. 2 (2012) 1204–1208. [47] S. Mills, A. Christy, J.-M. Génin, T. Kameda, F. Colombo, Mineral. Mag. 76 (2012) 1289–1336. [48] S. Gago, M. Pillinger, T.M. Santos, I.S. Goncalves, Ceramics-Silikáty 48 (2004) 155–158. [49] S.B. Khan, S.A. Khan, A.M. Asiri, Appl. Surf. Sci. 370 (2016) 445–451. [50] M. Mansoob Khan, S.A. Ansari, J.H. Lee, M.O. Ansari, J. Lee, M.H. Cho, J. Colloid Interface Sci. 431 (2014) 255–263. [51] B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi, Mater. Sci. Semicond. Process. 16 (2013) 1070–1078. [52] L. Xu, J. Wang, Environ. Sci. Technol. 46 (2012) 10145–10153. [53] S. Hamoudi, F. Larachi, A. Sayari, J. Catal. 177 (1998) 247–258.
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Cerium based photocatalysts for the degradation of acridine orange in visible light Tofail Arshad a, Shahid Ali Khan b,c, M. Faisal d, Zarbad Shah a, Kalsoom Akhtar e, Abdullah M. Asiri b,c, Adel A. Ismail f, Basma G. Alhogbi c, Sher Bahadar Khan b,c,⁎ a
Department of Chemistry, Bacha Khan University, Charsadda, Khyber Pukhtunkhwa, Pakistan Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia d Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano-Research Centre, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia e Chemistry Department, Ewha Womans University, Seoul, Republic of Korea f Nanostructured Materials and Nanotechnology Division, Advanced Materials Department, Central Metallurgical R & D Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt b c
a r t i c l e
i n f o
Article history: Received 19 January 2017 Received in revised form 13 April 2017 Accepted 16 May 2017 Available online 24 May 2017 Keywords: Ce-Cd and Ce-Sr oxides Acridine orange dye Visible light FESEM DRS Band gap
a b s t r a c t In the present study, Ce based catalysts were synthesized for the degradation of acridine orange (AO). The Ce-Sr and Ce-Cd oxides were synthesized through sol-gel method and applied for the degradation of acridine orange dye (AO) under visible light influence for environmental remediation. 0.03 mM AO was prepared and 1 g/L of the Ce-Cd and Ce-Sr oxides were applied for degradation of AO. The structure and morphology of the synthesized materials were examined through field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), fourier transform infrared spectroscopy (FT-IR) and the HOMO and LUMO energy differences were determined through diffuse reflectance spectroscopy (DRS). Both the catalysts were grown in sheet morphology; and the average size of each sheet in Ce-Cd oxide is approximately equal to 790 nm while its average particle size is approximately equal to 20–25 nm, but Ce-Sr oxide average sheet size and particle size were smaller than Ce-Cd oxide. The band gap 2.65 and 2.7 eV for a Ce-Sr and Ce-Cd oxides signifies the high catalytic performance under visible light. The photocatalytic performances of Ce-Cd and Ce-Sr were evaluated against AO dye which resulted 90.2 and 86.6% degradation in 3 h under visible light. These studies showed the high efficiency of Ce based catalysts for environmental remediation. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Waste matter and effluents produced by different industries, contains different dyes and pollutants which have created major problems globally. These dyes are harmful for environment and have carcinogenic effects [1,2]. These dyes make water colorful and form foam like layer on the surface of water, which blocks the availability of oxygen for aquatic flora and fauna [3–6]. According to a survey, there are about 106 available dyes which are using by different industries and about 10–20% of these dyes are lost as waste materials in water streams. The main contributing sources for producing such types of wastes and heavy metals are printing, paint, leather and textile industries [7,8]. Usage of metal oxides for removal of heavy metals and toxic pollutants have attained a valuable attention. Metal oxides such as Fe2O3 and CeO2 are used for decomposition of water and also perform the mineralization of pollutants. CeO2 is a semiconductor that can absorb light in UV and visible ⁎ Corresponding author at: Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia. E-mail address: [email protected] (S.B. Khan).
http://dx.doi.org/10.1016/j.molliq.2017.05.079 0167-7322/© 2017 Elsevier B.V. All rights reserved.
range [9]. CeO2 is used for degradation of organic pollutants but it has many other applications such as polishing material, Ultraviolet blocking material, additives in ceramics and optical materials. Thus metal oxides play a vital role in environment cleaning and maintenance [10]. Cerium is mainly used as electronic promoter, and used industrially as three way catalyst as an oxygen buffer. Cerium is largely used as heterogeneous catalyst in many industrial applications [11–14]. Pollution is a major threat for the human civilization and population. Water pollution has adverse effects on all living things and has become the main concern and issue of treatment and detoxification [15,16]. Rose Bengal is a hazardous pollutant causing scaling reddening and irritation when it comes in contact with skin. It is water soluble dye and is used in medical diagnosis such as liver function testing and in cornea conjunctivitis [17–19]. Acridine orange dye is a common hazardous pollutant showing main role in water pollution.The remedy of such hazardous organic pollutant is an essential and most urgent requirement [20]. Many attempts and methods have been used for the effective detoxification such as coagulation, sedimentation, filtration, adsorption, biodegradation, but photocatalytic degradation is most effective and valuable for the removal of
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such hazardous dyes. Photocatalytic methods are less costly than other methods and comparably good because the other methods produce byproducts which requires further treatment [21–25]. Heterogeneous photocatalysis decomposes such harmful organic pollutants to water and carbon dioxide and make them eco-friendly. In recent studies, TiO2 and ZnO have been the most broadly used photocatalysts, and effectiveness of these two catalysts have been reported extensively. TiO2 have many properties such as low cost, powerful oxidative agent, nontoxic qualities and also showing absorption of visible light to some extent. The absorption of visible light is not high but only utilizes 5% of solar energy. The previous studies reveals that a lot of organic pollutants such as hydrocarbons, surfactants, herbicides, pesticides and insecticides can be effectively photo mineralized by TiO2 [26–31]. TiO2 was first used for the splitting of H2O into oxygen and hydrogen in 1970. In photocatalytic degradation, organic pollutants are attacked by hydroxyl radical and oxide radical generated on TiO2 surface by the reduction of dissolved oxygen in solution. TiO2 is known as semiconducting metal oxide with numerous properties and with band gap of 3.2 eV. TiO2 is the most used semiconductor and produces highly oxidant hydroxyl radicals for the removal of pollutants from environment effectively [32–38]. Photocatalytic degradation of organic dyes is carried out using semiconductors as catalysts [39]. The semiconductor is irradiated with a photon of sufficient energy which promotes electrons from valence band to conduction band. The electrons leave a vacant space leading to electron hole pairs. Highly reactive hydoxyl radicals are produced as a result of oxidation/reduction reactions. Dyes are soluble in water and colorize it in traces amount. This color cannot be removed easily by simple aerobic biological treatment. Many synthetic dyes are resistant to aerobic biological treatment and thus causes high COD
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(chemical oxygen demand) of water and create many environmental consequences [40–42]. The release of dyes in waste water leads to many chemical reactions such as hydrolysis, complex formation and oxidation. The corresponding reactions generates byproducts which are more dangerous than the primary pollutants [43]. The previous studies shows that ZnO is more efficient than TiO2 in UV light photocatalytic degradation for a number of organic pollutants [44]. The major photocatalysts used for the effective remedy of organic pollutants are TiO2, WO3, TiO3, Fe2O3, ZnO and ZnS. These photocatalysts are less expensive, nontoxic, high selective and in stable condition during reactions. These photocatalysts have also sufficient energies of their band gap for activating degradation reactions [45–48]. An inorganic nanoparticle such as CeO2 has attained a considerable attention due to photochemical properties. CeO2 Photocatalyst has shown a valuable output in the fields of environmental remediation, fuel cells, sensors, oxygen storage capacitors, solar energy conversion and photochemical processes. It has a wide band gap (3.4 eV) which allows using it as an electronic material for logic operations. CeO2 and cerium based doped material has several fascinating catalytic activity due to its low cost, plentiful existence and versatile catalytic nature. The aim of this study is to find a semiconductor or catalyst that will show good efficiency in visible light for the degradation of pollutants. For instance, Ce based photocatalysts, such as Ce-Cd and Ce-Sr oxides have shown an effective results for the degradation of pollutants. The present study shows degradation phenomenon of AO dye under visible light with Ce-Cd and Ce-Sr oxides. The corresponding catalysts degraded AO dyes under the influence of visible light irradiation in high percentage. The synthesized Ce-Cd and Ce-Sr oxides have low band gap energy. The doped Ce based photocatalysts exists in the two valence
Fig. 1. FESEM images of Ce-Cd oxides (a,b) and Ce-Sr oxides (c,d).
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states Ce3+ and Ce4+, which can act as the electron acceptor to improve the separation efficiency of the photogenerated electron-hole pairs. Moreover, the absorption of the visible light extended more apparently than that of other used photocatalysts. The photodegradation yield of Ce-Cd and Ce-Sr oxides were much higher than other available commercial photocatalysts. The doped Ce based photocatalysts were chosen because these have interesting economic and physiochemical properties with comparison to other metal oxides. 2. Experimental 2.1. Chemicals All the Chemicals such as NaOH, and Cd(NO3)2.4H2O, Sr(NO3)2, Ce(NO3)2.6H2O were purchased from Sigma-Aldrich, while acridine orange dye was purchased from BDH. 2.2. Synthesis of Ce-Cd and Ce-Sr oxides The Ce-Cd and Ce-Sr oxides were prepared through sol-gel method. The Cd(NO3)2.4H2O and Ce(NO3)2.6H2O salts were well mixed in equimolar ratio in 250 mL beaker and stirred at room temperature. After that, 0.1 M NaOH was added to the resultant solution in drop wise fashion till the pH reaches 9. The pH of the solution was carefully monitored through pH meter. After the addition of base, the reaction mixture was heated at 60 °C on hot plate for 24 h with constant stirring. On completion of the reaction, the supernatant was drained from the reaction by carefully separating the precipitate. The resultant precipitate was washed with C2H5OH:H2O mixture several times. The precipitate was
then kept at 60 °C for drying in an oven for overnight and then stored in clean tubes. The same methodology was employed for Ce-Sr oxide [5]. 2.3. Photocatalytic methodology For the photocatalytic reaction, 0.03 mM stock solution of acridine orange (AO) was prepared in a beaker. During the experiment, 1 g/L of the Ce-Cd and Ce-Sr oxides were used for the degradation of AO. In this typical experiment, 85 mg of the respective catalysts were used for the degradation of 85 mL of 0.03 M AO dye. The 250 W and 400 W visible lights lamps ware used for Ce-Cd and Ce-Sr respectively. The decrease in concentration of the AO was monitored through UV–Vis spectrophotometer by taking 4 mL of dye solution from reaction mixture at regular intervals. Initially the reaction mixtures were placed in dark for 30 min, so that loss of the dye can be taken into account due to adsorption phenomenon and then transferred the reaction mixture under visible light for degradation. The following equation was used for the adsorption and photocatalytic degradation of AO dye,
R:E:ð%Þ ¼
C−Co A−Ao 100 ¼ 100 C A
where C (mg/L) represents the initial concentration of dyes; Co (mg/L) is the final concentration at time t (min). Ao is the UV–Vis absorption of same solution at time t (min) and A is the UV–Vis absorption of the original solution [49].
Fig. 2. EDS spectrum of Ce-Cd oxides (a,b) and Ce-Sr oxides (c,d).
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Fig. 3. XRD (a) and FTIR (b) spectrum of Ce-Cd and Ce-Sr oxides.
3. Results and discussion 3.1. Physiochemical characterization The FESEM images suggested that Ce-Cd and Ce-Sr oxides were grown in the form of sheets. These sheets are formed by the aggregation of small particles. The average size of each sheet in Ce-Cd oxide is approximately equal to 790 nm while its average particle size is approximately equal to 25 nm. Similarly, Ce-Sr oxide is also in sheet morphology, but the sheet and the particle size are smaller than Ce-Cd oxide as shown in Fig. 1.
The EDS spectrum of Ce-Cd oxides showed that it is made of Ce, Cd and O elements as shown in the inset of Fig. 1a, b. The EDS analysis indicated the highest weight percentage of oxygen followed by Cd and then Ce in Ce-Cd oxides as shown in the inset of Fig. 2b. Similarly, the Sr element is in the highest weight percentage followed by O and Ce in Ce-Sr oxides catalyst as shown in the inset of Fig. 2d. The XRD spectrum of the respective catalysts is shown in Fig. 3a. The CeO2 peaks were appeared around 2θ = 28.2 (111), 33.2 (200), 47.6 (220), 56.2 (311), 58.2 (222) and 69.2 (400) showing the face centered cubic phase of CeO2 in the synthesized catalysts as shown in Fig. 3a. The remaining peaks are due to cadmium and strontium oxide while some
Fig. 4. (a,b) Diffuse reflectance spectrum of Ce-Cd oxides and (Fig.c,d) Diffuse reflectance spectrum of Ce-Sr oxides.
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Table 1 Comparison of the present work with the literature. S. No
Adsorbent
Dye type
pH/temp
Time/minutes
Yield
References
1 2 3 4 5
Ce-Ag-ZnO Fe3O4/CeO2 Mn/Ce oxides CeO2 Ce-Cd Oxides Ce-Sr Oxides
Reactive Red 120 Dye 4-Chlorophenol Phenol Methylene blue Acridine orange Acridine orange
7 3 80 °C 25 °C Ambient Ambient
20 min. 120 min. 120 min. 125 min. 180 min. 180 min.
93% 66% Complete Removal 97% 90% 86%
[51] [52] [53] [9] Present Work Present Work
peaks were originating from the precursors in the respective catalysts. The IR (ATR) spectrum of both catalysts are shown in Fig. 3b. The OH stretching vibration exhibited at 3332 cm− 1 in both catalysts while the OH bending vibration are displayed at 1628 cm−1 due to the presence of water vapors [50]. The peak at 1382 cm−1 is due to the presence anions in the synthesized catalyst, while the M-O-M and M = of NO−1 3 O absorptions were appeared at 460–863 nm as shown in 3b. The effective photo degradation of dyes by photocatalysts depends on the in-resonance effect of light and less rate of recombination of electron hole pair. The generation of free radicles requires specific band gap energy as compared to incident photons [11]. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy difference was measured through solid state diffuse reflectance spectrum (DRS). The DRS spectra indicated a measured band gap at 2.65 and 2.7 eV for Ce-Sr and Ce-Cd oxides which reflects their high efficiency under visible light influence as depicted in Fig. 4a–d. The comparison of the present work with the reported literature is shown in the table-1 (Table 1).
The change in absorbance on UV–Vis spectrum indicated the rate of degradation of AO dye. The absorption spectrum indicates well-defined peak at 495 nm. The spectrum clearly displayed the absorption intensity of AO dye decreased with contact time. The fact was further confirmed through decrease in concentration of UV–Vis light as shown in Fig. 5a and b. The decrease in concentration (C/Co) of AO and % degradation of AO under visible light irradiation with Ce-Sr oxide (Fig. 6a and b) and Ce-Cd oxide (Fig. 6b and d) were shown respectively. The Ce-Cd and Ce-Sr oxides showed 90.2 and 86.5% degradation after 180 min. The Ce-Cd oxide showed good performance than Ce-Sr oxide, however with increased irradiation time, the % degradation was also increased. For instance, the % degradation increases from 86.5 to 94.6, when irradiation time was increased from 180 to 240 min by using Ce-Sr oxide. This showed that by increasing the irradiation time, the dyes will completely degraded and thus the water purification will be increases.
4. Possible mechanism of the reaction 3.2. Photocatalytic degradation of dye The Ce-Cd and Ce-Sr oxides catalysts were evaluated for photocatalytic degradation of AO dye under visible light exposure. Initially the AO dye was placed under visible light without catalyst which showed no degradation. This showed that light has itself no role in the degradation and de-coloration of AO. In this experiment, 1 g/L of Ce-Cd and Ce-Sr oxides were used for the degradation of 0.03 mM solution of AO. Prior to visible light exposure, the reaction mixture was kept in dark for half an hour to know the loss of dye due to adsorption. After that, the reaction mixture was kept under visible light irradiation with constant stirring. During the reaction progress, 4 mL of the aliquots were separated through pipette after each 30 min and measured in UV–Vis spectrophotometers. During the first 30 min, the reaction mixture was kept in dark which showed a small decrease in the concentration of AO. This showed that the catalyst was not working well in dark condition as shown in Fig. 5a, b.
When light of appropriate frequency strikes the surface of catalysts, such as Ce-Cd and Ce-Sr oxides, it excite an electron from the valance band of catalyst to its conduction band leaving a positive hole in the valance band, thus generates an e−, h+ pair system. During resonance the excited electrons in the conduction band recombine to reduce the photolytic activity. Electrons are excited from the valance band of Ce to the conduction band of Sr and Cd thus preventing it from recombination and leaving a positive hole in the valance band as shown in Fig. 7. The doping is successful in preventing the recombination and leaving the electrons in the excited state for the degradation processes. The electron excited in the conduction band reacts with oxygen to produce superoxide free radical anions (O2•−) and the hole reacts with water molecule produces hydroxyl free radical (OH•). It might be possible that both e− and positive hole (h+) transferred to the surface defects of the catalysts, where they make OH• and O2•− [4,50]. In the photo excitation state of the catalysts, hydroxyl free radicals and superoxide free radicals
Fig. 5. (a) UV–Vis spectrum of acridine orange dye with Ce-Cd oxides (b) UV–Vis spectrum of acridine orange dye with Ce-Sr oxides showing the gradual decrease in concentration with time.
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Fig. 6. (a,b) Decrease in concentration and % degradation of acridine orange dye with Cd-Ce oxide (c,d) Decrease in concentration and % degradation of acridine orange dye with Sr-Ce oxide.
are the possible redox agent that can easily oxidize the organic pollutants (AO) to H2O and CO2. 5. Conclusion This study reported the effectiveness of Ce based photocatalysts for the degradation of AO dyes under visible light exposure. The synthesis of Ce based photocatalysts was confirmed by FESEM, XRD, EDS, FT-IR and DRS. These catalysts showed small band gaps which are 2.7 and 2.65 for Ce-Cd and Ce-Sr oxides respectively. The band gap showed the high efficiency of both catalysts in visible light. Ce-Sr and Ce-Cd oxides degraded the AO approximately to 86.6 and 90.0% respectively which showed the strongest catalytic performance of both catalysts. The proposed approach has given a novel rout for the degradation of pollutants and can be used for different visible light applications.
Fig. 7. Schematic sketch of photo degradation of acidine orange dye by taking the Ce-Cd oxide.
Conflict of interest The Authors confirm that the content of this manuscript has no conflict of interest.
Acknowledgment The authors highly acknowledge Center of Excellence for Advanced Materials Research King Abdul Aziz University Jeddah Saudi Arabia and Department of Chemistry Bacha Khan University Charsadda Khyber Pakhtunkhwa Pakistan for providing research facility and collaboration. References [1] A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, J. Colloid Interface Sci. 337 (2009) 345–354. [2] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 340 (2009) 16–26. [3] S.A. Khan, S.B. Khan, T. Kamal, M. Yasir, A.M. Asiri, Int. J. Biol. Macromol. 91 (2016) 744–751. [4] S.A. Khan, S.B. Khan, A.M. Asiri, RSC Adv. 6 (2016) 83196–83208. [5] S.A. Khan, S.B. Khan, A.M. Asiri, Sci. Rep. 6 (2016) 35107. [6] R. Saravanan, S. Karthikeyan, V.K. Gupta, G. Sekaran, V. Narayanan, A. Stephen, Mater. Sci. Eng. C 33 (2013) 91–98. [7] H. Soni, J.N. Kumar, K. Patel, R.N. Kumar, Desalin. Water Treat. 57 (2016) 6355–6364. [8] R. Saravanan, S. Karthikeyan, V.K. Gupta, E. Thirumal, P. Thangadurai, V. Narayanan, A. Stephen, Mater. Sci. Eng. C 33 (2013) 2235–2244. [9] H.R. Pouretedal, A. Kadkhodaie, Chin. J. Catal. 31 (2010) 1328–1334. [10] P. Trogadas, J. Parrondo, V. Ramani, Electrochem. Solid-State Lett. 11 (2008) B113–B116. [11] S.A. Khan, S.B. Khan, A.M. Asiri, J. Mater. Sci. Mater. Electron. 27 (2016) 5294–5302. [12] M. Mansoob Khan, S.A. Ansari, M. Omaish Ansari, B.K. Min, J. Lee, M. Hwan Cho, J. Phys. Chem. C 118 (2014) 9477–9484. [13] M. Mansoob Khan, S.A. Ansari, D. Pradhan, D. Hung Han, J. Lee, M. Hwan Cho, Ind. Eng. Chem. Res. 53 (23) (2014) 9754–9763. [14] S. Ahmed, T. Kamal, S.A. Khan, Y. Anwar, M.T. Saeed, A.M. Asiri, S.B. Khan, Curr. Nanosci. 12 (5) (2016) 569–575.
26
T. Arshad et al. / Journal of Molecular Liquids 241 (2017) 20–26
[15] S.A. Khan, S.B. Khan, T. Kamal, A.M. Asiri, K. Akhtar, Recent Pat. Nanotechnol. (2016) 181–188. [16] D. Balarak, J. Jaafari, G. Hassani, Y. Mahdavi, I. Tyagi, S. Agarwal, V.K. Gupta, Colloid Interf. Sci. Commun. 7 (2015) 16–19. [17] V.K. Gupta, S. Agarwal, T.A. Saleh, J. Hazard. Mater. 185 (2011) 17–23. [18] V.K. Gupta, A. Mittal, D. Jharec, J. Mittal, RSC Adv. 2 (2012) 8381–8389. [19] V.K. Gupta, A. Nayak, Chem. Eng. J. 180 (2012) 81–90. [20] H. Khani, M.K. Rofouei, P. Arab, V.K. Gupta, Z. Vafaei, J. Hazard. Mater. 183 (2010) 402–409. [21] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface Sci. 342 (2010) 518–527. [22] D. Swami, P. Pandit, Nat. Environ. Pollut. Technol. 12 (2013) 517. [23] M. Chauhan, R. Kumar, A. Umar, S. Chauhan, G. Kumar, M. Faisal, S. Hwang, A. AlHajry, J. Nanosci. Nanotechnol. 11 (2011) 4061–4066. [24] S. Karthikeyan, V.K. Gupta, R. Boopathy, A. Titus, G. Sekaran, J. Mol. Liq. 173 (2012) 153–163. [25] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, J. Colloid Interface Sci. 344 (2010) 497–507. [26] N. Mohammadi, H. Khani, V.K. Gupta, E. Amereh, S. Agarwal, J. Colloid Interface Sci. 362 (2011) 457–462. [27] A. Jamal, M.M. Rahman, M. Faisal, S.B. Khan, Mater. Sci. Appl. 2 (2011) 676. [28] M. Rauf, S.S. Ashraf, J. Hazard. Mater. 166 (2009) 6–16. [29] C.-S. Lu, C.-C. Chen, L.-K. Huang, P.-A. Tsai, H.-F. Lai, Catalysts 3 (2013) 501–516. [30] Ö. Kerkez-Kuyumcu, E. Kibar, K. Dayıoğlu, F. Gedik, A.N. Akın, Ş. Özkara-Aydınoğlu, J. Photochem. Photobiol. A Chem. 311 (2015) 176–185. [31] T.A. Saleh, V.K. Gupta, J. Colloid Interface Sci. 371 (2012) 101–106. [32] R. Saravanan, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 181 (2013) 133–141. [33] S.A.B. Asif, S.B. Khan, A.M. Asiri, Nanoscale Res. Lett. 10 (2015) 1–10. [34] V.K. Gupta, R. Jain, A. Nayak, S. Agarwal, M. Shrivastava, Mater. Sci. Eng. C 31 (2011) 1062–1067.
[35] V.K. Gupta, R. Jain, A. Mittal, T.A. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Mater. Sci. Eng. C 32 (2012) 12–17. [36] R. Saravanan, V.K. Gupta, T. Prakash, V. Narayanan, A. Stephen, J. Mol. Liq. 178 (2013) 88–93. [37] R. Saravanan, T. Prakash, V.K. Gupta, A. Stephen, J. Mol. Liq. 193 (2014) 160–165. [38] R. Saravanan, F. Gracia, Mohammad Mansoob Khan, V. Poornima, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 209 (2015) 374–380. [39] M.M. Khan, S.A. Ansari, D. Pradhan, M. Omaish Ansari, D. Hung Han, J. Lee, M. Hwan Cho, J. Mater. Chem. A 2 (2014) 637–644. [40] C. Sajan, B. Basavalingu, S. Ananda, K. Byrappa, J. Geol. Soc. India 77 (2011) 82–88. [41] J. Zhang, Z. Xiong, X. Zhao, J. Mater. Chem. 21 (2011) 3634–3640. [42] T.A. Saleh, V.K. Gupta, J. Colloid Interface Sci. 362 (2011) 337–344. [43] R. Saravanan, E. Thirumal, V.K. Gupta, V. Narayanan, A. Stephen, J. Mol. Liq. 177 (2013) 394–401. [44] H. Zhang, Y. Zhang, D. Zhang, Color. Technol. 123 (2007) 101–105. [45] S. Dalhatou, C. Pétrier, S. Laminsi, S. Baup, Int. J. Environ. Sci. Technol. 12 (2015) 35–44. [46] S.B. Gajbhiye, Int. J. Mod. Eng. Res. 2 (2012) 1204–1208. [47] S. Mills, A. Christy, J.-M. Génin, T. Kameda, F. Colombo, Mineral. Mag. 76 (2012) 1289–1336. [48] S. Gago, M. Pillinger, T.M. Santos, I.S. Goncalves, Ceramics-Silikáty 48 (2004) 155–158. [49] S.B. Khan, S.A. Khan, A.M. Asiri, Appl. Surf. Sci. 370 (2016) 445–451. [50] M. Mansoob Khan, S.A. Ansari, J.H. Lee, M.O. Ansari, J. Lee, M.H. Cho, J. Colloid Interface Sci. 431 (2014) 255–263. [51] B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi, Mater. Sci. Semicond. Process. 16 (2013) 1070–1078. [52] L. Xu, J. Wang, Environ. Sci. Technol. 46 (2012) 10145–10153. [53] S. Hamoudi, F. Larachi, A. Sayari, J. Catal. 177 (1998) 247–258.