Preparation of Flower-Like ZnO Architecture for Photodegradation of Caffeine in Aqueous Solution - Le Thi Mai

Journal of Science & Technology 126 (2018) 016-020 16 Preparation of Flower-like ZnO Architecture for Photodegradation of Caffeine in Aqueous Solution Le Thi Mai, Nguyen Thi Men, Vu Anh Tuan* Hanoi University of Science and Technology – No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam Received: August 10, 2017; Accepted: May 25, 2018 Abstract The flower-like ZnO architecture was prepared by hydrothermal method. As-synthesized samples were characterized by XRD, SEM, and N2 adsorption/desorption isotherm. The composition of samples before and after calcination was analyzed by XRD, the precursor was completely transformed to ZnO at 400 °C. The catalytic performance of ZnO was evaluated by the degradation of caffeine in aqueous solution under different irradiation of lights. The degradation efficiency was 97.6% under UV light in 120 min and it was 100% under solar light in 60 min. The reaction kinetic of photodegradation of caffeine was studied by first- order kinetic model and the reusability of the ZnO sample was investigated through five cyclic tests. Keywords: ZnO, Caffeine, Photocatalyst, Flower-like structure, First-order kinetic 1.Introduction1 Pharmaceutical and personal care products (PPCPs) are increasingly important to our lives. However, they are classified as the most serious pollutants when discharged into the environment. Their presence in surface water pollutes the water environment and adversely effects on human health. Therefore, the treatment of PPCPs before emitting into environment has attracted the attention of many scientific researchers in the Global [1,2]. Caffeine, C8H10N4O2, an alkaloid belonging to the methylzanthine family [3], is an example of PPCPs that often appears in surface water. Caffeine present in beverages such as coffees, teas, soft drinks, chocolate, some drugs, which makes it a widely consumed substance in the world. The World Bank reported that with its potential, Vietnam’s market may consume 70,000 tons of coffees per year [4]. The presence of caffeine in surface water is due to sewage spills, leaky sewer pipes, poorly maintained septic systems, and other means of sanitary sewer flows. In addition, it may be due to attributable to storm water runoff containing wastewater influences, food waste or beverage containers from trash receptacles, recycled water over-irrigation, human waste at homeless encampments, or other anthropogenic activities [5]. Caffeine alone does not seem to be toxic to domestic organisms. However, the presence of too much of it in surface water, along with other organic pollutants such as pesticides, pharmaceuticals and other *Corresponding author: Tel.: (+84) 912.911.902 Email: tuan.vuanh@hust.edu.vn chemicals, has serious implications for organisms and humans [5]. Therefore, removal of caffeine from effluents is very necessary. Photocatalysis is an environmentally friendly method for water and wastewater treatment. The main advantage of advance oxidation process by photocatalyst is the mineralization of organic pollutants into CO2, H2O and simple inorganic acids [3,6]. There are many types of semiconductor oxides such as TiO2, ZnO, Bi2O3, and Nb2O5 were used as photocatalysts for the decomposition of organic pollutants. TiO2 has been widely used because of the large band gap (>3.2 eV). However, ZnO presented a relative lower cost, easier to synthesize, greater thermodynamic stability than TiO2. In addition, ZnO exhibited the great interest because of its excellent performance in photocatalyst, solar cell, paint manufacture and food additives [7]. In relation to structure, ZnO could be prepared with different morphologies and it strongly affected to the catalytic ability [8]. The architecture structure with the large surface area and high pore volume will give advantage for the adsorption, diffusion, and reaction of organic compounds on the surface of ZnO. Therefore, the objective of this research was to preparation of flower-like ZnO architecture for photodegradation of caffeine in water. 2. Experimental 2.1. Materials Zinc nitrate hexahydrate (Zn(NO3)2.6H2O, 99.5%) and urea (NH2)2CO, 99.5%) were purchased from China, caffeine was obtained from Sigma- Aldrich (99.0%). All the chemicals were used without any purification and distilled water was used Journal of Science & Technology 126 (2018) 016-020 17 throughout the experiments. 2.2. Synthesis of flower-like ZnO The hierarchical ZnO was fabricated by a simple hydrothermal process. In a first stage, 30 mL of zinc nitrate hexahydrate 0.5M, 0.03 mole urea, and 70 mL of distilled water were put in a bearker 250 mL. The mixed solution was stirred at 240 prm for 30 min. Then, the mixed solution was transferred into teflon- lined autoclave and heated at 90 °C for 24 h. Subsequently, the autoclave was cooled to room temperature, the precipitation was detached by vacuum pumping filtration, washed with distilled water for 4-5 times and dried at 90 °C for 24 h. Finally, the flower-like hierarchical ZnO were obtained by calcining the precipitate at 400 °C for 2 h with a heating rate of 2 °C/min. 2.3. Characterization The crystalline phase of samples was investigated by X-ray power diffraction. XRD patterns were obtained by using Bruker D8 Ax XRD- diffractometer (Germany) with Cu Kα irradiation (40kV, 40 mA). The 2θ ranging from 10° to 80° was selected to analyse the crystal structure. The morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, JEOL-7600F). The textural properties were measured via N2 adsorption/desorption isotherms using a Micromeritics (Gemini VII analyzer). The specific surface area, pore volume and pore diameter were obtained by using the Brunauer-Emmett-Teller (BET) method. Fig. 1. (a-c) SEM images of hierarchical flower-like ZnO with different scale bars and (d) the digital photo of the Persian rose. 3. Results and discussion 3.1. SEM analysis The SEM images of as-synthesized ZnO are depicted in Fig. 1. ZnO was observed with uniformly hierarchical flower and 1 μm in size, as seen in Fig. 1(a). The micro flower-like ZnO was composed of many nano sheets formed by many zinc oxide nanoparticles, as seen in Fig.s 1(b) and (c). In addition, as-synthesized sample showed a beautiful rose-like ZnO, as seen in Fig. 1(d). Thus, exception of the ZnO particles are shaped like nanorods, nanoneedles, nanowires, nanobelts, nanosheet, etc., the flower-like shape is expected as a great morphology for photocatalytic application process. 3.2. XRD analysis The crystal phase of the samples was characterized by XRD analysis and the result is shown in Fig. 2. The diffraction peaks of precursor before calcination were in good agreement with with zinc hydroxide carbonate, Zn4(CO3)(OH)6.H2O (JCPDS:11-0287). While, the diffraction peaks of precursor after calcination were attributed to zinc oxide, ZnO (JCPDS:19-1458) and no other characteristic peaks for impurities were detected, indicating precursor had completely transformed into the pure ZnO crystal at 400 °C in 2 h [9,10]. The ZnO formation mechanism was explained following: The mixing process was performed at room temperature. When urea was added to the Zn2⁺ solution, the urea molecules hydrolysis in aqueous slolution to form amonniac and carbon dioxit, and then form OH⁻ and CO32⁻ anions: (NH2)2CO+3H2O→2NH3.H2O+CO2 (1) NH3.H2O→NH4⁺+OH⁻ (2) CO2 + OH⁻ → CO32⁻ + H2O (3) Then the OH⁻ and CO32⁻ was continue to react with Zn2⁺ ion to generate zinc carbonate hydroxide 4Zn2⁺+CO32⁻+6OH⁻+H2O →Zn4(CO3)(OH)6.H2O (4) The precipitate was filted, washed with distilled water several times, dried at 90 °C over night, and then calcined at 400 °C for 2 h with ramping rate 2 °C/min to obtain ZnO powder: Zn4(CO3)(OH)6.H2O→4ZnO+CO2+4H2O (5) 3.3. N2 adsorption/desorption isotherm analysis The N2 adsorption/desorption isotherm was conducted to investigate the porosity of the material, including its specific surface area and pore sizes. The isotherm and pore size distribution curves flower-like ZnO sample are presented in Fig. 3. The isotherm curve was identified as type IV. When the relative pressure (P/Po) increased from 0.91 to 0.98, a sharp hysteresis loop was observed, indicating the presence of mesoporous material. In addition, when the Journal of Science & Technology 126 (2018) 016-020 18 relative pressure was higher than 0.98, an abrupt increase in the amount of adsorbed nitrogen was observed. The pore size distribution was relatively wide and most of pores in range from 15 to 100 nm. As a result, ability to diffuse and efficiently transport hydroxyl radicals in photochemical reactions enhances the catalytic activity of the ZnO material [9,11]. The surface area and average pore size of as- prepared ZnO were 24.4 m2/g and 0.280 cm3/g, respectively. Fig. 2. XRD patterns of the precursor before calcination and as-synthesized ZnO Fig. 3. N2 adsorption/desorption isotherm (inset: pore size distribution) of as-synthesized ZnO. 3.4. Photocatalytic degradation mechanism of caffeine on ZnO The degradation of caffeine was conducted with 0.3 g ZnO, 100 mL caffeine solution with a concentration of 5 mg/L under subdued light, tungsten lamp (100 W), UV light (15 W) and solar light (at 11 h-13 h in summer). The results are shown in Fig. 4(a). It was clearly seen that the degradation efficiency of caffeine was strongly affected by light irradiation. Under subdued light, the degradation efficiency in 60 min was negligible. Under tungsten lamp, the degradation of caffeine was 18.8% in 120 min, whereas the degradation efficiencies of caffeine significantly increased under UV light and solar light, these values were 97.6 and 100%, respectively. Particularly, the caffeine was completely degraded in 60 min, it could be attributed to simultaneous existent of visible and UV in solar light, this was agreed with previous report [12]. Fig. 4. (a) Effect of irradiation light on degradation of caffeine and (b) first-order curves. The photodegradation rate of caffeine by flower- like ZnO can be evaluated by using the pseudo-first- order model called Langmuir–Hinshelwood model as follow: ln C0 Ct = kt (6) Where C0 and Ct are the concentration of caffeine at initial (t =0) and time t (min), respectively. k is the pseudo first-order rate constant. The k value was calculated from the slope of the ln (C0/Ct) − t plots. It is obviously seen that in Fig. 4(b), the degradation rate of caffeine under subdued light was low with a small rate constant 0.001 min-1), the rate constant slightly increased to 0.003 min-1 with irradiation of tungsten lamp. Howerer, it was strongly increased to 0.017 to 0.041 min-1 for UV light and solar light, respectively. The directed comparison as-prepared ZnO with Journal of Science & Technology 126 (2018) 016-020 19 other samples in literature is a challenge since ZnO have been prepared by different methods for many applications such as cosmetic, paint, sensor, adsorption, and photocatalyst. However, it was clear showed that as-prepared ZnO had the hierarchical structure with the high reaction performance, the degradation efficiency was higher than that of other ZnO samples that had been reported, as seen in Fig. 1 and Table 1. Table 1. Comparison as-prepared ZnO with other samples. ZnO samples Particles Size Applicati -on Performance Ref. Flower 1 µm - - [13] Flower 1-2 µm Bromop- enol dye removal 96% within 120 min, concentration of 10 ppm [14] Rod 80-100 nm RhB removal 97 % within 120 min, concentration of 10 ppm [15] Nano- spheres 15-60 nm RR141 removal 78% within 240 min, concentration of 10 mg/L [16] Nano disks ~ 200 nm RhB removal 90 % within 90 min, concentration of 10 ppm. [17] Flower 1 µm Caffeine 100 %, within 60 min, concentration of 5 mg/L This study Fig. 5. UV-Vis spectra of caffeine aqueous solution as function of reaction time with ZnO as catalyst. Fig. 6. Mechanism of caffeine photodegradation using flower-like ZnO under UV light and solar light. Fig. 5 shows the absorption spectrum of the caffeine solution at the concentration of 5 mg/L with 0.3 g of ZnO under UV irradiation at different reaction times. The maximum absorption peak of the caffeine at 274 nm diminished gradually and even disappeared at 120 min. This result suggests that the caffeine molecules and intermediates are degraded almost completely by oxidation, hydroxylation and mineralization processes. It is therefore beneficial for water treatment to help protect the environment. The mechanism for photocatalyst activity of flower-like was proposed, as shown in Fig. 6. Upon the UV or solar light irradiation, the electrons in the valence band of ZnO can be excited to the conduct band, leaving holes in the valence band. The electrons can active molecular oxygen to form superoxide ion (O2-) and the photogenerated holes react with either water (H2O) or hydroxyl ions (OH⁻). The formation of •OH, •O2⁻ radicals water have created hydroxylation, oxidation and mineralization processes, as described by equations 7-12 [1,18-20]. Which are able to degrade caffeine to intermediated products, CO2 and H2O. Water is dissociated into ions H2O → H⁺ + OH⁻ (7) ZnO can absorb UV light (or solar light) and generate electron-hole pairs (Fig. 6) ZnO + hν → e⁻ + h⁺ (8) The electrons move to the surface of the catalyst and adsorbed O2 on the surface to form •O2⁻ e⁻ + O2 → •O2⁻ (9) The •O2⁻ can react with surface adsorbed H2O to form H2O2: •O2⁻ + H2O → H2O2 (10) Photoconversion of H2O2 gives OH radicals: H2O2 + hν → 2•OH (11) The holes react with OH⁻ ions in the water form OH radicals: h⁺ + OH⁻ → •OH (12) Fig. 7. Cycling stability of ZnO catalyst 3.5. Reusability of the catalyst In the practical application, the reusability of the catalyst is also the critical parameter. In this study, as-synthesized ZnO was reused for four times at the Journal of Science & Technology 126 (2018) 016-020 20 same condition, caffeine concentration of 5 mg/L and ZnO 0.3 g under UV light. Fig. 7 shows the degradation efficiency caffeine of ZnO at each cycle. The performance of catalyst only presented a little drop. At the first run, the degradation efficiency was 97.6%, and this value was remained over 80.0% after four cycles. This revealed that flower-like ZnO architecture had a good cycling stability and potential application in industry. 4. Conclusion In summary, flower-like ZnO architecture was successfully prepared by hydrothermal method. As- synthesized ZnO was mesoporous structure and composed from many nanosheets, it showed the large surface area (24.4 m2/g) and high pore volume (0.280 cm3/g). The degradation of caffeine on ZnO was fast and archived 97.6% under UV light at 120 min, particularly it was higher (100%) under solar light at 60 min. In addition, the flower-like ZnO exhibited a good cycling stability. Therefore, as-prepared sample was expected that could be potential material for cleaning industrial wastewater. Acknowledgments This work was supported by Vietnamese Ministry of Education and Training by the Grant number of B2017-BKA-53. References [1] R. Bedre Jagannatha, S. Rani Ramu, M. Padaki, R.G. Balakrishna, An efficient method for the synthesis of photo catalytically active ZnO nanoparticles by a gel- combustion method for the photo-degradation of Caffeine, Nanochemistry Research 2 (2017) 86-95. [2] A. Elhalil, R. Elmoubarki, A. Machrouhi, M. Sadiq, M. Abdennouri, S. Qourzal, N. Barka, Photocatalytic degradation of caffeine by ZnO-ZnAl2O4 nanoparticles derived from LDH structure, Journal of Environmental Chemical Engineering 5 (2017) 3719-3726. [3] B. Czech, M. Hojamberdiev, UVA- and visible-light- driven photocatalytic activity of three-layer perovskite Dion-Jacobson phase CsBa2M3O10 (M=Ta, Nb) and oxynitride crystals in the removal of caffeine from model wastewater, Journal of Photochemistry and Photobiology A: Chemistry 324 (2016) 70-80. [4] V.t.p.a.e.p. centre, Report on coffee sector in Vietnam, 2007. [5] L. Busse, Detection of Caffeine in the Streams and Rivers within the San Diego Region, Environmental Scientist Healthy Waters Branch, 2375 Northside Drive, Suite 100, San Diego, California 92108, 2015. [6] W. Fang, M. Xing, J. Zhang, Modifications on reduced titanium dioxide photocatalysts: A review, Journal of Photochemistry and Photobiology C: Photochemistry Reviews 32 (2017) 21-39. [7] L.N.B. Almeida, G.G. Lenzi, J.M.T.A. Pietrobelli, O.A.A.d. Santos, Performance Evaluation of Catalysts of ZnO in Photocatalytic Degradation of Caffeine Solution, CHEMICAL ENGINEERING TRANSACTIONS 57 (2017) 6. [8] C.B. Ong, L.Y. Ng, A.W. Mohammad, A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications, Renewable and Sustainable Energy Reviews 81 (2018) 536-551. [9] H. Zhou, H. Zhang, Y. Wang, Y. Miao, L. Gu, Z. Jiao, Self-assembly and template-free synthesis of ZnO hierarchical nanostructures and their photocatalytic properties, Journal of Colloid and Interface Science 448 (2015) 367-373. [10] J.N. Hasnidawani, H.N. Azlina, H. Norita, N.N. Bonnia, S. Ratim, E.S. Ali, Synthesis of ZnO Nanostructures Using Sol-Gel Method, Procedia Chemistry 19 (2016) 211-216. [11] B. Li, Y. Wang, Facile Synthesis and Enhanced Photocatalytic Performance of Flower-like ZnO Hierarchical Microstructures, The Journal of Physical Chemistry C 114 (2010) 890-896. [12] G.G.L. Lariana N. B. Almeida, Juliana M. T. A. Pietrobelli, Onelia A. A. dos Santos, Performance Evaluation of Catalysts of ZnO in Photocatalytic Degradation of Caffeine Solution, Engineering Transactions 57 (2017). [13] P. Ramasamy, J. Kim, Facile and fast synthesis of flower-like ZnO nanostructures, Materials Letters 93 (2013) 52-55. [14] S. Ameen, M. Shaheer Akhtar, H. Shik Shin, Speedy photocatalytic degradation of bromophenol dye over ZnO nanoflowers, Materials Letters 209 (2017) 150- 154. [15] M.A. Alvi, A.A. Al-Ghamdi, M. ShaheerAkhtar, Synthesis of ZnO nanostructures via low temperature solution process for photocatalytic degradation of rhodamine B dye, Materials Letters 204 (2017) 12-15. [16] S. Kakarndee, S. Nanan, SDS capped and PVA capped ZnO nanostructures with high photocatalytic performance toward photodegradation of reactive red (RR141) azo dye, Journal of Environmental Chemical Engineering 6 (2018) 74-94. [17] H.-K. Seo, H.-S. Shin, Study on photocatalytic activity of ZnO nanodisks for the degradation of Rhodamine B dye, Materials Letters 159 (2015) 265-268. [18] L. Saikia, D. Bhuyan, M. Saikia, B. Malakar, D.K. Dutta, P. Sengupta, Photocatalytic performance of ZnO nanomaterials for self sensitized degradation of malachite green dye under solar light, Applied Catalysis A: General 490 (2015) 42-49. [19] B. Krishnakumar, M. Swaminathan, Photodegradation of Acid Violet 7 with AgBr–ZnO under highly alkaline conditions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 99 (2012) 160-165. [20] K. Selvam, M. Muruganandham, I. Muthuvel, M. Swaminathan, The influence of inorganic oxidants and metal ions on semiconductor sensitized photodegradation of 4-fluorophenol, Chemical Engineering Journal 128 (2007) 51-57.