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The experimental results show that the highest photocatalytic activity was obtained from the Ag (5) doped Zn-0.75Al-0.25La-CO3 photocatalysts calcined at 500 0C with a degradation efficiency of 99,4 after 40 min of irradiation only. This study could provide a new route for the fabrication of high performance photocatalysts and facilitate their application in the environmental remediation issues.
Journal of Science: Advanced Materials and Devices (2019) 34e46 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Novel Ag-ZnO-La2O2CO3 photocatalysts derived from the Layered Double Hydroxide structure with excellent photocatalytic performance for the degradation of pharmaceutical compounds A Elhalil a, *, R Elmoubarki a, M Farnane a, A Machrouhi a, F.Z Mahjoubi b, M Sadiq a, S Qourzal c, M Abdennouri a, N Barka a a Laboratoire des Sciences des Mat eriaux, des Milieux et de la Mod elisation, Universit e Sultan Moulay Slimane, F.P Khouribga, B.P 145, 25000, Khouribga, Morocco b Universit e Sultan Moulay Slimane, Facult e des Sciences et Techniques, B eni Mellal, Laboratoire de Spectro-chimie Appliqu ee et Environnement (LSCAE), B.P: 523, B eni - Mellal, Morocco c Equipe de Catalyse et Environnement, D epartement de Chimie, Facult e des Sciences, Universit e Ibn Zohr, B.P.8106 Cit e Dakhla, Agadir, Morocco a r t i c l e i n f o a b s t r a c t Article history: Received 11 September 2018 Received in revised form 20 December 2018 Accepted January 2019 Available online January 2019 In this work, we have prepared the Ag-ZnO-La2O2CO3 nanomaterials as promising photocatalysts for the photocatalytic degradation of pharmaceutical pollutants Firstly, a series of ZnAl1-xLax(CO3) (0 x 0.5) layered double hydroxides (LDHs) were synthesized by the co-precipitation method at the component molar ratio of Zn/(Al þ La ¼ 3, where La/Al ¼ 0, 0.25 and 0.5) Photocatalysts were prepared by the calcination of the LDH precursors at different temperatures of 300, 400, 500, 600, 800 and 1000 C The effects of the La/Al molar ratio and the calcination temperature on the photocatalytic activity of the catalysts were evaluated by the degradation of caffeine as a model pharmaceutical pollutant in aqueous solutions under the UV irradiation Thereafter, in order to increase the photocatalytic activity, the catalysts obtained at the optimal La/Al molar ratio and calcination temperature were doped with the Ag noble metal at various concentrations (i.e 1, and wt%) using the ceramic preparation process to obtain the desired Ag-ZnO-La2O2CO3 catalysts The synthesized photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX) and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) Detailed photocatalytic experiments based on the effects of the irradiation time, the dopant amount, the catalyst dose, the initial solution pH and reuseability were performed and discussed in this study The Ag doped material showed significantly a higher photocatalytic activity compared to the undoped, pure ZnO and P25 catalysts The experimental results show that the highest photocatalytic activity was obtained from the Ag (5%) doped Zn-0.75Al-0.25La-CO3 photocatalysts calcined at 500 C with a degradation efficiency of 99,4% after 40 of irradiation only This study could provide a new route for the fabrication of high performance photocatalysts and facilitate their application in the environmental remediation issues © 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Layered double hydroxides Photocatalyst Doping Photocatalytic degradation Caffeine Introduction The occurrence of persistent pharmaceutical pollutants (PPhP) in the aquatic environment is a well-known environmental issue It is caused by the discharge of the untreated wastes of the pharmaceutical industry, by the secretion of non-metabolized drugs and * Corresponding author Fax: ỵ212 523 49 03 54 E-mail address: elhalil.alaaeddine@gmail.com (A Elhalil) Peer review under responsibility of Vietnam National University, Hanoi urine and feces discharged by human or animals onto the water bodies These pollutants at low concentrations produce negative effects on the human, aquatic organism and ecological environment due to their resistance to natural degradation and potential toxicity [1] A number of effects, such as the development of antibiotic-resistant microbes in the aquatic environment [2], fish reproduction changes due to the presence of estrogenic compounds [3] and the specific inhibition of photo-synthesis in algae caused by b-blockers [3] have been reported Caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione), is a natural alkaloid which is the main component of daily consumed https://doi.org/10.1016/j.jsamd.2019.01.002 2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 beverages and foods like coffee, tea, energetic drinks, coke and chocolate [4] Besides, caffeine is also used in several pharmaceutical formulations due to its psychoactive effects such as stimulation of the central nervous system, diuresis and gastric acid secretion [5] However, when excessively consumed, it can cause adverse mutation effects [6], such as mutagenic effects in the DNA repair and the cyclic AMP phosphodiesterase activity [7] Furthermore, it can be a cause of cancer, heart diseases and complications in pregnant women and aging [7] Because conventional treatments in municipal wastewater treatment facilities cannot degrade caffeine efficiently, it is necessary to look for alternatives Advanced Oxidation Processes (AOPs) seem to be very promising, although many other alternatives have been proposed in the recent years [8] The base of these methods is the formation of highly reactive chemical species, hydroxyl radicals (OH), which degrade the most persistent organic molecules and break them down into relatively less persistent organics and end products, such as CO2, H2O and mineral salts [9] Among the AOPs, heterogeneous photocatalysis has witnessed rapid progresses throughout the last few decades [10e13] This process uses a semiconductor photocatalyst, of which the electrons in the valence band can be promoted to the conduction band when it is excited by the adequate photoenergy, producing photogenerated electronehole (e/hỵ) pairs The generated e/hỵ pairs enable a series of reductive and oxidative reactions [14] During this process, hydroxyl radicals are formed from the water oxidation by holes (hỵ) [15] Zinc oxide (ZnO) is recognized as preferable photocatalysts due to its high photosensitivity, nontoxic nature, low cost, and its relative abundance in the earth crust [16] It is widely known for various applications, such as gas sensors [17], energy harvesting devices [18], light-emitting diodes [19] and photocatalysts [20] ZnO can absorb a larger part of UV spectrum and shows higher level photocatalytic properties In the ZnO semiconductor, the electronehole recombination is a major hindrance to the far reaching applications of its photocatalytic activity [21] Loading noble metal nanoparticles, such as Pt, Pd, Ag and Rh onto ZnO [22] surface is a good way to solve the problem Various methods for the preparation of ZnO materials have been reported, such as solegel [23], hydrothermal synthesis [24], chemical vapor deposition [25], photo-chemical reduction [26], coprecipitation [27], and microwave-assisted thermal decomposition [28] By using the layered double hydroxides (LDHs) as precursors for the preparation of ZnO it is possible to obtain a fine dispersion of the active components on the surface of the semiconductor, and as a consequence the formation of an intimate contact at atomic level between the generated semiconductor phases LDHs or even anionic clays are the subjects of a lively interest for various applications since the last years, because of their high anionic exchange capacity (2e5 mmol gÀ1), their high specific surface area (20e120 m2 gÀ1), the presence of fillers on the surface, and especially the tradability of interlayered anions [29] The general formula of a LDHs is: [MII1-xMIIIx(OH)2]xỵ (An-x/n).mH2O, where MII represents a divalent cation (Zn2ỵ, Ni2ỵ, Mg2ỵ, Mn2ỵ, Fe2ỵ ), MIII represents a trivalent cation (Al3ỵ, Fe3ỵ, Co3ỵ, Cr3ỵ, Mn3ỵ ), An the compensating anion (Cl, NO3, ClO2-4, CO2-3 … ), n is the charge of the anion, and m is the number of water molecules located in the interlayer region together with the anion The coefficient, x, is the molar fraction, expressed in terms of [MIII/ (MII ỵ MIII)] [30] Further, the Lanthanum element is very important in advanced photo-catalytic technologies due to its particular 4fe5d and 4fe4f electronic transitions which are different in the other elements [31] The Lanthanum with f-orbitals is renowned for being able to 35 form complexes with various Lewis bases, such as amines, aldehydes, and alcohols If it is combined with a ZnO matrix, this may provide means for pollutants to be adsorbed onto the semiconductor surface [32] In this work, a series of ZnAl1-xLax(CO3) (0 x 0.5) LDH materials at different (r) La/Al molar ratios (r ¼ 0, 0.25 and 0.5) was prepared by the co-precipitation method, calcined at different temperatures of 300, 400, 500, 600, 800 and 1000 C For the improvement of the photocatalytic performance, the samples were doped with different amounts of the Ag noble metal (namely: 1, and wt%) using the ceramic process The catalysts were characterized by several physico-chemical techniques, such as XRD, FTIR, TGA/DTA, ICP-AES and SEM/EDX The photocatalytic activity of the prepared photocatalysts was evaluated by the degradation of caffeine as a model of the pharmaceutical pollutants under the UV irradiation The effect of the Ag doping concentration on the photocatalytic activity was evaluated in detail Experimental 2.1 Reagents The starting chemicals: zinc nitrate (Zn(NO3)2$6H2O), aluminium nitrate (Al(NO3)3$9H2O), Lanthanum nitrate (La(NO3)3$6H2O), silver nitrate (Ag(NO3)), sodium carbonate (Na2CO3), sodium hydroxide (NaOH) and hydrochloric acid, 37% (HCl) and standard Degussa P-25 titanium dioxide have been acquired from SigmaeAldrich (Germany) Caffeine (C8H10N4O2) was a product of SigmaeAldrich (China) Nitric acid, 65%, extra pure was purchased from Scharlau Chemie (Spain) All the chemicals were of analytical grade and were used without further purification Bi-distilled water was used as the solvent throughout this study 2.2 Photcatalysts preparation A series of Zn-Al-La-CO3 LDH were prepared by the co-precipitation method The nitrates Zn(NO3)2$6H2O, Al(NO3)3$ 9H2O and La(NO3)3$6H2O were dissolved in 300 mL of double distilled water with the molar ratios Zn/(Al ỵ La) of and La/Al ẳ 0, 0.25 and 0.5 (at the total concentration of metal ions of mol/L) Then, the Na2CO3 (100 mL,1 mol/L) solution as a source of carbonate and the nitrates solutions were added dropwise in a backer containing 50 mL of bi-distilled water A solution of NaOH (2 mol/L) was added dropwise to the stirred salt solution until the final pH value reached 8.5 ± 0.2 for La/Al ¼ and 10 ± 0.2 for La/Al ¼ 0.25 and 0.5 The gel formed was stirred vigorously for h and then transferred into an autoclave and hydrothermally treated at 75 C for 16 h Finally the precipitates were filtered and washed several times with bi-distilled water to be neutralized and dried at 100 C for 24 h The photocatalysts were synthesized by calcination of the LDH materials at 300, 400, 500, 600, 800 and 1000 C for h in a muffle furnace The final catalysts were consecutively named as Zn-Al-T for LDH with r ¼ 0; Zn0.75Al-0.25La-T for r ¼ 0.25; and Zn-0.5Al-0.5La-T for r ¼ 0.5, where r represents the La/Al molar ratio and T the calcination temperature For comparison, the pure zinc oxide (ZnO) photocatalyst was prepared by the precipitation method as reported in our previous work [33] The photocatalysts were prepared via the deposition of Ag onto the LDH using the ceramic process, reported in our previous work [34] Desired amounts of the LDH precursor and AgNO3 were manually ground in an agate mortar for 30 After that the homogeneously mixed powder was transferred into a crucible and calcined in air at 500 C for h in a muffle furnace The entire process is free of solvent The obtained samples were denoted as x %-Ag-ZnO-La2O2CO3, where x% represents the weight percentage of Ag in the mixture (namely 1, 3, and wt%) 36 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 2.3 Characterization The XRD measurements were performed at room temperature on a D2 PHASER diffractometer, with the BraggeBrentano geometry, using CuKa target (l ¼ 0.15406 nm) operated at 30 KV and 10 mA The XRD scans were recorded in the 2q range 5e80 with step size 0.01 (0.5s/step) Fourier transform infrared (FTIR) spectra in KBr pellets were collected on a Perkin Elmer (FTIR2000) spectrophotometer, in the range of 4000e400 cmÀ1 Elemental analysis for the Zn/Al molar ratio was measured by an inductively coupled plasma-atomic emission spectrum (ICP-AES, JobinYvon Ultima2) after dissolving the samples in HNO3 acid Thermogravimetric and differential thermal analysis (TGA-DTA) curves were recorded on a SETARAM (SENSYSevo) instrument, in the temperature range from 30 to 700 C with a heating rate of 10 C/min under argon atmosphere The external surface of the sample was analyzed by scanning electron microscopy coupled to the energy dispersive X-ray spectroscopy (SEM/EDX) on a FEI Quanta 200 model, using an accelerating voltage of 20 kV The UVevis DRS spectra were recorded by a Lambda 35 in the range of 200e800 nm The point of zero charge (pHPZC) was determined by the pH drift method according to the method proposed by Noh and Schwarz [35] 2.4 Adsorption/photocatalytic degradation The photocatalytic performance of the photocatalysts was studied by the degradation of caffeine in a water solution Experiments were conducted using 0.3 g/L of each photocatalyst with an initial caffeine concentration equal to 20 mg/L The reaction was carried out in a cylindrical Pyrex photoreactor with a capacity of L and was initiated by an UV mercury lamp (400 W) placed in the center of the reactor The temperature was maintained at 25 ± C by connecting the reactor to the circulating water for preventing the lamp from overheating Before the irradiation, the mixtures were vigorously stirred for 60 in the darkness to establish an adsorption/desorption equilibrium on the surface of the catalysts The adsorbed quantity was calculated using the following equations: Qads ¼ ðC0 À CÞ R (1) where Qads (mg/g) is the quantity of caffeine adsorbed per unit mass of adsorbent, C0 (in mg/L) is the initial caffeine concentration, C (in mg/L) is the caffeine concentration after the adsorption and R (g/L) is the mass of the adsorbent per liter of aqueous solution During the irradiation, the mixture was stirred at a constant rate under a continuous O2 flow At given time intervals, mL aliquots were sampled and filtered to remove the solid particles The filtrates were analyzed using a double-beam scanning spectrophotometer (Shimadzu spectrophotometer, model Biochrom) at the maximum wavelength of 273 nm [36], which is characteristic to caffeine The percentage of degradation was calculated by C/C0, where C is the concentration of the remaining caffeine solution at each irradiated time interval, while C0 is the initial concentration precursors are shown in Fig The figure shows reticular planes (003), (006), (012), (015), (018), (110) and (113), which are typical of LDH structure The XRD pattern of the synthetic LDH was identical to that of the natural hydrotalcite (JCPDS card 22-700) [37] For the samples with La/Al ¼ 0, and 0.25, no impurities from any residual strange phases were observed When the La/Al molar ratio was equal to 0.5, the LaCO3OH phase (JCPDS n 49-0981) [38] appears due to the excess of Lanthanum The lattice parameters (a and c) calculated for each precursor are shown in Table The table shows also the molar ratio La/Al determined by the ICP-AES The table indicates a slight increase in the cell parameters and the cell volume with the increasing La/Al molar ratio from to 0.25 This result could be attributed to the insertion of lanthanum into the lattice, which has larger atomic radius than aluminum (1.03 nm for La and 0.53 nm for Al) The lattice parameters a and c remain unchanged when La/Al ¼ 0.5 This result may be due to the substitution limit of Al by La in the LDH matrix The exceeded lanthanum will be transformed to LaCO3OH Additionally, the crystallinity of the LDH precursors decreases with the increasing La proportion due to the distortion of the lattice caused by the substitution of Al by La The table also indicates a strong correlation between the theoretical and experimental La/Al molar ratios Fig shows the XRD patterns of the LDH precursors after the calcination at different temperatures Remarkable changes are observed after the calcination For all precursors, at the calcination temperature of T ¼ 300 C, the lamellar structure collapsed and new peaks corresponding to ZnO oxide appear The characteristic XRD peaks of ZnO oxide started to appear as indicated by the peaks at 2q ¼ 31.8 , 34.5 , 36.3 , 47.6 , 56.6 , 62.9 , 66.4 , 68 and 69.1 These peaks correspond to the reflections from the (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes, respectively This is also confirmed by the JCPDS data (Card No 36-1451) [39] There is no detection of signals corresponding to Al2O3 phase, implying that Al2O3 is amorphous [40] By increasing the calcination temperature of the Zn-Al-CO3 precursors, the characteristic reflections of the mixed composite ZnO-ZnAl2O4 appear at 600 C and they became sharper with the rise in the calcination temperature up to 1000 C, indicating the increase in the crystallite size in the sample These characteristic peaks are observed at 2q of 31.2 , 36.75 , 44.7, 49.1, 55.6 , 59.3 and 65.3 , corresponding to (220), (311), (400), (331), (422), (511), and (440) diffraction planes confirming the formation of the spinel ZnAl2O4 phase (JCPDS Card No 05-0669) [41] The characteristic diffraction peaks of the samples with La/ Al ¼ 0.25 and 0.5, calcined at 300 C matched those of the orthorhombic lanthanum hydroxycarbonate LaCO3OH phase At calcination temperatures of T ¼ 400, 500 and 600 C, the typical Results and discussion 3.1 Catalysts characterization 3.1.1 X-ray diffraction (XRD) study The XRD analysis was performed to identify the phase structure of the synthesized materials XRD patterns of the different LDH Fig XRD patterns of LDH precursors at different molar ratios La/Al A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 37 Table Molar ratio La/Al and cell parameters (a and c) LDH Theoretical molar ratio (La/Al) Experimental molar ratio (La/Al) a (nm) c (nm) Zn-Al-3 Zn-0.75Al-0.25La Zn-0.5Al-0.5La 0.25 0.5 0.24 0.53 0.3076 0.3084 0.3083 2.2775 2.2789 2.2789 Fig XRD patterns of the fresh and calcined LDH materials at different temperatures 38 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 patterns of the monoclinic lanthanum dioxycarbonate La2O2CO3 [38] are observed at the 2q angles of 13.11 ; 22.80 ; 29.57 ; 30.78 ; 31.32 ; 40.07 ; 44.45 These peaks correspond to the reflections from (020), (110), (130), (101), (101), (060), (200) planes, respectively, consistent with the JCPDS data (Card n 48-1113) [42] The formation of the La2O2CO3 phase is attributed to the transformation of the LaCO3OH phase as a function of the temperature At calcination temperatures of 800 C and above, the LaAlO3 perovskite phase appears, as observed by diffraction peaks at 23.57 (012); 33.52 (110); 41.32 (202); 48.06 (024); 54.18 (116); 59.81 (300); 70.26 (220); 77.07 (312), according to the JCPDS data (JCPDS n 310022) [43] The characteristic diffraction peaks of the synthesized pure ZnO match well with the patterns in the standard card of ZnO oxide [39] For the Ag-doped LDH samples (Fig 3), four additional peaks at 38.24 , 44.42 , 64.44 and 77.40 are observed These peaks can be assigned to (111), (200), (220) and (311) reflections of the face centered cubic metallic Ag nanoparticles (JCPDS card No 04-0783) [44] The peaks of the Ag nanoparticles are much sharper and the peak intensity increases with the increasing Ag content No peaks from other phases are detected, indicating the high phase purity of the products 3.1.2 Fourier transform infrared (FTIR) spectra The functional groups of the synthesized materials were characterized by the FTIR spectra Fig shows the FTIR spectra of the LDH precursors before and after the calcination at different temperatures The spectra of the LDH materials show a broad band between 3600 and 3200 cmÀ1, which is attributed to the stretching vibration of the OH groups of physically adsorbed and interlamellar water molecules [45] Another common band for the LDH materials is found at 1600 cmÀ1, due to the O-H bending vibrations of water molecules [45] The band at 1364 cmÀ1 is attributed to the stretching vibration of the carbonate anions CO2À In the lowfrequency region, the bands in the range 700 and 400 cmÀ1 are assigned to the metal-oxygen-metal vibrations [46] After calcinaÀ1 tion (T 600 C), the typical bands of CO2À (1364 cm ) still exist This band is due to the formation of the LaOHCO3 phase (T ¼ 300 C), which was further converted to La2O2CO3 at 400e600 C The XRD analysis also confirms their formation 3.1.3 Thermal analysis (TGA-DTA) The thermal stabilities of the LDH were determined as a function of the temperature by differential thermal analysis coupled with thermogravimetry (TGA-DTA) Fig shows the TGA-DTA curves of the as-prepared LDH products The TGA-DTA curves of all LDH show a first mass loss at ~100 C, which can be accredited to the removal of water at the surface It was followed by a second more pronounced and sharp endothermic phenomenon around Fig FTIR spectra of the fresh and calcined LDH materials at different temperatures 160e240 C This mass loss corresponds to the hydration water from the interlayer region A third step, extending up to 320 C, is assigned to the overlapped mass losses due to the decomposition of the carbonates The thermal decomposition of the LDH precursors with molar ratios La/Al of 0.25 and 0.5, however, is extending up to T~800 C The endothermic peak is observed in the temperature range of 325e525 C This peak is attributed to the loss of one H2O molecule and one of CO2 from 2LaOHCO3: 2LaOHCO3 / La2O2CO3 ỵ CO2 ỵ H2O The last mass loss is attributed to the loss of a further CO2 molecule and the formation of the LaAlO3 oxide: La2O2CO3 ỵ Al2O3 (amorphous phase) / 2LaAlO3 ỵ CO2 Fig XRD patterns of the fresh LDH precursor, undoped and Ag doped ZnOLa2O2CO3 The formation of LaAlO3 at 800 C was also confirmed by the XRD data Table compares the experimental and the calculated weight losses A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 39 Fig SEM-EDX images of undoped (a, b) and 5%Ag doped ZnO-La2O2CO3 (c, d) Fig TGA/DTA curves of the different LDH precursors 3.1.4 SEM/EDX observation The morphology and microstructure of the ZnO-La2O2CO3 and 5%Ag-ZnO-La2O2CO3 samples were investigated by SEM As shown in Fig 6(a,c), the surface morphology of these samples differs greatly from each other It is clearly seen that the photocatalysts have a heterogeneous surface with clearly observable porosity For the 5%Ag-ZnO-La2O2CO3, the Ag particles are homogeneously and highly dispersed on the surface of the ZnO-La2O2CO3 composite The Energy Dispersive X-ray Spectra (EDX) of the undoped and the Ag doped composite are shown in Fig 6(b,d) The results confirm the presence of Zn, Al, La, C and O in the undoped sample For the 5%Ag-ZnO-La2O2CO3 composite, the spectrum shows peaks corresponding to Ag along with the other constituent elements Zn, Al, La, C and O 3.1.5 UV-visible diffuse reflectance spectroscopy It is well known that the optical absorption properties of photocatalysts play a significant role in their photocatalytic activities Thus, the UV-Vis DRS technique was used to display Table Molar ratios La/Al and cell parameters (a and c) LDH 1st mass loss (~200 C) 2nd mass loss (~273 C) 3rd mass loss (~482 C) 4th mass loss (~800 C) Total mass loss Zn-Al Zn-0.75Al-0.25La-3 Zn-0.5Al-0.5La-3 22.65% 10.91% 6.36% 12.73% 12.27% e 5.45% 6.83% e 1.36% 2.72% 22.65% 30.45% 28.18% 40 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 However, the photoactivity drastically decreases by continuous calcination at temperatures of 600 up to 1000 C This result can be attributed to the transformation of a partition of ZnO to ZnAl2O4 for La/Al ¼ and of La2O2CO3 to LaAlO3 for La/Al ¼ 0.25 and 0.5, which is not beneficial in the photocatalytic activity It clearly indicates that the photocatalysts calcined at 500 C exhibite the best photocatalytic activity compared to the those calcinated at other temperatures Fig UVeVis DRS for the ZnO-Al2O3, ZnO-La2O2CO3 and 5%Ag-ZnO- La2O2CO3 photocatalysts optical properties of the ZnO-Al2O3, ZnO-La2O2CO3 and 5%AgZnO-La2O2CO3 photocatalysts and the results are shown in Fig The photocatalysts show a broader absorption in the UV region It can be clearly seen from figure that the maximum of the absorbance band increases with the incorporation of La into the lattice and while increasing the Ag doping to 5% Consequently, the 5%Ag-ZnO-La2O2CO3 nanocomposite could have reasonable activity under the UV-light irradiation compared with ZnO-La2O2CO3 and ZnO-Al2O3, respectively When the absorption intensity increases, the formation rate of the electronehole pairs on the photocatalyst surface also increases, leading to the higher photocatalytic activity [47] Likewise, the ZnO-La2O2CO3 and 5%Ag-ZnO-La2O2CO3 catalysts also show absorption in the zone of 420e800 nm This indicates that the presence of La and the metallic Ag can improve the visible light harvesting of the photocatalysts This result can be attributed to the formation of the synergic effect between Ag and ZnOLa2O2CO3 3.2 Photocatalytic study 3.2.1 Effect of calcination temperature on the adsorption/ photocatalytic degradation The effect the of calcination temperature on the photocatalytic activity of different as-prepared photocatalysts was assessed by monitoring the changes in the optical absorption spectra of the caffeine solution during the photodegradation process Fig shows the adsorption/photodegradation of the caffeine under UV irradiation in the presence of the catalysts calcined at different temperatures of 300, 400, 500, 600, 800 and 1000 C Notably, darkness adsorption tests were performed for the catalysts calcined at various temperatures before the photocatalytic degradation tests under irradiation The rate of adsorption increased with increasing the calcination temperature up to 500 C At T ! 600 C, the adsorption capacity was very low According to many results reportein literature [48], the reconstruction process avails oneself of the memory effect, the ability to recover the original LDH structure, obtained by the LDH precursor mildly calcinated around 500 C, and immersed in a solution of the anion to be intercalated This behavior was confirmed by the strong correlation between the adsorbed quantity and the photocatalytic degradation rate shown in Fig at T 500 C After the irradiation, as can be observed from Fig 8, the caffeine degradation efficiency of the catalysts increases with the increasing calcination temperature from 300 to 500 C This can be interpreted by the adsorption capacity and the beginning formation of the active phase, the ZnO oxide for the zero molar ratio (La/Al ¼ 0), and the formation of ZnO and La2O2CO3 for La/Al ¼ 0.25 and 0.5 3.2.2 Effect of the La/Al molar ratio on the photocatalytic reaction The effect of the La/Al molar ratios on the photocatalytic performance of the synthesized catalysts was evaluated The photocatalytic activity clearly increases with an increase of the La/Al molar ratio from to 0.25, and then decreases at La/Al ¼ 0.5 (Fig 9) Thus, the La/Al ¼ 0.25 catalysts have the highest photocatalytic efciency The reason could be explained as follows When La3ỵ is incorporated into the LDH structure, more surface defects could be produced and a space charge layer could be formed on the surface, which is beneficial to hindering the recombination of the photoinduced electron/hole pairs This result contributes to the improvement of the photocatalytic activity of the La/Al ¼ 0.25 catalysts, compared with that of La/Al ¼ samples [49] However, the further increase of the La3ỵ incorporation (La/Al ẳ 0.5) is likely to form more chemical bonds of Zn-O-La and the aggregation of LaOHCO3, and the role of the formed surface charge region is negatively influenced, which fail to efficiently separate the photoinduced electronehole pairs [49] From the above analysis, it can be concluded that the optimal concentration of lanthanum is La/ Al ¼ 0.25 in our work This composition is thought to be appropriate for the transfer of electrons and holes during the photocatalytic reaction 3.2.3 Effect of Ag doping For increasing the photocatalytic activity of the best catalyst with the molar ration La/Al ¼ 0.25, calcined at 500 C, the material was doped by Ag noble metal with different amounts (1, and wt %) using ceramic process The results illustrated in Fig 10 reveals that the doped catalysts display excellent photocatalytic performance compared to the undoped ones It can be seen from the figure that the degradation rate of caffeine slightly increased with the increase of Ag doping The experimental results indicate that the high amount of Ag (5%) shows the highest catalytic activity After 40 of irradiation, the complete degradation of caffeine was done The photocatalytic performance of 5%Ag-ZnO-La2O2CO3 was much higher than that of some photocatalysts cited in the literature Recently, Rimoldi et al [50] reported that the photocatalytic performance of TiO2 photocatalyst for the degradation of caffeine reached 90% disappearance after 360 of irradiation (with characteristic parameters of C0 ¼ 35 mg/L; 0.5 g/L of TiO2) In the work of Chuang et al [51] for example, the initial concentration of caffeine of 20 mg/L almost completely degraded within 360 in the presence of synthesized TiO2 With 1%Mg doped ZnO-Al2O3 catalyst, 98.9% of caffeine solution (20 mg/L) was removed after 70 of irradiation [52] Fig 11 also indicates that the adsorption rate decreases with the increasing Ag doping concentration Therefore, the increase in the photocatalytic activity of catalysts is mainly due to the synergistic effects between the Ag noble metal and ZnO oxide The relation between the amount of Ag in the catalyst and the photocatalytic degradation rate can be explained by the fact that Ag acts as an electron trap The electrons generated on the ZnO-La2O2CO3 surface by the UV light irradiation quickly move to the Ag particles to facilitate the effective separation of the photogenerated electrons and holes, resulting in the significant enhancement of A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 41 Fig Adsorption/photocatalytic degradation of caffeine in the presence of the synthesized catalysts (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L; pH of the natural solution (~7.5)) photocatalytic activity [53,54] Ag plays a positive role as an electron acceptor, more acceptor centers are provided with the increasing Ag-doping, and therefore, the degradation rate for caffeine increases with the increase of the Ag content The UV-Vis analysis results confirms the synergism between the ZnOLa2O2CO3 catalyst and the Ag nanoparticles for the photodegradation activities 3.2.4 Effect of the photocatalyst dose In order to avoid the excess catalyst and to ensure the total absorption of efficient photons, a series of experiments was carried out to assess the optimum catalyst loading by varying the amount of the best photocatalyst (5%Ag-ZnO-La2O2CO3) from 0.1 to 1.5 g/L Experiments were done in 20 mg/L caffeine aqueous solution at solution pH of 7.5 After 40 of the UV irradiation, the 42 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 Fig Correlation between the adsorbed quantity and the photocatalytic degradation of caffeine (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L; pH of the natural solution (~7.5)) photocatalytic degradation efficiency (%) was evaluated Results given in Fig 12 show that the increase of the catalyst dose from 0.1 to 0.3 g/L resulted in an increase in the photocatalytic degradation efficiency from 84.98 to 99.4% Beyond this dose, a slight decrease in the degradation efficiency with the rise of the catalyst dose was observed This can be explained by the fact that the excess photocatalyst dose resulted in an unfavorable light scattering and a reduction of the light penetration into the solution The same effect was observed by Elhalil et al [34] From a practical viewpoint, the optimum dosage of 0.3 g/L was chosen in further experiments 3.2.5 Effect of the initial solution pH The effect of the solution pH on the photocatalytic oxidation of caffeine in the presence of 5%Ag-ZnO-La2O2CO3 was studied at pH A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 43 Fig 10 Photocatalytic degradation of caffeine in the presence of undoped and Ag doped ZnO-La2O2CO3 (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L; pH of the natural solution (~7.5)) Fig 11 Correlation between adsorbed quantity and photocatalytic degradation of caffeine (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L; pH of the natural solution (~7.5)) of 3.5, 7.5 and 9.5 Fig 13 shows that solution pH affects significantly the percentage degradation of caffeine The photocatalytic activity was enhanced at pH of 9.5 and was dramatically decreased at pH of 3.5 Generally, the solution pH affects at the same time the surface charge of the photocatalyst and the ionization of caffeine molecules in the solution The pHpzc of 5%Ag-ZnO-La2O2CO3 catalyst was found to be 8.97 Therefore, at pH > 8.97 the surface acquires negative charge, favoring the adsorption of cationic molecules, while at pH < 8.97, the surface of the catalyst acquires positive charge, favoring the adsorption of anionic molecules The pKa of caffeine molecules is 10.4 which means that the molecule is fully protonated at pH < 10.4 Fig 12 Effect of catalyst dose on the photocatalytic degradation of caffeine after 40 of irradiation (Caffeine concentration: 20 mg/L; initial solution pH: ~7.5) Fig 13 Effect of the initial solution pH on the photocatalytic degradation of caffeine (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L) 44 A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 Fig 14 Photocatalytic degradation of caffeine over three cycles of regeneration of 5%Ag-ZnO-La2O2CO3 photocatalyst (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L; solution pH: ~7.5) Since the structure of caffeine was the same in the whole region of studied pH values, the observed behavior could only be due to the modification of the proprieties of the photocatalysts The observed trend of the photocatalytic activity observed at pH of 9.5 could be due to the favorably enhanced adsorption of caffeine on the photocatalyst at pH values between 8.97 and 10.4 and the more efficient formation of hydroxyl radicals In the acidic medium (pH ¼ 3.5), the decrease of the percentage degradation could be attributed to many phenomena simultaneously intervening: a) non favorable adsorption, b) the dissolution of the photocatalysts and c) the photodecomposition and dissolution of ZnO according to the following equations [55]: Dissolution: ZnO ỵ 2Hỵ / Zn2ỵ(aq) ỵ H2O (2) Photodecomposition: ZnO ỵ 2hỵVB / Zn2ỵ(aq) þ O* (3) 3.2.6 Efficiency of the regenerated photocatalyst Generally, recycling of the photocatalyst is crucially important for industrial applications [56,57] In order to determine the recyclability of the best catalyst (5%Ag-ZnO-La2O2CO3), we carried out a cycle of experiment under identical conditions In each experiment, after using the nanocomposite in one cycle, it was washed with distilled water and dried at 100 C for 24 h As illustrated in Fig 14, the photocatalytic activity of the prepared photocatalyst still maintains a high level even after times cycling As shown in the figure, the 5%Ag-ZnO-La2O2CO3 nanocomposite reveals a high photostability in these experiments, although a slight decrease of activity is observed compared to the first-run result (6.42%) This can be attributed to residual caffeine adsorbed on the surface of the catalyst The results suggest that the 5%Ag-ZnO-La2O2CO3 nanocomposite may have practical application potentials as an effective and stable photocatalyst for degradations of different pharmaceutical pollutants under UV irradiation 3.2.7 Comparison of the photoactivity with that of the pure ZnO and P-25 For comparison, the pure ZnO was prepared and studied under the same reaction conditions Titanium dioxide in powder form is largely used as a catalyst in the field of the photocatalytic degradation of organic pollutants in water One of the most efficient commercial photocatalysts is P-25 (TiO2) manufactured by flame hydrolysis, which was used in this study as a reference photocatalyst Fig 15 shows the comparison of the photocatalytic activity of the best photocatalyst 5%Ag-ZnO-La2O2CO3 with that of the pure ZnO and P-25 (TiO2) in the same experimental conditions After 40 of irradiation, the pure ZnO exhibits a moderate photocatalytic activity with the degradation percentage of 39.24% The reference P-25 catalyst exhibits a better photocatalytic efficiency than the pure ZnO, while reaching 48.89% disappearance of caffeine after 40 of irradiation The 5%AgZnO-La2O2CO3 photocatalyst synthesized from layered double hydroxides precursor enhances greatly the photocatalytic activities to 99.4% after 40 of irradiation This further confirms the judicious choice of the LDH structure as precursors of photocatalysts Conclusion Fig 15 Comparison of the photocatalytic activity of 5%Ag-ZnO-La2O2CO3 with that of the pure ZnO and the commercial Degussa P-25 (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L; pH of the natural solution (~7.5)) The Ag-ZnO-La2O2CO3 photocatalysts were prepared by a facile single step deposition of Ag noble metal onto La/Al ¼ 0.25 LDH precursor at different concentrations (1, and 5%) The prepared catalysts were characterized using several techniques such as XRD, FTIR, TGA/DTA, ICP-AES, SEM/EDX and UV-Vis DRS The photocatalytic activity of the catalysts was evaluated for the degradation of caffeine as a model of pharmaceutical pollutant in aqueous solution under UV irradiation The Ag-ZnO-La2O2CO3 photocatalysts exhibited an excellent photocatalytic activity A Elhalil et al / Journal of Science: Advanced Materials and Devices (2019) 34e46 toward the degradation of caffeine, in comparison to undoped ZnO-La2O2CO3, pure ZnO and commercial P-25 photocatalysts The maximum photocatalytic degradation of 99.4% of the caffeine was achieved by the 5%Ag-ZnO-La2O2CO3 catalyst after 40 of irradiation The enhanced photocatalytic activity was mainly attributed to the interfacial heterostructure in the 5%AgZnO-Al2O3 catalyst The optimum catalyst dosage for the degradation of a 20 mg/L of caffeine solution was found to be 0.3 g/L It was observed that the adsorption of the 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This further confirms the judicious choice of the LDH structure as precursors of photocatalysts Conclusion Fig 15 Comparison of the photocatalytic activity of 5 %Ag-ZnO-La2O2CO3 with that of the pure... provided with the increasing Ag-doping, and therefore, the degradation rate for caffeine increases with the increase of the Ag content The UV-Vis analysis results confirms the synergism between the