Novel Ag-ZnO-La2O2CO3 photocatalysts derived from the Layered Double Hydroxide structure with excellent photocatalytic performance for the degradation of pharmaceutical compounds

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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.

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Nội dung Text: Novel Ag-ZnO-La2O2CO3 photocatalysts derived from the Layered Double Hydroxide structure with excellent photocatalytic performance for the degradation of pharmaceutical compounds

Journal of Science: Advanced Materials and Devices 4 (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, Faculte des Sciences et Techniques, B eni Mellal, Laboratoire de Spectro-chimie Appliquee et Environnement (LSCAE), B.P: 523, B eni - Mellal, Morocco c Equipe de Catalyse et Environnement, D epartement de Chimie, Faculte 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: In this work, we have prepared the Ag-ZnO-La2O2CO3 nanomaterials as promising photocatalysts for the Received 11 September 2018 photocatalytic degradation of pharmaceutical pollutants. Firstly, a series of ZnAl1-xLax(CO3) (0 x 0.5) Received in revised form layered double hydroxides (LDHs) were synthesized by the co-precipitation method at the component 20 December 2018 molar ratio of Zn/(Al þ La ¼ 3, where La/Al ¼ 0, 0.25 and 0.5). Photocatalysts were prepared by the Accepted 4 January 2019 Available online 6 January 2019 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 cat- alysts were evaluated by the degradation of caffeine as a model pharmaceutical pollutant in aqueous Keywords: Layered double hydroxides solutions under the UV irradiation. Thereafter, in order to increase the photocatalytic activity, the catalysts Photocatalyst obtained at the optimal La/Al molar ratio and calcination temperature were doped with the Ag noble metal Doping at various concentrations (i.e. 1, 3 and 5 wt%) using the ceramic preparation process to obtain the desired Photocatalytic degradation Ag-ZnO-La2O2CO3 catalysts. The synthesized photocatalysts were characterized by X-ray diffraction (XRD), Caffeine 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 P- 25 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 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. © 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/). 1. Introduction urine and feces discharged by human or animals onto the water bodies. These pollutants at low concentrations produce negative The occurrence of persistent pharmaceutical pollutants (PPhP) effects on the human, aquatic organism and ecological environ- in the aquatic environment is a well-known environmental issue. It ment due to their resistance to natural degradation and potential is caused by the discharge of the untreated wastes of the phar- toxicity [1]. A number of effects, such as the development of maceutical industry, by the secretion of non-metabolized drugs and 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. * Corresponding author. Fax: þ212 523 49 03 54. E-mail address: elhalil.alaaeddine@gmail.com (A. Elhalil). Caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione), is a Peer review under responsibility of Vietnam National University, Hanoi. 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 4 (2019) 34e46 35 beverages and foods like coffee, tea, energetic drinks, coke form complexes with various Lewis bases, such as amines, alde- and chocolate [4]. Besides, caffeine is also used in several phar- hydes, and alcohols. If it is combined with a ZnO matrix, this may maceutical formulations due to its psychoactive effects such as provide means for pollutants to be adsorbed onto the semi- stimulation of the central nervous system, diuresis and gastric acid conductor surface [32]. secretion [5]. However, when excessively consumed, it can cause In this work, a series of ZnAl1-xLax(CO3) (0 x 0.5) LDH ma- adverse mutation effects [6], such as mutagenic effects in the DNA terials at different (r) La/Al molar ratios (r ¼ 0, 0.25 and 0.5) was repair and the cyclic AMP phosphodiesterase activity [7]. Further- prepared by the co-precipitation method, calcined at different more, it can be a cause of cancer, heart diseases and complications temperatures of 300, 400, 500, 600, 800 and 1000 C. For the in pregnant women and aging [7]. improvement of the photocatalytic performance, the samples were Because conventional treatments in municipal wastewater doped with different amounts of the Ag noble metal (namely: 1, 3 treatment facilities cannot degrade caffeine efﬁciently, it is neces- and 5 wt%) using the ceramic process. The catalysts were charac- sary to look for alternatives. Advanced Oxidation Processes (AOPs) terized by several physico-chemical techniques, such as XRD, FTIR, seem to be very promising, although many other alternatives have TGA/DTA, ICP-AES and SEM/EDX. The photocatalytic activity of the been proposed in the recent years [8]. The base of these methods is prepared photocatalysts was evaluated by the degradation of the formation of highly reactive chemical species, hydroxyl radicals caffeine as a model of the pharmaceutical pollutants under the UV (OH), which degrade the most persistent organic molecules and irradiation. The effect of the Ag doping concentration on the pho- break them down into relatively less persistent organics and end tocatalytic activity was evaluated in detail. products, such as CO2, H2O and mineral salts [9]. Among the AOPs, heterogeneous photocatalysis has witnessed 2. Experimental rapid progresses throughout the last few decades [10e13]. This process uses a semiconductor photocatalyst, of which the electrons 2.1. Reagents in the valence band can be promoted to the conduction band when it is excited by the adequate photoenergy, producing photogenerated The starting chemicals: zinc nitrate (Zn(NO3)2$6H2O), aluminium electronehole (e/hþ) pairs. The generated e/hþ pairs enable a nitrate (Al(NO3)3$9H2O), Lanthanum nitrate (La(NO3)3$6H2O), silver series of reductive and oxidative reactions [14]. During this process, nitrate (Ag(NO3)), sodium carbonate (Na2CO3), sodium hydroxide hydroxyl radicals are formed from the water oxidation by holes (hþ) (NaOH) and hydrochloric acid, 37% (HCl) and standard Degussa P-25 [15]. titanium dioxide have been acquired from SigmaeAldrich (Ger- Zinc oxide (ZnO) is recognized as preferable photocatalysts due many). Caffeine (C8H10N4O2) was a product of SigmaeAldrich to its high photosensitivity, nontoxic nature, low cost, and its (China). Nitric acid, 65%, extra pure was purchased from Scharlau relative abundance in the earth crust [16]. It is widely known for Chemie (Spain). All the chemicals were of analytical grade and were various applications, such as gas sensors [17], energy harvesting used without further puriﬁcation. Bi-distilled water was used as the devices [18], light-emitting diodes [19] and photocatalysts [20]. solvent throughout this study. ZnO can absorb a larger part of UV spectrum and shows higher level photocatalytic properties. In the ZnO semiconductor, the 2.2. Photcatalysts preparation electronehole recombination is a major hindrance to the far reaching applications of its photocatalytic activity [21]. Loading A series of Zn-Al-La-CO3 LDH were prepared by the noble metal nanoparticles, such as Pt, Pd, Ag and Rh onto ZnO [22] co-precipitation method. The nitrates Zn(NO3)2$6H2O, Al(NO3)3$ surface is a good way to solve the problem. 9H2O and La(NO3)3$6H2O were dissolved in 300 mL of double Various methods for the preparation of ZnO materials have been distilled water with the molar ratios Zn/(Al þ La) of 3 and La/Al ¼ 0, reported, such as solegel [23], hydrothermal synthesis [24], chem- 0.25 and 0.5 (at the total concentration of metal ions of 2 mol/L). Then, ical vapor deposition [25], photo-chemical reduction [26], co- the Na2CO3 (100 mL,1 mol/L) solution as a source of carbonate and the precipitation [27], and microwave-assisted thermal decomposition nitrates solutions were added dropwise in a backer containing 50 mL [28]. By using the layered double hydroxides (LDHs) as precursors of bi-distilled water. A solution of NaOH (2 mol/L) was added drop- for the preparation of ZnO it is possible to obtain a ﬁne dispersion of wise to the stirred salt solution until the ﬁnal pH value reached the active components on the surface of the semiconductor, and as a 8.5 ± 0.2 for La/Al ¼ 0 and 10 ± 0.2 for La/Al ¼ 0.25 and 0.5. The gel consequence the formation of an intimate contact at atomic level formed was stirred vigorously for 4 h and then transferred into an between the generated semiconductor phases. autoclave and hydrothermally treated at 75 C for 16 h. Finally the LDHs or even anionic clays are the subjects of a lively interest for precipitates were ﬁltered and washed several times with bi-distilled various applications since the last years, because of their high water to be neutralized and dried at 100 C for 24 h. The photo- anionic exchange capacity (2e5 mmol g1), their high speciﬁc catalysts were synthesized by calcination of the LDH materials at 300, surface area (20e120 m2 g1), the presence of ﬁllers on the surface, 400, 500, 600, 800 and 1000 C for 6 h in a mufﬂe furnace. The ﬁnal and especially the tradability of interlayered anions [29]. catalysts were consecutively named as Zn-Al-T for LDH with r ¼ 0; Zn- The general formula of a LDHs is: [MII1-xMIIIx(OH)2]xþ. 0.75Al-0.25La-T for r ¼ 0.25; and Zn-0.5Al-0.5La-T for r ¼ 0.5, where r (An-x/n).mH2O, represents the La/Al molar ratio and T the calcination temperature. where MII represents a divalent cation (Zn2þ, Ni2þ, Mg2þ, Mn2þ, For comparison, the pure zinc oxide (ZnO) photocatalyst was Fe2þ …), MIII represents a trivalent cation (Al3þ, Fe3þ, Co3þ, Cr3þ, prepared by the precipitation method as reported in our previous Mn3þ …), An the compensating anion (Cl, NO3, ClO2-4, CO2-3 … ), work [33]. The photocatalysts were prepared via the deposition of n is the charge of the anion, and m is the number of water molecules Ag onto the LDH using the ceramic process, reported in our previ- located in the interlayer region together with the anion. The coef- ous work [34]. Desired amounts of the LDH precursor and AgNO3 ﬁcient, x, is the molar fraction, expressed in terms of [MIII/ were manually ground in an agate mortar for 30 min. After that the (MII þ MIII)] [30]. homogeneously mixed powder was transferred into a crucible and Further, the Lanthanum element is very important in advanced calcined in air at 500 C for 6 h in a mufﬂe furnace. The entire photo-catalytic technologies due to its particular 4fe5d and 4fe4f process is free of solvent. The obtained samples were denoted as x electronic transitions which are different in the other elements %-Ag-ZnO-La2O2CO3, where x% represents the weight percentage of [31]. The Lanthanum with f-orbitals is renowned for being able to Ag in the mixture (namely 1, 3, and 5 wt%).
36 A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 2.3. Characterization precursors are shown in Fig. 1. The ﬁgure shows reticular planes (003), (006), (012), (015), (018), (110) and (113), which are typical of The XRD measurements were performed at room temperature LDH structure. The XRD pattern of the synthetic LDH was identical on a D2 PHASER diffractometer, with the BraggeBrentano ge- to that of the natural hydrotalcite (JCPDS card 22-700) [37]. For the ometry, using CuKa target (l ¼ 0.15406 nm) operated at 30 KV samples with La/Al ¼ 0, and 0.25, no impurities from any residual and 10 mA. The XRD scans were recorded in the 2q range 5e80 strange phases were observed. When the La/Al molar ratio was with step size 0.01 (0.5s/step). Fourier transform infrared (FTIR) equal to 0.5, the LaCO3OH phase (JCPDS n 49-0981) [38] appears spectra in KBr pellets were collected on a Perkin Elmer (FTIR- due to the excess of Lanthanum. 2000) spectrophotometer, in the range of 4000e400 cm1. The lattice parameters (a and c) calculated for each precursor are Elemental analysis for the Zn/Al molar ratio was measured by an shown in Table 1. The table shows also the molar ratio La/Al inductively coupled plasma-atomic emission spectrum (ICP-AES, determined by the ICP-AES. The table indicates a slight increase in JobinYvon Ultima2) after dissolving the samples in HNO3 acid. the cell parameters and the cell volume with the increasing La/Al Thermogravimetric and differential thermal analysis (TGA-DTA) molar ratio from 0 to 0.25. This result could be attributed to the curves were recorded on a SETARAM (SENSYSevo) instrument, in insertion of lanthanum into the lattice, which has larger atomic the temperature range from 30 to 700 C with a heating rate of radius than aluminum (1.03 nm for La and 0.53 nm for Al). The 10 C/min under argon atmosphere. The external surface of the lattice parameters a and c remain unchanged when La/Al ¼ 0.5. This sample was analyzed by scanning electron microscopy coupled to result may be due to the substitution limit of Al by La in the LDH the energy dispersive X-ray spectroscopy (SEM/EDX) on a FEI matrix. The exceeded lanthanum will be transformed to LaCO3OH. Quanta 200 model, using an accelerating voltage of 20 kV. The Additionally, the crystallinity of the LDH precursors decreases with UVevis DRS spectra were recorded by a Lambda 35 in the range the increasing La proportion due to the distortion of the lattice of 200e800 nm. The point of zero charge (pHPZC) was determined caused by the substitution of Al by La. The table also indicates a by the pH drift method according to the method proposed by Noh strong correlation between the theoretical and experimental La/Al and Schwarz [35]. molar ratios. Fig. 2 shows the XRD patterns of the LDH precursors after the 2.4. Adsorption/photocatalytic degradation calcination at different temperatures. Remarkable changes are observed after the calcination. For all precursors, at the calcination The photocatalytic performance of the photocatalysts was temperature of T ¼ 300 C, the lamellar structure collapsed and studied by the degradation of caffeine in a water solution. Experi- new peaks corresponding to ZnO oxide appear. The characteristic ments were conducted using 0.3 g/L of each photocatalyst with an XRD peaks of ZnO oxide started to appear as indicated by the peaks initial caffeine concentration equal to 20 mg/L. The reaction was at 2q ¼ 31.8 , 34.5 , 36.3 , 47.6 , 56.6 , 62.9 , 66.4 , 68 and 69.1. carried out in a cylindrical Pyrex photoreactor with a capacity of 2 L These peaks correspond to the reﬂections from the (100), (002), and was initiated by an UV mercury lamp (400 W) placed in the (101), (102), (110), (103), (200), (112) and (201) planes, respectively. center of the reactor. The temperature was maintained at 25 ± 2 C This is also conﬁrmed by the JCPDS data (Card No. 36-1451) [39]. by connecting the reactor to the circulating water for preventing There is no detection of signals corresponding to Al2O3 phase, the lamp from overheating. Before the irradiation, the mixtures implying that Al2O3 is amorphous [40]. were vigorously stirred for 60 min in the darkness to establish an By increasing the calcination temperature of the Zn-Al-CO3 adsorption/desorption equilibrium on the surface of the catalysts. precursors, the characteristic reﬂections of the mixed composite The adsorbed quantity was calculated using the following ZnO-ZnAl2O4 appear at 600 C and they became sharper with the equations: rise in the calcination temperature up to 1000 C, indicating the increase in the crystallite size in the sample. These characteristic ðC0 CÞ peaks are observed at 2q of 31.2 , 36.75 , 44.7, 49.1, 55.6 , 59.3 Qads ¼ (1) R and 65.3 , corresponding to (220), (311), (400), (331), (422), (511), where Qads (mg/g) is the quantity of caffeine adsorbed per unit and (440) diffraction planes conﬁrming the formation of the spinel mass of adsorbent, C0 (in mg/L) is the initial caffeine concentra- ZnAl2O4 phase (JCPDS Card No. 05-0669) [41]. tion, C (in mg/L) is the caffeine concentration after the adsorption The characteristic diffraction peaks of the samples with La/ and R (g/L) is the mass of the adsorbent per liter of aqueous Al ¼ 0.25 and 0.5, calcined at 300 C matched those of the ortho- solution. rhombic lanthanum hydroxycarbonate LaCO3OH phase. At calci- During the irradiation, the mixture was stirred at a constant nation temperatures of T ¼ 400, 500 and 600 C, the typical rate under a continuous O2 ﬂow. At given time intervals, 3 mL aliquots were sampled and ﬁltered to remove the solid particles. The ﬁltrates were analyzed using a double-beam scanning spec- trophotometer (Shimadzu spectrophotometer, model Biochrom) at the maximum wavelength of 273 nm [36], which is charac- teristic 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. 3. 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. 1. XRD patterns of LDH precursors at different molar ratios La/Al.
A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 37 Table 1 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 0 0 0.3076 2.2775 Zn-0.75Al-0.25La 0.25 0.24 0.3084 2.2789 Zn-0.5Al-0.5La 0.5 0.53 0.3083 2.2789 Fig. 2. XRD patterns of the fresh and calcined LDH materials at different temperatures.
38 A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (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, respec- tively, 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 calci- nation 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 31- 0022) [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 char- acterized by the FTIR spectra. Fig. 4 shows the FTIR spectra of the LDH precursors before and after the calcination at different tem- peratures. The spectra of the LDH materials show a broad band between 3600 and 3200 cm1, 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 cm1, due to the O-H bending vibrations of water molecules [45]. The band at 1364 cm1 is attributed to the stretching vibration of the carbonate anions CO2 3 . In the low- frequency region, the bands in the range 700 and 400 cm1 are assigned to the metal-oxygen-metal vibrations [46]. After calcina- 1 tion (T 600 C), the typical bands of CO2 3 (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 Fig. 4. FTIR spectra of the fresh and calcined LDH materials at different temperatures. thermogravimetry (TGA-DTA). Fig. 5 shows the TGA-DTA curves of the as-prepared LDH products. The TGA-DTA curves of all LDH 160e240 C. This mass loss corresponds to the hydration water show a ﬁrst mass loss at ~100 C, which can be accredited to from the interlayer region. A third step, extending up to 320 C, is the removal of water at the surface. It was followed by a second assigned to the overlapped mass losses due to the decomposition of more pronounced and sharp endothermic phenomenon around 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 þ CO2 þ H2O The last mass loss is attributed to the loss of a further CO2 molecule and the formation of the LaAlO3 oxide: La2O2CO3 þ Al2O3 (amorphous phase) / 2LaAlO3 þ CO2 The formation of LaAlO3 at 800 C was also conﬁrmed by the Fig. 3. XRD patterns of the fresh LDH precursor, undoped and Ag doped ZnO- XRD data. Table 2 compares the experimental and the calculated La2O2CO3. weight losses.
A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 39 Fig. 6. SEM-EDX images of undoped (a, b) and 5%Ag doped ZnO-La2O2CO3 (c, d). Fig. 5. TGA/DTA curves of the different LDH precursors. Ag doped composite are shown in Fig. 6(b,d). The results conﬁrm 3.1.4. SEM/EDX observation the presence of Zn, Al, La, C and O in the undoped sample. For the The morphology and microstructure of the ZnO-La2O2CO3 and 5%Ag-ZnO-La2O2CO3 composite, the spectrum shows peaks corre- 5%Ag-ZnO-La2O2CO3 samples were investigated by SEM. As shown sponding to Ag along with the other constituent elements Zn, Al, La, in Fig. 6(a,c), the surface morphology of these samples differs C and O. greatly from each other. It is clearly seen that the photocatalysts have a heterogeneous surface with clearly observable porosity. For 3.1.5. UV-visible diffuse reﬂectance spectroscopy the 5%Ag-ZnO-La2O2CO3, the Ag particles are homogeneously and It is well known that the optical absorption properties of highly dispersed on the surface of the ZnO-La2O2CO3 composite. photocatalysts play a signiﬁcant role in their photocatalytic The Energy Dispersive X-ray Spectra (EDX) of the undoped and the activities. Thus, the UV-Vis DRS technique was used to display Table 2 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 22.65% e e 22.65% Zn-0.75Al-0.25La-3 10.91% 12.73% 5.45% 1.36% 30.45% Zn-0.5Al-0.5La-3 6.36% 12.27% 6.83% 2.72% 28.18%
40 A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (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 0 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. 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 per- formance of the synthesized catalysts was evaluated. The photo- catalytic activity clearly increases with an increase of the La/Al Fig. 7. UVeVis DRS for the ZnO-Al2O3, ZnO-La2O2CO3 and 5%Ag-ZnO- La2O2CO3 molar ratio from 0 to 0.25, and then decreases at La/Al ¼ 0.5 (Fig. 9). photocatalysts. Thus, the La/Al ¼ 0.25 catalysts have the highest photocatalytic efﬁciency. The reason could be explained as follows. When La3þ is incorporated into the LDH structure, more surface defects could be optical properties of the ZnO-Al2O3, ZnO-La2O2CO3 and 5%Ag- produced and a space charge layer could be formed on the surface, ZnO-La2O2CO3 photocatalysts and the results are shown in which is beneﬁcial to hindering the recombination of the photo- Fig. 7. The photocatalysts show a broader absorption in the UV induced electron/hole pairs. This result contributes to the region. It can be clearly seen from ﬁgure that the maximum of improvement of the photocatalytic activity of the La/Al ¼ 0.25 the absorbance band increases with the incorporation of La into catalysts, compared with that of La/Al ¼ 0 samples [49]. However, the lattice and while increasing the Ag doping to 5%. Conse- the further increase of the La3þ incorporation (La/Al ¼ 0.5) is likely quently, the 5%Ag-ZnO-La2O2CO3 nanocomposite could have to form more chemical bonds of Zn-O-La and the aggregation of reasonable activity under the UV-light irradiation compared LaOHCO3, and the role of the formed surface charge region is with ZnO-La2O2CO3 and ZnO-Al2O3, respectively. When the negatively inﬂuenced, which fail to efﬁciently separate the photo- absorption intensity increases, the formation rate of the induced electronehole pairs [49]. From the above analysis, it can electronehole pairs on the photocatalyst surface also increases, be concluded that the optimal concentration of lanthanum is La/ leading to the higher photocatalytic activity [47]. Al ¼ 0.25 in our work. This composition is thought to be appro- Likewise, the ZnO-La2O2CO3 and 5%Ag-ZnO-La2O2CO3 catalysts priate for the transfer of electrons and holes during the photo- also show absorption in the zone of 420e800 nm. This indicates catalytic reaction. that the presence of La and the metallic Ag can improve the visible light harvesting of the photocatalysts. This result can be attributed 3.2.3. Effect of Ag doping to the formation of the synergic effect between Ag and ZnO- For increasing the photocatalytic activity of the best catalyst La2O2CO3. 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, 3 and 5 wt 3.2. Photocatalytic study %) using ceramic process. The results illustrated in Fig. 10 reveals that the doped catalysts display excellent photocatalytic perfor- 3.2.1. Effect of calcination temperature on the adsorption/ mance compared to the undoped ones. It can be seen from the photocatalytic degradation ﬁgure that the degradation rate of caffeine slightly increased with The effect the of calcination temperature on the photocatalytic the increase of Ag doping. The experimental results indicate that activity of different as-prepared photocatalysts was assessed by the high amount of Ag (5%) shows the highest catalytic activity. monitoring the changes in the optical absorption spectra of the After 40 min of irradiation, the complete degradation of caffeine caffeine solution during the photodegradation process. Fig. 8 shows was done. the adsorption/photodegradation of the caffeine under UV irradi- The photocatalytic performance of 5%Ag-ZnO-La2O2CO3 was ation in the presence of the catalysts calcined at different temper- much higher than that of some photocatalysts cited in the literature. atures of 300, 400, 500, 600, 800 and 1000 C. Recently, Rimoldi et al [50] reported that the photocatalytic perfor- Notably, darkness adsorption tests were performed for the cat- mance of TiO2 photocatalyst for the degradation of caffeine reached alysts calcined at various temperatures before the photocatalytic 90% disappearance after 360 min of irradiation (with characteristic degradation tests under irradiation. The rate of adsorption parameters of C0 ¼ 35 mg/L; 0.5 g/L of TiO2). In the work of Chuang et increased with increasing the calcination temperature up to 500 C. al. [51] for example, the initial concentration of caffeine of 20 mg/L At T 600 C, the adsorption capacity was very low. According to almost completely degraded within 360 min in the presence of many results reportein literature [48], the reconstruction process synthesized TiO2. With 1%Mg doped ZnO-Al2O3 catalyst, 98.9% of avails oneself of the memory effect, the ability to recover the caffeine solution (20 mg/L) was removed after 70 min of irradiation original LDH structure, obtained by the LDH precursor mildly cal- [52]. cinated around 500 C, and immersed in a solution of the anion to Fig. 11 also indicates that the adsorption rate decreases with the be intercalated. This behavior was conﬁrmed by the strong corre- increasing Ag doping concentration. Therefore, the increase in the lation between the adsorbed quantity and the photocatalytic photocatalytic activity of catalysts is mainly due to the synergistic degradation rate shown in Fig. 9 at T 500 C. effects between the Ag noble metal and ZnO oxide. The relation After the irradiation, as can be observed from Fig. 8, the caffeine between the amount of Ag in the catalyst and the photocatalytic degradation efﬁciency of the catalysts increases with the increasing degradation rate can be explained by the fact that Ag acts as an calcination temperature from 300 to 500 C. This can be interpreted electron trap. The electrons generated on the ZnO-La2O2CO3 surface by the adsorption capacity and the beginning formation of the by the UV light irradiation quickly move to the Ag particles active phase, the ZnO oxide for the zero molar ratio (La/Al ¼ 0), and to facilitate the effective separation of the photogenerated elec- the formation of ZnO and La2O2CO3 for La/Al ¼ 0.25 and 0.5. trons and holes, resulting in the signiﬁcant enhancement of
A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 41 Fig. 8. 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 elec- 3.2.4. Effect of the photocatalyst dose tron acceptor, more acceptor centers are provided with the In order to avoid the excess catalyst and to ensure the total increasing Ag-doping, and therefore, the degradation rate for absorption of efﬁcient photons, a series of experiments was carried caffeine increases with the increase of the Ag content. The UV-Vis out to assess the optimum catalyst loading by varying the amount analysis results conﬁrms the synergism between the ZnO- of the best photocatalyst (5%Ag-ZnO-La2O2CO3) from 0.1 to 1.5 g/L. La2O2CO3 catalyst and the Ag nanoparticles for the photo- Experiments were done in 20 mg/L caffeine aqueous solution at degradation activities. solution pH of 7.5. After 40 min of the UV irradiation, the
42 A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 Fig. 9. 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 reduction of the light penetration into the solution. The same effect given in Fig. 12 show that the increase of the catalyst dose from 0.1 was observed by Elhalil et al [34]. From a practical viewpoint, the to 0.3 g/L resulted in an increase in the photocatalytic degradation optimum dosage of 0.3 g/L was chosen in further experiments. 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 3.2.5. Effect of the initial solution pH observed. This can be explained by the fact that the excess photo- The effect of the solution pH on the photocatalytic oxidation of catalyst dose resulted in an unfavorable light scattering and a caffeine in the presence of 5%Ag-ZnO-La2O2CO3 was studied at pH
A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (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 so- lution (~7.5)). of 3.5, 7.5 and 9.5. Fig. 13 shows that solution pH affects signiﬁcantly found to be 8.97. Therefore, at pH > 8.97 the surface acquires the percentage degradation of caffeine. The photocatalytic activity negative charge, favoring the adsorption of cationic molecules, was enhanced at pH of 9.5 and was dramatically decreased at pH of while at pH < 8.97, the surface of the catalyst acquires positive 3.5. Generally, the solution pH affects at the same time the surface charge, favoring the adsorption of anionic molecules. The pKa of charge of the photocatalyst and the ionization of caffeine molecules caffeine molecules is 10.4 which means that the molecule is fully in the solution. The pHpzc of 5%Ag-ZnO-La2O2CO3 catalyst was protonated at pH < 10.4. Fig. 12. Effect of catalyst dose on the photocatalytic degradation of caffeine after Fig. 13. Effect of the initial solution pH on the photocatalytic degradation of caffeine 40 min of irradiation (Caffeine concentration: 20 mg/L; initial solution pH: ~7.5). (Caffeine concentration: 20 mg/L; photocatalyst dosage: 0.3 g/L).
44 A. Elhalil et al. / Journal of Science: Advanced Materials and Devices 4 (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 re- cycling. As shown in the ﬁgure, the 5%Ag-ZnO-La2O2CO3 nano- gion of studied pH values, the observed behavior could only be composite reveals a high photostability in these experiments, due to the modiﬁcation of the proprieties of the photocatalysts. although a slight decrease of activity is observed compared to The observed trend of the photocatalytic activity observed at pH the ﬁrst-run result (6.42%). This can be attributed to residual of 9.5 could be due to the favorably enhanced adsorption of caffeine adsorbed on the surface of the catalyst. The results caffeine on the photocatalyst at pH values between 8.97 and 10.4 suggest that the 5%Ag-ZnO-La2O2CO3 nanocomposite may have and the more efﬁcient formation of hydroxyl radicals. In the acidic practical application potentials as an effective and stable pho- medium (pH ¼ 3.5), the decrease of the percentage degradation tocatalyst for degradations of different pharmaceutical pollut- could be attributed to many phenomena simultaneously inter- ants under UV irradiation. vening: a) non favorable adsorption, b) the dissolution of the photocatalysts and c) the photodecomposition and dissolution of 3.2.7. Comparison of the photoactivity with that of the pure ZnO ZnO according to the following equations [55]: and P-25 For comparison, the pure ZnO was prepared and studied under Dissolution: ZnO þ 2Hþ / Zn2þ(aq) þ H2O (2) the same reaction conditions. Titanium dioxide in powder form is largely used as a catalyst in the ﬁeld of the photocatalytic degra- Photodecomposition: ZnO þ 2hþVB / Zn2þ(aq) þ O* (3) dation of organic pollutants in water. One of the most efﬁcient commercial photocatalysts is P-25 (TiO2) manufactured by ﬂame 3.2.6. Efﬁciency of the regenerated photocatalyst hydrolysis, which was used in this study as a reference photo- Generally, recycling of the photocatalyst is crucially important catalyst. Fig. 15 shows the comparison of the photocatalytic activity for industrial applications [56,57]. In order to determine the of the best photocatalyst 5%Ag-ZnO-La2O2CO3 with that of the pure recyclability of the best catalyst (5%Ag-ZnO-La2O2CO3), we car- ZnO and P-25 (TiO2) in the same experimental conditions. ried out a cycle of experiment under identical conditions. In each After 40 min of irradiation, the pure ZnO exhibits a moderate experiment, after using the nanocomposite in one cycle, it was photocatalytic activity with the degradation percentage of washed with distilled water and dried at 100 C for 24 h. As 39.24%. The reference P-25 catalyst exhibits a better photo- illustrated in Fig. 14, the photocatalytic activity of the prepared catalytic efﬁciency than the pure ZnO, while reaching 48.89% photocatalyst still maintains a high level even after 3 times disappearance of caffeine after 40 min of irradiation. The 5%Ag- ZnO-La2O2CO3 photocatalyst synthesized from layered double hydroxides precursor enhances greatly the photocatalytic ac- tivities to 99.4% after 40 min of irradiation. This further conﬁrms the judicious choice of the LDH structure as precursors of photocatalysts. 4. Conclusion 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, 3 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 Fig. 15. Comparison of the photocatalytic activity of 5%Ag-ZnO-La2O2CO3 with that of degradation of caffeine as a model of pharmaceutical pollutant the pure ZnO and the commercial Degussa P-25 (Caffeine concentration: 20 mg/L; in aqueous solution under UV irradiation. The Ag-ZnO-La2O2CO3 photocatalyst dosage: 0.3 g/L; pH of the natural solution (~7.5)). photocatalysts exhibited an excellent photocatalytic activity
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