# 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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## 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

2. 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 g1), their high speciﬁc catalysts were synthesized by calcination of the LDH materials at 300, surface area (20e120 m2 g1), 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, NO3, 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%).