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.
Trang 1Original Article
Double Hydroxide structure with excellent photocatalytic
performance for the degradation of pharmaceutical compounds
A Elhalila,*, R Elmoubarkia, M Farnanea, A Machrouhia, F.Z Mahjoubib, M Sadiqa,
a Laboratoire des Sciences des Materiaux, des Milieux et de la Modelisation, Universite Sultan Moulay Slimane, F.P Khouribga, B.P 145, 25000, Khouribga,
Morocco
b Universite Sultan Moulay Slimane, Faculte des Sciences et Techniques, Beni Mellal, Laboratoire de Spectro-chimie Appliquee et Environnement (LSCAE),
B.P: 523, Beni - Mellal, Morocco
c Equipe de Catalyse et Environnement, Departement de Chimie, Faculte des Sciences, Universite Ibn Zohr, B.P.8106 Cite Dakhla, Agadir, Morocco
a r t i c l e i n f o
Article history:
Received 11 September 2018
Received in revised form
20 December 2018
Accepted 4 January 2019
Available online 6 January 2019
Keywords:
Layered double hydroxides
Photocatalyst
Doping
Photocatalytic degradation
Caffeine
a b s t r a c t
In this work, we have prepared the Ag-ZnO-La2O2CO3nanomaterials 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/(Alþ 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 1000C 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 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, 3 and 5 wt%) using the ceramic preparation process to obtain the desired Ag-ZnO-La2O2CO3catalysts 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
P-25 catalysts The experimental results show that the highest photocatalytic activity was obtained from the
Ag (5%) doped Zn-0.75Al-0.25La-CO3photocatalysts calcined at 500C 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
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
phar-maceutical industry, by the secretion of non-metabolized drugs and
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 environ-ment 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
* Corresponding author Fax: þ212 523 49 03 54.
E-mail address: elhalil.alaaeddine@gmail.com (A Elhalil).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
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
Journal of Science: Advanced Materials and Devices 4 (2019) 34e46
Trang 2beverages and foods like coffee, tea, energetic drinks, coke
and chocolate[4] Besides, caffeine is also used in several
phar-maceutical 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]
Further-more, 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
neces-sary 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/hþ) pairs The generated e/hþ pairs enable a
series of reductive and oxidative reactions[14] During this process,
hydroxyl radicals are formed from the water oxidation by holes (hþ)
[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
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],
chem-ical vapor deposition [25], photo-chemical reduction [26],
co-precipitation[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 afine 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 g1), their high specific
surface area (20e120 m2g1), the presence offillers on the surface,
and especially the tradability of interlayered anions[29]
The general formula of a LDHs is: [MII1-xMIIIx(OH)2]xþ
(An-x/n).mH2O,
where MIIrepresents a divalent cation (Zn2þ, Ni2þ, Mg2þ, Mn2þ,
Fe2þ…), MIIIrepresents a trivalent cation (Al3þ, Fe3þ, Co3þ, Cr3þ,
Mn3þ…), An the compensating anion (Cl, NO
3, 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
coef-ficient, x, is the molar fraction, expressed in terms of [MIII/
(MIIþ 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
form complexes with various Lewis bases, such as amines, alde-hydes, and alcohols If it is combined with a ZnO matrix, this may provide means for pollutants to be adsorbed onto the semi-conductor surface[32]
In this work, a series of ZnAl1-xLax(CO3) (0 x 0.5) LDH ma-terials at different (r) La/Al molar ratios (r¼ 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, 3 and 5 wt%) using the ceramic process The catalysts were charac-terized 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 pho-tocatalytic activity was evaluated in detail
2 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 (Ger-many) 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
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/(Alþ La) of 3 and La/Al ¼ 0, 0.25 and 0.5 (at the total concentration of metal ions of 2 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 drop-wise to the stirred salt solution until the final pH value reached 8.5± 0.2 for La/Al ¼ 0 and 10 ± 0.2 for La/Al ¼ 0.25 and 0.5 The gel formed was stirred vigorously for 4 h and then transferred into an autoclave and hydrothermally treated at 75C for 16 h Finally the precipitates werefiltered and washed several times with bi-distilled water to be neutralized and dried at 100C for 24 h The photo-catalysts were synthesized by calcination of the LDH materials at 300,
400, 500, 600, 800 and 1000C for 6 h in a muffle furnace The final catalysts were consecutively named as Zn-Al-T for LDH with r¼ 0; Zn-0.75Al-0.25La-T for r¼ 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 previ-ous work[34] Desired amounts of the LDH precursor and AgNO3
were manually ground in an agate mortar for 30 min After that the homogeneously mixed powder was transferred into a crucible and calcined in air at 500 C for 6 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 5 wt%)
A Elhalil et al / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 35
Trang 32.3 Characterization
The XRD measurements were performed at room temperature
ge-ometry, using CuKa target (l¼ 0.15406 nm) operated at 30 KV
and 10 mA The XRD scans were recorded in the 2qrange 5e80
with step size 0.01(0.5s/step) Fourier transform infrared (FTIR)
spectra in KBr pellets were collected on a Perkin Elmer
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
Experi-ments 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 2 L
and was initiated by an UV mercury lamp (400 W) placed in the
center of the reactor The temperature was maintained at 25± 2C
by connecting the reactor to the circulating water for preventing
the lamp from overheating Before the irradiation, the mixtures
were vigorously stirred for 60 min in the darkness to establish an
adsorption/desorption equilibrium on the surface of the catalysts
The adsorbed quantity was calculated using the following
equations:
Qads¼ðC0 CÞ
where Qads (mg/g) is the quantity of caffeine adsorbed per unit
mass of adsorbent, C0(in mg/L) is the initial caffeine
concentra-tion, 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, 3 mL
aliquots were sampled andfiltered to remove the solid particles
Thefiltrates were analyzed using a double-beam scanning
spec-trophotometer (Shimadzu specspec-trophotometer, model Biochrom)
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
precursors are shown inFig 1 Thefigure 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/Al¼ 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 n49-0981)[38]appears due to the excess of Lanthanum
The lattice parameters (a and c) calculated for each precursor are shown in Table 1 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 0 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/Al¼ 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 2shows 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 T¼ 300C, 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 2q¼ 31.8, 34.5, 36.3, 47.6, 56.6, 62.9, 66.4, 68and 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 Al2O3is amorphous[40]
By increasing the calcination temperature of the Zn-Al-CO3
precursors, the characteristic reflections of the mixed composite ZnO-ZnAl2O4appear at 600C and they became sharper with the rise in the calcination temperature up to 1000C, indicating the increase in the crystallite size in the sample These characteristic peaks are observed at 2qof 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 ZnAl2O4phase (JCPDS Card No 05-0669)[41]
The characteristic diffraction peaks of the samples with La/
Al¼ 0.25 and 0.5, calcined at 300C matched those of the
ortho-rhombic lanthanum hydroxycarbonate LaCO3OH phase At
A Elhalil et al / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 36
Trang 4Table 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)
A Elhalil et al / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 37
Trang 5patterns of the monoclinic lanthanum dioxycarbonate La2O2CO3
[38]are observed at the 2qangles 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 n48-1113)[42] The
formation of the La2O2CO3phase 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, 64.44and 77.40are 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 4shows 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
stretching vibration of the carbonate anions CO3 In the
low-frequency region, the bands in the range 700 and 400 cm1are
assigned to the metal-oxygen-metal vibrations[46] After
calcina-tion (T 600C), the typical bands of CO3(1364 cm1) still exist.
(T ¼ 300 C), which was further converted to La2O2CO3 at
400e600C 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
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
160e240C This mass loss corresponds to the hydration water
from the interlayer region A third step, extending up to 320C, 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~800C The endothermic peak is observed in the temperature range of
325e525C This peak is attributed to the loss of one H2O molecule
and one of CO2from 2LaOHCO3: 2LaOHCO3/ La2O2CO3þ CO2þ H2O The last mass loss is attributed to the loss of a further CO2
molecule and the formation of the LaAlO3oxide:
La2O2CO3þ Al2O3(amorphous phase)/ 2LaAlO3þ CO2
The formation of LaAlO3at 800C was also confirmed by the XRD data.Table 2compares the experimental and the calculated weight losses
Fig 3 XRD patterns of the fresh LDH precursor, undoped and Ag doped
ZnO-Fig 4 FTIR spectra of the fresh and calcined LDH materials at different temperatures.
A Elhalil et al / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 38
Trang 63.1.4 SEM/EDX observation
The morphology and microstructure of the ZnO-La2O2CO3and
5%Ag-ZnO-La2O2CO3samples 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-La2O2CO3composite
The Energy Dispersive X-ray Spectra (EDX) of the undoped and the
Ag doped composite are shown inFig 6(b,d) The results confirm the presence of Zn, Al, La, C and O in the undoped sample For the 5%Ag-ZnO-La2O2CO3composite, the spectrum shows peaks corre-sponding 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
Fig 5 TGA/DTA curves of the different LDH precursors.
Table 2
Molar ratios La/Al and cell parameters (a and c).
LDH 1 st mass loss (~200 C) 2 nd mass loss (~273 C) 3 rd mass loss (~482 C) 4 th mass loss (~800 C) Total mass loss
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%
Fig 6 SEM-EDX images of undoped (a, b) and 5%Ag doped ZnO-La 2 O 2 CO 3 (c, d).
A Elhalil et al / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 39
Trang 7optical properties of the ZnO-Al2O3, ZnO-La2O2CO3 and
5%Ag-ZnO-La2O2CO3 photocatalysts and the results are shown in
Fig 7 The photocatalysts show a broader absorption in the UV
region It can be clearly seen fromfigure that the maximum of
the absorbance band increases with the incorporation of La into
the lattice and while increasing the Ag doping to 5%
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-La2O2CO3and 5%Ag-ZnO-La2O2CO3catalysts
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
ZnO-La2O2CO3
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 8shows
the adsorption/photodegradation of the caffeine under UV
irradi-ation in the presence of the catalysts calcined at different
temper-atures of 300, 400, 500, 600, 800 and 1000C
Notably, darkness adsorption tests were performed for the
cat-alysts calcined at various temperatures before the photocatalytic
degradation tests under irradiation The rate of adsorption
increased with increasing the calcination temperature up to 500C
At T 600C, 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
cal-cinated around 500C, and immersed in a solution of the anion to
be intercalated This behavior was confirmed by the strong
corre-lation between the adsorbed quantity and the photocatalytic
degradation rate shown inFig 9at T 500C.
After the irradiation, as can be observed fromFig 8, the caffeine
degradation efficiency of the catalysts increases with the increasing
calcination temperature from 300 to 500C 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/Al¼ 0), and
However, the photoactivity drastically decreases by continuous calcination at temperatures of 600 up to 1000C This result can be attributed to the transformation of a partition of ZnO to ZnAl2O4for La/Al¼ 0 and of La2O2CO3to LaAlO3for La/Al¼ 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 molar ratio from 0 to 0.25, and then decreases at La/Al¼ 0.5 (Fig 9) Thus, the La/Al¼ 0.25 catalysts have the highest photocatalytic
efficiency The reason could be explained as follows When La3þis 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 photo-induced 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/Al¼ 0 samples[49] However, the further increase of the La3þincorporation (La/Al¼ 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 photo-induced electronehole pairs[49] From the above analysis, it can
be concluded that the optimal concentration of lanthanum is La/
Al¼ 0.25 in our work This composition is thought to be appro-priate for the transfer of electrons and holes during the photo-catalytic reaction
3.2.3 Effect of Ag doping For increasing the photocatalytic activity of the best catalyst with the molar ration La/Al¼ 0.25, calcined at 500C, the material
was doped by Ag noble metal with different amounts (1, 3 and 5 wt
%) using ceramic process The results illustrated inFig 10reveals that the doped catalysts display excellent photocatalytic perfor-mance 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 min 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 perfor-mance of TiO2photocatalyst for the degradation of caffeine reached 90% disappearance after 360 min of irradiation (with characteristic parameters of C0¼ 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 min 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 min of irradiation
[52]
Fig 11also 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-La2O2CO3surface
by the UV light irradiation quickly move to the Ag particles
to facilitate the effective separation of the photogenerated
Fig 7 UVeVis DRS for the ZnO-Al 2 O 3 , ZnO-La 2 O 2 CO 3 and 5%Ag-ZnO- La 2 O 2 CO 3
photocatalysts.
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Trang 8photocatalytic activity[53,54] Ag plays a positive role as an
elec-tron 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
ZnO-La2O2CO3 catalyst and the Ag nanoparticles for the
photo-degradation 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 min of the UV irradiation, the
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)).
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Trang 9photocatalytic degradation efficiency (%) was evaluated Results
given inFig 12show 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
photo-catalyst 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-LaOCO was studied at pH
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)).
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Trang 10of 3.5, 7.5 and 9.5.Fig 13shows 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 10 Photocatalytic degradation of caffeine in the presence of undoped and Ag doped ZnO-La 2 O 2 CO 3 (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)).
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
A Elhalil et al / Journal of Science: Advanced Materials and Devices 4 (2019) 34e46 43