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Microwave-assisted polyol synthesis and characterization of pvp-capped cds nanoparticles for the photocatalytic degradation of tartrazine
Accepted Manuscript Title: Microwave-Assisted Polyol Synthesis and Characterization of PVP-Capped CdS Nanoparticles for the Photocatalytic Degradation of Tartrazine Author: Maher Darwish Ali Mohammadi Navid Assi PII: DOI: Reference:
S0025-5408(15)30194-X http://dx.doi.org/doi:10.1016/j.materresbull.2015.11.002 MRB 8482
To appear in:
MRB
Received date: Revised date: Accepted date:
16-6-2015 1-11-2015 3-11-2015
Please cite this article as: Maher Darwish, Ali Mohammadi, Navid Assi, MicrowaveAssisted Polyol Synthesis and Characterization of PVP-Capped CdS Nanoparticles for the Photocatalytic Degradation of Tartrazine, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.11.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microwave-Assisted Polyol Synthesis and Characterization of PVP-
1
Capped CdS Nanoparticles for the Photocatalytic Degradation of
2
Tartrazine
3
Maher Darwish1, Ali Mohammadi1, 2*, Navid Assi1
4 5
1
Department of Drug and Food Control, Faculty of Pharmacy, Tehran University of 6
Medical Sciences, Tehran, Iran 2
7
Nanotechnology Research Centre, Faculty of Pharmacy, Tehran University of 8
Medical Sciences, Tehran, Iran
9 10
Corresponding author:
11
Ali Mohammadi (Pharm.D, Ph.D)
12
Associate Professor of Pharmaceutical Analysis, Department of Drug & Food Control
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and Nanotechnology Research Centre, Faculty of Pharmacy, Tehran University of
14
Medical Sciences,
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P.O.Box:14155-6451, Tehran, Iran
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Phone/Fax: +98.21.88358801
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E-mail: [email protected]
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Maher Darwish (M.Sc, Ph.D Student)
19
Department of Drug & Food Control, Faculty of Pharmacy, Tehran University of
20
Medical Sciences.
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Phone: +98.93.87563368
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E-mail: [email protected]
23
Navid Assi (Analytical chemistry, Ph.D Student)
24
1
Phone: +98.91.22060649
1
E-mail: [email protected]
2 3
Graphical abstract
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5 6
2
Highlights PVP-stabilized CdS nanoparticles have been fabricated by a
1
2
polyol-microwave method.
3
CdS nanoparticles were characterized and the size was
4
approximately 48±10 nm.
5
Catalytic activity of our nanoparticles was examined for tartrazine
6
degradation.
7
Remarkable results were obtained under both UV and visible light
8
irradiations.
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10
3
ABSTRACT
1
Polyvinylpyrrolidone capped cadmium sulfide nanoparticles have been successfully 2 synthesized by a facile polyol method with ethylene glycol. Microwave irradiation 3 and calcination were used to control the size and shape of nanoparticles. 4 Characterization
with
scanning
electron
microscopy
revealed
a
restricted 5
nanoparticles growth comparing with the uncapped product, hexagonal phase and 48 6 nm average particle size were confirmed by X-ray diffraction, and finally mechanism 7 of passivation was suggested depending on Fourier transform infrared spectra.
8
The efficiency of nanoparticles was evaluated by the photocatalytic degradation of 9 tartrazine in aqueous solution under UVC and visible light irradiation. Complete 10 degradation of the dye was observed after 90 min of UVC irradiation under optimized 11 conditions. Kinetic of reaction fitted well to the pseudo-first-order kinetic and 12 Langmuir–Hinshelwood models. Furthermore, 85% degradation of the dye in 9h 13 under visible light suggests that cadmium sulfide is a promising tool to work under 14 visible light for environmental remediation.
15 16
KEYWORDS: B. chemical synthesis; A. nanostructures; A. chalcogenides; C. x-
17
ray diffraction; D. catalytic properties
18 19
4
1. INTRODUCTION
1
During the last two decades, photocatalytic materials received a large quantity of 2 research interest as they have a great potential to be applied in detoxification of 3 environmental organic pollutants and as a clean energy source [1]. Semiconductor 4 heterogeneous photocatalysts such as TiO2, ZnO, SnO2, WO3, Fe2O3, and CdS have 5 been intensively investigated in the area of water and air purification and in 6 remediation reactions [2].
7
Like other advanced oxidation procedures (AOPs), the common characteristic of 8 photocatalytic materials is the generation of very reactive species such as, principally 9 but not exclusively, hydroxyl radicals (HO•), which initiate a series of reactions 10 leading eventually to the destruction of the target pollutant [3]. Recently, 11 nanostructures of these photocatalysts have attracted more consideration as they are 12 expected to have higher photocatalytic activity than their bulk counterparts due to 13 their smaller size, higher surface area-to-volume ratio, and increased band-gab energy 14 which in turn lead to higher redox potentials [4].
15
Cadmium sulfide (CdS), a typical metal chalcogenide semiconductor with a direct 16 band gap Eg≈2.43 eV at room temperature [5], has become one of the most 17 considerable materials in research communities due to its diverse promising 18 applications in the field of solar cells, photoelectronic devices and photocatalysis [6]. 19 This material has shown better catalytic functions compared to those of TiO2 due to 20 the rapid generation of electron–hole pairs by photoexcitation [7]. Nevertheless, CdS 21 has the fatal photocorrosion problem due to the self-oxidation by the generated hole 22 [8]. To overcome such a problem, an effective approach is to cover the nanoparticle 23
5
core surface with a polymeric capping agent which might also help to stabilize the 1 surface, control the growth, and prevent agglomeration of the nanoparticles [9-11].
2
Many methods have been utilized for the synthesis of CdS nanostructures such as co- 3 precipitation [7, 12], polyol [13], solvothermal [14, 15], hydrothermal [16], non- 4 aqueous chemical method [17], and chemical bath deposition [18]. Among these 5 different processes, the polyol method appears as an easy to carry out with many other 6 advantages: a uniform shape, a narrow size distribution, and a low degree of 7 agglomeration [19]. This method generally uses poly-alcohol like Ethylene Glycol 8 (EG), Diethylene Glycol (DEG) or 1, 2-propanediol as both solvent and reducing 9 agent [20]. Furthermore, due to the relatively high dipole moment and loss factor of 10 polyol solvents, they are also suitable for microwave assisted route. In this regard, 11 using microwave in the synthesis of nanoparticles is considered a modern and rapidly 12 developing method. It has been evidenced that microwave irradiation gives a narrow 13 particle size distribution of nanocrystals with a high purity in a short reaction time in 14 addition of being cheap and environmentally friendly [21-23].
15
Tartrazine (Acid Yellow 23; FD&C Yellow No. 5), as an azo dye, has been chosen for 16 this study due to its extensively use as a colorant in food, cosmetics, pharmaceuticals 17 and textile industry [24], as well as its high stability against biodegradation and 18 conventional wastewater treatment procedures after disposal from industrial effluent 19 [25]. The toxic concentration of tartrazine on human has been reported to be 7.5 mg.k- 20 1
[26]. This compound is known to cause allergic reactions such as asthma and 21
urticaria and appears to cause more allergic and intolerance reactions than other azo 22 dyes [27].
23
6
Previously reported works on the removal of tartrazine from aqueous solutions 1 include mainly conventional methods like adsorption [28, 29], ecocoagulation [30], 2 and filtration [31] which are known not to effectively degrade pollutant but merely 3 transfer it to another phase where it is more concentrated [32]. Whereas other reports 4 have shown that tartrazine could be degraded employing AOPs like: ozonation [33], 5 electrochemical oxidation [34], photo Fenton oxidation [35], UV/H2O2 [36, 37], 6 photolytic [38], and photocatalytic oxidation [27, 39-46].
7
Most of photocatalytic methods have utilized TiO2, which was extensively studied 8 and known to have some drawbacks such as expensive precursors and inability to 9 absorb visible light [47, 48]. Hence, the aim of the present work was first, to fabricate 10 CdS nanoparticles by a chemical synthesis utilizing the polyol method with ethylene 11 glycol as a solvent and polyvinylpyrrolidone (PVP) as a capping agent. Microwave 12 irradiation followed by calcination was used for size and shape controlling. Further, to 13 investigate the structure, size, and morphology of the synthesized products using 14 various characterization techniques. In second, the work aimed to study the catalytic 15 properties of these nanoparticles for oxidation of tartrazine, as an organic pollutant, in 16 aqueous solution under UVC and visible light irradiations, taking in consider the 17 effect of different parameters such as initial pH of solution, amount of nanoparticles, 18 and concentration of pollutant in the degradation process.
19 20
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2. EXPERMENTAL
1 2
2.1. Materials and reagents
All reagents used in our experiments were of analytical grade and used as received 3 without any further purification. Tartrazine powder (≥85 %) was obtained from 4 Sigma-Aldrich (Germany). Cadmium chloride 2.5-hydrate (CdCl2 2.5H2O) and 5 thiourea (SC(NH2)2) were obtained from Scharlau (Spain). Polyvinylpyrrolidone 6 (MW≈25,000 g.mol-1), ethylene glycol, glacial acetic acid, and ammonia solution 7 (25%) were obtained from Merck (Germany). Double distilled water was used for the 8 preparation of all samples.
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2.2. Synthesis of nanoparticles
In a typical process, 9 g of cadmium chloride 2.5-hydrate and 3 g of PVP were 11 separately dissolved in 30 mL and 20 mL of EG, respectively, and then heated with 12 stirring for 60 min at 75 °C. The two solutions were then mixed slowly and heated 13 with stirring for 40 min at 100 °C. Concurrently, 3 g of thiourea was dissolved in 20 14 mL of EG and heated with stirring for 100 min at 75 °C and then added slowly to the 15 hot solution of (PVP-Cd2+). The temperature of the new mixture was gradually raised 16 to 170-180 °C within 45 min and kept at this temperature until most of the solvent 17 was evaporated. In this stage, the color of the solution changed from light yellow into 18 a dark orange suspension indicating the formation of CdS. Next, the suspension was 19 microwaved (LG MC-2820S microwave) for 5 min with a power of 720 W and 50 % 20 duty cycle (30 sec of irradiation and 30 sec stop). The obtained powder was then 21 calcined in a furnace (Sybron Thermolyne Type 1500) for 120 min at 450 °C and 22 finally, black crystallites of PVP-capped CdS nanoparticles were obtained. The 23 scheme of preparation is illustrated in Fig. 1. A control synthesis in the absence of 24
8
PVP and the exact other steps was carried out to evaluate the impact of PVP on the 1 yielded product.
2 3
2.3. Characterization of the CdS nanoparticles
The synthesized CdS nanoparticles were characterized using various analytical 4 techniques. Scanning electron microscopy (KYKY-EM3200 SEM) was used to 5 investigate the morphologies of nanoparticles. X-ray diffraction patterns of the 6 nanoparticles were recorded by a Bruker AXS D8-ADVANCE diffractometer fitted 7 with a (Cu Kα λ = 1.5418 Å) radiation tube. The average crystallite size of the 8 synthesized CdS was calculated with well-known Scherrer’s equation (Eq. (1)) with 9 full width at half maxima (FWHM) of X-ray diffraction pattern: D= 0.89 λ /β cos θ
10
(1)
11
Where λ is the wavelength of X-ray radiation source, β is FWHM of the peak and θ is 12 Bragg’s diffraction angle.
13
Furthermore, in order to determine the chemical composition of nanoparticles and 14 study the behavior of PVP, Fourier transform infrared spectra in the range of 400– 15 4000 cm-1 of many samples during the synthesis process was recorded in transmission 16 mode (Thermo Nicolet 8700 FTIR), the powder samples were grounded with KBr and 17 compressed into a pellet for analysis.
18 19 20
2.4. Photocatalytic activity under UVC light
Photocatalytic activity of the nanoparticles was evaluated by measuring the 21 degradation of tartrazine which was used as a test pollutant. An amount of (5-50) mg 22 of the catalyst was dispersed in 200 mL of tartrazine aqueous solution (10-100) mg.L- 23 9
1
. The pH of the suspension was adjusted to the desired value by ammonia or glacial 1
acetic acid solutions and determined by a digital pH meter (Metrohm pH lab 827). 2 The suspension in the photoreaction container was exposed to UV light source (CH 3 Lighting T8 30W UVC) positioned 10 cm above for 90-180 min. At given intervals of 4 illumination, 5 mL samples were taken out and centrifuged (Hettich EPA 12) at 2,500 5 rpm for 10 min in order to completely remove all nanoparticles. The degradation ratio 6 was monitored using UV–Vis spectrophotometer (T80+ UV-VIS Spectrometer PG 7 Instruments Ltd). According to the Beer-Lambert law, the concentration of tartrazine 8 is proportional to its absorbance. Hence, the degradation efficiency was calculated by 9 the following formula:
10
D%=[(C0-C)/C0]×100=[(A0-A)/A0]×100
(2)
11
C0 and A0 are the initial concentration and absorbance, respectively, while C and A 12 are the concentration and absorbance after intervals of illumination (t). The reaction 13 rate constant (k) was calculated using plots of ln(C/C0) versus illumination time 14 according to the following formula:
15
ln(C/C0)=-kt
(3)
16 17
2.5. Photocatalytic degradation and mineralization under visible light
After obtaining the optimal conditions for tartrazine degradation with the synthesized 18 nanoparticles under UVC, an assessment of the activity of these nanoparticles under 19 visible light was carried out using (OSRAM HWL 160W) lamp in the same 20 experimental setup used under UVC irradiation. In addition, a light protected 21 experiment under the same conditions of visible light experiment was carried out and 22 considered as a blank to assess the mineralization of tartrazine after same time of 23
10
irradiation. Samples were analyzed by a total organic carbon analyzer type (TOC-V, 1 Shimadzu).
2
3. RESULTS AND DISCUSSION
3 4
3.1. Morphology and structure
The SEM images presented in Fig. 2 show that PVP-capped CdS (PVP-CdS) particles 5 are in the range of nanometer with narrower particle size distribution (40-59 nm) and 6 less agglomeration comparing to those of the uncapped CdS which has an average 7 particle size of 115 nm (75-180) indicating clearly the advantages of PVP in 8 restricting the growth and preventing agglomeration. This is simply due to the stereo- 9 hindrance effect resulted from repulsive force among the polyvinyl groups of PVP 10 and due to the restrained Ostwald ripening kinetics in such a way that the growth rate 11 was decreased with the size of CdS nanoparticles. Consequently, smaller particles 12 with enhanced monodispersity were obtained.
13
X-ray powder diffraction patterns of the capped nanoparticles presented in Fig. 3 14 show a perfect match with the hexagonal phase of CdS (JCPDS No. 01-089-2944). 15 Diffraction peaks of monoclinic sulfur are also found (JCPDS No. 01-074-2108). The 16 average crystallite size of the capped nanoparticles calculated from XRD patterns 17 using the Scherrer’s equation was about 48 nm which is in agreement with the result 18 obtained from SEM image.
19
FTIR spectra of pure PVP and PVP-CdS prior to and after calcination are shown in 20 Fig. 4. In pure PVP spectrum, the O-H stretching vibration observed as a broad and 21 strong peak at 3517 cm-1 is due to H2O absorption on the surface of the sample. The 22 two bands at 2952 and 1423 cm-1 correspond to C-H asymmetric stretching vibration 23 and bending vibration, respectively. The broad peak with high intensity at 1665 cm-1 24 11
is ascribed to the C=O bond stretching and the band at 1286 cm-1 corresponds to C–N 1 bond stretching of the pyrrolidone structure.
2
Two observations can be found when comparing pure PVP spectrum with the other 3 two spectra. First, a decrease in all peaks intensities after exposing the sample to 4 microwave irradiation and more decrease after calcination. This can be explained in 5 the light of thermal degradation of PVP. Loría-Bastarrachea et. al [49], in agreement 6 with earlier reports, stated that at 450 °C the main product obtained from thermal 7 degradation of PVP is its corresponding monomer, i.e., vinyl pyrrolidone or less 8 unsaturated compounds such as PVP oligomers. The second observation is the shift to 9 lower wavenumber of some peaks, specifically C=O and C–N stretching vibrations. 10 These shifting indicate the interaction between PVP and Cd2+. The mechanism of this 11 interaction as suggested by Abdelghany et. al [50] implies the formation of coordinate 12 bonds between nitrogen and/or oxygen atoms of the PVP with cadmium ions in four 13 different probabilities as shown in Fig. 5. Hence, the preceding findings indicate the 14 formation of short polymeric chain-capped CdS nanoparticles.
15 16 17
3.2. Effect of tartrazine solution initial pH
The pH of the reaction medium is one of the most crucial parameters in photocatalytic 18 degradation of organic pollutants. It affects the surface charges of both the catalyst 19 and pollutant and consequently the adsorption of the pollutant on catalyst surface 20 [51].
21
In order to study the role of pH on the adsorption of tartrazine on the catalyst surface, 22 20 mg.L-1 solutions of the dye and a fixed amount of PVP-CdS (25 mg) were set at 23 pH 3.5, 5.5, 7 or 11 and kept in dark place with constant stirring for 90 min in order to 24 12
reach the equilibrium adsorption. In addition, photolysis experiments, in the absence 1 of catalyst and presence of UVC light with the same other experimental setup used in 2 the adsorption study, were carried out also over 90 min The role of pH on 3 photocatalytic degradation of tartrazine was investigated in the same conditions of 4 photolysis in the presence of 25 mg of PVP-CdS or uncapped CdS over 90 min. The 5 results are shown in Fig. 6.
6
As can be seen, a decrease in the dye concentration between 3-8% was obtained 7 among the different pH values in the presence of PVP-CdS only without UVC 8 irradiation. Whereas about 4.5-12% of the initial tartrazine concentration was 9 removed at different pH values in the presence of UVC irradiation alone without 10 nanoparticles. On the other hand, maximum photocatalytic degradation percentage of 11 PVP-CdS was observed in alkaline region. The degradation increased with increasing 12 pH and reached its maximum value at pH=11. For uncapped CdS, neutral medium 13 was favorable.
14
Our suggested interpretation is the following: tartrazine has a pKa=9.4 and so is 15 negatively charged in alkaline medium due to its ionization to the related phenyl salt. 16 likewise, CdS has a zero point charge at about pH=7 and is also negatively charged in 17 pH (8-12). As a result, the adsorption of tartrazine on CdS surface is minimum at 18 pH=11. Here, the enhancement of capped catalyst activity in alkaline medium could 19 be imputed to the attachment of CdS with pyrrolidone ring that might have acted as a 20 mediator to trap the carriers generated in the catalyst and adsorb the water or dye 21 molecules or even the abundant hydroxyl ions, which would subsequently facilitate 22 the generation of hydroxyl radicals that initiate the oxidization of the pollutant.
23
13
Unsurprisingly, PVP-CdS has shown a superior activity over uncapped CdS in 1 extreme acidic and basic conditions whereas in neutral conditions, the activities were 2 nearly equivalent. This is ascribed to two factors: first, the smaller particles size that 3 offered more surface area for photocatalytic reaction in the case of PVP-CdS and 4 second, the photocorrosin or dissolution that uncapped CdS might underwent in 5 extreme conditions which has reduced drastically its activity. Meanwhile, PVP-CdS 6 maintained an approximate percentages of degradation in acidic and neutral 7 conditions as a consequence of PVP passivation.
8 9 10
3.3. Effect of the catalyst loading
Catalyst loading is a critical parameter in photodegradation of organic pollutants. It 11 affects the reaction rate and consequently the cost of treatment [52].
12
The effect of PVP-CdS dosage on the degradation of tartrazine was investigated in the 13 presence of different amount of catalyst (5–50 mg) with 50 mg.L-1 initial 14 concentration of tartrazine at pH=11 for 90 min of irradiation. The results are shown 15 in Fig. 7.
16
As is noted, the photodegradation of the tartrazine was found to increase with the 17 increased catalyst loading within the range of study. Increasing the catalyst loading 18 gave a rise to a higher number and density of nanoparticles, which in turn caused 19 more photons to be absorbed and dye molecules to be adsorbed leading to better 20 degradation [53]. The opacity of the suspension was not greatly affected by the 21 increased amount of PVP-CdS and no hindering to light penetration was observed. On 22 the other hand, after 25 mg of catalyst amount there was no considerable enhancement 23
14
in the degradation profile. Hence, 25 mg of PVP-CdS nanoparticles was chosen to be 1 the optimal amount for further studies.
2 3 4
3.4. Effect of the initial concentration of tartrazine
The influence of pollutant concentration on photocatalytic degradation has also been 5 studied. Experiments have been carried out at different initial concentrations in the 6 range (10-100) mg.L-1 with constant amount of nanoparticles (25 mg) and initial 7 solution's pH=11. The results shown in Fig. 8 indicate that increasing the 8 concentration of tartrazine had decreased the degradation from 100% (10 mg.L-1) to 9 92.9% (50 mg.L-1) and 36.2% (100 mg.L-1) in 60 min.
10
This can be explained in the light of photocatalytic degradation mechanism; a single 11 layer of adsorbed water molecules exists on the surface of PVP-CdS. After photon is 12 adsorbed, the holes generated in valence band transfer to the surface and oxidize 13 water molecules and hydroxyl ions to produce HO• radicals. These radicals rapidly 14 attack tartrazine molecules and oxidize them to intermediates which in turn would be 15 mineralized with subsequent attacks of HO• radicals.
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PVP-CdS + hν → eCB- + hVB+
(4)
17
h+ + H2O → HO• + H+
(5)
18
HO• + TA → Intermediates
(6)
19
HO• + Intermediates → CO2 + H2O
(7)
20
Increasing the initial dye concentration means more tartrazine molecules to surround 21 the nanoparticles and decrease the path length of the photons entering the solution 22 resulting in lower photon absorption by the catalyst and reaction sequence is slowed 23 15
down. Consequently, rate of degradation is decreased. At this juncture, we believe 1 that PVP passivation has an important role in the separation of the photogenerated 2 carriers. This is attributed to the prohibition of nonradiative decay of carriers by 3 covering dangling bonds that may work as exciton traps. Also, the coordination of 4 nanoparticles surface atoms which minimizes the trap states lie within the band gap 5 and increases quantum yield by providing alternative pathways of electron-hole 6 separation. More importantly, complexation between Cd and the lone pair of 7 nitrogen/oxygen makes pyrrolidone moiety of PVP act as a hole acceptor that traps 8 the photogenerated holes and transfers them to the target objects. It is worth 9 mentioning here that partial decomposition of PVP, during the synthesis process, was 10 beneficial for exposing trapping sites of pyrrolidone to those targeted objects to 11 promote the catalytic reaction sequence.
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3.5. Kinetic study
In order to study the pattern of the photodegradation reaction of tartrazine, Langmuir– 15 Hinshelwood (L–H) model was used. This model has been successfully used for 16 heterogeneous photocatalysis to describe the exact relationship between the rate of 17 degradation and the concentration of pollutant in photocatalytic reaction [31]. The rate 18 constant (k) of degradation reaction was calculated according to the formula (3). As 19 shown in Fig. 9, a linear relation between tartrazine concentrations and illumination 20 time has been observed implying a pseudo-first order kinetics of the photodegradation 21 reaction. The pseudo-first order rate constant decreased dramatically with increased 22 initial concentration of tartrazine. This is because, as aforementioned, with high 23 concentrations of the pollutant, less hydroxyl radicals in active sites of the catalyst are 24
16
generated and the majority of photons are absorbed by pollutant molecules rather than 1 catalyst [54].
2
In order to cover the adsorption properties of the substrate on the catalyst surface, the 3 experimental data have been rationalized in terms of the modified form of L–H 4 kinetic model. The modified Langmuir-Hinshelwood expression that explains the 5 kinetics of heterogeneous catalytic systems is given by:
6
r=-dC/dt=krKC/(1+KC)
(8)
7
Where r represents the rate of reaction that changes with time (mg.L−1.min−1), kr is the 8 reaction rate constant (mg.L−1.min−1), and K is the adsorption coefficient of the 9 reactant onto the catalyst particles (L. mg −1).
10
The applicability of L–H equation for the degradation has been confirmed by the 11 linear plot obtained by plotting the reciprocal of initial rates (1/r0) against reciprocal 12 of initial concentrations of tartrazine (1/C0) as shown in Fig. 10. 1/r0=(1/kr)+(1/krKC0)
13
(9)
14
In this study, a reasonable agreement (R2 = 0.969) was obtained between the 15 experimental results and the linear form of the L-H expression.
16
The values of kr and K have been determined from the slope and intercept of this plot 17 and found to be 1.908 and 0.036, respectively, indicating that photocatalytic 18 degradation is a dominant factor when compared with pollutant adsorption onto the 19 surface of the catalyst.
20 21 22
17
1
3.6. Reusability
Reusability of capped catalyst was investigated in order to verify its stability and the 2 protective role of PVP in preventing catalyst degradation. After a first round of 3 catalytic reaction, the mixture (catalyst+slurry) was filtered and washed with a 4 mixture of water and ethanol (1:1) then dried in an oven at 65 °C for 6 h. The 5 recovered catalyst was used again to degrade tartrazine under same conditions used 6 before. A third round was carried out in the same procedure. Results depicted in Fig. 7 11 shows a very slight deference in catalytic activity among the first used and reused 8 catalysts. A small change in the kinetic could be seen but, the overall degradation in 9 90 min of study was not importantly affected. It is our believe that photocorrosion 10 suppression is due to prolonged life time of electron–hole pairs as a consequence of 11 hole trapping and transferring mediated by PVP.
12 13 14
3.7. Photodegradation and mineralization under visible light irradiation
A 50 mg.L-1 solution of tartrazine with pH=11 was mixed with 25 mg of PVP-CdS 15 nanoparticles and exposed to visible light irradiation in the same experimental setup 16 used under UVC irradiation. As can be seen in Fig. 12, approximately 85% of the dye 17 was degraded and about 69% of the total organic content compared with the blank 18 solution was removed after 9 hours.
19 20 21 22 23 18
4. CONCLUSION
1
Polyol synthesis with microwave irradiation and calcination have been used as an 2 efficient method to fabricate cadmium sulfide nanoparticles in an average crystallite 3 size of about 48 nm when PVP was used as a capping agent and about 115 nm 4 without capping. The prepared nanoparticles have been effectively used as a 5 photocatalyst for the degradation of tartrazine in aqueous solution. Total degradation 6 of tartrazine was obtained after 90 min using 25 mg of the PVP-CdS with initial dye 7 concentration of 50 mg.L-1 and pH=11 under UVC irradiation. The role of PVP in 8 controlling the growth and stabilizing the nanoparticles was demonstrated by the 9 compassion with the uncapped product which has shown more growing and less 10 catalytic activity.
11
The Langmuir-Hinshelwood models indicated that tartrazine underwent a pseudo first 12 order kinetics of the reaction and that photocatalytic degradation was prevailing 13 compared with pollutant adsorption on PVP-CdS surface. In addition, results has 14 shown that our nanoparticles can work sufficiently under visible light with about 85% 15 abatement of initial tartrazine concentration and 69% mineralization after 9h of 16 irradiation.
17
In conclusion, PVP-CdS nanoparticles synthesized in this work might be used 18 effectively in environmental treatment processes to remove organic pollutants from 19 water. These nanoparticles are promising to work under visible light sources to reduce 20 greatly the expense of treatment.
21 22
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Acknowledgment
1
The authors wish to thank Tehran University of Medical Sciences for the financial 2 and instrumental support of this research.
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Figure Captions
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Fig. 1. A schematic of the preparation of PVP-CdS nanoparticles.
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Fig. 2. SEM images of (a) uncapped and (b) PVP-capped CdS nanoparticles.
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Fig. 3. XRD patterns of the PVP-CdS nanoparticles.
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Position [°2Theta] (Copper (Cu))
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Fig. 4. FTIR spectra of (A) pure PVP, (B) PVP-CdS after microwave irradiation and, (C) PVP-CdS after calcination.
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Fig. 5. Suggested mechanism of interaction between PVP and Cd2+. (Reference No.
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Fig. 6. Effect of different pH values (3.5–11) on adsorption, photolysis and, 1 photocatalytic degradation of tartrazine over 90 min. 2 3
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Fig. 7. Photocatalytic degradation of tartrazine by varying the amount of PVP-CdS nanoparticles (5–50 mg) with Tartrazine concentration (50 mg.L-1) and pH=11.
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Fig. 8. Photocatalytic degradation of varying initial concentrations of tartrazine (10-
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100 mg.L-1) with pH=11 and 25 mg amount of PVP-CdS nanoparticles.
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Fig. 9. plots of -ln(C/C0) versus illumination time for different concentrations of
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tartrazine.
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Fig. 10. plot of 1/r0 versus 1/C0 of the photocatalytic degradation of tartrazine.
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Fig. 11. Reusability of PVP-CdS nanoparticles 2
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Fig. 12. Photocatalytic degradation and mineralization of 50 mg.L-1 tartrazine with 1 pH=11 and 25 mg of PVP-CdS nanoparticles under visible light irradiation. 2
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S0025-5408(15)30194-X http://dx.doi.org/doi:10.1016/j.materresbull.2015.11.002 MRB 8482
To appear in:
MRB
Received date: Revised date: Accepted date:
16-6-2015 1-11-2015 3-11-2015
Please cite this article as: Maher Darwish, Ali Mohammadi, Navid Assi, MicrowaveAssisted Polyol Synthesis and Characterization of PVP-Capped CdS Nanoparticles for the Photocatalytic Degradation of Tartrazine, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.11.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microwave-Assisted Polyol Synthesis and Characterization of PVP-
1
Capped CdS Nanoparticles for the Photocatalytic Degradation of
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Tartrazine
3
Maher Darwish1, Ali Mohammadi1, 2*, Navid Assi1
4 5
1
Department of Drug and Food Control, Faculty of Pharmacy, Tehran University of 6
Medical Sciences, Tehran, Iran 2
7
Nanotechnology Research Centre, Faculty of Pharmacy, Tehran University of 8
Medical Sciences, Tehran, Iran
9 10
Corresponding author:
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Ali Mohammadi (Pharm.D, Ph.D)
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Associate Professor of Pharmaceutical Analysis, Department of Drug & Food Control
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and Nanotechnology Research Centre, Faculty of Pharmacy, Tehran University of
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Medical Sciences,
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P.O.Box:14155-6451, Tehran, Iran
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Phone/Fax: +98.21.88358801
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E-mail: [email protected]
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Maher Darwish (M.Sc, Ph.D Student)
19
Department of Drug & Food Control, Faculty of Pharmacy, Tehran University of
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Medical Sciences.
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Phone: +98.93.87563368
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E-mail: [email protected]
23
Navid Assi (Analytical chemistry, Ph.D Student)
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1
Phone: +98.91.22060649
1
E-mail: [email protected]
2 3
Graphical abstract
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5 6
2
Highlights PVP-stabilized CdS nanoparticles have been fabricated by a
1
2
polyol-microwave method.
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CdS nanoparticles were characterized and the size was
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approximately 48±10 nm.
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Catalytic activity of our nanoparticles was examined for tartrazine
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degradation.
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Remarkable results were obtained under both UV and visible light
8
irradiations.
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10
3
ABSTRACT
1
Polyvinylpyrrolidone capped cadmium sulfide nanoparticles have been successfully 2 synthesized by a facile polyol method with ethylene glycol. Microwave irradiation 3 and calcination were used to control the size and shape of nanoparticles. 4 Characterization
with
scanning
electron
microscopy
revealed
a
restricted 5
nanoparticles growth comparing with the uncapped product, hexagonal phase and 48 6 nm average particle size were confirmed by X-ray diffraction, and finally mechanism 7 of passivation was suggested depending on Fourier transform infrared spectra.
8
The efficiency of nanoparticles was evaluated by the photocatalytic degradation of 9 tartrazine in aqueous solution under UVC and visible light irradiation. Complete 10 degradation of the dye was observed after 90 min of UVC irradiation under optimized 11 conditions. Kinetic of reaction fitted well to the pseudo-first-order kinetic and 12 Langmuir–Hinshelwood models. Furthermore, 85% degradation of the dye in 9h 13 under visible light suggests that cadmium sulfide is a promising tool to work under 14 visible light for environmental remediation.
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KEYWORDS: B. chemical synthesis; A. nanostructures; A. chalcogenides; C. x-
17
ray diffraction; D. catalytic properties
18 19
4
1. INTRODUCTION
1
During the last two decades, photocatalytic materials received a large quantity of 2 research interest as they have a great potential to be applied in detoxification of 3 environmental organic pollutants and as a clean energy source [1]. Semiconductor 4 heterogeneous photocatalysts such as TiO2, ZnO, SnO2, WO3, Fe2O3, and CdS have 5 been intensively investigated in the area of water and air purification and in 6 remediation reactions [2].
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Like other advanced oxidation procedures (AOPs), the common characteristic of 8 photocatalytic materials is the generation of very reactive species such as, principally 9 but not exclusively, hydroxyl radicals (HO•), which initiate a series of reactions 10 leading eventually to the destruction of the target pollutant [3]. Recently, 11 nanostructures of these photocatalysts have attracted more consideration as they are 12 expected to have higher photocatalytic activity than their bulk counterparts due to 13 their smaller size, higher surface area-to-volume ratio, and increased band-gab energy 14 which in turn lead to higher redox potentials [4].
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Cadmium sulfide (CdS), a typical metal chalcogenide semiconductor with a direct 16 band gap Eg≈2.43 eV at room temperature [5], has become one of the most 17 considerable materials in research communities due to its diverse promising 18 applications in the field of solar cells, photoelectronic devices and photocatalysis [6]. 19 This material has shown better catalytic functions compared to those of TiO2 due to 20 the rapid generation of electron–hole pairs by photoexcitation [7]. Nevertheless, CdS 21 has the fatal photocorrosion problem due to the self-oxidation by the generated hole 22 [8]. To overcome such a problem, an effective approach is to cover the nanoparticle 23
5
core surface with a polymeric capping agent which might also help to stabilize the 1 surface, control the growth, and prevent agglomeration of the nanoparticles [9-11].
2
Many methods have been utilized for the synthesis of CdS nanostructures such as co- 3 precipitation [7, 12], polyol [13], solvothermal [14, 15], hydrothermal [16], non- 4 aqueous chemical method [17], and chemical bath deposition [18]. Among these 5 different processes, the polyol method appears as an easy to carry out with many other 6 advantages: a uniform shape, a narrow size distribution, and a low degree of 7 agglomeration [19]. This method generally uses poly-alcohol like Ethylene Glycol 8 (EG), Diethylene Glycol (DEG) or 1, 2-propanediol as both solvent and reducing 9 agent [20]. Furthermore, due to the relatively high dipole moment and loss factor of 10 polyol solvents, they are also suitable for microwave assisted route. In this regard, 11 using microwave in the synthesis of nanoparticles is considered a modern and rapidly 12 developing method. It has been evidenced that microwave irradiation gives a narrow 13 particle size distribution of nanocrystals with a high purity in a short reaction time in 14 addition of being cheap and environmentally friendly [21-23].
15
Tartrazine (Acid Yellow 23; FD&C Yellow No. 5), as an azo dye, has been chosen for 16 this study due to its extensively use as a colorant in food, cosmetics, pharmaceuticals 17 and textile industry [24], as well as its high stability against biodegradation and 18 conventional wastewater treatment procedures after disposal from industrial effluent 19 [25]. The toxic concentration of tartrazine on human has been reported to be 7.5 mg.k- 20 1
[26]. This compound is known to cause allergic reactions such as asthma and 21
urticaria and appears to cause more allergic and intolerance reactions than other azo 22 dyes [27].
23
6
Previously reported works on the removal of tartrazine from aqueous solutions 1 include mainly conventional methods like adsorption [28, 29], ecocoagulation [30], 2 and filtration [31] which are known not to effectively degrade pollutant but merely 3 transfer it to another phase where it is more concentrated [32]. Whereas other reports 4 have shown that tartrazine could be degraded employing AOPs like: ozonation [33], 5 electrochemical oxidation [34], photo Fenton oxidation [35], UV/H2O2 [36, 37], 6 photolytic [38], and photocatalytic oxidation [27, 39-46].
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Most of photocatalytic methods have utilized TiO2, which was extensively studied 8 and known to have some drawbacks such as expensive precursors and inability to 9 absorb visible light [47, 48]. Hence, the aim of the present work was first, to fabricate 10 CdS nanoparticles by a chemical synthesis utilizing the polyol method with ethylene 11 glycol as a solvent and polyvinylpyrrolidone (PVP) as a capping agent. Microwave 12 irradiation followed by calcination was used for size and shape controlling. Further, to 13 investigate the structure, size, and morphology of the synthesized products using 14 various characterization techniques. In second, the work aimed to study the catalytic 15 properties of these nanoparticles for oxidation of tartrazine, as an organic pollutant, in 16 aqueous solution under UVC and visible light irradiations, taking in consider the 17 effect of different parameters such as initial pH of solution, amount of nanoparticles, 18 and concentration of pollutant in the degradation process.
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2. EXPERMENTAL
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2.1. Materials and reagents
All reagents used in our experiments were of analytical grade and used as received 3 without any further purification. Tartrazine powder (≥85 %) was obtained from 4 Sigma-Aldrich (Germany). Cadmium chloride 2.5-hydrate (CdCl2 2.5H2O) and 5 thiourea (SC(NH2)2) were obtained from Scharlau (Spain). Polyvinylpyrrolidone 6 (MW≈25,000 g.mol-1), ethylene glycol, glacial acetic acid, and ammonia solution 7 (25%) were obtained from Merck (Germany). Double distilled water was used for the 8 preparation of all samples.
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2.2. Synthesis of nanoparticles
In a typical process, 9 g of cadmium chloride 2.5-hydrate and 3 g of PVP were 11 separately dissolved in 30 mL and 20 mL of EG, respectively, and then heated with 12 stirring for 60 min at 75 °C. The two solutions were then mixed slowly and heated 13 with stirring for 40 min at 100 °C. Concurrently, 3 g of thiourea was dissolved in 20 14 mL of EG and heated with stirring for 100 min at 75 °C and then added slowly to the 15 hot solution of (PVP-Cd2+). The temperature of the new mixture was gradually raised 16 to 170-180 °C within 45 min and kept at this temperature until most of the solvent 17 was evaporated. In this stage, the color of the solution changed from light yellow into 18 a dark orange suspension indicating the formation of CdS. Next, the suspension was 19 microwaved (LG MC-2820S microwave) for 5 min with a power of 720 W and 50 % 20 duty cycle (30 sec of irradiation and 30 sec stop). The obtained powder was then 21 calcined in a furnace (Sybron Thermolyne Type 1500) for 120 min at 450 °C and 22 finally, black crystallites of PVP-capped CdS nanoparticles were obtained. The 23 scheme of preparation is illustrated in Fig. 1. A control synthesis in the absence of 24
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PVP and the exact other steps was carried out to evaluate the impact of PVP on the 1 yielded product.
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2.3. Characterization of the CdS nanoparticles
The synthesized CdS nanoparticles were characterized using various analytical 4 techniques. Scanning electron microscopy (KYKY-EM3200 SEM) was used to 5 investigate the morphologies of nanoparticles. X-ray diffraction patterns of the 6 nanoparticles were recorded by a Bruker AXS D8-ADVANCE diffractometer fitted 7 with a (Cu Kα λ = 1.5418 Å) radiation tube. The average crystallite size of the 8 synthesized CdS was calculated with well-known Scherrer’s equation (Eq. (1)) with 9 full width at half maxima (FWHM) of X-ray diffraction pattern: D= 0.89 λ /β cos θ
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(1)
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Where λ is the wavelength of X-ray radiation source, β is FWHM of the peak and θ is 12 Bragg’s diffraction angle.
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Furthermore, in order to determine the chemical composition of nanoparticles and 14 study the behavior of PVP, Fourier transform infrared spectra in the range of 400– 15 4000 cm-1 of many samples during the synthesis process was recorded in transmission 16 mode (Thermo Nicolet 8700 FTIR), the powder samples were grounded with KBr and 17 compressed into a pellet for analysis.
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2.4. Photocatalytic activity under UVC light
Photocatalytic activity of the nanoparticles was evaluated by measuring the 21 degradation of tartrazine which was used as a test pollutant. An amount of (5-50) mg 22 of the catalyst was dispersed in 200 mL of tartrazine aqueous solution (10-100) mg.L- 23 9
1
. The pH of the suspension was adjusted to the desired value by ammonia or glacial 1
acetic acid solutions and determined by a digital pH meter (Metrohm pH lab 827). 2 The suspension in the photoreaction container was exposed to UV light source (CH 3 Lighting T8 30W UVC) positioned 10 cm above for 90-180 min. At given intervals of 4 illumination, 5 mL samples were taken out and centrifuged (Hettich EPA 12) at 2,500 5 rpm for 10 min in order to completely remove all nanoparticles. The degradation ratio 6 was monitored using UV–Vis spectrophotometer (T80+ UV-VIS Spectrometer PG 7 Instruments Ltd). According to the Beer-Lambert law, the concentration of tartrazine 8 is proportional to its absorbance. Hence, the degradation efficiency was calculated by 9 the following formula:
10
D%=[(C0-C)/C0]×100=[(A0-A)/A0]×100
(2)
11
C0 and A0 are the initial concentration and absorbance, respectively, while C and A 12 are the concentration and absorbance after intervals of illumination (t). The reaction 13 rate constant (k) was calculated using plots of ln(C/C0) versus illumination time 14 according to the following formula:
15
ln(C/C0)=-kt
(3)
16 17
2.5. Photocatalytic degradation and mineralization under visible light
After obtaining the optimal conditions for tartrazine degradation with the synthesized 18 nanoparticles under UVC, an assessment of the activity of these nanoparticles under 19 visible light was carried out using (OSRAM HWL 160W) lamp in the same 20 experimental setup used under UVC irradiation. In addition, a light protected 21 experiment under the same conditions of visible light experiment was carried out and 22 considered as a blank to assess the mineralization of tartrazine after same time of 23
10
irradiation. Samples were analyzed by a total organic carbon analyzer type (TOC-V, 1 Shimadzu).
2
3. RESULTS AND DISCUSSION
3 4
3.1. Morphology and structure
The SEM images presented in Fig. 2 show that PVP-capped CdS (PVP-CdS) particles 5 are in the range of nanometer with narrower particle size distribution (40-59 nm) and 6 less agglomeration comparing to those of the uncapped CdS which has an average 7 particle size of 115 nm (75-180) indicating clearly the advantages of PVP in 8 restricting the growth and preventing agglomeration. This is simply due to the stereo- 9 hindrance effect resulted from repulsive force among the polyvinyl groups of PVP 10 and due to the restrained Ostwald ripening kinetics in such a way that the growth rate 11 was decreased with the size of CdS nanoparticles. Consequently, smaller particles 12 with enhanced monodispersity were obtained.
13
X-ray powder diffraction patterns of the capped nanoparticles presented in Fig. 3 14 show a perfect match with the hexagonal phase of CdS (JCPDS No. 01-089-2944). 15 Diffraction peaks of monoclinic sulfur are also found (JCPDS No. 01-074-2108). The 16 average crystallite size of the capped nanoparticles calculated from XRD patterns 17 using the Scherrer’s equation was about 48 nm which is in agreement with the result 18 obtained from SEM image.
19
FTIR spectra of pure PVP and PVP-CdS prior to and after calcination are shown in 20 Fig. 4. In pure PVP spectrum, the O-H stretching vibration observed as a broad and 21 strong peak at 3517 cm-1 is due to H2O absorption on the surface of the sample. The 22 two bands at 2952 and 1423 cm-1 correspond to C-H asymmetric stretching vibration 23 and bending vibration, respectively. The broad peak with high intensity at 1665 cm-1 24 11
is ascribed to the C=O bond stretching and the band at 1286 cm-1 corresponds to C–N 1 bond stretching of the pyrrolidone structure.
2
Two observations can be found when comparing pure PVP spectrum with the other 3 two spectra. First, a decrease in all peaks intensities after exposing the sample to 4 microwave irradiation and more decrease after calcination. This can be explained in 5 the light of thermal degradation of PVP. Loría-Bastarrachea et. al [49], in agreement 6 with earlier reports, stated that at 450 °C the main product obtained from thermal 7 degradation of PVP is its corresponding monomer, i.e., vinyl pyrrolidone or less 8 unsaturated compounds such as PVP oligomers. The second observation is the shift to 9 lower wavenumber of some peaks, specifically C=O and C–N stretching vibrations. 10 These shifting indicate the interaction between PVP and Cd2+. The mechanism of this 11 interaction as suggested by Abdelghany et. al [50] implies the formation of coordinate 12 bonds between nitrogen and/or oxygen atoms of the PVP with cadmium ions in four 13 different probabilities as shown in Fig. 5. Hence, the preceding findings indicate the 14 formation of short polymeric chain-capped CdS nanoparticles.
15 16 17
3.2. Effect of tartrazine solution initial pH
The pH of the reaction medium is one of the most crucial parameters in photocatalytic 18 degradation of organic pollutants. It affects the surface charges of both the catalyst 19 and pollutant and consequently the adsorption of the pollutant on catalyst surface 20 [51].
21
In order to study the role of pH on the adsorption of tartrazine on the catalyst surface, 22 20 mg.L-1 solutions of the dye and a fixed amount of PVP-CdS (25 mg) were set at 23 pH 3.5, 5.5, 7 or 11 and kept in dark place with constant stirring for 90 min in order to 24 12
reach the equilibrium adsorption. In addition, photolysis experiments, in the absence 1 of catalyst and presence of UVC light with the same other experimental setup used in 2 the adsorption study, were carried out also over 90 min The role of pH on 3 photocatalytic degradation of tartrazine was investigated in the same conditions of 4 photolysis in the presence of 25 mg of PVP-CdS or uncapped CdS over 90 min. The 5 results are shown in Fig. 6.
6
As can be seen, a decrease in the dye concentration between 3-8% was obtained 7 among the different pH values in the presence of PVP-CdS only without UVC 8 irradiation. Whereas about 4.5-12% of the initial tartrazine concentration was 9 removed at different pH values in the presence of UVC irradiation alone without 10 nanoparticles. On the other hand, maximum photocatalytic degradation percentage of 11 PVP-CdS was observed in alkaline region. The degradation increased with increasing 12 pH and reached its maximum value at pH=11. For uncapped CdS, neutral medium 13 was favorable.
14
Our suggested interpretation is the following: tartrazine has a pKa=9.4 and so is 15 negatively charged in alkaline medium due to its ionization to the related phenyl salt. 16 likewise, CdS has a zero point charge at about pH=7 and is also negatively charged in 17 pH (8-12). As a result, the adsorption of tartrazine on CdS surface is minimum at 18 pH=11. Here, the enhancement of capped catalyst activity in alkaline medium could 19 be imputed to the attachment of CdS with pyrrolidone ring that might have acted as a 20 mediator to trap the carriers generated in the catalyst and adsorb the water or dye 21 molecules or even the abundant hydroxyl ions, which would subsequently facilitate 22 the generation of hydroxyl radicals that initiate the oxidization of the pollutant.
23
13
Unsurprisingly, PVP-CdS has shown a superior activity over uncapped CdS in 1 extreme acidic and basic conditions whereas in neutral conditions, the activities were 2 nearly equivalent. This is ascribed to two factors: first, the smaller particles size that 3 offered more surface area for photocatalytic reaction in the case of PVP-CdS and 4 second, the photocorrosin or dissolution that uncapped CdS might underwent in 5 extreme conditions which has reduced drastically its activity. Meanwhile, PVP-CdS 6 maintained an approximate percentages of degradation in acidic and neutral 7 conditions as a consequence of PVP passivation.
8 9 10
3.3. Effect of the catalyst loading
Catalyst loading is a critical parameter in photodegradation of organic pollutants. It 11 affects the reaction rate and consequently the cost of treatment [52].
12
The effect of PVP-CdS dosage on the degradation of tartrazine was investigated in the 13 presence of different amount of catalyst (5–50 mg) with 50 mg.L-1 initial 14 concentration of tartrazine at pH=11 for 90 min of irradiation. The results are shown 15 in Fig. 7.
16
As is noted, the photodegradation of the tartrazine was found to increase with the 17 increased catalyst loading within the range of study. Increasing the catalyst loading 18 gave a rise to a higher number and density of nanoparticles, which in turn caused 19 more photons to be absorbed and dye molecules to be adsorbed leading to better 20 degradation [53]. The opacity of the suspension was not greatly affected by the 21 increased amount of PVP-CdS and no hindering to light penetration was observed. On 22 the other hand, after 25 mg of catalyst amount there was no considerable enhancement 23
14
in the degradation profile. Hence, 25 mg of PVP-CdS nanoparticles was chosen to be 1 the optimal amount for further studies.
2 3 4
3.4. Effect of the initial concentration of tartrazine
The influence of pollutant concentration on photocatalytic degradation has also been 5 studied. Experiments have been carried out at different initial concentrations in the 6 range (10-100) mg.L-1 with constant amount of nanoparticles (25 mg) and initial 7 solution's pH=11. The results shown in Fig. 8 indicate that increasing the 8 concentration of tartrazine had decreased the degradation from 100% (10 mg.L-1) to 9 92.9% (50 mg.L-1) and 36.2% (100 mg.L-1) in 60 min.
10
This can be explained in the light of photocatalytic degradation mechanism; a single 11 layer of adsorbed water molecules exists on the surface of PVP-CdS. After photon is 12 adsorbed, the holes generated in valence band transfer to the surface and oxidize 13 water molecules and hydroxyl ions to produce HO• radicals. These radicals rapidly 14 attack tartrazine molecules and oxidize them to intermediates which in turn would be 15 mineralized with subsequent attacks of HO• radicals.
16
PVP-CdS + hν → eCB- + hVB+
(4)
17
h+ + H2O → HO• + H+
(5)
18
HO• + TA → Intermediates
(6)
19
HO• + Intermediates → CO2 + H2O
(7)
20
Increasing the initial dye concentration means more tartrazine molecules to surround 21 the nanoparticles and decrease the path length of the photons entering the solution 22 resulting in lower photon absorption by the catalyst and reaction sequence is slowed 23 15
down. Consequently, rate of degradation is decreased. At this juncture, we believe 1 that PVP passivation has an important role in the separation of the photogenerated 2 carriers. This is attributed to the prohibition of nonradiative decay of carriers by 3 covering dangling bonds that may work as exciton traps. Also, the coordination of 4 nanoparticles surface atoms which minimizes the trap states lie within the band gap 5 and increases quantum yield by providing alternative pathways of electron-hole 6 separation. More importantly, complexation between Cd and the lone pair of 7 nitrogen/oxygen makes pyrrolidone moiety of PVP act as a hole acceptor that traps 8 the photogenerated holes and transfers them to the target objects. It is worth 9 mentioning here that partial decomposition of PVP, during the synthesis process, was 10 beneficial for exposing trapping sites of pyrrolidone to those targeted objects to 11 promote the catalytic reaction sequence.
12 13 14
3.5. Kinetic study
In order to study the pattern of the photodegradation reaction of tartrazine, Langmuir– 15 Hinshelwood (L–H) model was used. This model has been successfully used for 16 heterogeneous photocatalysis to describe the exact relationship between the rate of 17 degradation and the concentration of pollutant in photocatalytic reaction [31]. The rate 18 constant (k) of degradation reaction was calculated according to the formula (3). As 19 shown in Fig. 9, a linear relation between tartrazine concentrations and illumination 20 time has been observed implying a pseudo-first order kinetics of the photodegradation 21 reaction. The pseudo-first order rate constant decreased dramatically with increased 22 initial concentration of tartrazine. This is because, as aforementioned, with high 23 concentrations of the pollutant, less hydroxyl radicals in active sites of the catalyst are 24
16
generated and the majority of photons are absorbed by pollutant molecules rather than 1 catalyst [54].
2
In order to cover the adsorption properties of the substrate on the catalyst surface, the 3 experimental data have been rationalized in terms of the modified form of L–H 4 kinetic model. The modified Langmuir-Hinshelwood expression that explains the 5 kinetics of heterogeneous catalytic systems is given by:
6
r=-dC/dt=krKC/(1+KC)
(8)
7
Where r represents the rate of reaction that changes with time (mg.L−1.min−1), kr is the 8 reaction rate constant (mg.L−1.min−1), and K is the adsorption coefficient of the 9 reactant onto the catalyst particles (L. mg −1).
10
The applicability of L–H equation for the degradation has been confirmed by the 11 linear plot obtained by plotting the reciprocal of initial rates (1/r0) against reciprocal 12 of initial concentrations of tartrazine (1/C0) as shown in Fig. 10. 1/r0=(1/kr)+(1/krKC0)
13
(9)
14
In this study, a reasonable agreement (R2 = 0.969) was obtained between the 15 experimental results and the linear form of the L-H expression.
16
The values of kr and K have been determined from the slope and intercept of this plot 17 and found to be 1.908 and 0.036, respectively, indicating that photocatalytic 18 degradation is a dominant factor when compared with pollutant adsorption onto the 19 surface of the catalyst.
20 21 22
17
1
3.6. Reusability
Reusability of capped catalyst was investigated in order to verify its stability and the 2 protective role of PVP in preventing catalyst degradation. After a first round of 3 catalytic reaction, the mixture (catalyst+slurry) was filtered and washed with a 4 mixture of water and ethanol (1:1) then dried in an oven at 65 °C for 6 h. The 5 recovered catalyst was used again to degrade tartrazine under same conditions used 6 before. A third round was carried out in the same procedure. Results depicted in Fig. 7 11 shows a very slight deference in catalytic activity among the first used and reused 8 catalysts. A small change in the kinetic could be seen but, the overall degradation in 9 90 min of study was not importantly affected. It is our believe that photocorrosion 10 suppression is due to prolonged life time of electron–hole pairs as a consequence of 11 hole trapping and transferring mediated by PVP.
12 13 14
3.7. Photodegradation and mineralization under visible light irradiation
A 50 mg.L-1 solution of tartrazine with pH=11 was mixed with 25 mg of PVP-CdS 15 nanoparticles and exposed to visible light irradiation in the same experimental setup 16 used under UVC irradiation. As can be seen in Fig. 12, approximately 85% of the dye 17 was degraded and about 69% of the total organic content compared with the blank 18 solution was removed after 9 hours.
19 20 21 22 23 18
4. CONCLUSION
1
Polyol synthesis with microwave irradiation and calcination have been used as an 2 efficient method to fabricate cadmium sulfide nanoparticles in an average crystallite 3 size of about 48 nm when PVP was used as a capping agent and about 115 nm 4 without capping. The prepared nanoparticles have been effectively used as a 5 photocatalyst for the degradation of tartrazine in aqueous solution. Total degradation 6 of tartrazine was obtained after 90 min using 25 mg of the PVP-CdS with initial dye 7 concentration of 50 mg.L-1 and pH=11 under UVC irradiation. The role of PVP in 8 controlling the growth and stabilizing the nanoparticles was demonstrated by the 9 compassion with the uncapped product which has shown more growing and less 10 catalytic activity.
11
The Langmuir-Hinshelwood models indicated that tartrazine underwent a pseudo first 12 order kinetics of the reaction and that photocatalytic degradation was prevailing 13 compared with pollutant adsorption on PVP-CdS surface. In addition, results has 14 shown that our nanoparticles can work sufficiently under visible light with about 85% 15 abatement of initial tartrazine concentration and 69% mineralization after 9h of 16 irradiation.
17
In conclusion, PVP-CdS nanoparticles synthesized in this work might be used 18 effectively in environmental treatment processes to remove organic pollutants from 19 water. These nanoparticles are promising to work under visible light sources to reduce 20 greatly the expense of treatment.
21 22
19
Acknowledgment
1
The authors wish to thank Tehran University of Medical Sciences for the financial 2 and instrumental support of this research.
3 4
20
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Figure Captions
1 2
Fig. 1. A schematic of the preparation of PVP-CdS nanoparticles.
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4 5
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Fig. 2. SEM images of (a) uncapped and (b) PVP-capped CdS nanoparticles.
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2
3 4
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Fig. 3. XRD patterns of the PVP-CdS nanoparticles.
1
Counts
Cd S
10
20
30
40
50
60
70
80
Cd S
Cd S
Cd S
Cd S
Cd S
Cd S Cd S Cd S
Cd S
Cd S
S
S
S
S
S
S
100
Cd S; S Cd S
S
Cd S; S
200
Cd S; S
Cd S; S Cd S; S
300
Cd S; S
Cd S; S
S
400
90
100
Position [°2Theta] (Copper (Cu))
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Fig. 4. FTIR spectra of (A) pure PVP, (B) PVP-CdS after microwave irradiation and, (C) PVP-CdS after calcination.
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3
4
5 6
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Fig. 5. Suggested mechanism of interaction between PVP and Cd2+. (Reference No.
1
50)
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3 4 5
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Fig. 6. Effect of different pH values (3.5–11) on adsorption, photolysis and, 1 photocatalytic degradation of tartrazine over 90 min. 2 3
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Fig. 7. Photocatalytic degradation of tartrazine by varying the amount of PVP-CdS nanoparticles (5–50 mg) with Tartrazine concentration (50 mg.L-1) and pH=11.
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Fig. 8. Photocatalytic degradation of varying initial concentrations of tartrazine (10-
1
100 mg.L-1) with pH=11 and 25 mg amount of PVP-CdS nanoparticles.
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3 4
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Fig. 9. plots of -ln(C/C0) versus illumination time for different concentrations of
1
tartrazine.
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Fig. 10. plot of 1/r0 versus 1/C0 of the photocatalytic degradation of tartrazine.
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2 3
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1
Fig. 11. Reusability of PVP-CdS nanoparticles 2
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Fig. 12. Photocatalytic degradation and mineralization of 50 mg.L-1 tartrazine with 1 pH=11 and 25 mg of PVP-CdS nanoparticles under visible light irradiation. 2
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