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Role of L-cysteine and CdS as promoted agents in photocatalytic activity of TiO2 nanoparticles
Journal Pre-proof Role of L-cysteine and CdS as promoted agents in photocatalytic activity of TiO2 nanoparticles Sajad Karimzadeh, Kiumars Bahrami
PII:
S2213-3437(19)30577-9
DOI:
https://doi.org/10.1016/j.jece.2019.103454
Reference:
JECE 103454
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
2 June 2019
Revised Date:
2 October 2019
Accepted Date:
4 October 2019
Please cite this article as: Karimzadeh S, Bahrami K, Role of L-cysteine and CdS as promoted agents in photocatalytic activity of TiO2 nanoparticles, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103454
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Role of L-cysteine and CdS as promoted agents in photocatalytic activity of TiO2 nanoparticles
Sajad Karimzadeha, Kiumars Bahramia,b*, a
Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah 67149‐ 67346, Iran b
Department of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah
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67149‐ 67346 , Iran
*
Corresponding author: Kiumars Bahrami, Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah
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67149‐ 67346, Iran
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E-mail address: [email protected]; [email protected]
Abstract:
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The present investigation has developed new TiO2-based nanocomposites to increase its photocatalytic activity. L-cysteine doped TiO2/CdS with different weight percentages of L-cysteine
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(0.5, 1, 1.5, 2 and 2.5 wt. %) and CdS nanoparticles (5, 10 and 15 wt. %) were synthesized and their chemical properties were evaluated using XRD, FTIR, DRS, FESEM and PL analyses. The DRS results show that pure TiO2 and L-cysteine-TiO2/CdS are UV-active and visibleactive, respectively. Also, the PL analysis confirms low recombination rate of charge carriers for L-cysteine doped TiO2/CdS nanocomposites. Photo-catalyst activity of the prepared nanocomposites (pure TiO2, L-cysteine doped TiO2 and L-cysteine doped TiO2/CdS) was
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evaluated during degradation of methyl orang (MO), methylene blue (MB) and rhodamine B (RB) as azo dyes. As a result, L-cysteine (2 %) doped TiO2/CdS (10 %) was completely removed 10 mg of MO, MB and RB under visible light irradiation after 210, 200 and 180 minutes, respectively. Also, the kinetic study indicates that the rate constant of L-cysteine (2 %) doped TiO2/CdS (10 %) for RB, MO and MB photodegradation is 0.0261, 0.0124 and 0.137 min-1, respectively.
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Keywords: L-cysteine doped TiO2/CdS, photocatalysis, nonbiodegradable compounds, visible
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light.
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1. Introduction
Different dyes have been applied in various industries such as textile, paint, ink, plastics, and
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cosmetics. These compounds are an important group of pollutants that are present in industrial waste water [1]. Many conventional methods have been used for dye removal from industrial
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wastewaters, including adsorption, precipitation, electrochemical techniques, biological treatment, and advanced oxidation process (AOP) [2-4].
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The degradation of organic dyes with photocatalytic systems has attracted extensive attention owing to the increasing environmental crises. Titanium dioxide (TiO2) as a popular photocatalyst has been extensively applied in water and wastewater treatment due to high catalytic reactivity, chemical stability, non-toxic, low cost, and etc. [1]. However, TiO2 is only active in UVA light due to its wide band gap (3.2 and 3.02 eV for anatase and rutile TiO2, respectively). The UV light is only about 4 % of the solar irradiation in the earth surface which limited its light harvesting [2]. Also, the quick recombination of electron and hole pairs in pure TiO2 decreases its quantum efficiency thereby decreasing photo-catalytic activity [3]. In order 2
to overcome TiO2 limitations, it was modified by different approaches. Many efforts were done to develop of TiO2 photocatalysis in the visible light region, such as: (I) surface sensitization [4, 5], (II) doping extraneous elements into the lattices of TiO2 [6], (III) coupling with narrow band-gap semiconductors (CdS, Fe2O3, Bi2WO6, and Cu2O) [7, 8], and etc. The available reports indicate that TiO2 is mainly modified with metal and nonmetal atoms as a doping agent due to their effectiveness in modifying the structure-electronic properties of many semiconductor photocatalysts [3]. The doped metal ions replace Ti4+ sites in the TiO2 network
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which supply fast recombine rate of charge carriers and thermal instability. Therefore, metal doping is not suitable to modify TiO2 photocatalyst. Thus, nonmetal doping TiO2 with
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substituting nonmetal atoms into the TiO2 lattice was preferred to avoid these deficits. However,
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multi doping atoms (including nitrogen (N), carbon (C), sulfur (S), and fluorine (F)) into TiO2 structure finds more sensible due to low required energy for this substitution process and the
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considerable improvement effect on the TiO2 photocatalytic activity [9-10]. The
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electronegativity of F atom is more than oxygen which cause to form a new localized electronic state below the O2p levels. Therefore, the researches focus on the codoping or tripledoping of C, N and S atoms into TiO2 lattice [11-12]. The addition of L-cysteine in the TiO2 sol as C, N
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and S doping agents not only reduces the TiO2 band gap but also shows a considerable improvement in photo-catalytic activity of TiO2 under visible light [11-12]. The C-N-S triple
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doped TiO2 nanocomposites were prepared with different doping agents (L-cysteine, Thiourea, L-Cystine) and organic dye photodegradation results as shown in Table 1. The photo-electrochemical performance of semiconductors mainly depends on the (I) generation of photo-induced electron, (II) separation of electron-hole pairs, and (III) efficiency of charge carrier transfer [13]. CdS has been widely utilized in photo-catalytic degradation of organic pollutants and photo-catalytic water splitting [14]. This is an n-type semiconductor with a band gap of 2.42 eV which indicates high absorption potential in the visible light region.
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CdS-TiO2 heterojunction is a visible driven photocatalyst due to sensitizer effect of CdS as a narrow band gap semiconductor which indicates low recombination rate of electron and hole pairs [15, 16]. Therefore, CdS coupled with TiO2 photocatalyst can provide a suitable interface of charge transfer and improve the photo-catalytic activity under visible light irradiation. The list of C-N-S tripledoped-TiO2 and CdS-TiO2 nanocomposites which were applied for degradation of organic pollutant in water and wastewater are shown in the Table 1. As a result, the C-N-S triple doped TiO2 and CdS nanocomposites exhibit high photocatalytic activity for
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degradation of various nonbiodegradable organic pollutants. Accordingly, a novel visible active photocatalyst (L-cysteine doped TiO2/CdS with different
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loadings of amino acid and CdS) was synthesized by the sol gel method. The prepared
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photocatalysts were characterized and applied to degrade organic dyes (MO, MB and RB). The
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kinetic of adsorption and photodegradation process was also investigated.
Type of photocatalysts
Doping agent L-Cysteine
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Table 1. Photodegradation of xenobiotic compounds over C-N-S tripledoped TiO2 nanocomposites. Type of dye pollutants
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Brilliant Red X3B
Thiourea
Brilliant Red X3B
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Cystine
C-N-S tripledopedTiO2
Methyl orange
Thiourea
Thiourea
Rhodamine B Cyanotoxin microcystin-LR
L-Cysteine
Rhodamine B Thiourea
Cu@C-N-S triple doped TiO2
Cyanotoxin microcystin-LR
L-Cysteine Methyl orange
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Experimental conditions C0= 10-4 molL-1 Catalyst amount = 1 gL-1
Removal efficiency (%)
ref
100 %, 30 min
[17]
100 %, 120 min
[18]
100 %, 120 min
[19]
99 %, 180 min
[9]
100 %, 300 min
[20]
83 %, 120 min
[21]
75 %, 300 min
[22]
100 %, 180 min
[23]
visible light (λ>400 nm) C0= 50 mgL-1 Catalyst amount=0.2 gL-1 visible light (λ>420 nm) C0= 5 mgL-1 Catalyst amount=0.2 gL-1 350 W xenon lamp C0= 5 mgL-1 Catalyst amount=0.3 gL-1 300 W tungsten halogen lamp C0=0.5 µM Catalyst amount=0.5 gL-1 visible light (λ>400 nm) C0= 10 mgL-1 Catalyst amount=1 gL-1 visible light (470<λ<800 nm) C0=0.5 µM Catalyst amount=0.5 gL-1 visible light (λ>400 nm) C0=10 mgL-1 Catalyst amount= 1 gL-1 visible light (λ>400 nm)
C0=10 mgL-1 Methyl orange Catalyst amount= 1 gL-1 visible light (λ>400 nm) L-Cysteine C0=10 mgL-1 Ag@C-N-S Methyl orange Catalyst amount= 1 gL-1 tripledoped TiO2 visible light (λ>400 nm) L-Cysteine C0=10 mgL-1 Graphene-supported Methyl orange Catalyst amount= 1 gL-1 C–N–S tridoped TiO2 visible light (λ>400 nm) C0=10 mgL-1 C doped-TiO2/CdS Methylene blue Catalyst amount= 1 gL-1 visible light (λ>400 nm) C0= 20 mgL-1 Methylene blue Catalyst amount=0.5 gL-1 visible light (λ>420 nm) C0= 20 mgL-1 Rhodamine B Catalyst amount=1 gL-1 visible light (λ>420 nm) CdS/TiO2 C0= 15 mgL-1 Rhodamine B Catalyst amount=1 gL-1 visible light (λ>420 nm) C0= 15 mgL-1 Methylene blue Catalyst amount=1 gL-1 visible light (λ>400 nm) L-Cysteine: C3H7NO2S, Thiourea: CS(NH2)2 and Cystine: C6H12N2O4S2 L-Cysteine
[23]
100 %, 120 min
[24]
100 %, 240 min
[25]
100 %, 180 min
[26]
100 %, 180 min
[27]
100 %, 80 min
[28]
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90 %, 180 min
100 %, 120 min
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Co@C-N-S triple doped TiO2
2. Materials and methods
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2.1. Materials and instruments
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60 %, 180 min
Tetra-n-butylorthotitanate (Ti(OBu)4, 98 %), L-cysteine amino acid (C3H7NO2S, 99 %), ethanol
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(C2H5OH, 99 %) and acetic acid (CH3COOH, 99.8 %) were purchased from Merck, Germany. Sodium sulfide (Na2S, 99.9 %) and cadmium acetate dihydrate (Cd(CH3COO)2.2H2O, 99.9 %)
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were purchased from Sigma-Aldrich. Methyl orange (C14H14N3NaO3S), Methylene blue (C16H18ClN3S) and Rhodamine B (C28H31ClN2O3) were achieved from Alvan Co, Iran. The properties of synthesized nanocomposites were characterized using XRD (a Rigaku D-max CIII, X-ray diffractometer with Ni-filtered Cu Ka radiation), FT-IR (Nicolet Magna IR 550 spectrometer, USA), DRS (A Shimadzu 1800), PL (Perkin Elmer LS55), and FE-SEM/EDX (Philips XL-30ESM, Holland) analyses.
2.2. Preparation of cysteine doped TiO2/CdS nanocomposites
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[29]
[30]
First, Cd(CH3COO)2.2H2O aqueous solution (4 M) was slowly added into the Na2S (4 M) aqueous solution. The prepared CdS nanoparticle was washed with deionized (three times) [31]. Different amount of CdS nanoparticles (5, 10 and 15 wt. %) was dispersed in the ethanol and kept in the ultrasonic bath about 1 hours. Second, Ti(OBu)4 was dissolved in the mixture of ethanol and acetic acid. Then, different amount of L-cysteine TiO2 (0.5, 1, 1.5, 2, 2.5 wt. %) were dissolved in deionized water and then added slowly drop-wise to above solution under magnetic stirring for hydrolysis reaction.
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Then CdS ethanolic solution was added into the L-cysteine TiO2 sol. The volume ratio of Ti (OBu)4: ethanol: acetic acid: deionized water is 1:2.5:2.5:25 [23]. The resulting solution was
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kept on the ultrasonic bath for 30 minutes and then stirred at room temperature about 24 hours.
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The TiO2 sol was dried at 80°C for 12 h. The dried powder was calcined at 500°C for 2 h.
To prepare pure TiO2, the same route to prepare L-cycteine-TiO2 was carried out except the
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deionized water was used.
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stage of water addition at which instead of L-Cysteine dissolved deionized water, pure
2.3. Photo-catalytic activity measurements
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Photo-catalytic activities of the samples have been evaluated by the degradation of MO, MB and RB dyes with a concentration of 10 mgL-1 in a batch mode photoreactor (Fig. 1). 250 mL photo-reactor equipped with a fluorescent lamp (18 W, visible light source, λ > 420 nm, light intensity of 13 lumen per m2) has been used. Lamp chamber is surrounded by circulating water to maintain reaction temperature at 25 °C during photocatalysis process. 10 mgL-1 of organic dye aqueous solution and 1 gL-1 of modified TiO2 nanoparticles were mixed and kept in the dark conditions for 30 minutes to allow adsorption of the dyes on the catalyst surface. Then, the fluorescent lamp was switched on to start the photodegradation 6
process for 180 minutes. After definite irradiation times, suitable volumes of solution were sampled, then filtered to separate the photocatalyst. The filtered solution was analyzed by UVVis spectrophotometer (Rayleigh UV 2601 model) to measure the absorbance of organic dye and dye removal efficiencies [32]. The following equation was used to calculate the dye removal efficiency: Removel (%) = (1 −
At ) × 100 A0
(2)
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where, A0 and At are the initial absorption of dye solution and treated samples, respectively.
Fig. 1. Schematic image of the experimental photoreactor.
3. Results and discussion 3.1. Selection of optimum nanocomposites
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L-cysteine doped TiO2/CdS nanocomposites with different loadings of L-cysteine and CdS were synthesized and the optimum weight fraction of each components were determined during degradation of MO, MB and RB dyes under visible light. Fig. 2a-b illustrate changes of C/C0 versus L-cysteine and CdS loadings for MO, MB and RB dyes after 180 minutes irradiation. As a result, the optimum values of L-cysteine and CdS were achieved 2 and 10 %, respectively. Among all of the prepared samples, L-cysteine (2 %) doped
0.3
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TiO2/CdS (10 %) indicates higher photocatalytic activity compered other modified samples.
MO MB RB
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0.25
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C/C0
0.2
0.15
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0.1
0 5
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0.05
7
9
11
(a)
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CdS (wt. %)
8
13
15
1
MO
0.9
MB
0.8
RB
0.7
C/C0
0.6 0.5 0.4 0.3 0.2
0 0.5
1
1.5
2
2.5
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L-Cystein (wt. %)
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0.1
(b)
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Fig. 2. Change in dye concentration versus (a) L-cysteine weight fraction at L-cysteine doped
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TiO2 and (b) CdS weight fraction at L-cysteine (2 %) doped TiO2/CdS during photodegradation
180 minutes irradiation.
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process with a concentration of 10 mgL-1 of MO, MB and RB and 1 g.L-1 of photocatalyst after
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3.2. Structural characterization the TiO2 modified nanoparticles Fig. 3 illustrates typical XRD patterns of TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2
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%) doped TiO2/CdS (10 %) nanoparticles. As observed, XRD reflections were fully matched with the anatase TiO2. The patterns of the anatase TiO2 crystalline structure is appeared at 2θ = 25.1°, 37.8°, 48.1°, 53.9°, 55.0°, 62.5°, 62.8°, 68.8°, 70.5°, 75.5° correspond to the (101), (004), (200), (105), (211), (204), (116), (220), (215) planes of the anatase phase of TiO2 (JCPDS card 21-1272) [33].
Notably, no typical diffraction peaks belonging to the separate CdS is observed in the Lcysteine (2 %) doped TiO2/CdS (10 %) nanocomposite, which implies the CdS was highly dispersed throughout the TiO2 matrix in the nanocomposite or the small amount of CdS 9
quantum dots [32]. The average crystallite sizes of the TiO2 modified particles calculated using the Scherrer equation [34] at 2θ of 25.1°. The average crystallite is estimated 28, 20 and 15 nm for TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10 %) nanoparticles, respectively.
20
30
40
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Intensity (a.u.)
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L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 Pure TiO2
50
60
70
80
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2θ
Fig. 3. XRD patterns of the synthesized nanocomposites.
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The DRS spectra and Tauc plots of TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10 %) nanoparticles were displayed in Fig. 4a and b, respectively. The band
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gap of the prepared samples was estimated using Tauc plots [35]. . The intercept of the tangent to the plot of (αhν)1/2 versus hν gives a good approximation of band gap energy. TiO2 nanoparticles absorption edge is about 380 nm (band gap energy, Eg = 3.17 eV) corresponding to a band-gap energy in UVA region. The absorption edge of L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposite exhibits a red shift into 430 and 535 nm corresponding to Eg = 2.70 and 2.3 eV, respectively. With adding L-Cysteine into TiO2 sol, the nonmetal elements (C, N and S) introduces into TiO2 lattice resulting in the generation of new energy states between VB and CB of TiO2 by mixing of C2p, N2p and S3p with O2p of
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TiO2. Therefore, the absorption edge of L-Cysteine-TiO2 extends to the visible region [32]. The CdS nanoparticles as a photosensitizer agent leads to reduce the TiO2 band gap [32]. In order to investigate the recombination rate of photoinduced electron and hole pairs, PL analysis of all the prepared photocatalysts was done as depicted in the Fig. 5. When the PL emission intensity decreases, the photocatalytic activity of material increases due to suppress recombination rate of the excited electrons and hole pairs and improve lifetime of the photogenerated charge carriers [35].
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The PL results depict that the L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposite had the least intensity and therefore the least electron and hole pairs recombination. As a result, the PL
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intensity was decreased with the addition of CdS nanoparticles. The PL intensity was decreased
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with the addition of L-cysteine. Also, the addition of CdS nanoparticles into L-cysteine doped TiO2 sol cause to minimize recombination of photogenerated e-/h+ pairs, facility charge carriers
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separation [36].
L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 Pure TiO2
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1 0.9
0.7
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Absorbance
0.8
0.6 0.5 0.4 0.3 0.2 0.1
0 250
300
350
400
450
500
Wavelength (nm)
(a)
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550
600
650
L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2
2.5
(αhν) 0.5
2
1.5
1
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0.5
0 2.5
3.5
4.5
5.5
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1.5
Eg (eV)
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(b)
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Fig. 4. (a) DRS spectra and (b) Tauc plots of the synthesized nanocomposites.
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Under visible light irradiation, the L-cysteine doped TiO2 and CdS nanoparticles adsorbed photons and electron excited from VB to CB of TiO2 or CdS. The exited electrons of CdS (CB of CdS) injected to CB of TiO2 and the generated hole remained in VB of CdS. The generated
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hole from VB of TiO2 can also transfer to CB of CdS which promotes separation of the photoproduced charger carriers. Therefore, the excited electrons on the CB of TiO2 react with
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oxygen in the solution to generate O2•−, HOO•, •OH. The hole scavenged by oxidizing species such as H2O, OH− to the formation of hydroxyl radicals or oxidized the adsorbed dye or photodegradation intermediates [3, 31, 38, 39].
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Pure TiO2 L-Cysteine ( 2 % ) doped TiO2
380
430
480
Wavelength (nm)
530
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330
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Intensity
L-Cysteine (2 %) doped TiO2/CdS ( 10 %)
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Fig. 5. PL Spectra of the prepared samples.
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As you can see in Fig. 6, the FESEM images of the L-cysteine (2 %) doped TiO2/CdS (10 %)
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nanocomposite indicates spherical-like nanoparticles with uniform size distribution. In this work, the L-cysteine as a doping agent acts as a surfactant and supplies uniform morphology
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and shape of the TiO2 nanoparticles [40].
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Fig. 6. FE-SEM images of L-cysteine (2 %) doped TiO2/CdS (10 %) catalyst.
FT-IR spectra of TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10
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%) nanoparticles are represented in the Fig. 7. For all of the samples, nanoparticles, the peaks
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around of 1630 and 3420 cm-1 are bending and stretching vibrations of O−H are related to the adsorbed water molecules. The wide absorbance peak at 400-900 cm-1 is corresponding to the
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Ti−O−Ti or Ti-O-X (N, C and S) stretching vibration [23]. The peak at 1127 cm-1 is due to the C−N vibration bonds of L-cysteine [41]. The peak at 2300-2400 cm-1 shows atmospheric or
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dissolved or CO2 in the samples. L-cysteine (2 %) doped TiO2/CdS (10 %) spectrum, the peak at 1129 cm-1 is due to Cd-S and C-N band [32].
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4000 3600 3200 2800 2400 2000 1600 1200 800
400
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Wavenumber (cm-1)
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Transmittance (%)
TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %)
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Fig. 7. FT-IR spectra of the synthesized nanocomposites.
3.3. Photo-catalytic activity
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The L-cysteine doped TiO2/CdS nanocomposite with a weight fraction of 2 and 10 % for L-
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cysteine and CdS was selected as an optimum composition due to its high photocatalytic activity which is approved by DRS and PL results. The photodegradation of MO, MB and RB dyes
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were studied over of pure TiO2, L-cysteine (2 %) doped TiO2 and of L-cysteine (2 %) doped TiO2/CdS (10 %) at a dye concentration of 10 mgL-1, photocatalyst loading of 1 g.L-1. Figs. 8a-
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c display dye removal efficiency versus time during the photodegradation process for MO, MB and RB, respectively.
As observed, MO removal efficiencies are 26, 57 and 88 % for pure TiO2, L-cysteine (2 %) doped TiO2, and L-cysteine (2 %) doped TiO2/CdS (10 %), receptively. It is clearly demonstrated that L-cysteine (2 %)-doped TiO2/CdS (10 %) indicates better performance compared to L-cysteine (2 %)-doped TiO2. The similar trends were found for MB and RB observation with different values of removal efficiencies. The highest values of removal efficiencies are 93 and 99.9 % for MB and RB dyes over L-cysteine (2 %) doped TiO2/CdS (10 %). These results indicate that the addition of L-cysteine and CdS increases photo-catalytic 15
activity of TiO2 nanoparticles. Based on the DRS and PL results, L-cysteine and CdS not only reduced the band gap energy of the TiO2 photocatalyst but also suppressed recombination of photogenerated electron and hole pairs. The different values of photodegradation efficiency may be due to different molecular structures of used dyes which cause a different interaction between nanocomposites and organic dyes [42]. L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2 100
78
70
68
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60 53
50 40 30
30
20
0 0
30
60
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10 0
88
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85
80
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MO removal (%)
90
90
120
150
180
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Time (min)
(a)
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L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2
100
90
90
85
MB removal (%)
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80
93
77
70
67
60 50
44
40 30 20 10
0
0 0
30
60
90
Time (min)
(b)
16
120
150
180
L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2 100
99
85
80
RB removal (%)
97
94
90
70
70
60 54
50 40 30 20
0
0 0
30
60
90
120
150
180
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Time (min)
of
10
(c)
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Fig. 8. Dye removal efficiency of the prepared samples as a function of time with a concentration of 10 mgL-1 of (a) MO, (b) MB and (c) RB and 1 gL-1 of photocatalyst after 180
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minutes irradiation.
The photolysis is the decomposition or dissociation of chemical compounds caused by natural
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or artificial light. Two photo-induced processes are commonly applied: direct and indirect photolysis. In the first case, the organic compounds absorb visible light and may react with the
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constituents of the water matrix or suffer self-decomposition. Photolysis occurs when chemical substances absorb light. Therefore, it is important to know separately the effect of photolysis on dye removal. The MO, MB and RB photolysis process over L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposite is shown in the Fig. 9. From the Fig. 9, the MO, MB and RB removal efficiencies are 7, 10 and 14 % after 180 minutes under visible irradiation. The results indicated that the photolysis has not significant effect on dye removal. In addition, in order to find the contribution of dark adsorption in removal efficiencies of MO, MB and RB, this experiment was carried out and 31, 44 and 53 % after 180 minutes could be related to this phenomenon, respectively. 17
MO-photolysis MO-adsorbtion
MB-photolysis MB-adsorbtion
RB-photolysis RB-adsorbtion
Removal efficiency (%)
60 50 40 30 20
0 0
30
60
90
120
150
180
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Time (min)
of
10
Fig. 9. Dye removal efficiency of the prepared samples as a function of time with a
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concentration of 10 mgL-1 of (a) photolysis, (b) adsorption at 1 gL-1 of photocatalyst after 180
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minutes irradiation.
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3.4. Kinetic study
The kinetic of MO, MB and RB photodegradation was illustrated in the Figs. 10a-c,
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respectively. The experimental data follows the pseudo first-order kinetic model which its equation is given as follows [43]:
𝐶0 𝐿𝑛( ) = 𝐾𝑎𝑝 𝑡 𝐶𝑡
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(1)
where C0, Ct, Kap and t are concentrations at time zero and t, rate constant and irradiation time, respectively. The kinetic parameters were calculated as represented in Table 2. The photodegradation rate constant of L-cysteine (2 %) doped TiO2/CdS (10 %) is higher than that both of pure TiO2 and L-cysteine (2 %) doped TiO2 which is exhibited its more photocatalytic activity. The rate constant of L-cysteine (2 %) doped TiO2/CdS (10 %) for RB, MO and MB photodegradation is 0.0261, 0.0124 and 0.137 min-1, respectively.
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Table 2. Kinetic parameters for MO, MB and RB dyes photodegradation process. Photocatalyst Pure TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %) Pure TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %) Pure TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %)
MO
MB
RB
R2 0.987 0.9905 0.9943 0.9959 0.9863 0.9969 0.9864 0.9918 0.9847
Linear ( TiO2)
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Linear (L-Cysteine (2 %) doped TiO2) 2.5
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2
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1.5
1
0.5
0
60
90
120
Time ( min)
(a)
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30
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Ln C0/Ct)
Kap 0.0013 0.0042 0.0124 0.0014 0.0045 0.0137 0.0018 0.0064 0.0261
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Type of dyes
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150
180
Linear ( TiO2) Linear (L-Cysteine (2 %) doped TiO2) Linear (L-Cysteine (2 %) doped TiO2/CdS (10 %) )
3
Ln(C0/Ct)
2.5 2 1.5 1 0.5
30
60
90
120
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0 150
180
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Time (min)
(b)
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Linear ( TiO2) Linear (L-Cysteine (2 %) doped TiO2) Linear (L-Cysteine (2 %) doped TiO2/CdS (10 %) ) 5
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4.5 4
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Ln(C0/Ct)
3.5 3 2.5 2
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1.5
1
0.5
0
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30
60
90
120
150
180
Time (min)
(c)
Fig. 10. First order kinetics of (a) MO, (b) MB and (c) RB dyes photodegradation over optimum nanocomposites.
3.5. Reusability performance
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Reusability performance of the optimum photocatalysts was also evaluated to degrade MO, MB and RB which its results are indicated in Fig. 11. The used photocatalyst was centrifuged, washed with deionized water and dried at the oven. Then, the suspended aqueous solution of photocatalyst was irradiated under stirring conditions for 180 minutes to decompose adsorbed dyes on the nanoparticle surface. The significant changes in the dye removal efficiency were not found after three cycles for all of the dyes. However, the removal efficiency of MO decreases from 88 to 73 % (about 15 %) which is higher than that of MB (6 %) and RB (6%).
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As the prior section was observed MO degradation efficiency is lower than MB and RB which
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photodegradation process compared of MB and RB dyes.
Cycle 1
Cycle 2
100
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88
97
93
87
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82
91
73
60
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Remoival efficiency (%)
Cycle 3
99
93
80
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cause to remain MO molecules on the photocatalyst surface thereby limiting the
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20
0
MO
MB
RB
Type of dyes
Fig. 11. Reusability results of MO, MB and RB dyes photodegradation over L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposites.
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4. Conclusion In this study, L-cysteine-TiO2/CdS nanocomposites were successfully fabricated using a facile one-step method. The prepared nanoparticles were characterized by XRD, FT-IR, DRS, PL and FE-SEM analyses. The crystalline phase of TiO2 anatase was not changed with the addition of L-cysteine. The phase of CdS nanoparticles was not detected on the XRD pattern due to its small amount. With adding of L-cysteine as C, N and S atoms doping agent in the TiO2 sol, the
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energy transfer edge was transmitted from the UV to the visible region which was approved by DRS result. The CdS nanoparticles not only acts as sensitizer but also reduces the
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recombination rate of photoproduced charge carriers (DRS and PL result). The photoactivity
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results exhibit that the L-cysteine (2 %) doped TiO2/CdS (10 %) is optimum composite with more photocatalytic activity for all of the studied dyes (MO, MB and RB). The values of
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photodegradation efficiency for RB (99% after 180 minutes) is higher than MB (93%) and MO
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(88 %) due to its different molecular structures. Base on the results, it can be concluded that the prepared L-cysteine (2%) doped TiO2/CdS (10%) nanocomposites is highly promising
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and wastewater.
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environmental friendly photocatalyst with a great potential for organic dye removal from water
Acknowledgments
The authors acknowledge the Razi University Research Council for support of this work.
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PII:
S2213-3437(19)30577-9
DOI:
https://doi.org/10.1016/j.jece.2019.103454
Reference:
JECE 103454
To appear in:
Journal of Environmental Chemical Engineering
Received Date:
2 June 2019
Revised Date:
2 October 2019
Accepted Date:
4 October 2019
Please cite this article as: Karimzadeh S, Bahrami K, Role of L-cysteine and CdS as promoted agents in photocatalytic activity of TiO2 nanoparticles, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103454
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Role of L-cysteine and CdS as promoted agents in photocatalytic activity of TiO2 nanoparticles
Sajad Karimzadeha, Kiumars Bahramia,b*, a
Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah 67149‐ 67346, Iran b
Department of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah
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67149‐ 67346 , Iran
*
Corresponding author: Kiumars Bahrami, Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah
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67149‐ 67346, Iran
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E-mail address: [email protected]; [email protected]
Abstract:
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The present investigation has developed new TiO2-based nanocomposites to increase its photocatalytic activity. L-cysteine doped TiO2/CdS with different weight percentages of L-cysteine
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(0.5, 1, 1.5, 2 and 2.5 wt. %) and CdS nanoparticles (5, 10 and 15 wt. %) were synthesized and their chemical properties were evaluated using XRD, FTIR, DRS, FESEM and PL analyses. The DRS results show that pure TiO2 and L-cysteine-TiO2/CdS are UV-active and visibleactive, respectively. Also, the PL analysis confirms low recombination rate of charge carriers for L-cysteine doped TiO2/CdS nanocomposites. Photo-catalyst activity of the prepared nanocomposites (pure TiO2, L-cysteine doped TiO2 and L-cysteine doped TiO2/CdS) was
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evaluated during degradation of methyl orang (MO), methylene blue (MB) and rhodamine B (RB) as azo dyes. As a result, L-cysteine (2 %) doped TiO2/CdS (10 %) was completely removed 10 mg of MO, MB and RB under visible light irradiation after 210, 200 and 180 minutes, respectively. Also, the kinetic study indicates that the rate constant of L-cysteine (2 %) doped TiO2/CdS (10 %) for RB, MO and MB photodegradation is 0.0261, 0.0124 and 0.137 min-1, respectively.
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Keywords: L-cysteine doped TiO2/CdS, photocatalysis, nonbiodegradable compounds, visible
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light.
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1. Introduction
Different dyes have been applied in various industries such as textile, paint, ink, plastics, and
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cosmetics. These compounds are an important group of pollutants that are present in industrial waste water [1]. Many conventional methods have been used for dye removal from industrial
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wastewaters, including adsorption, precipitation, electrochemical techniques, biological treatment, and advanced oxidation process (AOP) [2-4].
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The degradation of organic dyes with photocatalytic systems has attracted extensive attention owing to the increasing environmental crises. Titanium dioxide (TiO2) as a popular photocatalyst has been extensively applied in water and wastewater treatment due to high catalytic reactivity, chemical stability, non-toxic, low cost, and etc. [1]. However, TiO2 is only active in UVA light due to its wide band gap (3.2 and 3.02 eV for anatase and rutile TiO2, respectively). The UV light is only about 4 % of the solar irradiation in the earth surface which limited its light harvesting [2]. Also, the quick recombination of electron and hole pairs in pure TiO2 decreases its quantum efficiency thereby decreasing photo-catalytic activity [3]. In order 2
to overcome TiO2 limitations, it was modified by different approaches. Many efforts were done to develop of TiO2 photocatalysis in the visible light region, such as: (I) surface sensitization [4, 5], (II) doping extraneous elements into the lattices of TiO2 [6], (III) coupling with narrow band-gap semiconductors (CdS, Fe2O3, Bi2WO6, and Cu2O) [7, 8], and etc. The available reports indicate that TiO2 is mainly modified with metal and nonmetal atoms as a doping agent due to their effectiveness in modifying the structure-electronic properties of many semiconductor photocatalysts [3]. The doped metal ions replace Ti4+ sites in the TiO2 network
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which supply fast recombine rate of charge carriers and thermal instability. Therefore, metal doping is not suitable to modify TiO2 photocatalyst. Thus, nonmetal doping TiO2 with
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substituting nonmetal atoms into the TiO2 lattice was preferred to avoid these deficits. However,
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multi doping atoms (including nitrogen (N), carbon (C), sulfur (S), and fluorine (F)) into TiO2 structure finds more sensible due to low required energy for this substitution process and the
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considerable improvement effect on the TiO2 photocatalytic activity [9-10]. The
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electronegativity of F atom is more than oxygen which cause to form a new localized electronic state below the O2p levels. Therefore, the researches focus on the codoping or tripledoping of C, N and S atoms into TiO2 lattice [11-12]. The addition of L-cysteine in the TiO2 sol as C, N
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and S doping agents not only reduces the TiO2 band gap but also shows a considerable improvement in photo-catalytic activity of TiO2 under visible light [11-12]. The C-N-S triple
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doped TiO2 nanocomposites were prepared with different doping agents (L-cysteine, Thiourea, L-Cystine) and organic dye photodegradation results as shown in Table 1. The photo-electrochemical performance of semiconductors mainly depends on the (I) generation of photo-induced electron, (II) separation of electron-hole pairs, and (III) efficiency of charge carrier transfer [13]. CdS has been widely utilized in photo-catalytic degradation of organic pollutants and photo-catalytic water splitting [14]. This is an n-type semiconductor with a band gap of 2.42 eV which indicates high absorption potential in the visible light region.
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CdS-TiO2 heterojunction is a visible driven photocatalyst due to sensitizer effect of CdS as a narrow band gap semiconductor which indicates low recombination rate of electron and hole pairs [15, 16]. Therefore, CdS coupled with TiO2 photocatalyst can provide a suitable interface of charge transfer and improve the photo-catalytic activity under visible light irradiation. The list of C-N-S tripledoped-TiO2 and CdS-TiO2 nanocomposites which were applied for degradation of organic pollutant in water and wastewater are shown in the Table 1. As a result, the C-N-S triple doped TiO2 and CdS nanocomposites exhibit high photocatalytic activity for
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degradation of various nonbiodegradable organic pollutants. Accordingly, a novel visible active photocatalyst (L-cysteine doped TiO2/CdS with different
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loadings of amino acid and CdS) was synthesized by the sol gel method. The prepared
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photocatalysts were characterized and applied to degrade organic dyes (MO, MB and RB). The
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kinetic of adsorption and photodegradation process was also investigated.
Type of photocatalysts
Doping agent L-Cysteine
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Table 1. Photodegradation of xenobiotic compounds over C-N-S tripledoped TiO2 nanocomposites. Type of dye pollutants
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Brilliant Red X3B
Thiourea
Brilliant Red X3B
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Cystine
C-N-S tripledopedTiO2
Methyl orange
Thiourea
Thiourea
Rhodamine B Cyanotoxin microcystin-LR
L-Cysteine
Rhodamine B Thiourea
Cu@C-N-S triple doped TiO2
Cyanotoxin microcystin-LR
L-Cysteine Methyl orange
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Experimental conditions C0= 10-4 molL-1 Catalyst amount = 1 gL-1
Removal efficiency (%)
ref
100 %, 30 min
[17]
100 %, 120 min
[18]
100 %, 120 min
[19]
99 %, 180 min
[9]
100 %, 300 min
[20]
83 %, 120 min
[21]
75 %, 300 min
[22]
100 %, 180 min
[23]
visible light (λ>400 nm) C0= 50 mgL-1 Catalyst amount=0.2 gL-1 visible light (λ>420 nm) C0= 5 mgL-1 Catalyst amount=0.2 gL-1 350 W xenon lamp C0= 5 mgL-1 Catalyst amount=0.3 gL-1 300 W tungsten halogen lamp C0=0.5 µM Catalyst amount=0.5 gL-1 visible light (λ>400 nm) C0= 10 mgL-1 Catalyst amount=1 gL-1 visible light (470<λ<800 nm) C0=0.5 µM Catalyst amount=0.5 gL-1 visible light (λ>400 nm) C0=10 mgL-1 Catalyst amount= 1 gL-1 visible light (λ>400 nm)
C0=10 mgL-1 Methyl orange Catalyst amount= 1 gL-1 visible light (λ>400 nm) L-Cysteine C0=10 mgL-1 Ag@C-N-S Methyl orange Catalyst amount= 1 gL-1 tripledoped TiO2 visible light (λ>400 nm) L-Cysteine C0=10 mgL-1 Graphene-supported Methyl orange Catalyst amount= 1 gL-1 C–N–S tridoped TiO2 visible light (λ>400 nm) C0=10 mgL-1 C doped-TiO2/CdS Methylene blue Catalyst amount= 1 gL-1 visible light (λ>400 nm) C0= 20 mgL-1 Methylene blue Catalyst amount=0.5 gL-1 visible light (λ>420 nm) C0= 20 mgL-1 Rhodamine B Catalyst amount=1 gL-1 visible light (λ>420 nm) CdS/TiO2 C0= 15 mgL-1 Rhodamine B Catalyst amount=1 gL-1 visible light (λ>420 nm) C0= 15 mgL-1 Methylene blue Catalyst amount=1 gL-1 visible light (λ>400 nm) L-Cysteine: C3H7NO2S, Thiourea: CS(NH2)2 and Cystine: C6H12N2O4S2 L-Cysteine
[23]
100 %, 120 min
[24]
100 %, 240 min
[25]
100 %, 180 min
[26]
100 %, 180 min
[27]
100 %, 80 min
[28]
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90 %, 180 min
100 %, 120 min
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Co@C-N-S triple doped TiO2
2. Materials and methods
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2.1. Materials and instruments
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60 %, 180 min
Tetra-n-butylorthotitanate (Ti(OBu)4, 98 %), L-cysteine amino acid (C3H7NO2S, 99 %), ethanol
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(C2H5OH, 99 %) and acetic acid (CH3COOH, 99.8 %) were purchased from Merck, Germany. Sodium sulfide (Na2S, 99.9 %) and cadmium acetate dihydrate (Cd(CH3COO)2.2H2O, 99.9 %)
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were purchased from Sigma-Aldrich. Methyl orange (C14H14N3NaO3S), Methylene blue (C16H18ClN3S) and Rhodamine B (C28H31ClN2O3) were achieved from Alvan Co, Iran. The properties of synthesized nanocomposites were characterized using XRD (a Rigaku D-max CIII, X-ray diffractometer with Ni-filtered Cu Ka radiation), FT-IR (Nicolet Magna IR 550 spectrometer, USA), DRS (A Shimadzu 1800), PL (Perkin Elmer LS55), and FE-SEM/EDX (Philips XL-30ESM, Holland) analyses.
2.2. Preparation of cysteine doped TiO2/CdS nanocomposites
5
[29]
[30]
First, Cd(CH3COO)2.2H2O aqueous solution (4 M) was slowly added into the Na2S (4 M) aqueous solution. The prepared CdS nanoparticle was washed with deionized (three times) [31]. Different amount of CdS nanoparticles (5, 10 and 15 wt. %) was dispersed in the ethanol and kept in the ultrasonic bath about 1 hours. Second, Ti(OBu)4 was dissolved in the mixture of ethanol and acetic acid. Then, different amount of L-cysteine TiO2 (0.5, 1, 1.5, 2, 2.5 wt. %) were dissolved in deionized water and then added slowly drop-wise to above solution under magnetic stirring for hydrolysis reaction.
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Then CdS ethanolic solution was added into the L-cysteine TiO2 sol. The volume ratio of Ti (OBu)4: ethanol: acetic acid: deionized water is 1:2.5:2.5:25 [23]. The resulting solution was
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kept on the ultrasonic bath for 30 minutes and then stirred at room temperature about 24 hours.
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-p
The TiO2 sol was dried at 80°C for 12 h. The dried powder was calcined at 500°C for 2 h.
To prepare pure TiO2, the same route to prepare L-cycteine-TiO2 was carried out except the
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deionized water was used.
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stage of water addition at which instead of L-Cysteine dissolved deionized water, pure
2.3. Photo-catalytic activity measurements
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Photo-catalytic activities of the samples have been evaluated by the degradation of MO, MB and RB dyes with a concentration of 10 mgL-1 in a batch mode photoreactor (Fig. 1). 250 mL photo-reactor equipped with a fluorescent lamp (18 W, visible light source, λ > 420 nm, light intensity of 13 lumen per m2) has been used. Lamp chamber is surrounded by circulating water to maintain reaction temperature at 25 °C during photocatalysis process. 10 mgL-1 of organic dye aqueous solution and 1 gL-1 of modified TiO2 nanoparticles were mixed and kept in the dark conditions for 30 minutes to allow adsorption of the dyes on the catalyst surface. Then, the fluorescent lamp was switched on to start the photodegradation 6
process for 180 minutes. After definite irradiation times, suitable volumes of solution were sampled, then filtered to separate the photocatalyst. The filtered solution was analyzed by UVVis spectrophotometer (Rayleigh UV 2601 model) to measure the absorbance of organic dye and dye removal efficiencies [32]. The following equation was used to calculate the dye removal efficiency: Removel (%) = (1 −
At ) × 100 A0
(2)
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-p
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where, A0 and At are the initial absorption of dye solution and treated samples, respectively.
Fig. 1. Schematic image of the experimental photoreactor.
3. Results and discussion 3.1. Selection of optimum nanocomposites
7
L-cysteine doped TiO2/CdS nanocomposites with different loadings of L-cysteine and CdS were synthesized and the optimum weight fraction of each components were determined during degradation of MO, MB and RB dyes under visible light. Fig. 2a-b illustrate changes of C/C0 versus L-cysteine and CdS loadings for MO, MB and RB dyes after 180 minutes irradiation. As a result, the optimum values of L-cysteine and CdS were achieved 2 and 10 %, respectively. Among all of the prepared samples, L-cysteine (2 %) doped
0.3
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TiO2/CdS (10 %) indicates higher photocatalytic activity compered other modified samples.
MO MB RB
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0.25
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C/C0
0.2
0.15
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0.1
0 5
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0.05
7
9
11
(a)
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CdS (wt. %)
8
13
15
1
MO
0.9
MB
0.8
RB
0.7
C/C0
0.6 0.5 0.4 0.3 0.2
0 0.5
1
1.5
2
2.5
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L-Cystein (wt. %)
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0.1
(b)
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Fig. 2. Change in dye concentration versus (a) L-cysteine weight fraction at L-cysteine doped
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TiO2 and (b) CdS weight fraction at L-cysteine (2 %) doped TiO2/CdS during photodegradation
180 minutes irradiation.
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process with a concentration of 10 mgL-1 of MO, MB and RB and 1 g.L-1 of photocatalyst after
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3.2. Structural characterization the TiO2 modified nanoparticles Fig. 3 illustrates typical XRD patterns of TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2
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%) doped TiO2/CdS (10 %) nanoparticles. As observed, XRD reflections were fully matched with the anatase TiO2. The patterns of the anatase TiO2 crystalline structure is appeared at 2θ = 25.1°, 37.8°, 48.1°, 53.9°, 55.0°, 62.5°, 62.8°, 68.8°, 70.5°, 75.5° correspond to the (101), (004), (200), (105), (211), (204), (116), (220), (215) planes of the anatase phase of TiO2 (JCPDS card 21-1272) [33].
Notably, no typical diffraction peaks belonging to the separate CdS is observed in the Lcysteine (2 %) doped TiO2/CdS (10 %) nanocomposite, which implies the CdS was highly dispersed throughout the TiO2 matrix in the nanocomposite or the small amount of CdS 9
quantum dots [32]. The average crystallite sizes of the TiO2 modified particles calculated using the Scherrer equation [34] at 2θ of 25.1°. The average crystallite is estimated 28, 20 and 15 nm for TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10 %) nanoparticles, respectively.
20
30
40
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Intensity (a.u.)
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L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 Pure TiO2
50
60
70
80
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2θ
Fig. 3. XRD patterns of the synthesized nanocomposites.
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The DRS spectra and Tauc plots of TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10 %) nanoparticles were displayed in Fig. 4a and b, respectively. The band
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gap of the prepared samples was estimated using Tauc plots [35]. . The intercept of the tangent to the plot of (αhν)1/2 versus hν gives a good approximation of band gap energy. TiO2 nanoparticles absorption edge is about 380 nm (band gap energy, Eg = 3.17 eV) corresponding to a band-gap energy in UVA region. The absorption edge of L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposite exhibits a red shift into 430 and 535 nm corresponding to Eg = 2.70 and 2.3 eV, respectively. With adding L-Cysteine into TiO2 sol, the nonmetal elements (C, N and S) introduces into TiO2 lattice resulting in the generation of new energy states between VB and CB of TiO2 by mixing of C2p, N2p and S3p with O2p of
10
TiO2. Therefore, the absorption edge of L-Cysteine-TiO2 extends to the visible region [32]. The CdS nanoparticles as a photosensitizer agent leads to reduce the TiO2 band gap [32]. In order to investigate the recombination rate of photoinduced electron and hole pairs, PL analysis of all the prepared photocatalysts was done as depicted in the Fig. 5. When the PL emission intensity decreases, the photocatalytic activity of material increases due to suppress recombination rate of the excited electrons and hole pairs and improve lifetime of the photogenerated charge carriers [35].
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The PL results depict that the L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposite had the least intensity and therefore the least electron and hole pairs recombination. As a result, the PL
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intensity was decreased with the addition of CdS nanoparticles. The PL intensity was decreased
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with the addition of L-cysteine. Also, the addition of CdS nanoparticles into L-cysteine doped TiO2 sol cause to minimize recombination of photogenerated e-/h+ pairs, facility charge carriers
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separation [36].
L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 Pure TiO2
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1 0.9
0.7
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Absorbance
0.8
0.6 0.5 0.4 0.3 0.2 0.1
0 250
300
350
400
450
500
Wavelength (nm)
(a)
11
550
600
650
L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2
2.5
(αhν) 0.5
2
1.5
1
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0.5
0 2.5
3.5
4.5
5.5
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1.5
Eg (eV)
-p
(b)
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Fig. 4. (a) DRS spectra and (b) Tauc plots of the synthesized nanocomposites.
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Under visible light irradiation, the L-cysteine doped TiO2 and CdS nanoparticles adsorbed photons and electron excited from VB to CB of TiO2 or CdS. The exited electrons of CdS (CB of CdS) injected to CB of TiO2 and the generated hole remained in VB of CdS. The generated
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hole from VB of TiO2 can also transfer to CB of CdS which promotes separation of the photoproduced charger carriers. Therefore, the excited electrons on the CB of TiO2 react with
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oxygen in the solution to generate O2•−, HOO•, •OH. The hole scavenged by oxidizing species such as H2O, OH− to the formation of hydroxyl radicals or oxidized the adsorbed dye or photodegradation intermediates [3, 31, 38, 39].
12
Pure TiO2 L-Cysteine ( 2 % ) doped TiO2
380
430
480
Wavelength (nm)
530
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330
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Intensity
L-Cysteine (2 %) doped TiO2/CdS ( 10 %)
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Fig. 5. PL Spectra of the prepared samples.
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As you can see in Fig. 6, the FESEM images of the L-cysteine (2 %) doped TiO2/CdS (10 %)
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nanocomposite indicates spherical-like nanoparticles with uniform size distribution. In this work, the L-cysteine as a doping agent acts as a surfactant and supplies uniform morphology
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and shape of the TiO2 nanoparticles [40].
13
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Fig. 6. FE-SEM images of L-cysteine (2 %) doped TiO2/CdS (10 %) catalyst.
FT-IR spectra of TiO2, L-cysteine (2 %) doped TiO2 and L-cysteine (2 %) doped TiO2/CdS (10
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%) nanoparticles are represented in the Fig. 7. For all of the samples, nanoparticles, the peaks
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around of 1630 and 3420 cm-1 are bending and stretching vibrations of O−H are related to the adsorbed water molecules. The wide absorbance peak at 400-900 cm-1 is corresponding to the
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Ti−O−Ti or Ti-O-X (N, C and S) stretching vibration [23]. The peak at 1127 cm-1 is due to the C−N vibration bonds of L-cysteine [41]. The peak at 2300-2400 cm-1 shows atmospheric or
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dissolved or CO2 in the samples. L-cysteine (2 %) doped TiO2/CdS (10 %) spectrum, the peak at 1129 cm-1 is due to Cd-S and C-N band [32].
14
4000 3600 3200 2800 2400 2000 1600 1200 800
400
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Wavenumber (cm-1)
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Transmittance (%)
TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %)
-p
Fig. 7. FT-IR spectra of the synthesized nanocomposites.
3.3. Photo-catalytic activity
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The L-cysteine doped TiO2/CdS nanocomposite with a weight fraction of 2 and 10 % for L-
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cysteine and CdS was selected as an optimum composition due to its high photocatalytic activity which is approved by DRS and PL results. The photodegradation of MO, MB and RB dyes
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were studied over of pure TiO2, L-cysteine (2 %) doped TiO2 and of L-cysteine (2 %) doped TiO2/CdS (10 %) at a dye concentration of 10 mgL-1, photocatalyst loading of 1 g.L-1. Figs. 8a-
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c display dye removal efficiency versus time during the photodegradation process for MO, MB and RB, respectively.
As observed, MO removal efficiencies are 26, 57 and 88 % for pure TiO2, L-cysteine (2 %) doped TiO2, and L-cysteine (2 %) doped TiO2/CdS (10 %), receptively. It is clearly demonstrated that L-cysteine (2 %)-doped TiO2/CdS (10 %) indicates better performance compared to L-cysteine (2 %)-doped TiO2. The similar trends were found for MB and RB observation with different values of removal efficiencies. The highest values of removal efficiencies are 93 and 99.9 % for MB and RB dyes over L-cysteine (2 %) doped TiO2/CdS (10 %). These results indicate that the addition of L-cysteine and CdS increases photo-catalytic 15
activity of TiO2 nanoparticles. Based on the DRS and PL results, L-cysteine and CdS not only reduced the band gap energy of the TiO2 photocatalyst but also suppressed recombination of photogenerated electron and hole pairs. The different values of photodegradation efficiency may be due to different molecular structures of used dyes which cause a different interaction between nanocomposites and organic dyes [42]. L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2 100
78
70
68
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60 53
50 40 30
30
20
0 0
30
60
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10 0
88
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85
80
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MO removal (%)
90
90
120
150
180
lP
Time (min)
(a)
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L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2
100
90
90
85
MB removal (%)
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80
93
77
70
67
60 50
44
40 30 20 10
0
0 0
30
60
90
Time (min)
(b)
16
120
150
180
L-Cysteine (2 %) doped TiO2/CdS (10 %) L-Cysteine (2 %) doped TiO2 TiO2 100
99
85
80
RB removal (%)
97
94
90
70
70
60 54
50 40 30 20
0
0 0
30
60
90
120
150
180
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Time (min)
of
10
(c)
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Fig. 8. Dye removal efficiency of the prepared samples as a function of time with a concentration of 10 mgL-1 of (a) MO, (b) MB and (c) RB and 1 gL-1 of photocatalyst after 180
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minutes irradiation.
The photolysis is the decomposition or dissociation of chemical compounds caused by natural
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or artificial light. Two photo-induced processes are commonly applied: direct and indirect photolysis. In the first case, the organic compounds absorb visible light and may react with the
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constituents of the water matrix or suffer self-decomposition. Photolysis occurs when chemical substances absorb light. Therefore, it is important to know separately the effect of photolysis on dye removal. The MO, MB and RB photolysis process over L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposite is shown in the Fig. 9. From the Fig. 9, the MO, MB and RB removal efficiencies are 7, 10 and 14 % after 180 minutes under visible irradiation. The results indicated that the photolysis has not significant effect on dye removal. In addition, in order to find the contribution of dark adsorption in removal efficiencies of MO, MB and RB, this experiment was carried out and 31, 44 and 53 % after 180 minutes could be related to this phenomenon, respectively. 17
MO-photolysis MO-adsorbtion
MB-photolysis MB-adsorbtion
RB-photolysis RB-adsorbtion
Removal efficiency (%)
60 50 40 30 20
0 0
30
60
90
120
150
180
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Time (min)
of
10
Fig. 9. Dye removal efficiency of the prepared samples as a function of time with a
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concentration of 10 mgL-1 of (a) photolysis, (b) adsorption at 1 gL-1 of photocatalyst after 180
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minutes irradiation.
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3.4. Kinetic study
The kinetic of MO, MB and RB photodegradation was illustrated in the Figs. 10a-c,
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respectively. The experimental data follows the pseudo first-order kinetic model which its equation is given as follows [43]:
𝐶0 𝐿𝑛( ) = 𝐾𝑎𝑝 𝑡 𝐶𝑡
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(1)
where C0, Ct, Kap and t are concentrations at time zero and t, rate constant and irradiation time, respectively. The kinetic parameters were calculated as represented in Table 2. The photodegradation rate constant of L-cysteine (2 %) doped TiO2/CdS (10 %) is higher than that both of pure TiO2 and L-cysteine (2 %) doped TiO2 which is exhibited its more photocatalytic activity. The rate constant of L-cysteine (2 %) doped TiO2/CdS (10 %) for RB, MO and MB photodegradation is 0.0261, 0.0124 and 0.137 min-1, respectively.
18
Table 2. Kinetic parameters for MO, MB and RB dyes photodegradation process. Photocatalyst Pure TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %) Pure TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %) Pure TiO2 L-cysteine (2 %) doped TiO2 L-cysteine (2 %) doped TiO2/CdS (10 %)
MO
MB
RB
R2 0.987 0.9905 0.9943 0.9959 0.9863 0.9969 0.9864 0.9918 0.9847
Linear ( TiO2)
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Linear (L-Cysteine (2 %) doped TiO2) 2.5
-p
2
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1.5
1
0.5
0
60
90
120
Time ( min)
(a)
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30
lP
Ln C0/Ct)
Kap 0.0013 0.0042 0.0124 0.0014 0.0045 0.0137 0.0018 0.0064 0.0261
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Type of dyes
19
150
180
Linear ( TiO2) Linear (L-Cysteine (2 %) doped TiO2) Linear (L-Cysteine (2 %) doped TiO2/CdS (10 %) )
3
Ln(C0/Ct)
2.5 2 1.5 1 0.5
30
60
90
120
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0 150
180
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Time (min)
(b)
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Linear ( TiO2) Linear (L-Cysteine (2 %) doped TiO2) Linear (L-Cysteine (2 %) doped TiO2/CdS (10 %) ) 5
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4.5 4
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Ln(C0/Ct)
3.5 3 2.5 2
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1.5
1
0.5
0
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30
60
90
120
150
180
Time (min)
(c)
Fig. 10. First order kinetics of (a) MO, (b) MB and (c) RB dyes photodegradation over optimum nanocomposites.
3.5. Reusability performance
20
Reusability performance of the optimum photocatalysts was also evaluated to degrade MO, MB and RB which its results are indicated in Fig. 11. The used photocatalyst was centrifuged, washed with deionized water and dried at the oven. Then, the suspended aqueous solution of photocatalyst was irradiated under stirring conditions for 180 minutes to decompose adsorbed dyes on the nanoparticle surface. The significant changes in the dye removal efficiency were not found after three cycles for all of the dyes. However, the removal efficiency of MO decreases from 88 to 73 % (about 15 %) which is higher than that of MB (6 %) and RB (6%).
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As the prior section was observed MO degradation efficiency is lower than MB and RB which
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photodegradation process compared of MB and RB dyes.
Cycle 1
Cycle 2
100
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88
97
93
87
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82
91
73
60
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Remoival efficiency (%)
Cycle 3
99
93
80
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cause to remain MO molecules on the photocatalyst surface thereby limiting the
40
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20
0
MO
MB
RB
Type of dyes
Fig. 11. Reusability results of MO, MB and RB dyes photodegradation over L-cysteine (2 %) doped TiO2/CdS (10 %) nanocomposites.
21
4. Conclusion In this study, L-cysteine-TiO2/CdS nanocomposites were successfully fabricated using a facile one-step method. The prepared nanoparticles were characterized by XRD, FT-IR, DRS, PL and FE-SEM analyses. The crystalline phase of TiO2 anatase was not changed with the addition of L-cysteine. The phase of CdS nanoparticles was not detected on the XRD pattern due to its small amount. With adding of L-cysteine as C, N and S atoms doping agent in the TiO2 sol, the
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energy transfer edge was transmitted from the UV to the visible region which was approved by DRS result. The CdS nanoparticles not only acts as sensitizer but also reduces the
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recombination rate of photoproduced charge carriers (DRS and PL result). The photoactivity
-p
results exhibit that the L-cysteine (2 %) doped TiO2/CdS (10 %) is optimum composite with more photocatalytic activity for all of the studied dyes (MO, MB and RB). The values of
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photodegradation efficiency for RB (99% after 180 minutes) is higher than MB (93%) and MO
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(88 %) due to its different molecular structures. Base on the results, it can be concluded that the prepared L-cysteine (2%) doped TiO2/CdS (10%) nanocomposites is highly promising
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and wastewater.
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environmental friendly photocatalyst with a great potential for organic dye removal from water
Acknowledgments
The authors acknowledge the Razi University Research Council for support of this work.
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[12] S.G. Kumar, L.G. Devi, Review on Modified TiO2 Photocatalysis under UV/Visible Light: Selected Results and Related Mechanisms on Interfacial Charge Carrier Transfer Dynamics, J.
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