Journal of Hazardous Materials 280 (2014) 723–733

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High performance sulfur, nitrogen and carbon doped mesoporous anatase–brookite TiO2 photocatalyst for the removal of microcystin-LR under visible light irradiation Said M. El-Sheikh a , Geshan Zhang b , Hamza M. El-Hosainy a , Adel A. Ismail a , Kevin E. O’Shea c , Polycarpos Falaras d , Athanassios G. Kontos d , Dionysios D. Dionysiou b,∗ a

Nanostructured Materials Lab., Advanced Materials Department, CMRDI, Cairo 11421, Egypt Environmental Engineering and Science Program, Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, United States c Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, United States d Institute of Advanced Materials, Physicochemical Processes, Nanotechnology and Microsystems (IAMPPNM), National Centre for Scientific Research “Demokritos“, Agia Paraskevi Attikis, 15310 Athens, Greece b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Synthesis of tailor-designed C, N and S doped titania anatase–brookite nano-heterojunction photocatalyst. • Microcystin-LR was completely removed in the presence of doped sample under visible light. • The MC-LR degradation rate achieved by the doped sample was much better than that of un-doped sample under visible light.

a r t i c l e

i n f o

Article history: Received 28 June 2014 Received in revised form 20 August 2014 Accepted 21 August 2014 Available online 30 August 2014 Keywords: Brookite–anatase heterojunction Microcystin-LR Non-metal doped Photodegradation Visible light Water treatment.

a b s t r a c t Carbon, nitrogen and sulfur (C, N and S) doped mesoporous anatase–brookite nano-heterojunction titania photocatalysts have been synthesized through a simple sol–gel method in the presence of triblock copolymer Pluronic P123. XRD and Raman spectra revealed the formation of anatase and brookite mixed phases. XPS spectra indicated the presence of C, N and S dopants. The TEM images demonstrated the formation of almost monodisperse titania nanoparticles with particle sizes of approximately 10 nm. N2 isotherm measurements confirmed that both doped and undoped titania anatase–brookite materials have mesoporous structure. The photocatalytic degradation of the cyanotoxin microcystin-LR (MC-LR) has been investigated using these novel nanomaterials under visible light illumination. The photocatalytic efficiency of the mesoporous titania anatase–brookite photocatalyst dramatically increased with the addition of the C, N and S non-metal, achieving complete degradation (∼100%) of MC-LR. The results demonstrate the advantages of the synthetic approach and the great potential of the visible light activated C, N, and S doped titania photocatalysts for the treatment of organic micropollutants in contaminated waters under visible light. © 2014 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +1 513 556 0724; fax: +1 513 556 4162. E-mail addresses: [email protected], [email protected] (D.D. Dionysiou). http://dx.doi.org/10.1016/j.jhazmat.2014.08.038 0304-3894/© 2014 Elsevier B.V. All rights reserved.

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1. Introduction

2. Experimental

Microcystins are extremely hazardous pollutant compounds of emerging concern in water. They are very stable and cannot be removed by common chemical treatments due to their cyclic structure and the presence of some unique aminoacids which contribute to their resistant toward thermal and hydrolytic degradation [1–3]. Until now nearly 90 variants of microcystins have been discovered [4,5], but most toxicological information is available for MC-LR [1]. In Brazil in 1996, about 76 patients at a dialysis center died due to acute hepatic failure as a result of exposure to the high doses of microcystins through contaminated intravenous fluids [6]. To overcome the limitations of conventional water treatment methods, advanced oxidation technologies (AOTs) are being explored as emerging treatment processes. Titanium dioxide (TiO2 ) photocatalysis has been employed for the removal of cyanotoxins [7]. Although TiO2 has a high photocatalytic activity under UV light, its limited photocatalytic activity in visible light is still one of its main drawbacks. Therefore, it is of great interest to find a way to extend the absorption wavelength range of TiO2 to the visible region. Actually, a huge emphasis has been put all over the world to generate a visible light active TiO2 [8]. However the success of these efforts is still modest. Recently, doping TiO2 with non-metals (C, N, S) is one of the most efficient methods to extend the spectral response from UV to visible light region [9–25]. Several attempts have been made using doping of TiO2 with nonmetals. For example, Asahi et al. [9] reported that N doping can achieve band gap narrowing through substitution lattice sites by mixing of N2p with O2p states in the valence band. Another point of view [10] proved that interstitial type of N-atoms are achieved by generation of inter-gap states induced by formation of NO bond with the ␲ character at interstitial lattice sites [11]. As for the examination of TiO2 doping with carbon, similar to N-doping, there has also been a dispute if the doped type of carbon is substitutional [12] or interstitial [13]. In case of sulfur doping, sulfur substitutes either the oxygen as anion or the titanium as a cation in the TiO2 photocatalysts [14]. The overlap of sulfur 3p states and oxygen 2p states facilitates the visible light catalytic activity of S-doped TiO2 [13]. Among non-metal doped TiO2 materials, codoped TiO2 with three kinds of non-metals such as N, S and C [15–21] shows higher photocatalytic activity in the visible range compared with single element doping into TiO2 because of the beneficial merits from each dopant. Anatase is the most popular phase to be employed as a photocatalyst because of its high photocatalytic activity [23]. On the other hand, the heterojunction of anatase/brookite biphase TiO2 can achieve higher photocatalytic activity than single phase TiO2 , due to the synergistic effect between the two phases. In addition, the doping of the anatase/brookite biphase TiO2 with one nonmetal element has attracted much attention due to the improvement of the photocatalytic activity under visible light [24,25]. However, to date, little information has been gathered with respect to the degradation of cyanotoxins using non-metal-doped anatase/brookite heterojunction TiO2 photocatalysts (NMABH TiO2 ). In fact, in our previous published paper, non-metal doped polymorphic TiO2 was studied for the photocatalytic oxidation of MC-LR under visible light and the photocatalytic degradation was 75% in 5 h [15]. In continuation of our interest in the photodegradation of cyanotoxins, the synthesis of non-metal- (S, N and C) co-doped mesoporous anatase/brookite heterojunction TiO2 photocatalysts is reported in this study. The newly synthesized photocatalysts were evaluated for the destruction of MC-LR in water under visible light. In the present work, complete (100%) photocatalytic degradation of the highly toxic MC-LR pollutant under visible light illumination was reached.

Materials. The block copolymer surfactant poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol) (P-123, M wt. ∼5800, Sigma–Aldrich), titanium butoxide (97%), thiourea (99%), concentrated hydrochloric acid (35%) and methanol were purchased from Sigma–Aldrich and used as received. 2.1. Synthesis of biphase brookite/anatase TiO2 Mixed phase of mesoporous (A/B) TiO2 was prepared via sol–gel method. To synthesize titanium oxide nanoparticles, titanium butoxide (10 ml) was added dropwise to a solution containing 25 ml of deionized H2 O and 3.5 g of P-123 (non-anionic surfactant) under vigorous stirring at 80 ◦ C for 30 min. Sol sample formed was aged in a sealed beaker at 80 ◦ C for 24 h in order to form TiO2 gel precipitates. Then the resultant white gel suspension was filtered, washed several times with deionized water and ethanol, and then dried under vacuum at room temperature for 24 h. The sample obtained was designated as un-doped TiO2 (S1). 2.2. Preparation of (S, N and C) doped mesoporous anatase/brookite heterojunction To prepare (NMABH TiO2 ) sample, the S1 sample mesoporous powder was thermally treated after being homogenized with thiourea. Powder mixture of titania/modifier in weight ratio of 1 was prepared and calcined in a covered vessel at 450 ◦ C for 1 h. Muffle furnace (Nabertherm with controller) was used in this work with 5 ◦ C/min as the heating rate. The sample obtained was named as codoped TiO2 (CD1S1) which refers to the N, S and C co-doped with S1-TiO2 sample. The above procedures were modified and based on the work of Lee et al. and Tedorova et al. [26,27]. 2.3. Characterization of S1 and CD1S1 samples The phases, relative crystallinity, and crystal size of the prepared samples were characterized by using X-ray diffractometry ˚ radia(XRD, Bruker AXS D8, Germany) with Cu-K␣ ( = 1.5406 A) tion and secondary monochromator in the range 2 from 20◦ to 80◦ . Crystallite size was calculated from XRD data using the Scherrer equation. Specific surface area (SBET ), pore volume, and pore size distribution of the powder were determined by BET surface area analyzer (Nova 2000 series, Quantachrome Instruments, UK). A transmission electron microscope with an acceleration voltage up to 200 kV, magnification power up to 600 kX and resolution power down to 0.2 nm (TEM, JEOL-JEM-1230, Tokyo, Japan) was used to examine the structure of the formed phases. Infrared (IR) spectra were recorded by FT-IR spectrometer using KBr tablets (JASCO 3600, Tokyo, Japan). Raman analysis was carried out on a Senterra Dispersive Micro Raman, Bruker, Germany), which uses a doubled Nd:YAG laser ( = 532 nm) 10 mW as the excitation source. UV–vis spectrometry was applied for diffuse reflectance measurements to investigate the optical properties and the band gaps of S1 and CD1S1 samples. The reflectance spectra of the samples over a range of 200–700 nm were recorded with a Varian Cary 100 Scan UV–vis system equipped with a Labsphere integrating sphere with BaSO4 as reference material. The reflectance data were converted to Kubelka–Munk units. Surface chemical composition of the powders was investigated by X-ray Photoelectron Spectroscopy (XPS). Thermo-Scientific K-Alpha XPS system with X-ray source–Al Ka micro-focused monochromator – variable spot size (30–400 ␮m in 5 ␮m steps), ion gun – energy range 100–4000 eV, vacuum system – 2× 220 l/s turbo molecular pumps for entry and analysis chambers – auto-firing, filament TSP was employed. The binding energies

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Fig. 1. XRD patterns of S1 and CD1S1 samples.

were calibrated to the C1s peak at 284.4 ± 0.1 eV of the surface adventitious carbon. 2.4. Photocatalytic experiments To study the photocatalytic efficiency of the S1 and CD1S1 TiO2 heterojunction catalyst for the removal of MC-LR under visible light, a set of experiments was carried out according to the previous report [15]. The photocatalytic reaction system consists of a borosilicate glass Petri dish (Pyrex, 60 mm (∅) × 15 mm (h)) sealed with a quartz cover and cooled with fans to avoid solution evaporation. As a visible light source, two 15 W fluorescent lamps (Cole-Parmer, having light intensity 1.33 mW/cm2 ) with a UV block filter (UV420, Opticology) were used [28]. The suspensions of samples S1 and CD1S1 were prepared using ultrasonication. Then MC-LR (99.3%, Cal-Biochem) stock solution was dissolved in Milli-Q water (pH 5.8) to achieve an initial concentration of 0.5 ␮M. The synthesized S1 and CD1S1 samples (0.5 g/l) applied in the reaction were prepared as suspensions with ultrasonication. The total volume of suspended reaction was 10.0 ml. Then, the reaction mixture was mixed with magnetic stirrer while irradiating under visible light. Samples of 0.2 ml were taken from time 0 to 5 h, and then diluted with methanol and filtered. A high-performance liquid chromatograph (HPLC, Series 1100, Agilent) with detection set at 238 nm with a C18 Discovery HS (Supelco) column was employed to determine the concentration of MC-LR in the samples [29]. 3. Results and discussion 3.1. Chemical and physical analysis of the produced samples Fig. 1 shows the XRD patterns of the S1 and CD1S1 samples. The A/B TiO2 phase contents of the prepared samples are summarized in Table 1. The weight percentage of each crystal phase can

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be estimated from individual diffraction peaks on the basis of formulas reported in the literature [30]. The XRD pattern of sample S1 revealed the anatase (1 0 1) phase and emerging peak of brookite (1 2 1), indicating the prepared material was a mixture of anatase and brookite. The percentage of brookite phase did not change by doping, calculated to be about 25–26% for both samples according to (JCPDS); file No. (04-014-8515) and file No. (00-016-0617). No significant change was detected in the lattice spacing of anatase and brookite phases of the samples before and after doping with nonmetals as shown in Table 2. The average crystal size was calculated from Scherrer equation and the results revealed that the crystal size of the anatase and brookite phases was slightly increased by the nonmetal doping from 6.8 nm to 8.9 nm and from 13.1 nm to 16.4 nm, respectively. Fig. 2(a and b) shows nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curve of producing S1 and CD1S1 samples. All the samples show the type IV isotherms according to IUPAC classification, indicating the presence of mesoporous structure. For samples S1 and CD1S1, a hysteresis loop was observed for a wide range of pressures (P/Po ). The surface areas of S1 and CD1S1 were 226.2 m2 g−1 and 85.1 m2 g−1 , respectively. The low-pressure portion of isotherm for S1 sample indicated the existence of micropores (1–2 nm). When the reaction mixture of S1 was heated to 80 ◦ C, the TiO2 with mesostructure was obtained, whose properties were affected by the calcination temperature. The pore size distributions of S1 and CD1S1 samples were obtained by using the BJH method for the desorption branch. The S1 sample exhibited average pore size at 2.2 nm and pore volume of 0.253 cm3 g−1 , whereas the CD1S1 sample possessed the average pore size at 3.6 nm and pore volume of 0.203 cm3 g−1 as shown in Fig. 2(b) and Table 2. This is because the CD1S1 sample was calcined at 450 ◦ C. During calcination, the organic residues in titania matrix decomposed, leaving space as pores leading to the production of highly porous material compared to the S1 sample. Moreover, the latter interpretation was supported by DSC thermal analysis (data are not shown) in which the two exothermic peaks at (230–370 ◦ C) appear in case of the S1 sample, which reflect the process of oxidation of organic residues and anatase/brookite crystallization. But, in case of CD1S1 sample, there was no appearance of any exothermic peaks in this region (230–370 ◦ C). This wide mesoporous nature of CD1S1 sample plays an important role in the photocatalytic activity [31]. Fig. 3(a and b) displays the Raman spectra of the CD1S1 sample in comparison with S1 sample by excitation at 532 nm. The prepared samples show Raman active modes corresponding to brookite and anatase phases [32,33]. The peak frequencies and their assignment are shown in Table 3 [15,33–35], whereas S1 brookite modes are not clearly resolved. The main anatase Raman band for sample S1 at around 146 cm−1 is sharpened and blue-shifted when CD1S1 sample calcined at high temperature as shown in Fig. 3(b). The frequency shifts of the anatase Raman modes show an increase of the mean anatase crystallite size from 6.8 to 8.9 nm, estimated by the corresponding correlation curve in [36], which is in a good agreement with the data obtained from XRD and TEM measurements. Fig. 4 demonstrates the IR spectra of prepared S1 and CD1S1 samples. The spectrum of S1 sample displays strong absorption bands at 620, 1616 as well as a broad band between 3000 and 3800 cm−1 .

Table 1 Phase content and type of doping for different samples of TiO2 . Sample ID

Reference sample CD-TiO2 sample

S1 CD1S1

Thiourea addition

Not applied Ex situ

Phase content fraction (wt%) Anatase

Brookite

76 75

24 25

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Table 2 Characteristic properties of reference and co-doped-TiO2 samples. Sample ID

S1 CD1S1

Anatase

Brookite

Crystal sizea (nm)

d-Spacingb (1 0 1) (Å)

Crystal size (nm)

d-Spacingc (1 2 1) (Å)

6.8 8.9

3.516 3.516

13.1 16.4

2.903 2.902

SBET (m2 g−1 )

Pore volume (cm3 g−1 )

Average pore size (nm)

Band gap (eV)

226.2 85.1

0.253 0.203

2.20 3.6

3.1 2.9

SBET , surface area. a Based on XRD analysis using the Scherrer equation: crystal size = 0.9/(ˇ × cos ), where  = 0.154 nm,  is the Bragg angle, and ˇ is the full wide at half the maximum intensity (FWHM) for the main peaks (1 0 1) and (1 2 1) for anatase and brookite phase, respectively. b,c Based on XRD analysis using Bragg’s law: lattice spacing D = /(2 × sin ).

Fig. 2. (a) N2 adsorption–desorption isotherms of the S1 and (CD1S1 samples). (b) BJH pore-size distribution.

Fig. 3. (a) Raman spectra of CD1S1 and S1 samples. (b) The main anatase Raman band of CD1S1 sample.

S.M. El-Sheikh et al. / Journal of Hazardous Materials 280 (2014) 723–733 Table 3 Raman vibration bands observed in the S1 and CD1S1 samples at 532 nm excitation wavelength. Peak position (cm−1 )

Assignment

N, S and C co-doped TiO2 at 532 nm

Un-doped TiO2 at 532 nm

146 198 199 294 325 366 399 518 639 641

150

400 516 638 640

Anatase (Eg ) Brookite (Ag ) Anatase (Eg ) Brookite (Ag ) Brookite (B1g ) Brookite (B2g ) Anatase (B1g ) Anatase (A1g , B1g ) Anatase (Eg ) Brookite (Ag )

Fig. 4. FT-IR spectra of sample CD1S1 and S1 samples.

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The appearance of the latter bands is ascribed to the surface (OH) stretching vibrations of the coordinated water molecule groups originating from dissociation chemisorbed and nondissociated water molecules [37]. The band located in the range 1616–1627 cm−1 can be attributed to the bending vibration of O H bond from surface adsorbed water [37]. All these bands in S1 sample are sharply decreased with CD1S1 sample as a result of calcination at 450 ◦ C. The band around 620–622 cm−1 belongs to the Ti O bending vibration [22] and is shown with similar intensity for both samples [22]. Furthermore, for the CD1S1 sample several low intensity modes are resolved at 480, 721, 840, 1048, 1125, 1400, 1430–1477, 1627, 2830, 2901 cm−1 . The absorption peaks at 1125 cm−1 and 1048 cm−1 are assigned to Ti O S bond and Ti S bond, respectively [38,39]. Moreover, the band at 1125 cm−1 is ascribed to the asymmetric and symmetric stretching frequencies of the S O bonds, which contribute to the TiO2 surface [40]. Moreover, the peak around 1400 cm−1 is assigned to the S O bond for sulfate group and it is typical of S6+ [41,42]. It should be mentioned however that most of the above peaks can be related to the N doping, too. Thus the weak band at 1400 cm−1 could be alternatively ascribed to the bending vibration of ammonium ion [43], the peaks at 1400 and 1048 cm−1 to TiO2 N and those at 1450 and 1125 cm−1 to nitrite [44]. One further peak at 840 cm−1 suggests that carbonate is present [45–47]. An additional peak at 1075 cm−1 demonstrated deformation and C O stretching of S1 [48]. The sharp absorption peak near 480 cm−1 is assigned to vibration of Ti–N bond [49]. FT-IR peaks at 2830, 2920 and 1477 cm−1 are characteristic of C H stretching, scissoring, and bending modes, respectively. Fig. 5(a) shows the UV–vis spectra of un-doped and doped TiO2 heterojunction. As shown in Fig. 5(a), when the S1 sample was doped, the absorption edge gradually shifted from 390 to 478 nm, reflecting that the band gap value of doped TiO2 obtained is decreased. The band gap (Eg ) can be estimated by plotting [F(R)h]1/2 , where F(R) is the Kubelka–Munk function, as a function of h. The analysis assumes indirect band gap and calculates Eg by extrapolating the linear part of the corresponding function curve to the energy axis (see Fig. 5(b)) [50–52]. The band gap energies of S1 and CD1S1 samples were 3.1 eV and 2.9 eV, respectively (Fig. 5(b) and Table 2). The red shift for doped sample is ascribed to the fact that N, S and C co-doping can narrow the effective band gap of the TiO2 [18,53]. In general, the significant increase of the absorption

Fig. 5. (a) UV–vis absorption spectra of CD1S1 and S1 samples. (b) Tauc plots of modified Kubelka–Munk function.

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Fig. 6. TEM images of samples (a) S1 and (b) CD1S1.

Fig. 7. (a) XPS survey spectra for samples CD1S1 and S1. XPS detailed scans in the energy regions of (b) N 1s, (c) S 2p and (d) C 1s for CD1S1.

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Table 4 Comparison of CD1S1 sample properties with literature data on C–N–S co-doped TiO2 materials. Photo-catalysts type (doping materials)

XRD analysis Phase content (wt%)

Crystal size (nm)

S–N–C–TiO2 (thiourea)

Anatase:brookite (75:25)

A = 8.9 B = 16.4

S–N–C–TiO2 (thiourea)

Anatase:brookite:rutile (59.5:34.3:6.2)

A = 6.4 B = 7.8 R = 12.7

C–N–S–TiO2 (thiourea)

Anatase

A = 11.7

C–N–S–TiO2 (cystine)

Anatase (100)

A = 3.4

C–N–S–TiO2 (cystine)

Anatase (100)

A = 5.23

C–N–S–TiO2 (thiourea)

Anatase (94)

A = 10.7

C–N–S–TiO2 (l-cysteine)

Anatase (99.7)

A = 10.5

C–N–S–TiO2 (thiourea)

Anatase –

–

XPS analysis Binding energy (eV)

- Two peaks for N1s at 399.7, 401.8 - One peak for S2p at 168.5 - Three peaks for C1s at 284.4, 286, 288.2 - Two peaks for N1s at 399.4, 401.1 - One peak for S2p at 168.3 - Three peaks for C1s at 284.6, 286, 288.4 - Two peaks for N1s at 396, 399.9 - Two peaks for S2p at 162.1 and 168.8 - Three peaks for C1s at 284.8, 286.4, 288.7 - Two peaks for N1s at 399.8, 401.8 - One peak for S2p at 168.4 - Three peaks for C1s at 284.54, 285.59, 288.60 - Two peaks for N1s at 399.8, 401.8 - One peak for S2p at 168.5 - Two peaks for C1s at 284.6, 288.2 - Two peaks for N1s at 399.7, 401.5 - Two peaks for S2p at 167.1, 168.4 - Four peaks for C1s at 282.6, 284.4, 286.4, 288.7 - Two peaks for N1s at 399.3, 401.1 - One peak for S2p at 163.3 - Three peaks for C1s at 284.8, 286.6, 288.4 - Three peaks for N1s at 397.8, 399.9, 401.9 - One peak for S2p at 168.5 - Three peaks for C1s at 284.8, 286.4, 288.8

in the visible irradiation range can be assigned to the formation of new energy levels that was obtained as a result of non-metal ion doping [15,24]. TEM measurements were used to investigate the particle size, crystallinity, and morphology of the synthesized heterojunctions A/B TiO2 as shown in Fig. 6(a and b). A relatively narrow size distribution of monodispersed nanoparticles with spheroid mor-

DRS analysis Wave length (nm)

Application

Reference

Band gap (eV)

478

2.9

Degradation of MC-LR (100%)

This work

420

2.95

Degradation of MC-LR (75%)

[15]

≈530

–

Degradation of X-3B (Reactive Brilliant Red dye, C.I. Reactive Red2)

[16]

≈545

–

Degradation of rhodamine B in UV and visible light

[17]

≈550

1.99

Degradation of phenol

[18]

≈490

2.67

Degradation of tetracycline

[19]

≈425

2.79

Removal of NO

[20]

≈420

2.88

Degradation of toluene

[21]

phology was revealed by the TEM analysis. Mean particle diameters of 5–10 nm and 10–15 nm were observed for samples S1 and CD1S1, respectively. The particle sizes of the prepared samples measured by TEM images are in agreement with XRD measurements by Scherrer equation. The selected area electron diffraction (SAED) data revealed two characteristic rings corresponding to brookite (1 2 1) and anatase (1 0 1) phases.

Table 5 C/Co values at t = 2 h as well as apparent removal constant (Kapp ) and removal rates (ro ) for adsorption in dark condition and visible light activated photocatalytic oxidation of MC-LR by the S1 and CD1S1 samples. Sample ID

C/Co

ln(C/Co )

Kapp (min−1 )

ro × 10−4 (␮M/min)

C/Co

Visible light irradation S1 CD1S1

0.90014 0.32138

−0.0985 −1.135

ln(C/Co )

Kapp (min−1 )

ro × 10−4 (␮M/min)

0.000548 0.0017

2.74 11.26

Dark condition 0.000821 0.009459

4.11 47.3

0.9363 0.76325

−0.0657 −0.2701

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Fig. 8. (a) Photocatalytic degradation under visible light illumination and (b) adsorption in the dark of microcystin-LR with S1 (blank) and CD1S1 samples. Corresponding time histograms showing (c) the visible light photocatalytic efficiency and (d) the adsorption of microcystin-LR with S1 (Blank) and CD1S1 samples.

The surface electronic states of the CD1S1 sample were investigated by XPS (Fig. 7). Fig. 7(a) compares the XPS survey spectra of Ti 2p, N1s, S 2p, C1s and O 1s core levels in undoped and doped mesoporous TiO2 sample. Fig. 7(b) shows the N1s peaks for N doped TiO2 sample CD1S1. It is clear that two peaks were noticed from curve fitting at about 399.7 and 401.8 eV. The peak at 399.7 is attributed to the binding energy of N atom in the chemical bond Ti N O. The peak at 401.8 eV was attributed to the N-doping in the TiO2 as an O Ti N structural feature [54,55]. The O Ti N is a substitutional N-doping, which resulted from the bonding of N atoms in sample CD1S1 to Ti atoms and replacement of lattice oxygen in the TiO2 [55]. The N content was found to be about ∼1.41%. Fig. 7(c) shows the peaks of S 2p for CD1S1 sample. The results revealed that the binding energy peaks located at 168.5 eV for S6+ [56–58]. The substitution of Ti4+ by S6+ is much easier and more favorable than the replacement of O2− with S2− [15,16,19,14].

The S content was found to be about ∼2.33%. Fig. 7(d) depicts the C 1s XPS spectrum of CD1S1 sample; the peak at 284.4 eV is attributed to C C and C H bonds (adventitious carbon) [59]. Moreover, peaks at 286 and 288.2 eV were assigned to C OH and C O (or carbon bound to the three oxygen atoms in the carbonate ions) [45,60]. XRD, XPS and DRS data for comparison between the codoped CD1S1 sample synthesized in the present work and C N S codoped TiO2 reported in the literature are summarized in Table 4. 3.2. Photocatalytic activity investigation under visible light In this work, S1 and CD1S1 samples were evaluated for the photocatalytic degradation of MC-LR under visible light irradiation (Fig. 8). The initial photocatalytic degradation rates of the MC-LR using CD1S1 and S1 samples were calculated

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Scheme 1. Proposed mechanism illustrating the action of the visible-light-induced C, N and S doped mesoporous anatase–brookite nano-heterojunction photocatalysts for the destruction of microcystin-LR (MC-LR); absorption of visible light by modified photocatalysts promotes the excitation of an electron from the valence band to the conduction band.

and summarized in Table 5. The results revealed that the removal (degradation + adsorption) rate of the CD1S1 sample was 47.3 × 10−4 ␮M/min, which was much better than that of the S1 sample (4.11 × 10−4 ␮M/min) under visible light. These results indicate that the CD1S1 sample has higher photocatalytic activity under visible light, although the surface area for CD1S1 sample was three times less than that of S1 sample. The adsorption of MC-LR in the dark was observed to be very small when compared with photocatalytic degradation data under visible light. Fig. 8(a–d) shows MC-LR degradation rate and efficiency with S1 and CD1S1 samples under visible light and in the dark for 5 h. Under visible light, we observed that S1 sample showed small removal of MC-LR (about 12.2%) However, a complete removal of MC-LR (100%) was achieved within 5 h in the presence of CD1S1 sample as shown in Fig. 8(a and c). On the other hand, it was found that the removal of MC-LR in the dark was about 37.5% after 5 h with CD1S1 sample compared to about 12% for the S1 sample as shown in Fig. 8(b and d). Fig. 9(a and b) shows rate constants for MC-LR degradation under visible irradiation and dark condition using doped and undoped mesoporous TiO2 for 5 h. The calculated rate constants for MC-LR degradation under visible light using doped and undoped TiO2 were 0.587 h−1 and 0.0232 h−1 , respectively. The calculated removal constants for MC-LR adsorption in the dark using doped and undoped TiO2 were 1.01 × 10−1 h−1 and 2.52 × 10−2 h−1 , respectively. The enhanced photocatalytic activity of doped samples is due to several possible factors, especially the presence of non-metals. For example, S6+ in SO4 2− anchored on the surface is favorable for trapping photoinduced electrons (e− ), which can produce more hydroxyl radicals by suppressing the recombination between electrons and holes [61]. Also, carbon doping can enhance the photocatalytic activity of A/B TiO2 due to the conductivity of the A/B TiO2 , allowing efficient charge transfer to the external site of the TiO2 nanoparticles, where the desired oxidation reactions take place. In addition to the above reasons, the high crystallinity and the mesoporosity of the new photocatalysts CD1S1 can help in the enhancement of the photocatalytic activity. This is explained by the high adsorption capacity as a result of faster and facile diffusion of the target MC-LR molecule to the active sites through the porous TiO2 network (Scheme 1) [31]. On the other hand, the electron transfer in the NMABH TiO2 is displayed in Scheme 1. It was suggested that, the electrons in the midgap levels formed from mixing of the O2p orbitals of TiO2 with orbitals of non-metal dopants in both anatase and brookite could be excited up to the individual conduction band, leaving holes in the localized states under the visible light irradiation. Meanwhile, the codoped carbon may form carbonaceous species on the

surface of TiO2 which acts as a photosensitizer like organic dyes [62]. As well as, the energy band matching of anatase and brookite facilitates the interfacial migration of photoinduced accumulated electrons from brookite to anatase depicted on the basis of the larger band gap of brookite compared to anatase [63–65]. Consequently, these electrons transferred to oxygen adsorbed on the TiO2 surface, producing the O2 •− which may help in MC-LR degradation [62].

Fig. 9. Plot of ln(C/Co ) versus time for degradation of microcystin-LR with CD1S1 and S1 (blank) samples under (a) visible light illumination and (b) dark condition.

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4. Conclusions Mesostructured undoped and C, N and S co-doped anatase–brookite nano-heterojunction titania photocatalysts have been successfully synthesized using a simple sol–gel method. Synthesized nonmetal doped TiO2 nanoparticles had an average diameter of about 10 nm and exhibited quite uniform size and shape. The visible-light-induced photocatalytic degradation of MC-LR using these materials was evaluated. The overall MC-LR removal (photocatalytic activity + adsorption) rate for the doped sample was higher than that of undoped sample. The superiority of C, N and S co-doped anatase–brookite nano-heterojunction titania photocatalysts prepared in the present work was explained by their high crystallinity and large surface area, in addition to heterojunction and mesoporous structure of the doped sample.

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Acknowledgments This work was supported by STDF under Grant no. ID 3727 and US-Egypt collaboration funded by the U.S. Department of Agriculture (58-3148-1-152). P.F. acknowledges co-financing by the European Union (European Social Fund – ESF) and Greek National Funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) – Research Funding Program: Thales “AOP-NanoMat” (MIS 379409). D.D. Dionysiou also acknowledges support from the University of Cincinnati through a UNESCO co-Chair Professor position on “Water Access and Sustainability”.

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