ZnO photocatalyst under visible light irradiation

Journal of Molecular Liquids 258 (2018) 354–365

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Evaluating the efficiency of nano-sized Cu doped TiO2/ZnO photocatalyst under visible light irradiation Mohammad Reza Delsouz Khaki a, Mohammad Saleh Shafeeyan b,⁎, Abdul Aziz Abdul Raman a, Wan Mohd Ashri Wan Daud a a b

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Chemical and Materials Engineering, Buein Zahra Technical University, Qazvin, Iran

a r t i c l e

i n f o

Article history: Received 28 July 2017 Received in revised form 2 November 2017 Accepted 3 November 2017 Available online 8 November 2017 Keywords: Photocatalyst Cu-TiO2/ZnO Band gap AOP Methyl orange Methylene blue

a b s t r a c t A visible light responsive photocatalyst, nano-sized copper doped TiO2/ZnO, was synthesized by sol gel method. It was characterized in terms of thermal stability, crystalline phase, crystal size, morphology, surface area, UV–Vis DRS and band gap. The results showed that the synergistic effect of copper ions considerably narrowed the band gap of the synthesized photocatalyst compared to TiO2/ZnO. Its photoactivity was then evaluated by measuring degradation efficiency of methyl orange (MO) and methylene blue (MB) in terms of colour, COD and TOC removal. The synergistic or antagonistic effects of different combinations of dye and catalyst concentrations, pH, intensity of light irradiation and reaction time on photoactivity of Cu-TiO2/ZnO were also investigated. The highest photoactivity achieved was 85.45% of colour, 70.56% of COD and 48.70% of TOC removal for MO and 73.20% of colour, 59.92% of COD and 38.77% of TOC removal for MB under optimal conditions. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent decades, advanced oxidation processes (AOPs), such as Fenton/photo-Fenton oxidation, ozonation and heterogeneous photocatalysis have been extensively investigated for their possible applications in wastewater treatment. Among them, photocatalysis has been confirmed as a stable and highly efficient alternative to traditional disinfection technique for a variety of wastes [1–6,72,73]. In photocatalysis, the photocatalyst absorbs enough energy, which is nearly equal to its band gap energy level to become excited. This process generates an electron-hole pair that reacts with water and oxygen molecules or hydroxyl groups to generate highly reactive oxygen species i.e. superoxide anion (•O2 −) and hydroxyl radical (•OH). Then, the oxyradical species attack vital organic components and decompose them through oxidation reactions. The aforementioned mechanisms are illustrated by Eqs. (1)–(6). Photocatalyst þ hν→e− þ h

ð1Þ

ðO2 Þads þ e→−O2 − ˙

ð2Þ

O2 − ˙ þ H þ →HOO˙

ð3Þ

⁎ Corresponding author. E-mail address: [email protected] (M.S. Shafeeyan).

https://doi.org/10.1016/j.molliq.2017.11.030 0167-7322/© 2017 Elsevier B.V. All rights reserved.

HO2 – þ Hþ →H 2 O2

ð4Þ

H 2 O2 þ e− →OH– þ OH˙

ð5Þ

þ

H 2 O þ h →Hþ þ OH˙

ð6Þ

Among different semiconductors, titanium dioxide (TiO2) and zinc oxide (ZnO) are the most successful and popular photocatalysts that have demonstrated a high photosensitivity and chemical stability [74]. They are non-toxic and of low cost. However, they suffer from some limitations regarding their optical and electronic properties [7]. TiO2 and ZnO are highly active photocatalysts under UV light irradiation since their photogenerated electrons and holes are efficient oxidizing and reducing agents. However, the large band gaps of TiO2 and ZnO (which are roughly 3.2 eV [8] and 3.37 eV [9]) restrict their photoelectrochemical application under visible light irradiation. Easy recombination of photo-induced holes and electrons also reduces their efficiency. Adding an oxide to these photocatalysts and supporting them on an oxide have been used to improve their photoactivity and thermal stability during catalysts preparation. Among various oxides such as TiO2, SnO2, SiO2, CeO2, ZnO, WO3 and ZrO2, previous works have demonstrated that the hetero-junction of TiO2 and ZnO can result in synergic effects due to the injection of conduction band electrons from ZnO to TiO2, decreasing the recombination rate and increasing the lifetime of the electron-hole pair.

Table 1 Application of ZnO-TiO2 hetro-junction in degradation of water pollutants. ZnO:TiO2 ratio

Morphology

Synthesis method

Synthesis conditions

Pollutant Light, λ (nm)

Time min

Dye ppm

Cat g/L

ZnO/TiO2 ZnO/TiO2

1.23:1 (w/w) 50:50%

NP, Ring SH NP, non-uniform

Pyrolysis Sol-Gel

MO B, T, X

SL, 320–800 VIS

120 120

5 100

– 1

ZnO/TiO2 Thin film

3:1 (w/w)

Spin coating

20,50,80 wt% 1:1 MR

SS Hydrothermal

450 °C, 6 h 180 °C, 2,6,12,24 h

ZnO/TiO2 ZnO/TiO2 ZnO/TiO2 TiO2/ZnO TiO2/ZnO

1:0.2 MR 10,20,30,40%M 1:1 (w/w) 1:10,2:10,3:10MR 1:1 MR

NP Porous thin film NF, flower SH NP, irregular NF, non-woven

MW Sol-Gel Sol-Gel Electrospinning Sol-Gel Electrospinning

500 °C, 7 h 500 °C, 0.5 h 500 °C, 4 h 500 °C, 5 h 500 °C, 2 h

210 210 75 25 25 30 280 70 90 40

50 10 100 5 10

TiO2/ZnO nanofiber

–

Poly-nanocrystal

Electrospinning

60

1e−6 M

ZnO/TiO2 ZnO/TiO2 ZnO/TiO2

Z:0.5,2,4,10%M 1:1 MR ZnO = 6 wt%

NP Mesoporous plate Composite

Wetness impregnation Tape casting & lamination Sol-Gel & SS

550,650,750, 850 °C, 2 h 400 °C, 2 h 600,650,700/2 h 500, 3 h

Orange II UV,315–400 VIS, 4CHl UV,303–578 MO UV, N400 VIS, RhB SS, VIS, MV MO SS ARS UV MB UV MO UV 4NPh RhB UV–vis,420

20

ZnO/TiO2 ZnO/TiO2

Z:nanowires T:nanoparticle NP, non-uniform NP, hollow spheres

400 °C 380 °C 500 °C 320 °C, 3 h

Cr(VI) RBR MO

2:0.1 M 1:1 (w/w) 1.97:1.11 (w/w) –

NP NP NP round-SH NP, nanorod

MW-Assisted Chemical Hydrothermal Sol-Gel

– 90 300 40 240 120 120 20

20 50 50

ZnO/TiO2 ZnO/TiO2 ZnO/TiO2 TiO2coated ZnO

UV–vis,365 UV VIS, N400 UV UV UV b 390 UV,365 UV, 365

ZnO/TiO2 ZnO/TiO2

0.36:0.66 (w/w) Z:15,30,45,60 wt % Z:T = − 1:1

NP/NF NF

12.8 1e−6 M

NP:Nano flower NP

60 40 40 120 120 120 15,30 45,60 120 180 150

10 5e−6 M

10 h

20

ZnO/TiO2 ZnO/TiO2 ZnO/TiO2 Core–shell TiO2/ZnO TiO2–ZnO ZnO/TiO2 ZnO/TiO2 NP TiO2/ZnO thin film

11.7:1 (Atomic R) NP Nano flower – Nano wires

Cr(VI) Diazinon Cr(VI) MB

Electrospinning Electrospinning

500, 0.5 h 100 °C, 3 h 180 °C, 20 h T: 70 °C, 2-3 h Z: 150 °C 500 °C, 4 h 500 °C, 4 h

Electrospinning Sol-Gel

250 °C, 3 h 100 °C, 3 h

MB Cr(VI)

Hydrothermal Pulsed laser deposition

140 °C, 2 h 450 °C, 1 h

RB5 RhB

UV,365 Visible,420 UV,365 UV,365 nm UV, 247.3 nm UV,UV, 320–400

Roll-coating Hydrothermal Microwave

120 °C, 12 h 140 °C, 2 h

Laser deposition

Z: 200 °C, 0.5 h T: 600 °C, 1 h 500,600,700 °C,2 h

MO MB MB Cr2O2− 7 MO

UV,300–600 UV,365 nm 671 nm UV VIS

TiO2/ZnO

Zn:Ti = 1:1

NP on glass film NP, Nano flower Tiny particle b10 nm TiO2:Nanorod on ZnO buffer layer NP irregular

ZnO–TiO2

7.5%:92.5%

NP

Ammonia-induced synthesis

450 °C, 4 h

TiO2/ZnO

1:7,1:5,1:3.5 wt%

NF

Electrospinning

500 °C, 5 h

ZnO/TiO2 TiO2/ZnO

– Zn:Ti = 1:3

NP b 10-20 nm NP irregular

Hydrolysis Sol–gel

ZnO coated TiO2

–

NP-amorphous

MW Sol-Gel

180–200 °C,350 °C, 1 h 150 °C, 24 h 120 °C, 3 h

1:1,1:2,1:3,1:4 96:4 atomic R 1:1,2:1 –

Sol-Gel

MB RhB

25 20

10 20 10 5

25 20

10 10 5e−6 M

−6

Efficiency (Y) ZnO/TiO2

Y ZnO

Y TiO2

Ref.

94.8 44.8, 45.7, 49.1 36.9, 39.5, 45,1 0.5 27.5 36.9 2 100, 87, 78 1 80.5,88.4,91,92 57.4,66,69,74.4 1.7 82.4, 68.8, 25.7 4 79.6,60.3,7.3,9.3 2.5 99.4 1 91.3, 94.9, 73.6 0.01 84.99 70.6 1 53.9, 86.8, 55.4, 36.2, 33.7 1 Max for ZnO:2% 1 76.9, 68.5, 31.5 1 11.7 45.38 1 29.5 0.5 12.7 0.5 16.3 0.5 98.9

28.9 –

19.1 –

[39] [40]

52.9 74.1 75 92.4 – 89.7,78.8,36.7 – 51.3, 67.9 – 49.9, 52.9 30.3, 29.5 –

–

[41]

84 29.3 – – 48.1 – 87.5 –

[30] [42]

35.2

[48]

– C700 = 55 – – 42 – 10.9 Z: 67.3

– – – 76.4 – – – –

[49] [50] [51] [52] [53] [54] [55]

NF:10 –

NF:7 94.2

[56] [57]

46.1 82.3

58 86.1

[58] [59]

47.2 59.7,76.5 54.8,59 0.01 57.6,65,73.5,80 0.8 69.7 14 96.7, 94.3 33.8, 31 – 32.9

52.6 60 min, 32.2

– 31.9

[60] [61]

46.5 47.7 Zn2TiO4:91.6 Zn2TiO4:4.7 25.5

87.8 66.6 –

[62] [63] [64]

19

[65]

0.5

30.8, 34.2, 54.8

–

–

[66]

0.8

41.2 68.9 44 84, 98.1, 90.1

–

[67]

–

[68]

81.7 27.9 47.5 98.9

76.3 – – 58.2

81.8 93.0 99.4 NF:80 NP:65 51.8 – –

[71]

2.5 0.5 1 0.8 1

NP:25,NF: 31.6 78, 93, 83.2, 81 79 68 99.9

0.3

MO

UV,254 nm

3h

MO SSY RHB RhB

UV-A,365 nm

30

25e M 50

UV,365 nm

24 h

8

0.5

MO MO

UV,365 nm UV,254 nm

20 3h

2.5 0.5

OG

UV,340 nm

90

20 25e−6 M –

–

[69] [70]

355

Note: MO: Methylene Orange, MB: Methyl Blue, 4CH: 4chloropheno, RhB: Rhodamine B. RBD: Remazol Brilliant Red, RB5: reactive black 5, OG: Orange G, 4NPh:4-nitrophenol, SSY: SunSet Yellow, T:TiO2, Z:ZnO. SS: Solid-State, MW:MicroWave, SH: Shaped, R:Raio. NF: Nano Fiber, NW: Nano Wire, NP: Nano Particle.

[43] [44] [45] [46] [47]

M.R. Delsouz Khaki et al. / Journal of Molecular Liquids 258 (2018) 354–365

Catalyst

356

M.R. Delsouz Khaki et al. / Journal of Molecular Liquids 258 (2018) 354–365

The most important works considering TiO2/ZnO photocatalyst in the past five years are listed in Table 1. This table summarises the most important results obtained by different researches aiming at investigating the effects of different variables on TiO2/ZnO lattice i.e. synthesis method, calcination temperature, TiO2:ZnO ratios, photocatalysis reaction time and light irradiation, etc. As per Table 1, TiO2/ZnO has mostly been synthesized at temperature ranging from 100 to 500 °C. Depending on the ratio of TiO2 to ZnO, increment of temperature to beyond 500 °C has reduced the photoactivity of TiO2/ZnO lattice in most cases. In terms of calcination time, it can be suggested that calcination at lower temperature but for a longer duration yields more favorable results. The ratio of TiO2 to ZnO is the other effective factor. As per Table 1, the optimum ratio of TiO2 to ZnO yields the highest TiO2/ZnO photoactivity, depending on the synthesis method and conditions. In short, TiO2/ZnO has demonstrated a better photocatalytic degradation of most organic contaminants compared to pure TiO2 or ZnO. However, it should be noted that most of the work have been conducted under UV irradiation, indicating that there are still rooms for improvement so that TiO2/ZnO lattice can be efficiently used for visible light photocatalysis. More details can be obtained from Table 1. On the other hands, doping and co-doping of photocatalysts with non-metals, transition-metals, noble metals or lanthanide ions are the methods employed for retarding the fast charge recombination and enabling visible light absorption by creating defect states in the band gap [75,76]. In the former case, the valance band holes or conduction band electrons are trapped in the defect sites, inhibiting the recombination of photo-induced holes and electrons and improving the interfacial charge transfer. In the latter case, the electronic transitions from the defect states to conduction band or from valance band to the defect states are allowed under sub-band gap irradiation. Among different transition metals such as Cr, Fe, Ni, Zn, Co, [10–13] and Cu, copper with redox potentials of 0.52 V (Cu2+/Cu) and 0.16 V (Cu2 +/Cu+) has been used as a suitable modifier for various visible light responsive photo-catalysts [14]. Cu2 + (0.73 Å), Zn2 + (0.83 Å) and Ti4 + (0.64 Å) have similar ionic radius parameters and therefore Cu2+ can easily penetrate into TiO2 and ZnO matrixes as a deep acceptor in conjunction with neighboring oxygen vacancy or substitute the positions of Zn2+ or Ti4+ [77]. In addition, doping of Cu shifts the absorption edges of both the photocatalysts towards the visible region [15]. Cu2+ directly traps the electron generated from the excitation of photocatalyst in Cu-TiO2 or Cu-ZnO. As such, doping reduces electronhole recombination rate during photocatalysis by generating charge trapping sites. Copper has been widely used as a dopant compared to the other transition metals. Sreethawong and Yoshikawa [16] compared the photocatalytic activities of Au-, Pd-, and Cu-loaded mesoporous TiO2 by a singlestep sol-gel process with surfactant template. Zhou et al. [17] also investigated the photocatalytic activity of meso-tetraphenylporphyrins with different metal centers (Fe, Co, Mn and Cu) on the surface of TiO2 (Degussa P25) and reported that CuP-TiO2 presented the highest activity. In the other work by Kaneco et al. [18], the authors investigated the photocatalytic hydrogen production from aqueous alcohol solution with ZnO/TiO2, SnO/TiO2, CuO/TiO2 and CuO/Al2O3/TiO2 where the maximal hydrogen production was obtained by using the latter. Few studies have also been conducted on codoping of TiO2 with copper and another metal/non-metal dopant [19–21]. Although incorporating of a dopant

into the integrated structure of ZnO/TiO2 may possess the improved physical and chemical properties, very few studies have focused on that. To the best of the authors' knowledge, no study has been conducted on integration of copper into the structure of TiO2/ZnO to further support the semiconductors. Accordingly, in this study, a hybrid composite of Cu-TiO2/ZnO was synthesized through sol-gel method and characterized subsequently. The catalyst was synthesized with the optimum quantity of Cu and it was calcined at the optimum calcination temperature. The main focus of the work was on evaluation of Cu-TiO2/ZnO photoactivity under visible light irradiation which was conducted through decomposition of MB and MO. Herein, the effects of five main process parameters: dye concentration, catalyst load, initial pH, intensity of visible light irradiation and reaction time were investigated using central composite design (CCD). 2. Materials and methods 2.1. Materials Titanium (IV) isopropoxide (TTIP, 97%), Zinc acetate dihydrate (Zn (CH3COO)2·2H2O, N98%), copper (II) nitrate trihydrate (Cu(NO3) 2.3H2O, 99%) and ethanol (C2H5OH N 99.8%) for catalyst synthesis were purchased from Sigma Aldrich and Chemical U.K. Acetic acid (CH3COOH, M = 60.051 g/mol and Purity = 98%) and hydrochloric acid (HCl, M = 36.45 g/mol, Purity = fuming 37%) and sodium hydroxide (NaOH, M = 40.0 g/mol, Purity = 99.99%) that were used to control the ambient pH were supplied by Merck. Methylene blue, and methyl orange were purchased from the same company. All the chemicals were used without further purification. Table 2 presents the main properties of the investigated dyes. 2.2. Synthesis of Cu-TiO2/ZnO In this study, Cu doped TiO2/ZnO photocatalyst was synthesized through sol-gel method. The sol–gel process has been applied in preparing supported metal catalysts and catalyst supports with higher thermal stability and resistance to deactivation while providing better flexibility in controlling catalyst properties, such as particle size, surface area and pore size distribution. Many authors have suggested sol–gel as the best means of dispersing catalytic metals in gels with fine texture [22]. The synthesis process comprises three steps, which are gel preparation, drying and calcination. In the first step, a predetermined amount of TTIP was mixed with ethanol and stirred for an hour at room temperature to get the first precursor solution (A). Then, a 30 mL solution of distilled water, ethanol, acetic acid and zinc acetate (according to the weight ratio of TiO2:ZnO = 70:30) were mixed together by continuous and tempestuous agitation to form a homogeneous solution (precursor B). Meanwhile, the solution pH value was controlled between 2.0 and 3.0 by adding certain quantity of hydrochloric acid solution. Afterwards, a certain amount of Copper (II) nitrate trihydrate was added to precursor B with vigorous stirring for metal doping. Then, precursor B was dropped into precursor A at a speed of one drop per second under strong stirring. Next, the mixture was magnetically stirred at room temperature for 12 h and placed in air for one day to form aged Cu-TiO2/ZnO sol. In the second step, the prepared sol was allowed to dry in an electric

Table 2 The main properties of the investigated dyes. Mw (g·mol−1)

λmax (nm)

Colour index

C16H18N3SCl

319.852

664

52.015

C14H14N3NaO3S

327.33

508

13,025

Investigated Dye

Formula

Methylene blue

Methyl orange

Molecular structure

M.R. Delsouz Khaki et al. / Journal of Molecular Liquids 258 (2018) 354–365

oven at 100 °C for 1 h and was then ground in a mortar-and-pestle into fine powder. In the final step, the prepared powder went through the calcination process in a Furnace (Thermconcept KL 15/11) with a heating rate of 10 °C/min. The calcination continued for 2 h with two desirable temperatures to make the molecular network and structure of the photocatalyst regular and remove excess solvent. The nano-sized Cu-TiO2/ZnO was then obtained through ball milling process. It should be noted that, 5 different concentrations of Cu (1 to 5 wt%) and the TiO2 to ZnO ratio of 7:3 (wt%) were investigated. The co-dopants ratio was: 1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%. The effects of two different calcination temperatures of 500 and 700 °C were considered as well. Cu content of 3 wt% along with calcination temperature of 500 °C provided the best characteristics for Cu-TiO2/ZnO. Accordingly, Cu (3 wt%)-TiO2/ZnO calcined at 500 °C was synthesized, characterized and evaluated in this study. In order to evaluate and compare the photocatalytic activity

Run No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Experimental design

Methyl orange removal (%)

Methylene blue removal (%)

A

B

C

D

E

UV

COD

TOC

UV

COD

TOC

4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 4 10 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

14 14 23 23 14 14 23 23 14 14 23 23 14 14 23 23 14 14 23 23 14 14 23 23 14 14 23 23 14 14 23 23 18.5 18.5 14 14 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5 18.5

20 20 20 20 50 50 50 50 20 20 20 20 50 50 50 50 20 20 20 20 50 50 50 50 20 20 20 20 50 50 50 50 35 35 35 35 20 50 35 35 35 35 35 35 35 35 35 35 35 35

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.5 0.5 0.5 0.5 0.5 0.5 0.3 0.7 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 80 80 80 80 80 80 80 80 40 120 80 80 80 80 80 80 80 80

33.00 25.95 40.00 35.00 13.66 17.60 24.88 20.46 57.00 42.50 63.00 47.00 36.40 19.20 45.60 28.80 45.95 41.55 59.00 49.50 36.44 22.50 52.60 39.76 79.80 58.00 83.35 64.55 55.68 33.20 56.80 34.88 68.00 47.00 59.40 81.57 70.85 48.00 48.00 64.00 60.97 78.91 69.54 68.62 68.02 67.17 66.94 67.17 68.65 68.31

25.90 18.63 31.54 24.27 11.00 14.83 15.83 15.00 44.72 32.00 52.18 37.63 33.00 16.33 39.83 23.16 32.27 28.09 49.09 39.72 30.16 16.83 41.36 31.33 69.72 49.09 73.54 52.90 43.00 24.66 43.83 25.50 53.15 35.57 48.63 64.68 50.81 38.83 41.07 51.84 50.68 64.73 58.68 57.57 56.78 55.68 56.73 56.89 57.84 59.94

18.32 13.91 18.92 18.84 3.30 4.96 6.79 5.64 29.59 17.79 31.38 19.18 23.43 6.89 26.50 10.88 25.24 22.66 35.09 25.92 20.23 13.28 25.40 22.56 53.26 30.09 54.46 31.40 30.56 16.96 29.66 16.16 37.45 25.34 42.88 50.95 40.12 28.05 38.88 41.29 43.02 51.19 47.99 48.22 47.92 48.12 48.29 47.85 47.82 47.95

24.50 31.00 33.25 38.75 13.82 21.76 21.00 24.84 34.85 49.95 38.90 56.00 19.34 36.80 26.20 42.94 33.45 37.50 41.00 46.35 18.80 30.46 29.88 38.86 44.15 71.00 59.00 75.50 25.98 48.00 32.00 54.20 28.00 42.14 51.51 54.77 57.00 39.80 45.00 50.00 50.28 57.37 53.80 53.00 53.14 52.71 52.91 52.65 53.48 53.37

16.00 24.00 18.00 24.00 7.09 17.00 10.00 20.90 28.00 44.00 32.00 48.00 18.00 25.63 23.00 31.45 26.00 28.00 22.00 34.00 12.00 26.36 16.90 31.45 36.00 66.00 48.00 68.00 16.90 38.72 21.81 43.63 19.25 31.75 38.00 41.25 40.00 27.72 36.75 28.25 33.50 42.50 34.50 34.45 35.00 34.25 35.75 35.50 34.52 35.55

9.47 14.13 5.26 16.22 4.63 5.31 4.95 6.44 9.85 31.84 11.62 33.23 2.75 12.16 6.70 14.20 14.17 19.32 7.49 19.33 7.66 20.57 8.95 24.89 19.41 45.64 27.24 46.41 3.12 25.98 5.22 28.20 7.16 19.90 21.95 23.08 17.83 12.70 22.12 18.22 21.15 32.18 27.38 27.95 27.98 27.02 28.57 28.08 27.32 26.15

A: Initial pH, B: Light irradiation intensity, C: Initial dye concentration, D: Catalyst concentration, E: Reaction time.

8.20

8.79%

10.39%

10.80

22.10% 29.39% 7.00% 4.18%

Fig. 1. TGA curves of the Cu-ZnO/TiO2 synthesized at 500 °C composite powder with Cu concentrations of ―: Cu (0 wt%), - - -: Cu (3 wt%). (Heating speed of 10 °C/min).

of Cu-ZnO/TiO2, pure ZnO/TiO2 was also synthesized separately. The detailed characterization of all synthesized samples were discussed and compared with that of pure TiO2 in the authors' other work.

2.3. Characterization of Cu-TiO2/ZnO The thermal stability of the synthesized photocatalyst was determined through the thermal process conducted in a ThermoGravimetric Analyser (TGA) (TG-Q500, Research instrument, USA) ranging from 25 to 1000 °C under a dynamic nitrogen atmosphere. After that, the obtained product was characterized in terms of crystalline phase and phase purity using X-Ray Diffractometer, XRD (7602 EA Almelo, analytical Empyrean, Netherlands). The XRD data were collected at a scanning rate of 0.026° s−1 from 20° to 80° (2θ). The morphology analysis was performed by Field Emission Scanning Electron Microscope (FESEM, model Quanta FEG 450, EDX-OXFORD). It operated at beam energy level of 10.00 kV under high vacuum level and spot size of 3.0. The surface area of the sample was determined using Brunauer Emmett Teller which is also known as BET surface area method (Micromeritics, tristar II 300). A spectrophotometer equipped with an integrated sphere (UV–Vis-NIR spectrophotometer Uv-3600, Shimadzu, Japan) was employed to determine the UV–vis diffuse reflectance spectra in the wavelength range of 200–800 nm. Accordingly, the band gap energy level was determined through the Kubelka-Munk's model.

22500

Intensity

Table 3 Experimental design matrix and the final results of methyl orange & methylene blue removals.

357

M .REZA _ R2 7 7 8

10000

2500

0 10

20

30 40 50 Diffraction angle (2 )

60

70

Fig. 2. XRD patterns of synthesized Cu (3 wt%)-ZnO/TiO2 calcined at 500 °C. ○:Anatase, ●: Rutile, ▲:Brookite, Δ:ZnO, ►:ZnTiO3, ■:Cu2O.

358

M.R. Delsouz Khaki et al. / Journal of Molecular Liquids 258 (2018) 354–365

Table 4 The main characteristics of Cu (3 wt%)-TiO2/ZnO. BET surface area (m2/g)

Crystal size (nm)

Band gap energy level (eV)

29.82

37.65

2.2

Adequate Precision ¼

2.4. Experimental design and statistical analysis In this study, a response surface methodology (RSM) based on a 3level, 5-factor central composite design (CCD) was used to investigate the effects of five independent numerical factors, develop regression models and optimize conditions for photocatalytic degradation. Design Expert (Version 9.0.3, Stat-Ease Inc., USA) was used. Initial dye concentration ranging from 20 to 50 ppm, initial pH from 4 to 10, catalyst concentration from 0.3 to 0.7 g/L, light irradiation intensity from 14 to 23 W and reaction time from 40 to 120 min were the studied range for evaluating the photocatalytic activity of the synthesized Cu-TiO2/ZnO on degradation of two types of dyes: methyl orange (MO) as an anionic- and methylene blue (MB) as a cationic-dye. The relevant range was selected according to the available literature of similar works and the properties of TiO2, ZnO and Cu dopant. The degradation responses and photocatalytic efficiency (%) were investigated in terms of Colour (YC), COD (YCOD) and TOC (YTOC) removal. The independent variables, Xi, were converted to a coded form - ‘xi’ for statistical calculations by Eq. (7): xi ¼

ðX i −X 0 Þ δX

ð7Þ

where, X0 refers the value of Xi at the center of the domain and δX refers to the step change. A total of 100 tests were conducted for both categorical variables of which 16 replications were at center points. The experimental conditions and the responses obtained for both MO and MB degradations are summarized in Table 3. Then, a second-order polynomial equation correlating the effect of variables in terms of linear, quadratic and cross product terms was used to evaluate the efficiency of the synthesized photocatalyst by showing the behavior of the selected response as a function of individual and interactive effects of independent variables [78]. Y ¼ b0 þ

n X i¼1

bi xi þ

n X i¼1

bii x2i þ

n X n X

bij xi x j þ ε

further evaluated by ANOVA (Analysis of Variance) and their fitness was expressed by the coefficients of determination, R2, R2adj and R2pred [79]. Fisher variation ratio (F-value), probability value (Prob N F) with 95% confidence level and adequate precision were also employed as the main indicators of the models [80].

ð8Þ

i¼1 j¼1

In this equation, Y is the predicted response, b0 is a constant term, bi, bii and bij are the coefficients for the linear, quadratic and cross product terms, respectively. xi denotes the coded variables and n refers to the number of variables. The model's significance and adequacy were

V ðY Þ ¼

maxðY Þ− minðY Þ qffiffiffiffiffiffiffiffiffiffiffi V ðY Þ

n 1X pσ 2 V ðY Þ ¼ n n i¼1

ð9Þ

ð10Þ

Here, Y is the predicted response, σ2 is the residual mean square, p and n refer to the number of model parameters and experiments. 2.5. Photocatalytic activity The photocatalytic activity of the synthesized photocatalyst was evaluated through photodegradation of the selected dyes under visible light irradiation. The experiments were conducted in a Pyrex cylindrical photoreactor (ID = 8 cm, H = 13 cm), as a batch photoreactor, covered with aluminum foil and equipped with an air distributor device and a magnetic stirrer to keep the photocatalyst suspended in the aqueous solution. The photoreactor was irradiated with a fluorescent lamp (provided by NEW ORALIGHT; light intensity: 18–23 W) with an emission wavelength range of N 400 nm, positioned at the center of the photoreactor. A specific volume of the catalyst was suspended in solutions with predetermined concentration of MB or MO. The pH of the solutions was adjusted to the desired level by using hydrochloric acid (HCl, 1.0 M) or sodium hydroxide (NaOH, 1.0 M) with the aid of a pHmeter (EUTECH, CyberScan pH 300). Prior to irradiation, the solution was magnetically stirred in dark for 30 min to reach sorption equilibrium. The photocatalytic reaction then initiated when exposed to visible light. Liquid samples were taken out with a disposable syringe at regular time intervals and filtered by a membrane filter to remove the remaining catalyst particle from the aqueous solution [81]. The photoactivity of the synthesized photocatalyst was then analyzed based on colour, COD and TOC removal. The absorbance of the dye solution was measured by a UV–Vis spectrophotometer (Spectroquant-Pharo 300) based on the calibration curve obtained using standard MB/MO solution that shows a linear relationship between absorbance and dye concentration. Chemical Oxygen Demand (COD, mg·L−1) was determined according to the standard method (APHA, AWWA, and WFE, 1998) using a COD test cell supplied by Merck in a thermoreactor (Spectroquant TR 420). Total Organic Carbon (TOC, mg·L− 1) was measured by Shimadzu TOC-VCSH analyser by using potassium phthalate solution as the calibration standard. The degradation efficiency in terms of colour, COD

Fig. 3. SEM images of micrographs of Cu-TiO2/ZnO calcined at a) 500 °C and b) 700 °C.

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and TOC removal percentage was calculated by the following equations [82]: Degredation Efficiency ð%Þ ¼

ðC 0 −C t Þ  100 C0

ð11Þ

where, C0 and Ct are the initial and the retained values of Colour, COD or TOC, respectively. 3. Results and discussion 3.1. Characterization of Cu-TiO2/ZnO According to the thermal stability results (Fig. 1), solvent evaporation, decomposition, oxidation/phase transformation and combustion are the main mass loss phases. The minor weight loss of 8.2–8.8% corresponded to solvent evaporation. It followed by decomposition phase occurred from 100 °C to 215 °C for TiO2/ZnO and from 155 °C to 285 °C for Cu-TiO2/ZnO resulting in the mass losses of about 10.39% and 10.80%, respectively. The next large mass reduction corresponded to oxidation and phase transformation which was up to 22.1% for CuTiO2/ZnO and 29.39% for TiO2/ZnO. The weight loss kept rising till 715 °C and 703 °C for Cu-TiO2/ZnO and TiO2/ZnO respectively, after that reached a plateau at the end. The final total weight losses were 51.25, 48.1% for TiO2/ZnO and Cu-TiO2/ZnO, respectively. It can be concluded that, the new hybrid photocatalyst was more stable under thermal tensions as Cu loaded in its matrix. Cu loading also retarded the phase transformation which might be in favor of improvement of photocatalytic activity [83]. The XRD spectrum of Cu-TiO2/ZnO is presented in Fig. 2. The XRD pattern indicated the development of mixed crystalline phase of TiO2, including strong peaks of brookite and anatase along with the traces of rutile according to the standard crystal phases of TiO2 which include anatase (Ref. Code:00-021-1272), rutile (Ref. Code:01-075-1750) and Brookite (Ref. Code:00-029-1360). On the basis of XRD spectrum, the quantity of different phases in the synthesized Cu-TiO2/ZnO was estimated through ⌊Contentphase(%) = Aphase/Atotal⌋. The results showed the quantity of 19.125%, 19.577% and 15.036% for anatase, brookite and rutile structures in the Cu-TiO2/ZnO, calcined at 500 °C, respectively. As the calcination temperature rose to 700 °C, these values reduced to 15.51% and 4.58% for anatase and brookite, whereas rutile structure increased to 35.74%. These observation confirmed higher activity of the photocatalyst synthesized at lower calcination temperature. It should be noted that, brookite is more reactive than anatase. However, preparing pure brookite without rutile or anatase is rather difficult [23]. After that, anatase has a higher photocatalytic activity compared to rutile because of its higher ECB (~0.2 eV) results in higher driving force for transferring electron to O2. Besides TiO2 crystals, ZnO with strong diffraction peak intensities (36.69%) were found in these photocatalysts. However, a number of ZnO peaks on the basis of standard peaks were not identifiable in this study. Meanwhile, the unwanted structure of ZnTiO3 (almost 2.88%) were clearly detected. Cu2O with the quantity of 2.245% was the other observed compound. However, no CuO peak was detected. It might be due to low composition of CuO or small dimensions of Cu components which were below the detection limit of the XRD. These observations were in-line with the standard JCPDS patterns of Zinc Oxide (Ref. Code:98-002-9272) Copper Nitrate Hydroxide (Ref. Code:00-003-0061), Cu2O (Ref. Code:00-05-0667) and CuO (Ref. Code:00-48-1548). The crystal size was estimated according to the XRD analysis. The average crystal size of nanoparticles were determined from the full-width of the half maximum (FWHW) of the most intense peaks of the respective crystals using the Scherrer's equation, D = Cs. k⁄β.cosθ, where D is equal to or smaller than the grain crystallite size, CS is the Scherrer constant, k is the wave length of the incident X-ray, θ is the Bragg diffraction angle and β is the full-width of the half maximum [24]. Data on the average crystal size and surface area of the

Fig. 4. The rate of light absorption spectra of Cu (3 wt%)-TiO2/ZnO calcined at 500 °C.

catalyst are summarized in Table 4. As observed, the Cu-TiO2/ZnO crystals were nanosized with their size being about 1.98 times greater than TiO2 crystals (18.98 nm) and within the range of ZnO (33 nm) crystals. SEM was also employed to investigate the morphology of the synthesized Cu-TiO2/ZnO as shown in Fig. 3. The micrographs clearly showed the nanorod structure of anatase with the length of 300–400 nm and diameter of 50–60 nm. The structure observed in this study was exactly similar to the pure tetragonal faceted nanorod structure, observed by Li et al. [25]. They also reported the same length with a diameter of 68 to 96 nm. As the calcination temperature increased to 700 °C, the crystallinity changed to approximately 100% of specific monodisperse abundant dumbbell shaped and nano-pores with a diameter ranging from 50 nm to 70 nm which was slightly above the estimation by the XRD diagrams. The rate of light absorption spectra of the synthesized Cu-TiO2/ZnO is presented in Fig. 4. As observed, incorporation of copper into TiO2/ ZnO lattice induced an optical absorption with a steep edge in the visible light region. Moreover, the absorption wavelength of the new photocatalyst was between 400 and 800 nm, enhancing its photocatalytic reaction under visible light. The visible light sensitivity of CuTiO2/ZnO to the visible light is attributed to appearance of oxygen vacancies within the band gaps of TiO2 and ZnO due to copper doping and additional energy levels by the dopant orbitals (2p or 3d). The band gap was approximated through the model presented by Perkin Elmer [26].   m ðFhvÞ ¼ A hv−Eg

ð12Þ

Fig. 5. Estimation of the band gap energy of Cu (3 wt%)-TiO2/ZnO calcined at 500 °C by Perkin Elmer.

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In this model, F is proportional to the absorption coefficient known as Kubelka-Munk value (F = (1 − R)2 / 2R) where R is diffuse reflection of the semiconductor. A and Eg denote the constant parameter and band gap respectively. m represents the type of transition which is equal to 1/ 2 for direct band gap semiconductor and 2 for indirect band gap

semiconductor. (αhν)2 versus energy curve is observed in Fig. 5 and the result is also presented in Table 4. As observed, the band gap of the synthesized photocatalyst was remarkably decreased compared to the pure TiO2 (3.2 eV) and ZnO (3.37 eV). Its band gap was also smaller than that of Cu (1.50), Cu-TiO2 (3.22 eV) [27,28], Cu-ZnO (3.34 eV) [29]

Fig. 6. The response surface plot of Colour, COD and TOC removal efficiency (%) as a function of BA, BC, BD and BE effects. (a)(c)(e)(g): MO and (b)(d)(f)(h):MB.

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and ZnO/TiO2 (2.82–2.89 eV) [30]. The value of the band gap obtained for Cu-TiO2/ZnO warrants application of this photocatalyst under visible light irradiation. 3.2. Photocatalytic activity of Cu-TiO2/ZnO The capability of Cu-TiO2/ZnO to degrade dye was evaluated through its application for degradation of methylene orange and methylene blue under visible light irradiation. Combinations of different levels of pH, light irradiation intensities, dye concentrations, catalyst loadings and reaction time were studied to evaluate the efficacy of the photocatalyst on colour, COD and TOC removal. Among a total of 60 different 3-D graphs for the three responses (20 for each), 8 graphs were chosen to cover all important aspects while neglecting the similar ones. Moreover, since higher light irradiation highlights the impact of each factor, the combined synergistic or antagonistic effects of each factor with the intensity of light irradiation while the other factors kept at their middle values, are presented in Fig. 6(a–h). 3.2.1. Effect of light irradiation intensity Fig. 6(a–h) shows the interactive effects between light irradiation intensity and other operating parameters. As observed, the photocatalysis reaction was more efficient under more intensive light irradiation since more electron-hole pairs are activated, directly affecting the reaction efficiency. On average, the increment was about 8% for MO and 6.5% for MB within 80 min. Besides, the effect of light irradiation slightly reduced as the dye concentration increased. It was attributed to the darkness of the media that reduces the penetration depth of the photons. 3.2.2. Effect of solution acidity The surface charge of a catalyst plays a vital role for photocatalytic reactions that take place on the surface of a catalyst. The surface charge is defined as the electrical potential difference between the surface of the catalyst and the media. It is dependent on the presence of hydroxyl groups on the catalyst surface. Basicity or acidity of the solution dictates the surface charge properties of the photocatalyst which directly affects its photoactivity. The point at which the surface charge of photocatalyst is null is known as zero charge point (pHPZC), making the catalyst less attractive to the dye molecules. According to the literature, pHPZC is about 6.0 and 9.0 for TiO2 [31] and ZnO [32,33], respectively. Therefore, at pH b pHPZC, protonation reaction takes place and the catalyst surface is positively charged. On the other hand, deprotonation reaction occurs at pH N pHPZC which makes the catalyst surface negatively charged. Accordingly, in this study, methyl orange as an anionic dye and methylene blue as a cationic dye were employed to determine the exact effect of pH on photoactivity of the synthesized Cu-TiO2/ZnO. The results are demonstrated in Fig. 6a and b. As observed, higher degradation efficiency of methyl orange was observed at acidic pH, while the opposite was observed for methylene blue. According to the aforementioned information, Cu-TiO2/ZnO is negatively charged in alkaline solution and it adsorbed MB molecules by electrostatic attraction. In such conditions, the adsorption of MO becomes weaker due to repulsive forces in alkaline solution [84]. The adsorption of MB molecules thus becomes stronger in alkaline condition. However, in this study, the desirable degradation values were obtained over a wide range of pH, which was observed in Fig. 6a and b, demonstrating the stability of copper species inside the catalyst structure. Similar results were observed by [34] when they evaluated the application of Cu doped TiO2 for degradation of methyl orange within the pH range: 3.0–10.0. They reported the initial solution pH of 3.0 as the optimum value. The increase of methylene blue decomposition with an increase of pH value using nanoparticles of anatase TiO2 was also perceived by Bubacz et al. [35]. The highest reactivity was obtained by basic reaction. Accordingly, the pH values ranging 4–6 and 7–10 were chosen as target values in the optimization process for MO and MB degradation, respectively.

361

3.2.3. Effects of dye and catalyst concentrations Fig. 6c and d illustrates the degradation percentage as a function of dye initial concentrations in the presence of 0.5 g/L catalyst after 80 min of light irradiation. It was observed that the reduction percentage of dye in aqueous solution depends dominantly on its initial concentration. In other words, higher absolute degradation ratio could be obtained at lower dye concentration or higher amount of catalyst. By increasing the initial concentration of dye, not only the surface of photocatalyst is saturated earlier but the photons also get intercepted before they can reach the surface of the catalyst. There are fewer active sites for adsorption of hydroxyl ions when the catalyst surface is saturated, reducing the efficiency of the catalyst [85]. Interception of photons reduces the absorption of photons by the catalyst and thus the dye reduction percentage decreases. In this study, the colour removal efficiency (%) decreased by about 22.85% and 17.20% with the rise in the dye concentration of MO and MB from 20 to 50 ppm within 80 min. The degradation efficiency reduced with increased dye concentration for both MO and MB, especially for MB from 35 ppm to 50 ppm. In such condition, even higher intensity of light irradiation did not contribute to significant change in photocatalytic degradation, as shown in Fig. 6d. It is important to employ an optimal catalyst amount to keep the treatment efficiency to the maximum level. In order to determine the effect of the amount of Cu-TiO2/ZnO on the photocatalysis reaction under visible light irradiation, the catalyst amount in a range of 0.3 g/L to 0.7 g/L was studied. The corresponding results are depicted in Fig. 6e and f. It revealed that the catalyst concentration of 0.6 g/L was in synergistic interaction with higher levels of light irradiation and at the solution pH of 5.5 to 6, leading to the maximum photocatalytic degradation of methylene orange within 2 h of reaction. Similar observation was obtained at pH 7.5 to 8 for methylene blue. Further increase of catalyst did not significantly improve the reaction and the reaction actually decreased slightly when the catalyst concentration was increased to above 0.6 g/L. For instance, reaction conditions of [catalyst]: 0.6 g/L, [dye]: 35 ppm, pH: 5.5, light irradiation intensity: 18.5 W and time: 120 min, brought about 78.45%, 63.68% and 51.81% removal of colour, COD and TOC of the methyl orange solution. However, the corresponding removals only rose to 77.15%, 63.85% and 51.86% when the catalyst concentration rose to 0.7 g/L. Similar conditions except for pH, which was equal to 7.5, brought about 66.13%, 51.90%, 40.41 removals of methylene blue that reached to the values of 64.72%, 51.67% and 40.30 at catalyst concentration of 0.7 g/L. It can be concluded that increase of catalyst amount corresponds with availability of active sites on the catalyst surface till an optimum amount [36]. Catalyst loading which is beyond the optimum amount increases the turbidity of the suspension and is in antagonistic interaction with penetration of visible

Fig. 7. Effect of irradiation time on ⃟ : Colour, □: COD Δ: TOC removals (%), Orange colour: MO, Blue Colour: MB. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 5 Analysis of individual and interaction effects of operational parameters. Investigated dye

Removal response

Final model in terms of coded forms

Eq.

Methyl orange

YColour YCOD YTOC YColour YCOD YTOC

−62.11 − 3.57B + 1.42C + 259.14D − 0.081E − 1.42A2 − 0.023C2 − 183.63D2 − 0.729CD −34.57 − 5.34B + 1.86C + 208.80D − 0.14E − 1.09A2 − 0.988CD −51.36 + 2.199C + 120.58D − 0.27E − 1.18A2 − 0.037C2 − 0.699CD −94.04 − 3.76B + 1.55C + 254.83D − 0.157E − 1.42A2 − 0.023C2 − 183.63D2 − 0.729CD −66.10 − 5.70B + 1.93C + 208.94D − 0.19E − 1.09A2 − 0.988CD −83.50 + 2.67C + 117.83D − 0.31E − 1.18A2 − 0.037C2 − 0.699CD

(13) (14) (15) (16) (17) (18)

Methylene blue

3.2.4. Effect of irradiation time on photocatalysis reaction The trends of TOC removal over 120 min for both MO and MB ([dye]: 35 ppm) are presented in Fig. 6g and h which follow the same trend as colour and COD removals. As observed, over 41% and 30% of TOC was removed within 40 min for MO and MB, respectively and the removal percentage continued to increase to above 53% and 39% at 2 h. Besides, the COD removal of MO using Cu-TiO2/ZnO under visible light was found to be about 52%, 57% and 66% within 40, 80 and 120 min, respectively. The COD removals of MB were 43%, 45% and 53% at the same times. The same trend was also observed for colour removal with the values of 65%, 71% and 82% for MO and 58%, 61% and 68% for MB within 40, 80 and 120 min, respectively. All three responses followed a linear trend with time. Under visible light irradiation, the degradation rate of MO was found to be 7.75% (as the average value) higher compared to MB. In order to assess the reaction kinetics of photocatalytic degradation using Cu-TiO2/ZnO, the variation of –ln Ct/C0 was plotted versus reaction time, as shown in Fig. 7. It was found that the degradation reactions of both MO and MB basically followed the second-order reaction kinetics. In other words, the degradation results implied a sharp increase as the reaction time increased to 40 min. After that, the degradation almost linearly increased with the reaction time with slower rate. This behavior indicates the effect of fresh catalyst with fresh activated sites. The results showed that the reaction became slower as the activated sites became saturated.

their fitness versus coefficients of determination were presented in Table 6. In these models, positive coefficients indicate a synergistic effect, while negative coefficients indicate an antagonistic effect between or among the variables. Based on Tables 5 and 6, it was observed that all parameters individually played effective roles during photocatalysis reaction in the presence of Cu-TiO2/ZnO while their interactive effects were negligible. Among the interactive effects, the interaction effect of catalyst and dye concentration played the most prominent antagonistic role in both systems containing methyl orange and methylene blue. In other words, visible light could hardly pass through the media as the dye concentration increased and therefore the photocatalysis efficiency reduced. On the other hand, higher concentration of catalyst (N 0.6 g/lit) was not as effective. The quality of the developed equations, presented in Table 6, also indicates the desirability of the suggested models. The adequate precision measures the signal to noise ratio. A signal to noise ratio N 4 indicates that the model is able to navigate the design space. As observed, the adequate precisions were 27.90, 21.39 and 16.64 (≫ 4) for the studied responses. Aside from the photocatalytic activity, the photocatalyst stability is another determinant factor in practical application. In order to investigate the stability of Cu-ZnO/TiO2, three runs of cycling photodegradation of 300 mL methyl orange and methyl blue 20 ppm with 0.5 g of photocatalyst under the light intensity of 23 W and pH of 7 have been carried out. Accordingly six extra tests were performed and the results were presented in Fig. 8. As observed, the photocatalytic performance of Cu-ZnO/TiO2 decreases by about 15–20% for each repeated use. However, the performance reduction is more highlighted after second cycling degradation experiment under methyl blue. It may attribute to the impact of methyl blue on reduction of light absorption on the photocatalyst surface.

3.3. ANOVA analysis

3.4. Process optimization

Based on the obtained results from the photocatalysis reaction in the presence of Cu-TiO2/ZnO, a series of regression models were suggested by CCD analysis in which the reaction efficiency (Colour, COD and TOC removals %) was illustrated as a function of independent variables: pH, light irradiation intensity, [Dye], [Catalyst] and reaction time. The equations were obtained according to Eq. (8) which include a constant value, linear and quadratic terms, presenting the individual effect of each parameter and cross product terms that assess the interactive effects of the parameters on responses (Y, YCOD, YTOC). The regression models for each categorical variable were obtained after elimination of insignificant terms and the results presented in Table 5 and the related

In order to identify the conditions at which Cu-TiO2/ZnO can demonstrate its highest removal efficiency or be more applicable/effective, optimization of operational factor levels was performed based on the experiments done for both MO and MB removals and the quadratic models obtained. Accordingly, the selected criteria to achieve the maximum desirability were as “maximize” for photocatalyst efficiency (Colour, COD and TOC), “within the range” for dye/catalyst concentrations, visible light irradiation and reaction time and “target” pH value ranging 4–6 for MO and 7–10 for MB. Among 20 proposed solutions for each category, the one with the highest desirability was selected and two additional tests were conducted to evaluate the validity of the procedure.

light, leading to decrease in photoactivation procedure. The other reason attributes to scavenging of OH radicals over the surface of the catalyst. The same observation was reported by [37] for degradation of azo dye acid red 14 in water using ZnO. Wang et al. [38] also obtained the same results for solar photocatalytic degradation of various dyes of methyl orange, rhodamine B, azo fuchsine, congo red and methyl blue, in the presence of Er3+:YAlO3/ZnO-TiO2 composite.

Table 6 Analysis of Variance (ANOVA) results for responses. Removal response

Final model in terms of coded forms

R2

R2adj

R2pred

PVa

APb

YColour YCOD YTOC

+59.84 + 3.96B − 8.19C + 7.33D + 6.78E − 4.34F − 12.81A2 − 5.18C2 − 7.35D2 + 6.55AF − 2.19CD − 1.50EF +45.50 + 2.78B − 6.78C + 7.25D + 5.69E − 5.06F − 9.85A2 − 5.44C2 + 6.42AF − 2.97CD +35.43 − 5.13C + 4.12D + 5.16E − 5.63F − 10.61A2 − 8.40C2 + 5.81AF − 2.10CD

0.9285 0.8766 0.8502

0.9031 0.8326 0.7968

0.8555 0.7676 0.7056

b0.0001 b0.0001 b0.0001

27.900 N 4 21.397 N 4 16.642 N 4

a b

P value, Prob N F. Adequate precision.

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MB molecules, the negatively charged MO molecules were strongly adsorbed on the surface of the synthesized photocatalyst and therefore, the overall degradation of methyl orange was about 7.75% higher than methylene blue (on average). According to the results, the decomposition efficiency of Cu-TiO2/ZnO photocatalyst also increased by about 8% and 6.5% with light intensity (ranging from 18 to 23 W) and reaction time (ranging from 40 to 120 min) for MO and MB, respectively. The results showed that the new Cu-TiO2/ZnO photocatalyst could be efficiently used for water treatment reactors and the catalyst particles could be easily removed from the system using simple filtration. Acknowledgements

Fig. 8. Cycling photocatalytic degradation of MO (Orange Colour) and MB (Blue Colour) using Cu (3 wt%)-ZnO/TiO2 calcined at 500 °C. Δ: First Cycle, ⃝ : Second Cycle, ⃟ : Third Cycle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The identified optimum conditions for MO and MB were: [Cu-TiO2/ ZnO]: 0.61 g/L, [Dye]: 20 ppm, pH: 7.8 (for MB); 4.5 (for MO), light intensity: 23 W and reaction time: 120 min, respectively. The values of the predicted and experimentally obtained colour, COD and TOC removal along with the value of discrepancy among them are summarized in Table 7. As observed, the maximum deviation between the predicted and experimental values was 4.18% which confirmed the validity of the obtained regression models and desirable photoactivity of the synthesized Cu-TiO2/ZnO with respect to decolourization, degradation and mineralization of both methyl orange and methylene blue under visible light irradiation. 4. Conclusion In this study, a new hybrid nano-sized photocatalyst, Cu-TiO2/ZnO was synthesized through sol-gel method. The catalyst was characterized in terms of thermal stability, crystalline phase, phase purity, morphology, crystal size, surface area, UV–vis diffuse reflectance spectra and band gap energy level. The results indicated a significant improvement in the characteristics of Cu-TiO2/ZnO compared to TiO2 and ZnO, especially in band gap energy level: 2.2 eV versus 3.2 eV and 3.37 eV, respectively. Evaluation of the photo-activity of Cu-TiO2/ZnO under visible light irradiation was the main objective of the work. Accordingly, its performance in reducing and degrading two different dyes; methyl orange and methylene blue, with different dye concentrations, catalyst loadings, pH of the medium, intensities of light irradiation and reaction times was statistically investigated. The results indicated that under the optimum conditions of [Cu-TiO2/ZnO]: 0.6 g/lit, [Dye]: 20 ppm, pH: 4.5 for MO; 7.8 for MB, light intensity: 23 W and reaction time: 120 min, the photocatalyst presented the maximum removal efficiency: colour: 85.45%, COD: 70.56% and TOC: 48.70% for MO and colour: 73.20%, COD: 59.92% and TOC: 38.77% for MB. The Cu-TiO2/ZnO presented different characteristics depending on the charge of the dye molecules. Cu-TiO2/ZnO was found to exhibit the highest photoactivity at pH~4.5 for methyl orange and pH ~ 7.8 for methylene blue. Compared to the Table 7 Predicted and experimental values of the studied responses at optimum conditions. Response

Predicted values

Experimental results

Error%

MO Colour removal efficiency (%) COD removal efficiency (%) TOC removal efficiency (%)

87.43

MB

MO

MB

MO

MB

76.08

85.45

73.20

2.27

2.78

73.15 50.82

60.96 40.19

70.56 48.70

59.92 38.77

3.54 4.18

1.7 3.54

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