A comprehensive study on the photocatalytic activity of coupled copper oxide-cadmium sulfide nanoparticles

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 196 (2018) 334–343

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

A comprehensive study on the photocatalytic activity of coupled copper oxide-cadmium sulfide nanoparticles Samaneh Senobari a,b, Alireza Nezamzadeh-Ejhieh a,b,c,⁎ a b c

Department of Chemistry, Shahreza Branch, Islamic Azad University, P.O. Box 311-86145, Shahreza, Isfahan, Islamic Republic of Iran Young Researchers and Elite Club, Shahreza Branch, Islamic Azad University, Shahreza, Islamic Republic of Iran Razi Chemistry Research Center (RCRC), Shahreza Branch, Islamic Azad University, Isfahan, Islamic Republic of Iran

a r t i c l e

i n f o

Article history: Received 13 November 2017 Received in revised form 5 February 2018 Accepted 12 February 2018 Available online 15 February 2018 Keywords: CuO-CdS semiconductors Nanoparticles Methylene blue Photocatalysis Clacination temperature

a b s t r a c t Coupled CdS-CuO nanoparticles (NPs) subjected in the photocatalytic degradation of Methylene blue (MB) aqueous solution. The calcination temperature and the crystallite phase of CuO had a significant role on the photocatalytic activity of the coupled system and CuO200/2h-CdS catalyst (containing CuO calcined at 200 °C for 2 h) showed the best photocatalytic activity. The coupled system showed increased activity with respect to the monocomponent semiconductors. The prepared catalysts characterized by x-ray diffraction (XRD), scanning electron microscope equipped with energy dispersive X-ray (EDX) analyzer, x-ray mapping, Fourier transform infrared (FTIR) spectroscopy, diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS) techniques. The best degradation extent of MB was obtained at: CMB: 1 mg L−1, pH 5, 80 min irradiation time and 0.8 g L−1 of the CuO200/2h-CdS catalyst. The chemical oxygen demand (COD) confirmed about 83% of MB molecules can be mineralized at the optimum conditions. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Rapid growth of industries counts as a major source of pollution of water, air and soil. Among them, water pollution by different organic compounds such as dyes has counted a major problem for aquatic life [1]. A dye with wide uses in some important industries such as textile and printing is methylene blue (MB) as a member of thiazine dyes. It has some harmful effects in humans including vomiting, heartbeat increase, jaundice, shock, cyanosis, quadriplegia, and tissue necrosis etc. [2,3]. Hence, removal or degradation of MB and other toxic organic pollutants is important in the environmental protection and public health point of views. So far, different techniques such as adsorption [4], ozonation [5], Fenton and photo-Fenton (as sub-techniques of the advanced oxidation processes (AOP)) [6,7] etc. have been applied for the removal of MB and other organic pollutants from water and waste water samples. However, the low adsorption capacities and poor cycling capabilities of the adsorbents, transferring of the pollutants to other phase/ media, pH-dependence of Fenton processes etc. have limited their applications. In contrast, a very useful and effective method with low side effects and cost, with respect to adsorption, ozonation, precipitation etc., for removing of different organic pollutants from water/wastewater is ⁎ Corresponding author at: Department of Chemistry, Shahreza Branch, Islamic Azad University, P.O. Box 311-86145, Shahreza, Isfahan, Islamic Republic of Iran. E-mail address: [email protected] (A. Nezamzadeh-Ejhieh).

https://doi.org/10.1016/j.saa.2018.02.043 1386-1425/© 2018 Elsevier B.V. All rights reserved.

semiconducting based photodegradation process. In this process, irradiating a semiconducting material by UV or Vis photons with sufficient energy excites it and the photogenerated electrons and holes form in its conduction band (Cb) and valence band (Vb), respectively. The resulted electron/hole (e/h) pairs react with dissolved oxygen and water (or hydroxyl radicals) and produce super oxide and hydroxyl radicals, respectively. Immediately, the produced powerful radicals attack to organic materials in solution and destroy them to photodegradation intermediates and finally to water and carbon dioxide and inorganic ions [8–10]. One major drawback of such heterogeneous photodegradation processes is recombination of e/h pairs that decreases efficiency of the process. The effective strategies to overcome this problem are coupling of two or more semiconductors with suitable energy levels, doping and supporting of semiconductors onto a suitable support [11–13]. Coupling of semiconductors causes the immigration of photogenerated electrons from the more negative Cb level of a semiconductor to the more positive Cb level of the other semiconductor. This significantly decreases the e/h recombination. According to above discussion, so far, different coupled and supported semiconducting systems have used for photodegradation of MB or other organic pollutants in aquatic systems [14–30]. The degradation intermediates of MB have also detected by GC–MS and LC-MS [31–34] which of results will be discus in the latest sections. In the present work, CdS nanoparticles (NPs) mixed with CuO NPs for increasing their photocatalytic activity in the photodegradation of MB. The p-type CuO is a cheap and available abundantly semiconductor

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The pH of point of zero charge (pHpzc) of CuO-CdS catalyst, was determined by the typical procedure described in literature [44,45]. For this goal, initial pH (pHI) of the suspensions containing 0.1 M NaCl and 0.1 g of the catalyst adjusted in the range of 2 to 12. After 24 shaking, the final pHs (pHF) were recorded. The plots of pHI - pHI (as bisector or linear plot in the curve, see curve ‘b’ in Fig. 7B) and pHF - pHI were constructed and their intersection was considered as pHPZC of the CuO-CdS catalyst.

with a narrow band gap (Eg = 1.2 eV) [35,36]. Although, CdS (Eg = 2.42 eV) is one of the best photocatalyst with a moderate energy gap. Unfortunately, the poor adsorption ability and the photocorrosion of pure CdS have limited its application in the photocatalytic processes. To enhance its photocatalytic activity, different technologies including supporting, coupling and hybridization of CdS within organic matrix materials have been used [37–40]. Among the two wurtzite and zincblende phase crystallite phases of CdS, the wurtzite CdS is more stable with easy preparation method. Hence, this crystallite form has been widely used in photocatalytic processes [41]. Hence, such CuO-CdS coupled system decreases the photocorrosion of CdS. This system was selected by the matched potentials of their Cb and Vb levels (Eo: CdSCb: −0.6 V, CuO-Cb: 0.27 V, CdS-Vb: 1.6 V, CuO-Vb: 1.62 V with respect to NHE). This effectively immigrates the photogenerated electrons in CdS-Cb to CuO-Cb, and the hole transfer in the opposite trend between their valence bands. These matched energy levels strongly prevent the e/h recombination and increase the photodegradation activity. In addition of easy preparation of CdS, it has also low solubility in water (pKSP = 13.5) which increases its stability. In addition, the effect of calcination temperature of CuO has studied on the photocatalytic activity of the resulted coupled systems. It has believed that changing in calcination temperature changes the crystallite phase of CuO that in turn changes the photocatalytic activity of the resulted systems. In more published works, there are little studies on the effect of calcination temperature.

In a typical photocatalytic experiment, 10 mL of 1 mg L−1 MB solution containing 8 mg CuO-CdS catalyst (equal to 0.8 g L−1 of the catalyst) was irradiated by a moderate pressure Hg-lamp (35 W, Philips, type G-line with maximum emission at 435.8 nm, positioned 10 cm above the sample) under magnetic stirring. Before, irradiation process, suspensions were shaken at dark for 30 min to reach equilibrium adsorption/desorption processes. The blank solution (without the catalyst) was also irradiated at the same conditions for studying direct photolysis. After centrifugation of suspensions (N13,000 rpm), absorbance of sample before (Ao) and after (A) irradiation process at the maximum absorption peak of MB (λmax = 664 nm) was recorded and used for calculation of C/Co or degradation extent of MB based on the following equation.

2. Experimental

MB Degradation ð%Þ ¼ ½ðAo −AÞ=Ao   100

2.1. Chemicals and Preparations Analytical grade copper(II) acetate, acetic acid, cadmium nitrate, ammonium sulfide and other chemicals were purchased from Merck. Commercial methylene blue dye (molecular formula: C16H18ClN3S) was prepared from local chemical companies. Natural clinoptilolite tuff was prepared from the Semnan region (Iran). All solutions/suspensions were prepared in distilled water and the pH of solutions adjusted by NaOH or HCl solution. 2.1.1. Preparation of CuO NPs A 5 mL glacial acetic acid portion added to 10 mL 0.2 mol L−1 of copper acetate (in a round bottom flask) and heated to 100 °C on a magnetic stirrer. Then, 30 mL 3 mol L−1 of NaOH added to the resulted solution to adjust pH at 6–7. The resulted black precipitate was centrifuged (N13,000 rpm) and re-suspended again in water to remove adsorbed materials (repeated 4 times). The obtained precipitate was air dried (24 h) and calcined at 200 °C (or other temperature will mention later) to obtain CuO nanoparticles [42]. The CdS NPs were prepared by adding 100 mL 0.085 mol L−1 Cd (NO3)2. 4H2O drop wise to 100 mL 0.1 mol L−1 (NH4)2S under vigorous stirring (1200 rpm) during 5 h. The production was dark yellow CdS NPs [43]. To prepare mixed CuO/CdS catalysts with different mole ratios, definite amount of each semiconductor was hand mixed in an agate mortar for 30 min to obtain a homogeneous powder. 2.2. Instruments and Characterization XRD diffractometer model X'PertPro (with Ni-filtered Cu-Kα radiation source at 1.5406 Å, 40 kV, i 30 mA; Netherland) was used for recording XRD patterns. A PerkinElmer Spectrum 65 FT-IR spectrophotometer, UV–Vis diffuse reflectance spectrophotometer (JASCO V 670, using BaSO4 as reference, Japan), MIRA3LMU scanning electron microscope (TESCAN Co Czech Republic) are other instruments used for the characterization of samples. A Jenway pH meter (model 3505) used for pH adjustment. An Ivium electrochemical impedance spectrometer (EIS) (IEC 61326, Netherland) used for recording EIS spectra.

2.3. Photodegradation Experiments

3. Results and Discussion 3.1. Characterization 3.1.1. X-ray Diffraction To study the effect of calcination temperature on photodegradation activity of the coupled CuO-CdS system, non calcined CuO NPs (prepared by the used method in ref. [42]) and the clacined CuO samples at different temperatures (for 2 h) were mixed with CdS NPs. XRD patterns of the prepared CuO-CdS catalysts are shown in Fig. 1A. All XRD peaks of CuO and CdS are assigned by corresponding hkl planes. Copper oxide has two characteristic XRD peaks at 2θ values of 35.5° and 39o that are present in the obtained patterns. It has reported that Tenorite CuO phase has the typical diffraction peaks at 38.73°, 38.92°, and 68.14° correspond to (111), (200) and (220) planes (according to JCPDS 05-0661), respectively [46]. By increasing the calcination temperature, the intensity of these peaks was significantly increased. In addition, some new peaks at 2θ values of 48.9°, 62.0°, 66.5° were appeared at calcination temperatures of 400 and 800 °C for monoclinic CuO phase [47], that are absent in the pattern of non calcined CuO and the calcined one at 200 °C. In general, particle sintering, ordering of structure and crystallite growth can cause to increase in the peak intensity at higher temperatures [48]. Cuprites' structure of Cu2O showed a diffraction peak at 2θ of 29.4° correspond to (110) plane. This peak disappeared at higher calcination temperatures, confirming changing of Cu2O to CuO. The formation of hexagonal wurtzite phase of CdS (JCPDS card 411049) can be confirmed by the peaks at 2θ values of 24.7°, 26.5°, 28.3°, 36.6° and 43.8°. The peak of cubic phase of CdS is appeared at 2θ degree of 30.3° that disappeared by mixing with calcined CuO at higher temperatures by broadening the peak that centered at 2θ of 27°. However, hexagonal phase of CdS has reported as the dominant phase of CdS crystallite phase [49]. Using the common and modified Scherrer equations [50,51], average crystallite sizes of the catalysts were estimated. In the common Scherrer equation, when d values decrease and 2θ values increase the nano crystallite size increases, because the β.cosθ value cannot be maintained as a constant [51]. As shown in Table 1 SD (see Supplementary data), all the prepared catalysts have nano dimension

c: CuO400 (2h)-CdS

220 Cu

222 CuO

022 CuO

d: CuO600 (2h)-CdS 113 CuO

112 CdS, 020 CuO

acetic acid and acetate salt used in the synthesis procedure are present. The peak located around 2998 cm−1 belongs to asymmetric vibration mode of CH3 . The absorption peaks in the range of 1620–1380 cm −1 belong to asymmetric and symmetric vibration modes of carboxylate anion, respectively. These peaks showed some shifts in different samples.

b: CuO200 (2h)-CdS 202 CdS,202 CuO

200 CuO 111 Cu

a: CuOn.c-CdS

d c

Cu2O

Intensity (a.u.)

101 CdS

A

111 CuO, 102 CdS

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111 CuO, 102 CdS

336

A

1.6

d

1.2

c

Abs.

a

20

30

40

50

60

70

a 0.8 c d e f b a

80

o

2-theta ( )

1574

1423

3444

2983

c

511

790 664

1148

0.4

B

d

b

0.0

925

200

618

400

600

3362

3000

532 460 437

B a

515

609

657

1007

b

534

1118

2000

40

30

1416 1380

d: CuO200 - CdS c: CuO200 b: CdS a: CuOnc

1613

1581

1576

1620

3413

1345

a

1014 932 852

1424

3421

2998

b

4000

800

Wavelength ( nm )

2998

Transmittance (a.u.)

f

e b

c

20 625

1000

a : CdS c :CuO 400(2h) +CdS b :CuO 600(2h) +CdS

10

Wavenumber (cm-1) 0

Fig. 1. A) X-ray diffraction patterns of CdS-CuO NPs with calcined CuO at different temperatures; B) FTIR spectra of some selected catalysts.

varied in the range of 15–28 nm and 10.7–14.5 nm obtained by the common and modified Scherrer equations, respectively. 3.1.2. FT-IR Study As shown later, the CuO-CdS catalyst containing calcined CuO at 200 °C for 2 h (CuO200/2h-CdS) has shown the best photodegradation activity and hence it has subjected in more characterization techniques used in this work. Accordingly, FTIR spectra of CdS, CuO and CuO-CdS samples are shown in Fig. 1B. In general, all samples include the absorption peak of adsorbed water at about 3500 cm−1. Characteristics absorption peaks of CuO at 532–534 cm−1 are present in the spectra of non-calcined CuO and clacined one at 200 °C (spectra of ‘a’ and ‘c’). The peak appeared at 625 cm−1 belongs to Cu2O absorption. This peak is weaker for the calcined sample, because Cu2O has converted to CuO at higher temperatures [52]. The stretching peak of CuO showed a red shift to 511 cm−1 in the coupled systems because of interactions between CuO and CdS. Unfortunately, our laboratory does not equipped to XPS instrument to detect CuO and Cu2O species. In general, CdS has the absorption peaks in the range of 400–650 cm−1 [53]. Here, these appeared in the range of 437–609 cm−1 (spectrum ‘b’). These were shifted to 618 cm−1 or overlapped with CuO absorption bands in this region in the spectra of the coupled system. In all spectra, some characteristics peaks of

1

30

2

3

4

5

6

C

25

d

20 15

e

10

f e :CuO n.c +CdS f :CuO n.c d :CuO 200(2h) +CdS

5 0 1

2

3

4

5

6

Fig. 2. A) Typical UV–Vis absorption DRS spectra of the CuO, CdS and CdS-CuO200 NPs; Typical Tauc plots for allowed indirect (B–C) and direct (D–E) transitions of the catalysts.

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5000

337

Table 1 Estimated band gaps for the catalysts.

D

Catalysts

a

4000

b

3000

CdS CuOn.c CuOn.c + CdS CuO200 + CdS CuO400 + CdS CuO600 + CdS

c

2000

Eg (eV) calculated from Kubelka-Munc Eq. for n values of:

Calculated from Eg = 1242.4/λ

1/2

3/2

2

3

λ (nm)

Eg (eV)

2.19 1.24 2.19 2.49 2.53 2.10

2.35 2.06 2.68 2.9 2.16 2.75

2.37 2.54 2.80 2.85 2.45 2.43

2.44 2.62 2.82 2.96 2.49 2.67

596 978 700 716 705 752

1.78 1.27 1.47 1.72 1.58 1.54

1000 a :CdS b : CuO 600 (2h) + CdS c : CuO 400 (2h) + CdS

0 1

2

4

3

5

6

800 d

E 600

e

400 4 f

200 d : CuO 200(2h) + CdS e : CuO n.c+ CdS f : CuO n.c

0 1

2

3

4

5

6

Fig. 2 (continued).

3.1.3. DRS Study Optical properties of the mono-component and the coupled catalysts studied by recording UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS) (see Fig. 2A). From the reflectance results, the KubelkaMunk and Tauc plots were plotted for the calculation of band gap energy by the following equation: ðαhνÞ ¼ β hν−Eg


n

In this equation, Eg is the energy band gap of the semiconductor (eV), β is the absorption constant, α is the absorption coefficient defined by the Beer-Lambert's law, while h (J s) and ν (s−1) have their common meaning. The index n has different values of 1/2, 2, 3/2, and 3 for the allowed direct and indirect, forbidden direct and indirect electronic transitions, respectively [54,55]. Typical Tauc plots for the allowed direct and indirect transitions are shown in Fig. 2B–E. The linear portion of the (αhν)n − hν curves was extrapolated and the band gap energy for all transitions was calculated. The band gaps were also calculated based on the absorption edges of the catalysts (Fig. 2A) by using the Eg (eV) = 1234.5/λ (nm). The estimated band gap values are summarized in Table 1. 3.1.4. FESEM-EDX and X-ray Maps Studies The surface morphology of CuO 200/2h (A, B), CdS (C, D) and CuO 200/2h -CdS (E, F) was studied by FESEM and corresponding

images are shown in Fig. 3. Images A, C and E have magnification of 75 kx while images B, D and F have magnification of 350 kx. As shown in images A–B (especially in ‘B’), CuO formed nearly dens nano-rods that their diameters are nearly same across the length. As shown in image ‘B’ the diameter of CuO nano-rods varied in the range of 14.3 to 18.7 nm. Images ‘C’ and ‘D’ confirms formation of spherical CdS nano-particles that are nearly uniform with diameters in the range of 6–8.8 nm. The uniform spherical CdS NPs seems as a flat surface as shown in image ‘C’. Comparison of E–F images for the coupled CuO-CdS with those of single components (A–D) confirms that CdS NPs relatively well dispersed around the CuO nano-rods. In these images, especially image ‘F’, around the CuO nano-rods is more covered. In general, SEM images confirm formation of nanoparticles during the synthesis process and the nanodimension retained after coupling of the semiconductors. Fig. 4A shows typical EDX spectrum of CuO-CdS catalyst, confirming the presence of all Cu, Cd, O and S elements as constituent elements of the catalyst. Typical X-ray mapping images of the CuO-CdS catalyst, showing well distribution of each constituent element, are shown in Fig. 4B. Individual X-ray mapping images in SD1 (Supplementary data) show distribution of each constituent element of the catalysts. 3.1.5. Electrochemical Impedance Characterizations Charge transfer in semiconductors is very important in semiconducting based photodegradation processes, because a higher charge transfer process causes a lower e/h recombination and finally a higher degradation efficiency. Hence, modified carbon paste electrodes (CPE) by the prepared semiconductors were prepared by the procedure reported in our previous works [56], and the electrochemical impedance spectra (EIS) were recorded (Fig. 4C). As shown, the modified CPE with CuO and CdS monocomponents have higher arc radius in the corresponding Nyquist plots, showing a higher resistance to charge transfer [57]. In contrast, when CPE modified with the coupled CuO-CdS a smaller arc radius observed that shows resistance of the interface layer at the electrode surface decreased. This confirms that coupling of CdS and CuO caused a charge transfer between the energy level of the semiconductors. These observations are in agreement with photodegradation results, so the CuO-CdS catalyst showed the best photocatalytic activity with respect to the monocomponent systems. 3.2. Photodegradation Studies 3.2.1. Effects of Coupling, Calcination Time and Temperature At the first step, photocatalytic activity of the as-prepared CdS NPs compared with that of CuO NPs calcined at different temperatures. As shown in Fig. 5A, after CdS NPs, CuO NPs calcined at 200 °C had the better activity in the photodegradation of MB. Hence, to enhance their photodegradation activities, their coupled systems were used. The results together with the corresponding UV–Vis absorption spectra are shown in Fig. 5B–C. Decrease in UV–Vis absorption confirms degradation of MB molecules to smaller fragments. No appearance of new absorption peaks at shorter wavelengths confirms that MB molecules mineralized relatively during the photodegradation process. As shown

338

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Fig. 3. FESEM images of CuO NPs (A, B), CdS NPs (C, D) and CdS-CuO200 NPs (E, F).

in Fig. 2B, all coupled CdS-CuO systems had better photodegradation activity with respect to monocomponent systems. Also, among the coupled systems, coupled CdS with calcined CuO at 200 °C had the best activity and hence it was used for further investigations. In the next step, effect of calcination time on the activity of these catalysts was studied. As shown in Fig. 5D the best activity was observed for the coupled catalyst containing calined CuO at 200 °C for 2 h. Dependence of the photocatalytic activity of the catalysts to calcination temperature of CuO, confirms that tenorite phase of CuO is more active

than its monoclinic phase, because XRD results showed that at higher calcination temperatures the related XRD peaks of monoclinic phase appeared. Increasing in the photocatalytic activity of the coupled CdS-CuO catalysts with respect to monocomponent systems is due to better charge transfer in the coupled systems as shown graphically in Fig. 5E. During the illumination of the system, the photo-generated electrons in the conduction band of CdS (CdSCb) can easily immigrate to the CuO conduction band (CuOCb), because the CdSCb with standard potential of −

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339

trend was observed in MB photodegradation. These observations confirm that e/h recombination is the rate limiting factor for the photocatalytic activity of the coupled CdS/CuO system. When, moles of CdS increased, it would be expected that a higher photo-generated electrons to be produced, causing a higher degradation activity. While, when lesser moles of CuO are present, the photo-generated electrons tend to

1.0 A 0.8 A

C/Co

0.6

CuO 800

CuO 600

CuO 400

CuO n.c

CdS

0.2

CuO 200

0.4

0.0

1.0 B 0.8 B

0.6 C/Co

300 C

0.4

250

-Z'' (ohm)

CuO n.c + CdS CuO 200 + CdS CuO 400 + CdS CuO 600 + CdS CuO 800 +CdS

CuO n.c CdS CuO 200 CuO 200 + CdS

200

0.2

150

0.0 100 0.25

50

C 0.20

100

200

300

400

Z' (ohm) Fig. 4. A) Typical EDX spectrum and (B) X-ray mapping of CdS-CuO200 NPs; C): Nyquist plots for the modified CPE electrodes with CuO, CdS and CdS-CuO NPs as modifiers (modifier: 15%) in phosphate buffer supporting electrolyte (pH 3.5), Frequency: 1–1000 kHz, Potential: 0.5 V, Amplitude: 0.08 V.

b

a : blank e : CuO n.c + CdS

0.15 C/Co

0

a

f : CuO

0.10

200

+ CdS

d : CuO

400

+ CdS

c : CuO

600

+ CdS

b : CuO

800

+ CdS

c

0.05

d e

0.6 V (vs SHE) is enough negative than the CuOCb with standard potential of 0.27 V. This significantly prevents from e/h recombination and hence causes a significant increase in the photocatalytic activity of the coupled systems. All the above experiments were done by the coupled catalysts containing 1:2 mol ratio of CuO/CdS. To study of the effect of mole ratio of CuO/CdS on the photocatalytic activity of the coupled catalysts, different mole ratios of CuO/CdS were examined and the results are summarized in Table 2. As shown, by fixing the CdS moles and changing CuO moles no significant changes were observed in degradation of MB, while when CuO mole hold constant and the CdS mole changed, a decreased

f

0.00 300

400

500

600

700

Wavelength (nm) Fig. 5. A) Effect of calcination temperature on the photocatalytic activity of CuO NPs alone; B) Photocatalytic activity the coupled CdS-CuO NPs containing calcined CuO NPs at different temperatures for 4 h; C) Decrease in UV–Vis absorbance of MB solution during the photodegradation process by the catalysts mentioned in Fig. 5B; D) Effect of calcination time of CuO NPs on the photocatalytic activity of CdS-CuO200; Conditions applied to all experiments (A to D): CMB: 2 ppm, irrad. time: 120 min, pH: 6.5, catalyst dosage: 0.8 g L−1; E) Typical Z-Scheme to describe the charge transfer in the coupled CdS-CuO system.

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0.6

0.5

D 0.5 0.4

A CuO n.c + CdS CuO 200 (2h) + CdS CuO 200 (4h) + CdS CuO 200 (6h) + CdS

0.4

C /Co

C/Co

0.3

0.3

0.2

0.2 0.1

0.1 0.0

0.0

1

0.5

0.4

4

2 CMB (ppm)

B -1 y = -0.65 - 0.028x r2 = 0.9520

-2

C/Co

ln C / Co

0.3

-3 -4 -5

0.2

-6 0

40

80

120

160

Irrad. Time (min)

0.1

0.0 0

40

80

120

160

Irrad. time (min) Fig. 6. A) Effect of MB concentration on its photodegradation extent, irrad. time: 120 min, pH: 6.5, CdS-CuO200 dosage: 0.8 g L−1; B) Effect of irradiation time on MB degradation; pH: 6.5, CMB: 1 ppm, CdS-CuO200 dosage: 0.8 g L−1 (Inset: Plot of lnC/Co vs time for calculation of rate constant).

E Fig. 5 (continued).

re-combine with the holes. Hence, balancing the mole ratio of CuO and CdS is important, so the best degradation extent of MB achieved at mole ratio of 1:1. 3.2.2. Effects of MB Concentration and the CdS/CuO Dosage As shown in Fig. 6A, at lower and higher MB concentrations (0.5 and 4 ppm), lower degradation extents were obtained for MB, while the best results were obtained at 1–2 ppm MB. In 0.5 ppm MB solution, collision Table 2 Results of MB degradation by changing mole ratio of CuO/CdS. Mole ratio of the semiconductors

CuO200/2h/CdS CuO200/2h/CdS CuO200/2h/CdS CuO200/2h/CdS CuO200/2h/CdS CdS/CuO200/2h CdS/CuO200/2h CdS/CuO200/2h CdS/CuO200/2h

1:1 1:2 1:3 1:4 1:5 1:2 1:3 1:4 1:5

MB degradation C/Co

Deg.%

0.157 0.485 0.353 0.382 0.515 0.279 0.240 0.255 0.402

84.1 51.5 64.7 61.8 48.5 72.1 76.0 74.5 59.8

probability of MB molecules and hydroxyl radicals is very low and OH radicals participate in the side reactions because of their very short lifetime in the range of nano second. At 4 ppm MB concentration, high amounts of MB molecules present in solution that absorb a part of photons and hence smaller part of the photocatalyst surface can be activated. Hence, production of hydroxyl radicals decreases [58]. Effect of the catalyst dose was studied in 1 ppm MB solution at pH 5 during 80 min. The C/Co values of 0.085 ± 0.002, 0.011 ± 0.001 and 0.079 ± 0.002 obtained for 0.6, 0.8 and 1.0 g L−1 of the CdS/CuO catalyst, respectively. At higher dosages of the catalyst, scattering effect and aggregation of the catalyst particles caused the decrease in the degradation efficiency. 3.2.3. Kinetics and pH Studies Effect of irradiation time on the photocatalytic activity of the CdSCuO catalyst in the degradation of MB is shown in Fig. 6B. As shown, degradation extent was increased rapidly during 80 min and no considerable increase obtained thereafter. Plot of lnC/Co versus irradiation time was constructed and a first order kinetic model, with rate constant of 0.028 min−1 observed for the process [59]. Effect of initial pH of the solution on the photodegradation of MB is shown in Fig. 7A. Also, pHpzc of monocomponent and the coupled CdS and CuO systems was calculated by two reported methods [44,45]

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molecules near the catalyst surface and the degradation extent of MB was increased. At pHs N pHpzc, repulsive force between the free electron pairs of N and S atoms of MB molecules and the negatively charged surface of the catalyst repels MB molecules and degradation extent was decreased. 3.2.4. Effect of Scavengers In the optimized conditions, effect of NaCl, NaHCO3 and isopropanol on the photodegradation extent of MB studied (Fig. 8A). As the results show, in the presence of these compounds and with increasing their concentrations, the degradation extent of MB was decreased. It has

A 0.4

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100 200 270 oC 100 200 270 oC

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Fig. 7. A) Effect of pH of MB solution on the degradation activity of CdS-CuO200–catalyst, Inset: Molecular structure of MB, (CMB: 1 ppm, irrad. time: 80 min, CdS-CuO200 dosage: 0.8 g L−1); B, C) Typical plots for calculation of pHpzc of some catalysts.

Degradation (%)

80 c

0

270 oC

100 200

60 40 20

(Figs 7B–C). As shown, all the catalysts have pHpzc around 7 which the charge of the catalysts surfaces was neutralized at this pH by the surrounded environment. It would be expected that, lower degradation extent to be achieved at acidic pHs because the positive charge of the catalysts surface at pH b pHpzc should repels the cationic MB molecules. While, higher degradation extent was obtained at pH 3–5. MB has pKa value of 3.8 and hence at pH N 3.8 the major part of MB is present as neutral (deprotonated) form (involving 2 N and 1 S atoms). In this condition, attractive force between the free electron pairs of N or S atoms in MB molecules and the positively charged catalyst surface bring MB

0 1

2 Reusing Runs

3

Fig. 8. A) Effect of NaCl, NaHCO3, and isopropanol on the photodegradation of MB (CMB: 1 ppm, irrad. time: 80 min, pH: 5, CdS-CuO200: 0.8 g L−1); B) Decrease in UV–Vis absorption of MB solution during the photodegradation process (pH: 6.5, CMB: 1 ppm, CdS-CuO200: 0.8 g L−1); C) Results of reusability experiments of the CdS-CuO200 photocatalyst in photodegradation of MB, (CMB: 1 ppm, irrad. time: 80 min, pH: 5, CdSCuO200: 0.8 g L−1).

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reported that chloride, carbonate, bicarbonate and isopropanol are strong hydroxyl radical scavengers [60]. Hence, the results confirm that OH radicals have more important role in MB degradation at the applied conditions than the superoxide radicals.

at the applied conditions. Lower decrease in the activity would be expected at higher drying temperatures because it can remove more adsorbed intermediate species from the catalyst surface.

3.2.5. UV–Vis and COD Studies As shown in Fig. 8B, the intensity of absorption peak for the peaks located at 664 and 291 nm was decreased during the MB photodegradation. The spectra show that no new peaks appeared at UV region, confirming much mineralization of MB molecules during the process. Monomers and oligomers (dimmer and trimmer) of MB in aqueous solutions have the UV–Vis absorption maxima at 668 and 624 nm, and at 606 nm and 565 nm, respectively with some overlapping [61]. Our results have well agreement with these observations. Intense decrease in the intensity of absorbance at 664 nm in the present work confirms that MB monomer can be broken rapidly while its dimmer or trimmer can be broken relatively hard and should be degraded at longer times. This is true, because MB dimmer has higher molar absorptivity than its monomer, and hence has higher stability due to its higher resonance [61]. In another work, it has reported that the main absorption peak at 664 nm belongs to absorbance of the conjugation system between the two dimethylamine substituted aromatic rings through the sulfur and nitrogen. The shoulder at 615 nm belongs to dyes' dimmer. The absorption band in the ultraviolet region (at 288 nm) relates to the substituted benzenes rings [31]. It has shown by LC/MS that at initial steps of MB photodegradation the main peak of MB (belong to 660 nm) has rapidly diminished and then the corresponding shoulder at 615 nm. This confirms the destruction of the parent MB molecules to some new intermediates that are structurally close to parent MB molecules [31]. It has reported that by the cleavage of one or more of the methyl groups substituent on the amine groups, some new intermediates/compounds such as azure A, B and C and thionin have formed that detected by GC/MS and LC/MS [31–33,62–64]. It has also confirmed by LC/MS that the N-demethylation route does not form the major degradation pathway, while oxidation of sulfur to sulfone followed, N-demethylation and hydroxylation of aromatic rings yields a compound that can rapidly mineralize to carbon, sulfur and nitrogen dioxides and ammonium chloride [33]. As we know, chemical oxygen demand (COD) shows the extent of total organic pollutant of solution. Hence, COD values of the MB solutions before and after photodegradation process (80 min) was evaluated. The initial COD of 768 mgO2/L was decreased to 128 mg L−1 during 80 min photodegradation process. This corresponds to degradation of 83% of MB molecules to smaller fragments especially the mineralized species. In photodegradation process, OH radicals can get electron from free electron pair, carbon carbon double bond and single bond (sigma bond) sequence. As shown in molecular structure of MB, it has high resonance and high stability. Hence, getting the electrons from free electron pairs and carbon carbon double bond cannot happen at initial steps. At initial steps of photodegradation process, N\\CH3 bond begin to break. It has reported that in oxidation of MB by hydrogen peroxide, in 1:1 MB/H2O2, the N\\CH3 bond began to break and in 1:2 MB/H2O2 the aromatic rings began to cleavage [65]. Accordingly, during the progress of the photodegradation process, after cleavage of N\\CH3 bonds, OH radicals can attack to aromatic rings and destroy the MB molecules. Some photodegradation intermediates during the photocatalytic degradation of MB, determined by GC-Mass, have illustrated in literature [34].

3.2.7. Comparison of the Catalyst With Some Previous Works Comparison of the work with some previous published works confirms relatively preference of the used catalyst in this work. In the present work, the best degradation extent of MB molecules (N98%) was obtained in 1 ppm MB solution at pH 5 by 0.1 g L−1 of the CuO-CdS NPS during 120 min with rate constant of 0.028 min−1. The rate constants of 0.0025 min−1 and 0.015–0.022 min−1 have obtained by Cu2O/rGO [19] and TiO2 [33] catalysts, respectively. The degradation extent of 90% has also obtained for MB by g-C3N4/TiO2 catalyst during 360 min [21]. The rate constant of 0.0052 min−1 has also obtained for MB photodegradation by Ti/Cr binary oxides (CMB: 5 μM, 0.64 g L−1 of the catalyst during 240 min). This comparison confirms preferentially of the used catalyst in this work regarding its faster rate and needing its lower dosages.

3.2.6. Reusability of the Photocatalyst The CdS-CuO catalyst was used in successive runs in photodegradation of MB at the optimized conditions (Fig. 8C). After each run, the recovered catalyst was dried at temperatures of 100, 200 and 270 °C and used again. Decrease values of 11, 25 and 29% in the photodegradation efficiency were observed for the catalysts dried at 100, 200 and 270 °C, respectively after 3 successive runs, confirming reasonable stability of the catalyst

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