Synthesis of novel heterostructured ZnO-CdO-CuO nanocomposite: Characterization and enhanced sunlight driven photocatalytic activity

Materials Chemistry and Physics 249 (2020) 122983

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Synthesis of novel heterostructured ZnO-CdO-CuO nanocomposite: Characterization and enhanced sunlight driven photocatalytic activity Tauseef Munawar a, Sadaf Yasmeen a, Fayyaz Hussain b, Khalid Mahmood c, Altaf Hussain a, M. Asghar d, Faisal Iqbal a, * a

Department of Physics, The Islamia University of Bahawalpur, Bahawalpur, 63100, Pakistan Materials Simulation Research Laboratory (MSRL), Department of Physics, Bahauddin Zakariya University Multan, 66000, Pakistan Department of Physics, Government College University Faisalabad, Faisalabad, 3800, Pakistan d Department of Physics, Khwaja Fareed University of Engineering & Information Technology Rahim Yar Khan, 64200, Pakistan b c

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

� A novel heterostructured ZnO-CdO-CuO nanocomposite was synthesized. � Enhanced photocatalytic activity of nanocomposite against MB dye than pure oxides. � CR dye was degraded more as compared to MB, RhB and MO dye under similar conditions. � The main role of O2*- and HO* radicals in degradation reaction. � The PL spectra shown NBE and DLE were due to intrinsic defects.

A R T I C L E I N F O

A B S T R A C T

Keywords: X-ray diffraction Volume fractions Synthetic dyes Raman spectroscopy Photoluminescence (PL) Photocatalyst

In this study, a novel photocatalytic material heterostructured ZnO-CdO-CuO nanocomposite along with pristine ZnO, CdO, and CuO nanoparticles were synthesized by the facile co-precipitation method. The grown nano­ composite was characterized by XRD, FTIR, Raman, SEM, IV, UV–vis and PL techniques. The XRD pattern exhibited the diffraction peaks of ZnO (hexagonal), CdO (cubic) and CuO (monoclinic) with ZnO, CdO, and CuO phases 65%, 16%, and 19%, respectively. The microstructural analysis was carried out using Scherrer plot, W-H and SSP methods. The FTIR and Raman spectra also inveterate the successful formation of ZnO-CdO-CuO. The IV measurements revealed the high electrical response of nanocomposite. The SEM images are shown agglomerated rod-shaped morphology and the elemental analysis also confirmed the higher atomic concentration of Zn. PL spectra shown strong NBE and DLE emissions related to extrinsic defects which could act as the trap centers for charge carriers and enhance photocatalytic activity. The energy bandgap (Eg) was 2.9 eV, specified that the grown nanocomposite could be an excellent photocatalyst. The photocatalytic activity was performed against methylene blue (MB) revealed higher degradation efficiency of 94% as compared to pristine ZnO (60.0%), CdO (41.0%), and CuO (61.0%). The photodegradation of other synthetic dyes i.e. rhodamine-B, methyl orange, and cresol red was also assessed by grown nanocomposite under sunlight, exhibits degradation efficiencies, 87%, 89% and 99% in 100 min illumination, respectively. Furthermore, the species trapping experiment along with the recyclability test was carried out against cresol red dye using the nanocomposite catalyst. A schematic model was also designed to illustrate the photodegradation reaction mechanism.

* Corresponding author. E-mail address: [email protected] (F. Iqbal). https://doi.org/10.1016/j.matchemphys.2020.122983 Received 28 November 2019; Received in revised form 10 February 2020; Accepted 28 March 2020 Available online 5 April 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.

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Materials Chemistry and Physics 249 (2020) 122983

In the present work, a novel ZnO-CdO-CuO nanocomposite along with pristine ZnO, CdO, and CuO were synthesized. The grown products were characterized by XRD, FTIR, Raman, UV–vis, PL, I-V measurements and SEM. The micro-strain (ε) and average crystallite size (D) were calculated by Scherrer, W-H, and SSP methods. The volume fraction of individual oxides phases (ZnO, CdO, CuO) in nanocomposite was calculated using XRD data. The photocatalytic activity was carried out against MB dye under sunlight irradiations. The photocatalytic perfor­ mance of grown ZnO-CdO-CuO nanocomposite was also evaluated using other synthetic dyes including RhB, MO, and CR. The species trapping experiment was also performed along with the recyclability test against CR dye by the ZnO-CdO-CuO catalyst. A promising schematic model is designed to illustrate the photocatalytic reaction mechanism of the ZnOCdO-CuO catalyst. Furthermore, the results imply that grown nano­ composite is a proficient compound for photocatalytic, photonic and optoelectronic applications.

1. Introduction In the past decade, cumulative global energy and environmental pollution problems posed severe threats for aquatic ecosystems and public health [1,2]. The release of toxics synthetic dyes from textile industries and medical laboratories have a diverse effect on the human and aquatic organisms [3,4]. Different approaches such as biological treatment, chemical, and physical have been utilized for environment remediation [5]. In recent years, the semiconductor-mediated solar photocatalysis is considered very often because it is eco-friendly, sus­ tainable and green technology for the degradation of dyes [6,7]. As the potential photo-catalysts, ZnO is an outstanding material for the decomposition of synthetic dyes due to its low cost, non-toxic, high chemical stability, and high photosensitivity [8,9]. Nevertheless, the fast rate of photo-induced electron (e )-hole (h þ ) pairs recombination and wide bandgap of 3.37 eV that greatly perturbs photo-degradation re­ action and limits its visible light response [10]. To cope with these limitations, persuasive chemical modification has been implemented such as tuning the microstructure, modifying its size, coupling with unlike semiconductors, ion doping, and co-doping, etc. [11,12] have so far established to alter the photo-response capability of ZnO. Among these approaches, combining ZnO with other dissimilar energy-level materials to build a heterojunction is an appropriate technique for improving its photocatalytic activity for maximum use of the solar spectrum [13–15]. Among various metals oxides, cadmium oxide (CdO) and copper oxide (CuO) have widely attracted for photocatalytic applications because of their low cost, abundance, chemical, and physical stability. CdO is an n-type semiconductor with 2.2–2.5 eV direct optical bandgap and has unique chemical, electrical, and optical properties that made it suitable for the fabrication of photo-catalysts [16,17]. CuO with narrow bandgap (1.2 eV) is a p-type semiconductor with photochemical and photoconductive properties owning significant applications to use as photocatalyst [18,19]. The different researchers have been synthesized the individual ZnO, CdO, and CuO and their binary composites ZnO-CdO, ZnO-CuO, and CdO-CuO for enhancing photocatalytic activity and investigated the physical properties in detail. C.V Reddy et al. [20] reported the photocatalytic degradation of RhB dye by CdO, ZnO, and CdO/ZnO nanocomposite under UV light and explained their physical properties by using characterization techniques i.e. XRD, UV–visible, HRTEM, Raman spectroscopy, FTIR, PL, and XPS. G. Somasundaram et al. [21] studied the structural, morphological, and optical properties of CdO nanoparticles along with it’s photocatalytic/antibacterial ac­ tivity and reported the 80.41% photodegradation of MB under UV light. S. Harish et al. [22] reported the photocatalytic mineralization of MB dye by CuO/ZnO nanocomposite under UV light. K.H Kim et al. [23] reported the photocatalytic activity of NB dye under sunlight by copper oxide nanoparticles. P. Biswas et al. [24] reported the photocatalytic degradation of MO dye under visible light using pristine zinc oxide. The coupling of n-type ZnO with n-type CdO and p-type CuO impact strongly on the physical properties of the ZnO and enhanced its photocatalytic performance for sunlight. A few reports on the synthesis of ZnO coupled with two metal oxides could be found such as AgO-NiO-ZnO [25], CdO-NiO-ZnO [26], NiO⋅CeO2⋅ZnO [25], ZnO-TiO2-SiO2 [27], CeO2⋅CuO⋅ZnO [28], Ag2O⋅CeO2⋅ZnO [29], Bi2O3-CeO2-ZnO [30], La2O2Co3.CeO2.ZnO [31], and so on for enhancing photocatalytic activity. The presence of double heterojunction can efficiently hinder the electron (e )-hole (h þ ) recombination and also improves charge separation/transfer ability. A literature study shows that nanocomposites of metal oxides have been prepared by various methods such as micro-emulsion synthesis, sol-gel technique, co-precipitation method, hydrothermal processing, microwave-assisted synthesis, wet chemical method, and solid-state reaction, etc. Among these, the co-precipitation method is used because it is effective, requires low growth temperature, simple, fast and low cost [32].

2. Experimental procedure 2.1. Materials Zinc acetate dihydrate Zn(CH3COO)2.2H2O, nitrate salts of cadmium (Cd(NO3)2.6H2O) and copper (Cu(NO3)2.6H2O), sodium hydroxide (NaOH), methylene blue (MB), cresol red (CR), rhodamine-B (RhB), methyl orange (MO), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), isopropanol (IPA), silver nitrate (AgNO3), 1,4-benzoqui­ none (PBQ), and distilled water. All materials from a commercial source (Sigma Aldrich) and used without extra purification. 2.2. Sample synthesis The synthesis procedure is provided in supplementary data. 2.3. Photocatalytic activity The photocatalytic activity of pristine ZnO, CdO, CuO, and ZnO-CdOCuO nanocomposite is assessed for the photodegradation of MB dye under sunlight illumination. First of all, 10 mg powder of each catalyst was separately dispersed in 60 ml dye aqueous solution at a specific concentration (5 ppm). Preceding to the photocatalytic test, the mixture solution was stirred magnetically in dark for 1 h to attain adsorption/ desorption equilibrium between dye and catalyst. A specific solution (5 ml) of initial concentration (Co) was taken out and exposed to sunlight. During the reaction process, reactant mixtures were continuously stirred and samples were extracted after regular intervals (20 min) to define the degradation %age of dye. The removed suspensions were centrifuged (6000 rpm for 10 min) for solid-liquid separation. Temporal concen­ trations variation of MB dye was monitored by investigating the change in absorption peaks using the UV–vis spectrometer. The photo­ degradation efficiency (η) was calculated using relation [33]: Degradation efficiency (η) ¼ [(Co

Ct)/Co] � 100%

(1)

Where Co and Ct are the initial and final concentrations of dye. Furthermore, the photodegradation of other synthetic dyes RhB, MO, and CR was also evaluated using the ZnO-CdO-CuO catalyst. The same photocatalytic experiment procedure was used to degrade all the dyes. The species trapping experiment was also performed using EDTA-2Na, IPA, AgNO3, and PBQ scavengers together with recyclability tests under similar experimental conditions. The chemical oxygen demand (COD) measurements were performed to confirm the degradation of MB, RhB, MO, and CR dyes by the ZnO-CdO-CuO catalyst. The COD removal (%) was calculated using relation [34]: COD removal ð%Þ ¼ ½ðCODo CODf Þ =CODo � � 100, Where CODo and CODf are the chemical oxygen demand values of dye solution before and after treatment. 2

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perceived in all grown samples. The intense and sharp diffraction peaks are showing the highly crystalline nature of grown nanocomposite. The pristine ZnO peaks appeared at diffraction angles of 31.9, 34.5, 36.3, 47.6, 56.7, 62.9, 66.5, 68.0, and 69.2� corresponding to reflection from (100), (002), (101), (102), (110), (103), (200), (112), and (202) planes (JCPDS card No. 01-089-7102), the pristine CdO diffraction peaks located at angles 2θ ¼ 33.0, 38.3, 55.3, 65.9 and 69.3� ascribable to the planes (111), (200), (220), (311) and (222), respectively, (JCPDS card No. 01-073-2245), and the pristine CuO peaks observed at 31.72, 35.6, 38.9, 45.4, 48.9, 56.5, 61.7, 66.2, 75.2 and 83.9� angles belong to the crystal facets (110), (002), (111), ( 112), ( 202), (202), ( 113), ( 311), ( 222) and (400), respectively, (JCPDS card No.00-041-0254). In the nanocomposite, the ZnO peaks appeared at diffraction angles of 31.7, 34.4, 36.2, 47.5, 56.6, 62.7, 69.2, 72.4, 75.3 and 80.1� are corresponded to reflection from (100), (002), (101), (102), (110), (103), (201), (004), (202) and (104) planes, reliable with the JCPDS card No. 01-089-1397. The diffraction peaks located at angles 2θ ¼ 33.1, 38.9, 55.3, 68.7 and 81.1� are ascribed to the planes (111), (200), (220), (222) and (400), respectively, related to the CdO phase (JCPDS card No. 00005-0640). The peaks observed at 35.5, 45.5, 48.6, 53.7, 58.3, 61.5, 66.0 and 81.3� angles are belong to the crystal facets (002), ( 112), ( 202), (020), (202), ( 113), ( 300) and ( 313), respectively, consistent to CuO (JCPDS card No.00-005-0661). The structural parameters for pristine ZnO hexagonal, CdO cubic and CuO monoclinic structure and in the nanocomposite were calculated [35,36] and summarized in Table 1. The texture coefficient (TC(hkl)) values are calculated using the following equation [37]:

Fig. 1. XRD patterns of (a) ZnO, (b) CdO, (c) CuO, and (d) ZnO-CdO-CuO nanocomposite.

2.4. Characterization

IðhklÞ =Io ðhklÞ � TCðhklÞ ¼ � � 1 IðhklÞ Io ðhklÞ N

The XRD (2θ ¼ 30-85� , Bruker-D8 Advance Laboratory Diffractom­ eter), FTIR (400-4000 cm 1, Tensor-27 FTIR spectrometer), Raman (60–1100 cm 1, Confocal micro-Raman) were used for structural anal­ ysis. The UV–vis (200–700 nm, Cary-60 Agilent Technologies) and PL (330–750 nm, He-Cd laser line) for optical, IV ( 5 to 5 V, Keithley Picometer 6487) for electrical, SEM (JEOL 3500) for morphological, and EDX for elemental analysis. The COD measurements were carried out using COD rector (Lovibond ET 125 SC) at 150 � C for 2 h and analysis of the data using COD Lovibond PCCHECKIT analyzer.

(2)

where, I(hkl) is the peak intensity measured from XRD, Io (hkl) is the standard intensity corresponding to the same plane and N is the number of measured prominent diffraction peaks. The deviation in TC(hkl) values for different planes of pristine ZnO, CdO and CuO and in nanocomposite are shown in Fig. S2 (a, b) (supplementary data). The crystallite size (D) and micro-strain (ε) were calculated using Scherrer (Fig. 2(a)), W-H (Fig. 2(b)), and SSP (Fig. 2(c)) methods dis­ cussed in the previous report [33]. The calculated values of D and ε are summarized in Table 1. The crystallite size (D) of ZnO and CdO decrease while CuO increase in nanocomposite as compared to pristine nano­ particles. The measured value of D for ZnO was higher as compared to CdO and CuO revealed improved crystallite growth in the nano­ composite. The dislocation density ‘δ’ was calculated using the equation: δ ¼ 1/D2 and given in Table 1. Furthermore, the calculated value of D is less for the Scherrer method as compared to other methods because it did not consider the contribution due to strain. The small alteration in the values of D may be due to the difference in averaging the particle size distribution. It is noteworthy to mention that the SSP was more accurate as compared to others.

3. Results and discussion 3.1. X-ray analysis The phase composition and crystalline structure of the grown sam­ ples were analyzed by X-ray diffraction. The XRD diffraction pattern of pristine ZnO, CdO, CuO, and ZnO-CdO-CuO nanocomposite are pre­ sented in Fig. 1(a–d). All diffraction peaks of pristine ZnO, CdO, CuO and in nanocomposite are well-matched with ZnO (hexagonal wurtzite, space group: P63mc), CdO cubic FCC structure (Monteponite, space group: Fm-3m) and CuO monoclinic crystal structure (Tenorite, space group: C2/c). No extra peak due to secondary phases or hydroxides is

Table 1 Geometric parameters of pristine ZnO, CdO, CuO nanoparticles and in ZnO-CdO-CuO nanocomposite determined from XRD analysis. Sample

a(Å)

Pristine ZnO 3.2445 CdO 4.6938 CuO 4.6709 In nanocomposite ZnO 3.2702 CdO 4.7207 CuO 4.6139

b(Å)

c(Å)

Volume (Å3)

dspacing

Scherrer method

Scherrer plot method

Williamson-Hall method

Size-Strain Plot method

D (nm)

D(nm)

D (nm)

ε(�

10 4)

D (nm)

ε(�

Dislocation Density δ (nm) 2 � 10

10 3)

– – 3.4509

5.1935 – 5.2099

47.3465 103.4135 83.1574

2.0356 1.8977 1.8469

41 40 35

58 60 48

51 63 46

4.52 7.43 7.42

46 48 32

4.94 4.87 3.90

5.89 6.05 8.07

– – 3.4345

5.2486 – 5.1823

48.6083 105.1990 81.0062

2.0387 1.8478 1.7902

40 39 38

42 46 48

48 48 45

8.65 4.19 7.63

43 44 45

4.75 3.57 5.91

6.25 6.57 6.93

3

4

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Fig. 2. (a) Scherrer plot, (b) W-H plot and (c) SSP to estimate crystallite size and micro-strain.

patterns. The calculated value for ZnO, CdO, and CuO phase was 65%, 16%, and 19% respectively, in the nanocomposite. The highest value of Zn (Zn > Cu > Cd) in nanocomposite was also confirmed from elemental analysis (section 3.4). The difference in the volume fraction calculated from the quantitative approach can not directly interrelated with the amount of materials used in the synthesis process. The alteration in the volume fractions of each phase is due to the modification in the nano­ particle precipitation yield among the three oxides.

Table 2 Parameters used to calculate the volume fractions of the ZnO, CdO and CuO phases in ZnO-CdO-CuO nanocomposite. Parameters Reflection (hkl) Integrated Intensity I 2θ Unit cell volume (nm3) sin θ/λ (Å 1) Structure factor ǀFǀ2 Multiplicity p Lorentz-polarization factor Temperature factor e 2M R (nm 6) Volume fraction (%)

ZnO-CdO-CuO nanocomposite ZnO phase

CdO phase

CuO phase

(100) 17 31.7 0.0486083 0.177 1087 6 24 0.9750 64645071 65

(200) 15 38.7 0.1051990 0.215 31759 6 15 0.9449 251768080 16

(-111) 12 35.5 0.0810062 0.198 13607 4 19 0.9774 152221591 19

3.2. FTIR analysis The FTIR spectrum of the ZnO-CdO-CuO nanocomposite is shown in Fig. 3(a). From the spectra, the bands positioned at 498, 451 and 474 cm 1 corresponded to characteristic Zn–O, Cd–O and Cu–O bonds vi­ brations [39–41] (Fig. 3(a)(insert)), further inveterate the formation of ZnO-CdO-CuO nanocomposite. The prominent vibrational bands observed at 520, 537, 582 and 645 cm 1 were due to M–O–M (M ¼ Zn, Cd or Cu) lattice vibrations [42–44]. Additional bands due to other chemical bonding were observed around 1629 and 1777 cm 1 might be due to the bending and deformation vibration of physisorbed and/or chemisorbed H2O molecules [45]. Furthermore, to strengthen the FTIR analysis, the bond length of the metal-oxygen (M O) bond was calculated. By using the observed vibration Zn–O, Cd–O and Cu–O

3.1.1. Crystal volume fraction To support the structural analysis, the volume fraction of each phase in nanocomposite was calculated by direct comparison method [38]. The parameters used for this estimation are provided in Table 2. The integrated intensity (IZnO, ICdO, and ICuO) for (100), (200) and ( 111) plane of ZnO, CdO and CuO respectively were obtained from XRD 4

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Materials Chemistry and Physics 249 (2020) 122983

Fig. 3. (a) FTIR spectra in the wavenumber range from 400 to 4000 cm 1 and insert shows enlarge view in the wavenumber range from 400 to 800 cm 1, (b) Raman spectra, and (c) I–V characteristics curves in a range of 5 to þ5 V of grown ZnO-CdO-CuO nanocomposite. Table 3 Wavenumber, effective mass, force constant, bond-length, optical phonon frequency and Debye temperature values for ZnO, CdO and CuO in nanocomposite. Oxides

Wavenumber (cm 1)

Effective mass (10 kg)

ZnO CdO CuO

498 451 474

2.1343 2.3256 2.1224

26

Force constant, k (N/ cm)

Bond-length (Å) (FTIR)

Bending optical phonon υo (Hz) 1013

Bending Debye temperature θD (K)

1.8807 1.6807 1.6942

2.0830 2.1626 2.1569

1.494 1.353 1.422

717 650 683

bonds, the optical phonon frequency (υo) and Debye temperature θD were calculated using relations: c ¼ λυo ¼ υo/υ and hυo ¼ kBθD where h and kB are the Planck’s constant and Boltzmann constant [46]. The measured values of vibrational parameters are listed in Table 3.

formation of the ZnO-CdO-CuO nanocomposite was further confirmed by Raman spectra. The weak and broad bands have anticipated increasing numerous lattice or host-lattice defects i.e. oxygen vacancy, zinc/cadmium/copper interstitials or their complexes (consistent with PL study presented later) that can support the detention of photo-induced electrons and hinder electron/hole recombination.

3.3. Raman analysis

3.4. Current-voltage (I-V) measurements

Raman spectroscopy was employed for identifying disorders and defects in the grown nanocomposite. Fig. 3(b) showed the Raman 1 spectra of the nanocomposite. The bands at 106 cm 1 (Elow 2 ), 433 cm 1 1 1 high (E2 ), 457 cm (2LA), 893 cm (LA þ LO) and 1020-1085 cm (TO þ LO) corresponds to ZnO phase [47] while CuO observed at 279 (A1g) cm 1, 293 (Ag) cm 1, 686 cm 1 (B2g) cm 1 [48], and CdO at 249 cm 1 (TA þ TO), and five peaks in the range of 765–965 (2LO) cm 1 [20,49]. The shifting of peaks and change in their intensity are due to structural defects generated by the interaction among ZnO, CdO and CuO. The

Fig. 3(c) exhibits the I-V characteristic curve of nanocomposite ( 5 to 5 V). It is observed that nanocomposite has ohmic nature. From the forward biased region, it can be perceived that the ZnO-CdO-CuO nanocomposite has higher conductivity, suggested that the electrical conductivity of metal oxide can be improved by forming ternary oxides nanocomposite.

5

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Fig. 4. Variation of (a) Absorbance, the insert shows Tauc’s plot for energy bandgap (Eg) and (b) Transmittance as a function of incident photon wavelength; SEM images of ZnO-CdO-CuO nanocomposite at (c) 5 μm and (d) 1 μm.

3.5. Optical analysis

has revealed the rough rod-shaped particles with random orientation. It can be seen that nanocomposite has high agglomeration which is due to the interaction between three kinds of oxides that altered the kinetics and morphology of grown particles. The sheet-like morphology was also obtained by K. Karthik et al. [26] for CdO-NiO-ZnO nanocomposite. The elemental analysis revealed the presence of zinc, cadmium, and copper with an atomic percentage of 47%, 16%, and 20% respectively, oxygen 17%.

Fig. 4(a) exhibits the absorption spectra of the nanocomposite. The nanocomposite has absorption maxima at 271 nm, 343 nm, 396 nm in UV-region and at 427 nm and 586 nm in the visible region. The corre­ sponding energy of these absorption peaks can be calculated from energy-wavelength relation [50]: Eg ¼ hc=λðabsorptionÞ . The three ab­ sorption peaks at 396 nm, 427 nm and 586 nm (3.1 eV, 2.9 eV and 2.1 eV) were due to scattering from individual oxide phase ZnO, CdO and CuO, proposed that as-synthesized nanocomposite is visible light sen­ sitive. The absorption peaks at 343 nm (3.6 eV) and 271 nm (4.6 eV) were due to Zn/Cd/Cu interstitial or oxygen-related defects (discussed in detail in section 3.7). The transmittance spectrum of nanocomposite (Fig. 4(b)) has exhibited an opposed trend to the absorption spectrum. It is important to mention that transmittance is increased in the visible range which can be related to the native defects or distortions/disorder due to interaction between oxides in the nanocomposite. The optical energy bandgap (Eg) calculated using Tauc’s relation [51] (Fig. 4(a)(insert)) was 2.9 eV which lies between ZnO (3.37eV), CdO (2.5 eV) and CuO (1.2 eV). The Eg value in the visible region sug­ gested that as-prepared nanocomposite has enhanced photocatalytic activity under sunlight.

3.7. Photoluminescence (PL) PL spectroscopy was used to investigate optical emission and defect states (vacancies, interstitials, and antisite, etc.) in the grown nano­ composite. Fig. 5(a and b) shows the PL spectrum of nanocomposite and the possible transition scheme of all emission peaks. The near-band emissions (NBE) in the UV region at 376 nm (3.3 eV), 381 nm (3.2 eV) and 395 nm (3.1 eV) are ascribed to the transitions from recombi­ nation of shallow trapped electron/hole pairs and/or excitons. In addi­ tion to NBE, nanocomposite has exhibited deep-level emission (DLE) in the visible region (400–600 nm), suggested the presence of impurities/ defects within the nanocomposite [52]. Fig. 5(b) shows that the in­ tensity of excitonic and visible emission is different, signifying that the defect density is affected by the interaction of metal oxides. The emis­ sion peaks in spectral range 500–600 nm are related to the green emission corresponds to different defects states. Two green emission at 509 nm (2.4 eV) and 534 nm (2.3 eV) are correspond to native defects like zinc/cadmium/copper interstitials or transition from CB to OZn (antisite) [53]. The green emission at 577 nm (2.1 eV) is appeared due to neutralized, singly and doubly ionized oxygen vacancies (Vo*, Voþ, and

3.6. SEM analysis SEM analysis was carried out to examine the surface morphology of the grown sample. Fig. 4(c and d) shows the SEM images of the ZnOCdO-CuO nanocomposite. The surface morphology of nanocomposite 6

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Here, k ¼ first-order rate constant. By plotting ln(Co/Ct) on the y-axis and irradiation time (t) on the x-axis gives the straight line shown in Fig. 6(f), k is taken from the slope. The calculated values of rate constant are 0.0082, 0.0084, 0.00504, and 0.02732 for MB dye by ZnO, CuO, CdO, and ZnO-CdO-CuO, respectively. The rate constant of the catalyst is increased with increasing irradiation time and is greater for MB and CR, indicating that ZnO-CdO-CuO nanocomposite is an excellent photocatalyst for MB and CR under sunlight. The effectiveness of the grown nanocomposite catalyst was further tested on other dyes i.e. RhB, MO, and CR under similar experimental conditions. The variation in absorption spectra of RhB, MO, and CR in the presence of ZnO-CdO-CuO nanocomposite are exhibited in Fig. 7 (a–c). The degradation efficiency of nanocomposite toward RhB, MO, and CR after 100 min sunlight irradiation is 87%, 89%, and 99% respectively (Fig. 7(d)), with rate constants 0.01952, 0.01964 and 0.06039, respectively, shown in Fig. 7(e) with the highest degradation efficiency against CR. The % age COD removal of MB, RhB, MO, and CR dyes were 76%, 59%, 52%, and 84%, respectively (Fig. 8(a)). The trapping species experiments were also carried out by adding EDTA2Na, IPA, AgNO3, and PBQ scavengers, against CR dye using ZnOCdO-CuO catalyst to detect the role of major species involved in pho­ todegradation reaction. The results clearly illustrate (Fig. 8(b)) that the degradation is quenched more by adding IPA and PBQ as compared to AgNO3 and EDTA-2Na, signifying that both O2*- and HO* radicals are the foremost reactive species in the degradation process of CR by ZnOCdO-CuO catalyst. The reusability test (Fig. 8(c)) shows that the degradation efficiency decrease from 99.0% to 96.75 after the 5th cycle, recommends that the synthesized nanocomposite catalyst is highly sta­ ble and reusable. Fig. 5. (a) Schematic illustration of the emission scheme for synthesized nanocomposite and (b) PL emission peaks.

3.9. Photodegradation mechanism

Voþþ). E. N. Epie el al [54]. and R. Khokhra et al. [55] have also been reported the green emissions are due to oxygen vacancies. These oxygen vacancies can induce lattice distortion and raise O2 molecules that can trap photo-excited electrons faster and yield more superoxide radical groups, ultimately enhance photocatalytic activity (discussed in sec. 3.8). The two red emission peaks at 701 nm (1.8 eV) and 730 nm (1.7 eV) are ascribed to transition from zinc or oxygen interstitial (Zni, Oi) defect levels. PL study is specifying that grown nanocomposite can significantly enhance photocatalytic activity due to the presence of mid-gap states and the formation of heterojunction that can favor electron-hole pair’s separation/transformation and facilitating photons excitation/absorption under the sunlight. The visible emission is a distinct characteristic of grown nanocomposite that is essential for biological fluorescence labeling, luminescent, nano-photonics and op­ toelectronic applications [26].

When light is irradiated on photo-catalyst, an e- from filled VB excite to empty CB with the simultaneous creation of e--hþ pairs. At the photocatalyst surface oxidation and reduction takes place. The photo­ generated e- and hþ transferred to the surface of catalyst and participates in redox reactions, where photogenerated e- react with oxygen (O2) to the form less toxic superoxide anions radical reductive process whereas hþ react with water or hydroxide ions to generate the most reactive hydroxyl radicals by oxidative process and finally hydrogen peroxide. The superoxide radicals react with hydrogen peroxide to form hydroxyl radicals. During the degradation reaction, these hydroxyl and superox­ ide radicals react with dyes and converted into intermediate com­ pounds. These compounds are finally transformed/decomposed into green complexes (CO2 and H2O). A model proposed by A. Hezam et al. [56] and Lei et al. [57] was used to illustrate the mechanism of electron/hole separation. On the basis of this model, it is supposed that ZnO, CdO and CuO are contiguous to each other to form a double heterojunction interface. The band edge potentials (EVB and ECB) of ZnO, CdO, and CuO were calculated using relations as:

3.8. Photocatalytic activity The photo-degradation of MB was performed to evaluate the pho­ tocatalytic performances of pristine ZnO, CdO, CuO, and ZnO-CdO-CuO nanocomposite under sunlight illumination. The spectral variation in absorption of MB dye at a different time interval is shown in Fig. 6(a–d). It is imperative to mention here that all grown samples have degraded against MB dye. The degradation efficiency of grown samples follows the trend ZnO-CdO-CuO (94.0%) > CuO (61.0%) > ZnO (60.0%) > CdO (41.0%), respectively (Fig. 6(e)). Notably, grown nanocomposite has high degradation efficiency as compared to pristine oxides. The pseudofirst-order kinetic model was used to study photo-degradation reaction quantitatively given as [33]: kt

(3)

¼ > lnðCo = Ct Þ ¼ kt

(4)

Ct ¼ Co e

ECB ¼ X

Ee

EVB ¼ ECB þ Eg

0:5Eg

(5) (6)

where, X refers to electronegativity of semiconductor (X ¼ 5.79 eV for ZnO, 5.71 eV for CdO and 5.80 eV for CuO [5,57,58]), Ee ¼ 4.5 eV (free-electron energy) and Eg ¼ optical energy bandgap of ZnO, CdO and CuO 3.37 eV, 2.5 eV and 1.2 eV respectively, taken from literature [50, 59,60]. To understand the photocatalytic mechanism a schematic model is suggested as shown in Fig. 9. The EVB and ECB values calculated from equation (5) and (6) are 0.395 eV and 2.975 eV for ZnO, 0.04 eV and 2.46 eV for CdO and 0.7 eV and 1.7 eV for CuO, respectively. Under sunlight or Uv-light irradiation, ZnO, CdO and CuO can excite and create photo-generated e /hþ pairs. The photo-generated electrons (e ) from 7

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Materials Chemistry and Physics 249 (2020) 122983

Fig. 6. Absorption spectra of MB dye with (a) ZnO, (b) CdO, (c) CuO, (d) ZnO-CdO-CuO nanocomposite, (e) Degradation efficiency (%), and (f) Degradation kinetics plots, at different time intervals.

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Fig. 7. Absorption spectra of different dyes with ZnO-CdO-CuO catalyst (a) RhB, (b) MO, (c) CR, (d) Decolourization efficiency (%), and (e) Degradation kinetics plots, at different time intervals.

CB of ZnO can easily flow to the CB of CuO via the interface as the CB position of ZnO is more positive as compared to CdO, also CB of CdO is more positive than CuO. Simultaneously, holes (hþ) tend to move from VB of CuO to VB of ZnO via CdO, as the VB of ZnO is negative enough than CdO and the VB of CdO is negative enough than CuO. This resulted in an efficient e /hþ pairs separation and repressed the recombination by elongating the lifetime of charge carriers and thus led to improve photocatalytic activity. The comparison of different dyes degradation with some other reported metal oxides and nanocomposites is given in Table 4. The higher absorbance in the visible region and low energy bandgap can ease in the excitation of more electrons from VB to CB which also fulfil the requirement for sunlight photo-catalysis, resulted in improved photo-degradation. The presence of heterojunction in the ZnO-CdO-CuO catalyst facilitate the charge carrier’s separation (as suggested in Fig. 9) resulted in improved degradation efficiency. The presence of double heterojunction in ZnO-CdO-CuO photo-catalyst enhanced the charge carrier’s transformation through the interface be­ tween ZnO, CdO and CuO. The interface’s states or surface defects (confirmed by PL spectra) could act as the trap centers for charge car­ riers which can speed up the interfacial charge separation/transition

mechanism, notably improve the photocatalytic activity. 4. Conclusion In summary, pristine ZnO, CdO, CuO, and ZnO-CdO-CuO nano­ composite was successfully synthesized and characterized to investigate their physical properties and photocatalytic activity. The phase composition was estimated using XRD revealed a greater volume frac­ tion of the ZnO phase in the nanocomposite. Peak broadening analysis (Scherrer plot, W-H and SSP methods) shown that the grown nano­ composite has a tensile strain and SSP was more precise for the esti­ mation of crystalline size. The FTIR and Raman spectra also inveterated the successful formation of ZnO-CdO-CuO. The energy bandgap value was 2.9 eV calculated using UV–vis spectra. The PL spectra have shown NBE and DLE were due to intrinsic defects which may help to capture the photo-induced electrons and hamper electron/hole recombination. The IV measurements exhibited a higher electrical response of nano­ composite. SEM images presented agglomerated rod-shape morphology and elemental analysis have shown that Zn has a higher atomic per­ centage as observed in XRD (volume fraction). The grown ZnO-CdO-CuO 9

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Materials Chemistry and Physics 249 (2020) 122983

Fig. 8. (a) Comparison of degradation efficiency (%) and COD removal efficiency, (b) Effect of different scavengers, and (c) Recyclability plots of CR dye degradation.

Fig. 9. The scheme represents the photocatalytic reaction mechanism, for MB, RhB, MO and CR dyes with the ZnO-CdO-CuO catalyst.

nanocomposite exhibited the enhanced photocatalytic degradation 94% against MB as compared to pristine ZnO (60.0%), CdO (41.0%), and CuO (61.0%). The synthesized nanocomposite catalyst also showed 87%, 89% and 99% degradation against RhB, MO and CR dyes under sunlight illumination. Notably, CR dye was degraded more as compared to MB,

RhB and MO dye under similar conditions. The species trapping exper­ iment revealed the main role of O2*- and HO* radicals in degradation reaction and recyclability test exhibited the greater reusability of nanocomposite after the 5th cycle. This work introduces a novel sunlight driven photo-catalyst for the degradation of synthetic dyes. 10

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Table 4 Comparison of different dyes degradation with some other metal oxide and nanocomposites. Photocatalysts

Dyes

Degradation efficiency (%)

References

Bare ZnO Commercial ZnO ZnO CuO/ZnO CdO CuO ZnO CuO-PMMA, Fe3O4-PMMA CdO–NiO–ZnO CdO–ZnO–MgO ZnO-CdO-CuO Bi2WO6/SnS ZnO, CdO, CdO/ZnO Cerium oxide/yttrium oxide Flower-like SnO2/Ag ZnO-CdO-CuO SnS/SnS2 heterostructures ZnO CuO/MX, CuO/NX Polyimide/heterostructured NiO–Fe2O3–ZnO ZnO-CdO-CuO ZnO-CdO-CuO

MB – – – – – – – – –

83.02 38.0 75.0 96.57

– –

93, 90 86.0 91.0 94.0 66.0 49.5, 60.2, 97.6 52.0 70.0 87.0 83.25 40.0 29.0, 59.0 81.4

[61] [62] [63] [22] Present Present Present [64] [26] [65] Present [66] [20] [67] [68] Present [2] [24] [69] [57]

– CR

89.0 99.0

Present Present

RhB – – – MO

CRediT authorship contribution statement Tauseef Munawar: Writing - original draft. Sadaf Yasmeen: Writing - original draft. Fayyaz Hussain: Writing - original draft. Khalid Mahmood: Writing - original draft. Altaf Hussain: Writing original draft. M. Asghar: Writing - original draft. Faisal Iqbal: Writing - original draft. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2020.122983. References [1] R. Gusain, K. Gupta, P. Joshi, O.P. Khatri, Adsorptive removal and photocatalytic degradation of organic pollutants using metal oxides and their composites: a comprehensive review, Adv. Colloid Interface Sci. 272 (2019), 102009, https:// doi.org/10.1016/j.cis.2019.102009. [2] K. Yao, J. Li, S. Shan, Q. Jia, One-step synthesis of urchinlike SnS/SnS2 heterostructures with superior visible-light photocatalytic performance, Catal. Commun. 101 (2017) 51–56, https://doi.org/10.1016/j.catcom.2017.07.019. [3] S.A. Khayatian, A. Kompany, N. Shahtahmassebi, A. Khorsand Zak, Enhanced photocatalytic performance of Al-doped ZnO NPs-reduced graphene oxide nanocomposite for removing of methyl orange dye from water under visible-light irradiation, J. Inorg. Organomet. Polym. Mater. 28 (2018) 2677–2688, https://doi. org/10.1007/s10904-018-0940-6. [4] A. Chachvalvutikul, J. Jakmunee, S. Thongtem, S. Kittiwachana, S. Kaowphong, Novel FeVO4/Bi7O9I3 nanocomposite with enhanced photocatalytic dye degradation and photoelectrochemical properties, Appl. Surf. Sci. 475 (2019) 175–184, https://doi.org/10.1016/j.apsusc.2018.12.214. [5] A. Naseri, M. Samadi, N.M. Mahmoodi, A. Pourjavadi, H. Mehdipour, A. Z. Moshfegh, Tuning composition of electrospun ZnO/CuO nanofibers: toward controllable and efficient solar photocatalytic degradation of organic pollutants, J. Phys. Chem. C 121 (2017) 3327–3338, https://doi.org/10.1021/acs. jpcc.6b10414. [6] J. You, Y. Guo, R. Guo, X. Liu, A review of visible light-active photocatalysts for water disinfection: features and prospects, Chem. Eng. J. 373 (2019) 624–641, https://doi.org/10.1016/j.cej.2019.05.071. [7] Q. Liu, E. Liu, J. Li, Y. Qiu, R. Chen, Rapid ultrasonic-microwave assisted synthesis of spindle-like Ag/ZnO nanostructures and their enhanced visible-light photocatalytic and antibacterial activities, Catal. Today 339 (2020) 391–402, https://doi.org/10.1016/j.cattod.2019.01.017. [8] W. Raza, M.M. Haque, M. Muneer, Synthesis of visible light driven ZnO: characterization and photocatalytic performance, Appl. Surf. Sci. 322 (2014) 215–224, https://doi.org/10.1016/j.apsusc.2014.10.067.

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