La or Ce codoped ZnO nanostructures: Morphological, optical, magnetic and electrical properties studies

Journal Pre-proof Enhanced visible light photocatalytic activity of C/La or Ce codoped ZnO nanostructures: morphological, optical, magnetic and electrical properties studies A.M. Youssef, S.M. Yakout

PII:

S2213-3437(19)30688-8

DOI:

https://doi.org/10.1016/j.jece.2019.103565

Reference:

JECE 103565

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

23 October 2019

Revised Date:

18 November 2019

Accepted Date:

24 November 2019

Please cite this article as: Youssef AM, Yakout SM, Enhanced visible light photocatalytic activity of C/La or Ce codoped ZnO nanostructures: morphological, optical, magnetic and electrical properties studies, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103565

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Enhanced visible light photocatalytic activity of C/La or Ce codoped ZnO nanostructures: morphological, optical, magnetic and electrical properties studies

A. M. Youssef, S. M. Yakout 1

Inorganic Chemistry Department, National Research Centre (NRC), El Buhouth St., Dokki,

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Cairo, Postal Code 12622, Egypt

*Corresponding author: [email protected], [email protected]

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Graphical abstract

Highlights

C/Ce or La codoped ZnO nanoparticles have been prepared by sol-gel method.

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(C, La) codoped ZnO exhibited superior visible light photocatalytic activity.

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Fast degradation in 80 minutes with perfect reusability.

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C/La enhances the electrical conductivity and dielectric constant of ZnO.

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Ferromagnetic properties were detected in C/Ce codoped ZnO.

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Abstract

New and effective visible light photocatalysts composed of C/La or Ce codoped ZnO

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nanostructures were prepared by sol gel method. Zinc nitrate salt produced very homogenous and finer ZnO spherical nanoparticles compared to zinc acetate. XRD and Rietveld refinement

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confirmed the formation of single phase ZnO wurtzite structure in all samples and suggested the actual incorporation of C/La or Ce into ZnO lattice. The crystallite size of pure ZnO-N was

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estimated to be 37.6 nm and it greatly reduced to 21 nm after codoping by C/La. The dual dopants of C/La decreased the band gap of ZnO-N (3.12 eV) to visible light absorption (2.95 eV) while C/Ce prolonged it to 3.2 eV. Notable room temperature ferromagnetism with

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magnetization of 0.014 emu/g and coercivity of 205 Oe was found in C/Ce codoped ZnO. Unusual diamagnetic performance was observed for C/La codoped ZnO with negative hysteresis

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loop. The electrical conductivity and dielectric constant of the pure ZnO was enhanced by C/La codoping. The nanostructured of C/La and C/Ce codoped ZnO exhibited notable visible light photocatalytic activities for methylene blue (MB) degradation in 80 and 100 minutes, respectively. Also, perfect reusability and photostability for MB degradation until four cycles was detected. Based on the band gap, dye-sensitized and the photogenerated electron-hole pairs

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mechanisms were used to explain the process of MB degradation. Grain size, crystallinity, trap centers and the ability of visible light absorption are the main factors which greatly enhanced the photocatalytic properties of C/La codoped ZnO.

Keywords: ZnO; Dual dopants; C/La or Ce; Room temperature ferromagnetism; Electrical properties; Visible light photocatalytic

1. Introduction

Nanostructures materials have attracted a great deal of research interest in many branches of science such as medicine [1], gas sensors [2], solar cell [3, 4], spintronics technologies [5] and water pollution treatment [6]. Particularly, the metal oxides nanostructures have showed unique chemical and physical properties and strong candidates to be utilized in these fields [7-9]. The demand for pure water is global issue and has been growing during this century and already a number of world regions are chronically water short [10, 11]. Giant global efforts have been done to solve this problem and satisfy the supplying of pure water. The wastewater treatment can

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considered as a main source to provide large quantity of good quality water [10, 11]. Industrial wastewater holding various organic dyes which harm water quality, obstruct sunlight

penetration, decrease photosynthetic reaction and cause disease to mankind and dealing with it is

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necessary [10-12]. These dyes are colored toxic organic substances and hard to handle [10].

Many techniques including physical or biological treatment are proposed to deal with dyes in

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wastewater [13]. Especially, the membrane bioreactor (MBR) has showed notable properties to apply in water reuse applications in wastewater treatment. The Influence of nitrifiers

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community on fouling mitigation and nitrification efficiency in a membrane bioreactor was investigated by Sepehri et al. [14]. The obtained results showed that an appropriate C/N

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ratio can control the microbial population and help the nitrifier significantly mitigate fouling in MBR. On the other hand, photocatalysis is also considered as simple and effective technique for wastewater treatment with complete disposed of these organic pollutants [10-12].

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In photocatalysis, light and catalysts are used together to speed up a chemical reaction for dyes degradation into small non-harmful molecules. Numerous nanostructures metal oxides semiconductor such as TiO2 [15], SnO2, CuO, NiO and ZnO [16] have been investigated and showed positive results for this goal. Among all oxides, ZnO nanostructures have been regarded as very efficient and cheap catalysts with large surface areas and unique spatial architectures

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[16]. However, pure ZnO as a photocatalyst still has two drawbacks which prevent it from reaching to the ideal state [17, 18]. The rapid recombination of the photogenerated electron-hole pairs limits its photocatalytic performance and increase the reaction time. Likewise, ZnO can only be activated under ultraviolet radiation which represents 5% of the solar light energy reaching to earth. Different strategies were reported to remove these difficulties including preparation methods, nanocomposites formation, doping or multidoping [17]. Doping or multidoping using selected elements can cause lattice defects and form trap centers to hinder the

recombination of the electron–hole pairs or modify the band gap to improve utilization under visible light. Currently, nonmetallic elements such as C [17], S [19], and N [20] have been considered as appropriate dopants to enhance the band gap and the visible light photocatalytic activity of ZnO. The insertion of carbon into ZnO host lattice has showed relative photocatalytic enhancements [18, 21, 22]. In addition, C-doped ZnO exhibited desirable performance for water splitting and photoelectrochemical cells [22]. Also, the rare earth metals such as La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+ and Er3+ are known for their ability to trap the electrons which efficiently

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inhibit the recombination of photogenerated electron-hole pairs [23, 24]. Many researches reported the operative roles of the rare earth on the photocatalytic activity of ZnO nanostructure [23-26]. Both La and Ce have wonderful electronic configurations with 4f orbitals partially filled

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by electrons [27, 28]. The combination between the properties of the nonmetallic carbon with that of La or Ce may induce desirable optical and photocatalytic properties in ZnO. The

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integration between the visible-light responsive properties of carbon and the rare earth as trap centers can prompt more efficiency for practical applications. Except the investigation performed

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by Luu et al [29] on (C, Ce) codoped ZnO, there is no available researches on (C, La) and (C, Ce) codoped ZnO nanoparticles as photocatalysts for organic dyes degradation. Herein,

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comprehensive study on the photocatalytic properties of pure, (C, La) and (C, Ce) codoped ZnO nanoparticles were carried out toward methylene blue dye Furthermore, the effect of the pH on the photocatalytic activity was studied in the pH range of 3-11. The influence of nitrate ions and

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acetate ions of zinc salts on the morphology and size of pure ZnO was examined. Besides that, the structure, optical, magnetic, electrical and dielectric properties of the prepared samples were investigated and discussed.

2. Materials and methods

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2.1. Preparation of samples

All starting materials are of analytical reagents grade (Sigma-Aldrich Inc., Steinheim,

Germany) and used as received without further purification. The influence of zinc salts on morphology and size of ZnO particles was investigated by using two different sources for zinc including zinc nitrate (ZnO-N) and zinc acetate (ZnO-A). Pure ZnO was synthesized by dissolving separately appropriate amounts of 0.1 mol/L Zn(NO3)2 and 0.1 mol/L Zn(CH3COO)2·2H2O in 100 ml deionized water with continuous stirring for 1/2 hours. After

that, 0.1 mol/L citric acid was added to the prepared solutions, followed by the addition of 0.1 mol/L ethylene glycol under continuous stirring at 70 oC for 2 hours. Next, the temperature was raised to 170 oC until dry gel was obtained. The obtained gel was heated at 250 oC to form xerogel. The obtained powders were transferred to desired pure ZnO powders by calcination at 500 oC for 3 hours. Based on SEM investigation, pure ZnO synthesized from Zn(NO3)2 exhibited homogenous and small nanosized compared to zinc acetate. Thus, Zn(NO3)2 was chosen as starting material to prepare the codoped ZnO powders. (C, Ce) and (C, La) codoped ZnO

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nanoparticles were prepared by adding appropriate amounts of 0.1 mol/L glucose, 0.1 M Ce(NO3)3, and 0.1 M La(NO3)3·6H2O to Zn(NO3)2 solutions with continuous stirring and

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followed by the same steps used in the preparation of pure ZnO.

2.2. Characterization and measurements

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The microstructural and chemical compositions of the prepared samples were carried out using a scanning electron microscope (SEM model Quanta 250 FEG) with attached EDX unit.

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Phases and crystallite size of the prepared samples were determined by X-ray diffraction (XRD) using Cu-Kα radiations (λ=0.15406 nm) in 2θ range from 20o to 80 o (PANalytical X-Ray

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diffraction equipment model X′Pert PRO). UV-Vis-NIR diffuse reflectance analysis was performed using a double beam spectrophotometer-JASCO (model V-570 UV-Vis-NIR). Magnetic hysteresis loops were measured by using a vibrating sample magnetometer (VSM,

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LakeShore Model 7410). Electrical conductivity and dielectric properties of the prepared samples were carried out by using LCR meter (Hitester, model Hioki 3532-50, made in Japan).

2.3. Photocatalytic test

The photocatalytic activity of pure, (C, Ce) or (C, La) codoped ZnO samples was evaluated

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for degradation of methylene blue (MB) as model pollutant under visible light radiation. The photocatalytic experiments were carried out in Pyrex beaker type reactor to which 100 mL of methylene blue dye solution (10 ppm) and 0.1 g of catalyst was added for each experiment. After mixing of the catalyst and MB dye solution, the mixture was stirred for 45 min under dark conditions to ensure the establishment of adsorption/desorption equilibrium between MB molecules and the surface of the ZnO photocatalyst. After that, the mixture solution was exposed to visible light irradiation and analytical solution samples for dye decomposition were withdrawn

at regular time intervals (20 minutes), and subsequently centrifuged at 4000 rpm for 5 min, to remove the catalyst particles. The degradation efficiency was determined by measuring the absorbance of the analytical solution samples after removing of the catalyst using JASCO V630 UV-Vis spectrophotometer. The characteristic absorption peak of the MB at 664 nm was selected to monitor the photocatalytic degradation process. The relative concentration (C/C0) at different irradiation time i.e. the photocatalytic degradation of MB was calculated from the following relation:

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Degradation = C/Co = A/Ao Where C0 and A0 are the initial concentration and absorbance of MB before irradiation while, C and A are the concentration and absorbance of MB after certain irradiation time. The influence of

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the pH on the photocatalytic activity was studied in the pH range of 3–11.

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3. Results and discussion

3.1. Influence of starting materials on morphology and size of pure ZnO

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The morphology, size and homogeneity of the particles are vital factors which influence on the ZnO properties such as photocatalytic activity [30, 31]. Fig. 1 displays the SEM micrographs

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of pure ZnO synthesized from zinc nitrate (ZnO-N) and zinc acetate (ZnO-A). The image of pure ZnO-N shows very homogenous spherical nanoparticles with small grain size of 0.06 µm. ZnOA particles have non-uniform blocks shape and large grain size of approximately 1.85 µm. it

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seems that the fine nanoparticles of pure ZnO-A were aggregated together to form these large non-uniform masses particles. Generally, the particles with higher surface energies have a tendency to grow in faster way compared to those with lower surface energies [32]. Thus, the particles tend to decrease their surface free energy by non-uniform growth into larger particles. The citric acid is fuel and nitrate is an oxidizing agent which may be supported the good

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crystallization of ZnO-N into small particles. The existence of acetate ions may be caused aggregation of ZnO particles to form large grain. Based on the homogenous morphology and size of ZnO-N particles, the Zn(NO3)2 was used as source material to prepare the (C, Ce) and (C, La) codoped ZnO nanopowders. The SEM images of (C, Ce) and (C, La) codoped ZnO samples showed homogenous spherical particles with small grain size compared to pure ZnO-N, Fig 1. The binary dopants of (C, Ce) or (C, La) restricted the grain growth of ZnO particles. The EDX analysis of (C, Ce) and (C, La) codoped ZnO powders are illustrated in Fig. 1. Both images

revealed the main elements of Zn and O besides C, Ce and La as dopants without any sign for other impurities. The weight percent of the dopants elements detected by EDX are in reliable with the amount used in the doping process of 2 wt% for each dopant.

3.2. XRD and Rietveld refinements results The XRD patterns of pure, (C, Ce) or (C, La) codoped ZnO nanoparticles were shown in Fig. 2. The diffraction peaks of all samples can be clearly indexed to ZnO hexagonal wurtzite

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structure (JCPDS. No: 36-1451, space group: p63mc). The patterns of undoped and codoped ZnO nanoparticles illustrate nine peaks with the same relative intensities which were indexed to (100), (002), (101), (102), (110), (103), (200), (112) and (201) planes. No secondary peaks

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corresponding to any impurity phases were detected. The intensity of pure ZnO-A peaks is

somewhat higher than that of pure ZnO-N. As well, the incorporation of (C, Ce) or (C, La) ions

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into ZnO lattice increased the intensity of the XRD peaks especially for La, implying the improvement in crystallinity. The peaks become more breadth which indicated to crystallite size

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decreasing [33]. For all samples, the (101) diffraction peak of ZnO was much stronger than the (002) peak, indicating that most nanocrystals of ZnO samples have a preferential

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crystallographic in (101) orientation. Noteworthy shifts for the main diffraction peaks toward lower angle was observed for pure ZnO-A, (C, Ce) and (C, La) codoped samples in comparing with pure ZnO-N, Fig 3. The shift in XRD peaks of pure ZnO-A compared to ZnO-N may be due

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to the lattice strain contribution which induced the changes in the lattice parameters. For codoped ZnO samples, these shifts can be attributed to the incorporation of (C, Ce) and (C, La) ions of higher ionic radii. The crystallite sizes of the pure and codoped ZnO were estimated from the XRD pattern by using Scherrer-equation [34]: D = 0.9λ/βcos θ

(1)

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Where λ is the X-ray wavelength (1.5406 Å for Cu Kα1), β is the full width at half maximum in radian and θ is the Bragg angle of diffraction. The Fullprof software was used for Rietveld refinement analysis and also the lattice parameters (a, c) of the prepared samples were determined. The unit cell volume (V) and the U parameter of the hexagonal ZnO samples were calculated from the relations [34]: V = 0.866a2c

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U = (1/3) (a2/c2) + (1/4)

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Furthermore, the nearest neighbor bond length "L" along "c" axis was calculated from the following relation: L = cu

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The Rietveld refinement demonstrated perfect agreement between the experimental and calculated diffraction peaks. All samples were good simulated as pure hexagonal ZnO, without observation of any impurities related to dopants. The obtained values of the lattice parameters, c/a ratio, unit cell volume, average crystallite size, U parameter and bond length are tabulated in

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Table 1. Based on lattice parameters and atomic positions, the crystal packing structures of the pure and codoped ZnO were drawn using VESTA software as shown in Fig. 4. The average crystallite size of pure ZnO-N and ZnO-A samples was found to be 37.6 and 46.2 nm,

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respectively. After incorporation of (C, Ce) and (C, La) into ZnO lattice, the crystallite size was decreased to reach to 32.6 and 21.2 nm, respectively. Both dopants have the ability to limit the

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crystallite growth of ZnO. The binary dopants induced notable increases in the lattice parameters (a, c) of the pure ZnO, Fig. 5. The ionic radius of Zn2+ ions is 0.74 Å while that of O2− is 1.4 Å in

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ZnO structure [35]. Thus, the incorporation of Ce3/4+ (1.01/0.87 Å) or La3+ (1.032 Å) ions of larger ionic radii into Zn2+ (0.74 Å) sites increased the lattice parameter and expanded the ZnO

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lattice [36, 37]. The carbon dopant can substitute the Zn2+ or O-2 sites into ZnO structure to form O-C and Z-C bonds [35]. In case of O2- sites substitution (1.4 Å), the C4- with ionic radius of 2.60 Å will increases the lattice parameters. While, the substitution of Zn2+ (0.74 Å) by C4+

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which has ionic radius of 0.16 Å will induce a lattice contraction. Furthermore, the carbon can be partially incorporated into the interstitial sites, leading to a larger lattice constant [35]. The increases in the lattice parameters a and c for (C, Ce) codoped ZnO sample are within 0.003 and 0.0048 Å, respectively. While, the growth in the lattice parameters of (C, La) codoped ZnO were 0.0015 and 0.0034 Å for a and c, respectively. Both samples exhibited sensible lattice expansion

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which suggested the effective incorporation of dopants. The C ions may be partially incorporated into the oxygen sites or at interstitial sites with no excluded for the partial replacement for Zn2+ ions. The integration effect between the two dopants may be leaded to these expansions. With respect to bond length (L), the calculated values for pure and codoped ZnO are slightly lower than that of bulk ZnO (1.99 Å) which can be correlated to structural defects like oxygen vacancies, Table 1. Insertion of (C, Ce) or (C, La) ions into ZnO lattice induced increases in the

bond length of the ZnO structure. These increases can be attributed to the substitution of Zn2+ ions by La3+and Ce3/4+ ions of higher ionic radii.

3.3. FTIR analysis Fig. 6 displays the FTIR spectra of pure and codoped ZnO samples calcined at 500 oC. Generally, analogous FTIR spectra were obtained for pure and codoped ZnO samples. The pure ZnO-N exhibited three absorption bands located at 3440, 1631 and 441 cm-1 [38]. The two bands positioned at 3440 and 1631 cm−1 were recognized to stretching and bending vibration modes of

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the adsorbed H2O. The band at 441 cm-1 represents the characteristic absorption band of ZnO which attributed to stretching vibration mode of Zn-O bond [38]. The pure ZnO-A has spectra

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comparable to ZnO-N with slight shift for the main absorption band to lower wavenumber (439 cm-1) and also this band become more intensive. The difference between the two pure ZnO

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samples can be attributed to the larger particles size of ZnO-A compared to ZnO-N. The insertion of (C, Ce) and (C, La) ions into ZnO lattice induced red shift in the main absorption

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band to centered at 437 and 433 cm-1, respectively. In addition, the main bands of both samples extended over wide wavenumber region as shown in Fig. 6 (inset). These shifts to lower

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wavenumber can be attributed to the changes in bond length of ZnO structure due to dopants implantation. Also, it was reported that the shape of the basic Zn-O band was depend on the size

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and geometry of the particles and/or the aggregate formation [38].

3.4. Optical properties

Fig. 7 shows the diffuse reflectance spectra of the prepared samples in the wavelength range of 200–1000 nm. With respect to pure ZnO-N nanoparticles, strong absorption observed in the wavelength region of 420-380 nm. This absorption edge refers to the optical band gap which

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represents the transition of electrons from valence band (VB) to conduction band (CB). The absorption edges of pure ZnO-A and (C, Ce) codoped ZnO nanoparticles exhibited blue shift to lower wavelengths compared to pure ZnO-N, signifying band gap expansion. In contrast, red shift on the absorption edge was seen for (C, La) codoped ZnO nanoparticles which indicated to band gap narrowing as well as the improvement in the light absorption in the entire visible region. The optical band gap (Eg) of all samples was calculated using the well- known equation F (R) = (1-R)2/2R (Kubelka–Munk rule). Where, F(R) is the Kubelka–Munk function and R is

the absolute value of reflectance. By plotting [F (R) hν]2 versus hν and by extrapolating the linear part of the curve to cut energy axis the optical band gap was determined as shown in Fig. 8. The curves in band gap region for both codoped samples are linear which is similar to that of pure ZnO, revealing that (C, Ce) or (C, La) as codopants did not changed the direct electron transition characteristic of ZnO [39]. Better light harvesting and direct transition characteristics are highly required for substances used in opticoelectronic, photocatalysis and solar cell fields [39]. The band gap of the pure ZnO-N and ZnO-A nanoparticles was found to be 3.12 and 3.2

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eV, respectively. Both samples have lower value than that of the intrinsic value of bulk ZnO (3.37 eV). This can be attributed to the presence of defects like oxygen vacancies or to strain effects where the strains has large influences on the optical band gap [40]. The (C, Ce) and (C,

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La) as binary dopants have showed opposite effects on the optical band gap of ZnO

nanoparticles. The incorporation of (C, Ce) ions into ZnO lattice increased the band gap from

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3.12 to 3.2 eV (→ 0.08 eV). The [4f, 5d, 6s] orbitals of Ce4+ ions are empty and can be contributed to the conduction band of ZnO. These levels can be exist slightly above the levels in

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conduction band, and therefore reveal a small increase in band gap of ZnO. Also, the increase in the band gap may be attributed to Burstein-Moss effect which originated from the lifting of

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Fermi level into the conduction band due to the increases in the charge carrier concentration. On contrast, the (C, La) ions reduced the band gap of ZnO nanoparticles to reach to 2.95 eV (← 0.17 eV). The formation of intermediate impurity levels within band gap region generated by defects

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and codoping may be leaded to this decrease [41]. The results suggested that the carbon may be more incorporated into ZnO matrix in case of La rather than laid down on ZnO surface. The C in ZnO lattice would revise the band structure of ZnO to introduce impurity mid gap and thus tune its band gap.

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3.5. Magnetic properties

Fig. 9 shows the room temperature magnetic hysteresis loops (M-H curves) of pure and

codoped ZnO samples in applied magnetic field of ±8000 Oe. The M-H loop of pure ZnO-N displayed very weak ferromagnetic like behavior at low magnetic field, ±2000 Oe, with magnetization (Ms) of 0.002 emu/g. At high magnetic field, the main diamagnetic behavior of ZnO structure was predominant. The weak ferromagnetic performance may be attributed to the uncompensated spins on the surface or defects like oxygen vacancies [42]. The diamagnetic

performance reflected the basic diamagnetic nature embedded in the nanoparticles core which induced at high magnetic field. While, pure ZnO-A exhibited diamagnetic performance without any hysteresis. The difference in this magnetic behavior between the two pure samples comes from the difference in particle size and agglomeration [43]. The pure ZnO-A has larger particles size compared to ZnO-N nanoparticles which reduced its surface defects required to induce the weak ferromagnetic behavior at low field. The dual dopants of (C, Ce) prompted ferromagnetic behavior in ZnO with magnetization of 0.014 emu/g and coercivity of 205 Oe. It can be seen that

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at high magnetic field there is a small superposition between ferromagnetic and diamagnetic properties in the sample. Unusual diamagnetic performance was observed for (C, La) ZnO

sample with negative hysteresis loop [44]. The XRD and diffuse reflectance results proved the

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single phase nature for the prepared ZnO samples with effective incorporation of dopants into ZnO lattice in substitution and/or interstitial sites. No sign for any secondary phases or any other

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impurities was observed in the samples. Thus, the ferromagnetic behavior observed in (C, Ce) ZnO could be assigned to the exchange interaction mediated either by free carriers or bound

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magnetic polarons [45]. The incorporation of dopants into ZnO lattice may be induced defects formation which stabilized the ferromagnetism. The Ce as second dopant with carbon have

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valuable role in promoting the ferromagnetism. The oxygen vacancy present in (C, Ce) ZnO structure may be contributing electron with the empty d-orbital of Ce4+ ion leading to Ce3+ (4d1) ion. The exchange interaction between the Ce3+ (4d1) ions through the electron trapped in the

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oxygen vacancy form the F-centers which may be induced the ferromagnetism.

3.6. Electrical conductivity and dielectric properties The variations of AC conductivity with frequency for pure and codoped ZnO samples at different temperatures are shown in Fig. 10. The electrical conductivity of all ZnO samples was

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increased with increasing the frequency at all temperatures. There are various factors which have influences on the conductivity including the number of charge carriers, mobility of free charge and the availability of connecting polar domain as the conduction pathway [46]. With respect to pure ZnO-N, the increase in the electrical conductivity can be assigned to structural defects such as oxygen vacancies. The formation of these defects can generate donor states in the forbidden band which located below the conduction band. The electrical conductivity of pure ZnO can be controlled by intrinsic defects such as oxygen vacancy and interstitial zinc atoms generated

during synthesis and also by doping technique [47]. In case of (C, Ce) codoped ZnO, the conductivity values are lower compared to pure ZnO. It seems that the Ce doping has influenced on the defect chemistry of ZnO. Therefore, Ce dopant may be decreased the concentration of the intrinsic donors [48]. Thus, the decreases in the intrinsic donor concentration reduce the electrical conductivity. On contrast, the incorporation of (C, La) ions into ZnO lattice increased the electrical conductivity due to the increases in the charge carrier concentrations [49]. Figs. 11 display the changes in the dielectric constant (εʹ) and dielectric loss (tan δ) with

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frequency for the prepared ZnO samples. The dielectric constant (εʹ) was found to decrease with increasing the applied frequency and at higher frequencies it becomes constant for both codoped ZnO samples while it decreased at high frequencies for pure ZnO. From Fig. 11, there is a

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relaxation process in the dielectric constant (εʹ) take place in all ZnO samples and supported by the peaks observed in the dielectric loss plots, Fig. 11. It was reported that ZnO structure

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exhibited strong ionic polarization due to Zn2+ and O2– ions and therefore possesses a high static permittivity value [50]. The dielectric constant (εʹ) can be assigned to different polarizations

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mechanisms under the influence of the electric field and it is directly proportional to the polarizability in the substances. At low frequency, all types of polarization can contribute and the

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quick increase in the dielectric constant is essentially due to the space charge and dielectric polarizations, which are strongly temperature dependent [51]. The accumulation of charges at the grain boundary prompted space charge polarization which increased with the growth of more

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charges at the grain boundary with temperature rising. The value of the dielectric constant was increased by (C, Ce) codoping due to the electron exchange between Ce4+, Ce3+ with Zn2+. This recommended that Ce ions were penetrated at Zn sites and as a result the electrons were generated by the charge transfer (Ce4+ + e– ←→ Ce3+). The presence of C dopant increased the number of oxygen vacancies and Zn interstitials in the crystal, which leaded to an increase in the

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dipole moment and, in turn, the orientation polarization. Referring to (C, La) codoping ZnO sample, maximum dielectric constant is observed in comparison with the other samples due to the electron exchange between La3+, C4+ with Zn2+. The presence of dopant ions (C, La) increased the number of the oxygen vacancies and Zn interstitials in the crystal, which leaded to the increases in the dipole moment and, in turn, the orientation polarization. The dielectric loss tan (δ) for pure ZnO was decreased by (C, Ce) or (C, La) codoping, Fig. 11. This is because of the fact that (C, Ce) and (C, La) as dopants can act as deep donors which decreased the

concentration of intrinsic donors. From the above results, the higher electrical conductivity and dielectric constant was noticed for ZnO codoped with (C, La) dopants.

3.7. Photocatalytic activity Degradation of methylene blue (MB) was carried to investigate and compare the photocatalytic performance of pure and codoped ZnO samples. Fig. 12 shows the effect of pure and codoped ZnO catalysts on the absorbance of 10 ppm MB solution under visible light

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irradiation for 80 minutes. In the absence of catalysts or under dark conditions, the intensity of the main absorption peak of MB situated at 664 nm is not affected. Notable decreases in the

absorption peak were noticed in the presence of ZnO catalysts, signifying that methylene blue

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can be effectively degraded by different ZnO catalysts under visible light irradiation. The

degradation rate after 80 minutes of visible light irradiation is 63%, 89%, and 99 % for ZnO-N,

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(C, Ce) and (C, La) codoped ZnO, respectively. Fig. 13 displays the C/C0 against time of irradiation for all ZnO samples. Where, C is the concentration after different illumination times

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and C0 is the dye concentration before irradiation. In case of pure ZnO, complete degradation of MB is attained after 140 minutes of visible light irradiation. The photocatalytic performance was

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obviously enhanced with incorporation of both (C, Ce) and (C, La) dopants and the total degradation is achieved in 100 and 80 minutes, respectively. The influence of the pH of dye solution on the photocatalytic activity of (C, La) codoped ZnO nanoparticles are shown in

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Fig. 14. The degradation of methylene blue (10ppm) was performed at different pH of 3, 7, 9 and 11 in the presence of (C, La) codoped ZnO under visible light irradiation for 80 minutes. At pH 3, the MB solution exhibited weak degradation due to the similar surface positive charge of MB and (C, La) codoped ZnO catalyst. The photocatalytic degradation of MB was increased with increasing the pH up to 9. At pH 7 and 9, the degradation of MB was

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completed in 80 and 60 minutes due to the electrostatic attraction between the negatively charged (C, La) codoped ZnO catalyst and the positively charged MB. By increasing the pH to 11, the photocatalytic activity was decreased because the positively charged MB became negatively charged which cause the repulsion with the negative charged nanoparticles of the catalyst. Based on Kubelka–Munk plots, the band gap of pure and (C, Ce) codoped ZnO are 3.12 and 3.2 eV respectively, which represent high band gap to be excited by visible light. In contrast,

(C, La) codoped ZnO possesses band gap of 2.95 eV and can be excited by some wavelengths in visible light. However, all ZnO samples exhibited noteworthy photocatalytic activities when irradiated by visible light irradiations. These results suggested that the mechanism of MB photodegradation in pure and (C, Ce) codoped ZnO is attributed to dye-sensitized process [39, 52]. While, the mechanism for (C, La) codoped ZnO sample can assigned for both dye-sensitized and photogenerated electron-hole pairs in ZnO structure [52]. In regard to dye-sensitized mechanism, the methylene blue molecules (D) are stimulated under visible light irradiation to

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form excited dye molecules (D*), then transfer the photoexcited electrons into ZnO conduction band (CB). The electrons injected into the ZnO conduction band can react with dissolved oxygen molecules (O2) to form active oxygen species (O2˙ˉ) which can decompose the MB dyes. With

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respect to (C, La) codoped ZnO sample, the mechanism of MB degradation can be assigned to the integration between dye-sensitized as above mentioned plus photo-generated electron-hole

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pairs in ZnO. Under visible light illumination, the electron also can transfer from the valence band (VB) to the conduction band (CB) with generation of hole in the valence band. The

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photogenerated electron-hole pair may be recombined or interacted separately with other molecules in solution. The electrons in the conduction band can react with the dissolved oxygen

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molecules to form superoxide radical anion (O2˙ˉ). The formed holes in the valence band can react with water (H2O) on the surface of ZnO or hydroxide ions (OHˉ) to form highly reactive hydroxyl radicals (˙OH). These reactive species are powerful oxidizing agent and can attacks the

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dye molecules to give the oxidized product. Fig 15 represents schematic diagram of the proposed mechanisms for methylene blue dye degradation under visible light irradiation. The dyesensitized photocatalytic and electron-hole pairs photogenerated mechanism can be summarized as follow [39, 52, 53]:

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Dye + hν → dye· (D* excited dye molecules) Dye· + ZnO (CB) → ZnO (CB)· ZnO (CB)· + O2 → O2˙ˉ ZnO (CB)· + Ce4+ → Ce3+ Ce3+ + O2 → O2˙ˉ ZnO (CB)· + La3+ → La2+ La2+ + O2 → O2˙ˉ

(C, La) ZnO + hν → eˉCB + h+VB h+ + eˉ → heat (recombination) h+VB + H2O → OH˙ + H+ h+ + OHˉ → OH˙ eˉ + O2 → O2˙ˉ 3O2˙ˉ + 2H2O + 2H+ → 3H2O2 + O2

of

OH˙/O2˙ˉ + MB molecules → non-harmful products (CO2+H2O)

The significant improvements in the photocatalytic activity of (C, La) codoped ZnO may

ro

be mainly attributed to its lower band gap with ability of visible light to excite the electrons from the VB to the CB. La3+ ions and the oxygen vacancies may be acted as electron acceptors which

-p

trap the photogenerated electrons and temporarily reduce the surface recombination of the electrons and holes. It seems that both the La3+ ions and oxygen vacancies prolong the lifetime of

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electron−hole pairs and increase the chance for photocatalytic degradation. The insertion of carbon may be enhanced the visible light absorption by generation of more oxygen vacancies in

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ZnO structure which improved the photocatalytic properties. For a practical application, it is essential to evaluate the reusability and photostability of the synthesized ZnO photocatalysts. The reusability of (C, La) codoped ZnO as best sample was investigated for the degradation of

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MB dyes under same reaction conditions. After degradation of MB, the (C, La) codoped ZnO catalyst was filtered and washed with deionized water twice. The recovered photocatalyst was dried in air atmosphere at 100 °C for 60 min and used again for a second degradation. Fig. 16 displays the results of MB degradations up to four degradation cycles. (C, La) codoped ZnO as photocatalyst exhibited a remarkable photostability for MB degradation with efficiencies of 99,

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96, 94, and 91 % in the first, second, third and fourth runs under visible light irradiation for 80 minutes, respectively. The reusability and photostability were accredited to the low photo corrosive influence and high catalytic stability. Furthermore, La3+ and oxygen vacancies due to carbon dopant could act as good traps for the photogenerated electrons and preventing them from recombination. These results indicated that the (C, La) codoped ZnO photocatalyst is sufficiently stable and not deactivated during the photodegradation of MB dye. In future research, the effects

of changes the carbon and La concentrations will perform which may induce more optimistic photocatalytic results in ZnO nanoparticles.

4. Conclusions The dual dopants based on (C, La) enhanced the visible light photocatalytic activity of ZnO nanoparticles for MB dye degradation with high reusability. The architecture and particles sizes of pure ZnO were greatly depend on zinc salts. The band gap energy of pure, (C, Ce) and (C, La)

of

codoped ZnO were estimated to be 3.12, 3.2 and 2.95 eV, respectively. Room temperature ferromagnetic performance was detected for (C, Ce) codoped ZnO structure with saturation

magnetization of 0.014 emu/g and coercivity of 205 Oe. Binary doping with C and La obviously

ro

increased the electrical conductivity and dielectric constant of pure ZnO. The mechanism of MB degradation for pure and (C, Ce) codoped ZnO was ascribed to dye-sensitized photocatalysis.

-p

For (C, La) codoped ZnO sample, the mechanism was assigned for both dye-sensitized and photo-generated electron-hole pairs in ZnO structure. Both C and La play main roles in

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improving the properties of ZnO through decreasing the grain size, oxygen vacancies formation,

Declarations of interest: none Author contributions section

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acting as trap centers and extend the band gap to the visible region.

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A. M. Youssef: Conceived and designed the analysis, Collected the data, Contributed data or analysis tools, Performed the analysis, Wrote the paper.

S. M. Yakout: Conceived and designed the analysis, Collected the data, Contributed data or analysis

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tools, Performed the analysis, Wrote the paper.

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Fig. 1. SEM micrographs of pure and codoped ZnO samples calcined at 500 oC, EDX spectra of

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codoped samples (bottom).

Intensity (a. u.)

(d)

(c)

50

of

112 201

200

60

70

ro

40

80

-p

30

103

(a) 20

110

102

101

100

002

(b)

2 (degree)

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Fig. 2. XRD pattern of (a): ZnO-N, (b): ZnO-A, (c): (C, Ce) ZnO and (d): (C, La) ZnO samples

ZnO(N) ZnO(A) (C, Ce) ZnO-N (C, La) ZnO-N

ZnO-N

(C, Ce, La)

ZnO-N

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ur na

Intensity (a. u.)

ZnO-N

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calcined at 500 oC.

31

32

33

34 35 2 (degree)

36

37

Fig. 3. Change in intensity and shift in peaks position of (100), (002) and (101) planes.

of ro -p

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Fig. 4. Crystals packing structure based on Rietveld refinement data drawn by using VESTA

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ur na

lP

software.

Fig. 5. Lattice parameters (a, c) and unit cell volume for the different ZnO samples.

437

439 433 1000

900

800

700

600

500

400

(d) (c)

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Intensity (a. u.)

441

ro

(b)

-OH

H-OH 3600

3200

2800

2400

2000

1600

Zn-O

1200

re

4000

-p

(a)

800

400

-1

Wavenumber (cm )

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Fig. 6. FTIR spectra of (a): ZnO-N, (b): ZnO-A, (c): (C, Ce) ZnO and (d): (C, La) ZnO samples,

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magnified pattern of the main absorption band for different ZnO samples (inset).

80 ZnO-N ZnO-A (C, Ce) ZnO-N (C, La) ZnO-N

70

Intensity (%)

60 50 40

of

30

10

300

400

500 600 700 Wavelength (nm)

800

900

1000

-p

0 200

ro

20

Fig. 7. Diffuse reflectance spectra of pure and codoped ZnO nanoparticles calcined at 500 oC.

210

150 120 90

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[F(R) hv]

2

180

KubelKa-Munk plot

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ZnO-N ZnO-A (C, Ce) ZnO-N (C, La) ZnO-N

re

240

3.12 eV

60

2.95 eV

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30

3.2 eV

3.2 eV

0

2.4

2.8

3.2

3.6

4.0

4.4

Energy (eV)

Fig. 8. Kubelka-Munk plots ([F(R) hν]2 versus hν) of pure and codoped ZnO samples for optical band gap determination.

0.02

0.02

ZnO-A 0.01

0.00

0.00

-0.01

-0.01

-0.02 -8000

-4000

0

4000

0.02

-0.02 -8000 8000

0

0.02

0.01

0.00

0.00

-0.01

-0.01

-4000

0

H (Oe)

4000

-0.02 8000 -8000

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-0.02 -8000

re

0.01

4000

-4000

0

4000

H (Oe)

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Fig. 9. M-H hysteresis loops of pure and codoped ZnO samples calcined at 500 oC.

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8000

ro

(C, La) ZnO-N

(C, Ce) ZnO-N

Ms (emu/g)

-4000

of

0.01

-p

Ms (emu/g)

ZnO-N

8000

-3

2.5x10 2.0x10 -1

s m 

ZnO-N

25 75 125 175 225 275

-3

-3

1.5x10

-3

1.0x10

-4

5.0x10

0.0 1x10

6

2x10

6

3x10

6

4x10

6

5x10

of

6

0

f (Hz) -3

-3

2.0x10

-1

-3

1.0x10

(C, La) ZnO-N

25 75 125 175 225 275

-3

2.0x10

-p

1.5x10

s m 

(C, Ce) ZnO-N

25 75 125 175 225 275

-3

ro

3.0x10

-3

re

1.0x10

-4

5.0x10

0.0 0

6

1x10

6

2x10

6

3x10

6

4x10

6

5x10

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f (Hz)

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0.0

0

6

1x10

6

2x10

6

3x10

6

4x10

6

5x10

f (Hz)

Fig. 10. Variation of the ac conductivity with frequency for pure and codoped ZnO samples

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measured at different temperatures.

4

2

10

10

ZnO- N (Ce, C) ZnO-N (La, C) ZnO-N

ZnO- N (Ce, C) ZnO-N (La, C) ZnO-N 3

10

1

2

10

0

Tan 

10

of

1

10

-1

10

ro

dielectric constant 

\

10

0

10

-2

-1

10

2

10

3

10

4

10

5

10

6

10

7

10

f z

1

10

2

10

3

10

re

1

10

-p

10

4

5

10 10 f z

6

10

7

10

Jo

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codoped ZnO samples.

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Fig. 11. Change in the dielectric constant and dielectric loss with frequency for pure and

Without catalyst ZnO-N (C, Ce) ZnO-N (C, La) ZnO-N

600

620

640 660 Wavelength (nm)

680

re

580

-p

ro

of

Absorbance (a. u.)

Visible irradiation 80 minutes

700

Fig. 12. Absorbance of methylene blue in presence and absence of different ZnO catalysts after

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visible light irradiation for 80 minutes.

1.2 ZnO-N (C, Ce) ZnO-N (C, La) ZnO-N

1.0

C/CO

0.8 0.6

of

0.4

0.0 20

40

60

80 100 Time (min)

120

140

160

-p

0

ro

0.2

110

pH = 3 pH = 7 pH = 9 pH = 11

100

80

ur na

Degradation (%)

90

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degradation at different irradiation times.

re

Fig. 13. Photocatalytic activity (C/C0) of pure and codoped ZnO samples for methylene blue

70 60 50 40

Jo

30 20 10

0

0

20

40

60

80

Time (minutes)

Fig. 14. Influence of pH on the photocatalytic activity of (C, La) codoped ZnO for methylene blue degradation at different irradiation times.

of ro

-p

Fig 15. Schematic diagram of the proposed mechanisms for methylene blue dye degradation.

120

60 40

0

120

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20 0

20

40

60

80

Cycle 3 100 80 60

100 0

94 %

20

40

60

80

100

91 %

Cycle 4

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Degradation (%)

96 %

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Degradation (%)

80

Cycle 2

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99 %)

Cycle 1 100

40 20 0

0

20

40

60

80

Time (minutes)

100 0

20

40

60

80

100

Time (minutes)

Fig. 16. Efficiency of (C, La) codoped ZnO photocatalyst for methylene blue degradation for four cycles, irradiation time 80 minutes.

Table 1 Crystallite size (d), lattice parameters (a, c), lattice parameters ratio (c/a), unit cell volume (V), U parameter, bond length (L) and band gap of pure and codoped ZnO samples. d

a

c

c/a

V

u

L

nm

Å

Å

ZnO-N

37.6

3.2458

5.2004

1.6021

47.445

0.3798

1.975

3.12

ZnO-A

46.2

3.2459

5.2016

1.6028

47.441

0.3797

1.975

3.20

(C, Ce) ZnO

32.6

3.2488

5.2052

1.6021

47.577

0.3798

1.977

3.20

(C, La) ZnO

21.2

3.2473

5.2038

1.6025

47.520

0.3798

1.976

2.95

Å3

Band gap

Jo

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lP

re

-p

ro

of

Samples