Selective metal ions doped CeO2 nanoparticles for excellent photocatalytic activity under sun light and supercapacitor application

Inorganic Chemistry Communications 109 (2019) 107577

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Short communication

Selective metal ions doped CeO2 nanoparticles for excellent photocatalytic activity under sun light and supercapacitor application

T

Govindhasamy Murugadossa, , Jianling Mab, Xuefeng Ningc, Manavalan Rajesh Kumard ⁎

a

Centre of Nanoscience and Technology, Sathyabama Institute of Science and Technology, Chennai 600119, India School of Physics and Electronic Engineering, Taishan University, Taian 271000, China c School of Physics and Electronic Engineering, Linyi University, Linyi 276005, China d Institute of Natural Science and Mathematics, Ural Federal University, Yekaterinburg 620002, Russia b

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Defect Dopant Photocatalyst Supercapacitor Recombination

Herein, photocatalytic activity of pure CeO2 and different metal ions (Ag, Bi, Cd and Pb) doped CeO2 nanoparticles (NPs) is investigated using degradation of methylene blue (MB) dye under direct sunlight irradiation. In addition, electrochemical supercapacitor was performed for selected metal ion doped CeO2 NPs. All the NPs are synthesized by one-step chemical precipitation method in aqueous medium at room temperature. The prepared samples were characterized by XRD, FE-SEM, TEM, FT-IR, Laser Raman, UV–Vis, PL, EPR, TGA and electrochemical techniques. According to XRD analysis, the synthesized pure CeO2 and doped CeO2 NPs are cubic crystal structure. Among the dopants, Ag doped CeO2 exhibited excellent photocatalytic activity by degrading of more than 99.5% of the MB dye within 120 min irradiation under sunlight. The enhanced photocatalytic activity of doped CeO2 NPs was ascribed to the significant suppression of the recombination rate of photo-generated electron–hole pairs due to charge transfer, increasing oxygen vacancy and the smaller optical band-gap. The reason behind the photocatalytic activity enhancement is studied through photoluminescence (PL) and the results indicated that the emission peak intensity of pure CeO2 decreased compared to the doped CeO2. The PL quenching was associated with a decrease in the electron–hole recombination rate. Electrochemical supercapacitor was performed for the best photocatalyst of Ag doped CeO2 NPs, the result found to be specific capacitance of 456 F g−1 at 3 A g−1 current density. Among the different metal ions, the Ag doped CeO2 showed as an excellent catalyst for dye degradation and electrochemical supercapacitor applications.

⁎

Corresponding author. E-mail address: [email protected] (G. Murugadoss).

https://doi.org/10.1016/j.inoche.2019.107577 Received 14 June 2019; Received in revised form 30 August 2019; Accepted 7 September 2019 Available online 07 September 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.

G. Murugadoss, et al.

Inorganic Chemistry Communications 109 (2019) 107577

1. Introduction

CeO2. The CeO2 was chosen as a core compound due to its favourable physical and chemical properties. Luckily, the band gap the pure CeO2 was significantly tuned by the dopants. The doped semiconductor photocatalysts were successfully synthesized by a facile chemical precipitation method and its photocatalytic properties was evaluated via photodegradation of MB, under visible-light irradiation and further studied electrochemical supercapacitor performance. These investigations can promote the further development of CeO2–Ag based devices for future energy and environment-related applications.

The rapid increase of organic pollutants in agriculture land and ground water by textile industry has become a serious issue for environment safety and human health. Textile dyes are the most common organic pollutants that are being released by industry into the environment. Semiconductor based metal oxides nanoparticles have shown as promising candidates for water and air purifications [1]. The semiconductor metal oxides are widely used in dealing of waste-water, especially textile dyes, due to their low cost, earth abundant, less toxicity, recyclability, and the ability to facilitate multi-electron transfer processes. Therefore, removal of organic dyes using suitable catalyst is an important target in waste-water treatment and recovery the toxic molecules from water. Metal oxide based catalyst having favourable optical and electronic properties moreover; it can be modified by reducing size and substitution of suitable dopants [2,3]. In general, the photocatalysis activity is depends on the photogenerated of charge carriers and efficient charge transfer. The photocatalyst efficiency is determined by the recombination rate of the charge carriers. The recombination rate in a photocatalyst is generally much faster (nanoseconds) than the interfacial transfer rate (microseconds to milliseconds) where many charge carriers recombine and releasing the resulting energy as heat, thereby limiting the overall quantum efficiency of the photocatalyst [4]. To avoiding or controlling the unwanted recombination of charge carriers, the photocatalytic activity of semiconductor metal oxides can be significantly improved by tuning electronic properties. More importantly, the catalytic activity of nanostructures strongly depends on their surface structure. It can be reducing the recombination of a catalyst by introducing defects on the surface. The oxygen vacancies on the surface of catalyst can act as charge capture centers resulting in generating superoxide radicals [5–7]. CeO2 as an n-type semiconductor with a band-gap of 2.9–3.2 eV, presents many advantages like the most popular photocatalyst (TiO2) such as chemical inertness, inexpensive, stability against photoirradiation, and non-toxicity. It is believed that non-toxic CeO2 is to be one of the key materials for future hydrogen production technology and photodegradation. Mainly, the doping approach on CeO2 nanostructures have drawn favourable consideration towards the excessively induced optical functionalities due to the transformation of Ce4+ to Ce3+ at low-temperature oxidation reactions [7–9]. The mixed conductivity of the ceria nanoparticles might be enhanced when doping transition metal ions which are lower than 4+ valances, it can be increasing oxide vacancies. Several methods have been used for preparing doped CeO2 nanoparticles such as forced hydrolysis [10], microemulsion method [11], solvothermal synthesis [12], two-stage precipitation process [13], nonhydrolytic sol-gel synthesis [14], solid-state reactions [15], mechanochemical reactions [16], sonochemical synthesis [17,18], microwave irradiation [19] and so on. Apart from the photocatalytic dye-degradation, CeO2 has been applied various energy applications, such as oxygen ion conductor in solid oxide fuel cells, UV absorbent, catalysts, polishing material, gas sensor, catalytic support, oxide storage capacity and fluorescent material. Interestingly, the CeO2 nanoparticles can also be considered as one of the promising redox supercapacitor materials but it has a major drawback that is poor electronic conductivity. Hence, carbon materials such as activated carbon, carbon black, carbon nanotubes and graphene have been used with CeO2 to improve the electrical conductivity [20–22]. But the carbon-based materials suffer from relatively low power density. Even though, the CeO2-graphene composite is performing as good photoanode material, the graphene-based materials are more expensive [23]. It is well-known that transition metal ions are alternative candidates for industrial application owing to their high catalysis, stability and low cost. In the present study, we have investigated the photocatalytic dye degradation and supercapacitor for pure and different metal ions doped

2. Experimental section 2.1. Materials Cerium nitrate hexahydrate (99.5%, Alfa Aesar), polyvinylpyrrolidone (M.W. 40,000, Alfa Aesar), sodium hydroxide (98%, SRL Pvt Ltd, India), silver nitrate (99.9%, Alfa Aesar), bismuth nitrate pentahydrate (98%, Aldrich), cadmium acetate dihydrate (98%, Alfa Aesar) and lead (II) acetate trihydrate (99%, Alfa Aesar) were used for the synthesis. All the chemicals were of analytical grade and used without further purification. Deionized (DI) water was used throughout the synthesis process. 2.2. Synthesis of pure and doped CeO2 nanoparticles A facile one-step chemical precipitation method was employed to synthesize pure and doped CeO2 nanoparticles (NPs). In a typical synthesis of CeO2 NPs, 0.5 M of cerium nitrate (Ce (NO3)3·6H2O) was dissolved in 50 mL of DI water. To prepare homogeneous nanoparticles, 1 g of polyvinylpyrrolidone (PVP) was used as a surface agent. The PVP was directly added into the above precursor solution. Then, the mixed solution was stirred vigorously for 10 min to obtain a homogeneous solution. Thereafter, 1 M of NaOH solution in 50 mL was added dropwise into the solution under vigorous magnetic stirring. Then, the mixed solution was again stirred vigorously for 2 h to ensure the homogeneous mixing of the solution. After 2 h, the colloidal solution was allowed to stand for 6 h at room temperature for complete settle down of the particles. Then, the colloidal solution was centrifuged for separate the product and then washed several times with distilled water until the pH became neutral. The collected solid wet sample was then dried at 120 °C for 2 h to obtain powder form. For the preparation of doped nanoparticles, silver nitrate (AgNO3), bismuth nitrate pentahycadmium acetate dihydrate (Cd drate (Bi(NO3)3·5H2O), (CH3COO)2·2H2O) and lead (II) acetate trihydrate (Pb (CH3COO)2·3H2O) were used as precursors. Following the above experimental procedure, 5% (wt%) of the dopant was added into cerium nitrate solution before the addition of PVP. 2.3. Photocatalytic activity measurement Methylene blue was used as a model dye to investigate the photocatalytic activity of the different metal ions doped CeO2 photocatalysts. The degradation was performed under natural sunlight (~32 °C). The amount of catalyst dispersed in the dye solution, and reaction time were changed to study their effect on MB degradation. Certain amount (50 mg) of catalyst in 50 mL aqueous solution with a constant amount of MB (10 mg/L) was used for photocatalytic reaction. Prior to irradiation, the suspensions were magnetically stirred in the dark for 20 min. Then, the measurement of photocatalytic activity was conducted under sun light. At particular irradiation time intervals, 5 mL of the suspension was collected and centrifuged to separate the catalysts. The residual MB concentration was measured using Hitachi (Model U4100) UV/Vis-NIR spectrophotometer equipped with 1 cm quartz cavette holder for liquid samples. 2

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2.4. Electrochemical measurements Electrochemical performances of the Ag-doped CeO2 thin film electrodes were tested using AUTOLAB PGSTAT 302N electrochemical work station at ambient condition in a standard three-electrode system. A platinum slice was used as the counter electrode with an Ag/AgCl electrode as the reference electrode. All electrochemical measurements were carried out in a conventional three-electrode cell containing 6 M KOH aqueous electrolyte. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements were done at room temperature under atmospheric pressure. 2.5. Characterization Structure and crystalline features of the synthesized powders were characterized by X-ray diffraction (XRD) using a powder diffractometer (PAN Analytical X' pert PRO Model X-ray diffractometer) with monochromatic Cu-Ka radiation (λ = 1.5418 Å). The sample was loaded onto an indented glass plate and the diffracted signal was recorded for 2θ range of 20 to 80° at a scan rate of 5°/min. The morphology, size and shape of the synthesized powder samples were determined using transmission electron microscopy (TEM, JEM 2100 F) performed with an acceleration voltage of 200 kV. For the TEM measurements, the samples were prepared by dispersing the powder sample in absolute ethanol and setting this liquid drop-wise on carbon-coated copper grids, then drying in air at 60 °C. Fourier transform infrared (FT-IR) analysis was conducted using a Bruker optic GmbH TENSOR-27 spectrometer from Thermos electron at room temperature. For the FT-IR measurement, the sample was prepared by mixing the powder sample and KBr (1:30 ratio) for making pellet formation. Raman spectra were recorded using LabRAM HR Evolution Laser-Raman spectrometer with an excitation wavelength of 514 nm from Oxxius laser source (100 mW). UV–Vis diffuse reflectance spectra were recorded in the wavelength range of 200–900 nm using a UV/VIS-NIR double beam spectrophotometer (VARIAN, Cary 5000). The fluorescence spectrophotometer was used to record the photoluminescence (PL) spectra (VARIAN, Cary Eclipse) with an excitation wavelength of 380 nm. Electron paramagnetic resonance (EPR) measurements were performed using a Bruker EMX plus system with a microwave frequency of 9.8645 GHz and a microwave power of 10 mW at room temperature. Thermogravimetric analysis was performed for the samples using simultaneous thermal analyser (TG/DTA, SDT Q 600 V20) within the range of room temperature to 1000 °C under nitrogen atmosphere.

Fig. 1. (a) XRD spectra of the pure and Ag, Bi, Cd and Pb doped CeO2 NPs and (b) Samples versus crystallite size.

Yang et al., the local symmetry can be change by doping heteroatoms [24] but it is also depending the ionic radius of the dopants. Smaller ionic radius (compared to pure CeO2) of the heteroatoms dopants causes changed the equilibrium lattice constants of the doped nanoparticles results broken the local symmetry [24]. But in the present work, ionic radius of the doped heteroatoms (Ag and Bi) is higher than the ceria atom. This is good agreements with XRD result, no changes observed in the diffraction pattern after doping. The strong diffraction peaks of Ag doped CeO2 demonstrates high crystalline nature. Comparatively lower intensity of other metal ion doped CeO2 NPs dictated reduced the crystalline nature. Due to the doping of the metal ions in CeO2 lattice, the concentration of Ce4+ ions would be reduced with increasing Ce3+ ions, which plays a dominant role in the lattice distortion. Fig. 1(b) shows variation of size of crystallite with respect to the dopants. Fig. 2(a–f) shows the FE-SEM images of pure and doped CeO2 nanoparticles synthesized from one-step solution method. As seen in the pure CeO2 (Fig. 2(a, b)), the particles are in small size with spherical shape and monodispersion. The average particles size of the nanoparticles is found to be ~30 nm. Due to limited magnification, the estimated particles size showed bigger than actual size. As seen in the FESEM images of Ag and Bi doped CeO2, tiny rods like structures observed whereas spherical shape obtained for the Cd and Pb doped CeO2 nanoparticles The FE-SEM observation demonstrates that the doped nanoparticles did not alter the morphology of the NPs but facilitates agglomeration..

3. Results and discussion In this present study, we used different metal ions for doping in CeO2 in order to obtain better photocatalytic activity. In this aspect, selected metal ions such as Ag, Bi, Cd and Pb were used as dopants with constant doping amount. Fig. 1(a–b) shows XRD spectra of pure and different metal ions doped CeO2 nanoparticles and a plot of doping concentration versus crystallite size, respectively. The dopant concentration was fixed at 5 (wt%) for all samples. The diffraction characteristic peaks at (2θ) = 28.2, 32.8, 47.2, 56.1, 58.8 and 69.2° correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) planes of CeO2 in face centered cubic structure. The results are in good agreement with the bulk CeO2 (JCPDS NO. 34-0394). The strong diffraction peaks indicated good crystalline nature of the samples. No impurity peaks observed in pure as well as doped CeO2 NPs, it indicates high purity of the synthesized NPs. The average grain size of pure and doped CeO2 NPs has been calculated using the Scherrer formula and the estimated crystallite size to be 10.7, 37.7, 10.2, 9.6 nm and 9.3 nm for pure and Ag, Bi, Cd and Pb doped CeO2 nanoparticles, respectively. The size of the samples changed for the doped nanoparticles, it may be different ionic radius of the metal dopants (Ce3+ (1.02 Å), Ag+ (1.15 Å), Bi3+ (1.03 Å), Cd2+ (0.95 Å) and Pb2+ (1.19 Å)). According to 3

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Fig. 2. FE-SEM micrographs of the pure CeO2 (a, b), Ag (c), Bi (d), Cd (e) and Pb (f) doped CeO2 nanoparticles.

nitrate [26]. The peak at 1650 cm−1 corresponds to C]O stretching motion of PVP [27]. The small peak observed at 1053 cm−1 is responsible for the CeO component [25]. The frequencies in the range of 900 cm−1 to 820 cm−1 it is due to CeOeO stretching vibration. The peak at 503 cm−1 corresponds to the asymmetric OeCeeO stretching mode of vibration [22]. Absence of the other functional peaks related to secondary product, the FT-IR study illustrates only primary nanoparticles developed during the adopted one-step chemical method. Raman spectroscopy performed for further investigation of structural phases of pure and different metal ions doped CeO2 NPs as shown in Fig. 5. The Raman spectra of the pure CeO2 exhibits a single active mode centered at 458 cm−1 characteristics of the cubic fluorite structure, which is attributed to a triple degenerated F2g mode [28]. The spectra show broadening as well as a changing in the intensity of bands suggestive of dimension variation of dopant with respect to the ceria. In general, the shift in Raman peak occurs due to the alternation in the structure, the particle size, the nature of defects and so on [29]. Compared to other dopants, Ag and Pb doped CeO2 spectra were significantly shifted towards lower wavelength side, it obviously reflected in the XRD result by significantly changes the particles size. Moreover, metals ions doped CeO2 NPs preserved the cubic structure which suggests that metal ions are substitutional doped into CeO2 structure. The absence of the secondary crystalline phases of dopants in the doped

In order to determine exact shape and size of the synthesized NPs, all the samples were further characterized using TEM as shown in Fig. 3(a–f). The morphology of pure CeO2 (Fig. 3(a–b)) shows sphericalshaped nanoparticles of different magnifications with diameter in the range of 10–15 nm whereas the Ag doped CeO2 (Fig. 3(c)) was found to be rod-like structure with ~10 nm dimensions and ~50 nm length with highly homogenous. The Bi doped CeO2 (Fig. 3(d)) showed mixed structure of both rod and spherical like morphology. The Cd and Pb doped nanoparticles (Fig. 3(e and f)) showed in spherical nature with reducing size. The selected area diffraction (SAED) pattern provided (as inset) for the corresponding samples. The morphology variation is due to different orientation of the particles due different dopants. The particles size determined by TEM result resembles the crystallite size calculated using Scherer's formula. To identify the functional groups presence on the surface of the NPs, FT-IR measurements was performed. The FT-IR spectra of pure and different metal ions doped CeO2 NPs are shown in Fig. 4. From these spectra, we observed broad band which is centered at 3435 cm−1 and 1639 cm−1 was attributed to the OeH stretching vibration [25]. The absorption band at 2890 cm−1 is related to the CeH stretching vibrations, which is occurred by organic compounds used in the synthesis of CeO2 nanoparticles [25]. The strong absorption band at 1384 cm−1 represents vibration mode of NeO3 stretch due to the presence of 4

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Fig. 3. TEM images of CeO2 (a, b), Ag (c), Bi (d), Cd (e) and Pb (f) doped CeO2 nanoparticles. The corresponding SAED pattern is included insert of the images.

samples indicates that it is finely dispersed in the CeO2 matrix. The results of the phase composition revealed from the Raman spectra also supported the XRD results. In order to investigate the optical properties of pure and doped CeO2 NPs, UV–visible absorption spectra were recorded for all the samples

and the results are displayed in Fig. 6(a). The corresponding band gap values are also indicated by Tauc plots in Fig. 6(b). The optical band gap energy of pure and doped CeO2 NPs has been determined by extrapolating the linear region of the plot of hν verses (αhν)2 as shown in Fig. 6(b). The incorporation of metal ions into the CeO2 lattice result in

Fig. 4. FT-IR spectra of the pure and doped CeO2 nanoparticles. 5

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Fig. 5. Laser Raman spectra of the pure and metal ions doped CeO2 nanoparticles.

a reduction of band gap energy. Absorption spectra of all the doped samples were shifted towards longer wavelength side with respect to the pure CeO2. Particularly, the Ag and Pb doped samples were significantly shifted. These results are presented in Fig. 6(c). The estimated band gap energy for the doped CeO2 samples is to be between 1.86 and 2.84 eV, the values are lower than that of the bulk CeO2 (3.2 eV) [30]. The reduction in the band gap may be due to the impurity defect level created by the influence of metal ions doping. When doping metal ions in CeO2, number of photons will be absorbed in the higher wavelength range. As a consequence, the utility range of light will be extended in visible range, which results in the considerable improvement in photocatalytic activity of doped CeO2 NPs. The optical result revealed importance of the metal ions incorporation into CeO2 framework. The PL emission spectra of the different metal ions doped CeO2 NPs are illustrated in Fig. 7 with an excitation wavelength of 380 nm. An intense PL emission observed from violet to orange region, however, maximum intensity displayed in green region from 450 to 500 nm. Along with the intense emission, several shoulder peaks located in the pale blue region (~430 nm) due to surface related defects, the other peaks at 452 nm, 470 and 495 nm are due to dislocation or oxygen vacancies, and an emission at 525 nm due to oxygen vacancies related defects [31]. It can be observed that emission peaks of pure CeO2 are almost similar to those of metal ions doped CeO2, but the PL intensity of the doped CeO2 is lower than that of pure CeO2. Particularly, the PL intensity of Ag doped CeO2 was highly reduced, next low intensity PL emissions observed for Bi and Pb doped samples, respectively. It was clearly observed from the PL result that a longer life time takes the excited electron of the doped samples compared to the pristine CeO2 which may be attributed to (i) surface defect on the doped CeO2, that captures and delays the charge recombination process. (ii) The energy levels of doped CeO2 presence below and above the conduction band (CB) and valance band (VB) of CeO2 facilitates the charge carrier movement, which delays the recombination process [32]. In addition, the intensity of PL spectra is directly related to the recombination of electrons and holes, therefore lower PL intensity suggest the delay in recombination rate which also implies that large number of photogenerated electrons and holes are participate in the photochemical transformation thereby enhancing the photocatalytic activity of Agdoped CeO2 NPs and then Bi and Pb doped samples. Electron paramagnetic resonance (EPR) was used to study the metal ions incorporation sites in the CeO2 nanoparticles (Fig. 8). From the EPR spectra, we suggested that dopant occupy interstitial sites in the

Fig. 6. (a–c) UV–vis absorption spectra, Tauc plot for determination of the energy band gap and doping elements versus band gap, respectively.

lattice of the CeO2. The increased catalytic activity of doped CeO2 nanoparticles is explained by enhanced concentration and mobility of oxygen vacancies, and in addition to the enhanced redox pair in the doped CeO2 system. All the samples revealed the formation of one type of paramagnetic species. This result corroborates the assumption of the 6

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Fig. 9. TGA curves of the pure and metal ions doped CeO2 nanoparticles.

Fig. 7. Photoluminescence spectra of the pure and doped CeO2 nanoparticles.

presence of Ce3+ ions and oxygen vacancies in the samples. The multiple peaks in the pure and doped system indicates presence of Ce3+, O2−, and Ce3+, Ce4+ type defect sites. Noticeable additional signals observed about 3375G and 3575G for Ag and Pb doped CeO2 indicates formation of the Ce3+ defect sites. The above result clearly showed that possibility of defects enhancement by substitution of the impurities. Several studies have been performed on the process of water adsorption on CeO2 and it is well accepted that water molecules strongly and dissociatively bind on oxygen vacancy sites [33]. Thus, it is possible that the formation of ∙OH radicals is dependent on the activation of water molecules on the oxygen vacancy sites through Ce3+/Ce4+ redox cycle [34,35]. Thermogravimetric analyses (TGA) were conducted for pure and doped CeO2 NPs to study the stability of materials during annealing, and the results are presented in Fig. 9. The thermal study was performed for the samples in the temperature range from RT-950 °C under nitrogen atmosphere with temperature gradients of 10 °C /min. The first weight loss occurs before 200 °C which corresponds to the loss of physically adsorbed water. The second weight loss showed in the temperature range between 200 and 300 °C, is related to the loss of structural water molecules. There is no major weight loss observed at higher temperatures (300–600 °C), which indicates the absence of additional phases or other structural changes. A small weight gain and weight loss showed in the TGA curve for Ag, Cd and Pb doped samples between 300 and 600 °C which may be due to oxidation and release of the dopant at the higher temperature, but the Bi doped CeO2 showed no weight loss event after the 800 °C, it showed high thermal stability of cubic CeO2. The photocatalytic activity of synthesized catalysts was examined by degradation of aqueous solution of methylene blue dye under direct sunlight irradiation. The UV–Vis spectra of neat MB dye and its aqueous solution mixed with the catalyst samples (pure and metal ions doped CeO2) were recorded for the spectral range 500–800 nm as a function of irradiation time. For all the experimental work, the dye concentration of 30 mg/L and catalyst concentration of 50 mg/L were used. The UV absorption was measured for dye solution in the absence and presence of synthesized photocatalysts after irradiation. From the absorption spectra, we found that the intensity of the main absorbance band centered at 664 nm gradually decreased with the increase in irradiation time. The decrease in the intensity of the absorbance band indicates the degradation of the MB dye. Fig. 10(a–e) shows UV–vis absorption spectra of MB dye degradation under sun light at different time interval in the presence of pure, Ag, Bi, Cd and Pb doped CeO2 catalyst,

Fig. 8. (a) EPR spectra of the pure and Ag, Bi, Cd and Pb doped CeO2 nanoparticles and (b) magnified image of the EPR spectra of (a).

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Fig. 10. Time-based spectral changes of MB in aqueous (a) pure CeO2 (b) CeO2-Ag (c) CeO2-Bi (d) CeO2-Cd and (e) CeO2-Pb dispersion under visible irradiation. (f) Photodegradation efficiency vs. time plot for the pure and doped nanostructures as photocatalyst in the dark and under visible light irradiation. (g) Proposed schematic diagram of the proposed mechanism for the photoexcited electron-hole separation and charge transport process at the CeO2-Ag nanostructures as a photocatalyst under the visible light irradiation. (h) Plot of ln (C/C0) vs irradiation time at room temperature for all the samples and (i) kinetics of the photocatalytic degradation of MB by pure and doped nanoparticles.

These separated e− and h+ pairs have a tendency of fast recombination [36]. Thus, only low dye degradation efficiency obtained for pure CeO2. However, significant enhancement of the photocatalytic activity showed for Ag doped CeO2 about 99.58% under similar reaction conditions. The photodegradation enhancement may be occurred by the following way: when irradiated with the light on the Ag doped CeO2, the electrons are excited and transformed to adsorbed oxygen forming O2− ions. The Ag nanoparticles are consequently oxidized by O2− to colourless Ag+ ions. In presence of CeO2 these Ag+ ions are reduced by the excited electrons and Ag nanoparticles are reformed. In general, the measured differences in photocatalytic activities of CeO2 containing different metal nanoparticles can be explained as the effect of difference in work functions between the noble metal and the semiconductor [36]. The mechanistic photocatalytic reaction scheme diagram for efficient catalyst of Ag-CeO2 is shown in Fig. 10g. All the degradation curves of C/C0 vs time displayed in Fig. 10(h) are already normalized and calibrated from the photodegradation of MB dye in water in the absence of catalyst under identical conditions. The rate of photo induced dye degradation was calculated in terms of the change in absorbance of MB using the relation:

respectively. The decrease in absorption at 664 nm is due to the destruction of benzene rings and the heteropolyaromatic linkage (Fig. 10(f)). Decrease in the intensity of absorption peak implies a decrease in concentration of MB dye under light irradiation. The disappearing peaks in the UV-spectra for the catalyst indicate completely degradation of the dye molecules under the natural sunlight. As shown in Fig. 10(f), the dye degradation results revealed that the doped samples exhibited a higher photocatalytic activity towards degradation of MB than pure CeO2. It is normally believed that more oxygen vacancies may be created when metal ions incorporated into CeO2. The amount of oxygen vacancies present in the catalyst is related to electron-hole pair (e−-h+) separation when introducing the metal ions. These oxygen vacancies could create deep trap states in the doped samples under the light irradiation. The developed trap again would restrict (e− - h+) recombination resulting in enhanced photocatalytic performance. Hence, the photoluminescence result showed reducing intensity for the doped samples than pure CeO2 (Fig. 8). However, the creation of oxygen vacancy and followed by the trap levels in a metal oxide system is mainly depends on the ionic radius of the dopant. It was found that higher ionic radius of Ag and Pb doping elements are more favourable for improving photocatalysis efficiency in CeO2-Ag and CeO2-Pb system. But it was found that only about 10% of MB degraded by using pure CeO2 catalyst as shown in Fig. 10(f). When CeO2 is exposed to light irradiation, the electrons present in its valence band are excited to conduction band, leaving behind holes in the valence band.

Degradation efficiency = [1

(Ct / C0 )] × 100

where C0 and Ct is absorption of initial and after a specific time of MB exposed by sunlight. Generally, when two materials with different work function contact each other, the Schottky barrier will be formed, and 8

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electrons will transfer from the material with lower work function to the material with higher work function. The work function of the selected metal ions, such as Ag (4.26–4.74 eV), Bi (4.31 eV), Cd (4.08 eV) and Pb (4.21 eV) is higher than CeO2 (2.90 eV). For metal doped CeO2 nanoparticles, excited photoinduced electrons from CeO2 will be transported to the dopant and will be scavenged by the electron acceptor, commonly the O2 species absorbed on the surface [37,38]. Under this circumstance, the Ag doped CeO2 thin film shows more effective electron-acceptor properties due to the larger work function of Ag. The work function for the selective metals and core ceria have been ordered in the following way, Ag > Bi > Pb > Cd > Ce. The kinetics of decolourisation of MB dye using different catalyst is illustrated in Fig. 10(i), and the results showed that the degradation process can be simulated by the pseudo-first-order kinetic equation as follow −ln (Ct / Co) = −kt where C0 and Ct are concentrations of dye initially and at time ‘t’ respectively and ‘k’ is the apparent first order photoreaction rate constant (min−1). The results from the kinetic plot shown in Fig. 10(i), clearly exhibits that the Ag-CeO2 NPs shows the highest photocatalytic performance with photoreaction rate constant k = 0.466 min−1 and it was found to be about 2 times higher than that of other metal ions (Bi, Cd and Pb) doped CeO2 NPs. Moreover, the photocatalytic activity of the doped NPs is more than 20 times higher than that of pure CeO2. From this result, we can conclude that the photocatalytic activity of CeO2 can be significantly improved in the presence of dopants. Generally, the absorption of photon energy by photocatalyst and rate of recombination of electron hole pairs in photocatalyst plays major role in the photocatalytic performance [39]. The high absorption of photon energy and extension of excitation wavelength by Ag doped CeO2 than pure CeO2 was verified by above UV–Vis DRS characterization (Fig. 6). As is known, when sun light is irradiating on dye suspension, the electron–hole pairs are generated due to the ejection of electron from VB that creates a hole in the VB. The decreased band gap of Ag doped CeO2 is more favourable to absorb the exciting light and generated more number of electron–hole pairs compared to undoped CeO2 sample. The doped ion energy level can act as a trapping site for photoinduced electrons and holes, which mainly slow down the recombination rate of electron–hole pairs. Therefore, Ag doping may be more effective for the production of those materials that can delay electron–hole recombination rate. Furthermore, active species of hydroxyl radicals (%OH) may be first yielded on the photocatalyst surface, formed by the surface hydroxyl groups (OH−) trapping generated holes, and then the electrons will be transferred to the conduction band and trapped by the dissolved oxygen molecules producing superoxide anions (%O2−) [40]. After that, the formed superoxide radical anion (% O2−) may either attack the organic molecules directly or generate a hydroxyl radical (%OH) by reacting with hydrogen (H+) and photogenerated electrons. Together, the resulting %O2− and %OH radicals can play a vital role in degrading the organic contaminant, acting as two very strong oxidizing agents. More importantly, the stability of a photocatalyst is always regarded as another important feature for its application, besides activity. Therefore, to evaluate the reusability of the samples, a cycling test was performed repetitively for five cycles for the degradation of most efficient catalyst of Ag doped CeO2 as shown in Fig. 11. It is clearly observed that it possessed a relatively stable photocatalysis with no significant decrease in activity (Fig. 11). In order to study the electrochemical behaviour of the selected catalyst (Ag doped CeO2), cyclic voltammetry (CV), galvanic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS) analyses were conducted using nickel foam coated with the CeO2-Ag nanoparticles as a working electrode, platinum wire as a counter electrode, and Ag/AgCl as a reference electrode in a freshly prepared 3 M potassium hydroxide (KOH) electrolyte solution. To estimate the capacity of prepared electrode, CV and GCD analysis have been carried out within the potential range of −0.1 to 0.45 V. The CV measurements of the prepared electrodes at different scan rates are shown in Fig. 12(a). The CV curves of the CeO2-Ag are shown asymmetric in

Fig. 11. Cycling runs of MB decolourisation using Ag-doped CeO2 catalyst.

nature. The redox peaks of CeO2-Ag nanoparticles indicate the oxidation of Ag. It is also noted that the CV curves of the CeO2-Ag shows much improved capacitance when compared to the pure CeO2 modified electrodes [41–44]. The increase in specific current with the scan rate displays better reversibility of the prepared electrodes during ion diffusion process. The Ag doped with CeO2 nanostructures produces effective surface kinetics and improves the electronic conductivity of the material. There exists a synergistic effect between the Ag and CeO2 nanoparticles accounting for higher capacitance in the GCD curve (Fig. 12(b)) of doped sample owing to its larger surface area (due to the smaller size, nano regime), high electrical conductivity. Specific capacitance obtained at different current density is showed in Fig. 12(c). To estimate the specific capacitance of the prepared electrode, the following equation can be used

Cs = I t/m v where I is the discharge current (mA), Δt is the discharging time (s), m is the mass of active material (g), and Δv is the potential window (V). Interestingly, the specific capacitance value remains higher (448 F g−1) even at a much higher scan rate (20 mV s−1). The excellent rate capacity is due to the excellent conductivity during effective charge transport of an Ag incorporated into the CeO2 nanostructure. Electrochemical impedance spectroscopy analysis was used to gain detailed study of ion transport kinetics of the electrode materials (catalyst). It is an important factor for supercapacitor study. Fig. 12(d) displays the EI spectra of Ag doped CeO2 electrode. The data was fitted to an equivalent circuit model, which was shown in inset of Fig. 12(d). EIS measurements were conducted over the frequency range from 0.01 Hz to 100 kHz. The EIS plots of Ag doped CeO2 electrode, 9

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Fig. 12. (a) CV plots CeO2-Ag, (b) charge-discharge curves at different scan rates and different current densities, respectively, (c) specific capacitance obtained at different current density and (d) Nyquist plot of electrodes initial and after cycling and corresponding equivalent circuit provided inset of the figure.

the maintenance of excellent stability during the electrochemical measurements. Cycling stability of the Ag doped CeO2 NPs, is of great importance to their practical applicability to supercapacitors. Fig. 13 shows the long-term cycling stability of the CeO2-Ag NPs/Ni foam electrode at the current densities of 3 A g−1. The capacitance retention observed about 97.5% of the initial cycle capacitance at the current density of 3 A g−1. The results presented in this work show that the Ag doped CeO2 NPs prepared by a facile chemical method could be a good candidate for photocatalyst as well as electrode material in supercapacitor applications. 4. Conclusion Different metal ions doped CeO2 NPs were synthesized using a simple, low-cost, one-step chemical precipitation method. All the synthesized samples were characterized by XRD, FE-SEM, TEM, Raman, UV–Vis, PL, EPR, and TGA. FE-SEM and TEM revealed uniform with different shape like, spherical and rod-like structure. UV–Vis study showed that the oxygen vacancy/defects in the doped CeO2 with decreasing band gap. Among the different dopants, Ag is a suitable candidate for photocatalytic and supercapacitor applications. It was believed that the photocatalytic enhancement was mainly due to ionic radius and work function of the dopant. Within 120 min, ~99.5% of the MB dye degraded under sun light. The enhanced physical and chemical properties of the rod-like structure Ag doped CeO2 NPs helped in achieving the high specific capacitance of 456 F g−1 with a capacitance retention ratio of 97.5% at the constant current densities of 3 A g−1. Finally, the doped CeO2 NPs showed excellent photocatalytic and supercapacitor performance for water treatment and energy storage applications. Based on the improved optical, photocatalytic, and supercapacitor performance, this type of doped CeO2 NPs can be used for real applications, such as real water treatment devices and supercapacitor.

Fig. 13. Cyclic performance of CeO2-Ag catalyst.

measured before and after cycling. The symbols ‘Rs’, Rct and ‘W’ represented solution resistance, charge transfer resistance and Warburg resistance, respectively. Both EIS spectra consisted of a semicircle in the high-frequency region and an inclined line in the low-frequency region, which were characterized by the frequency response of the electrode/ electrolyte system by examining the imaginary component (−Z″) of the impedance compared to the real component (Z′). From Nyquist plots, the Rct values of the Ag doped CeO2 before and after cycling were calculated to be 1.17 and 1.33 Ω, respectively. The lower Rct values of the material further indicated that it had lower resistance and aids in 10

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Acknowledgement

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