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Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)11789-6
10.1016/j.heliyon.2024.e35758
e35758
Research Article
Synthesis and Characterization of Cr doped CeO2 nanoparticles for Rhodamine B dye degradation
B Veena
M Seetha seetha.phy@gmail.com
⁎
Department of Physics, Kongunadu Arts and Science college, Coimbatore - 641029, Tamil Nadu, India
⁎ Corresponding author. seetha.phy@gmail.com
03 8 2024
30 8 2024
03 8 2024
10 16 e3575831 5 2024
29 7 2024
2 8 2024
© 2024 Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
The main aim of this work is to synthesis and study Cr doped CeO2 nanoparticles for Rhodamine B dye degradation. In this regard, 2 wt% and 4 wt% Cr doped CeO2 nanoparticles were successfully synthesized through a simple chemical precipitation method. The structural characteristics and elemental composition of the synthesized samples were analyzed using XRD and XPS techniques. The cubic fluorite structure with space group Fm 3m was confirmed through XRD and the presence of Ce, O and Cr atoms in the samples were identified through XPS. Spindle shaped structures were observed from FESEM analysis for 2 % Cr doped sample. Confocal Raman Spectroscopy was used to confirm the CeO2 stretching vibrational mode at 469 cm−1. The metal oxygen band was obtained at 447.49 cm−1 from FTIR spectroscopy. The band gap values were calculated from the Tauc plot and the values were found to be 2.0 eV, 2.85 eV and 2.88 eV for CeO2, 2 % Cr and 4 % Cr doped samples. The prepared nanoparticles were subjected to photocatalytic degradation of Rhodamine B dye at 5 ppm concentration and highest efficiency of 98.3 % was observed by the 4 % Cr doped CeO2 sample.

Graphical abstract

Image 1

Keywords

Rhodamine B
CeO2
Chromium
Tauc plot
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pmc1 Introduction

Cerium oxide is one of the most abundant and versatile among the rare earth oxides. It has excellent properties like oxygen storage capacity, high mechanical strength, good transport capability, non – toxicity, chemical stability, fine tunable bandgap etc [1,2]. Such properties of CeO2 make it available for various applications like Solid oxide fuel cells, UV absorbers and shields, sunscreens, gas sensors, three way catalysts, abrasives and many more [3]. The enhanced surface to volume ratio, excellent stability and the nature of switching between its oxidation states serves as significant features for nanoceria in various forms like nanorods, spheres, nanoflowers and nanotubes. The changes in oxidation states between Ce3+ and Ce4+ yields more oxygen vacancies, which strengthen its catalytic properties. It exhibits cubic fluorite structure with O2− ions in tetrahedral position. Much effort is taken in fine tuning its morphology, and size by appropriate synthesis procedures.

Reducing environmental pollution and providing sustainable surroundings is one of the biggest responsibilities of society in these times. The addition of impurities and contaminants to various sources, specifically water, is increasing alarmingly. Various techniques are available for treating the contaminants by converting them into an ecofriendly material before disposal. Photocatalysis is one of the widespread methods used in the treatment of contaminated water caused by various means. Photocatalysis is a simple and cost effective method that generally utilizes metal oxide/composite nanoparticles in the presence of some light illumination. Synthetic dyes are used in a variety of ways in the textile industry, like printing, coloring, bleaching, and for finishing purposes. Synthetic dyes, which are usually derived from chemical compounds, are broadly classified into three main types, namely cellulose fiber dyes, synthetic fiber dyes, and protein fiber dyes. Synthetic fiber dyes are further classified into direct, disperse, and basic dyes [4]. Rhodamine B falls under the category of basic dyes and contains four N- ethyl groups on the either side of Xanthene Ring [5]. Many ill effects, including irritation to skin, eyes and several respiratory problems, and carcinogenic conditions, are observed with temporary and prolonged exposure to Rhodamine B. Hence, degradation of such a highly stable industrial effluent is necessary for a better environment.

Several degradation mechanisms using CeO2 in various morphology and different dopants are reported. Er doped CeO2 nanoparticles synthesized through hydrothermal method exhibited a degradation efficiency of ∼94 % [6]. Polyaniline coated CeO2 nanoparticles prepared through oxidative polymerization shows a degradation efficiency of 91 % over Rhodamine B dye [7]. ZrO2 – PANI and CeO2 – PANI composites degraded Rhodamine B dye to 35 % in 60 min [8]. Hierarchical Yttria (Y2O3) nanosphere decorated CeO2 nanorods exhibited a degradation efficiency of 95.8 % over the RB dye [9]. In this work, Cr doped CeO2 nanoparticles were prepared through a chemical precipitation method and their photocatalytic degradation were studied over Rhodamine B.

2 Materials and methods

2.1 Required chemicals

All the chemicals of analytical grade were purchased and used. Cerium nitrate hexahydrate (Ce(NO3)3. 6H2O), chromium (III) nitrate nonahydrate (CrN3O9.9H2O) and anhydrous glucose powder (C6H12O6) were purchased from Himedia. Other chemicals like sodium hydroxide pellets and distilled water were used as received without further purification.

2.2 Synthesis of CeO2 and Cr doped CeO2 particles

2 wt% and 4 wt% Cr doped CeO2 particles were prepared with 0.04 M of Cerium nitrate hexahydrate precursor and 0.04 M of chromium nitrate hexahydrate. The appropriate molar powders were stirred in 30 ml of distilled water for an hour after which NaOH drops were added. Later, 0.06M of anhydrous glucose powder was added, and NaOH drops were added again. A brown colored precipitate was obtained, after which the solution was left to age for 24 h. The precipitate was then centrifuged at 5000 rpm and calcined at 600 °C for 3 h. Light brown colored Cr doped CeO2 particles were obtained. A Bare sample was prepared in a similar manner without the dopant precursor.

2.3 Dye degradation

The photocatalytic behavior of the prepared samples was studied for 50 mL of Rhodamine B dye solution at 5 ppm concentration in distilled water. 50 mg of active photocatalyst was added to the dye solution, and it was left to stir for 30 min to attain adsorption-desorption equilibrium. A Suitable amount of H2O2 solution was then added to the beaker, and the solution was kept for illumination produced by a 500 W tungsten filament. The samples were collected at every 15 min interval for CeO2 and at 10 min intervals for doped CeO2. The collected samples were studied using a UV- VIS spectrophotometer.

2.4 Characterization techniques

The crystalline properties of the synthesized materials were calculated using X-ray diffractometer (XRD) from Smartlab, Rigaku using CuKα radiation at 40 kV, 30 mA. Field Emission Scanning Electron Microscopy (FE- SEM) of model JSM – 6335F from JEOL Ltd was utilized for detrmining the morphology of the samples. The X-ray photoeletron spectroscopic analysis were taken using K-Alpha – KAN9954133, Thermo Scientific with miro-focused monochromated aluminium as the radiation source. The energy range of ion gun was from 100 to 4000 eV. Confocal Raman Microscope with AFM imaging of WiTec alpha 300, was used to determine the Raman active modes which operates in the range of 50 cm−1 - 4000 cm−1. . The compositional analysis was carried using FT–IR spectroscopy –SHIMADZU. The photocatalytic studies and optical studies were carried out by UV- VIS spectrophometer from CYBERLAB.

3 Results and discussion

3.1 XRD analysis

The structural characterizations of the prepared samples were investigated using X-ray diffraction analysis. The XRD patterns of CeO2, 2 % and 4 % Cr doped samples were studied from 20 to 110° and are shown in Fig. 1. The scattered angle positions, Miller planes, and structure obtained are consistent with the standard JCPDS Card No: 34–0394 [10]. Cubic fluorite structured ceria with space group Fm-3-m with no additional peaks corresponding to secondary phase impurities were observed. It also confirms that the cubic fluorite phase is not affected by doping. The 2θ values were obtained at 28.51°, 33.05°, 47.45°, 56.34°, 59.10°, 69.43°, 76.73°, 79.10°, 88.47°, 95.45° corresponding to (111), (200), (220), (311), (222), (400), (331), (420), (422), (511) Miller planes. The calculated average crystallite size was found to be 16 nm for doped and undoped CeO2. Increase in shift of angles by 0.01° is observed for 2 % Cr and 0.02° is observed for 4 % Cr doped CeO2 which indicates good incorporation of Cr ions into the ceria lattice without disrupting it. The average crystallite size is calculated using Debye Scherrer's formula,(1) D=0.941λβcosθ

Where λ is the wavelength of the X-ray used, β is the Full Width Half Maximum and θ is the diffracted angle.Fig. 1 XRD pattern of CeO2, 2 % Cr and 4 % Cr doped CeO2.

Fig. 1

3.2 FESEM analysis

The Field Emission Scanning Electron Microscopy technique was employed to study the morphology of the samples. The micrographs exhibit uniformly sized spherical aggregates for CeO2 and 4 % Cr doped CeO2 which are shown in Fig. 2. The image at 100 nm is shown in the inset of Fig. 2b. Similar spherical shaped agglomerated structures were reported in chromium doped CeO2 samples by Habib et al. 11. The particle sizes calculated for the samples were found to be 35.2 nm and 36.4 nm and a slight increase in particle size is observed upon doping. Fig. 3a shows the spindle shaped structures with sharp edges observed for the 2 % Cr doped CeO2 samples. The spindle type structures are observed along with spherical particles for 2 % Cr sample, which are shown in Fig. 3b. This structure formation may be due to the agglomeration of the spherical structures to form a spindle shape. The average diameter and length of those structures were found to be 24 nm and 103 nm respectively.Fig. 2 FESEM images of CeO2 and 4 % Cr doped CeO2.

Fig. 2

Fig. 3 FESEM images of 2 % Cr doped CeO2.

Fig. 3

3.3 Raman analysis

Raman analysis is used to study the various symmetries associated with the different structures of the samples. The Raman active modes obtained for CeO2 and 4 % Cr doped CeO2 were found to be 469.5 cm−1 and 469 cm−1 and are shown in Fig. 4a and b [11]. This peak is attributed to the triply degenerate F2g mode corresponding to the symmetric stretching vibrations of oxygen ions in CeO8 vibrational unit which is very sensitive to lattice disorder [12]. As indicated from the peak values of Raman active modes, a slight peak shift is observed which indicates the introduction of disorders in lattice thereby increasing the oxygen vacancies [13]. The peak values obtained from Raman analysis also confirm the cubic fluorite structure formation, which is well in accordance with the XRD results.Fig. 4 Raman spectra showing Raman Active Modes of (a) CeO2 and (b) 4 % Cr doped CeO2.

Fig. 4

3.4 X-ray Photoelectron spectroscopy

The compositional and the relative proportion of the elements in the CeO2 and 4 % Cr doped CeO2 sample are analyzed using XPS technique. The results confirm the presence of Ce, O and Cr atoms in the synthesized samples and the data is compatible with earlier reports. Survey scan of CeO2 exhibits Ce 3d and O 1s spectra. The Ce 3d core level spectra shows binding energy peaks at 882.3 eV, 898.1 eV, 900 eV and 916.6 eV which corresponds to Ce3+ 3d3/2, Ce4+ 3d5/2, Ce3+ 3d3/2 and Ce4+ 3d3/2 indicating the d orbital transitions (Fig. 5). Thus, Ce 3d3/2 and Ce 3d5/2 spin orbit coupling is confirmed from the Ce 3d spectra. The first peak at 529 eV of O 1s spectra indicates the lattice oxygen ions in the sample whereas the peak at 532.1 eV specifies the adsorbed oxygen ions to the surface [14].Fig. 5 (a) Survey spectra of CeO2, (b) Ce 3d spectra and (c) O 1s spectra CeO2.

Fig. 5

Similar binding energy values of Ce 3d and O 1s spectra obtained for 4 % Cr doped sample is shown in Fig. 6. The O 1s spectra peaks are obtained at 529.5 and 531.8 eV. The Ce 3d spectra peak values are obtained at 883.9 eV, 889.7 eV, 900.94 eV, 908.29 eV, 917.9 eV and 898.3 eV. The highest binding energy peak values obtained at 883.9 eV and 900.94 eV corresponds to the Ce 3 d9 4f1 O 2p6 final state. The lower binding energy value located at 889.7 eV corresponds to the Ce 3 d9 4f2 O 2p5 state [15]. The presence of Cr in the sample is confirmed through the Cr 2p spectra. The Cr 2p3/2 spin orbital splitting is observed at 579.7 eV and 589 eV and shows the presence of Cr 3+ ions [16].Fig. 6 (a) Survey Spectra, (b) Ce 3d spectra, (c) O1s spectra and (d) Cr 2p spectra of 4 % Cr doped CeO2.

Fig. 6

3.5 FTIR spectroscopy

The phase purity and the functional groups of the prepared samples are studied by FTIR spectroscopy and the transmittance graph is shown in Fig. 7. The metal oxygen band is obtained at 447.49 cm−1 indicating the Ce–O stretching [17]. The OH stretching vibrations of water molecules is observed at 3718.7 cm−1 19. The atmospheric carbon dioxide present in the sample is identified through band obtained at 1080.14 cm−1 as it readily adsorbs water and air [18]. The phonon band of CeO2 is also observed at 848.68 cm−1 12. Absence of broad peaks between 3200 and 3600 cm−1 indicates the absence of OH molecules from the base used for synthesizing [19].Fig. 7 FTIR transmittance spectra of CeO2 and 4 % Cr doped CeO2.

Fig. 7

3.6 UV Vis spectroscopy

Under the absorption of UV light, each element exhibits its characteristic chemical structure. To determine the absorption wavelength, the samples were dispersed in deionized water and studied in the range of 200–700 nm. Fig. 8 shows the UV spectra of the synthesized samples. The samples exhibit a broad absorption band at 328, 362, and 366 nm for CeO2, 2 % Cr, and 4 % Cr doped CeO2 respectively. This band originates from the charge transfer process between O 2p and Ce 4f states. The band gap values are determined by the Tauc formula,(2) (αhν)n=B(hν−Eg)

Where hν is the photon energy, α is the absorption co-efficient and B is a constant and n is 2 for direct and ½ for indirect transition [20]. The band gap value is determined by plotting a graph between (αhν) [2] and hν and extrapolating the linear part of absorption spectra to zero. The calculated direct band gap values were found to be 2.0 eV, 2.85 eV and 2.88 eV for CeO2, 2 % Cr and 4 % Cr samples. A red shift is observed from the UV spectra for the doped samples, indicating an increase in particle size, which is consistent with values obtained from FESEM analysis. The broadening of spectra from the ultraviolet to the visible light region takes place upon doping, which is favorable for photocatalytic applications.Fig. 8 (a) UV absorption spectra, Tauc plot of (b) CeO2, (c) 2 % Cr and (d) 4 % Cr doped CeO2.

Fig. 8

3.7 Photocatalytic dye degradation studies

The effect of chromium doping (2 % and 4 %) in CeO2 over degradation of Rhodamine B dye was studied and is shown in Fig. 9. The photocatalytic tests were performed with 5 ppm dye solution using 50 mg of catalyst and Hydrogen peroxide solution. The absorption curves were obtained at every 15 min interval for CeO2 and 10 min interval for doped samples.Fig. 9 Dye degradation graphs of Rhodamine B at periodic intervals by (a) CeO2 (b) 2 % Cr and (c) 4 % Cr doped CeO2.

Fig. 9

An absorption peak was observed at 553 nm which is the characteristic peak of Rhodamine B. It is observed from the graph that the peak intensity decreases with time indicating the degradation of the dye upon exposure to light. The degradation efficiency gradually increased with an increase in dopant concentration. This could account for the defects created upon doping leading to more oxygen vacancies as mentioned in the Raman analysis. The degradation efficiencies were found to be 93 %, 97.2 %, and 98.3 % for CeO2, 2 % Cr and 4 % Cr doped CeO2 nanoparticles. The degradation efficiencies were calculated using the following formula,(3) D.E=A0−AA0X100

Where, Aₒ is the initial Rhodamine B absorption and A is the Absorption after UV light irradiation [14]. Under H2O2 assisted conditions, a maximum degradation efficiency of 98.3 % was observed for 4 % Cr doped sample within a very short time of 30 min unlike bare sample where degradation was achieved over 90 min. A comparison of degradation efficiencies obtained from previous literature is reported in Table 1. It is seen from the table that a maximum degradation efficiency in a minimum period of time of about minutes is achieved by the 4 % Cr doped CeO2 nanoparticles. Thus, the effect of chromium doping has effectively reduced the time of degradation and also increased the degradation efficiency. Moreover, earlier reports confirm that the Ce3+ state readily oxidizes to the Ce4+ state upon reacting with H2O2 solution, creating more oxygen vacancies on the ceria surface [21]. The key factor for an effective degradation process is the formation of OH− radicals with a minimum electron-hole recombination. When a photon of energy hν is incident on the surface of CeO2, electrons and holes are released and these combine with oxygen molecules to form OH− and HO2 radicals. These radicals in turn combine with H2O molecules to form hydrogen peroxide H2O2 radicals [22]. The introduction of chromium ions into the crystal lattice increases the oxygen vacancies in the sample which results from continuous changes of states between Ce3+ - Ce4+ and Cr2+ - Cr3+. These vacancies aid in trapping the photogenerated electrons and holes by Cr3+ as it possesses high Lewis acidity. Such a process of proper trapping and releasing can increase the charge carrier separation and decrease electron–hole recombination thereby enhancing the photocatalytic efficiency [16]. In addition to these, OH− radicals are also released when H2O2 reacts with water. Among various treatments used in degradation of dyes, usage of H2O2 assisted systems (Fenton systems) proves to be more efficient as heavily charged wastewater can be effectively treated. Most heterogeneous catalysis systems use H2O2 as it produces peroxide radicals readily on the surface. The formation of such species is through strong binding of H2O2 on the ceria sites. Thus, the release of OH− radicals is increased by the use of H2O2 solution [delina]. H2O2 is one of the primary oxidant and affects the surface of the nanoparticles making it more polar. The mechanism can be summarized using the following equations from (4), (5), (6), (7), (8),(4) CeO2+hν→e−+h+

(5) e−+H2O→HO2+OH−

(6) H++H2O→OH−+H+

(7) OH_+RB→CO2+H2O

(8) H2O2+H2O→OH−+HO2

Table 1 Comparison of degradation efficiency of various metal doped CeO2.

Table 1Sample	Dye used	Degradation efficiency	Exposure time	Reference	
6 % Sm doped CeO2	Rhodamine B	89.04	90 min	[23]	
7 % Co doped CeO2	Acid orange 7	95.4 %	180 min	[3]	
Activated carbon – CeO2	Methylene Blue	94 %	90 min	[22]	
Ag doped CeO2	Rose Bengal	96 %	180 min	[19]	
8 % La doped CeO2	Rhodamine B	86.7 %	90 min	[13]	
1 wt % Pd doped CeO2	Methyl orange	92 %	120 min	[24]	
15 wt% Bi3+ doped CeO2	Methylene Blue	complete	75 min	[1]	
3 % Cr/CeO2 H2O2 assisted	Methylene Blue	59 %	100 min	[21]	
4 % Cr doped CeO2+ H2O2	Rhodamine B	98.3 %	30 min	This work	

The first order kinetics of the CeO2 and Cr doped CeO2 nanoparticles were calculated using the following equation(9) ln(CtC0)=−Kt

Where Ct and C0 represent the final and the initial dye concentration, K represents the rate constant and t the irradiation time [25]. The rate constants calculated were found to be 0.006499 min−1, 0.0496 min−1, 0.0524 min−1 for CeO2, 2 % Cr and 4 % Cr respectively and are shown in Fig. 10. The rate constant is found to decrease as the dopant concentration increases, indicating better photocatalytic activity of the doped samples. The enhanced photocatalytic activity is mainly due to the increased oxygen vacancies which increases the surface hydroxyl ions thereby increasing the degradation efficiencies.Fig. 10 Linear fit of ln (C/Co) with irradiation time of (a) CeO2, (b) 2 % Cr, (c) 4 % Cr, (d) radical scavenging of 4 % Cr by isopropyl alcohol.

Fig. 10

To identify the active radical participating in the Rhodamine B degradation reaction, radical scavenging tests were performed for 4 % Cr doped CeO2. The tests were performed using isopropyl alcohol which acts as a best OH− radical scavenger. It was found that the degradation efficiency was greatly reduced upon the addition of isopropyl alcohol. The addition of isopropyl alcohol neutralizes the active OH – radicals which are highly responsible for the degradation process. On neutralizing OH− radicals more H2O2 molecules are produced. H2O2 molecules possess a self-scavenging property when not added in appropriate amounts thus, producing per hydroxyl radicals [22]. Thus, the radicals scavenging test done using isopropyl alcohol confirms that OH− radicals effectively take part in the degradation mechanism.

4 Conclusion

Cr doped CeO2 nanoparticles were successfully prepared by a simple cost effective chemical precipitation method and were characterized by a number of techniques. Cubic fluorite structure with space group Fm 3m was confirmed from XRD for all the samples. The Raman active mode at 469 cm−1 also confirmed the cubic fluorite structure of the synthesized samples. Spindle shaped structures with sharp edges were observed specifically for 2 % Cr doped CeO2 which were examined through FESEM analysis. The bandgap values are calculated from the Tauc plot and a slight red shift in wavelength is noted for doped samples indicating increase in particle size. The Ce–O stretching vibrations were confirmed through a band obtained at 447.49 cm−1 from FTIR. Highest degradation efficiency of about 98.3 % was observed for the 4 % Cr doped CeO2 nanoparticles in a very short time interval of about 30 min. The highlight of this work is almost complete degradation of Rhodamine B dye is achieved over a small irradiation time indicating enhanced photocatalytic performance of Cr doped CeO2 nanoparticles. Thus, Cr doped CeO2 nanoparticles serves as a potential candidate in degrading contaminants over minimal irradiation time.

CRediT authorship contribution statement

Veena B: Methodology, Conceptualization. Seetha M: Validation, Supervision, Project administration.

Declaration of competing interest

There are no conflicts of interest to declare.
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