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Vol. 8. Issue 3.
Pages 2898-2909 (May - June 2019)
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Vol. 8. Issue 3.
Pages 2898-2909 (May - June 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.11.019
Open Access
Improved photocatalytic decomposition of aqueous Rhodamine-B by solar light illuminated hierarchical yttria nanosphere decorated ceria nanorods
C. Maria Magdalanea,b,
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Corresponding authors.
, K. Kaviyarasuc,d,
Corresponding author

Corresponding authors.
, G. Maria Assuntha Priyadharsinib, A.K.H. Bashirc,d, N. Mayedwac,d, N. Matinisec,d, Abdulgalim B. Isaeve, Naif Abdullah Al-Dhabif, Mariadhas Valan Arasuf, S. Arokiyarajg,
Corresponding author

Corresponding authors.
, J. Kennedyc,h, M. Maazac,d
a Department of Chemistry, St. Xavier's College (Autonomous), Tirunelveli 627002, India
b LIFE, Department of Chemistry, Loyola College (Autonomous), Chennai 600034, India
c UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology Laboratories, College of Graduate Studies, University of South Africa (UNISA), Muckleneuk Ridge, P O Box 392, Pretoria, South Africa
d Nanosciences African network (NANOAFNET), Materials Research Department (MRD), iThemba LABS-National Research Foundation (NRF), 1 Old Faure Road, 7129, P O Box 722, Somerset West, Western Cape Province, South Africa
e Department of Environmental Chemistry and Technology, Dagestan State University, M. Gadjieva, 43a, 367001 Makhachkala, Russian Federation
f Department of Botany and Microbiology, College of Science, King Saud University, P. O. Box 2455, Riyadh 11451, Saudi Arabia
g Department of Food Science and Biotechnology, Sejong University, Seoul, Republic of Korea
h National Isotope Centre, GNS Science, Lower Hutt, New Zealand
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We report the new synthesis route of hierarchical yttria (Y2O3) nanosphere decorated ceria (CeO2) nanorods from the precursor cerium nitrate and yttrium nitrate under hydrothermal method (HM). Synthesized nanorods (NRs) were analyzed by different techniques to investigate their textural morphology, size and shape of nanorods and crystalline structures, morphology growth, optical activity and sizes of the samples. The luminescence blue peaks were observed in the range of 450–490nm and green emission appeared at 533nm at the room temperature (RT), the presence of oxygen vacancies was confirmed. The photodecomposition nature of the prepared nanosamples was investigated by using the industrial effluent Rhodamine-B (RhB) dye in aqueous medium by the illumination of solar light. The photocatalytic degradation efficiency showed 95.8% for RhB after 240min. On illumination of visible light, the catalyst was found to tend to produce reactive oxygen species (ROS), which might account for the decay of dyes into small fractions. The enhanced photocatalytic effect of the synthesized NRs was primarily ascribed to the rising in the separation efficiency of electrons and holes.

Photocatalytic effect
Electron microscopy
Rhodamine-B (RhB)
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The industrial progress causes numerous severe environmental problems by releasing wide range of toxic and highly hazardous effluents to the environment [1]. Analysis of these contaminated materials in the surroundings is significant from pollution controlling point of observation. Dyes produce major dangerous polyaromatic effluent frequently available in the environment [2]. In the recent years, the continuous release of the toxic compounds from industries such as paints, leather, textile, printing inks, rubber, paper, plastics, art and craft, food, drug and cosmetics uses different types of synthetic dyes and pigments for coloring [3–5]. The dye effluents have unpleasant effect on living organism's due to their carcinogenic, cytotoxic and embryo-toxic to mammals and aquatic ecosystem [6]. The polyaromatic compounds and their derivatives have adverse effect on environment. Therefore, the degradation of such a compound is necessary and it is a challenge in the fields of catalytic and environmental science [7,8]. Advanced oxidation processes (AOPs) are efficient techniques for detecting and degrading the industrial effluents in aqueous solutions to protect the water resources and food provisions [9,10]. Recognizing and monitoring the toxic effluents and decolorization of organic pollutants are essential for industrial applications and ecological pollution control [11–13]. Nanomaterials have produced a core of consideration owing to their typical properties and distinguishing performance in a variety of fields such as human health and green application [14,15]. The widespread applications of nanomaterials are primarily measured due to their grain shape, tiny size, high reactive surface to volume ratio, and high surface movement [16–19]. All these properties make nanomaterials attractive in numerous fields [20]. Among the rare earth metal oxides (REMO), ceria (CeO2) is a mostly available noble metal oxide, because it has tendency to liberate and attract oxygen in the alternating redox reactions and becomes an exciting oxygen buffer source and its potent applications in the fields of green protection and in semiconductor industries [21]. Specifically, ceria in one-dimensional nanostructures has reached such potential owing to its shapes, grain size, phase and crystallographic attitudes [22]. To improve the structure and size of the ceria to utilize it in terms of environmental and other issues, a huge number of preparation procedures and mechanisms have been developed [23,24]. There are few reports on the synthesis of ceria-based nanomaterials and their photocatalytic effects of various industrial effluents which is Kasinathan et al. has reported the photodecomposition of organic synthetic pollutants RhB dye using UV simulated sunlight on ceria-based TiO2 nanomaterials. The photo-decolorization of dye using Ce–TiO2 nanocrystals occurred after 8h and the most intense absorption peak at 579nm was vanished and 99.89% of synthetic RhB dye degraded under solar irradiation [25]. Muduli et al. have discussed the mesoporous cerium oxide nanospheres and their visible-light driven photocatalytic degradation of RhB dyes [26]. Zhang et al. also synthesized CeO2 hierarchical nanorods and nanowires by electrochemical method with excellent photocatalytic activities. Both the synthesized cerium oxide nanorods and nanowires show higher photocatalytic nature toward methyl orange dye using visible light after 180min due to their larger surface area [27].

Gu et al., also reported CeO2 nanoparticles sensitized CdS nanorods as a photocatalyst using improved visible-light photocatalytic degradation of Rhodamine-B. The absorbance was decreased due to the adsorption of dye on CdS/CeO2 which also leads to degradation of dye nearly 96.6% after 48min by the direct illumination of visible light source [28]. Ma et al. has reported the preparation of Squama-like cerium-doped titania hierarchically nanostructured material for the degradation of RhB synthetic dye and catalytic CO oxidation and reported that the doping of ceria with titanium oxide increases the photocatalytic effect after 120min in the presence of visible light [29]. Maria Magdalane et al. have reported thermally stable La2O3 garlanded ceria nanomaterial and demonstrated the decomposition of RhB dye under visible light illumination. The synthesized photocatalyst considerably decompose the dye nearly 72% within 3h due to the higher adsorption capacity of catalyst and better electron hole pair separation in the presence of visible light [30]. Zhao et al. investigated the photocatalytic degradation of Rhodamine-B and safranin-T using MoO3:CeO2 nanofiber materials by continuous flowing mode using air (O2) as an oxidant [31]. The results confirm that the Rhodamine-B and safranin-T are decomposed effectively, and the removal efficiencies are 98.3% and 98.5%, respectively. In addition, the organic dyes are totally mineralized to simple organic species. Also, in our previous result, we have reported the heterostructured yttria sphere with cerium oxide nanoneedles for the photocatalytic degradation and catalytic reduction under UV light [32]. However, in the current result the preparation of ceria NRs incorporated by yttria in the form of 1-D nanomaterials by using hydrazine as reducing agent with more active morphology and its mechanism has not been reported so far.

In the present study, we have examined the preparation of hierarchical (Y2O3) yttria nanosphere decorated ceria (CeO2) NRs using hydrazine hydrochloride as reducing agent which changes the morphology than the previous work with the selected properties and applications [33–36]. Specifically, addition of inner transition metal oxide (IRMO) in the lattice ceria can enhance the strain inside the lattice and stimulate a creation of crystal defect, which can convert the material to a good semiconductor. Yttrium oxide (Y2O3) belongs to inner rare earth sequioxides series and exhibit extraordinary property, which shows potential material for radiation dosimetry. Yttria has efficient properties like a large bandgap, high refractive index, low dominant phonon energy, excellent thermal conductivity [37], and an extensive transparency range and can be easily doped with rare earth ions such as cerium. Both the metal oxide has a cubic crystal structure with very comparable lattice constants and trivalent environment. They have almost the same ionic radius, creates doping easier due to its little lattice mismatch among the two materials. Further, Y2O3 applications include luminescence [38], catalysis [39], sintering aid [40], electrical [41], mechanical [42], electronic [41], and thermal [41] resources. Even though Y2O3 presents great applicability, there is a lack of investigations on photocatalytic effect of yttrium oxide decorated ceramic ceria.

Many chemical and physical methods for the preparation of nanomaterials have been reported. Among them, a few are fashionable methodologies for generation of nanomaterials such as co-precipitation [42], sol–gel [43], hydrothermal [44], combustion synthesis [45], etc. Above all methods, the hydrothermal method has many advantages like high purity and efficiency of the sample, which has a major role in controlling active surface area, grain size and shape of nanomaterials. Hence in our present investigation, we have chosen hydrothermal method, for the synthesis of hierarchical (Y2O3) yttria nanosphere decorated ceria (CeO2) nanorods. Also, we report the photocatalytic decomposition of the highly stable industrial effluent RhB dye. We used the world wide cheapest energy source for the decomposition of industrial effluent and prove that the binary oxide nanostructure can be used for solar light harvesting.

2Experimental procedure2.1Materials and methods

All the chemicals used in this investigation were purchased from (E-Merck 99.9%). Metal hydroxide was prepared from soluble precursor using simple chemical precipitation followed by hydrothermal method. For the preparation of Y2O3 decorated ceria heterostructured binary metal oxide NRs, the respective metal nitrate and hydrazine hydrochloride were taken as the starting material. 0.1M cerium nitrate [Ce(NO3)3] and 0.1M yttrium nitrate [Y(NO3)3] dissolved separately in double deionized water and stirred for 1h to obtain a clear solution. Equal quantity of both the prepared solution was mixed by constant stirring to get homogeneous solution. To this mixture solution 0.5M hydrazine hydrochloride was added as precipitant until the pH reaches about ∼10.5 which leads to the formation of corresponding metal hydroxide [46–48]. The pH value was recorded for the resulting solution using a digital pH meter. The concentration of hydrazine hydrochloride and pH of the mixture solution increase the position of yttrium ion in the crystal lattice of ceria and crystal homogeneity. This mixture solution was loaded into stainless steel autoclave (150ml) and maintained at 150°C for 5h. At the end, the autoclave was cooled, and the solution was removed from the yellowish precipitate carefully by filtration process, followed by washing with ethanol and distilled water. The obtained precipitate was calcined at 800°C for 3h in a high temperature muffle furnace to get a highly crystalline anhydrous heterostructured ceria/yttria binary metal oxide NRs. The resulting pale yellow solid precipitate was then collected by centrifugation followed by washing with deionized water and finally with ethanol. The sample was dried in desiccator for 2h and then it was collected.

2.2Characterization studies

The crystalline structure and phases of pure and Y2O3 decorated ceria heterostructured binary metal oxide NRs were examined on a Siemens D-500 diffractometer using Ni-filtered Cu-Kα radiation at λ=1.540Å in the 2θ ranges from 20° to 90° with a scan speed of 2° per minute via Bragg–Brantano configuration. Fourier transform infrared spectrometer (FTIR) (Excalibur Series FTIR) was used to identify the functional groups in the synthesized samples. The morphological structure of the synthesized nanorods was done by scanning electron micrograph (SEM) (JEOL JSM-5300 microscope with acceleration voltage15kV). The optical property of samples was examined by using UV–vis (Diffuse Reflection Spectroscopy) spectra of nanomaterials were recorded on a Shimadzu spectrophotometer (UV-2450) in the collection of 200–800nm range. The elemental nature of individual nanocrystals was determined by energy dispersive X-ray spectroscopy (EDX) analysis in scanning transmission electron microscopy (STEM) mode with a focused electron probe. High resolution transmission electron microscopy (HRTEM) was used to establish the crystal facets in individual Y2O3 garlanded ceria heterostructured binary metal oxide NRs. Measurement of the emission spectra of pure and Y2O3 decorated ceria heterostructured binary metal oxide NRs has been recorded using a Lab RAM HR (UV) spectrophotometer equipped with 325nm laser sources with CCD detector.

2.3Photocatalytic characterization

Decolorization of Rhodamine-B was investigated by using Y2O3 decorated ceria heterostructured binary metal oxide NRs in aqueous solution under sunlight. In this experimental procedure, 50mg catalyst was dispersed in 100ml of aqueous Rhodamine-B solution at preferred concentration, pH and irradiated with sunlight. The catalyst dispersed RhB solution was kept in the dark for few hours to attain adsorption–desorption equilibrium. H2O2 was extra added to boost the creation of more OH radicals during photodegradation, resulted in rapid decoloration of dye. The experiment was passed out by concurrent experience of sunlight through the 100ml of the catalysts-loaded Rhodamine-B aqueous solution under stimulated circumstances and air was purged to maintain catalyst well dispersed in the solution mixture. The catalyst-loaded RhB solution was illuminated under sunlight was done at 30min intervals. At given times, the photo-reacted solution of the centrifuged sample, from the sunlight illuminated solution 2ml were with-drawn at different time intervals which was analyzed by recording the changes in the absorption band maximum, using a UV–visible spectrophotometer [49].

3Results and discussion3.1Crystalline structure and phase diffraction of Y2O3 decorated ceria NRs

Crystalline structure and phase diffraction of hydrothermal route synthesized Y2O3 decorated ceria heterostructured binary metal oxide NRs in 1:1 were examined by X-ray diffraction (XRD) as shown in Fig. 1. From the diffraction pattern results, the obtained facets of both the metal oxide were sharp and the highly intense facets prove the highly crystalline nature of the sample. In our present results, ceria NRs have the (111) plane as a dominant and high intensity peak, which requires more energy to form oxygen vacancies on this major plane than the other less intense peak as mentioned in our previous work [42] and becomes highly stable. XRD pattern of the sample does not show any extra satellite peaks belonging to metal, metal hydroxide and Ce2O3, thus signifying a whole exchange of the system into cubic fluorite metal oxide forms. Insertion of yttrium oxide into the crystal structure of ceria does not modify the phases [50]. The observed peaks were representing the facets of either ceria or yttria. It proves the obtained diffraction pattern is well consistent with the cubic lattice phases for both metal oxides. The 2 theta values in the diffraction peaks for ceria appeared in the prepared samples at 28.68°, 33.36°, 47.56°, 56.4°, 59.44°, 69.81°, 76.57°, 79.89° and 88.92° with the various planes of (111), (200), (220), (311), (222), (400), (331), (420) and (422) (JCPDS No.43-0394) [43]. Yttria was detected in the resultant XRD pattern, at two theta values 20.1°, 33.37°, 35.32°, 37.9°, 40.01°, 41.91°, 43.7°, 57.01°, 59.44° and 79.9°, which correspond to the yttria phases (200), (400), (411), (420), (332), (422), (134), (622), (136) and (752) respectively [JCPDS No. 89-5591]. The average size of the particle of synthesized sample is evaluated by using Scherrer's formula, d=0.89λ/ß Cosθ; where, d is average particle size, ß is full width half maxima (FWHM), θ is Bragg's angle, λ is the wavelength of Cu-Kα radiation. Combine NRs show the higher peak intensity with less FWHM leading to small crystallite size. Calculations based on the (111) diffraction peaks in the XRD patterns correspond to that the grain sizes of Y2O3 decorated ceria heterostructured binary metal oxide NRs are 15nm. In this binary system yttria has less intense peak and its major peak at 29.4° merges with the peak of ceria at 28.6° and peak positions are slightly changed, which indicates that the formation of ceria becomes uncomplicated and expanded form cubic lattice rod like structure due to its small size. The obtained results show that the incorporation of rare earth metal oxide (yttria) in ceria matrix leads to changes in the ceria crystal growth with more active surface morphology [51–55].

Fig. 1.

XRD pattern of Y2O3 decorated ceria nanorods.

3.2Fourier Transform Infrared Spectroscopy (FTIR) analysis

FTIR analysis was carried out for synthesized Y2O3 decorated ceria heterostructured binary metal oxide NRs prepared by hydrothermal method to inspect the nature of functional groups is presented in Fig. 2. The broad peak at 522cm−1 is mainly due to CeO bond. The peaks in the lower region in between 450cm−1 and 1332cm−1 correspond to MO bond (MCe and Y) indicating the formation of ceria/yttria composite [29]. The broad band in the higher region at 3400cm−1 is chiefly due to the OH stretching vibration mode of adsorbed water molecules. This is due to the presence of more active surface area in the ceria/yttria sample leading to absorption of moisture [42,49]. The band around 1527cm−1 may be attributed to bending vibration of water molecule.

Fig. 2.

FTIR spectra of Y2O3 decorated ceria nanorods.

3.3Morphological property of Y2O3 decorated ceria NRs (HRSEM)

Heterostructured CeO2/Y2O3 NRs were examined by HRSEM analysis to identify size, morphology and microstructure which is shown in Fig. 3(a–f). The HRSEM micrographs of NRs show that the sphere like Y2O3 metal oxide is embed on the ceria matrix NRs. We observe from Fig. 3(a–c) at higher magnification that the well-aligned hierarchical (Y2O3) yttria nanosphere decorated ceria (CeO2) NRs were successfully produced. In our previous work, we reported the yttria nanosphere embedded on the needle shaped ceria [29], but the present report CeO2/Y2O3 binary metal oxides NRs are thicker which also indicates that the nanosphere of yttria is decorated above the surface of CeO2 NRs, however, the surface morphology was found to be different with high active surface area by using hydrazine as reducing agent than our previous work. HRSEM picture of the CeO2/Y2O3 NRs evidently shows that there is an aggregation as well as agglomeration of particles which indicate that yttrium ions are healthy dispersed on the ceria nanorod matrix. Fig. 3(d–f) sample established that the yttrium atoms were well and consistently isolated in the cerium nanorod matrix. The shape, size and morphology of the nanocomposites depend on the synthesis temperature, pH and the surfactant. Furthermore, OH ion concentration was found to play an important role in engineering matrix with the addition of lattice constants and oxygen variations by the Y2O3 decorated ceria heterostructured binary metal oxide NRs.

Fig. 3.

(a–f) HRSEM images of Y2O3 decorated ceria nanorods.

3.4HRTEM and SAED analysis of Y2O3 decorated ceria NRs

HRTEM image of the Y2O3 decorated ceria heterostructured binary metal oxide NRs is illustrated in Fig. 4(a–f). Ceria-based binary metal oxides prove the formation of (Y2O3) yttria nanosphere decorated ceria (CeO2) NRs with the size in the range of 10–40nm. Fig. 4(e) is the HRTEM image of CeO2/Y2O3 (1:1) at higher magnification proving the clear formation of ceria NRs on which yttria nanosphere is observed and the yttrium nanoparticles are anchored on the surface of CeO2 nanorod with a particle size of 10nm. It is indicated that the Y2O3 sphere is decorated on the outer surface of the ceria heterostructured binary metal oxide NRs in Selected Area Electron Diffraction (SAED) and indicates that the nanomaterials are in high crystalline order. Specifically, it is proved that the lattice planes of CeO2 NRs in cubic structure are shown in Fig. 4(f); HRTEM nature reveals the crystallite direction naturally exhibited in the nanostructures as shown in Fig. 4(b–d).

Fig. 4.

(a–f) HRTEM images of Y2O3 decorated ceria nanorods.

3.5Optical properties (diffuse reflectance spectra) of Y2O3 decorated ceria NRs

To determine the optical properties of Y2O3 decorated ceria heterostructured binary metal oxide NRs were analyzed by UV–vis (DRS) measurements and results are given in Fig. 5(a and b). The charge transfer from 2p orbital of oxygen atom to 4f orbital of cerium atom leads to strong absorption band as show in the sample. Tauc's relation was used to calculate the bandgap of synthesized samples. From the Kubelka–Munk function model, the reflectance (R) was converted into equivalent absorption coefficient by using the Eq., F(R)=a(1R2)/2R; where F(R) is Kubelka–Munk function, R, the reflectance and α, the absorption coefficient. Therefore, the Tauc's relation is given by Eq., [F(R)=A(Eg)n]; where n=1/2 corresponds to indirect bandgap and n=2 represents the direct bandgap. The plot between photon energy and (F(R) )1/2 gives indirect bandgap and (F(R) )2 vs. signifying a direct bandgap transition. Our results show the direct and indirect band gap value for synthesized sample is found to be ∼2.9eV and 2.7eV respectively. Y2O3 decorated ceria heterostructured binary metal oxide NRs show the high opacity in the blue shift and high transparency in the red shift. The addition of yttria content leads to decrease in the optical bandgap of ceria. This may be due to the changes in the structural defects or oxygen vacancies in the crystal structure in the synthesized samples [42,49].

Fig. 5.

DRS spectrum of Y2O3 decorated ceria nanorods (a) direct bandgap and (b) indirect bandgap.

3.6Photoluminescence (PL) spectra of Y2O3 decorated ceria NRs

The photoluminescence (PL) spectra of Y2O3 decorated ceria heterostructured binary metal oxide NRs was carried out to investigate the optical property of the sample in the excitation wavelength of 325nm at normal temperature as shown in Fig. 6. The broad and high intensity peaks were observed in the range of 400–600nm, which provide the information about the defects of oxygen level, surface defects, excitation and emission spectra of the pure NRs. The luminescence blue emission peaks were noticed in the range of 460–485nm and green emission peaks appeared at 531nm and small peak near 573nm at the RT. The blue emission may be due to the transitions from different defect levels of the 2p orbital of oxygen atom to 4f orbital of metal atom. The highly intense and strong peak at 531nm might be due to the green emission and it could denote a profound level of visible emission that is interrelated to the confined to a small area levels in the bandgap energy. A weak emission band observed at 573nm may be qualified to the oxygen positions. This green emission is emitted, due to the recombination course of photogenerated hole with the ionized charge of the defect [56].

Fig. 6.

Photoluminescence spectrum of Y2O3 decorated ceria nanorods.

3.7Degradation of Rhodamine-B using Y2O3 decorated ceria heterogeneous catalyst

Solar irradiation is a cost effective and environmentally – friendly energy source to overcome current environmental challenges which are faced by humankind. In our previous result, we discussed the heterostructured cerium oxide/yttrium oxide nanocomposite for the photocatalytic degradation and catalytic reduction under UV light [29]. In this present experimental work, the same industrial effluent Rhodamine-B was chosen as a pollutant for the degradation from textile industries due to its high stability. Hierarchical yttria nanosphere decorated ceria (CeO2) nanorod (50mg catalyst/5ml of 10% H2O2) with 100ml of Rhodamine-B (10ppm) on illumination of sunlight leads to decomposition of dye. The absorbance of dye solution at various time intervals was measured using UV–vis spectrophotometer [50,51]. The pH of the mixture solution was maintained as neutral. The UV absorption spectrum of Rhodamine-B (RhB) in the presence of catalysts shows at 554nm. Decolorization of Rhodamine-B dye with and without catalyst was carried out, recorded with time (t) in min. The wavelength absorption band was observed at 554nm and gradually decreases under sunlight as shown in Fig. 7(a–c), which corroborates the photodegradation process. Strength of the pollutant RhB after each 30min at various intervals was carried out for the comparison of the absorbance intensity [42]. The plot between C/Co vs. the time under solar light exposure proves decomposition of dye and compares the efficiency of the decolorization under diverse circumstances. The efficiency of degradation of dye was nearly 95% after 240min by irradiation of light Fig. 7(b and c), thus signifying the possible use of these resources as catalyst. The degradation efficiency for the blank and with hydrogen peroxide is less than 5%. Gu et al. used CeO2 nanoparticles sensitized CdS nanorods as a for the degradation of Rhodamine-B. The absorbance was decreased nearly 96.6% after 48min by the presence of visible light source [28]. Ma et al. has reported the preparation of Squama-like Cerium-Doped Titania hierarchically nanostructured material for the degradation of RhB synthetic dye after 120min in the presence of visible light [29]. Kaviyarasu et al. degraded the same organic pollutants RhB dye after 8h using UV simulated sunlight on ceria-based TiO2 nanomaterials [51]. In the percent study, we examined the photocatalytic nature of the sample in the presence of the solar light which is a worldwide cheapest energy source [57,58]. The catalyst degrades the RhB pollutant after 240min. The catalytic properties of the synthesized 1-D ceria-based samples depend on the ability of the catalyst to distinct the electron–hole pairs [59]. The separation of rate of electron–hole pair and recombination is the further most essential feature in the photocatalytic nature. Our results show that there is a reduction in the absorption maximum for the degradation of aqueous medium of RhB solution due to the catalytic nature induced by solar light irradiation. The shape and size of the NRs stimulate the feasibility of photocatalytic degradation effect of organic pollutants, due to synergistic approach. The photocatalytic activities of the materials were evaluated by the photodegradation of Rhodamine-B (RhB) dye and in aqueous solution under simulated solar light irradiation. The result of photodegradation of RhB showed that the samples exhibit much higher photocatalytic activity. Moreover, the results indicated that the synthesized materials had good stability and reusability [60,61]. The excellent photocatalytic nature of yttria decorated ceria nanorods for RhB degradation may possibly due to the (i) higher oxygen absorbing or releasing capacity of yttria decorated ceria nanorods, (ii) the synergic interactions between yttria and ceria effectively prohibited the photo-corrosion and leaching of both the metal oxide and (iii) efficient charge partition reduced the recombination rates of excitons.

Fig. 7.

Effect of photocatalytic degradation plot for RhB dye: (a) absorption changes of RhB dye with catalyst-H2O2 at pH 7.0 under sunlight irradiation and in the dark at different time intervals, (b) C/Co vs. time (min) for the decolorization of dye, and (c) percentage of efficiency, in the presence of catalyst-H2O2 at pH 7.0.

3.8Mechanism of photocatalysis

In photocatalytic industrial effluent degradation process, Y2O3 decorated ceria heterostructured binary metal oxide NRs semiconductor catalyst was illuminated with sunlight source with higher energy photons than the bandgap. In the case, heterogeneous catalyst system, the formation of an electric field at the interface between the catalyst leads to the adsorption of pollutant on the surface of the catalyst. The first step is the higher energy photons absorbed by catalyst, which results in the generation of electron–hole (e/h+) pairs on the photocatalyst[62]. The valence band electrons (VB) absorb photon and move to the higher energy level conduction band (CB) by leaving positive hole in the valance band. These electron/hole pairs are utilized for next redox reaction in the whole system[63]. The hole in the VB is captured by the water molecules in the mixture solution thereby forming reactive hydroxyl radicals, which is a second strongest oxidizing agent. The generated reactive hydroxyl radicals are responsible for organic pollutant degradation. On the other hand, the photoexcited electron in the CB reacts with the dissolved oxygen forming superoxide which again intermingles with the proton, yielding the hydroperoxyl radical followed by the creation of hydrogen peroxide. Then hydroxyl radical is formed by attack of the photogenerated electron to the hydrogen peroxide [61]. Muduli et al. proposed the mechanism for the degradation of RhB after 6h under visible light in the presence of ceria, which involve the hydroxyl radicals as the active species for pollutant decomposition [26]. All the generated ROS hydroxyl radicals oxidized the organic contaminant into non-toxic small units. Usually the varieties of organic dyes experience the photocatalytic degradation by the influence of sunlight irradiation, which is ascribed to their oxidation by the generated ROS[64]. Yttria changes the morphology of ceria and increases the active surface area for the effective generation of excitons among the binary metal oxide system[65]. Hence, the RhB photodegradation mechanism could be explained by the electron–hole (e/h+) separation between conduction and the valance band of the photocatalyst as shown in Fig. 8. Also, the entire photodegradation course of action is summarized in the following equations:

h+ (VBY2O3)+H2OY2O3+H++HO
RhB+(O2+h++HO) (ROS)mineralized productsCO2+H2O+NO2

Fig. 8.

Photocatalytic mechanism of degradation dye of RhB under visible light irradiation.


The electron–hole pair generated by absorption of photons may undergo recombination. To reduce the recombination process, the electron acceptors like hydrogen peroxide was added, which was ready to accept the photoexcited electron and decomposed to hydroxyl radicals leading to decrease the recombination. The photogenerated hydroxyl radicals will degrade the synthetic dye pollutant

H2O2+e (CB,CeO2)OH+OH

The hydroxyl radical arrangement will lead to the generation of weak oxidant HO2, which merge with hydroxyl radical and forms oxygen molecule and water. On the further way hydroxyl radicals come together and form H2O2. This phenomenon reduces the effectiveness of decomposition



Y2O3 decorated ceria heterostructured binary metal oxide NRs were synthesized by using hydrothermal method followed by calcination. XRD dimensions support that these synthesized materials are made up of the cubic crystalline phase with high active surface area. The HRTEM image ceria-based binary metal oxides prove the formation of (Y2O3) yttria nanosphere decorated ceria (CeO2) NRs with the size in the range of 10–40nm. Photocatalytic degradation of dye was carried out on NRs surfaces. The significant outstanding properties such as remarkable charge carrier transport features make NRs promising candidates for their use in catalytic applications. The Y2O3 decorated ceria heterostructured binary metal oxide NRs exhibited significant changes in photocatalytic activities in neutral medium due to their higher-level generation of negatively charged hydroxyl radicals, which increase the degradation of Rhodamine-B dye. We believe that the binary nanocomposites will find more applications in the field of bio-electronics and for the treatment of contaminant wastewater applications.

Conflicts of interest

The authors declare no conflicts of interest.

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