One-dimensional oxide material and its nano structure have attracted great attention because of their remarkable applications in many fields of opto-electronic and electrochemical devices^[1]. Great progress has been achieved on the different synthesis approaches for one dimensional nano structure and usually they require surfactant and precipitant in a series of complicated procedures. Cerium oxide (CeO₂) is one of the excellent oxides materials and most important for its promising application as catalyst^[2].

Various controlled synthetic agents were already reported and indicate that various formation process of CeO₂ nano structure and it involves nucleation and aggregation of different sizes of this oxide nano structure^[3]. This cerium (IV) oxide source materials possess high ionic and electronic conductivity and have attracted a lot of attention because its desired size and shape can be easily produced to control processing parameter like pH, reaction temperature, reaction time, precipitants concentration and types of solvents^[4].

CeO₂ and its doped nano structure have attracted much attention due to improvement in redox properties, transport properties and the surface-to-volume ratios with respect to bulk material^[5]. There is a lack of information on the effect of different precipitants to study the structure, morphology and optical properties by hydrothermal processing and how precursor concentration affects the different doped ceria and its oxide nano particles^[6]. Like those composites, catalysts are usually provided to prepare active materials independently its emission and excitation wavelength^[7]. Elimination of organic pollutants through reduction and oxidation reaction has been applied to provide outstanding photo chemical activity in a number of reactions, CO₂ reduction as well as in a series of degradation reaction concerning several pollutants such as formaldehyde, phenol, toluene and organic dyes^[8-10]. Controlled morphology is a facile method to prepare CeO₂ sphere with high efficiency on photo catalysis under visible light. Different catalysts are widely used to control photo catalytic behaviour and that materials act as an oxygen buffer by releasing and up taking oxygen through redox process and increases efficiency of catalyst by enlarging the air to fuel operating window^[11].

The active molecules were prepared by various chemical routes. In recent years, along with, the application of nanoscience using various metals such as magnesium oxide (MgO), nanosilicon (Si), silver oxide (Ag₂O), zinc oxide (ZnO), copper oxide (CuO), gold (Au), titanium dioxide (TiO₂) and calcium oxide (CaO) were studied^[12]. The one of the anatase phase TiO₂ metal oxide nano particles was synthesised facile solutions phase method without use of surfactant from titanium isopropoxide and different alcoholic medium (methanol, ethanol, propanol and butanol) and also demonstrated their excellent performance in photo degrading methylene blue and bisphenol aqueous solution by using UV-A light irradiation^[13].

Qi et al. synthesized a series of fullerene (C₆₀)-modified anatase TiO₂ (a-TiO₂) nanocomposites by an effortless solution phase method and also investigated photocatalytic degradation of methylene blue (MB) under UV-A light irradiation^[14]. Zhang et al. successfully reported the photodegradation of methylene blue (MB) solution under a simulated solar light irradiation by Er³⁺:YAlO₃ loaded. The relationship between the volume of loading and the rate of deterioration was also studied^[15]. Sim et al. compared unimodal and bimodal nano porous silica structures in Ag NPs and discovered that the bimodal nano porous structure had a higher absorption potential and faster adsorption rate^[16].

Recently, ceria nanoparticles were synthesised by various methods such as combustion^[17], the spray diffusion method^[18], the reverse micelles/surfactant assisted method^[19], flame spray pyrolysis^[20], the sol-jel method^[21], homogeneous precipitation^[22], the sono chemical-microwave method^[23] and hydrothermal method^[24-27]. The hydrothermal method has fascinated and created a lot of interest to produce nanoparticles of a desired size and shape, owing to the ease controlling of processing parameter like solution pH, reaction time, reaction temperature, solute concentration and solvent used^[4]. Nanoparticles with small grains and high purity can be achieved due to homogeneous nucleation in hydrothermal synthesis^[28].

The successful preparation method of nanoparticles consolidates its atmosphere that small particles condense from the vapour and then filled with oxygen which promotes rapid oxidation of oxide nanoparticles. It has strong particles which has agglomeration during the oxidation step and difficulty on controlling the concentration of the precursor, although its different morphological nano structure such as nano rods, nano wire, and nano cube has already been reported and synthesized by various methods^[29].

This work describes hydrothermal synthesis which is an attractive method for the preparation of crystalline cerium (IV) oxide nanoparticles. It has a simple crystal structure and it has good solid solubility and many dopants^[30]. To the best of our knowledge one step-controlled synthesis of CeO₂ and its doping nano structure has not been reported to date. Hence, the paper describes a simple surfactant-free, hydrothermal synthesis route to the synthesis of CeO₂ nano structure^{[31, 32]}.

Many researchers focus on the selection of active components and the role of rare-earth oxides as catalysts. However, there is little research on the effects of pore structure, particle size, crystal surface area and other adsorption mechanisms on the performance of catalysts and the structure–activity relationship. Such improvement mechanisms could be due to the improved light absorption in the visible-light region induced by localized surface plasmon resonance (LSPR) and the efficient interfacial separation and transport of charge carriers in Ag/Ag₂S/Bi₂MoO₆^[33]. The extraordinary photocatalytic property is ascribed to the Z-scheme hetero-structure with unique core–shell architecture that realizes compactly interfacial contact between the components for efficient separation of photo-excited carriers, strong visible-light absorption, as well as possesses the strong oxidation ability of the photo-excited hole, and the high reduction capacity of photo-excited electrons^[34]. Superoxide free radicals ( ) and photo-excited holes (h⁺) were determined to be the main active species in degrading MB dyes by the results of absorption of photo catalysts^[35]. The study on photocatalytic mechanism demonstrates that superoxide free radicals ( ) and photo-generated hole (h⁺) are dominantly responsible for the MB dye degradation, therefore, huge absorption potential is utilized as a durable and highly active photocatalyst for wastewater treatment^[36].

Cerium oxide NPs were synthesized using analytical-grade cerium (III) nitrate (Ce(NO₃)₃·6H₂O, Sigma Aldrich, 99%) and sodium hydroxide (NaOH, Sigma Aldrich, 99%) without further purification and ultra-pure water.

The use of hydrothermal synthesis reduced the number of intermediate products. The technique of ceria synthesis has a significant impact on CeO₂'s photocatalytic activity. In the photocatalytic process carried out under visible and solar light, a rise in the specific surface area, a decrease in the pore size, and an increase in the quantity of oxygen vacancies resulted in a high rate of hydrogen generation.

Analytical grade cerium nitrate hexa hydrate (Ce(NO)₃·6H₂O) source material was dissolved in 75 mL of ultra-pure water (1 mmol solution of 0.4342 g was added), and the solution was stirred at room temperature for 15 min. 0.24, 0.20 and 0.16 g of NaOH pellets dissolved in ultra-pure water while the drop wise addition of NaOH solution with continuous stirring (stirring was about 30 min) been made. After 30 min, white precipitant was obtained and the pH noted was 12.8 ue to the dissolved oxygen present in air changes Ce(III) to Ce(IV)^[37]. By using hydrothermal synthesis we got the fine powder under the autoclave with a Teflon lining, which was kept at 160 °C for 12 h. After that autoclave was cooled to room temperature, the yellow precipitate was collected by centrifugation for 6 min with 7500 rpm. And the collected powder was washed with ultra-pure water and ethanol and dried at 80 °C for 12 h. The collected powder was calcined at 350 °C for 1 h to obtain for further crystallization. Different molar concentrations of NaOH (4MMOL, 5MMOL and 6MMOL) were prepared and its mechanisms were reported here^[38].

${\rm{C}}{{\rm{e}}^{3 + }} + 3{\rm{O}}{{\rm{H}}^ - } \to {\rm{Ce}}{({\rm{OH}})_3}, $ (1)

$4{\rm{Ce}}{({\rm{OH}})_3} + 2{{\rm{H}}_2}{\rm{O}} + {{\rm{O}}_2} \to 4{\rm{Ce}}{({\rm{OH}})_4}, $ (2)

${\rm{Ce}}{({\rm{OH}})_4} \to {\rm{Ce}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}}. $ (3)

The crystalline properties of the synthesized nanoparticles were studied by XRD using a Bench X MiniFlex600 equipped with graphite monochromatized Cu Kα radiation (λ = 1.540 Å).

An accelerating voltage of 40 kV and emission current of 30 mA were adopted for the measurements. In addition to XRD, Raman spectroscopy measurements were also performed to confirm the structure of the nanoparticles. The dispersive Raman spectrometer (Bruker SENTERA 200LX model) employs a solid-state laser with an excitation wavelength of 532 nm. Typical Raman measurements were carried out in the spectral range between wave number 200 and 1100 cm^–1. The morphology of the crystal was characterized by a field emission scanning electron microscope (FESEM). The FESEM measurements were performed by a JSM-7800F (JOEL, JAPAN). The maximum working voltage of 15 kV and 10 mm working distance was used during the measurement. The FESEM equipment was also furnished with an EDX spectrometer that was used for elemental analysis. X-ray photoelectron spectroscopy (XPS) measurements carried on a multi-probe X-ray photoelectron spectroscopy (XPS) (Omicron Nanotechnology, Germany). XPS measurements were analysed as individual components using Casa XPS software (Casa Software Ltd). The binding energies of the obtained data were calibrated with respect to carbon C 1s peak at 284.6 eV. Absorption spectra of the samples in the diffused reflectance spectrum (UV–vis DRS) mode were recorded in the wavelength range of 200-1000 nm using a spectrophotometer (Lambda 650S- perk Elmer). From the adsorption edge, the band gap values were calculated by extrapolation.

Photocatalytic activity of pure CeO₂ nano particle were examined by the rate of degradation of MB under the effect of UV-B irradiation. All photocatalytic reactions were performed out in a photocatalytic reactor system, which consists of a cylindrical borosilicate glass reactor vessel with volume of 250 mL, a cooling water jacket, and a UV-B lamp (8 watts medium pressure mercury lamp, Institute of Electric Light Source, Beijing) positioned axially at the centre as a visible light. The reaction temperature was kept at 25 °C by circulating the cooling water. A special glass frit as an air diffuser was fitted at the reactor to uniformly spread the air into the solution. The reaction suspension was prepared freshly for each cycle by mixing 250 mg of catalyst with 250 mL of methylene blue at a starting concentration of 5 mg/L. After the degradation reaction, filtration was done for all samples using a syringe and syringe filter 0.45 µm to remove any precipitated particles. The filtrate was analysed by an UV–vis spectrometer (UC-2450-SHIMADZU). The MB’s maximum characteristic absorption wavelength was positioned^[39].

The synthesized CeO₂ NPs was characterized by using X-ray diffractometer to study the structural properties of the sample. The XRD patterns of as synthesized sample was shown in Fig. 1. By using JCPDS file no: 81-0792, it was in good agreement with results and its diffractions are equal to (111), (200), (220), (400) and (311) planes. The diffraction peak absorbed in Fig. 1 can be indexed to pure cubic CeO₂ and its lattice parameter is a = 5.5425 Å. By using Debye Scherrer’s formula, the average grain size was calculated^[40].

$D = \alpha \lambda /\beta \cos \theta ,$ (4)

where D is the average grain size, α is a geometric factor (here, α = 0.9), λ is the wavelength of X-rays used for the measurements (λ = 1.54056 Å), β is the FWHM of the diffraction peaks, which can be measured from the XRD peaks and θ is the diffraction angle.

Table 1 displayed the crystalline structure and lattice parameters of CeO₂ NPs from XRD results. From Eq. (4) the calculated grain size are 8.2, 6.2 and 6.6 nm for different molar ratio (1 : 6), (1 : 5), (1 : 4) of NaOH respectively. The crystal size was reduced to 6.2 nm by lowering the sodium hydroxide concentration, but when the concentration is lowered higher, the crystal size gradually increases, it shows that 5 mmol concentration of NaOH is suitable to get the optimized crystal size of nano particles. The optimized concentration of NaOH acts as the oxidizing agent to get Ce (IV) oxide, which was confirmed by JCPDS file no: 81-0792 and elemental X-ray analysis.

The morphological studies of as synthesized CeO₂ NPs sample and the nano structures of the sample were examined by SEM shown in Fig. 2.

The mechanism of CeO₂ nano particles gives poly crystalline pore evaluation with crystal size ranges from 8.2 to 6.6 nm for different concentrations of NaOH solution. Figs. 2(a) and 2(c) show the uniform and large grain size with a sphere-like structure in the (1 : 6) and (1 : 4) concentration. It can be seen that there is some agglomeration due to strong attractive force among the particles, which develops thicker and longer nano particles that increases the size^[41]. Another reason for the agglomeration is a fall on the presence of a high concentration of the hydroxyl group on the surface of its hydrothermal synthesis^[42-44]. The proportion of cerium nitrate hexahydrate source material, continuously dehydrate and oxidize with the support of NaOH concentration, the ion regulates the release of oxygen by Ce(IV) during the synthesis. As a result, Fig. 2(b) depicts the particle size distribution with CeO₂ NPs as the average size.

As seen in Fig. 3, the EDAX analysis showed the peaks of the elements and percentages of cerium and oxygen concentrations for the different concentrations of NaOH. There was no such a difference in the elemental analysis and its proportions, which helps in determining the stoichiometry elements in CeO₂ NPs. The information ensured that the cerium oxide formed by hydrothermal synthesis was CeO₂ nanoparticles.

Optical properties of the synthesized CeO₂ NPs studied by using UV–vis DRS. Fig. 4 shows the absorbance spectrum of synthesized CeO₂ NPs 200–800 nm wavelength range. The absorption edges of the CeO₂ nano cube were found near 440–443 nm range and a strong red shift is shown by the spectra.

The optical absorption near band gap energy E_g was calculated by using Tauc’s formula is^[45]:

${(\alpha h\nu )^2} = A(h\nu - {E_{\rm g}}), $ (5)

where α is the absorption coefficient, hν is the incident photon energy, A is the constant.

For direct transition, the plots of ( )² versus for CeO₂ NPs are shown in Figs. 5(a)–5(c). The direct band gap energy E_g was found from the intersection of extra polated linear portion. Figs. 5(a)–5(c) displayed the band gap energy values are 2.88, 2.82 and 2.87 eV for CeO₂ NPs with molar ratios (1 : 4), (1 : 5) and (1 : 6) respectively. The bandgap values suggested the semiconducting nature of synthesized CeO₂ NPs^[46].

Figs. 5(a)–5(c) depicted that the incorporated oxygen has improper light absorption of capacity of synthesized ceria nano particles. Stronger absorption lies in the region of 200 to 450 nm and the higher absorption of this region clearly states that ceria nano particle has a lower dye loading ability; its photocatalytic activity conforms with this improvement in light absorption of capacity in our second sample, which is excellent, and is mainly due to the incorporation of oxygen inside the sites^[47].

The composition and purity of the as-prepared ceria sample were studied by XPS analysis as shown in Fig. 6. Peaks of Ce 3d, O 1s, C 1s, and Ce 4d can be analysed using CASAXPS software. There were no peaks obtained for Na 2p, which indicate that there were no impurities of NaOH. XPS analysis also confirmed that as-synthesized nano particle contains Ce⁴⁺ ion and oxygen vacancies, which is shown in the normal mode of Fig. 6(a). However, the following figure specifically defined the binding energy peaks, which were consistent with previously published findings labeled in our Fig. 6(b) – the final state of Ce^4+[48]. From Fig. 6(b) the core level Ce 3d spectra of CeO₂ shows six contributions in which two types of Ce that is Ce(III) and Ce(IV) are presented. The peaks appeared at 925 and 891 eV, together with 922 and 897 eV, are weaker than 916 and 885 eV, and confirms the presence of Ce(IV). The high intensity and area of 925 eV can be attributed to the interaction between Ce and its surrounding atoms. Ce(III) can be considered as an indicator for the photo reduction reaction^[44]. From Fig. 6(c), it is noted that the O 1s spectra of the products consist of three peaks at 529, 530.5, and 531.5 eV respectively. The peak at 529 eV can be attributed to lattice oxygen ion in CeO₂ like that the peak at 530.5 eV originated from absorbed oxygen another peak at 531.5 eV assigned to lattice oxygen ion if the synthesized cerium oxide nano particles is as CeO₂^[49].

Fig. 7 exhibits that the Ce(IV) ion and oxygen vacancy contribute to the most intense peak around 466 cm^–1 of the Raman peak position because there is no weaker peaks where detected for this CeO₂ nanoparticles. It can be attributed there is no ascribed broad band spectrum due to oxygen vacancy and defects caused by CeO₂ nanoparticles. The peak at 466 cm^–1 is assigned to first-order Raman mode (F_2g) features of super oxides, which is already reported in the literature^[50]. The reported results matched well with our intense peak.

Among the three samples, the concentration of oxygen vacancy is bigger than the higher concentration of Ce(IV) ion, which is well matched with spherical formula detected for the second sample. So, we can conclude that even the energy band is nearly 2.5 eV and red shift results of Ce(IV) ion and oxygen vacancy lead to a decrease in the binding of nanoparticles, which is the main reason of hydrothermal synthesis and its reaction time of 12 h. It is believed that there is no degree of oxygen defect for these three samples and it confirms that the increase in reaction time from 6 to 12 h enhances the above said properties of CeO₂ nanoparticles^[51].

In order to investigate the photo catalytic degradation properties of as-prepared CeO₂ nano structured samples, MB was used as model pollutants to study the photo catalytic performance of CeO₂ NPs. The utmost absorptive energy of methylene blue was at 664 nm. Fig. 8 shows the schematic illustration of the possible photocatalytic reaction mechanism over CeO₂ NPs. The absorptive intensity of MB gradually decreases with the prolonging of the irradiation time when the mixed solution of MB and CeO₂ nanoparticles were exposed to UV-B light irradiation at room temperature. This result indicates that MB dye attains optimum degradation behaviour under the catalysis of CeO₂ nanoparticles.

More importantly, the CeO₂ (1 : 4, 1 : 5, 1 : 6 molar ratio of NaOH) nanoparticles dramatically enlarged the specific surface area calculated by BET analysis to 488, 492 and 490 m²/g, together with an increase in micropore volume from 0.010, 0.310 and 0.206 cm³/g, which is, in fact, much higher than the reported semiconductor-graphene composites or most of the porous photocatalysts^[52].

The improved photocatalytic activity of CeO₂ (1 : 4, 1 : 5, 1 : 6 molar ratio of NaOH) nanoparticles under UV irradiation is attributed to high-surface-area, enhanced absorption of MB dyes, and more efficient separation of photogenerated electron–hole pairs. The transfer of photogenerated electrons from the surface of the samples reduces the possibility of decomposing Ce⁴⁺ for the recycling of CeO₂ photocatalyst^{[53, 54]}.

Figs. 9(a)–9(c). Photocatalytic degradation of absorbance spectrum of MB in the presence of CeO₂ (1 : 4, 1 : 5, 1 : 6 molar ratio), (catalyst dosage – 250 mg/250 mL, concentration of MB = 5 mg/L; light source = UV-B). The photo-catalyst produces surface oxidation to eliminate the colour effect of MB dye such as organic compounds when it is exposed to UV light. By applying the principle of degradation, dissolving in the air, removing the colour is possible. Fig. 9(a) shows the shift in the absorbance wavelength is the energy that makes electrons jump up by UV light.

In order to transfer electrons from the valence band (VB) to the conduction band (CB) in CeO₂, the energy of light photons should be at least equal or higher than the energy of a band gap in the photocatalyst. CeO₂ has some energy (2.88 eV = 3.2 × 1.6 × 10^–19 J) for (1 : 6) ratio, the necessary wavelength is nearly 430 nm, which tells us that the UV light needed this much of amount of energy to activate the photo catalyst for the degradation^[55].

CeO₂ nanoparticles as photocatalysis effectively detoxify noxious organic pollutants. UV is used to create electron–hole pairs in the semiconducting oxide of CeO₂ NPs.

Electrons can react with oxygen adsorbed on the photocatalyst surface and then the formation of superoxide anion radicals is observed according to the following reaction:

$ {\rm{Ce}}{{\rm{O}}_2}\left( {{\rm{e}}^ - _{{\rm{CB}}}} \right){\rm{ }} + {\rm{ }}{{\rm{O}}_2}^ - \to {\rm{Ce}}{{\rm{O}}_2} + {\rm{ }}{{\rm{O}}^{2 \bullet - }} $  ()

The radical species then attack the organic molecule, which is eventually oxidized to CO₂, H₂O, and HCl if the organic molecule contains chlorine^[56].

One of the most critical controlling elements in the generation of reactive oxide species is the location of edges in CB and VB (ROS). Because the locations of CB and VB in CeO₂ include the possibility for the generation of hydroxyl radicals and superoxide anion radicals (Fig. 8). The major reactive oxygen species (ROS) produced in the photocatalytic system in water oxidation by photogenerated holes (h⁺) and oxygen reduction by photoexcited electrons (e) in CB are hydroxyl radicals (OH•) and superoxide anion radicals (O^2•). These species, like hydroperoxide radicals (HO^2•), are extremely reactive and can contribute to the photocatalytic degradation of contaminants in industrial effluent^{[57, 58]}. The absorbance of photocatalytic efficiency of CeO₂ NPs have more surface energy because of its smaller particle size, which could be already observed 8.2 nm. The band gap energy of CeO₂ nearly equal to 2.88 eV and its band edges shifted for the different NaOH concentrations of its preparation to yield larger redox potential. This indicates that the electronegative CeO_{2 surface} creates a stronger dipole moment with the oxygen molecules; the dye added for the degradation technique is MB. A hybridization effect can retard the recombination of the electron–hole pair and increases the inhibition properties of the photo catalytic nano particles of the prepared solution. Here, we concluded that the prepared CeO₂ nanoparticles have a synergetic effect of quantum confinement and a large surface-to-volume ratio which contributes to the enhanced photo degradation of MB. Fig. 10 shows the absorption characteristics of MB (C₀ = 5 mg/L) onto the surface of CeO₂ nanoparticles for the concentration of MB after dark adsorption and UV–vis absorption were investigated. After equilibrium in the dark adsorption, for every 30 min only 5% of MB was adsorbed on the CeO₂ NPs surface, which were already reported^[59].

Under light irradiation, the dyes are excited and then photo-generated electrons could easily transfer into the CeO₂ surface and effectively participated in the redox reaction of CeO₂ nanoparticles. Fig. 10 shows the variation of normalized concentration of MB (C₀ = 5 mg/L) with irradiation time under UV–light condition. This figure revealed the effect of absorptivity and photo reactivity resulting in an appreciable improvement in the photo degradation of MB with CeO₂ nanoparticles. Here CeO₂ nanoparticles served as an electron acceptor and conductor due to its one-dimensional structure, which helps or suppress charge recombination, enhancing charge separation efficiency and improving photo catalytic activities. The excellent electron factors can enhance the photo catalytic activity of nano-sized CeO₂, high dye adsorptivity and high electron conduction to remove the dye molecules by the process of photo degradation.

When the particle size of CeO₂ is decreased to a nanoscale and applied in the form of nanostructures, the characteristics of the material can be drastically altered. CeO₂ features such as lattice symmetry, cell parameters, and structural features are all affected by this process. In their bulk form, phases have a high surface energy and are unstable. The surface energy of nanostructured CeO₂ materials dramatically reduces as the particle size is decreased to the nanoscale, and nanostructured materials have a high level of stability^[60]. In addition, hydrothermally prepared ceria nanoparticles have a high catalytic activity, which is due to the rapid mobility of surface oxygen vacancies.

CeO₂ NPs were successfully synthesized using an efficient hydrothermal process with three different NaOH precipitating agent molar ratios. Depending on the reaction time and NaOH concentration, CeO₂ NPs had different crystal sizes and lattice parameters. Under MB, the prepared CeO₂ NPs' photodegradation activity was characterized and evaluated. The crystal size was reported to be 8.2, 6.2, and 6.6 nm, with a cubic structure. The band gap values of 2.87, 2.82, and 2.88 eV were measured using a UV–vis spectrometer to describe the optical properties. Its value is lower than the bulk value of CeO₂. In the (1 : 6) and (1 : 4) molar ratios, these CeO₂ NPs are all spherical, with heavy agglomeration. The grain size of the (1 : 5) sample is unique in nature, and its best morphological structure justified that this composition is stronger in terms of Ce(IV) and oxygen ion composition, and XRD tests concluded that it is a single crystalline cubic cerium (IV) oxide nano particle. Oxygen defects decrease as reaction time (6–12 h) increases in two samples (1 : 6) and (1 : 4), respectively. Under UV light irradiation, the photocatalytic degradation behavior of MB dye shows that hydrothermally prepared CeO₂ NPs are an efficient photo catalyst.