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Vol. 9. Issue 2.
Pages 1819-1830 (March - April 2020)
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Vol. 9. Issue 2.
Pages 1819-1830 (March - April 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.12.014
Open Access
Influence of synthesis conditions on physico-chemical and photocatalytic properties of rare earth (Ho, Nd and Sm) oxides
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Katabathini Narasimharaoa,
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nkatabathini@kau.edu.sa

Corresponding author.
, Tarek T. Alia,b
a Chemistry Department, Faculty of Science, King Abdulaziz University, 21589 Jeddah, P.O. Box 80203, Saudi Arabia
b Chemistry Department, Faculty of Science, Sohag University, Sohag 82524, Egypt
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Tables (3)
Table 1. Crystalline phases and their sizes obtained from XRD analysis.
Table 2. Textural properties of the synthesized rare earth samples.
Table 3. The rate constants and t1/2 values for the synthesized rare earth oxide catalysts.
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Abstract

Three different rare earth metal (samarium, neodymium and holmium) oxides were synthesized by adapting organic and inorganic routes. The influence of synthesis route over the physico-chemical characteristics of the rare earth metal oxides was studied using X-ray diffraction, FT-IR, thermogravimetric analysis, microscopy (FESEM and HRTEM), N2-physisorption and diffusive reflective ultraviolet-visible spectroscopy techniques. The XRD, electron microscopy and N2-physisorption results indicated that the samples synthesized by organic route possessed smaller crystallite/particle size and high surface area with macro size pores compared to the samples synthesized by inorganic route. The synthesis conditions also influenced the morphology of the samples. The samples synthesized by organic route possessed sheets like morphology with large spaces in between the sheets, in contrast highly agglomerated particles were observed in case of samples synthesized by inorganic route. All the synthesized rare earth oxides were utilized as photocatalysts for degradation of crystal violet dye under visible light irradiation. The samples synthesized by organic route exhibited high photocatalytic efficiencies. Samarium oxide synthesized using organic route offered the superior photocatalytic performance as this sample possessed low band gap energy, high surface area, pore volume and presence of surface reactive −OH groups. In addition, the synthesized rare earth metal oxide catalysts exhibited excellent recyclability for photocatalytic crystal violet degradation.

Keywords:
Rare earth metal oxide
Organic route
Inorganic route
Structural properties
Photocatalytic activity
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1Introduction

Rare earth oxides have been broadly utilized in different research areas due to their unique and interesting properties [1]. Samarium, neodymium and holmium oxides are important rare earth materials because of their suitability for optical, ceramic, solar cells, nano-electronics, semiconductor, sensors and catalytic applications [2,3]. The diluted magnetic semiconductors are widely studied materials due to their high thermal stability and interesting thermal, electrical, and magnetic properties [4–6]. These materials are utilized in a wide range of applications starting from household materials to high-tech space applications [7]. The diluted magnetic semiconductors are semiconductors doped with rare earth materials to offer the semiconducting characteristics in addition to their distinguished electronic, optical and magnetic properties [8]. The samarium and neodymium oxides showed excellent catalytic activities in numerous chemical processes, including NH3 synthesis and oxidative coupling of CH4[9]. Rare earth metal oxides with nano porous structure of are particular interest; this is due to the lowered crystallite size to nanometers and increase in surface to bulk atoms ratio. These two characteristics could contribute to enhancement in their catalytic functionality [10–14]. The rare earth metal incorporated luminescent nanomaterials, which were synthesized by conventional precipitation method have been extensively reported [15–20], however, very few studies are devoted to study the synthesis and physicochemical properties of pure rare earth oxide nanomaterials [21].

It is known that the trivalent state is the most stable in rare earth oxides and therefore sesquioxides (R2O3) exists for all rare earth metals. However, some preparation conditions allow lower or higher oxidation state [22]. The R2O3 normally crystallize in three forms, A-type (hexagonal, e.g. Nd2O3), B-type (monoclinic, e.g. Sm2O3) and C-type (cubic, e.g. Y2O3) structures, depending on the ionic radius of the rare earth metal. However, if there is any foreign metal ion presents in the precursor, A-type rare earth metals form the C-type structure [23]. The phase transformation between A- and B-type or C- and B-type structure of R2O3 can form the two separate crystals share some of the same crystal lattice points in a symmetrical manner [24].

Different methods have been utilized to synthesize the nanosized rare earth oxides, including the thermal decomposition of inorganic precursors (chlorides, nitrates, sulfates and hydroxides) and organic precursors (carbonates, oxalates and acetates) [22]. It was observed that lower decomposition temperatures lead to the formation of nano sized rare earth metal oxides [23]. Reverchon et al. [24] prepared highly crystalline and non-porous samarium oxide by thermal decomposition of samarium acetate, and it was observed that the average particle size of synthesized samarium oxide is still high (on average 100nm). Researchers adapted low temperature hydrolysis method to prepare samarium oxide nanosized materials with mean size of 40nm [25]. However, XRD results indicated that the synthesized rare earth oxide samples exhibited some other crystalline phases. Liu et al. [2] synthesized bulk samarium and neodymium oxide nanoparticles via two step synthesis method. This method involves hydrogen plasma-metal reaction of metal precursor to fabricate hydride nanoparticles followed by oxidation treatment to generate rare earth metal oxides. Several decades ago, Glushkova et al. [26] synthesized holmium oxide by decomposing the holmium nitrate at 660°C and it was reported that elevated temperature (around 740°C) is needed to decompose carbonate and oxalate salts [27].

It is well known that aqueous precipitation and sol-gel methods are frequently used techniques to prepare bulk metal oxides [28], however these methods exhibited few limitations when they were utilized to synthesize same materials in nanoscale [29–32]. The aqueous sol-gel chemistry become complex to understand due to the dual role of H2O molecule as ligand and solvent. During the aqueous synthesis of metal oxides, three different types of processes such as hydrolysis, condensation, and aggregation occur simultaneously. And it was also observed that these process are very difficult to control individually. Moreover, small changes in preparation conditions results a different particle morphology, which is a major drawback to obtain reproducibility [33]. Non-aqueous synthesis methods are capable to overcome main drawbacks of aqueous preparation techniques. The advantages of non-aqueous methods are majorly due to moderate reactivity of the CO bond [34] and stabilizing effect of the organic agents [35]. Therefore, many researchers utilized organic precursors and solvents for preparation of nanoscale catalysts [36].

In the present study, nanosized of samarium, neodymium and holmium oxide powders were synthesized by using inorganic and organic synthesis routes. The obtained materials were characterized by means of thermogravimetric analysis, X-ray diffraction, FT-IR spectroscopy, SEM, TEM, DR UV-vis spectroscopy and N2-physisorption techniques to investigate the structural, morphology, electronic and textural properties. The synthesized materials were also utilized for the photocatalytic degradation of crystal violet under visible light irradiation.

2Experimental2.1Materials

Rare earth metal (Ho, Nd and Sm) nitrates and acetates (>99.5 %) were purchased from Merck (Germany). Aqueous ammonia solution and tetrapropyl ammonium hydroxide were obtained from Aldrich, U.K. The received chemicals were utilized without any purification. Other solvents and chemicals were analytical-grade and used as received.

2.2Synthesis of nano sized rare earth metal oxides2.2.1Organic synthesis route

In organic synthesis route, calculated amount of rare earth metal acetate were dissolved in 50mL of methanol in a glass beaker. A known amount of tetrapropyl ammonium hydroxide solution was added drop wise via burette until the pH of the solution reached to 9. Then the total solution was placed in an electric oven at 80°C to remove the excess solvent. After complete removal of the solvent, a dried material was obtained. The dried materials were thermally treated at 500°C for 4h and the final product was labelled as M-org (M=Ho, Nd and Sm).

2.2.2Inorganic synthesis route

In this method, rare earth metal oxides were obtained by drop wise addition of ammonium hydroxide solution to aqueous solution containing stoichiometric quantity of rare earth metal nitrate. The pH of the solution was maintained at 9 to obtain the precipitate and the obtained precipitate was filtered and washed with distilled water for five times. The filtered precipitate was dried at 100°C for 6h and then the materials were thermally treated at 500°C for 4h. The samples prepared by using this method were labelled as M-inorg (Ho, Nd and Sm).

2.3Characterization of samples

The thermogravimetric measurements of as synthesized samples were carried out using TA60 Shimadzu Thermal Analyzer. Powder X-ray diffraction analysis of the rare earth metal oxide samples was carried out using Philips PW1700 diffractometer. A detailed methodology was outlined in our previous publication [37]. The crystallite size of the rare earth metal oxide phases were calculated using Scherer’s equation.

The morphology of synthesized rare earth metal oxide samples was investigated by SEM and TEM techniques. A JEOL microscope, equipped with a field emission gun (200kV) was used for SEM analysis and the TEM images were obtained using JEOL 2010 transmission electron microscope. Detailed experimental procedures were outlined in our previous publications [38,39]. DR UV-vis spectral data was obtained using Thermo-Scientific evolution spectrophotometer. The band gap energy of the samples were calculated using Kubelka-Munk method. The Kubelka-Munk factor (K) was determined by following Eq. (1)

K=(1-R)2/2R
where ‘R’ is the percentage reflectance. The wavelengths (nm) were translated into energies (E) and a plot was drawn between (K*E) 0.5 and E to obtain a curve. The bandgap energy (eV) was determined from the intersection point of the two slopes in the curve. The textural properties (specific surface area, pore size and pore volume) of rare earth metal oxide samples were obtained from N2-physisorption measurements. The Quantachrome ASiQ instrument was used to obtain the N2 adsorption-desorption isotherms for all the samples. A detailed experimental procedure was described in our previous publication [40].

2.4Photocatalytic degradation of crystal violet (CV)

The photocatalytic CV degradation experiments were performed as described in our previous publication [41]. A Pyrex glass reactor was used to conduct the photocatalytic degradation measurements of the synthesized catalysts. Calculated amount of catalyst (100mg) was added to the 100mL of an aqueous CV solution (100ppm) under stirring. The contents were equilibrated for 45min under dark to stabilize the adsorption of CV on the surface of catalyst. The concentration of CV was measured at this stage and it was considered as initial concentration. Then the reactor was exposed to visible light irradiation to initiate the photochemical process. The photocatalytic degradation of CV was monitored by measuring the absorbance of CV for every 10min using a UV-vis spectrophotometer. The degradation efficiency of the catalyst was determined by using the following Eq. (2)

η=[1−(C/C0)]×100
where, ‘C0’ is the initial concentration of CV and ‘C’ is the concentration of CV after photocatalytic degradation at particular reaction time.

3Results and discussion

The XRD patterns (Fig. 1) of both Ho-inorg and Ho-org exhibited all the reflections due to cubic structure of Ho2O3, which is accordance with the reference [JCPDS file No. 01-074-1829]. However, the reflections observed in Ho-org sample are broader compared to reflections observed in Ho-inorg sample. This observation indicating that Ho2O3 synthesized using organic route yielded smaller size crystallites.

Fig. 1.

Powder XRD patterns of calcined rare earth oxides by organic and inorganic routes.

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It is interesting to note that the XRD patterns of Nd-org and Nd-inorg samples composed of crystalline NdO2 [JCPDS file No. 00-046-1074], Nd2O3 [JCPDS file No. 00-041-1089] and Nd2O4 [JCPDS file No. 01-078-1090] phases. However, the Nd-inorg sample predominately exhibited reflections due to NdO2 phase, in contrast. Nd-org sample showed major reflections due to Nd2O3 phase. The XRD patterns of Sm-org and Sm-inorg samples exhibited the presence of crystalline Sm(OH)3 [JCPDS file no. 01-083-2036] and Sm2O3 [JCPDS file no. 42-1461] phases. It is clear that Sm(OH)3 phase is predominantly appeared in case of Sm-inorg sample, while the Sm-org sample possessed majorly Sm2O3 phase. The crystallite size of different phases (Table 1) were obtained using Debye-Scherer formula and the FWHM of the strongest reflection of the corresponding structure. The results clearly indicating that the crystallite sizes of the phases synthesized using organic route were smaller compared to the phases observed in the samples synthesized using inorganic route. Therefore, it is possible to argue that preparation route influenced the crystallinity, size and phase composition in rare earth oxide samples. It is known that stabilized organic species not only involves in suppression of crystal growth but they also bind with crystal facets, which results anisotropic crystal growth [42].

Table 1.

Crystalline phases and their sizes obtained from XRD analysis.

Sample  Crystalline phase  Crystallite size (nm) 
Ho-inorg  Ho2O3  22.6 
Ho-org  Ho2O3  12.6 
Nd-inorg  NdO2  30.2 
  Nd2O3  27.8 
  Nd2O4  35.3 
Nd-org  NdO2  26.1 
  Nd2O3  21.0 
  Nd2O4  30.5 
Sm-inorg  Sm(OH)3  42.8 
  Sm2O3  32.6 
Sm-org  Sm(OH)3  34.4 
  Sm2O3  24.8 

The FT-IR spectroscopy was used to understand the structural difference between the samples synthesized using organic and inorganic routes. The sharp IR absorption band at 560cm−1 is due to the Ho-O species [43], which clearly appeared in the both Ho-org and Ho-inorg samples. The two absorption bands appeared at 1390 and 840cm−1 in both samples could be attributed to stretching and out-of-plane bending vibrations of carbonate species, which are adsorbed on the surface of holmium oxide. Another band appeared at 1515cm−1 is due to bonded carbonate ions in a different coordination [44]. The both Nd samples exhibits the presence of bands at 520cm−1 and 643cm−1 corresponds to Nd-O vibrations of Nd oxides [45]. The existence of IR absorption bands at 524cm−1 and 685cm−1 were observed in both Sm samples; these two bands could be attributed to the stretching vibration of Sm2O3 species and bending vibration of Sm-O-H groups respectively [46]. A noticeable characteristic IR absorption band at 863cm−1 due to the stretching vibration of Sm3+-O groups in Sm2O3 phase was clearly observed in case of Sm-org sample. This observation indicates that this sample possessed majorly Sm2O3 phase as observed in XRD results.

It is observed from Fig. 2 that IR absorption bands are broad in case of samples synthesized using organic route as a result of the small particle size [2]. It is also clear from the FTIR spectral analysis that Ho, Nd and Sm samples synthesized using both organic and inorganic routes shows major absorption bands due to the presence carbonate ions. However, the samples synthesized using organic route possessed intense bands due to carbonate ion. This is mainly due to an efficient sorption of atmospheric carbon dioxide on the surface of rare earth metal oxides synthesized using organic route.

Fig. 2.

FT-IR spectra of calcined rare earth oxides by organic and inorganic routes.

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The precalcined samples were analyzed using TG analysis and the results are presented in Fig. 3. The TG patterns of three (Ho, Nd and Sm) samples synthesized using organic route showed three weight loss stages. The first weight loss appeared below 100°C could be due to loss of physisorbed methanol molecules on the surface. The two major weight losses occurred in the temperature range of 150−300°C, in which the transformation of rare metal hydroxide to oxides occurred in case of the samples synthesized using organic route. On the other hand, the TG patterns of samples synthesized using inorganic route exhibited six weight loss stages in the temperature range of 50−700°C. The three minor weight loss stages below 200°C could be attributed to loss of physically adsorbed moisture and the release of molecular water. The other three weight loss stages in the range of 200−550°C corresponds to the conversion of rare metal hydroxides to oxides and the transformation of amorphous oxide phase into crystalline phases. These results are clearly indicating that the samples synthesized using organic route does not require high temperature treatment to transform into oxides. Treating the samples at lower temperatures could allow the materials to preserve their crystal and pore structure without losing the surface area.

Fig. 3.

TG patterns of as synthesized rare earth oxides by organic and inorganic routes.

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The morphology of synthesized rare metal oxide samples studied by SEM and TEM analyses. The SEM images of the samples are shown in Fig. S1 (supplementary information). As shown in the figure, Sm and Nd samples synthesized using inorganic route possessed agglomerated micro sized crystalline particles with no definite shape were observed. However, the Ho-inorg sample is composed of large size flake type particles. On other hand, the Sm and Nd samples synthesized using organic route composed of particles with sponge type morphology, but the Ho-org sample possessed nanosized particles with flake type morphology. The steric hindrance due to the presence of organic moieties might be the reason for the flake type morphology observed in case of the samples synthesized by organic route. The organic moieties could provide a different reaction interfaces for nanoparticles and induce the nanoparticles to assemble in a definite direction [28], therefore flake type nanostructures were obtained. The TEM images of the samples (Fig. 4) exhibited similar results as shown in SEM analysis. The TEM image of Ho-org sample clearly showed the presence of the particles appear as thin flakes of a uniform thickness, similarly Nd-org and Sm-org samples showed existence of the flake like particles with large size.

Fig. 4.

TEM images of calcined rare earth oxides by organic and inorganic routes.

(1.13MB).

The extent of particle aggregation is high in case of the three samples synthesized using inorganic route. The sizes of irregular plate like shaped particles are higher than the average crystallite size measured using XRD data. It is known that XRD results provides average crystallite size over a large volume including the aggregates of small particles [8]. It is clear that the formed aggregates consists of particles with different sizes with crystallites (Table 2).

Table 2.

Textural properties of the synthesized rare earth samples.

Sample  SBET (m2g−1Pore volume (ccg−1Pore width (nm) 
Ho-inorg  24.7  0.066  4.9 
Ho-org  21.6  0.141  67.2 
Nd-inorg  3.6  0.018  6.1 
Nd-org  29.3  0.149  12.6 
Sm-inorg  4.5  0.032  6.1 
Sm-org  25.5  0.157  15.5 

The N2 adsorption-desorption isotherms of rare earth metal oxides synthesized by organic and inorganic routes are shown in Fig. 5(A). It is clear from the figure that all the samples exhibited Type-III isotherm (as per IUPAC classification) [47] and it was reported that there is no identifiable monolayer formation as the isotherms of the samples have not exhibited any curvature at lower relative lower pressure [48]. Exhibition of Type-III isotherms is an indication that all the samples are either non-porous or macro porous in nature, as a weak adsorbent-adsorbate interaction on the surface of samples is clearly evident.

Fig. 5.

(A) N2 adsorption-desorption isotherms (B) pore size distribution of rare earth oxides synthesized by organic and inorganic routes.

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In addition, the isotherms of the samples showed Type H3 hysteresis loop. The main feature of the Type H3 hysteresis loop is that the samples contained the flexible aggregates of plate-like particles and also the pore structure of sample consists of macro size pores. To confirm the pore structure of the samples, the pore size distribution was measured using NLDFT method and the results are shown in Fig. 5(B). As shown in the figure, all the samples synthesized using organic route exhibited meso and macro pores, particularly Ho-org sample. In contrast, the samples synthesized using inorganic route consists of mainly meso pores. The changes observed in the samples is due to the variation in particle size and big particles have tendency to form voids on the surface as well as inside of the agglomerated particles.

The DR UV-vis spectroscopy was used to study the optical properties of the synthesized samples (Fig. 6). The holmium oxide samples showed the presence of multiple absorption peaks in the range of 190−700nm. It was previously reported that holmium oxide has various band gaps of approximately 2.60eV, 3.25eV and 4.07eV, which corresponds to absorption edges at 477, 382 and 305nm respectively [49]. This is possibly arises from an intrinsic property of the synthesized samples. Similarly, the Nd-inorg and Nd-org samples also exhibited various UV–vis absorption peaks in the range of 190−900nm, and this observation is in accordance with the previous reported results that UV-vis diffuse reflectance spectrum of exhibits eight absorption bands at 434, 470, 532, 594, 612, 686, 756 and 800nm. The multiple peaks could be due to the fact that the samples have different crystalline phases. The Sm-inorg and Sm-org samples showed presence of absorption peaks found between 200 and 520nm. The broad absorption below 300nm could be attributed to the O2−→Sm3+ charge transfer [50]. Zhang et al. reported very similar UV-vis absorption spectrum in the range of 200−600nm for mesoporous Sm2O3[51].

Fig. 6.

DR UV-vis spectra of the synthesized samples.

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The band gap energies (Eg) of the samples were obtained from the Tauc’s plots for all the synthesized samples (Fig. S2, supplementary information). The calculated band gap energy for Ho-org and Ho-inorg samples is observed as 3.8eV and 4.1eV respectively. The Ho samples possessed different band gap energy as both samples have the same crystalline phase.

This is essentially due to decrease in the grain size resulted change in the band gap energy. Interestingly, Nd-inorg and Nd-org samples exhibited band gap energy of 3.8eV and 4.2eV respectively, as the samples have different composition. The calculated band gap energies were found to be around 3.2eV and 3.9eV for Sm-org and Sm-inorg nanomaterials, which are lowest found among the investigated samples. The band gap energies of the synthesized rare earth metal oxide samples are in the visible region making them photocatalysts under visible irradiation.

Prior to determine the photocatalytic efficiencies of the synthesized rare earth oxides, the adsorption of CV solution over the catalysts was studied. The results of CV adsorption of CV solution (100ppm) with rare earth metal oxides (100mg), are shown in Fig. 7. The synthesized Sm, Nd and Ho samples exhibited from 14 to 20 % adsorption after 120min in absence of any light. The direct photolysis of CV solution in absence of a rare earth oxide have not offered any degradation even after 120min. The photocatalytic activities of all the synthesized samples are evaluated for CV photo degradation under visible light irradiation for 200min.

Fig. 7.

Crystal violet photocatalytic degradation efficiencies of rare earth oxides.

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The photo degradation efficiencies were calculated from Eq. (2), and the obtained values were plotted against to irradiation time, for rare earth metal samples (Fig. 7). Insignificant degradation efficiency was observed without catalyst, however presence of rare earth metal oxide offers enhanced photocatalytic activities under visible light after 200min, the photodegradation efficiency is 93.3 %, 85.1 % and 80.3 % for Sm-org, Nd-org and Ho-org, respectively. However, the Sm-inorg, Nd-inorg and Ho-inorg samples offered the photodegradation efficiency of 70.1 %, 67.3 % and 63.1.5 % respectively even after the 400min time under visible irradiation.

The variation in concentration of CV solution (initial concentration; 100ppm, the first spectrum) determined by the UV-vis absorption spectral analysis during the photocatalytic degradation experiments is depicted in Fig. 8. The decrease in the intensity of absorbance peak of the CV solution was due to the destruction of the structural aromatic rings of the CV molecules. From the results, it is clear that Sm-org sample offered better photocatalytic efficiency compared to other samples and the results also indicated the rare earth metal oxides synthesized using organic route effective photo catalysts than the samples prepared using inorganic route.

Fig. 8.

UV-vis spectra of crystal violet during photocatalytic degradation over rare earth oxides.

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Fig. 9 shows the photodegradation kinetics of CV degradation using rare earth metal oxides synthesized using both organic and inorganic routes. It was reported that CV photodegradation reaction follow the pseudo first order kinetics [52], therefore we used pseudo-first-order kinetics Eq. (3) to determine the efficacy of the investigated rare earth metal oxide catalysts.

ln (C0/C)=kt
Where ‘k’ is the apparent rate constant for the first-order reaction (min−1), ‘C0’ is initial concentration of CV and ‘C’ is the final concentration of CV at given reaction time.

Fig. 9.

Plot of ln(C0/C) versus irradiation time for synthesized rare earth metal oxide catalysts under visible irradiation.

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The plots drawn between irradiation time verses ln(C0/C) are shown in Fig. 9. As shown in the figure, straight lines were observed and the slope equals to the apparent first-order rate constant (kapp). Also, the half-life value (t1/2) were determined for all the investigated catalysts and the determined values are presented in Table 3. The obtained results again reveal that the best photocatalytic activities were observed for samples synthesized using organic route; in particular, Sm-org photocatalyst under visible irradiation, with the rate constant (kapp) and half-time (t1/2) of 0.0117min−1 and 64min respectively. The kinetic study indicated that the rare earth oxides are promising photocatalyst for the degradation of CV dye.

Table 3.

The rate constants and t1/2 values for the synthesized rare earth oxide catalysts.

Catalysts  Rate constant [min−1t1/2 [min] 
Ho-inorg  0.0034  106 
Ho-org  0.0049  90 
Nd-inorg  0.0050  88 
Nd-org  0.0075  75 
Sm-inorg  0.0044  84 
Sm-org  0.0117  64 

In general, the photocatalytic activity of a catalyst influenced by the band gap energy, crystallite size, number of surface −OH groups and surface area [53]. Therefore, the Sm-org has the highest photocatalytic performance among the investigated rare earth oxides. The organic solvent used in organic synthesis method is able to assist to overcome some of the major drawbacks of aqueous synthesis method. The organic agents acts as the oxygen-donor for the metal oxide, and also they could greatly influence the particle size, morphology, surface and agglomeration characteristics, and in some cases, even composition and crystalline structure of the product [54]. More often, formation of highly crystalline metal oxides with uniform particle morphologies were observed in case of organic synthesis method [55]; this is due to the slow reaction rates (moderate reactivity of the CO bond), and also stabilizing role of the organic moieties.

Fig. 10 shows the photodegradation efficiency of rare earth metal oxide catalysts synthesized using organic route under visible irradiation for five cycles. Indeed, under visible irradiation after five cycles, the photodegradation efficiencies are 85.2 %, 79.4 % and 74.8 % for Sm-org, Nd-org and Ho-org, respectively. The results clearly indicating that after five cycles, the photo-degradation activity is slightly decreased. Thus, the synthesized rare earth metal oxide samples exhibited a negligible decrease in their photocatalytic performances, revealing that these photocatalysts were reusable and retained good photodegradation efficiency.

Fig. 10.

Reusability of rare earth metal oxide catalyst synthesized by organic route.

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The plausible photocatalytic degradation mechanism over rare earth metal oxide is represented in Scheme 1. In presence of the visible irradiation and the rare earth metal oxide with energy equal or higher than band gap energy, results transport of an electron from the valence band (VB) to the conduction band (CB) of rare earth oxide. The combination of positive hole (h+) with H2O molecules yields *OH radicals. Simultaneously, oxygen molecules presented on the surface of the photocatalyst are reduced by the electrons in the CB to produce *O2− ions [56,57]. The produced *OH radicals and *O2− ions are effective oxidant for the degradation of harmful organic contaminants [58–62].

Scheme 1.

Photocatalytic degradation mechanism of rare earth oxide.

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4Conclusions

In conclusion, two different (inorganic and organic) routes are used to synthesize samarium, neodymium and holmium oxides. A systematic characterization of the synthesized materials was performed using X-ray diffraction, FT-IR, TG analysis, FESEM, HRTEM, N2-physisorption and DR UV-vis techniques. The characterization results indicated that the samples synthesized by organic route possessed smaller crystallite/particle size and high surface area with macro size pores compared to the samples synthesized by inorganic route. The samples synthesized by organic route possessed sheets like morphology, in contrast highly agglomerated particles were observed in case of samples synthesized by inorganic route. The photocatalytic activity of the synthesized samples were tested for degradation of crystal violet dye under visible light irradiation. The samples synthesized by organic route exhibited high photocatalytic efficiencies; among the studied samples samarium oxide synthesized using organic route offered the superior photocatalytic performance. This is majorly due to the fact that Sm-org sample possessed low band gap energy, high surface area, pore volume and presence of surface reactive −OH groups. In addition, the synthesized rare earth metal oxide catalysts exhibited excellent recyclability for photocatalytic crystal violet degradation.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no. G-129-130-1439. The authors, therefore, acknowledge with thanks DSR for technical and financial support.

Appendix A
Supplementary data

The following are Supplementary data to this article:

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