Study of obtaining thin films of CeO₂ doped with 2 and 4 mol% of europium, terbium and thulium by spin coating: photocatalytic properties

Estudo da obtenção de filmes finos de CeO ₂ dopados com 2 e 4 %mol de európio, térbio e túlio por spin coating: propriedades fotocatalíticas

Abstract

In this study, thin films of Eu³⁺, Tb³⁺ and Tm³⁺ doped CeO₂ in the proportions of 2 and 4 mol% were obtained by spin coating and calcined at 700 °C. The films were characterized by XRD, SEM, AFM and UV-vis techniques. The photocatalytic activity of the thin films was measured by the degradation of the methylene blue (MB) dye. The thin films were submitted to 4 photocatalytic cycles to analyze the capacity to be reused. XRD patterns showed no secondary phase formation, where all characteristic peaks were related to CeO₂. SEM micrographs indicated that doping at 4 mol% promoted the reduction in the thickness and surface porosity of the thin films. AFM images indicated the increase in surface roughness with rare earths doping. Doping with Eu³⁺, Tb³⁺ and Tm³⁺ increased the degradation of the methylene blue dye by at least 15% for thin films doped at 4 mol%. The reuse tests indicated that the photocatalytic activity remained practically constant even with the application of four consecutive cycles.

Keywords: Eu³⁺; Tb³⁺ and Tm³⁺ doped CeO₂; thin films; spin coating; photocatalytic reuse

Resumo

Neste estudo, filmes finos de CeO₂ dopados com Eu³⁺, Tb³⁺ e Tm³⁺ nas proporções de 2 e 4 %mol foram obtidos por spin coating e calcinados em 700 °C. Os filmes foram caracterizados pelas técnicas de DRX, MEV, MFA e UV-vis. As propriedades fotocatalíticas dos filmes finos foram determinadas pela degradação do corante azul de metileno (AM). Os filmes finos foram submetidos a 4 ciclos fotocatalíticos para analisar a capacidade de serem reusados. Os difratogramas de raios X mostraram que não houve a formação de fases secundárias, onde todos os picos foram referentes ao CeO₂. As micrografias de MEV indicaram que a dopagem com 4 %mol promoveu a redução da espessura e porosidade superficial dos filmes finos. As imagens de MFA indicaram o aumento da rugosidade superficial com a dopagem utilizando terras raras. A dopagem com Eu³⁺, Tb³⁺ e Tm³⁺ aumentaram a atividade fotocatalítica em pelo menos 15% para os filmes dopados com 4 %mol. Os testes de reuso indicaram que a atividade fotocatalítica se manteve praticamente constante após a aplicação de quatro ciclos consecutivos.

Palavras-chave: CeO₂ dopado com Eu³⁺; Tb³⁺ e Tm³⁺; filmes finos; spin coating; reuso fotocatalítico

INTRODUCTION

The growing increase in waste generation from the industries causes the need for constant studies on fast and efficient treatment methods. Waste from textile industries receives special attention because they contain a high amount of organic dyes, which are difficult to treat ¹. Dyes discarded in the wild without proper treatment may alter the local fauna and flora, preventing the marine environment from receiving an adequate amount of light ^{1), (2}. The use of heterogeneous photocatalysis for the treatment of these dyes is considered the most efficient method, as it converts these organic compounds into H₂O, CO₂ and mineral salts ³. Heterogeneous photocatalysis is usually used with a semiconductor material acting as a catalyst, where it acts to produce reactive oxygen species (ROS), such as hydroxyl radicals (˙OH) and superoxides (˙O₂), which have a high oxidative capacity against organic contaminants ^{4), (5}.

Recent studies indicate CeO₂ as an effective photocatalyst ⁶. Among its main characteristics are the high chemical resistance and the photocorrosion, high capacity of absorption of UV radiation and an excellent capacity of liberation and storage of oxygen ⁷. Moreover, the ease of conversion between cerium valences (Ce⁴⁺/Ce³⁺) makes it a material with high redox potential ^{8), (9}. However, in comparison to other materials used in photocatalysis, such as TiO₂, it presents low mobility of charges on its surface, which makes the photocatalytic process difficult ⁹. Modifications in the electronic structure of CeO₂, such as doping, generate defects that alter the mobility of the charges, increasing its photocatalytic activity ¹⁰. The introduction of rare earth ions into the CeO₂ lattice potentiates their intrinsic properties due to the replacement of Ce⁴⁺ by trivalent cations, which increase the amount of oxygen vacancies and introduce intermediate levels between the valence and conduction bands ^{11), (12}. The increase in the oxygen vacancy number provides the impediment in the e^-/h⁺ pair recombination generated in the photocatalytic process, increasing its effect ¹³.

Nanoparticulate materials have higher photocatalytic activity than the others due to the greater available surface area ¹⁴. However, due to the small scale, they become a problem for the separation after the catalytic process ¹⁵. The use of supported materials, such as thin films, eliminates this problem. In this work, the photocatalytic activity of CeO₂ thin films against methylene blue (MB) dye was optimized by doping with rare earth ions (Eu³⁺, Tb³⁺ and Tm³⁺) in the 2 and 4 mol% proportions. To analyze the ability of the films to be reused after a photocatalytic cycle, they were submitted to four consecutive cycles, without any treatment on its surface.

MATERIALS AND METHODS

Cerium nitrate [Ce(NO₃)₃, Sigma Aldrich], europium nitrate [Eu(NO₃)₃, Alfa Aesar], terbium nitrate [Tb(NO₃)₃, Alfa Aesar], thulium nitrate [Tm(NO₃)₃, Alfa Aesar], ethylene glycol (C₂H₆O₂, Alfa Aesar), citric acid (C₆H₈O₇, Synth), isopropyl alcohol (C₃H₈O, Alfa Aesar), ammonium hydroxide (NH₄OH, Synth) and deionized water were used to obtain the precursor solutions. For the preparation of the cerium precursor solution, citric acid was dissolved in deionized water at 70 °C; after complete dissolution, cerium nitrate was added. Finally, ethylene glycol was added and the temperature was maintained at 70 °C until the solution achieved a viscosity of 25 cps. Obtaining the europium, terbium and thulium doped cerium solutions occurred similarly, where the cerium cations were stoichiometrically substituted at 2 and 4 mol% (Ce_(1-x%)A_(x%)O₂ with A= Ce, Tb, Tm).

The silicon (100) substrates were washed with deionized water and isopropyl alcohol in an ultrasonic bath. Subsequently, the substrates were immersed in a solution of deionized water, ammonium hydroxide and hydrogen peroxide (H₂O₂, Synth), heated to 70 °C. The precursor solutions were deposited on the silicon substrates by spin coating technique, without heating. The rotation speed and spin time were fixed at 700 rpm for 3 s and 7200 rpm for 30 s, using a commercial spinner (Chemat Techn., KW-4B spin-coater). After deposition of each layer, the samples were taken to calcination at 700 °C (heating hate of 5 °C/min), where they remained for 1 h, and were cooled inside the furnace. In all samples, three layers of thin films were deposited, according to the scheme shown in Fig. 1.

Figure 1
Schemes of thin films obtained by spin coating.
Figura 1:
Esquemas dos filmes finos obtidos por spin coating.

In order to identify the present phases, the X-ray diffraction (XRD) technique was used, operating at a low angle, with a speed of 1 °/min and a 0.01° step in a Shimadzu diffractometer (XRD-6000) using CuKα radiation (1.5418 Å). For more information on the crystallinity of the thin films, the crystallite size was estimated using Scherrer’s equation ¹⁶. The morphological aspects of the films were observed by field emission scanning electron microscopy (SEM). The film thickness was determined with the use of ImageJ software ¹⁷. The chemical composition of the films was analyzed by energy-dispersive X-ray spectroscopy (EDX). Atomic force microscopy (AFM) was performed to obtain more information about the surface roughness of thin films. UV-vis spectroscopy was performed on a Shimadzu spectrophotometer (UV-2550) operating in the diffuse reflectance mode, with wavelength ranging from 200 to 900 nm. Kubelka-Munk function ¹⁸ was used to convert the reflectance data into absorbance and the Wood and Tauc method ¹⁹ was used for the estimation of band gap (E_gap). For these films, the direct transition was admitted.

The photocatalytic activity of the thin films was estimated by varying the concentration of the methylene blue dye by the test time. For this, the samples were illuminated by six UVC lamps (Phillips, 15 W, with maximum intensity at 254 nm=4.9 eV). Thin films with dimensions of 6x6 mm (receiving irradiance of 2.5 MW/m²) were submerged in a quartz beaker (15 mm diameter) containing 10 mL of the methylene blue dye (MB, C₁₆H₁₈ClN₃S, Mallinckrodt, with purity of 99.5%) without stirring, with a concentration of 10^-5 mol.L^-1 and pH 5, and 0.06 mL of H₂O₂. The temperature was monitored throughout the process and remained at about 27 °C. At 1 min intervals, an aliquot was withdrawn for analysis of the variation of the dye concentration. To eliminate the adsorption effects, the thin films remained for 24 h in contact with MB dye without contact with light sources. The reuse tests were performed by placing the thin films in contact with another MB solution with 10^-5 mol.L^-1 concentration. The photocatalytic tests were performed in duplicates and the mean values were used.

RESULTS AND DISCUSSION

Fig. 2 shows the XRD patterns obtained for CeO₂ thin films, pure and doped with Eu³⁺, Tb³⁺ and Tm³⁺, in the proportions of 2% (Fig. 2a) and 4% (Fig. 2b) obtained by spin coating. All peaks were related to cubic CeO₂ and spatial group Fm-3m, characterized by ICSD 72155 file. No secondary peaks were observed, indicating that Eu³⁺, Tb³⁺ and Tm³⁺ ions were well incorporated into the CeO₂ lattice. According to Fig. 2, the displacements of the peaks were observed with doping, indicating the introduction of Eu³⁺, Tb³⁺ and Tm³⁺ ions into CeO₂ lattice. Doping with materials of different valences and ionic radius tend to cause changes in the crystalline lattice that may go unnoticed when analyzed only visually, especially when doping occurs in low concentrations ²⁰.

Figure 2
XRD patterns for CeO₂ thin films, pure and doped with 2% (a) and 4% (b) of Eu³⁺, Tb³⁺ and Tm³⁺.
Figura 2:
Difratogramas de raios X dos filmes finos de CeO₂ puro e dopados com 2% (a) e 4% (b) de Eu³⁺, Tb³⁺ e Tm³⁺.

Table I presents the crystallite sizes obtained by the Scherrer equation; according to the values obtained, the doping with Eu³⁺ and Tb³⁺ promoted an increase in the size of the crystallite, whereas the doping with Tm³⁺ provided a small reduction. Rodrigues et al. ²¹ showed that the doping of CeO₂ with Eu³⁺ provides an increase in crystallite size due to the difference between the ionic radius of Ce⁴⁺ ions by Eu³⁺, which have an ionic radius of 0.87 and 0.95 Å, respectively. Sahoo et al. ²² also showed that the replacement of Ce⁴⁺ ions by Eu³⁺ generates oxygen vacancy in the crystalline structure of CeO₂ reducing the crystallite size. The addition of Tb³⁺, which has an ionic radius of 0.92 Å, increases the crystallite due to the generation of oxygen vacancies ²³. The reduction in the crystallite size of CeO₂ when doped with Tm³⁺ occurs because this ion has the same size of ionic radius and act on the surface of the crystal, hindering its growth ²⁴.

Table I
Crystallite size and microdeformation obtained by the Scherrer’s equation.
Tabela I
Tamanho de cristalito e microdeformação obtidos pela equação de Scherrer.

Sample	Crystallite size (nm)	Microdeformation, ε x10-4
Pure CeO2	17.54	4.20
CeO2:2%Eu	20.84	3.92
CeO2:2%Tb	22.41	3.62
CeO2:2%Tm	17.22	4.23
CeO2:4%Eu	20.84	3.91
CeO2:4%Tb	23.16	3.51
CeO2:4%Tm	17.37	4.22

Figs. 3 and 4 show the top view and transverse section of CeO₂ thin films doped with Eu³⁺, Tb³⁺ and Tm³⁺ and the CeO₂ and CeO₂:Eu:Tb:Tm thin films, respectively. The EDX analysis and the chemical mapping performed on the CeO₂:4%Eu:Tb:Tm thin film are shown in Figs. 4d and 4e. The thickness of the films was measured along its entire length. The mean value of film thickness, as well as the standard deviation, is shown in Table II. It was noticed that the films were dense and homogeneous, presenting low values of standard deviation for thickness. Doping of thin films of CeO₂ with rare earths promoted a reduction in its thickness; this reduction was more evident in the films doped with europium. According to ²⁵, the reduction in the thickness of CeO₂ thin film occurred due to the difference between the ionic radius of the Ce⁴⁺ and the rare earths. The increase in the rare earth concentration from 2% to 4% promoted higher densification of the films. Higher densification, coupled with reduced thin film thickness, provided less void space, making films ideal for sensitive applications such as gas sensors and photocatalysis ²⁶. The increase in the densification of thin films doped with 4% of rare earth occurred due to the interaction with the cerium ions, causing coalescence and grain growth to occur with the course of the calcination ²⁷. The EDX mapping for the CeO₂:4%Eu:Tb:Tm sample indicated that rare earth ions were properly incorporated.

Figure 3
Top view and transverse section SEM images of the thin films of CeO₂:2%Eu (a), CeO₂:4%Eu (b), CeO₂:2%Tb (c), CeO₂:4%Tb (d), CeO₂:2%Tm (e), and CeO₂:4%Tm (f).
Figura 3:
Imagens de MEV de vista de topo e seção transversal dos filmes finos de CeO₂:2%Eu (a), CeO₂:4%Eu (b), CeO₂:2%Tb (c), CeO₂:4%Tb (d), CeO₂:2%Tm (e) e CeO₂:4%Tm (f).

Figure 4
Top view and transverse section SEM images of the thin films of CeO₂ (a), CeO₂:2%Eu:Tb:Tm (b), CeO₂:4%Eu:Tb:Tm (c), and elemental mapping (d) and EDX spectrum (e) of CeO₂:4%Eu:Tb:Tm thin film.
Figura 4:
Imagens de MEV de vista de topo e seção transversal dos filmes finos de CeO₂ (a), CeO₂:2%Eu:Tb:Tm (b), CeO₂:4%Eu:Tb:Tm (c), e mapeamento elementar (d) e espectro de EDX (e) para o filme fino CeO₂:4%Eu:Tb:Tm.

Table II
Average thickness of the CeO₂ thin films obtained by spin coating.
Tabela II
Espessura média dos filmes finos de CeO₂ obtidos por spin coating.

Sample	Average thickness of thin film (nm)
Pure CeO2	556±8
CeO2:2%Eu	307±6
CeO2:2%Tb	325±2
CeO2:2%Tm	409±11
CeO2:2%Eu:Tb:Tm	346±9
CeO2:4%Eu	139±2
CeO2:4%Tb	142±6
CeO2:4%Tm	388±19
CeO2:4%Eu:Tb:Tm	262±7

Fig. 5 shows the 3D micrographs obtained by AFM for pure CeO₂ and doped with the rare earths. It was observed that doping increased the surface roughness and promoted the appearance of more CeO₂ sites on the surface of the films, indicating that rare earth ions assisted nucleation and grain growth ²⁸. Rare earth ions are chemically active and easily adsorbed onto the substrate surface, then attracting the CeO₂ molecules by van der Waals forces for rapid nucleation ²⁹. The thin films composed by Eu:Tb:Tm formed more bonds with CeO₂, obtaining smaller grains and, consequently, greater roughness ³⁰.

Figure 5
AFM images for the thin films of CeO₂ (a), CeO₂:2%Eu (b), CeO₂:4%Eu (c), CeO₂:2%Tb (d), CeO₂:4%Tb (e), CeO₂:2%Tm (f), CeO₂:4%Tm (g), CeO₂:2%Eu:Tb:Tm (h), and CeO₂:4%Eu:Tb:Tm (i).
Figura 5:
Imagens de MFA para os filmes finos de CeO₂ (a), CeO₂:2%Eu (b), CeO₂:4%Eu (c), CeO₂:2%Tb (d), CeO₂:4%Tb (e), CeO₂:2%Tm (f), CeO₂:4%Tm (g), CeO₂:2%Eu:Tb:Tm (h) e CeO₂:4%Eu:Tb:Tm (i).

Fig. 6 shows the absorbance extrapolation curves by photon energy for E_gap estimation. According to the values shown in Fig. 6, it is observed that the E_gap of the thin films obtained in this study ranged between 2.78 and 3.15 eV. These values were in accordance with the E_gap reported in the literature for CeO₂^{31), (32}. From Fig. 6, it can be seen that the increase in dopant concentration led to the reduction in E_gap of the CeO₂ films. E_gap of semiconductor materials is directly related to the presence of defects in the crystalline lattice. The oxygen vacancies act to form intermediate levels below the states of the band 4f, where this occupation depends on the temperature and defects concentration ³². Wang et al. ³³ studied the effect of the calcination temperature in the presence of oxygen vacancies in the CeO₂ lattice and showed that increasing this temperature from 550 to 850 °C increases the concentration of oxygen vacancies. The defects introduced into the CeO₂ lattice by the rare earths together with the temperature of 700 °C used in the calcination of thin films in this study can be attributed as those responsible for the presence of oxygen vacancies.

Figure 6
UV-vis absorbance spectra for the thin films of CeO₂ (a), CeO₂:2%Eu (b), CeO₂:2%Tb (c), CeO₂:2%Tm (d), CeO₂:2%Eu:Tb:Tm (e), CeO₂:4%Eu (f), CeO₂:4%Tb (g), CeO₂:4%Tm (h), and CeO₂:4%Eu:Tb:Tm (i).
Figura 6:
Espectros de absorbância UV-vis para os filmes finos de CeO₂ (a), CeO₂:2%Eu (b), CeO₂:2%Tb (c), CeO₂:2%Tm (d), CeO₂:2%Eu:Tb:Tm (e), CeO₂:4%Eu (f), CeO₂:4%Tb (g), CeO₂:4%Tm (h) e CeO₂:4%Eu:Tb:Tm (i).

Fig. 7 shows the curves of the variation of MB dye concentration by time, assembled from the absorbance curves. From Figs. 7a and 7b, it can be seen that doping with the rare earths provided the increase in the photocatalytic activity of the thin films. The photocatalytic activity of semiconductor materials is directly related to the generation of reactive oxygen species (ROS), which have a high oxidative capacity ⁴. The generation of ROS occurs when the material receives enough energy to excite the electron of the valence band for the conduction band, forming electron/hole pairs (e^-/h⁺) ^{34), (35}. The photocatalytic process may be described by a first-order kinetic model with relation to the absorbance of methylene blue ³⁶, and the results obtained are shown in Figs. 7c and 7d. The graphs show the linear relationship of ln(C/C_o) by the irradiation time. From this linear relationship, Eq. A was use and the kinetic constant determined:

$-\ln \frac{C_t}{C_O}=k·t$ (A)

where C_t is the absorbance of methylene blue at time t, C_o the initial absorbance, t the irradiation time, and k the kinetic constant. As discussed previously, the doping with Eu³⁺, Tb³⁺ e Tm³⁺ provided the formation of oxygen vacancies due to the differences in valence and ionic radius with Ce^{4+ (22)-(25}. Oxygen vacancies act in a way to restrict the recombination of the e^-/h⁺ pairs, increasing the amount of ROS available and, consequently, the photocatalytic activity of the thin films ³⁷. The doping of CeO₂ with rare earths proved to be very efficient, where the 4% terbium sample reduced the MB dye concentration by 96%, while the pure CeO₂ sample reduced by 75%. Correia et al. ³⁸ obtained TiO₂ and Bi₂O₃ thin films which reduced approximately 10% of the initial MB dye concentration after 310 min. The kinetic constants, k, allowed the observation of the first wave behavior of the curves, where it was evident the increase of this by doping with the rare earths.

Figure 7
Variation of the MB dye concentration with time for the samples doped with 2% (a) and 4% (b) of Eu³⁺, Tb³⁺ and Tm³⁺, and respective first-order kinetic plots (c,d).
Figura 7:
Variação da concentração do corante AM com o tempo para as amostras dopadas com 2% (a) e 4% (b) de Eu³⁺, Tb³⁺ e Tm³⁺ e respectivos gráficos de cinética de primeira ordem (c,d).

The use of thin films for the treatment of effluents is important because it avoids the generation of secondary residues after the photocatalytic process, since the catalyst is fixed in the substrate. Thus, it is important to analyze the ability of these materials to be reused in consecutive processes without the need for treatment. According to ³⁹, the evaluation of the reusability of the catalyst consists of two criteria: i) maintenance of the photocatalytic activity with the course of the cycles; and ii) ease of the catalyst being recycled from the solution. As the catalyst is immobilized on the substrate, the evaluation of the reusability of the ceria thin films is related to the capacity to maintain the photocatalytic activity with the course of the cycles. Fig. 8 shows the curves of the variation of the concentration of the MB dye by the time, during four cycles of the test without any treatment between them. As can be seen, all the films maintained their photocatalytic activity during the course of four test cycles, indicating their viability for photocatalytic applications in several cycles.

Figure 8
Photocatalytic reuse for the thin films of CeO₂ (a), CeO₂:4%Eu (b), CeO₂:4%Tb (c), CeO₂:4%Tm (d), and CeO₂:4%Eu:Tb:Tm (e).
Figura 8:
Reuso fotocatalítico para os filmes finos de CeO₂ (a), CeO₂:4%Eu (b), CeO₂:4%Tb (c), CeO₂:4%Tm (d), and CeO₂:4%Eu:Tb:Tm (e).

CONCLUSIONS

Spin coating technique, allied with the temperature of 700 °C used in the calcination, was efficient to obtain thin films of pure CeO₂ and doped with rare earths (Eu, Tb and Tm), without the formation of secondary phases. The increase in doping from 2 to 4 mol% produced thinner films, with lower surface porosity and greater roughness. The defects generated by the doping reduced the band gap (E_gap) of CeO₂ thin films. The increase of the defects increased the ionic conduction of the films, favoring the photocatalytic activity, which increased about 5% for films doped with 2 mol% and 15% for films doped with 4 mol% of rare earths. The reuse tests showed that the films maintained their photocatalytic activity after 4 consecutive cycles, without considerable losses, being indicated for long term applications.

ACKNOWLEDGMENTS

This study was partially financed by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES/PROCAD) - Finance Code 2013/2998/2014. The authors also thank the financial support from the Brazilian research funding institution: CNPq No. 307546/2014.

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Publication Dates

• Publication in this collection
  14 Nov 2019
• Date of issue
  Oct-Dec 2019

History

• Received
  04 Jan 2019
• Reviewed
  08 Mar 2019
• Reviewed
  27 Mar 2019
• Accepted
  02 Apr 2019