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Effect of rare earth doping on the enhancement of photocatalytic performance of ceria nanocrystals under natural sunlight

  • Madhu Karl Chinnu
  • Pandurangan Anandan
  • Mukannan Arivanandhan
  • Arumugam Venkatesan
  • Rangasamy Mohan Kumar
  • Ramasamy Jayavel
Article
  • 72 Downloads

Abstract

The photocatalytic properties of CeO2, Gd:CeO2 and Sm:CeO2 nanocrystals have been investigated under natural sunlight. The structural and morphological analyses of the synthesized samples were carried out by X-ray powder diffraction, field emission scanning electron microscopy and high-resolution transmission electron microscopy. The UV–Vis absorption of pure and doped ceria confirmed the reduction in the band gap values. Fluorescence spectra of CeO2 show strong emission peaks variation at 424 and 466 nm and for rare earth doped ceria at 466 nm. XPS analysis revealed in the composition, Ce4+ to Ce3+ ratio and energy states of the prepared samples. The photocatalytic performance of Sm:CeO2 was relatively high compared to pure and Gd:CeO2 in the degradation of methylene blue dye. The variation in the photocatalytic performance was discussed based on the oxygen vacancy and Ce3+ concentrations. The present study confirmed that the incorporation of rare earth ions increased the oxygen vacancies, which caused changes in Ce4+/Ce3+ ratio thereby enhancing the visible photocatalytic activities.

1 Introduction

The growing energy demand and environmental pollution are the main challenges for the present and future generation due to increasing world population. The emerging field of nanotechnology primarily aims to solve both the issues as many investigations are focused. In particular, nanostructures of metal oxides like TiO2, ZnO and CeO2 have been extensively investigated for solar cells, fuel cells and photocatalytic applications [1, 2, 3]. Among the metal oxide nanomaterials, TiO2 and CeO2 are widely studied for photocatalytic applications, such as hydrogen production by water splitting, degradation of pollutants and so on. CeO2 is an efficient catalyst due to its excellent redox behavior and hence processes a reasonable oxygen storage/release capacity (OSC) [4, 5, 6]. Moreover, it has high resistance against chemical and photo-corrosions along with strong light absorption in the UV region due to its wide band gap (3.19 eV). However, the wide band gap of CeO2 makes it inactive in the visible light. It is well known that, UV light only composes 3–5% of the photon flux reaching the earth surface from the solar spectrum (AM 1.5), while it is around 45% in the visible light range. Therefore, it would be highly beneficial to extend the light absorption of CeO2 to the visible region by following the green approach. Efforts have been made to enhance the photocatalytic activity of CeO2 in the visible region by proper doping [7, 8, 9, 10, 11]. For instance, doping of transition metal ions of different valencies into the CeO2 lattice shifts the optical absorption edge to lower energies and thereby enhancing the photocatalytic activity in the visible region [8]. Rare earth ions like Sm3+ and La3+ were doped into the CeO2 lattice and their photocatalytic activities studied [9]. Electrochemically active bio film was introduced into the CeO2 lattice with the intention to increase its photocatalytic activity in the visible region [10] and for Plasmon-Enhanced Catalytic Reactions [11]. From the reported results, it is obvious that the photocatalytic activity of CeO2 mainly depends on the oxygen vacancy, diffusion of oxygen and OSC which depends on Ce4+/Ce3+ reduction process [12, 13]. Although considerable efforts were made to increase the photocatalytic activity of CeO2 in the visible region, the formation of defects which are responsible for visible light catalytic activity has not been studied in detail. The photocatalytic property of CeO2 under natural sunlight is very essential for industrial waste water treatment and green environments. Moreover, it has been reported that, the dye decolorization study alone is not enough to ascertain the photocatalytic activity of a material as the other parameters related to various interactions between dye molecules and photocatalyst such as the dye sensitization of catalyst particles, electrostatic interaction, and the spectral overlap may influence the degradation process [14, 15, 16]. Therefore, a systematic investigation is needed on the physical characteristics of the photocatalyst such as structural (X-ray diffraction), compositional (X-ray photoelectron spectroscopy), morphological (electron microscopic analysis) and optical absorption (UV–Vis absorption spectroscopy) along with the study of photocatalytic activity [15].

In the present investigation, sol–gel synthesized (i) pure CeO2, (ii) Gd-doped (GC) and (iii) Sm-doped CeO2 (SC) nanocrystals were studied for their photocatalytic properties. A simple template sol–gel based sonochemical technique was adapted to synthesize CeO2, GC and SC nanocrystals from non-aqueous solution. The structure and morphology of the prepared nanocrystals were studied by XRD, FE-SEM and HR-TEM. Based on the X-ray photoelectron spectroscopy (XPS) and UV absorption studies, the presence of surface oxygen, mixed valance state and band gap of the materials were investigated. The photocatalytic activity of the prepared samples was confirmed through the degradation of methylene blue (MB) in an aqueous solution under natural sunlight at Anna University, India. The photocatalytic performance of CeO2, GC and SC samples were comparatively analyzed. A possible charge transfer mechanism responsible for the degradation process is proposed based on XPS and UV analyses.

2 Experiment

Analytical Reagent grade Ce(NO3)2·6H2O (99%), Gd(NO3)3·6H2O (99.9%), Sm(NO3)3·6H2O (99.9%) and NaOH (97%) procured from Aldrich were used without further purification. Pure CeO2 and rare earth doped (GC and SC) nanoparticles were synthesized by sonochemical method. At the precipitation stage, the homogeneous solution of 4 mol% of NaOH was added drop-wise to the solution of 0.2 mol% Ce(NO3)3·6H2O dissolved in isopropyl alcohol followed by heating at 70 °C until the pH of the solution was stabilized to 11. The hexamethylene tetramine (HMTA) surfactant was added with dissolved cerium(III) nitrate solution and allowed to continuous stirring for 1 h. Similarly, GC and SC were synthesized, by adding 0.02 mol% of gadolinium(III) nitrate or/and samarium(III) nitrate with 0.18 mol% of cerium(III) nitrate solution. The synthesized samples were centrifuged and washed several times using distilled water and ethanol. The prepared samples were dried in air oven at 100 °C for 24 h and annealed at 300 °C for 8 h.

The products were characterized for structural confirmation by using Siemens 5005 X-ray diffractometer (XRD) with Cu Kα radiation. The field emission scanning electron microscopic (FE-SEM) images were recorded by using a JEOL 6300F microscope operated at an acceleration voltage of 3–5 kV to study the surface morphology. High-resolution transmission electron microscopy (HR-TEM) images and selected-area electron diffraction (SAED) were taken using a JEOL (3010) TEM with a UHR pole-piece operated at an accelerating voltage of 300 kV for particle size determination and to find the crystalline nature. UV–Vis absorption spectra were recorded using Scan UV–Vis–NIR spectrophotometer (CARY 500) to estimate the band gap values of the synthesized samples. Fluorescence emission spectra were recorded at room temperature using Perkin Elmer LS 5B fluorescence spectrophotometer over the range 250–600 nm. XPS analysis was performed using Shimadzu ESCA 3400 electron spectrophotometer. The photocatalytic studies of pure and doped ceria nanoparticles were carried out under natural sunlight at Anna University with longitude and latitude of 80:15 E and 13:4 N, between 11.00 a.m. and 2.00 p.m. during the month of May. Based on the longitude and latitude, the solar light intensity was noted as 5.78 kWh/m2/day from Synergy Enviro Engineers Pvt ltd, India.

3 Results and discussion

The XRD patterns for CeO2, GC and SC nanocrystals are shown in Fig. 1. All the diffraction peaks have been indexed, to confirm the cubic structure of ceria. The crystallite size has been estimated by Scherrer’s Equation. The average crystallite size of the pure and rare earth doped nanoparticles is in the range of 10–20 nm (Table 1). Further, it has been observed that the crystallite size of the rare earth ion doped ceria is larger than the pure sample. The sharp peak at 45° in all the patterns might have appeared due to instrumental error as it has appeared for all samples. The lattice constants of the cubic ceria, GC and SC have been calculated from the predominant (111) diffraction peaks. The calculated lattice constant values for ceria, GC and SC are 5.421, 5.438 and 5.438 Å, respectively. The lattice constants of all samples have been observed to be higher than that of bulk ceria (5.403 Å) (JCPDS 65-5923). The increase in the lattice constant values is due to the presence of high concentrations of oxygen vacancies and Ce3+ state, which has led to the lattice expansion. During annealing process at high temperature, the diffusion of oxygen to surface has induced charged vacancies at the oxygen lattice site. In order to balance the charge, the free electrons released at the oxygen vacancy site have been captured by Ce4+ and transformed into Ce3+ ions. The large lattice constants of GC and SC samples indicate that the concentration of oxygen vacancies and Ce3+ ions are higher than ceria.

Fig. 1

XRD patterns of (a) CeO2, (b) Gd:CeO2 and (c) Sm:CeO2 nanoparticles

Table 1

Average crystallite size of CeO2 and rare earth doped CeO2 nanoparticles prepared using isopropyl alcohol

Sample

2 theta (degree)

‘d’ spacing (nm)

FWHM

Crystallite size (nm)

Average crystallite size (nm)

CeO2

28.49

3.13

1.86

08.38

10.63

47.40

1.93

1.61

12.56

56.12

1.64

2.24

10.97

Gd:CeO2

28.55

3.13

0.97

16.07

17.00

47.41

1.92

1.29

15.69

56.03

1.64

1.27

19.29

Sm:CeO2

28.24

3.14

0.89

17.30

18.06

47.32

1.96

1.26

16.00

56.23

1.67

1.18

20.88

47.26

1.92

0.98

20.06

56.07

1.63

1.2

20.40

Figure 2a–c shows FE-SEM images of HMTA capped CeO2 and GC and SC nanoparticles. It was observed from the FE-SEM image of uncapped sample (not shown) that the particles are highly agglomerated, whereas the particles are mono dispersed with spherical shape for pure and RE doped samples prepared with HMTA surfactant. From the FE-SEM images, it is observed that the capping agent has significantly controlled the agglomeration, which led to the uniform growth of nanoparticles as shown in Fig. 3. TEM and HR-TEM images of pure ceria, GC and SC samples confirm the highly mono dispersed spherical particles of about 10 nm (Fig. 3b). The high crystalline nature of the samples was confirmed through the HR-TEM images (inset of Fig. 3b and e). The inter-planar spacing of pure and GC samples have been measured 0.31 nm, which corresponds to the (111) plane of the fluorite structure of CeO2 (JCPDS No.43-1002). The GC and SC nanoparticles are also mono dispersed as observed from the TEM images in Fig. 3d and g. The HRTEM images of GC and SC samples (Fig. 3e and h) illustrate that the prepared materials are highly crystalline as the lattice fringes have been clearly observed. The SAED patterns of pure ceria, GC and SC (Fig. 3c, f and i) confirm the poly crystalline nature, as these patterns exhibit discontinuous rings, with different orientations [13].

Fig. 2

FE-SEM images of HMTA capped a CeO2, b Gd:CeO2, and c Sm:CeO2 nanoparticles

Fig. 3

HR-TEM images of a CeO2, d Gd:CeO2 and g Sm:CeO2 synthesized from isopropyl alcohol as a solvent, (b, e, h) high resolution images of CeO2, Gd:CeO2 and Sm:CeO2 nanoparticles, (c, f, i) SAED pattern showing the diffraction rings of CeO2, Gd:CeO2 and Sm:CeO2 respectively

As an ultraviolet blocking material, ceria based nanocrystals have strong absorption properties in the ultraviolet range. Figure 4 shows the UV–Vis spectra (Fig. 4a) and band gap plot (Fig. 4b) of ceria and RE doped ceria nanocrystals. The strong absorption in the UV inferred that the materials are capable of absorbing UV light, and hence the band gaps of the synthesized materials have been obtained from the plot to analyze the optical properties of the pure and RE doped ceria. It has been observed that the band gap of pure ceria (3.01 eV) is much smaller than the bulk ceria (3.36 eV) [17], probably due to the synthesis process of ceria in non-aqueous medium. The SC nanomaterial has shown further decrease in the band gap energy from 3.01 to 2.95 eV. This red shift in the band gap is attributed due to the presence of oxygen vacancies and Ce3+ concentration [18]. Hence, the band gap narrowing has occurred due to the formation of intermediate states caused by oxygen vacancies and Ce3+. Further, it is clearly understood that the increased oxygen vacancies and Ce3+ concentrations due to the incorporation of RE ions decreased the band gap in GC and SC samples.

Fig. 4

a UV–Vis absorbance spectra and b Tauc plot of CeO2, Gd: CeO2 and Sm:CeO2 nanoparticles

Different energy states exist between valence and conduction bands, which are responsible for radiative recombination, have been derived from the fluorescence spectral study. The spectra were recorded at room temperature for CeO2, GC and SC nanoparticles at an excitation wavelength of 350 nm. The fluorescence emission peaks were observed (Fig. 5) at 424 and 466 nm for CeO2. The emission at 424 nm corresponds to the band edge emission and the emission at 466 nm represents the intermediate trap states related to defects. On the other hand, the fluorescence spectra of GC and SC show a strong emission at 466 nm with the suppressed of band edge emission. It confirms the presence of intermediate trap states, which dominated the band edge emission in the doped material. The oxygen vacancies and other related defects are responsible for the formation of trap states in the material. The trap level emission is strong for RE doped material compared to pure ceria, possibly due to variation in the concentration of oxygen vacancy and related defect content in the material.

Fig. 5

Fluorescence spectra of CeO2, Gd: CeO2 and Sm:CeO2 nanoparticles

Figure 6a shows 3d core level XPS spectra of ceria, GC and SC samples in the region 880–925 eV. The spectra consist of two groups of distinct peaks of Ce4+ and Ce3+ with different spin states (3d5/2 and 3d3/2). The characteristic peaks of Ce4+ 3d5/2 and Ce4+ 3d3/2 were observed at the binding energies of 898.5 and 917 eV, respectively. The peaks at binding energies of 885.5 and 905 eV represent Ce3+ 3d5/2 and Ce3+ 3d3/2, respectively [19, 20]. The recorded spectra show the presence of mixed valence states of Ce3+ and Ce4+ in all samples. The main peaks observed at 885.5 and 898.5 eV represent the relative amount of Ce3+ and Ce4+ present in the sample. From the area of the respective peaks, it is obvious that the concentration of Ce3+ is relatively higher in all the samples. From the deconvoluted patterns, it is observed that the intensity of peak at 901.5 eV has decreased relatively in the GC and SC samples compared to pure sample, confirming that the conversion of Ce4+ into Ce3+ is relatively higher in the GC and SC samples [21, 22, 23, 24, 25]. From the results, it is inferred that the incorporation of RE ions reduces the Ce4+ into Ce3+. The presence of high concentration of Ce3+ has reasonably influenced the narrowing of band gap of ceria and established charge imbalance which, in turn, created oxygen vacancies [26, 27, 28, 29, 30, 31].

Fig. 6

XPS spectra of a Ce, b oxygen, c Gd and d Sm recorded for Ceria, GC and SC nanoparticles

The XPS O 1s spectra of the synthesized ceria, GC and SC samples are shown in Fig. 6b. All spectra are composed of two peaks. The main peak at low binding energy is assigned to lattice oxygen and the second low intense peak at higher binding energy corresponds to the surface oxygen [32]. The intensity of both the peaks has been decreased in the GC and SC samples compared to pure sample. Hence, the ratio of lattice oxygen and surface oxygen is relatively decreased in GC and SC samples compared to pure sample, which confirms high oxygen vacancy concentration in GC and SC samples. Moreover, in the SC sample, the high energy peak is shifted towards higher binding energy compared to pure and GC samples. The shift confirms that the Sm3+ bond with ceria through oxygen (Ce–O–Sm) reduces the Ce–O interaction results in loosely bonded oxygen which may further contribute to the OSC of ceria [21, 22, 23, 24, 25]. Figure 6c and d shows the core level spectra of Gd 3d and Sm 3d, respectively, which confirms the presence of dopants.

The photocatalytic activity of ceria, GC and SC nanostructures has been studied by analyzing the degradation of methylene blue (MB) dye under natural sunlight. Since the MB dye has strong absorption around 660 nm, it can be used to study the photocatalytic activity of ceria based photocatalysts as the absorption wavelength of ceria is far from dye absorption and thus overlapping of spectra can be avoided [18]. Absorption spectra of MB aqueous solution in the presence of ceria, GC and SC nanoparticles after exposing visible light for various irradiation times are shown in Fig. 7a–c. During irradiation, the initial concentration of dye solution was low; as a result, the absorption of dye solution decreases with irradiation time. Moreover, in the presence of SC sample, the absorption of MB has drastically decreased within a short period of irradiation time (180 min) compared to other two samples. From the absorption spectra, the rate of degradation of dye (C/C0) has been comparatively analyzed for various irradiation time as shown in Fig. 8. The kinetics of MB degradation process in the presence of ceria, GC and SC samples. From the kinetics plot, it has been found that the degradation follows the first order reaction as shown in Eq. (1);

Fig. 7

Photo-degradation of MB in the presence of a CeO2, b Gd:CeO2 and c Sm:CeO2 nanoparticles for various time intervals

Fig. 8

Rate of degradation as a function of irradiation time for Ceria, GC and SC nanoparticles

$$- {\text{ln }}\left( {{\text{C}}/{{\text{C}}_0}} \right)={\text{kt}}$$
(1)
where, C is the concentration of dye at time t, C0 is the initial concentration of dye, k is the rate constant (min−1). It is observed that the rare earth doped ceria has the strong photocatalytic activity than pure sample. The SC sample shows superior photocatalytic activity compared to other samples as the rate constant is relatively larger than that of other samples (Fig. 9). Moreover, the half-life time of the reaction has been calculated from the kinetics plot. The calculated rate constants and half-life time of the degradation reactions have been tabulated in Table 2. The half -life time of the degradation reaction in the presence of SC is relatively shorter than pure and GC samples, which further confirm the stronger photocatalytic activity.
Fig. 9

Kinetics plot for the first order reaction of degradation of MB dye in the presence of ceria, GC and SC nanoparticles

Table 2

Calculated rate constants and half-life time of degradation process for pure, GC and SC samples

Samples

Rate constant (min−1)

Half-life time, t1/2 (min)

Ceria

0.0122

56.7

GC

0.0132

52.4

SC

0.0179

38.6

Based on the experimental results, the degradation process is schematically shown in Fig. 10. During illumination, the electrons in the ceria gets excited by absorbing photons from the sun light and moves to the conduction band leaving holes in the valance band. The oxygen molecules adsorbed on the surface of ceria and in the dye solution were transformed into oxide radicals (.O2) by capturing the excited free electrons. On the other hand, holes are captured by OH group of water molecule and form the hydroxyl radicals (.OH). These oxide (at conduction band) and hydroxyl (at valance band) radicals are responsible for the degradation of dye molecules [9].

Fig. 10

Proposed mechanism for the photo-excited electron–hole separation and transport process at the ceria, GC and SC samples

The photocatalytic process mainly depends on the photoexcitation (generation of electron–hole pairs), separation of photogenerated charge carriers and surface redox reaction sites like oxygen vacancies [33]. Therefore, the oxygen vacancies and Ce3+ concentration also determine the photocatalytic performances of ceria [34, 35, 36]. In the present work, all the three samples are annealed at 300 °C. During the annealing process, the oxygen has diffused into the surface of the samples by creating oxygen vacancies at the lattice site, which has increased the Ce3+ concentrations in all the samples. The incorporation of rare earth ions has increased the RE–O–Ce bonding as confirmed through XPS analysis. Therefore, the interaction between Ce and oxygen has been decreased due to the presence of rare earth ions. Thus, the concentration of loosely bonded oxygen has been increased in the GC and SC samples thereby a large number of oxygen diffused to the surface during annealing process. As a consequence, the oxygen vacancy and Ce3+ concentrations are relatively higher in GC and SC samples as confirmed in the XPS analysis and hence, high photocatalytic performance has been observed in both the samples compared to ceria. Among the rare earth ions, the interaction of Sm with Ce may be stronger compared to Gd due to the variation in the ionic radii, which could enhance the oxygen vacancy and Ce3+ concentrations. As a result, the band gap of SC has been further narrowed with enhanced photocatalytic performance.

4 Conclusion

Effect of Gd and Sm doping on the photocatalytic activity of ceria was studied. XRD studies reveal the lattice expansion for the doped samples. UV–Vis absorption of pure, GC and SC confirmed that the band gap of the materials has been narrowed. Fluorescence studies confirm the presence of defect related intermediate trap states. The presence of Sm and Gd in CeO2 nanoparticles was confirmed by XPS analysis. XPS analysis demonstrated the changes in the Ce4+ to Ce3+ ratio and oxygen vacancy defects. The photocatalytic process was discussed based on the oxygen vacancy and Ce3+ concentrations. The photocatalytic performance of Sm:CeO2 was relatively higher compared with pure and Gd:CeO2 in the degradation of methylene blue dye, because of the variation in oxygen vacancy and Ce3+ concentrations. The present experimental results confirm that the RE doped ceria nanocrystals are potential catalytic materials for green environment.

Notes

Acknowledgements

One of the authors M. Karl Chinnu is grateful to Prof. Forest Shish-Sen Chien, Department of Applied Physics, Songhai University and Ministry of Taiwan Government for Postdoctoral fellowship. The authors are thankful to Nanomaterials laboratory, Research Institute of Electronics, Shizuoka University, Japan for HR-TEM and XPS studies.

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of PhysicsDr. Ambedkar Government Arts CollegeChennaiIndia
  2. 2.Department of PhysicsThiru Kolanjiappar Government Arts CollegeVriddhachalamIndia
  3. 3.Centre for Nanoscience and TechnologyAnna UniversityChennaiIndia
  4. 4.Department of PhysicsPanimalar Engineering CollegeChennaiIndia
  5. 5.Department of PhysicsPresidency CollegeChennaiIndia

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