Synthesis, Characterization, and Photocatalytic Properties of Flower-like Mn-doped Ceria

Abstract

Mn-doped CeO₂ flower-like microstructures have been synthesized by a facile one-step composite-hydroxide-mediated method. The structure, morphology, optical and the surface properties of Mn doped CeO₂ have been investigated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), UV-Vis absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). The XRD results confirmed the successful incorporation of Mn into the CeO₂ lattice through the formation of face-centered cubic solid solution. The photocatalytic activities of the catalysts were evaluated by measuring the photodegradation efficiency of Rhodamine B (RhB) under ultraviolet light irradiation. With an optimal molar ratio of 1% in Mn/CeO₂ the highest rate photodegradation was achieved under the experimental conditions. The enhanced photocatalytic activity can be attributed to the incorporation of multivalent Mn in CeO₂ promoted the separation of photogenerated charges, inhibited the recombination of photogenerated carriers, and thus prolonged the charges lifetime to participate in the photocatalytic reaction.

Keywords:
Cerium oxide; Manganese doped; Photocatalysis

1. Introduction

Semiconductor-based photocatalysis has emerged as one of the most attractive techniques to convert sunlight into chemical energy so as to remove organic pollutants from wastewater for environmental remediation. Among various semiconductor materials, as a high-efficiency, nontoxicity, abundant, photochemical stability and low-cost promising photocatalyst, CeO₂ has been an extensively investigated material for environmental pollution removal and photocatalytic hydrogen evolution^1-3. Nevertheless, some drawbacks limit its practical application in the photocatalyst region, including rapid recombination rate of photogenerated electron-hole pairs, low quantum yield in the reactions, and very poor response to visible light. Therefore, from the viewpoint of photochemistry, numerous methods for suppression of these drawbacks have been attempted to improve CeO₂-based photocatalytic activities. To the best of our knowledge, one of approaches is to develop CeO₂ based heterostructure semiconductor systems, such as TiO₂@Pt@CeO₂⁴, Au/CeO₂^5,6, CdS/CeO₂⁷, SrTiO₃/CeO₂⁸, Bi₂O₃/CeO₂⁹, TiO₂/CeO₂^10,11 and Cu₂O/CeO₂¹². On the other hand, there have been modifications of CeO₂ by doping non-metallic species^13,14 and crystal facet engineering^15,16. For instance, Zhu et al.¹³ reported the synthesis of N-doped CeO₂ nanoparticles with controllable doping levels at the nanoscale through a reliable wet chemical approach, showing enhanced visible-light sensitivity and photocatalytic activity. Fuertes et al.¹⁴ have doped nitrogen into ceria powder by sintering CeO₂ in NH₃ flow at very high temperature. Recent reports have shown that CeO₂ nanostructures with highly active exposed crystal planes such as {100} and {110} can significantly enhance their catalytic activity¹⁵. Meanwhile, the selective metal doping of CeO₂ to improve its performance started to appear in the literature. Recently, the metal doping of CeO₂ has also been examined in solar cell devices¹⁷, but the corresponding photocatalytic investigations are relatively few.

It has been reported that doping with multivalent transition-metal (TM) cations was considered an effective method to inhibit the recombination of photogenerated carriers in semiconductors¹⁸. Theoretical investigation showed that among the 3d metals, Mn has the greatest potential in permitting significant optical absorption in the visible or even the infrared solar light, through the combined effects of narrowed band gap and the introduction of intermediate bands within the forbidden gap^19,20. Recently, there have been many reports on Mn-doped oxide photo-activity under UV and visible light, such as Mn-ZnO^21-24, Mn-TiO₂^25,26, where Mn exists in the bivalence oxidation state.

Inspired by above-mentioned investigations, flower-like Mn-doped CeO₂ photocatalyst was obtained via a simple one-step composite-hydroxide-mediated method. In addition, we demonstrated the enhanced photocatalytic performance of the Mn doped CeO₂ flower-like nanostructures by degradation of Rhodamine B (RhB) solutions, and further investigated the impact of Mn doping concentrations of doped CeO₂ on the resulting photocatalytic activities under ultraviolet light.

2. Experimental

2.1 Material preparation

Pure and flower-like Mn-doped CeO₂ nanostructure were obtained by a simple one-step composite-hydroxide-mediated method according to our group previously report with tiny modification²⁷. In a typical preparation process, (1) a total of 20 g of KOH and KI was mixed at a ratio of 70.6:29.4 and placed in a 25mL Teflon vessel. (2) Different molar ratio of MnSO₄·H₂O and CeO₂ was used as the raw material for reaction, and was placed on the top of the hydroxide in the vessel. (3) The Teflon vessel was put into a furnace preheated to 235 ^oC. (4) After the hydroxide was totally molten (30 min later), the molten reactants were mixed uniformly by shaking the covered vessel. (5) 24 hours later, the vessel was taken out and cooled to room temperature naturally. The as-prepared samples were removed from the solution, rinsed thoroughly several times with deionized water and ethanol to remove residual salts, and subsequently dried for 12h at vacuum circumstance. The as-prepared samples in the subsequent discussion in this paper are denoted as x% Mn/CeO₂, where x% refers to the Mn/(Mn+Ce) molar ratio.

2.2 Catalyst characterization

The crystal structure of the resultant products was characterized by X-ray powder diffraction (XRD) by using a Bruker AXS D8 advance powder diffractometer with Cu Kα radiation (λ = 0.154056 nm). Field-emission scanning electron microscope (FESEM, S-4800) was employed to characterize the morphologies and size of the synthesized Mn doped CeO₂ samples. UV-vis diffuse reflectance spectra were obtained for the dry-pressed disk samples by using a Shimadzu UV 2550 recording spectrophotometer, which was equipped with an integrating sphere, and BaSO₄ was used as a reference. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific Escalab 250 spectrometer with monochromatized Al Kα excitation, and C_1s (284.6 eV) was used to calibrate the peak positions of the elements. The Raman-scattering experiments were carried out using NEXUS 670 Raman spectrometer at room temperature. The 473 nm line of the solid-state laser was used for excitation.

2.3 Catalyst activity

The photocatalytic performance of the as-prepared samples was characterized by decomposing RhB under ultraviolet light irradiation at room temperature. The photocatalytic experiments were carried out by adding 100 mg photocatalysts into 300 mL of 10 mg L^-1 RhB solution in the vessel. A 300 W Hg arc lamp (XPA-1, Nanjing XuJiang Electromechanical Plant) was used as the light source. Prior to the irradiation, the suspensions were magnetically stirred in the dark for 30 min to establish the adsorption/desorption equilibrium. After different irradiation time, the concentration of the RhB solution was measured on a UV-Vis spectrophotometer (Hitachi UV-3100).

3. Results and Discussion

The X-ray diffraction (XRD) patterns of Mn_xCe_1-xO₂ photocatalysts are shown in Figure 1 along with pure CeO₂. The diffraction peaks of pure CeO₂ can be indexed to the fluorite cubic phase of CeO₂ (JCPDF card: 65-2975) with lattice constant a =0.5411 nm. The diffraction pattern of Mn_xCe_1-xO₂ photocatalysts was similar to that of pure CeO₂. The XRD pattern does not show any impurity/additional peaks for any Mn doping concentration. This clearly confirms that the Mn ions occupy positions within the fluorite-lattice. Furthermore, the extended X-ray diffraction pattern of Mn_xCe_1-xO₂ photocatalysts reveals a slight shift in the peak positions towards larger angles with increasing Mn-concentration, which may be due to the lattice reduction of CeO₂ upon Mn ions doping. Because the ionic radius of Mnⁿ⁺ (Mn⁴⁺=0.53 Å, Mn³⁺=0.645 Å, Mn²⁺=0.83 Å) is smaller than that of Ce⁴⁺ (1.01 Å), when Mnⁿ⁺ embedded in CeO₂ lattice and takes the place of Ce⁴⁺, the contraction and distortion of the ceria lattice occur, leading to the decrease of the cell parameter²⁸. In present study, the existence of the Mnⁿ⁺ oxidation state in Mn_xCe_1-xO₂ photocatalysts will be discussed in the later section.

Figure 1
XRD patterns of Mn-doped CeO₂ from different mole concentrations of Mn source

Figure 2 shows the morphology and size of the source material CeO₂ (a) and the flower-like 1% Mn-doped CeO₂ sample (b) obtained by typical field-emission scanning electron microscope (FESEM). The anomalous particles morphology of the source material CeO₂ can be seen with the diameter ranging from 2-8 µm (Figure 2 a). From the high-magnification SEM images shown in Figure 2 b, it can be seen that the flower-like micro-/nano-architectures are built from dozens of flake-like nanopetals with the average diameter of 1-4 µm.

Figure 2
FESEM images of (a) pure CeO₂ raw materials; (b) 1% Mn/CeO₂ sample

The effect of Mn substitution on cubic fluorite structure of CeO₂ lattice is further confirmed using UV-visible optical spectroscopy measured in the range of 200-800 nm. Figure 3a shows the room-temperature optical absorption spectra of undoped and flower-like Mn-doped CeO₂ photocatalyst. The absorbance spectra for 1 % Mn-doped CeO₂ sample was found to increase when compared to the undoped CeO₂ sample. However, the absorbance intensity for the 3 and 5% Mn-doped CeO₂ sample were found to decrease when compared to the undoped CeO₂ sample. This may be due to the electron-electron, electron-donor atom and electron-hole interactions which increase drastically as doping is increased beyond a critical limit^29,30. These interactions dominate the electron-photon interactions. A plot of variation of (αhν)²versus hν , which is obtained according to the Kubelka-Munk function transformation, is shown in Figure 3 b. The evaluated band gap values for the pure and the 1% Mn-doped CeO₂ sample were observed to be 3.05 eV and 2.88 eV, respectively. It could be seen that the band gap value slightly decreased when the dopant concentration of Mn was increased.

Figure 3
(a) UV-Vis absorption spectra of Mn/CeO₂ with varying Mn/Ce molar ratio; (b) Plot of (αhν)² versus photon energy of pure and Mn-doped CeO₂ samples

Raman scatting is an effective tool for the investigation of the effects of doping on nanomaterials, as the incorporation of dopants leads to shifts of the lattice Raman vibrational peak positions. Figure 4 displays the Raman spectra of pure CeO₂ as well as Mn-doped CeO₂ samples with different Mn contents. For the pure CeO₂ sample, a strong peak at 461 cm^-1 can be assigned to the F_2g Raman active mode of the cubic fluorite structure of CeO₂, which is due to the symmetric breathing mode of the oxygen atoms around cerium ions. Compared to the pure CeO₂, the peak intensity decreased greatly and became broader and red-shifted for the Mn-doped CeO₂ samples. The red-shift could be attributed to the changes in lattice parameter with crystallite size, as it was previously explained by phonon confinement model^31,32. Another reason of shifting and broadening may be the increase in oxygen vacancies, which is related to structural defects derived from partially incorporation of manganese into CeO₂ lattice. In the Mn-doped CeO₂ samples, the extra oxygen vacancies were generated by the incorporation of Mn ion into the ceria fluorite lattice to compensate for the valence mismatch between the Mnⁿ⁺ and Ce⁴⁺ ions.

Figure 4
Raman spectra of Mn/CeO₂ with varying Mn/Ce molar ratio

The chemical states of the Mn doping in the as-prepared samples were then investigated by XPS, as presented in Figure 5a, the XPS survey spectra indicated that the as-prepared Mn doped CeO₂ nanoflowers were composed of Ce, O and Mn elements. Figure 5b shows the XPS spectra of the Mn 2p region. The double peaks with binding energies of ca. 641.0 eV and 653.1 eV correspond to the characteristic of Mn 2p_3/2 and Mn 2p_1/2 signals, respectively. Since XPS signals for Mn²⁺ and Mn³⁺ are very close to each other ～641.0 eV for the BE of Mn 2p_3/2,³³ thus the coexistence of Mn²⁺/Mn³⁺ ion couple in the Mn-doped CeO₂ samples. Here, the Mn2P_3/2 peak is deconvoluted with the Gaussian-Lorenz model functions, and two peaks at 641.2 eV and 642.3 eV can be assigned to the Mn²⁺ and Mn³⁺ ions, respectively, according to the standard binding energy and previous literature.³ This results suggest the coexistence of Mn²⁺ and Mn³⁺ ions on the surface of samples. In addition, we can observe that the XPS peaks show some noise, implying that the content of Mn ions at the surface of the sample is low and Mn ions have been doped into the interior of the nanostructure.

Figure 5
XPS spectra (a) survey spectrum and (b) Mn 2p for 3%Mn/CeO₂ sample

The effective incorporation of Mn²⁺ ions into CeO₂ crystalline lattices can greatly enhance the light photocatalytic performance. The UV-vis spectral changes of RhB solution over 1% Mn doped CeO₂ samples during the photodegradation are shown in Figure 6a, clearly show that the characteristic absorption peaks corresponding to RhB decrease rapidly as the exposure time increases, indicating the decomposition of RhB and the significant reduction in the RhB concentration. Figure 6 b shows the results of RhB photodegradation over Mn doped CeO₂ samples with different Mn doping concentrations. After 210 min of irradiation, the photodegradation efficiencies of RhB were about 65, 40 and 30% for 1, 3 and 5% Mn doped CeO₂ samples, respectively. It was evident that 1% Mn doped CeO₂ samples exhibited excellent photocatalytic activities for the RhB degradation. However, the photocatalytic performance of Mn doped CeO₂ samples decreased with the increase of Mn doped amount. The results show that there was an optimal Mn doping concentration in CeO₂ samples for the ultraviolet light photocatalysis (1% Mn dopant). When the Mn doping concentration was further increased, the Mn dopant sites could be also act as efficient recombination centers with increased recombination rate due to the reduced average distance between trapped carriers³⁴. The excess Mn dopant sites could greatly decrease the number of charge carriers and deteriorate the photocatalytic performance of doped CeO₂ samples, as identified from Figure 6b.

Figure 6
(a) UV-vis spectral variations of RhB solution over 1% Mn doped CeO₂ under ultraviolet light irradiation; (b) Degradation of RhB solutions over CeO₂ and Mn doped CeO₂ with different Mn concentrations under ultraviolet light

4. Conclusions

In summary, we have synthesized a series of Mn/CeO₂ photocatalysts by a facile one-step composite-hydroxide-mediated approach. This work focuses on understanding of the effects of the Mn addition in Mn-doped CeO₂ nanoflowers. Noticeably, there was an optimal Mn doping concentration in CeO₂ samples for the ultraviolet light photocatalysis. Experiments showed that 1% Mn-doped CeO₂ had higher photocatalytic capability compared with that of the pure CeO₂ or 3% and 5% Mn ion-doping catalyst. The enhancement in redox efficiency of CeO₂ samples upon Mn doping may be due to the increase in charge transport rate.

5. Acknowledgements

This research was financially supported by the Natural Science Foundation of AnHui Provincial Education Department (KJ2016A102), Anhui Provincial Natural Science Foundation (1808085ME138), Graduate student Innovation Fund of Anhui University of Technology and National Undergraduate Training Programs for Innovation and Entrepreneurship (201710360024, 201710360026)

6. References

• ¹
  Li L, Wang HR, Zou L, Wang X. Controllable synthesis, photocatalytic and electrocatalytic properties of CeO₂ nanocrystals. RSC Advances 2015;5(52):41506-41512.
• ²
  Lu X, Zhai T, Cui H, Shi J, Xie S, Huang Y, et al. Redox cycles promoting photocatalytic hydrogen evolution of CeO₂ nanorods. Journal of Materials Chemistry 2011;21(15):5569-5572.
• ³
  Rangaswamy A, Venkataswamy P, Devaiah D, Ramana S, Reddy BM. Structural characteristics and catalytic performance of nanostructured Mn-doped CeO₂ solid solutions towards oxidation of benzylamine by molecular O₂ Materials Research Bulletin 2017;88:136-147.
• ⁴
  Li SX, Cai JB, Wu XQ, Liu BW, Chen QY, Li YH, et al. TiO₂@Pt@CeO₂ nanocomposite as a bifunctional catalyst for enhancing photo-reduction of Cr (VI) and photo-oxidation of benzyl alcohol. Journal of Hazardous Materials 2018;346:52-61.
• ⁵
  Li BX, Zhang BS, Nie SB, Shao LZ, Hu LY. Optimization of plasmon-induced photocatalysis in electrospun Au/CeO₂ hybrid nanofibers for selective oxidation of benzyl alcohol. Journal of Catalysis 2017;348:256-264.
• ⁶
  Tanaka A, Hashimoto K, Kominami H. Selective photocatalytic oxidation of aromatic alcohols to aldehydes in an aqueous suspension of gold nanoparticles supported on cerium(IV) oxide under irradiation of green light. Chemical Communications 2011;47(37):10446-10448.
• ⁷
  Zhang P, Liu Y, Tian BZ, Luo YS, Zhang JL. Synthesis of core-shell structured CdS @CeO₂ and CdS @TiO₂ composites and comparison of their photocatalytic activities for the selective oxidation of benzyl alcohol to benzaldehyde. Catalysis Today 2017;281(Pt 1):181-188.
• ⁸
  Song S, Xu L, He Z, Ying H, Chen J, Xiao X, et al. Photocatalytic degradation of C.I. Direct Red 23 in aqueous solutions under UV irradiation using SrTiO₃/CeO₂ composite as the catalyst. Journal of Hazardous Materials 2008;152(3):1301-1308.
• ⁹
  Zou ZJ, Xie CS, Zhang SS, Yu XL, Zou T, Li J. Preparation and photocatalytic activity of TiO₂/CeO₂/Bi₂O₃ composite for Rhodamine B degradation under visible light irradiation. Journal of Alloys and Compounds 2013;581:385-391.
• ¹⁰
  Lu XW, Li XZ, Qian JC, Miao NW, Yao C, Chen ZG. Synthesis and characterization of CeO₂/TiO₂ nanotube arrays and enhanced photocatalytic oxidative desulfurization performance. Journal of Alloys and Compounds 2016;661:363-371.
• ¹¹
  Pavasupree S, Suzuki Y, Pivsa-Art S, Yoshikawa S. Preparation and characterization of mesoporous TiO₂-CeO₂ nanopowders respond to visible wavelength. Journal of Solid State Chemistry 2005;178(1):128-134.
• ¹²
  Hu S, Zhou F, Wang L, Zhang J. Preparation of Cu₂O/CeO₂ heterojunction photocatalyst for the degradation of Acid Orange 7 under visible light irradiation. Catalysis Communications 2011;12(9):794-797.
• ¹³
  Mao C, Zhao Y, Qiu X, Zhu J, Burda C. Synthesis, characterization and computational study of nitrogen-doped CeO₂ nanoparticles with visible-light activity. Physical Chemistry Chemical Physics 2008;10(36):5633-5638.
• ¹⁴
  Belén Jorge A, Fraxedas J, Cantarero A, Williams AJ, Rodgers J, Attfield JP, et al. Nitrogen doping of ceria. Chemistry of Materials 2008;20(5):1682-1684.
• ¹⁵
  Zhou K, Wang X, Sun X, Peng Q, Li Y. Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes. Journal of Catalysis 2005;229(1):206-212.
• ¹⁶
  Ren J, Liu X, Gao RH, Dai WL. Morphology and crystal-plane effects of Zr-doped CeO₂ nanocrystals on the epoxidation of styrene with tert-butylhydroperoxide as the oxidant. Journal of Energy Chemistry 2017;26(4):681-687.
• ¹⁷
  Corma A, Atienzar P, Garcia H, Chane-Ching JY. Hierarchically mesostructured doped CeO₂ with potential for solar-cell use. Nature Materials 2004;3(6):394-397.
• ¹⁸
  Prabaharan DDM, Sadaiyandi K, Mahendran M, Sagadevan S. Investigating the effect of Mn-doped CeO₂ nanoparticles by co-precipitation method. Applied Physics A 2018;124(2):86.
• ¹⁹
  Shao G. Electronic structures of manganese-doped rutile TiO₂ from first principles. The Journal of Physical Chemistry C 2008;112(47):18677-18685.
• ²⁰
  Shao G. Red shift in manganese- and iron-doped TiO₂: a DFT+U analysis. The Journal of Physical Chemistry C 2009;113(16):6800-6808.
• ²¹
  Yang Y, Li Y, Zhu L, He H, Hu L, Huang J, et al. Shape control of colloidal Mn doped ZnO nanocrystals and their visible light photocatalytic properties. Nanoscale 2013;5(21):10461-10471.
• ²²
  Ullah R, Dutta J. Photocatalytic degradation of organic dyes with manganese-doped ZnO nanoparticles. Journal of Hazardous Materials 2008;156(1-3):194-200.
• ²³
  Abdollahi Y, Abdullah AH, Gaya UI, Zainal Z, Yusof NA. Enhanced photodegradation of o-cresol in aqueous Mn(1%)-doped ZnO suspensions. Environmental Technology 2012;33(10-12):1183-1189.
• ²⁴
  Lu Y, Lin Y, Xie T, Shi S, Fan H, Wang D. Enhancement of visible-light-driven photoresponse of Mn/ZnO system: photogenerated charge transfer properties and photocatalytic activity. Nanoscale 2012;4(20):6393-6400.
• ²⁵
  Chen Z, Li Y, Guo M, Xu F, Wang P, Du Y, et al. One-pot synthesis of Mn-doped TiO₂ grown on graphene and the mechanism for removal of Cr(VI) and Cr(III). Journal of Hazardous Materials 2016;310:188-198.
• ²⁶
  Deng QR, Xia XH, Guo ML Gao Y, Shao G. Mn-doped TiO₂ nanopowders with remarkable visible light photocatalytic activity. Materials Letters 2011;65(13):2051-2054.
• ²⁷
  Tan J, Zhang W, Lv YH, Xia AL. Facile preparation of Mn-doped CeO₂ submicrorods by composite-hydroxide-salt-mediated approach and their magnetic property. Materials Research 2013;16(4):689-694.
• ²⁸
  Murugan B, Ramaswamy AV, Srinivas D, Gopinath CS, Ramaswamy V. Nature of manganese species in Ce_1-xMnxO_2-d solid solutions synthesized by the solution combustion route. Chemistry of Materials 2005;17(15):3983-3993.
• ²⁹
  Van Overstraeten RJ, Mertens RP. Heavy doping effects in silicon. Solid State Electronics 1987;30(11):1077-1087.
• ³⁰
  Pavan Kumar CHSS, Pandeeswari R, Jeyaprakash BG. Structural, morphological and optical properties of spay deposited Mn-doped CeO₂ thin films. Journal of Alloys and Compounds 2014;602:180-186.
• ³¹
  Spanier JE, Robinson RD, Zhang F, Chan SW, Herman IP. Size-dependent properties of CeO_2-y nanoparticles as studied by Raman scattering. Physical Review B 2001;64(24):245407.
• ³²
  Fernández-García M, Martínez-Arias A, Hanson JC, Rodriguez JA. nanostructured oxides in chemistry: characterization and properties. Chemical Reviews 2004;104(9):4063-4104.
• ³³
  Cong CJ, Liao L, Liu QY, Li JC, Zhang KL. Effects of temperature on the ferromagnetism of Mn-doped ZnO nanoparticles and Mn-related Raman vibration. Nanotechnology 2006;17(5):1520.
• ³⁴
  Choi WY, Termin A, Hoffmann MR. The role of metal ion dopants in quantum-sized TiO₂: correlation between photoreactivity and charge carrier recombination dynamics. The Journal of Physical Chemistry 1994;98(51):13669-13679.

Publication Dates

• Publication in this collection
  16 July 2018
• Date of issue
  2018

History

• Received
  05 Mar 2018
• Reviewed
  22 May 2018
• Accepted
  08 June 2018