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Visible light driven nanocrystal anatase TiO2 doped by Ce from sol–gel method and its photoelectrochemical water splitting properties
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Available online at www.sciencedirect.com
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Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties Mengkui Tian a,b,*, Huimin Wang a, Dongshan Sun a, Wenjie Peng a, Wenliang Tao a a
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou Province 550025, PR China Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
b
article info
abstract
Article history:
Visible light driven nanocrystal anatase TiO2 was prepared by doping rare earth element Ce
Received 31 October 2013
through solegel method. UVeVis diffusion reflectance spectrum indicated its absorption
Received in revised form
edge extended to about 550 nm, red shifting about 170 nm compared with that without
7 March 2014
doping. Ce doping TiO2 showed obvious anodic photocurrent effect for water splitting
Accepted 3 April 2014
under visible light irradiation (l > 420 nm) in photoelectrochemical measurement with
Available online xxx
three electrodes configuration. Ce doping TiO2 showed higher photocurrent density than
Keywords:
tures for CeO2 and TiO2 were analyzed theoretically based on the first principle calculation.
Ce doping TiO2
As a result, the electronic structure for Ce doping TiO2 is proposed as the overlap and some
Photoelectrochemical
degree of hybridization among splitting occupied Ce 4f and unoccupied Ce 4f with O 2p and
Water splitting
Ti 3d respectively. The visible light responsive property is mainly due to the transition from
Photoanode
O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d.
Electronic structure
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
that of without doping TiO2 under full arc irradiation. Furthermore, the electronic struc-
reserved.
Introduction The evolution of H2 by water splitting on semiconductor photocatalyst under solar irradiation is well recognized as one of the most promising ways to develop hydrogen as energy carrier considering resource, environment and cost issues [1,2]. For this solar-to-hydrogen process, the most important work is to design semiconductor photocatalysts with appropriate band gap of about 2.0 eV to make the best use of solar
energy in visible light region. Meanwhile, electrochemical potential for conduction band and valence band edges of a candidate photocatalyst must satisfy water reduction (0 V Vs. Normal Hydrogen Energy) and water oxidation (1.23 V Vs. Normal Hydrogen Energy) requirements respectively. Furthermore, the semiconductor materials must be stable in water or electrolyte solution and the above-mentioned properties must be satisfied simultaneously [3,4]. However, these requirements are the only prerequisite conditions for these high functional photocatalysts to split water rather than
* Corresponding author. School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou Province 550025, PR China. E-mail addresses: [email protected], [email protected] (M. Tian). http://dx.doi.org/10.1016/j.ijhydene.2014.04.036 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
sufficient condition. Actually, the above-mentioned properties for a semiconductor photocatalyst are contradictory in some degree and difficult to get a good trade-off among them. For instance, most oxides such as TiO2, ZrO2, ZnO, NaTaO3 are stable in solution and show high activities for water decomposition under ultraviolet region, but they fail to make the best use of solar energy for their wide band gaps [5,6]. On the contrary, these oxy(nitrides) and oxy(sulfides), such as Ta3N5, TaON and CdS, show good visible light responsive ability, but usually they are unstable in solution for the photocorrosion reaction of photocatalyst itself [7e9]. TiO2 is a well-known photocatalyst not only for its high activity, high stability and low cost as well non-toxic properties, but also it is the pioneer materials to photocatalytical science and technology field. Consequently, up to now, TiO2 is the only widely utilized photocatalyst in mass production and will be in the future [10e12]. Although TiO2 is a successful photocatalyst in many aspects, the wide band gap (3.2 eV for anatase phase) is the fatal shortcoming limiting its utilization and efficiency. There are so many efforts focusing on extending its absorption edge to visible light region and at the same time try to remain its high stability and high quantum yield. Among these efforts, doping with cations such as Niþ, Fe3þ, Cr3þ, Sb5þ, and anions such as N, C, S, and F, Cl etc were extensively studied, but in most cases, they demonstrated limited visible light responsive ability and low efficiency resulting from the discrete donor or acceptor energy levels from doping [7,13,14]. In author’s previous study, visible light driven photocatalysts K4Ce2M10O30 (M ¼ Ta, Nb) were presented and theoretical analysis revealed that Ce 4f had great contribution to the visible light responsive mechanism [15]. Similar phenomenon also happened on other rare earth including compounds LnTaO4 (Ln ¼ La, Ce, Pr, Nd, and Sm) [16]. In this paper, there presented new effort to extend TiO2 absorption edge by doping rare earth element Ce, and its photoelectrochemical properties for water splitting on photoelectrode film under visible light were investigated closely. Although there were ever reports about TiO2 doped by Ce [17,18], here focused on its photoelectrochemical water splitting properties under visible light irradiation and its visible light driven mechanism based on the theoretical calculation analysis.
Experimental Powder sample preparation Nanocrystal anatase TiO2 doped by Ce (denoted as CeeTiO2) and without doping was prepared by conventional solegel methods. 10 ml of TiCl4 was dipped into Ce(NO3)$6H2O solution (Ce 0.5% molar ratio) slowly with stirring, and adjusted pH to about 7.0 by NH3$H2O. The gel was aged at 70 C for 24 h, finally heat treated at 500 C in air for 24 h, and TiO2 without doping followed the same procedure. The as-prepared powder was identified by X-ray powder diffraction on Geiger-flex RADB (Rigaku; Cu Ka). UVeVis diffuse reflectance spectrum was recorded by a spectrophotometer (JASCO, V-670). SEM images were obtained on field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi).
Preparation of electrodes Porous film electrodes were prepared by electrophoretic deposition method on conductive fluorine doped tin oxide glass (FTO, Asahi Glass Co.). The electrophoretic deposition was carried out in an acetone solution (40 ml) containing powder sample (40 mg) and iodine (15 mg), which was dispersed by sonication for 3 min. Two FTO electrodes (1.5 5 cm) were immersed parallel in the solution with 10 mm of distance, and then 40 V of bias potential to guarantee 15 mA current was applied between them for 1 min by a potentiostat (PARSTAT 2263) with stirring. The coated area was controlled to be ca. 1.5 4 cm. Finally, the electrodes prepared were calcinated at 350 C in air for 1 h to fasten the adhesion of film. This procedure resulted in formation of porous CeeTiO2 and TiO2 layer with uniform thickness of ca. 2 mm, with good reproducibility.
Loading of IrO2 Colloidal IrO2$nH2O was deposited on the surface of the electrode as a co-catalyst to prompt water oxidation. The colloidal IrO2$nH2O aqueous was prepared by hydrolysis of Na2IrCl6, with the pH of the solution adjusted to 11e12 using 0.1 M NaOH aq. Then the solution was heated at 70 C for 30 min, and then cooled to room temperature by immersion in a cold water bath, followed by pH adjusted to 9 using 0.1 M HNO3 aq. Subsequently, heating at 70 C for 30 min resulted in a deep blue solution containing colloidal IrO2$nH2O, and denoted as IrO2 hereafter. The loading of IrO2 was carried out by immersing photoelectrode into the colloidal IrO2$nH2O solution for 1 h, washed with distilled water and dried in air. The amount of IrO2 loaded (ca. 0.3 wt%) was determined by the change in absorption spectrum of IrO2-colloidal solution.
Photoelectrochemical measurements The photoelectrochemical measurement was performed by three-electrodes configuration mode consisting of a working electrode (as prepared electrode), a counter electrode (Pt mesh) and a reference electrode (Ag/AgCl) as well as electrolyte (0.1 M aqueous Na2SO4 solution), and the pH of the electrolyte solution was adjusted to 4.0 by 0.1 M H2SO4. The bias potential on working electrode was controlled by a Potentiostat (HSV-100). The solution was purged with Ar gas for over 10 min before the measurement. The electrode was irradiated through silicon glass window by a Xe lamp (300 W, Cermax) fitted with cut-off filters under visible light irradiation, and that without any filter was denoted as full arc irradiation. A shutter controlled by a magnetic controller set 3s for light on and off intermediately.
Results and discussions Characterizations of CeeTiO2 and TiO2 The powder X-ray diffraction patterns of as prepared CeeTiO2 and TiO2, as well as standard references for anatase nanocrystalline TiO2 and CeO2 were shown in Fig. 1. From their XRD
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 1 e X-ray diffusion patterns for as-prepared samples and references.
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that the particle size is of micrometer level for aggregation. The UVeVis diffusion reflectance spectrums for these Ce doping TiO2 and without doping were shown in Fig. 3. From Fig. 3, it can be seen that the absorption tail edge of Ce doping TiO2 is about 550 nm, red shifting 170 nm compared with that of without doping TiO2(380 nm). This red shifting is consistent with their apparent colors, from white for without doping to yellow color for the doping with Ce. Additionally, in order to compare with CeO2, which shows some degree of visible light responsive ability (absorption edge is about 440 nm), the UVeVis diffusion reflectance spectrum of CeO2 was also illustrated in Fig. 3. Since there was no CeO2 phase in the CeeTiO2 sample, which can be confirmed from their XRD patterns shown in Fig. 1, so the visible light responsive ability of CeeTiO2 is attributed for the Ce4þ doped into TiO2 in intergranular. The mechanism for visible light responsive ability will be discussed in detail in later section.
Photoelectrochemical water splitting properties
patterns, it can be seen that the as-prepared samples showed good agreement with anatase nanocrystalline TiO2 for both doping with Ce and without. Since the ionic radius of Ce4þ (1.02 A) is much bigger than A), it is difficult to form lattice substitution. that of Ti4þ (0.68 Furthermore, as there showed no obvious shifting of the main diffraction peaks of TiO2 and Ce doped TiO2, it is proposed that the Ce4þ doped into TiO2 in intergranular rather than lattice substitution [19]. Their SEM images were shown in Fig. 2. Although their crystals sizes are typical nano size of 20e30 nm, it is obvious
The photoelectrochemical water splitting for CeeTiO2 and TiO2 was performed on FTO photoelectrodes with three electrodes configuration. The photocurrent effect under 300 W Xe lamp irradiation without filter was shown in Fig. 4(a). Both TiO2 and the CeeTiO2 demonstrated anodic photocurrent under positive bias potential, which revealed that TiO2 and CeeTiO2 are typical N-type semiconductors, acting as photoanodes under irradiation. The anodic current was resulting from the oxidation of H2O on the surface of photoelectrode by photo-excited holes, while the photo-excited electrons transferred to counter electrode through external circuit and involved the reaction of reducing water. Moreover, under this
Fig. 2 e SEM images for TiO2 (a) and (b), CeeTiO2 (c) and (d) under different magnification. Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Fig. 3 e UVeVis diffusion reflectance spectrum for Ce doped TiO2 and without.
full arc irradiation, Ce doping TiO2 showed higher current density than that of TiO2 without doping, which maybe ascribes to the Ce doping TiO2 can absorb more efficient photons with energy larger than the energy gap to generate more charge carriers to evolve redox reaction for its red shifting absorption edge. It is difficult to determine precisely the onset potential just from voltageecurrent curve. In this case, it is proposed the onset potential is less than 0 V (Vs. RHE(Reverse Hydrogen
Energy), RHE ¼ EAg/AgClþ0.059 pH þ E0Ag/AgCl, E0Ag/ AgCl ¼ 0.196 V Vs. NHE) since there still presented obvious current at 0 V (Vs. RHE) as shown in Fig. 4. The onset potential is regarded as equally to the semi-Fermi energy, which is near to the bottom of conduction band for N-type semiconductor, so it indicated that TiO2 and CeeTiO2 have appropriate conduction band edges to meet reducing water requirement without bias potential. Under visible light irradiation (l > 420 nm), for this without doping TiO2, there was no detectable current, which is consistent with its absorption edge. For Ce doping TiO2, the photocurrent effect under visible light (l > 420 nm) was shown in Fig. 4(b). CeeTiO2 demonstrated obvious anodic current under visible light irradiation and the photocurrent reached 300 mA/cm2 without any modification. When CeeTiO2 photoelectrode was loaded with IrO2, an extensively used co-catalyst to prompt the oxidation of water in photocatalysis [20], the photocurrent was enhanced greatly as shown in Fig. 4(b). The reason for the enhancement from IrO2 loading maybe ascribed to the facilitation transfer of photo-generated hole from the bulk to the surface and act as active sites for water oxidation. Furthermore, with series of filters under different wavelength at l > 420, 455, 475, 515 nm, the photocurrent on IrO2 loading CeeTiO2 become less, and showed agreement in some degree with its absorption edge, as shown in Fig. 5.
The visible light driven mechanism for CeeTiO2 The electronic structure of a photocatalyst is a dominate factor affecting its photophysical and photoelectrochemical properties, so the study of it is well recognized as an efficient way to connect the relationship between its inherent structure and its apparent properties [21,22], and also the basis for band engineering method to tailor and develop new semiconductor materials. As for the study of electronic structure of a semiconductor, the first principle calculation based on density functional theory is a powerful tool. Based on the calculated results, it can be proposed the band configuration of a semiconductor, especially the top of the valence band and the bottom of conduction band as well the band edge positions. The first principle calculation using CASETP package on
Fig. 4 e Photocurrent effect on (a): TiO2 and Ce doping TiO2 under 300 W Xe lamp full arc and (b) CeeTiO2 with loading of IrO2 and without under visible light irradiation (l > 420 nm).
Fig. 5 e The dependence of photocurrent effect for IrO2loading CeeTiO2 under full arc and different wavelength filters (l > 420, 455, 475, 515 nm) on its absorption edge.
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Materials Studio with Plane-wave pseudopotential method and Generalized Gradient Approximation (GGA) were carried on TiO2 and CeO2. Based on the theoretical results, the visible light driven mechanism of CeeTiO2 was proposed [23]. The core orbitals were replaced by the ultrasoft core potentials, and the Ti 3s23p63d24 s2, O 2s22p4, Ce 4f15s25p65d16 s2 electrons were treated explicitly. The kinetic energy cutoff was set at 330 eV. The partial density of states for anatase TiO2 was shown in Fig. 6(a). It revealed that theoretical band gap for TiO2 is about 2.2 eV, which is smaller than that from experimental results (3.2 eV) and regarded as the common features of GGA method. The band structure of TiO2 is mainly consisted by Ti 3d and O 2p to conduction band and valence band respectively. At the same time, the hybridization bonding of O 2p and Ti 3d is obvious and dominant to the band structure. Since the top of the valence band is mainly due to O 2p, which is located at 3.0 V Vs. NHE, and the bottom of conduction band is located nearly to the potential for reduction of Hþ/H2(0 V Vs. NHE), so the band gap for TiO2 is about 3.2 eV, which is good agreement with the absorption property. The sharp absorption edge at 380 nm is resulting from the band gap transition from O 2p to
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Ti 3d. For CeO2, its partial density of states was shown in Fig. 6(b). It indicated that the top of valence band ranging from 4 to 0 eV was mainly consisting of O 2p, and the conduction band is dominated by Ce 4f. The theoretical band gap for CeO2 is about 1.6 eV. Although there showed some degree of splitting of Ce 4f into partial occupied Ce 4f and unoccupied Ce 4f, the sharp absorption edge of CeO2 at 440 nm ascribed to the band gap transition from O 2p to Ce 4f. When it comes to the band structure of CeeTiO2, since it formed the TieOeCe bonding, combined with the absorption properties and band structure of TiO2 and CeO2, it can be proposed the conduction band is mainly composed by Ti 3d and localized unoccupied Ce 4f, while the valence band is mainly composed by O 2p. Meanwhile, these splitting partial occupied Ce 4f and unoccupied Ce 4f also hybridized with O 2p and Ti 3d in some degree. The sharp absorption edge of CeeTiO2 at 380 nm can be ascribed to the band transition from O 2p to Ti 3d, which is consistent with that of TiO2 without doping. For the absorption tail extending to visible light region of 550 nm, it is due to the energy gap transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f, which is located lower than that of Ti 3d, and they overlap rather than overall hybridization. Since the localized properties of Ce 4f, the transfer ability for photo-generated charges on it is limited, resulting in the poor agreement between photocurrent under different filters and absorption edge, as shown at Fig. 5. Additionally, the visible light driven mechanism for Ce doping TiO2 is different from those doped either by Fe3þ, Niþ, Co2þ etc. cations or N, C, S anions. For TiO2 doped by these cations, the doped cations formed discrete energy gap between the band gap and usually acted as recombination center for photogenerated electrons and holes, so they showed limited visible light responsive ability and poor photocatalytical activities [24,25]. On the contrary, for those doped by N, C, S anions, their visible light driven mechanism was ascribed to the hybridization of N 2p (S 2p) with O 2p, and the top of valence band became negatively with the valence band extension, resulting in the narrow of band gap, but there existed photocorrosion reaction for those anions doping [26,27].
Conclusions
Fig. 6 e Partial density states for anatase TiO2(a) and for CeO2(b).
In conclusion, there presented visible light driven nanocrystal anatase photocatalyst TiO2 doped by Ce through solegel method and its absorption edge shifted to about 550 nm. Under visible light irradiation (l > 420 nm), it showed obvious anodic photocurrent resulted from the oxidation of H2O to O2 at the surface of this photoanode. Based on the first principle calculation on the electronic structures of TiO2 and CeO2, the electronic structure for Ce doping TiO2 was proposed as the overlap and some degree of hybridization among splitting unoccupied Ce 4f and occupied Ce 4f with O 2p and Ti 3d. The visible light driven mechanism was mainly due to the transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d. The results presented in this paper not only enrich the visible light driven TiO2, but also further identify the mechanism for these visible light responsive Ce including photocatalysts and provide reference for developing new visible light driven photocatalyst.
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Acknowledgment This work was supported financially by the National Natural Science Foundation of China (NO. 21103028), Cultivating Program (NO. [2011]024, Education Department of Guizhou Province) and Key Program for Science and Technology (NO. [2012] 3052, Science and Technology Department of Guizhou Province).
references
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Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties Mengkui Tian a,b,*, Huimin Wang a, Dongshan Sun a, Wenjie Peng a, Wenliang Tao a a
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou Province 550025, PR China Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
b
article info
abstract
Article history:
Visible light driven nanocrystal anatase TiO2 was prepared by doping rare earth element Ce
Received 31 October 2013
through solegel method. UVeVis diffusion reflectance spectrum indicated its absorption
Received in revised form
edge extended to about 550 nm, red shifting about 170 nm compared with that without
7 March 2014
doping. Ce doping TiO2 showed obvious anodic photocurrent effect for water splitting
Accepted 3 April 2014
under visible light irradiation (l > 420 nm) in photoelectrochemical measurement with
Available online xxx
three electrodes configuration. Ce doping TiO2 showed higher photocurrent density than
Keywords:
tures for CeO2 and TiO2 were analyzed theoretically based on the first principle calculation.
Ce doping TiO2
As a result, the electronic structure for Ce doping TiO2 is proposed as the overlap and some
Photoelectrochemical
degree of hybridization among splitting occupied Ce 4f and unoccupied Ce 4f with O 2p and
Water splitting
Ti 3d respectively. The visible light responsive property is mainly due to the transition from
Photoanode
O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d.
Electronic structure
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
that of without doping TiO2 under full arc irradiation. Furthermore, the electronic struc-
reserved.
Introduction The evolution of H2 by water splitting on semiconductor photocatalyst under solar irradiation is well recognized as one of the most promising ways to develop hydrogen as energy carrier considering resource, environment and cost issues [1,2]. For this solar-to-hydrogen process, the most important work is to design semiconductor photocatalysts with appropriate band gap of about 2.0 eV to make the best use of solar
energy in visible light region. Meanwhile, electrochemical potential for conduction band and valence band edges of a candidate photocatalyst must satisfy water reduction (0 V Vs. Normal Hydrogen Energy) and water oxidation (1.23 V Vs. Normal Hydrogen Energy) requirements respectively. Furthermore, the semiconductor materials must be stable in water or electrolyte solution and the above-mentioned properties must be satisfied simultaneously [3,4]. However, these requirements are the only prerequisite conditions for these high functional photocatalysts to split water rather than
* Corresponding author. School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou Province 550025, PR China. E-mail addresses: [email protected], [email protected] (M. Tian). http://dx.doi.org/10.1016/j.ijhydene.2014.04.036 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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sufficient condition. Actually, the above-mentioned properties for a semiconductor photocatalyst are contradictory in some degree and difficult to get a good trade-off among them. For instance, most oxides such as TiO2, ZrO2, ZnO, NaTaO3 are stable in solution and show high activities for water decomposition under ultraviolet region, but they fail to make the best use of solar energy for their wide band gaps [5,6]. On the contrary, these oxy(nitrides) and oxy(sulfides), such as Ta3N5, TaON and CdS, show good visible light responsive ability, but usually they are unstable in solution for the photocorrosion reaction of photocatalyst itself [7e9]. TiO2 is a well-known photocatalyst not only for its high activity, high stability and low cost as well non-toxic properties, but also it is the pioneer materials to photocatalytical science and technology field. Consequently, up to now, TiO2 is the only widely utilized photocatalyst in mass production and will be in the future [10e12]. Although TiO2 is a successful photocatalyst in many aspects, the wide band gap (3.2 eV for anatase phase) is the fatal shortcoming limiting its utilization and efficiency. There are so many efforts focusing on extending its absorption edge to visible light region and at the same time try to remain its high stability and high quantum yield. Among these efforts, doping with cations such as Niþ, Fe3þ, Cr3þ, Sb5þ, and anions such as N, C, S, and F, Cl etc were extensively studied, but in most cases, they demonstrated limited visible light responsive ability and low efficiency resulting from the discrete donor or acceptor energy levels from doping [7,13,14]. In author’s previous study, visible light driven photocatalysts K4Ce2M10O30 (M ¼ Ta, Nb) were presented and theoretical analysis revealed that Ce 4f had great contribution to the visible light responsive mechanism [15]. Similar phenomenon also happened on other rare earth including compounds LnTaO4 (Ln ¼ La, Ce, Pr, Nd, and Sm) [16]. In this paper, there presented new effort to extend TiO2 absorption edge by doping rare earth element Ce, and its photoelectrochemical properties for water splitting on photoelectrode film under visible light were investigated closely. Although there were ever reports about TiO2 doped by Ce [17,18], here focused on its photoelectrochemical water splitting properties under visible light irradiation and its visible light driven mechanism based on the theoretical calculation analysis.
Experimental Powder sample preparation Nanocrystal anatase TiO2 doped by Ce (denoted as CeeTiO2) and without doping was prepared by conventional solegel methods. 10 ml of TiCl4 was dipped into Ce(NO3)$6H2O solution (Ce 0.5% molar ratio) slowly with stirring, and adjusted pH to about 7.0 by NH3$H2O. The gel was aged at 70 C for 24 h, finally heat treated at 500 C in air for 24 h, and TiO2 without doping followed the same procedure. The as-prepared powder was identified by X-ray powder diffraction on Geiger-flex RADB (Rigaku; Cu Ka). UVeVis diffuse reflectance spectrum was recorded by a spectrophotometer (JASCO, V-670). SEM images were obtained on field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi).
Preparation of electrodes Porous film electrodes were prepared by electrophoretic deposition method on conductive fluorine doped tin oxide glass (FTO, Asahi Glass Co.). The electrophoretic deposition was carried out in an acetone solution (40 ml) containing powder sample (40 mg) and iodine (15 mg), which was dispersed by sonication for 3 min. Two FTO electrodes (1.5 5 cm) were immersed parallel in the solution with 10 mm of distance, and then 40 V of bias potential to guarantee 15 mA current was applied between them for 1 min by a potentiostat (PARSTAT 2263) with stirring. The coated area was controlled to be ca. 1.5 4 cm. Finally, the electrodes prepared were calcinated at 350 C in air for 1 h to fasten the adhesion of film. This procedure resulted in formation of porous CeeTiO2 and TiO2 layer with uniform thickness of ca. 2 mm, with good reproducibility.
Loading of IrO2 Colloidal IrO2$nH2O was deposited on the surface of the electrode as a co-catalyst to prompt water oxidation. The colloidal IrO2$nH2O aqueous was prepared by hydrolysis of Na2IrCl6, with the pH of the solution adjusted to 11e12 using 0.1 M NaOH aq. Then the solution was heated at 70 C for 30 min, and then cooled to room temperature by immersion in a cold water bath, followed by pH adjusted to 9 using 0.1 M HNO3 aq. Subsequently, heating at 70 C for 30 min resulted in a deep blue solution containing colloidal IrO2$nH2O, and denoted as IrO2 hereafter. The loading of IrO2 was carried out by immersing photoelectrode into the colloidal IrO2$nH2O solution for 1 h, washed with distilled water and dried in air. The amount of IrO2 loaded (ca. 0.3 wt%) was determined by the change in absorption spectrum of IrO2-colloidal solution.
Photoelectrochemical measurements The photoelectrochemical measurement was performed by three-electrodes configuration mode consisting of a working electrode (as prepared electrode), a counter electrode (Pt mesh) and a reference electrode (Ag/AgCl) as well as electrolyte (0.1 M aqueous Na2SO4 solution), and the pH of the electrolyte solution was adjusted to 4.0 by 0.1 M H2SO4. The bias potential on working electrode was controlled by a Potentiostat (HSV-100). The solution was purged with Ar gas for over 10 min before the measurement. The electrode was irradiated through silicon glass window by a Xe lamp (300 W, Cermax) fitted with cut-off filters under visible light irradiation, and that without any filter was denoted as full arc irradiation. A shutter controlled by a magnetic controller set 3s for light on and off intermediately.
Results and discussions Characterizations of CeeTiO2 and TiO2 The powder X-ray diffraction patterns of as prepared CeeTiO2 and TiO2, as well as standard references for anatase nanocrystalline TiO2 and CeO2 were shown in Fig. 1. From their XRD
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Fig. 1 e X-ray diffusion patterns for as-prepared samples and references.
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that the particle size is of micrometer level for aggregation. The UVeVis diffusion reflectance spectrums for these Ce doping TiO2 and without doping were shown in Fig. 3. From Fig. 3, it can be seen that the absorption tail edge of Ce doping TiO2 is about 550 nm, red shifting 170 nm compared with that of without doping TiO2(380 nm). This red shifting is consistent with their apparent colors, from white for without doping to yellow color for the doping with Ce. Additionally, in order to compare with CeO2, which shows some degree of visible light responsive ability (absorption edge is about 440 nm), the UVeVis diffusion reflectance spectrum of CeO2 was also illustrated in Fig. 3. Since there was no CeO2 phase in the CeeTiO2 sample, which can be confirmed from their XRD patterns shown in Fig. 1, so the visible light responsive ability of CeeTiO2 is attributed for the Ce4þ doped into TiO2 in intergranular. The mechanism for visible light responsive ability will be discussed in detail in later section.
Photoelectrochemical water splitting properties
patterns, it can be seen that the as-prepared samples showed good agreement with anatase nanocrystalline TiO2 for both doping with Ce and without. Since the ionic radius of Ce4þ (1.02 A) is much bigger than A), it is difficult to form lattice substitution. that of Ti4þ (0.68 Furthermore, as there showed no obvious shifting of the main diffraction peaks of TiO2 and Ce doped TiO2, it is proposed that the Ce4þ doped into TiO2 in intergranular rather than lattice substitution [19]. Their SEM images were shown in Fig. 2. Although their crystals sizes are typical nano size of 20e30 nm, it is obvious
The photoelectrochemical water splitting for CeeTiO2 and TiO2 was performed on FTO photoelectrodes with three electrodes configuration. The photocurrent effect under 300 W Xe lamp irradiation without filter was shown in Fig. 4(a). Both TiO2 and the CeeTiO2 demonstrated anodic photocurrent under positive bias potential, which revealed that TiO2 and CeeTiO2 are typical N-type semiconductors, acting as photoanodes under irradiation. The anodic current was resulting from the oxidation of H2O on the surface of photoelectrode by photo-excited holes, while the photo-excited electrons transferred to counter electrode through external circuit and involved the reaction of reducing water. Moreover, under this
Fig. 2 e SEM images for TiO2 (a) and (b), CeeTiO2 (c) and (d) under different magnification. Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Fig. 3 e UVeVis diffusion reflectance spectrum for Ce doped TiO2 and without.
full arc irradiation, Ce doping TiO2 showed higher current density than that of TiO2 without doping, which maybe ascribes to the Ce doping TiO2 can absorb more efficient photons with energy larger than the energy gap to generate more charge carriers to evolve redox reaction for its red shifting absorption edge. It is difficult to determine precisely the onset potential just from voltageecurrent curve. In this case, it is proposed the onset potential is less than 0 V (Vs. RHE(Reverse Hydrogen
Energy), RHE ¼ EAg/AgClþ0.059 pH þ E0Ag/AgCl, E0Ag/ AgCl ¼ 0.196 V Vs. NHE) since there still presented obvious current at 0 V (Vs. RHE) as shown in Fig. 4. The onset potential is regarded as equally to the semi-Fermi energy, which is near to the bottom of conduction band for N-type semiconductor, so it indicated that TiO2 and CeeTiO2 have appropriate conduction band edges to meet reducing water requirement without bias potential. Under visible light irradiation (l > 420 nm), for this without doping TiO2, there was no detectable current, which is consistent with its absorption edge. For Ce doping TiO2, the photocurrent effect under visible light (l > 420 nm) was shown in Fig. 4(b). CeeTiO2 demonstrated obvious anodic current under visible light irradiation and the photocurrent reached 300 mA/cm2 without any modification. When CeeTiO2 photoelectrode was loaded with IrO2, an extensively used co-catalyst to prompt the oxidation of water in photocatalysis [20], the photocurrent was enhanced greatly as shown in Fig. 4(b). The reason for the enhancement from IrO2 loading maybe ascribed to the facilitation transfer of photo-generated hole from the bulk to the surface and act as active sites for water oxidation. Furthermore, with series of filters under different wavelength at l > 420, 455, 475, 515 nm, the photocurrent on IrO2 loading CeeTiO2 become less, and showed agreement in some degree with its absorption edge, as shown in Fig. 5.
The visible light driven mechanism for CeeTiO2 The electronic structure of a photocatalyst is a dominate factor affecting its photophysical and photoelectrochemical properties, so the study of it is well recognized as an efficient way to connect the relationship between its inherent structure and its apparent properties [21,22], and also the basis for band engineering method to tailor and develop new semiconductor materials. As for the study of electronic structure of a semiconductor, the first principle calculation based on density functional theory is a powerful tool. Based on the calculated results, it can be proposed the band configuration of a semiconductor, especially the top of the valence band and the bottom of conduction band as well the band edge positions. The first principle calculation using CASETP package on
Fig. 4 e Photocurrent effect on (a): TiO2 and Ce doping TiO2 under 300 W Xe lamp full arc and (b) CeeTiO2 with loading of IrO2 and without under visible light irradiation (l > 420 nm).
Fig. 5 e The dependence of photocurrent effect for IrO2loading CeeTiO2 under full arc and different wavelength filters (l > 420, 455, 475, 515 nm) on its absorption edge.
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Materials Studio with Plane-wave pseudopotential method and Generalized Gradient Approximation (GGA) were carried on TiO2 and CeO2. Based on the theoretical results, the visible light driven mechanism of CeeTiO2 was proposed [23]. The core orbitals were replaced by the ultrasoft core potentials, and the Ti 3s23p63d24 s2, O 2s22p4, Ce 4f15s25p65d16 s2 electrons were treated explicitly. The kinetic energy cutoff was set at 330 eV. The partial density of states for anatase TiO2 was shown in Fig. 6(a). It revealed that theoretical band gap for TiO2 is about 2.2 eV, which is smaller than that from experimental results (3.2 eV) and regarded as the common features of GGA method. The band structure of TiO2 is mainly consisted by Ti 3d and O 2p to conduction band and valence band respectively. At the same time, the hybridization bonding of O 2p and Ti 3d is obvious and dominant to the band structure. Since the top of the valence band is mainly due to O 2p, which is located at 3.0 V Vs. NHE, and the bottom of conduction band is located nearly to the potential for reduction of Hþ/H2(0 V Vs. NHE), so the band gap for TiO2 is about 3.2 eV, which is good agreement with the absorption property. The sharp absorption edge at 380 nm is resulting from the band gap transition from O 2p to
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Ti 3d. For CeO2, its partial density of states was shown in Fig. 6(b). It indicated that the top of valence band ranging from 4 to 0 eV was mainly consisting of O 2p, and the conduction band is dominated by Ce 4f. The theoretical band gap for CeO2 is about 1.6 eV. Although there showed some degree of splitting of Ce 4f into partial occupied Ce 4f and unoccupied Ce 4f, the sharp absorption edge of CeO2 at 440 nm ascribed to the band gap transition from O 2p to Ce 4f. When it comes to the band structure of CeeTiO2, since it formed the TieOeCe bonding, combined with the absorption properties and band structure of TiO2 and CeO2, it can be proposed the conduction band is mainly composed by Ti 3d and localized unoccupied Ce 4f, while the valence band is mainly composed by O 2p. Meanwhile, these splitting partial occupied Ce 4f and unoccupied Ce 4f also hybridized with O 2p and Ti 3d in some degree. The sharp absorption edge of CeeTiO2 at 380 nm can be ascribed to the band transition from O 2p to Ti 3d, which is consistent with that of TiO2 without doping. For the absorption tail extending to visible light region of 550 nm, it is due to the energy gap transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f, which is located lower than that of Ti 3d, and they overlap rather than overall hybridization. Since the localized properties of Ce 4f, the transfer ability for photo-generated charges on it is limited, resulting in the poor agreement between photocurrent under different filters and absorption edge, as shown at Fig. 5. Additionally, the visible light driven mechanism for Ce doping TiO2 is different from those doped either by Fe3þ, Niþ, Co2þ etc. cations or N, C, S anions. For TiO2 doped by these cations, the doped cations formed discrete energy gap between the band gap and usually acted as recombination center for photogenerated electrons and holes, so they showed limited visible light responsive ability and poor photocatalytical activities [24,25]. On the contrary, for those doped by N, C, S anions, their visible light driven mechanism was ascribed to the hybridization of N 2p (S 2p) with O 2p, and the top of valence band became negatively with the valence band extension, resulting in the narrow of band gap, but there existed photocorrosion reaction for those anions doping [26,27].
Conclusions
Fig. 6 e Partial density states for anatase TiO2(a) and for CeO2(b).
In conclusion, there presented visible light driven nanocrystal anatase photocatalyst TiO2 doped by Ce through solegel method and its absorption edge shifted to about 550 nm. Under visible light irradiation (l > 420 nm), it showed obvious anodic photocurrent resulted from the oxidation of H2O to O2 at the surface of this photoanode. Based on the first principle calculation on the electronic structures of TiO2 and CeO2, the electronic structure for Ce doping TiO2 was proposed as the overlap and some degree of hybridization among splitting unoccupied Ce 4f and occupied Ce 4f with O 2p and Ti 3d. The visible light driven mechanism was mainly due to the transition from O 2p hybridizing with occupied Ce 4f to unoccupied Ce 4f overlapping with Ti 3d. The results presented in this paper not only enrich the visible light driven TiO2, but also further identify the mechanism for these visible light responsive Ce including photocatalysts and provide reference for developing new visible light driven photocatalyst.
Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036
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Acknowledgment This work was supported financially by the National Natural Science Foundation of China (NO. 21103028), Cultivating Program (NO. [2011]024, Education Department of Guizhou Province) and Key Program for Science and Technology (NO. [2012] 3052, Science and Technology Department of Guizhou Province).
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Please cite this article in press as: Tian M, et al., Visible light driven nanocrystal anatase TiO2 doped by Ce from solegel method and its photoelectrochemical water splitting properties, International Journal of Hydrogen Energy (2014), http://dx.doi.org/ 10.1016/j.ijhydene.2014.04.036