Journal of Colloid and Interface Science 440 (2015) 162–167
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Dysprosium, Holmium and Erbium ions doped Indium Oxide nanotubes as photoanodes for dye sensitized solar cells and improved device performance Chuang Miao, Cong Chen, Qilin Dai ⇑, Lin Xu, Hongwei Song ⇑ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
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Article history: Received 4 August 2014 Accepted 17 October 2014 Available online 4 November 2014 Keywords: Dye sensitized solar cells In2O3 Rare earth Doping
a b s t r a c t In this work, rare earth (RE) ion RE3+ (RE3+ = Dy3+, Ho3+ and Er3+) doped and undoped In2O3 nanotubes are synthesized by the electrospinning method and the band gap of In2O3 is systemically controlled, depending on the order of doped elements. Dye-sensitized solar cells (DSSCs) based on In2O3:RE3+ nanotubes are also fabricated, and significantly improved performance of In2O3–DSSC is observed due to the modulation of the band gap, larger recombination charge transfer resistance and longer electron lifetime. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction DSSCs have attracted significant interest since the breakthrough work of Graetzel in 1991 due to the low cost and possibilities to create flexible devices [1–7]. The highest efficiency to date of a DSSC is 12.7% [3]. Photoanode is an important part of a DSSC that transport electrons generated by dye molecules to fluorine doped tin oxide (FTO) [2,6]. Among all the semiconductor materials, the mesoporous TiO2 nanocrystals have been widely used as photoanode in DSSCs due to its high power conversion efficiency and easy preparation [3]. However, the development of promising photoanode that exhibits better solar cell performance is still critical for the application of DSSCs. Some semiconductor materials with lower conduction band edge and higher electron mobility have the potential application in DSSCs since lower conduction band edge may sometimes be required to match with some infrared dye molecules having lower lowest unoccupied molecular orbital (LUMO) levels [8,9]. In the recent years, many progresses have been made in other semiconductor nanostructure materials for DSSCs, such as ZnO, SnO2 and some ternary metal oxides such as Zn2SnO4 and BaSnO3 [10–14]. In2O3 with a band gap of 3.55–3.85 eV is an important semiconductor, which has been widely investigated and applied in gas sensors, photocatalysis, etc [15–17]. Until now, only limited num⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Dai), [email protected] (H. Song). http://dx.doi.org/10.1016/j.jcis.2014.10.055 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
ber of reports about In2O3 with low power conversion efficiency (PCE) of about 0.32% can be found [9,18,19], and the poor device performance was attributed to the potential dependent current transients [9]. Doping has been performed in TiO2 to improve the device performance [20,21]. In particular, the energy level of the oxide can be elevated leading to the increase of the open-circuit voltage (Voc) of the DSSCs [22,23]. In this work, one dimensional In2O3 nanotubes, which are believed to have the capability of providing direct transport pathway for photogenerated electrons leading to enhanced charge collection efficiency in the DSSCs, were prepared by electrospinning [24]. In addition, the In2O3 nanotubes made by electrospinning have large surface area for dye loading which can result in higher overall power conversion efficiencies of DSSCs [25,26]. The DSSCs based on In2O3 nanotube photoanodes were fabricated and characterized. The objective of the paper was to fabricate and characterize the Dy3+, Ho3+ and Er3+ doped In2O3 Nanotubes as photoanodes for DSSCs. It would be demonstrated in the paper that considerable enhancement of the performance was observed in RE3+ (RE = Dy, Ho, Er) doped In2O3-DSSCs compared to the pure In2O3-DSSC. 2. Experimental details 2.1. Materials In(NO3)345H2O, polyvinylpyrrolidone (PVP-K90) and polyethylene glycol (PEG600) were purchased from Sinopharm Chemical Reagent Co., Ltd. Rare earth nitrate hexahydrate (RE(NO3)36H2O)
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was purchased from National Engineering Research Centre of RE Metallurgy and Functional Materials Co., Ltd., China. DMF (N,Ndimethylformamide) was purchased from Tianjin Tiantai Fine Chemicals Co., Ltd. All chemicals were of analytical grade and used without further purification. 2.2. Synthesis of In2O3:RE3+ nanotubes In2O3:RE3+ nanotubes were prepared by electrospinning as described previously [16]. 1.5 mmol of In(NO3)345H2O and 0.075 mmol of RE(NO3)36H2O (RE = Dy, Ho and Er, 5% molar ratio) were added into 10 mL of DMF with magnetic stirring until dissolved completely. Then, appropriate amounts of PVP-K90 were added into the above solution to make the weight ratio of the total inorganic (In(NO3)3 and RE(NO3)3) to PVP-K90 is 0.36. The final viscous precursor solution was obtained after vigorous stirring. In the electrospinning process of precursor fibers, an applied steady voltage of 16 kV was used and the distance between the spinneret tip and the collector was 15 cm. The rate of the precursor ejected from the tip of the spinneret was 0.5 mL/h controlled by an ejector jet pump. The precursor fibers were annealed in a tube furnace at 600 °C for 3 h with a rising rate of 1 °C/min from the room temperature after dried for overnight in an oven. The final In2O3: RE3+ nanotubes were obtained when the furnace cooled down to room temperature. From our previous study, the real RE doping concentration should be almost the same as the starting concentration (5%) [15]. 2.3. Preparation of In2O3–DSSCs An FTO substrate was first ultrasonically cleaned in a detergent solution and then in acetone, isopropyl alcohol and deionized water for 30 min successively followed by drying in a 60 °C oven for 10 min. 20 mg In2O3:RE3+ nanotube powders and 20 mg PEG600 were dispersed ultrasonically into 1 mL ethanol for 30 min, and then the paste of In2O3:RE3+ was obtained after stirring for 30 min. The In2O3:RE3+ film was prepared from the obtained paste through doctor blading method on the FTO substrate and the area of the In2O3:RE3+ was controlled to be 0.25 cm2 (0.5 cm 0.5 cm). Then the FTO coated with the In2O3:RE3+ electrode was obtained by annealing at 500 °C for 30 min with a rising rate of 2 °C/min. To sensitize the In2O3:RE3+ nanotubes, the electrode was immersed into a 0.5 mM C58H86N8O8RuS2 (N719) dye solution for 24 h at room temperature. Finally, the FTO substrate with dye coated In2O3:RE3+ nanotubes and a Pt counter electrode were bonded together through a hot-melt spacer to form a sandwich structure, and a drop electrolyte consisting of 0.60 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.10 M Guanidine thiocyanate and 0.05 M 4-tertbutylpyridine in a mixture of acetonitrile (85%) and valeronitrile (15%) was injected into the device through pre-drilled holes on the Pt counter electrode [27]. DSSCs based on pure In2O3 were also fabricated through the same progress. 2.4. Characterization and measurement The morphology of the In2O3:RE3+ was characterized using a JEOL JSM-7500F field emission scanning electron microscope (FESEM) operating at an accelerating voltage of 15 kV with gold sputtered on the samples. TEM image was acquired by a JEM2010 transmission electron microscope under a working voltage of 200 kV. Electrochemical measurements were carried out on a Zahner electrochemical workstation (IM6ex) in the frequency range of 50 mHz–100 kHz with a potential signal of 10 mV. All measurements were taken in the dark. XRD patterns of the samples were collected with a Rigaku D/max 2550 X-ray diffractometer using a Cu target radiation resource (k = 1.5045 Å), and the lattice constants
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of the samples were calculated by MDI Jade 5.0 software. The UV–vis absorption spectra were obtained with a UV-3101PC UV– vis–NIR scanning spectrophotometer (Shimadzu 1700). The current density–voltage (JV) characteristics were measured with a computer programmed Keithley 2400 Source Meter under AM 1.5G illumination at 100 mW/cm2 with SS150 solar simulator (150 W from Zolix Instruments Co., Ltd.) which was corrected by a standard silicon solar cell. Incident-photon-to-electron conversion efficiency (IPCE) was acquired with a Zolix solar cell Scan100 IPCE Measurement System. The electrical resistivity values of In2O3 and In2O3:Er3+ films (1 lm thick) prepared by spin coating of the corresponding precursors were determined by four-point electrical resistivity measurements. 3. Results and discussion 3.1. Morphology and structure of In2O3:RE3+ nanotubes Fig. 1 shows the FESEM images of the In2O3, In2O3:Dy3+, In2O3: Ho3+ and In2O3:Er3+ nanotubes. It can be seen that the nanotubes have a diameter of 70–120 nm with typical tube structure which can be confirmed through the cross-sectional examination. The diameters of four samples are nearly the same which indicates that the RE3+ doping does not affect the nanotube sizes. The hollow and porous structure of the nanotube (see inset of Fig. 1a) can allow more dye molecules to be adsorbed on the surface of In2O3, and the one dimensional structure can result in enhanced charge collection efficiency [24,28,29]. The nanotube diameters do not change much with the dopant according to our previous results [15]. The structure of the four samples was investigated by X-ray diffraction (Fig. 2a). The diffraction peaks of all the samples were well matched with the cubic phase standard card (JCPDS 06-0416) without any additional diffraction peaks. The lattice constants of the four samples based on the XRD patterns were also calculated. For the pure cubic phase In2O3 nanotubes, the lattice constant was 10.117 ± 0.002 Å. The lattice constants for Dy3+, Ho3+ and Er3+ doped samples were determined to be 10.129 ± 0.002 Å, 10.123 ± 0.002 Å and 10.139 ± 0.002 Å, respectively. The increase of the lattice constant due to the different radii of In3+ (0.80 Å) and RE3+ (0.89–0.912 Å) also confirmed that the Dy3+, Ho3+ and Er3+ ions were doped into the crystal lattice of In2O3 [30,31]. The UV–vis absorption spectra of the In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes are shown in Fig. 2c. The absorption peak of the pure In2O3 nanotubes is about 3.84 eV, which is consistent with the literatures [16,32], and it increases to 3.90 eV, 3.91 eV and 3.99 eV, respectively, as Dy3+, Ho3+ and Er3+ ions are doped into In2O3 nanotubes. As a matter of fact, many studies indicated that the RE3+ doping had a great impact on the band gap of the semiconductor [33–36]. The systematic and regular blueshifts with different dopants changing from Dy3+ to Er3+ suggested that the introduction of dopant ions, rather than size effects, brings about these changes. The reasons for the shifts can be usually explained as follows: (1) the charge-transfer transition between the electrons of RE3+ ions and In2O3 conduction or valence band [36]; (2) the formation of localized band edge states at the doped sites as the doped ions enter the In2O3 lattice [34]. 3.2. The enhanced properties of In2O3–DSSCs The photocurrent density–voltage curves (J–V) of DSSCs based on In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes are shown in Fig. 3a and the detailed photovoltaic parameters are summarized in Table 1. It can be observed that the short-circuit current (Jsc) and Voc of the devices are improved significantly by RE3+ doping. For the pure In2O3-DSSC, the values of Jsc, Voc and fill
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Fig. 1. The FESEM images of (a) In2O3, the inset is the high-magnification TEM image of one In2O3 tube. (b) In2O3:Dy3+, (c) In2O3:Ho3+ and (d) In2O3:Er3+ nanotubes. All the scale bars are 100 nm.
Fig. 2. (a) The XRD patterns of In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes. (b) UV–vis absorption spectra of In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes.
factor (FF) were 4.6 mA/cm2, 0.4 V and 26.2%, respectively. PCE is the ratio of power output versus power input. PCE ¼ JSC VI0OC FF. Jsc is the short-circuit current. Voc is open-circuit voltage, FF is fill-factor and I0 is incident light flux. The PCE is 0.5% for the pure In2O3– DSSC. Note that the performance of pure In2O3–DSSC is still much better than that of the previous report (PCE = 0.32%), which is probably because more dye molecules were adsorbed on the nanotubes due to their hollow and porous structure. Another possible reason for that is the improved charge collection efficiency due to their one dimensional structure [24]. The Jsc of In2O3–DSSC was enhanced dramatically to 6.5–8.6 mA/cm2 as RE3+ ions were doped into In2O3, while the values of Voc were increased moderately from 0.4 to 0.5 V. The Jsc was improved to 8.6 mA/cm2 for the DSSC based on In2O3:Er3+ nanotubes. The highest PCE of 1.4% was obtained in this study, which is still low compared to the
previously results, and further optimization of the DSSC fabrication parameters is needed to improve the device performance. In addition, the improvement of FF from 26.2% to 34.4% can be observed in Table 1. It can be clearly seen that Jsc was significantly enhanced for Er3+ doped In2O3–DSSC and almost doubled compared to that for the pure In2O3–DSSC. All the parameters were improved leading to a three-time increase in the PCE compared I I (k) = monochroto the pure In2O3–DSSC. IPCE ¼ 1240 I0ðkÞSCðkÞ kðnmÞ SC matic short circuit current. I0(k) = monochromatic light flux. The IPCE spectra of DSSCs based on RE3+ doped and undoped In2O3 nanotubes are shown in Fig. 3b. The 532 nm peaks in the IPCE spectra for all the four devices match with the absorption of the N719 dye. The same trend of improvement can be observed for the IPCE spectra. It can be seen that a maximum IPCE of 43% can be obtained for the In2O3:Er3+–DSSC.
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Fig. 3. (a) The J–V characteristics of DSSCs based on In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes measured under simulated solar illumination (AM 1.5G) with an intensity of 100 mW/cm2. (b) The IPCE spectra of DSSCs based on In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes.
Fig. 4. (a) Nyquist diagrams of the EIS spectra of DSSCs based on In2O3 and In2O3:Er3+ nanotubes. The lines are the fitting curves. The dots are the measured data. The inset is the equivalent circuit. (b) The phase plots of EIS spectra of DSSCs based on In2O3 and In2O3:Er3+ nanotubes.
Table 1 Photovoltaic parameters of DSSCs based on different photoanodes.
(inset) according to Ref. [40]. Each semicircle indicates a charge transfer processes which is exhibited by a resistance capacitance (R–C) parallel circuit. For a more precise fitting, the capacitance element is replaced by a constant phase element (CPE). Rs is the ohmic series resistance of the cell. R1 is the charge transfer resistance at the electrolyte/counter electrode interface [40]. R2 is the recombination charge transfer resistance at the In2O3/dye/electrolyte interface [40]. The higher R2 signifies the less recombination of electrons between the conduction band of In2O3 and the electrolyte which is expected [40]. Rs, R1 and R2 can be obtained by fitting according to the equivalent circuit (Table 2). The R2 values of the pure In2O3–DSSC and Er3+ doped In2O3–DSSC were determined to be 34.3 X and 106.2 X respectively. This result confirmed that the recombination of electrons in In2O3–DSSC was reduced by doping with Er3+, leading to the improvement of Jsc in Er3+ doped In2O3–DSSC. We think the recombination of the devices based on In2O3:Dy3+, In2O3:Ho3+, In2O3:Er3+ decreased as the order of doped elements increased from Dy to Er. The phase plots of EIS spectra (Fig. 4b) show the characteristic frequency peaks of the charge transfer process for the devices. The lifetime of electrons in In2O3 can be related to the inverse of the characteristic frequency and estimated by 1/2pfmax, where fmax is the maximum frequency of the peak [41–44]. The calculated lifetime was 26 and 122 ms for
Device
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
In2O3 In2O3:Dy3+ In2O3:Ho3+ In2O3:Er3+
4.6 6.6 6.7 8.6
0.4 0.5 0.5 0.5
26.2 33.4 33.2 34.4
0.5 1.1 1.1 1.4
3.3. The analysis of the improved device performance Different origins were proposed to explain the improvement of Jsc induced by doping or in a hybrid structure in previous works [37–39]. It was reported that TiO2/La2O3 hybrid photoanode possessed lower electron transfer resistance than the original TiO2 photoanodes, which alleviated the electron recombination rate and lead to higher Jsc [37]. Boschloo et al. found that significantly more charge could be extracted from TiO2 photoanode modified by insertion with aluminum ions than that from TiO2 [38]. Noticeably improved charge collection efficiency for Nb5+ doped TiO2 photoanode compared to TiO2 was reported by Graetzel et al. [39]. The higher electron transfer rate in the Sn doped TiO2 films than in the undoped TiO2 films was proposed by Lin et al., which resulted in higher Jsc [37]. In order to investigate the electron transfer process in the In2O3:RE3+–DSSCs, electrochemical impedance spectroscopy (EIS) measurements were taken. Fig. 4a presents the typical Nyquist plots of both the pure In2O3–DSSC and Er3+ doped In2O3–DSSC. Two semicircles locate at relative high and low frequency regions can be observed. The impedance spectra were fitted with Zview software based on the equivalent circuit
Table 2 Fitting results of the Nyquist plot. Photoanode
Rs (X)
R1 (X)
R2 (X)
In2O3 In2O3:Er3+
46.7 33.7
2.6 6.5
34.3 106.2
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In2O3 and In2O3:Er3+ respectively. It can be observed that In2O3: Er3+ has longer electron lifetime caused by Er3+ doping, which favors the electron transport leading to enhanced Jsc. Similar phenomenon has also been observed in iodine-doped TiO2 [44]. The fill factor was also improved by the doping which was probably due to the reduced recombination according to the literature reports [45,46]. Graetzel et al. observed that RE3+ doping in TiO2 decreased the resistivity, and used RE3+ doped in TiO2 as photoanode to improve the power conversion efficiency of the photovoltaic devices. However, the mechanism of that is still not clear [47]. Yang Yang also reported that Y doped TiO2 increased the conductivity, and the series resistance in the device was also reduced. The improved conductivity of Y–TiO2 matched that of the hole transport material, which balanced the carrier transport to reduce nonideal space charge distribution [48]. Lin et al. found that Sn4+ doped TiO2 photoanode for dye-sensitized solar cells improved the power conversion efficiency which was attributed to the higher transfer rate of electrons in Sn-doped TiO2 without a mechanism [37]. In our case, the electrical resistivity values were determined to be 6.3 104 X cm 4.3 104 X cm for In2O3 and In2O3:Er3+ films respectively by four-point resistivity measurements. Thus Er3+ doping in In2O3 decreased the resistivity. In EIS study, the device based on Er3+ doped in In2O3 has reduced the recombination and increased the lifetime of electrons leading to the improvement of the device performance. In addition, we also observed the series resistance in the device was reduced by Er3+ doping in EIS measurement (Table 2) which is similar to Yang Yang’s results. Further experiments are underway to better understand the mechanism. The Voc is determined by the energy difference between the red-ox potential of the electrolyte and the flat band potential (Efb) of the electrode material [22]. As mentioned above, the band gap of In2O3:RE3+ increased gradually with the atomic number of RE3+, which was evidenced by the UV–vis absorption spectra (Fig. 2b). It was reported that the band-gap energy increased with the decreasing of the refractive index (n0) for the oxides in the form of ApOq [49]. From Dy to Er, the value of n0 decreases gradually [50], indicating that the band gap of RE2O3 gradually increases. Thus the band-gap energy increases with the order of doped elements can be understood by the band gap of RE2O3 gradually increases. It was reported that incorporation of Mg ions in ZnO resulted in an apparent variation in the conduction band and the valence band remained almost the same as that of ZnO [51,52]. It is believed that the increase of the band gap is by the raised conduction band and the valence band remains the same although the real mechanism is still not clear. One explanation is that the effective mass of the holes is much larger than that of the electrons so the increased band gap is mainly caused by the upward shift of the conduction band edge [53]. The other is that the valence band of In2O3 is determined by the 2p orbit of oxygen ions, and RE3+ doping does not change the valence band. Therefore, it is possible that the increase of the band gap is caused by the higher conduction band of In2O3:RE, which leads to Efb shifting away from the redox potential and the improvement of the Voc of the DSSCs. Similar results were reported in conventional TiO2–DSSCs [22,23,37]. It can be concluded that all the photovoltaic parameters have been improved by the RE3+ doping for In2O3–DSSCs. The energy band alignment between In2O3:RE3+ nanotubes and a dye molecule (N719) is shown in Fig. 5 according to the Refs. [9,54]. It can be seen that the conduction band edge of the In2O3 nanotube is about 4.3 eV below the vacuum level. It shifted to 4.28, 4.26 and 4.18 eV when Dy3+, Ho3+ and Er3+ ions were doped in In2O3, respectively. N719 LUMO state is about 3.85 eV below the vacuum level, which is higher than the conduction band edge of In2O3:RE3+ allowing the photo-generated electrons to be easily injected into the conduction band of In2O3:RE3+.
Fig. 5. Energy band alignment between an In2O3:RE3+ nanotube and a dye molecule.
4. Conclusions In summary, In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes were successfully fabricated by the electrospinning method. The RE3+ doping shifted the absorption spectra of the In2O3 toward higher energy indicating increased band gap. DSSCs based on In2O3:RE3+ were also fabricated and compared with undoped devices. J–V and IPCE measurements exhibited significantly improved performance with the RE3+ doping. The improvement of Jsc was due to the increased recombination charge transfer resistance and electron lifetime caused by doping. The increased Voc can be attributed to the modulation of the band gap. As a result, a PCE of 1.4% was obtained for In2O3:Er3+–DSSC which was nearly three times that of the pure In2O3–DSSC. Acknowledgments This work was supported by the Major State Basic Research Development Program of China (973 Program) (No. 2014CB643506), the National Natural Science Foundation of China (Grant Nos. 11374127, 11304118, 61204015, 81201738, 81301289, 61177042, and 11174111), Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT13018). The China Postdoctoral Science Foundation Funded Project (2012M511337 and 2013T60327). References [1] B. O’regan, M. Grfitzeli, Nature 353 (1991) 737. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595. [3] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629. [4] T. Kinoshita, J.T. Dy, S. Uchida, T. Kubo, H. Segawa, Nat. Photon. 7 (2013) 535. [5] Z. Yang, M. Liu, C. Zhang, W.W. Tjiu, T. Liu, H. Peng, Angew. Chem. Int. Ed. 52 (2013) 3996. [6] M. Ye, D. Zheng, M. Lv, C. Chen, C. Lin, Z. Lin, Adv. Mater. 25 (2013) 3039. [7] X. Dang, J. Qi, M.T. Klug, P.-Y. Chen, D.S. Yun, N.X. Fang, P.T. Hammond, A.M. Belcher, Nano Lett. 13 (2013) 637. [8] K. Sayama, H. Sugihara, H. Arakawa, Chem. Mater. 10 (1998) 3825. [9] S. Mori, A. Asano, J. Phys. Chem. C 114 (2010) 13113. [10] M. McCune, W. Zhang, Y. Deng, Nano Lett. 12 (2012) 3656. [11] H. Wang, B. Li, J. Gao, M. Tang, H. Feng, J. Li, L. Guo, CrystEngComm 14 (2012) 5177. [12] Q. Dai, J. Chen, L. Lu, J. Tang, W. Wang, Nano Lett. 12 (2012) 4187. [13] Y. Zhang, H. Zhang, Y. Wang, W. Zhang, J. Phys. Chem. C 112 (2008) 8553. [14] X. Wei, G. Xu, Z. Ren, C. Xu, G. Shen, G. Han, J. Am. Ceram. Soc. 91 (2008) 3795. [15] L. Xu, B. Dong, Y. Wang, X. Bai, Q. Liu, H. Song, Sens. Actuat. B 147 (2010) 531. [16] L. Xu, B. Dong, Y. Wang, X. Bai, J. Chen, Q. Liu, H. Song, J. Phys. Chem. C 114 (2010) 9089. [17] L.Y. Chen, Y. Liang, Z.D. Zhang, Eur. J. Inorg. Chem. 2009 (2009) 903. [18] R. Sharma, R.S. Mane, S.-K. Min, S.-H. Han, J. Alloys Compd. 479 (2009) 840. [19] A. Furube, M. Murai, S. Watanabe, K. Hara, R. Katoh, M. Tachiya, J. Photochem. Photobiol. A: Chem. 182 (2006) 273. [20] J.-H. The, J. Mater. Chem. 20 (2010) 6505. [21] J. Zhang, W. Peng, Z. Chen, H. Chen, L. Han, J. Phys. Chem. C 116 (2012) 19182. [22] K.H. Ko, Y.C. Lee, Y.J. Jung, J. Colloid Interface Sci. 283 (2005) 482.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Dysprosium, Holmium and Erbium ions doped Indium Oxide nanotubes as photoanodes for dye sensitized solar cells and improved device performance Chuang Miao, Cong Chen, Qilin Dai ⇑, Lin Xu, Hongwei Song ⇑ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 August 2014 Accepted 17 October 2014 Available online 4 November 2014 Keywords: Dye sensitized solar cells In2O3 Rare earth Doping
a b s t r a c t In this work, rare earth (RE) ion RE3+ (RE3+ = Dy3+, Ho3+ and Er3+) doped and undoped In2O3 nanotubes are synthesized by the electrospinning method and the band gap of In2O3 is systemically controlled, depending on the order of doped elements. Dye-sensitized solar cells (DSSCs) based on In2O3:RE3+ nanotubes are also fabricated, and significantly improved performance of In2O3–DSSC is observed due to the modulation of the band gap, larger recombination charge transfer resistance and longer electron lifetime. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction DSSCs have attracted significant interest since the breakthrough work of Graetzel in 1991 due to the low cost and possibilities to create flexible devices [1–7]. The highest efficiency to date of a DSSC is 12.7% [3]. Photoanode is an important part of a DSSC that transport electrons generated by dye molecules to fluorine doped tin oxide (FTO) [2,6]. Among all the semiconductor materials, the mesoporous TiO2 nanocrystals have been widely used as photoanode in DSSCs due to its high power conversion efficiency and easy preparation [3]. However, the development of promising photoanode that exhibits better solar cell performance is still critical for the application of DSSCs. Some semiconductor materials with lower conduction band edge and higher electron mobility have the potential application in DSSCs since lower conduction band edge may sometimes be required to match with some infrared dye molecules having lower lowest unoccupied molecular orbital (LUMO) levels [8,9]. In the recent years, many progresses have been made in other semiconductor nanostructure materials for DSSCs, such as ZnO, SnO2 and some ternary metal oxides such as Zn2SnO4 and BaSnO3 [10–14]. In2O3 with a band gap of 3.55–3.85 eV is an important semiconductor, which has been widely investigated and applied in gas sensors, photocatalysis, etc [15–17]. Until now, only limited num⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Dai), [email protected] (H. Song). http://dx.doi.org/10.1016/j.jcis.2014.10.055 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
ber of reports about In2O3 with low power conversion efficiency (PCE) of about 0.32% can be found [9,18,19], and the poor device performance was attributed to the potential dependent current transients [9]. Doping has been performed in TiO2 to improve the device performance [20,21]. In particular, the energy level of the oxide can be elevated leading to the increase of the open-circuit voltage (Voc) of the DSSCs [22,23]. In this work, one dimensional In2O3 nanotubes, which are believed to have the capability of providing direct transport pathway for photogenerated electrons leading to enhanced charge collection efficiency in the DSSCs, were prepared by electrospinning [24]. In addition, the In2O3 nanotubes made by electrospinning have large surface area for dye loading which can result in higher overall power conversion efficiencies of DSSCs [25,26]. The DSSCs based on In2O3 nanotube photoanodes were fabricated and characterized. The objective of the paper was to fabricate and characterize the Dy3+, Ho3+ and Er3+ doped In2O3 Nanotubes as photoanodes for DSSCs. It would be demonstrated in the paper that considerable enhancement of the performance was observed in RE3+ (RE = Dy, Ho, Er) doped In2O3-DSSCs compared to the pure In2O3-DSSC. 2. Experimental details 2.1. Materials In(NO3)345H2O, polyvinylpyrrolidone (PVP-K90) and polyethylene glycol (PEG600) were purchased from Sinopharm Chemical Reagent Co., Ltd. Rare earth nitrate hexahydrate (RE(NO3)36H2O)
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was purchased from National Engineering Research Centre of RE Metallurgy and Functional Materials Co., Ltd., China. DMF (N,Ndimethylformamide) was purchased from Tianjin Tiantai Fine Chemicals Co., Ltd. All chemicals were of analytical grade and used without further purification. 2.2. Synthesis of In2O3:RE3+ nanotubes In2O3:RE3+ nanotubes were prepared by electrospinning as described previously [16]. 1.5 mmol of In(NO3)345H2O and 0.075 mmol of RE(NO3)36H2O (RE = Dy, Ho and Er, 5% molar ratio) were added into 10 mL of DMF with magnetic stirring until dissolved completely. Then, appropriate amounts of PVP-K90 were added into the above solution to make the weight ratio of the total inorganic (In(NO3)3 and RE(NO3)3) to PVP-K90 is 0.36. The final viscous precursor solution was obtained after vigorous stirring. In the electrospinning process of precursor fibers, an applied steady voltage of 16 kV was used and the distance between the spinneret tip and the collector was 15 cm. The rate of the precursor ejected from the tip of the spinneret was 0.5 mL/h controlled by an ejector jet pump. The precursor fibers were annealed in a tube furnace at 600 °C for 3 h with a rising rate of 1 °C/min from the room temperature after dried for overnight in an oven. The final In2O3: RE3+ nanotubes were obtained when the furnace cooled down to room temperature. From our previous study, the real RE doping concentration should be almost the same as the starting concentration (5%) [15]. 2.3. Preparation of In2O3–DSSCs An FTO substrate was first ultrasonically cleaned in a detergent solution and then in acetone, isopropyl alcohol and deionized water for 30 min successively followed by drying in a 60 °C oven for 10 min. 20 mg In2O3:RE3+ nanotube powders and 20 mg PEG600 were dispersed ultrasonically into 1 mL ethanol for 30 min, and then the paste of In2O3:RE3+ was obtained after stirring for 30 min. The In2O3:RE3+ film was prepared from the obtained paste through doctor blading method on the FTO substrate and the area of the In2O3:RE3+ was controlled to be 0.25 cm2 (0.5 cm 0.5 cm). Then the FTO coated with the In2O3:RE3+ electrode was obtained by annealing at 500 °C for 30 min with a rising rate of 2 °C/min. To sensitize the In2O3:RE3+ nanotubes, the electrode was immersed into a 0.5 mM C58H86N8O8RuS2 (N719) dye solution for 24 h at room temperature. Finally, the FTO substrate with dye coated In2O3:RE3+ nanotubes and a Pt counter electrode were bonded together through a hot-melt spacer to form a sandwich structure, and a drop electrolyte consisting of 0.60 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M I2, 0.10 M Guanidine thiocyanate and 0.05 M 4-tertbutylpyridine in a mixture of acetonitrile (85%) and valeronitrile (15%) was injected into the device through pre-drilled holes on the Pt counter electrode [27]. DSSCs based on pure In2O3 were also fabricated through the same progress. 2.4. Characterization and measurement The morphology of the In2O3:RE3+ was characterized using a JEOL JSM-7500F field emission scanning electron microscope (FESEM) operating at an accelerating voltage of 15 kV with gold sputtered on the samples. TEM image was acquired by a JEM2010 transmission electron microscope under a working voltage of 200 kV. Electrochemical measurements were carried out on a Zahner electrochemical workstation (IM6ex) in the frequency range of 50 mHz–100 kHz with a potential signal of 10 mV. All measurements were taken in the dark. XRD patterns of the samples were collected with a Rigaku D/max 2550 X-ray diffractometer using a Cu target radiation resource (k = 1.5045 Å), and the lattice constants
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of the samples were calculated by MDI Jade 5.0 software. The UV–vis absorption spectra were obtained with a UV-3101PC UV– vis–NIR scanning spectrophotometer (Shimadzu 1700). The current density–voltage (JV) characteristics were measured with a computer programmed Keithley 2400 Source Meter under AM 1.5G illumination at 100 mW/cm2 with SS150 solar simulator (150 W from Zolix Instruments Co., Ltd.) which was corrected by a standard silicon solar cell. Incident-photon-to-electron conversion efficiency (IPCE) was acquired with a Zolix solar cell Scan100 IPCE Measurement System. The electrical resistivity values of In2O3 and In2O3:Er3+ films (1 lm thick) prepared by spin coating of the corresponding precursors were determined by four-point electrical resistivity measurements. 3. Results and discussion 3.1. Morphology and structure of In2O3:RE3+ nanotubes Fig. 1 shows the FESEM images of the In2O3, In2O3:Dy3+, In2O3: Ho3+ and In2O3:Er3+ nanotubes. It can be seen that the nanotubes have a diameter of 70–120 nm with typical tube structure which can be confirmed through the cross-sectional examination. The diameters of four samples are nearly the same which indicates that the RE3+ doping does not affect the nanotube sizes. The hollow and porous structure of the nanotube (see inset of Fig. 1a) can allow more dye molecules to be adsorbed on the surface of In2O3, and the one dimensional structure can result in enhanced charge collection efficiency [24,28,29]. The nanotube diameters do not change much with the dopant according to our previous results [15]. The structure of the four samples was investigated by X-ray diffraction (Fig. 2a). The diffraction peaks of all the samples were well matched with the cubic phase standard card (JCPDS 06-0416) without any additional diffraction peaks. The lattice constants of the four samples based on the XRD patterns were also calculated. For the pure cubic phase In2O3 nanotubes, the lattice constant was 10.117 ± 0.002 Å. The lattice constants for Dy3+, Ho3+ and Er3+ doped samples were determined to be 10.129 ± 0.002 Å, 10.123 ± 0.002 Å and 10.139 ± 0.002 Å, respectively. The increase of the lattice constant due to the different radii of In3+ (0.80 Å) and RE3+ (0.89–0.912 Å) also confirmed that the Dy3+, Ho3+ and Er3+ ions were doped into the crystal lattice of In2O3 [30,31]. The UV–vis absorption spectra of the In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes are shown in Fig. 2c. The absorption peak of the pure In2O3 nanotubes is about 3.84 eV, which is consistent with the literatures [16,32], and it increases to 3.90 eV, 3.91 eV and 3.99 eV, respectively, as Dy3+, Ho3+ and Er3+ ions are doped into In2O3 nanotubes. As a matter of fact, many studies indicated that the RE3+ doping had a great impact on the band gap of the semiconductor [33–36]. The systematic and regular blueshifts with different dopants changing from Dy3+ to Er3+ suggested that the introduction of dopant ions, rather than size effects, brings about these changes. The reasons for the shifts can be usually explained as follows: (1) the charge-transfer transition between the electrons of RE3+ ions and In2O3 conduction or valence band [36]; (2) the formation of localized band edge states at the doped sites as the doped ions enter the In2O3 lattice [34]. 3.2. The enhanced properties of In2O3–DSSCs The photocurrent density–voltage curves (J–V) of DSSCs based on In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes are shown in Fig. 3a and the detailed photovoltaic parameters are summarized in Table 1. It can be observed that the short-circuit current (Jsc) and Voc of the devices are improved significantly by RE3+ doping. For the pure In2O3-DSSC, the values of Jsc, Voc and fill
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Fig. 1. The FESEM images of (a) In2O3, the inset is the high-magnification TEM image of one In2O3 tube. (b) In2O3:Dy3+, (c) In2O3:Ho3+ and (d) In2O3:Er3+ nanotubes. All the scale bars are 100 nm.
Fig. 2. (a) The XRD patterns of In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes. (b) UV–vis absorption spectra of In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes.
factor (FF) were 4.6 mA/cm2, 0.4 V and 26.2%, respectively. PCE is the ratio of power output versus power input. PCE ¼ JSC VI0OC FF. Jsc is the short-circuit current. Voc is open-circuit voltage, FF is fill-factor and I0 is incident light flux. The PCE is 0.5% for the pure In2O3– DSSC. Note that the performance of pure In2O3–DSSC is still much better than that of the previous report (PCE = 0.32%), which is probably because more dye molecules were adsorbed on the nanotubes due to their hollow and porous structure. Another possible reason for that is the improved charge collection efficiency due to their one dimensional structure [24]. The Jsc of In2O3–DSSC was enhanced dramatically to 6.5–8.6 mA/cm2 as RE3+ ions were doped into In2O3, while the values of Voc were increased moderately from 0.4 to 0.5 V. The Jsc was improved to 8.6 mA/cm2 for the DSSC based on In2O3:Er3+ nanotubes. The highest PCE of 1.4% was obtained in this study, which is still low compared to the
previously results, and further optimization of the DSSC fabrication parameters is needed to improve the device performance. In addition, the improvement of FF from 26.2% to 34.4% can be observed in Table 1. It can be clearly seen that Jsc was significantly enhanced for Er3+ doped In2O3–DSSC and almost doubled compared to that for the pure In2O3–DSSC. All the parameters were improved leading to a three-time increase in the PCE compared I I (k) = monochroto the pure In2O3–DSSC. IPCE ¼ 1240 I0ðkÞSCðkÞ kðnmÞ SC matic short circuit current. I0(k) = monochromatic light flux. The IPCE spectra of DSSCs based on RE3+ doped and undoped In2O3 nanotubes are shown in Fig. 3b. The 532 nm peaks in the IPCE spectra for all the four devices match with the absorption of the N719 dye. The same trend of improvement can be observed for the IPCE spectra. It can be seen that a maximum IPCE of 43% can be obtained for the In2O3:Er3+–DSSC.
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Fig. 3. (a) The J–V characteristics of DSSCs based on In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes measured under simulated solar illumination (AM 1.5G) with an intensity of 100 mW/cm2. (b) The IPCE spectra of DSSCs based on In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes.
Fig. 4. (a) Nyquist diagrams of the EIS spectra of DSSCs based on In2O3 and In2O3:Er3+ nanotubes. The lines are the fitting curves. The dots are the measured data. The inset is the equivalent circuit. (b) The phase plots of EIS spectra of DSSCs based on In2O3 and In2O3:Er3+ nanotubes.
Table 1 Photovoltaic parameters of DSSCs based on different photoanodes.
(inset) according to Ref. [40]. Each semicircle indicates a charge transfer processes which is exhibited by a resistance capacitance (R–C) parallel circuit. For a more precise fitting, the capacitance element is replaced by a constant phase element (CPE). Rs is the ohmic series resistance of the cell. R1 is the charge transfer resistance at the electrolyte/counter electrode interface [40]. R2 is the recombination charge transfer resistance at the In2O3/dye/electrolyte interface [40]. The higher R2 signifies the less recombination of electrons between the conduction band of In2O3 and the electrolyte which is expected [40]. Rs, R1 and R2 can be obtained by fitting according to the equivalent circuit (Table 2). The R2 values of the pure In2O3–DSSC and Er3+ doped In2O3–DSSC were determined to be 34.3 X and 106.2 X respectively. This result confirmed that the recombination of electrons in In2O3–DSSC was reduced by doping with Er3+, leading to the improvement of Jsc in Er3+ doped In2O3–DSSC. We think the recombination of the devices based on In2O3:Dy3+, In2O3:Ho3+, In2O3:Er3+ decreased as the order of doped elements increased from Dy to Er. The phase plots of EIS spectra (Fig. 4b) show the characteristic frequency peaks of the charge transfer process for the devices. The lifetime of electrons in In2O3 can be related to the inverse of the characteristic frequency and estimated by 1/2pfmax, where fmax is the maximum frequency of the peak [41–44]. The calculated lifetime was 26 and 122 ms for
Device
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
In2O3 In2O3:Dy3+ In2O3:Ho3+ In2O3:Er3+
4.6 6.6 6.7 8.6
0.4 0.5 0.5 0.5
26.2 33.4 33.2 34.4
0.5 1.1 1.1 1.4
3.3. The analysis of the improved device performance Different origins were proposed to explain the improvement of Jsc induced by doping or in a hybrid structure in previous works [37–39]. It was reported that TiO2/La2O3 hybrid photoanode possessed lower electron transfer resistance than the original TiO2 photoanodes, which alleviated the electron recombination rate and lead to higher Jsc [37]. Boschloo et al. found that significantly more charge could be extracted from TiO2 photoanode modified by insertion with aluminum ions than that from TiO2 [38]. Noticeably improved charge collection efficiency for Nb5+ doped TiO2 photoanode compared to TiO2 was reported by Graetzel et al. [39]. The higher electron transfer rate in the Sn doped TiO2 films than in the undoped TiO2 films was proposed by Lin et al., which resulted in higher Jsc [37]. In order to investigate the electron transfer process in the In2O3:RE3+–DSSCs, electrochemical impedance spectroscopy (EIS) measurements were taken. Fig. 4a presents the typical Nyquist plots of both the pure In2O3–DSSC and Er3+ doped In2O3–DSSC. Two semicircles locate at relative high and low frequency regions can be observed. The impedance spectra were fitted with Zview software based on the equivalent circuit
Table 2 Fitting results of the Nyquist plot. Photoanode
Rs (X)
R1 (X)
R2 (X)
In2O3 In2O3:Er3+
46.7 33.7
2.6 6.5
34.3 106.2
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In2O3 and In2O3:Er3+ respectively. It can be observed that In2O3: Er3+ has longer electron lifetime caused by Er3+ doping, which favors the electron transport leading to enhanced Jsc. Similar phenomenon has also been observed in iodine-doped TiO2 [44]. The fill factor was also improved by the doping which was probably due to the reduced recombination according to the literature reports [45,46]. Graetzel et al. observed that RE3+ doping in TiO2 decreased the resistivity, and used RE3+ doped in TiO2 as photoanode to improve the power conversion efficiency of the photovoltaic devices. However, the mechanism of that is still not clear [47]. Yang Yang also reported that Y doped TiO2 increased the conductivity, and the series resistance in the device was also reduced. The improved conductivity of Y–TiO2 matched that of the hole transport material, which balanced the carrier transport to reduce nonideal space charge distribution [48]. Lin et al. found that Sn4+ doped TiO2 photoanode for dye-sensitized solar cells improved the power conversion efficiency which was attributed to the higher transfer rate of electrons in Sn-doped TiO2 without a mechanism [37]. In our case, the electrical resistivity values were determined to be 6.3 104 X cm 4.3 104 X cm for In2O3 and In2O3:Er3+ films respectively by four-point resistivity measurements. Thus Er3+ doping in In2O3 decreased the resistivity. In EIS study, the device based on Er3+ doped in In2O3 has reduced the recombination and increased the lifetime of electrons leading to the improvement of the device performance. In addition, we also observed the series resistance in the device was reduced by Er3+ doping in EIS measurement (Table 2) which is similar to Yang Yang’s results. Further experiments are underway to better understand the mechanism. The Voc is determined by the energy difference between the red-ox potential of the electrolyte and the flat band potential (Efb) of the electrode material [22]. As mentioned above, the band gap of In2O3:RE3+ increased gradually with the atomic number of RE3+, which was evidenced by the UV–vis absorption spectra (Fig. 2b). It was reported that the band-gap energy increased with the decreasing of the refractive index (n0) for the oxides in the form of ApOq [49]. From Dy to Er, the value of n0 decreases gradually [50], indicating that the band gap of RE2O3 gradually increases. Thus the band-gap energy increases with the order of doped elements can be understood by the band gap of RE2O3 gradually increases. It was reported that incorporation of Mg ions in ZnO resulted in an apparent variation in the conduction band and the valence band remained almost the same as that of ZnO [51,52]. It is believed that the increase of the band gap is by the raised conduction band and the valence band remains the same although the real mechanism is still not clear. One explanation is that the effective mass of the holes is much larger than that of the electrons so the increased band gap is mainly caused by the upward shift of the conduction band edge [53]. The other is that the valence band of In2O3 is determined by the 2p orbit of oxygen ions, and RE3+ doping does not change the valence band. Therefore, it is possible that the increase of the band gap is caused by the higher conduction band of In2O3:RE, which leads to Efb shifting away from the redox potential and the improvement of the Voc of the DSSCs. Similar results were reported in conventional TiO2–DSSCs [22,23,37]. It can be concluded that all the photovoltaic parameters have been improved by the RE3+ doping for In2O3–DSSCs. The energy band alignment between In2O3:RE3+ nanotubes and a dye molecule (N719) is shown in Fig. 5 according to the Refs. [9,54]. It can be seen that the conduction band edge of the In2O3 nanotube is about 4.3 eV below the vacuum level. It shifted to 4.28, 4.26 and 4.18 eV when Dy3+, Ho3+ and Er3+ ions were doped in In2O3, respectively. N719 LUMO state is about 3.85 eV below the vacuum level, which is higher than the conduction band edge of In2O3:RE3+ allowing the photo-generated electrons to be easily injected into the conduction band of In2O3:RE3+.
Fig. 5. Energy band alignment between an In2O3:RE3+ nanotube and a dye molecule.
4. Conclusions In summary, In2O3, In2O3:Dy3+, In2O3:Ho3+ and In2O3:Er3+ nanotubes were successfully fabricated by the electrospinning method. The RE3+ doping shifted the absorption spectra of the In2O3 toward higher energy indicating increased band gap. DSSCs based on In2O3:RE3+ were also fabricated and compared with undoped devices. J–V and IPCE measurements exhibited significantly improved performance with the RE3+ doping. The improvement of Jsc was due to the increased recombination charge transfer resistance and electron lifetime caused by doping. The increased Voc can be attributed to the modulation of the band gap. As a result, a PCE of 1.4% was obtained for In2O3:Er3+–DSSC which was nearly three times that of the pure In2O3–DSSC. Acknowledgments This work was supported by the Major State Basic Research Development Program of China (973 Program) (No. 2014CB643506), the National Natural Science Foundation of China (Grant Nos. 11374127, 11304118, 61204015, 81201738, 81301289, 61177042, and 11174111), Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT13018). The China Postdoctoral Science Foundation Funded Project (2012M511337 and 2013T60327). References [1] B. O’regan, M. Grfitzeli, Nature 353 (1991) 737. [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595. [3] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Grätzel, Science 334 (2011) 629. [4] T. Kinoshita, J.T. Dy, S. Uchida, T. Kubo, H. Segawa, Nat. Photon. 7 (2013) 535. [5] Z. Yang, M. Liu, C. Zhang, W.W. Tjiu, T. Liu, H. Peng, Angew. Chem. Int. Ed. 52 (2013) 3996. [6] M. Ye, D. Zheng, M. Lv, C. Chen, C. Lin, Z. Lin, Adv. Mater. 25 (2013) 3039. [7] X. Dang, J. Qi, M.T. Klug, P.-Y. Chen, D.S. Yun, N.X. Fang, P.T. Hammond, A.M. Belcher, Nano Lett. 13 (2013) 637. [8] K. Sayama, H. Sugihara, H. Arakawa, Chem. Mater. 10 (1998) 3825. [9] S. Mori, A. Asano, J. Phys. Chem. C 114 (2010) 13113. [10] M. McCune, W. Zhang, Y. Deng, Nano Lett. 12 (2012) 3656. [11] H. Wang, B. Li, J. Gao, M. Tang, H. Feng, J. Li, L. Guo, CrystEngComm 14 (2012) 5177. [12] Q. Dai, J. Chen, L. Lu, J. Tang, W. Wang, Nano Lett. 12 (2012) 4187. [13] Y. Zhang, H. Zhang, Y. Wang, W. Zhang, J. Phys. Chem. C 112 (2008) 8553. [14] X. Wei, G. Xu, Z. Ren, C. Xu, G. Shen, G. Han, J. Am. Ceram. Soc. 91 (2008) 3795. [15] L. Xu, B. Dong, Y. Wang, X. Bai, Q. Liu, H. Song, Sens. Actuat. B 147 (2010) 531. [16] L. Xu, B. Dong, Y. Wang, X. Bai, J. Chen, Q. Liu, H. Song, J. Phys. Chem. C 114 (2010) 9089. [17] L.Y. Chen, Y. Liang, Z.D. Zhang, Eur. J. Inorg. Chem. 2009 (2009) 903. [18] R. Sharma, R.S. Mane, S.-K. Min, S.-H. Han, J. Alloys Compd. 479 (2009) 840. [19] A. Furube, M. Murai, S. Watanabe, K. Hara, R. Katoh, M. Tachiya, J. Photochem. Photobiol. A: Chem. 182 (2006) 273. [20] J.-H. The, J. Mater. Chem. 20 (2010) 6505. [21] J. Zhang, W. Peng, Z. Chen, H. Chen, L. Han, J. Phys. Chem. C 116 (2012) 19182. [22] K.H. Ko, Y.C. Lee, Y.J. Jung, J. Colloid Interface Sci. 283 (2005) 482.
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