- Email: [email protected]
Preparation of TiO2 hollow spheres for DSSC photoanodes
Journal of Physics and Chemistry of Solids 75 (2014) 38–41
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Preparation of TiO2 hollow spheres for DSSC photoanodes Chang-Yu Liao a, Shih-Ting Wang b, Fang-Chih Chang a, H. Paul Wang a,c,n, Hong-Ping Lin d a
Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Material Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Sustainable Environment Research Laboratories, National Cheng Kung University, Tainan 70101, Taiwan d Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan b c
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2013 Received in revised form 16 July 2013 Accepted 9 August 2013 Available online 19 August 2013
High crystallinity mesoporous TiO2 hollow spheres (MHS-TiO2) were prepared using the mesoporous carbon hollow sphere template. The MHS-TiO2 contains mainly nanostructured anatase. The mesopore of the MHS-TiO2 has a pore opening in the range of 400–600 nm. The refined extended X-ray absorption fine structure spectra indicate that the MHS-TiO2 possesses less the 1st-shell Ti–O coordination numbers than the nano-TiO2. More surface active species (A2 ((Ti ¼O)O4)) on the MHS-TiO2 are also observed by the component fitted X-ray absorption near edge structure spectroscopy. The MHS-TiO2 photoanode has a better DSSC conversion efficiency than the nano-TiO2 one by at least 40%. Note that the N3 dye molecules are accessible to the mesopores of the MHS-TiO2, and the loading time for N3 can be reduced by at least 70% if compared with those of the nano-TiO2. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis C. XAFS (EXAFS and XANES) D. Microstructure
1. Introduction Speciation data such as bond distance, coordination number (CN), and chemical identity of select elements in the disordered matrixes can be determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. With the benefits of refined EXAFS data, for example, migration routes and extraction pathways of copper and zinc species can be elucidated during electrokinetic treatments of copper-rich sludges [1]. The recovery process of the nanosize zinc from phosphor wastes using an ionic liquid was also observed by EXAFS [2]. The CN of the 1st-shell Cu–Cu decreased with a decrease of the Cu size in the Cu@C nanoparticles deposited on the counter electrode of a dye-sensitized solar cell (DSSC) [3–5]. An enhancement of 80% in the conversion efficiency with the use of smaller Cu@C particles (e.g., 20 vs. 7 nm) was found. Oxidation states, coordination geometry, and local bonding environment of hard X-ray absorbing atoms existed in complicated solid matrixes can also be determined by the informative X-ray absorption near edge structure (XANES) spectroscopic method. By XANES, it was found that a small amount of dispersed Cu@C in the DSSC electrolyte could highly enhance the DSSC efficiencies [6,7]. Hsiung and coworkers also observed by XANES that the A2 active
n Corresponding author at: Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan. Tel.: þ 886 6 276 3608; fax: þ886 6 275 2790. E-mail address: [email protected] (H.P. Wang).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.08.005
species were the mainly surface photoactive sites in the nano-TiO2 during photocatalytic degradation of methylene blue [8]. A better understanding of select elements in complicated solid and liquid phases can be provided by synchrotron X-ray absorption spectroscopy (XANES and EXAFS). Titanium-based microporous molecular sieves have been used in the photoanodes of DSSCs [9–11]. The DSSC photoanodes play the key functions on the loading of sufficient sunlight-absorbing dyes and effective transportation of photoexcited electrons [12]. The high surface area TiO2 can be coated on the transparent conducting oxide (TCO) glass, which can be used in the photoanodes of DSSCs [13]. However, the more active mesoporous TiO2 hollow spheres (MHS-TiO2) has seldom been applied in the DSSC photoanodes [14]. Thus, the main objective of the present work was to study chemical structure of the photoactive species in the MHS-TiO2 which was prepared using the mesoporous carbon hollow sphere template method. The photoactive species in the MHS-TiO2 were also investigated by synchrotron EXAFS and XANES. 2. Experimental Titanium(IV) n-butoxide (Ti(OBu)4) (Acrös) (0.4 g) was added in an acidic ethanolic solution containing 20 g of ethanol (95%) and 1 g of concentrated nitric acid to yield an acidic ethanolic titanium precursor having the Ti/C molar ratio of 0.02. The precursor was mixed with 0.4 g of mesoporous carbon hollow sphere templates at 298 K for 24 h. The titanium xerogel coated with the mesoporous
C.-Y. Liao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 38–41
carbon hollow sphere was dried at 333 K for solvent removal. The MHS-TiO2 was formed by calcination at 873 K for 8 h to remove the carbon templates. Chemical structure of the MHS-TiO2 samples was characterized on an X-ray diffraction (XRD) spectrometer (Rigaku Ultima þ ) scanned from 101 to 801 (2θ) at a scanning rate of 31/min. The sectional morphologic images of the MHS-TiO2 were determined by high-resolution transmission electron microscopy (HR-TEM) (FEI E.O, Tecnai F20 G2). The nano-beam diffraction (NBD) patterns of the MHS-TiO2 were also measured. The diffuse reflectance ultraviolet–visible (DR UV–vis) spectra of the MHS-TiO2 between 200 and 800 nm were measured on a spectrophotometer (Varian, Cary 100 Conc). The EXAFS and XANES spectra of the MHS-TiO2 were collected on the Wiggler beam line (17C) at the Taiwan National Synchrotron Radiation Research Center. The electron storage ring was operated at an energy of 1.5 GeV. A Si(111) double-crystal monochromator was used for energy selection at an energy resolution (ΔE/E) of 1.9 10–4 (eV/eV). The beam energy was calibrated against the absorption edge of a titanium foil at 4966 eV. The EXAFS spectrum of the MHS-TiO2 was refined and analyzed using the UWXAFS 3.0 and FEFF 8.0 simulation programs [15,16]. The Fourier transform coupled with a Bessel function was performed to convert the k3-weighted EXAFS oscillation from k-space (3.6–12.3 Å 1) to R-space. The many-body factor (S20 ) was fixed at 0.9 to reduce the fit variables during the simulation. The TiO2 slurry was prepared by mixing MHS-TiO2 (3 g) or nano-TiO2 (Merck) (3 g), with Triton X-100 (Sigma-Aldrich) (0.1 mL), acetylacetone (Fluka) (0.2 mL), and polyethylene glycol (Fluka) (1.5 g) in water (10 mL). The MHS-TiO2 or nano-TiO2 photoanode was made by coating the viscous TiO2 slurry onto a fluorine-doped tin oxide (FTO) glass (8 Ω/square, Solaronix) (0.5 cm 0.5 cm), which was dried and calcined at 723 K for 30 min. The N3 (Solaronix) ethanolic solution (3 10–4 M) was adsorbed in a saturated manner on the MHS-TiO2 and nano-TiO2 for 6 and 24 h, respectively. Three drops of a H2PtCl6 (Alfa Caesar) ethanolic solution (5 10–3 M) were spread onto a FTO glass which was calcined at 658 K for 10 min to yield the counter electrode. The iodide/triiodide (I /I3 ) electrolyte was composed of iodide (I2, Riedel-de Haën) (0.05 M), 1,2-dimethyl-3-propylimidazolium iodide (DMPII, Solaronix) (0.6 M), lithium iodide (LiI, Aldrich) (0.1 M), and 4-tert-butylpyridine (TBP, Aldrich) (0.5 M) in acetonitrile (Mallinckrodt). The photoanode and counter electrode were separated by a 60 μm-thick thermal foil (SO-SX1170, Solaronix). The I /I3 electrolyte was injected into the cell by means of the capillary force. An epoxy gel was then used to seal the cell. The assembled DSSCs were tested on a 300 W solar simulator (91160A, Oriel) combined with an AM 1.5 G filter (59044, Oriel). The power density of illumination of the class A simulator was calibrated and fixed to 100 mW/cm2 with a reference solar cell and meter (91150, Oriel). The current–voltage (I–V) data of the DSSC were recorded and analyzed with the I–V test station program (Oriel).
39
A: anatase A
A
A
A
Anatase Standard (PDF#21-1272)
20
30
40
50
60
Fig. 1. X-ray diffraction pattern of the MHS-TiO2.
3. Results and discussion The XRD pattern of the MHS-TiO2 is shown in Fig. 1. The broadened diffraction peaks broadened at 25.31, 36–391, and 53–56o (2θ) are evident for the existence of the nanostructured TiO2. It seems that the MHS-TiO2 contains mainly nano-anatase TiO2 crystallites. The crystalline size of the MHS-TiO2 calculated by the Scherrer equation using the full-width at half-maximum of the peak at 25.31 is 8 nm approximately. The HR-TEM image of the MHS-TiO2 shown in Fig. 2 indicates that the TiO2 crystallites (5–8 nm) are aggregated to form the mesopores. The MHS-TiO2 has the wall thicknesses of 20–60 nm,
Fig. 2. (a) Transmission and (b) high-resolution transmission electron microscopic images of the MHS-TiO2.
which is mainly composed of the TiO2 crystallites. The existence of mesopores within the MHS-TiO2 are also observed. The NBD pattern of the MHS-TiO2 shows distinct diffraction peaks at
C.-Y. Liao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 38–41
1.0
200 160
Absorbance (a.u.)
0.8
120 80
0.6
40
0.4
0 3.2
3.4
3.6 3.8 Band Gap (eV)
4.0
0.2
Normalized Absorbance (a.u.)
40
1.2
D
1.0 0.8
C
0.6
A3 B A2 A1
0.4 0.2 0.0 4.96
4.97
4.98
4.99
5.00
5.0
Photon Energy (keV)
300
400 500 600 Wavelength (nm)
700
800
Fig. 3. (a) Diffuse reflectance ultraviolet–visible and (b) Kubelka–Munk (inserted in the upper corner) spectra of the MHS-TiO2.
(101), (004), (200), (105), and (211), which are attributed to the anatase crystallites. It is worth to note that the MHS-TiO2 contains TiO2 nano-crystallites sized in the range of 5–10 nm, which is similar to the XRD observations (see Fig. 2). The DR UV–vis spectra of the MHS-TiO2 were also measured to reveal its optical characteristics. Fig. 3 shows the absorption wavelength of the MHS-TiO2. The (F(R)hv)2 as a function of the band gap denoting the Tauc plot is shown in the inset of Fig. 3. The F(R) and hv represent the Kubelka–Munk function and photon energy, respectively obtained from the dividing irradiative wavelength by a constant of 1240. The absorption band gap of the MHSTiO2 is determined by the fitting of the linear part of the Tauc plot to a straight line and extrapolated to the x-axis, which denotes the band gap. The direct band gap of 3.44 eV for the MHS-TiO2 is consistent with that of an anatase reported in the literature [17]. The XANES and EXAFS spectra of titanium in the MHS-TiO2 are shown in Fig. 4. The main features of four-fold tetrahedral (TiO4), five-fold square pyramid ((Ti¼O)O4), and six-fold octahedral (TiO6) titanium oxide structures arisen from 1s–3d dipole forbidden electron transitions in the pre-edge XANES can be attributed to A1 (4969.5 eV), A2 (4970.5 eV), and A3 (4971.5 eV), respectively (see Fig. 4(a)). The intense features at 4970–4975 eV yielded from 3d–4p dipole forbidden electron transitions were assigned to A3 and B features [18,19]. In addition, two distinct features at 4980–5010 eV of the post-edge XANES are generally referred to C and D traits which are induced by the 3s–np dipole allowed electron transitions [20]. By the Gaussian–Lorentzian curve fitting, the conjugated A1–A3 signals can be deconvoluted and semi-quantified. It is found that the fraction of the A2 active sites in the MHS-TiO2 is in the range of 22–55% (see Fig. 4(a)). Note that the A2 species generally have a high activity in photocatalytic degradation of organic pollutants [8]. To better understand the molecule-scale data of titanium in the MHS-TiO2, its EXAFS spectra were also collected and refined. Fig. 4(b) shows the EXAFS oscillation in terms of the FT magnitude and χ(k)k3 in R and k domain, respectively. The experimental spectra were theoretically simulated by the FEFF 8.0 program. The speciation data of the MHS-TiO2 are summarized in Table 1. The MHS-TiO2 possesses the 1st-shell Ti–O and 2nd-shell Ti–(O)–Ti bond distances of 1.98 and 3.07 Å with CNs of 2.9 and 0.7, respectively. Notably, the much less CNs of the Ti–O and Ti–(O)–Ti in the MHS-TiO2 compared to those in the nano-TiO2 are observed. More A2 surface species on the MHS-TiO2 are also found. Since the A2 photoactive sites play the major rule in photocatalysis, it is of interest to see if the A2 surface active site is also effective in DSSCs. The conversion efficiency of the DSSCs having the MHS-TiO2 coated photoanodes has been studied on an AM
0.05
FT Magnitude (a.u.)
0.0 200
0.04 0.03 0.02 0.01 0.00 0
1
2
3
4
5
6
7
8
R (Å) Fig. 4. The (a) XANES (and the component fitting) and (b) EXAFS (and the k3-weighted χ(k) curve) spectra of the MHS-TiO2.
Table 1 Speciation data of the MHS-TiO2 studied by EXAFS and XANES. Speciation data
MHS-TiO2 nano-TiO2
Shells
Ti–O Ti–(O)–Ti Ti–O Ti–(O)–Ti
CNa
2.9 0.7 6.7 2.6
R (Å)b
1.98 3.07 1.95 3.06
s2 (Å2)c
0.008 0.001 0.007 0.002
Photoactive species (%)d A1
A2
A3
22
23
55
22
20
58
a
Coordination number. Bond distance. c Debye–Waller factor. d Determined by XANES (A1: TiO4; A2: (Ti ¼O)O4; A3: TiO6). b
1.5 G solar simulator. Fig. 5 shows the current–voltage curves of the DSSC with the MHS-TiO2 and nano-TiO2 photoanodes under the light-illuminated and dark conditions. The photovoltaic data of the DSSC containing the MHS-TiO2 photoanode in the open-circuit voltage, short-circuit current density, and fill factor are 0.714 V, 2.53 mA/cm2, and 0.666, respectively, yielding a conversion efficiency of 1.2% illuminated by an AM 1.5 G solar power (100 mW/cm2). Note that the N3 dye is more accessible to the mesopores of the MHS-TiO2, and the loading time for N3 can be reduced by at least 70% if compared with those of the nano-TiO2.
4. Conclusions The mesopore of the MHS-TiO2 has pore openings between 400 and 600 nm which are accessible for the N3 dye molecules. The I–V curve of the DSSC having the MHS-TiO2 photoanodes demonstrates
C.-Y. Liao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 38–41
Center (NSRRC) are gratefully acknowledged. We also thank Dr. J.-F. Lee of the NSRRC for the experimental assistances in the measurements of EXAFS and XANES spectra.
3.0
Current Density (mA/cm2)
41
2.5 2.0 1.5
References
1.0 0.5 0.0 -0.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Voltage (V) Fig. 5. Current–voltage (I–V) curves of the DSSC photoanode containing the MHSTiO2 (solid line and triangle) and nano-TiO2 (dash line and square) under the (a) light illumination and (b) dark condition.
0.714 V, 2.53 mA/cm2, and 0.666 in open-circuit voltage, short-circuit current density, and fill factor, respectively, and a conversion efficiency of 1.2% is obtained. The photovoltaic performances for the DSSC having the MHS-TiO2 coated photoanode are attributed to A2 active sites observed by XANES spectroscopy. The MHS-TiO2 photoanode has a better DSSC conversion efficiency than the nano-TiO2 one by 41%. Note that the N3 dye is more accessible to the mesopores of the MHS-TiO2, and the loading time for N3 can be reduced by at least 70% if compared with the nano-TiO2. Acknowledgments The financial supports of the Taiwan National Science Council, Bureau of Energy and National Synchrotron Radiation Research
[1] S.H. Liu, H.P. Wang, C.H. Huang, T.L. Hsiung, J. Synchrot. Radiat. 17 (2010) 202–206. [2] H.L. Huang, H.P. Wang, E.M. Eyring, J.E. Chang, Environ. Chem. 6 (2009) 268–272. [3] C.H. Huang, H.P. Wang, J.E. Chang, E.M. Eyring, Chem. Commun. 31 (2009) 4663–4665. [4] Y.M. Chiu, C.H. Huang, F.C. Chang, H.Y. Kang, H.P. Wang, Sustain. Environ. Res. 21 (2011) 279–282. [5] H.Y. Kang, H.P. Wang, Appl. Energy 100 (2012) 144–147. [6] C.Y. Liao, H.P. Wang, F.L. Chen, C.H. Huang, Y. Fukushima, J. Nanomater. 698501 (2009) 1–4. [7] F.L. Chen, I.W. Sun, H.P. Wang, C.H. Huang, J. Nanomater. 472950 (2009) 1–4. [8] T.L. Hsiung, H.P. Wang, H.P. Lin, J. Phys. Chem. Solids 69 (2008) 383–385. [9] P. Atienzar, S. Valencia, A. Corma, H. Garcia, ChemPhysChem 8 (2007) 1115–1119. [10] P. Atienzar, M. Navarro, A. Corma, H. Garcia, ChemPhysChem 10 (2009) 252–256. [11] M. Ziόłzek, C. Martín, B. Cohen, H. Garcia, A. Douhal, J. Phys. Chem. C 115 (2011) 23642–23650. [12] B. O'Regan, M. Grätzel, Nature 353 (1991) 737–740. [13] H. He, C. Liu, K.D. Dubois, T. Jin, M.E. Louis, G.H. Li, Ind. Eng. Chem. Res. 51 (2012) 11841–11849. [14] C.Y. Chang-Chien, C.H. Hsu, T.Y. Lee, C.W. Liu, S.H. Wu, H.P. Lin, C.Y. Tang, C.Y. Lin, Eur. J. Inorg. Chem. (2007) 3798–3804. [15] E.A. Stern, M. Newville, B. Ravel, Y. Yacoby, D. Haskel, Physica B 208–209 (1995) 117–120. [16] A.L. Ankudinov, B. Ravel, J.J. Rehr, S.D. Conradson, Phys. Rev. B 58 (1998) 7565–7576. [17] H. Tang, F. Levy, H. Berger, Phys. Rev. B 52 (1995) 7771–7774. [18] F. Farges, G.E. Brown, J.J. Rehr, Phys. Rev. B 56 (1997) 1809–1819. [19] Z.Y. Wu, G. Ouvrard, P. Gressier, C.R. Natoli, Phys. Rev. B 55 (1997) 10382–10391. [20] V. Luca, S. Djajanti, R.F. Howe, J. Phys. Chem. B 102 (1998) 10650–10657.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Preparation of TiO2 hollow spheres for DSSC photoanodes Chang-Yu Liao a, Shih-Ting Wang b, Fang-Chih Chang a, H. Paul Wang a,c,n, Hong-Ping Lin d a
Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan Department of Material Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Sustainable Environment Research Laboratories, National Cheng Kung University, Tainan 70101, Taiwan d Department of Chemistry, National Cheng Kung University, Tainan 70101, Taiwan b c
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2013 Received in revised form 16 July 2013 Accepted 9 August 2013 Available online 19 August 2013
High crystallinity mesoporous TiO2 hollow spheres (MHS-TiO2) were prepared using the mesoporous carbon hollow sphere template. The MHS-TiO2 contains mainly nanostructured anatase. The mesopore of the MHS-TiO2 has a pore opening in the range of 400–600 nm. The refined extended X-ray absorption fine structure spectra indicate that the MHS-TiO2 possesses less the 1st-shell Ti–O coordination numbers than the nano-TiO2. More surface active species (A2 ((Ti ¼O)O4)) on the MHS-TiO2 are also observed by the component fitted X-ray absorption near edge structure spectroscopy. The MHS-TiO2 photoanode has a better DSSC conversion efficiency than the nano-TiO2 one by at least 40%. Note that the N3 dye molecules are accessible to the mesopores of the MHS-TiO2, and the loading time for N3 can be reduced by at least 70% if compared with those of the nano-TiO2. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis C. XAFS (EXAFS and XANES) D. Microstructure
1. Introduction Speciation data such as bond distance, coordination number (CN), and chemical identity of select elements in the disordered matrixes can be determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. With the benefits of refined EXAFS data, for example, migration routes and extraction pathways of copper and zinc species can be elucidated during electrokinetic treatments of copper-rich sludges [1]. The recovery process of the nanosize zinc from phosphor wastes using an ionic liquid was also observed by EXAFS [2]. The CN of the 1st-shell Cu–Cu decreased with a decrease of the Cu size in the Cu@C nanoparticles deposited on the counter electrode of a dye-sensitized solar cell (DSSC) [3–5]. An enhancement of 80% in the conversion efficiency with the use of smaller Cu@C particles (e.g., 20 vs. 7 nm) was found. Oxidation states, coordination geometry, and local bonding environment of hard X-ray absorbing atoms existed in complicated solid matrixes can also be determined by the informative X-ray absorption near edge structure (XANES) spectroscopic method. By XANES, it was found that a small amount of dispersed Cu@C in the DSSC electrolyte could highly enhance the DSSC efficiencies [6,7]. Hsiung and coworkers also observed by XANES that the A2 active
n Corresponding author at: Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan. Tel.: þ 886 6 276 3608; fax: þ886 6 275 2790. E-mail address: [email protected] (H.P. Wang).
0022-3697/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2013.08.005
species were the mainly surface photoactive sites in the nano-TiO2 during photocatalytic degradation of methylene blue [8]. A better understanding of select elements in complicated solid and liquid phases can be provided by synchrotron X-ray absorption spectroscopy (XANES and EXAFS). Titanium-based microporous molecular sieves have been used in the photoanodes of DSSCs [9–11]. The DSSC photoanodes play the key functions on the loading of sufficient sunlight-absorbing dyes and effective transportation of photoexcited electrons [12]. The high surface area TiO2 can be coated on the transparent conducting oxide (TCO) glass, which can be used in the photoanodes of DSSCs [13]. However, the more active mesoporous TiO2 hollow spheres (MHS-TiO2) has seldom been applied in the DSSC photoanodes [14]. Thus, the main objective of the present work was to study chemical structure of the photoactive species in the MHS-TiO2 which was prepared using the mesoporous carbon hollow sphere template method. The photoactive species in the MHS-TiO2 were also investigated by synchrotron EXAFS and XANES. 2. Experimental Titanium(IV) n-butoxide (Ti(OBu)4) (Acrös) (0.4 g) was added in an acidic ethanolic solution containing 20 g of ethanol (95%) and 1 g of concentrated nitric acid to yield an acidic ethanolic titanium precursor having the Ti/C molar ratio of 0.02. The precursor was mixed with 0.4 g of mesoporous carbon hollow sphere templates at 298 K for 24 h. The titanium xerogel coated with the mesoporous
C.-Y. Liao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 38–41
carbon hollow sphere was dried at 333 K for solvent removal. The MHS-TiO2 was formed by calcination at 873 K for 8 h to remove the carbon templates. Chemical structure of the MHS-TiO2 samples was characterized on an X-ray diffraction (XRD) spectrometer (Rigaku Ultima þ ) scanned from 101 to 801 (2θ) at a scanning rate of 31/min. The sectional morphologic images of the MHS-TiO2 were determined by high-resolution transmission electron microscopy (HR-TEM) (FEI E.O, Tecnai F20 G2). The nano-beam diffraction (NBD) patterns of the MHS-TiO2 were also measured. The diffuse reflectance ultraviolet–visible (DR UV–vis) spectra of the MHS-TiO2 between 200 and 800 nm were measured on a spectrophotometer (Varian, Cary 100 Conc). The EXAFS and XANES spectra of the MHS-TiO2 were collected on the Wiggler beam line (17C) at the Taiwan National Synchrotron Radiation Research Center. The electron storage ring was operated at an energy of 1.5 GeV. A Si(111) double-crystal monochromator was used for energy selection at an energy resolution (ΔE/E) of 1.9 10–4 (eV/eV). The beam energy was calibrated against the absorption edge of a titanium foil at 4966 eV. The EXAFS spectrum of the MHS-TiO2 was refined and analyzed using the UWXAFS 3.0 and FEFF 8.0 simulation programs [15,16]. The Fourier transform coupled with a Bessel function was performed to convert the k3-weighted EXAFS oscillation from k-space (3.6–12.3 Å 1) to R-space. The many-body factor (S20 ) was fixed at 0.9 to reduce the fit variables during the simulation. The TiO2 slurry was prepared by mixing MHS-TiO2 (3 g) or nano-TiO2 (Merck) (3 g), with Triton X-100 (Sigma-Aldrich) (0.1 mL), acetylacetone (Fluka) (0.2 mL), and polyethylene glycol (Fluka) (1.5 g) in water (10 mL). The MHS-TiO2 or nano-TiO2 photoanode was made by coating the viscous TiO2 slurry onto a fluorine-doped tin oxide (FTO) glass (8 Ω/square, Solaronix) (0.5 cm 0.5 cm), which was dried and calcined at 723 K for 30 min. The N3 (Solaronix) ethanolic solution (3 10–4 M) was adsorbed in a saturated manner on the MHS-TiO2 and nano-TiO2 for 6 and 24 h, respectively. Three drops of a H2PtCl6 (Alfa Caesar) ethanolic solution (5 10–3 M) were spread onto a FTO glass which was calcined at 658 K for 10 min to yield the counter electrode. The iodide/triiodide (I /I3 ) electrolyte was composed of iodide (I2, Riedel-de Haën) (0.05 M), 1,2-dimethyl-3-propylimidazolium iodide (DMPII, Solaronix) (0.6 M), lithium iodide (LiI, Aldrich) (0.1 M), and 4-tert-butylpyridine (TBP, Aldrich) (0.5 M) in acetonitrile (Mallinckrodt). The photoanode and counter electrode were separated by a 60 μm-thick thermal foil (SO-SX1170, Solaronix). The I /I3 electrolyte was injected into the cell by means of the capillary force. An epoxy gel was then used to seal the cell. The assembled DSSCs were tested on a 300 W solar simulator (91160A, Oriel) combined with an AM 1.5 G filter (59044, Oriel). The power density of illumination of the class A simulator was calibrated and fixed to 100 mW/cm2 with a reference solar cell and meter (91150, Oriel). The current–voltage (I–V) data of the DSSC were recorded and analyzed with the I–V test station program (Oriel).
39
A: anatase A
A
A
A
Anatase Standard (PDF#21-1272)
20
30
40
50
60
Fig. 1. X-ray diffraction pattern of the MHS-TiO2.
3. Results and discussion The XRD pattern of the MHS-TiO2 is shown in Fig. 1. The broadened diffraction peaks broadened at 25.31, 36–391, and 53–56o (2θ) are evident for the existence of the nanostructured TiO2. It seems that the MHS-TiO2 contains mainly nano-anatase TiO2 crystallites. The crystalline size of the MHS-TiO2 calculated by the Scherrer equation using the full-width at half-maximum of the peak at 25.31 is 8 nm approximately. The HR-TEM image of the MHS-TiO2 shown in Fig. 2 indicates that the TiO2 crystallites (5–8 nm) are aggregated to form the mesopores. The MHS-TiO2 has the wall thicknesses of 20–60 nm,
Fig. 2. (a) Transmission and (b) high-resolution transmission electron microscopic images of the MHS-TiO2.
which is mainly composed of the TiO2 crystallites. The existence of mesopores within the MHS-TiO2 are also observed. The NBD pattern of the MHS-TiO2 shows distinct diffraction peaks at
C.-Y. Liao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 38–41
1.0
200 160
Absorbance (a.u.)
0.8
120 80
0.6
40
0.4
0 3.2
3.4
3.6 3.8 Band Gap (eV)
4.0
0.2
Normalized Absorbance (a.u.)
40
1.2
D
1.0 0.8
C
0.6
A3 B A2 A1
0.4 0.2 0.0 4.96
4.97
4.98
4.99
5.00
5.0
Photon Energy (keV)
300
400 500 600 Wavelength (nm)
700
800
Fig. 3. (a) Diffuse reflectance ultraviolet–visible and (b) Kubelka–Munk (inserted in the upper corner) spectra of the MHS-TiO2.
(101), (004), (200), (105), and (211), which are attributed to the anatase crystallites. It is worth to note that the MHS-TiO2 contains TiO2 nano-crystallites sized in the range of 5–10 nm, which is similar to the XRD observations (see Fig. 2). The DR UV–vis spectra of the MHS-TiO2 were also measured to reveal its optical characteristics. Fig. 3 shows the absorption wavelength of the MHS-TiO2. The (F(R)hv)2 as a function of the band gap denoting the Tauc plot is shown in the inset of Fig. 3. The F(R) and hv represent the Kubelka–Munk function and photon energy, respectively obtained from the dividing irradiative wavelength by a constant of 1240. The absorption band gap of the MHSTiO2 is determined by the fitting of the linear part of the Tauc plot to a straight line and extrapolated to the x-axis, which denotes the band gap. The direct band gap of 3.44 eV for the MHS-TiO2 is consistent with that of an anatase reported in the literature [17]. The XANES and EXAFS spectra of titanium in the MHS-TiO2 are shown in Fig. 4. The main features of four-fold tetrahedral (TiO4), five-fold square pyramid ((Ti¼O)O4), and six-fold octahedral (TiO6) titanium oxide structures arisen from 1s–3d dipole forbidden electron transitions in the pre-edge XANES can be attributed to A1 (4969.5 eV), A2 (4970.5 eV), and A3 (4971.5 eV), respectively (see Fig. 4(a)). The intense features at 4970–4975 eV yielded from 3d–4p dipole forbidden electron transitions were assigned to A3 and B features [18,19]. In addition, two distinct features at 4980–5010 eV of the post-edge XANES are generally referred to C and D traits which are induced by the 3s–np dipole allowed electron transitions [20]. By the Gaussian–Lorentzian curve fitting, the conjugated A1–A3 signals can be deconvoluted and semi-quantified. It is found that the fraction of the A2 active sites in the MHS-TiO2 is in the range of 22–55% (see Fig. 4(a)). Note that the A2 species generally have a high activity in photocatalytic degradation of organic pollutants [8]. To better understand the molecule-scale data of titanium in the MHS-TiO2, its EXAFS spectra were also collected and refined. Fig. 4(b) shows the EXAFS oscillation in terms of the FT magnitude and χ(k)k3 in R and k domain, respectively. The experimental spectra were theoretically simulated by the FEFF 8.0 program. The speciation data of the MHS-TiO2 are summarized in Table 1. The MHS-TiO2 possesses the 1st-shell Ti–O and 2nd-shell Ti–(O)–Ti bond distances of 1.98 and 3.07 Å with CNs of 2.9 and 0.7, respectively. Notably, the much less CNs of the Ti–O and Ti–(O)–Ti in the MHS-TiO2 compared to those in the nano-TiO2 are observed. More A2 surface species on the MHS-TiO2 are also found. Since the A2 photoactive sites play the major rule in photocatalysis, it is of interest to see if the A2 surface active site is also effective in DSSCs. The conversion efficiency of the DSSCs having the MHS-TiO2 coated photoanodes has been studied on an AM
0.05
FT Magnitude (a.u.)
0.0 200
0.04 0.03 0.02 0.01 0.00 0
1
2
3
4
5
6
7
8
R (Å) Fig. 4. The (a) XANES (and the component fitting) and (b) EXAFS (and the k3-weighted χ(k) curve) spectra of the MHS-TiO2.
Table 1 Speciation data of the MHS-TiO2 studied by EXAFS and XANES. Speciation data
MHS-TiO2 nano-TiO2
Shells
Ti–O Ti–(O)–Ti Ti–O Ti–(O)–Ti
CNa
2.9 0.7 6.7 2.6
R (Å)b
1.98 3.07 1.95 3.06
s2 (Å2)c
0.008 0.001 0.007 0.002
Photoactive species (%)d A1
A2
A3
22
23
55
22
20
58
a
Coordination number. Bond distance. c Debye–Waller factor. d Determined by XANES (A1: TiO4; A2: (Ti ¼O)O4; A3: TiO6). b
1.5 G solar simulator. Fig. 5 shows the current–voltage curves of the DSSC with the MHS-TiO2 and nano-TiO2 photoanodes under the light-illuminated and dark conditions. The photovoltaic data of the DSSC containing the MHS-TiO2 photoanode in the open-circuit voltage, short-circuit current density, and fill factor are 0.714 V, 2.53 mA/cm2, and 0.666, respectively, yielding a conversion efficiency of 1.2% illuminated by an AM 1.5 G solar power (100 mW/cm2). Note that the N3 dye is more accessible to the mesopores of the MHS-TiO2, and the loading time for N3 can be reduced by at least 70% if compared with those of the nano-TiO2.
4. Conclusions The mesopore of the MHS-TiO2 has pore openings between 400 and 600 nm which are accessible for the N3 dye molecules. The I–V curve of the DSSC having the MHS-TiO2 photoanodes demonstrates
C.-Y. Liao et al. / Journal of Physics and Chemistry of Solids 75 (2014) 38–41
Center (NSRRC) are gratefully acknowledged. We also thank Dr. J.-F. Lee of the NSRRC for the experimental assistances in the measurements of EXAFS and XANES spectra.
3.0
Current Density (mA/cm2)
41
2.5 2.0 1.5
References
1.0 0.5 0.0 -0.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Voltage (V) Fig. 5. Current–voltage (I–V) curves of the DSSC photoanode containing the MHSTiO2 (solid line and triangle) and nano-TiO2 (dash line and square) under the (a) light illumination and (b) dark condition.
0.714 V, 2.53 mA/cm2, and 0.666 in open-circuit voltage, short-circuit current density, and fill factor, respectively, and a conversion efficiency of 1.2% is obtained. The photovoltaic performances for the DSSC having the MHS-TiO2 coated photoanode are attributed to A2 active sites observed by XANES spectroscopy. The MHS-TiO2 photoanode has a better DSSC conversion efficiency than the nano-TiO2 one by 41%. Note that the N3 dye is more accessible to the mesopores of the MHS-TiO2, and the loading time for N3 can be reduced by at least 70% if compared with the nano-TiO2. Acknowledgments The financial supports of the Taiwan National Science Council, Bureau of Energy and National Synchrotron Radiation Research
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