Single Crystalline-Like TiO2 Nanotube Fabrication with Dominant (001

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Single Crystalline-like TiO Nanotube Fabrication with Dominant (001) Facets using Poly(vinylpyrrolidone) Mi-Hee Jung, Kyoung Chul Ko, and Jin Yong Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5039078 • Publication Date (Web): 07 Jul 2014 Downloaded from http://pubs.acs.org on July 9, 2014

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The Journal of Physical Chemistry

Single Crystalline-like TiO2 Nanotube Fabrication with Dominant (001) Facets using Poly(vinylpyrrolidone) Mi-Hee Jung,*,† Kyoung Chul Ko, ‡ Jin Yong Lee‡ †

Solar Cell Technology Research Section, IT Components and Materials Industry Technology Research Department, IT Materials and Components Laboratory, Electronics and

Telecommunications Research Institute (ETRI), 218 Gajeong-no, Yuseong-gu, Daejeon 305-700, Republic of Korea. Fax: +82 (42) 860-6495; Tel: +82 (42) 860-5201; E-mail: [email protected]. ‡

Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea.

ABSTRACT The single crystalline-like TiO2 NTs (SC-TiO2 NTs) was prepared in the anodization process containing poly(vinylpyrrolidone) (PVP). The PVP in the electrolyte solution functions as a surfactant and controller of the crystal growth. The PVP is favorably adsorbed onto the (001) surfaces rather than the (101) facet during the TiO2 nanotubes (TiO2 NTs) synthesis due to the high absorption energy (81.1 Kcal/mol) on the (001) facets. PVP adsorbed (001) facets was protected during the synthesis and a single crystalline anatase that primarily exposes the (001) plane is prepared. Furthermore, the overall synthetic mechanism of the fabricated crystalline anatase is clarified using computational calculations. It is clear that

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there is difference in the binding interactions between the PVP and TiO2 facets depending on the type of surface. It is concluded that the differences in the binding energies might cause the generation of the SC-TiO2 NTs with exposed (001) facets. In the photovoltaic performance results, the dye-sensitized solar cells (DSSCs) based on the SC-TiO2 NTs show the higher photocurrent density due to the large amount of adsorbed dye and high crystallinity in comparison with that of TiO2 NTs. Since most of crystalline SC-TiO2 NTs with the active (001) facets have low recombination sites, it resulted in the effective charge separation and electron transport in the SC-TiO2 NTs DSSCs, leading to the high efficiency solar cell devices. KEYWORDS TiO2 nanotube, single crystalline, poly(vinyl pyrrolidone), solar cells

1. Introduction Titanium (IV) dioxide (TiO2) as a larger band gap material has been widely used for dyesensitized solar cells (DSSCs) and photocatalysts. To meet the requirements of high efficiency of solar cells, TiO2 should have large surface area for light harvesting efficiency and low resistance for electron transport and charge separation. To this end, TiO2 nanoparticles have been widely used for DSSCs owing to their large surface area for more dye adsorption. However, TiO2 nanoparticles composed of small sized nanoparticles produced grain boundary and are insufficient to induce the scattering effect for the light harvesting. This limits to improve the DSSCs efficiency. Recently, to overcome disadvantages of TiO2 nanoparticles, it was reported that TiO2 nanotubes (TiO2 NTs) is the most promising material to apply the photoelectronics and photocatalysis due to their specific properties such as one dimensional (1D) structure and ordered morphology. Since 1D TiO2 NTs were perpendicularly grown on the collecting electrode, TiO2 NTs could effectively separate the electron and hole pair at the TiO2/dye interface and,


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transport the electron to the collecting electrode. However, even though recombination reaction of TiO2 NTs is significantly slower than that of mesoporous TiO2 nanoparticles, ordered TiO2 NTs are also composed of crystalline nanoparticles, which also hinders electron transport as in a traditional TiO2 nanoparticle. Because of this drawback, TiO2 NTs could not significantly improve the power conversion efficiency (PCE).1 Anatase TiO2 crystalline was grown in the form of the truncated octahedron structure surrounded with the (101) and (001) facets in the solution process.2 Since the (001) facet has higher surface energy (0.90 J/m2) than the (101) facet (0.44 J/m2), the (001) facet shows the higher chemical reactivity for the application process. However, most of the synthesized TiO2 crystalline were dominated by the (101) facets (more than 94%)3 because the (001) facet was easily recombined with other reactive species during the synthesis and disappeared in TiO2 crystalline quickly4 due to the high surface energy.5 It causes TiO2 crystalline to show the poor performance for the potential applications.6,7 Therefore it is very challenging to synthesize TiO2 with high percentage of (001) facets for the industrial process. The highly exposed (001) TiO2 facets were synthesized by addition of the hydrofluoric acid (HF) as a crystal controlling agent into the TiO2 precursor solution (TiF4).3 Since the bonding energy (Do) of Ti and F is high (DoF-Ti = 569.0 kJ/mol), it exhibits very stable bond during the synthesis, hence resulted in high yield of 47% of (001) TiO2 facets and could be increased up to 60% by using the 2-propanol together HF. Furthermore, the sheet like anatase TiO2 with 89% of exposed (001) facet was also prepared by controlling the amount of TiO2 precursor, (Ti(OBu)4 and hydrofluoric acid (HF).8 However, considering environmental problems of HF, Submicrometer sized TiO2 sphere, which was consisted of the large ultrasthin TiO2 nanosheet, was prepared using the principle of spontaneous self-assembly of nanosheets during the synthesis.9


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However, for application purpose, the crystalline size of the prepared TiO2 should be further reduced to increase the surface area leading to increase the device performance. In this study, the preparation of TiO2 NTs with exposed (001) facets through an oriented attachment mechanism with surfactant-assistant processes using poly(vinylpyrrolidone) (PVP). The PVP in the electrolyte solution functions as a surfactant and controller of the crystal growth. Furthermore, the PVP was preferentially adsorbed onto the (001) surfaces and protected the (001) facets during the fabrication process and the single crystalline-like TiO2 NTs (SC-TiO2 NTs) that primarily exposed the (001) plane were prepared after the annealing process.10 In order to illustrate the reason for the high coverage of (001) facets of the TiO2 NTs due to the PVP during the synthesis, density functional theory (DFT) calculations were conducted for the PVP absorption on the TiO2 (101) and (001) facets. The calculation results demonstrate that the PVP was absorbed on the TiO2 (101) surface in parallel directions only for the electrostatic interaction, whereas it was absorbed on the TiO2 (001) facet with square and parallelogram directions for the electrostatic interaction, which led to an increase in the binding energy between the PVP and TiO2 (001) facet, as well as protecting the (001) facet from secondary reactions. When SC-TiO2 NTs were applied to the DSSCs, they effectively separated the charge at the TiO2/dye interfaces and exhibited the higher photocurrent density, larger diffusion coefficient and longer electron lifetime compared to the TiO2 NTs.


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2. Experimental Section

TiO2 NTs Fabrication. TiO2 NTs were prepared with the anodization process. The anodization process was performed in the electrochemical cell of two electrodes configuration with titanium foil (Ti, 100 mm × 100 mm × 0.05 mm, 99.6%; Goodfellow, England) as a working electrode and Pt electrode (100 cm2 size) as a counter electrode. The DC power was supplied on the Ti working electrode from 50 V to 100 V according to the experimental conditions. The conventional Ti anodization generally produced the formation of debris on the top of TiO2 NTs. To avoid such drawback and obtain the vertically well-aligned TiO2 NTs, the anodization process was performed with two step process as reported by Wang et al.11 First, Ti anodizaion process was carried out in the electrolyte solution, which is composed of 0.25 wt% NH4F and 0.75 wt% H2O dissolved in the ethylene glycol, for 3 h at 5 °C. After anodization process, the TiO2 NTs was peeled with the 1M HgCl2 solution. Second, the anodization process was performed with the same condition as the first process with the 1% NH4F at 0 °C. For the SC-TiO2 NT fabrications, 0.1 M acetic acid and 1 ~ 4 wt% PVP (Mw ≈ 1.3 × 106) were added into the electrolyte solution of Ti anodization. The electrolyte solution was well mixed to dissolve the PVP at 160 oC for 2h. The SC-TiO2 NTs was synthesized with the same method as the TiO2 NTs fabrication condition. The synthesized TiO2 NTs and SC-TiO2 NTs were sonicated for 1 ~ 5 s in the ultrasonification to remove residues of electrolyte solution and followed by drying at ambient air, and then annealed at 550 oC for 30 min to remove the organic compounds from anodization process.

DSSCs Fabrication and Characterization. The prepared TiO2 NTs and SC-TiO2 NTs were sensitized in a 0.3 mM (Bu4N)2-[Ru(4,4′-(COOH)-2,2′-bipyridine)2(NCS)2] (“N719”, Solaronix) for 30 min. The dye adsorbed photoanode was rinsed with the ethanol to remove the physically binding dye on the TiO2. The Pt precursor solution was prepared by dissolving 10 mM hydrogen


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hexachloroplatinate (IV) hydrate (H2PtCl6, Aldrich, 99.9%) in a 2-propanol solution. Counter electrode was obtained by spin coating (3000 rpm for 30s) of Pt precursor solution on the fluorine-tin-oxide (FTO, 3 mm, 10 Ω/square, Pilkington TEC8) glass and then annealed at 450 o

C for 30 min. The working and counter electrode were assembled by the Surlyn polymer film

(thickness of 50 µm, Dupont 1702) like the sandwich-type device. The electrolyte was composed of the 0.6 M tetra-butylammonium iodide, 0.1 M lithium iodide, 0.1 M iodide and 0.5 M 4-tertbutyl pyridine in acetonitrile and infiltrated through the hole of counter electrode of the sandwich device by the capillary force. The active area of all cells was 0.15-0.25 cm2. The photovoltaic characteristics were measured by employing a 1000 W xenon lamp (Oriel, 91193) under simulated solar light at AM 1.5. The one-sun light intensity (100 mW/cm2) was fixed by the Si reference solar cell (Fraunhofer Institute for Solar Energy System: Mono-Si + KG filter). The incident photon to current efficiency (IPCE) measurement was conducted by the QEXL Solar Cell Quantum Efficiency Measurement System (PV measurements, lnc.) which was calibrated with a silicon photodiode (NIST-calibrated photodidode G425) at the 360 nm ~ 1100 nm wavelength range. The IPCE was measured at chopping speed of 5 Hz because of response time of DSSCs. All data were collected in the lock in amplifier to the reference cell. The electrochemical impedance spectroscopy (EIS) was measured by the potentiostat (Solartron 1287) equipped with a frequency response analyzer (Solartron 1260) in the frequency region of 10-2 ~ 106 Hz. The SC-TiO2 NTs morphology and crystalline lattice were measured by the field emission scanning electron microscope (FE-SEM, Model: Sirion, Netherlands) and fieldemission transmission electron microscopy (FE-TEM, JEM-2100F, JEOL Ltd., Japan), respectively. Crystalline orientation of SC-TiO2 NTs was measured by X-ray diffraction (XRD, RIGAKU, D/MAX-2500) with Cu Kα radiation (40 kV, 30 mA and λ = 1.54056 Å) in a 2θ range


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of 20 ~ 80o. The amount of dye adsorption was measured using UV-VIS-NIR spectrometer (Lambda 750, Perkinelmer Co.). The Brunauer-Emmett-Teller (BET, ASAP 2010, Micrometritics Co, Inc.,) surface area was determined by the adsorbed amount of nitrogen for one monolayer of sample at STP state (0 oC, 1 atm). All samples were evacuated at 100 oC for 2 h and then, annealed at 300 oC for 4 h before measurement. Refractive index (n) and extinction coefficient (k) were measured by the ellipsometer (WVASE32, Woollam Co, Inc.). The polarized incident light to the surface was tilted at 75o. The cauchy model (nλ = A + ⁄λ + ⁄λ, where A is constant for the refractive index, B and C are constant for the dispersion) was used for the calcalution of n and k in the wavelength range of 350 nm ~ 1000 nm. The intensity modulated photovoltage spectroscopy (IMVS) and intensity modulated photocurrent spectroscopy (IMPS) were carried out using the ZAHNER CIMPS system. The light emitting diode (LED) was emitted at green light (λ max at 525 nm). The LED light was controlled by potentiostatic system in the frequency range of 0.1~300 Hz at amplitude 200 mV. The IMPS was measured at short circuit while IMVS was measured at open circuit state. The real and imaginary values for the photocurrent of IMPS and photovoltage of IMVS were simulated with the Levenberg-Marquardt algorithm, respectively.

Computational details. In order to perform the periodic density functional theory (DFT) calculations, the DMol3 package within the generalized gradient approximation (GGA) was used.12 The full geometry optimizations were performed using the Perdew, Burke, and Ernzerhof (PBE) functional and Double Numerical with d polarization (DND) basis set. Because of the large supercell for the anatase TiO2 slab models, the k-point grid was chosen as gamma point (1×1×1), which ensured the convergence of the molecular system. Furthermore, the convergence criteria of self-consistent field (SCF) energy was set to 1 × 10-5 Hartree, and that of displacement


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was 5 × 10-3 Å. The lattice constants of the anatase TiO2, which had tetragonal structures with lattice parameters of a = b = 3.776 Å and c = 9.486 Å, were used to build periodic slab models for the anatase TiO2 surfaces.13-17 These lattice constants are in good agreement with the experimental values 18 and have been used in several previous theoretical studies.15,19-22 For the anatase (101) and (001) surfaces, the slab models with the (120) and (100) TiO2 units that are presented in Figures 1(b) and 1(c) were used, respectively. For (101) surface, the slab thickness and the dimensions of supercells were 10.53 Å and 18.88 Å × 21.77 Å × 29.35 Å, respectively and for (001) surface, those values were 9.49 Å and 18.88 Å × 18.88 Å × 27.91 Å, respectively. All slabs were separated by a vacuum spacing of about 20 Å, and this guaranteed that there were no interactions between the slabs. For the optimizations of these two slab models, all atomic positions were fully relaxed. In the simulations for the PVP adsorption on the anatase (101) and (001) TiO2 surfaces, only the PVP and first layers of the TiO2 slab models were fully relaxed. However, the layers below the second layer were fixed at the corresponding the atomic positions of the fully optimized slab models. In Figure 2, it must be noted that the fully relaxed PVP and the first layer of the TiO2 slab models are represented by ball-and-sticks and lines, respectively, to aid in understanding. However, in Figures 3 to 5, the fully relaxed atoms are indicated using ball-and-sticks and the fixed atoms are represented using lines for clarity. Finally, the adsorption energy was calculated as follows: Eads = Epvp + Esurface - Epvp/surface, where Epvp and Esurface are the energies of an isolated PVP molecule and a clean TiO2 anatase surface, respectively. Epvp/surface is the total energy of the PVP adsorbed on the TiO2 anatase surfaces. It should be noted that a positive value of Eads denotes an energetically favorable adsorption.


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3. Results and discussion 3.1. Computational calculation for SC-TiO2 NTs growth mechanism SC-TiO2 NTs with exposed (001) facets can be synthesized when PVP is present in the Ti substrate anodized electrolyte solution. In order to investigate the adsorption details (i.e. binding modes, interactions, and energies) of the PVP on the TiO2 surfaces, the PVP isomer, which is stable, must be determined in advance. In this study, the DFT calculations were performed in order to locate the most stable geometrical isomer of PVP with a M06/6-31G(d) level using the Gaussian 09 program.23 Three types of isomers (PVP(a), (b), and (c)), were designed as PVP polymers (m = 4), as predicted in a previous study24, and these are depicted in Figure S1. Herein, the number of arrangements for the C=O bonds around the hydrocarbon backbone was considered. Then, the optimized structures and relative energies (in Kcal/mol) were obtained for the designed PVP (m = 4) conformers (Figure S2). It was found that PVP(c) was the most stable conformer. Therefore, PVP(c) was used as the representative PVP polymer (m = 4), and the PVP(c) conformer was used to investigate the binding characters between the PVP and TiO2 surfaces. In order to predict the binding modes between the PVP and anatase TiO2 surface, the electrostatic potential of PVP was analyzed as shown in Figure 1(a). At first, in order to predict the binding modes for the PVP adsorptions on the TiO2 surface, the electrostatic potential map of an optimized PVP was analyzed. Figure 1 presents the electrostatic potential map of the optimized PVP (m = 4). As shown in Figure 1(a), four –CH2 are located in the outer part of the PVP and the negatively charged –C=O, which is represented using red in the electrostatic potential map, is located in the inner area of the PVP. It is well known that the surface of anatase TiO2 is composed of 3-coordinate oxygen atoms (O3c), 2-coordinate oxygen atoms (O2c), and 5-


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coordinate Ti (Ti5c) for the (101) plane (Figure 1(b)).7 For the (001) plane, there are O2c and Ti5c, as shown in Figure 1(c).7 Based on this, it is expected that the intramolecular hydrogen bonds between the –CH2 of the PVP and the O2c of the TiO2 surfaces might be induced through adsorption. In addition to the hydrogen bond between the PVP and TiO2, it was expected that a strong electrostatic interaction between the –C=O of the PVP and the unsaturated Ti5c of the TiO2 surfaces could also be induced through adsorption. It should be noted that the four expected hydrogen bond sites of PVP form a parallelogram shape, as denoted by the blue lines in Figure 1(a) and that the electrostatic interaction sites (–C=O) of the PVP have a linear chain shape, as represented by the yellow rectangles in Figure 1(a). In this study, the initial geometries for adsorption of the PVP on the (101) and (001) TiO2 surfaces were designed with consideration of the above two type of interactions, i.e. the hydrogen bond and electrostatic interaction, as well as their geometrical differences. Figure 2 presents the optimized adsorption geometries of the PVP on the TiO2 surfaces obtained from a DMol3 package. For the (101) anatase surfaces, it was found that six hydrogen bonds were formed. In contrast, for the (001) anatase surfaces, four hydrogen bonds were formed. Interestingly, the absorption energies were calculated to be 31.7 and 81.1 Kcal/mol for the (101) and (001) surfaces, respectively. These results might be due to the differences in the electrostatic interactions of the adsorptions between the (101) and (001) TiO2 surfaces. The calculated two bond distances of –C=O···Ti5c were 4.68 and 4.77 for the (101) TiO2 surface, whereas they were 3.86 and 3.84 Å for the (001) TiO2 surface, as indicated by the yellow line in Figure 2. This denotes that the shorter bond distance of the electrostatic interactions for the (001) TiO2 surface might induce stronger adsorption energies than those for the (101) TiO2 surface. It was


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considered that these differences might result from the geometrical correspondence of the interaction sites between the PVP and the type of TiO2 surface. As seen in Figure 1, Figure 2, and Figures 3-5, the interaction sites for the hydrogen bonds, which are parallelograms, are well matched for both the PVP and (101)/(001) TiO2 surfaces. However, for the electrostatic interactions, the (101) surfaces have the unfavourable structural condition, which are denoted using yellow rectangles in Figure 1(b), Figure 2(a), and Figure 3. Therefore, we estimated that (101) surfaces dominantly have the binding of H-bonding. However, the PVP can be preferentially adsorbed onto the (001) surfaces because it has the same directional orientations for the electrostatic interaction sites (–C=O). Moreover, there are two adsorption configurations of the PVP on the (001) surfaces when considering the directions for the electrostatic interactions, as denoted by the two yellow rectangles in Figure 1(c). Figure 4 and Figure 5 present the types of PVP adsorptions on the (001) TiO2 surface and the adsorption energies of these two types were 81.1 and 79.7 Kcal/mol, respectively, and were higher than that of the (101) TiO2 surface (31.7 Kcal/mol) by a factor of 2.5. Therefore, The adsorption energies of PVP on (001) surfaces were composed of hydrogen bonds and electrostatic interactions. We suggest that the high adsorption of PVP on anatase (001) surface was due to the high polarity of PVP which was developed by the the resonance structure due to the C=O and C-N moieties of coplanar pyrrolidone ring of PVP.24 It was reported that –COOH of the organic acid on the (110) TiO2 facet was mainly occurred through the monodentate or bridge coordination mode which can be explained with the binding of carboxylate and proton of –COOH for the Ti and O of TiO2 surface, respectively. 25 However, since the chelate adsorptions as a type of dissociation adsorption have the low adsorption energy compared to the other adsorption, they might not occur.25 Vittadini et al.26 investigated the adsorption of both HCOOH and HCOONa on the (101)


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surface of anatase TiO2, and concluded that the HCOOH was adsorbed with the monodentate form while HCOONa was preferentially adsorbed with the dissociated bidentate configuration. The geometries of these molecular adsorptions were affected by the environmental state of TiO2 surface. Therefore, considering these results and high reactivity of (001) TiO2 surfaces27and that PVP has the stable molecular geometry for adsorption on the (001) TiO2 facets, we estimated that PVP was molecularly adsorbed on the (001) TiO2 surface and the high adsorption energy of PVP on (001) surface is due to the (001) surface state of TiO2 and high polarity of PVP. The anodization of Ti film containing F was dependent on the interaction of TiO2 and F. The role of F is not only the dissolution of TiO2 by the formation of [TiF6]2- complexation28 but also the controlling of the crystal growth by the binding with Ti on the TiO2 surface.3 We expected that the PVP and F association in the anodized reactions could increase the growth of (001) TiO2 surfaces through the synergistic effect because the adsorption energy of fluorine (F) on (001) surfaces (4.4 eV) is higher than that on (101) surfaces (2.8 eV).29

3.2. Application of SC-TiO2 NTs to DSSCs The TiO2 NTs formation in the fluoride (F) containing anodic corrosion process can be explained by the competition reaction between the continuous growth and dissolution of TiO2 oxide by F ions at the electrolyte solution and metal oxide interface. The morphology of TiO2 NTs was determined by the anodic corrosion conditions of Ti film.30 Figure 6(a) presents the SEM image of SC-TiO2 NTs with uniform pore diameter of 150 ~200 nm. The length and thickness of tube were about 12 µm and 20 nm, respectively. The hollow shape of SC-TiO2 NTs with relatively smooth wall was shown in the HR-TEM image (Figure 6(b)). The selected-area electron diffraction (SAED) pattern (inset of Figure 6(b)) reveals that the prepared SC-TiO2 NTs were


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mainly consisted of the (001) orientation of the anatase TiO2 crystalline structure. To confirm these results, the large area of SC-TiO2 NTs was selected to identify the crystalline lattice of the (001) facet of TiO2. As shown in HR-TEM image of Figure 6(c), the lattice distance of (001) facet is 0.237 nm, indicating that the surface of SC-TiO2 NTs was mainly composed of the active (001) facets. Figure 6 (d) shows the XRD results for the TiO2 NT and SC-TiO2 NTs after the heat treatment at 550 °C. The XRD also indicated that the relative intensity of (001) facets of TiO2 NT , which was confirmed by the anatase phase TiO2(JCPDS No. 21-1272), is 30% while that of the SC-TiO2 NTs is 92%, which is three folds for the TiO2 NT. Figure 7 presents photocurrent density (Jsc) - photovoltage (Voc) characteristics of TiO2 NT and SC-TiO2 NTs. The TiO2 NTs grown in the presence of 1, 2, 3, and 4 wt% PVP were designated as SC-TiO2 NT-1, SC-TiO2 NT-2, SC-TiO2 NT-3, and SC-TiO2 NT-4, respectively. The results of the photovoltaic property were summarized in Table 1. All devices showed little difference for the Voc. It is around 0.7+/-0.0 to 0.8+/-0.0 mV. However, the large differences were observed in Jsc, fill factor (FF) and power conversion efficiency (η). The Jsc (6.0+/-0.1~ 7.7+/-0.1 mAcm-2) of the SC-TiO2 NTs is larger than that (5.7+/-0.2 mAcm-2) of the TiO2 NT and the effective mass transport in SC-TiO2 NTs improved the FF due to the high crystallinity. These factors improved the solar cells efficiency. The highest η of the DSSCs achieved with SC-TiO2 NT-3 reached 3.5+/-0.1%, which was higher than that of TiO2 NT electrode (2.4+/-0.1%). We confirmed that it is related to the highly exposed reactive (001) facets of SC-TiO2 NT-3. However, at higher PVP concentrations, this binding specificity may decrease due to the ion diffusion process being hindered in the electrolyte. It was found that the cell efficiency was decreased in the SC-TiO2 NT-4 cell. For highly efficient DSSCs, It is very crucial to have an efficient charge separation and electron transport at the dye and TiO2 interface. It is mainly


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related to the photocurrent density which is generally proportional to the amount of dye adsorption, conductivity and effective charge collection. We measured the amount of adsorbed dye on the TiO2 NTs and SC-TiO2 NTs using UV-VIS-NIR spectrometer. The amount of dye adsorption is 2.2+/-0.1 × 10-5 mol/g for TiO2 NTs and 2.3+/-0.1 × 10-5, 3.5+/-0.2 × 10-5, 4.3+/-0.3 × 10-5 and 2.3+/-0.1 × 10-5 mol/g for the SC-TiO2 NT-1, SC-TiO2 NT-2, SC-TiO2 NT-3, and SCTiO2 NT-4, respectively (Table 1). It is clear that the amount of dye adsorption of all SC-TiO2 NTs is larger than that of the TiO2 NT. It was noted that the SC-TiO2 NT-3 exhibited a 2-fold increased dye loading than the TiO2 NTs. However, compared with the SC-TiO2 NT-3, the amount of dye adsorption of SC-TiO2 NT-4 was decreased as a result of limitation of the ionic transport due to the large amount of PVP during the synthesis. As indicated in Table 1, after calcination, the (001)/(101) ratios of the SC-TiO2 NT-1, SC-TiO2 NT-2, and SC-TiO2 NT-3 samples increased by approximately 44+/-1%, 86+/-1.5%, and 91+/-1.5%, respectively. It suggests that PVP played an important role to produce the active (001) facet in the TiO2 NT fabrication. On the contrary, the reduction of (001) in SC-TiO2 NT-4 could be attributed to limitation of ion diffusion due to the large amount of PVP. The BET surface areas of the asprepared TiO2 NTs, SC-TiO2 NT-1, SC-TiO2 NT-2, SC-TiO2 NT-3 and SC-TiO2 NT-4 samples were determined to be 38+/-1.2, 30+/-0.6, 26+/-1.2, 24+/-0.4 and 35+/-0.7 m2/g, respectively, Even though the BET surface area of the SC-TiO2 NTs was smaller than that of the TiO2 NTs, the amount of adsorbed dye in the SC-TiO2 NTs was higher than that of the TiO2 NTs. Figure 8 presents the external quantum efficiency (EQE) of the TiO2 NT and SC-TiO2 NTs DSSCs in the wavelength range of 400 ~ 800 nm. The quantum efficiency of wavelength range of 400 ~ 600 nm is correlated with the amount of dye adsorption while that of long wavelength range of 600-800 nm is attributed to the scattering effect. Considerable efforts have been made to


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improve the absorption range with the mainly two representative methods. The one is improved by nanostructures of sub-micrometer-sized photo-anode, which could increase the light path in the devices, leading to increase the light harvesting and the solar cell efficiency31 32 The other is the employment of two mode dyes for visible and near-IR regions, respectively.33 Since the total adsorption is the sum of the absorption of the each dye, it could increase the solar cell efficiency owing to the increase of the adsorption range. Figure 8 shows that All SC-TiO2 NTs DSSCs show higher EQE in compared to TiO2 NTs DSSCs in the range of all wavelengths. In particular, DSSCs based on SC-TiO2 NTs-3 shows the highest quantum efficiency, which was related to the highest percentage of (001) facets. We estimated that because highly crystalline SC-TiO2 NTs-3 has the lower defect sites in compared to the other photo-anodes, it reduces the recombination reaction between electron and hole and increases the photocurrent density in the cell. We have conducted EIS measurements to investigate the resistance and capacitance at the interfaces of solar cells. Figure 9 shows the Nyquist plot of TiO2 NT and SC-TiO2 NTs in the frequency range from 0.1 Hz to 105 Hz. The Nyquist plot result of DSSCs generally exhibits three hemispheres. The high frequency at the interception of real axis indicates the ohmic serial resistance (Rs). The low frequency range is the charge transfer resistance at the electrolyte/Pt counter electrode (R1) and intermediate frequency range is regarded as the electron transport resistance at the TiO2/dye/electrolyte interfaces (R2). The high frequency range is interpreted as the Warburg diffusion (W1) of redox coupling (I-/I3-) of electrolyte. The Nyquist plot of Figure 9 shows two hemispheres which were interpreted and fitted as the equivalent circuit (inset of Figure 9). The fitting parameters were summarized in the Table 2. The R2 and CPE2 values in the SC-TiO2 NTs-1~3 DSSCs show lower than that in TiO2 NTs DSSCs. It indicated that SCSC-TiO2 NTs-1~3 electrodes have the lower resistance and higher conductivity than TiO2 NTs


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electrode. Frome the fitting result using the equivalent circuit, we calculated the electron transit time with equation τn′ = R2 × CEP2, where CEP2 is the electrical double layer capacitance at the TiO2/dye/electrolyte interfaces. τn means time that the electron generated in the photo-electrode was transported to the collecting electrode. As shown in Table 2, τn in SC-TiO2 NTs DSSCs was shorter than that in TiO3 NT DSSC. Since SC-TiO2 NTs have the lower defect sites in compared to the TiO2 NTs due to the high crystallinity, it could decrease the charge recombination at the TiO2 and dye interfaces and increase the charge collection efficiency in the collecting electrode. Figure 10 presents the measured refractive index for the TiO2 NT and SC-TiO2 NT films for wavelengths from 350 nm to 1000 nm. The top surface of nanotubes was consisted of orderly arrayed nano-pores which induce the light adsorption rather than the light diffraction. The refractive index of the tubes exhibits in the range of from 1.00 to 1.83, which was less than that of the dense anatase (i.e. 2.5)34 In the meanwhile, it was notice that the SC-TiO2 NT-3 electrode exhibited the highest refractive index among the TiO2 NTs. This represented that a more crystalline structure produced a high transmittance film when compared with the noncrystalline structure of the as-prepared pristine TiO2 NTs electrodes. Figures 11(a) and 11(b) present the IMPS plots for wavelengths at 525 nm, 570 nm, and 670 nm for TiO2 NTs and SC-TiO2 NT-3, respectively. The IMPS measurements were measured under AC pulse illumination in the 0.1 ~ 100 Hz frequency range at short circuit condition. Since IMPS measurement is related to the charge recombination and electron transport in the cell, the electron transit time (τd), which means travel time of photo-generated-electron to collecting electrode, was calculated from the inverse equation (τd = (2πfmin(IMPS))-1)35 of frequency minimum of the IMPS response. At wavelengths of 570 nm and 670 nm, a slight increase in the


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electron transit time was noted for all light intensities. However, when the 525 nm wavelength light was applied, the electron transport rates depended on the light intensity. When an intensified background light was applied, the electron injection rate increased, which significantly increased the electron transport into the TiO2 electrode. At the same time, the τd for the SC-TiO2 NT-3 was shorter than that for the TiO2 NT devices, which implies that fast photoelectron collection occurred in the SC-TiO2 NT-3 compared with the TiO2 NT devices. Since the IMPS response corresponds to modulated light of the small-amplitude, the perturbation of current was dependent on the Fermi level and electron transport in the photo-anode. Therefore, the electron diffusion coefficient (Dn) was related to the τd and calculated from the Dn = d2/4τd,35 where d is the film thickness of photo-anode. Figure 11(c) shows the Dn of the SC-TiO2 NT-3 cells was also higher compared with that of the TiO2 NT cells. This is caused by the effective charge screening in the SC-TiO2 NT-3 cell, which results from a decrease in the recombination sites and the resulting fast moving of the electron. It appears that the diffusion coefficient is a strong function of the irradiated light intensity and wavelength. In the meanwhile, Since the IMVS measurements were measured at the open circuit voltage, the pure recombination in the cell was used for the measurement of electron life time (τn) which was calculated from inverse equation (τn = (2πfmin(IMVS))-1) 35 of the frequency minimum of IMVS plot. From Figure 11(c) and Figure 11(d) results, τn shows an order magnitude higher than τd. It indicated that the charge diffusion process fast proceeded in comparison with the recombination process under the illumination. Figure 11(d) shows that τn of the SC-TiO2 NT-3 cell measured using IMVS was much longer than that of the TiO2 NT cells. It should be noted that the electron lifetime increased as the wavelength decreased in contrast to the results of the electron transit time. As the wavelength increased, the kinetic energy of the electron decreased. This led to a


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decrease in the collision process in relation to the recombination process. Furthermore, the concentration of the migration charge carriers increased in order to obtain the collect electrode; this increased the electron lifetime, which increased the cell efficiency.

4. Conclusion In summary, we presented that Ti anodization process in the presence of PVP results in the synthesis of SC-TiO2 NTs with predominantly exposed, chemically active (001) facets. The binding energy of the PVP for (101) and (001) TiO2 surfaces was calculated, and that of the PVP for the (001) TiO2 was approximately two-fold higher than that of PVP for the (101) TiO2. The PVP was favorably absorbed on the (001) facet of TiO2 and protected the elimination of the (001) facet from TiO2 during the reaction. Thus, it is possible that the difference in the binding energies might generate SC-TiO2 NTs. In the photovoltaic measurement, DSSCs based SC-TO2 NTs exhibit the higher photocurrent density and lower electron transport resistance in compared to that based TiO2 NTs, which is attributed to the SC-TiO2 NTs with highly active (001) facet as photo-electrode. Therefore, the highly crystalline SC-TiO2 NTs could reduce the recombination of charge and effectively increase the diffusion coefficient and electron lifetime, resulting in the high efficiency solar cells.


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Figure 1. (a) Electrostatic potential map (left) and the structure (right) of the optimized PVP (m = 4) with an M06/6-31G(d) level at Gaussian09. Atoms are represented as blue (N), red (O), grey (C) and white (H). The expected hydrogen bonding sites (–CH2) are denoted using green circles and the directional orientation for the electrostatic interaction sites (–C=O) are represented using yellow rectangles. This notation is used throughout this paper. The optimized structures of the anatase (b) (101) surface and (c) (001) surface with the PBE/DND level at DMol3. The gray spheres and red spheres correspond to the Ti atoms and O atoms, respectively. The expected hydrogen bonding sites (O2c) are denoted using green circles and the directional orientation for the electrostatic interaction sites (Ti5c) are represented using yellow rectangles.


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Figure 2. PVP adsorption configurations on anatase (a) (101) and (b) (001) surfaces. The hydrogen bonding interactions and electrostatic interactions are denoted using green dotted lines and yellow lines, respectively. The bond lengths are given in Å.


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Figure 3. Optimized adsorption geometries of PVP on an anatase (101) surface: (a) top view of anatase (101) surface and (b, c) side views of anatase (101) surface. Hydrogen bonding interactions and electrostatic interactions are expressed by green dot lines and yellow line, respectively. The bond lengths are given in Å.


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Figure 4. Optimized geometries of PVP on an anatase (001) surface for adsorption type- I : (a) top view of the anatase surface and (b, c) side views anatase (001) surface. Hydrogen bonding interactions and electrostatic interactions are expressed by green dot lines and yellow line, respectively. The bond lengths are given in Å.


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Figure 5. Optimized geometries of PVP on an anatase (001) surface for adsorption type- II : (a) top view of the anatase surface and (b, c) side views anatase surface. Hydrogen bonding interactions and electrostatic interactions are expressed by green dot lines and yellow line, respectively. The bond lengths are given in Å.


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Figure 6. (a) SEM and (b) TEM images of SC- TiO2 NT array formed on a Ti substrate. The inset of (b) is the SAED pattern of the SC-TiO2 NT. (c) High-resolution TEM image of the vertical TiO2 NTs. (d) XRD patterns of the TiO2 NT and SC-TiO2 NT.


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9 TiO2 NT SC-TiO2 NT-1 SC-TIO2 NT-2 SC-TiO2 NT-3 SC-TiO2 NT-4

8 Photocurrent density/mA/cm2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7 6 5 4 3 2 1 0 0.0

0.1

0.2

0.3

0.4 0.5 0.6 Photovoltage/V

0.7

0.8

0.9

Figure 7. Photocurrent density (Jsc)-photovoltage (Voc) results for the TiO2 NT and SC-TiO2 NTs DSSCs under AM 1.5 irradiation of 100 mW cm-2.


25


8 TiO2 NT

7 External Quantum Efficiency/%

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SC-TiO2 NT-1 SC-TiO2 NT-2

6

SC-TiO2 NT-3 SC-TiO2 NT-4

5 4 3 2 1 0 300

400

500 600 Wavelength/nm

700

800

900

Figure 8. The external quantum efficiency of TiO2 NT and SC-TiO2 NTs DSSCs from the IPCE measurements.


26

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25 TiO2 NT SC-TiO2 NT-1 SC-TiO2 NT-2 SC-TiO2 NT-3 SC-TiO2 NT-4

20

15

lm

-Z'' /ohm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

5

0 0

5

10

15

20

25

30

35

40

45

Z'Re/ohm Figure 9. The Nyquist plot of EIS spectra for the TiO2 NT and SC- TiO2 NTs DSSCs with the different PVP weight percentages under the illumination of AM 1.5 (100 mW cm-2). All devices were interpreted with the equivalent circuit (inset).


27


2.2 TiO2 NT SC-TiO2 NT-1 SC-TiO2 NT-2 SC-TiO2 NT-3 SC-TiO2 NT-4

2.0

Refractive index, n

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.8 1.6 1.4 1.2 1.0 0.8 400

500

600

700

800

900

1000

1100

Wavelength/nm Figure 10. The refractive index of the TiO2 NT and SC-TiO2 NTs films as a function of the wavelength as measured using ellipsometry.


28

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1

1

0

-1 2 Photocurrent"/µ µAW m

2 -1

Photocurrent''/µ µAW m

5W 10 W 15 W 20 W

20 W 40 W 60 W 80 W

-1

525 nm

-2

670 nm

(b)

570 nm

670 nm

(a)

20 W 40 W 60 W 80 W

TiO2 NT

570 nm

20 W 40 W 60 W 80 W

0

5W 10 W 15 W 20 W

-1

-2

525 nm 20 W 40 W 60 W 80 W

-3 SC-TiO2 NT-3

-3

-4 -1

0

1

2

3

4

5

6

-1

7

1.4

(c)

525 nm TiO2 NT 670 nm TiO2 NT 525 nm SC-TiO2 NT-3 670 nm SC-TiO2 NT-3

0.08

6 5

0.06

525 nm TiO2NT 670 nm TiO2 NT 525 nm SC-TiO2 NT-3 670 nm SC-TiO2 NT-3

0.04

4 3 2

0.02 1 0.00 10

20

30

40

50

60

70

80

0 90

0

1

2

3

(d)

1.2

4

5

Photocurrent'/µ µAW-1m2

6

7

525 nm TiO2 NT 570 nm TiO2 NT 670 nm TiO2 NT 525 nm SC-TiO2 NT-3 570 nm SC-TiO2 NT-3 670 nm SC-TiO2 NT-3

Electron life time/s

0.10

Diffusion constant, Dminx10-2/µ m2s-1

Photocurrent'/µ µAW-1m2

Electron transit time/s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8 0.6 0.4 0.2 0.0 0

20

Light intensity/W m-2

40

60

80

100

Light Intensity/Wm-2

Figure 11. (a) The IMPS of the TiO2 NT and (b) SC-TiO2 NT-3 solar cells with N719 used as the sensitizer. The wavelength ranged from 525 nm to 670 nm. (c) Electron transit time and diffusion coefficient of the TiO2 NT and SC-TiO2 NT-3 solar cells with dependence of wavelength from the IMPS experiments. (d) The electron lifetime of the TiO2 NT and SC-TiO2 NT-3 solar cells with the dependence of the light intensity from the IMVS experiments.


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Table 1. Photovoltaic characteristics of TiO2 NT and SC-TiO2 NTs DSSCs and the amount of adsorbed dye, specific surface area and relative intensity (001) facet of the TiO2 NT and SC-TiO2 NTs films.

Devices

Jsc

Voc

FF

η

[mA/cm2]

[V]

[%]

[%]

Adsorbed dye amount × 105

Specific surface area

Relative intensity of (001)

(m2/g)

(%)

(mol/g) TiO2 NT

5.7+/-0.2

0.8+/-0.0

55+/-3

2.4+/-0.1

2.2+/-0.1

38+/-1.2

28+/-1.5

SC-TiO2 NT-1

6.7+/-0.1

0.7+/-0.0

60+/-3

2.9+/-0.1

2.3+/-0.1

30+/-0.6

44+/-1

SC-TiO2 NT-2

7.6+/-0.6

0.7+/-0.0

58+/-4

3.1+/-0.1

3.5+/-0.2

26+/-1.2

86+/-1.5

SC-TiO2 NT-3

7.7+/-0.1

0.7+/-0.0

61+/-1

3.5+/-0.1

4.3+/-0.3

24+/-0.4

91+/-1.5

SC-TiO2 NT-4

6.0+/-0.1

0.7+/-0.0

64+/-0.3

2.8+/-0.1

2.3+/-0.1

35+/-0.7

34+/-1


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Table 2. The fitted parameters of Nyquist plots (Figure 9) of the TiO2 NT and SC-TiO2 NTs DSSCs using the equivalent circuit. Photoelectrodes R1 (Ω)

CPE1 (µ µF)

R2 (Ω)

CPE2 (µ µF)

τn′ (ms)

TiO2 NT

1.7+/-0.3

350+/-1

34+/-0.6

1243+/-0.2

43+/-0.7

SC-TiO2 NT-1

1.3+/-0.0

161+/-0.2

24+/-0.9

499+/-0.5

12+/-0.5

SC-TiO2 NT-2

1.8+/-0.0

71+/-0.1

21+/-0.9

160+/-0.1

3.3+/-2

SC-TiO2 NT-3

1.9+/-0.1

68+/-0.0

18+/-0.5

150+/-0.1

2.7+/-0.1

SC-TiO2 NT-4

2.2+/-0.2

311+/-0.0

30+/-0.5

1590+/-0.0

48+/-0.9


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ASSOCIATED CONTENT Supporting Information. Designed PVP (m = 4) conformers and reference 24. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Mi-Hee Jung Solar Cell Technology Research Section, IT Components and Materials Industry Technology Research

Department,

IT

Materials

and

Components

Laboratory,

Electronics

and

Telecommunications Research Institute (ETRI), 218 Gajeong-no, Yuseong-gu, Daejeon 305-700, Republic of Korea. Fax: +82 (42) 860-6495; Tel: +82 (42) 860-5201; E-mail: [email protected] ACKNOWLEDGMENT This work was supported by Government funded R&D program under the Ministry of Strategy and Finance, Republic of Korea.


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BRIEFS It is demonstrated that the use of poly(vinylpyrrolidone) (PVP) during the synthesis of TiO2 nanotubes (NTs) results in the synthesis of single crystalline-like anatase TiO2 NTs with exposed, chemically active (001) facets

SYNOPSIS


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Single Crystalline-Like TiO2 Nanotube Fabrication with Dominant (001

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