Anionic Ligand Assisted Synthesis of 3-D Hollow TiO2 Architecture ...

Article pubs.acs.org/Langmuir

Anionic Ligand Assisted Synthesis of 3‑D Hollow TiO2 Architecture with Enhanced Photoelectrochemical Performance Seong Sik Shin,†,‡ Dong Wook Kim,⊥ Jong Hoon Park,† Dong Hoe Kim,† Ju Seong Kim,†,‡ Kug Sun Hong,*,†,‡ and In Sun Cho*,§,∥ †

Department of Materials Science and Engineering, and ‡WCU Hybrid Materials Program, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea § Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States ∥ Department of Materials Science & Engineering, Ajou University, Suwon 443-749, Korea ⊥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Hollow structured materials have shown great advantages for use in photoelectrochemical devices. However, their poor charge transport limits overall device performance. Here, we report a unique 3-D hollow architecture of TiO2 that greatly improves charge transport properties. We found that citric acid (CA) plays crucial roles in the formation of the 3-D hollow architecture. First, CA controls the hydrolysis rate of Ti ions and facilitates surface hydrolysis on templates during hydrothermal synthesis. Second, CA suppresses the growth of the carbon template at the initial reaction stage, resulting in the formation of comparatively small hollow fibers. More importantly, a prolonged hydrothermal reaction with CA enables a hollow sphere to grow into entangled hollow fibers via biomimetic swallowing growth. To demonstrate advantages of the 3-D hollow architecture for photoelectrochemical devices, we evaluated its photoelectrochemical performance, specifically the electrolyte diffusion and electron dynamics, by employing dye-sensitized solar cells as a model device. A systemic analysis reveals that the 3-D hollow architecture greatly improves both the electrolyte diffusion and electron transport compared to those of the nanoparticle and hollow sphere due to the elongated porous hollow morphology as well as the densely interconnected nanoparticles at the wall layer.

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INTRODUCTION Hollow micro/nanostructures with controllable sizes, shapes, compositions, and shell/internal structures have received extensive amounts of attention due to the controllability of their density, specific surface area, surface permeability and light-scattering capability, and because they can be applied to a wide range of applications, with examples being catalysis,1 sensors,2,3 dye-sensitized solar cells,4−6 photoelectrochemical (PEC) water-splitting cells,7,8 Li-ion batteries,9 and supercapacitors,10 to improve device performance levels. For Li-ion battery and supercapacitor applications, the hollow structures improve the electrolyte diffusion properties due to their mesoporous nature (superior surface permeability), enabling them to improve their charge/discharge rates.11−13 In particular, when these types of hollow structures are employed as photoelectrodes in dye-sensitized solar cells (DSSCs) or PEC water-splitting cells, they can improve (i) the light© 2014 American Chemical Society

harvesting properties via the effective scattering/trapping of incident light and (ii) light absorber (dyes and quantum-dots) loading due to the large surface area, ultimately improving the photovoltaic or PEC water-oxidation performance.14−16 Thus, far, various hollow structured materials, especially metal oxides, have been studied to improve the performances of photoelectrochemical devices. Most of these studies focused on the mesoporosity levels of these materials to improve the electrolyte permeability and the light-scattering capabilities in photoelectrochemical devices.13,14 However, in addition to these improvements, there remains a significant challenge, i.e., the poor charge collection level, before PEC performance levels can be improved further. Recently, one-dimensional (1-D) Received: September 11, 2014 Revised: November 27, 2014 Published: December 3, 2014 15531

dx.doi.org/10.1021/la503641n | Langmuir 2014, 30, 15531−15539

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structures such as nanorods,17 nanowires,18 nanotubes arrays,19 hollow fibers,20 and their composites with nanoparticles21 have been implemented in efforts to enhance the electron transport properties.17,19,22−24 However, while these 1-D nanostructures improve the electron transport property to some extent, their limited specific surface area leads to poor surface reactivity and low amount loading of light absorbers (dyes and quantum dots), thus hindering any significant improvement in their photoelectrochemical performance.25,26 Accordingly, to improve the photoelectrochemical performance further, it is necessary to design and synthesize a hollow structure that takes into account the aforementioned capabilities simultaneously, i.e., mesoporosity, a large surface area, efficient light-scattering ability, and rapid electron transport capability. The most common method used for the synthesis of metaloxide hollow structures is the one-pot hydrothermal method, which relies on a solution containing metal salts and a sacrificial template, whose hydrolysis rate of the precursor metal ions is critical to synthesize hollow morphology.27,28 Thus, far, various metal oxides, e.g., SnO2, In2O3, Fe2O3, and ZnO, have been successfully synthesized in attempts to realize hollow structures through this hydrothermal method.29−32 For instance, Jiang et al. realized a multishelled ZnO hollow sphere with a carbonaceous microsphere template by controlling the heating process.33 Lou et al. synthesized a Fe2O3 hollow sphere with a sheet-like subunit using a quasi-emulsion-templated method.34 Recently, Liu et al. reported a coral-like SnO2 hollow architecture created using SnCl4 and sucrose as a Sn precursor and as a carbonaceous template source, respectively.14 Interestingly, they showed that a carbon template formed from sucrose can grow into 1-D fiber morphology under specific hydrothermal conditions via a biomimetic swallowing growth process. However, unlike these metal oxides, the synthesis of a TiO2 hollow structure with a controllable morphology is limited due to the rapid hydrolysis rate of Ti ions during the hydrothermal growth process.9,35,36 Additionally, although there are a few reports of the synthesis of TiO2 hollow structures, their feature size is large, and even the morphology is limited to sphere- and fiber-type hollow structures.37−39 In this study, we report the strategic anionic-ligand-assisted hydrothermal synthesis of a TiO2 three-dimensional (3-D) hollow architecture consisting of entangled hollow fibers via biomimetic swallowing growth. In order to control the hydrolysis rate of Ti ions, which is critical for the formation of the hollow structure, citric acid (CA) is introduced because its multidentate ligands (three carboxylic acid groups and one alcohol group) can coordinate with Ti ions in a variety of ways, leading to high stability against rapid hydrolysis and/or growth.40 First, we investigate the effects of CA on the formation of the TiO2 hollow architecture by electron microscopy and nuclear magnetic resonance (NMR) analysis. Second, through controlled experiments, we show that the TiO2 hollow spheres formed at the intermediate growth stage grow further into elongated fiber morphology via the biomimetic swallowing growth process, eventually forming a 3-D hollow architecture. Lastly, to elucidate the advantages of the TiO2 3-D hollow architecture for photo/electrochemical devices, we fabricate a dye-sensitized solar cell as a model device and investigate its electrolyte diffusion, pore-size distribution, and light-scattering and electron transport properties and compare these properties with those of a simple hollow

sphere structure and nanoparticle synthesized by the same method.

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EXPERIMENTAL SECTION

Synthesis of TiO2 Hollow Architectures. All of the TiO2 hollow architectures are synthesized by a simple hydrothermal process. In a typical procedure, 10 mL of TiCl3 (20% in HCl, Kanto Chemicals) was added to 100 mL of deionized (DI) water under vigorous stirring, and 0.96 g of citric acid (C6H8O7, 99%, Sigma-Aldrich Chemicals) and 10.95 g of sucrose (C12H22O11, 99.5%, Sigma-Aldrich Chemicals) were then added to the TiCl3 aqueous solution. After 10 min of stirring, the mixture was transferred to a 130 mL Teflon-lined stainless steel autoclave, which was maintained at 170 °C for 12 h in an oven. After the reaction, the black precipitates (TiO2-carbon composite) were collected by centrifugation (12000 rpm for 7 min) and washed with distilled water and absolute ethanol several times, after which they were freeze-dried overnight. Finally, the product was annealed at 500 °C for 1 h in air to burn away carbonaceous species (Figure S1, Supporting Information). There was no detectable amount of carbon residues after the annealing process (Figure S2, Supporting Information). For controlled experiments, the amounts of citric acid and the reaction times were varied. Characterizations and Measurements. The crystal structures of the prepared powders were determined using an X-ray powder diffractometer (XRD; New D8 Advance, Bruker). The powder morphologies and structures were investigated using field-emission scanning electron microscopy (FESEM; SU70, Hitachi) and highresolution transmission electron microscopy (HR-TEM; JEM3000F, JEOL). The specific surface areas and pore-size distribution of the film samples were measured using a Brunauer−Emmett−Teller (BET) surface area analyzer (Belsorp-mini II, BEL). The diffuse reflectance spectra of the film samples were recorded by UV−vis spectroscopy (Lambda 650, PerkinElmer) with an integrating sphere accessory. The films prepared on FTO were placed in the analysis chamber and scanned from 400 to 800 nm wavelengths. The FTO glass substrate was used as the blank. Importantly, the escaped lights must be thoroughly blocked in the integrating sphere for accurate measurement. A 13C NMR analysis was performed by nuclear magnetic resonance spectroscopy at 500 MHz (Avance 500, Bruker) to confirm the interactions between the citric acid and the Ti3+. The photocurrent-voltage characteristics of each cell were measured with a potentiostat (CHI 608C, CH Instruments) under simulated solar light illumination (AM 1.5G, 100 mW/cm2). The incident photon-tocurrent conversion efficiency (IPCE) spectra were recorded as a function of the wavelength (400−800 nm) using a specially designed IPCE system (K3100, MC Science) for DSSCs. The electrical impedance spectra were measured using a potentiostat (CHI 608C, CH Instruments) with a frequency ranging from 10−1 and 105 HZ. The amplitude of the alternative signal was 10 mV. Impedance parameters were determined by the fitting of the impedance spectra using Z-MAN software. Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were conducted on an electrochemical workstation (Zahner, Zennium) with a frequency response analyzer under modulated red light-emitting diode (667 nm). Fabrication of DSSCs. The respective TiO2 (HA (3-D hollow architecture)-, HS (hollow sphere)-, and NP (nanoparticle)-TiO2) films were prepared on fluorine-doped tin-oxide glass substrates (FTO, TEC8, Pilkington) with a compact layer from a homemade paste which included HA, HS, and NP-TiO2 powders via a doctor-blade method. The coated films were annealed using multiple heating steps (325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and then 500 °C for 15 min) under an ambient atmosphere. The hollow-structurebased films maintained their original morphology (porous structure) even after consecutive film formation processes (Figure S5, Supporting Information). The annealed films were immersed in 0.05 M TiCl4 aqueous solution for 30 min at 70 °C, after which they were sintered again at 500 °C for 30 min. To ensure dye adsorption, the films were soaked in a dye solution (0.5 mM of purified N719 in absolute 15532


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Figure 1. Typical morphology of the synthesized HA-TiO2. SEM images: (a) low-magnification, (b) high-magnification, and (c) a fractured surface. TEM images: (d) low-magnification, (e) high-magnification (the inset shows the selected area diffraction (SAED) pattern), and (f) highmagnification at the rectangular region in panel e showing a mesoporous structure and the shell thickness (∼50 nm). The inset shows a highresolution TEM image of a single nanoparticle. ethanol) at room temperature for 20 h. After the dye absorption process, the films were thoroughly rinsed with a mild stream of absolute ethanol to remove any physically adsorbed N719 dye molecules. Sandwich-type DSSCs were then assembled using the dyeadsorbed TiO2 films and a platinized FTO substrate (by sputtering) with a hot-melt film (∼60 μm, Surlyn) between them. Finally, an iodide-based liquid electrolyte (SI16 L1535-01, Merck) was infiltrated into the cell through a hole from the counter electrode side. The active area of the dye-coated TiO2 films is 0.16 cm2.

by simply controlling the reaction conditions. Figure 2 shows two typical morphologies of NP-TiO2 and HS-TiO2. As shown

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RESULTS AND DISCUSSION Synthesis of 3-D TiO2 Hollow Architectures. Figure 1 shows the typical morphology of the TiO2 3-D hollow architecture (HA-TiO2) synthesized by the CA-assisted hydrothermal method followed by air annealing. The SEM images (Figure 1a and b) show that HA-TiO2 has a 3-D architecture composed of entangled 1-D hollow fibers with an average outer diameter of 300 nm and a length of ∼2.0 μm. Interestingly, a fractured cross-sectional SEM image of the HA-TiO2 (Figure 1c) shows its hollow nature throughout the architecture. The transmission electron microscopy (TEM) images also (Figure 1d−e) confirm that the HA-TiO2 has a hollow and porous structure. The selective-area electron diffraction (SAED) pattern (inset of Figure 1e) indicates that the HA-TiO2 is polycrystalline with an anatase phase. Further investigation of the high-resolution TEM image (Figure 1f) reveals that the wall of the HA-TiO2 consists of closely interconnected nanoparticles with an average size of 15 nm and a thickness of approximately 50 nm. Additionally, the lattice fringe shown in the inset of Figure 1f indicates that individual nanoparticles of HA-TiO2 have high crystallinity, where the lattice fringe spacing (0.35 nm) corresponds to the interlayer spacing of the (101) of anatase TiO2. In addition to the above HA-TiO2, nanoparticles (NP-TiO2) and a hollow-sphere architecture (HS-TiO2) were synthesized

Figure 2. SEM (a,c) and TEM (b,d) images of two TiO2 samples prepared under different reaction conditions: (a and b) without CA; (c and d), with CA (0.05 mol); growth time = 2 h.

in Figures 2a and b, without the addition of CA, nanoparticles with a size range of 10−20 nm are obtained. However, when an appropriate amount of CA (0.05 mol) is added, a hollow-sphere architecture with a size range of 100−200 nm is formed (Figure 2c and d) with a short reaction time (2 h). Its shell thickness is 15533


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annealing, respectively. First, the crystallinity of all three samples increases after the air-annealing process, as revealed in Figures 1f and 3b. Notably, regardless of the annealing characteristics, the HS- and HA-TiO2 synthesized with the CA possess a pure anatase phase, while the nanoparticles (NPTiO2) synthesized without the CA have a mixed phase of anatase and rutile. Consequently, the addition of CA leads to a pure anatase phase from a mixed phase of anatase and rutile. Typically, the hydrothermal synthesis of TiO2 using Ti salt and sucrose favors the formation of either a mixed (anatase and rutile) or rutile phase.27 In contrast, our method, i.e., CAassisted hydrothermal synthesis, gives a pure anatase phase. During the hydrothermal synthesis process, anatase and rutile TiO2 are constructed by linking TiO6 octahedra in different bonding modes (edge-shared bonding and corner-shared bonding), and the linking between TiO6 is carried out by dehydration reactions between OH ligands in hydroxo complexes like [Ti(OH)nClm]2−.40,43 The linking between TiO6 by the dehydration reaction is affected considerably by the ligand field strength of the anion in the complexes, e.g., OH− and Cl−.40,43 The larger the ligand field strength is, the more predominant the edge-shared bonding of TiO6 is compared to corner-shared bonding.43 The ligand field strength of the citrate anion is larger compared to the levels associated with OH− and Cl−.40 Therefore, although the acidity of the solution is low (pH 2 μm) due to the fast growth of the carbon template during the hydrothermal reaction.27,41 In contrast, as shown in Figure 2c and d, the HS-TiO2 synthesized by the CA-assisted hydrothermal method has a comparatively small feature size (∼200 nm), which may be attributed to a stabilization effect by the CA. In general, during the hydrothermal process, the dehydrated sucrose (or glucose) dispersion, prior to final carbonization, forms small droplets, which grow further via the carbonization process, resulting in the formation of a large microsized carbon sphere template.42 However, in our system, the CA appears to stabilize the initially formed small droplets, thus suppressing their further growth, unlike in the pure glucose or sucrose cases. Figure 3a and b shows the X-ray diffraction (XRD) patterns of the HA-, HS-, and NP-TiO2 samples before and after air-

Figure 3. X-ray diffraction patterns of (a) as-prepared and (b) airannealed TiO2 samples. HA, 3-D hollow architecture; HS, hollow sphere; NP, nanoparticles. A and R represent the anatase phase and the rutile phase, respectively.

Figure 4. Effects of CA addition: (a) 13C NMR spectra of a pure CA solution (black line) and a CA/Ti mixture solution (red line). SEM images of as-prepared samples (b) without CA and (c) with a CA addition, showing that nanoparticles are attached onto the surface of a carbon sphere, i.e., surface hydrolysis of Ti ions with an addition of CA. SEM images showing the growth-time-dependent morphology variation: (d) 2 h, (e) 4 h, and (f) 8 h. When the growth time increases, hollow spheres become aggregated and then form the fiber morphology by swallowing growth. 15534


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Figure 5. Schematic illustration of the formation process of the TiO2 hollow architecture.

Formation Process and Growth Mechanism. To understand the formation process and related growth mechanism of HA-TiO2, two important hydrothermal reaction parameters, i.e., the amount of CA and the reaction time, are controlled (Figure 4). First, the interaction of CA with the Tiprecursor (TiCl3) is investigated by a C NMR technique. Figure 4a shows the 13C NMR spectra of pure CA and CA/ TiCl3 mixture solutions at room temperature. For the pure CA solution, four resonance signals at 42.95, 72.97, 173.06, and 176.39 ppm are assigned to methylene carbon (CH2), alcoholic carbon (COH), the carbons in the two terminal carboxylic acid groups (t-COOH) and a carbon in the middle carboxylic acid group (m-COOH), respectively.40,44 Interestingly, in the case of the CA/TiCl3 mixture solution, slight upfield shifts in the four resonance signals (42.70, 72.85, 172.94, and 176.24 ppm for CH2, COH, m-COOH, and t-COOH, respectively) are observed. These shifts were also observed in a TiCl4/citric acid mixture aqueous solution, indicating that the t-COOH and/or m-COOH group(s) in the citric acid interact with TiCl4 (or a partially hydrolyzed species).40 Additionally, Nakamura et al. confirmed that a titanium peroxo complex with citric acid forms a tight coordination of titanium with t-COOH and OH.45 Similarly, the TiCl3 interacts with CA by coordinating the titanium ion, i.e., the chelation of the Ti3+ ion, resulting in the formation of a Ti−citrate complex with high stability toward hydrolysis.40 As shown in Figure S3 (Supporting Information), the color change from violet to blue-black after the mixing of TiCl3 with CA supports the formation of a complex of Ti ions with the CA by the coordination of the titanium ion. Second, the morphologies of the intermediate products (growth time = 1 h) during the hydrothermal synthesis process are examined. Figure 4b and c show SEM images of intermediate products without and with an addition of CA, respectively. Without the CA addition, only a mixture of carbon spheres (a smooth surface) and TiO2 nanoparticles can be obtained (Figure 4b). In contrast, with a CA addition, TiO2 nanoparticles are attached onto the surfaces of the carbon spheres. Generally, it has been reported that the hydrolysis of titanium ions occurs rapidly during the hydrothermal process.9,27,40 Specifically, the hydrolysis constant (pKa) of Ti3+ (aq) is ∼2.2, which is quite low compared to those of other metal ions such as Zn, Sn, Cu, and In.35,36 Therefore, without an addition of CA, the Ti ions rapidly hydrolyze to Ti(OH)x aggregates prior to the formation of the functionalized carbon sphere template during the hydrothermal process, i.e., favoring the bulk hydrolysis of Ti ions into the solution and resulting in the formation of nanoparticles and carbon spheres separately

(Figure 4b). However, with CA addition, the CA forms the Ti− citrate complex due to the Ti/CA interaction, which slows the hydrolysis rate of Ti ions and thus facilitates surface hydrolysis at the carbon template surface. However, it should be noted that an excessive addition of CA (>0.05 mol) leads to a crumbled structure (Figure S4, Supporting Information). The excess addition of carboxylate can lead to a massive stabilization of the formed carbon droplets before carbonization, which can no longer assemble into perfectly spherical shapes due to the steric effect. Therefore, the imperfect formation of the carbonaceous template can be a result of the crumbled structure.42 Finally, a series of time-dependent experiments at a fixed amount of CA (0.05 mol) were conducted to understand the growth process of the elongated fiber morphology in the HATiO2. Figure 4d−f shows SEM images of the structural evolution induced by the 2, 4, and 8 h hydrothermal reactions, respectively. At the initial growth stage (2 h), an aggregatedsphere architecture (TiO2 nanoparticles onto carbon spheres) with an individual sphere size of 200−300 nm is formed (Figure 4d). When increasing the hydrothermal reaction time to 4 h, the individual sphere begins to grow into an ellipsoidal morphology (Figure 4e). As the reaction continues (8 h), the ellipsoidal morphology grows further, finally forming the elongated fiber morphology of HA-TiO 2 (Figure 4f). Interestingly, the initially formed spheres evolve into a 1-D structure without an increase in their lateral size. On the basis of the series of experiments described above, a formation process and growth mechanism are proposed, as illustrated in Figure 5, to rationalize the formation of HA-TiO2. The synthesis of the TiO2 3-D architecture breaks down into five processes: (1) the formation of a carbonaceous sphere template, (2) the surface hydrolysis of titanium ions, (3) aggregation/crystallization, (4) encapsulation/dissolution (biomimetic swallowing growth) toward the 3-D architecture, and (5) formation of a hollow architecture with the burning away of the carbonaceous template. In the first step, dehydration/ carbonization reactions of sucrose occur under a hydrothermal process, resulting in the formation of a carbonaceous sphere with a hydrophilic surface due to functionalization with hydroxyl and carboxyl groups.41,42,46 For the CA-TiCl3 mixture, a CA-chelated titanium complex is formed in the solution, followed by hydrolysis at the surface of the carbonaceous sphere to form carbon−TiO2 nuclei composites. As the reaction proceeds (4 h), the sphere-like carbon−TiO2 nuclei composites become linked to each other and then grow further into elongated fibers by encapsulation. In the final stage, the interfacial TiO2 nanoparticles in the elongated fiber are dissolved (selective dissolution), eventually forming entangled hollow fibers after the annealing process. Interestingly, in contrast to the findings of previous reports (e.g., multishelled TiO2, ZnO, and SnO2 hollow spheres), the formed carbon− TiO2 sphere composites grow further into an elongated structure with the selective dissolution of their inner channels, which is in good agreement with a previously reported biomimetic swallowing growth process.14 However, without an addition of CA, the Ti ions are hydrolyzed and condensed in the solution, i.e., bulk hydrolysis, due to the faster hydrolysis rate of the Ti ions relative to the speed of the dehydration/ carbonization reaction of the sucrose (carbon sphere template formation). Finally, the TiO2 nanoparticles and the microsized carbonaceous sphere are separately formed, as shown in Figure 4b. Advantages of the 3-D Hollow Architecture. Pore Structure and Light-Scattering Properties. Figure 6a shows

Figure 6. Comparison of (a) the pore-size distributions and (b) the diffused reflectance spectra of thick films prepared from NP, HS, and HA-TiO2 (3-D hollow architecture).

the pore-size distribution curves of the HA-, HS-, and NPTiO2-based films (∼20 um thickness), as fabricated by a doctor blade method. The HA- and HS-TiO2-based films display narrow pore-size distributions with average pore sizes of 16 and 20 nm, respectively, indicating that the individual nanoparticles in the hollow structure are well organized even after the thin film processing step. In contrast, the NP-TiO2-based film shows a broad pore-size distribution with a large proportion of detrimental pores larger than the size of the nanoparticles. Such large pores are generated by the localized aggregation of particles during the calcination step, which can reduce the amount of dye adsorption due to an amount equivalent to the volume loss.49 Furthermore, SEM cross-sectional images of each film indicate that the hollow-structure-based films (HAand HS-TiO2) are considerably porous compared to the NPTiO2-based film (Figure S5, Supporting Information). The surface area of the HA-, HS-, and NP-TiO2 are 68.5, 66.4, and 49.5 m2/g, respectively, obtained from the N2 adsorption− desorption measurements. The isotherm loop (Figure S6, Supporting Information) of hollow structure (HA- and HSTiO2) presents a type IV isotherm with H3 types of hysteresis, indicating a mesoporous structure, which enables a narrow pore-size distribution of HA- and HS-TiO2. Additionally, the amounts of dye adsorbed on the three films were determined with UV−vis absorption spectroscopy.50 The adsorbed dye

Figure 7. Nyquist plots from impedance spectra of NP-, HS-, and HATiO2 (3-D hollow architecture) based on the equivalent circuit model shown in the inset.

illumination (100 mW/cm2) and an open circuit condition. The EIS spectra exhibit three semicircles with contact series resistance (Rs) on the FTO substrate of approximately 9 Ω for the three DSSCs. The semicircle in the high-frequency region (105−102 Hz) represents the resistance corresponding to the electrolyte/Pt-FTO interface (R 1 ), while those at the intermediate frequencies (102−10 Hz) and in the lowfrequency region give information pertaining to the resistance at TiO2/dye/electrolyte interface (R2) and the Nernst diffusion of the electrolyte (R3), respectively. Interestingly, the R3 value decreases in the order of NP-, HS-, and HA-TiO2. This can be ascribed to mesopores generated from the hollow structure (Figure 6a) and the subsequent efficient diffusion of I3− via the mesopores, i.e., high diffusion coefficients of I3−, implying the acceleration of the redox activity of the electrolyte.52−54 These results were also supported by reduced R2 values.49 15536


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and NP-TiO2-based DSSCs, respectively. As a result, the multifunctionality of the TiO2 3-D hollow architecture (i.e., a large surface area, mesoporosity, high light-scattering, and fast electron transport; see Figure S8, Supporting Information) contributes to the higher values of Jsc and Voc, leading to the superior photovoltaic performance (8.6%) compared to that of the hollow sphere (7.5%) and nanoparticle (6.8%) counterparts. The device performance (i.e., photocurrent density− voltage curves and IPCE spectra) and the corresponding solar cell parameters are summarized in Figure S9 (Supporting Information) and Table S1 (Supporting Information). By using the bilayer structure, i.e., the hollow architecture TiO2 layer on commercial TiO2 nanoparticle layer, we were able to further improve the device performance, where a Jsc of 18.97 mA/cm2, Voc of 0.79, FF of 0.67, and η of 10.07% (Figure S10, Supporting Information) were observed.

Figure 8a compares the electron transport time constant (τt) for the three DSSC as a function of the short-circuit current

Figure 8. (a) Electron transport (IMPS) time constant (τt) as a function of the short-circuit current density and (b) the electron recombination (IMVS) time constant (τr) as a function of the opencircuit voltage for the NP, HS, and HA-TiO2-based DSSCs.

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CONCLUSIONS In summary, we synthesized a unique TiO2 3-D hollow architecture composed of entangled hollow fibers via a CAassisted hydrothermal method. Through a systematic analysis, we found that CA plays a key role in the formation of the 3-D hollow architecture and its crystallization process. First, the CA controls the hydrolysis rate by forming a CA-chelated titanium complex, which enables surface hydrolysis onto a carbonaceous template. Second, the CA suppresses the growth of the carbonaceous template by stabilizing small carbon droplets, enabling the creation of small hollow spheres. Moreover, the CA has an effect on the formation of the pure anatase phase of HA-TiO2 due to the strong field ligand of the citrate anion. More importantly, a prolonged hydrothermal reaction with the CA encourages the hollow spheres, which form at an intermediate growth stage to grow into elongated fiber morphology via the biomimetic swallowing growth process, i.e., a 3-D hollow architecture from hollow spheres. Additionally, as an example of a feasible photoelectrochemical application, we demonstrate a dye-sensitized solar cell (DSSC) with the TiO2 3-D hollow architecture. Compared to a simple hollow sphere structure with nanoparticles, it shows superior electron transport, light-scattering, and electrolyte diffusion properties, resulting in high power conversion efficiency (8.6%). We believe that the synthesis method presented in this work can be extended to the fabrication of other hollow structures with different compositions and that the resultant hollow materials can be applied to other electrochemical devices, such as Li-batteries and supercapacitors, to improve their performance levels when used in devices.

density (Jsc). All samples show typical power-law dependence of the transport time constant (τt) on Jsc. The HS-TiO2-based DSSC shows poor electron transport (higher τt), poorer than even that of a NP-based DSSC. This can be ascribed to the low connectivity between the hollow spheres, constraining the rapid electron transport, i.e., the longer transport path. Interestingly, compared to the HS- and NP-TiO2-based DSSC, the HA-TiO2based DSSC shows a reduced τt value by approximately 40% in the entire range of Jsc, indicating its faster electron transport rate. This electron transport enhancement is attributed to the tightly interconnected nanoparticles in the wall of the individual hollow fibers, shortening the electron path length by the vertical arrangement of the individual hollow fibers, and to the high crystallinity of the HA-TiO2 (Figures 1f and 3b). The tightly interconnected (densely packed) nanoparticles act as a “highway” network for fast electron transport. Moreover, the 3D entangled architecture of the HA-TiO2 enhances the verticality of individual fibers and thereby provides a direct electron transport path to the conducting substrate (FTO) without significant grain boundary recombination, as in a nanowire system.24,55 Additionally, the high crystallinity of HATiO2 leads to fewer transport-limiting traps, reducing the probability of an electron trapping/detrapping process.56 Here, it should be noted that the electron transport property in the hollow structure can be improved by constructing a 3-D architecture such that a higher short-circuit current density (Jsc) is created compared to that of simple hollow spheres. Also, the recombination time constant (τr) of the HA-TiO2-based DSSC increases compared to that of its HS- and NP-TiO 2 counterparts, suggesting fewer electron recombination with I3− (Figure 8b). This can be mainly ascribed to the superior diffusion of the electrolyte (I3−) through the mesopores, as mentioned above. The superior diffusion of I3− accelerates the redox activity, which can retard recombinations at the TiO2/ electrolyte interface, i.e., a longer electron lifetime.49,57 The longer lifetime induces a higher open-circuit voltage (Voc). Additionally, compared to the HS-TiO2-based DSSC, the slight increase in τr can be understood considering its unique structure; i.e., the HA-TiO2 has not only a hollow structure but also a 3-D architecture entangled with hollow fibers, thus possessing more pores in the gaps among each entangled structure to permeate the electrolyte effectively. Finally, the charge collection efficiency (ηcc) was estimated from the equation ηcc = 1 − τt/τr,58 as shown in Figure S7 (Supporting Information). The estimated value ηcc for the HA-TiO2-based DSSC (97.2%) is ∼4% and ∼5% larger than those of the HS-

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ASSOCIATED CONTENT

* Supporting Information S

TEM and SEM images, photograph of TiCl3 solution, charge collection efficiency, I−V curves, solar cell parameter, and IPCE spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(K.S.H.) E-mail: [email protected]. *(I.S.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. 15537


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ACKNOWLEDGMENTS This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2012-054871). This work was partially supported by the new faculty research fund of Ajou University (S-2014-G0001-00309). We thank Hye Na Bae at National Center for Inter-University Research Facilities (NCIRF) for measuring nuclear magnetic resonance spectroscopy at 500 MHz (Avance 500, Bruker). We also thank Hye-Eun Nam at Kaywon School of Art & Design for help with the table of contents graphic. This paper is dedicated to Prof. Kug Sun Hong, a beloved professor and mentor who passed away on March 14, 2014.

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