Preparation and Characterization of Anatase TiO2 Nanosheets-Based

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Preparation and Characterization of Anatase TiO2 Nanosheets-Based Microspheres for Dye-Sensitized Solar Cells Yali Wang,† Weiguang Yang,*,‡ and Weimin Shi*,‡ †

Nano-Science and Nano-Technology Research Center, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, People's Republic of China ‡ Department of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, People's Republic of China ABSTRACT: The one-pot synthesis of hierarchically structured anatase TiO2 nanosheets-based microspheres with controlled sizes is successfully achieved through the hydrolysis of titanium tetrafluoride in polyol medium with heating at 170 °C. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) are used to characterize the products. The effect of heat treatment on F
adsorbed on the surface of the nanosheets-based microspheres is investigated. In our work, fluorine ions from TiF4, diethylene glycol (DEG) as a solvent as well as a protective capping agent, and excess acetic acid play important roles in the formation of the dominantly exposed {001} facets and hierarchical structure. Due to the dominant exposure of high surface energy {001} facets, enlarged surface area, and improved light-scattering effect of the anatase TiO2 nanosheets-based microspheres, the dye-sensitized solar cell (DSSC) based on such materials exhibits cell performance superior to that of P25 TiO2.

1. INTRODUCTION During the past decades, the synthesis of semiconductor nanoparticles with controlled size, morphology, and crystal structure has attracted much attention because shape, size, and crystal structure are crucial factors in determining their chemical, optical, and photo-electrochemical properties.1
5 In addition, the main focus of nanochemistry shifts more and more toward the use of these semiconductor nanoparticles as building blocks for the fabrication of complex three-dimensional hierarchical architecutures.6
9 As one of the most promising semiconductors, anatase TiO2 plays an important role in many applications, including dye-sensitized solar cells (DSSCs), photocatalysis, sensors, and photonic crystals.10
12 Due to the higher surface energy of {001} facets compared to {101} facets,13 most of the reported anatase TiO2 have been dominated by {101} facets, rather than {001} facets.14
17 Recently, Lu and co-workers made an important breakthrough in preparing sheetlike anatase TiO2 single crystals with 47% of {001} facets by hydrothermal method using HF as a capping agent.18 Soon after, several studies by research groups worldwide have developed new routes to prepare more anatase TiO2 sheets with exposed {001} facets.19
24 However, to our knowledge, the synthesis of 3D hierarchical architectures self-assembled with sheetlike anatase TiO2 as building blocks is, to date, extremely sparse.25 Recently, we have reported a communication on the fabrication of anatase TiO2 nanosheets-based hierarchical spheres with over 90% {001} facets by diethylene glycol
solvothermal method.26 Herein, the present paper explores a facile one-pot synthesis of sizecontrolled hierarchically structured anatase TiO2 nanosheetsbased microspheres with dominantly exposed {001} facets by a polyol method instead of solvothermal method and, furthermore, explores the effect of heat treatment on adsorbed F
and the roles of diethylene glycol, fluorine ion, and excess acetic acid in the formation of the dominantly exposed {001} facets and r 2011 American Chemical Society

hierarchical structure. In addition, we demonstrate that such hierarchically structured anatase TiO2 nanosheets-based microspheres exhibit improved photo-electrochemical properties when they are used as photoanode material in DSSCs and show good application potential.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The anatase TiO2 nanosheets-based microspheres (ATNMs) were prepared by the hydrolysis of titanium tetrafluoride in diethylene glycol (DEG) with heating at 170 °C. In a typical experiment, titanium tetrafluoride was added to DEG to give concentrations of 0.01
0.03 M and then magnetically stirred for 1 h at room temperature. A 5 mL aliquot of acetic acid was added to 40 mL of the above TiF4 solution under vigorous stirring for 1 h. The mixed solution was heated in an oil bath at a rate of 5 °C/min. The reaction continued for 7 h at 170 °C with continual stirring. Reflux with water cooling was employed to prevent solvent evaporation. After reaction, the ATNMs were obtained by centrifugation, followed by rinsing with ethanol and deionized water several times. 2.2. ATNM Film Preparation. For preparation of the ATNM paste, hydroxypropyl cellulose was added to DEG with a concentration of about 8 wt %. The mixed solution was added into the dried ATNM powder with heat treatment at 600 °C for 2 h and was stirred for 2 days to yield the slurry. The resulting slurry was spread onto FTO (fluorine-doped tin oxide) glass substrates (Nippon Sheet Glass) by doctor blading using Scotch tape as frame and spacer, dried at 200 °C for 10 min, and subsequently heated at 470 °C in air for 40 min. Received: July 26, 2011 Accepted: September 26, 2011 Revised: September 15, 2011 Published: September 26, 2011 11982

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Figure 2. Low-magnification TEM (a) and high-resolution TEM image (b) of the sample prepared with the TiF4 concentration of 10 mM after annealing at 600 °C for 2 h. The insets show the corresponding fast Fourier transform (FFT) patterns.

Figure 1. FESEM images (with the larger magnification in the inset) of the ATNMs prepared with different concentrations of TiF4: (a) 30, (b) 20, and (c) 10 mM. (d) XRD pattern of the ATNMs prepared with 30 mM of TiF4.

2.3. Fabrication of DSSCs. After sintering at 470 °C for 40 min, the ATNM films were cooled to 80 °C and immersed in dry ethanol containing 0.3 mM of cis-di(thiocyanate) bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) bis(tetrabutylammonium) overnight for dye adsorption. To assemble the solar cells, a Pt-coated conducting glass was placed on the dye-sensitized ATNM film separated by a 50 μm thin membrane spacer. The assembled cell was then clipped together as an open cell. An electrolyte, which was made with 0.1 M LiI, 0.1 M I2, 0.6 M dimethylpropylimidazolium iodide, and 0.5 M tert-butylpyridine in dry acetonitrile was injected into the open cell from the edges by capillarity. 2.4. Characterization. The ATNMs were characterized with use of field-emission scanning electron microscope (FESEM, Hitachi S-4800), high-resolution TEM (HRTEM, Phillips, Tecnai F30 operated at 300 kV), X-ray photoelectron spectroscopy (XPS, Kratos, AXIS Ultra), X-ray diffraction (XRD, Rigaku D/ max-2500 diffractometer with Cu Kα radiation, λ = 0.154 06, 40 kV, 100 mA), and BET (Brunauer
Emmett
Teller, Micrometrics ASAP 2020). Photocurrent
voltage measurements were performed using simulated AM 1.5 sunlight with an output power of 100 mW/cm2. The diffuse-reflectance spectra were measured by using a U-4100 spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. FESEM and XRD Studies. TiF4 concentration plays an important role in the formation of ATNMs. We investigated a series of samples with different TiF4 concentrations while acetic acid concentrations remained the same. Shown in Figure 1 are the FESEM images of prepared samples with different concentrations of TiF4 from 10 to 30 mM. Figure 1 indicates that microspherical TiO2 particles obtained through the

Figure 3. Comparison of the XPS survey spectra of the ATNMs with and without heat treatment at 600 °C for 2 h.

hydrolysis of titanium tetrafluoride in polyol medium have a hierarchical structure that is composed of numerous randomly arranged TiO2 nanosheets. When the TiF4 concentration was 30 mM, the nanosheets-based microspheres with an average size of 210 nm in diameter and building blocks, 2D TiO2 nanosheets of 8 nm thickness, are obtained. The hierarchically structured nanosheets-based microspheres are composed of nanosheet backbones of 210 nm diameter and a nanosheet branch of 40
90 nm diameter. By reducing the TiF4 concentration to 20 mM, the average diameter of nanosheet-based microspheres decreases from 210 to 160 nm; while the thickness of the constituent nanosheets remains about 8 nm, the amounts of constituent nanosheets tended to reduce. With a further decrease of the TiF4 concentration to 10 mM, the diameter and thickness of the hierarchically structured nanosheets of 150 nm diameter and 8 nm thickness have the same change tendency. At the same time, the amounts of constituent nanosheets are inclined to decrease. The crystallographic structure of the as-prepared ATNMs is confirmed by XRD. From the XRD pattern (Figure 1d), it is clear that the as-synthesized product is crystalline and consists of the pure anatase phase TiO2 without any indication of other crystalline byproduct. 3.2. TEM Studies. The microstructures of the ATNMs were further examined by TEM (shown in Figure 2). Figure 2a reveals that the entire structure of the architecture is built from several dozens of self-organized TiO2 nanosheets. A representative 11983

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Figure 4. High-resolution XPS spectra of Ti 2p (a), F 1s (b), O 1s (c), and C 1s (d) of the ATNMs with and without heat treatment at 600 °C for 2 h.

HRTEM image of a single nanosheet is given in Figure 2b. The two sets of the lattices with the fringe spacings of 0.098 and 0.183 nm and an interfacial angle of 90° agree well with the (400) and (020) lattice planes of anatase TiO2.18 The corresponding fast Fourier transform (FFT) pattern (Figure 2b, inset) can be indexed to diffraction spots of the [001] zone. On the basis of the above structural information, the percentage of exposed {001} facets of the anatase TiO2 nanosheets-based spheres can be estimated to be up to 90%. 3.3. XPS Analysis. The XPS measurement on the ATNMs with and without calcination at 600 °C for 2 h reveals the presence of Ti, O, F, and C elements (Figure 3). From Figure 3, the XPS peaks show that the binding energies of Ti 2p, O 1s, F 1s, and C 1s for the as-prepared ATNMs are located at 458.84, 529.94, 684.29, and 286.24 eV, respectively. The atomic ratio of Ti:O:F is 1:2.18:0.52, which is in good agreement with the nominal atomic composition of TiO2. To investigate the influence of heat treatment on the surface compositions and chemical status of the as-prepared ATNMs, the as-prepared products have been heated at 600 °C for 2 h. After calcination at 600 °C for 2 h, the binding energies for Ti 2p, O 1s, F 1s, and C 1s are located at 458.66, 529.87, 683.72, and 284.86 eV, respectively. The atomic ratio of Ti:O:F becomes 1:2.17:0.06. The C element is attributed to the residual carbon from the samples and adventitious hydrocarbon from the XPS instrument itself. The Ti 2p XPS spectra of the ATNMs with and without heat treatment at 600 °C for 2 h are shown in Figure 4a. Before heat treatment, the Ti 2P3/2 and 2P1/2 binding energies at 458.8 and

464.5 eV correspond to the energies of the photoelectrons of Ti4+, which agree well with that of TiO2 in the literature.27 After heat treatment, the intensity of the Ti 2p peaks at 458.6 and 464.4 eV shows an increase compared to the anatase TiO2 nanosheetsbased spheres without heat treatment, which is attributed to the replacement of the surface fluoride by surface hydroxyl groups caused by heat treatment at 600 °C for 2 h. Figure 4b shows the XPS spectra of F 1s core electrons for the ATNMs with and without calcination at 600 °C for 2 h. The asprepared sample shows only one peak at 684.3 eV, originating from surface fluoride formed by ligand exchange between F
and surface hydroxyl groups.28 When the as-prepared ATNMs were annealed at 600 °C for 2 h to remove the physically adsorbed F
, in addition to the peak 1 at 684.1 eV, a new peak at higher binding energy (688.1 eV, peak 2) with a much lower intensity compared to peak 1 appears; the peak intensity for peak 1 becomes significantly weaker. Peak 2 can be assigned to lattice F.28,29 From the XPS results, we can conclude that the heat treatment at 600 °C for 2 h can remove F
adsorbed on the surface of TiO2 (physically adsorbed F
or F
replacing surface hydroxyl groups), and, at the same time, trigger the substitution of F
for O2
ions in the lattice of TiO2. Figure 4c shows the high-resolution XPS spectra for the O 1s region taken on the surface of the ATNMs with and without calcination at 600 °C for 2 h. The O 1s region is composed of two peaks. The main peak at 529.9 eV is designated as lattice oxygen atoms of TiO2. The other minor peak at 531.1 eV is attributed to the OH in Ti
OH.30 Although the surface of TiO2 easily adsorbs 11984

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Figure 5. Comparison of the current
voltage characteristics of dyesensitized solar cells based on the 210-nm-sized ATNM and P25 TiO2 films of the same thickness (about 12 μm).

H2O, the physically absorbed H2O is easily desorbed under the ultrahigh-vacuum condition of the XPS system. Consequently, the hydroxyl on TiO2 is ascribed to Ti
OH. After annealing at 600 °C for 2 h, there is an increase in the peak intensities of Ti
O (529.8 eV) and OH of Ti
OH (531.0 eV) in TiO2. This is due to the fact that the F
on the surface of the ATNMs was replaced by OH, caused by high-temperature heat treatment. To investigate the carbon states in the samples before and after annealing, the C 1s core levels were measured, as shown in Figure 4d. There are three peaks at the binding energies of 284.8, 286.3, and 288.8 eV for the sample before annealing. The main peak at 286.3 eV is ascribed to C
OH (and C
O
C),31
33 which may come from DEG absorbed on the surface of TiO2. The small peak at 288.8 eV is attributed to CdO (and COO),34,35 which may come from acetic acid absorbed on the surface of TiO2. The second peak at 284.8 eV is attributable to other reduced carbons (C
C/C
H) from the XPS instrument itself.36 After calcination at 600 °C for 2 h, a large diminution in the peak located at 286.3 eV together with a large increase in the 284.8 eV and a slight increase in the 288.9 eV is observed, which demonstrates that the organic-like carbons from DEG and acetic acid absorbed on the surface of the sample can mainly be removed by heat treatment at 600 °C. 3.4. Formation Mechanism of Nanosheets-Based Structure. The synergistic functions of fluorine ions to markedly reduce the surface energy of the {001} facets to a level lower than that of {101} facets, based on the first-principles calculations,18 with acetic acid and DEG to act as protective capping agents lead to the formation of the ATNMs. As a result, the fluorine ions play a critical role in the formation of the exposed (001) facets. To understand the role of the fluorine ions in forming {001} exposed anatase TiO2, a series of experiments were carried out by introducing different amounts of TiF4 used as fluorine ion sources. When the concentration of TiF4 was decreased from 30 to 10 mM, the average diameter of nanosheet-based microspheres decreased from 210 to 150 nm while the amounts of constituent nanosheets decreased. Furthermore, DEG and acetic acid also play important roles in the formation of the exposed (001) facets and the hierarchical structure. The unsaturated Ti4+ cations on the (001) and (101) surfaces are inclined to be coordinatively bound to DEG or deprotonated DEG molecules.37 The excess acetic acid in the precursor solution has

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Figure 6. Diffuse-reflectance spectra of the two films of 210-nm-sized ATNMs and P25 TiO2 with the similar thickness of about 12 μm.

higher adsorption with respect to DEG through using two O in its carboxyl group binding to the 5-fold Ti on the surface of TiO2.38 The presence of absorbed DEG and acetic acid is confirmed by XPS results. Therefore, the growth of anatase TiO2 nanocrystals along the (001) direction relative to the (101) is significantly retarded because of more obvious selective adhesion of DEG, and acetic acid contributed to the higher density of 5-fold Ti on the (001) surfaces,22 leading to the formation of TiO2 nanosheets mainly dominated by {001} facets. As a result, fluorine ion is not only the mechanism of (001) surface stabilization; on the TiO2 surface, absorbed DEG and acetic acid can be very effective as well. As to the formation of hierarchically structured microspheres, a two-stage growth process may be proposed: at the initial reacting stage, DEG molecules can be used as a complexing agent to control the nucleation and growth rate of TiO2, at the same time, fluorine ions, DEG, and acetic acid retarded the growth of anatase TiO2 along the (001) direction, which is helpful in forming tiny nanoplates. The surfaces of tiny nanoplate building blocks stabilized by organic coating tended to oriented attachment into 2D nanosheets; at the following secondary stage, to further decrease the surface energy of the reacting system and under the directing action of DEG or acetic acid used as a directing agent for self-assembly of primary nanosheets, the primary 2D nanosheets aggregated into microspheres.39,40 3.5. Photovoltaic Performance. DSSCs consisting of 210nm-sized nanosheets-based TiO2 microspheres were characterized by measuring the current
voltage behavior under illumination with 100 mW/cm2 intensity and compared to P25 TiO2 nanoparticles. Figure 5 shows the solar cell response of ATNM film and P25 TiO2 film. The current
voltage curves reveal that the DSSC based on the ATNMs presents significantly superior short-circuit current (Jsc) and open-circuit voltage (Voc) over the P25 TiO2. The DSSC based on the ATNMs shows Jsc = 15.2 mA/cm2 and Voc = 0.65 V with the power conversion efficiency (PCE) of 6.64%, respectively, whereas the DSSC based on P25 TiO2 nanoparticles gives Jsc = 11.2 mA/cm2 and Voc = 0.63 V with the PCE = 4.71%, respectively, indicating 36 and 41% increase in Jsc and PCE compared to the P25 photoanodes of similar thickness, respectively. It is well-known that an increase in the light-scattering ability plays an important role in the light-harvesting efficiency. Lightscattering effect can be investigated by diffuse-reflection spectrum. 11985

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Industrial & Engineering Chemistry Research From Figure 6, the two films have high diffuse reflection in the visible range of 400
500 nm, but a distinctly rapid decline in light-scattering capability is observed for P25 TiO2 film in the wavelength range from 500 to 850 nm. The 210-nm-sized ATNMs film has a better lightscattering ability in the visible and near-infrared regions compared with P25 TiO2 film, which can be attributed to the higher Jsc of the ATNMs. The nanosheet-based hierarchically structured TiO2 has a higher surface area (74 m2/g, determined from N2 sorption isotherm at 77 K) with dominant exposure of high surface energy {001} facets than that of P25 (52 m2/g), which can favor electron injection from the excited state of the sensitizer into the TiO2 conduction band and increase dye loading, eventually leading to higher photocurrents.26 Besides, the unique hierarchical structure built from nanosheets with large particle sizes connected with each other favors electron transport because the architectures can effectively minimize the grain interface effect, causing the enhancement of Jsc.

4. CONCLUSIONS In summary, we have demonstrated a facile and nontoxic approach for one-pot synthesis of ATNMs with controlled sizes and dominantly exposed {001} facets through using DEG as a solvent as well as a protective capping agent together with acetic acid. The effect of heat treatment on F
adsorbed on the surface of ATNMs has been investigated. The roles of fluorine ions from TiF4, DEG, and excess acetic acid in forming such nanosheetsbased hierarchical structure have also been investigated in detail. Used as photoanode material of DSSC, the ATNMs show Jsc = 15.2 mA/cm2 and PCE = 6.64%, indicating 36 and 41% increase in Jsc and PCE compared to the P25 TiO2 photoanodes of similar thickness, respectively, which is due to their dominantly exposed {001} facets favoring electron injection, high surface area increasing dye loading, 210-nm-sized particle diameter enhancing light harvesting, and hierarchical structure favoring electron transport. Recently, through using a diblock copolymer assisted sol
gel process, Mueller-Buschbaum and co-workers fabricated hierarchically structured TiO2 films which have a low reflectivity of visible light.41,42 Introducing such hierarchically structured TiO2 films used as the underlayers of DSSCs derived from the ATNMs as light-scattering overlayers to reduce the light reflectivity may further improve the cell performance. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (W.Y.); [email protected] (W.S.).

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the project from the National Science Foundation of China (Grant Nos. 10775096 and 51072112), the China Postdoctoral Science Foundation (Grant No. 20100480579), the Key Subject of Shanghai Municipal Education Commission (Grant No. J50102), the Innovation Foundation of Shanghai University, Nature Science Foundation of Shanghai (Grant No. 06ZR14035), and the Shanghai Leading Academic Disciplines (Grant No. T0101). ’ REFERENCES (1) Jun, Y. W.; Lee, S. M.; Kang, N. J.; Cheon, J. Controlled synthesis of multi-armed CdS nanorod architectures using monosurfactant system. J. Am. Chem. Soc. 2001, 123, 5150.

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