Synthesis of Cuboid-Shaped Single-Crystalline TiO2 Nanocrystals

Mar 7, 2014 - Kyung-Hee Park , Sujan Mondal , Soumen Ghosh , Sibani Das , Asim Bhaumik. Microporous and Mesoporous Materials 2016 225, 255-260 ...
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Synthesis of Cuboid-Shaped Single-Crystalline TiO2 Nanocrystals with High-Energy Facets {001} and Its Dye-Sensitized Solar Cell Application Astam K. Patra, Arghya Dutta, and Asim Bhaumik* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India ABSTRACT: A new facile chemical approach has been developed to produce cuboidshaped TiO2 nanocrystals with high-energy facets {001} under stream-assisted ionothermal method. The structural features, crystallinity, purity, mesophase, and morphology of the nanostructured TiO2 were investigated by powder X−ray diffraction, high-resolution transmission electron microscopy, selected-area electron diffraction, Brunauer−Emmett− Teller N2 sorption measurement, ultraviolet−visible light diffuse reflectance spectroscopy studies, and photoluminescence spectroscopy. N2 adsorption−desorption studies showed high surface areas (118.5−124.3 m2 g−1) for these TiO2 samples. These mesoporous structure facilities the high dye-loading property and light harvesting in dye-sensitized solar cells. Short−circuit current density (Jsc) of 12.6 mA cm−2 and an overall power conversion efficiency of 4.21% have been achieved by utilizing these cuboid-shaped TiO2 photoelectrode in the DSSC with an open−circuit voltage (Voc) of 0.67 V.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have been recognized as promising devices for energy applications because of their advantages like low cost, ease to manufacture, potential for large-scale solar energy conversion, and involvement of lowtoxicity materials.1−6 A simple DSSC is assembled of photoanode and counter electrode with an electrolyte solution. The photoanode is constructed by casting a dye-sensitized semiconductor nanomaterial film on a transparent conducting glass and to absorb and convert photons to electrons, to transport, and then to collect the photoinduced electrons.7 Titanium dioxide (TiO2) is recognized as one of most promising wide band gap semiconducting materials for dyesensitized solar cells application since 1991.8 To date, the highest photoconversion efficiency in the DSSC with power conversion efficiency over 11% has been achieved with 20 nm titanium dioxide nanocrystalline film sensitized by rutheniumbased dyes.9 In particular, TiO2 nanostructured materials have attracted much attention because of their extensive uses in photocatalysis, hydrogen production, dye-sensitized solar cells, selfcleaning agents, Li ion battery, gas sensing, supercapacitors, catalysis, and also commercially used in a white pigment and in sunscreens, paints, ointments, toothpaste, and in many other applications.10−23 TiO2 nanocrystals with a particular shape and size are studied intensively.24 The properties of these nanocrystals strongly depend on their exposed surfaces, which are related to their surface atomic arrangement.25 It has been reported that anatase TiO2 with the {001} surface is much more reactive than the thermodynamically more stable {101} surface.26,27 So there has been recent increasing interest in the preparation of anatase TiO2 single crystals with exposed {001} facets. In 2008, Lu and coworkers successfully synthesized © 2014 American Chemical Society

anatase TiO2 microcrystals with a large percentage of reactive {001} facets using TiF4 as titanium source and hydrofluoric (HF) acid as a capping and shape-controlling agent.28 Since then, several research groups have prepared anatase TiO2 single crystals with exposed {001} facets from different titanium source and HF used as a shape-controlling agent,29−33 but HF is an environmentally toxic reagent (both liquid and vapor forms) and extremely corrosive chemical.34 Then, Liu et al. synthesized anatase TiO2 sheets with a high percentage of {001} facets from titanium nitride.35 Yu and coworkers synthesized single-crystalline anatase TiO2 microsheets with 80% exposed {001} facets by the hydrothermal method by using tetrafluoroborate-based ionic liquid.36 Ohtani and coworkers developed a more efficient route to prepare {001} facets-dominated anatase TiO2 via gas-phase hydrolysis of TiCl4 with rapid heating and quenching.37 It is still a challenge to synthesize nanometer-sized anatase TiO2 single crystals with environmentally friendly process and exposed {001} facets. Very recently Pradhan and coworkers synthesized cuboid TiO2 nanocrystals with high-energy {001} facets by varying the reaction duration using diethanolamine.38 In this context, we have synthesized few-nanometer-sized cuboid-shaped TiO2 nanocrystals with {001} facets by stream-assisted ionothermal method. Here ionic liquids control the shape of the materials, and they are environmental friendly solvent because of their unique properties such as extremely low volatility, good thermal stability, high dissolving ability, wide liquid temperature range, Special Issue: Michael Grätzel Festschrift Received: December 27, 2013 Revised: February 28, 2014 Published: March 7, 2014 16703

dx.doi.org/10.1021/jp412674g | J. Phys. Chem. C 2014, 118, 16703−16709

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the both electrode. The average active area of the electrode was ∼0.25 cm2. A Keithley source meter (model 2420) was used to examine the I−V curves. The DSSC was illuminated under 150 W xenon lamp source (Newport USA; model no. 69907), a simulated sunlight source. For all data, the corresponding illumination was 100 mW cm−2. The amount of dye loading was measured by the spectroscopic technique employing a UV−visible spectrophotometer (Shimadzu model UV 2401PC) in the quantitative mode. We measure the concentration of dye after and before being dipped in a 0.5 mM ethanolic solution for 6 h along with three standard solutions. Here we use the 530 nm wavelength as λmax. for N719 dye. All data are calibrated to produce the best linear fit. From this absorbance versus concentration data, the unknown concentration of the dye is measured. From this experiment, the dye adsorbed on the cuboid surface in the DSSC is 3.05 × 10−5 mmol/cm2. Characterization Techniques. The shape and crystal structures of the resulting ionic liquid template anatase TiO2 single crystals were investigated by different characterization techniques. Powder X-ray diffraction patterns of the samples were recorded on a Bruker D-8 Advance diffractometer operated at 40 kV voltage and 40 mA current using Cu Kα (λ = 0.15406 nm) radiation. TEM images were recorded in a JEOL 2010 TEM operated at 200 kV. A JEOL JEM 6700F field-emission scanning electron microscope (FE SEM) was used for the determination thickness of the layer of the materials. Nitrogen sorption isotherms were obtained using a Beckman Coulter SA 3100 surface area analyzer at 77 K. Prior to the measurement, the samples were degassed at 423 K for 3 h. UV−visible diffuse reflectance spectra were recorded on a Shimadzu UV 2401PC with an integrating sphere attachment. BaSO4 was used as background standard. PL studies were carried out in a Horiba Jobin Yvon, USA Fluoromax 3 instrument. The I−V between the two electrodes was measured using a Keithley source meter (model 2420). The DSSC efficiency was measured by illuminating with white light from a 150 W xenon lamp source (Newport USA; model no. 69907).

excellent microwave absorbing ability, designable structures, high ionic conductivity, and wide electrochemical window.39−41 Herein, we first report a novel synthetic strategy for one-pot synthesis of uniform size and shape anatase TiO2 single crystals with exposed {001} using titanium isopropoxide as Tiprecursor and ionic liquids (([bmim][Cl] and [omim][Cl]) as templating agent under stream-assisted ionothermal method. The product nanocrystals are stable upon heating at 673 K temperature. The materials are characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) studies. N2 adsorption−desorption studies showed high surface areas (118.5−124.3 m2g−1) for these TiO2 nanomaterials. The mesoporous TiO2 nanocrystals showed high efficiency to absorb and convert photons to electrons and to transport and collect the photoinduced electrons. The nanocrystals showed that the DSSC efficiency is 4.21% in the presence of the N719 dye.

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EXPERIMENTAL SECTION Synthesis. Self-assembled cuboid-shaped TiO2 nanomaterial was synthesized by employing the following procedure. In this typical synthesis, 3.5 g 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) was taken in a 50 mL round-bottomed flask and placed in an oil bath at 343 K under vigorous stirring for 30 min. Then, 2.84 g titanium isopropoxide (Ti(OiPr)4) was taken in 2 g isopropyl alcohol, and this solution was slowly added to the ionic liquid. The mixture was heated to 343 K with vigorous stirring again for 1 h. Then, the mixture was transferred to small glass vials and kept in room temperature for 12 h. Gel formation started during the aging process. Then, the glass bottles and water was kept inside the Teflon-lined stainless-steel autoclave very cautiously. Here TiO2 gel and water were separated by glass contact, and the Teflon-lined stainless-steel autoclave was sealed. Then Teflon-lined stainlesssteel autoclave was treated at 423 K for 24 h. The white fine solid was collected, washed with water, and dried at room temperature under vacuum. Furthermore, 1-octyl-3-methylimidazolium chloride ([omim][Cl]) was also used to synthesize cuboid-shaped TiO2 nanomaterial. We follow the same reaction procedure and kept the oil bath temperature at 373 K for this ionic liquid. All as-synthesized samples were calcined at 673 K for 6 h to remove the template ionic liquid molecules and thus to generate mesoporosity. The materials have been designated as TO-1 and TO-2, respectively. DSSC Fabrication. The fabrication of cuboid-shaped TiO2 nanostructure-based DSSCs was performed as follows: the ITO glass (Sigma Aldrich, surface resistivity 8−12 Ω/sq) was cleaned by standard procedure. The ITO glass was coated with the TiO2 materials by dropcast method and dried at 423 K in hot plate. The thickness of the TiO2 coating was ∼12 μm (measured from the FESEM image). The TiO2-coated ITO glass was immersed in a 0.5 mM methanolic solution of a ruthenium-based dye N719 for 6 h. This dye is chemically named (cis−bis(isothiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium (N719, Sigma Aldrich). Then, the electrode was washed with ethanol to remove excess dye adsorbed. The counter electrode was a graphite-coated ITO glass plate. The iodide−triiodide-based electrolyte solution, consisting of 0.5 M KI and 0.05 M I2 in γbutarolactone, was placed on the active electrode area in a dropwise fashion. The electrodes were sealed by paraffin hotmelt sealing foil and sandwiched together with clips. A small space of bare ITO glass was uncovered for wire connection on

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RESULTS AND DISCUSSION The synthesis process for the self-assembly cuboid-shaped TiO2 nanocrystal via stream-assisted ionothermal synthesis is shown in Scheme 1. At first, ionic liquid, 2-propanol, and titanium isopropoxide were mixed in oil bath, and sol formation takes place in the aging process. In general, the hydrolysis and condensation reactions of titanium alkoxide precursors are too fast, resulting in uncontrolled shape of TiO2, but here the presence of ionic liquids (([bmim][Cl]/[omim][Cl]), reduced hydrolysis rate of titanium isopropoxide, and slow hydrolysis rate make it possible to achieve a longer aging time for the formation of a stable sol−gel network, possibly with a particular arrangement.42 In the hydrothermal process, the presence of water vapor causes the titanium isopropoxide to be completely hydrolyzed. Here again ionic liquid controls direct hydrolysis of titanium isopropoxide and nucleation of TiO2 particle. Thus, TiO2 nanocrystals of uniform size and shape are dispersed in sol−gel network throughout the entire aging process and hydrothermal process. The calcination processes burn the ionic liquid molecule to generate the mesopore and further crystallize the TiO2 nanocuboids. The wide-angle XRD profiles of the samples showed sharp peaks (Figure 1), suggesting high crystallinity of the material. Crystalline planes corresponding to the peaks for single16704

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length 23 nm and width 17 nm, and FFT is shown in the inset of this Figure 2b. The selected-area electron diffraction (SAED) pattern is shown in Figure 2c, suggesting that the TiO2 nanocrystals have well-define diffraction spots for anatase TiO2. When 1-octyl-3-methylimidazolium chloride ([omim][Cl]) is used as ionic liquid, similarly self-assembled TiO2 nanocrystals are formed (Figure 2d). Here we have also seen that the well-faceted cuboid nanocrystals are self-assembled and generate the interparticle porosity. Furthermore, individual nanocrystals (Figure 2e) are seen clearly with {101} lattice fringes with 20 nm in length and 16 nm in width. Furthermore, a more close-up view of these images (Figure 2f) revealed the height (1.2 nm) of the nanocrystals. These results suggested the formation of self-assembled mesoporous cuboid-shaped TiO2 nanocrystals with well-defined lattice planes. The diffraction spots are indexed corresponding to an anatase TiO2 structure. Interestingly, the size of nanocrystals was limited within 30 nm after calcination at 673 K for 6 h. The high-resolution TEM image of TO-1 material (Figure 3) shows the presence of lattice fringes very clearly. A closer view of Figure 3a reveals the {101} and {011} atomic planes clearly. As shown in Figure 3b, the interfacial angle between {101} and {011} plane is 82.8°, and the corresponding fast-Fourier transform (FFT) pattern shows the {101} and {011} lattice planes. Here also the interfacial angle between {101} and {011} planes is 82.8°.44 On the basis of this HRTEM image analysis, the geometry of single-crystalline anatase TiO2 is predicted as cuboid with dominant {001} facets.44 Nitrogen adsorption−desorption studies are carried out on the self-assembled mesoporous TO-1 and -2 samples at 77 K, and the results are shown in Figure 4. These isotherms could be classified as typical type-IV isotherm with H1-type hysteresis loop in the 0.78 to 0.96 P/P0 range, characteristic of the mesoporous materials with large pore.43,45 In these isotherms between P/P0 of 0.01 and 0.78, the adsorption amount slowly increases for both samples, but after P/P0 > 0.78, the N2 uptake sharply increased due to the large interparticle mesopores. The Brunauer−Emmett−Teller (BET) surface areas for samples TO-1 and TO-2 were 118.5 and 124.3 m2 g−1, respectively. Their respective pore volumes were 0.621 and 0.695 ccg−1. Pore size distributions of these samples employing NLDFT model (N2 adsorption on silica as reference)43,45 suggested that TO-1 has an average pore width of ca. 9.6 nm and that for sample TO-2 is of ca. 10.4 nm. UV−visible spectroscopy is one of the most important analytical tools for characterizing the optical properties of the TiO2 nanocrystals. UV−visible diffuse reflectance spectra of mesoporousTiO2 samples are shown in Figure 5. Both samples show an absorption band at 340 nm that corresponds to bandgap energy of 3.15 eV. The peak observed corresponds to O→ Ti charge transfer in titania nanocrystal.11,43 Here the absorption maximum of this material is considerably blueshifted, and this could be related to nanoscale porosity.46 The photoluminescence spectra of both the samples are shown in Figure 6. The room-temperature photoluminescence emission spectra of these TiO2 nanomaterials show broad emissions between 400 and 600 nm. Both materials show emissions maxima in the range of 437−480 nm. These peaks correspond well to the band-edge transitions of the anatase TiO2 nanocrystals,47−49 and the broadening of these peaks could be attributed to the surface or bulk defect sites.50,51 Long emission tail with further emissions at 510−560 nm is present

Scheme 1. Synthetic Route and Formation Mechanism of Self-Assembled Cuboid-Shaped TiO2 Nanocrystal via Stream-Assisted Ionothermal Synthesisa

a

(i) The heated mixture is transferred into glass sample vials and cooled for gel formation. (ii) Stream-assisted ionothermal treatment of the gel inside the Teflon−lined stainless-steel autoclave. (iii) The material has been calcined at 673 K to remove the ionic liquid template, and this resulted in cuboid-shaped particles with interparticle mesopores.

Figure 1. Wide-angle XRD pattern of the highly crystalline calcined (a) TO-1 and (b) TO-2 materials and all of the peaks are indexed to anatase TiO2.

crystalline anatase TiO2 have been indexed. The samples show major peaks at 2θ values of 25.3, 36.9, 37.8, 38.5, 48.0, 53.9, 55.0, 62.7, 68.7, 70.3, and 75.0°, which correspond to anatase (101), (103), (004), (112), (200), (105), (211), (204), (116), (220), and (215) crystal planes, respectively (JCPDS PDF number 01-084-1285).43 All of the peaks are well-indexed to a pure body-centered tetragonal structure of TiO2 with an I41/ amd space group and lattice parameters a = 3.78 and c = 9.51. Thus these powder XRD results revealed that we have synthesized highly stable and crystalline TiO2 nanocrystals through this new synthesis method employing ionic liquid as a templating agent. A TEM image of representative cuboid-shaped TiO 2 materials is shown in Figure 2. As shown in Figure 2a, selfassembled TiO2 nanocrystals are formed in this stream-assisted ionothermal synthesis method using titanium tetraisopropoxide as a precursor and 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) as ionic liquid. Well-faceted cuboid nanocrystals with uniform particle size around 15−25 nm are seen throughout the specimen. Furthermore, individual nanocrystals (Figure 2b) are seen clearly with with{101} lattice fringes with 16705

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Figure 2. TEM images of (a) self-assembly structure of TO-1 nanocrystal and (b) single cuboid-shaped particle with {101} lattice fringes. (FFT is shown in the inset of this Figure.) (c) SEAD pattern of the TO-1 nanocrystal showing different lattice plane of anatase TiO2 and (d) self-assembly structure of TO-2 nanocrystals. (e) Single cuboid-shaped particle with {101} crystal lattice fringes and (f) more close-up part of panel e, showing the thickness of the particle.

calculated the short−circuit current density (Jsc), the open− circuit voltage (Voc), the fill factor (FF), and the overall power conversion efficiency (η). All data from the J−V curves are summarized in Table 1. In this DSSC, the photoanode is fabricated by TiO2 nanomaterials layer (12 μm) deposited on the ITO glass; then, the dye molecule (N719) is attached on the surface of the TiO2 materials. This photoanode is coupled to a graphite-coated ITO glass in the presence of iodide− triiodide-based electrolyte solution. This electrolyte is needed because any electric field usually needed for charge transport should be compensated by the ions, and electrons have to travel across many grain boundaries before they reach ITO collector. Hence, nanoscale porosity in these TiO2 nanomaterials is favorable for their application in electrodes. The DSSC efficiency was achieved as the dye molecules adsorbed at the particle surface were excited in the presence of white light. After absorbing the photon energy from the illuminated white light, the dye molecules become excited and injected electrons to the TiO2 nanomaterials. The electron transfer from dye to TiO2 nanomaterials occurs due to favorable energy difference between the lowest unoccupied molecular orbital (LUMO) of the dye and the conduction band of TiO2. Here the forward reactions are very fast; namely, electron injection from photoexcited dyes to the surface of TiO2 materials and dye regeneration by receiving the electron from I− ions occur very fast, whereas the backward transfer of electron from semi-

Figure 3. (a) High-resolution TEM image of a single cuboid particle along the {001} axis and inset is {001}-projected geometrical model of the cuboid-shaped anatase TiO2 single crystal. (b) Close-up view of panle a showing the direction of {101} and {011} lattice fringes (interfacial angle between them is 82.4°); corresponding FFT pattern is shown in the inset with indexing lattice planes.

simultaneously, and this could be strongly affected by surface chemistry of the TiO2 nanocrystals. The photovoltaic properties of the DSSCs fabricated with cuboid-shaped TiO2 nanocrystals are presented in Figure 7 and Table 1. The resulting current density versus applied voltage (J−V) curves are measured for these two DSSCs under simulated solar light illumination at 100 mW cm−2 from a xenon lamp. The performances of the TiO2 nanocrystals as a photoanode material are investigated by measuring the current density versus applied voltage (J−V) curves, and we have 16706

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Figure 6. Photoluminescence spectra of (a) TO-1 and (b) TO-2. All materials were excited at 330 nm.

Figure 4. (A) N2 adsorption (●)−desorption (○) isotherms of (a) TO-1 and (b) TO-2 at 77 K. Y axis of plot b has been enhanced by 50 for clarity. (B) Their pore-size distribution using the nonlocal density functional theory (NLDFT) model is shown, respectively.

Figure 7. Photocurrent voltage curves of DSSCs based on (a) TO-1 and (b) TO-2.

Table 1. Photovoltaic Parameters of the TiO2 DSSCs under Simulated White Light (100 mW cm−2) sample

Jsc/mA cm−2

Voc/mV

FF

efficiency (%)

TO-1 TO-2

12.62 12.16

672 671

0.497 0.486

4.21 3.96

4.21% in the presence of the N719 dye. The ionothermal strategy for the synthesis of TiO2 nanomaterials with high energy facets and its use in light-harvesting may motivate the researchers for the synthesis of high-surface-area semiconductor nanomaterials and their use in DSSCs.

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Figure 5. UV−visible diffuse reflectance spectra of (a) TO-1 and (b) TO-2 and the corresponding band gap of TO-1 sample is shown in inset.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 91-33-2473-4971 (207). Fax: 91-33-2473-2805.

conductor surface to I3− ions is very slow, which makes overall DSSC efficiency reasonably good.

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Notes

The authors declare no competing financial interest.

CONCLUSIONS From the previously described experimental results, we can conclude that cuboid-shaped TiO2 nanocrystals can be synthesized by a new stream-assisted ionothermal method. Ionic liquid molecules controlled the morphology of the cuboid-shaped nanoparticles with exposed {001} surface. The high-surface-area mesoporous TiO2 cuboid nanocrystals synthesized herein showed excellent photoluminescence property. The mesoporous TiO2 nannocrystals help to absorb and convert photons to electrons and to transport and collect the photoinduced electrons, resulting in DSSC efficiency of

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ACKNOWLEDGMENTS A.K.P. and A.D. thank CSIR, New Delhi for their respective senior research fellowships. A.B. wishes to thank DST New Delhi for providing instrumental facility through DST Unit on Nanoscience and DST-SERB projects.

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