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Tailored Synthesis of Porous TiO2 Nanocubes and Nanoparallelepipeds with Exposed {111} Facets and Mesoscopic Void Space: A Superior Candidate for Efficient Dye-Sensitized Solar Cells Vipin Amoli, Shekha Bhat, Abhayankar Maurya, Biplab Banerjee, Asim Bhaumik, and Anil Kumar Sinha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07954 • Publication Date (Web): 10 Nov 2015 Downloaded from http://pubs.acs.org on November 16, 2015

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ACS Applied Materials & Interfaces

Tailored Synthesis of Porous TiO2 Nanocubes and Nanoparallelepipeds with Exposed {111} Facets and Mesoscopic Void Space: A Superior Candidate for Efficient Dye-Sensitized Solar Cells Vipin Amoli† , Shekha Bhat†, Abhayankar Maurya†, Biplab Banerjee‡, Asim Bhaumik‡, Anil Kumar Sinha*,† †

CSIR- Indian Institute of Petroleum, Dehradun – 248005 & Network Institute of Solar

Energy-CSIR-NISE, New Delhi, India. ‡

Department of Material Science, Indian Association for the Cultivation of Science,

Jadavpur, Kolkata, 700032, India. *Corresponding Author. Tel.:+91-1352525842; fax:+91-135-266-203. E-mail: [email protected]

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ABSTRACT Anatase TiO2 nanocubes and nano-parallelepipeds, with highly reactive {111} facets exposed, were developed for the first time through a modified one pot hydrothermal method, through the hydrolysis of tetrabutyltitanate in the presence of oleylamine as the morphologycontrolling capping-agent and using ammonia/hydrofluoric acid for stabilizing the {111} faceted surfaces. These nanocubes/nano-parallelepipeds were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADFSTEM). Accordingly, a possible growth mechanism for the nanostructures is elucidated. The morphology, surface area and the pore size distribution of the TiO2 nanostructures can be tuned simply by altering the HF and ammonia dosage in the precursor solution. More importantly, optimization of the reaction system leads to the assembly of highly crystalline, high surface area, {111} faceted anatase TiO2 nanocubes/nano-parallelepipeds to form uniform mesoscopic void space. We report the development of a novel double layered photoanode for dye sensitized solar cells (DSSCs) made of highly crystalline, self assembled faceted TiO2 nanocrystals as upper layer and commercial titania nanoparticles paste as under layer. The bilayered DSSC made from TiO2 nanostructures with exposed {111} facets as upper layer shows a much higher power conversion efficiency (9.60%), than DSSCs fabricated with commercial (P25) titania powder (4.67%) or with

anatase TiO2

nanostructures having exposed {101} facets (7.59%) as upper layer. The improved performance in bilayered DSSC made from TiO2 nanostructures with exposed {111} facets as upper layer is attributed to high dye adsorption and fast electron transport dynamics owing to the unique structural features of the {111} facets in TiO2. Electrochemical Impedance Spectroscopy (EIS) measurements conducted on the cells supported these conclusions, which showed that the bilayered DSSC made from TiO2 nanostructures with exposed {111} facets 2 ACS Paragon Plus Environment

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as upper layer possessed lower charge transfer resistance, higher electron recombination resistance, longer electron lifetime and higher collector efficiency characteristics, compared to DSSCs fabricated with commercial (P25) titania powder or with anatase TiO2 nanostructures having exposed {101} facets as upper layer. KEYWORDS: Faceted Titania Nanocrystals, High surface area, Mesoscopic void space, Dye-sensitized solar cell, Retardation of charge recombination.



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INTRODUCTION Surface-structural engineering of semiconductor nanostructures has grown as an important area of research and development. This is because various applications including heterogeneous catalysis, gas sensing, energy conservation and storage are surface driven processes and these are very sensitive to surface atomic structures.1-3 TiO2, as one of the most promising semiconductor materials, has become a topic of intense research due to its applications in a wide range of fields, such as photocatalysis, dye-sensitized solar cells, and gas sensing.4-6 Since the pioneering work of Grätzel,6 dye-sensitized solar cells (DSSCs) based on mesoporous nanocrystalline TiO2 have emerged as a viable alternative to conventional silicon solar cells.7 Since then several strategies such as use of a thin blocking layer on the photoanode,8 development of various metal-oxide nanostructures including 1-D,9 2-D10 and 3-D structures,11 introduction of scattering layer,11 TiCl4 post-treatment on mesoporous TiO2 layer12 and use of photonic crystals13 have been developed to increase the efficiency of DSSCs. Recently, special attention has been paid on the crystal facets engineering of TiO2 to develop efficient solar driven devices and significant advancements in this area have been achieved.14 Among various anatase TiO2 nanostructures, {101} and {001} exposed facets nanostructures have been explored mostly till date and TiO2 nanostructures with higher percentages of exposed {001} facets are considered as the most promising candidate for energy related applications.14,15 Various types of TiO2 nanostructures with exposed {001} facets such as hierarchical fastener-like spheres, self assembled hierarchical nanosheets and yolk@shell hierarchical spheres have been proven to be very effective for light harvesting, dye adsorption and retardation of charge recombination leading to better performance of the devices.16-18 The predicted equilibrium shape for anatase TiO2 is a slightly truncated tetragonal bipyramidal, exposing eight isosceles trapezoidal {101} facets and two top squared {001} facets based on the Wulff construction from surface energy 4 ACS Paragon Plus Environment

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considerations.14 Despite nonexistence of certain facets like {010} and {111} in anatase TiO2 predicted on the basis of Wulff construction model, such faceted TiO2 nanostructures have also been obtained, even though only very few examples have been reported up to now.19-21 This is because the predicted equilibrium shape (truncated tetragonal bipyramid) for anatase TiO2 does not include surface tension effects and usually refers to the calculations obtained under relatively extreme conditions, such as in vacuum at absolute zero temperature, which are clearly different from practical experimental conditions.22 Recently Xu et al. reported the superior performance of anatase TiO2 single crystals with {111} exposed facets for photocatalytic hydrogen evolution, which is due to the superior atomic and electronic band structures of this material.21 However, very low surface area (11.6 m2/g) of these TiO2 single crystals limits severely the diversity of this method and the material in various applications like dye-sensitized solar cell and catalysis, where high surface area of the material is highly desirable for developing more efficient systems. In another report, Zhang et al.23 developed the high surface area TiO2 nanostructures (98m2/g) with exposed {111} faceted, but the obtained structures in their work are aggregated nanoparticles without any well defined shape and morphology. Moreover, nothing has been stated related to porous nature and pore size distribution of the nanostructures. The aggregated nanostructures obtained according to Zhang et al. report, resulted in non-uniform pore size distribution lying mostly in the microporous range; If such type of material is applied in DSSCs, dye molecules cannot diffuse in micropores resulting in lower dye loading of the photoanode.24 Moreover, photoanode films made up of such nanoparticles aggregates would exhibit a larger number of grain boundaries to be overcome by the electrons injected by excited dye molecules, which may result in increased probability of electron recombination losses at the semiconductor/dye interfaces.25 It is well known that surface area is by no means the only physical property which is important for various surface driven applications like catalysis and energy storage,



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equally important property is the porous nature of the material and more importantly is the pore size distribution of the material.26─28 Several reports have claimed that high surface area materials with non-uniform pore size distribution do not give activity proportional to the surface area in many catalytic reactions.29 Moreover shape selective performance of the nanomaterials in various applications is also well known phenomena as claimed in many reports.30─31 The order of average surface energy for different TiO2 facets is γ {111} (1.61 Jm-2) > γ {110} (1.09 Jm-2) > γ {001} (0.90 Jm-2) > γ {010} (0.57 Jm-2) > γ {101} (0.44 Jm2 21

).

Literature predicts that TiO2 nanostructures with high energy facets exposed are the

dominant sources of active sites resulting in favourable dye adsorption and also act as a superior candidate for effective retardation of charge recombination in DSSCs.16-20 Till date mesoporous TiO2 structures with tailored pore sizes and high specific surface area have proven the most efficient candidate for DSSCs.32,6 Therefore it is quite reasonable to believe that development of TiO2 nanocrystals with exposed {111} facets having high crystallinity, high surface area and uniform mesoscopic pore size distribution can be a key in order to improve the performance of the energy storage devices driven by solar energy. Here in we report a novel one pot method for the synthesis of porous anatase TiO2 nanocubes and nano-parallelepipeds structures with exposed {111} facets and explore the possible applications of these nanostructures as a photoanode material in DSSCs. The novelty of our work lies in the formation of well defined, highly crystalline nanostructures having high surface area (50─99 m2/g) and with mesoscale pore size distribution, making them potential candidate for application to surface driven processes. Optimization of synthesis conditions leads to the self assembly of well faceted anatase TiO2 nanocrystals with uniform mesoscopic void space, for which morphology, surface area and porosity integrity of TiO2 nanocrystals can be tuned simply by altering the HF and ammonia dosage of the precursor solution. Development of a novel double layered dye-sensitized solar cell (DSSC) made of highly 6 ACS Paragon Plus Environment

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crystalline, self assembled faceted TiO2 nanocrystals as upper layer and commercial material [Ti-Nanoxide, Solaronix (SLP)] as under layer (SLP/TiO2{111}), resulted in much higher power conversion efficiency (nearly 1.25 times higher) as compared to double layered DSSC made of anatase TiO2 nanostructures with exposed {101} facets as upper layer (SLP/TiO2{101}) and commercially available TiO2 powder. The great improvements of photovoltaic parameters in SLP/TiO2{111} bilayered DSSCs can be attributed to unique surface atomic structure of {111} facets TiO2 nanostructures (TNF{111}), leading to high dye loading, fast electron transport and effective retardation of charge recombination compared to SLP/TiO2{101} and P25 DSSCs.

EXPERIMENTAL SECTION Materials and Reagents. Tetrabutyl titanate, (TBT, Merck), oleylamine (OA, Sigma), NH3 solution (25%, Merck), absolute ethanol (Merck), deionized (DI) water, hydrofluoric acid (48%, Merck), TiO2 nanoparticles (P25, Degussa), flourine-doped SnO2 (FTO) coated transparent (Solaronix) glass, ruthenium535 dye (Ru(bpy)2(NCS)2H4, (Solaronix), Iodolyte TG-50 (Solaronix), Platisol (Solaronix) and Ti-Nanoxide T/SC (Solaronix) were used. All chemicals were used as received without further purification. Synthesis of {111} faceted TiO2 nanocrystals (TNF{111}). In a typical experiment, 2 mL tetrabutyl titanate was added to 90 ml ethanol solution of 2 ml oleylamine. Then 60 ml mixture of H2O and ethanol (1:1) was added dropwise to the above mixture under constant stirring and allowed to stir for 2 hours. The resulting white suspension was centrifuged and white precipitates were collected and washed with excess ethanol. For hydrothermal treatment, white precipitates (obtained in a single run) were dispersed in a mixture of 160 mL ethanol and 80 mL DI water. To this suspension, 2 mL ammonia solution (25%) and 1mL of 1M HF were added dropwise under stirring. Then the mixture was transferred to Teflon-line 7 ACS Paragon Plus Environment


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stainless steel autoclave of 500 ml which was loaded in an electric oven, and the treatment was carried at 160 ºC for 20 hours. After that the autoclave was cooled to room temperature and the mixture was filtered, washed with ethanol, and dried at 80 oC. The dried power was calcined at 500 oC (10 oC/min) for 2.5 hrs to obtain final product. A series of samples were prepared using HF solution with concentrations of 1, 3, and 5M, and denoted as TNF{111}-1, TNF{111}-3, and TNF{111}-5, respectively in the last step of the synthesis procedure, while leaving the molar ratio of tetrabutyl titanate (TBT), oleylamine (OA), solvent (H2O/C2H5OH) and NH3 unchanged. Synthesis of {101} faceted TiO2 nanocrystals (TN{101}). The TiO2 nanostructures with dominant {101} facets (TN{101}) were synthesized in the similar way to that for preparing TNF{111} nanostructures (TNF{111}-1, TNF{111}-3, and TNF{111}-5) without using HF in the final step of the synthesis procedure. Paste Preparation. TiO2 paste was prepared according to our previous work,33 by mixing the synthesized TiO2 powder (0.25 g) with acetic acid (0.5 mL) and grinding mechanically for 15 mins. After that, a mixture of water and ethanol (1:3) (15 mL) was added drop-wise, and ground mechanically. Finally, Triton X-100 (0.5 mL) was added to facilitate the spreading of the paste on the substrate. The resultant slurry was ground for another 15 mins to obtain a homogeneous paste. A similar method was adopted for the preparation of a paste with P25 nanoparticles. Device Fabrication. To prepare the DSSC working electrode, a fluorine-doped SnO2 (FTO) conducting glass substrate was first washed in a detergent solution using a sonic bath for 10 mins and then rinsed with water and ethanol. The resulting washed FTO glass substrates were then coated with TiO2 paste using doctor-blade technique and sintered at 450 0

C for 30 mins. For sensitization, the films were dipped in 0.3 mM N719 dye in ethanol for 8 ACS Paragon Plus Environment

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12 hrs at room temperature. The counter electrode was obtained by depositing a thin layer of platisol T/SP, and heat-treated for 30 mins at 450 oC. One drop of iodolyte TG-50 electrolyte was put on the surface of the dye-loaded TiO2 photo anode, which penetrated inside the porous TiO2 via capillary action. A Pt-coated fluorine–tin oxide electrode was then clipped onto the top of the TiO2 working electrode to form the complete DSSC. For comparison, DSSCs assembled with photoanodes made with commercially available TiO2 nanoparticles (P25) and commercially available paste (Ti-Nanoxide T/SC (SLP), Solaronix) were also fabricated adopting similar procedure as described above. The active area of each device was about 0.16 cm2. Dye loading Calculations. For dye-loading measurements,33 photoanodes with a dyeloaded area of 4 cm2 were dipped into 10 mL of a 0.1M solution of NaOH in ethanol/H2O (1:1 v/v), until complete desorption of the N719 dye occurred. The alkaline solution containing the fully desorbed dye was carefully measured by using UV/Vis spectra (Hitachi U-2900, UV/Vis Double Beam spectrophotometer); the Beer–Lambert law (A = εcl) was used to calculate the number of adsorbed N719 dye molecules by using the absorption value at 515 nm. Where A is the absorption of the UV/Vis spectra at 515 nm, ε is the molar absorptivity (14100M-1cm-1) of N719, l is the path length of light beam, and c is the concentration of the dye. Characterization. X-ray diffraction (XRD) patterns of the samples were recorded on Bruker D8 Advance Diffractometer operating in the reflection mode with Cu-Kα radiation (40 kV, 40 mA). Surface morphology of the samples was obtained using field emission scanning electron microscopy (FESEM, Quanta 200F, Netherlands).Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed on a Field Emission Gun Transmission Electron Microscope JEOL-TEM-2010, operating at 200 kV. The samples for the TEM measurements were prepared by depositing the ethanol 9 ACS Paragon Plus Environment


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dispersed TiO2 particles on the amorphous carbon-coated grids. High angle annular dark field-scanning transmission electron microscopic (HAADF-STEM) images were obtained by an ultrahigh resolution field emission gun coupled with a HAADF detector. The Brunauer−Emmett−Teller (BET) specific surface area, Barrett−Joyner−Halenda (BJH) calculated average pore size, and pore volume of the samples were obtained from the N2 adsorption/desorption isotherms recorded on BELSORP max (Japan) at 77 K. The samples were degassed and dried under a vacuum system at 150 °C for 2−3 h prior to the measurement. Photovoltaic performance of assembled DSSC with an active area 0.16 cm2 was measured by completely integrated I–V Test station (PVIV-211V) with Kiethley 2420 Source meter and 94023A Oriel Sol3A, class AAA Solar Simulator (power output: 100 mW/cm2, lamp power: 450 W) equipped with an AM 1.5 filter. J–V curves were used to calculate short circuit current (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency (ɳ) of the DSSC. Electrochemical Impedance Spectroscopy (EIS) measurement were performed using a cyclic voltammeter (Princeton Applied Research) with Versa Stat software with a frequency range of 0.01─100 kHz in the dark using 10mV ac signal superimposed on 0.7V applied potential. The impedance measurements were made at a room temperature. All of the impedance spectra were fitted using appropriate equivalent circuit models built in the Z-View software (Scribner Associates Inc.).

RESULTS AND DISCUSSION Figure 1 shows TEM and HRTEM images of the TiO2 nanostructures (TNF{111}-1) synthesised for TBT/OA/Solvent/NH3/HF system with NH3:HF molar ratio 1:1. Low magnification TEM image (Figure 1a) shows the formation of discrete nanoparticles. High magnification TEM image (Figure 1b) shows that the nanoparticles are of two shapes, nanocube and nano-parallelepiped.


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Figure 1. TEM and HRTEM images of the TiO2 nanostructures synthesised for TBT/OA/Solvent/NH3/HF system with NH3: HF molar ratio 1:1 (TNF{111}-1); (a) Low resolution TEM image, (b) High magnification TEM image of the sample, (c) High magnification TEM image of an individual TiO2 nanocube (Inset shows a model of cube shape as exhibited by the sample), (d) Lattice resolved HRTEM image and (e) The corresponding square-symmetric SAED pattern showing single crystalline nature of the nanostructure, (f) HAADF-STEM image showing porous nanocube/nano-parallelepiped shaped structure, (Inset is the magnified image). Parallelepipeds represent a family of shapes to which cube, cuboids belong, as represented in supporting information (SI, Figure S1). In our case nanostructures having an angle of 90º between two adjacent edges of equal lengths are referred as nanocubes. Nanostructures having an angle different from 90º between two adjacent edges are termed as nanoparallelepipeds (SI, Figure S1). High magnification TEM image (Figure 1c) of single nanoparticle confirms the cube shape of nanoparticles. The line drawing shows a model of cube shape exhibited by the nanoparticle. Lattice resolved HRTEM image (Figure 1d) directly presents the (101) and (011) crystal planes with 0.35 nm inter-planar spacing each, 11 ACS Paragon Plus Environment


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and 82° interfacial angle between them. The corresponding square-symmetric selected-area electron diffraction (SAED) pattern (Figure 1e) indicates single crystalline nature of the nanoparticle in which diffraction spots could be assigned to (101) and (011) faces.21 Zone axis is indexed to be [111]. These observations confirm that the exposed crystal plane is {111} facet. Aberration-corrected high-angle annular dark-field scanning TEM (HAADFSTEM) image presented in Figure 1f clearly shows that these structures are porous nanocube/nano-parallelepiped shaped structures, with 3-5 nm size intracrystalline mesopores. A magnified image is depicted in the inset, where the nanocube/nano-parallelepiped shape is prominently visible. Figure 2 represents the X-ray diffraction pattern of the sample (TNF{111}-1). A series of diffraction peaks at 25.32, 37.93, 48.02, 53.98, 55.04, 62.74, 68.91, 70.36, 75.42o correspond to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes of anatase TiO2 respectively, according to the standardized JCPDS (84−1286) card.

Figure 2. XRD of the nanostructures for TBT/OM/Solvent/NH3/HF system for NH3: HF molar ratio 1:1 (TNF{111}-1).


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No characteristic peak for {111} TiO2 facet can be observed in the XRD pattern. Structurally, anatase TiO2 crystallizes in a tetragonal lattice, belonging to the space group of I41/amd, hence only those diffraction peaks for which h + k + l = even number, will occur in the XRD pattern. Hence absence of (111) crystal plane is according to the extinction rules in the X-ray diffraction measurements.34 Figure 3 shows TEM and HRTEM images of the TiO2 nanostructures synthesised for TBT/OA/Solvent/NH3/HF system for NH3: HF molar ratio 1:3 (TNF{111}-3), while leaving the molar ratio of tetrabutyl titanate (TBT) and Oleylamine (OA) unchanged. Low resolution TEM image (Figure 3a) shows a dense network of ultra fine nanoparticles without noticeable agglomeration. High magnification TEM image (Figure 3b) reveals the nanocube/nanoparallelepiped shaped structure of the nanoparticles. High magnification TEM image (Figure 3c) of single nanoparticle confirms that obtained product contains cuboidal (parallelepiped) shaped nanoparticles. Lattice resolved HRTEM image of the nanostructure (Figure 3d) shows two set of lattice fringes with 0.35 nm inter-planar spacing each, and 82° interfacial angle between them which correspond to the (101) and (011) crystal planes of anatase TiO2. The spot type selected-area electron diffraction (SAED) pattern signifies single crystalline nature of the nanostructures (Figure 3e). The diffraction spots could be assigned to (101) and (011) faces with [111] zone axis, which confirm that the exposed crystal plane in the nanostructures is {111} facet. Aberration-corrected high-angle annular dark-field scanning TEM (HAADFSTEM) image presented in Figure 3f clearly shows that these structures are nanocuboidal (nano-parallelepiped) shaped structures. A magnified image is depicted in the inset, where the nanocuboidal shape is prominently visible.



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Figure 3. TEM and HRTEM images of the TiO2 nanostructures synthesised for TBT/OA/Solvent/NH3/HF system with NH3:HF molar ratio 1:3 (TNF{111}-3); (a) Low resolution TEM image, (b) High magnification TEM image, (c) High magnification TEM image of an individual TiO2 nanocube (Inset shows a model of cube shape as exhibited by the sample), (d) Lattice resolved HRTEM image and (e) The corresponding spot type SAED pattern showing single crystalline nature of the nanostructure, (f) HAADF-STEM image showing nanocube/nano-parallelepiped shaped structure, (Inset is the magnified image). Figure 4 shows TEM and HRTEM images of the TiO2 nanostructures (TNF{111}-5) synthesised for TBT/OA/Solvent/NH3/HF system for NH3: HF molar ratio 1:5, while leaving the molar ratio of tetrabutyl titanate (TBT) and Oleylamine (OA) unchanged. In low resolution TEM image (Figure 4a), spherical structures of various sizes and TiO2 nanoparticles (marked with white square) can be observed. The electron-density difference between the dark edge and pale centre confirms the hollow interior of the spherical structures (Figure 4b). The formation of hollow spheres in the presence of high HF concentration (used for the synthesis of TNF-5 in our work) is a common process reported frequently in many


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reports.35-37 These spheres have a less compact surface where the extruding crystallites can be observed as also reported by Yang et al.38 on the basis of Ostwald ripening under hydrothermal conditions. High magnification TEM image (Figure 4c) of the region marked with white square in Figure 4a reveals that most of the particles are nanocube/nanoparallelepiped shaped structures similar to TNF{111}-1 and TNF{111}-3 samples.

Figure 4. TEM and HRTEM images of the TiO2 nanostructures synthesised for TBT/OA/Solvent/NH3/HF system with NH3: HF molar ratio 1:5 (TNF{111}-5); (a) Low resolution TEM image, (b) High magnification TEM image, (c) High magnification TEM image of the region marked with white square in Figure 4a, (d) HR-TEM image showing individual nanocrystallite, Inset is corresponding SAED pattern. HRTEM image (Figure 4d) represents highly crystalline nano-parallelepiped shaped nanoparticle with two set of lattice fringes with 0.35 nm inter-planar spacing each, and 82°



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interfacial angle between them which correspond to the (101) and (011) crystal planes of anatase TiO2. The selected-area electron diffraction (SAED) pattern (inset) implies single crystalline nature of the nanostructures in which diffraction spots could be assigned to (101) and (011) faces with [111] zone axis. These observations confirm that exposed crystal plane in the nanostructures is {111} facet, similar to TNF{111}-1 and TNF{111}-3. Figure 5 shows TEM images of the sample synthesised using TBT/OA/Solvent/NH3 system (TN{101}). These samples were synthesised using same procedure as that used for the synthesis of TiO2 nanostructures with {111} facets but without using HF in the final step of the synthesis procedure.

Figure 5. (a) Low resolution TEM image and (b) HR-TEM image of the sample synthesised for TBT/OA/Solvent/NH3 system with NH3: HF molar ratio 1:0 (TN{101}), showing the presence of agglutinated structures that are made of interpenetrating nanoparticles. (c) HRTEM image showing individual nanocrystallites elongated in shapes, (d) magnified images of the circled portions of the Figure 5c, Inset shows corresponding SAED pattern. 16 ACS Paragon Plus Environment

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From Low magnification TEM image (Figure 5a,b) one can observe the formation of agglutinated structures that are made of interpenetrating nanoparticles (SI, Figure S2) of irregular shapes similar to Yan et al.39 High resolution TEM image (Figure 5c) shows the presence of randomly oriented nanocrystallites having elongated shape with clear lattice fringes. Crystallinity of the material is not so good throughout the product. The magnified images of the circled portions of the Figure 5c are shown in Figure 5d. The observed interplanar spacing of 0.35 nm in individual nanocrystallite corresponds to the (101) plane of anatase TiO2. Inset shows selected-area electron diffraction pattern (SAED) in which diffraction pattern can be indexed to randomly oriented {101} crystallites. These observations indicate that obtained product under these conditions (for TBT/OA/Solvent/NH3 system) is made up of TiO2 nanoparticles with {101} exposed facets. Figure 6 shows TEM images of the sample synthesised using TBT/Solvent/NH3/HF system. These samples were synthesised using same procedure as that used for the synthesis of TiO2 nanostructures with {111} facets but without using oleylamine. Low resolution TEM images (Figure 6a, b) indicate that the shape and size of nanoparticles are not well defined. In addition they appear to be aggregate and give rise to form large undefined and uncontrolled structures. High magnification TEM reveals that nanoparticle shape is not uniform (Figure 6c). In the high-resolution image (Figure 6d), two set of lattice fringes with 0.35 nm interplanar spacing each, correspond to the (101) and (011) crystal planes of anatase TiO2 with an interfacial angle 82o which confirm that the exposed crystal plane for the nanostructures is {111} facet. These observations confirm that exposed crystal plane in the nanostructures is {111} facet despite of shape selective development of TiO2 nanostructures. These results are consistent with the findings reported by Zhang et al.23 In their work they used tetrabutyl titanate (Ti(OBu)4), NH4F, HF and deionized water as starting materials and the obtained



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TiO2 nanostructures obtained in their work were exposed {111} facets without having any well defined shape as also detected in our case.

Figure 6. (a, b) Low resolution TEM images, (c) HRTEM image of the sample synthesised for TBT/H2O:C2H5OH/NH3/HF system (without oleylamine), (d) Lattice resolved HR-TEM image shows two set of lattice fringes with 0.35 nm inter-planar spacing each correspond to the (101) and (011) crystal planes of anatase TiO2. These above experimental results suggest that nanosized anatase TiO2 crystals with exposed {111} facets are successfully synthesised in the TBT/OA/H2O:C2H5OH/NH3/HF system. The central features of our approach are: (1) use of tetrabutyl titanate (TBT) as titanium source owing to its slow hydrolysis rate, which is favourable in forming the TiO2 nanostructures with controlled morphology;40 (2) use of oleyamine (OA) for effective shape controlled synthesis of TiO2 nanostructures, as also reported by Dinh et al.41, wherein TiO2


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nanostructures with different shapes such as rhombic and nanodot were obtained, (3) use of HF and NH3 for tailored synthesis of porous TiO2 nanostructures with {111} exposed facets. It

is

important

to

note

that

in

our

work,

the

use

of

oleylamine

for

TBT/OA/H2O:C2H5OH/NH3/HF system is necessary to obtain nanocube/nano-parallelepiped shaped TiO2 structures and use of NH3/HF is necessary to obtain {111} exposed facets in porous TiO2 nanoparticles. Oleylamine plays an important role in the morphology of the product; if no oleylamine was used with other experimental parameters kept constant, nonuniform and uncontrolled structures could be formed (Figure 6) which illustrated that OA contributed greatly towards the formation of nanocube/nano-parallelepiped particles and prevented random agglomeration of the nanoparticles. On the basis of experimental results, the formation process of nano-parallelepiped/nanocubes with exposed {111} facets in our work is described in Scheme 1.

Scheme 1. Proposed mechanism for the formation of nanocubes and nano-parallelepipeds with {111} exposed facets under our experimental conditions. 19 ACS Paragon Plus Environment


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During the early stage of synthesis when H2O/ethanol was added dropwise in tetrabutyl titanate/oleylamine, hydrolysis of tetrabutyl titanate occurred resulting in the formation of TiO6 octahedra, which are basic structural units of TiO2.42 The obtained phase of TiO2 depends on configuration and connectivity of these TiO6 octahedral units during polymerization and nucleation.42 In our case anatase phase is obtained so it is reasonable to consider edge sharing connections between these TiO6 octahedra. During nucleation OA tends to adhere on the TiO2 precursor surface leading to formation of discrete spherical particles of less than 500 nm size (SI, Figure S3), as also reported previously.41 The UVvisible spectra recorded for the product at this stage (SI, Figure S4) shows absorption bands in the range between 220─300 nm, with strong absorptions close to 270 nm attributed to penta- or hexacoordinated polymeric Ti species that were generated through the hydrolysis of tetrabutyl titanate, while a weak absorption near 220 nm is observed which may be assigned to a ligand-to-metal charge transfer (LMCT) transition from O to Ti in isolated TiO4 or HOTiO3 units as reported previously.43 Absence of absorption in 300–400 nm range is indicative of the absence of larger TiO2 clusters at this stage. In the next step, when NH3 and HF were added simultaneously, dissolution/recrystallization occurs. The morphology of the product changes at this stage. Figure S5 in SI, shows the SEM image of the product after addition of NH3 and HF to the TBT/OA/solvent mixture. Two noticeable changes in the morphology at this stage can be observed. First, sheet like structures begin to emerge in the solution (Figure S5) and second is the hollowing and size-reduction of pre-existed spherical structures (Figure S5 inset). We believe that both F- ions and ammonia together played the important role in the growth of {111} crystals facets of TiO2 during hydrothermal treatment. It is known that in case of different adsorbing species having different binding strengths, the relative competition between the facets surface energy determines the percentages of different exposed crystal facets during the nanostructure evolution. It is considered that


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during nanoparticles facets evolution, both HF and ammonia preferentially adsorbed on {111} facets due to the large number of under-coordinated atoms present on {111} facets of TiO2, compared to thermodynamically stable TiO2 {101} facets. This is in agreement with the previous experimental observations and theoretical calculations reported by Xu et al.21 The stabilization of {111} TiO2 facets by HF and NH3, hindered further growth along direction resulting in {111} facets exposed TiO2 nanostructures (Scheme 1). During the first crystallization process under hydrothermal condition, both F- anions (from HF, used in synthesis) and ammonia together played the important role to adsorb on the crystal facet of TiO2 to form Ammonium Titanium Oxide Fluoride (NH4)2(TiF4O), then after a calcination TiO2 nanocubes and nano-parallelepipeds structures are obtained. The formation of Ammonium Titanium Oxide Fluoride (NH4)2(TiF4O) phase as an intermediate product in our work is confirmed from the XRD (JCPDS No. 82-1330)44 analysis of the material before calcination (SI, Figure S6). These results are consistent with those of Xu et al.21 wherein Fanions and ammonia together adsorb on the crystal facet of TiO2 to form Ammonium Titanium Oxide Fluoride (NH4)2(TiF4O), then after a calcination process, left the TiO2 plates with exposed {111} facets. The formation of intracrystalline mesopores in TiO2 nanocubes/nano-parallelepiped may be due to the surface etching of TiO2 spherical structures, as a result of F- ions (released from HF during NH3/HF addition). The presence of F─ ions on the surface of TiO2, is supported by formation of Ammonium Titanium Oxide Fluoride (NH4)2(TiF4O) as an intermediate product (SI, Figure S6). In a recent report, Yu et al. also developed TiO2 photocatalysts with various morphologies including porous TiO2 nanocubes by hydrothermally etching process of pre-synthesised TiO2 spherical structures using [Bmim][BF4] and NH4F, where [Bmim][BF4] act as a source of F− ions.45 We observed that the presence of both HF and ammonia was necessary for the synthesis of nanosized anatase TiO2 crystals with exposed {111} facets. To establish this point, controlled 21 ACS Paragon Plus Environment


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experiments were carried out in different conditions. The use of oleylamine and ammonia solution (TBT/OA/H2O:C2H5OH/NH3 system) resulted in the formation of TiO2 agglutinated spherical structures (not perfect spheres) that are made up of irregular shaped {101} exposed facets TiO2 nanoparticles (Figure 5), while the product was found to be composed of ultrafine nanoparticles for TBT/OA/Solvent system (without using NH3 and HF) after identical hydrothermal and calcination processes as for {111} faceted TiO2 nanostructures (SI, Figure S7). In case of TBT/OA/Solvent/HF(1M) system (after identical hydrothermal and calcination processes as for {111} faceted TiO2 nanostructures) formation of solid sphere (SI, Figure S8) consisting of a solid core and relatively rough outer surface take place, as also reported frequently in many reports.36-38 Many reports claimed that under hydrothermal condition fluorides ions not only induce the outward hollowing of the spherical TiO2 aggregates, but also promote mass transfer from the core to the external surface and the crystallization of primary anatase TiO2 nanocrystals.46,47 In contrast to above three cases, the use of both HF and ammonia in equimolar ratio yielded porous TiO2 nanocube/nanoparallelepipeds shaped structures with exposed {111} facets (Figure 1). Figure 7 shows the N2 adsorption–desorption isotherms and corresponding pore size distributions of the different TiO2 nanostructures. For TN{101} nanostructures (Figure 7a), type IV isotherm with H3 type hysteresis loop, typical characteristic of mesoporous materials was observed, starting at relatively lower partial pressure (P/Po = 0.65). This type of behaviour is generally reported for aggregates of nanoparticles forming slit-shaped pores.47 As already shown in TEM results that TN{101} nanostructures were spherical aggregates formed by self assembly of fine nanoparticles that are elongated in shapes and the interparticle pores in the material may be responsible for such type of porous structures.


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Figure 7. Nitrogen adsorption−desorption isotherms of: (a) TN{101}, (b) TNF{111}-1, (c) TNF{111}-3, (d) TNF{111}-5, insets show corresponding pore size (BJH) distributions. BJH curve (inset, Figure 7a) displayed peaks in mesoporous as well as in macroporous region, which is a clear indication of nonuniform pore size distribution in the material. Surface area, mean pore diameter and the pore volume of the sample were 93 m2/g, 40.0 nm, and 0.93 cm3/g respectively. For TNF{111}-1 nanostructures, type IV isotherm with H3 type hysteresis loop (Figure 7b), typical characteristic of mesoporous materials was observed at relatively high partial pressure (P/Po = 0.8) compared to TN{101}. This type of hysteresis loop is generally reported for aggregates of plate-like or square shaped particles.48 BJH pore size distribution curve (inset, Figure 7b) displayed nearly bimodal pore size distribution, with mean pore sizes of ~5 nm (due to intracrystalline mesoporous) and ~60 nm (due to intercrystalline macropores). Presence of intracrystalline mesopores and intercrystalline macropores was clearly observed in the HAADF-STEM (Figure 1f) and TEM image 23 ACS Paragon Plus Environment


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discussed earlier (Figure 1a). Surface area, mean pore diameter and the pore volume of the sample were 67 m2/g, 21.7 nm, and 0.34 cm3/g respectively. For TNF{111}-3 nanostructures, type IV isotherm with H3 type hysteresis loop was observed (Figure 7c). Interestingly, for sample TNF{111}-3, pore size distribution curve (inset, Figure 7c) contained only a single broad peak centred at 15 nm. Thus, it can be inferred that TNF{111}-3 material possesses mesoporous network of homogeneously distributed nanostructures of equivalent dimensions. Well organised mesoscale pores with uniform pore sizes is essential requirement in several applications. Surface area, mean pore diameter and the pore volume of the TNF{111}-3 sample were 99 m2/g, 17.2 nm, and 0.43 cm3/g respectively. For TNF{111}-5 nanostructures, type IV isotherm with H1 type hysteresis loop (Figure 7c) was observed, which is the typical property of mesoporous materials. Surface area, mean pore diameter and the pore volume of the sample were 50 m2/g, 30.0 nm, and 0.37 cm3/g respectively. Lower surface area of the TNF{111}-5 sample compared to other samples (Table S1) resulted from the self assembly of nanoparticles to form hollow spherical structures filled with nanocrystallites, observed in TEM image of the material (Figure 4), as a result of higher HF concentration used for the synthesis of TNF{111}-5 material. The peak (inset, Figure 7d) indicates bimodal pore size distribution, the mesopore region in the BJH pore size distribution curve is due to the interparticle pores and peaks in the macropore region may be due to the pores between the individually assembled spherical structures. These above experiments evidenced that, well shaped, highly crystalline anatase TiO2 nanostructures with exposed {111} facets with tailored surface area and pore size distribution are obtained for TBT/OA/H2O:C2H5OH/NH3/HF system in this work. The synergetic effect of the F− and ammonia as the capping reagents resulted in the effective stabilization of anatase TiO2 {111} facets, as also reported by Xu et al.21 More importantly, the optimised reaction system consisting of titaniumbutoxide/oleylamine/H2O:C2H5OH/NH3/HF yielded 24 ACS Paragon Plus Environment

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nanocube/nano-parallelepiped shaped anatase TiO2 nanostructures with uniform pore size distribution, and intracrystalline mesoscopic void space, for the first time as illustrated in our work. Different combinations of ammonia and HF during the synthesis conditions resulted in TiO2 nanostructures with different shapes, sizes, surface area and pore-size distributions. For TBT/OM/H2O:C2H5OH/NH3/HF system with NH3:HF molar ratio 1:3, TiO2 nanocrystals with {111} exposed facets, highest surface area and narrow pore size distribution are obtained, indicating effective interplay between dissolution and selective adsorption of the F─ and ammonia on the {111} crystal facets for this reaction system, resulting in the formation of the uniform, monodisperse oxide nanoparticles. Sample prepared with NH3:HF ratio 1:5 has lowest surface area and broader pore-size distribution. The dye loading capacity and photovoltaic parameters of different photoanodes fabricated with different materials are listed in Table 1. It is interesting to note that despite of lower surface area of the TNF{111}-1 (67 m2/g) material than TN{101} (93 m2/g), the amount of dye adsorbed on TNF{111}-1 photoanode is 1.6 times of the dye loaded on TN{101} photoanode (from the table 1). Recent report predicted the existence of large number of undercoordinated Ti atoms on {111} (25% tri-coordinated Ti atoms and 75% penta-coordinated Ti atoms) surfaces than {101} (50% penta-coordinated Ti atoms and 50% hexa-coordinated Ti atoms) and (001} (100% penta-coordinated Ti atoms) surfaces, resulting in higher surface energy of {111} surfaces than {101} and {001} surfaces.21 It is known that crystal facet with a higher percentage of undercoordinated atoms is usually more reactive in surface driven applications. Therefore large number of undercoordinated Ti atoms on {111} facet can be considered the dominant source of active sites resulting in favourable dye adsorption. Among all the photoanodes, the highest Jsc in TNF{111}-1 DSSC is due to the superior dye loading capability of the photoanode resulted from large number of undercoordinated Ti atoms in TiO2 {111} faceted nanostructures. The high dye loading increases the light harvesting efficiency of the 25 ACS Paragon Plus Environment


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photoanode, which increases the Jsc of the device. It is observed that TiO2 photoelectrode made from TNF{111}-1 shows a short-circuit current density (Jsc) of 17.23 mA cm-2 and a photovoltage (Voc) of 754 mV with a power conversion efficiency (η) of 8.63%, TiO2 photoelectrode made from TN{101} shows a Jsc of 15.67 mA cm-2 and a Voc of 726 mV with a η of 6.72%, TiO2 photoelectrode made from Solaronix paste shows a Jsc of 11.52 mA cm-2 and a Voc of 739 mV with a η of 5.06%, whereas P25 nanoparticles cell only gives a Jsc of 11.18 mA cm-2 and a Voc of 732 mV with a η of 4.67%. The J-V curves for the DSSCs fabricated with different photoanodes are shown in Figure 8. It is important to note that in addition to highest Jsc, TNF{111}-1 photoanode also has highest Voc among all the photoanodes. The highest Voc in TNF{111}-1 can be attributed to a reduction in recombination losses at TNF-1/dye interfaces according to the diode equation Voc = (KT/nq) ln (Jmax/Jo), where the Jmax and Jo are the maximum current density and dark current density, respectively.49 This equation predicts that the suppression of the dark current density (Jo) (which arises as a result of recombination of charge carriers at the interface) results in a higher Voc. Literature predicts that dark current density in the DSSCs supplies qualitative information on dye coverage on the photoelectrodes surface and dark current density can be lowered by efficient dye coverage, as well as proper electrolyte penetration in the device.50 From Table S1, it is noted that porosity of TNF{111}-1 is less than TN{101} material, which cannot account for high open circuit voltage in TNF{111}-1 DSSC. Therefore it is reasonable to think that suppression of dark current due to efficient dye coverage on the TNF{111}-1 photoanode (due to large number of under-coordinated Ti and O atoms on the surface) is responsible to a great extent for increase in Voc. Higher dye loading fosters more charge injection from the excited state of the dye to the conduction band of TiO2, resulting in an upward shift in the TiO2 quasi-Fermi level, thus enhancing the potential difference between TiO2 and the redox species.51 The highest Jsc and Voc values in TNF{111}-1 photoanode are


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solely responsible for its superior performance, compared to other devices. It is observed that when a bilayered photoanode is formed by depositing a thin transparent layer of commercial material (Solaronix Ti-Nanoxide T/SC) prior to the TNF{111}-1 and TN{101} nanocrystals layer, photovoltaic parameters of the devices are improved significantly. The current density for SLP/TNF{111}-1 bilayered DSSC increases from 17.23 to 18.97 mA/cm2 and for SLP/TN{101} bilayered DSSC device it increases from 15.67 to 16.28 mA/cm2. These results suggest a rapid charge injection and transportation of photoexcited electrons in these bilayered architecture photoanode DSSCs. The main function of this under layer is to provide a compact layer/blocking layer for improving charge transport efficiency of the device reported frequently in many reports.52,53 The bilayered DSSCs, SLP/TNF{111}-1 and SLP/TN{101} show power conversion efficiency of 9.15% and 7.59% respectively due to improved Jsc and Voc in these devices compared to single-layer DSSCs (TNF{111}-1 and TN{101}).

Figure 8. Current density (J)─Voltage (V) curves of the DSSCs fabricated with different photoanodes. 27 ACS Paragon Plus Environment


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Electrochemical Impedance Spectroscopy (EIS) measurements were conducted on symmetrical cells, and the results are plotted in the form of Nyquist plots (Figure 9). Previous EIS studies on DSSCs suggest that the three semicircles from high to low frequency in the Nyquist plot correspond to the charge transfer resistance at the Pt/electrolyte interface, photoanode/electrolyte interfaces, and diffusion resistance electrolyte in the cell.43 The EIS parameters listed in Table 2, were obtained by fitting EIS plots using an equivalent circuit (right inset, Figure 9).

Figure 9. Nyquist plots of the bilayered DSSCs fabricated with different photoanodes (see figure labels). Inset is the equivalent circuit used to fit the Nyquist plots. An Ohmic resistance (Rs, starting point of the semicircle at high frequency) includes the resistance of the CE materials, FTO film, and the contact resistance at the counter electrode/FTO interface. Rct (arc in the middle frequency range) represents the charge transfer resistance between the dye-adsorbed photoanode and electrolyte interface and CPE represents the corresponding chemical capacitance. Zd is the Warburg impedance arises from the 28 ACS Paragon Plus Environment

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limitations in the mass transportation of the electrolyte. The diffusion impedance information appears on the low frequency side of the Nyquist plots, and is exhibited as various shapes, such as semicircles, depressed arcs, and straight lines. For better fitting, all capacitor elements were replaced by constant phase elements that, in any case, keep the constant phase element (CPE) exponent, by keeping the phase element (CPE) exponent quite close to the perfect capacitor value. From the fitted data, we could calculate the charge transport parameters such as the electron diffusion coefficient De = L2/(Rt × Cµ) where L is TiO2 film thickness, electron lifetime τn = Rct × Cµ, electron transit time τt = Rt × Cµ. The electron transport resistance (Rt), which is evaluated from the X-intercept of a line drawn from the linear-onset of the second semicircle in the Nyquist plots (left inset, Figure 9) as described in previous reports.54 From Table 2 it is noted that diffusion resistance (Zd) for P25-based DSSC is much higher than SLP/ TNF{111}-1 or SLP/TN{101} based solar cells, which can be ascribed to nonporous nature of

P25 material, not allowing the electrolyte penetration and ion

transport in the electrolyte. Larger recombination resistance (Rct) and longer electron life time (τn) were obtained for SLP/TNF{111}-1 DSSC indicating suppressed charge recombination process with more efficient charge transfer process compared to other DSSCs. The SLP/TNF{111}-1 photoanode exhibited a faster electron transport time than SLP/TN{101} and P25 photoanodes due to which its electron lifetime (τn) was significantly increased due to which photo induced electrons were prevented from recombination, which is reflected in its comparatively high Rct. Electron average diffusion length, Ln = (De × τn)1/2 and charge collection efficiency, Фcc = 1- (τt / τn) are two important parameters, useful for estimating the competition between electron diffusion and recombination in the device.55 It is noted (Table 2) that SLP/TNF{111}-1 DSSC exhibited the largest Ln and Фcc compared to the other devices (SLP/TN{101} and P25 DSSCs), suggesting that SLP/TNF{111}-1 DSSC should exhibit the highest Jsc and power conversion efficiency. The origin of higher Jsc in SLP/TNF{111}-1



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DSSC is attributed to the charge transport dynamics determined by the electron life time (τn) and charge recombination resistance (Rct). These experimental evidences prove that DSSCs fabricated with TiO2 nanostructures with {111} exposed facets, developed in this work resulted in more effective electron transport pathway and reduces the possibility of electron trapping for charge recombination, which in turn improves its Jsc and Voc compared to other devices, consistent with the J-V results. EIS and photovoltaic efficiency measurement tests on the similar bilayered DSSCs prove that SLP/TNF{111}-1 bilayered DSSC is the most efficient among all the devices. Since the under layer in the both the bilayered DSSCs (SLP/TNF{111}1 and SLP/TN{101}) is same, the upper layer (made up of different materials) is believed to be responsible for the variation in the performance of the devices. These findings support the conclusion that the upper TNF{111}-1 layer in SLP/TNF{111}-1 DSSC plays the multiple roles of high dye adsorption, fast electron transport, effective electrolyte permeation, and effective retardation of charge carriers as also observed (by dye loading calculations and EIS experiments) experimentally in our work. This work signifies that anatase TiO2 nanostructures with exposed {111} facets developed through a facile chemical route, is a very promising photoanode material for DSSCs. Photovoltaic parameters and the amount of dye loaded for different bilayered photoanodes fabricated with different {111} exposed facets TiO2 nanostructures (TNF{111}-1, TNF{111}-3, and TNF{111}-5 developed under different synthesis conditions in our work) are listed in Table 3. The under layer for all the DSSCs is made of commercial paste (Solaronix) about 4µm in thickness and the over layer is built from different TiO2 nanostructures with {111} exposed facets (obtained as a result of altering the HF and ammonia molar ratio in our work) about 8 µm in thickness. The amount of dye loaded on the SLP/TNF{111}-3

bilayered

photoanode is the maximum due to high surface area of upper TNF{111}-3 (99 m2/g) nanostructures compared to TNF{111}-1 (67 m2/g) and TNF{111}-5 (50 m2/g) materials, 30 ACS Paragon Plus Environment

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implying highest Jsc for SLP/TNF{111}-3 photoanode. SLP/TNF{111}-1 DSSC had a Jsc of 18.97 mA cm-2, a Voc of 769 mV, FF of 62.71, and η of 9.15%, SLP/TNF{111}-3 DSSC had a Jsc of 19.07 mA cm-2, a Voc of 760 mV, FF of 66.21, and η of 9.60%, and SLP/TNF{111}-5 DSSC had a Jsc of 17.70 mA cm-2, a Voc of 723 mV, FF of 63.58, and η of 8.14%.

Figure 10. Current density (J)─Voltage (V) curves of the bilayered DSSCs fabricated with different photoanodes. Figure 10 shows the current density–voltage (J–V) characteristics for DSSCs with bilayered nanostructured photoanodes. A high value of photocurrent current and fill factor in SLP/TNF{111}-3 bilayered photoanode DSSC, as compared to SLP/TNF{111}-1 and SLP/TNF{111}-5 photoanodes is believed to play a significant role in determining the overall efficiency of the device. The small decrease in Voc in SLP/TNF{111}-3 DSSC, compared to SLP/TNF{111}-1 DSSC is compensated by the large increase in the fill factor. Large porosity of upper TNF{111}-3 layer (Table S1) due to existence of uniform mesoscopic pore size distribution in SLP/ TNF{111}-3 DSSC, favours the effective diffusion of the redox electrolyte



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(I-/I3-) through the nanosized pores in the upper mesoporous TiO2 film, resulting in an increased fill factor in SLP/TNF{111}-3 device compare to SLP/TNF{111}-1 and SLP/TNF{111}-5 devices.56 Electrochemical Impedance Spectroscopy is employed to investigate the dynamics of charge transport and recombination in the different bilayered photoanode DSSCs. Nyquist impedance plots of DSSCs (Figure 11) with different working electrodes and corresponding electrochemical parameters, obtained by fitting EIS plots with an equivalent circuit (right inset, Figure 11), are listed in Table 4. Rt is evaluated from the x-intercept of a line drawn from the linear-onset of the second semicircle in the Nyquist plots (left inset, Figure 11) The Rs and Rct for different bilayered photoanode DSSCs follows the order as SLP/TNF{111}-1 > SLP/TNF{111}-3 > SLP/TNF{111}-5 and Rt in bilayered photoanode DSSCs follows the order as SLP/TNF{111}-5 > SLP/TNF{111}-3 > SLP/TNF{111}-1. The advantage of larger Rct in SLP/TNF{111}-1 DSSC is diminished due to larger Rs, similarly advantage of lower Rs in SLP/TNF{111}-5 is diminished due to lower Rct. It is interesting to note that the impedance parameters (Rs, Rt and Rct) for SLP/TNF{111}-3 are intermediate between SLP/TNF{111}-1 and SLP/TNF{111}-5. Lower Rct (large recombination) in SLP/TNF{111}-5 photoanode DSSC may be due to the surface traps or defects in upper TNF{111}-5 nanostructure layer resulting from the structure of TNF{111}-5 material (composed of spherical structures of various sizes and TiO2 nanoparticles). From Table 4, it is noted that in SLP/TNF{111}-3, DSSC electron transport is faster and also electron is longer-lived compared to SLP/TNF{111}-1 and SLP/TNF{111}-5 DSSCs. Previous EIS studies on nanostructured DSSCs explain that electron recombination or electron life time depends on various properties of the material such as crystallinity, porosity, interconnectivity of the nanoparticles, surface defects57,28 and we may not expect all of them to be effective at the same time. In certain conditions some effects could be detrimental. SLP/TNF{111}-3 bilayered DSSC exhibited the largest Ln and Фcc 32 ACS Paragon Plus Environment

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compared to the other devices, indicating fast, and efficient electron transport dynamics in SLP/TNF{111}-3 DSSC. These results are consistent with J-V results as SLP/TNF{111}-3 exhibited the highest Jsc and energy conversion efficiency. These experimental findings support the conclusion that TNF{111}-3 material with {111} exposed facets possesses high dye loading, fast electron transport, effective electrolyte diffusion and suppressed charge recombination. These results obtained in this work support the conclusion that the synthesised TiO2 nanocubes and nano-parallelepipeds with exposed {111} facets have great potential in developing efficient DSSCs.

Figure 11. Nyquist plots of the bilayered DSSCs fabricated with different photoanodes (see figure labels). Inset is the equivalent circuit used to fit the Nyquist plots.

The effective retardation in charge recombination in {111} faceted TiO2 nanostructures based DSSCs compared to other structures, can be explained on the basis of effective dye loading 33 ACS Paragon Plus Environment


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on {111} faceted TiO2 nanostructures (due to large number of under-coordinated Ti and O atoms on the surface) in the devices made from {111} faceted TiO2 nanostructures. The high Voc and fill factor in {111} faceted TiO2 nanostructures DSSC (SLP/TNF{111}-1 and SLP/TNF{111}-3, Tables 1 and 3) compared to other devices (P25, SLP/TN{101}) is a clear indication of suppressed charge recombination in these devices. According to the diode equation Voc = (KT/nq) ln (Jmax/Jo), where the Jmax and Jo are the maximum current density and dark current density respectively, suppression of the dark current density (Jo) (which arises as a result of recombination of charge carriers at the interface) results in a higher Voc.49 Dark current density in the DSSCs can be lowered by efficient dye coverage, as well as proper electrolyte penetration in the device.50 Higher dye loading in photoanodes fabricated with {111} faceted TiO2 nanostructures (SLP/TNF{111}-1 and SLP/TNF{111}-3) fosters more charge injection from the excited state of the dye to the conduction band of TiO2, resulting in an upward shift in the TiO2 quasi-Fermi level, and suppression of the interfacial electron recombination of the injected electrons with the oxidised electrolyte ions.58 The longer diffusion path length of the electrons in {111} faceted TiO2 DSSCs (SLP/TNF{111}-1 and SLP/TNF{111}-3, Table 4) compared to other devices (SLP/TN{101}, P25, Table 2), means that the electrons travel farther in {111} faceted TiO2 nanostructured film before they recombine with the electrolyte solution, thus decreasing the recombination reaction.

CONCLUSIONS Well defined anatase TiO2 nanocubes and nanoparallelepipeds with exposed {111} facets were prepared through a modified one pot hydrothermal method. Optimization of the synthesis conditions leads to the assembly of high surface area, {111} faceted anatase TiO2 nanocrystals to give uniform mesoscopic void space. Development of novel bilayered DSSCs fabricated with titania nanocrystals with exposed {111} facets demonstrated an overall conversion efficiency of 9.60%, much higher as compared to photoanodes fabricated with 34 ACS Paragon Plus Environment

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commercially available titania powder (4.67%), or with anatase TiO2 nanostructures with exposed {101} facets (7.59%). Higher efficiency can be attributed to high dye loading, fast electron transport, effective electrolyte permeation and effective retardation of charge recombination in the photoanodes fabricated with upper layer of titania nanocrystals having exposed {111} facets. The present approach will stimulate further investigations on the development of faceted nanocrystals with exposed high energy facets, and their applications for energy devices.

ASSOCIATED CONTENT Supporting Information Classification of obtained TiO2 nanostructures in our work, Surface and porous characteristics measured from N2 adsorption/desorption isotherms for the different TiO2 nanostructures, TEM image of TiO2 nanostructures for TBT/OA/Solvent/NH3 system (TN{101}) showing connected structures with interpenetrating particles, SEM image of the intermediate product (TBT/OA/Solvent), The UV-visible spectra recorded for intermediate stage (TBT/OA/Solvent), SEM image of the intermediate stage (TBT/OA/Solvent/NH3/HF) before hydrothermal treatment, XRD of the intermediate product (TNF{111}-3, before calcination). The diffraction pattern corresponds to Ammonium Titanium Oxide Fluoride (NH4)2(TiF4O) phase, according to the standardized JCPDS No. 82-1330, TEM images of TiO2 nanostructures for TBT/OA/Solvent system (without using ammonia and HF), TEM image of TiO2 nanostructures for TBT/OA/Solvent/HF system (oleylamine and 1M HF)."This information is available free of charge via the internet at "http://pubs.acs.org/."

AUTHOR INFORMATION

Corresponding author 35 ACS Paragon Plus Environment


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*

Tel.: 91-135-2525842. Fax: 91-135-266-203. E-mail: [email protected] (A.K.S.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Director, IIP is acknowledged for approving the work. VA thanks Council of Scientific and Industrial Research, India for fellowship. The authors thank Analytical Science Division, Indian Institute of Petroleum for analytical services. The Ministry of New and Renewable Energy, New Delhi is acknowledged for research funding.


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Table 1. Photovoltaic performance parameters of DSSCs fabricated with different types of photoanodes.

Jsc (mA/cm2)

Voc (mV)

FF (%)

η (%)

0.93 X 10-7

11.18

732

57.04

4.67

8

2.4 X 10-6

11.52

739

59.40

5.06

TN{101}

8

5.2 X 10-6

15.67

726

59.04

6.72

TNF{111}-1

8

8.32 X 10-6

17.23

754

66.46

8.63

SLP/TN{101}

12

7.51 X 10-6

16.28

757

61.57

7.59

SLP/TNF{111}-1

12

1.06 X 10-5

18.97

769

62.71

9.15

Photoanodes

Thickness

Adsorbed

(µm)

Dye [molcm-2]

P25

8

Solaronix Paste (SLP)



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Table 2. Electron dynamic parameters estimated by the Nyquist impedance plots of DSSCs fabricated with of different photoanodes.

Photoanode

Rs(Ω)a

Rt(Ω)b

Rct (Ω)c

Cµ(F)d

Zd (Ω)e

τt (ms)f

τn (ms)g

De(cm2/s)h

Ln (µm)i

Фc(%)j

P25

27.76

23.47

54.61

8.99× 10-5

69.27

2.11

4.11

3.03× 10-4

3.52

48.7

SLP/TN{101}

35.2

21.13

57.2

8.21 × 10-5

14.30

1.73

4.70

8.32× 10-4

6.25

63.2

30.55

91.10

5.52 × 10-5

15.69

1.69

5.03

8.54 × 10-4

6.55

66.5

SLP/TNF{111}-1 42.65

a

Series resistance.

b

Transport resistance.

c

Charge recombination resistance.

d

Chemical

capacitance of the electrons in the photoanode. eWarburg impedance arises from the electrolyte diffusion impedance of redox species in the electrolyte. fElectron transport time. g

Electron lifetime. hDiffusion coefficient. iDiffusion length of electrons. jCharge collection

efficiency of the photoanode.


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Table 3. Photovoltaic performance parameters of bilayered DSSCs fabricated with different types of photoanodes.

Photoanodes

Adsorbed

Voc (mV)

Jsc (mA/cm2)

FF (%)

η (%)

Dye [molcm-2] SLP (4 µm)/TNF{111}-1 (8 µm)

1.06 X 10-5

769

18.97

62.71

9.15

SLP (4 µm) /TNF{111}-3 (8 µm)

1.21 X 10-5

760

19.07

66.21

9.60

SLP (4 µm) /TNF{111}-5 (8 µm)

7.32 X 10-6

723

17.70

63.58

8.14



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Table 4. Electron dynamic parameters estimated by the Nyquist impedance plots of DSSCs fabricated with of different photoanodes.

Photoanode

Rs(Ω)a

Rt(Ω)b

Rct (Ω)c

Cµ(F)d

Zd (Ω)e

τt (ms)f

τn (ms)g

De(cm2/s)h

Ln (µm)i

Фc(%)j

SLP/ TNF{111}-1

42.65

30.55

91.10

5.52 × 10-5

15.69

1.69

5.03

8.54 × 10-4

6.55

66.5

SLP/ TNF{111}-3

32.17

14.34

74.80

8.84 × 10-5

6.69

1.27

6.62

11.4 × 10-4

8.69

80.8

SLP/ TNF{111}-5

27.69

13.56

41.85

1.56 × 10-4

4.04

2.12

6.53

6.80 × 10-4

6.66

67.6

a

Series resistance.

b

Transport resistance.

c

Charge recombination resistance.

d

Chemical

capacitance of the electrons in the photoanode. eWarburg impedance arises from the electrolyte diffusion impedance of redox species in the electrolyte. fElectron transport time. g

Electron lifetime. hDiffusion coefficient. iDiffusion length of electrons. jCharge collection

efficiency of the photoanode.


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