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Mesoporous Assembly of Cuboid Anatase Nanocrystals Into Hollow Spheres: Realizing Enhanced Photoactivity of High Energy {001} Facets Sivaram Illa, Ramireddy Boppella, Sunkara V. Manorama, and Pratyay Basak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04318 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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The Journal of Physical Chemistry

Mesoporous Assembly of Cuboid Anatase Nanocrystals into Hollow Spheres: Realizing Enhanced Photoactivity of High Energy {001} Facets† Sivaram Illa, #,1,2,3 Ramireddy Boppella, #,1,2,3 Sunkara V Manorama1,2,3,*and Pratyay Basak,1,2,3,*

† 1

Nanomaterials Laboratory, Inorganic and Physical Chemistry Division,

Council of Scientific & Industrial Research-Indian Institute of Chemical Technology (CSIR-IICT), 2

CSIR-Network Institutes for Solar Energy (CSIR-NISE), &

3

Academy of Scientific and Innovative Research (AcSIR), Hyderabad-500 007, India. Email: [email protected]; [email protected]

#

Both the authors have contributed equally to the study

*To whom all Correspondences should be addressed

-1- Environment ACS Paragon Plus


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KEYWORDS: anatase, hollow spheres, hierarchical aggregates, mesoporosity, photoactivity.

ABSTRACT Herein, we showcase a unique one-step synthetic strategy to obtain a mesoporous assembly of cuboid anatase nanocrystals into hollow spheres that offers not only high surface area but also predominantly tenders the reactive high energy {001} facets. The detailed experimental studies, parameter optimization and characterizations reveal the crucial role played by the shape and structure directing agents during nucleation and growth. Concentration of F--ions plays a determining role in stabilizing the size and shape at the initial stage of formation while an optimal balance of SO42- anions is critical in generating the hollow porous assemblies while retaining the morphology of the primary nanocuboid subunits. The single crystalline anatase TiO2 nanocuboids and their hollow spherical assemblies; both show substantial enhancement in photocatalytic and photoelectrochemical activity when compared with commercial P25. Photoelectrodes fabricated using cuboid nanocrystals demonstrate a superior DSSC efficiency when compared to Degussa P25. In combination with the mesoporous hollow spherical assemblies of nanocuboids as an overlayer, the power conversion efficiency of these photoelectrodes considerably increased owing to enhanced light harvesting. A confluence of physicochemical parameters, i.e, high surface area, reactive {001} facets, high dye-loading capacities, enhanced light scattering and confinement, low electron transfer resistance, improved electrodeelectrolyte interface, all positively impact the overall photoconversion efficiency. To exemplify the photocatalytic performance of the synthesized nanocuboids, oxidation of terephthalic acid was studied as a model system. A significantly superior photocatalytic activity mediated by •OH radicals draws strong support from the high photocurrents observed for these mesoporous hollow architectures.


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Introduction Solar light is an abundant source of energy, which effectively tapped has immense potential to address the global challenges and provide sustainable solutions.1 Semiconducting materials plays a key role in solar energy conversion to various other forms (generate electricity, produce chemical fuels and generate free radical species) by facilitating the significant absorption of photons and subsequent generation of photoelectrons and holes (charge carriers).2–4 The applications of these semiconductors apart from tendering good charge transport properties depends equally on effective adsorption or interaction of molecules and ions (water, O2, organic dyes, etc.). Amongst many semiconductor materials studied, titania (TiO2) has drawn significant attention owing to its high electron mobility, photochemical stability, and abundance. However, the catalytic efficiency of nanocrystalline TiO2 is largely dependent on its crystal phase, surface area, morphology, architecture, defect sites and surface non-stoichiometry.5,6 As the catalytic reactions occur at the surface of NPs, the catalytic activity is largely dependent upon crystalline structure and exposed facets.7,8 Recently, particular emphasis has been devoted to synthesize single crystalline anatase TiO2 nanocrystals (NCs) that can offer higher percentage of reactive {001} facets. Investigations on the facet-dependent reactivity of anatase TiO2 single-crystals have established that photocatalytic performance substantially increases on the {001} facets possessing higher surface energy when compared to {101} facets.9,10 Theoretical studies supports further with the findings that anatase (001) surface allows the dissociative adsorption of water, whereas anatase (101) surface allows only for the molecular adsorption of water.11-13 Since the breakthrough demonstration of the higher activity of {001} facets by Yang et al., considerable progress has been witnessed in the crystal facets engineering of TiO2 towards developing efficient solar driven devices.9,14 Concurrently, in DSSCs, a mesoporous TiO2 film accepts the electrons from the sensitized dye molecules attached on titania, thereby facilitating the charge separation.2,3 The efficiency of chargeseparation is largely dependent on the competing rates of electron injection and recombination losses, -3- Environment ACS Paragon Plus


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which in turn relies on the interaction of chromophore with TiO2.15,16 Till date, the strategy to obtain high efficiency DSSCs photoanodes primarily employed mesoporous larger particles or hollow structures build by subunits. They offer both high internal surface area for efficient dye loading and enhanced visible light scattering by tapping the confinement effects.17,18 However, most of these structures are built with polycrystalline or amorphous subunits, which significantly restricts their performance in optical, electrical, and optoelectronic applications, due to poor crystal quality and insufficient contact between subunits.19 Additionally these polycrystalline subunits increase grain boundary recombination thus reducing the carrier diffusion length. Recent report in the literature suggests that {001} facets of anatase single crystals can appreciably suppress the back recombination (TiO2 to dye molecule) leading to higher efficiencies when compared to the {101} surface.20 Thus, anatase TiO2 NCs with high ratio of the exposed {001} facets have shown great promise especially in applications such as, photocatalysis and dye-sensitized solar cells (DSSCs). In the light of these findings, a reasonable and alternate approach is to realize titania hollow structures that comprises of single crystalline anatase subunits enclosed with reactive {001} facets, which might effectively overcome the limitations. A combination of highly ordered reactive facets with the unique characteristics of hollow architectures i.e. large surface area, low density, potential for high dye loading capacity, kinetically favorable electrolyte diffusion, strong light scattering and so on, can make them attractive materials for optoelectronic applications in general. Herein, we demonstrate a one step template-free method that allows generating divergent structures of anatase titania of cuboid shape nanocrystals (NCs) solely enclosed with high energy reactive {001} facets. Specifically, the strategy showcases the ease of obtaining well-defined mesoporous assemblies of cuboid anatase nanocrystals into hollow spheres and the synergistic enhancement of the material properties. Comprehensive characterization of the synthesized morphologies and an understanding of growth mechanism are detailed followed by feasibility studies as photocatalysts and photoanodes to exemplify their significant potential. -4- Environment ACS Paragon Plus

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Experimental Materials: The chemicals used for hydrothermal synthesis are Titanium (IV) tetrachloride (99%, Merck), ammonium fluoride (Aldrich), anhydrous sodium sulfate (99%, Merck), and deionized water (18 MΩ-cm). For fabrication of TiO2 paste, ethyl cellulose (Sigma-Aldrich), α-terpineol (SigmaAldrich) were used with ethanol (s.d. Fine Chemicals) as the solvent. Cis-bis (isothiocyanato) - bis (2,2’-bipyridyl-4,4’-dicarboxylato) - ruthenium(II) bistetrabutyl ammonium (N719 dye, SigmaAldrich), was used as a dye, acetonitrile (Sigma-Aldrich), valeronitrile (Sigma-Aldrich), tert-butanol (>99.0%, Sigma–Aldrich), 4-tert-butylpyridine (Sigma-Aldrich), Iodine (Sigma-Aldrich), 1-butyl-3methyl-imidazolium iodide (BMII, Sigma-Aldrich), guanidinium thiocyanate (GSCN Sigma-Aldrich) were used to prepare the electrolyte. Commercially available titanium dioxide nanopowders (P25, 21nm, Sigma-Aldrich) were used as a control and Surlyn (25µ) obtained from Dyesol was used as separator and sealant. The chemicals were used as received without any further purification. Synthesis: The cuboid shaped titania nanocrystals with exposed {001} facets were synthesized by a simple one step hydrothermal route. In a typical method, 13.6 mmoles of TiCl4 was added drop-wise to a 50 mL of water containing NH4F. The reaction mix was immediately transferred into a 75mL Teflonlined stainless steel hydrothermal vessel and autoclaved at 180 oC for 24 hours. Following the completion of reaction time, the autoclave was cooled to room temperature, the products washed several times with de-ionized water, followed by ethanol and then dried at 60 oC in an oven. The synthesized product was named as T1. To synthesize the hollow and porous hierarchical spheres, various amount of sodium sulfate was additionally used in the reaction mixture while all other synthetic protocols and experimental condition were maintained as described above. The hierarchical structures synthesized based on different sodium sulfate concentration (7 mM and 10.5 mM) are denoted as T2, T3, etc. in the text.



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Photocatalytic Performance: Terephthalic acid (TA) was used as a probe molecule to evaluate the photocatalytic efficiency of synthesized TiO2 NCs. In a typical reaction, 20 mg of photocatalyst was suspended in 50 mL of 0.01M NaOH containing 3 mM of TA. The suspension was sonicated in the dark for 30 min before irradiating with a 300 W UV light source. Aliquots of 5 mL reaction mix were withdrawn for every 30 min interval and centrifuged for fluorescence spectroscopy measurements. During the photoreactions, no oxygen was bubbled into the suspension. A fluorescence spectrophotometer (Spex Fluorolog-3 (HORIBA JOBIN YVON) was used to measure the fluorescence signal of the 2-hydroxy terephthalic acid generated. The excitation light used in recording fluorescence spectra was 320 nm. Fabrication of Photoanodes and DSSC test cells: Titanium dioxide photoanodes were deposited from the as prepared T1, T2, T3 and standard nanocrystalline titania (P25) pastes and used in various combinations. These pastes were formulated using ethylcellulose and α-terpineol in absolute ethanol by grinding in a mortar as per the literature reported procedures.21 The so prepared viscous paste was deposited by spin coating technique, on Fluorine doped tin oxide coated (FTO) glass (Sigma Aldrich, 7 Ω/sq, 2.3 mm thick) pre-treated with TiCl4 solution followed by heating at 500 oC for 30 min to obtain a titania film of desired thickness. In certain cases multiple layers in various combinations were also achieved. Finally, the resulting TiO2 films were again treated with 40 mM TiCl4 (aq.) solution at 70 oC for 30 min, rinsed with ethanol and annealed at 500 oC for 30 min. The prepared samples were oven cooled to 80 oC and soaked in a solution of 0.5 mM N719 dye dissolved in 50:50 v/v tert-butanol and acetonitrile for 24h to obtain the DSSC photoanode. For preparation of the electrolyte; 0.7 M BMII, 0.1 M GSCN, 0.03 M Iodine and 0.5 M tertiary-butyl pyridine were freshly dissolved in 85:15 v/v of acetonitrile and valeronitrile. Thereafter, all the dye-adsorbed photoanodes as described above were assembled into DSSC test cells using 25µm thick Surlyn as a spacer, the ionic liquid mix as an electrolyte and sputtered platinum coated FTO as a counter electrode for evaluating their performance.


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The active area of these fabricated cells was 0.13cm2 and multiple cells (N > 3) of each type were fabricated to assess reproducibility. Photoelectrochemical Evaluation: A photocurrent measurement experiment was performed in a standard three-electrode system using Pt sheet as the counter electrode, Ag/AgCl (saturated with KCl) as the reference electrode and synthesized samples as the working electrodes. A 1.0 M NaOH aqueous solution was used as the electrolyte. Working electrodes were prepared by the same procedure for preparation of DSSCs photoanode as described above. All the electrodes had similar thickness (10µm) and the active area was kept for 1x1 cm2. The generated photocurrent was measured on an electrochemical workstation (Zahner Zennium) and the potential of the working electrode was kept at 1.2 V against RHE. The light source was simulated sunlight from a 300 W xenon solar simulator (6258, Newport Corp.) through an Air Mass filter (AM1.5 Global, 81094) with a measured intensity equivalent to standard AM1.5 sunlight (100 mW/cm2) at the sample face. Characterization: Morphology and surface analysis of the as prepared T1 and T2 samples were evaluated using field-emission scanning electron microscope (FESEM, JEOL 7610F) and transmission electron microscopy (JEOL JSM-2100F operated at 200kV). Powder X-ray diffraction studies were carried out on PANalytical Empyrean equipped with CuKα source in 2θ region: 10o to 70o to elucidate pertinent information on the crystallinity, phase and crystallite size using Scherrer’s equation. The pore size distribution was estimated from nitrogen physisorption isotherms at liquid nitrogen temperature on a Quantachrome BET instrument. The current-voltage (I-V) characteristics were performed employing a Keithley 2400 Source Meter under solar simulator (Newport Model: 94023A) using 100mW/cm2 illumination simulating one sun at AM 1.5G. A 400 W Xenon arc lamp (Newport Model: 6280NS) was used as a light source and the intensity was calibrated by a NREL standardized Si-solar cell equipped with an optical filter before each measurement. The incident photon to current conversion efficiency (IPCE) spectra was acquired with a spectral resolution of 5 nm on the basis of a spectral -7- Environment ACS Paragon Plus


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product Zolix DSC300PA as a function of wavelength in the range 300 to 800 nm. The diffuse reflectance measurements were performed on a Varian-Cary-5000 UV-Vis-NIR spectrophotometer for estimating scattering efficiency. The amount of dye-loading onto photoanodes was estimated by following dye-desorption protocol. Typically the dye loaded samples were dipped in a 0.1M NaOH water and ethanol (50:50 v/v) solution for dye-desorption followed by UV-Vis absorption measurements. A pre-calibration plot (Beer-Lambert) provided the concentration of the total amount of dye desorbed.

Results and Discussion Self-assembly of cuboid-shaped anatase TiO2 nanostructures with exposed {001} high energy facets yielded hierarchical hollow/porous sub-micron sized spheres under relatively mild hydrothermal conditions. The synthesis involves TiCl4 as precursor in the presence of ammonium fluoride and sodium sulfate as the structure directing agents under strongly acidic conditions (pH < 2). The corresponding XRD pattern of synthesized samples are provided in Figure 1(a) exhibits well resolved diffraction peaks that can be exclusively ascribed to TiO2 crystals with the tetragonal anatase phase (JCPDS No. 21-1272). The peak resolution, intensity and FWHM obtained suggest that the synthesized materials are well crystallized and no further heat treatment is required. The crystallinity of samples T2 is observed to significantly increase with addition of sodium sulfate (7 mmoles), indicating that sodium sulfate plays a crucial role during the grain growth. Addition of excess sodium sulfate (14 mmoles) however showed a considerable decrease in the observed peak intensity for T3, suggesting a role reversal and competitive effects partaking in the reaction medium and product formation. Full-width at half-maxima for the (101) reflection plane was used in the Scherrer’s equation to estimate the average crystallite sizes of T1, T2 and T3 as ca. 35 nm, 44 nm, and 24 nm respectively. In addition, average thickness of the TiO2 cuboid NCs in the direction of [001] and their length in the direction of [100] can be obtained from the “full width at half-maximum” of the (004) and (200) diffraction peaks , to -8- Environment ACS Paragon Plus

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estimate the percentage of the exposed (001) facets (see SI for calculation)9. On the basis of the above structural information, the percentage of highly reactive (001) facets in the T1 and T2 are 83.4 and 76.3%, respectively. Micro-Raman spectroscopy was also carried out to assess the sample quality and confirm the phase purity. The peaks observed at 144 cm-1 and 636 cm−1 are assigned to the Eg bands while 394 cm-1 and 514 cm-1 peaks are ascribed to the B1g and A1g bands of phase pure anatase (Figure S1). Figure 1(b)−(d) presents a series of representative field emission scanning electron microscopy (FESEM) images that shows the hierarchical structures generated from the one-step hydrothermal technique with varied ammonium fluoride and sodium sulfate. SEM image of Figure 1(b) shows the uniform TiO2 nanocrystals obtained when 30 mmoles of NH4F only was used as the structure directing agent along with TiCl4 precursor (designated as T1). The TiO2 nanocrystals have cuboid type morphology bounded with high energy {001} crystal facets according to shape symmetry and previous reports.9 The size of cuboid TiO2 NCs varies from 30 to 50 nm and most of cuboids are enclosed solely with {001} facets.22 Interestingly, hierarchical hollow porous structures were obtained when Na2SO4 was used along with NH4F as a co-additive in the reaction mixture as shown in Figure 1(c) (designated as T2). The hierarchical hollow porous structures have relatively non-uniform spherical shape, enclosing a hollow interior. The external surface of these assembled hollow microspheres was apparently rough and possesses a porous texture due to an interconnected mesoporous network. A closer look of high resolution FESEM image of this T2 sample (Figure 1(d)) indicates that the external shell of the microspheres are composed primarily with cuboid nanocrystalline subunits enclosed by high energy {001} facets. It is noteworthy that a significant fraction of particles were in close contact together and tends to fuse to form these complex structures. In contrast, when sodium sulfate concentration was increased further macroporous spheres devoid of hollow interior with evident loss of crystalline subunits was obtained (T3, Figure 1(e)). The size of the TiO2 structures however remains similar possessing an interconnected macro-porous surface. -9- Environment ACS Paragon Plus


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The morphology of the as-synthesized nanostructure was further characterized by transmission electron microscopy (TEM) and is presented in Figure 2. The TEM image in Figure 2(a) clearly reveals the near perfect cuboid shaped crystals showing sharp edges and corners with sizes ranging from 30 to 50 nm. The corresponding high-resolution TEM (HRTEM) image (Figure 2(b)) exhibits well resolved lattice spacing further concluding the high crystallinity of as synthesized anatase TiO2 material. The HRTEM on a single particle show the diffraction spot of the (001) zone and the (200) atomic lattice spacing of 1.9 Å. The bright intense spots in FFT (inset, Figure 2(b)) indicate the singlecrystalline nature of the cuboid TiO2 NCs. The findings clearly suggests that the cubic TiO2 NCs are solely enclosed by high-energy {001} facets. According to the symmetries of anatase TiO2 crystals, the two flat and square surfaces should be (001) facets, and the eight quadrilateral surfaces should be (101) facets of these crystals. The angle between different planes as designated in Figure 2(b) exactly matches the theoretical values.22,23 TEM images of the T2 sample (Figure 2(c)) revealed that it is composed of interconnected smaller primary nanoparticles that assemble into a hollow morphology with excellent porosity. The dark edges and bright centers clearly indicate the hollow interior of these structures, which is in good agreement with the SEM observations. Post ultrasonication of these hollow spheres, the TEM images undoubtedly reveal that the primary particles are nanocuboids with exclusively {001} facets on the exposed geometry (Figure 2(d)). The corresponding HRTEM image shown in Figure 2(e) displays two sets of lattice fringes taken at different places of cuboid TiO2 nnaocrystals showing the interplanar spacing 0.19 nm and 0.35 nm, which are in accordance with the (001) and (101) facets, respectively, indicating that the top surface exposed by a {001} facet.24,25 It is hence inferred that the TiO2 hollow spheres composed with single crystalline cuboid shape NCs are predominately enclosed by the thermodynamically stable {001} facets. For the T3 samples, the TEM images shown in Figure S2(a) showed the porous assemblies and Figure 2(f) obtained after ultrasonication clearly suggest that these porous spheres are also composed of nanocuboids albeit of smaller size ranges. The SAED pattern (Figure S2(b)) further confirms a good anatase crystal ACS Paragon -10Plus Environment

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structure of these TiO2 microspheres, and the diffraction ring in the SAED pattern reveals that the individual NCs in TiO2 microsphere is polycrystalline. The observations from XRD, FESEM and HRTEM are in good agreement with each other. The internal porosity of these particles as revealed by the TEM image indicates the existence of the porous structure inside the particle, supporting the formation of hollow spheres. The Brunauer– Emmett–Teller (BET) method was used to estimate the specific surface areas and calculate the pore size distribution curves using the Barrett–Joyner–Halenda (BJH) model from desorption and adsorption branches of the isotherm (summarized in Table-2). The representative BET measurements and pore size distribution curves suggest that all the synthesized TiO2 NCs exhibit type IV isotherms with H3 type hysteresis loop with a sharp capillary condensation implying bimodal pore size distributions in the mesoporous and macroporous regions.26 For T1 sample, typical characteristics of mesoporous nature was observed, starting at relatively lower partial pressure (P/Po = 0.6), which can be probably ascribed to the aggregation/coalescence of the nanocuboids.26,27 The surface area, mean pore diameter and the pore volume of the T1 sample were estimated to 43.36 m2.g-1, 2.55 nm and 0.133 cm3.g-1, respectively. After introduction of SO4-2 as a co-additive, the shapes of nitrogen adsorption and desorption isotherms underwent several obvious changes, implying a significant variation of pore size and structures. For the sample T2 and T3, the adsorption amount slowly increases between P/Po of 0.01 to 0.6, and then the observed hysteresis loop sharply shifts to a higher relative pressure on approaching P/Po ≈ 1. The corresponding BJH pore size distribution curve (inset, Figure 3(b)) displayed nearly bimodal pore size distribution, implying that both samples possess mesoporous and macroporous pores. This kind of pore texture is generally an effect of intra-aggregated and interaggregated pores (in lower vapor pressure and higher vapor pressure range, respectively) resulting from aggregation of primary and secondary particles. For the T2 sample, the mean pore diameter is centered at 3.2 nm and the corresponding surface area and pore volumes are 40.64 m2.g-1 and 0.1 cm3.g-1, respectively. Whereas, the T3 sample has the highest mean pore diameter of 11.3 nm and the ACS Paragon -11Plus Environment


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corresponding surface area and pore volumes are 12.38 m2.g-1 and 0.06 cm3.g-1, respectively. It is interesting note that the hysteresis loops in higher relative pressure range increased gradually for T2 sample while the hysteresis loops in higher relative pressure decrease for sample T3 eventually accounting for the lower surface area and pore volume of T3. The reduction in surface area and pore volume can be attributed to the decrease of the amount of pores caused by the fusion of the adjacent nanosubunits and the erosion by F- that resulted in these hollow architectures.26 Growth Mechanism For a complete understanding of the formation process and growth mechanism of the hierarchical hollow porous TiO2 structures composed of cuboid subunits, series of reactions have been performed varying the control parameters. Based on the observations and experimental results, a plausible growth mechanism is proposed to explain the formation of these exotic hierarchical hollow porous architectures composed of TiO2 nanocuboids as the primary building blocks. It should be referred that the conventional hydrolysis of TiCl4 in acidic medium yields phase-pure rutile TiO2 nanostructures in spindle or rod-like morphologies, and has been extensively investigated.28,29 This is attributed to the corner-sharing of [TiO62-] octahedra that nucleates and grows along the [011] axis yieldng the needle like rutile TiO2 structures. The structure directing effect of Cl--ions is also pivotal in the favorable formation of rutile TiO2 in highly acidic condition.30 However, it has now been verified that the addition of fluoride ion (F-) or sulfate ions (SO42-) to the TiCl4 aqueous solution favors the formation of anatase phase at the expense of the preferential phase, i.e., the rutile form.31–34 It is now well understood that the presence of F- can act as the capping reagent in order to minimize the surface energy of {001} TiO2 facets by forming strong Ti-F bond.9 As is evident from Figure 1(b), cuboid shaped nanocrystals (average size ~30-50 nm) bounded by {001} facets are formed when 30 mmoles of ammonium fluoride is used. That the samples synthesized are phase pure anatase titania was confirmed by X-ray diffraction studies with an estimated 83.4% of {001} facets present. The morphology of TiO2 NCs obtained with merely NH4F as additive could not be further tuned effectively ACS Paragon -12Plus Environment

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because non-uniform polydisperse TiO2 nanostructures were formed at lower concentration of NH4F (not shown here) while the {001} surface of TiO2 NCs are noticeably eroded at higher concentration of NH4F as witnessed in Figure S3. It is understood that increasing F- can reduce the surface energy below zero and surfaces with negative surface energy cannot maintain a stable morphology, thus the selective etching phenomenon appears on the {001} facets.35 Hence, 30 mmoles of NH4F is found to be an optimum concentration to obtain TiO2 nanocuboids. Further experiments thereafter were conducted with a fixed concentration of NH4F with various amounts of sodium sulfate as a co-additive. As evidenced the cuboid NCs self-assemble into hierarchical hollow porous microspheres with addition of sodium sulfate into the reaction mixture (refer Figure 1(c) and (d)). To investigate the effect of sodium sulfate on the formation of these hollow porous architectures, another series of experiments were performed. At lower concentration of SO42- (i.e., 3.5 mmoles), no obvious difference was observed in shape and size of cuboid NCs as observed in the case of F- environment. With increasing SO42- concentration (7 mmoles), the cuboid NCs self-assembled into large spherical structures with well-defined porous network. It is interesting to note that the cuboid nanocrystals retain their morphology in hollow porous structures, signifying that the SO42- does not damage the initial morphology of cuboid NCs while aiding in the self-assembly process. In order to have a deeper insight on the role of Na2SO4, time dependent experiments were carried out by taking 30 mmoles of NH4F and 7 mmoles of Na2SO4 as structure directing agents (Figure S4). At an initial stage (0 hrs), no distinct shape could be made out post nucleation. As shown in Figure S4(a), reaction at hydrothermal condition for 3 hrs gave rise to TiO2 NCs of cuboid shape with diameter of less than 50 nm, which were confirmed to consist of anatase phase from XRD analysis (showed in Figure S5). It is necessary to discuss here that the synergetic effects of the additional capping agents such as PO43- and (CH3)2CHO- groups are also found to stabilize the {001} by strengthening the Ti-F bond.19,36 It is well documented that both the F- and SO42− anions are considered to preferentially adsorb on anatase {001} to stabilize {001} facets due to the higher density of 5-fold Ti on the (001) ACS Paragon -13Plus Environment


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surfaces.19,37,38 Therefore it is reasonable to believe that the high electronegativity of F- ions can stabilize the {001} facet by lowering the surface energy while SO42− ions act as capping agent that stabilize the shape and size of the {001} facet of anatase at this stage. Due to the large concentration of F- ions in reaction and its structure directing ability, F- ions play a much more crucial role than SO42− ions in the formation of cuboid like-shaped nanostructures in the this stage of the formation process. When the time was increased to 6 hrs and 12 hrs, a combined presence of NCs and spheres composed of NCs were clearly observed (Figure S4(b)-(c)). Interestingly, the self-assembly of NCs is seen to be initiated without showing any obvious change in the cuboid shape. According to Bahnemann et al., TiO2 formed under acidic conditions naturally carry surface positive charge and it shows strong tendency to adsorb SO42-.28,31,39 The adsorbed SO42- further reduce the cuboid NCs as showed in TEM and XRD crystallite measurements and indicates highly energetic surfaces. Under these conditions, TiO2 NCs tends to aggregate in order to minimize their high surface energies yielding a relatively stable spherical architecture. Increasing the reaction time further to 18 hrs (Figure S4(d)), all the NCs is seen to assemble into hierarchical porous structures and then slowly these porous assemblies are finally transformed into hollow porous structures after 24 hrs as shown in Figure 1(c) and (d). Several mechanisms such as the Ostwald ripening, oriented attachment, coarsening effect and solid evacuation (solid-solid in-situ transformation) have been proposed to explain such growth mechanism in the literature.40–42 In the case of hollow porous structures, a subsequent etching process after the NCs agglomeration is probably responsible for the hollow core formation (solid-evacuation). It has been proposed that during the process of self-agglomeration smaller primary particles get entrapped within the core of the secondary hierarchical structure and are more prone to the etching mechanism (dissolution) and they tend to re-deposit on the surface of primary particles during aging thus creating a hollow core.42 In the case of using higher SO42- concentration, macro-porous spheres with irregular pores were obtained devoid of hollow interior nature. This is possibly due to the increasing concentration of SO42ACS Paragon -14Plus Environment

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ions, which replaces the F- from the surface thereby forming the HF in solution under hydrothermal condition. Consequently, the excess F- ions will start to etch or dissolute the surface immediately after formation, and thus induce the irregular porosity in the structure. This observation is consistent with the findings of other investigations in which F- induce dissolution in acidic medium.26,43,44 The process continues until a saturation of F- ions replacement is reached. Further increasing the sodium sulfate in reaction caused deformation of the spherical structures. However, the self-aggregation process is not observed when solely the F--ion is present in the reaction mixture. It is possibly due to the fact that in the fluoride environment, with large numbers of F- are adsorbed on the surface provides effective repulsive forces to stabilize the samples.45 It is also worth mentioning, that neither cuboid like structure nor hollow structures could be obtained with only sodium sulfate as an additive, and the samples obtained were similar to the previous studies reported for formation of TiO2 NCs agglomerates in presence of SO42- ions.31,46 We believe that both the F− and SO42− in the present acidic reaction condition play crucial roles to counter balance the formation of hierarchical structures. Thus, F- ions although determine the shape and size in the initial stages, the SO42− ions plays a key role during the self-assembly process. Hence, an optimum balance of both F− and SO42− ions are crucial for the formation of these hollow porous structures with dominant presence of high energy {001} facets. In addition, a significant fraction of titania particles tends to fuse together in form of dimers and trimers, which is similar to findings in previous studies. The dimerization process should be induced by titania condensation as two separate titania microspheres consisting of Ti–OH group on the surface contact each other.26,43 Evaluating Performances as DSSC photoanodes To evaluate the cell efficiency, performance and enhancement in scattering for the synthesized materials, different combinations of photoanodes were examined. The set of samples made into pastes for photoanode fabrications are: (i) commercial P25 paste as a control or reference material (16µm), (ii) T1, (iii) T2, and (iv) T3 was directly utilized as active layer (16µm). Considering the overall size ACS Paragon -15Plus Environment


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and hierarchical assembly of nanocuboids the mesoporous hollow texture of T2 is expected to offer multi-functionality. Hence, T2 was utilized as light scattering layer in two separate combinations with P25 and T1 as active layer and further evaluated for its light harvesting efficiency. Thus bilayered photoanodes (v) P25 paste as the active layer (10 µm) with T2 paste as the scattering layer (6 µm) (P25+T2) and (vi) T1 paste as the active layer (10 µm) with T2 paste as the scattering layer (6 µm) (T1+T2) were obtained. The films were soaked in N719 dye solution for a day and I-V characteristics were measured under simulated A.M1.5 conditions with a power density of 100mW/cm2 (one sun) using a solar simulator. The typical current-voltage plots are provided in Figure 4(a) and the corresponding photovoltaic parameters are listed in Table-2. The results indicate that the DSSCs made with T1 sample consisting cuboid type NCs with exposed {001} facets shows highest power conversion efficiency (PCE) of 7.25% with a short-circuit current density (Jsc) of 12.3 mA.cm-2, fill factor (FF) of 72% and an open-circuit voltage (Voc) of 0.82V. Whereas the reference DSSCs made with P25 showed the overall PCE of 6.58% with a Jsc of 12.4 mA.cm-2, FF of 69% and Voc of 0.77V, which is lower than that of T1. Although, the Jsc value of P25 was slightly larger than the T1, the substantial increment in VOC and FF leads to an improved overall PCE. In comparison with the P25 and T1, T2 and T3 displays lower PCE of 6.35% and 5.36% respectively, due to the lower short-circuit photocurrent density of 11.4 and 9.96 mA.cm-2, respectively. Nonetheless, the photoanode made with T2 sample consisting of relatively large sized mesoporous hollow spheres shows nearly same power conversion efficiency when compared with P25. It is to be noted that the DSSCs fabricated with T1, T2 and T3 samples based on the pre-evaluation of their respective physical and optical properties (i.e., BET and scattering properties) yields significantly different structural properties and film morphologies. The enhanced Jsc is often considered to be the primary reason for the variation in PCE for all photoanodes, and it is understood that Jsc strongly depends on the dye adsorption capability of the photoanodes. To substantiate, we perform quantitative dye-desorption experiments for each photoelectrode as presented in Figure S6. Interestingly, the dye loading capacity was found to be quite ACS Paragon -16Plus Environment

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similar for both P25 and T1 samples, slightly lower for T2 and least for T3 sample. Although, T1 and T2 samples possess lower surface area (46.36 m2.g-1 and 40.59 m2.g-1, respectively) than P25 (55 m2.g1

), T1 possess similar dye adsorption (loading capacity) to that of P25 while only slightly lower dye

loading capacity is observed for T2. It is often anticipated that the larger particles with lower effective surface area eventually reduce the number of absorption sites on the photoanode the dye molecules to be adsorbed. Nonetheless, the competitive dye loading capacities to that of P25 observed for T1 and T2 can be attributed to the dominant presence of high energy {001} crystal facets in these morphologies. The higher percentage (~ 5-fold) of unsaturated Ti(IV)-atoms available on the exposed surface offers the requisite active sites that makes up for the loss in surface area resulting in favorable dye adsorption.15 In case of T2, the evidently porous texture and hollow nature of these assemblies comprising nanocuboid subunits essentially helps to retain the attributes of T1 with minimal loss in surface area. Understandably, the T3 samples show the lowest dye loading capacity among all samples owing to its lower surface area along with predominant presence of low energy {010} facets. The generated photocurrent is significantly related to the light-harvesting efficiency (LHE) of TiO2 photoanode.47 The LHE of photoelectrode is in turn strongly dependent on the extinction coefficient of the dye, multiple scattering, confinement and absorption of the light along with the transport behavior (charge extraction process) of the photoelectrode film. To estimate the LHE of photoanodes, light scattering effect was investigated using diffuse reflectance studies and the profiles are presented in Figure 4(b). As observed, all the samples show good reflection characteristics beyond the 400 to 450 nm range. The behavior originates due to the large refractive index maximum occurring close to the band edge of the wide band gap semiconductor (TiO2, ZnO, SnO2) leading to maximum reflection at wavelengths in the 400-450 nm spectral range. Apart from that, all the photoanodes except P25 showed much more effective light scattering efficiency especially in 450-800 nm region. The reflectivity of the P25 decreased rapidly past 500 nm while the other photoanodes maintained quite high reflectivity throughout the measurement range, indicating the synthesized photoanodes have ACS Paragon -17Plus Environment


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better light scattering ability than the P25. This is reasonable considering that the light scattering effect increases aided by light trapping phenomena when the particle size is comparable with incident light wavelength (Mie theory). Understandably, the particle dimensions of T2 and T3 can significantly increase the light absorption in longer wavelength region due to optical confinement effects. A close look into the reflectance spectra reveals another interesting feature; the T1 photoanode particualrly shows the high photonic strength in the entire visible range. This high reflectivity of T1 photoanode can be attributed to the top and bottom planes of cuboid NCs (exposed high energy {001} facets) that are possibly aligned in a way, which acts as a reflective mirror in the visible range.48 The results clearly imply that the synthesized samples have good LHE and hence significantly contribute to the overall power conversion efficiency. Even though the T2 photoanode have comparable dye adsorption like P25, the large light scattering efficiency of T2 probably reflects some portion of light before the dye molecule can absorb, and hence reduce the PCE. Based on the above observations considering the size and shape of synthesized hierarchical structures and the marked performance of solely T1 and T2, we adopted a bilayer photoanode structure in order to further boost the power conversion efficiencies. The conversion efficiency of 6.58% for P25 is improved to 7.59% after deposition of T2 scattering overlayer. As shown in Table 1, with the introduction of scattering layer in the photoanode, Jsc of the P25 is also increased from 12.4 to 14.9 mA.cm-2. An overall enhancement of 16% in power conversion efficiency was observed. Concurrently, the electrodes made with double layers, (T1+T2), revealed even higher photo-current density of 15.1 mA.cm-2 with a corresponding conversion efficiency of 8.3%, which is remarkably higher than those of P25 and T1 working electrodes assessed. The power-conversion efficiency (η) is significantly boosted primarily owing to the enhancement in photocurrent density on introducing the additional scattering overlayer. This enhancement in photocurrent density due to better light harvesting offered by T2 scattering layer is also supported by both IPCE (Figure 4(c)) along with diffuse reflectance measurements (Figure 4(b)). To investigate the correlation between device efficiency and the ACS Paragon -18Plus Environment

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measured Jsc values, IPCE spectra were collected for all devices. From Figure 4(c), we can see significant difference at lower wavelength region (300 to 570 nm) for all the single layer photoanodes, T1 and T2 based photoanodes shows much higher IPCE especially in the longer wavelength range from 570–800 nm than P25. This observation is direct fallout of better reflectivity as captured in the characteristic DR-UV spectra of these samples. In fact, their superior light scattering and harvesting ability can be clearly evidenced in IPCE. Introducing a secondary layer composed of as synthesized hollow spheres on a nanocrystalline active layer increases the light harvesting capability owing to enhanced absorption in the available spectral zone. The light scattering phenomenon thus enhances the interaction between incident photons and dye molecules. The high IPCE hence originates from the combination of better dye loading and high light scattering ability of T1 and T2 photoanodes.17,18 Particularly, the double layer structure, (T1+T2) device showed a marked performance enhancement in the longer wavelength range from 570–800 nm over P25+T1 device. It is now well understood that in a solar cell, the competitive kinetics of electron transport and charge recombination have profound influence the photovoltaic performance especially the Voc. It has been reported that the smaller particle size with the large surface area results in more recombination opportunities and an increased inner series resistance of the cell. Whereas in case of larger particles, the narrow interfacial contact between the TiO2 sub-hollow microspheres and FTO glass limits the electron transport (charge extraction) when electron passes through the TiO2 sub-microsphere substrate interface.49 However, it is noticeable that the synthesized samples T1 and T2 samples displays relatively higher Voc (enhanced open circuit potential, ∆VOC = 20 - 50 mV) than the P25, which may be ascribed to considerably reduced charge recombination in these films. Similar observations were reported by Laskova et al.50, where they have put forth a strong hypothesis and rationalized the corresponding shift of flatband potential for the (001) faces/(101) faces as one of the reasons for obtaining positive values of ∆Voc. Electrochemical impedance spectroscopy (EIS) is a powerful technique to investigate the behaviors of electron transport and recombination processes at ACS Paragon -19Plus Environment


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photoanodes. EIS measured at Voc under one sun illumination are summarized in Figure 4(d). Detailed EIS analysis on normalized complex-plane plots obtained at various potentials in both dark and light conditions were also carried out as a function of bias potential applied across the cells. All samples show two distinct regions in the complex-plane plots. The smaller and larger semicircles in the Nyquist plots are attributed to the charge transfer at the counter electrode/electrolyte interface and the TiO2/dye/electrolyte interface, respectively.51,52 The charge recombination resistance was analyzed using the Randles’ equivalent circuit (provided in Figure S7) comprising of several distributed elements appropriately connected in series and parallel that has been used to simulate the data generated in the EIS studies. The charge-transfer resistance associated with recombination of electrons at the TiO2/electrolyte interface (RCt) and charge-transfer resistance for electron recombination from the uncovered layer of the TCO to the electrolyte (RTCO) as a function of bias potential applied across the cells are summarized in Figure S13. As can be seen from the Figure S13, films of T1 and T2 showed relatively higher charge transfer and recombination resistance (Rct) than the film with P25. This is probably due to the single crystalline nature of the cuboid shaped NCs that also acts as building blocks of hollow porous hierarchical architectures. The lower fraction of grain boundaries present along with excellent ordering in the bulk possibly provides a pathway of least resistance for electron transport. Specifically, the hollow porous subunits (T2) shows superior charge transfer as well as higher recombination resistance among all the samples. Assessing Activity for Photocatalytic Oxidation: The underlying principle of photocatalysis is the absorption of photons with energies higher than the band gap of semiconductor in use that subsequently leads to electron (e-)/hole (h+) pair generation. These electron (e-)/hole (h+) pair depending on their lifetime and carrier diffusion length migrates to the surface and effectively reacts with adsorbed species before they recombine. The photodecomposition processes usually involves one or more radicals or intermediate species, such as, •OH and O2•- radical intermediate, which play important roles in the overall photocatalytic reaction ACS Paragon -20Plus Environment

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mechanisms. Therefore, the surface of a catalyst also plays an equally important role in determining the recombination lifetime of the photogenerated e-/h+ pairs. Consequently, the photo-reactivity of a crystal facet must be related to both its surface atomic structure and optical properties. The photocatalytic activity of the as-prepared cuboids shaped NCs and the hollow porous spheres with fluorine-free surfaces (clean anatase cuboids obtained by washing with 0.01M NaOH followed by calcination at 600 °C) were measured and compared with commercial Degussa P25 TiO2 as the control. The optical properties of the synthesized samples were investigated and shown in Figure 5(a). As can be seen from Figure 5(a), T1 and T2 shows the absorption edges at 405 nm and 398 nm which corresponds to band gap of 3.06 and 3.1 eV respectively, and these bandgap values are slightly lower than the theoretical band gap (3.2 eV) of anatase TiO2 (with 101 surface).53 It has been reported that crystals facets with high energy {001} facets possess a smaller band gap than {101} facets.53 The lower band gap of our samples can be ascribed to the different atomic configurations of {001} surface and the percentage of {001} facets in the samples.53 However, the absorbance studies indicates that the absorption edge of T1 and T2 sample shows a blue shift of about 20 nm with respect to those of P25, It indicates in our studies that the T1 and T2 samples have slightly higher band gap energies than the P25. The photoactivity of respective samples was monitored by measuring the formation of active hydroxyl radicals (•OH) upon irradiation using terephthalic acid (TA) as a probe molecule.36,54 TA reacts spontaneously with •OH to generate 2-hydroxyterephthalic (TAOH) acid, which shows a unique fluorescence signal with its emission peak at around 426 nm. Figure 5(b) and Figure S14 displays the fluorescence spectra of T1 and T2, P25 samples respectively, in 3 mM terephthalic acid and 0.01 M NaOH solution at different irradiation times. As observed in the figures, the fluorescence emission intensity increase with irradiation time for all the samples confirming the capability of forming •OH radicals increases with exposure time. The concentration of •OH radical generated from different photocatalyst was normalized with BET surface area as provided in Figure 5(c), and clearly demonstrates the superior photoactivity of the synthesized T1 and T2 over P25. These result suggests ACS Paragon -21Plus Environment


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that theTiO2 nanocuboids and assembled hollow porous structures with predominant presence of {001} facets exhibits a much higher photocatalytic activity than the P25. The observation is consistent with the photocurrent measurements (Figure 5(d)), which shows the photocurrent response of the synthesized samples are better when compared to commercial P25 TiO2 nanoparticles. The as synthesized cuboid TiO2 (T1) exhibits a photocurrent of ~0.39 mA cm-2, whereas the hollow porous TiO2 spheres (T2) showed a photocurrent value of ~0.36 mA.cm-2 both having higher photocurrent density than the P25 (~0.28 mA.cm-2). This higher photocurrent density of synthesized samples indicate that the sample can appreciably separate the photo-generated electron (e-)/hole (h+) pair, which is eventually transfer to the adsorbed water molecule at the interface to generate the •OH radicals. The findings strongly suggest the proposed hypothesis that the anatase TiO2 cuboids possessing predominantly {001} facets can exhibit a considerably higher photocatalytic activity than the P25.

Conclusions In summary, we have successfully demonstrated a novel and facile approach for synthesis of single crystalline anatase nanocuboids enclosed with high energy {001} facets and their mesoporous assembly into hollow spheres. The template-free method allows generating divergent structures of anatase titania with cuboid shaped nanocrystals solely enclosed with high energy reactive {001} facets. Specifically, the strategy showcases the ease of obtaining well-defined mesoporous assemblies of cuboid anatase nanocrystals into hollow spheres. The one-step hydrothermal route using TiCl4 as precursor along with NH4F and Na2SO4 as shape directing agents promises a versatile strategy that can possibly be extended to other metal oxides. The detailed experimental studies, parameter optimization and characterizations reveal the crucial role played by the shape and structure directing agents during the stages of nucleation and growth. Comprehensive characterization of the synthesized morphologies provided critical insights in understanding the mechanism of growth and assembly. Concentration of F-ions plays a determining role in stabilizing the size and shape at the initial stage of formation while an ACS Paragon -22Plus Environment

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optimal balance of SO42- anions is critical in generating the hollow porous assemblies while retaining the morphology of the primary nanocuboid subunits. A combination of highly ordered reactive facets with the unique characteristics of hollow architectures i.e. large surface area, low density, potential for high dye loading capacity, kinetically favorable electrolyte diffusion, strong light scattering and so on, can make them attractive materials for optoelectronic applications in general. To exemplify, we have showcased their application potential as photoanodes of DSSCs and in photocatalytic oxidation. The single crystalline anatase TiO2 nanocuboids and their hollow spherical assemblies; both show substantial enhancement in the photocatalytic and photoelectrochemical activity when compared with commercial P25. Photoelectrodes fabricated using cuboid nanocrystals demonstrate a superior DSSC efficiency compared to Degussa P25. When fabricated in combination with the mesoporous hollow spherical assemblies of nanocuboids as an overlayer, the power conversion efficiency of these photoelectrodes considerably increased owing to enhanced light harvesting. A convergence of key physico-chemical properties, i.e, high surface area, reactive {001} facets, high dye-loading capacities, enhanced light scattering and confinement, low electron transfer resistance, improved electrodeelectrolyte interface, all positively influence the overall photoconversion efficiency. Oxidation of terephthalic acid studied as a model system showed better conversion over the mesoporous hollow spherical substrates when compared to P25. The significantly superior photocatalytic activity mediated by •OH radicals is strongly supported by the impressively high photocurrents measured for these mesoporous hollow structures. Overall, the feasibility studies demonstrate the potential to effectively harness the promising combination of highly ordered reactive facets paired with the unique characteristics of hollow architectures.

Acknowledgements. SI and BR acknowledge the University Grants Commission (UGC), India and Council of Scientific Industrial Research (CSIR), India, and for the Senior Research Fellowship (SRF). PB and SVM duly acknowledges the strong support of MNRE-CSIR TAPSUN Project on Dye

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Sensitized Solar Cells (DyeCell: GAP-0366) and the CSIR XII-FYP Project M2D (CSC-0134) for the grants received. Supporting Information. Additional FESEM and HRTEM images, XRD scans, UV-Vis, fluorescence spectra, and details of impedance analysis are provided. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table 1. Estimated crystallite size and its physico-chemical parameters of the synthesized samples

Sample

Crystallite size

Pore diameter

Pore Volume

dXRD (nm)

(nm)

(cm3.g-1.nm-1)

T1

~35

2.6

0.13

46.4

T2

~44

3.2

0.10

40.6

T3

~24

11.3

0.06

12.4

P25

~21

--

--

55.0


Surface area (m2.g-1)

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


Table 2. Photovoltaic parameters of different DSSCs based on single layer and double layer photoanodes fabricated.

Sample Name

P25

T1

T2

T3

P25+T2

T1+T2

16

16

16

16

10+6

10+6

Voc (V)

0.77

0.82

0.79

0.77

0.75

0.79

Jsc (mA.cm-2)

12.4

12.3

11.49

9.96

14.9

15.1

Fill Factor (FF)

69

72

70

70

68

70

Efficiency (η %)

6.58

7.26

6.35

5.36

7.59

8.35

Thickness (µ µm)



(101)

(a)

(204)

T2

(118)

(200)

(105) (201)

(004)

T3

Intensity (a.u.)

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

Page 34 of 40

T1 10

20

30

40

50

60

70

2Theta (2θ)

Figure 1. (a) XRD patterns of the as-synthesized crystalline anatase titania nanocuboids and their hierarchical assemblies into hollow porous spheres. Corresponding FESEM images of as synthesized (b) TiO2 nanocuboids (T1), (c) hollow and porous microspherical assemblies of these TiO2 nanocuboids (T2), (d) magnified image of (c). Retention of the exposed high energy {001} facets of the cuboid geometry post-coalescence is quite evident in the magnified image. (e) macroporous spheres (T3) formed with increased amount of sodium sulphate depicts a significant loss of crystalline order and oriented attachment.


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


Figure 2. (a) TEM images of cuboid shaped T1 sample, (b) Single cuboid-shaped particle with {001} crystal lattice fringes. The inset shows the FFT pattern of T1, (c) Hollow porous TiO2 spheres composed of NCs subunits, (d) HRTEM image of T2 sample showing the cuboid shaped NCs, (e) corresponding single particle with lattice fringes, (f) TEM image of T3 sample showing the NCs morphology. The data showed in (d) and (f) were obtained from the samples after ultrasonic treatment.


0.016

T1 T2 T3

100

0.014 0.012 0.010 0.008

80

0.006 0.004

60

0.002

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Pore volume (cm3/gm)

0.018

120

3

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

Volume adsorbed (cm /gr)


0.000 10

40

20

30

40

50

Pore diameter (nm)

20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 3. Nitrogen sorption isotherms of the as prepared TiO2 samples (T1, T2 and T3) were carried out to estimate the surface area. Inset shows the pore size distributions calculated using the BJH method. The results are summarized in Table 1.


(a)

12

8

4

0 0.0

P25 T1 T2 T3 P25+T1 T1+T2

0.2

0.4

0.6

Diffuse Reflectance (%R)

-2

16

0.8

100 80 60 40

P25 T1 T2 T3 P25+T2 T1+T2

20 0 300

400

500

600

700

800

40

100

(c)

60 40

P25 T1 T2 T3 P25+T2 T1+T2

20

P25 P25+T2

-Z"(Ohms)

80

0 300

(b)

Wavelength (nm)

Voltage (V)

IPCE (%)

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


Current Density (mA.cm )

Page 37 of 40

T1

T2 T1+T2

(d)

T3

30

20

10

0 400

500

600

700

800

10

20

Wavelength (nm)

30

40

50

60

70

Z'(Ohms)

Figure 4. (a) J–V curves of DSSCs based on different photoanodes; measurements were performed under AM1.5 one sun illumination on an active area of 0.13 cm-2, (b) diffuse reflectance spectra of corresponding photoanodes before dye adsorption, (c) Incident photon to current conversion efficiency (IPCE) spectra of various photoanodes, (d) Nyquist impedance plots of DSSCs on the different photoanodes measured under simulated light at Voc. The symbols in plot (d) represent the experimental data and the solid lines are the results obtained with equivalent circuit fitting.



1.0

Emission Intensity

Absorbance

1000

(a)

0.8

P25 T1 T2

0.6 0.4 0.2 0.0

(b)

800

40 min 30 min 20 min 10 min 0 min

600 400 200 0

300

400

500

600

700

800

350

400

Wavelength (nm)

15 10 5

(c)

0 0

10

20

30

40

Current density (mA.cm-2)

P25 T1 T2

20

450

500

550

600

Wavelength (nm)

25

Emission Intensity

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

Page 38 of 40

0.5 Light on

P25 T1 T2

Light off

0.4

(d)

0.3 0.2 0.1 0.0 0

50

Time (sec)

100

150

200

250

300

Time (sec)

Figure 5. (a) Absorption spectra of different samples depicting the absorption band edge, (b) Fluorescence spectra of T1 sample in 3 mM terephthalic acid and 0.01 M NaOH solution at different irradiation times under simulated light, (c) Normalized fluorescence intensity per unit surface area with different photocatalysts and (d) the photocurrent measurements of the corresponding samples in 1M NaOH solution at 1.2 V normalized against RHE.


Page 39 of 40

TOC Graphical Abstract ----------------------------------------------------------------------------------------------------------------------------

Hierarchical Self-Assembly of Anatase Nanocuboids Emission Intensity

16

-2

J (mA.cm )

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


12 8 4 0 0.0

0.2

0.4

V

0.6

0.8

900 600

100 nm


300 0

400 500 600 Wavelength (nm)

----------------------------------------------------------------------------------------------------------------------------



Hierarchical Self-Assembly of Anatase Nanocuboids Emission Intensity

16

-2

J (mA.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

12 8 4 0 0.0

0.2

0.4

V

0.6

0.8

100 nm

900


600 300 0

400 500 600 Wavelength (nm)

ACS Paragon Plus Environment

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Mesoporous Assembly of Cuboid Anatase ... - ACS Publications

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