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Mar 6, 2017 - Nanowires as Building Blocks Deposited on Fluorine-Doped Tin ... sensitized solar cells (DSSC), where the optimized 3D-TMSAs were used ... Among transition metal oxide semiconductors, the design and ... TiO2 is obtained by the calcinations of anatase phase at high ..... the following possible steps:40.
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Physical Mechanism Behind Enhanced Photoelectrochemical and Photocatalytic Properties of Superhydrophilic Assemblies of 3D-TiO2 Microspheres with Arrays of Oriented, Single-Crystalline TiO2 Nanowires as Building Blocks Deposited on Fluorine-Doped Tin Oxide Subha Sadhu,†,‡ Preeti Gupta,†,‡ and Pankaj Poddar*,†,‡ †

Physical & Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411 008, India Academy of Scientific and Innovative Research, Anusandhan Bhawan, Rafi Marg, New Delhi 110001, India



S Supporting Information *

ABSTRACT: In comparison to the one-dimensional (1D) semiconductor nanostructures, the hierarchical, three-dimensional (3D) microstructures, composed of the arrays of 1D nanostructures as building blocks, show quite unique physicochemical properties due to efficient photon capture and enhanced surface to volume ratio, which aid in advancing the performance of various optoelectronic devices. In this contribution, we report the fabrication of surfactant-free, radially assembled, 3D titania (rutile-phase) microsphere arrays (3D-TMSAs) composed of bundles of single-crystalline titania nanowires (NWs) directly on fluorine-doped conducting oxide (FTO) substrates with tunable architecture. The effects of growth parameters on the morphology of the 3D-TMSAs have been studied thoroughly. The 3D-TMSAs grown on the FTO-substrate showed superior photon-harvesting owing to the increase in light-scattering. The photocatalytic and photon to electron conversion efficiency of dyesensitized solar cells (DSSC), where the optimized 3D-TMSAs were used as an anode, showed around 44% increase in the photoconversion efficiency compared to that of Degussa P-25 as a result of the synergistic effect of higher surface area and enhanced photon scattering probability. The TMSA film showed superhydrophilicity without any prior UV irradiation. In addition, the presence of bundles of almost parallel NWs led to the formation of arrays of microcapacitors, which showed stable dielectric performance. The fabrication of single-crystalline, oriented, self-assembled TMSAs with bundles of titania nanowires as their building blocks deposited on transparent conducting oxide (TCO) substrates has vast potential in the area of photoelectrochemical research. KEYWORDS: titania microsphere self-assembly, solvothermal reaction, superhydrophilicity, DSSC, photocatalyticactivity



INTRODUCTION Among transition metal oxide semiconductors, the design and fabrication of TiO2 nanostructures have been extensively studied for several decades owing to its rich optical, dielectric, catalytic, and antimicrobial properties.1 These promising physicochemical properties of titania led to a variety of uses in solar photovoltaics, photoelectrochemical water-splitting, fuel cells, pigments, paints, sunscreens, antimicrobial surfaces, nanomedicines, superhydrophobic/hydrophilic materials, and so on.2−15 To date, among all the photocatalysts, titania is the optimum one, because of its high oxidizing efficiency, chemical and biological inertness, low cost, and long-term stability.16 The crystalline TiO2 exhibits three polymorphs, i.e., rutile, anatase, and brookite. Among these polymorphs, the rutile is the most thermodynamically stable one, and it possesses higher refractive index (2.6), opacity, and photon scattering efficiency and better photocatalytic properties.1 Usually, due to the higher positive © 2017 American Chemical Society

conduction band edge potential of the rutile, it exhibits less open-circuit potential than anatase phase. However, the electron transport rate is also slower in rutile phase, but the low electron transport rate produces higher electron densities in the conduction band, resulting in an increase in quasi-Fermi level and thus achieves almost similar open circuit voltage potential as anatase.17 The (110) surface of rutile-TiO2 has been extensively studied experimentally for its photocatalytic activity; photocatalytical properties under high vacuum conditions have also been simulated.18 Generally, the rutileTiO2 is obtained by the calcinations of anatase phase at high temperature. Due to the complex chemistry of titania surface, one-step synthesis of template-free rutile titania with specific Received: December 1, 2016 Accepted: March 6, 2017 Published: March 6, 2017 11202

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concentrated (35%) HCl were mixed together and stirred followed by sequential addition of 1 mL of titanium butoxide and 1 mL of titanium tetrachloride to the solution. Before keeping the substrate, the solution was transferred into a 25 mL Teflon vessel and kept inside an autoclave at 180 °C for 2−12 h. Here, the role of HCl is to control the rate of hydrolysis of the precursors to avoid very fast precipitation of titanium hydroxide. The hydrolyzed titanium precursor in the reaction solution plays a major role to determine the shape of the TiO2 crystals. With an increase in the reaction temperature, H+ ions are released from the hydrolyzed precursor and aid in forming hydrated titanyl ions through intramolecular oxolation.24 After that, further condensation of hydrated titanyl ions continues, and they share the opposite edges in the equatorial position and form TiO6 octahedrons. Finally, the rutile crystals are formed by the polymerization of the octahedrons. Rutile structures possess 42 screw axes and usually it crystallizes in 1D structure along the crystallographic c-axis. Moreover, lattice mismatch between the FTO and rutile structure is only ∼2%. Thus, epitaxial growth of rod-shaped rutile titania is favored on FTO substrate.22 In order to minimize the total free energy, the nanorods tried to aggregate into microspheres on (001) plane as the surface energy of the (001) plane is the highest.23 With an increase in the reaction time, the densification of the microspheres takes place. The nanorods possess some defect sites on their edges which further aids the nucleation of newer nanorods; thus, denser 3D-self-assembled microspheres consist of ordered nanorods formed through oriented self-assembly. After synthesis, the substrates were washed and dried in air. To study the effect of precursor, only 2 mL of titanium butoxide and 2 mL of titanium tetrachloride were added individually in two separate reactions.

morphology and size, at relatively lower temperature, and through a simple chemical method, is still difficult. The geometric shape and size of TiO2 shows significant effect in dictating its physical properties; for example, in 1D-single crystalline nanostructures of titania, the charge transport is preferred compare to mesoporous structure because of the reduction in grain boundaries and lattice imperfactions.6,19,20 1D nanostructures are known for slower electron−hole (e-h) recombination rate and faster electron transport,whereas 3D nanoarrays provide larger effective surface area for dye adsorption and excellent photon scattering.21 Due to the combination of micro- and nanometer-scaled building blocks, 3D-hybrid structures, composed of several 1D units, show unique properties in comparison to those of their building blocks. Combination of hierarchal, micro- and nanostructures provide larger surface to volume ratio, which aids to improve the performance for various optoelectronic applications. The immobilization of 1D or 3D structures on solid substrate is required for device fabrication and various applications. Direct hydrothermal growth of titania 3D structures on solid surfaces eradicates the need of postsynthesis fixation of the material for further use. In this contribution, to the best of our knowledge, we have for the first time synthesized oriented template-free superhydrophilic rutile 3D microsphere, composed of bundle of single crystalline nanowires, directly on FTO substrates through a simple low-temperature, surfactant-free hydrothermal method. The morphology of the 3D-TMSAs can be modified with different growth time. The titania precursors have huge effect in directing and controlling the surface topography of the 3D-TMSAs which is also studied in this work. Thus, we have also studied the effect of titanium precursor on the formation morphology of the microspheres. Because of comparatively higher surface area of these hierarchical structures and their superior light-scattering ability in comparison to that of the polycrystalline materials, these structures are preferred for solar cell application.22 The submicrometer area of the 3D-TMSAs is composed of numerous NWs, which is proved to produce effective light scattering as a result of their size compatibility to the wavelength of visible-light spectrum.21 The as-grown 3DTMSA film on FTO substrate possesses superhydrophilic properties without any UV irradiation treatment. These selfassembled 3D-TMSAs also show superior dielectric behavior as the 3D-TMSAs consist of bunches of NWs accompanied by nanoscale boundary cavities which can produce large polarization.23,24 The solar cell performance of the dye-sensitized solar cells (DSSC) fabricated from 3D-TMSAs has also been measured, where the dye-adsorbed 3D-TMSAs were used as photoanode. The photocatalytic activity of the as-synthesized microsphere was also explored for the first time, and we found that the activity remained unaltered for up to three consecutive cycles proving the stability of the material.





RESULTS AND DISCUSSION Structural Studies. The XRD study was carried out to confirm the crystallinity and phase purity of the 3D-TMSAs. All diffraction peaks were found to match with tetragonal rutile phase of TiO2, and no additional diffraction peaks corresponding to anatase was observed. Figure 1 compares the XRD patterns of 3D-TMSAs grown on FTO substrate after

MATERIALS AND METHODS

Materials. Titanium butoxide (TBOT) (purity ≥97%), titanium tetra chloride (TiCl4) purity (≥98%), and 1,4-dioxane were purchased from Sigma-Aldrich Inc. FTO-coated glass and N-719 dye (cisbis(isothiocyanato)bis(2,2′-bipyrridyl-4-4′-dicarboxylato)-ruthenium(II)bis tetrabutylammonium) were received from Solaronix SA Switzerland. Synthesis of 3D-TMSAs on FTO Substrates. Hierarchical 3DTMSAs, in which TiO2 NWs were assembled as nanoflowers, were synthesized by solvothermal method. The FTO substrates were cleaned and dried in N2 flow. Then, 10 mL of dioxane and 1 mL of

Figure 1. Comparison of XRD patterns of 3D-rutile TiO2 microspheres directly grown on FTO substrate at different reaction time and PDF no. 21−1276 (JCPDS, 2011). 11203

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Figure 2. (A, B) SEM images of 3D rutile TiO2 microspheres grown on FTO substrate after 8 h of hydrothermal reaction at different magnifications.

observe any diffraction peaks corresponding to anatase phase. Thus, the as-synthesized 3D-TMSAs are of pure rutile phase. In order to know the details of the formation morphology of the 3D-TMSAs and to get acquainted with the probable growth mechanism, a series of time-dependent growth studies were done for the microspheres. From the SEM images, it was found that after 2 h of reaction, a seed layer started to grow, and the surface coarsened. As the reaction solution becomes supersaturated, a TiO2 seed layer formed on the FTO surface (Figure 3A,B). The formation of these microspheres, as shown in Figure 3G, can be schematically represented as a blooming flower for the sake of visualization in order to understand the nucleation and growth of the titania microspheres on the FTO substrate. After 6 h of reaction, ordered NW-based flower-like 3D-TMSAs with uniform morphology started to blossom (Figure 3C). With prolonged reaction time, the density of the NWs, in the bunch of a microsphere, started to increase, and when the growth time was 8 h, the formation of stacking of spheres on top of the microspheres was initiated, leading to a decrease in the size of the solid core (Figure 3D). As the reaction progresses, the architecture of the microspheres turned from spherical to quasi-spherical. Finally, a hierarchical seaurchin-like structure having an average diameter ∼4 μm is formed (Figure 3E). After 12 h of solvothermal growth, the NWs adhered with each other and produced a textured film (Figure 3F). To further probe the effect of precursor on the synthesis of 3D-TMSAs, titania nanoparticles were synthesized on FTO substrates by hydrothermal reaction after 4 h of reaction in two different syntheses where either Ti(OBu)4 or TiCl4 was used as a precursor. It was seen from the images that when only the TiCl4 is used as a precursor that faceted truncated bypyramidal nanocrystals are formed (Figure S4A,B) whereas an agglomerated film was formed when only Ti(OBu)4 was used (Figure S4C,D). It is known that the rate of hydrolysis of TiCl4 is very fast; thus, it generates in situ hydrochloric acid, which caps {001} surface and reduces the surface energy. As a consequence, the growth of (001) surface with respect to other planes is decelerated so that TiO2 crystals continued to grow along [100] and [010] directions and {001} faceted growth of rectangular parallelepiped is favored.26 Optical Studies. The optical properties of the 3D-TMSAs were studied through photoluminescence (PL) and Raman spectroscopy. Figure S5 shows the room-temperature PL spectra of the 3D-TMSAs grown on FTO for 2 and 10 h of hydrothermal reaction, respectively. The 3D-TMSAs showed strong and broad PL signal, attributed to binding excitons at a

hydrothermal synthesis for varying time scales ranging from 2 to 12 h with the XRD patterns of −FTO (Powder Diffraction File (PDF) no. 21−1276, Joint Committee on Powder Diffraction Standards (JCPDS), 2011). The characteristic peaks of SnO2 were observed in all the samples, proving that 3D-TMSAs were grown directly on the FTO substrates. The intensity of the diffraction peaks increased with prolonged reaction time, indicating the formation of a better crystalline phase over time. Moreover, after analyzing the diffraction patterns, it was found that the intensity of the (002) peak increased by almost three times, i.e., from 10 to 30% compared to that in the standard PDF file, which confirmed that the 3DTMSAs were clustered by bunches of 1D-NWs grown along the 002 direction. From the diffraction pattern, it was observed that the samples grown at 4 h showed a left-handed shift, i.e., toward lower 2θ values, of ∼0.6° for each diffraction peak. Usually, the diffraction peaks shift to lower degree due to the distortion of crystal lattice, but as the shift for each peak is same (∼0.6°), we think it is caused by the sample displacement. Morphological Studies. The morphology of the 3DTMSAs was studied through SEM. Figure 2 shows the SEM images of the 3D-TMSAs grown on FTO after 8 h of reaction at different magnifications. It was observed from these images that the entire substrate was uniformly covered by titania microspheres, indicating a continuous growth. From the highmagnification images, it was confirmed that the bunch of NWs constitutes the 3D-TMSAs. The NWs were found to arrange densely on the surface having a diameter of ∼5 μm with no irregular aggregation. The films adhered homogeneously throughout the substrate, and no cracks were seen on the surface in the SEM images. The 3D-TMSAs were assembled by single-crystalline rutile NWs radially growing in the outward direction from the center of the sphere. From the EDX data (Figure S1), the atomic ratio of Ti to O is found to be 1:2. The image confirms that each microsphere consists of bunch of NWs (diameter ≈ 10 nm). The TEM images also revealed that the 3D-TMSAs were made of loosely packed bunches of thin NWs (Figure S2A−D). To further get acquainted with the microstructure, TEM study of a single thin NW was done (Figure S3). The observed lattice fringes and FFT pattern (Figure S3C,D) confirmed the single-crystallinity of the each NW. The interplanar distances of 0.32 and 0.29 nm corresponding to parallel and perpendicular to the NW wall, respectively, correspond well with the rutile phase of titania.25 It can be noted that interplanar spacing of (004) plane in anatase phase is 0.24 nm, but from the XRD data we did not 11204

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regarded as the asymmetric bending and symmetric stretching of O−Ti−O bonds in the {001} and {110} planes, respectively, due to the movement of O atoms across O−Ti−O bond. Besides an increase in the intensity, the peak height to halfwidth ratio also gets enhanced for TiO2 microspheres grown on FTO substrate for 10 h (Figure S7). This outcome might be considered as an effect of grain size and crystalline properties attributable to phonon confinement behavior.33,34 Surface Area Analysis Using BET. Materials possessing a larger specific surface area are ideal to facilitate light scattering. The surface area of the microspheres was measured through Brunauer−Emmett−Teller (BET) adsorption analysis. The presence of more surface active sites enhances the adsorption capacity of the dye as well as the light-harvesting and -scattering probabilities. According to Brunauer−Demming−Demming− Teller (BDDT) classification, the N2 adsorption−desorption isotherms (Figure 4) of the 3D-TMSAs showed type-IV

Figure 4. N2 adsorption and desorption isotherm of TiO2 microsphere arrays grown on FTO substrate for 2 and 10 h of hydrothermal reaction.

isotherms with H2 type hysteresis loop, signifying the presence of mesopores.35,36 Using BET multipoint method, the specific surface area of the 3D-TMSAs grown on substrate after 2 and 10 h of hydrothermal reaction were found to be 33 and 46 m2/ g, respectively. The surface areas of the controlled Degussa P25 TiO2 powder and as-grown TMSAs with different reaction times were tabulated in Table T1. It was observed that due to the formation of stacking of spheres on top of the microspheres with longer reaction time the surface area increases which also favors the rapid diffusion of charges. The high surface area can provide good electronic conductivity throughout the single crystalline structure and thus results in faster electronic transport, allowing fast diffusion of electrolyte component and increased overall light harvesting efficiency.37 The as-synthesized 3D-TMSAs directly grown on conducting FTO substrates have huge application as photoanode in DSSCs. As the 3D-TMSAs consist of bunches of singlecrystalline NWs, rapid and efficient transfer of electrons takes place from the sensitizer to the collecting conducting substrate through these NWs. In addition, the microspheres are directly grown on FTO substrate, which will effectively pass and scatter the incident light through the backside of the photoelectrode and thus is able to generate more photoelectrons compared to other fabrication methods where the microspheres are not directly grown on the substrates. The UV−vis absorption spectra of the dye molecules adsorbed on the 3D-TMSAs surfaces (Figure S8) showed an absorption peak around 515

Figure 3. (A−F) SEM images of 3D-TMSAs grown on FTO substrates with varying reaction time. (G) Schematic of the solvothermal growth of 3D-TMSAs with reaction time.

range from ∼400 to 600 at 380 nm excitation wavelength.27−29 Corresponding PLE spectra exhibited absorption at 410 nm (Figure S5). The peak at ∼415 nm (∼2.98 eV) is due to interband electron transition in rutile TiO2 NWs from the conduction band to the valence band.28 The characteristic peak of rutile TiO2 at ∼464 nm corresponds to metal−ligand charge transfer.1 The PL emission intensity can be associated with the recombination dynamics of the excited excitons.30,31 Room-temperature Raman spectroscopy was performed to identify the vibrational properties and phase purity of the rutile 3D-TMSAs grown on FTO substrate. The two Raman active fundamental vibration modes, Eg (∼444 cm−1) and A1g (∼610 cm−1), and the second-order effect at ∼244 cm−1, caused by multiple phonon vibration of B1g mode, can be visibly spotted (Figure S6). These Raman active fundamental modes are attributed to the motions of O2− anions with respect to stationary central Ti4+ cation either perpendicular to (B1g and A1g) or along (Eg) the c-axis.32 The Eg and A1g modes are 11205

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of the cell further increased. The better performance of the cell is mainly due to the better adsorption of the dye which can be proved from the increase in photocurrent density. Here it is worth mentioning that titania is also used as a photocatalyst to degrade dyes, but the dyes which are used in DSSC have much better photostability to resist the fast degradation during the sunlight exposure. In addition, unfavorable dye aggregation on the titania surface is avoided through optimization of the molecular structure of the dye.39 Researchers have found that the sensitizer has sustained more than 107 turnovers without significant decomposition since beginning of illumination. One of the major reasons behind the selection of N719 is that it is quite photostable (hence its use for the DSSC) in contrast to the dyes that were tested for the photodegradation studies. Photocatalytic Studies. Photodegradation of Rhodamine B without As-Synthesized 3D-TMSAs. Rhodamine B (RhB) is selected as a representative organic pollutant to demonstrate the photocatalytic performance of the assynthesized 3D TMSAs photocatalyst under UV light irradiation. Figure S10A represents the sequential degradation of the dye without 3D-TMSAs as determined by UV−vis spectroscopy with respect to the irradiation time. As seen from the figure, RhB is not photodegraded to a greater extent. The concentration only decreases by about 10% during the experiment as shown in Figure S10B. The photodegradation mechanism of RhB without as-synthesized 3D-TMSAs includes the following possible steps:40

nm. The absorption spectra of the titania microspheres grown on FTO before dye absorption had also been shown for comparison. The most intense absorption peak was found for microspheres grown on FTO substrate for 10 h compared to that with 2 h of growth. The microspheres have stacking of spheres on top exhibited a strong absorption peak, implying the most successful consumption and effective trapping of incident photons inside the film to increase the absorption. Consequently, a large number of incoming photons will scatter back inside the film, and the light harvesting property of the photoanode will be improved. The roughness and effective surface area of the microspheres increased with the increase in reaction time; thus, the amount of dye adsorption is also enhanced which can also be confirmed from the BET results. The diffused reflectance spectra of 10 h grown titania microspheres also showed an increase in reflectance (Figure S9), indicating higher light scattering ability and an increase in optical path length inside the film due to more light scattering. Photovoltaic Studies. The photocurrent density (JSC) of the titania microsphere, grown on FTO after 10 h of reaction, increased from 6.4 to 9.4 mA cm−2 compared to that with 2 h of growth, whereas there was a small change in the open-circuit voltage (VOC) as the conduction band edge position remains unaltered (Figure 5). The photon to electron conversion

RB + ℏν → RB*

(1)

RB* + O2 → RB+• + O2−•

(2)

O2−• + H+ → OOH•

(3)

O2

RB•+ → Rhodamine → Products

(4)

Here, the RhB dye radical is degraded under UV illumination to form carbon dioxide, mineral acids, and water via rhodamine as an intermediate (eq 4). However, the intermediate, i.e., rhodamine, was not detected in the UV−vis spectroscopy, but a decrease in concentration was observed as shown in Figure S10B. Similar phenomena were observed earlier by Wihelm et.al.40 Though they were unable to detect the rhodamine intermediate by UV−vis spectroscopy, but a decrease in concentration of RhB was observed. In our study, we have also observed the decrease in the RhB concentration with increase in illumination time but were not able to detect the rhodamine peak in UV−vis absorption spectra at 498 nm. It was reported that the degradation of rhodamine through OOH• is very fast compared to the formation of the intermediate through N-de-ethylation. Thus, it is very hard to detect the intermediate. With the use of catalyst, i.e., 3D TMSAs, dye can be degraded to a greater extent which is shown in the next section. Photodegradation of Rhodamine B with As-Synthesized 3D-TMSAs. Titania is an excellent photocatalyst owing to its sufficient positive valence band edge aiding to oxidize the dyes effectively. When the surface of TiO2 is illuminated with photons having more energy than its bandgap, the excitons are formed.41 The electrons and holes in the conduction and valence band respectively are very powerful reducing and oxidizing agents. The electrons reduce the oxygen present at the surface of the catalyst, and the holes oxidize the water to produce hydroxyl radicals. These radicals further degrade the

Figure 5. Current density vs potential curves for DSSC fabricated from TiO2 microspheres grown on FTO substrate after 2 and 10 h of hydrothermal reaction. Schematic in inset represents the as prepared cell for photovoltaic measurements.

efficiency and other photovoltaic parameters of all the solar cells containing titania microsphere anode with different growth times and anodes fabricated with Degussa P-25 were tabulated in Table T2. It seems that the gradual increase in the enhanced photon conversion efficiency of the DSSC fabricated with 3DTMSAs anode is due to the increase in photocurrent density. JSC depends on various factors like the area of the solar cell, the spectrum of the incident light, the electron and hole diffusion lengths, and so on. As the other factors except the surface area of the microspheres remain unchanged and the surface area of the microsphere increases with increases in reaction time, we think that the increase in surface area can cause an increase in dye adsorption and thus increase in Jsc. We have also observed that the pore size distribution of the microspheres synthesized under different conditions varied from 3 to 4 nm. Thus, higher dye loading might be due to the increase in BET surface area. The efficiency of the cells can be improved further by treating the cells either with TiCl4 or NbCl5.19,38 After treating the TiO2 anode with 0.2 M TiCl4 aqueous solution, the average efficiency 11206

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Figure 6. (A) UV−vis absorption spectra, (B) photocatalysis degradation profiles of RhB as a function of time in UV light, (C) photocatalytic degradation of RhB dye up to three cycles, and (D) kinetic plot of photocatalytic degradation of only RhB, Degussa P-25, and titania microsphere. (E) N-de-ethylation of rhodamine B under UV illumination.

dye. In addition to this, OH− groups can also trap more photogenerated holes, thus increasing the electron−hole separation which results in the enhancement of photocatalysis. The photocatalytic activity of as-synthesized TiO2 microspheres was evaluated by measuring the decoloration of RhB aqueous solution as shown in Figure 6. The sequential decoloration of the dye in the presence of TiO2 and the change in concentration as a function of irradiation time up to three cycles is represented in Figures 6A,B respectively. As seen from the figure, without catalyst, the concentration of RhB does not exhibit any significant change, whereas in the presence of as-synthesized titania microspheres, the decoloration is faster. The absorption peak maximum at 552 nm gradually decreases during the UV illumination, and the concentration follows an exponential decay. In presence of as-synthesized titania microspheres under UV illumination, additional reactions can occur:40,42 2TiO2− → TiO2 (e−) + TiO2 (h+)

(5)

RhB + TiO2 (h+) → RB+• + TiO2

(6)

H 2O + TiO2 (h+) → OH• + H+ + TiO2

(7)

The direct excitation of titania through UV irradiation also leads to the cationic dye radical RhB+• and OH• (eqs 5−7). It is well-known from the literature that the complete degradation step of RhB involved the adsorption of the dye onto the titania surface followed by N-de-ethylation (Figure 6E) in the presence of hydroxyl radical.42,43 From the time-dependent UV−vis spectra, it was found that after an irradiation time of 100 min that 79% of the RhB was decolored producing a colorless solution. For comparison with the control sample, the dye-degradation study was also done with Degusaa P-25, and we found that after 100 min of irradiation that 65% of the RhB got decolored (Figure S11). The photocatalyst experiment was performed up to three cycles to verify the sustainability of the as-grown titania microspheres on FTO. A 4 nm shift of the maximum absorption peak toward lower wavelength from 552 to 548 nm was observed due to step-by-step degradation of the RhB to rhodamine through Nde-ethylation of RhB, similar to the earlier result using BiOCl as catalyst.44 It was found that even in the third cycle the degradation capability of the catalyst was almost similar to that of the first cycle (Figure 6C) which proved the stability of the material. Although for each cycle ∼75% of RhB got degraded, we note that for the first cycle the dye degradation is low until 11207

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ACS Applied Materials & Interfaces 60 min. During the course of 60 min of UV irradiation, the dye degrades, but it forms intermediate products and the chromophore does not yet degrade so much (less decoloration). However, with a further increase in time, the intermediate products can further photooxidize and lead to degradation of the chromophore moiety, which leads to faster decoloration and degradation.The degradation of the dye in the presence of UV irradiation is confirmed by the decrease in the intensity of the absorption maximum at 552 nm with time. Additionally, it is known that photodegradation of RhB follows a pseudo-first-order kinetics according to Langmuir−Hinshelwood (L-H) model, which is well-established for heterogeneous photocatalyst at low dye concentration.45 The relevant equation is ln(C0/C) = kt

(8)

where C0 is the initial concentration of the dye, C is the concentration of the dye after illumination time t, and k is the rate constant.46 From the slope of the graph (Figure 6D), the k values for the as-synthesized titania microspheres grown on FTO, Degussa P-25, and only RhB without catalyst were found to be 0.08, 0.3, and 0.05 h−1 respectively Study of Superhydrophilic Properties. The other phenomenon, superhydrophilicity in 3D-TMSAs film, has been studied in detail. Similar to photocatalysis, in photoinduced hydrophilicity electrons and holes are generated. To achieve a hydrophilic surface, the electrons tend to reduce the Ti(IV) cations to the Ti(III) state, and the holes oxidize the O2− anions, creating oxygen vacancies on the surface. Water molecules dissociatively adsorbed on these vacancies produced adsorbed OH groups to produce hydrophilic surfaces. Superhydrophilic surfaces can be used for fabrication of antifogging material, for biomolecular immobilization, for drag reduction, and so on.47,48 Furthermore, by changing the thickness of the film; it can also be used in a solar cell as antireflective coating. To study the wettability of the as-synthesized film, the water contact angle (CA) was measured. The as-grown 3D-TMSAs film on FTO substrate showed superhydrophilicity without UVirradiation treatment. The water CA of the film within 2.5 s after dropping the water droplet was ∼7°. Beyond that, it was not possible to measure the CA as the CA showed extremely small value exhibiting superhydrophilic behavior (Figure 7). To prove the better superhydrophilic effect of the titania microsphere, we have also measured the CA of Degussa P25. The water CA of Degussa P-25 was found to be 12° after dropping a water droplet at 9 s, as shown in Figure S12. Thus, it proves that Degussa P-25 is more hydrophobic than the assynthesized TMSA film. The combination of superhydrophilic surface with good photocatalytic activity makes the 3D-TMSAs film a very good self-cleaning material. The superhydrophylicity of the film without UV radiation might be caused by the presence of dangling bonds and a high concentration of oxygen vacancies at the surfaces.47 The nanosized roughness of the films and the presence of large amount of surface OH− group are also known to increase the wettability.42,47,49 It is also known from literature that surface energy and surface roughness have huge impact on wettability of the surface.50−52 For a hydrophilic surface, the surface tension of solid−vapor interface is larger than that at the solid−liquid interface; thus, solid−liquid contact is favored.50 The Cassie model (a modified version of the Wenzel model) describes the wettability of rough surface by considering surface pores according to the following equation

Figure 7. (A) Water contact angle measurement on TiO2 microspheres grown on FTO substrate after 10 h of hydrothermal treatment. (B) Schematic illustration of synergetic effect of superhydrophilic and photocatalytic mechanism in 3D TMSAs assembly.

cos θ* = f 1 cos θ + f 2

(9)

where θ* is the apparent contact angle and θ is the intrinsic contact angle. f1 and f 2 denote fractions of the solid and area of the droplet in contact with completely filled pores, respectively.51 Complete filling of the pores with water leads to superhydrophilic behavior. Although the mechanism of hydrophilicity and photocatalytic phenomenon are different, the synergetic relation can be observed between them. It is known that for a superhydrophilic surface the Ti−O bond length is larger which can also enhance the photocatalytic oxidation.41 The hefty amount of OH− groups on the surface favors both the phenomena and can also enhance the adsorption of more contamination which will turn the surface hydrophobic, but after photocatalysis, the contaminated organic compounds are decomposed which will again restore the hydrophilic property.53 The XPS study was performed to know the amount of hydroxyl group present on the as-synthesized titania microsphere. Figure S13 shows the O 1s XPS spectra of the titania microsphere and Degussa P-25, respectively. The peaks at 530.5, 531.8, and 533 eV, respectively, correspond to lattice oxygen, Ti−OH, and molecularly adsorbed water.54 It was found that for the as-synthesized titania microspheres the amount of hydroxyl groups is ∼23% compared to that of Degussa P-25 which was ∼12%. We have also measured the FTIR spectra of the as-synthesized titania microspheres and Degussa P-25. In comparison to the FTIR spectra of Degussa P-25, that of the TiO2 microsphere shows a broad O−H stretch between 3200 and 3500 cm−1 which might be due to the presence of more hydroxyl groups (Figure S14). Self-Cleaning Property of 3D-TMSAs Film: Role of Photocatalysis and Superhydrophilicity. Superhydrophilicity introduces an interesting self-cleaning property which is 11208

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ACS Applied Materials & Interfaces indeed important for photovolatic (PV) applications. The transparency of a PV module is reduced by the accumulation of dust and other particle contaminants, thus consequently decreasing the electrical performance of the modules. The superhydrophilic surface of 3D-TMSAs film can be advantageous as it can act as a self-cleaning surface. The self-cleaning property of 3D-TMSAs film can be understood by considering the presence of chemisorbed H2O layer on the film due to its hydrophilicity which further adsorbs water by van der Waals forces and hydrogen bonds.53 The formation of water layers on the surface obstructs the close contact between surface and adsorbed contaminants. The contaminants will decompose due to the photocatalysis effect of titania, and the hydrophilicity as well as the self-cleaning effect will be regenerated, as shown in Figure 7B. Thus, the self-cleaning effect is retained longer due to the synergetic effect of photocatalysis and hydrophilicity.53 Furthermore, the self-cleaning effect can be monitored by studying the degradation of organic dirt, e.g., stearic acid, or decomposition of any colorant on titania surface. We have studied dye degradation along with contact angle measurement. The as-grown titania microspheres showed good photocatalytic property, and from different measurements, the presence of a hefty amount of hydroxyl groups on titania microsphere surface has also been revealed. The titania microsphere film also exposed superhydrophilicity which can be proved from contact angle measurement. Thus, we believe that the as-synthesized titania microsphere have good self-cleaning properties. Dielectric Studies. Titania is a wide bandgap semiconductor (∼3 and 3.2 eV for rutile and anatase, respectively) and thus displays leaky behavior due to space-charge limited conduction in comparison to other oxides. To overcome this leaky behavior and further the material’s use as a gate insulator, usually the thickness of the film is increased or capped with a poly(α-methylstyrene) (PAMS).55 Thus, among simple binary oxides, rutile phase TiO2 is commonly used as gate insulator and capacitive energy storage for its high permittivity among all simple oxides and low dielectric loss.56 Figure 8A,B shows the room-temperature dielectric permittivity and loss tangent as a function of frequency for the TiO2 microspheres grown on FTO substrates for 10 h. As seen from the figure, ε′ strongly depends on the frequency and decreases with an increase in frequency. This behavior can be explained as follows: At low frequency, the dipoles follow the alternating electric field giving a large ε′ which is mainly due to the combined contribution from the interfacial, dipolar, atomic, ionic, and electronic polarization and can be explained by the Maxwell−Wagner effect,57,58 whereas at higher frequency, the dipoles no longer follow the field. Hence, ε′ decreases. The as-synthesized microspheres consist of bunches of aligned 1D-NWs having nanoscale boundary cavities and surface defect dipoles.59 The dielectric constant (ε′) for 3D TMSAs can be closely linked to the nanoscale cavities at the grain boundaries, as indicated by TEM (Figure S1B). These nanocavities in 3D TMSAs act as an insulating barriers which may lead to a relatively high boundary resistance and a large charge carrier accumulation is expected at interfaces. When an external electric field is applied, the carrier conducting path is likely to be blocked by these nanocavities and opposite charges would thus accumulate at two edges of cavities to create a microparallel capacitor as shown at the top right panel of Figure 8B. Thus, the cavities act as a subminiature capacitors in between the parallel wires and together with the internally localized interfacial polarization give large permittivity.59,60 These TiO2 nanoflowers show a permittivity value of 80

Figure 8. (A) Room-temperature dielectric permittivity and (B) loss tangent as a function of frequency for as-prepared TiO2 microspheres grown on FTO substrate after 10 h of hydrothermal reaction. Inset of panel B shows the illustration of parallel plate capacitor formation in 3D-TMSAs when an external electric field (Eeff) is applied.

at 10 Hz with a dissipation factor (tan δ) ≈ 0.4. These microspheres can be used to confine multiple polarons for designing and fabricating better storage devices.61



CONCLUSION In summary, in this contribution, we have shown the fabrication of superhydrophilic, self-assembled, 3D arrays of rutile titania microspheres consisting of self-assembled bunches of singlecrystalline nanowires grown directly on the FTO substrate. To learn the plausible growth mechanism, the effects of time and precursor on the formation of the microsphere have been studied thoroughly. After investigating the optical properties of the as-synthesized microspheres, it was found that the microspheres grown on FTO substrate after 10 h of hydrothermal reaction showed better optical properties due to the presence of stacking layer of spheres on top of microsphere and favor effective light-scattering and harvesting of photons. The facile, one-step, template-free, low-temperature method for synthesis of rutile TiO2 microspheres has huge applications for creation of photocatalyst as well as of photovoltaic solar cells. The microsphere film showed superhydrophilicity prior to any UV irradiation. The as-synthesized microsphere on FTO for 10 h showed significant improvement of photoconversion efficiency (44%) because of the synergistic effect of higher surface area and scattering layer compared to those of Degussa P-25. This current one-pot, surfactant-free synthesis method can also be used for synthesis of various sizes and shapes of metal oxide structure by controlling the various reaction parameters. Moreover, due to the simple synthetic method, low-cost industrial-scale synthesis can also be achieved 11209

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Crystalline TiO2Nanorods on Transparent Conducting Oxide Coated Glass Substrates. RSC Adv. 2013, 3, 1933−1940. (7) Braun, J. H.; Baidins, A.; Marganski, R. E. TiO2Pigment Technology: A Review. Prog. Org. Coat. 1992, 20, 105−138. (8) Pfaff, G.; Reynders, P. Angle-Dependent Optical Effects Deriving from Submicron Structures of Films and Pigments. Chem. Rev. 1999, 99, 1963−1981. (9) Zallen, R.; Moret, M. P. The Optical Absorption Edge of Brookite TiO2. Solid State Commun. 2006, 137, 154−157. (10) Zhang, Z. B.; Wang, C. C.; Zakaria, R.; Ying, J. Y. Role of Particle Size in Nanocrystalline TiO2-Based Photocatalysts. J. Phys. Chem. B 1998, 102, 10871−10878. (11) O'Regan, B.; Grätzel, M. A Low-cost, High-efficiency Solar Cell Based on Dye- sensitized Colloidal TiO2 Thin Films. Nature 1991, 353, 737−740. (12) Zhang, Y. Y.; Ma, X. Y.; Chen, P. L.; Li, D. S.; Yang, D. R. Electroluminescence from TiO2/p+-Si+ Heterostructure. Appl. Phys. Lett. 2009, 94, 061115. (13) Benkstein, K. D.; Semancik, S. Mesoporous Nanoparticle TiO2 Thin Films for Conductometric Gas Sensing on Micro hotplate Platforms. Sens. Actuators, B 2006, 113, 445−453. (14) Zhang, X.; Kono, H.; Liu, Z.; Nishimoto, S.; Tryk, D. A.; Murakami, T.; Sakai, H.; Abe, M.; Fujishima, A. A Transparent and Photo-patternable Superhydrophobic film. Chem. Commun. 2007, 4949−4951. (15) Hosono, E.; Matsuda, H.; Honma, I.; Ichihara, M.; Zhou, H. Synthesis of a Perpendicular TiO2Nanosheet Film with the Superhydrophilic Property without UV Irradiation. Langmuir 2007, 23, 7447−7450. (16) Yu, J.; Ma, T.; Liu, S. Enhanced Photocatalytic Activity of Mesoporous TiO2 Aggregates by Embedding Carbon Nanotubes as Electron-transfer Channel. Phys. Chem. Chem. Phys. 2011, 13, 3491− 3501. (17) Lin, J.; Heo, Y.; Nattestad, A.; Sun, Z.; Wang, L.; Kim, J.; Dou, S. 3D Hierarchical Rutile TiO2 and Metal-free Organic Sensitizer Producing Dye-Sensitized Solar Cells 8.6% Conversion Efficiency. Sci. Rep. 2014, 4, 5769. (18) Sang, Y.; Geng, B.; Yang, J. Fabrication and Growth Mechanism of three-dimensional Spherical TiO2 Architectures Consisting of TiO2nanorods with {110} Exposed Facets. Nanoscale 2010, 2, 2109− 2113. (19) Feng, X. J.; Shankar, K.; Varghese, O. K.; Paulose, M. T.; Latempa, J.; Grimes, C. A. Vertically Aligned Single Crystal TiO2 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis Details and Applications. Nano Lett. 2008, 8, 3781−3786. (20) Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO2Nanorods on Transparent Conducting Substrates for DyeSensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 3985−3990. (21) Liu, Z.; Su, X.; Hou, G.; Bi, S.; Xiao, Z.; Jia, H. Spherical TiO2 Aggregates with Different Building Units for Dye-sensitized Solar Cells. Nanoscale 2013, 5, 8177−8183. (22) Ye, M.; Liu, H.; Lin, C.; Lin, Z. Hierarchical Rutile TiO2 Flower Cluster-Based High Efficiency Dye-Sensitized Solar Cells via Direct Hydrothermal Growth on Conducting Substrates. Small 2013, 9, 312− 321. (23) Zhou, J.; Zhao, G.; Song, B.; Han, G. Solvent-controlled Synthesis of Three-dimensional TiO2 Nanostructures via a One-step Solvothermal Route. CrystEngComm 2011, 13, 2294−2302. (24) Hu, W.; Li, L.; Tong, W.; Li, G. Supersaturated Spontaneous Nucleation to TiO2 Microspheres: Synthesis and Giant Dielectric Performance. Chem. Commun. 2010, 46, 3113−3115. (25) Sun, Z.; Kim, J.; Zhao, Y.; Bijarbooneh, F.; Malgras, V.; Lee, Y.; Kang, Y.-M.; Dou, S. X. Rational Design of 3D TiO2 Nanostructures with Favorable Architectures. J. Am. Chem. Soc. 2011, 133, 19314− 19317. (26) Liu, B.; Aydil, E. S. Anatase TiO2 Films with Reactive {001} Facets on Transparent Conductive Substrate. Chem. Commun. 2011, 47, 9507−9509.

by following this elegant path to fabricate rutile TiO 2 microsphere. The as-synthesized microspheres have remarkable possibility for exploitation in not only photovoltaics or photocatalysis but also hydrogen storage, lithium-ion batteries, photonic crystals, self-cleaning membranes, and design of optoelectronics devices.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15420. Details of characterization techniques, TEM images of TiO2 nanowires, SEM images of rutile 3D-TMSAs grown on FTO substrate using different precursors at different magnifications, PLE−PL, UV−vis absorption spectra, and Raman spectra of 3D-TMSAs, BET tabulated for control and 3D TMSAs, photovoltaic data of P-25 and TiO2, photocatalytic data of P-25 and TiO2, XPS spectra of P-25 and TiO2 and contact angle measurement of P25. (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pankaj Poddar: 0000-0002-2273-588X Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS P.P. acknowledges the Centre for Excellence in Surface Science at the CSIR-National Chemical Laboratory, and network project Nano-Safety, Health & Environment (SHE) funded by the Council of Scientific and Industrial Research (CSIR), India, and the Department of Science & Technology (DST), India, through an Indo-Israel grant to develop materials for solar-voltaic energy devices (DST/INT/ISR/P-8/2011). S.S. acknowledges support from the Council of Scientific and Industrial Research, India (CSIR), for providing the Senior Research Fellowship. P.G. acknowledges the support from the Council of Scientific and Industrial Research (CSIR), India, for providing Senior Research Fellowship (SRF).



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