Physical Mechanism Behind Enhanced Photoelectrochemical and

Subscriber access provided by The Bodleian Libraries of The University of Oxford

Article

The physical mechanism behind enhanced photoelectrochemical & photocatalytic properties of superhydrophilic assemblies of 3D-TiO2 microspheres with arrays of oriented, singlecrystalline TiO2 nanowires as building blocks deposited on FTO Subha Sadhu, Preeti Gupta, and Pankaj Poddar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15420 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 40

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

ACS Applied Materials & Interfaces

The physical mechanism behind enhanced photoelectrochemical & photocatalytic properties of superhydrophilic assemblies of 3D-TiO2microspheres with arrays of oriented, single-crystalline TiO2 nanowires as building blocks deposited on FTO Subha Sadhu1,2, Preeti Gupta1,2 and Pankaj Poddar1,2,*

1

Physical & Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune - 411 008, India,

2

Academy of Scientific and Innovative Research, Anusandhan Bhawan, Rafi Marg, New Delhi110001, India,

Abstract

In comparison to the one-dimensional (1D) semiconductor nanostructures, the hierarchical, three-dimensional (3D) microstructures, composed of the arrays of 1D nanostructures as the building blocks, show quite unique physicochemical properties due to efficient photon capture, enhanced surface to volume ratio, which aid to advance the performance of various optoelectronic devices. In this contribution, we report the fabrication of surfactant-free, radiallyassembled, 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 dye sensitized solar cells (DSSC), where the 1

ACS Paragon Plus Environment


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

optimized 3D-TMSAs were used as an anode, showed around 44% increase in the photoconversion efficiency compare to 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 micro-capacitors, 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 ___________________________________________________________________________ * Corresponding author Email: [email protected]

2


Page 2 of 40

Page 3 of 40

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


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 etc.2-15 To date, among all the photocatalysts, titania is the maximum apposite one, because of its high oxidizing efficiency, chemical, and biological inertness, low-cost and longterm 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, photon scattering efficiency and better photocatalytic properties.1 Usually, due to the higher positive conduction band edge potential of the rutile, it exhibits less open circuit potential than anatase phase. On the other hand, the electron transport rate is also slower in rutile phase. But the low electron transport rate produces higher electrondensities in the conduction-band resulting in an increase in quasi Fermi-level and thus achieve almost similar open circuit voltage potential as anatase.17 The (110) surface of rutile-TiO2 has been extensively studied experimentally for its photocatalytic activity, in addition, photocatalytical properties under high vacuum conditions have also been simulated.18 Generally, the rutile-TiO2 is obtained by the calcinations of anatase phase at high temperature. Due to the the complex chemistry of titania surface, one-step synthesis of template-free rutile titania at relatively lower temperature, through simple chemical method, with specific morphology and size, is still difficult.

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

The geometrical 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 One dimensional 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 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, for the first time, to the best of our knowledge, we have 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 the polycrystalline materials, these structures are preferred for solar cell application.22 The sub-micron area of the 3D-TMSAs is composed of numerous NWs, 4


Page 4 of 40

Page 5 of 40

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


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 3D-TMSA 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 are consist of bunches of NWs accompanied with nanoscale boundary cavities which can produce large polarization.23,24 The solar cell performance of the 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 found that the activity remained unaltered upto three consecutive cycles proving the stability of the material. Materials and method: 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 3D-TMSAs, in which, TiO2 NWs were assembled as nanoflowers, were synthesized by solvothermal method. The FTO substrates were cleaned and dried in N2 flow. Ten mL dioxane and 1 mL concentrated (35%) HCl were mixed together and stirred followed by sequential addition of 1 mL titanium butoxide and 1 mL titanium tetrachloride to the solution. Before 5



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

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 in controlling 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 them to form 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 1-D 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 aid the nucleation of newer nanorods and thus denser 3D-self assembled microspheres consist of ordered nanorods are formed through oriented self-assembly.

After synthesis, the substrates were washed and dried in air. To study the effect of precursor, only 2 mL titanium butoxide and 2 mL titanium tetrachloride were added individually in two separate reactions.

6


Page 6 of 40

Page 7 of 40

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


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. Fig. 1 compares the XRD patterns of 3D-TMSAs grown on FTO substrate after hydrothermal synthesis for varying time-scale ranging from 2 h to 12 h with the XRD patterns of -FTO, and the PDF file # 21-1276. The characteristic peaks of SnO2 were observed in all the samples, proving that the 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 the standard PDF file which confirmed that the 3D-TMSAs 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 ~ 0.6 0 left-handed (i.e. towards lower 2θ values) shift of 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 0 ) ,so we think it is caused by the sample displacement.

Morphological studies:

The morphology of the 3D-TMSAs was studied through SEM. Fig. 2 showed the SEM images of the 3D-TMSAs grown on FTO after 8 h 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 high magnification images, it was confirmed that the 7



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

bunch of NWs constitutes the 3D-TMSAs. The NWs were found to arrange densely on the surface having diameter ~ 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 bunch of thin NWs (Figure S2 (A)-(D)). To further get acquainted with the microstructure, TEM study of a single thin NW was done (Figure S3). The observed latticefringes and FFT pattern (Figure S3(C) & (D)) confirmed the single-crystallinity of the each NW. The interplanar distance of 0.32 and 0.29 nm corresponding to parallel and perpendicular to the NW wall respectively corresponds well with the rutile phase of titania.25 It can be noted that though interplanar spacing of (004) plane in anatase phase is 0.24 nm, but from the XRD data we did not observe any diffraction peaks correspond to anatase phase. Thus the as-synthesized 3DTMSAs are of pure rutile phase.

In order to know the niceties 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 2h of reaction, a seed layer started to grow and the surface coarsened. As the reaction solution becomes supersaturated, TiO2 seed layer gets formed on the FTO surface (Fig. 3(A-B)). The formation of these microspheres, as shown in Fig. 3(G), can be schematically represented as a blooming flower for the sake of visualization in order to understand the nucleation and growth of the titania 8


Page 8 of 40

Page 9 of 40

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


microspheres on the FTO substrate. After 6 h of reaction, ordered NW based flower-like 3DTMSAs with uniform morphology started to blossom (Fig. 3(C)). 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 initiated, leading to decrease in the size of the solid core (Fig. 3(D)). As the reaction progresses, the architecture of the microspheres turned from spherical to quasi-spherical. Finally, hierarchical sea-urchin like structure having average diameter ~ 4 µm are formed (Fig. 3(E)). After 12 h of solvothermal growth, the NWs adhered with each other and produced a textured film (Fig. 3(F)). 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 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, faceted truncated bypyramidal nanocrystals are formed (Figure S4 (A), (B)), whereas, an agglomerated film was formed when only Ti(OBu)4 was used (Figure S4 (C), (D)). It is known that the rate of hydrolysis of TiCl4 is very fast and 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 and TiO2 crystals continued to grow along [100] and [010] directions and {001} faceted growth of rectangular parallelepiped is favoured.26

Optical studies:

The optical properties of the 3D-TMSAs were studied through photoluminescence (PL) and Raman spectroscopy. S5 showed 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 9



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 10 of 40

broad PL signal, attributed to binding excitons at a range from ~ 400 to 600 nm 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 conduction band to 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 to 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 (B1g, A1g) or along (Eg) the caxis.30 The Eg and A1g modes are 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 half-width 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 behaviour.33-34

Surface area analysis using BET:

Materials, possessing larger specific surface area, are ideal to facilitate light-scattering. The surface area of the microspheres was measured through Brunauer–Emmett–Teller (BET) 10


Page 11 of 40

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


adsorption analysis. The presence of more surface active sites enhances the adsorption capacity of the dye as well as the light-harvesting and scattering probability. According to Brunauer– Demming–Demming–Teller (BDDT) classification, the N2 adsorption-desorption isotherms (Fig. 4) of the 3D-TMSAs showed type-IV isotherms with H2 type hysteresis loop, signifying the presence of mesopores.35-36 Using BET multipoint method, the specific surface area of the 3DTMSAs grown on substrate after 2 and 10 h of hydrothermal reaction were found to be 33 and 46 m2/g, respectively. The surface area of the controlled Degussa P-25 TiO2 powder and as-grown TMSAs with different reaction time was 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 into faster electronic transport and also allows fast diffusion of electrolyte component and thus 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 single-crystalline NWs, rapid and efficient transfer of electrons takes place from the sensitizer to the collecting conducting substrate through these NWs. In addition, since 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, 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 absorption peak around 515 nm. The absorption spectra of the titania microspheres 11



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

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 2 h of growth. The microspheres having stacking of spheres on top, exhibited 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 get improved. The roughness and effective surface area of the microspheres increased with the increase in reaction time and 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 thus 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 reaction, increased from 6.4 to 9.4 mA cm-2 compared to 2 h of growth, whereas, there was a small change in the open-circuit voltage (VOC) as the conduction band edge position remains unaltered (Fig.5). The photon to electron conversion efficiency and other photovoltaic parameters of all the solar cells contain titania microsphere anode with different growth time and anode fabricated with Degussa P-25 was tabulated in Table T2. It seems that the gradual increase in the enhanced photon conversion efficiency of the DSSC, fabricated with 3D-TMSAs 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 etc. As the other factors except the surface area of the microspheres remain unchanged and the surface area of the 12


Page 12 of 40

Page 13 of 40

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


microsphere increase with increases in reaction time. So, 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 in different condition 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 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 dye which are used in DSSC have much better photostablity 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 used for the DSSC) in contrast 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 as-synthesised 3D TMSAs photocatalyst under UV-light irradiation, respectively. The Figure S10 (A) represents the sequential degradation of the dye, without 3D-TMSAs determined by UV–vis spectroscopy with respect to the irradiation time. As 13



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 14 of 40

seen from the figure, the RhB is not photodegraded to a greater extent. The concentration only decreases by about 10% during the experiment as shown in Figure S10 (B). The photodegradation mechanism of Rhodamine B without as-synthesized 3D-TMSAs includes the following possible steps:40

RB+hν→RB*...........................................(1)

RB∗ +O2→ RB+• + O2−•.................................(2) O2−• + H+→ OOH•.......................................(3)

RB•+→Rhodamine → Products.............(4)

Here, the RhB dye radical is degraded under UV-illumination to form to 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 S10 (B). 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 compare to the formation of the intermediate through Nde-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.

14


Page 15 of 40

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


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 energy more 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 dye. In addition to this, OH- groups can also trap more photogenerated holes thus, increase 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 Fig. 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 Figs. 6(A) and (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)

15



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 16 of 40

H2O + 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-deethylation (Fig. 6 (E)) 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, 79% of the RhB got decolored, producing a colorless solution. For comparing with the control sample the dye-degradation study was also done with Degusaa P-25 and found that after 100 min of irradiation 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 towards lower wavelength from 552 nm to 548 nm was observed due to step-by-step degradation of the RhB to rhodamine through N-de-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 the first cycle (Fig. 6(c)), which proved the stability of the material. Here it can be noted that though for each cycle ~ 75 % of RhB got degraded but for the first cycle the dye degradation is low until 60 minutes. During the course of 60 min UV irradiation, the dye degrades but it forms intermediate products and the chromophore does not degrade yet so much (less decolouration). However, with further increase in time, the intermediate products can further photooxidize and lead to degradation of the chromophore moiety which leads to faster decolouration and degradation.The degradation of the dye in presence of UV-irradiation is confirmed by the decrease in the intensity of the absorption maximum at 552 nm with time. Additionally, it is 16


Page 17 of 40

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


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( /) = ............................. (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 (Fig. 6 (D)), 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 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 to fabricate antifogging material, for biomolecular immobilization, drag reduction etc.47,48 Further, by changing the thickness of the film; it can also be used in 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 UV-irradiation treatment. The water CA of the film within 2.5 s after dropping the water droplet was ~ 70. Beyond that, it was not possible to measure the CA, as the CA showed extremely small value exhibiting 17



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 18 of 40

superhydrophylic behavior (Fig.7). To prove the better superhydrophylic effect of the titania microsphere, we have also measured the CA of Degussa P 25. The water CA of Degussa P 25 was found to be 120 after dropping water droplet at 9 s and shown in Figure S12. Thus it proves that Degussa P 25 is more hydrophobic than the as-synthesized TMSA film. The combination of superhydrophylic 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 high concentration of oxygen vacancies at the surfaces.47 The nano-sized 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 hydrophilic surface the surface tension of solid vapor interface is larger than solid-liquid interface and thus solid liquid contact is favored.50 Cassie model (a modified version of Wenzel model) describes the wettability of rough surface by considering surface pores according to the following equation

∗ = 1 + 2................................ (9)

where θ* is the apparent contact angle and θ is the intrinsic contact angle. f1 and f2 denote fraction 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 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 18


Page 19 of 40

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


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 showed the O1s XPS spectra of the titania microsphere and Degussa P-25 respectively. The peaks at 530.5, 531.8, and 533 eV respectively corresponds to lattice oxygen, Ti-OH and molecularly adsorbed water respectively.54 It was found that for the as-synthesized titania microspheres, the amount of hydroxyl groups are ~ 23 % compare to the 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 the TiO2 microsphere shows a broad O-H stretch between 3200 – 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 interesting self cleaning property which is indeed important for Photovolatic (PV) application. The transparency of PV module gets reduced by the accumulation of dust and other particle contaminant, thus, consequently decreases the electrical performances 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.53The formations of 19



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 20 of 40

water layers on the surface obstruct 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 Fig. 7(B). Thus, the self-cleaning effect can stay longer due to the synergetic effect of photocatalysis and hydrophilicity.53 Further 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 the dye degradation along with contact angle measurement. The as-grown titania microspheres showed good photocatalytic property and from different measurements, the presence of hefty amount of hydroxyl groups on titania microsphere surface has also been revealed. The titania microsphere film also exposed superhydrophillicity 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 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 Fig.8 (A, B) showed the room temperature dielectric permittivity and loss tangent as a 20


Page 21 of 40

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


function of frequency for the TiO2 microspheres grown on FTO substrates for 10 h. As seen from the figure, the 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 large ε' which is mainly due to the combined contribution from the interfacial, dipolar, atomic, ionic and electronic polarization and can be explained by theMaxwell–Wagner effect.57,58 Whereas, at higher frequency, the dipoles no longer follow the field and hence the ε' deceases. 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 (S1 (B)). 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 micro-parallel capacitor as shown at the top right panel of Fig. 8 (B). Thus, the cavities act as a subminiature capacitors in between the parallel wires and together with the internally localized interfacial polarization gives large permitivity.59,60 These TiO2 nanoflowers show permittivity value of 80 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 device.61

Conclusion

In summary, in this contribution, we have shown the fabrication of superhydrophilic, selfassembled, 3D-arrays of rutile titania microspheres consisting of self-assembled bunches of 21



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 22 of 40

single-crystalline nanowires grown directly on the FTO substrate. To know 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, favors effective light scattering and harvesting of photon. This facile, one-step, template-free, low-temperature method for synthesis of rutile TiO2 microspheres, has huge applications as a photocatalyst as well as a photovoltaic solar cell. The microsphere film showed superhydrophillicity prior to any UV irradiation. The as-synthesized microsphere on FTO for 10 h showed significant improvement of photo conversion efficiency (44%) because of the synergistic effect of higher surface area and scattering layer compare to Degussa P-25. This current one pot surfactant- free, synthesis method can also be used for synthesis of various size and shaped metal oxide structure by controlling the various reaction parameters. Moreover, due to simple synthetic method, low cost industrial scale synthesis can also be achieved by following this elegant path to fabricate rutile TiO2 microsphere. The as-synthesized microspheres have remarkable possibility for exploit not only in photovoltaics or photocatalysis but also in hydrogen storage, lithium ion batteries, photonic crystals, self-cleaning membranes and designing optoelectronics devices.

Acknowledgement

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 & 22


Page 23 of 40

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


Technology (DST), India through and Indo-Israel grant to develop materials for solar-voltaic energy devices (DST/INT/ISR/P-8/2011). S.S acknowledges the 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).

Supporting Informattion

Supporting information contains the 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 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 P 25.

References

1. Zhou, W.; Liu, X.; Cui, J.; Liu, D.; Li, J.; Jiang, H.; Wang, J.; Liu, H. Control Synthesis of Rutile TiO2 Microspheres, Nanoflowers, Nanotrees and Nanobelts via AcidHydrothermal Method and Their Optical Properties. CrystEngComm 2011, 13, 45574563. 2. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. 3. Armstrong, A. R.; Armstrong, G.; Canales, J.; Garcia, R.; Bruce, P. G. Lithium-Ion Intercalation in TiO2-B Nanowires. Adv.Mater.2005, 17, 862-865.

23



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 24 of 40

4. Bang, J.; Kamat, P. Solar Cells by Design: Photoelectrochemistry of TiO2 Nanorod Arrays Decorated with CdSe. Adv. Funct. Mater. 2010, 20, 1970–1976. 5. Wang, H.; Bai, Y.; Zhang, H.; Zhang, Z.; Li, J.; Guo, L. CdS Quantum Dots-Sensitized TiO2 Nanorod Array on Transparent Conductive Glass Photoelectrodes. J. Phys. Chem. C 2010,114, 16451–16455. 6. Sadhu, S.; Jaiswal, A.; Adyanthaya, S.; Poddar, P. Surface Chemistry and Growth Mechanism of Highly Oriented, Single 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. Oregan, B.; Gratzel, 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, 1125-1127. 13. Benkstein, K. D.; Semancik, S. Mesoporous Nanoparticle TiO2 Thin Films for Conductometric Gas Sensing on Micro hotplate Platforms. Sens. Actuators, B, Chemical 2006, 113, 445-453. 24


Page 25 of 40

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


14. Zhang, X.; Kono, H.; Liu, Z.; Nishimoto, S.; D. A. Tryk,; Taketoshi, M. H.; Sakai, H.; Abe, M.; A. Fujishima. 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 and Dou, S. 3D Hierarchical Rutile TiO2 and Metal-free Organic Sensitizer Producing Dye-Sensitized Solar Cells 8.6% Conversion Efficiency. Scientific Reports 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 Letters 2008, 8, 3781-3786; 20. Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO2Nanorods on Transparent Conducting Substrates for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009,131, 3985-3990.

25



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 26 of 40

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. Cryst Eng Comm. 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. Rational Design of 3D TiO2 Nanostructures with Favorable Architectures. J. Am. Chem. Soc. 2011, 133, 1931419317. 26. Liu, B. ; Aydil, E. S. Anatase TiO2 Films with Reactive {001} Facets on Transparent Conductive Substrate. Chem. Commun.2011, 47, 9507-9509. 27. Liquiang, J.; Xiaojun, S.; Baifu, X.; Baiqi, W.; Weimin, C.; Honggang, F. The Preparation and Characterization of La doped TiO2 Nanoparticles and Their Photocatalytic Activity J. Solid State Chem. 2004,177, 3375-3382. 28. Yang, L.; Zhang, Y.; Ruan, W.; Zhao, B.; Xu, W.; Lombardi, J. Improved SurfaceEnhanced Raman Scattering Properties of TiO2 Nanoparticles by Zn DopantJ. Raman Spectrosc. 2010, 41, 721-726.

26


Page 27 of 40

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


29. Sadhu, S.; Poddar, P. Growth of Oriented Single Crystalline La-doped TiO2Nanorod Arrays Electrode and Investigation of Optoelectronic Properties for Enhanced Photoelectrochemical Activity RSC Adv. 2013, 3, 10363-10369. 30. Wu, J. M.; Shih H. C.; Wu, W. T. Formation and Photoluminescence of SingleCrystalline

Rutile

TiO2

Nanowires

Synthesized

by

Thermal

Evaporation.

Nanotechnology 2006, 17, 105-109. 31. Naphade, R.; Tathavadekar, M.; Jog,J.; Agarkar S.; Ogale, S.Plasmonic Light Harvesting of Dye Sensitized Solar Cells by Au-nanoparticle Loaded TiO2 Nanofibers J. Mater. Chem. A 2014, 2, 975–984. 32. Zhang, Y.; Harris. C. X; Wallenmeyer, P.; Murowchick, J.; Chen, X.; Asymmetric Lattice Vibrational Characteristics of Rutile TiO2 as Revealed by Laser Power Dependent Raman Spectroscopy J. Phys. Chem. C 2013,117, 24015-24022. 33. Ameen S.; Akhtar, M. S.; Kim, Y. S.; Shin, H. S.; Controlled Synthesis and Photoelectrochemical Properties of Highly Ordered TiO2 Nanorods. RSC Adv. 2012, 2, 4807-4813. 34. Cheng, H.; Ma, J.; Zhao, Z.; Qi, L.; Hydrothermal Preparation of Uniform Nanosize Rutile and Anatase Particles Chem. Mater. 1995, 7, 663-671. 35. Yu, J.; Fan, J.; Cheng, B.; Dye-sensitized Solar Cells Based on Anatase TiO2 Hollow Spheres/carbon Nanotube Composite Films J. of. power. Sources. 2011, 196, 7891-7898. 36. Sadhu, S.; Poddar, P. Template Free One Pot Synthesis of Oriented Single Crystalline1D Rutile TiO2-MWCNT Composite for Enhanced Photoelectrochemical Activity J. Phys. Chem. C 2014, 118, 19393-19373.

27



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 28 of 40

37. Lin, J.; Zhao, L.; Heo, Y.; Wang, L.; Bijarbooneh, F.; Mozer, A.; Nattestad. A.; Yamauchi. Y.; Dou, S. and Kim, J. Mesoporous Anatase Single Crystals for Efficient Co(2+/3+)-based Dye-Sensitized Solar Cells. Nano Energy 2015, 11, 557-567. 38. Wang, J.; and Lin, Z. Dye-Sensitized TiO2 Nanotube Solar Cells with Markedly Enhanced Performance via Rational Surface Engineering. Chem. Mater. 2010, 22,579584. 39. Hagfeldt, A.; Boschloo, L; Sun, L; Kloo, L; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110 , pp 6595–6663. 40. Wilhelm, P., Stephan, D. Photodegradation of Rhodamine B in Aqueous Solution via SiO2@TiO2 Nano-spheres. Journal of Photochemistry and Photobiology A: Chemistry 2007, 185, 19–25. 41. Hashimoto, K.; Irie,H.; Fujishima, A; TiO2Photocatalysis: A Historical Overview and Future Prospects. Jpn. J. Appl. Phys. 2005, 44, 8269-8285. 42. Watanabe, T.; Takizawa, T.; Honda,K. Photocatalysis through Excitation of Adsorbates. Highly Efficient N-Deethylation of Rhodamine B Adsorbed to CdS. J. Phys. Chem. 1977 ,81, 1845. 43. Schmid, G. Nanoparticles, Wiley-VCH, Weinheim, 2004. 44. Biswas, A.; Das, R.; Dey, C.; Banerjee, R.; P. Poddar. Ligand-Free One-Step Synthesis of {001} Faceted Semiconducting BiOCl Single Crystals and Their Photocatalytic Activity. Cryst. Growth Des. 2014, 14, 236–239. 45. Sarkar, D.; Ghosh, C. K.; Mukherjee, S.; Chattopadhyay, K. K.; Three Dimensional Ag2O/TiO2 Type-II (p−n) Nano hetero junctions for Superior Photocatalytic Activity ACS Appl. Mater. Interfaces 2013, 5, 331–337. 28


Page 29 of 40

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


46. Hazra, C.; Samanta, T.; Asaithambi, A. V.; Mahalingam. V. Bilayer stabilized Ln3+doped CaMoO4Nanocrystals with High Luminescence Quantum Efficiency and Photocatalytic Properties. Dalton Trans, 2014, 43, 6623–6630. 47. Zhang, L.; Zhao, N.; and Xu, J.; Fabrication and Application of Superhydrophilic Surfaces: a ReviewJ. Adh. Sci. technol., 28: 8-9 769-790. 48. Huang, W. ; Chen, Y. ; Yang, C. ; Situ, Y. ; Huang, H.; pH-driven Phase Separation: Simple Routes for Fabricating Porous TiO2 Film with Superhydrophilic and Anti-fog Properties. Ceramics International. 2015, 41, 7573-7581. 49. Han, Y.; Wu, G.; Wang, M.;

Chen, H. The Growth of a c-axis Highly Oriented

Sandwiched TiO2 Film with Superhydrophilic Properties without UV Irradiation on SnO:F Substrate Nanotechnology 2009, 20, 235605 (6pp). 50. Bico, J.; Thiele, U; Que´re, D. Wetting of textured surfaces. Colloids and Surfaces A: Physicochem. Eng. Aspects 2002, 206, 41–46. 51. Jarn, M. ; Xu, Q. ; Linden, M. Wetting Studies of Hydrophilic-Hydrophobic TiO2 @ SiO2 Nanopatterns Prepared by Photocatalytic Decomposition. Langmuir 2010, 26, 1133011336. 52. Zhang, X.; Jin, M. Liu, Z.; Nishimoto, S.; Saito, H.; Murakami, T.; Fujishima, A.Preparation

and

Photocatalytic

Wettability

Conversion

of

TiO2-Based

Superhydrophobic Surfaces. Langmuir 2006, 22, 9477-9479. 53. Guan, K. Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO2/SiO2 films. Surface & Coating Technology 2005, 191, 155-160. 54. Ketteler, G. ; Yamamoto, S.; Bluhm, H. ; Andersson, K. ; Starr, E.; Ogletree, D. F.; Ogasawara, H. ; Nilsson, A. and Salmeron, M. The Nature of Water Nucleation Sites on 29



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 30 of 40

TiO2 (110) Surfaces Revealed by Ambient Pressure X-ray Photoelectron Spectroscopy J. Phys. Chem. C 2007, 111, 8278-8282. 55. Majewski, L. A.; Schroeder, R.; and Grell, M. Low-Voltage, High-Performance Organic Field-Effect Transistors with an Ultra-Thin TiO2 Layer as Gate Insulator Adv. Functional Material 2005,15,1017-1022. 56. Wanbiao, H.; Liping. L.; Wenming, T.; Guangshe, Li.; Supersaturated Spontaneous Nucleation to TiO2 Microspheres: Synthesis and Giant Dielectric Performance. Chem. Commun. 2010, 46, 3113–3115. 57. Intatha, U.; Eitssayeam, S.;Wang J.; Tunkasiri, T. Impedance Study of Giant Dielectric Permittivity in BaFe0.5Nb0.5O3 Perovskite Ceramic, Curr. Appl. Phys. 2010, 10, 21. 58. Gupta, P.; Poddar, P. Using Raman and Dielectric Spectroscopy to Elucidate the Spin Phonon and Magnetoelectric Coupling in DyCrO3 Nanoplatelets. RSC Adv. 2015, 5, 10094-10101. 59. Thongbai, P.; Tangwancharoen, S.; Yamwong T.; Maensiri, S. Dielectric Relaxation and Dielectric Response Mechanism in (Li, Ti)-doped NiO Ceramics. J. Phys.: Condens. Matter 2008, 20, 395227-1-11. 60. Maensiri, S.; Thongbai. P.; Yamwong, T. Giant dielectric response in (Li, Ti)-doped NiO Ceramics Synthesized by the Polymerized Complex Method. Acta Mater.2007, 55, 28512861. 61. Hu, W.; Li, L.; Tong, W.; Li, G.; Wan, T. Tailoring the Nanoscale Boundary Cavities in Rutile TiO2 Hierarchical Microspheres for Giant Dielectric Performance. J.Mater. Chem. 2010, 20, 8659-8667.

30


Page 31 of 40

12 h 10 h 8h

Intensity (a.u.)

6h

4h

2h

10

(002) (310) (221) (301) (112) (311) (320) (202)

(211) (220)

(101) (200) (111) (210)

FTO (110)

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


20 30 40 50 60 70 80 2θ (degree)

Fig. 1: Comparison of XRD patterns of 3D-rutile TiO2 microspheres directly grown on FTO substrate at different reaction time and PDF file # 21-1276.

31



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

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

32


Page 32 of 40

Page 33 of 40

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


Fig. 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

33



10 h 2h

60

3

Vol adsorbed (cm /g STP)

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

50 40 30 20 10 0 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/Po)

1.0

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

34


Page 35 of 40

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


. Fig. 5: Current density vs. potential curves for dye sensitized solar cell fabricated from TiO2 microspheres grown on FTO substrate after 2 and 10 h of hydrothermal reaction. Schematic at the bottom left corner represent the as prepared cell for photovoltaic measurements.

35



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

Fig. 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.

36


Page 36 of 40

Page 37 of 40

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


Fig. 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.

37



.

2.0 (B) Loss tangent

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

1.5 1.0 0.5 0.0 0

10

10

1

2

3

4

10 10 10 Frequency (Hz)

10

5

6

10

Fig. 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. Top right of panel (B) shows the illustration of parallel plate capacitor formation in 3DTMSAs when an external electric field (Eeff) is applied 38


Page 39 of 40

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


TOC (Graphics):

Multifunctional 3D TMSA assembly showing self-cleaning, photocatalytic and photovoltaic application.

39



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 40 of 40

TOC (Graphics):

O2eH2O

H+

O2

TiO2

OH-

Multifunctional 3D TMSA assembly showing self-cleaning, photocatalytic and photovoltaic application.

1


Physical Mechanism Behind Enhanced Photoelectrochemical and

Recommend Documents