Template-Free Fabrication of Highly-Oriented Single-Crystalline 1D

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Template-Free Fabrication of Highly-Oriented Single-Crystalline 1D-Rutile TiO-MWCNT Composite for Enhanced Photoelectrochemical Activity 2

Subha Sadhu, and Pankaj Poddar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5023983 • Publication Date (Web): 16 May 2014 Downloaded from http://pubs.acs.org on May 30, 2014

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The Journal of Physical Chemistry

Template-Free Fabrication of Highly-Oriented Single-Crystalline 1D-Rutile TiO2MWCNT Composite for Enhanced Photoelectrochemical Activity Subha Sadhu1,2 and Pankaj Poddar1,2,3*

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 Delhi-110001, India,

3

Center of Excellence on Surface Science, CSIR-National Chemical Laboratory, Pune – 411008, India

Abstract

Template-free synthesis of phase pure one-dimensional (1D), single crystalline rutile titania nanorods or wires at low temperature still remains a challenging task due to its complex nature of surface chemistry. In these 1D structures, charge transport is highly favoured. To further modify the electrical conductivity and optoelectronic properties of these 1D nanostructures, various methods such as doping of TiO2 with metal and non-metal, synthesis of branched and hybrid structures are developed. If these hybrid structures can directly synthesize on substrate, the transport of electron will improve due to reduce grain boundary and exciton recombination. In this contribution, for the first time, we have simultaneously synthesized 1D-rutile TiO2-multiwalled carbon nanotube (MWCNT) composite film directly grown on fluorine dope conducting oxide (FTO) substrate along-with 1D-rutile TiO2MWCNT composite powder. The as-grown nanorods films were single-crystalline and oriented vertically with respect to the substrate, having average height of ~ 2 µm. The well connected network of TiO2 with MWCNTs was observed through electron microscopy. The 1 ACS Paragon Plus Environment


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composite film shows positive movement of the flat-band edge and increase in charge carrier density. The TiO2-MWCNT composite was successfully used as photoanode in a dye sensitized solar cell (DSSC) and exhibits a 60% increase in energy-conversion efficiency compared with only TiO2 nanorods. Keywords: 1-D TiO2, composite, charge transfer, dye sensitized solar cell. ___________________________________________________________________________ * Corresponding author Email: [email protected]

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Introduction For several decades, among transition metal oxide semiconductors,TiO2 is widely used in both research and industrial field due to its diverse application in solar cell, photoelectrochemical water splitting, fuel cells, pigment, paint, sunscreen etc.1-12 Due to its rich properties TiO2, in various polymorphic forms, morphologies and doping, remain one of the most favourite compound till date for material scientists, engineers, physicists as rich applications span from energy, health, environment, devices, consumer good etc. It is well known that crystalline TiO2 can exist in three polymorphs i.e. rutile (P42/mnm), anatase (I42/amd) and brookite (Pcab). Among these polymophs, rutile is thermodynamically more stable and exhibits higher refractive index (2.609) than anatase.13 Among all binary oxides rutile TiO2 has high dielectric constant (k ~115) where as for anatase TiO2 has moderate dielectric constant (k ~50). Materials exhibiting large dielectric properties are extensively used in electronic devices and energy storage such as stabilizing the electric grid, integration of variably renewable power sources, imbedded capacitors and microelectro-mechanical systems. Thus, rutile TiO2 is preferred for various applications such as power-circuits, dielectric-ceramics, capacitors, Li-ion battery materials etc.14,15 Template free synthesis of pure rutile phase TiO2 nanostructures with control over size, shape and crystallinity at low temperature still remains a challenging task due to its complex nature of surface chemistry. As geometrical shape of TiO2 plays an important role in dictating its physical properties so researchers have tried to synthesize various one-dimensional (1D) TiO2 structures such as, nanorods, nanotubes, nanowires etc.16 In these 1D-structures charge transport is highly favoured due to less grain boundary and high aspect ratio. The potential use of singlecrystallite 1D structure is also supposed to abridge the electron transport pathway and thus



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enhance the probability of electrode accessibility to hole-transporting materials.17 1D structure can also scatter light effectively and thus competent for better light harvesting.18 To further improve the charge transport properties and thus the performance of associated devices, various methods such as doping of TiO2 with metal and non-metal, synthesis of branched and hybrid structures are developed.19-22 Carbon nanotubes (CNT) are used as semiconductor support due to their unique electrical and electronic properties and wide electrochemically stable window. Kongakanad et.al. fabricated single walled carbon nanotube (SWCNT) on conducting carbon fibre and glass electrode in order to disperse TiO2 nanoparticles in nanowire network and thus improve the photoelectrochemical performance.23 Later they also explained the mechanism of superior photon harvesting ability of SWCNT through electron charging and discharging property.24 Moreover, CNTs also have high surface area, good adsorption, mechanical and thermal properties.25 At room temperature, CNT can conduct electrons with almost no electrical resistance.26 Besides, due to the ease of surface functionalization and superior electronic properties of multiwalled carbon nanotube (MWCNT), incorporation of MWCNTs into TiO2 has attracted the attention of researchers to fabricate various optoelectronic devices.27,28 The TiO2-MWCNT composites are also supposed to produce synergetic effect between the metal oxide and MWCNTs. Several studies have been done to synthesize and understand physicochemical properties of TiO2-MWCNT nanocomposites. For example, Lee et.al. have synthesized TiO2 porous film along with various amount of MWCNTs through sol-gel method and measured charge transport resistance.27 Peining et.al. have fabricated rice-grain shaped TiO2-CNT nanocomposites by electrospinning and found 32% enhancement in the energy conversion efficiency in dye-sensitized-solar-cells (DSSC).29 If these composite structures can be directly grown on substrate, the transport of electron will be far better due to reduce grain boundary and exciton recombination. Transport 4 ACS Paragon Plus Environment

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and recombination studies on the oriented nanotubes showed smooth charge transport while suppressing recombination.30 In this contribution, for the first time, we report a one-step fabrication of 1D-rutile (1D-R) TiO2-MWCNT composite film directly grown on fluorine dope conducting oxide (FTO) substrate along with 1D-R-TiO2-MWCNT composite powder. The nanorods in as-grown nanorod-films were single-crystalline and oriented vertically with respect to the substrate. The as-synthesized TiO2-MWCNT composite powders were rod shaped with average length of ~ 2 µm. The composite was synthesized by hydrolysis of Titanium butoxide precursor in a mixture of water and acid solution in the presence of carboxylic acid functionalized MWCNT where it is assumed that the formation of TiO2 and its anchoring with MWCNT to form composite happened almost simultaneously via the polar oxygenated groups present in MWCNT. To the best of our knowledge, there is no study of synthesis of rutile TiO2 nanorods-MWCNT composites. The phase purity of the assynthesized material were proved through X-ray diffraction (XRD) and Raman study while the inclusion of MWCNTs into TiO2 were confirmed through X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis (TGA). The well connected network of TiO2 with MWCNT was observed through FESEM and TEM micrographs. Furthermore, the optoelectronic properties of the as-synthesized composites have been studied in detail through various experiments such as UV-Vis-DRS, photoluminescence spectroscopy and electrochemical impedance measurement. The beneficial effect of MWCNTs addition in the charge carrier density of TiO2 nanorods was calculated through Mott-Schotky (M-S) measurements. Finally, the positive influence of intrusion of MWCNTs to improve photoelectrochemical performance was measured after constructing a liquid junction DSSC and the efficiency of the cell was increased ~ 60% compared with only TiO2 nanorods.



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Materials and method Materials All chemicals were of analytical grade and used as-received without any further purification, unless otherwise described. Titanium butoxide (TBOT) (purity ≥97%) and carboxylic acid functionalized MWCNT (average diameter and length 9.5 nm and 1.5 µm respectively) were purchased from Sigma Aldrich INC. FTO coated glass (fluorine doped tin oxide, sheet resistance = 8 ohm/square) substrate (FTO:Glass) and N-719 dye (cisbis(isothiocyanato)bis(2,2′-bipyrridyl-4-4′-dicarboxylato)-ruthenium(II)bistetrabutylammonium) were received from Solaronix SA Switzerland . (A) Synthesis of TiO2 and TiO2- MWCNT composites TiO2 nanorods and 1D-TiO2-MWCNT composites were synthesized through a simple hydrothermal method. First the substrates were ultrasonically cleaned in a mixed solution of deionised water, acetone and isopropanol and then dried in N2 flow. A mix of 30 mL H2O and 30 mL concentrated (35%) HCl was stirred for 10 min in a beaker followed by the addition of 1 mL titanium butoxide and 5 mg of MWCNT. The solution was then sonicated for 2 h and transferred into a 100 mL Teflon vessel and the substrates were placed horizontally within the Teflon vessel. Then the vessel was loaded inside an autoclave and heated at 180 oC for 20 h for hydrothermal reaction. After synthesis, the substrates and the resulted product were washed vigorously with DI water and then dried in air to produce greyish TiO2-MWCNT film and powder respectively. A schematic of the synthesis method has been shown in Fig. 1. To synthesize TiO2 nanorods same method was followed without adding MWCNTs.


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(B) Fabrication of DSSC To fabricate a solar cell, at first, the as-synthesized TiO2-MWCNT composite film on FTO substrate was annealed in air at 400 °C for 30 min. Then it was cooled to 80 °C and was immersed in a 0.5 mM ethanolic solution of N-719 dye for 24 h in order to enable sufficient adsorption of dye which will act as the sensitizer in the presence of light. After complete dye adsorption, the substrate was washed very carefully with ethanol and dried for 1 h. After that, a platinum coated FTO substrate, which will be used as the counter electrode, was placed above the dye adsorbed FTO substrate and an electrolyte containing 0.6 M 1-hexyl-2-3dimethylimidazolium iodide, 0.1 M LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine in methoxyacetonitrile was injected into the space between the anode and the cathode to complete the assembly of the DSSC. (C) Characterization techniques In order to confirm the crystalline phase of TiO2 nanorods, X-ray diffraction (XRD) study was performed using a PANalytical X’PERT PRO instrument and the iron-filtered CuKα radiation (λ = 1.5406 Å) in the 2θ range of 10-80° with a step size of 0.02°. To further confirm the crystalline phase and formation of composite material, Raman spectroscopy measurements were recorded at room temperature on an HR 800 Raman spectrophotometer (Jobin Yvon, Horiba, France) using monochromatic radiation (achromatic Czerny-Turner type monochromator with silver treated mirrors) emitted by a He-Ne laser (633 nm), operating at 20 mW and with accuracy in the range between 450 nm and 850 nm ± 1 cm–1, equipped with thermoelectrically cooled (with Peltier junctions), multi-channel, spectroscopic grade charge coupled detector (1024×256 pixels of 26 microns) with dark current lower than 0.002 electrons pixel-1 s-1. An objective of 50 X LD magnification was used both to focus and collect the signal from the samples. X-ray photoelectron spectroscopy (XPS) was done with an ESCALab spectrometer having Al kα x-ray source (hν =1486.6 eV) operating at 150 W



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using a Physical Electronics 04-548 dual Mg/Al anode and in a UHV system with a base pressure of ≤5 × 10−9 Torr. Both the incident and take off angles are 55o with respect to the surface normal. The analysis depth was around 10 to 15 Å. The spectra were energy-analyzed with an Omicron Nanotechnology EA 125 Hemispherical Energy Analyzer in pulse-count mode with pass energies of 50 eV for broad scan data and 25 eV for specific, narrow-range core-level transitions. The XPS data were curve-resolved using XPSPEAK 4.123 after removal of a Shirley background. The curve resolved spectra were fit with the minimum number of peaks needed to reproduce the spectral features using a Gaussian−Lorentzian product function. In order to study the thermal decomposition profile of the composite thermo gravimetrical analysis (TGA) was done using SDT model Q600 of TA Instruments Inc. USA at a heating rate of 10 °C/m under air flow at 100 mL/m. To analyze the shape and size of the synthesized nanorods field emission scanning electron microscopy (FESEM: Hitachi S-4200) was done. The specific structure details and morphology of the prepared nanocomposite particles were obtained by using FEI Tecnai F30 high resolution transmission electron microscope (HRTEM) equipped with a super-twin lens (s-twin) operated at 300 keV accelerating voltage with Schottky field emitter source with maximum beam current (> 100 nA) and small energy spread (0.8 eV or less). The point to point resolution of the microscope is 0.20 nm and line resolution of 0.102 nm with a spherical aberration of 1.2 mm and chromatic aberration of 1.4 mm with 70 µm objective aperture size. The powders obtained were dispersed in hexane and then drop-casted on carbon-coated copper TEM grid with 200 mesh and loaded to a single tilt sample holder. Optical properties of the as-synthesized TiO2 nanorods were investigated by UV-visible spectrophotometer. UV-Vis-DRS spectroscopy measurements were performed on a Jasco UV-Vis-NIR (Model V570) dual beam spectrometer operated at a resolution of 2 nm. PL spectra were acquired using a Fluorolog Horiba Jobin Yvon fluorescence spectrophotometer,


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equipped with a 400 W Xe lamp as an excitation source and a Hamamatsu R928 photomultiplier tube (PMT) as a detector. M-S measurements were obtained in 3M KCl solution at 298 K at frequency 10 KHz using a Solartron S1287 Electrochemical Interface and 1255B Frequency Response Analyzer (Solartron Analytical, UK). Current-voltage characteristics were calculated by irradiated the cell with 100 mW/cm2 (450 W xenon lamp, Oriel instrument), 1 sun AM 1.5 G filter was used to simulate the solar spectrum. The active area of the cell was 0.5 cm2. The photocurrent was measured by using a Series 2400 Source Measure Unit from Keithley Instruments, Inc., Cleveland, Ohio. Results and Discussion To confirm the crystallinity and phase purity of the as-prepared material, XRD study was carried out. Fig. 2 shows the XRD pattern of as-synthesized 1D TiO2-MWCNT nanocomposite film directly grown on FTO:Glass substrate and TiO2-MWCNT powders along with FTO:Glass substrate, only MWCNTs and PDF file # 21-1276 for comparison. From the data the synthesis of phase pure rutile structure can be confirmed. After analyzing the diffraction pattern, it was also confirmed that the highly oriented 1D-nanocomposite film were directly synthesized on the substrate along the (002) plane as the relative intensity of the (002) peak was enhanced ~ 10 times from ~ 10% to 100% after compared with the standard data file (PDF data file # 21-1276). The intensity ratio of (I002/I110) for the standard data, the TiO2-MWCNT powder and the TiO2-MWCNT film were 0.1, 0.09 and 5.5 respectively. To further confirm the degree of orientation, the texture coefficient (TC) for (002) peak of the oriented nanocomposite film were determined and compared with the TC value of the assynthesized TiO2-MWCNT powder. The semi-quantitative estimation of the texture can be defined through TC, where TC

I /I

n ∑ I /I 9

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I(hkl) is the observe relative peak intensity, I0(hkl) is the standard intensity of the plane taken from data file, n is the number of Bragg reflections considered for the sum in the denominator.31 Deviation of TC values from unity implies preferred growth. A higher value of TC denotes preferred orientation of grains. The calculated TC(002) values of as-synthesized nanocomposite powder and film were 1.1 and 7.9 respectively. Strong preferential alignment of the as-grown composite film along the [001] direction normal to substrate can be confirmed from the TC value which can be further established from FESEM micrograph. In the TiO2-MWCNT powder, the diffraction peak at 2θ =25.5 ° is attributable to MWCNT. Due to the higher intensity of (002) peak and presence of less amount of MWCNT, it is very hard to elicit the characteristic peak of the MWCNTs in as-synthesized TiO2-MWCNT composite. The presence of other phases like TiO, Ti2O3 can be ruled out as no diffraction peak corresponding to TiO or Ti2O3 are observed. Further, in Raman analysis also only subsequent peaks of rutile TiO2 and MWCNT are found and as well as In XPS study too Ti2+ and Ti3+ oxidation state are not observed. To further explore the effect of MWCNT loading on the local structure of the assynthesized rutile TiO2 composite, Raman spectroscopy was used to analyze the vibrational properties of the samples. The appearance of two Raman active fundamental modes for characteristic rutile TiO2 nanocrystals at ~ 444 cm-1 and 610 cm-1 due to Eg and A1g optical phonon respectively can be clearly distinguished in as-synthesized TiO2 nanorods, TiO2MWCNT film and TiO2-MWCNT powder (Fig. 3 (A), (B), and (D)).32,33 In addition to that, two peaks at 1356 and 1572 cm-1 correspond to D and G band of MWCNTs can also be noticed in the composite samples () which confirms the successful incorporation of MWCNTs on the surface of TiO2 (Fig. 3 (A), (B)).34 Fig. 3(C) shows the Raman spectra of as purchased MWCNT having D and G band at 1350 and 1571 cm-1 respectively for comparison. The presence of D-band, which is attributed to sp3-hybridization of carbon, 10 ACS Paragon Plus Environment

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indicates internal structural defects and dangling bonds. The presence of G-band is corresponding to sp2-hybridization originating from in-plane symmetric stretching of C-C bond.35 The introduction of functionalized hydroxyl groups result into the broadening of D (43 to 51 cm-1) and G (57 to 65 cm-1 ) bands, respectively. It was also seen that the full width at half maxima (FWHM) of Eg mode in the nanocomposites has been varied from 50 to 60 cm-1. Such broadening of peak was observed due to confinement of optical phonons, oxygen vacancies and surface strain effect due to interface integration.36 XPS study of the as-prepared composite nanorods was done in order to know the chemical states of the surface elements. From the survey spectra of the nanorods (Fig. 4A), the characteristics energy peaks of C, O and Ti were detected. By comparing the peaks of assynthesized TiO2 and TiO2-MWCNT composite in Fig. 4A, enhanced intensity of the carbon peak in the nanocomposite proved the successful attachment of MWCNTs ontoTiO2. The C 1s core level spectra of the nanocomposite can be deconvulated into two peaks (Fig. 4B). The main peak at 284.6 eV is due to the presence of sp2 hybridized C atom and the other peak around 285.8 eV can be assigned to carbon atom bound to oxygen atom.37The absence of any peak at ~ 281 eV confirms that C-doped TiO2 has not been formed. The O 1s peak can also be deconvulated into two peaks (Fig. 4C). The peak around 530.6 eV is due to the presence of lattice oxygen in TiO2.38The broad shoulder at 532.2 eV is due to O-C bond.38XPS spectrum of Ti 2p shows two peak at 459.2 and 464.9 eV corresponding to spin-orbital splitting of Ti 2p3/2 and Ti 2p1/2 respectively, attributable to the presence of Ti4+ in an octahedral environment (Fig. 4D).39The binding energy of Ti4+ is slightly shifted towards higher side than bulk rutile which proves that the surrounding electronic environment of titania inside the composite material is different than bulk rutile.29 No peaks around 455 and 456.7 eV due to Ti2+ or Ti3+ states were observed.40 So from the XPS peak profile absence of remnant phases for e.g. TiO and Ti2O3 can be confirmed which supports the XRD data. 11 ACS Paragon Plus Environment


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To know the amount of MWCNTs present in the as-synthesized composite, TGA was done in constant air flow. Fig. 5 shows the TGA plot of MWCNTs, TiO2 and TiO2-MWCNT composite material. In case of MWCNTs, the curve began to decline at 380 °C and at 620 °C there was almost complete weight loss due to the combustion of carbon in air. For only TiO2 the weight-loss around 50 °C is due to the vaporization of water and there was no further weight-loss. Similar weight-loss at ~ 50 °C was also observed in TiO2-MWCNT composite following the oxidation of MWCNTs around 350 °C and continued till 620 °C, forming CO2 attributable to the 21% weight loss. Above 620 °C, the thermo gravimetric curve exhibits nearly flat characteristic and no weight loss was further observed which proves that no residue of MWCNTs was left to burn out, indicating the presence of only TiO2 residue in the composite. In order to identify the morphology of the as-synthesized composites, FESEM was done. In Fig. 6 (A) and (B) the tilted cross-sectional and top-view FESEM micrographs of the MWCNT-TiO2 film on FTO substrate can be viewed, respectively.

The as-grown 1D-

composite was found to be closely packed and strongly aligned out-of-plane direction with respect to the substrate. The square shaped top facets of the rods were the inveterate proof of the synthesis of tetragonal rutile structure. The whole substrate was uniformly covered with the 1D- composite film and the average length and diameter of the as-grown rods were 2 µm and 250 nm, respectively. The zoom view of the composite film (Fig. 6 (C)) clearly shows the entanglement of MWCNTs into the rods. Fig. 6 (D) showed the FESEM micrograph of as- synthesized TiO2-MWCNT powders which also revealed the well connected network of MWCNT with TiO2 rods. To further get acquainted with the microstructure and formation niceties of the composite in more detail, TEM study was carried out. Fig. 7 (A)-(D) showed the TEM


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images of the as-synthesized 1D-composite with different magnifications. It is clear that in the composites, MWCNTs were interwoven with TiO2 rods having diameter ~100 nm. The MWCNTs properly bridged contact with the TiO2 rods. The lattice fringes with inter planar distance of d220 = 0.16 nm corresponding to the rutile-phase of TiO2 can be clearly distinguished (Fig. 8(A)). Further the fast Fourier transform (FFT) (Fig. 8(A) inset) and sharp selected area electron diffraction (SAED) pattern of the nanorods can be visualized along [1 1 0] zone axis which proves the single crystalline nature of the nanorods. In Fig. 8(C) the reciprocal lattice points corresponding to the c-plane were masked and inverse-FFT (IFFT) image was generated (Fig. 8(D)) to acquire clearer structure with high crystallinity and minimal distortion. Further, the optical properties of the as-synthesized samples were studied through UVVis diffused reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). In DRS spectra, both the samples exhibit an intense transition around ~ 400 nm as a consequence of electron transition from valence band to conduction band (O2p-Ti3d) (Fig. 9).41 It was clear from the figure that the absorption capability of TiO2 was enhanced after incorporation of MWCNTs. The enhancement of light absorption capability indicated the betterment of light harvesting potential.42 It can be suggested that the surface electric charge on the oxides of the composite has been increased due to the enhancement of absorption in the visible range which can modify the fundamental process of exciton formation. The spectrum of TiO2-MWCNT underwent a ~10-nm red-shift (from ~410 nm in TiO2 to ~420 nm in TiO2-MWCNT composite) due to the interaction between TiO2 and MWCNT and also implied an ease of charge transfer between CNT and TiO2.40Assuming that the rutile TiO2 nanorods possess a direct transition, the band edge energy of the as-synthesized nanorods are calculated by plotting (αhν)2 against hν (where α is absorption coefficient and hν is photon



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energy). 43,44 After calculating the band-gap from the intercepts (Fig. 9 inset), it is found that with inclusion of MWCNTs a little amount of red shift takes place from ~3.0 eV to ~2.9 eV. As PL emission occurs from the recombination of photo generated charge carrier, so the study of PL spectra is useful to divulge the behaviour of charge carrier trapping, separation and to understand the fate of excitons in semiconductor materials. Fig. 10 shows the room temperature PL spectra of TiO2 and TiO2-MWCNT composite at a range from 400 to 650 nm, excited at the wavelength of 380 nm. Both the samples showed three emission peaks situated at ~ 414, 434 and 460 nm respectively due to band-band emission and metalligand charge transfer transition.20 The shapes of all the emission spectra were similar. There was a certain decrease in the intensity of the PL signal after incorporation of MWCNTs which could be due to the electron transfer process from the conduction band of TiO2 to MWCNTs attributed to efficient charge separation.45 The emission peaks were also slightly broadened in the composite. Several reasons could be found to explain the decrease in PL intensity. There might be a decline of radiative recombination process which is attributed to weak PL signal for the composite.46 MWCNTs encompasses light harvesting property similar to dyes and they possess π-conjugated structure which could work as an efficient electron accepting material and could obstruct the recombination of electron–hole pairs.35 The efficient charge separation in the composite system will aid both the photovoltaic and photocatalytic properties of the material. Electrochemical impedance measurement is a common tool to study the transport properties of the semiconducting materials. The M-S plot, calculated from the electrochemical measurements provides a deep understanding of the materials composition and the nature of the donor levels. Fig. 11 shows the M-S plot of as-synthesized TiO2 and TiO2-MWCNT nanorods on FTO substrate. Both the samples exhibit positive slopes due to ntype semiconducting properties of TiO2.19 Compared to the only TiO2 nanorods, the 14 ACS Paragon Plus Environment

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composite materials showed much smaller slopes which signify the increase in charge carrier density with inclusion of MWCNTs. The flat-band potential (VFB ) and carrier density of the doped and undoped nanorods were calculated from M-S equation 1 2 "# $ where e0 is the fundamental charge constant, ε the dielectric constant of TiO2, ε0 is the permittivity of vacuum, Nd the donor density, V the electrode applied potential, VFB the flatband potential and kT/e0 is a temperature-dependent correction term. The VFB value of only TiO2 and TiO2-MWCNT were determined to be -0.717 and -0.314 V respectively. The positive movement of the flat-band edge probably due to the shift of the conduction band edge of the as-synthesized composite towards lower potential attributable to the incorporation of MWCNTs. Thickness of the space-charge layer (W) can also be calculated from the given equation /

2 %& '

The charge carrier or donor density (Nd) and thickness of the space-charge layer at the semiconductor/electrolyte interface of TiO2 and TiO2-MWCNT was calculated to be 6.1 x 1016 and 2.2 x 1017 cm-3 and 340 and 257 nm respectively. A potential of 1.0 V was chosen to calculate the space charge region. As the thickness of the space-charge region is smaller than the film thickness so there is a possibility to increase the photocurrent.42 Oxides having higher specific surface area are always preferred for harvesting light due to the presence of more surface active sites leading to the enhancement of adsorption capacity of sensitized molecules. Hence, to study the effect of MWCNTs intrusion on the specific surface area of the as-synthesized composite materials Brunauer–Emmett–Teller (BET) adsorption analysis was performed. According to Brunauer–Demming–Demming–Teller (BDDT) 15 ACS Paragon Plus Environment


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classification, the N2 adsorption-desorption isotherms (Fig. 12) of TiO2 and composite nanorods can be attributed to type IV isotherms with H2 type hysteresis loop, indicating the presence of mesopores.42Using BET multipoint method, the specific surface area of TiO2 and TiO2-MWCNT were found to be 24 and 50 m2/g, respectively. After incorporation of MWCNTs, the surface area of TiO2 was increased upto two times which also aid the rapid diffusion of charges.39 To study the light harvesting property of the as-synthesized 1D-TiO2-MWCNT composite film, the material was sensitized with N719 dye and photoelectric conversion measurements were done by fabricating a DSSC. These 1D-highly-crystalline oriented TiO2-MWCNT composites, grown on FTO:Glass substrates can effectively act as photoanode in sensitized solar cells. As the composite film were directly grown on the transparent substrate, so there is an enormous possibility of increasing the generation of photoelectrons as more incident light can pass through the backside of the photoelectrode. Fig.13 shows the current density-voltage curves of the assembled solar cell under 100 mW/cm2 illumination using dye-adsorbed TiO2 and TiO2-MWCNT as photoanode. It was found that after incorporation of MWCNTs, the photocurrent density (JSC) increased from 3.94 to 7.49 mA cm-2, the open-circuit voltage (VOC), decreased from 0.78 to 0.69 V. The fill-factor and the photon to electron conversion efficiency of the composite increased from 0.49 to 0.52 and from 1.5 to 2.4 %, respectively. Several reasons can be found behind the enhancement of Jsc and hence the efficiency of the solar cell. The composite formation of CNT with the TiO2 nanorods favours faster transport and collection of photo excited electrons from the dye to the electrode surface through the TiO2 network and hence increases the photocurrent density. Further the rate of exciton recombination is also decreased due to better charge separation and transfer of a fraction of electrons from TiO2 to CNT.26 Due to the more positive VFB of the composite materials, calculated from the M-S plot, the effective driving force of the photoelectrons enhanced 16 ACS Paragon Plus Environment

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which led to the augmentation of electron injection efficiency from the excited state of the dye to the semiconductor.48 Moreover, due to the increase in the specific surface area of the material after incorporation of CNT, the amount of dye loading is also increased leading to the increase in JSC value. The decrease in VOC value is due to the positive shift of the Fermilevel which can be known from the value of the VFB. But, it is notable that despite the decrease in the VOC, the overall efficiency of the DSSC increased by 60% in the composite material compared to only TiO2 which support the beneficial effect of inclusion of MWCNTs into TiO2 nanorods.

Conclusion In summary, in this contribution, we have reported the synthesis of oriented single crystalline 1D rutile TiO2-MWCNT composite film on FTO:Glass substrates along with 1Drutile-TiO2-MWCNT powder all at once through a simple low temperature hydrothermal method. Formation of pure rutile phase and incorporation of MWCNTs into TiO2 nanorods were confirmed through XRD, XPS and Raman analysis. Detailed optical studies revealed that the TiO2-MWCNT composite efficiently act as quencher and with incorporation of MWCNT, rate of recombination decreases and thus it can be efficiently used in photovoltaics and photocatalysis applications. Electrochemical impedeance measurement was done to calculate the band edge position and electron carrier density of the composite material and it was found that the donor density increased almost ~10 times with the incorporation of MWCNTs in TiO2 nanorod network. The composite film was successfully used as photoanode in a DSSC and the photon to electron conversion efficiency was increased by 60% after inclusion of MWCNT. This simple hydrothermal synthesis of composite material can find a number of applications in not only photovoltaics or photocatalysis but also in



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lithium ion batteries, photonic crystals, self cleaning membranes and designing optoelectronics devices.

Acknowledgement PP acknowledges support from Young Scientist Award grant from Council for Scientific and Industrial Research (CSIR) in Physical Sciences and a separate grant from Department of Science & Technology (DST), India (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. Authors also acknowledge the seed funds from their institute.


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Fig. 1: Schematic of the simultaneous synthesis of 1D-R-TiO2-MWCNT film and 1D-RTiO2-MWCNT powder.


TiO2-MWCNT powder TiO2-MWCNT film

20

40 50 2θ (degree)

(310)

60

(301)

(211) (220)

(101)

30

(200) (111) (210)

FTO MWCNT PDF21 1276

(110)

Intensity (a.u.)

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

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(002)


70

Fig. 2: The XRD patterns of 1D-R-TiO2-MWCNT powder and film directly grown on FTO:Glass.


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Eg TiO -MWCNT powder 2 B1g

Intensity (a.u.)

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


A1g

Eg

D

TiO2-MWCNT film A1g

G

D

G

B1g

1200 1400 1600 1800

1200 1400 1600 1800

D D 500

G

G

1000

1500

2000

MWCNT

D

500

1000

Eg

1500

2000

TiO2 A1g

B1g G

500

1000

1500

2000

500

1000

1500

2000

-1

Raman shift (cm )

Fig. 3: Raman spectra for (A) 1D-R-TiO2-MWCNT powder, (B) 1D-R-TiO2-MWCNT film directly grown on FTO:Glass, (C) MWCNT and (D) 1D-R -TiO2.



TiO2-MWCNT

1000

800

600

400

C 1s

C 1s

(B)

TiO2 C 1s

Ti 2p O 1s

Ti 2p O 1s

(A)

Intensity (a.u.)

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

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0 290

200

O 1s

(C)

288

286

284 2p3/2

(D)

282

280

Ti 2p

2p1/2

5.7 eV 538

536

534

532

530

528

468 466 464 462 460 458 456 454 452

Binding Energy (eV)

Fig. 4: (A) XPS survey spectra of 1D-R-TiO2 and 1D-R -TiO2-MWCNT powder, (B) C 1s, (C) O 1s and (D) Ti 2p core level spectra of TiO2-MWCNT powder.


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21.4%

100 Weight (%)

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


80 60 40

TiO2- MWCNT

20

MWCNT

TiO2

0 150

300 450 o 600 Temperature ( C)

750

Fig. 5: TGA graph of 1D-R-TiO2, 1D-R-TiO2-MWCNT powder and MWCNT.



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: FESEM images of 1D-R-TiO2-MWCNT film directly grown on FTO:Glass (A) tilted cross-sectional view, (B) top view (C) zoom view of (B) and (D) 1D-R-TiO2-MWCNT powder.


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


Fig. 7: (A)–(D) TEM images of 1D-R-TiO2-MWCNT powder with different magnification.



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. 8: (A) The HRTEM image with inset showing FFT pattern, (B) SAED pattern of 1D-RTiO2-MWCNT powder, (C) masked reciprocal lattice points corresponding to (110) plane and (D) Results of IFFT based on the lattice points shown in (C).


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TiO2-MWCNT TiO2

350

(αhν)

2

Absorbance (a.u.)

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


TiO2-MWCNT TiO2

2.6 2.8 3.0 3.2 3.4 hν (eV)

400

450 500 550 600 Wavelength (nm)

650

Fig. 9 : (A) UV-Vis-DRS spectra with inset showing the dependence of the absorption coefficient as a function of energy plot for 1D-R-TiO2 and 1D-R-TiO2-MWCNT film.



TiO2-MWCNT TiO2 Intensity (a.u.)

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

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400

450

500 550 600 Wavelength (nm)

650

Fig. 10: The PL-spectra of 1D-R-TiO2 and 1D-R-TiO2-MWCNT film.


700

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2 TiO2-MWCNT TiO2 1

2

12 -2

4

1/C (10 F cm )

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


0 -1.0

-0.9

-0.8 -0.7 -0.6 Potential (V)

-0.5

-0.4

Fig.11: Mott-Schottky plots measured at a frequency of 10 kHz at 298 K in 3M-KCl solution for 1D-R-TiO2 and 1D-R-TiO2-MWCNT film.


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

Vol adsorbed (cm /g STP)


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80 70 60

TiO2-MWCNT TiO2

50 40 30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Fig.12: N2 adsorption and desorprtion isotherm of 1D-R-TiO2 and 1D-R-TiO2-MWCNT powder.


2

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


Current Density (mA/cm )

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TiO2-MWCNT TiO2

8 6 4 2 0 0.0

0.2

0.4 0.6 Potential (V)

0.8

Fig. 13: Current density vs. potential curves for dye-sensitized solar cell fabricated from 1DR-TiO2 and 1D-R-TiO2-MWCNT film.



TOC (Graphics):

2

Current Density (mA/cm )

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

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TiO2-MWCNT

8

TiO2

6 4 2 0 0.0

CNT

eTiO2

e e e - - ‐

e -e e

Dye

- -

0.2

0.4 0.6 Potential (V)

ACS Paragon Plus Environment

0.8

Template-Free Fabrication of Highly-Oriented Single-Crystalline 1D

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