Synthesis, Characterization of Hydrothermally Grown

ECS Journal of Solid State Science and Technology, 1 (2) M15-M23 (2012) 2162-8769/2012/1(2)/M15/9/$28.00 © The Electrochemical Society

M15

Synthesis, Characterization of Hydrothermally Grown MWCNT–TiO2 Photoelectrodes and Their Visible Light Absorption Properties Sawanta S. Mali,a,z Chirayath A. Betty,b Popatrao N. Bhosale,c and P. S. Patila,z a Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India b Chemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai 85, India c Materials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India

We have synthesized multiwalled carbon nanotubes (MWCNT)-TiO2 hybrid nanocoral thin films by hydrothermal route with titanium tetrachloride (TiCl4 ) and functionalized MWCNT as the precursor. The new MWCNT-TiO2 hybrid nanocoral material was characterized by scanning electron microscopy (SEM), Fourier Transform Infrared (FT-IR) spectroscopy, Fourier Transform Raman spectroscopy (FT-Raman), X-ray diffraction (XRD), and X-Ray photoelectron spectroscopy (XPS) etc. The study indicates that the deposition process alters the typical structures of the nanocoral MWCNT-TiO2 hybrid material. The MWCNT-TiO2 hybrid film shows drastically changed morphology; i.e. the coral like morphology of TiO2 gets disturbed and transforms into compactly arranged spherical ball like particles with size ranging from ∼550–650 nm. The MWCNT–TiO2 hybrid thin film exhibit high photocurrent density (0.693 mA/cm2 ) and yields overall conversion efficiency (η) of 2.37%. © 2012 The Electrochemical Society. [DOI: 10.1149/2.004202jss] All rights reserved. Manuscript submitted February 1, 2012; revised manuscript received April 19, 2012. Published July 20, 2012.

Among the new materials being developed for solar cells, photocatalytic and other applications, titanium dioxide (TiO2 ) remains one of the most promising materials because of its high efficiency, low cost, chemical inertness, ecofriendly nature and photostability.1 However, the widespread use of TiO2 is hindered by its low utilization of solar energy in the visible region (about 3–5%) because of the wide bandgap (3.0 eV for rutile and 3.2 eV for anatase crystalline phase) restricting the effective utilization of pure TiO2 nanoparticles for solar excitation or conversion. Therefore, many efforts were concentrated on narrowing the optical bandgap of titania. To improve the photoelectrochemical efficiency of the material, it is desirable to red-shift the photoelectrochemical onset so as to include the less energetic but more intense visible part of the solar spectrum. Traditionally this has been achieved by anchoring organic dyes to the surface, and this approach has been successful in dye sensitized solar cells (DSSCs).1 For efficient DSSCs TiO2 nanocorals2–5 oriented single-crystalline nanorods (TNR) and nanoflowers,6 microspheres,7 would be most desirable, but achieving these structures is a challenging task. In dye sensitized solar cells, this is being implemented successfully by anchoring low bandgap ruthenium dye on to the surface of TiO2 . But the ruthenium dyes are expensive and undergo degradation.8 Another way is by doping of impurities (cationic as well as anionic) into the TiO2 matrix for narrowing the bandgap. However, cationic impurities such as transition metals have drawbacks of thermal instability, higher probability to form charge carrier recombination centers, and expensive synthesis protocols. Therefore, anion impurities such as nitrogen (N),9,10 carbon (C)11–16 or sulfur (S)17 have been extensively tried for narrowing the bandgap. After the discovery of carbon nanotubes by Iijima in 1991,18 carbon nanotubes (CNTs) have attracted significant attention in a variety of scientific fields because of their unique properties; structural, chemical, thermal, mechanical, electronic properties and potential applications.19–21 Their structure can be rationalized as resulting from the folding of a graphene sheet, or several of them aligned in a concentric way, giving rise, respectively, to fullerene (C60), single(SWCNTs) or multi-walled (MWCNTs) carbon nanotubes. Fullerenes are zero dimensional molecular compounds while carbon nanotubes and graphene are one and two dimensional materials respectively. All these carbon forms contain sp2 hybridization between the carbon atoms. In case of SWCNTs, the metallic, insulating or semiconducting properties depend on the folding angle of graphene sheet and the diameter of the tube, whereas MWCNTs are all conductive.22 Recently, with the development of technology for the large-scale synthesis of MWCNTs, efforts have also been devoted to MWCNTs on z

E-mail: [email protected]; [email protected]

TiO2 to form binary nanocomposites with synergetic combination of their intrinsic properties.23 Moreover, MWCNTs coated with metal oxides are expected to exhibit different physical properties than those of pure nanotubes, and they may be proven to be key components in the next generation of nano-optical and electronic devices such as quantum memory elements, high density magnetic storage media, semiconducting devices, field electron emitters, magnetic field sensors and scanning probe microscopes. Recently, efforts has been made to combine TiO2 with activated carbon24,25 or carbon nanotubes (CNTs) in simple mixtures or as nanocomposite materials to obtain high surface area and better electrical connectivity which will lead to highly efficienct dye sensitized solar cells, catalysts.26,27 In MWCNTTiO2 hybrid nanostructures CNT acts a catalyst-support to increase the reactive surface of the nanostructured TiO2 . It is also observed that interfacial contact between the CNTs and TiO2 other synergistic photochemical effects. Due to the surface bridging Oxygen vacancy of TiO2 , it is known to participate in the surface photochemical processes resulting in the oxidation/reduction of the adsorbed substrate. Considering the carbon nanotube as a smaller bandgap material it can act as a sensitizer when it attaches to the surface of TiO2 resulting in photo-induced charge transfer from carbon nanotubes to TiO2 . Many researchers have investigated the preparation of TiO2 /CNTs nanocomposites using different methods.28,29 Hydrothermal process is a well-known processing route for synthesizing hierarchical metal oxide materials.30,31 Hydrothermal synthesis is widely employed, which involves reaction under controlled temperature and/or pressure, and is usually performed in steel autoclaves. The novelty of this work lies in the development of a one step synthesis of MWCNT-TiO2 hybrid thin films at 120◦ C using hydrothermal route. To the best of our knowledge, for the first time we employed the hydrothermal technique to synthesize TiO2 nanocorals and hybrid nanocoral thin films of MWCNT–TiO2 by direct mixing of functionalized MWCNT with TiO2 which was grown on the fluorine doped tin oxide (FTO) coated glass substrate. Its photoelectrochemical performance under visible light illumination was studied subsequently. The MWCNT-TiO2 hybrid thin films obtained were characterized by a range of techniques including SEM, XRD, FT-IR and FT-Raman spectroscopy. All chemical reagents used in this study were analytical grade (AR) and were used without further purification, unless stated otherwise. Experimental In a typical experiment, the hydrothermal inorganic precursor solution was prepared by mixing 0.05 M TiCl4 carefully with

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ECS Journal of Solid State Science and Technology, 1 (2) M15-M23 (2012)

absolute ethanol in ice-cold bath forming yellow colored titanium chloroalkoxide [TiCl2 (OEt)2 (HOEt)2 )] solution. A small amount of 0.001M concentrated hydrochloric acid (HCl, 36%, Thomas Beaker) was added as a catalyst and pH of solution was adjusted to 2.1 using nitric acid (HNO3 , 38%, Thomas Beaker) as a peptizer. The transparent solution thus prepared was further mixed with saturated solution of 6.8 M NaCl (Extra pure, Thomas Baker). Detailed experimental procedure we published in our earlier report.2–4 The MWCNTs (diameter 95 wt%) were purchased from Monad Nanotech (Mumbai, India). The functionalization process of MWCNTs was done as per our earlier report.32 In typical experiment the pristine MWCNT were refluxed for 12 h in mixture of H2 SO4 and HNO3 (3:1) solution for –COOH surface functionalization. The functionalized MWCNTs were dispersed by sonication in the above TiO2 precursor solution and transferred into a polytetrafluoroethylene (Teflon)-coated 25 mL vessel. The cleaned glass or FTO substrate was immersed into this solution vertically. It was then sealed and maintained at 120◦ C for 3 h (at 1.5 kg/cm2 pressure). Upon completion of the reaction, the autoclave was allowed to cool at room temperature and the deposited film was rinsed in double distilled water and finally dried in oven at 70◦ C. Initially we vary the amount of functionalized MWCNT in TiO2 precursor from 0.005 to 0.02 g and it is observed that the samples deposited at 0.02 g MWCNT concentration shows good photoelectrochemical performance, therefore, the samples deposited at 0.02 g MWCNT concentration were used for further characterization.

troscopy (EDS) (at 20 kV) is used for elemental analysis. UV–vis spectra were obtained with a (Shimadzu −1800 UV) UV–vis–NIR spectrophotometer. The FT-IR spectra on pellets of the samples with KBr were recorded on a FTIR-8201PC spectrometer (Perkim Elemner). The FT-Raman spectra were obtained by a MultiRAM Raman spectrometer (Bruker Optics, Germany, software OPUS 6.5) at a resolution of 4 cm−1 using the 1064 nm line of a Nd:YAG laser as the excitation source. X-ray photoelectron spectra were recorded by using XPS, VG Multilab 2000, Thermo VG Scientific, UK, for phase evaluation. The current-voltage (J–V) characteristics were measured using a VersaStat-II (EG&G) potentiostat/galvanostat, computer-controlled by M270 software and the potential was swept between anodic (+0.9 V) to cathodic (–0.9 V) with respect to SCE. A 500W tungsten lamp was used as the light source, the incident light intensity calibrated by using a photometer. The photocurrent density versus measured potential for FTO/MWCNT-TiO2 photoanode characteristics were carried out using electrochemical workstation with a three-electrode system. The MWCNT-TiO2 film electrode, platinum coated FTO, and an aqueous SCE served as the working, counter, and reference electrodes, respectively. The electrolyte solution consisted of i) 0.5M NaOH ii) Iodolyte AN-50 (Solaronix) which were used for pure TiO2 and MWCNT-TiO2 samples respectively. The currentvoltage characteristics of the cell were measured under 5 mW/€cm UV illumination (400 nm) for TiO2 and MWCNT-TiO2 samples respectively. All experiments were carried out at room temperature.

Results and Discussion Characterizations The surface images were obtained by means of SEM observation on a JEOL-6360 JSM electron microscope. Energy dispersive spec-

After functionalization in the sulfuric acid and nitric acid, carboxyl (–COOH) can be generated on the surfaces of MWCNTs (eq. 1) at low pH.32

[1]

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ECS Journal of Solid State Science and Technology, 1 (2) M15-M23 (2012) where,

M17

A1g

0.08 0.07

TiO2

During the hydrothermal process dispersed functionalized MWCNT-COOH (i.e. negatively charged due to –COOH group) attracts Ti4+ ions from titanium chloroalkoxide [TiCl2 (OEt)2 (HOEt)2 )] solution (eq. 2). Eventually, it leads to the growth of TiO2 nanocorals directly on the surface of the functionalized MWCNTs and a tight combination between MWCNTs and TiO2 nanoparticles can be formed. Typical UV–Vis absorption spectra of the pristine TiO2 and functionalized MWCNT and MWCNT-TiO2 samples are presented in Fig. 1. Although the pristine TiO2 has no absorption above its fundamental absorption sharp edge, rising at 400 nm, an apparent enhancement of absorption can be observed for the MWCNT-TiO2 . The absorption band observed at around 289 nm can be assigned to the pure MWCNTs. This absorption is characteristic of dispersed MWCNTs whereas strongly bundled CNTs show a different absorption edge.33–38 Thus the occurrence of well-dispersed MWCNTs in the water solution makes it possible for the molecular TiO2 precursor to access the bare surface of the MWCNTs for decomposition and subsequent TiO2 formation. A significant shift toward visible region in the spectral photoresponse (514 nm) is observed for MWCNT-TiO2 sample. Also the absorption spectrum shows scattering in the range 362–289 nm range due to formation nanoclusters of MWCNTs in the TiO2 matrix. It clearly illustrates visible light absorption by the MWCNT-TiO2 electrode. The FT-Raman spectra of MWCNT–TiO2 nanocoral sample, pure TiO2 and pristine MWCNT are compared in Fig. 2. Characteristic bands at 391, 510 and 629 cm−1 in TiO2 corresponding to the B1g , A1g + B2g and Eg modes of anatase, respectively, are presented.39 For the hybrid film, the Raman band corresponding to the Eg mode of the anatase phase is shifted to a lower energy, indicating a decrease in crystal lattice vibration energy and therefore a decrease in the crystallinity. The peaks at 1343 and 1584 cm−1 in MWCNT–TiO2 are assigned to the D-band (disordered carbon induced) and G-band

0.03 0.02

0.00

Absorbance (a.u.)

0.5 0.0 200

MWCNT 300

400

500

600

700

800

900

1000

Wavelength (nm) Figure 1. Optical absorption spectra of (a) MWCNT (b) TiO2 (c) MWCNTTiO2 thin films.

1500

1000

500

Figure 2. FT-Raman spectra of (a) MWCNT (b) TiO2 (c) MWCNT-TiO2 thin films.

(graphite carbon related) of the MWCNTs, respectively due to sp2 hybridization in carbon.40 The relative intensity ratio of the D-band to the G-band is known as an index of graphitization to determine the CNT microstructure.41 The ID /IG ratio of MWCNT–TiO2 is about 0.89 compared to that of the pure MWCNTs (0.76), indicating that the graphitization structure of MWCNTs in the nanocomposites did not change much after functionalization in nitric acid or the reflux treatment. The observed Raman shift of TiO2 , MWCNT, and MWCNTTiO2 samples are summarized in Table I.42–44 The phonon lifetime (τ) can be derived from the Raman spectra via the energy time

Table I. Observed Raman shift of TiO2 , MWCNT and MWCNTTiO2 samples.

Peaks (cm−1 ) D band in MWCNT G band in MWCNT G band in MWCNT

Standard Peak Observed (cm−1 )

Observed (cm−1 )

Raman Shift (cm−1 )

1343

1286

57

1584

1604

20

2595

2564

31

References 34, 35

TiO2

A1g B1g B1g E1g C1g D1g

143 197 397 447 516 637

144 194 396 447 515 639

1 3 1 0 1 2

36

MWCNTTiO2

D band in MWCNT G band in MWCNT G band in MWCNT A1g B1g B1g E1g C1g D1g

1343

1291

52

In this Paper

1584

1607

23

In this Paper

2595

2562

33

In this Paper

143 197 397 447 516 637

155 Merge 396 439 518 615

12 – 1 8 2 22

In this Paper In this Paper In this Paper In this Paper In this Paper In this Paper

MWCNT-TiO2

TiO2

2000

Raman Shift (cm )

2.5

1.0

2500

-1

3.5

1.5

MWCNT 3000

4.0

2.0

TiO2

0.01

MWCNT

3.0

E1g

MWNT-TiO2

Sample 4.5

D1g

0.04

D-Band

Ti

G-Band

-COOH

MWCNT

0.05

G'-Band

CNT

Raman Intensity (a.u.)

0.06


M18


Table II. Phonon lifetimes of the Eg and A1g modes of TiO2 , MWCNT and MWCNT-TiO2 thin films at room temperature. The characteristic decay time associated with deposition time. Phonon mode

(10−12 s)

MWCNT

D-Band G-Band G -Band

0.1163 0.1418 0.1104

TiO2

A1g B1g B1g E1g C1g D1g

0.4251 0.5345 0.2451 0.1530 0.2759 0.1950

MWCNT-TiO2

A1g E1g D1g D-Band G-Band G -Band

0.1698 0.1108 0.1102 0.1080 0.2430 0.1045

Transmitance (T%)

Sample

54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 4000

MWCNT MWCNT-TiO2 TiO2

Ti-O-Ti

3000

2000

1000 -1

Wavenumber (cm ) Figure 3. FT-IR spectra of functionalized TiO2 , MWCNT and MWCNT-TiO2 samples.

20

30

40

50

20

(215)

(204) 60

(116)

MWCNT-TiO2

70

80

f-MWCNT

30

40

(004)

The results are summarized in Table II. From Table II it is observed the phonon lifetime (τ) of the MWCNT-TiO2 hybrid nanocoral material decreases drastically. This is due to the interaction of MWCNTs with TiO2 molecules. It is known that the chemical oxidation of carbon materials is frequently used as a method to obtain a more hydrophilic surface structure with a relatively large number of oxygen containing surface groups. These oxygen-containing groups behave as acids or bases that exhibit ion-exchange properties and improve the dispersibility. Our previous reports32,44 indicated the oxidation of CNTs with HNO3 and H2 SO4 which suggested the introduction of many functional groups, such as hydroxyl (–OH), carboxyl (–COOH), and carbonyl (>C=O), onto the surface of CNTs. In our experiments, the nature of the pristine and functionalized MWCNT surface groups was investigated using FT-IR spectroscopy. Details of functional group attachments are explained in our earlier report.32,44 Figure 3 shows the FT-IR spectrum of functionalized MWCNT, pristine TiO2 nanocorals and MWCNT–TiO2 hybrid nanocorals. The band at 664 cm−1 is due to titanium oxide and assigned to the stretching of Ti–O–Ti. The peak centered at ∼1012 cm−1 due to characteristic O-O stretching vibration. The sharp peak at 1400 cm−1 can be attributed to the lattice vibrations of TiO2 . The absorption band at 1627 cm−1 was caused by a bending vibration of coordinated H2 O as well as Ti–OH. It can be observed that there are broad peaks at 3400 and 1638 cm−1 , which correspond to the surface adsorbed water and hydroxyl groups.45,46 For pure MWCNTs and MWCNT– TiO2 nanocomposites, the band at 1564 cm−1 is assigned to C=C stretching originating from the inherent structure of MWCNTs, and

(200) (004) (105)

[4]

(100)

1 1 1 = + τ τA τI

(103) (004) (100)

where E is the uncertainty in the energy of the phonon mode, is the Planck’s constant, and is the full width at half maximum (FWHM) of the Raman peak in units of cm−1 . Phonon lifetime is mainly limited by two mechanisms: i) anharmonic decay of the phonon into two or more phonons so that energy and momentum are conserved, with a characteristic decay time τA and ii) perturbation of the translational symmetry of the crystal by the presence of impurities, defects and isotopic fluctuations, with a characteristic decay time τI . The phonon lifetime deduced from the Raman measurements is therefore written as,

the band at 1701 cm−1 can be attributed to the vibration of C=O stretching.47,48 Fig. 4 gives XRD pattern of functionalized MWCNTs (denoted red triangle: symbol) and MWCNT/TiO2 samples (denoted red triangle: and pink star ★ symbol) respectively. The XRD results demonstrated that the MWCNT-TiO2 nanoparticles were of homogeneous anatase structure with crystallinity, and the average crystallite sizes calculated from Scherrer equation (particle size, d = 0.9λ/βcosθ, where λ is the characteristic X-ray wavelength applied (0.15 nm), β is the half width of the peak at the 2θ value) were found to be about 12 nm. Fig. 5 demonstrates the scanning electron microscopic (SEM) images of TiO2 , MWCNT and MWCNT-TiO2 . The fibrous like morphology was observed for pure MWCNT as shown in Fig. 5a.

(101) (002)

[3]

(002)

E 1 = = 2πc τ h¯

Intensity (a.u.)

uncertainty relation,

50

60

70

80

2 θ (Degree) Figure 4. X-ray diffraction patterns of the functionalized MWCNT and TiO2 MWCNT thin films.



M19

(a)

(b)

(c)

Figure 5. SEM micrographs of (a) functionalized MWCNTs at different magnification (b) hydrothermally grown TiO2 nanocorals and (c) MWCNT-TiO2 hybrid nanocorals at different magnification. Last figure shows higher magnified SEM image (x 25000 magnification) of MWCNT-TiO2 hybrid nanocorals.

Without MWCNT precursor, TiO2 was formed almost exclusively in a nanocoral like morphology of ∼400–500 nm diameter, as seen in Fig. 5b. While Fig. 5c shows MWCNT-TiO2 sample with drastically changed morphology; i.e. the coral like morphology got disturbed and the compactly arranged spherical ball like particles were formed with size ranging from ∼550–650 nm. The pure TiO2 shown in Figure 5b and See Fig. S149 have coral surfaces, however, MWCNTTiO2 shown in Fig. 5c have compactly arranged morphology. This observation demonstrates that a relatively thick layer of TiO2 nanoparticles was deposited on the MWCNTs which are consistent with the Raman observation in Fig. 2. This is consistent with our EDS data (Fig. 6): the percentage of MWCNT i.e. carbon in TiO2 nanocorals is approximately 11%. Quantitative analyzes of the electronic structures and chemical properties of the TiO2 -MWCNT films were performed by XPS. Fig. 7a illustrates the survey spectrum of MWCNT-TiO2 sample. Fig. 7b shows the Ti(2p) high resolution XPS spectrum with two bands in this region. The bands centered at binding energies of 464.3 and 458.38 eV were attributed to the Ti(2p1/2) and Ti(2p3/2) spinorbital splitting photoelectrons in Ti4+ respectively (Fig. 7b).50 In addition, the separation between these two bands was found 5.9 eV, consistent with presence of the normal state of Ti4+ in the anatase TiO2 film. The same sample also exhibited an O1s peak at 529.8 eV, characteristic of the lattice oxide and a minor one at 531.6 eV due to the contribution of extrinsic contamination was assigned to –OH bond, such as adsorbed water molecules on the film surface introduced during film transfer (Fig. 7c), which corresponds to the results of FT-IR spectra. It can be seen from Fig. 7d that, the C1s peak at 284.8 eV is usually assigned to conjugated carbon in CNTs. The peak at 282.35 eV is very close to the C1s peak can be ascribed to carbon substituting for oxygen atom in the lattice of TiO2 , which resulted in

Figure 6. EDS spectra of the TiO2 and MWCNT-TiO2 thin films.


M20

ECS Journal of Solid State Science and Technology, 1 (2) M15-M23 (2012) 400000

(a)

Ti 2P

40000

Counts/ s

250000 200000 150000

30000

Ti2p 1/2

20000

Si2p

C1s

100000 50000

10000

0 1200

1000

(b)

Ti2p 3/2

50000

Survey Spectrum Ti2P

Counts/ s

300000

60000

O1S

MWCNT-TiO2 350000

800

600

400

200

0 470

0

468

466

Binding Energy (eV)

464

462

460

458

456

454

Binding Energy (eV)

70000

(c)

O 1s

O1s 60000

C=C & C-C

C1s

(d)

C=O

40000

Counts/s

Counts/s

50000

Ti-O

30000

O-C=O

Ti-C 20000

10000

0 540

-OH

538

536

534

532

530

528

526

296

294

292

290

288

286

284

282

280

Binding Energy (eV)

Binding Energy (eV)

5.07 eV

6.08 eV

TiO2-MWCNT

Intensity/ a.u.

(e)

12

10

8

6

4

2

0

-2

Binding Energy/ eV Figure 7. (a) XPS survey spectrum of MWCNT-TiO2 sample (b) The XPS spectrum of the Ti2P band (c) The XPS spectrum of the O1s band for TiO2 samples (d) The XPS spectrum of the C1s band (e) fits for valence band (VB) XPS for of MWCNT-TiO2 .

formation of O–Ti–C bonds.51 The peak at 286.6 eV can be ascribed to Ti–C–O bonds, forming carbonate species.52,53 In addition, the bands at 286.3 and 288.6 eV were assigned to the C–O and O–C=O bonds, respectively.54,55 These results also suggest presence of Ti–O–C carbonaceous bonds at the interface.

Modification of the TiO2 valence band (VB) caused by the MWCNT is ascertained from the VB XPS, as shown in Fig. 7e.56,57 If carbon is doped in the TiO2 lattice it provides additional electronic states above the valence band edge of pure TiO2 and the substitutional carbon states exist above the VB.58 As seen in the XPS valence


2

0.1

0

0.0

-1

M21

MWCNT-TiO2

-2

-0.1

-0.2

TiO2 -0.3

-0.4

MWCNT -e

TiO2

-3

Energy Level (eV)

Current density (mA/cm )


-4

CB

-5

e 5.2 eV TCO

--

-6

e

4.2 eV

100

200

300

400

500

600

700

7.4 eV

+

-8 0

I -

MWCNT-TiO2 -0.6

-

+

h

-7

-0.5

Visible Light

--

h

VB

I3

800

-9

Applied voltage (mV) Figure 8. J-V characteristics of (a) TiO2 under UV (400 nm) light in 0.1M NaOH and LiI/Iodine electrolyte respectively.

band spectrum there is no significance of carbon or MWCNT doping, while FT-Raman studies reveals that the presence of MWCNT in TiO2 MWCNT film. This evidence directly supports the theoretical DFT predictions and experimental results indicating that the MWCNTs are not doped in the TiO2 lattice.59 However, the additional diffused states can be considered as a direct electronic origin of red-shifted absorption and subsequent visible light photoactivities of the TiO2 -MWCNT nanowires.56,57 The observed “shoulder” and “tail-like” features in the UV–vis spectra are directly related to the above modification of the diffused states from the MWCNTs. Fig. 8 shows a comparison of the photocurrent density versus applied potential curves of the FTO/TiO2 and FTO/MWCNT-TiO2 photoanode under UV and visible illumination respectively. The observed dark current densities were found to be negligible. The corresponding cell parameters are summarized in Table III. The MWCNTs used in this work can be assumed as small gap semiconductors. For a better understanding of the proposed mechanism of MWCNT–TiO2 , the terminology of conduction band (CB)/ valence (VB) band is used for the MWCNTs although the band-gap structure of MWCNTs is not defined yet. It is well known that MWCNTs contain both holes (h+ ) and electrons (e− ), but at room temperature generally show metallic behavior with electrons as a majority carriers due to the overlapping CB and VB which vary with nanotube diameter and helicity.60 Schematic representation of energy levels MWCNT-TiO2 electrode is represented in Fig. 9. MWCNTs can also show conductivity values in the semiconducting range, with the energy band overlap varying according to the nanotube chirality and the interaction between the different walls of the MWCNT61 within the nanocrystalline TiO2 photoanode is carried by the redox mediator I − anion and its counter-ion, Li+ . Detailed mechanism is schematically illustrated with energy levels of MWCNT–TiO2 under visible light in Fig. 9. The sequence of events occurring at the MWCNT-TiO2 photoanode can be summarized as follows: − + h+ M W C N T + hν → M W C N T (eC.B. V.B. )

Figure 9. Schematic illustration of energy levels of MWCNT–TiO2 under visible light.

− + − M W C N T (eC.B. + h+ V.B. ) → M W C N T (h V.B. ) + T i O2 (eC.B. )

− 2M W C N T (h + V.B. ) + 2I → I2

[6]

[7]

In the case of MWCNT-TiO2 hybrid nanocorals which are subjected to bandgap excitation (>380 nm), they undergo electron-hole pair production and charge separation. Photoinduced electrons from MWCNT are easily transferred to the TiO2 conduction band and subsequently transferred to TCO, since the conduction band of TiO2 is intermediate state between TCO and MWCNT. Simultaneously, the positively charged hole (h+ ) from the TiO2 valence band can migrate to MWCNT/electrolyte interface. With this understanding, the role played by CNTs can be illustrated by injecting electrons into the TiO2 conduction band under visible light irradiation, triggering the formation of very reactive radicals such as superoxide radical ions (O2 •− ) and hydroxyl radicals (HO• ), which are then responsible for the degradation of the organic compound.62–65 In this regenerative cell a redox mediator, the I− ion, is reduced at the photoanode into I3 − . I2 + I − → I3−

[8]

Overall reaction written as, at photoanode T i O2 − M W C N T + hν + 3I − → I3− + 2e−

[9]

at electrolyte I3− + 2e− → 3I −

[10]

To simplify the scheme, the drift of Li+ cations is not shown. The power conversion efficiency η of solar cells were calculated using equation 11 η=

[5]

Jsc Voc × F F × 100 Pin

[11]

Table III. Solar cell parameters for TiO2 and MWCNT-TiO2 samples. Rs is the series resistance and Rsh is the shunt resistance.

Sr. No. 1. 2.

Sample

Thickness (μm)

Jsc (μA/cm2 )

Voc (mV)

Jmax (μA/cm2 )

Vmax (mV)

Rs ()

Rsh (k)

Ideality factor (nd )

FF

Efficiency (η)%

TiO2 (UV light) MWCNT-TiO2 (Visible light)

0.485 0.536

154 451

652 693

96 379

305 556

281 147

19 42

1.86 2.13

0.29 0.68

0.58 2.37


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ECS Journal of Solid State Science and Technology, 1 (2) M15-M23 (2012) Acknowledgment

where FF is the fill factor and is calculated using equation 12 FF =

Jmax × Vmax Jsc × Voc

[12]

where Jmax is maximum current density, Vmax is maximum voltage, Voc is the open circuit voltage, Jsc is the short circuit current density and Pin the power density of the incident light. The FF depends on the series (Rs ) and shunt (Rsh ) resistance. The Rs is due to the resistance of the metal contacts, ohmic losses in the front surface of the cell, impurity concentration and junction depth. Ideally, the RS should be ∼0 . The RSh represents the loss due to surface leakage along the edge of the cell or due to crystal defects. Ideally, the Rsh should be infinite. A lower Rs means that higher current will flow through the device and high Rsh corresponds to fewer shorts or leaks in the device. The Rs can be estimated from the inverse slope at a positive voltage where the J–V curves become linear. The Rsh can be derived by taking the inverse slope of the J–V curves around zero (0) voltage.66 The resistances Rs and Rsh were analyzed from the J–V curves of the film (Fig. 8) using the relation (13 and 14 respectively). 1 dI [13] = d V I =0 Rs

dI dV

v=0

=

1 Rsh

[14]

The value of Rs and Rsh are calculated from J-V curves and are given in Table I. From the above equation the calculated Rs are 758 and 530 , while Rsh is 19 and 42 k for TiO2 and MWCNT-TiO2 samples respectively. Note that Rs Rsh. From the Table III and equation 12, it is clear that the drastic change in FF of MWCNT-TiO2 electrode compared to bare TiO2 electrode explains the increased efficiency. When photons are incident on TiO2 samples the electrons are excited and flow from VB to CB of TiO2 . Due to influence of MWCNT in TiO2 the conductivity of the film increases and reduction of Rsh is observed. The ideality factor ‘nd ’ of prepared films are determined from following diode equation (eq. 15) as, I = I0 (eq V /nd kT − 1)

[15]

where, I0 is the reverse saturation current, V is forward bias voltage, k is Boltzmann’s constant, T is ambient temperature in Kelvin and nd is an ideality factor. The ideality factor is determined under forward bias and is normally found to be in between 1 to 2 depending up on the relation between diffusion current and recombination current. When diffusion current is more than recombination current then ideality factor becomes 1 and it becomes 2 in the opposite case. The ideality factor was found to be 1.86 and 2.13 for the TiO2 and MWCNT-TiO2 electrodes respectively. Conclusions TiO2 -MWCNTs hybrid nanocoral thin films were fabricated successfully by hydrothermal process. MWCNTs were fully and homogenously coated with TiO2 . The deposition of the nanoparticles onto the MWCNTs takes place at a temperature around 120◦ C and the composite structure was confirmed by XPS and Raman spectroscopy. The MWCNT-TiO2 hybrid film shows drastically changed morphology; i.e. the coral like morphology of TiO2 gets disturbed and transforms into compactly arranged spherical ball like particles with size ranging from ∼550–650 nm. The MWCNT–TiO2 hybrid thin film exhibit high photocurrent density (0.693 mA/cm2 ) and yields overall conversion efficiency (η) of 2.37%. Furthermore, the experimental facts indicated that the MWCNT–TiO2 hybrid nanocoral show visible-light-driven photo activity, which is crucial for high conversion efficiency of solar light to the solar cell applications.

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