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Structural and Optical Characterization of CdS/TiO2 Nanocomposite

Archana Maurya, Pratima Chauhan

PII: S1044-5803(11)00028-3DOI: doi: 10.1016/j.matchar.2011.01.014Reference: MTL 6930

To appear in: Materials Characterization

Received date: 22 May 2010Revised date: 17 January 2011Accepted date: 19 January 2011

Please cite this article as: Maurya Archana, Chauhan Pratima, Structural and OpticalCharacterization of CdS/TiO2 Nanocomposite, Materials Characterization (2011), doi:10.1016/j.matchar.2011.01.014

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Structural and Optical Characterization of CdS/TiO2 Nanocomposite

Archana and Pratima Chauhan*

UGC Centre of Advanced Studies, Department of Physics, University of Allahabad,

Allahabad-211002, India.

E-mail: v.arch17@gmail.com, mangu167@gmail.com*

Abstract: Nanocomposites are advantageous because they can compensate for the

disadvantages of the individual component, and induce a synergistic effect, such as an

efficient charge separation and improvement of photostability. Present study reveals the

advantage of nanocomposite CdS/TiO2 over TiO2 which have been prepared by a simple

Co-precipitation method. The particle size of CdS/TiO2 estimated from X-ray diffraction

pattern comes out to be ~ 5.8 nm which is in good agreement with that of transmission

electron microscopy. High resolution transmission electron microscopy showed a

reduction in defect levels in the as prepared CdS/TiO2 nanocomposite in comparison to

TiO2. In our method titanium monoxide (TiO) phase has appeared at room temperature

against the earlier reports. Williamson-Hall plot analysis revealed the presence of tensile

strain in both the CdS/TiO2 and TiO2 samples. New Raman bands for rutile TiO2 have

also been observed which have not been previously reported. UV-visible study reveals

an enhanced absorption capability of the composite sample (~383.85 nm) in comparison

to pure TiO2 (~331.37 and 324.40 nm).The optical and electrical results have shown the

activation of the excited TiO2 in the presence of CdS.

Keywords: CdS/TiO2 Nanocomposite, Structural characterization, Optical

characterization.

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1 Introduction

In recent decades, nanostructured materials have attracted much attention due to their

special structure and excellent properties in optics, electronics, magnetics, chemistry, etc

[1-4]. Discovery and study of new materials whose properties can be tailored made

constitute the core development of solid state technology. Among various nanostructures,

including nanoparticles, nanotubes, nanorods and nanospheres; nanoparticles have

attracted wide attention due to their size-dependent properties. To fully harness the

potential capabilities of nanoparticles, we need to develop new methods to assemble them

into useful patterns or structures [5]. The development of new heterostructure is still a

challenging research subject. It’s challenging to modulate the properties by tailoring the

nucleation of one phase on the surface of the other [6].

In particular titanium dioxide (TiO2), as an important IV-VI group semiconductor,

has been widely applied in chemical industry [7], electronic industry [8, 9],

environmental protection [10], cosmetic industry [11], medical science [12], and so on. In

its pure form it is an n-type semiconductor with indirect band gaps of 3.2 eV for anatase

and 3.02 eV for rutile between the full oxygen 2p valence band and titanium 3d states at

the bottom of the conduction band. Wurtzite CdS, a direct band gap semiconductor with

band gap energy of 2.42 eV, is one of the first discovered semiconductors which have

promising applications in photochemical catalysis, gas sensor, detectors for laser and

infrared, solar cells, non-linear optical materials, various luminescence devices, and

optoelectronic devices [13-15]. One of the major factors that limit the efficiency of TiO2

photocatalysis is fast recombination of photogenerated electron/hole pairs [16].The large

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bandgap of TiO2 also significantly limits its light harvesting ability. To overcome this

drawback composites of two or more semiconducting materials with different band gaps

and band positions are made as this produces a long distance charge separated state with

electron and holes at sites far from each other.

Many methods have already been reported for generating CdS/TiO2 composites [17-21].

The preparation of CdS semiconducting quantum dots (QDs) by the chemical

precipitation method using Thioglycerol (TG) as the capping agent has already been

reported by our group [22]. Herein, we report an approach to efficiently synthesize

CdS/TiO2 composite nanoparticles by a simple co-precipitation method at room

temperature using CTAB as capping agent. Our experimental results clearly give the

evidence of an effective transfer of photogenerated electron from conduction band of CdS

to that of TiO2 and also show a good coupling between CdS and TiO2.

2 Experimental Procedures

2.1 Materials

Cadmium Sulphate (3CdSO4.8H2O) of M.W. = 769.49 and Min. Assay of 99.0 %,

Sodium Thiosulfate Pentahydrate (Na2S2O3 . 5H2O) of Molarity = 248.18 g/mol and Min.

Assay of 99.5-100.5 %, N-Cetyl-N ,N,N-trimethyl ammonium bromide (C19H42 Br N),

M.W. = 364.46, Minimum assay(iodometry)= 99.0% and Titanium (IV) chloride (TiCl4)

(anhydrous), M.W. = 189.71, density (d420

) = 1.729-1.730 and Min. Assay

(Argentometric) of 99.0 % had been used as starting materials. Ethanol (Merck), density

(d 20oC/20

oC) = 0.790-0.793 and purity of 99.9 % being commercially available. Double

distilled water had been used for all studies. All the used chemicals were analytic grade

reagents without further purification.

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2.1 Preparation of TiO2

Titanium tetrachloride was used as the starting material to prepare pure TiO2. 6.0 ml

titanium tetrachloride (TiCl4) was dropped into 100 ml double distilled water at room

temperature under vigorous stirring until the white fog disappeared. The solution

gradually turned into transparent soft gel, which continued to change with time, from

translucent to milky white. The process took 4–5 days, including the formation of gel (2–

3 days). The final gel was neutral (pH about 7). The gel was filtered under suction and

washed with distilled water until Cl− could not be measured (with 0.1 mol/l AgNO3). The

final white precipitate was washed with ethanol and double distilled water several times

and then dried at room temperature. In order to get finally divided TiO2 nanopowder the

samples were grinded in mortar with pestle.

2.3 Preparation of CdS/TiO2 nanocomposite

CdSO4 and Na2S2O3 were used as the reactant materials, and N-Cetyl-N, N, N-trimethyl

ammonium bromide (C19H42 Br N) (CTAB) was added as the capping agent. The

concentrations of CdSO4, Na2S2O3 and CTAB were 2mM, 100 mM and 0.2 M

respectively. In a simple procedure 100 ml aqueous solution of the reactants (CdSO4 and

Na2S2O3) and capping agent CTAB were prepared by Co- precipitation route. 6.0 ml of

TiCl4 was also slowly added dropwise to the solution under constant stirring. A colorless

solution was obtained which had been left at room temperature. A white precipitate

comes out after leaving the solution for 4 days. The solid product was then recovered by

filtration, washed with deionized water and air dried at room temperature. Finally divided

CdS/TiO2 nanopowder was obtained by grinding the samples in mortar and pestle. We

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have observed that the color of white precipitate turned to light yellow in the process of

drying.

Reaction mechanism:

The following reaction mechanism may be expected to occur between CdSO4 and

Na2S2O3 when CTAB was used as capping agent:

CdSO4 + Na2S2O3 + H2O CdS + Na2SO4 + H2SO4

Also, TiCl4 reacts with deionized water to give TiO2 as:

TiCl4 + 2H2O TiO2 + 4HCl

Finally, the reaction between CdS and TiO2 takes place as [23]:

CdS (e- + h

+) + TiO2 CdS (h

+) + TiO2 (e

-)

CdS (e- + h

+) + TiO2 (e

- + h

+) CdS (h

++ h

+) + TiO2 (e

- + e

-)

2.4 Characterization

Rigaku D-max XRD with Cu-Kα line (λ = 1.54A˚) was used for recording X-ray

diffraction (XRD) pattern operating at 30kV and 30mA in the 2 range 20o – 60

o.

Transmission electron microscopy (TEM) and high resolution transmission electron

microscopy (HRTEM) were performed by using Tecnai 30 G2 S-Twin electron

microscope operating at 300 kV accelerating voltage. Samples for TEM were prepared by

ultrasonically dispersing the product in ethanol, and the droplets were placed on carbon

coated Cu grids. Raman spectra were obtained using a RENISHAW invia Raman

spectrometer (RM2000) integrated with a Lica microscope. All the Raman spectra were

recorded by 514 nm Ar+ laser at room temperature. The UV-visible spectra were recorded

on Lambda 35 Perkin Elmer in the range 200-800 nm. Photoluminescence (PL) spectra of

the prepared samples were measured at room temperature using LS 55 Perkin Elmer

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spectrophotometer with 340 nm and 375 nm excitation line of Xenon discharge lamp,

equivalent to 20kV for 8µs duration with pulse width at half height < 10µs. Current-

voltage (I-V) curves of pure TiO2 and CdS/TiO2 pallets were taken under illumination

with a solar simulator (Oriel). The intensity of the Xenon lamp was adjusted to 100

mW/cm2.

3 Results and Discussion

Pure TiO2 and CdS/TiO2 composite were analyzed using XRD, TEM and HRTEM to

evaluate the structural characteristics. Figure 1 (a) shows the powder XRD pattern of pure

TiO2, which can be indexed to tetragonal rutile phase (JCPDS file no. 21-1276). In the

XRD spectra of CdS/TiO2 composite (Figure 1(b)) additional diffraction peaks marked

with #, $ and * also appears which corresponds to the powder diffraction patterns for

cubic CdS (JCPDS file no.772415), hexagonal CdS (JCPDS file no. 800006) and

titanium monoxide (TiO) (JCPDS file no.851061), respectively. The coexistence of both

the phases of CdS in the CdS/TiO2 composite attributes the particle size to be in the nano-

size regime [24-27]. In our case the TiO phase appears at room temperature itself against

the earlier report [28]. The possible explanation for this can be given by assuming the

formation of TiO molecules as a result of chemical reaction between Ti and TiO2 under

such experimental condition. The particle size (D) for both samples has been calculated

from (110) crystal plane located at 27.23o, using Debye-Scherrer formula ([D= 0.89 λ/ β

cos(θ)], where λ is the wavelength of X-ray used, β is the full width at half maximum

(FWHM) value and θ is the scattering angle).The calculated particle size of the pure TiO2

and CdS/TiO2 composite are ~ 6.3 nm and ~ 5.8 nm respectively. Furthermore, it is

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known that FWHM can be interpreted in terms of lattice strain and crystalline size [29]

which can be expressed by the following equation,

sin1cos

Where, ε and η are the effective particle size and the effective strain. The effective

particle size taking strain into account can be estimated by plotting β cosθ/λ versus sinθ/λ.

We have calculated the effective particle size and strain for both the samples by

Williamson-Hall plot, which are shown in Figure 2(a) and 2 (b). A negative slope in the

plot indicates the presence of compressive strain as has been reported by several authors

[30, 31]. In our case the appearance of positive slope has ensured the possibility of the

presence of tensile strain [32]. Williamson-Hall plot gives lattice strain of 4.29 % for pure

TiO2 and 4.97 % for CdS/TiO2. Effective particle size calculated by Williamson-Hall plot

for TiO2 and CdS/TiO2 are ~ 7.5 nm and ~ 5.6 nm respectively. These particle size values

appear to be in good agreement with the one calculated by Scherrer formula.

TEM image of both the samples are shown in Figure 3(a) and 3(b)

respectively. These figures correspond to the formation of uniform spherical

nanoparticles with slight prolate deviation. Some aggregations among the nanoparticles

have been observed for the case of CdS/TiO2. The particle size of TiO2 lies in the range

of ~ 2.79 to 6.55 nm whereas of CdS/TiO2 ~ 3.49 to 5.08 nm. These results are found to

be consistent with the XRD and Williamson-Hall plot results. Figure 4(a) and 4(b) show

the HRTEM images of the prepared pure TiO2 and CdS/TiO2 composite. The images

clearly show lattice fringes indicating good crystallinity of the samples. From figure 4(a),

the lattice fringes most frequently observed at d0.33, d0.22, d0.25 and d0.17 correspond to the

(1 1 0), (1 1 1), (1 0 1) and (2 1 1) crystal planes of pure rutile phase of TiO2. The

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formation of some planar defects including twinned crystal plane and line imperfection

has also been observed in TiO2. These defect sites act as trap states for the electrons and

holes. Meanwhile, the HRTEM image of composite CdS/TiO2 (figure 4(b)) shows the

formation of mixed phases of crystal planes. The most frequently observed fringes at

d0.28, d0.25, and d0.19 corresponds to (0 1 5), (1 0 9) and (2 1 1) crystal plane of TiO and

d0.33 is the (1 1 0) crystal plane of rutile TiO2. However, in addition we have also

observed fringes of cubic CdS at d0.15 corresponding to (2 0 0) crystal plane and fringes

of hexagonal CdS at d0.34 which corresponds to (0 0 2) crystal plane. Thus CdS particles

being in close contact with TiO2 particles are easily observed in HRTEM images of

CdS/TiO2, indicating a good coupling between the distinct phase domains of CdS and

TiO2. A reduction in planar defect can easily be seen from the HRTEM image of

CdS/TiO2.

Young-wook Jun, Yung-tak Jang et al. have reported that nanocrystals can have

potentially a variety of crystalline phases from isotropic structures of cubic to anisotropic

structure of hexagonal or monoclinic [33]. Among various crystalline phases, the stable

phase of materials is highly dependent on its environmental conditions such as

temperature and choice of capping molecules. The surface energy can significantly

modify the phase relationship when the particle size is very small. Adsorption of ions

onto surfaces and lattice strain can further modify phase relations through their effect on

surface energy. Given the particle size effect on the phase relationship and the small

volumes of the materials involved; formation of titanium monoxide and rutile TiO2 phase

from anatase TiO2, and hexagonal and cubic phases of CdS may be possible by a solid

state reaction. The nucleation of rutile may be promoted by the fact that twin surfaces

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contain a unit cell of rutile. This surface can act as a nucleus, and may grow as a stable or

nearly stable phase, driven by random fluctuations. From the above discussion we may

arrive to an important conclusion that oriented attachment and growth can lead to the

formation of planar defects including twin planes and other interfaces. The structurally

different regions may serve as nucleation site for new phases.

Typical Raman spectra of TiO2 and CdS/TiO2 samples at room

temperature using 514.5 nm excitation wavelength of Ar+ laser are shown in Figure 5. To

see the differences between the spectra more clearly, the wavenumbers with their

corresponding full width half maxima (FWHM) are given in table 1. Rutile (tetragonal,

P42/mnm) consist of two units and four Raman active modes at 143 cm-1

(B1g), 447 cm-

1(Eg), 612 cm

-1(A1g) and 826 cm

-1(B2g) [34]. As can be seen from the table that the

Raman bands in our case occurs at position of higher wavenumber side compared to that

of the reported value for rutile TiO2. Since, XRD pattern has confirmed the pure rutile

phase of TiO2, with no significance of any other phase, all the observed Raman bands can

be assigned to the Raman bands of the rutile single phase. These Raman bands of rutile

TiO2 observed in our case have not been previously reported to the best of our

knowledge. The peak at 220.08 cm-1

in CdS/TiO2 nanocomposite resembles the peak of

CdS as observed by D. Mohanta et al. [35]. Earlier reports show that the rutile peaks of

TiO2 appear at higher temperature [36]. But at high temperature the particle size also

increases and exhibit a large frequency redshift with remarkable decrease in linewidth

[37] .Hyun Chul Choi et al, have reported that for TiO2 the Raman band shifts towards

higher wavenumber and their intensities relatively decrease as the particle size decreases

[38]. The same result has been reported by several authors for different types of

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nanoparticles [39-44]. The variation in Raman band can be attributed due to the small

size and room temperature preparation of the nanoparticles. Furthermore, while

comparing the shifts in band position between TiO2 and CdS/TiO2 we observe that for

CdS/TiO2 nanocomposite the Raman band at 262.28 cm-1

shifts to 263.97 cm-1

and

436.82 cm-1

shifts to 437.86 cm-1

. It has been reported earlier that such a blue shift could

be attributed to phonon confinement caused by a decrease in crystal size [45] or an

increased surface strain of CdS/TiO2 caused by the adsorption of surfactant [46].

However, the band at 594.26 cm-1

possess less variation (594.14 cm-1

) and that at 687.07

cm-1

suffers large variation (684.72 cm-1

) towards lower wavenumber for the case of

CdS/TiO2. These effects occur because CdS/TiO2 sample contains a range of particles

from ~ 3.49 to 5.08 nm as obtained from TEM analysis. When the particle size decreases

towards the nanometer scale a volume contraction occurs within the nanoparticle due to

the size-induced radial pressure and this leads to increase in the force constants as a result

of the decrease in the interatomic distances. In vibrational transitions, the wavenumber

vary approximately in proportion to k1/2

, where k is the force constant. Consequently, the

Raman band shifts towards higher wavenumber due to the increasing force constants

[47]. The contraction effect also induces a decrease in the vibrational amplitudes of the

nearest neighbour bonds due to the increase of the static disorder and thermal vibrational

disorder of a material [48, 49]. This decrease in vibrational amplitude with decrease in

particle size is responsible for the intensity variation of the Raman bands observed in

figure 5. Also, as far as FWHM value is considered for composite sample we observe its

value decreases for some band positions and increases for others. H. Cao et al. have

reported the decrease of FWHM to the improvement of the crystallinity of the

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synthesized material [50]. A measurement of FWHM value implies that the crystallinity

of composite sample is somehow less perfect than TiO2 but not in great amount.

O ptical properties of nanocomposite could be different from the

corresponding components. Also, different sizes and shapes have different effect on

the properties of the material as well. The variation of any physical properties

caused by the incorporation of CdS in TiO 2 can be elucidated by considering the

optical properties: UV-visible and PL spectroscopy.

The UV-visible absorption spectra of TiO2 and CdS/TiO2 composite samples are shown

in Fig.6 (a) and 6 (b) respectively. The absorption band gap energy can be determined by

the following equation [51]:

(αhν)n = B (hν – Eg)

Where hυ is the photon energy, α is the absorption coefficient, B is a constant relative to

the material and n is a value that depends on the nature of transition (2 for a direct

allowed transition, 2/3 for direct forbidden transition, and 1/2 for indirect allowed

transition). However, in the nanocrystal there may be some deviation from a bulk-like

transition. So, we have determined the nature of transition by the equation [52]

d{ln(αhν)}/d(hν) = n/(hν − Eg) (2)

Figure 7(a) and 7 (b) show the differential graphs of TiO2 and CdS/TiO2 obtained by

considering equation (2). We observed that absorption edge of CdS/TiO2 nanocomposite

occurs at energy of 3.22 eV (~383.85 nm) which matches neither the optical edge of bulk

rutile TiO2, (3.02 eV) (~409 nm) nor that of bulk CdS (2.42 eV) (~508 nm). Also, as far

as prepared pure TiO2 sample is considered, the absorption edge gives double transitions

at the energies of 3.73 eV (~331.37 nm) and 3.81 eV (~324.40 nm) which is larger than

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the value of 3.02 eV for bulk rutile TiO2 suggesting a possibility of degeneracy of

valence band in TiO2. It can be seen that the absorption of both the samples lies in the

ultraviolet region and the absorption edge is less than 400 nm. The enhanced absorption

capability of CdS/TiO2 nanocomposite in comparison to pure TiO2 in the UV-visible

region of the spectra is attributed due to a coupling between TiO2 and CdS

heterostructure.

Figure 8 shows the PL spectra of TiO 2 and CdS/ TiO 2 nanocomposite at excitation

wavelengths of 3 4 0 and 3 7 5 nm. For PL measurements an excitation wavelength of

3 4 0 nm has been selected since it is close to the maximum absorption spectra for

pure TiO 2. Band gap (Eg) excitation originates a broad emission band centered at

4 0 5 nm (3 .0 5 eV) in addition to two minor bands at 45 7 and 49 8 nm for both

the cases of pure TiO 2 and CdS/ TiO 2. It has been reported earlier that there is no

PL peak in pure anatase phase, but a weak PL peak in the rutile phase of TiO 2 [5 3 ,

5 4 ]. But in our case we have observed a fine PL for rutile phase TiO 2 and an

enhanced PL for the CdS/ TiO 2 which is contradicting the earlier reports. Also, it

can be seen that the main peak in the high-energy side and minor peaks in the low-

energy side remain at same position upon combination of TiO 2 with CdS. O nly the

overall intensity of PL gets increased. Several authors have reported quenching

behavior of CdS/ TiO 2 nanocomposite which indicates deactivation of the excited

CdS in the presence of TiO 2 via electron transfer to TiO 2 nanoparticles [5 5 - 5 9 ].

An enhanced PL intensity in our case can be attributed to an activation of the

excited TiO 2 in the presence of CdS rather than deactivation. Also, on increasing

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excitation wavelength 37 5 nm, not only the number of minor PL peaks in the low-

energy side decreases, but the peak position also shifts as well. This happens because

at lower energies, several different recombination pathways involving trap states

exist viz.; (1 ) recombination of a conduction band (CB) electron with a trapped

hole; (2 ) recombination of trapped electron with a valence band (VB) hole; (3 )

recombination of a trapped electron with a trapped hole. The region is referred to

as trap-state PL [6 0 ].

The current density-voltage (J-V) curves of the pure TiO 2 and

CdS/ TiO 2 under illumination are shown in Figure 9 (a) and 9 (b). It is observed

that larger values of Jsc occur for CdS/ TiO 2 nanocomposite, which varied from

0 .6 5 to 9 .7 4 mA/ cm2,indicating that the composite sample is more conductive.

The values of Jsc for TiO 2, on the other hand, varied in the range of microamperes

from 1 .6 5 to 4 9 .89 μA/ cm2,which can be attributed due to the high molecular

defect density and large electron-hole recombination. Considering that the

crystallite size and oxygen vacancies are similar for both the samples, the difference

in electrical behavior of CdS/ TiO 2 with respect to that of TiO 2 can be attributed

due to a mixture of crystalline phases. It has been established that in the CdS/ TiO 2

composite, CdS and TiO 2 serve for light collection and photoinduced charge

transfer respectively. The carrier of CdS can be promoted from the valence band to

the conduction band by light excitation due to its direct band gap in the visible

wavelength region. The photogenerated electrons in CdS are transferred into the

TiO 2 particles while the hole remains in the CdS particles (Figure 1 0 ). This helps in

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charge separation by isolating electron and hole in two distinct particles and hence

causing an increased Jsc in case of CdS/ TiO 2.

4 Conclusions

Present paper demonstrates the detailed study of the structural and optical

properties of pure TiO 2 and that of CdS/ TiO 2 composite nanoparticles. XRD

investigation has confirmed the formation of CdS/ TiO 2 composite. The particle size

of pure TiO 2 and CdS/ TiO 2 composite synthesized at room temperature are ~ 6 .3

nm and ~ 5 .8 nm. TEM image of pure TiO 2 and CdS/ TiO 2 shows the formation of

nearly spherical nanoparticles. The particle size as estimated by Williamson-Hall plot

also appeared to be consistent with the XRD and TEM results. The appearance of

positive slope has ensured the possibility of the presence of tensile strain. New Raman

bands for rutile TiO2 is observed in our case which has not been previously reported to

the best of our knowledge. Such variation in Raman band could be attributed due to

phonon confinement caused by decrease in crystal size. UV-visible study reveals an

enhanced absorption capability of the composite sample in comparison to TiO2 in the

UV-visible region of the spectra. PL characterization attributes an activation of the

excited TiO 2 in the presence of CdS rather than deactivation. An enhanced Jsc for

CdS/ TiO 2 indicates that the composite sample is more conductive than TiO 2.

Electrical and optical properties of CdS / TiO 2 nanocomposite reveals that sample

could have its major application in white light nanodevices such as light emitting

diodes (LEDs) and photocatalysis.

Acknowledgement

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The authors wish to thank to Prof. A.C. Pandey, Nanotechnology Application Center,

University of Allahabad, Allahabad, India for providing XRD, TEM, HRTEM, RAMAN

and PL facilities. We are thankful to Prof. Ram Gopal, Deptt of Physics, University of

Allahabad, Allahabad, India for providing the UV-Visible absorption facility. One of the

authors Ms Archana would like to acknowledge UGC for granting research fellowship.

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Figure Caption

Figure 1. X-ray dif f raction patterns of pure rutile TiO 2, and CdS/ TiO 2

nanocomposite prepared via simple Co- precipitation method.

Figure 2. Williamson-Hall plots of (a) pure TiO2 and (b) CdS/TiO2 for the size and strain

analysis of the prepared nanoparticles.

Figure 3. TEM image of (a) pure TiO 2 ;(b) CdS/ TiO 2 nanocomposite indicating the

formation nearly spherical nanoparticles in shape.

Figure 4. HRTEM image of (a) TiO 2 showing crystal planes of pure rutile phase, and

(b) CdS/ TiO 2 nanocomposite showing crystal planes of rutile (R) titanium dioxide

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(TiO2), titanium monoxide (TiO), and hexagonal (H) and cubic (C) cadmium sulfide

(CdS).

Figure 5. Raman spectra of TiO2 and CdS/TiO2 composite sample prepared via Co-

precipitation method.

Figure 6. Comparison of UV-visible absorption spectra between (a)TiO 2 and (b)

CdS/ TiO 2. The nominal concentration of TiCl4 in both the samples is same 6 .0 ml.

Figure 7. Plot of d{ln(αhν)}/d(hν) vs. hν for the (a) pure TiO2 nanoparticles ,and (b)

CdS/TiO2 nanocomposite.

Figure 8. PL spectra of TiO 2 and CdS/ TiO 2 nanoparticles at excitation wavelength of

3 4 0 and 3 7 5 nm.

Figure 9. Current density vs. applied voltage curves (J-V) of the (a) pure TiO 2 and

(b) CdS/ TiO 2 under illumination of 1 0 0 mW/ cm2.

Figure 10. Energy level diagram of CdS/ TiO 2 nanocomposite.

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Figure 1. X-ray dif f raction patterns of pure rutile TiO 2, and CdS/ TiO 2

nanocomposite prepared via simple Co-precipitation method.

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Figure 2 Williamson-Hall plots of (a) pure TiO2 and (b) CdS/TiO2 for the size and strain

analysis of the prepared nanoparticles.

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Figure 3. TEM image of (a) pure TiO 2 ;(b) CdS/ TiO 2 nanocomposite indicating the

formation nearly spherical nanoparticles in shape.

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Figure 4. HRTEM image of (a) TiO 2 showing crystal planes of pure rutile phase, and

(b) CdS/ TiO 2 nanocomposite showing crystal planes of rutile (R) titanium dioxide

(TiO2), titanium monoxide (TiO), and hexagonal(H) and cubic (C) cadmium sulfide

(CdS).

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Figure 5. Raman spectra of TiO2 and CdS/TiO2 composite sample prepared via Co-

precipitation method.

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Figure 6. Comparison of UV-visible absorption spectra between (a)TiO 2 and (b)

CdS/ TiO 2. The nominal concentration of TiCl4 in both the samples is same 6 .0 ml.

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Figure 7. Plot of d{ln(αhν)}/d(hν) vs. hν for the (a) pure TiO2 nanoparticles ,and (b)

CdS/TiO2 nanocomposite.

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Figure 8. PL spectra of TiO 2 and CdS/ TiO 2 nanoparticles at excitation wavelength of

3 4 0 and 3 7 5 nm.

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Figure 9. Current density vs. applied voltage curves (J-V) of the (a) pure TiO 2 and

(b) CdS/ TiO 2 under illumination of 1 0 0 mW/ cm2.

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Figure 10. Energy level diagram of CdS/ TiO 2 nanocomposite

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Table 1 Raman peak position and FWHM value for the as-synthesized pure rutile TiO2

and CdS/TiO2 nanocomposite sample.

Raman Peak Positions (cm-1

) and FWHM value

TiO2 FWHM CdS/TiO2 FWHM

262.28 91.35 220.08 4.788

436.82 44.70 263.97 73.51

594.26 44.51 437.86 47.42

687.07 76.59 470.18 16.90

594.14 47.49

684.71 84.41

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Graphical Abstract

Structural and Optical Characterization of CdS/TiO2 Composite Nanoparticles

Archana Mauryaa and Pratima Chauhan

a*

UGC Centre of Advanced Studies, Department of Physics, University of Allahabad,

Allahabad-211002, India.

E-mail: v.arch17@gmail.com, mangu167@gmail.com*

Abstract: Nanocomposites are advantageous because they can compensate for the

disadvantages of the individual component, and induce a synergistic effect, such as an

efficient charge separation and improvement of photostability. Present study reveals the

advantage of nanocomposite CdS/TiO2 over TiO2 which have been prepared by a simple

Co-precipitation method. The particle size of CdS/TiO2 estimated from X-ray diffraction

pattern comes out to be ~ 5.8 nm which is in good agreement with that of transmission

electron microscopy. High resolution transmission electron microscopy showed a

reduction in defect levels in the as prepared CdS/TiO2 nanocomposite in comparison to

TiO2. In our method titanium monoxide (TiO) phase has appeared at room temperature

against the earlier reports. Williamson-Hall plot analysis revealed the presence of tensile

strain in both the CdS/TiO2 and TiO2 samples. New Raman bands for rutile TiO2 have

also been observed which have not been previously reported. UV-visible study reveals

an enhanced absorption capability of the composite sample (~383.85 nm) in comparison

to pure TiO2 (~331.37 and 324.40 nm).The optical and electrical results have shown the

activation of the excited TiO2 in the presence of CdS.

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Research Highlights

Advantage of nanocomposite CdS/TiO2 over TiO2 prepared by a simple Co-

precipitation method.

Appearance of positive slope has ensured the possibility of the presence of tensile

strain.

New Raman bands for rutile TiO2 which has not been previously reported.

Activation of the excited TiO 2 in the presence of CdS rather than

deactivation