Synthesis of γ-Fe2O3-CdS nanocomposites with enhanced photocatalytic activity by thermal decomposition approach

Article

Journal of

Nanoscience and Nanotechnology

Vol. 16, 9072–9084, 2016

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Printed in the United States of America

Synthesis of ꢀ-Fe₂O₃–CdS Nanocomposites with

Enhanced Photocatalytic Activity by Thermal

Decomposition Approach

Sudheer Kumar Yadav and P. Jeevanandam^∗

Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

ꢀ-Fe₂O₃–CdS nanocomposites have been prepared by a single step thermal decomposition method

which involves thermal decomposition of iron acetylacetonate, cadmium acetate and thiourea in

diphenyl ether at 200 ^ꢀC. The nanocomposites were characterized by powder X-ray diffraction,

IR spectroscopy, atomic absorption spectroscopy, thermal gravimetric analysis, energy dispersive

X-ray analysis, transmission electron microscopy, surface area measurements, diffuse reﬂectance

spectroscopy, photoluminescence spectroscopy and magnetic measurements. The effect of vary-

ing concentration of iron acetylacetonate, cadmium acetate and thiourea during synthesis of the

nanocomposites on the crystal phases, microstructure and properties of the nanocomposites was

investigated. The XRD results conﬁrm the presence of CdS nanocrystals (1.2 to 2.9 nm) in the

ꢀ-Fe₂O₃–CdS nanocomposites and the nanocomposites exhibit superparamagnetic behaviour at

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room temperature. The nanocomposites show good absorption in visible region (1.8 eV to 2.5 eV)

IP: 46.161.63.44 On: Wed, 08 Mar 2017 08:33:28

and they exhibit visible-light-driven photocatalytic degradation of congo red in an aqueous solution

Copyright: American Scientific Publishers

with good stability.

Keywords: Nanoparticles, ꢀ-Fe₂O₃–CdS

Nanocomposites,

Thermal

Decomposition,

Photocatalytic Activity.

1. INTRODUCTION

chemical stability, iron oxide nanoparticles have been

extensively investigated for a wide variety of applications

such as catalysis, water treatment, sensors, magnetic mate-

rials, pigments, drug delivery and lithium ion batteries.^14–20

With low band gap (about 2.0 to 2.2 eV), ꢁ-Fe₂O₃absorbs

in the visible region and it is one of the most promis-

ing photocatalysts in this region.²¹However, the photo-

induced electron–hole pairs in ꢁ-Fe₂O₃easily recombine

which inhibits its application as an efﬁcient photocatalyst.

To overcome this drawback, composites of ꢁ-Fe₂O₃with

Nanocomposites have been intensively investigated due

to their diverse applications.^1–4Metal oxide-metal sul-

ﬁde nanocomposites are of current technological interest

due to their wide applications in solar energy conver-

sion, optoelectronic devices, and photocatalysis.⁵Metal

sulﬁde semiconductor nanocrystals such as CdS, ZnS, CuS

and PbS have been coupled with metal oxide semicon-

ductors such as TiO₂, SnO₂, ZnO and Fe₂O₃to extend

their photo response to visible region. The coupling facil-

itates charge separation by accumulating electrons and

holes in separate particles.⁶Some of the recently reported

metal oxide-metal sulﬁde nanocomposites that are useful

for the photocatalytic degradation of organic dye pollu-

tants include CdS–TiO₂, Fe₃O₄–ZnS, CuS–ZnO, Fe₂O₃–

CdS and PbS–TiO₂.^7–11

22

other semiconductors such as CdS, SnO₂,²⁰and TiO₂

have been prepared. CdS is a semiconductor with a direct

band gap of 2.42 eV and regarded as one of the most

important photocatalysts in the visible region. However,

CdS is susceptible to photo corrosion in aqueous media

due to which it is not preferred for applications in envi-

ronmental remediation.²³In recent times, attempts have

been made to prepare CdS based nanocomposites.¹⁰CdS

is coupled with another semiconductor with lower but

closely lying conduction band and appropriate valence

band. Charge separation occurs at the interface of the two

Synthesis of iron oxide based semiconductor nanocom-

posites is of immense interest.^12ꢀ13Because of their low

cost, easy production, environmental friendliness and good

^∗Author to whom correspondence should be addressed.

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J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 9

1533-4880/2016/16/9072/013

doi:10.1166/jnn.2016.12902

Yadav and Jeevanandam

Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Table I. Designation of various ꢁ-Fe₂O₃–CdS nanocomposites pre-

semiconductors and the photo generated carriers interact

with the reactants efﬁciently at the interface.²⁴CdS–Fe₂O₃

heterojunctions are good photocatalysts for the degradation

of hazardous organic dyes such as methylene blue, and

congo red, photocatalytic reduction of Cr(VI) and hydro-

gen production.^25–28

pared using different synthetic conditions.

Sample

code

Fe(acac)₃

(mmol)

Cd(ac)₂

(mmol)

Thiourea

(mmol)

Thermal decomposition

time (min)

Fe₂O₃

CdS

FC1

FC2

FC3

FC4

FC5

FC6

1

–

1

1ꢃ25

1

–

1

0ꢃ1

0ꢃ25

1

0ꢃ1

0ꢃ25

1

–

1

0ꢃ1

0ꢃ25

1

0ꢃ1

0ꢃ25

1

35

70

Fe₂O₃–CdS nanocomposites with different morpholo-

gies such as CdS nanowires decorated with Fe₂O₃

nanoparticles have been prepared by a solvothermal

method.¹⁰Wang et al. have synthesized CdS nanowires

decorated with Fe₃O₄microspheres by the same method.²⁵

Shi et al. have grown CdS nanoparticles on single crys-

talline ꢂ-Fe₂O₃nanorods by hydrothermal and solution

routes.²⁹Wu et al. have synthesized CdS urchin-like

micro-spheres decorated with hematite/magnetite nanopar-

ticles by the solvothermal method.³⁰Zhang et al.

have synthesized hierarchical ꢂ-Fe₂O₃/CdS microﬂow-

ers with ꢂ-Fe₂O₃core and CdS shell through chem-

ical bath deposition.²⁷Preethi et al. have synthesized

Fe₂O₃@CdS@ZnS core–shell nanoparticles by a multi-

step solution route.²⁸Chen et al. have deposited CdS

1

1ꢃ25

or 70 min. Then, 30 mL methanol was added to the con-

tents to get the precipitate. The precipitates obtained were

centrifuged, washed with methanol and dried overnight

under vacuum to obtain the ꢁ-Fe₂O₃–CdS nanocomposite

powders. For comparison purpose, pure CdS and Fe₂O₃

nanoparticles were also prepared using the similar syn-

thetic procedure.

The analysis of crystal phases of all the nanocompos-

ites was carried out by powder X-ray diffraction using

a Bruker AXS D8-Advance X-ray diffractometer with

Cu-Kꢂ radiation (ꢄ = 1.5406 Å) with a scan speed,

0.2^ꢀ/min. A Thermo Nicolet Nexus (FT-IR) spectrome-

ter was used for recording IR spectra of the nanocom-

posites using KBr pellets. Thermo gravimetric analysis

nanoparticles on Fe₂O₃nanorods via

a three step

hydrothermal method.³¹Kwon et al. have synthesized

hetero-structured nanocrystals of ꢁ-Fe₂O₃and CdS by

airless thermal decomposition of Fe(CO)₅in octyl ether

in the presence of oleic acid.³²Liu et al. have synthe-

sized Zn_1−xCd_xS/ꢁ-Fe₂O₃nanocomposites (x = 0 to 1)

by a solution combustion method.³³Most of the reported

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(TGA) was carried out on an EXSTAR TG/DTA 6300

IP: 46.161.63.44 On: Wed, 08 Mar 2017 08:33:28

instrument under air (ﬂow rate = 200 mL/min) from 25

to 1000 ^ꢀC with a heating rate of 10 ^ꢀC min⁻¹. The

elemental analysis of the ꢁ-Fe₂O₃–CdS nanocomposites

was carried out using a ﬁeld emission scanning elec-

tron microscope (Carl Zeiss ULTRA plus) operating at

an accelerating voltage of 15 kV coupled with an energy

dispersive X-ray analysis (EDXA) facility. The cadmium

content in the nanocomposites and in the supernatant solu-

tions after photocatalytic experiments was analyzed using

a Perkin–Elmer AAnalyst 800 atomic absorption spec-

trometry (AAS) instrument. For the AAS measurements,

all the composites of known amount were digested in

HCl and then, Cd²⁺solutions were prepared in volu-

metric ﬂasks and then analyzed. Transmission electron

micrographs and selected area electron diffraction pat-

terns of the nanocomposites were recorded using a FEI

TECHNAI G²electron microscope operating at 200 kV.

For the TEM measurements, the sample powders were

dispersed in ethanol with the help of a sonicator and

a drop each from the dispersions was allowed to dry

in air on carbon coated copper grids. For the study of

optical properties of the ꢁ-Fe₂O₃–CdS nanocomposites,

a Shimadzu UV-2450 UV-visible spectrophotometer was

used along with a diffuse reﬂectance accessory and BaSO₄

was used as the reference. Photoluminescence spectra

were recorded using a HORIBA Fluoromax-4 spectroﬂu-

orophotometer (ꢄ_excitation= 360 nm). The emission spectra

were recorded at room temperature for the suspensions

methods for the preparation of Fe O –CdS nanocompos-

ites consist of multi-steps with long reaction time and the

₂Copyright: American Scientific Publishers

3

methods often require expensive reagents. In the present

study, ꢁ-Fe₂O₃–CdS nanocomposites were synthesized by

a single step thermal decomposition of cadmium acetate,

thiourea and iron acetylacetonate in diphenyl ether at about

200 ^ꢀC for 35 min. The ꢁ-Fe₂O₃–CdS nanocomposites

were characterized by different techniques and their optical

and magnetic properties were studied. The nanocomposites

were then explored as catalysts for photocatalytic degra-

dation of congo red, a toxic dye, in an aqueous solution.

2. EXPERIMENTAL DETAILS

The chemicals used were cadmium acetate (Hime-

dia), thiourea (Rankem), iron acetylacetonate (ACROS),

diphenyl ether (Aldrich) and congo red (SRL). All the

chemicals were used as received. Millipore^®water was

used for the preparation of the solutions. The ꢁ-Fe₂O₃–

CdS nanocomposites were synthesized by the thermal

decomposition method and more details are as follows.

Six different nanocomposites (Table I) were prepared

by varying the concentration of cadmium acetate, thiourea

and iron acetylacetonate and also by varying the thermal

decomposition time. The reagents (iron acetylacetonate,

cadmium acetate and thiourea) were added to 10 mL of

diphenyl ether (boiling point = 257 ^ꢀC) in a round bot-

ꢀ

tom ﬂask and reﬂuxed in air at about 200 C for 35 min

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

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Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Yadav and Jeevanandam

prepared by dispersing about 5 mg each of ꢁ-Fe₂O₃–CdS

Fe₂O₃

* CdS

nanocomposite powders in about 5 mL water by soni-

cation for 10 minutes. Surface area of the nanocompos-

ites was measured using a NOVA 2200e (Quantachrome)

instrument by BET method using nitrogen physisorption

at 77 K. Prior to the analysis, the samples were degassed

at about 100 ^ꢀC for about 2 h. Magnetic measurements

(M–H curves at 300 K and 5 K) of pure Fe₂O₃nanopar-

ticles and the ꢁ-Fe₂O₃–CdS nanocomposites were carried

out on a 3 Tesla cryogen free mini Vibrating Sample

Magnetometer with a pulse cryocooler (CRYOGENICS

Ltd., UK).

CdS

FC1 (1:0.1:0.1)

FC2 (1:0.25:0.25)

FC3 (1.25:1:1)

FC4 (1:0.1:0.1)

FC5 (1:0.25:0.25)

FC6 (1.25:1:1)

*

Congo red (sodium salt of benzidine diazo-bis-1-

naphthyl amine-4-sulphonic acid) is an azo dye which

is often used as a coloring agent in textile and paper

10

20

30

40

50

60

70

80

90

industries.³⁴Congo red can cause serious environmental

hazards and endanger human life due to its toxic, muta-

genic and carcinogenic nature.³⁵Hence, it is necessary

to develop efﬁcient and economical methods to remove

congo red from the waste water. Semiconductors based

photocatalysis is considered as one of the most promising

methods for waste water treatment. This is because of the

strong destructive power of semiconductors to mineralize

toxic pollutants into CO₂and H₂O. In the present study,

photocatalytic activity of the ꢁ-Fe₂O₃–CdS nanocompos-

ites was tested using aqueous solution of congo red under

sunlight irradiation. About 50 mg of each nanocomposite

powders was dispersed in 50 mL of congo red solution

(concentration = 50 mg L⁻¹ꢅ. The suspension was soni-

cated for about 10 minutes and kept in dark for 10 minutes

to reach adsorption–desorption equilibrium. Afterwards,

the suspension was irradiated in sunlight at room tem-

perature for various time intervals (0 to 60 minutes).

Aliquots were taken from the suspension every 5 minutes

and the photocatalyst powder was separated by centrifu-

gation. The concentration of congo red in the supernatant

solutions was determined by measuring the absorbance at

495 nm using a UV-visible spectrophotometer (Shimadzu

UV-2450); congo red shows an intense absorption band at

495 nm and two other bands at 236 and 341 nm.¹⁷Pure

CdS and ꢁ-Fe₂O₃nanoparticles were also tested as the

photocatalysts to compare their photocatalytic activity with

that of the ꢁ-Fe₂O₃–CdS nanocomposites.

2θ (Degrees)

Figure 1. XRD patterns of pure Fe₂O₃, CdS nanoparticles and

ꢁ-Fe₂O₃–CdS nanocomposites (FC1–FC6).

(SAED) results discussed later. Pure CdS nanoparticles

and all the ꢁ-Fe₂O₃–CdS nanocomposites show peaks at

2ꢆ values of 24.8^ꢀ, 26.53^ꢀ, 28.2^ꢀ, 36.28^ꢀ, 43.9^ꢀ, 47.9^ꢀ,

51.9^ꢀand 70.8^ꢀcorresponding to (100), (002), (101),

(102), (110), (103), (112) and (211) reﬂections of wurtzite

CdS (JCPDS no. 41-1049). The crystallite size of CdS in

the nanocomposites calculated using Debye-Scherrer for-

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mula, varies from 1.2 to 2.9 nm while the crystallite size

IP: 46.161.63.44 On: Wed, 08 Mar 2017 08:33:28

of pure CdS nanoparticles is about 5.7 nm. In general,

Copyright: American Scientific Publishers

the crystallite size of CdS in the nanocomposites increases

with an increase in the concentration of cadmium acetate

and thiourea used during their synthesis.

The IR spectra of pure Fe₂O₃, CdS nanoparticles and

all the nanocomposites are shown in Figure 2. All the

samples show bands near 3400 cm⁻¹and 1630 cm⁻¹

attributed to O–H stretching and bending, respectively,

due to physisorbed water molecules.³⁷The IR bands in

FC6

FC5

FC4

FC3

FC2

FC1

3. RESULTS AND DISCUSSION

The X-ray diffraction (XRD) patterns of pure Fe₂O₃,

CdS nanoparticles and all the ꢁ-Fe₂O₃–CdS nanocompos-

ites (FC1–FC6) are shown in Figure 1. Pure iron oxide

nanoparticles do not show any feature in the XRD pattern.

Since the thermal decomposition of iron acetylacetonate

was carried out in the presence of air and under reﬂuxing

conditions in diphenyl ether, the product obtained is pro-

posed to be ꢁ-Fe₂O₃.³⁶Further evidence for the presence

of ꢁ-Fe₂O₃comes from transmission electron microscopy

(TEM) measurement and selected area electron diffraction

Fe₂O₃

CdS

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm^–1

)

Figure 2. IR spectra of pure Fe₂O₃, CdS nanoparticles and ꢁ-Fe₂O₃–

CdS nanocomposites (FC1–FC6).

9074

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

Yadav and Jeevanandam

Table II. EDX analysis data for the ꢁ-Fe₂O₃–CdS nanocomposites.

Cd

Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

S

Fe

O

Nanocomposite

FC1

Wt%

At%

Wt%

At%

Wt%

At%

Wt%

At%

Comment

Uniform

11ꢃ1

11ꢃ8

11ꢃ0

6ꢃ0

2ꢃ7

2ꢃ6

9ꢃ0

5ꢃ3

8ꢃ9

8ꢃ3

4ꢃ2

7ꢃ6

39ꢃ9

39ꢃ8

38ꢃ8

15ꢃ7

16ꢃ0

15ꢃ0

37ꢃ9

47ꢃ9

41ꢃ2

69ꢃ8

77ꢃ0

70ꢃ6

FC2

FC3

FC4

FC5

FC6

60ꢃ7

60ꢃ2

60ꢃ5

27ꢃ1

31ꢃ4

31ꢃ1

23ꢃ6

24ꢃ8

24ꢃ2

36ꢃ9

43ꢃ2

43ꢃ8

5ꢃ9

5ꢃ6

6ꢃ1

5ꢃ3

6ꢃ6

6ꢃ2

9ꢃ7

5ꢃ4

5ꢃ8

30ꢃ6

18ꢃ8

17ꢃ8

Uniform

70ꢃ9

71ꢃ6

71ꢃ9

34ꢃ7

40ꢃ4

39ꢃ5

14ꢃ9

18ꢃ5

18ꢃ9

25ꢃ6

35ꢃ6

36ꢃ0

2ꢃ6

2ꢃ2

2ꢃ4

3ꢃ6

2ꢃ5

2ꢃ6

10ꢃ5

5ꢃ5

5ꢃ7

35ꢃ9

21ꢃ4

21ꢃ9

9ꢃ3

14ꢃ1

15ꢃ5

3ꢃ5

1ꢃ3

5ꢃ8

4ꢃ8

3ꢃ8

11ꢃ5

6ꢃ3

4ꢃ3

14ꢃ8

71ꢃ9

69ꢃ9

69ꢃ6

53ꢃ8

54ꢃ8

45ꢃ8

23ꢃ9

22ꢃ1

12ꢃ7

36ꢃ4

49ꢃ5

33ꢃ6

Non-uniform

Uniform

45ꢃ8

46ꢃ1

46ꢃ2

16ꢃ6

31ꢃ2

18ꢃ2

16ꢃ2

21ꢃ2

16ꢃ2

20ꢃ8

36ꢃ7

21ꢃ4

17ꢃ0

18ꢃ9

17ꢃ7

14ꢃ8

18ꢃ9

13ꢃ5

18ꢃ5

6ꢃ7

17ꢃ7

47ꢃ7

23ꢃ2

46ꢃ9

28ꢃ7

28ꢃ8

28ꢃ6

11ꢃ2

11ꢃ1

12ꢃ8

14ꢃ1

14ꢃ2

14ꢃ7

19ꢃ3

19ꢃ2

18ꢃ2

44ꢃ8

43ꢃ8

45ꢃ7

35ꢃ9

34ꢃ2

34ꢃ1

12ꢃ4

13ꢃ2

34ꢃ1

35ꢃ4

35ꢃ7

Uniform

the 3000–2500 cm⁻¹range in the nanocomposites are

attributed to C–H stretching. The IR bands in the range

2000–2500 cm⁻¹are attributed to CO₂present in the

chamber of IR spectrometer during the spectral mea-

surement and the band at 1049 cm⁻¹is attributed to

C–O–C stretching.³⁸The band at 1421 cm⁻¹is attributed

acetylacetonate, methanol etc.) from the nanocomposites.

The nanocomposites FC1, FC2, FC3, FC4, FC5 and FC6

exhibited % wt loss of 30.8, 18.4, 6.6, 45.5, 6.7 and

3.9, respectively. The nanocomposites FC1 and FC4 show

more weight loss compared to the other nanocomposites

and this is attributed to the presence of more organic moi-

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to the stretching vibration of COO . These bands sug-

eties and some undecomposed precursors (e.g., iron acety-

lacetonate) in these nanocomposites.

Copyright: American Scientific Publishers

gest the presence of organic content adsorbed on the sur-

face of the nanocomposites.^39ꢀ40The band at 503 cm⁻¹

is attributed to Cd–S stretching⁴¹and the IR bands at

about 553 and 480 cm⁻¹are attributed to the character-

istic vibrations of ꢁ-Fe₂O₃.²¹TGA measurements were

carried out for pure ꢁ-Fe₂O₃, CdS and the nanocompos-

ites (FC1–FC6). Pure iron oxide showed a weight loss of

23.1% and pure CdS showed a weight loss of 12.2% in

Table II shows the energy dispersive X-ray (EDX) anal-

ysis results on the ꢁ-Fe₂O₃–CdS nanocomposites. The

nanocomposites FC1, FC2, FC3, FC5 and FC6 show fairly

uniform elemental distribution and nanocomposite FC4

exhibits non-uniform elemental distribution. The particle

size and morphology of the ꢁ-Fe₂O₃–CdS nanocompos-

ites were investigated by transmission electron microscopy

(TEM) analysis. The TEM images for pure ꢁ-Fe₂O₃and

CdS nanoparticles are shown in Figure 3. The TEM

ꢀ

the range 25 C to 400 C. The weight loss is attributed

to the removal of organic molecules (e.g., diphenyl ether,

Figure 3. TEM images and SAED patterns of (a) pure Fe₂O₃and (b) CdS nanoparticles.

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

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Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Yadav and Jeevanandam

image of Fe₂O₃(Fig. 3(a)) shows that the nanoparti-

cles are very small (∼2 nm) and they are agglomerated.

The selected area electron diffraction (SAED) pattern of

ꢁ-Fe₂O₃nanoparticles (inset of Fig. 3(a)) shows rings

that are attributed to (311), (410), (511) and (440) planes

of ꢁ-Fe₂O₃(JCPDS no. 39-1346). The TEM image

of CdS nanoparticles (Fig. 3(b)) shows agglomerated

nanoparticles with no particular shape. The inset of

Figure 3(b) shows the SAED pattern of CdS nanopar-

ticles and the observed rings are attributed to (103),

(100), (110) and (300) planes of polycrystalline hexago-

nal CdS. Figure 4 shows the TEM images of ꢁ-Fe₂O₃–

CdS nanocomposites FC1, FC2, FC3, FC5 and FC6. The

images reveal that small nanoparticles are agglomerated

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Figure 4. TEM images and SAED patterns of ꢁ-Fe₂O₃–CdS nanocomposites; (a) FC1, (b) FC2, (c) FC3, (d) FC5 and (e) FC6.

9076

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Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

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Copyright: American Scientific Publishers

Figure 5. HRTEM images of ꢁ-Fe₂O₃–CdS nanocomposites; (a) FC1, (b) FC2 and (c) FC3.

Figure 5(c) corresponds to (210), (211) and (311) planes

of ꢁ-Fe₂O₃, respectively.

The diffuse reﬂectance spectra (DRS) of pure ꢁ-Fe₂O₃,

CdS nanoparticles and the ꢁ-Fe₂O₃–CdS nanocomposites

to produce particles with close to spherical morphol-

ogy. The SAED pattern of nanocomposite FC1 (inset of

Fig. 4(a)) shows rings attributed to (311) and (440) planes

of ꢁ-Fe₂O₃. The SAED patterns of nanocomposites FC2

and FC3 (insets of Figs. 4(b and c)) show rings that are

attributed to (103), (100), (101) planes of polycrystalline

hexagonal CdS and (311), (440) planes of ꢁ-Fe₂O₃. The

SAED patterns of nanocomposites FC5 and FC6 (insets of

Figs. 4(d and e)) show rings that are attributed to (103),

(100), (101) planes of polycrystalline hexagonal CdS and

(311), (511), (440) planes of ꢁ-Fe₂O₃.

Fe₂O₃

Nanocomposites FC4, FC5 and FC6 were prepared in

order to understand the effect of thermal decomposition

time on the morphology of ﬁnal products. It was found

from electron microscopy studies that there is not much

difference in the morphology of nanocomposites when

the thermal decomposition time is increased from 35 to

70 minutes. The high resolution TEM images (HRTEM) of

nanocomposites FC1, FC2 and FC3 are shown in Figure 5.

The observed interplanar spacing of 0.35 nm in nanocom-

posites FC1, FC2 and FC3 corresponds to (100) plane

of hexagonal CdS. The interplanar spacing of 0.37 nm

in Figure 5(a), 0.34 nm in Figure 5(b) and 0.25 nm in

CdS

FC1

FC2

FC3

FC4

FC5

FC6

300

400

500

600

700

Wavelength (nm)

Figure 6. DRS spectra of ꢁ-Fe₂O₃, CdS nanoparticles and the

ꢁ-Fe₂O₃–CdS nanocomposites (FC1–FC6).

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Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Yadav and Jeevanandam

CdS

Fe₂O₃

1.9 eV

2.45 eV

1.6

1.8

2.0

2.2

hν (eV)

2.4

2.6

2.8

3.0

1.6

1.8

2.0

2.2

hν (eV)

2.4

2.6

2.8

3.0

FC1

FC2

1.8 eV

2.1 eV

2.4

1.8

2.0

2.2

hν (eV)

2.4

2.6

2.8

3.0

1.6

1.8

2.0

2.2

hν (eV)

2.6

2.8

3.0

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FC3

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2.45 eV

2.2 eV

2.4

1.8

2.0

2.2

2.4

2.6

2.8

3.0

1.6

1.8

2.0

2.2

2.6

2.8

3.0

hν (eV)

FC5

FC6

2.1 eV

2.35 eV

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

hν (eV)

Figure 7. Tauc plots of pure ꢁ-Fe₂O₃, CdS nanoparticles and the ꢁ-Fe₂O₃–CdS nanocomposites (FC1–FC6).

9078

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

Yadav and Jeevanandam

Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

estimated by plotting (ꢂhꢇꢅ²versus hꢇ (Tauc plots) using

1.2

529 nm

560 nm

504 nm

510 nm

Fe₂O₃

the relation:

CdS

ꢂhꢇ = Aꢈhꢇ −E_gꢅ^1/2

1.0

0.8

0.6

0.4

0.2

0.0

475 nm

465 nm

FC1

FC2

Where ꢂ is the absorption coefﬁcient, hꢇ is the pho-

ton energy, E_gthe optical band gap and A is a constant.

FC3

FC4

From the Tauc plots (Fig. 7), E_gwas estimated by extrap-

FC5

olating a straight line from the absorption curve to the

FC6

X-axis. Pure ꢁ-Fe₂O₃and CdS nanoparticles have band

gap of 1.9 eV and 2.45 eV, respectively. The nanocom-

posites FC1, FC2, FC3, FC4 and FC6 show band gap

absorption at about 1.8 eV, 2.1 eV, 2.45 eV, 2.2 eV, and

2.35 eV, respectively. Nanocomposite FC5 possesses the

same band gap as that of FC2 (2.1 eV). In general, the

400

450

500

550

600

650

700

band gap of nanocomposites increases with an increase in

the concentration of CdS (FC1 to FC3). Figure 8 shows the

photoluminescence spectra of pure ꢁ-Fe₂O₃, CdS nanopar-

ticles and the ꢁ-Fe₂O₃–CdS nanocomposites (FC1–FC6).

The spectra for the nanocomposites were normalized with

respect to the amount of CdS present in the nanocompos-

ites, as determined by the atomic absorption spectroscopy

(AAS) analysis. Pure ꢁ-Fe₂O₃shows a small emission

hump at 470 nm which is attributed to the presence of

Wavelength (nm)

Figure 8. Photoluminescence spectra of pure ꢁ-Fe₂O₃, CdS nanoparti-

cles and the ꢁ-Fe₂O₃–CdS nanocomposites (FC1–FC6).

(FC1–FC6) are shown in Figure 6. The ꢁ-Fe₂O₃–CdS

nanocomposites exhibit optical absorption in the vis-

ible region. The optical band gap of ꢁ-Fe₂O₃, CdS

nanoparticles and the ꢁ-Fe₂O₃–CdS nanocomposites was

Fe₂O₃

8

FC1(1:0.1:0.1)

MH–300K

4

MH–300K

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MH–

w

Y^5Kork at Binghamton

MH–5K

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4

Mar 2017 08:33:28

2

0

Copyright: American Scientific Publishers

2

0

–2

–4

–6

–8

–2

–4

–3

–2

–1

0

1

2

3

–3

–2

–1

0

1

2

3

Magnetic field (T)

FC2 (1:0.25:0.25)

0.8

0.4

MH–300K

MH–5K

0.0

–0.4

–0.8

–3

–2

–1

0

1

2

3

Magnetic field (T)

Figure 9. Typical M–H curves for pure ꢁ-Fe₂O₃nanoparticles and ꢁ-Fe₂O₃–CdS nanocomposites (FC1 and FC2) at 300 K and 5 K.

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

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Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Yadav and Jeevanandam

oxygen vacancies, defects and other structural impurities.⁴²

Pure CdS nanoparticles show band edge emission at about

529 nm with a broad shoulder at 560 nm attributed to

Cd²⁺and S²⁻vacancies.⁴³Nanocomposites FC1 and FC5

show band edge emission due to CdS at about 510 nm

and nanocomposites FC3 and FC6 show band edge emis-

sion at about 475 nm. Nanocomposite FC2 shows band

edge emission due to CdS at 465 nm and nanocomposite

FC4 shows band edge emission due to CdS at 504 nm

and a broad shoulder at 560 nm attributed to vacancies.

There is a blue shift of the CdS band edge emission

in the ꢁ-Fe₂O₃–CdS nanocomposites compared to pure

CdS nanoparticles. The blue shift is attributed to quan-

tum size effect due to decrease in the particle size of

CdS in the nanocomposites. The blue shift is more in

the case of nanocomposites FC2 and FC3 as compared

to that in FC1 and FC5 in spite of the larger crystal-

lite size of CdS and this is attributed to the presence

of defects.^27ꢀ43The ꢁ-Fe₂O₃–CdS nanocomposites con-

tain CdS content (from AAS analysis) of 0.93%, 26.3%,

60.3%, 7.2%, 25.9% and 10.9% for FC1, FC2, FC3, FC4,

FC5 and FC6, respectively. In general, the PL intensity of

the ꢁ-Fe₂O₃–CdS nanocomposites decreases as compared

to pure CdS nanoparticles which is attributed to an efﬁcient

charge separation in the nanocomposites.^10ꢀ27Nanocom-

posite FC3 exhibits the highest PL emission intensity

among the nanocomposites. The nanocomposites FC2 and

FC5 exhibit less emission intensity compared to FC3 due

to decrease in the CdS content. The nanocomposites FC1,

FC4 and FC6 have less CdS content and thus show lower

emission intensity compared to the other nanocomposites

(FC2, FC3 and FC5). The presence of a broad emission

band at 475 nm in the nanocomposite FC6 is attributed

to the transfer of excited electrons from the conduction

band of CdS to the empty surface oxygen vacancies on the

surface of iron oxide. Although nanocomposites FC1 and

FC4 contain appreciable amount of iron oxide, they pos-

sess less amount of CdS compared to FC6 (see Table II).

The decrease in emission intensity at 475 nm in nanocom-

posite FC6 compared to FC1 and FC4 is attributed

3.0

(a)

3.0

(b)

CR(50 mg L^–1

Fe₂O₃

)

FC5

0

2.5

2.0

1.5

1.0

0.5

0.0

2.5

2.0

CdS

FC1

FC2

FC3

FC5

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1.5

FC6

Copyright: American Scientific Publishers

1.0

0.5

0.0

60 min

700

200

300

400

500

600

700

800

200

300

400

500

600

800

Wavelength (nm)

(c)

1.0

Fe₂O₃

CdS

FC5

0.8

0.6

0.4

0.2

0.0

0

10

20

30

Time (min)

40

50

60

Figure 10. (a) UV-visible spectra indicating photocatalytic degradation of congo red in an aqueous solution (time = 30 min) using pure ꢁ-Fe₂O₃,

CdS nanoparticles and the ꢁ-Fe₂O₃–CdS nanocomposites as the catalysts, (b) kinetics of photocatalytic degradation of congo red using ꢁ-Fe₂O₃–

CdS nanocomposite FC5 and (c) comparison of photocatalytic degradation of congo red with pure ꢁ-Fe₂O₃, CdS nanoparticles and ꢁ-Fe₂O₃–CdS

nanocomposite FC5.

9080

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

Yadav and Jeevanandam

Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

to the presence of smaller amount of CdS in these

nanocomposites.

moments across the anisotropy barrier.^44ꢀ45Nanocompos-

ite FC1 shows higher coercivity (0.05 T) compared to

pure iron oxide nanoparticles which is attributed to larger

particle size of iron oxide in the nanocomposite which

increases the anisotropic energy in the single domain.³⁹On

the other hand, nanocomposite FC2 exhibits lower coerciv-

ity (0.033 T) compared to nanocomposite FC1 due to the

formation of particles in multi-domain size range.⁴⁶The

remanent magnetization (M_rꢅ for the nanocomposite FC1

(1.65 emu/g) is higher compared to that of pure ꢁ-Fe₂O₃

nanoparticles. Nanocomposite FC2 shows decrease in the

remanent magnetization (0.23 emu/g) compared to that of

pure iron oxide nanoparticles. At room temperature, the M_s

values for the nanocomposites FC1 (4.4 emu/g) and FC2

(0.65 emu/g) are higher compared to that of pure ꢁ-Fe₂O₃

nanoparticles. In general, the ꢁ-Fe₂O₃–CdS nanocompos-

ites show lower saturation magnetization values com-

pared to that of bulk ꢁ-Fe₂O₃(M_s= 76 emu/g). This is

attributed to the presence of disordered or canted spins

Typical ﬁeld dependent magnetization (M–H) plots

for pure iron oxide nanoparticles and the ꢁ-Fe₂O₃–CdS

nanocomposites with uniform elemental distribution (FC1

and FC2) at 300 K and 5 K are shown in Figure 9. Pure

ꢁ-Fe₂O₃nanoparticles show superparamagnetic behaviour

at 300 K, while at 5 K, weak ferromagnetic behaviour is

observed with M_rand H_cvalues of 0.32 emu/g and 0.03 T,

respectively. At 300 K, the magnetization of ꢁ-Fe₂O₃

nanoparticles saturates with a M_svalue of 0.61 emu/g

while at 5 K, the magnetization does not saturate at a

maximum ﬁeld of 3 Tesla. The ꢁ-Fe₂O₃–CdS nanocom-

posites FC1 and FC2 show superparamagnetic behaviour

at 300 K, weak ferromagnetic behaviour at 5 K and satu-

ration of magnetization is not observed in these nanocom-

posites at 5 K. Pure iron oxide nanoparticles and all the

nanocomposites show coercivity at 5 K but not at 300 K

and this is attributed to thermal ﬂuctuation of the blocked

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Scheme 1. Proposed mechanism of photodegradation of congo red.

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

9081

Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Yadav and Jeevanandam

on the surface of nanocomposites and superparamagnetic

relaxation.⁴⁷

the presence of ꢁ-Fe₂O₃–CdS nanocomposites as the cata-

lyst. Based on the literature,^26ꢀ27ꢀ30the mechanism of pho-

todegradation is described as follows (Scheme 2):

The photocatalytic activity of the ꢁ-Fe₂O₃–CdS

nanocomposites was examined by studying the degradation

of congo red in an aqueous solution under sunlight irra-

diation. Figure 10(a) shows the UV-visible spectral results

on the photodegradation of congo red by pure ꢁ-Fe₂O₃,

CdS nanoparticles and all the nanocomposites (FC1–FC6).

It can be seen from Figure 10(a) that nanocomposite

FC5 shows maximum photodegradation compared to pure

Fe₂O₃, CdS nanoparticles and all the other nanocom-

posites. It is proposed that when the amount of CdS

in the nanocomposites is too low (e.g., FC1), an efﬁ-

cient interfacial electron transfer from CdS to ꢁ-Fe₂O₃

does not occur. Also, too much CdS on the surface of

ꢁ-Fe₂O₃(e.g., nanocomposite FC6) will block irradiation

of Fe₂O₃and nanocomposite FC5 contains optimum con-

centration of CdS and it exhibits the maximum degrada-

tion. The BET surface area values for the ꢁ-Fe₂O₃–CdS

nanocomposites FC1, FC2, FC3, FC4, FC5 and FC6 are

29.8 m²/g, 52 m²/g, 59.8 m²/g, 30.7 m²/g, 66.1 m²/g and

65.2 m²/g, respectively. The ꢁ-Fe₂O₃–CdS nanocompos-

ite FC5 possesses higher surface area than that of pure

ꢁ-Fe₂O₃(51.1 m²/g), CdS nanoparticles (58.9 m²/g) and

all the other nanocomposites. Thus, due to optimum con-

centration of CdS and high surface area, nanocomposite

FC5 shows better photocatalytic activity compared to pure

hꢇ

CdS–Fe₂O₃→CdSꢈe⁻+h⁺ꢅ+Fe₂O₃ꢈe⁻+h⁺ꢅ

CdSꢈe⁻+h⁺ꢅ+Fe₂O₃ꢈe⁻+h⁺ꢅ → CdSꢈh⁺ꢅ+Fe₂O₃ꢈe⁻ꢅ

e⁻+O₂→ O^ꢁ₂⁻

2e⁻+O₂+2H⁺→ H₂O₂

H₂O₂+O^ꢁ₂⁻→OH^ꢁ+OH⁻+O₂

On sunlight irradiation, electrons in the valence bands

of CdS and ꢁ-Fe₂O₃are excited to their respective con-

duction bands. The electrons on the CB of CdS are trans-

ferred to that of ꢁ-Fe₂O₃while the photo generated holes

on the VB of ꢁ-Fe₂O₃are transferred to that of CdS.⁴⁹

The photogenerated electron–hole pairs migrate easily to

the surface and are separated efﬁciently at the interface of

(a)

Fe₂O₃(0.009 min^–1

CdS (0.024 min^–1

FC5 (0.035 min^–1

)

3.0

2.5

2.0

)

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ꢁ-Fe₂O₃, CdS nanoparticles and other nanocomposites.

1.5

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Figure 10(b) shows the kinetics of photo degradation

Copyright: American Scientific Publishers

of congo red using ꢁ-Fe₂O₃–CdS nanocomposite FC5 as

1.0

0.5

0.0

the catalyst. It can be seen that the absorbance at 495 nm

decreases with time, and it almost approaches zero within

60 min. On the other hand, pure Fe₂O₃and CdS nanopar-

ticles are unable to complete the degradation of congo red

even after 60 minutes of irradiation (Fig. 10(c)). Accord-

ing to the reported literature^34ꢀ35ꢀ48the mechanism for the

photo degradation of congo red involves cleavage of C–S

bond, aromatic ring opening, cleavage of –N N– dou-

ble bond and cleavage of various C–N and C–C bonds in

addition to decarboxylation. Scheme 1 shows the possible

species formed during the degradation of congo red in

0

10

20

30

40

50

60

Time (min)

(b)

Fe₂O₃(0.006 mg^–1min^–1

CdS (0.022 mg^–1min^–1

FC5 (0.088 mg^–1min^–1

)

6

5

4

3

2

1

0

)

0

10

20

30

Time (min)

40

50

60

Figure 11. Kinetics plots using (a) pseudo ﬁrst order and (b) pseudo

second order kinetic models for photo degradation of congo red. Symbols

are experimental values and solid lines are the ﬁts using the two kinetic

models.

Scheme 2. Band alignment and photo-induced electron transfer in

ꢁ-Fe₂O₃–CdS nanocomposites.

9082

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

Yadav and Jeevanandam

Synthesis of ꢁ-Fe₂O₃–CdS Nanocomposites with Enhanced Photocatalytic Activity

Table III. Estimated pseudo ﬁrst order and second-order kinetic parameters for the photocatalytic degradation of congo red in an aqueous solution

using pure ꢁ-Fe₂O₃, CdS nanoparticles and ꢁ-Fe₂O₃–CdS nanocomposite (FC5) as the catalysts.

Pseudo ﬁrst-order kinetics

)

Pseudo second-order kinetics

k₂(mg⁻¹min⁻¹

Catalyst

k₁(min⁻¹

R²

)

R²

ꢁ-Fe₂O₃

CdS

Nanocomposite (FC5)

0ꢃ009

0ꢃ024

0ꢃ035

0ꢃ52

0ꢃ89

0ꢃ71

0ꢃ006

0ꢃ022

0ꢃ088

0ꢃ93

0ꢃ97

0ꢃ98

the nanocomposites with low recombination.⁵⁰The trans-

ferred electrons are trapped by the adsorbed surface oxy-

gen molecules to give O^ꢁ₂⁻and H₂O₂to form active species

(OH^ꢂradicals) which attack chromophore of the congo red

leading to its degradation.^26ꢀ51The pseudo-ﬁrst-order and

pseudo-second-order kinetics models were applied to study

the kinetics of congo red photodegradation by pure Fe₂O₃,

CdS nanoparticles and ꢁ-Fe₂O₃–CdS nanocomposite FC5

(Eqs. (1) and (2)).⁵²

decreased from 93% to 85% after three cycles. From

atomic absorption spectroscopy studies, after the photo-

catalytic experiments, on the supernatant solutions, it was

found that no detectable amount of cadmium is leaching

into the solution. These studies prove that the ꢁ-Fe₂O₃–

CdS nanocomposites can be used to purify water.

4. CONCLUSION

ꢁ-Fe₂O₃–CdS nanocomposites were successfully prepared

by a simple and facile thermal decomposition approach.

The nanocomposites were characterized using different

analytical techniques. XRD results conﬁrm the presence

of CdS nanocrystals (1.2 to 2.9 nm) in the nanocom-

posites and TEM results indicate formation of agglomer-

ated sphere-like particles in the nanocomposites. Diffuse

reﬂectance spectral studies of ꢁ-Fe₂O₃–CdS nanocom-

posites indicate absorbance in the visible region (1.8 to

Pseudo ﬁrst order kinetics model:

ꢀ

ꢁ

C_o

C

ln

= k₁t

(1)

(2)

Pseudo second order kinetics model:

1

ꢉCꢊ

1

ꢉC_oꢊ

= k₂t +

Where C_oand C represent the concentration of congo

red at t = 0 and at any time t. k₁and k₂are pseudo-

ﬁrst order and pseudo-second order rate constants, respec-

tively. Figures 11(a and b) show the ﬁtting of kinetics

data for the photodegradation of congo red using pure

Fe₂O₃, CdS nanoparticles and ꢁ-Fe₂O₃–CdS nanocompos-

ite FC5 as the catalyst. The kinetic parameters obtained

by linear regression using the two kinetic models are

summarized in Table III. The higher correlation coefﬁ-

cient values (R²ꢅ indicate a better ﬁtting of the kinetic

data using the pseudo-second-order kinetics model rather

than the pseudo-ﬁrst-order one. The value of second

order rate constant of photodegradation of congo red

degradation using ꢁ-Fe₂O₃–CdS nanocomposite FC5 is

0.088 mg⁻¹min⁻¹which is higher than the values

for pure ꢁ-Fe₂O₃(0.006 mg⁻¹min⁻¹ꢅ and pure CdS

(0.022 mg⁻¹min⁻¹ꢅ. The enhanced photocatalytic activ-

ity of the ꢁ-Fe₂O₃–CdS nanocomposites is attributed to an

efﬁcient charge separation at the CdS–Fe₂O₃interface.

The ability to recycle a photocatalyst is very impor-

tant. To investigate the reusability ꢁ-Fe₂O₃–CdS nanocom-

posites, the photocatalytic degradation experiment was

repeated four times using the same catalyst (FC5) again

and again. The concentration of congo red was kept con-

stant (50 mg L⁻¹ꢅ with 60 minutes as the irradiation time.

The photocatalyst was recovered in each cycle from the

sun light irradiated solutions by centrifugation followed

by subsequent washing with double distilled water three

times and the recovered catalyst was reused for the catal-

ysis. The degradation efﬁciency of nanocomposite FC5

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2.5 eV) and the nanocomposites show blue shift of band

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edge emission compared to pure CdS due to quantum size

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effect. The ꢁ-Fe₂O₃–CdS nanocomposites exhibit super-

paramagnetic behavior at room temperature and they are

promising catalysts for the photodegradation of congo red

in an aqueous solution with good recyclability.

Acknowledgments: The award of Senior Research Fel-

lowship (SRF) to Mr. Sudheer Kumar Yadav by the

University Grants Commission, Government of India, is

gratefully acknowledged. The authors are thankful to the

Institute Instrumentation Centre, Indian Institute of Tech-

nology Roorkee for providing the XRD, FE-SEM and

TEM facilities.

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Yadav and Jeevanandam

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Received: 26 September 2015. Accepted: 7 November 2015.

9084

J. Nanosci. Nanotechnol. 16, 9072–9084, 2016

Article Doi

Synthesis of γ-Fe2O3-CdS nanocomposites with enhanced photocatalytic activity by thermal decomposition approach

DOI: 10.1166/jnn.2016.12902

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Full text of DOI:10.1166/jnn.2016.12902

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