INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6, pp. 937-942 JUNE 2013 / 937

© KSPE and Springer 2013

Ultrasound-Assisted Synthesis of Hybrid NanostructuresUsing RAFT Polymerization from the Surface ofQuantum Dots

Long Giang Bach1, Md. Rafiqul Islam1, Mai Thanh Binh1, Do Hoon Kim1, and Kwon Taek Lim1,#

1 Department of Imaging System Engineering, Pukyong National University, Busan, Korea, 608-737# Corresponding Author / E-mail: ktlim@pknu.ac.kr; TEL: +82-51-629-6409, FAX: +82-51-629-6408

KEYWORDS: Quantum Dots, Poly(N-vinylcarbazole), RAFT Polymerization, Ultrasound, Photoluminescence

Poly(N-vinylcarbazole) (PVK) grafted ZnS quantum dots (QDs) (ZnS-g-PVK) involving covalent bonds between the inorganic and

organic segments were synthesized by a facile ultrasonic irradiation technique. The hydroxyl group-coated ZnS QDs were first

prepared using zinc chloride and sodium sulfide in the presence of 2-mercaptoethanol. The ZnS QDs were then coupled with the

reversible addition-fragmentation chain transfer agent 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DTMPA) by

condensation reaction. Subsequently, the surface initiated graft polymerization of N-vinylcarbazole from the DTMPA functionalized

ZnS QDs surface was accomplished upon exposure to ultrasonic irradiation to deliver covalently attached PVK with ZnS QDs. The

chemical structure of the nanocomposites was investigated by FT-IR and XPS analyses. Molecular weight and polydispersity index

of the cleaved PVK from the nanocomposites were determined by GPC. The physical structure and thermal properties of the

nanocomposites were studied by XRD and TGA. TEM images suggested that ZnS QDs were uniformly distributed in PVK matrices.

The nanocomposites showed good optical properties as studied by photoluminescence spectroscopy.

Manuscript received: September 12, 2012 / Accepted: April 26, 2013

1. Introduction

Nanostructured materials are of great interest because they can

bridge the gap between the bulk and molecular levels and leads to

dramatically new horizons for applications in electronics,

optoelectronics and biology.1-5 In particular, semiconductor quantum

dots (QDs) exhibit unique optical, electrical, and magnetic properties,

which are strongly dependent on the cluster shape and size. Zinc sulfide

(ZnS) has attracted much interest because of its excellent physical

properties, for example, wide band gap energy of 3.7 eV, high

refractive index and low absorption coefficient over a broad

wavelength range.6 Thus, ZnS QDs have been widely used in flat-panel

displays, electroluminescence devices, light-emitting diodes, nonlinear

optical devices and infrared window materials.7,8 In order to enhance

the functional diversity in ZnS QDs, the QDs are incorporated into

polymer matrices using various strategies.9-11 Most of those strategies

are based on physical adsorption or simple blending. In the polymer/

QDs nanoblends, a partial incompatibility of QDs and polymer

matrices usually leads to difficulties in achieving homogeneous

dispersions and ultimately phase separation at high loadings which

hinder both intra- and inter chain charge transfer processes resulting in

a lower device performance. In contrast, chemical immobilization of

polymer onto nanoparticles provides superior chemical, mechanical,

and optoelectronics properties.12,13 The preparation of nanocomposites

by covalent attachment of inorganic nanoparticles with an organic

polymer matrix is highly desirable and may find wide applications in

the electronics, optics and energy sectors. Poly (N-vinylcarbazole)

(PVK) has attracted great academic and industrial interests since

1950 s. Having photoconductivity and hole transport property PVK

plays a significant role in the current researches on electrical and

optical properties for advanced polymeric materials.14,15 Incorporation

of ZnS QD into the PVK matrices is expected to display improved

optoelectronic and optical properties.

Recent advances in nanostructured materials have been led

scientists’ to develop versatile and generalized synthetic methods for

the preparation of a variety of nanostructured materials.16-19 The

importance of choosing a proper synthetic route in designing

nanostructured materials has been a driving force for the development

DOI: 10.1007/s12541-013-0123-x

938 / JUNE 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6

of new methodologies. The utilization of high intensity ultrasound

methodology offers a facile and versatile synthetic tool for the

preparation of nanostructured materials that is often unavailable by

conventional methods. The use of the ultrasonic irradiation in chemical

reaction has been found to be advantageous due to the remarkable

decrease in the time necessary to carry out reaction, to the considerable

improvement in yield of the target product, and most importantly as

environmentally benign technology.20 In the past several years,

ultrasound treatment has widely been used in the preparation of many

inorganic nanomaterials such as CdS, CdSe, Pd, SiO2, TiO2, ZnO, and

others.21,22 All of these materials exhibited evenly distributed size, good

dispersion, and good performance. The aim of the current research is

designing and synthesizing PVK grafted ZnS QDs nanocomposites in

which two components directly connected with a covalent bond

employing a facile ultrasonic assisted bulk polymerization technique.

In this contribution, bulk polymerization is carried out in the absence

of any solvent or dispersant and is thus the simplest in terms of

formulation. A green and versatile synthetic protocol to prepare ZnS-

g-PVK nanocomposites by surface initiated reversible addition

fragmentation chain transfer (RAFT) polymerization is illustrated in

Scheme 1. The nanocomposites were prepared upon ultrasonic

irradiation using (2-(dodecylthiocarbonothioylthio)-2-methylpropionic

acid (DTMPA) immobilized ZnS QDs as the RAFT agent. The as-

synthesized nanocomposites were well-investigated by relevant

physical and spectral analyses.

2. Experimental procedures

2.1 Materials

N-vinylcarbazole (VK) was recrystallized from methanol, and

stored in vacuo in the dark. 2,2’-Azobisisobutyronitrile (AIBN) was

purified by recrystallization from methanol. Zinc chloride (ZnCl2), 2-

mercaptoethanol (ME), sodium sulfide (Na2S.9H2O), N,N’-

dimethylamino-pyridine (DMAP), 1,3-dicyclohexylcarbodiimide (DCC),

dodecane thiol, potassium phosphate (K3PO4), carbon disulfide (CS2), 2-

bromo isobutyric acid, N,N-dimethylformamide (DMF), acetone,

dichloromethane (CH2Cl2), ethyl acetate, methanol, and hydrochloric

acid (HCl) were used as received. All of the above mentioned

chemicals were purchased from Aldrich, Yongin, Korea.

2.2 Synthesis of 2-(dodecylthiocarbonothioylthio)-2-methylpropionic

acid (DTMPA), RAFT agent

Dodecane thiol (1.34 g) was added to a stirred suspension of K3PO4

(1.02 g) in acetone (20 ml) and the mixture was stirred for 10 min. CS2

(1.37 g) was added to the mixture and the solution turned bright yellow.

After stirring for 10 min, 2-bromo isobutyric acid (1.00 g) was added

and stirred for 13 h. The solvent was removed under reduced pressure

and the residue was extracted with CH2Cl2 using 1M HCl. The organic

extracts were washed with water and brine. The solvent was removed

under reduced pressure and the residue was purified by column

chromatography on silica using ethyl acetate to yield DTMPA as bright

yellow oil, which crystallized upon standing. The product was

confirmed by 1H NMR analysis. 1H NMR (400 MHz, CDCl3): δ 3.25

(2H, t, SCH2(CH2)10CH3), 1.91 (2H, m, SCH2CH2(CH2)9CH3), 1.67

(6H, s, C(CH3)2), 1.35-1.23 (18H, t, SCH2CH2(CH2)9CH3), 0.85 (3H, t,

SCH2(CH2)10CH3).

2.3 Preparation of hydroxyl group-coated ZnS QDs (ZnS-OH)

5.0 mmol of ZnCl2 in 40 ml of DMF was continuously stirred and

mixed with 10.0 mmol of ME in 4 ml of deionized water. After 10 min,

4 ml of aqueous solution of Na2S (3 mmol) was slowly added drop-

wise into the above solution upon stirring. Once added, the color of the

solution immediately turned white. Then, the reaction mixture was

stirred for additional 6 h at room temperature. Finally, the white

solution gradually turned transparent. The water and salts in the

transparent ZnS-OH suspension were removed and then washed with

fresh DMF for several times.

2.4 Immobilization of DTMPA onto ZnS QDs (ZnS- DTMPA)

To a mixture of ZnS-OH suspension (50 mmol) and DTMPA

(50 mmol) in DMF was added DMAP (30 mmol) under a nitrogen

atmosphere at 0oC. After stirring for 80 min, DCC (50.3 mmol) was

added to the above system at the same temperature, and then continue

stirring for an additional 48 h at room temperature. The reaction

mixture was poured into methanol to yield ZnS-DTMPA QDs as a

yellow powder.

2.5 Preparation of ZnS-g-PVK nanocomposites

A degassed mixture of N-vinylcarbazole (1 ml), AIBN (10 mg) and

ZnS-DTMPA (337.0 mg) was heated in a water bath at 75oC in an

ultrasonic generator for 4 h. The ultrasonic irradiation was applied to

the mixture in order to form homogeneous system at given temperature

higher than the melting point of VK. Without ultrasonic irradiation, the

RAFT polymerization required a longer reaction time (~ 12 h). After

the reaction, the melted mixture was allowed to cool to room

temperature, and then dissolved in DMF. The transparent light-yellow

solution obtained was poured into 200 mL of methanol to give cream-

colored solid powder ZnS-g-PVK, which was further dried in vacuum

at 50oC for several hours before characterization.

2.6 Instrumentation

Fourier-transformed infrared spectrophotometry (FT-IR) was

employed to characterize the chemical change in the surface of ZnS

Scheme 1 Synthetic protocol for the preparation of ZnS-g-PVK

nanocomposite via covalent linkage

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6 JUNE 2013 / 939

using a BOMEM Hartman & Braun FT-IR spectrometer. Surface

composition was investigated using an X-ray Photoelectron

Spectroscopy (XPS) (Thermo VG Multilab 2000) in an ultra high

vacuum with Al Kα radiation. Gel permeation chromatography (GPC)

was performed employing an Agilent 1200 Series equipped with PLgel

5 μm MIXED-C columns, with tetrahydrofuran as the solvent and

calibration was carried out by PS standard. Thermogravimetric analysis

(TGA) was conducted with Perkin-Elmer Pyris 1 analyzer (USA). High

Resolution Transmission Electron Microscopy (HR-TEM) images were

recorded using a Joel JEM 2010 instrument (Japan) with an accelerating

voltage of 200 kV. The physical structure of nanocomposites was

determined by a Philips X’pert-MPD system diffractometer (Netherlands)

with Cu Kα radiation. Room temperature photoluminescence (PL)

spectrum were recorded on a Hitachi F-4500 spectrophotometer.

3. Results and discussion

The chemical anchoring of PVK with ZnS QDs was investigated by

FT-IR (Fig. 1). The strong absorption bands at 3392 cm-1 suggest that

abundant hydroxyl groups are attached on the surface of ZnS-OH. The

absorption bands at 2925 and 1421 cm-1 indicate the existence of the

methylene group originated from ME molecules. In this case, there are

no characteristic bands for the mercapto group (2500 to 2600 cm-1)

indicating that the free mercapto groups disappeared and the robust

bonding between Zn2+ and ME organic ligand was formed. The

hydroxyl group thus introduced onto the surface of ZnS not only allows

the ZnS QDs to be dispersed well in the solvent but also offers further

scope for modification. The FT-IR spectrum of DTMPA functionalized

ZnS QDs led to the appearance of some new absorption bands at 1627,

1578, 1304, 1243 and 826 cm-1, among which the first band is ascribed

to the C = O stretching absorption of newly formed ester linkage

between DTMPA and ZnS-OH. The C = S band and C-S observed at

1243 and 826 cm-1 confirms the attachment of DTMPA.23 In the FT-IR

spectrum of ZnS-g-PVK nanocomposites, the bands at 3046, 1593,

1478 and 1449 cm-1 are corresponding to the C-H carbazyl ring. The

absorption band at 1327 cm-1 indicates the existence of the C-N group.

The comparison data of the different vibrational modes of ZnS-OH,

ZnS-DTMPA, and ZnS-PVK nanocomposites are summarized in Table 1.

For the chemical analysis and identifying the nature and specific

sites of interaction in the nanocomposites, XPS survey spectra of the

samples were recorded as shown in Fig. 2. The detailed scan for Zn2p

spectrum comprises of two peaks with BE at 1020.1 eV and 1043.7 eV

are due to Zn2p3 and Zn2p1, respectively. The peak component at the

binding energies (BE) of about 531.9, 285.10 and 163.2 eV are

assigned to the O1s, C1s, and S2p of the ME functionalities on QDs,

as shown in Fig. 2(c1), 2(e1) and 2(f1), respectively. The BE for the

Zn2p, O1s, C1s, S2s and S2p levels for the ZnS-g-PVK

nanocomposites were observed to be comparable with the ZnS-OH

QDs except the C1s peak which resulted in high intensity with minor

shift to higher BE, indicating that the polymeric chains were directly

grafted from the surfaces of ZnS-OH QDs (Fig. 2(a2)). In Fig. 2(e2), the

C1s core-level spectra of ZnS-g-PVK can be curve-fitted into four peak

components with BEs at about 284.7, 286.3, 287.2, and 288.4 eV

which are attributable to the C-C/C-H, C-N, C-O and O = C-O species,

respectively. In fact, there should be a fact of overlap between the BEs

of C-N and S-C(=S)-S species. In addition, the S2p core-level spectrum

of ZnS-g-PVK can be curve-fitted into two peak components with BEs

at about 1632.9 and 164.3 eV attributable to the S-C and S = C species,

respectively (Fig. 2(f2)). It should be noted that the BE of 400.1 eV is

ascribed to the N1s (Fig. 2(d2)). These results suggest that the ZnS QDs

were directly anchored with PVK polymer.

GPC analyses provided useful information for the inclusion of ZnS

QDs in the PVK matrices as shown in Fig. 3. PVK polymer was

cleaved from the ZnS QDs surface upon acid treatment. The number

average molecular weight (Mn) and molecular weight distribution

(PDI) of the cleaved PVK were found to be 18.7 kg/mol and 1.67,

respectively. For the comparison, a DTMPA-PVK polymer

(Mn = 20.6 kg/mol, PDI = 1.34) was prepared under the similar

condition by using DTMPA as RAFT agent. Cleaved PVK from the

ZnS QDs gave slightly broader PDI (1.67) than referenced DTMPA-

PVK (1.34), which might be due to the stronger steric hindrance of

covalently attached DTMPA to the ZnS QDs backbone.

Fig. 4 demonstrates the XRD patterns of ZnS-OH, DTMPA-PVK

Fig. 1 FT-IR spectra of (a) ZnS-OH, (b) DTMPA anchored ZnS QDs

(ZnS-DTMPA), and (c) ZnS-g-PVK nanocomposites in the frequency

range of 4000 - 400 cm-1 (with KBr pellet)

Table 1 Assignment of FT-IR spectra of ZnS-OH, ZnS-DTMPA, and

ZnS-g-PVK

SamplesStretching

frequency (cm-1)Vibrational mode

ZnS-OH 3392 v (O–H)

2925 v (C–H)

1421 δ (C–H)

1056, 1004 v (C–O)

690-740 δ (C–H) out-of-plane

ZnS-DTMPA 2945, 2876 vasym CH3 , vasym CH2

1627 v (C=O)

1578 vasym (C-O)

1423, 1304 v (C-O), δ (C–H) bend in plane

1243 v (C=S)

826 δ (C–S)

ZnS-g-PVK 3048 v (C-H) carbazyl ring

2,941, 2,842 vas (C–H) and vs (C–H)

1594, 1478, 1449 v (C=C) carbazyl ring

1327 v (C-N)

940 / JUNE 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6

and ZnS-g-PVK nanocomposite. ZnS-OH QDs exhibit three diffraction

peaks at 2θ = 28.8o, 47.9o and 55.3o corresponding to (111), (220) and

(311) planes, which are in accordance with the zinc blende crystal

structure. For DTMPA-PVK, the pattern shows broad peaks at 2θ =

13.4o and a shoulder at 2θ = 20.5o, suggesting the sample is typical

semicrystalline material. On the other hand, when ZnS QDs are

incorporated in PVK, the characteristic diffraction peaks of ZnS QDs

are also observed. The pattern of ZnS-g-PVK nanocomposites exhibits

detectable peaks arising from (111), (220), and (311) planes of ZnS

QDs. The X-ray patterns further confirmed the successful synthesis of

ZnS-g-PVK nanocomposites.

Thermal properties of the ZnS-g-PVK nanocomposites were

evaluated using TGA. The TGA scans of the QDs and nanocomposites

are shown in Fig. 5. The TGA scans of both ZnS-g-PVK and DTMPA-

PVK show two plateaus. ME-capped ZnS QDs started to decompose at

ca. 200oC due to the presence of the unstable ME on particle surfaces.

The onset temperature for the thermal degradation of ZnS-OH QDs

was ca. 318oC. The decomposed ZnS residues were found to be stable

up to 425oC. In the nanocomposites, the DTMPA moieties first

decomposed at 100-260oC and then PVK moieties underwent for

decomposition to leave ZnS residues. The onset decomposition

temperature of ZnS-g-PVK nanocomposites was found to be decreased,

resulting in char yield ca. 18% at 800oC. The phenomena can be

explained in such a way that in situ formation of free radicals from

PVK decomposed ZnS QDs consequently lowering the degradation

temperature.

TEM was used to observe the morphology of ZnS-OH QDs and the

Fig. 2 XPS spectra of (a1, a2) wide-scan, (b1, b2) Zn2p, (c1, c2) O1s,

(d1, d2) N1s, (e1, e2) C1s, (f1, f2) S2p core-level spectrum of (1) ZnS-

OH, and (2) ZnS-g-PVK nanocomposites

Fig. 3 GPC scans of (a) cleaved PVK from ZnS-g-PVK

nanocomposites (Mn, GPC = 18.7 kg/mol, PDI = 1.67), and (b) DTMPA-

PVK (Mn, GPC = 20.6 kg/mol, PDI = 1.34) (using PS standard)

Fig. 4 XRD patterns of (a) ZnS-OH, (b) DTMPA-PVK, and (c) ZnS-g-

PVK nanocomposites

Fig. 5 TGA curves of (a) ZnS-OH QDs, (b) DTMPA-PVK, (C) ZnS-g-

PVK nanocomposites at a heating rate of 10oC min-1 under the

continuous N2 flow

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6 JUNE 2013 / 941

as-prepared ZnS-g-PVK nanocomposites. Fig. 6(a) shows a typical

TEM overview of ZnS-OH QDs nanoparticles. The average size of the

ZnS-OH nanoparticles is ca. 4 nm, as estimated from the TEM image.

It reveals that the small ZnS-OH nanoparticles aggregated strongly into

secondary particles, which might form because of the high surface

energy of the nanometer-sized crystals. The well-resolved lattice

fringes of the ZnS QDs as captured by selected area electron diffraction

(SAED) indicate the good crystallinity of the ZnS QDs (insert Fig. 6(a)).

In the ZnS-g-PVK nanocomposites, grafting of the polymer conferred

a spherical morphology of QDs, thereby reducing the aggregation of

particles.

Upon covalent functionalization, the fluorescence intensity of PVK

was expected to be quenched by incorporation of ZnS-OH QDs. The

photoluminescence (PL) spectrum of DTMPA-PVK exhibits a strong

emission peak at ca. 375 nm, whereas the characteristic emission for

ZnS-OH QDs is observed at ca. 498 nm. On the other hand, for the

chemically hybridized ZnS-g-PVK nanocomposites, the PL spectrum is

nearly similar to that of the DTMPA-PVK, but the emission intensity

was decreased in compare with the latter, suggesting the formation of

an effective carrier trapping center in ZnS-g-PVK nanocomposites.

4. Conclusions

ZnS-g-PVK nanocomposites were synthesized by the simple

technique. Initially, the surface of ZnS QDs was functionalized by

DTMPA to give ZnS-DTMPA as a strategic RAFT agent. Upon

ultrasonic irradiation, the surface initiated RAFT reaction was

employed to polymerize N-vinylcarbazole to afford PVK-anchored

ZnS QDs. The FT-IR and XPS results clearly suggested that ZnS QDs

were ligated with the PVK moiety through ester linkage. TGA results

showed that the onset thermal decomposition temperature of ZnS-g-

PVK nanocomposites was decreased leaving char yield ca. 18% upon

the inclusion of ZnS QDs. Encapsulation of ZnS QDs within PVK

matrices considerably prevented the aggregation of QDs as suggested

by TEM analyses. The PL intensity of PVK was found to be decreased

upon incorporation of ZnS QDs, which may be attributed to the

quenching effect through the interfacial charge transfer to the

luminescence center. This simple green protocol offers a great promise

for fabricating functionalized optical nanohybrids based on QDs for

versatile applications.

ACKNOWLEDGEMENT

This work was financially supported by the Joint Program of

Cooperation in Science and Technology through NRF grant funded by

the MEST (No. 2011-0025680).

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