synthesis and nonlinear optical properties of reduced graphene oxide hybrid material covalently...

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Synthesis and nonlinear optical properties ofreduced graphene oxide hybrid material covalentlyfunctionalized with zinc phthalocyanine

http://dx.doi.org/10.1016/j.carbon.2014.06.0180008-6223/� 2014 Elsevier Ltd. All rights reserved.

* Corresponding authors: Fax: +86 451 8667 3647.E-mail addresses: [email protected] (C. He), [email protected] (Y. Wu).

Please cite this article in press as: Song W et al. Synthesis and nonlinear optical properties of reduced graphene oxide hybrid materialfunctionalized with zinc phthalocyanine. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.06.018

Weina Song a,b, Chunying He a,*, Wang Zhang c, Yachen Gao a, Yixiao Yang a,Yiqun Wu a,d,*, Zhimin Chen a, Xiaochen Li a, Yongli Dong a,b

a Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials,

Heilongjiang University, Harbin 150080, PR Chinab College of Environmental and Chemical Engineering, Heilongjiang University of Science and Technology, Harbin 150022, PR Chinac Research Center for Space Optics Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR Chinad Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China

A R T I C L E I N F O A B S T R A C T

Article history:

Received 2 March 2014

Accepted 10 June 2014

Available online xxxx

A reduced graphene oxide–zinc phthalocyanine (RGO–ZnPc) hybrid material with good dis-

persibility has been prepared by covalent functionalization method, based on the initial

covalent linkage of ZnPc to GO and subsequent in situ reduction of GO moiety to RGO dur-

ing mild thermal treatment in DMF solvent. The microscopic structure, morphology and

photophysical properties of resultant RGO–ZnPc hybrid are characterized. The nonlinear

optical (NLO) properties of the RGO–ZnPc hybrid are also investigated using the Z-scan

technique at 532 nm with 4 ns laser pulses. The results show that the efficient functiona-

lization and reduction of GO make RGO–ZnPc hybrid possess much larger NLO properties

and optical limiting performance than those of individual GO, ZnPc and the GO–ZnPc

hybrid. It can be ascribed to a combination of different NLO absorption mechanisms for

RGO–ZnPc hybrid, including two-photon absorption originating from the sp3 domains,

saturable absorption from the sp2 carbon clusters and excited state absorption from numer-

ous localized sp2 configurations in RGO moiety, reverse saturable absorption arising from

ZnPc moiety and the contribution of efficient photo-induced electron transfer or energy

transfer process between ZnPc and RGO.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Materials with large optical nonlinearities and fast nonlinear

optical (NLO) response are usually considered to be promising

candidates for optical communication, optical limiting, opti-

cal storage, information processing and so on [1–3]. Among

all the NLO applications, optical limiting (OL) has attracted

considerable attention because high intensity laser beams

present hazards to delicate optical instruments and human

eyes. A successful optical limiter should strongly attenuate

intense, potentially dangerous laser beams, while exhibiting

high transmittance for low-intensity ambient light [4].

However, the preparation of single nonlinear and optically

active material required for such practical applications still

covalently

http://dx.doi.org/10.1016/j.carbon.2014.06.018

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2 C A R B O N x x x ( 2 0 1 4 ) x x x – x x x

represents a significant challenge. Therefore, much work

have been done to integrate materials with different NLO

mechanisms by means of the multiple plane p–p interaction

[5,6] or covalent bonding [7,8] in order to improve the OL

performance.

Graphene with a super p electron conjugation system is an

ideal ultrabroadband and fast saturable absorber derived from

the ultrafast carrier dynamics, large absorption and Pauli

blocking [9–12]. Graphene oxide (GO), holding some character-

istics of graphene due to the presence of pristine graphitic

nanoislands, exhibits some heterogeneous optical transition

and nonlinear dynamics because small sp2 carbon nanois-

lands are isolated by the sp3 matrix [13]. Therefore, NLO

response of GO usually displays a flip behavior from saturable

absorption (SA) to reverse saturable absorption (RSA) with the

increasing pump intensity [14–18]. Moreover, the NLO perfor-

mance can be enhanced during the reduction of GO to

reduced graphene oxide (RGO) in a certain extent owing to

the partial restoration of sp2 p-conjugated network of

graphene [19]. However, the low solubility and rather poor

processability in kinds of solvents are apparently obstacles

for the application of graphene as NLO materials. Phthalocya-

nine complexes (Pcs) are a class of soluble NLO materials with

RSA properties originating from the occurrence of intersys-

tem crossing from the lowest excited singlet state to the

lowest triplet state and the subsequent increase in the

population of the strongly absorbing triplet state with

nanosecond dynamics [20]. The architectural flexibility of

Pcs facilitates the tuning of photophysical and nonlinear

optical properties over a very broad range by changing the

peripheral substituents and the central metal ion of the

macrocycle [21–23]. Therefore, it is meaningful to graft gra-

phene with Pcs materials and study the photophysical and

NLO properties of the composites. Recently, Zhang et al.

reported the photo-induced electron transfer process of non

covalent ZnTSPc-graphene composite [24]. The preparation

of Pcs-graphene covalent functionalization materials and

their photo-induced transient behavior in picosecond time

scale are also investigated in Refs. [25,26]. Chen et al. reported

the OL response of covalently functionalized Pcs-GO

composites in the excitation of 6 ns pulse laser of 532 and

1064 nm [8,27]. However, to the best of our knowledge, few

studies on the preparation of RGO hybrid material covalently

functionalized with zinc phthalocyanine (ZnPc) and

the NLO mechanism of this hybrid material have been

reported so far.

Encouraged by these considerations, in this study, an easy

covalent functionalization method for the fabrication of

RGO–ZnPc hybrid material, based on the initial covalent

bonding of GO with soluble ZnPc by esterification and the

subsequent in situ reduction of GO to RGO during mild

thermal treatment, is developed in the DMF solvent without

any reductant. Special attentions are paid on the structural,

photophysical and nonlinear optical properties of the

RGO–ZnPc hybrid material. RGO–ZnPc exhibits much larger

NLO properties and OL performance than those of individual

GO, ZnPc and the GO–ZnPc hybrid, ascribed to a combination

of different NLO absorption behaviors originating from RGO,

ZnPc and the photo-induced electron transfer or energy

transfer (PET/ET) process between the two moieties.

Please cite this article in press as: Song W et al. Synthesis and nonlinear ofunctionalized with zinc phthalocyanine. Carbon (2014), http://dx.doi.org

2. Experimental section

2.1. Synthesis of GO

GO was prepared by oxidation of graphite according to the

Hummers method [28]. Graphite powder (2 g) and sodium

nitrate (1 g) were first mixed and stirred in concentrated sul-

furic acid (50 mL) at 0 �C. Potassium permanganate (8 g) was

added gradually to above solution with vigorous mechanical

stirring. Then the stirring was continued for 1.5 h at 0 �Cand 2 h at 35 �C. Followed by the addition of de-ionized water

(100 mL), the temperature was raised to 98 �C and maintained

for 15 min. Subsequently, de-ionized water (1 L) and hydrogen

peroxide (30%, 10 mL) were added to terminate the reaction.

Finally, the resulting suspension was filtered and washed

with 10% HCl (500 mL) and de-ionized water. The obtained

solid product was dried under vacuum at 40 �C for 24 h.

2.2. Synthesis of RGO–ZnPc

The triethyleneglycol-substituted Zn(II) phthalocyanine

(ZnPc) was firstly synthesized using 3-(2-[2-(2-hydroxyethox-

y)ethoxy]ethoxy)phthalonitrile according to the method

reported by Ahsen and coworkers [29]. The synthesis of

RGO–ZnPc hybrid material is based on the initial covalent

linkage of ZnPc to GO by an esterification reaction and subse-

quent in situ reduction of GO moiety to RGO during mild ther-

mal treatment in DMF solvent. In a typical synthetic

procedure, GO (40 mg) was dispersed in dry DMF (40 mL) by

ultrasonic (400 W) for 30 min. Then, a solution of Dicyclohex-

ylcarbodiimide (DCC, 40 mg) dissolved in DMF (5 mL) was

added to convert the ACOOH groups at the edge and the

defect position of GO into active carbodiimide esters. After

the active reaction system was stirred vigorously for 1 h,

another DMF solution (10 mL) of ZnPc (160 mg) was added,

and then this esterification reaction was performed for 4 days

at room temperature. The solid product was filtered and

further washed by DMF for three times in order to remove

excess unreacted Pcs or adsorbed Pcs on the GO sheets. Sub-

sequently, the obtained product was redispersed in dry DMF

(40 mL) with sonication for 15 min, and then the suspension

was stirred at 120 �C for 24 h in atmosphere. Finally, the pro-

duct was filtered and washed with ethanol thoroughly, and

dried at 80 �C for 6 h. A brown–green RGO–ZnPc sample was

obtained. Meanwhile, a GO–ZnPc hybrid sample was also

prepared by the same covalent functionalization method,

but without the following thermal treatment process. The

RGO reference sample was prepared by the same thermal

reduction route from GO.

2.3. Characterization

X-ray diffraction (XRD) measurements were performed on a

Bruker D8 Advance X-ray diffractometer with Cu Ka

(k = 1.5418 A) radiation (40 kV, 40 mA). Raman spectra were

carried out using a HR800 (JY) spectrometer with an Ar+ ion

laser (457.9 nm). FT-IR spectra were obtained with a Perkin

Elmer instruments Spectrum One FT-IR Spectrometer in KBr

disks. X-ray photoelectron spectroscopy (XPS) was recorded

on a Thermo ESCALAB 250 spectrometer using a

ptical properties of reduced graphene oxide hybrid material covalently/10.1016/j.carbon.2014.06.018


C A R B O N x x x ( 2 0 1 4 ) x x x – x x x 3

monochromatic Al Ka X-ray source (15 kV, 150 W) and analy-

zer pass energy of 100 eV. Binding energies (BE) are referred

to the C (1s) binding energy of carbon taken to be 284.6 eV. Ele-

mental analysis of C, H, O, N were obtained on a Elemental

Vario EL Element Analyzer. Scanning electron microscope

(SEM) micrographs were acquired using a Hitachi S-4800

instrument operating at 5.0 kV. Transmission electron micro-

scopy (TEM) was taken on a JEM-2100 electron microscope

with an acceleration voltage of 200 kV. Sample preparation

was involved in sonicating material in DMF for 30 min and

dropping the resulting suspension onto carbon-coated copper

grids. Atomic force microscopy (AFM) images were obtained

on a Digital Instruments Nanoscope IIIa using tapping mode

with a Si cantilever. UV–vis spectra were recorded on a Jena

SPECORD S600 spectrophotometer using a quartz cell with a

path length of 10 mm. Fluorescence spectra measurements

were carried out on an Edinburgh instruments FL900. The

absorption of sample at excitation wavelength 635 nm was

adjusted to 0.15 Abs.

2.4. Nonlinear optical measurement

In Z-scan measurement systems, the second harmonic of a

Q-switched Nd: YAG laser (1064 nm, 4 ns) was used as the

laser source. The laser beam with repetition rate of 10 Hz

was firstly adjusted by an inverted telescope system including

a fluence attenuator and a Glan–Taylor prism, and then

focused by f/100 mm convex (Zolix OLB50–100, U50, f 100) to

a beam waist radius x0 of 50 lm. After entered the sample,

the laser beam was divided by a beam splitter: the reflected

beam was used as open-aperture signal and the transmitted

one passed through a small hole (s = 0.11) as a close-aperture

signal. Both laser pulses were monitored per 850 ms by energy

detectors (PE9-ROHS energy probes, OPHIR Laser Measure-

ment Group). A computer was used to collect and process

the data that were sent from the energy detectors through a

Zolix SC300–2A Motion Controller. The mobile speed of

motion controller was 0.5 mm/s in the process of Z-scan

measurement. DMSO solutions of GO, ZnPc, GO–ZnPc and

RGO–ZnPc with 0.13 mg/mL were placed in 2 mm quartz cells.

In the determination of the nonlinear absorption coefficient b

of the samples, the corresponding Z-scan recordings were

fitted by using the intensity variation equation and adopting

an intensity-dependent absorption coefficient, owing to the

bleaching of sample transmission at lower pump intensity

region [30]. Details of numerical simulations of Z-scan can

be seen in Supporting Information. In optical limiting

experiments, the input fluence-dependent transmittance

at 532 nm was extracted from the Z-scan measurement

results.

3. Results and discussion

3.1. Synthesis and characterization

The preparation of the RGO–ZnPc hybrid material is based on

the initial formation of covalently functionalized GO–ZnPc

and the subsequent in situ reduction of the GO moiety to

RGO during mild thermal treatment in DMF solvent, as

illustrated in Fig. 1. The schematic representation for the


structure of GO moiety contains the sp2 carbon clusters and

smaller sp2 carbon configurations dispersed in an insulating

sp3 carbon matrix (represented by grey honeycomb lattice),

where a large fraction of carbon is bonded with oxygen (oxy-

gen atoms are not shown) [13,31–33]. After the thermal treat-

ment at 120 �C, a large amount of oxygen functional groups of

the GO moiety in GO–ZnPc could be removed due to the deox-

ygenation. The result of elemental analysis shows that the

content of C and O species are 47.5 wt% and 49.1 wt% for

GO, 72.1 wt% and 20.7 wt% for RGO, respectively. As shown

in Fig. 1, the sp2 carbon domains have been divided into sp2

carbon clusters with larger size and smaller sp2 carbon config-

urations [31,33]. Since the additional sp2 carbon clusters can-

not be formed and the exiting sp2 carbon clusters do not grow

much under such mild reduced condition, only smaller sp2

carbon configurations would be created and increased in

number for resultant RGO–ZnPc hybrid [31–33]. The high effi-

cient covalent linkage of the ZnPc to GO, structure evolution

of GO moiety to RGO and synergistic photophysical properties

between RGO and ZnPc moieties of the synthesized RGO–

ZnPc hybrid material are discussed below.

3.1.1. X-ray diffractionThe XRD was carried out to investigate the structure of the

RGO–ZnPc hybrid. Fig. 2 shows the XRD patterns of graphite,

GO, GO–ZnPc, RGO–ZnPc and RGO. While the characteristic

(002) diffraction peak of graphite presents at about 26.6� with

a d-spacing of 0.34 nm, a prominent (002) diffraction peak of

GO is observed at around 11.2� with the interlayer spacing of

0.78 nm. The increase of d-spacing of GO with respect to that

of graphite can be attributed to the introduction of oxyge-

nated functional groups [34,35]. The XRD pattern of GO–ZnPc

displays a relative low diffraction peak at about 10.2� with the

d-spacing of 0.87 nm, indicating a further expansion of the

(002) inter-planar spacing of GO owing to the incorporation

of the Pcs molecules. In addition, one additional weak peak

at around 22.9� is also observed, which is probably due to par-

tial stacking of phthalocyanine, either with itself or with the

GO sheets during the introduction of ZnPc molecules onto

GO sheets.

After the thermal treatment of GO–ZnPc, RGO–ZnPc exhi-

bits a similar pattern to the RGO reference sample. The char-

acteristic peak assigned to GO disappears due to the

deoxygenation reaction. Three broad weak peaks belonging

to RGO appear at about 18.2�, 22.8� and 26.4�, respectively,

suggesting that the mild thermal treatment is an efficient

method for the reduction of GO moiety to the limited few-

layers stacking of reduced GO sheets. The multiple peaks in

the spectrum may be resulted from the partial reduction of

GO, which gives rise to the uneven interlayer spacing at the

edges or in the whole sample of the RGO sheets [36].

3.1.2. Raman spectraThe Raman spectroscopy is considered as an effective techni-

que for studying the carbon framework of various graphene

materials. The Raman spectra of graphite, GO, GO–ZnPc,

RGO–ZnPc and RGO are illustrated in Fig. 3. The graphite dis-

plays a characteristic G band at 1586 cm�1 with a weak D band

at 1372 cm�1, corresponding to the ordered sp2-bonded car-

bon atoms and the disordered modes, respectively [35,37]. In



(GO-ZnPc) (RGO-ZnPc)

N

N N

N

OOO

O

O

O

O

OH

O O OH

O

O

HO

Zn

O

HO

sp3-C matrix sp2-C cluster sp2-C configuration

N

N N

NOOO

O

O

O

O

OH

OO OH

O

O

HO

Zn

O

O

1) DCC activated graphene oxide, DMF, 25 oC, 96 h

2) Thermal reduction

DMF, 120 oC, 24 h

C OO

CO

O

CO

O

CO O

PcZn

ZnPc

PcZn

ZnPc

CO

O ZnPc

CO O ZnPc

CO

OPcZn

C O

Fig. 1 – The synthesis scheme of RGO–ZnPc. (A colour version of this figure can be viewed online.)

1200 1500 1800

0.06

0.84

0.77

0.85

0.87

ID/IG

GO

RGO

GO-ZnPc

RGO-ZnPc

Graphite

Inte

nsity

/ a.

u.

Raman shift / cm-1

Fig. 3 – Raman spectra of graphite, GO, GO–ZnPc, RGO–ZnPc

and RGO. (A colour version of this figure can be viewed

online.)

10 20 30 40 50 60 70

G(002)GO(002)

RGO

RGO-ZnPc

GO-ZnPc

GO

Graphite

Inte

nsity

/ a.

u.

2 Theta / Degree

Fig. 2 – XRD patterns of parent graphite, GO, GO–ZnPc, RGO–

ZnPc and RGO. (A colour version of this figure can be viewed

online.)


the spectrum of GO, the G band at 1586 cm�1 is broadened

while the intensity of D band increases substantially. It could

be attributed to the significant reduction in size of the in-

plane sp2 domains due to the oxidation [38]. In contrast to

the case of GO, the G and D bands of GO–ZnPc appearing at

1596 and 1378 cm�1, respectively, are found to be slightly

shifted to high wavenumbers. Furthermore, a similar result

of red-shift for G and D bands is also obtained when compar-

ing the spectrum of RGO–ZnPc with those of GO and RGO

reference samples, which implies that the red-shift may


result from the effect of covalently bonded ZnPc molecules

on the carbon framework of the RGO moiety in the RGO–ZnPc

hybrid [38–40].

The D to G band intensity ratio (ID/IG) often affords infor-

mation about the structural changes and covalent modifica-

tion [32,41]. Usually functionalization of GO would lead to

the enhancement of the ID/IG ratio. But, in this study, the

intensity ratio (ID/IG) decreases from 0.84 to 0.77 after the

covalent functionalization of GO with ZnPc. The reason may

be that the ZnPc molecules grafted onto GO sheets contain



0.0 0.5 1.0 1.5 2.0 2.5 3.0-1012345

Hei

ght /

nm

Position / µm

(d)

(a) (b)

(c)

Fig. 4 – SEM images (a and b), TEM image (c) and AFM image (d) of RGO–ZnPc, with the inset of (d) showing the thickness of

RGO–ZnPc sheet. (A colour version of this figure can be viewed online.)

3000 2000 1500 1000 500

(methylene)(methylene) C-H

(primary alcohols)

(e)

(d)

(c)

(b)

C=O

Tran

smitt

ance

/ a.

u.

Wavenumber / cm-1

(ester)

(a)

C-H

C-OO-H

Fig. 5 – FT-IR spectra of GO (a), RGO (b), GO–ZnPc (c), ZnPc (d)

and RGO–ZnPc (e). (A colour version of this figure can be

viewed online.)


a large amount of sp2 aromatic carbon atoms [41]. Followed by

the mild thermal treatment, the intensity ratios (ID/IG)

increase up to 0.85 for RGO–ZnPc and 0.87 for the RGO refer-

ence sample, which suggests a decrease in the average size

of sp2 carbon domains upon reduction of the GO moiety. This

change can be explained that if additional graphitic sp2

domains (sp2 configurations) were created, they should be

smaller in size than the ones (sp2 clusters) presenting in GO

before reduction, but more numerous in number [32]. These

results essentially support our hypothesis of schematic repre-

sentation for the structure changes of the GO moiety to RGO,

as illustrated in Fig. 1.

3.1.3. Morphological analysisFurther insight into the morphology of RGO–ZnPc hybrid has

been gained from the SEM, TEM and AFM measurements. The

SEM images of the RGO–ZnPc hybrid are shown in Fig. 4a and

b, which demonstrates the characteristics of turbostratic

stacked flakes of graphene. It is likely to evolve from re-aggre-

gation and concomitant folding of few-layer graphene sheets.

In addition, the surface of the RGO–ZnPc hybrid sheet exhibits

a wrinkled texture with slightly scrolled edges, a typical char-

acteristic of graphene, which can be obviously seen from the

TEM image in Fig. 4c. As illustrated in Fig. 4d, a typically flake

with a thickness of ca. 4 nm is also observed in the AFM

image, indicating few-layer graphene with 4–8 layers [42,43].

3.1.4. FT-IR spectraThe FT-IR spectrum can provide essential and useful informa-

tion for the covalent functionalization of RGO with ZnPc


moieties. The FT-IR spectra of GO, RGO, GO–ZnPc, ZnPc and

RGO–ZnPc are demonstrated in Fig. 5. The main characteristic

absorption peaks of GO are located at 1731 cm�1 (mC@O) and

1415 cm�1 (dOAH) from carbonyl and carboxyl groups,

3416 cm�1 (mOAH) and 1046 cm�1 (mCAO) from hydroxyl groups,

1633 cm�1 (mC@C) corresponding to skeletal vibrations from

unoxidized graphitic domains and 1222 cm�1 (mCAOH) from

epoxy/ether groups [38,44,45]. In comparison with GO, the

characteristic absorption peaks of ZnPc with the primary

alcohols at 1065 cm�1 (mCAO) and 1266 cm�1 (dOAH), methylene



C-CC-O

C=O

C(O)O

GO

GO-PcZn

inte

nsity

/ a.

u. C-N

294 291 288 285 282 279

Rel

ativ

e

RGO-ZnPc

RGO

Bending energy / eV

Fig. 7 – C 1s XPS spectra of GO, GO–ZnPc, RGO–ZnPc and RGO.

(A colour version of this figure can be viewed online.)


at 2922 cm�1 (mCAH) and 1487 cm�1 (dCAH), ether at 1715 cm�1

(mC@O) and CAN stretching vibrations at 1332 cm�1 are

observed in the spectrum of GO–ZnPc, which suggests that

the ZnPc molecules were introduced onto GO sheets. How-

ever, these characteristic groups of ZnPc show relative low

resolution because of their overlapping with various vibra-

tions of CAO, C@O and OAH bonds in GO.

Following mild thermal treatment of GO–ZnPc at 120 �C, a

large amount of hydroxyl, carbonyl/carboxyl and epoxy/ether

groups of the GO moiety would be removed owing to the

deoxygenation reaction [46]. As shown in the spectrum of

the RGO reference sample, the intensities of all absorption

peaks corresponding to oxygen functional groups show a sig-

nificant decrease. However, the structure of ZnPc molecules

can be well maintained because the temperature of thermal

treatment is much lower than that for its synthesis (150 �C).

Therefore, the characteristic peaks coming from the ZnPc

moiety exhibit relative high resolution, which can be identi-

fied more accurately in the spectrum of RGO–ZnPc. Moreover,

an obvious C@O stretching vibration of ester bond is observed

at 1715 cm�1, which can be contributed to the esterification of

the carboxylic ends activated by DCC in GO with the primary

alcohols in the periphery substituent of ZnPc. These results

further corroborate the synthesis of RGO–ZnPc hybrid by an

efficient covalent functionalization and subsequent thermal

reduction strategy.

3.1.5. X-ray photoelectron spectroscopyThe elemental speciation of the RGO–ZnPc hybrid has been

analyzed by XPS. The XPS survey spectra of GO, RGO, GO–ZnPc

and RGO–ZnPc are shown in Fig. 6. It is clear that only two

main peaks corresponding to the C and O species can be

observed in the spectra of GO and RGO. After the covalent

functionalization and subsequent thermal treatment, three

additional peaks of Zn 2p1/2, Zn 2p3/2 and N 1s obviously pre-

sent in the spectra of GO–ZnPc and RGO–ZnPc at around

1021.4 eV, 1044.5 eV and 399.4 eV, respectively, indicating a

successful incorporation of ZnPc into the hybrid. On the other

hand, after the thermal treatment the peak intensity ratio of

O1s to C1s for RGO–ZnPc is obvious smaller than that of

1000 800 600 400 200

N 1s

Zn Auger

Zn 2p3/2

Zn 2p1/2

C 1sO 1s

RGO-ZnPc

GO-ZnPc

RGO

GO

Inte

nsity

/ a.

u.

Binding Energy / eV

Fig. 6 – XPS survey spectra of GO, RGO, GO–ZnPc and

RGO–ZnPc. (A colour version of this figure can be viewed

online.)


GO–ZnPc, suggesting an effective reduction of the GO moiety

to RGO by the mild thermal treatment route.

In addition, owing to the introduction of the ZnPc mole-

cules by the covalent ester bonds (C(O)O) and efficient reduc-

tion of GO to RGO, as demonstrated in the XRD, Raman and

FT-IR results, the bonding state of carbon may be changed sig-

nificantly and provide useful information. So, the C 1s XPS

spectra of the RGO–ZnPc hybrid together with GO, GO–ZnPc

and RGO are further investigated, as shown in Fig. 7. The C

1s XPS spectrum of GO can be fitted into four peaks corre-

sponding to different carbon species, CAC (sp2 carbon) at

284.6 eV, CAO at 286.0 eV, C@O at 287.3 eV and C(O)O at

288.6 eV, suggesting a considerable degree of oxidation for

the GO nanosheets [32,33,46,47]. Followed by the introduction

of ZnPc, one additional CAN species coming from the Pcs

macrocycle appears at 285.8 eV in the spectrum of GO–ZnPc

[32,41]. The peak area ratios of carbon-containing bonds to

total area are also calculated on the basis of XPS results, as

shown in Table 1. The amount of CAO species (30.4%) of

GO–ZnPc increases obviously compared to that of GO

(23.6%), which should be attributed to the considerable

increase of the ether and hydroxyl groups arising from the

ZnPc molecules. After the thermal treatment, the amount of

oxygen-containing groups such as CAO, C@O and C(O)O exhi-

bits an obvious decrease in the spectra of RGO and RGO–ZnPc

along with significant increase of CAC species (sp2 carbon),

which further provides strong evidence for the effective

reduction of the GO moiety in the hybrid. Furthermore, com-

pared with the RGO reference sample, the amount of CAO

(30.7%) and C(O)O (9.1%) species in the RGO–ZnPc hybrid is

higher than that in RGO (19.2% and 7.9%, respectively), which

suggests that the CAO groups in the ZnPc moiety and the

ester bonds formed in the RGO–ZnPc hybrid could be well

maintained during the thermal treatment.



400 600 800 10000.0

0.2

0.4

0.6

0.8

637

724(b)

(c)

(d)

(a)

Abs

orba

nce

/ a.u

.

Wavelength / nm

703

Fig. 8 – UV–vis absorption spectra of GO (a), GO–ZnPc (b),

RGO–ZnPc (c) and ZnPc (d) in DMSO. (A colour version of this

figure can be viewed online.)

400 600 800 10000.0

0.5

1.0

1.5

2.0

10 20 30 40 50 600.0

0.5

1.0

Abs

at 7

24 n

m

Concentration / mg L-1

Abs

orba

nce

/ a.u

.

Wavelength / nm

Fig. 9 – UV–vis absorption spectra of RGO–ZnPc in DMSO

(concentrations, bottom to top: 5, 10, 20, 30, 40 and 60 mg/L).

The inset is the plot of optical density at 724 nm versus

concentration. (A colour version of this figure can be viewed

online.)

Table 1 – The peak area (A) ratios of carbon-containing bonds to total area (AT) according to the XPS results.

Sample ACAC/AT (%) ACAO/AT (%) AC@C/AT (%) AC(O)O/AT (%) ACAN/AT (%)

GO 33.6 23.6 23.4 19.4GO–ZnPc 35.9 30.4 14.6 10.8 8.3RGO–ZnPc 45.6 30.7 5.3 9.1 9.3RGO 62.6 19.2 10.3 7.9


3.1.6. UV–vis absorption spectraThe UV–vis absorption spectra of GO, ZnPc, GO–ZnPc and

RGO–ZnPc are illustrated in Fig. 8. The absorption spectrum

of GO displays a strong broad absorption at around

270–400 nm, attributed to p! p* transitions of aromatic C@C

bonds [48]. The spectrum of ZnPc exhibits typical characteris-

tic absorption of metal phthalocyanines (MPcs) with an

intense S0–S1 transition band (Q-band) centered at 703 nm fol-

lowed by a smaller shoulder at 637 nm and a low broad Soret

band at 320–380 nm [22]. Following the covalent attachment

of GO with ZnPc, the absorption peak at 706 nm with a

shoulder at 637 nm observed in the spectrum of GOAZnPc

should be assigned to the Q band of the ZnPc moiety. A strong

broad UV absorption band at 270–400 nm can be attributed to

the combination of the Soret band of ZnPc and p! p* transi-

tions of GO. Compared with the spectrum of ZnPc, the slight

red shift of Q band with the broadening of UV absorption band

suggests the ground-state electronic interactions between the

two moieties within the hybrid [8,25,26]. Moreover, after the

reduction treatment of GO–ZnPc, the Q band of the ZnPc moi-

ety becomes broader and red shifts significantly to 724 nm in

the spectrum of RGO–ZnPc, and the relative intensity of the

UV absorption band decreases clearly. These changes of

the Q band and the UV absorption band for RGO–ZnPc can

be ascribed not only to the reduction of the GO moiety to

RGO, but also to the alteration of the electronic state of ZnPc

caused by the electronic interactions between the ZnPc and

RGO moieties.

Good dispersion is of particular importance for graphene

processability and applications because most of their attrac-

tive properties are only associated with individual graphene

sheets. Solution-phase UV–vis spectra has been reported to

demonstrate a linear relationship between the absorbance

and the relative concentrations of various graphene oxide

hybrids, which obeys Beer’s law at low concentrations, and

has been used to determine the solubility of the hybrids


[7,49]. Fig. 9 shows the absorption spectra of RGO–ZnPc hybrid

in DMSO with different concentrations. The absorption inten-

sities at 724 nm were plotted against the mass concentrations

(inset of Fig. 9), displaying a very good linear relationship that

obeys Beer’s law at low concentration. According to Beer’s law,

we can estimate the effective extinction coefficient of

RGO–ZnPc from the slope of the linear least-squares fit to be

0.018 L mg�1 cm�1, with an R value of 0.998. The absorbance

of RGO–ZnPc in DMSO solutions at other wavelengths was

also in line with Beer’s law. These results indicate that the

RGO–ZnPc hybrid has been homogenously dispersed in

DMSO.

3.1.7. Fluorescence spectroscopyThe steady state fluorescence spectra of ZnPc, GO–ZnPc and

RGO–ZnPc with the absorption at excitation wavelength

matching to 0.15 are shown in Fig. 10. Upon excitation at

635 nm, the spectrum of ZnPc exhibits an emission peak at

715 nm, corresponding to the fluorescence of the S1! S0 tran-

sition [22,29]. The emission peaks of GO–ZnPc and RGO–ZnPc

observed at around 710 nm can also be attributed to the lumi-

nescence of the Pcs moiety in the hybrid [8]. Compared with

the maximum absorption at 703 nm of ZnPc, the Stokes shift

of ZnPc is 12 nm whereas that of the hybrids GO–ZnPc and

RGO–ZnPc is only 7 nm, which reflects the alternation of the

electronic state of Pcs induced by graphene as demonstrated

in the UV–vis spectra. Since electrons can move ballistically

through graphene even at room temperature without or with

much less energy loss [9], the Stokes shift in the hybrid sys-

tem resulted from the energy loss during the process from

S0! S1 transition absorption to S1! S0 transition emission,

is thus less than that of pristine ZnPc [24]. Moreover, the

emission intensities of GO–ZnPc and RGO–ZnPc relative to

that of ZnPc are decreased obviously with the sequence:



0.5

1.0

Nor

m. t

rans

mitt

ance

ZnPc Experiment Theoretical fit

0.5

1.0

Nor

m. t

rans

mitt

ance

GO Experiment Theoretical fit

0.5

1.0

Nor

m. t

rans

mitt

ance

GO-ZnPc Experiment Theoretical fit

-80 -40 0 40 80

0.5

1.0

Nor

m. t

rans

mitt

ance

Z-position / mm

RGO-ZnPc Experiment Theoretical fit

Fig. 11 – Open aperture Z-scan of GO, ZnPc, GO–ZnPc and

RGO–ZnPc excited by an input intensity of 0.43 J/cm2. (A

colour version of this figure can be viewed online.)

600 700 800 9000.0

4.0k

8.0k

12.0k ZnPc

RGO-ZnPc

GO-ZnPcEm

issi

on in

tens

ity /

a.u.

Wavelength / nm

Fig. 10 – Steady state fluorescence spectra of ZnPc, GO–ZnPc

and RGO–ZnPc in DMSO. (A colour version of this figure can

be viewed online.)


ZnPc > GO–ZnPc > RGO–ZnPc. This may be explained by the

fluorescence quenching arising from two competitive pro-

cesses, photo-induced electron transfer (PET) and energy

transfer (ET), from Pcs to graphene because graphene is excel-

lent acceptor for energy and electron, while Pcs can act as an

energy absorbing and electron transporting antenna [24].

Considering such a fact that RGO possesses better capability

of electron and energy transfer than GO because of the

incomplete restoration of sp2 p-conjugated network for gra-

phene after the reduction, the PET/ET process between Pcs

and graphene would be further enhanced. It is thus reason-

able that more efficient fluorescence quenching in RGO–ZnPc

has been observed than that of GO–ZnPc.

3.2. Nonlinear optical properties

The nonlinear optical (NLO) properties of the RGO–ZnPc

hybrid were investigated using Z-scan technique. To test the

NLO response, the samples were individually dispersed in

DMSO at a concentration of 0.13 mg/mL. All the samples exhi-

bit very good dispersibility in DMSO solution. Upon excitation

by 4 ns laser pulses of 532 nm with input intensity of 0.43 J/

cm2, the absolute starting transmittance of GO, ZnPc,

GO–ZnPc and RGO–ZnPc is 0.76, 0.84, 0.72 and 0.54, respec-

tively. Fig. 11 gives open aperture Z-scan curves of GO, ZnPc,

GO–ZnPc and RGO–ZnPc. The normalized transmittance

curve of GO exhibits two weak shoulder peaks along with a

valley corresponding to a transformation from SA to RSA with

the increase of the pump intensity, which should be closely

dependent on its structure characteristics of sp2/sp3 carbon

hybridization [17,19,50,51]. The unique atomic and electronic

structure of GO has been elucidated so that the sp2 carbon

clusters and small sp2 configurations are isolated by the sp3

matrix [13,52–54]. The presence of pristine graphitic nanois-

lands which are sp2-hybridized carbon clusters, makes the

GO possess some characteristics of graphene, including ultra-

fast carrier dynamics and Pauli blocking, which results in fast

SA in ultra broad spectra region [10,11]. Therefore, after

excited by 532 nm laser, the SA originating from Pauli block-

ing dominates the NLO absorption at low pump intensities

owing to the state filling of the interband transitions in the

sp2 clusters [15,16,19]. On the other hand, the two photo

absorption (TPA) originating from the sp3 domains dominates

the NLO absorption at high pump intensities due to the high


energy gap of sp3-bonded carbon (2.7–3.1 eV) [15,55]. The con-

tribution of excited state absorption (ESA) arising from small

localized sp2 configurations to the nonlinear absorptive valley

should be minor in comparison with the TPA owing to the

small amount of the sp2 configurations in GO [14,17,19].

The curve of ZnPc displays a typical valley of RSA behavior,

corresponding to the absorption of the triplet excited state

[20]. Following the covalent functionalization of GO with

ZnPc, the GO–ZnPc shows a much deeper valley than that of

GO, reflecting a combination of NLO absorption arising from

the GO and ZnPc moieties. In addition, the PET/ET process

between ZnPc and GO documented in fluorescence analysis

may also devote to the NLO absorption by the fluorescence

quenching and energy releasing [56,57]. Moreover, the non-

linear absorptive valley of RGO–ZnPc is further deepened

and broadened obviously compared with that of GO–ZnPc,

suggesting an enhancement of NLO properties, which should

be mainly attributed to the thermal reduction of the GO

moiety to RGO. After the reduction, the small localized sp2

configurations may increase numerously in number but not

interconnect to form new sp2 carbon clusters in RGO moiety

[31,33], as represented in synthesis scheme (Fig. 1) and con-

firmed by the results of Raman and XPS. Therefore, it is the

RSA valley, rather than the SA peak, that has been enlarged.

Additionally, the PET/ET process between ZnPc and RGO has



0.4 0.6 0.8 1.0

0

50

100

300

600

900

1200

1500

ZnPc

GO

GO-ZnPc

RGO-ZnPc

Input intensity / J cm-2

ß/ c

m G

W-1

Fig. 12 – The nonlinear absorption coefficient b as a function

of the input intensity for nanosecond pulses. (A colour

version of this figure can be viewed online.)

11.00.0

0.3

0.6

0.9

1.2

T = 50%

Nor

mal

ized

tran

smitt

ance

Pulse energy density / J cm-2

RGO-ZnPc GO-ZnPcZnPc GO

Fig. 13 – The optical limiting of RGO–ZnPc, GO–ZnPc, ZnPc

and GO excited at 532 nm with 4 ns pulses. (A colour version

of this figure can be viewed online.)


also been enhanced due to the partial restoration of sp2

p-conjugated network (see fluorescence spectra). As a result,

the combination of NLO absorption originating from numer-

ous small localized sp2 configurations and the contribution

of improved PET/ET process may lead to the significant

enhancement of NLO properties.

The nonlinear absorption coefficient b of these materials

were investigated at different input intensities from 0.32 to

1.01 J/cm2. As shown in Fig. 12, after covalent functionaliza-

tion with ZnPc, the GO–ZnPc hybrid exhibits much higher

value of b than that of GO at different input intensities,

although the ZnPc only showed very low value of b. It can

be attributed not only to the combined NLO performance of

the GO and ZnPc moieties but also to the contribution of

the PET/ET process between ZnPc and GO. Furthermore, the

RGO–ZnPc hybrid shows significantly larger value of b with

respect to that of GO–ZnPc hybrid, and gives a highest non-

linear absorption coefficient b of 1500 cm/GW, which should

be devoted to efficient reduction of GO moiety to RGO as dis-

cussed above. The theoretically fitted nonlinear optical para-

meters (saturation intensity IS and nonlinear absorption

coefficient b) can be seen in Table S1 in detail.

In general, the value of b for the RSA behavior decreases

with the increasing input intensity because of the saturation

of RSA at higher input intensities [57], as shown in the curve

of ZnPc (see Fig. 12). However, the curves of other materials

show different trends for nonlinear absorption coefficient b,

which should be owing to their complicated NLO response

mechanisms. Since the value of b should be constant for

TPA and decreased for RSA behavior, the increased trend of

nonlinear absorption coefficient b from 55 to 100 cm/GW for

GO with the increase of input intensity implies that the

observed NLO performance is not only depended on the non-

linear absorption, but also influenced by nonlinear scattering

in the higher intensity regime [57]. The similar phenomena

has been observed in the previous work upon GO for nanose-

cond pulses [17]. The nonlinear absorption coefficient b of

GO–ZnPc decreases firstly from 300 to 200 cm/GW when the

input intensity increases from 0.32 to 0.43 J/cm2, then it

shows a slight increase at relative high input intensities. Such


changes imply an intricate competition mechanism of NLO

response between the nonlinear absorption and nonlinear

scattering. At relative low input intensities, nonlinear

absorption, especially the RSA behavior, dominates the NLO

performance so that the value of b decreases with the

increase of input intensity. At relative high input intensities,

the influence of nonlinear scattering will be enhanced, and

thus the value of b will be increased.

It can be noticed that the value of b for the RGO–ZnPc

hybrid decreases clearly from 1500 to 1050 cm/GW with the

increasing input intensity, but it is still much larger than that

of individual GO, ZnPc and GO–ZnPc hybrid. The result indi-

cates that the nonlinear scattering of RGO–ZnPc should be

depressed at a certain extent due to the covalent functionali-

zation of RGO with soluble ZnPc and the significantly

improved solubility of RGO–ZnPc as demonstrated in the

UV–vis analysis. Moreover, the nonlinear absorption, espe-

cially RSA behaviors originating from ZnPc, RGO and the

PET/ET process between the ZnPc and RGO moieties, should

play more important role in comparison with nonlinear scat-

tering and dominate the NLO performance of the RGO–ZnPc

hybrid. Therefore, the value of b would be decreased with

the raising input intensity owing to the saturation of the

excited state at high input intensities. In summary, although

the NLO response are observed for all four materials, the lar-

ger value of b observed for RGO–ZnPc suggests that it should

have competitively better optical limiting performance.

The optical limiting (OL) performance of different materi-

als was investigated in DMSO at same linear transmittance.

Pure DMSO solvent displayed no detectable OL performance

under the same condition, suggesting that the observed OL

response should be attributed solely to the samples. Fig. 13,

in which the normalized transmittance was plotted as

functions of the input energy densities, presents OL behavior

of RGO–ZnPc, GO–ZnPc, GO and ZnPc. The optical-limiting

threshold values (F50, defined as the input energy density

at which the transmittance falls to 50% of the linear

transmittance) for different samples are also investigated. It

can be clearly seen that at the same level of linear transmit-

tance of 80%, the RGO–ZnPc hybrid exhibits a lower F50 value

(0.98 J/cm2) in comparison with the GO (2.12 J/cm2), ZnPc

(1.97 J/cm2) and GO–ZnPc hybrid (1.74 J/cm2), indicative of

much better OL performance. These results further prove that

the preparation of RGO–ZnPc hybrid by the initial covalent




functionalization of GO with ZnPc and the subsequent in situ

thermal reduction of GO to RGO, not only improves the solu-

bility of hybrid material but also enhances its NLO and OL

performance.

4. Conclusion

We have reported the synthesis, structure and nonlinear opti-

cal properties of RGO–ZnPc hybrid material. The results of

XRD, Raman, FT-IR, XPS, UV–vis and morphological studies

(SEM, TEM, AFM), confirm the successful fabrication of

RGO–ZnPc hybrid material, based on the initial covalent func-

tionalization of GO with ZnPc and the subsequent in situ

reduction of GO to RGO during mild thermal treatment. The

considerable covalent functionalization of ZnPc significantly

improves the dispersibility of RGO in organic solvent. An

enhancement of PET/ET process with more efficient fluores-

cence quenching and energy release is also observed after

the reduction of initial GO–ZnPc to RGO–ZnPc hybrid material.

As expected, upon excitation by a 532 nm laser of 4 ns pulses,

RGO–ZnPc exhibits much larger NLO absorption coefficient b

and better OL performance than those of individual GO, ZnPc

and GO–ZnPc hybrid, which can be attributed to the combina-

tion of different NLO mechanisms in RGO–ZnPc. Such com-

bined mechanisms contain the ESA arising from numerous

localized sp2 carbon configurations, TPA from the sp3 domains

and SA from the sp2 clusters in the RGO moiety, the RSA ori-

ginating from the ZnPc moiety and the contribution of the

efficient PET/ET process between ZnPc and RGO. Considering

the easy preparation of covalently bonded RGO–ZnPc hybrid,

this work may provide some insight into the design of other

novel graphene-based materials, and the present RGO–ZnPc

hybrid is expected to afford good candidate for optoelectronic

devices, such as optical limiting, optical switching and solar

energy conversion applications.

Acknowledgments

This work is supported by the National Natural Science

Foundation of China (61137002, 21203058 and 61275117),

Natural Science Foundation of Heilongjiang Province of China

(B201308, F201112), Foundation of Educational Commission of

Heilongjiang Province of China (12521399 and 12531579).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2014.06.018.

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synthesis and nonlinear optical properties of reduced graphene oxide hybrid material covalently...

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