Synthesis and nonlinear optical properties of reduced graphene oxide hybrid material covalently functionalized with zinc phthalocyanine

CARBON

x x x ( 2 0 1 4 ) x x x –x x x

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Synthesis and nonlinear optical properties of reduced graphene oxide hybrid material covalently functionalized with zinc phthalocyanine 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 China b College of Environmental and Chemical Engineering, Heilongjiang University of Science and Technology, Harbin 150022, PR China c Research Center for Space Optics Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, PR China d 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:

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

Received 2 March 2014

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

Accepted 10 June 2014

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

Available online xxxx

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 functionalization 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 numerous 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. Ó 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, optical 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

* Corresponding authors: Fax: +86 451 8667 3647. E-mail addresses: [email protected] (C. He), [email protected] (Y. Wu). http://dx.doi.org/10.1016/j.carbon.2014.06.018 0008-6223/Ó 2014 Elsevier Ltd. All rights reserved.

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CARBON

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 characteristics of graphene due to the presence of pristine graphitic nanoislands, exhibits some heterogeneous optical transition and nonlinear dynamics because small sp2 carbon nanoislands 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 performance 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. Phthalocyanine complexes (Pcs) are a class of soluble NLO materials with RSA properties originating from the occurrence of intersystem 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 graphene 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.

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 sulfuric 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 °C and 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-hydroxyethoxy)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 subsequent in situ reduction of GO moiety to RGO during mild thermal 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 Dicyclohexylcarbodiimide (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. Subsequently, 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 product 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 ˚ ) radiation (40 kV, 40 mA). Raman spectra were (k = 1.5418 A 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

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monochromatic Al Ka X-ray source (15 kV, 150 W) and analyzer pass energy of 100 eV. Binding energies (BE) are referred to the C (1s) binding energy of carbon taken to be 284.6 eV. Elemental 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 microscopy (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 Measurement 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

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 (oxygen atoms are not shown) [13,31–33]. After the thermal treatment at 120 °C, a large amount of oxygen functional groups of the GO moiety in GO–ZnPc could be removed due to the deoxygenation. 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 configurations [31,33]. Since the additional sp2 carbon clusters cannot 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 efficient 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 diffraction

The 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 (0 0 2) diffraction peak of graphite presents at about 26.6° with a d-spacing of 0.34 nm, a prominent (0 0 2) 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 oxygenated 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 (0 0 2) 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 partial 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 exhibits a similar pattern to the RGO reference sample. The characteristic 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 fewlayers 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.

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

3

Raman spectra

The Raman spectroscopy is considered as an effective technique 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 displays a characteristic G band at 1586 cm1 with a weak D band at 1372 cm1, corresponding to the ordered sp2-bonded carbon atoms and the disordered modes, respectively [35,37]. In

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x x x ( 2 0 1 4 ) x x x –x x x

CARBON

OH O O OH

O HO

O

O

O

O

N Zn N

N

O

N

O

O

O

O

OH

O O

O

HO

O

PcZn

O

PcZn

2) Thermal reduction O C O

C O

O C O

DMF, 120 oC, 24 h

O C O

ZnPc

O

O ZnPc

(GO-ZnPc) sp3-C matrix

OH

O

O O

O C O

C

O

O

C O

O

O

N Zn N N

N

O

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

HO PcZn O

O

C

ZnPc

O ZnPc

(RGO-ZnPc)

sp2-C cluster

sp2-C configuration

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

GO(002)

G(002)

ID/IG

RGO RGO-ZnPc

Intensity / a.u.

Intensity / a.u.

0.87

RGO

0.85

RGO-ZnPc

0.77

GO-ZnPc

0.84

GO

GO-ZnPc GO Graphite

10

20

30

40

50

60

70

2 Theta / Degree

0.06 1200

Graphite

1500

1800 -1

Raman shift / cm

Fig. 2 – XRD patterns of parent graphite, GO, GO–ZnPc, RGO– ZnPc and RGO. (A colour version of this figure can be viewed online.)

Fig. 3 – Raman spectra of 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 cm1 is broadened while the intensity of D band increases substantially. It could be attributed to the significant reduction in size of the inplane 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 cm1, 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 comparing 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 information about the structural changes and covalent modification [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

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(a)

(b)

(c)

(d)

Height / nm

5 4 3 2 1 0 -1 0.0

0.5

1.0 1.5 2.0 Position / µm

2.5

3.0

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.) 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 reference 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 representation for the structure changes of the GO moiety to RGO, as illustrated in Fig. 1.

Morphological analysis

Further 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-aggregation 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 characteristic 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.

(a)

FT-IR spectra

The FT-IR spectrum can provide essential and useful information for the covalent functionalization of RGO with ZnPc

(b) (c)

Transmittance / a.u.

3.1.3.

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 cm1 (mC@O) and 1415 cm1 (dOAH) from carbonyl and carboxyl groups, 3416 cm1 (mOAH) and 1046 cm1 (mCAO) from hydroxyl groups, 1633 cm1 (mC@C) corresponding to skeletal vibrations from unoxidized graphitic domains and 1222 cm1 (mCAOH) from epoxy/ether groups [38,44,45]. In comparison with GO, the characteristic absorption peaks of ZnPc with the primary alcohols at 1065 cm1 (mCAO) and 1266 cm1 (dOAH), methylene

(d)

(e)

C=O C-H

(ester) C-H

(methylene)

(methylene)

O-H

C-O

(primary alcohols)

3000 2000

1500

1000

500

-1

Wavenumber / cm

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.)

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at 2922 cm1 (mCAH) and 1487 cm1 (dCAH), ether at 1715 cm1 (mC@O) and CAN stretching vibrations at 1332 cm1 are observed in the spectrum of GO–ZnPc, which suggests that the ZnPc molecules were introduced onto GO sheets. However, these characteristic groups of ZnPc show relative low resolution because of their overlapping with various vibrations 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 significant 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 identified more accurately in the spectrum of RGO–ZnPc. Moreover, an obvious C@O stretching vibration of ester bond is observed at 1715 cm1, 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 spectroscopy

The 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 present 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

C 1s

Zn 2p1/2

O 1s

Zn 2p3/2 N 1s

Intensity / a.u.

RGO-ZnPc Zn Auger

GO-ZnPc

RGO

GO

1000

800

600

400

200

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.)

C-O C-C

C=O C(O)O

GO

Relative intensity / a.u.

6

C-N

GO-PcZn

RGO-ZnPc

RGO

294

291

288

285

282

279

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.) 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 molecules by the covalent ester bonds (C(O)O) and efficient reduction of GO to RGO, as demonstrated in the XRD, Raman and FT-IR results, the bonding state of carbon may be changed significantly 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 corresponding 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 exhibits 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, compared 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.

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CARBON

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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 GO–ZnPc RGO–ZnPc RGO

33.6 35.9 45.6 62.6

23.6 30.4 30.7 19.2

23.4 14.6 5.3 10.3

19.4 10.8 9.1 7.9

8.3 9.3

Abs at 724 nm

2.0

Absorbance / a.u.

Absorbance / a.u.

0.8 (a) 703

0.6 (b)

724 637

0.4 (c) (d) 0.2

0.5

0.0

10

20

30

40

50

Concentration / mg L-1

60

1.0

0.5

0.0

0.0 400

600

800

400

1000

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.)

UV–vis absorption spectra

The 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 characteristic absorption of metal phthalocyanines (MPcs) with an intense S0–S1 transition band (Q-band) centered at 703 nm followed 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* transitions 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 moiety 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 attractive 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

600

800

1000

Wavelength / nm

Wavelength / nm

3.1.6.

1.5

1.0

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.) [7,49]. Fig. 9 shows the absorption spectra of RGO–ZnPc hybrid in DMSO with different concentrations. The absorption intensities 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 mg1 cm1, 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 spectroscopy

The 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 transition [22,29]. The emission peaks of GO–ZnPc and RGO–ZnPc observed at around 710 nm can also be attributed to the luminescence 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 system 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:

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Norm. transmittance

ZnPc GO-ZnPc RGO-ZnPc

8.0k

4.0k

0.0 600

700

800

900

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 processes, photo-induced electron transfer (PET) and energy transfer (ET), from Pcs to graphene because graphene is excellent 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 graphene after the reduction, the PET/ET process between Pcs and graphene would be further enhanced. It is thus reasonable 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 exhibit 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, respectively. 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 nanoislands which are sp2-hybridized carbon clusters, makes the GO possess some characteristics of graphene, including ultrafast 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 blocking 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

Norm. transmittance

12.0k

x x x ( 2 0 1 4 ) x x x –x x x

Norm. transmittance

Emission intensity / a.u.

CARBON

Norm. transmittance

8

1.0

GO Experiment Theoretical fit

0.5

1.0

ZnPc Experiment Theoretical fit

0.5

1.0

GO-ZnPc Experiment Theoretical fit

0.5

1.0

RGO-ZnPc Experiment Theoretical fit

0.5

-80

-40

0

40

80

Z-position / mm 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.)

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

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CARBON

9

xxx (2014) xxx–xxx

RGO-ZnPc

ß / cm GW -1

1200 900 600

GO-ZnPc 300

Normalized transmittance

1.2

1500

0.9

0.6 T = 50% RGO-ZnPc GO-ZnPc ZnPc GO

0.3

0.0

GO

0.1

100

1

Pulse energy density / J cm-2

50

ZnPc

0 0.4

0.6

0.8

1.0

Input intensity / J cm-2 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.) 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 numerous 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 functionalization 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 nonlinear absorption coefficient b of 1500 cm/GW, which should be devoted to efficient reduction of GO moiety to RGO as discussed above. The theoretically fitted nonlinear optical parameters (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 nonlinear 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 nanosecond 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

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.)

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 indicates that the nonlinear scattering of RGO–ZnPc should be depressed at a certain extent due to the covalent functionalization of RGO with soluble ZnPc and the significantly improved solubility of RGO–ZnPc as demonstrated in the UV–vis analysis. Moreover, the nonlinear absorption, especially 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 scattering 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 larger value of b observed for RGO–ZnPc suggests that it should have competitively better optical limiting performance. The optical limiting (OL) performance of different materials 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 transmittance 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

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10

CARBON

x x x ( 2 0 1 4 ) x x x –x x x

functionalization of GO with ZnPc and the subsequent in situ thermal reduction of GO to RGO, not only improves the solubility of hybrid material but also enhances its NLO and OL performance.

4.

Conclusion

We have reported the synthesis, structure and nonlinear optical 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 functionalization 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 fluorescence 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 combination of different NLO mechanisms in RGO–ZnPc. Such combined 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 originating 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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