chemical communications volume 46 issue 39 2010 [doi 10.1039_c0cc02374d] mei, qingsong; zhang, kui;...

8/13/2019 Chemical Communications Volume 46 Issue 39 2010 [Doi 10.1039_c0cc02374d] Mei, Qingsong; Zhang, Kui; Guan,

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 73197321 7319

Highly efficient photoluminescent graphene oxide with tunable surfacepropertiesw

Qingsong Mei,ab Kui Zhang,ab Guijian Guan,a Bianhua Liu,a Suhua Wang*a and

Zhongping Zhang*ab

Received 5th July 2010, Accepted 18th August 2010DOI: 10.1039/c0cc02374d

A bright blue fluorescent graphene oxide that originates from

passivation of surface reactive sites by amide formation and

ring-opening amination of epoxide has been prepared. The

surface polarity and charges of the fluorescent graphene oxide

can synchronously be tuned by varying the used alkylamines.

Highly efficient and stable luminescent materials are of

fundamental and technological importance in optoelectronic

devices, biological labeling and sensing.14 Intensive efforts

have been made in the exploration of new efficient emitterssuch as semiconductor quantum dots,1 silicon nanoparticles,2

gold nanodots,3 and carbon-based nanomaterials including

carbon nanotubes, nanodiamond, and carbon dots (C-dots).4

Among those materials, the fluorescent carbon-based nano-

materials have been attracting much more attention as they

show more stable emissions, lower cytotoxicity and give rise to

less environmental concern. The origin of photoluminescence

(PL) in these carbon-based nanomaterials is tentatively

proposed to be from isolated polyaromatic structures or

passivated surface defects.4 However, the preparation of these

carbon nanomaterials usually shows a very low yield and is

carried out under extreme conditions,e.g.laser ablation, high

temperature and high pressure.

4

Graphene oxide (GO) nano-sheet, a two-dimensional oxidized derivative of graphene, also

contains isolated polyaromatic clusters and can be easily

exfoliated from graphite with a high yield under simple

oxidizing conditions. While it has been widely studied in

regard to electrical conductivity, drug delivery, self-assembly,

and surface functionalization,5 the PL properties of GO have

rarely been explored due to its low emission efficiency.6

GO possesses a finite electronic bandgap generated by the

disruption of p networks due to the formation of oxygen-

containing groups. It is well documented that GO nanosheet

bears phenol hydroxyl and epoxide groups at the basal plane

and carboxylic groups at the lateral edge.6 Recently, an

extremely weak broad PL from GO was reported, which wasbelieved to originate from the carbon sp2 domains/clusters,6,7

but it was invisible under UV irradiation. Even though the PL

intensity was slightly improved after moderate reduction

using hydrazine, the quantum yield (QY) was too low to be

measured accurately.6 Moreover, the weak PL was quenched

upon further reduction with hydrazine due to the formation of

non-fluorescent graphene that is known as a zero-bandgap

semiconductor.

As we know, the epoxy and carboxylic groups usually

induce non-radiative recombination of localized electronhole

(eh) pairs, which leads to the nonemissive property of GO.6

However, alkylamines are reactive to both the epoxy and

carboxylic groups through nucleophilic reaction. The removal

of non-radiative recombinative sites is expected to transform

GO nanosheet into a highly efficient emitter. Here we report a

bright blue fluorescent GO which was achieved by the surface

amide formation and ring-opening amination of epoxide of an

original GO by using various alkylamines. The functionalized

GO exhibits a maximum QY up toB13% and tunable surface

properties such as hydrophilic, hydrophobic and negative

charges. The highly fluorescent GO nanosheets with diverse

surface properties could facilitate and expand their applications

in optoelectronics, biolabeling, and bioimaging.

GO nanosheet was first prepared by the oxidation of

graphite using a modified Hummers method.8 The fluorescent

GO was obtained through a facile and effective chemical

approach consisting of the acylation reaction to form alkylamides

and the ring-opening aminations of epoxides to yield

1,2-amino alcohols at the lateral edge and basal surface of

GO nanosheets, respectively (see ESI). Typically, the original

GO was activated first using dichlorosulfoxide to transform

carboxylic groups (at the lateral edge) into acyl chloride under

anhydrous conditions. The further treatment with n-butylamine

(C3Me) led to the occurrence of acylation and ring-opening

reactions at the GO nanosheets. The product was separated

from the mixture and was redispersed in appropriate solvents

for further studies. The n-butylamine modified GO is called

GO-C3Me here for easy communication.

The GO-C3Me aqueous suspension exhibits strong blue

fluorescence under UV irradiation which can be easily seen

with the naked eye and recorded with a digital camera, as

shown in Fig. 1a. The PL spectrum of GO-C3Me shows an

emission maximum at 430 nm under the excitation wavelength

of 350 nm (Fig. 1b). Unlike GO-C3Me, the starting GO shows

no visible fluorescence when irradiated under the same UV

lamp (Fig. 1a, bottom) and only a very weak PL maximum at

525 nm (Fig. 1b-B). The PL quantum yield of GO-C3Me was

measured to be B13% (see ESI), which is about six hundred

fold that of the original GO nanosheets (0.02%). The fluorescence

enhancement by n-butylamine treatment could be attributed

to the surface passivation instead of n-butylamine molecules

themselves because they contain no any visible or near-UV

a Institute of Intelligent Machines, Chinese Academy of Sciences,Hefei, Anhui, 230031, China. E-mail: [email protected],[email protected]

b Department of Chemistry, University of Science & Technology ofChina, Hefei, Anhui, 230026, China

w Electronic supplementary information (ESI) available: Preparationand characterization of alkylamine-modified graphene oxide. SeeDOI: 10.1039/c0cc02374d

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7320 Chem. Commun., 2010, 46, 73197321 This journal is c The Royal Society of Chemistry 2010

fluorophores. The surface passivation removes reactive sites

such as epoxy and carboxylic groups by nucleophilic reactions

and hence improves the emission efficiency of the sp2 domains

on GO nanosheets. This can be evidenced by the changes of

absorption and excitation spectra of GO before and after

alkylamine treatment (Fig. 1c). Clearly, the absorption peak

around 230 nm of GO is assigned to the pp* transitions of

CQC. After alkylamine treatment, absorption bands at

276 nm and 350 nm become more pronounced and resolved.

Meanwhile, the corresponding excitation spectrum exhibits a

strong exciting band centered at 350 nm. The results suggest

the formation and increase of new luminescent centers at the

surface of GO-C3Me nanosheets.9

Interestingly, the emission maxima of the GO-C3Me

aqueous suspension are dependent on the excitation wavelengths

(Fig. 2). The emission maxima shift from 430 nm to 515 nm

accompanied by the gradual decrease in emission intensity as

the excitation wavelength varies from 350 nm to 470 nm.

Furthermore, the integrated emission intensities are correlated

with the absorbance in a parallel way (Fig. 2b), indicating

similiar PL quantum yield among those emissive sites. The

results further suggest the existence of heterogeneous electronic

structures attributed to the polydistribution of sp2 cluster sizes

within GO-C3Me.

In general, GO shows very weak PL because of the isolated

sp2 domains generated by oxidation.6,7 These sp2 domains

have opened heterogeneous electronic band gaps which are

intrinsically correlated to their sizes, shapes, and fractions. In

principle, large sp2 domains have narrower energy gaps than

those smaller ones and emit longer wavelengths when excited

at appropriate wavelengths. However, the epoxide groups on

the basal plane and carboxylic groups at the edge of GO often

induce non-radiative recombination of localized eh pairs,

leading to a very low quantum yield. After reacting withalkylamines, these groups are removed and this results in a

bright photoluminescence.

Similar photoluminescent characteristics were also observed

when GO was modified with other alkylamines such as

1,6-hexylenediamine (C6NH2), octylamine (C7Me), dodecylamine

(C11Me) and diamine-terminated poly(ethylene glycol)

(PEG1500N). These longer alkylamine-passivated GO also

exhibit bright blue fluorescence under UV light. The emission

maxima show the same dependence on the excitation

wavelengths (see ESI). For example, the emission maximum

of GO-PEG1500N suspension shifts from 430 nm to 570 nm

when the excitation wavelength varies accordingly. The PL

quantum yields of these different surface passivated GO aremeasured using the same procedure and presented in Table 1.

It is clear that the PL quantum yields obviously decrease with

the increase of carbon chain length in the alkylamine

molecules. GO-PEG1500N still exhibits a 4% quantum yield

that is comparable to the PEG1500N capped C-dots (B5%).4

Meanwhile, the alkylamine functionalized GO also become

less hydrophilic as the the length of carbon chain increases,

consistent with their molecular polarity. Furthermore, the

surface charges are dependent not only on pH values but also

on the properties of amines (see ESI). Therefore, the surface

properties of the alkylamine functionalized GO can be easily

tuned by altering the passivation molecular properties.

Fig. 1 (a) Photographs of GO-C3Me (above) and GO (below)

aqueous solutions under 350 nm irradiation. (b) Photoluminescence

spectra of (A) GO-C3Me and (B) GO in water (18 mg ml1). (c) The

UV-vis absorption spectra (A) of GO (dashed line, 0.02 mg ml1) and

GO-C3Me (solid line, 0.5 mg ml1) aqueous solution, and the excitation

spectra (B) of GO (dashed line, 18 mg ml1) and GO-C3Me (solid line,

18 mg ml1) aqueous solutions.

Fig. 2 (a) Photoluminescence spectra of GO-C3Me aqueous solution

at the different excitation wavelengths. (b) The correlation between the

integrated PL intensities and the absorbance at the excitation

wavelengths. The excitation beam intensities are calibrated for the

integration of PL intensity.

Table 1 The nitrogen/carbon ratios in the alkylamines and thequantum yields and solubility of various amine functionalizedgraphene oxides

Sample NH2/C F (%)a Solubility

GO 0 0.02 H2O, DMF, ethanolGO-C3Me 1/4 12.8 H2O, DMF, ethanolGO-C6NH2 1/3 11.9 H2O, DMF, ethanolGO-C7Me 1/8 5.0 DMF, ethanol, cyclohexaneGO-C11Me 1/12 5.6 DMF, ethanol, cyclohexaneGO-PEG1500N 1/34 4.0 H2O, DMF, ethanol

a The quantum yields were measured in H2O, H2O, H2O, ethanol,

ethanol and H2O under excitation of 350 nm, respectively.

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 73197321 7321

The surface covalent attachment of n-butylamine is

supported by FT-IR and X-ray spectroscopy (XPS) data, as

shown in Fig. 3. For GO-C3Me, a new vibration band around

1648 cm1 due to the CQO stretching of primary amide

emerges (Fig. 3a),10 but the carboxylic group bands at 1733

and 1225 cm1 of original GO disappear after the chemical

treatment. The results immediately suggest the covalent

attachment of n-butylamine to the GO surface through theformation of an amide bond. It can also be seen that the

epoxide band at 1055 cm1 of the GO completely disappears

in GO-C3Me, accompanied by the appearance of a new band

at 1126 cm1 that is assigned to the CNC asymmetric

stretching of the attached alkylamines. Clearly, n-butylamine

not only successfully removes the epoxy groups but also

covalently attaches on the surface of GO by ring-opening

aminations of epoxides to yield 1,2-amino alcohols. The

double bands around 740 cm1 and 818 cm1 of the NH2vibration in pure n-butylamine disappear in the GO-C3Me

sample, which strongly excludes the physical adsorption of

n-butylamine. The band at 1626 cm1 owing to the CQC

vibration of aromatic rings is observed in both GO-C3Me andGO samples, implying the retention of most of the sp2

characteristic structures in GO-C3Me even after the alkylamine

treatment. XPS results also suggest the covalent attachment of

butylamine through amide formation (Fig. 3b). The result of

deconvolution treatment for the N 1s spectrum revealed two

peaks at 400.6 and 399.5 eV, which are attributed to the N 1s

of the NC bond of the amide linkage and the NC bond of

1,2-amino alcohols of GO nanosheets,10 respectively. Based on

the FT-IR and XPS analysis, a structure of the GO-C3Me is

modeled as presented in Fig. 3c. The GO-C3Me nanosheet

consists of small sp2 conjugated aromatic domains isolated by

sp3 carbon.

In summary, we have prepared a bright blue photo-

luminescent GO by surface alkylamine functionalization

under mild conditions. The fluorescence quantum yields of

the alklyamine-functionalized GO are remarkably enhanced

up to six hundred times compared with the original GO.

Meanwhile, the surface properties of the modified GO can

be easily tuned from hydrophilic to hydrophobic by changing

the molecular structures of the used amines. These will greatly

expand the applications of GO in many fields.

This work was supported by Natural Science Foundation

of China (20925518, 20875090, 20807042, 30901008) and

ChinaSingapore Joint Project (2009DFA51810) and 863

project of China (2007AA10Z434) and Innovation Project of

Chinese Academy of Sciences (KSCX2-YW-G-058).

Notes and references

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Fig. 3 (a) FT-IR spectra of (A) GO-C3Me, (B) n-butylamine and (C)

GO. (b) Nitrogen 1s XPS spectrum obtained from GO-C3Me showing

the oxidation states of nitrogen atoms. (c) The structure of alkylamine

functionalized GO (R represents various hydrocarbon chains).

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chemical communications volume 46 issue 39 2010 [doi 10.1039_c0cc02374d] mei, qingsong; zhang, kui;...

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