Bright Green Photoluminescence in Aminoazobenzene

Article pubs.acs.org/JPCC

Bright Green Photoluminescence in AminoazobenzeneFunctionalized Graphene Oxide Abhisek Gupta, Bikash Kumar Shaw, and Shyamal K. Saha* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, West Bengal, India S Supporting Information *

ABSTRACT: Graphene oxide (GO) enriched in oxygen functionalities is limited in optoelectronic application because of its poor optical behavior. One of the major strategies for developing the optical properties of GO is functionalization. Here, GO sheets are functionalized by aminoazobenzene to achieve an intercalated type structure with interlayer separation of 9.3 Å. Bright green emission is observed in this aminoazobenzene-functionalized GO (AAB-GO). Remarkable enhancement in photoluminescence via surface passivation and excited-state intramolecular proton transfer is noticed in the AAB-GO composite. Density functional theory calculations are also carried out to investigate the stability of the modified structure along with its interlayer separation, the results of which agree well with the experimental results. The estimated energy gap (∼2.73 eV) between the highest occupied molecular orbital and lowest unoccupied molecular orbital is also in agreement with the experimental results (∼2.85 eV) of UV−vis absorption data.

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In our previous work,14 we have carried out edge selective functionalization of GO by azo-pyridine ligand which involves the active carbon centers of the phenolic moieties present at the edges of GO. But in this reaction, the epoxy groups on the basal plane of GO responsible for nonradiative recombination are not involved. To overcome this drawback, in the present work, we have functionalized GO by aminoazobezene (AAB) ligand in such a manner that the diazonium cation binds to the active carbon centers of the phenolic moieties located at the edges, and amino groups are attached to the active carbon centers of the epoxy moieties on the basal plane of the GO15 nanosheets resulting in the formation of a layered type structure. It is seen that the as-synthesized layered type AAB-GO exhibits strong and stable green fluorescence emission via surface passivation and an excited-state intramolecular proton transfer (ESIPT) process.14,16 Density functional theory (DFT) is used to investigate the stability of the modified structure along with its interlayer separation and the estimated highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gaps are compared to the experimental results. Therefore, this type of modification of graphene surface with different functional groups will enable researchers to engineer the chemical and physical properties of graphene for fabrication of optoelectronic devices and fluorescent sensors.

INTRODUCTION Graphene, as a novel carbon-based material discovered in 20041 obeying Dirac physics, has attracted great attention because of its outstanding structural, mechanical, and electronic properties and potential applications in nanoelectronics, energy storage, and photovoltaics.2,3 However, because of its intrinsic zero band gap and hydrophobic properties, graphene possesses limited applications in optoelectronic and sensing devices. In recent years, the chemical modification or functionalization of graphene has been investigated intensively to modulate its electrical, optical, catalytic, and mechanical properties and expand its applications in the related areas.4−6 Graphene oxide (GO), a chemically modified oxygen functionalities-enriched derivative of graphene, has received great interest7 because of its superior dispersion ability in water and a finite electronic band gap different than that of graphene.8 According to the literature, graphene is not a fluorescent material; however, GO possesses a very weak fluorescence property.9 GO exhibits a very weak photoluminescence (PL) property in the visible and near-infrared region10 because of the presence of oxygen functionalities, e.g., hydroxyl and epoxy groups, which usually induce nonradiative recombination as a result of transfer of their electrons to the holes present in sp2 clusters, producing nonradiative localized electron−hole (e−h) pairs.10,11 Recently, superior luminescent graphene or functionalized graphene with blue or red emissions have been obtained using oxidation or reduction treatments.11,12 Mei et al.13 have recently reported a bright blue fluorescent GO that arises from passivation of surface reactive sites by amide formation and ring-opening amination of epoxides. Hence, the PL property in GO originates from the isolated polyaromatic structures and/or passivated surface defects prepared at different conditions. © 2014 American Chemical Society

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RESULTS AND DISCUSSION Figure 1a shows the XRD pattern for the as-synthesized AABGO composite. For AAB-GO composite, the main peak Received: December 12, 2013 Revised: March 10, 2014 Published: March 10, 2014 6972

dx.doi.org/10.1021/jp412156x | J. Phys. Chem. C 2014, 118, 6972−6979

The Journal of Physical Chemistry C

Article

N = t /d

(2)

Taking t as 54 Å, the number of layers for AAB-GO composite has been found to be ∼6. Figure 1b shows TEM images in which individual layers are indicated by dotted curves. In the figure, 6−7 layers, as estimated from XRD analysis, are clearly identified. It is also noticed that the edge of one GO sheet is connected to the basal plane of another GO sheet as proposed in the mechanism. The Raman spectra of graphene and GO exhibit two main bands. The G band corresponds to the first-order scattering of the E2g phonon of in-plane vibration mode of sp2 carbon atom domains and the D band assigned to the vibrations of sp3 hybridized carbon atoms of disordered graphene, i.e., the structural defects and partially disordered structures of the sp2 domains.17 Figure 1c shows the Raman spectra of GO and AAB-GO composite. In the Raman spectra of GO, the D band appears at 1356 cm−1 and the G band appears at 1602 cm−1; however, for the AAB-GO composite, the G band is red-shifted to 1588 cm−1 and the D band is shifted upward to 1363 cm−1. Such shifting of binding energies of D and G bands has also been reported in some functionalized graphitic materials.18 This type of shifting is ascribed to the modification of graphene surface structure caused by the additional chemical bonds between carbon atoms of phenolic moieties of graphene and the functional groups. The intensity ratio of D band and G band (ID/IG) reveals the degree of in-plane defects and edge defects in the carbon materials.19 In the present study, the ID/IG ratio of the AAB-GO composite is calculated to be ∼0.91, which is slightly smaller than that of the initial GO (∼0.93). This observation implies the increase of the domains of sp2 carbon in the AAB-GO composite. The functionalization of GO with aminoazobenzene is confirmed by FTIR measurements. Figure 2a shows the FTIR spectra for GO and AAB-GO composite samples. For pure dried GO, the presence of different types of oxygen functionalities is observed. The usual peaks at 3432, 1702, 1628, 1400, and 1067 cm−1 correspond to hydrogen-bonded

Figure 1. (a) XRD peaks of AAB-GO composite. The main peak appears at a 2θ value of 9.5° along with the usual peak at 12.3°. (b) TEM image at different magnifications of the AAB-GO composite material showing a layered type structure (inset: indication of different GO layers). (c) RAMAN spectra of GO and AAB-GO composite.

appears at a 2θ value of 9.5° with the usual peak at 12.3° (interlayer separation, ∼0.72 nm) corresponding to GO (Figure S1 of the Supporting Information). The major peak at a 2θ value of 9.5° indicates the intercalated structure with an interlayer separation of 0.93 nm. For AAB-GO composite, another peak appears at a 2θ value of 23.2°, which corresponds to the multilayered graphene with interlayer separation 0.34 nm.15 It is also to be noted that the composite structure synthesized in the present case is not 100% pure ; rather, a fraction of GO is still present in the sample, which needs to be purified for device applications. Here, the mean crystallite thickness of the AAB-GO composite has been estimated using the following equation:

t = Kλ /(fwhm cos θ )

(1)

where t is the mean crystallite thickness, K a dimensionless shape factor of crystallite with a value of about 0.9, λ the wavelength of the X-ray used (λ = 1.5418 Å), θ the angular position of the peak, and fwhm the full width at half-maximum extracted from the graph (expressed in radians). The value of the average crystallite thickness for GO is ∼73 Å (for detailed calculation, see the Supporting Information) and for AAB-GO composite is ∼54 Å. The number of layers N, considering the lattice spacing as d, is

Figure 2. (a) FTIR spectra of GO and AAB-GO composite. (b) TGA profile of GO and AAB-GO composite. 6973



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O−H stretching, carbonyl (CO) stretching, CC stretching, O−H deformation, and C−O stretching of the epoxides (C−O−C), respectively. In the AAB-GO composite, the broad peak in the region centered at 3430 cm−1 is due to the presence of both the −OH and −NH groups . The benzenoid CC vibrations are observed at 1599 and 1505 cm−1. The peak at 1460 cm−1 indicates the presence of the NN group, i.e., the azo group stretching vibration.20 The peaks between 1277 and 1219 cm−1 are due to C−N stretching vibrations. In addition, the band centered at 1055 cm−1 represents the C−O group. The additional peak at 742 cm−1 can be ascribed to the out-ofplane deformation of aromatic C−H bond. TGA analysis has been carried out for both the GO and AAB-GO composites, which are shown in Figure 2b. The TGA curve of GO shows a weight loss of ∼18% below 100 °C which is due to the evaporation of adsorbed water molecules. The sharper mass loss of 25% between 170° and 230 °C is due to the decomposition of labile oxygen groups, e.g., carboxylic, anhydride, or lactone groups, and the gradual mass decrease above 230 °C (20%) is attributed to the removal of more stable oxygen-containing functional groups, e.g., phenol, carbonyl, and quinine.21 The TGA curve of the AAB-GO composite shows a small weight loss (∼ 9%) below 100 °C, whereas a sharp weight loss is observed between 145° and 245 °C (∼ 37%) as a result of decomposition of the labile oxygen and nitrogen groups. Above 250 °C, the gradual decrease in mass follows the same behavior as that of GO. To investigate the binding energies of different functional groups in AAB-GO composite, we have performed an X-ray photoelectron spectroscopy (XPS) study. The XPS spectrum of AAB-GO composite (Figure 3a) shows the characteristic C 1s, N 1s, and O 1s core-level photoemission peaks at ∼285, ∼400, and ∼432 eV, respectively. From the XPS analysis, the C:O:N ratio has been found to be approximately ∼0.60:0.32:0.08, which is close to the C:N ratio (∼0.56:0.10) estimated from elemental analysis. For GO (XPS spectra shown in Figure S2 of the Supporting Information), the low-range XPS spectra (Figure S2a) show the presence of only C and O 1s corelevel photoemission peaks. The high-resolution carbon 1s XPS spectrum of GO (Figure S2b of the Supporting Information) shows three peaks at 284.8, 286.8, and 288.1 eV corresponding to C−C, C−O (hydroxyl and epoxide), and CO (carboxyl) groups of GO, whereas the high-resolution carbon 1s XPS spectrum of the composite shown in Figure 3b is conveniently fitted using four components. The lowest binding energy peak at 284.8 eV is mainly assigned to the C−C bonds of the graphitic network. The peak at 286.1 eV originates from the C−N bonds. Another peak at 286.8 eV accounts for the C−O bonds, and the binding energy component at 288.5 eV is typically attributed to the presence of carboxylic groups. Figure 3c represents the highresolution nitrogen 1s XPS spectrum of the composite which appears at ∼400 eV. This peak arises because of the combined effect of the C−N bonds and the azo groups (NN bonds) present in the AAB-GO composite. On the basis of the above characterizations, we propose the formation mechanism of the layered type AAB-GO composite structure by functionalization of GO with o- aminodiazobenzene molecules as shown in Scheme 1. GO nanosheets possess quite a large number of phenolic and epoxy moieties at the edges and basal plane which provide some active sites for chemical reactions. Here, the in situ grown aryl aminodiazonium cation reacts easily with the active sites on GO

Figure 3. (a) Low-range XPS spectra for AAB-GO composite. (b) Deconvoluted XPS peaks for C 1s in AAB-GO composite. (c) Highresolution XPS peak of N 1s of AAB-GO composite.

through the formation of carbon−nitrogen covalent linkage.14,15 To investigate the stability of AAB-GO composite we have carried out DFT calculations. In the case of aminoazobenzene linking with the graphene sheet, its all-chemical properties like bond length, bond angle, and stabilization energy are accordingly changed. We estimate the total energy and molecular orbital (MO) calculation of the optimized composite structure with B3LYP/3-21G, 6-31G levels of theory using the standard program in the Gaussian 03 software package. The optimized structure shows the energy value of −1 848 520 kcal/ mol for all the basis sets shown in Figure 4. For this optimized structure, the two-dimensional graphene sheets are slightly displaced outward from the plane because of the bonding with amino and azo groups of the ligand. The bond lengths and bond angles of the optimized doped structure are summarized in Table 1. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are −5.75 and −3.02 eV, respectively, as shown in Figure 5, and the band gap (∼2.73 eV) of these two is in close agreement with the value obtained from experimental study. The AAB-GO composite is further subjected to optical measurements to understand the mode of functionalization in the composite. Figure 6a shows the UV−vis absorption spectra of pristine GO and the AAB-GO composite. GO dispersions show a maximum absorption at 230 nm and a shoulder between ∼290−300 nm, which are assigned to the π−π* transition of aromatic CC bonds and n−π* transition of the 6974



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Scheme 1. Schematic of the Formation of AAB-GO Composite

CO bonds, respectively. For o-phenylenediamine, the UV− vis absorption spectrum exhibits two bands at ∼240 and ∼293 nm corresponding to π−π* and n−π * transitions, respectively.

The UV−vis absorption spectra of the AAB-GO composite comprises two peaks at ∼260 and 433 nm which correspond to the π−π* and n−π * transitions of the composite, respectively. 6975



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indicating the increase of sp2 domains, and a new band appears at 433 nm with respect to GO.14 Meanwhile, the excitation spectrum of AAB-GO composite (Figure 6b) exhibits a strong band centered at ∼430 nm. This result again indicates the formation of new luminescent centers in the AAB-GO composite which are basically the azo groups bonded to the α-position of the phenolic −OH groups present at the edges of GO nanosheets. The excellent PL in the composite is generated because of the ESIPT between the −OH group of the phenol moiety and the azo group. It has been reported by Alarcón et al. that the fluorescence emission for substituted hydroxyl benzaldehydes comes only from the keto form.23 The enol− azo (EA) form of the AAB-GO undergoes rapid ESIPT resulting in the tautomeric keto−hydrazo (KH) form. This KH tautomeric form exhibits bright PL. To explain the observed spectral behavior, we have demonstrated the following simplified scheme which involves both the keto−enol tautomers as shown in Scheme 2. The excited EA (EA*) form undergoes a fast transformation to the excited KH form (KH*), and the emission results from the KH form. The PL property of the AAB-GO composite can also be tuned by controlling the pH value of the solution. Dilute HCl and NaOH as well as buffer solutions are used to adjust the pH of the composite solutions. As observed in Figure 6c, the PL intensity of the composite gradually increases with increase in the [H+] ion concentration of the solution, and any change in peak positions have not been observed with the decrease in pH. This observation suggests that the tuning in pH mainly influences the GO surface and not the size of the nanosheets.24 This type of pH response of the PL is related to the passivation of radiative defects embedded in the AAB-GO composite (shown in Scheme S1 of the Supporting Information). The carboxylic, hydroxyl, amino, and azo groups of the AAB-GO composite get protonated as well as deprotonated because of the changes in pH which causes electrostatic doping or charging in the AABGO composite, resulting in shifting in the Fermi level similar to that of carboxylate single-walled carbon nanotubes (SWCNT).25 With decrease in pH, the −OH, COOH, −NH, and −NN− groups of the composite get protonated, which results in the destabilization of the n−π* state and stabilization of the π−π* state. Consequently, the energy gap between the states gets reduced, which in turn decreases the vibronic interaction. This increases the π−π * state symmetry and thereby decreases the Franck−Condon factor, resulting in a decrease in radiationless decay which explains the bright PL intensity of the AAB-GO composite with decrease in pH.14 We have investigated the PL of the AAB-GO composite at different excitation wavelengths as shown in Figure 6d. With an increase in the excitation wavelength from 400 to 460 nm, a very small red shift in PL peak is observed but a dramatic change in PL intensity is noticed. With change in excitation wavelengths from 400 to 430 nm, the PL intensity increases, whereas from 430 to 460 nm, the PL intensity decreases. This observation is totally different from the other GO-function-

Figure 4. Optimized structure of the AAB-GO composite system.

This observation of shifting the π−π* transition and appearance of a new peak compared to that of GO indicates the successful functionalization of GO by aminoazobenzene. Figure 6b shows the PL and photoluminescence excitation (PLE) spectra of the AAB-GO composite. In the inset of Figure 6b we have shown that the PL spectra of the GO solution in the visible range display a broad weak emission band exhibiting maximum intensity at ∼565 nm for an excitation wavelength at 416 nm.14 The reasons behind the weak PL of GO were discussed in the introduction briefly. In the present work, for the AAB-GO composite, the PL maximum is blue-shifted to 536 nm and the fluorescence intensity of the composite increases remarkably by 1200% with respect to GO. The PL quantum yield (see the Supporting Information for detailed calculation) of AAB-GO composite was measured to be ∼21.2% using Rhodamine 6G dissolved in ethanol as reference. It is to be noted that the reagent used in this work, i.e., ophenylenediamine possess excitation and emission peak wavelengths at 305 and 345 nm,22 which are different from the present PL emission maxima. By nucleophilic substitution reaction of amino groups, the surface passivation removes the reactive sites such as basal epoxy groups which induce nonradiative recombination of localized e−h pairs, resulting in quenching of PL and hence improves the PL emission efficiency of the sp2 domains of GO nanosheets. This is further verified by the changes in the absorption and excitation spectra of GO before and after functionalization. After functionalization, the absorption band at 228 nm is red-shifted to 260 nm,

Table 1. Bond Lengths and Bond Angles of the Optimized AAB-GO Composite Structure optimized structure bond length (Å) bond angle (deg)

N −N , N −N 1.2086 C50−N104−N103 109.61 84

85

103

104

N −C , N −C 1.4871 C24−N85−N84 113.98 82

56

97

C30−C46 7.2216

10

6976

C11−C65 9.2731

C44−C7 9.0171

C5−C58 9.77

C15−C42 9.42



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Figure 5. Spatial distributions of HOMO and LUMO for the optimized AAB-GO composite structure.

Figure 6. (a) UV−VIS absorption spectra of GO and AAB-GO composite. (b) PL and PLE spectra of AAB-GO composite (insets: comparison of PL spectra of GO and AAB-GO composite and digital image of AAB-GO composite (40 mg/L) under UV light of 365 nm). (c) PL spectra of AABGO composite with variation of pH of the solutions. (d) PL spectra of AAB- GO composite with different excitation wavelengths.

alized materials and graphene quantum dots in which the PL peaks are excitation-dependent, i.e., a major shifting in PL peaks has been observed with change in the excitation wavelengths. Figure 7a shows the concentration-dependent PL spectra of AAB-GO composite. It has been observed that at lower concentration (5−50 mg/L) the PL intensity gradually increases with concentration, but at higher concentrations (>50 mg/L) PL intensity gradually decreases. To investigate the photostability of AAB-GO composite, the time-dependent fluorescence intensity (Figure 7b) was measured at an interval of 120 s over the total period of 1200 s under constant

irradiation. No significant change in PL intensity was observed during the experiment. To obtain insight into the excitonic dynamics, we have measured the fluorescence lifetimes of GO before and after functionalization. The average lifetimes together with the component decay times obtained from the fitted decay curves (see Figure S3 in the Supporting Information), are summarized in Table 2. Average fluorescence lifetimes τ for exponential fitting are calculated from the decay times τi and the relative amplitudes ai using the following relation: 6977



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Scheme 2. Schematic of the ESIPT Mechanism

Figure 7. (a) PL spectra of AAB-GO composite at different concentrations. (b) Variation of PL intensities with increase in time.

Table 2. Lifetime Values of GO and AABGO (in Different pH) system

τ1 (ns)

a1

τ2 (ns)

a2

τ3 (ns)

a3

⟨τ⟩ (ns)

GO AABGO (pH 1) AABGO (pH 7) AABGO (pH 10)

1.62 0.811 0.134 1.367

11.36 40.35 0.49 50.40

0.343 2.837 2.60 5.566

42.33 44.68 73.87 41.79

0.047 0.093 4.59 0.146

46.31 14.97 25.64 7.81

0.994 2.40 3.357 4.586

n

⟨τ ⟩ =

n

excitons with the holes, and the excitons decay through a different path.

∑ aiτi 2/∑ aiτi i

■

i

CONCLUSIONS This work describes a convenient approach for the preparation of intercalated layered type structure by functionalizing GO sheets with aminoazobenzene. In this composite, the GO sheets are attached through amino and azo moieties to achieve a layered type structure with an interlayer separation of 0.93 nm. The formation of AAB-GO composite is further supported by a detailed series of structural and optical characterizations, such as XRD; Raman spectroscopy; FTIR, TGA, and XPS analyses; and UV−vis absorption spectroscopy. Structural stability and interlayer separation is confirmed using DFT calculation, and the estimated HOMO−LUMO gap is in conformation with the

From Table 2, it is seen that the average lifetime of GO increases in the AAB-GO composite, indicating the formation of a more stable excited state. It is also to be mentioned that average lifetime increases with increase in pH values, suggesting that the excitons become more stabilized at higher pH though the considerable decrease in PL intensity. At these higher pH values comparatively less fluorescent complex is produced at the ground state because of the formation of deprotonated groups (O−, COO−) in the composite surface which facilitate the nonradiative recombination of the ions to the nearby holes. This results in the decrease in radiative recombination of the 6978



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experimental results. This intercalated layered type functionalized GO composite shows superior optical properties with bright green emission via surface passivation and ESIPT. The present study provides an effective method for making GO luminescent as well as expanding the applications of GO in optoelectronic devices, visible-light-emitting diodes, and solar cells with this functionalized GO as an active material.

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ASSOCIATED CONTENT

S Supporting Information *

Further information regarding experimental sections, quantum yield (QY) measurement, XRD of GO, calculation of mean crystallite thickness of graphene oxide, XPS of GO, decay curves of GO and the composite, and the schematic diagram of the pH effect on the composite. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-33-2473 4971 (Ext 227). Fax: +91-33-2473-2805. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.G. and B.K.S. acknowledge CSIR, New Delhi, for awarding of a fellowship and S.K.S. acknowledges DST, Project SR/S2/ CMP-0097/2012 and DST unit of Nano Science for providing XPS facilities.

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REFERENCES

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Bright Green Photoluminescence in Aminoazobenzene

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