Photoluminescence of Graphene Oxide in Visible Range Arising from

Aug 5, 2015 - Graphene oxide (GO) has attracted considerable attention due to its interesting structure and properties. The photoluminescence (PL) of ...
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Photoluminescence of Graphene Oxide in Visible Range Arising from Excimer Formation Donghe Du,† Haiou Song,‡ Yuting Nie,§ Xuhui Sun,§ Lei Chen,† and Jianyong Ouyang*,† †

Department of Materials Science & Engineering, National University of Singapore, 117576 Singapore State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, Jiangsu 210023, China § Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano & Soft Materials, Soochow University, Suzhou, Jiangsu 215123, China ‡

S Supporting Information *

ABSTRACT: Graphene oxide (GO) has attracted considerable attention due to its interesting structure and properties. The photoluminescence (PL) of GO is much stronger than that of graphene owing to the opening of an energy band gap. However, the origin of the PL bands in the ultraviolet and visible ranges remains controversial. In this paper, we report the dependence of the PL spectrum of GO on the pH value and concentration of GO aqueous solutions. It was discovered that PL in the visible range becomes prominent when the pH value is low and/or the GO concentration is high. As revealed by the time-resolved photoluminescence, the lifetime of the PL in the visible range is longer than that in the UV range. These results evidence the formation of excimers and prove that the PL band at the long wavelength is caused by the GO excimers.

1. INTRODUCTION Graphene has been one of the most popular research topics in the past decade due to its unique structure and properties, since it was reported for the first time from mechanical exfoliation of graphite in 2004.1−8 Graphene has potential applications in many areas, e.g., electronics, catalysis, and energy storage, owing to its high charge carrier mobility and high specific surface area. Nevertheless, graphene has limited applications in optics, as it has a metallic structure without a bandgap. After the oxidation of graphene, a bandgap is opened. The bandgap renders graphene oxide (GO) interesting optical properties.9−12 For example, the photoluminescence (PL) of GO is much stronger than that of graphene. GO exhibits PL in the visible and ultraviolet ranges. There are mainly two PL bands for GO, one in the blue region (blue band) and another in the range of 500−650 nm (long wavelength, or LW band). In comparison to organic compounds, the PL spectrum of GO is complicated due to the presence of different domains and various functional groups. The PL bands have not been well understood, and their origin has been controversial. Eda et al. identified the blue band as the π−π* transition of the isolated sp2 domains within the carbon−oxygen sp3 matrix.12 Chien et al. claimed that the π−π* transition gives rise to the broad LW band.13 They considered the broad band to be the result of the disorder-induced variation in the bandgap of the sp2 domains. Galande et al. observed the pH dependence of PL and attributed the PL bands to the polycyclic aromatic carboxylic acids.14 They proposed that the PL of GO resembled that of a © 2015 American Chemical Society

mixture of polycyclic aromatic compounds in alkaline solution, and that the PL arises from the quasi-molecular fluorophores because of the coupling of the carboxylic acid groups electronically with nearby atoms of the graphene sheet. They observed that the blue band was predominant at pH >8, and assigned it to the optical transition from (G−COO−)* to G− COO−. However, the LW band became significant at pH 12).21 Thus, the GO samples with pH 10 and 12 did not undergo such separation process. To minimize the re-absorption effect of GO solutions on the PL spectra, a cuvette of 10 mm × 1 mm was used. The optical path for the excitation light is 10 mm, while it is 1 mm or less for the emission light. The PL spectra of the five groups of GO samples are presented in Figure S5. Figure 4 shows the PL spectra of acidic (pH 2), neutral (pH 7), and alkaline (pH 12) GO solutions with different concentrations. In order to verify the re-absorption effect to the spectra shape, PL spectra were

The XPS spectrum indicates that GO is highly oxidized. Functionalized groups on GO is confirmed by FTIR (Figure 2b). The IR bands at 1220, 1054, and 1732 cm−1 correspond to the C−OH stretching, C−O stretching, and CO carbonyl stretching, respectively.8,26 In terms of the Rourke’s model for the GO structure, ODs are attached to graphene-like plane through π−π overlapping. In principle, the π−π overlapping can affect the optical transition of GO. At first, we studied on the PL spectra of the two components of GO. GO was separated into basewashed GO (bwGO), which were mainly graphene-like planes, and ODs according to Rourke’s method with slight modification (illustrated in Figure 3a).19 A co-solvent of water/ethanol (50/50 vol %) was used to replace water in our work. We observed that water/ethanol co-solvent system was more effective in separating GO into bwGO and ODs. The black flocculation of bwGO was observed after vigorous stirring for only a few seconds. This separation in such a short time at room temperature can avoid the damage of the functional groups on GO. Both the bwGO and ODs aqueous solution were then re-protonated to the same pH value (pH 5) as the pristine GO solution. ODs were characterized by SEM (Figure S1), EDX (Figure S2), FTIR (Figure S3a), and XPS (Figure S3b,c). Dried ODs were porous as observed by SEM. The EDX analysis and elemental mapping indicate that these ODs are highly oxidized. The strong Si peak in the EDX spectrum originates from silicon substrate used for the sample preparation. The strong FTIR absorption band at 1080 cm−1 suggests that the highly oxidized structure of ODs is mainly C− O−C group. These results were further confirmed by XPS. The XPS survey spectrum indicates that ODs are highly oxidized. The strong C1s band at 286.3 eV evidences that the C−O−C group is the dominant oxidized structure in ODs. The normalized PL spectra of bwGO, OD, and pristine GO solutions are shown in Figure 3b. The pristine GO exhibits two 20087

DOI: 10.1021/acs.jpcc.5b04529 J. Phys. Chem. C 2015, 119, 20085−20090

Article

The Journal of Physical Chemistry C

concentration, while the LW broad band becomes predominant at high concentration. The PL spectra were analyzed with the Lorentz models (Figure S7). The intensity ratios of the two bands for the solutions with different pH values are plotted versus the GO concentration (Figure 5). The LW band

Figure 5. Variation of the peak area ratios of the blue band to the LW band versus concentration at various pH values.

becomes increasingly predominant as concentration increased especially in acidic and neutral solutions, and the trend became less obvious in alkaline solutions. The concentration effect on the PL of GO had not been discussed in literature,12−16,21,30,31 and none of the models and theories presented in literature for the PL of GO could be used to interpret this concentrationdependent PL behavior because the change in concentration should not affect the size of sp2 clusters,12 amount of disorderinduced states,13 free zigzag sites,31 or the protonation and deprotonation of carboxyl and hydroxyl groups.15,16 It is interesting to point out that the concentration effect on the two PL bands is similar to that of many polyaromatic compounds like pyrene, naphthalene, and conjugated polymers.23,24,32 For those polyaromatic compounds, the band in the shorter wavelength is due to the monomer PL while the broad band in the longer wavelength is assigned to the excimer PL. The graphene-like plane and polyaromatic ODs of GO contain a large number of conjugated planar structures that are similar to naphthalene and pyrene; thus, it is reasonable to attribute the LW band to be the excimeric emission.22,33 Furthermore, the excimer is formed by the interaction between an excited conjugated plane with a ground state neighbor, and the absorption spectra should not be affected by the formation of the excimer.34 As shown in Figure 4, the normalized absorption spectrum of GO does not change with the pH value or the GO concentration. Therefore, by combining the pH and concentration effect on the PL and the UV absorption spectra of GO, we confirm that the LW band originates from the excimer of GO. The effect of the pH value on the PL spectra can be understood in term of the effect of the protonation and deprotonation of carboxylic group on the excimer formation. As evidenced by the zeta-potential results, the zeta-potential of GO solutions decreases with the increase in the pH value. It is −0.03 mV at pH 2, and increases to −56.77 mV when pH increases to 12. In alkaline solution, the carboxylic acid groups present as −COO−. The surfaces of GO plane as well as ODs were negatively charged, and the electrostatic repulsion prevents the formation of excimer. The carboxylic groups are

Figure 4. UV absorption and PL spectra (λex ≈ 320 nm) of GO samples with various concentrations at (a) pH 2, (b) pH 7, and (c) pH 12.

also measured with cuvette of 10 mm × 10 mm (Figure S6). The re-absorption was corrected through a theoretical calculation according to the absorbance at each wavelength. By comparing the PL spectra, though the re-absorption effect can affect the spectrum shape, it does not change the overall trend of the PL bands. The PL spectra of the GO solutions are sensitive to the solution pH value and GO concentration.21 With the increase in the pH value, the intensity of the LW band decreases, while the intensity of the blue band remains almost the same. The PL spectrum of GO is also strongly dependent on the GO concentration. The blue PL band is pronounced at low 20088

DOI: 10.1021/acs.jpcc.5b04529 J. Phys. Chem. C 2015, 119, 20085−20090

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in neutral state in acidic solution, so that the GO planes and ODs can interact and lead to the excimer formation. Time-resolved photoluminescence (TRPL) is a powerful tool to study the excimer fluorescence. The TRPL results at 440 and 510 nm were collected (Figure 6). The decays at the two

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04529. PL spectra of aqueous solution of bwGO, ODs, pristine GO, and a sample re-mixing separated bwGO and ODs; characterizations of ODs; PL spectra of GO aqueous solution with various concentrations at different pHs; deconvolution of PL spectra of GO with various concentrations at different pHs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a research grant from the Ministry of Education, Singapore (R-284-000-125-112).

Figure 6. Time-resolved photoluminescence (TRPL) of GO at blue (440 nm) and LW (510 nm) band (λex ≈ 370 nm).

Table 1. Time-Resolved Photoluminescence of Graphene Oxide at Blue and LW Bands blue band (440 nm) LW band (510 nm)

τ1 (ns)

ω1 (%)

τ2 (ns)

ω2 (%)

0.806 ± 0.005 0.972 ± 0.005

95.98 94.94

6.6 ± 0.4 12.2 ± 0.4

4.02 5.06

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4. CONCLUSION In conclusion, the PL band in the range of 500−650 nm for GO is attributed to the excimer PL. There are two components in GO, namely bwGO and ODs. The LW band is related to the π−π overlapping of the two components. The PL spectrum of GO is sensitive to the pH value and concentration of GO aqueous solution. The LW band becomes predominant at high GO concentration and low pH value. The excimer PL is supported by the effects of the pH value and GO concentration on the PL spectrum of GO and the long lifetime of the LW band. 20089

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