Article pubs.acs.org/JPCC
Epoxy to Carbonyl Group Conversion in Graphene Oxide Thin Films: Effect on Structural and Luminescent Characteristics J. R. Rani,†,⊥ Juhwan Lim,†,⊥ Juyeong Oh,† Jung-Woo Kim,‡ Hyeon Suk Shin,‡ Jae Hun Kim,§ Seok Lee,§ and Seong Chan Jun*,† †
NEMD Lab., School of Mechanical Engineering, Yonsei University, Seoul 120 749, Republic of Korea Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Banyeon-ri 100, Ulsan 689-805, Korea § Korea Institute of Science and Technology, Seoul 130 650, Republic of Korea ‡
ABSTRACT: The conversion of epoxy to carbonyl group in graphene oxide (GO) thin films has been carried out via oxygen plasma treatment, and the effects of this conversion on structural and optical properties were investigated. Hydrophilicity of the prepared GO solution allows it to be uniformly deposited onto substrates in the form of thin films. Highresolution transmission electron microscopy and electron diffraction analysis confirmed 4−5 layers of the graphene oxide layers which are polycrystalline in structure, and the oxygen plasma treatment results in short-range order crystallization in graphene oxide films with an increase in interplanar spacing which can be attributed to the presence of oxygen functional groups on the graphene oxide layers. Electron energy loss spectroscopy (EELS) and Raman spectroscopy confirm the presence of the sp2 and sp3 hybridization due to the disordered crystal structures of the carbon atoms results from oxidation, and XPS analysis shows that epoxy pairs convert to more stable CO and O−CO groups with oxygen plasma treatment. The broad energy level distribution resulting from the broad size distribution of the sp2 clusters produces excitation-dependent photoluminescent (PL) emission in a broad wavelength range from 400 to 650 nm. Our results suggest that as oxygen pressure increases, there is a change from epoxide to carbonyl linkages which also resulted in variation in PL emission. graphite to more than 1 nm for GO.5 In GO, the ratio of sp2/ sp3 fractions opens up possibilities for new functionalities and possesses a finite electronic band gap generated by the disruption of π-networks due to the formation of oxygencontaining groups. Thus, it is possible to modify the electronic structure by means of chemical or physical treatments with different gases to reduce the connectivity of the electron network. Graphite oxide can undergo complete exfoliation in water, and other solvents yielding colloidal suspensions of almost entirely individual graphene oxide and several efforts have been made to assemble graphene oxide platelets from their water suspensions to large-area films sheets. GO films can be prepared by different methods such as chemical techniques, vacuum filtration, dip-coating, spin-coating, Langmuir−Blodgett assembly, and direct chemical vapor deposition.6,7 Several studies show that GO shows significant optical absorption and broadband fluorescence in the UV−vis and near-infrared (NIR) spectral regions, which has been attributed to π-electrons confined in localized sp2 domains.8 The partial disruption of the pristine order of the π-network and the reorganization of the
1. INTRODUCTION Graphene, a single-atom-thick sheet of hexagonally arrayed sp2bonded carbon atoms, has received significant attention due to its unique electronic, mechanical, and thermal properties, all deriving from the unique details of its electronic band structure.1 Recent efforts have been devoted to exploring the applications of graphene and related materials, such as functionalized graphene, graphene oxide (GO), reduced GO, and graphene nanoribbons in electronic and optical devices due to its tunable optoelectronic properties and excellent electrical, mechanical, and thermal properties.2 Bao et al. reported graphene−polymer nanofiber membrane as efficient photonic materials for the generation of ultrashort pulses in fiber lasers.3 It is one of the hottest areas of ongoing research activity to get over current functional limitation in silicon and compound semiconductor electronics.4 GO consists of phenol hydroxyl and epoxide functional groups on the top and bottom surfaces of each sheet and sp2-hybridized carbons containing carboxyl and carbonyl groups mostly at the sheet edges, and these groups offer tremendous opportunities for access to functionalized graphene-based materials. The oxidation of graphite to GO breaks up the sp2-hybridized structure of the stacked graphene sheets, and the disruption of graphene lattice is reflected in an increase in interlayer spacing from 0.335 nm for © 2012 American Chemical Society
Received: May 23, 2012 Revised: August 6, 2012 Published: August 6, 2012 19010
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
Figure 1. (a) AFM image of GO-1 film. (b) (i) and (ii) depict two typical emission photos taken from GO solution at two different excitation wavelengths of 300 and 420 nm showing green and red emission, and (iii) and (iv) depict emission photos taken from GO-1 film at two different excitation wavelengths of 200 and 275 nm showing bluish green and green emission. (c) TEM images of GO solution, and inset shows the photograph of the as-prepared GO solution. (d) SEM images of GO-1 film.
suspensions. Also, oxygen plasma treatment is considered to be an important fabrication method for making highly hydrophilic surfaces, and this method subsequently improves the adhesion strength between two different material systems. We further did oxygen plasma treatment of the deposited film by varying the plasma pressure, and further analysis of the film using Raman and photoluminescence (PL) spectroscopy measurements shows evidence of the modification of the electronic properties of graphene oxide. We observed visible PL in a wide range of 440−650 nm and demonstrate that by appropriately controlling the oxygen plasma pressure and time, PL emission wavelength was varied. The experimental results are explained in terms of a functionalization of the pristine sp2 graphene lattice with chemisorbed epoxy groups; we find evidence of a structural modification of GO samples. Since the PL emission depends on the size and fraction of the sp2 domains, PL emission in the visible region can be achieved by controlling the nature of sp2 sites. Our results suggest that an oxygen plasma treatment represents a valid approach to control the variation from epoxy to carbonyl group. Since the previous reports are based on the oxygen plasma treatment of graphene, it was our curiosity to observe what happens to a GO thin film when it is treated with oxygen plasma. Moreover, to our knowledge the structural variation of oxygen plasma-treated GO thin films and their correlation with luminescent emission are unknown, and here in the present
graphene lattice give rise to photoluminescence in GO, and reports show that the collective band gap of GO increases with oxygen contents.9 In contrast to pristine graphene, in which all atoms are sp2 hybridized, GO also contains sp3 carbon atoms covalently bonded to oxygen-bearing functional groups, and the photoluminescence (PL) in such carbon systems usually is a consequence of recombination of localized e−h pairs in sp2 clusters. Excitation wavelength-dependent PL emission from the GO and graphene quantum dots were also observed, and the PL peak shifted from 430 to 515 nm when the excitation wavelength was changed from 320 to 420 nm.10 Oxygen plasma treatment was applied to introduce band gap opening in graphene, and the degree of band gap opening is proportional to the degree of oxidation. The effects of oxygen plasma treatments on the photoluminescence properties of single- and few-layer graphene growth by chemical vapor deposition (CVD) process were described by Gokus et al.,11 and they observed visible luminescence from oxygen plasma-treated graphene which has been attributed to the emission to carbon− oxygen related localized states. Several of the above applications are still not feasible because the large-scale production of pure graphene and GO sheets remains challenging. Here we prepared graphene oxide solutions in aqueous media, and thin films of the GO solutions are made by spin-coating on oxygen plasma-treated Si/SiO2 substrate. Since GO is hydrophilic, it can readily disperse in water to form stable colloidal 19011
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
Figure 2. (a−d) FT-IR spectra of GO-1, GO-2, GO-3, and GO-4 films, respectively, (e) shows the FTIR spectrum of GO films in O−H vibration region, and (f) the UV−vis absorption spectrum of GO solution. (g) Comparison of Raman spectra at 514 nm for GO films. (h) corresponds to proposed possible atomic structure of graphene oxide before and after oxygen treatment with carbon atoms in green, oxygen atoms in blue, and hydrogen atoms in orange.
spectroscopy (FTIR) (Bruker Vertex70), Raman spectroscopy (inVia Raman microscope excitation at 532 nm), X-ray photoemission spectroscopy (XPS) (VG Scientific Instruments, ESCALAB 220i-XL), transmission electron microscopy (TEM/ EELS) (JEM.ARM.200F), and photoluminescence (PL) (PerkinElmer LS-55) measurements.
study we explore the effect of epoxy and carbonyl group on the optical properties of graphene oxide thin films.
2. EXPERIMENTAL SECTION Graphite oxide was synthesized by a modified Hummer’s method12 and exfoliated to give a brown dispersion of graphene oxide under ultrasonication. The resulting graphene oxide (GO) is negatively charged over a wide pH condition since the GO sheet has chemical functional groups such as carboxylic acids. Si/SiO2 substrate was cleaned and treated with oxygen plasma at 100 W and 300 mbar oxygen partial pressure (PO2) for 30 s to make it a hydrophilic surface and to improve the strength of the film adhesion to the substrate. GO thin films were deposited on Si/SiO2 substrate using the repeated spincoating technique at 300, 500, 800, 1600 rpm for 30 s each (the total time of deposition for each film was kept constant at 120 s). An equal amount of GO solution (∼0.15 mL) was dropped onto the substrate for each coating, and a total of ∼0.6 mL of GO solution was used for each film deposition. The samples are then exposed to oxygen RF plasma. Four thin films has been prepared in which GO-1 represents the film that was not treated with oxygen plasma, GO-2 (70 mbar of PO2, 10 s), GO3 (80 mbar of PO2, 10 s), and GO-4 (90 mbar of PO2, 30 s). The structural and optical changes are monitored by atomic force microscopy (AFM) (multimode, Veeco), scanning electron microscopy (SEM) (JEOL, JSM-6700F), UV−vis absorption spectra (JASCO V-650), Fourier transform infrared
3. RESULTS AND DISCUSSION Figure 1a shows the AFM image of GO-1 film, (i) and (ii) in Figure 1b depict two typical emission photos taken from GO solution at two different excitation wavelengths of 300 and 420 nm showing green and red emission, and (iii) and (iv) in Figure 1b depict emission photos taken from GO-1 film at two different excitation wavelengths of 200 and 275 nm showing bluish green and green emission. Figure 1c gives TEM images of GO solution, and the inset shows the photograph of the asprepared GO solution. Figure 1d shows SEM images of GO-1 film. TEM and PL analysis are discussed later. Figure 2h corresponds to proposed possible atomic structure of graphene oxide before and after oxygen treatment with carbon atoms in green, oxygen atoms in blue, and hydrogen atoms in orange. The Fourier-transformed infrared (FT-IR) spectra of graphene oxide films (GO-1 to GO-4) are shown in Figure 2a−d, respectively. The band around 1000 cm −1 corresponds to C−C vibrations. The intensity of O−H vibration (3000− 3400 cm−1) (Figure 2e) decreases with oxygen plasma treatment, and the presence of C−O (νC−O at 1100 cm−1) 19012
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
Figure 3. XPS analysis of GO-1, GO-2, GO-3, and GO-4 films.
increases with oxygen pressure. The band around 1630 cm−1 corresponds to CC bonds. Because of oxygen treatment, the peak around 1478 cm−1 increases which could be due to the increase of O−CO vibrations (discussed later). Also, there is an increase in vibration of 1700 cm−1 which indicates the increase of CO bonds with oxygen plasma treatment. Thus, FTIR spectroscopy provides evidence of the presence of different types of oxygen functionalities on the graphite oxide material. The synthesized GO also displays absorption properties as shown in UV−vis spectra (inset of Figure 2f). Specifically, the broad absorption peak from 200 to 305 nm and the absorption peak around 207−264 nm correspond to π−π* transition of the CC bond of GO in different sized aromatic sp2 clusters and the shoulder in the range 295−305 nm to the n to π transitions due to the presence of epoxide (C−O−C) and peroxide (R−O−O−R) like linkages, and these positions can be bathochromically shifted by conjugation.13 Thus, the characteristics of the optical absorption can be used to find the oxidation degree of GO samples and that can be tuned by simple chemical reactions. In order to further characterize the oxygen plasma-treated samples and to study the nature of sp2 domains, Raman analyses were performed and presented in Figure 2g. There are prominent spectral features, called G at ∼1586 cm−1, D ∼1350 cm−1 bands, and 2D at 2697 cm−1.15 Raman spectra of all disordered carbons are dominated by the relatively sharp G and D features of the sp2 sites. The G and 2D peaks represent the E2g vibrational and out-of-plane modes within aromatic carbon rings, respectively. The G band is a degenerate optical phonon mode at the Brillouin zone center and is induced by a single
resonance process. The peak near 1350 cm−1 is denoted with the D band (in-plane carbon ring breathing mode (A1g mode)), which is forbidden in perfect graphite.15 This process requires a scattering at defect sites in order to conserve the momentum. Previous reports shows that the dominant peaks in the D mode spectra originate from phonons between the K and M points of the Brillouin zone. The D mode is dispersive; it varies with photon excitation energy, even when the G peak is not dispersive, and its intensity is strictly connected to the presence of 6-fold aromatic rings. The G band corresponds to graphitelike sp2 carbon, and D band corresponds to disordered sp2 carbon induced by the linking with sp3 carbon atoms. The D peak ultimately gives a relative measure of the amount of sp3 carbons in the surrounding. Defects such as impurity atoms, functional groups, heptagon−hexagon pairs, folding, etc., of the graphene layers give rise to the D-band. The cooperation between D and G peaks gives rise to a G0 peak near 2931 cm−1. The D mode grows in as oxidation proceeds. Because of plasma treatment, G band intensity remains almost the same, but there is an increase in D band intensity. The prominent D peak in the Raman spectra is from the structural imperfections created by the attachment of hydroxyl and epoxide groups on the carbon basal plane. The ID/IG ratio is found to be 0.8996, 0.9137, 0.9197, and 0.9235 respectively for GO-1, GO-2, GO-3, and GO-4 films, indicating the increased density of structural defects due to oxygen plasma treatment. The present observations are in agreement with previous work regarding UV-light-assisted oxidation of graphene.16 Local bonding distortions should result in some sp3 C orbital character and π-orbital misalignment. This is expected to lead to increased ID/IG values and as has been observed in O2 oxidation of 19013
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
carbon nanotubes.17 After oxygen plasma treatment G peak intensity remains almost constant and D peak intensity increases, which is attributed to enhance sp3 hybridization by oxidation implying that the increase of sp3 hybridization is caused not by the deterioration of carbon atoms in the graphene plane but by the chemisorption of oxygen atoms onto the carbon network which is in consistent with previous results.16 It is also interesting to observe the (D + G) band near 2931 cm−1 known as a defect-related peak, which occurs from two phonons with different momentum. Generally, there are several different carbon groups in chemically synthesized graphene oxide which are characterized by the appearance of several XPS spectral peaks. The XPS results of the samples in Figure 3 support the conclusions that the films were oxidized and the graphene structure was disordered slightly with oxygen plasma treatment. The C1s spectra consist of four peaks that correspond to sp2 carbon at 284.8 eV, O−H/O−C−O at 286.2 eV, CO at 287.8 eV, O− CO at 288.5 eV, and COOH groups at 289.3 eV.18 The peak at 290.2 eV is from plasmon which is a collective behavior of delocalized valence electrons of GO. The peak positions of all films were almost constant at 284.6 eV. It can be considered that the main bonding configuration of carbon atoms in the film is C−C. A sheet of graphene oxide consists of a hexagonal ring based carbon network having both sp2-hybridized carbon atoms and sp3-hybridized carbons bearing hydroxyl and epoxide functional groups on either side of the sheet. Thus, the sp2 clusters in graphene oxide vary in size and are separated by an amorphous and highly disordered sp3 bonded matrix, which forms a high tunnel barrier between the clusters. The peaks in Figure 3 were fitted with Gaussian functions. Plasma treatment might change the natural structure of functional oxygen groups in graphene. Figure 3 shows that as from GO-1 to GO-4, the peak intensity of C−C bond, C−OH, and COOH group decreases while that of CO (287.8 eV) and O−CO (288.5 eV) increases relatively. The peak at 289.3 eV almost vanishes for GO-4 while that at 286.2 eV does not vanish completely due to the presence of epoxy groups. However, its relative intensity decreases which depicts the change from epoxy to carbonyl and/or O−CC with oxygen plasma treatment. As pressure and time increase, carbonyl components increase more than epoxy components, and previous reports show that carbonyls were formed by rearrangement of epoxy groups.19 Epoxy groups tend to form a line on a carbon lattice, and the cooperative alignment induces a rupture of the underlying C− C bonds. Once an epoxy chain appears, it is energetically preferable for it to be further oxidized into epoxy pairs that then convert to more stable carbonyl and O−CO groups as oxygen plasma pressure increases.19 Moreover, the sp2 to sp3 fraction mainly takes place in the basal plane where epoxy and hydroxyl groups coexist.20 These results are also consistent with FTIR analysis. The peak at 283.0 eV in the XPS spectrum of all the films (Figure 3) is assigned to carbon bound to silicon, which indicates the adhesion of films to the substrate. Graphene oxide sample was deposited on the standard holeycarbon-film-covered copper grids and loaded into the microscope for TEM measurements to study graphene oxide lattice. HRTEM images of GO-1 (Figure 4a), GO-3 (Figure 4b), and GO-4 (Figure 4c) films illustrate that some atomic structures become ordered at an optimum oxygen pressure for GO-3 film and getting disordered thereafter. TEM images show that during oxygen plasma treatment the thermal energy favors the further clustering of the sp2 phase stimulating connection
Figure 4. HR-TEM images of (a) GO-1, (b) GO-3, and (c) GO-4 films. Inset shows the corresponding SAED pattern.
between ordered rings and oxygen atoms attach to graphene sites randomly and convert sp2 carbon bonds into sp3 bonds. Thus, initially the sp2 clusters in graphene oxide (GO-1) are small and separated by an amorphous and highly disordered sp3-bonded matrix, which forms a high tunnel barrier between the clusters. During heat treatment, the thermal energy favors the further clustering of the sp2 phase stimulating connection between ordered rings and moving from polycrystalline to the two-dimensional nanocrystalline graphene (GO-3 and GO-4). From the selective area electron diffraction (SAED) pattern, a typical sharp, polycrystalline ring pattern is obtained for GO-1 film (inset of Figure 4a). Strikingly, clear diffraction spots are 19014
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
291 eV that corresponds to transitions from the 1s to the σ* states (1s−σ*). Similar spectra have been reported by Saxena et al. in carbon K-edge of pristine GO films.22 The inset of Figure 5a shows 1s−π* transitions from GO-2 and GO-3 films. The higher-order π* and σ* resonances in GO at ∼292.5 and ∼293.5 eV may be assigned mainly to the presence of carbon atoms bonded to oxygen atoms by epoxide and carbonyl bonds.22 Lower energy plasmon excitations are observed from the films in Figure 5b. Previous reports shows that for monolayer GO plasmon energy of pristine graphene oxide sheet is at 24.5 eV, and plasmon energy decreases almost linearly with sp3 fraction. The sp3 hybridization induced by oxygen containing groups increases this value in graphene oxide. The experimentally recorded π* + σ* plasmon energies for a single-layer and five-layer graphene are 16.6 and 18 eV, respectively.23 Thus, in the present study 19.2 eV plasmon energy corresponds to 4−6 layers of GO. Thus, GO-3 and GO-4 samples in Figure 5b give peaks around 21 and 22 eV, respectively, and reports show that plasmon energy of an amorphous carbon of ∼45% fraction of sp3 bond is around 24 eV, which confirms that oxygen plasma treatment increases sp3 bonding. The low-energy plasma excitation of π* electrons in the GO film is ∼5.2 eV (inset of Figure 5b), and that of the π* electrons in single layer GO is ∼5 eV, which shows that our film consist of 3−5 layers of GO. In the present study GO-1 shows a peak around 539.7 eV in Figure 5c may possibly be due to higher-order π* resonance of all oxygen atoms and higher-order σ* resonance of the 1,2epoxy linkages in the GO sample. In GO-3 film the absorption corresponds to peak at ∼536.7 eV seems to arise form σ* and higher-order π* resonances of only the epoxy bond.22 The peak corresponding to 545 eV arises from the oxygen atom from CO and O−CO groups; the intensity of this peak increases, which is in consistent with the XPS results, which shows the dominance of carbonyl groups in the advanced oxidation stages. Figure 6 shows PL emission spectra from GO-1, at various excitations ranging from 200 to 500 nm. The PL peak emission red-shifts monotonically from 480 to 650 nm as the excitation wavelength is increased from 200 to 500 nm. For 200 nm excitation PL measurements shows emission peaked at ∼440, 460, 532, 545, and 620 nm. Like most luminescent carbon material, GO also exhibit excitation-dependent PL behavior. GO suspensions in water as well as films are photoluminescent under illumination by visible and UV light sources.24 Reports show that mainly there are two distinct types of photoluminescence for GO. The first one is a broad PL covering visible to near-IR range. The second type is blue emission, centered around 390−440 nm, and is observed upon excitation with UV light. Recent work shows that possible origin of the blue PL emission is the radiative recombination of electron− hole pair (e−h pairs) generated within localized states, and the broad PL was believed to originate from the carbon sp2 domains/clusters but it was invisible under UV irradiation.25 The origin of the two different kinds of PL is still being debated. In the present study, absorption and EELS measurements show π−π* transitions in the GO film, and from Raman and XPS analysis, it has been suggested that GO consists of sp2 clusters isolated within sp3 carbon matrix. GO possesses a finite electronic band gap generated by the disruption of π-networks due to the formation of oxygen containing groups. The π states
observed (GO-3) characteristic of internal short-range crystalline order; the 6-fold pattern is consistent with a hexagonal lattice21 (inset of Figure 4). This simple hexagonal pattern of sharp spots is similar to those obtained from graphite oxide and leads immediately to several conclusions that the GO films are not completely amorphous; the lack of any diffraction spots other than those corresponding to the graphite structure shows that any oxygen-containing functional groups present do not form superlattice-type ordered arrays, and this implies that appreciable fraction of oxygen atoms is involved in linkage between carbon atoms. However, for GO-4 (inset of Figure 4c) the pattern again shows disordered nanocrystalline pattern when compared to GO-3. Also, the existence of some disorder is obvious from the broadening of the diffraction spots. This broadening shows that a disturbance exists in the stacking of the close-packed planes. Similar behavior is also observed for the electron diffraction patterns of graphite.21 First diffraction rings in GO-1 have a lattice spacing of 0.76 nm, which have been identified to be the (110) reflections. For GO-3 and GO-4 films the corresponding spacing is 2.1 and 2.4 nm, respectively, which is larger than the interplanar spacing in GO-1, and this can be attributed to the presence of oxygen functional groups on the graphene oxide layers. The EELS spectrum, as shown in Figure 5a, in the carbon Kedge region shows a peak at 285 eV that corresponds to transitions from the 1s to the π* states (1s−π*) and a peak at
Figure 5. (a) EELS analysis of C K-edge, (b) lower energy plasmons resonance of GO-1, GO-3, and GO-4 films, and (c) shows O K-edge spectrum of GO films. Inset of (a) shows 1s−π* transitions from GO2 and GO-3 films. Inset of (b) shows low-energy plasma excitations of π* electrons in the GO-1 and GO-3 films. 19015
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
Figure 6. PL emission spectra for (a) GO-1, (b) GO-2, (c) GO-3, and (d) GO-4 films at different excitation wavelengths.
For oxygen-plasma-treated films the PL emission spectrum is distinct from that of GO-1 film. For all films the peak emission wavelength at 487 nm remains almost constant with 200 nm excitation. However, the shoulder at 530 nm disappears gradually with increasing oxygen pressure (GO-2 to GO-4), resulting in a broad emission. The emission intensity of higher wavelength emission (550−650 nm) from the films decreases considerably with oxygen partial pressure. From the XPS and EELS analysis it is clear that when oxygen pressure increases sp3 hybridization increases due to the incorporation of oxygen resulting in O−C−O, CO, and O−CC linkage. However, the variation is gradual as GO-2 film contains more epoxy group than CO and O−CC linkages, while GO-3 and GO-4 contain more CO and O−CC linkages. Larger clusters are more susceptible to oxidation introducing more nonradiative paths and dangling bonds resulting in quenching of emission at longer wavelength (550−650 nm) for GO-2, GO-3, and GO-4 films. As we know, epoxy and carbonyl groups usually induce nonradiative recombination of localized electron−hole (e−h) pairs, which leads to the nonemissive property of GO. The oxidation increases reactive sites such as epoxy and carbonyl groups and hence decreases the emission efficiency of the sp2 domains on GO nanosheets. Moreover, XPS analysis shows that as oxygen pressure increases carbonyl groups increases. For oxygen-treated films it can be inferred that due to the presence of increased number of carbonyl groups; most of the electrons excited to the high levels relax nonradiatively which decreases the PL emission in higher wavelength region. Moreover, for these samples, the nonuniformity of graphene edges and the potential for dangling bonds are thought to have significant influence on their chemical properties and reactivity which may also decrease the PL emission in higher wavelength region. We compared the integral intensity with constant absorbance for different samples as per ref 27. It is observed that integral
of the sp2 sites form the valence-band and conduction-band edges which lie closest to the band gap. Hence, optical excitation creates electron−hole pairs in the π and π* states. GO is characterized by the nanometer-scale sp2 carbon clusters, and electron confinement in such clusters is possible because sp3 carbon sites act as large repulsive barriers for carriers. The energy gap giving rise to PL between the π and π* states generally depends on the size of sp2 clusters26 or conjugation length and may thus arise from even smaller fragments of sp2 carbon. Thus, the presence of isolated sp2 clusters within the carbon−oxygen sp3 matrix leads to the localization of e−h pairs, facilitating radiative recombination of small clusters after losing their excess energy through the thermalization processes. Thus, GO is expected to possess local wide range of energy gaps at least in an energy region of 2−3 eV, thus giving rise to the broad PL. In such cases, the energy gap of the cluster is inversely related to its size. The narrow π−π* gap of sp2 sites surrounded by the wider gap of sp3 sites creates large bandedge fluctuations which tend to strongly localize tail states. Also, sp2 clusters contain different structural units with double conjugated CC bonds, which can also act as recombination centers. Previous studies shows that it is essentially a quantum confinement effect related to the size of the sp2 clusters, and so a red-shift is observed from the PL spectra with increasing excitation wavelengths.27 In the present case also clusters in GO have a large size range, and thus there is size distribution of sp2 cluster and the energy gap of the localized sp2 clusters caused by quantum confinement effect gives excitationdependent emission. Since the energy gap of the cluster is inversely related to its size, larger clusters will emit longer wavelength light. So when excited using 200 nm radiation, emissions from all the clusters is observed resulting in a broad emission. However, when excited using higher wavelength radiation, emission from selected clusters (large clusters) is only observed, thereby resulting in reduction PL emission intensity. 19016
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
The Journal of Physical Chemistry C
Article
(5) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457−460. (6) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett. 2008, 8, 1704−1708. (7) De, S.; King, P. J.; Lotya, M.; Neill, A. O.; Doherty, E. M.; Hernandez, Y.; Duesberg, G. S.; Coleman, J. N. Small 2010, 6, 458− 464. (8) Navarro, C. G.; Weitz, A. M.; Bittner, M.; Scolari, A.; Mews, M.; Burghard, K. Nano Lett. 2007, 7, 3499−3503. (9) Liu, Z.; Robinson, J. T.; Sun, X.; Dai, H. J. Am. Chem. Soc. 2008, 130, 10876−10877. (10) Loh, K. P.; Bao, Q. L.; Eda, G.; Chhowalla, M. Nat. Chem. 2010, 2, 1015−1024. (11) Gokus, T.; Nair, R. R.; Bonetti, A.; Böhmler, M.; Lombardo, A.; Novoselov, K. S.; Geim, A. K.; Ferrari, A. C.; Hartschuh, A. ACS Nano 2009, 3, 3963−3968. (12) Hirata, M.; Gotou, T.; Horiuchi, S.; Fujiwara, M.; Ohba, M. Carbon 2004, 42, 2929−2937. (13) Li, D.; Mueller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101−105. (14) Lucas, C. H.; Peinado, A. J. L.; Gonzalez, J. D.; Cervantes, M. L.; Martin-Aranda, R. M. Carbon 1995, 33 (11), 1585−1592. (15) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Nano Lett. 2007, 7, 238−242. (16) Gunes, F.; Han, G. H.; Shin, H. Y.; Lee, S. Y.; Jin, M.; Duong, D. L.; Seung, S. J.; et al. Nano: Brief Rep. Rev. 2011, 6, 409−418. (17) Yudasaka, M.; Zhang, M.; Iijima, S. Chem. Phys. Lett. 2003, 374, 132−136. (18) Chiang, T. C.; Seitz, F. Ann. Phys. 2001, 10 (1−2), 61−74. (19) Bagri1, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Nat. Chem. 2010, 2, 581−587. (20) Jeong, H. K.; Lee, Y. P.; Lahaye, R. J. W. E.; Park, M. H.; An, K. H.; Kim, I. J.; Yang, C.-W.; Park, C. Y.; Ruoff, R. S.; Lee, Y. H. J. Am. Chem. Soc. 2008, 130, 1362−1366. (21) Kushita, K. N.; Hojou, K. Ultramicroscopy 1991, 35, 289−293. (22) Saxena, S.; Trevor, A.; Tyson; Negusse, E. J. Phys. Chem. Lett. 2010, 1, 3433−3437. (23) Zhi, X.; Bando, Y.; Liu, L.; Wang, W.; Bai, X.; Golberg, D. ACS Nano 2011, 5, 4401−4406. (24) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Adv. Mater. 2010, 22, 734− 738. (25) Eda, G.; Lin, Y. Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. A.; Chen, I. S.; Chen, C. W.; Chhowalla, M. Adv. Mater. 2009, 22, 505− 509. (26) Robertson, J.; O’Reilly, E. P. Phys. Rev. B 1987, 35, 2946−2957. (27) Jose, R.; Biju, V.; Yamaoka, Y.; Nagase, T.; Makita, Y.; Shinohara, Y.; Baba, Y.; Ishikawa, M. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 1833−1838.
intensity for constant absorbance for increases from GO-2 to GO-4 compared to GO-1, which shows enhanced PL quantum yield for the latter sample. The PL from the GO solution and film is intense that luminescence is visible to the naked eyes even when excited by a Xe lamp.
4. CONCLUSION Preparation of high-quality graphene oxide thin films on the desired scale is essential for many applications. In the present study hydrophilicity of the as-prepared GO solution allows it to be uniformly deposited onto substrates in the form of thin films. During oxygen plasma treatment, the thermal energy favors the further clustering of the sp2 phase stimulating connection between ordered rings and oxygen atoms attach to graphene sites randomly convert sp2 carbon bonds in to sp3 bonds. The structural modifications of graphene oxide are dependent on the level of oxidation. As oxygen pressure and time increases carbonyl components increases than epoxy components and the increases in carbonyls groups may be due to the rearrangement of epoxy groups. The presence of isolated sp2 clusters within the carbon−oxygen sp3 matrix leads to localization of electron−hole pairs, facilitating radiative recombination for small clusters in GO which results in excitation-dependent emission. The emission intensity of higher wavelength emission (550−650 nm) from the films decreases considerably with oxygen partial pressure due to the presence of increased number of carbonyl groups. The PL emission from GO is intense that luminescence is visible to the naked eyes even when excited by a Xe lamp. Our results suggest that an oxygen plasma treatment represents a valid approach to control the variation from epoxy to carbonyl group which may find application in the design of graphene-based nanooptoelectronic devices.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ⊥
Both authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful to Prof. K. T. Lee for his great support in photoluminescent measurements. This work was partially supported by the Priority Research Centers Program (20090093823), the Pioneer Research Center Program (20100019313), and the Basic Science Research Program (2011-80856) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) of the Korean government.
■
REFERENCES
(1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183−191. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666−669. (3) Bao, B. Q.; Han, Z.; Yang, J. X.; Wang, S.; Tang, D. Y.; Jose, R.; Ramakrishna, S.; Lim, C. T.; Loh, K. P. Adv. Funct. Mater. 2010, 20, 782−791. (4) Rani, J. R.; Sandeep, S. C .S.; Philp, R.; Pillai, V. P. M. Electrochem. Solid-State Lett. 2008, 11 (8), K73−K77. 19017
dx.doi.org/10.1021/jp3050302 | J. Phys. Chem. C 2012, 116, 19010−19017
