Investigations on optical properties of ZnO decorated graphene oxide ([email protected]) and reduced graphene oxide ([email protected])

Investigations on optical properties of ZnO decorated graphene oxide ([email protected]) and reduced graphene oxide ([email protected])

Journal of Alloys and Compounds 744 (2018) 64e74 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://...

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Journal of Alloys and Compounds 744 (2018) 64e74

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Investigations on optical properties of ZnO decorated graphene oxide ([email protected]) and reduced graphene oxide ([email protected]) Pushpendra Kumar a, 1, Sudipta Som b, 1, Mukesh K. Pandey c, Subrata Das d, Anupama Chanda e, Jai Singh e, * a

Physical & Materials Chemistry Division, CSIR-National Chemical Laboratory, Pashan Road, Pune 411008, India Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC Department of Physics, National Taiwan University, Taipei, Taiwan, ROC d Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, 695019, India e Department of Physics, Dr. H.S.G. University, Sagar, MP 470003, India b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 November 2017 Received in revised form 3 February 2018 Accepted 5 February 2018 Available online 7 February 2018

The present investigation is based on the production of reduced graphene oxide (r-GO) from the graphene oxide using Hummer's (GO) and improved Hummer's methods (IGO) at elaborated conditions, named as GO and IGO, respectively hereafter. In contrast to previously known techniques, the presented process does not generate toxic gas. Meanwhile, the reduction temperature can be easily controlled. This approach provides a more significant amount of hydrophilic oxidized graphene as compared to GO and IGO with the use of additional KMnO4. Thus synthesized IGO was used to produce r-GO by thermal treatment. The morphological characteristics show that the obtained samples have a wrinkled paper-like morphology with severely folded lines. However, r-GO has double layers and multilayer at the edges. All the products (GO, IGO, and r-GO) have been decorated with ZnO nanoparticles (NPs). The XRD patterns of [email protected] composites have confirmed the characteristic peaks of wurtzite ZnO indicating the formation of ZnO nanoparticles onto the surface of graphene. The microscopic studies confirm the random growth/ decoration of ZnO NPs on the surface of GO/IGO/r-GO sheets. However, in IGO and r-GO, loading/growth of ZnO NPs are less as compared to [email protected] Overall structural studies indicate the oxidation of graphite and reduction of graphene oxide into r-GO sheets and ZnO decoration. Upon UV excitations, a bright blue emission has been exhibited by the GO that originates from geminate recombination of localized e-h pairs in sp2 clusters those primarily act as the luminescent centers. The noteworthy enhancement in the emission intensities after the incorporation of ZnO nanoparticles on the surface of GO is observed. The improved synthesis method and low-temperature reduction technique of GO may be essential for the large-scale production of r-GO as well as the construction of devices composed of [email protected]/IGO/r-GO. © 2018 Published by Elsevier B.V.

Keywords: Graphene ZnO nanoparticles CIE diagram Optical properties Raman studies

1. Introduction Two-dimensional (2D) materials, a unique class of materials with stupendously attractive optical, electrical, and mechanical properties ranging from transparent nanoelectronics to medical applications. The knowledge of two-dimensional graphene and its analogue materials have recently qualified novel change due to the development of several large-scale growth methods [1e5]. Also, two-dimensional materials and graphene have renewed interest in

* Corresponding author. E-mail address: [email protected] (J. Singh). 1 Authors contributed equally. https://doi.org/10.1016/j.jallcom.2018.02.057 0925-8388/© 2018 Published by Elsevier B.V.

inorganic materials with unique electronic and optical functionalities [1e7]. The electronic band structure of graphene has a linear dispersion near the K point, and charge carriers behave as massless Dirac fermions, providing scientists with a profusion of new physics [1]. Graphene is a unique example of an extremely thin electrical and thermal conductor, with high carrier mobility, and surprising molecular barrier properties [1]. Graphene featuring high electrical conductivity, flexibility with sensibly excellent mechanical and thermal properties, has materialized as a new-generation material with countless applications in several electronic devices [8e10]. Graphene synthesized either from solution-processed reduced graphene oxide (r-GO) or by chemical vapor deposition (CVD) methods, have been used for organic photovoltaics [11], light-

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emitting diodes (OLEDs) [12], field-effect transistors (OFETs) [13,14] and photodetectors [15]. To obtain r-GO, chemical and thermal treatment of GO is required to remove the oxygen-containing groups partially and to restore the electrical properties. Chemical reduction by toxic hydrazine introduces impurities in r-GO [16] while high-temperature treatment is incompatible with glass pots. Also, Graphene oxide (GO) has a similar layered structure to graphene, but the plane of carbon atoms in graphene oxide is decorated by oxygen-containing groups [1e4,17]. Although the first experimental synthesis was done more than 150 years ago, the structure of GO is still indefinable due to its non-stoichiometry. So far, the growths have been proposed to elucidate the structure of GO by Hummer and Offeman, Staudenmaier, and Brodie [17]. The exact composition of GO is challenging to determine. The disruption of the lattice is reflected in an increase in interlayer spacing from 0.335 nm for graphite to more than 0.625 nm for GO. To date, chemical efforts at graphite exfoliation have been focused primarily on intercalation, chemical derivatization, thermal expansion, oxidation/reduction, the use of surfactants, or some combination thereof. Currently, Hummers' method (KMnO4, NaNO3, H2SO4) is the most common method used for preparing graphene oxide. In the present work, three different methods are being used to synthesize GO by using Hummer, modified Hummer and another method (developed by us) similar to modified Hummer's method followed by thermal exfoliation of GO achieved by modified Hummer method from natural graphite at elaborate conditions. The improved method present herein provides a greater amount of hydrophilic oxidized graphene material as compared to Hummers' method with additional KMnO4. In contrast to Hummers' method, the new process adopted by us does not generate any toxic gas and the temperature can be easily controlled. Due to its outstanding aqueous processability, surface enhanced Raman scattering (SERS), amphiphilicity, surface functionalizability, and fluorescence quenching ability, r-GO chemically exfoliated from oxidized graphite using our technique is considered a promising material for biological applications [1e17]. To explore the potential application of the as-synthesized graphene/graphene oxide/reduced graphene oxide in the field of photocatalysis and other low-cost optoelectronic devices, we have synthesized ZnO NPs decorated graphene oxide and reduced graphene oxide. Graphene that includes a two-dimensional macromolecular sheet of carbon atoms has excellent electrical conductivity and mechanical properties and act as a unique electron-transport material in the process of photocatalysis, and proved to be more superior than C60, polyaniline, graphite-like carbon [18]. Recently, ZnO NPs decorated with graphene oxides has also attracted a great deal of attention due to potential applications in photocatalysis [19e22]. It is expected that the hybridization of ZnO nanoparticles with graphene would reduce the recombination of charge carriers, and increase the photocatalytic efficiency. The presence of ZnO NPs prevents aggregation of graphene sheets upon drying. Under the UV excitations, these ZnO decorated reduced graphene oxide sheets exhibit distinctive optical behaviors as compared to the ZnO nanoparticles, and emit blue light. Such blue luminescence from the ZnO NPs adorned reduced graphene oxide is considerable for the growth of low-cost nanodevices [23,24]. As an inexpensive, efficient approach, several solution based methods have been employed to synthesize metal oxide-graphene nanocomposites. Herein, one-pot hydrolysis approach has been introduced to incorporate the spherical ZnO NPs decorated graphene oxide sheets. Hydrolysis approach has been proved to be very useful to yield spherical nanoparticles with the size ~10 nm [25]. Reportedly, the nanoparticles with a spherical morphology have appreciable packing density, useful slurry properties, and low light scattering

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properties [26]. Therefore, it was expected that the decoration of graphene oxide sheets with ZnO NPs via hydrolysis method could be significant to enhance the optical efficiency. The structural and photoluminescence of the ZnO NPs decorated graphene oxide sheets were investigated in detail for their potential applications as photocatalyst and in other low-cost optoelectronic devices. 1.1. Experimental procedure In the present investigation, synthesis of GO/IGO has been carried out by two different methods namely the traditional Hummer's and improved Hummer's method [17,27]. The production of r-GO has been carried out by adopting GO from improved Hummer's method. In the representative synthesis of GO by improved Hummer's method a 9:1 mixture of concentrated H2SO4/H3PO4 (450:50 mL) was added to a combination of graphite flakes (5.0 g, 1 wt% equivalent) and KMnO4 (30.0 g, 6 wt% equivalent), producing a slight exothermic about 40  C. The reaction was then heated to 50  C and stirred continuously for 24 h. The reaction was cooled to room temperature and poured into the solution mixture of ice (400 mL) þ 30% H2O2 (20 mL). The mixture was then filtered and centrifuged (5000 rpm) than the supernatant was decanted away. The remaining solid wet cake type material was then washed in succession with 500 mL DI water, 250 mL of 30% HCl, and 250 mL ethanol 3e4 times; in each wash, the mixture filtered with the filtrate being centrifuged and the supernatant decanted away. Then the product was sonicated to get GO sheets and was further washed several times with water to reach final pH ¼ 7. Thus obtained wet cake was kept on the filter and was vacuum-dried overnight at room temperature. After that, the obtained product was named IGO which was further thermally exfoliated at 300  C in open air atmosphere. The thermal shock under the presence of water molecule attached to GO sheets leads to the formation of reduced graphene oxide (r-GO) sheets as shown in Fig. 1. The ZnO NPs decorated GO/r-GO nanocomposites were synthesized by hydrolysis of zinc chloride. Zinc chloride (ZnCl3), was dissolved in ethanol and then the appropriate amount of GO/IGO/rGO was added to the solution. The concentration of ZnCl2 was fixed to 10 wt%. The solution was stirred for 10 min, the pH value of the solution was adjusted to 7 by adding ammonium hydroxide (NH4OH). After stirring for 30 min, the solution was kept at rest for few hrs and then filtered via a simple glass filtration assembly. The filtered slurry was dried at 80  C for few hrs. Finally, the nanocomposites of ZnO NPs decorated GO/IGO/r-GO were obtained after sintering the dried slurry at 400  C in air for 1 h. During the adjustment of pH value, zinc chloride reacted with NH4OH and converted to its hydroxide form via the reaction: ZnCl2 þ 2NH4OH ¼ Zn(OH)2 þ 2NH4Cl. The heat treatment of the filtered slurry in air convert Zn(OH)2 to ZnO NPS. 1.2. Material characterization The as-synthesized products were examined by X-ray diffraction (XRD) using a Rigaku D/Max-2550 diffractometer with Cu-Ka radiation (l ¼ 1.54056 Å) at 50 kV and 200 mA in the range of 10e40 (2q) at a scanning rate of 0.5 min1. Scanning electron microscopy (SEM) images were taken on a Tescan, VEGA-3 at an accelerating applied potential of 20 kV. The sample for SEM characterization was prepared by placing one drop of the dispersion on a bare indium tin oxide coated glass substrate (ITO) and then was air-dried at room temperature. Transmission electron microscopy (TEM) measurements were made on a TEM, Tecnai 20 G2 electron microscopy with an accelerating voltage of 200 kV. The sample for TEM studies was prepared by dispersing a small amount of the assynthesized material into ethanol and sonication for 10 min. Drops

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Fig. 1. Schematic representation of the synthesis of GO by Hummer's (GO), improved Hummer's (IGO) method and production of r-GO followed by thermal exfoliation of IGO at elaborated temperature.

of the dispersed material were placed onto a holey carbon grid and dried. Raman spectroscopy of the as-grown sample was carried out by Raman spectrometer (Renishaw, model no. H 45517) using an argon ion laser (l ¼ 514 nm). Fourier transforms infrared (FTIR) spectra of the samples were recorded using the Perkin-Elmer (Spectrum 100, USA) spectrometer. Emission and excitation spectra were measured using a fluorescence spectrophotometer (Fluorlog, HORIBA JOBINYVON). The Commission Internationale de l’Eclairage (CIE) coordinates were calculated using a CIE analyzer (Ocean Optics, USBe2000). 2. Results and discussion 2.1. X-ray diffraction X-ray diffraction (XRD) measurements were employed to investigate the phase and structure of the synthesized GO, IGO, and r-GO samples. Fig. 2a shows the typical XRD pattern for GO, IGO and r-GO and corresponding composite with the decoration of ZnO NPs samples. The XRD pattern of graphite oxides indicates a shift in the 2q peak of graphite from 25 to 11 due to the oxidation of layers. The broad peak observed around 11 corresponds to an interlayer distance of 0.82 nm (d002) for the AB-stacked of GO [18(a)]. The oxidation of graphite is accompanied by the increase of d-spacing which shifts the 2q peak position after oxidation of graphite. In the reduction atmosphere, the conversion GO into r-GO reduces the interlayer spacing and as a result the shift in the 2q peak position to the higher angle side (2q~26 ). This can be observed in case of the rGO sample for the (002) graphitic reflection. After reduction IGO shows an intense and broad peak seen at 26.7 (0.37 nm) which is

assigned to (002) refection plane that confirms the formation of reduced graphene oxide, being similar to other reports. Reduction in the inter-planar spacing of r-GO as compared to GO and IGO is due to the removal of the intercalated moisture molecules and the oxide groups that allow graphene sheets to be tightly packed. After the reduction, significant peak broadening with decreased intensity is observed as evident from Fig. 2a, indicating the loss of long-range ordering of the graphene planes. This is known as the exfoliation of layered structures of graphite oxide. The broad peak may form due to the partial restacking of exfoliated graphene layers. The number of graphene sheets in the r-GO samples obtained for exfoliation can be obtained by fitting the (002) profiles under different ambient. By Lorentzian fitting of the (002) reflection, the average number of sheets can be achieved using the Debye-Scherrer formula as described elsewhere. Fig. 2b shows representative ideal Lorentzian fit for the (002) reflection. From Fig. 2b, the example Lorentzian fits for the case of GO, IGO sample is shown. Similar Lorentzian fits have been obtained for the other samples r-GO (not shown here), and the calculated number of sheets in the exfoliated graphene stack is obtained as 35, 15 and 2 sheets, respectively. The reduction ambient dependent exfoliation of graphene sheets is schematically shown in Fig. 2c. Fig. 2a also shows the XRD patterns of the [email protected] composites with GO, IGO, and r-GO. The [email protected] composites show characteristic peaks of ZnO wurtzite structure (the red dot in Fig. 2a; JCPDS code: 00-036-1451). The coexistence of graphite (002) diffraction peak and ZnO diffraction peaks in [email protected] XRD pattern indicates that the surface of the GO is being covered with ZnO nanoparticles. The broad diffraction peak of GO in the range of 2q~20 e30 has disappeared in case of improved GO and r-GO. This might be attributed to the assembled ZnO clusters, which

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Fig. 2. (a) XRD patterns for graphene oxide, reduced graphene oxide, and [email protected] composites. (b) The Lorentzian fitting of the (002) reflection of GO sample. (c) A schematic diagram to elaborate the formation of GO sheets and the incorporation of ZnO nanoparticles upon the nanosheets.

effectively hindered the re-stacking of the graphene layers. The XRD patterns indicate that ZnO had been successfully nucleated, grown and incorporated onto the surface of graphene and they worked as spacers to keep the individual graphene sheets from restacking. 2.2. Morphological characterization Detail microstructural characterization of synthesized GO, IGO, and r-GO samples was done by TEM and High-Resolution TEM (HRTEM), and the images are shown in Fig. 3(aed). All as-synthesized samples have a wrinkled paper-like morphology with severely folded lines, as shown in Fig. 3 (a). Furthermore, r-GO has a few layered structures such as double layers and multilayer at the edges (Fig. 3 (d)). In particular, the flat area between the micro-folding regions shows the in-plane lattice ordering of a two-dimensional carbon network. This difference in morphology between the flat AB-stacked structure for GO, IGO, and r-GO implies that the thermal reduction process plays an essential role in this transformation. Figs. 4e6 shows the scanning electron micrograph (SEM) of the as-synthesized samples prepared by Hummer's method (GO), improved Hummer's method (IGO), after thermal treatment (r-GO) and ZnO NPs decoration ([email protected]/@IGO/@r-GO). It can be seen that the bulk product is highly porous as well as flaky. Also, the SEM images of GO/IGO/r-GO (Fig. 4 aec, Fig. 5 aeb, and Fig. 6 aeb respectively) have a rug-like structure, which may be due to residual moisture and hydroxyl/carboxyl groups attached to GO. All of the micrographs consist of individual graphene sheets randomly connected to each other held into a porous structure that typically extends over few microns, with the presence of numerous cavities, or holes. The graphene sheets are therefore initially obtained as fused sheets, weakly held into a foam-like structure which is then

quickly separated into individual layers by sonication in ethanol for several minutes. It can be seen that the individual graphene sheets stack together to form larger sheets in the form of flake-like structures. Some of these flakes are folded on the edges, while the remaining graphene sheets appear smooth. Figs. 4 (d), 5 c-d and 6 c-d represent the SEM images of the GO/IGO/r-GO respectively, after ZnO NPs decoration. We can see that the ZnO NPs growth/ decoration has taken place on the surface of GO/IGO/r-GO sheets randomly. In [email protected] (Fig. 4d), NPs got agglomerated and formed bigger crystals covering the whole GO surface, that may be because of availability of more preferable sites to grow ZnO. In IGO and r-GO, loading/growth of ZnO NPs ([email protected] and [email protected]) are less as compared to [email protected] (Fig. 5ced and 6 c-d). ZnO NPs still got agglomerated and formed bigger crystals covering the IGO/r-GO surface (see Fig. 7).

3. Growth/decoration mechanisms of the [email protected]/IGO/r-GO nanocomposites The results presented above indicate that ZnO NPs get nucleated and grow on the GO, IGO, and r-GO sheets, and their growth mechanisms can be explained as follows. When the pH value of the solution that contained GO/IGO/r-GO, and ZnCl2 has been tuned to 7e8, the ZnCl2 converted to hydroxides those may produce thin precipitate on the sheets and turned to oxide nanoparticles upon heating. The surfaces of GO, IGO, and r-GO exhibited a relatively rough morphology as observed in the TEM/SEM characterization. Also, the oxygen-containing groups between the graphene layers were dissociated from the GO, which leads to a gradual decrease in the d-spacing of the (0 0 2) plane with rising annealing temperature.

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Fig. 3. Transmission electron micrograph (TEM) images of (aeb) GO, (c) IGO (d) r-GO synthesized by Hummer's method, improved Hummer's method and thermal treatment of IGO obtained adopting improved Hummer's method.

4. Raman characterization Raman spectroscopy is widely used to characterize crystal structure, disorder, and defects in carbon-based materials. In the past decade, graphene has been studied more extensively using Raman spectroscopy to identify the number of graphene layers, the ratio of sp2/sp3 bonding, monitoring of doping concentration, electron-phonon interaction, etc. Fig. 8 (a) shows the comparison of Raman spectra of chemically obtained GO, IGO, and r-GO systems. The Raman spectra of GO, IGO sample show two distinct broad peaks at 1350 cm1 (D) and 1585 cm1 (G) [28]. The peak at 1350 cm1 is due to defects presented in the system and is a measure of the sp3 bonding in graphene [28]. The broadened G peak in the r-GO sample is slightly blue shifted as compared with that of G peak of GO and IGO. The broadening of G peak in the r-GO sample may be due to interruption of the unbroken hexagonal honeycomb crystal lattice of graphene. It has frequently been revealed that after the introduction of defects in graphene resulting in the breakdown of regular hexagonal crystal symmetry which hampers the condition of resonance and hence reduction in the intensity of 2D peak. It is well studied and documented that GO is associated with large defects sites originating mainly due to the presence of hydroxyl and epoxy groups and these functional defects account for the poor 2D peak in GO. For the r-GO sample decorated with ZnO NPs, along with the appearance of D and G peaks, four other peaks can be evidently seen in the Raman spectrum as demonstrated in Fig. 8 (b). They are located at around 328 cm1, 440 cm1, 570 cm1 and at 1130 cm1 respectively. These peaks could be ascribed to the ZnO NPs. The peak at 440 cm1 is a typical Raman active branch of wurtzite ZnO, which originates from its optical phonons of E2 (high) mode [29]. The peaks at 328 cm1 and 1130 cm1 can be attributed to the second-order Raman processes. The peak at 570 cm1 may

correspond to the surface optical phonons due to their smaller particle size [30]. The presence of the all these peaks indicates that ZnO NPs are well decorated/attached to the r-GO surface. 5. FTIR characterization It is well reported that the GO samples consist of covalently attached oxygen-containing groups such as hydroxyl, epoxy, carbonyl, and carboxyl groups. In the present investigation FTIR spectrum of GO illustrates the characteristic features including stretching vibration peak of carboxylic (-OH) groups at 3400 cm1, the C¼O stretching vibration peak in carboxyl and carbonyl at 1726 cm1, deformation peaks of carboxylic (-OH) groups at 1405 cm1, the epoxy (CeO-C) stretching vibration peak at 1224 cm1, and the alkoxy (CeO) stretching vibration peak at 1052 cm1 [31e33]. The peak at 1620 cm1 was assigned to skeletal (C¼C) vibrations of un-oxidized graphitic parts or may have been contributed from the intercalated water molecules [34]. We have prepared the r-GO by thermal exfoliation of GO at 300  C in ambient conditions and performed FTIR to find out how the functional groups evolve on GO in both situations. FTIR spectra show the progressive removal of the oxygen groups by thermal exfoliations. The carboxylic group initially indicates its presence throughout the spectra of GO in the form of a very broad eOH peak centered near at 3400 cm1. However after thermal exfoliations the -OH peak decreased significantly and seemed almost to be disappeared. Therefore, the -OH groups are particularly affected by the thermal treatment. The appearance of the weak shoulders at 28602970 cm1 corresponds to C-H stretching vibration, which may be due to the open-circle of the epoxide. The carbon backbone C¼C stretch can be seen at 1562 cm1 as in graphite. The decreasing peak at 1726 cm1was a strong indication of carbonyl reduction in

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Fig. 4. (aec) Shows the scanning electron micrograph (SEM) of the as-synthesized sample prepared by Hummers' methods. (d) SEM of the growth of ZnO NPs on GO surface ([email protected]) in liquid media by hydrolysis process at 80  C.

Fig. 5. (aeb) Shows the scanning electron micrograph (SEM) of the as-synthesized sample prepared by improved Hummers' methods. (ced) SEM of the growth of ZnO NPs on IGO surface ([email protected]) in liquid media by hydrolysis process at 80  C.

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Fig. 6. (aeb) SEM images of the as-synthesized r-GO sample prepared by improved Hummers' methods followed by thermal treatment. (ced) SEM of the growth of ZnO NPs on the r-GO surface ([email protected]) in liquid media by hydrolysis process at 80  C.

Fig. 7. Schematic for the synthesis of GO, IGO and r-GO by Hummer's, and production of r-GO followed by thermal exfoliation of GO and nucleation & growth of ZnO NPs on the GO/ IGO/r-GO surface in liquid media by hydrolysis process at 80  C.

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Fig. 8. Comparison of Raman spectra (a) Raman spectra of GO, IGO and r-GO samples synthesized by Hummer's, improved Hummer's method and after exfoliation at 300  C (b) Raman spectra of ZnO NPs decorated r-GO.

GO, suggesting that C¼O group of products can be easily converted. As the GO is reduced by thermal exfoliations, peaks for oxygen functional groups were significantly reduced and perhaps entirely removed, and two broad peaks at 1562 cm1 and 1186 cm1 were found for the r-GO. The peak at 1562 cm1 could be assigned to the aromatic C¼C stretch. The peak at 1186 cm1 could be attributed to the C-O stretch. These observations confirmed that most oxygen functionalities in the GO were removed by thermal exfoliations in the present case. The typical FTIR spectrums of as-synthesized GO and r-GO samples are shown in Fig. 9. 6. Photoluminescence The PL characterizations of the as-synthesized [email protected] nanocomposites were recorded to examine the potential, optical and electronic applications. Fig. 10 represents the PL excitation spectra of pure GO, IGO, and r-GO samples and their respective ZnO nanocomposites. All the samples exhibit efficient excitation at around 245 nm, which is the well-regarded excitation of the p-p* transition of atomic C-C bonds of graphitic structure [35]. Compared to the spectrum of pure GO sample, the composite displayed more intense excitation due to the further contribution from ZnO NPs. The UVeVis bandgap absorption of bulk ZnO was

Fig. 9. FTIR Spectrum of as-synthesized GO and r-GO obtained by thermal exfoliation of GO.

reported to appear at 375 nm. The composite shows a definite blueshift in its absorption edge, which might be due to the quantum size-effect of the fine structure in the nanometer regime. The PL spectra of the prepared GO, IGO, and r-GO samples, obtained at the UV excitation of 245 nm, shows a broad emission band peaked at around 418e421 nm. Such blue emission from the prepared GO, IGO, and r-GO samples is an outcome of geminate recombination of localized e-h pairs in sp2 clusters those mainly perform as the luminescent centers. The PL intensity increases with an increase in sp2 content in the disordered carbon systems. Herein, the PL peak position blue shifted slightly from 421 nm to 418 nm from GO to r-GO owing to the reduction of GO. Remarkable difference can be seen in the emission intensities after the incorporation of ZnO nanoparticles on the surface of GO those were grown at 300  C. The PL intensities are amplified in the [email protected] samples due to the superposition of the blue emission from ZnO. All emissions mainly consist of a broad peak in the visible region without any peak in the UV region. Recently, several reports on the photoluminescence properties of ZnO and graphene nanocomposites have been published. For example, recently, Li et al. [36] reported one-pot chemical route for the synthesis of r-GO loaded with ZnO nanoparticles. They observed that in comparison with the UV emission of ZnO at 390 nm, the emission of [email protected] sample was

Fig. 10. PL emission and excitation spectra of graphene oxide, reduced graphene oxide, and [email protected]/IGO/r-GO composites.

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Fig. 11. PL emission spectra and the corresponding CIE diagram of [email protected]/IGO/r-GO composites monitored at different UV excitations.

very less, since the interfacial connection between ZnO and carbon materials reduced the probability of recombination and led to an increased charge carrier separation. In another report, Pham et al. [37] investigated ZnO decorated GO synthesized via the selfassembly processes. They also observed that the band-edge emission of ZnO in GO-ZnO was decreased drastically as compared to ZnO, since photoexcited electron transfer from the conduction band of ZnO nanoparticles to GO. Azarang et al. [38] recently reported the photocatalytic activities of [email protected] nanocomposites synthesized via the sol-gel method. They observed very less PL emission intensity in [email protected] nanocomposites in comparison with ZnO nanoparticles because of the efficient transfer of photoinduced electrons between ZnO nanoparticles and r-GO. In the present case the nonappearance of any UV emission peak is because of the absence of near-band-edge (NBE) emission that is usually generated by the recombination of free excitons through an exciton-exciton collision process. The absence of NBE is observed possibly because of the charge transfer between ZnO and GO. The enhanced blue emission at 421 nm in all composites could also be attributed to the charge transfer between ZnO and GO, as described by Pham et al. [37]. In their report, the PL emission of ZnO nanoparticles exhibit a narrow emission band at around 377 nm (NBE) and a broadband emission peaked at 525 nm (defect emission). However, they observed that the emission intensity of the bandgap emissions are drastically quenched in [email protected] in comparison with ZnO NPs owing to the favored transfer of free electrons from the conduction band of ZnO NPs to GO which results in decrease in NBE emission intensity. In the present situation, the blue emission of GO is observed due to the geminate recombination of localized e-h pairs in sp2 clusters of GO host, as described above. While the ZnO nanostructures also exhibit blue emission at around 410 nm owing to the electron transition from the Zn interstitial levels to the top of the valence band [39]. As illustrated by Pham et al., the mutual energy transfer between graphene and the defect states of ZnO

could enhance the blue emission [37]. The optical results are in good agreement with the XRD and SEM results. Among the pure GO samples, the emission intensity of the r-GO is found to be maximum. It is well known that the carbon molecules have an adverse effect on luminescence intensity owing to its high phonon energy that enhances the nonradiative transitions, which decreased the luminescence efficiency. According to the XRD analysis, the intensity of graphite plane is observed to be maximum for r-GO sample and diminish in the order of GO, IGO respectively. Therefore, r-GO exhibits maximum luminescence intensity because of the less phonon energy from carbon molecules compared to GO and IGO. Meanwhile, among all [email protected] composites, [email protected] illustrates maximum PL intensity may be due to the more efficient energy transfer process from the ionized oxygen vacancies in ZnO to sp2 clusters of the r-GO host. Fig. 11 shows the PL emission spectra of [email protected] composites and the corresponding CIE diagram, monitored at different UV

Table 1 CIE coordinates of [email protected] composites monitored at different UV excitations. Sample

Wavelengths (nm)

CIE coordinates CIE X

CIE Y

[email protected]

2424 24555 nm 250 260 270 2424 24555 nm 250 260 270 2424 24555 nm 250 260 270

0.17 0.19 0.21 0.23 0.19 0.19 0.19 0.20 0.20 0.21 0.23 0.26

0.10 0.12 0.18 0.24 0.15 0.17 0.19 0.23 0.17 0.18 0.24 0.28

[email protected]

[email protected]

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excitations. An interesting fact to note here is that the emission color of all the composites tuned from pure blue to the bluish white region with the enhancement of UV wavelength. Such variation in the CIE or emission color has been observed due to the different saturation behavior of oxygen vacancies in ZnO and sp2 clusters of GO host. The corresponding CIE color coordinates are listed in Table 1. Meanwhile, the color coordinates of [email protected] composite, excited by 270 nm, is estimated to be (0.26, 0.28), which is close to the pure white coordinate (0.33, 0.33). It is worth to mention that in the current LED research, enormous effort has been paid to develop the color controlling ability of LEDs. Though the commercial LEDs with different colors are existing, however, the realization of color tuning ability in a single LED device is very difficult [40]. Meanwhile, the development of graphene-based blue LEDs has not been commercialized yet because of its zero bandgap nature [41]. The present study elaborates a significant enhancement in the blue emission of Go, r-GO, and IGO owing to the ZnO nanoparticle incorporations. In addition to the above, the color tuning ability of these low-temperature solution-processed composites making them suitable candidates for LEDs. 7. Conclusions The structure of GO, IGO, and r-GO are of significant interest, as their properties are dependent on the type and distribution of functional groups in various applications. r-GO has been successfully prepared without toxic hydrazine, and the main advantage of this method over other method is the mass production of r-GO by thermal exfoliation at low temperature. Investigation of photoluminescence properties, upon UV excitation, reveals a bright blue emission from the graphene oxide that originates from geminate recombination of localized eeh pairs in sp2 clusters. The PL emission intensity of this blue emission amplified in the [email protected] samples due to the mutual energy transfer between graphene and the defect states of ZnO. With the enhancement of UV wavelength, emission color of all the composites tuned from pure blue to the bluish white region might be due to the different saturation behavior of oxygen vacancies in ZnO and sp2 clusters of GO host. Meanwhile, the color coordinates of [email protected] composite, excited by 270 nm, is estimated to be (0.26, 0.28), which is close to the pure white coordinate (0.33, 0.33). Therefore, such low-temperature solution-processed [email protected] composite could also have broad applications in UV excited white LEDs. Acknowledgements Jai Singh gratefully acknowledges the University Grants Commission (UGC, New Delhi, India) for providing UGC-BSR Research Start-Up-Grant and DST fast track Grant to carry out this work. M. K. Pandey is grateful to the Ministry of Science and Technology in Taiwan, ROC for financial support. References [1] (a) V. Sahu, V.K. Maurya, G. Singh, S. Patnaik, R.K. Sharma, Enhanced ferromagnetism in edge enriched holey/lacey reduced graphene oxide nanoribbons, Mater. Des. 132 (2017) 295e301; (b) A.K. Geim, Graphene: status and prospects, Science 324 (2009) 1530; (c) A.H. Castro Neto, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Rev. Mod. Phys. 81 (2009) 109; (d) P. Kumar, A.K. Singh, S. Hussain, K.N. Hui, K.S. Hui, J. Eom, J. Jung, J. Singh, Graphene: synthesis, properties, and application in transparent electronic devices, Rev. Adv. Sci. Eng. 2 (2013) 238e258. [2] (a) R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308; (b) J. Hass, W.A. de Heer, E.H. Conrad, The growth and morphology of epitaxial multilayer Graphene, J. Phys. Condens. Matter 20 (2008), 323202.

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