Optical Property Characterization of Novel Graphene-X (X=Ag, Au and Cu) Nanoparticle Hybrids

The present investigation reports new results on optical properties of graphene-metal nanocomposites. These composites were prepared by a solution-based chemical approach. Graphene has been prepared by thermal reduction of graphene oxide (GO) at 90C by hydrazine hydrate in an ammoniacal medium.This ammoniacal solution acts as a solvent as well as a basic medium where agglomeration of graphene can be prevented. This graphene solution has further been used for functionalization with Ag, Au, and Cu nanoparticles (NPs). The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-Vis spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to reveal the nature and type of interaction of metal nanoparticles with graphene.The results indicate distinct shift of graphene bands both in Raman and UV-Vis spectroscopies due to the presence of the metal nanoparticles. Raman spectroscopic analysis indicates blue shift of D and G bands in Raman spectra of graphene due to the presence of metal nanoparticles except for the G band of Cu-G, which undergoes red shift, reflecting the charge transfer interaction between graphene sheets and metal nanoparticles. UV-Vis spectroscopic analysis also indicates blue shift of graphene absorption peak in the hybrids. The plasmon peak position undergoes blue shift in Ag-G, whereas red shift is observed in Au-G and Cu-G.


Introduction
Graphene is a unique allotrope of carbon characterized by honeycomb lattice of sp 2 -hybridized carbon atoms in which carbon atoms are packed in a two-dimensional (2D) hexagonal lattice [1].Graphene, often considered as a "miracle material" of the 21st century, has attracted tremendous attention in the academic community [2].Being one-atomic layer thick sheet of carbon extending infinitely in 2D, its properties encompass range of superlattices.This includes high value of Young's modulus (∼1100 GPa) [3], fracture strength (125 GPa) [3], thermal conductivity (∼5000 W/mK) [4], mobility of charge carriers (2 × 10 5 cm 2 /Vs) [5], and specific surface area 2630 (m 2 /g) [6].Owing to its excellent physical and chemical properties, graphene is considered as a potential candidate for large number of applications in many technological fields such as nanoelectronics [7], composites [8], energy storage devices [6], efficient lasers, photodetectors, and biomedical applications [9].
Preparation of high quality graphene is needed to study the unique properties for applications.A number of different ways of preparing graphene have been reported in the literature [10].This includes micromechanical exfoliation of graphite [11], chemical vapor deposition [12], and chemical methods to create colloidal suspension [13].The micromechanical method is time consuming, and the yield is low and thus limits our ability to have significant economic and technological impact [11].On the other hand, graphene is prepared via solution chemistry involving oxidation of graphite to prepare layered graphene oxide sheets (GOS), which can further be reduced to obtain graphene sheets (GS).GS can be considered as chemically modified sheets having controllable electronic properties.However, unless these sheets are well separated from each other, the graphene sheets tend to form agglomerates or even restack to form graphite due to Van der Waals interaction [14,15].Thus, the addition of any dispersoid which can bind tightly onto graphene sheets is of utmost necessity to achieve better dispersion of graphene sheets.In this context, decoration of graphene with metal nanoparticle becomes very important.Very recently, integration of graphene and metal nanoparticle to prepare new generation of hybrid nanomaterials has aroused extensive interest for large number of potential applications like chemical sensors, energy storage and catalysis, as well as hydrogen storage [16][17][18][19][20][21].Most notably, noble metal nanoparticles (NP) have widely been used as a catalyst to promote various chemical reactions [22].Catalyst support is found to be an important aspect affecting the catalytic activity of the catalyst [23].To enhance the catalytic activity, graphene-metal nanoparticle hybrid composite can be used due to its very high specific surface area.
In the light of recent studies carried out by different groups on decoration of NPs with graphene to prepare hybrid structure, we consider that it is important to investigate the influence of metal nanoparticles on the electronic structure of graphene.For this purpose, we have synthesized gold (Au-G), silver (Ag-G), and copper nanoparticle decoratedgraphene (Cu-G) via chemical synthesis.Almost all the previous works had generally dealt with synthesis of such hybrid structure [24,25].The nature of interaction between graphene and NPs is not properly studied.In this work, we have employed both Raman and UV-Vis spectroscopic techniques to obtain optical properties of the nanomaterials, and thereby electronic structures of the materials are brought about.), and Sodium hydroxide (NaOH) (Merck Millipore, India) were used for the preparation of graphene and nanocomposites.All the chemicals were used in the condition as received without further purification.The water used in all the experiments was purified through a Millipore system.

Preparation of Graphite Oxide.
Graphite oxide was prepared by modified Hummers method [26] by taking 2 g of natural graphite powder in a 250 mL beaker placed in an ice bath, maintaining the temperature at 20 ∘ C. Subsequently, 1 g of NaNO 3 , 46 mL of H 2 SO 4 , and 6 g of KMnO 4 were added slowly to the graphite powder and kept under stirring for 15 min.After that, the ice bath was removed, and the solution was heated at 35 ∘ C for 30 min.Then, 92 mL of distilled water was added slowly into the solution and stirred for further 30 min.Finally, 80 mL of hot distilled water was added, followed by the addition of 30% H 2 O 2 aqueous solution until the bubbling disappeared and the colour of the solution turned to deep yellow.The residue was collected after centrifugation, and warm water was utilized for subsequent washing to make it neutral (pH = 7).The resulting powder was redispersed into distilled water by ultrasonication for 30 min.The solution was freeze dried to get the desired brown coloured graphite oxide powder.

Exfoliation and Reduction of Graphene Oxide (GO).
The exfoliated GO was prepared by ultrasonicating 0.01 g of the synthesized graphite oxide powder in 100 mL of distilled water in a 250 mL round-bottom flask for 2 h.The pH of the so obtained yellow brown suspension of GO was adjusted to the value of 10 by dropwise addition of 315 L of 25% ammonia solution into it.For reduction of GO, 35 L of hydrazine hydrate was added slowly, and the whole suspension was refluxed in an oil bath at 90 ∘ C for 2 h while stirring.Finally, black coloured precipitate of graphene (G) obtained after centrifugation was redispersed in distilled water and kept as a stock solution for the synthesis of Ag-G, Au-G, and Cu-G.

Preparation of Ag-G, Au-G, and Cu-G. The synthesis of
Ag-G has been carried out according to the method described by Mao et al. [27].Equal volume (20 mL) of graphene stock solution and silver nitrate aqueous solution (0.001 M) were slowly mixed in a 250 mL round-bottom flask.The whole solution was heated to 90 ∘ C with subsequent addition of 2 mL sodium citrate (w/v 1%) aqueous solution into it while stirring for 15 min to complete the reaction.After cooling down to room temperature, the solution was centrifuged many times with ethanol and water until the undesired byproducts are removed, and the precipitate was redispersed in distilled water.Au-G was also prepared in similar fashion by mixing equal volume (20 mL) of graphene solution and HAuCl 4 aqueous solution (0.001 M) followed by the addition of 2 mL of 38.8 mM sodium citrate aqueous solution.Cu-G was prepared by the following method.2.4 g of dextrose was dissolved in 30 mL of water.60 L of 1.0 M NaOH solution and 1 mL of graphene stock solution were added to it.The whole mixture was heated to 80 ∘ C under stirring in argon atmosphere.1.2 mL of copper sulfate aqueous solution (0.01 M) was added to it.After five minutes of constant stirring at 80 ∘ C, Cu-G was obtained.

Characterizations
X-ray diffraction (XRD) of all the powder samples were recorded by a Bruker D8 focus X-ray diffractometer with Cu K  radiation ( = 0.154056 nm) operating at 40 KV and 20 mA.To investigate structural change during chemical processing, the Raman spectra were recorded with a WITec GmbH alpha 300 R Raman spectrometer in the back-scattering mode using an Ar-ion laser (514.5 nm) as an excitation source.The UV-Visible absorption spectra of aqueous solutions of GO, G, Ag-G, Au-G, and Cu-G were recorded in the range between 200 and 800 nm using a Jasco V-670 UV-Vis spectrophotometer.Scanning electron microscopic (SEM) and Transmission electron microscopic (TEM) micrographs of G, Ag-G, Au-G, and Cu-G were taken with a Carl Zeiss EVO 50 VP scanning electron microscope and FEI Tecnai G 2 U-twin 200 KV instrument, respectively, to observe the nanoscale microstructure.

Results and Discussions
In the following, we shall describe the results obtained during present investigation and discuss the results in the light of the available literature.

X-Ray Diffraction (XRD)
. X-ray diffraction patterns of graphene oxide (GO), graphene (G), and graphene silver (Ag-G) nanoparticles hybrid materials are presented in Figure 1.XRD pattern of GO shows a sharp diffraction peak at 2 = 14.03 ∘ corresponding to (001) of GO.The XRD pattern of G shows two distinct peaks at 2 = 24 ∘ and 44.8 ∘ , which are attributed to (002) and (100) reflections of graphene nanosheets.The calculated d-spacing of graphene using broad peak is 0.364 nm.The broadness of the peak signifies decrease in the crystallinity of graphene upon reduction from GO.On the other hand, XRD pattern of Ag-G reveals the peaks due to (111), ( 200), (220), and (311) of Ag nanocrystals as well as broad peak (002) due to graphene.Similar observation has been made for Au-G and Cu-G nanohybrids.No peaks corresponding to oxides of Ag, Au, or Cu are observed within the detectable limit of XRD.The peaks due to metal nanocrystals are broad, indicating nanocrystalline size and strain due to incorporation within graphene nanosheet.The position of (002) peak of graphene does not change for different metal hybrids.Using the peak position of graphene, the calculated d-spacing of Ag-G is 0.370 nm, whereas d 002 of graphite is 0.335 nm.The broadness of the peak is the indication of the presence of graphene nanosheets with varying d-spacing values.

Scanning (SEM) and Transmission Electron (TEM)
Microscopic Observation.The detailed microstructural characterization of graphene and graphene-metal nanoparticle hybrids has been carried out using SEM and TEM. Figure 2(a) shows high resolution SEM micrograph of graphene and reveals crumbled and scrolled morphology of graphene sheets.Thin sheet of graphene crumbled into different layers is clearly visible in the Figure 2(a).Figures 2(b), 2(c), and 2(d) are the SEM images of graphene-metal hybrids, that is, Ag-G, Au-G, and Cu-G.The figure shows that the metal nanoparticles appear as discrete bright dots, which are homogeneously distributed on the surface of the graphene sheets.The graphene and graphene-metal nanoparticle hybrids are characterized directly under SEM without coating by gold film, suggesting their conductive nature.The detailed morphology and structure of the graphene-metal nanoparticle hybrids have been characterized by TEM.It is to be noted that TEM samples were prepared by dropping the sonicated dispersion of sample in ultrapure methanol onto 600 mesh copper grids coated with lacy carbon and dried at room temperature for 3 hours.As observed in Figure 3(a), graphene nanosheets are randomly compact and stacked, revealing uniform laminar morphology such as crumbled silk veil waves.The selected area diffraction pattern (SADP) obtained from such nanosheets is shown as inset in Figure 3(a).The SADP reveals two diffraction rings, which can be indexed using (100) and (002) of graphene.Therefore, the formation of graphene by the reduction of graphene oxide is confirmed from the SADP pattern.The sharp diffraction rings can be indexed as those of Ag, while diffraction rings corresponding to (100) and (002) of graphene can also be deciphered.

Optical Property Characterization.
In the following, we shall describe and discuss the results obtained during optical property characterization of the graphene as well as graphene-metal nanoparticle hybrids.The optical properties of the graphene and hybrids are obtained using Raman and UV-Vis spectroscopy.From the earlier section, it is clear that the hybrids contain uniform distribution of Ag, Au, and Cu nanoparticles embedded in the graphene matrix.Therefore, it is worth to look into the optical properties of these hybrids.The discussion will be done by comparing the optical properties of monolithic graphene sheets with the hybrids.to in-plane stretching of sp 2 -bonded carbon atoms in the hexagonal lattice, whereas G band is due to small graphitic domains because of vibrations of sp 3 -bonded carbon atoms [28].The 2D band originates due to second order two phonon process.Table 1 shows the detailed value of different bands in GO and graphene.For GO, D band is located at 1346 cm −1 , whereas G and 2D bands are located at 1590 cm −1 and 2697 cm −1 , respectively.The reduction of GO to graphene shows significant red shift in position of G (1579 cm −1 ), D band (1340 cm −1 ), and 2D band (2677 cm −1 ) (Figure 4(a)).

Raman Spectroscopy.
The red shift of G band in graphene is mainly due to the restoration of conjugated double bonds and the increase in the number of sp 2 -bonded carbon atoms in the graphene during the reduction of GO.Table 1 also reports the ratio of the intensities of D and G bands ( D / G ) in GO (1.01) and graphene (1.09).It is to be noted that the ratio ( D / G ) is a measure of disordered carbon and normally expresses (sp 2 /sp 3 ) carbon ratio [29].It can be observed that  D / G value increases from GO to graphene.The increase in the value indicates the relative increase of sp 2 domains and decrease of average crystallite size [30].The ratio is found to be inversely proportional to the crystallite size (La): Using  laser = 514.5 nm, the calculated crystallite size of graphene is 18 nm.Let us now discuss the case of graphene-metal nanoparticle hybrids.Figure 4(b) reveals Raman spectra of Ag-G, Au-G, and Cu-G hybrids.Table 1 shows detailed results of the positions of different vibrational bands as well as  D / G values.The nature of the peaks of different bands, that is, D, G, 2D, and D + G bands, remains almost the same as that of monolithic graphene.
As indicated, G band is usually assigned to E 2g phonons of sp 2 -bonded carbon atoms, whereas D band is assigned to breathing mode of k-phonon of A 1g symmetry.The position of G band does not change (Table 1) after the incorporation of metal nanoparticles.However, nature of shift is distinctly different in case of Cu-G as compared to Ag-G and Au-G.In case of Cu-G, the G band has undergone slight red shift (1578 cm −1 ) as compared to graphene (1579 cm −1 ).This indicates that the vibrational nature of small graphitic domain (sp 3 -bonded carbon) has not been affected much by the presence of the metal nanoparticles.On the other hand, the position of D band in the hybrids has undergone blue shift as compared to graphene.The major shift has occurred in case of Cu-G (1351 cm −1 ) indicating that the vibrational energy due to in-plane stretching of sp 2 -bonded carbon is increased due to incorporation of metal nanoparticles.We believe that this is due to occupation of Cu atoms in specific position in the graphene structure, that is, in the hexagonal cage.These shifts in the position of G band may also be related to the position of nanoparticles that occupy the sites on the surface of graphene.As Cu has smaller atomic radius as compared to Ag and Au, we believe that Cu atoms may occupy the hexagonal cage of graphene, while Ag and Au occupy other adsorption sites T, (top on carbon atom) or B (bridge between two carbons).On the other hand, the shift in the position of D band is significantly larger in case of Cu-G as compared to that of Ag-G and Au-G.This may be due to larger distortion generated by occupation of Cu atoms in hexagonal cage of graphene than that generated by occupation of Ag and Au atoms on T or B site.Another important parameter characterizing the Raman spectra of hybrid is  D / G ratio.This ratio has been found to increase from 1.09 to 1.12 in case of Ag-G, whereas it decreases in case of Au-G (1.05) and 0.89.The enhancement in the ratio indicates decrease in the site of in-plane sp 2 domain and partially ordered crystal structure of graphene nanosheets (GNs).On the other hand, the significant decrease in  D / G ratio for Cu-G indicates that the new graphitic domains are more in number but smaller in size, formed upon reduction of GO to Cu-G.Another important parameter of Raman spectroscopy is full width at half maxima (FWHM) of the D and G bands.Table 2 shows the results of GO, G, and graphenemetal nanoparticle hybrids.It is clearly observed that FWHM decreases substantially for Cu-G, whereas it does not change  is UV-Vis spectroscopy.Figures 5 and 6 summarize the UV-Vis spectroscopic observations in the present investigation.Detailed results are summarized in Table 3. Figure 5 reveals UV-Vis absorption spectra of graphene oxide (GO), graphene (G), graphene-metal nanoparticle hybrids, Ag-G, Au-G, and Cu-G.For GO, two characteristic peaks are observed in UV-Vis spectrum, a maximum at 230 nm, due to  →  * transition of aromatic C-C bond and a shoulder at 303 nm, from n →  * transition of C=O bond [31].The original electronic conjugation is restored during reduction of GO to G by hydrazine hydrate, and a characteristic absorption peak of G has been observed at 266 nm, whereas the shoulder at 303 nm disappears.The incorporation of Ag NPs, in the G framework (Ag-G) is confirmed by the presence of absorption peak at 420 nm, which is considered to be due to surface plasmon resonance of Ag NPs.The bare Ag NPs derived from the reduction of Ag + by hydrazine hydrate under same condition is characterized by an absorption peak at 430 nm, which is consistent with previous studies [32,33].Clearly the incorporation of Ag NPs onto the stable aqueous graphene dispersion leads to blue shift of the surface plasmon resonance characteristics.It also has been observed that the absorption peak position of graphene for Ag-G hybrids changes to 260 nm, a blue shift of 6 nm as compared to aqueous graphene dispersion.The blue shift can be the result of charge transfer interaction between graphene and Ag NPs and has been confirmed by enhancement of Raman signals (Figure 4(b)).Similar measurements have been carried out for Au-G and Cu-G hybrids.The absorption peak of graphene in the Au-G is also blue shifted, by 4 nm.Therefore, charge transfer interaction between graphene and Au is poor as compared to Ag-G.The similar observation has been made for absorption peak of graphene in case of Cu-G.Table 3 also reveals that the surface plasmon resonance of Cu NPs is affected strongly by graphene nanosheets as plasmon resonance has red shifted by 65 nm.On the other hand, the surface plasmon resonance of Au NPs has undergone red shift only by 4 nm as compared to free Au NPs.Therefore, the behavior of metal NPs can be modified extensively by incorporating them into the graphene nanosheets.Obviously these interactions depend on the position of these NPs, where they occupy the hexagonal graphene framework.This will be investigated further.

Summary
The present investigation has categorically shown that it is possible to synthesize graphene-metal nanoparticle hybrids in aqueous media by chemical rule.The following conclusion can be drawn from the study.(c) The surface plasmon resonance of the metal nanoparticles can be altered by incorporation of nanoparticles in the graphene nanosheets.
(d) The electronic interaction between the metal nanoparticle and graphene depends on the type of metal as well as size of the atom.
Figures 3(b), 3(c), and 3(d) reveal graphenemetal nanoparticles hybrid composite with Ag, Au, and Cu used as nanoparticles to decorate the graphene nanosheets.Figure 3(b) shows scrolled morphology of graphene sheets with distributed Ag nanoparticles, embedded within the graphene sheets.The multiple stacking of graphene consisting of uniformly distributed Ag nanoparticles with average size of 25 ± 5 nm is distinctly observed.The histogram (as shown in the bottom inset) corroborates the narrow size distribution with maximum number of particles around 25 nm along with few larger particles of about 30 nm.The SADP obtained from the hybrid nanocomposite is shown as inset in Figure 3(b).

Figure 3 (
c) shows the results of the TEM investigation on graphene-Au hybrid prepared by chemical synthesis as outlined in Section 2.4.The bright field micrograph (Figure 3(c)) shows uniform distribution of Au nanocrystals (appeared as dark dots) in the crumbled nanosheets of graphene.It is clearly visible that highly dispersed Au nanoparticles may provide large available surface area and enhance activities towards different reactions.The distribution of Au nanoparticles embedded in the graphene is shown as bottom inset of Figure 3(c).The average size of the particle is 8 ± 4 nm.The histogram indicates broad size distribution.The SADP pattern obtained from the hybrid is shown as upper inset of Figure 3(c).It

Figure 4 (
a) shows the Raman spectra of GO and graphene.The spectra reveal the presence of characteristics D (defect) and G (graphite) bands, 2D band, and (D + G) band.It is to be noted that D band arises due

Figure 3 :
Figure 3: TEM micrographs and corresponding SAED patterns of (a) G, (b) Ag-G, (c) Au-G, and (d) Cu-G.The upper inset in each figure shows SAD pattern, whereas lower inset in figure (b) and (d) shows nanoparticle size distribution and in figure (c) shows low magnification bright field micrograph, revealing graphene sheets decorated with nanoparticles.

Figure 4 :
Figure 4: Raman spectra of (a) GO and G (b) Ag-G, Au-G, and Cu-G.

Table 1 :
Comparison of D, G, 2D, and (D + G) band positions and  D / G value of GO, G, Ag-G Au-G, and Cu-G.
(a) Fine nanoparticles (Ag, Au, and Cu) decorated graphene can be prepared by chemical synthesis.The size of the nanoparticles varies from 5 to 35 nm.(b) The G and D bands can be modified more extensively by Cu than Ag or Au.

Table 2 :
Comparison of FWHM of D and G band of GO, G, Ag-G, Au-G, and Cu-G.