Green synthesis of silver-graphene nanocomposite-based transparent conducting film

Physica E 90 (2017) 76–84

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Physica E journal homepage: www.elsevier.com/locate/physe

Green synthesis of silver-graphene nanocomposite-based transparent conducting film Pankaj Chamolia, Malay K. Dasb, Kamal K. Kara,b, a b

MARK

⁎

Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India

A R T I C L E I N F O

A BS T RAC T

Keywords: Graphene Silver Nanocomposite Reducing agent Sheet resistance Transmittance

In the present work, silver nanoparticles (Ag NPs)/graphene nanocomposite has been synthesized successfully by simple solvothermal method via green route. Citric acid is used as green reducing agent for the reduction of graphene oxide (GO) and Ag ions. Silver nitrate is used as a precursor material for Ag NPs. As synthesized Ag NPs/graphene nanocomposite has been characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infra-red spectroscopy, UV–vis spectroscopy, thermal gravimetric analysis, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. Experimental results confirm the reduction of GO and the successful formation of Ag NPs decorated graphene nanosheets. In addition, spray coating technique is employed for the fabrication of transparent conducting films. Enhancement in the optoelectrical signatures has been achieved using thermal graphitization of fabricated films. Thermal graphitization at 800 °C for 1 h marks the best performance of fabricated film with sheet resistance of ~3.4 kΩ/□ and transmittance (550 nm) of ~66.40%, respectively.

1. Introduction Transparent conducting films (TCFs) are an essential component of modern optoelectronic devices such as touch screen panels (TSPs), liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs) [1]. Indium tin oxide (ITO) is commercial material for TCFs having sheet resistance less than 100 Ω/□ along with high transparency ~90% (at 550 nm wavelength). ITO shows DC conductivity to optical conductivity (σDC/σOP) ratio more than 35, and frequently utilized in the above mentioned applications. However, alternative materials (e.g. doped metal oxides, single-walled carbon nanotubes (CNTs), metal nanowires (NWs), printable metal grids and nanocomposites, etc.) have been searched extensively to fulfill the huge commercial demand due to the less availability of indium, high brittleness and high refractive index of ITO film [2,3]. Since the discovery, graphene: a one atom thick planar sheet of sp2 hybridized carbon atoms in hexagonal lattice, has attracted scientific community worldwide by owing extraordinary properties including high optical transparency ~97.7%, high room temperature mobility ~2×105 cm2 V−1 s−1, high Young's modulus ∼1 TPa, excellent fracture strength ∼130 GPa, and enormous thermal conductivity ∼5000 W m−1 K−1. Thus, it is a wonder material for variety of applica-

tions in the modern optoelectronic devices [4–6]. So far, various methods have been adopted for the synthesis of graphene nanosheets (GNs) such as chemical vapor deposition (CVD), arc discharge, and wet chemical routes, etc. [7,8]. Among these methods, wet chemical routes are more versatile and cost-effective way to produce GNs at large scale, followed by chemical reduction of graphene oxide (GO) in the presence of reducing agents (e.g. hydrazine and its derivative, etc.). These reducing agents are toxic/explosive in nature and have long lasting environmental impacts. Hence, green methodologies are preferable to produce GNs during reduction of GO. In contrast, restacking of graphene layers due to the van der Waals interaction becomes main drawback of wet chemical routes, suppress inherent properties of GNs, and limits their applications. Moreover, combination of GNs with different nanomaterials (e.g. nano metal oxide, nanorods, and metal girds, etc.) provide surface modification as well as restricts restacking of graphene layers [9]. Therefore, silver nanostructures (Ag Ns) combined with GNs have been explored for various applications including galvanic replacement reaction, optoelectrical properties, surface-enhanced Raman scattering (SERS), antibacterial activity and TCFs [10–12]. Particularly, TCFs based on Ag Ns/graphene (G) have been reported using various techniques by different groups. For example, Lee et al. have fabricated silver nanowires (Ag NWs)/G based

⁎ Correspondence to: Indian Institute of Technology, Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering and Materials Science Programme, Kanpur 208016, India. E-mail address: [email protected] (K.K. Kar).

http://dx.doi.org/10.1016/j.physe.2017.03.015 Received 6 October 2016; Received in revised form 17 March 2017; Accepted 20 March 2017 Available online 22 March 2017 1386-9477/ © 2017 Elsevier B.V. All rights reserved.

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2. Experimental

TCFs, which exhibits optical transmittance (T) ~88.6% at 550 nm and sheet resistance (Rs) ~19.9 Ω/□ [13]. Whereas, Hsiao et al. have fabricated G/Ag NWs hybrid nanomaterial based TCFs with Rs ~71 Ω/□ and T ~85% [14]. Besides Ag NWs, silver nanoparticles (Ag NPs) are being used as top electrode in the modern optoelectronic devices mainly due to their high electrical conductivity combined with the excellent light-trapping and anti-reflecting properties. Thus, Ag NPs together with GNs as a hybrid composite film shows excellent optoelectrical signatures. For example, Zhou et al. have fabricated Ag NPs/G multilayer film by electrostatic self-assembly process, and found Rs ~97 kΩ/□, T ~86.3% [15]. However, rGO/Ag NPs film has been fabricated using high Ag (~0.6%) concentration by Zhou et al., and film shows Rs ~8.3 kΩ/□ and T~89.2%, respectively [16]. Moreover, different process have been explored to fabricate GNs based TCFs such as Langmuir–Blodgett deposition [17], transfer printing [18], spin coating [19], dip coating [20], vacuum filtration [21] and spray coating [22,23]. Most importantly, poor dispersion of rGO/GNs in aqueous medium limits their use in TCFs fabrication [24]. Organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), and N,N-dimethylformamide (DMF), etc. create homogenous dispersion with rGO/GNs, and mostly contains monolayers of rGO/GNs. Such homogenous dispersion are preferred, and coated directly over transparent substrate to fabricate better quality TCFs [25]. Additionally, thermal graphitization increases the crosslinking between sp2 C-C bonds of small grain size GNs, which enhances the optoelectrical properties of TCFs [26]. Among available techniques, spray coating is the most efficient way to produce uniform TCFs of small grain size GNs [27]. Herein, Ag NPs/G nanocomposite has been successfully synthesized by solvothermal method using citric acid as green reducing agent for GO and AgNO3 as precursor material for Ag NPs. Experimental results reveal the removal of oxygen species from GO by citric acid, and confirms the formation of Ag NPs /G nanocomposites. In addition, TCFs of Ag NPs/G nanocomposite have been fabricated by spray coating technique. Measurements of optoelectrical signatures have been carried out after thermal graphitization of fabricated TCFs. The schematic of various steps involved in the fabrication process of Ag NPs/G based TCFs is shown in Fig. 1.

2.1. Materials Graphite powder (Loba Chemie Pvt. Ltd., Mumbai, India) was used as graphite source for synthesis of GO. In addition, sulfuric acid (H2SO4, 98%), citric acid (C6H8O7), hydrogen peroxide (H2O2, 30%), silver nitrate (AgNO3), potassium permanganate (KMnO4), sodium nitrate (NaNO3), and N,N-Dimethylformamide (C3H7NO) were received from Qualigens Fine Chemicals, Mumbai, India. All chemicals were analytical grade and used without further purification. 2.2. Preparation of Ag NPs/G nanocomposite Firstly, GO was prepared by modified Hummer's method as reported elsewhere [28]. Then, as synthesized GO (500 mg) was mixed in the 100 ml of DI water, and sonicated for 3 h to get homogenous dispersion. Thereafter, 0.5 M AgNO3 and 0.1 M C6H8O7 were mixed into GO suspension, and stirred rigorously for 1 h. Further, formed blackish dispersion was transferred to 100 ml N2 purge teflon lined autoclave at 120 °C for 3 h. Finally, obtained grayish solution was filtered by 0.2 µm membrane filter, and washed several times with DI water to remove residual impurities; and dry at 80 °C for overnight in vacuum oven. Final product was dry powder of Ag NPs /G nanocomposite. For comparison GNs have been synthesized in the same condition (GO 500 mg in the 100 ml DI with 0.1 M C6H8O7). 2.3. Synthesis of thin nanocomposite film Spray coating was used to deposit TCFs of Ag NPs/G nanocomposite [29]. Ag NPs/G nanocomposite (two samples having concentrations of 1 and 3 mg/ml) was mixed in 10 ml DMF, sonicated for 1 h, and stirred for 8 h at 80 °C. The resultant dispersion was allowed to cool at room temperature, and centrifuged at 3000 rpm for 0.25 h. Then, top 5 ml of Ag NPs/G/DMF dispersion was sprayed onto a 25×25 mm2 preheated quartz substrates and named as AGF-1, AGF-2 for 1 and 3 mg/ml, respectively. Before coating, quartz substrates were ultrasonically cleaned with DI water, acetone subsequently and dried in vacuum oven. Then, quartz substrates dipped into piranha solution

Fig. 1. Schematic of various steps involved in the fabrication process of Ag NPs/G based TCFs.

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to (001) plane. This strongly indicates that the harsh oxidation of graphite powder has been carried out successfully, and oxygen functionalities are attached to carbon skeleton during the synthesis of GO by Hummers method as shown in Fig. 2(b) [30]. The XRD pattern of GNs shows that the GO peak is vanished and new board peak appears at 2θ of 25.30° (dspacing~3.50 Å), corresponds to (001) plane. This clearly shows the removal of oxygen functionalities form GO, lead to form GNs as shown in Fig. 2(c). Moreover, XRD pattern of Ag NPs/G nanocomposite exhibits three well defined diffraction peaks at 2θ of 38.16, 44.32 and 64.47° which can be assigned to the face centered cubic (fcc) structure of metallic silver, and corresponds to reflections at (111), (200) and (220), respectively as shown in Fig. 2(d). Meanwhile, GO peak disappears completely in Ag NPs/G nanocomposite, which confirms the reduction of oxygen species from GO sheets by citric acid. In addition, the lattice parameters of metallic silver (fcc structure) have been calculated from XRD pattern, and found a=b=c=4.087 Å, shows well agreement with literature value for lattice parameter ~4.086 Å. The corresponding dspacing have been calculated d111~2.36 Å, d200~2.05 Å and d220~1.44 Å, respectively. In addition, a less intense broad peak has been found at 2θ of 25.04° (dspacing~3.53 Å) of honeycomb network [31]. The all peaks of silver structure (fcc) have been well matched with JCPDS File No. 04-0783. The well-defined sharp peaks of silver and honeycomb network in XRD pattern confirm the successful formation of Ag NPs/G nanocomposite. Fig. 3 shows the Raman spectra of GO and Ag NPs/G nanocomposite. Raman spectrum of GO exhibits two characteristic peaks of carbon skeleton corresponds to D and G band at 1341 and 1611 cm−1, respectively. The D band provides the information of the disordered structure of graphite (breathing mode of κ-point), and the G band relates to the in-plane vibration of the carbon atoms, arises due to the doubly degenerate Zone centre E2g mode of graphite as shown in Fig. 3(a) [32]. In addition, the ratio of relative intensity of the Raman D and G bands (ID/IG) is found ~1.12. Meanwhile, ID/IG ratio is found ~1.20, confirms the reduction of oxygen functionalities from GO into GNs as shown in Fig. 3(b). Moreover, Raman spectrum of Ag NPs/G shows D and G band at 1347 and 1616 cm−1, respectively. This red shifting in spectral features (D and G band) is observed due to the removal of oxygen species from GO as shown in Fig. 3(c). Meanwhile, significant enhancement in D and G band intensities has been observed due to the decoration of Ag NPs on the surface of GNs, which signifies the surface-enhanced Raman scattering (SERS) activity as shown in Fig. 3(Inset). Thus, ID/IG ratio is decreased from 1.12 (GO) to 1.01. Additionally, new bands at 931 and 1110 cm−1 are observed, which reveal the Ag traces. The SERS activity may be attributed due to a short-range interaction between Ag NPs and GO sheets [33].

(3:1) to make quartz surface more adhesive. An airbrush system (nozzle diameter of 0.2 mm) with N2 as a carrier gas was used for coating in single action mode with 2 bar inlet pressure of N2 on to preheated quartz substrate ( > 200 °C). The distance between the tip of the nozzle and the quartz substrate were kept 12 cm. The spraying rate was 2 ml/min. The deposited films were dried in the vacuum oven for 3 h at 140 °C. Thereafter, thermal graphitization was carried out at 500 °C for 3 h and 800 °C for 1 h in 10−3 Torr vacuum. 2.4. Characterizations The crystalline phase of graphite powder, GO and Ag NPs/G nanocomposite were recorded with X-ray diffraction (XRD) patterns with X′Pert Powder PANalytical, Advanced X-Ray Diffractometer. Raman spectra were recorded by Raman microscope, Horiba scientific with λ–633 nm laser to characterize GO and Ag NPs/G nanocomposite. FTIR and UV–vis spectroscopy were carried out by PerkinElmer spectrum and PerkinElmer Lambda 1050 to find bond stretching and characteristic absorption, respectively of GO and Ag NPs/G nanocomposite. TGA of GO and Ag NPs/G nanocomposite were carried out in N2 atmosphere at a heating rate of 10 °C/min by Perkin-Elmer Diamond TG/DT analyzer. The morphology of GO and Ag NPs /G nanocomposite were examined by field scanning electron microscopy (FESEM) from JEOL JSM-7100F. X-ray photoelectron spectroscopy (XPS) measurements of GO and Ag NPs/G nanocomposite were carried out by using a Multifunctional XPS (ULVAC, PHI500 VersaProbe II with 1486.6 eV Kα Al X-ray source) to test the surface composition. Electrical properties of fabricated Ag NPs/G TCFs were measured by four probe setup with PID controlled oven (Scientific Equipment Roorkee, India) attached with current and voltage sources of Keithley 6221, Keithley 2182 A respectively. Optical properties of Ag NPs/G based TCFs were obtained in visible range by PerkinElmer Lambda 1050. The film thickness (t) and average roughness (Rz) were measured by BRUKER (Contour GTK0X-14–150) optical profilometer. 3. Results and discussion 3.1. Structural characteristics Fig. 2 shows the XRD pattern of graphite powder, GO and Ag NPs/ G nanocomposite. XRD pattern of graphite powder exhibits sharp intense peak (002) at 2θ of 26.49° (dspacing~3.36 Å), and denotes the hexagonal crystalline structure of graphite as shown in Fig. 2(a). The XRD pattern of GO shows that the graphite crystalline peak is vanished and new peak appears at 2θ of 12.13° (dspacing~7.28 Å), corresponds

Fig. 3. Raman spectra of (a) GO, (b) GNs and (c) Ag NPs/G nanocomposite; Inset: Raman intensity of GNs and Ag NPs/G.

Fig. 2. XRD pattern of (a) graphite (b) GO (c) GNs and (d) Ag NPs/G nanocomposite.

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Fig. 4. FTIR spectra of (a) GO (b) GNs and (c) Ag NPs/G nanocomposite. Fig. 6. TGA analysis of (a) GO and (b) Ag NPs/G nanocomposite.

Fig. 4 shows the FTIR spectra of GO and Ag NPs/G nanocomposite. FTIR spectrum of GO exhibits the stretching vibration peaks at 3410, 1717, 1569, 1214 and 1040 cm−1, are assigned to O-H stretching vibration, C˭O stretching vibration, C˭C skeletal vibration of graphitic domain, O-H deformations of the C-OH groups, and C-O stretching vibrations, respectively. In addition, the stretching vibration peaks at 2913 and 2825 cm−1 are assigned to the C-H stretching vibrations as shown in Fig. 4(a). Meanwhile, FTIR spectrum of GNs shows that the stretching vibration peaks at 3410, 1040, 1214, and 1717 cm−1 have suppressed/flattened, strongly indicates that the reduction oxygen spices from GO shown in Fig. 4(b). Moreover, FTIR spectrum of Ag NPs/G shows that the stretching vibration peaks at 1040, 1214, and 1717 cm−1 are suppressed, strongly suggests the removal of oxygen spices from GO. Meanwhile, the stretching vibration peak at 1569 cm−1 becomes stronger. This attributes the restoration of the honeycomb structure as shown in Fig. 4(c) [34]. Therefore, FTIR spectrum of Ag NPs/G confirms that the GO has been successfully exfoliated and reduced into GNs, and strong interactions between Ag NPs and hydroxyl groups facilitates to remain on the GNs surface. Fig. 5 shows UV–visible spectra of GO and Ag NPs/G nanocomposite. UV spectrum of GO exhibits 230 nm band absorption, ascribes a characteristic feature of π–π* transition of C–C bond. The shoulder at 305 nm is attributed to n–π* transition of C-O bond as shown in Fig. 5(a). UV–visible spectrum of GNs exhibits red shifted band absorption at 271 nm, strongly indicates the change in electronic configuration of honeycomb lattice as shown Fig. 5(b). Meanwhile, UV–visible spectrum of Ag NPs/G exhibits red shifted band absorption

at 271 nm as shown Fig. 5(c). The red shifting is mainly due to the change in electronic configuration of GNs compared to GO. Whereas, 305 nm shoulder is absent, which also indicates the reduction of GO into GNs [35]. Additionally, a new absorption band appears at 410 nm, which is attributed to the presence of Ag NPs. Thermal stability of GO and Ag NPs/G nanocomposite have been assessed by TGA and shown in Fig. 6. The TGA trace of GO shows that the weight loss ~11.42 wt% occurs below 120 °C. This weight loss is mainly due to the absorbed water molecules in the GO galleries. Above 120 °C, weight loss starts at 160 °C, and occurs ~14.35 wt%. This weight loss is due to the decomposition of oxygen-containing functional groups attached during oxidation of graphite, which yields CO, CO2 and steam. The total weight loss ~42.83 wt% is noticed, which indicates the lower thermal stability of GO as shown Fig. 6(a). However, TGA plot of Ag NPs/G shows less weight loss (~0.4%) below 120 °C, attributes to the evaporation of absorbed water molecules. Above 120 °C, the weight loss starts at 160 °C, and ~4.33 wt%. This is assigned to the decomposition of oxygen functional groups attached to the GNs as shown in Fig. 6(b). In comparison to GO, Ag NPs/G nanocomposite shows reduced weight loss. The total weight loss is found ~10.30%, which confirms that the Ag NPs/G nanocomposite is more thermally stable. This is happened due to the removal of the thermally labile oxygen functional groups, which are reduced by citric acid from GO to produce GNs, which increases the thermal stability of nanocomposite [36]. Surface morphological analysis of GO and Ag NPs/G nanocomposite has been carried out by FESEM and shown in Fig. 7. Fig. 7(a) shows the FESEM image of GO, clearly depicts the restacked, aggregated arrangement of oxygenated GNs. These sheets resemble plateletlike, porous and three-dimensional interlinked large number of GNs. Moreover, FESEM image of Ag NPs/G nanocomposite shows the Ag NPs anchored uniformly and densely distributed over both side of GNs, and prevents aggression of GNs as shown in Fig. 7(b). XPS analysis has been carried out to investigate the surface composition of GO and Ag NPs/G nanocomposite as shown in Fig. 8. Fig. 8(a) shows XPS survey of GO and Ag NPs/G nanocomposite. XPS survey of GO shows the harsh oxidation of graphite by Hummers method. In addition, XPS survey of Ag NPs/G nanocomposite reveals the formation of Ag NPs over the carbon skeleton and C/O ratio is increased (~3.01) as compared to GO (~2.23), confirms the removal of oxygen functional groups from GO and reduced into GNs. Fig. 8(b) shows high resolution C1s XPS spectrum of GO with Shirley background. The spectrum clearly indicates harsh oxidation of graphite. The four carbon atomic configurations of different functional groups have been found at binding energies ~284.6, 286.6, 287.1, and 288.5 eV, attributes to C-C, C-O, C˭O and O-C˭O, respectively [37]. After reduction of GO by citric acid, C1s spectrum of Ag NPs/G nanocom-

Fig. 5. UV–Vis spectra of (a) GO and (b) GNs (c) Ag NPs/G nanocomposite.

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Fig. 7. FESEM morphology of (a) GO and (b) Ag NPs /G nanocomposite.

3.2. Optoelectronic characteristics

posite shows substantial decrease in the peak intensities of oxygencontaining functional groups compared to the C1s spectrum of GO. This indicates the removal of the hydroxyl, epoxyl and carboxyl functional groups from GO to produce GNs as shown in Fig. 8(c). Additionally, high resolution XPS spectrum of Ag3d exhibits two sharp peaks at the binding energies ~367.8, 373.8 eV, which are identified for Ag electrons doublet 3d5/2 and 3d3/2, respectively, as shown in Fig. 8(d) [38].

TCFs of Ag NPs/G nanocomposite have been deposited by simple spray coating technique on quartz substrates. The optoelectrical properties of deposited films (AGF-1 and AGF-2) have been measured after thermal graphitization as shown in Fig. 9. The photograph of fabricated films at 500 °C (3 h) is shown in the inset of Fig. 9(a). Fig. 9(a) shows the T (%) of deposited films. The average transmittance

Fig. 8. XPS analysis: (a) XPS survey of GO and Ag NPs/G nanocomposite, (b) high resolution spectrum C1s of GO, (c) high resolution spectrum C1s of Ag NPs/G nanocomposite and (d) high resolution spectrum of Ag3d.

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Fig. 9. Optoelectrical signatures of deposited films (A) transmittance of AGF-1 and AGF-2, Inset: photograph of films (B) temperature dependent sheet resistance behavior of AGF-1 and AGF-2, annealed at 500 °C; (C) transmittance of AGF-1 and AGF-2, Inset: photograph of films (D) temperature dependent sheet resistance behavior of AGF-1 and AGF-2, annealed at 800 °C.

films is tested by XRD and shown in Fig. 10(a). XRD pattern shows dominant peak, which corresponds to the graphite phase (002) at 2θ of 25.15°(dspacing~3.522 Å) for AGF-1, 25.21°( dspacing~3.514 Å) for AGF2, respectively when films are annealed at 500 °C for 3 h as shown in Fig. 10(a). In addition, three less intense diffraction peaks are found at 2θ of 38.13° (111), 44.39° (200) and 64.42° (220); attributed to the face centered cubic (fcc) structure of metallic silver [16]. After the annealing at 800 °C for 1 h, the dominant peak of graphite phase (002) is shifted towards higher degree; and observed at 2θ of 25.99° (dspacing~3.410 Å), 25.80° (dspacing~3.435 Å) for AGF-1 and AGF-2, respectively as shown in Fig. 10(a). This is mainly due to the restoration of sp2 bonds thus improves the quality of graphene film. Furthermore, Raman analysis of films has been carried out and shown in Fig. 10(b). Raman spectrum exhibits the prominent spectral features of carbon skeleton such as D band at 1331, 1338 cm−1 and G bands at 1606, 1601 cm−1 for AGF-1 and AGF-2, respectively (when films are annealed at 500 °C for 3 h) as shown in Fig. 10(b). In addition, the ID/ IG ratio has been found ~0.98, 0.99 for AGF-1 and AGF-2, respectively. Moreover, the red shifting has been observed in the prominent spectral features of carbon skeleton after annealing (800 °C for 1 h), and found D bands at 1339, 1344 cm−1 and G bands at 1614, 1607 cm−1 for AGF1 and AGF-2, respectively as shown in Fig. 10(b). Whereas, the ID/IG ratio is decreased and found ~0.94, 0.96 for AGF-1 and AGF-2, respectively. This strongly indicates that the sp2 phase restoration occurs thereby reducing the defect quality due to the decomposition of oxygen functionalities [40]. Thus, structural studies of the films reveal that the increased annealing temperature improves the quality of films due to the restoration in the sp2-carbon network and the possibility of

(Tav) has been found ~66.69, 26.23% for AGF-1 and AGF-2, respectively in the visible light region (380–780 nm). Whereas, the T at 550 nm wavelength has been found ~61.16, 16.43% for AGF-1 and AGF-2, respectively. Then, Rs of deposited films has been measured and found ~8.01, 1.04 kΩ/□ for AGF-1 and AGF-2, respectively at room temperature. In addition, temperature dependent behavior of Rs has been investigated over the range of 25–180 °C, using the hot air oven attached with the current-voltage source as shown in Fig. 9(b). Temperature dependent behavior reveals that the Rs value decreases with temperature. This signifies that the temperature dependent electron transport takes place in honeycomb lattice due to the increased ionization of impurity states and/or the increased charge carrier mobility at high temperature, enhances conductivity of deposited films [39]. To enhance the optoelectrical properties of deposited films, thermal graphitization has been further carried out at 800 °C for 1 h. The photograph of annealed films has been shown in Fig. 9(c: Inset). It has been observed that the Tav is increased and found ~70.25, 22.23% for AGF-1 and AGF-2, respectively. Meanwhile, T at 550 nm wavelength is found ~66.40, ~19.88% for AGF-1 and AGF-2, respectively as shown in Fig. 9(c). It has also been found that the Rs of annealed films decreases, and found ~3.40, 0.81 kΩ/□ for AGF-1 and AGF-2, respectively. Afterwards, temperature dependent Rs behavior of annealed films is investigated. It is found that Rs of annealed films also decreases with temperature as shown in Fig. 9(d). Structural studies of AGF-1 and AGF-2 have been examined to determine the influence of thermal graphitization on sp2 domains, and shown in Fig. 10. The crystalline phase of Ag NPs/G nanocomposite 81

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Fig. 10. Structural study of AGF-1 and AGF-2 films at different annealing temperature (A) XRD analysis (B) Raman analysis. Table 1 Performance evaluation of deposited films. Film name

AGF-1 AGF-2

t* nm

174 438

Annealed film @500 °C for 3 h

Annealed film @800 °C for 1 h

Rs* kΩ/□

T*%

σ* S/cm

FOM* ×10−2

Rs* kΩ/□

T*%

σ* S/cm

FOM*

8.01 1.04

61.16 16.43

7.17 21.95

8.38 12.07

3.40 0.81

66.40 19.88

16.90 28.18

0.24 0.18

*t=thickness of film,*RS (sheet resistance), *T=Transmittance@550 nm, *σ (conductivity), *FOM (Figure of Merit)=

ZO 1 −1) T

where free space impedance (ZO)=377 Ω.

2 . RS . (

Fig. 11. Surface roughness image of (a) AGF-1 (b) AGF-2 films; 2D and 3D profile after annealing at 800 °C.

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Table 2 Comparison of transparency and conductivity of Ag NPs/G based TCFs deposited by different processes using different reducing agents. Method

Precursor material

Reducing agent

T (%)

Rs (kΩ/□)

Issue of reducing agent

Ref.

Electrostatic self-assembly Electrostatic self-assembly Vacuum filtration Vacuum filtration Spin coating Spray coating Spray coating

Ag/GO Ag/GO Ag/GO Ag/GO Ag/GO Ag/GO Ag/GO

Thermal reduction Thermal reduction C5H13N3 NaBH4 NaBH4 NaBH4 C6H8O7

86.30 89.20 55.45 78.00 47.00 82.20 66.40

97.00 8.30 43.00 0.093 83.00 0.012 3.40

– – Hazardous Hazardous Hazardous Hazardous Green

Zhou et al. [15] Zhou et al. [16] Yun et al. [43] Tien et al. [44] Moaven et al. [45] Voronin et al. [46] Present work

linkage formation between sp2-carbon networks [41]. Therefore, the Rs of films decreases and electrical conductivity of films increases. The conductivity of films has been found ~16.90, ~28.18 S cm−1 for AGF-1 and AGF-2, respectively. The performance of TCFs is evaluated by figure of merit (FOM) based on the Rs and T values. High value of FOM is considered as a sign of better performance, which requires a high transparency and low sheet resistance of film [42]. The performance evaluation has been tabulated of AGF-1 and AGF-2 in the Table 1. It has been found that the AGF-1 shows better performance compared to AGF-2 for the application of TCFs. Finally, the surface roughness analysis has been carried out by optical profiler. The analysis shows that the films have been deposited uniformly over quartz substrate. The average surface roughness (Rz) has been found ~6.39, 9.35 µm for AGF-1 and AGF-2, respectively as shown in Fig. 11(a,b). In addition, film thickness (t) has been measured by step profile, and found ~174, 438 nm for AGF-1 and AGF-2, respectively. It is observed that the surface roughness and film thickness of the films gradually increase with the concentration of Ag NPs/G nanocomposite. The comparison of transparency and conductivity of Ag NPs/G based TCFs deposited by different processes using different reducing agents have been tabulated in Table 2. Hence, by simple spray coating approach, TCFs of Ag NPs/G nanocomposite have been made. AGF-1 shows best performance TCF after thermal graphitization 800 °C for 1 h, and shows Rs ~3.40 kΩ/□, T~66.40% with FOM ~0.24, respectively.

[4] [5] [6] [7] [8] [9] [10]

[11]

[12]

[13]

[14]

[15]

[16]

4. Conclusions In summary, Ag NPs/G nanocomposite has been synthesized via solvothermal method, employing AgNO3 as a precursor material for Ag NPs and C6H8O7 as a green reducing to reduce GO into GNs. Elemental analysis indicates the removal of oxygen related groups from GO, and attainment of the C/O ratio of ~3.01. Further, TCFs of synthesized Ag NPs/G nanocomposite have been fabricated over quartz substrates by spray coating. Thermal treatment of the spray coated TCFs (at 500 °C for 3 h and at 800 °C for 1 h) is found to enhance their optoelectrical signatures significantly. It has been observed that optoelectrical properties of the deposited films improve with increase in the thermal treatment temperature. AGF-1, after thermal graphitization at 800 °C for 1 h, shows best optoelectronic performance with Rs of ~3.40 kΩ/□ and T of 66.40%.

[17]

[18]

[19]

[20] [21]

[22]

Acknowledgements

[23]

The authors acknowledge the financial support provided by the Indian Institute of Technology Kanpur, India for carrying out this research work.

[24]

[25]

References

[26]

[1] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 809–817. [2] Q. Zheng, Z. Li, J. Yang, J.K. Kima, Graphene oxide-based transparent conductive films, Prog. Mater. Sci. 64 (2014) 200–247. [3] K.K. Kar, S. Rana, J.K. Pandey, Handbook of Polymer Nanocomposites. Processing,

[27]

[28]

83

Performance and Application, Volume B: Carbon Nanotube Based Polymer Nanocomposites, Springer Heidelberg New York Dordrecht, London, 2015. K.S. Novoselov, V.I. Falko, L. Colombo, P.R. Gellert, M.G. Schwab, K. Kim, A roadmap for graphene, Nature 490 (2012) 192 (00). P. Chamoli, M.K. Das, K.K. Kar, Green synthesis of less defect density bilayer graphene, Graphene 3 (2015) 56–60. K.K. Kar, A. Hodzic, Developments in Nanocomposites, Research Publishing Services, Singapore, 2014. P. Avouris, C. Dimitrakopoulos, Graphene: synthesis and applications, Mater. Today 15 (2012) 86–97. Y.L. Zhong, Z. Tian, G.P. Simon, D. Li, Scalable production of graphene via wet chemistry: progress and challenges, Mater. Today 18 (2015) 73–78. V. Dhand, K.Y. Rhee, H.J. Kim, D.H. Jung, A comprehensive review of graphene nanocomposites: research status and trends, J. Nanomater. 2013 (2013) 763953. L. Fu, M.M. Sokiranskya, J. Wang, G. Lai, Aimin Yu, Development of Ag dendritesreduced graphene oxide composite catalysts via galvanic replacement reaction, Physica E 83 (2016) 146–150. S. Khan, J. Alia, Harsh, M. Husaina, M. Zulfequar, Synthesis of reduced graphene oxide and enhancement of its electrical and optical properties by attaching Ag nanoparticles, Physica E 81 (2016) 320–325. W. Shao, X. Liu, H. Min, G. Dong, Q. Feng, S. Zuo, Preparation, characterization, and antibacterial activity of silver nanoparticle-decorated graphene oxide nanocomposite, ACS Appl. Mater. Interfaces 7 (12) (2015) 6966–6973. D. Lee, H. Lee, Y. Ahn, Y. Lee, High-performance flexible transparent conductive film based on graphene/AgNW/graphene sandwich structure, Carbon 81 (2015) 439–446. S.T. Hsiao, H.W. Tien, W.H. Liao, Y.S. Wang, S.M. Li, C.C.M. Ma, Y.H. Yub, W.P. Chuangc, A highly electrically conductive graphene–silver nanowire hybrid nanomaterial for transparent conductive films, J. Mater. Chem. C 2 (2014) 7284–7291. Y. Zhou, J. Yang, X. Cheng, N. Zhao, Lei Sun, Hongbo Sun, Dan Li, , Electrostatic self-assembly of graphene–silver multilayer films and their transmittance and electronic conductivity, Carbon 50 (2012) 4343–4350. Y. Zhou, J. Yang, X. Cheng, N. Zhao, H. Sun, D. Li, Transparent and conductive reduced graphene oxide/silver nanoparticles multilayer film obtained by electrical self-assembly process with graphene oxide sheets and silver colloid, RSC Adv. 3 (2013) 3391–3398. X. Li, G. Zhang, X. Bai, X. Sun, X. Wang, E. Wang, H. Dai, Highly conducting graphene sheets and Langmuir–Blodgett films, Nat. Nanotechnol. 3 (2008) 538–542. C. Mattevi, G. Eda, S. Agnoli, S. Miller, K.A. Mkhoyan, O. Celik, D. Mastrogiovanni, G. Granozzi, E. Garfunkel, M. Chhowalla, Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films, Adv. Funct. Mater. 19 (2009) 2577–2583. J.B. Wu, H.A. Becerril, Z.N. Bao, Z.F. Liu, Y.S. Chen, P. Peumans, Organic solar cells with solution-processed graphene transparent electrodes, Appl. Phys. Lett. 92 (2008) 263302. L. Zhao, Y.X. Xu, T.F. Qiu, L.J. Zhi, G.Q. Shi, Polyaniline electrochromic devices with transparent graphene electrodes, Electrochim. Acta 55 (2009) 491–497. A.K. Sundramoorthy, Y. Wang, J. Wang, J. Che, Y.X. Thong, A.C.W. Lu, M.B. ChanPark, Lateral assembly of oxidized graphene flakes into large-scale transparent conductive thin films with a three-dimensional surfactant 4-sulfocalix[4]arene, Sci. Rep. 5 (2015) 10716. X. Li, D. Zhang, C. Yang, Y. Shang, Direct and efficient preparation of graphene transparent conductive films on flexible poly carbonate substrate by spray-coating, J. Nanosci. Nanotechnol. 15 (2015) 9500–9508. H. Shi, C. Wang, Z. Sun, Y. Zhou, K. Jin, G. Yang, Transparent conductive reduced graphene oxide thin films produced by spray coating, Sci. China Phys. Mech. Astron. 58 (2015) 1–5. H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463–470. J.I. Paredes, S. Villar-Rodil, A. Martınez-Alonso, J.M.D. Tascon, Graphene oxide dispersions in organic solvents, Langmuir 24 (2008) 10560–10564. X. Wang, L.J. Zhi, K. Mullen, Transparent, conductive graphene electrodes for dyesensitized solar cells, Nano Lett. 8 (2008) 323–327. F.A. Chowdhury, T. Morisaki, J. Otsuki, M.S. Alam, Annealing effect on the optoelectronic properties of graphene oxide thin films, Appl. Nanosci. 3 (2013) 477–483. P. Chamoli, M.K. Das, K.K. Kar, Green reduction of graphene oxide into graphene by cow urine, Curr. Nanomater. 1 (2) (2016) 110–116.

Physica E 90 (2017) 76–84

P. Chamoli et al. [29] P. Chamoli, R. Sharma, M.K. Das, K.K. Kar, Mangifera indica Ficus religiosa and Polyalthia longifolia leaf extract-assisted green synthesis of graphene for transparent highly conductive film, RSC Adv. 6 (2016) 96355–96366. [30] H. Feng, R. Cheng, X. Zhao, X. Duan, J. Li, A low-temperature method to produce highly reduced graphene oxide, Nat. Commun. 4 (2013) 1539. [31] T. Jiao, H. Guo, Q. Zhang, Q. Peng, Y. Tang, X. Yan, B. Li, Reduced graphene oxidebased silver nanoparticle-containing composite hydrogel as highly efficient dye catalysts for wastewater treatment, Sci. Rep. 5 (2015) 11873. [32] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nanotechnol. 8 (2013) 235–246. [33] B. Yang, Z. Liu, Z. Guo, W. Zhang, M. Wan, X. Qin, H. Zhong, In situ green synthesis of silver–graphene oxide nanocomposites by using tryptophan as a reducing and stabilizing agent and their application in SERS, Appl. Surf. Sci. 316 (2014) 22–27. [34] I. Roy, D. Rana, G. Sarkar, A. Bhattacharyya, N.R. Saha, S. Mondal, S. Pattanayak, S. Chattopadhyay, D. Chattopadhyay, Physical and electrochemical characterization of reduced graphene oxide/silver nanocomposites synthesized by adopting a green approach, RSC Adv. 5 (2015) 25357–25364. [35] S.K. Bhunia, N.R. Jana, Reduced graphene oxide-silver nanoparticle composite as visible light photocatalyst for degradation of colorless endocrine disruptors, ACS Appl. Mater. Interfaces 6 (2014) 20085–20092. [36] J. Shen, M. Shi, Bo Yan, Hongwei Ma, N. Li, M. Ye, One-pot hydrothermal synthesis of Ag-reduced graphene oxide composite with ionic liquid, J. Mater. Chem. 21 (2011) 7795–7801. [37] S. Some, Y. Kim, Y. Yoon, H.J. Yoo, S. Lee, Y. Park, H. Lee, High-quality reduced graphene oxide by a dual-function chemical reduction and healing process, Sci. Rep. 3 (2013) 1929. [38] Y. Yoon, K. Samanta, H. Lee, K. Lee, A.P. Tiwari, J.H. Lee, J. Yang, H. Lee, Highly

[39]

[40] [41]

[42]

[43]

[44]

[45]

[46]

84

stretchable and conductive silver nanoparticle embedded graphene flake electrode prepared by In situ dual reduction reaction, Sci. Rep. 5 (2015) 14177. X.-Y. Fang, X.-X. Yu, H.-M. Zheng, H.-B. Jin, L. Wang, M.-S. Cao, Temperatureand thickness-dependent electrical conductivity of few-layer graphene and graphene nanosheets, Phys. Lett. A 379 (2015) 2245–2251. S.J. Wang, Y. Geng, Q. Zheng, J.K. Kim, Fabrication of highly conducting and transparent graphene films, Carbon 48 (2010) 1815–1823. S.H. Tamboli, B.S. Kim, G. Choi, H. Lee, D. Lee, U.M. Patil, J. Lim, S.B. Kulkarni, S.C. Juna, H.H. Cho, Post-heating effects on the physical and electrochemical capacitive properties of reduced graphene oxide paper, J. Mater. Chem. A 2 (2014) 5077–5086. Q. Zheng, W.H. Ip, X. Lin, N. Yousefi, K.K. Yeung, Z. Li, J.K. Kim, Transparent conductive films consisting of ultra large graphene sheets produced by Langmuir– Blodgett assembly, ACS Nano 5 (2011) 6039–6051. S.W. Yun, J.R. Cha, M.S. Gong, Easy preparation of nanosilver-decorated graphene using silver carbamate by microwave irradiation and their properties, Bull. Korean Chem. Soc. 35 (2014) 2251–2256. H.W. Tien, Y.L. Huang, S.Y. Yang, J.Y. Wang, C.C.M. Ma, The production of graphene nanosheets decorated with silver nanoparticles for use in transparent, conductive films, Carbon 49 (2011) 1550–1560. S. Moaven, L. Naji, F.A. Taromi, F. Sharif, Effect of bending deformation on photovoltaic performance of flexible graphene/Ag electrode-based polymer solar cells, RSC Adv. 5 (2015) 30889–30901. A.S. Voronin, F.S. Ivanchenko, M.M. Simunin, A.V. Shiverskiy, A.S. Aleksandrovsky, I.V. Nemtsev, Y.V. Fadeev, D.V. Karpova, S.V. Khartov, High performance hybrid rGO/Ag quasi-periodic mesh transparent electrodes for flexible electrochromic devices, Appl. Surf. Sci. 364 (2016) 931–937.