Restructural confirmation and photocatalytic applications of graphene oxide–gold composites synthesized by Langmuir–Blodgett method

Accepted Manuscript Restructural comfirmation and Photocatalytic Applications of graphene oxidegold composites synthesized by Langmuir-Blodgett method Veeresh Kumar, Nupur Bahadur, Divya Sachdev, Shweta Gupta, G.B. Reddy, Renu Pasricha PII: DOI: Reference:

S0008-6223(14)00807-0 http://dx.doi.org/10.1016/j.carbon.2014.08.067 CARBON 9265

To appear in:

Carbon

Received Date: Accepted Date:

5 May 2014 19 August 2014

Please cite this article as: Kumar, V., Bahadur, N., Sachdev, D., Gupta, S., Reddy, G.B., Pasricha, R., Restructural comfirmation and Photocatalytic Applications of graphene oxide-gold composites synthesized by LangmuirBlodgett method, Carbon (2014), doi: http://dx.doi.org/10.1016/j.carbon.2014.08.067

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Restructural comfirmation and Photocatalytic Applications of graphene oxide-gold composites synthesized by Langmuir-Blodgett method Veeresh Kumara,b,Nupur Bahadur a,c, Divya Sachdeva,d, Shweta Guptaa,eG. B.Reddyb, Renu Pasrichaa,* a

*CSIR-National Physical Laboratory, Council of Scientific and Industrial Research, Dr. K.S.Krishnan Road , New Delhi-110012. [*]Renu Pasricha- Corresponding-Author, Presently at National Centre for Biological Sciences Tata Institute of Fundamental Research, Bangalore-560065 b

Department of Physics , IIT Delhi

c

Amity Institute of Nanotechnology, Amity University Uttar Pradesh, Noida-201303, India.

d

National Institute of FoodTechnology Entrepeneurship and Management- Kundli,Sonepat

e

Department of Chemistry, Delhi University

Abstract We present a first report on the use of Langmuir Blodgett (LB) technique for the synthesis of edge decorated graphene oxide gold (GO-Au) nanostructures by simple manipulation of electrostatic interactions. The GO-Au nanostructures when characterized using spectroscopy, surface, chemical and micro structural techniques, displayed unique physical and structural properties. The results re-established the theoretical corroboration that the carboxyl groups are primarily located at the edges of the 2D sheets of GO. The exploitation of air-water interface platform makes this process novel and fundamentally different from existing protocols for synthesis of GO-metal composites. These GO-Au hybrid materials favoured visible-light driven plasmonic photo catalysis together with enhanced charge separation and transportation properties, resulting in the augmentation of photocatalytic activity and conductivity with high transmittance. A plausible reaction mechanism for the degradation of pollutant dye and the role of gold nanoparticles (NPs) on GO has been established. [*Corresponding author] Ph. +91-80-2366-6390, Fax. +91-80-23666396, E-mail: [email protected], [email protected] (Renu Pasricha)

1

1. Introduction Graphene oxide (GO) is considered as an insulating and disordered analogue of the highly conducting crystalline graphene. GO unlike, graphene is an electrical insulator wherein insertion of oxygen imparts significant changes in its electronic properties, alters the band gap, interrupt the conjugated network and induces surface defects. In order to manipulate the electrical [1], structural [2] and interfacial properties of GO for enhanced performances, functionalization and doping of GO with varied nano-hybrid have been envisaged using routes varying from adsorption, electrochemical and physical [3-5]. However the reported methods of fabricating graphene and garaphene oxide based composites have their drawbacks often in the form of a high remainder of unattached metal nanoparticles. Such remainders diminish the active performance of the device, so it is of no surprise that the pursuit of methods for efficiently attaching metal nanoparticles to GO is a serious business. In this regard, we have already worked on a simple, green, room temperature mechanism for the UV assisted synthesis of graphene and simultaneously targeted reduction of the metal (Au, Ag and Pt) nanoparticles onto the graphene matrix [6-7]. Moreover our group is working towards increasing the conductivity of GO based composite materials without compromising the transmittance. The first and paramount step towards tailoring the properties of GO is synthesis of thin controllable films on any substrate. Thin films have been obtained by various techniques like spin coating, [8-9]chemical vapour deposition (CVD) [10-11] mechanical cleavage [12]. However LB technique for the formation of GO thin films is not frequently used [1314]. Through our experience and cited works in the area of Langmuir-Blodgett method we believe that this technique is superior to other film deposition techniques as it is versatile for not only generating ordered mono layers but it facilitates the deposition of any number of uniform multilayers with an efficient control over area and thickness. This further helped us in achieving our goal of conducting dynamic experiment and perceiving the alignment of gold nanoparticles on the surface of GO. Although recent easy methods like layer by layer (LBL) method for the deposition of graphene films with metal nanoparticles decorated over it have been reported [15], however we would like to emphasise the fact that the films deposited by LB method are well packed and structurally better organized as 2

compared to the one deposited by LBL method. The densely packed nature of LB method is envisioned to have better optical and electrical functions [16]. Methods like drop casting for generating thin films are also known but the uncontrollable thickness and non-uniform nature results in generation of structural defects which may hamper its efficiency. In previous reported work on fabrication of thin GO films using LB technique, research groups have utilized only the passive role of air-water interface [13-14, 17] whereas here we represent a dynamic model of the air-water interface, wherein GO acts as an excellent interface component for specific metal nanoparticle attachment. Prior functionalization of metal nanoparticles enabled the strong interfacial interactions between GO and metal NPs making

this process fundamentally different from other routes of metal nanoparticle

interfacing [18-20] with graphene merely by the adsorption, electrochemical, or chemical routes. The oxygen containing functional groups on GO not only induces fluorescence but also act as an active site for depositing metal nanoparticles. In addition, the electronegative oxygen atoms and other “defects” give rise to energy gap [21-22] thus making graphene oxide nonconducting. Few groups have suggested and shown that the conductivity of GO can be enhanced by incorporation of metal nanoparticles [18, 23-25], however the conductivity results were mostly demonstrated for non-uniform distribution of metal nanoparticles over GO[26]. We envisaged that conductivity can be raised without any substantial change in transmittance by linear alignment of metal nanoparticles on GO via LB method. Keeping this objective in view, an attempt was made to anchor amine functionalized gold nanoparticles (AuNPs) to the oxygen-containing groups (presumed to be at the edges only) of GO films. This not only amplified the conductivity values which are at par to the values achieved for reduced graphene systems, furthermore these results and HRTEM (High resolution transmission electron microscopy) images helped us to re-establish the site of carboxyl groups in GO which is more likely to be located at the edges, as per the widely accepted 2D model proposed by Lerf and co-workers [13, 27-29]. In addition the large surface area of the GO material synthesized by LB method supports the metal nanoparticles for achieving exceptionally high optical and electronic properties [25, 30-31].

3

So far, the prevalent environmental problem requires highly efficient and durable photocatalysts for pollution abatement, along with the conventional semiconductors like ZnO and TiO2 [32-34], although there have been advancement in terms of newly developed composites of Ag/AgCl/TiO2 [35], GO/TiO2 [36-37], GO/ZnO [38], GO/Ag/AgBr (and AgCl) [23, 25] . This shift in the choice of conventional material from TiO2 to GO based materials reinforces the effective utilization of abundantly available visible light of the solar spectrum rather than restricting to a small UV region. As a result the noble-metal nanoparticle (Ag, Au and Cu) based composites have been explored as a plasmonic photocatalyst for photodegradation of rhodamine B (RhB) under visible light. Interestingly, this has been demonstrated with the use of powdered form of GO/reduced GO (RGO), which by our own experience poses additional difficulty of having unstable suspensions in aqueous mediums. We instead experimented with graphene oxide based films uniformly edge decorated with AuNPs. To the best of our knowledge, this is the first report depicting the GO-Au composites LB films as plasmonic photocatalysts towards degradation of a non-biodegradable dye, Methylene blue (MB) under visible light irradiation. The investigation might open up new avenues for the development of high performance, large area, highly conducting and stable GO-based plasmonic photocatalysts that utilize visible light as an energy source. 2. Experimental 2.1 Synthesis of gold nanoparticles (AuNP) All the reagents used were purchased from Sigma Aldrich and were used as received without further purification. Pure Millipore water was used throughout the course of this investigation. Gold nanoparticles were synthesized as described in the literature [39]. In a typical experiment 0.01g of sodium borohydride (NaBH4) was dissolved in 10 ml water and was transferred drop wise to 100ml of 10−4 M hydroaurochloric acid (HAuCl4) aqueous solution at room temp 30°C and pH 9. The gold solution in flask changed to rubyred colour indicating the formation of gold nanoparticles (AuNPs) [40]. 2.2 Synthesis of 4-aminothiophenol (ATP) modified gold nanoparticles AuNPs Assemblies 4

In order to synthesize aminothiophenol conjugated gold nanoparticles, 2ml of 10-5 M ethanolic 4-ATP solution was added to 9 ml of AuNP solution (as prepared above) under vigorous stirring. After about 10 minutes of stirring the solution turned dark red from ruby red indicating the formation of 4-ATP masked AuNPs. The colour change at the end of the reaction is probably due to the change in the interparticle-gap of nanoparticles on addition of 4-ATP [41-42]. The synthesized nanomaterial was named as Au-ATP 2.3 Synthesis of Graphene oxide (GO) GO was synthesized from natural graphite by a modified Hummers method [43]. In a typical reaction, 2g of graphite flakes was stirred in 35 ml 98% H2SO4 for 2 h. 6g of KMnO4 was gradually added to the above solution below 20°C. The mixture was then stirred at 35°C for 2 h in an oil-bath. The resulting solution was diluted by adding 90 ml of water under vigorous stirring, a dark brown suspension was obtained and stirring was continued for another 1 h. The suspension was treated again by adding 30% H2O2 solution drop wise until the color of the solution became bright yellow. The resulting GO suspension was washed by repeated centrifugation, first with 5% HCl aqueous solution to remove excess of manganese salt and then with millipore water until the pH of the solution becomes neutral. The purified GO was finally dispersed in water (0.5 mg/ml) and ultrasonically exfoliated in an ultrasonic bath. The dispersion was found to be stable for a long time (as shown in Fig S1 supplementary information (SI) for a period of three months)[44]. 2.4 Langmuir Blodgett Assemblies In order to prepare uniform film of GO by LB method [14], as prepared GO sheets were centrifuged and purified repeatedly. This ensures uniformity in the size of GO sheets which helps in preventing the overlapping of nanosheets during compression at the airwater interface. In order to transfer GO as uni-sized sheets onto a water surface, a volatile spreading solvent (1:4) water/methanol was added to the GO solution and sonicated for 30min. Repetitive centrifugation steps both at 5000 rpm and at 2500 rpm were performed for further purification of the sample which removed larger aggregates of GO sheets (2 µm and 3-4 µm respectively). After disposing off the larger aggregates of GO sheets, the 5

supernatant containing well-dispersed GO sheets was collected. For the assembly, the trough was carefully cleaned with chloroform and filled with distilled water. The GO dispersion was carefully spread on the water surface drop by-drop using a glass syringe. Appearance of a faint brown colour at the interface confirms the formation of GO monolayer. The monolayer was left for about 20 min for stabilization before the isothermal compression. As the monolayer gets compressed, slight darkening of the interfacial layer was observed, a feature consistent with increased material density at the water surface. As hydrophilic surfaces are necessary for effectively collecting the GO sheet from LB film, silicon wafers were treated with 1:1:5 NH4OH:H2O2:H2O solution for 15 min to induce wetting. The monolayers were transferred to different substrates (hydrophilic silicon wafer, copper grids, Quartz) at various points during the compression by vertically dipping the substrate into the trough and slowly pulling it up (2mm/min). Double layer GO was prepared by depositing the first layer using the method described above and drying the substrate in an oven at 80 °C, and then repeating the process. 2.5 Preparation of functionalized gold-aminothiophenol-graphene oxide (Au-ATPGO) films using LB technique The experiment conducted above as shown in Fig.1 was repeated with Au-ATP as subphase instead of water and keeping GO as monolayer. The monolayers were transferred to copper grids, silicon wafers, glass slides, or mica disks by vertical dip-coating for characterization.

Fig. 1Formation of Langmuir-Blodgett assemblies of GO with Au-ATP on LB trough. 6

2.6 Materials Characterization Langmuir Blodgett trough: Apex LB trough, Model 220 was used for the present work. Pressure–area (π–A) isotherms were recorded at room temperature as a function of time from spreading the monolayer at a compression and expansion rate of 10 Å 2 molecule -1 min -1 . A standard Wilhelmy plate was used for surface pressure sensing. Monolayer and Multilayer films of the GO and Au-ATP-GO composites were formed by the LB technique (vertical dip-coating) at a constant surface pressure at a deposition rate of 2mm/min with a waiting time for drying between dips on different substrates. The IR spectrum (400–4,000 cm-1) was measured using a Nicolet 5700 FT-IR spectrometer with pure KBr as the background. The UV-Vis spectrum (190–1100 nm) was measured using a UV 1800 Shimadzu UV spectrophotometer. Microstructural characterization was conducted using Tecnai G2 F30 S-Twin (FEI; Super Twin lens with Cs =1.2 mm) instrument operated at an accelerating voltage at 300 kV, having a point resolution of 0.2nm and lattice resolution of 0.14nm. Program Digital Micrograph (Gatan) was used for image processing. The TEM samples were prepared by vertical dip coatings method using LB technique for both GO and Au-ATP-GO composites on a 400 mesh carbon coated grid. All AFM measurements were performed in the contact mode on a VEECO Digital Instruments multimode V scanning probe microscope equipped with a Nanoscope V Controller. I-V measurements were done using a Keithley 4200 (Semiconductor characterization system) wherein the measurements were performed in linear sweep mode from -10 V to 10 V. For the electrical characterization, the GO and Au-ATP–GO complex was deposited onto quartz substrates by vertical dip-coating and dried completely at slightly elevated temperatures (80 to 100 °C). Electrodes of 1 mm width were painted at the opposite ends of the films with gold electrodes 5 µm apart to ensure proper electrical contact. The measurements were performed in ambient atmosphere at room temperature. The photo-catalytic activities of GO and Au-ATP-GO films were measured by the photodegradation of MB as a model reaction under visible light irradiation (λ ~500 nm) and was monitored by the UV-Vis absorption spectra of MB in aqueous solutions. The detailed set up and evaluation of photocatalytic activity is provided in supplementary information

7

3. Results and Discussion Density of COOH/OH groups on GO cannot be controlled during the exfoliation progression as the extent of oxidation of sheets is expected to be higher for the sheets exfoliated at an early stage, therefore the sheets were exposed to the oxidizing medium for a longer time. Consequently, in a particular set of experiment, diverse GO sheets were obtained with different oxy-group densities, thus making it tricky to determine the arrangement of oxy-groups on GO. To ensure equivalent oxidation on GO, oxidation environment is needed to be controlled and monitored and therefore in order to recognise the accuracy in the structure of synthesized GO, preliminary investigations were done to propose the structure. The Ultra-violet Visible (UV) absorption spectrum in Fig. 2(a) of as synthesized GO solution shows an absorption peak at 272 nm, which can be attributed to the π–π* transitions. The shoulder around 300nm is due to the n-π* transitions because of oxygenation in graphitic layer [21]. Fourier Transform Infrared (FT-IR) spectrum of as synthesized GO (Fig. 2(b)) indicates the presence of different functional groups. The peak at 1410 cm-1 and 1730 cm−1 can be attributed to the O-H deformations vibrations and C=O stretching vibration of carboxylic group respectively [43, 45]. The broad peak at 3350 cm−1 can be assigned to O-H stretching vibrations in hydroxyl groups however the peak at 1590 cm−1 is indicative of C=C stretching of aromatic carbon which forms an important part of the GO skeleton [46].

Fig.2(a) The UV absorption spectrum of GO, (b) FTIR spectrum of GO with peaks marked (c) XRD pattern of GO-LB film

8

X-ray Diffraction (XRD,(Fig.2(c)) pattern of synthesised GO showed sharp peak at 2θ=10.6 corresponding to (002) reflection of GO with interlayer spacing of 8.33 Å which is nearly 2.5 times the interlayer spacing of (002) plane of pristine graphite (3.34 Å) [47]. This increased interlayer spacing is due to the breaking of van der Waals interactions with in the layer and also due to the presence of functional groups (-COOH, -OH) on the layers [14].Systematic X-ray Photoelectron Spectroscopy (XPS) investigations, Fig. 3 carried out on GO films depicts the presence of significant amount of oxygen moieties (55.9%), ascertaining the formation of GO [48]. Deconvoluting the peaks into Gaussian components reveal C in four different environment (i) non-oxygenated ring C (C–C, 284.71 eV), (ii) C in C–O bonds, (286.39 eV), (iii) carbonyl C (C=O, 287.79 eV), and (iv) Carbon (–O– C=O, 289.26 eV). Here it is worth mentioning that the XPS deconvoluted compnent [species (ii) & (iii)] are in well agreement [48] which comes from sp2 and epoxy/hydroxy groups respectively. 40

C(1s)

b C -O 20

3

Counts (10 arb units)

a

C=O -O -C = O C -C 0 300

295

290

285

280

275

270

B inding E nergy (eV )

Fig.3 (a) XPS of GO (b) C(1s) pattern of GO synthesized by LB method Physical analysis data of synthesised GO was compared with the already published results of various researchers which enable us to conclude that the synthesized GO sheets consists of a hexagonal network of covalently linked carbon atoms with oxygen containing functional groups attached to various sites as depicted later in Fig 5(a) . To study the behaviour of the GO monolayers on air water interface the Langmuir Blodgett studies were conducted with water as sub-phase and GO monolayer; thereafter changing 9

only the sub-phase to Au-ATP aqueous solution with the GO monolayer. A typical pressure-area isotherm for GO on aqueous subphase and GO on Au-ATP as subphase is shown in Fig. 4(a) and Fig 4(b). The isotherm was recorded at a constant compression rate of 10 Å2 molecule-1min-1. The arrows in the isotherms indicate the compression and expansion cycles of the monolayer. It was observed that during compression, the surface pressure builds up to a limiting value of ~35 mN/m. Further compression resulted in fall in the surface pressure, indicating collapse/overlap of the edge decorated gold nanoparticle GO monolayer. From the π –A isotherm measurements (Fig.4), a region of reasonably large incompressibility is seen to occur up to surface pressure of ~25 mN/m and therefore, multilayer films of GO-capped gold nanoparticles of different thickness were transferred onto different substrates at this surface pressure by the LB technique for further studies. During expansion of the monolayer, some hysteresis is observed. The hysteresis curves indicate the changes that have taken place with the repeated compression and expansion of the monolayer. The observed hysteresis is not unexpected as the gold nanoparticles cannot be considered to be truly amphiphilic. The hysteresis observed may also arise due to rearrangement of the gold nanoparticles within domains (here the GO films) and also due to reorganization of the domains themselves on release of surface pressure with a timescale larger than the experimentally controlled expansion rate of the monolayer. In case of GO film on aqueous subphase the initial and final surface areas were around 160 and 55 cm2, respectively. On compression, phase transitions from liquid extended (LE) to liquid condensed (LC) to final condense (C) phase followed. A gradual increase in surface pressure was recorded as the area under the curve decreased and the GO sheets were pushed closer. This is evident in the isothermal surface pressure-area plot. Subsequent compression and expansion cycles follow the same pattern.

10

Fig.4 Pressure-Area Isotherm of a) GO on water as subphase. b) GO on Au-ATP as subphase. The compression and expansion curves are marked with arrows and the phases (LE, LC and C) are marked.

On comparison, the π-A isotherm of systems shown in

Fig. 4(a) and (b) a

prominent difference in take-off area for GO monolayer at 180 cm2 on aqueous subphase and at 210 cm2 for GO on Au-ATP subphase can be noticed. The prominent red shift observed in case of GO on Au-ATP subphase can be attributed to the binding of amine terminated gold nanostructures with –COOH functionalized GO sheets which may be a plausible reason for the expansion of the monolayer resulting in increase in takes off value. The scheme, in Fig.5(b) is depicting the picked up films on substrate after complexation of Au-ATP with GO (model of GO shown in Fig 5(a)) at a certain compression. The figure shows the binding of AuNPs to carbon network including arrangement of benzene rings with carboxyl group mainly located at the edges. This 2D structure is also indicative of very high aspect ratio and nominal surface area, since a single layer is essentially the complete surface.

11

Fig.5 (a) Proposed model of Graphene oxide (b) Schematic showing complexation of GO and Au-ATP due to electrostatic interaction at the air-water interface. (Schematic not to scale)

The arrangement of nanostructures in the films was viewed by Atomic Force Microscopy (AFM). Fig.6 illustrates the arrangement of the GO nanosheets with the variation of surface pressure at the interface. During the initial gas phase or liquid expanded phase (LE), the surface pressure remains constant on compression since the GO sheets at the subphase are well-isolated and this can be noticed by the AFM image Fig. 6(a). It is interesting to note that flat GO sheets of ~1.5 nm height and 8-10 µm in size (as deduced by the AFM studies) were collected which showed a slight increase in the height as compared to reduced graphene sheets [14], this may be due to the presence of oxygen moieties (-COOH, -OH) attached to the planar structure [22].With rise in surface pressure to 14mN/m, the GO sheets are pushed closer to each other, and there is a decrease in the distance between the sheets which is also evident from Fig 6(b).

12

Fig.6 AFM images of GO on silicon wafer deposited by dip coating. a) deposited from LE region surface pressures 2 mN/m, b) from LC region surface pressures 14 mN/m, c) solid region surface pressures 25mN/m and d) multilayer surface pressures 30 mN/m. An additional increase in surface pressure resulted on further compression of the monolayer to the close-packed region denoted by “C”, observed. The film was picked up at 25 mN/m and was dried before characterizing. Since the single layers are soft and flexible, the increased surface pressure can sometimes lead to folding of the sheets in addition to the overlapping of sheets (marked with arrow) which can be observed from the Fig. 6(c) and (d) wherein few wrinkles and folds can be distinctly viewed. In total, a set of three cycles of compression-expansion were performed and it was observed that the isotherms have nearly the same shape and attain almost the same final pressure. The hysteresis observed during the expansion cycle in Fig. 4(a) indicates the loss of small amount of material from the monolayer which might be due to the slight solubility of the monolayer in aqueous subphase. The π-t curve shows a stable formation of monolayer over a time period of 3 h after an initial hysteresis was observed (graph not shown). Double layers were also fabricated by sequential, layer-by-layer dip-coating however, the second layer of GO sheets experiences a repulsion from the groups both on 13

their neighbours and those in the under layer. As a result, the newly deposited second layer normally tends to be wrinkled. This is evident from AFM images which were collected by multiple dipping as shown in Fig. 6(d), a continuous film of multilayered GO was picked at 30 mN/m. While many researchers have alleged on the location of the oxy groups mainly at the edges but the experimental confirmation is yet to be investigated. Therefore in-order to reestablish the existence of charges at the edges, Au-ATP-GO composites sheets were synthesized by LB method to ensure the exposure of the edge located groups to the functionalized nanoparticles. GO having a hydrophilic and hydrophobic moiety forms a monolayer at the air-water interface which can facilitate electrostatic interactions between edged carboxylic groups of GO and amine (of ATP) groups of gold. Since GO is known to behave as a cross linked monolayer and the moving barrier of LB trough can be effectively used to control the monolayers of GO in terms of distance between the sheets as well as the number of monolayers being deposited [13-14]. As the GO approach each other on compression of the monolayer at the interface, electrostatic repulsion and van der Waals forces of attraction both come into existence which leads to change in the comparative size while controlling the overlapping of the sheets. Therefore it is important to ensure the uniformity of size in the synthesized GO by repeating the centrifugation steps. Au-ATP-GO composites synthesis is pH dependant since complexation can be achieved only when amine group of gold and carboxylic group of GO are in ionized forms. Therefore control experiment was performed, using LB assembly containing Au-ATP colloidal solution as subphase held at different pH values with GO monolayer at the airwater interface. Complexation was observed under acidic and basic conditions as a function of time after spreading the monolayer by measuring the π–A isotherms.

14

(a)

(b)

Fig.7π–A isotherms of GO monolayer spread on Au-ATP as subphase. a) pH = 6 and b) pH = 12. (black lines, curve 1 indicates isotherms measured immediately after spreading the GO monolayer (t = 20 min) and the red lines, curve 2 indicates isotherms measured after 2 h of spreading)

At pH = 6, in order to maintain the pH, the amine(NH2) coated on gold clusters gets ionized to -NH3+ (pKb 10.5) and –COOH on GO forms –COO- which leads to maximum attractive electrostatic interaction, that can be seen as a large increase in the langmuir monolayer area with time in Fig. 7(a). On the other hand, at pH = 12, –COOH gets ionized while NH2 remains unionized and as a result the electrostatic interaction between the AuATP and the GO monolayer is considerably weaker. Almost negligible complexation occurs at pH=12 and as a result π–A isotherms showed no change in the area with time (Fig. 7(b)). Even though the Au-ATP nanoparticles-GO interaction is negligible at pH 12, there was a slight change in colour of the monolayer at this pH. This indicates that a small percentage of Au-ATP forms complex with GO monolayer under these conditions which may be due to the hydrogen bonding and ion-dipole interactions between ionized –COO- of GO monolayer and neutral NH2 of Au-ATP. These results are similar to the one shown by Serra et al. where hydrogen bonding interaction play a significant role in the complexation and film formation of an ODA titanyl oxylate system [49].Thus by simple variation of the colloidal solution pH, it was possible to modulate the electrostatic interaction between the colloidal particles and the Langmuir monolayer and thus vary the nanoparticle

15

concentration at the air–water interface. The synthesized Au-ATP-GO composites synthesized by LB-method was investigated and characterized. UV-Vis spectroscopy was used as a preliminary tool to investigate the synthesis and attachment of Au-ATP-GO sheets. Comparative UV measurements of AuNPs (Curve 2, Fig.8a) and Au-ATP (Curve 3, Fig. 8a) shows a shift of λmax value to higher wavelength region on addition of ligand [40] with a distinctive change in colour to dark red indicating the formation of 4-ATP capped Au-NPs .A covalent bond is formed between the thiol (−SH) group of ATP and AuNPs due to the high chemical affinity of thiol to Au and the −NH2 group remains free with a residual positive charge [39, 42].

C=C

(b) 3 -OH

C=C

2

Transmittance (%)

S-H

1 -OH

C =O

1500

-NH2

2000 2500 3000 3500 Wavenumber (cm -1)

4000

Fig.8a) The UV-Vis absorption spectra of Curve 1: GO, Curve 2: gold nanoparticles, Curve 3: AuATP and Curve 4: Au-ATP-GO LB Film. b) FTIR spectra recorded from curve 1: GO, Curve 2: Au-ATP and Curve 3: GO after binding with Au-ATP.

Au-ATP-GO composites exhibited a peak in visible region at 525 nm (curve 4) which is slightly broad and shifted as compared to AuNPs (curve 2) as the complexation of Au-ATP with GO can lead to increase in the dielectric constant on Au resulting in broadening and shifting of the plasmon band. In addition the absorption peak of GO shows a marked red 16

shift from 272 nm to 290nm(Curve 4, Fig. 8a) [44] indicating π–π* transitions due to the charge transfer interactions between the high charge carrier density of AuNPs to GO [50]. Further, it can be seen that pure GO has no absorption in the visible region, whereas GOAu LB Film display strong absorption in the visible region. This suggests the existence of metallic Au species in these composites, and thereby shows a strong absorptions in the visible region. In addition to this the blue shift in the plasmon band on formation of the composite occurs on account of the electron transfer between the metal and graphene levels driven by the work function difference to equilibrate the Fermi levels, as well as the chemical interaction between graphene and the metal. This is also controlled by the distribution of metal nanoparticles on GO. Recent investigations depicting the decrease in work function values from GO to GO-AuNPs also supports our above discussion and confirms the charge transfer [51]. Furthermore, curve 4 with no shoulder or sideband around 300 nm suggests that binding of functional groups C=O in GO with Au-ATP induces no n- π* transitions. In order to verify that Au-ATP-GO composites have been formed, their Fourier transform infrared spectra (FT-IR) together with that of bare GO is presented in Fig. 8(b). The peak at 3350 cm−1 for O-H stretching in GO (curve 1) completely vanishes on formation of the Au-ATP-GO composites spectrum (curve3), clearly indicating the complexation between Au-ATP and GO. It is known from the equation υ= 1/2π √ k/m (where υ= frequency, k is the force constant and m as the effective mass of the system) that the vibration frequency of C=O depends on the π-electron density available for polarization [52], thus the slight shift of C=O to higher wavenumber from 1638 cm−1 (Au-ATP) to1730 cm−1 (Au-ATP-GO composites) indicates the evident electrostatic interactions between Au-ATP and GO. Moreover in case of Au-ATP-GO composites, the metal Au induces an electron releasing effect near the -COO- group thus strengthening the bond and increasing the k value and thereby the wavenumber. Where as in the case of GO, C=C group in conjugation to -COOgroup delocalizes the π-electron density, reducing the k value followed by wavenumber. The sharp peak of C=O absorption at 1730cm−1 in Au-ATP-GO can be assigned to the change in the environment of GO in vicinity to Au nanoparticles, as greater the change in charge distribution, the stronger is the absorption. 17

Fig.9HRTEM micrograph showing binding of Au-ATP with GO at edges. a) Arrows show the different sheets marked (b-c) Aligned Au-ATP on edges of GO, inset of (b) shows diffraction pattern which can be indexed to fcc gold. (d) Marked lattice of one fcc Au nanoparticle.

HRTEM micrographs shown in Fig. 9 elucidate almost transparent Au-ATP decorated on GO sheet. The wavy darker and lighter areas in Fig.9 (a) suggest that there might be possibility of existence of multi-layers. This may be due to the overlapping of sheets on compression. As mentioned earlier, the sheets, if uneven in size, will overlap due to the van-der Waals interactions. This micrograph clearly elucidates that the Au nanoparticles are indeed attached to the edges of GO sheets. Fig. 9(b-c) (additional TEM micrographs shown in Fig S2, SI) shows the magnified micrograph of the high density of Au-ATP on the sheets. Another remarkable observation that comes to light is that no nanoparticles were found scattered outside the GO sheets, which indicates that Au-ATP are strongly bound to the edge groups via electrostatic interaction [28-29]. At pH 6 of the gold solution 18

carboxyl groups of GO located at the edges get ionized with negative charge and the AuATP are left with free amine groups with residual positive charge. The HRTEM image (Fig. 9d) clearly shows that AuNPs attached to GO edges are in crystalline state with lattice space of 0.234 nm and indicating a clear polycrystalline diffraction pattern. Inset of Fig. 9b shows the diffraction pattern of AuNPs which can be indexed to fcc gold. The metal NPs supported on larger surface areas GO films have properties which are advantageous for optical and electronic properties [23, 53] .Therefore we investigated these gold-GO composites for conductivity and photocatalytic behaviour. 3.1 Electrical Conductivity Measurements of Au-ATP-GO films Fig. 10(a) shows the electrical measurements of Au-ATP-GO film and the bare-GO film at room temperature. The immobilization of the sheets was achieved by simple dip coating mechanism on quartz with gold electrodes 5 µm apart. Since it has been proven that GO films are mostly insulating [14], the confirmatory tests to prove the restoration of the sp2 network on reduction of GO to graphene is the increase in conductivity. In case of the AuATP-GO composites, only –COOH groups present at the edges are occupied in binding with ATP modified Au nanoparticles, it is worth mentioning that, this simplistic binding approach results insignificant increase in the conductivity values as compared to pure GO films. The achieved conductivity values are at par with the values wherein few other groups were able to achieve ,either with the use of harsh chemicals (hydrazine, NaBH4) or with intense thermal treatment for restoration of sp2 network [22]. The conductivity of AuATP-GO film was an order of 109 magnitude higher than that for GO, which can be clearly attributed to the formation of low-resistance nanogold chain like structure at the edges of GO. Thus it can be inferred that the aligned and dense Au nanoparticles provides an alternative path for charge transfer thus compensating the degradation of the electrical network on oxidation of GO sheets [50]. This smooth and continuous transfer of electrons further leads to percolation of charges through the sheets achieved due to LB method and as a result high conductivity is achieved.

19

Fig.10 (a) Current vs. voltage (I-V) plot of a film of: Curve 1: GO and Curve 2: Au-ATP-GO. (b) Transmittance spectra of GO (curve 1) and Au-ATP-GO (curve 2) film as a function of wavelength.

The application of electric field for long durations did not change the conductivity; this further shows that the chemical binding on GO was a result of compensation of the positive charge of amine groups on Au-ATP with the negative charge on the carboxylic acid groups on GO [54]. Fig. 10(b) shows the transmission spectra for GO and Au-ATP-GO films wherein the presence of Au-nanoparticles in GO films reduces the transmittance nearly 12% as the Au nanoparticles act as scattering centre for the light[26, 55]. The GO film, Fig. 10 (b) curve 1 shows high transparency of 98%, whereas the Au-ATP-GO film, curve 2 shows transparency of around 86%, thus providing us with the potential application of replacing ITO with Au-ATP-GO films. One of the other important observations was the change in band gap of GO on complexation with Au-ATP. The band gap of GO was calculated by Tauc method and found to be 3.35 eV [56]. This band gap slightly was red shifted on formation of GO-ATP-Au composite as shown in Fig 10 (b) curve 2 which is analogous to the shift observed in UV in Fig 8(a) for the composites. This can be explained by taking into account that chemical interactions of Au with GO might result in the changes of structure of GO[57] as well as due to charge transfer between GO and Au occurs which can also be verified by Raman spectra discussed in the later part of Section 3.2, 20

wherein a shift in the G-band of GO is observed on the formation of composite resulting in a subsequent electron transfer [25]. 3.2 Photocatalytic application of GO-Gold Langmuir Blodgett film towards degradation of Methylene Blue (MB) The photocatalytic activities of GO and Au-ATP-GO were probed by monitoring the degradation of MB dye as a model reaction under visible light irradiation. The strong absorption band of MB was observed at 667 nm which is due to the presence of MB dimer [58] (shown in Fig S3, supplementary information). The intensity of the peak decreases with increase in the degradation of MB under the visible light irradiation due to the blocking of photocatalytically active sites on the catalyst and thus reducing the interaction of photons with the sites. A plot of photocatalytic performance of GO and Au-ATP-GO (shown in Fig. 11) in terms of normalized concentration change (C/C0) of degraded MB versus irradiation time was found to be proportional to the maximum absorbance (A/A0) derived from the changes in absorption profile of MB (λ=667 nm) at a given time interval. It was observed that the degradation of MB alone (without photocatalyst) as a blank experiment Fig 11(curve a) is quite low whereas degradation of MB is quite effective in presence of GO (curve b) and Au-ATP-GO (curve c) with Au-ATP-GO exhibiting an improved photocatalytic performance with 50% of degradation of MB in 90 min compared to only 12% of decay in presence of GO. The kinetics of MB decay can be fitted to pseudo-first order reaction in both the cases and the rate constant for both the reactions can be estimated from the slope of the of the curves from equation(1) ln(C/ C0)=kt,

(1)

where C is initial and measured concentrations at a particular time and k is the overall degradation rate constant in min-1.

21

1.0 a 0.9

b

C/C0

0.8

0.7

0.6

c

0.5 0

15

30

45 Time (min)

60

75

90

Fig.11Photo-degradation curves of pure MB dye (a) with GO (b) and with GO-Au composites film (c), respectively

The k value calculated in the presence of Au-ATP-GO composites and in the case of only GO comes out to be 7.4x10-2 min-1and 1.92x10-2 min-1respectively which infers that the composites films show a fourfold increase in rate constant as compared to sole GO films. The k value calculated for the GO-Au composites film is also higher than the recently reported rate constant values for graphene-gold composites by Xiong et. al (4.1x10-2 min-1) [59] and also few other graphene based composites e.g graphene oxide-Ag2O as 3.5x10-2 min-1[60] ,poly-ophenylene diammine/TiO2 composites as 1.98x10-2 min-1[61], ZnO-Graphene (0.1 wt%) [62] as 4.7x10-2 min-1, 3.5x10-2 min-1 for InNbO4-graphene [63] and for CdS nanoparticles as 3.61x10-3 min-1[64], under visible light irradiation for MB degradation. Almost two fold increase in MB degradation in presence of Au-ATP-GO LB thin films as compared to the earlier report corroborates our explanation of charge transfer of electrons from Au to GO resulting in separation of charges. This indicates that GO is an efficient transporter and acceptor of electrons which is further explained in the mechanism [60]. Photocatalytic Mechanism Here in the case of GO-Au Langmuir-Blodgett films as visible-light driven plasmonic photocatalysts, we propose that the, AuNPs gets photo-excited in the visible region of solar spectrum 22

due to plasmon resonance and this light irradiation generates electron-hole pair in the GO system. The subsequent charge transfer / separation leads to an enhanced photocatalytic activity on the basis of relative work function values of GO compared to those of Au and MB respectively. The photo-oxidation is elucidated schematically in Fig.12 and photochemical splitting of MB schematically is illustrated in Fig 13. The redox potential of MB is 1.17 V versus standard hydrogen electrode (SHE) [65]. The work function (E) of MB versus vacuum can be calculated to be about –5.67 eV by using equation E(eV)= – 4.5 – ESHE(V) [59]. Primarily MB molecules gets adsorbed over GO due to the strong interactions via π-conjugated planar structure of GO and as a result there is increase in uptake of MB molecules from water. It has been recently shown that with incorporation of the Au nanoparticles (with different sizes) over GO the work function reduces from (GO= -5.9eV) [66] to (Au-ATP-GO composites= -5.35eV) [51]. This further explains the fact that different locations of adsorbed MB molecules on the catalyst surface would lead to different degradation efficiencies. When Au-ATP-GO film is irradiated with visible light (of wavelength~ 550nm= 2.25eV) the excited plasmons on the surface of the gold interact and generates an electron-hole pair in AuATP-GO composites. Further in order to equilibrate the Fermi levels of Au and GO (the work function of Au metal is = -4.75eV) [59] the charges from Au migrate to the GO interface resulting in a charge separation wherein the subsequent migration of electrons and holes takes place on the absorbed species. Methylene blue (which has a work function of about –5.67 eV, as compared to

-5.90 eV of GO) thus gets oxidized by reactive species( holes, OH● and O2-) to

MB* in presence of visible light. The electrons thus produced during the degradation are accepted by GO which transfers it to the reactive species and form (Au, OH, holes and O2) and a network is formed; resulting in enhanced photocatalytic behaviour.

23

Fig. 12 Schematic diagram for the charge separation and transportation for enhanced dye degradation in the visible light irradiated GO-Au system.

Further it is important to mention here that the LB approach to synthesize GO-Au composite improves the quality of films, provide orderliness to GO and prevents the aggregation of Au nanoparticles which in turn helps in a smooth network of electron movement and also favours higher adsorption of MB molecules over Au-ATP-GO. In order to ascertain that the films deposited by LB method were well packed and structurally better organized, we characterized these films using Raman spectroscopy (Fig. S4) and the results supports our view wherein the shift in G-band of GO was observed indicating that a hybrid material is formed via electron transfer [27]. G-band observed at 1600 cm-1(curve1,2), is the characterstic feature of all sp2 carbon systems in GO while indicating higher degree of oxidation of graphite that arises due to the C-C bond stretching whereas the D-band indicates disorderliness in the system [67]. AuATP-GO composites (curve3), showed red shift in G band of 12 cm-1 (1580 cm-1), as compared to GO , which is due to the presence of electron donor property of Au nanoparticles [25]. Mostly D band arises due to edges of graphene oxide samples[68] and the ratio of ID/IG i.e. ratio of intensities of D and G bands, is the measure of disorder degree [69] and the quality of graphene structure. Lower the ID/IG ratio, better the structure of films. Au-ATP-GO film exhibit broader D-band with lower relative intensities compared to that of GO film (curve2) supports our view

24

that Au in GO (mostly at the edges of GO sheets) has changed the GO structure and brought orderliness in the material or in other words has improved the quality of material. Thus the higher adsorption of MB dye facilitated the separated holes and electrons to be fully involved in photocatalytic reaction of MB which evades the recombination process and contributes to the photo-catalytic performance. Au-ATP-GO system has an additional advantage wherein MB does not behave as a sensitizer which otherwise may have posed problems in electron transfers resulting in a low quantum yield for photocatalytic reaction. In order to check the recyclability of Au-ATP-GO films for photocatalysis, after every use, AuATP-GO films were washed with deionised water under sonication and then vacuum dried to be reused. The photocatalyst was subjected for fresh reaction (up to three cycles) towards the degradation of MB. Initial rate of photo-degradation of MB was studied in all cases and no significant difference in the activity was observed, clearly indicating the reproducible performance of Au-ATP-GO films. After complete treatment we visually ensured that the films were intact and in good condition to be reused till the third cycle, after which we observed some sought of etching off these films from the substrate. Thus these Au-ATP-GO films had reusability up to three cycles without any appreciable change in the rate constant value.

25

CH3 N

CH3

CH3 +

S

N

H3C

Cl

-

+

+

NH

+

S

NH

H3C

CH3 N

CH3

+

h

CH3

+

N

N H3C

+

++ NH

H3C

+

S h

+

CH3

HN

+

CH3 Demethylation CH3

H3C OH

+HO

H+ N

H3C

CH3

+

H2N

S

+ HO O 2 ,

NH2

-

splitting bond

+

+ OH

NH2

SO 3H

+

-

2-

CO2, H 2O, NH4 , NO 3 , SO 4 , Cl

-

Fig. 13 Plausible photocatalytic degradation mechanism for MB dye under visible light irradiation in the presence of Au-ATP-GO composites.

26

4. Conclusions Highly conducting, large area GO and Au-ATP-GO LB films have been fabricated using the principle of electrostatic binding of GO with Au-ATP-GO at the air-water interface. With high resolution electron microscopy analysis, the attachment of gold at the edges was confirmed and this in turn gives an experimental evidence and support Lerf model of presence of carboxylic group at the edges. The excellent conductivity parameters demonstrated by the GO-Au system are at par with conductivity of chemically or thermally reduced graphene. Further, the high transmittance value obtained in this case provides us with an alternative idea to substitute to replace ITO in organic solar cells. On checking the photocatalytic activity of the GO and AuATP-GO LB films for the photodegradation of MB dye, it was found that the Au decorated GO films displayed enhanced photocatalytic activity. This can be attributed to the presence of gold NPs, which act as an electron relay in the presence of visible light, passing the excited electrons from the GO sheets to the mediators (OH, O2) and leading to continuous generation of reactive oxygen species (ROSs) for the degradation of MB. We can say that the Au on GO LB-films results in extended photo response range in the visible spectrum; improves the quality of films by providing more orderliness to GO structure which in turn favours good adsorptive capacity of MB molecules on to GO-Au. This facilitated charge transfer and suppressed recombination of electron-hole pairs in Au-ATP-GO composite and finally contributes to the enhanced photocatalytic performance To the best of our knowledge, this is the first report where the novel reaction mechanism and MB photo-oxidation over Au-ATP-GO composites have been explored. These findings open up the possibilities for the controlled synthesis of highly efficient Au-ATP-GO composites assemblies via LB method towards opto-electronic and stable GO-metal based Hybrid Plasmonic Photocatalytic applications that utilize visible light as an energy source.

Acknowledgements We acknowledge the support and guidance of Prof. R.C. Budhani, director NPL. Veeresh Kumar, Nupur Bahadur and Divya Sachdev, thanks the University Grants Commision (UGC), Department of Science and Technology (DST) and Council of Scientific Industrial Research (CSIR) Govt. of India for the research funds and fellowship. We would like to thank Dr Rajiv

27

Kumar for carrying out conductivity measurements and Mr Sandeep for the AFM characterization studies. . References [1]

Dikin D, Jung I, Dommett G, Stankovich S, Ruoff R. Electrical conductivity of graphene

oxide sheets and networks of such sheets. BullAmeChemSoc. 2008;53. [2]

Mohanty N, Berry V. Graphene-based single-bacterium resolution biodevice and DNA

transistor: interfacing graphene derivatives with nanoscale and microscale biocomponents. Nano Lett. 2008;8(12):4469-76. [3]

Liu L, Ryu S, Tomasik MR, Stolyarova E, Jung N, Hybertsen MS, et al. Graphene

oxidation: thickness-dependent etching and strong chemical doping. Nano Lett. 2008;8(7):196570. [4]

Liu J, Tang J, Gooding JJ. Strategies for chemical modification of graphene and

applications of chemically modified graphene. J Mater Chem. 2012;22(25):12435-52. [5]

Gómez-Navarro C, Weitz RT, Bittner AM, Scolari M, Mews A, Burghard M, et al.

Electronic transport properties of individual chemically reduced graphene oxide sheets. Nano Lett. 2007;7(11):3499-503. [6]

Pasricha R, Gupta S, Srivastava AK. A Facile and Novel Synthesis of Ag–

Graphene‐Based Nanocomposites. Small. 2009;5(20):2253-9. [7]

Pasricha R, Gupta S, Joshi AG, Bahadur N, Haranath D, Sood KN, et al. Directed

nanoparticle reduction on graphene. Mater Today. 2012;15(3):118-25. [8]

Becerril HA, Mao J, Liu Z, Stoltenberg RM, Bao Z, Chen Y. Evaluation of solution-

processed reduced graphene oxide films as transparent conductors. ACS nano. 2008;2(3):463-70. [9]

Robinson JT, Zalalutdinov M, Baldwin JW, Snow ES, Wei Z, Sheehan P, et al. Wafer-

scale reduced graphene oxide films for nanomechanical devices. Nano Lett. 2008;8(10):3441-5. [10]

Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern growth of

graphene films for stretchable transparent electrodes. Nature. 2009;457(7230):706-10. [11]

Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V, et al. Large area, few-layer graphene

films on arbitrary substrates by chemical vapor deposition. Nano Lett. 2008;9(1):30-5. [12]

Novoselov K, Geim AK, Morozov S, Jiang D, Grigorieva MKI, Dubonos S, et al. Two-

dimensional gas of massless Dirac fermions in graphene. Nature. 2005;438(7065):197-200. 28

[13]

Kim J, Cote LJ, Kim F, Yuan W, Shull KR, Huang J. Graphene oxide sheets at interfaces.

J Am Chem Soc. 2010;132(23):8180-6. [14]

Cote LJ, Kim F, Huang J. Langmuir− Blodgett assembly of graphite oxide single layers. J

Am Chem Soc. 2008;131(3):1043-9. [15]

Kong B-S, Geng J, Jung H-T. Layer-by-layer assembly of graphene and gold

nanoparticles by vacuum filtration and spontaneous reduction of gold ions. Chem Commun. 2009(16):2174-6. [16]

Ariga K, Yamauchi Y, Mori T, Hill JP. 25th Anniversary Article: What Can Be Done

with the Langmuir‐Blodgett Method? Recent Developments and its Critical Role in Materials Science. AdvMat. 2013;25(45):6477-512. [17]

Gao Y, Chen X, Xu H, Zou Y, Gu R, Xu M, et al. Highly-efficient fabrication of

nanoscrolls from functionalized graphene oxide by Langmuir–Blodgett method. Carbon. 2010;48(15):4475-82. [18]

Muszynski R, Seger B, Kamat PV. Decorating graphene sheets with gold nanoparticles.

The Journal of Physical Chemistry C. 2008;112(14):5263-6. [19]

Sundaram G, Monsma D, Becker J. Leading Edge Atomic Layer Deposition

Applications. ECS Transactions. 2008;16(4):19-27. [20]

Williams G, Seger B, Kamat PV. TiO2-graphene nanocomposites. UV-assisted

photocatalytic reduction of graphene oxide. ACS nano. 2008;2(7):1487-91. [21]

Eda G, Chhowalla M. Chemically Derived Graphene Oxide: Towards Large‐Area

Thin‐Film Electronics and Optoelectronics. AdvMat. 2010;22(22):2392-415. [22]

Zhu Y, Murali S, Cai W, Li X, Suk JW, Potts JR, et al. Graphene and graphene oxide:

synthesis, properties, and applications. AdvMat. 2010;22(35):3906-24. [23]

Xu C, Wang X, Zhu J. Graphene− metal particle nanocomposites. The JPhyChemC.

2008;112(50):19841-5. [24]

Liu L, Liu J, Sun DD. Graphene oxide enwrapped Ag3PO4 composite: towards a highly

efficient and stable visible-light-induced photocatalyst for water purification. CatalSci&Tech. 2012;2(12):2525-32. [25]

Zhu M, Chen P, Liu M. Graphene oxide enwrapped Ag/AgX (X= Br, Cl) nanocomposite

as a highly efficient visible-light plasmonic photocatalyst. ACS nano. 2011;5(6):4529-36.

29

[26]

Kim KK, Reina A, Shi Y, Park H, Li L-J, Lee YH, et al. Enhancing the conductivity of

transparent graphene films via doping. Nanotechnology. 2010;21(28):285205. [27]

Mkhoyan KA, Contryman AW, Silcox J, Stewart DA, Eda G, Mattevi C, et al. Atomic

and electronic structure of graphene-oxide. Nano Lett. 2009;9(3):1058-63. [28]

Lerf A, He H, Forster M, Klinowski J. Structure of graphite oxide revisited. The

JPhyChemB. 1998;102(23):4477-82. [29]

Cai W, Piner RD, Stadermann FJ, Park S, Shaibat MA, Ishii Y, et al. Synthesis and solid-

state

NMR

structural

characterization

of

13C-labeled

graphite

oxide.

Science.

2008;321(5897):1815-7. [30]

Wang Y, Zhen SJ, Zhang Y, Li YF, Huang CZ. Facile Fabrication of Metal

Nanoparticle/Graphene Oxide Hybrids: A New Strategy To Directly Illuminate Graphene for Optical Imaging. The JPhyChemC. 2011;115(26):12815-21. [31]

Murphy SJ. Metal Nanoparticle-graphene Oxide Composites: Photophysical Properties

and Sensing Applications. University of Notre Dame, 2013. [32]

Lu F, Cai W, Zhang Y. ZnO hierarchical micro/nanoarchitectures: solvothermal synthesis

and structurally enhanced photocatalytic performance. Adv Funct Mater. 2008;18(7):1047-56. [33]

Kansal S, Singh M, Sud D. Studies on TiO< sub> 2/ZnO photocatalysed

degradation of lignin. J Hazard Mater. 2008;153(1):412-7. [34]

Liao D, Badour C, Liao B. Preparation of nanosized TiO< sub> 2/ZnO composite

catalyst and its photocatalytic activity for degradation of methyl orange. J Photochem Photobiol, A. 2008;194(1):11-9. [35]

Cao Y, Tan H, Shi T, Tang T, Li J. Preparation of Ag‐doped TiO2 nanoparticles for

photocatalytic degradation of acetamiprid in water. J Chem Technol Biotechnol. 2008;83(4):54652. [36]

Wang X, Zhi L, Müllen K. Transparent, conductive graphene electrodes for dye-

sensitized solar cells. Nano Lett. 2008;8(1):323-7. [37]

Zhang LW, Fu HB, Zhu YF. Efficient TiO2 Photocatalysts from Surface Hybridization of

TiO2 Particles with Graphite‐like Carbon. Adv Funct Mater. 2008;18(15):2180-9. [38]

Williams G, Kamat PV. Graphene− Semiconductor Nanocomposites: Excited-State

Interactions between ZnO Nanoparticles and Graphene Oxide†. Langmuir. 2009;25(24):1386973. 30

[39]

Sastry M, Mayya K, Patil V, Paranjape D, Hegde S. Langmuir-Blodgett films of

carboxylic acid derivatized silver colloidal particles: role of subphase pH on degree of cluster incorporation. J Phys Chem B. 1997;101(25):4954-8. [40]

Vigderman L, Zubarev ER. Therapeutic platforms based on gold nanoparticles and their

covalent conjugates with drug molecules. AdvDrugDelRev. 2012. [41]

Gole A, Sainkar S, Sastry M. Electrostatically controlled organization of carboxylic acid

derivatized colloidal silver particles on amine-terminated self-assembled monolayers. Chem Mater. 2000;12(5):1234-9. [42]

Sharma J, Mahima S, Kakade BA, Pasricha R, Mandale A, Vijayamohanan K. Solvent-

assisted one-pot synthesis and self-assembly of 4-aminothiophenol-capped gold nanoparticles. The JPhyChemB. 2004;108(35):13280-6. [43]

Dikin DA, Stankovich S, Zimney EJ, Piner RD, Dommett GH, Evmenenko G, et al.

Preparation and characterization of graphene oxide paper. Nature. 2007;448(7152):457-60. [44]

Paredes J, Villar-Rodil S, Martinez-Alonso A, Tascon J. Graphene oxide dispersions in

organic solvents. Langmuir. 2008;24(19):10560-4. [45]

Zhang D-D, Zu S-Z, Han B-H. Inorganic–organic hybrid porous materials based on

graphite oxide sheets. Carbon. 2009;47(13):2993-3000. [46]

Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z, Slesarev A, et al. Improved

synthesis of graphene oxide. ACS nano. 2010;4(8):4806-14. [47]

Wang G, Yang J, Park J, Gou X, Wang B, Liu H, et al. Facile synthesis and

characterization of graphene nanosheets. The JPhyChemC. 2008;112(22):8192-5. [48]

Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem

Soc Rev. 2010;39(1):228-40. [49]

Serra A, Genga A, Manno D, Micocci G, Siciliano T, Tepore A, et al. Synthesis and

characterization of TiO2 nanocrystals prepared from n-octadecylamine-titanyl oxalate LangmuirBlodgett films. Langmuir. 2003;19(8):3486-92. [50]

Guardia L, Villar-Rodil S, Paredes J, Rozada R, Martínez-Alonso A, Tascón J. UV light

exposure of aqueous graphene oxide suspensions to promote their direct reduction, formation of graphene–metal nanoparticle hybrids and dye degradation. Carbon. 2012;50(3):1014-24.

31

[51]

Khoa NT, Kim SW, Yoo D-H, Kim EJ, Hahn SH. Size-dependent work function and

catalytic performance of gold nanoparticles decorated graphene oxide sheets. Applied Catalysis A: General. 2014;469:159-64. [52]

Smith BC. Fundamentals of Fourier transform infrared spectroscopy: CRC press; 2011.

[53]

Goncalves G, Marques PA, Granadeiro CM, Nogueira HI, Singh M, Gracio J. Surface

modification of graphene nanosheets with gold nanoparticles: the role of oxygen moieties at graphene surface on gold nucleation and growth. Chem Mater. 2009;21(20):4796-802. [54]

Zhu T, Fu X, Mu T, Wang J, Liu Z. pH-dependent adsorption of gold nanoparticles on p-

aminothiophenol-modified gold substrates. Langmuir. 1999;15(16):5197-9. [55]

Chandramohan S, Kang JH, Katharria Y, Han N, Beak YS, Ko KB, et al. Chemically

modified multilayer graphene with metal interlayer as an efficient current spreading electrode for InGaN/GaN blue light-emitting diodes. J Phys D: Appl Phys. 2012;45(14):145101. [56]

Seema H, Kemp KC, Chandra V, Kim KS. Graphene–SnO2 composites for highly

efficient photocatalytic degradation of methylene blue under sunlight. Nanotechnology. 2012;23(35):355705. [57]

He F, Fan J, Ma D, Zhang L, Leung C, Chan HL. The attachment of Fe< sub> 3

O< sub> 4 nanoparticles to graphene oxide by covalent bonding. Carbon. 2010;48(11):3139-44. [58]

Mills A, Sheik M, O’Rourke C, McFarlane M. Adsorption and photocatalysed

destruction of cationic and anionic dyes on mesoporous titania films: Reactions at the air–solid interface. ApplCatalB Envi. 2009;89(1):189-95. [59]

LiáZhang L. Photocatalytic degradation of dyes over graphene–gold nanocomposites

under visible light irradiation. Chem Commun. 2010;46(33):6099-101. [60]

Ji Z, Shen X, Yang J, Xu Y, Zhu G, Chen K. Graphene Oxide Modified Ag2O

Nanocomposites with Enhanced Photocatalytic Activity under Visible‐Light Irradiation. Eur J Inorg Chem. 2013;2013(36):6119-25. [61]

Wang H-L, Zhao D-Y, Jiang W-F. VIS-light-induced photocatalytic degradation of

methylene blue (MB) dye using PoPD/TiO2 composite photocatalysts. Desalination and Water Treatment. 2013;51(13-15):2826-35. [62]

Xu T, Zhang L, Cheng H, Zhu Y. Significantly enhanced photocatalytic performance of

ZnO via graphene hybridization and the mechanism study. ApplCatalB Envi. 2011;101(3):382-7. 32

[63]

Zhang X, Quan X, Chen S, Yu H. Constructing graphene/InNbO< sub> 4

composite with excellent adsorptivity and charge separation performance for enhanced visiblelight-driven photocatalytic ability. ApplCatalB Envi. 2011;105(1):237-42. [64]

Soltani N, Saion E, Hussein MZ, Erfani M, Abedini A, Bahmanrokh G, et al. Visible

light-induced degradation of methylene blue in the presence of photocatalytic ZnS and CdS nanoparticles. International journal of molecular sciences. 2012;13(10):12242-58. [65]

Shen T, Zhao Z-G, Yu Q, Xu H-J. Photosensitized reduction of benzil by heteroatom-

containing anthracene dyes. J Photochem Photobiol, A. 1989;47(2):203-12. [66]

Jeong HK, Yang C, Kim BS, Kim K-j. Valence band of graphite oxide. EPL

(Europhysics Letters). 2010;92(3):37005. [67]

Islam MR, Joung D, Khondaker SI. Schottky diode via dielectrophoretic assembly of

reduced graphene oxide sheets between dissimilar metal contacts. New Journal of Physics. 2011;13(3):035021. [68]

Ferrari A, Meyer J, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, et al. Raman spectrum

of graphene and graphene layers. Phys Rev Lett. 2006;97(18):187401. [69]

Ferrari AC. Raman spectroscopy of graphene and graphite: disorder, electron–phonon

coupling, doping and nonadiabatic effects. Solid State Commun. 2007;143(1):47-57.

33