1 Chapter 12 Graphene-based transparent thin films and nanocomposites for energy storage 12.1 Preparation of transparent conductive thin films 12.2 Large area flexible transparent conductive thin films 12.3 Flexible transparent conductive thin films for electrode 12.4 Graphene-based nanocomposites for energy storage

Preparation of transparent conductive thin films[12-1] Although carbon films possess high hardness, conductive carbon coatings do not provide sufficient optical transparency and often have poor adhesion to substrates. Either additional metal interlayers, periodic multilayers of carbon, or pretreatment of substrates with ion implantation is required to promote adhesion of the carbon films to the substrates. Moreover, the metal oxide coatings are susceptible to ion diffusion from the metal oxide films into the substrates, which can be unfavorable for long term device performance. Attempts to exfoliate graphene-based sheets by intercalation of graphite with potassium metal have been discussed. GO prepared via the Hummers method was exfoliated in a water/ethanol mixture to produce a stable suspension of individual graphene oxide sheets. Addition of TMOS into this dispersion yielded graphene oxide-containing sols that can be stored at room temperature for several days (weeks for the sols with high weight percentage of graphene oxide (11 wt %)). Thin composite films can be then prepared from these sols by spin-coating onto hydrophilic substrates of either borosilicate glass or SiOx/silicon. Solvent evaporation leads to quick gelation, and the resulting composite films

3 TMOS: Tetramethyl orthosilicate is the chemical compound with the formula Si(OCH3)4. This molecule consists of four methyl groups. Si(OCH3)4 + 2 H2O → SiO2 + 4 CH3OH TEOS: Tetraethyl orthosilicate (Less toxic as compared with TMOS) Prior to spin-coating, substrate surfaces were treated with an oxygen plasma in order to increase their hydrophilicity. This step helps improve the adhesion of the sol during spin-coating deposition. During the spin-coating process, composite sols experience a combination of in-plane centrifugal and counter-balancing viscous forces on the rotating stage, and we suggest this leads to stretching and flattening of graphene oxide sheets that might be to some degree crumpled/wrinkled in the colloidal suspension.

4 During spinning, the thickness of the films increased slightly with increasing graphene oxide concentration (Figure 2d). In all cases, the thickness of the films decreased after the high-temperature curing step, indicating densification and consolidation of the matrix. TEM images of the cross sections of the sol-gel derived composite films with (a) 0 wt %, (b) 11 wt % of graphene oxide before the H.T.curing, and (c) 11 wt % of graphene oxide after the H.T. curing. The layers are, from the bottom, the glass substrate, composite film, Pt layer, and carbon layer. (d) Film thickness of the same samples after curing obtained from both TEM and XRR and the surface roughness obtained from XRR before and after curing. XRR ： X-ray reflectivity (thickness)

5 Pure silica Peak disappear Peak appear I: The non-oxygenated C at eV, II. The carbon in C-O at eV, III. The carbonyl carbon (C=O) at eV IV. The carboxylate carbon (O-C=O) at eV. The C 1s XPS spectra of the hydrazine-treated film shows the presence of the same functionalities (Figure 3b,c) but with much smaller contribution of the oxygenated carbons (27.6% vs 81.8%),

6 The lowest measured conductivity (resistance <1 GΩ) could be observed at loadings as low as 3.9 wt % of graphene oxide. As a control, pure silica film that were exposed to hydrazine and cured showed no conductivity. The chemical reduction step appears to be essential for converting insulating graphene oxide sheets into conductive graphene-like sheets and inducing electrical conductivity in composite samples. Films subjected to curing without chemical reduction were simply nonconductive. Chemical reduction alone renders the uncured hydrazine-treated films sufficiently conductive to be measured with our testing system but only at higher loading levels of the graphene oxide filler. ((1.1±0.1) ×10 -3 S/cm at 11 wt % to (7.0±0.7) ×10 -5 S/cm at 9.1 wt %, The combination of chemical reduction and high-temperature treatment improved the overall conductivity of the samples. Presumably, the consolidation of the film upon curing increases the density of the graphene-based sheets inside the matrix, reducing the average intersheet distances (change of film thickness) and resulting in more pathways for electrical conduction.

7 However, even at the highest loading (11 wt %), the transmittance is consistently high, ranging from 0.94 to 0.96 in the wavelength range of nm. The transparency of the graphene oxide-silica composite is further reduced after chemical reduction and curing (Figure 5b), primarily due to the “graphenization” of the nanofiller, and the transparency at 650 nm drops at most by only 4% after chemical reduction and curing. 11wt % graphene-silica composite spun-cast films 0.45 S/cm. The bulk conductivity of ITO films reported in the literature is 1×10 4 S/cm.

8 Use sodium borohydride (NaBH4) to reduce graphene oxide to graphene nanosheets (GNS), which contain the carboxylic functional group that becomes carboxyl (–COOH) or carboxylate anion (–COO)-type in the acid or alkaline environment, respectively. The GNS with didodecyldimethylammonium bromide (DDAB) particles becomes hydrophilic (A-GNS) or hydrophobic (B-GNS) through control of the pH value, which can be dispersed efficiently in water or a water/THF medium to deposit on the hydrophilic (poly(acrylic acid-acryl amide) PAA-AAM) or hydrophobic (polystyrene) substrate for preparing the transparent, conductive film (TCF) by spin coating. Preparation of transparent, conductive films by graphene nanosheet deposition on hydrophilic or hydrophobic surfaces through control of the pH value [12-2] Natural graphite GNS -COOH in acid (-COO)- in alkaline + NaBH4 + DDAB A-GNS B-GNS PAA-AAA substrate Polystyrene substrate GNS treated with N2H4 or NaBH4 precipitated as a hydrophobic surface, which is insoluble in water and organic solvents, resulting in further processing difficulties. The problem can be solved by treating the GNS with alkaline materials. Treated by alkaline materials, the surface of the GNS is charged because the carboxylic acid functional groups (–COOH) formed carboxylate anion (–COO), which may also enable the formation of well-dispersed GNS colloids.

9 By placing surfactant particles in the GO solution before reduction, the reduced GO still remain exfoliated and disperse in an aqueous medium. The GNS can also be dispersed in a polar organic solution. When the pH value of the GNS solution is controlled to approximately 9, thecarboxylic acid (–COOH) becomes a carboxylate anion (–COO), which is negatively charged. The cationic surfactant is added into the solution, which may attract the negative charged functional group on the GNS, and the hydrophobic chains of the surfactant can then enable the GNS to disperse in organic solvent. However, the surfactant particles remain on the surface of the GNS, and obstruct the transmission of electrons and reduce electrical conductivity. To solve this problem, the surfactant particles can be removed after HNO3 treatment. When using spin coating to prepare TCFs, the surface of the substrate must be treated by chemical materials to allow the surface, which becomes hydrophilic or hydrophobic, to ‘‘catch’’ the solution of GNS.

10 GNS with DDAB particles forms A-GNS or B-GNS through control of pH value, enabling GNS disperse in water or water/THF medium to deposit on the PAA-AAM or PS substrate for preparing the TCF

11 The C1s peak in the XPS spectrum of (a) GO and (b) GNS. Before using NaBH4 to reduce GO, NH4OH was used to treat the GO solution, which sets the pH value of the GO solution to approximately 9. The carboxylic functional groups remain on the GO surface after reduction. The carboxylic functional groups with a pH value of approximately 9 supply the negative charges (COO - ) on the GNS surface to be mixed with the cathode surfactant (DDAB). The hydrophilic heads of DDAB carry positive charges, which can attract the carboxylic functional groups, and the hydrophobic tail of DDAB is then oriented towards the water/THF phase.

12 When GO was reduced to the GNS, the large interlayer spacing no longer existed, and an intense steep peak of GO will shift back to the graphite type, due to the vanishing of the functional groups at the surface. Fig. 3(b) demonstrates that the inclusion of DDAB within the interlayer spaces may separate the sheets, preventing the GNS from aggregating. After reduction, the sharp peak of GO was no longer present and does not shift back to the graphite type, and either becomes weak or disappears GO A-GNS B-GNS

13 For the GO, the G-band becomes broad and up-shifts to 1594 cm1 because the resonance at frequencies of the isolated double bonds is higher than that of Graphene. For few-layer GNS, the ID/IG ratio is related to the in-plane crystallite size (La), which is given as La =4.4 (IG/ ID). The La value of GO can be obtained from the Raman spectrum that is 5.06 The ID/IG ratio of B-GNS, which is 1.08 (La=4.07), is higher than that of GO due to The presence of unrepaired defects that remain after the removal of large number of oxygenic functional groups. La:

14 The static contact angles of water droplet on the (a) PAA-AAM, (b) A-GNS, (c) HA-GNS, (d) PS, (e) B-GNS and (f) HB-GNS film surfaces. Sheet surface resistance and (b) transmittance varies with different cycles of spin coating.

15 Transparent and conductive thin films of graphene/polyaniline nanocomposites prepared through interfacial polymerization [12-3] Excellent transparent and conductive thin films can be obtained using polyaniline as matrix and graphene as filler prepared based on an interfacial polymerization.

16 Polyaniline is especially attractive because it is relatively inexpensive, has three distinct oxidation states with different colors and has an acid/base doping response. This latter property makes polyaniline an attractive for acid/base chemical vapor sensors. The different colors, charges and conformations of the multiple oxidation states also make the material promising for applications such as actuators, supercapacitors and electrochromics. (from wiki) Attractive fields for current and potential utilization of polyaniline is in antistatics, charge dissipation or electrostatic dispersive (ESD) coatings and blends, (EMI), coatings, hole injection layers, transparent conductors, ITO replacements, actuators, chemical vapor and solution based sensors, electrochromic coatings (for color change windows, mirrors etc.), PEDOT-PSS replacements, toxic metal recovery, catalysis, fuel cells and active electronic components such as for non-volatile memory. (from wiki) The majority of the reports on the graphene/polyaniline nanocomposites take advantage of the in situ polymerization of aniline over a graphene (or graphite oxide) dispersion as the synthetic approach. The graphene prepared in their work is easily dispersed in toluene.

17 Synthesis of the nanocomposites: mg of graphene were dispersed in 20 mL of a toluene solution of aniline. Different graphene/aniline ratios have been used, by fixing the graphene amount and varying the amount of aniline (2.5; 5; 10; 20 and 60 mL, corresponding to graphene/aniline weight ratios of 1/4; 1/8; 1/16; 1/32 and1/100, respectively). 2. The resulting mixture was transferred to a 50 mL round-flask containing 20 mL of a 1 mol L1 HCl aqueous solution in which a suitable amount of ammonium persulfate was previously dissolved, according to the starting amount of aniline. The magnetic stirring was subsequently interrupted and immediately a continuous, self-standing, homogeneous and transparent film was spontaneously formed at the interface. The film was transferred from the water/oil interface and deposited on glass substrates.

18 The spectrum of the neat polyaniline presents the following PANI-ES bands: a shoulder at 1640 cm -1 (cross-linked chains), 1623 cm -1 ( C–C of the benzene rings), 1580 cm -1 ( CQC of the quinoid rings), 1517 cm -1 ( CQNH+ of the quinoid protonated di-imine units), 1485 cm -1 ( CQN of the quinoid non-protonated di- imine units), 1337 and 1318 cm -1 ( C–N+, characteristic band of the polaron radical cation), 1255 cm -1 ( C–N of benzenoid and quinoid rings), 1186 (sh) and 1168 cm -1 (C–H bending of the benzenoid and quinoid rings, respectively). Raman spectra of the films: (a)neat polyaniline; (b)GR/PANI-1/32; (c)GR/PANI-1/16; (d)GR/PANI-1/8; (e)GR/PANI-1/4; (f) neat graphene.

19 Emeraldine PANI salt (PANI-SE) The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is neutral, if doped (protonated) it is called emeraldine salt (ES). The bands associated to the oxidized portions of polyaniline chains (quinoid rings and imine nitrogen, associated to the bi-polaron carrier in PANI-ES) are strongly modified by the presence of graphene, which can be an indicative that the interaction between the graphene and the polymer occurs through these segments: (ii) the bands at 1485 and 1517 cm -1 collapse to one single and broad band that is red shifted according to the increase in the amount of graphene (1480, 1472, 1464 and 1464 cm -1 for samples PANI/GR 1/32, 1/16, 1/8 and 1/4, respectively); (iii) the band at 1255 cm1 is red shifted to 1218 cm -1. (i) the band of polyaniline at 1168 cm-1 is red shifted to 1160 cm-1 by the presence of graphene. This shift has been usually associated to an increase in the bi- polaron segments on the polymer chain;

20 The low-frequency region of the Raman spectra (200– ) is very sensitive to the structure and conformation of the polyaniline chains. As showing in Fig. 2, right, the polymer bands become broader and less definite in the presence of graphene, showing less uniformity in the polymer structure. There are some indicatives that the polymer chain becomes less planar (more coiled) in the presence of graphene. (1)The band at 294 cm -1 present in the neat polyaniline is characteristic of a pseudo-orthorhombic crystalline structure of the polymer, which is not observable in the nanocomposites. (2) The bands at 810, 835 and 873 cm -1 are attributed to out-of-plane C–H motions and are very sensitive to the torsion angle between two aniline rings. (3) All three modes are clearly observed in the neat PANI and almost disappear in the samples containing graphene, corroborating the model in which the graphene induces the occurrence of less-planar polyaniline chains. (4) The intensity of the cross-linked band at 574 cm -1 decreases in the presence of graphene (and the band at 1640 cm -1 disappears), which indicates that the cross-linking degree of the samples decreases in the presence of graphene.

21 The interpretation of the Raman data indicates that apparently the interaction between the graphene and PANI-ES occurs through the oxidized segments of the polymer, inducing a more bipolaronic and less planar structure of the PANI. It is well known that to be used in optoelectronic devices transparent electrodes must have a sheet resistance lower than 100 Ω sq-1 coupled with optical transmittance at 550 nm of approximately 90%. The best sample (GR/PANI-1/32) presented a sheet resistance of 60.6 Ω sq -1 and transmittance of 89%.

Nano Letters, Watcharotone J of Materials Chemistry, Tien ChemComm, Dominggues Applied Physics Letters, Kim ACS nano, Zheng, Nature Nanotechnology Applied Physics Letters, Kobayashi Nano Letters, Wang Chemistry of Materials Energy Environ. Sci., Pumera Nano Letters, Li Nanoscale, Nguyen 2011 References