Reduced Graphene Oxide Thin Films Research Articles

Study of Single Diode Model of DSSC Based on Reduced Graphene Oxide as Counter Electrode

We studied the characteristic of reduced graphene oxide (RGO) as a counter electrode (CE) in Pt-free dye-sensitized solar cell (DSSC) using single diode model equation based on current density-voltage (J-V) measurement. The electrical characteristics of DSSC was investigated by electrochemical impedance spectroscopy (EIS) in order to knowing the catalytic properties of RGO as counter electrode. The RGO thin film as CE was deposited on fluorine-doped tin oxide (FTO) substrate and stacked with FTO/TiO2/ruthenium dye/mosalyte/RGO/FTO DSSC structure. The RGO film was obtained from GO (graphene oxide) film that was thermally reduced at 200°C for one hour under argon flow. The performance of DSSC with RGO as CE was characterized by current-voltage measurement and the interface between each layer was observed by Electrochemical Impedance Spectroscopy (EIS).   The device efficiency (η) of DSSC that used GO and RGO film as CE are 0.09% and 3.43%, respectively. DSSC parameters such as series resistance (RS) shunt resistance (RSH), and ideality factor (n) were obtained from J-V curve which analyzed using single diode model equation. All samples have a Rs value of 2 indicates that all devices have a good ohmic contact. The RSH of device using RGO-1mg/ml is increase (from 2850 to 3670 Ω.cm2) compared with GO indicate that thermal reduction is successes. The RGO film shows a comparable performance to Pt-conventional CE thus it is a good candidate as alternative of DSSC counter electrode.

Electrical and Optical Characterization of Graphene Oxide and Reduced Graphene Oxide Thin Films

Despite a growing interest in graphene, an aspect which is less studied is the electrical and optical characterization of graphene oxide (GO)-based transparent conductors obtained using thermal annealing. In addition, few research works have studied the electrical properties of GO and reduced graphene oxide (RGO) films using electrical impedance measurements. In this study, electric impedance measurements are performed on GO and thermally reduced GO films dip-coated on glass substrates. The electric resistance of RGO films decreases by about two orders of magnitude compared to GO films. Moreover, optical microscopy and variable angle spectroscopic ellipsometry (VASE) were carried out on the same samples. Thermal annealing increases the optical conductivity and the absorption coefficient of GO films. Such findings could be used in many optoelectronic applications, improving future GO applicability.

Effect of substrate temperature on electrical properties of RGO thin films deposited by atomized spray pyrolysis

Synthesis of reduced graphene oxide (RGO) by the modified Hummer's method is an excellent way for the bulk production of RGO powder. A novel atomized spray pyrolysis deposition (ASPD) method was used to deposit RGO thin films on borosilicate glass substrates. Deposition temperature was varied to optimize the electrical conductivity of RGO thin films. Turbostratic disorder and oxygen content of RGO powder decreased after depositing onto a thin film. Hall measurements and Raman analysis confirmed the deposition of p-type RGO thin films. Post-deposition annealing of the films decreased the oxygen content, recovering sp2 clusters. The conductivity of the RGO thin film was optimized at a substrate temperature of 150 °C at 16.8 × 102 S m−1.

A Comparative Investigation of Chemically Reduced Graphene Oxide Thin Films Deposited via Spray Pyrolysis.

We present a comparative investigation between thin films of graphene oxide (GO) and chemically reduced graphene oxide (rGO) deposited onto glass substrates via spray pyrolysis. Two reduction techniques are investigated: (1) the exposition of a sprayed layer of GO to vapors of hydrazine hydrate to produce rGOV and (2) the addition of liquid hydrazine hydrate to a suspended GO solution, which is then sprayed onto a substrate to produce rGOL. Three different spectroscopy techniques, Raman, Fourier transform infrared, and UV–Vis–NIR, show that the two reduced samples have less lattice disorder in comparison to GO, with rGOL having the highest degree of reduction. Interestingly, topography characterization by atomic force microscopy reveals a morphological change occurring during the exposure to hydrazine hydrate vapors, resulting in a thickness of 110 nm for the rGOV film, which is a factor of 16 larger than rGOL and GO. Finally, I–V measurements show a significant decrease of the GO’s resistivity after the reduction process, where rGOL features a resistivity 90 times lower than rGOV, confirming that rGOL has the highest degree of reduction. Our results indicate that the reduction process for rGOV is susceptible to introducing intercalated water molecules in the material while the fabrication technique for rGOL is a suitable route to obtain a material with minimal lattice disorder and properties approaching those of graphene.

Deposition of Reduced Graphene Oxide Thin Film by Spray Pyrolysis Method for Perovskite Solar Cell

The Perovskite absorber layer, the electron transport layer (ETL), the hole transport layer (HTL), and the transparent conducting oxide layer (TCO) are the major components that make up a Perovskite solar cell. Between ETL and HTL, the absorber layer is sandwiched, on which electron-hole pairs are created after absorption of solar radiation. Despite substantial progress toward efficiency, long-term stability still remains a serious concern. Present work focuses toward contributing on the later issue by adopting Titanium dioxide (TiO2) as ETL and reduced graphene oxide (rGO) as HTL. Specifically, in the present work, we report our efforts on the preparation of compact titanium dioxide (C-TiO2) and mesoporous titanium dioxide (M-TiO2) layers as an ETL and a reduced graphene oxide thin film as a HTL. The C-TiO2 film was spin casted on FTO glass followed by casting of M-TiO2 film using Doctor Blading technique. Similarly, the rGO film was produced by spray casting over the glass substrate. The asprepared ETL and HTL layers were characterized by measuring their optical properties (transmittance and reflectance of thin films). Then, the bandgap, Eg was extracted from reflectance and transmittance curves for ETL and HTL respectively. In the case of rGO, we found the value of Eg to be 2.1 eV, which varies between 2.7eV and 0.02eV depending upon its reduction level based on the previously reported values. Similarly, the bandgap of the C-TiO2 was 4.51 eV which was reduced to 4.12 eV after the addition of M-TiO2 , which are 0.9 to 1.1 eV higher than previously reported values. However, bandgap shows decreasing trend after employing M- TiO2 over C-TiO2. In a Perovskite solar cell, both ETL and HTL will be investigated.

Effect of annealing temperature on electrical properties of RGO thin films deposited by Atomized Spray Pyrolysis

The modified Hummers method is an ideal method for the bulk production of reduced graphene oxide (RGO) using Sri Lankan natural vein graphite. Deposition of RGO as a thin film (TF) on a substrate is essential for electronic applications in various industries. A novel, improved spray pyrolysis technique called Atomized Spray Pyrolysis deposition method was used to deposit RGO-TFs at a substrate temperature of 150 °C. Thermal annealing (TA) was carried out after deposition of RGO-TFs since post-deposition treatment is essential to enhance its intrinsic properties. Then TA was done for 30 min at different temperatures from 230 to 430 °C. Electrical, morphological and structural characterizations suggested that TA at 330 °C yields optimum intrinsic properties of RGO-TFs. Annealing temperature up to 330 °C is adequate to eliminate most defects such as fabrication residues, residual oxygen functional groups, and folds at edges of RGO flakes. TA at 330 °C retains maximum possible hexagonal lattice structure and carbon to oxygen atomic ratio. However, TA beyond 330 °C exhibits doping of oxygen atoms to vacant carbon atoms at the hexagonal structure of RGO, increasing bandgap since oxygen atoms localize some electrons. At higher annealing temperatures, the formation of buckling, ripples, and wrinkles on the RGO sheets due to induced compressive strain was observed. This results in hindering charge transportation since they behave as scattering sites.

Hydrophobic behaviour of reduced graphene oxide thin film fabricated via electrostatic spray deposition

Graphene-based materials such as graphene oxide (GO) and chemically reduced graphene oxide (rGO) thin films have been fabricated using electrostatic spray deposition (ESD), followed by thermal annealing under Ar/H2 atmosphere. The thickness and surface morphology of thin films of GO and rGO on silicon substrates were controlled by varying deposition time and content of GO/rGO in tetrahydrofuran (THF) solution. Here, we present a comparative analysis between GO and rGO thin films. Water contact angle (WCA) measurements of these thin films were ~88° for the GO films and >127° for the rGO films. We discuss how their hydrophobic behaviour is influenced by removal of oxygen-containing functional groups during the reduction process.

Fabrication of Reduced Graphene Oxide Thin Films on Corona Treated Silicon Substrates

Graphene is a material with excellent properties. However, the manufacture of a single-layer graphene film with a large area is challenging. It requires modern and expensive equipment. In this study, we suggested another approach to improve these problems. The raw material graphite was used to prepare graphene oxide solutions. The resulting graphene oxide solutions were then dispersed onto silicon substrates first treated with corona plasma, which increases the adhesion between materials and substrates. In the next step, these covered substrates were deoxygenated by heating at very high temperatures for a short time to form reduced graphene oxide, which has graphene-like properties. The reduced graphene oxide films formed had a uniform structure over a large area of 2 × 2 cm2 with an oxygen content of about 4%, a film roughness of less than 5 nm, and a sheet resistance of 4-5 ohm/square.

Microstructure and electrothermal characterization of transparent reduced graphene oxide thin films manufactured by spin-coating and thermal reduction

We herein report the microstructure, optical and electrothermal properties of a series of reduced graphene oxide (RGO) thin films, which are manufactured by spin-coating (1 and 3 layers) of graphene oxide (GO) dispersion on a quartz substrate and thermal reduction at different temperatures of 800–1000 °C. The energy dispersive X-ray spectra reveal that the relative carbon content of RGO films increases from 84.3 at% to 93.5 at% with increasing the thermal reduction temperature from 800 °C and 1000 °C, whereas the oxygen content decreases from 15.7 at% to 6.5 at%. The optical transmittance of RGO1 (1 spin-coating layer) and RGO3 (3 spin-coating layers) films at 550 nm is characterized 32.7–40.8% and 6.3–12.4%, respectively, although it decreases with increasing the thermal reduction temperature. The sheet resistance of RGO1 films decreases from 5160 Ω/sq to 1220 Ω/sq with the increment of the thermal reduction temperature, whereas the sheet resistance of RGO3 films decreased from 1080 Ω/sq to 350 Ω/sq. The decreases in optical transparency and sheet resistance values of RGO thin films are due to the successful reduction of GO films with oxygen-related functional groups to electrically conductive graphene films during the thermal treatment. The RGO thin films are characterized to exhibit high performance electrothermal behavior in terms of uniform temperature distribution, controllable steady-state saturated temperatures, rapid temperature responsiveness, and electric power efficiency at input power densities.

Functionalized Reduced Graphene Oxide Thin Films for Ultrahigh CO2 Gas Sensing Performance at Room Temperature.

The demand for carbon dioxide (CO2) gas detection is increasing nowadays. However, its fast detection at room temperature (RT) is a major challenge. Graphene is found to be the most promising sensing material for RT detection, owing to its high surface area and electrical conductivity. In this work, we report a highly edge functionalized chemically synthesized reduced graphene oxide (rGO) thin films to achieve fast sensing response for CO2 gas at room temperature. The high amount of edge functional groups is prominent for the sorption of CO2 molecules. Initially, rGO is synthesized by reduction of GO using ascorbic acid (AA) as a reducing agent. Three different concentrations of rGO are prepared using three AA concentrations (25, 50, and 100 mg) to optimize the material properties such as functional groups and conductivity. Thin films of three different AA reduced rGO suspensions (AArGO25, AArGO50, AArGO100) are developed and later analyzed using standard FTIR, XRD, Raman, XPS, TEM, SEM, and four-point probe measurement techniques. We find that the highest edge functionality is achieved by the AArGO25 sample with a conductivity of ~1389 S/cm. The functionalized AArGO25 gas sensor shows recordable high sensing properties (response and recovery time) with good repeatability for CO2 at room temperature at 500 ppm and 50 ppm. Short response and recovery time of ~26 s and ~10 s, respectively, are achieved for 500 ppm CO2 gas with the sensitivity of ~50 Hz/µg. We believe that a highly functionalized AArGO CO2 gas sensor could be applicable for enhanced oil recovery, industrial and domestic safety applications.

Organic sensitization of graphene oxide and reduced graphene oxide thin films for photovoltaic applications

In this work, a new organic compound (K-Azo) was introduced to enhance the electrical and optical performance of graphene oxide (GO) and reduced GO (rGO) nanostructured films. The improved and modified chemical vapour method was employed for the synthesis of GO and rGO. The photophysical characterization of thin films was performed by applying analytical techniques including X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible, and Raman spectroscopy. The electrical properties (I-V characteristic) of GO and rGO thin films displayed higher conductivity which was 4.07 × 10−7 and 1.10 × 10−3 S/cm, respectively in the presence of organic sensitizer. However, GO and rGO thin films showed 9.91 × 10−7 and 6.17 × 10−4 S/cm, respectively in the absence of K-Azo sensitizer. Optical and electrical investigations indicated that the characteristics of GO and rGO were improved due to the presence of organic sensitizer. The long-range π-electron delocalization in organic sensitizer contributed to higher conductivity for potential photovoltaic solar cell applications.

Characteristics of a Multiple-Layered Graphene Oxide Memory Thin Film Transistor with Gold Nanoparticle Embedded as Charging Elements

In this study, we report the highest mobility in the reduced graphene oxide- (rGO-) based TFTs embedded with Au NPs. In addition, we fabricated a reduced graphene oxide memory device (rGO-capacitor), a reduced graphene oxide thin film transistor (rGO-TFT), and a reduced graphene oxide memory thin film transistor (rGO-MTFT) and characterized their electrical performances. While the rGO-TFT device was investigated for nonambipolar channel performance, the rGO-capacitor and rGO-MTFT were examined for nonvolatile memory capabilities in a metal-graphene-insulator-silicon (MGIS) structure. The incorporation of the gold nanoparticles (Au NPs) between the rGO and an insulator silicon dioxide (SiO2) layer served as a charging element. The rGO-capacitor revealed the memory effect of hysteretic capacitance-voltage (C-V) loops, and the flat-band voltage shift ( Δ V FB ) was measured as 0.1375 V after 100 s retention time. The rGO-TFT shows the p-channel characteristics with high hole mobility of 16.479 cm2/V⋅s. The threshold voltage shift ( Δ V th ) of the rGO-MTFT was detected as 5.74 V from 10 V to -30 V sweep, demonstrating high mobility of 22.887 cm2/V⋅s.

Reduced Graphene Oxide Thin Film with Strong Optical Nonlinearity

Herein, a simple, low‐cost, and scalable process is developed to prepare the reduced graphene oxide (rGO) thin films (TFs) with strong optical nonlinearity. To do so, a chemical reduction method (using inorganic agent hydroiodic acid) and drop‐casting technique are used. Optical nonlinearity of the prepared thin rGO film is measured by the Z‐scan technique at low light intensity (continuous‐wave wavelength of 632 nm). The result of optical nonlinearity responses is illustrated by a remarkable nonlinear refraction index (n2 ≈ 10−4 cm2 W−1) as well as a considerable nonlinear absorption coefficient (β = 1.43 × 10−8 cm2 W–1) compared with similar reported results. Moreover, experimental results show that the prepared film enjoys a positive nonlinear refractive index with strong optical nonlinearity. Results also confirm that the changes made in optical absorption of rGO TFs are due to state filling effects and the optical nonlinearity proportional to the carriers’ density is in fact due to thermal effects of SiO2 substrate.

Multilayer Engineering of Polyaniline and Reduced Graphene Oxide Thin Films on a Plastic Substrate for Flexible Optoelectronic Applications Using NIR

This article describes the photoconductive properties of a multilayer uniform ultrathin film comprising alternating polyaniline (PANi) and reduced graphene oxide (RGO) layers, fabricated on a poly(ethylene terephthalate) (PET) sheet. The fabrication of the two electron-rich layers on the PET substrate is successfully completed using a layer-by-layer (LBL) deposition technique under mild conditions and HI/H2O vapor treatment at 100°C. The photocurrent under illumination of >300 nm light exhibits 80 µA compared with that of obtained in the dark. However, it decreases to 75 µA under >460 nm light illumination and further increases to 80 µA under >600 nm light illumination (0.64 mW) at an applied voltage of 1.0 V. The PET sheets coated with (PANi/RGO)20 films (d = 46.3 nm) exhibits a photoresponsivity of 125 mA/W at an illunination intensity of 0.64 mW using red light (> 600 nm). The extraordinary optoelectronic characteristics of the (PANi/RGO) films are ascribed to the charge transfer complex formation of the two PANi and RGO layer components and also to the structural uniformity of the LBL-assembled optoelectronic thin films, which tended to favor the rapid interfacial charge transfer to the electrodes.

Chemical-free transfer of patterned reduced graphene oxide thin films for large area flexible electronics and nanoelectromechanical systems

In this paper, a wet-dry hybrid technique to transfer patterned reduced graphene oxide (rGO) thin film to arbitrary substrates at predetermined locations without using any chemicals is reported. The transfer process involves water-assisted delamination of rGO, followed by dry transfer to an acceptor substrate using viscoelastic stamp. Patterned reduced graphene oxide films are transferred to silicon dioxide (SiO2/Si) substrate to begin with. Subsequently, the method is deployed to transfer rGO to different polymer substrates such as poly(methyl methacrylate) (PMMA), and crosslinked poly(4-vinylphenol) (c-PVP), which are commonly used as gate dielectric in flexible electronic applications. The credibility of the transfer process with precise spatial positioning on the target substrate leads to fabrication of freely suspended reduced graphene oxide membrane towards nanoelectromechanical systems (NEMS) based devices such as nanomechanical drum resonators.

Reduced graphene oxide layered WO3 thin film with enhanced electrochromic properties

The electrochromic (EC) properties of inorganic-organic hybrids of tungsten oxide/reduced graphene oxide (WO3/rGO) thin films were examined. Using hydrothermal method WO3 nanorods were deposited on a conducting Indium-doped tin oxide (ITO) glass substrate. In succession, the reduced graphene oxide thin film was coated on WO3 thin film using improved Hummers method. The structural, morphological, optical, and electrochromic responses of WO3, rGO, and WO3/rGO films are illustrated. Compared with pure WO3 film, WO3/rGO composite film demonstrates improved EC performance because of enhanced double insertion/extraction of ions and electrons. It realizes a large optical modulation (58.8% at 633 nm) with a significant increase in Coloration Efficiency (CE) and EC stability.

Tuning of reduced graphene oxide thin film as an efficient electron conductive interlayer in a proven heterojunction photoanode for solar-driven photoelectrochemical water splitting

Although bismuth vanadate (BiVO4) has shown excellent photoelectrochemical (PEC) properties and is a good candidate of photoanode materials, the solar-driven PEC water splitting performance is still remained below its full potential due to the fast recombination and sluggish charge mobility of photogenerated charge carriers. Previously, we have communicated a proven Type II staggered vanadium pentoxide (V2O5)/BiVO4 heterojunction photoanode that could improve the photocurrent density. This study aimed to examine the effect of introducing an rGO thin film as an efficient electron conductive interlayer in a proven V2O5/BiVO4 heterojunction photoanode, and subsequently tuning the rGO film thickness in achieving the optimum PEC performance. The resultant ternary photoanode structure of V2O5/rGO/BiVO4 was characterised by using field emission-scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), UV–vis spectroscopy, X-ray diffractometer (XRD), Raman spectroscopy and photoluminescence (PL) measurements. Results showed that the interlayer rGO thin film arising from the sequential drop cast and electrochemical reduction of 320 μL ultrasonicated GO solution resulted in the optimal photocurrent density of 2.1 mA/cm2 at 1.5 V vs. Ag/AgCl. Furthermore, the chemical physics surrounding the photogenerated charge carrier transfer for heterojunction V2O5/BiVO4 was validated for the structure with and without the rGO interlayer. In particular, the electrochemical impedance spectroscopy (EIS) was used to measure multiple resistances at the FTO/semiconductor, semiconductor/semiconductor and semiconductor/electrolyte interfaces. Additionally, the charge transfer (Kt) and recombination (Kr) rate constants for the heterojunction V2O5/BiVO4 with the rGO interlayer were quantified using intensity modulated photocurrent spectroscopy (IMPS). Finally, the PEC H2 evolution rate from the ternary V2O5/rGO/BiVO4 photoanode was measured to be 32.7 μ mol/hr, which was about 3-fold higher than the bare V2O5/BiVO4 heterojunction photoanode.

Thermally Conductive Reduced Graphene Oxide Thin Films for Extreme Temperature Sensors

AbstractReduced graphene oxide (RGO) films are promising in applications ranging from electronics to flexible sensors. Though high electrical and thermal conductivities have been reported for RGO films, existing thermal conductivity data for RGO films show large variations from 30 to 2600 W m−1 K−1. Further, there is a lack of data at low temperatures (<300 K), which is critical for the understanding of thermal transport mechanisms. In this work, a temperature‐dependent study of thermal (10–300 K) and electrical (10–3000 K) transport in annealed RGO films indicates the potential application of RGO films for sensing temperatures across an extremely wide range. The room‐temperature thermal conductivity increases significantly from 46.1 to 118.7 W m−1 K−1 with increasing annealing temperature from 1000 to 3000 K with a corresponding increase in the electrical conductivity from 5.2 to 1481.0 S cm−1. In addition, films reduced at 3000 K are promising for sensing extreme temperatures as demonstrated through the measured electrical resistivity from 10 to 3000 K. Sensors based on RGO films are advantageous over conventional temperature sensors due to the wide temperature range and flexibility. Thus, this material is useful in many applications including flexible electronics and thermal management systems.

Electrophoretic deposition of reduced graphene oxide thin films for reduction of cross-sectional heat diffusion in glass windows

Abstract Effective management of heat transfer, such as conduction and radiation, through glass windows is one of the most challenging issues in smart window technology. In this work, reduced Graphene Oxide (rGO) thin films of varying thicknesses are fabricated onto Fluorine-doped Tin Oxide (FTO) glass via electrophoretic deposition technique. The sample thicknesses increase with increasing number of deposition cycles (5, 10, 20 cycles). It is hypothesized that such rGO thin films, which are well-known for their high thermal conductivities, can conduct heat away laterally towards heat sinks and reduce near-infrared (NIR) transmittance through them, thus effectively slowing down the temperature increment indoors. The performance of rGO/FTO in reducing indoor temperatures is investigated with a solar simulator and a UV-Vis-NIR spectrophotometer. The 20-cycles rGO thin films showed 30% more NIR blocked at 1000 nm as compared to clean FTO, as well as the least temperature increment of 0.57 °C following 30 min of solar irradiation. Furthermore, the visible transmittance of the as-fabricated rGO films remain on par with commercial solar films, enabling up to 60% of visible light transmittance for optimal balance of transparency and heat reduction. These results suggest that the rGO thin films have great potential in blocking heat transfer and are highly recommended for smart window applications.

A microstructuring route to enhanced thermoelectric efficiency of reduced graphene oxide films

Microstructuring of nanocomposites by methods such as corrugation and wrinkling is explored as a new tool to improve their thermoelectric efficiency of energy harvesting. Its effects include modulating a given composite’s electrical conductivity and Seebeck coefficient. Here we demonstrate that corrugating and wrinkling a reduced graphene oxide thin film can give rise to an enhanced electrical conductivity as well as Seebeck coefficient which together results in an increased thermoelectric power factor. Specifically, wrinkling in two dimensions greatly increased both the Seebeck coefficient, up to 26%, and the power factor by almost an order of magnitude. The observed enhancements are attributable to an increased electrical network conductivity of the stacked reduced graphene-oxide flakes and a phonon drag effect associated with a higher packing density. This microstructuring route is simple yet effective in harvesting thermoelectric energies and can be naturally occurring in, highly scalable for, and applicable to flexible, stretchable and wearable electronics.