Claims:We Claim:
1. A nanocomposite comprising;
a) Graphene Oxide nanosheets, prepared by a method comprising acts of
i. oxidising graphite using potassium permanganate, hydrogen peroxide and deionized water in presence of sulphuric acid to obtain graphite oxide,
ii. optionally purifying the graphite oxide using hydrochloric acid and deionized water, and
iii. exfoliating the graphite oxide to obtain graphene oxide nanosheets.
and
b) metal nanoparticles,
Wherein the ratio of graphene oxide to metal nanoparticle is 1:1 w/w.
2. The nanocomposite as claimed in claim 1, wherein the graphene oxide nanosheet is of size ranging from about 300 nm to 600 nm, preferably about 400±50 nm.
3. The nanocomposite as claimed in claim 1, wherein the metal nanoparticle is of size ranging from about 100 nm to about 500nm, preferably about 400±20 nm.
4. The nanocomposite as claimed in claim1, wherein the metal nanoparticle is selected from a group comprising Ag, Au, Fe and Pt, preferably Pt.
5. A process for preparing the nanocomposite of claim 1, said process comprising acts of
a) synthesising graphene oxide nanosheets by a method comprising acts of
i. oxidising graphite using potassium permanganate, hydrogen peroxide and deionized water in presence of sulphuric acid to obtain graphite oxide,
ii. optionally purifying the graphite oxide using hydrochloric acid and deionized water, and
iii. exfoliating graphite oxide to obtain graphene oxide nanosheets.
b) dispersing graphene oxide nanosheets and metal nanoparticles in a solvent in a ratio¬¬¬ of 1:1 w/w to obtain the nanocomposite.
6. The process as claimed in claim 5, wherein the solvent for dispersing graphene oxide nanosheets and metal nanoparticles is selected from a group comprising N- Methyl-2-pyrrolidone (NMP), Dimethyl formamide (DMF), Tetrahydrofuran (THF), acetone and water , preferably N-Methyl-2-pyrrolidone.
7. A sensor comprising nanocomposite of claim 1, fabricated on a substrate wherein the substrate is a flexible substrate selected from a group comprising inorganic polyimide films (kapton), mylar, PEN, PET, thin metal / metal alloy substrate like Ni, Cu, Al / Stainless Steel with an insulating coating, preferably polyimide films (kapton).
8. A method of fabricating a sensor comprising a nanocomposite of claim 1, said method comprising acts of
i. coating the nanocomposite on a substrate to obtain a patterned substrate, and
ii. annealing (80°C) the nanocomposite coated on the patterned substrate toobtain a nanocomposite film.
9. The method of claim 8 , wherein the nanocomposite is annealed at temperature ranging from about 50°C to about 90°C preferably about 80°C.
10. The method of claim 8, wherein the thickness of nanocomposite film is of size ranging from about 30µm to 70µm preferably 50 ± 20µm.
11. The sensor as claimed in claim 7, wherein the sensor is for sensing temperature, pressure, humidity, gas and biological samples, preferably for temperature..
12. The sensor as claimed in claim 7, wherein the sensor measures the temperature ranging from about -60°C to about 85°C .
13. The sensor as claimed in 12, where in the sensor is completely coated with Parylene (thickness of 1.5 µm) in order to protect the device from moisture, dust, avoids the film from undergoing physical damage.
14. The method of claim 13, wherein the complete Parylene coated devices were annealed at 150°C for 4 hours. The purpose of annealing the device is for getting the rearrangement of atoms in order to obtain uniform films for achieving good stability.
, Description:Field of invention:
The present invention relates to the field of sensors. More specifically, the invention relates to piezoresistive nanocomposites for use in sensors. In particular the invention is in relation to Graphene Oxide (GO) – Platinum (Pt) nanocomposite (GO-Pt) for temperature sensor applications, based on the principle of negative temperature coefficient (NTC) resistivity. The invention discloses the synthesis of the nanocomposite and also the method of fabrication of the nanocomposite on a substrate.
Background:
In the recent past, sensor technology has attracted worldwide attention for various applications. The most frequently used types of sensors are based on pressure, thermal, electric current, magnetic, or radio sensors. The most commonly measured physical parameter is temperature whether in industrial applications, medical or in laboratory settings. Temperature sensors measure the temperature that is generated by an object or system. The different types of temperature sensors include thermostats, thermistors, resistance temperature detectors, thermocouples, radiation thermometers, thermal imagers, thermopiles and the like. These devices are made up of transition metal oxide (MnO2, NiO, CoO and the like) with ceramic process technology and semiconductor technology. Though the traditional sensors exhibit sensitivity, they have a low response time and a limited temperature sensing range. Also they undergo deformation when mounted on curved surfaces due to their restricted bendable or stretchable nature. Recently, nanoscale materials are found to be attractive candidates for temperature sensing resistive elements due to their unique properties such as large surface to volume ratio and dimensionality in their size. These materials lead to make a sensor with high performance while miniaturizing the device size and minimizing the power consumption. These sensors involve nanoparticles, nanotubes, nanowires and films.
Currently, Graphene has proved to be a beneficial nanomaterial due to profound thermal, mechanical and electrical properties of graphene. Graphene has a two dimensional (2D) single planar sheet of sp² bonded carbon atoms that are densely packed in honeycomb like crystal lattice. Research is being carried out on graphene and its derivatives for applications such as optoelectronics, environmental sensors, energy storage devices, biomedical devices and the like. Graphene and its derivatives are devised into films for use in sensing applications.
US 9178129discloses a sensor which comprises a graphene thin film that exhibits negative temperature coefficients (NTC), resulting in rapid decrease in electrical resistance as temperature increases, as well as a much faster response time.The electrical resistance of the graphene film also increases in response totheincrease in environmental humidity. The electrical resistance changes of the graphene film can also be used as a sensing mechanism for changes in chemical and biological parameters in the environment of the sensor.
Document titled “Electrocatalytically Active Graphene-Platinum Nanocomposites. Role of 2-D Carbon Support in PEM Fuel Cells” J. Phys. Chem. C, 2009, 113 (19), pp 7990–7995discloses the use of reduced graphene oxide and metal nanocomposites for use in electrocatalytic applications. The aforementioned document discloses the use of a 2-D carbon nanostructure, graphene, as a support material for the dispersion of Pt nanoparticles to provide new ways to develop advanced electrocatalyst materials for fuel cells. Platinum nanoparticles are deposited onto graphene sheets by means of borohydride reduction of H2PtCl6 in a graphene oxide (GO) suspension. The partially reduced GO-Pt catalyst is deposited as films onto glassy carbon and carbon Toray paper by drop cast or electrophoretic deposition methods. Nearly 80% enhancement in the electrochemically active surface area (ECSA) can beachieved by exposing partially reduced GO-Pt films with hydrazine followed by heat treatment (300°C, 8 h). The electrocatalyst performance as evaluated from the hydrogen fuel cell demonstrates the role of graphene as an effective support material in the development of an electrocatalyst.
The aforementioned two documents discloses the use of graphene and reduced graphene oxide for use in sensing and electrocatalytic applications respectively. The synthesis of the nanocomposite films and also the fabrication of the films are expensive with restricted temperature sensing range and limited flexibility when mounted onto large curved surfaces.
Hence there is a need for a sensor with improved sensitivity and response time, improved flexibility in terms of bendability, and which can be adapted for various sensing applications including temperature, pressure, speed, humidity, gas and biological applications.
The present invention provides for a nanocomposite sensor, which performs with enhanced sensitivity, response time and flexibility in terms of bendability and a simplified process of fabrication of the sensor onto a substrate; which allows for reducing the device size as well as minimizing the power consumption; which allows for mass production of the sensor at a low cost.
Summary of the invention:
Accordingly, the present invention is in relation to a nanocomposite comprising; graphene oxide nanosheets, and metal nanoparticles; wherein the graphene oxide is prepared by oxidizing graphite using potassium permanganate, hydrogen peroxide and deionized water in the presence of sulphuric acid to obtain graphite oxide, optionally purifying the graphite oxide using hydrochloric acid and deionized water, and exfoliating the graphite oxide obtained to beget graphene oxide nanosheets; wherein the ratio of graphene oxide nanosheets to metal nanoparticle is 1:1 w/w. The invention also provides for a process for preparing the nanocomposite and also a method of fabrication of the nanocomposite on a substrate. The sensor exhibits sensitivity at a temperature ranging from about -60°C to about 85°C.
Brief description of the Figures:
The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.

Figure 1 shows a schematic view of Graphene Oxide-Platinum nanocomposite based Temperature Sensor.
Figure 2 shows a photograph of the fabricated nanocomposite based Temperature Sensor.
Figure 3 displays the temperature setup of Hot-Cold measurement calibration system.
Figure 4 describes the relative change in resistance with respect to temperature resulted as Temperature Coefficient of Resistance (TCR).
Figure 5 explains R-T curve which exhibits the behavior of linear relationship with respect to each other, Non-linearity+ Hysteresis for R: 2.72% FSO, Repeatability: 97.67 % FSO, Hysteresis of R: 2.22% FSO.
Figure 6 explains theXRD phases of GO nanosheets and GO-Pt nanocomposite.
Figure 7 shows FE-SEM images: (a) GO nanosheets, (b) Dispersion of Pt nanoparticles with GO in the nanocomposite.
Figure 8 explains the Raman Spectroscopy of the GO nanosheets and GO-Pt nanocomposite system.
Figure 9 shows photograph of the commercial devices(4 no’s) with the present invention (GO-Pt nanocomposite) based temperature sensors mounted on isothermal base for comparative performance studyand benchmarking.
Figure 10(a – c) provides a comparative performance results of GO-Pt and Commercial Temperature sensors (Pt100 and Thermistor 471).
Detailed description of the invention:
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the Figures, description and claims. It may further be noted that as used herein and in the appended claims, the singular “a” “an” and “the” include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention discloses Graphene Oxide-Platinum nanoparticles based nanocomposite and a process for the synthesis of the nanocomposite. The invention also discloses the method of fabrication of the nanocomposite on a flexible substrate for use in sensors. The graphene oxide in the invention is synthesized by Modified Hummer’s method in which graphite is oxidized with potassium permanganate in the presence of concentrated sulphuric acid. The reaction is further completed by the addition of hydrogen peroxide and deionized water to obtain graphite oxide. The graphite oxide is purified using hydrochloric acid and deionized water. Thus obtained the graphite oxide is exfoliated as graphene oxide nanosheets. The synthesis of graphene oxide by Modified Hummer’s method is an improvement over the Hummer’s method to beget graphite oxide in highly oxidized form and exfoliation of the graphite oxide into individual sheets called as graphene oxide nanosheets. The method also eliminates the use of sodium nitrate in the process thereby stopping the release of toxic gases during the synthesis of graphene oxide and making the process of synthesis more efficient and environment friendly.The GO-Pt metal nanoparticles composite is then prepared by dispersing GO nanosheets and Pt nanoparticles in a solvent selected from a group comprising N- Methyl-2-pyrrolidone (NMP), Dimethyl formamide (DMF), Tetrahydrofuran (THF), acetone and water, preferably N-Methyl-2-pyrrolidone.The mixed solution is ultra-sonicated in order to achieve exfoliation of the GO nanosheets and dispersion of the Pt nanoparticles. The size of the Pt nanoparticle is ranging from about 100 nm to about 500nm, preferably about 400 ± 20 nm and the Pt metal particles are dispersed on the GO nanosheets. Pt is seen to provide for enhanced temperature sensing applications. Subsequently, this composition is used for the fabrication of resistive elements for temperature sensor applications.
The present invention is in relation to a nanocomposite comprising; graphene oxide nanosheets, prepared by aforesaid method comprising acts of oxidizing graphite using potassium permanganate, hydrogen peroxide and deionized water in presence of sulphuric acid to obtain graphite oxide, optionally purifying the graphite oxide using hydrochloric acid and deionized water, and exfoliating the graphite oxide to obtain graphene oxide nanosheets, and; and metal nanoparticles.
In another embodiment of the invention the nanocomposite comprises graphene oxide nanosheets and metal nanoparticle in a ratio of 1:1 w/w.
In another embodiment of the invention, the graphene oxide nanosheet is of size ranging from about300 nm to 600 nm, preferably about 400±50 nm.
In yet another embodiment of the invention, the metal nanoparticle is of size ranging from about 100 nm to about 500 nm, preferably about 400±20 nm.
In yet another embodiment the metal nanoparticle is selected from a group comprising Ag, Au, Fe and Pt, preferably Pt.
The present invention is also in relation to a process for preparing the nanocomposite wherein said process comprising acts of synthesizing graphene oxide nanosheets by a method comprising acts of oxidizing graphite using potassium permanganate, sulphuric acid, hydrogen peroxide and deionized water to obtain graphite oxide, optionally purifying the graphite oxide using hydrochloric acid and deionized water, and exfoliating graphite oxide to obtain graphene oxide nanosheets; dispersing graphene oxide nanosheets and metal nanoparticles in a solvent in a ratio¬¬¬ of 1:1 w/w to obtain the nanocomposite.
In an embodiment of the invention,the solvent for dispersing graphene oxide nanosheets and metal nanoparticles is selected from a group comprising N- Methyl-2-pyrrolidone (NMP), Dimethyl formamide (DMF), Tetrahydrofuran (THF), acetone andwater, preferably N-Methyl-2-pyrrolidone.
The present invention also provides for a sensor comprising the nanocomposite fabricated on a substrate, wherein the substrate is a flexible, selected from a group comprising inorganic polyimide films (kapton), mylar, PEN, PET, thin metal / metal alloy substrate like Ni, Cu, Al / Stainless Steel with an insulating coating, preferably polyimide films (kapton).
The present invention also provides for a method of fabricating a sensor comprising a nanocomposite, said method comprising acts of coating the nanocomposite on a substrate to obtain a patterned substrate, annealing the nanocomposite coated on the patterned substrate to obtain a nanocomposite film.
In an embodiment of the invention, the nanocomposite is annealed on the patterned substrate at temperature ranging from about 50°C to about 90°C preferably about 80°C.
In another embodiment of the invention, the thickness of nanocomposite film is of size ranging from about 30 µm to 70µm, preferably 50 ± 20 µm.
In yet another embodiment of the invention the sensory device comprising the nanocomposite can be used in temperature, pressure, humidity, gas and biological sensing applications preferably in thermal applications.
In yet another embodiment of the invention the sensitivity of the sensor is measured in the temperature ranging from about -60°C to about 85°C.
In yet another embodiment of the invention sensor is completely coated with Parylene (thickness of 1.5 µm) in order to protect the device from moisture, dust, avoids the film from undergoing physical damage.
In yet another embodiment of the invention the Parylene coated devices were annealed at 150°C for 4 hours. The purpose of annealing the device is for getting the rearrangement of atoms in order to obtain uniform film for good stability.
Figure. 1 & 2 shows the schematic representation and photograph image of Graphene Oxide-Pt nanocomposite based temperature sensor (1) fabricated on a flexible substrate (2) silver paste contacts (3) and attached copper wires (4) electrodes for regulating electric flow. The setup of nanocomposite fabricated is encapsulated using Parylene (5) to protect the sensor from external agents such as moisture, dust, humidity and the like. The sensor output can be measured by using digital multimeter (6).
The fabricated nanocomposite sensor, resistance versus temperature response is studied and the typical responses are plotted (Figure. 4&5).In order to study the performance of the nanocompositebased sensor (3d), the sensor is placed inside the Hot-Cold measurement system (3a) having digital temperature indicator & controller (3b) and a digital multimeter (3c) as shown in Figure.3; and the resistive element is subjected to thermal heating environment by using temperature calibration system.
The schematic view of the experimental set up is shown in Figure.3. The electrical leads of the temperature sensor are connected to a 6 ½ digital multimeter (Gwinstek GDM-8261) (Figure.3). The sensor is subjected to heating cycles using digitally controlled Hot–Cold enclosure. It is observed that, the conductance of nanocomposite film is highly sensitive to local electrical and chemical changes due to which, the resistance of the sensor varies with respect to the temperature. It is observed that the fabricated GO-Pt nanocomposite films behaves as a negative temperature coefficient (NTC) sensing element. The electrical resistance decreases with the increase of temperature linearly as shown in Figure.4 & Figure.5. The calculated NTC value of the Graphene Oxide-Pt nanocomposite films is found to be -4.72 × 10-3 ? / ? / Kas shown in Figure.4. The sensitivity of the temperature sensor is found to be around 2.306 ? / K in a temperature ranging from about-60°C to about 85°C(Figure.4).
The temperature coefficient of resistance (TCR) of the sensor is calculated using the following equation,
TCR(a)=(R_(T-) R_Te)/((T-T_e)R_Te )= ?R/R?T ? / ? / K…………………………………………………. (1)
Where a is the temperature coefficient of resistance, ?R is the change in resistance and R the initial resistance and ?T is the change in temperature.
The sensitivity of the sensor is calculated using the following equation,
Sensitivity = (R_(T- ) R_Te)/((T-T_e))= ?R/?T?/ K………………………………………………………… (2)
By using the above relations, the calculated values of negative temperature coefficient and the sensitivity of the fabricated GO-Pt nanocomposite sensors are as follows.
In the first time calibration, TCR: -4.72 ×10-3 ? /? / K, Sensitivity: 2.306 ? / K and
In the second time calibration, TCR: -5.02 × 10-3 ? /? / K, Sensitivity: 2.474 ? / K
Further, the resistance versus temperature characteristics (non-linearity, hysteresis and repeatability) of GO-Pt nanocomposite temperature sensor is studied. Using the straight fit curve method(method of least square), (Figure. 5) the combined non-linearity and hysteresis and hysteresis is found to be a maximum of 3.30 % full scale output (FSO) and a maximum of 4.78 % full scale output (FSO)respectively in the temperature range of-60°C to about 85°C. The calculated root mean square (RMS) error and root sum square (RSS) error, by considering the combined effect of maximum non-linearity and maximum hysteresis is about 4.10% FSO and 5.80 % of FSO respectively. However, for the repeatability of the GO-Pt nanocomposite temperature sensor, the above experiment is repeated. The measured repeatability of the sensor is about 97.67 % of FSO. Hence, it is concluded that the performance of the fabricated GO-Pt nanocomposite based temperature sensor is repeatable even after two cycles of measurement. Furthermore, the calculated maximum combined non-linearity & hysteresis and maximum hysteresis of the sensor when repeated second time are 2.72 % of FSO and 2.22 % of FSO respectively in the temperature range of -60°C to about 85°Crespectively. The calculated root mean square (RMS) error and root sum square (RSS) error by considering the combined effect of maximum non-linearity & hysteresis, maximum hysteresis and non-repeatability are about 2.43 % and 4.21 % of FSO respectively.
The temperature sensing property of the fabricated nanocomposite based GO-Pt temperature sensor is detailed. It has been tested experimentally and compared with the commercially available standard resistance based temperature sensors such as Pt 100, Thermistor 471. Also, the GO-Pt sensor is compared with LM 35 and Thermocouple (K-type). Figure 9 shows the photograph of mounting arrangement of commercial devices with the GO- Pt nanocomposite based temperature sensors on an isothermal base. The experiments are performed by keeping all the devices/sensors together in the Hot-Cold enclosure calibration set up. Same temperature and stabilization (1 hour) time are maintained for all the devices uniformly during the measurement. It is observed that the resistance of Pt 100 decreases with the decrease in temperature indicating the positive temperature coefficient (PTC) behavior. Thermistor 471 and GO-Pt nanocomposite show the increase of resistance with the decrease in temperature indicating the negative temperature coefficient (NTC) behavior. The present invention addresses the NTC behavior, the resistance changes linearly with respect to the applied temperature in the range of -60°C to about 85°C. The sensitivity (2.306 ?/°C) of the present device is found to be an improvement over the standard Pt 100 temperature sensor (0.38 ?/°C), which is shown in Figure 10(a). Although, the thermistor (Figure 10 (b)) shows better sensitivity (14.85 ?/°C) when compared to Pt 100 and GO-Pt nanocomposite sensor, it exhibits non-linear behavior. However, LM 35 and Thermocouple K-type cannot be compared with the present invention, due to the different type of sensing mechanism. In order to study the response time of the temperature sensors such as Pt 100, Thermistor and GO-Pt, experiments are conducted by dipping the sensors in ice bath and hot water bath for cold and hot responses. Care is taken to ensure that the device is in contact with thermal equilibrium between ice bath/hot bath and the device environment. The observed response time for the above sensors (Pt 100, Thermistor and GO-Pt) in cold bath are 22s,49.3s and 18.8s respectively. Similarly, the observed response time in hot water bath for the same sensors are 17.5s, 2.7s and 11.5s respectively. The fabricated GO-Pt sensor is seen to have improved response time (18.8s) in comparison with the commercial Pt 100 (22.0s) and thermistor 471 (49.3s) sensors when it is in cold bath. Also, the fabricated GO-Pt sensor has better response time (11.5s) in comparison with commercial Pt 100 (17.5s) when it is in hot bath.
The comparative performance study results for GO-Pt and commercial temperature sensors such as Pt 100, thermistor 471 are shown in Figure 10 (c). It is observed that Pt 100 and GO-Pt sensors exhibit linear response compared to thermistor type sensor. Moreover, the TCR value (-4.72 × 10-3 ?/?/K) for the GO-Pt sensor is higher than the Pt 100 (3.85 × 10-3 ?/ ? / K) and Thermistor 471 (-3.9 × 10-3 ?/?/K).
As observed, the response of the GO – Pt nanocomposite based temperature sensor is linear and repeatable with respect to the temperature variation and hence the electronic circuitry becomes simpler.
The detailed comparative performance study data of both the commercial and GO-Pt nanocomposite temperature sensors are given in Table 1 below.
Table 1:
S.No Parameters Pt 100 SS Encapsulated Thermistor Bare GO-Pt Al Encapsulated LM35 Bare Thermocouple K-Type SS Encapsulated
1. Sensing type PTC NTC NTC Semiconductor Seebeck
2. Sensitivity 0.38 ?/°C 14.85 ?/°C 2.306 ?/°C 10 mV/°C 41 µV/ °C
3. TCR 3.85 × 10-3 ?/?/K -3.90 × 10-3 ?/?/K -4.72 × 10-3 ?/?/K - -
4. Linearity Linear Non-linear Linear Non-linear Linear
5. Temperature Range -50 °C to 230 °C -40 °C to 125 °C -60 °C to 170 °C -55 °C to 150 °C -300 °C to 1350 °C
6. Weight 36.87 gm 0.18 gm 0.4 gm 0.17 gm 56.29 gm
7. Type of device Metal Ceramics Nanocomposite Semiconductor Alloy
8. Resistance/ Voltage at 0 °C 100 ? 1532 ? 836 ? 1.733 mV 0.0105 mV
9. Response Time Cold- 22.0 Sec Hot -17.5 Sec Cold- 49.3 Sec Hot – 2.7 Sec Cold- 18.8 Sec Hot – 11.5 Sec - -
10. Cost High Low Very Low Moderate High

Structural and surface morphology characterization of the GO nanosheets and GO-Pt nanocomposite:
X-ray diffraction (XRD) patterns used to determine the crystalline phases of the GO nanosheets and GO-Pt nanocomposite samples are shown in Figure. 6. The existence of XRD peak at 2? -25.61°, corresponding to the (002) plane of non-crystalline nature of GO nanosheets, appear to be arising from stack of graphene layers before exfoliation of graphite. The same peak when shifted to 11.2°, corresponding to the (001) plane after oxidation treatment, indicating that the interlayer spacing in the Graphite Oxide has increased. The interlayer distance is increased in the GO nanosheets due to the presence of the epoxide, carboxyl groups and water molecules between the graphene oxides layers. These functional groups make the graphene oxide hydrophilic in nature. Also, these surface functional groups act as attaching sites for metal complexes. The diffraction peak at 42.82º corresponding to the (100) plane, represent the reformation of graphite microcrystals in the GO system. In the GO-Pt nanocomposite system, Pt nanoparticles are decorated on their surfaces and planes of the GO nanosheets. The peak at 39.79° indicate the corresponding plane of (111) face centered cubic (FCC) Pt nanoparticles. The additional peaks at 46.19°, 67.62°, 81.38° and 86.01° indicate the corresponding planes of (200), (220), (311) and (222) of Pt nanoparticles respectively. Other small intensity peaks at 13.17° and 25.61° indicate the corresponding planes of (001) and (002), which are more functionalof Pt nanoparticles in the GO nanosheets system. The presence of high intensity sharp peaks corresponding to Pt nanoparticles is seen inFigure. 6.
The surface morphology analysis of GO nanosheets and GO-Pt nanocomposite is examined using field emission- scanning electron microscopy (FE-SEM (Carl Zeiss), ULTRA 55). Figure.7 (a) shows the prepared GO nanosheets surface morphology, confirming the strong interlinking sheet like structure. The distribution of Pt nanoparticles in the GO-Pt nanocomposite system can be clearly seen in Figure7 (b). The presence of Pt nanoparticles distribution in the GO matrix prevents the agglomeration of the nanosheets in the entire medium. In the nanocomposite, the GO sheets with uneven arrangementof Pt nanoparticlesare dispersed on their surfaces as well as in the planes of the nanosheets. The Pt nanoparticles are non-homogeneously distributed and intercalated between the GO nanosheets and this arrangement improves the electrical performance of the nanocomposite system. The presence of Pt nanoparticles in the GO nanosheets plays an important role as a dispersing agent, in reinforcement and formation of conduction path between the GO nanosheets. Also, it enhances the piezoresistive property of the nanocomposite system.
Further, the structural characteristics and properties of graphene based nanocomposite materials is also studied using Raman Spectroscopy. The spectrum of GO and GO-Pt nanocomposite showed the existence of the D, G and 2D peaks (Figure.8). In the GO, the presence of two peaks corresponding to the D-band at 1344.35 cm-1 and G-band at 1586.54 cm-1, indicate the structural defects and disorder structure of the GO nanosheets. The Gband line gives rise to the first order scattering of the E2g phonon vibration mode of sp2 bonded C atoms and the D-band line is the breathing mode of the K-point phonons of A1g symmetry. The ratio of D to G bands is inversely proportional to the average size of sp2 domains. The 2D peak at 2709.75 cm-1 initiate from second order double resonant Raman Scattering and it can be varied depending on the number of layers present in the material. The peak position of 2D is similar to the monolayer graphene prepared from the mechanical cleavage method. Raman intensities of the D peak at 1344.35 cm-1 and G peak at 1591.97 cm-1 indicate the defects in the nanocomposite system and the nature of graphite in Graphene Oxide nanosheets respectively. The intensities of these peaks increases due to the adsorption of Pt nanoparticles with surface enhanced Raman scattering effect. The peak position of 2D at 2923.08 cm-1 represents graphitic nature in the composite system after functionalization of Pt nanoparticles. Furthermore, the presence of Pt nanoparticles in GO-Pt nanocomposite sharpens the relative intensity ratio (a) Graphene Oxide (GO) nanosheets (b) Graphene Oxide (GO) nanosheets and Pt nanoparticles in a ratio of 1:1 w/w of D/G. The ratio of D/G changes by proper dopants, which will be an ideal tool for the degree of disorder in the graphene for nano electronics applications.
Experimental:
The synthesis of the graphene oxide nanosheets, graphene oxide –platinum (Pt) nanocomposite and the fabrication of the nanocomposite on a substrate is discussed in the examples provided below:

Example 1
Synthesis of Graphene Oxide (GO): Graphite Oxide is synthesized by Modified Hummer’s method. The graphite powder (2gm) is mixed with potassium permanganate (KMnO4, 6gm) in the presence of sulphuric acid (H2SO4, 54 ml) under constant magnetic stirring. The potassium permanganate (KMnO4, 6gm) is slowly added to the solution for 20 min duration. 30 ml hydrogen peroxide (H2O2) is added to the mixture. The stirring of the mixture is continued for 40 min. Deionized (DI) water is then added to the solution in a dropwise manner, stirring for 90 min duration. The colour of the reaction mixture changes from pinkish black to reddish brown (towards the end of the reaction). The mixture is then washed two or three times with HCl and deionized water in order to remove metal ions and unreacted graphite from the reaction mixture. The mixture is filtered using vacuum filtration and the residue is collected at the end of the process. Finally GO residue is annealed at temperature ranging from about 85°C for 2 hours and subjected to grinding, resulting in the formation of GO sheets wherein the graphene oxide nanosheet is of size ranging from about 300 nm to 600 nm, preferably about 400±50 nm.
Example 2:
Procedure for the synthesis of GO-Pt nanocomposite: The GO-Pt metal nanoparticles composite is prepared by dispersing GO nanosheets and Pt nanoparticles in N-Methyl-2-pyrrolidone in the ratio of 1:1 w/w. The mixed solution is ultra-sonicated (the operating frequency is about 33 KHz ± 3%)in order to achieve exfoliation of the GO nanosheets and dispersion of the Pt nanoparticles.
Example 3:
Procedure for the fabrication of GO-Pt nanocomposite based temperature sensor: The piezoresistive graphene oxide-Pt nanocomposite thin film temperature sensor having a structural active area of about 4 mm × 1 mm, is integrated on a polyimide film (kapton) of dimensions 15 mm × 5 mm × 0.175 mm. Astructural mask is fabricated from a stainless steel sheet of thickness of about 50 µm by chemical etching process technology. The GO-Pt nanocomposite solution is used to achieve the patterns on flexible kapton substrate by drop casting method. Further, the GO-Pt nanocomposite sensing film is kept at temperature of about 80°C for 60 min,to anneal the film.The annealed sensing film provides feasibility for the removal of the solvent and rearrangement of the atoms in the nanocomposite films. The thickness of active area varies with respect to the spacing between the adjacent ink droplets (nanocomposite solution) and the number of nanocomposite drops deposited. In other words, the number of nanocomposite drops deposited on the substrate material is directly proportional to the thickness of the film.The thickness of GO-Pt nanocomposite film is about 50 ± 20µm.As described, the GO-Pt composition in verified by using XRD, FE-SEM and Raman Spectroscopy methods. The thickness is measured using a micrometer having an accuracy ± 1 µm.
The nanocomposite films can influence on their electrical resistance. The electrical leads are taken outwith thin double enameled copper wires (dia: 70 µm) using silver paste on top corner side of the nanocomposite patterned films. Also, the fully fabricated sensing film is annealed for the purpose of curing of the electrical contacts Furthermore, the fabricated device is encapsulated with Parylene coating (thickness of 1.5 µm)in order to protect the device from moisture, dust, avoids the film from undergoing any form of physical damage. Parylene coating gives conformal, uniform thickness as well as pinhole free even greater than 0.5 µm. Once Parylene is coated onto the device, it has the stability as well as long life period for about 10 years. Moreover, the complete Parylene coated devices were annealed at 150°C for 4 hours for the purpose of getting the rearrangement of atoms for obtaining uniform film for good stability.
The table below provides a summary of a typical sensor:
Parameter Specification
Type of sensing film Nanocomposite sensing film
Dimensions 15 × 5 × 0.175 mm
Active Area 4 × 1 mm
Thickness ~ 50 µm
Weight ~ less than one gram
Tested temperature Range -60°C to 85°C
Moisture proof material Parylene
Application Temperature Sensor
Negative Temperature Coefficient of resistance (NTC) -4.72 × 10-3 ? /? / K ± 3 %
Sensitivity 2.306 ? / K
Type of nanostructures 1D nanoparticles & 2D nanosheets
Type of material Graphene Oxide & Platinum nanocomposite on kapton membrane
Deposition process Drop casting method
Thus, the present invention is in relation to peizoresistive nanocomposites of Graphene Oxide and Platinum nanoparticle in a ratio of 1:1 w/w. The nanocomposite wherein the graphene oxide is synthesized by Modified Hummer’s method and the metal nanoparticles are dispersed on the graphene oxide nanosheets. The nanocomposite is fabricated onto a flexible substrate for use in sensor applications. The nanocomposite film operates on the basis of negative temperature coefficient of resistance, which provides a TCR of -4.72 × 10-3 ? /? / K ± 3 %. The nanocomposite fabricated substrate is encapsulated to provide protection from environmental factors. In the present invention, the effectiveness of nanocomposite fabricated on a flexible has been demonstrated to enable the possibility of using it as a temperature sensor. The fabricated sensor is compact in size, low cost and flexible. The same sensor can be applied for the measurement of strain, pressure, force, acoustic, speed, humidity, gas and biological sensing applications.