KR101710798B1 - High-performance moisture sensor based on ultralarge graphene oxide - Google Patents

Abstract

The present invention relates to a high-performance moisture sensor based on ultralarge graphene oxide (UGO) and, more specifically, to an electrode for a moisture sensor which coats and uses ultralarge graphene oxide (UGO) whose lateral side reaches 47.422.2 m as a moisture sensing film of an electrode for a moisture sensor to greatly increase sensitivity and stability in comparison to conventional small-size graphene oxide (SGO) and realize quick response time and recovery time in comparison to reduced graphene oxide (rGO), a method for easily and inexpensively manufacturing the same, and a high-performance moisture sensor including the same. According to the present invention, the moisture sensor can be widely applied to various fields requiring a moisture sensor such as a next generation cutting-edge element, moisture monitoring through artificial skin, a noncontact screen directly connected to a smartphone or a smart watch, precision air conditioning controls for an electronic device, breathing air systems, and moisture alarms for pressurized telecommunications transmission cables.

Description

HIGH-PERFORMANCE MOISTURE SENSOR BASED ON ULTRALARGE GRAPHENE OXIDE BACKGROUND OF THE INVENTION [0001]

The present invention relates to a high performance moisture sensor based on a large area graphene oxide, and more particularly to a high performance moisture sensor using a large area graphene oxide (UGO) having a lateral size of 47.4 ± 22.2 μm, (SGO), the contrast sensitivity and stability are greatly increased, and very fast response time and recovery time can be achieved compared with the case of using reduced graphene oxide (rGO) or the like An electrode for a moisture sensor, a method for easily and cheaply manufacturing the electrode, and a high-performance moisture sensor including such an electrode.

The moisture sensor according to the present invention can be used as a next-generation advanced device, moisture monitoring through artificial skin, non-contact screen for direct connection to a smartphone or smart watch, precision air conditioning controls for an electronic device, systems, and Moisture alarms for pressurized telecommunications transmission cables, which can be used in a wide range of applications requiring moisture sensing.

Background of the Invention [0002] Humidity sensors (hereinafter also referred to as "moisture sensors") are important sensors widely used in scientific research laboratories including the automotive, biomedical, These humidity sensors are commonly applied to precision air conditioning controls, breathing air systems, and moisture alarms for pressurized telecommunications transmission cables.

Most current humidity sensors use inorganic or organic-based humidity sensing materials to precisely measure the moisture levels of the surrounding environment.

Of these inorganic-based sensing materials _{(e.g., Al 2 O 3, In 2} O 3, MnWO 4, SiO 2, SnO 2, TiO 2, WO 3 and ZrTiO ₃₎ is sensitive to moisture is excellent, but the high processing temperature (1000 占 폚) is required, the utilization in the microchip fabrication industry is limited, which is not suitable for the semiconductor manufacturing field.

Such inorganic materials can be incorporated into high-soluble organic or polymer-based sensing materials such as polyvinyl alcohol, poly (dimethyldiallylammonium chloride), poly (sodium p -styrenesulfonate), poly (vinylpyrrolidone) ≪ / RTI > and their derivatives), and these materials have the advantage that they can be solution-processed at low temperatures.

However, in some cases, the high water solubility of such organic materials is a major drawback, especially when such polymeric materials are to be used under high humidity conditions.

In addition, most conventional resistive sensors have a decisive weakness that response times (typically in the range of 10 to 30 seconds) are relatively slow, and some low-cost humidity sensors have long response times Which is virtually impractical.

Therefore, these sensors are not suitable for integration into wearable and mobile electronic devices requiring high sensitivity, fast response and high energy efficiency.

Therefore, it is necessary to continuously develop a high-performance sensing material that meets the needs of the next-generation high-end innovative products under the effort to design a more improved humidity sensor.

Meanwhile, research on the development of various types of devices using graphene oxide (GO) is underway.

Graphen oxide (GO) is a superior amphipathic soft material with a variety of oxygenated functional groups attached to the basal plane and sheet edges. Due to these oxygen functionalities, the GO can be used in proton conductors.

However, despite the various experimental and theoretical studies on GO, the water sensing mechanism associated with the oxygenated portion of the GO remains controversial. Some researchers speculated that the GO would not exhibit any ionic conductivity, while other researchers speculated that the epoxide groups on the GO basal plane, according to the Grotthuss mechanism (H ₂ O + H ₃ O ⁺ → H ₃ O ⁺ + H ₂ O) Suggesting that hydroxyl groups can form an excellent network of proton hopping sites acting as proton conduction pathways.

This assumption has been recently supported by several experimental studies. Humidity sensors composed of reduced GO (rGO), graphene quantum dots and H-doped graphene showed poor humidity sensing characteristics due to insufficient epoxide and hydroxyl groups. On the other hand, GO (functionalized with epoxide groups and hydroxyl groups) has recently been shown to be a very good moisture sensing material due to its high surface area / volume ratio and hygroscopicity.

However, the influence of the side dimension of the GO sheet on the humidity sensing characteristics among the various characteristics of the GO has not been studied to date.

Accordingly, the correlation between the lateral size (area) of the GO sheet and the humidity sensing characteristic of the GO-based moisture sensor is positively identified and proved, and an optimal GO sheet is applied to the sensor based on the result, It is time to develop new technologies that can maximize all of the required properties such as recovery time and stability.

K. Hatakeyama, M. R. Karim, C. Ogata, H. Tateishi, A. Funatsu, T. Taniguchi, M. Koinuma, S. Hayami and Y. Matsumoto, Angew. Chem. Int. Edit., 2014, 53, 6997-7000.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems of the prior art, and it is an object of the present invention to provide an electrode for a moisture sensor coated with a large area graphene oxide (UGO) having a lateral size of 47.4 ± 22.2 μm, The present invention also provides a high-performance moisture sensor comprising such an electrode.

Specifically, the present inventors have conducted intensive studies based on the proton conduction mechanism and have confirmed and confirmed the correlation between the lateral size of the GO and the water-sensing characteristics. That is, the characteristics of the GO element are greatly influenced by the size of the GO sheet used. Particularly, in the case of the GO moisture sensor, increasing the size of the GO significantly improves the performance (conductance, sensitivity, response / recovery speed and stability) And reached the present invention.

In order to accomplish the above object, the present invention provides an electrode for a moisture sensor coated with ultralarge graphene oxide (UGO) having a lateral size of 47.4 ± 22.2 μm.

The large area graphene oxide sheet used in the present invention has a very large size such that the average lateral size is 47.4 ± 22.2 μm and the side size of the individual sheet is at most 150 μm or more.

In the present invention, the lateral size refers to a length of a piece of graphene oxide sheet separated from graphite, which is relatively long in length and length or a random length of a piece of graphene oxide sheet Can be interpreted as the longest distance between the both ends of the link.

The present invention is to investigate the influence of the size of graphene oxide (GO) on the moisture sensing characteristics. It is known that the effective length of the proton conduction path (= proton is slow at the contact boundary of the GO sheet or A distance that can be moved without interruption) increases, and a favorable condition for proton conduction is provided, and the humidity sensing characteristic such as sensitivity to moisture is greatly improved.

Specifically, the large-area graphene oxide (UGO) has epoxide and hydroxyl groups on the basal plane and a carboxylic acid group on the edges, Sized sheet of small-size graphene oxide (SGO, side size = 0.8 +/- 0.5 mu m level) sheet coated on a substrate having the same area of the number of carboxyl groups present on the edge as the sheet is formed, As compared with that of the first embodiment.

According to the GO structure model originally proposed by Lerf and Klinowski, the basal plane of the GO mainly contains an epoxide group and a hydroxyl group (a small number of carbonyl groups are present in the vicinity of the hole edge in the sheet), whereas the sheet edge mainly contains a carboxyl group . It is important that the hydroxyl group (attached to the carbon atom) always coexists with the epoxide group bound to the adjacent carbon atom. This close proximity of the epoxide and hydroxyl groups produces a well aligned proton exchange site on the basal surface of the GO and can favor proton conduction. However, the proton conduction pathway tends to be blocked by the carboxyl groups scattered at the edge of the GO sheet.

Based on this proton conduction mechanism, the present inventors have found that the UGO sheet (side size ~ 47 mu m) provides a longer uninterrupted travel distance for proton conduction on the basal surface compared to SGO (side size < 1 mu m) Proton conductivity. It is believed that in the case of a large UGO sheet, the energy barrier for hopping the protons from one sheet to another in the vicinity is reduced.

The electrical conductance at the GO can be understood in relation to the Grotthuss mechanism, which describes the proton hopping between the H ₃ O ⁺ moiety and the nearest neighboring free-flowing molecule ). Proton transport along the plane of the GO sheet is associated with rapid exchange of protons through the formation of hydrogen bonds between the proton donor (water) and the proton acceptor (the epoxide group or the alcohol group on the GO). Thus, the strength of hydrogen bonds formed between water molecules and oxygen functional groups (e.g., epoxide groups, hydroxyl groups and carboxyl groups) greatly affects the proton conduction in GO. Among these oxygen functional groups (for example, epoxide group, hydroxyl group and carboxyl group), the carboxyl groups located at the edges and boundaries of the GO sheet form strongest hydrogen bonds with water molecules, which causes water molecules to be strongly bonded to the edge of the GO sheet The proton conduction is adversely affected. Actually, when the number of the carboxyl group moiety at the GO edge is increased, the proton conductivity is decreased. That is, when the size of the GO sheet is reduced (in the case of SGO), the number of edges or grain boundaries increases, thereby increasing the number of carboxyl groups, thereby blocking the proton transport path (FIG. 1B) The result is to reduce the overall proton conductivity.

Unlike SGO, the UGO sheet according to the present invention provides favorable conditions for long-distance proton conduction along its wide basal plane, and it is effective to provide a moving distance of about 47 to 150 mu m.

In addition, in the large area graphene oxide (UGO) of the present invention, the activation energy (E _a ) for proton conduction may be 0.63 eV.

The effect of GO size on the overall proton conductivity was clearly confirmed from the proton conduction activation enthalpy (E _a ) measured on the GO sheet. The conductivity of UGO and SGO was determined as a function of temperature (Fig. 2a), and the Arrhenius plots of each were plotted (Fig. 2b). As expected, the activation energy (E _a = 0.63 eV) for proton conduction in UGO was about half that of SGO (E _a = 1.14 eV) because the carboxyl groups scattered at the edges and grain boundaries of the GO were flattened by proton conduction (In-plane proton conduction).

In one embodiment, the conductance of an electrode for a moisture sensor coated with UGO according to the present invention is about 10 ⁴ times greater as the relative humidity (RH) varies from 7 to 100%, for example, (RH) 7% to 2.8 x ^{10 -11} S, and relative humidity (RH) 100% to 1.2 x ^{10 -7} S.

In addition, the surface roughness R _a of the large area graphene oxide (UGO) film coated on the electrode according to the present invention may be about 9 nm. Considering that it is not easy to obtain a uniform film in the case of drop-casting, unlike spin-coating, roughness of about 9 nm corresponds to a very uniform film.

According to another aspect of the present invention, there is provided a method for peeling off an optimum graphene oxide sheet (i.e., a UGO sheet) from graphite in an intact state without cleavage or damage, as a premise for obtaining the electrode for a moisture sensor .

Specifically, the present invention provides

(a) dipping graphite in a sulfuric acid solution to obtain a graphite intercalation compound (GIC);

(b) heating the graphite intercalation compound (GIC) to 1000 to 1200 캜 to obtain thermally expanded graphite;

(c) introducing an oxidizing agent and performing centrifugal separation at a rotational speed of 3900 to 4100 rpm to separate the graphene oxide layer;

(d) Centrifugation was performed on the separated graphene oxide layer at a rotation speed of 7900 to 8100 rpm and 3900 to 4100 rpm, respectively, so that a small area graphene oxide (SGO) sheet and a lateral size of 47.4 占 22.2 占 퐉 Separating the large area graphene oxide (UGO) sheet; And

(e) coating a separated large-area graphene oxide (UGO) sheet on an electrode substrate to form a humidity-sensitive film.

In the present invention, graphite composed of large-area graphenes is immersed in a sulfuric acid solution, heated at a high temperature of about 1000 ° C for a short time (for example, about 10 to 30 seconds) After the graphite layer is inflated, the GO layer is separated from the graphite by performing an oxidation process and centrifugation without ultrasonic treatment. Then, the separated GO layer is sequentially centrifuged by the rotation speed to obtain a large area Remove the pin oxide sheet. That is, in the entire manufacturing process, physical ultrasonic treatment as in the Hamers method is excluded, so that a large-area graphene oxide sheet can be obtained in an intact state without damaging the sheet, thereby efficiently manufacturing an electrode for a moisture sensor .

In the method for manufacturing an electrode for a moisture sensor according to the present invention, the step (a) is a step of immersing graphite composed of large-area large-area graphene as a raw material into a sulfuric acid solution to obtain a graphite intercalation compound (GIC).

In one embodiment, step (b) may be to heat the graphite intercalation compound (GIC) to 1000 占 폚 for 10 seconds to obtain thermally expanded graphite.

Further, the step (c) may be carried out using an oxidizing agent such as H ₂ SO ₄ + KMnO ₄ ) and centrifuging at a rotation speed of 4000 rpm for 20 minutes to separate the graphene oxide layer (= GO precursor).

In the step (d), the separated graphene oxide layer is centrifuged at 8000 rpm for 40 minutes to separate the small area graphene oxide (SGO) sheet, and then the precipitate is dispersed again in the aqueous solution Followed by centrifugation again at 40,000 rpm for 40 minutes at 4000 rpm to separate the large area graphene oxide (UGO) sheet.

In the step (e), the separated large-area graphene oxide (UGO) sheet may be drop-coated on a gold electrode to finally form a humidity film.

According to another aspect of the present invention, there is provided a high performance moisture sensor including such an electrode.

According to the present invention, when a moisture sensor (specifically, a resistive-type moisture sensor) is fabricated using an electrode coated with a large area graphene oxide (UGO), the maximum sensitivity ( S ) 433, the response time and the recovery time of the moisture sensor are extremely short as 0.2 seconds and 0.7 seconds, respectively, so that the performance of the conventional moisture sensor is greatly improved. Response time and recovery time are very important factors for implementing more advanced sensing products. Here, the response time and recovery time are 90% of the final equilibrium value, And the time required for reaching the target. Furthermore, the moisture sensor according to the present invention can be used in a case where the deviation of the values measured over 5 days (for example, the DC conductance and the AC conductivity value) in a constant temperature and humidity condition (for example, a humidity chamber of RH 97% %. &Lt; / RTI &gt;

Further, the present inventors investigated the non-contact-mode moisture sensing of the resistive UGO moisture sensor according to the present invention, and as a result, confirmed that the present invention can be applied to the non-contact position interface.

In one embodiment, in the case of the moisture sensor according to the present invention, i) the sensitivity is 17.4, the response time is 0.6 sec, and the response time is 0.5 sec when the distance between the fingertip and the large area graphene oxide (UGO) The recovery time was 1.3 seconds, and the response time was 0.4 seconds and the recovery time was 0.7 seconds when the distance between the fingertip and the large area graphene oxide (UGO) coated on the electrode was 2.5 mm. As a result, Water sensitivity.

The moisture sensor of the present invention can be widely used in various fields requiring a water sensing function.

For example, i) a food waste disposer, a household appliance, a dehumidifier, an air conditioner, a microwave oven, a mobile communication terminal, a clothes dryer, a wet cleaner, a flowerpot requiring water supply to a bandit, a diaper, Or non-contact touchscreen screens, artificial skin, medical devices, and the like, as well as existing applications, such as, for example, non-contact temperature sensors, , Iii) water monitoring through artificial skin, or a smart phone, such as a smart phone, or a smart phone, or a smart phone, Non-contact screen that can be connected directly to smart watch, iv) precision air conditioning for electronic equipment (Pre cision air conditioning controls, breathing air systems, or moisture alarms for pressurized telecommunications transmission cables.

The electrode for the moisture sensor using the UGO (side dimension = 47.4 占 22.2 占 퐉) according to the present invention has an in-plane conductance value increased by about 10 ⁴ times as the relative humidity (RH) is changed from 7 to 100% . (* SGO = about 10 ³ times increase)

In addition, the UGO moisture sensor according to the invention exhibits a very high maximum sensitivity ( S ) of the order of 4339 + 433. (* For SGO = 1982 ± 122)

Also, the UGO moisture sensor according to the present invention exhibits a very short response / recovery time of 0.2 sec / 0.7 sec, respectively, compared to the conventional sensing material. (* rGO = 4 sec / 10 sec; graphene quantum dot = 10 sec / 20 sec; carbon nanotube = 6 sec / 120 sec; SnO ₂ = 120 sec / 20 sec; Graphen = 3 minutes / several hours; for VS ₂ = 30 seconds / 12 seconds)

In addition, the UGO moisture sensor according to the invention is also excellent in stability with a deviation of only +/- 4.6% over 5 days. (* For SGO = deviation 48.9%)

Further, the present invention makes it possible to manufacture a high-performance moisture sensor electrode relatively easily and inexpensively using GO, and there is no need to undergo a high-temperature condition such as an inorganic-based sensor or an ultrasonic treatment process for GO production.

The high-performance UGO moisture sensor according to the present invention can be widely applied to various industrial fields requiring moisture sensing such as a next-generation advanced device, and has excellent performance in terms of finger end water sensing. Thus, potential use of the non-contact display position interface It is expected to be very large.

Figure 1 (a) schematically illustrates the Grotthuss mechanism for proton conduction; (b) is a digital image of UGO (upper left image) and SGO (upper right image) films drop coated on a pair of gold electrodes. (* 3D images show the alignment of UGO and SGO sheets on gold electrodes, SGO boundaries or edges acting as scattering centers to block the proton-transport pathway, thereby reducing overall proton conductivity.)
2 (a) is a graph showing the temperature-dependency of conductivity at 97% RH for each of UGO (O) and SGO (); (b) is an Arrhenius plot of UGO (◯) and SGO (□) applied thereto. (* The activation energies (E _a ) of UGO and SGO are found to be 0.63 eV and 1.14 eV, respectively).
3 is a diagram illustrating a setup for dynamically measuring the humidity sensing characteristics of UGO and SGO sensors. (* The gap between the gold electrode and the electrode is fixed to 0.3 cm and 50 μm, respectively.)
4 is an FE-SEM image of UGO (left side) and SGO (right side). (* UGO and SGO sheets are drop-coated on Si / SiO ₂ substrates using a stable dispersion (5 μl).)
Fig. 5 is a histogram of FE-SEM image and side size distribution of UGO sheets (a, c) and SGO sheets (b, d).
Figures 6 (a), 6 (b) and 6 (c) are survey scans and high energy decomposition XPS scans for C 1s, UGO and SGO, respectively. (XPS curve of (b) and (c) = (experimental XPS curve, (-) = fitting XPS curve derived by Gaussian-Lorentzian function after Shirley background correction).
Figure 7 compares the Visible Raman spectra (? _Ex = 532 nm) of (a) UGO and (b) SGO. (*) = Experimental Raman spectrum, (-) = Fitting Raman curve using Gaussian-Lorentzian function The chemical components used in the fitting are shown in curves 1 to 5. Peak 1 -> D band, peak 2 Stokes combination (layer-breathing mode) of the G band, peak 3 -> D 'band, peak 4 -> anti-Stokes, peak 5 -> E _2g LO phonon and B _2g ZO' phonon.
Figure 8 is a surface precision SEM and AFM image of UGO (upper two images) and SGO (lower two images). (* The thicknesses of UGO and SGO are measured as 617 ± 20 nm and 711 ± 27 nm, respectively.)
9 is a diagram showing AFM tapping mode images of UGO (upper left image) and SGO (upper right image) and their corresponding height profile measurements.
Figure 10 (a) shows the conductance response of UGO (red) and SGO (black) films exposed to various RH values; (b) shows the conductance response of UGO (○) and SGO (□) exposed to a controlled humidifier flow (32.7% RH) (* all measurements are made by applying a DC voltage of 1V at 20 ° C); (c) shows the stability results of UGO (○) and SGO (□) humidity sensors performed by measuring the DC conductance of the device at 20 ° C humidity chamber (97 RH%); (d) are the stability results of UGO (∘) and SGO (□) humidity sensors performed by measuring the AC conductivity of the device at 20 ° C humidity chamber (97 RH%).
11 (a) is a graphical image of a non-contact-mode UGO humidity sensor that dynamically responds to fingertip moisture; (b) shows the change in conductance as a function of the distance between the fingertip and the UGO electrode; (c) is the corresponding sensitivity as a function of fingertip and UGO electrode distance; (d) is a graph depicting the response and recovery time of the UGO humidity sensor according to fast pulse moisture stimulation on the fingertip (* fast approaching or separating finger tips on the UGO electrode surface);

Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples. It should be understood, however, that these examples are for illustrative purposes only and are not intended to limit the scope of the invention in any way.

Example

(1) Material

(93%) from Matsunoen Chemicals Ltd. (Japan), ammonium hydroxide (29%) from Mallinckrodt Baker Inc (Japan), from Alfa Aesar, potassium permanganate from Sigma-Aldrich, natural graphite flakes (NJ, USA), lithium chloride monohydrate (98.2%) was added to Shinyo Pure Chemicals Co. Sodium bicarbonate dihydrate (99.0%) was obtained from Junsei Chemical Co., Ltd. (Japan). Concentrated sulfuric acid (95%), hydrogen peroxide (34.5%), hydrochloric acid (35%) and potassium chromate (98.5%) were obtained from Samchun Pure Chemical Co. (98.0%), sodium hydroxide (97.0%) and sodium chloride (99.5%) from Daejung Chemical Co., Ltd. (South Korea) Chemicals and Metals. Co. Ltd. (South Korea).

All chemicals were of analytical grade without further purification.

DI water (18 MΩ / cm) was used for all experimental steps including the wash procedure.

A silicon wafer (d = 100 mm, Libby Owens Ford, USA) was immersed in a pyran solution (H ₂ SO ₄ : H ₂ O ₂ = 7: 3) and an RCA solution (H ₂ O: H ₂ O ₂ : NH ₄ OH = 5: 1: 1) were sequentially washed at 70 ° C. for 1 hour and rinsed with distilled water.

Transparent PET film was purchased from Saehan Industries (South Korea) and washed with digital UV ozone system (PSD Series, Novascan) for 1 hour to remove molecular organic contaminants.

(2) Production of graphene oxide (GO) from thermally expanded graphite (EG)

A precursor GO was synthesized as follows based on the Hummers method using thermally expanded graphite.

First, the natural graphite flakes (2 g) and concentrated sulfuric acid (60 mL) were stirred well for 24 hours.

Fuming nitric acid (20 mL) was added to the mixture followed by continued stirring at room temperature for 24 hours. Distilled water (80 mL) was added slowly and stirred for 1 hour. The mixture was washed three times with distilled water, centrifuged at 4000 rpm for 20 minutes, and then the precipitate was dried at 60 DEG C for 2 days to obtain a graphite intercalation compound (GIC).

The compound was placed in a ceramic boat and then inserted into a long quartz tube. The quartz tube was filled with argon and sealed.

The sealed quartz tube was placed in a preheated ceramic furnace to 1000 DEG C for 30 seconds to obtain an expanded graphite (EG) compound.

After this heat treatment, the physical appearance of the high density graphite flakes changed abruptly in the form of highly expanded graphite powder.

To synthesize GO from EG, EG powder (1 g) was stirred in 200 mL of concentrated sulfuric acid, potassium permanganate (10 g) was slowly added, and the mixture was stirred for 24 hours. Distilled water (200 mL) and hydrogen peroxide (50 mL, 35%) were then slowly added to the mixture and stirred for 30 minutes. Through this process, the color of the suspension changed from dark green to light brown.

The suspension was repeatedly washed with 10% HCl (v / v) solution and centrifuged three times for 20 minutes at 4000 rpm.

The suspension was then washed with distilled water until the pH of the suspension reached 5-6.

(3) Large area Grapina  oxide( UGO ) And Small area Grapina  oxide( SGO )

The prepared GO suspension was diluted with distilled water and centrifuged at 8000 rpm for 40 minutes to collect the supernatant (small area graphene oxide; SGO).

The precipitate was again dispersed in distilled water and centrifuged at 4000 rpm for 40 minutes to collect precipitates containing large area graphene oxide (UGO) and redispersed in distilled water.

That is, by excluding the ultrasonic treatment in the entire manufacturing process, the side size of the GO sheet can be prevented from being significantly reduced.

(4) Characterization method

The lateral dimensions of the UGO and SGO sheets and the surface morphology of the film were characterized using field emission scanning electron microscopy (FE-SEM) (JEOL JSM-7800F).

Surface roughness and film thickness of UGO and SGO films were measured using atomic force microscopy (AFM) (Park Systems, XE-100).

The elemental composition of the UGO and SGO films was investigated using an X-ray photoelectron spectrometer (PHI 5000 VersaProbe II).

Structural features of the UGO and SGO films were investigated using a Raman-LTPL (532 nm excitation laser).

At different relative humidities (RH), the sensor's DC conductance was measured using a Keithley 2400 source meter with a 1 V DC voltage. A detailed measurement setup is shown in FIG. RH was adjusted by bubbling air through a saturated salt solution. The humidifier was blown directly onto the sample at room temperature (20 &lt; 0 &gt; C) for 30 minutes. RH was adjusted using different salts of each; _{NaOH.H 2 O (7% RH)} , LiCl.H 2 0 (11.3% RH), MgCl 2 .6H 2 O (32.7% RH), Na 2 Cr 2 O 7 .2H 2 O (53.7% RH), NaNO ₂ (64.4% RH), NaCl (75.1% RH), K ₂ CrO ₄ (86.5% RH), and K ₂ SO ₄ (97.0% RH). Deionized water (DI water) was used for 100% RH. The distance between the air nozzle and the sample was fixed at 2 mm.

To calculate the enthalpy of activation of proton conduction in UGO and SGO sensors, a humidity chamber (20 ° C, 97% RH) was fabricated using electrochemical impedance spectroscopy (Ivium-StatTechnologies Compactstat) (frequency range 1 MHz to 50 Hz, ), The AC proton conductivity of the sensor was measured. Prior to measurement, the sample was placed in a humidity chamber at 20 DEG C for 24 hours under constant humidity to reach equilibrium. The proton conductivity σ of the sample was calculated using the equation "σ = L / (R × T × D) ". Here, the impedance R was measured from the diameter of the corresponding Nyquist plot, and the Nyquist plot was obtained by plotting the fitting on the equivalent circuit using the ZView program (version 2.3d, Scribner Associates Inc.). The distance L between the two electrodes was fixed at 5 x ^10-3 cm for all samples. Film thickness T was measured using AFM. The electrode length D was fixed at 0.3 cm.

Experimental Example

(One) UGO  And SGO of FE - SEM  analysis

Side dimensions of the drop coated UGO and SGO sheets on Si / SiO ₂ substrates were measured from each FE-SEM image (see FIG. 4).

The analysis of 183 pieces of UGO sheet and 162 pieces of SGO sheet (see FIG. 5), the average side sizes of the UGO and SGO sheets were measured to be 47.4. + -. 22.2 .mu.m and 0.8. + -. 0.5 .mu.m, respectively. In some cases, the side dimensions of the UGO sheet exceeded 150 μm.

This large area portion accounted for about 7% of the UGO dispersion, and this large area was larger than the maximum size of SGO (several hundred nm to several 탆).

(2) UGO  And SGO X-ray photoelectron spectroscopy ( XPS ) result

Surface chemical analysis of UGO and SGO was performed using X-ray photoelectron spectroscopy (XPS).

The XPS survey scan of the UGO and SGO films showed two distinct peaks corresponding to C 1s (287.2 eV) and O 1s (534.8 eV) (see FIG. 6A).

The high-resolution XPS spectrum of the GO showed several relatively well-degraded peaks corresponding to carbon atoms in different chemical environments.

The chemical states of C 1s and corresponding area compositions were obtained by peak-fitting each high energy XPS signal (FIG. 6b, 6c and Table 1).

Interestingly, peaks at 284.5 eV corresponding to aromatic carbon atoms were found not to be present in both UGO and SGO, which is similar to the previous report that fully oxidized GO loses directionality.

Carbon / oxygen (C / O) ratio is a useful parameter for investigating the degree of oxidation of GO. The C / O value (0.85) of UGO was slightly higher than that of SGO (0.75).

The C 1s peak of GO consists of three different chemically transferred components, C-O (287.3 eV); C = O (289.2 eV); And C (= O) - (OH) (290.3 eV), which is in good agreement with the reported contents.

The C-O bond is due to the alcohol, phenol and ether groups of the basal plane, and the C = O bond is mainly derived from a single ketone and quinone which adorns the edge of the GO sheet. On the other hand, C (= O) - (OH) is mainly present at the edge of the GO sheet.

Based on the XPS results (Table 1), two very important conclusions can be gained, insightfully explaining the outstanding performance of UGO as a proton conductor.

First, the percent area of CO in UGO (40.0%) was higher than that of SGO (31.1%). Hatakeyama et al . , The CO portion of this basal plane can provide an important pathway for proton conduction.

Second, the percentage of area of C (═O) - (OH) in UGO (18.2%) was lower than that of SGO (25.3%). This supports the fact that smaller SGO sheets have a relatively higher amount of edge sites. This edge-to-edge bonding in SGO films is undesirable in that the proton requires additional energy to hop from one sheet to the adjacent sheet and consequently blocks proton transport.

[Table 1] High Energy Decomposition of UGO and SGO Thin Films Bond energy (eV) and corresponding area composition (%) of C 1s core-level peak chemical state in the XPS spectrum

(3) UGO  And SGO Raman spectroscopy results

Raman spectroscopy is a powerful tool that can characterize defect density in graphene oxide, and defect density is considered to be proportional to the degree of oxidation of GO.

In application to humidity sensing, such defects derived from oxygen functional groups are an essential element that governs the proton conduction properties in the GO.

The D peak at ~ 1350 cm ^-1 is due to the breaching modes of the 6-atomic ring activated by structural defects. In addition, the high degree of disorder G peak is confirmed by the presence of (~ 1590cm ^-1) and overlap D 'peak (~ 1615cm ^-1) is, as a result - a single upwardly moving a large peak at 1600cm ^-1 in the GO appear. It is therefore more convenient to regard the two overlapping peaks as a single broad peak.

The Raman spectra of fully oxidized UGO and SGO can be de-convoluted to five peaks (see FIG. 7). Average distance between defects (L _D) is proportional to the expression _{"I D / I G = C '} (λ) L D 2 " D-to-G intensity ratio as shown in (I _D / I _G).

Calculation results (Table 2), L _D for UGO (1.78 nm) and SGO (1.73 nm) The values were very similar, indicating a high degree of structural defects on the basal plane of fully oxidized UGO and SGO. In addition, the fact that the full width at half height (FWHM (G)) of the G peak is large indicates that the degree of structural disorder is high in UGO and SGO.

[Table 2] The D-to-G ratio ( I _D / I _G ) of UGO and SGO, the average distance between defects ( L _D ) and the full width at half maximum of G peak (FWHM

(4) UGO  And SGO Surface of Morphology  And Humidity Sensing  characteristic

The humidity sensing characteristics of the sheet were evaluated by drop coating UGO and SGO sheets (see Fig. 3) on two gold electrodes (inter-electrode gap: 50 mu m) formed on the PET substrate.

In addition, surface morphology of UGO and SGO films was investigated using FE-SEM and AFM (Fig. 8).

The UGO film showed less corrugated flat morphology than SGO. Shen et al . Reported that the corrugated structure in the SGO film is caused by edge-to-edge interaction between adjacent individual GO sheets.

It is believed that the sheet in the UGO film has a smaller edge-to-edge interaction than the SGO film, resulting in a less corrugated and uniform film.

The surface roughness ( R _a ) of UGO (determined from AFM image analysis) was 9 nm, which is only one-third of the SGO surface roughness ( R _a = 27 nm) (Fig.

The sensitivity (based on 20 ° C) of the UGO and SGO electrodes when exposed to humidity was evaluated based on 1 V DC voltage application and electrical characteristics at 7 to 100% RH (FIG. 10).

Both UGO and SGO electrodes exhibited low electrical conductance (2.8x10 ^-11 S) at 7% RH (FIG. 10A). However, as RH varied from 7% to 100%, the conductance of UGO (1.2x10 ^-7 S) increased about 10 ⁴ times. Whereas the conductance (5.7x10 ^-8 S) of SGO stayed increase of about 10 ³ times.

As the RH increased from 7% to 100%, the electrical conductance increased exponentially. The UGO sensor showed a sensitivity ( S ) value of 4339 ± 433, which was more than twice as sensitive as the SGO sensor ( S = 1982 ± 122) . (* Sensitivity S = [ G _{100 % RH} - G _{7 % RH} ] / G _{7 % RH} ).

That is, UGO electrode according to the invention is H- doped graphene (S = 0.8), graphene quantum dots (S = 0.5) or the reduction of graphene oxide (S = 0.06), such as less than 10 ^3-10 The reported conventional sensing material ^4- fold higher humidity sensitivity (Table 3).

[Table 3] Performance comparison of resistance type humidity sensor

The poor performance of the conventional graphene-based humidity sensor described above can be explained for two reasons: (a) the epoxide and hydroxyl groups that are critical for proton transport are not on the graphene bases; (b) on the edge of the reduced GO And that the existing carboxyl groups interfere with proton conduction.

The effect of GO size on the overall proton conductivity was clearly confirmed from the proton conduction activation enthalpy (E _a ) measured on the GO sheet. The results are shown in FIG. 2B. The activation energy (E _a = 0.63 eV) for proton conduction in UGO was determined by SGO (E _a = 1.14 eV), which supports the hypothesis that the carboxyl groups scattered at the edges and grain boundaries of the GO block the in-plane proton conduction.

The response time and recovery time (time required to reach 90% of the steady state) of the humidity sensor were measured to be 0.2 / 0.7 sec for the UGO electrode and 0.1 / 0.3 sec for the SGO electrode (Fig. 10b).

UGO electrode showed slightly slower recovery time than SGO electrode but reduced GO (4 sec / 10 sec), graphene quantum dot (10 sec / 20 sec), carbon nanotube (6 sec / 120 sec), SnO ₂ Sec / 20 sec) or H-doping graphene (3 min / hr), which is significantly shorter than the conventional sensing material.

The stability of UGO and SGO humidity sensors was also evaluated based on DC conductance and AC conductivity as a result of storage over 5 days in a humidity chamber (97% RH, 20 ° C) (FIG. 10c, d).

As a result of the evaluation, the UGO humidity sensor shows an excellent stability compared to the SGO humidity sensor (± 48.9%), which is clearly compared with ± 4.6% for 5 days.

(5) UGO  The fingertip of the sensor Humidity Sensing  characteristic

As shown in the graphical diagram of FIG. 11A, the usability of the UGO electrode for finger-tip water sensing was investigated.

The UGO electrode exhibited excellent real-time conductance response to fingertip humidity (Fig. 11B) as the fingertip position moved away from the UGO surface (0.5 -> 2.5 mm).

Specifically, at a fingertip-UGO distance of 0.5 mm, the UGO humidity sensor exhibited a very fast response / recovery time of 0.6 sec / 1.3 sec with a sensitivity of 17.4 (Fig. 11c, d). Sensitivity and recovery / response time gradually decreased as the fingertip - UGO distance increased from 0.5mm to 2.5mm. It should be noted that the fastest response / recovery time (0.4 sec / 0.7 sec) of the UGO humidity sensor was obtained at the fingertip-UGO distance of 2.5 mm.

This, UGO- based fingertip of the present invention the humidity sensor rGO- and VS ₂ - as compared with the sensor based on the sensitivity _{(rGO = 0.06; VS 2 =} 3) , and response / recovery time (rGO = 4 sec / 10 sec; VS ₂ = 30 seconds / 12 seconds).

Review results

In the present invention, a pair of gold electrodes (inter-electrode gap = 50 μm) drop-coated with either UGO (side dimension = 47.4 ± 22.2 μm) or SGO sheet (side size = 0.8 ± 0.5 μm) The influence of the size of the pin oxide on the humidity sensing performance was examined.

As the RH increased from 7% to 100%, the plane conductance of the UGO-coated electrode increased 10 ⁴ times, while the plane conductance of the SGO-coated electrode increased 10 ³ times. The maximum sensitivity values of the UGO and SGO humidity sensors were 4339 ± 433 and 1982 ± 122, respectively, which means that the size of GO affects the plane conductance.

The effect of this GO size was supported by the hypothesis that large GOs provide a favorable condition for proton conduction through the basal plane by reducing the proton hopping barrier height (located between the GO sheets).

The effect of GO size on the overall proton conductivity was confirmed by calculating the activation enthalpy (E _a ) required for proton conduction in UGO and SGO sheets. The activation enthalpy (E _a = 0.63 eV) of proton conduction in UGO was about half of that measured in SGO (E _a = 1.14 eV). Here, the value of E _a was calculated from the Arrhenius plot based on the temperature-dependence of UGO and SGO electrode conductivity.

In addition, the UGO humidity sensor (± 4.6% deviation) showed excellent stability over the SGO humidity sensor (± 48.9% deviation). Here, the sensor stability was evaluated by measuring DC conductance and AC conductivity by storing in a humidity chamber (97% RH, 20 ° C) for 5 days.

Interestingly, the response time and recovery time of the humidity sensor did not depend heavily on the GO sheet size (UGO electrode = 0.2 / 0.7 sec; SGO electrode = 0.1 / 0.3 sec).

Using the inventive resistive UGO humidity sensor (driven at a low DC voltage of 1V) for non-contact-mode humidity sensing results in a sensitivity of 17.4 to the fingertip 0.5 mm away from the sensor and a response / recovery time of 0.6 sec / .

The response and recovery times of UGO electrodes were much faster than inorganic disulfide vanadium (VS ₂ ) (response / recovery time = 30 sec / 12 sec).

The UGO-based humidity sensor according to the present invention has an excellent moisture sensing characteristic, and can be used for real-time skin moisture monitoring and non-contact on / off switch sensors as well as clinical devices such as a high- And the like.

Claims (24)

An electrode for a moisture sensor coated with a large area ultrafine graphene oxide (UGO) having a lateral size of 47.4 ± 22.2 μm.
The method according to claim 1,
The large-area graphene oxide (UGO) has an epoxide group and a hydroxyl group on the base and a carboxyl group on the edge,
Size graphene oxide (SGO) sheet having a lateral size of 0.8 +/- 0.5 mu m which is coated on a substrate having the same area as the number of carboxyl groups existing on the edge of the large area sheet, Wherein the electrode for the moisture sensor has a reduced thickness.
The method according to claim 1,
Wherein an activation energy (E _a ) for proton conduction in the large area graphene oxide (UGO) is 0.63 eV.
The method according to claim 1,
Wherein the conductance of the electrode is 1.2x10 &lt; ^-7 &gt; S at a relative humidity (RH) of 7% to 2.8x10 &lt; ^-11 &gt; S and a relative humidity (RH) of 100%.
The method according to claim 1,
Wherein the surface roughness ( R _a ) of the large area graphene oxide (UGO) film coated on the electrode is 9 nm.
(a) dipping graphite in a sulfuric acid solution to obtain a graphite intercalation compound (GIC);
(b) heating the graphite intercalation compound (GIC) to 1000 to 1200 캜 to obtain thermally expanded graphite;
(c) introducing an oxidizing agent and performing centrifugal separation at a rotational speed of 3900 to 4100 rpm to separate the graphene oxide layer;
(d) Centrifugation was performed on the separated graphene oxide layer at a rotation speed of 7900 to 8100 rpm and 3900 to 4100 rpm, respectively, so that a small area graphene oxide (SGO) sheet and a lateral size of 47.4 占 22.2 占 퐉 Separating the large area graphene oxide (UGO) sheet; And
(e) coating a separated large-area graphene oxide (UGO) sheet on the electrode substrate to form a humidity-sensitive film.
The method according to claim 6,
Wherein the moisture sensor electrode is manufactured by separating a large area graphene oxide sheet without ultrasonic treatment.
The method according to claim 6,
Wherein the step (b) comprises heating the graphite intercalation compound (GIC) at 1000 DEG C for 10 seconds to obtain thermally expanded graphite.
The method according to claim 6,
Wherein the step (c) comprises injecting an oxidizing agent and performing centrifugal separation at a rotation speed of 4000 rpm to separate the graphene oxide layer.
The method according to claim 6,
In the step (d), the separated graphene oxide layer is centrifuged at a rotation speed of 8000 rpm for 40 minutes to separate a small-area graphene oxide sheet. The precipitate is dispersed again in an aqueous solution, And then separating the large area graphene oxide sheet by performing centrifugal separation again for 40 minutes.
The method according to claim 6,
Wherein the electrode is a gold electrode.
The method according to claim 6,
Wherein the coating of step (e) is performed by drop-coating.
A moisture sensor comprising an electrode according to any one of claims 1 to 5.
14. The method of claim 13,
Wherein the moisture sensor is a resistive-type moisture sensor.
15. The method of claim 14,
Wherein the maximum sensitivity S of the moisture sensor is 4339 433.
15. The method of claim 14,
And the response time of the moisture sensor is 0.2 seconds.
15. The method of claim 14,
Wherein the moisture sensor has a recovery time of 0.7 seconds.
15. The method of claim 14,
Wherein the moisture sensor has a deviation of within ± 4.6% measured over five days under constant temperature and humidity conditions.
19. The method of claim 18,
Wherein the moisture sensor has a deviation of the DC conductance and the AC conductivity value measured within 5 days in a humidity chamber under conditions of relative humidity (RH) of 97% and 20 占 폚 within ± 4.6%.
15. The method of claim 14,
Sensitivity is 17.4, response time is 0.6 seconds, and recovery time is 1.3 seconds when the distance between the large area of graphene oxide (UGO) coated on the electrode and the fingertip is 0.5 mm,
Wherein the response time is 0.4 second and the recovery time is 0.7 second when the distance between the large-area graphene oxide (UGO) coated on the electrode and the fingertip is 2.5 mm.
15. The method of claim 14,
The moisture sensor can be used in various applications such as touchless position interface applications, noncontact-mode moisture sensing, noncontact touchscreen screens, artificial skin, medical devices, indoor humidity control systems, moisture alarms, biochemical incubators, A flexible device, a flexible transparent electrode, or a wearable device.
22. The method of claim 21,
Wherein the moisture sensor is applied to a non-contact screen capable of monitoring moisture through an artificial skin or directly connecting to a smartphone or a smart watch.
15. The method of claim 14,
Characterized in that the moisture sensor is applied to precision air conditioning controls, breathing air systems, or moisture alarms for pressurized telecommunication transmission cables for pressurized telecommunication transmission cables Moisture sensor.
15. The method of claim 14,
The moisture sensor may be a food waste disposer, a household appliance, a dehumidifier, an air conditioner, a microwave oven, a mobile communication terminal, a clothes dryer, a wet cleaner, a plant requiring water supply to a bandit, a diaper, Wherein the sensor is applied to an indoor temperature / humidity sensor.

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