KR101598017B1 - Catalyst for hydrogen evolution reaction including n-doped graphene quantum sheet - Google Patents

Abstract

There is provided a catalyst for hydrogen-forming reaction comprising the N-doped graphene quantum sheet formed by applying a nitrogen plasma to a single-layer graphene formed on a metal catalyst layer by chemical vapor deposition.

Description

CATALYST FOR HYDROGEN EVOLUTION REACTION INCLUDING N-DOPED GRAPHENE QUANTUM SHEET <br> <br> <br> Patents - stay tuned to the technology CATALYST FOR HYDROGEN EVOLUTION REACTION INCLUDING N-DOPED GRAPHENE QUANTUM SHEET

The present invention relates to a catalyst for hydrogen-forming reaction, comprising an N-doped graphene quantum sheet.

Hydrogen production from solar energy is an important energy and environmental issue. Silicon, one of the largest elements, has been extensively studied as a cathode for the production of photoelectrochemical hydrogen. However, there is a need for a catalyst to improve conversion efficiency by reducing high overvoltage for the production of hydrogen from water. Carbon-based catalysts are attractive candidates due to their low cost and high stability. Graphene quantum dots (GQD) exhibit excellent potential for a variety of optoelectronic applications due to their size-dependent and edge-sensitive photoluminescence properties. Korean Patent No. 10-1361132 discloses a method for producing graphene quantum dots. Traditionally, GQD has been synthesized from graphene oxide by a multistage lithographic method, including chemical stripping or masking, patterning, and lift-off, in strongly acidic environments, which hindered the efficient production of high quality GQD for practical applications. On the other hand, nitrogen-functionalization or doping is known to be very useful for controlling the intrinsic properties of GQD, but requires more complex wet-chemical reactions. An atom-thick GQD can be called a graphene quantum sheet (GQS), which is much smaller than the GQD obtained from graphene oxide (GO) or carbon fiber Are expected to exhibit stronger quantum effects induced.

The discovery of effective catalysts is one of the most important challenges for the implementation of photoelectrochemical hydrogen production. Important conditions for good catalysts in photoelectrochemical cells (PEC) are not only the ability to increase the kinetics of chemical reactions, but also the resistance to electrochemical and mineral oil degradation. In general, noble metals such as platinum exhibit excellent performance in these demands. However, since precious metals are expensive, they have disadvantages in terms of economy. Thus, intensive development of efficient, durable, and inexpensive alternative catalysts is being made.

PEC is the most promising system for producing hydrogen fuel in the long run. The PEC requires semiconductor photoelectrodes to generate electron-hole pairs with absorbed photons and to promote charge transfer to the semiconductor-conductive contact interface. In order to improve the dynamics of charge transport, heterogeneous catalysts are added to the surface of the semiconductor. Many attempts to develop high-efficiency catalysts have achieved limited success. The development of abundant catalysts operating at low overvoltage and pH 7 is a challenge. In PEC, the negative effects of catalysts have to be considered: (1) reflection by the applied catalyst, (2) disadvantageous band structures such as the Schottky barrier, (3) photocorrosion, and Recombination sites at the contact interface.

The design (development) of carbon-based catalysts presents important research directions for investigating non-noble metal, environmentally friendly, and corrosion resistant catalysts. In particular, graphene has excellent light transmittance and excellent intrinsic carrier mobility: therefore, there have been various attempts to use graphene as a catalyst. Reduced graphene oxide (rGO), which contains catalytically active materials, can be produced by hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction Lt; / RTI &gt; (ORR). In most cases, however, the role of the carbon materials is limited to supports that improve the performance of electrically conductive substrates or other decorated active catalysts. Carbon-based catalysts are attracting attention due to their low cost and high stability in renewable energy technologies, but their inadequate activity is still a difficult problem.

The present application is intended to provide a catalyst for a hydrogen-forming reaction comprising an N-doped graphene quantum sheet.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

One aspect of the present invention provides a catalyst for hydrogen-forming reaction comprising an N-doped graphene quantum sheet formed by applying a nitrogen plasma to a single-layer graphene formed on a metal catalyst layer by chemical vapor deposition.

According to one embodiment of the present invention, the N-doped graphene quantum sheet acts as a catalyst for hydrogen production and is superior to other carbon catalysts in terms of overvoltage, current density, and long-term stability. Through defect and impurity control, the catalytic activity can be improved in the N-doped graphene quantum sheet. Moreover, these results can be applied to the use of a graphene catalyst for different electrochemical reactions in various energy conversion systems.

According to one embodiment of the present invention, graphene quantum sheets can be used as catalysts for the photovoltaic-induced hydrogen generation reaction in Si-photoelectrodes, and their catalytic activity can be amplified by plasma treatment in an N ₂ atmosphere . The plasma treatment induces abundant defects and mixing of nitrogen atoms into the structure of the graphene quantum sheet, which can act as a catalyst site in the graphene quantum sheet. The graphene quantum sheet containing nitrogen impurities exhibits a significant increase in exchange current density and induces a significant anodic displacement of the photocurrent onset from the Si-photoelectrode. In addition, the graphene quantum sheet shows the passivation effect of suppressing the surface oxidation of Si, thereby enabling operation of the Si-photoelectrode in neutral water. The present application shows that the graphene quantum sheet itself can be applied as a catalyst with good activity and chemical stability in photoelectrochemical systems.

According to one embodiment of the present invention, metal-free carbon-based catalysts with high efficiency and resistance to hydrogen fuel production through solar-water splitting can be developed.

Figure 1 is a graph of the photocurrent density versus potential for a p-Si electrode deposited with Gr (graphene) and N-doped graphene quantum sheet (N-GQS) (B) polarization curve of Gr and N-GQS on n ⁺ -type Si electrodes heavily doped with arsenic under a dark and JE curve.
FIG. 2 is (a) a CV (cyclic voltammetry) curve and (b) a Tafel plot showing the electrochemical activity of Gr on a glassy carbon (GC) electrode in one embodiment of the present invention.
Figure 3 is a graphical representation of the relationship between the CV of the Si photoelectrode at (a) pH 0 and (b) pH 6.8, and (c) J _init Is the initial current density in a chronoamperometry operation.
FIG. 4 is a graph showing the results of a comparison of the Ag / AgCl reference electrode using a Pt wire as a counter electrode for (a) Pt foil for Si PEC battery test and (b) RDE (rotating disk electrode) Lt; / RTI &gt;
Figure 5 is an impedance spectroscopy analysis for the resistance of bare vitreous carbon (GC), Gr on GC, and N-GQS on GC, in one embodiment of the invention.
FIG. 6 is a log of the exchange current density calculated by extrapolation to the x-axis in one embodiment of the invention.
FIG. 7 is a graph showing the relationship between (a) polarization curves for the photoelectrochemical performance of the N-GQS-Si photoelectrode with varying amounts of Pt solution and (b) representative data from the polarization curves to be.
8 is a variation of the onset potential of pure Si (black line), Gr-Si (red line), N-GQS-Si (blue line) electrodes in one embodiment of the invention.
Figure 9 shows that in one embodiment of the invention, the increase in time in (a) pH 0 (1 M HClO ₄ ) and (b) pH 6.8 (0.6 M NaH ₂ PO ₄ and 0.4 M Na ₂ HPO ₄ ) At pH 0, the N-GQS-Si electrode is a change in photocurrent density at 0 V (vs. RHE) of the electrodes according to the present invention, and because the N-GQS-Si electrode stuck to the hydrophobic surface until the hydrogen bubble suddenly bursts, Shows a spiky plot.
Figure 10 shows the CV results of Si photoelectrodes for 300 cycles using (a) pH 0, (b) pH 3.8, and (c) a scan rate of 0.05 V / sec at pH 6.8 FIG.
11 is a large time current test of photoelectrodes at pH 3.8 in one embodiment of the present invention. (a) change in photocurrent density with time and (b) change in normalized photocurrent density with time ( J / J _init ).
12 is a large time current test of photoelectrodes at pH 6.8 in an embodiment of the present invention, wherein (a) the change in photocurrent density with increasing time and (b) the normalized photocurrent density with increasing time ( J / J _init ).
13 shows, in one embodiment of the present invention, an SEM image of (a) pure Si and (b) porous Si, (c) a photocurrent density- potential ( JE ) curve, and (d) Activity.
14 is a schematic view and a photograph of (b) of a photoelectrode hydrogen generation reaction according to N-GQS / Si in one embodiment of the present invention.
15 is a graph showing the electrochemical activity of N-GQS on GC using an RDE system in one embodiment of the present invention.
16 is a graph showing the N 1s peak (a) Raman spectra of Gr and N-GQS and (b) the high resolution XPS spectrum of Gr and N-GQS, It is also the XPS spectrum of each sample before (c) before the large time current method test and (d) after the time current method test.
FIG. 17 is a graph of an AFM image of (a) a single layer Gr and (b) an N-GQS, and (c) a Raman spectrum of the samples, according to the exposure time, And the images all have a size of 6.5 x 6.5 μm ² .

Hereinafter, embodiments and examples of the present invention will be described in detail with reference to the accompanying drawings, which will be readily apparent to those skilled in the art to which the present invention pertains. It should be understood, however, that the present invention may be embodied in many different forms and is not limited to the embodiments and examples described herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout this specification, when a part is referred to as being "connected" to another part, it is not limited to a case where it is "directly connected" but also includes the case where it is "electrically connected" do.

Throughout this specification, when a member is "on " another member, it includes not only when the member is in contact with the other member, but also when there is another member between the two members.

Throughout this specification, when an element is referred to as "including " an element, it is understood that the element may include other elements as well, without departing from the other elements unless specifically stated otherwise.

As used herein, the terms "about," " substantially, "and the like are used herein to refer to or approximate the numerical value of manufacturing and material tolerances inherent in the stated sense, Accurate or absolute numbers are used to prevent unauthorized exploitation by unauthorized intruders of the mentioned disclosure.

The term &quot; step &quot; or &quot; step of ~ &quot; as used throughout the specification does not imply &quot; step for.

Throughout this specification, the term "combination (s) thereof " included in the expression of the machine form means a mixture or combination of one or more elements selected from the group consisting of the constituents described in the expression of the form of a marker, Quot; means at least one selected from the group consisting of the above-mentioned elements.

Throughout this specification, the description of "A and / or B" means "A or B, or A and B".

Throughout this specification, the term "quantum dot" refers to a nanosized material having a bandgap due to the quantum confinement effect.

Throughout this specification, the term graphene quantum sheet (GQS) may refer to an atom-thick graphene quantum dot (GQD) made from a single-layer graphene, but is not limited thereto .

Throughout the present specification, N ₂ plasma treatment the graphene sheet both (N-GQS) will be to sense the graphene _{(N 2 -plasma-treated graphene,} NGr) the N ₂ plasma treatment according to the time of the nitrogen plasma treatment However, the present invention is not limited thereto.

Hereinafter, embodiments of the present invention are described in detail, but the present invention is not limited thereto.

The first aspect of the present application provides a catalyst for hydrogen-forming reaction comprising an N-doped graphene quantum sheet formed by applying a nitrogen plasma to a single-layer graphene formed on a metal catalyst layer by chemical vapor deposition.

In one embodiment of the present invention, the metal catalyst layer comprises at least one metal selected from the group consisting of Ni, Co, Fe, Pt, Au, Al, Cr, Cu, Mg, Mn, Mo, Rh, Si, Ta, Ti, W, But are not limited to, those selected from the group consisting of brass, bronze, stainless steel, Ge, and combinations thereof.

According to one embodiment of the present invention, the metal catalyst layer may include, but not limited to, a metal catalyst layer formed on a substrate.

In one embodiment of the present invention, the chemical vapor deposition method may include, but not limited to, growing a graphene on the catalyst layer by supplying a carbon source and reacting. For example, the carbon source may be selected from the group consisting of carbon monoxide, carbon dioxide, methane, ethane, ethylene, ethanol, acetylene, propane, butane, butadiene, pentane, pentene, cyclopentadiene, hexane, cyclohexane, benzene, But the present invention is not limited thereto.

According to one embodiment of the present invention, the carbon source may be gaseous or liquid, but may not be limited thereto.

In one embodiment of the invention, the graphene may include, but is not limited to, heating the chemical vapor deposition process at a temperature of from about 400 DEG C to about 1,100 DEG C, and then cooling the resultant. For example, the temperature may range from about 400 ° C to about 500 ° C, from about 400 ° C to about 600 ° C, from about 400 ° C to about 700 ° C, from about 400 ° C to about 800 ° C, From about 500 ° C to about 700 ° C, from about 500 ° C to about 800 ° C, from about 500 ° C to about 900 ° C, from about 500 ° C to about 900 ° C, from about 400 ° C to about 1,100 ° C, From about 600 ° C to about 1000 ° C, from about 500 ° C to about 1,100 ° C, from about 600 ° C to about 700 ° C, from about 600 ° C to about 800 ° C, from about 600 ° C to about 900 ° C, About 700 ° C to about 900 ° C, about 700 ° C to about 900 ° C, about 700 ° C to about 1,000 ° C, about 700 ° C to about 1,100 ° C, about 800 ° C to about 900 ° C, From about 800 占 폚 to about 1,100 占 폚, from about 900 占 폚 to about 1,000 占 폚, from about 900 占 폚 to about 1,100 占 폚, or from about 1,000 占 폚 to about 1,100 占 폚.

In one embodiment of the invention, the N-doped graphene quantum sheet may be, but not limited to, increased defect formation and N-doping in the single-layer graphene by applying the nitrogen plasma .

In one embodiment of the present invention, the N-doped graphene quantum sheet is formed by applying a nitrogen plasma to a single-layer graphene formed on a metal catalyst layer by chemical vapor deposition to form an increased defect portion and an increased N- But may not be limited thereto.

In one embodiment of the present invention, the N-doped graphene quantum sheet may include an N-doped amount to induce a significant current density increase and a significant anodic displacement of the photocurrent onset from the photoelectrode, .

In one embodiment of the present invention, the N-doped graphene quantum sheet may increase the efficiency of the hydrogen-forming reaction through photoelectrochemistry by reducing the overvoltage, but may not be limited thereto.

In one embodiment of the present invention, the defect sites and the N-doped amount may improve the catalytic activity of the N-doped graphene quantum sheet, but the present invention is not limited thereto.

In one embodiment of the present invention, the N-doped graphene quantum sheet may have activity against a hydrogen-forming reaction through photoelectrochemistry or electrochemistry as a photocatalyst, but the present invention is not limited thereto.

In one embodiment of the present invention, the photocatalyst may have activity against hydrogen-forming reaction (for example, hydrogen generation through water-decomposing reaction) through photoelectrochemistry or electrochemistry under neutral condition, .

In one embodiment of the present invention, the N-doped graphene quantum sheet may increase the efficiency of the hydrogen-forming reaction through photoelectrochemistry by reducing the overvoltage, but may not be limited thereto.

In one embodiment of the present invention, the N-doped graphene quantum sheet is transferred to the surface of a photoelectrode used for a hydrogen-forming reaction through photoelectrochemistry to prevent or reduce surface oxidation of the photoelectrode, But the present invention is not limited thereto.

In one embodiment of the present invention, the photoelectrode may include Si but may not be limited thereto.

In one embodiment of the present invention, the photoelectrode may be spin-coated with a polymer such as poly (methyl methacrylate) (PMMA) on the N-doped graphene quantum sheet, But it may be, but not limited to, removed by the same organic solvent.

In one embodiment, the photoelectrode may comprise spin-coating a polymer such as PMMA onto the N-doped graphene quantum sheet and then removing the catalyst layer by an etchant, . For example, the etchant of ammonium persulfate _{[(NH 4) 2 S 2} O 8], benzimidazole (benzimidazole, BI), acid (acid), an etchant (buffered oxide etchant, BOE) in a buffer oxide, Fe ( NO ₃ ) ₃ , Iron (III) Chloride, FeCl ₃ , CuCl ₂ , and combinations thereof. In other embodiments, the acid may comprise an organic or inorganic acid, for example, but not exclusively, a fluorine-containing acid such as HF.

In one embodiment of the present invention, the photocatalyst may be such that the PMMA is removed after the N-doped graphene quantum sheet is transferred onto another photoelectrode substrate (for example, Si), but the present invention is not limited thereto no.

In one embodiment of the present invention, the photoelectrode is dispersed in a common organic solvent to remove the metal layer without the PMMA and then transferred onto a substrate using the solvent extraction technique of the floating N-doped graphene quantum sheet , But may not be limited thereto.

The organic solvent is not particularly limited and includes, for example, acetone, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, cyclohexanone, methyl cellosolve, ethyl cellosolve, Butanol, 2-butanol, isobutanol, isobutyl acetate, isobutanol, isobutyl acetate, isobutyl acetate, isobutyl acetate, isobutyl acetate, , Isobutyl alcohol, isopropyl alcohol, dichloromethane, chloroform, dichloroethane, trichloroethane, tetrachloroethane, dichloroethylene, trichlorethylene, tetrachlorethylene, chlorobenzene, ortho-dichlorobenzene, n- And may include at least one member selected from the group consisting of hexanol, methylcyclohexanol, benzene, toluene, and xylene, There can not be.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are given for the purpose of helping understanding of the present invention, but the present invention is not limited to the following Examples.

[ Example ]

[ Example  One]

N- Doped Grapina  Synthesis of quantum sheets

Single layer graphenes were grown on Cu foil by chemical vapor deposition (CVD) and then transferred to the SiO ₂ surface. In the first step of graphene synthesis, a copper foil was placed in a quartz reactor in a CVD system and then heated at 1,000 DEG C with H ₂ flowing at 70 mTorr. Additionally, the sample was annealed for 20 minutes without changing the conditions. A mixed gas of H ₂ and CH ₄ was injected at a rate of 5 SCCM and 50 SCCM for 30 minutes at 8 Torr. Finally, the sample was rapidly cooled to room temperature while flowing H ₂ . After growth, graphene on Cu was placed in a plasma chamber (SNTEK) to remove graphene on one side of the Cu foil. Pressure in the chamber was lowered to the pump 50 mTorr, O ₂ gas of 100 W for 10 seconds radio - by applying a frequency (13.56 MHz) power incident (forward power) was injected into the chamber. A low density N ₂ -plasma was also generated by applying a 10 W power to create a vacancy site on the front of the Cu and an N-doped graphene quantum sheet. The flow rate of N ₂ was 20 SCCM and the working pressure of the chamber was 120 mTorr. Under these conditions, the plasma treatment was performed using various exposure times for N ₂ plasma from 0 to 16 seconds to test the electrochemical reaction of the graphene surface. Finally, poly (methyl methacrylate) (PMMA) was spin-coated on the graphene and then the copper foil was removed in 0.1 M ammonium sulfate solution. After washing with deionized water, the graphene was transferred onto the Si substrate and then the PMMA was removed from the acetone for 30 minutes.

Si The  Ready

[02] In order to form an ohmic contact between the copper wire and the un-polished surface (back surface) of the Si wafer, a gallium-indium eutectic alloy was incorporated and then treated with a silver paste . Epoxy was used for insulation and protection of the Si back contact except for the intended light irradiated area (0.25 cm ² ) of the front side of the Si. Gr was transferred from the Cu foil to the Si surface. PMMA was spin-coated onto the Gr phase and then the copper foil was removed from the ammonium persulfate solution. After washing with deionized water, the Gr was transferred onto the Si substrate and then the PMMA was removed from the acetone for 30 minutes.

Electrochemical measurements

Electrochemical measurements were performed in a 3-electrode cell using an electrochemical analyzer (CHI 600D, CH Instruments, Inc.). A Pt foil was used as a counter electrode, and an Ag / AgCl / 3 M NaCl electrode was used as a reference electrode. The reference electrode was calibrated carefully for the reversible hydrogen electrode (RHE) in a 1 M perchloric acid aqueous solution with high purity H ₂ saturation at 25 ° C (FIG. 4). RHE was measured at -0.201 V and -0.203 V for the Ag / AgCl reference electrode. To measure photoelectrochemical behavior, visible light from a 300 W Xe lamp equipped with an Air Mass 1.5 Global controlled glass filter was irradiated onto the substrate with a light intensity of 100 mW cm & lt ^; &quot; ^{2 &} gt ;. The RDE system was purchased from EG & G PARC Inc, and a glass carbon tip (3 mm diameter) for RDE measurements was purchased from BASi. A Pt catalyst mixed with 5 wt% Nafion was synthesized from the method of the prior art. 10 [mu] L of the Pt catalyst ink was loaded onto the GC and dried at 110 [deg.] C. RDE measurements were performed at a rotation rate of 1,000 rpm using a scan rate of 5 mV s ^-1 .

Correction to relative hydrogen electrode

The Ag / AgCl / 3 M NaCl electrode (BASi) was used as the reference electrode. A Pt foil (2 cm x 2 cm x 0.1 mm, purity 99.997%, Alfa Aesar) was used as the counter electrode. For the RDE experiments, a Pt wire (diameter 0.1 mm, Dr Bob's cell) was used as the counter electrode. (RHE) in a 1 M perchloric acid aqueous solution (Sigma Aldrich) with high purity H ₂ saturation at 25 ° C. The RHE was measured at -0.201 V and -0.203 V for the Ag / AgCl reference electrode (FIG. 4). Since the potential difference between Ag / AgCl and RHE is dependent on the pH and temperature of the electrode, the potential was measured carefully prior to setting each measurement, and the measurement was performed at least three times for each condition. The potential was adjusted using a potentiostat (CHI 600D, Ch Instrument) at a scan rate of 5 mV s ^{&lt; -1 &} gt ;.

Character analysis

Raman spectra were obtained using a Renishaw micro-Raman spectrometer using an excitation wavelength of 514.5 nm, which is an Ar laser. The spot diameter was about 2 μm using a 50 × objective lens. Oxygen plasma treatment (SNTEK) was performed with a radio-frequency (rf) power of 100 W for 13 seconds at 140 mTorr and a nitrogen plasma at a power of 10 W at various exposure times (0 to 16 seconds) under 120 mTorr . The XPS spectra were collected by AXIS Ultra DLD (Kratos, Inc) using a monochromatic Al K (1,486.6 eV), a 150 W source at Korea Basic Science Institute (KBSI). Narrow-scan data were collected using 40 eV per step and 0.05 eV pass energy.

Monolayer graphene (Gr) was grown on Cu foil by chemical vapor deposition (CVD) and was transferred to p-type Si wafers. In order to investigate the photocathodic behavior of Gr in the Si (Gr-Si) electrode, the current density measurement was carried out in the same manner as the potential irradiated from 0.4 V to -1.0 V with respect to the reversible hydrogen electrode (RHE) It was swept. 300 W Xe lamp light source of the air mass 1.5 global (Air Mass 1.5 Global) 100 mW cm by adjusting the filter under aqueous 1 M perchloric acid solution (pH 0) ^- was irradiated with light for Si photo-electrode having a light intensity of the ^two.

In the measurement of photoelectrochemical performance, Gr shows catalytic activity against HER. As shown in Fig. 1 (a), the current density of pure Si (bare Si) gradually increases from -0.2 V with respect to RHE, and increases to about -32 mA cm ^{&lt; 2} to -1.0 V for RHE. Interestingly, in the Gr-Si measurements, the overall current density-potential ( JE ) curve was shifted by about 0.2 V towards the positive potential. Onset potential (potential onset) is -1 mA cm ^- is defined as the potential in the photoelectric current density of ^2. The onset potential of Gr-Si is 0.01 V for RHE, and the potential is a positive displacement by 0.18 V compared to pure Si (-0.17 V vs. RHE). Figure 1 (b) also shows the dark current density of n ⁺ type Si electrodes doped with arsenic. In the dark state, the positive displacement at 0.14 V of the onset potential (-0.49 V versus RHE for Gr-Si) is also lowered to HER compared to the pure Si (-0.63 V vs. RHE) High activity. This result shows that the single layer Gr acts as an effective catalyst for HER at the Si photoelectrode.

To investigate the electrocatalytic activity of Gr, cyclic voltammetry (CV) was performed without light irradiation using a rotating disk electrode (RDE). To produce a working electrode, Gr was transferred to a glassy carbon (GC) tip, which is inert in aqueous solution. As shown in Figure 2 (a), in the JE curves of the RDE measurements, the current density from the electrolysis of water increases rapidly as the potential sweeps from 0 V to -0.5 V. To compare the onset potential for HER in the RDE system, -5 mA cm of HER current density ^- the potential to reach the pure ² (bare) was measured for the GC and GC-Gr. The potential for reaching -5 mA cm &lt; ^{2 &} gt ^; of HER current density was -0.33 V for the pure GC. Similar to the behavior of the photocurrent response on the Gr-Si electrode, a positive displacement in the overall JE curve was also observed for Gr-GC. The potential for Gr-GC at -5 mA cm ^{- 2} is 0.28 V (vs. RHE); The potential will be positively displaced to 50 mV relative to the potential for the bare GC. This result means that the Gr monolayer has electrocatalytic activity towards HER regardless of the substrate materials.

To obtain more quantitative information on the catalytic activity of the Gr, the JE curve in Figure 2 (a) is converted to a plot of potential as a logarithm of J ; The plot is called a Tafel plot. The measured potentials are corrected for the loss of the resistance potential drop (i R) resulting from the surface resistance between the substrate and the electrolyte. Impedance spectroscopy analysis shows that the resistances of the pure GC and Gr-GC are 7.1 ohms (Ω) and 7.2 ohms (Ω), respectively (FIG. 5). The Tafel plot provides two parameters to measure the electrocatalytic activity: the Tafel slope and the exchange current density. The tendell slope is defined as a measure of the potential increase required for an increase by an order of magnitude of the resulting current. The pure GC showed a Tappel slope of 85 mV / decade and the Gr-GC showed 9 mV / decade, a lower Tappel slope (74 mV / decade) than pure GC. For comparison with known catalysts, Pt particles were deposited on GC and analyzed for electrocatalytic activity. -5 mA cm ^- the applied potential in order to obtain a ² is 0.04 V (vs. RHE), which is displaced by an amount (positive) as compared to 0.24 V Gr-GC. The Ptell GC tarpaulin slope is 42 mV / decade, which is 32 mV / decade lower than the Gr-GC. For the catalyst 2D MoS _2, -5 mA cm ^- potential for obtaining the ^two is -0.19 V (vs. RHE), and was tapel slope is 60 mV / decade when deposited as a mixture with Nafion to GC.

Tappel slope is an inherent characteristic of the catalyst as measured by the rate-limiting step for HER. Mechanically, for HER in acidic solutions, the following possible reaction steps have been proposed:

H ₃ O ⁺ + e ^- → H _ads + H ₂ O (1)

H _ads + H ₃ O ⁺ + e ^- → H ₂ + H ₂ O (2)

H _ads + H _{ads -} > H ₂ (3)

Where H _ads is adsorbed hydrogen. (1) is a discharge step (Volmer reaction), (2) is a desorption step [Heyrovsky reaction], and (3) is a recombination step [Tafel reaction]. The Tappel slope value is also related to the hydrogen coverage (&amp;thetas; _H ) adsorbed on the surface of the electrode. If the recombination of the adsorbed hydrogen (tapel reaction) is a rate-determining step for HER and the coverage is very high (? _H ? 1), the measured Telfel slope is 30 mV / decade. However, if the electrochemical desorption step (the Heilovsky reaction) is a rate-determining step, the inclination of the Telfel of 40 mV / decade to 118 mV / decade is measured and is subject to the value of the above-mentioned θ _H (0 to 1). The observed Tappel slope of ~ 80 mV / decade here indicates that the rate of HER at the pure GC and Gr-GC electrodes is determined by the Hirovsky reaction, since? _H has an intermediate value.

The exchange current density ( J ₀ ) is defined as the current density at zero (0) overvoltage. The catalytic effect is derived from either increasing the rate of charge transfer at the interface between the electrode and the electrolyte or lowering the activation energy barrier to the chemical reaction; These catalytic effects are represented by J ₀ . High J ₀ Indicates that adsorption / desorption of protons in the electron transfer or electrode / electrolyte can be more easily caused by lower kinetic barriers. In the tapel plot, J _o can be obtained by extrapolating the plot in Figure 2 (d) and extracting the current density at 0 V (vs. RHE). The Gr-GC electrode is 2.73 × 10 ^-6 A cm ^- represents an improved J ₀ of Figure ^2, this is higher than J ₀ of the pure ^{GC (1.63 × 10 -6 A cm} -2) ( Fig. 6). The J ₀ of the single layer Gr is also compared to that of the MoS ₂ nanoparticles. Exact J ₀ Can be calculated considering the number of active sites. For example, J ₀ of MoS ₂ for HER Is experimentally measured as 1.3 × 10 ^-7 A cm _geometric- ² to 3.1 × 10 ^-7 A cm _geometric- ² . The active site of MoS ₂ nanoparticles for the HER is known to be the edge region (edge site) of MoS ₂ nanoparticles. For direct comparisons of site-to-site between MoS ₂ nanoparticles and Pt catalysts, T. Jaramillo et al. Measured the exchange current per site of MoS ₂ including STM analysis. The exchange current per site was then multiplied by the region density of Pt for a fair comparison with the transition metal catalyst to obtain an exchange current density of 7.9 x 10 &lt; ^{-6 &} gt ^; A cm &lt;&quot; ^{2 &} gt ;. The value is still close to the exchange current density of Gr (2.7 × 10 ^-6 A cm ^-2 ). The present inventors have recently conducted an investigation to identify and quantify the active sites of a single layer Gr. In the TAPPEL assay, Gr had catalytic activity for HER compared to that of MoS ₂ nanoparticles.

The catalytic activity of the Gr can be further improved by generating more active sites. The present inventors have anticipated that the N ₂ plasma treatment will induce nitrogen doping and abundant defects in the graphitic carbon structure, and that such doping and defect sites can be catalyst sites for HER. In a previous study, the Dai group showed that nitrogen impurities and defective sites in the disclosed CNT-Gr complex promoted an improvement in the electrocatalytic activity for the oxygen reduction reaction (ORR). The surface of Gr grown on Cu foil was modified with N ₂ plasma in Reactive Ion Etching (RIE), and the catalytic activities of N ₂ -plasma-treated Gr (NGr) Respectively.

The N ₂ -plasma treatment improved the catalytic activity of the Gr. The present inventors optimized the duration of the plasma treatment and obtained the best form Gr at a duration of 14 seconds (Fig. 1 (a) and Fig. 7]. As shown in Figure 1 (a), additional positive displacements of the onset potential were observed for N-GQS under light illumination. The onset potential for N-GQS-Si is 0.12 V (vs. RHE) and the above onset potential is improved by more than 0.11 V over that for Gr-Si. Compared to pure Si with an onset of -0.17 V (vs. RHE), the onset potential was positively displaced by 0.29 V. A more significant improvement by Gr and N-GQS was observed at a current density of -10 mA cm ^-2 . To obtain a photocurrent of -10 mA cm ^-2 , potentials of -0.42 V, -0.21 V, and -0.04 V (vs. RHE) were required for each of pure Si, Gr-Si, and N-GQS-Si . The potential for -10 mA cm ^-2 was displaced in the anodic direction by 0.38 V with respect to N-GQS-Si, compared to pure Si. Cathodic currents from N-GQS-Si rapidly increased at potentials that are more negative than the onset potential and at current densities less than -5 mA cm ^-2 . In the dark state, N-GQS-Si also exhibits a positive displacement at 50 mV (-0.44 V vs. RHE) relative to the Gr-Si (FIG. 1 (b) The N-GQS deposition on Si resulted in a light voltage of 0.56 V (Table 1). In order to compare the catalytic activity of N-GQS with that of the best catalyst, the CV of Si with non-electrolytically-deposited Pt (Pt-Si) was also measured (FIG. The onset of the Pt-Si is 0.24 V (vs. RHE), and the potential for -10 mA cm ^-2 is 0.11 V (vs. RHE), which is 0.12 V versus the N-GQS-Si electrode RHE is a positive displacement. N-GQS-Si was also compared with Ni-decorated Si electrodes, which are abundant catalysts on earth. The Ni-Si electrode exhibited a 0.4% solar-to-hydrogen (STH) conversion efficiency with an onset potential of 0.34 V (vs. RHE) and a J of 10 mA cm ^-2 at 0 V (vs. RHE) . N-GQS-Si exhibited an onset potential (0.12 V vs. RHE) and J (-5.5 mA cm ^-2 ) at 0 V (vs. RHE). As a result, STH conversion efficiency (+ 0.16%) was achieved by using N-GQS catalyst (Table 1). N-GQS-Si was also compared with Si electrodes with MoS ₂ catalysts. The photocurrent density (-5.5 mA cm ^-2 ) at 0 V (vs. RHE) of N-GQS in the Si electrode was much higher than the reported value of MoS ₂ (about -0.5 mA cm ^-2 ).

[Table 1]

The light voltage is defined as the difference between the dark potential and the onset potential under the light irradiation condition. The values were calculated taking into account the following references; YW Chen, JD Prange, S. Duhnen, Y. Park, M. Gunji, CE Chidsey and PC McIntyre, Nature Mater., 2011, 10, 539-544.

Additional positive displacements at the onset potential by N-GQS were also observed in RDE measurements. The potential to achieve a current density of -5 mA cm ^-2 is 0.26 V (vs. RHE); The value was about 70 mV greater than that of pure GC (-0.33 V vs. RHE) and about 20 mV greater than that of Gr-GC (-0.28 V vs. RHE). Improvement in catalytic activity using N-GQS was also confirmed by comparison of J ₀ in the Tafel plots (FIG. 2 (b)). The J ₀ of the N-GQS-GC was 7.29 × 10 ^-6 A cm ^-2 , which was about 2.8 times higher than that of Gr-GC. This is similar to that of MoS ₂ nanoparticles (7.9 × 10 ^-6 A cm ^-2 ).

In Figure 16, the inventors describe changes in the chemical state of Gr after N ₂ plasma treatment for various durations. The AFM image of the surface of the N-GQS treated with N ₂ for 14 seconds indicated that the AFM image of the single layer Gr was smooth while many defects and edges were generated in the Gr (FIG. 17). In Raman spectrum (Fig. 16 (a)), generation of defects and edges by N ₂ treatment was also confirmed. In the Raman spectrum of the single layer Gr, the D-peak is due to the breathing mode of the six-atom rings, which is around the K or K ₀ point in the first Brillouin zone, transverse optical phonon). This includes an intervalley double resonance process. The absence of significant D peaks is evidence of good crystallization of a single Gr layer. In single-layer graphenes with good crystallinity, the D-peak is negligible in the single layer Gr, but the D-peak is still as high as about 2% of the G-peak, which may be an excellent active site for hydrogen production. However, after plasma treatment, a significant increase in D peak intensity, ~ 1,620 cm ^- D in the ^first "peak, that is, K or K 'that connects two points belonging to the same cone (cone) of the surrounding, inter-valley double resonance process Lt; / RTI &gt; Also, the D + G combination mode at ~ 2,950 cm ^-1 requires a defect for its activation. Thus, this change in Raman spectra indicates the formation of rich edge and defect sites in N-GQS.

The passivation effect of Gr was also investigated. The CV of the Si photoelectrodes was measured for 300 cycles using a scan rate of 0.05 V s ^-1 at pH 0 and pH 6.8 (the charge-on time of each electrode was about 20,000 seconds). As shown in FIGS. 3A and 3B, the CV curves of all the electrodes are negatively displaced with respect to RHE as the CV cycle is increased. However, pure Si has the largest negative value even in a very low cycle, Displacement. The change in the onset potential was measured as the main parameter for measuring the passivation performance of Gr and N-GQS for Si surface oxidation (Fig. 8). At pH 0, the Gr-Si and N-GQS-Si electrodes exhibited only displacements at the onset potential of 0.1 V and 0.035 V, respectively, whereas the onset potential of the pure Si electrode was negative by 0.38 V within 100 cycles ) (Fig. 8). At pH 6.8, both the Gr-Si and N-GQS-Si electrodes exhibit a negative displacement below 0.24 V at the onset potential, while the onset potential of pure Si is sharply reduced and only about- Saturated at values of 1.1 V (vs. RHE).

Chronoamperometry tests of pure Si, Gr-Si, and N-GQS-Si were also performed at 0 V (vs. RHE) and normalized current densities by their initial values are shown in FIG. 3 As a function of time in (c). Gr-Si, and N-GQS-Si also showed inhibited inhibition of the performance in photocurrent density compared to that of pure Si in both pH 0 and pH 6.7 conditions. The performance of the pure Si was completely lost only after 1000 seconds at pH 6.8 (Fig. 3 (c) and Fig. 9]. The N-GQS-Si electrode maintained more than 30% higher than the normalized current and -4.8 mA cm ^-2 even in 10,000 seconds. Thus, it has been found from the changes in current density at the onset potential and 0 V (vs. RHE) that Gr and N-GQS suppress the reduction of photoelectrochemical performance by oxidation of the Si surface.

To confirm the passivation effect of Gr and N-GQS on the Si surface, the surface state of Si was analyzed before and after the large time current test at 0 V (vs. RHE) for 10,000 seconds (Figure 16 (c) and (d)]. Pure Si, XPS spectrum of Gr-Si, and N-GQS-Si was obtained from the Si 2p _3/2 region. Si peaks can be assigned at 99.3 eV, and SiO ₂ peaks can be assigned at 103.3 eV. From the XPS spectrum of pure Si, the peak of Si-O was increased after the large time current method test. For the Gr-Si and N-GQS-Si samples, after long term testing, they only show a slight increase in the SiO ₂ peak. These results indicate that graphene inhibits oxidation of the Si surface during photoelectrolysis.

In order to further improve the electrocatalytic activity of Gr, Pt nanoparticles were electrodeposited from the N-GQS-Si (Pt-N-GQS-Si). The present inventors also anticipated that during the photoelectrochemical decomposition, the passivation effect of Gr which inhibits Si oxidation could make a synergistic advantage with other catalysts. As a proof of concept, Pt was chosen to investigate these effects. The catalytic activity of Pt-N-GQS-Si was significantly improved as compared with N-GQS-Si as shown in Fig. 1 (a). From CV measurements, the onset of Pt-N-GQS-Si was 0.35 V (vs. RHE) at -1 mA cm ^-2 and the potential for reaching -10 mA cm ^-2 was 0.25 V (vs. RHE) . STH conversion efficiency increased to 3.05%. As expected, long-term stability was also achieved by combining Pt and Gr, as shown in Figs. 11 and 12. The current versus time test method is Pt-N-Si-GQS each -4.7 mA cm ^-2 and -4.0 mA cm at pH 3.8 and pH 6.8 even after 8,000 ^seconds showed that the maintenance of a stable current density of ^2.

Recently, graphene based catalysts have been an interesting candidate for photoelectrochemical reactions. 13 shows the catalyst application of N-GQS on flat, pure Si-based and porous Si-based substrates for hydrogen production. The scanning electron microscope (SEM) images of FIGS. 13 (a) and 13 (b) show cross sections of the pure Si substrate and the porous Si substrate, with each internal view showing a top-view of the sample. To evaluate photocathodic behavior, the N-GQS was loaded onto pure Si using dry transfer and loaded onto porous Si using solution drop-casting. The photocurrent density was measured by a potential sweep from 0.4 V to -0.8 V against a reversible hydrogen electrode (RHE) in a three electrode cell. A light source with a 300 W Xe lamp (100 mW cm ^-2 ) equipped with an Air Mass 1.5 Global condition filter was irradiated onto the samples in a 1 M perchloric acid aqueous solution (pH 0) (FIG. 14). Interestingly, the N-GQS showed excellent catalytic activity for HER. As shown in Fig. 13 (c), N-GQS / photocurrent density of the pure Si - potential (JE) curve photocurrent density of the pure Si - potential (JE) potential direction of sharply compared to the curve amount (positive) And the positive potential of the N-GQS was also 0.29 V (Table 2). Compared to pure Si, the porous Si exhibited a positive displacement of the threshold current density and the onset potential improved by the light trapping effect. In N-GQS on a porous Si substrate, the positive displacement at an onset potential of 0.09 V also shows a higher activity for HER compared to that of porous Si. Figure 13 (d) shows the electrocatalytic activity of N-GQS, and the CV curves of the samples were obtained using a rotating disk electrode (RDE) system without light irradiation. For the working electrode, N-GQS was transferred to a glassy carbon tip which is inert in aqueous solution. The JE curve was swept from 0.1 V to -0.35 V and the onset potential obtained a HER current density of -5 mA cm ^-2 for the prepared graphene and N-GQS (Table 2). The N-GQS's onset potential was -0.22 V versus RHE and had a positive displacement by 0.07 V compared to the onset potential of the formed graphene. The results are similar to the photoelectrochemical behavior of the JE curve, which is a positive displacement by N-GQS in the overall JE curve. From the above results, we have come to the conclusion that the N-GQS exhibits an excellent electrocatalytic effect for hydrogen production when combined with a Si photoelectrode having a particular morphology.

[Table 2]

In summary, the present application demonstrated the formation of N-GQS from a single-layer graphene formed on copper using a nitrogen plasma. The N-GQS could be transferred to the photoelectrode surface of a specific shape to enhance catalytic activity for photoelectrochemical hydrogen generation that could be useful for a variety of display, energy, and biological applications.

In this example, our studies have shown that N-doped graphene quantum sheets act as catalysts for hydrogen production and are superior to other carbon catalysts in terms of overvoltage, current density and long-term stability. The method shows the importance of controlling defects and impurities to improve catalytic activity in graphene. Moreover, these results could be applied as the use of a graphene catalyst for different electrochemical reactions in various energy conversion systems.

The present example also shows that the porous Si-cathode decorated with N-GQS exhibits improved photochemical and electrochemical activity, which is advantageous for solar-driven hydrogen generation reaction (HER) Proved. Thus, the N-GQS converted from the single-layer graphene is expected to be useful for a wide range of optoelectronic, electrochemical, and energy storage applications in the future. The present example also confirmed that the N-GQS transferred onto flat Si or porous Si could be an excellent photo-electrochemical catalyst for the hydrogen generation reaction.

In this example, the inventors have demonstrated that graphene quantum sheets can catalyze a hydrogen evolution reaction (HER); Thus, the graphene quantum sheet could improve the performance of the silicon (Si) photoelectrodes through a significant reduction in the overvoltage. The catalytic activity could be improved in graphene quantum sheets prepared by N ₂ doping and defective N ₂ plasma treatments. In addition, surface passivation with transparent graphene quantum sheet catalyst enabled the Si photoelectrode to react efficiently and stably by neutralization of oxidation in neutral water.

In this example we have investigated new possibilities for graphene quantum sheets, such as electrochemical catalysts for efficient HER, and found that the nitrogen doping and defects achieved through N ₂ plasma treatment improved catalytic activity . As known to the present inventors, there has been no prior report on the application of graphene quantum sheets to hydrogen production.

As a proof of concept, the Si photoelectrode was used to investigate the catalytic activity of the graphene quantum sheet and the nitrogen doping effect in the photoelectrochemical HER. Si is the most promising photoelectrode material due to its small band gap of 1.12 eV and precise operability; However, this can not be sustained under the aqueous electrolyte due to surface oxidation. Therefore, the passivation of the Si surface is essential for the continuous operation of the Si photoelectrode in neutral water. The single-layer graphene in Si acted as a passivation layer without reducing the photon generation rate against surface oxidation.

The present inventors have proposed an N-doped graphene quantum-sheet catalyst which improves the PEC performance of Si-photoelectrodes. The onset potential for the photocurrent from Si has shifted significantly in the anodic direction without any change in the saturation current density. N-GQS or N-GQD is a passivation layer that has excellent catalytic activity for photoelectrochemical HER at the Si photoelectrode and maintains higher onset potential and current density even at neutral pH. The approach in this example has developed a strategy for developing metal-free carbon-based catalysts with high efficiency and resistance to solar-induced hydrogen fuel production.

It will be understood by those of ordinary skill in the art that the foregoing description of the embodiments is for illustrative purposes and that those skilled in the art can easily modify the invention without departing from the spirit or essential characteristics thereof. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims (7)

1. A catalyst for hydrogen-forming reaction comprising an N-doped atom-thick graphene quantum dot formed by applying a nitrogen plasma to a single-layer graphene formed on a metal catalyst layer by chemical vapor deposition,
The N-doped atom-thick graphene quantum dot contains increased defect sites and increased N-doping quantities by the nitrogen plasma treatment,
Wherein the N-doped atom-thick graphene quantum dot has an activity toward a hydrogen-forming reaction through photoelectrochemistry or electrochemistry as a photocatalyst.
Catalyst for hydrogen-forming reaction.
delete delete The method according to claim 1,
Wherein the photocatalyst has an activity for hydrogen-forming reaction through photoelectrochemistry or electrochemistry under neutral conditions.
The method according to claim 1,
Wherein the N-doped atom-thick graphene quantum dot increases the efficiency of the hydrogen-forming reaction through photoelectrochemistry by reducing overvoltage.
The method according to claim 1,
The N-doped atom-thick graphene quantum dot is transferred to the surface of the photoelectrode used for the hydrogen-forming reaction through photoelectrochemistry to prevent or reduce surface oxidation of the photoelectrode, thereby improving the stability of the photoelectrode &Lt; / RTI &gt;
The method according to claim 6,
Wherein the photoelectrode comprises Si.

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