KR102532405B1 - Photoelectrochemical device with increased photostability by using a surface oxide layer and a co-catalyst - Google Patents

Abstract

The present invention relates to a photoelectrochemical device with improved stability using a GaN film whose surface is modified with an oxide layer and a cocatalyst. The photoelectrochemical device according to the present invention may implement a photoelectrochemical device in which stability is secured by a passivation effect by introducing an oxide naturally grown by a heat treatment process. In addition, there is an advantage in that a photoelectrochemical device having excellent efficiency can be manufactured by additionally using a catalyst.

Description

Photoelectrochemical device with increased photostability by using a surface oxide layer and a co-catalyst

The present invention relates to a photoelectrochemical device with improved stability using a GaN film whose surface is modified with an oxide layer and a cocatalyst.

For practical use of hydrogen production technology through solar light, a solar-to-hydrogen conversion efficiency (STH) of 10% or more and a low cost equivalent to the unit cost of fossil fuel-based hydrogen production are required. can be broadly classified into three categories. The first is a powder-type photocatalyst-based system. The highest efficiency reported so far is as low as 1% of STH, and it is difficult to separate the generated oxygen and hydrogen, making it difficult to put into practical use. Currently, the photovoltaic-electrolysis (PV-EC) system that utilizes the voltage obtained from the solar cell, which is the most mature technology, has achieved a high efficiency of up to 30% of STH, but the high unit cost due to the complex system is an obstacle to practical use. becomes On the other hand, in the case of a photoelectrode-based photoelectrochemical (PEC) system in which an absorber layer and a catalyst are integrated into a single device, the production cost can be reduced by using low-cost materials and is considered intermediate in terms of efficiency and complexity.

Photoelectrochemical water splitting (PEC) is an environmentally friendly method that can generate hydrogen energy using water and solar energy. In order to construct an efficient photoelectrochemical system, attempts have been made to increase the efficiency by changing the size and structure of the electrode or by synthesizing a catalyst together with the electrode.

PEC cells generally use semiconductors that can absorb light and form electron-hole pairs to undergo redox reactions. However, most anodes in semiconductors operate at high potentials for water splitting. In addition, in the PEC process, the design of durable devices is very important because it plays an important role in determining the cost. However, it is very difficult to achieve long-term stability due to corrosion and segregation of constituent materials.

To solve this problem, it is necessary to develop materials that can be used for photoelectrodes, and metal nitrides are emerging as promising applications in photo(electro)chemical reactions due to their excellent electronic structure, high electrical conductivity and moderate corrosion resistance. Research and development for commercialization is still required.

An object of the present invention is to provide a photoelectrochemical device having a long duration by securing stability.

An object of the present invention is to provide a photoelectrochemical device exhibiting high photocurrent density by reducing recombination of electrons and holes.

An object of the present invention is to provide a method for manufacturing a photoelectrochemical device having excellent stability and efficiency through a simple method.

The present invention is a first metal nitride thin film layer located on the substrate; a first metal oxide layer positioned on the first metal nitride thin film layer; and a second metal phosphate layer positioned on the first metal oxide layer.

The first metal of the photoelectrochemical device according to an embodiment of the present invention is gallium (Ga), tantalum (Ta), barium (Ba), titanium (Ti), lanthanum (La), strontium (Sr), niobium ( Nb) may be selected from the group consisting of. In this case, the first metal nitride thin film layer may have porosity.

The first metal oxide of the photoelectrochemical device according to an embodiment of the present invention may have a monoclinic crystal, may be included in an amount of 3 to 15 at% with respect to the first metal nitride, and the thickness of the first metal oxide layer may be 1 to 15 at%. 100 nm, and the first metal oxide may be derived from an oxidation reaction of the first metal nitride thin film layer.

The second metal of the photoelectrochemical device according to an embodiment of the present invention is nickel (Ni), cobalt (Co), manganese (Mn), chromium (Cr), copper (Cu), zinc (Zn), aluminum (Al) And it may be selected from the group consisting of magnesium (Mg). Accordingly, the first metal of the photoelectrochemical device according to an embodiment of the present invention may be gallium (Ga), and the second metal may be nickel (Ni).

The second metal phosphate layer of the photoelectrochemical device according to an embodiment of the present invention may be dispersed as discontinuous nanoparticles on the first metal oxide layer, and the particle size of the second metal phosphate in the form of nanoparticles is It may be 1 to 10 nm, and when depth profiling is measured by X-ray photoelectron spectroscopy, the first metal may be simultaneously detected at the initial depth at which the second metal is detected.

In addition, the present invention comprises the steps of (a) preparing a first metal nitride thin film layer; (b) etching the thin film layer prepared in step (a); and (c) heat-treating the thin film layer etched in step (b) in air.

In the method for manufacturing a photoelectrochemical device according to an embodiment of the present invention, the etching of step (b) may be performed for 20 to 40 seconds, and the heat treatment of step (c) may be performed at 400 to 800 ° C. can proceed in

In the method for manufacturing a photoelectrochemical device according to an embodiment of the present invention, a step of photoelectrodepositing a second metal phosphate layer may be additionally included after the step (c), and the photoelectrodeposition is carried out for 100 to 300 seconds. may proceed during

In addition, the present invention provides a method of decomposing water by irradiating light to the photoelectrochemical device.

The photoelectrochemical device according to the present invention may implement a photoelectrochemical device in which stability is secured by a passivation effect by introducing an oxide naturally grown by a heat treatment process. In addition, there is an advantage in that a photoelectrochemical device having excellent efficiency can be manufactured by additionally using a catalyst.

1 is a diagram showing a manufacturing process of a photoelectrochemical device according to the present invention.
2 is a graph showing an XRD spectrum of a photoelectrochemical device according to the present invention.
3 is a diagram showing a FE-SEM image of a photoelectrochemical device according to the present invention.
4 is a graph showing the EDX spectrum of the photoelectrochemical device according to the present invention.
5 is a LSV graph of a photoelectrochemical device according to the present invention.
6 is a LSV graph when light is repeatedly turned ON/OFF in the photoelectrochemical device according to the present invention.
7 is an IPCE graph of a photoelectrochemical device according to the present invention.
8 is an OCVD graph of a photoelectrochemical device according to the present invention.
9 is a Mott-Schottky graph of a photoelectrochemical device according to the present invention.
10 is a graph showing light stability of the photoelectrochemical device according to the present invention.
11 is a graph showing the amount of oxygen generated by the PEC reaction using the photoelectrochemical device according to the present invention.
12 is a diagram showing a FE-SEM image of a photoelectrochemical device according to the present invention.
13 is a graph showing the EDX spectrum of the photoelectrochemical device according to the present invention.
14 is a graph showing XPS spectra before and after PEC reaction of a photoelectrochemical device according to the present invention.
15 is a LSV graph of a photoelectrochemical device according to the present invention.
16 is a LSV graph when light is repeatedly turned ON/OFF in the photoelectrochemical device according to the present invention.
17 is a graph showing the amount of hydrogen and oxygen generated by the PEC reaction using the photoelectrochemical device according to the present invention.
18 is a graph showing light stability of the photoelectrochemical device according to the present invention.

Hereinafter, the photoelectrochemical device of the present invention will be described in detail with reference to the accompanying drawings.

The drawings introduced below are provided as examples to sufficiently convey the spirit of the present invention to those skilled in the art. Therefore, the present invention may be embodied in other forms without being limited to the drawings presented below, and the drawings presented below may be exaggerated to clarify the spirit of the present invention.

At this time, unless there is another definition in the technical terms and scientific terms used, they have meanings commonly understood by those of ordinary skill in the art to which this invention belongs, and the gist of the present invention in the following description and accompanying drawings Descriptions of known functions and configurations that may unnecessarily obscure are omitted.

Also, the singular forms used in the specification and appended claims may be intended to include the plural forms as well, unless the context dictates otherwise.

In this specification and the appended claims, terms such as first and second are used for the purpose of distinguishing one element from another, not in a limiting sense.

In this specification and the appended claims, terms such as include or have mean that features or elements described in the specification exist, and unless specifically limited, one or more other features or elements may be added. It does not preclude the possibility that it will happen.

In this specification and the appended claims, when a part of a film (layer), region, component, etc. is on or on another part, not only when it is in contact with and directly on top of another part, but also when another film ( layer), other areas, other components, etc. are interposed.

A photoelectrochemical device of the present invention includes a first metal nitride thin film layer disposed on a substrate; a first metal oxide layer positioned on the first metal nitride thin film layer; and a second metal phosphate layer positioned on the first metal oxide layer.

In one embodiment, the first metal is aluminum (Al), tungsten (W), gallium (Ga), tantalum (Ta), barium (Ba), titanium (Ti), lanthanum (La), strontium ( It may be selected from the group consisting of Sr) and niobium (Nb), preferably gallium.

The first metal nitride may be selected from the group consisting of GaN, Ta ₃ N ₅ , BaTaO ₂ N, LaTiO ₂ N and SrNbO ₂ N, and may preferably be GaN, but is not limited thereto.

Since the first metal nitride has a wide band gap, it can be used for a water oxidation reaction, and GaN is particularly suitable for a PEC reaction because it has a wide band gap of ~3.4 eV. In addition, the conduction band of GaN has a negative value compared to that of photoelectrodes such as Ta ₃ N ₅ , BaTaO ₂ N, LaTiO ₂ N and SrNbO ₂ N, and has the advantage of reducing charge recombination by preventing photo-generated electrons from moving to GaN. It has advantages such as photostability, efficient light collection in the visible light spectrum, fast water oxidation rate, and non-toxicity.

The first metal nitride thin film layer may have porosity. The porous first metal nitride thin film layer may have a pore size sufficient to adsorb water molecules in a PEC reaction, and may specifically have a size of 1 to 20 nm, preferably a size of 1 to 15 nm. It may have, and most preferably may have a size of 1 to 10 nm, but is not limited thereto. Due to the porous nature of the first metal nitride, it may exhibit excellent adsorption to water molecules in the PEC reaction.

In one embodiment, the first metal oxide is from the group consisting of Al ₂ O ₃ , WO ₃ , Ga ₂ O ₃ , Ta ₂ O ₅ , BaO, TiO ₂ , La ₂ O ₃ , SrO and Nb ₂ O ₃ It may be selected, preferably Ga ₂ O ₃ It may be, but is not limited thereto.

The first metal oxide is a semiconductor having a high permittivity, and when the surface of the first metal nitride is passivated with the first metal oxide, charge separation and charge transfer at an interface are improved, thereby suppressing recombination rate. Through this, corrosion as the PEC reaction progresses can be suppressed or delayed. In particular, since β-Ga ₂ O ₃ has a band gap of 4.8 eV and a dielectric constant of 10.2-14.2, and is a natural oxide that is chemically, thermally, and catalytically stable, it serves as a protective layer on the surface of GaN to prevent the PEC reaction from occurring. Stable reaction is possible by preventing corrosion.

In one embodiment, the first metal oxide may be derived by an oxidation reaction of the first metal nitride thin film layer. Since the photoelectrochemical device can grow the first metal oxide on the first metal nitride itself, contamination of the first metal oxide/first metal nitride film can be prevented. In addition, since the first metal oxide layer is naturally formed by the oxidation reaction of the first metal nitride layer, the first metal oxide layer and the first metal nitride layer are chemically bonded to each other, thereby exhibiting excellent photoelectrochemical properties. can

In one embodiment, the first metal oxide may have a monoclinic crystal structure. Here, the monoclinic crystal means a crystal form that has three crystal axes of different lengths, of which the left and right axes and the top and bottom axes are orthogonal, but the front and rear axes form an oblique angle with the left and right axes. Since the first metal oxide has a monoclinic crystal structure, excellent light stability is exhibited when the electrolyte is basic, and the first metal oxide blocks corrosion of the first metal nitride, so that the PEC reaction can finally proceed stably. there is.

In one embodiment, the first metal oxide may be included in 3 to 15 at%, preferably 4 to 12 at%, and most preferably 5 to 10 at% based on the first metal nitride. there is. When the first metal oxide in the above range is included, the first metal oxide having insulating properties does not hinder the movement of electrons during the PEC reaction and at the same time prevents corrosion of the first metal nitride, which is great for the PEC reaction. It has the advantage of being able to apply photovoltage.

In one embodiment, the thickness of the first metal oxide layer may be 1 to 100 nm, preferably 3 to 80 nm, and most preferably 5 to 50 nm. In the case of including the first metal oxide layer having the above thickness range, the first metal oxide layer having insulating properties does not hinder the movement of electrons during the PEC reaction and at the same time prevents corrosion of the first metal nitride, thereby preventing the PEC reaction. It has the advantage of increasing the efficiency of

In one embodiment, the metal of the second metal phosphate layer is nickel (Ni), cobalt (Co), manganese (Mn), chromium (Cr), copper (Cu), zinc (Zn), aluminum (Al) and magnesium (Mg), preferably nickel (Ni).

In addition, the metal phosphate of the second metal phosphate layer is Ni ₃ (PO ₄ ) ₂ , Co ₃ (PO ₄ ) ₂ , Mn ₃ (PO ₄ ) ₂ , Cr ₃ (PO ₄ ) ₂ , Cu ₃ (PO ₄ ) ₂ , Zn ₃ (PO ₄ ) ₂ , Mg ₃ (PO ₄ ) ₂ and AlPO ₄ It may be selected from the group consisting of Ni ₃ (PO ₄ ) ₂ It may be, but is not limited thereto .

In one embodiment, the second metal phosphate layer may be dispersed as discontinuous nanoparticles on the first metal oxide layer. At this time, the particle diameter of the second metal phosphate may be 1 to 10 nm, preferably 1 to 8 nm, but is not limited thereto as long as it is easy to use as a cocatalyst for the PEC reaction.

In one embodiment, in the photoelectrochemical device, when depth profiling is measured by X-ray photoelectron spectroscopy (XPS), the first metal may be simultaneously detected at an initial depth at which the second metal is detected. That is, since the second metal phosphate nanoparticles exist in the form of particles on the first metal oxide layer, a part of the first metal oxide layer is exposed to the surface, and a part of the first metal oxide layer is not exposed to the surface, so that the first metal oxide layer is not exposed to the surface in the XPS spectrum. and the second metal can be simultaneously detected.

A method of manufacturing a photoelectrochemical device of the present invention includes the steps of (a) preparing a first metal nitride thin film layer; (b) etching the thin film layer prepared in step (a); and (c) heat-treating the thin film layer etched in step (b) in air.

An oxidation mechanism occurring in the first metal nitride thin film layer will be described. A first metal oxide layer is introduced through heat treatment of the first metal nitride thin film layer, and a porous structure is formed on the surface of the first metal nitride thin film layer. During oxidation through heat treatment, the first metal nitride thin film layer is decomposed into MO _x and NO _x gas as shown in Equation (1) below, and nitrogen is removed in the form of NO _x . That is, since nitrogen oxide is removed from the first metal nitride thin film layer through heat treatment, the first metal oxide is formed, and the first metal oxide moves and accumulates near the defect of the first metal nitride thin film layer to form nuclei near the defect on the surface. can form

MN ( s ) + O ₂ ( g ) → MO _x ( s ) + NO _x ( s ) (M means the first metal.)

In one embodiment, in the method of manufacturing the photoelectrochemical device, the etching of step (b) may be performed for 20 to 40 seconds, preferably 25 to 35 seconds. By the etching time in the above range, excellent PEC performance is achieved by controlling the increase in reactivity due to the increase in the porosity of the first metal nitride thin film layer and the interference with the movement of electrons during PEC operation according to the increase in the first metal oxide having insulating properties. There are advantages that can be shown.

In one embodiment, in the photoelectrochemical device manufacturing method, the heat treatment of step (c) may be heat treatment at 400 to 800 ° C, preferably at 450 to 750 ° C, most preferably may be heat treated at 500 to 700 ° C. Since the crystallinity of the first metal oxide can be increased by the heat treatment temperature in the above range and the first metal oxide can be naturally grown in the first metal nitride itself, excellent photostability is exhibited when the electrolyte is basic, The first metal oxide blocks corrosion of the first metal nitride and prevents contamination of the film, so that the PEC reaction can finally proceed stably.

In one embodiment, the method of manufacturing the photoelectrochemical device may further include photoelectrodepositing a second metal phosphate layer after the step (c).

In one embodiment, in the method for manufacturing a photoelectrochemical device, the photoelectrodeposition may be performed for 100 to 300 seconds, preferably for 150 to 250 seconds, most preferably can be photoelectrodeposited for 180 to 220 seconds. With the photoelectrodeposition time within the above range, separation of the second metal phosphate layer can be prevented and aggregation of the second metal phosphate layer can be minimized.

Since the manufacturing method of the photoelectrochemical device simply goes through the steps of etching and heat-treating a metal nitride thin film layer, it is possible to form a metal oxide layer in a very simple way. ALD (Atomic Layer Deposition) is generally used for coating metal oxides, but ALD requires the use of expensive metal-organic compounds and a vacuum treatment process, which is a problematic method for manufacturing photoelectrochemical devices on a large scale. am.

In addition, the present invention provides a method of producing hydrogen and oxygen by irradiating light to the photoelectrochemical device to decompose water through a PEC reaction.

Hereinafter, the present invention will be described in detail through examples. However, these are for explaining the present invention in more detail, and the scope of the present invention is not limited by the following examples.

<Example 1> Preparation of β-Ga ₂ O ₃ /GaN-30s

GaN and Pt were etched by applying a voltage of 20V for 30 seconds to an electrode composed of anode and cathode and an electrolyte containing 2 wt% water and 2 wt% NH ₄ F salt in 50 mL ethylene glycol. Thereafter, after washing the residue with an ethanol solution, heat treatment was performed at 600° C. for 1 hour in an air atmosphere.

<Example 2> Preparation of Ni-Pi/β-Ga ₂ O ₃ /GaN-30s

β-Ga ₂ O ₃ /GaN and Pt prepared in Example 1 were composed of 0.02M nickel nitrate in 0.1 M potassium phosphate buffer solution (pH 7) electrolyte in three electrodes composed of an anode, a cathode, and a reference electrode, and 300 W It was photoelectrodeposited for 200 seconds under 0.2V Ag/AgCl condition under Xe lamp.

<Comparative Example 1> Preparation of β-Ga ₂ O ₃ /GaN-10s

It was prepared in the same manufacturing method as in Example 1, except that etching was performed for 10 seconds.

<Comparative Example 2> Preparation of β-Ga ₂ O ₃ /GaN-1min

It was manufactured in the same manufacturing method as in Example 1, except that etching was performed for 1 minute.

<Comparative Example 3> Preparation of Ni-Pi/β-Ga ₂ O ₃ /GaN-10s

It was prepared in the same manufacturing method as in Example 2, except that etching was performed for 10 seconds.

<Comparative Example 4> Preparation of Ni-Pi/β-Ga ₂ O ₃ /GaN-1min

It was prepared in the same manufacturing method as in Example 2, except that etching was performed for 1 minute.

<Experimental Example 1> Structural analysis of GaN and β-Ga ₂ O ₃ /GaN

First, X-ray diffraction spectroscopy (XRD) analysis was performed to analyze the structures of GaN and β-Ga ₂ O ₃ /GaN.

Referring to FIG. 2, the XRD spectrum of β-Ga ₂ O ₃ /GaN formed only by a heat treatment process without an etching process is shown in b1, and c1 to e1 are 600° in air after etching for 10 seconds, 30 seconds, and 1 minute. The XRD spectrum of β-Ga ₂ O ₃ /GaN heat-treated at C is shown. Through this, it can be seen that an oxide was formed on the surface of GaN, and that the oxide was a polycrystalline monoclinic β-Ga ₂ O ₃ , and the XRD spectrum of β-Ga ₂ O ₃ /GaN that had undergone an etching process and only the heat treatment process formed It can be seen that there is no significant difference in the XRD spectrum of β-Ga ₂ O ₃ /GaN.

Next, surface structures of GaN and β-Ga ₂ O ₃ /GaN were observed through FE-SEM.

Referring to FIG. 3 , in the case of GaN, it can be seen that a GaN thin film with a smooth surface and a thickness of about 1 μm was grown on the substrate. However, in the case of samples etched for a certain period of time, it can be seen that the surface structure of the GaN thin film is changed to a porous structure, and the porosity becomes stronger as the etching time increases.

In addition, nanometer-sized islands were observed in Comparative Example 1 (β-Ga ₂ O ₃ /GaN-10s) after heat treatment at 600 ° C, and the size of the islands increased as the etching time increased. . Therefore, it was confirmed that as the etching time increased, the nanoparticles became larger aggregates and the voids between the nanoparticles decreased. This is because GaN is decomposed by a fast oxidation rate as the etching time increases.

Next, EDX analysis was performed to analyze the type and content of materials constituting GaN and β-Ga ₂ O ₃ /GaN.

Referring to FIG. 4, the ratio of Ga (52.59 at %), N (46.40 at %), and O (1.2 %) in GaN indicates that the ratio of Ga/N is about 1, and through this, GaN It can be seen that oxygen is contained in a negligibly small amount. In contrast, Comparative Example 1 (β-Ga ₂ O ₃ /GaN-10s), Example 1 (β-Ga ₂ O ₃ /GaN-30s) and Comparative Example 2 (β-Ga ₂ O ₃ /GaN-1min) It can be seen that the amounts (at%) of Ga, N and O elements are (51.69, 45.61 and 3.01), (50.18, 43.82 and 6.0) and (46.05, 43.02 and 10.93), respectively. That is, it can be seen that the O content increases as the etching time increases, which means that the thickness of β-Ga ₂ O ₃ increases as the etching time increases.

As analyzed above, as the etching time increases, the reactivity may also increase because the porosity of the GaN thin film increases, but since the thickness of β-Ga ₂ O ₃ also increases, the excess β-Ga ₂ O ₃ having insulating properties During PEC operation, the movement of electrons may be hindered, resulting in poor PEC performance of GaN.

<Experimental Example 2> Analysis of PEC characteristics of GaN and β-Ga ₂ O ₃ /GaN

In order to analyze the PEC characteristics of GaN and β-Ga ₂ O ₃ /GaN, linear sweep voltammogram (LSV) analysis was performed.

Referring to FIG. 5 , the onset potential (V _on ) of GaN was 0.12V _RHE , and it was confirmed that a photocurrent density (J) of 0.95 mA·cm ⁻² was exhibited at 1.23V _RHE . On the other hand, Comparative Example 1 (β-Ga ₂ O ₃ /GaN-10s), Example 1 (β-Ga ₂ O ₃ /GaN-30s) and Comparative Example 2 (β-Ga ₂ O ₃ /GaN-1min) It was confirmed that V _on showed 0.195V, 0.209V and 0.232V, and J showed 0.99 mA·cm ^-2 , 1.2 mA·cm ^-2 and 0.86 mA·cm ^-2 respectively. That is, it can be seen that the J value is maximum in Example 1 (β-Ga ₂ O ₃ /GaN-30s), and through this, the passivation effect occurs due to the formation of β-Ga ₂ O ₃ on the GaN surface. Could know.

It was found that the J value increased as the etching time increased until it reached 30 seconds, and the J value decreased as the etching time further increased. This can be understood as the effect of the thick passivation layer (Comparative Example 2 (β-Ga ₂ O ₃ layer etched for 1 minute)), and the thicker passivation layer increases the series resistance and blocks the movement of carriers at the interface. Thus, J is finally reduced.

Referring to FIG. 6, all samples exhibited excellent photostability and fast photoresponse. That is, surface defects can be resolved by the generation of oxide (β-Ga ₂ O ₃ ), it is possible to suppress the recombination of photo-generated carriers on the GaN surface, and the efficiency of the PEC reaction through efficient supply of carriers at the interface. can improve

The incident photon-to-current efficiency (IPCE), defined as the ratio of the number of photogenerated electrons to the number of incident photons, was measured to determine the relationship between the passivation effect and J for the photoelectrode. The IPCE value is described by the following formula.

IPCE = (1240 * J) / (λ * P _light )

Here, J is the photocurrent density measured at a specific wavelength (mA·cm ^-2 ), λ is the wavelength of the incident light, and P _light is the incident light (mW·cm ^-2 ).

Referring to FIG. 7, GaN shows an IPCE of 15%, Comparative Example 1 (β-Ga ₂ O ₃ /GaN-10s) is 36%, Example 1 (β-Ga ₂ O ₃ /GaN-30s) and comparison It can be seen that Example 2 (β-Ga ₂ O ₃ /GaN-1min) exhibits IPCEs of -57% and -25%, respectively, at the same wavelength. That is, Example 1 (β-Ga ₂ O ₃ /GaN-30s) showed the highest IPCE in proportion to the J value of all samples, but Comparative Example 2 (β-Ga ₂ O ₃ /GaN-30s) 1 min), it was found that IPCE decreased again.

Referring to FIG. 8, in order to understand the cause of electrode and recombination loss, GaN and β-Ga ₂ O ₃ /GaN are irradiated with light (OCV _light ) and open terminals when light is not irradiated (OCV _dark ). The voltage (open-circuit voltage; OCV) was measured. V _ph of GaN is measured as 0.5V _RHE , β-Ga ₂ O ₃ /GaN-30s (1.2 V _RHE ) > β-Ga ₂ O ₃ /GaN-10s (0.88 V _RHE ) > β-Ga ₂ O ₃ The value of /GaN-1min (0.67 V _RHE ) was shown, and it was confirmed that V _ph was greater than that of GaN. The high photovoltage observed in the thin film of Example 1 (β-Ga ₂ O ₃ /GaN-30s) means that the charge recombination is reduced, which means that the photo-generated electrons are quickly separated from the GaN surface. Therefore, the β-Ga ₂ O ₃ /GaN thin film can promote charge separation on the surface of the GaN thin film and reduce recombination of electrons and holes.

Referring to FIG. 9, electrical properties of GaN and β-Ga ₂ O ₃ /GaN thin films were investigated through Mott-Schottky plots in 0.5M NaOH (pH: 13.6) electrolyte and different frequency ranges (0.5 kHz and 1 kHz) did

A positive slope in all samples represents the characteristics of an n-type semiconductor. GaN, Comparative Example 1 (β-Ga ₂ O ₃ /GaN-10s), Example 1 (β-Ga ₂ O ₃ /GaN-30s) and Comparative Example 2 (β-Ga ₂ O ₃ /GaN-1min) thin films The flat-band potential (E _FB ) measured at is -0.36, -0.45, -0.53 and -0.40 V _RHE , respectively. This means that the low E _FB causes strong band bending in the interfacial region, which not only promotes the fast separation of photogenerated charges but also suppresses charge recombination at the solid/liquid interface. In addition, the carrier density of the thin film is GaN, Comparative Example 1 (β-Ga ₂ O ₃ /GaN-10s), Example 1 (β-Ga ₂ O ₃ /GaN-30s) and Comparative Example 2 (β-Ga ₂ O ₃ /GaN) for the thin film, which measures about 1.34 × 10 ²³ , 6.38 × 10 ²³ , 3.08 × 10 ²³ and 5.75 × 10 ²³ . Therefore, it was confirmed that the PEC activity of the thin film of Example 1 (β-Ga ₂ O ₃ /GaN-30s) was improved compared to the PEC activity of the electrode without etching because the carrier density was high through low resistance and strong band bending.

Referring to FIG. 10, when a constant light was irradiated at 1.23V _RHE , the photocurrent density over time was measured to investigate the photostability of GaN and Example 1 (β-Ga ₂ O ₃ /GaN-30s). The GaN thin film initially showed a high J _ph , but the J _ph rapidly decreased due to corrosion by the accumulated holes. However, Example 1 (β-Ga ₂ O ₃ /GaN-30s) was confirmed to have excellent stability in the PEC reaction by maintaining about 99% of the initial J _ph value during the reaction for 9 hours. It can be seen that an oxidation reaction occurs in the GaN thin film, and Ga is dissolved in an alkaline solution to decompose the GaN thin film. However, due to the porous β-Ga ₂ O ₃ oxide layer on the GaN thin film, the β-Ga ₂ O ₃ /GaN thin film exhibits excellent stability in the PEC reaction.

Referring to FIG. 11, through GC analysis at a constant potential of 1.23V _RHE , the generation rate of O ₂ gas for the initial 30 minutes is ~1.45 μmol, and when the reaction proceeds for 3 hours, it is confirmed that ~9.5 μmol is calculated It was confirmed that O ₂ gas was sufficiently generated, and excellent stability of Example 1 (β-Ga ₂ O ₃ /GaN-30s) was confirmed.

<Experimental Example 3> Structure analysis of Ni-Pi deposited GaN and β-Ga ₂ O ₃ /GaN-30s thin films

The surface morphology and microstructure of the material photoelectrodeposited with nickel phosphate (Ni ₃ (PO ₄ ) ₂ ; Ni-Pi) on a β-Ga ₂ O ₃ /GaN-30s thin film were observed using FE-SEM.

Referring to FIG. 12 , it can be seen that Ni-Pi nanoparticles having a diameter of ~10 nm are uniformly deposited on the surface of the β-Ga ₂ O ₃ /GaN-30s thin film. In addition, it was confirmed that the Ni-Pi nanoparticles were present in a uniform spherical shape and there was no aggregation under the optimal conditions (t = 200 seconds) for Ni-Pi photoelectrodeposition.

Referring to FIG. 13, through the EDX spectrum of Ni-Pi/β-Ga ₂ O ₃ /GaN-30s, Ga, N, O, Ni, and P elements are present in the film, and the ratio of each element present in the film is Looking at it, it can be seen that 45.72%, 40.98%, 8.46%, 3.6%, and 1.24% of Ga, N, O, Ni, and P were included, respectively, and Ni-Pi was formed.

In addition, XPS analysis was performed to investigate the chemical states of Ni and P in Ni-Pi/β-Ga ₂ O ₃ /GaN-30s.

Referring to FIG. 14, the Ni _2p spectrum of Ni-Pi/β-Ga ₂ O ₃ /GaN-30s showed peaks at 855.57, 857.38, 873.46, and 874.90 eV, and the peaks centered at 856.04 and 873.70 eV were phosphate and Ni _2p3/2 and Ni _2p1/2 capable of interacting with hydroxide ions. That is, peaks at 857.38 eV and 874.90 eV in the Ni _2p spectrum indicate the presence of hydrated Ni ₃ (PO ₄ ) ₂ and Ni(OH) ₂ . The P _2p spectrum shows peaks at 133.22 eV and 134.05 eV, indicating PO bonding at Ni ₃ (PO ₄ ) ₂ . In particular, the peak at 133.22 eV constitutes 80-85% of the total area of the P _2p spectrum in Ni-Pi/β-Ga ₂ O ₃ /GaN-30s, which is the major component of the Ni ₃ (PO ₄ ) ₂ surface. PO ₄ ^3- . In addition, the formation of nickel phosphate was confirmed through the O _1s spectrum. The O _1s spectrum shows one broad spectrum, with a peak at 531.2 eV representing Ni-OH and PO bonds.

In addition, XPS analysis was performed on Ni-Pi/β-Ga ₂ O ₃ /GaN-30s to find out the change after the PEC test. After the PEC test, the peaks of Ni _2p1/2 and Ni _2p3/2 were observed at 855.52 and 873.36 eV, respectively, and these peaks showed a positive shift (0.5 eV) without a change in area, indicating that Ni donated electrons to P and partially indicates the formation of an electron-deficient state with a positive charge. This indicates that the Ni ²⁺ on the Ni-Pi surface was reduced during the PEC test in the electrolyte. In addition, a small negative shift (0.4 eV) was observed in the P _2p spectrum, although it indicated that the electron distribution was reorganized during the PEC reaction. The range is small, indicating that Ni ₃ (PO ₄ ) ₂ is stable when acting as a catalyst. In addition, the O _1s spectrum shows almost no change, proving that Ni ₃ (PO ₄ ) ₂ is not greatly affected by the PEC reaction.

<Experimental Example 4> PEC Characteristic Analysis of Ni-Pi Deposited GaN and β-Ga ₂ O ₃ /GaN

To understand the effect of the Ni-Pi cocatalyst on PEC, Ni-Pi nanoparticles were deposited on GaN and β-Ga ₂ O ₃ /GaN-30s thin films and the PEC performance was tested.

Referring to FIG. 15 , the photoelectrodeposited Ni-Pi layer improved J, especially in the low potential region around 0V _RHE . That is, Ni-Pi/GaN exhibits a J value of 0.04 mA cm ^-2 at 0 V _RHE , while Ni-Pi/β-Ga ₂ O ₃ /GaN exhibits a J value of 1.4 mA cm ^-2 at 0 V _RHE . indicate The photocurrent increases as the applied voltage increases, but does not increase any more above the applied voltage of about 0.5 V _RHE . That is, even if the applied voltage increases, the number of charge carriers does not greatly increase, and the movement of charge carriers reaches a stable state in the space charge layer. These photocurrents converge to 1.2 mA cm ^-2 and 1.48 mA cm ^-2 at 1.23V _RHE . In other words, β-Ga ₂ O ₃ not only improves charge separation in the GaN thin film but also delays charge recombination, and the Ni-Pi layer catalyzes the water oxidation reaction at the interface between the Ni-Pi layer and the electrolyte. Ni-Pi, which acts as a cocatalyst for PEC water oxidation, promotes the reaction on the surface of the β-Ga ₂ O ₃ /GaN thin film to improve PEC performance.

Referring to FIG. 16, the V _ph of Ni-Pi/GaN and Ni-Pi/β-Ga ₂ O ₃ /GaN-30s are about 1.03 and 1.4 V, respectively, which corresponds to the β-Ga ₂ O ₃ /GaN-30s thin film. This means that the Ni-Pi layer provides a V _ph of 0.2 V compared to the V _ph (1.2 V) observed in . That is, it can be seen that Ni-Pi/β-Ga ₂ O ₃ /GaN-30s can promote charge transfer at the interface to promote the PEC reaction.

Referring to FIG. 17 , the effect of the Ni-Pi layer on the β-Ga ₂ O ₃ /GaN film can be confirmed through the amount of H ₂ /O ₂ generated over time. The PEC cell used a Ni-Pi/β-Ga ₂ O ₃ /GaN film as a working electrode, Ag/AgCl as a reference electrode, and a Pt plate as a counter electrode. The amount of released H ₂ and O ₂ was detected by gas chromatography under 0 V _RHE conditions. The Ni-Pi/β-Ga ₂ O ₃ /GaN-30s film showed a stable current density of ~1.3 mA cm ^-2 , and the Faraday efficiency was calculated by dividing the detected gas amount by the calculated gas amount from the total charge. It is calculated according to the following formula.

where z is the released O ₂ (mol) gas, n is the electron involved in the reaction (n=2 in H ₂ , n=4 in O ₂ ), F is the Faraday constant (96485 C mol ^-1 ), and Q is It means the amount of charge passing through the working electrode.

The H ₂ and O ₂ outgassing rates of Ni-Pi/β-Ga ₂ O ₃ -30 s/GaN were measured to be 4.2 μmol and 2.3 μmol every 20 min, respectively, and reached 36 μmol and 16 μmol after 3 h. Ni-Pi/β-Ga ₂ O ₃ /GaN exhibited high photostability and high H ₂ production rate. The maximum H ₂ :O ₂ ratio was nearly 2:1, indicating that the water splitting reaction was stoichiometric and the Faradaic efficiency was 100%. The rate of gas evolution was very stable until 3 cycles were run and no other oxidation/reduction reactions occurred.

Referring to FIG. 18, the stability of the Ni-Pi/β-Ga ₂ O ₃ /GaN film was evaluated. The film did not show any appreciable collapse during the long-term photostability test, indicating that the β-Ga ₂ O ₃ oxide combined with the cocatalyst can effectively improve the photostability of the GaN photoanode.

As described above, the photoelectrochemical device according to the present invention can implement a photoelectrochemical device in which stability is secured by a passivation effect by introducing an oxide naturally grown through a heat treatment process. In addition, there is an advantage in that a photoelectrochemical device having excellent efficiency can be manufactured by additionally using a catalyst.

Claims (18)

A first metal nitride thin film layer located on the substrate;
a first metal oxide layer positioned on the first metal nitride thin film layer; and
A photoelectrochemical device comprising a second metal phosphate layer positioned on the first metal oxide layer.
According to claim 1,
The first metal is selected from the group consisting of gallium (Ga), tantalum (Ta), barium (Ba), titanium (Ti), lanthanum (La), strontium (Sr), and niobium (Nb), photoelectrochemical device.
According to claim 1,
The first metal nitride thin film layer has a porous, photoelectrochemical device.
According to claim 1,
The first metal oxide is derived from the oxidation reaction of the first metal nitride thin film layer, the photoelectrochemical device.
According to claim 1,
The first metal oxide has a monoclinic crystal, a photoelectrochemical device.
According to claim 1,
The first metal oxide is contained in 3 to 15 at% with respect to the first metal nitride, the photoelectrochemical device.
According to claim 1,
A photoelectrochemical device having a thickness of the first metal oxide layer of 1 to 100 nm.
According to claim 1,
The second metal is selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), chromium (Cr), copper (Cu), zinc (Zn), aluminum (Al) and magnesium (Mg) , photoelectrochemical devices.
According to claim 1,
Wherein the first metal is gallium (Ga) and the second metal is nickel (Ni).
According to claim 1,
The second metal phosphate layer is a photoelectrochemical device that is dispersed as discontinuous nanoparticles on the first metal oxide layer.
According to claim 1,
A photoelectrochemical device wherein the first metal is simultaneously detected at an initial depth at which the second metal is detected during depth profiling measurement by X-ray photoelectron spectroscopy.
According to claim 10,
A particle diameter of the second metal phosphate in the form of nanoparticles is 1 to 10 nm, a photoelectrochemical device.
(a) preparing a first metal nitride thin film layer;
(b) etching the thin film layer prepared in step (a);
(c) heat-treating the thin film layer etched in step (b) in air; and
(d) a method of manufacturing a photoelectrochemical device comprising the step of photoelectrodepositing a second metal phosphate layer.
delete According to claim 13,
The etching step of step (b) proceeds for 20 to 40 seconds, a method of manufacturing a photoelectrochemical device.
According to claim 13,
The heat treatment of step (c) is performed at 400 to 800 ° C., a method for manufacturing a photoelectrochemical device.
According to claim 13,
Wherein the photoelectrodeposition proceeds for 100 to 300 seconds, a method for manufacturing a photoelectrochemical device.
A method of decomposing water by irradiating light to the photoelectrochemical device according to any one of claims 1 to 12.

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