KR102511780B1 - Preparation method of photoanode comprising porous TaN by electrochemical anodization and nitridation - Google Patents

Abstract

The present invention relates to a porous tantalum oxide substrate; and crystalline tantalum nitride covering the surface of tantalum oxide. In the photoelectrode of the present invention, the photo-oxidation activity can be improved through increased scattering of light due to the morphology of the surface concavo-convex shape. In addition, through nitriding and heat treatment, light absorption including sunlight in the 600 nm wavelength region can be increased by reducing the band gap of the photoelectrode, and photocatalytic activity can be improved by increasing the surface area of the photoelectrode due to the reduction in pore size. can Furthermore, improved charge transfer characteristics of the photoelectrode can be provided by improving the properties between the tantalum oxide substrate and the tantalum nitride. A photoelectrode with improved photoelectrochemical properties can be used in a water splitting device to improve the decomposition efficiency of water and hydrogen.

Description

Preparation method of photoanode comprising porous TaN by electrochemical anodization and nitridation

The present invention relates to a photoelectrode that reacts to visible light and has chemical stability.

Photoelectrochemical water splitting technology using solar energy without fear of depletion is considered as an essential means to solve global environmental problems and energy depletion crisis. In particular, in a photoelectrochemical cell, a photoelectrode that makes direct contact with an aqueous electrolyte solution, absorbs irradiated sunlight to produce charge carriers, and effectively separates the produced charge carriers to cause oxidation and reduction reactions of water. ) manufacturing technology is the most important.

A material used as a photoelectrode has an appropriate band gap and absorbs light energy corresponding to the band gap to produce charge carriers. A typical example is titanium dioxide (TiO ₂ ), and TiO ₂ is a highly efficient photocatalytic activity reaction. It is known to be chemically stable. However, since TiO ₂ has a large band gap, it has a disadvantage in that it absorbs only light in the ultraviolet region, which accounts for only about 5% of sunlight. Therefore, in terms of efficient use of sunlight, photocatalysts absorb light in the visible light range and manufacture semiconductors that respond to light in the visible light range.

In accordance with this demand, a photocatalyst with a complex structure containing a metal oxide and a transition metal oxynitride thin film has been developed, as in Korean Patent Publication No. 10-2014-0121003, but the incident photon electron conversion efficiency (Incident Photon Electron The maximum value of Conversion Efficiency (IPEC) is highest in the 380~500nm region. In order to increase the efficiency of sunlight use, the need for IPCE to be extended to the long-wavelength visible light region is required. In addition, since the process of manufacturing a transition metal oxide followed by a process of manufacturing a metal oxide of a specific morphology shape according to the configuration of the composite structure and nitriding treatment are required, the process is complex and requires a lot of time for manufacturing.

Republic of Korea Patent Publication No. 10-2014-0121003

An object of the present invention is to provide a photoelectrode having an excellent photocatalytic effect by absorbing light in the visible light region.

Another object of the present invention is to provide a photoelectrode having excellent conductivity, heat resistance and chemical stability.

In order to solve the above problems, according to one aspect of the present invention, a porous tantalum oxide substrate; and crystalline tantalum nitride covering the surface of the porous tantalum oxide substrate.

The morphology of the porous tantalum oxide substrate according to an embodiment of the present invention may include surface irregularities formed by an electrochemical reaction.

The thickness of crystalline tantalum nitride according to an embodiment of the present invention may be 1 μm to 3 μm.

An atomic ratio of tantalum to nitrogen contained in crystalline tantalum nitride according to an embodiment of the present invention may be 1:0.4 to 1.5.

Crystalline tantalum nitride according to an embodiment of the present invention may include an orthorhombic structure.

According to another aspect of the present invention, preparing a porous tantalum oxide substrate by oxidizing a tantalum substrate by electrochemical anodic oxidation; and nitriding the porous tantalum oxide substrate at a heat treatment temperature of 700 to 1200° C. to form crystalline tantalum nitride.

In the manufacturing method according to an embodiment of the present invention, the electrochemical anodization may be performed at a constant voltage of 25 to 35V at room temperature.

In the manufacturing method according to an embodiment of the present invention, the heat treatment temperature may be performed at 900 to 1100 ℃.

In the manufacturing method according to an embodiment of the present invention, tantalum oxide may be tantalum pentoxide (Ta ₂ O ₅ ).

In the manufacturing method according to an embodiment of the present invention, the thickness of crystalline tantalum nitride may be 1 to 3 micrometers.

In the manufacturing method according to an embodiment of the present invention, the size of pores formed in the porous tantalum oxide substrate may be changed according to the heat treatment temperature.

In the manufacturing method according to an embodiment of the present invention, the gas used for nitriding treatment may be a mixture of nitrogen gas and ammonia gas.

According to another aspect of the present invention, the porous tantalum oxide substrate described above; and crystalline tantalum nitride covering the surface of the porous tantalum oxide substrate.

In the photoelectrode of the present invention, photo-oxidation activity can be improved through increased light scattering due to the pore-formed morphology.

In addition, light absorption, including sunlight in the 600 nm wavelength range, can be increased through band gap reduction through nitriding and heat treatment, and photocatalytic activity can be improved by increasing the surface area of the photoelectrode due to the reduction in pore size.

Furthermore, improved charge transfer characteristics of the photoelectrode can be provided by improving the properties between the tantalum oxide substrate and the tantalum nitride.

1 is a diagram showing X-ray diffraction (XRD) pattern characteristics of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2.
2 is a view showing field emission scanning electron microscope (FE-SEM) images of the surface and cross-sectional area of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2.
3 is a view showing high-magnification transmission electron microscope (HR-TEM) images of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2.
FIG. 4 is a diagram showing bright field images specifying the degree of dispersion of elements included in each of the TaN-based photoelectrodes manufactured in Example 1, Comparative Example 1, and Example 2;
5 is a view showing visible light absorbance and tauc plots of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2.
6 is a view showing the results of X-ray photoelectron spectroscopy analysis of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2.
7 is a diagram illustrating photoelectrochemical characteristics of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2.

Hereinafter, a photoelectrode including porous tantalum nitride through an electrochemical anodic oxidation method and nitriding heat treatment according to the present invention and a water splitting device including the same 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 well-known functions and configurations that may be unnecessarily obscure are omitted.

A photoelectrode according to the present invention comprises a porous tantalum oxide substrate; and crystalline tantalum nitride on the surface of the porous tantalum oxide substrate.

The photoelectrode according to the present invention has a structure in which tantalum nitride is included on the surface of a tantalum oxide substrate, and tantalum oxide and tantalum nitride may exist simultaneously. Tantalum oxide and tantalum nitride have different bandgap energies, and accordingly, the wavelength range of light that causes a photoreaction is different. Therefore, since the two materials exist at the same time, they can react with light in a wider wavelength range than before, and thus can exhibit a synergistic effect in terms of photoelectrochemical performance.

The photoelectrode according to the present invention may have a large specific surface area due to the porosity of the tantalum oxide substrate, and thus may have an effect of increasing a photoreaction that may occur in the photoelectrode by irradiated light.

Tantalum nitride included on the surface of the photoelectrode according to the present invention may have crystallinity. When a photoelectrode is used in a water splitting device, the surface of the photoelectrode comes into contact with a water-soluble electrolyte. As tantalum nitride having crystallinity is formed on the surface of the photoelectrode, the heat resistance and chemical stability of the photoelectrode can be improved. .

As described above, the porous tantalum oxide substrate according to the present invention; and tantalum nitride on the surface of the porous tantalum oxide base material. A photoelectrode including tantalum oxide and tantalum nitride can promote photocatalytic activity due to an increase in specific surface area, and synergistic photoelectrochemical reactions due to the mixture of tantalum oxide and tantalum nitride having different bandgap energies. effects can be expected, and since crystalline tantalum nitride is formed on the surface of the photoelectrode according to the present invention, a thermally and chemically stable photoelectrode can be provided.

In one embodiment of the present invention, the morphology of the porous tantalum oxide substrate may include surface irregularities formed by an electrochemical reaction.

The surface irregularities may be formed through an etching action by an electrochemical reaction and a tantalum oxide nucleation reaction, and thus may have an irregular shape.

Furthermore, surface irregularities may be further included by aggregation of tantalum nitride particles. The size of the nitride particles may be 5 to 100 nm, specifically 10 to 80 nm, and more specifically 20 to 50 nm. The smaller the particle size, the higher the surface energy, so that the surface energy is lowered through aggregation of small-sized particles to maintain a thermodynamically stable state, and the agglomerated shape of these particles may appear irregular. These irregularly agglomerated particles can form irregularities on the surface.

The shape of the surface irregularities may appear irregular, and when light particles reach the irregularly formed rough surface, some of these surface irregularities may absorb light energy, and other light particles may be elastically scattered at various angles. The light energy irradiated by scattering can be used as another energy source to increase the probability of reacting with the photoelectrode of the present invention, thereby increasing the activity of the photoreaction.

In one embodiment of the present invention, the atomic ratio of tantalum:nitrogen contained in crystalline tantalum nitride may be 1:0.2 to 1.8, preferably 1:0.4 to 1.5.

According to one embodiment of the present invention, the band gap of the photoelectrode included in the tantalum nitride may be changed according to the concentration of nitrogen contained in the crystalline tantalum nitride, and as a result, light in a wider wavelength range may be changed. can absorb

The band gap energy (E _g ) of the photoelectrode may be 2.0 to 2.3 eV, specifically 2.0 to 2.2 eV, depending on the nitrogen concentration contained therein. When the band gap energy is reduced by 0.1 eV based on 2.3 eV (ie, E _g = 2.2 eV), the theoretical absorption limit wavelength range can be extended to the long wavelength region from 557 nm to 582 nm. Furthermore, when the bandgap energy is adjusted to 2.0 eV, the theoretically absorbable wavelength can be extended to 640 nm, thereby improving the photoelectrochemical effect of the photoelectrode using visible light.

According to one embodiment of the present invention, the photoelectrode may include a structure of tantalum oxide-tantalum oxynitride-tantalum nitride having a concentration gradient of nitrogen.

Tantalum nitride can be produced by the substitution of nitrogen atoms with oxygen atoms in tantalum oxide and is derived from a tantalum oxide substrate. The degree of substitution of nitrogen may have a nitrogen concentration gradient in which the nitrogen concentration decreases from the surface of the photoelectrode toward the inside.

According to one embodiment of the present invention, tantalum oxide, tantalum oxynitride, and tantalum nitride may be provided as a structure having intermixed regions without distinct layer distinction. Therefore, the photoelectrode according to the present invention can provide a photoelectrode having various band gaps of tantalum oxide, tantalum oxynitride, and tantalum nitride having a nitrogen concentration gradient.

As described above, since the band gap energy varies in the wavelength range of light that can be absorbed even with a difference of 0.1 eV, the components having various band gaps in the photoelectrode according to the present invention allow a wider wavelength than the conventional technology. Since it can react with light in the range, a synergistic effect can be expected in terms of the photoelectrochemical effect of the photoelectrode.

Crystalline tantalum nitride according to an embodiment of the present invention may be at least one selected from TaN, Ta ₂ N _, and Ta ₃ N ₅ .

As a specific example, the chemical shift of the Ta4p peak in the XPS spectrum of the photoelectrode may be 0.5 to 1.1 eV, preferably 0.6 to 1.0 eV, in the direction of high binding energy based on 403.64 eV, and the N1s peak The chemical shift may be 1.1 to 1.9 eV in the high binding energy direction based on 396.27 eV, preferably 1.2 to 1.8 eV. The chemical shift of the Ta4p and N1s peaks in tantalum nitride toward the higher binding energy means that the proportion of nitrogen in tantalum nitride increases.

According to one embodiment of the present invention, the photoelectrode may include tantalum oxynitride TaO _x N _y (0<x<1 and 0<y<1). Since tantalum nitride is derived from tantalum oxide and has the highest nitrogen concentration on the surface of the upper layer of the photoelectrode and may have a nitrogen concentration gradient that decreases toward the inside of the tantalum oxide substrate in the thickness direction, the TaO _x N _y is tantalum oxide substrate means the average value in the thickness direction of tantalum oxynitride located between crystalline tantalum nitrides from

A photoelectrode according to one embodiment of the present invention may include oxygen vacancies.

In the case of the O1s peak, a chemical shift from 1.0 to 1.5 eV may be included in the high binding energy direction based on 530.2 eV. The presence of tantalum oxide can be seen from the reference value of the O1s peak, and the chemical shift in the direction of high binding energy indicates that there are oxygen vacancy defects in tantalum oxide.

Oxygen vacancies may be generated in the process of forming tantalum oxide on the surface of the tantalum oxide substrate or in the process of nitriding the tantalum oxide. These oxygen vacancy defects act like new free charges, resulting in an increase in free charge concentration, which can improve photoelectric efficiency. In addition, the oxygen vacancy defect may serve to delay or reduce their recombination by temporarily trapping free electrons and holes generated by the reaction between the irradiated light and the photoelectrode. Accordingly, the efficiency of the photoelectrochemical reaction may be increased by increasing the time during which electrons and holes generated by the photoreaction may exist in a separated state.

Crystalline tantalum nitride according to an embodiment of the present invention may include an orthorhombic structure.

In the case of the orthorhombic system, it is composed of a layered structure, and the [-Ta-N-N-Ta-] layer is connected around the N element, so it is a structure that can give chemical stability. Accordingly, since the crystalline tantalum nitride has an orthorhombic structure and the crystalline tantalum nitride having the orthorhombic structure is located on the surface, the heat resistance and chemical stability of the photoelectrode can be improved. It is preferable in that an electrode can be provided.

The thickness of the crystalline tantalum nitride according to an embodiment of the present invention may be 100 nm to 30 μm, specifically 500 nm to 15 μm, and more specifically 1 to 3 μm.

a porous tantalum oxide substrate according to the present invention; and crystalline tantalum nitride covering the surface of the porous tantalum oxide substrate.

Next, the photoelectrode manufacturing method of the present invention will be described.

A method of manufacturing a photoelectrode according to the present invention comprises the steps of preparing a porous tantalum oxide substrate by oxidizing a tantalum substrate by electrochemical anodic oxidation; and forming crystalline tantalum nitride by nitriding the porous tantalum oxide substrate at a heat treatment temperature of 700 to 1200 °C.

In the method for manufacturing a photoelectrode according to an embodiment of the present invention, a constant voltage applied during electrochemical anodization; time during which anodization is performed; and the heat treatment temperature at which the nitriding treatment is performed; by controlling one or more factors selected from the morphological characteristics of the photoelectrode, the thickness of tantalum oxide and tantalum nitride formed on the surface, and the atomic ratio of tantalum: nitrogen. can be adjusted

In the method for manufacturing a photoelectrode according to an embodiment of the present invention, tantalum oxide is formed on a tantalum substrate through an anodizing method, and tantalum nitride is formed on the surface of the previously formed substrate through a nitriding heat treatment process. It may have a structure composed in the order of tantalum-tantalum oxide-tantalum nitride. Unlike coating a new material on an existing substrate, tantalum oxide is produced by anodic oxidation from a tantalum substrate, and tantalum nitride is produced by substitution of oxygen atoms and nitrogen atoms in tantalum oxide. It can be manufactured as a structure having (intermixed) regions, and has the advantage of improving the binding between regions.

The tantalum substrate according to an embodiment of the present invention may have a rectangular shape with a thickness of 0.1 to 0.2 mm, but is not limited thereto.

The porous tantalum oxide substrate according to an embodiment of the present invention may be manufactured by an electrochemical anodic oxidation method.

The electrochemical anodic oxidation method according to an embodiment of the present invention may be performed at a constant voltage of 20 to 40V in an electrolyte at room temperature, preferably at a constant voltage of 25 to 35V. If it is out of the range of the previously applied positive voltage, pores may not be formed, or a morphology other than the form of pores may be formed.

Room temperature according to an embodiment of the present invention may be 18 to 35 ° C, preferably 20 to 32 ° C, more preferably 22 to 28 ° C.

The electrolyte according to an embodiment of the present invention may include non-aqueous organic solvents such as ethylene glycol and dimethyl sulfoxide, and aqueous solutions of sulfuric acid, oxalic acid, phosphoric acid, chromic acid, and mixed aqueous solutions thereof may be used. A mixture of organic solvents may be used, but is not limited thereto.

An electrolyte according to an embodiment of the present invention may include sulfuric acid, nitric acid, phosphoric acid, sodium fluoride or ammonium fluoride in an organic solvent.

In one embodiment, phosphoric acid contained in the organic solvent may be 0.1 to 10% by weight, specifically 0.5 to 5% by weight, and more specifically 1 to 3% by weight, but is not limited thereto.

In one embodiment, ammonium fluoride contained in the organic solvent may be 0.1 to 10% by weight, 0.5 to 5% by weight, specifically 0.5 to 1.5% by weight, but is not limited thereto.

The electrochemical anodic oxidation method according to an embodiment of the present invention may be performed for 30 seconds to 30 minutes, 1 minute to 20 minutes, preferably 2 minutes to 10 minutes, more preferably 4 minutes to 6 minutes. can

Tantalum oxide according to an embodiment of the present invention may be made of tantalum pentoxide (Ta ₂ O ₅ ).

The heat treatment temperature according to an embodiment of the present invention may be preferably performed at 800 to 1100 ° C, more preferably at 900 to 1100 ° C.

Depending on the heat treatment temperature, the size of pores formed in the porous tantalum oxide substrate of the present invention may change. As the heat treatment temperature increases, the size of pores decreases, which can increase the surface area that causes photoreaction. On the other hand, due to the morphological characteristics of surface irregularities, the photoreaction efficiency can be increased by scattering light. It has a duality that can influence it. The temperature range according to an embodiment of the present invention has an effect of reducing irregular reflection of light by minimizing surface energy while maintaining morphological characteristics of surface irregularities.

The crystallinity of tantalum nitride according to an embodiment of the present invention may change according to the heat treatment temperature during the nitriding process, and the higher the temperature, the higher the crystallinity.

The atomic ratio of tantalum:nitrogen contained in the crystalline tantalum nitride of the present invention may change according to the heat treatment temperature according to an embodiment of the present invention.

Tantalum nitride through nitriding treatment is produced by substitution of oxygen atoms and nitrogen atoms in the tantalum oxide of the present invention. At this time, a mixture of nitrogen and ammonia may be used. First, for the substitution of two atoms, N ₂ , a diatomic molecule, must be decomposed, and energy must be applied to the lattice structure existing on tantalum oxide to break the Ta-O bond. Energy supply for this is delivered by the temperature applied during nitriding treatment, and the temperature range of the present invention can be adjusted so that the atomic ratio of tantalum:nitrogen in tantalum nitride can be 1:0.2 to 1.8.

Depending on the amount of nitrogen included, the band gap of the photoelectrode according to the present invention is reduced, so that the degree of activation of the photocatalyst can be improved by absorbing visible light in a long wavelength region compared to the prior art.

Gas used for nitriding treatment according to an embodiment of the present invention may be used as a mixture of nitrogen gas and ammonia gas.

The ratio of ammonia:nitrogen in the mixed gas used according to an embodiment of the present invention may be 1:4 to 9, preferably 1:6 to 9, but is not limited thereto. Nitrogen in ammonia can contribute to doping or formation of tantalum nitride, and the present invention is to provide a photoelectrode with a higher nitrogen content than the prior art by replacing most of the oxygen atoms in tantalum oxide with nitrogen atoms, so that a mixed gas is used for nitriding treatment. it is desirable

The flow rate of the mixed gas supplied to the nitriding process according to an embodiment of the present invention may be 10 to 70 mL/min, specifically 20 to 60 mL/min. More specifically, it may be 30 to 50mL/min, but is not limited thereto.

Nitriding treatment according to an embodiment of the present invention may be performed for 1 to 5 hours, specifically, for 2 to 4 hours, but is not limited thereto.

The tantalum substrate according to an embodiment of the present invention may include a surface treatment step of the tantalum substrate before performing the electrochemical anodic oxidation method.

Surface treatment according to an embodiment of the present invention may be performed by one or more methods selected from mechanical polishing, ultrasonic treatment, and electrolytic polishing.

In one embodiment, mechanical polishing may be performed using soft sandpaper.

In one embodiment, ultrasonication may be performed using acetone, ethanol, and water for 1 to 30 minutes, specifically, 2 to 20 minutes, respectively, and specifically, 5 to 15 minutes. It is not limited.

Electrolytic polishing according to an embodiment of the present invention includes being performed by applying a voltage to an alkaline solution.

The alkaline solution may be a solution of any one of NaOH, KOH, CaO, NH3, Ca(OH)2, Cu(OH)2, or a combination of two or more.

In one embodiment, the alkali solution may be 0.1 to 0.9 M, specifically 0.4 to 0.6 M, but is not limited thereto.

In one embodiment, the voltage applied to the electropolishing may be 5 to 60V, specifically 20 to 40V, but is not limited thereto.

(Example 1)

Tantalum foil (Alfa Aesar, 0.127mm thickness, 99.95% purity) was cut into 1.5cm x 2.0cm in size, mechanically polished with soft sandpaper before anodization, followed by ultrasonic cleaning with acetone, ethanol, and water for 10 minutes each, In addition, electrochemical polishing was performed by applying a voltage of 30V in a 0.5 M NaOH solution.

Anodization was performed using ethylene glycol (Alfa Aesar, 99% purity) containing 2 wt% H ₃ PO ₄ (SAMCHUN, 98% purity) and 1 wt% NH ₄ F (SIGMA-ALDRICH, 98% purity). ) by applying a constant voltage of 30 V for 5 min in the electrolyte. A platinum (Pt) wire was used as the counter electrode and the distance from the tantalum foil was maintained at 2 cm. During anodization, the preceding electrolyte was stirred at 100 rpm using a magnetic rod.

Next, the porous tantalum substrate was washed with ethanol and water and dried using nitrogen gas.

In order to improve the crystal properties, the prepared tantalum oxide was annealed and nitrided in a tube furnace under a nitrogen:ammonia (90:10) mixed fluid at a flow rate of 40mL/min at 1000 °C for 3 hours to prepare a photoelectrode. .

(Example 2)

A photoelectrode was prepared in the same manner as in Example 1, except that nitriding was performed at 800°C.

(Comparative Example 1)

A photoelectrode was prepared in the same manner as in Example 1, except that nitriding was performed at 600°C.

Referring to FIG. 1, it can be confirmed that the tantalum nitride phase was successfully synthesized, and it can be seen that the intensity of the tantalum nitride peak increased in Example 1 compared to Comparative Examples 1 and 2, which indicates that the heat treatment temperature during the nitriding process It can be confirmed that the higher the amount of doped nitrogen, the greater the amount. In addition, according to the standard data (JCPDS 79-1533), it can be seen that tantalum nitride has an orthorhombic structure.

As shown in FIG. 2, the morphology of tantalum nitride confirmed the porous shape consisting of a plurality of pores, and it could be confirmed that the pores were closed pores. In addition, it can be confirmed that the size of the pores changes according to the nitriding temperature. It can be confirmed that the particle diameter of the formed pores is smaller in Example 1 (FIG. 2-C ₁ ) compared to Comparative Example 1 and Example 2, which can result in improving photoelectrochemical performance by increasing the active surface area. .

The cross-sectional thicknesses of Comparative Example 1, Example 2, and Example 1 can be confirmed through a2, b2, and c2 of FIG. 2, respectively. It was confirmed that the thicknesses of the tantalum nitride prepared in Comparative Example 1, Example 2, and Example 1 were 3.72 μm, 2.87 μm, and 2.5 μm, respectively, indicating that the thickness of the tantalum nitride was changed according to the nitriding treatment temperature.

Referring to FIG. 3 , the interplanar distance of tantalum nitride in Example 1 was measured as 0.372 nm, and in Comparative Example 1 and Example 2, it was confirmed as 0.281 and 0.307 nm, respectively. This confirms again that the tantalum nitride previously identified in FIG. 1 is an orthorhombic phase.

It can be seen from FIG. 4 that nitrogen, oxygen, and tantalum elements are uniformly distributed throughout the photoelectrode. In particular, in the case of nitrogen, in Example 1 (FIG. 4-C ₄ ), a relatively increased color can be confirmed, which means that more nitrogen is present at a higher temperature.

Additionally, as summarized in Table 1 through energy dispersive X-ray (EDX) analysis, the atomic ratios of oxygen and tantalum elements contained in tantalum nitride, including nitrogen element, Example 1, Comparative Example 1 and Example 2 compared.

atomic percentage (%) element Ta O N Example 1 29 32 39 Comparative Example 1 26 55 19 Example 2 27 40 33

As shown in Table 1, it can be confirmed that the atomic ratio of nitrogen is relatively high at high temperature, and the atomic ratio of oxygen is decreased. It can be seen that nitrogen doping is more active at high temperatures.

As shown in FIG. 5(a), the optical properties of the photoelectrodes prepared in Example 1, Comparative Example 1 and Example 2 were analyzed using a UV-vis spectrophotometer. It can be confirmed that the light absorption of the photoelectrodes prepared in Example 1, Comparative Example 1 and Example 2 is in the 450 - 610 nm wavelength region. In the case of Example 1, it was confirmed that light absorption started actively at a wavelength of 605 nm, and in the case of Comparative Example 1 and Example 2, a rapid change in light absorption was confirmed at a wavelength of 585 nm and 598 nm, respectively. Therefore, it can be seen that the increase in the doped nitrogen element content due to the high heat treatment temperature causes a narrowing of the band gap, resulting in a red shift in the absorption limit.

It can be inferred that the red shift of the absorption limit confirmed in FIG. 5 (a) is caused by a phenomenon in which the band gap narrows due to nitrogen doping in tantalum oxide, and the exact bands of Example 1, Comparative Example 1, and Example 2 above. The band gap energy was determined from the tau graph in FIG. 5(b). The calculated bandgap energy values of Example 1, Comparative Example 1 and Example 2 were observed to be 2.01 eV, 2.12 eV and 2.06 eV, respectively.

Therefore, it can be seen that the heat treatment temperature during the nitriding process affects the doping degree of the nitrogen element, which in turn has a difference in the band gap of tantalum nitride, and causes a red shift in the light absorption limit by the difference in the band gap. there is.

6 is a view showing the X-ray photoelectron spectroscopy (XPS) analysis results of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Comparative Example 2. FIG. b) shows the Ta4f spectrum, FIG. 6(c) shows the N1s spectrum, and FIG. 6(d) shows the O1s spectrum.

As can be seen in FIG. 6, as the heat treatment temperature increases during the nitriding process, a core-level shift of Ta4f toward a higher binding energy was confirmed, and this chemical shift indicates the presence of an abundant nitrogen phase. In the case of Comparative Example 1 (600° C.), deconvolution (˜26 eV) of the broad peak indicates the presence of elemental oxygen, which was not observed in the XRD analysis shown in FIG. 1 . However, in the case of Example 2 (800 ° C) and Example 1 (1000 ° C), the peak binding energies of Ta4f _7/2 and Ta4f _5/2 were confirmed to be 25.68 eV, 26.12 eV, 27.53 eV, and 28.04 eV, respectively. , which indicates the presence of Ti-N bonds. As can be seen in FIG. 6(c), as the annealing temperature increases, the binding energies of the Ta4p and N1s peaks shift toward higher binding energies. shows a gradual increase in

Meanwhile, in FIG. 6(d), a chemical shift of the O1s peak toward a higher binding energy can be observed as the heat treatment temperature increases. This indicates that oxygen vacancies were formed because oxygen was not sufficiently supplied from the outside during the heat treatment process.

FIG. 7(a) is a diagram showing the photoelectrochemical characteristics of each of the TaN-based photoelectrodes prepared in Example 1, Comparative Example 1, and Example 2, and is a linear scanning potential (LSV) curve. Photoelectrochemical properties were measured using a potentiostat under light irradiation. As the light, a 150W Xe lamp having an intensity of 100mW/cm ² was used and sunlight was simulated with an AM 1.5 filter. A 0.1M Na ₂ SO ₄ aqueous solution was prepared, and the prepared TaN-based photoelectrode was used as a working electrode with an active area of 0.2 cm ² and platinum with an area of 10 cm ² was used as a counter electrode. , Ag/AgCl in saturated KCl solution was used as the reference electrode. The current of the photoelectrodes was measured under a chopping condition to measure photocurrent and dark current values according to the presence or absence of irradiation with simulated sunlight. While the light was being chopped, the amount of current versus the applied voltage was measured using the linear scanning potential method. All potential values measured for the photoelectrochemical properties were converted into values relative to the reversible hydrogen electrode (RHE) using Equation 1 below, and the pH of the electrolyte under the experimental conditions was 11.

(Equation 1)

E _RHE = E _{Ag/AgCl (sat.)} + E° _{Ag/AgCl (sat.)} + 0.0591 × pH

As shown in FIG. 7(a), it was observed that all photoelectrodes showed a fast photoreaction, and in the case of Example 1, a relatively high photocurrent of 0.253 mA/cm ² at 1.23 V _RHE compared to Comparative Example 1 and Example 2 density was shown. The photocurrent densities of Comparative Example 1 and Example 2 were 0.067 mA/cm ² and 0.185 mA/cm ² , respectively.

The electrical properties of the interface of the photoelectrodes were investigated by Mott-Schottky analysis through electrochemical impedance measurement, and the analysis results are shown in FIG. 7(b). Mott-Schottky analysis was performed at a frequency of 1 kHz and a modulating applied potential under dark conditions in 0.1 M Na ₂ SO ₄ aqueous solution.

Information on the flat-band potential (V _FB ) of the photoelectrode can be known through the x-axis extrapolation line of the Mott-Schottky graph, and all photoelectrodes have a positive slope. slop), which shows that the photoelectrodes have n-type conductivity. The exact quantitative flat band potential values and carrier density (N _D ) of the fabricated photoelectrodes were calculated through Equations 2 and 2.

(Equation 2)

(Equation 3)

As summarized in Table 2 below, the flat band potential of Comparative Example 1 and Example 2 shows a larger negative shift from 0.16 V to 0.14 V, whereas the flat band potential of Example 1 shows a smaller shift. . In addition, the increased density of charge carriers increases the degree of band bending, which makes it possible for electrons and holes generated inside the semiconductor to be easily separated and move to the interface region between the photoelectrode and the electrolyte. let it In particular, heat treatment and nitriding treatment performed under a mixed gas atmosphere of nitrogen gas and ammonia gas cause an increase in the concentration of charge carriers by more than 10 times compared to the conventional method, which is due to partial crystal amorphization occurring near the crystal phase.

V _FB (V _RHE ) ND(cm ^-3 ) Example 1 0.07 2.08 x 10 ⁸ Comparative Example 1 0.16 4.41 x 10 ⁷ Example 2 0.15 1.09 x 10 ⁸

7(c) shows the incident photoelectric conversion efficiency (IPCE) of the photoelectrodes manufactured according to Example 1, Comparative Example 1, and Example 2, and was evaluated at a potential of 1.23 V _RHE .

It can be seen that the IPCE profile of Example 1 is wider and stronger throughout the visible light region (400 to 650 nm) compared to Comparative Example 1 and Example 2, which means that it has higher quantum efficiency. At the 480 nm wavelength of Comparative Example 1 and Example 2, the IPCE showed 1.5% and 2.2%, respectively, while the IPCE of Example 1 showed an efficiency of 3%.

7(d) shows Nyquist plots of the photoelectrodes prepared according to Example 1, Comparative Example 1, and Example 2, from the results measured by electrochemical impedance spectroscopy (EIS). got it The EIS was measured under open-circuit potential (OCP) conditions under light irradiation.

Referring to FIG. 7(d), it was observed that the photoelectrode manufactured according to Example 1 had the smallest arc radius compared to the photoelectrodes manufactured according to Comparative Example 1 and Example 2. As a result of heat treatment at a high temperature of 1000 ° C, the interfacial charge resistance of the photoelectrode due to the improvement of the charge transfer process at the interface between tantalum oxide and tantalum nitride is lower than that of heat treatment at 600 ° C and 800 ° C. It means.

Claims (7)

a porous tantalum oxide substrate and
Including crystalline tantalum nitride covering the surface of the porous tantalum oxide substrate,
It includes a structure of sequentially located tantalum oxide-tantalum oxynitride-tantalum nitride, has a concentration gradient in which nitrogen concentration decreases from the surface of the tantalum nitride to the inside in the structure, and the porous tantalum oxide by an electrochemical reaction. A method for manufacturing a photoelectrode having irregular surface irregularities resulting from a substrate,
a) preparing a porous tantalum oxide substrate by oxidizing the tantalum substrate by electrochemical anodic oxidation; and
b) nitriding the porous tantalum oxide substrate at a heat treatment temperature of 700 to 1200° C. to form crystalline tantalum nitride; Including,
The anodic oxidation method is performed for 2 to 10 minutes at a temperature of 20 to 32 ° C and a constant voltage of 25 to 35 V, and the nitriding treatment in step b) uses a nitrogen and ammonia mixture gas. According to claim 1,
The heat treatment temperature is 900 to 1100 ℃ photoelectrode manufacturing method. According to claim 1,
A method for manufacturing a photoelectrode, wherein the thickness of the crystalline tantalum nitride is 1 to 3 micrometers. According to claim 1,
A method for manufacturing a photoelectrode in which the tantalum oxide is tantalum pentoxide (Ta ₂ O ₅ ). According to claim 1,
A method for manufacturing a photoelectrode in which the size of pores formed in the porous tantalum oxide substrate is controlled by the heat treatment temperature in step b). According to claim 1,
The method of manufacturing a photoelectrode, wherein the atomic ratio of tantalum:nitrogen contained in crystalline tantalum nitride included in the photoelectrode is 1:0.4 to 1.5. According to claim 6,
The method of manufacturing a photoelectrode, wherein the crystalline tantalum nitride has an orthorhombic structure.

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