Photoanode for photoelectrochemical water-splitting device and method of fabricating photoanode

Abstract

Proposed are a photoanode and method for fabricating a photoanode used as a photoelectrochemical water-splitting device. According to these specific implementation embodiments of this technique, a BiOI photoactive material is electrodeposited on the surface of the FTO photoanode through an electrodeposition process. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the FTO photoanode, and then the FTO photoanode is annealed at a temperature of 500° C. with a heating rate of 2° C. / min., thus fabricating a BiVO4 photoanode. The surface of the fabricated BiVO4 photoanode is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO / Co-DTPMP photoanode. Through this, it is possible to greatly improve photoelectrochemical performance and oxygen evolution reaction (OER) rate.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority to Korean Patent Application No. 10-2024-0074182, filed Jun. 7, 2024, the entire contents of which is incorporated herein for all purposes by this reference.BACKGROUND OF THE DISCLOSURE1. Field of the Disclosure

[0002] The present disclosure relates to a system and a method for fabricating a photoanode. More specifically, the present disclosure relates to a technique for fabricating a photoanode used as an electrochemical water-splitting device by depositing a diethylenetriamine penta (methylene phosphonic acid) (DTPMP) cocatalyst cross-linked with cobalt (Co) on a surface of a BiVO4 (BVO) photoanode.2. Description of the Related Art

[0003] Bismuth vanadate (BiVO4) has a small bandgap energy (up to 2.4 eV) and an appropriate band position. Bismuth vanadate is considered one of the most promising photocatalysts for water oxidation to generate oxygen.

[0004] However, the photocatalytic potential of BiVO4 is limited in photoanode applications due to low electron mobility of BiVO4 (up to 10−2 cm2 / Vs), high charge recombination, and unsuitable water oxidation kinetics.

[0005] Although BiVO4 for photoelectrochemical (PEC) water oxidation has a low starting potential, BiVO4 has a photocurrent density much smaller than the theoretical expectation (7.5 mA cm−2). Due to that, research has been actively conducted to increase oxygen evolution reaction (OER) activity of a BiVO4 photoanode through element doping, heterostructure construction, and structure control.

[0006] However, the research has mainly focused on charge separation of the BiVO4 photoanode, but this approach has encountered a bottleneck, resulting in a slow OER reaction rate at the electrode interface.

[0007] Accordingly, the present inventors have proposed an approach for improving PEC performance and OER reaction rate, enhancing stability and durability, and preventing photocorrosion by forming a metal-organic complex Co-DTPMP layer. To achieve this, a surface of a fluorine-doped tin oxide (FTO) photoanode is subjected to an electrodeposition process to grow a Nano array structure of bismuth oxoiodide (BiOI), followed by drop-casting to fabricate a BiVO4 photoanode. The surface of the BiVO4 photoanode has a metal-organic complex Co-DTPMP layer formed thereon through a successive ionic layer adsorption and reaction (SILAR) process technique.RELATED ART DOCUMENTPatent Document

[0008] (Patent Document 1) Korean Patent No. 10-2055409 (Publication date: Dec. 12, 2019)SUMMARY OF THE DISCLOSURE

[0009] A technical solution of the present disclosure is to provide a photoanode used as an electrochemical water-splitting device and a method of fabricating the same photoanode, the water-splitting device having improved photoelectrochemical (PEC) water-splitting performance and an improved oxygen evolution reaction (OER) rate by coating a surface of the photoanode to have a metal-organic complex co-diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) layer.

[0010] In addition, another technical solution of the present disclosure is to provide a photoanode used as an electrochemical water-splitting device and a method of fabricating the photoanode, the method enabling the photoanode to have improved stability and durability while preventing photocorrosion.

[0011] The purposes of the present disclosure are not limited to the purposes mentioned above. Other purposes and advantages of the present disclosure that are not mentioned may be understood from the following description and will be more clearly understood by the embodiments of the present disclosure. In addition, it will be readily apparent to those skilled in the art that the purposes and advantages of the present disclosure may be realized by means and combinations thereof as set forth in the claims.

[0012] According to one embodiment of the present disclosure, a photoanode used as an electrochemical water-splitting device is a photoanode that absorbs light and causes a water oxidation reaction to generate oxygen.

[0013] The photoanode is fabricated by coating the surface of a photoelectrode on which has a nanoporous material grown with a metal-organic complex cocatalyst through a successive ionic layer adsorption and reaction (SILAR) process.

[0014] Preferably, the photoelectrode may include a bismuth vanadate (BiVO4).

[0015] Preferably, the cocatalyst may include diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

[0016] Preferably, the SILAR process may involve continuously performing a process of immersing the photoelectrode in a 15 mM DTPMP solution for 5 minutes, washing the photoelectrode with deionized water (DI water), and then immersing the photoelectrode in 15 mM Co (NO3)26H2O for 5 minutes a predetermined number of times.

[0017] According to another embodiment of the present disclosure, the method of fabricating a photoanode may include:

[0018] fabricating a photoelectrode on which has a nanoporous material grown by electro-depositing a bismuth oxoiodide (BiOI) activating material on a surface of a fluorine-doped tin oxide (FTO), then cast-dropping a dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) on the surface, followed by annealing; and

[0019] coating a BiVO4 by coating the surface of the photoelectrode on which has the nanoporous material grown with a metal-organic complex cocatalyst through a successive ionic layer adsorption and reaction (SILAR) process.

[0020] Preferably, the photoelectrode may include a bismuth vanadate (BiVO4).

[0021] Preferably, the fabricating of a photoelectrode may include:

[0022] a first step of producing a bismuth oxoiodide (BiOI) photoactive material by completely dissolving 1.66 g of Potassium Iodide KI and 0.485 g of Bismuth Nitrate (Bi(NO3)3) in 25 mL of De-Ionized water (DI), and then adding nitric acid (HNO3) dropwise to the solution for a pH concentration to be adjusted to 1.7, and adding 10 mL of 0.23 M benzoquinone solution to the pH adjusted solution while stirring with ethanol; and

[0023] a second step of fabricating a photoelectrode on which has a nanoporous material grown by electro-depositing BiOI photoactive material on a surface of an FTO through an electrodeposition process, and then drop-casting 60 μL of dimethylsulfoxide solution containing vanadyl acetylacetonate (VO(acac)2), followed by annealing.

[0024] Preferably, the second step may involve:

[0025] electrodepositing the BiOI photoactive material on the surface of the FTO photoanode by applying a constant potential of −0.1 V with respect to an Ag / AgCl reference electrode for 400 seconds at room temperature; and

[0026] fabricating a BiVO4 by drop-casting 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) on the surface of the FTO on which the BiOI photoactive material has been electro-deposited, followed by annealing at a temperature of 500° C. at a heating rate of 2° C. / min.

[0027] The second step may further involve removing excess vanadium pentoxide (V2O5) by immersing the fabricated BiVO4 in 1 M sodium hydroxide (NaOH) solution, and then washing the BiVO4 with DI water and ethanol a predetermined number of times, followed by natural drying.

[0028] Preferably, the cocatalyst may include metal-organic diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

[0029] Preferably the coating of a photoelectrode may involve fabricating a BVO / Co-DTPMP through coating of the surface of the photoelectrode with a metal-organic complex (Co-DTPMP) cocatalyst by repeating the successive ionic layer adsorption and reaction (SILAR) process a predetermined number of times.

[0030] Preferably the SILAR process may involve continuously performing a process of immersing the photoelectrode in a 15 mM DTPMP solution for 5 minutes, washing the photoelectrode with deionized water (DI water), and then immersing the photoelectrode in 15 mM Co(NO3)26H2O for 5 minutes a predetermined number of times.

[0031] According to these embodiments, a BiOI photoactive material is electrodeposited on the surface of an FTO photoanode through an electrodeposition process. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the FTO photoanode, and then the FTO photoanode is annealed at a temperature of 500° C. with a heating rate of 2° C. / min., thus fabricating a BiVO4. The surface of the fabricated BiVO4 is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO / Co-DTPMP photoanode. Through this, it is possible to greatly improve photoelectrochemical performance and the oxygen evolution reaction (OER) rate.

[0032] In addition, on the basis of the present disclosure, long-term stability and durability of the BVO / Co-DTPMP photoanode are improved and photocorrosion is prevented.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The following drawings attached to this specification illustrate preferred embodiments of the present disclosure. The drawings serve to further help understand the technical idea of the present disclosure along with the detailed description of the disclosure described later. The present disclosure should not be construed as limited to the matters described in the drawings.

[0034] FIG. 1 shows a flow chart showing a BVO / Co-DTPMP photoanode fabricating process according to one embodiment;

[0035] FIG. 2 shows a diagram showing a detailed process of fabricating the BVO / Co-DTPMP photoanode of FIG. 1;

[0036] FIG. 3 shows a photograph showing crystallinity of the BVO / Co-DTPMP photoanode of FIG. 2;

[0037] FIGS. 4A to 4M show images and a graph related to structures of photoanodes including the BVO / Co-DTPMP photoanode of FIG. 2;

[0038] FIGS. 5A to 5I show images and graphs showing composition and surface chemical state of photoanodes including the BVO / Co-DTPMP photoanode of FIG. 2;

[0039] FIGS. 6A to 6B show graphs showing UV-vis absorbance spectra of photoanodes including the BVO / Co-DTPMP photoanode of FIG. 2;

[0040] FIGS. 7A to 7F show graphs showing PEC performance of photoanodes including the BVO / Co-DTPMP photoanode of FIG. 2;

[0041] FIG. 8 shows a graph showing OER performance of photoanodes including the BVO / Co-DTPMP photoanode of FIG. 2;

[0042] FIGS. 9A to 9D show graphs showing the electrochemical (PEC) performance of a pure BiVO4 (BVO) photoanode and a BVO / Co-DTPMP photoanode under dark conditions;

[0043] FIG. 10 shows a graph showing IPCE efficiency of photoanodes including the BVO / Co-DTPMP photoanode of FIG. 2;

[0044] FIG. 11 shows a graph showing current density of various BVO photoanodes bonded with different cocatalysts including the BVO / Co-DTPMP photoanode of FIG. 2;

[0045] FIGS. 12A to 12E show graphs and an image showing stability and durability of the BVO / Co-DTPMP photoanode of FIG. 2 by comparison with various photoanodes; and

[0046] FIG. 13 shows a diagram showing an SEM analysis result of the BVO / Co-DTPMP photoanode of FIG. 12.DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] Herein below, with reference to the attached drawings, embodiments of the present disclosure will be described in detail so that those skilled in the art can easily implement the present disclosure. However, the present disclosure may be implemented in many different forms and is not limited to the embodiments described herein. To clearly explain the present disclosure in the drawings, parts unrelated to the description are omitted, and similar parts are given similar reference numerals throughout the specification.

[0048] Herein below, one embodiment will describe a configuration of fabricating a BVO / Co-DTPMP photoanode in detail. The configuration is as follows: a BiOI photoactive material is electrodeposited on the surface of an FTO photoanode through an electrodeposition process.

[0049] Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the FTO photoanode, and then the FTO photoanode is annealed at a temperature of 500°° C. with a heating rate of 2° C. / min., thus fabricating a BiVO4 photoelectrode. The surface of the fabricated BiVO4 photoelectrode is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO / Co-DTPMP photoanode.

[0050] FIG. 1 shows a flow chart showing a BVO / Co-DTPMP photoanode fabricating process according to one embodiment, and FIG. 2 shows a diagram showing concept images for each step of the photoanode fabricating process of FIG. 1.

[0051] Referring to FIGS. 1 and 2, the method of fabricating a BVO / Co-DTPMP photoanode in one embodiment is configured as follows: a BiOI photoactive material is electrodeposited on the surface of an FTO photoanode through an electrodeposition process. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the BiOI photoanode, and then the BiOI photoanode is annealed at a temperature of 500° C. with a heating rate of 2° C. / min., thus fabricating a BiVO4 photoelectrode. Finally, the surface of the fabricated BiVO4 photoelectrode is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO / Co-DTPMP photoanode. The method may include fabricating a BiVO4 100 and coating a BiVO4 200.

[0052] Herein, in the fabricating of a BiVO4 100, a BiOI photoactive material is electrodeposited on the surface of a fluorine-doped tin oxide (FTO) photoanode through an electrodeposition process, the FTO photoanode being a photoanode material for the photoelectrochemical water-splitting device. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the BiOI photoanode, followed by annealing to fabricate the BiVO4.

[0053] That is, referring to FIG. 2, the fabricating of a BiVO4 100 includes the 11th to 14th steps 110 to 140. In the 11th step 110, 1.66 g of potassium iodide (KI) and 0.485 g of bismuth nitrate (Bi(NO3)3) were completely dissolved in 25 mL of de-ionized water (DI). Then, nitric acid (HNO3) is added dropwise to the solution for the pH concentration to be adjusted to 1.7. 0.23 M benzoquinone in 10 mL ethanol solution is added to the pH adjusted solution.

[0054] In the 12th step 120, a BiOI photoactive layer is formed by electro-depositing the generated BiOI photoactive material on the surface of the FTO photoanode through an electrodeposition process. Herein, the electrodeposition process is a process of applying a constant potential of −0.1 V to the Ag / AgCl reference electrode for 400 seconds at room temperature. At this point, as shown in FIG. 2, the BiOI photoactive layer 120 is a nanoarray grown with a nanoporous material.

[0055] Next, in the 13th step 130, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the surface of the FTO photoanode on which the BiOI photoactive material has been electro-deposited, followed by annealing to fabricate a BiVO4.

[0056] Afterward, in the 14th step 140, the fabricated BiVO4 is immersed in a 1 M sodium hydroxide (NaOH) solution to remove excess vanadium pentoxide (V2O5) from the BiVO4 and washed several times with deionized water and ethanol, followed by natural drying in the air.

[0057] Meanwhile, the coating of the BiVO4 200 involves coating the surface of the BiVO4 with a metal-organic complex diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) cocatalyst bonded with cobalt (Co) by continuously performing a successive ionic layer adsorption and reaction (SILAR) process on the surface of the naturally dried BiVO4 (sample) a predetermined number of times to fabricate a BVO / Co-DTPMP (sample).

[0058] Herein, the SILAR process involves immersing the naturally dried BiVO4 in 15 mM DTPMP solution for 5 minutes, washing the BiVO4 with deionized water (DI water), and then immersing the BiVO4 in 15 mM cobalt (II) nitrate hexahydrate (Co(NO3)26H2O) for 5 minutes.

[0059] The DTPMP solution is an organic derivative of phosphoric acid rich in N and P elements. Thus, the DTPMP solution may have multiple coordination sites capable of forming a stable complex by combining with the BiVO4 and cobalt (Co) cations.

[0060] That is, referring to FIG. 2, in the coating of the BiVO4 200, the BiVO4 is immersed in 15 mM DTPMP solution for 5 minutes, washed with deionized water (DI water), and immersed in 15 mM Co(NO3)26H2O for 5 minutes. The SILAR process is performed a predetermined number of times in succession to coat the BiVO4 with the metal-organic complex Co-DTPMP cocatalyst, thus fabricating a BVO / Co-DTPMP photoanode.

[0061] Due to this Co-DTPMP cocatalyst, tunneling of charge carriers is promoted, the OER reaction rate is improved, and photo-corrosion may be prevented.

[0062] FIG. 3 shows a photograph showing crystallinity of the BVO / Co-DTPMP photoanode analyzed using scanning electron microscopy (SEM) in a further embodiment. Referring to FIG. 3, when looking at the BiOI photoanode analyzed using scanning electron microscopy (SEM), it may be confirmed that a nanoarray is well aligned on the surface of the FTO without surface aggregation.

[0063] FIGS. 4A to 4M show images and a graph showing crystallinity of the BVO / Co-DTPMP photoanode analyzed in scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in a yet further embodiment. Referring to FIGS. 4A and 4B, the pure BiVO4 is made of soft nanoporous particles with an average size of 150 nm, so the BiVO4 has a somewhat rough surface according to the SEM analysis results. From this, it may be confirmed that even after coating the surface of the BiVO4 with the Co-DTPMP cocatalyst, the result does not change significantly.

[0064] FIGS. 4C and 4E show TEM analysis results for the BiVO4, while FIGS. 4D and 4F show the TEM analysis results for the BVO / Co-DTPMP. Referring to BVO / Co-DTPMP shown in FIGS. 4E and 4F, it may be confirmed that the BVO photoanode coated with Co-DTPMP cocatalyst has a lattice spacing of 0.31 nm, consistent with planar d-spacing while exhibiting amorphous properties.

[0065] FIGS. 4G to 4M show images showing the results of HAADF-STEM analysis of the BVO / Co-DTPMP shown in FIGS. 4E and 4F. Referring to FIGS. 4G to 4M, the presence of Bi, V, O, C, Co, and P may be confirmed in the element mapping image and EDS spectrum results.

[0066] FIGS. 5A and 5B show photographs showing crystal structure and crystal phase of a pure BVO photoanode and a BVO / Co-DTPMP photoanode in a still yet further embodiment. FIG. 5C shows a graph showing X-ray diffraction (XRD) patterns of the pure BVO photoanode and the BVO / Co-DTPMP photoanode.

[0067] Referring to FIGS. 5A and 5B, it may be confirmed that wettability of the BVO / Co-DTPMP photoanode is significantly improved after the deposition of the Co-DTPMP layer. For example, referring to FIGS. 5A and 5B, it may be confirmed that an contact angle of the BVO / Co-DTPMP photoanode decreases from 106.9°, which is the contact angle of the BVO, to 38.2°. From this, it may be confirmed that the coating layer of the BVO / Co-DTPMP photoanode has hydrophilicity.

[0068] Referring to FIG. 5C, it may be confirmed that characteristic peaks of the BVO / Co-DTPMP photoanode were detected in the monoclinic phase corresponding to (110), (011), (121), (040), (002), (211), (150), (240) and (042) planes in the results of X-ray diffraction (XRD) analysis.

[0069] FIGS. 5D to 5I are graphs showing the composition and surface chemical state of a pure BVO photoanode and a BVO / Co-DTPMP photoanode according to the analysis results of X-ray photoelectron spectroscopy (XPS) in a still yet further embodiment. The presence of Bi and V and Co, N, and P related to the BVO photoanode may be confirmed in the XPS results. Accordingly, it may be confirmed that the layer of metal-organic complex Co-DTPMP was successfully grown as an amorphous material on the surface of the BVO photoanode.

[0070] For example, referring to FIG. 5D, Bi 4f of the BVO / Co-DTPMP photoanode appears in the form of two peaks, Bi 4f7 / 2 and Bi 4f5 / 2, at 158.5 eV and 163.9 eV, respectively. Referring to FIG. 5E, the corresponding binding energy bands of V 2p are shown in the peaks of V 2p3 / 2 and V 2p1 / 2, appearing at 516 eV and 523 eV, respectively. Referring to FIG. 5F, it may be confirmed that the O 1s spectrum of the BVO sample shows only two peaks that correlate with lattice oxygen and adsorbed OH groups, respectively. Meanwhile, for the BVO / Co-DTPMP sample, the O 1s spectrum shows three main peaks attributed to the oxygen lattice, P—O species, and adsorbed OH groups, respectively.

[0071] Thus, in the BVO spectrum, the binding energy peaks of Bi 4f and V 2p are shifted to the right. This means that there is a strong interaction between the BVO and the thin layer of the cocatalyst.

[0072] Meanwhile, referring to FIG. 5G, the Co 2p spectrum shows two peaks at 782.9 eV (Co 2p3 / 2) and 796.5 eV (Co 2p1 / 2), respectively, and the corresponding two satellite peaks at 784 eV and 805 eV, respectively. These peaks appear when Co is in a Co2+ state in the Co-DTPMP cocatalyst. In addition, N 2p and P 2p are identified in the presence of the organic acid DTPMP containing P and N in the phosphate group. For P 2p, the two main peaks are correlated with P 2p3 / 2 and P 2p1 / 2. That is, referring to FIGS. 5H and 5I, in the case of N 2p, it may be confirmed that the two main peaks correspond to N 2p3 / 2 and N 2p1 / 2 at 398 eV and 401.5 eV, respectively.

[0073] FIGS. 6A and 6B show graphs showing the results of analyzing the UV-vis absorbance spectra for pure BVO photoanode and BVO / Co-DTPMP photoanode in a still yet further embodiment. Referring to 6A, UV analysis shows that the band gap of the BVO and the BVO / Co-DTPMP is similar at about 2.5 eV, so the difference in absorbance between the two electrodes is small. Referring to 6B, it may be confirmed that the photon flux absorption capacity of the BVO and BVO / Co-DTPMP is similar. Accordingly, it may be confirmed that the total theoretical photocurrent absorbed by the pure BVO photoanode and the BVO / Co-DTPMP photoanode is 4.5 and 4.3 mA / cm2, respectively.

[0074] FIGS. 7A to 7F show graphs showing the results of linear sweep voltammetry (LSV) analysis for each of pure BiVO4, BiVO4 / Co, BVO / DTPMP, and BVO / Co-DTPMP photoanodes in a still yet further embodiment. FIG. 8 shows a graph showing LSV curves for each of pure BiVO4, FTO / Co-DTPMP, and BVO / Co-DTPMP photoanodes of a still yet further embodiment. Referring to 7A, the current density obtained from the BVO photoanode is 0.5 mA / cm2 at 1.23 V based on the reversible hydrogen electrode (RHE) standard, which is similar to the current density obtained from the BVO coated with a DTPMP cocatalyst. It may be confirmed that when the BVO photoanode is coated with the Co-DTPMP cocatalyst, the current density increases to 4.0 mA / cm2, confirming that the current density of the photoanode is improved by 8 times compared to the BVO photoanode and the BVO / Co photoanode, and the current increases significantly after coating the BVO photoanode with the Co-DTPMP cocatalyst.

[0075] In addition, referring to FIG. 8, it may be confirmed that the current obtained from the FTO / Co-DTPMP is negligibly small, confirming that the Co-DTPMP cocatalyst cannot absorb light.

[0076] Referring to FIG. 7B, the photocurrent density generated by the BVO / Co-DTPMP photoanode may be confirmed by applying various coating cycles of 2, 4, and 6 cycles. Among them, a BVO / Co-DTPMP 4c (4 cycle) shows the best PEC performance. From this, it may be confirmed that as the number of coating cycles increases, the number of photons absorbed decreases due to the shielding effect.

[0077] Herein, referring to FIG. 7C, current density-time (J-t) curves may be used to determine the photocurrent response and stability of the electrode over several cycles at approximately up to 0.6 V relative to the reversible hydrogen electrode (RHE) standard. In the process, the photocurrent response of the pure BVO photoanode and the BVO / Co-DTPMP photoanode may be confirmed.

[0078] That is, it may be confirmed that the 4-coated BVO / Co-DTPMP (4c) photoanode showed a faster photo response and more stable characteristics than other electrodes as a result of using the current density-time (J-t) curve. In addition, it may be confirmed that the BVO / Co-DTPMP photoanode still generates the highest photocurrent density among photoanodes even after 10 on-off cycles.

[0079] Referring to FIG. 7D, it may be confirmed that the BVO / Co-DTPMP photoanode shows 1.37% (based on 0.7V relative to the RHE standard) in an applied bias photon to current efficiency (ABPE) analysis, which is greatly improved compared to 0.15% (based on 1 V relative to the RHE standard) for pure BiVO4 film.

[0080] Referring to FIG. 7E, when an open-circuit photovoltaic (OCP) transient decay curve and derived carrier lifetime were analyzed to further investigate the charge separation performance, the BVO / Co-DTPMP photoanode exhibited a longer carrier lifetime and higher ΔOCP value (ΔOCP=OCP dark−OCP light) than the BiVO4 photoanode (sample). Accordingly, it may be confirmed that the driving force for charge separation under the illumination of the BVO / Co-DTPMP photoanode is stronger than that of the BiVO4 photoanode (sample).

[0081] Referring to FIG. 7F, when electrochemical impedance spectroscopy (EIS) is used to investigate the charge-transfer resistance (Rct) of the electrode surface, the charge transfer resistance (Rct) of the BiVO4 (sample) and the BVO / Co-DTPMP photoanode (sample) measured at OCP was expressed as the radius of a semicircle under AM 1.5G spectrum conditions, which is the standard illumination for solar cell testing to evaluate the photoelectrochemical properties of electrodes. Compared with the BVO / Co-DTPMP photoanode, the pure BiVO4 photoanode had a larger Rct. This shows that the BVO / Co-DTPMP photoanode is more efficient in separating and transferring charges at the electrolyte / photoanode interface, and that the charge transfer resistance (Rct) is reduced by the Co-DTPMP cocatalyst.

[0082] The best way to increase the charge transfer efficiency of the BiVO4 is to load the Co-DTPMP cocatalyst to optimize the oxygen evolution reaction (OER) rate at the electrode / electrolyte interface. The Co-DTPMP cocatalyst efficiently uses holes generated by light and provides additional active sites, thereby the OER rate for PEC water oxidation is effectively improved.

[0083] Herein below, to demonstrate the improved PEC performance of the BVO / Co-DTPMP photoanode, the effect of the Co-DTPMP cocatalyst as a catalyst of the OER electrode is confirmed under dark conditions.

[0084] FIGS. 9A to 9D show graphs showing the electrochemical (PEC) performance of a pure BiVO4 (BVO) photoanode and a BVO / Co-DTPMP photoanode under dark conditions in a still yet further embodiment. Referring to 9A, under dark conditions, the minimum potential of the BVO / Co-DTPMP photoanode shifted in the negative direction compared to FTO / Co-DTPMP and pure BiVO4 photoanodes. Accordingly, it may be confirmed that the water oxidation reaction rate of the BVO / Co-DTPMP photoanode is fast.

[0085] Referring to FIG. 9B, the Tafel slope results derived from the LSV curve for OER show that the Tafel slope of the BVO / Co-DTPMP photoanode is lower than that of the pure BVO photoanode. The lower Tafel slope value is due to the higher OER catalytic activity, and the increase in water oxidation reaction is due to the hydrophilic nature of the metal-organic complex (Co-DTPMP) layer. Thus, it may be confirmed that the contact angle of the BiVO4 photoanode is greatly reduced after coating with the Co-DTPMP cocatalyst according to these contact angle measurement analysis results, and surface wettability of the BVO / Co-DTPMP photoanode is greatly increased.

[0086] Meanwhile, electrochemically active surface area (ECSA) is determined from a cyclic voltammetry (CV) plot using the capacitance of the electric double layer (Cdl) at the interface between the photoanode and the electrolyte. Accordingly, the CV is performed in a neutral electrolyte at scan rates in the range of 20-500 mV / s. The Cdl value is determined by plotting the anode-cathode current density difference versus scan rate and then calculating the linear slope of the data.

[0087] Referring to FIG. 9C, according to the ECSA analysis results for the BVO photoanode and BVO / Co-DTPMP photoanode, the BVO / Co-DTPMP photoanode (sample) shows a higher slope than the pure BVO (sample). From this, it may be confirmed that the ECSA of the BVO / Co-DTPMP photoanode is improved after loading the Co-DTPMP cocatalyst on the surface of the BVO photoanode.

[0088] Referring to FIG. 9D, when the flat band potential and charge separation efficiency are calculated by the Mott-Schottky method for the BVO and BVO / Co-DTPMP photoanodes, the flat band potential and charge separation efficiency curves for the BVO and BVO / Co-DTPMP photoanodes show positive slopes. This shows that the photoanodes are an n-type semiconductor and may be used as photoanodes in PEC. Compared to the BVO photoanode, the flat band potential and the slope of the charge separation efficiency curve of the BVO / Co-DTPMP photoanode are smaller, confirming that the charge separation efficiency for the BVO / Co-DTPMP photoanode is higher.

[0089] FIG. 10 shows a graph showing the photoconversion capacity of BVO photoanode and BVO / Co-DTPMP photoanode samples according to the analysis results of incident photon to current efficiency (IPCE). Referring to FIG. 10, the IPCE spectrum obtained in the wavelength range of 300 to 700 nm shows that the IPCE of the BVO / Co-DTPMP photoanode (up to 73%) at a wavelength of 400 nm is much higher than that of the BiVO4 photoanode (up to 28%).

[0090] FIG. 11 shows a graph showing the current density for BVO photoanodes when cocatalysts such as Co—Pi and Co(OH)2 are deposited on the surface thereof, respectively. Referring to FIG. 11, it may be confirmed that the BVO / Co-DTPMP photoanode combination shows significantly higher PEC efficiency than the combination of other cocatalysts.

[0091] FIGS. 12A to 12E shows graphs and an image showing the stability and durability of the BVO / Co-DTPMP photoanode of a still yet further embodiment by comparison with various photoanodes. Referring to FIG. 12A, the effect of the Co-DTPMP cocatalyst on surface reaction and charge transfer may be confirmed. This is based on the LSV analysis results for pure BVO and BVO / Co-DTPMP photoanodes, respectively, in the presence of hole scavenger H2O2.

[0092] Referring to FIG. 12B, based on the charge injection curve, the resulting value of the BVO / Co-DTPMP increases up to 91%, while the value of the pure BiVO4 is only 20% at 1.23V relative to the RHE standard. The improved charge injection efficiency by the BVO / Co-DTPMP photoanode shows that the cocatalyst successfully limits hole recombination and increases the fraction of holes participating in the oxidation reaction.

[0093] Referring to FIG. 12C, a photoelectrode shows no change in charge separation efficiency, meaning that the presence of a cocatalyst coating on the surface of the photoelectrode has minimal effect on the overall bulk charge separation within the semiconductor. It may be confirmed that considering the negligible changes in light absorption and charge separation, the enhancement in photoelectrochemical (PEC) activity observed for the BVO / Co-DTPMP photoanode mainly comes from the increased water oxidation kinetics promoted by the Co-DTPMP cocatalyst.

[0094] Referring to FIG. 12D, the long-term stability (current density vs. time) of the photoelectrode measured at 0.6 V (relative to the RHE standard) for the BVO / Co-DTPMP photoanode shows that even after 11000 seconds of measurement, the current density of the BVO / Co-DTPMP photoanode is still high, confirming that coating the BiVO4 photoanode with the Co-DTPMP cocatalyst ensures long-term stability of an electrode. Additionally, the current density gradually increases after 2000 seconds and reaches a plateau, confirming effective charge separation and hole transfer.

[0095] In addition, referring to FIG. 12D, the possibility of photo corrosion of the pure BiVO4 photoanode may be confirmed in areas not coated by the Co-DTPMP cocatalyst. It may be seen that a pure BiVO4 photoanode lacks stability due to light-induced dissolution of VO43− ions.

[0096] FIG. 13 shows a photograph showing the SEM of the BVO / Co-DTPMP photoanode after verifying the stability of the BVO / Co-DTPMP photoanode. Referring to FIG. 13, it may be confirmed that photo-corrosion of the BVO / Co-DTPMP photoanode is prevented by the Co-DTPMP layer compared to the pure BVO photoanode.

[0097] Accordingly, in a still yet further embodiment, a BiOI photoactive material is electrodeposited on the surface of the FTO photoanode through an electrodeposition process. Then, 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) is drop-cast on the FTO photoanode, and then the FTO photoanode is annealed at a temperature of 500° C. with a heating rate of 2° C. / min., thus fabricating a BiVO4 photoelectrode. Finally, the surface of the fabricated BiVO4 photoelectrode is coated with a metal-organic complex (Co-DTPMP) cocatalyst through a SILAR process to fabricate a BVO / Co-DTPMP photoanode. Through this, it is possible to greatly improve photoelectrochemical (PEC) performance and the oxygen evolution reaction (OER) rate.

[0098] As described above, although the embodiments have been described with limited examples and drawings, various modifications and variations can be made by those skilled in the art from the above description. For example, the described techniques are performed in a different order than the described method, and / or components of the described system, structure, device, and circuit are bonded or bonded in a different form than the described method, or other components are used. Alternatively, appropriate results may be achieved even if substituted or substituted by an equivalent.

[0099] Therefore, the scope of the present disclosure should not be limited to the described embodiments, but should be determined by the claims described below as well as equivalents to these claims.

Claims

1. A photoanode used as an electrochemical water-splitting device, for absorbing light and causing a water oxidation reaction to generate oxygen, the photoanode comprising:a photoelectrode on which a nanoporous material is grown a metal-organic complex cocatalyst coated on the surface of the photoelectrode by a successive ionic layer adsorption and reaction (SILAR) process.

2. The photoanode of claim 1, wherein the photoelectrode comprises a bismuth vanadate (BiVO4).

3. The photoanode of claim 1, wherein the cocatalyst comprises a metal-organic diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

4. The photoanode of claim 1, wherein the SILAR process comprises continuously performing a process of immersing the photoelectrode in a 15 mM DTPMP solution for 5 minutes, washing the photoelectrode with deionized water (DI water), and then immersing the photoelectrode in 15 mM Co(NO3)26H2O for 5 minutes a predetermined number of times.

5. A method of fabricating a photoanode used as an electrochemical water-splitting device, the method comprising:fabricating a photoelectrode on which has a nanoporous material grown by electro-depositing a bismuth oxoiodide (BiOI) activating material on a surface of a fluorine-doped tin oxide (FTO), then cast-dropping a dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) on the surface, followed by annealing; andcoating a BiVO4 by coating the surface of the photoelectrode with a metal-organic complex cocatalyst through a successive ionic layer adsorption and reaction (SILAR) process.

6. The method of claim 5, wherein the photoelectrode comprises a bismuth vanadate (BiVO4).

7. The method of claim 6, wherein the fabricating of a photoelectrode comprises:a first step of producing a bismuth oxoiodide (BiOI) photoactive material by completely dissolving 1.66 g of Potassium Iodide KI and 0.485 g of Bismuth Nitrate (Bi(NO3)3) in 25 ml of de-ionized water (DI), and then adding nitric acid (HNO3) dropwise to the solution for a pH concentration to be adjusted to 1.7, and adding 0.23 M benzoquinone in 10 mL ethanol solution is added to the pH adjusted solution; and a second step of fabricating a photoelectrode on which has a nanoporous material grown by electro-depositing a BiOI photoactive material on a surface of an FTO photoanode through an electrodeposition process, and then drop-casting 60 μL of dimethylsulfoxide solution containing vanadyl acetylacetonate (VO(acac)2), followed by annealing.

8. The method of claim 7, wherein the second step comprises:electrodepositing the BiOI photoactive material on the surface of the FTO photoanode by applying a constant potential of −0.1 V with respect to an Ag / AgCl reference electrode for 400 seconds at room temperature; andfabricating a BiVO4 by drop-casting 60 μL of dimethyl sulfoxide (DMSO) solution containing vanadyl acetylacetonate (VO(acac)2) on the surface of the FTO on which the BiOI photoactive material has been electro-deposited, followed by annealing at a temperature of 500° C. at a heating rate of 2° C. / min.

9. The method of claim 8, wherein the second step further comprises removing excess vanadium pentoxide (V2O5) by immersing the fabricated BiVO4 in 1 M sodium hydroxide (NaOH) solution, and then washing the BiVO4 with DI water and ethanol a predetermined number of times, followed by natural drying.

10. The method of claim 5, wherein the cocatalyst comprises a metal-organic diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

11. The method of claim 10, wherein the coating of a photoelectrode comprises fabricating a BVO / Co-DTPMP through coating of the surface of the photoelectrode with a metal-organic complex (Co-DTPMP) cocatalyst by repeating a successive ionic layer adsorption and reaction (SILAR) process a predetermined number of times.

12. The method of claim 11, wherein the SILAR process comprises continuously performing a process of immersing the photoelectrode in a 15 mM DTPMP solution for 5 minutes, washing the photoelectrode with deionized water (DI water), and then immersing the photoelectrode in 15 mM Co(NO)26H2O for 5 minutes a predetermined number of times.

13. The photoanode of claim 2, wherein the cocatalyst comprises a metal-organic diethylenetriamine penta (methylene phosphonic acid) (Co-DTPMP) bonded with cobalt (Co).

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