WO2023054947A1 - Core-shell α-fe2o3@ws2/wox composition, photocatalyst for photoelectrochemical cell comprising same, and method for manufacturing same - Google Patents

Abstract

A heterojunction with an α-Fe2O3 nanorod is formed by using WS2 nanosheets prepared through a liquid phase exfoliation process, and a fabricated core-shell α-Fe2O3@WS2/WOx structure can be optimized by the concentration, layer, and thickness parameters of the WS2 nanosheets by means of simple, low-cost synthesis in order to improve the efficiency of PEC performance. In this core-shell α-Fe2O3@WS2/WOx structure, the penetration of WS2 into the pores of a α-Fe2O3 film can be examined by means of a TOF-SIMS analysis. An optimal α-Fe2O3@4WS2/WOx electrode having a 5.7 nm-thick WS2 shell exhibits a photocurrent density of 0.98 mA.cm-2 at 1.23 VRHE, which is 14 times higher than that of a pure α-Fe2O3 electrode.

Description

Core-shell α-FE2O3@WS2/WOX composition, photocatalyst for photoelectrochemical cell containing the same, and manufacturing method thereof

The present application relates to a core-shell α-Fe ₂ O ₃ @WS ₂ /WOx composition, a photocatalyst for a photoelectrochemical cell including the same, and a method for preparing the same.

Photoelectrochemical (PEC) water splitting is one of the direct ways to convert solar energy into chemical energy that can be converted into electricity. In the future, PEC water splitting is expected to play a remarkable role in sustainable energy production. Although a considerable amount of research has already been done in this field, research on highly efficient water splitting cells for fuel conversion from sunlight to hydrogen is still needed. Various n-type semiconductors such as TiO ₂ , BiVO ₄ , ZnO and α-Fe ₂ O ₃ have been applied as photoanode materials to improve PEC performance.

Among them, hematite (α-Fe ₂ O ₃ ) has a suitable band gap (2.1 eV), eco-friendliness, abundance, ca. It is getting a lot of attention due to its high absorption range of 40% solar radiation. Also, α-Fe ₂ O ₃ is 1.23 V vs. 1.5 G in AM. At RHE 12, it has a maximum solar-to-hydrogen conversion efficiency (15.3%) corresponding to a photocurrent density of about 12.6 mA cm ^-2 . However, α-Fe ₂ O ₃ also has the disadvantage of significant charge recombination and short hole diffusion length (LD = ~(2-4) nm). Therefore, research on new and efficient methods for improving PEC performance is still a demanding field. One of the most effective ways to overcome charge recombination is to form an appropriate heterojunction on the semiconductor surface.

Currently, two-dimensional materials have exclusive potential and properties for various applications. Among two-dimensional materials, transition metal dichalcogenides (TMDs) are attracting attention due to their appropriate band positions and light absorption of (5-10)% in the visible region. TMDs, particularly MoS ₂ and WS ₂ , have excellent catalytic activity as well as stability for energy applications. It has a variety of active sites, both active metal sites (W or Mo) and covalent (S or Se) sites. The exfoliation layer of WS ₂ has high thermal and chemical stability, a low coefficient of resistance and an appropriate band edge for enhanced visible light absorption. The electronic properties of 2D TMD materials are highly dependent on their shape and fabrication method. Exfoliated WS ₂ in bulk form has an indirect band gap (1.35 eV), and in the form of a single layer or few layers, it becomes a direct band semiconductor (2.05 eV) and exhibits excellent optoelectronic properties. Depending on the type of solvent used, it can be obtained with a yield of 36% by a direct liquid exfoliation method, and research on this is continuously needed.

According to the present application, WS ₂ is synthesized using a liquid phase exfoliation procedure in a solvent mixture (ethanol/water) with advantages of low toxicity, no additives and biocompatibility. WS ₂ nanosheets can be easily deposited on the surface of α-Fe ₂ O ₃ electrodes by a simple and inexpensive drop casting process to fabricate core-shell structures. Concentration, length, thickness and other parameters are controlled for the prepared WS ₂ nanosheet precursor. The thickness and uniformity of WS ₂ as a shell is optimized by the time it takes to drop on the surface of α-Fe ₂ O ₃ nanorods, and through this structure, PEC performance can be greatly improved. The flat band potential (Vfb), space charge layer (WSCL), donor concentration (ND), geometry of the nanorods and charge transfer resistance (Rct) values for the composition of the present application were systematically confirmed by Mott-Schottky and EIS analyses. can By measuring the PL and TRPL spectra, it is possible to calculate the average decay time of the charge carriers and to confirm the decrease in the recombination rate in the core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx nanorod. In addition, using Time of Flight-secondary ion mass spectrometry (ToF-SIMS), the degree of penetration of WS ₂ into the pores of the α-Fe ₂ O ₃ film can be investigated, and the depth of heat treatment and its effect on the surface can be confirmed.

One aspect of the present application relates to a method for manufacturing an electrode of a photoelectrochemical cell.

In one example, the manufacturing method includes preparing a substrate; Forming an α-Fe ₂ O ₃ nanorod film on the prepared substrate; and drop-casting a WS ₂ nanosheet precursor on the formed α-Fe ₂ O ₃ nanorod film to heterocouple the core-shell α-Fe ₂ O ₃ and WS ₂ .

In one example, forming the α-Fe ₂ O ₃ nanorod film may include mixing FeCl ₃ .6H ₂ O and NaNO ₃ ; and calcining at 500 to 600 °C for 3 to 5 hours.

In one example, the WS ₂ nanosheet precursor may be prepared by liquid phase exfoliation (LPE).

In one example, the WS ₂ nanosheet precursor may be prepared by dissolving the WS ₂ powder in a mixed solvent of ethanol and water; Sonicating the mixed solution; and centrifuging at 3000 to 4000 rpm for 30 to 90 minutes.

In one example, the hetero coupling step may include dropping a WS ₂ nanosheet precursor onto the α-Fe ₂ O ₃ nanorod film; and heat-treating at 400 to 500 °C for 1 to 3 hours.

In one example, the step of dropping the WS ₂ nanosheet precursor on the α-Fe ₂ O ₃ nanorod film may be repeated 6 times or less.

One aspect of the present application relates to a composition for a photocatalyst.

In one example, as a core-shell photocatalyst composition, the core may include α-Fe ₂ O ₃ nanorods, and the shell may include mixed nanosheets of WS ₂ and WO _x .

In one example, the length of the mixed nanosheet may be 200 nm or less.

In one example, the average thickness of the mixed nanosheets may be 12 nm or less.

Another aspect of the present application relates to an electrode for a photoelectrochemical cell.

In one example, the electrode may be laminated on a substrate and a composition for photocatalysis of a photoelectrochemical cell in the form of a film on the substrate.

Another aspect of the present application relates to photoelectrochemical cells.

In one example, the photoelectrochemical cell may include a photoelectrochemical cell electrode.

A heterojunction is formed with α-Fe ₂ O ₃ nanorods using WS ₂ nanosheets prepared through a liquid-phase exfoliation process, and the prepared core-shell α-Fe ₂ O ₃ @WS ₂ /WOx structure has excellent PEC performance. It can be optimized by the concentration, layer and thickness parameters of WS ₂ nanosheets using a low-cost and simple synthetic process to improve efficiency. In this core-shell α-Fe ₂ O ₃ @WS2/WOx structure, penetration of WS ₂ into the pores of the α-Fe ₂ O ₃ film can be investigated by TOF-SIMS analysis. The optimal α-Fe ₂ O ₃ @4WS ₂ /WOx electrode with a 5.7 nm-thick WS ₂ shell exhibits a photocurrent density of 0.98 mA.cm ⁻² at 1.23 VRHE, which is 14 times lower than the pure α-Fe ₂ O ₃ electrode. times higher than that of the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode due to the increased efficiency of photogenerated charge carriers, the facile hole extraction through the fabricated core-shell heterojunction, and the reduced resistance due to the reduced recombination rate of electrons. PEC performance can be improved.

Figure 1 shows (a) UV-Vis absorption spectrum (inside is a schematic diagram of the ultrasonic effect in the liquid-phase exfoliation method), (b) Raman scattering excitation spectrum, and (c) AFM images and height profiles of exfoliated WS ₂ nanosheets. it is a drawing

FIG. 2 is a diagram showing AFM images and highest _profiles of WS ₂ nanosheets doped 2 times, 4 times, and 8 times with WS 2 .

Figure 3 is a high-resolution TEM image of the selected area, the inset in (c) shows a hexagonal arrangement of atoms with zigzag edges, and (d) is a TEM image of a typical WS ₂ sample and the inset is a WS ₂ nanosheet. It is a diagram of the Fast Fourier Transform (FFT) pattern for

4 is a plan view FE-SEM image of (a) pure α-Fe ₂ O ₃ and (b) α-Fe ₂ O ₃ @4WS ₂ /WOx, (c) α-Fe ₂ O ₃ , (d) α -Fe ₂ O ₃ @4WS ₂ /WOx cross-sectional FE-SEM images, (e) and (f) α-Fe ₂ O ₃ @4WS ₂ /WOx HRTEM images of core-shell nanorod structures, (g) α-Fe HRTEM images of ₂ O ₃ @2WS ₂ /WOx photoanode, (h) α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode, (i) α-Fe ₂ O ₃ @8WS ₂ /WOx photoanode and (j ) SEM mapping and EDX analysis of the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode.

5 is (a) XRD, (b) Raman spectrum, (c) UV-vis absorbance, and (d) LHE result graphs for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx, (e) α-Fe ₂ O ₃ and (f) WS ₂ It is a graph of UV-vis absorbance spectrum and band gap energy dp.

6 shows XPS spectral results for (a) Fe _2p (b) O _1s (c) W _4f and (d) S _2p for pure α-Fe _{2 O 3 and α-Fe 2 O 3 @} 4WS ₂ /WOx Graphs, XPS for (e) Fe _2p and (f) O ₁ of pure α-Fe ₂ O ₃ electrodes, and (g) pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes. This is a graph of the survey analysis result.

Figure 7 is a time-of-flight secondary ion mass spectrometry (Time-of-Flight Secondary Ion Mass Spectrometry, TOF-SIMS) results, (a) to (d) are α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes It is a surface analysis result graph for the separation of WS ₂ to WOx in , and (e) a depth profile and (f) a 3D image of ions in an α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode.

8 is a graph of EPR spectrum analysis results for pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes.

9 shows (a) Linear scan voltammetry (LSV) and chopped LSV versus RHE at potentials from 0.3 to 1.5 V for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ samples. Graphs of the results, (c) photocurrent response, (d) photocurrent stability (inset shows long-term (24 hours) photocurrent stability for α-Fe ₂ O ₃ @4WS ₂ ).

10 shows the relationship between (a) OCP, (b) Mott-Schottky, (c) ND and Vfb, and (d) IPCE for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes. This is the resulting graph for the plot.

11 shows Nyquist plots for (a) α-Fe ₂ O ₃ and (b) Fe ₂ O ₃ @4WS ₂ /WOx at potentials of 0.9, 1, and 1.1 V vs. RHE, and (c) at potentials of 1.1 V vs. RHE. Nyquist plot for (d) α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes for Time-Resolved Photoluminescence (TRPL) results.

12 shows (a) linear scan voltammetry (LSV), (b) charge injection, (c) charge separation efficiency, 1.23 V vs RHE measured in 1M NaOH and 1M NaOH + 0.5MH ₂ O ₂ electrolytes. Graphs of the results of (d ₎ H ₂ and (e) O 2 generation versus reaction time per illumination area (1 cm ² ) for the α-Fe ₂ O ₃ and α-Fe ₂ O 3 @4WS ₂ /WOx electrodes at the potential _. .

13 shows (a) UPS spectra for pure α-Fe ₂ O ₃ and WS ₂ , schematic diagrams of electronic band structures of α-Fe ₂ O ₃ and WS ₂ before (b) and after (c) heterojunction, and (d) ) is a schematic diagram of the mechanism and structure for a stable core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode.

Terms used in this application are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this application, terms such as "include" or "have" are intended to designate that the features, components, etc. described in the specification exist, but one or more other features or components may not exist or be added. That doesn't mean there aren't any.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related art, and unless explicitly defined in this application, it should not be interpreted in an ideal or excessively formal meaning. don't

In the present application, the term "nano" may refer to a size in a nanometer (nm) unit, for example, from 1 to 1,000 nm, but is not limited thereto. In addition, the term "nanoparticle" in this specification may mean a particle having an average particle diameter in nanometer (nm) units, for example, may mean a particle having an average particle diameter of 1 to 1,000 nm, but It is not limited.

Hereinafter, a core-shell α-Fe ₂ O ₃ @WS ₂ /WOx composition of the present application, a photocatalyst for a photoelectrochemical cell including the same, and a manufacturing method thereof will be described in detail with reference to the accompanying drawings. However, the accompanying drawings are illustrative, and the scope of the core-shell α-Fe ₂ O ₃ @WS ₂ /WOx composition, photocatalyst for photoelectrochemical cell including the same, and manufacturing method of the present application is limited by the accompanying drawings. It is not.

1. Preparation of α-Fe ₂ O ₃ @4WS ₂ /WOx Photoelectrode

Hematite (α-Fe ₂ O ₃ ) precursor is prepared by mixing 0.15M FeCl ₃ .6H ₂ O and 1M NaNO ₃ . HCl is added dropwise to the mixture to adjust the pH of the mixture to 1.5. After this, 80 mL of the solution-washed FTO is placed in a Teflon container placed on the bottom. The autoclave is set at 100° C. for 6 hours. Thereafter, the surface of the prepared β-FeOOH sample is washed with deionized water, and finally the electrode is fired at 500 to 600 ° C for 3 to 5 hours, for example, placed in an air furnace at 550 ° C for 4 hours, α -Manufacture Fe ₂ O ₃ nanorods.

The exfoliated WS ₂ nanosheet precursor is prepared through the LPE (liquid phase exfoliation) method. WS ₂ bulk powder (300mg) is mixed with a mixed solvent of ethanol (35mL) and water (55mL). The precursor is sonicated continuously for 5 days. To separate the WS ₂ nanosheets, the dispersion is centrifuged at 3000 to 4000 rpm for 30 to 90 minutes, for example, 3,500 rpm for 60 minutes. After this, the supernatant containing the thin WS ₂ nanosheets is collected from the top of the solution.

The core-shell structures of α-Fe ₂ O ₃ and WS ₂ are fabricated by a convenient and inexpensive drop casting procedure. WS ₂ nanosheet precursor (100 μL) is dropped onto the top surface of the α-Fe ₂ O ₃ electrode and placed in a furnace at 450 °C for 5 min. The dropping process may be performed 10 times or less, for example, 2, 4, and 8 times, respectively, α-Fe ₂ O ₃ @2WS ₂ /WOx, α-Fe ₂ O ₃ @4WS ₂ /WOx, and α- It is expressed as Fe ₂ O ₃ @8WS ₂ /WOx. Finally, all of the prepared α-Fe ₂ O ₃ /WS2 samples are subjected to heat treatment at 400 to 500° C. for 1 to 3 hours, for example, in a furnace at 450° C. for 2 hours.

2. How to measure physical properties

Morphological investigations of α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx thin films were conducted using scanning electron microscopy (SEM, Model Quanta 250 FEG) and transmission electron microscopy (TEM, JEOL, JEM- 2100F). investigate The structural and crystalline properties of the fabricated thin films were characterized by X-ray diffraction (XRD), a Bruker D8Advance, using Raman spectra using monochromatic Cu Kα radiation (λ=1.5406 Å) and a laser in the 2θ range of 10–55° and an excitation source at room temperature. with a 785 nm line (Bruker, Model: Senteraa 2009, Germany). Chemical compositions and components were analyzed with a Thermo Scientific Sigma Probe spectrometer using a monochromatic AlKα source (photon energy 1486.6 eV), spot size 400 μm, energy step size 1.0 eV, pass energy 200 eV, and ToF-SIMS (Time-of- Flight secondary ion mass spectrometry is utilized by a pulsed primary ion beam and time-of-flight mass spectrometer for the detection of molecular ions in the mass range > 9,000 amu in a single spectrum. This technique can provide three-dimensional images of chemicals within complex samples with lateral resolution of less than 500 nm. Electron paramagnetic resonance (EPR) measurements are performed in the X-band (9.64GHz) using the CW/Pulse EPR System (QM09). 1 mW of power absorbed by the sample is recorded at room temperature. The optical properties of the photoanode are measured by a Perkin Elmer UV-Vis-NIR model Lambda 950. Time-resolved photoluminescence (TRPL) is measured with the second harmonic beam of a Ti:sapphire laser (800 nm wavelength, 100 fs pulse width and 82 MHz repetition rate). The collected luminescence signal is dispersed by a monochromator (MS3504i, SOLAR TII) and then detected by a photomultiplier tube (PMT-100, Becker & Hickl).

3. Photoelectrochemical measurements

A standard three-electrode cell is used to measure the PEC performance. α-Fe ₂ O ₃ , α-Fe ₂ O ₃ @2WS ₂ /WOx, α-Fe ₂ O ₃ @4WS ₂ /WOx and α-Fe ₂ O ₃ @8WS ₂ /WOx photoanodes are applied as working electrodes. . The counter electrode is a platinum wire. Ag / AgCl is used as a reference electrode in 1M NaOH (PH ~ 12) electrolyte, and is calculated as RHE through the following relational expression 1.

[Relationship 1]

The PEC performance is measured at 100 mW/cm ² under 300 W Xe lamp illumination on top of the photoelectrode in the voltage range of 0.3 to 1.5 V (vs. RHE). EIS is measured at different potentials (0.9, 1, 1.1 V vs. RHE) by the same electrode formation.

4. Measurement of hydrogen and oxygen evolution

α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx Total water dissociation at the photoanode is evaluated by measuring H ₂ and O ₂ evolution at 1.23 V versus RHE under 100 mW.cm ⁻² irradiation in 1 M NaOH electrolyte. The amount of hydrogen and oxygen gas produced is measured using a gas chromatography (GC) system (YL Instrument, 6500GC System). Prior to measurement, the water decomposition reaction nitrogen gas was purged through the cell for 2 hours to remove air remaining in the reaction vessel. Turn on the light source and measure the amount of oxygen and hydrogen released with a sealed syringe every 20 minutes using a gas chromatograph for 2 hours. A gas sample is injected into the GC and the resulting peak areas (AreaH ₂ , AreaO ₂ ) are recorded, and the emitted hydrogen-oxygen gas is calculated using the following relationship Equation 2.

[Relationship 2]

The detailed calculation process for Faraday efficiency is as follows.

Faradic Efficiency = Actual Photocurrent Density/Theoretical Photocurrent Density

Actual photocurrent density = N×nH ₂ /O ₂ ×F

F is the Faraday constant equal to 0.096487 C/μmol, nH ₂ /O ₂ (μmol) is the amount of H ₂ or O ₂ generated as determined by gas chromatography, and N is the electron required to generate one molecule of H ₂ or O ₂ is the number of Assume that 2 electrons are required to produce 1 molecule of H ₂ , and 4 electrons are required for 1 molecule of O ₂ .

Theoretical photocurrent density = Q = I × t (5)

Q is the charge (electricity) in coulombs (C), I is the current (amps, A), and t is the time (seconds).

Hereinafter, the present application will be described in more detail through experimental examples.

[Experimental Example]

1. Structural characteristics of prepared WS ₂

Figure 1 shows (a) UV-Vis absorption spectrum (inside is a schematic diagram of the ultrasonic effect in the liquid-phase exfoliation method), (b) Raman scattering excitation spectrum, and (c) AFM images and height profiles of exfoliated WS ₂ nanosheets. it is a drawing

To measure the concentration, length and thickness of the exfoliated WS ₂ nanosheets, atomic force microscopy (AFM) and near-infrared (NIR) to ultraviolet (UV) regions are used. The light absorption of the nanosheets can be seen at three major excitation points, A (~631 nm), B (~522 nm) and C (~423 nm), corresponding to the electronic band structure, which are observed in the exfoliated WS ₂ nanosheets. means the presence of (Fig. 1a). The A and B excitation peaks as shown in FIG. 1A can be shifted by layer accumulation under sonication time and centrifuge speed. The energy change between A and B may increase by enhancing the accumulation of layers. Peak C is due to van der Waals interactions and is related to monolayer or multilayer materials. The values indicated for peaks A, B and C indicate a significant contribution of several layered nanosheets. Raman analysis of the WS ₂ nanosheets shows strong signals at around 355 and 425 (cm ⁻¹ ), which correspond to both in-plane E ¹ _2g and out-of-plane A _1g vibrations (FIG. 1b). These two peaks coincide with the peaks observed in the exfoliated WS ₂ nanosheets. As shown in Figure 1b, the AFM results indicate three points (P1, P2 and P3) where the highest profile diagram of these three exfoliated WS ₂ nanosheets shows a thickness of about 10 nm and a length of 140 nm ( Fig. 1c). In order to find out more experimental results, the length, thickness, and concentration of the exfoliated WS ₂ nanosheets were estimated by the formula. The following relational expression 2 can be used to calculate the thickness of the WS ₂ nanosheet.

[Relationship 2]

λ _A is the wavelength of peak A shown in Fig. 1a. In the prepared precursor, the thickness of the few-layer WS ₂ nanosheets was calculated to be about 10 nm, indicating an exfoliated multilayer, and it could be confirmed that the AFM results approached the thickness. In addition, the length (~140 nm) and concentration (~0.04 mg.mL ^-1 ) of the WS ₂ nanosheets are calculated by relational expressions 3 and 4.

[Relationship 3]

[Relationship 4]

In Equation 3, L is the average size of nanosheets and is estimated from the EXT _B and EXT ₃₄₅ values of the UV-Vis data. In relational equation 8, the concentration of nanosheets depends on the absorbance value at 265 nm and the extinction coefficient at 235 nm, which are invariant (ε _{235 nm} = 47.7 Lg ^-1 cm ^-1 ). In addition, the amount of WS ₂ precursor per photoelectrode area is calculated through relational expression 9, which is shown in Table 1.

[Relationship 5]

Sample's Name	Time of Drops	Volume of WS2 precursor:0.1mlxTimes of drop	Concentration of WS2 nanpsheets in precursor (mg.ml)	Amount of WS2 nanosheets on each photoelectrode (mg)	Amount of nanosheets in photoelectrode's area (mg/cm2)(FTO area=25mmx25mm)
α-Fe2O3@2WS2/WOX	2	2x0.1=0.2	0.04	0.008	0.0013
α-Fe2O3@4WS2/WOX	4	4x0.1=0.4	0.04	0.016	0.0026
α-Fe2O3@8WS2/WOX	8	8x0.1=0.8	0.04	0.032	0.0051

The inset of Figure 1a shows the role of sonication, which applies continuous ultrasound or vibration to an aqueous surfactant solution or organic solvent. This can be used to prevent changes in the electronic and chemical behavior of WS ₂ during conversion from bulk to nanosheets. Ultrasonic waves of the LPE method form small bubbles or pores in the solvent. During sonication, the cavity is continuously imploded to form bursts of energy. This energy generated by the standing wave pattern as organic solvent molecules pass through the cavities can hold the WS ₂ nanosheets together to prevent reaggregation. Therefore, the interfacial reaction between the solvent molecules and the WS ₂ nanosheets is greater than the interlayer force binding the WS ₂ sheets in the bulk crystal.

FIG. 2 is a diagram showing AFM images and highest _profiles of WS ₂ nanosheets doped 2 times, 4 times, and 8 times with WS 2 .

As shown in FIG. 2, AFM analysis of the WS ₂ nanosheet precursor dropped 2, 4, and 8 times on a clean surface confirmed the thickness and uniformity of the deposited film. Through this, it can be confirmed that the uniform WS ₂ nanosheets on the surface can be stabilized during the 2nd and 4th drop casting. However, in the case of drop casting 8 times, in some parts of the process, the thickness of WS ₂ exceeds 10 nm, and it can be seen that the uniformity of the surface is disturbed. This uniformity may be the reason for the reduced PEC performance in the α-Fe ₂ O ₃ @8WS ₂ /WOx photoanode.

Figure 3 is a high-resolution TEM image of the selected area, the inset in (c) shows a hexagonal arrangement of atoms with zigzag edges, and (d) is a TEM image of a typical WS ₂ sample and the inset is a WS ₂ nanosheet. Shows the Fast Fourier Transform (FFT) pattern for

The lattice structure of the WS ₂ nanosheets can be confirmed through high-resolution TEM (HRTEM) images (FIGS. 3a to 3c) and 2-dimensional fast Fourier transform (FFT) analysis (FIG. 3d). Figure 3a shows random WS ₂ nanosheet structures prepared using continuous sonication in the LPE method. 3b clearly confirms the existence of a two-dimensional form of WS ₂ having a length of (100-200) nm. As shown in Figure 3c, it can be seen that the hexagonal crystal structure of the WS ₂ nanosheets is not damaged during the long-term sonication process. Referring to the FFT inset of FIG. 3C, a hexagonal arrangement of atoms with zigzag edges can be seen. As shown in Fig. 3d, the FFT image and electron diffraction pattern of the inner part of one nanosheet show a hexagonal arrangement with two close lattice distances of (0.27 and 0.16) nm, which are respectively _the (100) and the standard interplanar distance of the (110) plane. As can be seen in the TEM images of the prepared WS ₂ nanosheets, it can be confirmed that the structure is due to the uniform distribution of tungsten and sulfur elements in the entire individual WS ₂ nanosheets.

2. Characteristics of α-Fe ₂ O ₃ @WS ₂ /Wox photoanode

4 is a plan view FE-SEM image of (a) pure α-Fe ₂ O ₃ and (b) α-Fe ₂ O ₃ @4WS ₂ /WOx, (c) α-Fe ₂ O ₃ , (d) α -Fe ₂ O ₃ @4WS ₂ /WOx cross-sectional FE-SEM images, (e) and (f) α-Fe ₂ O ₃ @4WS ₂ /WOx HRTEM images of core-shell nanorod structures, (g) α-Fe HRTEM images of ₂ O ₃ @2WS ₂ /WOx photoanode, (h) α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode, (i) α-Fe ₂ O ₃ @8WS ₂ /WOx photoanode and (j ) SEM mapping and EDX analysis of the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode.

As shown in FIGS. 4A to 4D , the morphologies of the α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx structures can be confirmed by SEM analysis. As shown in the SEM images of FIGS. 4a and 4c, α-Fe ₂ O ₃ is a structure of rice-shaped nanorods of random sizes arranged vertically on the FTO substrate. In contrast, as shown in the SEM images of Figs. 4b and 4d, the core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx nanorods become longer, denser, and wider after being fabricated as a heterojunction. Also, as shown in Fig. 4d, the thickness of the film of α-Fe ₂ O ₃ @4WS ₂ /WOx increases to about 10 nm. As shown in FIG. 4j, EDAX and elemental mapping are shown, and it can be confirmed that Fe, O, W, and S exist on the surface of the photoelectrode.

4e to 4i show high-resolution TEM images of pure α-Fe ₂ O ₃ and core-shell α-Fe ₂ O ₃ @WS ₂ /WOx nanorods, where the confirmed lattice for α-Fe ₂ O ₃ The spacing is ~ 0.25 nm, which is related to the grating 110; It is also shown in the XRD analysis results (see FIG. 5a). As shown in FIG. 4e, it can be clearly seen that WS2 completely surrounds the α-Fe ₂ O ₃ nanorods and forms a core-shell structure. The exfoliated WS ₂ nanosheet concentration value increases by repeating the drop-casting on the α-Fe ₂ O ₃ electrode surface 2 to 8 more times with 2WS ₂ /WOx, 4WS ₂ /WOx, and 8WS ₂ /WOx, respectively ( see Table 1). The thickness created by the concentration of the exfoliated WS ₂ nanosheets on the surface of the α-Fe ₂ O ₃ nanorods affects the light harvesting and charge separation of the PEC performance. Moreover, increasing the concentration of WS ₂ nanosheets on the outer surface of α-Fe ₂ O ₃ nanorods has a direct effect on increasing the thickness of the shell. Therefore, high resolution of α-Fe ₂ O ₃ @2WS ₂ /WOx (Fig. 4g), α-Fe ₂ O ₃ @4WS ₂ /WOx (Fig. 4h) and α-Fe ₂ O ₃ @8WS ₂ /WOx (Fig. 4i) TEM images show different thicknesses of WS ₂ nanosheets on α-Fe ₂ O ₃ nanorods of about 2.5, 5.7 and 11.8 nm. Finally, a shell with an appropriate thickness of about 5.7 nm could be fabricated in the optimal state where the WS ₂ nanosheet precursor was dropped onto the α-Fe ₂ O ₃ electrode (α-Fe ₂ O ₃ @4WS ₂ /WOx) four times. And through this, it can be confirmed that it can be provided with the best PEC performance. Therefore, the α-Fe ₂ O ₃ @4WS ₂ /WOx photoelectrode can generate more electron-hole pairs, providing a denser core-shell structure for efficient light harvesting, which can increase the PEC performance.

5 is (a) XRD, (b) Raman spectrum, (c) UV-vis absorbance, and (d) LHE result graphs for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx, (e) α-Fe ₂ O ₃ and (f) WS ₂ It is a graph of UV-vis absorbance spectrum and band gap energy dp.

5a shows X-ray diffraction (XRD) analysis of pure α-F _e 2 O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes. The peaks at 63.96°, 35.78° and 33.85° correspond to the (300), (110) and (104) planes of the hematite crystal structure in the two fabricated electrodes, respectively. This indicates that the crystallographic structure of hematite in the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode is the same as that in the pure α-Fe ₂ O ₃ electrode. The diffraction peaks at 33.75° and 13.96° correspond to the (100) and (200) planes, respectively, and may be related to the hexagonal WS ₂ structure. The other peaks are attributed to FTO since substrates were detected in all samples. Raman spectra of pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode are shown in FIG. 5B. The peaks observed at 221.3, 289.9, 404.2, 607.4, and 1,306.5 cm ^-1 indicate the Raman active mode of α-Fe ₂ O ₃ . The spectrum of the α-Fe ₂ O ₃ @4WS ₂ /Wox film shifts to higher wavelengths compared to the α-Fe ₂ O ₃ sample. compared to the α-Fe ₂ O ₃ sample. Shifting to higher wavenumbers indicates improved crystallinity of the material after Z-way heterojunction.

UV-vis absorption spectra of pure α- _{Fe 2 O 3 and} α-Fe ₂ O ₃ @4WS ₂ /WOx thin films to evaluate the effect of WS ₂ shell on the optical effect of α-Fe 2 O 3 nanorods. is measured (Fig. 5c). Compared to α-Fe ₂ O _{3 ,} the absorbance of the α-Fe ₂ O ₃ @4WS ₂ /WOx thin film was somewhat improved by loading the WS ₂ shell on the α-Fe ₂ O ₃ thin film. In addition, a slight redshift indicates the presence of WS ₂ nanosheets, which means that the absorbance intensity is improved. The absorbance and bandgap energy of the pure α-Fe ₂ O ₃ and pure WS ₂ photoanode are shown in FIGS. 5E and FI, respectively. The band gap of the prepared α-Fe ₂ O ₃ and WS ₂ is calculated by the Tauc equation (relational expression 6).

[Relationship 6]

hν is the photon energy, and Eg and A are proportionality constants.

The light-harvesting efficiency (LHE) of the α-Fe ₂ O ₃ @4WS ₂ /WOx thin film shows enhancement of light absorption, which is effective to capture more photons and generate more charge carriers. As shown in FIG. 4D, LHE is determined by relation 7.

[Relationship 7]

A(λ) is the absorbance at different wavelengths. The higher the absorbance of the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode, the higher the LHE value. The LHE enhancement is attributed to the fact that the combination of WS ₂ nanosheets and α-Fe ₂ O ₃ heterojunctions has a notable complementary effect on optical properties, and that WS ₂ nanosheets as shells play an important role in light capture efficiency.

6 shows XPS spectral results for (a) Fe _2p (b) O _1s (c) W _4f and (d) S _2p for pure α-Fe _{2 O 3 and α-Fe 2 O 3 @} 4WS ₂ /WOx Graphs, XPS for (e) Fe _2p and (f) O ₁ of pure α-Fe ₂ O ₃ electrodes, and (g) pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes. This is a graph of the survey analysis result.

X-ray photoelectron spectroscopy (XPS) spectra were measured for the α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /Wox thin films to confirm the elemental composition of the surface ( FIGS. 6a to 6g ). It can be confirmed that O, Fe, and W elements exist on the surface of the prepared electrode. Carbon contamination and extraneous carbon in the XPS instrument appear as a C1s peak at 289.1 eV (Fig. 6g). The XPS spectrum of Fe _2p shows two peaks at about 710 and 723 eV for α-Fe ₂ O ₃ @4WS ₂ /Wox thin film (Fig. 6a) and 711 and 724 eV for pure α-Fe ₂ O ₃ (Fig. 6a). 6e), which correspond to Fe _2p1/2 and Fe _2p3/2 respectively. Figure 6b shows the binding energies of two distinct diffraction O1 peaks at around 532 and 530 eV of -OH and Fe-O. The area of the deconvoluted O1s peak in the α-Fe ₂ O ₃ @4WS ₂ /WOx sample is increased compared to that _of the pure α-Fe ₂ O ₃ electrode (FIG. 6f), which is a product of heat treatment (annealing). It is due to the conversion to The XPS of W shows two peaks at about 35 and 37 eV for W _4f7/2 and W _4f5/2 respectively for the α-Fe ₂ O ₃ /WS ₂ sample (FIG. 6c). The presence of sulfur can be confirmed through the S _2p peak of the α-Fe ₂ O ₃ @4WS ₂ /WOx sample. S _2p peaks appear at about 161.9 and 163.3 eV representing S _2p1/2 and S _2p3/2 respectively (FIG. 6d). XPS analysis results confirm that all thin film structures were successfully fabricated.

Figure 7 is a time-of-flight secondary ion mass spectrometry (Time-of-Flight Secondary Ion Mass Spectrometry, TOF-SIMS) results, (a) to (d) are α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes It is a surface analysis result graph for the separation of WS ₂ to WOx in , and (e) a depth profile and (f) a 3D image of ions in an α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode.

By examining the TOF-SIMS of the optimized sample, it is possible to obtain detailed information about what happened at different depths. 7e to 7f show the depth profile, 3D image and surface analysis of the TOF-SIMS spectrum, respectively. The intensities of FeO ⁺ , Fe ₂ O ₃₊ , WS ₂₊ , W ^{+ ,} S ^- , O ^- and Sn ⁺ ions were detected as a function ^of sputtering time. The results clearly show some WS ₂ oxidation to WOx (x=2,3) after the calcination process. The TOF-SIMS depth profile in Fig _. 7e shows the surface WS ₂₊ , S ^- and W ⁺ ions and the α-Fe ₂ O ³ structure, respectively. The WS ₂₊ , S ⁻ and W ⁺ ion intensities ^gradually decrease with increasing sputtering time. This means that WS ₂ is evenly distributed as a shell in the upper region of the α-Fe ₂ O ₃ @4WS ₂ /WOx nanorod. Therefore, the TOF-SIMS depth profile clearly confirms that WS ₂ ⁻ , W ⁺ , and S ⁻ (WS ₂ ) are intercalated and chemically combined with the α-Fe ₂ O ₃ photoanode. Therefore, it can be confirmed that WS ₂ is predominantly connected to the thin film, and there is a small amount of WOx (oxidized from WS ₂ ) generated at the electrode, which can be detected due to the high sensitivity of the TOF-SIMS measurement. TOF-SIMS spectra of α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes were collected from cathode and anode in the 0-240 mass range. This clearly confirms the presence of a condensed form of tungsten oxide (WOx, x = 2, 3), and the WOW bridge in this sample is present at the outer surface. Uniform distribution of ions in the α-Fe ₂ O ₃ @4WS ₂ /WOx thin film can be confirmed through three-dimensional (3D) visualization of the SIMS profile of the heterojunction (FIG. 7f). In ^structures for morphochemical applications, Fe ³⁺ , Fe ²⁺ , O ²⁺ , W ⁺ , WS ₂₊ , and S ^2- ions are detected. Through this, WS ₂₊ and S ^2- existing as shells on the surface, penetration of W ions into the ^middle layer, and Fe ²⁺ at the internal interface between α-Fe ₂ O ₃ and WS ₂ /WOx can be confirmed.

8 is a graph of electron spin resonance (EPR) spectral analysis results for pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes.

The pore density of pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx samples was investigated using ESR spectral analysis (FIG. 8). The intensity of the paramagnetic signal increases significantly after making the heterojunction. Therefore, these results indicate that the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode has more defects than the pure α-Fe ₂ O ₃ electrode. Also, g values of pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ photoanode are detected at about 2.002 and 1.989, respectively. The g value shifts to lower numbers, which may be related to electron deficient capacitance and high oxidation reaction. In particular, the ESR results indicate the presence of sulfur vacancies because anions can donate unpaired electrons, which can enhance the photoelectrochemical performance for hydrogen production. Therefore, the ultra-thin WS ₂ shell on the surface of the α-Fe ₂ O ₃ nanorods can promote hole migration to the electrode/electrolyte interface and enhance the oxidation reaction.

3. PEC performance of photoanode

9 shows (a) Linear scan voltammetry (LSV) and chopped LSV versus RHE at potentials from 0.3 to 1.5 V for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ samples. Graphs of the results, (c) photocurrent response, (d) photocurrent stability (inset shows long-term (24 hours) photocurrent stability for α-Fe ₂ O ₃ @4WS ₂ ).

Photocurrent density versus potential for α-Fe ₂ O ₃ and all α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes was measured by linear scanning voltammetry (LSV) between (0.3–1.5) V versus RHE in 1 M NaOH solution as electrolyte. ) and chopping LSV (Fig. 9a and b). The photocurrent density of the pure α-Fe ₂ O ₃ electrode was ~0.07 mA.cm ^-2 at 1.23 V vs. RHE. After depositing the WS ₂ nanosheets on the α-Fe ₂ O ₃ electrode, _{1.23 V} vs. 1.23 _V vs. The highest photocurrent density at RHE reaches 0.98 mA.cm ^-2 (FIG. 9a). The photocurrent density of core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx is increased by a factor of 14 using a heterojunction between α-Fe ₂ O ₃ and WS ₂ . This structure can simultaneously improve charge separation and reduce recombination of charge carriers. However, the photocurrent density value is greatly reduced due to WS ₂ added to α-Fe ₂ O ₃ . This decrease may be due to excessive WS ₂ blocking the light absorption of the photoanode (FIGS. 9A-9B). The heterojunction of WS ₂ nanosheets on the α-Fe ₂ O ₃ photoanode surface significantly reduces the onset potential with a lower applied voltage (0.38 VRHE).

The photocurrent density and onset potential values for all fabricated photoelectrodes can be clearly compared in Fig. 9e. The choking LSV in Fig. 9b was measured at 2 sec light on/off, which is consistent with the LSV result (Fig. 9a). As a photoanode with excellent performance, it can be confirmed that the core-shell α-Fe ₂ O ₃ @WS ₂ /WOx nanorod structure can rapidly respond to light and quickly generate a photocurrent from 0 to an equilibrium value. Fig. 9c shows the chronoamperometry scanned for 4 cycles under intermittent light illumination (lighting on/off). Identical responses to light on and off intervals indicate good reproducibility of the α-Fe ₂ O ₃ @WS ₂ /WOx photoanode. The stability of the photocurrent for different α-Fe ₂ O ₃ @WS ₂ /WOx samples was measured under continuous illumination (FIG. 9d). All fabricated photoelectrodes show stable photocurrent without significant drop for 15 min. Furthermore, the long-term stability of the photocurrent for the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode is shown in the inset of Fig. 9d under continuous illumination for 24 h. WOx is formed on the upper surface to prevent photocorrosion with a thin protective layer, improve the stability of alkali-weak WS ₂ on the surface of α-Fe ₂ O ₃ photoanode in alkaline electrolyte (NaOH) solution, and improve alkali-weak MoS ₂ and Effective separation between NaOH electrolytes can be achieved.

The photocurrent densities and starting potential values for all the fabricated photoelectrodes can be compared in FIG. 9e. Comparative results for photocathodes fabricated by different methods are shown in FIG. 9E , which confirms that holes with long lifetime appear at lower bias in the Fe ₂ O ₃ @4WS ₂ /WOx photoanode. The negative shift in onset potential is in good agreement with the TRPL and long-term stability results. Also, referring to FIG. 9F, the ABPE value of the α-Fe ₂ O ₃ @WS ₂ /WOx electrode is higher than that of pure α-Fe ₂ O ₃ in the potential range measured in 1M NaOH (pH≒14) as an electrolyte. The highest efficiency was obtained for the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode (0.194%) at the lowest potential (0.9 VRHE), which was 19.4% higher than that of 0.01% pure α-Fe ₂ O ₃ @4WS ₂ /WOx. times higher Therefore, the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode shows the highest PEC water breakdown among the fabricated α-Fe ₂ O ₃ based photoelectrodes.

The applied bias photon-to-current conversion efficiency (ABPE) at various applied potentials shown in FIG. 9F is calculated by the following relational expression 8.

[Relationship 8]

J is the photocurrent density at various applied potentials, Vb means applied bias versus RHE, and P is the power density of the incident light (100 mW/cm ² ). The ABPE value of the α-Fe ₂ O ₃ @WS ₂ /WOx electrode is higher than pure α-Fe2O3 in the potential range measured in 1M NaOH (pH=12) as an electrolyte. At the lowest potential (0.9 VRHE), the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode can provide the highest efficiency (0.194%), which is 19.4 times higher than that of 0.01% pure α-Fe ₂ O ₃ . Higher. Therefore, the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode shows the highest PEC water splitting effect among the prepared α-Fe ₂ O ₃ based photoelectrodes.

10 shows (a) open circuit photovoltage (OCP), (b) Mott-Schottky, (c) ND for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes. and Vfb and (d) the resulting graph for the IPCE plot.

Details of the photogenerated charge carrier behavior are investigated using open circuit photovoltage (OCP) measurements (Fig. 10a). In the OCP plot, a lower potential at illumination time indicates more photogenerated electron generation and higher conductance. The photoelectrode has a negative voltage after turning on the lamp, indicating the direction of the photogenerated electrons towards the FTO, and another photoelectrochemical analysis confirms the anodic photocurrent. Also, the α-Fe ₂ O ₃ @4WS ₂ /Wox photoanode takes about 44 seconds longer to return to dark equilibrium from the illuminated state. This can be attributed to the greatly extended lifetime of the photogenerated charge carriers after fabrication of the heterojunction. Thus, the conductivity of the core-shell α-Fe ₂ O ₃ @4WS ₂ /Wox nanorod structure is due to increased available photogenerated electrons as well as reduced recombination of charge carriers at the α-Fe ₂ O ₃ @4WS ₂ /Wox photoelectrode. increase due to concentration.

Mott-Schottky (MS) plots are shown in FIG. 10B, α-Fe ₂ O ₃ , α-Fe ₂ O ₃ @2WS ₂ /WOx, α-Fe ₂ O ₃ @4WS ₂ /WOx and α-Fe ₂ O _{The 3} @8WS ₂ /WOx photoanode shows a positive slope before and after the heterojunction, which means the characteristics of an n-type semiconductor with electrons as charge carriers. MS parameters, flat band potential (V _fb ), space charge region width (W _SCL ) and conduction band electron density (N _D ) are calculated by relations 9, 10 and 11, respectively.

[Relational Expression 9]

where V is the CB potential (V), V _fb is the flat band potential (V), k is the Boltzmann constant, T is the temperature (K), e is the charge of electrons (C), ε is the relative permittivity, ε ₀ dielectric constant, ND is the donor concentration per unit volume (cm ³ ), and C _sc is the surface charge capacitance (F/cm ² ). Qualitative information about the donor concentration can be obtained based on the Mott-Schottky plot, where the positive slope of the electrode indicates an n-type semiconductor. The width of the space charge layer (W _SCL ) can be calculated as shown in Equation 10 by solving Poisson's equation depending on N _D and V _fb .

[Relational Expression 10]

The Mott-Schottky equation and constant volume for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @WS ₂ /WOx electrodes are:

[Relationship 11]

Based on the following relationships for α-Fe ₂ O _3, α-Fe ₂ O ₃ @2WS ₂ /WOx, α-Fe ₂ O ₃ @4WS ₂ /WOx, and α-Fe ₂ O ₃ @8WS ₂ /WOx photoanodes , the measurement bag is shown in Table 2.

Sample	slope	Intercept (x-axis)	ND (cm3)	Vfb(v)	W(nm)
α-Fe 2 O 3	4.63E+09	0.7	3.05E+20	0.67	6.86
α-Fe 2 O 3 @2WS 2 /WO x	2.11E+09	0.63	6.70E+20	0.60	4.54
α-Fe 2 O 3 @4WS 2 /WO x	1.04E+09	0.55	1.35E+21	0.52	3.12
α-Fe 2 O 3 @8WS 2 /WO x	1.68E+09	0.61	8.39E+20	0.58	4.03

As shown in FIG. 10B, the slope of the MS curve is greatly reduced, and the applied potential shows a cathodic shift. The highest N _D and lowest V _fb of the α-Fe ₂ O ₃ @4WS ₂ /WOx photoelectrode are calculated to be 1.35E+27m ³ and 0.52V, respectively (Fig. 10c), which is attributed to the increased electron density in the conduction band. and charge carrier separation is improved in the α-Fe ₂ O ₃ @4WS ₂ /WOx thin film by the optimal shell thickness. The highest value of W _SCL in the α-Fe ₂ O ₃ @4WS ₂ /WOx sample is about 54% of pure α-Fe ₂ O ₃ (6.86 nm). Through this, the PEC performance can be improved by increasing the charge carrier transfer. The photoelectrochemical performances of all samples in the visible light region are investigated using relational equation 12 to determine the incident-photon-to-current-efficiencies (IPCE). ) can be confirmed by measurement.

[Relationship 12]

The highest IPCE values of α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode are 1.65 and 15.5% at 325 nm, respectively (FIG. 10d). The highest IPCE value is the heterojunction effect with WS ₂ nanosheets. The IPCE found for α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode is 9.4 times higher than that for pure α-Fe ₂ O ₃ . The enhanced IPCE curve corresponds to better results of the optical absorption analysis spectrum for the α-Fe ₂ O ₃ @4WS ₂ /WOx sample under visible light. Therefore, the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode can greatly enhance the PEC capacity of the α-Fe ₂ O ₃ based photoanode even without removing holes.

11 shows Nyquist plots for (a) α-Fe ₂ O ₃ and (b) Fe ₂ O ₃ @4WS ₂ /WOx at potentials of 0.9, 1, and 1.1 V vs. RHE, and (c) at potentials of 1.1 V vs. RHE. Nyquist plot for (d) α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx electrodes for Time-Resolved Photoluminescence (TRPL) results.

Photogenerated electron transfer at the α-Fe ₂ O ₃ :WS ₂ :electrolyte interface by measuring electrochemical impedance spectroscopy (EIS) of α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WO x photoelectrodes. Investigate the role of the WS ₂ /WOx shell in the process. 11A-11C show Nyquist plots at different potentials (0.9, 1.1.1 V vs. RHE) for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoelectrodes. . These are essentially constant and small series resistance (Rs), interfacial charge transfer resistance (R) and Randles circuit model including the space charge capacitance (C) of the α-Fe ₂ O ₃ :WS ₂ and electrolyte interface (Fig. 11c inset). ) fits well. The table inside the graph shows the resistance and space charge capacitance values, indicating that the lowest resistance and highest capacitance was obtained for the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode. As shown in the internal tables of FIGS. 11A and 11B , the charge transfer resistance decreases with an increase in overpotential due to an increase in electron transfer rate. As shown in FIG. 11C, the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode has a 1.1V vs. interfacial charge transfer resistance of the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode. It shows that the lowest semicircle diameter greatly decreases at the voltage of RHE. This behavior can be attributed to the fabrication of core _- shell heterojunctions using WS ₂ nanosheets with improved hole extraction from α-Fe ₂ O ₃ to WS 2 thin films as shells. As a result, the α-Fe ₂ O ₃ @4WS ₂ /WOx core-shell can reduce the recombination rate of charge carriers in the α-Fe ₂ O ₃ thin film, which is in perfect agreement with the TRPL data.

To investigate more information on the effect of charge transfer, the electron transfer of pure α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx is investigated by measuring the PL lifetime (Fig. 11d). The normalized TRPL of the α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode fits is a double exponential represented by the light gray curve, which is calculated by relation (13).

[Relational Expression 13]

Here, τ ₁ and τ ₂ are the lifetimes of long and short components, respectively, and A ₁ and A ₂ are the scaling factors of each component. The average lifetime (τ) is calculated through relational expression 14.

[Relational Expression 14]

Table 3 shows the TRPL parameters of the photoanode fitted in the equivalent circuit model.

	α-Fe 2 O 3	α-Fe 2 O 3 @4WS 2 /WO x
A1	0.30579	0.12725
t1	1.96626	2.81576
A2	0.65808	0.30107
t2	0.51369	0.50204

The TRPL spectrum confirms that the average decay time of α-Fe ₂ O ₃ @4WS ₂ /WOx charge carriers (τave = 2.13 ns) is longer than pure α-Fe ₂ O ₃ (τ _ave = 1.44 ns). This significantly improved calculation of τ _ave shows that the use of WS ₂ nanosheets as a heterojunction means that efficient charge transfer occurs between the α-Fe ₂ O ₃ and WS ₂ semiconductors. As a result, electron-hole pairs are better In isolation, better transfer of charge carriers at the α-Fe ₂ O ₃ @4WS ₂ /WOx electrode can increase the PEC water oxidation efficiency. This effectively suppresses the trapping of charge carriers on the surface of the photoelectrode, reduces charge recombination at the α-Fe ₂ O ₃ @4WS ₂ /WOx/electrolyte interface, and reduces the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode. 12 shows (a) linear scan voltammetry (LSV) measured in 1M NaOH and 1M NaOH + 0.5MH ₂ O ₂ electrolyte, (b) charge injection, ( c) Charge separation efficiency, versus response time per illuminated area (1 cm ² ) for α-Fe ₂ O ₃ and α-Fe _{2 O 3} @4WS ₂ /WOx electrodes at 1.23 V vs. e) This is the result graph for O ₂ generation.

The reaction barrier can be overcome by loading WS ₂ nanosheets onto α-Fe ₂ O ₃ nanorods to improve hole transport through the electrode/electrolyte interface. As shown in Fig. 12a, the photocurrent density and apparent onset potential shift in the presence of H ₂ O ₂ for α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoelectrodes due to faster oxidation kinetics. This is greatly improved. Photocurrent is typically generated through light absorption in semiconductors, separation of photogenerated charge carriers, and surface charge injection for PEC performance. The surface charge injection efficiency (η _inj ) and charge separation efficiency (η _sep ) of the pure α-Fe ₂ O ₃ photoanode and the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode are shown in FIGS. 12A and 12B . The charge separation efficiency of the photogenerated carriers (η _sep ) and the charge injection efficiency into the electrolyte (η _inj ) are calculated by the following relationships 15, 16 and 17 at different applied biases using well-recognized hole removal methods.

[Relational Expression 15]

[Relational Expression 16]

[Relational Expression 17]

Here, J _H2O and J _H2O2 denote photocurrent densities measured in electrolytes of 1M NaOH and 1M NaOH + 0.5MH ₂ O ₂ , respectively.

By loading WS ₂ nanosheets on the surface of α-Fe ₂ O ₃ nanorods as shells, η _inj of α-Fe ₂ O ₃ increases from about 0.15 to 0.88 at 0.6 V _RHE (FIG. 12b). η _sep and η _inj are enhanced in α-Fe ₂ O ₃ @4WS ₂ /WOx nanorod core-shell by fabricating a heterojunction with 2D-WS ₂ nanosheets. For α-Fe ₂ O ₃ photoanode and α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode, the value of ηsep is about 0.003 to 0.09 at V _RHE (FIG. 12c).

For comparison, the total water dissociation of α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoanodes generated H ₂ and O ₂ at 1.23 V vs RHE under 100 mW.cm ^-2 irradiation in 1 M NaOH electrolyte. Measure and evaluate. As shown in FIGS. 12c and 12d, the total H ₂ generated after 2 hours of irradiation for the α-Fe ₂ O ₃ and α-Fe ₂ O ₃ @4WS ₂ /WOx photoanodes was 1.39 and 32 μmol.cm, respectively. ⁻² and the total O ₂ produced were 0.69 and 15.3 μmol.cm ^{−2 ,} respectively, indicating a 2:1 ratio of the water splitting reaction. The H ₂ /O ₂ gas evolution process is accompanied by a stable photocurrent density as shown by chronoamperometry in FIG. 9d . In addition, the Faraday efficiency of O ₂ and H ₂ generation for the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode is calculated to be 82% to 86%, which means that most of the photogenerated holes are used for the water oxidation reaction. do.

13 shows (a) UPS spectra for pure α-Fe ₂ O ₃ and WS ₂ , schematic diagrams of electronic band structures of α-Fe ₂ O ₃ and WS ₂ before (b) and after (c) heterojunction, and (d) ) is a schematic diagram of the mechanism and structure for a stable core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode.

The energy band bending of α-Fe ₂ O ₃ nanorods achieved equilibrium in the core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx nanorod structure, which can be confirmed using UPS analysis and work function calculations ( Figure 13a). The schematic electronic band structures of α-Fe ₂ O ₃ and WS ₂ before and after the heterojunction can be confirmed by UPS spectra (FIGS. 13b and c). In the core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx structure, this band alignment results in higher light absorbance in visible light compared to α-Fe ₂ O ₃ thin films because the charge separation increases and promotes charge transfer. . 13D shows a schematic diagram of the charge carrier transport pathways in the α-Fe ₂ O ₃ @4WS ₂ /WOx photoanode. In addition, the electrons of VB gained energy from visible light illumination and transferred from the valence band (VB) to the conduction band (CB) in both α-Fe ₂ O ₃ and WS ₂ semiconductors. Charge separation increases after fabrication of core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx nanorod structures with improved PEC performance. The recombination rate of electron-hole pairs decreases significantly with decreasing resistance at the interface and increasing charge carrier lifetime. Electrons can also move from the CB of WS ₂ to the CB of α-Fe ₂ O ₃ at different energy levels. For the same reason, holes move simultaneously from VB of α-Fe ₂ O ₃ to VB of WS ₂ by the electrostatic field. This internal electrostatic field can extract more charge carriers from the space charge region. As a result, the core-shell α-Fe ₂ O ₃ @4WS ₂ /WOx nanorod structure can generate more electrons and holes on the surface of the photoelectrode, promote electron-hole separation, and improve PEC performance.

Although the above has been described with reference to the preferred embodiments of the present application, those skilled in the art can variously modify and change the present application within the scope not departing from the spirit and scope of the present invention described in the claims below. You will understand that you can.

Claims (11)

1. Preparing a substrate;

   Forming an α-Fe ₂ O ₃ nanorod film on the prepared substrate; and

   A photoelectrochemical cell comprising the step of hetero-bonding core-shell α-Fe ₂ O ₃ and WS ₂ by drop casting a WS ₂ nanosheet precursor on the formed α-Fe ₂ O ₃ nanorod film. Method for manufacturing electrodes.
2. According to claim 1,

   The step of forming the α-Fe ₂ O ₃ nanorod film,

   mixing FeCl ₃ .6H ₂ O and NaNO ₃ ; and

   A method comprising the step of calcining at 500 to 600 ° C. for 3 to 5 hours.
3. According to claim 1,

   The WS ₂ nanosheet precursor is prepared by liquid phase exfoliation (LPE).
4. According to claim 1,

   The WS ₂ nanosheet precursor is prepared by dissolving the WS ₂ powder in a mixed solvent of ethanol and water;

   Sonicating the mixed solution; and

   A method prepared by a method comprising the step of centrifuging at 3000 to 4000 rpm for 30 to 90 minutes.
5. According to claim 1,

   The hetero linking step is,

   Dropping a WS ₂ nanosheet precursor on the α-Fe ₂ O ₃ nanorod film; and

   A method comprising heat treatment at 400 to 500 ° C. for 1 to 3 hours.
6. According to claim 5,

   The step of dropping the WS ₂ nanosheet precursor on the α-Fe ₂ O ₃ nanorod film is repeated 6 times or less.
7. As a core-shell photocatalyst composition,

   A composition in which the core includes α-Fe ₂ O ₃ nanorods, and the shell includes a mixed nanosheet of WS ₂ and WO _x .
8. According to claim 7,

   A composition in which the length of the mixed nanosheet is 200 nm or less.
9. According to claim 7,

   A composition in which the average thickness of the mixed nanosheet is 12 nm or less.
10. An electrode for a photoelectrochemical cell in which a substrate and the composition for photocatalysis of a photoelectrochemical cell according to claim 7 are laminated on the substrate in the form of a film.
11. A photoelectrochemical cell comprising the electrode for a photoelectrochemical cell according to claim 10.

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